WO2025197349A1 - Release film - Google Patents

Abstract

This release film is provided with a resin layer A on at least one surface of a copolyester film that contains a copolyester. The storage modulus of the release film is 1000 MPa or less at 80°C, and is 30 MPa or more at 180°C. The ratio of the storage modulus at 80°C with respect to the storage modulus at 180°C is 13 or less.

Description

Release film

The present invention relates to a release film.

Polyethylene terephthalate (PET) film, a typical polyester film, especially biaxially oriented PET film, is used in a variety of fields, including industrial materials, optical materials, electronic component materials, and battery packaging, due to its excellent transparency, mechanical strength, heat resistance, and flexibility.

Incidentally, semiconductor chips are usually sealed with resin to protect them from the outside air, and are mounted on a substrate in the form of a molded product called a package.
On the other hand, in the fields of BGA and WL-CSP, due to the demand for smaller packages and more pins, there has been a shift in molding methods from the conventional transfer molding method to the compression molding method, with the aim of increasing the size and efficiency of one shot.
Conventionally, fluororesin films (for example, ETFE (ethylene-tetrafluoroethylene copolymer) films) have been widely used as release films. However, there are concerns about the recent restrictions on organic fluorine compounds (PFAS; Polyfluoroalkylsubstances), and alternative films are being investigated.

Patent No. 6414345 Patent No. 7170975 Patent No. 7287273

As described above, polyester films have various properties and are promising alternatives to fluororesin films.
However, with ordinary polyester films for molding, for example, when the polyester film is preheated to acclimate it to the molding temperature, the Tg (glass transition temperature) of the polyester film is exceeded, and in the temperature range of around 80°C, the film does not stretch as much as fluororesin films. On the other hand, in the high temperature range of over 160°C, which is the actual molding temperature range, fluororesin films retain a certain degree of strength, whereas general-purpose polyester films for molding show a significant decrease in strength. As the film properties are completely different, it has been difficult to use polyester films for molding as a substitute for fluororesin films.

The object of the present invention is to provide a release film for semiconductor compression molding that can be used as a replacement for fluororesin film, for example, when molding a semiconductor package using resin compression molding, allowing the encapsulant to be easily released from the mold without damaging the semiconductor package.

The present invention provides the following.
[1] A release film comprising a resin layer A on at least one surface of a copolymer polyester film containing a copolymer polyester, wherein the release film has a storage modulus at 80°C of 1000 MPa or less, a storage modulus at 180°C of 30 MPa or more, and a ratio of the storage modulus at 80°C to the storage modulus at 180°C of 13 or less.
[2] The release film according to the above [1], wherein the copolymerized polyester comprises a copolymerized polyester (z1) of terephthalic acid (X1), a dicarboxylic acid component (X2) having 4 to 10 carbon atoms, and an alcohol component (Y2) other than ethylene glycol (Y1), and the proportion of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms in all the polyesters (Z) contained in the copolymerized polyester film is 1 to 30 mol % and the proportion of the other alcohol component (Y2) in the alcohol components is 15 to 60 mol %.
[3] The release film according to the above [2], wherein the dicarboxylic acid component (X2) having 4 to 10 carbon atoms includes an aliphatic dicarboxylic acid.
[4] The release film according to the above [3], wherein the aliphatic dicarboxylic acid includes adipic acid.
[5] The release film according to any one of the above [2] to [4], wherein the other alcohol component (Y2) includes 1,4-butanediol.
[6] The release film according to any one of the above [2] to [5], wherein the other alcohol component (Y2) includes 1,4-butanediol and 1,6-hexanediol.
[7] The release film according to any one of the above [1] to [6], which is a copolymer polyester film having a copolymer polyester layer (A1) containing a copolymer polyester (a1) as the copolymer polyester.
[8] The release film according to the above item [7], wherein the copolymer polyester layer (A1) is a polyester other than the copolymer polyester (a1) and further contains a polyester (a2) containing terephthalic acid as a dicarboxylic acid component and ethylene glycol as an alcohol component.
[9] The release film according to any one of the above [7] to [8], which comprises a polyester layer (B1) and a polyester layer (B2) on both the front and back surfaces of the copolymer polyester layer (A1), respectively.
[10] The release film according to the above [9], wherein the temperature difference between the copolymer polyester layer (A1) and at least one of the polyester layer (B1) and the polyester layer (B2) is 5°C or more, as determined by softening temperature measurement using nano TA.
[11] The release film according to any one of the above [1] to [10], wherein the resin layer A contains a silicone-based resin.
[12] The release film according to any one of the above [1] to [10], wherein the resin layer A contains a non-silicone resin.
[13] The release film according to any one of the above [1] to [12], which is used in semiconductor manufacturing.
[14] The release film according to the above [13], which is for use in a compression mold.

The present invention provides a release film for semiconductor compression molding that can be used as a replacement for fluororesin film, allowing for easy release of the encapsulant from the mold without damaging the semiconductor package, for example, when molding a semiconductor package using resin compression molding.

The data curves are plots of the change in storage modulus versus the temperature change in the TD direction for Examples 1-1 and 1-2, Comparative Examples 1 and 2, and Reference Example (fluororesin film).

Next, an example of an embodiment of the present invention will be described. However, the present invention is not limited to the embodiment described below.

[Release film]
The release film of the present invention is characterized in that it comprises a resin layer A on at least one surface of a copolymer polyester film containing a copolymer polyester, and the release film has a storage modulus of 1000 MPa or less at 80°C, a storage modulus of 30 MPa or more at 180°C, and a ratio of the storage modulus at 80°C to the storage modulus at 180°C of 13 or less.

<Copolymer polyester film>
The copolymerized polyester film constituting the release film according to one embodiment of the present invention (hereinafter referred to as the "copolymerized polyester film") contains a copolymerized polyester. The copolymerized polyester film is, for example, a single-layer or multilayer film having a copolymerized polyester layer (A1) containing a copolymerized polyester (a1) as the copolymerized polyester.

The present copolymer polyester film is preferably a uniaxially or biaxially stretched film, and may be a uniaxially or biaxially stretched film, with a biaxially stretched film being preferred in terms of excellent balance of mechanical properties and flatness.
When the copolymer polyester film is such a stretched film, it becomes easy to make the storage modulus of the release film at 80°C 1000 MPa or less and the storage modulus at 180°C 30 MPa or more.

<Copolymer Polyester Layer (A1)>
The copolymer polyester layer (A1) is a layer containing a copolymer polyester (a1). The copolymer polyester layer (A1) preferably contains a polyester (a2) in addition to the copolymer polyester (a1). The copolymer polyester layer (A1) may also contain a resin (a3) in addition to the copolymer polyester (a1) and polyester (a2).

(Copolymer polyester (a1))
The copolymer polyester (a1) is a copolymer of terephthalic acid (X1), a dicarboxylic acid component (X2) having 4 to 10 carbon atoms, and an alcohol component (Y2) other than ethylene glycol (Y1). In this specification, such a copolymer polyester is sometimes referred to as copolymer polyester (z1), and the following description of copolymer polyester (a1) also refers to copolymer polyester (z1). The copolymer polyester (a1) may be crystalline or amorphous. The dicarboxylic acid component (X2) having 4 to 10 carbon atoms refers to a dicarboxylic acid component having 4 to 10 carbon atoms excluding terephthalic acid (X1).
The copolymer polyester (a1) (copolymer polyester (z1)) is a polycondensate of a dicarboxylic acid including terephthalic acid (X1) and a dicarboxylic acid component (X2) having 4 to 10 carbon atoms, and an alcohol component including an alcohol component (Y2) other than ethylene glycol (Y1). The alcohol component is generally a diol component.

Examples of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms include aromatic dicarboxylic acids, alicyclic dicarboxylic acids, aliphatic dicarboxylic acids, and polyfunctional acids. Among these, aliphatic dicarboxylic acids are preferred from the viewpoint of improving the storage modulus at room temperature and improving flexibility and elongation. The aliphatic dicarboxylic acid may be used alone as the dicarboxylic acid component (X2) having 4 to 10 carbon atoms, or may be used in combination with other dicarboxylic acid components having 4 to 10 carbon atoms.
Examples of the aliphatic dicarboxylic acid having 4 to 10 carbon atoms include saturated aliphatic dicarboxylic acids such as succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, and sebacic acid. Among these, from the viewpoint of ease of reaction during polymerization, adipic acid and sebacic acid are more preferred, and adipic acid is even more preferred.

The proportion of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms in the dicarboxylic acid components constituting the copolymer polyester (a1) (copolymer polyester (z1)) is not particularly limited as long as the proportion of (X2) relative to all of the polyesters (Z) described below falls within a predetermined range, but is, for example, 1 to 30 mol %, preferably 1 to 25 mol %, more preferably 1 to 20 mol %, and of these, particularly 4 to 15 mol %.
The proportion of terephthalic acid (X1) in the dicarboxylic acid components constituting the copolymer polyester (a1) (copolymer polyester (z1)) is, for example, 65 to 95 mol %, preferably 75 to 92 mol %, and more preferably 80 to 90 mol %.

In addition, in the copolymer polyester (a1) (copolymer polyester (z1)), the dicarboxylic acid component may consist of terephthalic acid (X1) and a dicarboxylic acid component (X2) having 4 to 10 carbon atoms, or may contain a dicarboxylic acid component (other dicarboxylic acid component (X3)) other than terephthalic acid (X1) and the dicarboxylic acid component (X2) having 4 to 10 carbon atoms as a copolymer component, provided that the gist of the present invention is not impaired. Examples of such dicarboxylic acid components include dodecanedioic acid, eicosanoic acid, dimer acid, and derivatives thereof.
The proportion of the other dicarboxylic acid component (X3) in the dicarboxylic acid components constituting the copolymer polyester (a1) is, for example, 10 mol % or less, preferably 5 mol % or less, more preferably 3 mol % or less, and most preferably 0 mol %.

Other alcohol components (Y2) include aliphatic diols such as 1,4-butanediol, 1,4-hexanediol, 1,6-hexanediol, diethylene glycol, trimethylene glycol, pentamethylene glycol, octamethylene glycol, decamethylene glycol, neopentyl glycol, and 2-ethyl-2-butyl-1,3-propanediol; 1,2-cyclohexanediol, 1,1-cyclohexanedimethylol, 1,4-cyclohexanedimethanol, and 2,5-norbornanediol. Examples include alicyclic diols such as methylol, aromatic diols such as xylylene glycol, 4,4'-dihydroxybiphenyl, 2,2-bis(4'-hydroxyphenyl)propane, 2,2-bis(4'-β-hydroxyethoxyphenyl)propane, bis(4-hydroxyphenyl)sulfone, and bis(4-β-hydroxyethoxyphenyl)sulfonic acid, ethylene oxide adducts or propylene oxide adducts of 2,2-bis(4'-hydroxyphenyl)propane, and dimer diol. These may be used alone or in combination.
Among the above, aliphatic diols are preferred from the viewpoints of flexibility and crystallinity. The aliphatic diol is preferably an aliphatic diol having 4 to 8 carbon atoms, more preferably an aliphatic diol having 4 to 6 carbon atoms.

More specifically, the aliphatic diol is preferably at least one selected from diethylene glycol, 1,4-butanediol, 1,4-hexanediol, and 1,6-hexanediol. It is more preferable that the alcohol component contains at least 1,4-butanediol, and it is even more preferable that the alcohol component in the copolymerized polyester (a1) contains both 1,4-butanediol and 1,6-hexanediol.
Furthermore, the copolymer polyester (a1) (copolymer polyester (z1)) may contain ethylene glycol (Y1) as an alcohol component (copolymer component). It is preferable that any of the polyesters (A) constituting the copolymer polyester layer (A1) contains ethylene glycol (Y1) as an alcohol component (copolymer component). Therefore, when the copolymer polyester (a1) does not contain ethylene glycol (Y1) as an alcohol component (copolymer component), for example, the polyester (a2) may contain ethylene glycol (Y1) as an alcohol component.

The copolymer polyester (a1) (copolymer polyester (z1)) may be used alone or in combination of two or more kinds.
The proportion of the other alcohol component (Y2) in the alcohol components constituting the copolymer polyester (a1) (copolymer polyester (z1)) is not particularly limited as long as the proportion of (Y2) relative to all of the polyesters (A) described below falls within a predetermined range, and is, for example, 50 to 100 mol %, preferably 70 to 100 mol %, and more preferably 90 to 100 mol %.

Furthermore, when the copolymer polyester film (a1) contains a certain amount or more of a polyester having, as a structural unit, terephthalic acid, a dicarboxylic acid having 4 to 10 carbon atoms, and one or both of 1,4-butanediol and 1,6-hexanediol, the copolymer polyester layer (A1) is flexible, has excellent elongation at low temperatures, and also has strength and heat resistance.

(Intrinsic Viscosity (IV) of Copolymer Polyester (a1))
The intrinsic viscosity (IV) of the copolymer polyester (a1) (copolymer polyester (z1)) (as a polyester mixture when two or more copolymer polyesters are used) is preferably 0.40 dL/g to 1.20 dL/g, more preferably 0.45 dL/g or more, and even more preferably 0.48 dL/g or more, and is more preferably 1.15 dL/g or less, and even more preferably 1.10 dL/g or less.
When the intrinsic viscosity of the copolymerized polyester (a1) is within this range, it is possible to obtain a polyester having excellent moldability without deteriorating productivity.
The intrinsic viscosity (IV) of a polyester is a value measured at 30°C by precisely weighing 1 g of polyester from which components incompatible with the polyester have been removed, dissolving the 1 g of polyester in 100 ml of a mixed solvent of phenol/tetrachloroethane = 50/50 (by weight).

The content of copolymer polyester (a1) may be at least a certain percentage of the resin components constituting copolymer polyester layer (A1) so that the proportions of each component in all polyesters (Z) or polyesters (A) described below fall within a predetermined range. In copolymer polyester layer (A1), copolymer polyester (a1) may account for, for example, 30% by mass or more, preferably 35% by mass or more, and more preferably 50% by mass or more, of the resin components constituting copolymer polyester layer (A1), and may account for, for example, 80% by mass or more. Furthermore, copolymer polyester (a1) may account for 100% by mass or less of the resin components constituting copolymer polyester layer (A1), but may also account for, for example, 70% by mass or less, or 60% by mass or less.

(Polyester (a2))
The copolymer polyester layer (A1) may be composed solely of the copolymer polyester (a1), but preferably contains a polyester (a2) in addition to the copolymer polyester (a1).
Here, polyester (a2) is a polyester other than copolymer polyester (a1), and is a homopolyester or copolymer polyester containing terephthalic acid (X1) as a dicarboxylic acid component and ethylene glycol (Y1) as an alcohol component. Note that a polyester containing terephthalic acid (X1) as a dicarboxylic acid component and ethylene glycol (Y1) as an alcohol component is sometimes referred to as polyester (z2), as described below.

When the polyester (a2) (polyester (z2)) is a homopolyester, the homopolyester is polyethylene terephthalate in which the dicarboxylic acid component is terephthalic acid (X1) and the alcohol component is ethylene glycol (Y1). It is preferable to use a homopolyester as the polyester (a2). However, when referring to a homopolyester, the alcohol component may contain diethylene glycol, which is inevitably mixed in. Specifically, in polyethylene terephthalate (homopolyester), the proportion of diethylene glycol in the alcohol component may be, for example, 5 mol% or less, or may be 3 mol% or less. The same applies to the polyester (B) described below.
When polyester is produced (polycondensed) using ethylene glycol as one of the raw materials, part of the ethylene glycol is modified to become diethylene glycol, which is then introduced into the polyester skeleton.

When the polyester (a2) (polyester (z2)) is a copolymer polyester, examples of the dicarboxylic acid component other than the terephthalic acid (X1) include aromatic dicarboxylic acids, alicyclic dicarboxylic acids, aliphatic dicarboxylic acids, polyfunctional acids, etc. Examples of these dicarboxylic acid components include the same dicarboxylic acids as those listed for the copolymer polyester (a1).
When the polyester (a2) (polyester (z2)) is a copolymer polyester, the proportion of dicarboxylic acid components other than terephthalic acid in the dicarboxylic acid components is preferably 1 to 30 mol %, more preferably 3 mol % or more, and even more preferably 5 mol % or more, and is more preferably 20 mol % or less, and even more preferably 10 mol % or less.

When the polyester (a2) (polyester (z2)) is a copolymer polyester, the alcohol component (Y2) other than ethylene glycol (Y1) can be appropriately selected from the compounds listed for the copolymer polyester (a1), and preferred examples include 1,4-butanediol, 1,6-hexanediol, 1,4-hexanediol, diethylene glycol, trimethylene glycol, neopentyl glycol, and 1,4-cyclohexanedimethanol.
When the polyester (a2) (polyester (z2)) is a copolymer polyester, the proportion of the alcohol component (Y2) other than ethylene glycol in the alcohol components in the copolymer polyester is preferably 1 mol % or more and less than 100 mol %, more preferably 3 mol % or more, even more preferably 5 mol % or more, more preferably 90 mol % or less, even more preferably 50 mol % or less, even more preferably 30 mol % or less, and particularly preferably 10 mol % or less.

The intrinsic viscosity (IV) of the polyester (a2) (polyester (z2)) (as a polyester mixture when two or more types of polyester (a2) are used) is preferably 0.40 dL/g to 1.20 dL/g, more preferably 0.45 dL/g or more, even more preferably 0.48 dL/g or more, and more preferably 1.15 dL/g or less, even more preferably 1.10 dL/g or less.
When the intrinsic viscosity of the copolymer polyester (a2) is within this range, it is possible to obtain a polyester having excellent moldability without deteriorating productivity.

The polyester (a2) (polyester (z2)) may be used alone or in combination of two or more kinds.
The content of the polyester (a2) may be any content as long as the proportion of each component in the polyester (A) falls within the ranges described below. Of the resin components constituting the copolymerized polyester layer (A1), the content of the polyester (a2) is, for example, 70% by mass or less, preferably 65% by mass or less, and more preferably 50% by mass or less, and is also preferably 30% by mass or more, and more preferably 40% by mass or more.

(Resin (a3))
The copolymer polyester layer (A1) may be a layer containing a resin (a3) other than the copolymer polyester (a1) and the polyester (a2). As the resin (a3), a resin compatible with the copolymer polyester (a1) may be used, and when the polyester (a2) is used, the resin may also be compatible with the polyester (a2).
When the copolymer polyester layer (A1) is a layer in which a sea-island structure is formed by the copolymer polyester (a1) or the copolymer polyester (a1) and the polyester (a2), and the resin (a3), shielding properties and heat resistance can be imparted by selecting, as the resin (a3), a polyester such as polyolefin, polystyrene, an acrylic resin, a urethane resin, or polybutylene terephthalate (PBT).

In the copolymer polyester layer (A1), the mass ratio ((a1+a2):a3) of the total amount of copolymer polyester (a1) and polyester (a2) to resin (a3) is preferably 98:2 to 50:50, more preferably 95:5 to 60:40, and even more preferably 90:10 to 65:35.

Furthermore, the resin (a3) is preferably a resin compatible with the copolymer polyester (a1) or the copolymer polyester (a1) and the polyester (a2), and has a melting point of 270°C or less and a glass transition temperature of 30 to 120°C. By selecting such a resin (a3), the glass transition temperature of the copolymer polyester layer (A1) can be increased, thereby improving heat resistance. Examples of such a resin include, but are not limited to, polybutylene terephthalate (PBT).
The resin (a3) may be used alone or in combination of two or more kinds.

(Proportion of each component in the copolymer polyester layer (A1))
In the present invention, the proportion of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms in all the polyesters (A) contained in the copolymer polyester layer (A1) is preferably 1 to 30 mol %. Note that "all the polyesters (A)" here refers to all the polyesters contained in the copolymer polyester layer (A1). Therefore, the above proportion refers to the proportion based on the dicarboxylic acid components constituting all the polyesters (A), and similar terms will be used with the same meaning hereinafter.

When the proportion of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms is 1 mol % or more, the effects of using the dicarboxylic acid component (X2) having 4 to 10 carbon atoms can be fully obtained, and flexibility, elongation, etc. at low temperatures can be ensured. When the proportion is 30 mol % or less, the heat shrinkage rate increases, and heat resistance can be ensured.
From the viewpoint of ensuring heat resistance while improving flexibility, elongation, and the like at low temperatures, the proportion of the C4 to C10 dicarboxylic acid component (X2) is more preferably 2 mol% or more, even more preferably 3 mol% or more, and of these, particularly 4 mol% or more, and is more preferably 25 mol% or less, and of these, particularly preferably 20 mol% or less, and particularly preferably 15 mol% or less.

As described above, it is preferable to use adipic acid as the dicarboxylic acid component (X2) having 4 to 10 carbon atoms. When adipic acid is used, adipic acid may be used alone as the dicarboxylic acid component (X2) having 4 to 10 carbon atoms, or may be used in combination with a dicarboxylic acid having 4 to 10 carbon atoms other than adipic acid.
The proportion of adipic acid in the dicarboxylic acid components in all of the polyesters (A) is, for example, 1 to 30 mol %, preferably 2 mol % or more, more preferably 3 mol % or more, and particularly 4 mol % or more, and also preferably 25 mol % or less, more preferably 20 mol % or less, and even more preferably 15 mol % or less.

Furthermore, from the viewpoint of maintaining various performance properties such as heat resistance, the proportion of terephthalic acid (X1) in the dicarboxylic acid components of all polyesters (A) is, for example, 65 to 98 mol%, but is preferably 97 mol% or less, and more preferably 96 mol% or less, and is also preferably 75 mol% or more, more preferably 80 mol% or more, and of these, particularly 85 mol% or more.

Furthermore, the proportion of other dicarboxylic acids (X3) in the dicarboxylic acid components in all of the polyesters (A) is, for example, 10 mol % or less, preferably 5 mol % or less, more preferably 3 mol % or less, and most preferably 0 mol %. In other words, it is most preferable that the dicarboxylic acid components in the polyesters (A) do not contain other dicarboxylic acids (X3).

The dicarboxylic acid component of the polyester (A) contained in the copolymer polyester layer (A1) can be quantified by measuring the ^1H -NMR spectrum. The same applies to the dicarboxylic acid component of the polyester contained in the polyester layers (B1) and (B2) and the copolymer polyester film described below.

In the present invention, the proportion of the other alcohol component (Y2) in the alcohol components of all the polyesters (A) contained in the copolymer polyester layer (A1) is preferably 15 to 60 mol%. When it is 15 mol% or more, flexibility, elongation, etc. at low temperatures are more likely to be ensured. Furthermore, when it is 60 mol% or less, an increase in the thermal shrinkage rate is prevented, and heat resistance is more likely to be ensured. From the perspective of ensuring heat resistance while maintaining good flexibility, elongation, etc. at low temperatures, the proportion of the other alcohol component (Y2) is more preferably 20 mol% or more, even more preferably 25 mol% or more, even more preferably 30 mol% or more, and more preferably 55 mol% or less.

From the viewpoint of ensuring flexibility and high elongation at low temperatures, the other alcohol component (Y2) preferably contains at least 1,4-butanediol, and more preferably contains both 1,4-butanediol and 1,6-hexanediol.
The proportion of 1,4-butanediol and 1,6-hexanediol in the alcohol components of all polyesters (A) is preferably 15 to 60 mol %, more preferably 20 mol % or more, even more preferably 25 mol % or more, even more preferably 30 mol % or more, and more preferably 55 mol % or less.
The proportion of 1,4-butanediol and 1,4-hexanediol means, for example, the proportion of 1,4-butanediol when only 1,4-butanediol is used, and means the total proportion of 1,4-butanediol and 1,6-hexanediol when both are used.
When 1,4-butanediol and 1,6-hexanediol are contained, the ratio of the molar amount of 1,6-hexanediol to the molar amount of 1,4-butanediol is, for example, 0.5 or more, preferably 0.7 or more, more preferably 0.8 or more, even more preferably 0.9 or more, or for example, 2.5 or less, preferably 2.0 or less, more preferably 1.6 or less, even more preferably 1.4 or less.

In all polyesters (A), the proportion of ethylene glycol (Y1) in the alcohol component is 40 to 85 mol%, preferably 45 mol% or more, and preferably 80 mol% or less, more preferably 75 mol% or less, and even more preferably 70 mol% or less.

The alcohol components of the polyester (A) contained in the copolymer polyester layer (A1) can be quantified by measuring the ¹ H-NMR spectrum. The same applies to the polyester layers (B1) and (B2) and the copolymer polyester film described below.

Among the above, a particularly preferred embodiment is one in which the copolymer polyester (a1) is a crystalline copolymer polyester (hereinafter, may be referred to as "copolymer polyester (Za)") composed of a copolymer of terephthalic acid and an aliphatic dicarboxylic acid having 4 to 10 carbon atoms with one or both of 1,4-butanediol and 1,6-hexanediol, and in which the proportion of the aliphatic dicarboxylic acid having 4 to 10 carbon atoms in the dicarboxylic acid components in all of the polyesters (A) is 3 to 50 mol %, and the total proportion of 1,4-butanediol and 1,6-hexanediol in the alcohol components is 15 to 60 mol %.
Usually, when the ratio of copolymerization components of a copolymer polyester is increased to reduce the elastic modulus, the crystallinity decreases and the polyester becomes amorphous. The copolymer polyester (Za) has a high ratio of copolymerization components and can achieve a low elastic modulus, but still maintains its crystallinity, and can be heat-set by heat treatment after stretching. As a result, the copolymer polyester (Za) is flexible, yet has good elongation and strength, and can also suppress thermal shrinkage.

As shown in Figure 1, a conventional general-purpose copolymer polyester film (Comparative Example 1) has a high storage modulus around 80°C, poor flexibility, and an extremely low storage modulus around 180°C, meaning it is unable to maintain strength. Under these circumstances, even those skilled in the art would be forced to think too hard to figure out how to approximate the storage modulus curve of the fluororesin film (Reference Example) shown in the Reference Example. However, the inventors discovered that by using the above-mentioned polyester (A) or the polyester (Z) described below, it is possible to draw a fitting curve for the storage modulus close to that of a fluororesin film; specifically, they discovered that it is possible to further reduce the storage modulus around 80°C compared to conventional films while maintaining a storage modulus around 180°C at a certain level or higher (Examples), thereby completing the present invention.

In addition, the copolymer polyester film also exhibits good solvent resistance by containing the above-mentioned copolymer polyester (a1), or copolymer polyester (a1) and polyester (a2) (or copolymer polyester (z1), or copolymer polyester (z1) and polyester (z2) described below). Therefore, as described below, when a resin layer is formed using an organic solvent, the copolymer polyester film can be prevented from being dissolved by the solvent.

(Polyester blend)
As described above, the polyester (A) contained in the copolymer polyester layer (A1) may be one type of polyester or a blend of two or more types of polyester. When the polyester (A) is made of one type of polyester, that is, when the copolymer polyester layer (A1) contains one type of polyester as the copolymer polyester (a1), the polyester is preferably a copolymer polyester containing, as the dicarboxylic acid component, a dicarboxylic acid having 4 to 10 carbon atoms and terephthalic acid, and, as the alcohol component, ethylene glycol and one or both of 1,4-butanediol and 1,6-hexanediol.

On the other hand, when the polyester (A) contained in the copolymer polyester layer (A1) is composed of two or more types of polyesters, that is, when the copolymer polyester layer (A1) contains a polyester blend composed of two or more types of polyesters as the polyester (A), it is preferable that the polyester blend contains, as the dicarboxylic acid component, a dicarboxylic acid having 4 to 10 carbon atoms and terephthalic acid, and, as the diol component, ethylene glycol and one or both of 1,4-butanediol and 1,6-hexanediol.
In this case, it is sufficient that the polyester blend contains the above-mentioned structural units. For example, when the polyester blend is a mixed resin of a first polyester and a second polyester, it is not necessary for each of the first polyester and the second polyester to contain all of the above-mentioned structural units.

In the copolymer polyester layer (A1), the polyester (A) is a main component resin. The "main component resin" means a resin that is contained in the largest proportion among the resin components constituting the copolymer polyester layer (A).
The content of the polyester (A) relative to the resin components contained in the copolymer polyester layer (A1) is, for example, 50% by mass or more, preferably 60% by mass or more, more preferably 80% by mass or more, and is not particularly limited as long as it is 100% by mass or less.
The content of the polyester (A) is the total amount of all polyesters contained in the copolymer polyester layer (A1).

The copolymer polyester layer (A1) may contain particles. Specific examples of the particles include inorganic particles such as silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, aluminum oxide, and titanium oxide, and organic particles such as acrylic resin, styrene resin, urea resin, phenolic resin, epoxy resin, and benzoguanamine resin. Furthermore, precipitated particles obtained by precipitating and finely dispersing a part of a metal compound such as a catalyst during the polyester production process can also be used.
The particles may be used alone or in combination of two or more kinds.
The shape of the particles to be used is not particularly limited, and any of spherical, blocky, rod-like, flat, etc. may be used. Furthermore, there are no particular limitations on the hardness, specific gravity, color, etc. Two or more types of these particles may be used in combination as needed.

The average particle size of the particles used is preferably 5 μm or less, more preferably in the range of 0.1 to 4 μm. By using particles with an average particle size in the above range, an appropriate surface roughness can be imparted to the film, ensuring good slipperiness and smoothness.
The average particle size of the particles can be determined by measuring the diameters of 10 or more particles arbitrarily selected from the copolymer polyester layer (A1) using a scanning electron microscope (SEM) and calculating the average value of the diameters. In the case of non-spherical particles, the average value of the longest and shortest diameters ((minor diameter + major diameter)/2) can be measured as the diameter of each particle.
The copolymer polyester layer (A1) may contain a combination of two or more types of particles having different particle sizes.
The particle content in the copolymer polyester layer (A1) is preferably 5% by mass or less, more preferably in the range of 0.0003 to 3% by mass, and even more preferably in the range of 0.001 to 0.2% by mass. By setting the particle content within the above range, it is possible to easily impart slipperiness to the substrate film while ensuring the transparency of the substrate film. Furthermore, by keeping the particle content low, it is possible to easily increase the tensile elongation at break.

In addition to the particles described above, the copolymer polyester layer (A1) may optionally contain at least one additive selected from the group consisting of a crystal nucleating agent, antioxidant, color inhibitor, pigment, dye, UV absorber, release agent, lubricant, flame retardant, antistatic agent, etc.

(Method for producing copolymer polyester (a1))
The method for producing the copolymerized polyester (a1) (polyester (z1)) is not particularly limited, and a conventional method can be applied. For example, first, a dicarboxylic acid component containing terephthalic acid or an ester-forming derivative thereof, a C4-C10 dicarboxylic acid or an ester-forming derivative thereof, and a diol component containing ethylene glycol, preferably further containing diethylene glycol, are mixed in a predetermined ratio under stirring to prepare a raw material slurry (raw material slurry preparation step). Next, the raw material slurry is heated under normal pressure or pressure to cause an esterification reaction to produce a polyester low polymer (hereinafter sometimes referred to as an "oligomer") (oligomer preparation step). Thereafter, a C4-C10 dicarboxylic acid or an ester-forming derivative thereof, and an alcohol component other than ethylene glycol and diethylene glycol, such as 1,4-butanediol or 1,6-hexanediol, are added to the obtained oligomer, and the mixture is gradually reduced in pressure and heated in the presence of a transesterification catalyst or the like to cause a melt polycondensation reaction to obtain a polyester (polyester preparation step). The obtained polyester may be further subjected to a solid-phase polycondensation reaction, if necessary (solid-phase polycondensation step).

However, the method for producing polyester (a1) is not limited to the above-mentioned method, and the dicarboxylic acid having 4 to 10 carbon atoms or an ester-forming derivative thereof does not need to be added to both the raw material slurry and the oligomer as described above, and may be added only to the raw material slurry or only to the oligomer.
Similarly, the alcohol components other than ethylene glycol and diethylene glycol are not limited to being added only to the oligomer as described above, but may be added to both the raw material slurry and the oligomer, or may be added only to the raw material slurry.

Examples of the transesterification catalyst include antimony compounds such as diantimony trioxide; germanium compounds such as germanium dioxide and germanium tetroxide; titanium compounds such as titanium alcoholates such as tetramethyl titanate, tetraisopropyl titanate, and tetrabutyl titanate, and titanium phenolates such as tetraphenyl titanate; dibutyltin oxide, methylphenyltin oxide, tetraethyltin, hexaethylditin oxide, cyclohexahexylditin oxide, didodecyltin oxide, triethyltin hydroxide, triphenyltin hydroxide, and triisobutyltin. Examples of suitable tin compounds include tin acetate, dibutyltin diacetate, diphenyltin dilaurate, monobutyltin trichloride, tributyltin chloride, dibutyltin sulfide, butylhydroxytin oxide, methylstannoic acid, ethylstannoic acid, and butylstannoic acid; magnesium compounds such as magnesium acetate, magnesium hydroxide, magnesium carbonate, magnesium oxide, magnesium alkoxide, and magnesium hydrogen phosphate; and calcium compounds such as calcium acetate, calcium hydroxide, calcium carbonate, calcium oxide, calcium alkoxide, and calcium hydrogen phosphate.
These catalysts may be used alone or in combination of two or more.

Furthermore, during the production of polyesters, it is preferable to use a stabilizer in combination with the transesterification catalyst. Examples of the stabilizer include orthophosphoric acid, polyphosphoric acid, pentavalent phosphorus compounds such as trimethyl phosphate, triethyl phosphate, tri-n-butyl phosphate, trioctyl phosphate, triphenyl phosphate, tricresyl phosphate, tris(triethylene glycol) phosphate, ethyl diethyl phosphonoacetate, methyl acid phosphate, ethyl acid phosphate, isopropyl acid phosphate, butyl acid phosphate, monobutyl phosphate, dibutyl phosphate, dioctyl phosphate, and triethylene glycol acid phosphate, phosphorous acid, hypophosphorous acid, and trivalent phosphorus compounds such as diethyl phosphite, trisdodecyl phosphite, trisnonyldecyl phosphite, and triphenyl phosphite.
Among these, trivalent phosphorus compounds generally have stronger reducing properties than pentavalent phosphorus compounds, and there is a risk that the metal compound added as a polycondensation catalyst will be reduced and precipitated, which may cause the generation of foreign matter, and therefore pentavalent phosphorus compounds are preferred.

The reaction pressure in the melt polycondensation reaction is preferably 0.001 kPa to 1.33 kPa in terms of absolute pressure, and the reaction temperature in the melt polycondensation reaction is preferably 220°C to 280°C, more preferably 230°C or higher and 260°C or lower.
The solid-phase polycondensation reaction may be carried out under reduced pressure or in an inert gas atmosphere, and the reaction temperature is preferably 180° C. to 220° C. The reaction time of the solid-phase polycondensation reaction is preferably 5 hours to 100 hours. By adopting the above melt polycondensation reaction conditions and solid-phase polycondensation reaction conditions, a polyester having a desired intrinsic viscosity can be obtained.

<In the case of a multi-layer structure>
As described above, the present copolymer polyester film may have a multilayer structure comprising the copolymer polyester layer (A1) and other layers.

When the copolymer polyester film has a multilayer structure, it may have, for example, a copolymer polyester layer (A1) and polyester layers (B1) and (B2) on both sides of the copolymer polyester layer (A1). Each of the polyester layers (B1) and (B2) contains a polyester (B) as a main component resin.
Here, the term "main component resin" refers to the resin that is contained in the largest proportion of the resin components constituting each of the copolymer polyester layers (B1) and (B2). The main component resin preferably accounts for 50% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more of the resin components constituting each of the polyester layers (B1) and (B2), and may also account for, for example, 100% by mass or less.

When the copolymer polyester (a1) is crystalline, the polyester (B) contained in each polyester layer (B1) and (B2) preferably contains a polyester having a melting point higher than that of the copolymer polyester (a1). Such a polyester may account for 50% by mass or more of the resin components constituting each of the polyester layers (B1) and (B2). Furthermore, when the copolymer polyester (a1) is amorphous, the polyester (B) may contain a polyester having a melting point higher than the glass transition point of the copolymer polyester (a1). Such a polyester may account for 50% by mass or more of the resin components constituting each of the polyester layers (B1) and (B2).
In the case of a multilayer structure having a configuration in which polyester layers (B1) and (B2) containing polyester (B) as a main component resin are laminated, raw material resin compositions are laminated by coextrusion or the like so as to form polyester layer (B1)/copolymer polyester layer (A1)/polyester layer (B2), stretched, and then heat-set at a higher temperature than in the case of a single layer of copolymer polyester layer (A1). Therefore, it is possible to achieve a level of flexibility that cannot be achieved with a single layer of copolymer polyester layer (A1), and to further prevent heat shrinkage.
Specifically, the storage modulus of the release film at 80° C. can be set to 1000 MPa or less, preferably 800 MPa or less, and more preferably 100 MPa or more and 400 MPa or less.

In the above multilayer structure, the thickness of each of the polyester layers (B1) and (B2) is preferably 1 to 20% of the thickness of the copolymer polyester layer (A1).
When the thickness of each of the polyester layers (B1) and (B2) is 1% or more of the thickness of the copolymer polyester layer (A1), film formation is possible without significantly impairing productivity, and when it is 20% or less, the required flexibility can be sufficiently ensured.
From this viewpoint, the thickness of each of the polyester layers (B1) and (B2) is more preferably 3% or more and 15% or less of the thickness of the copolymer polyester layer (A1), and even more preferably 5% or more and 12% or less.
The thickness of each of the polyester layers (B1) and (B2) present on both the front and back sides of the copolymer polyester layer (A1) may be different on the front and back sides or may be the same.
However, in the multi-layer structure, the number of polyester layers is not limited to three, and may be two, or four or more.

One or both of the polyester layers (B1) and (B2) may contain particles. Specific examples, shapes, average particle diameters, content, and other details of the particles are as described for the polyester layer (A). However, the content should be interpreted as the content of particles in each of the polyester layers (B1) and (B2).
In addition to the particles described above, one or both of the polyester layers (B1) and (B2) may contain at least one additive, the details of which are as described above.
In the case of a multi-layer structure, one or both of the polyester layers (B1) and (B2) may contain particles, while the polyester layer (A) may not contain particles.

When the copolymer polyester (a1) is crystalline, the polyester (B) preferably contains a polyester having a melting point higher than that of the copolymer polyester (a1) by 10 to 100° C., more preferably by 20° C. or more, even more preferably by 40° C. or more, and more preferably by 90° C. or less, and even more preferably by 70° C. or less. Such a polyester should account for 50% by mass or more of the resin components constituting each of the polyester layers (B1) and (B2).
On the other hand, when the copolymer polyester (a1) is amorphous, the polyester (B) preferably contains a polyester having a melting point higher than the glass transition point of the copolymer polyester (a1) by preferably 120 to 260° C., more preferably 140° C. or more, even more preferably 160° C. or more, and more preferably 230° C. or less, and even more preferably 200° C. or less. Such a polyester should account for 50% by mass or more of the resin components constituting each of the polyester layers (B1) and (B2).
The polyester (B) as the main component of each of the polyester layers (B1) and (B2) present on both the front and back sides of the copolymer polyester layer (A1) may be different or the same on the front and back sides, but it is preferable that the melting points of the polyesters (B) on the front and back sides do not differ significantly.

As polyester (B), for example, a homopolyester or copolymer polyester (also referred to as "polyester (z2)") containing terephthalic acid as the dicarboxylic acid component and ethylene glycol as the alcohol component can be suitably used. However, the polyester in polyester (B) is not limited to this. The homopolyester is polyethylene terephthalate.

When the polyester (B) is a copolymer polyester, examples of the dicarboxylic acid component other than terephthalic acid include aromatic dicarboxylic acids, alicyclic dicarboxylic acids, aliphatic dicarboxylic acids, polyfunctional acids, etc. Examples of these dicarboxylic acid components include the same dicarboxylic acids as those listed for the copolymer polyester (a1).
When the polyester (B) is a copolymer polyester, the proportion of dicarboxylic acid components other than terephthalic acid in the dicarboxylic acid components is preferably 1 to 30 mol %, more preferably 5 mol % or more, and even more preferably 10 mol % or more, and is more preferably 25 mol % or less, and even more preferably 20 mol % or less.

When the polyester (B) is a copolymer polyester, examples of the alcohol component other than ethylene glycol include 1,4-butanediol, 1,6-hexanediol, ethylene glycol, diethylene glycol, trimethylene glycol, neopentyl glycol, 1,4-cyclohexanedimethanol, bisphenol, and derivatives thereof.
In the polyester (B), the proportion of alcohol components other than ethylene glycol in the alcohol components is preferably 1 mol % or more and less than 100 mol %, more preferably 5 mol % or more, even more preferably 10 mol % or more, and more preferably 95 mol % or less, even more preferably 90 mol % or less.
However, the polyester in the polyester (B) preferably contains the polyester (z2) as described above, and the detailed description thereof is as described above for the polyester (a2).

Furthermore, as the copolymer polyester in polyester (B), a copolymer polyester containing terephthalic acid as the dicarboxylic acid component and an alcohol component (Y2) other than ethylene glycol (Y1) as the alcohol component can also be suitably used. Among these, as the copolymer polyester for polyester (B), it is preferable to use a copolymer polyester (z1) which is a copolymer of terephthalic acid (X1), a dicarboxylic acid component (X2) having 4 to 10 carbon atoms, and an alcohol component (Y2) other than ethylene glycol (Y1). Details of such copolymer polyester (z1) are as described above for copolymer polyester (a1), and therefore a detailed description thereof will be omitted.

(Proportion of each component in polyester layers (B1) and (B2))
The content of each polyester in the polyester (B) contained in the polyester layers (B1) and (B2) may be appropriately adjusted so that the proportion of each component in all the polyesters (Z) contained in the copolymer polyester film falls within the range described below.
However, the content of the copolymerization component of the polyester (B) contained in each of the polyester layers (B1) and (B2) is preferably equal to or less than, and more preferably lower than, the content of the copolymerization component in the polyester layer (A) on a mol % basis. By reducing the content of the copolymerization component in each of the polyester layers (B1) and (B2), it becomes easier to obtain good heat resistance while ensuring flexibility, high elongation, etc.

Hereinafter, the case where the above-mentioned copolymer polyester (z1), polyester (z2), or both of them are used as the polyester (B) contained in each of the polyester layers (B1) and (B2) will be specifically described in more detail. The copolymer polyester (z1) in each of the polyester layers (B1) and (B2) is particularly preferably the above-mentioned copolymer polyester (Za). The polyester (z2) is preferably polyethylene terephthalate.
The proportion of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms in all the polyesters (B) contained in each of the polyester layers (B1) and (B2) is preferably equal to or less than the proportion of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms in all the polyesters (A) contained in the polyester layer (A), and more preferably lower than this proportion, on a mol % basis. When the proportion is lower than this proportion, the difference in proportion is, for example, 1 to 30 mol %, preferably 2 to 20 mol %, and more preferably 3 to 10 mol %.
The term "all polyesters (B)" as used herein refers to all polyesters contained in each of the polyester layers (B1) and (B2). Therefore, the proportion in all polyesters (B) refers to the proportion based on the dicarboxylic acid component constituting the polyester (B) in the polyester layer (B1) or (B1). Similar terms will be used in the following with similar meanings.

As the dicarboxylic acid component (X2) having 4 to 10 carbon atoms, adipic acid is preferably used as described above. When adipic acid is used, the proportion of adipic acid in the dicarboxylic acid components of all the polyesters (B) contained in each of the polyester layers (B1) and (B2) is preferably equal to or less than the proportion of adipic acid in the dicarboxylic acid components of all the polyesters (A) contained in the polyester layer (A), on a mol % basis, and more preferably lower than that proportion. When the proportion is lower than that proportion, the difference in proportion is, for example, 1 to 30 mol %, preferably 2 to 20 mol %, and more preferably 3 to 10 mol %.
On the other hand, the proportion of terephthalic acid (X1) in the dicarboxylic acid components in all of the polyesters (B) contained in each of the polyester layers (B1) and (B2) is preferably equal to or greater than the proportion of terephthalic acid (X1) in the dicarboxylic acid components in all of the polyesters (A) contained in the polyester layer (A), on a mol % basis, and more preferably higher than this proportion.

The proportion of the other alcohol component (Y2) in the alcohol components of all the polyesters (B) contained in each of the polyester layers (B1) and (B2) is preferably equal to or less than the proportion of the other alcohol component (Y2) in the alcohol components of all the polyesters (A) contained in the polyester layer (A), and more preferably lower than this proportion, on a mol % basis. When the difference is lower than this proportion, the difference is, for example, 1 to 50 mol %, preferably 5 to 40 mol %, and more preferably 10 to 35 mol %.
As the other alcohol component (Y2), it is preferable to use 1,4-butanediol, 1,6-hexanediol, or both of them, but the proportions of 1,4-butanediol and 1,6-hexanediol in the alcohol components of all the polyesters (B) contained in each of the polyester layers (B1) and (B2) are preferably equal to or less than the proportions of 1,4-butanediol and 1,6-hexanediol in the alcohol components of all the polyesters (A) contained in the polyester layer (A) on a mol % basis, and more preferably lower than that proportion. When the difference is lower than that proportion, the difference is, for example, 1 to 50 mol %, preferably 5 to 40 mol %, and more preferably 10 to 35 mol %.
On the other hand, the proportion of ethylene glycol (Y1) in the alcohol components of all the polyesters (B) contained in each of the polyester layers (B1) and (B2) is preferably equal to or greater than the proportion of ethylene glycol (Y1) in the alcohol components of all the polyesters (A) contained in the polyester layer (A), on a mol % basis, and more preferably higher than this proportion.

(Proportion of each component in the entire film)
The polyester used in the copolymer polyester film preferably contains the copolymer polyester (z1), particularly preferably the copolymer polyester (Za). Details of the copolymer polyester (z1) and the copolymer polyester (Za) are as described in the copolymer polyester layer (A1).
The proportion of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms in all the polyesters (Z) contained in the copolymer polyester film is preferably 1 to 30 mol %. Note that "all the polyesters (Z)" here refers to all the polyesters contained in the copolymer polyester film. Therefore, the above proportion refers to the proportion based on the dicarboxylic acid components constituting all the polyesters (Z) contained in the copolymer polyester film, and similar terms will be used with the same meaning hereinafter.
When the proportion of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms is 1 mol % or more, the effects of using the dicarboxylic acid component (X2) having 4 to 10 carbon atoms are sufficiently obtained, and the storage modulus is reduced, making it easier to ensure flexibility at low temperatures, high elongation, etc. When the proportion is 30 mol % or less, the heat shrinkage rate is increased, making it easier to ensure heat resistance.
The proportion of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms in the polyester (Z) is more preferably 2 mol% or more, even more preferably 3 mol% or more, and particularly preferably 4 mol% or more, and is more preferably 20 mol% or less, even more preferably 15 mol% or less, and particularly preferably 10 mol% or less.
The proportion of adipic acid in the dicarboxylic acid components in all of the polyesters (Z) is, for example, 1 to 30 mol %, preferably 2 mol % or more, more preferably 3 mol % or more, and particularly preferably 4 mol % or more, and is also preferably 20 mol % or less, more preferably 15 mol % or less, and even more preferably 10 mol % or less.

The proportion of terephthalic acid (X1) in the dicarboxylic acid components of all the polyesters (Z) is, for example, 65 to 98 mol %, preferably 97 mol % or less, more preferably 96 mol % or less, and also preferably 75 mol % or more, more preferably 80 mol % or more, and particularly 85 mol % or more, from the viewpoint of maintaining various performance properties such as heat resistance in a satisfactory manner.
The proportion of the other dicarboxylic acid (X3) in the dicarboxylic acid components in all of the polyesters (Z) is, for example, 10 mol % or less, preferably 5 mol % or less, more preferably 3 mol % or less, and most preferably 0 mol %. That is, it is most preferable that the dicarboxylic acid components in the polyester (A) do not contain the other dicarboxylic acid (X3).

Furthermore, the proportion of the other alcohol component (Y2) in the alcohol components of all the polyesters (Z) contained in the copolymer polyester film is, for example, 10 to 60 mol, but is preferably 15 mol or more, more preferably 20 mol or more, even more preferably 23 mol or more, still more preferably 25 mol or more, and is preferably 60 mol or less, more preferably 55 mol% or less, even more preferably 45 mol% or less, and still more preferably 35 mol% or less. When the proportion of the alcohol component (Y2) in the polyester (Z) is a certain level or higher, it becomes easier to reduce the storage modulus and ensure flexibility and high elongation at low temperatures. Furthermore, when the proportion of the alcohol component (Y2) is a certain level or lower, it becomes easier to prevent an increase in heat shrinkage and ensure heat resistance.

The proportion of 1,4-butanediol and 1,6-hexanediol in the alcohol component of the polyester (Z) is, for example, 10 to 60 mol, but is preferably 15 mol or more, more preferably 20 mol or more, even more preferably 22 mol or more, still more preferably 24 mol or more, and is preferably 60 mol or less, more preferably 55 mol or less, even more preferably 44 mol or less, and still more preferably 34 mol or less.
Furthermore, when polyester (Z) contains 1,4-butanediol and 1,6-hexanediol, the ratio of the molar amount of 1,6-hexanediol to the molar amount of 1,4-butanediol in polyester (Z) is, for example, 0.5 or more, preferably 0.7 or more, more preferably 0.8 or more, and even more preferably 0.9 or more, or for example, 2.5 or less, preferably 2.0 or less, more preferably 1.6 or less, and even more preferably 1.4 or less.

In all polyesters (Z), the proportion of ethylene glycol (Y1) in the alcohol component is, for example, 40 to 85 mol%, preferably 40 mol% or more, more preferably 45 mol% or more, even more preferably 55 mol% or more, even more preferably 60 mol% or more, and preferably 82 mol% or less, more preferably 80 mol% or less, even more preferably 77 mol% or less. Even more preferably 75 mol% or less.

<Thickness of the present copolymer polyester film>
The thickness of the present copolymer polyester film is not particularly limited, and an appropriate thickness can be selected depending on the application.
In particular, from the viewpoint of fully exhibiting the characteristics of the present copolymerized polyester film, it is preferable that the total thickness of the film exceeds 5 μm.
Although the stiffness of a film is said to be proportional to the cube of its thickness, the present copolymer polyester film is characterized by being weak in stiffness and flexible even when it has a thickness of more than 5 μm, and can further enjoy the benefits of the present invention.
From this viewpoint, the total thickness of the present copolymer polyester film is preferably more than 5 μm, more preferably 12 μm or more, and even more preferably 30 μm or more.
The total thickness of the copolymer polyester film is not particularly limited, but is, for example, 200 μm or less, preferably 150 μm or less, and more preferably 100 μm or less.

<Method of manufacturing the present copolymer polyester film>
As an example of the method for producing the present copolymer polyester film, a method for producing the present copolymer polyester film as a biaxially stretched film will be described below, although the present copolymer polyester film is not limited to the method described here.

First, using a known method, raw materials, such as polyester chips, are fed into a melt extrusion device and heated to above the melting point of each polymer. The molten polymer is then extruded through a die and cooled to a temperature below the glass transition point of the polymer on a rotating cooling drum, solidifying it to obtain an unoriented sheet in a substantially amorphous state.

The unoriented sheet is then stretched in one direction using a roll or tenter type stretching machine at a stretching temperature of usually 25 to 120°C, preferably 35 to 100°C, and at a stretching ratio of usually 2.5 to 7 times, preferably 2.8 to 6 times.
The film is then stretched in a direction perpendicular to the first stretching direction at a temperature of usually 50 to 140° C. and a stretch ratio of usually 3.0 to 7 times, preferably 3.5 to 6 times.
In the above stretching, a method of stretching in one direction in two or more stages can also be employed.

After the stretching, the film is subsequently heat-set at a temperature of 130 to 270°C under tension or under relaxation of 30% or less to obtain the present copolymer polyester film as a biaxially oriented film. By subjecting the present copolymer polyester film to heat-setting, the flexibility, heat resistance, etc. can be improved.
In the case of a single-layer copolymer polyester layer (A1), the heat setting treatment (also referred to as "heat treatment") is preferably carried out at a temperature 5 to 70°C lower than the melting point of the polyester for forming the copolymer polyester layer (A1). In the case of a multilayer structure, the heat setting treatment is also preferably carried out at a temperature 5 to 70°C lower than the melting point of the polyester for forming the copolymer polyester layer (A1), and is also preferably carried out at a temperature 5 to 70°C lower than the melting point of the polyester for forming the polyester layer (B).

When the present copolymer polyester film has a laminated structure of polyester layer (A1) and polyester layers (B1) and (B2), the polyester layer (A1) and the polyester layers (B1) and (B2) can be co-extruded, and then the resulting integrated film can be stretched and heat-set as described above. Note that the present copolymer polyester film can also be produced by co-extrusion in the same manner when it has a laminated structure other than the above-described layers (A1), (B1), and (B2).

<Release film characteristics>
(Storage modulus at 80°C)
The release film of the present invention must have a storage modulus of 1000 MPa or less at 80° C. When the storage modulus at 80° C. is 1000 MPa or less, flexibility can be ensured and curved surface conformability and the like can be improved, and for example, when the film is mounted on a mold, the film can sufficiently conform to the shape of the mold.
From this viewpoint, the storage modulus of the release film at 80° C. is more preferably 1000 MPa or less, even more preferably 800 MPa or less, and particularly preferably 400 MPa or less.

On the other hand, the storage modulus at 80° C. is preferably 100 MPa or more, more preferably 200 MPa or more, from the viewpoint of ease of handling in each step.
The storage modulus at 80°C and 180°C described later are values obtained by the measurement method described in the examples described later.

In the release film, the storage modulus at 80°C can be adjusted to the above range by adjusting the type and content of the copolymerization components of the copolymerized polyester film. From this viewpoint, the copolymerization components of the copolymerized polyester in the copolymerized polyester film preferably contain, for example, an aliphatic dicarboxylic acid having 4 to 10 carbon atoms and one or both of 1,4-butanediol and 1,6-hexanediol, and the proportions of these in the dicarboxylic acid component and the alcohol component in the polyester (Z) are preferably 1 to 30 mol% and 15 to 60 mol%, respectively, as described above. More preferably, the aliphatic dicarboxylic acid having 4 to 10 carbon atoms is adipic acid.
Among these, the other alcohol component (Y2) is preferably 1,4-butanediol and 1,6-hexanediol, and in this case, the total amount (mol %) of the other alcohol component (Y2) is preferably 15 to 60 mol %.

The storage modulus at 80°C can also be easily adjusted to fall within the above range by laminating polyester layers (B1) and (B2) containing the polyester (B) as a main component resin on both the front and back sides of the copolymer polyester layer (A1) to form a multilayer structure.
Furthermore, the storage modulus at 80° C. can also be adjusted by the stretching conditions during production of the present copolymerized polyester film and the conditions for subsequent heat setting.

(Storage modulus at 180°C)
The release film of the present invention has a storage modulus at 180° C. of 30 MPa or more, preferably 40 MPa or more, more preferably 50 MPa or more, and even more preferably 60 MPa or more.
When the storage modulus at 180° C., i.e., near the processing temperature, is 30 MPa or more, particularly 40 MPa or more, the heat resistance is improved. Furthermore, the improved heat resistance makes it easier to suppress heat shrinkage and the like.
The storage modulus at 180° C. is not particularly limited, but from the viewpoint of flexibility, it is, for example, 400 MPa or less, preferably 200 MPa or less, and more preferably 150 MPa or less.

(Storage modulus at 80°C/Storage modulus at 180°C)
The release film of the present invention has a ratio of the storage modulus at 80°C to the storage modulus at 180°C (storage modulus at 80°C/storage modulus at 180°C) of 13 or less. If the storage modulus ratio is higher than 13, it is difficult to increase the storage modulus at around 180°C while decreasing the storage modulus at around 80°C, making it difficult to use as a substitute for a fluororesin film. The storage modulus ratio is preferably 12 or less, and more preferably 10 or less. The lower the storage modulus ratio, the better, and it is sufficient if it is 1 or more, but in practice it may be, for example, 2 or more, or 3 or more.

(Softening temperature measured by NanoTA)
When the copolymer polyester film has a multilayer structure, the release film preferably has a temperature difference of 5° C. or more between the copolymer polyester layer (A1) and at least one of the polyester layer (B1) and the polyester layer (B2), as measured by softening temperature measurement with Nano TA, and more preferably a temperature difference of 5° C. or more between the copolymer polyester layer (A1) and both the polyester layer (B1) and the polyester layer (B2). By ensuring that the difference in softening temperature between the layer (A) and the layer (B1), (B2), or both of these is at least a certain level, the amount of copolymer component in the layer (A) and the layer (B1), (B2), or both of these tends to differ, making it easier to improve various performances.
The softening temperature of at least one of the polyester layer (B1) and the polyester layer (B2), preferably both, is preferably higher than the softening temperature of the copolymer polyester layer (A1). When the softening temperature of one or both of the polyester layer (B1) and the polyester layer (B2) is higher than the softening temperature of the copolymer polyester layer (A1) and there is a predetermined temperature difference between them, the copolymer polyester layer (A1) contains more copolymer components while the polyester layers (B1) and (B2) contain less copolymer components, which tends to improve heat resistance while ensuring flexibility and high elongation.

The temperature difference is preferably 10°C or more, more preferably 15°C or more, and even more preferably 40°C or more. Furthermore, from the perspective of making it easier to appropriately adjust various physical properties, the temperature difference is preferably 100°C or less, more preferably 90°C or less, even more preferably 80°C or less, and preferably 70°C or less.

Furthermore, the softening temperature of the copolymer polyester layer (A1) measured by nano TA is preferably 160°C or higher, more preferably 170°C or higher, and even more preferably 180°C or higher from the viewpoint of heat resistance, and is preferably 230°C or lower, more preferably 220°C or lower, and even more preferably 215°C or lower from the viewpoint of flexibility, elongation, etc.
Furthermore, the softening temperature of each of the polyester layer (B1) and the polyester layer (B2) measured by NanoTA is preferably 180°C or higher, more preferably 200°C or higher, and even more preferably 210°C or higher, from the viewpoint of heat resistance, and is preferably 265°C or lower, more preferably 260°C or lower, and even more preferably 255°C or lower, from the viewpoint of flexibility, elongation, etc.

<Release film details>
A release film according to one embodiment of the present invention has the above-described copolymer polyester film and a resin layer A provided on at least one surface of the copolymer polyester film.
The resin layer may be stretched together with the copolymer polyester film in a state where it is laminated on the copolymer polyester film, whereby the resin layer A is thinned and formed into a functional sheet.
The copolymer polyester film is flexible and has a high elongation rate at room temperature, making it easy to stretch and suitable for forming into functional sheets by stretching. Furthermore, its excellent heat resistance reduces thermal shrinkage even when the laminated film is heat-treated.
In the release film of the present invention, the resin layer A functions as a release layer.

<<Resin layer A>>
The resin layer A is preferably a layer formed by curing a release layer composition (sometimes referred to as the "present release layer composition") containing a silicone resin, preferably a curable silicone resin (sometimes referred to as the "present curable silicone resin") as a main component resin, and is disposed on at least one side of the present copolymer polyester film described above. The resin layer A can also be said to be a release layer containing a cured product formed by curing the release layer composition.
The resin layer A may also contain a non-silicone resin. Examples of the non-silicone resin include alkyd resins, olefin resins, acrylic resins, long-chain alkyl group-containing compounds, rubbers, waxes, and other non-silicone resins.
The resin layer A may be composed solely of a non-silicone resin, or may be a combination of a non-silicone resin with other components such as a binder resin, a crosslinking agent, etc. By using a non-silicone resin for the resin layer A, the release layer can also be a non-silicone release layer.

When resin layer A contains a non-silicone resin, the content of the non-silicone resin in the resin layer composition is preferably in the range of 5 to 90 mass%, more preferably 10 to 70 mass%, and even more preferably 10 to 50 mass%, relative to the total mass of non-volatile components in resin composition A. By keeping the content of the non-silicone resin within the above range, resin layer A will have good releasability from the mold.

(wax)
Examples of waxes include natural waxes, synthetic waxes, and modified waxes.
Natural waxes include vegetable waxes, animal waxes, mineral waxes and petroleum waxes.
Examples of vegetable waxes include candelilla wax, carnauba wax, rice wax, Japan wax, and jojoba oil.
Animal waxes include beeswax, lanolin, and spermaceti.
Examples of mineral waxes include montan wax, ozokerite, and ceresin.
Petroleum waxes include paraffin wax, microcrystalline wax, and petrolatum.

Synthetic waxes include synthetic hydrocarbons, hydrogenated waxes, fatty acids, acid amides, amines, imides, ester waxes, and ketones, as well as Fischer-Tropsch wax (also known as Sasol wax) and polyethylene wax. Other examples include the following low-molecular-weight polymers (specifically, polymers with a number-average molecular weight of 500 to 20,000): polypropylene, ethylene-acrylic acid copolymer, polyethylene glycol, polypropylene glycol, and block or graft bonded polyethylene glycol and polypropylene glycol.

Examples of modified waxes include montan wax derivatives, paraffin wax derivatives, and microcrystalline wax derivatives. The derivatives referred to here are compounds obtained by any of the following processes: refining, oxidation, esterification, and saponification, or a combination of these. Examples of hydrogenated waxes include hydrogenated castor oil and hydrogenated castor oil derivatives.

Among these, synthetic waxes are preferred from the viewpoint of excellent release performance, with polyethylene wax being more preferred and oxidized polyethylene wax being even more preferred.

From the standpoint of handling and the ability to form irregularities through phase separation, the number average molecular weight of the synthetic wax is preferably 500 to 30,000, more preferably 1,000 to 15,000, and even more preferably 2,000 to 8,000.

Furthermore, considering that heating is performed for crosslinking and other purposes when forming resin layer A, the melting point or softening point of the wax is preferably 80°C or higher, and more preferably 110°C or higher. On the other hand, from the perspective of controlling phase separation performance after heat treatment, the melting point or softening point of the wax is preferably 200°C or lower, more preferably 170°C or lower, and even more preferably 150°C or lower.

(Long-chain alkyl group-containing compound)
The long-chain alkyl group-containing compound is a compound having a straight-chain or branched alkyl group with 6 or more carbon atoms, preferably 8 or more carbon atoms, and more preferably 12 or more carbon atoms.
Examples of alkyl groups include hexyl, octyl, decyl, lauryl, octadecyl, and behenyl groups. Examples of compounds having an alkyl group include various long-chain alkyl group-containing polymeric compounds, long-chain alkyl group-containing amine compounds, long-chain alkyl group-containing ether compounds, and long-chain alkyl group-containing quaternary ammonium salts. In consideration of heat resistance, polymeric compounds are preferred, and from the viewpoint that a small content can effectively achieve roughness-forming performance through appropriate phase separation, polymeric compounds having a long-chain alkyl group in the side chain are more preferred.

A polymeric compound having a long-chain alkyl group on the side chain can be obtained by reacting a polymer having a reactive group with a compound having an alkyl group capable of reacting with the reactive group. Examples of the reactive group include hydroxyl groups, amino groups, carboxyl groups, and acid anhydrides. Examples of compounds having these reactive groups include polyvinyl alcohol, polyethyleneimine, polyethyleneamine, reactive group-containing polyester resins, and reactive group-containing poly(meth)acrylic resins. Of these, polyvinyl alcohol is preferred due to its ease of handling.

Examples of compounds having an alkyl group capable of reacting with the above-mentioned reactive groups include long-chain alkyl group-containing isocyanates such as hexyl isocyanate, octyl isocyanate, decyl isocyanate, lauryl isocyanate, octadecyl isocyanate, and behenyl isocyanate; long-chain alkyl group-containing acid chlorides such as hexanoyl chloride, octanoyl chloride, decanoyl chloride, lauroyl chloride, octadecanoyl chloride, and behenoyl chloride; long-chain alkyl group-containing amines; and long-chain alkyl group-containing alcohols. Among these, considering ease of handling, long-chain alkyl group-containing isocyanates are preferred, with octadecyl isocyanate being particularly preferred.

In addition, polymeric compounds having long-chain alkyl groups in the side chains can also be obtained by polymerizing long-chain alkyl (meth)acrylates or copolymerizing long-chain alkyl (meth)acrylates with other vinyl group-containing monomers. Examples of long-chain alkyl (meth)acrylates include hexyl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, lauryl (meth)acrylate, octadecyl (meth)acrylate, and behenyl (meth)acrylate.

(binder resin)
A binder resin may be used in combination to improve coatability onto polyester film. The binder resin is preferably a polymer compound having a number-average molecular weight (Mn) of 1,000 or more as measured by gel permeation chromatography (GPC) in accordance with the "Flow Scheme for the Safety Evaluation of Polymeric Compounds" (November 1985, sponsored by the Chemical Substances Council), and also having film-forming properties. There are no particular limitations on such binder resins, and conventionally known binder resins can be used. Examples include (meth)acrylic resins, polyvinyl alcohol, polyester resins, and polyurethane resins. Among these, from the viewpoint of high hydrophilicity and film formation, the (B) binder resin is preferably at least one selected from (meth)acrylic resins and polyvinyl alcohols, and more preferably a (meth)acrylic resin. Note that the (B) binder resin may be used alone or in combination of two or more.

(Crosslinking agent)
A crosslinking agent may be used in combination to impart durability to the resin layer A. The crosslinking agent is not particularly limited, and any conventionally known crosslinking agent can be used. Examples include melamine compounds, oxazoline compounds, epoxy compounds, carbodiimide compounds, isocyanate compounds, and silane coupling compounds. Among these, from the viewpoint of imparting durability to the resin layer A, it is preferable to include at least one selected from melamine compounds and oxazoline compounds.

<Release Layer Composition>
The release layer composition is preferably a composition containing a curable silicone resin as a main component resin. Here, the term "main component resin" refers to the resin with the largest mass ratio among the resins constituting the release layer composition, and is assumed to account for 50 mass % or more, 75 mass % or more, 90 mass % or more, or 100 mass % of the resins constituting the release layer composition.

(curable silicone resin)
In the present invention, the term "curable silicone resin" refers to a silicone resin that has the property of being able to be cured.
The curing method of the curable silicone resin is optional, and may be, for example, a resin that cures by a condensation reaction in response to moisture in the air, a resin that cures by an addition reaction caused by heating, or a resin that cures by addition polymerization or radical polymerization caused by light.
Among these, thermosetting silicone resins that cure by an addition reaction caused by heating are preferred, since they do not produce by-products in the curing reaction and the physical properties of the cured film are stable.
When the curable silicone resin is a thermosetting type, it is characterized in that an alkenyl group such as a vinyl group (Vi group) or a hexenyl group is introduced into the side chain and/or the terminal of the main chain consisting of a siloxane bond in the structure.

The curable silicone resin may be any polymer having a main chain of siloxane bonds consisting of silicon and oxygen.
Examples of the curable silicone resin include those in which various substituents have been introduced into the side chains or terminals of the main chain consisting of siloxane bonds, such as methyl groups, phenyl groups, polyethers, epoxy groups, amines, carboxyl groups, aralkyl groups, and the like, or combinations of two or more of these groups.

Another example of a curable silicone resin is a silicone resin in which an Si—H group and an alkenyl group such as a vinyl group are introduced into the side chain and/or terminal of a linear main chain formed of siloxane bonds.
When the curable silicone resin has Si—H groups in the silicone resin, the content of Si—H groups is preferably 0.8 to 2.2 mol % relative to the total amount of siloxane components, more preferably 1.2 mol % or more, particularly preferably 1.4 mol % or more, and more preferably 2.1 mol % or less, particularly preferably 2.0 mol % or less.
Furthermore, when the curable silicone resin into which alkenyl groups have been introduced does not itself have Si—H groups and Si—H groups are added as a crosslinking agent, the content of Si—H groups is preferably 0.8 to 2.2 mol % of the total amount of siloxane components in the curable silicone resin, more preferably 1.2 mol % or more, particularly preferably 1.4 mol % or more, more preferably 2.1 mol % or less, and particularly preferably 2.0 mol % or less. Note that the silicone crosslinking agent described below is preferred as the crosslinking agent.
The content of vinyl groups in the curable silicone resin is preferably 0.4 to 1.5 mol % of the total amount of siloxane components, more preferably 0.5 mol % or more, even more preferably 0.6 mol % or more, and even more preferably 1.3 mol % or less.

The curable silicone resin may be a combination of two or more types of curable silicone resins. In this case, the content ratio of each of the above-mentioned functional groups is preferably such that the content of vinyl groups and the content of Si—H groups in the total of the two or more types of curable silicone resins are within the above-mentioned ranges.
When two or more types of curable silicone resins are used, for example, a curable silicone resin (main component) having an alkenyl group such as a vinyl group and a silicone resin (also called a silicone crosslinking agent) having a Si—H group can be used in combination. The release layer composition can be appropriately cured by mixing the main component and the silicone crosslinking agent and causing an addition reaction by heating.
The content of vinyl groups in the curable silicone resin constituting the main component is preferably 0.4 to 2.5 mol%, more preferably 0.5 mol% or more, even more preferably 0.6 mol% or more, and more preferably 2.0 mol% or less, even more preferably 1.5 mol% or less.
An example of a silicone crosslinking agent is a silicone resin in which an Si—H group and an alkenyl group such as a vinyl group are introduced into the side chain and/or terminal of a linear main chain formed of siloxane bonds. The silicone crosslinking agent may constitute a part of the curable silicone resin.

The number average molecular weight (Mn) of the curable silicone resin is preferably 9,000 or more and 350,000 or less.
When the number average molecular weight (Mn) of the curable silicone resin is set to the above lower limit or more, when an adhesive layer is laminated on a release film, the amount of low molecular weight silicone resin that elutes or migrates into the adhesive layer can be reduced, and by applying a thick release layer, it becomes easier to obtain a light release effect. On the other hand, when the number average molecular weight (Mn) of the curable silicone resin is set to the above upper limit or less, it prevents the viscosity from increasing and the fluidity of the release layer composition from decreasing. Therefore, when the release layer composition is applied, it is possible to prevent streaky coating unevenness from occurring, and it is easy to smooth the surface of the release layer.
From this viewpoint, the number average molecular weight (Mn) of the curable silicone resin is more preferably 10,000 or more, even more preferably 20,000 or more, and particularly preferably 30,000 or more. On the other hand, it is more preferably 50,000 or less, and even more preferably 40,000 or less.

From the same perspective as the number average molecular weight, the mass average molecular weight (Mw) of the curable silicone resin is preferably 10,000 to 500,000, more preferably 20,000 or more, even more preferably 50,000 or more, and especially preferably 80,000 or more, and more preferably 250,000 or less, even more preferably 100,000 or less.

The ratio (Mw/Mn) of the mass average molecular weight (Mw) to the number average molecular weight (Mn) of the curable silicone resin is preferably 1.7 to 2.7, more preferably 1.9 or more, and even more preferably 2.5 or less. By satisfying this range, it is expected that the crosslinking reaction will proceed efficiently.
The number average molecular weight (Mn) and mass average molecular weight (Mw) are values determined by gel permeation chromatography (GPC) measurement using polystyrene as a standard. Specifically, a chromatogram is measured using a GPC measurement device, and the number average molecular weight (Mn) and mass average molecular weight (Mw) are determined based on a calibration curve using standard polystyrene. Specifically, 4 mg of the measurement sample is dissolved in 4 mL of THF to prepare a measurement solution, and 100 μL of the measurement solution is injected into the GPC measurement device for measurement. Tetrahydrofuran (THF) was used as the eluent. For the analysis, an "Ecosec 8320" manufactured by Tosoh Corporation was used, and a "TSKgel guard column HXL-L" manufactured by Tosoh Corporation and four "TSKgel GMHXL" columns manufactured by Tosoh Corporation were used in conjunction. The analysis was carried out under conditions of an oven temperature of 40° C. and a THF flow rate of 1.0 mL/min, and RI was used for detection.

The curable silicone resin may be a combination of two or more types of curable silicone resins, in which case the average molecular weight of the two or more types of curable silicone resins is preferably within the above range, where the average is a weighted average weighted by the mass of each resin.
Furthermore, when a base resin and a silicone crosslinking agent are used as the curable silicone resin, it is preferable that the number average molecular weight, mass average molecular weight, and Mw/Mn of the base resin are within the above ranges.

The curable silicone resin preferably has a viscosity of 1 to 400 mcps at 25° C. when diluted with n-heptane solvent to a concentration of 15% by mass.
If the viscosity of the present curable silicone resin is 1 mcps or more, the appropriate viscosity of the coating liquid suppresses repelling and results in a good coating appearance, which is preferable. If the viscosity is 400 mcps or less, the fluidity of the release layer composition can be maintained, the occurrence of streaky coating unevenness when the release layer composition is applied can be suppressed, and the release layer surface can be made smooth.
From this viewpoint, the viscosity of the curable silicone resin is more preferably 1.0 mcps or more, more preferably 10 mcps or more, and more preferably 300 mcps or less, more preferably 200 mcps or less.
The viscosity was measured by diluting the curable silicone resin with n-heptane to 15% by mass, and measuring the viscosity of this solution at 25° C. using an E-type viscometer (TVE-22L, manufactured by Toki Sangyo Co., Ltd.).

The curable silicone resin may be either a solventless curable silicone or a solvent curable silicone, or a combination of both.
Here, "solventless curable silicone" refers to a silicone with a viscosity that allows it to be applied without diluting it with a solvent, and is a silicone resin with a relatively low molecular weight that is made up of short polysiloxane chains.
On the other hand, "solvent-curable silicone" is a silicone resin that is so viscous that it cannot be applied without being diluted with a solvent, and is a silicone that has a relatively high molecular weight compared to solventless-curable silicone.
Among these, it is preferable that the curable silicone resin has a medium number average molecular weight (Mn) and a medium viscosity, as described above, and furthermore, it is preferable that it is a solvent-type curable silicone, from the viewpoints that it has good adhesion to the substrate film, has a good coating appearance without uneven coating, and makes it easy to adjust the film thickness of the release layer.

When the release layer composition contains a certain amount of vinyl groups relative to the total amount of siloxane components, the release layer is sufficiently cured, and on the other hand, by not containing an excessive amount of alkenyl groups, i.e., vinyl groups, it is possible to prevent the peel force after exposure to air from becoming too strong. For these reasons, the release layer composition preferably contains 0.4 to 1.0 mol% of vinyl groups relative to the total amount of siloxane components. Among these, it is more preferable that the release layer composition contains 0.5 mol% or more of vinyl groups, and even more preferably 0.6 mol% or more, and even more preferably 0.9 mol% or less of vinyl groups.
When the release layer composition contains a certain amount of Si-H groups relative to the total amount of siloxane components, the release layer will be sufficiently cured, and on the other hand, if the amount of Si-H groups is not excessive, the Si-H groups remaining after curing will be prevented from reacting with the adhesive layer, and heavy peeling of the release film can be suppressed. For these reasons, the release layer composition preferably contains 0.8 to 2.2 mol% of Si-H groups relative to the total amount of siloxane components, more preferably 1.2 mol% or more, particularly preferably 1.4 mol% or more, and more preferably 2.1 mol% or less, and particularly preferably 2.0 mol% or less of Si-H groups.

The release layer composition may contain components other than the curable silicone resin, such as catalysts, reaction control agents, crosslinking agents, polymerization initiators, diluting solvents, light release agents, release control agents, and other additives, as needed.

The total content of siloxane components in the release layer composition can be measured, for example, by ^1H -NMR from the integral ratio of the dimethylsiloxane unit in the main chain to other units. The vinyl group and Si—H group contents represent the ratio to the total amount of functional groups bonded to the siloxane chain, and can be evaluated by measuring ^1H -NMR. However, the present invention is not limited to this method.

The release layer, i.e., the release layer composition, is substantially free of particles. By being substantially free of particles, the release layer can stabilize release properties while reducing migration. Note that "substantially free" means that the release layer (release layer composition) may contain particles in such a small amount that the effects of the present invention are not impaired; for example, it may contain particles that are unavoidably mixed in. The specific particle content in the release layer is, for example, less than 0.05% by mass, preferably less than 0.01% by mass, and more preferably less than 0.0001% by mass, based on non-volatile components. Note that the range of the particle content in this release layer composition based on non-volatile components is the same as the particle content described above.

(Light release agent)
The release layer composition may contain, in addition to the curable silicone resin, a light release agent, which is preferably a silicone oil having a dimethylsiloxane skeleton (DM) represented by the following formula (I) and a methylphenylsiloxane skeleton (MP) represented by the following formula (II):

By having a dimethylsiloxane skeleton (DM) and a methylphenylsiloxane skeleton (MP), for example, when the other surface to which the light release agent is attached is an adhesive layer, even if the light release agent migrates to the adhesive layer, it is able to penetrate into the adhesive layer, thereby reducing a decrease in adhesive strength.
The molar ratio (DM:MP) of the dimethylsiloxane skeleton (DM) represented by formula (I) above to the methylphenylsiloxane skeleton (MP) represented by formula (II) above is preferably in the range of 98:2 to 70:30, more preferably 95:5 to 80:20, and particularly preferably 92:8 to 85:15.
By setting DM:PM within the above range, the releasability of the present release film can be ensured.
The light release agent preferably has a weight average molecular weight of less than 10,000. A light release agent having a weight average molecular weight of less than 10,000 is advantageous in terms of migration and light release properties.

The content of the light release agent in the release layer composition is preferably in the range of 0.1 to 10 parts by mass, more preferably in the range of 0.5 to 8 parts by mass, and even more preferably in the range of 0.8 to 5 parts by mass, relative to 100 parts by mass of the curable silicone resin.
When the content of the light release agent is equal to or greater than the above lower limit, light release can be achieved, and when it is equal to or less than the above upper limit, adhesion to the substrate is good.

On the other hand, when it is desired to increase the peel strength of the release layer, a release control agent (MQ resin (B)) can be used in combination.
The MQ resin (B) is a silicone resin containing siloxane units represented by M units [R ₃ SiO _1/2 (wherein R is a monovalent hydrocarbon group)] and siloxane units represented by Q units [SiO _4/2 ]. The MQ resins may be used alone or in combination of two or more, and may contain D units [R ₂ SiO _2/2 (wherein R is a monovalent hydrocarbon)] or T units [RSiO _3/2 (wherein R is a monovalent hydrocarbon)]. Furthermore, it is preferable that the functional groups contained in the molecule do not contain either alkenyl groups or Si—H groups. If the MQ resin contains either alkenyl groups or Si—H groups, the amount of unreacted components (Si—H groups or alkenyl groups) remaining in the release layer is likely to increase, making it more likely to experience heavy release.

In MQ resin (B), the ratio of M units to Q units (molar ratio) is preferably 0.1:1.0 to 1.0:1.0, more preferably 0.7:1.0 to 1.0:1.0. If the M units (molar ratio) is less than 0.1, the crosslink density will decrease and the release force may not be sufficiently strong. On the other hand, if the Q units (molar ratio) is greater than 1.0, the coating film may become too hard and brittle.

Specific commercially available examples of MQ resin (B) include KS-3800 and X-92-183 manufactured by Shin-Etsu Chemical Co., Ltd., and SD7292, BY24-843, and BY24-4980 manufactured by Dow Corning Toray Co., Ltd.

The mass ratio ((B)/(A)) of the curable silicone resin (A) to the MQ resin (B) is preferably in the range of 0.01 to 1.5, and even more preferably 0.05 to 1.0. When (B)/(A) is 0.01 or greater, the peel force can be made sufficiently strong. On the other hand, when (B)/(A) is 1.5 or less, poor appearance of the release layer caused by the poor film-forming properties of the MQ resin does not occur.

<Other ingredients>
In addition to the curable silicone resin, the release layer composition may contain, as necessary, for example, a curing catalyst, a diluent solvent, a reaction control agent, a crosslinking agent other than the silicone crosslinking agent described above, a polymerization initiator, an adhesion promoter, and other additives.
For example, an example of a release layer composition may include a composition containing a curable silicone resin (main component) having an alkenyl group such as a vinyl group, a silicone crosslinking agent having a hydrosilyl group (Si-H group) on the side chain and/or end of the main chain consisting of a siloxane bond, a catalyst containing platinum (Pt) (platinum-based catalyst), and a solvent.

(catalyst)
As described above, the release layer composition may optionally contain a curing catalyst, i.e., a catalyst for promoting the hydrosilylation addition reaction between the alkenyl group bonded to the silicon atom of the curable silicone resin and the hydrogen silane (SiH) group of the silicone crosslinker.
Examples of the curing catalyst include, but are not limited to, platinum black, platinic chloride, chloroplatinic acid, reaction products of chloroplatinic acid with monohydric alcohols, complexes of chloroplatinic acid with olefins, platinum-based catalysts such as platinum bisacetoacetate, palladium-based catalysts, and rhodium-based catalysts.

The content of the curing catalyst in the release layer composition is preferably 0.5 to 500 ppm by mass, more preferably 5 ppm by mass or more, and even more preferably 10 ppm by mass or more, more preferably 300 ppm by mass or less, and even more preferably 200 ppm by mass or less, in terms of metal equivalent, relative to the curable silicone resin.

(Dilution solvent)
As described above, the release layer composition may contain a diluent solvent, if necessary. Examples of diluent solvents include aromatic hydrocarbons such as toluene, aliphatic hydrocarbons such as hexane, heptane, and isooctane, esters such as ethyl acetate and butyl acetate, ketones such as ethyl methyl ketone (MEK) and isobutyl methyl ketone, alcohols such as ethanol and 2-propanol, and ethers such as diisopropyl ether and dibutyl ether. These are preferably used alone or in combination, taking into consideration solubility, coatability, boiling point, and the like. The diluent solvent is a volatile component that volatilizes upon drying after application.

(Reaction inhibitor)
As described above, the release layer composition may contain a reaction inhibitor, if necessary. Examples of reaction inhibitors include acetylene alcohol. One type of reaction inhibitor may be used, or two or more types may be used in combination, if necessary. The content of the reaction inhibitor is preferably 0.001 to 5.0 parts by mass per 100 parts by mass of the total amount of the release layer composition (based on non-volatile components), more preferably 0.01 parts by mass or more, even more preferably 0.05 parts by mass or more, and more preferably 1.0 part by mass or less, and even more preferably 0.5 parts by mass or less.

<Thickness of Resin Layer A>
The thickness of the resin layer A is preferably in the range of 0.05 to 1.5 μm to achieve a predetermined peel strength. If the thickness of the release layer is less than 0.05 μm, it may be difficult to control the peel strength of the resin layer A, as described below. On the other hand, if the thickness is greater than 1.5 μm, it may increase the migration of the release agent component to the opposing adherend (e.g., adhesive tape, etc.) to which it is attached, or blocking may not be sufficiently prevented. From the viewpoint of preventing blocking and suppressing the increase in migration, the thickness of the resin layer A is preferably in the range of 0.1 to 1.3 μm, more preferably 0.1 to 1.0 μm.

Resin layer A can be formed, for example, by applying the release layer composition to at least one side of a copolymer polyester film and then drying and curing it as appropriate. The release layer composition can be applied by in-line coating on the molding line during the molding of the copolymer polyester film, or by offline coating after the film has been molded.

<<Other demographics>>
The present release film may have "another layer" between the present substrate film and the present resin layer A.
Examples of the "other layer" include layers having various functions such as an antistatic layer, an oligomer sealing layer, etc. Furthermore, the layer provided between the present copolymer polyester film and the present resin layer A is preferably a cured resin layer.

<Uses of release film>
As described above, the release film has excellent flexibility at room temperature, and is not only flexible but also has a large elongation rate, and yet can exhibit heat resistance sufficient for practical use.
The release film has the above-mentioned properties and is therefore suitable for use in various stretching processes, and is particularly suitable for use in stretch molding processes in which various molded products are obtained by stretching.

The release film of the present invention has excellent flexibility in the low temperature range, and is not only flexible but also has a high elongation rate. At the same time, it has a moderate storage modulus in the high temperature range, which is an opposing characteristic, and is therefore heat resistance sufficient for practical use. Therefore, it is useful for semiconductor manufacturing, and is particularly useful as a release film for semiconductor compression molding, used in compression molding.

<Explanation of terms, etc.>
In the present invention, the term "film" includes the term "sheet", and the term "sheet" includes the term "film".
Furthermore, when the term "panel" is used, such as an image display panel or a protective panel, it encompasses a plate, a sheet, and a film.

In this invention, when it is stated that a value is "X to Y" (X and Y are arbitrary numbers), unless otherwise specified, it means "X or more and Y or less," and also includes the meanings "preferably larger than X" or "preferably smaller than Y."

Furthermore, when it is stated that the quantity is "X or more" (X is any number), it also means that the quantity is "preferably greater than X" unless otherwise specified, and when it is stated that the quantity is "Y or less" (Y is any number), it also means that the quantity is "preferably smaller than Y" unless otherwise specified.

Next, the present invention will be explained in more detail using examples. However, the present invention is not limited to the examples described below.

<Evaluation method>
In the following, measurements and evaluations of various physical properties were carried out as follows.

(1) Film Thickness After fixing a small piece of film with epoxy resin, it was cut with a microtome and the cross section of the film was observed using a transmission electron microscope. Two interfaces were observed in the cross section, roughly parallel to the film surface, as light and dark regions. The distances between the two interfaces and the film surface were measured from 10 photographs, and the average value was taken as the film thickness.

(2) Tensile storage modulus E'
Based on JIS K 7244, a dynamic viscoelasticity measuring device DVA-200 manufactured by IT Measurement & Control Co., Ltd. was used to measure the length (MD) direction and width (TD) direction of the sample films obtained in the Examples and Comparative Examples from -100°C to 200°C at a vibration frequency of 10 Hz, a strain of 0.1%, and a heating rate of 1°C/min. From the obtained data, the tensile storage moduli E' at 80°C and 180°C were obtained, and the average values in the length (MD) and width (TD) were used to determine the storage moduli at 80°C and 180°C. In addition, the ratio of the storage moduli was also calculated from the storage moduli at 80°C and 180°C.

(3) Softening Temperature Measurement The softening temperature of each polyester resin layer of the sample film was measured using a nano TA measurement device (AFM5300E manufactured by Hitachi High-Tech Science Corporation). Measurements were performed on the surface layer and the intermediate layer under the following measurement conditions, and the average value of N=3 was used as the softening temperature of each layer.
Regarding the measured values, tangent lines were drawn to the temperature rise curve and the temperature fall curve from the obtained measurement chart, and the intersections of the tangent lines were determined.
Next, the point where the line passing through the obtained intersection point and perpendicularly intersecting with the measurement temperature axis was determined as the softening temperature.
<<Measurement conditions>>
Measurement mode: Nano TA mode Probe: Thermal cantilever AN2-300
Measurement temperature range: room temperature (23°C)
Measurement atmosphere: atmospheric pressure

(4) Solvent Resistance After immersing the sample film in each solvent for 24 hours, the condition was visually observed and judged according to the following criteria.
(Judgment criteria)
A: No change in appearance or flatness.
B: The film curls.
C: Poor solvent resistance, film dissolves.

(5) Stretchability (substitute evaluation of mold step-following ability)
The sample film was stretched at a stretching ratio of 3.5 (corresponding to an elongation rate of 250%) using a film biaxial stretching device manufactured by Imoto Seisakusho.
The stretchability of the obtained film was judged according to the following criteria.
(Judgment criteria)
A: The stretchability is particularly good (elongation at break at room temperature is 300% or more).
B: Good stretchability (elongation at break at room temperature of 250% or more).
C: Poor stretchability (elongation at break at room temperature is less than 250%).

(raw materials)
The following raw materials were used in the examples and comparative examples.

(1) Copolymer polyester (a1-1)
A copolymer polyester (a1-1) (intrinsic viscosity (IV) 0.70 dl/g) was prepared, which contained terephthalic acid and adipic acid (having 6 carbon atoms) as dicarboxylic acid components, with the terephthalic acid content being 85% by mass and the adipic acid content being 15% by mass, and the alcohol components being 45% by mass of 1,4-butanediol and 55% by mass of 1,6-hexanediol.

(2) Copolymer polyester (a1-2)
A copolymer polyester (a1-2) (intrinsic viscosity (IV) 0.70 dL/g) was prepared, which contained terephthalic acid and isophthalic acid as dicarboxylic acid components, with the terephthalic acid content being 78% by mass and the isophthalic acid content being 22% by mass, and which contained ethylene glycol 98% by mass and diethylene glycol 2% by mass as alcohol components.

(3) Homopolyester (a2-1)
A homopolyester (a2-1) was prepared, which was a polyester having a dicarboxylic acid component of terephthalic acid and an alcohol component of 98% by mass of ethylene glycol and 2% by mass of diethylene glycol, and had an intrinsic viscosity (IV) of 0.64 dl/g.

(4) Homopolyester (a2-2)
A particle-containing homopolyester (a2-2) (particle-containing homoPET) was prepared. The polyester had a dicarboxylic acid component of terephthalic acid and alcohol components of 98% by mass of ethylene glycol and 2% by mass of diethylene glycol, and had an intrinsic viscosity (IV) of 0.62 dL/g and contained 0.2% by mass of silica particles having an average particle size of 3 μm.

[Example 1-1]
Chips of a polyester composition containing 35% by mass of copolymer polyester (a1-1), 50% by mass of homopolyester (a2-1), and 15% by mass of homopolyester (a2-2) were fed into a main vented twin-screw extruder set at 270°C.
The extrusion was performed through a die and rapidly cooled and solidified on a cooling roll whose surface temperature was set at 25° C. using an electrostatic application adhesion method, to obtain an unstretched sheet.

The unstretched sheet was then stretched 3.0 times in the machine direction (MD) at 60°C, introduced into a tenter, and then stretched 4.3 times in the width direction (TD) at 85°C, after which it was heat-treated at 230°C for 10 seconds to obtain a copolymer polyester film (sample) having a thickness of 50 μm.
Next, a resin layer A made of the following resin layer composition was applied to a thickness (after drying) of 0.1 μm, and after drying at 120° C. for 20 seconds, a release film (sample film) with a thickness of 50 μm was obtained.
(Resin composition)
Curable silicone resin (Dow Toray LTC856): 100 parts by mass Curing agent (Dow Toray SRX-212): 1 part by mass Solvent Toluene: 600 parts by mass MEK: 600 parts by mass Hexane: 600 parts by mass

[Example 1-2]
A release film (sample film) having a thickness of 50 μm was obtained in the same manner as in Example 1, except that the type of resin layer A was changed.
(Resin composition)
Curable silicone resin (X-62-5039 manufactured by Shin-Etsu Chemical Co., Ltd.): 80 parts by mass Curable silicone resin (KS-3800 manufactured by Shin-Etsu Chemical Co., Ltd.): 20 parts by mass Curing agent (PL-5000 manufactured by Shin-Etsu Chemical Co., Ltd.): 2 parts by mass Solvent Toluene: 600 parts by mass MEK: 600 parts by mass Hexane: 600 parts by mass

[Example 2-1]
Chips of a polyester composition containing 35% by mass of copolymer polyester (a1-1) and 65% by mass of homopolyester (a2-1) were fed into a main vented twin-screw extruder set at 270°C for the intermediate layer (copolymer polyester layer (A1)), and chips of a polyester composition containing 35% by mass of copolymer polyester (a1-1), 35% by mass of homopolyester (a2-1), and 30% by mass of homopolyester (a2-2) were fed into another vented twin-screw extruder set at 270°C for the surface layers (polyester layers (B1) and (B2)). The polyester raw materials were co-extruded from the die and rapidly solidified on a cooling roll set at 25°C using an electrostatic adhesion method, to obtain an unstretched sheet.

The resulting unstretched sheet was then stretched 3.0 times in the machine direction (MD) at 60°C, introduced into a tenter, and then stretched 4.3 times in the width direction (TD) at 85°C, after which it was heat-treated at 230°C for 10 seconds to obtain a 50 μm thick copolymer polyester film (sample) with a multilayer structure of polyester layer (B1)/copolymer polyester layer (A1)/polyester layer (B2). Next, resin layer A, consisting of the resin layer composition used in Example 1-1, was applied to a thickness (after drying) of 0.1 μm, and after drying at 120°C for 20 seconds, a 50 μm thick release film (sample film) was obtained.

[Example 2-2]
A release film (sample film) having a thickness of 50 μm was obtained in the same manner as in Example 2-1, except that the resin composition for forming the resin layer A was changed to that used in Example 1-2.

[Examples 2-3 and 2-5]
The same procedure as in Example 2-1 was carried out except that the compositions of the polyesters for the intermediate layer and surface layer were changed as shown in Table 1, to obtain release films (sample films) having a thickness of 50 μm.

[Examples 2-4 and 2-6]
The same procedure as in Example 2-1 was carried out, except that the compositions of the polyesters for the intermediate layer and the surface layer were changed as shown in Table 1 and the resin composition for forming the resin layer A was changed to that used in Example 1-2, to obtain a release film (sample film) having a thickness of 50 μm.

[Comparative Example 1]
Chips of a polyester composition containing 85% by mass of copolymer polyester (a1-2) and 15% by mass of homopolyester (a2-2) were fed into a main vented twin-screw extruder set at 270°C.
The extruded material was extruded from a die and rapidly cooled and solidified on a cooling roll whose surface temperature was set at 25° C. using an electrostatic application adhesion method, to obtain an unstretched sheet.

The resulting unstretched sheet was then stretched 3.4 times in the machine direction (MD) at 80°C, introduced into a tenter, and then stretched 3.9 times in the transverse direction (TD) at 105°C, after which it was heat-treated at 185°C for 10 seconds to obtain a 50μm-thick copolymer polyester film (sample).

[Comparative Example 2]
Chips of a polyester composition containing 92% by mass of homopolyester (a2-1) and 8% by mass of homopolyester (a2-2) were fed into a main vented twin-screw extruder set at 280°C.
The extruded material was extruded from a die and rapidly cooled and solidified on a cooling roll whose surface temperature was set at 25° C. using an electrostatic application adhesion method, to obtain an unstretched sheet.

The resulting unstretched sheet was then stretched 3.4 times in the machine direction (MD) at 90°C, introduced into a tenter, and then stretched 4.5 times in the transverse direction (TD) at 145°C, after which it was heat-treated at 222°C for 10 seconds to obtain a polyester film (sample) with a thickness of 50 μm.

[Reference example]
The following film samples were also evaluated as reference examples.
ETFE: Aflex manufactured by AGC, 50 μm

*The thickness of each layer is the thickness of the entire copolymer polyester film for single-layer copolymer polyester films, and the thickness of each layer, including the surface layer, intermediate layer, and surface layer, for multi-layer structures.

*In Table 2, the mol% of the dicarboxylic acid component and the alcohol component are the proportions relative to the dicarboxylic acid component and the alcohol component, respectively, in all of the polyester (A) (or polyester (Z)) contained in the copolymer polyester film (copolymer polyester layer (A1)).

*In Table 3, the mol % of the dicarboxylic acid component and the alcohol component is the ratio to the dicarboxylic acid component and the alcohol component, respectively, in all the polyesters (Z) contained in the copolymer polyester film.

From the above examples and the results of tests conducted by the inventors, it has been found that in a release film comprising a resin layer A on at least one side of a copolymer polyester film containing a copolymer polyester, if the storage modulus at 80° C. is 1000 MPa or less, the film has excellent flexibility at an ambient temperature of about 80° C. Furthermore, not only is the film flexible, but by satisfying that the storage modulus at 180° C. is 30 MPa or more and the ratio of the storage modulus at 80° C. to the storage modulus at 180° C. is 13 or less, the film can maintain an appropriate level of strength and have heat resistance sufficient for practical use, and can also be used as a substitute for a fluororesin film release film.
In this example, the copolymerized polyester film contains a copolymerized polyester, and the copolymerized polyester is a copolymerized polyester (z1) of terephthalic acid (X1), a dicarboxylic acid component (X2) having 4 to 10 carbon atoms, and an alcohol component (Y2) other than ethylene glycol (Y1). In all the polyesters (Z) contained in the copolymerized polyester film, the proportion of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms in the dicarboxylic acid components is 1 to 30 mol %, and the proportion of the other alcohol component (Y2) in the alcohol components is 15 to 60 mol %, thereby achieving the above storage modulus.

The release film of the present invention is suitable for use in compression molding of epoxy resin sealants used in semiconductor manufacturing.

Claims (14)

A release film comprising a copolymer polyester film containing a copolymer polyester and a resin layer A on at least one side thereof, wherein the storage modulus of the release film at 80°C is 1000 MPa or less, the storage modulus at 180°C is 30 MPa or more, and the ratio of the storage modulus at 80°C to the storage modulus at 180°C is 13 or less. The copolymer polyester comprises a copolymer polyester (z1) of terephthalic acid (X1), a dicarboxylic acid component (X2) having 4 to 10 carbon atoms, and an alcohol component (Y2) other than ethylene glycol (Y1),
2. The release film according to claim 1, wherein the proportion of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms in all the dicarboxylic acid components of all the polyesters (Z) contained in the copolymerized polyester film is 1 to 30 mol %, and the proportion of the other alcohol component (Y2) in all the alcohol components is 15 to 60 mol %. The release film according to claim 2, wherein the dicarboxylic acid component (X2) having 4 to 10 carbon atoms includes an aliphatic dicarboxylic acid. The release film according to claim 3, wherein the aliphatic dicarboxylic acid includes adipic acid. The release film according to any one of claims 2 to 4, wherein the other alcohol component (Y2) includes 1,4-butanediol. The release film according to any one of claims 2 to 4, wherein the other alcohol component (Y2) includes 1,4-butanediol and 1,6-hexanediol. The release film according to claim 1, which is a copolymer polyester film having a copolymer polyester layer (A1) containing a copolymer polyester (a1) as the copolymer polyester. The release film according to claim 7, wherein the copolymer polyester layer (A1) is a polyester other than the copolymer polyester (a1) and further contains a polyester (a2) containing terephthalic acid as a dicarboxylic acid component and ethylene glycol as an alcohol component. The release film according to any one of claims 7 to 8, comprising a polyester layer (B1) and a polyester layer (B2) on both the front and back surfaces of the copolymer polyester layer (A1). The release film according to claim 9, wherein the temperature difference between the copolymer polyester layer (A1) and at least one of the polyester layer (B1) and polyester layer (B2), as measured by softening temperature measurement using NanoTA, is 5°C or more. The release film according to claim 1, wherein resin layer A contains a silicone-based resin. The release film according to claim 1, wherein resin layer A contains a non-silicone resin. The release film according to any one of claims 1 to 4, 7, 8, 11, and 12, which is used in semiconductor manufacturing. The release film according to claim 13, which is for use in compression molding.