TWI875841B - Copolyester film, laminated film and methods of using the same - Google Patents

Abstract

The copolyester film of the present invention comprises a copolyester layer (A1) containing a copolyester (a1), wherein the copolyester (a1) is a copolymer comprising terephthalic acid (X1), a dicarboxylic acid component having 4 to 10 carbon atoms (X2), and an alcohol component (Y2) other than ethylene glycol (Y1), and the difference (ΔTcg) between the cold crystallization temperature (Tcc) and the glass transition temperature (Tg) exceeds 60°C.

Description

Copolyester film, laminated film and methods of using the same

The present invention relates to a copolyester film having a copolyester layer containing copolyester, a laminated film having the copolyester film, and methods of using the same.

The representative polyester film is polyethylene terephthalate (PET) film, especially biaxially stretched PET film, which is excellent in transparency, mechanical strength, heat resistance, flexibility, etc., and is therefore used in a wide range of fields such as industrial materials, optical materials, electronic component materials, and battery packaging materials.

Regarding such a polyester film, for example, in Patent Document 1, a softened polyester film is proposed as a soft polyester film that exhibits softness that previous polyester films do not have and has excellent formability at relatively low temperatures and low pressures. The characteristics of the softened polyester film are: the elastic modulus E' of the film is less than 20 MPa at 120°C and less than 5 MPa at 180°C, the film haze is less than 1.0%, and it contains 29 to 32 mol% of 1,4-cyclohexanedimethanol units as a diol component and does not contain isophthalic acid units as a dicarboxylic acid component.

In recent years, as image display devices, computers that are small enough to be worn on the body (wearable computers) have attracted much attention as mobile terminals have become smaller and more powerful. It is ideal to have an electronic device (wearable terminal) used for a wearable computer installed in a person's belongings such as a card, bag, watch, clothes, and shoes (Patent Document 2).

In addition, as the next generation of image display devices, flexible displays that can be bent freely are attracting attention. Organic electroluminescent (organic EL) displays are mainly used in flexible displays. Since flexible displays use thinner glass substrates or plastic substrates, the polyester film used in the components of such image display devices requires not only the optical properties and durability required of previous flat display panels, but also flexibility such as not generating creases even when subjected to bending tests.

Furthermore, as one of the expanded uses of soft polyester films, multilayer films with various functions have attracted much attention. The multilayer films adopt the so-called multilayer stretching technology, that is, heterogeneous materials are formed into a multilayer structure and stretched (non-patent document 1). For example, as a combination of heterogeneous materials, EVA resin-PVC, EVA-polyester, EVA-low-density polyethylene, PVA-PVC, PVA-polyester, PVA-low-density polyethylene, etc. are exemplified. As for the functions given, gas barrier properties, heat sealing properties, scratch resistance, printability, moisture resistance, etc. are exemplified.

In recent years, the following situation has emerged: With the high performance of communication equipment such as smart phones and electronic equipment such as laptops, the electronic components they carry also need to be miniaturized and have higher performance. For example, when focusing on ceramic multilayer capacitors, there is a tendency for the thickness of the green sheets used to be thinner, and it is necessary to stably produce thin film green sheets of about 0.5 μm to 1 μm. In the previous coating method, a relatively low viscosity slurry diluted with a solvent is usually coated on a base film and dried to form a sheet.

However, after coating with a low-concentration slurry and drying it, the obtained green sheet may have pinholes, etc., and the performance may not be sufficient. Therefore, as another method, as described in Patent Document 3, it is disclosed that in the process of coating and drying the slurry on the substrate film, the substrate film and the slurry coating layer are stretched together to obtain a green sheet. When the heterogeneous material (for example, the material constituting the slurry) as described above is layered on the substrate film and stretched in the state of forming a layered body, there is an advantage that the pinhole generation can be reduced by adjusting (for example, increasing the concentration) the slurry concentration.

Furthermore, in addition to the use of manufacturing green sheets, research is also underway to form a multilayer body by stacking a resin layer or other heterogeneous materials on a substrate film, and to perform a stretching process on the multilayer body, thereby stretching the resin layer provided on the substrate film. Previous technical literature Patent literature

Patent document 1: Japanese Patent Publication No. 2014-169371 Patent document 2: Japanese Patent Publication No. 2002-174688 Patent document 3: Japanese Patent Publication No. 10-250014 Non-patent document

Non-patent document 1: "Fibers and Industry" Biaxial Stretching Technology of Multilayer Films by Mutsuo Kuga (Vol.35, No.7 (1979))

[The problem the invention is trying to solve]

However, the stretching process of a substrate film in which a foreign material is layered is subject to the following conditions: the processing conditions are limited by the mechanical properties of the substrate film or the foreign material disposed on the substrate film, especially the elongation. For example, when a foreign material sheet is obtained by stretching a foreign material layer with a relatively thick thickness in a state where the foreign material layer is layered before the stretching process, the substrate film is required to be able to be stretched at a higher stretching ratio. The polyester film commonly used in the past, such as the copolyester film using isophthalic acid, sometimes has insufficient elongation at room temperature and is difficult to stretch.

Furthermore, as for materials that can be used as substrate films and that can be expected to have high elongation at room temperature, nylon, PVC, and PP (polypropylene) films are exemplified. However, these films often have high elongation but lack heat resistance. For example, after a heat treatment step, the shrinkage rate of the film increases, making it difficult to use for applications that require dimensional accuracy.

Therefore, the subject of the present invention is to provide a copolyester film which can take into account opposite characteristics, that is, on the one hand, it has excellent flexibility and high elongation at room temperature, and on the other hand, it has good heat resistance. In addition, a further limiting subject of the present invention is to provide a copolyester film, a laminated film and a method of using them, which is suitable for manufacturing a functional sheet of a thin film by stretching a functional layer laminated on a substrate film. [Technical means for solving the problem]

The gist of the present invention is as follows. [1] A copolyester film having a copolyester layer (A1) containing a copolyester (a1), the copolyester (a1) being a copolymer comprising terephthalic acid (X1), a dicarboxylic acid component having 4 to 10 carbon atoms (X2), and an alcohol component (Y2) other than ethylene glycol (Y1), and having a difference (ΔTcg) between a cold crystallization temperature (Tcc) and a glass transition temperature (Tg) exceeding 60°C. [2] A copolyester film having a copolyester layer (A1) containing a copolyester (a1), wherein the copolyester (a1) is a copolymer of terephthalic acid (X1), a dicarboxylic acid component (X), and an alcohol component (Y2) other than ethylene glycol (Y1), wherein the other alcohol component (Y2) comprises two or more, and wherein the difference (ΔTcg) between the cold crystallization temperature (Tcc) and the glass transition temperature (Tg) exceeds 60°C. [3] A copolyester film as described in [1] or [2] above, wherein the ratio of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms in the dicarboxylic acid components of the total polyester (A) contained in the copolyester layer (A1) is 3 to 50 mol%. [4] The copolyester film as described in any one of the above [1] to [3], wherein the ratio of the other alcohol component (Y2) to the alcohol component in the total polyester (A) contained in the above copolyester layer (A1) is 15 to 60 mol%. [5] The copolyester film as described in any one of the above [1] to [4], wherein the tensile elongation at break at 25°C is 295% or more. [6] The copolyester film as described in any one of the above [1] to [5], wherein the storage elastic modulus at 25°C is 2500 MPa or less and the storage elastic modulus at 120°C is 10 MPa or more. [7] The copolyester film as described in any one of the above [1] and [3] to [6], wherein the dicarboxylic acid component (X2) having 4 to 10 carbon atoms comprises an aliphatic dicarboxylic acid. [8] The copolyester film as described in [7] above, wherein the aliphatic dicarboxylic acid comprises adipic acid. [9] The copolyester film as described in any one of [1] to [8] above, wherein the other alcohol component (Y2) comprises 1,4-butanediol. [10] The copolyester film as described in any one of [1] to [9] above, wherein the other alcohol component (Y2) comprises 1,4-butanediol and 1,6-hexanediol. [11] The copolyester film as described in any one of [1] to [10] above, wherein the copolyester layer (A1) further comprises a polyester (a2) which is a polyester other than the copolyester (a1) and comprises terephthalic acid as a dicarboxylic acid component and ethylene glycol as an alcohol component. [12] A copolyester film as described in any one of [1] to [11] above, which has a polyester layer (B1) and a polyester layer (B2) on the front and back sides of the copolyester layer (A1), respectively. [13] A laminated film, which has a copolyester film as described in any one of [1] to [12] above, and a functional layer disposed on at least one side of the copolyester film. [14] A laminated film as described in [13] above, wherein the functional layer is a resin layer comprising a resin containing a vinyl alcohol structural unit. [15] A laminated film as described in [13] or [14] above, wherein the functional layer constitutes a blank. [16] The laminated film described in [13] or [14], wherein the functional layer constitutes a polarizing element. [17] The laminated film described in [13], wherein the functional layer is an adhesive layer. [18] The laminated film described in [17], wherein the adhesive layer contains a conductive material. [19] The laminated film described in [13], wherein the functional layer is a conductive layer. [20] A method for using a copolyester film or a laminated film, comprising the steps of: stretching the copolyester film described in any one of [1] to [12] or the laminated film described in any one of [13] to [19]. [21] A method for using a copolyester film or a laminated film as described in [20] above, wherein the stretching is performed in air or water. [22] A method for using a copolyester film or a laminated film as described in [20] or [21] above, wherein the stretching is performed at a stretching ratio of 2.0 to 6.0 times. [23] A copolyester film as described in any one of [1] to [12] above, which is used for any one of an adhesive tape, a surface protection film, and a semiconductor cutting protective tape. [24] A laminated film as described in any one of [13] to [19] above, which is used for any one of an adhesive tape, a surface protection film, and a semiconductor cutting protective tape. [25] The method of use as described in [20] or [21] above, wherein the copolyester film or laminated film is used for any one of an adhesive tape, a surface protection film, and a semiconductor cutting protection tape. [25] The copolyester film as described in any one of [1] to [12] above, which is used for any one of a wearable terminal, a bioelectrode substrate, and a biosensor substrate. [26] The laminated film as described in any one of [13] to [19] above, which is used for any one of a wearable terminal, a bioelectrode substrate, and a biosensor substrate. [27] The method of use as described in [20] or [21] above, wherein the copolyester film or laminated film is used for any one of a wearable terminal, a bioelectrode substrate, and a biosensor substrate. [28] The copolyester film as described in any one of [1] to [12] above, which is used for manufacturing electronic components. [29] The laminated film as described in any one of [13] to [19] above, which is used for manufacturing electronic components. [30] The method of use as described in [20] or [21] above, wherein the copolyester film or laminated film is used for manufacturing electronic components. [31] The copolyester film as described in any one of [1] to [12] above, which is used for manufacturing optical components. [32] The laminated film as described in any one of [13] to [19] above, which is used for manufacturing optical components. [33] The method of use as described in [20] or [21] above, wherein the copolyester film or laminated film is used to manufacture an optical component. [Effect of the invention]

The copolyester film of the present invention has excellent flexibility at room temperature. It is not only flexible but also has a large elongation. It can also have a moderately low heat shrinkage characteristic as an opposite characteristic, and has heat resistance sufficient for practical use. In addition, according to the present invention, for example, a copolyester film suitable for manufacturing a functional sheet of a thin film by stretching a functional layer laminated on a substrate film, a laminated film, and a method of using the same can be provided.

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

<Present copolyester film> The copolyester film of one embodiment of the present invention (referred to as "present copolyester film") is a single-layer film or a laminated film having a copolyester layer (A1) containing a copolyester (a1).

The copolyester film is preferably a stretched film stretched in a uniaxial direction or a biaxial direction, and may be a uniaxial stretched film or a biaxial stretched film. Among them, a biaxial stretched film is preferred in order to achieve a balance of mechanical properties and excellent planarity. If the copolyester film is such a stretched film, it is easy to make the storage elastic modulus at 120°C to be above 10 MPa, or the tensile elongation at break at 25°C to be above 295%.

<Copolyester layer (A1)> The copolyester layer (A1) is a layer containing copolyester (a1). The copolyester layer (A1) preferably contains polyester (a2) in addition to copolyester (a1). Furthermore, the copolyester layer (A1) may contain resin (a3) in addition to the copolyester (a1) and polyester (a2).

(Copolyester (a1)) In one embodiment of the present invention, the copolyester (a1) is a copolymer comprising terephthalic acid (X1), a dicarboxylic acid component having 4 to 10 carbon atoms (X2), and an alcohol component (Y2) other than ethylene glycol (Y1). The copolyester (a1) may be crystalline or amorphous. Furthermore, the dicarboxylic acid component having 4 to 10 carbon atoms (X2) refers to a dicarboxylic acid component having 4 to 10 carbon atoms other than terephthalic acid (X1). The copolyester (a1) in this embodiment is a condensation product of a dicarboxylic acid comprising terephthalic acid (X1) and a dicarboxylic acid component having 4 to 10 carbon atoms (X2) and an alcohol component comprising an alcohol component (Y2) other than ethylene glycol (Y1). The alcohol component is usually a diol component. In the present invention, by using a dicarboxylic acid component having 4 to 10 carbon atoms as the dicarboxylic acid, it is easy to ensure flexibility and elongation at low temperatures.

As the dicarboxylic acid component (X2) having 4 to 10 carbon atoms, there can be listed aromatic dicarboxylic acids, alicyclic dicarboxylic acids, aliphatic dicarboxylic acids, polyfunctional acids, etc. Among them, aliphatic dicarboxylic acids are preferred in order to achieve a good storage elastic modulus at room temperature and to improve flexibility and elongation. Aliphatic dicarboxylic acids can be used alone as the dicarboxylic acid component (X2) having 4 to 10 carbon atoms, or they can be used in combination with other dicarboxylic acid components having 4 to 10 carbon atoms. As the aliphatic dicarboxylic acids having 4 to 10 carbon atoms, there can be listed saturated aliphatic dicarboxylic acids such as succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, and sebacic acid. Among them, from the viewpoint of the ease of reaction during polymerization, adipic acid and sebacic acid are more preferred, and adipic acid is further preferred. Moreover, as an aromatic dicarboxylic acid, isophthalic acid etc. can be mentioned.

In one embodiment, the ratio of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms in the dicarboxylic acid component constituting the copolyester (a1) can be adjusted in a manner such that the ratio of (X2) in the following total polyester (A) becomes a specific range, and is not particularly limited, for example, 5 to 35 mol%, preferably 8 to 25 mol%, and more preferably 10 to 20 mol%. In another embodiment, the ratio of terephthalic acid (X1) in the dicarboxylic acid component constituting the copolyester (a1) is, for example, 65 to 95 mol%, preferably 75 to 92 mol%, and more preferably 80 to 90 mol%.

In one embodiment of the copolyester (a1), the dicarboxylic acid component may include terephthalic acid (X1) and a dicarboxylic acid component (X2) having 4 to 10 carbon atoms, and may also include a dicarboxylic acid component (other dicarboxylic acid component (X3)) other than terephthalic acid (X1) and a dicarboxylic acid component (X2) having 4 to 10 carbon atoms as a copolymer component without prejudice to the subject matter of the present invention. Examples of such dicarboxylic acid components include dodecanedioic acid, eicosanoic acid, dimer acid, and derivatives thereof. In the above embodiment, the ratio of the other dicarboxylic acid component (X3) to the dicarboxylic acid component constituting the copolyester (a1) is, for example, 10 mol% or less, preferably 5 mol% or less, more preferably 3 mol% or less, and most preferably 0 mol%.

As other alcohol components (Y2), there can be listed: aliphatic diols such as 1,4-butanediol, 1,4-hexanediol, 1,6-hexanediol, diethylene glycol, trimethylene glycol, 1,5-pentanediol, 1,8-octanediol, 1,10-decanediol, neopentyl glycol, and 2-ethyl-2-butyl-1,3-propanediol; 1,2-cyclohexanediol, 1,1-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, 2,5- Alicyclic diols such as norethanedimethanol; aromatic diols such as benzyl alcohol, 4,4'-dihydroxydiphenyl, 2,2-bis(4'-hydroxyphenyl)propane, 2,2-bis(4'-β-hydroxyethoxyphenyl)propane, bis(4-hydroxyphenyl)sulfone, bis(4-β-hydroxyethoxyphenyl)sulfonic acid; ethylene oxide adducts or propylene oxide adducts of 2,2-bis(4'-hydroxyphenyl)propane, dimer alcohol, etc. They may contain one type alone or two or more types in combination. Among them, aliphatic diols are preferred from the viewpoint of softness and crystallinity. Aliphatic diols are preferably aliphatic diols having 4 to 8 carbon atoms, and more preferably aliphatic diols 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. Furthermore, the other alcohol component (Y2) preferably contains at least 1,4-butanediol. Furthermore, the alcohol component (Y2) is preferably used in two or more kinds. When using two or more kinds, it can be appropriately selected from the above examples. Among them, the alcohol component in the copolyester (a1) is further preferably 1,4-butanediol and 1,6-hexanediol. By using two or more other alcohol components (Y2), it has the advantage that when combined with the acid component, it is easier to adjust the crystallinity than before. In addition, the copolyester (a1) may also contain ethylene glycol (Y1) as an alcohol component (copolymer component). Furthermore, any of the polyesters (A) constituting the copolyester layer (A1) may contain ethylene glycol (Y1) as an alcohol component. Therefore, when the copolyester (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 (copolymer component).

The ratio of the other alcohol component (Y2) in the alcohol component constituting the copolyester (a1) can be adjusted so that the ratio of (Y2) in the entire polyester (A) described below is within a specific range, for example, 50 to 100 mol%, preferably 70 to 100 mol%, and more preferably 90 to 100 mol%.

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

Furthermore, in another aspect of the present invention, the copolyester (a1) is a copolymer comprising terephthalic acid (X1), a dicarboxylic acid component (X), and other alcohol components (Y2) other than ethylene glycol (Y1), and other alcohol components (Y2) include two or more kinds. Therefore, the copolyester (a1) in another aspect of the present invention is a condensation product of terephthalic acid (X1) and a dicarboxylic acid of the dicarboxylic acid component (X) and an alcohol component comprising other alcohol components (Y2) other than ethylene glycol (Y1), and other alcohol components (Y2) include two or more kinds of alcohol components. Furthermore, the alcohol component is usually a diol component. Furthermore, the dicarboxylic acid component (X) refers to a dicarboxylic acid component other than terephthalic acid (X1). However, when it is described as "dicarboxylic acid" in this specification, the concept includes terephthalic acid (X1). In another embodiment, the other alcohol component (Y2) is as described above except that two or more kinds are used. The details when two or more kinds are used are also as described above.

In the present invention, by using two or more alcohol components (Y2), the following advantages are achieved: in combination with an acid component, it is easier to adjust the crystallinity than before, and the difference between Tcc and Tg described below is also easier to adjust. In particular, in the present invention, a significant effect is achieved in combination with a dicarboxylic acid component (X2) having 4 to 10 carbon atoms. Therefore, in the present invention, in another embodiment, it is also preferred that the dicarboxylic acid component (X) includes a dicarboxylic acid component (X1) having 4 to 10 carbon atoms. In this case, the details of the copolyester (a1) are as described in the copolyester (a1) of the above embodiment. However, in another embodiment, the dicarboxylic acid used in the copolyester (a1) may also include terephthalic acid (X1) and other dicarboxylic acid components (X3). In this case, the details of the other dicarboxylic acid components (X3) are as described above. In each aspect, the copolyester (a1) may be used alone or in combination of two or more.

(Intrinsic viscosity (IV) of copolyester (a1)) The intrinsic viscosity (IV) of copolyester (a1) (when two or more copolyesters (a1) are used as a polyester mixture) is preferably 0.40 dL/g to 1.20 dL/g, more preferably 0.45 dL/g or more, more preferably 0.48 dL/g or more, and more preferably 1.15 dL/g or less, more preferably 1.10 dL/g or less. If the intrinsic viscosity of copolyester (a1) is within this range, polyester with excellent molding processability can be produced without deteriorating productivity. Furthermore, regarding the intrinsic viscosity (IV) of polyester, 1 g of polyester from which components incompatible with the polyester have been removed is accurately weighed, dissolved in 100 ml of a mixed solvent of phenol/tetrachloroethane = 50/50 (weight ratio), and measured at 30°C.

The content of the copolyester (a1) in the resin component constituting the copolyester layer (A1) may be a certain ratio or more so that the ratio of each component in the following total polyester (A) is within a specific range. The copolyester (a1) may account for, for example, 20% by mass or more of the resin component constituting the copolyester layer (A1), preferably 30% by mass or more, more preferably 35% by mass or more, and more preferably 50% by mass or more, and may also account for, for example, 80% by mass or more. Furthermore, the copolyester (a1) may account for 100% by mass or less of the resin component constituting the copolyester layer (A1), for example, 70% by mass or less, and may also account for 60% by mass or less.

(Polyester (a2)) Regarding the copolyester layer (A1), the constituent resin may be only the copolyester (a1), but preferably contains the polyester (a2) in addition to the copolyester (a1). Herein, in each of the above-mentioned aspects, the polyester (a2) is a polyester other than the copolyester (a1) in each of the above-mentioned aspects, and is a homopolyester or copolyester containing terephthalic acid (X1) as a dicarboxylic acid component and ethylene glycol (Y1) as an alcohol component.

The above-mentioned homopolyester is polyethylene terephthalate in which the dicarboxylic acid component is terephthalic acid (X1) and the alcohol component is ethylene glycol (Y1). As polyester (a2), it is preferable to use homopolyester. However, when describing the case of homopolyester, the alcohol component may also contain diethylene glycol which is inevitably mixed. Specifically, in polyethylene terephthalate (homopolyester), the ratio of diethylene glycol in the alcohol component may be, for example, 5 mol% or less, and may also be 3 mol% or less. The same is true for the polyester (B) described below. Furthermore, diethylene glycol is a part of ethylene glycol modified into diethylene glycol and introduced into the polyester skeleton when ethylene glycol is used as one of the raw materials to produce (polycondensation) polyester.

In the case where the polyester (a2) is a copolyester, the copolyester may be, for example, a copolymer of a dicarboxylic acid component including terephthalic acid (X1) and a dicarboxylic acid component other than terephthalic acid (X1) and an alcohol component including ethylene glycol. In this case, the alcohol component may also contain diethylene glycol inevitably mixed as described above, and the content ratio of diethylene glycol in this case is as described above. Furthermore, in this case, the copolymer polyester in which the alcohol component includes ethylene glycol and diethylene glycol inevitably mixed is set as polyester (a2). In addition, in the case where the polyester (a2) is a copolyester, the copolyester may be, for example, a copolymer of terephthalic acid (X1) and an alcohol component including ethylene glycol and an alcohol component (Y2) other than ethylene glycol (Y1). In addition, in the above-mentioned aspects, as long as the polyester (a2) is a polyester other than the polyester (a1), it may also be a polyester other than the polyester (a1).

When the polyester (a2) is a copolyester, as the dicarboxylic acid component other than terephthalic acid (X1), aromatic dicarboxylic acids, alicyclic dicarboxylic acids, aliphatic dicarboxylic acids, polyfunctional acids, etc. can be listed. As these dicarboxylic acid components, the same ones as those listed in the copolyester (a1) can be listed. Here, the dicarboxylic acid component other than terephthalic acid (X1) can be used alone or in combination of two or more. Furthermore, as the dicarboxylic acid component other than terephthalic acid (X1), a dicarboxylic acid component having 4 to 10 carbon atoms can be used, and dicarboxylic acid components other than the above dicarboxylic acid components such as dimer acid can also be used, and these can also be used in combination. As the dicarboxylic acid component, aromatic dicarboxylic acids are preferred, and isophthalic acid is more preferably included. When the polyester (a2) is a copolyester and the dicarboxylic acid component contains a dicarboxylic acid component other than terephthalic acid, the ratio of the dicarboxylic acid component other than terephthalic acid in the dicarboxylic acid component is preferably 1 to 30 mol%, more preferably 3 mol% or more, further preferably 5 mol% or more, further preferably 20 mol% or less, further preferably 15 mol% or less.

When the polyester (a2) is a copolyester, as the alcohol component (Y2) other than ethylene glycol (Y1), the compounds listed in the copolyester (a1) can be appropriately selected and used, preferably 1,4-butanediol, 1,6-hexanediol, 1,4-hexanediol, diethylene glycol, trimethylene glycol, neopentyl glycol, 1,4-cyclohexanedimethanol, etc. When the polyester (a2) is a copolyester and the alcohol component includes an alcohol component (Y2) other than ethylene glycol (Y1), the ratio of the alcohol component (Y2) other than ethylene glycol in the alcohol component of the copolyester is preferably 1 mol% or more and less than 100 mol%, more preferably 3 mol% or more, further preferably 5 mol% or more, more preferably 90 mol% or less, further preferably 50 mol% or less, further preferably 30 mol% or less, and particularly preferably 10 mol% or less.

The intrinsic viscosity (IV) of the polyester (a2) (when two or more polyesters (a2) are used as a polyester mixture) is preferably 0.40 dL/g to 1.20 dL/g, more preferably 0.45 dL/g or more, more preferably 0.48 dL/g or more, and more preferably 1.15 dL/g or less, more preferably 1.10 dL/g or less. If the intrinsic viscosity of the copolyester (a2) is within this range, a polyester having excellent molding processability can be produced without deteriorating productivity.

The polyester (a2) may be used alone or in combination of two or more. The content of the polyester (a2) may be such that the ratio of each component in the polyester (A) is within the range described below, for example, 80% by mass or less, preferably 70% by mass or less, more preferably 65% by mass or less, more preferably 50% by mass or less, more preferably 30% by mass or more, more preferably 40% by mass or more, in the resin component constituting the copolyester layer (A1).

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

In the copolyester layer (A1), the mass ratio of the total amount of the copolyester (a1) and the polyester (a2) to the resin (a3) ((a1+a2):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 that is compatible with the copolyester (a1) or the copolyester (a1) and the polyester (a2), 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 copolyester layer (A1) can be increased, and the heat resistance can be improved. As such a resin, polybutylene terephthalate (PBT) can be cited, but it is not limited to PBT. The resin (a3) can be used alone or in combination of two or more.

(Ratio of each component) In the present invention, the ratio of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms in the dicarboxylic acid components of the entire polyester (A) contained in the copolyester layer (A1) is, for example, 3 to 50 mol%. Furthermore, the entire polyester (A) mentioned here refers to the entire polyester contained in the copolyester layer (A1). Therefore, the above ratio refers to the ratio based on the dicarboxylic acid components constituting the entire polyester (A), and similar terms are used in the following with the same meaning.

Here, by setting the ratio of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms to 3 mol% or more, the effect of using the dicarboxylic acid component (X2) having 4 to 10 carbon atoms can be fully obtained, and the flexibility and elongation at low temperatures can be ensured. By setting it to 50 mol% or less, the heat shrinkage rate is reduced, and heat resistance can be ensured. From the perspective of making the flexibility and elongation at low temperatures good and ensuring heat resistance, the ratio of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms is preferably 5 mol% or more, more preferably 7 mol% or more, and more preferably 45 mol% or less, more preferably 40 mol% or less, more preferably 25 mol% or less, and more preferably 12 mol% or less.

As described above, adipic acid is preferably used 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 ratio of adipic acid to the dicarboxylic acid components in the entire polyester (A) is, for example, 3 to 50 mol%, preferably 4 mol% or more, more preferably 5 mol% or more, further preferably 7 mol% or more, and further preferably 40 mol% or less, more preferably 25 mol% or less, further preferably 12 mol% or less, further preferably 10 mol% or less.

Furthermore, from the viewpoint of maintaining good various properties such as heat resistance, the ratio of terephthalic acid (X1) to the dicarboxylic acid components in the entire polyester (A) is, for example, 50 to 97 mol%, preferably 95 mol% or less, further preferably 93 mol% or less, further preferably 60 mol% or more, more preferably 65 mol% or more, further preferably 75 mol% or more, further preferably 88 mol% or more.

The ratio of other dicarboxylic acid (X3) to the dicarboxylic acid components in the entire polyester (A) is, for example, 10 mol% or less, preferably 5 mol% or less, further preferably 3 mol% or less, and most preferably 0 mol%. That is, the dicarboxylic acid component in the polyester (A) preferably does not contain other dicarboxylic acid (X3).

Furthermore, the dicarboxylic acid component of the polyester (A) contained in the copolyester layer (A1) can be quantitatively determined by measuring 1H-NMR spectra.

In the present invention, the ratio of other alcohol components (Y2) to the alcohol components in the entire polyester (A) contained in the copolyester layer (A1) is, for example, 15 to 60 mol%. If it is 15 mol% or more, it is easy to ensure flexibility and elongation at low temperatures. If it is 60 mol% or less, the heat shrinkage rate becomes low and it is easy to ensure heat resistance. From the perspective of making the flexibility and elongation at low temperatures good and ensuring heat resistance, the ratio of other alcohol components (Y2) is preferably 20 mol% or more, more preferably 25 mol% or more, further preferably 30 mol% or more, and more preferably 55 mol% or less.

From the perspective of ensuring flexibility at low temperatures and high elongation, the other alcohol component (Y2) preferably contains at least 1,4-butanediol. As mentioned above, it is preferred to use two or more kinds. It is particularly preferred to contain both 1,4-butanediol and 1,6-hexanediol. By using two or more other alcohol components (Y2), the following advantages are achieved: in combination with an acid component, it is easier to adjust the crystallinity than before. In particular, it has a significant effect in combination with a dicarboxylic acid component (X2) having 4 to 10 carbon atoms in the present invention. The ratio of 1,4-butanediol and 1,6-hexanediol in the alcohol components of the entire polyester (A) is preferably 15 to 60 mol%, more preferably 20 mol% or more, more preferably 25 mol% or more, more preferably 30 mol% or more, and more preferably 55 mol% or less. In addition, the ratio of 1,4-butanediol and 1,6-hexanediol refers to the ratio of 1,4-butanediol when only 1,4-butanediol is used, the ratio of 1,4-hexanediol when only 1,6-hexanediol is used, and the total ratio of 1,4-butanediol and 1,6-hexanediol when both are used. In the case of containing 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 is, for example, 0.5 or more, preferably 0.7 or more, more preferably 0.8 or more, and further preferably 0.9 or more, or, for example, 2.5 or less, preferably 2.0 or less, more preferably 1.6 or less, and further preferably 1.4 or less.

The ratio of ethylene glycol (Y1) to the alcohol components in the entire polyester (A) is, for example, 40 to 85 mol%, preferably 45 mol% or more, more preferably 80 mol% or less, more preferably 75 mol% or less, and even more preferably 70 mol% or less.

Furthermore, the amount of each alcohol component of the polyester (A) contained in the copolyester layer (A1) can be quantified by measuring 1H-NMR spectrum.

As a particularly preferred embodiment of the above, the following embodiment can be cited: the copolyester (a1) is a crystalline copolyester (Aa) comprising a copolymer of terephthalic acid and an aliphatic dicarboxylic acid having 4 to 10 carbon atoms and one or both of 1,4-butanediol and 1,6-hexanediol, wherein the ratio of the aliphatic dicarboxylic acid having 4 to 10 carbon atoms in the dicarboxylic acid component of the entire polyester (A) is 3 to 50 mol%, and the total ratio of 1,4-butanediol and 1,6-hexanediol in the alcohol component is 15 to 60 mol%. In general, when the ratio of the copolymer component is increased in order to reduce the elastic modulus, the crystallinity of the copolyester decreases and becomes amorphous. Although the above copolyester (Aa) has a high ratio of the copolymer component and can achieve a low elastic modulus, it can be thermally fixed by heat treatment after stretching because the crystallinity is maintained. As a result, the copolyester (Aa) is soft and has good elongation and strength, and further, thermal shrinkage can be suppressed.

Furthermore, the copolyester film contains copolyester (a1) or copolyester (a1) and polyester (a2), so that solvent resistance is also improved. Therefore, when the resin layer is formed using an organic solvent, the copolyester film can be prevented from being dissolved by the solvent, as described below.

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

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

In the copolyester layer (A1), polyester (A) is the main component resin. "Main component resin" refers to the resin with the highest content ratio in the resin components constituting the copolyester layer (A). The content of polyester (A) relative to the resin components contained in the copolyester layer (A1) is, for example, 50% by mass or more, preferably 60% by mass or more, more preferably 80% by mass or more, and there is no particular limitation as long as it is 100% by mass or less. Furthermore, the content of polyester (A) refers to the total amount of all polyesters contained in the copolyester layer (A1).

The copolyester layer (A1) may also contain particles. Specific examples of particles include inorganic particles such as silicon dioxide, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, aluminum oxide, titanium oxide, and organic particles such as acrylic resin, styrene resin, urea resin, phenol resin, epoxy resin, and benzoguanamine resin. Furthermore, in the process of polyester production, precipitated particles obtained by partially precipitating and microdispersing a metal compound such as a catalyst may also be used. The particles may be used alone or in combination of two or more. There is no particular limitation on the shape of the particles used, and any of spherical, blocky, rod-shaped, flat, etc. may be used. In addition, there is no particular limitation on hardness, specific gravity, color, etc. Two or more types of particles may be used in combination as needed.

In addition, the average particle size of the particles used is preferably less than 5 μm, and more preferably in the range of 0.1 to 4 μm. By using the average particle size within the above range, the film can be given an appropriate surface roughness and good sliding and smoothness can be ensured. Furthermore, regarding the average particle size of the particles, 10 or more particles present in the copolyester layer (A1) can be arbitrarily selected by scanning electron microscope (SEM), and the diameter of each particle can be measured and obtained as its average value. At this time, in the case of non-spherical particles, the average value of the longest diameter and the shortest diameter ((short diameter + long diameter)/2) can be measured as the diameter of each particle. The copolyester layer (A1) can also be a combination of two or more particles with different particle sizes. The content of particles in the copolyester layer (A1) is preferably 5% by mass or less, more preferably in the range of 0.0003 to 3% by mass, and further preferably in the range of 0.001 to 0.2% by mass. By setting the content of particles within the above range, the transparency of the substrate film is ensured while the slipperiness is easily imparted to the substrate film. In addition, by suppressing the content of particles to a low level, the tensile elongation at break is easily increased.

In addition to the above-mentioned particles, the copolyester layer (A1) may further contain at least one selected from a crystal nucleating agent, an antioxidant, an anti-coloring agent, a pigment, a dye, an ultraviolet absorber, a mold release agent, a lubricant, a flame retardant, an antistatic agent, etc. as an additive as needed.

(Method for producing copolyester (a1)) The method for producing copolyester (a1) is not particularly limited, and a conventional method can be applied. For example, first, a dicarboxylic acid component including terephthalic acid or its ester-forming derivatives, and a dicarboxylic acid having 4 to 10 carbon atoms other than terephthalic acid or its ester-forming derivatives, and an alcohol component including an alcohol component other than ethylene glycol are mixed in a specific ratio under stirring to prepare a raw material slurry (raw material slurry preparation step). Then, the raw material slurry is heated under normal pressure or pressure to cause an esterification reaction to occur to obtain a polyester oligomer (hereinafter sometimes referred to as an "oligomer") (oligomer preparation step). Subsequently, a dicarboxylic acid other than terephthalic acid or an ester-forming derivative thereof, and the above-mentioned alcohol component are further added to the obtained oligomer, and the pressure is gradually reduced in the presence of an ester exchange catalyst, and the mixture is heated to undergo a melt polycondensation reaction to obtain a polyester (polyester preparation step). The obtained polyester may also be further subjected to a solid phase polycondensation reaction (solid phase polycondensation step) as required.

However, the method for producing polyester (a1) is not limited to the above method. Dicarboxylic acids other than terephthalic acid or their ester-forming derivatives such as dicarboxylic acids having 4 to 10 carbon atoms do not need to be added to both the raw material slurry and the oligomer, and may be added only to the raw material slurry or only to the oligomer. In addition, terephthalic acid or its ester-forming derivatives may be added only to the raw material slurry or to the oligomer. Similarly, the alcohol component 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 ester exchange catalyst include: antimony compounds such as antimony 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; dibutyl tin oxide, methylphenyl tin oxide, tetraethyl tin, hexaethyl ditin oxide, hexahexyl ditin oxide, didodecyl tin oxide, triethyl tin hydroxide, triphenyl tin hydroxide; Tin, triisobutyltin acetate, dibutyltin diacetate, diphenyltin dilaurate, monobutyltin trichloride, tributyltin chloride, dibutyltin sulfide, butylhydroxytin oxide, methyltin acid, ethyltin acid, butyltin acid and other tin compounds; magnesium compounds such as magnesium acetate, magnesium hydroxide, magnesium carbonate, magnesium oxide, magnesium alkoxide, magnesium hydrogen phosphate, and calcium compounds such as calcium acetate, calcium hydroxide, calcium carbonate, calcium oxide, calcium alkoxide, calcium hydrogen phosphate, etc. Furthermore, these catalysts can be used alone or in combination of two or more.

Furthermore, when manufacturing polyester, it is preferred to use an ester exchange catalyst and a stabilizer together. As stabilizers, there can be listed: orthophosphoric acid, polyphosphoric acid, and pentavalent phosphorus compounds such as trimethyl phosphate, triethyl phosphate, tri-n-butyl phosphate, trioctyl phosphate, triphenyl phosphate, tricresyl phosphate, tri(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, tri(dodecyl) phosphite, trinonyldecyl phosphite, and triphenyl phosphite. Among them, the reducing property of trivalent phosphorus compounds is usually stronger than that of pentavalent phosphorus compounds, which may cause the metal compound added as a condensation catalyst to be reduced and precipitated to produce foreign matter, so pentavalent phosphorus compounds are preferred.

The reaction pressure in the melt polycondensation reaction is preferably 0.001 kPa to 1.33 kPa in absolute pressure. In addition, the reaction temperature in the melt polycondensation reaction is preferably 220°C to 280°C, preferably 230°C or higher, and more preferably 260°C or lower. The solid phase polycondensation reaction can 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-mentioned melt polycondensation reaction conditions and solid phase polycondensation reaction conditions, a polyester having a desired inherent viscosity can be obtained.

<Multi-layer structure> As described above, the copolyester film may have a multi-layer structure including a copolyester layer (A1) and other layers.

When the copolyester film has a multi-layer structure, for example, it may have a copolyester layer (A1), and a polyester layer (B1) and a polyester layer (B2) respectively laminated on the front and back sides of the copolyester layer (A1). Each polyester layer (B1) and polyester layer (B2) contains polyester (B) as a main component resin. Here, "main component resin" refers to the resin with the highest content ratio in the resin components constituting the copolyester layers (B1) and (B2). The main component resin may account for more than 50% by mass, more than 70% by mass, more than 80% by mass, and, for example, may account for less than 100% by mass in the resin components constituting the polyester layers (B1) and (B2).

When the copolyester (a1) is crystalline, the polyester (B) contained in each polyester layer (B1) and (B2) may be a polyester having a higher melting point than the copolyester (a1). When the copolyester (a1) is amorphous, the polyester (B) may be a polyester having a higher melting point than the glass transition point of the copolyester (a1). If the multilayer structure is a structure in which polyester layers (B1) and (B2) containing polyester (B) as the main component resin are stacked, the raw material resin composition can be stacked and stretched in the form of polyester layer (B1)/copolyester layer (A1)/polyester layer (B2) by co-extrusion, etc., and then heat-fixed at a temperature higher than that of a single layer containing the copolyester layer (A1). Therefore, it can be softened to a level that cannot be achieved by a single layer of the copolyester layer (A1), or heat shrinkage can be further prevented. Specifically, the storage elastic modulus of the copolyester film at 25° C. can be set to 300 to 2500 MPa, preferably 500 MPa or more or 2000 MPa or less, and further preferably 800 MPa or more or 1500 MPa or less.

In the above multi-layer structure, the thickness of each layer of the polyester layer (B1) and (B2) is preferably 1-20% of the thickness of the copolyester layer (A1). If the thickness of each layer of the polyester layer (B1) and (B2) is 1% or more of the thickness of the copolyester layer (A1), the film can be produced without significantly impairing productivity. If it is 20% or less, the required softness can be fully ensured, which is preferred. From this point of view, the thickness of each layer of the polyester layer (B1) and (B2) is 3% or more of the thickness of the copolyester layer (A1), and more preferably 15% or less, more preferably 5% or more, and more preferably 12% or less. Furthermore, the thickness of each layer of the polyester layers (B1) and (B2) existing on the front and back sides of the copolyester layer (A1) may be different or the same on the front and back sides.

When the copolyester (a1) is crystalline, the polyester (B) is a polyester having a melting point that is preferably 10 to 100°C higher than the melting point of the copolyester (a1), more preferably 20°C higher, more preferably 40°C higher, more preferably 90°C higher, more preferably 70°C higher. On the other hand, when the copolyester (a1) is amorphous, the polyester (B) is a polyester having a melting point that is preferably 120 to 260°C higher than the glass transition point of the copolyester (a1), more preferably 140°C higher, more preferably 160°C higher, more preferably 230°C higher, more preferably 200°C higher. Furthermore, the main component of each of the polyester layers (B1) and (B2) on the front and back sides of the copolyester layer (A1), namely the polyester (B), may be different or the same on the front and back sides, but the melting points of the polyester (B) on the front and back sides are preferably not much different.

As the polyester (B), for example, a homopolyester or copolyester containing terephthalic acid as a dicarboxylic acid component and ethylene glycol as an alcohol component is preferably used, but the present invention is not limited thereto. In addition, the homopolyester is polyethylene terephthalate.

When the polyester (B) is a copolyester, as the dicarboxylic acid component other than terephthalic acid, aromatic dicarboxylic acids, alicyclic dicarboxylic acids, aliphatic dicarboxylic acids, polyfunctional acids, etc. can be listed. As these dicarboxylic acid components, the same ones as those listed in the copolyester (a1) can be listed. When the polyester (B) is a copolyester, the ratio of the dicarboxylic acid component other than terephthalic acid in the dicarboxylic acid component is preferably 1 to 30 mol%, more preferably 5 mol% or more, more preferably 10 mol% or more, more preferably 25 mol% or less, and further preferably 20 mol% or less.

When the polyester (B) is a copolyester, the alcohol component other than ethylene glycol includes 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 ratio of the alcohol component other than ethylene glycol in the alcohol component is preferably 1 mol% or more and less than 100 mol%, more preferably 5 mol% or more, more preferably 10 mol% or more, more preferably 95 mol% or less, more preferably 90 mol% or less.

<Thickness of the copolyester film> The thickness of the copolyester film is not particularly limited, and an appropriate thickness can be selected according to the application. Among them, from the viewpoint of further exerting the characteristics of the copolyester film, the total thickness of the film is preferably more than 5 μm. The plastic strength of the film is proportional to the cube of the thickness. However, even if the copolyester film has a thickness of more than 5 μm, it has the characteristics of weak plasticity and softness, and can further enjoy the benefits of the present invention. From this viewpoint, the total thickness of the copolyester film is preferably more than 5 μm, more preferably 12 μm or more, and more preferably 30 μm or more. In addition, the total thickness of the copolyester film is not particularly limited, for example, it is 200 μm or less, preferably 150 μm or less, and more preferably 100 μm or less.

(Difference between Tcc and Tg (ΔTcg)) The difference (ΔTcg) between the cold crystallization temperature (Tcc) and the glass transition temperature (Tg) of the copolyester film of the present invention exceeds 60°C. If the difference (ΔTcg) between Tcc and Tg is below 60°C, the heat resistance is reduced, and it is difficult to take into account the opposite characteristics, that is, on the one hand, excellent flexibility and high elongation at room temperature, and on the other hand, good heat resistance. From the perspective of improving heat resistance, the difference (ΔTcg) between Tcc and Tg is preferably above 63°C, and further preferably above 70°C. The difference between Tcc and Tg (ΔTcg) is not particularly limited, for example, it can be below 150°C, and it can also be below 100°C. Furthermore, the cold crystallization temperature (Tcc) is higher than the glass transition temperature (Tg), and the difference between Tcc and Tg (ΔTcg) is defined as "Tcc-Tg". Furthermore, the difference between Tcc and Tg can be adjusted according to the type and amount of the dicarboxylic acid component (X2), the type and amount of the other alcohol component (Y2), the presence or absence of stretching, the stretching ratio, the stretching temperature, and the amount of the copolyester (a1).

The cold crystallization temperature (Tcc) of the copolyester film is not particularly limited, and is, for example, 90 to 150° C., preferably 105 to 135° C. The glass transition temperature (Tg) of the copolyester film is not particularly limited, and is, for example, 10 to 60° C., preferably 25 to 45° C.

(Stretching elongation at break) The stretching elongation at break of the copolyester film at 25°C is preferably 295% or more. If the stretching elongation at 25°C is 295% or more, it is easy to have a high elongation at room temperature, and the following stretching process can be properly performed. In addition, the softness of the copolyester film can also be ensured. In order to ensure softness at room temperature and ensure excellent elongation, the stretching elongation at break of the copolyester film at 25°C is preferably 300% or more, and further preferably 320% or more. In addition, the above-mentioned stretching elongation at break is not particularly limited, and in order to easily give the copolyester film a certain mechanical strength, it is, for example, 600% or less, preferably 500% or less.

(Tensile strength at break) The tensile strength at break of the copolyester film at 25°C is preferably 30 MPa or more. By setting the tensile strength at break to 30 MPa or more, it is easy to give the copolyester film a certain mechanical strength. From this point of view, the tensile strength at break at 25°C is more preferably 40 MPa or more, and more preferably 50 MPa or more. From the point of view of easily improving the above-mentioned tensile elongation at break, the tensile strength at break at 25°C is, for example, 300 MPa or less, preferably 200 MPa or less, more preferably 150 MPa or less, and more preferably 100 MPa or less.

In the present copolyester film, the tensile elongation at break and the tensile strength at break can be adjusted to the above ranges by adjusting the type and content of the copolymer components of the copolyester (a1). Alternatively, the elongation at break and the tensile strength at break can be adjusted by adjusting the presence or absence of stretching, the stretching ratio, etc. Furthermore, the stretching temperature can also be adjusted.

Furthermore, when the tensile elongation at break of the copolyester film is known in MD and TD, the tensile elongation at break in these directions is measured and the higher value is adopted. On the other hand, when MD and TD are unknown, the tensile elongation at break in the direction with the highest tensile elongation at break is adopted. Moreover, the tensile strength at break is the value measured in the direction where the tensile elongation at break is adopted.

(Storage elastic modulus at 25°C) The storage elastic modulus of the polyester film at 25°C is preferably 2500 MPa or less. By making the storage elastic modulus at 25°C, i.e., room temperature, less than 2500 MPa, flexibility can be ensured, and the curved surface tracking property can be improved, for example, when wearing a wearable terminal, it can fully follow the skin. From this point of view, the storage elastic modulus of the polyester film at 25°C is more preferably 2000 MPa or less, more preferably 1800 MPa or less, and particularly preferably 1600 MPa or less.

On the other hand, from the viewpoint of the processing properties in each step, the storage elastic modulus at 25°C is preferably 500 MPa or more, more preferably 800 MPa or more, and even more preferably 1000 MPa or more. Furthermore, the storage elastic modulus at 25°C and the following 120°C is a value obtained by the measurement method described in the following embodiment.

In the present copolyester film, in order to adjust the storage elastic modulus at 25°C to the above range, the type and content of the copolymer components of the copolyester (a1) can be adjusted. From this point of view, the copolymer components of the copolyester (a1) preferably include aliphatic dicarboxylic acids having 4 to 10 carbon atoms, and one or both of 1,4-butanediol and 1,6-hexanediol, and the ratios of the dicarboxylic acid component and the alcohol component in the polyester (A) are preferably 3 to 50 mol% and 15 to 60 mol% as described above. It is further preferred that the aliphatic dicarboxylic acid having 4 to 10 carbon atoms is adipic acid. Among them, the other alcohol components (Y2) are preferably 1,4-butanediol and 1,6-hexanediol, and in this case, the total amount (mol%) of the other alcohol components (Y2) is preferably 15 to 60 mol%.

In addition, the storage elastic modulus at 25°C can be easily adjusted to the above range by laminating polyester layers (B1) and (B2) containing polyester (B) as a main component resin on the front and back sides of the copolyester layer (A1) to form a multilayer structure. Furthermore, the storage elastic modulus at 25°C can be adjusted according to the stretching conditions when manufacturing the copolyester film and the subsequent heat setting conditions.

(Storage modulus at 120°C) The storage modulus at 120°C of the copolyester film is, for example, 10 MPa or more, preferably 20 MPa or more, more preferably 25 MPa or more, and particularly preferably 30 MPa or more. By making the storage modulus near 120°C, i.e., near the processing temperature, 10 MPa or more, especially 20 MPa or more, it is better to improve the heat resistance. In addition, by improving the heat resistance, it is also easy to suppress heat shrinkage, etc. In addition, the storage modulus at 120°C is not particularly limited, and from the viewpoint of flexibility, it is, for example, 400 MPa or less, preferably 200 MPa or less, and further preferably 150 MPa or less.

(Crystallization melting enthalpy ΔHm) When the polyester contained in the copolyester layer (A1) of the copolyester film includes a crystalline polyester, the crystallization melting enthalpy ΔHm of the polyester (in the case of two or more polyesters, a polyester composition including the two or more polyesters) is preferably 15.0 J/g or more, more preferably 20.0 J/g or more, and more preferably 25.0 J/g or more. ΔHm is an index of crystallinity. When it is 15.0 J/g or more, sufficient heat resistance can be obtained, and the heat shrinkage of the copolyester film can be suppressed in the processing steps including the heat treatment step. The crystallization melting enthalpy ΔHm is not particularly limited, and from the viewpoint of flexibility, for example, it is 50 J/g or less, preferably 40 J/g or less, and more preferably 32 J/g or less.

(Young's modulus) The Young's modulus of the copolyester film is preferably 5.0 GPa or less, more preferably 4.0 GPa or less, and further preferably 3.5 GPa or less. By setting the Young's modulus to be below the above upper limit, appropriate softness can be ensured. In addition, the Young's modulus is not particularly limited, but from the perspective of ensuring a certain mechanical strength, it is preferably 0.5 GPa or more, and more preferably 0.8 GPa or more. Furthermore, when the Young's modulus of the copolyester film mentioned above is known in MD and TD, the Young's modulus in these directions is measured and the higher value is adopted. On the other hand, when MD and TD are unknown, the Young's modulus in the direction with the highest Young's modulus is adopted.

<Method for producing the copolyester film> As an example of the method for producing the copolyester film, the method for producing the copolyester film in the case of a biaxially stretched film is described. However, the method is not limited to the method described here.

First, a raw material, such as a polyester chip, is supplied to a melt extruder by a known method, heated to a temperature above the melting point of each polymer, the molten polymer is extruded from a mold, and cooled and solidified on a rotating cooling drum to a temperature below the glass transition point of the polymer, thereby obtaining a substantially amorphous non-oriented sheet.

Then, the unoriented sheet is stretched in one direction by a roller stretching machine or a tenter stretching machine. At this time, the stretching temperature is usually 25-120°C, preferably 35-100°C, and the stretching ratio is usually 2.5-7 times, preferably 2.8-6 times. Then, it is stretched in a direction orthogonal to the stretching direction of the first stage. At this time, the stretching temperature is usually 50-140°C, and the stretching ratio is usually 3.0-7 times, preferably 3.5-6 times. Furthermore, in the above stretching, a method of stretching in one direction in more than two stages can also be adopted.

After stretching, the copolyester film is heat-fixed at a temperature of 130 to 270°C under stretching or under relaxation of less than 30%, and the copolyester film as a biaxially oriented film can be obtained. The copolyester film can be improved in flexibility and heat resistance by heat-fixing. In the case of a single layer including a copolyester layer (A1), the heat-fixing treatment (also referred to as "heat treatment") is preferably performed at a temperature 5 to 70°C lower than the melting point of the polyester used to form the copolyester layer (A1). In the case of a multi-layer structure, the reaction may be carried out at a temperature 5 to 70°C lower than the melting point of the polyester used to form the copolyester layer (A1), but is preferably carried out at a temperature 5 to 70°C lower than the melting point of the polyester used to form the polyester layer (B).

When the copolyester film has a laminated structure of the polyester layer (A1) and the polyester layers (B1) and (B2), the polyester layer (A1) and the polyester layers (B1) and (B2) may be co-extruded and then stretched and heat-fixed as a single film as described above.

<Laminated film> The laminated film of one embodiment of the present invention has the above-mentioned copolyester film and a functional layer provided on at least one side of the copolyester film. The functional layer is not particularly limited, and a resin layer may be cited. The functional layer such as the resin layer is preferably stretched together with the copolyester film when laminated on the above-mentioned copolyester film. Thereby, the thickness of the functional layer is thinned and formed into a functional sheet. The copolyester film is soft and has a higher elongation at room temperature, so it is easy to stretch and is suitable for forming into a functional sheet by stretching. In addition, due to its good heat resistance, when the laminated film is subjected to heat treatment, heat shrinkage is also reduced.

(Resin layer) The resin layer is a layer laminated on the copolyester film. The type of the resin layer is not particularly limited, and a resin having stretch-following properties is preferably used. The resin layer preferably has an elongation of 295% or more. Furthermore, a resin layer having an elongation of 295% or more means that the resin layer will not break even if it is stretched at an elongation of 295%. In other words, it means that when laminated on the copolyester film, the resin layer will not break even if it is stretched at least 295% (i.e., at least 3.0 times the stretching ratio) together with the copolyester film. The elongation of the resin layer is more preferably 300% or more, more preferably 350% or more, and particularly preferably 400% or more. The elongation of the resin layer is not particularly limited, and is, for example, 800% or less, preferably 600% or less. By satisfying the above range, a relatively thick resin sheet can be stably formed by stretching from a relatively thick state.

The resin layer may contain ceramic particles in addition to the above-mentioned resins. By using ceramic particles, for example, the resin layer after extension can be used as a green sheet. As ceramic particles, known ceramic particles used for green sheets can be used. Specifically, barium titanate, Pb(Mg _1/3 , Nb _2/3 )O ₃ , Pb(Sm _1/2 , Nb _1/2 )O ₃ , Pb(Zn _1/3 , Nb _2/3 )O ₃ , PbThO ₃ , PbZrO ₃ and the like can be cited, but are not particularly limited to these, as long as the dielectric properties required for the ceramic multilayer capacitor can be obtained. When the resin layer contains ceramic particles, the resin used may include polyurethane resins, urea resins, melamine resins, water-based polymer-isocyanate resins, epoxy resins, vinyl acetate resins, acrylic resins, etc. In addition, the following resins containing vinyl alcohol structural units may also be used. When ceramic particles are contained, the ceramic particles may be the main component of the resin layer (for example, 50% by mass or more of the resin layer), and the mass ratio of ceramic particles to resin (ceramic particles)/(resin) is preferably in the range of 70/30 to 95/5. Furthermore, the resin layer may contain additives such as a dispersant, a plasticizer, an antistatic agent, and a defoaming agent in addition to the resin and the ceramic particles.

When the resin layer contains ceramic particles in addition to the above-mentioned resin, it can be formed by applying a slurry containing ceramic particles and resin and dispersing the ceramic particles in any solvent such as water or an organic solvent to a copolyester film, followed by heating and drying.

In addition, the resin layer is preferably in a state containing a resin as a main component. Here, "containing ... as a main component" means that the content of the resin is based on the total amount of the resin layer, for example, 50% by mass or more, preferably 70% by mass or more, preferably 80% by mass or more and 100% by mass or less. Furthermore, in the case of containing a resin as a main component resin, as the resin, it is preferred to use the following resin containing a vinyl alcohol structural unit, wherein a vinyl alcohol polymer is preferred. When the resin layer contains a resin as a main component, it can be formed by diluting a resin composition containing a resin as a main component with an organic solvent, water, etc. as needed, applying the diluted resin composition to the copolyester film, and heating and drying the diluted resin composition as needed. The resin composition may contain additives such as a crosslinking agent in addition to the resin.

(Resin containing vinyl alcohol structural units) In the present invention, the resin used as the resin layer is preferably a resin containing vinyl alcohol structural units. The vinyl alcohol structural unit is a structural unit composed of -(CH ₂ CHOH)-. Specifically, examples include vinyl butyral polymers and other vinyl alcohol acetal polymers, vinyl alcohol polymers, etc. The vinyl alcohol polymer is obtained by saponifying a polymer, and the polymer is obtained by polymerizing a vinyl ester monomer, or by copolymerizing a vinyl ester monomer with a monomer having an ethylenic double bond other than the vinyl ester monomer. In addition, the vinyl alcohol polymer may be unmodified vinyl alcohol, or may be a modified vinyl alcohol polymer by using a copolymer with a monomer having an ethylenic double bond as described above. Furthermore, the vinyl alcohol acetal polymer can also be introduced with a functional group by a subsequent reaction. As the modified vinyl alcohol polymer, preferably, an acetyl acetyl modified vinyl alcohol polymer can be mentioned.

The vinyl alcohol acetal polymer can be obtained by acetalizing the above-mentioned vinyl alcohol polymer. The vinyl alcohol acetal polymer has an unacetalized hydroxyl group, thereby having a vinyl alcohol structural unit. In the case of the vinyl alcohol acetal polymer, the above-mentioned monomer having an ethylene double bond is preferably a polyfunctional monomer having two or more ethylene double bonds (hereinafter sometimes referred to as a polyfunctional monomer). In order to acetalize the vinyl alcohol copolymer to obtain the vinyl alcohol acetal polymer, the vinyl alcohol copolymer is preferably water-soluble. Therefore, the vinyl alcohol acetal polymer can be obtained, for example, by acetalizing a vinyl alcohol copolymer having an ethylene double bond in its side chain and being water-soluble. Here, the ethylenic double bonds on the side chains refer to the unreacted ethylenic double bonds in the multifunctional monomers copolymerized in the vinyl alcohol-based polymer.

When the resin layer contains ceramic particles as the main component, the resin containing vinyl alcohol structural units is preferably a vinyl alcohol acetal polymer, and particularly preferably a vinyl alcohol acetal polymer containing ethylene double bonds on the side chains. By using a vinyl alcohol acetal polymer containing ethylene double bonds on the side chains, the dispersibility of ceramic particles in the slurry is excellent, and the storage stability of the slurry is also improved. In addition, when the resin layer is a resin as the main component, the resin containing vinyl alcohol structural units is preferably a vinyl alcohol polymer. By using a vinyl alcohol polymer, the resin sheet obtained after stretching can be suitably used in optical components such as polarizing elements.

Examples of the vinyl ester monomer include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl carbonate, vinyl caproate, vinyl octanoate, vinyl laurate, vinyl palmitate, vinyl stearate, and vinyl benzoate. Among them, vinyl acetate is preferred from the viewpoint of easy production.

Examples of the polyfunctional monomer include monomers containing a vinyl ether group such as 1,4-butanediol divinyl ether and triethylene glycol divinyl ether, monomers containing an allyl group such as 1,9-decadiene, polyethylene glycol diallyl ether and pentaerythritol diallyl ether, and monomers having a plurality of (meth)acrylates in one molecule such as ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, glycerol tri(meth)acrylate and pentaerythritol tri(meth)acrylate.

The saponification degree of the vinyl alcohol copolymer is preferably 60 to 99.9 mol%. If the saponification degree is set to 60 mol% or more, the water solubility of the vinyl alcohol copolymer is good. From this point of view, the saponification degree is more preferably 65 mol% or more. On the other hand, if the saponification degree is set to 99.9 mol% or less, industrial production is easier, and the viscosity stability of the vinyl alcohol copolymer aqueous solution is good and easy to handle. From this point of view, the saponification degree is more preferably 99.5 mol% or less.

The viscosity average degree of polymerization Pη of the vinyl alcohol copolymer is preferably 100 to 8000. By setting the viscosity average degree of polymerization Pη to 100 or more, the industrial production of the vinyl alcohol copolymer becomes easy. From this point of view, the viscosity average degree of polymerization Pη is more preferably 200 or more. On the other hand, by setting the viscosity average degree of polymerization Pη to 8000 or less, the industrial production of the vinyl alcohol copolymer becomes easy. In addition, the viscosity of the aqueous solution of the vinyl alcohol copolymer is prevented from being too high, and its operation becomes easy. From these viewpoints, the viscosity average degree of polymerization Pη of the vinyl alcohol copolymer is more preferably 5000 or less, and further preferably 2500 or less.

The viscosity average polymerization degree Pη can be measured according to JIS K6726. Specifically, the vinyl alcohol copolymer is saponified again, and the remaining carboxylic acid residues are completely saponified. After the re-saponified vinyl alcohol copolymer is purified and dried, 1 g of the dried sample is added to 100 ml of water to be heated and dissolved, and then cooled to 30°C. The obtained aqueous solution is weighed with a viscometer, and the limiting viscosity [η] (unit: L/g) in water at 30°C is measured. Based on the measured limiting viscosity [η], the following formula (1) can be used to calculate.

[Formula 1] Viscosity average degree of polymerization Pη = ([η] × 10000/8.29) ^(1/0.62) (1)

(Adhesive layer) The functional layer may also be an adhesive layer. The functional layer is an adhesive layer, so the laminated film becomes an adhesive tape. The adhesive layer is a layer having pressure-sensitive adhesiveness and can be formed by a known adhesive. The adhesive is not particularly limited as long as it is a material that can follow the elongation at break even when the copolyester film is stretched to the tensile fracture elongation. Previously known materials such as acrylic, silicone, polyurethane, and polyester can be used.

The adhesive layer may also contain a conductive material in the layer. The adhesive layer becomes a conductive adhesive layer (conductive layer) having conductivity by containing a conductive material. The conductive adhesive layer can be formed by an adhesive mixed with a conductive material. As conductive materials, there can be listed: metal powder particles such as gold, silver, copper, nickel, aluminum, etc., conductive carbon particles such as carbon black and graphite, resins, solid glass beads, hollow glass beads, etc., and conductive particles such as particles with metal coating on the surface of the core material. As a conductive material, in addition to conductive particles, conductive polymers can also be used. As a conductive material, conductive polymers can also be used. As the conductive polymer, there is no particular limitation as long as it is an organic polymer whose main chain is composed of a π-conjugated system. For example, compounds such as polythiophenes, polypyrroles, polyacetylenes, polyphenylenes, polyphenylene vinylenes, polyanilines, polybenzones, polythiophene vinylenes, and copolymers thereof can be listed. The conductive material can be used alone or in combination of two or more. In addition, the conductive material is preferably at least one selected from the carbon black and polythiophenes mentioned above. Polythiophenes are unsubstituted or substituted thiophene polymers. As a specific example of substituted thiophene polymers, poly(3,4-ethylenedioxythiophene) can be listed. In addition, the conductive layer is not limited to the conductive adhesive layer, but can also be a conductive material blended in the resin layer.

<Application of this copolyester film> As mentioned above, this copolyester film has excellent flexibility at room temperature. It is not only flexible but also has a large elongation and can also exhibit heat resistance sufficient for practical use. Since this copolyester film has the above properties, it is suitable for various stretching processes, especially stretching forming processes to obtain various molded products through stretching processes.

Furthermore, specifically, the copolyester film and the laminated film are suitable for battery packaging materials, surface protection films, cutting protective tapes, adhesive tapes, image display components such as flexible displays, wearable terminals, bioelectrode substrates, biosensor substrates and other bio-related components, manufacturing electronic components, such as blank sheet forming used in manufacturing ceramic laminated components, manufacturing optical components, such as manufacturing polarizing elements constituting polarizing plates, etc. Moreover, among the above, any one of adhesive tapes, surface protection films, semiconductor cutting protective tapes, wearable terminals, bioelectrode substrates, biosensor substrates, manufacturing electronic components, and manufacturing optical components is more suitable. In particular, when used in wearable terminals, bioelectrode substrates, and biosensor substrates, it is preferred to provide a conductive layer containing the above-mentioned conductive material on the copolyester film of the present invention.

First, the following will describe in more detail the case where the copolyester film is used for stretching processing. The stretching processing is not limited as long as it includes a step of stretching the copolyester film (stretching step), but it is preferably a step of stretching the laminated film as described above. By stretching the laminated film, the functional layer laminated on the copolyester film is also stretched, so the functional layer can also be stretched. Thereby, the functional layer is formed into a functional sheet, for example, by making the thickness thinner. The functional sheet can be peeled off from the copolyester film and used as needed.

In the stretching step, the stretching can be performed in a uniaxial direction or in a biaxial direction. The stretching ratio is preferably 2.0 to 6.0 times, more preferably 3.0 to 5.5 times. By setting the stretching ratio within the above range, the thickness can be made sufficiently thin without causing defects or cracks in the functional layer and the copolyester film, and appropriate stretching processing can be performed. In addition, the stretching ratio refers to the stretching ratio of each axis when stretching in the biaxial direction.

Furthermore, the above-mentioned stretching can be performed in a dry manner or in a wet manner. The dry manner is, for example, a method of performing stretching processing in the atmosphere. It can be performed at a stretching temperature in the dry manner, for example, 25 to 70°C, preferably around 30 to 60°C. In addition, the wet manner is a method of stretching a copolyester film or a laminated film immersed in a liquid such as water, for example, at a liquid temperature of 25 to 70°C, preferably 30 to 60°C. The copolyester film has a high elongation near room temperature and can be appropriately stretched.

As described above, when the functional layer is formed into a functional sheet by stretching, the functional layer is a resin layer and can be formed into various resin sheets by stretching. For example, when the resin layer contains resin and ceramic particles, the functional sheet (resin sheet) can be used as a green sheet. By laminating a plurality of green sheets, a ceramic laminate capacitor can be obtained. In addition, when the resin layer contains a resin as a main component, the functional sheet can be used for optical components, etc., and when the resin layer contains, for example, a vinyl alcohol polymer as a main component, the functional sheet can be used as a polarizing element.

As described above, the copolyester film is suitable for use in the manufacture of electronic components, especially green sheet forming used in the manufacture of ceramic laminated components, when subjected to stretching processing. It is also suitable for use in the manufacture of optical components, such as the manufacture of polarizing elements constituting polarizing plates.

Furthermore, when the functional layer is an adhesive layer, the laminated film can be used as an adhesive tape. The adhesive tape can be used for household adhesive tape, industrial adhesive tape, packaging adhesive tape, electronic component adhesive tape, etc., but it is preferably used for surface protection film, semiconductor cutting protective tape, etc. The surface protection film is attached to the adherend such as electronic parts, optical components, semiconductor wafers, etc., and is used to protect the adherend. The semiconductor cutting protective tape is attached to the back of the semiconductor wafer, etc., and supports the semiconductor wafer during cutting. In addition, after cutting, the cutting protective tape is extended, thereby separating the singulated semiconductor chips from each other and picking them up. Since the copolyester film is excellent in softness at low temperature and has a large elongation, it can follow the adherend even when the adherend to which the adhesive tape is attached has a curved or concave-convex shape. Furthermore, when the adhesive tape is used as a cutting protective tape, it can be easily extended when picking up semiconductor chips. In addition, due to its high heat resistance and resistance to heat shrinkage, the reliability of the adhesive tape is also improved, and when used as a surface protective film, for example, it has good protective performance.

In addition, the copolyester film can be used for bioelectrode substrates, biosensor substrates, etc., and can also be used for bio-related components such as substrates for wearable terminals. The copolyester film is soft and has high elongation at low temperatures, so it can easily follow the skin surface and is suitable for use in bio-related components. Furthermore, the bioelectrode substrate or biosensor substrate is a substrate that supports batteries or various sensors. Specifically, an electrode or sensor element can be set on one surface of the substrate containing the copolyester film. In addition, the electrode can also be composed of a conductive adhesive layer, so an adhesive tape can also be used.

Furthermore, the copolyester film or laminated film can be used as a battery packaging material such as an outer packaging material for lithium-ion batteries, and can also be used as an image display component such as a flexible display. Although battery packaging materials and image display components may be used in a bent state, the copolyester film is flexible and has a high elongation at low temperatures, so it can be bent to follow the shape of the battery or display. In addition, in such uses, the copolyester film will reach a high temperature, but due to its excellent heat resistance, it can also prevent heat shrinkage.

<Explanation of Words and Phrases> In the present invention, "film" also includes "sheet", and "sheet" also includes "film". In addition, when an image display panel, a protective panel, etc. are expressed as a "panel", it includes a plate, a sheet, and a film.

In the present invention, when "X to Y" (X and Y are arbitrary numbers) is described, unless otherwise specified, it also includes the meaning of "greater than X and less than Y", and the meaning of "preferably greater than X" or "preferably less than Y".

Furthermore, when "X or more" (X is an arbitrary number) is recorded, unless otherwise specified, it includes the meaning of "preferably greater than X", and when "Y or less" (Y is an arbitrary number) is recorded, unless otherwise specified, it also includes the meaning of "preferably less than Y". Implementation Example

Next, the present invention is further described in detail by using embodiments. However, the present invention is not limited to the embodiments described below.

<Evaluation method> The following methods are used to measure and evaluate various physical properties.

(1) Tensile storage modulus E' Based on JIS K 7244, the dynamic viscoelasticity measuring device DVA-200 manufactured by IT Meter. and Control (Co., Ltd.) was used to measure the width direction (TD) of the polyester film (sample) obtained in the embodiment and comparative example at a vibration frequency of 10 Hz, a strain of 0.1%, and a heating rate of 1°C/min from -100°C to 200°C. The tensile storage modulus E' at 25°C and 120°C was obtained from the obtained data and used as the storage modulus at 25°C and 120°C.

(2) Crystallization melting enthalpy ΔHm Based on JIS K7141-2 (2006), the sample was measured by differential scanning calorimetry (DSC). The temperature was raised from 30°C to 280°C at 10°C/min and maintained for 1 minute. The temperature was then lowered from 280°C to 30°C at 10°C/min and maintained for 1 minute. The temperature was then raised from 30°C to 280°C again at 10°C/min. At this time, the crystallization melting enthalpy (ΔHm) was calculated based on the crystallization melting peak area during the reheating process. In the case of a single layer, the polyester film was used as the measurement sample, and in the case of a laminated structure, the copolyester layer (A1) was used as the measurement sample.

(3) Young's modulus For the polyester films (samples) obtained in the examples and comparative examples, a tensile testing machine (manufactured by Intesco Co., Ltd., Intesco model 2001) was used to stretch the polyester films (samples) with a length of 300 mm and a width of 20 mm at a strain rate of 10%/min in a room adjusted to a temperature of 23°C and a humidity of 50% RH. The initial straight line portion of the tensile stress-strain curve was used to calculate the film's length direction (MD) and width direction (TD) using the following formula. E＝Δσ/Δε (In the above formula, E is Young's modulus (GPa), Δσ is the stress difference of the original average cross-sectional area between two points on the straight line (GPa), and Δε is the strain difference between the same two points/initial length) Measure 5 points in the length direction (MD) and width direction (TD) of the film, and calculate the average value respectively.

(4) Tensile strength at break For the polyester films (samples) obtained in the examples and comparative examples, a tensile testing machine (manufactured by Intesco Co., Ltd., Intesco Model 2001) was used. In a room adjusted to a temperature of 25°C and a humidity of 50% RH, a 15 mm wide polyester film (sample) was placed in the testing machine with a chuck interval of 50 mm. The film was stretched at a strain rate of 200 mm/min. The tensile strength at break in the longitudinal direction (MD) and the width direction (TD) of the film was determined by the following formula. Tensile strength at break (MPa) = F/A In the above formula, F is the load at break (N), and A is the original cross-sectional area of the test piece (mm ² ). Five points were measured in the longitudinal direction (MD) and the width direction (TD) of the film, and the average values were calculated.

(5) Elongation at break The polyester film (sample) obtained in the embodiment and comparative example was subjected to the same test as the tensile strength at break as described above, and the tensile elongation at break in the length direction (MD) and width direction (TD) of the film was calculated using the following formula. Elongation at break (%) = 100 × (L-L0)/L0 In the above formula, L is the distance between the points at break (mm), and L0 is the original distance between the points (mm). Five points were measured in the length direction (MD) and width direction (TD) of the film, and the average values were calculated.

(6) Heat shrinkage rate The polyester film (sample) obtained in the embodiment and comparative example was treated in an oven maintained at 120°C for 5 minutes without tension. The length of the sample before and after was measured, and the heat shrinkage rate in the longitudinal direction (MD) and the width direction (TD) of the film was calculated by the following formula. Heat shrinkage rate (%) = {(L0-L1)/L0}×100 (In the above formula, L0 is the length of the sample before the heat treatment, and L1 is the length of the sample after the heat treatment) Five points were measured in the longitudinal direction (MD) and the width direction (TD) of the film, and the average values were calculated respectively.

(7) Difference between cold crystallization temperature (Tcc) and glass transition temperature (Tg) (ΔTcg) Using a differential scanning calorimeter (DSC8500 manufactured by PerkinElmer), the specific heat change caused by the transition from the glass state to the rubber state is read at a heating rate of 10°C/min to obtain Tg (glass transition temperature). In addition, the exothermic peak temperature accompanying crystallization is set as the cold crystallization temperature (Tcc). The difference (ΔTcg (°C)) between the cold crystallization temperature (Tcc) and the glass transition temperature (Tg) is calculated by the following formula. ΔTcg = Tcc - Tg In the present invention, ΔTcg must exceed 60°C. The larger the value, the higher the crystallinity. In the case of a single layer, the polyester film was used as a measurement sample, and in the case of a laminated structure, the copolyester layer (A1) was used as a measurement sample.

(8) Solvent resistance After the sample film was immersed in each solvent for 24 hours, the state was visually observed and judged according to the following judgment criteria. (Judgment criteria) A: No change in appearance or flatness. B: The film curled. C: Lack of solvent resistance, the film dissolved.

(9) Laminated film stretching tracking property A resin layer composition containing the following components was applied to a sample film and dried at 80°C for 5 minutes to form a functional layer with a thickness of 3.6 μm, thereby preparing a sample containing a laminated film. Subsequently, the film was stretched at a stretching ratio of 3.5 times (equivalent to an elongation of 250%) at 25°C in air using a tensile testing machine (AUTOGRAPH AG-I) manufactured by Shimadzu Corporation. The stretching tracking property of the obtained laminated film was determined using the following criteria. (Resin layer composition) "GOHSENX Z-200" manufactured by Mitsubishi Chemical Corporation 100 parts by mass "Safelink SPM-01" manufactured by Mitsubishi Chemical Corporation 5 parts by mass (Judgment criteria) A: The extension tracking property is particularly good. B: The extension tracking property is good. C: The extension tracking property is poor.

(Raw materials) The following raw materials were used in the examples and comparative examples. (1) Copolyester (a1-1) A copolyester (a1-1) (intrinsic viscosity (IV) 0.70 dl/g) was prepared. The copolyester (a1-1) contained terephthalic acid and adipic acid (carbon number 6) as dicarboxylic acid components, the terephthalic acid content was 85 mol%, the adipic acid content was 15 mol%, and contained 45 mol% of 1,4-butanediol and 55 mol% of 1,6-hexanediol as alcohol components.

(2) Copolyester (a1-2) Copolyester (a1-2) (intrinsic viscosity (IV) 0.70 dl/g) was prepared. The copolyester (a1-2) contained terephthalic acid and isophthalic acid as dicarboxylic acid components, the content of terephthalic acid was 78 mol%, the content of isophthalic acid was 22 mol%, and the content of ethylene glycol as alcohol components was 98 mol% and diethylene glycol was 2 mol%.

(3) Copolyester (a1-3) Copolyester (a1-3) (intrinsic viscosity 0.64 dl/g) was prepared. The copolyester (a1-3) contained terephthalic acid, isophthalic acid, and hydrogenated dimer acid having 36 carbon atoms as dicarboxylic acid components, 88 mol% of terephthalic acid, 5 mol% of isophthalic acid, and 7 mol% of hydrogenated dimer acid. As alcohol components, 95 mol% of ethylene glycol and 5 mol% of diethylene glycol were used.

(4) Copolyester (a1-4) Copolyester (a1-3) (intrinsic viscosity 0.72 dl/g) was prepared. The copolyester (a1-4) contained terephthalic acid and hydrogenated dimer acid having 36 carbon atoms as dicarboxylic acid components, terephthalic acid was 88 mol%, hydrogenated dimer acid was 12 mol%, and ethylene glycol as alcohol components was 67 mol% and 1,4-butanediol was 33 mol%.

(5) Copolyester (a1-5) Copolyester (a1-5) (intrinsic viscosity 1.6 dl/g) was prepared, wherein the copolyester (a1-5) contained terephthalic acid as a dicarboxylic acid component and contained 1,4-butanediol as an alcohol component.

(6) Copolyester (a1-6) Copolyester (a1-6) (intrinsic viscosity 0.69 dl/g) was prepared. The copolyester (a1-6) contained terephthalic acid, isophthalic acid, and sebacic acid as dicarboxylic acid components, terephthalic acid was 56 mol%, isophthalic acid was 12 mol%, and sebacic acid was 32 mol%. As alcohol components, ethylene glycol was 95 mol% and diethylene glycol was 5 mol%.

(5) Homopolyester (a2-1) A homopolyester (a2-1) having an intrinsic viscosity (IV) of 0.64 dl/g was prepared. The homopolyester (a2-1) was a polyester having a dicarboxylic acid component including terephthalic acid, and 98 mol% of ethylene glycol and 2 mol% of diethylene glycol as an alcohol component.

(6) Homopolyester (a2-2) A homopolyester (a2-2) having an intrinsic viscosity (IV) of 0.82 dl/g was prepared. The homopolyester (a2-2) was a polyester having a dicarboxylic acid component including terephthalic acid, and 98 mol% of ethylene glycol and 2 mol% of diethylene glycol as an alcohol component.

(7) Homopolyester (a2-3) Prepare a homopolyester (a2-3) containing particles (PET containing particles). The homopolyester (a2-3) containing particles is a polyester having a dicarboxylic acid component including terephthalic acid, 98 mol% of ethylene glycol as an alcohol component and 2 mol% of diethylene glycol, an intrinsic viscosity (IV) of 0.62 dl/g, and containing 0.2 mass% of silica particles having an average particle size of 3 μm.

(8) Homopolyester (a2-4) Prepare a homopolyester (a2-4) containing particles (PET containing particles). The homopolyester (a2-4) containing particles is a polyester having a dicarboxylic acid component including terephthalic acid and an alcohol component of 98 mol% ethylene glycol and 2 mol% diethylene glycol, an intrinsic viscosity (IV) of 0.63 dl/g, and containing 1.0 mass% of silica particles having an average particle size of 3 μm.

[Example 1] A chip of a polyester composition containing 50% by mass of copolyester (a1-1), 35% by mass of homopolyester (a2-1) and 15% by mass of homopolyester (a2-3) was fed into a double-spindle extruder with a main exhaust port set at 270°C. The chip was extruded from the nozzle and rapidly cooled and solidified on a cooling roll set at a surface temperature of 25°C using an electrostatic bonding method to obtain an unstretched sheet.

Then, the obtained unstretched sheet was stretched 3.0 times in the length direction (MD) at 75°C, introduced into a tenter, and then stretched to 4.5 times in the width direction (TD) at 95°C, and then heat-treated at 240°C for 10 seconds to obtain a copolyester film (sample) with a thickness of 50 μm.

[Example 2] A chip of a polyester composition containing 35% by mass of copolyester (a1-1), 50% by mass of homopolyester (a2-1) and 15% by mass of homopolyester (a2-3) was fed into a double-spindle extruder with a main exhaust port set at 270°C. The chip was extruded from the nozzle and rapidly cooled and solidified on a cooling roll set at a surface temperature of 25°C using an electrostatic bonding method to obtain an unstretched sheet.

Then, the obtained unstretched sheet was stretched 3.0 times in the length direction (MD) at 75°C, introduced into a tenter, and then stretched to 4.3 times in the width direction (TD) at 95°C, and then heat-treated at 240°C for 10 seconds to obtain a copolyester film (sample) with a thickness of 50 μm.

[Example 3] A chip of a polyester composition containing 50% by mass of copolyester (a1-1), 20% by mass of homopolyester (a2-2) and 30% by mass of homopolyester (a2-4) was fed into a double-spindle extruder with a main exhaust port set at 270°C. The chip was extruded from the nozzle and rapidly cooled and solidified on a cooling roll set at a surface temperature of 25°C using an electrostatic bonding method to obtain an unstretched sheet.

Then, the obtained unstretched sheet was stretched 3.3 times in the length direction (MD) at 65°C, introduced into a tenter, and then stretched to 4.7 times in the width direction (TD) at 90°C, and then heat-treated at 230°C for 10 seconds to obtain a copolyester film (sample) with a thickness of 25 μm.

[Example 4] A chip of a polyester composition containing 25% by mass of copolyester (a1-1), 50% by mass of copolyester (a1-3) and 25% by mass of homopolyester (a2-2) was fed into a double-spindle extruder with a main exhaust port set at 270°C. It was extruded from the nozzle and rapidly cooled and solidified on a cooling roll set at a surface temperature of 25°C using an electrostatic bonding method to obtain an unstretched sheet. Then, the obtained unstretched sheet was stretched 3.0 times in the length direction (MD) at 90°C, introduced into a tenter, and then stretched to 3.0 times in the width direction (TD) at 90°C, and then heat-treated at 160°C for 15 seconds to obtain a copolyester film (sample) with a thickness of 50 μm.

[Comparative Example 1] A chip of a polyester composition containing 85% by mass of copolyester (a1-2) and 15% by mass of homopolyester (a2-3) was fed into a double-spindle extruder with a main exhaust port set at 270°C. The chip was extruded from the nozzle and rapidly cooled and solidified on a cooling roll set at a surface temperature of 25°C using an electrostatic bonding method to obtain an unstretched sheet.

Then, the obtained unstretched sheet was stretched 3.3 times in the length direction (MD) at 90°C, introduced into a tenter, and then stretched to 3.7 times in the width direction (TD) at 100°C, and then heat-treated at 190°C for 10 seconds to obtain a copolyester film (sample) with a thickness of 75 μm.

[Comparative Example 2] A chip of a polyester composition containing 92% by mass of homopolyester (a2-1) and 8% by mass of homopolyester (a2-3) was fed into a double-spindle extruder with a main exhaust port set at 280°C. The chip was extruded from the nozzle and rapidly cooled and solidified on a cooling roll set at a surface temperature of 25°C using an electrostatic bonding method to obtain an unstretched sheet.

Then, the obtained unstretched sheet was stretched 3.4 times in the length direction (MD) at 90°C, introduced into a tenter, and then stretched to 4.5 times in the width direction (TD) at 135°C, and heat-treated at 223°C for 10 seconds to obtain a polyester film (sample) with a thickness of 50 μm.

[Comparative Example 3] In Comparative Example 2, a 200 μm thick unstretched polyester film (sample) was obtained in the same manner except that the film was not stretched.

[Comparative Example 4] The copolyester (a1-4) was fed as an intermediate layer into a double-spindle extruder with a primary exhaust port set at 280°C. In addition, a wafer of a polyester composition containing 50% by mass of the homopolyester (a2-1) and 50% by mass of the copolyester (a1-2) was fed as a surface layer into a double-spindle extruder with a secondary exhaust port set at 280°C. The polymer from the main extruder was used as the middle layer, and the polymer from the sub-extruder was used as the surface layer. The two layers of three layers (surface layer/middle layer/surface layer) were co-extruded and extruded from the nozzle. The sheet was rapidly cooled and solidified on a cooling roll with the surface temperature set to 30°C using an electrostatic bonding method to obtain an unstretched sheet. Then, the unstretched sheet was stretched 3.5 times in the length direction (MD) at 85°C, introduced into a tenter, and then stretched to 4.4 times in the width direction (TD) at 100°C, and then heat treated at 200°C for 10 seconds to obtain a copolyester film (sample) with a thickness of 38 μm. In Table 1, the crystallization melting enthalpy ΔHm, the cold crystallization temperature (Tcc), and the glass transition temperature (Tg) are the values of the middle layer.

[Comparative Example 5] A chip of a polyester composition containing 10% by mass of copolyester (a1-1), 30% by mass of copolyester (a1-5), and 60% by mass of copolyester (a1-6) was fed into a double-spindle extruder with a main exhaust port set at 270°C. The chip was extruded from the nozzle and rapidly cooled and solidified on a cooling roll set at a surface temperature of 25°C using an electrostatic bonding method to obtain an unstretched sheet (sample) with a thickness of 200 μm.

[Reference Examples 1-4] As reference examples, the following film samples were also evaluated. Unstretched PVC: GR379 manufactured by DiaPlus ONY (Stretched Nylon Film): SANTONYL manufactured by Mitsubishi Chemical OPP (Stretched Polypropylene Film): PYLEN P2161 manufactured by Toyobo CPP (Unstretched Polypropylene Film): ARTPLY manufactured by DiaPlus

[Table 1] Unit Embodiment 1 Embodiment 2 Embodiment 3 Embodiment 4 Comparison Example 1 Comparison Example 2 Comparison Example 3 Comparison Example 4 Comparison Example 5 Reference Example 1 Reference Example 2 Reference Example 3 Reference Example 4 Dicarboxylic acid component Terephthalic acid (TA) mol% 91.6 94.0 91.6 92.1 81.3 100.0 100.0 88.0 74.5 Unstretched PVC ONY OPP CPP Dicarboxylic acid component with 4-10 carbon atoms Adipic acid(AG) mol% 8.4 6.0 8.4 4.3 0.0 0.0 0.0 0.0 1.8 Isophthalic acid (IPA) mol% 0.0 0.0 0.0 2.6 18.7 0.0 0.0 0.0 7.4 Sebacic acid(SA) mol% 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 16.3 Total mol% 8.4 6.0 8.4 6.9 18.7 0.0 0.0 0.0 25.5 other Dimer acid (DA) mol% 0.0 0.0 0.0 1.0 0.0 0.0 0.0 12.0 0.0 Alcohol content Ethylene glycol (EG) mol% 61.6 74.6 61.6 81.2 98.8 98.8 98.8 67.0 66.7 Other alcohols Diethylene glycol (DEG) mol% 0.7 0.9 0.7 2.0 1.2 1.2 1.2 0.0 2.1 1,4-Butanediol mol% 19.5 12.7 19.5 8.7 0.0 0.0 0.0 33.0 27.8 1,6-Hexanediol mol% 18.2 11.8 18.2 8.1 0.0 0.0 0.0 0.0 3.4 Total mol% 38.4 25.4 38.4 18.8 1.2 1.2 1.2 33.0 33.3 Manufacturing conditions and results Extension ratio MD - 3.0 3.0 3.3 3.0 3.3 3.4 - 3.5 - TD - 4.5 4.3 4.7 3.0 3.7 4.5 - 4.4 - Extension temperature MD ℃ 75 75 65 90 90 90 - 85 - TD ℃ 95 95 90 90 100 135 - 100 - Heat treatment temperature ℃ 240 240 230 160 190 223 - 200 - Film thickness μm 50 50 25 50 75 50 200 38 200 80 25 25 80 Young's modulus MD GPa 1.1 2.2 0.9 0.7 3.5 5.0 2.2 0.3 0.6 0.2 2.4 2.2 0.6 TD GPa 1.1 2.9 1.0 - 4.1 5.4 2.4 0.3 - 0.2 1.6 4.8 Tensile breaking strength MD MPa 58 80 73 39 144 219 67 20 13 31 269 140 twenty three TD MPa 91 145 92 - 177 248 20 - Tensile elongation at break MD % 459 318 252 362 219 155 6 270 318 312 175 208 599 TD % 285 166 221 - 205 134 - 290 - Thermal shrinkage (120℃×5min) MD % 2.9 2.0 2.8 0.1 3.1 0.4 - 1.5 0.3 0.4 3.2 1.5 TD % -0.2 -1.1 0.5 - 1.4 0.4 - 0.0 - 0.9 -0.1 Crystallization melting energy (∆H) J/g 17.9 18.9 25.1 29.1 12.2 41.1 42.8 22.8 7.5 54.4 41.1 Store elastic modulus 25℃ MPa 1300 2300 1500 630 3500 4300 2000 410 318 290 3900 2200 630 120℃ MPa 31 120 29 27 60 1300 5 49 13 - 930 270 5 Tcc ℃ 106 113 102 114 - 144 147 96 51 - - - - Tg ℃ 33 49 34 37 74 79 80 twenty four 6 - 57 57 61 Tcc－Tg ℃ 73 64 68 77 - 65 67 72 45 - - - - Solvent resistance Toluene - A A A A A A A - - B A A A MEK - A A A A A A A - - C A A A Ethyl acetate - A A A A A A A - - B A A A Laminated Stretch Formability Extended tracking - A A B A C C C - - A C C A ※The mol% of the dicarboxylic acid component and the alcohol component are the ratios of the dicarboxylic acid component and the alcohol component in the entire polyester (A) contained in the copolyester film (copolyester layer (A1)).

According to the above-mentioned embodiments and the test results conducted by the inventors, the copolyester film has a copolyester layer (A1) containing a copolyester (a1), wherein the copolyester (a1) is a copolymer comprising terephthalic acid (X1), a dicarboxylic acid component (X), and other alcohol components (Y2) other than ethylene glycol (Y1), wherein the dicarboxylic acid component (X) comprises a dicarboxylic acid component (X2) having 4 to 10 carbon atoms, and the other alcohol components (Y2) comprise two or more, and the difference between the cold crystallization temperature (Tcc) and the glass transition temperature (Tg) exceeds 60°C. Thus, the copolyester film has excellent flexibility at room temperature, and is not only flexible, but also has elongation and strength, and thus has heat resistance sufficient for practical use. [Industrial Applicability]

The copolyester film of the present invention can be used for battery packaging materials, surface protection films, cutting protection tapes, adhesive tapes, image display components such as flexible displays, wearable terminals, bioelectrode substrates, biosensor substrates and other bio-related components, manufacturing electronic components, such as blanks used in manufacturing ceramic laminated components, manufacturing optical components, such as manufacturing polarizing elements constituting polarizing plates, etc. Among them, it is suitable for stretching processing in a state of being laminated with heterogeneous materials, especially for the use of having a step of stretching the film at or near room temperature by stretching.

Claims (38)

A copolyester film, which has a copolyester layer (A1) containing a copolyester (a1), wherein the copolyester (a1) is a copolymer comprising terephthalic acid (X1), a dicarboxylic acid component (X2) having 4 to 10 carbon atoms, and an alcohol component (Y2) other than ethylene glycol (Y1), wherein the dicarboxylic acid component (X2) having 4 to 10 carbon atoms accounts for 3 to 50 mol% of the dicarboxylic acid components in all the polyesters (A) contained in the copolyester layer (A1), and the other alcohol component (Y2) accounts for 15 to 60 mol% of the alcohol components in all the polyesters (A) contained in the copolyester layer (A1), and the total thickness exceeds 5 μm, and the difference between the cold crystallization temperature (Tcc) and the glass transition temperature (Tg) exceeds 60°C. A copolyester film having a copolyester layer (A1) containing a copolyester (a1), wherein the copolyester (a1) is a copolymer comprising terephthalic acid (X1), a dicarboxylic acid component (X), and other alcohol components (Y2) other than ethylene glycol (Y1), wherein the other alcohol components (Y2) comprise two or more, the total thickness exceeds 5 μm, and the difference between the cold crystallization temperature (Tcc) and the glass transition temperature (Tg) exceeds 70°C. The copolyester film of claim 2, wherein the copolyester (a1) contains a dicarboxylic acid component (X2) having 4 to 10 carbon atoms, and the ratio of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms to the dicarboxylic acid components in the entire polyester (A) contained in the copolyester layer (A1) is 3 to 50 mol%. For example, in the copolyester film of claim 2 or 3, the ratio of other alcohol component (Y2) to the alcohol component of all polyesters (A) contained in the copolyester layer (A1) is 15-60 mol%. A copolyester film as claimed in claim 1, wherein the difference between the cold crystallization temperature (Tcc) and the glass transition temperature (Tg) exceeds 70°C. A copolyester film as claimed in any one of claims 1 to 3, wherein the ratio of the dicarboxylic acid component (X2) having 4 to 10 carbon atoms in the dicarboxylic acid components of the entire polyester (A) contained in the copolyester layer (A1) is 3 to 12 mol%. For the copolyester film of any one of claims 1 to 3, its tensile elongation at break at 25°C is 295% or more. For the copolyester film of any one of claims 1 to 3, the storage elastic modulus at 25°C is less than 2500 MPa, and the storage elastic modulus at 120°C is more than 10 MPa. A copolyester film as claimed in any one of claims 1 to 3, wherein the dicarboxylic acid component (X2) having 4 to 10 carbon atoms comprises an aliphatic dicarboxylic acid. As in claim 9, the copolyester film, wherein the aliphatic dicarboxylic acid comprises adipic acid. As in claim 10, the copolyester film, wherein the ratio of adipic acid in the dicarboxylic acid components of the total polyester (A) contained in the copolyester layer (A1) is 7-40 mol%. A copolyester film as claimed in any one of claims 1 to 3, wherein the other alcohol component (Y2) comprises 1,4-butanediol. A copolyester film as claimed in any one of claims 1 to 3, wherein the other alcohol component (Y2) comprises 1,4-butanediol and 1,6-hexanediol. For example, the copolyester film of claim 13, wherein the ratio of the molar amount of 1,6-hexanediol to the molar amount of 1,4-butanediol is greater than 0.5 and less than 1.4. A copolyester film as claimed in any one of claims 1 to 3, wherein the copolyester layer (A1) further comprises a polyester (a2), which is a polyester other than the copolyester (a1) and comprises terephthalic acid as a dicarboxylic acid component and ethylene glycol as an alcohol component. A copolyester film as claimed in any one of claims 1 to 3, which has a polyester layer (B1) and a polyester layer (B2) on the front and back sides of the copolyester layer (A1), respectively. A laminated film comprising the copolyester film of any one of claims 1 to 16, and a functional layer disposed on at least one side of the copolyester film. As in claim 17, the functional layer is a resin layer comprising a resin containing vinyl alcohol structural units. A laminated film as claimed in claim 17 or 18, wherein the functional layer is formed into a blank. A laminated film as claimed in claim 17 or 18, wherein the functional layer constitutes a polarizing element. As in claim 17, the laminated film, wherein the functional layer is an adhesive layer. As in claim 21, the laminated film, wherein the adhesive layer contains a conductive material. As in claim 17, the multilayer film, wherein the functional layer is a conductive layer. A method for using a copolyester film or a laminated film, comprising the following steps: stretching the copolyester film of any one of claims 1 to 16 or the laminated film of any one of claims 17 to 23. A method for using a copolyester film or a laminated film as claimed in claim 24, wherein the stretching is performed in the atmosphere or in water. The method for using the copolyester film or laminated film of claim 24 or 25, wherein the stretching is performed at a stretching ratio of 2.0 to 6.0 times. A copolyester film as claimed in any one of claims 1 to 3, which is used for any one of an adhesive tape, a surface protection film, and a semiconductor cutting protection tape. The laminated film of claim 17 or 18 is used for any of adhesive tape, surface protection film, and semiconductor cutting protection tape. The method of use of claim 24 or 25, wherein the copolyester film or laminated film is used for any of adhesive tape, surface protection film, and semiconductor cutting protection tape. A copolyester film as claimed in any one of claims 1 to 3, which is used for any one of a wearable terminal, a bioelectrode substrate, and a biosensor substrate. The laminated film of claim 17 or 18 is used for any of a wearable terminal, a bioelectrode substrate, and a biosensor substrate. The method of use of claim 24 or 25, wherein the copolyester film or laminated film is used for any of a wearable terminal, a bioelectrode substrate, and a biosensor substrate. A copolyester film as claimed in any one of claims 1 to 3, which is used for manufacturing electronic components. The laminated film of claim 17 or 18 is used for manufacturing electronic components. The method of use of claim 24 or 25, wherein the copolyester film or laminated film is used to manufacture electronic components. A copolyester film as claimed in any one of claims 1 to 3, which is used for manufacturing optical components. A multilayer film as claimed in claim 17 or 18, which is used to manufacture optical components. The method of use of claim 24 or 25, wherein the copolyester film or laminated film is used to manufacture optical components.