Polyester film and application thereof

Abstract

To provide a polyester film excellent in handleability while having extremely excellent surface smoothness.SOLUTION: The present invention relates to a polyester film having one surface (A) and the other surface (C), wherein the maximum peak height (Sp) of the one surface (A) is 101 nm or less, the arithmetic average height (Sa) of the other surface (C) is 10 nm or less, and the air leakage index is 20000 seconds or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyester film, a release film, a polyester film with a ceramic green sheet, use of the polyester film as a support for a ceramic green sheet, and a method for producing a ceramic green sheet. [Background technology]

[0002] Polyester films, typified by polyethylene terephthalate films and polyethylene naphthalate films, have excellent properties such as mechanical properties, dimensional stability, flatness, heat resistance, chemical resistance, and optical properties, and are also excellent in cost performance, and are therefore used in a variety of applications. For example, by utilizing the smoothness of the film surface, polyester films are suitably used in a variety of applications, such as release films for molding green sheets for multilayer ceramic capacitors, substrates for releasing interlayer insulating resins, and substrates for dry film resists.

[0003] For example, polyester films are used as supports for release films used to mold green sheets for multilayer ceramic capacitors. In recent years, progress has been made in miniaturizing and increasing the capacity of multilayer ceramic capacitors, leading to the thinning of ceramic green sheets. As ceramic green sheets become thinner, any minute protrusions on the surface of the release film acting as a carrier film can cause pinholes and other defects in the ceramic green sheets. For this reason, release films used to manufacture ceramic green sheets are required to have a high degree of surface smoothness.

[0004] Conventionally, as a support for this type of release film, Patent Document 1 discloses a release film for producing ceramic green sheets, which comprises a substrate having a first side and a second side, a smoothing layer provided on the first side of the substrate, and a release agent layer provided on the side of the smoothing layer opposite the substrate, wherein the smoothing layer is formed by heating and curing a composition for forming a smoothing layer that contains a thermosetting compound having a weight-average molecular weight of 950 or less, and wherein the arithmetic mean roughness Ra1 of the outer surface of the release agent layer is 8 nm or less and the maximum protrusion height Rp1 of the outer surface of the release agent layer is 50 nm or less.

[0005] Patent Document 2 also describes a polyester film for release that has excellent surface smoothness and particularly few fine defects on the film surface, with the number of depression defects of 0.5 μm or more in depth being 5 / m 2 and a release polyester film having a center line average roughness SRa of 15 to 35 nm and a ten-point average roughness SRz of 1000 nm or less on at least one surface thereof.

[0006] Patent Document 3 also describes a polyester film roll obtained by winding up a polyester film, in which slack defects present in the polyester film are removed within 100 m. 2 A polyester film is disclosed in which the number of particles per unit area is less than 5.

[0007] Furthermore, Patent Document 4 discloses a polyester film in which one surface (A) contains at least particles (a1) and particles (a2), the particles (a1) are alumina particles, and the particles (a2) are particles other than the particles (a1), and the surface (A) satisfies the following requirements (1) and (2): (1) The arithmetic mean height (Sa) is 1 to 15 nm. (2) The maximum peak height (Sp) is 150 nm or less. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-177093 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-7054 [Patent Document 3] Japanese Patent Application Publication No. 2018-90803 [Patent Document 4] Japanese Patent Publication No. 2024-046615 Summary of the Invention [Problem to be solved by the invention]

[0009] As described above, development of polyester films with excellent smoothness has been promoted as supports for release films for producing ceramic green sheets. However, in recent ceramic green sheet production processes, a higher level of surface smoothness is sometimes required. Therefore, there is a demand for the development of polyester films with extremely excellent surface smoothness.

[0010] On the other hand, if the surface smoothness of a polyester film is increased, the films may adhere to each other when laminated together, resulting in problems such as poor handling and processability of the film. In particular, since polyester films are stored and distributed in rolled form, it is important from the viewpoint of increasing the production efficiency of ceramic green sheets that the films do not adhere excessively to each other and have good unwinding and handling properties.

[0011] Therefore, in order to solve the problems of the conventional techniques, the present inventors have carried out investigations with the aim of providing a polyester film that has excellent surface smoothness and also excellent handleability. [Means for solving the problem]

[0012] Examples of specific embodiments of the present invention are given below.

[0013] [1] Having one surface (A) and the other surface (C), A polyester film having a maximum peak height (Sp) of one surface (A) of 101 nm or less, an arithmetic mean height (Sa) of the other surface (C) of 10 nm or less, and an air leakage index of 20,000 seconds or less. [2] The polyester film according to [1], wherein the other surface (C) has a maximum peak height (Sp) of 50 nm or less. [3] The polyester film according to [1] or [2], wherein the arithmetic mean height (Sa) of one surface (A) is 50 nm or less. [4] The polyester film according to any one of [1] to [3], wherein the arithmetic mean height (Sa) of one surface (A) is 1 nm or more. [5] The polyester film according to any one of [1] to [4], wherein the ratio of maximum peak height (Sp) to arithmetic mean height (Sa) on one surface (A) is 3 to 20. [6] The polyester film according to any one of [1] to [5], wherein the maximum peak height (Sp) / arithmetic mean height (Sa) of the other surface (C) is 67 or less. [7] The polyester film according to any one of [1] to [6], wherein the layer forming one surface (A) contains at least particles (a1) and particles (a2). [8] The polyester film according to [7], in which the zeta potential of the particles (a1) and the particles (a2) at pH 7 satisfies the following condition (1) or (2): (1) The zeta potential of particle (a1) is positive and the zeta potential of particle (a2) is negative; (2) The zeta potential of the particles (a1) is negative, and the zeta potential of the particles (a2) is positive. [9] The polyester film according to [7] or [8], wherein the particles (a1) have an average particle size of 0.01 to 1 μm.

[10] The polyester film according to any one of [7] to [9], wherein the particles (a2) have an average particle size of 0.05 to 1.5 μm.

[11] The polyester film according to any one of [7] to

[10] , wherein the average particle size of the particles (a1) is smaller than the average particle size of the particles (a2).

[12] The polyester film according to any one of [7] to

[11] , wherein the value obtained by dividing the average particle size of the particles (a2) by the average particle size of the particles (a1) is from 1.2 to 15.

[13] The polyester film according to any one of [7] to

[12] , wherein at least one of the particles (a1) and the particles (a2) has a Mohs hardness of 8 or more.

[14] The polyester film according to any one of [7] to

[13] , wherein the particles (a1) are alumina particles.

[15] The polyester film according to any one of [7] to

[14] , wherein the particles (a2) are either organic particles or silica particles.

[16] The polyester film according to any one of [1] to

[15] , wherein the intrinsic viscosity of the polyester resin constituting the layer forming one surface (A) is 0.64 dL / g or more.

[17] The polyester film according to any one of [1] to

[16] , wherein the intrinsic viscosity of the polyester resin constituting the layer forming the other surface (C) is 0.63 dL / g or more.

[18] The polyester film according to any one of [1] to

[17] , wherein the intrinsic viscosity of the polyester resin constituting the layer forming the other surface (C) divided by the intrinsic viscosity of the polyester resin constituting the layer forming one surface (A) is 0.7 to 1.3.

[19] The polyester film according to any one of [1] to

[18] , wherein the layer forming the other surface (C) contains particles.

[20] The polyester film according to any one of [7] to

[19] , wherein the total content of the particles (a1) and the particles (a2) is 1,000 to 10,000 ppm based on the total mass of the layer forming one surface (A).

[21] The polyester film according to any one of [7] to

[20] , wherein the content of the particles (a1) is 100 to 5000 ppm based on the total mass of the layer forming one surface (A).

[22] The polyester film according to any one of [7] to

[21] , wherein the content of the particles (a2) is 100 to 8000 ppm based on the total mass of the layer forming one surface (A).

[23] The polyester film according to any one of [1] to

[22] , which has a three-layer structure.

[24] The polyester film according to

[23] , wherein the intermediate layer of the three-layer structure is substantially free of particles.

[25] The polyester film according to

[23] or

[24] , wherein the intermediate layer of the three-layer structure contains recycled polyester resin, and the content of recycled polyester resin is 10% by mass or more based on the total mass of the resins constituting the intermediate layer.

[26] The polyester film according to any one of [1] to

[25] , which is used as a support for a ceramic green sheet in a process for producing a multilayer ceramic capacitor.

[27] A release film comprising the polyester film according to any one of [1] to

[26] , further comprising a release layer on at least one surface thereof.

[28] A polyester film with a ceramic green sheet, obtained by laminating a ceramic green sheet on the polyester film according to any one of [1] to

[26] .

[29] Use of the polyester film according to any one of [1] to

[26] as a support for a ceramic green sheet in a process for producing a multilayer ceramic capacitor.

[30] A method for producing a ceramic green sheet, comprising a step of applying a ceramic slurry containing a ceramic component to the surface (C) side of the polyester film according to any one of [1] to

[26] . [Effects of the Invention]

[0014] According to the present invention, a polyester film having excellent surface smoothness and excellent handleability can be obtained. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a cross-sectional view illustrating the structure of the polyester film of this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention will be described in detail below. The following description may be based on representative embodiments or specific examples, but the present invention is not limited to such embodiments. In this specification, when "X to Y" (X and Y are arbitrary numbers) is used, it means "X or more and Y or less" unless otherwise specified, and also includes "preferably greater than X" or "preferably less than Y." Furthermore, when "X or more" (X is an arbitrary number) or "Y or less" (Y is an arbitrary number), it also includes the meaning "preferably greater than X" or "preferably less than Y." In the following description, the terms "film" and "sheet" are not clearly distinguished from each other, and the term "film" includes the term "sheet," and the term "sheet" includes the term "film."

[0017] (polyester film) This embodiment relates to a polyester film (hereinafter also referred to as the present film) having one surface (A) and another surface (C), in which the maximum peak height (Sp) of one surface (A) is 101 nm or less, the arithmetic mean height (Sa) of the other surface (C) is 10 nm or less, and the air leakage index is 20,000 seconds or less. This embodiment provides a polyester film that has excellent surface smoothness and excellent handleability.

[0018] In this specification, one surface (A) has a larger arithmetic mean height (Sa) than the other surface (C) and is designed to be rougher than the other surface (C). In other words, the other surface (C) has superior smoothness. For this reason, for example, when the present film is used as a support for a release film for producing a ceramic green sheet, it is preferable to laminate a ceramic green sheet on the other surface (C). In recent years, ceramic green sheets have become thinner in order to reduce the size and increase the capacity of multilayer ceramic capacitors, and support materials are required to have high surface smoothness on the side on which the ceramic green sheet is laminated. On the other hand, in order to impart slipperiness and handleability while maintaining surface smoothness, it has been considered to design the side opposite to the side on which the ceramic green sheet is laminated to be rougher than the side on which the ceramic green sheet is laminated. However, if the relatively rough surface (A) has coarse protrusions, the uneven shape caused by the coarse protrusions will be transferred to the surface (C), impairing the smoothness of the smooth surface (C). In this regard, the present film has the above-mentioned structure, and therefore can achieve both high surface smoothness of the smooth surface (C) and excellent handleability.

[0019] In the present film, the maximum peak height (Sp) of one surface (A) is preferably 101 nm or less, more preferably 98 nm or less, and even more preferably 95 nm or less. The lower limit of the maximum peak height (Sp) of one surface (A) is not particularly limited, and the maximum peak height (Sp) of one surface (A) may be, for example, 5 nm or more, 10 nm or more, or 20 nm or more. By setting the maximum peak height (Sp) of one surface (A) to the above-mentioned upper limit or less, it is possible to prevent the unevenness caused by minute protrusions on surface (A) from being transferred to surface (C) (described below) when the polyester film is laminated or wound into a roll. On the other hand, by setting the maximum peak height (Sp) of surface (A) to the above-mentioned lower limit or more, the necessary roughness is provided on the back surface of the polyester film, thereby improving the handleability of the polyester film.

[0020] The arithmetic mean height (Sa) of one surface (A) of the present film is preferably 50 nm or less, more preferably 45 nm or less, even more preferably 40 nm or less, even more preferably 35 nm or less, even more preferably 30 nm or less, even more preferably 25 nm or less, even more preferably 20 nm or less, particularly preferably 15 nm or less, and most preferably 10 nm or less. The arithmetic mean height (Sa) of one surface (A) is preferably 1 nm or more, more preferably 3 nm or more, and even more preferably 5 nm or more. By setting the arithmetic mean height (Sa) of one surface (A) to the above upper limit or less, a polyester film with superior surface smoothness can be obtained. The arithmetic mean height (Sa) of one surface (A) is preferably the above lower limit or more, and the polyester film surface is preferably provided with a predetermined amount of roughness or more. This provides the polyester film with the necessary roughness to improve its handleability and reduce its air leakage index. As a result, the polyester film can exhibit appropriate slip properties and improve its handleability. For example, a polyester film having a suitable surface roughness can be easily wound into a roll.

[0021] The maximum peak height (Sp) of the other surface (C) of the present film is preferably 50 nm or less, more preferably 45 nm or less, even more preferably 40 nm or less, even more preferably 35 nm or less, even more preferably 30 nm or less, and particularly preferably 25 nm or less. The lower limit of the maximum peak height (Sp) of the other surface (C) is not particularly limited, and the maximum peak height (Sp) of the other surface (C) may be, for example, 5 nm or more or 10 nm or more. By setting the maximum peak height (Sp) of the other surface (C) to the above upper limit or less, it is possible to effectively prevent protrusions of unintended sizes from occurring on the surface of the polyester film, even on surface (C), and a polyester film with even better surface smoothness can be obtained.

[0022] The arithmetic mean height (Sa) of the other surface (C) of the present film is preferably 10 nm or less, more preferably 5 nm or less, even more preferably 3 nm or less, even more preferably 2 nm or less, and particularly preferably 1.6 nm or less. The lower limit of the arithmetic mean height (Sa) of the other surface (C) is not particularly limited, and the arithmetic mean height (Sa) of the other surface (C) may be, for example, 0.1 nm or more, 0.3 nm or more, or 0.5 nm or more. By setting the arithmetic mean height (Sa) of the other surface (C) to the above upper limit or less, a polyester film with excellent surface smoothness can be obtained. When the present film is used as a support for a release film for producing a ceramic green sheet, it is preferable that a ceramic green sheet be laminated on the other surface (C). In this case, a smaller arithmetic mean height (Sa) of the other surface (C) is preferred because this can suppress the occurrence of defects such as repelling and pinholes during the formation of the ceramic green sheet.

[0023] The value (Sp / Sa) obtained by dividing the maximum peak height (Sp) of one surface (A) by the arithmetic mean height (Sa) of said surface (A) is preferably 50 or less, more preferably 30 or less, even more preferably 20 or less, and particularly preferably 15 or less. There are no particular restrictions on the lower limit of the value of Sp / Sa, and the value of Sp / Sa of one surface (A) is, for example, preferably 1.5 or more, more preferably 2 or more, even more preferably 3 or more, still more preferably 4 or more, and particularly preferably 5 or more.

[0024] The value (Sp / Sa) obtained by dividing the maximum peak height (Sp) of the other surface (C) by the arithmetic mean height (Sa) of the surface (C) is preferably 67 or less, more preferably 60 or less, even more preferably 50 or less, even more preferably 40 or less, and particularly preferably 30 or less. There is no particular limitation on the lower limit of the Sp / Sa value, and the value of Sp / Sa of the other surface (C) is, for example, preferably 1 or more, more preferably 2 or more, even more preferably 3 or more, even more preferably 4 or more, and particularly preferably 5 or more.

[0025] The methods for measuring the maximum peak height (Sp) and arithmetic mean height (Sa) of each surface of the film are the same as those for measuring the maximum peak height (Sp) and arithmetic mean height (Sa) of each surface layer, as described below.

[0026] In this film, by controlling the maximum peak height (Sp) of one surface (A) to be low, it is possible to prevent the transfer of unevenness caused by coarse protrusions to the other surface (C). On the other hand, it is preferable to maintain the arithmetic mean height (Sa) of the surface (A) within a certain numerical range. This provides the rough surface necessary to improve the handleability of the polyester film and reduce the air leakage index of the polyester film. Furthermore, in this film, by controlling the arithmetic mean height (Sa) and maximum peak height (Sp) of the other surface (C) to be as low as possible, it is possible to obtain a polyester film with an extremely smooth surface (C). In this way, this film has succeeded in achieving high smoothness, particularly on the surface (C), while ensuring processability and handleability. Therefore, this film is suitable as a support (substrate) for ceramic green sheets in the manufacturing process of multilayer ceramic capacitors, and its use facilitates the formation of thin ceramic green sheets.

[0027] To achieve a predetermined range for the maximum peak height (Sp) of each surface of the present film and a predetermined range for the air leakage index, for example, the type and physical properties of particles added to the layers forming each surface may be appropriately controlled, and the intrinsic viscosity of the polyester resin constituting each layer may be further controlled. More specifically, the layer forming one surface (A) may contain two or more types of particles with different average particle sizes and zeta potentials, and the intrinsic viscosity of the polyester resin constituting that layer may be adjusted to a predetermined value or higher. When particles are blended, the surface properties can be adjusted by adjusting the content in consideration of the type, composition, average particle size, particle size distribution, hardness, affinity with the polyester to be contained, etc. In addition, adjusting the type and content of particles in consideration of the type of polyester to be contained, such as composition, viscosity, molecular weight, thermal properties, and the presence or absence of a copolymerization component, is also useful for adjusting the surface properties. Another method for keeping the maximum peak height (Sp) of each surface of the present film within a predetermined range and the air leakage index below a predetermined numerical range is to use recycled polyester raw materials for the polyester film. Furthermore, during the production of the polyester film, it is also effective to control, for example, the stretching ratio (in the case of biaxial stretching, the longitudinal and transverse stretching ratios), the stretching temperature, the heat treatment temperature and treatment time (in the case of biaxial stretching, the heat treatment temperature and treatment time, particularly after transverse stretching). The preferred conditions for the particles and polyester resin contained in the layer forming one surface (A) are as explained in the section on the structure of each layer.

[0028] Another method is to appropriately control the manufacturing conditions of the polyester film so that the maximum peak height (Sp) and arithmetic mean height (Sa) of each surface of the present film fall within a predetermined range and the air leakage index falls within a predetermined range. More specifically, as described below, this method includes appropriately controlling the kneading conditions and extrusion rate of the molten polyester resin in the polyester film manufacturing process.

[0029] The present film may be a single-layer polyester film, but is preferably a laminated polyester film having two or more layers, more preferably a laminated polyester film having a three-layer structure, in which case the present film is particularly preferably a laminated polyester film having a surface layer (A), an intermediate layer (B), and a surface layer (C).

[0030] When the present film is a laminated polyester film having a surface layer (A), an intermediate layer (B), and a surface layer (C), as shown in Figure 1, the laminated polyester film 10 of this embodiment has a surface layer (A) 12 on one side of the intermediate layer (B) 14 and a surface layer (C) 16 on the other side of the intermediate layer (B) 14. In this embodiment, the intermediate layer (B) 14 and the surface layer (A) 12 are preferably laminated so as to be in direct contact with each other, but another layer may be provided between the intermediate layer (B) 14 and the surface layer (A) 12. Similarly, the intermediate layer (B) 14 and the surface layer (C) 16 are preferably laminated so as to be in direct contact with each other, but another layer may be provided between the intermediate layer (B) 14 and the surface layer (C) 16.

[0031] This embodiment may also relate to a roll (rolled body) obtained by winding the present film. As described above, the present film has appropriate slip properties, so it can be stored or distributed as a roll.

[0032] <Surface layer (A)> The present film preferably has a surface layer (A) as a layer forming one surface (A). Hereinafter, the layer forming one surface (A) may be referred to as the "surface layer (A)." The surface layer (A) is a layer containing a polyester resin. When the present film is used as a support for a release film for producing a ceramic green sheet, the surface layer (A) is a layer disposed on the side opposite to the side on which the ceramic green sheet is laminated.

[0033] In this embodiment, the maximum peak height (Sp) of the surface layer (A) is preferably 101 nm or less, more preferably 98 nm or less, and even more preferably 95 nm or less. The lower limit of the maximum peak height (Sp) of the surface layer (A) is not particularly limited, and the maximum peak height (Sp) of the surface layer (A) may be, for example, 5 nm or more, 10 nm or more, or 20 nm or more. By setting the maximum peak height (Sp) of the surface layer (A) to the above-mentioned upper limit or less, it is possible to prevent the unevenness caused by minute protrusions on the surface layer (A) from being transferred to the surface layer (C) described below when the polyester film is laminated or wound into a roll. On the other hand, by setting the maximum peak height (Sp) of the surface layer (A) to the above-mentioned lower limit or more, a rough surface necessary for improving the slip properties and handleability of the polyester film is provided, thereby improving the handleability of the polyester film.

[0034] The arithmetic mean height (Sa) of the surface layer (A) is preferably 50 nm or less, more preferably 45 nm or less, even more preferably 40 nm or less, even more preferably 35 nm or less, even more preferably 30 nm or less, even more preferably 25 nm or less, even more preferably 20 nm or less, particularly preferably 15 nm or less, and most preferably 10 nm or less. The arithmetic mean height (Sa) of the surface layer (A) is preferably 1 nm or more, more preferably 3 nm or more, and even more preferably 5 nm or more. By setting the arithmetic mean height (Sa) of the surface layer (A) to the above upper limit or less, it is possible to prevent the uneven shape of the surface layer (A) from being transferred to the surface layer (C). On the other hand, by setting the arithmetic mean height (Sa) of the surface layer (A) to the above lower limit or more, a rough surface necessary for improving the slipperiness and handleability of the polyester film is provided, thereby improving the handleability of the polyester film. For example, when a polyester film is wound into a roll, the polyester film can exhibit appropriate slip properties, making it easy to wind into a roll.

[0035] The value (Sp / Sa) obtained by dividing the maximum peak height (Sp) of the surface layer (A) by the arithmetic mean height (Sa) of the surface layer (A) is preferably 50 or less, more preferably 30 or less, even more preferably 20 or less, and particularly preferably 15 or less. There are no particular restrictions on the lower limit of the value of Sp / Sa, and Sp / Sa of the surface layer (A) is, for example, preferably 1.5 or more, more preferably 2 or more, even more preferably 3 or more, still more preferably 4 or more, and particularly preferably 5 or more.

[0036] Here, arithmetic mean height (Sa) is one of the surface roughness parameters (ISO 25178), and is a three-dimensional extension of the two-dimensional Ra (arithmetic mean roughness of a line), and is found by dividing the volume of the area enclosed by the surface shape curve and the mean surface by the measured area, and is calculated using the following formula (1): When the surface is the XY plane and the height direction is the Z axis, A is the defined area (the entire image), and Z(x, y) is the height from the surface at height 0 of the image point (x, y), then it can be expressed as in the following formula (1).

[0037]

number

[0038] The maximum peak height (Sp) is one of the surface roughness parameters (ISO 25178) and represents the maximum value of the height from the mean plane of the surface, and is expressed by the following formula (2).

[0039]

number

[0040] The root mean square height (Sq) of the surface layer (A) of the present film is preferably 40 nm or less, more preferably 35 nm or less, and even more preferably 30 nm or less. The lower limit of the root mean square height (Sq) of the surface layer (A) is not particularly limited, and the root mean square height (Sq) of the surface layer (A) is, for example, preferably 1 nm or more, more preferably 3 nm or more.

[0041] From the viewpoint of achieving both surface smoothness and slip properties, the value (SqA / SqC) obtained by dividing the root mean square height (SqA) of the surface layer (A) described below by the root mean square height (SqC) of the surface layer (C) is preferably 7 or more, more preferably 8 or more, and even more preferably 9 or more. On the other hand, SqA / SqC is preferably 20 or less, more preferably 18 or less, and even more preferably 17 or less.

[0042] The root mean square height (Sq) is one of the surface roughness parameters (ISO 25178) and is a three-dimensional extension of the two-dimensional Rq. It is the root mean square value of the height data in a defined area, and is a parameter equivalent to the standard deviation of the distance from the mean surface, and can be calculated using the following formula:

[0043]

number

[0044] The kurtosis (Sku) of the surface layer (A) of the present film is preferably 18 or less, more preferably 16 or less, even more preferably 14 or less, and particularly preferably 12 or less. The lower limit of the kurtosis (Sku) of the surface layer (A) is not particularly limited, and the kurtosis (Sku) of the surface layer (A) is, for example, preferably 1 or more, more preferably 2 or more, and may be 3 or more.

[0045] Kurtosis (Sku) is one of the surface roughness parameters (ISO 25178) that can be used to evaluate the peakiness (kurtosis) of a height distribution histogram and can be calculated using the following formula:

[0046]

number

[0047] The skewness (Ssk) of the surface layer (A) of the present film is preferably 4 or less, more preferably 3.5 or less, even more preferably 3 or less, even more preferably 2.5 or less, and particularly preferably 2 or less. On the other hand, the lower limit of the skewness (Ssk) of the surface layer (A) is not particularly limited, and for example, the skewness (Ssk) of the surface layer (A) is preferably 0.5 or more, more preferably 0.8 or more, and may be 1 or more.

[0048] Skewness (Ssk) is one of the surface roughness parameters (ISO 25178) and can be calculated using the following formula:

[0049]

number

[0050] The arithmetic mean height (Sa), maximum peak height (Sp), root mean square height (Sq), kurtosis (Sku), and skewness (Ssk) can be adjusted, for example, by adjusting the content of particles in consideration of the type, composition, average particle size, particle size distribution, hardness, affinity with the polyester to be contained, etc. Also, adjusting the type and content of particles in consideration of the type of polyester to be contained, such as composition, viscosity, molecular weight, thermal properties, and the presence or absence of copolymerization components, is also useful for adjusting surface properties. When two or more types of particles are used in combination, it is preferable to adjust the content ratio in consideration of the type of particles and polyester to be used. In addition, when adjusting the arithmetic mean height (Sa), maximum peak height (Sp), root mean square height (Sq), kurtosis (Sku), and skewness (Ssk), it is also effective to control, during the production of the polyester film, for example, the stretching ratio (longitudinal and transverse stretching ratios in the case of biaxial stretching), the stretching temperature, the heat treatment temperature and treatment time (particularly the heat treatment temperature and treatment time after transverse stretching in the case of biaxial stretching). Another example is a method of appropriately controlling the kneading conditions and extrusion rate of the molten polyester resin in the process of producing the polyester film, as will be described later.

[0051] The surface layer (A) preferably contains particles. The presence of particles in the surface layer (A) can provide easy slippage and prevent scratches during each process. Furthermore, the presence of particles in the surface layer (A) makes it easy to control the maximum peak height (Sp) and arithmetic mean height (Sa) within a desired range. The maximum peak height (Sp) and arithmetic mean height (Sa) may be controlled within a desired range by subjecting the surface layer (A) to a surface treatment or coating.

[0052] The type of particles contained in the surface layer (A) is not particularly limited as long as they are particles that can impart slipperiness. Specific examples include inorganic particles such as silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, aluminum oxide, titanium oxide, and barium sulfate, as well as organic particles obtained by polymerizing acrylic acid ester monomers, styrene monomers, silicone monomers, etc., or organic particles obtained by copolymerizing these monomers, acrylic resin particles, melamine resin particles, silicone resin particles, and cross-linked polystyrene particles. Among these, organic particles, calcium carbonate, silica, aluminum oxide, etc. are preferably used. Furthermore, precipitated particles obtained by precipitating and finely dispersing a portion of a metal compound such as a catalyst during the polyester production process can also be used.

[0053] The shape of the particles in the surface layer (A) is not particularly limited, and any of spherical, blocky, rod-like, flat, etc. may be used. There are also no particular limitations on the hardness, specific gravity, color, etc.

[0054] The surface layer (A) preferably contains two or more types of particles. The surface layer (A) more preferably contains at least particles (a1) and particles (a2). Here, particles (a1) and particles (a2) are different types of particles. In this embodiment, by using a combination of two or more types of particles in the surface layer (A), it becomes possible to precisely control the fine unevenness of the film surface, and as a result, it becomes easy to control the arithmetic mean height (Sa) and maximum peak height (Sp) of the surface layer (A) within the desired range.

[0055] The zeta potential at pH 7 of the particles (a1) and particles (a2) contained in the surface layer (A) preferably satisfies the following condition (1) or (2). (1) The zeta potential of the particles (a1) is positive, and the zeta potential of the particles (a2) is negative. (2) The zeta potential of the particles (a1) is negative, and the zeta potential of the particles (a2) is positive. The zeta potential can be measured by electrophoretic light scattering.

[0056] That is, at pH 7, the difference in charge between particle (a1) and particle (a2) causes particle (a1) and particle (a2) to be electrically attracted to each other, and for example, particle (a1) can be positioned around particle (a2). In such particles in which particle (a1) is electrically positioned around particle (a2) (hereinafter, sometimes referred to as "composite particles"), the presence of particle (a1) with the same charge on the outside causes the composite particles to repel each other, and it is thought that the composite particles do not aggregate and maintain high dispersibility.

[0057] In such composite particles, the average particle size of particles (a1) is preferably smaller than that of particles (a2). That is, in composite particles, the average particle size of particles (a1) located on the outside is preferably smaller than that of particles (a2) located on the inside. Furthermore, the value (a2 / a1) obtained by dividing the average particle size of particles (a2) by the average particle size of particles (a1) is preferably 1.2 or more, more preferably 2 or more, and even more preferably 3 or more. The value (a2 / a1) obtained by dividing the average particle size of particles (a2) by the average particle size of particles (a1) is preferably 15 or less, more preferably 12 or less, and even more preferably 10 or less. In this way, a structure is formed in which relatively small particles (a1) are positioned so as to surround relatively large particles (a2), and the composite particles are electrically repelled from each other, which is thought to prevent aggregation of particles (a2) and further improve dispersibility.

[0058] Examples of particles having a positive zeta potential at pH 7 include alumina, cation-modified silica, and rare earth compounds such as ytterbium trifluoride, yttrium fluoride, lanthanum fluoride, yttrium oxide, lanthanum oxide, and ytterbium oxide, among which alumina is preferred. Examples of particles having a negative zeta potential at pH 7 include inorganic particles such as metal oxides such as silica, titanium oxide, ceria, zirconium oxide, barium oxide, chromium oxide, iron oxide, and tungsten oxide, composite oxides such as silica-zirconium oxide, silica-titanium oxide, silica-titanium oxide-barium oxide, silica-titanium oxide-zirconium oxide, borosilicate glass, aluminosilicate glass, and fluoroaluminosilicate glass, and organic particles having a carboxy group or a sulfonic acid group, among which organic particles and silica are preferred.

[0059] The zeta potential of particles (a1) and particles (a2) at pH 7 may be the zeta potential of the particles themselves, or may be adjusted by modifying the particle surface with a surface treatment agent, etc. Examples of surface treatment agents include silane coupling agents such as vinyltriethoxysilane, vinyltrimethoxysilane, vinyl-tris(β-methoxyethoxy)silane, γ-methacryloyloxypropyltrimethoxysilane, κ-methacryloyloxydodecyltrimethoxysilane, β-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane, γ-ureidopropyltriethoxysilane, γ-chloropropyltrimethoxysilane, methyltrimethoxysilane, ethyltrimethoxysilane, and methyltriethoxysilane, as well as titanate-based coupling agents. The type and amount of treatment agent may be appropriately adjusted to achieve the desired zeta potential.

[0060] The amount of the surface treatment agent used for surface treatment is usually 1 to 30 parts by mass, preferably 3 to 15 parts by mass, per 100 parts by mass of particles. Furthermore, the method for treatment with the surface treatment agent is not particularly limited, and known methods can be used. Examples include a method in which the particles and the surface treatment agent are dispersed and mixed in an appropriate solvent using a ball mill or the like, dried in an evaporator or air-dried, and then heated to 50 to 150°C, a method in which the particles and the surface treatment agent are heated and refluxed in a solvent such as alcohol for several hours, and a method in which the surface treatment agent is graft-polymerized onto the particle surfaces.

[0061] In particular, in this embodiment, it is preferable that the particles (a1) are alumina particles. Also, in this embodiment, it is preferable that the particles (a2) are either organic particles or silica particles. When the particles (a1) are alumina, the zeta potential of the particles (a1) at pH 7 is positive, so particles (a2) with a negative zeta potential are selected. Suitable examples of particles with a negative zeta potential at pH 7 include organic particles and silica particles. Note that when composite particles are formed, the same phenomenon occurs even if the particles (a1) are not alumina, and it is believed that the effects of the present invention can be achieved by satisfying the above requirements (1) and (2).

[0062] The average particle size of the particles (a1) contained in the surface layer (A) is preferably 0.01 μm or more, more preferably 0.03 μm or more, and even more preferably 0.05 μm or more, and is preferably 1 μm or less, more preferably 0.7 μm or less, and even more preferably 0.5 μm or less.

[0063] The average particle size of the particles (a2) contained in the surface layer (A) is preferably 0.05 μm or more, more preferably 0.07 μm or more, even more preferably 0.1 μm or more, and even more preferably 0.3 μm or more. The average particle size of the particles (a2) is preferably 5 μm or less, more preferably 3 μm or less, even more preferably 2 μm or less, even more preferably 1.5 μm or less, and particularly preferably 1 μm or less.

[0064] In this embodiment, by setting the average particle size of the particles (a1) and particles (a2) contained in the surface layer (A) within the above range and further setting the relationship between the average particle sizes of the respective particles within the above range, particle dispersion in the polyester raw material during the melt extrusion process for forming the surface layer (A) can be promoted and particle aggregation can be suppressed. Aggregation of the particles contained in the surface layer (A) is undesirable because it induces the formation of coarse protrusions, leading to defects. However, in this embodiment, particle aggregation is effectively suppressed, thereby suppressing the formation of coarse protrusions in the surface layer (A). Furthermore, by setting the average particle size of the particles (a1) and particles (a2) contained in the surface layer (A) within the above range, the surface roughness of the surface layer (A) does not become too high, making it easy to control the maximum peak height (Sp) and arithmetic mean height (Sa) within the desired range. Furthermore, by setting the average particle size of the particles (a1) and particles (a2) within the above range, haze is suppressed, making it easier to ensure the transparency of the entire film.

[0065] In the case of powder particles, the average particle size can be determined by measuring the powder using a centrifugal sedimentation particle size distribution analyzer (e.g., Shimadzu Corporation's "SA-CP3 Model") and determining the particle size at an integrated volume fraction of 50% (d50) in the equivalent spherical distribution. The average particle size of particles in a film, layer, or resin can be determined by observing 10 or more particles with a scanning electron microscope (SEM) to measure their diameters and calculating the average value. In this case, for non-spherical particles, the average of the longest and shortest diameters can be measured as the diameter of each particle.

[0066] In this embodiment, the Mohs hardness of at least one of the particles (a1) and (a2) is preferably 8 or more, more preferably 8.5 or more, and even more preferably 9 or more. The upper limit of the Mohs hardness of at least one of the particles (a1) and (a2) is not particularly limited, and the Mohs hardness of the at least one of the particles may be, for example, 10 or less or 9.5 or less. By setting the Mohs hardness of the particles within the above range, the conformability of the particles to the polyester can be effectively improved when the film is stretched, etc., and this makes it easier to suppress the occurrence of coarse protrusions and voids. The Mohs hardness scale is a numerical representation of hardness based on how a material is scratched against a standard material. Standard materials are designated in order of softness from 1 to 10, and the Mohs hardness can be measured using a Mohs hardness tester in the usual way.

[0067] In particular, in this embodiment, the Mohs hardness of the particles (a1) is preferably within the above range. As described above, the average particle size of the particles (a1) is preferably smaller than the average particle size of the particles (a2). If the Mohs hardness of the particles (a1) with a smaller average particle size is equal to or greater than the lower limit, when a composite particle is formed, the particles (a1) located on the outer side are likely to be inserted between the particles (a2) located on the inner side. Furthermore, the high Mohs hardness of the particles (a1) makes the particles (a1) themselves less likely to be crushed. This is thought to enable the particles (a1) to maintain their shape while more easily suppressing the aggregation of the particles (a2). This further improves the dispersibility of the particles.

[0068] In this embodiment, the ratio of the content of particles having a Mohs hardness of 8 or more (content of particles having a Mohs hardness of 8 or more contained in the surface layer (A) / content of all particles contained in the surface layer (A)) is preferably 0.2 or more, more preferably 0.3 or more, even more preferably 0.4 or more, and particularly preferably 0.5 or more, by mass. The ratio of the content of particles having a Mohs hardness of 8 or more (content of particles having a Mohs hardness of 8 or more contained in the surface layer (A) / content of all particles contained in the surface layer (A)) is preferably 1 or less, more preferably 0.9 or less, and even more preferably 0.88 or less.

[0069] The total content of particles (a1) and particles (a2) contained in the surface layer (A) is preferably 1000 ppm or more, more preferably 2000 ppm or more, even more preferably 3000 ppm or more, even more preferably 4000 ppm or more, and particularly preferably 5000 ppm or more, based on the total mass of the surface layer (A). Furthermore, the total content of particles (a1) and particles (a2) contained in the surface layer (A) is preferably 10000 ppm or less, more preferably 9000 ppm or less, and even more preferably 8000 ppm or less, based on the total mass of the surface layer (A). By setting the particle content at or above the lower limit, it is possible to effectively impart slipperiness and prevent scratches in each process. Furthermore, by setting the particle content at or below the upper limit, it is possible to effectively prevent the unevenness caused by minute protrusions on the surface layer (A) from being transferred to the surface layer.

[0070] The content of particles (a1) contained in the surface layer (A) is preferably 100 ppm or more, more preferably 500 ppm or more, even more preferably 1000 ppm or more, even more preferably 1500 ppm or more, and particularly preferably 2000 ppm or more, based on the total mass of the surface layer (A). The content of particles (a1) contained in the surface layer (A) is preferably 5000 ppm or less, more preferably 4500 ppm or less, and even more preferably 4000 ppm or less, based on the total mass of the surface layer (A).

[0071] The content of particles (a2) contained in the surface layer (A) is preferably 100 ppm or more, more preferably 500 ppm or more, even more preferably 1000 ppm or more, still more preferably 1500 ppm or more, and particularly preferably 2000 ppm or more, based on the total mass of the surface layer (A). The content of particles (a2) contained in the surface layer (A) is preferably 8000 ppm or less, more preferably 7000 ppm or less, even more preferably 6000 ppm or less, and particularly preferably 5000 ppm or less, based on the total mass of the surface layer (A).

[0072] The method for adding particles to the polyester film is not particularly limited, and any conventionally known method can be used. For example, particles can be added at any stage in the production of the polyester constituting the surface layer (A), but it is preferable to add them after the completion of the esterification or transesterification reaction. Another method is to prepare masterbatch pellets containing particles in advance, blend them with polyester resin pellets, and then melt-knead and form a film.

[0073] The intrinsic viscosity of the polyester resin contained in the surface layer (A) is preferably 0.64 dL / g or more, more preferably 0.65 dL / g or more, and even more preferably 0.66 dL / g or more. The intrinsic viscosity of the polyester resin contained in the surface layer (A) is preferably 0.9 dL / g or less, more preferably 0.8 dL / g or less, even more preferably 0.75 dL / g or less, even more preferably 0.72 dL / g or less, and particularly preferably 0.7 dL / g or less. By setting the intrinsic viscosity at or above the lower limit, the dispersibility of particles is more easily improved, and the maximum peak height (Sp) and arithmetic mean height (Sa) of the surface layer (A) can be more easily controlled within the specified range. Furthermore, the air leakage index can be more easily controlled within the specified numerical range. More preferably, by appropriately controlling the type and physical properties of the particles added to the surface layer (A) and then controlling the intrinsic viscosity of the polyester resin constituting the surface layer (A), the desired maximum peak height (Sp) and arithmetic mean height (Sa) of the surface layer (A) can be achieved. In this regard, it is believed that by setting the intrinsic viscosity of the polyester resin contained in the surface layer (A) to a predetermined value or higher, shear force is more easily applied to each particle during melt-kneading of the polyester resin to form the surface layer (A), resulting in improved particle dispersion and easier control of the surface properties. On the other hand, by setting the intrinsic viscosity below the upper limit, excessive pressure buildup in the film-forming extruder can be suppressed, thereby improving the stability of film formation. The intrinsic viscosity of the polyester resin contained in the surface layer (A) was measured at 30°C using an intrinsic viscosity (IV) measuring device after precisely weighing 1 g of polyester resin and dissolving it in 100 mL of a 50 / 50 (mass ratio) phenol / tetrachloroethane mixed solvent. When two or more polyester resins with different intrinsic viscosities (IV) are used for the surface layer (A), the intrinsic viscosity (IV) refers to the intrinsic viscosity (IV) of the mixed resins. The intrinsic viscosity of the surface layer (A) may also be within the above range.

[0074] In this embodiment, the surface layer (A) may contain a recycled polyester resin. In particular, the recycled polyester resin is preferably a chemically recycled polyester resin. When the surface layer contains a chemically recycled polyester resin, the content of the chemically recycled polyester resin is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, relative to the total mass of the resin components contained in the surface layer. The content of the chemically recycled polyester resin may be 50% by mass or more, 70% by mass or more, or 90% by mass or more.

[0075] In this embodiment, by setting the content of recycled polyester resin in the surface layer (A) within the above range, for example, it is possible to reduce CO2 emissions and contribute to reducing the burden on the environment. Furthermore, by using a chemically recycled polyester resin as the recycled polyester resin, it is possible to impart the required roughness to the surface layer (A) while keeping the maximum peak height (Sp) of the surface layer (A) below a predetermined value, thereby more effectively improving the processability and handleability of the polyester film.

[0076] The recycled polyester resin contained in the surface layer (A) is obtained by recycling polyester, which is a recycled raw material. The recycled (recycled) polyester may be, for example, polyester derived from polyester containers (e.g., PET bottles) or polyester derived from polyester films (e.g., processing films). Thus, in this embodiment, the recycled polyester resin constituting the surface layer may be recycled polyester resin derived from polyester containers (PET bottles) or recycled polyester resin derived from polyester films. When recycled polyester resin derived from polyester films is used, the recycled polyester film may be a polyester film used as a support (substrate) for ceramic green sheets in the manufacturing process of multilayer ceramic capacitors.

[0077] The thickness of the surface layer (A) is preferably 0.5 μm or more, more preferably 0.8 μm or more, even more preferably 1 μm or more, and particularly preferably 1.2 μm or more. The thickness of the surface layer (A) is preferably 10 μm or less, more preferably 8 μm or less, even more preferably 6 μm or less, and particularly preferably 4 μm or less.

[0078] The thickness of the surface layer (A) is preferably 1 to 20% of the total thickness of the laminated polyester film, more preferably 2 to 15%, even more preferably 2.5 to 10%, still more preferably 3 to 8%, and particularly preferably 3 to 6%.

[0079] <Middle layer (B)> The present film has an intermediate layer (B). The intermediate layer (B) is a layer containing a polyester resin. The intermediate layer (B) functions as the thickest main layer in the present film.

[0080] The intrinsic viscosity of the polyester resin contained in the intermediate layer (B) is preferably 0.60 dL / g or more, more preferably 0.62 dL / g or more, and even more preferably 0.64 dL / g or more. The intrinsic viscosity of the polyester resin contained in the intermediate layer (B) is preferably 1 dL / g or less, more preferably 0.9 dL / g or less, even more preferably 0.85 dL / g or less, even more preferably 0.8 dL / g or less, even more preferably 0.75 dL / g or less, and particularly preferably 0.7 dL / g or less. Setting the intrinsic viscosity to the above lower limit or higher tends to stabilize film formation. On the other hand, setting the intrinsic viscosity to the above upper limit or lower is preferred because it facilitates the prevention of excessive pressure buildup in the film-forming extruder and the reduction of the heat shrinkage rate of the film. The intrinsic viscosity of the polyester resin contained in the intermediate layer (B) is measured at 30°C using a viscosity (IV) measuring device after precisely weighing 1 g of polyester resin and dissolving it in 100 mL of a 50 / 50 (mass ratio) phenol / tetrachloroethane solvent. When two or more polyester resins with different intrinsic viscosities (IV) are used in the intermediate layer (B), the intrinsic viscosity (IV) refers to the intrinsic viscosity (IV) of the mixed resins. The intrinsic viscosity of the intermediate layer (B) may also be within the above range.

[0081] The intermediate layer (B) may contain particles, but preferably does not substantially contain particles. In this case, the particle content in the intermediate layer (B) is preferably 1% by mass or less, more preferably 0.5% by mass or less, and even more preferably 0.1% by mass or less, relative to the total mass of the intermediate layer (B). When the intermediate layer (B) contains particles, the particles may be exemplified by the same particles that may be contained in the surface layer (A) described above. When the intermediate layer contains a recycled polyester resin as described below, particles contained in polyester, which is a recycled raw material, may be contained in the intermediate layer (B).

[0082] In this embodiment, the intermediate layer (B) may contain recycled polyester resin. In this case, the content of recycled polyester resin is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, based on the total mass of the resins constituting the intermediate layer (B). The content of recycled polyester resin may be 50% by mass or more, 70% by mass or more, or even 90% by mass or more. The upper limit of the recycled polyester resin content is not particularly limited and may be 100% by mass based on the total mass of the resins constituting the intermediate layer (B). By containing recycled polyester resin in the intermediate layer (B) within the above range, for example, CO2 emissions can be reduced, contributing to a reduction in environmental impact. Furthermore, by containing recycled polyester resin in the intermediate layer (B), the surface layer (A) can be provided with the necessary roughness while maintaining the maximum peak height (Sp) of the surface layer (A) at a predetermined value or less, thereby more effectively improving the processability and handleability of the polyester film.

[0083] The recycled polyester resin contained in the intermediate layer (B) may be a material recycled polyester resin obtained by material recycling of recycled polyester raw materials, or may be a chemically recycled polyester resin obtained by chemically recycling recycled polyester raw materials. In particular, the recycled polyester resin contained in the intermediate layer (B) is preferably a material recycled polyester resin obtained by material recycling of recycled polyester raw materials. The recycled (regenerated) polyester may be, for example, a polyester derived from a polyester container (e.g., a PET bottle) or a polyester film (e.g., a processing film). Thus, in this embodiment, the recycled polyester resin constituting the intermediate layer (B) may be a polyester resin derived from a polyester container or a polyester film. When a recycled polyester resin derived from a polyester film is used, the recycled polyester film may be a polyester film used as a support (substrate) for a ceramic green sheet in the manufacturing process of a multilayer ceramic capacitor.

[0084] The intermediate layer (B) functions as the thickest main layer. The thickness of the intermediate layer (B) is preferably 8 μm or more, more preferably 10 μm or more, even more preferably 12 μm or more, even more preferably 14 μm or more, even more preferably 16 μm or more, and particularly preferably 18 μm or more. The thickness of the intermediate layer (B) is preferably 50 μm or less, more preferably 45 μm or less, even more preferably 40 μm or less, and particularly preferably 35 μm or less.

[0085] The thickness of the intermediate layer (B) is preferably 50 to 95% of the total thickness of the laminated polyester film, more preferably 55 to 93%, and even more preferably 58 to 90%.

[0086] <Surface layer (C)> This film has a surface layer (C). The surface layer (C) is a layer containing a polyester resin. When this film is used as a support (substrate) for ceramic green sheets in the manufacturing process of a multilayer ceramic capacitor, the surface layer (C) is a layer disposed on the side on which the ceramic green sheets are laminated. In the manufacturing process of a multilayer ceramic capacitor, for example, a release layer is formed on the surface layer (C) and then the ceramic green sheets are laminated.

[0087] The maximum peak height (Sp) of the surface layer (C) is preferably 50 nm or less, more preferably 45 nm or less, even more preferably 40 nm or less, even more preferably less than 38 nm, even more preferably 35 nm or less, even more preferably 30 nm or less, particularly preferably 25 nm or less, and most preferably 22 nm or less. The lower limit of the maximum peak height (Sp) of the surface layer (C) is not particularly limited, and the maximum peak height (Sp) of the surface layer (C) may be, for example, 5 nm or more or 10 nm or more. By setting the maximum peak height (Sp) of the surface layer (C) to the above upper limit or less, it is possible to effectively prevent protrusions of unintended sizes from occurring on the surface of the polyester film, even on the surface (C), and a polyester film with extremely excellent surface smoothness can be obtained.

[0088] The arithmetic mean height (Sa) of the surface layer (C) is preferably 10 nm or less, more preferably 5 nm or less, even more preferably 3 nm or less, even more preferably 2 nm or less, and particularly preferably 1.6 nm or less. The lower limit of the arithmetic mean height (Sa) of the surface layer (C) is not particularly limited, and the arithmetic mean height (Sa) of the surface layer (C) may be, for example, 0.1 nm or more, 0.3 nm or more, or 0.5 nm or more. By setting the arithmetic mean height (Sa) of the surface layer (C) to the above upper limit or less, a polyester film with excellent surface smoothness can be obtained. When this film is used as a support for a release film for producing a ceramic green sheet, it is preferable that a ceramic green sheet be laminated on the surface layer (C) side. In this case, a smaller arithmetic mean height (Sa) of the surface layer (C) is preferred because this can suppress the occurrence of defects such as repelling and pinholes during the formation of the ceramic green sheet.

[0089] The value (Sp / Sa) obtained by dividing the maximum peak height (Sp) of the surface layer (C) by the arithmetic mean height (Sa) of the surface layer (C) is preferably 67 or less, more preferably 60 or less, even more preferably 50 or less, even more preferably 40 or less, and particularly preferably 30 or less. There is no particular limitation on the lower limit of the value of Sp / Sa, and Sp / Sa of the surface layer (C) is, for example, preferably 1 or more, more preferably 2 or more, even more preferably 3 or more, even more preferably 4 or more, and particularly preferably 5 or more.

[0090] The root mean square height (Sq) of the surface layer (C) of the present film is preferably 5 nm or less, more preferably 4 nm or less, even more preferably 3.5 nm or less, even more preferably 3 nm or less, and particularly preferably 2.5 nm or less. On the other hand, the lower limit of the root mean square height (Sq) of the surface layer (C) is not particularly limited, and the root mean square height (Sq) of the surface layer (C) is, for example, preferably 0.1 nm or more, more preferably 0.3 nm or more, and even more preferably 0.5 nm or more.

[0091] The kurtosis (Sku) of the surface layer (C) of the present film is preferably 100 or less, more preferably 80 or less, even more preferably 60 or less, even more preferably 40 or less, still more preferably 20 or less, particularly preferably 15 or less, and most preferably 10 or less. On the other hand, the lower limit of the kurtosis (Sku) of the surface layer (C) is not particularly limited, and the kurtosis (Sku) of the surface layer (C) is, for example, preferably 0.5 or more, more preferably 1 or more.

[0092] The skewness (Ssk) of the surface layer (C) of the present film is preferably 5 or less, more preferably 4.5 or less, even more preferably 4 or less, even more preferably 3.5 or less, and particularly preferably 3 or less. On the other hand, the lower limit of the skewness (Ssk) of the surface layer (C) is not particularly limited, and the skewness (Ssk) of the surface layer (C) is, for example, preferably 0.2 or more, more preferably 0.4 or more.

[0093] The arithmetic mean height (Sa), maximum peak height (Sp), root mean square height (Sq), kurtosis (Sku), and skewness (Ssk) can be adjusted, for example, by adjusting the content in consideration of the type of particles, composition, average particle size, particle size distribution, hardness, affinity with the polyester to be contained, etc. Adjusting the type and content of particles in consideration of the type of polyester to be contained, for example, composition, viscosity, molecular weight, thermal properties, presence or absence of copolymerization components, etc. is also useful for adjusting the surface properties. In addition, when adjusting the arithmetic mean height (Sa), maximum peak height (Sp), root mean square height (Sq), kurtosis (Sku), and skewness (Ssk), it is also effective to control, during the production of the polyester film, for example, the stretching ratio (longitudinal and transverse stretching ratios in the case of biaxial stretching), the stretching temperature, the heat treatment temperature and treatment time (particularly the heat treatment temperature and treatment time after transverse stretching in the case of biaxial stretching). Another example is a method of appropriately controlling the kneading conditions and extrusion rate of the molten polyester resin in the process of producing the polyester film, as will be described later.

[0094] The surface layer (C) preferably contains particles. The presence of particles in the surface layer (C) can provide easy slippage and prevent scratches during each process. Furthermore, the presence of particles in the surface layer (C) makes it easy to control the maximum peak height (Sp) and arithmetic mean height (Sa) within a desired range. The maximum peak height (Sp) and arithmetic mean height (Sa) may be controlled within a desired range by applying a surface treatment or coating to the surface layer (C).

[0095] The type of particles contained in the surface layer (C) is not particularly limited as long as they are particles that can impart slipperiness. Specific examples include inorganic particles such as silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, aluminum oxide, titanium oxide, and barium sulfate, as well as organic particles obtained by polymerizing acrylic acid ester monomers, styrene monomers, silicone monomers, etc., or organic particles obtained by copolymerizing these monomers, acrylic resin particles, melamine resin particles, silicone resin particles, and cross-linked polystyrene particles. Among these, organic particles, calcium carbonate, silica, aluminum oxide, etc. are preferably used. Furthermore, precipitated particles obtained by precipitating and finely dispersing a portion of a metal compound such as a catalyst during the polyester production process can also be used.

[0096] The shape of the particles in the surface layer (C) is not particularly limited, and any of spherical, blocky, rod-like, flat, etc. may be used. There are also no particular limitations on the hardness, specific gravity, color, etc.

[0097] The average particle size of the particles contained in the surface layer (C) is preferably 0.01 μm or more, more preferably 0.03 μm or more, and even more preferably 0.05 μm or more, and is preferably 3 μm or less, more preferably 2 μm or less, even more preferably 1.5 μm or less, and even more preferably 1 μm or less.

[0098] In the case of powder particles, the average particle size can be determined by measuring the powder using a centrifugal sedimentation particle size distribution analyzer (e.g., Shimadzu Corporation's "SA-CP3 Model") and determining the particle size at an integrated volume fraction of 50% (d50) in the equivalent spherical distribution. The average particle size of particles in a film, layer, or resin can be determined by observing 10 or more particles with a scanning electron microscope (SEM) to measure their diameters and calculating the average value. In this case, for non-spherical particles, the average of the longest and shortest diameters can be measured as the diameter of each particle.

[0099] The total content of particles contained in the surface layer (C) is preferably 100 ppm or more, more preferably 200 ppm or more, even more preferably 300 ppm or more, even more preferably 400 ppm or more, and particularly preferably 500 ppm or more, based on the total mass of the surface layer (C). The total content of particles contained in the surface layer (C) is preferably 5000 ppm or less, more preferably 3000 ppm or less, even more preferably 2000 ppm or less, even more preferably 1500 ppm or less, and particularly preferably 1000 ppm or less, based on the total mass of the surface layer (C). By setting the particle content at or above the lower limit, it is possible to effectively impart slipperiness and prevent scratches in each process. By setting the particle content at or below the upper limit, it is easy to control the maximum peak height (Sp) and arithmetic mean height (Sa) of the surface layer (C) within the desired range.

[0100] The method for adding particles to the polyester film is not particularly limited, and any conventionally known method can be used. For example, particles can be added at any stage in the production of the polyester constituting the surface layer (C), but it is preferable to add them after the completion of the esterification or transesterification reaction.

[0101] The intrinsic viscosity of the polyester resin contained in the surface layer (C) is preferably 0.63 dL / g or more, more preferably 0.64 dL / g or more, even more preferably 0.65 dL / g or more, and particularly preferably 0.66 dL / g or more. The intrinsic viscosity of the polyester resin contained in the surface layer (C) is preferably 1 dL / g or less, more preferably 0.9 dL / g or less, even more preferably 0.85 dL / g or less, even more preferably 0.8 dL / g or less, even more preferably 0.76 dL / g or less, and particularly preferably 0.73 dL / g or less. By setting the intrinsic viscosity at or above the lower limit, it becomes easier to control the maximum peak height (Sp) and arithmetic mean height (Sa) of the surface layer (C) within a predetermined range. On the other hand, by setting the intrinsic viscosity at or below the upper limit, excessive pressure buildup in the film-forming extruder can be suppressed, thereby improving the stability of film formation. The intrinsic viscosity of the polyester resin contained in the surface layer (C) is measured at 30°C using an intrinsic viscosity (IV) measuring device after precisely weighing 1 g of polyester resin and dissolving it in 100 mL of a 50 / 50 (mass ratio) phenol / tetrachloroethane solvent. When two or more polyester resins with different intrinsic viscosities (IV) are used in the surface layer (C), the intrinsic viscosity (IV) refers to the intrinsic viscosity (IV) of the mixed resins. The intrinsic viscosity of the surface layer (C) may also be within the above range.

[0102] In this embodiment, the value obtained by dividing the intrinsic viscosity of the polyester resin constituting the surface layer (C) by the intrinsic viscosity of the polyester resin constituting the surface layer (A) is preferably 0.7 or more, more preferably 0.8 or more, even more preferably 0.85 or more, even more preferably 0.9 or more, and particularly preferably 0.95 or more. Furthermore, the value obtained by dividing the intrinsic viscosity of the polyester resin constituting the surface layer (C) by the intrinsic viscosity of the polyester resin constituting the surface layer (A) is preferably 1.3 or less, more preferably 1.2 or less, even more preferably 1.15 or less, even more preferably 1.1 or less, and particularly preferably 1.05 or less. In other words, the intrinsic viscosities of the polyester resin constituting the surface layer (C) and the polyester resin constituting the surface layer (A) are preferably similar in value. By keeping the value obtained by dividing the intrinsic viscosity of the polyester resin constituting the surface layer (C) by the intrinsic viscosity of the polyester resin constituting the surface layer (A) within the above range, curling of the film when heated can be effectively suppressed. For example, when the present film is used as a support for a release film for producing a ceramic green sheet, a release layer may be laminated thereon as described below, but the film may curl due to the heat generated when the release agent composition is coated and heated. In this regard, in this embodiment, the polyester resins constituting the surface layer have approximately the same intrinsic viscosities, and the ratio is within the above range, thereby making it possible to effectively prevent the film from curling during heat treatment.

[0103] In this embodiment, the surface layer (C) may contain a recycled polyester resin. In particular, the recycled polyester resin is preferably a chemically recycled polyester resin. When the surface layer contains a chemically recycled polyester resin, the content of the chemically recycled polyester resin is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, relative to the total mass of the resin components contained in the surface layer. The content of the chemically recycled polyester resin may be 50% by mass or more, 70% by mass or more, or 90% by mass or more.

[0104] The recycled polyester resin contained in the surface layer (C) is obtained by recycling polyester, which is a recycled raw material. The recycled (recycled) polyester may be, for example, polyester derived from polyester containers (e.g., PET bottles) or polyester derived from polyester films (e.g., processing films). Thus, in this embodiment, the recycled polyester resin constituting the surface layer may be recycled polyester resin derived from polyester containers (PET bottles) or recycled polyester resin derived from polyester films. When recycled polyester resin derived from polyester films is used, the recycled polyester film may be a polyester film used as a support (substrate) for ceramic green sheets in the manufacturing process of multilayer ceramic capacitors.

[0105] The thickness of the surface layer (C) is preferably 0.5 μm or more, more preferably 0.8 μm or more, even more preferably 1 μm or more, and particularly preferably 1.2 μm or more. The thickness of the surface layer (C) is preferably 10 μm or less, more preferably 8 μm or less, even more preferably 6 μm or less, and particularly preferably 4 μm or less.

[0106] The thickness of the surface layer (C) is preferably 1 to 20% of the total thickness of the laminated polyester film, more preferably 2 to 15%, even more preferably 2.5 to 10%, still more preferably 3 to 8%, and particularly preferably 3 to 6%.

[0107] <Polyester> The polyester constituting each layer of the present film may be a homopolyester or a copolymer polyester. Specific examples include polyesters obtained by polycondensation of a dicarboxylic acid component and a diol component. In the present film, it is preferable to use a polyester containing more than 50 mol% of aromatic dicarboxylic acid or aliphatic dicarboxylic acid units, assuming that the dicarboxylic acid units are 100 mol%.

[0108] Examples of the dicarboxylic acid component include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, and 4,4'-diphenylsulfonedicarboxylic acid, and aliphatic dicarboxylic acids such as adipic acid, suberic acid, sebacic acid, dimer acid, dodecanedioic acid, cyclohexanedicarboxylic acid, and ester derivatives thereof.

[0109] Examples of diol components include ethylene glycol, 1,2-propanediol, 1,3-propanediol, neopentyl glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-hexanedimethanol, diethylene glycol, triethylene glycol, polyalkylene glycol, 2,2-bis(4-hydroxyethoxyphenyl)propane, isosorbate, and spiroglycol.

[0110] When the polyester is a homopolyester, it is preferably one obtained by polycondensation of an aromatic dicarboxylic acid and an aliphatic glycol. In this case, examples of the aromatic dicarboxylic acid include terephthalic acid and 2,6-naphthalenedicarboxylic acid, and examples of the aliphatic glycol include ethylene glycol, diethylene glycol, and 1,4-cyclohexanedimethanol. Representative examples of the polyester include polyethylene terephthalate (PET) and polyethylene-2,6-naphthalenedicarboxylate (PEN), with PET being preferred. Furthermore, examples of the polyester that can be used include polyethylene terephthalate, which is composed of 80 mol % or more, preferably 90 mol % or more, of ethylene terephthalate units, and polyethylene-2,6-naphthalate, which is composed of ethylene-2,6-naphthalate units.

[0111] On the other hand, when the polyester is a copolymer polyester, it is preferably a copolymer containing 30 mol% or less of a third component. The third component is a component other than the compound that constitutes the main dicarboxylic acid component of the polyester and the compound that constitutes the main diol component. For example, in the case of polyethylene terephthalate, it is a component other than terephthalic acid and ethylene glycol. Examples of the dicarboxylic acid component of the copolymer polyester include one or more of isophthalic acid, phthalic acid, terephthalic acid, 2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid, and oxycarboxylic acid. Examples of the glycol component of the copolymer polyester include one or more of ethylene glycol, diethylene glycol, propylene glycol, butanediol, 1,4-cyclohexanedimethanol, and neopentyl glycol.

[0112] Typically, when polyester is produced (polycondensed) using ethylene glycol as one of the raw materials, diethylene glycol is by-produced from ethylene glycol. In this specification, this diethylene glycol is referred to as by-product diethylene glycol. The amount of diethylene glycol by-produced from ethylene glycol varies depending on the polycondensation method, etc., but is approximately 5 mol% or less of the ethylene glycol. In the present invention, 5 mol% or less of diethylene glycol is defined as by-product diethylene glycol, and by-product diethylene glycol is also included in ethylene glycol and is distinguished from a copolymerization component. On the other hand, depending on the diethylene glycol content, more specifically, when diethylene glycol is contained in excess of 5 mol%, diethylene glycol is treated as a copolymerization component rather than as a by-product diethylene glycol.

[0113] In this embodiment, at least one selected from the dicarboxylic acid component and the diol component constituting the polyester may be a biomass-derived raw material. In particular, it is preferable that the diol component is a biomass-derived raw material. Biomass-derived ethylene glycol is made from ethanol (biomass ethanol) produced from biomass as a raw material. For example, biomass-derived ethylene glycol can be obtained by converting biomass ethanol into ethylene oxide using a conventionally known method to produce ethylene glycol. Alternatively, commercially available biomass ethylene glycol may be used; for example, biomass ethylene glycol commercially available from India Glycoal Limited can be preferably used.

[0114] <<Polycondensation catalyst>> Examples of polycondensation catalysts used in polycondensing the polyester include antimony compounds, germanium compounds, aluminum compounds, and titanium compounds. Among these, it is preferable to use at least one selected from antimony compounds and titanium compounds, and it is more preferable to use a titanium compound. By using a titanium compound as a polycondensation catalyst, it is possible to reduce the amount of polycondensation catalyst added, which makes it easier to suppress the generation of foreign matter or protrusions derived from the polycondensation catalyst.

[0115] <<Others>> In this embodiment, in order to suppress the amount of oligomer component precipitation, the polyester film may be produced using a polyester with a low oligomer component content as the raw material. Various known methods can be used to produce a polyester with a low oligomer component content, such as a method of solid-state polymerization after polyester production. Alternatively, the polyester may be obtained by esterification or transesterification, followed by melt polycondensation under reduced pressure at a higher reaction temperature. For example, it is also preferable to suppress the amount of oligomer component precipitation by forming the surface layer of the present film using a polyester raw material with a low oligomer component content.

[0116] In addition to the above-mentioned components, conventionally known ultraviolet absorbers, antioxidants, antistatic agents, heat stabilizers, lubricants, dyes, pigments, etc. may be added to the present film as needed.

[0117] <Recycled polyester> The present film may contain recycled polyester resin. The recycled polyester resin may be a material-recycled polyester resin or a chemically recycled polyester resin. For example, only the intermediate layer (B) may contain recycled polyester resin. Alternatively, the surface layer (A) and the surface layer (C) may contain chemically recycled polyester resin, and only the intermediate layer (B) may contain material-recycled polyester resin.

[0118] [Material: Recycled polyester] The recycled polyester resin may be a material recycled polyester. In material recycling, collected used PET bottles and polyester films are first crushed into flakes. Since these flakes often contain foreign matter, they are preferably washed, and alkaline washing is more preferable.

[0119] In the process of pelletizing the flakes, an extruder is used to melt, extrude, cool, and granulate the flakes. In the melting process in the extruder, melt kneading is usually carried out at 260 to 300°C. It is preferable that the flakes are sufficiently dried in advance. In addition, the extruder preferably has at least one vacuum vent in the resin melting zone as a degassing means.

[0120] It is also preferable that a filtering means is provided downstream of the extruder, and the filtering means preferably has a filter capable of filtering out solid foreign matter contained in the molten resin.

[0121] The molten resin that passes through the filter passes through a die, is cooled in water, and then cut into pellets of the desired shape and granulated, yielding recycled polyester resin.

[0122] In addition, in the process of cleaning recovered PET bottles and polyester film, or in the process of melting these raw materials, the polyester may be partially hydrolyzed by the cleaning components or heat, which reduces the degree of polymerization of the recycled polyester resin. Depending on the intended use, a reduced degree of polymerization may result in poor moldability, strength, transparency, heat resistance, and other properties. Therefore, a solid-state polymerization process may be performed to restore the reduced degree of polymerization. In the solid-state polymerization process, flakes may be melt-extruded and pelletized, and then continuously solid-state polymerized in an inert gas such as nitrogen gas or a rare gas at 180 to 245°C.

[0123] [Chemically recycled polyester] The recycled polyester resin may be chemically recycled polyester. Examples of methods for producing chemically recycled polyester resin include sorting, crushing, and washing collected PET bottles and polyester films to remove foreign matter, followed by depolymerization to break them down into raw materials or intermediate materials for polyester resin, purifying them, and then repolymerizing these materials. Depolymerization involves adding ethylene glycol (EG) in the presence of a catalyst to return them to bis-2-hydroxyethyl terephthalate (BHET), an intermediate material used in resin production, which is then purified and repolymerized into PET. Alternatively, polyethylene terephthalate may be heated in a non-aqueous organic solvent in the presence of a catalyst containing oxidized iron to produce terephthalic acid and ethylene glycol, which are then repolymerized. A distinctive feature of chemically recycled polyester resin is that foreign matter and other materials are removed during the depolymerization and repolymerization process, allowing it to be recycled into a polyester resin of similar quality to virgin resin.

[0124] The collected used PET bottles and polyester films are washed and then crushed into flakes. Crushing may be performed underwater, or the washing and crushing steps may be performed simultaneously. Furthermore, a foreign matter removal step may be performed before or after these steps.

[0125] Next, the polyethylene terephthalate flakes are depolymerized, melted, and simultaneously hydrolyzed to produce a polyethylene terephthalate melt with a low degree of polymerization. Furthermore, it is preferable to depolymerize the flakes using excess ethylene glycol to obtain a two-component mixture solution of crude BHET and crude ethylene glycol. After the depolymerization reaction is complete, the two-component mixture solution of crude BHET and crude ethylene glycol is cooled and filtered to remove solid foreign matter. Further, colored substances and dissolved ions may be removed by adsorption and ion exchange treatment.

[0126] Next, the mixed solution of crude BHET and crude ethylene glycol is preferably subjected to distillation and evaporation to separate and distill off the ethylene glycol to obtain concentrated BHET. Alternatively, the mixed solution may be cooled to 10°C or below to crystallize BHET, followed by solid-liquid separation of the ethylene glycol and BHET to obtain concentrated BHET. Purified bis-β-hydroxyethyl terephthalate is obtained by evaporating the concentrated BHET under specified conditions under vacuum. After obtaining highly purified BHET as described above, the purified BHET can be charged into a melt polycondensation reactor to repolymerize the polyester.

[0127] <Physical properties of polyester film> The air leakage index of the present film is preferably 20,000 seconds or less, more preferably 18,000 seconds or less, even more preferably 16,000 seconds or less, even more preferably 15,000 seconds or less, even more preferably 14,000 seconds or less, and particularly preferably 13,000 seconds or less. There are no particular restrictions on the lower limit of the air leakage index, and the air leakage index is, for example, preferably 100 seconds or more, and may be 500 seconds or more, 1,000 seconds or more, or 2,000 seconds or more. The air leakage index of the present film is measured using a DigiBec smoothness tester (manufactured by Toyo Seiki Co., Ltd., "DB-2") in accordance with JIS P8119 under an atmosphere of 23°C and 50% relative humidity. The pressure of the pressure device was 100 kPa, and the vacuum container was a 38 ml container. The time (seconds) for 1 mL of air to flow, i.e., the time (seconds) for the pressure inside the container to change from 50.7 kPa to 48.0 kPa, was measured, and the air leakage index was calculated by multiplying the obtained number of seconds. The film sample size was 70 mm square, and 20 sheets were stacked with the front and back sides (film surfaces (A) and (C)) overlapping) to form a test laminate film. A 5 mm diameter hole was drilled in the center of this test laminate film to measure the air leakage index. In this specification, a higher air leakage index value indicates that it takes longer for air to leak through the gaps between the films, indicating that the films are more closely spaced. Therefore, an air leakage index below the upper limit indicates that there is an appropriate gap between the films, which improves the slipperiness of the film when wound into a roll and reduces the risk of wrinkles when rolled into a film.

[0128] In this embodiment, both surfaces of the film are highly smooth, but the air leakage index has been successfully reduced to the above upper limit or less. The reduction in the air leakage index can be achieved by roughening the surface (A) to a certain extent, as well as by appropriately controlling the type and physical properties of the particles contained in the surface layer (A) and the type and physical properties of the polyester resin. More specifically, by using a combination of particles with a relatively high Mohs hardness and particles with a relatively low Mohs hardness, the surface (A) is appropriately roughened while suppressing particle aggregation (by keeping Sp low and adjusting Sa to an appropriate size). In other words, by appropriately controlling the Mohs hardness of the particles and the ratio of Sp / Sa, high smoothness and a reduced air leakage index can both be achieved.

[0129] The haze of the present film is preferably 15% or less, more preferably 10% or less, even more preferably 7% or less, and even more preferably 5% or less. The lower limit of the haze of the present film is not particularly limited and may be 0%, 0.5%, 1%, 1.5%, or 2.1%. The haze of the film is measured using a haze meter in accordance with JIS K7136:2000.

[0130] The total light transmittance of the present film is preferably 70% or more, more preferably 75% or more, even more preferably 80% or more, and even more preferably 85% or more. The upper limit of the total light transmittance of the present film is not particularly limited and may be 100%, 97%, or 95%. The total light transmittance of the film is measured in accordance with JIS K7136:2000 using a haze meter under a D65 light source.

[0131] The total thickness of the present film is not particularly limited as long as it is within a range that allows film formation, but from the viewpoints of mechanical strength, handleability, productivity, etc., it is preferably 10 μm or more, more preferably 15 μm or more, and even more preferably 18 μm or more. The total thickness of the present film is preferably 150 μm or less, more preferably 100 μm or less, even more preferably 80 μm or less, even more preferably 50 μm or less, even more preferably 38 μm or less, and particularly preferably 32 μm or less.

[0132] <Coating layer> In this embodiment, a coating layer may be further provided on the surface layer (C). Also, a coating layer may be further provided on the surface layer (A). For example, the polyester film of this embodiment may further have a coating layer provided on the surface (C). In this embodiment, the coating layer is preferably a layer formed by applying a coating layer-forming composition (coating liquid) onto the surface (C). The coating layer can be formed by in-line coating or offline coating, but is preferably formed by in-line coating. This can improve the production efficiency of the polyester film.

[0133] The thickness of the coating layer is preferably 5 nm or more, more preferably 10 nm or more, and even more preferably 15 nm or more, and is preferably 500 nm or less, more preferably 400 nm or less, and even more preferably 300 nm or less.

[0134] The coating layer-forming composition preferably contains a binder resin and a crosslinking agent. The total content of the binder resin and crosslinking agent contained in the coating layer-forming composition is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more, as non-volatile components. The coating layer-forming composition also preferably contains particles, a catalyst, etc.

[0135] <<Binder resin>> The coating layer-forming composition preferably contains a binder resin. The binder resin is a polymer compound having a number-average molecular weight (Mn) of 1,000 or more as measured by gel permeation chromatography (GPC) in accordance with the "Flow Scheme for the Safety Evaluation of Polymeric Compounds" (November 1985, sponsored by the Chemical Substances Council). Among these, those with film-forming properties are preferred. There are no particular limitations on the binder resin, and conventionally known binder resins such as polyester resins, polyurethane resins, (meth)acrylic resins, polyvinyl resins (e.g., polyvinyl alcohol, vinyl chloride vinyl acetate copolymers), polyalkylene glycols, polyalkyleneimines, methyl cellulose, hydroxycellulose, and starches can be used. From the viewpoints of film-forming properties and adhesion to polyester films, the composition preferably contains at least one selected from the group consisting of polyester resins, polyurethane resins, and (meth)acrylic resins, and more preferably at least one selected from the group consisting of polyester resins and polyurethane resins. In the resin composition, one binder resin may be used alone, or two or more binder resins may be used in combination.

[0136] Examples of polyester resins, polyurethane resins, (meth)acrylic resins, and polyvinyl resins used as binder resins include compounds described in WO 2023 / 145952.

[0137] The content of the binder resin in the coating layer-forming composition is preferably 5 to 95% by mass, more preferably 10 to 80% by mass, even more preferably 20 to 70% by mass, and even more preferably 30 to 60% by mass, as a proportion of all nonvolatile components in the coating layer-forming composition. By keeping the content within the above range, it is possible to easily form a film that has film-forming properties and contains particles. Furthermore, by improving adhesion to the polyester film, it is possible to prevent the coating film from falling off.

[0138] <<Crosslinking agent>> The coating layer-forming composition preferably contains a crosslinking agent. The crosslinking agent is not particularly limited, and conventionally known crosslinking agents can be used. Examples of crosslinking agents include melamine compounds, isocyanate compounds, oxazoline compounds, epoxy compounds, carbodiimide compounds, and silane coupling compounds. Among these, it is preferable to contain a melamine compound from the viewpoint of increasing the strength of the coating layer and improving adhesion to the polyester film. In the coating layer-forming composition, the crosslinking agent may be used alone or in combination of two or more types.

[0139] Examples of the melamine compound and isocyanate compound used as the crosslinking agent include the compounds described in WO 2023 / 145952.

[0140] The content of the crosslinking agent in the coating layer-forming composition is preferably 5 to 50 mass %, more preferably 8 to 40 mass %, even more preferably 10 to 35 mass %, and particularly preferably 15 to 30 mass %, as a proportion of all nonvolatile components in the coating layer-forming composition. By keeping the content within the above range, it is possible to easily form a film containing particles with film-forming properties. Furthermore, by improving adhesion to the polyester film, it is possible to prevent the coating film from falling off.

[0141] <<Particle>> The coating layer-forming composition preferably contains particles. Examples of the particles include inorganic particles such as silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, zirconium oxide, aluminum oxide, and titanium oxide, as well as crosslinked polymers such as crosslinked silicone resin particles, crosslinked acrylic resin particles, crosslinked styrene-acrylic resin particles, and crosslinked polyester particles, and organic particles such as calcium oxalate and ion exchange resins. Among these, zirconium oxide, titanium oxide, and silica are preferred, and zirconium oxide and silica are more preferred. The particles may be used alone or in combination of two or more types.

[0142] The shape of the particles used may be spherical, blocky, rod-like, flat, chain-like, etc. Among these, spherical particles are preferred from the viewpoint of facilitating uniform distribution in the resin composition.

[0143] The average particle size is preferably 0.5 to 300 nm, more preferably 1 to 250 nm, even more preferably 2 to 200 nm, even more preferably 2.5 to 200 nm, even more preferably 3 to 150 nm, even more preferably 3.5 to 100 nm, even more preferably 4 to 60 nm, and particularly preferably 4.5 to 30 nm. An average particle size within this range can prevent the generation of coarse protrusions due to particle aggregation and process contamination due to particle dropout. The average particle size can be measured by a method that calculates it from the specific surface area measured by a specific surface area measuring device and the particle density, a method that calculates the particle diameter after observation with a transmission electron microscope (TEM) or scanning electron microscope (SEM), or a method that determines it by measurement using dynamic light scattering. The average particle size can be measured by a method that is appropriate for the particle size.

[0144] The content of the particles in the composition for forming a coating layer is preferably in the range of 0.01 to 20 mass%, more preferably 0.05 to 15 mass%, and even more preferably 0.1 to 10 mass%, as a ratio of the total nonvolatile components in the composition for forming a coating layer. By setting the content within the above range, the elastic deformation power (η it ) can be easily controlled within the desired range.

[0145] <Application> This film can be suitably used for various release applications, such as dry film resist (DFR), multilayer circuit boards, and the production of ceramic green sheets for multilayer ceramic capacitors. In release and process applications, this film can be used, for example, as a support, onto which various materials such as ceramic slurries can be applied or laminated.

[0146] In particular, as described above, the present film has excellent surface smoothness, allowing for the thinning of ceramic green sheets, and is therefore preferably used as a support for ceramic green sheets in the manufacturing process of multilayer ceramic capacitors. That is, the polyester film of this embodiment is preferably a polyester film for manufacturing multilayer ceramic capacitors. In the polyester film of this embodiment, the maximum peak height (Sp) of one surface (A) is suppressed to 101 nm or less, so that the unevenness caused by coarse protrusions is not transferred to the surface (C), and the smoothness of the smooth surface (C) is not impaired. Therefore, even when manufacturing thin ceramic green sheets, defects such as pinholes in the ceramic green sheets can be suppressed. Furthermore, surface defects such as pinholes in ceramic green sheets are problematic because they significantly affect quality and reliability in terms of short-circuit defects, capacitance variations, and so on.

[0147] Furthermore, as electrification continues to increase in automobiles, it is predicted that the ceramic green sheets used will become thinner as capacitors become smaller and higher capacity. Therefore, this film is suitable for use as a support for ceramic green sheets in the manufacturing process of automotive ceramic capacitors.

[0148] (Production method of polyester film) The present embodiment may relate to a method for producing the above-mentioned polyester film, which includes a step of supplying a polyester resin to an extruder, melting the polyester resin, and then extruding the melted polyester resin, and is a method for producing a polyester film having a maximum peak height (Sp) of 101 nm or less on one surface (A) and an air leakage index of 20,000 seconds or less.

[0149] When the polyester film of this embodiment is a laminated polyester film having a surface layer (C), an intermediate layer (B), and a surface layer (A), the method for producing the laminated polyester film includes a step of laminating a polyester layer C constituting the surface layer (C), a polyester layer B constituting the intermediate layer (B), and a polyester layer A constituting the surface layer (A). Alternatively, the method for producing the laminated polyester film of this embodiment includes a step of feeding the polyester resin C constituting the surface layer (C), the polyester resin B constituting the intermediate layer (B), and the polyester resin A constituting the surface layer (A) into respective extruders, melting them, and then co-extruding them. In each extruder, each polymer is heated to above its melting point to form a molten polymer. The molten polymer is then extruded through a die and cooled and solidified on a rotating cooling drum to a temperature below the glass transition point of the polymer, thereby obtaining an unstretched polyester film.

[0150] In this embodiment, a step of stretching an unstretched polyester film may be provided. In the stretching step, the unstretched polyester film is first stretched in one direction using a roll or tenter-type stretching machine. In this case, the stretching temperature is usually 25 to 120°C, preferably 35 to 100°C, and the stretching ratio is usually 2.5 to 7 times, preferably 2.7 to 6 times, and more preferably 2.9 to 5 times. Next, it is preferable to stretch the film in a direction perpendicular to the first-stage stretching direction. In this case, the stretching temperature is usually 50 to 140°C, and the stretching ratio is usually 3 to 7 times, preferably 4 times or more, and more preferably 4.5 to 5 times. In the stretching step, a method in which unidirectional stretching is performed in two or more stages may also be used.

[0151] Subsequently, it is preferable to carry out a heat setting treatment at a temperature of 180 to 240°C, preferably 190 to 235°C, under tension or under relaxation of 30% or less. In this way, a biaxially stretched polyester film is obtained. The heat setting treatment may be carried out in two or more steps at different temperatures. Furthermore, cooling may be carried out in a cooling zone after the heat setting treatment. The cooling temperature is preferably higher than the glass transition temperature (Tg) of the polyester resin constituting the polyester film, more specifically, preferably in the range of 100 to 160°C. This cooling may be carried out in two or more steps at different temperatures.

[0152] When the polyester film is a laminated polyester film having a surface layer (C), an intermediate layer (B), and a surface layer (A), the extruder used to form the surface layer (C) and the surface layer (A) is preferably a vented twin-screw extruder having a raw material supply port and a vent opening in the cylinder, and having at least one decompressed vent. The diameter (cylinder inner diameter) D mm and the length L mm of the twin-screw extruder used in this embodiment are not particularly limited. The diameter and length of the twin-screw extruder should be selected so that the maximum peak height (Sp) and arithmetic mean height (Sa) of each surface of the film fall within the specified range, and the thermal degradation of the raw polyester is effectively suppressed while the raw polyester is sufficiently plasticized.

[0153] When the polyester film is a laminated polyester film having a surface layer (C), an intermediate layer (B), and a surface layer (A), it is preferable to appropriately control the extrusion conditions for the surface layer (C) and the surface layer (A), namely, the extrusion rate Q kg / h of the polyester raw material constituting each layer and the ratio Q / N of the extrusion rate Q kg / h of the polyester raw material constituting each layer to the screw rotation speed N rpm.

[0154] By controlling the extrusion rate Q kg / h of the polyester raw material constituting each layer, particle aggregation in the polyester raw material can be suppressed, making it easier to achieve the desired surface characteristics of each surface layer (such as the maximum peak height (Sp) and arithmetic mean height (Sa) of the polyester film). Furthermore, the screw rotation speed can be set within an appropriate range relative to the extrusion rate of the polyester resin, allowing for sufficient degassing, facilitating the formation of polyester films with the desired surface characteristics.

[0155] In this way, by appropriately adjusting the shear rate based on the screw rotation speed and extrusion rate in the melt extrusion process of the polyester raw material, it is possible to apply an appropriate shear stress to the particles in the polyester resin, which is thought to promote the dispersion of the particles.

[0156] In the present embodiment, when a coating layer is provided on the surface layer, a step of forming the coating layer may be provided. In the step of forming the coating layer, the coating layer is formed by applying a coating layer-forming composition (coating liquid) onto the surface layer.

[0157] (Release film) This embodiment may also relate to a release film further comprising a release layer on at least one surface of the above-described polyester film. Preferably, the release layer is provided on the surface (C) side of the polyester film. When the present film is a laminated polyester film, the release layer is provided on the surface layer (C) side. In this case, the release film has a structure of release layer / surface layer (C) / intermediate layer (B) / surface layer (A). When the present film has a coating layer, it may have a structure of release layer / coating layer / surface layer (C) / intermediate layer (B) / surface layer (A).

[0158] The release layer is laminated on the polyester film directly or via another layer. Examples of other layers include an easy-adhesion coating layer for improving adhesion to the film, an antistatic layer, an antiblocking layer, etc. By providing a release layer on the polyester film in this way, when the film is used as a support (substrate) for a ceramic green sheet in the manufacturing process of a multilayer ceramic capacitor, it is possible to easily peel off the ceramic green sheet laminated on the release layer.

[0159] The release layer is formed from a release agent composition containing a release agent, and the release agent composition preferably contains a silicone-based release agent or a non-silicone-based release agent.

[0160] Examples of silicone-based release agents include release agents containing a curable silicone resin as a main component, modified silicone release agents obtained by graft polymerization with an organic resin such as a urethane resin, an epoxy resin, or an alkyd resin, and fluorosilicone release agents. Of these, it is more preferable that the silicone-based release agent contains a curable silicone resin.

[0161] As the curable silicone resin, any of the existing curing reaction types can be used, such as heat-curable types such as addition types and condensation types, and electron beam-curable types such as ultraviolet-curable types, and multiple types of curable silicone resins can also be used in combination.

[0162] Examples of non-silicone release agents include waxes, compounds containing long-chain alkyl groups, and fluorine compounds.

[0163] The waxes include natural waxes, synthetic waxes, and modified waxes. Natural waxes include vegetable waxes, animal waxes, mineral waxes and petroleum waxes. Examples of vegetable waxes include candelilla wax, carnauba wax, rice wax, Japan wax, and jojoba oil. Animal waxes include beeswax, lanolin, and spermaceti. Examples of mineral waxes include montan wax, ozokerite, and ceresin. Petroleum waxes include paraffin wax, microcrystalline wax, and petrolatum. Synthetic waxes include synthetic hydrocarbons, modified waxes, hydrogenated waxes, fatty acids, acid amides, amines, imides, ester waxes and ketones.

[0164] The long-chain alkyl group-containing compound is a compound having a linear or branched alkyl group with 6 or more carbon atoms, preferably 8 or more carbon atoms, and more preferably 12 or more carbon atoms. Examples of the alkyl group include hexyl, octyl, decyl, lauryl, octadecyl, and behenyl groups. Examples of compounds having an alkyl group include various long-chain alkyl group-containing polymeric compounds, long-chain alkyl group-containing amine compounds, long-chain alkyl group-containing ether compounds, and long-chain alkyl group-containing quaternary ammonium salts. Polymeric compounds having a long-chain alkyl group in the side chain can be obtained by reacting a polymer having a reactive group with a compound having an alkyl group capable of reacting with the reactive group. Examples of the reactive group include hydroxyl, amino, carboxy, and acid anhydrides. Examples of compounds having these reactive groups include polyvinyl alcohol, polyethyleneimine, polyethyleneamine, reactive group-containing polyester resins, and reactive group-containing poly(meth)acrylic resins. Among these, polyvinyl alcohol is preferred for ease of handling.

[0165] The fluorine compound is a compound containing fluorine atoms. As the fluorine compound, an organic fluorine compound is preferably used, for example, a perfluoroalkyl group-containing compound, a polymer of an olefin compound containing a fluorine atom, an aromatic fluorine compound such as fluorobenzene, etc.

[0166] There are no particular limitations on the form of application of the release agent composition when forming the release layer. The release agent composition preferably contains a solvent in addition to the release agent. The release agent composition may be in the form of a solution in an organic solvent, in the form of an aqueous emulsion, or in the form of a solventless composition.

[0167] The release agent composition for forming the release layer may further contain, as necessary, a binder, an antifoaming agent, a coatability improver, a thickener, inorganic particles, organic particles, an organic lubricant, an antistatic agent, a conductive agent, an ultraviolet absorber, an antioxidant, a foaming agent, a dye, a pigment, or the like.

[0168] The release layer is provided by coating the present film with a release agent composition. Either in-line coating, which is carried out during the film production process, or so-called off-line coating, in which the release agent composition is applied outside the system onto a film that has already been produced, may be employed.

[0169] The release layer can be provided on the film by any of the conventional coating methods such as reverse gravure coating, direct gravure coating, roll coating, die coating, bar coating, and curtain coating.

[0170] The curing conditions for forming the release layer are not particularly limited. When forming the release layer by offline coating, the heat treatment is usually carried out at 80°C or higher for 10 seconds or more, preferably at 100 to 200°C for 3 to 40 seconds, and more preferably at 120 to 180°C for 3 to 40 seconds.

[0171] The coating amount of the release layer (after drying) is usually 0.005 to 5 g / m from the viewpoint of coating properties. 2 , preferably 0.005 to 1 g / m 2 , more preferably 0.005 to 0.1 g / m 2 The coating amount (after drying) is in the range of 0.005 g / m 2 When the amount is 5 g / m or more, good stability can be obtained in terms of coating properties, and a uniform coating film can be obtained. 2If it is below this level, the release layer itself can have good coating adhesion, curability, etc.

[0172] (Polyester fill with ceramic green sheet) This embodiment may relate to a polyester film with a ceramic green sheet, in which a ceramic green sheet is laminated on the above-mentioned polyester film, or to a release film with a ceramic green sheet used in the manufacturing process of an automotive ceramic capacitor. The release film with a ceramic green sheet is obtained in the manufacturing process of a multilayer ceramic capacitor. Since the polyester film of this embodiment is suitable for manufacturing thin ceramic green sheets, for example, the thickness of the ceramic green sheet after drying may be 2 μm or less, 1 μm or less, or 0.5 μm or less.

[0173] This embodiment may also relate to use of the polyester film as a support for a ceramic green sheet in a process for producing a multilayer ceramic capacitor. This embodiment may also relate to a method for producing a ceramic green sheet, which includes a step of applying a ceramic slurry containing a ceramic component to the surface (C) side of the polyester film.

[0174] When manufacturing the polyester film with ceramic green sheet of this embodiment, a ceramic slurry containing ceramic components and a binder resin can be applied to the surface layer or coating layer of the above-mentioned polyester film, or to the release layer of the above-mentioned release film, and then dried to produce a ceramic green sheet (dielectric sheet). [Example]

[0175] The features of the present invention will be explained in more detail below with reference to examples and comparative examples. The materials, amounts used, ratios, treatment contents, treatment procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below.

[0176] [Polyester raw materials] The polyester raw materials used in the examples and comparative examples are shown in Table 1. In addition, "ppm" in Tables 1, 2 and 4 is the content ratio based on mass (ppm by mass).

[0177] [Table 1]

[0178] The polyesters in polyester raw materials A to I listed in Table 1 are all homopolyethylene terephthalate. Polyester raw material I is a recycled raw material (intrinsic viscosity 0.58 dL / g) obtained by recovering, pulverizing, and re-pelletizing offcuts of titanium compound catalyst-based polyester film containing 50 to 1000 mass ppm of particles (alumina particles, silica particles, organic particles, calcium carbonate particles) with a particle diameter of 0.05 to 1.0 μm.

[0179] Example 1 The surface layer (C) was made from a blend of 90% polyester A and 10% polyester B by mass. The intermediate layer (B) was made from 100% polyester D. The surface layer (A) was made from a blend of 20% polyester E, 30% polyester F, and 50% polyester B by mass. These materials were fed into a vented extruder and melt-extruded at 280°C. The surface layer (C) and the surface layer (A) were then co-extruded in a three-layer structure (surface layer (C) / intermediate layer (B) / surface layer (A)). The thickness ratio of the co-extrusion was C / B / A = 1.6 / 27.8 / 1.6. The mixture was cooled and solidified on a cooling roll with a surface temperature set at 20°C using an electrostatic adhesion method to obtain an amorphous film. Next, the film was stretched 3.5 times in the machine direction (MD) at a film temperature of 86°C using the difference in roll peripheral speed, and then this machine-stretched film was introduced into a tenter and stretched 4.2 times in the transverse direction (TD) at 105°C. It was then heat-treated at 170°C, 230°C, and 230°C in heat treatment (fixing) zones 1, 2, and 3 within the tenter, respectively, and cooled to 140°C at a relaxation rate of 2%, resulting in a polyester film with an overall thickness of 31 μm.

[0180] Example 2 The surface layer (C) was made from a blend of 90% polyester A and 10% polyester B by mass. The intermediate layer (B) was made from a blend of 50% polyester C and 50% polyester I by mass. The surface layer (A) was made from a blend of 55% polyester E, 25% polyester F, and 20% polyester B by mass. These materials were fed into a vented extruder and melt-extruded at 280 °C. The surface layer (C) and the surface layer (A) were then co-extruded in a three-layer structure (surface layer (C) / intermediate layer (B) / surface layer (A)). The thickness ratio of the co-extrusion was C / B / A = 4.0 / 25.5 / 1.5. The mixture was cooled and solidified on a cooling roll with a surface temperature set at 20 °C using an electrostatic adhesion method to obtain an amorphous film. Next, the film was stretched 3.5 times in the machine direction (MD) at a film temperature of 86°C using the difference in roll peripheral speed, and then this machine-stretched film was introduced into a tenter and stretched 4.2 times in the transverse direction (TD) at 105°C. It was then heat-treated at 170°C, 230°C, and 230°C in heat treatment (fixing) zones 1, 2, and 3 within the tenter, respectively, and cooled to 140°C at a relaxation rate of 2%, resulting in a polyester film with an overall thickness of 31 μm.

[0181] Example 3 The procedure was the same as in Example 2, except that the raw material for the surface layer (A) was changed to a blend of 50% polyester A, 30% polyester F, and 20% polyester B by mass, and a polyester film with a total thickness of 31 μm was obtained.

[0182] Example 4 The procedure was the same as in Example 2, except that the raw material for the surface layer (A) was changed to a blend of 30% polyester A, 30% polyester F, and 40% polyester B by mass, and a polyester film with a total thickness of 31 μm was obtained.

[0183] Example 5 A polyester film having a total thickness of 31 μm was produced in the same manner as in Example 4, except that the raw material for the intermediate layer (B) was changed to 100% polyester C.

[0184] (Comparative Example 1) The procedure was the same as in Example 1, except that the raw material for the surface layer (A) was changed to a blend of 20% polyester C, 30% polyester F, and 50% polyester B by mass, and a polyester film with a total thickness of 31 μm was obtained.

[0185] (Comparative Example 2) A polyester film having a total thickness of 31 μm was produced in the same manner as in Example 1, except that the raw material for the surface layer (A) was changed to a raw material obtained by blending 30% Polyester F and 70% Polyester B by mass.

[0186] (Comparative Example 3) The raw material for the surface layer (C) was changed to a material blended at a mass ratio of 90% polyester C and 10% polyester B, the raw material for the intermediate layer (B) was changed to a material blended at a mass ratio of 10% polyester C, 50% polyester F, and 40% polyester B. The raw material for the surface layer (A) was changed to a material blended at a mass ratio of 10% polyester C, 50% polyester F, and 40% polyester B. The materials were co-extruded to a thickness composition ratio of C / B / A = 2.0 / 27.0 / 2.0, and stretched 4.5 times in the transverse direction (TD). Except for this, a polyester film with an overall thickness of 31 μm was produced in the same manner as in Example 1.

[0187] Comparative Example 4 The raw material for the surface layer (C) was changed to a blend of 87% polyester C and 13% polyester B by mass, and the raw material for the surface layer (A) was changed to a blend of 55% polyester E, 17% polyester A, and 28% polyester F by mass. Except for this, the materials were co-extruded to a thickness composition ratio of C / B / A = 4.0 / 25.5 / 1.5 and stretched 4.5 times in the transverse direction (TD), and a polyester film with an overall thickness of 31 μm was obtained in the same manner as in Example 1.

[0188] (Comparative Example 5) The raw material for the surface layer (C) was changed to 100% polyester C, the raw material for the intermediate layer (B) was changed to 100% polyester C, and the raw material for the surface layer (A) was changed to a blend of 70% polyester C and 30% polyester G by mass. These were co-extruded to a thickness composition ratio of C / B / A = 3.0 / 26.5 / 1.5, and then stretched 3.5 times in the machine direction (MD) at a film temperature of 86°C using the roll peripheral speed difference, and 4.5 times in the transverse direction (TD) at 140°C. A polyester film with an overall thickness of 31 μm was produced in the same manner as in Example 1.

[0189] (Comparative Example 6) The raw material for the surface layer (C) was changed to 100% polyester C, the raw material for the intermediate layer (B) was changed to 100% polyester C, and the raw material for the surface layer (A) was changed to a blend of 87% polyester C and 13% polyester B by mass, and the materials were co-extruded so that the thickness composition ratio was C / B / A = 1.5 / 28.0 / 1.5. Except for this, the same production procedure as in Example 1 was carried out to obtain a polyester film with an overall thickness of 31 μm.

[0190] (Comparative Example 7) A polyester film having a total thickness of 31 μm was produced in the same manner as in Example 2, except that the raw material for the surface layer (A) was changed to a raw material obtained by blending 60% polyester E and 40% polyester H by mass.

[0191] (Comparative Example 8) The procedure was the same as in Example 2, except that the raw material for the surface layer (A) was changed to a blend of 30% polyester C, 40% polyester H, and 30% polyester B by mass, and a polyester film with a total thickness of 31 μm was obtained.

[0192] Comparative Example 9 A polyester film having a total thickness of 31 μm was produced in the same manner as in Example 2, except that the raw material for the surface layer (A) was changed to a raw material obtained by blending 60% polyester C and 40% polyester H by mass.

[0193] <Measurement and evaluation methods> (1) Arithmetic mean height (Sa), maximum peak height (Sp), kurtosis (Sku), skewness (Ssk), and root mean square height (Sq) The surfaces of the surface layer A and the surface layer C of the films of the examples and comparative examples were measured using a surface roughness measuring device (Ametec Co., Ltd., "NewView" (registered trademark)), and the arithmetic mean height (Sa), maximum peak height (Sp), kurtosis (Sku), skewness (Ssk), and root mean square height (Sq) were determined from the obtained surface profile curves. Specifically, measurements were taken using the above-mentioned surface roughness measuring instrument under conditions of an objective lens magnification of 10x, a zoom magnification of 2.0x, and a viewing angle of 0.44mm x 0.44mm, and the arithmetic mean height (Sa), maximum peak height (Sp), kurtosis (Sku), skewness (Ssk), and root mean square height (Sq) were determined after the following processing. Measurements were taken at 12 points, and the average of the 10 points excluding the maximum and minimum values ​​was taken as the measured value. FilterType:Spline Filter: High Pass Type:Robust Gaussian Spline Fixed Cutoffs Mode: Period Long Period: 200 μm

[0194] (2) Intrinsic viscosity (IV) 1 g of polyester was precisely weighed and dissolved in 100 mL of a 50 / 50 (mass ratio) phenol / tetrachloroethane mixed solvent, and the viscosity (IV) was measured at 30°C using a VMS-022UPC·F10 viscosity (IV) measuring device (manufactured by Rigo Co., Ltd.).

[0195] (3) Air leakage index The air leakage index was measured using a DigiBec smoothness tester ("DB-2" manufactured by Toyo Seiki Co., Ltd.) in accordance with JIS P8119 at a temperature of 23°C and a relative humidity of 50%. The pressure of the pressure device was 100 kPa, and the vacuum container was a 38 ml container. The time (seconds) for 1 mL of air to flow, i.e., the time (seconds) for the pressure inside the container to change from 50.7 kPa to 48.0 kPa, was measured, and the air leakage index was calculated by multiplying the measured time by 10. The polyester film sample size was 70 mm square, and 20 sheets were stacked with the front and back sides (film surfaces (A) and (C)) overlapping to form a test laminate film. A 5 mm diameter hole was then drilled in the center of this test laminate film, and the air leakage index was measured as described above. A higher air leakage index value indicates a longer time for air to leak through the gaps between the films, indicating a tighter contact between the films and a greater likelihood of wrinkling when rolled.

[0196] (4) Heat curl The evaluation film (sampled at 150 mm in the machine direction (MD) × 25 mm in the transverse direction (TD) from the center of the transverse direction (TD) of the film-formed film) was placed on a stainless steel pad in an untensioned state, with the surface (A) side facing up or the surface (C) side facing up. It was then placed in a hot air oven maintained at a predetermined temperature (120°C) and heat-treated for 1 minute or 5 minutes. The height of the curled film edge of the evaluation film after heat treatment was measured.

[0197] [Table 2]

[0198] [Table 3]

[0199] [Table 4]

[0200] [Table 5]

[0201] In the examples, the surface (A) has no coarse protrusions, and excellent surface smoothness is achieved. Furthermore, in the examples, the surface (C) also has extremely excellent surface smoothness. Furthermore, in the examples, the air leakage index of the polyester film is small. Thus, in the polyester films obtained in the examples, both surfaces are highly smooth, yet the air leakage index is small, demonstrating good handleability of the polyester film. In addition, in Examples 2 to 4, recycled polyester is used for the intermediate layer (B), and the surface (A) has no coarse protrusions, and excellent surface smoothness is achieved. Furthermore, in the examples, the value obtained by dividing the intrinsic viscosity of the polyester resin constituting the surface layer (C) by the intrinsic viscosity of the polyester resin constituting the surface layer (A) was close to 1.00, and thus curling during heating tended to be suppressed.

[0202] On the other hand, in Comparative Examples 1 to 5 and 7 to 9, the Sp of the surface (A) exceeded 101 nm, resulting in the formation of coarse protrusions on the surface layer. Such coarse protrusions are undesirable because they directly lead to pinholes in the ceramic sheet. Furthermore, in Comparative Examples 4, 5 and 7 to 9, the air leakage index was large, resulting in poor handling of the polyester film. Furthermore, in Comparative Example 6, although the surface (A) was smooth, the air leakage index was 55 times or more that of Example 1, resulting in very poor handling of the polyester film.

[0203] Example 101 The raw material for the surface layer (A) was changed to a blend of 50% polyester A, 30% polyester F, and 20% polyester B by mass, and the layers were co-extruded to a thickness composition ratio of C / B / A = 4.0 / 25.5 / 1.5. Except for this, the same procedure as in Example 1 was repeated to obtain a polyester film with an overall thickness of 31 μm.

[0204] Example 102 The procedure for production was the same as in Example 101, except that the raw material for the surface layer (A) was changed to a blend of 30% Polyester A, 30% Polyester F, and 40% Polyester B by mass, to obtain a polyester film with a total thickness of 31 μm.

[0205] (Example 103) The procedure for production was the same as in Example 101, except that the raw material for the surface layer (A) was changed to a blend of 55% polyester A, 25% polyester F, and 30% polyester B by mass, and a polyester film with a total thickness of 31 μm was obtained.

[0206] Example 104 A polyester film with a total thickness of 31 μm was produced in the same manner as in Example 103, except that the raw material for the intermediate layer (B) was changed to a raw material obtained by blending 50% Polyester D and 50% Polyester H by mass. Polyester H was a recycled raw material (intrinsic viscosity 0.58 dL / g) made by recovering, crushing, and re-pelletizing polyester film scraps containing 50 to 1000 mass ppm of particles with a particle size of 0.05 to 1.0 μm.

[0207] It is estimated that in Examples 101 to 104, polyester films having excellent surface smoothness and a small air leakage index can also be obtained. [Industrial Applicability]

[0208] The polyester film of the present invention has excellent surface smoothness and excellent handleability. Therefore, it is useful, for example, as a support (substrate) for ceramic green sheets in the manufacturing process of multilayer ceramic capacitors. When used as a support for ceramic green sheets in the manufacturing process of multilayer ceramic capacitors, a uniform thin dielectric layer can be formed with reduced pinhole defects. The polyester film of the present invention is particularly suitable for use as a support for ceramic green sheets used in multilayer ceramic capacitors for automobiles. [Explanation of symbols]

[0209] 10 Polyester film 12 Surface layer (A) 14 Middle layer (B) 16 Surface layer (C)

Claims

1. It has one surface (A) and the other surface (C), A polyester film having a maximum peak height (Sp) of 101 nm or less on one surface (A), an arithmetic mean peak height (Sa) of 10 nm or less on the other surface (C), and an air leakage index of 20,000 seconds or less.

2. The polyester film according to claim 1, wherein the other surface (C) has a maximum peak height (Sp) of 50 nm or less.

3. The polyester film according to claim 1, wherein the arithmetic mean height (Sa) of the one surface (A) is 50 nm or less.

4. The polyester film according to claim 1, wherein the one surface (A) has an arithmetic mean height (Sa) of 1 nm or more.

5. 2. The polyester film according to claim 1, wherein the ratio of the maximum peak height (Sp) to the arithmetic mean peak height (Sa) on the one surface (A) is 3 to 20.

6. 2. The polyester film according to claim 1, wherein the ratio of the maximum peak height (Sp) to the arithmetic mean peak height (Sa) of the other surface (C) is 67 or less.

7. The polyester film according to claim 1 , wherein the layer forming the one surface (A) comprises at least particles (a1) and particles (a2).

8. 8. The polyester film according to claim 7, wherein the zeta potentials of the particles (a1) and the particles (a2) at pH 7 satisfy the following condition (1) or (2): (1) The zeta potential of the particles (a1) is positive, and the zeta potential of the particles (a2) is negative; (2) The zeta potential of the particles (a1) is negative, and the zeta potential of the particles (a2) is positive.

9. The polyester film according to claim 7, wherein the particles (a1) have an average particle size of 0.01 to 1 μm.

10. The polyester film according to claim 7, wherein the particles (a2) have an average particle size of 0.05 to 1.5 μm.

11. The polyester film according to claim 7 , wherein the particles (a1) have an average particle size smaller than the average particle size of the particles (a2).

12. 8. The polyester film according to claim 7, wherein a value obtained by dividing the average particle size of the particles (a2) by the average particle size of the particles (a1) is 1.2 to 15.

13. The polyester film according to claim 7 , wherein at least one of the particles (a1) and the particles (a2) has a Mohs hardness of 8 or more.

14. The polyester film according to claim 7 , wherein the particles (a1) are alumina particles.

15. The polyester film according to claim 7 , wherein the particles (a2) are either organic particles or silica particles.

16. 2. The polyester film according to claim 1, wherein the layer (A) on one surface of the polyester film has an intrinsic viscosity of 0.64 dL / g or more.

17. 2. The polyester film according to claim 1, wherein the layer (C) forming the other surface (C) has an intrinsic viscosity of 0.63 dL / g or more.

18. 2. The polyester film according to claim 1, wherein a value obtained by dividing the intrinsic viscosity of the polyester resin constituting the layer forming the other surface (C) by the intrinsic viscosity of the polyester resin constituting the layer forming one surface (A) is 0.7 to 1.

3.

19. The polyester film according to claim 1 , wherein the layer forming the other surface (C) contains particles.

20. 8. The polyester film according to claim 7, wherein the total content of the particles (a1) and the particles (a2) is 1,000 to 10,000 ppm based on the total mass of the layer forming the one surface (A).

21. 8. The polyester film according to claim 7, wherein the content of the particles (a1) is 100 to 5000 ppm based on the total mass of the layer forming the one surface (A).

22. 8. The polyester film according to claim 7, wherein the content of the particles (a2) is 100 to 8,000 ppm based on the total mass of the layer forming the one surface (A).

23. 2. The polyester film according to claim 1, which has a three-layer structure.

24. 24. The polyester film of claim 23, wherein the middle layer of the three-layer configuration is substantially free of particles.

25. 24. The polyester film according to claim 23, wherein the intermediate layer of the three-layer structure contains a recycled polyester resin, and the content of the recycled polyester resin is 10% by mass or more based on the total mass of the resins constituting the intermediate layer.

26. 2. The polyester film according to claim 1, which is used as a support for a ceramic green sheet in the production process of a multilayer ceramic capacitor.

27. A release film comprising the polyester film according to any one of claims 1 to 26, further comprising a release layer on at least one surface thereof.

28. A polyester film with a ceramic green sheet, comprising the polyester film according to any one of claims 1 to 26 and a ceramic green sheet laminated thereon.

29. Use of the polyester film according to any one of claims 1 to 26 as a support for a ceramic green sheet in the process of producing a multilayer ceramic capacitor.

30. A method for producing a ceramic green sheet, comprising: applying a ceramic slurry containing a ceramic component to the surface (C) side of the polyester film according to any one of claims 1 to 26.

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