Polyester film for film capacitor, method for producing polyester film for film capacitor, and metal laminate film

Abstract

An object of the present invention is to provide a polyester film having a high dielectric breakdown strength even in a case of being thinned.SOLUTION: A polyester film for a film capacitor contains a polyester resin (X) and a polypropylene resin (Y), and has a particle content of 0.001% by mass or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyester film for a film capacitor, a method for producing a polyester film for a film capacitor, and a metal laminated film. [Background technology]

[0002] Polyethylene terephthalate (PET) film, a representative polyester film, especially biaxially oriented PET film, is widely used in a variety of fields, including industrial materials, optical materials, electronic component materials, and battery packaging, due to its excellent transparency, mechanical strength, heat resistance, and flexibility.

[0003] Furthermore, due to their excellent electrical properties, such as low dielectric loss, and high moisture resistance, resin films such as biaxially oriented polypropylene films are used as dielectric films for capacitors, such as filter capacitors and smoothing capacitors for high-voltage capacitors, various switching power supplies, converters, inverters, etc. (e.g., Patent Document 1).

[0004] In recent years, there has been a growing need for further miniaturization and higher capacity of capacitors. For example, when using resin films in capacitors for inverter power supply devices that control the drive motors of electric vehicles, hybrid vehicles, etc., small size, light weight, and high capacity are required. As the capacity of capacitors increases, for example, high voltage resistance characteristics (capacitance stability) over long periods of time are sometimes required in temperature ranges exceeding 120°C. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-231584 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when polypropylene is blended with polyester to improve heat resistance, the dielectric breakdown strength of the polyester film tends to decrease, especially in the extremely thin region of 12 μm or less. If the dielectric breakdown strength of a polyester film decreases, it will no longer be able to maintain good electrical properties, so improvements are needed.

[0007] The present invention has been made in view of the above problems, and an object of the present invention is to provide a polyester film that has high dielectric breakdown strength even when made thin. [Means for solving the problem]

[0008] As a result of extensive research, the present inventors have found that the above problems can be solved by employing a polyester film having a specific configuration, and have completed the present invention as described below. That is, the present invention provides the following [1] to

[18] . [1] A polyester film for film capacitors, comprising a polyester resin (X) and a polypropylene resin (Y), and having a particle content of 0.001% by mass or less. [2] The polyester film for a film capacitor according to [1], wherein at least one surface of the polyester film satisfies the following formula (1): Sv / Sa<34 (1) (In the above formula (1), Sv represents the maximum valley depth, and Sa represents the arithmetic mean height.) [3] The polyester film for film capacitors according to [1] or [2], wherein the maximum valley depth (Sv) of at least one surface of the polyester film is 5000 nm or less. [4] The polyester film for film capacitors according to any one of [1] to [3], wherein the arithmetic mean height (Sa) of at least one surface of the polyester film is 200 nm or less. [5] The polyester film for film capacitors according to any one of [1] to [4], wherein the polyester resin (X) is at least one selected from polyethylene terephthalate and polyethylene-2,6-naphthalate. [6] The polyester film for film capacitors according to any one of [1] to [5], wherein the polycondensation catalyst for the polyester resin (X) is a Ti-based catalyst. [7] The polyester film for film capacitors according to any one of [1] to [6], wherein the polyester resin (X) has an intrinsic viscosity of 0.80 dL / g or less. [8] The polyester film for film capacitors according to any one of [1] to [7], which contains 1 to 30 parts by mass of polypropylene resin (Y) per 100 parts by mass of polyester resin (X). [9] The polyester film for a film capacitor according to any one of [1] to [8], which has a thickness of 0.5 to 12.0 μm.

[10] The polyester film for film capacitors according to any one of [1] to [9], which has a dielectric breakdown strength of 110 kV / μm or more.

[11] The polyester film for film capacitors according to any one of [1] to

[10] , further comprising a compatibilizer (Z).

[12] The polyester film for film capacitors according to

[11] , wherein the compatibilizer (Z) is a resin or ionomer having an acid anhydride structure [—C(═O)—O—C(═O)—].

[13] The polyester film for film capacitors according to

[11] or

[12] , which contains 0.01 to 40 parts by mass of a compatibilizer (Z) per 100 parts by mass of the polyester resin (X).

[14] A metal laminated film for a film capacitor, comprising the polyester film according to any one of [1] to

[13] , and a metal layer provided on at least one surface of the polyester film.

[15] A method for producing a polyester film for a film capacitor, comprising obtaining an amorphous film from a resin composition containing a polyester resin (X), a polypropylene resin (Y), and a compatibilizer (Z), and satisfying the following (2) and (3), and then stretching the film in the longitudinal and transverse directions, wherein the particle content in the resin composition is 0.001% by mass or less. (2) The polypropylene resin (Y) has a melt flow rate (MFR) of 7.0 g / 10 min or less. (3) The compatibilizer (Z) is a resin or ionomer having an acid anhydride structure [—C(═O)—O—C(═O)—].

[16] The method for producing a polyester film for a film capacitor according to

[15] , wherein the resin composition contains 0.01 to 40 parts by mass of the compatibilizer (Z) per 100 parts by mass of the polyester resin (X).

[17] The method for producing a polyester film for a film capacitor according to

[15] or

[16] , wherein the resin composition contains 1 to 30 parts by mass of the polypropylene resin (Y) per 100 parts by mass of the polyester resin (X).

[18] The method for producing a polyester film for a film capacitor according to any one of

[15] to

[17] , wherein the polyester resin (X) has an intrinsic viscosity of 0.80 dL / g or less. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a polyester film that has high dielectric breakdown strength even when made thin. DETAILED DESCRIPTION OF THE INVENTION

[0010] An example of an embodiment of the present invention will be described below, however, the present invention is not limited to the embodiment described below. In the present invention, the term "film" includes the term "sheet", and the term "sheet" includes the term "film". In the present invention, when it is stated that "X to Y" (X and Y are any numbers), unless otherwise specified, it means "X or more and Y or less", and also means "preferably larger than X" or "preferably smaller than Y". Furthermore, when it is stated that the amount is "X or more" (X is any number), it also means that the amount is "preferably greater than X" unless otherwise specified, and when it is stated that the amount is "Y or less" (Y is any number), it also means that the amount is "preferably smaller than Y" unless otherwise specified.

[0011] [Polyester film] The present invention relates to a polyester film for film capacitors (hereinafter sometimes referred to as "the polyester film") that contains a polyester resin (X) and a polypropylene resin (Y) and has a particle content of 0.001% by mass or less. The particle content in the polyester film is 0.001% by mass or less, meaning that the polyester film is substantially particle-free. The particle content is 0.001% by mass or less, preferably 0.0005% by mass or less, and more preferably 0% by mass or less. Conventionally, particles have been added to polyester films for film capacitors primarily for the purposes of imparting slipperiness and preventing scratches during various processes. However, the present invention has discovered that particle-free polyester films can more effectively increase dielectric breakdown strength and exhibit favorable electrical properties. Therefore, the polyester film is suitable for use in film capacitors.

[0012] In the present invention, in order to accommodate capacitor applications, the film thickness is preferably thin, specifically, in the ultrathin range (0.5 to 12 μm). Therefore, unlike other applications (e.g., to provide cushioning as a paper replacement), in which voids are intentionally formed using an incompatible polypropylene resin with a polyester resin, the polypropylene resin (Y) is used, for example, as a capacitor film, particularly for the purpose of improving voltage resistance. Furthermore, in order to obtain a film with good electrical properties (e.g., dielectric breakdown strength) at 150°C, it is preferable that the film does not contain particles.

[0013] In the present invention, the polypropylene resin (Y) is a resin that is incompatible with the polyester resin (X). Since the present polyester film contains the polypropylene resin (Y) that is incompatible with the polyester resin (X), fine irregularities are formed on the film surface. Therefore, the particle content in the film can be set to 0.001% by mass or less, and even in such a case, slipperiness can be imparted to the film.

[0014] Because of the above-described structure, the polyester film exhibits high dielectric breakdown strength even when thinned. Therefore, the polyester film can maintain good electrical properties. Furthermore, because of the above-described structure, the polyester film has excellent heat resistance and can be made thin, making it useful for capacitors. Furthermore, the polyester film has excellent heat resistance, flatness, optical properties, and strength.

[0015] The polyester film may be a single layer or a multilayer film having two or more layers with different properties (i.e., a laminate film). When the polyester film is a laminate film, it is preferable that at least one layer contains polyester resin (X) and polypropylene resin (Y) and has a particle content of 0.001% by mass or less.

[0016] The polyester film may be either an unstretched film or a stretched film. Among these, the polyester film is preferably a uniaxially or biaxially stretched film. Among these, a biaxially stretched film is more preferred from the viewpoint of the balance of mechanical properties and flatness. That is, the polyester film is preferably a biaxially stretched polyester film.

[0017] The thickness of the polyester film is preferably 0.5 to 12.0 μm, more preferably 0.5 to 10.0 μm, even more preferably 1.0 to 8.0 μm, and particularly preferably 1.0 to 4.0 μm. By controlling the thickness of the polyester film within the above range, it is suitable for use in, for example, capacitors. Furthermore, the polyester film can be thinned, and even within the above thickness range, it can exhibit high dielectric breakdown strength.

[0018] The polyester film preferably further contains a compatibilizer (Z). The polyester film preferably contains a polyester resin (X), a polypropylene resin (Y), and a compatibilizer (Z), and the compatibilizer (Z) is a resin having an acid anhydride structure [-C(=O)-OC(=O)-] or an ionomer. The polyester resin (X), the polypropylene resin (Y), and the compatibilizer (Z) are described in detail below.

[0019] <Polyester resin (X)> The polyester film contains a polyester resin (X). The polyester resin (X) is the main component resin among the resins constituting the polyester film. The main component resin refers to the resin that accounts for the largest mass percentage among the resin components constituting the polyester film, and is preferably 50 mass% or more, 60 mass% or more, 70 mass% or more, or 80 mass% or more of the total mass of the resin components.

[0020] The polyester resin (X) may be a homopolyester or a copolymer polyester.

[0021] The homopolyester is preferably one obtained by polycondensation of an aromatic dicarboxylic acid and an aliphatic glycol, such as terephthalic acid or 2,6-naphthalenedicarboxylic acid, or an aliphatic glycol, such as ethylene glycol, diethylene glycol, 1,4-butanediol, or 1,4-cyclohexanedimethanol. Typical examples of homopolyesters include polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene-2,6-naphthalate (PEN). In the present invention, at least one selected from polyethylene terephthalate (PET) and polyethylene-2,6-naphthalate (PEN) is particularly preferred, and these can also be used in combination.

[0022] On the other hand, when the polyester resin (X) is a copolymer polyester, it is preferably a copolymer containing 30 mol % or less of a third component. 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, etc., and examples of the glycol component include one or more of ethylene glycol, diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, etc.

[0023] Among these, the polyester film is preferably a polyethylene terephthalate film containing 60 mol % or more, preferably 80 mol % or more of ethylene terephthalate units, or a polyethylene-2,6-naphthalate film containing 60 mol % or more, preferably 80 mol % or more of ethylene-2,6-naphthalate units.

[0024] (intrinsic viscosity) The intrinsic viscosity of the polyester resin (X) is preferably 0.80 dL / g or less, more preferably 0.70 dL / g or less. On the other hand, there is no particular restriction on the lower limit of the intrinsic viscosity, but it is preferably 0.45 dL / g or more, more preferably 0.50 dL / g or more, and even more preferably 0.52 dL / g or more. When the intrinsic viscosity is equal to or less than the upper limit, stability of melt extrusion is ensured. On the other hand, when the intrinsic viscosity is equal to or more than the lower limit, productivity during film production is maintained and sufficient mechanical strength of the film is ensured.

[0025] (Polyester polycondensation catalyst) Examples of polycondensation catalysts used to obtain the polyester resin (X) by polycondensation include antimony compounds, germanium compounds, aluminum compounds, and titanium compounds. Among these, the polycondensation catalyst is preferably at least one selected from antimony compounds and titanium compounds, and particularly preferably a titanium compound. That is, when polycondensing the polyester resin (X), it is preferable to use a Ti-based polycondensation catalyst. Therefore, the polyester film preferably contains at least one selected from antimony compounds and titanium compounds, and more preferably a titanium compound (Ti-based). By using a titanium compound as a polycondensation catalyst when polycondensing the polyester resin (X), it is possible to suppress the generation of metal-containing aggregates, or so-called coarse foreign matter, derived from the titanium compound in the film.

[0026] (particle) Although particles have conventionally been blended into polyester resins primarily for the purposes of imparting lubricity and preventing scratches during each process, in the present embodiment, the polyester resin (X) does not substantially contain particles. Here, "substantially not containing particles" means that particles are not intentionally contained, and specifically refers to a particle content (particle concentration) of 200 ppm by mass or less, more preferably 150 ppm by mass or less.

[0027] <Polypropylene resin (Y)> The polypropylene resin (Y) is a resin that is incompatible with the polyester resin (X), that is, the polypropylene resin (Y) used in the present invention is an incompatible polypropylene resin.

[0028] The polypropylene resin (Y) may be a homopolymer of propylene or a copolymer of propylene with ethylene and / or an α-olefin having 4 to 20 carbon atoms. When the polypropylene resin (Y) is a copolymer, the copolymerization component may be one type, or two or more types may be used in combination, as necessary.

[0029] The method for producing the polypropylene resin (Y) is not particularly limited, and any conventionally known method, such as a method of polymerizing propylene and an optional comonomer using a Ziegler-Natta catalyst, can be used. This method is also described in Section 2.3.1 (pages 20-57) of "Polypropylene Handbook," edited by Edward P. Moore Jr., translated and supervised by Tetsuo Yasuda and Nobu Sakuma, published by the Industrial Research Institute (1998).

[0030] (MFR (Melt Flow Rate)) The MFR of the polypropylene resin (Y) is preferably 7.0 g / 10 min or less, more preferably 5.0 g / 10 min or less, and even more preferably 3.0 g / 10 min or less. The lower limit of the MFR is not particularly limited, but is preferably 0.1 g / 10 min or more, more preferably 0.2 g / 10 min or more, and even more preferably 0.3 g / 10 min or more. When the MFR is equal to or less than the upper limit, good fluidity can be ensured during film molding. The MFR is measured in accordance with ISO 1133-1:2011.

[0031] (Melting Point) The melting point of the polypropylene resin (Y) is preferably 150°C or higher, more preferably 160°C or higher. When the melting point is equal to or higher than the above lower limit, the desired heat resistance can be ensured. The upper limit of the melting point is not particularly limited, but is preferably 170°C or lower. The melting point in the present invention can be determined by differential scanning calorimetry (DSC). Specifically, the temperature is first raised from room temperature to 200°C, the thermal history is erased, and then the temperature is lowered to 40°C at a rate of 10°C / min, and the temperature is again measured at a rate of 10°C / min, and the melting point is defined as the temperature at the top of the endothermic peak.

[0032] <Compatibilizer (Z)> The polyester film preferably contains a compatibilizer (Z), which is preferably a resin having an acid anhydride structure [—C(═O)—O—C(═O)—] or an ionomer.

[0033] (Resin having an acid anhydride structure [-C(=O)-OC(=O)-]) The compatibilizer (Z) is preferably a resin having an acid anhydride structure [—C(═O)—O—C(═O)—]. Having such a structure, the compatibilizer exhibits excellent compatibility with the polyester resin (X) and polypropylene resin (Y). Various resins can be used as the resin having the acid anhydride structure [—C(═O)—O—C(═O)—]. Specifically, a polyolefin resin, such as polyethylene or polypropylene, having the acid anhydride structure, or a polystyrene resin having the acid anhydride structure can be used. Among these, the resin having the acid anhydride structure [—C(═O)—O—C(═O)—] is preferably a polyolefin resin having the acid anhydride structure, and more preferably a polypropylene resin having the acid anhydride structure. That is, the compatibilizer (Z) is preferably a modified polypropylene resin having the acid anhydride structure (a polypropylene resin modified with a carboxylic acid anhydride). A modified polypropylene resin having an acid anhydride structure refers to a resin synthesized using a monomer having an acid anhydride structure in addition to propylene as a raw material. In addition, as the modified polypropylene resin having an acid anhydride structure, a modified polyethylene resin having an acid anhydride structure can be used together with the modified polypropylene resin having an acid anhydride structure or in place of the modified polypropylene resin.

[0034] The monomer having an acid anhydride structure is preferably a compound having an acid anhydride structure and an ethylenically unsaturated bond. Specific examples include maleic anhydride, citraconic anhydride, aconitic anhydride, itaconic anhydride, etc. Among them, maleic anhydride is preferred because it has good compatibility with the polyester resin (X) and the polypropylene resin (Y).

[0035] If the acid value of the resin having an acid anhydride structure [-C(=O)-OC(=O)-] is too small, the compatibility between the polyester resin (X) and the polypropylene resin (Y) may not be fully exhibited. On the other hand, if the acid value is too large, the compatibility with the resin decreases. Therefore, the acid value is preferably 0.5 to 50, more preferably 0.5 to 10. The MFR is preferably 2 to 20 g / 10 min, more preferably 3 to 10 g / 10 min. The acid value is measured in accordance with JIS K 0070.

[0036] Commercially available resins having an acid anhydride structure can also be used. For example, when a modified polypropylene resin is used, Admer (trade name, manufactured by Mitsui Chemicals, Inc.) or OREVAC (trade name, manufactured by Arkema) can be used. When a modified polyethylene resin is used, Admer (trade name, manufactured by Mitsui Chemicals, Inc.) LF128 (MFR: 2.7 g / 10 min) can be used. When a styrene-based thermoplastic resin modified with maleic anhydride is used, examples include "Tufprene 912" manufactured by Asahi Kasei Corporation, "FG1901" and "FG1924" manufactured by Kraton Polymer Japan Co., Ltd., and "Tuftec M1911," "Tuftec M1913," and "Tuftec M1943" manufactured by Asahi Kasei Corporation.

[0037] (ionomer) Ionomers are random, block, or graft copolymers of ethylene and acidic vinyl monomers such as unsaturated carboxylic acids that are partially neutralized with metal salts, and these polymer chains are made into aggregates using the cohesive force of the metal ions. For example, there are ionomer resins made by neutralizing at least a portion of the carboxyl groups of an ethylene-unsaturated carboxylic acid copolymer resin with metal ions.

[0038] The ionomer used is preferably an ethylene-unsaturated carboxylic acid copolymer resin in which 10 mol % or more, preferably 10 to 90 mol %, and more preferably 15 to 80 mol % of the carboxyl groups have been neutralized with metal ions. Examples of metal ions include polyvalent metal ions such as alkali metals such as lithium and sodium, zinc, and alkaline earth metals such as magnesium and calcium. Among these, polyvalent metal ions such as zinc are preferred because of their excellent compatibilizing effect.

[0039] (Amount of polypropylene resin (Y)) The blending amount of polypropylene resin (Y) per 100 parts by mass of polyester resin (X) is preferably 1 to 30 parts by mass, more preferably 5 to 30 parts by mass, even more preferably 10 to 30 parts by mass, and particularly preferably 10 to 25 parts by mass. By blending the polypropylene resin (Y) within the above range, the polyester film can have good electrical properties, for example, as a polyester film for capacitors. When the polyester film is a multilayer film, the above blending amount refers to the blending amount in the entire polyester film. Therefore, even in the case of a multilayer polyester film formed using resin compositions with different blending amounts of polypropylene resin (Y), for example, the blending amount refers to the blending amount of polypropylene resin (Y) in the entire laminate film.

[0040] (Compatibilizer (Z) content) The amount of the compatibilizer (Z) blended relative to 100 parts by mass of the polyester resin (X) is preferably 0.01 to 40 parts by mass. More preferably, it is 0.1 to 20 parts by mass, and particularly preferably 1 to 20 parts by mass. By blending the amount of the compatibilizer (Z) within the above range, the compatibility between the polyester resin (X) and the polypropylene resin (Y) becomes good, and a polyester film with good electrical properties can be obtained as a polyester film for capacitors. When the polyester film is a multilayer film, the above blending amount refers to the blending amount in the entire polyester film. Therefore, even in the case of a multilayer polyester film formed using resin compositions with different blending amounts of the compatibilizer (Z), for example, the blending amount refers to the blending amount of the compatibilizer (Z) in the entire laminate film.

[0041] [Polyester film manufacturing method] The present invention relates to a method for producing a polyester film for film capacitors (hereinafter sometimes referred to as "the present production method"), which comprises obtaining an amorphous film from a resin composition containing a polyester resin (X), a polypropylene resin (Y), and a compatibilizer (Z) and satisfying the following (2) and (3), and then stretching the film in the longitudinal and transverse directions. The particle content in the resin composition used in the production method of the present invention is 0.001 mass% or less. (2) The polypropylene resin (Y) has a melt flow rate (MFR) of 7.0 g / 10 min or less. (3) The compatibilizer (Z) is a resin or ionomer having an acid anhydride structure [—C(═O)—O—C(═O)—]. The polyester resin (X), polypropylene resin (Y) and compatibilizer (Z) contained in the resin composition may be the polyester resin (X), polypropylene resin (Y) and compatibilizer (Z) described above.

[0042] As an example of the present production method, a production method in which the present polyester film is a biaxially stretched film will be described below, although the present invention is not limited to the production method described here.

[0043] In this manufacturing method, for example, raw materials such as polyester chips, polypropylene chips and a compatibilizer are fed into a melt extrusion device by a known method, heated to above the melting point of each polymer, the molten polymer is extruded from a die, and cooled and solidified on a rotating cooling drum to a temperature below the glass transition point of the polymer, to obtain a substantially amorphous unoriented sheet (amorphous film). practiceIt is advisable to improve the dispersion by extrusion.

[0044] Next, the unoriented sheet is stretched in one direction using a roll or tenter-type stretching machine. The stretching temperature is typically 25 to 120°C, preferably 35 to 100°C, and the stretching ratio is typically 2.5 to 7 times, preferably 2.8 to 6 times. Next, the sheet is stretched in a direction (TD, transverse direction) perpendicular to the first-stage stretching direction (MD, machine direction). The stretching temperature is typically 50 to 140°C, and the stretching ratio is typically 3.0 to 7 times, preferably 3.5 to 6 times. In this specification, stretching in the machine direction may be referred to as machine direction stretching, and stretching in the transverse direction may be referred to as transverse stretching. It is also possible to employ a method in which unidirectional stretching is performed in two or more stages. In the present invention, as described above, it is preferable to increase the stretching ratio in the transverse direction (TD), and it is preferable to set the stretching ratio to 4.0 times or more in either the first or second stage.

[0045] After stretching, the polyester film can be heat-set at a temperature of 130 to 270°C under tension or relaxation of 30% or less to obtain the present polyester film as a biaxially oriented film. By subjecting the present polyester film to heat-setting, the heat resistance and other properties can be improved.

[0046] When producing a laminated polyester film having a multilayer structure as the present polyester film, the resin compositions constituting each layer are prepared and co-extruded to obtain a multilayer film. When co-extruding, all layers may be extruded under the above conditions, but at least one layer needs to be produced under the above extrusion conditions. For example, in the case of a three-layer structure of surface layer / middle layer / surface layer, only the middle layer may be extruded under the above conditions, or all layers may be extruded under the above conditions. Subsequent stretching, etc. may be performed in the same manner as described above.

[0047] <Characteristics of this polyester film> (dielectric breakdown strength) The dielectric breakdown strength of the polyester film is preferably 110 kV / μm or more, more preferably 130 kV / μm or more, and particularly preferably 150 kV / μm or more. The upper limit of the dielectric breakdown strength is not particularly limited, but is preferably 300 kV / μm or less, for example. In this specification, the dielectric breakdown strength is the value obtained by dividing the dielectric breakdown voltage (kV) by the thickness (μm) of the polyester film. By making the dielectric breakdown strength of the polyester film equal to or greater than the lower limit, the polyester film can have good insulating properties in a high-temperature atmosphere.

[0048] Specific methods for achieving the desired dielectric breakdown strength of the polyester film include adjusting the types and amounts of the polyester resin (X) and polypropylene resin (Y) appropriately and maintaining a particle content of 0.001% by mass or less. Other methods that can appropriately control the dielectric breakdown strength include allowing sufficient mixing time when mixing the raw materials, setting the stretching ratio, particularly the transverse stretching ratio, to 4.0 times or more, selecting a polypropylene resin with an MFR of 0.1 to 10 g / 10 min, and using a compatibilizer. These methods may be used alone or in combination.

[0049] In the polyester film, the polypropylene resin (Y) is preferably uniformly dispersed in the polyester resin (X). Since the polypropylene resin (Y) is incompatible with the polyester resin (X), from a microscopic point of view, the polyester resin (X) and the polypropylene resin (Y) are uniformly dispersed while being phase-separated.

[0050] In the present invention, it is preferable that at least one surface of the polyester film satisfies the following formula (1). Sv / Sa<34 (1) (In the above formula (1), Sv represents the maximum valley depth, and Sa represents the arithmetic mean height.)

[0051] In the present invention, the value of Sv / Sa is preferably less than 34, more preferably 33 or less, even more preferably 32 or less, even more preferably 31 or less, and particularly preferably 30 or less. The lower limit of the value of Sv / Sa is not particularly limited, but is preferably 10 or more, for example.

[0052] Conventionally, unevenness was formed on the film surface by incorporating particles into the film. However, in this case, the depth of the recesses became deeper (the depth of the recesses became greater), which is thought to have resulted in a thinner core resin layer and a lower dielectric breakdown strength. In contrast, in the present invention, unevenness is formed by blending a polyester resin (X) with a polypropylene resin (Y) that is incompatible with the polyester resin (X), utilizing the incompatibility between the resins. Therefore, by utilizing the fact that unevenness is formed by an incompatible system without incorporating particles into the film, the depth of the recesses can be reduced (the depth of the recesses can be reduced) in the surface roughness of the unevenness. Furthermore, in the present invention, we focused on the maximum depth (Sv) as an index representing the depth of such recesses. When Sv is appropriately small, the thickness of the core resin layer (the thickness from the bottom of the recesses on one side to the bottom of the recesses on the other side) is sufficiently secured, thereby increasing the dielectric breakdown strength and, as a result, exhibiting good electrical properties.

[0053] The maximum valley depth (Sv) of at least one surface of the polyester film is preferably 5000 nm or less, more preferably 4500 nm or less, even more preferably 4000 nm or less, even more preferably 3800 nm or less, and particularly preferably 3500 nm or less. The maximum valley depth (Sv) of at least one surface of the polyester film is preferably 1500 nm or more, more preferably 2000 nm or more, and even more preferably 2500 nm or more. By ensuring the maximum valley depth (Sv) is within the above range, the thickness of the core resin layer (the thickness from the bottom of the concave portion of the unevenness on one surface to the bottom of the concave portion of the unevenness on the other surface) is sufficiently ensured, thereby increasing the dielectric breakdown strength and, as a result, exhibiting good electrical properties.

[0054] The arithmetic mean height (Sa) of at least one surface of the polyester film is preferably 200 nm or less, more preferably 180 nm or less, even more preferably 160 nm or less, and particularly preferably 140 nm or less. The arithmetic mean height (Sa) of at least one surface of the polyester film is preferably 20 nm or more, more preferably 40 nm or more, and even more preferably 60 nm or more. By keeping the arithmetic mean height (Sa) within the above range, the dielectric breakdown strength can be increased more effectively, resulting in excellent electrical properties.

[0055] <Metal laminated film> The present invention may also relate to a metal laminated film for film capacitors (hereinafter, sometimes referred to as a "metal laminated film"), which comprises the above-mentioned polyester film and a metal layer provided on at least one side thereof. The metal laminated film is useful for capacitors.

[0056] Examples of metals constituting the metal layer include copper, silver, chromium, aluminum, nickel, and zinc. Of these, aluminum and zinc are preferred from the standpoints of cost and environmental friendliness. The thickness of the metal layer is preferably in the range of 10 to 5000 Å, more preferably in the range of 100 to 4000 Å, and even more preferably in the range of 100 to 2000 Å. A thickness within the above range is advantageous in terms of electrical properties.

[0057] <Cured resin layer> The polyester film of the present invention may have a cured resin layer provided on its surface. That is, the present invention may relate to a polyester film with a cured resin layer. The cured resin layer preferably contains a fluorine-containing compound. By providing a cured resin layer containing a fluorine-containing compound, the electrostatic potential per thickness is improved compared to the polyester film alone, and the attenuation of the electrostatic potential can be further suppressed, and dust and oil adhering to the polyester film surface can be easily removed.

[0058] In particular, the cured resin layer is preferably formed by curing a cured resin layer composition containing a fluorine-containing compound (A), a crosslinking agent (B), and a binder resin (C). By using a cured resin layer composition containing these components (A) to (C), the cured resin layer can improve the electrostatic potential per thickness and further suppress attenuation of the electrostatic potential, thereby preventing scratching of the cured resin layer and improving solvent resistance, and improving the film-forming properties and transparency of the cured resin layer.

[0059] (Fluorine-containing compound (A)) The fluorine-containing compound is preferably a fluorine-containing resin from the viewpoint of increasing the strength of the cured resin layer. Specific examples of fluorine-containing resins include fluoroolefin-based copolymer resins containing vinyl fluoride, vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene, chlorotrifluoroethylene, etc. as monomers; fluoromethylene ether, difluoromethylene ether, fluoroethylene ether, difluoroethylene ether, tetrafluoroethylene ether, hexafluoropropylene ether, etc., fluorine-containing copolymer resins obtained by polymerizing polyalkylene ethers in which some or all of the hydrogen atoms have been substituted with fluorine atoms with other monomers; fluorine-containing copolymer resins obtained by graft polymerization of a hydroxyl group-containing fluororesin copolymer with a (meth)acrylic acid ester compound or other monomer; and vinyl polymers having perfluoroalkyl groups. Among these, fluoroolefin-based copolymer resins and fluororesin copolymers containing polyalkylene ether groups in which some or all of the hydrogen atoms have been fluorinated are preferred from the viewpoint of excellent improvement in electrostatic potential and excellent wiping properties of dust and oil.

[0060] Among the monomers of fluoroolefin copolymer resins, vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene are preferred. Furthermore, among polyalkylene ethers in which some or all of the hydrogen atoms are substituted with fluorine atoms, difluoromethylene ether, difluoroethylene ether, and tetrafluoroethylene ether are preferred. When the fluorine-containing resin is composed of a fluorine-containing monomer, it is preferably a mixed dispersion with other components from the viewpoints of dispersibility in solvents and compatibility with other resins. The other components will be described later.

[0061] Examples of fluorine-based copolymer resins obtained by polymerizing polyalkylene ethers in which some or all of the hydrogen atoms have been substituted with fluorine atoms with other monomers include urethane resins having a polyfluoroalkylene ether group, polyester resins having a polyfluoroalkylene ether group, and acrylic resins having a polyfluoroalkylene ether group. Other monomers constituting the urethane resin having a polyfluoroalkylene ether group include isocyanate compounds. Examples of the isocyanate compound include aromatic diisocyanates such as tolylene diisocyanate, xylylene diisocyanate, methylene diphenyl diisocyanate, phenylene diisocyanate, naphthalene diisocyanate, and tolidine diisocyanate; aliphatic diisocyanates having an aromatic ring such as α,α,α',α'-tetramethylxylylene diisocyanate; aliphatic diisocyanates such as methylene diisocyanate, propylene diisocyanate, lysine diisocyanate, trimethylhexamethylene diisocyanate, and hexamethylene diisocyanate; and alicyclic diisocyanates such as cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, dicyclohexylmethane diisocyanate, and isopropylidenedicyclohexyl diisocyanate.

[0062] Other monomers constituting the urethane resin include polyols not containing fluorine atoms, and polyols having a carboxy group such as dimethylolpropanoic acid, dimethylolbutanoic acid, bis-(2-hydroxyethyl)propionic acid, and bis-(2-hydroxyethyl)butanoic acid. Among these, it is preferable to include a polyol having a carboxy group, from the viewpoint of being able to self-emulsify when water is used as the dispersion solvent, and among these, dimethylolpropanoic acid is more preferable.

[0063] The cured resin layer is preferably formed from a cured resin layer-forming composition containing a fluorine-containing compound. The cured resin layer-forming composition (hereinafter referred to as "cured resin layer composition") may contain a fluorine-containing compound as a non-volatile component, or may contain other components. The content of the fluorine-containing compound in the cured resin layer composition is preferably 5 to 100% by mass, more preferably 20 to 98% by mass, and even more preferably 45 to 95% by mass, based on the non-volatile components in the cured resin layer composition. By making it 5% by mass or more, it is possible to improve the charge potential per thickness, further suppress decay of the charge potential, and easily remove dust and oil adhering to the polyester film surface.

[0064] (Crosslinking agent (B)) As described above, the cured resin layer composition for forming the cured resin layer preferably contains a crosslinking agent (B). By including a crosslinking agent in the cured resin layer composition, a dense cured resin layer with a high crosslink density can be formed. In addition, scratches on the cured resin layer can be prevented, and solvent resistance can be easily improved. There are no particular restrictions on the crosslinking agent, and conventionally known crosslinking agents can be used.

[0065] Examples of crosslinking agents include melamine compounds, oxazoline compounds, epoxy compounds, isocyanate compounds, carbodiimide compounds, and silane coupling compounds. Among these, the crosslinking agent is preferably at least one selected from melamine compounds, oxazoline compounds, and isocyanate compounds, and from the viewpoint of curability, a melamine compound is preferred. These crosslinking agents may be used alone or in combination of two or more. Any polymerizable monomer may be included in the cured resin layer composition as a component that cures together with these crosslinking agents.

[0066] (melamine compounds) The melamine compound used as the crosslinking agent refers to a compound having a melamine skeleton within the compound. Examples of such compounds include alkylolated melamine derivatives, compounds obtained by reacting alkylolated melamine derivatives with alcohol to partially or completely etherify them, and mixtures thereof. Examples of alkylolation include methylolation, ethylolation, isopropylolation, n-butylolation, and isobutyrolation. Among these, methylolation is preferred from the viewpoint of reactivity. Furthermore, alcohols used for etherification include methyl alcohol, ethyl alcohol, isopropyl alcohol, n-butanol, and isobutanol. From the viewpoint of improving the coating strength of the cured resin layer and improving the adhesion between the cured resin layer and the polyester film, a partially or completely etherified alkylolated melamine derivative is preferred, and an alkylol etherified with methyl alcohol is more preferred. The amount of the partially etherified alkylol group relative to the unetherified alkylol group is preferably 0.5 to 5 equivalents, more preferably 0.7 to 5 equivalents. The melamine compound may be a monomer, a dimer or higher polymer, or a mixture thereof. Furthermore, a melamine compound obtained by co-condensing melamine with urea or the like may also be used.

[0067] In order to increase the reactivity of the melamine compound, the cured resin layer composition may contain a crosslinking catalyst in addition to the melamine compound. Various known catalysts can be used as the crosslinking catalyst, including, for example, amine compounds, salts of amine compounds, organic acids such as aromatic sulfonic acid compounds (e.g., paratoluenesulfonic acid) and phosphoric acid compounds, and their salts, imine compounds, amidine compounds, guanidine compounds, organometallic compounds, and metal salts such as zinc stearate, zinc myristate, aluminum stearate, and calcium stearate. Among these, amine compounds, salts of amine compounds, and paratoluenesulfonic acid are preferred, and amine compounds and salts of amine compounds are more preferred.

[0068] (Oxazoline compounds) The oxazoline compound is a compound having an oxazoline group in the molecule, and a polymer containing an oxazoline group is particularly preferred. The polymer can be prepared by polymerizing an addition-polymerizable oxazoline group-containing monomer alone or with other monomers. Among these, an acrylic polymer that is a copolymer of an addition-polymerizable oxazoline group-containing monomer and an acrylic monomer that is a monomer having a (meth)acryloyl group is preferred, and the acrylic polymer may have a polyalkylene oxide chain. In this specification, when the expression "(meth)acryloyl group" is used, it means one or both of "acryloyl group" and "methacryloyl group", and the same applies to other similar terms. Examples of the addition-polymerizable oxazoline group-containing monomer include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, and 2-isopropenyl-5-ethyl-2-oxazoline. One or a mixture of two or more of these can be used. Among these, 2-isopropenyl-2-oxazoline is industrially readily available and is therefore preferred. The addition-polymerizable oxazoline group-containing monomer can be used alone, or two or more monomers can be used.

[0069] The other monomers are not limited as long as they are copolymerizable with the addition-polymerizable oxazoline group-containing monomer, and examples thereof include (meth)acrylic acid esters such as alkyl(meth)acrylates (the alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, 2-ethylhexyl, and cyclohexyl groups); unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, and styrenesulfonic acid and salts thereof (salts include sodium salts, potassium salts, ammonium salts, and tertiary amine salts); unsaturated nitriles such as acrylonitrile and methacrylonitrile; (meth)acrylic acid Examples of suitable monomers include unsaturated amides such as acrylamide, N-alkyl(meth)acrylamide, and N,N-dialkyl(meth)acrylamide (the alkyl group can be, for example, a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a t-butyl group, a 2-ethylhexyl group, or a cyclohexyl group); vinyl esters such as vinyl acetate and vinyl propionate; vinyl ethers such as methyl vinyl ether and ethyl vinyl ether; α-olefins such as ethylene and propylene; halogen-containing α,β-unsaturated monomers such as vinyl chloride, vinylidene chloride, and vinyl fluoride; and α,β-unsaturated aromatic monomers such as styrene and α-methylstyrene.

[0070] Furthermore, as the other monomer, a monomer having a polyalkylene oxide chain can also be used. Preferred examples of the monomer having a polyalkylene oxide chain include esters in which a polyalkylene oxide is added to the carboxy group of unsaturated carboxylic acids such as acrylic acid and methacrylic acid. Preferred examples of the polyalkylene oxide chain include polymethylene oxide, polyethylene oxide, polypropylene oxide, and polybutylene oxide. The number of repeating units of the polyalkylene oxide chain may be, for example, in the range of 3 to 100. The other monomers used in the oxazoline compound may be used alone or in combination of two or more.

[0071] The amount of oxazoline groups in the oxazoline compound is preferably in the range of 0.5 to 10 mmol / g, more preferably 1 to 9 mmol / g, even more preferably 3 to 8 mmol / g, and particularly preferably 4 to 6 mmol / g. By using it in the above range, the durability of the coating film (cured resin layer) tends to be improved.

[0072] (epoxy compounds) The epoxy compound is a compound having an epoxy group in the molecule, and examples thereof include condensates of epichlorohydrin with ethylene glycol, polyethylene glycol, glycerin, polyglycerin, bisphenol A, and other hydroxyl group- or amino group-containing compounds. Examples of the epoxy compound include polyepoxy compounds, diepoxy compounds, monoepoxy compounds, and glycidylamine compounds. Examples of polyepoxy compounds include sorbitol polyglycidyl ether, polyglycerol polyglycidyl ether, pentaerythritol polyglycidyl ether, diglycerol polyglycidyl ether, triglycidyl tris(2-hydroxyethyl)isocyanate, glycerol polyglycidyl ether, and trimethylolpropane polyglycidyl ether. Examples of diepoxy compounds include neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, resorcinol diglycidyl ether, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and polytetramethylene glycol diglycidyl ether. Examples of the monoepoxy compound include allyl glycidyl ether, 2-ethylhexyl glycidyl ether, and phenyl glycidyl ether. Examples of the glycidylamine compound include N,N,N',N'-tetraglycidyl-m-xylylenediamine, 1,3-bis(N,N-diglycidylamino)cyclohexane, etc. The epoxy compounds may be used alone or in combination of two or more.

[0073] (Isocyanate compounds) The isocyanate compound is a compound having an isocyanate or an isocyanate derivative structure, such as a blocked isocyanate. Examples of the isocyanate include aromatic isocyanates such as tolylene diisocyanate, xylylene diisocyanate, methylene diphenyl diisocyanate, phenylene diisocyanate, and naphthalene diisocyanate; aliphatic isocyanates having an aromatic ring such as α,α,α',α'-tetramethylxylylene diisocyanate; aliphatic isocyanates such as methylene diisocyanate, propylene diisocyanate, lysine diisocyanate, trimethylhexamethylene diisocyanate, and hexamethylene diisocyanate; and alicyclic isocyanates such as cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, methylenebis(4-cyclohexyl isocyanate), and isopropylidenedicyclohexyl diisocyanate. Further examples include polymers and derivatives of these isocyanates, such as biuretized products, isocyanurated products, and urethodionated products. These isocyanates may be used alone or in combination. Among the above isocyanates, aliphatic isocyanates or alicyclic isocyanates are more preferred in order to prevent yellowing due to ultraviolet rays.

[0074] When used in the form of a blocked isocyanate, examples of the blocking agent include bisulfites such as sodium bisulfite; phenolic compounds such as phenol, cresol, and ethylphenol; alcoholic compounds such as propylene glycol monomethyl ether, ethylene glycol, benzyl alcohol, methanol, and ethanol; active methylene compounds such as dimethyl malonate, diethyl malonate, methyl acetoacetate, ethyl acetoacetate, and acetylacetone; mercaptan compounds such as butyl mercaptan and dodecyl mercaptan; lactam compounds such as ε-caprolactam and δ-valerolactam; amine compounds such as diphenylaniline, aniline, and ethyleneimine; acid amide compounds such as acetanilide and acetic acid amide; and oxime compounds such as formaldehyde, acetaldoxime, acetone oxime, methyl ethyl ketone oxime, and cyclohexanone oxime. These may be used alone or in combination of two or more.

[0075] The isocyanate compound may be used alone or in combination with various polymers. Furthermore, the isocyanate compound may be blended into the cured resin layer composition as a mixture with various polymers. From the viewpoint of improving the dispersibility and crosslinking ability of the isocyanate compound, it is preferable to use a mixture or combination with a polyester resin or a urethane resin. The isocyanate compound may be used alone or in combination with two or more types. The amount of crosslinking agent when an isocyanate compound is used includes the amount of the blocking agent, the polymer to be blended in combination, or the like.

[0076] (Carbodiimide compounds) A carbodiimide compound is a compound having a carbodiimide structure. The use of a carbodiimide compound can improve the moist heat resistance of the cured resin layer. Carbodiimide compounds can be synthesized using conventionally known techniques, and generally involve the condensation reaction of a diisocyanate compound. The diisocyanate compound is not particularly limited, and both aromatic and aliphatic diisocyanates can be used. Specific examples include tolylene diisocyanate, xylene diisocyanate, diphenylmethane diisocyanate, phenylene diisocyanate, naphthalene diisocyanate, hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, dicyclohexyl diisocyanate, and dicyclohexylmethane diisocyanate. The carbodiimide compounds may be used alone or in combination of two or more.

[0077] (Silane coupling compounds) Silane coupling compounds are organosilicon compounds that contain an organic functional group and a hydrolyzable group such as an alkoxy group in one molecule. For example, epoxy group-containing compounds such as 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; vinyl group-containing compounds such as vinyltrimethoxysilane and vinyltriethoxysilane; styryl group-containing compounds such as p-styryltrimethoxysilane and p-styryltriethoxysilane; (meth)acrylic group-containing compounds such as 3-(meth)acryloxypropyltrimethoxysilane, 3-(meth)acryloxypropyltriethoxysilane, 3-(meth)acryloxypropylmethyldimethoxysilane, and 3-(meth)acryloxypropylmethyldiethoxysilane; 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, and N-2-(aminoethyl)- Examples of suitable compounds include amino group-containing compounds such as 3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldiethoxysilane, 3-triethoxysilyl-N-(1,3-dimethylbutylidene)propylamine, N-phenyl-3-aminopropyltrimethoxysilane, and N-phenyl-3-aminopropyltriethoxysilane; isocyanurate group-containing compounds such as tris(trimethoxysilylpropyl)isocyanurate and tris(triethoxysilylpropyl)isocyanurate; and mercapto group-containing compounds such as 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 3-mercaptopropylmethyldimethoxysilane, and 3-mercaptopropylmethyldiethoxysilane. Among the above compounds, from the viewpoint of maintaining the strength of the cured resin layer, epoxy group-containing silane coupling agents, silane coupling agents containing a double bond such as a vinyl group or a (meth)acrylic group, and amino group-containing silane coupling agents are more preferred. The silane coupling agent may be used alone or in combination of two or more kinds.

[0078] The crosslinking agent contained in the cured resin layer composition may be designed to react during the drying process when forming the cured resin layer or during the film-forming process to improve the performance of the cured resin layer. It can be assumed that the cured resin layer formed from the cured resin layer composition contains unreacted crosslinking agents, reacted compounds, or mixtures thereof.

[0079] The content of the crosslinking agent in the cured resin layer composition is preferably in the range of 5 to 60% by mass relative to the non-volatile components in the cured resin layer composition. By setting the content of the crosslinking agent relative to the non-volatile components in the cured resin layer composition to 5 to 60% by mass, it becomes easier to suppress the decay of the electrostatic potential. It also improves the strength of the cured resin layer and makes it easier to improve scratch resistance. From the above viewpoints, the content is more preferably 10 to 50% by mass, even more preferably 15 to 40% by mass, and particularly preferably 20 to 40% by mass.

[0080] Furthermore, when a crosslinking catalyst is contained in the cured resin layer composition, the content of the crosslinking catalyst is preferably in the range of 0.4 to 10 mass % relative to the non-volatile components in the cured resin layer composition, as this tends to improve the strength of the cured resin layer and improve scratch resistance, etc. From the above viewpoints, the content of the crosslinking catalyst is preferably 0.6 to 8 mass %, more preferably 0.8 to 5 mass %.

[0081] (Binder resin (C)) The binder resin is a polymer component contained in the cured resin layer composition other than the polymer formed by crosslinking with the crosslinking agent (B). When the cured resin layer composition contains a binder resin, the film-forming properties and transparency of the cured resin layer are improved.

[0082] Specific examples of binder resins include acrylic resins, polyvinyl alcohol, polyester resins, urethane resins, polyalkylene glycols, polyalkyleneimines, methyl cellulose, hydroxycellulose, starches, etc. Among these, from the viewpoint of coatability and the like, acrylic resins, polyester resins, and urethane resins are preferred, from the viewpoint of improving the durability of the cured resin layer itself, acrylic resins and polyester resins are more preferred, and from the viewpoint of further improving coatability and the like, acrylic resins are even more preferred.

[0083] (acrylic resin) The acrylic resin is a polymer made of polymerizable monomers including acrylic and methacrylic monomers. These may be homopolymers or copolymers, or copolymers with polymerizable monomers other than acrylic and methacrylic monomers. Also included are copolymers of these polymers with other polymers (such as polyesters and polyurethanes), such as block copolymers and graft copolymers. That is, the acrylic resin may be an acrylic-modified polyester resin or an acrylic-modified polyurethane resin. Further included are polymers (and in some cases polymer mixtures) obtained by polymerizing polymerizable monomers in a polyester solution or polyester dispersion. Similarly, they also include polymers (and in some cases polymer mixtures) obtained by polymerizing polymerizable monomers in a polyurethane solution or polyurethane dispersion. Similarly, they also include polymers (and in some cases polymer mixtures) obtained by polymerizing polymerizable monomers in other polymer solutions or dispersions, and these are also referred to as acrylic-modified polyester resins and acrylic-modified polyurethane resins in this specification. The polyesters and polyurethanes used in the acrylic resins can be appropriately selected from the polyesters and polyurethanes exemplified below as being used in binder resins. The acrylic resin may contain a hydroxyl group or an amino group in order to further improve adhesion to the polyester film.

[0084] The polymerizable monomer is not particularly limited, but particularly representative compounds include, for example, various carboxy group-containing monomers such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid, and citraconic acid, and salts thereof; various hydroxy group-containing monomers such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, monobutylhydroxyfumarate, and monobutylhydroxyitaconate; methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, lauryl (meth)acrylate, and the like. Examples include various (meth)acrylic acid esters such as acrylates; various nitrogen-containing compounds such as (meth)acrylamide, diacetone acrylamide, N-methylolacrylamide, or (meth)acrylonitrile; various styrene derivatives such as styrene, α-methylstyrene, divinylbenzene, and vinyltoluene; various vinyl esters such as vinyl propionate; various silicon-containing polymerizable monomers such as γ-methacryloxypropyltrimethoxysilane and vinyltrimethoxysilane; phosphorus-containing vinyl monomers; various vinyl halides such as vinyl chloride and vinylidene chloride; and various conjugated dienes such as butadiene.

[0085] (polyester resin) Examples of polyester resins include those composed of the following polycarboxylic acids and polyhydroxy compounds as main constituents: Polycarboxylic acids include terephthalic acid, isophthalic acid, orthophthalic acid, phthalic acid, 4,4'-diphenyldicarboxylic acid, 2,5-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 2-potassium sulfoterephthalic acid, 5-sodium sulfoisophthalic acid, adipic acid, azelaic acid, sebacic acid, dodecanedicarboxylic acid, glutaric acid, succinic acid, trimellitic acid, trimesic acid, pyromellitic acid, trimellitic anhydride, phthalic anhydride, p-hydroxybenzoic acid, monopotassium trimellitic acid, and ester-forming derivatives thereof. Examples of polyhydroxy compounds that can be used include ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 2-methyl-1,5-pentanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, p-xylylene glycol, bisphenol A-ethylene glycol adduct, diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, polytetramethylene oxide glycol, dimethylolpropionic acid, glycerin, trimethylolpropane, sodium dimethylolethylsulfonate, and potassium dimethylolpropionate. One or more of these compounds can be selected as appropriate and used to synthesize a polyester resin via a conventional polycondensation reaction. The polyester resin can also be prepared as a water dispersion, in which case hydrophilic functional groups can be introduced into the polyester resin.

[0086] (Polyvinyl alcohol) Polyvinyl alcohol is a compound having a polyvinyl alcohol moiety. For example, conventionally known polyvinyl alcohols can be used, including modified compounds in which polyvinyl alcohol has been partially acetalized or butyralized. The degree of polymerization of polyvinyl alcohol is not particularly limited, but is usually 100 or higher, preferably in the range of 300 to 40,000. A degree of polymerization of 100 or higher can prevent a decrease in the water resistance of the cured resin layer. The degree of saponification of polyvinyl alcohol is not particularly limited, but saponified polyvinyl acetates having a degree of saponification of 70 mol % or higher, preferably 70 to 99.9 mol %, more preferably 80 to 97 mol %, and particularly preferably 86 to 95 mol %, are practically used.

[0087] (urethane resin) A urethane resin is a polymeric compound having a urethane bond in the molecule. Urethane resins are usually produced by the reaction of a polyol with an isocyanate. Examples of polyols include polycarbonate polyols, polyester polyols, polyether polyols, polyolefin polyols, and acrylic polyols. These compounds may be used alone or in combination. The urethane resin may be a water dispersion. In this case, for example, a hydrophilic functional group may be appropriately introduced into the polyol.

[0088] The content of the binder resin in the cured resin layer composition is preferably in the range of 10 to 70% by mass relative to the non-volatile components in the cured resin layer composition. By setting the content of the binder resin to 10 to 70% by mass, it becomes easier to suppress the attenuation of the coating property and the electrostatic potential. It also becomes easier to improve the appearance and transparency of the cured resin layer. From the above viewpoints, the content is more preferably 20 to 65% by mass, even more preferably 30 to 60% by mass, and particularly preferably 30 to 55% by mass. In addition to the components described above, the cured resin layer composition may contain additives such as a reaction adjuster, an adhesion promoter, a surfactant, an antistatic agent, and particles, as appropriate.

[0089] The cured resin layer composition may be applied to a polyester film as a liquid coating solution, and then dried and cured as necessary. The cured resin layer composition is preferably diluted with a solvent to form a coating solution. The components (e.g., components (A) to (C)) constituting the cured resin layer composition may be dissolved in a solvent or dispersed in the solvent. The solvent used in the cured resin layer composition is not limited, and either water or an organic solvent may be used. However, from the viewpoint of environmental protection, it is preferable to use an aqueous coating liquid using water as the solvent. The aqueous coating liquid may contain a small amount of organic solvent. The specific amount of organic solvent should be less than that of water on a mass basis, for example, less than 30 mass %, preferably less than 20 mass %, more preferably less than 10 mass % of the solvent. Examples of organic solvents used in combination with water include alcohols such as ethanol, isopropanol, ethylene glycol, and glycerin; ethers such as ethyl cellosolve, t-butyl cellosolve, propylene glycol monomethyl ether, and tetrahydrofuran; ketones such as acetone and methyl ethyl ketone; esters such as ethyl acetate; and amines such as dimethylethanolamine. These can be used alone or in combination. By appropriately selecting and adding these organic solvents to the aqueous coating solution as needed, the stability and coatability of the coating solution can sometimes be improved.

[0090] When an organic solvent is used alone as the solvent, examples of the organic solvent include aromatic hydrocarbons such as toluene; aliphatic hydrocarbons such as hexane, heptane, and isooctane; esters such as ethyl acetate and butyl acetate; ketones such as ethyl methyl ketone (MEK) and isobutyl methyl ketone; alcohols such as ethanol and 2-propanol; and ethers such as diisopropyl ether and dibutyl ether. These may be used alone or in combination, taking into consideration solubility, coatability, boiling point, and the like.

[0091] <Method for forming a cured resin layer> The method for forming the cured resin layer will be described in detail below. The cured resin layer may be formed by in-line coating or off-line coating. In-line coating is a method in which a coating liquid of a cured resin layer composition is applied to the surface of a polyester film on the production line in which the polyester film is produced. Off-line coating is a method in which a coating liquid is applied to a polyester film that has already been produced outside the system (outside the production line). From the viewpoint of ease of processing, the cured resin layer is preferably formed by in-line coating.

[0092] Specifically, in-line coating is a method of applying a coating liquid of a cured resin layer composition to a polyester film at any stage from melt-extrusion of the polyester to stretching, heat-setting, and winding up. Usually, the coating liquid is applied to the polyester film in any of the following stages: an unstretched sheet obtained by melting and quenching, a stretched uniaxially stretched film, a biaxially stretched film before heat-setting, and a film after heat-setting and before winding up.

[0093] Examples of the method for applying the coating liquid include air doctor coating, blade coating, rod coating, bar coating, knife coating, squeeze coating, impregnation coating, reverse roll coating, transfer roll coating, gravure coating, kiss roll coating, cast coating, spray coating, curtain coating, calendar coating, and extrusion coating.

[0094] Furthermore, although not particularly limited, for example, in sequential biaxial stretching, a method in which a coating solution is applied to a uniaxially stretched film that has been stretched in the longitudinal direction (machine direction) and then stretched in the transverse direction is particularly preferred. This method has the advantage of reducing production costs because it allows the polyester film and the cured resin layer to be formed simultaneously. Furthermore, since stretching is performed after coating, the thickness of the cured resin layer can be changed by adjusting the stretch ratio, making thin-film coating easier than offline coating. Furthermore, the thickness of the cured resin layer can be made more uniform.

[0095] Furthermore, by applying a coating liquid of the cured resin layer composition onto the polyester film before stretching, the cured resin can be stretched together with the polyester film, thereby allowing the cured resin layer to adhere firmly to the polyester film. Furthermore, in the production of biaxially stretched films, stretching while gripping the film edges with clips or the like allows the polyester film to be restrained in both the longitudinal and transverse directions, and in the heat setting step, the film can be heated to a high temperature while maintaining its flatness and without wrinkles, etc. Therefore, the heat treatment performed after application of the cured resin layer composition can be performed at a high temperature that cannot be achieved by other methods, allowing for stronger adhesion between the cured resin layer and the polyester film.

[0096] In addition, the coating solution of the cured resin layer composition applied to the polyester film may be subjected to either heat treatment or irradiation with active energy rays such as ultraviolet light, or both, whether by offline coating or inline coating, but at least heat treatment is preferably performed. The cured resin layer composition may be cured by one or both of heat treatment and irradiation with active energy rays. The heat treatment may be performed by heating, for example, in a heat setting step as described above, but may also be performed by other methods. When the coating solution of the cured resin layer composition contains a solvent, it is dried as appropriate, but it is preferably dried by the heat treatment described above. Furthermore, in order to improve the applicability of the coating liquid for forming the cured resin layer to the polyester film and the adhesion of the cured resin layer to the polyester film, the surface of the polyester film on which the cured resin layer is to be formed may be subjected to a surface treatment such as chemical treatment, corona discharge treatment, plasma treatment, ozone treatment, chemical treatment, or solvent treatment before application of the coating liquid.

[0097] As described above, the polyester film with a cured resin layer may have another layer between the polyester film and the cured resin layer, such as an antistatic layer, an easy-adhesion layer, or an oligomer sealing layer, which has various functions.

[0098] The thickness of the cured resin layer is preferably 0.005 to 1 μm. By making the thickness 1 μm or less, the components constituting the cured resin layer are prevented from migrating to the cured resin layer, and further, blocking during winding is more easily prevented. On the other hand, by making the thickness 0.005 μm or more, it is easier to suppress attenuation of the electrostatic potential. From these viewpoints, the thickness of the cured resin layer is more preferably 0.01 μm or more, even more preferably 0.02 μm or more, and is more preferably 0.2 μm or less, even more preferably 0.1 μm or less, and particularly preferably 0.06 μm or less. [Example]

[0099] The present invention will now be described in more detail with reference to examples, although the present invention is not limited to the examples described below.

[0100] <Evaluation method> The methods for measuring and evaluating various physical properties and characteristics are as follows.

[0101] (1) Intrinsic viscosity (IV) 1 g of polyester was precisely weighed, dissolved in 100 ml of a mixed solvent of phenol / tetrachloroethane = 50 / 50 (weight ratio), and measured at 30°C.

[0102] (2) Melting point of polypropylene resin (Y) Polypropylene resin (Y) was measured by differential scanning calorimetry (DSC) by heating from room temperature to 200°C, erasing the thermal history, then lowering the temperature to 40°C at a rate of 10°C / min, scanning again at a rate of 10°C / min, and the temperature at the top of the endothermic peak when the temperature was raised again was taken as the melting point.

[0103] (3) Dielectric breakdown strength Using a Yamayo Testing Instruments Co., Ltd. AC 50kV / DC 10kV dielectric breakdown tester YST-243AT-B model, an AC voltage was applied to the polyester film in air at a rate of 100V / sec using equal-diameter electrodes (25mm x 25mm cylinders). The dielectric breakdown voltage (kV) was measured by dividing the dielectric breakdown voltage by the film thickness (μm).

[0104] The raw materials of the polyester films in the examples and comparative examples are as follows. (polyester film) (a) Polyester resin (X1): Polyethylene terephthalate homopolymer (polycondensation catalyst: titanium) with an intrinsic viscosity of 0.64. (b) Polyester resin (X2): Polyethylene terephthalate homopolymer (polycondensation catalyst: titanium) containing 0.5% by mass of resin particles with an average particle size of 0.3 μm and having an intrinsic viscosity of 0.61. (c) Polypropylene resin (Y1): melting point 167°C, MFR = 0.4 (Japan Polypropylene Corporation: EA9HD) (d) Compatibilizer having an acid anhydride structure (Z1): maleic anhydride-modified polypropylene resin (Admer QE800, melt flow rate: 9.1 g / 10 min, acid value: 4.3, manufactured by Mitsui Chemicals, Inc.)

[0105] [Example 1] 87.5% by mass of polyester resin X1, 10% by mass of polypropylene resin Y1, and 2.5% by mass of compatibilizer Z1 were mixed and melt-extruded into a film at 294°C using a vented twin-screw extruder. The extruded film was then rapidly cooled and solidified on a rotating cooling drum at 25°C using an electrostatic adhesion method to obtain an amorphous film. This film was stretched 3.8 times in the longitudinal direction while passing through a group of heated rolls at 86°C to obtain a uniaxially oriented film. This film was then introduced into a tenter stretching machine, stretched 4.2 times in the width direction at 97°C, and further heat-treated at 218°C, followed by a 3.1% relaxation treatment in the width direction to obtain a polyester film with a thickness of 2.5 μm.

[0106] [Comparative Example 1] A polyester film having a thickness of 2.5 μm was produced in the same manner as in Example 1, except that the raw material composition was changed to the conditions shown in Table 1 (a resin containing particles was added).

[0107] [Table 1]

[0108] In Example 1, since the film does not contain particles, it was found that the dielectric breakdown strength was good. On the other hand, in Comparative Example 1, the dielectric breakdown strength was poor due to the inclusion of particles in the film. In Comparative Example 1, when focusing on the formation of irregularities on the surface of the obtained film, it is presumed that because the Sv value was large, the depressions were significantly hollowed out when the irregularities on the film surface were formed, resulting in many locations where the resin layer was thin in the film thickness direction. [Industrial Applicability]

[0109] The polyester film of the present invention is a polyester film that has good heat resistance while maintaining good electrical properties and can be thinned, and is therefore extremely useful for capacitors and has great industrial value.

Claims

1. A polyester film for a film capacitor, comprising a polyester resin (X) and a polypropylene resin (Y), and having a particle content of 0.001% by mass or less.

2. The polyester film for a film capacitor according to claim 1, wherein at least one surface of the polyester film satisfies the following formula (1): Sv / Sa<34...(1) (In the above formula (1), Sv represents the maximum valley depth, and Sa represents the arithmetic mean height.)

3. 2. The polyester film for a film capacitor according to claim 1, wherein a maximum valley depth (Sv) of at least one surface of the polyester film is 5000 nm or less.

4. 2. The polyester film for a film capacitor according to claim 1, wherein the arithmetic mean height (Sa) of at least one surface of the polyester film is 200 nm or less.

5. The polyester film for film capacitors according to claim 1, wherein the polyester resin (X) is at least one selected from polyethylene terephthalate and polyethylene-2,6-naphthalate.

6. 2. The polyester film for a film capacitor according to claim 1, wherein the polycondensation catalyst for the polyester resin (X) is a Ti-based catalyst.

7. 2. The polyester film for a film capacitor according to claim 1, wherein the intrinsic viscosity of the polyester resin (X) is 0.80 dL / g or less.

8. The polyester film for a film capacitor according to claim 1, wherein the polypropylene resin (Y) is 1 to 30 parts by mass relative to 100 parts by mass of the polyester resin (X).

9. The polyester film for film capacitors according to claim 1, having a thickness of 0.5 to 12.0 μm.

10. 2. The polyester film for a film capacitor according to claim 1, which has a dielectric breakdown strength of 110 kV / μm or more.

11. The polyester film for a film capacitor according to claim 1 , further comprising a compatibilizer (Z).

12. The polyester film for film capacitors according to claim 11, wherein the compatibilizer (Z) is a resin or ionomer having an acid anhydride structure [-C(=O)-O-C(=O)-].

13. The polyester film for film capacitors according to claim 11, wherein the compatibilizer (Z) is contained in an amount of 0.01 to 40 parts by mass relative to 100 parts by mass of the polyester resin (X).

14. A metal laminated film for a film capacitor, comprising the polyester film according to any one of claims 1 to 13, and a metal layer provided on at least one surface of the polyester film.

15. A method for producing a polyester film for a film capacitor, comprising obtaining an amorphous film from a resin composition containing a polyester resin (X), a polypropylene resin (Y), and a compatibilizer (Z), and satisfying the following (2) and (3), and then stretching the film in the longitudinal and transverse directions, wherein the particle content in the resin composition is 0.001 mass% or less. (2) The polypropylene resin (Y) has an MFR (melt flow rate) of 7.0 g / 10 min or less. (3) The compatibilizer (Z) is a resin or ionomer having an acid anhydride structure [—C(═O)—O—C(═O)—].

16. The method for producing a polyester film for a film capacitor according to claim 15, wherein the resin composition contains 0.01 to 40 parts by mass of the compatibilizer (Z) relative to 100 parts by mass of the polyester resin (X).

17. The method for producing a polyester film for a film capacitor according to claim 15, wherein the resin composition contains 1 to 30 parts by mass of the polypropylene resin (Y) relative to 100 parts by mass of the polyester resin (X).

18. The method for producing a polyester film for a film capacitor according to claim 15, wherein the polyester resin (X) has an intrinsic viscosity of 0.80 dL / g or less.

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