Polyester film

Abstract

To provide a novel polyester film for a capacitor, wherein the polyester film retains current electric characteristics while having excellent thermostability and allowing for film thinning.SOLUTION: A polyester film contains a polyester resin (X) and a polypropylene resin (Y). For a relaxation time T of the polypropylene resin (Y) determined by pulse NMR, the relationship between a relaxation time (T2) (μs) after one-minute heating at 150°C and a relaxation time (T1) (μs) before the heating satisfies the following formula (1): (T2) / (T1)≥1.20 (1)

Description

Technical Field

[0001] The present invention relates to a polyester film, and particularly to a polyester film suitable for use as a capacitor.

Background Art

[0002] In various fields such as industrial materials, optical materials, electronic component materials, and battery packaging materials, a typical polyester film such as polyethylene terephthalate (PET) film, particularly a biaxially oriented PET film, is widely used because of its excellent transparency, mechanical strength, heat resistance, flexibility, and the like.

[0003] Also, due to excellent electrical properties such as low dielectric loss characteristics 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, and inverters. In recent years, there has been a need for further miniaturization and high capacitance of capacitors. For example, when using a resin film for a capacitor in an inverter power supply device that controls a drive motor of an electric vehicle or a hybrid vehicle, small size, light weight, and high capacitance are required (Patent Document 1). With the increase in the capacitance of capacitors, for example, in a temperature range exceeding 120°C, there is a tendency to require high withstand voltage characteristics (stability of capacitance) over a long period of time.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, for example, in the case of a general-purpose biaxially stretched polypropylene film, although the electrical properties are good, when the use temperature range is in the temperature region of 120 °C or higher, the heat resistance of the film itself reaches the limit level, making it difficult to cope with the situation. Therefore, the present invention has been made in view of the above problems, and as a polyester film for a capacitor, it newly proposes a polyester film for a capacitor that maintains conventional electrical properties, has good heat resistance, and can be made thinner.

Means for Solving the Problems

[0006] As a result of intensive studies, the present inventors have found that by using a polyester film containing a polyester resin (X) and a polypropylene resin (Y) and satisfying a specific range for the relationship between the relaxation time (T2) (μs) of the polypropylene resin (Y) component obtained by the pulse NMR method after heat treatment at 150 °C for 1 minute and the relaxation time (T1) (μs) before heat treatment, the above problems can be solved, and the following present invention has been completed. That is, the present invention provides the following [1] to

[14] . [1] A polyester film containing a polyester resin (X) and a polypropylene resin (Y), and for the relaxation time T of the polypropylene resin (Y) component obtained by the pulse NMR method, the relationship between the relaxation time (T2) (μs) after heat treatment at 150 °C for 1 minute and the relaxation time (T1) (μs) before heat treatment satisfies the following formula (1). (T2) / (T1)≧1.20···(1) [2] The polyester film according to [1] above, containing 1 to 30 parts by mass of the polypropylene resin (Y) with respect to 100 parts by mass of the polyester resin (X). [3] The polyester film according to [1] or [2] above, containing 0.01 to 40 parts by mass of a compatibilizer (Z) with respect to 100 parts by mass of the polyester resin (X). [4] The polyester film according to [3] above, wherein the compatibilizer (Z) is a resin or an ionomer having an acid anhydride structure [-C(=O)-O-C(=O)-]. [5] The polyester film according to any one of [1] to [4] above, wherein the polycondensation catalyst of the polyester resin (X) is a Ti-based or Sb-based catalyst. [6] The polyester film according to any one of [1] to [5] above, wherein the polyester resin (X) is at least one selected from polyethylene terephthalate and polyethylene-2,6-naphthalate. [7] The polyester film according to any one of [1] to [6] above, wherein a cured resin layer is provided on at least one surface of the polyester film. [8] The polyester film according to any one of [1] to [7] above, wherein the film thickness is 0.5 to 12.0 μm. [9] The polyester film according to any one of [1] to [8] above, wherein the dielectric tangent (tanδ) at 1 kHz is 0.55 or less.

[10] A metal laminated film in which a metal layer is provided on at least one side of the polyester film according to any one of [1] to [9] above.

[11] The polyester film according to any one of [1] to [9] above, which is for a capacitor.

[12] The metal laminated film according to

[10] above, which is for a capacitor.

[13] The polyester film according to

[11] above, which is for a capacitor mounted on an automobile.

[14] The metal laminated film according to

[12] above, which is for a capacitor mounted on an automobile. [Effect of the Invention]

[0007] In the present invention, as a polyester film for a capacitor, a polyester film is proposed which maintains conventional electrical properties and yet has good heat resistance. [Embodiments for Carrying Out the Invention]

[0008] Next, an example of an embodiment of the present invention will be described. However, the present invention is not limited to the embodiments described below.

[0009] [Polyester Film] The polyester film of the present invention (hereinafter sometimes referred to as "this polyester film") contains a polyester resin (X) and a polypropylene resin (Y). This polyester film is excellent in physical properties such as heat resistance, flatness, optical properties, and strength. The above polyester film may be a single layer or a multilayer film (i.e., a laminated film) having two or more layers with different properties. In the case of a multilayer film, any one of the layers may have a polypropylene resin (Y), but it is preferable that all layers have a polypropylene resin (Y). For example, in the case of a three-layer multilayer film of surface layer / middle layer / surface layer, any layer may contain a polypropylene resin (Y), but it is preferable that the middle layer contains a polypropylene resin (Y), and it is more preferable that all layers contain a polypropylene resin (Y). Therefore, in the case of a multilayer, any one of the layers may contain a polypropylene resin (Y) described later, and it is preferable that the layer containing the polypropylene resin (Y) appropriately contains a compatibilizer (Z) described later. Further, the polyester film may be an unstretched film (sheet) or a stretched film. Among them, a stretched film stretched in a uniaxial direction or a biaxial direction is preferable. Among them, from the viewpoint of the balance of mechanical properties and flatness, a biaxially stretched film is more preferable. Therefore, a biaxially stretched polyester film is even more preferable.

[0010] (Polyester Resin (X)) The polyester resin (X), which is the main component resin of this polyester film, may be a homopolyester or a copolyester. The main component resin means the resin having the largest mass ratio among the resins constituting the polyester film, and it may occupy 50% by mass or more, or 75% by mass or more, or 90% by mass or more, or 100% by mass of the resins constituting the polyester film.

[0011] As the above-mentioned homopolyester, those obtained by polycondensing an aromatic dicarboxylic acid and an aliphatic glycol are preferred. 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, 1,4-butanediol, 1,4-cyclohexanedimethanol, etc. Examples of typical homopolyesters include polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene-2,6-naphthalate (PEN), etc. In the present invention, polyethylene terephthalate (PET) and polyethylene-2,6-naphthalate (PEN) are particularly preferred, and they can also be used in combination.

[0012] On the other hand, when the above polyester is a copolyester, it is preferably a copolymer containing 30 mol% or less of a third component. Examples of the dicarboxylic acid component of the copolyester 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. Among them, as the present polyester film, polyethylene terephthalate in which 60 mol% or more, preferably 80 mol% or more is ethylene terephthalate units or polyethylene-2,6-naphthalate in which 60 mol% or more, preferably 80 mol% or more is ethylene-2,6-naphthalate units is preferred.

[0013] (Polyester polycondensation catalyst) Examples of the polycondensation catalyst used to obtain the above polyester by polycondensation include antimony compounds, germanium compounds, aluminum compounds, titanium compounds, and the like. Among these, at least one of an antimony compound and a titanium compound is preferable, and in particular, it is preferable to use a polyester obtained using a titanium compound. Therefore, the polyester film preferably contains at least one of an antimony compound (Sb-based) and a titanium compound, and more preferably contains a titanium compound (Ti-based). By using the titanium compound, the number of metal-containing aggregates, so-called coarse foreign matters, derived from the titanium compound in the film can be reduced.

[0014] The polyester constituting the outermost layer of this film (also referred to as the "surface layer", for example, the surface layer on which the cured resin layer described later is laminated) preferably uses a titanium compound as its polycondensation catalyst. For example, it is preferable that the surface layer contains a titanium compound. Note that the outermost layer is the outermost layer among a plurality of layers in the case of a laminated film, and is the surface layer of the layer in the case of a single layer. The content of titanium element derived from the titanium compound in the outermost layer is preferably 3 mass ppm or more and 40 mass ppm or less, and more preferably 4 mass ppm or more and 35 mass ppm or less. Within the above range, foreign matters caused by the catalyst can be reduced without reducing the production efficiency of the polyester. From the same viewpoint, the content of the antimony compound in the outermost layer of this film is preferably 100 mass ppm or less.

[0015] (Particles) Particles can be blended into the polyester resin (X) mainly for the purpose of imparting easy slippage and preventing scratches in each process. When particles are blended, the type of particles to be blended is not particularly limited as long as they are particles capable of imparting easy slippage, and specific examples include inorganic particles such as silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, aluminum oxide, and titanium oxide, and organic particles such as acrylic resin, styrene resin, urea resin, phenolic resin, epoxy resin, and benzoguanamine resin. Furthermore, precipitated particles obtained by precipitating and finely dispersing a part of a metal compound such as a catalyst during the polyester production process can also be used.

[0016] On the other hand, the shape of the particles to be used is not particularly limited, and any of spherical, blocky, rod-like, flat, etc. may be used. In addition, there is no particular limit to the hardness, specific gravity, color, etc. Two or more kinds of these particles may be used in combination as necessary. The average particle size of the particles used is preferably 3 μm or less, more preferably 0.1 to 2 μm, and particularly preferably in the range of 0.1 to 1 μm. By using particles having an average particle size in the above range, a suitable surface roughness can be imparted to the polyester film, ensuring good slipping properties and smoothness. When particles are blended, for example, a surface layer and an intermediate layer may be provided, and the particles may be contained in the surface layer. In this case, a multi-layer structure may be formed having a surface layer containing particles, an intermediate layer, and a surface layer containing particles in this order.

[0017] In addition, an embodiment in which the polyester resin (X) does not substantially contain particles is also preferred. Here, "substantially does not contain particles" means that particles are not intentionally contained, and specifically means that the particle content (particle concentration) is 200 ppm by mass or less, more preferably 150 ppm by mass or less. Since the polyester film of the present invention contains a polypropylene resin (Y) that is incompatible with the polyester resin (X) constituting the polyester, the polypropylene resin (Y) can form fine irregularities on the film surface. Therefore, by setting the amount of particles in the film within the above range, it becomes easier to impart slipperiness to the film while ensuring the transparency of the film. When the polyester film does not contain particles or the content is small, a polyester film with high transparency of the base film and good appearance can be obtained, but the slipperiness may be insufficient. In such a case, it is advisable to improve the slipperiness by blending particles in the cured resin layer described later.

[0018] (Polypropylene resin (Y)) The polypropylene resin (Y) may be a homopolymer of propylene or a copolymer of propylene and ethylene and / or an α-olefin having 4 to 20 carbon atoms. In the present invention, the copolymerization component may be one type, or two or more types may be used in combination as necessary.

[0019] The production method of the polypropylene resin (Y) is not particularly limited, and for example, a conventionally known method such as a method of polymerizing propylene and an optional comonomer with a Ziegler-Natta catalyst can be adopted. It is also described in Section 2.3.1 (pages 20 to 57) of "Polypropylene Handbook" edited by Edward P. Moore Jr., translated and supervised by Tetsuo Yasuda and Noboru Sakuma, Industrial Research Institute (1998).

[0020] (MFR (Melt Flow Rate)) In the present invention, the MFR is preferably in the range of 0.1 to 10 g / 10 min, more preferably 0.5 to 8 g / 10 min, and even more preferably 1 to 8 g / 10 min. By satisfying the above range, good fluidity can be ensured during film forming processing. The MFR is measured in accordance with ISO 1133:1997.

[0021] (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 satisfies the above range, the desired heat resistance can be ensured. The upper limit of the melting point is not particularly limited, but is usually 170°C. The melting point in the present invention can be determined by differential scanning calorimetry (DSC). Specifically, it is defined as the temperature of the endothermic peak top when the temperature is once raised from room temperature to 200°C to erase the thermal history, then lowered to 40°C at a temperature-lowering rate of 10°C / min, and measured again at a temperature-raising rate of 10°C / min.

[0022] (Blending amount) The blending amount of the polypropylene resin (Y) with respect to 100 parts by mass of the polyester resin (X) is preferably 1 to 30 parts by mass. More preferably, it is 5 to 20 parts by mass, and particularly preferably 5 to 15 parts by mass. By satisfying the above range, the polyester film for capacitors can have good electrical properties. When the present polyester film is a multilayer film, the above blending amount is the blending amount in the entire polyester film. Therefore, for example, even in a multilayer polyester film having layers with different blending amounts of polypropylene resin, it refers to the blending amount of polypropylene resin in the entire laminated film.

[0023] (Compatibilizer (Z)) The polyester film of the present invention preferably contains a compatibilizer (Z) for compatibilizing the polyester resin (X) and the polypropylene resin (Y). The compatibilizer (Z) is preferably a resin having an acid anhydride structure or an ionomer.

[0024] (Resin having an acid anhydride structure [-C(=O)-O-C(=O)-]) As the resin having an acid anhydride structure [-C(=O)-O-C(=O)-], any of polyolefin resins such as polyethylene or polypropylene having an acid anhydride structure [-C(=O)-O-C(=O)-], or a polystyrene resin having an acid anhydride structure can be used. In particular, a modified polypropylene resin having an acid anhydride structure (a polypropylene resin modified with a carboxylic anhydride) is preferred. The modified polypropylene resin having an acid anhydride structure refers to a resin synthesized using, in addition to propylene as a raw material, a monomer having an acid anhydride structure. Further, 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.

[0025] As the monomer having an acid anhydride structure, a compound having an acid anhydride structure and an ethylenically unsaturated bond is preferred. Specific examples include maleic anhydride, citraconic anhydride, aconitic anhydride, itaconic anhydride, etc. Among them, maleic anhydride is preferred in terms of good compatibility between the polyester resin (X) and the polypropylene resin (Y).

[0026] If the acid value of the resin having an acid anhydride structure [-C(=O)-O-C(=O)-] is too small, the interfacial adhesion force between the resins may not be fully exerted. On the other hand, if the acid value is too large, the compatibility with the resin may decrease, making it difficult to suppress the maximum diameter of the voids within a predetermined range. Therefore, the acid value is preferably 0.5 to 50, more preferably 0.5 to 10. Also, the MFR is preferably 2 to 20 g / 10 min, more preferably 3 to 10 g / 10 min. Note that the acid value is the value measured in accordance with JIS K 0070.

[0027] Commercially available products can also be used as the resin having an acid anhydride structure. For example, when using a modified polypropylene resin, Admer (trade name, manufactured by Mitsui Chemicals, Inc.), OREVAC (trade name, manufactured by Arkema, Inc.), etc. can be used. When using a modified polyethylene resin, Admer (trade name, manufactured by Mitsui Chemicals, Inc.) LF128 (MFR 2.7), etc. can be used. Also, when using a styrene-based thermoplastic resin modified with maleic anhydride, for example, "Tufprene 912" manufactured by Asahi Kasei Corporation, "FG1901", "FG1924" of Clayton Polymer Japan Co., Ltd., "Tuftec M1911", "Tuftec M1913", "Tuftec M1943" of Asahi Kasei Corporation, etc. are exemplified.

[0028] (Ionomer) An ionomer is a random, block, or graft copolymer of ethylene and an acidic vinyl monomer such as an unsaturated carboxylic acid, which is made into a partial neutralized metal salt, and these polymer chains are aggregated using the cohesive force of metal ions. For example, an ionomer resin obtained by neutralizing at least a part of the carboxyl groups of an ethylene-unsaturated carboxylic acid copolymer resin with metal ions can be mentioned.

[0029] Those obtained by neutralizing 10 mol% or more, preferably 10 - 90 mol%, more preferably 15 - 80 mol% of the carboxyl groups of the ethylene-unsaturated carboxylic acid copolymer resin with metal ions are preferably used. Examples of the metal ions include alkali metals such as lithium and sodium, and polyvalent metal ions such as zinc or alkaline earth metal ions such as magnesium and calcium. Among them, polyvalent metal ions such as zinc are preferred in terms of having a good compatibilizing effect.

[0030] (Blending amount) The blending amount of the compatibilizer (Z) with respect to 100 parts by mass of the polyester resin (X) is preferably 0.01 - 40 parts by mass. More preferably, it is 0.1 - 20 parts by mass, and among them, 1 - 20 parts by mass is particularly preferred. By satisfying the above range, the compatibility between the polyester resin (X) and the polypropylene resin (Y) becomes good, and as a polyester film for capacitors, a polyester film having good electrical properties can be obtained. In addition, when this polyester film is a multilayer film, the above blending amounts are the blending amounts in the entire polyester film. Therefore, for example, even in a multilayer polyester film having layers with different blending amounts of a compatibilizer, it refers to the blending amount of the compatibilizer in the entire laminated film.

[0031] [Cured resin layer] It is preferable to provide a cured resin layer on at least one surface of the polyester film of the present invention. The cured resin layer only needs to be provided on at least one side of the polyester film, and may be provided on both sides. As the cured resin layer, it is preferable to contain a fluorine-containing compound. By providing a cured resin layer containing a fluorine-containing compound, the charging potential per thickness is improved compared to the polyester film alone, the attenuation of the charging potential can be more suppressed, and dust and oil adhering to the surface of the polyester film can be easily removed. 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) for the cured resin layer, it becomes possible to improve the charging potential per thickness and more suppress the attenuation of the charging potential, prevent damage to the cured resin layer, have good solvent resistance, and improve the film-forming property, transparency, etc. of the cured resin layer.

[0032] (Fluorine-containing compound (A)) The fluorine-containing compound is preferably a fluorine-containing resin from the viewpoint of enhancing the strength of the resin layer. Specific examples of the fluorine-containing resin include fluoroolefin copolymer resins having monomers such as vinyl fluoride, vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene, chlorotrifluoroethylene, etc.; fluorine-based copolymer resins formed by polymerizing polyalkylene ethers in which some or all of the hydrogen atoms are substituted with fluorine atoms, such as fluoromethylene ether, difluoromethylene ether, fluoroethylene ether, difluoroethylene ether, tetrafluoroethylene ether, hexafluoropropylene ether, etc., with other monomers; fluorine-based copolymer resins formed by graft-polymerizing a hydroxy group-containing fluororesin copolymer with a (meth)acrylate-based compound or other monomers; vinyl polymers having a perfluoroalkyl group, etc. Among them, from the viewpoints of excellent improvement in charging potential and excellent wiping properties of dust and oil, fluoroolefin copolymer resins and fluororesin copolymers containing a polyalkylene ether group in which some or all of the hydrogen atoms are fluorinated are preferred.

[0033] Among the monomers of the fluoroolefin copolymer resin, vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene are preferred. Also, among the 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 fluorine-containing monomers, it is preferably a mixed dispersion with other components from the viewpoints of dispersibility in a solvent and compatibility with other resins. Other components will be described later.

[0034] Examples of the fluorine-based copolymer resin formed by polymerizing a polyalkylene ether in which some or all of the hydrogen atoms are substituted with fluorine atoms with other monomers include urethane resins having a polyfluoroalkylene ether group, polyester resins having a polyfluoroalkylene ether group, acrylic resins having a polyfluoroalkylene ether group, etc. Examples of other monomers constituting the urethane resin having a polyfluoroalkylene ether group include isocyanate compounds. Examples of the isocyanate compounds 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 isopropylidene dicyclohexyl diisocyanate.

[0035] Examples of other monomers constituting the urethane resin include polyols not containing fluorine atoms, polyols having a carboxy group such as dimethylolpropanoic acid, dimethylolbutanoic acid, bis-(2-hydroxyethyl)propanoic acid, and bis-(2-hydroxyethyl)butanoic acid. Among these, from the viewpoint of self-emulsifiability when water is used as a dispersion solvent, it is preferable to contain a polyol having a carboxy group, and dimethylolpropanoic acid is more preferable.

[0036] 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 consist of 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-forming composition is preferably in the range of 5 to 100% by mass, more preferably 20 to 98% by mass, and still more preferably 45 to 95% by mass with respect to the non-volatile components in the cured resin layer composition. By setting it to 5% by mass or more, it becomes possible to improve the charging potential per thickness and further suppress the attenuation of the charging potential, and it also becomes possible to easily remove dust and oil adhering to the surface of the polyester film.

[0037] (Crosslinking agent (B)) The cured resin layer composition for forming the cured resin layer preferably contains the crosslinking agent (B) as described above. By including a crosslinking agent in the cured resin layer composition, it is possible to form a dense cured resin layer with a high crosslinking density. Also, damage to the cured resin layer can be prevented, and solvent resistance and the like can be easily improved. The crosslinking agent is not particularly limited, and conventionally known crosslinking agents can be used. Examples of the crosslinking agent include melamine compounds, oxazoline compounds, epoxy compounds, isocyanate-based compounds, carbodiimide-based compounds, silane coupling compounds, and the like. Among these, the crosslinking agent is preferably at least one selected from melamine compounds, oxazoline compounds, and isocyanate-based compounds, and is preferably a melamine compound from the viewpoint of peelability. Also, these crosslinking agents may be used alone or in combination of two or more. Further, as a component that cures together with these crosslinking agents, an arbitrary polymerizable monomer may be included in the cured resin layer composition.

[0038] (Melamine compound) The melamine compound used as a crosslinking agent refers to a compound having a melamine skeleton in the compound. For example, an alkylolated melamine derivative, a compound obtained by reacting an alkylolated melamine derivative with an alcohol to partially or completely etherify it, and a mixture thereof can be used. Examples of alkylolation include methylolation, ethylolation, isopropylolation, n-butylation, isobutylation, etc. Among these, from the viewpoint of reactivity, methylolation is preferred. Also, as the alcohol used for etherification, methyl alcohol, ethyl alcohol, isopropyl alcohol, n-butanol, isobutyl alcohol, etc. are preferably used. From the viewpoint of improving the coating film strength of the cured resin layer and improving the adhesion between the cured resin layer and the polyester film, it is preferably a partially or completely etherified alkylolated melamine derivative, and more preferably an alkylol etherified with methyl alcohol. The partially etherified alkylol group is preferably 0.5 to 5 equivalents, more preferably 0.7 to 5 equivalents, relative to the unetherified alkylol group. Also, the melamine compound may be a monomer, a dimer or higher multimer, or a mixture thereof. Further, a co-condensed product of urea or the like with a part of melamine may be used.

[0039] To increase the reactivity of the melamine compound, the cured resin layer composition may contain a crosslinking catalyst in addition to the melamine compound. As the crosslinking catalyst, various known catalysts can be used. For example, amine compounds, salts of amine compounds, aromatic sulfonic acid compounds such as p-toluenesulfonic acid, organic acids such as phosphoric acid compounds and their salts, imine compounds, amidine compounds, guanidine compounds, organometallic compounds, metal salts such as zinc stearate, zinc myristate, aluminum stearate, calcium stearate, etc. Among these, amine compounds, salts of amine compounds, and p-toluenesulfonic acid are preferred, and amine compounds and salts of amine compounds are more preferred.

[0040] (oxazoline compound) An 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 polymerization of an addition-polymerizable oxazoline group-containing monomer alone or with other monomers. Among them, an acrylic polymer which is a copolymer of an addition-polymerizable oxazoline group-containing monomer and an acrylic monomer which is a monomer having a (meth)acryloyl group is preferred, and the acrylic polymer may have a polyalkylene oxide chain. In the present specification, when the expression (meth)acryloyl group is used, it means one or both of an acryloyl group and a 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, 2-isopropenyl-5-ethyl-2-oxazoline, etc., and one or a mixture of two or more of these can be used. Among these, 2-isopropenyl-2-oxazoline is industrially easily available and suitable. The addition-polymerizable oxazoline group-containing monomer may be used alone or two or more monomers may be used.

[0041] Other monomers are not limited as long as they are copolymerizable with the addition polymerizable oxazoline group-containing monomer. For example, (meth)acrylic acid esters such as alkyl (meth)acrylates (as the alkyl group, methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, t-butyl group, 2-ethylhexyl group, cyclohexyl group, etc.); unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, styrenesulfonic acid and its salts (as the salts, sodium salt, potassium salt, ammonium salt, tertiary amine salt, etc.); unsaturated nitriles such as acrylonitrile, methacrylonitrile; unsaturated amides such as (meth)acrylamide, N-alkyl (meth)acrylamide, N,N-dialkyl (meth)acrylamide (as the alkyl group, methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, t-butyl group, 2-ethylhexyl group, cyclohexyl group, etc.); vinyl esters such as vinyl acetate, vinyl propionate; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether; α-olefins such as ethylene, propylene; halogen-containing α,β-unsaturated monomers such as vinyl chloride, vinylidene chloride, vinyl fluoride; α,β-unsaturated aromatic monomers such as styrene, α-methylstyrene, etc. can be mentioned.

[0042] In addition, as other monomers, monomers having a polyalkylene oxide chain can also be used. As the monomer having a polyalkylene oxide chain, for example, esters obtained by adding a polyalkylene oxide to the carboxyl group of unsaturated carboxylic acids such as acrylic acid and methacrylic acid can be preferably exemplified. Here, as the polyalkylene oxide chain, for example, polymethylene oxide, polyethylene oxide, polypropylene oxide, polybutylene oxide, etc. can be preferably exemplified. The repeating unit of the polyalkylene oxide chain is preferably in the range of, for example, 3 to 100. The other monomers used in the oxazoline compound may be used alone or in combination of two or more.

[0043] 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, still more preferably 3 to 8 mmol / g, and particularly preferably 4 to 6 mmol / g. By using within the above range, the durability of the coating film (cured resin layer) is likely to be improved.

[0044] (Epoxy compound) An epoxy compound is a compound having an epoxy group in the molecule. Examples include condensates of epichlorohydrin with hydroxyl group- or amino group-containing compounds such as ethylene glycol, polyethylene glycol, glycerin, polyglycerin, and bisphenol A. Examples of epoxy compounds 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, resorcin 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 monoepoxy compounds include allyl glycidyl ether, 2-ethylhexyl glycidyl ether, and phenyl glycidyl ether. Examples of glycidylamine compounds include N,N,N’,N’-tetraglycidyl-m-xylylenediamine and 1,3-bis(N,N-diglycidylamino) cyclohexane. The epoxy compound may be used alone or in combination of two or more.

[0045] (Isocyanate compound) An isocyanate compound is a compound having an isocyanate or an isocyanate derivative structure typified by a blocked isocyanate. Examples of isocyanates include aromatic isocyanates such as tolylene diisocyanate, xylylene diisocyanate, methylene diphenyl diisocyanate, phenylene diisocyanate, naphthalene diisocyanate, etc., 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, hexamethylene diisocyanate, etc., and alicyclic isocyanates such as cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, methylene bis(4-cyclohexyl isocyanate), isopropylidene dicyclohexyl diisocyanate, etc. Also included are polymers and derivatives such as biuretized products, isocyanurated products, and uretdione products of these isocyanates. Isocyanates may be used alone or in combination of multiple types. Among the above isocyanates, aliphatic isocyanates or alicyclic isocyanates are more preferable in order to avoid yellowing due to ultraviolet rays.

[0046] When used in the state of blocked isocyanate, examples of the blocking agent include bisulfites such as sodium bisulfite, phenolic compounds such as phenol, cresol, and ethylphenol, alcohol 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, acetaldehyde oxime, acetone oxime, methyl ethyl ketone oxime, and cyclohexanone oxime. These may be used alone or in combination of two or more.

[0047] Also, the isocyanate compound may be used alone or in the form of a conjugate with various polymers. Further, the isocyanate compound may be blended into the cured resin layer composition as a mixture of various polymers. From the viewpoint of improving the dispersibility and crosslinkability of the isocyanate compound, it is preferable to use a mixture or conjugate with a polyester resin or a urethane resin. The isocyanate compound may be used alone or in combination of two or more. Note that the amount of the crosslinking agent when the isocyanate compound is used includes the amount of the above-mentioned blocking agent, the polymer to be conjugated or mixed, and the like.

[0048] (Carbodiimide compound) A carbodiimide compound is a compound having a carbodiimide structure. When a carbodiimide compound is used, the moisture and heat resistance of the cured resin layer can be improved. Carbodiimide compounds can be synthesized by conventionally known techniques, and generally, a condensation reaction of a diisocyanate compound is used. The diisocyanate compound is not particularly limited, and either an aromatic type or an aliphatic type can be used. Specifically, tolylene diisocyanate, xylene diisocyanate, diphenylmethane diisocyanate, phenylene diisocyanate, naphthalene diisocyanate, hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, dicyclohexyl diisocyanate, dicyclohexylmethane diisocyanate, etc. can be mentioned. The carbodiimide compound may be used alone or in combination of two or more.

[0049] (Silane coupling compound) A silane coupling compound is an organosilicon compound having 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, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; vinyl group-containing compounds such as vinyltrimethoxysilane, vinyltriethoxysilane; styryl group-containing compounds such as p-styryltrimethoxysilane, p-styryltriethoxysilane; (meth)acrylic group-containing compounds such as 3-(meth)acryloxypropyltrimethoxysilane, 3-(meth)acryloxypropyltriethoxysilane, 3-(meth)acryloxypropylmethyldimethoxysilane, 3-(meth)acryloxypropylmethyldiethoxysilane; amino group-containing compounds such as 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-2-(aminoethyl)-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, N-phenyl-3-aminopropyltriethoxysilane; isocyanurate group-containing compounds such as tris(trimethoxysilylpropyl)isocyanurate, tris(triethoxysilylpropyl)isocyanurate; mercapto group-containing compounds such as 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 3-mercaptopropylmethyldimethoxysilane, 3-mercaptopropylmethyldiethoxysilane, etc. can be mentioned. Among the above compounds, from the viewpoint of maintaining the strength of the cured resin layer, epoxy group-containing silane coupling compounds, double bond-containing silane coupling compounds such as vinyl groups and (meth)acrylic groups, and amino group-containing silane coupling compounds are more preferable. The silane coupling compound may be used alone or in combination of two or more kinds.

[0050] The crosslinking agent contained in the cured resin layer composition may be designed to react during the drying process or the film-forming process when forming the cured resin layer so as to improve the performance of the cured resin layer. It can be presumed that unreacted substances of these crosslinking agents, compounds after reaction, or mixtures thereof are present in the cured resin layer formed from the cured resin layer composition.

[0051] The content of the crosslinking agent in the cured resin layer composition is preferably in the range of 5 to 60% by mass based on the non-volatile components in the cured resin layer composition. By setting the content of the crosslinking agent in the cured resin layer composition to 5 to 60% by mass, it becomes easier to suppress the attenuation of the charging potential. Also, the strength of the cured resin layer is improved, and it becomes easier to improve the scratch resistance and the like. From the above viewpoints, more preferably it is 10 to 50% by mass, still more preferably 15 to 40% by mass, and particularly preferably 20 to 40% by mass.

[0052] Also, when the cured resin layer composition contains a crosslinking catalyst, when the content of the crosslinking catalyst is in the range of 0.4 to 10% by mass based on the non-volatile components in the cured resin layer composition, the strength of the cured resin layer is improved, and the scratch resistance and the like tend to be easily improved, which is preferable. From the above viewpoints, the content of the crosslinking catalyst is preferably 0.6 to 8% by mass, more preferably 0.8 to 5% by mass.

[0053] (Binder resin (C)) The binder resin is a polymer component contained in the cured resin layer composition other than the polymer formed by crosslinking the above crosslinking agent (B). By the cured resin layer composition containing the binder resin, the film-forming property, transparency, etc. of the cured resin layer are improved.

[0054] Specific examples of the binder resin include acrylic resins, polyvinyl alcohol, polyester resins, urethane resins, polyalkylene glycols, polyalkyleneimines, methyl cellulose, hydroxycellulose, starches, and the like. 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. From the viewpoint of further improving coatability and the like, acrylic resins are even more preferred.

[0055] (Acrylic resin) An acrylic resin is a polymer composed of polymerizable monomers containing acrylic and methacrylic monomers. These may be homopolymers, copolymers, or even copolymers with polymerizable monomers other than acrylic and methacrylic monomers. Also included are copolymers of these polymers with other polymers (such as polyester, polyurethane, etc.). For example, block copolymers and graft copolymers. That is, the acrylic resin may be an acrylic-modified polyester resin or an acrylic-modified polyurethane resin. Furthermore, also included are polymers (in some cases, mixtures of polymers) obtained by polymerizing polymerizable monomers in a polyester solution or a polyester dispersion. Similarly, polymers (in some cases, mixtures of polymers) obtained by polymerizing polymerizable monomers in a polyurethane solution or a polyurethane dispersion are also included. Similarly, polymers (in some cases, polymer mixtures) obtained by polymerizing polymerizable monomers in other polymer solutions or dispersions are also included, and these are also regarded as acrylic-modified polyester resins and acrylic-modified polyurethane resins in this specification. The polyester and polyurethane used in the acrylic resin can be appropriately selected from those exemplified as the polyester and polyurethane used in the binder resin described later. Also, the acrylic resin may contain a hydroxyl group or an amino group in order to further improve the adhesion to the polyester film.

[0056] The polymerization monomer is not particularly limited, but typical compounds include, for example, various carboxyl group-containing monomers such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid, and citraconic acid, and their salts; various hydroxyl group-containing monomers such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, monobutyl hydroxy fumarate, and monobutyl hydroxy itaconate; various (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, and lauryl (meth)acrylate; various nitrogen-containing compounds such as (meth)acrylamide, diacetone acrylamide, N-methylol acrylamide, or (meth)acrylonitrile; various styrene derivatives such as styrene, α-methylstyrene, divinylbenzene, and vinyltoluene, and 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.

[0057] (Polyester resin) A polyester resin is, for example, composed of the following polyvalent carboxylic acids and polyvalent hydroxy compounds as main constituent components. That is, as polyvalent carboxylic acids, 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 trimellitate and their ester-forming derivatives, etc. can be used. As polyvalent hydroxy compounds, 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 dimethylol ethyl sulfonate, potassium dimethylolpropionate, etc. can be used. One or more of these compounds can be appropriately selected from among them, and a polyester resin can be synthesized by a conventional polycondensation reaction. Further, the polyester resin may be in the form of an aqueous dispersion, and in that case, a hydrophilic functional group or the like may be appropriately introduced into the polyester resin.

[0058] (Polyvinyl alcohol) Polyvinyl alcohol is a compound having a polyvinyl alcohol moiety. For example, conventionally known polyvinyl alcohols can be used, including modified compounds that are partially acetalized or butyralized with respect to polyvinyl alcohol. The degree of polymerization of polyvinyl alcohol is not particularly limited, but is usually 100 or more, preferably in the range of 300 to 40,000. By setting the degree of polymerization to 100 or more, it is possible to prevent a decrease in the water resistance of the cured resin layer. Also, the saponification degree of polyvinyl alcohol is not particularly limited, but polyvinyl acetate saponified products that are usually 70 mol% or more, preferably in the range of 70 to 99.9 mol%, more preferably 80 to 97 mol%, and particularly preferably 86 to 95 mol% are practically used.

[0059] (Urethane resin) A urethane resin is a high molecular weight compound having a urethane bond in the molecule. Usually, urethane resins are produced by the reaction of polyols and isocyanates. Examples of polyols include polycarbonate polyols, polyester polyols, polyether polyols, polyolefin polyols, and acrylic polyols, and these compounds may be used alone or in combination of multiple types. The urethane resin may be an aqueous dispersion, and in that case, for example, a hydrophilic functional group may be appropriately introduced into the polyol.

[0060] The content of the binder resin in the cured resin layer composition is preferably in the range of 10 to 70% by mass with respect 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 coating property and the attenuation of the charging potential. Also, it becomes easier to improve the appearance and transparency of the cured resin layer. From the above viewpoints, it is more preferably 20 to 65% by mass, still more preferably 30 to 60% by mass, and particularly preferably 30 to 55% by mass. In addition to the above-described components, additives such as reaction regulators, adhesion enhancers, surfactants, antistatic agents, and particles may be appropriately blended in the cured resin layer composition.

[0061] The curable resin layer composition may be applied to a polyester film as a liquid coating solution, dried and cured as necessary. The curable resin layer composition is preferably made into a coating solution by diluting with a solvent. Each of the above components ((A) to (C) components, etc.) constituting the curable resin composition may be dissolved in a solvent or dispersed in a solvent. There is no limitation on the solvent used for the curable resin layer composition, and either water or an organic solvent may be used. From the viewpoint of environmental protection, it is preferable to use an aqueous coating solution using water as a solvent. The aqueous coating solution may contain a small amount of an organic solvent. The specific amount of the organic solvent is preferably less than that of water on a mass basis. For example, it is less than 30% by mass, preferably less than 20% by mass, and more preferably less than 10% by mass in the solvent. Examples of the organic solvent 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 of two or more. Appropriately selecting and containing these organic solvents in the aqueous coating solution as necessary may improve the stability and coatability of the coating solution.

[0062] When using only an organic solvent as the above 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 methyl ethyl 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 of multiple types in consideration of solubility, coatability, boiling point, etc.

[0063] <Method for forming curable resin layer> The method for forming the cured resin layer will be described in detail below. The formation of the cured resin layer may be performed by in-line coating or by off-line coating. In-line coating is a method of applying a coating liquid of the cured resin layer composition onto the surface of the polyester film on the production line for manufacturing the polyester film. Off-line coating is a method of applying the coating liquid outside the system (outside the above production line) onto the polyester film once it has been manufactured. From the viewpoint of ease of processing, it is preferable to form the cured resin layer by in-line coating.

[0064] Specifically, in-line coating is a method of applying a coating liquid of the cured resin layer composition to the polyester film at any arbitrary stage from when the polyester is melt-extruded until after stretching and heat-setting and being wound up. Usually, it is advisable to apply the coating liquid to the polyester film using any one of an unstretched sheet obtained by melting and quenching, a uniaxially stretched film that has been stretched, a biaxially stretched film before heat-setting, or a film before winding up after heat-setting.

[0065] As a method for applying the coating liquid, for example, conventionally known coating methods such as 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, extrusion coating, etc. can be used.

[0066] Also, although not particularly limited, for example, in sequential biaxial stretching, a method of applying the coating liquid to a uniaxially stretched film stretched particularly in the longitudinal direction (vertical direction) and then stretching it in the lateral direction is preferable. According to such a method, since the film formation of the polyester film and the formation of the cured resin layer can be performed simultaneously, there are advantages in terms of manufacturing cost. Also, since stretching is performed after coating, the thickness of the cured resin layer can be changed according to the stretching ratio, and thin film coating can be performed more easily compared to off-line coating. Furthermore, the thickness of the cured resin layer can be made more uniform.

[0067] Further, by applying a coating solution of a curable resin composition onto the polyester film before stretching, the curable resin can be stretched together with the polyester film, whereby the curable resin layer can be firmly adhered to the polyester film. In the production of a biaxially stretched film, by stretching while gripping the film end with a clip or the like, the polyester film can be restrained in the longitudinal and lateral directions, and in the heat setting step, it can be heated to a high temperature while maintaining flatness without wrinkles. Therefore, since the heat treatment applied after applying the curable resin layer composition can be set to a high temperature that cannot be achieved by other methods, the curable resin layer and the polyester film can be adhered more firmly.

[0068] Also, the coating solution of the curable resin composition applied to the polyester film may be subjected to only one of heat treatment and active energy ray irradiation such as ultraviolet irradiation, or both may be used in either off-line coating or in-line coating, but it is preferable to perform at least heat treatment. Further, the curable resin layer composition may be cured by one or both of heat treatment and active energy ray irradiation. The heat treatment may be performed by heating, for example, by the heat setting step as described above, but may also be performed by other methods. Further, when the coating solution of the curable resin layer composition contains a solvent, it is appropriately dried, but it is preferably dried by the above heat treatment. In order to improve the coatability of the coating solution for forming the curable resin layer and the adhesiveness of the curable resin layer to the polyester film, before applying the coating solution, a surface treatment such as chemical treatment, corona discharge treatment, plasma treatment, ozone treatment, chemical treatment, solvent treatment, etc. may be performed on the surface of the polyester film where the curable resin layer is to be formed.

[0069] As described above, the polyester film with a curable resin layer may be provided with another layer between the polyester film and the curable resin layer. Examples of the other layer include layers having various functions such as an antistatic layer, an easy adhesion layer, and an oligomer sealing layer.

[0070] The thickness of the cured resin layer is preferably 0.005 to 1 μm. By setting it to 1 μm or less, migration of the components constituting the cured resin layer to the cured resin layer or the like is suppressed, and blocking during winding can be more easily prevented. On the other hand, by setting it to 0.005 μm or more, attenuation of the charging potential can be more easily suppressed. From these viewpoints, 0.01 μm or more is more preferable, 0.02 μm or more is even more preferable, 0.2 μm or less is more preferable, 0.1 μm or less is even more preferable, and among them, particularly 0.06 μm or less is good.

[0071] <Metal laminated film> The polyester film of the present invention can be a metal laminated film having a metal layer provided on at least one side thereof. The metal layer may be provided on one side of the polyester film or on both sides. Further, it may be provided on the above-mentioned cured resin layer. Examples of the metal include copper, silver, chromium, aluminum, nickel, zinc, etc. Among these, from the viewpoints of cost and environmental compatibility, aluminum and zinc are preferable. 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. Being within the above range is advantageous in terms of electrical characteristics.

[0072] <Manufacturing method of the present polyester film> As an example of the manufacturing method of the present polyester film, the manufacturing method in the case where the present polyester film is a biaxially stretched film will be described. However, it is not limited to the manufacturing method described here.

[0073] First, by a known method, a raw material, for example, a polyester chip, is supplied to a melt extrusion apparatus, heated to a temperature above the melting point of each polymer, the molten polymer is extruded from a die, and cooled and solidified on a rotary cooling drum to a temperature below the glass transition point of the polymer so as to obtain a substantially amorphous and unoriented sheet.

[0074] Next, the non-oriented sheet is stretched by a roll or tenter-type stretcher in one direction. At this time, 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 2.8 to 6 times. Next, it is stretched in a direction perpendicular to the stretching direction of the first stage. At this time, the stretching temperature is usually 50 to 140°C, and the stretching ratio is usually 3.0 to 7 times, preferably 3.5 to 6 times. In addition, in the above stretching, a method of performing the stretching in one direction in two or more stages can also be adopted. Also, in the present invention, it is preferable to increase the stretching ratio in the lateral direction. Therefore, it is preferable that the stretching ratio is 4.5 times or more in either the first stage or the second stage.

[0075] After stretching, a heat setting treatment is continuously performed at a temperature of 130 to 270°C under tension or under relaxation within 30%, and the present polyester film as a biaxially oriented film can be obtained. By performing the heat setting treatment, the heat resistance and the like of the present polyester film can be improved. In addition, although the above manufacturing method has been described on the premise of a single-layer film, in the case of a multilayer film, for example, an unoriented sheet may be produced by coextrusion or the like, and then the same procedure may be performed.

[0076] (Film thickness) The film thickness is preferably 0.5 to 12.0 μm. When the thickness is within this range, it is suitable for use as a capacitor. From the above viewpoints, the thickness of the present polyester film is more preferably 0.5 to 10.0 μm, and particularly preferably 1.0 to 8.0 μm.

[0077] <Properties of the present polyester film> Regarding the relaxation time T of the polypropylene resin (Y) component obtained by the pulse NMR method for the polyester film of the present invention, the relationship between the relaxation time (T2) (μs) after heat-treating the film at 150°C for 1 minute and the relaxation time (T1) (μs) before heat-treatment needs to satisfy the following formula (1). (T2) / (T1) ≥ 1.20 ··· (1)

[0078] The value of (T2) / (T1) is preferably 1.40 or more, more preferably 2.00 or more, most preferably 4.00 or more, and particularly preferably 6.00 or more. The higher this value is, the higher the dielectric breakdown voltage is even at high temperatures, and it is considered that when it is used as a capacitor, reliability can be exhibited for a long time in a high-temperature environment. As a method for increasing the value of (T2) / (T1), it is preferable to use a compatibilizer (Z) having a specific structure, which will be described in detail later, but it is not limited to the configuration using the compatibilizer (Z). For example, by appropriately adjusting the kneading conditions to sufficiently disperse the polypropylene resin in the polyester resin, (T2) / (T1) can be increased. Also, in the film stretching step, by setting the transverse stretching ratio to 4.5 times or more, preferably 4.8 times or more, the polypropylene resin can be dispersed over a wider range, which can contribute to increasing (T2) / (T1).

[0079] Satisfying the above formula (1) suggests that when the film is heat-treated, the dispersion state of the polypropylene resin in the film is good. That is, it is presumed that the better the dispersion state of the polypropylene resin and the wider the dispersion range, the longer the relaxation time (T2) will be as the molecular motion of the polypropylene resin is activated by heat treatment. Particularly in a high-temperature environment of 120°C or higher, it is presumed that good electrical characteristics can be exhibited due to the active molecular motion of the polypropylene resin.

[0080] In the present invention, in order to cope with capacitor applications, the film thickness is different from normal, and the ultra-thin region (0.5 to 12 μm) is the mainstream. Therefore, it is different from the case of intentionally forming voids by using a polypropylene resin incompatible with the polyester resin for other applications (for example, for the purpose of imparting cushioning properties in paper substitution). The reason for using polypropylene resin is that it is used as a film for capacitors, particularly with the intention of improving electrical properties in the low-frequency region. From this perspective, based on the design concept that better electrical properties can be achieved by minimizing the formation of voids derived from polypropylene resin in the film after film forming processing, as described above, the film structure was observed from a microscopic viewpoint, and the dispersibility of polypropylene resin was replaced with physical properties, specifically the relaxation time measured by pulsed NMR, and by focusing on the relationship with electrical properties (dielectric tangent), it was found that the two showed a good correlation, leading to the completion of the present invention.

[0081] Also, although the mechanism for expressing the above characteristics is unclear, it is presumed that when stretching a film having a thickness in the ultra-thin region (0.5 to 12 μm), forces are applied from above and below in the thickness direction of the film. The force applied from above and below in the thickness direction of the film, combined with the synergistic effect with the compatibilizer, improved the adhesion between the polyester resin layer (X) and the polypropylene resin layer (Y). As a result, for example, by extruding the air present at the interface between the two, it is also presumed to be involved in forming a structure that is less likely to form voids.

[0082] (Dielectric tangent (tanδ)) The polyester film of the present invention preferably has a tanδ of 0.55 or less at 1 kHz, more preferably 0.50 or less, still more preferably 0.45 or less, and particularly preferably 0.40 or less among them. By satisfying the above range of tanδ, the electrical properties of the film become good, making it suitable for use in capacitors.

[0083] <Use> The polyester film and the metal laminated film of the present invention have excellent electrical properties such as a low dielectric tangent (tanδ), and thus are useful for capacitors. Particularly, due to their thin film form, they are useful for capacitors mounted in automobiles that require miniaturization, weight reduction, and high capacitance, such as hybrid automobiles and electric automobiles.

[0084] <Explanation of terms, etc.> In the present invention, when referring to "film", it includes "sheet", and when referring to "sheet", it includes "film". In the present invention, when described as "X to Y" (X and Y are arbitrary numbers), unless otherwise specified, it includes the meaning of "X or more and Y or less", as well as the meaning of "preferably greater than X" or "preferably less than Y". Also, when described as "X or more" (X is an arbitrary number), unless otherwise specified, it includes the meaning of "preferably greater than X", and when described as "Y or less" (Y is an arbitrary number), unless otherwise specified, it includes the meaning of "preferably less than Y".

Examples

[0085] Next, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the examples described below.

[0086] <Evaluation method> The measurement and evaluation methods for various physical properties and characteristics are as follows.

[0087] (1) Intrinsic viscosity (IV) 1 g of polyester was precisely weighed, 100 ml of a mixed solvent of phenol / tetrachloroethane = 50 / 50 (mass ratio) was added and dissolved, and the measurement was carried out at 30°C.

[0088] (2) Film thickness After fixing and molding a small film piece with an epoxy resin, it was cut with a microtome, and the cross-section of the film was observed with a transmission electron micrograph. Two interfaces that are substantially parallel to the film surface and whose interfaces can be observed by light and darkness are present in the cross-section. The distances from these two interfaces to the film surface were measured from 10 photographs, and the average value was taken as the laminate thickness.

[0089] (3) Melting point of polypropylene resin (Y) It is defined as the temperature at the top of the endothermic peak when the temperature is raised from room temperature to 200 °C by differential scanning calorimetry (DSC) to eliminate the thermal history, then the temperature is lowered to 40 °C at a cooling rate of 10 °C / min, and then measured again at a heating rate of 10 °C / min.

[0090] (4) NMR relaxation times (T2) (μs) and (T1) (μs) and their ratio (T2) / (T1) The method of heat-treating the film at 150 °C for 1 minute uses a square metal frame with a width of 20 mm that has been punched out to a thickness of 2 mm, an outer dimension of 300 mm × 300 mm, and an inner dimension of 280 mm × 280 mm. Double-sided tape ("Nice Tack" NW-H15 adhesion 02 manufactured by Nichiban Co., Ltd.) is attached to the four sides of the frame surface. The film is attached so that it covers the entire surface of the metal frame, and then the film is sandwiched between metal frames of the same dimensions. At this time, the film is attached so that no wrinkles are formed. Next, a sample is created by sandwiching and fixing the four sides of the frame with clips in the state of metal frame / double-sided tape / film / metal frame, and the sample is left in an oven heated to 150 °C for 1 minute. After 1 minute, the sample is taken out and left at room temperature for 5 minutes, and then the film is cut out along the inner frame of the metal frame to obtain the film after heat treatment at 150 °C for 1 minute. When the film cannot be obtained in a size of 300 mm × 300 mm, a metal frame of a size that can be attached is used.

[0091] Next, the relaxation time (T2) (μs) of the polypropylene resin (Y) component in the polyester film by the pulsed NMR method before heat treatment at 150 °C for 1 minute and the relaxation time (T1) (μs) of the polypropylene resin (Y) component in the polyester film by the pulsed NMR method after treatment were determined under the following apparatus and conditions, and their ratio (T2) / (T1) was calculated. Apparatus: mq20 manufactured by Bruker Biospin Temperature: 40 °C Observation frequency: 20 MHz 90° pulse width: 2.74 μs Pulse repetition time: 2.0 s Pulse mode: Solido Echo method The measurement was carried out as follows for each of the films before and after the heat treatment. The film was cut and packed into a glass tube with an outer diameter of 10 mm up to a height of 1 cm. The spin-spin relaxation time T2 of the 1H nucleus of the polypropylene resin (Y) component in the polyester film was determined. The measurement was started after the film was put into the apparatus and kept warm for 15 minutes. The obtained attenuation curve was separated into a Gaussian function component with a short T2 and an exponential function component with a long T2 by the least squares method.

[0092] (5) Dielectric loss tangent (tanδ) A sample with circular Al vapor deposition on both sides of the sample film in advance is placed on an apparatus (manufactured by HP (HEWLETT PACKARD), model: 4284A), and electrodes are brought into contact from above and below. The tanδ when the current frequency is set to 1 kHz is measured. Specifically, when an alternating voltage is applied to a capacitor, power loss occurs. At this time, the loss angle is defined as δ, and tanδ is defined as the dielectric loss tangent. The smaller the value of tanδ, the better the capacitor.

[0093] The raw materials of the polyester films in each of the examples and comparative examples are as follows. (Polyester film) (a) Polyester resin (X1): Polyethylene terephthalate homopolymer with an intrinsic viscosity of 0.63 (polycondensation catalyst; antimony). (b) Polyester resin (X2): Polyethylene terephthalate homopolymer with an intrinsic viscosity of 0.65 containing 0.5% by mass of silica particles with an average particle diameter of 0.8 μm (polycondensation catalyst; antimony). (c) Polypropylene resin (Y): Melting point 163 °C, MFR = 7.5 (manufactured by Sumitomo Chemical Co., Ltd.: FLX80E4) (d) Resin (Z1) having an acid anhydride structure: Maleic anhydride-modified polypropylene resin (Admer QE800, melt flow rate: 9.1 g / 10 min, acid value 4.3, manufactured by Mitsui Chemicals, Inc.) (e) Resin (Z2) having an acid anhydride structure: maleic anhydride-modified polystyrene resin (Taftec M1943, melt flow rate: 8.0 g / 10 min, manufactured by Asahi Kasei Corporation)

[0094] [Example 1] A mixed raw material obtained by mixing polyester resin X1, polypropylene resin Y, and Z1 as a compatibilizer at ratios of 87.5% by mass, 10% by mass, and 2.5% by mass, respectively, was fed into an extruder, melted at 285°C, and then extruded and cooled and solidified on a cooling roll set at 25°C to obtain an unstretched sheet. Subsequently, using the roll peripheral speed difference, it was stretched 3.2 times in the longitudinal direction (MD) at a film temperature of 85°C. This longitudinally stretched film was led into a tenter and stretched 4.9 times in the transverse direction (TD) at 100°C. After heat treatment at 220°C, it was relaxed 0.5% in the transverse direction to obtain a polyester film with a thickness of 6.9 μm.

[0095] [Examples 2 - 3, Comparative Example 1] A polyester film was obtained in the same manner as in Example 1 except that the conditions were changed as shown in Table 1.

[0096]

Table 1

[0097] Examples 1 to 3 using a compatibilizer with a specific structure had good compatibility despite having a higher polypropylene content compared to Comparative Example 1, and the ratio of NMR relaxation times (T2 / T1) tended to be large before and after heat-treating the film. Moreover, it was also found that in the relationship with the dielectric loss tangent, the larger the ratio (T2 / T1), the better the dielectric loss tangent. Considering from the film structure aspect, when the film is heat-treated, it is presumed that as the dispersibility of the polypropylene resin in the film is better and it is more widely dispersed, the relaxation time (T2) becomes even longer as the molecular motion of the polypropylene resin becomes more active. For example, from the comparison between Example 1 and Example 3 where the blending ratio of the polypropylene resin to the polyester resin is the same, it is suggested that Example 1 has better dispersibility of the polypropylene resin in the film, which also conforms to the tendency of the dielectric loss tangent. Also, when stretching a film having a thickness in the ultra-thin region (0.5 to 12 μm) as in the present invention, it is presumed that forces are applied from above and below in the thickness direction of the film. Due to the forces applied from above and below in the thickness direction of the film, the synergistic effect with the compatibilizer is also added, and as a result, the adhesion between the polyester resin layer (X) and the polypropylene resin layer (Y) is improved. For example, by extruding the air present at the interface between the two, it is presumed that a structure that is less likely to form voids is obtained. Furthermore, it was also found that if a compatibilizer made of a resin having an acid anhydride structure is used, the compatibility is particularly good, and a further improvement effect on the electrical properties can be expected.

Claims

1. A polyester film comprising a polyester resin (X) and a polypropylene resin (Y), containing 0.01 to 40 parts by mass of a compatibilizer (Z) with respect to 100 parts by mass of the polyester resin (X), wherein the compatibilizer (Z) is a resin or an ionomer having an acid anhydride structure [-C(=O)-O-C(=O)-], and regarding the relaxation time T of the polypropylene resin (Y) component obtained by the pulse NMR method, the relationship between the relaxation time (T2) (μs) after heat treatment at 150°C for 1 minute and the relaxation time (T1) (μs) before heat treatment satisfies the following formula (1). (T2) / (T1) ≥ 1.20... (1)

2. The polyester film according to Claim 1, containing 1 to 30 parts by mass of a polypropylene resin (Y) with respect to 100 parts by mass of the polyester resin (X).

3. The polyester film according to Claim 1 or 2, wherein the polycondensation catalyst of the polyester resin (X) is a Ti-based or Sb-based catalyst.

4. The polyester film according to any one of Claims 1 to 3, wherein the polyester resin (X) is at least one selected from polyethylene terephthalate and polyethylene-2,6-naphthalate.

5. The polyester film according to any one of Claims 1 to 4, wherein a cured resin layer is provided on at least one surface of the polyester film.

6. The polyester film according to any one of Claims 1 to 5, having a film thickness of 0.5 to 12.0 μm.

7. The polyester film according to any one of Claims 1 to 6, having a dielectric loss tangent (tanδ) of 0.55 or less at 1 kHz.

8. A metal laminated film having a metal layer provided on at least one side of the polyester film according to any one of Claims 1 to 7.

9. The polyester film according to any one of Claims 1 to 7, for use in a capacitor.

10. The metal laminated film according to Claim 8, for use in a capacitor.

11. The polyester film according to Claim 9, for use in a capacitor mounted on an automobile.

12. The metal laminated film according to Claim 10, for use in a capacitor mounted on an automobile.

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