JP2022182944A - polyester film - Google Patents

Abstract

To provide a new polyester film as a polyester film for a capacitor which has good heat resistance while maintaining electric characteristics equivalent to a conventional film, and can correspond to the conversion into the thin film.SOLUTION: A polyester film contains a polyester resin (X) and a polypropylene resin (Y), wherein a film cross section structure has a layered structure, and has a polypropylene resin layer (Yt) having a ratio (y/x) of a length (y) in a width direction (TD) to a length (x) in a thickness direction is 17 or more.

Description

TECHNICAL FIELD The present invention relates to a polyester film, and more particularly to a polyester film suitable for capacitors.

Polyethylene terephthalate (PET) film, especially biaxially oriented PET film, is a typical polyester film in various fields such as industrial materials, optical materials, electronic component materials, and battery packaging materials. It is widely used due to its excellent flexibility.

In addition, due to its excellent electrical properties such as low dielectric loss properties and high moisture resistance, it is used as a dielectric film for capacitors such as high voltage capacitors, various switching power supplies, filter capacitors such as converters and inverters, and smoothing capacitors. As such, a resin film such as a biaxially oriented polypropylene film is used.
In recent years, there has been a need for further miniaturization and higher capacity of capacitors. For example, when a resin film is used for a capacitor for an inverter power supply device that controls a drive motor of an electric vehicle, a hybrid vehicle, or the like, a small size, light weight, and high capacity are required (see Patent Document 1). As the capacity of capacitors increases, there is a tendency to require high withstand voltage characteristics (capacitance stability) over a long period of time, for example, in a temperature range exceeding 120°C.

JP 2014-231584 A

However, for example, in a general-purpose biaxially oriented polypropylene film, although the electrical properties are good, if the temperature range of use is 120 ° C. or higher, the heat resistance of the film itself reaches the limit level, making it difficult to handle. there were.

Therefore, the present invention has been made in view of the above problems. An object of the present invention is to propose a polyester film particularly suitable for use.

As a result of extensive studies, the present inventors have found that the film contains polyester resin (X) and polypropylene resin (Y), has a layered structure in the cross-sectional structure of the film, and has a length in the thickness direction (x) and a width direction (TD ) has a length (y) ratio (y/x) of 17 or more, it was found that the above problems can be solved, and the following invention was completed.
That is, the present invention provides the following [1] to [19].
[1] A polyester film containing a polyester resin (X) and a polypropylene resin (Y), wherein the cross-sectional structure of the film has a layered structure, and the length in the thickness direction (x) and the width direction (TD) A polyester film having a polypropylene resin layer (Yt) having a length (y) ratio (y/x) of 17 or more.
[2] The polyester film according to [1] above, wherein the polypropylene resin layer (Yt) is 0.08 pieces/μm ² or more.
[3] The polyester film according to [1] or [2] above, wherein the ratio (y/x) divided by the film thickness (t (unit: μm)) is 1.5 or more.
[4] The polyester according to any one of the above [1] to [3], wherein the polypropylene resin layer (Yt) has a continuous structure from end to end in the width direction (TD) of the observation field of 19 μm × 25 μm. the film.
[5] Having a plurality of polypropylene resin layers (Yt), having a structure in which a polyester resin layer (Xt) is interposed between the polypropylene resin layers (Yt) adjacent in the thickness direction. , the polyester film according to any one of the above [1] to [4].
[6] A polyester film containing a polyester resin (X) and a polypropylene resin (Y), wherein the apparent occupancy of the polypropylene resin (Y) in the film by FIB (focused ion beam)-SEM is 10% or more. polyester film.
[7] The polyester film according to any one of [1] to [6] 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).
[8] The polyester film according to any one of [1] to [7] above, containing 0.01 to 40 parts by mass of the compatibilizer (Z) with respect to 100 parts by mass of the polyester resin (X).
[9] The polyester film of [8] above, wherein the compatibilizer (Z) is a resin or ionomer having an acid anhydride structure [-C(=O)-OC(=O)-].
[10] The polyester film according to any one of [1] to [9] above, wherein the polycondensation catalyst of the polyester resin (X) is Ti-based.
[11] The polyester film according to any one of [1] to [10] above, wherein the polyester resin (X) is at least one selected from polyethylene terephthalate and polyethylene-2,6-naphthalate.
[12] The polyester film according to any one of [1] to [11] above, wherein a cured resin layer is provided on at least one surface of the polyester film.
[13] The polyester film according to any one of [1] to [12] above, which has a thickness of 0.5 to 12.0 μm.
[14] The polyester film according to any one of [1] to [13] above, which has a dielectric loss tangent (tan δ) at 1 kHz of 0.54 or less.
[15] A metal laminate film comprising the polyester film of any one of [1] to [14] and a metal layer provided on at least one side thereof.
[16] The polyester film according to any one of [1] to [14] above, which is used for capacitors.
[17] The metal laminate film according to [15] above, which is for a capacitor.
[18] The polyester film according to [16] above, which is for a capacitor to be mounted on an automobile.
[19] The metal laminate film according to [17] above, which is for a capacitor to be mounted on an automobile.

According to the present invention, as a polyester film for capacitors, the heat resistance is good while maintaining the same electrical properties as before, and the heat resistance that can be used even if the temperature range of the capacitor is 120 ° C. or higher. It is possible to provide a polyester film for capacitors that has properties and can be made thin.

1 is a film cross-sectional photograph (magnification: 5000 times) of a polyester film (sample film) obtained in Example 1. FIG. 1 is a film cross-sectional photograph (magnification: 5000 times) of a polyester film (sample film) obtained in Example 2. FIG. 1 is a film cross-sectional photograph (magnification: 5000 times) of a polyester film (sample film) obtained in Example 3. FIG. 1 is a film XY plane image of a polyester film (sample film) obtained in Example 1. FIG. 2 is a film XY plane image of a polyester film (sample film) obtained in Example 2. FIG. 4 is a film XY plane image of the polyester film (sample film) obtained in Example 3. FIG. FIG. 2 is a conceptual diagram showing X-, Y-, and Z-axes in FIB (focused ion beam)-SEM measurement.

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

[Polyester film]
The polyester film of the present invention (hereinafter sometimes referred to as "the present polyester film") contains a polyester resin (X) and a polypropylene resin (Y), has a layered structure in the cross-sectional structure of the film, and has a thickness of It has a polypropylene resin layer (Yt) in which the ratio (y/x) of the length (x) in the direction and the length (y) in the width direction (TD) is 17 or more.

The present polyester film has excellent physical properties such as heat resistance, flatness, optical properties and strength. The polyester film may be a single layer or a multilayer film having two or more layers with different properties (that is, laminated film).
Further, the present polyester film may be a non-stretched film (sheet) or a stretched film. Among them, stretched films uniaxially or biaxially stretched are preferred. Among them, a biaxially stretched film is more preferable from the viewpoint of balance of mechanical properties and flatness. Therefore, biaxially oriented polyester films are even more preferred.

In the case of a multilayer film, any of the layers may be the polyester film of the present invention described above (that is, a layer having a layered structure having a polypropylene resin layer (Yt)), and all layers are the polyester film is preferably For example, in the case of a multilayer film having a three-layer structure of surface layer/middle layer/surface layer, any layer may be the present polyester film, but the middle layer is preferably the present polyester film, and all layers are the present polyester film. is more preferred. Therefore, in the case of multiple layers, any layer may contain the polypropylene resin (Y) described later, and the layer containing the polypropylene resin (Y) may contain the compatibilizer (Z) as appropriate. is preferred.

<Polyester resin (X)>
The polyester resin (X), which is the main component resin of the present polyester film, may be a homopolyester or a copolymer polyester. In addition, the main component resin means the resin having the largest mass ratio among the resins constituting the polyester film, and the resin constituting the polyester film is 50% by mass or more, or 75% by mass or more, or 90% by mass. above, or 100% by mass.

The homopolyester is preferably obtained by polycondensation of an aromatic dicarboxylic acid and an aliphatic glycol. Examples of aromatic dicarboxylic acids include terephthalic acid and 2,6-naphthalene dicarboxylic acid, and examples of aliphatic glycols include ethylene glycol, diethylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, and the like. be able to.
Representative homopolyesters include polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene-2,6-naphthalate (PEN), and the like.
In the present invention, polyethylene terephthalate (PET) and polyethylene-2,6-naphthalate (PEN) are particularly preferred, and they can be used in combination.

On the other hand, when the above polyester is a copolymerized polyester, it is preferably a copolymer containing 30 mol % or less of the 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, and the like, and the glycol component includes ethylene glycol, One or more of diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol and the like can be used.
Among them, the present polyester film is polyethylene terephthalate in which 60 mol % or more, preferably 80 mol % or more is ethylene terephthalate units, or 60 mol % or more, preferably 80 mol % or more is ethylene-2,6-naphthalate units. Polyethylene-2,6-naphthalate is preferred.

(Polyester polycondensation catalyst)
Antimony compounds, germanium compounds, aluminum compounds, titanium compounds and the like can be used as polycondensation catalysts for polycondensation of the polyester. Among these, at least one of an antimony compound and a titanium compound is preferred, and it is particularly preferred to use a polyester obtained using a titanium compound.
Therefore, the polyester film preferably contains at least one of an antimony compound and a titanium compound, and more preferably contains a titanium compound (Ti-based).
By using the titanium compound, it is possible to reduce the number of metal-containing aggregates derived from the titanium compound, that is, so-called coarse foreign matter, in the film.

It is preferable to use a titanium compound as a polycondensation catalyst for the polyester that constitutes the outermost layer of the film (also referred to as a "surface layer", for example, a surface layer on which a cured resin layer described later is laminated). Preferably the layer contains a titanium compound.
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 titanium element content derived from the titanium compound in the outermost layer is preferably 3 mass ppm or more and 40 mass ppm or less, more preferably 4 mass ppm or more and 35 mass ppm or less.
If it is within the above range, it is possible to reduce the amount of foreign matter caused by the catalyst without lowering the production efficiency of polyester.
Also, from the same point of view, the content of the antimony compound in the outermost layer of the present film is preferably 100 mass ppm or less.

(particle)
Particles may be blended into the polyester resin (X) for the main purpose of imparting slipperiness and preventing scratching in each step. When particles are blended, the type of particles to be blended is not particularly limited as long as they are particles capable of imparting lubricity. Specific examples include silica, calcium carbonate, magnesium carbonate, barium carbonate, and sulfuric acid. Examples include inorganic particles such as calcium, calcium phosphate, magnesium phosphate, kaolin, aluminum oxide and titanium oxide, and organic particles such as acrylic resin, styrene resin, urea resin, phenol 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 production process of polyester can also be used.

On the other hand, the shape of the particles to be used is not particularly limited either, and any of spherical, massive, rod-like, flattened and the like may be used. Moreover, there are no particular restrictions on its hardness, specific gravity, color, and the like. Two or more types of these series of particles may be used in combination, if necessary.
The average particle diameter of the particles used is preferably 3 μm or less, more preferably 0.1 to 2 μm, and particularly preferably 0.1 to 1 μm. By using particles having an average particle size within the above range, the polyester film can be given an appropriate degree of surface roughness, and good lubricity and smoothness can be ensured.
When particles are blended, for example, a surface layer and an intermediate layer can be provided and the particles can be contained in the surface layer. In this case, it is also possible to have a multi-layer structure having a surface layer containing particles, an intermediate layer, and a surface layer containing particles in this order.

An embodiment in which the polyester resin (X) contains substantially no particles is also preferred. Here, "substantially free of particles" means that it is not intentionally contained, and specifically, the content of particles (particle concentration) is 200 mass ppm or less, more preferably 150 mass ppm It refers to ppm or less.
Since the polyester film of the present invention contains the polyester resin (X) and the polypropylene resin (Y) that are incompatible with the polyester resin (X) that constitutes the polyester, the polypropylene resin (Y) can form fine unevenness on the film surface. can. Therefore, the amount of particles in the film can be made equal to or less than the above upper limit, and it becomes easy to impart slipperiness to the film while ensuring the transparency of the film.
When the polyester film contains no particles or when the content is small, the transparency of the base film is high and a polyester film having a good appearance can be obtained, but the slipperiness may be insufficient. In such a case, it is preferable to improve the lubricity by, for example, adding particles to the cured resin layer, which will be described later.

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

The method for producing the polypropylene resin (Y) is not particularly limited, and conventionally known methods such as a method of polymerizing propylene and an optional comonomer with a Ziegler-Natta catalyst can be employed. "Polypropylene Handbook" Edward P. Moore Jr. It is also described in section 2.3.1 (pages 20-57) of Industrial Research Institute (1998) edited and edited by Tetsuo Yasuda and Nobu Sakuma.

(MFR (melt flow rate))
In the present invention, MFR is preferably in the range of 0.1 to 10 g/10 minutes, more preferably 0.5 to 8 g/10 minutes, still more preferably 1 to 8 g/10 minutes. Satisfying the above range ensures good fluidity during film molding.
The MFR is measured according to ISO 1133:1997.

(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 at least the lower limit, the desired heat resistance can be ensured. Although the upper limit of the melting point is not particularly limited, it is usually 170°C. The melting point in the invention can be determined by differential scanning calorimetry (DSC). Specifically, once the temperature is raised from room temperature to 200° C. to erase the thermal history, the temperature is lowered to 40° C. at a temperature drop rate of 10° C./min, and then again at a temperature rise rate of 10° C./min. It is defined as the temperature of the endothermic peak top when measured by

(Combination amount)
The blending amount of the polypropylene resin (Y) is preferably 1 to 30 parts by mass with respect to 100 parts by mass of the polyester resin (X). More preferably, it is 5 to 20 parts by mass, especially 5 to 15 parts by mass.
By satisfying the above range, the polyester film for capacitors can have good electrical properties.
In addition, when this polyester film is a multilayer film, the said compounding quantity is a compounding quantity in the whole this polyester film. Therefore, for example, even in the case of a multilayer polyester film having layers with different polypropylene resin blending amounts, it refers to the blending amount of the polypropylene resin as the entire laminated film.
However, if only a part of the layer has a polypropylene resin (Y) and only a part of the layer has a polypropylene resin layer (Yt), and the layer has a layered structure, The blending amount of the polypropylene resin (Y) may be as described above, and the total blending amount does not necessarily have to be within the above range.

<Compatibilizer (Z)>
The polyester film of the present invention preferably contains a compatibilizer (Z). The compatibilizer (Z) is preferably a resin or ionomer having an acid anhydride structure.

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

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, and itaconic anhydride. Maleic anhydride is preferred.

If the acid value of the resin having the acid anhydride structure [-C(=O)-OC(=O)-] is too small, the interlayer adhesion between the resins may not be sufficiently exhibited. On the other hand, if the acid value is too high, the compatibility with the resin may decrease, making it difficult to keep the maximum diameter of voids within a predetermined range. Therefore, the acid value is preferably 0.5-50, more preferably 0.5-10. Also, MFR is preferably 2 to 20 g/10 min, more preferably 3 to 10 g/10 min.
In addition, the value measured based on JISK0070 is employ|adopted for an acid value.

A commercial product can also be used as resin which has an acid anhydride structure. For example, when a modified polypropylene resin is used, ADMER (trade name, manufactured by Mitsui Chemicals, Inc.), OREVAC (trade name, manufactured by Arkema), or the like can be used. When a modified polyethylene resin is used, ADMER (trade name, manufactured by Mitsui Chemicals, Inc.) LF128 (MFR2.7) or the like can be used.
Further, when using a styrenic thermoplastic resin modified with maleic anhydride, for example, Asahi Kasei Co., Ltd. "Toughprene 912", Kraton Polymer Japan Co., Ltd. "FG1901", "FG1924", Asahi Kasei Co., Ltd. "Tuftec M1911", "Tuftec M1913", "Tuftec M1943" and the like.

(Ionomer)
Ionomers are random, block, or graft copolymers of ethylene and acidic vinyl monomers such as unsaturated carboxylic acids, which are partially neutralized metal salts. Examples thereof include ionomer resins obtained by neutralizing at least part of the carboxyl groups of an ethylene/unsaturated carboxylic acid copolymer resin with metal ions.

A metal ion obtained by neutralizing 10 mol % or more, preferably 10 to 90 mol %, more preferably 15 to 80 mol % of the carboxy groups of the ethylene/unsaturated carboxylic acid copolymer resin is preferably used. Examples of the metal ions include alkali metal ions such as lithium and sodium, zinc, and polyvalent metal ions such as alkaline earth metals such as magnesium and calcium. Among them, polyvalent metal ions such as zinc are preferable because they have a good compatibilizing effect.

(Combination amount)
The amount of the compatibilizing agent (Z) to be blended is preferably 0.01 to 40 parts by mass with respect to 100 parts by mass of the polyester resin (X). More preferably, it is 0.1 to 20 parts by mass, and particularly preferably 1 to 20 parts by mass.
By satisfying the above range, the compatibility between the polyester resin (X) and the polypropylene resin (Y) is improved, and a polyester film having good electrical properties can be obtained as a polyester film for capacitors.
In addition, when this polyester film is a multilayer film, the said compounding quantity is a compounding quantity in the whole this polyester film. Therefore, for example, even in the case of a multilayer polyester film having layers with different amounts of compatibilizing agents, it refers to the amount of compatibilizing agents in the laminated film as a whole.
However, when only a part of the layer contains the polypropylene resin (Y) and only the part of the layer is a layer having a layered structure, the part of the layer contains the compatibilizer (Z) It is preferable to contain The amount of the compatibilizing agent (Z) to be blended in the layer may be as described above, and the overall blending amount does not necessarily have to be within the above range.

<Cured resin layer>
The polyester film of the present invention preferably has a cured resin layer on at least one surface. The cured resin layer may be provided on at least one side of the polyester film, and may be provided on both sides.
The cured resin layer preferably contains a fluorine-containing compound. By providing a cured resin layer containing a fluorine-containing compound, the charge potential per thickness is improved compared to the polyester film alone, making it possible to further suppress the attenuation of the charge potential, and dust adhering to the polyester film surface. and oil 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 cross-linking 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 charge potential per thickness and further suppress the attenuation of the charge potential. The cured resin layer is prevented from being scratched, the solvent resistance is improved, and the film formability and transparency of the cured resin layer are improved.

(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 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., in which some or all of the hydrogen atoms are substituted with fluorine atoms, and other monomers are polymerized. Fluorine-based copolymer resin; fluorine-based copolymer resin obtained by graft-polymerizing a hydroxyl group-containing fluororesin copolymer with a (meth)acrylic acid ester compound or other monomer; vinyl having a perfluoroalkyl group A polymer etc. are mentioned. Among them, fluororesins containing fluoroolefin-based copolymer resins and polyalkylene ether groups in which some or all of the hydrogen atoms are fluorinated, from the viewpoint of being excellent in improving the charging potential and being excellent in wiping off dust and oil. Copolymers are preferred.

Vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene are preferable among the monomers of the fluoroolefin copolymer resin. 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 viewpoint of dispersibility in solvents and compatibility with other resins. Other components will be described later.

Examples of fluorine-based copolymer resins obtained by polymerizing polyalkylene ether in which some or all of the hydrogen atoms are substituted with fluorine atoms and other monomers include urethane resins and polyfluoroalkylene ethers having polyfluoroalkylene ether groups. Examples include polyester resins having a group and acrylic resins having a polyfluoroalkylene ether group.
Other monomers constituting the urethane resin having a polyfluoroalkylene ether group include isocyanate compounds. Isocyanate compounds include aromatic diisocyanates such as tolylene diisocyanate, xylylene diisocyanate, methylenediphenyl diisocyanate, phenylene diisocyanate, naphthalene diisocyanate, and tolidine diisocyanate; aromatic rings such as α, α, α', α'-tetramethylxylylene diisocyanate. aliphatic diisocyanates such as methylene diisocyanate, propylene diisocyanate, lysine diisocyanate, trimethylhexamethylene diisocyanate and hexamethylene diisocyanate; Examples include cyclic diisocyanates.

Other monomers constituting the urethane resin include fluorine-free polyols, dimethylolpropanoic acid, dimethylolbutanoic acid, bis-(2-hydroxyethyl)propionic acid, bis-(2-hydroxyethyl)butane. Examples thereof include polyols having a carboxyl group such as acids. Among these, from the viewpoint of self-emulsification when water is used as a dispersion solvent, it is preferable to contain a polyol having a carboxyl group, and among these, dimethylolpropanoic acid is more preferable.

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 composition is preferably 5 to 100% by mass, more preferably 20 to 98% by mass, still more preferably 45 to 95% by mass, based on the non-volatile components in the cured resin layer composition. % range. By making it 5% by mass or more, it is possible to improve the electrification potential per thickness, to further suppress the attenuation of the electrification potential, and to easily remove dust and oil adhering to the polyester film surface. Become.

(Crosslinking agent (B))
The cured resin layer composition for forming the cured resin layer preferably contains the cross-linking agent (B) as described above. By including a cross-linking agent in the cured resin layer composition, a dense cured resin layer having a high cross-linking density can be formed. In addition, the cured resin layer can be prevented from being damaged, and the solvent resistance can be easily improved. The cross-linking agent is not particularly limited, and conventionally known cross-linking agents can be used.
Examples of cross-linking agents include melamine compounds, oxazoline compounds, epoxy compounds, isocyanate compounds, carbodiimide compounds, silane coupling compounds, and the like. Among these, the cross-linking agent is preferably at least one selected from melamine compounds, oxazoline compounds and isocyanate compounds, and is preferably a melamine compound from the viewpoint of curability. Moreover, these crosslinking agents may be used individually by 1 type, and may use 2 or more types together. In addition, any polymerizable monomer may be contained in the cured resin layer composition as a component that cures together with these cross-linking agents.

(melamine compound)
The melamine compound used as a cross-linking agent is a compound having a melamine skeleton in the compound. For example, an alkylolated melamine derivative or an alkylolated melamine derivative is partially or completely etherified by reacting it with an alcohol. compounds, and mixtures thereof can be used. Examples of alkylolation include methylolation, ethylolation, isopropylolation, n-butyrolation, and isobutyrolation. Among these, methylolation is preferable from the viewpoint of reactivity. As the alcohol used for etherification, methyl alcohol, ethyl alcohol, isopropyl alcohol, n-butanol, isobutanol and the like are preferably used. 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, it is preferably a partially or completely etherified alkylolated melamine derivative, and methyl alcohol is an ether. It is more preferable that it is a modified alkylol. The partially etherified alkylol group is preferably 0.5 to 5 equivalents, more preferably 0.7 to 5 equivalents, relative to the non-etherified alkylol group. The melamine compound may be a monomer, a dimer or higher polymer, or a mixture thereof. Further, a part of melamine co-condensed with urea or the like may be used.

In order to increase the reactivity of the melamine compound, the cured resin layer composition may contain a cross-linking 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 salts thereof, Examples include 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 p-toluenesulfonic acid are preferred, and amine compounds and salts of amine compounds are more preferred.

(oxazoline compound)
The oxazoline compound is a compound having an oxazoline group in the molecule, and a polymer containing an oxazoline group is particularly preferable. The polymer can be prepared by polymerization of an addition polymerizable oxazoline group-containing monomer alone or with another monomer. Among them, 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 preferable, 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.
Addition polymerizable oxazoline group-containing monomers 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 and the like can be mentioned, and one or a mixture of two or more thereof can be used. Among these, 2-isopropenyl-2-oxazoline is suitable because it is easily available industrially. The addition polymerizable oxazoline group-containing monomer may be used alone, or two or more monomers may be used.

Other monomers are not limited as long as they are copolymerizable with addition-polymerizable oxazoline group-containing monomers. , n-butyl group, isobutyl group, t-butyl group, 2-ethylhexyl group, cyclohexyl group); acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, Unsaturated carboxylic acids such as styrenesulfonic acid and salts thereof (salts such as sodium salt, potassium salt, ammonium salt, tertiary amine salt); unsaturated nitriles such as acrylonitrile and methacrylonitrile; (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 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; α,β-unsaturated aromatic monomers such as styrene and α-methylstyrene;

As other monomers, monomers having polyalkylene oxide chains can also be used. Preferred examples of the monomer having a polyalkylene oxide chain include esters obtained by adding polyalkylene oxide to the carboxy group of unsaturated carboxylic acids such as acrylic acid and methacrylic acid. Here, as the polyalkylene oxide chain, for example, polymethylene oxide, polyethylene oxide, polypropylene oxide, polybutylene oxide and the like can be preferably exemplified. The number of repeating units of the polyalkylene oxide chain may range from 3 to 100, for example.
Other monomers used in the oxazoline compound may be used singly or in combination of two or more.

The oxazoline group content of 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, particularly preferably 4 to 6 mmol/g. By using it in the above range, the durability of the coating film (cured resin layer) is likely to be improved.

(epoxy compound)
An epoxy compound is a compound having an epoxy group in the molecule. mentioned. Epoxy compounds include polyepoxy compounds, diepoxy compounds, monoepoxy compounds, glycidylamine compounds, and the like.
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, trimethylolpropane. Polyglycidyl ethers are mentioned.
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. glycidyl ether, polytetramethylene glycol diglycidyl ether;
Examples of monoepoxy compounds include allyl glycidyl ether, 2-ethylhexyl glycidyl ether, and phenyl glycidyl ether.
Glycidylamine compounds include N,N,N',N'-tetraglycidyl-m-xylylenediamine, 1,3-bis(N,N-diglycidylamino)cyclohexane, and the like. An epoxy compound may be used individually by 1 type, and may use 2 or more types together.

(Isocyanate compound)
The isocyanate-based compound is a compound having an isocyanate derivative structure represented by isocyanate or blocked isocyanate. Examples of isocyanates include aromatic isocyanates such as tolylene diisocyanate, xylylene diisocyanate, methylene diphenyl diisocyanate, phenylene diisocyanate, and naphthalene diisocyanate; Aliphatic isocyanates; aliphatic isocyanates such as methylene diisocyanate, propylene diisocyanate, lysine diisocyanate, trimethylhexamethylene diisocyanate, and hexamethylene diisocyanate; and the like are exemplified. Polymers and derivatives of these isocyanates such as burettes, isocyanurates and uretdione compounds are also included. These isocyanates may be used alone or in combination of two or more. Among the above isocyanates, aliphatic isocyanates or alicyclic isocyanates are more preferable in order to avoid yellowing due to ultraviolet rays.

When used in the state of blocked isocyanate, the blocking agent includes, for example, bisulfites such as sodium bisulfite, phenolic compounds such as phenol, cresol, and ethylphenol; propylene glycol monomethyl ether, ethylene glycol, benzyl alcohol, methanol, alcohol compounds such as 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; ε-caprolactam, δ-valerolactam lactam compounds such as diphenylaniline, aniline, amine compounds such as ethyleneimine; acetanilide, acid amide compounds of acetic acid amide; These may be used alone or in combination of two or more.

Also, 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 of various polymers. From the viewpoint of improving the dispersibility and crosslinkability of the isocyanate compound, it is preferable to use a mixture or combination with a polyester resin or a urethane resin. An isocyanate compound may be used individually by 1 type, and may use 2 or more types. The amount of the cross-linking agent when the isocyanate-based compound is used is the amount including the blocking agent and the polymer that is combined or mixed.

(carbodiimide compound)
A carbodiimide-based 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. A carbodiimide-based compound can be synthesized by a conventionally known technique, and generally a condensation reaction of a diisocyanate compound is used. The diisocyanate compound is not particularly limited, and both aromatic and aliphatic compounds can be used. methylene diisocyanate, trimethylhexamethylene diisocyanate, cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, dicyclohexyl diisocyanate, dicyclohexylmethane diisocyanate and the like. A carbodiimide type compound may be used individually by 1 type, and may use 2 or more types.

(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, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, 2-(3,4- epoxy group-containing compounds such as ethyltrimethoxysilane; vinyl group-containing compounds such as vinyltrimethoxysilane and vinyltriethoxysilane; styryl group-containing compounds such as p-styryltrimethoxysilane and p-styryltriethoxysilane; 3-(meth)acryloxypropyltrimethoxysilane, 3-(meth)acryloxypropyltriethoxysilane, 3-(meth)acryloxypropylmethyldimethoxysilane, 3-(meth)acryloxypropylmethyldiethoxysilane, etc. (Meth) acrylic group-containing compounds; 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 Amino group-containing compounds such as -(1,3-dimethylbutylidene)propylamine, N-phenyl-3-aminopropyltrimethoxysilane, N-phenyl-3-aminopropyltriethoxysilane; tris(trimethoxysilylpropyl) Isocyanurate, isocyanurate group-containing compounds such as tris(triethoxysilylpropyl) isocyanurate; and mercapto group-containing compounds such as ethoxysilane.
Among the above compounds, from the viewpoint of maintaining the strength of the cured resin layer, epoxy group-containing silane coupling agents, double bond-containing silane coupling agents such as vinyl groups and (meth)acrylic groups, and amino group-containing silane coupling agents are used. more preferred.
A silane coupling agent may be used individually by 1 type, and may use 2 or more types together.

The cross-linking agent contained in the cured resin layer composition is preferably designed so as to be reacted during the drying process or film-forming process for forming the cured resin layer to improve the performance of the cured resin layer. It can be assumed that unreacted products of these cross-linking agents, compounds after reaction, or mixtures thereof are present in the cured resin layer formed from the cured resin layer composition.

The content of the cross-linking 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 cross-linking agent to 5 to 60% by mass with respect to the non-volatile components in the cured resin layer composition, it becomes easier to suppress the attenuation of the charging potential. In addition, the strength of the cured resin layer is improved, and scratch resistance and the like are easily improved. From the above viewpoints, it is more preferably 10 to 50% by mass, still more preferably 15 to 40% by mass, and particularly preferably 20 to 40% by mass.

Further, when the cured resin layer composition contains a cross-linking catalyst, the content of the cross-linking catalyst is in the range of 0.4 to 10% by mass with respect to the non-volatile components in the cured resin layer composition. It is preferable because the strength of the resin layer is improved and the scratch resistance tends to be improved. From the above viewpoints, the content of the cross-linking catalyst is preferably 0.6 to 8% by mass, more preferably 0.8 to 5% by mass.

(Binder resin (C))
The binder resin is a polymer component contained in the cured resin layer composition other than the polymer obtained by cross-linking the cross-linking agent (B). By including a binder resin in the cured resin layer composition, the film formability, transparency, etc. of the cured resin layer are improved.

Specific examples of the binder resin include acrylic resin, polyvinyl alcohol, polyester resin, urethane resin, polyalkylene glycol, polyalkyleneimine, methylcellulose, hydroxycellulose, starches and the like. Among these, acrylic resins, polyester resins, and urethane resins are preferable from the viewpoint of coatability, etc., and acrylic resins and polyester resins are more preferable from the viewpoint of improving the durability of the cured resin layer itself. From the point of view, acrylic resin is more preferable.

(acrylic resin)
Acrylic resins are polymers composed of polymerizable monomers including acrylic and methacrylic monomers. These may be homopolymers, copolymers, or copolymers with polymerizable monomers other than acrylic and methacrylic monomers.
Copolymers of these polymers with other polymers (eg, polyester, polyurethane, etc.) are also included. Examples are block copolymers and graft copolymers. That is, the acrylic resin may be an acrylic-modified polyester resin or an acrylic-modified polyurethane resin.
Furthermore, polymers obtained by polymerizing polymerizable monomers in polyester solutions or polyester dispersions (sometimes mixtures of polymers) are also included. Also included are polymers obtained by polymerizing polymerizable monomers in polyurethane solutions and polyurethane dispersions (mixtures of polymers in some cases). Similarly, other polymer solutions or polymers obtained by polymerizing polymerizable monomers in dispersions (polymer mixtures in some cases) are also included, and these are also referred to herein as acrylic-modified polyester resins and acrylic-modified Polyurethane resin is used. The above polyester and polyurethane used in the acrylic resin can be appropriately selected and used from those exemplified as the polyester and polyurethane used in the binder resin described later.
Moreover, the acrylic resin may contain a hydroxyl group and an amino group in order to further improve adhesion to the polyester film.

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

(polyester resin)
Examples of polyester resins include those composed of polyvalent carboxylic acids and polyvalent hydroxy compounds as shown below as main constituents. That is, polyvalent carboxylic 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, trimellitic acid monopotassium salts and ester-forming derivatives thereof can be used. Examples of polyhydroxy compounds 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, dimethylolpropionic acid Potassium or the like can be used. One or more of these compounds may be appropriately selected, and a polyester resin may be synthesized by a conventional polycondensation reaction. Moreover, the polyester resin may be in the form of an aqueous dispersion, in which case a hydrophilic functional group or the like may be appropriately introduced into the polyester resin.

(polyvinyl alcohol)
Polyvinyl alcohol is a compound having a polyvinyl alcohol moiety. For example, conventionally known polyvinyl alcohols including partially acetalized or butyralized modified compounds can be used. Although the degree of polymerization of polyvinyl alcohol is not particularly limited, it 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 the water resistance of the cured resin layer from deteriorating. The degree of saponification of polyvinyl alcohol is not particularly limited, but is usually 70 mol% or more, preferably in the range of 70 to 99.9 mol%, more preferably 80 to 97 mol%, particularly preferably 86 to 99.9 mol%. 95 mol % saponified polyvinyl acetate is practically used.

(urethane resin)
A urethane resin is a polymer compound having a urethane bond in its molecule. Urethane resins are usually produced by the reaction of polyols and isocyanates. Polyols include polycarbonate polyols, polyester polyols, polyether polyols, polyolefin polyols, and acrylic polyols, and these compounds may be used singly or in combination. The urethane resin may be an aqueous dispersion, in which case a hydrophilic functional group may be appropriately introduced into the polyol, for example.

The content of the binder resin in the cured resin layer composition is preferably in the range of 10 to 70% by mass based on 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 coating properties and attenuation of charging potential. In addition, 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.
Additives such as a reaction modifier, an adhesion enhancer, a surfactant, an antistatic agent, and particles may be added to the cured resin layer composition as appropriate in addition to the components described above.

The cured resin layer composition is preferably applied to the polyester film as a liquid coating solution, dried and cured as necessary. The cured resin layer composition is preferably diluted with a solvent to form a coating liquid. Each of the components (components (A) to (C), etc.) constituting the cured resin composition may be dissolved in a solvent, or may be dispersed in the solvent.
The solvent used for the cured resin layer composition is not particularly limited, and either water or an organic solvent may be used. From the viewpoint of environmental protection, however, it is preferable to use an aqueous coating liquid using water as a solvent. The aqueous coating liquid may contain a small amount of organic solvent. The specific amount of the organic solvent should be less than water on a mass basis, for example less than 30% by mass, preferably less than 20% by mass, more preferably less than 10% by mass in the solvent.
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; ethyl acetate. and amines such as dimethylethanolamine. These can be used singly or in combination. By appropriately selecting and including these organic solvents in the aqueous coating liquid as necessary, the stability and coatability of the coating liquid can sometimes be improved.

When an organic solvent alone is used as the solvent, the organic solvent includes 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; ethers such as diisopropyl ether and dibutyl ether. These may be used singly or in combination of two or more in consideration of solubility, coatability, boiling point, and the like.

<Method for forming cured resin layer>
A method for forming the cured resin layer will be described in detail below. 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 a cured resin layer composition to the surface of a polyester film on a production line for producing the polyester film. Off-line coating is a method of applying a coating liquid outside the system (outside the production line) on a once produced polyester film. The cured resin layer is preferably formed by in-line coating from the viewpoint of ease of processing.

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

Examples of methods 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, and cast coating. , spray coating, curtain coating, calendar coating, extrusion coating and the like can be used.

Although not particularly limited, for example, in sequential biaxial stretching, a method of applying a coating liquid to a uniaxially stretched film stretched in the longitudinal direction (vertical direction) and then stretching in the transverse direction is preferable. According to such a method, the film formation of the polyester film and the formation of the cured resin layer can be performed at the same time, which is advantageous in terms of manufacturing cost. can also be varied by, making thin film coating easier to perform than off-line coating. Furthermore, the thickness of the cured resin layer can be made more uniform.

In addition, by applying the 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 firmly adhering the cured resin layer to the polyester film. can.
In the production of biaxially stretched film, the polyester film can be restrained in the vertical and horizontal directions by holding the ends of the film with a clip or the like during the stretching process. It can be heated to a high temperature while maintaining flatness. Therefore, the heat treatment applied after applying the cured resin layer composition can be performed at a high temperature that cannot be achieved by other methods, so that the cured resin layer and the polyester film can be adhered more firmly.

Further, the coating liquid of the cured resin layer composition applied to the polyester film may be subjected to either one of heat treatment and active energy ray irradiation such as ultraviolet irradiation in either offline coating or in-line coating, Although both may be used together, it is preferable to perform at least heat treatment. Further, the cured 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 by, for example, a heat setting process as described above, but may be performed by other methods. In addition, when the coating liquid 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.
In addition, in order to improve the coatability of the coating liquid for forming the cured resin layer to the polyester film and the adhesiveness of the cured resin layer to the polyester film, a cured resin layer of the polyester film is formed before the coating liquid is applied. Surface treatment such as chemical treatment, corona discharge treatment, plasma treatment, ozone treatment, chemical treatment, and solvent treatment may be applied to the surface.

The polyester film with a cured resin layer may have another layer between the polyester film and the cured resin layer, as described above. Examples of the other layer include layers having various functions such as an antistatic layer, an easy adhesion layer, and an oligomer sealing layer.

The thickness of the cured resin layer is preferably 0.005 to 1 μm. When the thickness is 1 μm or less, the components constituting the cured resin layer are prevented from migrating to the cured resin layer and the like, and blocking during winding can be easily prevented. On the other hand, when the thickness is 0.005 μm or more, it becomes easier to suppress the attenuation of the charging potential.
From these viewpoints, the thickness of the cured resin layer is more preferably 0.01 μm or more, more preferably 0.02 μm or more, more preferably 0.2 μm or less, further preferably 0.1 μm or less, and particularly 0.06 μm. The following are good.

<Metal laminated film>
The polyester film of the present invention can be a metal-laminated film having a metal layer on at least one side thereof. The metal layer may be provided on one side or both sides of the polyester film. Alternatively, it may be provided on the cured resin layer described above.
Examples of metals include copper, silver, chromium, aluminum, nickel, and zinc. Among these, aluminum and zinc are preferred from the viewpoint 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, even more preferably in the range of 100 to 2000 Å. If it is within the above range, it is advantageous in terms of electrical properties.

<Method for producing the present polyester film>
As an example of the method for producing the present polyester film, a method for producing the present polyester film 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.

First, by a known method, raw materials such as polyester chips are supplied to a melt extruder, heated to the melting point of each polymer or higher, the molten polymer is extruded from a die, and cooled to a temperature below the glass transition point of the polymer on a rotating cooling drum. It may be cooled and solidified so as to obtain a non-oriented sheet in a substantially amorphous state.

Next, the unoriented sheet is stretched in one direction by a roll or tenter type stretching machine. 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, the film is stretched in a direction orthogonal 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 stretching in one direction in two or more stages can also be employed.
Further, in the present invention, as described above, it is preferable to increase the draw ratio in the horizontal direction. Therefore, it is preferable to set the draw ratio to 4.5 times or more in either the first stage or the second stage.

After the stretching, the polyester film can be obtained as a biaxially oriented film by heat setting at a temperature of 130 to 270° C. under tension or under relaxation of 30% or less.
The present polyester film can be improved in heat resistance and the like by performing a heat setting treatment.
In the above description, the production method is described assuming a single-layer film, but in the case of a multi-layer film, it is preferable to prepare a non-oriented sheet by, for example, co-extrusion, and then carry out the same process.

(film thickness)
The thickness of the polyester film of the present invention is preferably 0.5 to 12.0 μm. A thickness within this range is suitable for use in capacitors. From the above viewpoints, the thickness of the present polyester film is more preferably 0.5 to 10.0 μm, especially 1.0 to 8.0 μm.

<Characteristics of this polyester film>
(Cross-sectional structure of film)
The polyester film of the present invention has a cross-sectional structure having a layered structure, and a polypropylene having a ratio (y/x) of the length (x) in the thickness direction and the length (y) in the width direction (TD) of 17 or more. It has a resin layer (Yt).
Preferably, in addition to the above, at least one of the following additional requirements 1) to 4) should be satisfied at the same time.
1) The number of polypropylene resin layers (Yt) in which the ratio (y/x) of the length (x) in the thickness direction and the length (y) in the width direction (TD) is 17 or more is 0.08/μm ² It should be more than that. It is more preferably 0.16 pieces/μm ² or more, and most preferably 0.20 pieces/μm ² or more.
2) The value obtained by dividing the ratio (y/x) by the film thickness (t) is preferably 1.5 or more, more preferably 2.0 or more, still more preferably 4.0 or more, and especially 5.0. That's all.
3) It is preferable that the polypropylene resin layer (Yt) has a continuous structure from end to end in the width direction (TD) of the observation field. The size of the observation field is 19 μm×25 μm.
4) It is preferable to have a layered structure in which there are a plurality of polypropylene resin layers (Yt) and a polyester resin layer (Xt) is interposed between the polypropylene resin layers (Yt) adjacent in the thickness direction. .
By satisfying at least one or more, preferably two or more, more preferably three or more, and most preferably all of the above 1) to 4) at the same time, it is possible to obtain a polyester film having a good dielectric loss tangent. .

Specific means for forming the above structure include, for example, a) taking a sufficient kneading time at the time of blending the raw materials, and b) considering the stretching conditions, the stretching ratio, especially the lateral stretching ratio, should be 4.5 times or more. c) selecting a type of polypropylene resin having an MFR of 0.1 to 10 g/10 min or less; and d) using a compatibilizer in combination.
These techniques may be used singly or in combination.

In the present invention, the thickness of the film is different from usual, and the ultra-thin region (0.5 to 12 μm) is the mainstream in order to correspond to the use of capacitors. Therefore, it is different from intentionally forming voids by using polypropylene resin, which is incompatible with polyester resin, as in other applications (for example, for the purpose of imparting cushioning properties in substitute for paper). different.
The reason for using the polypropylene resin is that it is used as a film for capacitors with the intention of improving electrical properties, particularly in the low frequency range. From this point of view, after the film molding process, from the design concept that it is better to avoid forming voids derived from the polypropylene resin in the film as much as possible to improve the electrical characteristics, as described above, the cross-sectional structure and electrical characteristics (dielectric The present invention was completed by paying attention to the relationship between the tangent and the tangent.

Compared to Comparative Example 1, Examples 1 to 3 in which a compatibilizer having a specific structure was used in combination had good compatibility despite having a high polypropylene content, and the film had a flat layered structure in cross section. I found out. Moreover, the layered structure was derived from the polypropylene resin (Y), and was confirmed in large numbers in the observation field region. Regarding the relationship with electrical properties (dielectric loss tangent), there was a tendency for the electrical properties to be better as the number of layers in the layered structure increased in the thickness direction of the film.
In addition, although the mechanism for forming the layered structure is unknown, it is speculated that force is applied from above and below in the thickness direction of the film when stretching a film having a thickness in the ultra-thin region (0.5 to 12 μm). . A synergistic effect with the compatibilizer is also added by the force applied from above and below in the thickness direction of the film, and as a result of improving the adhesion between the polyester resin layer (Xt) and the polypropylene resin layer (Yt), for example, at the interface between the two It is presumed that by pushing out the existing air, a structure in which it is difficult to form voids is obtained.

(Apparent occupancy of polypropylene resin in film (%))
The apparent occupancy of the polypropylene resin (Y) in the film by FIB (focused ion beam)-SEM is preferably 10% or more, more preferably 20% or more, and particularly preferably 30% or more.
The purpose of this method is to continuously take 200 two-dimensional SEM images, connect them, and visualize them three-dimensionally. As a result, it has the advantage of being able to three-dimensionally visualize the degree of dispersion of the polypropylene resin (Y) in the film, ie, the degree of dispersion in the film.
In the conceptual diagram shown in FIG. 7, the present inventors have found that even if the cross-sectional structure (XZ plane) is at the same level, the dispersibility in the depth (XY plane) does not differ with respect to the dispersibility of the polypropylene resin (Y). Verification was performed by assuming that It has also been found that the electrical properties (dielectric loss tangent) are further improved by satisfying the above apparent occupation rate of the polypropylene resin (Y).

(Dielectric loss tangent (tan δ))
The polyester film of the present invention preferably has a tan δ at 1 kHz of 0.54 or less, more preferably 0.50 or less, still more preferably 0.45 or less, especially 0.40 or less.
When tan δ satisfies the above range, the film has good electrical properties and is suitable for capacitors.

<Application>
The polyester film and metal laminate film of the present invention have excellent electrical properties such as a low dielectric loss tangent (tan δ), and are therefore useful for capacitors. In particular, since it is in the form of a thin film, it is useful for capacitors mounted in automobiles such as hybrid automobiles and electric automobiles, where miniaturization, weight reduction, and high capacity are required.

<Explanation of terms, etc.>
In the present invention, the term "film" includes the "sheet", and the term "sheet" includes the "film".
In the present invention, when "X to Y" (X and Y are arbitrary numbers), unless otherwise specified, it means "X or more and Y or less" and "preferably larger than X" or "preferably larger than Y". It also includes the meaning of "small".
In addition, when described as "X or more" (X is any number), unless otherwise specified, it includes the meaning of "preferably greater than X", and when described as "Y or less" (Y is any number) , also includes the meaning of "preferably smaller than Y" unless otherwise specified.

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.

<Evaluation method>
Methods for measuring and evaluating various physical properties and characteristics are as follows.

(1) Intrinsic viscosity (IV)
1 g of polyester was precisely weighed, dissolved in 100 ml of a mixed solvent of phenol/tetrachloroethane=50/50 (mass ratio), and measured at 30.degree.

(2) Film thickness A piece of film was fixed and molded with an epoxy resin, cut with a microtome, and the cross section of the film was observed with a transmission electron microscope. Among the cross sections, two interfaces are observed by light and shade almost parallel to the film surface. The distance between the two interfaces and the film surface was measured from 10 photographs, and the average value was taken as the film thickness.

(3) Melting point of polypropylene resin (Y) By differential scanning calorimetry (DSC), the temperature was raised from room temperature to 200 ° C. After erasing the heat history, the temperature was lowered to 40 ° C. at a temperature drop rate of 10 ° C./min. defined as the endothermic peak top temperature measured again at a heating rate of 10° C./min.

(4) Ratio (y/x) of the length (x) in the thickness direction and the length (y) in the width direction (TD) of the polypropylene resin layer (Yt)
The cross-sectional structure of the sample film was observed with an SEM (manufactured by Hitachi High-Tech Co., model: FE-SEM SU8220) (observation field: 19 μm×25 μm, magnification: 5000 times). After that, using the photographed cross-sectional image, the ratio (y/x) of the length (x) in the thickness direction and the length (y) in the width direction (TD) of the polypropylene resin layer (Yt) was measured. In addition, the number (pieces/μm ² ) of polypropylene resin layers (Yt) having a ratio (y/x) of 17 or more in the observation field was determined.

(5) FIB (focused ion beam)-Apparent occupancy of polypropylene resin (Y) in the film by SEM For each sample, XZ layer (cross section) data shown in FIG. Acquired. An XY image (image viewed from directly above) was reconstructed from the above data using ImageJ, and then the polypropylene resin (Y) area within the film plane was calculated using WinRooF2018 software.
FIB-SEM measurement conditions Equipment: Thermo Fisher Scientific, Helios G4 UX
Accelerating voltage: FIB 30 kV, SEM 3 kV
Observation image: Backscattered electron image Observation field: 50 μm×7 μm
Display software: Thermo Fisher Scientific, Avizo
Image analysis software: ImageJ

(6) Dielectric loss tangent (tan δ)
A sample in which Al was vapor-deposited in a circle on both sides of the sample film was placed on a device (model: 4284A manufactured by HP (HEWLETT PACKARD)), electrodes were brought into contact from above and below, and tan δ was measured when the current frequency was set to 1 kHz. did.
Specifically, power loss occurs when an AC voltage is applied to a capacitor. A smaller value of tan δ indicates a better capacitor.

The raw materials of the polyester film in each example and comparative example are as follows.
[Polyester film]
(a) Polyester resin (X1): A polyethylene terephthalate homopolymer (polycondensation catalyst; antimony) having an intrinsic viscosity of 0.63.
(b) Polyester resin (X2): A polyethylene terephthalate homopolymer (polycondensation catalyst: antimony) containing 0.5% by mass of silica particles having an average particle size of 0.8 µm and having an intrinsic viscosity of 0.65.
(c) polypropylene resin (Y): melting point 163°C, MFR = 7.5 (manufactured by Sumitomo Chemical Co., Ltd.: FLX80E4)
(d) Resin 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.)
(e) Resin having an acid anhydride structure (Z2): maleic anhydride-modified polystyrene resin (Tuftec M1943, melt flow rate; 8.0 g/10 min, manufactured by Asahi Kasei Corporation)

[Example 1]
A mixed raw material obtained by mixing polyester X1, polypropylene Y, and Z1 as a compatibilizer at a ratio of 87.5% by mass, 10% by mass, and 2.5% by mass, respectively, is supplied to an extruder and melted at 285 ° C., It was extruded onto a cooling roll set at 25° C. and solidified by cooling to obtain an unstretched sheet. Then, the film was stretched 3.2 times in the machine direction (MD) at a film temperature of 85° C. using a roll peripheral speed difference. This longitudinally stretched film was guided to a tenter and stretched in the transverse direction (TD) at 100° C. by a factor of 4.9. After heat treatment at 220° C., the polyester film was relaxed by 0.5% in the transverse direction and had a thickness of 6.9 μm.
FIG. 1 shows a cross-sectional photograph (magnification: 5000 times) of the polyester film (sample film) obtained in Example 1. As shown in FIG. In addition, Table 1 shows the physical property values measured by the above method.

[Examples 2 and 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. 2 and 3 show cross-sectional photographs (magnification: 5000 times) of the polyester films (sample films) obtained in Examples 2 and 3, respectively. Tables 1 and 2 show the physical property values measured by the above method for Examples 2 and 3 and Comparative Example 1.

Compared to Comparative Example 1, Examples 1 to 3, in which a compatibilizer having a specific structure was used in combination, had good compatibility despite the fact that the polypropylene content was high, and the film had a flat layered structure with a cross-sectional structure. was found to have Moreover, the layered structure was derived from the polypropylene resin (Y), and was confirmed in large numbers in the observation field region. Regarding the relationship with electrical properties (dielectric loss tangent), the more the number of layers in the layered structure is present in the thickness direction of the film, the better the electrical properties tend to be.
In addition, although the mechanism for forming the layered structure is unknown, when stretching a film having a thickness in the ultrathin region (0.5 to 12 μm), especially at a horizontal stretching ratio of 4.5 times or more, It is presumed that a stronger force is applied from above and below in the thickness direction of the film.
The synergistic effect of the force applied from above and below in the thickness direction of the film and the compatibilizer is also added, and as a result of further improving the adhesion between the polyester resin layer (Xt) and the polypropylene resin layer (Yt), for example, at the interface between the two It is presumed that the extruding of the existing air results in a layered structure in which voids are less likely to form.
Furthermore, it has been found that use of a compatibilizing agent comprising a resin having an acid anhydride structure results in particularly good compatibility, and further improvement in electrical properties can be expected.
On the other hand, the apparent occupancy of the polypropylene resin (Y) in the film by FIB (focused ion beam)-SEM captures the dispersibility of the polypropylene resin (Y) in the film three-dimensionally, and how much the corners of the film It is an index that indicates the degree of dispersibility, whether it is distributed to
The present inventors verified the dispersibility of the polypropylene resin (Y) by assuming that the dispersibility in the depth (XY plane) differs even if the cross-sectional structure (XZ plane) is at the same level. . From the results of FIGS. 4 to 6 (where the black areas are polypropylene resin (Y)), it can be seen that there is a correlation between the apparent occupancy of polypropylene resin (Y) and the electrical properties (dielectric loss tangent). It was also found that the larger the value of , the better the electrical characteristics (dielectric loss tangent).

Claims (19)

A polyester film containing a polyester resin (X) and a polypropylene resin (Y), wherein the cross-sectional structure of the film has a layered structure, and the length in the thickness direction (x) and the length in the width direction (TD) ( A polyester film having a polypropylene resin layer (Yt) in which the ratio (y/x) of y) is 17 or more. The polyester film according to claim 1, wherein the polypropylene resin layer (Yt) is 0.08 pieces/μm ² or more. The polyester film according to claim 1 or 2, wherein the value obtained by dividing the ratio (y/x) by the film thickness (t (unit: µm)) is 1.5 or more. The polyester film according to any one of claims 1 to 3, wherein the polypropylene resin layer (Yt) has a continuous structure from end to end in the width direction (TD) of an observation field of 19 µm × 25 µm. A structure comprising a plurality of the polypropylene resin layers (Yt) and having a polyester resin layer (Xt) interposed between the polypropylene resin layers (Yt) adjacent in the thickness direction. The polyester film according to any one of 1 to 4. A polyester film containing a polyester resin (X) and a polypropylene resin (Y), wherein the apparent occupancy of the polypropylene resin (Y) in the film by FIB (focused ion beam)-SEM is 10% or more. The polyester film according to any one of claims 1 to 6, comprising 1 to 30 parts by mass of the polypropylene resin (Y) with respect to 100 parts by mass of the polyester resin (X). The polyester film according to any one of claims 1 to 7, comprising 0.01 to 40 parts by mass of the compatibilizer (Z) with respect to 100 parts by mass of the polyester resin (X). The polyester film according to claim 8, wherein the compatibilizer (Z) is a resin or ionomer having an acid anhydride structure [-C(=O)-OC(=O)-]. The polyester film according to any one of claims 1 to 9, wherein the polycondensation catalyst for the polyester resin (X) is Ti-based. The polyester film according to any one of claims 1 to 10, wherein the polyester resin (X) is at least one selected from polyethylene terephthalate and polyethylene-2,6-naphthalate. The polyester film according to any one of claims 1 to 11, wherein a cured resin layer is provided on at least one surface of said polyester film. The polyester film according to any one of claims 1 to 12, which has a film thickness of 0.5 to 12.0 µm. The polyester film according to any one of claims 1 to 13, which has a dielectric loss tangent (tan δ) at 1 kHz of 0.54 or less. A metal laminate film comprising a metal layer provided on at least one side of the polyester film according to any one of claims 1 to 14. The polyester film according to any one of claims 1 to 14, which is used for capacitors. 16. The metal laminate film according to claim 15, which is used for capacitors. 17. The polyester film according to claim 16, which is used for capacitors mounted on automobiles. 18. The metal laminate film according to claim 17, which is used for a capacitor mounted on an automobile.

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