Polyimide precursor solution, porous polyimide film, and insulated wire

Abstract

To provide a polyimide precursor solution that can give a porous polyimide film having a low dielectric constant and high mechanical strength.SOLUTION: A polyimide precursor solution includes: a polyimide precursor that is a polymer of a tetracarboxylic dianhydride and a diamine compound; particles; a solvent; and a compound having at least four carboxy groups per molecule.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyimide precursor solution, a porous polyimide film, and an insulated wire. [Background technology]

[0002] Patent Document 1 discloses an insulated wire comprising a linear conductor and an insulating coating arranged to surround the outer peripheral surface of the conductor, the insulating coating including a polyimide layer having a molecular structure including specific repeating units at a specific molar ratio, and the polyimide layer having a plurality of pores at a specific ratio.

[0003] Patent Document 2 discloses an insulated wire including a linear conductor and one or more insulating layers laminated on the outer surface of the conductor, in which at least one of the insulating layers contains a plurality of hollow inorganic particles, and the hollow inorganic particles have a pressure resistance of 10 MPa or more as measured by the glycerol method in accordance with ASTM D3102-78. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2018 / 230706 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-45662 Summary of the Invention [Problem to be solved by the invention]

[0005] Polyimide films used as insulating coatings in insulated wires are required to have a low dielectric constant in order to increase the corona discharge inception voltage of the wire. Methods for obtaining a polyimide film with a low dielectric constant include, for example, making the polyimide film porous and increasing the porosity of the polyimide film. On the other hand, if the porosity is increased in order to lower the dielectric constant of the porous polyimide film, the mechanical strength of the polyimide film itself may be reduced.

[0006] An object of the present invention is to provide a polyimide precursor solution that can give a porous polyimide film that has both a low dielectric constant and high mechanical strength, as compared to a solution containing only a polyimide precursor, particles, and a solvent. [Means for solving the problem]

[0007] The above problems are solved by the following means:

[0008] <1> a polyimide precursor which is a polymer of a tetracarboxylic dianhydride and a diamine compound; Particles and A solvent, a compound having four or more carboxy groups in one molecule; A polyimide precursor solution containing <2> the content of the compound having four or more carboxy groups in one molecule is 0.3 parts by mole or more and 2.0 parts by mole or less relative to 100 parts by mole of the tetracarboxylic dianhydride; <1> The polyimide precursor solution according to claim 1. <3> the content of the compound having four or more carboxy groups in one molecule is 0.5 parts by mole or more and 1.5 parts by mole or less with respect to 100 parts by mole of the tetracarboxylic dianhydride; <2> The polyimide precursor solution according to claim 1.

[0009] <4> The compound having four or more carboxy groups in one molecule is an aromatic tetracarboxylic acid. <1> ~ <3> 1. The polyimide precursor solution according to claim 1 . <5> The aromatic tetracarboxylic acid has five or less aromatic rings in one molecule. <4> The polyimide precursor solution according to claim 1. <6> The polyimide precursor is a polymer of an aromatic tetracarboxylic dianhydride and an aromatic diamine compound. <1> ~ <5> 1. The polyimide precursor solution according to claim 1 .

[0010] <7> The particles are resin particles. <1> ~ <6> 1. The polyimide precursor solution according to claim 1 . <8> the content of the particles is 50% by volume or more and 70% by volume or less with respect to the total volume of the polyimide precursor and the particles; <1> ~ <7> 1. The polyimide precursor solution according to claim 1 . <9> The solvent comprises water. <1> ~ <8> 1. The polyimide precursor solution according to claim 1 . <10> Further containing an organic amine compound, <9> The polyimide precursor solution according to claim 1.

[0011] <11> <1> ~ <10> 1. A porous polyimide film, which is a porous fired product having pores obtained from the polyimide precursor solution according to any one of 1 to 8. <12> The porosity is 50% by volume or more and 70% by volume or less. <11> The porous polyimide film according to claim 1. <12> The wire body, provided on the surface of the wire body <11> or <12> a porous polyimide film according to An insulated wire having [Effects of the Invention]

[0012] <1> According to the present invention, there is provided a polyimide precursor solution that can give a porous polyimide film that has both a low dielectric constant and high mechanical strength, compared to a solution containing only a polyimide precursor, particles, and a solvent. <2> According to the present invention, there is provided a polyimide precursor solution that can give a porous polyimide film that has both a low dielectric constant and high mechanical strength, as compared to when the content of the compound having four or more carboxy groups per molecule is less than 0.3 parts by mole per 100 parts by mole of the tetracarboxylic dianhydride. <3> According to the present invention, there is provided a polyimide precursor solution that can give a porous polyimide film that has both a low dielectric constant and high mechanical strength, as compared to when the content of the compound having four or more carboxy groups per molecule is less than 0.5 parts by mole per 100 parts by mole of the tetracarboxylic dianhydride.

[0013] <4> According to the present invention, there is provided a polyimide precursor solution from which a porous polyimide film having higher mechanical strength can be obtained, compared to when the compound having four or more carboxy groups in one molecule is an aliphatic tetracarboxylic acid. <5> According to the present invention, there is provided a polyimide precursor solution that can give a porous polyimide film that has both a low dielectric constant and high mechanical strength, as compared to when the number of aromatic rings in one molecule of the aromatic tetracarboxylic acid is greater than five. <6> According to the present invention, there is provided a polyimide precursor solution from which a porous polyimide film having higher mechanical strength can be obtained, as compared with a case in which the polyimide precursor is a polymer of an aliphatic tetracarboxylic dianhydride and an aliphatic diamine compound.

[0014] <7> According to the present invention, a polyimide precursor solution is provided that allows a porous polyimide film to be obtained at lower cost than when the particles are inorganic particles. <8> According to the present invention, a polyimide precursor solution is provided that can produce a porous polyimide film that has both a low dielectric constant and high mechanical strength, compared to when the particle content is less than 50% by volume or more than 70% by volume relative to the total volume of the solid content of the polyimide precursor and the particles. <9> According to the present invention, there is provided a polyimide precursor solution that can give a porous polyimide film that has both a low dielectric constant and high mechanical strength, compared to when the solvent does not contain water. <10> According to the present invention, there is provided a polyimide precursor solution that can give a porous polyimide film having higher mechanical strength than a film not containing an organic amine compound.

[0015] <11> According to the present invention, a porous polyimide film is provided which has both a low dielectric constant and high mechanical strength compared to a film formed by using a polyimide precursor solution containing only a polyimide precursor, particles, and a solvent. <12> According to the present invention, a porous polyimide film is provided which has both a low dielectric constant and high mechanical strength, compared to a film having a porosity of less than 50% by volume or more than 70% by volume. <13> According to the present invention, an insulated wire having a porous polyimide film is provided that has both a low dielectric constant and high mechanical strength, as compared to a case in which a polyimide precursor solution containing only a polyimide precursor, particles, and a solvent is used. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic diagram showing the configuration of a porous polyimide film obtained using the polyimide precursor solution of the present embodiment. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an embodiment of the present invention will be described. These descriptions and examples are for illustrating the embodiment, and are not intended to limit the scope of the embodiment. In the present specification, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range. In addition, in the present specification, the upper or lower limit of a numerical range may be replaced with a value shown in the examples.

[0018] In this specification, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved. Each component may contain multiple types of the corresponding substance. When referring to the amount of each component in a composition, if there are multiple substances corresponding to each component in the composition, the amount refers to the total amount of those multiple substances present in the composition, unless otherwise specified.

[0019] In this embodiment, the term "membrane" is a concept that encompasses not only what is generally called a "membrane," but also what is generally called a "film" and a "sheet."

[0020] [Polyimide precursor solution] The polyimide precursor solution according to this embodiment contains a polyimide precursor which is a polymer of a tetracarboxylic dianhydride and a diamine compound, particles, a solvent, and a compound having four or more carboxy groups in one molecule (hereinafter also referred to as a "specific compound"). The polyimide precursor solution according to the present embodiment, having the above-described structure, can provide a porous polyimide film that has both a low dielectric constant and high mechanical strength. The reason for this is unclear, but is presumed to be as follows.

[0021] As described above, a method for obtaining a polyimide film with a low dielectric constant can be, for example, a method of making the polyimide film porous and increasing the porosity of the polyimide film. On the other hand, increasing the porosity to lower the dielectric constant of the porous polyimide film can sometimes result in a decrease in the mechanical strength of the polyimide film itself.

[0022] In contrast, the present embodiment contains a specific compound, which is a compound containing four or more non-anhydrous carboxyl groups in one molecule. Here, the porous polyimide film is produced, for example, by applying a polyimide precursor solution onto a substrate to form a coating film, drying the coating film to form a film, and baking the film (i.e., imidization) to remove the particles. It is believed that the specific compound is anhydrous due to dehydration of the carboxyl group during baking of the film, converting to a carboxylic acid anhydride. The anhydride formed by the anhydride of the specific compound is then thought to react with units derived from diamine in the polyimide precursor to form a crosslinked structure. In other words, it is presumed that the specific compound acts as a crosslinking agent, increasing the mechanical strength even when the porosity is high, thereby achieving both a low dielectric constant and high mechanical strength in the resulting porous polyimide film. Each component contained in the polyimide precursor solution according to this embodiment will be described below.

[0023] <Polyimide precursor> The polyimide precursor solution of this embodiment contains a polyimide precursor, which is a polymer of a tetracarboxylic dianhydride and a diamine compound. The polyimide precursor is, for example, a polymer obtained by polymerizing a tetracarboxylic dianhydride and a diamine compound in a molar ratio of 1:1, and is a polymer containing units derived from the tetracarboxylic dianhydride and units derived from the diamine compound. The polyimide precursor may be, for example, a resin (polyimide precursor) having a repeating unit represented by general formula (I).

[0024] [ka]

[0025] (In general formula (I), A represents a tetravalent organic group, and B represents a divalent organic group.)

[0026] Here, in the general formula (I), the tetravalent organic group represented by A is a residue obtained by removing four carboxyl groups from the starting tetracarboxylic dianhydride. On the other hand, the divalent organic group represented by B is the residue obtained by removing two amino groups from the diamine compound used as the raw material.

[0027] That is, the polyimide precursor having the repeating unit represented by general formula (I) is a polymer of a tetracarboxylic dianhydride and a diamine compound.

[0028] The tetracarboxylic dianhydride may be either an aromatic compound or an aliphatic compound, but from the viewpoint of obtaining a porous polyimide film having high mechanical strength, the tetracarboxylic dianhydride is preferably an aromatic compound, i.e., the tetravalent organic group represented by A in general formula (I) is preferably an aromatic organic group.

[0029] Examples of aromatic tetracarboxylic dianhydrides include pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-biphenylsulfonetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 3,3',4,4'-biphenylethertetracarboxylic dianhydride, and 3,3',4,4'-dimethyldiphenylsilanetetracarboxylic acid. Acid dianhydrides, 3,3',4,4'-tetraphenylsilane tetracarboxylic dianhydride, 1,2,3,4-furan tetracarboxylic dianhydride, 4,4'-bis(3,4-dicarboxyphenoxy)diphenyl sulfide dianhydride, 4,4'-bis(3,4-dicarboxyphenoxy)diphenyl sulfone dianhydride, 4,4'-bis(3,4-dicarboxyphenoxy)diphenylpropane dianhydride, 3,3',4,4'-perfluoroisopropylidenediphthalic dianhydride, 3,3',4,4'-Biphenyltetracarboxylic dianhydride, 2,3,3',4'-biphenyltetracarboxylic dianhydride, bis(phthalic)phenylphosphine oxide dianhydride, p-phenylene-bis(triphenylphthalic)dianhydride, m-phenylene-bis(triphenylphthalic)dianhydride, bis(triphenylphthalic)-4,4'-diphenylether dianhydride, bis(triphenylphthalic)-4,4'-diphenylmethane dianhydride, 1,3,3a, 4,5,9b-hexahydro-2,5-dioxo-3-furanyl)-naphtho[1,2-c]furan-1,3-dione, 1,3,3a,4,5,9b-hexahydro-5-methyl-5-(tetrahydro-2,5-dioxo-3-furanyl)-naphtho[1,2-c]furan-1,3-dione, 1,3,3a,4,5,9b-hexahydro-8-methyl-5-(tetrahydro-2,5-dioxo-3-furanyl)-naphtho[1,2-c]furan-1,3-dione, and the like.

[0030] Examples of the aliphatic tetracarboxylic dianhydride include butane tetracarboxylic dianhydride, 1,2,3,4-cyclobutane tetracarboxylic dianhydride, 1,3-dimethyl-1,2,3,4-cyclobutane tetracarboxylic dianhydride, 1,2,3,4-cyclopentane tetracarboxylic dianhydride, 2,3,5-tricarboxycyclopentylacetic dianhydride, 3,5,6-tricarboxynorbornane-2-acetic dianhydride, 2,3,4,5-tetrahydrofuran tetracarboxylic dianhydride, 5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic dianhydride, and bicyclo[2,2,2]-oct-7-ene-2,3,5,6-tetracarboxylic dianhydride.

[0031] Among these, the tetracarboxylic acid dianhydride is preferably an aromatic tetracarboxylic acid dianhydride, specifically, for example, pyromellitic acid dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 2,3,3',4'-biphenyltetracarboxylic acid dianhydride, 3,3',4,4'-biphenylethertetracarboxylic acid dianhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, further, pyromellitic acid dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride is more preferable, and 3,3',4,4'-biphenyltetracarboxylic acid dianhydride is particularly preferable.

[0032] The tetracarboxylic dianhydrides may be used alone or in combination of two or more. When two or more kinds are used in combination, aromatic tetracarboxylic dianhydrides or aliphatic tetracarboxylic acids may be used in combination, or an aromatic tetracarboxylic dianhydride and an aliphatic tetracarboxylic dianhydride may be used in combination.

[0033] On the other hand, the diamine compound is a diamine compound having two amino groups in its molecular structure. The diamine compound may be either an aromatic compound or an aliphatic compound, but from the viewpoint of obtaining a porous polyimide film having high mechanical strength, the diamine compound is preferably an aromatic compound. That is, in the general formula (I), the divalent organic group represented by B is preferably an aromatic organic group.

[0034] Examples of the diamine compound include p-phenylenediamine, m-phenylenediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylethane, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfide, 4,4'-diaminodiphenyl sulfone, 1,5-diaminonaphthalene, 3,3-dimethyl-4,4'-diaminobiphenyl, 5-amino-1-(4'-aminophenyl)-1,3,3-trimethylindane, 6-amino-1-(4'-aminophenyl)-1,3 ,3-Trimethylindane, 4,4'-diaminobenzanilide, 3,5-diamino-3'-trifluoromethylbenzanilide, 3,5-diamino-4'-trifluoromethylbenzanilide, 3,4'-diaminodiphenyl ether, 2,7-diaminofluorene, 2,2-bis(4-aminophenyl)hexafluoropropane, 4,4'-methylene-bis(2-chloroaniline), 2,2',5,5'-tetrachloro-4,4'-diaminobiphenyl, 2,2'-dichloro-4,4'-diamino-5,5'-dimethicone 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, 1,4-bis(4-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)-biphenyl, 1,3'-bis(4-aminophenoxy)benzene, 9,9-bis(4-aminophenyl)fluorene aromatic diamines such as 4,4'-(p-phenyleneisopropylidene)bisaniline, 4,4'-(m-phenyleneisopropylidene)bisaniline, 2,2'-bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]hexafluoropropane, and 4,4'-bis[4-(4-amino-2-trifluoromethyl)phenoxy]-octafluorobiphenyl; aromatic diamines having two amino groups bonded to an aromatic ring and a heteroatom other than the nitrogen atom of the amino groups, such as diaminotetraphenylthiophene;1,1-meta-xylylenediamine, 1,3-propanediamine, tetramethylenediamine, pentamethylenediamine, octamethylenediamine, nonamethylenediamine, 4,4-diaminoheptamethylenediamine, 1,4-diaminocyclohexane, isophoronediamine, tetrahydrodicyclopentadienylenediamine, hexahydro-4,7-methanoindanidinediamine, tricyclo[6,2,1,0; 2.7 ]-undecylenedimethyldiamine, 4,4'-methylenebis(cyclohexylamine), and other aliphatic diamines and alicyclic diamines.

[0035] Among these, the diamine compound is preferably an aromatic diamine compound, specifically, for example, p-phenylenediamine, m-phenylenediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfide, or 4,4'-diaminodiphenyl sulfone, and particularly, 4,4'-diaminodiphenyl ether or p-phenylenediamine is preferable.

[0036] The diamine compounds may be used singly or in combination of two or more. When two or more diamine compounds are used in combination, aromatic diamine compounds or aliphatic diamine compounds may be used in combination, or an aromatic diamine compound and an aliphatic diamine compound may be used in combination.

[0037] The polyimide precursor may be any of a polymer of an aromatic tetracarboxylic dianhydride and an aromatic diamine compound, a polymer of an aromatic tetracarboxylic dianhydride and an aliphatic diamine compound, a polymer of an aliphatic tetracarboxylic dianhydride and an aromatic diamine compound, and a polymer of an aliphatic tetracarboxylic dianhydride and an aliphatic diamine compound. Among these, the polyimide precursor is preferably a polymer of an aromatic tetracarboxylic dianhydride and an aromatic diamine compound, from the viewpoint of obtaining a porous polyimide film having high mechanical strength.

[0038] The weight average molecular weight of the polyimide precursor used in this embodiment is preferably 5,000 or more and 300,000 or less, and more preferably 10,000 or more and 150,000 or less.

[0039] The weight-average molecular weight of the polyimide precursor is measured by gel permeation chromatography (GPC) under the following measurement conditions. Column: Tosoh TSKgel α-M (7.8 mm ID x 30 cm) Eluent: DMF (dimethylformamide) / 30mM LiBr / 60mM phosphoric acid ·Flow rate: 0.6mL / min ·Injection volume: 60μL Detector: RI (Differential Refractive Index Detector)

[0040] The content of the polyimide precursor contained in the polyimide precursor solution according to this embodiment may be 0.1% by mass or more and 40% by mass or less, and preferably 1% by mass or more and 25% by mass or less, relative to the total mass of the polyimide precursor solution.

[0041] <particle> The polyimide precursor solution according to this embodiment contains particles. The material of the particles is not particularly limited as long as they are dispersed in the polyimide precursor solution without dissolving and can be removed in the particle removal step described below when producing a porous polyimide film. The particles are broadly classified into resin particles and inorganic particles described below. In this specification, "the particles are insoluble" means that the particles are not dissolved in the target liquid at 25°C, and also means that the particles are soluble to a degree of 3% by mass or less.

[0042] The volume average particle diameter D50v of the particles is not particularly limited. The volume average particle diameter D50v of the particles can be, for example, in the range of 0.1 μm to 30 μm, preferably 0.15 μm to 10 μm, more preferably 0.2 μm to 5 μm, and even more preferably 0.25 μm to 1 μm. When the volume average particle diameter of the particles is within this range, particle aggregation is easily suppressed, uneven distribution of pores in the porous polyimide film is suppressed, and a porous polyimide film that further combines low dielectric constant and high mechanical strength is easily obtained. Furthermore, when the particles are resin particles, productivity of the resin particles is easily improved. The particle volume size distribution index (GSDv) is preferably 1.30 or less, more preferably 1.25 or less, and most preferably 1.20 or less. The particle volume size distribution index is calculated from the particle size distribution of the particles in the polyimide precursor solution by the following formula: (D84v / D16v) 1 / 2 It is calculated as:

[0043] The particle size distribution of particles in the polyimide precursor solution according to this embodiment is measured as follows: The solution to be measured is diluted, and the particle size distribution of the particles in the solution is measured using a Coulter Counter LS13 (manufactured by Beckman Coulter, Inc.). Based on the measured particle size distribution, a volume cumulative distribution is drawn for each divided particle size range (channel) from the smallest diameter side, and the particle size distribution is measured. In the volume cumulative distribution drawn from the small diameter side, the particle size at 16% of the cumulative diameter is defined as the volume particle size D16v, the particle size at 50% of the cumulative diameter is defined as the volume average particle size D50v, and the particle size at 84% of the cumulative diameter is defined as the volume particle size D84v.

[0044] When it is difficult to measure the volume particle size distribution of the particles in the polyimide precursor solution of this embodiment by the above-mentioned method, it may be measured by a method such as dynamic light scattering.

[0045] The particles are preferably spherical in shape, and when a porous polyimide film is produced using spherical particles, a porous polyimide film having spherical pores is obtained. In this specification, the term "spherical" refers to particles that are both spherical and nearly spherical (shapes close to a sphere). Specifically, this means that 90% or more of the particles have a ratio of major axis to minor axis (major axis / minor axis) of 1 to 1.5. The closer the ratio of major axis to minor axis is to 1, the closer the particles are to a perfect sphere.

[0046] The particles may be either resin particles or inorganic particles, but it is preferable to use resin particles. When resin particles are used as the particles, the particles are removed by heating when the film of the polyimide precursor solution is baked in the process of forming the porous polyimide film, and therefore, when resin particles are used as the particles, a separate operation for removing the particles is not required as in the case of using inorganic particles, and a porous polyimide film can be easily obtained at low cost. In addition, since both the resin particles and the polyimide precursor are organic materials, the use of resin particles tends to improve particle dispersibility in a polyimide precursor solution or a coating film of the polyimide precursor solution, interfacial adhesion with the polyimide precursor, etc., compared with the use of inorganic particles. Furthermore, in the imidization step for producing a polyimide film, the resin particles tend to absorb volumetric shrinkage, making it less likely that cracks will occur in the polyimide film due to this volumetric shrinkage.

[0047] Specific materials for the resin particles and inorganic particles will be described below.

[0048] -Resin particles- Specific examples of resin particles include vinyl polymers such as polystyrenes, poly(meth)acrylic acids, polyvinyl acetate, polyvinyl alcohol, polyvinyl butyral, and polyvinyl ether; condensation polymers such as polyester, polyurethane, and polyamide; hydrocarbon polymers such as polyethylene, polypropylene, and polybutadiene; and fluorine-based polymers such as polytetrafluoroethylene and polyvinyl fluoride. Here, "(meth)acrylic" means both "acrylic" and "methacrylic." Furthermore, (meth)acrylic acids include (meth)acrylic acid, (meth)acrylic acid esters, and (meth)acrylamides.

[0049] The resin particles may be crosslinked or not. When crosslinked, a bifunctional monomer such as divinylbenzene, ethylene glycol dimethacrylate, nonane diacrylate, or decanediol diacrylate, or a polyfunctional monomer such as trimethylolpropane triacrylate or trimethylolpropane trimethacrylate may be used in combination.

[0050] When the resin particles are vinyl resin particles, they are obtained by polymerizing a monomer, such as the following monomers: Examples of the vinyl resin unit include vinyl resin units obtained by polymerizing monomers such as styrene, alkyl-substituted styrenes (e.g., α-methylstyrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, 2-ethylstyrene, 3-ethylstyrene, 4-ethylstyrene, etc.), halogen-substituted styrenes (e.g., 2-chlorostyrene, 3-chlorostyrene, 4-chlorostyrene, etc.), and styrenes having a styrene skeleton, such as vinylnaphthalene; (meth)acrylic acid esters, such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, lauryl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; vinyl nitriles, such as acrylonitrile and methacrylonitrile; vinyl ethers, such as vinyl methyl ether and vinyl isobutyl ether; vinyl ketones, such as vinyl methyl ketone, vinyl ethyl ketone, and vinyl isopropenyl ketone; acids, such as (meth)acrylic acid, maleic acid, cinnamic acid, fumaric acid, and vinylsulfonic acid; and bases, such as ethyleneimine, vinylpyridine, and vinylamine. As other monomers, monofunctional monomers such as vinyl acetate may be used in combination. The vinyl resin may be a resin using one of these monomers alone, or may be a resin that is a copolymer using two or more types of monomers.

[0051] From the viewpoints of manufacturability and adaptability to the particle removal step described below, the resin particles are preferably polystyrenes or poly(meth)acrylic acids. Specifically, polystyrene, styrene-(meth)acrylic acid copolymers, and poly(meth)acrylic acids are more preferred, and polystyrene and poly(meth)acrylic acid esters are most preferred. These resin particles may be used alone or in combination of two or more.

[0052] The resin particles preferably maintain their particle shape during the process of preparing the polyimide precursor solution according to this embodiment, and during the process of applying the polyimide precursor solution to prepare a porous polyimide film and drying the coating before removing the resin particles. From this viewpoint, the glass transition temperature of the resin particles is preferably 60°C or higher, more preferably 70°C or higher, and even more preferably 80°C or higher.

[0053] The glass transition temperature is determined from a DSC curve obtained by differential scanning calorimetry (DSC), more specifically, from the "extrapolated glass transition onset temperature" described in JIS K 7121-1987 "Method for measuring transition temperatures of plastics."

[0054] -Inorganic particles- Specific examples of inorganic particles include silica (silicon dioxide) particles, magnesium oxide particles, alumina particles, zirconia particles, calcium carbonate particles, calcium oxide particles, titanium dioxide particles, zinc oxide particles, and cerium oxide particles. As described above, the particle shape is preferably spherical. From this perspective, preferred inorganic particles include silica particles, magnesium oxide particles, calcium carbonate particles, magnesium oxide particles, and alumina particles, more preferred inorganic particles include silica particles, titanium oxide particles, and alumina particles, and even more preferred inorganic particles include silica particles. These inorganic particles may be used alone or in combination of two or more types.

[0055] If the inorganic particles have insufficient wettability and dispersibility in the solvent of the polyimide precursor solution, the inorganic particles may be surface-modified as necessary. Examples of surface modification methods include treating with an alkoxysilane having an organic group, such as a silane coupling agent; or coating with an organic acid, such as oxalic acid, citric acid, or lactic acid.

[0056] In this embodiment, the particle content is preferably 50% by volume or more and 70% by volume or less, more preferably 52% by volume or more and 68% by volume or less, and even more preferably 55% by volume or more and 65% by volume or less, relative to the total volume of the polyimide precursor and the particles. The polyimide precursor solution according to this embodiment contains the specific compound, and therefore, even if the particle content is within the above range, a porous polyimide film having both a low dielectric constant and high mechanical strength can be obtained. Furthermore, by ensuring that the particle content is within the above range, a porous polyimide film having a lower dielectric constant can be obtained compared to when the particle content is less than the above range, and a porous polyimide film having a higher mechanical strength can be obtained compared to when the particle content is greater than the above range.

[0057] Here, an example of a method for determining the volume of particles contained in a specific amount of polyimide precursor solution is to filter the polyimide precursor solution and determine the difference between the volume of the polyimide precursor solution before filtration and the volume of the filtrate after filtration. Another method for determining the volume of the polyimide precursor contained in a specific amount of polyimide precursor solution is to apply the polyimide precursor solution to a substrate, dry it at 200°C for 1 hour, measure the volume of the dried film using a laser volume meter, and determine the volume of the polyimide precursor from the difference between the volume of the dried film obtained and the volume of the particles determined by the above-mentioned method.

[0058] Furthermore, the particle content is preferably 0.1% by mass or more and 20% by mass or less, more preferably 0.5% by mass or more and 20% by mass or less, and even more preferably 1% by mass or more and 20% by mass or less, relative to the total mass of the polyimide precursor solution.

[0059] <Solvent> The polyimide precursor solution of the present embodiment contains a solvent. The solvent is not particularly limited as long as it dissolves the polyimide precursor in the polyimide precursor solution and leaves the particles in a dispersed state without dissolving. The solvent preferably contains water. A water-containing solvent tends to have a higher solubility of the specific compound than a water-free solvent, and increasing the concentration of the specific compound makes it easier to obtain a porous polyimide film that has both a low dielectric constant and high mechanical strength.

[0060] Examples of water include distilled water, ion-exchanged water, ultrafiltered water, and pure water. The content of water in the entire solvent is preferably 50% by mass or more and 100% by mass or less, and more preferably 70% by mass or more and 100% by mass or less, from the viewpoint of increasing the concentration of the specific compound. Hereinafter, a solvent having a water content of 50% by mass or more relative to the total solvent will also be referred to as an "aqueous solvent," and a solvent having a water content of less than 50% by mass relative to the total solvent and containing an organic solvent will also be referred to as an "organic solvent."

[0061] -Water-based solvent- The aqueous solvent may contain a solvent other than water. Examples of the solvent other than water include water-soluble organic solvents and aprotic polar solvents. As the solvent other than water, a water-soluble organic solvent is preferable in terms of the mechanical strength of the porous polyimide film. Here, water-soluble means that the target substance dissolves in water at 1% by mass or more at 25°C.

[0062] When resin particles are used as the particles and an aqueous solvent containing a water-soluble organic solvent is used as the solvent, the content of the water-soluble organic solvent is preferably 40% by mass or less, and preferably 30% by mass or less, of the total aqueous solvent to suppress dissolution and swelling of the particles in the polyimide precursor solution. Furthermore, to suppress dissolution and swelling of the resin particles when the coating of the polyimide precursor solution is dried to form a film, the content of the water-soluble organic solvent is preferably 3% by mass or more and 50% by mass or less, preferably 5% by mass or more and 40% by mass or less, and more preferably 5% by mass or more and 35% by mass or less, of the total amount of the particles and polyimide precursor in the polyimide precursor solution.

[0063] Examples of the water-soluble organic solvent include the water-soluble ether solvents, water-soluble ketone solvents, and water-soluble alcohol solvents shown below.

[0064] A water-soluble ether solvent is a water-soluble organic solvent having an ether bond in one molecule. Examples of water-soluble ether solvents include tetrahydrofuran (THF), dioxane, trioxane, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, and diethylene glycol diethyl ether. Among these, tetrahydrofuran and dioxane are preferred as water-soluble ether solvents.

[0065] A water-soluble ketone solvent is a water-soluble organic solvent having a ketone group in one molecule. Examples of the water-soluble ketone solvent include acetone, methyl ethyl ketone, and cyclohexanone. Among these, acetone is preferred as the water-soluble ketone solvent.

[0066] The water-soluble alcoholic solvent is a water-soluble organic solvent having an alcoholic hydroxyl group in one molecule. Examples of the water-soluble alcoholic solvent include methanol, ethanol, 1-propanol, 2-propanol, tert-butyl alcohol, ethylene glycol, monoalkyl ethers of ethylene glycol, propylene glycol, monoalkyl ethers of propylene glycol, diethylene glycol, monoalkyl ethers of diethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, 2-butene-1,4-diol, 2-methyl-2,4-pentanediol, glycerin, 2-ethyl-2-hydroxymethyl-1,3-propanediol, and 1,2,6-hexanetriol. Among these, preferred water-soluble alcohol solvents are methanol, ethanol, 2-propanol, ethylene glycol, monoalkyl ethers of ethylene glycol, propylene glycol, monoalkyl ethers of propylene glycol, diethylene glycol, and monoalkyl ethers of diethylene glycol.

[0067] To improve the properties of the porous polyimide film (e.g., transparency, mechanical strength, heat resistance, electrical properties, solvent resistance, etc.), the aqueous solvent may contain an aprotic polar solvent. In this case, to suppress dissolution and swelling of particles in the polyimide precursor solution, the content of the aprotic polar solvent is preferably 40% by mass or less, and preferably 30% by mass or less, of the total aqueous solvent. Furthermore, to suppress dissolution and swelling of resin particles when the polyimide precursor solution is dried and formed into a film, the content of the aprotic polar solvent is preferably 3% by mass or more and 200% by mass or less, preferably 3% by mass or more and 100% by mass or less, more preferably 3% by mass or more and 50% by mass or less, of the total content (solids content) of particles and polyimide precursor in the polyimide precursor solution. The aprotic polar solvents may be used alone or in combination of two or more.

[0068] When an aprotic polar solvent other than water is contained as the aqueous solvent, the aprotic polar solvent used in combination can be an organic solvent having a boiling point of 150° C. or higher and 300° C. or lower and a dipole moment of 3.0 D or higher and 5.0 D or lower. Specific examples of the aprotic polar solvent include N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), hexamethylene phosphoramide (HMPA), N-methylcaprolactam, N-acetyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone (DMI), N,N'-dimethylpropyleneurea, tetramethylurea, trimethyl phosphate, triethyl phosphate, etc.

[0069] When a solvent other than water is contained as the aqueous solvent, the solvent used in combination should have a boiling point of 270° C. or lower, preferably 60° C. or higher and 250° C. or lower, and more preferably 80° C. or higher and 230° C. or lower. When the boiling point of the solvent used in combination is within the above range, solvents other than water are less likely to remain in the polyimide film, and a polyimide film with high mechanical strength is more likely to be obtained.

[0070] -Organic solvents- The organic solvent is selected so that the polyimide precursor dissolves and the particles remain dispersed in the polyimide precursor solution. The organic solvent is preferably a mixed solvent of a good solvent (S1) for the polyimide precursor and a solvent (S2) other than the good solvent (S1).

[0071] A good solvent (S1) for a polyimide precursor refers to a solvent in which the solubility of the polyimide precursor is 5% by mass or more. Specific examples of good solvents (S1) include aprotic polar solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, N-ethylpyrrolidone, tetramethylurea, 1,3-dimethyl-2-imidazolidinone, dimethylpropyleneurea, dimethyl sulfoxide, γ-butyrolactone, β-propiolactone, γ-valerolactone, δ-valerolactone, and γ-caprolactone. Among these, N,N-dimethylacetamide, N-methylpyrrolidone, tetramethylurea, 1,3-dimethyl-2-imidazolidinone, dimethyl sulfoxide, and γ-butyrolactone are preferred, N,N-dimethylacetamide, N-methylpyrrolidone, tetramethylurea, dimethyl sulfoxide, and γ-butyrolactone are more preferred, and N,N-dimethylacetamide, N-methylpyrrolidone, and γ-butyrolactone are even more preferred.

[0072] As the solvent (S2) other than the good solvent for the polyimide precursor, a solvent having a low solubility for the particles to be used is selected. For example, a method of adding particles to the target solvent and selecting a solvent having a dissolution amount of 3 mass % or less can be mentioned.

[0073] Examples of the solvent (S2) other than a good solvent for the polyimide precursor include hydrocarbon solvents such as n-decane and toluene; alcohol solvents such as isopropyl alcohol, 1-propanol, 1-butanol, 1-pentanol, and phenethyl alcohol; glycol solvents such as ethylene glycol, propylene glycol, ethylene glycol monomethyl ether, 1-methoxy-2-propanol, diethylene glycol monomethyl ether, and triethylene glycol monomethyl ether; ether solvents such as diglyme, triglyme, tetraglyme, and methyl cellosolve acetate; and phenolic solvents such as phenol and cresol.

[0074] When the resin particles described above are used as the particles, the solvent (S1) is often highly polar, and therefore, solvent (S1) alone may dissolve not only the polyimide precursor but also the resin particles. Therefore, the mixing ratio of solvent (S1) and solvent (S2) should be determined so that the polyimide precursor dissolves but the resin particles do not. Furthermore, to prevent the resin particles from dissolving during the heating process of the coating film of the polyimide precursor solution, which would result in, for example, the shape of the pores becoming distorted, the boiling point of solvent (S2) is preferably at least 10°C higher than that of solvent (S1), and more preferably at least 20°C higher.

[0075] <Specific compound> The polyimide precursor solution of the present embodiment contains a specific compound. The specific compound is not particularly limited as long as it is a compound having four or more non-anhydrous carboxy groups in one molecule. The number of carboxy groups that the specific compound has in one molecule is, for example, in the range of 4 to 6, preferably in the range of 4 to 5, and more preferably 4.

[0076] The specific compound may be a compound having an aromatic ring or a compound not having an aromatic ring, but from the viewpoint of obtaining a porous polyimide film having high mechanical strength, the specific compound is preferably a compound having an aromatic ring, and more preferably an aromatic tetracarboxylic acid. The specific compound that is an aromatic compound may have a benzene ring, an aromatic heterocycle, or both a benzene ring and an aromatic heterocycle as the aromatic ring. The aromatic heterocycle is an aromatic ring containing an element other than carbon within the ring, and examples thereof include a thiophene ring, a thiophine ring, a pyrrole ring, a furan ring, heterocycles in which the carbon atoms at the 3- and 4-positions of these rings are further substituted with nitrogen, and a pyridine ring.

[0077] The specific compound that is an aromatic compound may be a compound having one aromatic ring per molecule, or may be a compound having two or more aromatic rings per molecule. The number of aromatic rings in the specific compound that is an aromatic compound is preferably 1 to 8, more preferably 1 to 5, and even more preferably 1 to 3. The specific compound is preferably a tetracarboxylic acid having 1 to 8 aromatic rings, more preferably 1 to 5 aromatic rings, and even more preferably 1 to 3 aromatic rings.

[0078] Examples of aromatic compounds having two or more aromatic rings include compounds having polynuclear aromatic rings in which the atoms constituting each ring are bonded to each other, compounds having fused aromatic rings in which two or more atoms constituting a ring are shared with other rings, etc. In the polynuclear aromatic ring, the atoms constituting each ring may be bonded directly to each other by a covalent bond or may be bonded via a linking group. Examples of compounds having a polynuclear aromatic ring include compounds having a skeleton such as biphenyl, terphenyl, stilbene, triphenylethylene, etc. Examples of compounds having a condensed aromatic ring include compounds having a skeleton such as naphthalene, anthracene, phenanthrene, pyrene, perylene, fluorene, etc. The aromatic compound having two or more aromatic rings may be a compound having both a polynuclear aromatic ring and a condensed aromatic ring.

[0079] Specific examples of the specific compound that is an aromatic compound include, for example, pyromellitic acid, 3,3',4,4'-benzophenonetetracarboxylic acid, 3,3',4,4'-biphenylsulfonetetracarboxylic acid, 1,4,5,8-naphthalenetetracarboxylic acid, 2,3,6,7-naphthalenetetracarboxylic acid, 3,3',4,4'-biphenylethertetracarboxylic acid, 3,3',4,4'-dimethyldiphenylsilanetetracarboxylic acid, 3,3',4,4'-tetraphenylsilanetetracarboxylic acid, 1,2,3,4-furantetracarboxylic acid, 4,4'-bis(3,4-dicarboxyphenoxy)diphenyl sulfide, 4,4'-bis(3,4-dicarboxyphenoxy)diphenyl sulfone, 4,4'-bis(3,4-dicarboxyphenoxy)diphenylpropane, 3,3',4,4'-perfluoroisopropylidenediphthalic acid, and 3,3',4,4'-biphenylether. tetracarboxylic acid, 2,3,3',4'-biphenyltetracarboxylic acid, bis(phthalic acid)phenylphosphine oxide, p-phenylene-bis(triphenylphthalic acid), m-phenylene-bis(triphenylphthalic acid), bis(triphenylphthalic acid)-4,4'-diphenyl ether, bis(triphenylphthalic acid)-4,4'-diphenylmethane, 1,3,3a,4,5,9b-hexahydro-2,5-dioxo-3-furan

[0033] Examples of aromatic tetracarboxylic acids include 1,3,3a,4,5,9b-hexahydro-5-methyl-5-(tetrahydro-2,5-dioxo-3-furanyl)-naphtho[1,2-c]furan-1,3-dione, 1,3,3a,4,5,9b-hexahydro-8-methyl-5-(tetrahydro-2,5-dioxo-3-furanyl)-naphtho[1,2-c]furan-1,3-dione, and the like.

[0080] Specific examples of the specific compound that is an aliphatic compound include aliphatic tetracarboxylic acids such as butane tetracarboxylic acid, 1,2,3,4-cyclobutane tetracarboxylic acid, 1,3-dimethyl-1,2,3,4-cyclobutane tetracarboxylic acid, 1,2,3,4-cyclopentane tetracarboxylic acid, 2,3,5-tricarboxycyclopentyl acetic acid, 3,5,6-tricarboxynorbornane-2-acetic acid, 2,3,4,5-tetrahydrofuran tetracarboxylic acid, 5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic acid, and bicyclo[2,2,2]-oct-7-ene-2,3,5,6-tetracarboxylic acid.

[0081] Among these, the specific compound is preferably an aromatic tetracarboxylic acid, more preferably a tetracarboxylic acid having at least one of a benzene ring and a naphthalene ring, even more preferably at least one selected from the group consisting of pyromellitic acid, 3,3',4,4'-benzophenonetetracarboxylic acid, 3,3',4,4'-biphenylsulfonetetracarboxylic acid, 1,4,5,8-naphthalenetetracarboxylic acid, and 2,3,6,7-naphthalenetetracarboxylic acid, and particularly preferably at least one selected from the group consisting of pyromellitic acid, 3,3',4,4'-benzophenonetetracarboxylic acid, 1,4,5,8-naphthalenetetracarboxylic acid, and 2,3,6,7-naphthalenetetracarboxylic acid.

[0082] The molecular weight of the specific compound is not particularly limited, and may be in the range of 150 or more and 500 or less, preferably in the range of 180 or more and 480 or less, and more preferably in the range of 200 or more and 450 or less.

[0083] The content of the specific compound is preferably 0.3 parts by mol or more and 2.0 parts by mol or less, more preferably 0.4 parts by mol or more and 1.7 parts by mol or less, and even more preferably 0.5 parts by mol or more and 1.5 parts by mol or less, relative to 100 parts by mol of units derived from tetracarboxylic dianhydride contained in the polyimide precursor. When the content of the specific compound is within the above range, the effect of the specific compound as a crosslinking agent can be more easily obtained, and a porous polyimide film that combines a low dielectric constant and high mechanical strength can be more easily obtained, compared with when the content is less than the above range. Furthermore, when the content of the specific compound is within the above range, unreacted specific compound is less likely to remain after baking a film of the polyimide precursor solution, and a decrease in strength of the porous polyimide film due to the remaining unreacted specific compound is less likely to occur, compared with when the content is greater than the above range.

[0084] <Organic amine compounds> When the solvent contains water, the polyimide precursor solution may further contain an organic amine compound as needed to dissolve the polyimide precursor in the solvent. In particular, when the solvent is an aqueous solvent, the polyimide precursor solution preferably contains an organic amine compound. The polyimide precursor is made water-soluble by containing the organic amine compound in the polyimide precursor solution. The organic amine compound converts the carboxyl groups of the polyimide precursor into amine salts, thereby increasing the solubility in solvents containing water and also functioning as an imidization accelerator. Therefore, by using a solvent containing water as the solvent and further containing an organic amine compound in the polyimide precursor solution, a porous polyimide film with high mechanical strength is more easily obtained. Specifically, the organic amine compound is preferably an amine compound having a molecular weight of not more than 170. The organic amine compound is preferably a compound other than a diamine compound that is a raw material for a polyimide precursor. The organic amine compound is preferably a water-soluble compound, which means that the target substance dissolves in water at 25°C in an amount of 1% by mass or more.

[0085] The organic amine compound includes a primary amine compound, a secondary amine compound, and a tertiary amine compound. Among these, the organic amine compound is preferably at least one selected from the group consisting of secondary amine compounds and tertiary amine compounds, and more preferably a tertiary amine compound. When at least one selected from the group consisting of secondary amine compounds and tertiary amine compounds is used as the organic amine compound, the solubility of the polyimide precursor in a solvent is likely to be increased, film-forming properties are likely to be improved, and the storage stability of the polyimide precursor solution is likely to be improved.

[0086] In addition to monovalent amine compounds, examples of the organic amine compound include divalent or higher polyvalent amine compounds. The use of divalent or higher polyvalent amine compounds facilitates the formation of a pseudo-crosslinked structure between polyimide precursor molecules and also facilitates the improvement of the storage stability of the polyimide precursor solution.

[0087] Examples of primary amine compounds include methylamine, ethylamine, n-propylamine, isopropylamine, 2-ethanolamine, and 2-amino-2-methyl-1-propanol. Examples of secondary amine compounds include dimethylamine, 2-(methylamino)ethanol, 2-(ethylamino)ethanol, and morpholine. Examples of tertiary amine compounds include 2-dimethylaminoethanol, 2-diethylaminoethanol, 2-dimethylaminopropanol, pyridine, triethylamine, picoline, N-methylmorpholine, N-ethylmorpholine, 1,2-dimethylimidazole, and 2-ethyl-4-methylimidazole.

[0088] From the viewpoint of the pot life and film thickness uniformity of the polyimide precursor solution, a tertiary amine compound is preferred. In this respect, at least one selected from the group consisting of 2-dimethylaminoethanol, 2-diethylaminoethanol, 2-dimethylaminopropanol, pyridine, triethylamine, picoline, N-methylmorpholine, N-ethylmorpholine, 1,2-dimethylimidazole, 2-ethyl-4-methylimidazole, N-methylpiperidine, and N-ethylpiperidine is more preferred. Furthermore, at least one selected from the group consisting of 2-dimethylaminoethanol, 2-diethylaminoethanol, 2-dimethylaminopropanol, triethylamine, N-methylmorpholine, N-ethylmorpholine, 1,2-dimethylimidazole, 2-ethyl-4-methylimidazole, N-methylpiperidine, and N-ethylpiperidine is most preferred.

[0089] Here, as the organic amine compound, from the viewpoint of film-forming properties, an amine compound having a nitrogen-containing heterocyclic structure (particularly a tertiary amine compound) is also preferred. Examples of the amine compound having a nitrogen-containing heterocyclic structure (hereinafter referred to as "nitrogen-containing heterocyclic amine compound") include isoquinolines (amine compounds having an isoquinoline skeleton), pyridines (amine compounds having a pyridine skeleton), pyrimidines (amine compounds having a pyrimidine skeleton), pyrazines (amine compounds having a pyrazine skeleton), piperazines (amine compounds having a piperazine skeleton), triazines (amine compounds having a triazine skeleton), imidazoline, methyl ... Examples of the amine compounds include amine compounds having an imidazole skeleton, morpholines, polyaniline, polypyridine, and polyamine.

[0090] From the viewpoint of film-forming properties, the nitrogen-containing heterocyclic amine compound is preferably at least one selected from the group consisting of morpholines, pyridines, piperidines, and imidazoles, and more preferably a morpholine (an amine compound having a morpholine skeleton). Among these, at least one selected from the group consisting of N-methylmorpholine, N-methylpiperidine, pyridine, 1,2-dimethylimidazole, 2-ethyl-4-methylimidazole, and picoline is more preferred, and N-methylmorpholine is more preferred.

[0091] Among these, the organic amine compound is preferably a compound having a boiling point of 60° C. or higher (preferably 60° C. or higher and 200° C. or lower, more preferably 70° C. or higher and 150° C. or lower). When the boiling point of the organic amine compound is 60° C. or higher, the organic amine compound is prevented from volatilizing from the polyimide precursor solution during storage, and a decrease in the solubility of the polyimide precursor in the solvent is more easily prevented.

[0092] The organic amine compound is contained in an amount of 50 mol % to 500 mol % relative to the carboxyl groups (—COOH) of the polyimide precursor in the polyimide precursor solution, preferably 80 mol % to 250 mol %, and more preferably 90 mol % to 200 mol %. When the content of the organic amine compound is within the above range, the solubility of the polyimide precursor in aqueous solvents is likely to be increased, film-forming properties are likely to be improved, and the storage stability of the polyimide precursor solution is likely to be improved.

[0093] The organic amine compounds may be used alone or in combination of two or more.

[0094] <Other additives> The polyimide precursor solution according to this embodiment may contain a catalyst for accelerating the imidization reaction, a leveling material for improving the quality of the film formation, and the like. As a catalyst for promoting the imidization reaction, a dehydrating agent such as an acid anhydride, an acid catalyst such as a phenol derivative, a sulfonic acid derivative, or a benzoic acid derivative may be used.

[0095] Furthermore, for example, a conductive material may be added to impart conductivity. The conductive material has conductivity (for example, a volume resistivity of 10 7 The material may be semiconductive (e.g., volume resistivity less than 10 Ω·cm) or 7 Ω cm or more 10 13 The material may be a material with a resistivity of Ω·cm or less. Examples of conductive materials include carbon black (e.g., acidic carbon black having a pH of 5.0 or less), metals (e.g., aluminum, nickel, etc.), metal oxides (e.g., yttrium oxide, tin oxide, etc.), ion-conductive substances (e.g., potassium titanate, LiCl, etc.), etc. These conductive materials may be used alone or in combination of two or more. The polyimide precursor solution according to this embodiment may also contain LiCoO2, LiMn2O, or the like, which are used as electrodes for lithium ion batteries.

[0096] <Method of producing polyimide precursor solution> Examples of a method for producing the polyimide precursor solution according to this embodiment include the following method (i) and method (ii). (i) A method in which a solution of a polyimide precursor is prepared and then particles and a specific compound are added (ii) A method in which a particle dispersion is prepared, a polyimide precursor is synthesized in the dispersion, and then a specific compound is added. Among these, the method for preparing the polyimide precursor solution according to this embodiment is preferably the above method (ii) from the viewpoint of improving the dispersibility of the particles.

[0097] (i) A method in which particles and a specific compound are added after preparing a solution of a polyimide precursor. First, a solution of the polyimide precursor before dispersing the particles is obtained by a known method, for example, by polymerizing a tetracarboxylic dianhydride and a diamine compound in a solvent to produce a polyimide precursor, thereby obtaining a solution of the polyimide precursor. When an aqueous solvent is used as the solvent, polymerization may be performed in the presence of an organic amine to obtain a solution of a polyimide precursor. In another example, a tetracarboxylic dianhydride and a diamine compound are polymerized in an organic solvent such as an aprotic polar solvent (e.g., N-methylpyrrolidone (NMP)) to produce a polyimide precursor, and then the polyimide precursor is precipitated by adding the polymer to an aqueous solvent. Thereafter, the polyimide precursor and the organic amine compound are dissolved in the aqueous solvent to obtain a solution of the polyimide precursor.

[0098] Next, the particles and the specific compound are added to the obtained polyimide precursor solution. The order of adding the particles and the specific compound to the obtained polyimide precursor solution is not particularly limited. The particles may be added to the polyimide precursor solution before the specific compound is added, or the specific compound may be added after the particles are added, or the particles may be added simultaneously with the specific compound.

[0099] Regarding the particles, for example, when the particles are vinyl resin particles, they may be prepared in an aqueous solvent by a known polymerization method (a radical polymerization method such as emulsion polymerization, soap-free emulsion polymerization, suspension polymerization, miniemulsion polymerization, or microemulsion polymerization). For example, when emulsion polymerization is applied to the production of vinyl resin particles, a monomer such as a styrene or a (meth)acrylic acid is added to an aqueous solvent in which a water-soluble polymerization initiator such as potassium persulfate or ammonium persulfate has been dissolved, and then, if necessary, a surfactant such as sodium dodecyl sulfate or a diphenyloxide disulfonate is added, and the mixture is heated with stirring to polymerize, thereby obtaining vinyl resin particles.

[0100] When the vinyl resin particles are added to a solution of a polyimide precursor containing an aqueous solvent, the particles are added, for example, by mixing and stirring the aqueous solvent dispersion of the resin particles obtained by the above method with the polyimide precursor solution obtained above.

[0101] When the vinyl resin particles are added to a polyimide precursor solution containing an organic solvent, the resin particles are extracted as a powder from an aqueous solvent dispersion of the resin particles by a known method such as reprecipitation or freeze-drying, and then mixed and stirred with the polyimide precursor solution obtained above. Alternatively, the extracted resin particle powder may be redispersed in an organic solvent that does not dissolve the resin particles, and then mixed and stirred with the polyimide precursor solution. The methods of mixing, stirring, and dispersing are not particularly limited. In order to improve the dispersibility of the particles, a known nonionic or ionic surfactant may be added.

[0102] When using commercially available particles (resin particles or inorganic particles), if the particles are obtained as a powder, the particles are mixed and dispersed at the desired concentration regardless of whether the solvent of the polyimide precursor solution is an organic solvent or an aqueous solvent. If the particles are obtained as a particle dispersion, the particle dispersion and the polyimide precursor solution obtained above are mixed and dispersed in the same manner as in the case of preparing the above-mentioned particles, and the particles are added.

[0103] In the addition of the specific compound, the specific compound may be added as it is to the solution of the polyimide precursor to which the particles have been added, or a solution in which the specific compound is dissolved in a solvent may be added. Furthermore, the specific compound may be added as it is to the solution of the polyimide precursor before the particles have been added, or may be added in the form of a solution in which the specific compound is dissolved in a solvent, or may be added in the form of a specific-compound-containing particle dispersion in which the specific compound is dissolved in a particle dispersion.

[0104] (ii) A method in which a particle dispersion is prepared, a polyimide precursor is synthesized in the dispersion, and then a specific compound is added. When an organic solvent is used as the solvent for the polyimide precursor solution, a dispersion of particles is first prepared in an organic solvent in which the particles are not dissolved but the polyimide precursor is dissolved. Next, a tetracarboxylic dianhydride and a diamine compound are polymerized in the dispersion to produce a polyimide precursor, thereby obtaining a polyimide precursor solution in which the particles are dispersed. The specific compound is then added directly to the resulting solution, or a solution in which the specific compound is dissolved in a solvent is added, thereby obtaining the polyimide precursor solution according to this embodiment.

[0105] When an aqueous solvent is used as the solvent for the polyimide precursor solution, first, an aqueous solvent dispersion of particles is prepared. Next, in the dispersion, a tetracarboxylic dianhydride and a diamine compound are polymerized in the presence of an organic amine to produce a polyimide precursor, thereby obtaining a polyimide precursor solution in which the particles are dispersed. Thereafter, the specific compound is added directly to the obtained solution, or a solution in which the specific compound is dissolved in a solvent is added, thereby obtaining the polyimide precursor solution according to this embodiment.

[0106] When resin particles are used as the particles, the surfaces of the resin particles may be coated with a resin having a chemical structure different from that of the original resin in order to improve dispersibility in the polyimide precursor solution according to this embodiment. The resin to be coated may be changed depending on the solvent used and the chemical structure of the polyimide precursor. Examples of the resin to be coated include resins having acidic or basic groups. For example, when vinyl resin particles are produced by emulsion polymerization, a method of coating the surfaces of resin particles with a resin includes, after completing the polymerization of the monomer derived from the original resin particles, further adding a small amount of a monomer having an acidic or basic group, such as methacrylic acid or 2-dimethylaminoethyl methacrylate, to continue the polymerization.

[0107] [Porous polyimide film] The porous polyimide film according to this embodiment is a porous fired product having pores obtained from the polyimide precursor solution described above. The porous polyimide film according to this embodiment contains a reaction product of an imidized product of the polyimide precursor contained in the polyimide precursor solution and a specific compound contained in the polyimide precursor solution.

[0108] <Method of manufacturing porous polyimide film> The porous polyimide film according to this embodiment can be obtained by the following manufacturing method. The method for producing a porous polyimide film according to this embodiment includes a first step of applying the polyimide precursor solution described above to a substrate to form a coating film, and then drying the coating film to form a film containing the polyimide precursor, the specific compound, and the particles; and a second step of heating the film to imidize the polyimide precursor to form a polyimide film, the second step including a treatment to remove the particles. According to the method for producing a porous polyimide film according to this embodiment, spherical particles are used, thereby making it possible to obtain a porous polyimide film having spherical pores.

[0109] Hereinafter, an example of a suitable method for producing the porous polyimide film according to this embodiment will be described with reference to the drawings. FIG. 1 is a schematic diagram showing the structure of a porous polyimide film obtained by the method for producing a porous polyimide film according to this embodiment. In FIG. 1, 31 represents the substrate, 10A represents the holes, and 10 represents the porous polyimide film.

[0110] (1st step) In the first step, the polyimide precursor solution is prepared, and then the polyimide precursor solution is applied to a substrate to form a coating film, which is then dried to form a film containing the polyimide precursor and particles.

[0111] The coating film is formed by applying the polyimide precursor solution obtained by the above-described method onto a substrate. The obtained coating film contains at least the polyimide precursor, particles, the specific compound, and a solvent.

[0112] The substrate (substrate 31 in FIG. 1) to which the polyimide precursor solution is applied is selected depending on the application of the resulting porous polyimide film. When the porous polyimide film is used alone, a substrate for forming a polyimide film may be used as the substrate. When the porous polyimide film is used as a coating film for covering the surface of a member, the member itself may be used as the substrate.

[0113] Examples of substrates for forming a polyimide film include resin substrates such as polystyrene and polyethylene terephthalate; glass substrates; ceramic substrates; metal substrates such as iron and stainless steel (SUS); and composite material substrates that combine these materials. If necessary, the substrate for forming a polyimide film may be provided with a release layer by performing a release treatment using, for example, a silicone-based or fluorine-based release agent, etc. It is also effective to roughen the surface of the substrate for forming a polyimide film to a size approximately equal to the particle diameter of the particles, thereby facilitating exposure of the particles at the surface that comes into contact with the substrate.

[0114] When a porous polyimide film is used as a coating film for coating the surface of a member, specific examples of members that can be used as a substrate include the wire body of an insulated wire described below; various substrates used in liquid crystal elements; semiconductor substrates on which integrated circuits are formed, wiring substrates on which wiring is formed, and substrates of printed circuit boards on which electronic components and wiring are provided; and the like.

[0115] The method for applying the polyimide precursor solution onto the substrate is not particularly limited, and examples thereof include various methods such as spray coating, spin coating, roll coating, bar coating, slit die coating, and inkjet coating. The amount of polyimide precursor solution to be applied may be set to an amount that will give a predetermined film thickness.

[0116] The coating is formed by drying the coating formed on the substrate. The coating contains at least a polyimide precursor, a specific compound, and particles. The method for drying the coating film formed on the substrate is not particularly limited, and examples thereof include various methods such as heat drying, natural drying, vacuum drying, etc. More specifically, it is preferable to form the coating by drying the coating so that the solvent remaining in the coating is 50 mass % or less (preferably 30 mass % or less) relative to the solid content of the coating.

[0117] (2nd process) The second step is a step of heating the film obtained in the first step to imidize the polyimide precursor to form a polyimide film, and includes a step of removing particles, resulting in a porous polyimide film.

[0118] In the second step, specifically, the film obtained in the first step is heated to promote imidization, thereby forming a polyimide film. As the imidization progresses and the imidization rate increases, the polyimide film becomes less soluble in a solvent.

[0119] Then, in the second step, a process for removing the particles is carried out. By removing the particles, the regions where the particles were present become pores (pores 10A in FIG. 1), and a porous polyimide film (porous polyimide film 10 in FIG. 1) is obtained. The particles may be removed during the process of heating the film to imidize the polyimide precursor, or may be removed from the polyimide film after imidization.

[0120] In terms of particle removability, the particle removal treatment is preferably carried out when the imidization rate of the polyimide precursor in the polyimide film is 10% or more during the process of imidizing the polyimide precursor. When the imidization rate is 10% or more, the shape of the film is easily maintained.

[0121] -Particle removal- Next, the process of removing particles will be described. First, the process for removing the resin particles will be described. Examples of the treatment for removing the resin particles include a method for removing the resin particles by heating, a method for removing the resin particles with an organic solvent that dissolves the resin particles, a method for removing the resin particles by decomposition with a laser, etc. Among these, the method for removing the resin particles by heating and the method for removing the resin particles with an organic solvent that dissolves the resin particles are preferred.

[0122] As a method for removing the resin particles by heating, for example, the resin particles may be decomposed by heating to promote imidization during the process of imidizing the polyimide precursor. In this case, there is no need to perform an operation of removing the resin particles with an organic solvent, which is advantageous in terms of reducing the number of steps. When the resin particles are removed by heating to form a porous film, the resin particles do not decompose at the drying temperature after application, but are thermally decomposed at a temperature that imidizes the polyimide precursor film. From this perspective, the thermal decomposition temperature of the resin particles is preferably 150°C or higher and 320°C or lower, more preferably 180°C or higher and 300°C or lower, and even more preferably 200°C or higher and 280°C or lower.

[0123] Examples of the method for removing the resin particles using an organic solvent that dissolves the resin particles include a method in which the resin particles are brought into contact with an organic solvent that dissolves the resin particles (for example, immersed in the organic solvent) to dissolve and remove the resin particles. The method of immersing in an organic solvent is preferred because it increases the efficiency of dissolving the resin particles. The organic solvent for removing the resin particles is not particularly limited as long as it does not dissolve the polyimide film or the polyimide film after imidization, but dissolves the resin particles. Examples of the organic solvent include ethers such as tetrahydrofuran (THF), aromatic compounds such as toluene, ketones such as acetone, and esters such as ethyl acetate. When resin particles are removed by dissolution to form a porous film, it is preferable to use resin particles that dissolve in a general-purpose solvent such as tetrahydrofuran, acetone, toluene, ethyl acetate, etc. Depending on the types of resin particles and polyimide precursor used, water may be used as the solvent for removing the resin particles.

[0124] Next, the treatment for removing inorganic particles will be described. Examples of treatments for removing inorganic particles include a method using a liquid (hereinafter sometimes referred to as a "particle removal liquid") that dissolves inorganic particles but not polyimide precursors or polyimides. The particle removal liquid is selected depending on the type of inorganic particles used. Examples of particle removal liquids include aqueous solutions of acids such as hydrofluoric acid, hydrochloric acid, hydrobromic acid, boric acid, perchloric acid, phosphoric acid, sulfuric acid, nitric acid, acetic acid, trifluoroacetic acid, and citric acid; and aqueous solutions of bases such as sodium hydroxide, potassium hydroxide, tetramethylammonium hydroxide, sodium carbonate, potassium carbonate, ammonia, and the above-mentioned organic amines. Furthermore, depending on the types of inorganic particles and polyimide precursors used, water alone may be used as the particle removal liquid.

[0125] -Imidization- In the second step, the heating for imidizing the polyimide precursor in the coating is preferably performed in multiple stages, for example, two or more stages, under the heating conditions shown below. The heating conditions for the first stage are preferably a temperature at which the particle shape is maintained. The heating temperature for the first stage is preferably in the range of 50°C to 150°C, and more preferably in the range of 60°C to 140°C. The heating time for the first stage is preferably in the range of 10 minutes to 60 minutes. The higher the heating temperature for the first stage, the shorter the heating time for the first stage can be. The heating conditions for the second stage include, for example, a temperature of 150°C to 450°C (preferably 200°C to 400°C) for 20 minutes to 120 minutes. Heating conditions within this range allow the imidization reaction to proceed further. During the heating reaction, it is advisable to gradually increase the temperature stepwise or at a constant rate before reaching the final heating temperature. The heating conditions are not limited to the two-stage heating method described above, and a one-stage heating method may also be used. In the case of a one-stage heating method, the imidization may be completed only under the heating conditions shown in the second stage above.

[0126] When the porous polyimide film is used alone, in the second step, the polyimide film-forming substrate used in the first step may be peeled off when a dried film is formed, when the polyimide precursor in the polyimide film is in a state where it is difficult to dissolve in an organic solvent, or when the polyimide film is in a state where imidization is completed.

[0127] A porous polyimide film is obtained through the above steps, and the porous polyimide film may be post-processed depending on the intended use.

[0128] The polyimide precursor solution of the present embodiment may be subjected to a degassing treatment before forming a coating film of the polyimide precursor solution, which is preferable because a porous polyimide film having fewer defects can be obtained more easily than when the degassing treatment is not performed. The defoaming method is not particularly limited, and may be defoaming under reduced pressure (vacuum defoaming) or defoaming under normal pressure. Examples of defoaming under normal pressure include methods that apply centrifugal force such as rotation or revolution. Whether defoaming under normal pressure or defoaming under reduced pressure, the defoaming may be performed while adding treatments such as stirring or heating, as necessary. Defoaming under reduced pressure is preferred because it is simple and has a high defoaming ability. The conditions for the defoaming treatment may be set depending on the degree of remaining bubbles.

[0129] -Imidization rate- Here, the imidization rate of the polyimide precursor will be described. Examples of the partially imidized polyimide precursor include precursors having a structure having units represented by the following general formula (I-1), (I-2), and (I-3).

[0130] [ka]

[0131] In general formulas (I-1), (I-2), and (I-3), A represents a tetravalent organic group, B represents a divalent organic group, l represents an integer of 1 or more, and m and n each independently represent an integer of 0 or 1 or more.

[0132] In addition, A and B in the general formulae (I-1), (I-2), and (I-3) have the same meanings as A and B in the above-mentioned general formula (I).

[0133] The imidization ratio of a polyimide precursor represents the ratio of the number of imide ring-closed bonds (2n+m) to the total number of bonds (2l+2m+2n) in the bonds of the polyimide precursor (reaction sites between tetracarboxylic dianhydride and diamine compound). In other words, the imidization ratio of a polyimide precursor is expressed as "(2n+m) / (2l+2m+2n)".

[0134] The imidization rate of the polyimide precursor (the value of "(2n+m) / (2l+2m+2n)") is measured by the following method.

[0135] -Measurement of imidization rate of polyimide precursor- Preparation of polyimide precursor samples (i) The polyimide precursor solution to be measured is applied to a silicon wafer to a film thickness of 1 μm to 10 μm to prepare a coating sample. (ii) The coating sample is immersed in tetrahydrofuran (THF) for 20 minutes to replace the solvent in the coating sample with tetrahydrofuran (THF). The solvent for immersion is not limited to THF, but can be selected from solvents that do not dissolve the polyimide precursor and are miscible with the solvent components contained in the polyimide precursor solution. Specifically, alcohol solvents such as methanol and ethanol, and ether compounds such as dioxane are used. (iii) The coating sample is removed from the THF, and the THF adhering to the surface of the coating sample is removed by blowing N2 gas. The coating sample is then dried under a reduced pressure of 10 mmHg or less at a temperature of 5 to 25°C for 12 hours or more to prepare a polyimide precursor sample.

[0136] Preparation of 100% imidized standard sample (iv) In the same manner as in (i) above, a polyimide precursor solution to be measured is applied to a silicon wafer to prepare a coating sample. (v) The coating sample is heated at 380°C for 60 minutes to carry out an imidization reaction, and a 100% imidized standard sample is prepared.

[0137] Measurement and analysis (vi) Using a Fourier transform infrared spectrophotometer (FT-730, manufactured by Horiba, Ltd.), the infrared absorption spectra of the 100% imidized standard sample and the polyimide precursor sample are measured. -1 The absorption peak (Ab') derived from the aromatic ring near 1500 cm -1 )) to 1780cm -1 The absorption peak (Ab') (1780 cm) derived from the imide bond -1 )) to find the ratio I'(100). (vii) Similarly, measurements were carried out on the polyimide precursor sample, and the -1 The absorption peak (Ab) derived from the aromatic ring near 1500 cm -1 )) to 1780cm -1 The absorption peak (Ab (1780 cm)) derived from the imide bond near -1 )) to find the ratio I(x).

[0138] Then, the imidization rate of the polyimide precursor is calculated using the measured absorption peaks I'(100) and I(x) based on the following formula. Formula: Imidization rate of polyimide precursor = I(x) / I'(100) ·Formula: I'(100)=(Ab'(1780cm -1 )) / (Ab'(1500cm -1 )) ·Formula: I(x)=(Ab(1780cm -1 )) / (Ab(1500cm -1 ))

[0139] The measurement of the imidization rate of this polyimide precursor is also applied to the measurement of the imidization rate of an aromatic polyimide precursor. When measuring the imidization rate of an aliphatic polyimide precursor, a peak derived from a structure that remains unchanged before and after the imidization reaction is used as an internal standard peak instead of the absorption peak of the aromatic ring.

[0140] <Characteristics of porous polyimide film> (vacancy) The porosity of the porous polyimide film is not particularly limited, but is preferably 45% by volume to 70% by volume, more preferably 50% by volume to 70% by volume, even more preferably 52% by volume to 68% by volume, and particularly preferably 55% by volume to 65% by volume. When the porosity of the porous polyimide film is in the above range, the dielectric constant of the porous polyimide film is lower than when the porosity is lower than the above range, and the mechanical strength of the porous polyimide film is higher than when the porosity is higher than the above range.

[0141] Here, the porosity of the porous polyimide film is determined from the apparent density and true density of the porous polyimide film. The apparent density d is the mass (g) of the porous polyimide film divided by the volume (cm) of the porous polyimide film including the pores. 3 The apparent density d is the mass per unit area of ​​the porous polyimide film (g / m 2 ) by the thickness (μm) of the porous polyimide film. The true density ρ is the mass (g) of the porous polyimide film divided by the volume (cm) of the porous polyimide film excluding the pores (i.e., the volume of only the resin skeleton). 3 ) is the value divided by

[0142] The porosity of the porous polyimide film is calculated by the following formula (II). ·Formula (II) Porosity (volume %)={1-(d / ρ)}×100=[1-{(w / t) / ρ)}]×100 d: apparent density of the porous polyimide film (g / cm 3 ) ρ: True density of porous polyimide film (g / cm 3 ) w: mass per unit area of ​​porous polyimide film (g / m 2 ) t: Thickness of porous polyimide film (μm)

[0143] The pores preferably have a spherical or nearly spherical shape, and preferably have a shape in which the pores are connected to each other.

[0144] The average pore size is not particularly limited, but is preferably in the range of 0.1 μm to 0.5 μm, more preferably in the range of 0.25 μm to 0.48 μm, and even more preferably in the range of 0.25 μm to 0.45 μm.

[0145] The average pore diameter is a value observed and measured using a scanning electron microscope (SEM). Specifically, a porous polyimide film is first cut in the thickness direction to prepare a measurement sample with the cut surface as the measurement surface. This measurement sample is then observed and measured using a KEYENCE VE SEM with the image processing software that comes standard with the measurement sample. Observation and measurement are performed on 100 pore portions of the cross section of the measurement sample, the pore diameter distribution is determined, and the average pore diameter is calculated by averaging these values. If the shape of the pore is not circular, the longest part is used as the diameter.

[0146] (film thickness) The average thickness of the porous polyimide film is not particularly limited and is selected depending on the application. The average thickness of the porous polyimide film may be, for example, 10 μm or more and 1000 μm or less, 20 μm or more, or 30 μm or more, or 500 μm or less, or 400 μm or less. For example, when the porous polyimide film is used as an insulating coating, which is a coating film for an insulated wire described below, from the viewpoint of maintaining sufficient insulation without impairing the volumetric efficiency of the insulated wire, the average thickness of the porous polyimide film is preferably 5 μm or more and 200 μm or less, more preferably 7 μm or more and 150 μm or less, and even more preferably 10 μm or more and 100 μm or less. The average thickness of the porous polyimide film is calculated by measuring the thickness of the porous polyimide film at five points using an eddy current film thickness meter CTR-1500E manufactured by Sanko Electronics Co., Ltd., and calculating the arithmetic mean of the measurements.

[0147] (Dielectric constant) The dielectric constant of the porous polyimide film at 1 kHz is not particularly limited. For example, when the porous polyimide film is used as an insulating coating, which is a coating film for an insulated electric wire described below, from the viewpoint of increasing the corona discharge inception voltage of the electric wire, the dielectric constant of the porous polyimide film at 1 kHz is preferably 2.5 or less, more preferably 2.3 or less, and even more preferably 2.1 or less. The lower limit of the dielectric constant is not particularly specified, but it is preferably larger than 1, which is the dielectric constant of air.

[0148] The relative permittivity at 1 kHz is calculated using the following formula from the capacitance measurement results when an AC electric field of 1 V and 1 kHz is applied using an LCR meter (ZM2353 manufactured by NF Corporation). In the following formula, ε r is the relative permittivity, ε is the dielectric constant, C is the capacitance, l is the thickness, A is the electrode area when measuring the capacitance, and ε0 is the dielectric constant of a vacuum.

[0149]

number

[0150] <Applications of porous polyimide film> Examples of applications of the porous polyimide film include insulating coatings, which are coatings for insulated wires described below; battery separators for lithium batteries and the like; separators for electrolytic capacitors; electrolyte membranes for fuel cells and the like; battery electrode materials; gas or liquid separation membranes; low-dielectric-constant materials; and filtration membranes.

[0151] [Insulated wire] The insulated wire according to the present embodiment includes a wire body and the porous polyimide film provided on the surface of the wire body. In the insulated wire according to the present embodiment, the porous polyimide film is used as an insulating coating that is a coating film that covers the wire body.

[0152] Examples of the electric wire body include wires, rods, and plates made of metals or alloys such as soft copper, hard copper, oxygen-free copper, chromium ore, aluminum, aluminum alloys, nickel, silver, soft iron, steel, and stainless steel. The electric wire body may also be a stranded wire made by twisting together a plurality of wires. The thickness of the wire body is not particularly limited and may be, for example, in the range of 0.1 mm to 5.0 mm. The thickness of the wire body refers to the major axis in a cross section perpendicular to the longitudinal direction of the wire body.

[0153] The porous polyimide film is provided, for example, so as to surround the outer peripheral surface of the wire body. The porous polyimide film may cover the entire outer peripheral surface of the wire body, or may cover only a part of the outer peripheral surface of the wire body. The porous polyimide film may be provided in contact with the surface of the wire body, or may be provided via another layer, such as an inner semiconducting layer, that may be provided between the wire body and the porous polyimide film. Furthermore, other layers may be provided on the outer peripheral surface of the porous polyimide layer, such as an outer semiconducting layer.

[0154] The porous polyimide film serving as the insulating coating may be formed by applying the aforementioned polyimide precursor solution to the outer peripheral surface of the wire body, followed by drying, imidization, and particle removal. Alternatively, the insulating coating may be formed by applying the polyimide precursor solution to the surface of a polyimide film-forming substrate, drying the resulting film, baking the film to be imidized, or baking the film to be imidized and particle removal to be removed from the polyimide film-forming substrate, and then providing the film on the outer peripheral surface of the wire body, followed by heating, etc., as necessary. [Example]

[0155] Examples will be described below, but the present invention is not limited to these examples. In the following description, unless otherwise specified, all "parts" and "%" are by mass.

[0156] [Preparation of Resin Particle Dispersion] -Resin particle dispersion (1)- 360 parts by weight of styrene, 11.9 parts by weight of surfactant Dowfax 2A1 (47% by weight solution, Dow Chemical Company), and 150 parts by weight of deionized water were mixed and emulsified using a dissolver at 1,500 rpm for 30 minutes to produce a monomer emulsion. Next, 0.9 parts by weight of Dowfax 2A1 (47% by weight solution, Dow Chemical Company) and 446.8 parts by weight of deionized water were added to a reaction vessel. After heating to 75°C under a nitrogen stream, 24 parts by weight of the monomer emulsion were added. A polymerization initiator solution consisting of 5.4 parts by weight of ammonium persulfate dissolved in 25 parts by weight of deionized water was then added dropwise over 10 minutes. After reacting for 50 minutes, the remaining monomer emulsion was added dropwise over 180 minutes. After reacting for another 180 minutes, the mixture was cooled to obtain resin particle dispersion (1). The solid content of the resin particle dispersion (1) was 36.0% by mass, and the volume average particle size of the resin particles was 0.38 μm.

[0157] [Preparation of Polyimide Precursor-Containing Liquid] -Polyimide precursor-containing liquid (A)- 560.0 parts by mass of ion-exchanged water was heated to 50°C under a nitrogen stream, and 53.75 parts by mass (50 mol parts) of p-phenylenediamine and 146.25 parts by mass (50 mol parts) of 3,3',4,4'-biphenyltetracarboxylic dianhydride were added with stirring. A mixture of 150.84 parts by mass (150 mol parts) of N-methylmorpholine (hereinafter also referred to as "MMO") and 89.16 parts by mass of ion-exchanged water was added over 20 minutes with stirring at 50°C under a nitrogen stream. The reaction was carried out at 50°C for 15 hours to obtain a polyimide precursor-containing solution (A). The solids concentration of the polyimide precursor-containing solution (A) was 20.0% by mass.

[0158] Example 1 [Preparation of Resin Particle-Dispersed Polyimide Precursor Solution (PAA-1)] 289.65 parts by mass of polyimide precursor-containing liquid (A), 172.41 parts by mass of resin particle dispersion liquid (1), and 137.94 parts by mass of aqueous solvent (a mixed solution of NMP and water, mass ratio = 30.41:107.53) were mixed. The mixture was subjected to ultrasonic dispersion at 50°C for 30 minutes. 0.37 parts by mass (0.14 mol) of pyromellitic acid, a specific compound, was added as a crosslinking agent to obtain a polyimide precursor solution (PAA-1). The weight average molecular weight of the polyimide precursor (A1) contained in the polyimide precursor solution (PAA-1) was 30,000. The resin content relative to the total amount of polyimide precursor and particles ("particle content (vol %)" in the table) was measured by the above-mentioned method, and the results are shown in Table 1. Table 1 also shows the content of the specific compound relative to 100 parts by mole of units derived from tetracarboxylic dianhydride contained in the polyimide precursor ("content ratio (parts by mole)" in the table).

[0159] <Examples 2 to 7> Polyimide precursor solutions (PAA-2) to (PAA-7) were obtained in the same manner as in Example 1, except that the particle content relative to the total amount of polyimide precursor and particles ("Particle content (vol %)" in the table), the type of specific compound ("Type" in the table), and the content of the specific compound relative to 100 molar parts of units derived from tetracarboxylic dianhydride contained in the polyimide precursor ("Content ratio (molar parts)" in the table) were set as shown in Table 1. In Table 1, specific compounds 1 to 4 refer to the following compounds. Specific compound 1: Pyromellitic acid Specific compound 2: 1,4,5,8-naphthalenetetracarboxylic acid Specific compound 3: 1,2,3,4-cyclobutanetetracarboxylic acid Specific compound 4: 3,3',4,4'-benzophenonetetracarboxylic acid

[0160] <Comparative Example 1> A polyimide precursor solution (PAA-8) was obtained in the same manner as in Example 1, except that the specific compound was not used. In Table 1, "-" indicates that the corresponding component was not added.

[0161] <Comparative Example 2> A polyimide precursor solution (PAA-9) was obtained in the same manner as in Example 1, except that 7.1 parts by mass (0.14 parts by mole) of an aqueous urethane resin (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., product name: Elastron BN-P17) was used as a crosslinking agent instead of the specific compound.

[0162] <Evaluation> The polyimide precursor solutions obtained in each example were used to produce porous polyimide films.

[0163] (Production of porous polyimide films) The polyimide precursor solution obtained in each example was applied to a 1.0 mm thick glass substrate over an area of ​​10 cm x 10 cm using an applicator, and then dried in an oven at 80°C for 30 minutes to obtain a coating. The applicator gap was adjusted so that the average thickness of the dried coating would be 30 μm. The glass substrate on which the film was formed was left to stand in an oven heated to 400°C for 2 hours to bake the film, and then the film was immersed in ion-exchanged water to peel it off from the glass substrate and dried to obtain a porous polyimide film.

[0164] (Porosity measurement) The porosity (vol %) of the resulting porous polyimide film was measured by the above-mentioned method, and the results are also shown in Table 1.

[0165] (Measurement of dielectric constant) The dielectric constant of the resulting porous polyimide film was measured by the method described above, and the results are shown in Table 1.

[0166] (Evaluation of tensile strength) The tensile strength of the resulting porous polyimide film at 25° C. was measured using a tensile tester (Strograph VI-C, manufactured by Toyo Seiki Seisakusho) and evaluated according to the following criteria. The results are shown in Table 1. -Evaluation criteria- A: Tensile strength is 80 MPa or more B: Tensile strength is 60 MPa or more and less than 80 MPa C: Tensile strength less than 60 MPa

[0167] (Evaluation of bending resistance) The resulting porous polyimide film was folded in half, a 500 g load was applied to the fold, and the film was left standing for 24 hours. The load was then released, and the folded portion was visually inspected and evaluated according to the following criteria. The results are shown in Table 1. -Evaluation criteria- A: There are traces of bending, but no damage to the film is observed. B: There are traces of folding and some cracks in the film are visible in the folded area. C: The film cracks at the bent part.

[0168] [Table 1]

[0169] The results shown in Table 1 indicate that the porous polyimide film prepared using the polyimide precursor solution obtained in this example has both a lower dielectric constant and higher mechanical strength than the film prepared in the comparative example. [Explanation of symbols]

[0170] 10 Porous polyimide film 10A hole 31 Base material

Claims

1. a polyimide precursor which is a polymer of a tetracarboxylic dianhydride and a diamine compound; Particles and A solvent, a compound having four or more carboxy groups in one molecule; Contains the particles are selected from the group consisting of resin particles, silica particles, magnesium oxide particles, alumina particles, zirconia particles, calcium carbonate particles, calcium oxide particles, titanium dioxide particles, zinc oxide particles, and cerium oxide particles; The particles have a volume average particle size D50v of 0.1 μm or more and 30 μm or less, The polyimide precursor solution has a content of the particles in a volume ratio of 50% by volume to 70% by volume with respect to the total volume of the polyimide precursor and the particles.

2. A polyimide precursor solution as described in claim 1, wherein the particle content is 50 volume % or more and 60 volume % or less relative to the total volume of the polyimide precursor and the particles.

3. A polyimide precursor solution as described in Claim 2, wherein the volume average particle diameter D50V of the particles is 0.25 μm or more and 1 μm or less.

4. 4. The polyimide precursor solution according to claim 1, wherein an amount of the compound having four or more carboxy groups in one molecule is 0.3 parts by mole or more and 2.0 parts by mole or less with respect to 100 parts by mole of the tetracarboxylic dianhydride.

5. 5. The polyimide precursor solution according to claim 4, wherein the content of the compound having four or more carboxy groups in one molecule is 0.5 parts by mole or more and 1.5 parts by mole or less with respect to 100 parts by mole of the tetracarboxylic dianhydride.

6. 6. The polyimide precursor solution according to claim 1, wherein the compound having four or more carboxy groups in one molecule is an aromatic tetracarboxylic acid.

7. 7. The polyimide precursor solution according to claim 6, wherein the aromatic tetracarboxylic acid has five or less aromatic rings in one molecule.

8. 8. The polyimide precursor solution according to claim 1, wherein the polyimide precursor is a polymer of an aromatic tetracarboxylic dianhydride and an aromatic diamine compound.

9. 9. The polyimide precursor solution according to claim 1, wherein the solvent contains water.

10. The polyimide precursor solution according to claim 9 , further comprising an organic amine compound.

11. A porous polyimide film, which is a porous fired product having pores obtained from the polyimide precursor solution according to any one of claims 1 to 10.

12. The porous polyimide film according to claim 11, having a porosity of 50% by volume or more and 70% by volume or less.

13. The wire body, the porous polyimide film according to claim 11 or 12 provided on the surface of the electric wire body; An insulated wire having

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