Porous polyimide film, porous polyimide film production method, and varnish for porous polyimide film formation

Abstract

Provided are a porous polyimide film that has small pore size and excellent fluid permeability, a production method therefor, and a varnish for porous polyimide film formation that is preferably used in said production of the porous polyimide film.　The average pore size of the porous polyimide film as measured by a half-dry method is 30 nm or less. A polyimide resin constituting the porous polyimide film is formed by heating a polyamide acid (A). The tensile modulus of elasticity of a film of the polyamide resin as measured by a specific method is 2.0-5.0 GPa.

Description

Porous polyimide film, method for producing porous polyimide film, and varnish for forming porous polyimide film

[0001] The present invention relates to a porous polyimide film, a method for producing a porous polyimide film, and a varnish for forming a porous polyimide film.

[0002] Various porous membranes have been used for a long time as filters for separating gases or liquids, etc. In recent years, porous membranes have also been increasingly used as separators for secondary batteries such as lithium batteries.

[0003] For example, a known method for producing a porous polyimide film involves coating a substrate with a varnish in which silica particles are dispersed in a solution of polyamic acid or polyimide, and then heating the coated film as necessary to obtain a polyimide film containing silica particles, and then eluting and removing the silica in the polyimide film with hydrogen fluoride water to make it porous (see Patent Document 1).

[0004] Patent No. 5605566

[0005] As the porous membrane described above, porous membranes with small pore diameters are required. For example, in semiconductor manufacturing, porous membranes are used as filters to remove foreign matter contained in cleaning solutions, coating solutions, etc. As semiconductors become increasingly miniaturized, the size of foreign matter that causes defects also becomes smaller. In order to capture small foreign matter, porous membranes with small pore diameters are required. Note that porous membranes with small pore diameters can be manufactured using a porous membrane manufacturing composition (varnish) containing fine particles such as silica with small particle diameters. However, reducing the pore diameter of a porous membrane causes shrinkage during the membrane formation process, resulting in a problem of a decrease in the flow rate of liquids and gases passing through the porous membrane.

[0006] The present invention has been made in view of the above circumstances, and aims to provide a porous polyimide film having a small pore size and excellent fluid permeability, a method for producing the same, and a varnish for forming a porous polyimide film that is suitable for use in producing the porous polyimide film.

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that the above-mentioned problems can be solved by a porous polyimide film in which the average pore size of the porous polyimide film measured by the half-dry method is 30 nm or less, the polyimide resin constituting the porous polyimide film is formed by heating a polyamic acid (A), and the tensile modulus of the polyimide resin film measured by a specific method is 2.0 GPa or more and 5.0 GPa or less, and have completed the present invention. Specifically, the present invention provides the following.

[0008] A first aspect of the present invention is a porous polyimide film, wherein the average pore size of the porous polyimide film measured by a half-dry method is 30 nm or less, and the polyimide resin constituting the porous polyimide film is formed by heating polyamic acid (A), and the porous polyimide film comprises the following steps 1) to 5): 1) preparing a solution in which polyamic acid (A) is dissolved in an amide-based organic solvent and the concentration of polyamic acid (A) is 20 mass %, 2) applying the solution to a glass substrate so that the film thickness after drying is 50 μm to form a coating film, and then drying the coating film at 90° C. for 150 minutes using a hot plate to form a cured film, 3) peeling the cured film from the glass substrate and heating at 400° C. for 30 minutes, 4) cutting a strip-shaped test piece having a width of 10 mm and a length of 90 mm from the cured film, and 5) performing a tensile test on the test piece under conditions of a grip distance of 50 mm and a tensile speed of 10 mm / min, The porous polyimide film has a tensile modulus of elasticity of the polyimide resin film measured by a method including the steps of:

[0009] A second aspect of the present invention is a method for producing a porous polyimide film according to the first aspect, comprising the steps of preparing a varnish containing polyamic acid (A) and fine particles (B); applying the varnish onto a substrate to form a composite film containing polyamic acid (A) and fine particles (B); heating the composite film; and removing the fine particles (B) from the heated composite film, and comprising the steps of: 1) preparing a solution in which polyamic acid (A) is dissolved in an amide-based organic solvent, and the concentration of polyamic acid (A) is 20% by mass; 2) applying the solution onto a glass substrate so that the film thickness after drying is 50 μm to form a coating film, and then drying the coating film at 90° C. for 150 minutes using a hot plate to form a cured film; 3) peeling the cured film from the glass substrate and heating it at 400° C. for 30 minutes; 4) cutting a strip-shaped test piece having a width of 10 mm and a length of 90 mm from the cured film; 5) conducting a tensile test on the test piece under conditions of a gripping distance of 50 mm and a tensile speed of 10 mm / min, wherein the tensile modulus of the polyimide resin film measured by a method including the steps of: is 2.0 GPa or more and 5.0 GPa or less.

[0010] A third aspect of the present invention is a porous polyimide film-forming varnish comprising polyamic acid (A) and fine particles (B), wherein the tensile modulus of the polyimide resin film measured by a method including the following steps 1) to 5): 1) preparing a solution in which the polyamic acid (A) is dissolved in an amide-based organic solvent, the polyamic acid (A) having a concentration of 20 mass %, 2) applying the solution to a glass substrate so that the film thickness after drying is 50 μm to form a coating film, and then drying the coating film at 90°C for 150 minutes using a hot plate to form a cured film, 3) peeling the cured film from the glass substrate and heating it at 400°C for 30 minutes, 4) cutting a strip-shaped test piece having a width of 10 mm and a length of 90 mm from the cured film, and 5) conducting a tensile test on the test piece under conditions of a gripping distance of 50 mm and a tensile speed of 10 mm / min, is 2.0 GPa or more and 5.0 GPa or less, and the average particle size of the fine particles (B) is 80 nm or less.

[0011] According to the present invention, it is possible to provide a porous polyimide film having a small pore size and excellent fluid permeability, a method for producing the same, and a varnish for forming a porous polyimide film that is suitably used for producing the porous polyimide film.

[0012] Hereinafter, the embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments and can be practiced with appropriate modifications within the scope of the object of the present invention.

[0013] <<Varnish for Forming Porous Polyimide Film>> The varnish for forming a porous polyimide film (hereinafter also referred to as “varnish”) contains a polyamic acid (A), fine particles (B), and a solvent (S). The varnish for forming a porous polyimide film may also contain a polyimide as a resin component.

[0014] [Polyamic Acid (A)] As described above, the varnish for forming a porous polyimide film contains polyamic acid (A). The polyamic acid (A) will be described below.

[0015] [Polyamic Acid] The polyamic acid may be any product obtained by polymerizing any tetracarboxylic dianhydride and diamine. The amounts of the tetracarboxylic dianhydride and diamine used are not particularly limited, but the amount of the diamine used is preferably 0.50 to 1.50 mol, more preferably 0.60 to 1.30 mol, and particularly preferably 0.70 to 1.20 mol, per mol of the tetracarboxylic dianhydride.

[0016] The tetracarboxylic dianhydride can be appropriately selected from tetracarboxylic dianhydrides that have conventionally been used as raw materials for synthesizing polyamic acids. The tetracarboxylic dianhydride may be an aromatic tetracarboxylic dianhydride or an aliphatic tetracarboxylic dianhydride. From the viewpoint of the heat resistance of the resulting polyimide resin, it is preferable to use an aromatic tetracarboxylic dianhydride as the tetracarboxylic dianhydride. One type of tetracarboxylic dianhydride may be used alone, or two or more types may be used in combination.

[0017] Specific examples of suitable aromatic tetracarboxylic dianhydrides include pyromellitic dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4'-biphenyltetracarboxylic dianhydride, and 2,2,6,6-biphenyltetracarboxylic dianhydride. Carboxylic acid dianhydrides, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis(2,3-dicarboxyphenyl)propane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride, 2,2-bis(2,3-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride, 3,3',4,4'-benzophenone tetraanhydride Carboxylic acid dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride, bis(2,3-dicarboxyphenyl)ether dianhydride, 2,2',3,3'-benzophenonetetracarboxylic acid dianhydride, 4,4-(p-phenylenedioxy)diphthalic dianhydride, 4,4-(m-phenylenedioxy)diphthalic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic acid Examples of the aliphatic tetracarboxylic dianhydride include 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,2,3,4-benzenetetracarboxylic dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride, 2,3,6,7-anthracenetetracarboxylic dianhydride, 1,2,7,8-phenanthrenetetracarboxylic dianhydride, 9,9-bisphthalic anhydride fluorene, 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride, etc. Examples of the aliphatic tetracarboxylic dianhydride include ethylene tetracarboxylic dianhydride, butane tetracarboxylic dianhydride, cyclopentane tetracarboxylic dianhydride, cyclohexane tetracarboxylic dianhydride, 1,2,4,5-cyclohexane tetracarboxylic dianhydride, and 1,2,3,4-cyclohexane tetracarboxylic dianhydride.Among these, 3,3',4,4'-biphenyltetracarboxylic dianhydride and pyromellitic dianhydride are preferred in terms of price, availability, etc. These tetracarboxylic dianhydrides may be used alone or in combination of two or more.

[0018] The diamine can be appropriately selected from diamines conventionally used as raw materials for synthesizing polyamic acid. The diamine may be an aromatic diamine or an aliphatic diamine. From the viewpoint of the heat resistance of the resulting polyimide resin, aromatic diamines are preferred. These diamines may be used alone or in combination of two or more.

[0019] Examples of aromatic diamines include diamino compounds having one or about 2 to 10 phenyl groups bonded to one another, such as phenylenediamine and its derivatives, diaminobiphenyl compounds and their derivatives, diaminodiphenyl compounds and their derivatives, diaminotriphenyl compounds and their derivatives, diaminonaphthalene and its derivatives, aminophenylaminoindan and its derivatives, diaminotetraphenyl compounds and their derivatives, diaminohexaphenyl compounds and their derivatives, and cardo-type fluorenediamine derivatives.

[0020] Phenylenediamines include m-phenylenediamine, p-phenylenediamine, etc. Phenylenediamine derivatives include diamines to which alkyl groups such as methyl groups and ethyl groups are bonded, such as 2,4-diaminotoluene and 2,4-triphenylenediamine.

[0021] In diaminobiphenyl compounds, two aminophenyl groups are bonded to each other, such as 4,4'-diaminobiphenyl, 4,4'-diamino-2,2'-dimethylbiphenyl, and 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl.

[0022] A diaminodiphenyl compound is a compound in which two aminophenyl groups are bonded together via another group. The bond may be an ether bond, a sulfonyl bond, a thioether bond, a bond via an alkylene or its derivative group, an imino bond, an azo bond, a phosphine oxide bond, an amide bond, or a ureylene bond. The number of carbon atoms in the alkylene bond is approximately 1 to 6. A derivative group of an alkylene group is an alkylene group substituted with one or more halogen atoms, etc.

[0023] Examples of the diaminodiphenyl compound include 3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl sulfone, 3,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenylmethane, 3,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl ketone, 3,4'-diaminodiphenyl ketone, 2,2-bis(p-aminophenyl)propane, 2,2'-bis(p-aminophenyl)hexafluoropropane, 4-methyl-2,4-bis(p-aminophenyl)-1-pentene, 4-methyl-2,4-bis(p-aminophenyl)

[0033] Examples of the bis(4-aminophenoxy)phenyl compound include 1,4-bis(4-aminophenoxy)phenyl, 2,4-bis(4-aminophenoxy)phenyl, 4-methyl-2,4-bis(p-aminophenyl)pentane, bis(p-aminophenyl)phosphine oxide, 4,4'-diaminoazobenzene, 4,4'-diaminodiphenylurea, 4,4'-diaminodiphenylamide, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 4,4'-bis(4-aminophenoxy)biphenyl, bis[4-(4-aminophenoxy)phenyl]sulfone, bis[4-(3-aminophenoxy)phenyl]sulfone, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane.

[0024] Among these, p-phenylenediamine, m-phenylenediamine, 2,4-diaminotoluene, and 4,4'-diaminodiphenyl ether are preferred in terms of cost, availability, and the like.

[0025] A diaminotriphenyl compound is a compound in which two aminophenyl groups and one phenylene group are bonded via other groups. The other groups are selected from the same groups as those in the diaminodiphenyl compound. Examples of diaminotriphenyl compounds include 1,3-bis(m-aminophenoxy)benzene, 1,3-bis(p-aminophenoxy)benzene, and 1,4-bis(p-aminophenoxy)benzene.

[0026] Examples of diaminonaphthalenes include 1,5-diaminonaphthalene and 2,6-diaminonaphthalene.

[0027] Examples of aminophenylaminoindan include 5 or 6-amino-1-(p-aminophenyl)-1,3,3-trimethylindan.

[0028] Examples of the diaminotetraphenyl compound include 4,4'-bis(p-aminophenoxy)biphenyl, 2,2'-bis[p-(p'-aminophenoxy)phenyl]propane, 2,2'-bis[p-(p'-aminophenoxy)biphenyl]propane, and 2,2'-bis[p-(m-aminophenoxy)phenyl]benzophenone.

[0029] Examples of the cardo-type fluorenediamine derivatives include 9,9-bisanilinefluorene.

[0030] The number of carbon atoms of the aliphatic diamine is preferably, for example, about 2 to 15. Specific examples of the aliphatic diamine include pentamethylenediamine, hexamethylenediamine, and heptamethylenediamine.

[0031] The diamine may be a compound in which the hydrogen atom of the diamine is substituted with at least one substituent selected from the group consisting of halogen atoms, methyl groups, methoxy groups, cyano groups, phenyl groups, and the like.

[0032] There are no particular limitations on the means for producing the polyamic acid, and any known method can be used, such as reacting an acid and a diamine component in a solvent.

[0033] The reaction between the tetracarboxylic dianhydride and the diamine is usually carried out in a solvent. The solvent used for the reaction between the tetracarboxylic dianhydride and the diamine is not particularly limited as long as it can dissolve the tetracarboxylic dianhydride and the diamine and does not react with the tetracarboxylic dianhydride and the diamine. One solvent may be used alone, or two or more solvents may be used in combination.

[0034] Examples of solvents used in the reaction of tetracarboxylic dianhydride with diamine include nitrogen-containing polar solvents such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-diethylacetamide, N,N-dimethylformamide, N,N-diethylformamide, N-methylcaprolactam, and N,N,N',N'-tetramethylurea; lactone-based polar solvents such as β-propiolactone, γ-butyrolactone, γ-valerolactone, δ-valerolactone, γ-caprolactone, and ε-caprolactone; dimethyl sulfoxide; acetonitrile; fatty acid esters such as ethyl lactate and butyl lactate; ethers such as diethylene glycol dimethyl ether, diethylene glycol diethyl ether, dioxane, tetrahydrofuran, methyl cellosolve acetate, and ethyl cellosolve acetate; and phenolic solvents such as cresols and xylene-based mixed solvents. These solvents may be used alone or in combination of two or more. There is no particular limitation on the amount of solvent used, but the amount of solvent used is preferably an amount that results in a polyamic acid content of 5 to 50% by mass.

[0035] Among these solvents, nitrogen-containing polar solvents such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-diethylacetamide, N,N-dimethylformamide, N,N-diethylformamide, N-methylcaprolactam, and N,N,N',N'-tetramethylurea are preferred in terms of the solubility of the polyamic acid produced.

[0036] The polymerization temperature is generally −10 to 120° C., preferably 5 to 30° C. The polymerization time varies depending on the raw material composition used, but is usually 3 to 24 hours. The polyamic acid may be used alone or in combination of two or more types.

[0037] When a polyimide resin film (cured film) is formed using the polyamic acid (A) by the method described below, the tensile modulus of the polyimide resin film is preferably 2.0 GPa or more and 5.0 GPa or less, and more preferably 2.5 GPa or more and 4.5 GPa or less.

[0038] In order to obtain a polyimide resin that can provide a film exhibiting the above-mentioned tensile modulus, it is preferable that the polyamic acid (A) be a polymer obtained using two or more diamines. Specific examples include a polyamic acid (A) composed of 3,3',4,4'-biphenyltetracarboxylic dianhydride, p-phenylenediamine, and 4,4'-diaminodiphenyl ether, and a polyamic acid (A) composed of pyromellitic dianhydride, p-phenylenediamine, and 4,4'-diaminodiphenyl ether. Furthermore, even when polyamic acids (A) having different monomer compositions are used in combination, a polyimide resin film exhibiting the above-mentioned tensile modulus can be easily formed. As a specific example, a polyimide resin film exhibiting the above-mentioned tensile modulus can be formed by using a combination of a polyamic acid made of 3,3',4,4'-biphenyltetracarboxylic dianhydride and p-phenylenediamine with a polyamic acid made of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether, or a combination of a polyamic acid made of pyromellitic dianhydride and p-phenylenediamine with a polyamic acid made of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether.

[0039] (Method for measuring the tensile modulus of a polyimide resin film) Polyamic acid (A) is dissolved in an amide-based organic solvent to prepare a solution with a polyamic acid (A) concentration of 20% by mass. The solution is applied to a glass substrate so that the film thickness after drying is 50 μm to form a coating film, and the coating film is then dried at 90° C. for 150 minutes using a hot plate to form a cured film. The cured film is peeled off from the glass substrate and heated at 400° C. for 30 minutes. This gives a cured film (polyimide resin film) for measuring the tensile modulus.

[0040] For the measurement of the tensile modulus, a rectangular test piece 10 mm wide x 90 mm long is cut out from the cured film for measuring the tensile modulus. A tensile test is performed on the test piece using a Shimadzu Autograph (500 N) at a grip distance of 50 mm and a tensile speed of 10 mm / min to measure the tensile modulus of the polyimide resin film.

[0041] [Fine particles (B)] The fine particles (B) may be a single fine particle, not a mixture of multiple types of fine particles having different particle size distributions and average particle sizes, or a mixture of multiple types of fine particles having different particle size distributions and average particle sizes. When the fine particles (B) contain multiple types of fine particles having different particle size distributions and average particle sizes, the fine particles (B) contain small fine particles (B1) and fine particles (B2) having an average particle size larger than the fine particles (B1).

[0042] The pores (spherical pores) in a porous polyimide film are formed by removing individual microparticles present in a composite film (resin-particle composite film) formed using a varnish in a subsequent microparticle removal step. The porous polyimide film has a structure in which spherical pores are interconnected (hereinafter referred to as interconnected pores). The interconnected pores are formed in a method for producing a porous polyimide film by removing, in a subsequent microparticle removal step, a plurality of microparticles (B) that are present in contact with each other in a composite film (resin-particle composite film) formed using a varnish. The locations where the spherical pores in the interconnected pores are interconnected originate from the locations (contact points) where the plurality of microparticles (B) come into contact with each other before being removed. When two types of fine particles (B) of different sizes, i.e., small fine particles (B1) and fine particles (B2) having an average particle size larger than that of the fine particles (B1), are used, the diameter of the communicating holes formed at the points where the fine particles come into contact with each other (contact points) depends on the particle size of the small fine particles (fine particles (B1)), and the diameter of the communicating holes formed at the contact points between the small fine particles (fine particles (B1)) becomes equal to that of the communicating holes formed at the contact points between the small fine particles (fine particles (B1)).

[0043] When a single type of fine particles is used as the fine particles (B) instead of a mixture of multiple types of fine particles having different particle size distributions and average particle sizes, the average particle size of the fine particles (B) is preferably 80 nm or less, more preferably 50 nm or less, and even more preferably 30 nm or less. The average particle size of the fine particles (B) may be 10 nm or more.

[0044] When the microparticles (B) contain the above-mentioned microparticles (B1) and microparticles (B2) as two types of microparticles having different particle size distributions and average particle sizes, the average particle size of the microparticles (B1) is preferably 80 nm or less, more preferably 50 nm or less, and even more preferably 30 nm or less. The average particle size of the microparticles (B1) may be 10 nm or more. The average particle size of the microparticles (B2) is preferably 100 nm or less, more preferably 80 nm or less, and even more preferably 60 nm or less. The average particle size of the microparticles (B2) may be 40 nm or more.

[0045] The difference between the average particle size of the fine particles (B1) and the average particle size of the fine particles (B2) is not particularly limited and may be, for example, 5 nm to 80 nm, 10 nm to 70 nm, or 20 nm to 60 nm.

[0046] In this specification, the average particle size of the fine particles (B) is D50, which means the particle size at an integrated value of 50% in the volume-based particle size distribution determined by a laser diffraction / scattering method.

[0047] The ratio (D2 / D1) of the average particle size (D2) of the fine particles (B2) to the average particle size (D1) of the fine particles (B1) is preferably 1.2 to 6.0.

[0048] The ratio of the mass of the fine particles (B2) to the mass of the fine particles (B1) is preferably 0.10 to 0.90, more preferably 0.20 to 0.80, even more preferably 0.30 to 0.70, and particularly preferably 0.40 to 0.60.

[0049] The material of the fine particles (B) is not particularly limited, and any known material can be used, and may be either an inorganic material or an organic material, as long as it is insoluble in the solvent (S) contained in the varnish and can be removed from the composite film (resin-particle composite film) formed using the varnish in the fine particle removal step. Note that the material of the fine particles (B1) and the material of the fine particles (B2) may be different, but are preferably the same.

[0050] Examples of the inorganic material fine particles (B) include inorganic oxide fine particles. Specific examples of the inorganic material fine particles (B) include silica (silicon dioxide) fine particles, titanium oxide fine particles, and alumina (Al ２ O ３ Examples of silica include colloidal silica. Among them, monodispersed spherical silica particles are preferred because they can form uniform pores.

[0051] Examples of fine particles of organic materials include fine particles of organic polymers such as high molecular weight olefins (polypropylene, polyethylene, etc.), polystyrene, epoxy resins, cellulose, polyvinyl alcohol, polyvinyl butyral, polyesters, and polyethers.

[0052] Furthermore, it is preferable that the fine particles (B) have a high sphericity and a small particle size distribution index. Fine particles satisfying these conditions (B) have excellent dispersibility in the varnish and can be used in a state where they do not aggregate with each other.

[0053] [Solvent (S)] The solvent (S) is not particularly limited as long as it can dissolve the polyamic acid (A) and polyimide contained in the varnish and does not dissolve the fine particles (B). Examples of the solvent (S) include the solvents exemplified as solvents used in the reaction between tetracarboxylic dianhydride and diamine. The solvent (S) may be used alone or in combination of two or more.

[0054] [Dispersant] A dispersant may be added together with the fine particles (B) to uniformly disperse the fine particles (B) in the varnish. Adding a dispersant allows the fine particles (B) to be mixed more uniformly in the varnish and to be uniformly distributed in the film formed from the varnish. As a result, dense openings can be formed on the surface of the finally obtained porous polyimide film, and the front and back surfaces can be efficiently connected, thereby improving the air permeability of the porous polyimide film. Furthermore, adding a dispersant tends to improve the drying properties of the varnish and the peelability of the formed unsintered composite film from the substrate, etc.

[0055] The dispersant is not particularly limited, and known dispersants can be used. For example, anionic surfactants such as coconut fatty acid salts, castor sulfated oil salts, lauryl sulfate salts, polyoxyalkylene allyl phenyl ether sulfate salts, alkyl benzene sulfonic acid, alkyl benzene sulfonate salts, alkyl diphenyl ether disulfonate salts, alkyl naphthalene sulfonate salts, dialkyl sulfosuccinate salts, isopropyl phosphate, polyoxyethylene alkyl ether phosphate salts, and polyoxyethylene allyl phenyl ether phosphate salts; cationic surfactants such as oleylamine acetate, lauryl pyridinium chloride, cetyl pyridinium chloride, lauryl trimethyl ammonium chloride, stearyl trimethyl ammonium chloride, behenyl trimethyl ammonium chloride, and didecyl dimethyl ammonium chloride; amphoteric surfactants such as coconut alkyl dimethyl amine oxide, fatty acid amidopropyl dimethyl amine oxide, alkyl polyaminoethyl glycine hydrochloride, amido betaine surfactants, alanine surfactants, and lauryliminodipropionic acid; Nonionic surfactants of polyoxyalkylene primary alkyl ethers or polyoxyalkylene secondary alkyl ethers, such as ethylene octyl ether, polyoxyethylene decyl ether, polyoxyethylene lauryl ether, polyoxyethylene laurylamine, polyoxyethylene oleylamine, polyoxyethylene polystyrylphenyl ether, and polyoxyalkylene polystyrylphenyl ether; other polyoxyalkylene-based nonionic surfactants, such as polyoxyethylene dilaurate, polyoxyethylene laurate, polyoxyethylenated castor oil, polyoxyethylenated hydrogenated castor oil, sorbitan laurate, polyoxyethylene sorbitan laurate, and fatty acid diethanolamides; fatty acid alkyl esters, such as octyl stearate and trimethylolpropane tridecanoate; and polyether polyols, such as polyoxyalkylene butyl ether, polyoxyalkylene oleyl ether, and trimethylolpropane tris(polyoxyalkylene) ether, but are not limited to these.The above dispersants may also be used in combination of two or more.

[0056] In the varnish, the content of the dispersant is, for example, preferably 0.01% by mass or more and 5% by mass or less, more preferably 0.05% by mass or more and 1% by mass or less, and even more preferably 0.1% by mass or more and 0.5% by mass or less, relative to the mass of the microparticles (B), from the viewpoint of film-forming properties.

[0057] The method for producing the porous polyimide film-forming varnish (varnish) is not particularly limited. The varnish is typically produced by a step of preparing a particle dispersion liquid by dispersing the particles (B) in a solvent, a step of preparing a resin solution containing the polyamic acid (A), and a step of kneading the particle dispersion liquid and the resin solution together to adjust the concentration.

[0058] The solid content of the varnish is preferably 10% by mass or more and 50% by mass or less. The varnish is produced by kneading for preferably 2 minutes to 10 hours, more preferably 2 minutes to 60 minutes. For kneading the varnish, a rotation / revolution mixer (product name: Awatori Rentaro, manufactured by Thinky Corporation), a planetary mixer, a bead mill, or the like can be used. In the kneading process, a mixture of a mixture of a cross-sectional area of ​​1960 μm ２ 785,000 μm or more ２ A dispersion process may be performed by using a dispersion device having the following flow path, and passing a mixed liquid (slurry) containing a microparticle dispersion and a resin solution pressurized to 50 MPa or more through the flow path. An example of a method for performing a dispersion process by passing the mixed liquid through such a flow path is the method described in JP 2020-104105 A.

[0059] <<Method for Producing Porous Polyimide Film>> The method for producing a porous polyimide film includes a composite film-forming step of forming a composite film made of the aforementioned porous polyimide film-forming varnish on a substrate, and a particle removal step of removing particles from the composite film. The method for producing a porous polyimide film may also include a baking step of baking the composite film using polyamic acid (A) as a resin component after the composite film-forming step and before the particle removal step.

[0060] [Composite Film Formation Step (Production of Unbaked Composite Film)] In the composite film formation step, a composite film made of the aforementioned porous polyimide film-forming varnish (varnish) is formed on a substrate. The unbaked composite film can be formed, for example, by applying the varnish to a substrate and then drying it at 0 to 100°C under normal pressure or in vacuum, preferably at 10 to 100°C under normal pressure. Examples of the substrate include a PET film, a SUS substrate, and a glass substrate.

[0061] The unsintered composite film is then baked to obtain a polyimide-fine particle composite film, which is then subjected to a baking step. When the unsintered composite film is formed on a substrate, it may be baked as is, or the unsintered composite film may be peeled off from the substrate before the baking step.

[0062] When peeling the composite film (unfired composite film) from the substrate, a substrate pre-formed with a release layer can be used to further enhance the film's releasability. When a release layer is pre-formed on the substrate, a release agent is applied to the substrate and dried or baked before applying the varnish. The release agent used here can be any known release agent, such as an alkyl ammonium phosphate salt, a fluorine-based agent, or a silicone-based agent, without any particular restrictions. When peeling the dried unfired composite film from the substrate, a small amount of release agent remains on the peeled surface of the unfired composite film, which can cause discoloration during firing and adversely affect the electrical properties, so it is preferable to remove it as much as possible. To remove the release agent, a cleaning step can be introduced in which the unfired composite film peeled from the substrate is washed with an organic solvent.

[0063] On the other hand, when the substrate is used as is without providing a release layer for forming the green composite film, the above-mentioned release layer forming step and the above-mentioned cleaning step can be omitted. Furthermore, in the production of the green composite film, the following optional steps may be provided before the firing step described below: a step of immersing in a solvent containing water, a pressing step, and a drying step after the immersion step.

[0064] [Baking Step (Production of Resin-Particle Composite Film)] The baking step is a step of baking the composite film after the composite film formation step and before the particle removal step. By subjecting the varnish containing the polyamic acid (A) to a post-treatment by heating (baking step), a composite film (resin-particle composite film) consisting of a polyimide resin and particles (B) is formed.

[0065] The baking temperature in the baking step varies depending on whether or not a condensing agent is used, but is preferably 120°C or higher and 450°C or lower, more preferably 150°C or higher and 420°C or lower. When an organic material is used for the fine particles (B), the temperature must be set lower than the thermal decomposition temperature. The imidization of the polyamic acid (A) contained in the varnish is preferably completed in the baking step.

[0066] The firing method may be, for example, a method in which the temperature is raised from room temperature to 400° C. or lower over 3 hours and then held at 400° C. for 20 minutes, or a stepwise drying-thermal imidization method in which the temperature is raised from room temperature to 400° C. in steps of 50° C. (each step held for 20 minutes) and finally held at 400° C. for 20 minutes. When an unfired composite film is formed on a substrate and then temporarily peeled off from the substrate, a method in which the edges of the unfired composite film are fixed to a stainless steel mold or the like to prevent deformation can be used.

[0067] The thickness of the resulting resin-particle composite film can be determined by measuring the thickness at multiple locations with, for example, a micrometer and averaging the measured thicknesses. The thickness of the resin-particle composite film is appropriately set taking into consideration the thickness of the porous polyimide film to be finally obtained.

[0068] [Particle Removal Step (Making Resin-Particle Composite Film Porous)] In the particle removal step, the particles (B) are removed from the composite film after the composite film formation step (or from the composite film (resin-particle composite film) after the baking step, if a baking step has been performed). By selecting an appropriate method to remove the particles (B), a porous polyimide film can be produced with good reproducibility.

[0069] When silica is used as the material of the fine particles (B), for example, the resin-fine particle composite film can be treated with low-concentration hydrogen fluoride water or the like to dissolve and remove the silica.

[0070] An organic material can also be selected as the material for the fine particles (B). There are no particular limitations on the organic material, as long as it decomposes at a lower temperature than the resin contained in the resin-particle composite film. For example, resin fine particles made of a linear polymer or a known depolymerizable polymer can be used. In a normal linear polymer, the molecular chain of the polymer is randomly cut upon thermal decomposition, while in a depolymerizable polymer, the polymer is decomposed into monomers upon thermal decomposition. In either case, the polymer is a low molecular weight substance or a CO ２ By decomposing the resin particles to a temperature of 200°C or higher, they disappear from within the porous polyimide film. The decomposition temperature of the resin particles used is preferably 200°C or higher and 320°C or lower, and more preferably 230°C or higher and 260°C or lower. If the decomposition temperature is 200°C or higher, film formation can be performed even when a high-boiling point solvent is used in the varnish, and the range of selection of baking conditions for the resin-particle composite film becomes wider. Furthermore, if the decomposition temperature is lower than 320°C, only the resin particles can be eliminated without thermally damaging the resin contained in the resin-particle composite film.

[0071] The total thickness of the porous polyimide film is not particularly limited. When the porous polyimide film is used as a filter, the thickness of the porous polyimide film is preferably 5 μm or more and 500 μm or less, more preferably 10 μm or more and 300 μm or less, and even more preferably 15 μm or more and 100 μm or less. The thickness can be determined by measuring the thickness at multiple locations with, for example, a micrometer or the like and averaging the thicknesses, as in the case of measuring the resin-particle composite film.

[0072] [Resin Removal Step] The method for producing a porous polyimide film may include a resin removal step of removing at least a portion of the resin portion of the resin-particle composite film before the particle removal step, or removing at least a portion of the porous polyimide film after the particle removal step. By removing at least a portion of the resin portion of the resin-particle composite film or at least a portion of the porous polyimide film, it is possible to improve the porosity of the final porous polyimide film compared to when the resin portion is not removed.

[0073] The step of removing at least a portion of the resin portion or the step of removing at least a portion of the porous polyimide film can be carried out by a conventional chemical etching method, a physical removal method, or a combination of these methods.

[0074] Examples of chemical etching methods include treatment with a chemical etching solution such as an inorganic alkaline solution or an organic alkaline solution. Inorganic alkaline solutions are preferred. Examples of inorganic alkaline solutions include hydrazine solutions containing hydrazine hydrate and ethylenediamine; solutions of basic alkali metal compounds such as potassium hydroxide, sodium hydroxide, sodium carbonate, sodium silicate, and sodium metasilicate; ammonia solutions; and etching solutions containing alkali metal hydroxides, hydrazine, and 1,3-dimethyl-2-imidazolidinone as main components. Examples of organic alkaline solutions include solutions of organic bases such as primary amines such as ethylamine and n-propylamine; secondary amines such as diethylamine and di-n-butylamine; tertiary amines such as triethylamine and methyldiethylamine; alkanolamines such as dimethylethanolamine and triethanolamine; quaternary ammonium salts such as tetramethylammonium hydroxide and tetraethylammonium hydroxide; and cyclic amines such as pyrrole and piheridine.

[0075] The solvent contained in each of the above solutions can be appropriately selected from pure water and organic solvents such as alcohols. Two or more solvents may be used in combination. An appropriate amount of surfactant may be added to the alkaline etching solution. The concentration of the basic compound relative to the mass of the alkaline etching solution is, for example, 0.01% by mass or more and 20% by mass or less.

[0076] As a physical method, for example, dry etching using plasma (oxygen, argon, etc.), corona discharge, etc., or a surface treatment method in which an abrasive (e.g., alumina (hardness 9), etc.) is dispersed in a liquid and irradiated onto the surface of the film at a speed of 30 to 100 m / s can be used.

[0077] The above-mentioned method is preferable because it can be applied to the resin removal step either before or after the fine particle removal step.

[0078] On the other hand, as a physical method applicable only to the resin removal process performed after the particle removal process, a method can be employed in which the target surface is pressed against a mount film (e.g., a polyester film such as a PET film) wetted with a liquid, and then the porous polyimide film is peeled off from the mount film either without drying or after drying. Due to the surface tension or electrostatic adhesion of the liquid, the porous polyimide film is peeled off from the mount film with only the surface layer of the porous polyimide film remaining on the mount film.

[0079] According to the method for producing a porous polyimide film described above, a porous polyimide film having a small pore size and a high flow rate can be produced.

[0080] In the produced porous polyimide film, the IPA flow rate measured by the method described later in [IPA flow rate (FR)] is 0.01 mL / (min cm ２ ) or more, and 0.025 mL / (min cm ２In addition, the average pore diameter of the produced porous polyimide film, as measured by the half-dry method described later in [Average pore diameter (nm)], is preferably 30 nm or less, more preferably 25 nm or less, and even more preferably 21 nm or less.

[0081] The porous polyimide film thus produced contains a structure in which the spherical pores are interconnected (communicating pores). The openings in the porous polyimide film are the portions of the surface of the porous polyimide film where the communicating pores open.

[0082] The spherical shape of a hole is a concept that includes a perfect sphere, but is not necessarily limited to a perfect sphere. A spherical shape may be substantially a perfect sphere, and may also include a shape that can be recognized as a nearly perfect sphere when a magnified image of the hole is visually observed. Specifically, in a spherical hole, the surface that defines the hole is a curved surface, and the curved surface may define a perfect or nearly perfect spherical hole.

[0083] Individual spherical pores are typically formed by removing individual particles (B) present in the composite film (resin-particle composite film) in a subsequent particle removal step. Furthermore, interconnected pores are formed in the method for producing a porous polyimide film by removing multiple particles (B) present in contact with each other in the resin-particle composite film in a subsequent particle removal step. The interconnected spherical pores are formed at locations where multiple particles (B) contact each other before removal.

[0084] The diameter of the openings in the porous polyimide film may be adjusted, for example, within a range of 10 nm to 5000 nm, depending on the intended use of the porous polyimide film. The diameter of the openings is equal to or approximately equal to the diameter of the spherical holes that make up the communicating holes. The communicating holes, formed by a series of spherical holes of such diameter, allow fluid to pass smoothly through the porous polyimide film. The porous polyimide film has communicating holes that penetrate the film in the thickness direction as fluid flow paths. This allows fluid to pass from one main surface of the porous polyimide film to the other main surface. Furthermore, when the laminate is used as a filter, the fluid passes through the interior of the porous polyimide film while contacting the curved surfaces that define the individual spherical holes. The contact area of ​​the fluid within the porous polyimide film is quite large due to the presence of communicating holes made up of spherical holes. Therefore, when a fluid passes through a laminate including a porous polyimide film, it is thought that minute substances present in the fluid are easily adsorbed by the spherical holes in the porous polyimide film.

[0085] <Porous Polyimide Film> The porous polyimide film is made of polyimide. The porous polyimide film is produced by a production method including forming a composite film made of the aforementioned porous polyimide film-forming varnish (varnish) on a substrate and removing the fine particles (B) from the composite film. That is, the porous polyimide film can be produced by the aforementioned porous polyimide film production method. Since the porous polyimide film is produced using the aforementioned porous polyimide film-forming varnish (varnish), it is possible to provide a porous polyimide film with a small pore size and an increased fluid flow rate.

[0086] As described above, the present inventors have provided the following (1) to (7): (1) a porous polyimide film, wherein the average pore size of the porous polyimide film measured by the half-dry method is 30 nm or less, and the polyimide resin constituting the porous polyimide film is formed by heating polyamic acid (A), and the following steps 1) to 5) are performed: 1) preparing a solution in which polyamic acid (A) is dissolved in an amide-based organic solvent and the concentration of polyamic acid (A) is 20% by mass, 2) applying the solution to a glass substrate so that the film thickness after drying is 50 μm to form a coating film, and then drying the coating film at 90°C for 150 minutes using a hot plate to form a cured film, 3) peeling the cured film from the glass substrate and heating it at 400°C for 30 minutes, 4) cutting a strip-shaped test piece having a width of 10 mm and a length of 90 mm from the cured film, and 5) performing a tensile test on the test piece under conditions of a grip distance of 50 mm and a tensile speed of 10 mm / min, (2) The porous polyimide film according to (1), having a tensile modulus of elasticity of the polyimide resin film of 2.0 GPa or more and 5.0 GPa or less, as measured by a method comprising the steps of: (1) forming a porous polyimide film having spherical or nearly spherical pores interconnected with each other;(3) A method for producing the porous polyimide film according to (1) or (2), comprising: preparing a varnish containing polyamic acid (A) and fine particles (B); applying the varnish onto a substrate to form a composite film containing polyamic acid (A) and fine particles (B); heating the composite film; and removing the fine particles (B) from the heated composite film, and comprising the steps of: 1) preparing a solution in which polyamic acid (A) is dissolved in an amide-based organic solvent, and the concentration of polyamic acid (A) is 20% by mass; 2) applying the solution onto a glass substrate so that the film thickness after drying is 50 μm to form a coating film, and then drying the coating film at 90° C. for 150 minutes using a hot plate to form a cured film; 3) peeling the cured film from the glass substrate and heating it at 400° C. for 30 minutes; 4) cutting out a strip-shaped test piece having a width of 10 mm and a length of 90 mm from the cured film; (5) A method for producing a porous polyimide film, wherein the tensile modulus of the polyimide resin film measured by a method including: conducting a tensile test on a test piece under conditions of a gripping distance of 50 mm and a tensile speed of 10 mm / min is 2.0 GPa or more and 5.0 GPa or less. (4) A method for producing a porous polyimide film according to (3), wherein the fine particles (B) have an average particle size of 80 nm or less. (5) A method for producing a porous polyimide film according to (3) or (4), wherein the fine particles (B) are spherical or approximately spherical fine particles.(6) A porous polyimide film-forming varnish comprising polyamic acid (A) and fine particles (B), wherein the tensile modulus of the polyimide resin film measured by a method including the following steps 1) to 5): 1) preparing a solution in which the polyamic acid (A) is dissolved in an amide-based organic solvent and the concentration of the polyamic acid (A) is 20% by mass; 2) applying the solution to a glass substrate so that the film thickness after drying is 50 μm to form a coating film, and then drying the coating film at 90°C for 150 minutes using a hot plate to form a cured film; 3) peeling the cured film from the glass substrate and heating it at 400°C for 30 minutes; 4) cutting a strip-shaped test piece having a width of 10 mm and a length of 90 mm from the cured film; and 5) conducting a tensile test on the test piece under conditions of a gripping distance of 50 mm and a tensile speed of 10 mm / min., is 2.0 GPa or more and 5.0 GPa or less; and wherein the average particle size of the fine particles (B) is 80 nm or less. (7) The varnish for forming a porous polyimide film according to (6), wherein the fine particles (B) are spherical or nearly spherical fine particles.

[0087] The present invention will be described in more detail below with reference to examples, but the scope of the present invention is not limited to the following examples.

[0088] Polyamic acid (A) was prepared using the following tetracarboxylic dianhydrides and diamines as shown in Table 1. Tetracarboxylic dianhydrides: 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) (a1-1) Pyromellitic dianhydride (PMDA) (a1-2) Diamines: p-phenylenediamine (PDA) (a2-1) 4,4'-diaminodiphenyl ether (ODA) (a2-2) 4,4'-diamino-2,2'-dimethylbiphenyl (a2-3)

[0089] The tensile modulus of elasticity of the cured film (polyimide resin film) formed using the prepared polyamic acid (A) was measured according to the following method. The results are shown in Table 1.

[0090] [Tensile Modulus] Each polyamic acid (A) listed in Table 1 was dissolved in an amide-based organic solvent to prepare a solution containing 20% ​​by mass of polyamic acid (A). The solution was applied to a glass substrate to form a coating film having a dry thickness of 50 μm. The coating film was then dried at 90°C for 150 minutes using a hot plate to form a cured film. The cured film was peeled from the glass substrate and heated at 400°C for 30 minutes. A strip-shaped test piece measuring 10 mm wide and 90 mm long was cut from the cured film (polyimide resin film). A tensile test was performed on the test piece using a Shimadzu Corporation autograph (500 N) under conditions of a grip distance of 50 mm and a tensile speed of 10 mm / min to measure the tensile modulus of the polyimide resin film.

[0091]

[0092] [Examples 1 to 6 and Comparative Examples 1 to 3] A silica dispersion (containing 0.5% by mass of dispersant relative to silica) was added to a polyamic acid solution so that the ratio of the mass of polyamic acid (A) to the mass of silica (fine particles (B)) was as shown in Table 2. Furthermore, organic solvents (1) and (2) were added separately so that the solvent composition in the entire final composition (porous polyimide film-forming varnish (varnish)) was organic solvent (1):organic solvent (2) = 45:55 by mass. The resulting mixture was stirred at 4000 rpm for 30 minutes in a 1000 mL container for dispersion. Thereafter, the mixture was dispersed using a stirring blade of 60 μm diameter (cross-sectional area 2826 μm) manufactured by Yoshida Kikai Kogyo Co., Ltd. ２A dispersion treatment was carried out 10 times by passing the mixed solution through the flow path at 200 MPa using a dispersion device equipped with a flow path of 1000 MPa, to prepare a varnish for forming a porous polyimide film (varnish) with a solids concentration of 20% by mass. The following polyamic acid solutions, organic solvents, dispersants, and fine particles were used. Polyamic acid solution: A solution of polyamic acid (A) shown in Table 1 (solids content 20% by mass (organic solvent: N,N-dimethylacetamide)) Organic solvent (1): N-methyl-2-pyrrolidone Organic solvent (2): N,N-dimethylacetamide (DMAc) Dispersant: Polyoxyethylene secondary alkyl ether-based dispersant Fine particles (B): Silica with an average particle size of 30 nm, silica with an average particle size of 50 nm, silica with an average particle size of 80 nm, or silica with an average particle size of 100 nm

[0093] The obtained porous polyimide film-forming varnish was applied to a polyethylene terephthalate (PET) film substrate using an applicator and dried at 90°C for 5 minutes to form a composite film on the substrate (composite film formation step). This composite film (unbaked composite film) was peeled from the substrate and placed in an oven and baked at 380°C for 15 minutes to complete the imidization and obtain a resin-particle composite film (baking step). The peeled resin-particle composite film was immersed in hydrogen fluoride (HF) for 10 minutes to remove the silica particles contained in the film (particle removal step). After washing with water and drying, porous polyimide films of Examples 1 to 6 and Comparative Examples 1 to 3, each with a film thickness of 20 μm, were obtained.

[0094] The average pore size (nm), IPA flow rate (FR), shrinkage rate, and tensile elongation of the obtained porous polyimide films of the Examples and Comparative Examples were measured according to the following methods. The results are shown in Table 2.

[0095] [Average pore diameter (nm)] The average pore diameter (nm) was measured using a liquid porometer (manufactured by Porometer). Specifically, each porous polyimide film was cut into a membrane filter size of 25 mm diameter and attached to a measurement holder. The secondary side of the membrane (the downstream side of the flow path of the porous polyimide film) was wetted with isobutyl alcohol, and pressure was gradually applied from the primary side with a fluorinated solvent (Porefil, manufactured by Aptco), and the average pore diameter was measured in accordance with the half-dry method (ASTM E1294-89). The measurement was carried out at room temperature (25°C).

[0096] [IPA Flow Rate (FR)] Each porous polyimide film was cut into a membrane filter size of 47 mm in diameter and attached to an in-line filter holder. Next, isopropyl alcohol (IPA) was pressurized with air at 0.1 MPa from the primary side (upstream side of the flow path of the porous polyimide film) to perform filtration, and the flow rate was measured. The measurement was carried out at 25°C. In Table 1, ２ Expressed in mL per minute.

[0097] [Shrinkage Rate] The thickness Th1 of the resin-particle composite film obtained by the above method and the thickness Th2 of the porous polyimide film obtained by the above method were measured. The shrinkage rate (%) was calculated based on the measured film thickness using the following formula: Shrinkage rate (%) = (1 - Th2 / Th1) x 100

[0098] [Tensile elongation] Each polyimide porous film was cut into a dumbbell-shaped No. 6 test piece to prepare it. A small tabletop tester EZ Test EZ-SX (manufactured by Shimadzu Corporation) was used as the tester, and the tensile elongation (%) of the test piece was measured under the measurement conditions of 25°C and 5 mm / min in accordance with JIS K 7127. The tensile elongation (%) was calculated by dividing the length (20 mm) of the parallel part of the test piece by L ０ The length of the parallel part of the test piece until it broke (the length of the parallel part of the test piece when it broke: 20 mm + α) was defined as L, and the tensile elongation was calculated using the following formula: Tensile elongation (%) = {(L - L ０ ) / L ０} x 100

[0099] *1 Measurement impossible due to film shrinkage *2 Measurement impossible due to poor film formation

[0100] According to Tables 1 and 2, in Comparative Example 2, in which a polyamic acid (A) having a tensile modulus of elasticity of the polyimide resin film measured by the above-mentioned specified method of less than 2.0 GPa was used, it was found that the use of small particle size silica (fine particles (B)) made it difficult to form a good film, and the IPA flow rate of the obtained porous polyimide film was low. Note that, as shown in Comparative Example 1, when a large particle size silica (fine particles (B)) was used, a good film could be formed even if a polyamic acid (A) having a tensile modulus of elasticity of the polyimide resin film measured by the above-mentioned specified method of less than 2.0 GPa was used. Furthermore, in Comparative Example 3, in which a polyamic acid (A) having a tensile modulus of elasticity of the polyimide resin film measured by the above-mentioned specified method of more than 5.0 GPa was used, it was found that the use of small particle size silica (fine particles (B)) made it difficult to form a film. On the other hand, in Examples 1 to 6, which used polyamic acid (A) having a polyimide resin film with a tensile modulus of elasticity of 2.0 GPa or more and 5.0 GPa or less as measured by the above-mentioned predetermined method, it is found that, compared to Comparative Example 2, porous polyimide films exhibiting a high IPA flow rate (FR) can be successfully formed even when silica (fine particles (B)) with a small particle size is used.

Claims

1. A porous polyimide film, wherein the average pore size of the porous polyimide film measured by a half-dry method is 30 nm or less, and the polyimide resin constituting the porous polyimide film is formed by heating polyamic acid (A), and the following steps 1) to 5) are performed: 1) preparing a solution in which the polyamic acid (A) is dissolved in an amide-based organic solvent, and the concentration of the polyamic acid (A) is 20% by mass; 2) applying the solution to a glass substrate so that the film thickness after drying is 50 μm to form a coating film, and then drying the coating film at 90°C for 150 minutes using a hot plate to form a cured film; 3) peeling the cured film from the glass substrate and heating it at 400°C for 30 minutes; 4) cutting a strip-shaped test piece having a width of 10 mm and a length of 90 mm from the cured film; and 5) performing a tensile test on the test piece under conditions of a grip distance of 50 mm and a tensile speed of 10 mm / min.

4. A porous polyimide film, wherein the tensile modulus of the polyimide resin film measured by a method comprising the steps of:

2. The porous polyimide film according to claim 1, which has interconnected pores in the form of spherical or nearly spherical pores.

3. A method for producing a porous polyimide film according to claim 1, comprising: preparing a varnish containing polyamic acid (A) and fine particles (B); applying the varnish onto a substrate to form a composite film containing the polyamic acid (A) and the fine particles (B); heating the composite film; and removing the fine particles (B) from the heated composite film, and comprising the steps of: 1) preparing a solution in which the polyamic acid (A) is dissolved in an amide-based organic solvent, and the concentration of the polyamic acid (A) is 20% by mass; 2) applying the solution onto a glass substrate so that the film thickness after drying is 50 μm to form a coating film, and then drying the coating film at 90°C for 150 minutes using a hot plate to form a cured film; 3) peeling the cured film from the glass substrate and heating it at 400°C for 30 minutes; 4) cutting a strip-shaped test piece having a width of 10 mm and a length of 90 mm from the cured film; 5) conducting a tensile test on the test piece under conditions of a gripping distance of 50 mm and a tensile speed of 10 mm / min. The method for producing a porous polyimide film, wherein the tensile modulus of the polyimide resin film measured by a method comprising the steps of:

4. The method for producing a porous polyimide film according to claim 3, wherein the average particle size of the fine particles (B) is 80 nm or less.

5. The method for producing a porous polyimide film according to claim 3 or 4, wherein the fine particles (B) are spherical or nearly spherical fine particles.

6. A polyimide resin film comprising polyamic acid (A) and fine particles (B), wherein the tensile modulus of the polyimide resin film measured by a method including the following steps 1) to 5): 1) preparing a solution in which the polyamic acid (A) is dissolved in an amide-based organic solvent and the concentration of the polyamic acid (A) is 20% by mass; 2) applying the solution to a glass substrate so that the film thickness after drying is 50 μm to form a coating film, and then drying the coating film at 90°C for 150 minutes using a hot plate to form a cured film; 3) peeling the cured film from the glass substrate and heating it at 400°C for 30 minutes; 4) cutting a strip-shaped test piece having a width of 10 mm and a length of 90 mm from the cured film; and 5) conducting a tensile test on the test piece under conditions of a gripping distance of 50 mm and a tensile speed of 10 mm / min. The porous polyimide film-forming varnish, wherein the fine particles (B) have an average particle size of 80 nm or less.

7. The varnish for forming a porous polyimide film according to claim 6, wherein the fine particles (B) are spherical or nearly spherical fine particles.

Images