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

Abstract

To provide a porous polyimide film with 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 in the production of the aforementioned porous polyimide film. [Solution] 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, the polyimide resin constituting the porous polyimide film is formed by heating 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.

Description

Technical Field

[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.

Background Art

[0002] Conventionally, various porous membranes have been used in applications such as filters that are membranes for separating gases or liquids. In recent years, the application of porous membranes to separators for secondary batteries such as lithium batteries has also been progressing.

[0003] For example, as a method for producing a porous membrane of polyimide, a varnish in which silica particles are dispersed in a solution of polyamic acid or polyimide is applied onto a substrate, and then, if necessary, the coated film is heated to obtain a polyimide film containing silica particles. Next, a method of eluting and removing silica in the polyimide film with hydrofluoric acid water to make it porous is known (see Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] As the above-mentioned porous membrane, a porous membrane with a small pore diameter is required. For example, in the manufacture of semiconductors, a porous membrane is used as a filter for removing foreign substances contained in cleaning liquids, coating liquids, etc. As semiconductor miniaturization progresses, the size of foreign substances that cause defects also becomes smaller. In order to capture small foreign substances, a porous membrane with a small pore diameter is required. Incidentally, a porous membrane with a small pore diameter can be manufactured by using a composition (varnish) for producing a porous membrane containing fine particles such as silica with a small particle diameter. However, reducing the pore size of a porous membrane can lead to shrinkage during the film formation process, resulting in 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 suitably used in the production of the aforementioned porous polyimide film. [Means for solving the problem]

[0007] The inventors diligently conducted research to solve the above problems. As a result, they discovered that the above 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 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 thus completed the present invention. Specifically, the present invention provides the following.

[0008] A first aspect of the present invention is a porous polyimide film, 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 polyamic acid (A), 1)~5) below) 1) Prepare 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) After applying the solution to a glass substrate so that the dry film thickness is 50 μm, the coated film is dried using a hot plate at 90°C for 150 minutes to form a cured film. 3) Peel the cured film from the glass substrate and heat it at 400°C for 30 minutes. 4) Cut out strip-shaped test pieces measuring 10 mm wide x 90 mm long from the cured film, and 5) The test specimen shall be subjected to a tensile test under the conditions of a gripping distance of 50 mm and a tensile speed of 10 mm / min. This is a porous polyimide film in which the tensile modulus of the polyimide resin film, as measured by a method including [specific method], is between 2.0 GPa and 5.0 GPa.

[0009] A second aspect of the present invention is a method for producing a porous polyimide film according to the first aspect, The manufacturing method involves preparing a varnish containing polyamic acid (A) and fine particles (B), The process involves applying varnish to a substrate to form a composite film containing polyamic acid (A) and fine particles (B), Heating the composite film, This includes removing fine particles (B) from a heated composite film, 1)~5) below) 1) Prepare 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) After applying the solution to a glass substrate so that the dry film thickness is 50 μm, the coated film is dried using a hot plate at 90°C for 150 minutes to form a cured film. 3) Peel the cured film from the glass substrate and heat it at 400°C for 30 minutes. 4) Cut out strip-shaped test pieces measuring 10 mm wide x 90 mm long from the cured film, and 5) The test specimen shall be subjected to a tensile test under the conditions of a gripping distance of 50 mm and a tensile speed of 10 mm / min. This is a method for producing a porous polyimide film in which the tensile modulus of the polyimide resin film, as measured by a method including [specific method], is 2.0 GPa or more and 5.0 GPa or less.

[0010] A third aspect of the present invention comprises a polyamic acid (A) and fine particles (B), 1)~5) below) 1) Prepare 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) Apply the solution onto a glass substrate so that the film thickness after drying is 50 μm to form a coating film, and then dry the coating film at 90 °C for 150 minutes using a hot plate to form a cured film. 3) Peel the cured film from the glass substrate and heat it at 400 °C for 30 minutes. 4) Cut out a strip-shaped test piece with a width of 10 mm and a length of 90 mm from the cured film, and 5) Conduct a tensile test on the test piece under the conditions of 50 mm between gripping tools and a tensile speed of 10 mm / min. The tensile modulus of the polyimide resin film measured by the method including the above is 2.0 GPa or more and 5.0 GPa or less, and it is a varnish for forming a porous polyimide film in which the average particle size of the fine particles (B) is 80 nm or less.

Advantages of the Invention

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

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments in any way, and can be implemented 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 polyamic acid (A), fine particles (B), and a solvent (S). Further, the varnish for forming a porous polyimide film may contain polyimide as a resin component.

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

[0015] [Polyamic acid] As the polyamic acid, any product obtained by polymerizing any tetracarboxylic dianhydride and a diamine can be used without particular limitations. The amounts of tetracarboxylic dianhydride and diamine used are not particularly limited, but it is preferable to use 0.50 to 1.50 moles of diamine per mole of tetracarboxylic dianhydride, more preferably 0.60 to 1.30 moles, and particularly preferably 0.70 to 1.20 moles of diamine.

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

[0017] Suitable specific examples of 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-biphenyltetra 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 tetra Carboxylic acid dianhydrides, bis(3,4-dicarboxyphenyl) ether dianhydrides, bis(2,3-dicarboxyphenyl) ether dianhydrides, 2,2',3,3'-benzophenonetetracarboxylic acid dianhydrides, 4,4-(p-phenylenedioxy)diphthalic acid dianhydrides, 4,4-(m-phenylenedioxy)diphthalic acid dianhydrides, 1,2,5,6-naphthalenetetracarbone dianhydrides, 1,4,5,8-naphthalenetetracarboxylic acid Examples of dianhydrides include 2,3,6,7-naphthalenetetracarboxylic acid dianhydride, 1,2,3,4-benzenetetracarboxylic acid dianhydride, 3,4,9,10-perylenetetracarboxylic acid dianhydride, 2,3,6,7-anthracenetetracarboxylic acid dianhydride, 1,2,7,8-phenanthrenetetracarboxylic acid dianhydride, 9,9-bisphthalic anhydride fluorene, and 3,3',4,4'-diphenylsulfonetetracarboxylic acid dianhydride. Examples of aliphatic tetracarboxylic acid dianhydrides include ethylenetetracarboxylic acid dianhydride, butanetetracarboxylic acid dianhydride, cyclopentanetetracarboxylic acid dianhydride, cyclohexanetetracarboxylic acid dianhydride, 1,2,4,5-cyclohexanetetracarboxylic acid dianhydride, and 1,2,3,4-cyclohexanetetracarboxylic acid dianhydride. Among these, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride and pyromellitic acid dianhydride are preferred due to their price, availability, etc.Furthermore, these tetracarboxylic dianhydrides can be used individually or in combination of two or more types.

[0018] The diamine can be appropriately selected from diamines that have conventionally been used as raw materials for the synthesis of polyamic acids. The diamine may be an aromatic diamine or an aliphatic diamine, but an aromatic diamine is preferred in terms of the heat resistance of the resulting polyimide resin. These diamines may be used individually or in combination of two or more.

[0019] Examples of aromatic diamines include diamino compounds in which one or two to ten phenyl groups are bonded. Specifically, these include 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 full orangeamine derivatives.

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

[0021] In diaminobiphenyl compounds, two aminophenyl groups are bonded together. Examples include 4,4'-diaminobiphenyl, 4,4'-diamino-2,2'-dimethylbiphenyl, and 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl.

[0022] Diaminodiphenyl compounds are compounds in which two aminophenyl groups are linked together via other groups. The linkages include ether bonds, sulfonyl bonds, thioether bonds, alkylene or its derivative groups, imino bonds, azo bonds, phosphine oxide bonds, amide bonds, and ureylene bonds. The number of carbon atoms in the alkylene bond is approximately 1 to 6. Alkylene derivative groups are alkylene groups substituted with one or more halogen atoms.

[0023] Examples of diaminodiphenyl compounds 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) Examples include 2-pentene, iminodianiline, 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 due to their price, availability, etc.

[0025] Diaminotriphenyl compounds are compounds in which two aminophenyl groups and one phenylene group are bonded together via other groups. The other groups are selected to be the same as those used in diaminodiphenyl compounds. 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] An example of aminophenylaminoindanes is 5 or 6-amino-1-(p-aminophenyl)-1,3,3-trimethylindanes.

[0028] Examples of diaminotetraphenyl compounds 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 cardo-type fluororangeamine derivatives include 9,9-bisanilinefluorene.

[0030] The number of carbon atoms in aliphatic diamines is preferably around 2 to 15. Specific examples of aliphatic diamines include pentamethylenediamine, hexamethylenediamine, and heptamethylenediamine.

[0031] Furthermore, these diamines may be compounds in which the hydrogen atoms are substituted with at least one substituent selected from the group consisting of halogen atoms, methyl groups, methoxy groups, cyano groups, phenyl groups, etc.

[0032] There are no particular restrictions on the means used to produce polyamic acid; for example, known methods such as reacting an acid and a diamine component in a solvent can be used.

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

[0034] Examples of solvents used in the reaction between tetracarboxylic dianhydrides and diamines 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 cellulose acetate, and ethyl cellulose acetate; and phenolic solvents such as cresols and xylene-based mixed solvents. These solvents may be used individually or in combination of two or more. There are no particular restrictions on the amount of solvent used, but it is desirable that the resulting polyamic acid content be 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 due to their solubility of the resulting polyamic acid.

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

[0037] When forming a polyimide resin film (cured film) using 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] To obtain a polyimide resin that gives a film exhibiting the above tensile modulus, it is preferable that the polyamic acid (A) is a polymer obtained using two or more diamines. Specific examples include polyamic acid (A) composed of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, p-phenylenediamine, and 4,4'-diaminodiphenyl ether, and polyamic acid (A) composed of pyromellitic acid dianhydride, p-phenylenediamine, and 4,4'-diaminodiphenyl ether. Furthermore, even when using a combination of polyamic acids (A) with different monomer compositions, it is easy to form a polyimide resin film exhibiting the above-mentioned tensile modulus. Specifically, by using a combination of a polyamic acid consisting of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride and p-phenylenediamine and a polyamic acid consisting of pyromellitic acid dianhydride and 4,4'-diaminodiphenyl ether, or by using a combination of a polyamic acid consisting of pyromellitic acid dianhydride and p-phenylenediamine and a polyamic acid consisting of pyromellitic acid dianhydride and 4,4'-diaminodiphenyl ether, a polyimide resin film exhibiting the above-mentioned tensile modulus can be formed.

[0039] (Method for measuring the tensile modulus of polyimide resin films) A solution is prepared by dissolving polyamic acid (A) in an amide-based organic solvent to obtain a solution with a concentration of 20% by mass of polyamic acid (A). This solution is applied to a glass substrate to form a coating film with a dry film thickness of 50 μm. The coating film is then dried on a hot plate at 90°C for 150 minutes to form a cured film. The cured film is peeled off the glass substrate and heated at 400°C for 30 minutes. This yields a cured film (polyimide resin film) for tensile modulus measurement.

[0040] To measure the tensile modulus, a strip-shaped test piece measuring 10 mm wide x 90 mm long is cut from the cured film for tensile modulus measurement. A tensile test is then performed on this test piece using a Shimadzu Autograph (500N) under the 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.

[0041] [Fine particles (B)] The fine particles (B) may be a single fine particle, or a mixture of multiple fine particles having different particle size distributions and average particle sizes. If the fine particles (B) include multiple fine particles having different particle size distributions and average particle sizes, then the fine particles (B) include small fine particles (B1) and fine particles (B2) with an average particle size larger than that of fine particles (B1).

[0042] The pores (spherical pores) in a porous polyimide film are formed when individual microparticles present in a composite film (resin-microparticle composite film) formed using varnish are removed in a subsequent microparticle removal process. A porous polyimide film contains a structure in which spherical pores are interconnected (hereinafter abbreviated as interconnected pores). In the manufacturing method of a porous polyimide film, interconnected pores are formed when multiple microparticles (B) present in contact with each other in a composite film (resin-microparticle composite film) formed using varnish are removed in a subsequent microparticle removal process. The locations where the spherical pores in the interconnected pores are connected originate from the locations (contact points) where the multiple microparticles (B) were in contact with each other before removal. When two types of fine particles (B) of different sizes are used, namely small fine particles (B1) and fine particles (B2) with an average particle size larger than that of fine particles (B1), the diameter of the communication 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 will be the same diameter as the communication holes formed at the contact points between the small fine particles (fine particles (B1)).

[0043] When using a single particle as the fine particle (B), rather than a mixture of multiple types of fine particles with different particle size distributions and average particle sizes, the average particle size of the fine particle (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 particle (B) may also be 10 nm or more.

[0044] When the fine particles (B) include the fine particles (B1) and fine particles (B2) described above as two types of fine particles with different particle size distributions and average particle sizes, the average particle size of fine particles (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 fine particles (B1) may also be 10 nm or more. The average particle size of fine particles (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 fine particles (B2) may also 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, but for example, it may be between 5 nm and 80 nm, between 10 nm and 70 nm, or between 20 nm and 60 nm.

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

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

[0048] The ratio of the mass of fine particles (B2) to the mass of 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, as long as it is insoluble in the solvent (S) contained in the varnish and can be removed from the composite film (resin-fine particle composite film) formed using the varnish in the fine particle removal process. It may be an inorganic material or an organic material. The materials of the fine particles (B1) and the fine particles (B2) may be different, but it is preferable that they be the same.

[0050] Examples of inorganic material nanoparticles (B) include inorganic oxide nanoparticles, and specific examples include metal oxide nanoparticles such as silica (silicon dioxide) nanoparticles, titanium oxide nanoparticles, and alumina (Al2O3) nanoparticles. Colloidal silica is one example of silica. Among these, monodisperse spherical silica particles are preferred because they can form uniform pores.

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

[0052] Furthermore, it is preferable that the fine particles (B) have a high sphericity and a small particle size distribution index. Fine particles that meet these conditions (B) exhibit excellent dispersibility in varnish and can be used without agglomerating 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). An example of solvent (S) is the solvent exemplified as the solvent used in the reaction between tetracarboxylic dianhydride and diamine. Solvent (S) may be used alone or in combination of two or more types.

[0054] [Dispersant] To uniformly disperse the fine particles (B) in the varnish, a dispersant may be added along with the fine particles (B). By adding a dispersant, the fine particles (B) can be mixed more uniformly into the varnish, and furthermore, the fine particles (B) can be uniformly distributed in the film into which the varnish has been formed. As a result, dense openings can be created on the surface of the final porous polyimide film, and the front and back surfaces can be efficiently connected, improving the air permeability of the porous polyimide film. Furthermore, by adding a dispersant, the drying properties of the varnish are improved, and the peelability of the formed unfired composite film from the substrate is also improved.

[0055] The dispersant is not particularly limited and any known dispersant can be used. For example, anionic surfactants such as coconut fatty acid salt, castor sulfate, lauryl sulfate, polyoxyalkylene allylphenyl ether sulfate, alkylbenzene sulfonic acid, alkylbenzene sulfonate, alkyldiphenyl ether disulfonate, alkylnaphthalene sulfonate, dialkyl sulfosuccinate, isopropyl phosphate, polyoxyethylene alkyl ether phosphate, polyoxyethylene allylphenyl ether phosphate; cationic surfactants such as oleylamine acetate, laurylpyridinium chloride, cetylpyridinium chloride, lauryltrimethylammonium chloride, stearyltrimethylammonium chloride, behenyltrimethylammonium chloride, didecyldimethylammonium chloride; amphoteric surfactants such as coconut alkyldimethylamine oxide, fatty acid amidopropyl dimethylamine oxide, alkylpolyaminoethylglycine hydrochloride, amide betaine type surfactant, alanine type surfactant, lauryliminodipropionic acid; polyoxy Examples of nonionic surfactants include, but are not limited to, ethylene octyl ether, polyoxyethylene decyl ether, polyoxyethylene lauryl ether, polyoxyethylene laurylamine, polyoxyethylene oleylamine, polyoxyethylene polystyrylphenyl ether, polyoxyalkylene polystyrylphenyl ether, and other polyoxyalkylene primary alkyl ethers or polyoxyalkylene secondary alkyl ethers; polyoxyethylene dilaurate, polyoxyethylene laurate, polyoxyethylene-derived castor oil, polyoxyethylene-derived hydrogenated castor oil, sorbitan laurate, polyoxyethylene sorbitan laurate, and fatty acid diethanolamide; 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.Furthermore, the above-mentioned dispersants can also be used in combination of two or more types.

[0056] In the varnish, the dispersant content is preferably 0.01% to 5% by mass, more preferably 0.05% to 1% by mass, and even more preferably 0.1% to 0.5% by mass, relative to the mass of the fine particles (B), in terms of film-forming properties.

[0057] The method for producing the aforementioned porous polyimide film varnish (varnish) is not particularly limited. Varnish is typically manufactured by a process of preparing a particulate dispersion, in which particulate matter (B) is dispersed in a solvent; preparing a resin solution containing polyamic acid (A); and a kneading process, in which these particulate dispersions and resin solutions are combined and kneaded to adjust the concentration.

[0058] For the varnish, the solid content concentration is preferably 10% by mass or more and 50% by mass or less. The varnish is manufactured by kneading for preferably 2 minutes or more and 10 hours or less, more preferably 2 minutes or more and 60 minutes or less. For mixing the varnish, a rotating / revolving mixer (product name: Awatori Rentaro, manufactured by Shinky Co., Ltd.), a planetary mixer, a bead mill, etc., can be used. Furthermore, in the mixing process, the cross-sectional area is 1960 μm². 2 More than 785000μm 2 A dispersion device equipped with the following flow channels may be used to perform a dispersion treatment by passing a mixed liquid (slurry) containing a fine particle dispersion and a resin solution, under pressure of 50 MPa or more, through the flow channels. An example of a method for performing such a dispersion treatment by passing through flow channels is the method described in Japanese Patent Application Publication No. 2020-104105.

[0059] ≪Method for manufacturing porous polyimide film≫ A method for manufacturing a porous polyimide film includes a composite film formation step of forming a composite film made of the aforementioned porous polyimide film forming varnish on a substrate, and a fine particle removal step of removing fine particles from the composite film. Furthermore, the method for producing a porous polyimide film may include a firing step in which polyamic acid (A) is used as the resin component, and the composite film is fired after the composite film formation step but before the particulate matter removal step.

[0060] [Composite film formation process (manufacturing of unfired composite films)] In the composite film formation process, a composite film consisting of the aforementioned porous polyimide film formation varnish (varnish) is formed on the substrate. An unfired composite film can be formed, for example, by applying varnish to a substrate and performing a drying process at 0 to 100°C, preferably 10 to 100°C under normal pressure or vacuum. Examples of substrates include PET film, SUS substrates, and glass substrates.

[0061] Furthermore, the process proceeds to a firing step in which the unfired composite film is fired to obtain a polyimide-fine particle composite film. If the unfired composite film is deposited on a substrate, it may be fired as is, or the unfired composite film may be peeled off the substrate before proceeding to the firing step.

[0062] When peeling a composite film (unfired composite film) from a substrate, a substrate with a pre-applied release layer may be used to further improve the peelability of the film. When a pre-applied release layer is applied to 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 alkyl ammonium phosphate, fluorine, or silicon, without any particular limitations. 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 adverse effects on electrical properties, so it is preferable to remove it as much as possible. To remove the release agent, a cleaning step may be introduced in which the unfired composite film peeled from the substrate is cleaned with an organic solvent.

[0063] On the other hand, when forming an unfired composite film, if the substrate is used as is without a release layer, the above-mentioned release layer formation step and cleaning step can be omitted. Furthermore, in the manufacturing of an unfired composite film, an immersion step in a solvent containing water, a pressing step, and a drying step after the immersion step may be added as optional steps before the firing step described later.

[0064] [Firing process (manufacturing of resin-microparticle composite film)] The firing process is a step in which the composite film is fired after the composite film formation process and before the particulate matter removal process. By performing a post-treatment (sintering process) by heating on a varnish containing polyamic acid (A), a composite film (resin-microparticle composite film) is formed consisting of a resin made of polyimide and fine particles (B).

[0065] The firing temperature in the firing process varies depending on whether or not a condensing agent is used, but it is preferably between 120°C and 450°C, and more preferably between 150°C and 420°C. Furthermore, when an organic material is used for the fine particles (B), it is necessary to set the temperature lower than the thermal decomposition temperature of that material. It is preferable that the imidation of polyamic acid (A) contained in the varnish is completed during the firing process.

[0066] As for the firing method, for example, a method of raising the temperature from room temperature to 400°C over 3 hours and then holding it at 400°C for 20 minutes can be used, or a stepwise drying-thermal imidization method can be used, such as raising the temperature from room temperature in 50°C increments to 400°C (holding each step for 20 minutes) and finally holding it at 400°C for 20 minutes. When an unfired composite film is deposited on a substrate and the unfired composite film is to be peeled off the substrate, a method can be adopted in which the edges of the unfired composite film are fixed to a SUS mold or the like to prevent deformation.

[0067] The thickness of the resulting resin-microparticle composite film can be determined by measuring the thickness at multiple points using a micrometer, for example, and averaging the results. The thickness of the resin-microparticle composite film is set appropriately, taking into consideration the thickness of the final porous polyimide film.

[0068] [Microparticle removal process (porosification of resin-microparticle composite film)] In the particulate removal process, particulate matter (B) is removed from the composite film after the composite film formation process (or, if a firing process is performed, from the composite film after the firing process (resin-particulate matter composite film)). By removing particulate matter (B) using an appropriate method, porous polyimide films can be manufactured with high reproducibility.

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

[0070] Organic materials can also be selected as the material for the fine particles (B). The organic material is not particularly limited as long as it decomposes at a lower temperature than the resin contained in the resin-fine particle composite film. Examples include resin fine particles made of linear polymers or known depolymerizable polymers. Conventional linear polymers undergo random cleavage of their molecular chains during thermal decomposition, while depolymerizable polymers decompose into monomers during thermal decomposition. Both types of polymers disappear from the porous polyimide film by decomposing into low molecular weight substances or CO2. The decomposition temperature of the resin fine particles used is preferably between 200°C and 320°C, and more preferably between 230°C and 260°C. A decomposition temperature of 200°C or higher allows for film formation even when using high-boiling point solvents in the varnish, broadening the range of selectable firing conditions for the resin-fine particle composite film. Furthermore, a decomposition temperature below 320°C allows for the disappearance of only the resin fine particles without causing thermal damage to the resin contained in the resin-fine particle composite film.

[0071] The overall film thickness of the porous polyimide film is not particularly limited. When the porous polyimide film is used as a filter, the film thickness is preferably 5 μm to 500 μm, more preferably 10 μm to 300 μm, and even more preferably 15 μm to 100 μm. The above film thickness can be determined by measuring the thickness at multiple points using a micrometer, for example, and averaging the results, similar to the measurement of resin-fine particle composite films.

[0072] [Resin removal process] A method for manufacturing a porous polyimide film may include a resin removal step in which at least a portion of the resin portion of the resin-microparticle composite film is removed before the microparticle removal step, or at least a portion of the porous polyimide film is removed after the microparticle removal step. By removing at least a portion of the resin portion of the resin-microparticle 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 no removal is performed.

[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 thereof.

[0074] Chemical etching methods include treatment with chemical etching solutions such as inorganic alkaline solutions or organic alkaline solutions. 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 mainly composed of alkali metal hydroxides, hydrazine, and 1,3-dimethyl-2-imidazolidinone. Examples of organic alkaline solutions include solutions of organic bases such as primary amines like ethylamine and n-propylamine; secondary amines like diethylamine and di-n-butylamine; tertiary amines like triethylamine and methyldiethylamine; alkanolamines like dimethylethanolamine and triethanolamine; quaternary ammonium salts like tetramethylammonium hydroxide and tetraethylammonium hydroxide; and cyclic amines like pyrrole and pyreridine.

[0075] The solvents 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 included in combination. In addition, 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] Furthermore, physical methods such as dry etching using plasma (oxygen, argon, etc.) or corona discharge, and surface treatment methods that involve dispersing an abrasive (e.g., alumina (hardness 9), etc.) in a liquid and irradiating the film surface with this liquid at a speed of 30 to 100 m / s can be used.

[0077] The method described above is preferable because it can be applied to either the resin removal step before or after the particulate removal step.

[0078] On the other hand, as a physical method applicable only to the resin removal process performed after the particulate matter removal process, a method can be employed in which the target surface is moistened with a liquid, the porous polyimide film is pressed onto a backing film (e.g., a polyester film such as PET film), and then peeled off the porous polyimide film from the backing film either before or after drying. Due to the surface tension or electrostatic adhesion force of the liquid, the porous polyimide film is peeled off the backing film with only the surface layer of the porous polyimide film remaining on the backing film.

[0079] According to the method for producing porous polyimide films described above, it is possible to produce porous polyimide films with small pore sizes and high flow rates.

[0080] In the porous polyimide film produced, the IPA flow rate measured by the method described in [IPA flow rate (FR)] below is 0.01 mL / (min·cm). 2 ) or more is preferable, and 0.025 mL / (min·cm) 2 The above is preferable. Furthermore, in the porous polyimide film produced, the average pore diameter measured by the half-dry method described in [Average Pore Diameter (nm)] below 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 produced contains a structure in which spherical pores are interconnected (connecting pores). An opening in a porous polyimide film is a portion on the surface of the porous polyimide film where the above-mentioned connecting pores open.

[0082] The term "spherical" in relation to the shape of a hole includes a perfect sphere, but is not necessarily limited to a perfect sphere. A hole is considered spherical if it is substantially spherical; a shape that can be visually recognized as approximately spherical when a magnified image of the hole is observed is also included in the definition of spherical. Specifically, in the case of a spherical hole, it is sufficient that the surface defining the hole is a curved surface, and that this curved surface defines a perfectly spherical or nearly perfectly spherical void.

[0083] Individual spherical pores are typically formed when individual microparticles (B) present in a composite film (resin-microparticle composite film) are removed in a subsequent microparticle removal step. Similarly, interconnected pores are formed in the manufacturing method of porous polyimide films when multiple microparticles (B) present in contact with each other in the resin-microparticle composite film are removed in a subsequent microparticle removal step. The locations where spherical pores connect in interconnected pores originate from the locations where the multiple microparticles (B) were in contact with each other before removal.

[0084] The diameter of the openings in the porous polyimide film can be appropriately changed depending on the intended use of the porous polyimide film, for example, within the range of 10 nm to 5000 nm. The diameter of the opening is equal to or approximately equal to the diameter of the spherical holes that constitute the connecting holes. Connecting holes, which are composed of a series of spherical holes of this diameter, allow fluid to pass through well within the porous polyimide film. A porous polyimide film has internal communication holes that penetrate the film in the thickness direction, serving as fluid channels. This allows fluid to permeate 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 in contact with the curved surfaces that define the individual spherical pores. The contact area of ​​the fluid inside the porous polyimide film is quite large due to the presence of interconnected pores made up of spherical holes. Therefore, when a fluid passes through a laminate containing a porous polyimide film, it is thought that minute substances present in the fluid are easily adsorbed onto the spherical pores within the porous polyimide film.

[0085] ≪Porous polyimide film≫ Porous polyimide film is made of polyimide. A porous polyimide film is manufactured by a manufacturing method that includes forming a composite film on a substrate consisting of the aforementioned porous polyimide film forming varnish (varnish), and removing fine particles (B) from the composite film. In other words, a porous polyimide film can be manufactured by the aforementioned method for manufacturing a porous polyimide film. Since the porous polyimide film is manufactured using the aforementioned porous polyimide film forming varnish (varnish), it is possible to provide a porous polyimide film with a small pore size and increased fluid flow rate.

[0086] As described above, the inventors of the present invention provide the following (1) to (7). (1) A porous polyimide film, 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 polyamic acid (A), 1)~5) below) 1) Prepare 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) After applying the solution to a glass substrate so that the dry film thickness is 50 μm, the coated film is dried using a hot plate at 90°C for 150 minutes to form a cured film. 3) Peel the cured film from the glass substrate and heat it at 400°C for 30 minutes. 4) Cut out strip-shaped test pieces measuring 10 mm wide x 90 mm long from the cured film, and 5) The test specimen shall be subjected to a tensile test under the conditions of a gripping distance of 50 mm and a tensile speed of 10 mm / min. A porous polyimide film having a tensile modulus of elasticity of 2.0 GPa or more and 5.0 GPa or less, as measured by a method including [a specific method]. (2) A porous polyimide film according to (1), having interconnected pores in which spherical or substantially spherical pores are interconnected. (3) A method for producing a porous polyimide film as described in (1) or (2), The manufacturing method involves preparing a varnish containing polyamic acid (A) and fine particles (B), The process involves applying varnish to a substrate to form a composite film containing polyamic acid (A) and fine particles (B), Heating the composite film, This includes removing fine particles (B) from a heated composite film, 1)~5) below) 1) Prepare 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) After applying the solution to a glass substrate so that the dry film thickness is 50 μm, the coated film is dried using a hot plate at 90°C for 150 minutes to form a cured film. 3) Peel the cured film from the glass substrate and heat it at 400°C for 30 minutes. 4) Cut out strip-shaped test pieces measuring 10 mm wide x 90 mm long from the cured film, and 5) The test specimen shall be subjected to a tensile test under the conditions of a gripping distance of 50 mm and a tensile speed of 10 mm / min. A method for producing a porous polyimide film, wherein the tensile modulus of the polyimide resin film, as measured by a method including [a specific component], 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 average particle size of the fine particles (B) is 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 substantially spherical fine particles. (6) comprising polyamic acid (A) and fine particles (B), 1)~5) below) 1) Prepare 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) After applying the solution to a glass substrate so that the dry film thickness is 50 μm, the coated film is dried using a hot plate at 90°C for 150 minutes to form a cured film. 3) Peel the cured film from the glass substrate and heat it at 400°C for 30 minutes. 4) Cut out strip-shaped test pieces measuring 10 mm wide x 90 mm long from the cured film, and 5) The test specimen shall be subjected to a tensile test under the conditions of a gripping distance of 50 mm and a tensile speed of 10 mm / min. The tensile modulus of the polyimide resin film, measured by a method including the following, is 2.0 GPa or more and 5.0 GPa or less. A varnish for forming porous polyimide films, wherein the average particle size of the fine particles (B) is 80 nm or less. (7) The porous polyimide film forming varnish according to (6), wherein the fine particles (B) are spherical or substantially spherical fine particles. [Examples]

[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 polyamic acid with the tetracarboxylic dianhydride and diamine shown below, as shown in Table 1. • Tetracarboxylic acid dianhydride: 3,3',4,4'-biphenyltetracarboxylic acid dianhydride Item (BPDA) ... (a1-1) Pyromellitic dianhydride (PMDA) ... (a1-2) • Diamine: 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 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 of elasticity] Each polyamic acid (A) listed in Table 1 was dissolved in an amide-based organic solvent to prepare a solution with a concentration of 20% by mass of polyamic acid (A). This solution was applied to a glass substrate to form a coating film with a dry film thickness of 50 μm. The coating film was then dried on a hot plate at 90°C for 150 minutes to form a cured film. The cured film was peeled off the glass substrate and heated at 400°C for 30 minutes. A strip-shaped test piece measuring 10 mm wide x 90 mm long was cut from the cured film (polyimide resin film). A tensile test was performed on the test piece using a Shimadzu Autograph (500N) under the 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] [Table 1]

[0092] [Examples 1-6 and Comparative Examples 1-3] A silica dispersion (containing 0.5% by mass of dispersant relative to silica) was added to the 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, respectively, so that the solvent composition of the final composition (varnish for forming porous polyimide film) was in a mass ratio of organic solvent (1):organic solvent (2) = 45:55. The resulting mixture was dispersed in a 1000 mL container by stirring with a stirring blade at 4000 rpm for 30 minutes. Afterwards, it was dispersed using a 60 μm diameter (cross-sectional area 2826 μm) manufactured by Yoshida Machinery Industry Co., Ltd. 2 Using a dispersion apparatus equipped with a flow channel, a dispersion treatment was performed 10 times in which the mixed liquid was passed through the flow channel at 200 MPa to prepare a varnish for forming porous polyimide films with a solid content of 20% by mass. The following polyamic acid solution, organic solvent, dispersant, 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 varnish for forming porous polyimide films 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). After peeling this composite film (unfired composite film) from the substrate, it was placed in an oven and fired at 380°C for 15 minutes to complete the imidization and obtain a resin-microparticle composite film (firing step). The peeled resin-microparticle composite film was immersed in hydrogen fluoride (HF) for 10 minutes to remove the silica microparticles contained in the film (microparticle removal step). Subsequently, it was washed with water and dried to obtain porous polyimide films of Examples 1-6 and Comparative Examples 1-3, each with a film thickness of 20 μm.

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

[0095] [Average pore diameter (nm)] The average pore size (nm) was measured using a liquid porometer (Porometer Inc.). Specifically, each porous polyimide film was cut to the size of a 25 mm diameter membrane filter and mounted in a measuring holder. The secondary side of the membrane (downstream side of the flow channel of the porous polyimide film) was moistened with isobutyl alcohol, and pressure was gradually applied from the primary side with a fluorine-based solvent (Porefil, Aptco Inc.). The average pore size was measured according to the half-dry method (ASTM E1294-89). The measurements were performed at room temperature (25°C).

[0096] [IPA flow rate (FR)] Each porous polyimide film was cut to a 47 mm diameter membrane filter size and mounted in an in-line filter holder. Next, isopropyl alcohol (IPA) was filtered by pressurizing it with 0.1 MPa air from the primary side (upstream of the porous polyimide film's flow path), and the flow rate was measured. Measurements were performed at 25°C. In Table 1, 1 cm 2 Displayed in milliliters per minute.

[0097] [Shrinkage rate] The film thickness Th1 of the resin-microparticle composite film obtained by the above method and the film 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. Contraction rate (%) = (1 - Th2 / Th1) × 100

[0098] [Tensile elongation] Each polyimide porous membrane was cut into a dumbbell-shaped No. 6 specimen to prepare test pieces. A small benchtop testing machine, EZ Test EZ-SX (manufactured by Shimadzu Corporation), was used as the testing machine, and the tensile elongation (%) of the test pieces was measured in accordance with JIS K 7127 under measurement conditions of 25°C and 5 mm / min. The tensile elongation (%) was calculated using the following formula, where L0 is the length of the parallel section of the test specimen (20 mm) and L is the length of the parallel section of the test specimen until fracture (length of the parallel section of the test specimen at fracture: 20 mm + α): Tensile elongation (%) = {(L - L0) / L0} × 100

[0099] [Table 2] *1 Measurement impossible due to membrane contraction *2 Measurement impossible due to poor film deposition.

[0100] Tables 1 and 2 show that in Comparative Example 2, which uses polyamic acid (A) in which the tensile modulus of the polyimide resin film measured by the predetermined method described above is less than 2.0 GPa, it is difficult to achieve good film formation by using silica with small particle size (fine particles (B)), and the resulting porous polyimide film has a low IPA flow rate. Furthermore, as shown in Comparative Example 1, when using silica with a large particle size (fine particles (B)), good film formation is possible even when using polyamic acid (A) in which the tensile modulus of the polyimide resin film measured by the predetermined method described above is less than 2.0 GPa. Furthermore, in Comparative Example 3, which uses polyamic acid (A) in which the tensile modulus of the polyimide resin film measured by the aforementioned predetermined method is greater than 5.0 GPa, it is clear that it is difficult to form a film by using silica with a small particle size (fine particles (B)). On the other hand, in Examples 1 to 6, which use polyamic acid (A) having a tensile modulus of elasticity of 2.0 GPa or more and 5.0 GPa or less as measured by the predetermined method described above, it can be seen that porous polyimide films exhibiting a high IPA flow rate (FR) can be formed well even when using silica with a small particle size (fine particles (B)) compared to Comparative Example 2.

Claims

1. A porous polyimide film, The average pore size of the porous polyimide film, as measured by the half-dry method, is 30 nm or less. The polyimide resin constituting the porous polyimide film is formed by heating polyamic acid (A), 1) to 5) below): 1) Prepare 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) After applying the solution to a glass substrate so that the film thickness after drying is 50 μm, the coated film is dried using a hot plate at 90°C for 150 minutes to form a cured film. 3) Peel the cured film from the glass substrate and heat it at 400°C for 30 minutes. 4) Cutting out strip-shaped test pieces measuring 10 mm in width and 90 mm in length from the cured film, 5) The test specimen shall be subjected to a tensile test under the conditions of a gripping distance of 50 mm and a tensile speed of 10 mm / min. A porous polyimide film having a tensile modulus of elasticity of the polyimide resin film measured by a method including the above, which is 2.0 GPa or more and 5.0 GPa or less.

2. The porous polyimide film according to claim 1, having interconnected pores in which spherical or substantially spherical pores are interconnected.

3. A method for producing a porous polyimide film according to claim 1, The above manufacturing method involves preparing a varnish containing polyamic acid (A) and fine particles (B), The varnish is applied to the substrate to form a composite film containing the polyamic acid (A) and the fine particles (B), Heating the composite film, This includes removing the fine particles (B) from the heated composite film, 1) to 5) below): 1) Prepare 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) After applying the solution to a glass substrate so that the film thickness after drying is 50 μm, the coated film is dried using a hot plate at 90°C for 150 minutes to form a cured film. 3) Peel the cured film from the glass substrate and heat it at 400°C for 30 minutes. 4) Cutting out strip-shaped test pieces measuring 10 mm in width and 90 mm in length from the cured film, 5) The test specimen shall be subjected to a tensile test under the conditions of a gripping distance of 50 mm and a tensile speed of 10 mm / min. A method for producing a porous polyimide film, wherein the tensile modulus of the polyimide resin film, as measured by a method including [a specific component], is 2.0 GPa or more and 5.0 GPa or less.

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 substantially spherical fine particles.

6. It contains polyamic acid (A) and fine particles (B), 1) to 5) below): 1) Prepare 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) After applying the solution to a glass substrate so that the film thickness after drying is 50 μm, the coated film is dried using a hot plate at 90°C for 150 minutes to form a cured film. 3) Peel the cured film from the glass substrate and heat it at 400°C for 30 minutes. 4) Cutting out strip-shaped test pieces measuring 10 mm in width and 90 mm in length from the cured film, 5) The test specimen shall be subjected to a tensile test under the conditions of a gripping distance of 50 mm and a tensile speed of 10 mm / min. The tensile modulus of the polyimide resin film, measured by a method including the following, is 2.0 GPa or more and 5.0 GPa or less. A varnish for forming porous polyimide films, wherein the average particle size of the fine particles (B) is 80 nm or less.

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

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