Porous film, method for purifying liquid, filter medium, and filter device

Abstract

The present invention provides: a method for producing a porous film, with which it is possible to produce a porous film that has a small pore diameter and excellent permeability of a fluid; and a porous film which is produced by the method for producing a porous film.　Provided is a method for producing a porous film, the method including a specific step in which a resin composition that contains a resin component (A), fine particles (B), and a solvent (S) is used, wherein: the resin component (A) is at least one selected from the group consisting of a polyamide acid, a polyimide, a polyamide-imide precursor, and a polyamide-imide; the fine particles (B) include fine particles (B1) and fine particles (B2); the fine particles (B1) and the fine particles (B2) each have an average particle diameter of 100 nm or less; and the average particle diameter of the fine particles (B2) is larger than the average particle diameter of the fine particles (B1).

Description

Porous membrane, liquid purification method, filter medium, and filter device

[0001] The present invention relates to a porous membrane, a method for purifying a liquid, a filter medium, and a filter device.

[0002] Various porous membranes have been used in applications such as filters for separating gases or liquids. In recent years, porous membranes have also been increasingly applied to 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 a small particle size. However, reducing the pore diameter of a porous membrane reduces the permeability (liquid permeability and breathability) of the porous membrane, resulting in a problem of a reduced flow rate of liquids and gases passing through the porous membrane.

[0006] The present invention has been made in consideration of the above circumstances, and aims to provide a method for manufacturing a porous membrane that can produce a porous membrane having a small pore size and excellent fluid permeability, and a porous membrane manufactured by the method for manufacturing the porous membrane.

[0007] The present inventors have discovered that the above-mentioned problems can be solved by a method for producing a porous membrane, which includes a specific step using a resin composition containing a resin component (A), fine particles (B), and a solvent (S), and in which the resin component (A) is at least one selected from the group consisting of polyamic acid, polyimide, a polyamideimide precursor, and polyamideimide, the fine particles (B) include fine particles (B1) and fine particles (B2), each of which has an average particle size of 100 nm or less, and the average particle size of the fine particles (B2) is larger than that of the fine particles (B1), and have completed the present invention. Specifically, the present invention provides the following.

[0008] A first aspect of the present invention is a method for producing a porous film, comprising: a composite film formation step of forming a composite film made of a resin composition containing a resin component (A), fine particles (B), and a solvent (S) on a substrate; a fine particle removal step of removing the fine particles from the composite film to obtain an untreated porous film; and a chemical etching step of chemically etching the untreated porous film to obtain a porous film, wherein the resin component (A) is at least one selected from the group consisting of polyamic acid, polyimide, a polyamideimide precursor, and polyamideimide; the fine particles (B) include fine particles (B1) and fine particles (B2), each of which has an average particle size of 100 nm or less; and the average particle size of the fine particles (B2) is larger than the average particle size of the fine particles (B1).

[0009] A second aspect of the present invention is a porous membrane produced by the method for producing a porous membrane according to the first aspect.

[0010] According to the present invention, it is possible to provide a method for manufacturing a porous membrane that can produce a porous membrane having a small pore size and excellent fluid permeability, and a porous membrane manufactured by the method for manufacturing the porous membrane.

[0011] 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.

[0012] <Porous Film> The porous film is produced by a method including: a composite film formation step in which a composite film made of a resin composition containing a resin component (A), fine particles (B), and a solvent (S) is formed on a substrate; a fine particle removal step in which the fine particles are removed from the composite film to obtain an untreated porous film; and a chemical etching step in which the untreated porous film is chemically etched to obtain a porous film. Here, the "untreated porous film" refers to a porous film from which the fine particles have been removed but which has not been subjected to chemical etching. The untreated porous film may be imidized through a baking step described below. The resin component (A) is at least one selected from the group consisting of polyamic acid, polyimide, polyamideimide precursor, and polyamideimide. The fine particles (B) include fine particles (B1) and fine particles (B2). The fine particles (B1) and fine particles (B2) each have an average particle size of 100 nm or less. The average particle size of the fine particles (B2) is larger than that of the fine particles (B1).

[0013] As described above, the porous membrane is produced using a resin composition containing at least one selected from the group consisting of polyamic acid, polyimide, polyamideimide precursor, and polyamideimide as the resin component (A). As a result, the porous membrane is made of at least one resin component (A) selected from the group consisting of polyimide and polyamideimide, or a resin composition containing the resin component (A). The porous membrane is suitable for use as a filter.

[0014] The porous membrane preferably has interconnected pores. The interconnected pores are formed by adjacent, connected individual pores (hereinafter sometimes simply referred to as "pores") that impart porosity to the porous membrane. The individual pores are preferably pores having a curved inner surface, as described below, and more preferably spherical or nearly spherical pores, as described below. In the porous membrane, the portions where such individual pores are formed adjacent to each other are interconnected pores. The interconnected pores have a structure in which such individual pores are interconnected. Typically, a plurality of such pores are connected together to form a flow path for the liquid to be purified. The "flow path" is usually formed by a series of individual "pores" and / or "interconnected pores." The individual pores can also be said to be pores formed by removing individual particles present in the resin-particle composite membrane in a later process in the porous membrane manufacturing method described below. The interconnected pores can also be said to be adjacent individual pores formed by removing the particles in a later process at the portion where individual particles present in the resin-particle composite membrane were in contact with each other in the porous membrane manufacturing method described below.

[0015] The porous membrane preferably has communicating holes that have openings on the outer surface of the porous membrane, which communicate the interior of the porous membrane and also have openings on the outer surface on the opposite side (back side) of the porous membrane, ensuring a flow path for a fluid passing through the porous membrane.

[0016] The average pore size of the porous membrane is not particularly limited, but is preferably 5 nm or more and 40 nm or less, more preferably 5 nm or more and 30 nm or less. The average pore size is the average pore size measured by a porometer. The average pore size of the porous membrane can be adjusted, for example, by appropriately adjusting the particle size of the fine particles contained in the porous membrane production composition (varnish) in the porous membrane production method described below.

[0017] The pore size distribution of the porous membrane preferably satisfies the conditions described below. Here, the pore size distribution of the porous membrane is the volume-based distribution of pores of each size relative to the volume of all pores, measured every 1 nm in the pore size range of 5 nm or more. The upper limit of the pore size distribution range is not particularly limited, but may be 150 nm or less, or 120 nm or less. In the pore size distribution of the porous membrane, the total value of the distribution of pores in a total of four fractions (the first fraction with the smallest pore size, the second fraction with the smallest pore size, the third fraction with the third smallest pore size, and the fourth fraction with the fourth smallest pore size) within the pore size range from the minimum value of the pore size range of the smallest pore size fraction in which pores exist to the minimum value + 4 nm is preferably 51% or more. For example, when the pore size range of the smallest pore size fraction in which pores exist (the first fraction) is 5 nm or more but less than 6 nm, the pore size ranges of the first to fourth fractions are as follows: First fraction: 5 nm or more and less than 6 nm Second fraction: 6 nm or more and less than 7 nm Third fraction: 7 nm or more and less than 8 nm Fourth fraction: 8 nm or more and less than 9 nm

[0018] In the pore size distribution of porous membrane, the method of adjusting the total value of the pore distribution of the total 4 fractions in the pore size range of the minimum value of the pore size range of the fraction of the smallest pore size that pores exist in from the minimum value to the pore size range of the minimum value + 4nm can be, in the manufacturing method of porous membrane described later, the method of adjusting the particle size distribution of the microparticles contained in the varnish for manufacturing porous membrane.The method of adjusting the particle size distribution of microparticles can be, as microparticles, the method of using a combination of multiple kinds of microparticles with different average particle size.In this case, it is preferable to increase the proportion of the microparticles with small particle size in multiple kinds of microparticles.

[0019] In the pore size distribution of the porous membrane, the upper limit of the total value of the distribution of pores in a total of four fractions within the pore size range from the minimum value of the pore size range of the fraction with the smallest pore size in which pores exist to the minimum value + 4 nm is preferably 60% or less, more preferably 55% or less.

[0020] In the pore size distribution of the porous membrane, the difference between the maximum value of the pore size range of the largest pore size fraction in which the pores are distributed and the minimum value of the pore size range of the smallest pore size fraction (the first fraction) in which the pores are distributed is preferably 15 nm or more. The difference is calculated as the difference between D and A when the pore size range of the smallest pore size fraction is A nm or more and less than B nm, and the pore size range of the largest pore size fraction is C nm or more and less than D nm.

[0021] As a method for adjusting the difference in the porous membrane, the method for adjusting the particle size distribution width of the fine particles contained in the varnish for producing the porous membrane can be mentioned in the method for producing the porous membrane described later.As a method for adjusting the particle size distribution width of the fine particles, the method for using a combination of multiple types of fine particles with different average particle sizes can be mentioned.

[0022] The pore size distribution is a pore size distribution measured by a porometer. The upper limit of the difference between the maximum value of the pore size range of the maximum pore size fraction in which the pores are distributed and the minimum value of the pore size range of the minimum pore size fraction in which the pores are distributed is not particularly limited, but may be 90 nm or less, or may be 50 nm or less.

[0023] The average pore size measured by a porometer is the mean flow pore size determined by liquid-liquid phase displacement. The average pore size can be measured, for example, using a PMI Liquid Liquid Porometer LLP-1500A (ultra-low pressure pore size distribution permeability measurement device) in accordance with the pore size distribution measurement test method [half-dry method (ASTM E1294-89)]. Perfluoropolyester (trade name Galwick) and isopropyl alcohol (interfacial tension: 4.6 dyne / cm) are used as test solutions for measuring the average pore size. After filling the pores of the porous membrane with isopropyl alcohol, one main surface of the porous membrane is filled with perfluoropolyester. Compressed air is used to apply pressure to the main surface of the porous membrane filled with perfluoropolyester, gradually increasing the pressure. The measurement temperature is 25°C, and the measurement pressure is in the range of 50 to 500 psi.

[0024] As described above, the porous membrane is preferably a porous membrane containing pores having a curved inner surface, and more preferably, most (preferably substantially all) of the pores in the porous membrane are formed with a curved surface. In this specification, the phrase "having a curved inner surface" in relation to pores means that at least the inner surface of the pore that creates the porosity has a curved surface in at least a part of the inner surface.

[0025] It is preferable that the pores in the porous membrane have substantially entirely curved inner surfaces, and such pores are hereinafter sometimes referred to as "spherical or approximately spherical pores." In this specification, "spherical or approximately spherical pores" refers to pores whose inner surfaces form spherical or approximately spherical spaces. Spherical or approximately spherical pores can also be said to be pores formed when the microparticles (B) used in the porous membrane manufacturing method described below are spherical or approximately spherical. In this specification, "spherical or approximately spherical" refers to a concept that includes true spheres, but is not necessarily limited to true spheres, and includes particles that are substantially spherical. In this specification, "substantially spherical" refers to a particle with a sphericity of 1±0.3 or less, defined by the sphericity expressed by dividing the major axis by the minor axis. The sphericity of the spherical or approximately spherical pores in the porous membrane is preferably 0.9 to 1.1, more preferably 0.95 to 1.05.

[0026] By having the pores in the porous membrane have a curved inner surface, when a fluid is passed through the porous membrane, the fluid can be sufficiently distributed inside the pores in the porous membrane and can fully contact the inner surface of the pores. In some cases, it is possible that the fluid may undergo convection along the curved inner surface. For this reason, it is thought that minute substances such as metal particles present in the fluid are easily adsorbed into the pores in the porous membrane or into recesses that may exist on the inner surface of the pores. The spherical or approximately spherical pores may further have recesses on their inner surfaces. For example, the recesses may be formed by pores with a smaller pore diameter than the spherical or approximately spherical pores, which have openings on the inner surface of the spherical or approximately spherical pores.

[0027] From the viewpoint of excellent filtration rate, the thickness of the porous membrane is preferably 10 μm or more and 90 μm or less, and more preferably 50 μm or more and 80 μm or less.

[0028] As described above, the porous film is made of at least one resin component (A) selected from the group consisting of polyimide and polyamideimide, or a resin composition containing the resin component (A). The porous film is produced by a production method including forming a composite film made of a porous film production composition (hereinafter also referred to as "varnish") described below on a substrate, and removing the fine particles (B) from the composite film. The porous film can be produced by the porous film production method described below.

[0029] <Composition for producing porous membrane> The composition for producing porous membrane (hereinafter also referred to as "varnish") contains at least one resin component (A) selected from the group consisting of polyamic acid, polyimide, polyamideimide precursor, and polyamideimide, fine particles (B), and a solvent (S). The material and particle size of the fine particles (B) are not particularly limited as long as a porous membrane with the desired performance can be formed. The fine particles (B) include fine particles (B1) and fine particles (B2) having an average particle size larger than that of the fine particles (B1). The average particle sizes of the fine particles (B1) and the fine particles (B2) are 100 nm or less.

[0030] [Resin Component (A)] As described above, the composition for producing a porous membrane contains at least one resin component (A) selected from the group consisting of polyamic acid, polyimide, polyamideimide precursor, polyamideimide, and polyethersulfone. These resin components (A) will be described below.

[0031] [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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] In diaminobiphenyl compounds, two aminophenyl groups are bonded to each other, such as 4,4'-diaminobiphenyl and 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl.

[0038] 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.

[0039] 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.

[0040] Among these, p-phenylenediamine, m-phenylenediamine, 2,4-diaminotoluene, and 4,4'-diaminodiphenyl ether are preferred in terms of cost, availability, and the like.

[0041] 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.

[0042] Examples of diaminonaphthalenes include 1,5-diaminonaphthalene and 2,6-diaminonaphthalene.

[0043] Examples of aminophenylaminoindan include 5 or 6-amino-1-(p-aminophenyl)-1,3,3-trimethylindan.

[0044] 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.

[0045] Examples of the cardo-type fluorenediamine derivatives include 9,9-bisanilinefluorene.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] [Polyimide] The structure and molecular weight of the polyimide are not limited, and known polyimides can be used. The polyimide may have a condensable functional group such as a carboxyl group or a functional group that promotes a crosslinking reaction during baking in its side chain. In addition, a soluble polyimide that can be dissolved in the solvent (S) contained in the varnish is preferred.

[0054] In order to make the polyimide soluble in the solvent (S), it is effective to use a monomer for introducing a flexible, bent structure into the main chain, for example, an aliphatic diamine such as ethylenediamine, hexamethylenediamine, 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, or 4,4'-diaminodicyclohexylmethane; an aromatic diamine such as 2-methyl-1,4-phenylenediamine, o-tolidine, m-tolidine, 3,3'-dimethoxybenzidine, or 4,4'-diaminobenzanilide; a polyoxyalkylene diamine such as polyoxyethylenediamine, polyoxypropylenediamine, or polyoxybutylenediamine; a polysiloxane diamine; 2,3,3',4'-oxydiphthalic anhydride, 3,4,3',4'-oxydiphthalic anhydride, or 2,2-bis(4-hydroxyphenyl)propanedibenzoate-3,3',4,4'-tetracarboxylic dianhydride; It is also effective to use a monomer having a functional group that improves solubility in a solvent, for example, a fluorinated diamine such as 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl or 2-trifluoromethyl-1,4-phenylenediamine. Furthermore, in addition to the monomer for improving the solubility of the polyimide, the same monomer as the monomer listed in the polyamic acid section above can also be used in combination, as long as the solubility is not impaired. Each of the polyimide and its monomer may be used alone or in combination of two or more.

[0055] There are no particular limitations on the means for producing the polyimide. Known methods, such as chemical imidization or thermal imidization of polyamic acid, can be used to produce the polyimide. Examples of such polyimides include aliphatic polyimides (fully aliphatic polyimides) and aromatic polyimides, with aromatic polyimides being preferred. Examples of aromatic polyimides include those obtained by thermally or chemically ring-closing polyamic acids having repeating units represented by formula (1), or polyimides having repeating units represented by formula (2). In the formula, Ar represents an aryl group. These polyimides are then preferably dissolved in the solvent (S) to be used.

[0056] [Polyamideimide and polyamideimide precursor] The polyamideimide is not limited in structure or molecular weight, and known polyamideimides can be used. The polyamideimide may have a condensable functional group such as a carboxyl group or a functional group that promotes a crosslinking reaction during baking in the side chain. In addition, a soluble polyamideimide that can be dissolved in the solvent (S) contained in the varnish is preferred.

[0057] Polyamideimides that can be used are generally, without particular limitation, (i) resins obtained by reacting an acid having a carboxyl group and an acid anhydride group in one molecule, such as trimellitic anhydride, with a diisocyanate, or (ii) resins obtained by imidizing a precursor polymer (polyamideimide precursor) obtained by reacting a reactive derivative of the above acid, such as trimellitic anhydride chloride, with a diamine.

[0058] Examples of the above acid or its reactive derivative include trimellitic anhydride, trimellitic anhydride halides such as trimellitic anhydride chloride, and trimellitic anhydride esters.

[0059] Examples of the optional diamine include the diamines exemplified in the description of polyamic acid above. Diaminopyridine compounds can also be used.

[0060] The optional diisocyanate is not particularly limited, and examples thereof include diisocyanate compounds corresponding to the optional diamines. Specific examples of the diisocyanate include metaphenylene diisocyanate, p-phenylene diisocyanate, o-tolidine diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate, 4,4'-oxybis(phenylisocyanate), 4,4'-diisocyanatodiphenylmethane, bis[4-(4-isocyanatophenoxy)phenyl]sulfone, 2,2'-bis[4-(4-isocyanatophenoxy)phenyl]propane, 2,4- Examples of the isocyanate include tolylene diisocyanate, 2,6-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 3,3'-dimethyldiphenyl-4,4'-diisocyanate, 3,3'-diethyldiphenyl-4,4'-diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, m-xylene diisocyanate, p-xylene diisocyanate, and naphthalene diisocyanate.

[0061] In addition to the above, compounds described as general formulas in JP-A-63-283705 and JP-A-2-198619 can also be used as raw material monomers for polyamideimides. Furthermore, the imidization in the above method (ii) can be either thermal imidization or chemical imidization. Chemical imidization can be achieved by immersing a composite film (unbaked composite film) formed using a varnish containing a polyamideimide precursor or the like in acetic anhydride or a mixed solvent of acetic anhydride and isoquinoline. The polyamideimide precursor can also be referred to as a polyimide precursor from the perspective of a precursor before imidization.

[0062] The polyamideimide contained in the varnish may be, for example, (1) a polymer obtained by reacting an acid such as trimellitic anhydride with a diisocyanate, or (2) a polymer obtained by imidizing a precursor polymer obtained by reacting a reactive derivative of the acid such as trimellitic anhydride chloride with a diamine. In this specification and claims, "polyamideimide precursor" refers to a polymer (precursor polymer) before imidization. Each of the polyamideimide and polyamideimide precursor may be used alone or in combination of two or more. Furthermore, for polyamideimide, each of the polymers, raw material monomers, and oligomers may be used alone or in combination of two or more.

[0063] [Fine particles (B)] The fine particles (B) include fine particles (B1) and fine particles (B2) having an average particle size larger than that of the fine particles (B1). The average particle size of the fine particles (B1) and the average particle size of the fine particles (B2) are each 100 nm or less.

[0064] When the varnish contains, as the fine particles (B), fine particles (B1) and fine particles (B2) having an average particle size larger than that of the fine particles (B1), and the average particle sizes of the fine particles (B1) and the fine particles (B2) are each 100 nm or less, the pore size of the produced porous membrane can be reduced and the flow rate can be improved, as shown in the examples described below. This is believed to be due to the following reason. The pores (spherical pores) of the porous membrane are formed by removing individual fine particles present in a composite membrane (resin-particle composite membrane) formed using the varnish in a subsequent particle removal step. The porous membrane has a structure in which spherical pores are interconnected (hereinafter referred to as interconnected pores). The interconnected pores are formed by removing multiple fine particles (B) that are present in contact with each other in a composite membrane (resin-particle composite membrane) formed using the varnish in a subsequent particle removal step in the porous membrane manufacturing method. The points at which the spherical pores in the interconnected pores are interconnected are derived from the points (contact points) where multiple fine particles (B) come into contact with each other before removal. The pore size of the communicating holes is measured using the aforementioned porometer. When two types of fine particles (B) of different sizes are used, i.e., small fine particles (B1) with an average particle size of 100 nm or less and fine particles (B2) with an average particle size larger than the fine particles (B1) but 100 nm or less, the pore size of the communicating holes formed at the points (contact points) where the fine particles (B1) contact each other or the fine particles (B1) and the fine particles (B2) contact each other depends on the particle size of the small fine particles (fine particles (B1)). In other words, even when the fine particles (B1) and the fine particles (B2) contact each other, communicating holes of the same pore size as the communicating holes formed at the contact points between the small fine particles (fine particles (B1)) are formed. On the other hand, when two types of fine particles of different sizes (fine particles (B1) and fine particles (B2)) are used, the number of pores formed is smaller than when only small fine particles (fine particles (B1)) are used, and therefore the resistance to fluid flow through the porous membrane is reduced, thereby improving the flow rate of the fluid passing through the porous membrane per unit time. Therefore, by using, as the fine particles (B), small fine particles (B1) having an average particle size of 100 nm or less and fine particles (B2) larger than the fine particles (B1), it is easy to achieve both a small pore size in the porous membrane and an improved flow rate.

[0065] Furthermore, by carrying out a chemical etching process described later after the particle removal process, the average pore size of the pores derived from the small particles (B1) becomes smaller than the average pore size before the chemical etching, and the average pore size of the pores derived from the large particles (B2) becomes larger than the average pore size before the chemical etching. As a result, the average pore size of the porous membrane becomes smaller and the flow rate of the fluid through the porous membrane is further improved.

[0066] The average particle size of the fine particles (B1) is 100 nm or less. The average particle size of the fine particles (B1) may be, for example, 50 nm or less, 30 nm or less, or 15 nm or less. The average particle size of the fine particles (B1) may be, for example, 5 nm or more.

[0067] The average particle size of the fine particles (B2) is not particularly limited as long as it is 100 nm or less and larger than the average particle size of the fine particles (B1). The average particle size of the fine particles (B2) may be, for example, 20 nm or more, 50 nm or more, or 90 nm or more.

[0068] 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 90 nm, 10 nm to 80 nm, or 20 nm to 70 nm.

[0069] Specific examples of combinations of fine particles (B1) and fine particles (B2) include the following combinations 1) to 5): 1) fine particles (B1): fine particles with an average particle size of 40 nm to 50 nm, fine particles (B2): fine particles with an average particle size of 90 nm to 100 nm, 2) fine particles (B1): fine particles with an average particle size of 30 nm to 40 nm, fine particles (B2): fine particles with an average particle size of 80 nm to 100 nm, 3) fine particles (B1): fine particles with an average particle size of 20 nm to 30 nm, fine particles (B2): fine particles with an average particle size of 70 nm to 100 nm, 4) fine particles (B1): fine particles with an average particle size of 10 nm to 30 nm, fine particles (B2): fine particles with an average particle size of 70 nm to 90 nm, 5) fine particles (B1): fine particles with an average particle size of 10 nm to 30 nm, fine particles (B2): fine particles with an average particle size of 40 nm to 60 nm

[0070] 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.

[0071] 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.

[0072] Mass M of the fine particles (B1) Ｂ１ The mass M of the fine particles (B2) Ｂ２ The ratio (M Ｂ２ / M 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.

[0073] 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.

[0074] Examples of inorganic material fine particles (B) include inorganic oxide fine particles, and specific examples thereof include silica (silicon dioxide) fine particles, titanium oxide fine particles, alumina (Al ２ O ３ Examples of silica include metal oxide particles such as silica nanoparticles. Colloidal silica is an example of silica. Among these, monodispersed spherical silica particles are preferred because they can form uniform pores.

[0075] 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.

[0076] 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.

[0077] The fine particles (B) may be used alone or in combination of two or more kinds.

[0078] [Solvent (S)] The solvent (S) is not particularly limited as long as it can dissolve the resin component (A) comprised of polyamic acid, polyimide, polyamideimide precursor, polyamideimide, and / or polyethersulfone 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. When the resin component (A) is polyethersulfone, examples of the solvent (S) include polar solvents such as diphenyl sulfone, dimethyl sulfone, dimethyl sulfoxide, benzophenone, tetrahydrothiophene-1,1-dioxide, and 1,3-dimethyl-2-imidazolidinone, in addition to the nitrogen-containing polar solvents described above.

[0079] [Dispersant] A dispersant may be added together with the fine particles (B) for the purpose of uniformly dispersing the fine particles (B) in the varnish. By adding a dispersant, the fine particles (B) can be mixed more uniformly in the varnish, and further, the fine particles (B) can 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 film, and the front and back surfaces can be efficiently connected, thereby improving the air permeability of the porous film. Furthermore, the addition of a dispersant tends to improve the drying properties of the varnish and also tends to improve the peelability of the formed unfired composite film from the substrate, etc.

[0080] The dispersant is not particularly limited, and known dispersants can be used. Examples of dispersants include anionic surfactants such as coconut fatty acid salts, castor sulfated oil salts, lauryl sulfate salts, polyoxyalkylene allyl phenyl ether sulfate salts, alkyl benzene sulfonic acids, alkyl benzene sulfonates, alkyl diphenyl ether disulfonates, alkyl naphthalene sulfonates, 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 polyoxyethylene 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.

[0081] 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.

[0082] The method for producing the composition for producing a porous membrane (varnish) is not particularly limited. The varnish is typically produced by a step of preparing a fine particle dispersion by dispersing fine particles (B) in a solvent, a step of preparing a resin solution containing the resin component (A), and a step of kneading the fine particle dispersion and the resin solution together to adjust the concentration.

[0083] 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.

[0084] <<Method for producing porous film>> The porous film is produced by a method including: a composite film formation step of forming a composite film made of a resin composition containing a resin component (A), fine particles (B), and a solvent (S) on a substrate; a fine particle removal step of removing the fine particles from the composite film to obtain an untreated porous film; and a chemical etching step of chemically etching the untreated porous film to obtain a porous film. The resin component (A) and the fine particles (B) are as described above. In addition, the method for producing a porous film may include a step in which the resin component (A) contains at least one of a polyamic acid and a polyamide-imide precursor, and a baking step of baking the composite film after the composite film formation step and before the fine particle removal step.

[0085] [Composite Film Formation Step (Production of Unbaked Composite Film)] In the composite film formation step, a composite film made of the porous film production composition (varnish) described above is formed on a substrate. The composite film (hereinafter also referred to as "unbaked composite film") is preferably formed (deposited) directly on the substrate. The unbaked composite film can be formed, for example, by applying a varnish to the substrate and then performing a drying step in which the varnish is dried at 0 to 100°C under normal pressure or in a 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] [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. When the resin component (A) contained in the varnish is a polyamic acid or a polyamideimide precursor, the composite film (unbaked composite film) is subjected to a post-treatment (baking step) by heating to form a composite film (resin-particle composite film) consisting of a resin made of polyimide and / or polyamideimide and particles (B). When the resin component (A) contained in the varnish is polyimide or polyamideimide, the baking step may be included, but may be omitted.

[0090] The baking temperature in the baking step varies depending on the structure of the unbaked composite film and the presence or absence of a condensing agent, but is preferably 120°C to 450°C, more preferably 150°C to 420°C. When fine particles made of an organic material are used as the fine particles (B), the baking temperature must be set to a temperature lower than the thermal decomposition temperature of the organic material. When the resin component (A) contained in the varnish is a polyamic acid, it is preferable to complete imidization in the baking step.

[0091] 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 maintained 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 is maintained for 20 minutes) and finally maintained 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.

[0092] The thickness of the completed resin-particle composite film can be determined by measuring the thickness at multiple points 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 film to be finally obtained.

[0093] [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 (if a baking step is performed, the composite film (resin-particle composite film) after the baking step). By selecting an appropriate method to remove the particles (B), an untreated porous film can be produced with good reproducibility.

[0094] 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.

[0095] 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 and 320°C or lower, the resin particles disappear from within the porous 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.

[0096] [Chemical Etching Step] In the chemical etching step, the porous membrane is obtained by removing at least a portion of the resin portion of the untreated porous membrane by chemical etching. By removing at least a portion of the resin portion of the untreated porous membrane by chemical etching, the pore size of the porous membrane in the final product can be made small, and the filtration flow rate can be improved.

[0097] The chemical etching method is not particularly limited, and any conventionally known method can be used, but alkaline etching is preferred as the chemical etching method.

[0098] Examples of alkaline etching include treatment with an alkaline etching solution such as an inorganic alkaline solution or an organic alkaline solution. The alkaline etching solution is preferably an inorganic alkaline solution. 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.

[0099] 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.

[0100] After removing at least a portion of the resin portion of the untreated porous membrane by chemical etching, the etching solution is removed by rinsing with water. After rinsing the porous membrane with water, a re-baking step is preferably carried out in which the porous membrane is baked. By carrying out chemical etching, at least a portion of the imide rings are opened. By the re-baking step, the portions of the imide rings that have been opened by chemical etching can be re-cyclized. The baking conditions in the re-baking step are the same as those in the baking step.

[0101] By chemical etching, the average pore size of the pores that originate from small particles (B1) in untreated porous film tends to be small, and the average pore size of the pores that originate from large particles (B2) tends to be large.In the small pores that originate from small particles (B1), the inside of the pores is hard to contact with etchant, and the pore size is hard to expand by etching, while the pore size is strongly reduced by the shrinkage of porous film caused by chemical etching.On the other hand, in the large pores that originate from large particles (B2), the inside of the pores is good contact with etchant, so the expansion of pore size by etching is more significant than the effect of the shrinkage of porous film caused by chemical etching.

[0102] The rate of change in average pore diameter due to the chemical etching step between the average pore diameter (P1) of the untreated porous membrane before the chemical etching step and the average pore diameter (P2) of the porous membrane after the chemical etching step is calculated by the following formula: Rate of change in average pore diameter (%) = (P1 - P2) / P1 x 100 (Formula 1)

[0103] The rate of change in the average pore diameter calculated by the above formula is preferably 7% or more and 30% or less, more preferably 10% or more and 25% or less, and even more preferably 10% or more and 18% or less.

[0104] The filtration flow rate of the produced porous membrane is expressed as the IPA flow rate (mL / (min cm)) measured by the method described in [IPA Flow Rate (FR)] below. ２ The rate of change due to the chemical etching step between the filtration flow rate (F1) using an untreated porous membrane before the chemical etching step and the filtration flow rate (F2) using a porous membrane after the chemical etching step is calculated by the following formula: Rate of change in filtration flow rate (%) = (F2 - F1) / F1 × 100 (Formula 2)

[0105] The rate of change in filtration flow rate calculated by the above formula is preferably 15% or more and 60% or less, more preferably 20% or more and 55% or less, and even more preferably 25% or more and 50% or less.

[0106] According to the method for producing a porous membrane described above, a porous membrane having a small pore size and an improved flow rate can be produced.

[0107] The porous membrane produced by the above method includes a structure in which spherical pores are interconnected (communicating pores). The openings in the porous membrane are the portions on the surface of the porous membrane where the communicating pores open.

[0108] 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.

[0109] Individual spherical pores are typically formed by removing individual particles (B) present in the composite membrane (resin-particle composite membrane) in a subsequent particle removal step. Furthermore, interconnected pores are formed in a porous membrane manufacturing method by removing multiple particles (B) present in contact with each other in the resin-particle composite membrane in a subsequent particle removal step. The locations where the spherical pores in the interconnected pores are interconnected originate from the locations where multiple particles (B) contact each other before being removed.

[0110] The diameter of the openings in the porous membrane may be adjusted appropriately depending on the intended use of the porous membrane, for example, within the range of 5 nm to 100 nm. The diameter of the openings is equal to or approximately equal to the diameter of the spherical pores that make up the communicating pores. The communicating pores, which are formed by a series of spherical pores of such diameters, allow fluid to pass smoothly through the porous membrane. The porous membrane has communicating pores that penetrate the membrane in the thickness direction as a fluid flow path. This allows fluid to pass from one main surface of the porous membrane to the other main surface. Furthermore, when the laminate is used as a filter, the fluid passes through the interior of the porous membrane while contacting the curved surfaces that define the individual spherical pores. The contact area of ​​the fluid inside the porous membrane is quite large due to the presence of communicating pores made of spherical pores. Therefore, when a fluid passes through a laminate including the porous membrane, it is thought that minute substances present in the fluid are easily adsorbed by the spherical pores in the porous membrane.

[0111] <Filtration Treatment Method> A liquid is filtered using the porous membrane described above. The purpose of the filtration is not particularly limited. Examples of the purpose of filtration include separating solids contained in the liquid and purifying the liquid by removing solids contained in the fluid.

[0112] The liquid to be filtered is not particularly limited. Preferred liquids include various chemicals used in semiconductor device manufacturing. Specific examples of chemicals include organic solvents and liquid resist compositions. Liquid resist compositions include various resist compositions used in the manufacture of electronic components such as displays and semiconductor elements to form protective films, insulating films, etching masks, light-shielding films, colored films, high-refractive-index films, low-refractive-index films, plating molds, and the like. The resist composition may be a photosensitive composition or a non-photosensitive composition, and is preferably a photosensitive composition. Resist compositions typically contain components such as various resins; polymerizable monomers or crosslinkers such as (meth)acrylate compounds, epoxy compounds, vinyl ether compounds, and methylol compounds; radical polymerization initiators; photoacid generators; quenchers; sensitizers; and surfactants.

[0113] When performing filtration, the porous membrane can be attached to various filter devices. In this case, the porous membrane may be processed into various shapes. For example, the porous membrane may be folded to increase the surface area. The porous membrane may also be processed into a cylindrical or bag shape.

[0114] The filtration of the fluid is carried out appropriately by a method depending on the type of filter device in which the porous membrane is mounted.

[0115] Specific Methods for Purifying Liquids A method for purifying a liquid involves causing a portion or all of the liquid to permeate from one side of the porous membrane described above to the other side by means of a pressure difference.

[0116] In the method for purifying a liquid, a part or all of the liquid can be permeated from one side of the porous membrane to the other side by filtering the part or all of the liquid using the porous membrane as a separating or adsorbing material. The porous membrane used as a separating or adsorbing material may be incorporated into a filter device described below.

[0117] The porous membrane may be used in a liquid purification method in a flat shape or a pipe shape with opposing sides of the porous membrane joined together. A pipe-shaped porous membrane is preferably pleated to increase the contact area with the feed liquid. The porous membrane is appropriately sealed to prevent the feed liquid and the filtrate from mixing, as described below.

[0118] The purification of liquid can be carried out by using the above-mentioned porous membrane without differential pressure, that is, by gravity natural filtration, but preferably by differential pressure.Differential pressure is not particularly limited as long as it can be set up between one side and the other side of porous membrane with pressure difference, but usually, it can be mentioned that one side (feed liquid side) of porous membrane is pressurized (positive pressure), one side (filtrate side) of porous membrane is made negative pressure by decompression (negative pressure) etc., and pressurization is preferred.

[0119] Pressurization involves applying pressure to the side of the porous membrane (the supply liquid side) where the liquid (sometimes referred to herein as the "supply liquid") is present before being passed through the porous membrane. For example, pressure is preferably applied by utilizing the pressure generated by the circulation or delivery of the supply liquid or by utilizing positive gas pressure. Pressurization can be performed using an active pressurizing device such as a pump (a liquid delivery pump, a circulation pump, etc.). Specifically, a rotary pump, a diaphragm pump, a metering pump, a chemical pump, a plunger pump, a bellows pump, a gear pump, a vacuum pump, an air pump, a liquid pump, etc. can be used. When pressurizing with gas, the gas used for pressurization is preferably a gas that is inert or non-reactive with the supply liquid, specifically, nitrogen or a rare gas such as helium or argon. Pressurization is preferred in the manufacturing field of electronic materials, particularly semiconductors. In this case, the side where the liquid that has permeated the porous membrane is collected can be atmospheric pressure without reducing the pressure, and positive gas pressure is preferred. The above pressurization method may also be performed via a pressure valve, a pressure valve, a three-way valve, or other valve. The pressure is reduced on the side (filtrate side) where the liquid that has permeated the porous polyimide resin membrane is collected, and may be reduced by, for example, a pump, but it is preferable to reduce the pressure to a vacuum. When circulating or transporting the supply liquid by a pump, the pump is usually placed between the supply liquid tank (or circulation tank) and the porous membrane.

[0120] The pressurization may utilize both pressurization of the liquid by a device such as a pump and pressurization by gas. Furthermore, the differential pressure may be a combination of pressurization and depressurization, such as a combination of pressurization of the liquid by a device such as a pump and depressurization, a combination of pressurization by gas and depressurization, or pressurization of the liquid by a device such as a pump, and pressurization by gas and depressurization. When combining methods for creating a differential pressure, a combination of pressurization of the liquid by a device such as a pump and positive gas pressure, or a combination of pressurization of the liquid by a device such as a pump and depressurization, is preferred in terms of simplifying production, etc. In the above-described liquid purification method, a porous membrane is used, and therefore purification with excellent impurity removal performance can be achieved even if only one method for creating a differential pressure, such as positive gas pressure, is used.

[0121] The pressure difference applied before and after the porous membrane by providing a pressure difference can be appropriately set according to the membrane thickness, porosity or average pore size of the porous membrane used, or the desired degree of purification, flow rate, flow velocity, or the concentration or viscosity of the supply liquid, etc. For example, in the case of a so-called cross-flow method (the supply liquid flows parallel to the porous membrane), it is, for example, 3 MPa or less, and in the case of a so-called dead-end method (the supply liquid flows crosswise to the porous membrane), it is, for example, 1 MPa or less.The lower limit value is not particularly limited, and is, for example, 10 Pa.

[0122] In the method for purifying a liquid, when part or all of the liquid is permeated from one side to the other side of a porous membrane, if the liquid contains a solute, the feed liquid may be appropriately diluted with a diluent.

[0123] In the method for purifying a liquid, before passing the feed liquid through the porous membrane, a solution such as an alcohol (e.g., methanol, ethanol, isopropyl alcohol), a ketone (e.g., acetone, methyl ethyl ketone), water, a solvent contained in the feed liquid, or a mixture thereof may be contacted with the porous membrane and passed through in order to clean the porous membrane, improve its wettability to the feed liquid, or adjust the surface energy between the porous membrane and the feed liquid. When contacting the porous membrane with the solution before passing the feed liquid through the porous membrane, the porous membrane may be impregnated or immersed in the solution. By contacting the porous membrane with the solution, for example, the solution can penetrate into the pores inside the porous membrane. The contact between the solution and the porous membrane before passing the feed liquid through the porous membrane may be carried out by the above-mentioned differential pressure, and may be carried out under pressure, particularly when the solution is to penetrate into the pores inside the porous membrane.

[0124] In the liquid purification method, a porous membrane having the above-mentioned average pore size and surface area is considered to remove some or all of the impurities from the liquid before treatment by separation and / or adsorption. In this specification, "separation" may include at least one selected from the group consisting of filtration, isolation, removal, capture, purification, and sieving. The liquid purification method can also be suitably used for a process that involves both separation and adsorption, such as a process of separating microsubstances from a liquid that contains them by adsorbing the microsubstances into the pores and / or communicating pores of a porous membrane.

[0125] In the method for purifying a liquid, the porous membrane can be used, for example, as a filter medium or other filtering material, specifically, it may be used alone, or it may be used as a filtering material to which other functional layers (membranes) are added, or it may be used as a membrane to be combined with other filtering materials, for example, it can be used as a membrane to be used in a filter device, etc. The functional layer that can be used in combination with the porous membrane is not particularly limited, and examples thereof include nylon membranes, polytetrafluoroethylene (PTFE) membranes, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) membranes, or membranes modified from these, which have chemical or physicochemical functions.

[0126] In the liquid purification method of the present invention, the porous membrane can be used as a filter medium, such as an impurity removal filter used in the semiconductor manufacturing field, and can also be used as a laminate containing the filter medium and other filter materials, or as a filter device.The filter device is not particularly limited, but in the filter device, the porous membrane is arranged so that the feed liquid and the filtrate intersect.In relation to the liquid flow path, it may be arranged parallel to the flow path or so as to intersect.The regions before and after passing through the porous membrane are appropriately sealed so that the feed liquid is separated from the filtrate.For example, as a sealing method, the porous membrane may be processed, as necessary, by light (UV) curing adhesion or heat adhesion (including anchor effect adhesion (thermal welding, etc.)), or adhesion using an adhesive, or the porous membrane of the present invention and other filter materials (filters) can be bonded, for example, by an integration method, and these porous membranes can be further used with an outer container made of a thermoplastic resin such as polyethylene, polypropylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), polyethersulfone (PES), polyimide, polyamideimide, etc.

[0127] As described above, the present inventors have provided the following (1) to (7): (1) A method for producing a porous film, comprising: a composite film formation step of forming a composite film on a substrate, the composite film comprising a resin composition containing a resin component (A), fine particles (B), and a solvent (S); a fine particle removal step of removing the fine particles from the composite film to obtain an untreated porous film; and a chemical etching step of chemically etching the untreated porous film to obtain a porous film, wherein the resin component (A) is at least one selected from the group consisting of polyamic acid, polyimide, a polyamideimide precursor, and polyamideimide; the fine particles (B) comprise fine particles (B1) and fine particles (B2), each of which has an average particle size of 100 nm or less; and the average particle size of the fine particles (B2) is larger than that of the fine particles (B1). (2) The method for producing a porous film according to (1), wherein the chemical etching is alkaline etching. (3) The method for producing a porous film according to (1) or (2), wherein the resin component (A) comprises at least one of a polyamic acid and a polyamideimide precursor, and wherein a baking step of baking the composite film is included after the composite film-forming step and before the fine particle removal step. (4) The method for producing a porous film according to any one of (1) to (3), wherein the resin component (A) comprises at least one selected from the group consisting of a polyamic acid, a polyimide, a polyamideimide precursor, and a polyamideimide, and wherein a re-baking step of baking the porous film obtained in the chemical etching step is included. (5) The method for producing a porous film according to any one of (1) to (4), wherein the rate of change in average pore diameter calculated by the following formula 1: Rate of change in average pore diameter (%) = (D1 - D2) / D1 × 100 (Formula 1) (In Formula 1, D1 is the average pore diameter of the untreated porous film before the chemical etching step, and D2 is the average pore diameter of the porous film after the chemical etching step) is 7% or more and 20% or less. (6) The following formula 2 is used: Filtration flow rate change rate (%) = (F2 - F1) / F1 × 100 (Formula 2) (In Formula 2, F1 is the filtration flow rate (ml / min cm) using the porous membrane before the chemical etching process. ２ ), and F2 is the filtration flow rate (ml / min cm) using the porous membrane after the chemical etching process. ２) is 15% or more and 60% or less, and the filtration flow rate is measured by filtering isopropyl alcohol from one side of the porous membrane at 25°C under pressure at a pressure of 0.1 MPa (G). (7) A porous membrane produced by the method for producing a porous membrane according to any one of (1) to (6).

[0128] 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.

[0129] [Examples 1 to 3 and Comparative Examples 1 to 7] A silica dispersion (containing 0.5% by mass of dispersant relative to silica) was added to a polyamic acid solution so that the amount of polyamic acid was 30% by mass and the amount of silica was 70% by mass relative to the total mass of polyamic acid (resin component (A)) and silica (fine particles (B)). Furthermore, organic solvents (1) and (2) were added, respectively, so that the solvent composition in the entire final composition (porous membrane production composition (varnish)) was organic solvent (1):organic solvent (2) = 90:10. The resulting mixture was stirred at 4000 rpm for 30 minutes with a stirring blade in a 1000 mL container to disperse the mixture. Thereafter, a 60 μm diameter (cross-sectional area 2826 μm) dispersion was prepared using a Yoshida Kikai Kogyo Co., Ltd. ２) using a dispersing device equipped with a flow path, a dispersion process was performed 10 times, in which the mixed solution was passed through the flow path at 200 MPa, to prepare a porous film manufacturing composition (varnish) with a solids concentration of 30 mass%. The ratio of polyamic acid to silica (polyamic acid:silica) in the obtained varnish was 38:62 in volume ratio and 30:70 in mass ratio. The polyamic acid solution, organic solvent, dispersant, and fine particles shown below were used. The type and parts by mass of silica used are listed in Table 1. Table 1 also lists the average particle size ratio of the silica used (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)). Polyamic acid solution: reaction product of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether (solid content 20% by mass (organic solvent: N,N-dimethylacetamide)) Organic solvent (1): N,N-dimethylacetamide (DMAc) Organic solvent (2): gamma butyrolactone 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, and / or silica with an average particle size of 100 nm

[0130] The obtained porous film-forming composition 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 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 resin-particle composite film was then peeled off from the substrate. 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). The film was then washed with water and dried to obtain untreated polyimide porous films of Examples 1 to 3 and Comparative Examples 1 to 7, each with a film thickness of 40 μm. The untreated polyimide porous films of Examples 1 to 3 and Comparative Examples 1 to 4 were immersed in a 2.38% by mass aqueous solution of tetramethylammonium hydroxide (TMAH) as an etching solution for 30 seconds to remove a portion of the resin in the untreated porous film (chemical etching step). In Comparative Examples 5 to 7, the untreated polyimide porous membranes were washed with water without chemical etching and baked at 380°C for 15 minutes to obtain the baked membranes as polyimide porous membranes.The chemically etched porous membranes were then washed with water.The washed porous membranes were baked in an oven at 380°C for 15 minutes to complete the imidization and obtain polyimide porous membranes.The average pore size and pore size distribution of the polyimide porous membranes of Examples 1 to 3 and Comparative Examples 1 to 4 and Comparative Examples 5 to 7 were measured using the porometer described above.

[0131] The IPA flow rate (FR) of the obtained polyimide porous membranes of Examples 1 to 3 and Comparative Examples 1 to 4 and Comparative Examples 5 to 7 was measured according to the following method. The results are shown in Table 1. [IPA Flow Rate (IPA FR)] Each polyimide porous membrane 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 membrane) to perform filtration, and the flow rate of IPA passing through the polyimide porous membrane was measured. The flow rate was measured at 25°C. In Table 1, the IPA flow rate is expressed as 1 cm２ The blood pressure is reported as mL per minute.

[0132] *Measurement is not possible after the chemical etching process due to film shrinkage.

[0133] The rate of change in the average pore size and filtration flow rate (IPA flow rate) due to the chemical etching process for the polyimide porous membranes obtained in Examples 1 to 3 and the polyimide porous membranes obtained in Comparative Examples 5 to 7, which correspond to untreated polyimide porous membranes obtained as intermediates in the production of the polyimide porous membranes in Examples 1 to 3, was calculated using the above-mentioned formulas 1 and 2. The results are shown in Table 2.

[0134]

[0135] The pore size distributions of the obtained polyimide porous membranes of Examples 1 to 3 and Comparative Examples 5 to 7 were measured using the above-mentioned porometer. The results are shown in Tables 3 and 4.

[0136]

[0137]

[0138] Tables 1 to 4 show that the polyimide porous membranes of Examples, which were produced using varnishes containing the above-mentioned specified components and by the above-mentioned specified processes, allow fluids such as IPA to pass through well, despite their small average pore size. On the other hand, Tables 1 to 4 show that the polyimide porous membranes of Comparative Examples, which were produced using varnishes that do not fall under the category of varnishes containing the above-mentioned specified components or which were produced without a chemical etching process, had large average pore sizes or, as a fluid flow rate corresponding to the average pore size, had smaller fluid flow rates than the polyimide porous membranes of Examples.

Claims

1. A method for producing a porous film, comprising: a composite film formation step of forming a composite film made of a resin composition containing a resin component (A), fine particles (B), and a solvent (S) on a substrate; a fine particle removal step of removing the fine particles from the composite film to obtain an untreated porous film; and a chemical etching step of chemically etching the untreated porous film to obtain a porous film, wherein the resin component (A) is at least one selected from the group consisting of polyamic acid, polyimide, polyamideimide precursor, and polyamideimide, the fine particles (B) include fine particles (B1) and fine particles (B2), each of which has an average particle size of 100 nm or less, and the average particle size of the fine particles (B2) is larger than the average particle size of the fine particles (B1).

2. The method for producing a porous film according to claim 1, wherein the chemical etching is alkaline etching.

3. The method for producing a porous film according to claim 1, wherein the resin component (A) contains at least one of a polyamic acid and a polyamide-imide precursor, and the method further comprises a baking step of baking the composite film after the composite film formation step and before the fine particle removal step.

4. The method for producing a porous film according to claim 1, wherein the resin component (A) comprises at least one selected from the group consisting of polyamic acid, polyimide, polyamideimide precursor, and polyamideimide, and the method further comprises a baking step of baking the porous film obtained in the chemical etching step.

5. The method for producing a porous membrane according to claim 1, wherein the rate of change in average pore diameter calculated by the following formula 1: Rate of change in average pore diameter (%)=(D1-D2) / D1×100 (Formula 1) (In Formula 1, D1 is the average pore diameter of the untreated porous membrane before the chemical etching step, and D2 is the average pore diameter of the porous membrane after the chemical etching step), is 7% or more and 30% or less.

6. The rate of change in filtration flow rate (%) is calculated using the following formula 2: F2-F1) / F1×100 (in the formula 2, F1 is the filtration flow rate (ml / min cm) using the porous membrane before the chemical etching step). ２ ), and F2 is the filtration flow rate (ml / min cm) using the porous membrane after the chemical etching step. ２ ) The method for producing a porous membrane according to claim 1, wherein the rate of change in filtration flow rate calculated by (a) is 15% or more and 60% or less, and the filtration flow rate is measured by filtering isopropyl alcohol from one side of the porous membrane at 25°C under pressure at a pressure of 0.1 MPa (G).

7. A porous membrane produced by the method for producing a porous membrane according to any one of claims 1 to 6.

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