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

Abstract

To provide a method for producing a porous membrane, by which a porous membrane having a small pore diameter and excellent fluid permeability can be produced, and to provide a porous membrane produced by the method for producing a porous membrane.SOLUTION: A specific step using a resin composition containing a resin component (A), fine particles (B), and a solvent (S), wherein the resin component (A) is at least one selected from the group consisting of a polyamic acid, a polyimide, a polyamideimide precursor, and a polyamideimide; The fine particles (B) include fine particles (B1) and fine particles (B2), the fine particles (B1) and the fine particles (B2) are fine particles having a mean particle size of 100nm or less, and the mean particle size of the fine particles (B2) is larger than the mean particle size of the fine particles (B1).SELECTED DRAWING: None

Description

[Technical Field]

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

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

[0003] For example, a known method for producing a porous polyimide film involves applying a varnish, in which silica particles are dispersed in a solution of polyamic acid or polyimide, onto a substrate, and then heating the applied film as necessary to obtain a polyimide film containing silica particles.Then, the silica in the polyimide film is dissolved and removed with hydrogen fluoride water to make it porous (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5605566 Summary of the Invention [Problem to be solved by the invention]

[0005] There is a demand for the above-mentioned porous membrane to have a small pore size. For example, in the manufacture of semiconductors, porous films 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 films with small pore diameters are required. Note that porous films with small pore diameters can be manufactured using a porous film manufacturing composition (varnish) containing fine particles such as silica with a small particle size. However, reducing the pore size of a porous membrane reduces the permeability (liquid permeability and air permeability) of the porous membrane, which causes a problem of a decrease in the flow rate of liquids and gases passing through the porous membrane.

[0006] The present invention has been made in 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. [Means for solving the problem]

[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), wherein the resin component (A) is at least one selected from the group consisting of polyamic acid, polyimide, a polyamideimide precursor, and a 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 composite film by 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 particulate removal step of removing particulates from the composite membrane to obtain an untreated porous membrane; A chemical etching step is included in which an untreated porous film is chemically etched to obtain a porous film; 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) contain fine particles (B1) and fine particles (B2); the fine particles (B1) and the fine particles (B2) each have an average particle size of 100 nm or less, In this method for producing a porous membrane, 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. [Effects of the Invention]

[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. DETAILED DESCRIPTION OF THE INVENTION

[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 membrane≫ The porous film includes a composite film forming 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 particulate removal step of removing particulates from the composite membrane to obtain an untreated porous membrane; and a chemical etching step of chemically etching an untreated porous film to obtain a porous film. Here, the "untreated porous film" refers to a porous film from which fine particles have been removed but which has not been subjected to chemical etching. The untreated porous film may be imidized through a baking process 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 the 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 the average particle size of the fine particles (B1).

[0013] As described above, the porous membrane is produced using a resin composition containing at least one resin component (A) selected from the group consisting of polyamic acid, polyimide, polyamideimide precursor, and polyamideimide. 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 a resin-particle composite membrane in a later step in the manufacturing method for a porous membrane described below. The interconnected pores can also be said to be adjacent individual pores formed by removing individual particles present in a resin-particle composite membrane in a later step in the manufacturing method for a porous membrane described below, at portions where the individual particles present in the resin-particle composite membrane were in contact with each other.

[0015] The porous membrane preferably has communicating holes that have openings on the outer surface of the porous membrane, which connect 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 porous membrane preferably satisfies the following conditions regarding the pore distribution of the porous membrane. Here, the pore size distribution of the porous membrane is the distribution of pores of each size based on the volume of the total pore volume, measured in pore size ranges of 5 nm or more for every 1 nm. 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, it is preferable that 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 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 51% or more. For example, when the pore size range of the smallest pore size fraction (first fraction) in which pores exist is 5 nm or more and 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 4th fraction: 8nm or more and less than 9nm

[0018] In the pore size distribution of porous membrane, the total value of the pore distribution of a total of four fractions in the pore size range from the minimum value of the pore size range of the fraction with the smallest pore size where pores exist to the minimum value + 4 nm can be adjusted by the method of adjusting the particle size distribution of the microparticles contained in the varnish for manufacturing porous membrane, which will be described later.As the method of adjusting the particle size distribution of microparticles, the method of using a combination of multiple types of microparticles with different average particle sizes can be used as microparticles.In this case, it is preferable to increase the ratio of the microparticles with small particle size in the multiple types 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, it is preferable that 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 in which the pores are distributed (the first fraction described above) is 15 nm or more. The above difference is calculated as the difference between D and A when the pore size range of the smallest pore size fraction is from A nm to less than B nm, and the pore size range of the largest pore size fraction is from C nm to 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 measured by a porometer. The upper limit of 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 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 mean pore size measured by a porometer is the mean flow pore size measured by liquid-liquid phase displacement. The mean pore size can be measured, for example, using a PMI Liquid Liquid Porometer LLP-1500A (ultra-low pressure, pore size distribution, and permeability measurement device) according to 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 mean 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 applied 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 inner surfaces of the pores in the porous membrane are substantially entirely curved. 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 particles with a sphericity of 1±0.3 or less, defined as the sphericity of the particle divided by 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 speed, 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 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 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 porous membrane production> The composition for producing a 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 they can form a porous film with the desired properties. 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 fine particles (B2) is 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) are 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, 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.

[0033] Specific preferred examples of the aromatic tetracarboxylic dianhydride 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 dianhydrides, 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 suitable aliphatic tetracarboxylic dianhydrides 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, and 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride. Examples of suitable aliphatic tetracarboxylic dianhydrides include ethylenetetracarboxylic dianhydride, butanetetracarboxylic dianhydride, cyclopentanetetracarboxylic dianhydride, cyclohexanetetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, and 1,2,3,4-cyclohexanetetracarboxylic dianhydride. Among these, 3,3',4,4'-biphenyltetracarboxylic dianhydride and pyromellitic dianhydride are preferred in terms of price, availability, and the like.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, but aromatic diamines are preferred from the viewpoint of the heat resistance of the resulting polyimide resin. 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 and p-phenylenediamine, and 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] Diaminobiphenyl compounds have two aminophenyl groups bonded together, such as 4,4'-diaminobiphenyl and 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl.

[0038] Diaminodiphenyl compounds are compounds in which two aminophenyl groups are bonded to each other 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. The alkylene group derivative 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'-diaminodiphenyl methane, 3,4'-diaminodiphenyl methane, 4,4'-diaminodiphenyl methane, 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) bis(p-aminophenyl)-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, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, and the like.

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

[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 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.

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

[0046] The number of carbon atoms in 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 a halogen atom, a methyl group, a methoxy group, a cyano group, a phenyl group, 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 a method of reacting an acid and a diamine component in a solvent.

[0049] The reaction between tetracarboxylic dianhydride and diamine is usually carried out in a solvent. The solvent used for 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 the tetracarboxylic dianhydride and 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 phenol-based 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 it is desirable that the content of the produced polyamic acid be 5 to 50 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 composition of the raw materials used, but is usually 3 to 24 hours. The polyamic acid may be used alone or in combination of two or more kinds.

[0053] [Polyimide] The polyimide is not limited in structure or molecular weight, 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] 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 described in the polyamic acid section above can also be used in combination, as long as the solubility is not impaired. The polyimide and the monomer thereof may be used singly or in combination of two or more kinds.

[0055] There are no particular limitations on the means for producing polyimides. For example, known methods such as chemical imidization or thermal imidization of polyamic acid can be used. 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. [ka] [ka]

[0056] [Polyamideimide and polyamideimide precursors] 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 thereof 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'- ... Examples of suitable isocyanates include 2,4-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 Japanese Patent Application Laid-Open Nos. 63-283705 and 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 (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 above acid such as trimellitic anhydride chloride with a diamine. In this specification and claims, "polyamideimide precursor" means a polymer (precursor polymer) before imidization. The polyamideimide and polyamideimide precursor may each be used alone or in combination of two or more thereof.Furthermore, with respect to the polyamideimide, the polymer, raw material monomer, and oligomer may each be used alone or in combination of two or more thereof.

[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 size of the fine particles (B1) and the average particle size of the fine particles (B2) are each 100 nm or less, the pore size of the porous membrane produced can be reduced and the flow rate can be improved, as shown in the examples described later. This is thought to be due to the following reasons. The pores (spherical pores) in the porous membrane are formed by removing individual microparticles present in a composite membrane (resin-particle composite membrane) formed using varnish in a subsequent microparticle 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 in the porous membrane manufacturing method by removing multiple microparticles (B) that are present in contact with each other in a composite membrane (resin-particle composite membrane) formed using varnish in a subsequent microparticle removal step. The points at which the spherical pores in the interconnected pores are interconnected originate from the points (contact points) where multiple microparticles (B) come into contact with each other before being removed. The diameter of the communicating holes is measured by the above-mentioned 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 diameter of the interconnected pores formed at the contact points (contact points) between the fine particles (B1) or between the fine particles (B1) and the fine particles (B2) depends on the diameter 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, interconnected pores of the same diameter as the interconnected pores formed at the contact points between the small fine particles (fine particles (B1)) are formed. On the other hand, when two types of microparticles of different sizes (microparticles (B1) and microparticles (B2)) are used, the number of pores formed is smaller than when only small microparticles (microparticles (B1)) are used, and therefore the resistance to the fluid flowing 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 small particles (B1) with an average particle size of 100 nm or less and particles (B2) larger than the particles (B1) as the particles (B), 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, thereby further reducing the average pore size of the porous membrane and further improving the flow rate of the fluid through the porous membrane.

[0066] The fine particles (B1) have an average particle size of 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, and 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, but may be, for example, 5 nm or more and 90 nm or less, 10 nm or more and 80 nm or less, or 20 nm or more and 70 nm or less.

[0069] Specific examples of combinations of the fine particles (B1) and the 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 or more and 90 nm or less 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 50% of the integrated value 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 particle (B1) B1 Mass M of the particle (B2) B2 The ratio (M B2 / M B1 ) is preferably 0.10 to 0.90, more preferably 0.20 to 0.80, further 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 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 process, and either an inorganic material or an organic material can be used. The material of the fine particles (B1) and the material of the fine particles (B2) may be different, but it is preferable that they are the same.

[0074] Examples of inorganic material fine particles (B) include inorganic oxide fine particles, and specific examples include metal oxide fine particles such as silica (silicon dioxide) fine particles, titanium oxide fine particles, and alumina (Al2O3) fine particles. Examples of silica include colloidal silica. Among them, 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 together.

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

[0078] Solvent The solvent (S) is not particularly limited as long as it can dissolve the resin component (A) contained in the varnish, which is composed of polyamic acid, polyimide, polyamideimide precursor, polyamideimide, and / or polyethersulfone, 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 the above-mentioned nitrogen-containing polar solvents as well as polar solvents such as diphenylsulfone, dimethylsulfone, dimethylsulfoxide, benzophenone, tetrahydrothiophene-1,1-dioxide, and 1,3-dimethyl-2-imidazolidinone.

[0079] [Dispersant] A dispersant may be added together with the fine particles (B) to uniformly disperse the fine particles (B) in the varnish. Adding a dispersant allows the fine particles (B) to be mixed more uniformly in the varnish and to be uniformly distributed in the film formed from the varnish. As a result, dense openings can be formed on the surface of the final porous film, and the front and back surfaces can be efficiently connected, improving the air permeability of the porous film. Furthermore, adding a dispersant tends to improve the drying properties of the varnish and the peelability of the resulting unfired composite film from the substrate, etc.

[0080] The dispersant is not particularly limited, and known dispersants can be used. For example, anionic surfactants such as coconut fatty acid salts, castor sulfated oil salts, lauryl sulfate salts, polyoxyalkylene allyl phenyl ether sulfate salts, alkyl benzene sulfonic acid, alkyl benzene sulfonate salts, alkyl diphenyl ether disulfonate salts, alkyl naphthalene sulfonate salts, dialkyl sulfosuccinate salts, isopropyl phosphate, polyoxyethylene alkyl ether phosphate salts, and polyoxyethylene allyl phenyl ether phosphate salts; cationic surfactants such as oleylamine acetate, lauryl pyridinium chloride, cetyl pyridinium chloride, lauryl trimethyl ammonium chloride, stearyl trimethyl ammonium chloride, behenyl trimethyl ammonium chloride, and didecyl dimethyl ammonium chloride; amphoteric surfactants such as coconut alkyl dimethyl amine oxide, fatty acid amidopropyl dimethyl amine oxide, alkyl polyaminoethyl glycine hydrochloride, amido betaine surfactants, alanine surfactants, and lauryliminodipropionic acid; Nonionic surfactants of polyoxyalkylene primary alkyl ethers or polyoxyalkylene secondary alkyl ethers, such as ethylene octyl ether, polyoxyethylene decyl ether, polyoxyethylene lauryl ether, polyoxyethylene laurylamine, polyoxyethylene oleylamine, polyoxyethylene polystyrylphenyl ether, and polyoxyalkylene polystyrylphenyl ether; other polyoxyalkylene-based nonionic surfactants, such as polyoxyethylene dilaurate, polyoxyethylene laurate, polyoxyethylenated castor oil, polyoxyethylenated hydrogenated castor oil, sorbitan laurate, polyoxyethylene sorbitan laurate, and fatty acid diethanolamides; fatty acid alkyl esters, such as octyl stearate and trimethylolpropane tridecanoate; and polyether polyols, such as polyoxyalkylene butyl ether, polyoxyalkylene oleyl ether, and trimethylolpropane tris(polyoxyalkylene) ether, but are not limited to these.The above dispersants can 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 (varnish) for producing the porous film is not particularly limited. The varnish is typically produced by a microparticle dispersion preparation step in which microparticles (B) are dispersed in a solvent, a step in which a resin solution containing resin component (A) is prepared, and a kneading step in which the microparticle dispersion and the resin solution are mixed together and kneaded to adjust the concentration.

[0083] The solid content of the varnish is preferably 10% by mass to 50% by mass 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 (trade name: Awatori Rentaro, manufactured by Thinky Corporation), a planetary mixer, a bead mill, or the like can be used. In addition, in the kneading process, the cross-sectional area is 1960 μm 2 More than 785000μm 2 A dispersion treatment may be performed by using a dispersion device equipped with 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 treatment by passing the mixed liquid through such a flow path is the method described in JP 2020-104105 A.

[0084] <Method for manufacturing porous membrane> The porous membrane is a composite film forming 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 particulate removal step of removing particulates from the composite membrane to obtain an untreated porous membrane; and a chemical etching step of chemically etching an 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 membrane may include a step in which the resin component (A) contains at least one of a polyamic acid and a polyamide-imide precursor, and the method further includes a step of baking the composite membrane after the composite membrane formation step and before the fine particle removal step.

[0085] [Composite film formation process (manufacturing unsintered composite film)] In the composite film forming step, a composite film made of the above-mentioned porous film manufacturing composition (varnish) is formed on a substrate. The composite film (hereinafter also referred to as "unfired composite film") is preferably formed (deposited) directly on a substrate. The unsintered composite film can be formed, for example, by applying a varnish onto a substrate and carrying out a drying step in which the varnish is dried at 0 to 100°C under normal pressure or in vacuum, preferably at 10 to 100°C under normal pressure. Examples of the substrate include a PET film, a SUS substrate, and a glass substrate.

[0086] The unsintered composite film is then baked to obtain a polyimide-particle composite film (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. This can cause discoloration during firing or adversely affect the electrical properties, so it is preferable to remove it as much as possible. A washing step in which the unfired composite film peeled from the substrate is washed with an organic solvent can be introduced to remove the release agent.

[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 process (manufacturing resin-particle composite film)] The firing step is a step of firing the composite film after the composite film formation step and before the fine 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 a polyimide or a polyamideimide, a baking step may be included, but the baking step may be omitted.

[0090] The firing temperature in the firing step varies depending on the structure of the unfired 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 firing 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 above room temperature to below 400°C over 3 hours and then held at 400°C for 20 minutes, or a stepwise drying-thermal imidization method in which the temperature is raised from room temperature to 400°C in 50°C increments (each step held for 20 minutes) and finally held at 400°C for 20 minutes.When an unfired composite film is formed on a substrate and then temporarily peeled off from the substrate, a method in which the edges of the unfired composite film are fixed to a stainless steel mold or the like to prevent deformation can be used.

[0092] The thickness of the resulting resin-particle composite membrane can be determined by measuring the thickness at multiple locations with, for example, a micrometer and averaging the measured thicknesses. The thickness of the resin-particle composite membrane is appropriately set taking into account the thickness of the porous membrane to be finally obtained.

[0093] [Particle removal process (making the resin-particle composite membrane 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] Organic materials can also be selected as the material for the fine particles (B). Any organic material can be used, as long as it decomposes at a lower temperature than the resin contained in the resin-particle composite film. Examples include resin fine particles made of linear polymers or known depolymerizable polymers. Typical linear polymers undergo random cleavage of polymer molecular chains during thermal decomposition, while depolymerizable polymers decompose into monomers during thermal decomposition. Both types of fine particles disappear from the porous film by decomposing into low-molecular-weight compounds or CO2. The decomposition temperature of the resin fine 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. A decomposition temperature of 200°C or higher allows film formation even when a high-boiling-point solvent is used in the varnish, broadening the range of baking conditions for the resin-particle composite film. Furthermore, a decomposition temperature of less than 320°C allows the resin fine particles to disappear without thermally damaging the resin contained in the resin-particle composite film.

[0096] [Chemical etching process] In the chemical etching step, the resin portion of the untreated porous membrane is at least partially removed by chemical etching to obtain the porous membrane. By chemically etching the untreated porous membrane to remove at least a portion of the resin portion, the pore size of the final porous membrane can be reduced, thereby improving the filtration flow rate.

[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 a hydrazine solution containing hydrazine hydrate and ethylenediamine; a solution of a basic alkali metal compound such as potassium hydroxide, sodium hydroxide, sodium carbonate, sodium silicate, or sodium metasilicate; an ammonia solution; and an etching solution mainly containing an alkali metal hydroxide, hydrazine, and 1,3-dimethyl-2-imidazolidinone. 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 also 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 the chemical etching method, the average pore size of the pores derived from the small particles (B1) in the untreated porous membrane tends to become smaller, and the average pore size of the pores derived from the large particles (B2) tends to become larger. In the small pores resulting from the small particles (B1), the inside of the pores is less likely to come into contact with the etchant, and the pore diameter is less likely to expand due to etching, while the pore diameter is significantly reduced due to shrinkage of the porous film caused by chemical etching.On the other hand, in the large pores resulting from the large particles (B2), the inside of the pores is more likely to come into contact with the etchant, and the pore diameter is significantly increased due to etching, rather than the shrinkage of the porous film caused by chemical etching.

[0102] The rate of change in average pore size due to the chemical etching step between the average pore size (P1) of the untreated porous membrane before the chemical etching step and the average pore size (P2) of the porous membrane after the chemical etching step is calculated by the following formula. Rate of change in average pore size (%) = (P1 - P2) / P1 × 100 (Equation 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 manufactured porous membrane is the IPA flow rate (mL / (min cm)) measured by the method described below in [IPA Flow Rate (FR)]. 2 The rate of change due to the chemical etching process between the filtration flow rate (F1) using an untreated porous membrane before the chemical etching process and the filtration flow rate (F2) using a porous membrane after the chemical etching process is calculated using the following formula. Filtration flow rate change rate (%) = (F2 - F1) / F1 × 100 (Equation 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 the hole is a concept that includes a perfect sphere, but is not necessarily limited to a perfect sphere. The spherical shape may be substantially a perfect sphere, and also includes a shape that can be recognized as a nearly perfect sphere when visually observing an enlarged image of the hole portion. Specifically, in the case of a spherical hole, the surface defining the hole portion is a curved surface, and the curved surface may define a hole having a perfect sphere or a nearly perfect sphere.

[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 interconnected spherical pores are formed at locations where multiple particles (B) come into contact with each other before being removed.

[0110] The diameter of the openings in the porous membrane may be appropriately changed depending on the intended use of the porous membrane, for example, within the range of 5 nm to 100 nm. The diameter of the opening is equal to or approximately equal to the diameter of the spherical holes that make up the communicating holes. The communicating holes, which are formed by a series of spherical holes of such diameter, allow fluid to pass through the porous membrane efficiently. The porous membrane has internal communicating pores that run through the membrane in the thickness direction and serve as fluid flow paths, allowing the 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 coming into contact with 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 interconnected pores made up of spherical pores. Therefore, when a fluid passes through a laminate including a porous membrane, it is thought that minute substances present in the fluid are likely to be adsorbed by the spherical pores in the porous membrane.

[0111] <Filtration method> The porous membrane described above is used to filter a liquid. The purpose of the filtration is not particularly limited. Examples of the purpose of the 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 chemical liquids used in semiconductor device manufacturing. Specific examples of chemical liquids 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, etc. 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 such as (meth)acrylate compounds, epoxy compounds, vinyl ether compounds, and methylol compounds, or crosslinking agents; 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 method for refining liquid> The method for purifying a liquid involves forcing some or all of the liquid through a porous membrane as described above from one side to the other by differential pressure.

[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 liquid purification can be carried out by using the above-mentioned porous membrane without differential pressure, that is, by natural filtration by gravity, but it is preferable to carry out by differential pressure.The differential pressure is not particularly limited as long as it is a pressure difference between one side and the other side of the porous membrane, but usually includes pressurization (positive pressure) that applies pressure to one side (feed liquid side) of the porous membrane, and decompression (negative pressure) that applies negative pressure to one side (filtrate side) of the porous membrane, and pressurization is preferable.

[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 permeating the porous membrane. For example, pressure is preferably applied by utilizing the pressure generated by circulating or feeding the supply liquid or by utilizing positive gas pressure. Pressurization can be performed using an active pressurizing device such as a pump (a liquid feed 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 although this may be reduced by a pump, it is preferable to reduce the pressure to a vacuum. When the supply liquid is circulated or sent 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-mentioned 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 across the porous membrane by providing a pressure difference may be appropriately set depending on the membrane thickness, porosity or average pore size of the porous membrane used, or the desired degree of purification, flow rate, flow velocity, or concentration or viscosity of the feed liquid, but for example, in the case of a so-called cross-flow method (where the feed 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 (where the feed liquid flows crosswise to the porous membrane), it is, for example, 1 MPa or less. The lower limit 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 supply liquid through the porous membrane, a solution of an alcohol such as methanol, ethanol, or isopropyl alcohol, a ketone such as acetone or methyl ethyl ketone, water, a solvent contained in the supply liquid, or a mixture thereof may be brought into contact with the porous membrane and passed through in order to wash the porous membrane, improve the wettability of the porous membrane with the supply liquid, or adjust the surface energy between the porous membrane and the supply liquid. In contacting the porous membrane with the solution before the feed solution is passed through, the porous membrane may be impregnated or immersed in the solution, and by contacting the porous membrane with the solution, for example, the solution can be allowed to penetrate into the pores inside the porous membrane. The contacting of the porous membrane with the solution before the feed solution is passed through may be carried out by the above-mentioned pressure difference, and in particular, when the solution is allowed to penetrate into the pores inside the porous membrane, the contacting may be carried out under pressure.

[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 processes that involve both separation and adsorption, such as a process of separating microsubstances from a liquid that contained the microsubstances 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 another functional layer (membrane) is added, or it may be used as a membrane to be combined with another filtering material, 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, and 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 semiconductor manufacturing. It 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 a filter device, the porous membrane is arranged so that the feed liquid and the filtrate intersect. With respect to the liquid flow path, the porous membrane may be arranged parallel to the flow path or intersect the flow path. To separate the feed liquid from the filtrate, the regions before and after passing through the porous membrane are appropriately sealed. For example, as a sealing method, the porous membrane may be processed, as needed, by light (UV) curing or heat bonding (including anchoring effect bonding (thermal welding, etc.)), or bonding with an adhesive. Alternatively, the porous membrane of the present invention can be bonded to another filter material (filter) by, for example, an integration method. 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, or polyamideimide.

[0127] As described above, the present inventors provide the following (1) to (7). (1) a composite film forming 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 particulate removal step of removing particulates from the composite membrane to obtain an untreated porous membrane; A chemical etching step is included in which an untreated porous film is chemically etched to obtain a porous film; 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) contain fine particles (B1) and fine particles (B2); the fine particles (B1) and the fine particles (B2) each have an average particle size of 100 nm or less, A method for producing a porous membrane, wherein 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 (1), wherein the chemical etching is alkaline etching. (3) The resin component (A) contains at least one of a polyamic acid and a polyamide-imide precursor, The method for producing a porous film according to (1) or (2), further comprising a firing step of firing the composite film after the composite film forming step and before the fine particle removing step. (4) The resin component (A) contains at least one selected from the group consisting of polyamic acid, polyimide, polyamideimide precursor, and polyamideimide, The method for producing a porous film according to any one of (1) to (3), further comprising a re-firing step of firing the porous film obtained in the chemical etching step. (5) Formula 1 below: Rate of change in average pore diameter (%) = (D1 - D2) / D1 × 100 (Equation 1) (In Equation 1, D1 is the average pore size of the untreated porous membrane before the chemical etching step, and D2 is the average pore size of the porous membrane after the chemical etching step.) The method for producing a porous membrane according to any one of (1) to (4), wherein the rate of change in average pore diameter calculated by the above formula is 7% or more and 20% or less. (6) Formula 2 below: Filtration flow rate change rate (%) = (F2 - F1) / F1 × 100 (Equation 2) (In Equation 2, F1 is the filtration flow rate (ml / min cm) using a 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 ) The rate of change in the filtration flow rate calculated by is 15% or more and 60% or less, The method for producing a porous membrane according to any one of (1) to (5), wherein 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). [Example]

[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 a dispersant relative to silica) was added to the 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 the polyamic acid (resin component (A)) and silica (fine particles (B)). Furthermore, organic solvents (1) and (2) were added 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 dispersed in a 1000 mL container by stirring with a stirring blade at 4000 rpm for 30 minutes. 2 Using a dispersing device equipped with a flow path of 1000 psi, the mixture was passed through the flow path at 200 MPa for 10 cycles to prepare a composition for producing a porous membrane (varnish) with a solids concentration of 30% by mass. The ratio of polyamic acid to silica (polyamic acid:silica) in the obtained varnish was 38:62 by volume and 30:70 by mass. The following polyamic acid solution, organic solvent, dispersant, and fine particles were used. The types and parts by mass of silica used are shown in Table 1. Table 1 also shows the ratio of the average particle size 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 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 polyimide-untreated porous films of Examples 1 to 3 and Comparative Examples 1 to 7, each with a film thickness of 40 μm. The polyimide-untreated 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 part 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 being subjected to chemical etching, and baked at 380° C. for 15 minutes to obtain the baked membranes as polyimide porous membranes. The chemically etched porous membrane was then washed with water. The washed porous membrane was baked in an oven at 380°C for 15 minutes to complete the imidization, thereby obtaining a polyimide porous membrane. 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 the polyimide porous membranes of Comparative Examples 5 to 7 were measured using the porometer described above.

[0131] The IPA flow rate (FR) of each of the obtained porous polyimide 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 cm 2 Express as mL per minute.

[0132] [Table 1] *Measurement is not possible after chemical etching due to film shrinkage.

[0133] The rates of change due to the chemical etching step in the average pore size and filtration flow rate (IPA flow rate) of 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, were calculated using the above-mentioned formulas 1 and 2. The results are shown in Table 2.

[0134] [Table 2]

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

[0136] [Table 3]

[0137] [Table 4]

[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 steps, 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 performing a chemical etching step, 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 composite film forming 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 particle removal step of removing the particles from the composite membrane to obtain an untreated porous membrane; A chemical etching step of chemically etching the untreated porous film to obtain a porous film, 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 the fine particles (B2) each have an average particle size of 100 nm or less, The method for producing a porous membrane, wherein 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 resin component (A) contains at least one of a polyamic acid and a polyamide-imide precursor, The method for producing a porous film according to claim 1 , further comprising a firing step of firing the composite film after the composite film forming step and before the fine particle removing step.

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

5. Formula 1 below: Rate of change in average pore diameter (%)=(D1−D2) / D1×100 (Equation 1) (In the 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.) The method for producing a porous membrane according to claim 1, wherein the rate of change in average pore diameter calculated by the above formula is 7% or more and 30% or less.

6. Formula 2 below: Filtration flow rate change rate (%) = (F2 - F1) / F1 × 100 (Equation 2) (In the above 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 rate of change in the filtration flow rate calculated by is 15% or more and 60% or less, The method for producing a porous membrane according to claim 1, wherein the filtration flow rate is measured by pressure filtering isopropyl alcohol from one side of the porous membrane at 25 ° C. 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.

Images