Porous film and method for producing porous film

Abstract

Provided are: a porous film capable of having a reduced hole diameter and an improved flow rate; and a method for producing the porous film.　This porous film contains at least one resin component (A) selected from the group consisting of polyimides, polyamide imides, and polyether sulfones, and a solvent (S). The solvent (S) includes a nitrogen-containing polar solvent (S1) having a boiling point of at least 200°C under atmospheric pressure, and the contained amount of the nitrogen-containing polar solvent (S1) in the porous film is 10-2000 ppm.

Description

Porous membrane and method for producing porous membrane

[0001] The present invention relates to a porous membrane and a method for producing a porous membrane.

[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 small particle diameters. 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 view of the above circumstances, and aims to provide a porous membrane that can reduce the pore diameter and improve the flow rate, and a method for manufacturing the porous membrane.

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that the above-mentioned problems can be solved by providing a porous membrane containing at least one resin component (A) selected from the group consisting of polyimide, polyamideimide, and polyethersulfone, and a solvent (S), wherein the solvent (S) contains a nitrogen-containing polar solvent (S1) having a boiling point of 200°C or higher at atmospheric pressure, and the content of the nitrogen-containing polar solvent (S1) in the porous membrane is 10 ppm or more and 2000 ppm or less, thereby completing the present invention. Specifically, the present invention provides the following.

[0008] [1] A porous membrane comprising at least one resin component (A) selected from the group consisting of polyimide, polyamideimide, and polyethersulfone, and a solvent (S), wherein the solvent (S) contains a nitrogen-containing polar solvent (S1) having a boiling point of 200°C or higher under atmospheric pressure, and the content of the nitrogen-containing polar solvent (S1) in the porous membrane is 10 ppm or more and 2000 ppm or less.

[0009] [2] The nitrogen-containing polar solvent (S1) is at least one selected from the group consisting of nitrogen-containing heterocyclic compounds which may have a substituent and aromatic hydrocarbons substituted with a nitrogen-containing group, the porous membrane according to [1] above.

[0010] [3] The porous membrane according to the above [2], wherein the nitrogen-containing polar solvent (S1) is at least one selected from the group consisting of 1,3-dimethyl-2-imidazolidinone, N-methylpyrrolidone, N-ethyl-2-pyrrolidone, and nitrobenzene.

[0011] [4] The porous membrane according to any one of the above [1] to [3], wherein the solvent (S) comprises an organic solvent (S2) having a boiling point of less than 200°C under atmospheric pressure.

[0012] [5] The porous membrane according to the above [4], wherein the organic solvent (S2) is an aliphatic amide-based solvent.

[0013] [6] The porous membrane according to the above [5], wherein the organic solvent (S2) is dimethylacetamide.

[0014] [7] The porous membrane according to any one of [1] to [6] above, wherein the content of the solvent (S) in the porous membrane is 10 ppm or more and 2100 ppm or less.

[0015] [8] A method for producing a porous film according to any one of the above [1] to [7], comprising: a composite film-forming step of forming, on a substrate, a composite film made of a composition for producing a porous film, the composition comprising at least one resin component selected from the group consisting of polyamic acid, polyimide, a polyamideimide precursor, polyamideimide, and polyethersulfone, fine particles (B), and a solvent (S); a heating step of heating the composite film; and a fine particle removing step of removing the fine particles (B) from the composite film after the heating step, wherein the solvent (S) comprises the nitrogen-containing polar solvent (S1), and in the heating step, a part of the solvent (S) is removed from the composite film so that the content of the nitrogen-containing polar solvent (S1) in the produced porous film is 10 ppm or more and 2000 ppm or less.

[0016] According to the present invention, it is possible to provide a porous membrane that can reduce the pore diameter and improve the flow rate, and a method for manufacturing the porous membrane.

[0017] 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.

[0018] <Porous Membrane> The porous membrane contains at least one resin component (A) selected from the group consisting of polyimide, polyamideimide, and polyethersulfone, and a solvent (S). The solvent (S) contained in the porous membrane contains a nitrogen-containing polar solvent (S1) having a boiling point of 200°C or higher under atmospheric pressure. The content of the nitrogen-containing polar solvent (S1) in the porous membrane is 10 ppm or more and 2000 ppm or less.

[0019] The polyimide, polyamideimide, or polyethersulfone as the resin component (A) contained in the porous membrane is not particularly limited and can be appropriately selected depending on the application of the porous membrane.

[0020] The solvent (S) contained in the porous membrane includes a nitrogen-containing polar solvent (S1) having a boiling point (hereinafter simply referred to as "boiling point") of 200°C or higher under atmospheric pressure. The boiling point of the nitrogen-containing polar solvent (S1) may be 200°C or higher, or may be 210°C or higher, or 220°C or higher. Examples of the nitrogen-containing polar solvent (S1) include nitrogen-containing heterocyclic compounds which may have a substituent, and aromatic hydrocarbons substituted with a nitrogen-containing group. Examples of the nitrogen-containing heterocyclic compounds which may have a substituent as the nitrogen-containing polar solvent (S1) include 1,3-dimethyl-2-imidazolidinone (boiling point 220°C), N-methylpyrrolidone (boiling point 202°C), and N-ethyl-2-pyrrolidone (boiling point 218°C). Examples of the nitrogen-containing group include a nitro group. Examples of the aromatic hydrocarbons substituted with a nitrogen-containing group as the nitrogen-containing polar solvent (S1) include nitrobenzene (210°C). The nitrogen-containing polar solvent (S1) may be one kind or two or more kinds.

[0021] The content of the nitrogen-containing polar solvent (S1) in the porous membrane is 10 ppm or more and 2000 ppm or less. In this way, by the porous membrane containing 10 ppm or more and 2000 ppm or less of the nitrogen-containing polar solvent (S1) having a boiling point of 200 ° C. or more, the pore size of the porous membrane can be reduced and the flow rate can be improved (increased), as shown in the examples described later.

[0022] On the other hand, if the content of the nitrogen-containing polar solvent (S1) is less than 10 ppm, the flow rate will be low, and if the content of the nitrogen-containing polar solvent (S1) is more than 2000 ppm, the pores of the porous membrane will be large.

[0023] The content of the nitrogen-containing polar solvent (S1) in the porous membrane is preferably 10 ppm or more and 1500 ppm or less, more preferably 150 ppm or more and 1000 ppm or less.

[0024] The solvent (S) contained in the porous membrane may contain an organic solvent (S2) having a boiling point of less than 200°C under atmospheric pressure. The boiling point of the organic solvent (S2) may be less than 200°C, but is preferably 180°C or less, more preferably 170°C or less. The boiling point of the organic solvent (S2) is preferably 100°C or more, more preferably 120°C or more. Examples of the organic solvent (S2) include aliphatic amide-based solvents. Examples of the aliphatic amide-based solvent as the organic solvent (S2) include dimethylacetamide (boiling point 165°C). The organic solvent (S2) may be one type or two or more types.

[0025] The content of the solvent (S) in the porous membrane is not particularly limited, but is preferably 10 ppm or more and 2100 ppm or less, and more preferably 550 ppm or more and 2000 ppm or less.

[0026] The porous membrane has an IPA flow rate of, for example, 0.06 mL / (min cm) as measured by the method described in [IPA Flow Rate (FR)] below. ２ ) or more, and 0.22 mL / (min cm ２ ) or more. The porous membrane may have a bubble point, measured by the method described in the "Bubble Point (BP)" section below, of, for example, 90 psi or more, and may also have a bubble point of, for example, 95 psi or more. The bubble point is an index of the pore size of the interconnecting pores; the higher the bubble point, the smaller the pore size. The porous membrane produced may have a tensile strength, measured by the method described in the "Tensile Strength" section below, of, for example, 27 MPa or more, and may also have a tensile strength of, for example, 29 MPa or more.

[0027] The porous membrane includes a structure in which spherical pores are interconnected (hereinafter referred to as interconnected pores). The openings in the porous membrane are the portions on the surface of the porous membrane where the interconnected pores open.

[0028] 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.

[0029] Individual spherical pores are typically formed by removing individual particles (B) present in a composite membrane (resin-particle composite membrane) in a subsequent particle removal step in the method for producing a porous membrane, which will be described in detail later. Furthermore, continuous pores are formed by removing multiple particles (B) present in contact with each other in a resin-particle composite membrane in a subsequent particle removal step in the method for producing a porous membrane. The locations where the spherical pores in the continuous pores are interconnected originate from the locations where multiple particles (B) come into contact with each other before being removed.

[0030] 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 100 nm to 5000 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.

[0031] <<Method for Producing Porous Film>> The porous film described above can be produced by a method comprising the steps of: forming a composite film on a substrate; the composite film is composed of a porous film-producing composition containing at least one resin component selected from the group consisting of polyamic acid, polyimide, polyamideimide precursor, polyamideimide, and polyethersulfone; fine particles (B); and a solvent (S); heating the composite film; and removing the fine particles (B) from the composite film after the heating step. The solvent (S) comprises a nitrogen-containing polar solvent (S1). In the heating step, the solvent (S) is partially removed from the composite film so that the nitrogen-containing polar solvent (S1) content in the produced porous film is 10 ppm or more and 2000 ppm or less. The porous film can also be produced by a method comprising the steps of: forming a composite film on a substrate; forming a composite film on a substrate; heating the composite film; and removing the fine particles (B) from the composite film after the heating step; the solvent (S) comprises a nitrogen-containing polar solvent (S1); and removing the nitrogen-containing polar solvent (S1) from the composite film during the heating step; .... The porous film may also be produced by a method wherein the resin component of the porous film-producing composition comprises at least one of polyamic acid and polyamideimide precursor; and baking the composite film during the heating step or after the heating step and before the fine particle removal step. Each step is described below.

[0032] [Composite Film Formation Step (Production of Unbaked Composite Film)] In the composite film formation step, a composite film made of a porous film-forming composition is formed on a substrate. The porous film-forming composition (hereinafter also referred to as "varnish") contains at least one resin component (a) selected from the group consisting of polyamic acid, polyimide, polyamideimide precursor, polyamideimide, and polyethersulfone, fine particles (B), and a solvent (S). The solvent (S) preferably contains the nitrogen-containing polar solvent (S1) and the organic solvent (S2).

[0033] [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. When the composition for producing a porous membrane contains polyamic acid or polyimide, the porous membrane contains polyimide as the resin component (A). When the composition for producing a porous membrane contains polyamideimide precursor or polyamideimide, the porous membrane contains polyamideimide as the resin component (A). When the composition for producing a porous membrane contains polyethersulfone, the porous membrane contains polyethersulfone as the resin component (A). These resin components (a) are described below.

[0034] <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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] Among these, p-phenylenediamine, m-phenylenediamine, 2,4-diaminotoluene, and 4,4'-diaminodiphenyl ether are preferred in terms of cost, availability, and the like.

[0044] 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.

[0045] Examples of diaminonaphthalenes include 1,5-diaminonaphthalene and 2,6-diaminonaphthalene.

[0046] Examples of aminophenylaminoindan include 5 or 6-amino-1-(p-aminophenyl)-1,3,3-trimethylindan.

[0047] 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.

[0048] Examples of the cardo-type fluorenediamine derivatives include 9,9-bisanilinefluorene.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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 it is desirable that the content of the resulting polyamic acid be 5 to 50% by mass.

[0054] As the solvent used in the reaction between the tetracarboxylic dianhydride and the diamine, it is preferable to use the solvent (S) contained in the varnish, specifically the nitrogen-containing polar solvent (S1) or the organic solvent (S2) added as needed. Note that when the nitrogen-containing polar solvent (S1) contained in the varnish or the organic solvent (S2) added as needed is not used as the solvent used in the reaction between the tetracarboxylic dianhydride and the diamine, the solvent may be replaced with the nitrogen-containing polar solvent (S1) or the organic solvent (S2) after the reaction.

[0055] 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.

[0056] <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 nitrogen-containing polar solvent (S1) contained in the varnish or the organic solvent (S2) that is added as needed is preferred.

[0057] In order to make the polyimide soluble in the nitrogen-containing polar solvent (S1) and the organic solvent (S2) to be contained as needed, a monomer for introducing a flexible, bent structure into the main chain is used, for example, an aliphatic diamine such as ethylenediamine, hexamethylenediamine, 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, or 4,4'-diaminodicyclohexylmethane; 2-methyl-1,4-phenylenediamine, o-tolidine, m-tolidine, or 3,3'-dimethoxybenzene. Effective examples of the polyimide include aromatic diamines such as diamine and 4,4'-diaminobenzanilide; polyoxyalkylene diamines such as polyoxyethylene diamine, polyoxypropylene diamine, and polyoxybutylene diamine; polysiloxane diamines; 2,3,3',4'-oxydiphthalic anhydride, 3,4,3',4'-oxydiphthalic anhydride, and 2,2-bis(4-hydroxyphenyl)propane dibenzoate-3,3',4,4'-tetracarboxylic dianhydride. Also effective are monomers having functional groups that improve solubility in solvents, such as fluorinated diamines such as 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl and 2-trifluoromethyl-1,4-phenylenediamine. Furthermore, in addition to the monomers for improving the solubility of the polyimide, the same monomers as those listed in the polyamic acid section above can also be used in combination, provided that solubility is not impaired. The polyimide and the monomer thereof may be used singly or in combination of two or more kinds.

[0058] 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 dissolved in the nitrogen-containing polar solvent (S1) to be used or, if necessary, in an organic solvent (S2) to be added.

[0059] <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 nitrogen-containing polar solvent (S1) contained in the varnish or the organic solvent (S2) contained as needed is preferred.

[0060] 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.

[0061] Examples of the above acid or its reactive derivative include trimellitic anhydride, trimellitic anhydride halides such as trimellitic anhydride chloride, and trimellitic anhydride esters.

[0062] Examples of the optional diamine include the diamines exemplified in the description of polyamic acid above. Diaminopyridine compounds can also be used.

[0063] 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'- ...

[0033] 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.

[0064] 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.

[0065] 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.

[0066] <Polyethersulfone> The polyethersulfone is not particularly limited as long as it is soluble in the nitrogen-containing polar solvent (S1) used in the varnish and the organic solvent (S2) added as needed. The polyethersulfone can be appropriately selected depending on the application of the porous membrane to be produced, and may be either hydrophilic or hydrophobic. It may also be an aliphatic polyethersulfone or an aromatic polyethersulfone. The mass average molecular weight is, for example, 5,000 to 1,000,000, preferably 10,000 to 300,000.

[0067] [Fine Particles (B)] 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 subsequently removed from the composite film (resin-fine particle composite film). For example, inorganic materials include silica (silicon dioxide), titanium oxide, alumina (Al ２ O ３ Examples of organic materials include organic polymer particles such as high molecular weight olefins (polypropylene, polyethylene, etc.), polystyrene, epoxy resins, cellulose, polyvinyl alcohol, polyvinyl butyral, polyesters, and polyethers.

[0068] Specific examples of the fine particles (B) include colloidal silica. Among them, monodispersed spherical silica particles are preferred because they can form uniform pores.

[0069] Furthermore, it is preferable that the fine particles (B) have a high sphericity and a small particle size distribution index. Fine particles (B) that meet these conditions have excellent dispersibility in varnish and can be used in a state where they do not aggregate with each other. The average particle size of the fine particles (B) used is, for example, preferably 10 nm or more and 500 nm or less, more preferably 50 nm or more and 300 nm or less, and even more preferably 70 nm or more and 150 nm or less. 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 laser diffraction / scattering method. The size of the pores formed in the porous film due to the fine particles (B) is the same as or close to the average particle size of the fine particles (B). Fine particles (B) may be used alone or in combination of two or more types.

[0070] [Solvent (S)] The solvent (S) contained in the varnish contains the nitrogen-containing polar solvent (S1) described above. The solvent (S) contained in the varnish preferably contains the organic solvent (S2) described above. The boiling points and specific examples of the nitrogen-containing polar solvent (S1) and the organic solvent (S2) are as described above in the section "Porous Membrane."

[0071] The solvent (S), nitrogen-containing polar solvent (S1), and organic solvent (S2) contained in the varnish for forming the porous film are solvents that can dissolve the resin component (a) contained in the varnish, which is polyamic acid, polyimide, polyamideimide precursor, polyamideimide, and / or polyethersulfone, but do not dissolve the fine particles (B).

[0072] In the varnish, when the solvent (S) contains a nitrogen-containing polar solvent (S1) and a solvent other than the nitrogen-containing polar solvent (S1), such as an organic solvent (S2), the ratio of the mass of the nitrogen-containing polar solvent (S1) to the mass of the solvent (S) is preferably 1% by mass or more and 95% by mass or less, more preferably 5% by mass or more and 90% by mass or less, and even more preferably 10% by mass or more and 80% by mass or less. The content of the nitrogen-containing polar solvent (S1) in the varnish is preferably 100 parts by mass or more and 900 parts by mass or less, more preferably 200 parts by mass or more and 800 parts by mass or less, and even more preferably 300 parts by mass or more and 700 parts by mass or less, per 100 parts by mass of the resin component (a).

[0073] [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.

[0074] The dispersant is not particularly limited, and known dispersants can be used. For example, anionic surfactants such as coconut fatty acid salts, castor sulfated oil salts, lauryl sulfate salts, polyoxyalkylene allyl phenyl ether sulfate salts, alkyl benzene sulfonic acid, alkyl benzene sulfonate salts, alkyl diphenyl ether disulfonate salts, alkyl naphthalene sulfonate salts, dialkyl sulfosuccinate salts, isopropyl phosphate, polyoxyethylene alkyl ether phosphate salts, and polyoxyethylene allyl phenyl ether phosphate salts; cationic surfactants such as oleylamine acetate, lauryl pyridinium chloride, cetyl pyridinium chloride, lauryl trimethyl ammonium chloride, stearyl trimethyl ammonium chloride, behenyl trimethyl ammonium chloride, and didecyl dimethyl ammonium chloride; amphoteric surfactants such as coconut alkyl dimethyl amine oxide, fatty acid amidopropyl dimethyl amine oxide, alkyl polyaminoethyl glycine hydrochloride, amido betaine surfactants, alanine surfactants, and lauryliminodipropionic acid; Nonionic surfactants of polyoxyalkylene primary alkyl ethers or polyoxyalkylene secondary alkyl ethers, such as ethylene octyl ether, polyoxyethylene decyl ether, polyoxyethylene lauryl ether, polyoxyethylene laurylamine, polyoxyethylene oleylamine, polyoxyethylene polystyrylphenyl ether, and polyoxyalkylene polystyrylphenyl ether; other polyoxyalkylene-based nonionic surfactants, such as polyoxyethylene dilaurate, polyoxyethylene laurate, polyoxyethylenated castor oil, polyoxyethylenated hydrogenated castor oil, sorbitan laurate, polyoxyethylene sorbitan laurate, and fatty acid diethanolamides; fatty acid alkyl esters, such as octyl stearate and trimethylolpropane tridecanoate; and polyether polyols, such as polyoxyalkylene butyl ether, polyoxyalkylene oleyl ether, and trimethylolpropane tris(polyoxyalkylene) ether, but are not limited to these.The above dispersants may also be used in combination of two or more.

[0075] In the varnish, the content of the dispersant is, for example, from the viewpoint of film-forming properties, 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).

[0076] 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.

[0077] The viscosity of the varnish at 25°C is preferably 0.1 to 3 Pa·s or more, and the solid content 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. The viscosity of the varnish is measured using an E-type viscometer. A rotary / revolution mixer (product name: Awatori Rentaro, manufactured by Thinky Corporation), a planetary mixer, a bead mill, or the like can be used to knead the varnish. In the kneading process, 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.

[0078] The composite film (hereinafter also referred to as "unsintered composite film") may be formed (deposited) directly on a substrate, or may be formed (deposited) on an underlayer film different from the unsintered composite film. Alternatively, an unsintered composite film may be formed using the aforementioned varnish (porous film-producing composition), and then an upper layer film different from the unsintered composite film may be formed on top of the unsintered composite film. In the present application, both a method of providing an underlayer film on a substrate and a method of forming an unsintered composite film using the aforementioned varnish and then forming an upper layer film different from the unsintered composite film on top of the unsintered composite film are included in the methods of forming an unsintered composite film on a substrate. However, when the resin component (a) contained in the aforementioned varnish is a polyamic acid or polyamideimide precursor and a material that does not require a sintering step is used for the upper layer film, an upper layer film may be formed on the resin-particle composite film after sintering. The composite film (unsintered composite film) made of the aforementioned varnish is preferably formed as a single layer film on a substrate. Examples of the substrate include a PET film, a SUS substrate, and a glass substrate.

[0079] Examples of the lower layer film (or upper layer film) include a lower layer (or upper layer) unsintered composite film formed using a lower layer (or upper layer) film varnish containing a resin component consisting of polyamic acid, polyimide, polyamideimide precursor, polyamideimide, and / or polyethersulfone, fine particles, and a solvent, wherein the content of the fine particles is more than 40% by volume and not more than 81% by volume relative to the total of the resin component and the fine particles. The lower layer (or upper layer) unsintered composite film may be formed on a substrate. When the content of the fine particles is more than 40% by volume, the particles are uniformly dispersed. When the content of the fine particles is 81% by volume or less, the particles are dispersed without agglomeration, thereby enabling the formation of uniform pores in the porous film. Furthermore, when the content of the fine particles is within the above range, when the lower layer (or upper layer) unsintered composite film is formed on a substrate, release properties after film formation are easily ensured even if a release layer is not previously provided on the substrate.

[0080] The fine particles used in the varnish for the lower (or upper) layer film and the fine particles (B) used in the varnish may be the same or different. To make the pores in the unsintered lower (or upper) layer composite film denser, it is preferable that the fine particles used in the varnish for the lower (or upper) layer film have a particle size distribution index smaller than or equal to that of the fine particles (B) used in the varnish. Alternatively, it is preferable that the fine particles used in the varnish for the lower (or upper) layer film have a sphericity smaller than or equal to that of the fine particles (B) used in the varnish.

[0081] The average particle size of the fine particles used in the varnish for the lower (or upper) layer film may be the same as or different from the average particle size of the fine particles (B) of the varnish described above. The average particle size may be appropriately set within the range of 10 nm to 5000 nm depending on the application. When using a varnish for the lower (or upper) layer film, it is preferable to use the above-mentioned varnish as the upper layer in combination with varnishes for the lower layer film having different average particle sizes or particle size distribution indices of the fine particles.

[0082] Furthermore, the content of fine particles in the varnish for forming the lower layer (or upper layer) film may be greater or less than that in the aforementioned varnish. Suitable examples of components such as resin components, fine particles, and solvents contained in the varnish for forming the lower layer (or upper layer) film are the same as those in the aforementioned varnish. The varnish for forming the lower layer (or upper layer) film can be prepared by the same method as the aforementioned varnish. The lower layer unsintered composite film can be formed, for example, by applying the above-mentioned varnish for forming the lower layer film onto a substrate and drying it at 0 to 100°C under atmospheric pressure or in vacuum, preferably at 10 to 100°C under atmospheric pressure. The film formation conditions for the upper layer unsintered composite film are also similar.

[0083] Examples of the lower layer (or upper layer) film include a lower layer film made of a fibrous material such as a cellulose-based resin or a nonwoven fabric (e.g., a polyimide nonwoven fabric, etc., with a fiber diameter of, for example, about 50 nm to about 3000 nm), and a polyimide film.

[0084] Furthermore, the process enters a firing step in which the unfired composite film or the laminated film of the unfired composite film and the lower (or upper) film is fired to obtain a polyimide-particle composite film. When the unfired composite film or the lower unfired composite film is formed on a substrate, it may be fired as is, or the unfired composite film or the laminated film of the unfired composite film and the lower unfired composite film may be peeled off from the substrate before the firing step.

[0085] In addition, when the lower layer (or upper layer) film in the laminated film is a lower layer (or upper layer) unfired composite film formed using a lower layer (or upper layer) film varnish, and the composition of the lower layer (or upper layer) film varnish is the same as the composition of the varnish described above, the laminated film of the composite film (unfired composite film) and the lower layer (or upper layer) film is essentially one layer (single layer), but is referred to as a laminated film in this specification.

[0086] When peeling a composite film (unfired composite film) or a laminated film of an unfired composite film and a lower (or upper) unfired composite film from a 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, or a silicone-based release agent, without any particular restrictions. When peeling the 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 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 or the laminated film of the unfired composite film and the lower unfired composite film peeled from the substrate is washed with an organic solvent.

[0087] On the other hand, when the substrate is used as is without providing a release layer for forming the unsintered composite film or the lower-layer unsintered composite film, the above-mentioned release layer forming step and the above-mentioned cleaning step can be omitted. Furthermore, in the production of the unsintered 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.

[0088] [Heating Step] In the heating step, the composite film (unfired composite film) is heated to remove a portion of the solvent (S) from the composite film so that the content of the nitrogen-containing polar solvent (S1) in the produced porous film is 10 ppm or more and 2000 ppm or less. Since the solvent is removed, the heating step can also be called a drying step. By adjusting the conditions of the heating step according to the components contained in the varnish and their amounts, the content of the nitrogen-containing polar solvent (S1) in the produced porous film can be 10 ppm or more and 2000 ppm or less. The heating temperature is, for example, preferably 40 ° C or more and 170 ° C or less, more preferably 60 ° C or more and 145 ° C or less. The heating step may be performed under normal pressure, reduced pressure, or vacuum.

[0089] [Baking step (production of resin-particle composite film)] The baking step is a step of baking the composite film during or after the heating step and before the particle removal step. The baking step may be performed after the heating step (drying step), or may be performed during the heating step. In other words, the heating in the baking step may also serve as the heating (drying) in the heating step.

[0090] When the resin component (a) contained in the varnish contains a polyamic acid and / or a polyamideimide precursor, a composite film (unbaked composite film) is formed by subjecting the composite film (unbaked composite film) to a baking step, which comprises a resin comprising polyimide and / or polyamideimide and the fine particles (B). When the resin component (a) contained in the varnish is polyimide, polyamideimide, or polyethersulfone, the baking step may be included, but may be omitted.

[0091] In the composite film forming step, when the unsintered composite film is formed on an underlayer film different from the unsintered composite film, the underlayer film is also fired together with the unsintered composite film in the firing step.

[0092] The baking temperature in the baking step varies depending on the structures of the composite film and the underlayer 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 an organic material is used for the fine particles (B), the temperature must be set lower than the thermal decomposition temperature. When the resin component (a) contained in the varnish is a polyamic acid, it is preferable to complete imidization in the baking step.

[0093] The firing conditions may be, for example, a method in which the temperature is raised from room temperature to 400° C. or lower over 3 hours and then held at 400° C. for 20 minutes, or a stepwise drying-thermal imidization method in which the temperature is raised from room temperature to 400° C. in steps of 50° C. (each step held for 20 minutes) and finally held at 400° C. for 20 minutes. When an unfired composite film is formed on a substrate and then temporarily peeled off from the substrate, a method in which the edges of the unfired composite film are fixed to a stainless steel mold or the like to prevent deformation can be used.

[0094] The thickness of the completed resin-particle composite film can be determined by measuring the thickness at multiple locations with, for example, a micrometer and averaging the measured thickness. The preferred average film thickness varies depending on the application of the porous film, but for example, when used as a separator, the film thickness is preferably 5 μm to 500 μm, more preferably 10 μm to 100 μm, and even more preferably 15 μm to 30 μm. When used as a filter, the film thickness is preferably 5 μm to 500 μm, more preferably 10 μm to 300 μm, and even more preferably 20 μm to 150 μm.

[0095] [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 heating step (or from the composite film (resin-particle composite film) after the baking step, if a baking step has been performed). By selecting an appropriate method for removing the particles (B), a porous film can be produced with good reproducibility.

[0096] 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.

[0097] 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 The decomposition temperature of the resin particles used is preferably 230°C or higher and 320°C or lower.

[0098] The overall thickness of the porous membrane is not particularly limited, but for example, when used in a separator or the like, it is preferably 5 μm to 500 μm, more preferably 10 μm to 100 μm, and even more preferably 15 μm to 30 μm. When used in a filter or the like, it is preferably 5 μm to 500 μm, more preferably 10 μm to 300 μm, and even more preferably 20 μm to 150 μm. The above membrane thickness can be determined, as in the case of measuring a resin-particle composite membrane, by measuring the thickness at multiple locations with, for example, a micrometer and averaging the measured thickness.

[0099] When a porous membrane is formed from two or more types of porous membrane-forming compositions, the thickness ratio of the regions formed by each porous membrane-forming composition may be appropriately set depending on the application of the porous membrane. In the case of a porous membrane having two regions, a layer (I) formed by a porous membrane-forming composition and a layer (II) formed by another porous membrane-forming composition, the thickness ratio of each region ((I):(II)) may be, for example, 1:99 to 99:1, preferably 5:95 to 95:5. The thickness of each layer may be calculated by averaging the thicknesses of multiple locations on the cross section of the porous membrane observed using a scanning electron microscope (SEM) or the like.

[0100] [Resin Removal Step] The method for producing a porous membrane may include a resin removal step of removing at least a portion of the resin portion of the resin-particle composite membrane before the particle removal step, or removing at least a portion of the porous membrane after the particle removal step. By removing at least a portion of the resin portion of the resin-particle composite membrane or at least a portion of the porous membrane, it is possible to improve the pore size of the porous membrane in the final product compared to when no removal is performed.

[0101] The step of removing at least a portion of the resin portion or the step of removing at least a portion of the porous film can be carried out by a common chemical etching method or physical removal method, or a combination of these methods.

[0102] Examples of chemical etching methods include treatment with a chemical etching solution such as an inorganic alkaline solution or an organic alkaline solution. Inorganic alkaline solutions are preferred. Examples of inorganic alkaline solutions include hydrazine solutions containing hydrazine hydrate and ethylenediamine; solutions of alkali metal hydroxides such as potassium hydroxide, sodium hydroxide, sodium carbonate, sodium silicate, and sodium metasilicate; ammonia solutions; and etching solutions containing an alkali hydroxide, hydrazine, and 1,3-dimethyl-2-imidazolidinone as main components. Examples of organic alkaline solutions include alkaline solutions of primary amines such as ethylamine and n-propylamine; secondary amines such as diethylamine and di-n-butylamine; tertiary amines such as triethylamine and methyldiethylamine; alcohol amines such as dimethylethanolamine and triethanolamine; quaternary ammonium salts such as tetramethylammonium hydroxide and tetraethylammonium hydroxide; and cyclic amines such as pyrrole and piheridine.

[0103] The solvent for each of the above solutions can be selected appropriately from pure water or alcohols. It is also possible to use a solution containing an appropriate amount of surfactant. The alkali concentration is, for example, 0.01 to 20% by mass.

[0104] As a physical method, for example, dry etching using plasma (oxygen, argon, etc.), corona discharge, etc., or a surface treatment method in which an abrasive (e.g., alumina (hardness 9), etc.) is dispersed in a liquid and irradiated onto the surface of the film at a speed of 30 to 100 m / s can be used.

[0105] The above-mentioned method is preferable because it can be applied to the resin removal step either before or after the fine particle removal step.

[0106] On the other hand, as a physical method applicable only to the resin removal process performed after the particle removal process, a method can be adopted in which the target surface is pressed against a mount film (e.g., a polyester film such as a PET film) wetted with a liquid, and then the porous membrane is peeled off from the mount film either without drying or after drying. Due to the surface tension or electrostatic adhesion of the liquid, the porous membrane is peeled off from the mount film with only the surface layer of the porous membrane remaining on the mount film.

[0107] According to the method for manufacturing a porous membrane described above, it is possible to improve (increase) the flow rate of a fluid (liquid or gas) passing through the porous membrane. Therefore, it is possible to achieve both a small pore size and an increase in flow rate. Therefore, it is possible to manufacture a porous membrane with a small pore size and a high flow rate.

[0108] 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.

[0109] Examples 1 to 20 and Comparative Examples 1 to 8 A silica dispersion (containing 0.5% by mass of dispersant relative to the type of silica listed in Table 1) 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 the polyamic acid (resin component (a)) and silica (fine particles (B)). Furthermore, as the solvent (S), a nitrogen-containing polar solvent (S1), an organic solvent (S2), and / or an organic solvent (S') not corresponding to the nitrogen-containing polar solvent (S1) or the organic solvent (S2) were each added so that the composition of the solvent (S) in the entire final composition (porous membrane production composition (varnish)) was the mass % listed in Table 1. 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) porosity separator (manufactured by Yoshida Kikai Kogyo Co., Ltd.) was used. ２ Using a dispersing device equipped with a flow path, a dispersion process was carried out 10 times by passing the mixed solution through the flow path at 200 MPa to prepare a porous film manufacturing composition (varnish) with a solid content 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. Polyamic acid solution: reaction product of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether (solid content 20% by mass (organic solvent: N,N-dimethylacetamide)) Nitrogen-containing polar solvent (S1): N-methylpyrrolidone (NMP) or 1,3-dimethyl-2-imidazolidinone (DMI) Organic solvent (S2): N,N-dimethylacetamide (DMAc) Organic solvent (S'): gamma butyrolactone (GBL) Dispersant: polyoxyethylene secondary alkyl ether-based dispersant Fine particles (B): silica with an average particle size of 100 nm, silica with an average particle size of 80 nm, or silica with an average particle size of 50 nm

[0110] The obtained porous film-producing composition was applied to a polyethylene terephthalate (PET) film substrate using an applicator to form a composite film on the substrate (composite film formation step). Next, the composite film (unbaked composite film) was heated at 90°C for 5 minutes to remove a portion of the solvent (S) (heating step). After the heating step, the 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). Then, this resin-particle composite film was 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). Then, the film was washed with water and dried to obtain polyimide porous films of Examples 1 to 20 and Comparative Examples 1 to 8, each with a film thickness of 40 μm.

[0111] The content of nitrogen-containing polar solvent (S1), IPA flow rate (FR), bubble point (BP), and tensile strength of the obtained polyimide porous membranes of Examples and Comparative Examples were measured according to the following methods. The results are shown in Table 2.

[0112] [Content of Nitrogen-Containing Polar Solvent (S1)] 10 g of acetone was added to 0.1 g of a polyimide porous film, and the mixture was stirred for 3 hours. The polyimide porous film was then removed to prepare a test solution. This test solution was measured using a GC-MS (manufactured by Agilent, GC: 7820A, MS: 5977E) to determine the content of the nitrogen-containing polar solvent (S1).

[0113] [IPA Flow Rate (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 was measured. The measurement was carried out at 25°C. In Table 2, ２ Expressed in mL per minute.

[0114] [Bubble Point (BP)] The bubble point (BP) was measured in accordance with ASTM F316-86 and JIS K 3832. Specifically, each polyimide porous membrane was cut into a membrane filter size of 47 mm in diameter and mounted on a bubble point measurement holder. The secondary side of the membrane (the downstream side of the flow path of the porous membrane) was wetted with a fluorine-based solvent (Novec 7200, manufactured by 3M, hydrofluoroether), and air was gradually applied from the primary side to measure the pressure at which bubbles were generated on the secondary side. The measurement was carried out at room temperature (25°C).

[0115] [Tensile strength] Each polyimide porous film was cut into a dumbbell-shaped No. 6 test piece to prepare a test piece. Using a small tabletop tester EZ Test EZ-SX (manufactured by Shimadzu Corporation) as a tester, the stress at break (tensile strength (MPa)) of the test piece was measured under the measurement conditions of 25°C and 5 mm / min in accordance with JIS K 7127.

[0116]

[0117]

[0118] As shown in Tables 1 and 2, Examples 1 to 18 were produced using fine particles (B) having an average particle size of 100 nm, and the content of nitrogen-containing polar solvent (S1) in the porous film was within the range of 10 ppm to 2000 ppm. These examples had bubble points comparable to those of Comparative Example 5, which was produced using fine particles (B) having an average particle size of 100 nm and in which the porous film did not contain a nitrogen-containing polar solvent (S1). That is, Examples 1 to 18 can be said to be porous films with small pore sizes comparable to those of Comparative Example 5, which did not contain a nitrogen-containing polar solvent (S1). Furthermore, Examples 1 to 18 had higher IPA flow rates (FR) and higher flow rates than Comparative Example 5. From these results, it can be said that the porous films of Examples 1 to 18 had small pore sizes and improved flow rates. Furthermore, Examples 1 to 18 also had higher film strength than Comparative Example 5. On the other hand, it can be seen that the porous membranes of Comparative Examples 1 to 4, which were produced using fine particles (B) having an average particle size of 100 nm and in which the content of the nitrogen-containing polar solvent (S1) in the porous membrane was outside the range of 10 ppm to 2000 ppm, had a low flow rate or a large pore size. Furthermore, Example 19, which was produced using fine particles (B) having an average particle size of 80 nm and in which the content of the nitrogen-containing polar solvent (S1) in the porous membrane was within the range of 10 ppm to 2000 ppm, had a bubble point similar to that of Comparative Example 7, which was produced using fine particles (B) having an average particle size of 80 nm and in which the porous membrane did not contain the nitrogen-containing polar solvent (S1). In other words, it can be said that Example 19 is a porous membrane with a small pore size similar to that of Comparative Example 7, which did not contain the nitrogen-containing polar solvent (S1). Furthermore, Example 19 had a higher IPA flow rate (FR) and a higher flow rate than Comparative Example 7. From these, it can be said that the porous membrane of Example 19 had a small pore size and an improved flow rate.

[0119] In addition, Example 20, which is produced using fine particles (B) with an average particle size of 50 nm, and in which the content of nitrogen-containing polar solvent (S1) in the porous film is within the range of 10 ppm or more and 2000 ppm or less, is produced using fine particles (B) with an average particle size of 50 nm, and the porous film is produced using the fine particles (B) with an average particle size of 50 nm, and the bubble point is the same as that of Comparative Example 8, in which the porous film does not contain the nitrogen-containing polar solvent (S1). That is, it can be said that Example 20 is a porous film with a small pore size similar to that of Comparative Example 8, which does not contain the nitrogen-containing polar solvent (S1). In addition, Example 20 has a higher IPA flow rate (FR) and a larger flow rate than Comparative Example 8. From these, it can be said that the porous film of Example 20 has a small pore size and an improved flow rate.

Claims

1. A porous membrane comprising at least one resin component (A) selected from the group consisting of polyimide, polyamideimide, and polyethersulfone, and a solvent (S), wherein the solvent (S) contains a nitrogen-containing polar solvent (S1) having a boiling point of 200°C or higher under atmospheric pressure, and the content of the nitrogen-containing polar solvent (S1) in the porous membrane is 10 ppm or more and 2000 ppm or less.

2. The porous membrane described in claim 1, wherein the nitrogen-containing polar solvent (S1) is at least one selected from the group consisting of nitrogen-containing heterocyclic compounds which may have a substituent, and aromatic hydrocarbons substituted with a nitrogen-containing group.

3. The porous membrane of claim 2, wherein the nitrogen-containing polar solvent (S1) is at least one selected from the group consisting of 1,3-dimethyl-2-imidazolidinone, N-methylpyrrolidone, N-ethyl-2-pyrrolidone, and nitrobenzene.

4. The porous membrane according to claim 1, wherein the solvent (S) comprises an organic solvent (S2) having a boiling point of less than 200°C at atmospheric pressure.

5. The porous membrane according to claim 4, wherein the organic solvent (S2) is an aliphatic amide-based solvent.

6. The porous membrane according to claim 5, wherein the organic solvent (S2) is dimethylacetamide.

7. The porous membrane according to claim 1, wherein the content of the solvent (S) in the porous membrane is 10 ppm or more and 2100 ppm or less.

8. A method for producing a porous film according to any one of claims 1 to 7, comprising: a composite film formation step of forming a composite film on a substrate, the composite film being made of a composition for producing a porous film, the composition containing at least one resin component selected from the group consisting of polyamic acid, polyimide, a polyamideimide precursor, polyamideimide and polyethersulfone, fine particles (B) and a solvent (S); a heating step of heating the composite film; and a fine particle removal step of removing the fine particles (B) from the composite film after the heating step, wherein the solvent (S) contains the nitrogen-containing polar solvent (S1), and in the heating step, a part of the solvent (S) is removed from the composite film so that the content of the nitrogen-containing polar solvent (S1) in the produced porous film is 10 ppm or more and 2000 ppm or less.

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