Varnish for the manufacture of polyimide porous membranes

Abstract

[Problem] To provide: a varnish composition that has a uniform compositional makeup including organic fine particles from which it is possible to form a polyimide porous membrane having fine uniform spherical pores having a diameter equivalent to the median diameter of the particles; a production method for a precursor membrane of a polyimide porous membrane, using said varnish composition; and a production method for a polyimide porous membrane using said precursor membrane. [Solution] This varnish composition for forming a polyimide porous membrane is obtained by mixing a polyamic acid (A), organic fine particles (B), and a solvent (S). The organic fine particles (B) include vinyl-based resin particles of a polymer having a structural unit (a) derived from a vinyl-based monomer and a structural unit (b1) that is derived from a compound represented by general formula (I) and that is different from the structural unit (a). [In the formula, m represents an integer of 1-3, R represents a polymerizable unsaturated group, AO represents an alkyleneoxy group having 2-4 carbon atoms, n represents an integer of 0-100, and X represents a hydrogen atom or an anionic hydrophilic group.]

Description

[Technical Field]

[0001] This invention relates to a varnish for manufacturing polyimide porous membranes. [Background technology]

[0002] In recent years, polyimide and / or polyamide-imide porous membranes have been studied for use as filters for separating gases or liquids, separators in lithium-ion batteries, electrolyte membranes in fuel cells, or as low dielectric constant materials.

[0003] For example, a known method for producing porous polyimide films used as separators involves coating a substrate with a varnish containing fine particles such as silica particles dispersed in a polymer solution of polyamic acid or polyimide, then heating the coated film as needed to obtain a polyimide film containing the fine particles, and finally removing the fine particles such as silica particles from the polyimide film using hydrofluoric acid to make it porous (see Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 5605566 [Overview of the project] [Problems that the invention aims to solve]

[0005] The handling of hydrofluoric acid used when forming porous polyimide films by methods such as those described in Patent Document 1 is not easy. Therefore, the use of hydrofluoric acid increases the manufacturing cost of porous polyimide films, and there is a need for a method to manufacture porous films without using hydrofluoric acid. For example, it is conceivable to use other microparticles, such as organic microparticles, instead of the silica particles mentioned above. Organic microparticles are often prepared in an aqueous solvent and are frequently distributed as dispersions of microparticles containing water. Therefore, when using organic microparticles, if a varnish containing polyamic acid or polyimide is prepared using a dispersion of microparticles containing water, a varnish containing water will inevitably be obtained.

[0006] However, when the above varnish contains water and fine particles, there is a problem in that the poor compatibility between polyamic acid and water-containing solvents, and the presence of fine particles inhibit the orientation of the polyamic acid molecules, resulting in the formation of a mixture with a non-uniform composition that can cause poor coating film formation, such as the inclusion of polyamic acid clumps containing fine particles. This leads to defects that result in a decrease in film strength.

[0007] To avoid these problems, it is conceivable to manufacture a varnish that is substantially water-free using completely dry organic microparticles. However, dry organic microparticles have poor dispersion stability and solvent resistance in organic solvents that dissolve polyamic acid, and aggregates form, making it difficult to obtain a polyimide porous film with uniformly formed pores and good air permeability.

[0008] Therefore, there is a need for a varnish containing organic fine particles that can form a polyimide porous membrane with a high aperture ratio and has a uniform dispersion composition, as well as a method for producing a precursor membrane for a polyimide porous membrane using the varnish, and a method for producing a polyimide porous membrane.

[0009] The present invention has been made in view of the above problems and aims to provide a varnish composition with a uniform composition containing organic fine particles that can form a polyimide porous membrane having uniform, fine spherical pores with a diameter equivalent to the median diameter of the particles, a method for producing a precursor membrane for a polyimide porous membrane using the aforementioned varnish composition, and a method for producing a polyimide porous membrane using the precursor membrane. [Means for solving the problem]

[0010] The present invention applies to the following [1] to [7]. [1] A varnish composition for forming a polyimide porous film, obtained by mixing polyamic acid (A), organic fine particles (B), and a solvent (S), The aforementioned organic fine particles (B) Structural unit (a) derived from vinyl monomers, A varnish composition comprising vinyl resin particles, which are polymers having a structural unit (b1) derived from a compound represented by the following general formula (I), different from the aforementioned structural unit (a). [ka] [In the formula, m represents an integer between 1 and 3. R represents the group shown in formula (i) or formula (ii) below. [ka] (In the formula, R1 represents a hydrogen atom or a methyl group.) AO represents an alkylene oxy group with 2 to 4 carbon atoms, and n represents an integer from 0 to 100. X represents either a hydrogen atom or an anionic hydrophilic group selected from the group consisting of -SO3M, -COOM, and -PO3M (wherein M represents an alkali metal atom, an alkaline earth metal atom, an ammonium group, or an organic ammonium group). [2] The varnish composition of [1], wherein the structural unit (a) derived from the vinyl monomer includes a structural unit (a0) derived from a monofunctional vinyl monomer and a structural unit (a3) derived from a polyfunctional vinyl monomer. [3] A varnish composition for forming a polyimide porous membrane, obtained by mixing a polyamic acid (A), organic fine particles (B), and a solvent (S), wherein the organic fine particles (B) are polymer particles for producing a porous membrane, having a structural unit (a0) derived from a monofunctional vinyl monomer, a structural unit (a3) derived from a polyfunctional vinyl monomer, and a structural unit (b0) derived from a reactive emulsifier, and the varnish composition contains vinyl-based resin particles for producing a porous membrane, wherein the proportion of the structural unit (a0) is 88 to 99% by mass, the proportion of the structural unit (a3) is 0.9 to 10% by mass, and the proportion of the structural unit (b0) is 0.1 to 2% by mass. A coating film forming step of forming a coating film by applying the varnish composition according to any one of [1] to [3] on a substrate; A method for producing a precursor film of a polyimide porous membrane, including a precursor film forming step of removing the solvent (S) from the coating film to form a precursor film of the polyimide porous membrane. [5] The method for producing a precursor film of a polyimide porous membrane according to [4], including a peeling step of peeling the precursor film from the substrate after the precursor film forming step. [6] The method for producing a precursor film of a polyimide porous membrane according to [5], including a winding step of winding the precursor film into a roll after the peeling step. [7] A method for producing a polyimide porous membrane, including producing a precursor film of the polyimide porous membrane by the method according to any one of [4] to [6], and then including a removing step of removing the organic fine particles (B) from the precursor film.

[0011] ​​The varnish composition of the present invention can provide a polyimide porous membrane having spherical pores with a uniform distribution and a diameter equivalent to the median diameter of the organic fine particles contained in the varnish. Furthermore, the method for producing a porous membrane according to the present invention can produce a polyimide porous membrane having a uniform distribution of spherical pores. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 shows SEM images of porous membranes ((a) Example 1, (b) Example 2, (c) Example 3, (d) Example 4). [Figure 2] Figure 2 shows an SEM image of a porous membrane (Comparative Example 1). [Modes for carrying out the invention]

[0013] Varnish Composition The varnish composition covered by the present invention comprises a polyamic acid (A), specific organic fine particles (B), and a solvent (S). A precursor film for a polyimide porous film can be formed by removing the solvent (S) from a coated film formed using the varnish composition of the present invention. By imidizing the polyamic acid (A) contained in this precursor film and removing organic fine particles (B) from the precursor film, a polyimide porous film mainly composed of polyimide resin can be obtained. Therefore, the varnish composition of the present invention can be used to form a polyimide porous film.

[0014] The following describes the essential and optional components used in the preparation of the varnish composition.

[0015] <Polyamic acid (A)> As the polyamic acid (A), any product obtained by polymerizing any tetracarboxylic dianhydride with a diamine can be used without particular limitation. The amount of tetracarboxylic dianhydride and diamine used (charging amount) is not particularly limited, but it is preferable to use diamine in a ratio of 0.50 moles to 1.50 moles per mole of tetracarboxylic dianhydride, more preferably 0.60 moles to 1.30 moles, and particularly preferably 0.70 moles to 1.20 moles.

[0016] The above-mentioned tetracarboxylic dianhydride can be appropriately selected from compounds that have been conventionally used as raw materials for the synthesis of polyamic acids. The above-mentioned 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 and, consequently, the porous membrane, it is preferable to use an aromatic tetracarboxylic dianhydride. The above-mentioned tetracarboxylic dianhydride may be used alone or in combination of two or more types.

[0017] Suitable specific examples of aromatic tetracarboxylic dianhydrides include, but are not limited to, 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, 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'-benzo Phenonetetracarboxylic dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride, bis(2,3-dicarboxyphenyl)ether dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 4,4-(p-phenylenedioxy)diphthalic acid dianhydride, 4,4-(m-phenylenedioxy)diphthalic acid dianhydride, 1,2,5,6-naphthalenetetracarbone dianhydride, 1,4,5,8-naphthalenetetracarboxylate Examples include dianhydride of rubonate, 2,3,6,7-naphthalenetetracarboxylic acid dianhydride, 1,2,3,4-benzenetetracarboxylic acid dianhydride, 3,4,9,10-perylenetetracarboxylic acid dianhydride, 2,3,6,7-anthracenetetracarboxylic acid dianhydride, 1,2,7,8-phenanthrenetetracarboxylic acid dianhydride, 9,9-bisphthalic anhydride fluorene, and 3,3',4,4'-diphenylsulfonetetracarboxylic acid dianhydride. Examples of aliphatic tetracarboxylic 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 acid dianhydride and pyromellitic acid dianhydride are preferred due to their price, availability, etc.

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

[0019] Examples of aromatic diamines include diamino compounds containing one benzene ring, diamino compounds containing an aromatic skeleton in which two to ten benzene rings are linked by single bonds or via divalent linking groups, and diamino compounds containing an aromatic skeleton in which two to ten such benzene rings are fused. Specifically, examples include phenylenediamine compounds and their derivatives, diaminobiphenyl compounds and their derivatives, diaminodiphenyl compounds and their derivatives, diaminotriphenyl compounds and their derivatives, diaminonaphthalene compounds and their derivatives, aminophenylaminoindan compounds and their derivatives, diaminotetraphenyl compounds and their derivatives, diaminohexaphenyl compounds and their derivatives, and cardo-type full orangeamine derivatives.

[0020] Examples of phenylenediamine compounds include m-phenylenediamine and p-phenylenediamine, and their derivatives include diamines in which the hydrogen atoms on the benzene ring are substituted with alkyl groups such as methyl and ethyl groups, such as 2,4-diaminotoluene and 2,4-triphenylenediamine.

[0021] Diaminobiphenyl compounds and their derivatives have a structure in which two aminophenyl groups are linked by a single bond. Specific examples include 4,4'-diaminobiphenyl and 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl.

[0022] Diaminodiphenyl compounds and their derivatives have a structure in which two aminophenyl groups are bonded together via other groups (linking groups). Examples of such linking groups (bonds) include ether bonds, sulfonyl bonds, thioether bonds, carbonyl bonds, bonds by alkylene or its derivative groups, imino bonds, azo bonds, phosphine oxide bonds, amide bonds, and ureylene bonds. The number of carbon atoms in the alkylene bond is approximately 1 to 6, and may include some double bonds. Examples of alkylene derivative groups include alkylene groups substituted with one or more halogen atoms, and alkylene groups substituted with alkenyl groups.

[0023] Examples of diaminodiphenyl compounds and their derivatives 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, and 3,4'-diamino Examples include diphenyl ketones, 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)-2-pentene, iminodianiline, 4-methyl-2,4-bis(p-aminophenyl)pentane, bis(p-aminophenyl)phosphine oxide, 4,4'-diaminoazobenzene, 4,4'-diaminodiphenylurea, and 4,4'-diaminodiphenylamide.

[0024] Diaminotriphenyl compounds and their derivatives are compounds having a structure in which two aminophenyl groups are bonded via single bonds and / or linking groups, flanking one phenylene group. The linking group is selected to be the same as the group listed for diaminodiphenyl compounds and their derivatives. Examples of diaminotriphenyl compounds and their derivatives include 1,3-bis(m-aminophenoxy)benzene [also known as 1,3-bis(3-aminophenoxy)benzene], 1,3-bis(p-aminophenoxy)benzene [also known as 1,3-bis(4-aminophenoxy)benzene], 1,4-bis(p-aminophenoxy)benzene [also known as 1,4-bis(4-aminophenoxy)benzene], and 2,4-triphenylenediamine.

[0025] Examples of diaminonaphthalene compounds and their derivatives include 1,5-diaminonaphthalene and 2,6-diaminonaphthalene.

[0026] Examples of aminophenylaminoindan compounds and their derivatives include 5 or 6-amino-1-(p-aminophenyl)-1,3,3-trimethylindan.

[0027] Examples of diaminotetraphenyl compounds and their derivatives include 4,4'-bis(p-aminophenoxy)biphenyl, bis[4-(p-aminophenoxy)phenyl]sulfone [also known as bis[4-(4-aminophenoxy)phenyl]sulfone], bis[4-(m-aminophenoxy)phenyl]sulfone [also known as bis[4-(3-aminophenoxy)phenyl]sulfone], and 2,2'-bis[p-(p'-aminophenoxy)phenyl]pro Examples include pan[also known as 2,2-bis[4-(4-aminophenoxy)phenyl]propane], 2,2'-bis[p-(p'-aminophenoxy)phenyl]hexafluoropropane[also known as 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane], 2,2'-bis[p-(p'-aminophenoxy)biphenyl]propane, and 2,2'-bis[p-(m-aminophenoxy)phenyl]benzophenone.

[0028] Specific examples of cardo-type fluororangeamine derivatives include 9,9-bisanilinefluorene.

[0029] Among these aromatic diamines, p-phenylenediamine, m-phenylenediamine, 2,4-diaminotoluene, and 4,4'-diaminodiphenyl ether are preferred due to their price, availability, and other factors.

[0030] Examples of aliphatic diamines include diamine compounds having approximately 2 to 15 carbon atoms. Specific examples of aliphatic diamines include pentamethylenediamine, hexamethylenediamine, and heptamethylenediamine.

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

[0032] There are no particular restrictions on the means of producing polyamic acid (A), and known methods such as reacting a tetracarboxylic dianhydride component with a diamine component in a solvent can be used.

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

[0034] Examples of solvents used in the reaction between tetracarboxylic dianhydrides and diamines include nitrogen-containing polar solvents such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-diethylacetamide, N,N-dimethylformamide, N,N-diethylformamide, N-methylcaprolactam, and N,N,N',N'-tetramethylurea; lactone-based polar solvents such as β-propiolactone, γ-butyrolactone, γ-valerolactone, δ-valerolactone, γ-caprolactone, and ε-caprolactone; dimethyl sulfoxide; acetonitrile; fatty acid esters such as ethyl lactate and butyl lactate; ethers such as diethylene glycol dimethyl ether, diethylene glycol diethyl ether, dioxane, tetrahydrofuran, methyl cellulose acetate, and ethyl cellulose acetate; and phenolic solvents such as cresols and xylene-based mixed solvents. There are no particular restrictions on the amount of solvent used, but it is desirable to use it so that the content of the resulting polyamic acid (A) is between 5% by mass and 50% by mass.

[0035] Among these solvents, nitrogen-containing polar solvents such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-diethylacetamide, N,N-dimethylformamide, N,N-diethylformamide, N-methylcaprolactam, and N,N,N',N'-tetramethylurea are preferred due to the solubility of the resulting polyamic acid (A).

[0036] The temperature used during polyamic acid synthesis can generally be any temperature between -10°C and 120°C, preferably between 5°C and 30°C. The reaction time varies depending on the composition of the raw materials used, but it is usually between 3 and 24 hours. Furthermore, if the synthesis of polyamic acid is carried out in solvent (S) as described later, the reaction solution of polyamic acid can be used directly as a polyamic acid-containing solution in the preparation of the varnish composition.

[0037] The content of polyamic acid (A) in the varnish composition is not particularly limited and can be determined as appropriate, taking into account the viscosity, applicability, and solid content concentration of the varnish composition. Polyamic acid (A) may be used alone or in combination of two or more types.

[0038] <Organic fine particles (B)> The organic fine particles (B) used in the varnish composition of the present invention are vinyl resin particles, which are polymers having a structural unit (a) derived from a vinyl monomer and a structural unit (b1) derived from a compound represented by the general formula (I) described later. The organic fine particles (B) can be copolymers of monomer components (mixtures) containing vinyl monomers and compounds represented by general formula (I), which constitute each of the structural units described above.

[0039] Furthermore, in one embodiment of the present invention, vinyl resin particles can be used as the organic fine particles (B) used in the varnish composition, which are polymers having structural units (a0) derived from a monofunctional vinyl monomer described later, structural units (a3) ​​derived from a polyfunctional vinyl monomer described later, and structural units (b0) derived from a reactive emulsifier described later.

[0040] In this specification, (meth)acrylic monomers refer to both acrylic monomers and methacrylic monomers. For example, alkyl methacrylate refers to alkyl acrylate and alkyl methacrylate. Furthermore, in this specification, terms such as "structural units derived from vinyl monomers," "structural units derived from monofunctional styrene monomers," "structural units derived from monofunctional (meth)acrylic monomers," and "structural units derived from polyfunctional vinyl monomers" refer to structural units formed when vinyl monomers, monofunctional styrene monomers, monofunctional (meth)acrylic monomers, and polyfunctional vinyl monomers are polymerized, respectively, and do not represent the monomers themselves.

[0041] [Structural units derived from vinyl monomers (a)] Structural units (a) derived from vinyl monomers are distinct from structural units (b0) derived from reactive emulsifiers, which will be described later, and structural units (b1) derived from compounds represented by general formula (I). The aforementioned structural unit (a) may include a structural unit (a0) derived from a monofunctional vinyl monomer and a structural unit (a3) ​​derived from a polyfunctional vinyl monomer. Furthermore, the structural unit (a0) derived from a monofunctional vinyl monomer may include a structural unit (a1) derived from a monofunctional styrene monomer and a structural unit (a2) derived from a monofunctional (meth)acrylic monomer. In a preferred embodiment, structural unit (a) includes both structural unit (a0) derived from a monofunctional vinyl monomer and structural unit (a3) ​​derived from a polyfunctional vinyl monomer.

[0042] [Structural unit (a0) derived from monofunctional vinyl monomers] <Structural unit (a1) derived from monofunctional styrene monomers> The structural unit (a0) derived from the monofunctional vinyl monomer may include a structural unit (a1) derived from a monofunctional styrene monomer. In general, structural units derived from styrene monomers can contribute to the formation of uniform, perfectly spherical particles. Examples of monofunctional styrene monomers that constitute the structural unit (a1) include styrene and its derivatives such as styrene, α-methylstyrene, 3-methylstyrene, 4-methylstyrene, 2,4-dimethylstyrene, 2,5-dimethylstyrene, and 2,4,6-trimethylstyrene; and styrene sulfonates such as sodium styrenesulfonate and ammonium styrenesulfonate. Among these, styrene, α-methylstyrene, and sodium styrenesulfonate can be cited as preferred.

[0043] <Structural units (a2) derived from monofunctional (meth)acrylic monomers> Furthermore, the structural unit (a0) derived from the monofunctional vinyl monomer may include, in addition to the structural unit (a1) derived from the monofunctional styrene monomer, a structural unit (a2) derived from a monofunctional (meth)acrylic monomer. Structural units derived from (meth)acrylic monomers have the characteristics of being easily decomposed (depolymerized) at the monomer level, regardless of whether they are monofunctional or polyfunctional, and having excellent thermal decomposition properties, which can lower the thermal decomposition temperature of organic fine particles (B). Examples of monofunctional (meth)acrylic monomers that constitute the structural unit (a2) include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, n-pentyl (meth)acrylate, 3-methylbutyl (meth)acrylate, n-hexyl (meth)acrylate, cyclohexyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, decyl (meth)acrylate, lauryl (meth)acrylate, and other (meth)acrylic acid esters having 1 to 18 carbon atoms in the alkyl group. Among these, from the viewpoint of easily obtaining organic fine particles (B) with uniform particle size, methyl (meth)acrylate and ethyl (meth)acrylate can be listed as suitable (meth)acrylic acid monomers, with methyl (meth)acrylate being particularly preferred.

[0044] [Structural units derived from polyfunctional vinyl monomers (a3)] The aforementioned structural unit (a) may include not only a structural unit (a0) derived from a monofunctional vinyl monomer, but also a structural unit (a3) ​​derived from a polyfunctional vinyl monomer. By including the structural unit (a3) ​​derived from the polyfunctional vinyl monomer, the solvent resistance of the resulting organic fine particles (B) can be improved, and the decrease in viscosity of the varnish composition (polyimide varnish) due to swelling of the organic fine particles (B) can be suppressed. Furthermore, by including the structural unit (a3), it becomes easier to obtain organic fine particles (B) with high compressive strength and uniform particle size. Examples of the above structural units (a3) ​​include structural units derived from polyfunctional (meth)acrylic monomers (a3-1) and structural units derived from polyfunctional (poly)vinyl monomers (a3-2).

[0045] Specific examples of polyfunctional (meth)acrylic monomers that constitute the aforementioned structural unit (a3-1) include ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,3-butylene glycol di(meth)acrylate, 1,4-butylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, ethylene oxide-modified 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, and propylene oxide-modified neopentyl Di(meth)acrylates of polyhydric alcohols with 1 to 10 carbon atoms, such as glycol di(meth)acrylate and tripropylene glycol di(meth)acrylate; alkyl di(meth)acrylates with 2 to 50 added moles of alkylene oxide groups with 2 to 4 carbon atoms, such as polyethylene glycol di(meth)acrylate with 2 to 50 added moles of ethylene oxide, polypropylene glycol di(meth)acrylate with 2 to 50 added moles of propylene oxide, and tripropylene glycol di(meth)acrylate; etoxy Tri(meth)acrylates of polyhydric alcohols with 1 to 10 carbon atoms, such as glycerin tri(meth)acrylate, propylene oxide-modified glycerol tri(meth)acrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol monohydroxytri(meth)acrylate, trimethylolpropane triethoxytri(meth)acrylate; pentaerythritol tetra(meth)acrylate, dipentaerythritol tetra(meth)acrylate. Examples include, but are not limited to, tetra(meth)acrylates of polyhydric alcohols having 1 to 10 carbon atoms, such as (meth)acrylate and ditrimethylolpropanetetra(meth)acrylate; penta(meth)acrylates of polyhydric alcohols having 1 to 10 carbon atoms, such as pentaerythritol penta(meth)acrylate and dipentaerythritol (monohydroxy)penta(meth)acrylate; and hexa(meth)acrylates of polyhydric alcohols having 1 to 10 carbon atoms, such as pentaerythritol hexa(meth)acrylate.

[0046] Furthermore, specific examples of polyfunctional (poly)vinyl monomers that constitute the structural unit (a3-2) include: polyfunctional aliphatic vinyl monomers such as isoprene and butadiene; polyfunctional alicyclic vinyl monomers such as cyclopentadiene and cyclohexadiene; polyfunctional aromatic vinyl monomers such as divinylbenzene, divinyltoluene, and divinylnaphthalene; polyfunctional vinyl ester monomers such as divinyl adipate, divinyl maleate, divinyl phthalate, and divinyl isophthalate; polyfunctional allyl ester monomers such as diallyl maleate, diallyl phthalate, diallyl isophthalate, and diallyl adipate; divinyl ether, dieth Examples include, but are not limited to, polyfunctional vinyl ether monomers such as ethylene glycol divinyl ether and triethylene glycol divinyl ether; polyfunctional allyl ether monomers such as diallyl ether, diallyl oxyethane, and triallyl oxyethane; polyfunctional vinyl ketone monomers such as divinyl ketone and diallyl ketone; polyfunctional nitrogen-containing vinyl monomers such as diallylamine, diallyl isocyanurate, diallyl cyanurate, methylenebis(meth)acrylamide, and bismaleimide; and polyfunctional silicon-containing vinyl monomers such as dimethyldivinylsilane, divinylmethylphenylsilane, and diphenyldivinylsilane.

[0047] Among these, from the viewpoint of easily obtaining organic fine particles (B) with uniform particle size, ethylene glycol di(meth)acrylate, 1,3-butylene glycol di(meth)acrylate, 1,4-butylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, divinylbenzene, divinyltoluene, etc. are preferred as polyfunctional vinyl monomers constituting the above structural unit (a3). Furthermore, from the viewpoint of having excellent polymerization stability and easily obtaining organic fine particles (B) with few aggregates, ethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, and 1,3-butylene glycol di(meth)acrylate are mentioned, and among these, ethylene glycol di(meth)acrylate is preferred.

[0048] The structural unit (a3) ​​derived from the polyfunctional vinyl monomer is preferably 1% to 10% by mass relative to the total mass of the structural unit (a).

[0049] <Structural units derived from other polymerizable monomers> The polymer, which is the organic fine particle (B), may contain structural units derived from other vinyl monomers (polymerizable monomers) other than the above-mentioned structural units (a0) [(a1), (a2)] and (a3) ​​[(a3-1), (a3-2)], to the extent that the effects of the present invention are not impaired. In other words, the organic fine particle (B) can be a copolymer of monomer components (mixtures) containing other polymerizable monomers.

[0050] For example, other polymerizable monomers other than the above-mentioned monofunctional styrene monomers and monofunctional (meth)acrylic monomers include, but are not limited to, monofunctional (meth)acrylonitrile monomers such as (meth)acrylonitrile; monofunctional heterocycle-containing vinyl monomers such as N-vinylimidazole and N-vinyl-2-pyrrolidone; monofunctional vinyl ester monomers such as vinyl acetate, isopropenyl acetate, vinylpropionate, and vinyldecanoate; monofunctional vinyl ether monomers such as ethyl vinyl ether, propyl vinyl ether, butyl vinyl ether, cyclohexyl vinyl ether, and ethylene glycol vinyl ether; other monofunctional vinyl compound monomers such as vinylcyclopentane, vinylcyclohexane, and ethyl vinylbenzene; monofunctional (meth)acrylic acid monomers such as (meth)acrylic acid and itaconic acid; and monofunctional (meth)acrylamide monomers such as (meth)acrylamide and N,N-dimethyl(meth)acrylamide.

[0051] <Reactive emulsifiers and structural units derived from reactive emulsifiers (b0)> The reactive emulsifier is not particularly limited as long as it is an emulsifier that is reactive with the monomer or polymer thereof as described above, but examples include those that have a radically polymerizable double bond, a hydrophilic functional group, and a hydrophobic group in their molecular structure, and that have emulsifying, dispersing, and wetting functions similar to general emulsifiers.

[0052] Examples of radically polymerizable double bond structures in molecular structures include, for example, 1-propenyl group, 2-methyl-1-propenyl group, allyl group, methallyl group, vinyl group, acryloyl group, and methacryloyl group.

[0053] Examples of hydrophilic functional groups in molecular structures include anionic groups such as sulfate groups, nitrate groups, phosphate groups, boric acid groups, and carboxyl groups (-OSO3). - , -NO3 - ,-OPO3 - -B(OH)4 - , -COO - etc.); cationic groups such as amino groups (-NH3 + Examples include polyoxyalkylene chains such as polyoxyethylene, polyoxymethylene, and polyoxypropylene; and hydroxyl groups.

[0054] Examples of hydrophobic groups in molecular structures include alkyl groups, alkenyl groups, phenyl groups, alkylphenyl groups, styrene-phenyl groups, and naphthyl groups.

[0055] Reactive emulsifiers are classified into anionic emulsifiers, nonionic emulsifiers, cationic emulsifiers, amphoteric emulsifiers, etc., depending on the type of hydrophilic functional group contained in their molecular structure. Furthermore, the radical polymerizable double bonds, hydrophilic functional groups, and hydrophobic groups in the molecular structure of reactive emulsifiers can each have multiple types of structures and functional groups.

[0056] Among those mentioned above, reactive emulsifiers are preferably those that have at least a polyoxyalkylene chain and a sulfate group as hydrophilic functional groups within their molecular structure.

[0057] Examples of commercially available product names for such reactive emulsifiers are not limited to those listed above, but include Adekarya Soap SR, ER, SE, NE, PP (ADEKA Corporation), Aqualon HS, BC, KH (Daiichi Kogyo Seiyaku Co., Ltd.), Latemul PD (Kao Corporation), Eleminol JS, RS (Sanyo Chemical Industries, Ltd.), and Antox MS (Nippon Emulsifier Co., Ltd.).

[0058] [Structural unit (b1) derived from a compound represented by general formula (I)] As described above, the organic fine particles (B) may have a structural unit (b1) derived from a compound represented by the following general formula (I). Compounds represented by the following general formula (I) have both hydrophobic and hydrophilic groups in their molecules, as well as copolymerizable unsaturated groups. Therefore, compounds represented by the following general formula (I) also function as reactive (copolymerizable) emulsifiers (corresponding to the reactive emulsifiers mentioned above), and are expected to suppress and improve various problems in conventional emulsion polymerization, such as polymerization instability and foaming of the system during emulsion polymerization, and deterioration of the physical properties of the polymer obtained after polymerization. [ka] In the above general formula (I), m represents an integer from 1 to 3, and preferably represents 2 from the viewpoint of emulsification.

[0059] AO represents an alkylene oxy group having 2 to 4 carbon atoms. Examples of alkylene oxy groups having 2 to 4 carbon atoms include ethylene oxy group, propylene oxy group, and butylene oxy group. Among these, the ethylene oxy group is preferred as AO. The ethylene oxy group has higher hydrophilicity than other alkylene oxy groups and can form a resin emulsion with a dense hydration layer, thereby further improving the stability of resin particles in an aqueous dispersion medium. n represents the number of repeating alkylene oxy units (i.e., the number of moles of alkylene oxy groups added). n is an integer from 0 to 100, and from the viewpoint of the stability of resin particles in an aqueous dispersion medium, it is preferably an integer from 5 to 50, and more preferably an integer from 5 to 30.

[0060] X represents a hydrogen atom, or an anionic hydrophilic group selected from the group consisting of -SO3M, -COOM, and -PO3M (wherein M represents an alkali metal atom, an alkaline earth metal atom, an ammonium group, or an organic ammonium group). Examples of alkali metal atoms include sodium atoms and potassium atoms. Examples of alkaline earth metal atoms include calcium atoms and barium atoms. Considering emulsifying properties, X is preferably a hydrogen atom, -SO3NH4, -SO3Na, or -SO3K, and more preferably -SO3NH4.

[0061] R represents a polymerizable unsaturated group, specifically a group represented by formula (i) or formula (ii) below, where R1 represents a hydrogen atom or a methyl group. [ka]

[0062] A preferred example of a compound represented by the general formula (I) is the compound represented by the following formula (I-1). [ka] (In the formula, m, AO, n, and X are as defined in formula (I).)

[0063] The compound represented by the above general formula (I) can also be a commercially available product, for example, the Aqualon AR series (AR-10, AR-1025, AR-20, AR-2020) manufactured by Daiichi Kogyo Kagaku Co., Ltd.

[0064] In organic fine particles (B) (polymer), from the viewpoint of copolymerizability during polymerization, etc., when the total structural units of the polymer are set to 100% by mass, for example, the proportion of structural unit (a) can be 98.0% to 99.9% by mass, and the proportion of structural unit (b0) (for example, structural unit (b1)) can be 0.1% to 2.0% by mass. Furthermore, when the total structural units of organic fine particles (B) (polymer) are set to 100% by mass, the proportion of structural unit (a0) can be 88-99% by mass, the proportion of structural unit (a3) ​​can be 0.9-10% by mass, and the proportion of structural unit (b0) can be 0.1-2% by mass. The proportion of structural unit (b0) mentioned above may be interpreted as the proportion of structural unit (b1), or as the total proportion of structural unit (b1) and structural units other than structural unit (b0).

[0065] Furthermore, from the viewpoint of obtaining resin particles with uniform particle size and stability against solvents, for example, the proportion of structural units (a1) derived from monofunctional styrene monomers in structural unit (a) can be set to 10% to 99% by mass, the proportion of structural units (a2) derived from monofunctional (meth)acrylic monomers can be set to 0% to 80% by mass, the proportion of structural units (a3) ​​derived from polyfunctional vinyl monomers can be set to 1% to 10% by mass, and the proportion of structural units derived from other polymerizable monomers can be set to 0% to 5% by mass (totaling 100% by mass).

[0066] The content of organic fine particles (B) in the varnish composition is not particularly limited and can be determined as appropriate, taking into account the viscosity, applicability, and solid content concentration of the varnish composition. The organic fine particles (B) may be used individually or in combination of two or more types.

[0067] [Particle size of organic microparticles (B)] Organic microparticles (B) have a median diameter D 50 It is preferable that the particles are 0.05 μm to 2.0 μm in size. In this invention, the median diameter can be the 50% volume diameter value measured by dynamic light scattering. Generally, as particle size decreases, particle aggregation is more likely to occur, especially during polymerization. However, the organic fine particles (B) used in this invention exhibit an excellent aggregation suppression effect in their dispersion, allowing the particle size of the organic fine particles (B) to be kept within a relatively small range. By setting the median diameter within the above range, when producing a polyimide porous film from the varnish composition described later, fine pores can be formed in the polyimide, making it possible to provide the resulting polyimide porous film as a material with a low dielectric constant. However, if the median diameter is less than 0.2 μm, the particle size may be too small to contribute to the formation of sufficient pores. Conversely, if it exceeds 1.5 μm, there is a risk of reducing the mechanical strength of the polyimide resin to be pored, or the desired dielectric properties may not be achieved.

[0068] [Thermal decomposition temperature of organic fine particles (B)] The organic fine particles (B) preferably have a thermal decomposition temperature lower than that of the thermosetting resin described later, under atmospheric pressure. In this specification, the thermal decomposition temperature refers to the temperature at which the weight loss due to thermal decomposition of a sample begins, as measured by a thermogravimetry analyzer (TGA) under conditions compliant with JIS K7120 (Thermogravimetric analysis method for plastics). Depending on the type of thermosetting resin being used, the thermal decomposition temperature of organic fine particles (B) under a nitrogen atmosphere is, for example, 340 to 440°C, preferably 370 to 410°C.

[0069] [Method for producing organic microparticles (B)] Organic fine particles (B) can be obtained by emulsion polymerization of a monomer component containing the vinyl monomer and the reactive emulsifier (for example, a compound represented by general formula (I)). Emulsion polymerization is preferred because it easily yields particles with small particle sizes. Examples of the vinyl monomer include the various monomers mentioned above [monofunctional vinyl monomers (monofunctional styrene monomers, monofunctional (meth)acrylic monomers), polyfunctional vinyl monomers (polyfunctional (meth)acrylic monomers, polyfunctional (poly)vinyl monomers), and other polymerizable monomers], and examples of the reactive emulsifier include the compounds mentioned above. A preferred embodiment of emulsion polymerization includes an emulsion polymerization step in which a polymerization mixture containing the monomer components, a polymerization initiator, and optionally other additives (surfactants, protective colloids, chain transfer agents, pH adjusters, etc.) is subjected to emulsion polymerization, and optionally includes a maturation step in which the reaction solution obtained in the emulsion polymerization step is matured.

[0070] The emulsion polymerization is usually carried out in an aqueous dispersion medium, and the aqueous dispersion medium is not particularly limited, but can be water, a mixture of water and an alcohol-based solvent, etc. From the viewpoint of the stability (non-aggregation) of the organic fine particles (B) formed after emulsion polymerization, water is preferred as the aqueous dispersion medium. The amount of aqueous dispersion medium used can be appropriately set so that the content of organic fine particles (B) present in the system after emulsion polymerization is at a desired ratio. For example, the content of organic fine particles (B) present in the system can be set to 1% to 70% by mass, 10% to 60% by mass, 20% to 50% by mass, etc., and the amount of aqueous dispersion medium used can be appropriately set.

[0071] There are no particular restrictions on the polymerization initiator used in the emulsion polymerization, and known polymerization initiators can be used. Examples include azo compounds such as azobisisobutyronitrile, 2,2-azobis(2-methylbutyronitrile), 2,2-azobis(2,4-dimethylvaleronitrile), 2,2-azobis(2-diaminopropane)hydrochloride, 4,4-azobis(4-cyanovaleric acid), 2,2-azobis(2-methylpropionamidine), and 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate; persulfates such as potassium persulfate and ammonium persulfate; and peroxides such as hydrogen peroxide, benzoyl peroxide, parachlorobenzoyl peroxide, lauroyl peroxide, and ammonium peroxide, but the invention is not limited to these examples. Among these, azo compounds and peroxides can also function as decomposition accelerators, meaning they can accelerate the thermal decomposition of organic fine particles (B) when manufacturing the polyimide porous membrane described later, and are therefore preferred for use. The amount of polymerization initiator used is not particularly limited, but from the viewpoint of increasing the polymerization rate and reducing the amount of unreacted monomers remaining, it is preferably 0.05 parts by mass or more, more than 0.1 parts by mass or more, per 100 parts by mass of monomer components, and from the viewpoint of polymerization stability, it can be 5 parts by mass or less, for example.

[0072] The reactive emulsifier and the compound represented by the general formula (I) also act as emulsifiers, enabling successful initiation and completion of emulsion polymerization. However, surfactants (emulsifiers) commonly used in emulsion polymerization may also be used as additional additives. The above-mentioned surfactant may be an anionic surfactant, a cationic surfactant, or / or other nonionic surfactants in combination. For example, anionic surfactants (anionic emulsifiers) include fatty acid soaps; rosinate soaps; alkyl sulfates such as ammonium dodecyl sulfate and sodium dodecyl sulfate; alkyl sulfonates such as ammonium dodecyl sulfonate and sodium dodecyl sulfonate; alkylaryl sulfonates such as ammonium dodecylbenzenesulfonate, sodium dodecylbenzenesulfonate, and sodium dodecylnaphthalenesulfonate; polyoxyalkylene alkyl sulfates; polyoxyalkylene aryl sulfates; polyoxyalkylene alkylaryl sulfates; dialkyl sulfosuccinates; aryl sulfonic acid-formaldehyde condensates; and fatty acid salts such as ammonium laurylate and sodium stearate. Examples of cationic surfactants include stearyltrimethylammonium, cetyltrimethylammonium, and lauryltrimethylammonium. Examples of nonionic surfactants include polyoxyalkylene alkylphenyl ethers, polyoxyalkylene alkyl ethers, alkyl polyglucosides, polyglycerin alkyl ethers, polyoxyalkylene fatty acid esters, polyglycerin fatty acid esters, and total ruby ​​mono fatty acid esters. When a surfactant is used separately in the emulsion polymerization process, the amount used can be, for example, 0.05 parts by mass or more, 0.1 parts by mass or more, or 0.3 parts by mass or more, per 100 parts by mass of monomer components, with upper limits being, for example, 10 parts by mass or less, 8 parts by mass or less, or 5 parts by mass or less.

[0073] Furthermore, to improve polymerization stability during emulsion polymerization, other known protective colloidal agents may be used in combination as additives. Examples of such protective colloidal agents include fully saponified polyvinyl alcohol, partially saponified polyvinyl alcohol, hydroxyethylcellulose, carboxymethylcellulose, methylcellulose, polyacrylic acid, and gum arabic.

[0074] In addition, known chain transfer agents and pH adjusters may be used as other additives during emulsion polymerization. Examples of the chain transfer agent include octyl mercaptan, dodecyl mercaptan, mercaptoethanol, thioglycolic acid, allyl alcohol, isopropyl alcohol, and sodium hypophosphate. Examples of pH adjusting agents include inorganic acids such as hydrochloric acid, sulfuric acid, and phosphoric acid; organic acids such as citric acid, succinic acid, malic acid, and lactic acid; inorganic bases such as sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate; alkanolamines such as monoethanolamine, diethanolamine, triethanolamine, and isopropanol; aliphatic amines such as ethylenediamine, propylenediamine, hexamethylenediamine, diethylenetriamine, and triethylenetetramine; aromatic polyamines such as phenylenediamine and tolylenediamine; and organic bases such as heterocyclic polyamines such as piperazine and aminoethylpiperazine.

[0075] In the monomer components subjected to the emulsion polymerization described above, the amount of each monomer used (composition ratio) can be set as appropriate. For example, relative to the total amount of all monomers (100% total mass), the proportion of vinyl monomers can be 98.0% to 99.9% by mass, and the proportion of reactive emulsifiers (e.g., compounds represented by general formula (I)) can be 0.1% to 2.0% by mass. For example, relative to the total amount of all monomers (100% total by mass), the proportion of monofunctional vinyl monomers can be 88% to 99% by mass, the proportion of polyfunctional vinyl monomers can be 0.9% to 10% by mass, and the proportion of the reactive emulsifier can be 0.1% to 2% by mass. Furthermore, in the vinyl monomer (total 100% by mass), the monofunctional styrene monomer can be 10% to 99% by mass, the monofunctional (meth)acrylic monomer can be 0% to 80% by mass, the polyfunctional vinyl monomer can be 1% to 10% by mass, and the other polymerizable monomer can be 0% to 5% by mass.

[0076] The emulsion polymerization can be carried out using any known emulsion polymerization method, such as the monomer drop method, pre-emulsion method, or batch polymerization method. From the viewpoint of industrial productivity, the pre-emulsion method is preferable because it allows for stable polymerization and yields polymers (organic fine particles (B)) with few aggregates.

[0077] In the emulsion polymerization described above, there are no particular restrictions on the method of adding the monomer components, polymerization initiators, and other additives, and these can be set as appropriate. For example, the procedure for emulsion polymerization using the pre-emulsification method involves first emulsifying a vinyl monomer with a reactive emulsifier (e.g., a compound represented by general formula (I)) and an aqueous dispersion medium such as water to obtain a pre-emulsification. Then, the obtained pre-emulsification is added dropwise to a reaction vessel, and a polymerization initiator is added as appropriate to allow the emulsion polymerization reaction to proceed. Alternatively, emulsion polymerization may be initiated using a portion of the polymerization mixture, followed by the addition of the remaining polymerization mixture dropwise. Or, emulsion polymerization may be initiated using a mixture consisting of a portion of the total amount of monomer components and a portion of the polymerization initiator (and other additives), followed by the addition of the remaining monomer components and the polymerization initiator (and other additives) dropwise, either separately or mixed together.

[0078] Furthermore, the emulsion polymerization process can be repeated in two or more steps, for example, by including a first emulsion polymerization step and a second emulsion polymerization step. The first emulsion polymerization step forms a core, and the subsequent second emulsion polymerization step forms a shell on the surface of the core, thereby forming core-shell type resin particles. In this case, the second emulsion polymerization step may be performed multiple times. If the second emulsion polymerization step is performed a second time, resin particles can be obtained in which a new shell is formed on the surface of the shell formed by the first second emulsion polymerization step. When the process includes a first emulsion polymerization step and a second emulsion polymerization step, the composition of the monomer components used in each step can be changed, and the monomer components used in each step may be a single type of monomer. That is, different monomers (one type) may be used in the first emulsion polymerization step and the second emulsion polymerization step, or a mixture of monomers and a single type of monomer may be used, or a mixture of different monomers may be used in each step. When a mixture of the same type of monomer is used, mixtures with varying monomer mixing ratios can be used. For example, in the first emulsion polymerization step, a mixture containing a monofunctional styrene monomer, a polyfunctional vinyl monomer, and a reactive emulsifier (for example, a compound represented by general formula (I)) may be used, and in the subsequent second emulsion polymerization step, a mixture containing a monofunctional styrene monomer, a monofunctional (meth)acrylic monomer, a polyfunctional vinyl monomer, and a reactive emulsifier (for example, a compound represented by general formula (I)) may be used.

[0079] The polymerization temperature in the emulsion polymerization described above can be set appropriately depending on the polymerization initiator used, but for example, it can be 30°C to 90°C or 50°C to 80°C. The polymerization time can be set appropriately depending on the reaction rate determined from the amount of monomer components charged and the amount remaining in the reaction solution, but it is usually 1 to 12 hours, for example, 2 to 8 hours.

[0080] Next, an optional maturation step is performed after the emulsion polymerization step for the purpose of reducing unreacted monomers or stabilizing the polymer particles (organic fine particles (B)) obtained by emulsion polymerization. The maturation temperature in the aforementioned maturation process can be, for example, 50°C to 90°C, or for example, 70°C to 85°C. By setting the maturation temperature within this range, it is expected that the amount of unreacted monomer mixture can be reduced while suppressing particle aggregation. The maturation time can be appropriately set according to the reaction rate determined from the total amount of monomer components added and the amount of monomer components remaining in the reaction solution, but it is usually 1 to 12 hours, preferably 2 to 8 hours.

[0081] In the aforementioned maturation process, a surfactant may be added as needed for purposes such as making it easier to suppress the aggregation of organic fine particles (B) during maturation. As the surfactant used in the aforementioned maturation process, it is preferable to use the surfactants listed in the emulsion polymerization process described above, and it is also possible to use anionic surfactants or nonionic surfactants. The amount of surfactant used in the maturation step is, for example, 0.05 parts by mass or more, 0.1 parts by mass or more, 0.3 parts by mass or more, or 10 parts by mass or less, 8 parts by mass or less, or 5 parts by mass, based on the total amount of monomer components subjected to the emulsion polymerization step: 100 parts by mass.

[0082] After the emulsion polymerization step (and optionally a maturation step), organic fine particles (B) can be obtained in the form of a dispersion containing the formed polymer in an aqueous dispersion medium. In the varnish composition described later, the organic fine particles (B) can be used in the form of a fine particle dispersion containing the organic fine particles (B), or as dry organic fine particles (B).

[0083] When using organic fine particles (B) as dry powder organic fine particles (B), the organic fine particles (B) in the form of a dispersion in the aforementioned aqueous dispersion medium can be freeze-dried, hot-air dried, spray-dried, or otherwise processed to obtain a powder form. Furthermore, when using organic fine particles (B) in the form of a fine particle dispersion containing organic fine particles (B), the dispersion containing the organic fine particles (B) obtained through the emulsion polymerization process described above may be used as is, or the aqueous dispersion may be replaced with a solvent to form a fine particle dispersion, or the organic fine particles (B) in the powder form described above may be dispersed in a suitable solvent to form a fine particle dispersion. In this case, one or more solvents can be selected from water (SI) and organic solvents (S-III), as described later.

[0084] <Solvent (S)> The varnish composition contains a solvent (S). The solvent (S) may be, for example, water (SI), an organic solvent (S-III), or a combination thereof.

[0085] The above organic solvent (S-III) may be basic, but from the viewpoint of avoiding hydrolysis of polyamic acid (A), it is preferable that it be a compound that is neutral or weakly basic in water.

[0086] Suitable examples of organic solvents (S-III) include N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylisobutylamide, N,N-diethylacetamide, N,N-dimethylformamide (DMF), N,N-diethylformamide, N-methylcaprolactam, 1,3-dimethyl-2-imidazolidinone (DMI), pyridine, and N,N,N',N'-tetramethylurea (TMU). Nitrogen-containing polar solvents such as β-propiolactone, γ-butyrolactone, γ-valerolactone, δ-valerolactone, γ-caprolactone, and ε-caprolactone; dimethyl sulfoxide; hexamethylphosphoric triamide; acetonitrile; aromatic solvents such as benzene, toluene, and xylene; tetrahydrofuran (THF), dioxane, trioxane, 1,2-dimethoxyethane, diethylene glycol di Examples of solvents include ether-based solvents such as methyl ether and diethylene glycol diethyl ether; ketone-based solvents such as acetone, methyl ethyl ketone, and cyclohexanone; and alcohol-based solvents such as methanol, ethanol, 1-propanol, 2-propanol, tert-butyl alcohol, ethylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, 2-butene-1,4-diol, 2-methyl-2,4-pentanediol, glycerin, 2-ethyl-2-hydroxymethyl-1,3-propanediol, and 1,2,6-hexanetriol. In the specification of the present application, in classifying the above solvents, for example, a compound corresponding to a ketone or an ether and having an alcoholic hydroxy group can be classified as an alcohol-based solvent. Further, a compound corresponding to both a ketone and an ether can be classified as a ketone-based solvent.

[0087] From the viewpoints of the solubility or dispersion stability of the varnish composition and the ease of removing the solvent (S) from the coating film, the solvent (S) is an organic solvent (S-III), and particularly, the organic solvent (S-III) has the following formula (S1):

Chemical formula

Chemical formula

[0088] Among the compounds represented by the formula (S1), R S3Specific examples of cases where is a hydrogen atom or a group represented by formula (S1-1) include N,N-dimethylformamide, N,N-dimethylacetamide, N,N,2-trimethylpropionamide, N-ethyl-N,2-dimethylpropionamide, N,N-diethyl-2-methylpropionamide, N,N,2-trimethyl-2-hydroxypropionamide, N-ethyl-N,2-dimethyl-2-hydroxypropionamide, and N,N-diethyl-2-hydroxy-2-methylpropionamide.

[0089] Among the compounds represented by formula (S1), R S3 Specific examples of groups represented by formula (S1-2) include N,N,N',N'-tetramethylurea and N,N,N',N'-tetraethylurea.

[0090] Among specific examples of compounds represented by formula (S1), particularly preferred compounds include N,N-dimethylformamide, N,N-dimethylacetamide, N,N,2-trimethylpropionamide, and N,N,N',N'-tetramethylurea. Of these, N,N,2-trimethylpropionamide and N,N,N',N'-tetramethylurea are preferred.

[0091] N,N,2-trimethylpropionamide and N,N,N',N'-tetramethylurea are useful because they are low-toxicity substances, as they are not designated as SVHCs (Substances of Very High Concern) under the EU's REACH regulation.

[0092] The solvent (S) content in the varnish composition is not particularly limited as long as it does not hinder the objectives of the present invention. The solvent (S) content in the varnish composition is appropriately adjusted according to the solid content of the varnish composition.

[0093] <Dispersant> To uniformly disperse the organic fine particles (B) in the varnish composition, a dispersant may be added along with the organic fine particles (B). By adding a dispersant, the polyamic acid (A) and the organic fine particles (B) can be mixed more uniformly, and furthermore, the organic fine particles (B) can be uniformly distributed in the formed film. As a result, dense openings can be provided on the surface of the final polyimide porous film, and the front and back surfaces can be efficiently connected, improving the air permeability of the polyimide porous film. Furthermore, by adding a dispersant, the drying properties of the varnish composition are improved, and the peelability of the precursor film of the formed polyimide porous film from the substrate is also improved.

[0094] When a dispersant is used, the content of the dispersant in the varnish composition is preferably 0.01% to 5% by mass, more preferably 0.05% to 1% by mass, and even more preferably 0.1% to 0.5% by mass, relative to the fine particles, in terms of film-forming properties.

[0095] The varnish composition is manufactured by mixing the essential or optional components described above, in a predetermined composition that takes into account the applicability of the varnish composition and the various properties of the polyimide porous film produced.

[0096] ≪Method for manufacturing varnish composition≫ The method for producing the varnish composition involves mixing predetermined amounts of the various components mentioned above, and the specific procedure is not particularly limited. One preferred method for producing a varnish composition is to mix a polyamic acid-containing solution containing polyamic acid (A) and an organic solvent (S-III) with a fine particle dispersion or dry powder of organic fine particles (B) containing organic fine particles (B).

[0097] Regarding the polyamic acid-containing solution, the polyamic acid (A) and the organic solvent (S-III) are as described above. The polyamic acid-containing solution may be prepared by dissolving polyamic acid (A), which has been produced by a well-known method, in the organic solvent (S-III), or polyamic acid (A) may be synthesized in the organic solvent (S-III), and the reaction solution may be used as the polyamic acid-containing solution. The polyamic acid-containing solution may contain water (SI). Furthermore, the polyamic acid-containing solution may contain any other components besides polyamic acid (A), organic solvent (S-III), and water (SI).

[0098] Regarding the particulate dispersion containing organic fine particles (B), the organic fine particles (B) are as described above. The dispersion medium contained in the particulate dispersion is preferably one or more selected from water (SI) and organic solvents (S-III).

[0099] In the above method, when mixing the various materials constituting the varnish composition, the mixing may be carried out under heated conditions, provided that excessive decomposition or deformation of the polyamic acid (A) or organic fine particles (B) does not occur. Alternatively, various dispersion devices may be used to disperse organic fine particles (B) while mixing the various materials that constitute the varnish composition.

[0100] The viscosity of the varnish composition is not particularly limited as long as it can form a coating film of the desired thickness. For example, the viscosity of the varnish composition is preferably 300 cP to 20,000 cP, more preferably 1,000 cP to 15,000 cP, and even more preferably 1,500 cP to 12,000 cP. If the viscosity of the varnish composition is within this range, uniform film formation is easy.

[0101] The varnish composition preferably contains organic microparticles (B) and polyamic acid (A) such that the ratio of organic microparticles (B) to polyamic acid (A) is 0.5 to 4.0 (mass ratio) when used as a polyamic acid-microparticle composite film (precursor film) as described later, and more preferably contains organic microparticles (B) and polyamic acid (A) such that the (B) / (A) ratio is 0.7 to 3.5 (mass ratio). Furthermore, when a polyamic acid-microparticle composite film is formed using the varnish composition, it is preferable that the varnish composition contains organic microparticles (B) and polyamic acid such that the volume ratio of organic microparticles (B) to polyamic acid (A) in the composite film is 1.0 to 5.0. The aforementioned volume ratio is more preferably 1.2 to 4.5. If the mass ratio or volume ratio of organic fine particles (B) to polyamic acid (A) is above the lower limit value mentioned above, it is easy to form pores of appropriate density. If the mass ratio or volume ratio of organic fine particles (B) to polyamic acid (A) is below the upper limit value mentioned above, a stable film can be formed without causing problems such as an increase in the viscosity of the varnish composition or cracking in the film.

[0102] The solid content concentration of the varnish composition is not particularly limited, but is preferably 1% by mass or more, preferably 5% by mass or more, more preferably 10% by mass or more, with an upper limit of, for example, 60% by mass or less, and preferably 30% by mass or less. The solid content concentration referred to here means the concentration of components other than the solvent (S), and even liquid components are included in the weight as solid content.

[0103] Method for producing precursor membranes of polyimide porous membranes A method for producing a precursor film of a polyimide porous film includes a coating film formation step of applying the aforementioned varnish composition onto a substrate to form a coating film, and a precursor film formation step of removing the solvent (S) from the coating film to form a precursor film of a polyimide porous film.

[0104] Examples of the above-mentioned substrates include PET film, SUS substrate, and glass substrate. Furthermore, to remove the solvent (S) from the coating film, the aforementioned varnish composition can be applied to the substrate to form a coating film, and then dried at 0°C to 100°C, preferably 10°C to 100°C, under normal pressure or vacuum.

[0105] The following describes the procedure for forming a precursor film of a polyimide porous membrane (hereinafter also simply referred to as "precursor film"). The precursor film may be deposited directly onto the substrate, or it may be deposited on a different underlying film formed on the substrate. Alternatively, after depositing the precursor film using the aforementioned varnish composition, a different upper film may be deposited on top of it. In this application, both the method of forming a precursor film on the substrate and the method of forming an upper film on top of the precursor film are included in the method of forming a precursor film on the substrate.

[0106] Examples of the above-mentioned lower layer (or upper layer) include a varnish containing a resin selected from the group consisting of polyamic acid, polyimide, polyamideimide precursor, polyamideimide, and polyethersulfone, fine particles, and a solvent, wherein, for example, the content of the fine particles is more than 40% and 81% or less by volume relative to the total of the resin and the fine particles, and the lower layer (or upper layer) is formed using this varnish for lower layer (or upper layer) formation. The lower layer is formed on a substrate. In the above varnish, if the content of the above fine particles exceeds 40% by volume, the fine particles are uniformly dispersed in the varnish, and if the content of the above fine particles is 81% by volume or less, the fine particles are dispersed in the varnish without agglomerating with each other, so that pores can be uniformly formed in the layer derived from the above lower layer film (or upper layer film). Furthermore, if the content of the fine particles in the above varnish is within the above range, when forming an unfired composite film on a substrate, it is easier to ensure release properties after film formation, even if a release layer is not provided on the substrate beforehand.

[0107] The fine particles used in the varnish for forming the lower (or upper) layer film may be the same as or different from the organic fine particles (B) used in the varnish composition described above. To make the pores in the unfired lower (or upper) layer composite film denser, it is preferable that the fine particles used in the varnish for forming the lower (or upper) layer film have a particle size distribution index smaller than or the same as the organic fine particles (B) used in the varnish composition described above. Alternatively, it is preferable that the fine particles used in the varnish for forming the lower (or upper) layer film have a sphericity smaller than or the same as the fine particles used in the varnish composition described above.

[0108] Furthermore, the average particle size of the fine particles used in the varnish for forming the lower (or upper) layer film is preferably 5 nm to 1000 nm, and more preferably 10 nm to 600 nm.

[0109] Furthermore, the content of fine particles in the varnish for forming the lower (or upper) layer film may be greater or less than that of the aforementioned varnish composition. Preferred examples of components such as fine particles and solvents contained in the above-mentioned varnish for forming the lower (or upper) layer film are the same as those in the varnish composition described above. The varnish for forming the lower (or upper) layer film can be prepared by the same method as the varnish composition described above. The lower layer unfired composite film can be formed, for example, by applying the varnish for lower layer formation onto a substrate and drying it at 0°C to 100°C, preferably at 10°C to 100°C, under normal pressure or vacuum. The film formation conditions for the upper layer unfired composite film are similar.

[0110] Furthermore, examples of the lower (or upper) layer include films made of fibrous materials such as cellulose resins and nonwoven fabrics (for example, polyimide nonwoven fabrics (with fiber diameters of approximately 50 nm to approximately 3000 nm)), as well as polyimide films.

[0111] By the method described above, the precursor membrane is formed on the substrate either alone or, if necessary, together with the underlying (or upper) membrane.

[0112] A method for producing a precursor film of a polyimide porous membrane may include a peeling step after the precursor film formation step, in which the precursor film is peeled off from the substrate. When the precursor film is peeled off from the substrate, the substrate does not need to have the heat resistance to withstand the temperature at which the precursor film is fired.

[0113] When peeling a precursor film or a laminated film consisting of a precursor film and an unfired lower (or upper) layer composite film from a substrate, a substrate with a pre-applied release layer may be used to further improve the peelability of the film. When a pre-applied release layer is provided on the substrate, a release agent is applied to the substrate and dried or baked before applying the aforementioned varnish composition or the varnish for forming the lower layer film. The release agent used here can be any known release agent, such as alkyl ammonium phosphate, fluorine, or silicone-based agents, without particular limitations. However, when peeling the dried precursor film from the substrate, if even a small amount of release agent remains on the peeled surface of the precursor film, the remaining release agent may cause discoloration during firing or adverse effects on electrical properties. For this reason, it is preferable to remove as much of the release agent adhering to the peeled surface as possible. To remove the release agent, a washing step may be introduced in which the precursor film peeled from the substrate or the laminated film containing the precursor film is washed with an organic solvent.

[0114] If the substrate is used as is without providing a release layer, the above-mentioned peeling and washing steps can be omitted. Furthermore, in the method for manufacturing the precursor film, before the removal step for removing organic fine particles (B) in the method for manufacturing the polyimide porous film described later, an immersion step in which the precursor film is immersed in water or a water-containing solvent, a pressing step in which the precursor film is pressed after the immersion step, and a drying step in which the precursor film is dried after the immersion step may be provided as optional steps.

[0115] In a method for manufacturing a precursor film of a polyimide porous membrane, when the above-mentioned peeling step is performed, a winding step may be performed after the peeling step in which the precursor film is wound into a roll. When the precursor membrane is wound into a roll, it is possible to calcine the rolled precursor membrane in a small calcination furnace. Furthermore, the transport of the precursor membrane until calcination is easy, and storage space can be reduced. Furthermore, a roll-to-roll process can be applied to the precursor membrane firing process, enabling the efficient production of porous polyimide membranes.

[0116] Method for producing porous polyimide membranes The method for producing a polyimide porous membrane includes a removal step of removing organic fine particles (B) from the aforementioned polyimide porous membrane precursor. In this removal step, the removal of organic fine particles (B) may be carried out while imidizing the polyamic acid (A), or after imidizing the polyamic acid (A). Removal of organic fine particles (B) is preferably by heating, and may be carried out by heating after chemical imidization as described later, or may be carried out simultaneously with, during, or after imidization of the precursor membrane by calcination related to thermal imidization. By thermal decomposition of organic fine particles (B) due to heating, a polyimide porous membrane having spherical pores with a uniform distribution is obtained.

[0117] The method for imidizing polyamic acid (A) is not particularly limited. Imidization may be either thermal imidization or chemical imidization. For chemical imidization, a method such as immersing a precursor film containing polyamic acid (A) in acetic anhydride or a mixed solvent of acetic anhydride and isoquinoline can be used.

[0118] Among the imidation methods described above, thermal imidation, specifically calcination, is preferred because it eliminates the need to remove the imidating agent by washing. The calcination process for thermal imidation will be described below.

[0119] Furthermore, if a lower (or upper) layer is formed together with the precursor membrane during its manufacture, the lower (or upper) layer is fired at the same time as the precursor membrane. The firing temperature varies depending on the structure of the polyamic acid (A), but is preferably between 120°C and 500°C, more preferably between 150°C and 450°C, and even more preferably between 300°C and 450°C.

[0120] The firing conditions can include, for example, raising the temperature from room temperature to around 400°C to 450°C over about 3 hours and then holding it at the same temperature for about 2 to 30 minutes, or using a drying-thermal imidization method that involves continuous or stepwise temperature increases, such as raising the temperature from room temperature in increments of 50°C to 400°C to 450°C (holding each step for about 20 minutes), and finally holding it at 400°C to 450°C for about 2 to 30 minutes. When a precursor film is formed on a substrate, and the precursor film or laminated film containing the precursor film is peeled off the substrate and then fired, a method can be adopted in which the edges of the precursor film or laminated film are fixed to a stainless steel mold or the like to prevent deformation due to firing.

[0121] The thickness of the porous polyimide film obtained after firing can be determined by measuring the thickness at multiple points using a micrometer, for example, and averaging the results. The preferred average thickness varies depending on the application of the porous polyimide film. For example, when used as a separator, a thickness of 5 μm to 500 μm is preferred, 10 μm to 100 μm is more preferred, and 15 μm to 30 μm is even more preferred. When used as a filter, a thickness of 5 μm to 500 μm is preferred, 10 μm to 300 μm is more preferred, and 20 μm to 150 μm is even more preferred.

[0122] The polyimide porous membrane obtained in this way is an opaque or yellow or brownish porous membrane. Furthermore, regardless of the film thickness, the polyimide porous membrane is a porous membrane in which spherical pores are distributed in a continuous manner throughout the entire membrane, and the front and back surfaces are connected.

[0123] A method for manufacturing a polyimide porous membrane may include a resin removal step after a removal step for removing the organic fine particles (B) mentioned above, in which at least a portion of the polyimide porous membrane is removed. The resin removal step means a step of removing resin in the film thickness direction of the porous membrane (thinning the film thickness), and by removing at least a portion of the porous membrane after the removal step, it is possible to improve the porosity of the final polyimide porous membrane compared to a polyimide porous membrane in which at least a portion of the porous membrane is not removed. Furthermore, the process may include a step to remove at least a portion of the resin portion of the precursor film, for example, after the precursor film formation step, prior to the step to remove the organic fine particles (B). In this case, it is acceptable for some of the organic fine particles (B) contained in the precursor film to be removed. By including this step, when the organic fine particles (B) are removed in the subsequent fine particle removal step and pores are formed, it becomes possible to improve the porosity of the porous polyimide resin film of the final product compared to a process in which the resin portion of the precursor film is not removed.

[0124] The step of removing at least a portion of the resin portion, or the step of removing at least a portion of the polyimide porous membrane, can be carried out by a conventional chemical etching method, a physical removal method, or a combination thereof.

[0125] Chemical etching methods include treatment with chemical etching solutions such as inorganic alkaline solutions or organic alkaline solutions, with the use of inorganic alkaline solutions being 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 mainly composed of alkali metal hydroxide compounds, hydrazine, and 1,3-dimethyl-2-imidazolidinone. Examples of alkaline solutions for organic alkalis include 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 piperidine. The alkali concentration of these inorganic and organic alkaline solutions is, for example, 0.01% by mass or more and 20% by mass or less.

[0126] For the solvent in each of the above solutions, pure water or alcohols can be selected as appropriate. Alternatively, solutions with an appropriate amount of surfactant added can also be used.

[0127] Furthermore, physical removal methods include, for example, dry etching using plasma (oxygen, argon, etc.) or corona discharge; and surface treatment by dispersing an abrasive (e.g., alumina (hardness 9), etc.) in a liquid and irradiating the film surface with this at a speed of 30 m / s to 100 m / s.

[0128] Another physical removal method involves pressing the porous film onto a backing film (such as a polyester film like PET film) that has been moistened with a liquid, and then peeling the porous film off the backing film either before or after drying. In this method, the porous film is peeled off the backing film, with only the surface layer of the porous film remaining on the backing film, due to the surface tension or electrostatic adhesion of the liquid. [Examples]

[0129] The present invention will be described in more detail below with reference to examples, but the scope of the present invention is not limited thereto.

[0130] In the examples and comparative examples, the following tetracarboxylic dianhydrides, diamines, polyamic acids, and organic solvents were used. • Tetracarboxylic acid dianhydride: Pyromellitic acid dianhydride • Diamine: 4,4'-diaminodiphenyl ether • Polyamic acid solution: Reaction product of pyromellitic dianhydride and 4,4'-diaminodiphenyl ether (solids content 20% by mass (organic solvent: dimethylacetamide)) • Organic solvent: Dimethylacetamide (DMAc)

[0131] Synthesis Example 1 A 1.0L glass container equipped with a stirrer, thermometer, temperature controller, condenser, and dripping device was filled with 383.0g of deionized water, and nitrogen gas was introduced while stirring to perform nitrogen purging. The container was then heated with a mantle heater, and the temperature was controlled to 72±2°C to create the polymerization vessel. In a 1.0 L glass container equipped with a stirrer, 122.4 g of deionized water, 12.8 g of polyoxyethylene styrene-propenylphenyl ether sulfate ammonium salt (Aqualon AR-1025 (25% aqueous solution) manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) as a compound represented by general formula (I) (reactive emulsifier), 378.6 g of styrene (styrene monomer manufactured by Asahi Kasei Corporation) as a monofunctional monomer, and 22.2 g of ethylene glycol dimethacrylate (Acryester ED manufactured by Mitsubishi Chemical Corporation) as a polyfunctional monomer were added and stirred to obtain a monomer emulsion in which styrene and ethylene glycol dimethacrylate were emulsified in deionized water. In a 0.1L glass container equipped with a stirrer, 48.6g of deionized water and 3.1g of 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate (VA-057, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a polymerization initiator were added and stirred to dissolve, obtaining an aqueous polymerization initiator solution. 26.8 g of the prepared monomer emulsion and 5.0 g of the prepared polymerization initiator aqueous solution were placed in the polymerization container, and initial polymerization was carried out for 120 minutes. After initial polymerization for 120 minutes, the remaining monomer emulsion and polymerization initiator aqueous solution were delivered to the polymerization vessel via a liquid delivery pump over a period of 240 minutes, and dropwise polymerization was carried out. After the dropwise polymerization was complete, the liquid delivery line was rinsed with 9.0 g of deionized water. After continuing the polymerization reaction for 120 minutes, the mixture was cooled to 40°C to obtain an aqueous dispersion of a crosslinked polymer with a solid content of 40%.

[0132] Synthesis Example 2 Polymerization was carried out in the same manner as in Synthesis Example 1, except that 374.2 g of styrene and 4.4 g of methyl methacrylate were used instead of 378.6 g of styrene in Synthesis Example 1, and trimethylolpropane trimethacrylate was used instead of ethylene glycol dimethacrylate, to obtain a crosslinked polymer aqueous dispersion with a solid content of 40%.

[0133] Synthesis Example 3 Polymerization was carried out in the same manner as in Synthesis Example 1, except that 388.8 g of styrene was used instead of 378.6 g of styrene in Synthesis Example 1, and 12.0 g of a divinylbenzene mixture (DVB570, manufactured by Nippon Steel Chemical & Material Co., Ltd., containing 57% divinylbenzene and 43% ethyl vinylbenzene) (divinylbenzene: 6.84 g, ethyl vinylbenzene: 5.16 g) was used instead of 22.2 g of ethylene glycol dimethacrylate, to obtain a crosslinked polymer aqueous dispersion with a solid content of 40%.

[0134] Synthesis Example 4 Polymerization was carried out in the same manner as in Synthesis Example 1, except that 364.7 g of styrene and 4.0 g of methyl methacrylate were used instead of 378.6 g of styrene in Synthesis Example 1, and 32.1 g of 1,3-butylene glycol dimethacrylate was used instead of 22.2 g of ethylene glycol dimethacrylate, to obtain a crosslinked polymer aqueous dispersion with a solid content of 40%.

[0135] Synthesis Example 5 A 1.0L glass container equipped with a stirrer, thermometer, temperature controller, condenser, and dropping device was filled with 343.3g of deionized water, and nitrogen gas was introduced while stirring to perform nitrogen purging. After nitrogen purging, 0.6g of a 40% triethanolamine lauryl sulfate aqueous solution (Alscope LS-40T, manufactured by Toho Chemical Industry Co., Ltd.) was added as an emulsifier, and the mixture was heated with a mantle heater, with the temperature controlled at 72±2℃ to create a polymerization vessel. In a 1.0L glass container equipped with a stirrer, 169.7g of deionized water, 3.5g of a 40% triethanolamine lauryl sulfate aqueous solution as an emulsifier, 364.9g of styrene as a monofunctional monomer, and 11.1g of 2-hydroxyethyl methacrylate (Acryester HO, manufactured by Mitsubishi Chemical Corporation) were added and stirred to obtain a monomer emulsion in which styrene and 2-hydroxyethyl methacrylate were emulsified in deionized water. In a 0.1L glass container equipped with a stirrer, 49.1g of deionized water and 3.2g of 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate as a polymerization initiator were added and stirred to dissolve, obtaining an aqueous polymerization initiator solution. 28.4 g of the prepared monomer emulsion and 4.5 g of the prepared polymerization initiator aqueous solution were placed in the polymerization container, and initial polymerization was carried out for 120 minutes. After initial polymerization for 120 minutes, the remaining monomer emulsion and the remaining polymerization initiator aqueous solution were each delivered to the polymerization vessel via a liquid delivery pump over a period of 300 minutes, and dropwise polymerization was carried out. After continuing the polymerization reaction for 120 minutes, the mixture was cooled to 40°C to obtain an aqueous dispersion of a non-crosslinked polymer with a solid content of 40%.

[0136] Synthesis Example 6 Polymerization was carried out in the same manner as in Synthesis Example 1, except that 8.0 g of triethanolamine lauryl sulfate (40% aqueous solution) was used instead of 12.8 g of polyoxyethylene styrene-propenylphenyl ether sulfate ammonium salt (25% aqueous solution) in Synthesis Example 1, and 392.8 g of styrene and 8.0 g of ethylene glycol dimethacrylate were changed to obtain a crosslinked polymer aqueous dispersion with a solid content of 40%.

[0137] [Example 1] <Drying of aqueous dispersion of organic particulate matter> The crosslinked polymer aqueous dispersion (organic fine particle aqueous dispersion) obtained in Synthesis Example 1 was spray-dried using a spray dryer ADL-311S-A (manufactured by Yamato Scientific Co., Ltd.) to obtain powdered organic fine particles.

[0138] <Preparation of Varnish Composition> 10.7 parts by mass of powdered organic fine particles and 43.0 parts by mass of N,N-dimethylacetamide were stirred to form a dispersion. 46.3 parts by mass of polyamic acid (20% by mass solution of dimethylacetamide) was added to the dispersion and dispersed using a three-roll mill to obtain a varnish composition with a uniform composition.

[0139] <Manufacturing of polyimide porous membranes> The varnish composition was applied to a polyethylene terephthalate film and dried at 90°C for 5 minutes to obtain a precursor film for a porous polyimide film. After peeling the obtained precursor film from the polyethylene terephthalate film, the precursor film was fired in a firing furnace at 420°C for 5 minutes to thermally decompose the vinyl resin particles while imidizing the polyamic acid, thereby obtaining the porous polyimide film of Example 1. The surface of the obtained porous film (the film side and the air side of the substrate) was observed using a scanning electron microscope (SEM). The obtained SEM image of the air side is shown in Figure 1 (Figure 1(a)). From Figure 1(a), it can be seen that spherical pores of uniform size were formed in the polyimide porous film, and the diameter of the pores was measured using the SEM's length measuring tool. The results confirmed that pores of a size equivalent to the median diameter of the organic resin particles used were formed.

[0140] [Examples 2-4] Except for changing the type of organic microparticles to those listed in Table 1, the drying of the aqueous dispersion of organic microparticles, the preparation of the varnish composition, and the production of the polyimide porous membrane were carried out in the same manner as in Example 1. SEM observation was also performed on each porous membrane. The obtained SEM images from the air side are shown in Figure 1 (Figure 1(b): Example 2, Figure 1(c): Example 3, Figure 1(d): Example 4). As shown in Figures 1(b), (c), and (d), uniformly sized spherical pores were formed in the polyimide porous material. Measurement of the pore diameter using an SEM measuring tool confirmed that pores of a size equivalent to the median diameter of organic microparticles were formed.

[0141] [Comparative Examples 1-2] Except for changing the type of organic microparticles to those listed in Table 1, the drying of the organic microparticle dispersion, preparation of the varnish composition, and production of the polyimide porous film were carried out in the same manner as in Example 1. SEM observation was also performed on Comparative Example 1, and the obtained SEM images are shown in Figure 2. As shown in Figure 2, in this example, spherical pores of non-uniform size were formed in the polyimide porous material with a non-uniform distribution compared to the example. Measurement of the diameter of the pores using an SEM measuring tool confirmed that the pores were larger than the median diameter of the organic microparticles. Similarly, in Comparative Example 2, the pores were formed with a non-uniform distribution compared to the example, and it was confirmed that pores larger and smaller than the median diameter of the organic microparticles were sparsely distributed.

[0142] <Rating> The following evaluations were performed on each porous membrane prepared as described above. [Stress and elongation at fracture] Each of the prepared porous membranes was cut into strips measuring 3 cm x 3 mm to obtain rectangular samples. The stress (MPa; tensile strength) and elongation at break (%GL) of these samples were evaluated using the EZ Test (manufactured by Shimadzu Corporation).

[0143] [Air permeability] Each of the prepared porous membranes was cut into 5cm squares to serve as a sample for air permeability measurement. Using a Gurley-type densometer (manufactured by Toyo Seiki Seisakusho Co., Ltd.), the time it took for 100ml of air to pass through the sample was measured in accordance with JIS P 8117. As a guideline for air permeability, it can be, for example, within 250 seconds or within 200 seconds. A lower value is preferable, so no lower limit is specifically set, but considering the handling of porous membrane samples, it can be, for example, 30 seconds or more. If the Gurley air permeability is within 250 seconds, it can be judged that it is suitable for use as a filter for lithium-ion battery separators or gas or liquid separation membranes because it exhibits sufficiently high ion permeability.

[0144] [Table 1]

[0145] As shown in Table 1 and Figures 1-2, the varnish compositions of the examples containing the organic fine particles defined in the present invention were found to have better air permeability than the varnish compositions of the comparative examples, and it was confirmed that it was possible to produce polyimide porous membranes with spherical pores having a diameter equivalent to the median diameter of the organic fine particles in a uniform distribution. The polyimide porous membrane made from the varnish composition of Example 4 had uniform pore sizes on the surface and a generally uniform distribution of surface openings. The polyimide porous membranes made from the varnish compositions of Examples 1-3 had uniform pore sizes and a uniform distribution of surface openings, resulting in superior polyimide porous membranes.

Claims

1. A varnish composition for forming a polyimide porous film, obtained by mixing polyamic acid (A), organic fine particles (B), and a solvent (S), The organic fine particles (B) Structural unit (a) derived from vinyl monomers, A varnish composition comprising vinyl resin particles, which are a polymer having a structural unit (b1) derived from a compound represented by the following general formula (I) different from the aforementioned structural unit (a), wherein the polymer contains 0.1 to 2.0% by mass of the structural unit (b1) when the total structural units of the polymer are considered to be 100% by mass. 【Chemistry 1】 [In the formula, m represents an integer from 1 to 3. R represents a base represented by the following formula (i) or formula (ii). 【Chemistry 2】 (In the formula, R １ (where represents a hydrogen atom or a methyl group) AO represents an alkylene oxy group with 2 to 4 carbon atoms, and n represents an integer from 0 to 100. X represents a hydrogen atom, or -SO ３ M, -COOM and -PO ３ This represents an anionic hydrophilic group selected from the group consisting of M (wherein M represents an alkali metal atom, an alkaline earth metal atom, an ammonium group, or an organic ammonium group).

2. The varnish composition according to claim 1, wherein the structural unit (a) derived from the vinyl monomer includes a structural unit (a0) derived from a monofunctional vinyl monomer and a structural unit (a3) ​​derived from a polyfunctional vinyl monomer.

3. A varnish composition for forming a polyimide porous film, obtained by mixing polyamic acid (A), organic fine particles (B), and a solvent (S), The organic fine particles (B) Structural units (a0) derived from monofunctional vinyl monomers, Structural units (a3) ​​derived from polyfunctional vinyl monomers, Vinyl resin particles for manufacturing porous membranes, which are polymers having a structural unit (b0) derived from a reactive emulsifier having a radically polymerizable double bond, A varnish composition comprising vinyl resin particles for producing porous membranes, wherein the proportion of structural unit (a0) is 88 to 99% by mass, the proportion of structural unit (a3) ​​is 0.9 to 10% by mass, and the proportion of structural unit (b0) is 0.1 to 2% by mass.

4. A coating film forming step, comprising applying the varnish composition according to any one of claims 1 to 3 onto a substrate to form a coating film, A method for producing a precursor of a polyimide porous film, comprising a precursor formation step of removing the solvent (S) from the coated film to form a precursor of a polyimide porous film.

5. A method for producing a precursor film of a polyimide porous film according to claim 4, comprising a peeling step of peeling the precursor film from the substrate after the precursor film formation step.

6. A method for producing a precursor film of a polyimide porous membrane according to claim 5, comprising a winding step of winding the precursor film into a roll shape after the peeling step.

7. A method for producing a polyimide porous membrane, comprising: producing a precursor membrane of a polyimide porous membrane by the method described in any one of claims 4 to 6; and then a removal step of removing the organic fine particles (B) from the precursor membrane.

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