JP6404028B2 - Method for producing porous polyimide film, method for producing separator, and varnish - Google Patents

Description

  The present invention relates to a method for producing a porous polyimide film, a porous polyimide film, a separator comprising a porous polyimide film, and a varnish.

  Conventionally, various organic electrolyte secondary batteries are known as batteries capable of achieving high capacity, high voltage, and high energy density. In particular, in recent years, lithium ion secondary batteries (lithium ion batteries) in which lithium ions in the electrolyte are responsible for electrical conduction have been widely used.

  In these lithium ion batteries, an organic solvent such as ethylene carbonate and a lithium salt such as lithium hexafluorophosphate are used as the electrolyte, and between the positive electrode and the negative electrode arranged opposite to each other, ions between both electrodes are used. A separator made of a porous polymer that can be circulated is disposed. Here, as a material used for the separator, a film made of a polyolefin-based material such as a polyethylene film or a polypropylene film has been mainly used.

  These separators are required to have high heat resistance from the standpoint of safety, and to be thin from the viewpoint of weight reduction and space saving of the battery. However, the film made of the polyolefin-based material has not only poor heat resistance, but also has a problem that it is difficult to reduce the film while maintaining the required strength. Therefore, there is a possibility that inconvenience such as ignition may occur in the battery.

  On the other hand, a porous film made of an aromatic polyimide has substantially no melting point and high heat resistance, and has a high rigidity and is thin even when compared with a film made of a polyolefin-based material. Many studies have been made from the advantage of being able to reduce the thickness of the film because of its high strength (see Patent Documents 1 to 3).

JP 2007-2111136 A JP 2000-044719 A JP 2012-107144 A

  The porous film made of the aromatic polyimide may generate wrinkles and warpage depending on the film forming conditions. When a polyimide film having wrinkles is laminated on an electrode as a separator, there arises a problem that an unnecessary space is formed in the wrinkled portion.

  This invention is made | formed in view of the said situation, and it aims at providing the manufacturing method of the porous polyimide film which has sufficient intensity | strength, without generating a wrinkle.

  The present inventors have found that the film-forming property of the porous polyimide film can be improved by using a varnish containing a polyamic acid or polyimide and a fine particle solvent having a specific boiling point, and completed the present invention. It was.

  The first aspect of the present invention is a film forming step of an unfired composite film in which a varnish containing polyamic acid or polyimide, fine particles and a solvent is applied on a substrate and then dried to form a film, and the unfired composite film A peeling step of peeling off the substrate, a firing step of firing the unfired composite film after the peeling step to form a polyimide-fine particle composite film, and a fine particle removing step of removing fine particles from the polyimide-fine particle composite film A method for producing a porous polyimide film, wherein the solvent is a mixed solvent (S) containing a high boiling point solvent (S1) having a boiling point of 190 ° C. or higher at atmospheric pressure.

  The second aspect of the present invention is a porous polyimide membrane produced by the method of the first aspect of the present invention.

  The third aspect of the present invention is a separator comprising the porous polyimide film of the second aspect of the present invention.

  A fourth aspect of the present invention is a varnish containing polyamic acid or polyimide, fine particles and a solvent used in the method of the first aspect of the present invention, wherein the solvent has a boiling point of 190 ° C. or higher at atmospheric pressure. It is a mixed solvent (S) containing a high boiling point solvent (S1).

  According to the production method of the present invention, it is possible to provide a porous polyimide film having sufficient strength without generating wrinkles.

It is a figure which shows the influence of the varnish solvent with respect to the film | membrane intensity | strength of the porous polyimide film of this invention.

  Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments, and can be implemented with appropriate modifications within the scope of the object of the present invention. .

  The method for producing a porous polyimide film of the present invention comprises a step of forming an unfired composite film, in which a varnish containing polyamic acid or polyimide, fine particles and a solvent is applied on a substrate and then dried to form a film. A peeling step of peeling the fired composite film from the substrate, a firing step of firing the unfired composite film after the peeling step to form a polyimide-fine particle composite film, and a fine particle removing step of removing fine particles from the polyimide-fine particle composite film; , Wherein the solvent is a mixed solvent (S) containing a high boiling point solvent (S1) having a boiling point of 190 ° C. or higher at atmospheric pressure.

[Manufacture of varnish]
The varnish can be produced by mixing a solvent in which fine particles are dispersed in advance with polyamic acid or polyimide in an arbitrary ratio, or by polymerizing tetracarboxylic dianhydride and diamine in a solvent in which fine particles are previously dispersed to obtain polyamic acid. Further, imidization is performed to obtain polyimide. Finally, the viscosity is preferably 300 to 1500 cP, more preferably 400 to 700 cP. If the viscosity of the varnish is within this range, it is possible to form a film uniformly.

  In the varnish, resin fine particles and polyamic acid or polyimide can be mixed so that the fine particle / polyimide ratio is 1 to 3.5 (mass ratio) when the fine particles are baked into a polyimide-fine particle composite film. The fine particle / polyimide ratio is preferably 1.2 to 3 (mass ratio). Fine particles and polyamic acid or polyimide may be mixed so that the volume ratio of fine particles / polyimide is 1.5 to 4.5 when a polyimide-fine particle composite film is formed. The fine particle / polyimide ratio is more preferably 1.8 to 3 (volume ratio). If the fine particle / polyimide mass ratio or volume ratio is equal to or higher than the lower limit value, pores having an appropriate density can be obtained as a separator. It is possible to form a film stably without any problems. In the present specification, volume% and volume ratio are values at 25 ° C.

<Solvent>
The solvent used in the method for producing a porous polyimide membrane of the present invention is a high boiling point solvent (S1) having a boiling point of 190 ° C. or higher at atmospheric pressure and a mixed solvent (S) containing other solvents.

  As the high-boiling solvent (S1), a known solvent can be used without particular limitation as long as it has a boiling point at atmospheric pressure of 190 ° C. or higher and can dissolve polyamic acid or polyimide.

  The boiling point of the high boiling point solvent (S1) is preferably 190 ° C. or higher, and more preferably 200 ° C. or higher. The boiling point of the high boiling point solvent (S1) is preferably 250 ° C. or lower, and more preferably 230 ° C. or lower. If the boiling point of the high-boiling solvent (S1) is 190 ° C. or higher, the formed unfired composite film does not wrinkle and there is no problem. Moreover, if the boiling point of the high boiling point solvent (S1) is 250 ° C. or less, the prebaking conditions for the unfired composite film can be set within a good workability range.

  Examples of such high boiling point solvents (S1) include, for example, nitrogen-containing polar solvents such as N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone; γ-butyrolactone, γ-valerolactone, δ-valero Lactone polar solvents such as lactone, γ-caprolactone, and ε-caprolactone; Phenol solvents such as pyrrolidonephenol, o-, m- or p-cresol, xylenol, and catechol; Sulphoxide solvents such as diethyl sulfoxide . Among these, 1 type may be used and 2 or more types may be used together.

  Among these, it is preferable to use a nitrogen-containing polar solvent and / or a lactone polar solvent, and one of these may be used, or two or more may be used in combination.

  The mixed solvent (S) of the present invention contains a solvent (S2) having a boiling point of less than 190 ° C. in addition to the high boiling point solvent (S1). The solvent used here is not particularly limited as long as the boiling point at atmospheric pressure is less than 190 ° C., as long as it is compatible with the high boiling point solvent (S1) and dissolves polyamic acid or polyimide. Can be used.

  The boiling point of the solvent (S2) is preferably less than 190 ° C, and more preferably less than 170 ° C. Further, the boiling point of the solvent (S2) is preferably 60 ° C. or higher, and more preferably 80 ° C. or higher. If the boiling point of the solvent (S2) is less than 190 ° C., the prebaking conditions for the unfired composite film can be set within a good workability range. Moreover, if the boiling point of a solvent (S2) is 60 degreeC or more, the volatilization of the solvent in a varnish will be suppressed and the problem on a work environment can be improved.

  Examples of such a solvent (S2) include, for example, N, N-dimethylacetamide, N, N-diethylacetamide, N, N-dimethylformamide, N, N-diethylformamide, N, N, N ′, N Nitrogen-containing polar solvents such as' -tetramethylurea and N-vinyl-2-pyrrolidone; sulfoxide solvents such as dimethyl sulfoxide; acetonitrile; fatty acid esters such as ethyl lactate and butyl lactate; tetrahydrofuran, dioxane, dioxolane, diethylene glycol dimethyl ether, Examples include ethers such as diethylene glycol diethyl ether, methyl cellosolve acetate, and ethyl cellosolve acetate. Among these, 1 type may be used and 2 or more types may be used together.

  The content ratio of the high boiling point solvent (S1) and the solvent (S2) having a boiling point of less than 190 ° C. in the mixed solvent (S) is not particularly limited, and a wide range of solvents can be used. In the mixed solvent (S), the high boiling point solvent (S1) is preferably 5 to 80% by mass. It is more preferable to set it as 7-70 mass%, and it is most preferable to set a high boiling point solvent (S1) as 10-60 mass%. If the high-boiling solvent (S1) is contained in an amount of 5% by mass or more, there is no problem in the film formability and film properties of the porous polyimide film, and if S1 is 80% by mass or less, the coating and prebaking steps can be performed without any problem. It can be carried out.

  Of all the components in the varnish, the content of the mixed solvent (S) is preferably 50 to 95% by mass, more preferably 60 to 85% by mass. The solid content concentration in the varnish is preferably 5 to 50% by mass, more preferably 15 to 40% by mass.

<Fine particles>
The material of the fine particles used in the present invention is not particularly limited as long as it is insoluble in the organic solvent used in the varnish and can be selectively removed after film formation. For example, inorganic materials include metal oxides such as silica (silicon dioxide), titanium oxide, and alumina (Al ₂ O ₃ ), and organic materials include high molecular weight olefins (polypropylene, polyethylene, etc.), polystyrene, acrylic resins ( Examples thereof include organic polymer fine particles (resin fine particles) such as methyl methacrylate, isobutyl methacrylate, polymethyl methacrylate (PMMA), epoxy resin, cellulose, polyvinyl alcohol, polyvinyl butyral, polyester, and polyether.

  As for the inorganic material, silica such as colloidal silica is preferably used in the production of the porous polyimide film. Among them, it is preferable to select monodispersed spherical silica particles in order to form uniform and fine pores.

The resin fine particles used in the present invention can be selected from, for example, ordinary linear polymers and known depolymerizable polymers without particular limitation depending on the purpose. A normal linear polymer is a polymer in which polymer molecular chains are randomly cut during thermal decomposition, and a depolymerizable polymer is a polymer in which the polymer is decomposed into monomers during thermal decomposition. Any of them can be removed from the polyimide film by decomposition to a monomer, a low molecular weight substance, or CO ₂ at the time of heating. The decomposition temperature of the resin fine particles used is preferably 200 to 320 ° C, more preferably 230 to 260 ° C. When the decomposition temperature is 200 ° C. or higher, film formation can be performed even when a high-boiling solvent is used for the varnish, and the range of selection of the baking conditions for polyimide is widened. If the decomposition temperature is less than 320 ° C., only the resin fine particles can be lost without causing thermal damage to the polyimide.

  Among these depolymerizable polymers, methyl methacrylate or isobutyl methacrylate alone (polymethyl methacrylate or polyisobutyl methacrylate) having a low thermal decomposition temperature, or a copolymer having a main component thereof is preferable for handling during pore formation. .

  The fine particles used in the present invention preferably have a high sphericity and a small particle size distribution index. The fine particles having these conditions are excellent in dispersibility in the varnish and can be used in a state where they do not aggregate with each other. As the particle diameter (average diameter) of the fine particles to be used, for example, those having a particle diameter of 100 to 2000 nm can be used. By satisfying these conditions, the pore diameter of the porous film obtained by removing the fine particles can be made uniform, so that the applied electric field can be made uniform, particularly when used as a separator.

  In the production method described later, when the unfired composite film is formed as a two-layer unfired composite film, the fine particles (B1) used for the first varnish and the fine particles (B2) used for the second varnish are the same. A thing different from each other may be used. In order to make the holes on the side in contact with the substrate more dense, the fine particles of (B1) preferably have a smaller or the same particle size distribution index as the fine particles of (B2). Alternatively, it is preferable that the fine particles of (B1) have a smaller sphericity or the same as the fine particles of (B2). The fine particles of (B1) preferably have a smaller particle size (average diameter) than the fine particles of (B2), and in particular, (B1) is 100 to 1000 nm (more preferably 100 to 600 nm), (B2 ) Is preferably 500 to 2000 nm (more preferably 700 to 2000 nm). By using a particle size smaller than (B2) as the particle size of the fine particles of (B1), the opening ratio of the pores on the surface of the obtained porous polyimide film can be made high and uniform, and the entire porous polyimide film is ( The strength of the film can be increased as compared with the case of B1).

<Polyamide acid>
As the polyamic acid used in the present invention, those obtained by polymerizing any tetracarboxylic dianhydride and diamine can be used without any particular limitation. Although the usage-amount of tetracarboxylic dianhydride and diamine is not specifically limited, It is preferable to use 0.50-1.50 mol of diamine with respect to 1 mol of tetracarboxylic dianhydrides, and 0.60-1. It is more preferable to use 30 mol, and it is particularly preferable to use 0.70 to 1.20 mol.

  The tetracarboxylic dianhydride can be appropriately selected from tetracarboxylic dianhydrides conventionally used as raw materials for synthesizing polyamic acid. The tetracarboxylic dianhydride may be an aromatic tetracarboxylic dianhydride or an aliphatic tetracarboxylic dianhydride. From the viewpoint of the heat resistance of the resulting polyimide resin, the aromatic tetracarboxylic dianhydride may be used. Preference is given to using carboxylic dianhydrides. Tetracarboxylic dianhydride may be used in combination of two or more.

  Preferable specific examples of the aromatic tetracarboxylic dianhydride include pyromellitic dianhydride, 1,1-bis (2,3-dicarboxyphenyl) ethane dianhydride, bis (2,3-dicarboxy). Phenyl) methane dianhydride, bis (3,4-dicarboxyphenyl) methane dianhydride, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride, 2,3,3 ′, 4′- Biphenyltetracarboxylic dianhydride, 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-dicarboxy Enyl) -1,1,1,3,3,3-hexafluoropropane dianhydride, 3,3 ′, 4,4′-benzophenone tetracarboxylic dianhydride, bis (3,4-dicarboxyphenyl) Ether dianhydride, bis (2,3-dicarboxyphenyl) ether dianhydride, 2,2 ′, 3,3′-benzophenonetetracarboxylic dianhydride, 4,4- (p-phenylenedioxy) diphthal Acid dianhydride, 4,4- (m-phenylenedioxy) diphthalic dianhydride, 1,2,5,6-naphthalene tetracarboxylic dianhydride, 1,4,5,8-naphthalene tetracarboxylic acid di Anhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,2,3,4-benzenetetracarboxylic dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride , 2, 3 6,7-anthracenetetracarboxylic dianhydride, 1,2,7,8-phenanthrenetetracarboxylic dianhydride, 9,9-bisphthalic anhydride fluorene, 3,3 ′, 4,4′-diphenylsulfone Tetracarboxylic dianhydride etc. are mentioned. Examples of the aliphatic tetracarboxylic dianhydride include ethylene tetracarboxylic dianhydride, butane tetracarboxylic dianhydride, cyclopentane tetracarboxylic dianhydride, cyclohexane tetracarboxylic dianhydride, 1, 2,4,5-cyclohexanetetracarboxylic dianhydride, 1,2,3,4-cyclohexanetetracarboxylic dianhydride, and the like. Among these, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride and pyromellitic dianhydride are preferable from the viewpoint of price and availability. These tetracarboxylic dianhydrides can be used alone or in combination of two or more.

  The diamine can be appropriately selected from diamines conventionally used as a raw material for synthesizing polyamic acid. This diamine may be an aromatic diamine or an aliphatic diamine, but an aromatic diamine is preferred from the viewpoint of the heat resistance of the resulting polyimide resin. These diamines may be used in combination of two or more.

  Examples of the aromatic diamine include diamino compounds in which one phenyl group or about 2 to 10 phenyl groups are bonded. Specifically, phenylenediamine and derivatives thereof, diaminobiphenyl compounds and derivatives thereof, diaminodiphenyl compounds and derivatives thereof, diaminotriphenyl compounds and derivatives thereof, diaminonaphthalene and derivatives thereof, aminophenylaminoindane and derivatives thereof, diaminotetraphenyl Compounds and derivatives thereof, diaminohexaphenyl compounds and derivatives thereof, and cardo-type fluorenediamine derivatives.

  Phenylenediamine is m-phenylenediamine, p-phenylenediamine, etc., and phenylenediamine derivatives include diamines to which alkyl groups such as methyl group and ethyl group are bonded, such as 2,4-diaminotoluene, 2,4-triphenylene. Diamines and the like.

  The diaminobiphenyl compound is a compound in which two aminophenyl groups are bonded to each other. For example, 4,4'-diaminobiphenyl, 4,4'-diamino-2,2'-bis (trifluoromethyl) biphenyl, and the like.

  The diaminodiphenyl compound is a compound in which two aminophenyl groups are bonded to each other via other groups. The bond is an ether bond, a sulfonyl bond, a thioether bond, a bond by alkylene or a derivative group thereof, an imino bond, an azo bond, a phosphine oxide bond, an amide bond, a ureylene bond, or the like. The alkylene bond has about 1 to 6 carbon atoms, and the derivative group has one or more hydrogen atoms in the alkylene group substituted with halogen atoms or the like.

  Examples of diaminodiphenyl compounds include 3,3′-diaminodiphenyl ether, 3,4′-diaminodiphenyl ether, 4,4′-diaminodiphenyl ether, 3,3′-diaminodiphenyl sulfone, 3,4′-diaminodiphenyl sulfone, 4,4'-diaminodiphenylsulfone, 3,3'-diaminodiphenylmethane, 3,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl ketone 3,4′-diaminodiphenyl ketone, 2,2-bis (p-aminophenyl) propane, 2,2′-bis (p-aminophenyl) hexafluoropropane, 4-methyl-2,4-bis ( p-aminophenyl) -1-pentene, 4-methyl-2 4-bis (p-aminophenyl) -2-pentene, iminodianiline, 4-methyl-2,4-bis (p-aminophenyl) pentane, bis (p-aminophenyl) phosphine oxide, 4,4 ′ -Diaminoazobenzene, 4,4'-diaminodiphenylurea, 4,4'-diaminodiphenylamide, 1,4-bis (4-aminophenoxy) benzene, 1,3-bis (4-aminophenoxy) benzene, 1, 3-bis (3-aminophenoxy) benzene, 4,4′-bis (4-aminophenoxy) biphenyl, bis [4- (4-aminophenoxy) phenyl] sulfone, bis [4- (3-aminophenoxy) phenyl ] Sulfone, 2,2-bis [4- (4-aminophenoxy) phenyl] propane, 2,2-bis [4- (4-aminophen ) Phenyl] hexafluoropropane, and the like.

  Among these, p-phenylenediamine, m-phenylenediamine, 2,4-diaminotoluene, and 4,4'-diaminodiphenyl ether are preferable from the viewpoint of price and availability.

  In the diaminotriphenyl compound, two aminophenyl groups and one phenylene group are both bonded via other groups, and the other groups are the same as those of the diaminodiphenyl compound. Examples of diaminotriphenyl compounds include 1,3-bis (m-aminophenoxy) benzene, 1,3-bis (p-aminophenoxy) benzene, 1,4-bis (p-aminophenoxy) benzene, and the like. be able to.

  Examples of diaminonaphthalene include 1,5-diaminonaphthalene and 2,6-diaminonaphthalene.

  Examples of aminophenylaminoindane include 5 or 6-amino-1- (p-aminophenyl) -1,3,3-trimethylindane.

  Examples of diaminotetraphenyl compounds include 4,4′-bis (p-aminophenoxy) biphenyl, 2,2′-bis [p- (p′-aminophenoxy) phenyl] propane, 2,2′-bis [ p- (p′-aminophenoxy) biphenyl] propane, 2,2′-bis [p- (m-aminophenoxy) phenyl] benzophenone, and the like.

  Examples of the cardo type fluorenediamine derivative include 9,9-bisaniline fluorene.

  The aliphatic diamine has, for example, about 2 to 15 carbon atoms, and specific examples include pentamethylenediamine, hexamethylenediamine, and heptamethylenediamine.

  A compound in which the hydrogen atom of these diamines is substituted with at least one substituent selected from the group such as a halogen atom, a methyl group, a methoxy group, a cyano group, and a phenyl group may be used.

  There is no restriction | limiting in particular in the means to manufacture the polyamic acid used by this invention, For example, well-known methods, such as the method of making an acid and a diamine component react in an organic solvent, can be used.

  Reaction of tetracarboxylic dianhydride and diamine is normally performed in an organic solvent. The organic solvent used for the reaction of the tetracarboxylic dianhydride and the diamine is particularly capable of dissolving the tetracarboxylic dianhydride and the diamine and not reacting with the tetracarboxylic dianhydride and the diamine. It is not limited. An organic solvent can be used individually or in mixture of 2 or more types. Among the organic solvents, N-methyl-2-pyrrolidone, N, N-dimethylacetamide, N, N-diethylacetamide, N, N-dimethylformamide, N, N-diethylformamide are used because of the solubility of the resulting polyamic acid. Nitrogen-containing polar solvents such as N-methylcaprolactam and N, N, N ′, N′-tetramethylurea are preferably used. Although there is no restriction | limiting in particular in the usage-amount of an organic solvent, It is desirable for content of the polyamic acid to produce | generate to be 5-50 mass%.

  The polymerization temperature is generally -10 to 120 ° C, preferably 5 to 30 ° C. The polymerization time varies depending on the raw material composition to be used, but is usually 3 to 24 Hr (hour). Further, the intrinsic viscosity of the polyamic acid solution obtained under such conditions is preferably in the range of 1000 to 100,000 cP (centipoise), and more preferably in the range of 5000 to 70000 cP.

<Polyimide>
The polyimide used in the present invention is not limited to its structure and molecular weight as long as it is a soluble polyimide that can be dissolved in the organic solvent used in the varnish of the present invention. About a polyimide, you may have a functional group which accelerates | stimulates a crosslinking reaction etc. at the time of baking, or a functional group which can be condensed, such as a carboxy group.

  Use of a monomer to introduce a flexible bending structure into the main chain in order to obtain a polyimide soluble in an organic solvent, such as ethylenediamine, hexamethylenediamine, 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, Aliphatic diamines such as 4,4′-diaminodicyclohexylmethane; 2-methyl-1,4-phenylenediamine, o-tolidine, m-tolidine, 3,3′-dimethoxybenzidine, 4,4′-diaminobenzanilide, etc. Aromatic diamines; polyoxyalkylene diamines such as polyoxyethylene diamine, polyoxypropylene diamine, polyoxybutylene diamine; polysiloxane diamines; 2,3,3 ′, 4′-oxydiphthalic anhydride, 3,4,3 ′, 4′-oxydiphthalic anhydride, 2,2-bis (4- Rokishifeniru) propane dibenzoate-3,3 ', use of such 4,4'-tetracarboxylic dianhydride is valid. In addition, use of a monomer having a functional group that improves solubility in an organic solvent, for example, 2,2′-bis (trifluoromethyl) -4,4′-diaminobiphenyl, 2-trifluoromethyl-1,4 -It is also effective to use a fluorinated diamine such as phenylenediamine. Furthermore, in addition to the monomer for improving the solubility of the polyimide, the same monomers as those described in the column for the polyamic acid can be used in combination as long as the solubility is not inhibited.

There is no restriction | limiting in particular in the means to manufacture the polyimide which can be melt | dissolved in the organic solvent used by this invention, For example, well-known methods, such as the method of making a polyamic acid chemically imidize or heat-imidize and dissolve in an organic solvent, are used. be able to. Examples of such polyimide include aliphatic polyimide (total aliphatic polyimide), aromatic polyimide and the like, and aromatic polyimide is preferable. The aromatic polyimide is obtained by thermally or chemically obtaining a polyamic acid having a repeating unit represented by the formula (1) by a ring-closing reaction or by dissolving a polyimide having a repeating unit represented by the formula (2) in a solvent. Good. In the formula, Ar represents an aryl group.

  In the production method described later, when the unfired composite film is formed as a two-layer unfired composite film, the volume ratio of polyamic acid or polyimide (A1) and fine particles (B1) in the first varnish is 19:81. It is preferable to set it to -45: 65. If the fine particle volume is 65 or more when the total volume is 100, the particles are uniformly dispersed. If the particle volume is within 81, the particles are dispersed without agglomeration. It can be formed uniformly. Moreover, it is preferable that the volume ratio of polyamic acid or polyimide (A2) and fine particles (B2) in the second varnish is 20:80 to 50:50. If the fine particle volume is 50 or more when the total volume is 100, the single particles are uniformly dispersed, and if they are within 80, the particles do not aggregate with each other, and cracks and the like may occur on the surface. Therefore, a porous polyimide film having good electrical characteristics can be formed stably.

  Regarding the above volume ratio, the second varnish preferably has a lower content of fine particles than the first varnish, and by satisfying the above conditions, the fine particles are highly filled in the polyamic acid or polyimide. Even if it is made, the intensity | strength and the softness | flexibility of an unbaking composite film, a polyimide fine particle composite film, and a porous polyimide film can be ensured. Further, by providing a layer having a low fine particle content ratio, the film manufacturing cost can be reduced.

  In addition to the above-mentioned components, for the purpose of antistatic, imparting flame retardancy, low-temperature firing, etc., known components such as antistatic agents, flame retardants, chemical imidizing agents, condensing agents, etc. Can be contained in the varnish.

  Further, for the purpose of uniformly dispersing the fine particles in the varnish, a dispersant may be further added together with the fine particles. By adding the dispersant, the polyamic acid or polyimide and the fine particles can be mixed more uniformly, so that the front and back surfaces can be efficiently communicated with each other, and the air permeability of the separator made of the porous polyimide film or the porous polyimide film can be obtained. The degree is improved.

  The dispersing agent used by this invention is not specifically limited, A well-known thing can be used. For example, palm fatty acid salt, castor sulfate oil salt, lauryl sulfate salt, polyoxyalkylene allyl phenyl ether sulfate salt, alkylbenzene sulfonic acid, alkylbenzene sulfonate, alkyl diphenyl ether disulfonate, alkylnaphthalene sulfonate, dialkyl sulfosuccinate Anionic surfactants such as nate salt, isopropyl phosphate, polyoxyethylene alkyl ether phosphate salt, polyoxyethylene allyl phenyl ether phosphate salt; oleylamine acetate, lauryl pyridinium chloride, cetyl pyridinium chloride, lauryl trimethyl ammonium chloride, stearyl trimethyl ammonium chloride , Behenyltrimethylammonium chloride, didecyldimethyl Cationic surfactants such as ammonium chloride; amphoteric surfactants such as coconut alkyldimethylamine oxide, fatty acid amidopropyldimethylamine oxide, alkylpolyaminoethylglycine hydrochloride, amidobetaine type activator, alanine type activator, lauryliminodipropionic acid Agent: polyoxyethylene octyl ether, polyoxyethylene decyl ether, polyoxyethylene lauryl ether, polyoxyethylene lauryl amine, polyoxyethylene oleylamine, polyoxyethylene polystyryl phenyl ether, polyoxyalkylene polystyryl phenyl ether, polyoxy Nonionic surfactant of alkylene primary alkyl ether or polyoxyalkylene secondary alkyl ether, polyoxyethylene dilaur Polyoxyethylene laurate, polyoxyethylenated castor oil, polyoxyethylenated hydrogenated castor oil, sorbitan lauric acid ester, polyoxyethylene sorbitan lauric acid ester, fatty acid diethanolamide and other polyoxyalkylene nonions Surfactants; fatty acid alkyl esters such as octyl stearate and trimethylolpropane tridecanoate; and polyether polyols such as polyoxyalkylene butyl ether, polyoxyalkylene oleyl ether and trimethylolpropane tris (polyoxyalkylene) ether However, it is not limited to these. Moreover, the said dispersing agent can also be used in mixture of 2 or more types.

[Production of unfired composite film]
-Film formation step of unfired composite film The unfired composite film containing polyamic acid or polyimide and fine particles is formed by applying the above varnish on the substrate and performing at 50 to 100 ° C under normal pressure or vacuum. . This is called pre-baking. A release layer may be provided on the substrate as necessary. Prebaking conditions are preferably 60 to 95 ° C. under normal pressure, more preferably 65 to 90 ° C. under normal pressure. If the pre-baking temperature condition is 50 ° C. or higher, the solvent evaporates within 30 minutes, so there is no problem in workability, and if it is 100 ° C. or lower, the film strength of the final product is sufficient.

  Further, when forming as a two-layer unfired composite film, first, the first varnish is applied as it is on a substrate such as a glass substrate, and 0 to 100 ° C. (preferably 0 to 90 ° C.) under normal pressure or vacuum. C.), more preferably 10 to 100 [deg.] C. (more preferably 10 to 90 [deg.] C.) under normal pressure to form a first unfired composite film having a thickness of 1 to 5 [mu] m.

  Then, said 2nd varnish is apply | coated on the formed 1st unbaking composite film | membrane, 0-80 degreeC (preferably 0-50 degreeC) similarly, More preferably 10-80 degreeC (normal pressure) More preferably, drying is performed at 10 to 30 ° C. to form a second unfired composite film having a film thickness of 5 to 30 μm to obtain a two-layer unfired composite film.

-Peeling process for peeling the unfired composite film from the substrate After forming the unfired composite film, the unfired composite film is peeled from the substrate. The peeling method is not particularly limited, and may be automatically performed using a manipulator or the like, or may be performed manually.

  The varnish according to the present invention includes a mixed solvent (S) containing a high boiling point solvent (S1) having a boiling point of 190 ° C. or higher at atmospheric pressure, and is a non-fired composite film (or a two-layer unfired composite film, the same shall apply hereinafter). Since it is excellent in film formability and properties, it is possible to omit a step of providing a release layer in advance on the substrate and a step of cleaning the release agent when a release agent is used, and the generation of wrinkles can be suppressed.

[Production of polyimide-fine particle composite film (post-bake process)]
The unfired composite film after the film formation / peeling step can be post-treated by heating (post-bake) to form a composite film (polyimide-fine particle composite film) composed of polyimide and fine particles. In this post-baking step, it is preferable to complete imidization.

  The post-baking temperature varies depending on the structure of the polyamic acid or polyimide contained in the unfired composite film or the presence or absence of a condensing agent, but is preferably 120 to 375 ° C, more preferably 150 to 350 ° C.

  For example, when firing at 375 ° C., the temperature is raised from room temperature to 375 ° C. in 3 hours and then held at 375 ° C. for 20 minutes, or stepwise from the room temperature in increments of 50 ° C. It is also possible to use a stepwise drying-thermal imidization method such as raising the temperature to 0 ° C. (retaining each step for 20 minutes) and finally retaining at 375 ° C. for 20 minutes. In particular, it is more preferable to perform post-baking by stepwise temperature increase in order to improve the film strength. Further, at the time of post-baking, a method may be adopted in which the end of the unfired composite film is fixed to a SUS mold or the like to prevent deformation.

  The film thickness of the completed polyimide-fine particle composite film can be obtained by measuring and averaging the thickness of a plurality of locations with, for example, a micrometer. What average film thickness is preferable depends on the use of the polyimide-fine particle composite film or the porous polyimide film, but for example, when used for a separator or the like, it is preferably 5 to 500 μm, and 10 to 100 μm. More preferably it is.

[Particle removal step (Porosification of polyimide-particle composite film)]
By removing fine particles from the polyimide-fine particle composite film by selecting an appropriate method, a porous polyimide film having fine pores can be produced with good reproducibility. For example, when silica is employed as the fine particles, the polyimide-fine particle composite film can be made porous by dissolving and removing silica with a low concentration of hydrogen fluoride water (HF) or the like. Further, when the fine particles are resin fine particles, the resin fine particles can be removed by heating to a temperature not lower than the thermal decomposition temperature of the resin fine particles as described above and lower than the thermal decomposition temperature of the polyimide.

[Use of porous polyimide membrane]
The porous polyimide membrane produced by the production method of the present invention can be used as a fuel cell electrolyte membrane, a gas or liquid separation membrane, and a low dielectric constant material, in addition to a lithium ion battery separator.

EXAMPLES Hereinafter, although an Example is shown and this invention is demonstrated further more concretely, the scope of the present invention is not limited to these Examples. In addition, Example 1 and 2 shall be read as a reference example.

In the examples and comparative examples, the following polyamic acid, organic solvent, and fine particles were used. Unless otherwise specified, the proportion of the total organic solvent in the varnishes used in Examples and Comparative Examples was adjusted to 70% by mass.
Polyamic acid: Reaction product of pyromellitic dianhydride as tetracarboxylic dianhydride and 4,4′-diaminodiphenyl ether as diamine Organic solvent: N, N-dimethylacetamide (DMAc: boiling point 166 ° C. )
N, N, N ′, N′-tetramethylurea (TMU: boiling point 177 ° C.)
N-methyl-2-pyrrolidone (NMP: boiling point 202 ° C.)
γ-butyrolactone (GBL: boiling point 204 ° C.)
Fine particles: Silica (1): 700 nm silica
Silica (2): 200 nm silica / dispersant: polyoxyethylene secondary alkyl ether dispersant

<Example 1>
[Preparation of varnish]
A polyamic acid solution was obtained by adjusting 13.25 g of polyamic acid and a mixed solvent. To the obtained polyamic acid solution, 53 g of fine particles (silica (1)) were added and stirred to prepare a varnish.

  Here, NMP was selected as the high boiling point solvent (S1) and DMAc was selected as the other solvent (S2), and the mixture was mixed at DMAc: NMP = 60: 40 to obtain a mixed solvent.

<Example 2>
Example 2 was made in the same manner as Example 1 except that GBL was used as the high boiling point solvent (S1) during the preparation of the varnish.

<Comparative Example 1>
Comparative Example 1 was made in the same manner as in Example 1 except that only the DMAc was used without using the mixed solvent when preparing the varnish.

<Comparative example 2>
Comparative Example 2 was made in the same manner as Example 1 except that a solvent mixed with DMAc: TMU = 60: 40 was used when preparing the varnish.

[Deposition of unfired composite film and evaluation of peelability]
Each of the varnishes was formed into a film using an applicator on a glass plate coated with a release agent. Prebaking was performed under the conditions shown in Table 1 to produce an unfired composite film having a thickness of 25 μm. The unfired composite film was peeled off from the substrate and dried. Moreover, the peelability at this time was evaluated according to the following criteria and shown in Table 1. In Comparative Example 2, ◎ ... peels off naturally from the base material ○ ... peels off when pulled from the base material △ ... part is cut when peeled off from the base material

[Evaluation of film formability of unfired composite film]
Table 1 shows prebaking conditions and characteristics of the unfired composite film obtained above. An overview of the dried unfired composite film was evaluated according to the following evaluation criteria and shown in Table 1.
◎ ・ ・ ・ No wrinkles,
○ ・ ・ ・ Slight wrinkles at the end but no wrinkles △ ・ ・ ・ Slight wrinkles are seen × ・ ・ ・ Many wrinkles are seen

  As is clear from Table 1, when a mixed solvent containing a high-boiling solvent having a boiling point of 190 ° C. or higher is used for the varnish, a uniform and good film can be obtained without generation of wrinkles or the like. did it. In Examples 1 and 2, the peelability was also good.

[Imidization of unfired composite film]
The unbaked composite films of Examples 1 and 2 and Comparative Examples 1 and 2 that were pre-baked at 70 ° C. were further subjected to heat treatment (post-bake) at 320 ° C. for 15 minutes to complete imidization. .

[Formation of porous polyimide film]
The polyimide-fine particle composite film obtained above was immersed in a 10% HF solution for 10 minutes to remove fine particles contained in the film, and then washed with water and dried to obtain a porous polyimide film.

[Strength evaluation of porous polyimide film]
A strip-shaped test piece having a size of 10 mm × 50 mm is cut out from the porous polyimide film obtained above, and is pulled using a tensile tester (manufactured by A & D Co., Ltd.) at a tensile speed of 2 mm / min and a distance between chucks of 20 mm. The strength was measured. The result is shown in FIG.

  From FIG. 1, the membrane strength of the porous polyimide membranes of Examples 1 and 2 produced using a mixed solvent containing a high boiling point solvent (S1) having a boiling point of 190 ° C. or higher does not use the high boiling point solvent (S1). It was confirmed that it was not inferior to Comparative Examples 1 and 2 formed by a conventional manufacturing method.

<Evaluation of two-layer porous polyimide film>
[Preparation of Varnish-1]
In [Preparation of Varnish], the mixing ratio of the mixed solvent to be used was changed to the mass ratio shown in Table 2, and instead of silica (1), 75 g of silica (2) was used, and each varnish (first varnish) was It was adjusted. The volume ratio of polyamic acid to silica (2) in the polyamic acid solution is 22:78 (mass ratio is 15:85).

[Preparation of Varnish-2]
In [Preparation of varnish], each varnish (second varnish) was adjusted with the mixing ratio of the mixed solvent to be used as the mass ratio in Table 2. The volume ratio of polyamic acid to silica (1) in the polyamic acid solution is 28:72 (mass ratio is 20:80).

[Formation of polyimide-fine particle composite film]
The first varnish was formed on a glass plate using an applicator and then baked at 70 ° C. for 1 minute to obtain a first unfired film having a thickness of about 2 μm. Since the first layer is thin and the shrinkage stress to the substrate is weak, it does not peel off naturally.

  Subsequently, after the second varnish was formed on the first unfired film using an applicator, it was pre-baked at 70 ° C. for 5 minutes to form a two-layer unfired composite having an overall film thickness of about 25 μm. A film was formed. The obtained two-layer unfired composite film was evaluated for peelability and film formability according to the same criteria as described above. The results are shown in Table 2.

  As can be seen from Table 2, when a mixed solvent containing a high boiling point solvent having a boiling point of 190 ° C. or higher is used for the varnish, wrinkles and the like are generated even when a two-layer unfired composite film is formed. A uniform and good film was not obtained. In addition, Examples 4 to 9 had good peelability.

<Evaluation of dispersant-containing porous polyimide membrane>
<Example 10>
A single-layer porous polyimide film was obtained in the same manner as in Example 1 except that the second varnish in Example 5 was used and a film was formed on a PET film using an applicator.
<Example 11>
Furthermore, a single-layer porous polyimide film was obtained in the same manner as in Example 10 except that 0.5% by mass of a dispersant was added to the varnish with respect to the contained silica.

The film formability and peelability of Examples 10 and 11 and the following Gurley air permeability were evaluated. The results are shown in Table 3.
<Evaluation of air permeability>
・ [Gurley air permeability]
A sample having a thickness of about 25 μm was cut into a 5 cm square with respect to the porous polyimide films of Examples 10 and 11. Using a Gurley type densometer (manufactured by Toyo Seiki), the time required for 100 ml of air to pass through the sample was measured according to JIS P 8117.

The polyimide-fine particle composite films of Example 10 and Example 11 were all good in film formability and peelability, and no difference was observed depending on the presence or absence of a dispersant. On the other hand, the air permeability of both porous polyimide films was smaller for the porous polyimide film of Example 11 than for the porous polyimide film of Example 10. This is because the silica fine particles in the varnish are highly dispersed by the action of the dispersant, so that the silica in the polyimide-fine particle composite film formed using the varnish is uniformly dispersed and finally obtained. It shows that the connectivity of both sides of the porous polyimide film has been improved.

Claims (4)

After applying a varnish containing polyamic acid or polyimide, fine particles and a solvent on a substrate, and drying to form a film, a film forming step of an unfired composite film,
A peeling step of peeling the green composite film from the substrate;
A firing step of firing the unfired composite film after the peeling step to form a polyimide-fine particle composite film;
A fine particle removal step of removing fine particles from the polyimide-fine particle composite film;
I have a,
Before applying the varnish on the substrate in the film forming step, the method for producing a porous polyimide film does not include a step of previously providing a release layer on the substrate ,
The solvent is a mixed solvent (S) containing a high boiling point solvent (S1) having a boiling point of 190 ° C. or higher at atmospheric pressure,
A method for producing a porous polyimide film, wherein the high boiling point solvent (S1) contains a lactone polar solvent.   The manufacturing method of the porous polyimide membrane of Claim 1 whose content of the said high boiling point solvent (S1) in the said mixed solvent (S) is 5-80 mass%. A method for producing a separator including a porous polyimide film, the method comprising producing the porous polyimide film by the method for producing a porous polyimide film according to claim 1 or 2 . A varnish containing polyamic acid or polyimide, fine particles and a solvent used in the production method according to claim 1 or 2 , wherein the solvent is a high boiling point solvent (S1) having a boiling point of 190 ° C or higher at atmospheric pressure. A varnish containing a mixed solvent (S), wherein the high boiling point solvent (S1) contains a lactone polar solvent.

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