JPS621616B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a porous membrane for fluid separation made of a specific aromatic polyimide obtained by polymerizing a monomer whose main components are a biphenyltetracarboxylic acid component and an aromatic diamine component, and A polyimide composition in which aromatic polyimide is uniformly dissolved in a specific mixed solvent of a phenolic solvent in which the polymer is soluble and an aromatic solvent in which the polymer is insoluble is used as a dope liquid, and by a dry method, The present invention relates to a method of manufacturing a polyimide porous membrane. The polyimide porous membrane of the present invention is a heat-resistant gas separation material that can be used to separate and concentrate various mixed gases, or a solution that can separate and concentrate various mixed solutions by ultrafiltration. It is a separation material, and can be suitably used for separating and concentrating a mixed gas of hydrogen and carbon monoxide, for example, and is a composite in which a homogeneous membrane layer with gas separation performance is provided on this polyimide porous membrane. It can also be used as a material for type separation materials. Conventionally, as a separation membrane made of aromatic polyimide that can be used as a separation material for gases or solutions, at most a homogeneous membrane of aromatic polyimide obtained from pyromellitic acid and an aromatic diamine is known. However, the known homogeneous membrane of aromatic polyimide is produced by first producing polyamic acid of pyromellitic acid and aromatic diamine, and using a solution of the polyamic acid as a dope solution. Because uniform membranes were manufactured by performing membrane and imidization simultaneously or sequentially, the quality was extremely unstable.
The manufacturing method itself had become complicated. Further, known homogeneous membranes of aromatic polyimide clearly have the drawback of low permeation rates for hydrogen, carbon monoxide, and the like. In the past, aromatic polyimide porous membranes had been expected to emerge because they were expected to have good heat resistance and a high gas or liquid permeation rate. Only homogeneous membranes of polyimides of the group polyimides were known, and no porous membranes of aromatic polyimides had been specifically proposed, much less the method for producing them. It was. As a result of intensive research on heat-resistant porous membranes made of aromatic polyimide and methods for producing them, the inventors discovered a specific membrane obtained from monomers whose main components are biphenyltetracarboxylic acid and aromatic diamine. If a polyimide composition in which aromatic polyimide is uniformly dissolved in a specific mixed solvent of a phenolic solvent in which the polymer is soluble and an aromatic solvent in which the polymer is insoluble is used as a dope solution, the aroma can be produced by a dry method. The present invention was completed based on the discovery that a polyimide porous membrane of the group polyimide group can be produced. That is, the present invention is based on the general formula (However, R is a divalent aromatic residue excluding the amino group of an aromatic diamine.) Regarding a polyimide porous membrane for gas separation made of an aromatic polyimide having 90% or more of repeating units represented by: It is. Further, the present invention provides a base solvent 100 in which the aromatic polyimide is made of (a) a phenolic compound capable of dissolving the polyimide in an amount of 5% by weight or more;
and (b) an additive solvent 5 to 5 consisting of an aromatic solvent that does not dissolve 3% by weight or more of the polyimide and has a slower evaporation rate from the thin film of the dope during heating drying during film formation than the base solvent. A polyimide composition uniformly dissolved at a polymer concentration of about 3 to 30% by weight in a uniform mixed solvent with 150 parts by weight is used as a dope solution, and a thin film is formed with the dope solution. It relates to a method for producing a porous polyimide membrane, characterized in that each solvent is gradually removed by heating and drying at about 50 to 400°C. The aromatic polyimide porous membrane of the present invention is a heat-resistant separation membrane that has an excellent gas or liquid permeation rate and can separate and concentrate components of a mixed gas or mixed solution, and is particularly useful as a gas separation membrane. Are better. The aromatic polyimide porous membrane of the present invention has excellent heat resistance, chemical resistance, and mechanical performance. The method of the present invention uses a polyimide composition in which aromatic polyimide is uniformly dissolved in a specific mixed solvent as a dope liquid, and forms a film by a dry method by heating and drying, thereby producing aromatic polyimide with good reproducibility. This is a method that can produce porous membranes. Generally, when producing a gas-separating porous membrane from a polymer solution, a coagulating liquid is used to form a coagulating membrane and then drying. There is no need to use , and in that respect it is very practical. In the method of the present invention, flat membrane-like products can be suitably produced, but hollow fiber-like products can also be produced. The aromatic polyimide forming the polyimide porous membrane of this invention or the aromatic polyimide in the polyimide composition used in the method of this invention has the general formula (However, R is a divalent aromatic residue excluding the amino group of an aromatic diamine.) It is an aromatic polyimide having 90% or more, preferably 95% or more of repeating units. The above aromatic polyimide was heated at 30°C and at a concentration of 0.5.
If the logarithmic viscosity measured with g/100ml solvent (mixed solvent of 4 volumes of parachlorophenol and 1 volume of orthochlorophenol) is within a wide range of 0.3 to 7.0, particularly 0.4 to 5.0, more preferably 0.5 to 4.0. good. The aromatic polyimide mentioned above is 3, 3', 4, 4'-
Biphenyltetracarboxylic acid component, 2, 3, 3',
A biphenyltetracarboxylic acid component such as a 4'-biphenyltetracarboxylic acid component and a general formula H ₂ N-R
Any known method as long as it can be obtained from the aromatic diamine component represented by -NH ₂ (R has the same meaning as above) through a polymerization reaction and an imidization reaction (imide cyclization reaction) It may also be an aromatic polyimide manufactured by. As a method for producing the aromatic polyimide used in the method of the present invention, for example, a biphenyltetracarboxylic acid component and an aromatic diamine component are mixed into N-methyl-2-pyrrolidone, pyridine, N.N.
- Dissolved in approximately equimolar amounts in an organic polar solvent such as dimethylacetamide, N/N-dimethylformamide, dimethyl sulfoxide, tetramethylurea, phenol, or cresol, and polymerized at a temperature of about 120°C or less, particularly 0 to 60°C. Logarithmic viscosity (30℃,
Concentration: 0.5g/100mlN-methylpyrrolidone)
A polyamic acid having a molecular weight of 0.3 or more, particularly 0.5 to 7 is produced, and a tertiary amine compound such as trimethylamine, triethylamine, or pyridine is added to a solution of the polyamic acid in an organic polar solvent (the polymerization reaction solution may be used as it is), Either an imidization promoter such as acetic anhydride, thionyl chloride, or carbodiimide is added and the polyamic acid solution is imidized at 5 to 150°C, or the polyamic acid solution is heated at 100 to 300°C, preferably without adding an imidization promoter. 120~250
A preferred method is to heat the polymer to a temperature of 0.degree. C. to imidize the polymer to an imidization rate of 90% or more, thereby precipitating and isolating a powdery aromatic polyimide. In addition, as a manufacturing method for aromatic polyimide, the logarithmic viscosity (30°C, 0.5
Add a large amount of acetone or alcohol to a solution of polyamic acid with a ratio of 0.5 g/100 ml or more of polyamic acid to precipitate polyamic acid powder, or precipitate while removing the solvent from the polyamic acid solution by evaporation. Polyamic acid powder is precipitated and isolated by adding an agent, and the polyamic acid powder is heated to 150 to 300°C and imidized until the imidization rate of the polymer is 90% or more to produce polyimide. Here are some ways to do it. Furthermore, as a method for producing aromatic polyimide,
For example, 2, 3, 3', 4'- and/or 3,
Polymerizing and imidizing a 3', 4, 4'-biphenyltetracarboxylic acid component and an aromatic diamine in a melt of a phenolic compound at 120 to 400°C, particularly 150 to 300°C, in one step, Aromatic polyimides can also be produced. The biphenyltetracarboxylic acid component used in each of the above-mentioned aromatic polyimide manufacturing methods includes 3,3',4,4'-biphenyltetracarboxylic dianhydride (hereinafter abbreviated as S-BPDA). ), 2,3,3',4'-biphenyltetracarboxylic dianhydride is preferred, but 2,3,3',
4'- or 3,3',4,4'-biphenyltetracarboxylic acid or 2,3,3',4'- or 3,
It may also be a salt of 3', 4, 4'-biphenyltetracarboxylic acid or an esterified derivative thereof.
The biphenyltetracarboxylic acid component may be a mixture of the above biphenyltetracarboxylic acids. In addition to the above-mentioned biphenyltetracarboxylic acids, the above biphenyltetracarboxylic acid component also includes pyromellitic acid, 3,3',4,4'-benzophenonetetracarboxylic acid, 2・2-bis(3,4-dicarboxyphenyl)propane, bis(3,4-dicarboxyphenyl)sulfone, bis(3,4-dicarboxyphenyl)ether, bis(3,4-dicarboxyphenyl) (phenyl)thioether, butanetetracarboxylic acid, or their acid anhydrides, salts, or esterified derivatives, in a proportion of 10 mol% or less, especially 5 mol% or less, based on the total tetracarboxylic acid component. May contain. As the aromatic diamine component represented by the general formula H ₂ NR-NH ₂ used in the above-mentioned method for producing aromatic polyimide, for example, the aromatic diamine component represented by the general formula

[expression] or

[Formula] (However, in the above general formula, R ₁ or R ₂ is
A substituent such as hydrogen, lower alkyl, or lower alkoxy, and A is -O-, -S-, -CO-, _-SO2
-, _-SO2- , _-CH2- , -C( _CH3 ) _2- , and other divalent groups) are preferred. general formula

Examples of aromatic diamine compounds represented by the formula include 4.
4'-diaminodiphenyl ether (hereinafter referred to as DADE)
), 3,3'-dimethyl-
4,4'-diaminodiphenyl ether, 3,3'-
Dimethoxy-4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 3,
Diphenyl ether compounds such as 4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl thioether, 3,3'-dimethyl-4,4'-diaminodiphenyl thioether, 3,3'-dimethoxy-4, 4'-diaminodiphenyl thioether,
Diphenyl thioether compounds such as 3,3'-diaminodiphenyl ether, benzophenone compounds such as 4,4'-diaminobenzophenone, 3,3'-dimethyl-4,4'-diaminobenzophenone, 3,3 '-Diaminodiphenylmethane, 4.
4'-diaminodiphenylmethane (hereinafter sometimes abbreviated as DADM), 3,3'-dimethoxy-
Diphenylmethane compounds such as 4,4'-diaminodiphenylmethane, 3,3'-dimethyl-4,4'-diaminodiphenylmethane, 2,2-bis(4-aminophenyl)propane, 2,2-bis(3- bisphenylpropane compounds such as (aminophenyl) propane, 4,4'-diaminophenyl sulfoxide, 4,4'-diaminodiphenyl sulfone,
Examples include 3,3'-diaminodiphenyl sulfone. Also, the general formula

Examples of the aromatic diamine compound represented by the formula include benzidine, 3,3'-dimethylbenzidine,
Examples include 3,3'-dimethoxybenzidine (orthodianisidine) and 3,3'-diaminobiphenyl. The aromatic diamine component represented by the general formula _H2N -R- _NH2 is represented by the general formula

It may be a diaminopyridine represented by the formula, such as 2,6-diaminopyridine, 3,6-diaminopyridine, 2,5-diaminopyridine, 3,4-diaminopyridine, and the like. As the aromatic diamine component, 4,4'-diaminodiphenyl ether (DADE), 4,4'-
Diaminodiphenylthioether, 4,4'-diaminodiphenylmethane (DADM), 3,3'-dimethoxybenzidine (ortho-dianisidine, hereinafter sometimes abbreviated as O-DAN), 3,3'-dimethylbenzidine One or more aromatic diamines selected from the group consisting of diamines are preferred. In addition, aromatic diamines include the above DADE,
It may also be a mixture of DADMO-DAN and other aromatic diamines. In the method of this invention, the mixed solvent in which the aromatic polyimide is dissolved includes (a) 100 parts by weight of a base solvent consisting of a phenolic compound capable of dissolving 5% by weight or more of the polyimide; and (b) the mixed solvent in which the polyimide is dissolved. A homogeneous mixed solvent with 5 to 150 parts by weight of an aromatic solvent which does not dissolve 1% by weight or more and which has a slower evaporation rate from the thin film of the dope during heating drying during film formation than the base solvent. be. The above-mentioned phenolic compound may be any compound capable of dissolving the above-mentioned aromatic polyimide powder in an amount of 5% by weight or more, preferably 6% by weight or more, and particularly has a melting point of about 100°C or less, particularly preferably 80°C or more.
℃ or less and whose boiling point is about 300℃ or less, particularly preferably 280℃ or less at normal pressure, such as phenol, O-, m-P
Suitable examples include monovalent phenols such as -cresol, 3,5-xylenol, carvacrol, and thymol, and halogenated phenols in which hydrogen atoms in the benzene nucleus of the monovalent phenols are replaced with halogens. In particular, as a halogenated phenol, the general formula

[expression] or

[Formula] (However, R ₃ or R ₄ is hydrogen or has 1 to 1 carbon atoms.
3, X is a halogen atom), and has a melting point of about 100°C or less and a boiling point of about 300°C or less at normal pressure, which is biphenyltetracarboxylic. It is optimal because it has excellent ability to dissolve acid-based aromatic polyimides. In the method of this invention, the halogenated phenols include, for example, 3-chlorophenol, 4-
Chlorphenol (para-chlorphenol,
(sometimes abbreviated as PCP), 3-bromophenol, 4-bromophenol, 2-chloro-4-
Hydroquine toluene, 2-chloro-5-hydroxytoluene, 3-chloro-6-hydroxytoluene, 4-chloro-2-hydroxytoluene, 2-
Bromo-4-hydroxytoluene, 2-bromo-
Examples include 5-hydroxytoluene, 3-bromo-5-hydroxytoluene, 3-bromo-6-hydroxytoluene, and 4-bromo-2-hydroxytoluene. The aromatic solvent that is the added solvent of the mixed solvent contains the above-mentioned polyimide powder in an amount of 3% by weight or more, preferably
The aromatic solvent does not dissolve more than 1.5% by weight, and the rate of evaporation of the dope from the thin film during heating drying during film formation is slower than the base solvent, but it must be mutually compatible with the base solvent. It is natural that it is. The aromatic solvent may be any aromatic solvent that is insoluble in the polyimide and has a boiling point that is at least 5°C higher, sometimes preferably 10 to 200°C higher than the boiling point of the above-mentioned base solvent. , even if it has a boiling point in the range of 5°C higher than the boiling point of the base solvent to a boiling point 30°C lower than the boiling point of the base solvent, the ionization potential of the base solvent is 0.1ev or more, sometimes preferably 0.2ev or more lower than the base solvent. Any aromatic solvent that has an ionization potential and is insoluble in the polyimide may be used. Examples of the aromatic solvent include phenanthrene, O- or P-terphenyl, α-chlornaphthalene, triphenylphosphine, triphenylamine, aniline, anthracene, naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, 1 -2-Benzanthracene etc. can be mentioned. In the method of the present invention, the mixed solvent is a mixed solvent of 100 parts by weight of the base solvent (phenolic solvent) and 5 to 150 parts by weight, particularly preferably 10 to 140 parts by weight of the above-mentioned additive solvent. In the polyimide composition used in the method of the present invention, the above-mentioned aromatic polyimide is uniformly added to a mixed solvent of a base solvent and an additive solvent so that the polymer concentration is 3 to 30% by weight, preferably 5 to 25% by weight. It is a solution composition in which . The above polyimide composition may be prepared by any known method, but for example, as described in the production of polyimide, a biphenyltetracarboxylic acid component and an aromatic diamine component,
1 at 120-400℃ in a solution of phenolic compounds.
The polyimide composition can be prepared by polymerizing and imidizing the aromatic polyimide in a step to prepare a solution of the aromatic polyimide in a phenolic solvent, and adding the necessary amount of an additive solvent to the solution, or the above-mentioned preparation of the polyimide can be carried out. As described in , a polyimide powder is first produced and isolated, and then a polyimide composition is prepared by dissolving the polyimide powder in the above-mentioned mixed solvent of the base solvent and additive solvent. It can be prepared. The polyimide composition used in the method of this invention has a rotational viscosity of at least
It is preferably about 500 centipoise, particularly preferably about 10 to 10,000 poise, and is a uniform liquid composition as a whole, which can be used as a dope solution for film formation. In the method of the present invention, a dope solution made of the above-mentioned polyimide composition is used, a thin film is formed with the dope solution, and the thin film is heated and dried at about 50 to 400°C to gradually remove each solvent. A polyimide porous membrane is manufactured. In this invention, the dope solution is preferably used after being filtered and defoamed at a temperature of 20 to 180°C, particularly preferably 50 to 150°C, prior to film formation. In addition, in the method of the present invention, as a specific method for forming a film, any known film forming method that can form a thin film by a dry method from a dope solution of various polymer solution compositions may be used. for example,
After filtering and defoaming the dope solution of the polyimide composition described above, the temperature is about 50 to 180°C, particularly preferably 60°C.
Coating or casting on the surface of a substrate with a smooth surface at a temperature of ~150 °C to form a thin film of dope liquid,
In order to gradually remove each solvent from the thin film on the substrate, the thin film is heated and dried at about 50 to 400°C, particularly preferably 60 to 350°C, and more preferably 80 to 300°C, and the thin film is solidified. Finally, by peeling off the solidified thin film from the base material, a porous polyimide membrane can be manufactured. The purpose of the heat drying described above is to gradually remove each solvent from the thin film of the dope solution, so it can be carried out at a relatively low temperature (for example, 50 to 200 °C, especially 60 to 150 °C) over a long period of time. However, in order to improve productivity, it is preferable to increase the drying temperature in stages between 50 and 400°C. For example, in the first stage of drying, the drying temperature should be increased to about 50 to 150°C. Especially preferably 60
Drying is performed at a temperature range of ~120°C, followed by a second stage drying at a temperature approximately 30 to 80°C higher than the first stage drying temperature, and a second stage drying at a temperature approximately 30 to 80°C higher than the second stage drying temperature. Three stages of drying are performed, and if necessary, drying can be performed at a higher temperature. In the multi-stage drying described above, the heating drying time at each stage varies depending on the polymer of the dope, the type of solvent, the polymer concentration of the dope, each drying temperature, etc., so it should be determined appropriately for each specific example. good. Examples of the base material with a smooth surface used for the above-mentioned film formation include a glass plate, a copper plate with a smooth surface, a steel plate, a metal plate with an appropriate metal plating, or a heat-resistant and peelable resin plate. ,moreover,
Examples include metal rolls or belts with smooth surfaces. In the method of the present invention, a dope solution made of a polyimide composition having a rotational viscosity of about 500 to 100,000 centipoise at the time of discharge is passed through a slit having voids at intervals of about 0.1 to 1 mm at a temperature of about 50 to 150°C. The dope is discharged (extruded) from between the holes and received on a rotating metal roll or belt, and a doctor knife placed on the metal roll or belt is used to form a dope of uniform thickness by natural casting of the dope. A thin film is formed, and then the solvent is gradually removed by blowing a heated inert gas onto the thin film on a rotating metal roll or belt, or by irradiating it with a heating wire and drying it by heating at approximately 30 to 400°C. , by solidifying the thin film as a polyimide film and peeling off the solidified polyimide film from a metal roll or belt,
A polyimide porous membrane can be continuously produced by a dry method. Note that the polyimide porous membrane produced as described above is preferably thoroughly washed with a solvent such as methanol or acetone in order to remove residual solvent. In the method of this invention, as the aromatic polyimide,
A specific aromatic polyimide that is soluble in a phenolic solvent and has heat resistance and chemical resistance, which is obtained from the above-mentioned biphenyltetracarboxylic acid component and an aromatic diamine, is used as a solvent for the dope solution. A polyimide composition is prepared using a specific mixed solvent of a base solvent consisting of a phenolic solvent capable of dissolving the polyimide and an additive solvent consisting of an aromatic solvent insoluble in the polyimide, and as a dope solution, This was the first time that a porous polyimide membrane could be produced using a dry method. The method of the present invention involves a simple process of forming a thin film of the dope solution from a dope solution of a polyimide composition by a solution casting method, and gradually removing each solvent from the thin film by heating and drying. It is an advantageous method that can produce porous polyimide membranes and can industrially stably and continuously manufacture porous membranes. The polyimide porous membrane produced by the method of this invention has extremely high heat resistance, and has approximately
It is a porous membrane that can be used at temperatures between 50℃ and 350℃, has excellent chemical resistance and mechanical strength, and can also be used as a gas or solution separation membrane with a sufficiently high gas or liquid permeation rate. . Therefore, the polyimide porous membrane of the present invention can be used for separating and concentrating various mixed gases, for example, it can be suitably used for separating and concentrating a mixed gas of hydrogen and carbon monoxide. can. Examples and comparative examples are shown below. In the Examples and Comparative Examples, the gas permeation test was conducted by installing a polyimide membrane (porous membrane, etc.) in a stainless steel cell with an area of 14.65 cm ² and pressurizing it with hydrogen gas, carbon monoxide gas, and 3 kg/cm ^2. The gas was supplied at room temperature and the volume of gas passing through the porous membrane was measured using a flowmeter. The results of the above gas permeation test were expressed as gas permeability calculated according to the calculation formula for gas permeability (P) shown by the following formula. Gas permeability (P) = permeation gas amount (STP) / membrane area x permeation time x pressure difference (cm ³ /cm ²・sec ・cmHg) Examples 1 to 4 Separator equipped with a stirrer and nitrogen gas introduction tube In a blue flask, add 40 mmol of 3,3',4,4'-biphenyltetracarboxylic dianhydride (S-BPDA), 40 mmol of 4,4'-diaminodiphenyl ether (DADE), and 178 mmol of P-chlorophenol.
The temperature was raised to a reaction temperature of 180°C in about 1 hour while stirring and flowing nitrogen gas, and the polymerization reaction was continued at that temperature for 5 hours with stirring, and then the temperature was increased to room temperature. It cooled down to near. A large amount of methylene chloride and methanol were added to the polymerization reaction solution while stirring to precipitate polyimide, which was then filtered, washed, and dried to obtain polyimide powder. This polyimide powder has an imidization rate of 95% or more as measured by infrared absorption spectrum, and a logarithmic viscosity [30°C, concentration 0.5 g/100 ml solvent (4 volumes of parachlorophenol and 1 volume of orthochlorophenol). mixed solvent] was 2.18. Note that the logarithmic viscosity was calculated using the following formula. Logarithmic viscosity = natural logarithm (viscosity of solution / viscosity of solvent) / polymer concentration in solution 10 g of the above polyimide powder and 90 g of parachlorophenol (PCP) were placed in a separable flask equipped with a stirrer and heated to approximately 100°C. A polyimide solution in which polyimide is uniformly dissolved is prepared by heating and stirring, and an aromatic solvent having the boiling point and ionization potential shown in Table 1 is added in the amount shown in Table 1, A uniform polyimide composition was prepared, and the polyimide composition was filtered and defoamed to obtain a dope solution for film formation. The above dope solution was cast onto a glass plate at 60°C and made to a uniform thickness (0.2 mm) using a doctor blade to form a thin film of the dope solution, and the thin film was heated at 100°C.
The solvent was gradually removed by heating and drying for 3 hours.
A polyimide porous membrane with a thickness of about 10μ was produced. A gas permeation test was conducted using this polyimide porous membrane. The results are shown in Table 1. Comparative Examples 1-2 Same as Example 1 except that the amount shown in Table 1 was used as the additive solvent,
A polyimide membrane was manufactured. Table 1 shows the results of the gas permeation test for the polyimide membrane.

【table】

[Table] In addition, in the Examples and Comparative Examples, each aromatic solvent used as an additive solvent alone is equivalent to the aromatic polyimide powder (imidization rate: 95% or more, Logarithmic viscosity;
2.18) is a solvent that does not substantially dissolve. According to the results of Examples and Comparative Examples, when an aromatic solvent having a boiling point much higher than that of the base solvent was used as an additive solvent, excellent results were obtained as shown in Examples 1, 3, and 4. As shown in Example 2, even when an aromatic solvent with a boiling point slightly lower than that of the base solvent is used as an additive solvent, the ionization potential of the solvent is However, if it is much smaller than the base solvent, the added solvent will evaporate slower than the base solvent during heating drying during membrane formation, resulting in a polyimide porous membrane with excellent performance. However, when an aromatic solvent having a boiling point considerably lower than that of the base solvent was used as the additive solvent, as shown in Comparative Examples 1 and 2,
During heating and drying during membrane formation, the added solvent evaporates and is removed before the base solvent, making it impossible to obtain a polyimide porous membrane with a high gas permeation rate.
The polyimide membrane obtained in this comparative example shows measured values that correspond to a homogeneous membrane, based on the results of the gas permeation test. Furthermore, even if an aromatic solvent whose boiling point is slightly lower than that of the base solvent is used as an additive solvent, as shown in Comparative Example 3, if the ionization potential of the solvent is considerably larger than that of the base solvent, problems may occur during film formation. During heating drying, the added solvent evaporates and is removed faster than or simultaneously with the base solvent, making it impossible to obtain a polyimide porous membrane with a high gas permeation rate. Rather, it has the performance of a homogeneous membrane.

Claims (1)

[Claims] 1. General formula (However, R is a divalent aromatic residue excluding the amino group of an aromatic diamine.) A polyimide porous membrane for separation comprising an aromatic polyimide having 90% or more of repeating units represented by the following formula. 2 General formula (However, R is a divalent aromatic residue excluding the amino group of an aromatic diamine.) An aromatic polyimide having 90% or more of repeating units represented by (a) 5% or more of the above polyimide by weight Base solvent consisting of soluble phenolic compounds100
and (b) an additive solvent 5 to 5 consisting of an aromatic solvent that does not dissolve 3% by weight or more of the polyimide and has a slower evaporation rate from the thin film of the dope during heating drying during film formation than the base solvent. A polyimide composition uniformly dissolved at a polymer concentration of about 3 to 30% by weight in a uniform mixed solvent with 150 parts by weight is used as a dope solution, and a thin film is formed with the dope solution. A method for producing a porous polyimide membrane, which comprises heating and drying at about 50 to 400°C to gradually remove each solvent.