Aromatic polyimide separation membrane

Abstract: A process is disclosed for preparing an aromatic polyimide separation membrane which comprises fluorinating an aromatic polyimide membrane with active fluorine. The aromatic polyimide separation membrane thus prepared has high selectivity and gas permeability and is therefore useful as a membrane for separation of gas mixture or liquid mixtures.

Description: The present invention relates to a process for preparing an aromatic polyimide separation membrane, which membrane has an extremely high selectivity and is useful for separation of gas mixtures or liquid mixtures. JP-A-57-15189(1982), JP-A-58-3603(1983) and JP-A-58-8512(1983) describe gas separation membranes prepared from a polyimide obtained from a biphenyltetracarboxylic acid component and an aromatic diamine component. However, although the selectivity of hydrogen against carbon monooxide is relatively high, the permeation rate of hydrogen is not sufficiently high. JP-B-55-41802(1980), JP-A-63-111921(1988) and JP-A-63-123420(1988) describe polyimide separation membranes having high gas permeation rates. However, the membranes disclosed in these documents do not have sufficient separation efficiency. JP-A-62-155919(1987) and JP-A-63-171619(1988) describe separation membranes prepared by treating substituted polyacetylenes with fluorine, and JP-A-64-22320(1989) describes a treatment of silicone-based membranes with fluorine. However, the separation membranes of these documents have inferior durability such as heat resistance and solvent resistance. EP-A-194,546 describes a membrane for the purification of water by ultrafiltration prepared by subjecting a polyacrylonitrile, polyamide, polysulfone, acetylcellulose or polyvinylidenefluoride to plasma or corona discharge in the presence of a gas such as an inert gas, H₂, NH₃, O₂, CO₂, SO₂, H₂S, H₂O, Cl₂, NF₃ or BF₃. The present inventors have carried out extensive research to try to obtain a separation membrane which shows a higher selectivity than known separation membranes and satisfies the requirements for a useful separation membrane, i.e. a separation membrane having excellent heat resistance, solvent resistance, stability in membrane characteristics and other properties. The present invention provides a process for preparing an aromatic polyimide separation membrane which process comprises fluorinating an aromatic polyimide membrane with fluorine gas or HF gas. The modification of a polyolefin porous membrane by fluorine treatment has been known in the art. However, in this technique, it has been very difficult to avoid a deterioration in membrane properties due to the fluorine treatment. On the contrary, in the present invention, an aromatic polyimide membrane is subjected to fluorination without causing such deterioration. This makes it possible to carry out the fluorination under a wide range of conditions to secure a practical advantage. In has also been found that an aromatic polyimide membrane subjected to fluorination is especially improved in selectivity. An aromatic polyimide membrane having a hydrogen permeability coefficient larger than a specific level is found to be effectively fluorinated due to the large diffusion rate of active fluorine into the membrane, and therefore, the separation membrane produced by the present invention can be advantageously obtained by using such an aromatic polyimide membrane. The separation membrane produced according to the present invention has a good durability in addition to a high selectivity and a high gas permeability, and is highly useful for practical use. The aromatic polyimide separation membrane produced by the present invention which hereinafter may be referred to as "the present separation membrane" or "the present aromatic polyimide separation membrane") is preferably prepared from a membrane of a polyimide which comprises, or which more preferably consists essentially of, the constitutional repeating unit represented by the following formula (I): wherein R represents a tetravalent aromatic group and R' represents a divalent aromatic group. The polyimide membrane preferably has a hydrogen permeability coefficient at 30 °C of not less than 10⁻⁹ cm³(STP)·cm/cm²·sec·cmHg, STP being standard temperature and pressure. In formula (I), R is a tetravalent aromatic group, preferably (1) a group represented by the following formula: and (2) a group represented by the formula (II): wherein X represents CO, SO₂, C(CF₃)₂, Si(CH₃)₂, CH₂, C(CH₃)₂ or O, and ; and (3) a non-benzenoid aromatic group comprising a heterocycle, for example of the formula: A polyimide having the tetravalent aromatic group R in the main polymer chain is generally produced by a reaction of a tetracarboxylic acid dianhydride corresponding thereto and a diamine of the formula H₂N-R'-NH₂. Examples of such a tetracarboxylic acid dianhydride include pyromellitic acid dianhydride, 2,3,6,7-naphthalenetetracarboxylic acid dianhydride, 3,4,3',4'-diphenyltetracarboxylic acid dianhydride, 3,4,3',4'-benzophenonetetracarboxylic acid dianhydride, 3,4,3',4'-diphenylsulfone tetracarboxylic acid dianhydride, 3,4,3',4'-diphenylhexafluoroisopropylidenetetracarboxylic acid dianhydride, 3,4,3',4'-diphenyl(trifluoromethyl)methanetetracarboxylic acid dianhydride, 1,4,5,8-naphthalenetetracarboxylic acid dianhydride, pyrazinetetracarboxylic acid dianhydride and 3,4,3',4'-diphenyldimethylmethanetetracarboxylic acid dianhydride. Two or more of the dianhydrides may be used in the reaction. In formula (I), R' is a divalent aromatic group, preferably selected from groups of the following formulae (III), (IV) and (V) and isomeric groups thereof and derived groups therefrom, or a heterocycle containing non-benzenoid aromatic group represented by the following formulae: wherein each of R¹ to R¹⁸ independently represents a hydrogen atom, a halogen such as fluorine, chlorine, bromine and iodine, a nitro group or a C₁-C₁₂ organic group which is an alkyl group such as methyl, ethyl, propyl, butyl, i-propyl, i-butyl and t-butyl, an alkenyl group such as allyl, vinyl and butenyl, an alkynyl group such as acetylenyl and 2-propynyl, a cycloalkyl group such as cyclopropyl, cyclopentyl and cyclohexyl, an aryl group such as phenyl and naphthyl, an aralkyl group such as benzyl, an alkoxy group such as methoxy, ethoxy and propoxy, an aryloxy group such as phenoxy and naphthoxy, an acyl group such as acetyl and propionyl, an acyloxy group such as acetoxy and propionyloxy, an alkoxycarbonyl group such as carbomethoxy and carboethoxy, or a carboxyl group or metal salt thereof; n represents an integer from 1 to 4; Y represents CO, SO₂, C(CF₃)₂, Si(CH₃)₂, CH₂, C(CH₃)₂, O, S or NH; and each of Z¹ and Z² independently represents a direct bond, CH₂, CO, SO₂, C(CF₃)₂, C(CH₃)₂, Si(CH₃)₂, CH₂, O or S. The polyimide having the constitutional repeating unit of formula (I) is usually prepared by polycondensing a tetracarboxylic acid dianhydride corresponding to the group R and an aromatic diamine corresponding to the group R' to first prepare a polyamic acid comprising the constitutional repeating unit represented by the following formula (VI): wherein R and R' are as defined above, and subjecting the polyamic acid to chemical or thermal ring closing imidization. Any other preparation method may also be employed as long as a polyimide comprising the constitutional unit of formula (I) is prepared. In the preparation of the polyimide, two or more tetracarboxylic acid dianhydrides and/or two or more aromatic diamines may be used. The aromatic polyimide referred to in the present invention includes the copolyimides thus prepared. Examples of the aromatic diamine for preparing the aromatic polyimide, are metaphenylenediamine, paraphenylenediamine, 4-chlorometaphenylenediamine, 4-fluorometaphenylenediamine, 4-nitrometaphenylenediamine, 2,4-diaminotoluene, 2,6-diaminotoluene, 4,5-diaminometaxylene, diaminomesitylene, 2,3,5,6-tetramethylparaphenylenediamine, 4-phenoxymetaphenylenediamine, 4-naphthoxymetaphenylenediamine, 3,5-diaminobenzoic acid, sodium 3,5-diaminobenzoate, methyl 3,5-diaminobenzoate, ethyl 3,5-diaminobenzoate, isopropyl 3,5-diaminobenzoate, metaphenylenediamine-2-sulfonic acid, sodium metaphenylenediamine-2-sulfonate, 3,5-diaminodiphenylamine, 2,3,5-trimethylbenzidine, 3,3',5-trimethylbenzidine, 2,2',6,6'-tetramethylbenzidine, 3,3',5,5'-tetramethylbenzidine, 2,2',5,5'-tetramethylbenzidine, 2,2',3,3'-tetramethylbenzidine, 2,3',5,5'-tetramethylbenzidine, 2,2',3,5-tetramethylbenzidine, 2,3,5,6-tetramethylbenzidine, 2,2',4,4'-tetramethyl-3,3'-diaminodiphenyl, 2,2',6,6'-tetraethylbenzidine, 3,3',5,5'-tetraethylbenzidine, 5,5'-diethyl-3,3'-dimethylbenzidine, 3,3',5,5'-tetra-n-propylbenzidine, 2,2'-di-i-propyl-5,5'-dimethylbenzidine, 3,3',5,5'-tetrabromobenzidine, 2,2',6,6'-tetrabromobenzidine, 3,3',5,5'-tetrachlorobenzidine, 2,2',6,6'-tetrachlorobenzidine, 3,3',5-tribromobenzidine, 4,4'-diamino-5,5'-dimethyl-[1,1'-biphenyl]-3,3'-dicarboxylic acid, 4,4'-diamino-6'-nitro-[1,1'-biphenyl]-2,2',6-tricarboxylic acid, 3,3'-diphenyl-5,5'-dimethylbenzidine, 3,3'-diphenyl-5,5'-diethylbenzidine, 3,3',5,5'-tetramethoxybenzidine, 2,2',6,6'-tetraethoxybenzidine, 3,3-phenoxy-5,5'-dimethylbenzidine, 3,3',5,5'-tetracyclohexylbenzidine, 3,3'-diallyl-5,5'-dimethylbenzidine, 3,3'-diaminobenzophenone, 3,4'-diaminobenzophenone, 3,3'-diaminodipheyl sulfone, 4,4'-diaminodiphenyl sulfone, bis(3-(3-aminophenoxy)phenyl) sulfone, bis(4-(3-aminophenoxy)phenyl) sulfone, 2,2-bis(4-aminophenyl)hexafluoropropane, 2,2-bis(4-(4-aminophenoxy)phenyl)hexafluoropropane, 4,4'-diaminodiphenylmethane, 2,2-bis(4-aminophenyl)propane, 2,2-bis(4-(4-aminophenoxy)phenyl)propane, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl thioether, 4,4'-diaminodiphenylamine, 1,4-diaminoanthraquinone, 1,5-diaminoanthraquinone, 2,7-diaminofluorene, orthotolidine sulfone, 9,9-bis(4-aminphenyl)fluorene, 3,3'-bisaminophenylbisbenzothiazole, 2,6-diaminopyridine, 2,'4-diaminopyridine, 2,6-diaminopyrazine and 3,4-di(4-aminophenyl)-2,5-diphenylthiophene. An aromatic diisocyanate which is derived from each of the above aromatic diamines by substituting amino groups with isocyanato groups may be also used as the starting substance for the aromatic polyimide in place of the aromatic diamines. An aromatic polyimide derived from a benzidine corresponding to formula (III) wherein n is 2 is preferred because an aromatic polyimide membrane having a hydrogen permeability coefficient at 30 °C of not less than 10⁻⁹ cm³(STP)·cm/cm²·sec·cmHg can be comparatively easily prepared. The aromatic polyimide membrane to be subjected to fluorination may be prepared by coating or casting a dope solution such as a solution of the aromatic polyimide having the constitutional repeating unit of formula (I), a solution of a polyamic acid having the constitutional unit of the formula (VI): wherein R and R' are as defined above, or a solution of a mixture of both of them in a polar solvent on a substrate with a flat surface to form a thin layer of the solution, then evaporating the solvent under appropriate conditions, if said polyamic acid is present under such conditions as to cause ring closing imidation of said polyamic acid to form said aromatic polyamide, and, furthermore, drying it sufficiently. The concentration of the aromatic polyimide and/or polyamic acid in the dope solution depends on the molecular weight of the aromatic polyimide and/or polyamic acid, and is usually 2 to 40 % by weight. A method for preparing a dense membrane of aromatic polyimide by using a solution of a polyamic acid of formula (VI) in an organic polar solvent as the dope is now shown below in more detail. A dope solution is cast on, for example, a glass plate, teflon plate, stainless steel plate or steel plate having flat surface using, for example, a doctor blade to form a thin layer of about 1 to 40 mil (2.5 to 102 µm) thick at 0 to 100°C. Then, after gradually evaporating the solvent under normal pressure or a reduced pressure, the temperature is gradually elevated to 150 °C or higher, preferably 200 to 400 °C. At this temperature ring closing imidization of the polyamic acid is carried out to form the aromatic polyimide having the constitutional unit of formula (I) and, at the same time, the solvent is thoroughly removed and the resultant membrane is dried. A preferred result may be obtained by setting the drying temperature to a temperature at which the aromatic polyimide does not melt. The above ring closing imidization may be also carried out by any known chemical method. An unsymmetrical aromatic polyimide membrane comprising a dense layer and a porous layer, a mixed membrane of the aromatic polyimide and an organic material including other polyimides or inorganic material, and a composite membrane comprising an aromatic polyimide membrane and another porous membrane are also included in the aromatic polyimide membrane which is subjected to fluorination. As a method for forming a thin aromatic polyimide membrane described above, there can be mentioned, for example, a method of spreading a solution of a polyamic acid or an aromatic polyimide on the surface of water, a method of coating a solution of a polyamic acid or an aromatic polyimide on a supporting substrate membrane, a method of coating or impregnating a diamine and an aromatic tetracarboxylic acid dianhydride, which are monomers for the aromatic polyimide or the polyamic acid, on or into a supporting substrate membrane and polymerizing the monomers, or a method of using interfacial polymerization technique, as well as any other known method. The membrane obtained by the methods described above may be fabricated in the form of flat sheets or hollow fibers. The aromatic polyimide and the polyamic acid used for preparing an aromatic polyimide membrane are preferred to have a logarithmic viscosity of not less than 0.1 dl/g, preferably from 0.3 to 5 dl/g,when measured as an N-methylpyrrolidone solution at a 0.5 g/dl polymer concentration at 30 °C. The logarithmic viscosity is determined in accordance with the formula: Logarithmic viscosity = Natural Logarithm of (viscosity of solution/viscosity of solvent) Polymer concentration in solution As the organic polar solvent for the aromatic polyimide or polyamic acid, there can be mentioned, for example, N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, formamide, dimethylsulfoxide and tetramethylurea. The fluorination is carried out after the formation of the aromatic polyimide membrane. The fluorination is carried out with fluorine gas or HF gas. Preferred is fluorine gas only or a mixed gas of fluorine and another gas. The fluorination method is illustrated below wherein fluorine gas is used as an example. The aromatic polyimide membrane prepared as described above is subjected to contact treatment in a mixed gas containing fluorine gas in a concentration of not less than 0.001 % by volume, preferably from 0.005 to 80 % by volume, more preferably from 0.01 to 60 % by volume, at a temperature not higher than the glass transition temperature of the aromatic polyimide under reduced pressure or under pressure. The time period for the contact treatment depends on the fluorine concentration in the gas used, the treatment temperature and the form of the membrane to be treated, and is usually from a few second to 48 hours, generally within 10 hours. The aromatic polyimide membrane used in the present invention is advantageous because the physical properties thereof are not deteriorated by fluorine treatment, and therefore a wide range of treatment conditions may be employed. Examples of a mixture of fluorine gas with another gas are F₂/O₂, F₂/N₂, F₂/He, F₂/Ar, F₂/Cl₂, F₂/CO₂, F₂/SO₂, F₂/O₂/N₂, F₂/Cl₂/N₂, F₂/CO₂/N₂, F₂/SO₂/N₂ and F₂/SO₂/Cl₂/N₂. The increase in the fluorine content in the fluorinated aromatic polyimide membrane is not specifically restricted, but is preferably from 0.3 to 40 % by weight, and more preferably not more than 30 % by weight,based on elemental analysis. The present aromatic polyimide separation membrane may be used in any form such as a flat membrane, a hollow fiber and a spiral-wound element. The fluorination of the aromatic polyimide membrane is carried out in a gaseous phase, and therefore the degree of fluorination can be easily controlled on each side of the aromatic polyimide membrane. The aromatic polyimide separation membrane produced by the present invention is highly suitable for a separation membrane due to its extremely high selectivity and high gas permeability. Furthermore the aromatic polyimide separation membrane has a high heat stability and high mechanical strength. It can be used more effectively even in severe conditions under high temperature and high pressure, as compared with a conventional separation membrane obtained from other organic materials. For instance, it can be used as a membrane for separation of carbon dioxide in the third recovery of petroleum, separation of for example, water vapour, helium, carbon dioxide and carbon disulfid from natural gases, as well as recovery of hydrogen in petroleum refining and ammonia synthesis plants, separation of carbon monoxide and hydrogen from synthesis gases and iron making gases, separation of oxygen or nitrogen from air for the production of oxygen-enriched air used for combustion and medical application or production of nitrogen as an inert gas. Since the fluorinated polyimide separation membrane also has excellent organic solvent resistance, it can be used as a separation membrane for treatment of waste water containing organic solvents by means of reverse osmosis, ultrafiltration or precision filtration. In addition, it is advantageously adopted as a membrane for separation of organic liquids and water by means of pervaporation. The present invention is now further described in more detail in the following Examples. The gas permeability is represented by the gas permeability coefficient P represented by the formula: P = Gas permeation amount(cm³·STP) × Membrane thickness (cm) Membrane area(cm²) × Time(sec) × Pressure difference (cmHg) wherein STP represents standard temperature and pressure. The unit for P is indicated by cB (centi barrer). cB = 10⁻¹² cm³(STP)·cm cm²·sec·cmHg The gas permeability is measure by a gas permeation rate measuring device. This is a device in which a predetermined test gas is supplied at a constant pressure on one surface of a test membrane attached to a cell of the device and the amount of the gas which permeates to the other side through the membrane is analyzed by a pressure sensor or by gas chromatography. The selectivity of gas separation factor is represented as a ratio of permeability coefficients for the respective gases measured. Reference Example 1 : In a nitrogen atmosphere at room temperature, 5.08 g (21 mmol) of 3,3',5,5'-tetramethylbenzidine (hereinunder referred to as "TMB") was placed in a 300-ml four-necked flask equipped with a stirrer and a nitrogen inlet and then dissolved in 20 ml of N-methylpyrrolidone (hereinunder referred to as "NMP"). To the diamine solution was added 0.92 g (4.2 mmol) of pyromellitic acid dianhydride (hereinunder referred to as "PMDA") together with 20 ml of NMP and the mixture was stirred for 1 hour. To the resultant mixture, was added 7.47 g (16.8 mmol) of 3,4,3',4'-diphenylhexafluoroisopropylidenetetracarboxylic acid dianhydride (hereinunder referred to as "6FDA") together with 20 ml of NMP. The mixture was stirred for 10 hours at room temperature, thereby obtaining a viscous solution of a polyimide precursor (polyamic acid) having a polymer concentration of 18 wt%. A part of the solution was diluted with NMP to prepare a solution of a concentration of 0.5 g/dl. The logarithmic viscosity measured at 30°C was 1.53 dl/g. Reference Example 2 : To the polyamic acid solution prepared in Reference Example 1 were added 9 g (84 mmol) of acetic anhydride and 36 ml of NMP. After the mixture was stirred for 1 hour, 9 g (84 mmol) of triethylamine was added thereto and the mixture was stirred for 24 hours to obtain a polyimide solution. The polyimide solution was solidified in water into fibers, pulverized, washed with methanol and dried under reduced pressure to obtain a polyimide powder. The polyimide powder was dissolved in NMP to obtain a 10 wt% solution. The solution was filtered through a filter with a 5 µm pore size, and the filtrate was cast on a glass plate at a knife interval of 20 mil (51 µm) using a doctor blade. The glass plate was dried in an oven at 80°C for 4 hours in a nitrogen atmosphere, thereby obtaining a self-supporting membrane. The membrane was then separated from the glass plate and heat treated at 120°C for 4 hours under reduced pressure. The temperature of the oven was then raised from room temperature to 300°C, and heat treatment was carried out at 300°C for 2 hours to obtain a dense polyimide membrane of 1 mil (2.5 µm) thickness. The contact angle of the membrane with respect to water was 74°. Comparative Example 1 : Using the dense polyimide membrane obtained in Reference Example 2, the permeation of pure gases was measured at 30 °C. The results are: H₂ permeability coefficient 30000 cB CO₂ permeability coefficient 27700 cB CH₄ permeability coefficient 1100 cB H₂/CH₄ separation factor 27 CO₂/CH₄ separation factor 25 The permeations of air and a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) through the membrane at 35°C were measured. The results are: O₂ permeability coefficient 5820 cB O₂/N₂ separation factor 3.75 CO₂ permeability coefficient 29100 cB CO₂/CH₄ separation factor 23 Example 1 : The dense polyimide membrane obtained in Reference Example 2 was deaerated under a reduced pressure for 1 hour, and the deaerated polyimide membrane was subjected to fluorination treatment by contacting the membrane with an F₂/N₂ mixed gas having an F₂ concentration of 4 vol% for 1 hour at room temperature. The elemental analysis of the treated polyimide membrane showed that the fluorine content in the dense polyimide membrane increased by 4.22 wt% due to the fluorination treatment. The contact angle of the fluorinated membrane with respect to water was 31°. The permeation of pure gases through the fluorinated dense polyimide membrane at 30°C was measured. The results are: H₂ permeability coefficient 9980 cB CO₂ permeability coefficient 1330 cB CH₄ permeability coefficient 16.2 cB H₂/CH₄ separation factor 616 CO₂/CH₄ separation factor 82 It is evident that the separation factors increased due to fluorination. The permeations of air and a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) through the fluorinated dense polyimide membrane at 35°C were measured. The results are: O₂ permeability coefficient 910 cB O₂/N₂ separation factor 5.61 CO₂ permeability coefficient 2740 cB CO₂/CH₄ separation factor 90 Example 2 : The dense polyimide membrane was subjected to fluorine treatment in the same way as in Example 1 except for using an F₂/N₂ mixed gas having an F₂ concentration of 8 vol%. The elemental analysis of the treated polyimide membrane showed that the fluorine content in the dense polyimide membrane increased by 8.43 wt% due to the fluorine treatment. The contact angle of the membrane with respect to water was 42°. The permeations of air and a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) through the fluorinated dense polyimide membrane at 35°C were measured. The results are: O₂ permeability coefficient 535 cB O₂/N₂ separation factor 6.24 CO₂ permeability coefficient 2210 cB CO₂/CH₄ separation factor 100 Example 3 : The dense polyimide membrane was subjected to fluorine treatment in the same way as in Example 1 except for using an F₂/N₂ mixed gas having an F₂ concentration of 12 vol%. The elemental analysis of the treated polyimide membrane showed that the fluorine content in the dense polyimide membrane increased by 6.16 wt% due to the fluorine treatment. The contact angle of the membrane with respect to water was 56°. The permeation of pure gases through the fluorinated dense polyimide membrane at 30°C was measured. The results are: H₂ permeability coefficient 11200 cB CO₂ permeability coefficient 2600 cB CH₄ permeability coefficient 37.2 cB H₂/CH₄ separation factor 301 CO₂/CH₄ separation factor 70 The permeations of air and a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) of the fluorinated dense polyimide membrane at 35°C were measured. The results are: O₂ permeability coefficient 985 cB O₂/N₂ separation factor 5.76 CO₂ permeability coefficient 4550 cB CO₂/CH₄ separation factor 87 Reference Example 3 : A copolyimide was produced from 3,3'4,4'-benzophenonetetracarboxylic acid dianhydride (hereinunder referred to as "BTDA") and a diisocyanate mixture of 80 mol% of tolylene diisocyanate (a mixture of about 80 mol% of a 2,4-isomer and about 20 mol% of 2,6-isomer) and 20 mol% of 4,4'-diphenylmethane diisocyanate in accordance with the procedure described in Example 4 in US-A-3,708,458. N,N'-dimethylformamide was used as the polymerization solvent, and the resin concentration in the solution was 21 wt%. A part of the solution was diluted with NMP to prepare a solution having a concentration of 0.5 g/dl. The logarithmic viscosity of the polymer measured at 30°C was 0.60 dl/g. Reference Example 4 : The polyimide solution prepared in Reference Example 3 was filtered through a filter with a 5 µm pore size, and the filtrate was cast on a glass plate at a knife interval of 9 mil (23 µm) using a doctor blade. The glass plate was dried in an oven at 100°C for 1 hour in a nitrogen atmosphere, thereby obtaining a self-supporting membrane. The membrane was then separated from the glass plate and heat treated at 120°C for 4 hours under reduced pressure. The temperature of the oven was then raised from room temperature to 250°C, and heat treatment was carried out at 250°C for 1 hour. The contact angle of the membrane with respect to water was 72°. Comparative Example 2 : The permeations of air and a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) through the dense polyimide membrane obtained in Reference Example 4 were measured at 35°C. The results are: O₂ permeability coefficient 33.4 cB O₂/N₂ separation factor 7.84 CO₂ permeability coefficient 163 cB CO₂/CH₄ separation factor 74 Example 4 : The dense polyimide membrane obtained in Reference Example 4 was deaerated under a reduced pressure for 1 hour, and subjected to fluorine treatment by bringing the deaerated polyimide membrane into contact with an F₂/N₂ mixed gas having an F₂ concentration of 12 vol% at room temperature. The elemental analysis of the treated polyimide membrane showed that the fluorine content in the dense polyimide membrane increased by 2.80 wt% due to the fluorine treatment. The contact angle of the membrane with respect to water was 13°. The permeations of air and a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) through the fluorinated dense polyimide membrane were measured at 35°C. The results are: O₂ permeability coefficient 40.1 cB O₂/N₂ separation factor 9.31 CO₂ permeability coefficient 146 cB CO₂/CH₄ separation factor 97 Reference Example 5 : In a nitrogen atmosphere at room temperature, 5.08 g (21 mmol) of TMB was placed in a 300-ml four-necked flask equipped with a stirrer and a nitrogen inlet and 30 ml of NMP was added to dissolve the TMB. Into the diamine solution was added 9.33 g (21 mmol) of 6FDA together with 30 ml of NMP. The mixture was stirred for 10 hours at room temperature, thereby obtaining a viscous solution of a polyimide precursor (polyamic acid) having a polymer concentration of 20 wt%. A part of the solution was diluted with NMP to prepare a solution having a concentration of 0.5 g/dl. The logarithmic viscosity of the polymer measured at 30°C was 1.57 dl/g. Reference Example 6 : To the polyamic acid solution prepared in Reference Example 5 were added 9 g (84 mmol) of acetic anhydride and 36 ml of NMP. After the mixture was stirred for 1 hour, 9 g (84 mmol) of triethylamine was added thereto and the mixture was stirred for 24 hours to obtain a polyimide solution. The polyimide solution was solidified in water into fibers, pulverized, washed with methanol and dried under reduced pressure to obtain polyimide powder. The polyimide powder was dissolved in NMP to obtain a 10 wt% solution. The solution was filtered through a filter with a 5 µm pore size, and the filtrate was cast on a glass plate at a knife interval of 20 mil (51 µm) using a doctor blade. The glass plate was dried in an oven at 80°C for 4 hours in a nitrogen atmosphere, thereby obtaining a self-supporting membrane. The membrane was then separated from the glass plate and heat treated at 120°C for 4 hours under reduced pressure. The temperature of the oven was the raised from room temperature to 300°C, and heat treatment was carried out at 300°C for 2 hours to obtain a dense polyimide membrane of 1 mil (2.5 µm) thickness. The contact angle of the membrane with respect to water was 79°. Comparative Example 3 : Using the dense polyimide membrane obtained in Reference Example 6, the permeation of pure gases was measured at 30 °C. The results are: H₂ permeability coefficient 22700 cB CO₂ permeability coefficient 22300 cB CH₄ permeability coefficient 860 cB H₂/CH₄ separation factor 26 CO₂/CH₄ separation factor 26 The permeations of air and a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) through the dense polyimide membrane obtained in Reference Example 6 were measured at 35°C. The results are: O₂ permeability coefficient 3080 cB O₂/N₂ separation factor 4.25 CO₂ permeability coefficient 23500 cB CO₂/CH₄ separation factor 26 Example 5 : The dense polyimide membrane obtained in Reference Example 6 was deaerated under a reduced pressure for 1 hour, and subjected to fluorine treatment by bringing the deaerated polyimide membrane into contact with an F₂/N₂ mixed gas having an F₂ concentration of 0.05 vol% at room temperature. The elemental analysis of the treated polyimide membrane showed that the fluorine content in the dense polyimide membrane increased by 0.73 wt% due to the fluorine treatment. The contact angle of the membrane with respect to water was 69°. The permeation of pure gases through the fluorinated dense polyimide membrane at 30°C was measured. The results are: H₂ permeability coefficient 22500 cB CO₂ permeability coefficient 13800 cB CH₄ permeability coefficient 284 cB H₂/CH₄ separation factor 79 CO₂/CH₄ separation factor 49 The permeations of air and a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) through the fluorinated dense polyimide membrane at 35°C were measured. The results are: O₂ permeability coefficient 3990 cB O₂/N₂ separation factor 4.35 CO₂ permeability coefficient 16600 cB CO₂/CH₄ separation factor 48 Example 6 : The dense polyimide membrane was subjected to fluorine treatment in the same way as in Example 5 except for using an F₂/N₂ mixed gas having an F₂ concentration of 0.5 vol%. The elemental analysis of the treated polyimide membrane showed that the fluorine content in the dense polyimide membrane increased by 2.22 wt% due to the fluorine treatment. The contact angle of the membrane with respect to water was 54°. The permeation of pure gases through the fluorinated dense polyimide membrane at 30°C was measured. The results are: H₂ permeability coefficient 7690 cB CO₂ permeability coefficient 772 cB CH₄ permeability coefficient 7.3 cB H₂/CH₄ separation factor 1053 CO₂/CH₄ separation factor 106 The permeations of air and a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) through the fluorinated dense polyimide membrane at 35°C were measured. The results are: O₂ permeability coefficient 482 cB O₂/N₂ separation factor 6.77 CO₂ permeability coefficient 1450 cB CO₂/CH₄ separation factor 133 Example 7 : The dense polyimide membrane was subjected to fluorine treatment in the same way as in Example 5 except for using an F₂/N₂ mixed gas having an F₂ concentration of 4 vol%. The elemental analysis of the treated polyimide membrane showed that the fluorine content in the dense polyimide membrane increased by 9.45 wt% due to the fluorine treatment. The contact angle of the membrane with respect to water was 14°. H₂ permeability coefficient 7570 cB CO₂ permeability coefficient 1590 cB CH₄ permeability coefficient 33.9 cB H₂/CH₄ separation factor 223 CO₂/CH₄ separation factor 47 The permeations of air and a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) through the fluorinated dense polyimide membrane at 35°C were measured. The results are: O₂ permeability coefficient 1270 cB O₂/N₂ separation factor 5.10 CO₂ permeability coefficient 3530 cB CO₂/CH₄ separation factor 82 Reference Example 7 : In a nitrogen atmosphere at room temperature, 5.08 g (21 mmol) of TMB was placed in a 300-ml four-necked flask equipped with a stirrer and a nitrogen inlet and 20 ml of NMP was added to dissolve the TMB. To the diamine solution was added 2.03 g (6.3 mmol) of BTDA together with 20 ml of NMP and the mixture was stirred for 1 hour. To the resultant mixture, was added 6.54 g (14.7 mmol) of 6FDA together with 20 ml of NMP. The mixture was stirred for 10 hours at room temperature, thereby obtaining a viscous solution of a polyimide precursor (polyamic acid) having a polymer concentration of 19 wt%. A part of the solution was diluted with NMP to prepare a solution having a concentration of 0.5 g/dl. The logarithmic viscosity of the polymer measured at 30°C was 1.44 dl/g. Reference Example 8 : To the polyamic acid solution prepared in Reference Example 7 were added 9 g (84 mmol) of acetic anhydride and 36 ml of NMP. After the mixture was stirred for 1 hour, 9 g (84 mmol) of triethylamine was added thereto and the mixture was stirred for 24 hours to obtain a polyimide solution. The polyimide solution was solidified in water into fibers, pulverized, washed with methanol and dried under reduced pressure to obtain polyimide powder. The polyimide powder was dissolved in NMP to obtain a 10 wt% solution. The solution was filtered through a filter with a 5 µm pore size, and the filtrate was cast on a glass plate at a knife interval of 20 mil (51 µm) using a doctor blade. The glass plate was dried in an oven at 80°C for 4 hours in a nitrogen atmosphere, thereby obtaining a self-supporting membrane. The membrane was then separated from the glass plate and heat treated at 120°C for 4 hours under reduced pressure. The temperature of the oven was then raised from room temperature to 300°C, and heat treatment was carried out at 300°C for 1 hour to obtain a dense polyimide membrane of 1 mil (2.5 µm) thickness. The contact angle of the membrane with respect to water was 75°. Comparative Example 4 : The permeations of air and a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) through the dense polyimide membrane obtained in Reference Example 8 were measured at 35°C. The results are: O₂ permeability coefficient 2280 cB O₂/N₂ separation factor 4.59 CO₂ permeability coefficient 19800 cB CO₂/CH₄ separation factor 26 Example 8 : The dense polyimide membrane obtained in Reference Example 8 was deaerated under reduced pressure for 1 hour, and subjected to fluorine treatment by bringing the deaerated polyimide membrane into contact with an F₂/N₂ mixed gas having an F₂ concentration of 4 vol% at room temperature. The elemental analysis of the treated polyimide membrane showed that the fluorine content in the dense polyimide membrane increased by 5.02 wt% due to the fluorine treatment. The contact angle of the membrane with respect to water was 23°. The permeations of air and a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) of the fluorinated dense polyimide membrane at 35°C were measured. The results are: O₂ permeability coefficient 795 cB O₂/N₂ separation factor 5.48 CO₂ permeability coefficient 3190 cB CO₂/CH₄ separation factor 90 Example 9 : The dense polyimide membrane was subjected to fluorine treatment in the same way as in Example 8 except for using an F₂/N₂ mixed gas having an F₂ concentration of 30 vol%. The elemental analysis of the treated polyimide membrane showed that the fluorine content in the dense polyimide membrane increased by 8.60 wt% due to the fluorine treatment. The contact angle of the membrane with respect to water was 15°. The permeations of air and a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) of the fluorinated dense polyimide membrane at 35°C were measured. The results are: O₂ permeability coefficient 976 cB O₂/N₂ separation factor 5.37 CO₂ permeability coefficient 2970 cB CO₂/CH₄ separation factor 80 Reference Example 9 : In a nitrogen atmosphere at room temperature, 5.08 g (21 mmol) of TMB was placed in a 300-ml four-necked flask equipped with a stirrer and a nitrogen inlet and 30 ml of NMP was added to dissolve the TMB. To the diamine solution was added 4.58 g (21 mmol) of PMDA together with 30 ml of NMP and the mixture was stirred for 10 hours at room temperature, thereby obtaining a viscous solution of a polyimide precursor (polyamic acid) having a polymer concentration of 13 wt%. A part of the solution was diluted with NMP to prepare a solution having a concentration of 0.5 g/dl. The logarithmic viscosity of the polymer measured at 30°C was 1.72 dl/g. Reference Example 10 : The polyamic acid solution prepared in Reference Example 9 was filtered through a filter with a 5 µm pore size, and the filtrate was cast on a glass plate at a knife interval of 10 mil (25 µm) using a doctor blade. The glass plate was dried in an oven at 100°C for 1 hour in a nitrogen atmosphere. The temperature of the oven was then raised to 300°C and the glass plate was heat treated at 300°C for 1 hour. The glass plate was then gradually cooled, thereby obtaining a polyimide membrane of 1 mil (2.5 µm) thickness. The contact angle of the polyimide membrane with respect to water was 54°. Comparative Example 5 : The permeations of air and a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) through the polyimide membrane obtained in Reference Example 10 were measured at 35°C. The results are: O₂ permeability coefficient 1120 cB O₂/N₂ separation factor 3.70 CO₂ permeability coefficient 3530 cB CO₂/CH₄ separation factor 34 Example 10 : The dense polyimide membrane obtained in Reference Example 10 was deaerated under a reduced pressure for 1 hour, and subjected to fluorine treatment by bringing the deaerated polyimide membrane into contact with an F₂/N₂ mixed gas having an F₂ concentration of 0.05 vol% at room temperature. The elemental analysis of the treated polyimide membrane showed that the fluorine content in the dense polyimide membrane increased by 0.30 wt% due to the fluorine treatment. The contact angle of the membrane with respect to water was 56°. The permeations of air and a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) through the fluorinated dense polyimide membrane at 35°C were measured. The results are: O₂ permeability coefficient 757 cB O₂/N₂ separation factor 5.16 CO₂ permeability coefficient 3880 cB CO₂/CH₄ separation factor 52 Reference Example 11 : In a nitrogen atmosphere at room temperature, 5.08 g (21 mmol) of 2,2',6,6'-tetramethylbenzidine (2,6-TMB) was placed in a 300-ml four-necked flask equipped with a stirrer and a nitrogen inlet and 30 ml of NMP was added to dissolve 2,6-TMB. To the diamine solution was added 4.58 g (21 mmol) of PMDA together with 30 ml of NMP and the mixture was stirred for 10 hours at room temperature, thereby obtaining a viscous solution of a polyimide precursor (polyamic acid) having a polymer concentration of 13 wt%. A part of the solution was diluted with NMP to prepare a solution having a concentration of 0.5 g/dl. The logarithmic viscosity of the polymer measured at 30°C was 1.36 dl/g. Reference Example 12 : A dense polyimide membrane of 1 mil (2.5 µm) thickness was obtained from the solution prepared in Reference Example 11 in the same way as in Reference Example 10. The contact angle of the polyimide membrane with respect to water was 75°. Comparative Example 6 : The permeation of a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) through the dense polyimide membrane obtained in Reference Example 12 was measured at 35°C. The results are: CO₂ permeability coefficient 2010 cB CO₂/CH₄ separation factor 53 Example 11 : The dense polyimide membrane obtained in Reference Example 12 was deaerated under reduced pressure for 1 hour, and subjected to fluorine treatment by bringing the deaerated polyimide membrane into contact with an F₂/N₂ mixed gas having an F₂ concentration of 4 vol% at room temperature. The elemental analysis of the treated polyimide membrane showed that the fluorine content in the dense polyimide membrane increased by 1.32 wt% due to the fluorine treatment. The contact angle of the membrane with respect to water was 13°. The permeation of a mixed gas of carbon dioxide and methane (containing 20 vol% of carbon dioxide) through the fluorinated dense polyimide membrane at 35°C were measured. The results are: O₂ permeability coefficient 901 cB CO₂/CH₄ separation factor 167

Claims: A process for preparing an aromatic polyimide separation membrane which process comprises fluorinating an aromatic polyimide membrane with fluorine gas or HF gas. A process according to claim 1 wherein the hydrogen permeability coefficient of the aromatic polyimide membrane at 30°C is not less than 10⁻⁹ cm³(STP)·cm/cm²·sec·cmHg. A process according to claim 1 or 2 wherein the aromatic polyimide membrane comprises an aromatic polyimide consisting essentially of the constitutional repeating unit represented by the following formula (I): wherein R represents a tetravalent aromatic group and R' represents a divalent aromatic group. A process according to any one of the preceding claims which includes the steps of preparing the aromatic polyimide membrane by dissolving, in a polar organic solvent, an aromatic polyimide comprising the constitutional repeating unit of formula (I) as defined in claim 3; a polyamic acid comprising the constitutional repeating unit represented by the following formula (VI): wherein R and R' are as defined in formula (I), or a mixture of said aromatic polyimide and polyamic acid; forming a thin layer of said solution on a substrate; evaporating the solvent from the thin layer, if said polyamic acid is present under such conditions as to cause ring closing imidation of said polyamic acid to form said aromatic polyimide; and drying the evaporated thin layer to form an aromatic polyimide membrane. A process according to claim 3 or 4, wherein R in formula (I) is a tetravalent aromatic group selected from (1) a group represented by the following formulae: (2) a group represented by the formula (II): wherein X represents CO, SO₂, C(CF₃)₂, Si(CH₃)₂, CH₂, C(CH₃)₂ or O, ; and (3) a non-benzenoid aromatic group comprising a heterocycle of the formula: A process according to claim 3, 4 or 5 wherein R' in the formula (I) is a divalent aromatic group selected from groups of the following formulae (III), (IV) and (V) and isomeric groups thereof and derived groups therefrom: and heterocycle-containing non-benzenoid aromatic groups represented by the following formula: wherein each of R¹ to R¹⁸ independently represents hydrogen, a halogen, a nitro group or a C₁-C₁₂ organic group which is an alkyl, alkenyl, alkynyl, cycloalkyl, aryl, aralkyl, alkoxy, aryloxy, acyl, acyloxy, or alkoxycarbonyl group, or a carboxyl group or metal salt thereof; n represents an integer from 1 to 4; Y represents CO, SO₂, C(CF₃)₂, Si(CH₃)₂, CH₂, C(CH₃)₂, O, S or NH; and each of Z¹ and Z² independently represents a direct bond, CH₂, CO, SO₂, C(CF₃)₂, C(CH₃)₂, Si(CH₃)₂, CH₂, O or S. A method according to any one of the preceding claims wherein the fluorine gas is in the form of a mixture with at least one other gas.

Inventors: KASAI TETSUO, KOBAYASHI YOSHITERU, KASAI, TETSUO, KOBAYASHI, YOSHITERU
Applicants: MITSUBISHI CHEM IND, MITSUBISHI KASEI CORPORATION
Application year: 1990
Main classification: B01D67
IPC codes: C08J7, B01D71, B01D67, C08G73