SELECTIVE GAS PERMEATION MEMBRANES AND METHOD OF MANUFACTURING THEM

Abstract: The present invention provides a selective gas permeation membrane in which the inner walls of pores in a porous substrated are coated with organosiloxane polymer and voids are formed in the central portion of the pores. The selective gas permeation membrane is produced by impregnating the porous substrate with the siloxane polymer followed by a crosslinking treatment to such an extent that the penetration degree of the crosslinked organosiloxane polymer reaches a range from 20 to 150 and then removing the crosslinked organosiloxane polymer undergoing insufficient crosslinking by means of a solvent.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention concerns selective gas permeation membranes and a method of manufacturing them. More specifically, it relates to selective gas permeation membranes which are excellent, particularly, in the permeability and separatability of oxygen molecules, capable of obtaining oxygen-enriched gases or oxygen-reduced gases, such as nitrogen-enriched gases efficiently by removing oxygen from oxygen-containing gases such as air and capable of being used suitably to various gas separation membranes for use in combustion facilities, foodstuff industries, medical equipments and waste disposing facilities, as well as to a method of manufacturing such gas permeation membranes with practical advantage. (2) Description of the Related Art As the gas separation method, various methods have been employed long since such as cooling-condensation-evaporation, adsorption separation, solvent extraction separation, etc. By the way, these methods employed generally require a great amount of energy and complicate facilities. In view of the above, gas separation by using membranes made of polymeric material have been attempted generally intending to reduce the consumption of energy. Gas separation methods using such polymeric membranes have been known, for example : (1) Use of polyorganosiloxane of excellent selective gas permeability, particularly, selective permeability of oxygen molecules in the form of a copolymer or a blend with other polymeric material, for example, polycarbonate, polyurethane, polystyrene or polyphenylene oxide with an aim of improving the strength, etc. of the membranes (refer to Japanese Patent Laid-Open Nos. Sho 48-64199, 58-163403, 58-14926, etc.), (2) Use of porous polymeric material for the membranes, and (3) Use of composite membranes prepared by applying a thin film to the surface of a porous support (for instance refer to Japanese Patent Publication No. Sho 59-3201). However, the membranes in (1) and (3) above are not sufficient in the gas permeability, whereas the membranes in (2) above are insufficient for the selectivity. Further, although a method of impregnating pores of a porous polymeric material with polyorganosiloxane has also been attempted, it involves a problem that the gas permeability is low only by such means, failing to obtain a sufficient practical performance. On the other hand, it has also been proposed gas separating composite membranes in which thin films of silver or like other metal are formed at the surface or the inside of a polymeric membrane (Japanese Patent Application Laid-Open No. Sho 58-8510). However, in a case where air is caused to permeate through the composite membrane, although the separation coefficient for O₂/N₂ is increased to 3.0 as compared with that of 1.0 in a case where no metal thin films are formed, the permeation coefficient is rather reduced as low as 5.25 x 10⁻⁹ cm³.cm/cm².sec.kPa (7.0 x 10⁻⁹ cc-cm/cm².sec.cmHg) as compared with that of 0.82 x 10⁻⁷ cm³.cm/cm².sec.kPa (1.1 x 10⁻⁷ cc-cm/cm².sec.cmHg) in a case where no metal thin films are formed. Accordingly, it is not quite satisfactory in view of the permeation coefficient. It has further reported for the separation of oxygen-nitrogen by using Nafion-silver composite membranes in which silver is deposited by way of sulfone groups on the Nafion membranes having such groups (refer to "Journal of Membrane Science", vol. 31, - 227 (1987)). However, in the Nafion-silver composite membranes, although the O₂/N₂ separation coefficient is remarkably increased to 11.0 as compared with that of 2.0 in a case where silver is not deposited, the permeation coefficient is decreased, although little, to 0.75 x 10⁻⁹ cm³.cm/cm².sec.kPa (1.0 x 10⁻⁹ cc-cm/cm².sec.cmHg) as compared with that of 0.75 x 10⁻⁹ cm³.cm/cm².sec.kPa (1.0 x 10⁻⁹ cc-cm/cm².sec.cmHg) in a case where silver is not deposited and the value for the permeation coefficient is not also quite satisfactory. SUMMARY OF THE INVENTION The present invention has been made in view of the foregoing situations. Specifically, the object of the present invention is to provide selective gas permeation membranes which are highly gas permeable and excellent in the gas permeability and the separability, particularly, for the permeability and the separability of oxygen molecules. Another object of the present invention is to provide a method of producing such selective gas permeation membranes which is particularly advantageous from a practical point of view. The present inventors have made earnest studies for overcoming the foregoing problems and, as a result, have found that selective gas permeation membranes excellent both in the gas permeability and separability can be obtained and the foregoing objects can be attained by impregnating a film-like porous substrate with a siloxane polymer followed by drying, or with a crosslinkable organosiloxane polymer followed by crosslinking treatment to a predetermined crosslinking degree and, thereafter, extracting to remove the organosiloxane polymer not undergoing sufficient crosslinking by means of a solvent, and have accomplished the present invention based on such a finding. That is, the present invention concerns a selective gas membrane, in which the inner walls of pores in a porous substrate are coated with an organosiloxane polymer and voids are formed at the central portion of the pores. The present invention also relates to a process for producing a selective gas permeation membrane which comprises impregnating a porous substrate with a siloxane polymer followed by drying, or with a crosslinkable organosiloxane polymer followed by partial crosslinking treatment. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an electromicroscopic photograph by 3,000X illustrating one example of a selective gas permeation membrane according to the present invention; Figure 2 is an electromicroscopic photograph by 3,000X illustrating one example at the rearface of the selective permeation membrane; and Figure 3 is a cross sectional view illustrating a selective gas permeation membrane in which silver is deposited to the surface of pores. DESCRIPTION OF THE PREFERRED EMBODIMENTS - Selective Gas Permeation Membrane- (a) Porous substrate As the porous substrate, there can be mentioned an organic porous substrate and an inorganic porous substrate. As the organic porous substrate, there can be mentioned those polymeric porous substrates including homopolymers or copolymers of nitrocellulose, cellulose acetate, polysulfone, polyacrylonitrile, acrylic polymer such as polyacrylate, olefinic polymer such as polypropylene, polyvinyl chloride, polycarbonate, polyether sulfone, etc. or mixtures of them. As the inorganic porous substrate, there can be mentioned those film-like porous substrates using ceramics such as barium titanate, potassium titanate, silica, alumina and glass, metals such as stainless steels and coppers, either alone or containing them as the main ingredient. In any of the cases of using the abovementioned porous substrates those prepared from fiberous materials into sheet-like materials by means of binder are suitable. The average pore size of the porous substrate is within a range usually from 0.01 to 10 µm, preferably, from 0.1 to 5 µm. If the average pore size is less than 0.01 µm, permeability (permeation coefficient) of gas such as an oxygen gas may sometimes be reduced. On the other hand, if it exceeds 10 µm, the permeability and the separability, particularly, for oxygen molecules may sometimes be reduced. There is no particular restrictions for the configuration of the porous substrate and any of the shapes, for example, flat film-like shape, hollow thread shape or tubular shape may be used. Furthermore, depending on the requirement, a gas permeable support such as a porous support, for example, porous ceramic support, porous organic polymeric support, porous metal support, etc. may be used for supporting the porous substrate. (b) Silver In the present invention, the inner walls of the pores in the porous substrate are coated with an organosiloxane polymer. The inner walls of the porous substrate may be coated directly with the organosiloxane polymer. Further, silver also may be deposited to the inner walls of the pores at least near the surface of the porous substrate and the silver-deposited inner walls may be coated with the organosiloxane polymer, by which the separation performance for oxygen molecules can be improved. In this case, it is technically difficult to deposit silver to the inner walls of the pores only near the surface of the porous substrate. In view of the above, it is practical to deposit silver in a film-like manner to the entire surface of the porous substrate and to the inner walls of the pores near the surface of the porous substrate. For depositing the silver in a film-like manner, known methods, for example, vacuum deposition, ion plating, sputtering, etc. may be employed. In the case of forming a thin silver film by the vacuum deposition, a porous substrate and metal silver are set, for example, to a bell jar type vapor deposition device and a tungsten boat containing metal silver is heated under a sufficient vacuum of about 0.133 kPa (10⁻⁶ torr), so that silver vapors are brought into contact with the surface of the porous substrate and silver is deposited there. The thickness for the thin silver layer thus formed can be controlled optionally by varying the heating temperature and the vapor deposition time. The thickness of the thin silver layer is selected within a range usually from 50 to 5000 Å, preferably, from 100 to 1000 Å. In any of the cases, the thickness has to be controlled such that the fine pores of the porous substrate are not closed by the thin silver film. Accordingly, if the inner diameter of the pores of the porous substrate used is greater, the thickness of the film can also be made greater correspondingly. Other than the method as described above, a method of depositing silver to the substrate by coating silver nitrate and then removing the silver nitrate may also be used. Furthermore, a method of coating or impregnating a paste containing fine silver particles dispersed therein may also be used. Since the thickness of the thin silver film tends to be increased in this method, it is preferably applied to a porous substrate of a relatively large pore size. (c) Organosiloxane polymer The organosiloxane polymer can be classified into siloxane polymers and crosslinkable organosiloxane polymers. As the siloxane polymer, there can be mentioned, for example, a silicone rubber represented by the formula (1): where R¹ and R² represent respectively hydrogen atoms, alkyl group with 1 to 4 carbon atoms, phenyl group, vinyl group, etc. and n is a number showing the degree of polymerization, or a copolymer having a constitutional unit represents by the formula (2) : where R¹ and R² have the same meanings as described above, n and m represent respectively the polymerization degree and A represents the comonomer unit. Further, as the crosslinkable organosiloxane polymer, there can be used those employed customarily as the material for the separation membranes of gases such as oxygen. As the crosslinkable organosiloxane polymer, there can be mentioned those polymer or prepolymer represented by the formula (3) or the formula (4) : where Y and Z represent respectively functional groups such as vinyl group, amino group, hydroxy group and epoxy group, R¹ and R² and n and m have the same meanings as described above. As the crosslinkable organosiloxane polymer, there can be mentioned, for example, polyorganosiloxane, siloxane - polycarbonate block copolymer, dimethylsiloxane - α-methylstyrene copolymer, α,ω-polysiloxane - phenyl resin copolymer, α,ω-diaminoethylpolydimethylsiloxane - polyhydroxystyrene copolymer, siloxane - polyurethane copolymer and siloxane - polyether copolymer. The crosslinkable organosiloxane polymer is converted by means of crosslinking treatment into an organosiloxane polymer. As the crosslinking treatment, there can be mentioned those methods employed customarily, for example, heat treatment, UV-ray irradiation or method of adding a crosslinker. The crosslinkable organosiloxane polymer described above can be obtained, as required, by properly conducting polycondensation under the presence of a crosslinker such as a bi-functional organic silane compound, for example, dimethyldichlorosilane or a trifunctional organic silane compound such as methyltrichlorosilane. In the organopolysiloxane polymer used as the selective gas permeation membranes according to the present invention, it is possible to add additives such as a polymerization promotor, a molecular weight controller, an aging inhibitor, etc. or other polymer within such a range as not impair the object of the present invention. The kind of the polyfunctional group and the ratio of the crosslinkable organosiloxane polymer, the kind and the amount of the crosslinker used optionally may properly be selected depending on the desired properties of the gas permeation membranes. (d) Selective gas permeation membrane The selective gas permeation membrane according to the present invention has coating films of the organosiloxane polymer to the inner walls of the pores in the polymeric porous membrane and has voids at the central portion of the pores (refer for example to Figure 1 and Figure 2). Further, the present invention also includes within a scope thereof those selective gas permeation membranes in which silver is deposited to the inner wall of the pores in the porous substrate at least near the surface of the porous substrate, and the surface of the porous substrate deposited with the silver is coated with the organosiloxane polymer. The selective gas permeation membrane has a structure, for example, as shown in Figure 3, in which a thin silver layer 2 is formed to the surface of the porous substrate 1, and a thin layer 4 of the organosiloxane polymer is formed on the thin layer 2 and to the inner walls of the pores 3. The selective gas permeation membrane can be used alone, as a laminated structure in a stack of two or more of them depending on the purpose of use, or as a composite membrane supported on the support as described above. Since the selective gas permeation membrane according to the present invention has a coating membrane of the organosiloxane polymer of excellent permeation selectivity due to dissolution and diffusion of gases, particularly, oxygen molecules and has voids at the central portion of the pores, it can constitute a selective gas permeation membrane of excellent practical usefulness, which is excellent in the strength of membrane, etc., excellent in permeability and separability for gases, particularly, oxygen molecules. Accordingly, it can reduce the size of a separation device and extend the applicable range remarkably. The selective gas permeation membrane according to the present invention can preferably be utilized for various fields of gas separation membranes such as for the production of oxygen-enriched gases, as well as various other fields such as combustion, foodstuff industry, biochemical industry, medical equipments, analyzing equipments, waste disposals, etc. - Production Method for Selective Gas Permeation Membrane - Explanation is to be made in details for the second invention concerning a production process for the selective gas permeation membrane according to the present invention having practical usefulness. In the production process, the porous substrate may be used as it is, or silver may be deposited to the inner wall of the pores in the porous substrate near the surface of the porous substrate before use. As the porous substrate having silver deposited thereon, those porous substrates having silver deposited in a thin film-like manner at the surface of the porous substrate and to the inner walls of the pores near the surface of the porous substrate are employed practically. The method of forming the thin silver film has been detailed in the item (b) for "Selective Gas Permeation Membrane" as described above. The impregnating treatment can be conducted by the method of (1) impregnating a porous substrate with a siloxane polymer followed by drying, (2) impregnating a porous substrate with a crosslinkable siloxane polymer followed by partial crosslinking and (3) impregnating a crosslinkable siloxane polymer, conducting partial crosslinking treatment and, further, removing the crosslinked organosiloxane polymer undergoing insufficient cross linking by means of a solvent. In the case of using the method (1) above, impregnation can be conducted by those means employed generally, for example, dipping, spraying and various kind of coating methods. The impregnation can be applied to the entire portion or a desired portion of a porous substrate used. Further, the impregnation may be applied from one side of the porous substrate (inside or outer side in a case where the porous substrate is in a hollow thread-like form), or applied from both sides by dipping the entire porous substrate. Referring to the amount of the siloxane polymer used, although it can not be defined simply since it varies depending on the pore volume of the porous substrate used, the membrane thickness of the siloxane polymer formed on the porous substrate, etc., it is desirable to use a sufficient amount to impregnate the inside of the pores over the entire surface of the gas permeation cross section of the porous substrate. In a usual case, the siloxane polymer is desirably used by from 10 to 500 parts by weight, preferably, from 20 to 250 parts by weight based on 100 parts by weight of the porous substrate. The amount of the siloxane polymer used has to be determined so that the pores are not closed by the siloxane polymer. It is also possible to form a crosslinked organosiloxane polymer to a portion corresponding to the desired film thickness of the porous substrate used by properly selecting the amount of the siloxane polymer used for the method of impregnation. After applying impregnation of the siloxane polymer in this way, drying is applied by usual means, a solvent is removed if it is used, and a thin film of the organosiloxane polymer is formed to a desired thickness to the inner walls of the pores in the porous substrate. In the case of using the method (2) and (3) above, the porous substrate is impregnated with the crosslinkable organosiloxane polymer as it is or after properly diluting with a solvent if the crosslinkable organosiloxane polymer is liquious, or after dissolving into a solution of an appropriate concentration by using a solvent if the crosslinkable organosiloxane polymer is solid. The method and the amount of impregnation are the same as those in (1) above. After applying the impregnation as described above, crosslinking treatment is applied to the crosslinkable organosiloxane polymer impregnated into the pores. For the crosslinking treatment, customary means can be used such as heat treatment, UV-irradiation or addition of a crosslinker. In this case, it is desirable that the degree of crosslinking is controlled within a specific range. That is, the crosslinking treatment is desirably conducted such that the degree of the crosslinking for the crosslinkable organosiloxane polymer is such that the penetration degree is within a range from 20 to 150 and, preferably, within range from 50 to 80. The penetration degree is based on that for the crosslinked organosiloxane polymer obtained by applying crosslinking treatment under the same conditions for the crosslinkable organosiloxane polymer, separately, without impregnation to the porous substrate, which corresponds to 1/4 immiscible viscosity measured according to JIS-K2220 method. If the crosslinking degree is less than 20 expressed by the penetration degree, the crosslinkable organosiloxane polymer undergoing only insufficient crosslinkable in the porous substrate can not effectively be extracted even applying solvent extraction in the extraction step and, accordingly, the gas permeation performance of the resultant gas permeation membrane may not sometimes be sufficient. On the other hand, if the crosslinking is applied to such an extent as providing the penetration degree of greater than 150, films of the organosiloxane polymer may not sometimes be formed sufficiently to result in the reduction of the gas selectivity, particularly, for oxygen molecules. As the method used for the specific crosslinking treatment, there is no particular restriction and heat treatment, for example, by warm or hot blow can preferably be employed usually. Although the conditions for the heat treatment can not generally be defined, depending on the kind of the crosslinkable organosiloxane polymer used, the addition ratio of crosslinker and like other conditions, there are generally applicable those conditions as applying the treatment at a temperature from 10 to 150°C, preferably, from 50 to 100°C, usually for 1 min to 2 hours, preferably, from 10 to 30 min. In the case of using a solvent for the impregnation, the heat treatment may be applied after the removal of the solvent by the evaporation, etc. or heat treatment may be conducted under the presence of the solvent. After applying the crosslinking treatment as described above, extraction treatment using a solvent is applied to remove the not-crosslinked crosslinkable organosiloxane polymer from the crosslinked product. As the solvent usable for the extracting treatment, there can be used those not substantially dissolving the porous substrate used but capable of dissolving the not-crosslinked crosslinkable organosiloxane polymer. Usually, hydrocarbon solvents etc. can be used preferably. As specific examples of the hydrocarbon solvent, there can be mentioned aliphatic hydrocarbons such as hexane, heptane and octane, cycloaliphatic hydrocarbons such as cyclohexane and methylcyclohexane, aromatic hydrocarbons such as benzene, toluene and xylene and halogenated hydrocarbons such as carbon tetrachloride, dichloromethane, trichloroethane, tetrachloroethane, trichloroethylene and chlorobenzene. These extracting solvents may be used alone or as a mixture of two or more of them. Among the various solvents described above, aromatic hydrocarbons are preferred and, particularly, benzene, toluene and xylene are preferred. While customary means can be employed for the extracting treatment, a method of using a heated solvent, method of recycling the solvent and like other heating methods can preferably be employed. Although the conditions employed for the solvent extraction can not generally be defined since they vary depending on the kind of the crosslinkable organosiloxane polymer employed, the kind and the amount of solvent used, the conditions are, for example, a temperature range usually from 20 to 150°C, preferably from 50 to 100°C and a treating time usually from 1 min to 1 hour, preferably, from 10 min to 30 min. After applying the extracting treatment, washing is applied optionally by means of the solvent. The solvent contained in the resultant selective gas permeation membranes is dried up by usual drying method, for example, heat-drying, hot blow drying and reduced pressure drying, thereby enabling to obtain an aimed selective gas permeation membrane. As has been described above, it is possible to obtain a selective gas permeation membrane having crosslinked organosiloxane polymer in the pores of the porous substrate and having voids at the central portions of the pores easily and efficiently. ADVANTAGE OF THE INVENTION According to the first invention of the present application, it is possible to provide a selective gas permeation membrane of practical usefulness, which is excellent in mechanical property such as strength of membrane, remarkably improved gas permeability such as for gaseous oxygen without reducing the permeation selectivity to gases, particularly, gaseous oxygen. Further, according to the second invention of the present application, it is possible to provide a production process for the selective gas permeation membrane of remarkable practical usefulness having excellent performance as described above easily and efficiently by the impregnation treatment. EXAMPLE Example 1 After coating and impregnating a nitrocellulose membrane filter of 1.0 µm pore size with polysiloxane (KE1052, manufactured by Shinetsu Silicone Co.), crosslinking was applied in a hot blow at 70°C for 2 hours. Separately, penetration degree was measured for the crosslinked polysiloxane prepared by applying crosslinking to the polysiloxane alone in a hot blow at 70°C for 2 hours, according to JIS-K2220. As a result, the penetration degree was 65. Then, the resultant filter impregnated with the crosslinked polysiloxane was immersed in toluene at 93°C for 45 min to remove both not-crosslinked polysiloxane and the polysiloxane undergoing insufficient crosslinking by extraction. Then, the product was dried under a reduced pressure at 50°C to obtain a gas permeation membrane. For the resultant gas permeation membrane, the permeability was evaluated both for oxygen gas and nitrogen gas, respectively, by means of a pressure method (with differential pressure of 49 kPa [0.5 kg/cm²]). The results are shown in Table 1. Examples 2 - 7 After coating and impregnating the same filter as in Example 1 with various polysiloxanes respectively, crosslinking was applied in a hot blow at 100°C for 2 hours. Separately, the polysiloxane was applied with crosslinking treatment and the penetration degree was as shown in Table 1. The resultant filter impregnated with the crosslinked polysiloxane was subjected to the same solvent extraction and drying as in Example 1 and evaluated for the gas permeability. The results are shown in Table 1. Example 8 The procedures were the same as those in Example 1 except for using a barium titanate porous membrane (manufactured by Honshu Seishi Co.) as the porous substrate. The results are shown in Table 1. Example 9 The procedures were the same as those in Example 1 except for using an alumina substrate as the porous substrate. The results are shown in Table 1. Example 10 The procedures were the same as those in Example 1 except for using a stainless steel filter of 3.0 µm pore size (manufactured by Nippon Seisen Co.) as the porous substrate. The results are shown in Table 1. Comparative Example 1 The procedures were the same as those in Example 1 except for not conducting the toluene extraction. The results are shown in Table 1. Comparative Example 2 After coating and impregnating the same nitrocellulose membrane filter as used in Example 1 with polysiloxane (X82-7308 : manufactured by Shinethu Silicone Co.), crosslinking was applied by UV-irradiation for 3 min. In this case, the penetration degree of the crosslinked polysiloxane was less than 1. The product was applied with extraction treatment as and dried as in Example 1, and the permeability was evaluated. The results are shown in Table 1. Comparative Example 3 After coating and impregnating the same barium titanate porous membrane as used in Example 8 with polysiloxane (KE-103 : manufactured by Shinetsu Silicone Co.), crosslinking was applied by heating at 100°C for 2 hours. The penetration degree of the crosslinked polysiloxane under this condition was 4. The resultant selective gas permeation membrane was applied with the same solvent extraction and drying as in Example 1 and then the gas permeability was evaluated. The results are shown in Table 1. - Evaluation - From the results in Table 1, it can be seen that the selective gas permeation membranes prepared by impregnating the porous substrates with the crosslinkable organosiloxane polymer, applying crosslinking treatment to such an extent that the degree of the crosslinking reaches a range of the penetration degree for the crosslinked organopolysiloxane polymer of from 20 to 150, and removing the organosiloxane polymer undergoing insufficient crosslinking by using the solvent (Examples 1 - 10) have more excellent gas permeability than those selective gas permeation membranes produced in other methods (Comparative Examples 1 - 3). Examples 11 - 14 The procedures were the same as those in Example 1 except for using a nitrocellulose membrane filter having the pore size of the values shown in Table 2. The results are shown in Table 2. Example 15 The procedures were the same as those in Example 1 except for using an acetyl cellulose membrane filter (0.8 µm pore size) as the filter. The results are shown in Table 2. Example 16 The procedures were the same as those in Example 1 except for using a regenerated cellulose membrane filter (0.65 µm pore size) as the porous substrate. The results are shown in Table 2. Example 17 The procedures were the same as those in Example 1 except for using a polyvinyl chloride porous membrane (0.45 µm pore size) as the porous substrate. The results are shown in Table 2. Example 18 The procedures were the same as those in Example 1 except for using a polyacrylate porous membrane (1.2 µm pore size) as the porous substrate. The results are shown in Table 2. Example 19 The procedures were the same as those in Example 1 except for using a polypropylene porous membrane (0.65 µm, pore size) as the porous substrate. The results are shown in Table 2. Example 20 The procedures were the same as those in Example 1 except for using hexane as the extraction solvent at an extraction temperature of 69°C. The results are shown in Table 2. Example 21 The procedures were the same as those in Example 1 except for using carbon tetrachloride as the extraction solvent at an extraction temperature of 76°C. The results are shown in Table 2. Example 22 The procedures were the same as those in Example 1 except for using a polyether sulfone porous membrane (0.8 µm pore size) as the porous substrate using hexane as the extraction solvent and applying extraction at 69°C. The results are shown in Table 2. Example 23 The procedures were the same as those in Example 1 except for using a modified polysulfone porous membrane (0.4 µm pore size) as the porous substrate using hexane as the extraction solvent and applying extraction at 69°C. The results are shown in Table 2. Example 24 The procedures were the same as those in Example 1 except for using a polysiloxane having a methylphenylsiloxane skelton for the molecular structure. The results are shown in Table 2. Next, the permeation and the separation coefficient (N₂/O₂) for oxygen gas and nitrogen gas were measured for the selective gas permeation membrane according to the present invention in which silver was not deposited to the near the surface of the porous substrate. Example 25 After vapor depositing silver to a nitrocellulose porous membrane of 1.0 µm pore size to form a thin silver layer of 300 Å thickness, it was impregnated with a liquid composition comprising a mixture of a liquid organic siloxane compound A (containing -SiH bond in the molecule) and a liquid organic silicon compound B (containing -CH=CH₂ bond in the molecule) in 1 : 1 molar ratio for functional groups and applied with crosslinking under heating at 70°C for 2 hours. Then, the membrane was immersed in toluene and, after applying extraction treatment for not-crosslinked product by heating at 90°C for 30 min, dried to obtain a gas permeation membrane. The membrane was used for the same peremeation test as in Example 1 to obtain the following results : PO₂=(1.3 x 10⁻⁶ cc.cm/cm².sec.cmHg) 0.975 x 10⁻⁶ cm³.cm/cm².sec.kPa PO₂/PN₂ = 2.9 Example 26 After vapor depositing silver to an acrylic porous membrane of 0.8 µm pore size to form a thin silver layer of 250 Å thickness, it was impregnated with a liquid composition comprising a mixture of the liquid organic siloxane compound A and the liquid organic silicon compound B in 1 : 1 molar ratio for functional grops and applied with crosslinking under heating at 70°C for 2 hours. Then, the membrane was immersed toluene and, after applying extraction treatment for not-crosslinked product under heating at 90°C for 30 min, dried to obtain a gas permeation membrane. The membrane was used for the same peremeation test as in Example 1 to obtain the following results : PO₂ = (2.3 x 10⁻⁶ cc.cm/cm².sec.cmHg) 1.725 x 10⁻⁶ cm³.cm/cm².sec.kPa PO₂/PN₂ = 2.8 Comparative Example 4 The procedures were the same as those in Example 26 except for not forming the thin silver layer in Example 26. The results are : PO₂ = (4.3 x 10⁻⁶ cc.cm/cc².sec.cmHg) 3.22 x 10⁻⁶ cm³.cm/cm².sec.kPa PO₂/PN₂ = 1.9 Example 27 A paste containing fine silver particles dispersed in toluene was coated to a film thickness of 0.1 µm on a nitrocellulose porous membrane of 3.0 µm pore size and dried. Then, after impregnating the coated membrane with a liquid mixture of the liquid organic silicon compound A and the liquid organic silicon compound B in 1 : 1 molar ratio for functional groups, crosslinking was applied under heating at 70°C for 2 hours. Then, the membrane was immersed in toluene and applied with extraction for not-crosslinked product under heating at 90°C for 30 min and then dried to obtain a gas separation membrane. The membrane was used for the same permeation test as in Example 1 to obtain the following results : PO₂ = (1.0 x 10⁻⁶ cc.cm/cm².sec.cmHg) 0.75 x 10⁻⁶ cm³.cm/cm².sec.kPa PO₂/PN₂ = 2.5 Example 28 A paste containing fine silver particles dispersed in toluene was coated to a film thickness of 0.1 µm on a nitrocellulose porous membrane of 5.0 µm pore size and dried. Then, after impregnating the coated membrane with a liquid mixture of the liquid organic silicon compound A and the liquid organic silicon compound B in 1 : 1 molar ratio for functional groups, crosslinking was applied under heating at 70°C for 2 hours. Then, the membrane was immersed in toluene and applied with extraction for not-crosslinked product under heating at 90°C for 30 min and then dried to obtain a gas separation membrane. The membrane was used for the same permeation test as in Example 1 to obtain the following results : PO₂ = (3.4 x 10⁻⁶ cc.cm/cm².sec.cmHg) 2.55 x 10⁻⁶ cm³.cm/cm².sec.kPa PO₂/PN₂ = 2.7 - Evaluation - As apparent from the results of Examples 25 - 28 and Comparative Example 4, it can be seen that selective gas permeation membranes prepared by coating organosiloxane polymers to silver-deposited porous substrates, applying crosslinking and then removing those organosiloxane polymers undergoing insufficient crosslinking by means of solvent treatment, thereby forming voids in the central portions of the pores in the porous substrates are more excellent in the selectivity to oxygen molecules than those of not deposited with silver.

Claims: A selective gas permeation membrane, in which the inner walls of pores in a porous substrate are coated with an organosiloxane polymer and voids are formed to the central portion of said pores. A selective gas permeation membrane as defined in claim 1, wherein silver is deposited to the inner wall of the pores at least near the surface of the porous substrate, and the organosiloxane polymer is coated to the silver-deposited surface of said porous substrate. A selective gas permeation membrane as defined in claim 1, wherein the organosiloxane polymer is a siloxane polymer or a crosslinked organosiloxane polymer having a penetration degree from 20 to 150. A selective gas permeation membrane as defined in claim 1, wherein the inner diameter of the pore in the porous substrate is from 0.01 to 10 um. A method of producing a selective gas permeation membrane, which comprises steps of impregnating a porous substrate with a siloxane polymer followed by drying or with a crosslinkable organosiloxane polymer followed by crosslinking treatment to such an extent that the penetration degree of the crosslinked organosiloxane polymer reaches a range from 20 to 150 and, thereafter, removing the crosslinked organosiloxane polymer undergoing insufficient crosslinking by means of a solvent. A method of producing a selective gas permeation membrane as defined in claim 5, wherein the solvent is a hydrocarbon solvent. A method of producing selective gas permeation membrane as defined in claim 5, wherein silver is deposited to the inner wall of the pores in the porous substrate at least near the surface of said porous substrate.

Inventors: HATA, YASUO, TAKAHASHI, KENKICHI
Applicants: IDEMITSU KOSAN COMPANY LIMITED
Application year: 1988
Main classification: B01D71
IPC codes: B01D67, B01D71