JPH0378128B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a method for stabilizing membranes used for membrane separation of mixtures in liquid or gas phase, particularly for gas separation, and aims to improve the practicality of membranes. That is. <Prior Art> In recent years, membrane separation technology has been used in many fields due to its energy saving properties. Various methods have been developed for producing membranes used in such separations, among which a method that separates the membrane into a part that exhibits a separation function (separation functional layer) and a part that maintains mechanical strength (porous support), So-called composite membranes, in which each membrane is composed of individual materials that are optimal for each, are attracting attention as they dramatically increase the degree of freedom in membrane design. As mentioned above, in a composite membrane, a functional layer is provided on a porous membrane, but it does not matter whether the functional layer is formed directly on the porous membrane or whether separately prepared layers are laminated. Usually, the functional layer is formed as a thin layer (thin film) from a solution in which a separation membrane-forming substance is dissolved in an organic solvent. By the way, the composite membrane thus formed gradually deteriorates during membrane separation or while the membrane is being stored, such as a decrease in permeation flow rate or, in some cases, a decrease in selectivity. I saw that. In particular, it is a practical problem that performance deteriorates simply by storing the membrane without subjecting it to a separation operation, so there has been a need to develop a method for preventing such deterioration. The present invention was achieved under such circumstances through intensive research into methods for stabilizing membranes. <Structure of the Invention> The present invention provides a composite membrane consisting of a porous membrane and a separation functional layer formed from an organic solvent solution containing a separation membrane-forming substance present thereon. The pressure on the porous membrane side is lower than the pressure on the separation functional layer side.
This is a film stabilization method characterized by treatment at a temperature of 50 to 200°C for 10 minutes or more while maintaining the temperature at 0.1 Torr to 500 Torr. The porous membrane used in the present invention has a large number of uniformly distributed pores, and includes, for example, nonwoven fabric, synthetic paper, foam, filtration membrane, ultrafiltration membrane, porous film, porous glass, and sintered metal. The porous membrane used in the present invention has the characteristics of a surface pore diameter of 0.005μ to 0.1μ, a surface pore diameter of 0.005μ to 0.1μ, and Opening degree 5% ~ 40
% and air permeation rate 10 ^-4 ~1 c.c./cm ²・sec・cm
Hg (measured at 20°C) is preferred, and polysulfone-based porous membranes or aromatic polyamide-based porous membranes that can easily obtain such properties and are made from materials that are heat resistant and have excellent mechanical properties are preferred. used. Polysulfone, which is the material of the polysulfone porous membrane used in the present invention, is a polymer having -SO ₂ - bonding groups in its molecule in a broad sense, but it has particularly excellent mechanical strength and heat resistance. As the following formula (1) or (2) Examples include polymers containing 50 mol% or more of the repeating unit represented by: Furthermore, in a broad sense, aromatic polyamide, which is the material of the aromatic polyamide porous membrane used in the present invention, has a

It is a polymer having a bonding group represented by the formula: (R is a hydrogen atom, a lower alkyl group, or an aryl group), and at least 50% of the main chain is composed of aromatic moieties. In particular, the following formulas (3) and (4) have excellent mechanical strength, heat resistance, and film formability.
or (5) At least 50 mol% of repeating units represented by
Examples include polymers containing Polysulfone-based porous membranes or aromatic polyamide-based porous membranes are formed using a known wet film-forming method in which a solution of polyester sulfone-based resin or aromatic polyamide-based resin dissolved in a solvent is coagulated in a coagulating liquid. It will be carried out in Examples of such solvents include dimethylacetamide, dithymelformamide, N-methylpyrrolidone, and sulfuric acid. Further, commonly used additives such as pore opening agents and stabilizers can also be added to the polymer solution. In the present invention, a resin solution containing the resin as described above, its solvent, and optionally additives is used to form a film shape such as a flat film, a tubular film, or a hollow fiber by casting or spinning. When forming into a flat film or a tubular film, other supports may be used as necessary. Further, after forming by casting, spinning, etc., the solvent, etc. in the resin solution may be partially dried. In the present invention, film formation is performed by immersing the thus formed film in a coagulating liquid. The coagulating liquid used in forming the porous membrane of the present invention is water, at least one organic liquid that is freely miscible with water, or a mixture thereof. Examples of organic liquids that are freely miscible with water include methanol, ethanol, ethylene glycol, dimethylformamide, dimethylacetamide, N
- Methylpyrrolidone, sulfuric acid, etc. can be mentioned. The coagulation bath of the present invention is particularly preferably a liquid consisting essentially of water. The porous membrane of the present invention can be obtained by immersing a membrane formed from the resin solution in these coagulating liquids, substantially coagulating the membrane, and if necessary, washing with water to remove residual solvent and the like. The porous membrane of the present invention may be formed in the form of a flat membrane, a tubular membrane, a hollow fiber membrane, etc. depending on the purpose of use. In the case of flat membranes, tubular membranes, etc., other base materials such as nonwoven fabrics may be used as reinforcing materials. The separation functional layer of the composite membrane used in the present invention is a thin film made of an organic polymer compound, and such a thin film can be prepared by spreading on water, casting, coating, or other methods from an organic solvent solution containing a separation film-forming substance. It is formed by a well-known method such as an interfacial polymerization method. Therefore, the separation membrane-forming substance may be a substance that forms a membrane as it is, or it may be a substance that forms a membrane in a reaction. As the material for the separation membrane-forming substance that forms the membrane as it is, any known material having excellent selectivity for a specific gas can be appropriately used for gas separation. When the specific gas is, for example, oxygen, an addition polymer or main chain of at least one unsaturated compound selected from hydrocarbon compounds and/or silane compounds having a polymerizable double bond or triple bond between carbons. Polymers having siloxane units in side chains, aromatic polyethers, etc. are preferably used. Examples of such polymers include polymethylpentene, polymethylhexenepolybutadiene, polyvinyltrimethylsilane, polytrimethylsilylacetylene, poly(methylhexane-allyltrimethylsilane) copolymer, poly-t-butylacetylene, poly(allyltrimethylsilane-). allyl-t-butyldimethylsilane), polydimethylsiloxane, polysiloxane-polycarbonate copolymer, polysiloxane-polybutadiene copolymer, poly{methacryloxypropyltris(trimethylsiloxy)silane}, poly2,6-dimethylphenylene You can give things like ether. Any solvent may be used as long as it dissolves the membrane material, such as hydrocarbon solvents such as benzene, toluene, xylene, cyclohexane, cyclohexene, hexane, octane, hexadecene, and octadecene, and trichloroethylene. , halogenable hydrocarbon solvents such as tetrachloroethylene, chloroform, trichloroethylene, tetrahydrofuran,
Ether solvents such as dioxane, aprotic solvents such as dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethanol,
iso - Alcohol solvents such as propyl alcohol and butanol, either alone or in combination.
Used in mixed systems of more than one species. In addition to the solvent, a surfactant or a developing aid may be added to stabilize the film formation. In order to form a thin film functional layer, a solution of a separation membrane forming substance dissolved in such a solvent is prepared, and a thin film layer is formed by a commonly known method. That is, for example, this separation membrane forming material solution is spread on a water surface or casted on a flat solid surface to make a thin solution.
There are methods such as a method in which the solvent is then removed to form a thin film and then laminated on a porous membrane, or a method in which a separation membrane forming substance solution is coated on the porous membrane and the solvent is removed to form a thin film. In either case, the separation membrane-forming substance is dissolved in a solvent to form a solution, which is used to form a membrane, and then the solvent is removed to form a thin membrane. Note that the separation functional layer may be a single layer (single layer) or may be a plurality of layers consisting of at least two layers. In particular, a separation functional layer formed by laminating at least two thin films is preferably used because the defective parts of the thin films are filled by overlapping, improving the separation properties, and the strength of the separation functional layer can be improved. Of course, the thin films to be laminated may be made of the same material or may be made of different materials. As the separation membrane-forming substance of the present invention, a substance that forms a membrane through reaction can also be used. As a method for forming a film by reaction, there is a method using an interfacial polymerization method on a porous film. When used as a gas separation membrane, preferred examples of the interfacial polymerization method used for membrane formation include combinations of polyamine compounds and polyisocyanates or combinations of polyamine compounds and polycarboxylic acid chlorides. A combination of these is preferred because it facilitates the formation of a defect-free thin film. A polyamine compound is a compound that has at least two primary and/or secondary amino groups in its molecule, and if it also has a siloxane unit in its molecule, the gas permeability of the formed membrane increases. preferable. Examples of polyamine compounds containing such siloxane units include, for example: etc. can be given. The polyisocyanate compound that reacts with this is a compound having at least two isocyanate groups in its molecule, and examples of such polyisocyanate compounds include toluylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 4,4' -Diphenyl ether diisocyanate, isophorone diisocyanate, xylylene diisocyanate, and the like. In the interfacial polymerization method, two solutions are prepared: a solution of a polyamine compound dissolved in a solvent and a solution of a polyisocyanate compound dissolved in a solvent that forms an interface with the solvent, and one of the solutions is first impregnated into a porous membrane. Next, the other solution is brought into contact with the surface of the porous membrane to form a two-liquid interface, and a thin film is formed by interfacial polymerization. After the reaction, a composite membrane is obtained by washing with water or alcohol to remove the solvent. As the solvent used to dissolve the polyamine compound, for example, water or alcohols and glycols such as methanol, ethanol, propanol butanol, ethylene glycol glycerin, and diethylene glycol are preferably used. Examples of solvents used to dissolve the polyisocyanate compound include hexane, octane,
Hydrocarbon compounds such as hexadecene and octadecene are preferably used. In such interfacial polymerization, a polysiloxane compound having a silanol end, as previously proposed by the present inventor (Japanese Patent Application No. 59-125046), is added to the amine solution or polyisocyanate solution, or such polysiloxane is added to the amine solution or polyisocyanate solution. It is also possible to subsequently bring the dissolved solution into contact with the generated interfacial film to cause a reaction. By adding such a polysiloxane having a terminal silanol group, the thickness of the resulting interfacial polymerized film can be reduced without any defects. Other methods of forming films by reaction include:
There is a method of forming a thin film by adding a curing agent and/or a crosslinking agent to a solution in which a monomer or oligomer is dissolved, coating the porous membrane, and then causing a curing or crosslinking reaction. The used solvent is removed by washing or drying. Such separation membrane forming substances include, for example, polydimethylsiloxane having silanol ends, and triacetylmethylsilane or triethoxymethyl as a crosslinking agent. The thickness of the separation functional layer of the present invention is preferably as thin as possible in terms of gas permeability, unless defects occur in the membrane, and the thickness is 0.5μ or less, preferably 0.3μ.
It is more preferably 0.15μ or less. The thus obtained composite membrane is heated to a temperature of 50 to 200℃.
The composite membrane of the present invention is treated for 10 minutes or more under gas phase conditions at a reduced pressure of 0.1 Torr to 500 Torr, so that the flow rate and selectivity of the composite membrane are stabilized, especially the flow rate, by such heat treatment. The reason for this stabilization is, first, that a very small amount of the organic solvent used in the step of forming the separation functional layer usually remains in the composite membrane, and that the polysulfone porous membrane obtained by wet membrane formation or the aromatic This is thought to be because a trace amount of the solvent used in forming the group polyamide porous membrane remained, and this was almost completely removed by the heat treatment. Next, heat treatment may also be effective in stabilizing the structure of the separation functional membrane or the porous membrane that is its support. In any case, such heat treatment improves the durability of the composite membrane. The treatment of the composite membrane of the present invention is carried out under gas phase conditions at a reduced pressure of 0.1 to 500 Torr. The reduced-pressure gas phase conditions here refer to cases where the composite membrane of the present invention is placed in such a vacuum container and the entire membrane is treated, and cases where the composite membrane is assembled into a module shape that can be sucked under reduced pressure from one side of the membrane. 0.1~ on one side of the membrane
Aspirate to a reduced pressure of 500Torr (atmospheric pressure on the other side of the membrane)
There are two methods: one in which air or inert gas is passed through a composite membrane; Among these, the latter method is preferably used because a large amount of gas passes through the composite membrane and the treatment effect is large. The degree of reduced pressure is 0.1 to 500 Torr, preferably 10
~300 Torr, preferably 20-250 Torr.
If the temperature exceeds 500Torr, the effect of heat treatment will be small, and
Even if it is set to 0.1Torr or less, the effect is no different than if it is set to 0.1Torr or less. The temperature condition for this treatment is 50 to 200°C, but when assembled as a composite membrane member or module, the temperature is limited by the heat resistance of the member including the module member. That is, if the treatment is performed at a temperature higher than the allowable temperature limit of the member, the membrane or module will be deformed and damaged, resulting in inconvenience. The temperature is preferably lower than the heat resistant temperature of the member, but as high as possible. Generally preferably 65~
The temperature is 180°C, more preferably 70 to 150°C. The heat treatment time is 10 minutes or more, preferably 30 minutes or more, and more preferably 1 hour or more. There is no effect of heat treatment for less than 0.2 hours. The length of processing time is not particularly limited, but is usually within 24 hours. Even if you use it for more than 24 hours, the effect will not be any different than if you use it for less than 24 hours. The composite membrane obtained in the present invention can be used for gas separation by being stacked if it is in the form of a flat membrane, or bundled into multiple membranes if it is in the form of a tube or hollow fiber, and then assembled into a module and further as an oxygen enrichment device. Because it can be used to produce oxygen-enriched air from the atmosphere, it is useful for improving the combustion efficiency of engines and heating equipment, and it is also useful for caring for premature babies and treating people with respiratory diseases, and for artificial lungs and artificial gills. It can be used as Moreover, the composite membrane of the present invention is not only suitable for producing oxygen-enriched air from the atmosphere, but also for separating carbon dioxide from a gas mixture mainly consisting of carbon dioxide and air (e.g. combustion waste gas), and for separating helium or argon gas. separation of helium or argon from gas mixtures consisting primarily of liquefied helium or nitrogen gas (e.g. liquefied helium or argon mixed with air), enrichment of helium from natural gas, or separation of hydrogen and carbon monoxide or methane, etc. It can also be suitably used for the separation of hydrogen from gas mixtures (for example, water gas and process gas). Further, the composite membrane of the present invention can be used as a separation membrane for pervaporation, separation of ethanol and water, desalination of seawater, recovery of valuables from wastewater, etc. The present invention will be further explained below with reference to Examples, but the present invention is not limited to these Examples in any way. Example 1 Densely woven polyester nonwoven fabric (basis weight 180
g/m ² ) on top of polysulfone (Nissan Chemical Udel
p350) Cast a solution consisting of 15wt% and 85wt% N-methylpyrrolidone in a layer approximately 0.35mm thick,
Immediately, the polysulfone layer was gelled in a water bath at 20°C to obtain a nonwoven reinforced polysulfone porous membrane. Subsequently, the porous membrane was washed in running water for 2 days to remove residual solvent. The air permeation rate of this porous membrane after drying was measured to be 1 to 2 x 10 ^-2 cc/cm ² sec cmHg, and the surface scanning electron micrograph showed that the surface pore diameter was 0.01 to 0.02 microns.
The average degree of opening was 16%. On this polysulfone porous membrane, several thin films of poly-4-methylpentene were formed by spreading a solution of poly-4-methylpentene in cyclohexene (3 wt% of cyclohexenyl hydroperoxide was added as a developing aid) on the water surface. A composite membrane was created by stacking the layers to a thickness of approximately 0.12μ. Cut this composite membrane into a size of 25cm x 50cm,
The aluminum plate, net, and composite membrane were stacked in this order, a nozzle opening communicating with the nonwoven fabric side space was attached, and the four sides were solidified with epoxy resin to form a separation membrane element (module). After drying the membrane, air was separated by suction at 160 Torr from the nozzle opening, and oxygen-enriched air with an oxygen concentration of 40.5% was obtained at a rate of 210 c.c./min. Next, put this element in a hot air oven at 80℃,
The pressure was reduced to 130 Torr from the nozzle opening and the treatment was carried out for 3 hours. The performance of this composite membrane after heat treatment is oxygen concentration 40.5
%, and the enriched air amount was 202 c.c./min. Next, suck this element from the nozzle opening.
Suction was carried out continuously for 6,000 hours under a reduced pressure of 160 Torr in a clean air atmosphere at a temperature of 23°C, but the oxygen concentration remained unchanged and the flow rate remained at 95%. On the other hand, when a similar durability test was conducted on a composite membrane without heat treatment, the oxygen concentration did not change, but the flow rate retention rate was 87.
It was %. Comparative Example 1 The entire separation membrane element obtained in the same manner as in Example was placed in a vacuum dryer and heat treated at 80°C for 3 hours under a reduced pressure of 130 Torr with both sides of the membrane conductive. Ta. The performance of the composite membrane after this heat treatment is that the amount of enriched air is 203 c.c./min and the oxygen concentration is 40.4 c.c./min.
It was %. Then, when the same durability test as in Example 1 was conducted using this heat-treated element, the oxygen concentration of the enriched air remained unchanged, but the flow rate retention rate was as low as 91%. Example 2 Poly 4-methylpentene-1 (Mitsui Petrochemical Co., Ltd.)
A solution consisting of 3 parts by weight of cyclohexenyl hydroperoxide (Grade D
The needle tip was continuously supplied onto the water surface maintained at 5°C at a flow rate of cc/min while being in contact with the water surface. The polymer solution spread on the water surface, and the solvent and additives gradually disappeared by evaporation or dissolution in the water, forming a thin film on the water surface. The formed thin film was pulled up onto the polysulfone porous membrane by continuously pressing the polysulfone porous membrane used in Example 1 from above at a distance of 60 cm from the needle tip. Next, polydimethylsiloxane-polycarbonate copolymer (siloxane content 60 mol%, PO ₂ 2×10 ^-8 cc cm/cm ²
sec・cmHg, α: 2.2, tensile modulus 260Kg/
A solution consisting of 8 parts by weight of poly(4-methylpentene), 5 parts by weight of cyclohexenylhydroperoxide ^, and 87 parts by weight of benzene was spread on the water surface to form a thin film of the copolymer. It was laminated on top and layered again to form an intermediate layer with a thickness of 0.08μ. Next, a 0.017 μm thick thin film of poly-4-methylpentene-1 formed in the same manner as above was laminated on the outside of this intermediate layer to obtain a composite membrane having a laminated separation functional layer. Using this composite membrane, we conducted an air separation test by supplying air to the separation functional membrane side and reducing the pressure to 160 Torr on the opposite side, the porous support side.
% oxygen concentration was obtained at a rate of 4.10/m ² min. This was treated in an atmosphere at a temperature of 80° C. for 2 hours while suctioning under a reduced pressure of 130 Torr from a nozzle port communicating with the porous support side. After the treatment, air separation was performed in the same manner, and an amount of oxygen-enriched air with an oxygen concentration of 41.3% was obtained in an amount of 3.91/ ^m2 . In addition, when the treated membrane was subjected to a continuous suction test under the same conditions as in Example 1,
The flow rate retention rate after 6000 hours was over 94%, and the oxygen concentration was also stable. Example 3 Densely woven polyester nonwoven fabric (basis weight 135
g/m ² ) on top of polysulfone (Sumitomo Chemical
PES300P) 15wt%, N-methylpyrrolidone 85wt
A nonwoven fabric-reinforced polysulfone porous membrane was obtained by casting a solution consisting of 100% of the polysulfone into a layer with a thickness of about 0.3 mm, and immediately gelling the polysulfone layer in a 15°C water bath. This porous membrane was washed in running water for 2 days to remove residual solvent. On this polysulfone porous membrane, poly(allyl t)
-butyldimethylsilane-allyltrimethylsilane) copolymer (allyl t-butyldimethylsilane)
30 mol%) with a thickness of 0.024μ, and then a polydimethylsiloxane-polyvinyltrimethylsilane copolymer (dimethylsiloxane content 70 mol%).
0.10 μ thick thin film, then poly(aryl-t)
-butyldimethylsilane-allyltrimethylsilane) copolymer having a thickness of 0.024 μm were laminated in this order to obtain a composite film. When air separation was performed to examine the performance of this composite membrane (at reduced pressure of 50 Torr), the oxygen concentration was 38.8% and the enriched air amount was 1.45/m ² ·min. This composite membrane was heat-treated in a temperature atmosphere of 70° C. for 5 hours by suctioning to a reduced pressure of 120 Torr through a nozzle port communicating with the nonwoven fabric side. When air separation was performed under the same conditions as before, the oxygen concentration was 38.7% and the enriched air amount was 1.39/m ² ·min. As for the durability of this membrane, the flow retention rate after 4000 hours was 96% under the same conditions as in Example 1, which was an improvement in stability compared to the retention rate of 86% for the membrane that was not heat treated. Example 4 Polysulfone (Nissan Chemical, udel p3500) 20wt
%, N-methylpyrrolidone 57wt%, lithium chloride 3wt%, and 2-methoxyethanol 20wt% is discharged from an annular slit at 30°C using water as the core liquid and immersed in water at 25°C to solidify. A polysulfone hollow porous membrane with an outer diameter of 800 μm and an inner diameter of 500 μm was obtained. This hollow porous membrane was packed into a polycarbonate pipe, and both ends were hardened with adhesive to obtain a hollow fiber membrane module. The air permeation rate of this porous membrane is 1×10 ^-2 cc/cm ²・
sec·cmHg (25°C), and the pore diameter of the inner surface of the porous membrane is 0.01 to 0.02μ. Bis(3-
aminopropyl)tetramethyldisiloxane
A mixed solution consisting of 0.5 wt% ethanol, 50 wt% ethanol, and 49.5 wt% water was flowed for 10 minutes at a linear velocity of 1 m/min. After draining the inner liquid, a solution consisting of 0.5 wt% 4,4'-diphenylmethane diisocyanate and 99.5% hexane was flowed for 3 minutes at a linear velocity of 1 m/min, and the membrane was air-dried for 1 day to form a hollow fiber. A composite membrane was obtained. Then, suction was applied at a reduced pressure of 160 Torr from the suction port communicating with the outer space of the hollow fiber in the module in an atmosphere of 75° C., and heat treatment was performed for 5 hours. When air separation was performed at a reduced pressure of 160 Torr, enriched air with an oxygen concentration of 41.0% was obtained at a rate of 0.31/m ² ·min. Even after 5,000 hours of continuous operation, the oxygen concentration remained unchanged and the flow rate retention rate was 94%. For comparison, when no heat treatment is applied, the flow rate retention rate after 5000 hours is 86%, and the oxygen concentration is 40.5%.
% decreased slightly. Example 5 A solution consisting of 15 parts by weight of polym-phenylene isophthalamide and 85 parts by weight of N-methylpyrrolidone was applied to a thickness of 0.3 on a densely woven Dacron nonwoven fabric (basis weight 180 g/m ² ). A nonwoven-reinforced aromatic polyamide porous membrane was obtained by casting the membrane into a layer of mm thick and immediately gelling the aromatic polyamide layer in a water bath in a greenhouse. The surface pore diameter measured from surface scanning electron microscopy is
The average aperture was 0.015 μm and 17%. Also, the air permeation rate is 6×10 ^-3 cc/cm ²・sec・cm
It was Hg. Separately, a solution consisting of 1.5 parts by weight of poly-2,6-dimethylphenylene oxide, 1.5 parts by weight of cyclohexenyl hydroperoxide and 97 parts by weight of trichlorethylene was prepared, and the same procedure as in Example 2 was carried out to coat the poly-2,6-dimethylphenylene oxide on the water surface. A thin film of 2,6-dimethylphenylene oxide was formed and laminated on the aromatic polyamide porous film previously formed.
A plurality of thin films of poly-2,6-dimethylphenylene oxide were stacked to a total thickness of 0.14 μm. A separation module is made from this composite membrane, and suction is applied to 160 Torr from the nozzle port leading to the nonwoven fabric side.
(Atmospheric pressure on the membrane surface side) When air was separated, the oxygen concentration was 40.0% and the enriched air amount was 142 c.c./m ² ·min. Next, this film was applied through the nozzle opening in an atmosphere of 90℃.
The pressure was reduced to 130 Torr and heat treatment was performed for 2 hours. To check the performance of heat treatment, air separation was performed in the same way, and the oxygen concentration was 39.9%, and the enriched air amount was 137.
It was cc/ ^m2・min. When this composite membrane was subjected to a durability test under the same conditions as in Example 1, it was found that
After some time, the oxygen concentration remained stable and the flow rate retention rate was 95%.

Claims (1)

[Claims] 1. A composite membrane consisting of a porous membrane and a separation functional layer formed from an organic solvent solution containing a separation membrane-forming substance present thereon, is heated under gas phase conditions. With the pressure on the porous membrane side lower than the pressure on the separation functional layer side, the pressure on the porous membrane side is 0.1 Torr ~
10 at a temperature of 50-200℃ while maintaining at 500Torr
A film stabilization method characterized by heat treatment for more than a minute. 2. The membrane stabilization method according to claim 1, wherein the porous membrane is a wet-formed polysulfone-based porous membrane or an aromatic polyamide-based porous membrane.