JP2918137B2 - Porous membrane surface-treated with modified nylon and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrophilic porous membrane and a hydrophilic porous membrane.
And a method for producing the same. More specifically hydrophobic
Water-insoluble N-methyl on the microporous surface of polymer porous membraneLow
, N-alkoxymethyl and N-alkylthiomethyl
Coated with modified nylon selected from modified nylon
Related to hydrophilic porous membrane and method for producing the same
It is.

[0002]

2. Description of the Related Art Porous polymer membranes are used for water treatment of aqueous solutions and suspensions, filtration membranes or separation membranes used for production of ultrapure water , plasma separation membranes, artificial lungs, etc. In various fields such as medical materials or batteries, and battery separators used for electrolysis, etc., it is preferable that the membrane is hydrophilic in any case.

[0003] The hydrophilic porous membrane can be roughly classified into a porous membrane made of a hydrophilic polymer as a raw material, and a porous membrane made of a hydrophobic polymer as a raw material, the surface of which is subjected to a hydrophilic treatment.

As the former porous membrane, a membrane made of a water-insoluble hydrophilic polymer material derived from cellulose, polyvinyl alcohol, nylon or the like is well known. The feature of these porous membranes is that the membrane material itself is hydrophilic, so the interaction with water is large and it is easy to get wet with water,
Water or an aqueous solution penetrates into the membrane without special treatment of the membrane in advance, and filtration and separation can be easily performed.

[0005] However, these hydrophilic porous membranes have a high water absorption rate of the polymer material itself forming the membrane, so that not only the mechanical strength is reduced due to swelling, but also the pore size,
There is a disadvantage that the membrane performance is reduced due to a decrease in porosity. In addition, these hydrophilic porous membranes are often brittle when dried, and have a drawback of low mechanical strength.

Further, these hydrophilic porous membranes are prepared by forming a solution of a hydrophilic polymer as a raw material into a film, evaporating a part or all of the solvent, immersing it in a coagulation bath, and performing phase change. Since many methods are used to form a porous structure, the process is complicated, and the management and treatment of the organic solvent is not only complicated, but also the resulting film is often a film having irregular pores. Having.

On the other hand, as the latter porous membrane obtained by hydrophilizing the surface of the hydrophobic porous membrane, various membranes have been proposed depending on the treatment method.

(1) A membrane obtained by permeating a water-soluble organic solvent such as ethyl alcohol or acetone into the fine pores of a hydrophobic porous membrane, and replacing the water with water.

(2) Coatings of ionic or nonionic surfactants with or without polymerizable functional groups, lipids, etc., and membranes obtained by subsequent polymerization (JP-A-47-14269, JP-A-60-11536, JP-A-61-2743, JP-A-62-114
610, JP-A-63-141609, etc.)

(3) A film obtained by coating a water-soluble polymer having a polymerizable functional group and then insolubilizing the water-soluble polymer with water by irradiation with ionizing radiation such as ultraviolet rays or electron beams, or heat treatment (Japanese Patent Publication No. 55-3
5145, JP-A-62-14904, JP-A-63-97634, and the like. However, the films obtained by these methods have various disadvantages.

The membrane obtained in (1) is hydrophilic as long as the membrane surface and the inside of the micropores are in a wet state, but once the membrane is dried, the hydrophilic property is lost, and the water permeation phenomenon occurs unless the same operation is performed again. It is not allowed.

[0012] In the membrane obtained in (2), the coated surfactant or lipid elutes with the water permeation time, and the treatment liquid is gradually contaminated and the water permeation performance decreases.

In the film (3), the interaction between the water-soluble polymer used for coating and the hydrophobic film is small,
The water-soluble polymer is not uniformly coated on the surface and in the microporous surface of the hydrophobic membrane with good adhesiveness. In addition, when the water-insoluble polymer is insolubilized by the post-treatment, there is a problem that the pore size and the porosity decrease, and when the water-insolubilization does not sufficiently proceed,
The treatment solution is contaminated due to the elution of the water-soluble polymer. Further, post-treatment such as irradiation with ionizing radiation or heating involves deterioration of the film material itself. It is difficult to industrially employ a treatment by ionizing radiation irradiation that requires a large-scale facility for hydrophilizing a polymer porous membrane.

As described above, any of the conventionally known hydrophilic porous membranes has various disadvantages, and although it is sufficient as a microporous membrane suitable for separation and purification of an aqueous fluid having excellent water permeability. Absent.

Accordingly, an object of the present invention is to provide a filtration membrane or a separation membrane used for water treatment of an aqueous solution or an aqueous suspension without the above-mentioned disadvantages, production of ultrapure water, a plasma separation membrane, an artificial lung, etc. It is an object of the present invention to provide a hydrophilic porous film having uniform fine pores which can be suitably used in a field requiring hydrophilicity such as a medical material or a battery separator used for batteries and electrolysis.

[0016]

The inventors of the present invention have conducted intensive studies on a hydrophilic porous membrane overcoming the above-mentioned disadvantages and a method for producing the same. The membrane is made of water-insoluble N-methylol, N
A membrane obtained by contacting a solution of a modified nylon selected from -alkoxymethyl and N-alkylthiomethyl-modified nylon is a porous membrane in which the fine pore surface of the hydrophobic membrane is uniformly coated with the modified nylon; The present inventors have found that the porous membrane has excellent heat stability, has sufficient hydrophilicity necessary and sufficient for filtration of water or an aqueous solution, and has a wide range of filtration ability from microfiltration to ultrafiltration. That is, the present invention relates to a hydrophobic membrane having a large number of through micropores, wherein a part or all of the surface of the micropores is water-insoluble N-
An object of the present invention is to provide a porous membrane characterized by being coated with a modified nylon selected from methylol, N-alkoxymethyl and N-alkylthiomethyl-modified nylon, and a method for producing the same.

The hydrophobic polymer membrane having a large number of through micropores used in the present invention has a porosity of 10 to 90% and a porosity of 0.1 to 90%.
It refers to a membrane that does not wet the surface of the membrane and does not penetrate water into the interior of the membrane even when it comes into contact with water and an aqueous solution having an average pore diameter of 01 to 10 μm. For example, high density polyethylene, polypropylene, poly (4-methyl-pentene-
Examples thereof include porous membranes made of polyolefins such as 1), halogenated polyolefins such as polytetrafluoroethylene, polyvinylidene fluoride, and polyvinyl chloride, polysulfone, and polycarbonate.

The above-mentioned N-methylol, N-alkoxymethyl and N-alkylthiomethyl-modified nylons used in the present invention are described in J. Am. Am. Chem. Soc. 71 volumes, 651 pages, 1
949; Chemical and ind. 10
Volume, 985, 1951; US Patent 24
Nylon obtained by acting according to a known method described in, for example, 30860, 1947, in which some or all of the hydrogen atoms of an amide group of nylon are substituted with a methylol group, an alkoxymethyl group and an alkylthiomethyl group. Here, as the original nylon, nylon 6, nylon 66, nylon 610, nylon 11, nylon 11, nylon 61,
Nylon metaxylylenediamine 6, nylon bis (4
-Aminocyclohexyl) methane 6 and copolymers thereof. Of these, those in which the modified nylon is soluble in an aliphatic alcohol having 1 to 5 carbon atoms are particularly preferred. These can be used alone or as a mixture of two or more. The alcohols and thioalcohols are preferably monoalcohols and monothioalcohols, and particularly preferably aliphatic monoalcohols and monothioalcohols having 1 to 6 carbon atoms.

Next, the method for producing the porous membrane of the present invention will be described. A type of modified nylon selected from the above-mentioned N-methylol, N-alkoxymethyl and N-alkylthiomethyl-modified nylon is dissolved in a specific solvent to prepare a solution. The solvent is an aliphatic alcohol having 1 to 5 carbon atoms or an aqueous solution thereof, a hydrocarbon solution, a halogenated hydrocarbon solution, an ether solution, a ketone solution, an ester solution, an amine solution, a carboxylic acid solution, and a mixed solvent thereof. It is a good solvent for the modified nylon and wets the surface of the micropores of the hydrophobic membrane but does not substantially swell or dissolve the polymer forming the hydrophobic membrane.

For example, methanol, ethanol, methanol-water, ethanol-water, isopropanol-water, methanol-toluene, methanol-chloroform, methanol-diethyl ether, ethanol-acetone, methanol-ethyl acetate, ethanol-isopropylamine, ethanol- Formic acid, ethanol-toluene-water, methanol-methylene chloride-water and the like. These can be appropriately selected depending on the type of the hydrophobic membrane and the modified nylon.

Next, the modified nylon solution and the hydrophobic membrane are heated at 10 to 70 ° C., preferably at 20 to 50 ° C. for 1 hour.
For 120 to 120 seconds, preferably 5 to 60 seconds. The method of contact is not particularly limited, such as a method in which the hydrophobic membrane is immersed in the modified nylon solution, a method in which the modified nylon solution is supplied from one of the hydrophobic membranes and passed through the membrane, and a concentration gradient or pressure gradient. Is used as a driving force to permeate the modified nylon solution into the membrane. But the device,
Considering simplicity of operation, processing capacity, efficiency, etc.,
It is preferable to adopt a method of immersing the hydrophobic membrane in the modified nylon solution. Next, the hydrophobic membrane which has been subjected to the permeation treatment of the modified nylon solution is subjected to a drying treatment and / or a contact with water followed by a drying treatment to completely remove the solvent, thereby forming the hydrophobic membrane on the surface of the fine pores. A porous membrane uniformly coated with the modified nylon is obtained.

The porous membrane obtained in this manner is coated with only a very thin surface of the hydrophobic membrane and the surface of the micropores by appropriately adjusting the concentration of the modified nylon solution, the contact time, the number of contacts and the like. Shows sufficient wettability and water permeability, but the pore size is almost the same as the pore size before the contact with the modified nylon solution, the one having a non-porous sheet layer on at least one of the hydrophobic membrane surfaces, There are various types including those in which the pores are closed with the modified nylon. In addition, even if any of the films is immersed in water or repeatedly permeated and dried, the water permeability hardly decreases.

[0023]

The hydrophilized porous membrane of the present invention is a hydrophilic porous membrane whose surface is covered with a water-insoluble modified nylon made of a hydrophobic polymer. The hydrophilic porous membrane obtained by the present invention overcomes the various drawbacks of the hydrophilic porous membrane obtained by the above-described conventional method, while maintaining the physical, chemical, and mechanical properties inherent in the hydrophobic membrane. It is a film having excellent water permeability and durability that can withstand repeated use. Therefore, it is not only used as a separation membrane suitable for separation and purification of aqueous fluids utilizing high water permeability, but also used for batteries, electrolysis, etc. by taking advantage of the characteristics of a water-insoluble hydrophilic material as an immobilization layer. It can also be applied to battery separators and the like.

[0024]

The present invention will be described below in more detail with reference to examples. The present invention is, of course, not limited to the following embodiments.

(Example 1) Polypropylene (UBE-P)
P-F109K, trade name: Ube Industries, Ltd., MFI =
9 g / 10 min) using an extruder equipped with a die having an outer diameter of 150 mm, a molding temperature of 190 ° C., and a take-up speed of 3
An inflation film was formed under the condition of 0 mm / min.

The obtained polypropylene film was 145
Heat treatment in a heated air bath at 145 ° C. for 30 minutes, and then stretched in liquid nitrogen (-195.8 ° C.) by 20% with respect to the initial length. 2
Heat treatment was performed for minutes.

This film was placed in a heated air bath at 145 ° C.
Heat treatment for 0 minutes, then liquid nitrogen (-195.6 ° C)
In the medium, the film was stretched by 20% with respect to the initial length, and heat-treated in a heated air bath at 145 ° C. for 2 minutes while maintaining the stretched state.

This film was heated at 130 ° C. in an air atmosphere for 4 hours.
After performing hot stretching of 00%, the stretched state is maintained at 14%.
Heat treatment was performed for 15 minutes in a heated air tank at 5 ° C. to produce a porous polypropylene flat membrane.

The average pore size of the obtained porous polypropylene flat membrane was 0.27 μm as measured by a half dry method using ethanol. The porosity is a POROSIMETRO SERIES made by CARLOERBA (Italy).
When measured by a mercury intrusion method using 1500, 73.
6%. When the membrane surface and membrane cross section of this polypropylene porous flat membrane were observed with a scanning electron microscope, fine fibril groups were formed on the membrane surface at approximately equal intervals and in parallel with the stretching direction of the membrane. It was found that the voids spread two-dimensionally in the direction of the cross section of the film, and formed a large number of fine through holes. Next, N-methoxymethyl-modified nylon 6 was synthesized. Nylon 6 (U
To a formic acid solution (concentration: 30% by weight / volume) containing 30 g of BE NYLON 1013B (trade name: manufactured by Ube Industries, Ltd.), 60 ml of a 50% by weight / volume% methanol solution of paraformaldehyde was gradually added with stirring at 60 ° C.
After stirring for 30 minutes, 2 liters of acetone-water (1-
1, volume ratio), washed with aqueous ammonia, and dried to obtain a white granular material. The obtained white granular material was N-methoxymethyl-modified nylon 6 in which 40% of the hydrogen of the amide group of nylon 6 was substituted with a methoxymethyl group.

Subsequently, the porous polypropylene flat membrane was immersed in a methanol solution of the above-mentioned N-methoxymethyl-modified nylon 6 at a concentration of 2% by weight / volume for 1 minute at room temperature, and the solution was poured into the through pores of the porous polypropylene flat membrane. After thoroughly spreading, the mixture was air-dried at room temperature for 2 hours.

When the surface and the cross section of the porous membrane thus obtained were observed with a scanning electron microscope, it was observed that the porous membrane had essentially the same number of through micropores as the porous flat membrane before coating. When water was brought into contact with the porous flat membrane, the water easily permeated into the micropores and permeated. Permeation rate is 1
It was 60.9 liter / min · m ² · kgf / cm ² . When the porous flat membrane whose water permeation rate was measured was dried and contacted again with water, water easily permeated again into the micropores of the porous membrane as described above, and the change in water permeation rate was within 5%. This operation was repeated 10 times.
There was no change in the permeation phenomenon, and the change in the water permeation rate was also within ± 5%.

(Example 2) The procedure of Example 1 was repeated except that the modified nylon was changed as follows. Nylon 66 (UBE NYLON 2015
B, trade name: Ube Industries, Ltd.) 90 g formic acid 2 in 60 g
Add 00 ml and stir at 60 ° C. for 2 hours to prepare a formic acid solution of nylon 66. To this solution, 170 ml of a 35% by weight aqueous solution of paraformaldehyde was added little by little, and then 50 ml of water was added.
Pour into water (1-1, volume ratio), wash in the order of water, aqueous ammonia, and acetone, and dry to obtain a white granular material. The obtained white granules are 19% of the hydrogen of the amide group of nylon 66.
Was N-methylol-modified nylon 66 substituted by a methylol group.

Subsequently, the porous polypropylene flat membrane of Example 1 was immersed in a 1.5% by weight / volume methanol solution of the N-methylol-modified nylon 66 at room temperature for 1 minute to reach the inside of the through-pores of the porous polypropylene flat membrane. After the solution was thoroughly spread, it was air-dried at room temperature for 2 hours.

When the surface and the cross section of the porous membrane thus obtained were observed with a scanning electron microscope, it was found that the porous membrane had essentially the same number of through micropores as the porous flat membrane before coating. When water was brought into contact with the porous flat membrane, the water easily permeated into the micropores and permeated. Permeation speed is 13
It was 5 liter / min · m ² · kgf / cm ² .
When the porous flat membrane whose water permeation rate was measured was dried and contacted again with water, water easily permeated again into the micropores of the porous membrane as described above, and the change in water permeation rate was within 5%. This operation was repeated 10 times, but there was no change in the permeation and permeation phenomena of water, and the change in water permeation rate was within ± 5%.

(Example 3) The procedure of Example 1 was repeated, except that the modified nylon was changed as follows. Nylon 66 (UBE NYLON 2015
B, trade name: Ube Industries, Ltd.) 80 g of powder, 100 g of paraformaldehyde and 300 ml of pyridine were charged into a 1-liter stainless steel autoclave equipped with a stirrer and kept at 130 to 135 ° C for 30 minutes. After cooling to room temperature, the product was poured into 3 liters of water, washed with running water and dried to give a bulky white powder. The obtained white powder was N-methylol-modified nylon 66 in which 56% of the hydrogen of the amide group of nylon 66 was replaced by a methylol group.
Met.

Subsequently, the polypropylene porous flat membrane of Example 1 was immersed in a 1% by weight / volume methanol solution of the N-methylol-modified nylon 66 at room temperature for 30 seconds, and the solution was poured into the through pores of the polypropylene porous flat membrane. After thoroughly spreading, it was air-dried at room temperature for 2 hours.

When the surface and the cross section of the porous membrane thus obtained were observed with a scanning electron microscope, it was observed that the porous membrane had essentially the same number of through micropores as the porous flat membrane before coating. When water was brought into contact with the porous flat membrane, the water easily permeated into the micropores and permeated. Permeation speed is 15
It was 1 liter / min · m ² · kgf / cm ² .
When the porous flat membrane whose water permeation rate was measured was dried and contacted again with water, water easily permeated again into the micropores of the porous membrane as described above, and the change in water permeation rate was within 5%. This operation was repeated 10 times, but there was no change in the permeation and permeation phenomena of water, and the change in water permeation rate was within ± 5%.

(Example 4) The procedure of Example 1 was repeated, except that the modified nylon was changed as follows. Nylon 66 (UBE NYLON 2015
B, trade name: Ube Industries, Ltd.) 50 g of 90% formic acid 1
The mixture was dissolved in 20 ml, and a mixture of 120 g of ethyl mercaptan containing 0.1 g of sodium hydroxide and 60 g of paraformaldehyde was added little by little at room temperature with stirring. After 1 hour, the stirring was stopped, and the uniform layer was separated.
200 to 1500 ml of acetone was gradually added, and ammonia water was further added to this solution, followed by washing with water and drying to obtain a white granular material. The white granules obtained are nylon 66
Was N-ethylthiomethyl-modified nylon 66 in which 28% of the hydrogens of the amide groups were replaced by ethylthiomethyl groups.

Subsequently, Example 1 was added to a 2% w / v methanol solution of N-ethylthiomethyl-modified nylon 66.
Was immersed at room temperature for 1 minute to sufficiently spread the solution into the through-pores of the porous polypropylene flat membrane, and then air-dried at room temperature for 2 hours.

When the surface and the cross section of the porous membrane thus obtained were observed with a scanning electron microscope, it was found that the porous membrane had essentially the same number of through micropores as the porous flat membrane before coating. When water was brought into contact with the porous flat membrane, the water easily permeated into the micropores and permeated. Permeability is 12
It was 3 liters / min · m ² · kgf / cm ² .
When the porous flat membrane whose water permeation rate was measured was dried and contacted again with water, water easily permeated again into the micropores of the porous membrane as described above, and the change in water permeation rate was within 5%. This operation was repeated 10 times, but there was no change in the permeation and permeation phenomena of water, and the change in water permeation rate was within ± 5%.

(Example 5) In Example 1, Luckami was used as a solution of N-alkoxymethyl-modified nylon.
de 5003 (N-methoxymethyl-modified nylon 6,
The operation was performed in the same manner as in Example 1 except that a 2% by weight / volume% methanol solution was used.

When the surface and the cross section of the porous film thus obtained were observed with a scanning electron microscope, it was found that the porous film had essentially the same number of through micropores as the porous flat film before coating. When water was brought into contact with the porous flat membrane, the water easily permeated into the micropores and permeated. Permeability is 16
It was 4 liter / min · m ² · kgf / cm ² .
When the porous flat membrane whose water permeation rate was measured was dried and contacted again with water, water easily permeated again into the micropores of the porous membrane as described above, and the change in water permeation rate was within 5%. This operation was repeated 10 times, but there was no change in the permeation and permeation phenomena of water, and the change in water permeation rate was within ± 5%.

Example 6 Polypropylene (UBE-P)
P-J109K, trade name: Ube Industries, Ltd., MFI =
9 g / 10 min) was spun at a spinning temperature of 200 ° C. and a take-up speed of 116 m / min using a hollow fiber production nozzle equipped with a gas supply pipe having an outer diameter of 33 mm and an inner diameter of 27 mm. The obtained polypropylene hollow fiber was heat-treated in a heated air bath at 145 ° C. for 6 minutes, and then liquid nitrogen (−195.
(8 ° C.) in the heated air bath at 145 ° C. for 2 minutes while maintaining the stretched state.

The hollow fiber was placed in an air atmosphere at 140 ° C. for 40 hours.
After performing 0% thermal stretching, 145 ° C. while maintaining the stretched state
Was heated in a heated air tank for 15 minutes to produce a polypropylene porous hollow fiber membrane. The average pore size of the obtained polypropylene porous hollow fiber membrane was 0.41 μm, and the porosity was 76.5%. Observation of the membrane surface and membrane cross section of this polypropylene porous hollow fiber membrane with a scanning electron microscope revealed that microfibrils were formed on the membrane surface at substantially regular intervals at equal intervals in the stretching direction of the membrane, and the fibrils and fibrils were formed. It was found that the gaps between them spread two-dimensionally in the cross-sectional direction of the film, and formed a large number of fine through holes.

Next, the polypropylene porous hollow fiber membrane described above was used for N-methoxymethyl-modified nylon 6 used in Example 1.
Was immersed in a 2% by weight / volume% methanol solution at room temperature for 1 minute to sufficiently spread the solution into the through-holes of the porous polypropylene flat membrane, and then air-dried at room temperature for 2 hours. Observation of the surface and the cross section of the porous membrane thus obtained with a scanning electron microscope revealed a form in which essentially the same number of through micropores as the porous hollow fiber membrane before coating were retained.

When the porous flat membrane was brought into contact with water, the water easily penetrated into the micropores and permeated. Permeability is 172
Liter / min · m ² · kgf / cm ² . When the porous hollow fiber membrane whose water permeation rate was measured was dried and again contacted with water, water permeated into the pores of the porous membrane again as described above, and the change in water permeation rate was also within 5%. . This operation was repeated 10 times, but there was no change in the permeation and permeation phenomena of water, and the change in water permeation rate was within ± 5%.

(Comparative Example 1) When the polypropylene porous flat membranes described in Examples 1 to 5 were brought into contact with water without pretreatment with ethanol or the like, the porous flat membrane repelled water, and water was used as the porous flat membrane. The surface was hemispherical. Water pressure 2kgf / cm
Attempted water permeation at ² but the water permeation rate was 0 l / min
M ² · kgf / cm ² .

(Comparative Example 2) When the porous polypropylene hollow fiber membrane described in Example 6 was brought into contact with water without pretreatment with ethanol or the like, the porous flat membrane repelled water, and water was exposed on the surface of the porous flat membrane. Showed a hemispherical shape. Water pressure 2kgf / cm ²
Tried to permeate water, but the water permeation rate was 0 l / min.
m ² · kgf / cm ² .

Comparative Example 3 The porous polypropylene flat membranes described in Examples 1 to 5 were immersed in a 2% by weight / volume% methanol solution of poly-N-vinylpyrrolidone (average molecular weight 10,000) at room temperature for 30 seconds. After sufficiently spreading the solution to the inside of the through micropores of the porous flat membrane, it was air-dried at room temperature for 2 hours. Observation of the surface and the cross section of the porous membrane thus obtained with a scanning electron microscope revealed a form in which essentially the same number of through micropores as the porous hollow fiber membrane before coating were retained.

When the porous flat membrane was brought into contact with water, the water easily penetrated into the micropores and permeated. Permeability is 122
Liter / min · m ² · kgf / cm ² . When the porous flat membrane whose water permeation rate was measured was dried and brought into contact with water again, the porous flat membrane repelled water, and the water exhibited a hemispherical shape on the surface of the porous flat membrane. Water permeation was attempted at a water pressure of 2 kgf / cm ² , but the water permeation rate was 0 l / min · m ² · kgf
/ Cm ² .

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-84434 (JP, A) JP-A-4-187226 (JP, A) JP-A-61-283305 (JP, A) JP-A-4-283 265131 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B01D 53/22 B01D 61/00-71/82 510 C02F 1/44

Claims (2)

(57) [Claims] 1. A number of through the surface of the hydrophobic polymer membrane having micropores is water-insoluble N- methylcarbamoyl role, is covered by N- alkoxymethyl and N- alkylthiomethyl modified selected modified nylon than nylon A porous membrane, characterized in that: 2. A hydrophobic polymer porous membrane having a large number of through micropores is prepared, and the hydrophobic membrane is brought into contact with a solution of a water-insoluble modified nylon, and then heated and / or the modified nylon is insoluble. The method for producing a porous membrane according to claim 1, wherein the solvent forming the solution is brought into contact with a soluble liquid.