WO2011010690A1 - Process for producing porous film - Google Patents

Abstract

Provided is a process for producing a porous film which comprises a step in which a film-forming solution containing a film-forming polymer and an additive for controlling phase separation is coagulated in a coagulation liquid to obtain a porous-film precursor and a step in which the phase-separation control additive that remains in the porous-film precursor is removed, wherein as the phase-separation control additive, use is made of polyvinylpyrrolidone having an integrated molecular-weight distribution curve in which the proportion of the area of a high-molecular-weight region is 11% or less. In the process, the additive for controlling phase separation can be removed by a treatment conducted in a short time period, and a porous film having satisfactory permeability can be produced.

Description

Method for producing porous membrane

The present invention relates to a method for producing a porous membrane suitable as a microfiltration membrane or an ultrafiltration membrane in water treatment.
This application claims priority on July 22, 2009 based on Japanese Patent Application No. 2009-171121 filed in Japan, the contents of which are incorporated herein by reference.

In recent years, with increasing interest in environmental pollution and stricter regulations, water treatment using a membrane method using a filtration membrane has attracted attention because of its excellent separation completeness and compactness. In such applications for water treatment, filtration membranes are required to have excellent separation characteristics, permeation performance and mechanical properties.

Conventionally, filtration membranes manufactured by a wet or dry wet spinning method using a hydrophobic polymer such as polysulfone, polyacrylonitrile, cellulose acetate, or polyvinylidene fluoride as a membrane material forming polymer are known as filtration membranes having excellent permeation performance. It has been. These filtration membranes are manufactured by microphase separation of a polymer solution, and then solidifying the polymer solution in a non-solvent, and are provided with a dense layer and a support layer, and have a high porosity and an asymmetric property. It has a simple structure. As a specific film forming method, a method of coagulating a film forming solution containing a film material forming polymer and an additive for controlling phase separation in a coagulating solution is known. As an additive for controlling phase separation, for example, a hydrophilic polymer such as polyethylene glycol or polyvinyl pyrrolidone is used. The additive controlling phase separation is removed after coagulation.

As a method for removing the hydrophilic polymer after coagulation, Patent Document 1 proposes a method of decomposing the hydrophilic polymer with an oxidizing agent, and Patent Document 2 proposes a method of chemically treating the hydrophilic polymer. . Patent Document 3 proposes a method of removing a degradable polymer such as polyvinylpyrrolidone using a decomposing agent.

Japanese Patent No. 3196029 US Pat. No. 5,076,925 Japanese Patent No. 3169404

In a porous membrane made of a hydrophobic polymer, the permeation performance deteriorates if a hydrophilic polymer (additive that controls phase separation) remains in the porous portion after film formation. A method for removing the agent at a higher level by a simpler method is required.
However, the method described in Patent Document 1 is disadvantageous in terms of productivity, for example, it takes several hours to several tens of hours or more for the removal treatment of the hydrophilic polymer.
In addition, the methods described in Patent Documents 2 and 3 are not always satisfactory because good permeability may not be obtained depending on the type of hydrophilic polymer.
The present invention has been made in view of the above circumstances, and provides a method for producing a porous membrane that can remove an additive for controlling phase separation in a short time and can produce a porous membrane having good permeability. The purpose is to provide.

In order to solve the above problems, the method for producing a porous membrane of the present invention comprises:
A step of solidifying a film-forming solution containing a film-forming polymer and an additive for controlling phase separation in a coagulating solution to obtain a porous membrane precursor; and the phase separation remaining in the porous membrane precursor And a step of removing the additive for controlling the additive, wherein the additive for controlling the phase separation was determined for the ratio of the high molecular weight area in the integral molecular weight distribution curve by the following method. A method for producing a porous membrane, characterized in that the proportion of the high molecular weight area is 11% or less of polyvinylpyrrolidone;
Here, the ratio value of the area of the high molecular weight area is determined by measuring the molecular weight distribution of polyvinylpyrrolidone by gel permeation chromatography under the following conditions, and the horizontal axis (X axis) is LogM (where M is the molecular weight). An integrated molecular weight distribution curve with the vertical axis (Y axis) as the integral distribution value (mass%), and the value of X at the point where the integral molecular weight distribution curve reaches Y = 100 is defined as P, When the area of the region surrounded by the integral molecular weight distribution curve, the straight line representing X = P, and the straight line representing Y = 0 is 100%, the integral molecular weight distribution curve, the straight line representing X = 6, and X = P Obtained as a ratio of the area of the region surrounded by the straight line representing Y = 0 and the straight line representing Y = 0;
Here, the conditions of the gel permeation chromatography method are as follows:
Column: TSKgel α-M, 7.8 mm (ID) × 30.0 cm (L) 2 pieces (manufactured by Tosoh Corporation),
Column temperature: 30 ° C.
Mobile phase (eluent): 0.2 mol / L NaNO ₃ aqueous solution and acetonitrile mixed solution, the mixing ratio represented by NaNO ₃ aqueous solution / acetonitrile is 8/2 (vol / vol),
Flow rate: 0.6 ml / min,
Sample concentration: 1 mg / ml,
Detector: RI detector,
Injection volume: 20 μl,
Molecular weight calibration PEO: Polyethylene oxide [manufactured by Polymer Laboratories],
Calibration curve: Standard PEO (Polymer Laboratories) three-dimensional approximate curve, measuring device: Tosoh HLC-8020GPC, and sample filtered with cellulose acetate cartridge filter (fractionation performance 0.45 μm) immediately before measurement Including doing.

The ratio of the high molecular weight area is preferably 5% or more.
Moreover, it is preferable that the said polyvinyl pyrrolidone is a polyvinyl pyrrolidone whose ratio of the low molecular weight area | region in an integral molecular weight distribution curve is 5% or more and less than 13%.
In addition, the ratio of the low molecular weight area of polyvinylpyrrolidone can be measured by the following method.
That is, the molecular weight distribution of polyvinylpyrrolidone is measured by gel permeation chromatography under the following conditions, the horizontal axis (X axis) is LogM (M is molecular weight), and the vertical axis (Y axis) is the integral distribution value. An integral molecular weight distribution curve with (% by mass) is obtained. Let P be the value of X at the point where the integral molecular weight distribution curve reaches Y = 100. When the area of the region surrounded by the integral molecular weight distribution curve, the straight line representing X = P and the straight line representing Y = 0 is 100%, the integral molecular weight distribution curve, the straight line of X = 3.5, The ratio of the area of the region surrounded by the straight line of = 4.5 and the straight line representing Y = 0 is obtained as the value of the ratio of the low molecular weight area.
(Conditions for gel permeation chromatography)
Column: TSKgel α-M, 7.8 mm (ID) × 30.0 cm (L) 2 pieces (manufactured by Tosoh Corporation),
Column temperature: 30 ° C.
Mobile phase (eluent): 0.2 mol / L NaNO ₃ aqueous solution and acetonitrile mixed solution, the mixing ratio represented by NaNO ₃ aqueous solution / acetonitrile is 8/2 (vol / vol),
Flow rate: 0.6 ml / min,
Sample concentration: 1 mg / ml,
Detector: RI detector,
Injection volume: 20 μl,
Molecular weight calibration PEO: Polyethylene oxide [manufactured by Polymer Laboratories],
Calibration curve: Standard PEO [Polymer Laboratories (Polymer Laboratories)] three-dimensional approximate curve, measuring device: Tosoh HLC-8020GPC,
The sample is filtered through a cellulose acetate cartridge filter (fractionation performance 0.45 μm) immediately before measurement.
Furthermore, the polyvinyl pyrrolidone preferably has a K value of 82 or less.
The polyvinyl pyrrolidone preferably has a K value of 78 or more.

According to the present invention, a step of solidifying a film-forming solution containing a film-forming polymer and an additive for controlling phase separation in a coagulating liquid to obtain a porous film precursor; In the method for producing a porous membrane having a step of removing the additive that controls the phase separation remaining in the porous film, the additive that controls the phase separation can be removed in a short time, and the porous membrane has good permeation performance A membrane can be manufactured.

It is sectional drawing which shows an example of the annular nozzle used for manufacture of the porous membrane of this invention. It is an example of an integral molecular weight distribution curve.

<Film material forming polymer>
From the viewpoint of improving chemical resistance and heat resistance, it is preferable to use a fluororesin as the film-forming polymer. Of these, polyvinylidene fluoride resin is preferred. In particular, polyvinylidene fluoride (A) having a weight average molecular weight (hereinafter also referred to as Mw) of 100,000 to 1,000,000 and polyvinylidene fluoride (B) having a weight average molecular weight of 10,000 to 500,000. Are preferably used in combination so that the Mw of (A) is larger than the Mw of (B) and the difference between the two Mw is 30,000 or more. When (A) and (B) are used in combination, the mass ratio of (A) / (B) is preferably in the range of 0.5 to 10, and more preferably in the range of 1 to 3. When the mass ratio (A) / (B) is within the above range, the pore diameter of the membrane can be easily adjusted.

<Additive for controlling phase separation>
In the present invention, as an additive for controlling phase separation (hereinafter, simply referred to as additive), polyvinylpyrrolidone having a high molecular weight area ratio determined by the above method of 11% or less is used.
The ratio of the high molecular weight area can be specifically determined by the following procedure.
First, polyvinyl pyrrolidone was weighed, and the following eluent was added so that the concentration of polyvinyl pyrrolidone (sample concentration) would be 1 mg / ml, and allowed to stand for 16 hours. 0.45 μm). Using the obtained filtrate as a sample, the molecular weight distribution is measured under the above conditions to obtain an integrated molecular weight distribution curve.
FIG. 2 is an integral molecular weight distribution curve obtained by measuring the molecular weight distribution by the above method for an example of polyvinylpyrrolidone. The horizontal axis (X axis) is LogM (M is molecular weight), and the vertical axis (Y axis) is the integral distribution value (mass%). Let P be the value of X at the point where the integral molecular weight distribution curve reaches Y = 100. In the figure, symbol a is an integral molecular weight distribution curve, symbol b is a straight line representing X = P, symbol c is a straight line representing Y = 0, and symbol d is a straight line representing X = 6.
When the area of the region surrounded by the curve a, the straight line b, and the straight line c is 100%, the ratio of the area of the region (the hatched portion) surrounded by the curve a, the straight line d, the straight line b, and the straight line c is increased. Calculated as the ratio of the molecular weight area.

The ratio of the high molecular weight area determined by the above method represents the ratio of the total molecular weight with a molecular weight of 10 ⁶ or more to the total molecular weight. The ratio of the high molecular weight area of polyvinylpyrrolidone can be controlled by the polymerization time of vinylpyrrolidone.
By using polyvinylpyrrolidone having a high molecular weight area ratio of 11% or less as an additive, good detergency (removability) can be obtained. If the ratio of the high molecular weight area exceeds 11%, the cleaning property is lowered, the filtration performance in the porous membrane is lowered, or fine cracks are easily generated in the porous membrane, which is not preferable.
The ratio of the high molecular weight area may be zero, but is preferably 5% or more, more preferably 6% or more, and further preferably 7% or more. If the ratio of the area of the polymer region is less than 5%, the formed pore size becomes too small, and the filtration characteristics deteriorate when used as a filter membrane for sewage drainage.

As an additive, it is preferable to use polyvinyl pyrrolidone having a low molecular weight area ratio of 5% or more and less than 13% in the integral molecular weight distribution curve. When the proportion of the low molecular weight area is 5% or more and less than 13%, the water permeability of the resulting porous membrane is improved.
In addition, the ratio of the low molecular weight area of polyvinylpyrrolidone can be measured by the following method.
That is, the molecular weight distribution of polyvinylpyrrolidone is measured by gel permeation chromatography under the following conditions, the horizontal axis (X axis) is LogM (M is molecular weight), and the vertical axis (Y axis) is the integral distribution value. An integral molecular weight distribution curve with (% by mass) is obtained. Let P be the value of X at the point where the integral molecular weight distribution curve reaches Y = 100. When the area of the region surrounded by the integral molecular weight distribution curve, the straight line representing X = P and the straight line representing Y = 0 is 100%, the integral molecular weight distribution curve, the straight line of X = 3.5, The ratio of the area of the region surrounded by the straight line of = 4.5 and the straight line representing Y = 0 is obtained as the value of the ratio of the low molecular weight area.
(Conditions for gel permeation chromatography)
Column: TSKgel α-M, 7.8 mm (ID) × 30.0 cm (L) 2 pieces (manufactured by Tosoh Corporation),
Column temperature: 30 ° C.
Mobile phase (eluent): 0.2 mol / L NaNO ₃ aqueous solution and acetonitrile mixed solution, the mixing ratio represented by NaNO ₃ aqueous solution / acetonitrile is 8/2 (vol / vol),
Flow rate: 0.6 ml / min,
Sample concentration: 1 mg / ml,
Detector: RI detector,
Injection volume: 20 μl,
Molecular weight calibration PEO: Polyethylene oxide [manufactured by Polymer Laboratories],
Calibration curve: Standard PEO [Polymer Laboratories (Polymer Laboratories)] three-dimensional approximate curve, measuring device: Tosoh HLC-8020GPC,
The sample is filtered through a cellulose acetate cartridge filter (fractionation performance 0.45 μm) immediately before measurement.

The polyvinyl pyrrolidone used in the present invention preferably has a K value of 82 or less. If the K value exceeds 82, the cleaning property of the additive is lowered and the filtration performance is lowered, which is not preferable. Moreover, it is preferable that the K value of polyvinylpyrrolidone is 78 or more. When the K value is less than 78, the pore diameter in the porous membrane becomes too small, and the filtration characteristics deteriorate when used as a filtration membrane for sewage, which is not preferable.
The K value of polyvinylpyrrolidone is a viscosity characteristic value that correlates with the molecular weight, and is a value calculated by applying a relative viscosity value (25 ° C.) measured by a capillary viscometer to the Fikentscher equation shown below. The K value of polyvinylpyrrolidone can be controlled by the polymerization time of vinylpyrrolidone. Commercially available polyvinylpyrrolidone has a unique K value depending on the grade, and the K value is displayed for each product.

In this invention, you may use together other additives other than polyvinylpyrrolidone in the range which does not impair the effect of this invention as an additive. As the other phase separation inhibitor, a known hydrophilic polymer used in a method for producing a porous membrane through a step of coagulating a membrane-forming solution containing a hydrophobic polymer and a hydrophilic polymer in a coagulating solution is appropriately used. Can be used. For example, monool-based, diol-based, triol-based hydrophilic polymers represented by polyethylene glycol can be used.
When the total amount of additives to be used is 100% by mass, the proportion of other additives other than polyvinylpyrrolidone is preferably 5% by mass or less, more preferably 1% by mass or less, and most preferably zero.

<Solvent>
The film forming solution is prepared by dissolving a film material forming polymer and an additive in a solvent. As the solvent, an organic solvent is preferable. As the organic solvent, dimethylformamide, N, N-dimethylacetamide, dimethyl sulfoxide and the like are used. Among them, N, N-dimethylacetamide is more preferable from the viewpoint that the obtained porous body has a high water permeation flow rate.

<Method for producing porous membrane>
As an embodiment of the method for producing a porous membrane of the present invention, a method for producing a porous hollow fiber membrane will be described as an example.
In this embodiment, a dry and wet spinning method is used. That is, after the film-forming liquid is discharged from the annular nozzle, the film is idled for a predetermined time, and then immersed in the coagulating liquid to form a porous film material.

In this embodiment, a braid is used as a base material, a first film-forming liquid is applied to the braid using an annular nozzle, and solidified in a coagulation liquid to form a first porous layer, and then a ring is formed. A porous film precursor is obtained by applying a second film-forming liquid onto the surface of the first porous layer using a nozzle and coagulating it in a coagulating liquid to form a second porous layer.
The first film-forming solution preferably has a lower polymer concentration than the second film-forming solution. That is, it is preferable that the first film-forming solution has a polymer concentration such that the braid is easily impregnated. The second film-forming solution preferably has a polymer concentration suitable for the formation of the porous layer. By using the first film-forming liquid and the second film-forming liquid having different concentrations in this way, the main part of the braid can be sufficiently impregnated with the film-forming liquid, and the membrane material (porous layer) is removed from the braid. It can suppress peeling.

Considering the impregnation property into the braid, the total concentration of the film material forming polymer in the first film-forming liquid is preferably 12% by mass or less, more preferably 10% by mass or less, and further preferably 7% by mass or less. The lower limit is preferably 1% by mass or more, and more preferably 3% by mass or more.
By setting it as this range, the first film-forming solution is easily impregnated into the braid. The porosity of the braid used for the porous hollow fiber membrane is generally about 90 to 95%. In the obtained porous hollow fiber membrane, the proportion of the membrane material forming polymer in the void of the braid is the first product. The film material forming polymer concentration in the film liquid is approximately the same. Therefore, the water permeability of the membrane during filtration can be kept high. Furthermore, the membrane material can be attached to the braid with sufficient strength.
The concentration of the additive in the first film-forming solution is preferably 0.5% by mass or more, more preferably 1% by mass or more from the viewpoint of keeping the water permeability high. The upper limit is preferably 5% by mass or less, more preferably 3% by mass or less from the viewpoint of polyvinyl pyrrolidone detergency.

The second film-forming liquid preferably has a polymer concentration equal to or higher than that of the first film-forming liquid in order to obtain a good mechanical strength in which a void layer is hardly formed when a porous film is formed. Specifically, the total concentration of the film material forming polymer in the second film-forming solution is preferably 12% or more, and more preferably 15% or more. In order to increase the permeation flow rate, the polymer concentration is preferably in a range not exceeding 25%.
The concentration of the additive in the second film-forming solution is preferably 5% by mass or more, more preferably 7% by mass or more from the viewpoint of keeping the water permeability high. The upper limit is preferably 15% by mass or less, more preferably 12% by mass or less from the viewpoint of polyvinyl pyrrolidone detergency.

When the polyvinylidene fluoride (A) and (B) are used in combination as the film-forming polymer, the mass ratio of (A) / (B) in the first film-forming liquid and (A) / ( The mass ratio of B) may be the same or different. The same is preferable from the viewpoint of keeping the water permeability high.

As the annular nozzle, a known annular nozzle used in a method for producing a porous hollow fiber membrane by forming a first porous layer and a second porous layer on a string-like substrate may be appropriately used. it can.
FIG. 1 is a cross-sectional view showing an example of an annular nozzle preferably used in the method for producing a porous hollow fiber membrane of the present embodiment. The annular nozzle is schematically configured by laminating a distribution plate 10, a first distribution nozzle 9, and a second distribution nozzle 8 forming the tip of the annular nozzle in this order.

The distribution plate 10 is a substantially disk-shaped member, and a pipe line 1 through which a braid passes is formed at the center thereof. A first supply port 6 for supplying the first film-forming solution and a second supply port 7 for supplying the second film-forming solution are provided around the pipe line 1 of the distribution plate 10. Yes.

The first distribution nozzle 9 is a member having a substantially T-shaped cross section and a disk having a disk shape in plan view. At its center, a protruding tubular portion 13 protruding into the second distribution nozzle 8 is formed. The inside of the protruding tubular portion 13 is a hollow portion, and the hollow portion communicates with the pipe line 1 to form a braided passage 100. When the first distribution nozzle 9 and the distribution plate 10 are concentrically overlapped, a braid path 100 is formed at the center thereof.
A hollow part communicating with the first supply port 6 and a hollow part communicating with the second supply port 7 are provided around the braided passage 100 of the first distribution nozzle 9.

The first liquid pool portion 11 communicating with the first supply port 6 is formed in a state where the lower surface of the distribution plate 10 and the upper surface of the first distribution nozzle 9 are concentrically stacked so that they contact each other. In addition, grooves are formed in the lower surface of the distribution plate 10 and the upper surface of the first distribution nozzle 9, respectively. In addition, an annular slit is formed so that the first discharge port 2 is formed over the entire circumference of the peripheral wall of the braided passage 100 in a state where these are concentrically stacked. The first discharge port 2 communicates with the first liquid pool portion 11. Further, the first liquid pool part 11 and the first discharge port 2 communicate with each other.
When the distribution plate 10 and the first distribution nozzle 9 are concentrically overlapped and liquid is supplied to the first supply port 6, the supplied liquid is stored in the first pool portion 11, and then the braided passage 100 from the first discharge port 2. The liquid can be discharged toward

The second distribution nozzle 8 is also a disk-shaped member, and a second liquid pool portion 12 is formed at the center thereof, and a hollow portion communicating with the second liquid pool portion 12 is further formed. The hollow portion communicates with the second supply port 7 through a hollow portion that communicates with the second supply port 7 formed in the first distribution nozzle 9. By overlapping the second distribution nozzle 8 and the first distribution nozzle 9 concentrically, the second liquid pool portion 12 is formed around the protruding tubular portion 13 of the first distribution nozzle 9. Specifically, the space formed by the end surface of the first distribution nozzle 9, the outer wall of the protruding tubular portion 13, and the upper surface of the second distribution nozzle 8, which is connected to the base end of the protruding annular portion 13, is the second liquid pool. Part 12. The second liquid pool portion 12 is formed such that its cross-sectional area decreases toward the distal end of the protruding tubular portion 13 of the first distribution nozzle 9. That is, the inner wall of the second distribution nozzle 8 gradually protrudes toward the protruding annular portion 13.

Further, a second protruding port 3 is formed at the tip of the second pool liquid portion 12. That is, the second discharge port 3 is formed by the outer wall of the distal end portion of the protruding tubular portion 13 and the inner wall of the second distribution nozzle 8.
In particular, the front end surface of the protruding annular portion 13, that is, the front end surface 110 of the braided passage 100 is located more inward of the annular nozzle than the front end surface 5 of the second discharge port 3, that is, the front end surface 5 of the second distribution nozzle 8. It is preferable.
In other words, the distance 4 between the distal end surface of the projecting annular portion 13, that is, the braided passage distal end surface 110, and the distal end surface 5 of the second discharge port 3, that is, the distal end surface 5 of the second distribution nozzle 8 (hereinafter referred to as the liquid seal length). Is preferably 0.5 to 150 mm. The lower limit of the liquid seal length is more preferably 0.6 mm or more, and still more preferably 0.8 mm or more. When the liquid seal length is less than 0.5 mm, the second film-forming liquid coated on the surface of the first porous layer is discharged with almost no coating pressure. For this reason, even if there exists a part with a thin outer diameter of the film | membrane in which the 1st porous layer was formed, the 2nd porous layer will be discharged by the same diameter. As a result, a large gap may be generated between the first porous layer and the second porous layer. When the porous hollow fiber membrane is actually used for water treatment, a fixing member such as a synthetic resin is usually used to partition the primary side and the secondary side, but the first porous layer and the second porous If such a gap is formed with the porous layer, the resin constituting the fixing member enters the gap, and there is a high possibility that the water to be treated will not easily be impregnated into the entire porous membrane. When the liquid seal length is set to an appropriate length, the coating pressure of the discharged film forming liquid tends to increase. Therefore, it is possible to prevent a large gap from being formed between the first porous layer and the second porous layer.
The upper limit of the liquid seal length is not particularly limited from the viewpoint of coating pressure, but if it is too long, it tends to be difficult to manufacture the annular nozzle. Therefore, the upper limit of the liquid seal length is preferably 150 mm or less. The upper limit of the liquid seal length is preferably 100 mm or less, and more preferably 50 mm or less.

In the annular nozzle having such a configuration, when the liquid is supplied to the second supply port 7 in a state where the distribution plate 10, the first distribution nozzle 9, and the second distribution nozzle 8 are concentrically superimposed, the liquid is supplied. The liquid is stored in the second pool portion 12 through the hollow portion of the first distribution nozzle 9 and the hollow portion formed by the first distribution nozzle 9 and the second distribution nozzle 8, and then the braid from the second discharge port 3. The ink can be discharged toward the passage 100.

In order to manufacture a porous film using the annular nozzle having such a structure, first, a braid is supplied from the pipe line 1 to the braid path 100, and the first film is formed from the first supply port 6 to the first liquid pool section 11. The liquid is supplied, and the second film-forming liquid is supplied from the second supply port 7 to the second liquid pool section 12.
While supplying the braid to the pipe line 1, that is, while moving the braid in the braid passage 100, the first film forming liquid is discharged from the first discharge port 2 to impregnate the braid, and the second discharge port 3 Two film-forming liquids are discharged to impregnate the braid.

When the temperature of each film-forming solution when discharged is less than 20 ° C., the film-forming solution may be gelled at low temperature, which is not preferable. On the other hand, when the temperature is 40 ° C. or higher, it is difficult to control the pore diameter, and as a result, bacteria such as Escherichia coli and suspended substances are permeated, which is not practically preferable. Accordingly, the temperature at the time of discharging the first film-forming liquid and the second film-forming liquid is preferably in the range of 20 to 40 ° C.

Next, after running the film-forming solution applied on the braid, the first porous film precursor is formed by immersing in the coagulating solution and coagulating the first film-forming solution and the second film-forming solution. To do.
If the idling time is 0.01 seconds or less, the filtration performance is lowered, which is not preferable. There is no upper limit to the travel time, but 4 seconds is sufficient for practical use. Therefore, the idle time is preferably in the range of 0.01 to 4 seconds.
As the coagulation liquid, an aqueous solution containing a solvent used for the film forming liquid is preferably used. Although depending on the type of solvent used, for example, when N, N-dimethylacetamide is used as the solvent for the film forming solution, the concentration of N, N-dimethylacetamide in the coagulation solution is preferably 1 to 50%.
The temperature of the coagulation liquid is preferably low from the viewpoint of increasing the mechanical strength. However, if the temperature of the coagulating liquid is lowered too much, the water permeation flow rate of the resulting film is lowered, so that it is usually selected in the range of 90 ° C. or lower, more preferably 50 ° C. or higher and 85 ° C. or lower.

After coagulation, it is preferable to wash the solvent in hot water at 60 ° C to 100 ° C. At this stage, a part of the additive adhering to the film surface is removed.
It is effective to make this cleaning bath temperature as high as possible within a range in which the first porous membranes are not fused. From this viewpoint, the temperature of the washing bath is preferably 60 ° C. or higher.

It is preferable to perform chemical cleaning with hypochlorous acid after hot water cleaning. Thereby, the additive inside the film is decomposed and removed. At this stage, most of the additives that control phase separation can be removed.
When using a sodium hypochlorite aqueous solution, the concentration is preferably in the range of 10 to 120,000 mg / L. When the concentration of the sodium hypochlorite aqueous solution is less than 10 mg / L, it is not preferable because the water flow rate of the completed membrane decreases. There is no upper limit to the concentration of the aqueous sodium hypochlorite solution, but 120,000 mg / L is sufficient for practical use.
Next, it is preferable to wash the membrane after chemical cleaning in hot water at 60 ° C. to 100 ° C. Thereby, the additive which controls the remaining phase separation can be removed.

Thereafter, it is preferably dried at 60 ° C. or more and less than 120 ° C. for 1 minute or more and less than 24 hours. If it is less than 60 degreeC, since a drying process time will take too much and production cost will rise, it is unpreferable on industrial production. When the temperature is 120 ° C. or higher, the film is excessively shrunk in the drying step, and there is a possibility that minute cracks are generated on the film surface.
The dried film is preferably wound on a bobbin or a cassette.
Thus, a string-like body in which the first porous layer (multilayer film) is formed around the braid is obtained.

Next, a second porous layer is formed on the formed first porous layer, but water permeability decreases when the first porous layer and the second porous layer are completely bonded. In order to prevent this, it is preferable to attach a solution that does not dissolve the membrane material to the surface of the first porous layer before forming the second porous layer.
As the solution that does not dissolve the film material, an aqueous solution containing a solvent used for the film-forming solution is preferably used. For example, when N, N-dimethylacetamide is used as the solvent for the film-forming solution, the concentration of N, N-dimethylacetamide in the solution that does not dissolve the solvent is preferably 1 to 50%. In addition, as a solution that does not dissolve a preferable film material, an organic solvent, a mixture of an organic solvent and water, or a solution obtained by adding an additive containing glycerin or the like as a main component thereto is preferably used.

The step of attaching a solution that does not dissolve the membrane material to the surface of the first porous layer and the step of applying the second film-forming solution thereon are continuously performed using, for example, an annular nozzle having the structure shown in FIG. It is preferable to carry out. In addition, the annular nozzle used for forming the first porous layer is continuously used, and a solution that does not dissolve the membrane material is supplied instead of the first film-forming solution, and the already-supplied second film-forming solution is used as it is. May be.
That is, the string-like body having the first porous layer obtained above is supplied from the pipe line 1 to the braided string path 100, a solution that does not dissolve the membrane material is supplied to the first supply port 6, and the first discharge port A solution that does not dissolve the membrane material is discharged from 2 to apply the solution onto the surface of the first porous layer. Moreover, the 2nd film forming liquid supplied from the 2nd supply port 7 and stored in the 2nd liquid pool part 12 is again discharged from the 2nd discharge port 3, and is apply | coated to the surface of a 1st porous layer. .
Next, in the same manner as the step of forming the first porous layer, the second porous film precursor is formed by immersing in the coagulating liquid and coagulating the second film forming liquid.
Thereafter, in the same manner as in the step of forming the first porous layer, hot water and chemical cleaning are performed to remove the additive remaining in the second porous membrane precursor, and then drying and winding up are performed. Thus, a porous hollow fiber membrane in which the first porous layer and the second porous layer are formed around the braid is obtained.

According to the present embodiment, as an additive, by using polyvinyl pyrrolidone in which the ratio value of the high molecular weight area obtained by the above method is in a specific range, the additive remaining in the porous membrane precursor Detergency (removability) is improved. Therefore, the additive can be removed to a high degree by a short treatment, and a porous membrane having good permeability can be obtained.
It is a surprising finding that the proportion of the high molecular weight area is involved in the removability of the additive. The reason for this is not clear, but if the ratio of the polymer region area is too small, the water permeability decreases, and if the ratio of the polymer region area is too large, the decomposition / removability of the additive tends to decrease.

Further, in the present invention, as the additive, polyvinyl pyrrolidone having a ratio of the high molecular weight area in a specific range and a K value in a specific range is used to remain in the porous membrane precursor. The detergency (removability) of the additive is further improved.
It is a surprising finding that the K value is involved in the removability of the additive. The reason for this is not clear, but if the K value is too low, the water permeability decreases due to the pore size being formed too small, and if the K value is too high, the decomposition / removability of the additive tends to decrease. is there.

In the above embodiment, the porous hollow fiber membrane is formed by forming the braid so as to partially impregnate the membrane material, but the shape and structure of the porous membrane are not limited thereto.
In particular, a porous hollow fiber membrane is preferable in that the production cost can be reduced.
In the case of a porous membrane used for water treatment, it is necessary to cause the liquid on the primary side of the membrane to flow with respect to the membrane surface. Since the membrane is swung and pulled by this membrane surface flow, sufficient mechanical strength is required. In particular, a porous hollow fiber membrane using a braid as a base material has excellent mechanical strength because the braid bears this mechanical strength.

Moreover, in the said embodiment, although the film | membrane material demonstrated the porous film which consists of a 1st porous layer and a 2nd porous layer, film | membrane structure is not restricted to this.
The film material only needs to have at least one dense layer, but it is more preferable that a film material having two or more dense layers is provided in order to improve the durability of the film. Moreover, you may provide a multilayer dense layer further on the 2nd porous layer in this embodiment. In that case, what is necessary is just to form a porous layer sequentially like the procedure of forming a 2nd porous layer on a 1st porous layer.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. In the following, “%” used for notation of content and concentration represents mass%.

Each physical property value was measured by the following method.
[Ratio of high molecular weight area]
The ratio of the high molecular weight area of polyvinylpyrrolidone was measured by the following method.
That is, the molecular weight distribution of polyvinylpyrrolidone was measured by gel permeation chromatography under the following conditions, the horizontal axis (X axis) is LogM (M is molecular weight), and the vertical axis (Y axis) is the integral distribution value ( An integral molecular weight distribution curve (mass%) is obtained. Let P be the value of X at the point where the integral molecular weight distribution curve reaches Y = 100. When the area of the region surrounded by the integral molecular weight distribution curve, the straight line representing X = P, and the straight line representing Y = 0 is 100%, the integral molecular weight distribution curve, the straight line representing X = 6, and X = P The ratio of the area of the region surrounded by the straight line representing Y and the straight line representing Y = 0 is determined as the ratio value of the high molecular weight area.
(Conditions for gel permeation chromatography)
Column: TSKgel α-M, 7.8 mm (ID) × 30.0 cm (L) 2 pieces (manufactured by Tosoh Corporation),
Column temperature: 30 ° C.
Mobile phase (eluent): 0.2 mol / L NaNO ₃ aqueous solution and acetonitrile mixed solution, the mixing ratio represented by NaNO ₃ aqueous solution / acetonitrile is 8/2 (vol / vol),
Flow rate: 0.6 ml / min,
Sample concentration: 1 mg / ml,
Detector: RI detector,
Injection volume: 20 μl,
Molecular weight calibration PEO: Polyethylene oxide [manufactured by Polymer Laboratories],
Calibration curve: Standard PEO [Polymer Laboratories (Polymer Laboratories)] three-dimensional approximate curve, measuring device: Tosoh HLC-8020GPC,
The sample is filtered through a cellulose acetate cartridge filter (fractionation performance 0.45 μm) immediately before measurement.

[Ratio of low molecular weight area]
The ratio of the low molecular weight area of polyvinylpyrrolidone was measured by the following method.
That is, the molecular weight distribution of polyvinylpyrrolidone was measured by gel permeation chromatography under the following conditions, the horizontal axis (X axis) is LogM (M is molecular weight), and the vertical axis (Y axis) is the integral distribution value ( An integral molecular weight distribution curve (mass%) is obtained. Let P be the value of X at the point where the integral molecular weight distribution curve reaches Y = 100. When the area of the region surrounded by the integral molecular weight distribution curve, the straight line representing X = P and the straight line representing Y = 0 is 100%, the integral molecular weight distribution curve, the straight line of X = 3.5, The ratio of the area of the region surrounded by the straight line of = 4.5 and the straight line representing Y = 0 is obtained as the value of the ratio of the low molecular weight area.
(Conditions for gel permeation chromatography)
Column: TSKgel α-M, 7.8 mm (ID) × 30.0 cm (L) 2 pieces (manufactured by Tosoh Corporation),
Column temperature: 30 ° C.
Mobile phase (eluent): 0.2 mol / L NaNO ₃ aqueous solution and acetonitrile mixed solution, the mixing ratio represented by NaNO ₃ aqueous solution / acetonitrile is 8/2 (vol / vol),
Flow rate: 0.6 ml / min,
Sample concentration: 1 mg / ml,
Detector: RI detector,
Injection volume: 20 μl,
Molecular weight calibration PEO: Polyethylene oxide [manufactured by Polymer Laboratories],
Calibration curve: Standard PEO [Polymer Laboratories (Polymer Laboratories)] three-dimensional approximate curve, measuring device: Tosoh HLC-8020GPC,
The sample is filtered through a cellulose acetate cartridge filter (fractionation performance 0.45 μm) immediately before measurement.

[K value]
The K value was calculated by measuring the relative viscosity value (25 ° C.) with a capillary viscometer and applying it to the Fikentscher equation.
In addition, even if the K value is a product of the same standard, there is a slight difference depending on the production lot. In addition, the molecular weight may decrease over time due to autooxidation or the like.

[Bubble point]
The bubble point was measured in accordance with JIS K 3832 using ethyl alcohol as a measurement medium. The value of the bubble point is a value serving as an index of the maximum pore diameter, and the larger the value, the smaller the maximum pore diameter.
Further, if the cleaning property (removability) of the additive is poor, large defects such as fine cracks are generated on the film surface, and as a result, the bubble point value is lowered.
[Remaining PVP]
The measurement was carried out by the following procedure by infrared absorption analysis (IR method). The apparatus used was FTS-40 (product name) manufactured by Varian.
(1) First, the membrane is dissolved in DMAc (N, N-dimethylacetamide) and thinly spread on a glass slide to form a film.
(2) The IR spectrum of the film is measured, and the peak value around 1700 cm ⁻¹ (PVP value) and the peak value around 1400 cm ⁻¹ (PVDF value) are read from the waveform.
(3) A value is obtained by substituting into the following formula.
Residual PVP amount (%) = PVP value × a / PVDF value × 100
(A in the formula is a constant obtained from a calibration curve. In this case, 26.3.)

[Transmission performance]
The permeation performance was evaluated by measuring the value of permeation flux (permeability) per pressure difference by the following method. The larger this value, the better the transmission performance. This value is required to be 30 or more for use in sewage filtration.
(Measuring method)
A mini module (effective length of the hollow fiber membrane is about 4 cm) is manufactured using the hollow fiber membrane by the following method, and water is discharged from the foot cap below under the condition that a pressure of 200 kPa is applied to the hollow portion of the hollow fiber membrane. It press-fits, allows water to permeate in the direction from the inner wall portion to the outer wall portion of the hollow fiber membrane, and calculates the water flux from the outflow amount for 1 minute.
(Mini module manufacturing method)
(1) A cap is attached to the foot of a membrane having an effective length of about 4 cm.
(2) Potting agent (Coronate 4403 (manufactured by Nippon Polyurethane Industry Co., Ltd.) 52%: Nippon Run 4423 (manufactured by Nippon Polyurethane Industry Co., Ltd.) 48%) is stirred with a spatula.
(3) The prepared potting agent is hung on the foot of the cap.
(4) The potting agent is cured by leaving it in a dryer set at 40 ° C. for 3 hours.
(5) The tip is sealed with a potting agent prepared in the same manner as (2).
(6) The potting agent is cured in a dryer at 40 ° C. as in (4).
[Permeability performance rate]
A hollow fiber membrane collected in the same manner as the hollow fiber membrane used for the water permeability measurement was measured for 1 m, immersed in 1 L of a sodium hypochlorite aqueous solution having an effective chlorine concentration of 12% for 5 minutes, and then heated with hot water at 100 ° C. The operation of treating for 5 minutes is repeated twice, and then treated at 110 ° C. for 10 minutes and dried (sample B). When the water permeation performance evaluation value is defined as the water permeation value A and the water permeation evaluation value of the sample B is defined as the water permeation value B, the permeation performance expression rate is obtained by substituting it into the following calculation formula.
Permeability performance rate (%) = water permeability value A / water permeability value B × 100
If the water permeability performance is low and the water permeability performance rate is low, it means that the cleaning is poor.If the water permeability performance rate is high even though the water permeability performance is low, the water permeability performance is due to the small pore diameter formed. Means that is falling.

Example 1
As an additive, polyvinylpyrrolidone (trade name: K-90, manufactured by ISP Co., Ltd.) having a high molecular weight area ratio of 10.1% and a K value of 81.4 is used, and polyvinylidene fluoride A is used as a film material forming polymer. (Trade name: Kyner 301F, Mw 500,000, manufactured by Atofina Japan) and polyvinylidene fluoride B (product name, Kyner 9000LD, manufactured by Atfina Japan, Mw 20,000), and N, N-dimethylacetamide as a solvent. A first film-forming solution and a second film-forming solution having the compositions shown in Table 1 were prepared.

A porous hollow fiber membrane was manufactured using an annular nozzle having the configuration shown in FIG.
That is, a polyester multifilament monowoven braid (multifilament; total decitex 830/96 filament, 16 beats) is introduced into the pipe 1 of the annular nozzle having an outer diameter of 2.5 mm and an inner diameter of 2.4 mm and kept at 30 ° C., The first film forming liquid was discharged from the first discharge port 2, and the second film forming liquid was discharged from the second discharge port 3. The braid coated with the first and second film-forming solutions is led into a coagulation bath kept at 80 ° C. composed of 5% by mass of N, N-dimethylacetamide and 95% by mass of water. Was solidified to obtain a first porous membrane precursor.
This first porous membrane precursor was desolvated in 98 ° C. hot water for 1 minute, immersed in a 50,000 mg / L sodium hypochlorite aqueous solution, and then in 90 ° C. hot water for 10 minutes. It was washed, dried at 90 ° C. for 10 minutes, and wound up with a winder. Thus, a string-like body having the first porous layer was obtained.
Next, the string-like body having the first porous layer is introduced into the pipe line 1 of the annular nozzle shown in FIG. 1 and maintained at 30 ° C. having an outer diameter of 2.7 mm and an inner diameter of 2.6 mm. Glycerin (manufactured by Wako Pure Chemical Industries, Ltd., first grade) was discharged from the discharge port 2 as a solution that did not dissolve the film material, and the second film forming solution was discharged from the second discharge port 3. Thereby, the 2nd film forming liquid was apply | coated on the 1st porous layer. This was introduced into a coagulation bath made of 5% by mass of N, N-dimethylacetamide and 95% by mass of water and kept at 80 ° C., and the second film-forming solution was coagulated to obtain a second porous film precursor.
The second porous membrane precursor was desolvated in hot water at 98 ° C. for 1 minute, then immersed in a 50,000 mg / L aqueous sodium hypochlorite solution, and then in hot water at 90 ° C. for 10 minutes. It was washed, dried at 90 ° C. for 10 minutes, and wound up with a winder. A porous hollow fiber membrane was thus obtained.

The obtained porous hollow fiber membrane had an outer diameter / inner diameter of about 2.8 / 1.1 mm, a film thickness of 900 μm, and a thickness of the resin layer from the braid to the surface of 400 μm.
The bubble point, permeation performance, water permeation performance rate, and residual PVP amount were evaluated by the above methods. The results are shown in Table 2. Table 2 also shows the ratio of the high molecular weight area and the K value of the polyvinylpyrrolidone used (the same applies hereinafter).

(Example 2)
The same as Example 1 except that polyvinylpyrrolidone (trade name: K-80, manufactured by Nippon Shokubai Co., Ltd.) having a high molecular weight area ratio of 9.0% and a K value of 79.9 was used as an additive. Thus, a porous hollow fiber membrane was obtained.
The obtained porous hollow fiber membrane had an outer diameter / inner diameter of about 2.8 / 1.2 mm, a film thickness of 800 μm, and a thickness of the resin layer from the braid to the surface of 400 μm.
In the same manner as in Example 1, the bubble point, the permeation performance, the water permeation performance rate, and the residual PVP amount were evaluated. The results are shown in Table 2.

(Example 3)
The same as Example 1 except that polyvinylpyrrolidone (trade name: K-80, manufactured by Nippon Shokubai Co., Ltd.) having a high molecular weight area ratio of 7.9% and a K value of 78.5 was used as an additive. Thus, a porous hollow fiber membrane was obtained.
The obtained porous hollow fiber membrane had an outer diameter / inner diameter of about 2.8 / 1.2 mm, a film thickness of 800 μm, and a thickness of the resin layer from the braid to the surface of 400 μm.
In the same manner as in Example 1, the bubble point, the permeation performance, the water permeation performance rate, and the residual PVP amount were evaluated. The results are shown in Table 2.

Example 4
Porous as in Example 1 except that polyvinylpyrrolidone (trade name: K-90, manufactured by ISP Co.) having a high molecular weight area ratio of 9.2% and a K value of 84 was used as an additive. A hollow fiber membrane was obtained.
In the same manner as in Example 1, the bubble point, the permeation performance, the water permeation performance rate, and the residual PVP amount were evaluated. The results are shown in Table 2.
(Example 5)
Porous as in Example 1 except that polyvinylpyrrolidone (trade name: K-90, manufactured by ISP) having a low molecular weight area ratio of 8.9% and a K value of 81 was used as an additive. A hollow fiber membrane was obtained.
In the same manner as in Example 1, the bubble point, the permeation performance, the water permeation performance rate, and the residual PVP amount were evaluated. The results are shown in Table 2.

(Comparative Example 1)
The same procedure as in Example 1 was used except that polyvinylpyrrolidone (trade name: K-90, manufactured by ISP) having a high molecular weight area ratio of 13.2% and a K value of 82.9 was used as an additive. As a result, a porous hollow fiber membrane was obtained.
Fine cracks were observed on the surface of the obtained porous hollow fiber membrane. When the bubble point was measured in the same manner as in Example 1, it was reduced to 20 kPa, and the product pass rate was significantly reduced.

(Comparative Example 2)
A porous hollow fiber membrane was obtained in the same manner as in Example 1 except that polyvinylpyrrolidone (trade name K81 / 86, manufactured by ISP Co.) having a high molecular weight area of 11.4% was used as an additive. It was.
Fine cracks were observed on the surface of the obtained porous hollow fiber membrane. When the bubble point was measured in the same manner as in Example 1, it was reduced to 30 kPa, and the product pass rate was significantly reduced.
(Example 6)
Porous hollow in the same manner as in Example 1 except that polyvinylpyrrolidone (trade name K90, manufactured by ISP Co.) having a low molecular weight area ratio of 15.3% and a K value of 73.7 was used as an additive. A yarn membrane was obtained.

As shown in the results of Table 2, the porous hollow fiber membranes obtained in Examples 1 to 5 have a high water permeability performance rate, a small amount of residual PVP, a high bubble point, and a good permeation performance. . Although Example 6 had low water permeability, it showed a good water permeability performance rate. On the other hand, in Comparative Examples 1 and 2, the bubble point, water permeability performance, and water permeability performance expression rate were greatly reduced.

According to the method for producing a porous membrane of the present invention, a porous membrane having excellent cleaning properties of additives and excellent filtration performance can be obtained. Since the porous membrane obtained by the method of the present invention has high permeation performance, the membrane area used can be reduced and the equipment can be made compact.

DESCRIPTION OF SYMBOLS 1 Pipe line 2 1st discharge port 3 2nd protrusion port 4 Distance (liquid seal length) of the braid string channel | path front end surface and the front end surface of a 2nd distribution nozzle
5 Front end surface of second distribution nozzle 6 First supply port 7 Second supply port 8 Second distribution nozzle 9 First distribution nozzle 10 Distribution plate 11 First liquid pool section 12 Second liquid pool section 13 Projecting tubular section 100 Braided passage 110 Braid path front face

Claims (5)

1. A step of solidifying a film-forming solution containing a film-forming polymer and an additive for controlling phase separation in a coagulating solution to obtain a porous membrane precursor; and the phase separation remaining in the porous membrane precursor A method for producing a porous membrane having a step of removing an additive for controlling
   The additive for controlling the phase separation is polyvinylpyrrolidone having a high molecular weight area ratio of 11% or less when the ratio of the high molecular weight area in the integral molecular weight distribution curve is determined by the following method. A method for producing a porous membrane;
   Here, the value of the ratio of the high molecular weight area is measured by the gel permeation chromatography method under the following conditions, the molecular weight distribution of polyvinylpyrrolidone,
   Obtain an integrated molecular weight distribution curve in which the horizontal axis (X axis) is LogM (where M represents molecular weight) and the vertical axis (Y axis) is the integrated distribution value (mass%),
   The value of X at the point where the integral molecular weight distribution curve reaches Y = 100 is P, and the area of the region surrounded by the integral molecular weight distribution curve, the straight line representing X = P and the straight line representing Y = 0 is 100. %, It is determined as a ratio of the area of the region surrounded by the integral molecular weight distribution curve, a straight line representing X = 6, a straight line representing X = P, and a straight line representing Y = 0;
   Here, the conditions of the gel permeation chromatography method are as follows:
   Column: TSKgel α-M, 7.8 mm (ID) × 30.0 cm (L) 2 pieces (manufactured by Tosoh Corporation),
   Column temperature: 30 ° C.
   Mobile phase (eluent): 0.2 mol / L NaNO ₃ aqueous solution and acetonitrile mixed solution, the mixing ratio represented by NaNO ₃ aqueous solution / acetonitrile is 8/2 (vol / vol),
   Flow rate: 0.6 ml / min,
   Sample concentration: 1 mg / ml,
   Detector: RI detector,
   Injection volume: 20 μl,
   Molecular weight calibration PEO: Polyethylene oxide [manufactured by Polymer Laboratories],
   Calibration curve: Standard PEO (Polymer Laboratories) three-dimensional approximate curve, measuring device: Tosoh HLC-8020GPC, and sample filtered with cellulose acetate cartridge filter (fractionation performance 0.45 μm) immediately before measurement Including doing.
2. The method for producing a porous membrane according to claim 1, wherein the additive for controlling the phase separation is polyvinylpyrrolidone having a ratio of the high molecular weight area of 5% or more.
3. The polyvinyl pyrrolidone is a polyvinyl pyrrolidone in which the proportion of the low molecular weight region is 5% or more and less than 13% when the proportion of the low molecular weight region in the integral molecular weight distribution curve is determined by the following method. A method for producing a porous membrane;
   Here, the value of the ratio of the low molecular weight area is measured by the gel permeation chromatography method under the following conditions, the molecular weight distribution of polyvinylpyrrolidone,
   Obtain an integrated molecular weight distribution curve in which the horizontal axis (X axis) is LogM (where M represents molecular weight) and the vertical axis (Y axis) is the integrated distribution value (mass%),
   The value of X at the point where the integral molecular weight distribution curve reaches Y = 100 is P, and the area of the region surrounded by the integral molecular weight distribution curve, the straight line representing X = P and the straight line representing Y = 0 is 100. %, It is determined as a ratio of the area of the region surrounded by the integral molecular weight distribution curve, a straight line of X = 3.5, a straight line of X = 4.5, and a straight line representing Y = 0;
   Here, the conditions of the gel permeation chromatography method are as follows:
   Column: TSKgel α-M, 7.8 mm (ID) × 30.0 cm (L) 2 pieces (manufactured by Tosoh Corporation),
   Column temperature: 30 ° C.
   Mobile phase (eluent): 0.2 mol / L NaNO ₃ aqueous solution and acetonitrile mixed solution, the mixing ratio represented by NaNO ₃ aqueous solution / acetonitrile is 8/2 (vol / vol),
   Flow rate: 0.6 ml / min,
   Sample concentration: 1 mg / ml,
   Detector: RI detector,
   Injection volume: 20 μl,
   Molecular weight calibration PEO: Polyethylene oxide [manufactured by Polymer Laboratories],
   Calibration curve: Standard PEO (Polymer Laboratories) three-dimensional approximate curve, measuring device: Tosoh HLC-8020GPC, and sample filtered with cellulose acetate cartridge filter (fractionation performance 0.45 μm) immediately before measurement Including doing.
4. The method for producing a porous membrane according to claim 1 or 2, wherein the polyvinyl pyrrolidone has a K value of 82 or less.
5. The method for producing a porous membrane according to any one of claims 1 to 3, wherein the polyvinyl pyrrolidone has a K value of 78 or more.

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