KR101342187B1 - Process for producing porous film - Google Patents

Abstract

The present invention provides a step of coagulating a film forming liquid containing a film material forming polymer and an additive controlling a phase separation in a coagulating solution to obtain a porous membrane precursor, and a step of removing the additive controlling the phase separation remaining in the porous membrane precursor. It is a manufacturing method of the porous membrane which has, and relates to the said manufacturing method using polyvinylpyrrolidone whose ratio of the high molecular weight area area in an integrated molecular weight distribution curve is 11% or less as an additive which controls the said phase separation. According to the present invention, it is possible to provide a method for producing a porous membrane which can remove an additive added for controlling phase separation by a short time treatment and can produce a porous membrane having good permeation performance.

Description

Production method of porous membrane {PROCESS FOR PRODUCING POROUS FILM}

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 based on Japanese Patent Application No. 2009-171121 for which it applied in Japan on July 22, 2009, and uses the content here.

In recent years, with increasing interest in environmental pollution and tightening regulations, water treatment by a membrane method using a filtration membrane has been attracting attention because of its superiority in completeness and compactness. In the use for such water treatment, the filtration membrane requires excellent separation characteristics, permeation performance and mechanical properties.

Conventionally, as a filtration membrane excellent in permeation performance, a filtration membrane produced by a wet or dry wet spinning method using a hydrophobic polymer such as polysulfone, polyacrylonitrile, cellulose acetate, polyvinylidene fluoride as a film material forming polymer, and known is known. . These filtration membranes are prepared by coagulating the polymer solution in microphase and then coagulating the polymer solution in a non-solvent. The filter membrane includes a dense layer and a support layer, and has a high porosity and an asymmetric structure. As a specific film forming method, a film forming liquid containing a film material forming polymer and an additive for controlling phase separation is known. As an additive which controls phase separation, hydrophilic polymers, such as polyethyleneglycol and polyvinylpyrrolidone, are used, for example. The additives controlling the phase separation are removed after solidification.

As a method of removing a hydrophilic polymer after solidification, in patent document 1, the method of decomposing a hydrophilic polymer with an oxidizing agent is proposed, and in patent document 2, the method of chemically treating a hydrophilic polymer is proposed. Moreover, as patent document 3, the method of removing degradable polymers, such as polyvinylpyrrolidone, using a disintegrating agent is proposed.

Japanese Patent No. 3196029 U.S. Pat.No.5076925 Japanese Patent No. 3169404

In a porous membrane made of a hydrophobic polymer, the permeation performance is deteriorated when a hydrophilic polymer (additive controlling phase separation) remains in the porous portion after film formation, so that the additive controlling the phase separation is more easily removed by a simpler method. Method is required.

However, the method of the said patent document 1 has a problem in terms of productivity, such as requiring several hours to several tens hours or more for the removal process of a hydrophilic polymer.

Moreover, the method of patent documents 2 and 3 may not obtain favorable permeation | transmission performance depending on the kind of hydrophilic polymer, and is not necessarily a satisfactory method.

This invention is made | formed in view of the said situation, It aims at providing the manufacturing method of the porous membrane which can remove the additive which controls phase separation by a short process, and can manufacture the porous membrane which has favorable permeation | transmission performance.

In order to solve the said subject, the manufacturing method of the porous membrane of this invention,

A porous membrane having a step of coagulating a film forming liquid containing a film material forming polymer and an additive controlling a phase separation in a coagulating solution to obtain a porous membrane precursor, and a step of removing the additive controlling the phase separation remaining in the porous membrane precursor. As a manufacturing method, the additive which controls the said phase separation is polyvinylpyrrolidone whose ratio of the said high molecular weight area | region area is 11% or less when the ratio of the high molecular weight area | region area in an integrated molecular weight distribution curve is calculated by the following method. The manufacturing method of the porous membrane characterized by the above-mentioned.

[Here, the value of the ratio of the said high molecular weight area | region area | region measures the molecular weight distribution of polyvinylpyrrolidone by the gel permeation chromatography method of the following conditions, and the horizontal axis (X-axis) is LogM (where M is molecular weight ), The integral molecular weight distribution curve which makes the vertical axis | shaft (Y-axis) an integral distribution value (mass%), obtains the value of X of the point where the said integral molecular weight distribution curve reached Y = 100, and is said integral When the area of the region surrounded by the molecular weight distribution curve, the straight line representing X = P and the straight line representing Y = 0 is 100%, the integrated molecular weight distribution curve, the straight line representing X = 6, the straight line representing X = P and the Y Obtained as the ratio of the area of the area enclosed by a straight line representing = 0:

Here, the conditions of the gel permeation chromatography method,

Column: TSK gel (TSKgel) α-M, 7.8 mm (ID) x 30.0 cm (L) two (made by Tosoh),

Column temperature: 30 ° C.,

Mobile phase (eluent): A mixed solution of 0.2 mol / L NaNO ₃ aqueous solution and acetonitrile, 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: 20μl,

Molecular weight calibration PEO: polyethylene oxide (manufactured by Polymer Laboratories);

Calibration curve: Standard PEO [made by Polymer Laboratories] 3-dimensional approximation curve, measuring device: HLC-8020GPC manufactured by Tosoh, and

Filtering the sample with a cellulose acetate cartridge filter (fractional performance 0.45 mu m) immediately before measurement]

It is preferable that the ratio of the said high molecular weight area area is 5% or more.

Moreover, it is preferable that the said polyvinylpyrrolidone is polyvinylpyrrolidone whose ratio of the low molecular weight area area in an integrated molecular weight distribution curve is 5% or more and less than 13%.

In addition, the ratio of the low molecular weight area area of polyvinylpyrrolidone can be measured with the following method.

That is, the molecular weight distribution of polyvinylpyrrolidone is measured by the gel permeation chromatography method of the following conditions, A horizontal axis (X-axis) is LogM (M represents molecular weight), and a vertical axis (Y-axis) is integral distribution value. An integral molecular weight distribution curve is set to (mass%). Let P be the value of X of the point where the said integral molecular weight distribution curve reached Y = 100. When the area of the area surrounded by the integrated molecular weight distribution curve, the straight line representing X = P and the straight line representing Y = 0 is 100%, the integrated molecular weight distribution curve, the straight line of X = 3.5, the straight line of X = 4.5, The ratio of the area of the area | region enclosed by the straight line which shows Y = 0 is calculated | required as a value of the ratio of the area of low molecular weight area | region.

(Condition of Gel Permeation Chromatography)

Column: TSK gel α-M, 7.8 mm (ID) x 30.0 cm (L) (made by Tosoh),

Column temperature: 30 ° C.,

Mobile phase (eluent): A mixed solution of 0.2 mol / L NaNO ₃ aqueous solution and acetonitrile, 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: 20μl,

Molecular weight calibration PEO: polyethylene oxide [manufactured by Polymer Laboratories],

Calibration curve: Standard PEO [made by Polymer Laboratories] 3-dimensional approximation curve, measuring device: manufactured by Tosoh HLC-8020 GPC,

The sample is filtered with a cellulose acetate cartridge filter (fraction performance 0.45 mu m) immediately before measurement.

In addition, the polyvinylpyrrolidone preferably has a K value of 82 or less.

Moreover, it is preferable that the said polyvinylpyrrolidone has a K value of 78 or more.

According to the present invention, a film forming liquid comprising a film material forming polymer and an additive for controlling phase separation is coagulated in a coagulating solution to obtain a porous film precursor, and a step for removing an additive for controlling phase separation remaining in the porous film precursor. In the manufacturing method of the porous membrane which has a WHEREIN: The additive which controls phase separation can be removed by a short process, and the porous membrane which has favorable permeation | transmission performance can be manufactured.

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

<Film forming polymer>

From the viewpoint of improving chemical resistance and heat resistance, it is preferable to use a fluorine resin as the film material forming polymer. Especially, polyvinylidene fluoride resin is preferable. 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 is represented by Mw of (A) being (B) It is preferable to use in combination so that it is larger than Mw, and the difference of Mw of both becomes 30,000 or more. When using combining (A) and (B), it is preferable that the mass ratio of (A) / (B) exists in the range of 0.5-10, and the inside of the range of 1-3 is more preferable. If the mass ratio of (A) / (B) is within the above range, adjustment of the pore diameter of the film can be facilitated.

<Additives controlling phase separation>

In this invention, polyvinylpyrrolidone whose ratio of the high molecular weight area | region area calculated | required by said method is 11% or less is used as an additive (henceforth simply an additive) which controls phase separation.

The ratio of the high molecular weight region area can be specifically determined by the following procedure.

First, polyvinylpyrrolidone is weighed, and the following eluent is added so that the concentration of polyvinylpyrrolidone (sample concentration) is 1 mg / ml, and left to dissolve for 16 hours, and a cartridge filter made of cellulose acetate immediately before measurement Fraction performance 0.45 μm). Using the obtained filtrate as a sample, the molecular weight distribution is measured under the above conditions, and an integrated molecular weight distribution curve is obtained.

2 is an integrated molecular weight distribution curve obtained by measuring the molecular weight distribution by the above-described method for one example of polyvinylpyrrolidone. The horizontal axis (X axis) is LogM (M is molecular weight), and the vertical axis (Y axis) is integral distribution value (mass%). Let P be the value of X of the point where the integral molecular weight distribution curve reached Y = 100. In the figure, code | symbol a is an integrated molecular weight distribution curve, code | symbol b is a straight line which shows X = P, code c is a straight line which shows Y = 0, code | symbol d is a straight line which shows X = 6.

When the area of the area enclosed by the curve a, the straight line b, and the straight line c is 100%, the ratio of the area of the area enclosed by the curve a, the straight line d, the straight line b, and the straight line c (the hatched portion) is determined by the ratio of the high molecular weight area area. Obtained as the value.

The ratio of the high molecular weight area | region area calculated | required by said method represents the ratio which the molecular weight sum total of molecular weight ¹⁰⁶ or more occupies among the total molecular weight sum total. The proportion of the high molecular weight region area of polyvinylpyrrolidone can be controlled by the polymerization time of vinylpyrrolidone.

As an additive, favorable washability (removability) is obtained by using polyvinylpyrrolidone whose ratio of this high molecular weight area area is 11% or less. When the ratio of the high molecular weight region area exceeds 11%, the detergency is deteriorated, the filtration performance in the porous membrane is reduced, or fine cracks are easily generated in the porous membrane, which is not preferable.

Although the ratio of the said high molecular weight area | region area may be zero, 5% or more is preferable, 6% or more is more preferable, 7% or more is more preferable. If the ratio of the area of the polymer region is less than 5%, the pore formed is too small, and when used as a filtration membrane for wastewater, the filtration characteristics are lowered, which is not preferable.

As an additive, it is preferable to use the polyvinylpyrrolidone whose ratio of the low molecular weight area area in an integrated molecular weight distribution curve is 5% or more and less than 13%. When the ratio of the area of the low molecular weight region is 5% or more and less than 13%, the permeation performance of the obtained porous membrane is improved.

In addition, the ratio of the low molecular weight area area of polyvinylpyrrolidone can be measured with the following method.

That is, the molecular weight distribution of polyvinylpyrrolidone is measured by the gel permeation chromatography method of the following conditions, A horizontal axis (X-axis) is LogM (M represents molecular weight), and a vertical axis (Y-axis) is integral distribution value. An integral molecular weight distribution curve is set to (mass%). Let P be the value of X of the point where the said integral molecular weight distribution curve reached Y = 100. When the area of the area surrounded by the integrated molecular weight distribution curve, the straight line representing X = P and the straight line representing Y = 0 is 100%, the integrated molecular weight distribution curve, the straight line of X = 3.5, the straight line of X = 4.5, The ratio of the area of the area | region enclosed by the straight line which shows Y = 0 is calculated | required as a value of the ratio of the area of low molecular weight area | region.

(Condition of Gel Permeation Chromatography)

Column: TSK gel α-M, 7.8 mm (ID) x 30.0 cm (L) (made by Tosoh),

Column temperature: 30 ° C.,

Mobile phase (eluent): A mixed solution of 0.2 mol / L NaNO ₃ aqueous solution and acetonitrile, 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: 20μl,

Molecular weight calibration PEO: polyethylene oxide [manufactured by Polymer Laboratories],

Calibration curve: Standard PEO [made by Polymer Laboratories] 3-dimensional approximation curve, measuring device: manufactured by Tosoh HLC-8020 GPC,

The sample is filtered with a cellulose acetate cartridge filter (fraction performance 0.45 mu m) immediately before measurement.

In addition, the polyvinylpyrrolidone used in the present invention preferably has a K value of 82 or less. When K value exceeds 82, since the washability of an additive falls and filtration performance falls, it is unpreferable. Moreover, it is preferable that K value of polyvinylpyrrolidone is 78 or more. If the K value is less than 78, the pore size in the porous membrane becomes too small, and the filtration characteristics are lowered when used as a filtration membrane for drainage, which is not preferable.

In addition, the K value of polyvinylpyrrolidone is a viscosity characteristic value correlated with molecular weight, and it calculates by applying the relative viscosity value (25 degreeC) measured by a capillary viscometer to the formula of Fikentscher shown below. Is a value. The K value of polyvinylpyrrolidone can be controlled by the polymerization time of vinylpyrrolidone. Commercially available polyvinylpyrrolidone has its own K value depending on the grade, and the K value is indicated for each product.

In this invention, other additives other than polyvinylpyrrolidone can also be used together as an additive in the range which does not impair the effect of this invention. As said other phase separation inhibitor, the well-known hydrophilic polymer used in the method of manufacturing a porous membrane through the process of coagulating the film forming liquid containing a hydrophobic polymer and a hydrophilic polymer in a coagulation liquid can be used suitably. For example, hydrophilic polymers, such as the monool type, diol type, and triol type represented by polyethylene glycol, are mentioned.

When the total amount of the additive to be used is 100 mass%, the proportion of other additives other than polyvinylpyrrolidone is preferably 5 mass% or less, more preferably 1 mass% or less, and most preferably zero. .

<Solvent>

The film forming solution is prepared by dissolving the film forming polymer and the additive in a solvent. As a solvent, an organic solvent is preferable. Dimethyl formamide, N, N-dimethylacetamide, dimethyl sulfoxide, etc. are used as an organic solvent. Especially, N, N- dimethylacetamide is more preferable at the high permeation flow rate of the porous body obtained.

<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 is described as an example.

In this embodiment, a wet and dry spinning method is used. That is, after discharging the film forming liquid from the annular nozzle, the membrane is freed for a predetermined time and then immersed in the coagulating liquid to form a porous membrane material.

In this embodiment, using a braid as a base material roughly, apply | coating a 1st film forming liquid to the said braid using an annular nozzle, solidifying in a coagulation liquid, and forming a 1st porous layer, and using the said annular nozzle, A porous film precursor is obtained by applying a second film forming solution to the surface of the first porous layer and coagulating in the coagulating solution to form a second porous layer.

It is preferable that a 1st film forming liquid has a polymer density lower than a 2nd film forming liquid. That is, it is preferable that a 1st film forming liquid is a polymer concentration of the grade which is easy to impregnate in braid. It is preferable that a 2nd film forming liquid is a polymer concentration of the grade suitable for formation of a porous layer. Thus, by using the 1st film forming liquid and 2nd film forming liquid with which concentration differs, the film forming liquid can fully be impregnated in the main part of braid, and peeling of a film material (porous layer) from a braid can be suppressed.

In consideration of the impregnation in the braid, the concentration of the total of the film-forming polymer in the first film forming liquid is preferably 12 mass% or less, more preferably 10 mass% or less, and even more preferably 7 mass% or less. 1 mass% or more is preferable and, as for a lower limit, 3 mass% or more is more preferable.

By setting it as this range, a 1st film forming liquid impregnates easily in braid. In addition, the porosity of the braid used for the porous hollow fiber membrane is generally about 90 to 95%, and in the obtained porous hollow fiber membrane, the ratio of the film material forming polymer to the voids of the braid is equal to the concentration of the film material forming polymer in the first film forming liquid. It is about. Therefore, the permeability of the membrane during filtration can be maintained high. In addition, the membrane material can be attached to the braid with sufficient strength.

0.5 mass% or more is preferable and, as for the density | concentration of the additive in a 1st film forming liquid, permeability is high, 1 mass% or more is more preferable. 5 mass% or less is preferable from a viewpoint of polyvinylpyrrolidone washability, and, as for an upper limit, 3 mass% or less is more preferable.

When a 2nd film forming liquid is made into a porous film, it is difficult to form a void layer, and in order to acquire favorable mechanical strength, it is preferable to have a polymer concentration more than the said 1st film forming liquid. Specifically, 12% or more is preferable and, as for the density | concentration of the sum total of the film | membrane formation polymer in a 2nd film forming liquid, 15% or more is more preferable. In order to increase permeate flow rate, the polymer concentration is preferably in the range not exceeding 25%.

5 mass% or more is preferable and, as for the density | concentration of the additive in a 2nd film forming liquid, a water permeability is high, 7 mass% or more is more preferable. 15 mass% or less is preferable from a viewpoint of polyvinylpyrrolidone washability, and, as for an upper limit, 12 mass% or less is more preferable.

When using the said polyvinylidene fluoride (A) and (B) together as a film material forming polymer, the mass ratio of (A) / (B) in a 1st film forming liquid, and (A) / in a 2nd film forming liquid The mass ratio of (B) may be the same or different. The same thing is preferable at the point which keeps high water permeability.

As an annular nozzle, the well-known annular nozzle used for the method of forming a 1st porous layer and a 2nd porous layer on a string-like base material, and manufacturing a porous hollow fiber membrane can be used suitably.

BRIEF DESCRIPTION OF THE DRAWINGS It is sectional drawing which shows an example of the annular nozzle used preferably for the manufacturing method of the porous hollow fiber of this embodiment. As for this annular nozzle, the distribution plate 10, the 1st distribution nozzle 9, and the 2nd distribution nozzle 8 which form the front-end | tip part of an annular nozzle are laminated | stacked in this order, and are comprised schematically.

The distribution plate 10 is a substantially disk-like member, and a conduit 1 through which the braid passes is formed at the center thereof. Around the pipeline 1 of the distribution plate 10, a first supply port 6 for supplying the first film forming liquid and a second supply port 7 for supplying the second film forming liquid are provided. .

The first dispensing nozzle 9 is a substantially T-shaped member in cross-sectional shape and a disk-shaped member in planar shape. At the center thereof, a protruding tubular portion 13 protruding into the second dispensing nozzle 8 is formed. The inside of this protruding tubular part 13 is a hollow part, and this hollow part communicates with the said pipe path 1, and forms the braid passage 100. When the first dispensing nozzle 9 and the dispensing plate 10 overlap concentrically, a braid passage 100 is formed in the center of these.

In the circumference | surroundings of the braid passage | channel 100 of the 1st distribution nozzle 9, the hollow part which communicates with the 1st supply port 6 and the hollow part which communicates with the 2nd supply port 7 are provided, respectively.

The first liquid pool portion 11 communicating with the first supply port 6 is formed in a state in which the lower surface of the distribution plate 10 and the upper surface of the first dispensing nozzle 9 are in contact with each other concentrically. Grooves are formed in the lower surface of the distribution plate 10 and the upper surface of the first dispensing nozzle 9, respectively. Moreover, in the state which superimposed them concentrically, the annular slit is formed so that the 1st discharge port 2 may be formed over the perimeter of the circumferential wall of the braid passage 100. This first discharge port 2 communicates with the first liquid pool portion 11. Moreover, the said 1st liquid pool part 11 and the 1st discharge port 2 are in communication.

When the distribution plate 10 and the first dispensing nozzle 9 overlap concentrically, and the liquid is supplied to the first supply port 6, the supplied liquid is collected in the first pool 11, and then the first discharge port. The liquid can be discharged toward the braid passage 100 from (2).

The 2nd dispensing nozzle 8 is also a disk-shaped member, The 2nd liquid pool part 12 is formed in the center, and the hollow part which communicates with the 2nd liquid pool part 12 is formed. This hollow part is connected to the said 2nd supply port 7 through the hollow part which communicates with the 2nd supply port 7 formed in the 1st dispensing nozzle 9. By overlapping the 2nd dispensing nozzle 8 and the 1st dispensing nozzle 9 concentrically, the 2nd liquid pool part 12 is formed around the protrusion tubular part 13 of the 1st dispensing nozzle 9. Specifically, it is formed at the end face of the first dispensing nozzle 9, the outer wall of the protruding tubular part 13, and the upper face of the second dispensing nozzle 8, which is continuous to the proximal end of the protruding annular portion 13. The space becomes the second liquid pool 12. The said 2nd liquid pool part 12 is formed so that the cross-sectional area may become small toward the tip direction of the protrusion tubular part 13 of the 1st dispensing nozzle 9. That is, the inner wall of the 2nd dispensing nozzle 8 protrudes toward the protruding annular part 13 gradually.

Moreover, the 2nd protrusion part 3 is formed in the front-end | tip part of the 2nd liquid pool part 12. As shown in FIG. That is, the 2nd discharge port 3 is formed in the outer wall of the front-end | tip part of the protrusion tubular part 13, and the inner wall of the 2nd distribution nozzle 8. As shown in FIG.

In particular, the distal end surface of the protruding annular portion 13, that is, the distal end surface 110 of the braid passage 100, is the distal end surface 5 of the second discharge port 3, that is, the distal end surface of the second dispensing nozzle 8. It is preferable to locate inward of an annular nozzle rather than (5).

In other words, the tip end surface of the protruding annular portion 13, that is, the end face of the braid passage 110, and the tip surface 5 of the second discharge port 3, that is, the tip surface 5 of the second dispensing nozzle 8. It is preferable that the distance 4 to the bottom (hereinafter referred to as the liquid seal length) is configured to be 0.5 to 150 mm. As for the minimum of liquid sealing length, it is more preferable that it is 0.6 mm or more, and it is still more preferable that it is 0.8 mm or more. When the liquid sealing length is less than 0.5 mm, the second film forming liquid coated on the surface of the first porous layer is discharged with little coating pressure applied. For this reason, even if there exists a thin part of the 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, there exists a possibility that a big clearance may arise between a 1st porous layer and a 2nd 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. When such a gap is formed between the first porous layer and the second porous layer, the fixed side is fixed. The resin constituting the member enters the gap, so that the water to be treated becomes difficult to impregnate the entire porous membrane. When the liquid sealing length is set to an appropriate length, the coating pressure of the film forming liquid discharged tends to be increased. Therefore, it is possible to prevent the formation of a large gap between the first porous layer and the second porous layer.

In addition, the upper limit of the liquid sealing length is not particularly limited in terms of coating pressure, but if it is too long, it tends to be difficult to manufacture the annular nozzle. Therefore, it is preferable that the upper limit of the liquid sealing length is 150 mm or less. It is preferable that it is 100 mm or less, and, as for the upper limit of the liquid sealing length, it is more preferable that it is 50 mm or less.

In the annular nozzle of such a configuration, when the distribution plate 10, the first distribution nozzle 9, and the second distribution nozzle 8 are superposed concentrically, the liquid is supplied to the second supply port 7. , The supplied liquid is collected in the second pool portion 12 through the hollow portion of the first dispensing nozzle 9 and the hollow portion formed by the first dispensing nozzle 9 and the second dispensing nozzle 8, and then the second discharge port. It is possible to discharge from the braid passage 100 from (3).

In order to manufacture a porous membrane using the annular nozzle of such a structure, a braid is first supplied from the conduit 1 to the braid passage 100, and a first film forming from the first supply port 6 to the first liquid pool 11 is performed. The liquid is supplied, and the second film forming liquid is supplied from the second supply port 7 to the second liquid pool portion 12.

While supplying the braid to the conduit 1, that is, moving the braid in the braid passage 100, the first film forming solution is discharged from the first outlet 2 to be impregnated with the braid, and from the second outlet 3 2 The film-forming solution is discharged to impregnate the braid.

If the temperature of each film forming liquid at the time of discharge is less than 20 degreeC, there exists a possibility that film forming liquid may gelatinize at low temperature, and it is not preferable. On the other hand, if it is 40 degreeC or more, pore control is difficult, As a result, it causes permeation | transmission of bacteria, such as Escherichia coli, and a floating substance, and is not practically preferable. Therefore, as for the temperature at the time of discharge of a 1st film forming liquid and a 2nd film forming liquid, both the range of 20-40 degreeC is preferable.

Subsequently, after forming the film forming liquid coated on the braid, the first film forming liquid and the second film forming liquid are solidified by immersion in the coagulating liquid to form the first porous film precursor.

 If the freezing time is 0.01 seconds or less, the filtration performance is lowered, which is not preferable. There is no upper limit to the running time, but practically 4 seconds is sufficient. Therefore, the princess time is preferably in the range of 0.01 to 4 seconds.

As a coagulation liquid, the aqueous solution containing the solvent used for a film forming liquid is used preferably. Although it depends also on the kind of solvent to be used, For example, when N, N-dimethylacetamide is used as a solvent of a film forming liquid, the density | concentration of N, N- dimethylacetamide in a coagulation liquid is 1 to 50% is preferable. Do.

It is preferable that the temperature of the coagulating liquid is lower from the viewpoint of increasing the mechanical strength. However, if the temperature of the coagulating solution is too low, the permeate flow rate of the finished membrane is lowered, so it is usually selected in the range of 90 ° C or less, more preferably 50 ° C or more and 85 ° C or less.

After coagulation | solidification, it is preferable to wash a solvent in 60 degreeC-100 degreeC hot water. In this step, a part of the additive attached to the membrane surface is removed.

It is effective to make this washing | cleaning bath temperature high temperature as possible in the range which does not fuse together 1st porous membranes. From this viewpoint, 60 degreeC or more of the temperature of a washing bath is preferable.

It is preferable to perform chemical liquid washing with hypochlorous acid or the like after the hydrothermal washing. As a result, the additives inside the film are decomposed and removed. At this stage most of the additives controlling the phase separation can be removed.

When using an aqueous sodium hypochlorite solution, its concentration is preferably in the range of 10 to 120,000 mg / L. When the concentration of aqueous sodium hypochlorite solution is less than 10 mg / L, the permeate flow rate of the finished membrane is lowered, which is not preferable. Although there is no upper limit in the concentration of the sodium hypochlorite aqueous solution, 120,000 mg / L is enough practically.

Subsequently, it is preferable to wash the film | membrane after chemical liquid washing in hot water of 60 degreeC-100 degreeC. Thereby, the additive which controls the remaining phase separation can be removed.

Then, it is preferable to dry for 1 minute or more and less than 24 hours at 60 degreeC or more and less than 120 degreeC. If it is less than 60 degreeC, drying processing time is too long and production cost rises, and it is unpreferable on industrial production. If it is 120 degreeC or more, since a film | membrane may shrink too much in a drying process and a micro crack may generate | occur | produce on the film surface, it is unpreferable.

It is preferable to wind the film | membrane after drying in a bobbin or a dust.

In this way, the sticky body in which the 1st porous layer (multilayer film) was formed around the braid is obtained.

Next, a second porous layer is formed on the formed first porous layer. Since the water permeability decreases when the first porous layer and the second porous layer are completely adhered, the second porous layer is formed to prevent this. It is preferable to attach a solution which does not dissolve the membrane material on the surface of the first porous layer before.

As a solution which does not melt | dissolve such a film material, the aqueous solution containing the solvent used for a film forming liquid is used preferably. 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 in which the solvent is not dissolved is preferably 1 to 50%. In addition, as a solution which does not melt | dissolve a preferable film material, the solution which added the organic solvent, the mixture of an organic solvent, and water, or the additive which has glycerin etc. as a main component to these is used preferably.

The step of adhering a solution which does not dissolve the membrane material on the surface of the first porous layer and the step of applying the second film forming solution thereon are performed continuously using, for example, an annular nozzle having a structure shown in FIG. 1. desirable. Moreover, the annular nozzle used for formation of a 1st porous layer can be used continuously, and the solution which does not dissolve a film | membrane material can be supplied instead of a 1st film forming liquid, or the 2nd film forming liquid already supplied can be used as it is.

That is, the string body which has the 1st porous layer obtained above is supplied from the pipe line 1 to the braid channel | path 100, the solution which does not melt | dissolve a film material in the 1st supply port 6, and the 1st discharge port ( The solution which does not melt | dissolve a membrane material from 2) is discharged, and the said solution is apply | coated on the surface of a 1st porous layer. Moreover, the 2nd film forming liquid supplied from the 2nd supply port 7 and compared with the 2nd liquid pool part 12 is discharged from the 2nd discharge port 3 again, and is apply | coated to the surface of a 1st porous layer.

Subsequently, the second porous membrane precursor is formed by immersing in the coagulation liquid and coagulating the second film forming liquid in the same manner as the step of forming the first porous layer.

Thereafter, in the same manner as in the step of forming the first porous layer, the first porous layer and the second porous layer around the braid are obtained by hot water and drug washing to remove the additives remaining in the second porous membrane precursor, and to dry them. A porous hollow fiber membrane in which a porous layer is formed is obtained.

According to this embodiment, as an additive, the cleaning property (removability of the additive remaining in a porous membrane precursor is used by using polyvinylpyrrolidone whose value of the ratio of the high molecular weight area | region area calculated | required by said method is in a specific range. ) Is improved. Therefore, the additive can be removed highly by a short treatment, and a porous membrane having good permeation performance is obtained.

It is surprising that such a high proportion of high molecular weight area is involved in the removal of additives. Although the reason is not clear, when there are too few ratios of a polymer area | region area, water permeability falls, and when there are too many ratios of a polymer area area, there exists a tendency for the decomposition / removability of an additive to fall.

In addition, in the present invention, as the additive, the value of the ratio of the area of the high molecular weight region is not only in a specific range, but also by using polyvinylpyrrolidone having a K value in a specific range, the cleaning property of the additive remaining in the porous membrane precursor (Removability) is further improved.

It is surprising that the K value is involved in the removal of the additive. Although the reason is not clear, when K value is too low, water permeability falls by too small to be formed, and when K value is too high, there exists a tendency for the decomposition / removability of an additive to fall.

In addition, in the said embodiment, although the porous hollow fiber membrane was formed so that a braid might be made to impregnate a membrane material, the shape and structure of a porous membrane are not limited to this.

In particular, a porous hollow fiber membrane is preferable in that the production cost can be reduced.

Moreover, in the porous membrane used for water treatment use, it is necessary to make the liquid on the primary side of membrane permeation flow with respect to a membrane surface. Since the membrane is rocked and stretched by this membrane surface, sufficient mechanical strength is required. In particular, the porous hollow fiber using braid as a substrate has excellent mechanical strength because braid is responsible for this mechanical strength.

In addition, in the said embodiment, although the membrane material demonstrated the porous film which consists of a 1st porous layer and a 2nd porous layer, a membrane structure is not limited to this.

The membrane material should have at least one dense layer, but it is more preferable that the membrane material having two or more dense layers is disposed because it improves the durability of the membrane. Moreover, you may further provide a multilayer dense layer on the 2nd porous layer in this embodiment. In that case, the porous layer may be formed in the same manner as in the procedure of forming the second porous layer on the first porous layer.

[Example]

Although an Example is given to the following and this invention is demonstrated to it in more detail, this invention is not limited to these Examples. Hereinafter, "%" used for description of content rate and concentration shows the mass%.

Each physical property value was measured by the method shown below.

[Ratio of high molecular weight area]

The ratio of the high molecular weight area area of polyvinylpyrrolidone was measured by the following method.

That is, the molecular weight distribution of polyvinylpyrrolidone is measured by the gel permeation chromatography method of the following conditions, A horizontal axis (X-axis) is LogM (M represents molecular weight), and a vertical axis (Y-axis) is integral distribution value. An integral molecular weight distribution curve is set to (mass%). Let P be the value of X of the point where the said integral molecular weight distribution curve reached Y = 100. When the area of the area surrounded by the integrated molecular weight distribution curve, the straight line representing X = P and the straight line representing Y = 0 is 100%, the integrated molecular weight distribution curve, the straight line representing X = 6 and the straight line representing X = P And the ratio of the area of the area | region enclosed by the straight line which shows Y = 0 as a value of the ratio of the area of high molecular weight area | region.

(Condition of Gel Permeation Chromatography)

Column: TSK gel α-M, 7.8 mm (ID) x 30.0 cm (L) (made by Tosoh),

Column temperature: 30 ° C.,

Mobile phase (eluent): A mixed solution of 0.2 mol / L NaNO ₃ aqueous solution and acetonitrile, 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: 20μl,

Molecular weight calibration PEO: polyethylene oxide [manufactured by Polymer Laboratories],

Calibration curve: Standard PEO [made by Polymer Laboratories] 3-dimensional approximation curve, measuring device: manufactured by Tosoh HLC-8020GPC,

The sample is filtered with a cellulose acetate cartridge filter (fraction performance 0.45 mu m) immediately before measurement.

 [Ratio of low molecular weight area]

The ratio of the low molecular weight area area of polyvinylpyrrolidone was measured by the following method.

That is, the molecular weight distribution of polyvinylpyrrolidone is measured by the gel permeation chromatography method of the following conditions, A horizontal axis (X-axis) is LogM (M represents molecular weight), and a vertical axis (Y-axis) is integral distribution value. An integral molecular weight distribution curve is set to (mass%). Let P be the value of X of the point where the said integral molecular weight distribution curve reached Y = 100. When the area of the area surrounded by the integrated molecular weight distribution curve, the straight line representing X = P and the straight line representing Y = 0 is 100%, the integrated molecular weight distribution curve, the straight line of X = 3.5, the straight line of X = 4.5, The ratio of the area of the area | region enclosed by the straight line which shows Y = 0 is calculated | required as a value of the ratio of the area of low molecular weight area | region.

(Condition of Gel Permeation Chromatography)

Column: TSK gel α-M, 7.8 mm (ID) x 30.0 cm (L) (made by Tosoh),

Column temperature: 30 ° C.,

Mobile phase (eluent): A mixed solution of 0.2 mol / L NaNO ₃ aqueous solution and acetonitrile, 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: 20μl,

Molecular weight calibration PEO: polyethylene oxide [manufactured by Polymer Laboratories],

Calibration curve: Standard PEO [made by Polymer Laboratories] 3-dimensional approximation curve, measuring device: manufactured by Tosoh HLC-8020GPC,

The sample is filtered with a cellulose acetate cartridge filter (fraction performance 0.45 mu m) immediately before measurement.

[K value]

K value was calculated by measuring a relative viscosity value (25 degreeC) with a capillary viscometer, and applying it to the above formula of Pikentcher.

In addition, even if the value of K is a product of the same standard, there exist some differences with manufacture lot. In addition, the molecular weight may decrease over time due to self oxidation or the like.

[Bubble point]

The bubble point measured ethyl alcohol as a measuring medium according to JIS K 3832. The value of the bubble point is an index indicating the maximum pore size, and the larger the value, the smaller the maximum pore size.

In addition, when the additives are poor in washability (removability), large defects such as minute cracks occur on the surface of the film, and as a result, the value of the bubble point is lowered.

[Remaining PVP]

It was measured by the infrared absorption assay (IR method) by the following procedure. The apparatus used FTS-40 (product name) by the Varian company.

(1) First, the membrane is dissolved in DMAc (N, N-dimethylacetamide), and thinly extended on slide glass to form a film.

(2) The IR spectrum of the film was measured, and the peak value (PVP value) around 1700 cm ⁻¹ and the peak value (PVDF value) around 1400 cm ⁻¹ were read from the waveform.

(3) Substitute the following formula to find the value.

Remaining PVP amount (%) = PVP value × a / PVDF value × 100

(Where a is a constant from the calibration curve, in this case 26.3.)

[Permeability]

The evaluation of permeation | transmission performance measured the value of permeation flux (permeability) per pressure difference with the following method. The larger this value, the better the transmission performance. In the use of wastewater filtration, it is required that this value is 30 or more.

(How to measure)

A mini module (effective length of the hollow fiber membrane is about 4 cm) was manufactured using a hollow fiber membrane by the following method, and water was indented from the cap of the following foot under the conditions which the pressure of 200 kPa was applied to the hollow part of a hollow fiber membrane. Then, water is permeated from the inner wall portion of the hollow fiber membrane toward the outer wall portion, and the water flux is calculated from the flow rate for one minute.

(How to make a mini module)

(1) A cap is attached to the foot of the membrane about 4 cm in effective length.

(2) A potting agent (Coronate 4403 (manufactured by Nippon Polyurethane Co., Ltd.) 52%: Nippon 4423 (manufactured by Nippon Polyurethane Co., Ltd.) at a ratio of 48%) is stirred with a spatula.

(3) The combined potting agent is drooped on the foot of the cap.

(4) It is left to stand for 3 hours in the dryer set to 40 degreeC, and a potting agent is hardened.

(5) The tip portion is sealed with the potting agent combined in the same manner as in (2).

(6) In the same manner as in (4), the potting agent is cured in a dryer at 40 ° C.

[Permeability Performance Expression Rate]

The hollow fiber membrane collected in the same manner as the hollow fiber membrane used for the permeability performance measurement was taken by measuring 1 m, immersed in 1 L of sodium hypochlorite aqueous solution having an effective chlorine concentration of 12% for 5 minutes, and then treated with hot water at 100 ° C. for 5 minutes. The operation is repeated twice, followed by treatment at 110 ° C. for 10 minutes and drying (sample B). When the permeability evaluation value is defined as the permeability value A and the permeability evaluation value of the sample B as the permeability value B, the permeability performance expression rate is calculated by substituting the following formula.

Permeability performance expression rate (%) = Permeability value A / Permeability value B x 100

When the permeability performance is low and the permeability performance expression rate is also low, it means that the cleaning is poor. When the permeability performance expression rate is high despite the low permeability performance, it means that the permeability performance is reduced due to the small pore formed. .

(Example 1)

As the additive, polyvinylidene fluoride A (Atpina) was used as a film material forming polymer using polyvinylpyrrolidone (ISP, trade name: K-90) having a ratio of high molecular weight region area of 10.1% and K value of 81.4. Japan company make, brand name: Kyina 301F, Mw 500,000) and polyvinylidene fluoride B (Atpina Japan make, brand name China 9000LD, Mw 20,000), and N, N-dimethylacetamide as a solvent, The 1st film forming liquid and the 2nd film forming liquid which have a composition shown in following Table 1 were manufactured.

The porous hollow fiber was manufactured using the annular nozzle of the structure shown in FIG.

That is, polyester multifilament woven braid (multifilament; total desiccate 830/96 filament, 16 strokes) is introduced into the conduit 1 of the annular nozzle which was insulated at 30 ° C with an outer diameter of 2.5 mm and an inner diameter of 2.4 mm, 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 to which the first and second film forming solutions were applied was guided in a coagulation bath kept at 80 ° C. composed of 5% by mass of N, N-dimethylacetamide and 95% by mass of water to solidify the first and second film forming solutions. To obtain a first porous membrane precursor.

The first porous membrane precursor was desolvated for 1 minute in 98 ° C hot water, then immersed in 50,000 mg / L aqueous sodium hypochlorite solution, washed for 10 minutes in 90 ° C hot water, and dried at 90 ° C for 10 minutes. The winder was wound up. Thus, the string body which has a 1st porous layer was obtained.

Next, the string body which has the said 1st porous layer is introduce | transduced into the conduit 1 of the annular nozzle shown in FIG. 1 heated at 30 degreeC which consists of an outer diameter of 2.7 mm and an inner diameter of 2.6 mm, Glycerin (made by Wako Junyaku Kogyo Co., Ltd.) was discharged as a solution which 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 guide | induced in the coagulation | bath_bath which kept warm at 80 degreeC which consists of 5 mass% of N, N- dimethylacetamide and 95 mass% of water, and solidified the 2nd film forming liquid and obtained the 2nd porous film precursor.

The second porous membrane precursor was desolvated in hot water at 98 ° C. for 1 minute, then immersed in 50,000 mg / L aqueous sodium hypochlorite solution, washed for 10 minutes in hot water at 90 ° C., and dried at 90 ° C. for 10 minutes. The winder was wound up. In this way, a porous hollow fiber was obtained.

The outer diameter / inner diameter of the obtained porous hollow fiber membrane was about 2.8 / 1.1 mm, the film thickness was 900 micrometers, and the thickness of the resin layer from braid to the surface was 400 micrometers.

By the method described above, the bubble point, permeation performance, permeability performance expression rate, and remaining amount of PVP were evaluated. The results are shown in Table 2 below. In Table 2, the ratio and K value of the high molecular weight area | region of the used polyvinylpyrrolidone are put together (it is the same below).

(Example 2)

As an additive, it carried out similarly to Example 1 except having used polyvinylpyrrolidone (The Nihon Shokubai company make, brand name: K-80) whose ratio of high molecular weight area area is 9.0% and K value is 79.9. Got a desert.

The outer diameter / inner diameter of the obtained porous hollow fiber was about 2.8 / 1.2 mm, the film thickness was 800 micrometers, and the thickness of the resin layer from braid to the surface was 400 micrometers.

In the same manner as in Example 1, bubble points, permeation performance, permeability performance expression rate, and residual PVP amount were evaluated. The results are shown in Table 2.

(Example 3)

As an additive, it carried out similarly to Example 1 except having used polyvinylpyrrolidone (The Nihon Shokubai company make, brand name: K-80) whose ratio of high molecular weight area area is 7.9%, and K value is 78.5. Got a desert.

The outer diameter / inner diameter of the obtained porous hollow fiber was about 2.8 / 1.2 mm, the film thickness was 800 micrometers, and the thickness of the resin layer from braid to the surface was 400 micrometers.

In the same manner as in Example 1, bubble points, permeation performance, permeability performance expression rate, and residual PVP amount were evaluated. The results are shown in Table 2.

(Example 4)

A porous hollow fiber membrane was prepared in the same manner as in Example 1, except that polyvinylpyrrolidone (ISP, trade name: K-90) having a ratio of high molecular weight region area of 9.2% and K value of 84 was used. Got it.

In the same manner as in Example 1, bubble points, permeation performance, permeability performance expression rate, and residual PVP amount were evaluated. The results are shown in Table 2.

(Example 5)

As the additive, a porous hollow fiber membrane was prepared in the same manner as in Example 1 except that polyvinylpyrrolidone (ISP, trade name: K-90) having a proportion of the low molecular weight region area of 8.9% and a K value of 81 was used. Got it.

In the same manner as in Example 1, bubble points, permeation performance, permeability performance expression rate, and residual PVP amount were evaluated. The results are shown in Table 2.

(Comparative Example 1)

A porous hollow fiber membrane was prepared in the same manner as in Example 1 except that polyvinylpyrrolidone (ISP, trade name: K-90) having a ratio of high molecular weight region area of 13.2% and K value of 82.9 was used as the additive. Got it.

Fine cracks were observed on the surface of the obtained porous hollow fiber. When the bubble point was measured similarly to Example 1, it fell to 20 kPa and the product pass rate fell remarkably.

(Comparative Example 2)

As the additive, a porous hollow fiber membrane was obtained in the same manner as in Example 1 except that polyvinylpyrrolidone (ISP, trade name K81 / 86) having a high molecular weight area area ratio was 11.4%.

Fine cracks were observed on the surface of the obtained porous hollow fiber. When the bubble point was measured similarly to Example 1, it fell to 30 kPa and the product pass rate fell remarkably.

(Example 6)

As a additive, the porous hollow fiber membrane was obtained like Example 1 except having used the polyvinylpyrrolidone (ISP make, brand name K90) whose ratio of the low molecular weight area | region area is 15.3%, and K value 73.7.

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

According to the manufacturing method of the porous membrane of this invention, the porous membrane which is excellent in the washability of an additive and excellent in filtration performance is obtained. Since the porous membrane obtained by the method of the present invention has high permeability, the use membrane area is reduced, and the equipment can be made compact.

1 pipeline
2 first outlet
3 2nd protrusion
4 Distance (liquid seal length) between the end face of the braid passage and the end face of the second dispensing nozzle
5 End face of the second dispense nozzle
6 1st supply port
7 2nd supply port
8 Second Dispensing Nozzle
9 First Dispensing Nozzle
10 distribution plate
11 first liquid pool part
12 second liquid pool part
13 protrusion
100 braid aisle
110 Braid Pathway Tip

Claims (5)

A porous membrane having a step of coagulating a film forming liquid containing a film material forming polymer and an additive controlling a phase separation in a coagulating solution to obtain a porous membrane precursor, and a step of removing the additive controlling the phase separation remaining in the porous membrane precursor. As a manufacturing method,
The additive controlling the phase separation is polyvinylpyrrolidone having a ratio of the high molecular weight region area of 11% or less when the ratio of the high molecular weight region area in the integral molecular weight distribution curve is obtained by the following method. Method for producing a porous membrane.
[Here, the value of the ratio of the said high molecular weight area | region area measures the molecular weight distribution of polyvinylpyrrolidone by the gel permeation chromatography method of the following conditions,
Obtain an integral 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 an integral distribution value (mass%),
The value of X of the point where the said integral molecular weight distribution curve reached Y = 100 is set to P, and the area of the area | region enclosed by the said integrated molecular weight distribution curve, the line which shows X = P, and the line which shows Y = 0 is made into 100%. Is obtained as the ratio of the area of the integrated molecular weight distribution curve and the area surrounded by 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,
Column: TSK gel (TSKgel) α-M, 7.8 mm (ID) x 30.0 cm (L) two (made by Tosoh),
Column temperature: 30 ° C.,
Mobile phase (eluent): A mixed solution of 0.2 mol / L NaNO ₃ aqueous solution and acetonitrile, 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: 20μl,
Molecular weight calibration PEO: polyethylene oxide (manufactured by Polymer Laboratories);
Calibration curve: Standard PEO [made by Polymer Laboratories] 3-dimensional approximation curve, measuring device: HLC-8020 GPC manufactured by Tosoh, and
Filtering the sample with a cellulose acetate cartridge filter (fractional performance 0.45 mu m) immediately before measurement] The method for producing a porous membrane according to claim 1, wherein the additive controlling the phase separation is polyvinylpyrrolidone having a ratio of the high molecular weight region area of 5% or more. The polyvinyl pyrrolidone according to claim 1, wherein the polyvinylpyrrolidone has a polyolefin having a ratio of the low molecular weight region area of 5% or more and less than 13% when the ratio of the low molecular weight region area in the integral molecular weight distribution curve is obtained by the following method. The manufacturing method of the porous membrane which is vinylpyrrolidone.
[Here, the value of the ratio of the area of the low molecular weight area | region measures the molecular weight distribution of polyvinylpyrrolidone by the gel permeation chromatography method of the following conditions,
Obtain an integral 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 an integral distribution value (mass%),
The value of X of the point where the said integral molecular weight distribution curve reached Y = 100 is set to P, and the area of the area | region enclosed by the said integrated molecular weight distribution curve, the line which shows X = P, and the line which shows Y = 0 is made into 100%. Is obtained as the ratio of the area of the integrated molecular weight distribution curve and the area surrounded by 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,
Column: TSK gel α-M, 7.8 mm (ID) x 30.0 cm (L) (made by Tosoh),
Column temperature: 30 ° C.,
Mobile phase (eluent): A mixed solution of 0.2 mol / L NaNO ₃ aqueous solution and acetonitrile, 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: 20μl,
Molecular weight calibration PEO: polyethylene oxide [manufactured by Polymer Laboratories],
Calibration curve: Standard PEO [made by Polymer Laboratories] 3-dimensional approximation curve, measuring device: HLC-8020 GPC manufactured by Tosoh, and
Filtering the sample with a cellulose acetate cartridge filter (fractional performance 0.45 mu m) immediately before measurement] The method for producing a porous membrane according to claim 1 or 2, wherein the K value of the polyvinylpyrrolidone is 82 or less. The method for producing a porous membrane according to any one of claims 1 to 3, wherein the polyvinylpyrrolidone has a K value of 78 or more.

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