JP2014073487A - Porous membrane, water purifier incorporating porous membrane and method for producing porous membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous membrane which has both virus removability and water permeability, where a range having desired opposite performances such as the virus removability and the water permeability is found as the result of focusing on a short diameter and a long diameter of a pore on a surface and, especially, where the virus removability and the water permeability can be enhanced by making the variation of the short diameter of the pore small and making the variation of the long diameter of the pore large by the control of the variation of the pore.SOLUTION: A porous membrane having an average value of a short diameter of a pore at least on one surface of 10 nm or more and 50 nm or less, a standard deviation of the short diameter of the pore of 30 nm or less, a standard deviation of the long diameter of the pore of 30 nm or more and 150 nm or less and an average value of the long diameter of the pore of 2.5 times or more of the average value of the short diameter is provided.

Description

  The present invention relates to a porous membrane, a water purifier incorporating the porous membrane, and a method for producing the porous membrane. In particular, it relates to a porous membrane that removes viruses.

  Porous membranes are suitable for membrane separation in which substances in a liquid are sieved according to the size of the pores, medical applications such as hemodialysis and blood filtration, water treatment applications such as household water purifiers and water purification treatments, and beverages It is used in a wide range of applications such as food production processes such as sterilization and fruit juice concentration.

  In particular, in the field of household water purifiers, in areas where water and sewage systems are not complete and in developing countries, virus removal performance has been improved to avoid the risk of contamination by viruses and bacteria in drinking water. There is a need for household water purifiers. Norovirus causes food poisoning due to oral infection, among viruses that are at risk of mixing in water for beverage use. In food poisoning caused by norovirus, it is often difficult to identify the source of infection, but in many cases it is suspected to be caused by water used for beverages. Norovirus is as small as 38 nm in size. Since the porous membrane removes a substance by its size, the smaller the substance, the lower the removal performance. In addition, norovirus is highly infectious, and even a small amount of 10 to 100 can infect humans. Therefore, high removal performance is required to prevent food poisoning.

  That is, for household water purifier applications, there is a demand for a porous membrane that can remove 99.99% or more of substances of 38 nm or more.

  Water purifiers that remove impurities using porous membranes have been widely used in the past, but the purpose of removal is malodorous substances and bacteria contained in tap water, and those using activated carbon and microfiltration membranes as filter media. It has become mainstream. However, activated carbon has low virus adsorption performance, and the microfiltration membrane is targeted for removing bacteria and iron rust with a diameter of 100 nm or more, and cannot remove viruses with a diameter of 38 nm.

  If the pores of the porous membrane are made small in order to remove the virus, the water permeability performance is lowered, which has been a big problem in water purifier applications that require a large amount of water to be obtained in a short time. The virus removal performance and water permeation performance required for the porous membrane are greatly affected by the pore diameter on the surface of the porous membrane, and there is a contradictory relationship that the virus removal performance increases but the water permeation performance decreases when the pore diameter is small.

  Moreover, in household water purifier use, since it is used with a water pressure, the membrane structure which bears a high water pressure is required.

  In order to improve the water permeation performance and removal performance of the porous membrane, there is a technique for enlarging the pores on the surface and increasing the major axis of the pores relative to the minor axis. As a method of stretching the pores on the surface of the porous membrane, there are a method of stretching after the porous membrane is solidified and a method of applying a draft before the porous membrane is solidified.

  Patent Document 1 discloses a porous membrane manufactured by stretching.

  Patent Documents 2 and 3 disclose porous membranes manufactured by drafting.

  Patent Document 4 and Patent Document 5 describe porous membranes in which the surface pores are stretched by adjusting the composition of the film-forming stock solution and the film-forming temperature and controlling the growth and solidification of the pores by phase separation. ing.

JP-A 64-75015 International Publication No. 2010/029908 JP-A-6-165926 International Publication No. 2010/074136 Japanese Patent Laid-Open No. 9-308685

  Patent Document 1 describes a porous membrane in which the major axis of the surface pores is 1.5 times or more the minor axis by stretching, but there is no description or suggestion about variations in major axis. In addition, the minor diameter of the pores on the surface is as large as 3 to 30 μm, and the virus cannot be removed.

  Patent Document 2 has a description of a porous membrane for use in a water purifier having a large ratio of the short diameter and long diameter of the surface pores produced by drafting and a large open area ratio. However, the minor diameter of the pores on the surface is as large as 1 μm, and the virus cannot be removed. Moreover, there is no description about the variation in hole diameter.

  Patent Document 3 describes a porous film in which the short diameter of pores on the surface produced by drafting is 1 nm to 50 nm. However, there is no description about the major diameter of the surface pores, and there is no specification regarding variations in the pore diameter.

  Patent Document 4 describes a porous film that removes viruses by making holes on the surface into slits. However, there is no description regarding the minor axis, major axis, and variation of the surface holes. The performance of the porous membrane with slits on the surface is high in water permeability but low in virus removal performance. Unless the short diameter and long diameter of the surface pores are precisely controlled, a porous membrane having both high water permeability and virus removal performance cannot be obtained.

  Patent Document 5 describes a porous film having a major axis of surface pores that is twice or more the minor axis and a minor axis of 20 nm to 150 nm. The minor axis of the surface is small and viruses can be removed to some extent. Although it is described so that the minor axis of the surface hole is uniform, there is no description about the variation of the major axis of the hole. There is no description about stretching or drafting to stretch the surface holes, and there is a high possibility that variations in the major and minor diameters of the pores cannot be precisely controlled.

  An object of the present invention is to provide a porous membrane having both virus removal performance and water permeation performance.

  In order to achieve the above object, the present invention has the following configuration.

  That is, according to the present invention, the average value of the minor axis of the pores on at least one surface is 10 nm or more and 50 nm or less, the standard deviation of the minor axis of the pores is 30 nm or less, and the standard deviation of the major axis of the pores is 30 nm or more. There is provided a porous membrane having a length of 150 nm or less and an average value of the major axis of the pores being 2.5 times or more of the average value of the minor axis.

  Moreover, according to the preferable form of this invention, the porous film whose average value of the short diameter of the hole of one surface is smaller than the average value of the short diameter of the hole of the other surface is provided.

  Further, according to a preferred embodiment of the present invention, there is provided a porous membrane in which a layer having a pore size of 130 nm or less in the film thickness direction from the surface is 0.3 μm or more and 20 μm or less.

  Moreover, according to the preferable form of this invention, the porous membrane whose porosity of the said surface is 1% or more and 20% or less is provided.

  Moreover, according to the preferable form of this invention, the porous membrane which consists of an amorphous polymer is provided.

  Moreover, according to the preferable form of this invention, the porous membrane whose said non-crystalline polymer is a polysulfone type polymer is provided.

  Moreover, according to the preferable form of this invention, the porous membrane whose weight average molecular weight of the hydrophilic high molecular weight in a porous membrane is 20,000 or more and 80,000 or less is provided.

  Moreover, according to the preferable form of this invention, the porous membrane whose content of the hydrophilic polymer in a porous membrane is 1.5 weight% or more and 8 weight% or less is provided.

  Moreover, according to the preferable form of this invention, the porous membrane which is a hollow fiber membrane is provided.

  Moreover, according to the preferable form of this invention, the porous membrane whose average value of the short diameter of the hole of the inner surface of a hollow fiber membrane is smaller than the average value of the short diameter of the hole of an outer surface is provided.

  Moreover, according to the preferable form of this invention, the porous membrane used for the use which removes a virus is provided.

  Moreover, according to another form of this invention, the water purifier which incorporates the said porous membrane is provided.

  According to another aspect of the present invention, in the method for producing a porous film in which a film-forming stock solution is discharged from a slit and solidified in a coagulation bath after passing through a dry part, the draft ratio is 1.7 or more and 5.0 or less. A method for producing a porous membrane in which the weight-average molecular weight of the hydrophilic polymer in the membrane-forming stock solution is 20,000 or more and 80,000 or less is provided.

  Moreover, according to the preferable form of this invention, the manufacturing method of the porous membrane whose passage time of a dry-type part is 0.05-0.14 second is provided.

  Moreover, according to the preferable form of this invention, the manufacturing method of the porous membrane which the coagulation liquid containing a poor solvent and the film-forming stock solution contact in a dry part is provided.

  Moreover, according to the preferable form of this invention, the manufacturing method of the porous membrane whose concentration of the hydrophilic polymer in the film-forming stock solution is 30% or more and 70% or less of the concentration of the polymer mainly constituting the porous membrane Is provided.

  According to the present invention, as described below, a porous membrane having both virus removal performance and water permeability performance can be provided. For example, by incorporating it in a water purifier, it is possible to obtain a large amount of safe water excellent in compactness and free of pathogenic viruses in water in a short time.

2 is a scanning electron microscope (SEM) photograph of the inner surface of the porous membrane produced by the method of Example 1. FIG. 3 is a diagram obtained by binarizing an SEM image of an inner surface of a porous film manufactured by the method of Example 1. FIG.

  As a result of intensive studies in the present invention, the average value of the minor axis of the pores on at least one surface is 10 nm or more and 50 nm or less, the standard deviation of the minor axis of the pores is 30 nm or less, and the standard deviation of the major axis of the pores is 30 nm or more. It was found that a porous membrane having a length of 150 nm or less and an average value of the major axis of the pores is 2.5 times or more of the average value of the minor axis has high virus removal performance and water permeability.

  Since the porous membrane screens the substance to be removed according to the size, the virus removal performance depends on the minor axis of the surface. Since the size sifting by pores exerts an effect up to a size larger than the actual pore diameter, in order to sufficiently remove norovirus having a diameter of 38 nm, it is necessary that the average value of the minor diameter of the surface pores is 50 nm or less, 38 nm or less is more preferable. Furthermore, 30 nm or less is more preferable in consideration of variation. On the other hand, if the average value of the minor diameters of the pores on the surface is small, the water permeability is remarkably deteriorated, so that 10 nm or more is necessary, and 15 nm or more is preferable.

  There are variations in the minor diameters of the pores on the surface, and if there are pores with a large minor axis, the virus removal performance is reduced. When high removal performance is required, such as virus removal, it is necessary to reduce the variation. Therefore, in order to improve the virus removal performance, it is necessary to consider not only the average value of the minor diameter of the surface pores but also the variation, and the standard deviation indicating the variation of the minor diameter of the pores must be 30 nm or less, and 15 nm or less. Is more preferable. In order to reduce the variation in the minor diameter of the surface hole, it is effective to stretch the surface hole. When the surface hole is stretched, the larger the hole, the easier it is to deform. Therefore, when the amount of deformation is increased, the short diameter of the large hole becomes smaller, the short diameter of the small hole does not change much, and variations in the short diameter are reduced.

  By maintaining the short diameter of the pores on the surface and increasing the long diameter, the water permeation resistance is reduced and the water permeation performance is improved while maintaining the virus removal performance. The greater the average value of the major axis is relative to the average value of the minor axis, the greater is the water permeability of the virus removal performance. Therefore, the average value of the major axis of the surface holes needs to be 2.5 times or more of the average value of the minor axis, and more preferably 3.0 times or more. Further, from the viewpoint of the strength of the membrane structure, the average value of the major axis of the surface pores is preferably 10 times or less, more preferably 8 times or less of the average value of the minor axis.

  As a method of increasing the average value of the major axis of the surface pores relative to the average value of the minor axis, a method of stretching the pores is effective, and a stretching method that stretches after the porous membrane is solidified or a draft ratio is increased. There is a method of stretching before the porous membrane is solidified. The method of increasing the draft ratio is preferable because it can be widely applied without being limited by the method of forming the porous membrane and the material. Since the stretching method cannot be applied unless the strength of the porous membrane is strong, the membrane material is limited to a crystalline polymer or the like.

  The draft ratio is a value obtained by dividing the take-up speed of the porous film by the discharge linear speed from the slit. The discharge linear velocity is a value obtained by dividing the discharge flow rate by the sectional area of the slit. In order to increase the draft ratio, there are methods such as increasing the take-up speed, increasing the sectional area of the slit, and decreasing the discharge flow rate. A method of increasing the cross-sectional area of the slit, which can increase the draw ratio without changing the shape of the porous membrane, is preferable. In the method of increasing the take-up speed and the method of decreasing the discharge flow rate, the cross-sectional area of the porous film decreases, so there is a concern that the physical strength of the porous film will decrease.

  When the average value of the major axis is made larger than the average value of the minor axis of the surface holes, the structural strength of the surface is lowered. Since the surface hole is formed by stretching, the hole exists in parallel in one direction. When a large number of holes having a large major axis are arranged, since there are few connections in the minor axis direction, the structural portion becomes elongated and the structural strength is lowered. If the structural strength of the surface of the porous membrane is low, damage or expansion of the pore size may occur due to water pressure when the porous membrane is used, and the virus removal performance may be reduced. In particular, it is difficult to achieve virus removal performance at a high water pressure of 50 kPa or more. In order to increase the structural strength of the surface, it is effective to lower the surface area porosity, reduce the major axis relative to the minor axis of the hole, and increase the thickness of the dense layer in the cross-sectional direction of the film thickness. Water permeability will be reduced.

  In the prior art, it was difficult to obtain a membrane having high virus removal performance under high pressure conditions in a porous membrane having a larger average value of the major axis than the average value of the minor axis of the surface pores. The present inventors have found that it is effective to increase the variation of the major axis in order to increase the structural strength of the surface of the porous membrane. By coexisting a long hole with a long diameter and a short hole, the structure is connected to the short diameter direction to form a network, so that the structural strength is maintained. In this way, by controlling the ratio between the minor axis and major axis of the surface hole and the variation in major axis, it is possible to achieve both water permeability and structural strength. The standard deviation of the major diameter of the surface holes needs to be 30 nm or more, preferably 40 nm or more. On the other hand, if the variation in the major diameter of the pores is too large, the performance variation of the porous membrane increases, and a certain performance cannot be obtained. Therefore, the standard deviation of the major axis of the surface holes needs to be 150 nm or less, and more preferably 100 nm or less.

  In order to increase the variation in the major diameter of the surface hole, the hole having a large variation in the hole diameter may be stretched. When the surface hole is stretched, the larger the hole, the easier it is to deform. Therefore, when the amount of deformation is increased, the major axis of the larger hole becomes larger, the major axis of the smaller hole does not change much, and the variation of the major axis increases. In order to increase the variation in the pore diameter before stretching, the weight-average molecular weight distribution of the hydrophilic polymer added to the film-forming stock solution as a pore-forming agent is increased to make the phase separation non-uniform, or the phase separation proceeds. It is effective to promote the growth of surface pores. When the phase separation proceeds, the holes grow due to the fusion of the holes, so that the growth of the holes is biased depending on the probability of collision, and the size of the formed holes becomes non-uniform. In order to advance the phase separation, the composition of the film forming stock solution, the composition of the coagulating liquid, the temperature of the phase separation process, the time until solidification, etc. may be adjusted.

  It is effective that the average weight molecular weight of the hydrophilic polymer is large, and the average weight molecular weight of the hydrophilic polymer is preferably 20,000 or more. On the other hand, the concentration of the hydrophilic polymer added to the film-forming stock solution increases locally at the interface between the pores and the structure in the process of forming the porous film. When the surface pores are stretched, if the molecular weight of the hydrophilic polymer is small, the entanglement of the molecular chains between the polymer and the hydrophilic polymer in the structure is reduced, and deformation tends to occur when the pores are stretched. Therefore, the average weight molecular weight of the hydrophilic polymer contained in the porous membrane is preferably 80,000 or less, and more preferably 50,000 or less.

  The average weight molecular weight of the hydrophilic polymer contained in the porous membrane can be measured, for example, by the following method. The porous membrane is dissolved with a solvent, and extracted with a solvent having a high solubility of the hydrophilic polymer and a low solubility of the polymer that forms the porous membrane structure. The weight average molecular weight of the hydrophilic polymer in the extract is measured by gel filtration chromatography or the like. The extraction conditions and the like at this time may be changed as appropriate depending on the combination of the polymer that forms the porous membrane structure and the hydrophilic polymer, but by increasing the extraction rate of the hydrophilic polymer, a more accurate weight can be obtained. The average molecular weight can be measured.

  By adding a poor solvent to the film-forming stock solution, the progress of phase separation is promoted, and the variation in the major diameter of the pores is increased. The optimum range varies depending on the composition of the film forming stock solution and the type of the poor solvent, but when water is used as the poor solvent, the water content in the film forming stock solution is preferably 0.5% or more, and more preferably 0.8% or more. . On the other hand, when the amount of the poor solvent in the film-forming stock solution is large, the film-forming stock solution is solidified, and therefore the water content is preferably 3% or less.

  The major axis and minor axis of the hole can be measured from an image obtained by observing the surface with a scanning electron microscope (SEM). The minor axis is the longest diameter in the minor axis direction, and the major axis is the longest diameter in the major axis direction. With respect to holes that can be confirmed at a magnification of 50000 in SEM observation, all holes in the range of 1 μm × 1 μm are measured. When the total number of measured holes is less than 50, data in the range of 1 μm × 1 μm is repeated until the total number of measured holes reaches 50 or more, and data is added. The average value and standard deviation are calculated from the measurement results.

  The higher the porosity of the surface of the porous membrane, the higher the water permeability because the number of water channels increases. On the other hand, when the porosity is lowered, the structural strength of the surface is increased, and damage due to water pressure and expansion of the pore diameter when the porous membrane is used at a high water pressure of 50 kPa or more can be suppressed.

  Therefore, the porosity of the porous membrane surface is preferably 1% or more, more preferably 3% or more. On the other hand, the porosity is preferably 20% or less, and more preferably lower than 8%.

  In order to increase the open area ratio, it is effective to increase the amount of the hydrophilic polymer added to the film-forming stock solution.

  The surface porosity can be measured from an image obtained by observing the surface of the porous membrane with an SEM. The image observed at a magnification of 10,000 is subjected to image processing, binarized at the structure portion and the hole portion, and the percentage of the area of the hole portion with respect to the measurement area is calculated to obtain the hole area ratio.

  The structure of the cross section in the film thickness direction of the porous membrane is different from the symmetrical membrane structure in which the pore diameter does not substantially change, the pore diameter changes continuously or discontinuously, and the pore diameter is different on one surface, inside, and the other surface. Broadly divided into asymmetric membrane structures. Among them, the asymmetric membrane structure has a thin layer with a small pore diameter for size exclusion, and therefore has a low water permeation resistance and a high water permeation performance. Therefore, the structure of the cross section in the film thickness direction is preferably an asymmetric film structure.

  The higher the asymmetry, the more advantageous the water permeability. The asymmetry can be represented by the difference in average pore diameter between the surface layer and the central layer in the cross section in the film thickness direction. Unlike the surface, the hole diameter in the cross section in the film thickness direction does not need to be controlled to the short diameter and the long diameter. The area of the hole was calculated by image processing or the like, and the diameter when converted to a circle was taken as the hole diameter. The average pore size of the central layer of the porous membrane is preferably at least 1.5 times the average pore size of at least one of the surface layers, more preferably at least 2 times. The center layer is a layer having a thickness of 2 μm in total, 1 μm on each surface from the center, and the surface layer is a layer having a thickness of 2 μm in the film thickness direction from the surface.

  In the asymmetric membrane structure, in the cross section in the film thickness direction, the presence of a dense layer having a pore size that can contribute to virus removal on the surface side where the minor axis of the pore is small increases the virus removal performance. Deep-layer filtration in which the virus is removed stepwise in the thickness direction occurs, and the virus can be removed more than the effect of the pore size. In order to achieve a high virus removal rate of 99.99% with only one surface, it is necessary to have a small hole that suppresses variations in the short diameter of the hole, and it is difficult to control and the water permeability performance is significantly reduced. The virus removal performance required for the pores on the surface is reduced by removing the virus in the dense layer, and a certain degree of variation in pore diameter can be allowed and the pore diameter can be increased, so that the water permeability can be enhanced. In addition, when the dense layer is thick, the surface structure is also retained by the structure in the thickness direction, so that the structural strength of the surface is increased, and when using a porous membrane at a high water pressure of 50 kPa or more, damage due to water pressure and pore size Expansion is suppressed. Norovirus, which is mixed in drinking water and causes gastroenteritis, has a diameter of 38 nm. The pore diameter that can contribute to the removal of norovirus with a diameter of 38 nm is 130 nm. On the other hand, when the thickness of the dense layer is small, the permeation resistance is small and the water permeability is improved. In the present invention, the dense layer is a layer having a pore diameter of 130 nm or less. The layer having a pore diameter of 130 nm or less in the film thickness direction from the surface is preferably 0.3 μm or more. On the other hand, it is preferably 20 μm or less, and more preferably 10 μm or less.

  In order to fully exert the effect of virus removal in the dense layer, water should be membrane-filtered from the side with the larger average value of the minor diameter of the surface pores toward the side with the smaller average value of the minor diameter of the surface pores. Is preferred.

  The thickness of the dense layer can be measured from an image obtained by observing a cross section of the porous film with an SEM. Since the hole in the cross section is indefinite, the area of the hole observed by image processing is obtained and converted to a circle to obtain the hole diameter. A hole closest to the surface in the film thickness direction and having a hole diameter of 130 nm or more is specified, and the length from the surface to the hole is measured.

  In order to thicken the dense layer, mainly increasing the concentration of the polymer constituting the membrane in the film-forming stock solution to reduce the pore diameter of the entire porous membrane, or increasing the viscosity of the film-forming stock solution to increase the pore size due to phase separation. It is effective to reduce the pore size by suppressing the growth or promoting solidification of the film-forming stock solution.

  Moreover, the structure which has a dense layer on both sides may be sufficient, and if a dense layer exists on both sides, since a removal layer will be two layers, a virus removal rate will improve.

  As the material for the porous membrane, an amorphous polymer is preferably used. The non-crystalline polymer is a polymer that does not crystallize, and is a polymer that does not have an exothermic peak due to crystallization as measured by a differential scanning calorimeter.

  Since the amorphous polymer easily undergoes structural deformation, the effect of stretching the pores is enhanced. Further, the structure control in the film thickness direction becomes easy. A porous membrane made of an amorphous polymer can be obtained by inducing phase separation of a stock solution prepared by dissolving an amorphous polymer in a solvent by heat or a poor solvent and removing the solvent component. can get. The amorphous polymer dissolved in the solvent has high mobility, and agglomerates during phase separation to increase the concentration, resulting in a dense structure. By changing the phase separation speed in the film thickness direction, it is possible to obtain a film having a structure with different pore diameters in the film thickness direction.

  Examples of the non-crystalline polymer used as the material for the porous membrane include acrylic polymers, vinyl acetate polymers, polysulfone polymers, and the like. Among these, a polysulfone polymer is preferably used because it can easily control the pore diameter. The polysulfone polymer referred to in the present invention has an aromatic ring, a sulfonyl group and an ether group in the main chain. For example, polysulfone represented by the chemical formulas of the following formulas (1) and (2) is preferably used. The present invention is not limited to these. N in the formula is an integer such as 50 to 80.

  Specific examples of polysulfone include Udel (registered trademark) Polysulfone P-1700, P-3500 (manufactured by Solvay), Ultrason (registered trademark) S3010, S6010 (manufactured by BASF), Victrex (registered trademark) (Sumitomo Chemical) ), Radel (registered trademark) A (manufactured by Solvay), Ultrasulone (registered trademark) E (manufactured by BASF), and the like. The polysulfone used in the present invention is preferably a polymer composed only of the repeating units represented by the above formulas (1) and / or (2), but other monomers may be used as long as the effects of the present invention are not hindered. It may be copolymerized. Although it does not specifically limit, it is preferable that another copolymerization monomer is 10 weight% or less.

  By adding a hydrophilic polymer to the membrane-forming stock solution, the hydrophilic polymer is contained in the porous membrane, the water wettability is increased, and the water permeability is increased. Therefore, it is preferable that 1.5% by weight or more of the hydrophilic polymer is contained in the porous film. On the other hand, by reducing the content of the hydrophilic polymer in the porous membrane, the amount of eluate decreases, so 8 wt% or less is preferable.

  Although the content of the hydrophilic polymer needs to be selected depending on the type of polymer, it can be measured by a method such as elemental analysis.

  Although not particularly limited, specific examples of the hydrophilic polymer include polyethylene glycol, polyvinyl pyrrolidone, polyethylene imine, polyvinyl alcohol, and derivatives thereof. Moreover, you may copolymerize with another monomer.

  The material may be appropriately selected depending on the affinity for the material of the porous membrane and the solvent. However, when the material of the porous membrane is a polysulfone-based polymer, polyvinyl pyrrolidone is preferably used because of high compatibility.

  The form of the porous membrane may be appropriately selected from a flat membrane, a tubular membrane, a hollow fiber membrane, etc. according to the application. Although not particularly limited, a hollow fiber membrane that can increase the membrane area per volume and can accommodate a large-area membrane in a compact manner is preferable. The hollow fiber membrane is made by flowing an injection solution or injection gas from a circular tube inside the double tube cap and discharging a membrane forming raw solution from an outer slit. At this time, the structure of the inner surface of the hollow fiber membrane can be controlled by changing the poor solvent concentration or temperature of the injection solution or the temperature of the injection gas. In order to make it easier to control the structure of the surface on the side where the average value of the minor axis of the pore is small, which has a large effect on the virus removal performance, the average value of the minor axis of the pore on the inner surface of the hollow fiber membrane It is preferably smaller than the average value of the minor axis.

  The film thickness of the porous membrane may be appropriately determined depending on the pressure of the intended use, but in the application to the water purifier, the thickness is preferably 60 μm or more, more preferably 80 μm or more so as to withstand water pressure. On the other hand, the smaller the film thickness, the lower the water permeation resistance and the higher the water permeability, so the film thickness is preferably 200 μm or less, and more preferably 150 μm or less.

  When the porous membrane is a hollow fiber membrane, the pressure resistance has a correlation with the ratio between the film thickness and the inner diameter, and the pressure resistance increases when the ratio between the film thickness and the inner diameter is large. If the inner diameter is reduced, the water purifier can be reduced in size and pressure resistance is also improved. However, in order to reduce the inner diameter, it is necessary to narrow down at the time of film formation, and a star-shaped yarn having a wrinkled inner diameter tends to occur. In star-shaped yarns, the phase separation is non-uniform, resulting in large variations in pore size and reduced virus removal performance. In order to reduce the size of the water purifier and increase the virus removal performance, water permeability, and pressure resistance, the thickness of the hollow fiber membrane is preferably 50 μm or more, more preferably 60 μm or more. On the other hand, 240 micrometers or less are preferable and 190 micrometers or less are more preferable. The film thickness / inner diameter of the hollow fiber membrane is preferably 0.3 or more, and more preferably 0.5 or more. On the other hand, the film thickness / inner diameter of the hollow fiber membrane is preferably 1.00 or less, and more preferably 0.7 or less.

  Since the present invention is a porous membrane having high virus removal performance and water permeability, it is suitably used for the purpose of removing viruses. Moreover, it is used suitably for the use which processes a lot of water in a short time like the porous membrane incorporated in a water purifier.

  The porous membrane of the present invention is not particularly limited, but can be obtained by discharging a raw film forming solution from a slit and solidifying it in a coagulation bath after passing through a dry part consisting of a gas phase. In the case of inducing phase separation by heat, it is cooled in a dry part and then rapidly cooled in a coagulation bath to be solidified. When inducing phase separation with a poor solvent, the film-forming stock solution is discharged in contact with a coagulation liquid containing the poor solvent, and solidified in a coagulation bath made of the poor solvent. In the method of inducing phase separation with a poor solvent, since the poor solvent is supplied by diffusion in the film thickness direction, a difference in concentration of the poor solvent occurs in the film thickness direction, and an asymmetric structure is likely to occur. Therefore, it is preferable that the coagulating liquid containing the poor solvent and the film forming stock solution are in contact with each other in the dry part.

  If the concentration is adjusted by using the coagulation liquid as a mixture of a poor solvent and a good solvent, the coagulation property is changed and the diameter of the surface pores can be changed.

  The poor solvent is a solvent that does not dissolve mainly the polymer constituting the film at the film forming temperature. Although it does not specifically limit as a poor solvent, Water is used suitably. The good solvent is not particularly limited, but N, N-dimethylacetamide is preferably used.

  The preferred range of the composition of the coagulation liquid varies depending on conditions such as the composition of the film forming stock solution and the type of poor solvent and good solvent. For example, the concentration of the poor solvent is preferably 50% by weight or more, more preferably 60% by weight or more. . On the other hand, 80 weight% or less is preferable and 75 weight% or less is more preferable.

  By increasing the draft ratio and stretching before solidification, the major axis of the surface holes can be made longer than the minor axis. Since it is before solidification, it does not cause breakage or cracks, which is a problem in the stretching method. The draft ratio needs to be 1.7 times or more, and more preferably 2 times or more. On the other hand, if the draft ratio is too large, yarn breakage occurs, so the draft ratio needs to be 5 or less, and preferably 4 or less.

  When the viscosity is high, the stretching stress increases, and the effect of stretching the surface pores increases, so that the major axis can be made longer than the minor axis of the pores. Since the viscosity increases with the progress of the phase separation, it is possible to extend the viscosity in a state where the viscosity is increased by extending the passage of the dry part and extending the time for the phase separation to proceed. Although it depends on conditions affecting the progress of phase separation such as the composition and temperature of the film-forming stock solution, the passage time of the dry part is preferably 0.05 seconds or more, more preferably 0.14 seconds or more. On the other hand, 0.40 second or less is preferable, and 0.35 second or less is more preferable.

  In order to increase the viscosity of the film-forming stock solution, an increase in the amount of polymer and / or hydrophilic polymer constituting the porous film and / or a thickening agent may be added, or the discharge temperature may be lowered. The viscosity of the film-forming stock solution is preferably 0.5 Pa · s or more, more preferably 1.0 Pa · s or more at the discharge temperature. On the other hand, the lower the viscosity of the film-forming stock solution, the lower the discharge pressure and the more stable the discharge and the uniform porous film is obtained. Therefore, it is preferably 20 Pa · s or less, more preferably 15 Pa · s or less.

If a large amount of hydrophilic polymer is added to the film-forming stock solution, the amount of hydrophilic polymer at the interface between the pores and the structure increases in the pore formation process of the porous membrane, and the entanglement of the molecular chains of the polymer in the structure increases. The deformation of the hole is suppressed. On the other hand, by increasing the amount of the hydrophilic polymer, the number of pores increases and the porosity of the porous membrane surface increases. Therefore, the concentration of the hydrophilic polymer in the membrane-forming stock solution is preferably 70% or less, more preferably 60% or less of the concentration of the polymer constituting the porous membrane. On the other hand, by increasing the concentration of the hydrophilic polymer, the surface porosity increases, so that the concentration of the polymer mainly constituting the porous membrane is preferably 30% or more, more preferably 36% or more. The present invention will be described by way of examples, but the present invention is not limited to these examples.

(1) Measurement of water permeation performance An example of measurement when the porous membrane is a hollow fiber membrane is shown.

A hollow fiber membrane was filled in a housing having a diameter of 5 mm and a length of 17 cm so that the membrane area of the outer surface was 0.004 m ² . The membrane area is calculated by the following formula.

Membrane area (m ² ) = outer diameter (μm) × π × 17 (cm) × number of yarns × 0.00000001
A hollow fiber membrane module is produced by potting both ends with an epoxy resin chemical reaction type adhesive “Quick Mender” manufactured by Konishi Co., Ltd., cutting and opening. Next, the hollow fiber membrane of the module and the inside of the module were washed with distilled water at 100 ml / min for 1 hour. A water pressure of 13 kPa was applied to the outside of the hollow fiber membrane, and the amount of filtration per unit time flowing out to the inside was measured. The water permeability (UFR) was calculated by the following formula.

UFR (ml / hr / Pa / m ² ) = Q _w / (P × T × A)
Here, Q _w : Filtration amount (mL), T: Outflow time (hr), P: Pressure (Pa)
(2) Measurement of virus removal performance An example of measurement when the porous membrane is a hollow fiber membrane is shown.

  Evaluation was performed using the module for which the evaluation in (1) was completed.

The virus stock solution was prepared in distilled water so that bacteriophage MS-2 (Bacteriophage MS-2 ATCC 15597-B1) having a size of about 25 nm contained a concentration of about 1.0 × 10 ⁷ PFU / ml. The distilled water used here was distilled water from a pure water production apparatus Auto Still (manufactured by Yamato Kagaku) and subjected to high pressure steam sterilization at 121 ° C for 20 minutes. The virus stock solution was fed from the outer surface toward the hollow part under conditions of a temperature of about 20 ° C. and a filtration differential pressure of 50 kPa, and the whole was filtered. For collecting the filtrate, after discarding 150 ml of the permeate, about 5 ml of the permeate for measurement was collected and diluted with distilled water to 0, 100, 10000, and 100,000 times. Based on the method of Overlay agar assay, Standard Method 9211-D (APHA, 1998, Standard methods for the examination of water and wastewater, 18th.) The concentration of bacteriophage MS-2 was determined. Plaque is a group of bacteria that have been killed by infection with a virus, and can be counted as punctate lysis spots. Virus removal performance was expressed in terms of virus log removal rate (LRV). For example, LRV2 means -log ₁₀ x = 2, that is, 0.01, and means that the residual concentration is 1/100 (removal rate 99%). When no plaque was measured in the permeate, LRV 7.0 was set.

If the virus log removal rate is measured with bacteriophage MS-2, the removal performance of viruses mixed in drinking water having a larger diameter is ensured.
(3) Measurement of surface pore diameter The surface of the porous film was observed at 50000 times with a scanning electron microscope (SEM) (S-5500, manufactured by Hitachi High-Technologies Corporation), and the image was taken into a computer. When the inner surface of the porous membrane was a hollow fiber membrane, the hollow fiber membrane was cut into a semicircular shape and observed.

The short diameter of the hole was the longest diameter in the short axis direction, and the long diameter was the longest diameter in the long axis direction. Measurements were made for all holes in the range of 1 μm × 1 μm. The measurement was repeated in the range of 1 μm × 1 μm until the total number of measured holes reached 50 or more, and data was added. When the hole was observed twice in the depth direction, it was measured at the exposed part of the deeper hole. When a part of the hole was out of the measurement range, the hole was excluded. When the difference between the minor axis and the major axis is large and the major axis of all the holes does not fit in the measurement range, the measurement range was expanded to 2 μm × 1 μm. When the major axis did not fit in one SEM image, the measurement was performed by synthesizing two consecutive SEM images. The average value and standard deviation of the minor axis and major axis were calculated.
(4) Measurement of surface porosity The surface of the porous membrane was observed at a magnification of 10,000 with a scanning electron microscope SEM (S-5500, manufactured by Hitachi High-Technologies Corporation), and the image was taken into a computer. The sample used in the measurement of (3) was observed. The SEM image was cut out in a range of 6 μm × 6 μm, and image analysis was performed with image processing software. The threshold value was determined so that the structure portion became bright luminance and the other portions had dark luminance by binarization processing, and an image was obtained in which the bright luminance portion was white and the dark luminance portion was black. If the structure part cannot be separated from the other parts due to the difference in contrast in the image, the image is separated at the same contrast part and binarized, and then connected to the original one. Returned to the image. Alternatively, the image analysis may be performed by painting other than the structure portion in black. Since the image contains noise and the dark luminance portion where the number of consecutive pixels is 5 or less, noise and a hole cannot be distinguished from each other, so that it is treated as a bright luminance portion as a structure. As a method for eliminating noise, a dark luminance portion having 5 or less consecutive pixels was excluded when measuring the number of pixels. Alternatively, the noise portion may be painted white. The number of pixels in the dark luminance portion was measured, and the percentage with respect to the total number of pixels in the analysis image was calculated as the hole area ratio. The same measurement was performed on 10 images, and the average value was calculated.
(5) Measurement of pore diameter of cross section The cross section of the porous membrane was observed with a SEM (S-5500, manufactured by Hitachi High-Technologies Corporation) at a magnification of 10,000, and the image was taken into a computer. The porous membrane was soaked in water for 5 minutes, then frozen with liquid nitrogen and quickly folded to obtain a cross-sectional observation sample. When the hole in the cross section was closed as observed by SEM, the sample preparation was repeated. The clogging of the holes may occur due to the deformation of the porous film in the stress direction during the cutting process. The SEM image was cut into a range of 2 μm in the film thickness direction and 6 μm in parallel with the surface of the porous film, and image analysis was performed with image processing software. The cross-sectional center layer was measured in a range of 2 μm in total, 1 μm on each surface from the center in the film thickness direction. The surface layer was measured in the range of 2 μm from the surface in the film thickness direction. The threshold value was determined by binarizing the SEM image so that the structure portion had bright luminance and the other portions had dark luminance, and an image was obtained in which the bright luminance portion was white and the dark luminance portion was black. If the structure part cannot be separated from the other parts due to the difference in contrast in the image, the image is separated at the same contrast part and binarized, and then connected to the original one. Returned to the image. Alternatively, the image analysis may be performed by painting other than the structure portion in black. Since the image contains noise and the dark luminance portion where the number of consecutive pixels is 5 or less, noise and a hole cannot be distinguished from each other, so that it is treated as a bright luminance portion as a structure. As a method for eliminating noise, a dark luminance portion having 5 or less consecutive pixels was excluded when measuring the number of pixels. Alternatively, the noise portion may be painted white. When a hole was observed twice in the depth direction, the measurement was made with the shallower hole. When a part of the hole was out of the measurement range, the hole was excluded. The number of pixels of the scale bar indicating a known length in the image was measured, and the length per pixel number was calculated. The number of pixels in the dark luminance portion was measured, and the hole area was determined by multiplying the length per pixel. The average area of the holes was measured, and the hole diameter converted to a circle was calculated using the following formula.

Hole diameter = (hole area ÷ circumference) ^0.5 × 2
The same measurement was performed at five locations, and the average value was calculated. The ratio of the pore diameter of the central layer to the pore diameter of the surface layer was calculated.
(6) Measurement of dense layer thickness The cross section of the porous film was observed with a SEM (S-5500, manufactured by Hitachi High-Technologies Corporation) at a magnification of 10,000 times, and the image was taken into a computer. The sample used in the measurement of (5) was observed. The SEM image was cut out to be 6 μm parallel to the surface of the porous film and an arbitrary length in the film thickness direction, and image analysis was performed with image processing software. The length of the analysis range in the film direction may be a length that allows the dense layer to be accommodated. When the dense layer did not fit in the observation field at the measurement magnification, two or more SEM images were synthesized so that the dense layer could fit. The threshold value was determined so that the structure portion became bright luminance and the other portions had dark luminance by binarization processing, and an image was obtained in which the bright luminance portion was white and the dark luminance portion was black. If the structure part cannot be separated from the other parts due to the difference in contrast in the image, the image is separated at the same contrast part and binarized, and then connected to the original one. Returned to the image. Alternatively, the image analysis may be performed by painting other than the structure portion in black. When a hole was observed twice in the depth direction, the measurement was made with the shallower hole. When a part of the hole was out of the measurement range, the hole was excluded. Since the image contains noise and the dark luminance portion where the number of consecutive pixels is 5 or less, noise and a hole cannot be distinguished from each other, so that it is treated as a bright luminance portion as a structure. As a method for eliminating noise, a dark luminance portion having 5 or less consecutive pixels was excluded when measuring the number of pixels. Alternatively, the noise portion may be painted white. The number of pixels of the scale bar indicating a known length in the image was measured, and the length per pixel number was calculated. The number of pixels in the hole was measured, and the hole area was determined by multiplying the length per pixel. A hole having a diameter of 130 nm in terms of a circle and having a hole area of 1.3 × 10 ⁴ (nm ² ) or more is specified, and the layer without the hole is defined as a dense layer, and the thickness of the dense layer is increased in the vertical direction from the surface. It was measured. When the dense layer is in contact with the surface, the distance between the surface and the surface of the hole having a pore diameter of 130 nm or more closest to the surface. When the dense layer is not in contact with the surface and a hole having a pore diameter of 130 nm or more exists between the dense layer and the surface, the hole having the pore diameter of 130 nm or more closest to the surface in the vertical direction is the second closest hole having a pore diameter of 130 nm or more. And the distance. Five locations were measured in the same image. The same measurement was performed on 10 images, and an average value of a total of 50 measurement data was calculated.
(7) Elemental analysis 3 g of the porous membrane was freeze-dried, and the sample decomposition path 950 ° C., reduction furnace 500 ° C., helium flow rate 200 ml / min, oxygen flow rate 20 to 25 ml using a fully automatic elemental analyzer varioEL (Elemental). Measurement was performed at / min. When polysulfone is used as the structural polymer and polyvinylpyrrolidone is used as the hydrophilic polymer, from the measured nitrogen content (w _N (wt%)), the hydrophilic polymer content (w _C (height wt%)) is And calculated by the following formula.

w _C = w _N × 111/14
[Example 1]
20 parts by weight of polysulfone (Udelpolysulfone (registered trademark) P-3500 manufactured by Solvay) and 11 parts by weight of polyvinylpyrrolidone (K30 weight average molecular weight 40,000 manufactured by BASF) were added to 68 parts by weight of N, N′-dimethylacetamide and water 1 In addition to parts by weight of the mixed solvent, the mixture was dissolved by heating at 90 ° C. for 6 hours to obtain a film forming stock solution. This film-forming stock solution was discharged from an annular slit of a double-tube cylindrical die. The outer diameter of the annular slit was 0.59 mm, and the inner diameter was 0.23 mm. A solution composed of 72 parts by weight of N, N′-dimethylacetamide and 28 parts by weight of water was discharged from the inner tube as an injection solution. The base was kept at 40 ° C. The discharged film-forming stock solution passes through a dry part 110 mm having a dew point of 26 ° C. (temperature 30 ° C., humidity 80%) in 0.14 seconds, and is then solidified after being guided to a 40 ° C. water bath (coagulation bath). The first roller outside the bath was taken up at a speed of 40 m / min, washed with a 50 ° C. water bath, and wound around a casserole at 40.5 m / min. It was cut into 20 cm in the longitudinal direction, washed with hot water at 80 ° C. for 5 hours, and then heat treated at 100 ° C. for 2 hours. By adjusting the discharge amount of the stock solution and the discharge amount of the injection solution, a hollow fiber membrane-like porous membrane having an inner diameter of 180 μm and a film thickness of 90 μm after heat treatment was obtained. The draft ratio was 2.3. The viscosity of the film-forming stock solution was 4.1 Pa · s.

  In order to increase the major axis relative to the minor axis of the hole on the inner surface, the draft area was increased by increasing the cross-sectional area of the annular slit of the die.

  Water permeability performance measurement, virus removal performance measurement, inner surface pore diameter measurement, inner surface pore ratio measurement, cross-sectional pore diameter measurement, inner surface side dense layer thickness measurement, elemental analysis, and the results are shown in Table 1. It was. A scanning electron microscope (SEM) photograph of the inner surface of the porous membrane produced by the method of this example is shown in FIG.

The average value and standard deviation of the minor axis of the inner surface pores are small, the major axis is large relative to the minor axis of the pores, the standard deviation of the major axis of the pores is large, the open area ratio is high, the asymmetric membrane structure, and the dense layer A thick porous membrane was obtained. It was a porous membrane with excellent virus removal performance and water permeability performance under high pressure conditions. .
[Example 2]
An experiment similar to that of Example 1 was performed, except that a solution composed of 73 parts by weight of N, N′-dimethylacetamide and 27 parts by weight of water was used as the injection solution. The draft ratio was 2.3.

  Compared to Example 1, by increasing the good solvent concentration of the injection solution, the minor axis of the inner surface was increased to increase the water permeability.

  Water permeability performance measurement, virus removal performance measurement, inner surface pore diameter measurement, inner surface aperture ratio measurement, cross section maximum pore diameter, cross section pore diameter measurement, inner surface side dense layer thickness measurement, elemental analysis, results Are shown in Table 1.

Similar to Example 1, the porous membrane was excellent in virus removal performance and water permeability performance under high pressure conditions.
[Example 3]
The same experiment as in Example 1 was performed except that the temperature of the base was kept at 30 ° C. The draft ratio was 2.3. The viscosity of the film-forming stock solution was 5.9 Pa · s.

  Water permeability performance measurement, virus removal performance measurement, inner surface pore diameter measurement, inner surface pore ratio measurement, cross-sectional pore diameter measurement, inner surface side dense layer thickness measurement, elemental analysis, and the results are shown in Table 1. It was.

Similar to Example 1, the porous membrane was excellent in virus removal performance and water permeability performance under high pressure conditions.
[Example 4]
The outer diameter of the annular slit of the base is 0.73 mm, the inner diameter is 0.23 mm, a solution consisting of 75 parts by weight of N, N′-dimethylacetamide and 25 parts by weight of water is used as the injection solution, the dry part is made 70 mm, and taken off. The same experiment as in Example 1 was performed except that the speed was 30 m / min, the winding speed was 30.5 m / min, the inner diameter was 177 μm, and the film thickness was 92 μm. The draft ratio was 3.7.

  Compared to Example 1, the draft ratio was high because the sectional area of the annular slit was large, but on the other hand, the dry part was short, so the major axis was smaller than the minor axis of the hole on the inner surface. However, it has a hole shape that can sufficiently obtain virus removal performance and water permeability performance under high pressure conditions.

  Water permeability performance measurement, virus removal performance measurement, inner surface pore diameter measurement, inner surface aperture ratio measurement, cross-sectional pore diameter measurement, dense layer thickness measurement, and elemental analysis were performed, and the results are shown in Table 1.

Similar to Example 1, the porous membrane was excellent in virus removal performance and water permeability performance under high pressure conditions.
[Example 5]
The temperature of the base is kept at 30 ° C., the outer diameter of the annular slit of the base is 0.48 mm, the inner diameter is 0.23 mm, and the injection solution is 69 parts by weight of N, N′-dimethylacetamide and 31 parts by weight of water. The same experiment as in Example 1 was performed except that the dry part was 70 mm, the take-up speed was 30 m / min, the take-up speed was 30.5 m / min, the inner diameter was 180 μm, and the film thickness was 91 μm. The draft ratio was 1.8.

  Compared to Example 1, since the cross-sectional area of the annular slit of the die was small, the draft ratio was low and the dry part was also short, so that the major axis was smaller than the minor axis of the hole on the inner surface. However, it has a hole shape that can sufficiently obtain virus removal performance and water permeability performance under high pressure conditions.

  Water permeability performance measurement, virus removal performance measurement, inner surface pore diameter measurement, inner surface aperture ratio measurement, cross-sectional pore diameter measurement, dense layer thickness measurement, and elemental analysis were performed, and the results are shown in Table 1.

Similar to Example 1, the porous membrane was excellent in virus removal performance and water permeability performance under high pressure conditions.
[Example 6]
The temperature of the base is kept at 30 ° C., the outer diameter of the annular slit of the base is 0.62 mm, the inner diameter is 0.37 mm, and the injection solution is composed of 75 parts by weight of N, N′-dimethylacetamide and 25 parts by weight of water. The same experiment as in Example 1 was conducted except that the dry part was 70 mm, the take-up speed was 30 m / min, the take-up speed was 30.5 m / min, the inner diameter was 232 μm, and the film thickness was 71 μm. The draft ratio was 2.9.

  Compared to Example 1, the sectional area of the annular slit of the base is small, but the draft ratio is high because the sectional area of the membrane is small. On the other hand, since the dry part is short, the major axis is smaller than the minor axis of the hole on the inner surface. However, it has a hole shape that can sufficiently obtain virus removal performance and water permeability performance under high pressure conditions.

  Water permeability performance measurement, virus removal performance measurement, inner surface pore diameter measurement, inner surface aperture ratio measurement, cross-sectional pore diameter measurement, dense layer thickness measurement, and elemental analysis were performed, and the results are shown in Table 1.

Similar to Example 1, the porous membrane was excellent in virus removal performance and water permeability performance under high pressure conditions.
[Comparative Example 1]
The temperature of the die is kept at 30 ° C., the outer diameter of the annular slit of the die is 0.50 mm, the inner diameter is 0.23 mm, and the injection solution is composed of 75 parts by weight of N, N′-dimethylacetamide and 25 parts by weight of water. The same experiment as in Example 1 was performed except that the dry part was 70 mm, the take-up speed was 30 m / min, the take-up speed was 30.5 m / min, the inner diameter was 232 μm, and the film thickness was 93 μm. The draft ratio was 1.6.

  Compared to Example 1, the draft ratio was lowered by reducing the sectional area of the annular slit of the die and increasing the sectional area of the membrane. The major axis was smaller than the minor axis of the hole on the inner surface.

  Water permeability performance measurement, virus removal performance measurement, inner surface pore diameter measurement, inner surface aperture ratio measurement, cross-sectional pore diameter measurement, dense layer thickness measurement, and elemental analysis were performed, and the results are shown in Table 1.

Since the draft ratio was small, the major axis of the surface pore was 2.1 times smaller than the minor axis, and the virus removal performance was low.
[Comparative Example 2]
An experiment similar to that of Example 6 was performed except that the inner diameter was 230 μm and the film thickness was 151 μm. The draft ratio was 1.0.

  The draft ratio was low because the cross-sectional area of the membrane was large compared to Example 6, and the major axis was smaller than the minor axis of the hole on the inner surface.

  Water permeability performance measurement, virus removal performance measurement, inner surface pore diameter measurement, inner surface aperture ratio measurement, cross-sectional pore diameter measurement, dense layer thickness measurement, and elemental analysis were performed, and the results are shown in Table 1.

Since the draft ratio was small, the surface pore diameter was 1.5 times smaller than the minor diameter, the average value of the minor diameter of the surface and the standard deviation were low, and the virus removal performance was low.
[Comparative Example 3]
The same experiment as in Example 1 was performed at a take-up speed of 80 m / min. The discharge amount of the film forming stock solution was made the same as in Example 1 without adjusting the yarn diameter. The draft ratio was 6.0. A thread breakage occurred in the dry part, and a porous membrane could not be obtained.
[Comparative Example 4]
17 parts by weight of polysulfone (Udelpolysulfone (registered trademark) P-3500 manufactured by Solvay) and 7 parts by weight of polyvinylpyrrolidone (K90 weight average molecular weight 1,200,000 manufactured by BASF) were added to 75 parts by weight of N, N′-dimethylacetamide and water 1 In addition to parts by weight of the mixed solvent, the mixture was dissolved by heating at 90 ° C. for 6 hours to obtain a film forming stock solution. This film-forming stock solution was discharged from an annular slit of a double-tube cylindrical die. The outer diameter of the annular slit was 0.59 mm, and the inner diameter was 0.23 mm. A solution composed of 65 parts by weight of N, N′-dimethylacetamide and 35 parts by weight of water was discharged from the inner tube as an injection solution. The base was kept at 50 ° C. The discharged film-forming stock solution passes through a dry part 70 mm having a dew point of 26 ° C. (temperature 30 ° C., humidity 80%) in 0.14 seconds, and is then solidified after being guided to a 40 ° C. water bath (coagulation bath). The sample was taken out at a speed of 30 m / min with a first roller outside the bath, washed with a 50 ° C. water bath, and wound around a casserole at 30.5 m / min. It was cut into 20 cm in the longitudinal direction, washed with hot water at 80 ° C. for 5 hours, and then heat treated at 100 ° C. for 2 hours. By adjusting the discharge amount of the stock solution and the discharge amount of the injection solution, a hollow fiber membrane-like porous membrane having an inner diameter of 180 μm and a film thickness of 80 μm after heat treatment was obtained. The draft ratio was 2.4. The viscosity of the film forming stock solution was 6.1 Pa · s.

  Water permeability performance measurement, virus removal performance measurement, inner surface pore diameter measurement, inner surface aperture ratio measurement, cross-sectional pore diameter measurement, dense layer thickness measurement, and elemental analysis were performed, and the results are shown in Table 1.

  Since the hydrophilic polymer contained has a large weight average molecular weight, the draft ratio is high, but the major axis of the pore is not sufficiently large with respect to the minor axis. Therefore, it is a membrane having low water permeability against virus removal performance.

Claims (16)

The average value of the minor axis of the pores on at least one surface is 10 nm or more and 50 nm or less, the standard deviation of the minor axis of the pores is 30 nm or less, the standard deviation of the major axis of the pores is 30 nm or more and 150 nm or less, and the major axis of the pores The porous film is characterized in that the average value of is 2.5 times or more the average value of the minor axis. The porous membrane according to claim 1, wherein a cross section in the film thickness direction has an asymmetric membrane structure. 3. The porous film according to claim 1, wherein the layer having a pore diameter of 130 nm or less in the film thickness direction from the surface is 0.3 μm or more and 20 μm or less. The porous film according to any one of claims 1 to 3, wherein a porosity of the surface is 1% or more and 20% or less. The porous membrane according to any one of claims 1 to 4, wherein the porous membrane is made of an amorphous polymer. 6. The porous membrane according to claim 1, wherein the amorphous polymer is a polysulfone polymer. The porous membrane according to any one of claims 1 to 6, wherein the weight average molecular weight of the hydrophilic high molecular weight in the porous membrane is 20,000 or more and 80,000 or less. The porous membrane according to any one of claims 1 to 7, wherein the content of the hydrophilic polymer in the porous membrane is 1.5 wt% or more and 8 wt% or less. It is a hollow fiber membrane, The porous membrane in any one of Claim 1 to 8 characterized by the above-mentioned. 10. The porous membrane according to claim 9, wherein the average value of the minor diameters of the pores on the inner surface of the hollow fiber membrane is smaller than the average value of the minor diameters of the pores on the outer surface. The porous membrane according to claim 1, wherein the porous membrane is used for removing viruses. A water purifier comprising the porous membrane according to claim 1. In a method for producing a porous membrane in which a membrane-forming solution is discharged from a slit and solidified in a coagulation bath after passing through a dry part, the draft ratio is 1.7 or more and 5.0 or less, and the hydrophilic polymer in the membrane-forming solution The weight average molecular weight of 20,000 or more and 80,000 or less, The manufacturing method of the porous membrane characterized by the above-mentioned. 14. The method for producing a porous membrane according to claim 13, wherein the passage time of the dry part is 0.05 or more and 0.14 seconds or less. The method for producing a porous membrane according to claim 13 or 14, wherein the coagulating liquid containing a poor solvent and the film-forming stock solution are in contact with each other in the dry part. The porous polymer according to any one of claims 13 to 15, wherein the concentration of the hydrophilic polymer in the film-forming stock solution is 30% or more and 70% or less of the concentration of the polymer mainly constituting the porous membrane. A method for producing a membrane.