Hollow fiber membrane and method for manufacturing the same

Abstract

One aspect of the present invention relates to a porous hollow fiber membrane containing a vinylidene fluoride-based resin, which has an inclined structure in which the pore diameter of the pores inside the hollow fiber membrane gradually decreases toward the inner peripheral surface side or the outer peripheral surface side, and contains a crosslinked body of a polyvinylpyrrolidone-based resin, whereby the hollow fiber membrane is hydrophilized.

Description

Technical Field

[0001] The present invention relates to a hollow fiber membrane and a method for manufacturing the same. Background Art

[0002] Since the separation technology using hollow fiber membranes has advantages such as miniaturization of devices, it is widely used in various fields such as water treatment fields such as water purification treatment, drinking water production, industrial water production, and wastewater treatment, food industry fields, and pharmaceutical manufacturing fields.

[0003] The hollow fiber membranes used in such separation technologies are required to further improve permeation performance, fractionation performance, etc. Specifically, if the permeation performance of the hollow fiber membrane is improved, the required membrane area becomes smaller, and the device implementing the separation technology using the hollow fiber membrane can be further miniaturized. Therefore, equipment costs and membrane replacement costs can be reduced, which is also advantageous in terms of cost. In addition, if the fractionation characteristics of the hollow fiber membrane can be improved, there are advantages such as an expanded range of removal targets.

[0004] However, generally speaking, the permeation performance and fractionation characteristics of separation membranes such as hollow fiber membranes tend to have a so-called trade-off relationship, that is: if the permeation performance is improved, the fractionation characteristics are reduced, and if the fractionation characteristics are improved, the permeation performance is reduced. Therefore, it is difficult for hollow fiber membranes to improve both permeation characteristics and fractionation characteristics.

[0005] On the other hand, separation membranes using fluorine-based materials such as vinylidene fluoride-based resins are attracting attention due to their high chemical and physical durability. As such separation membranes using fluorine-based materials, for example, the hollow fiber membranes described in Patent Documents 1 to 3 can be cited.

[0006] Patent Document 1 describes the following fluorine-based hollow fiber membrane: including a filtration region, a support region, and a backwash region, wherein the filtration region is a sponge structure including pores with an average diameter of 0.01 μm to 0.5 μm, the support region is a sponge structure including pores with an average diameter of 0.5 μm to 5 μm, the backwash region is a sponge structure including pores with an average diameter of 2 μm to 10 μm, and the filtration region, the support region, and the backwash region are formed in this order in the direction from the outer surface toward the inner surface.

[0007] According to Patent Document 1, it is disclosed that it not only has excellent mechanical strength but also can exhibit excellent backwash performance and filtration performance.

[0008] In addition, Patent Document 2 describes a method for producing a porous membrane by discharging a membrane-forming stock solution containing at least polyvinylidene fluoride resin and a solvent and bringing it into contact with a coagulation liquid containing at least a non-solvent, and producing a porous membrane by a non-solvent induced phase separation method. Further, Patent Document 2 describes that in this production method, the discharge temperature of the membrane-forming stock solution is equal to or higher than the melting point of the polyvinylidene fluoride resin and lower than the decomposition temperature of the polyvinylidene fluoride resin, and the temperature of the coagulation liquid is higher than the porous structure formation start temperature of the membrane-forming stock solution.

[0009] According to Patent Document 2, a porous membrane is disclosed which has excellent water permeability and blocking performance against minute pathogens, has extremely high chemical resistance, and can stably and sufficiently remove minute pathogens for a long period of time.

[0010] In addition, Patent Document 3 describes a fluororesin-based polymer separation membrane having both a three-dimensional mesh structure and a spherical structure, wherein the three-dimensional mesh structure contains a hydrophilic polymer, and the hydrophilic polymer has at least one selected from cellulose ester, vinyl fatty acid ester, vinyl pyrrolidone, ethylene oxide, and propylene oxide.

[0011] According to Patent Document 3, improvement in various properties such as separation characteristics, water permeability, chemical strength (chemical resistance), physical strength, and fouling resistance is disclosed.

[0012] Prior Art Documents

[0013] Patent Documents

[0014] Patent Document 1: Japanese Patent Publication Gazette Special Table 2012-525966

[0015] Patent Document 2: Japanese Patent Laid-Open Gazette Tokkai 2013-202461

[0016] Patent Document 3: Japanese Patent Laid-Open Gazette Tokkai 2006-239680. Summary of the Invention

[0017] An object of the present invention is to provide a hollow fiber membrane having excellent permeation performance and fractionation characteristics and excellent strength.

[0018] One aspect of the present invention relates to a hollow fiber membrane which is a porous hollow fiber membrane containing a vinylidene fluoride-based resin, the hollow fiber membrane having an inclined structure in which the pore diameter of the pores inside the hollow fiber membrane gradually decreases toward the inner peripheral surface side or the outer peripheral surface side, and the hollow fiber membrane containing a crosslinked body of a polyvinylpyrrolidone-based resin, whereby the hollow fiber membrane is hydrophilized.

[0019] The above and other objects, features, and advantages of the present invention will become apparent from the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a partial perspective view of the hollow fiber membrane related to the embodiment of the present invention.

[0021] Figure 2 It is a schematic view showing an example of a nozzle for forming hollow fibers used in the manufacturing method related to the embodiment of the present invention.

[0022] Figure 3 It is a schematic view showing an example of a membrane filtration device including the hollow fiber membrane related to the embodiment of the present invention.

[0023] Figure 4 It is a figure showing a scanning electron microscope photograph of a cross section of the hollow fiber membrane related to Example 1.

[0024] Figure 5 It is a figure showing a scanning electron microscope photograph of the vicinity of the outer peripheral surface in the cross section of the hollow fiber membrane related to Example 1.

[0025] Figure 6 It is a figure showing a scanning electron microscope photograph of the vicinity of the central portion in the cross section of the hollow fiber membrane related to Example 1.

[0026] Figure 7 It is a figure showing a scanning electron microscope photograph of the vicinity of the inner peripheral surface in the cross section of the hollow fiber membrane related to Example 1.

[0027] Figure 8 It is a figure showing a scanning electron microscope photograph of the outer peripheral surface of the hollow fiber membrane related to Example 1.

[0028] Figure 9 It is a figure showing a scanning electron microscope photograph of the inner peripheral surface of the hollow fiber membrane related to Example 1.

[0029] Figure 10 It is a figure showing the evaluation results of the hydrophilicity of each hollow fiber membrane related to Example 1 and Comparative Example 1. Detailed Embodiments

[0030] According to the research of the present inventors, in the hollow fiber membranes described in Patent Document 1 and the porous membranes described in Patent Document 2, the permeation performance is not sufficiently improved with respect to the fractionation characteristics, and it is necessary to further improve the permeation performance.

[0031] In addition, according to the research of the present inventors, in the separation membrane described in Patent Document 3, the peeling between the three-dimensional mesh structure layer and the spherical structure layer and the occurrence of uneven thickness of the three-dimensional mesh structure layer cannot sometimes be sufficiently suppressed. In addition, in the separation membrane described in Patent Document 3, the uneven thickness of the three-dimensional mesh structure layer is large, and sometimes minute pores are formed in the three-dimensional mesh structure layer. This is considered to be caused by, for example, the following reasons. As a method for manufacturing this polymer separation membrane, Patent Document 3 describes a method in which a fluororesin-based polymer solution containing the hydrophilic polymer is coated on the surface of the spherical structure layer, and the spherical structure layer is covered with the three-dimensional mesh structure layer. It is considered that in this manufacturing method, when the polymer solution for forming the three-dimensional mesh structure layer is coated on the surface of the spherical structure layer, it cannot be coated evenly. This occurs significantly when attempting to thin the three-dimensional mesh structure layer. Therefore, it is considered that minute pores are sometimes formed in the three-dimensional mesh structure layer. In addition, this manufacturing method requires the three-dimensional mesh structure layer and the spherical structure layer to be formed separately, which is also disadvantageous in terms of manufacturing cost.

[0032] The present invention has been made in view of the above circumstances, and an object thereof is to provide a hollow fiber membrane having excellent permeation performance and fractionation characteristics and excellent strength, and a method for manufacturing the same.

[0033] In addition, as a hollow fiber membrane having excellent permeation performance and fractionation characteristics, a porous hollow fiber membrane is known. Moreover, as a method for manufacturing such a porous hollow fiber membrane, a method using phase separation is known. As a method for the hollow fiber membrane using this phase separation, for example, a nonsolvent-induced phase separation method (NIPS method), a thermally-induced phase separation method (TIPS method), etc. can be cited.

[0034] The NIPS method refers to a method in which a homogeneous polymer stock solution obtained by dissolving a polymer in a solvent is brought into contact with a nonsolvent that does not dissolve the polymer, and thus a phase separation phenomenon is caused by the replacement of the solvent of the polymer stock solution with the nonsolvent using the concentration difference between the polymer stock solution and the nonsolvent as a driving force. In general, the NIPS method changes the pore diameter of the formed pores according to the solvent exchange rate. Specifically, there is a tendency for the pores to become coarser as the solvent exchange rate becomes slower. In addition, the solvent exchange rate is the fastest at the contact surface with the nonsolvent in the manufacture of the hollow fiber membrane, and becomes slower as it approaches the inside of the membrane. Therefore, the hollow fiber membrane manufactured by the NIPS method has an asymmetric structure in which it is dense near the contact surface with the nonsolvent and the pores gradually become coarser as it approaches the inside of the membrane. However, in the portion far from the contact surface, the solvent exchange rate is too slow, and large pores called macrovoids are formed, and there is a tendency for the strength and chemical resistance to decrease.

[0035] On the other hand, the TIPS method refers to a method in which a polymer is dissolved in a poor solvent at a high temperature, and phase separation is caused by cooling the solution. In this method, the poor solvent can dissolve the polymer at a high temperature but is difficult to dissolve it when the temperature drops. Generally speaking, the heat exchange rate is faster than the solvent exchange rate of the NIPS method and it is difficult to control the rate. Therefore, the TIPS method easily forms uniform fine pores in the membrane thickness direction.

[0036] In addition, the permeation performance and fractionation characteristics of the hollow fiber membrane change according to the number, shape, size, etc. of the fine pores formed in the membrane. The present inventors focused on this point. Specifically, in order to improve the fractionation characteristics, the inventors focused on making the membrane fine. On the other hand, if the entire membrane is made fine, the permeation performance will decrease.

[0037] In response to this, the present inventors speculated that in order to obtain a hollow fiber membrane having excellent permeation characteristics and fractionation characteristics, first, it is important to thin the fine layered portion that exhibits fractionation characteristics in the membrane thickness direction, that is, the separation layer thin layer directly related to separation. And it was speculated that by making the hollow fiber membrane have an asymmetric structure in which the portion required to maintain the strength of the hollow fiber membrane, etc., that is, the portion other than the separation layer, is a thick porous body, both the permeation performance and the fractionation characteristics can be improved. Moreover, based on the study of the membrane material, the present inventors speculated that by controlling the structure inside the membrane as described above, the permeation performance and the fractionation characteristics can be controlled.

[0038] As a result of various studies by the present inventors, it was found that the above object of obtaining a hollow fiber membrane having excellent permeation performance, fractionation characteristics, and strength can be achieved by the present invention described below.

[0039] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these.

[0040] The hollow fiber membrane according to one aspect of the present invention is a porous hollow fiber membrane containing a vinylidene fluoride resin, and has an inclined structure in which the pore diameter of the air holes inside the hollow fiber membrane gradually becomes smaller toward the inner circumferential surface side or the outer circumferential surface side. That is, the hollow fiber membrane according to the present invention is a hollow fiber membrane having an asymmetric structure in the membrane thickness direction. Therefore, this hollow fiber membrane has an inclined structure in which the pore diameter of the air holes inside the membrane gradually becomes smaller toward the inner circumferential surface side or the outer circumferential surface side. Therefore, a fine layered portion related to fractionation characteristics and other portions having relatively large air holes (fine pores) are formed. For example, a fine layered portion related to fractionation characteristics is formed on the surface, etc., and in other portions, the air holes (fine pores) formed therein are relatively large. Therefore, it is considered that a decrease in permeation performance is suppressed.

[0041] In addition, the hollow fiber membrane according to the present embodiment contains a cross-linked body of a polyvinylpyrrolidone-based resin, whereby the hollow fiber membrane is hydrophilized. First, the hollow fiber membrane according to the present embodiment contains a vinylidene fluoride-based resin and thus has a tendency to have relatively high hydrophobicity. Even such a hollow fiber membrane can improve its hydrophilicity by containing a cross-linked body of a polyvinylpyrrolidone-based resin. In addition, instead of simply containing a polyvinylpyrrolidone-based resin, a cross-linked body of a polyvinylpyrrolidone-based resin is contained to suppress the shedding of the polyvinylpyrrolidone-based resin. Therefore, the effect of improving hydrophilicity can be maintained. By improving hydrophilicity in this way, the hollow fiber membrane can form suitable pores as described above and can further improve the permeability to a liquid containing water. In addition, since the hollow fiber membrane contains a vinylidene fluoride-based resin, a hollow fiber membrane with excellent strength can be obtained.

[0042] Therefore, it is considered that the hollow fiber membrane according to the present embodiment is a hollow fiber membrane with excellent permeation performance and fractionation characteristics and also with excellent strength. In addition, it is considered that the hollow fiber membrane according to the present embodiment can improve its fouling resistance by improving hydrophilicity.

[0043] In addition, the hollow fiber membrane has an inclined structure in which the pore diameter of the pores inside the membrane gradually becomes smaller toward the inner circumferential surface side or the outer circumferential surface side as described above. Specifically, it is not particularly limited as long as the diameter of the fine pores formed on the outer circumferential surface of the hollow fiber membrane (outer circumferential side fine pore diameter) is smaller than the diameter of the fine pores formed on the inner circumferential surface (inner circumferential side fine pore diameter). Specifically, the outer circumferential side fine pore diameter is preferably 0.01 to 1 μm, more preferably 0.1 to 0.5 μm, and further preferably 0.1 to 0.3 μm. In addition, the inner circumferential side fine pore diameter is not particularly limited either. Specifically, it is preferably 1 to 20 μm, more preferably 1 to 10 μm, and further preferably 2 to 8 μm. In addition, the ratio of the inner circumferential side fine pore diameter to the outer circumferential side fine pore diameter (inner circumferential side fine pore diameter / outer circumferential side fine pore diameter) is greater than 1, preferably 10 to 100, more preferably 20 to 50, and further preferably 30 to 50. Thus, the hollow fiber membrane has an inclined structure in which the size (pore diameter) of the pores inside the membrane gradually becomes smaller in the thickness direction from the inner circumferential surface side toward the outer circumferential surface side in a manner that satisfies the outer circumferential side fine pore diameter and the inner circumferential side fine pore diameter. In addition, the diameter here is the average value of the diameter, and for example, the arithmetic average value of the diameter can be cited.

[0044] The vinylidene fluoride-based resin contained in the hollow fiber membrane is the main component of the hollow fiber membrane. Specifically, it is preferably 85% by mass or more, more preferably 90 to 99.9% by mass.

[0045] The vinylidene fluoride resin is not particularly limited as long as it can form a hollow fiber membrane. Specific examples of the vinylidene fluoride resin include homopolymers of vinylidene fluoride, copolymers of vinylidene fluoride, etc. The vinylidene fluoride copolymer is not particularly limited as long as it is a copolymer having a repeating unit based on vinylidene fluoride. Specific examples of the vinylidene fluoride copolymer include copolymers of at least one selected from the group consisting of vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, and chlorotrifluoroethylene and vinylidene fluoride, etc. As the vinylidene fluoride resin, polyvinylidene fluoride, which is a homopolymer of vinylidene fluoride, is preferably used among the above examples. In addition, as the vinylidene fluoride resin, the resins exemplified above can be used alone or in combination of two or more.

[0046] The molecular weight of the vinylidene fluoride resin varies depending on the use of the hollow fiber membrane, etc. For example, a weight average molecular weight of 50,000 to 1,000,000 is preferred. If the molecular weight is too small, there is a tendency for the strength of the hollow fiber membrane to decrease. In addition, if the molecular weight is too large, there is a tendency for the film-forming property of the hollow fiber membrane to decrease. Further, when the hollow fiber membrane is used for water treatment applications washed with a medical solution, the hollow fiber membrane is required to have higher performance. Therefore, in order to obtain a hollow fiber membrane with excellent strength and further suitability, excellent film-forming property is required. Therefore, the weight average molecular weight of the vinylidene fluoride resin contained in the hollow fiber membrane is preferably 100,000 to 900,000, more preferably 150,000 to 800,000.

[0047] The hollow fiber membrane contains not only the vinylidene fluoride resin but also a crosslinked body of a polyvinylpyrrolidone resin as described above to be hydrophilized. The polyvinylpyrrolidone resin is not particularly limited as long as it is a resin containing vinylpyrrolidone in the molecule. Specific examples of the polyvinylpyrrolidone resin include polyvinylpyrrolidone, a copolymer of vinylpyrrolidone and vinyl acetate, a copolymer of vinylpyrrolidone and vinylcaprolactam, etc. As the polyvinylpyrrolidone resin, polyvinylpyrrolidone is preferably used among the above examples. In addition, as the polyvinylpyrrolidone resin, the resins exemplified above can be used alone or in combination of two or more.

[0048] The crosslinking degree of the crosslinked body of the polyvinylpyrrolidone resin is not particularly limited. As the crosslinking degree, for example, the crosslinking degree at which no polyvinylpyrrolidone resin can be detected in the filtrate when water is passed through the obtained hollow fiber membrane can be cited. The degree of not detecting the polyvinylpyrrolidone resin specifically refers to the following degree.

[0049] First, after flushing the hollow fiber membrane with pure water, a 40% by volume aqueous ethanol solution is circulated through the cleaned hollow fiber membrane at 40°C for one hour. The concentration of the polyvinylpyrrolidone resin in the circulated aqueous ethanol solution is measured. Based on the concentration of the polyvinylpyrrolidone resin and the membrane area of the used hollow fiber membrane, the extraction amount of the polyvinylpyrrolidone resin per 1 m 2 of the membrane area is calculated. It is preferable that the calculated extraction amount per 1 m 2 of the membrane area is 300 mg or less, more preferably 100 mg or less, and even more preferably 10 mg or less.

[0050] The content of the cross-linked body of the polyvinylpyrrolidone resin is not particularly limited as long as it is an amount that can fully exhibit the effect of the cross-linked body containing the polyvinylpyrrolidone resin, that is, an amount that can appropriately hydrophilize the hollow fiber membrane containing the vinylidene fluoride resin. Specifically, the content of the cross-linked body of the polyvinylpyrrolidone resin is preferably 0.1% by mass or more and less than 15% by mass, more preferably 0.1 to 10% by mass, and even more preferably 0.5 to 5% by mass with respect to the mass of the hollow fiber membrane. If the content is too small, there is a tendency that the hydrophilicity of the hollow fiber membrane cannot be sufficiently improved. Therefore, the fouling resistance cannot be sufficiently improved. In addition, the hollow fiber membrane cannot form suitable pores (fine pores), and there is a tendency that the permeability to the liquid containing water cannot be sufficiently improved. Further, if the content is too large, there is a tendency that the permeation performance decreases. First, this is because the moldability of the hollow fiber membrane decreases, and there is a tendency that it is difficult to form a suitable hollow fiber membrane. In addition, it is considered that the polyvinylpyrrolidone resin in the hollow fiber membrane swells, and the water permeability is likely to decrease due to clogging of the fine pores of the membrane. For these reasons, it is considered that if the content of the cross-linked body of the polyvinylpyrrolidone resin is within the above range, the hollow fiber membrane containing the vinylidene fluoride resin can be moderately hydrophilized, the decrease in water permeability due to clogging of the fine pores of the membrane can be suppressed, and the hydrophilicity can be improved. Therefore, it is considered that a hollow fiber membrane with excellent fractionation characteristics, more excellent permeation performance, and excellent fouling resistance can be obtained.

[0051] The method for measuring the content of the cross-linked body of the polyvinylpyrrolidone resin is not particularly limited. For example, it can be measured as follows. Specifically, a trace nitrogen analysis is performed on the obtained hollow fiber membrane, and it can be measured based on the amount of nitrogen (N) present. More specifically, first, a trace nitrogen analysis is performed on the obtained hollow fiber membrane and the polyvinylpyrrolidone resin monomer, respectively, and the amount of nitrogen (N) present is measured. Based on this amount, the content of the cross-linked body of the polyvinylpyrrolidone resin is calculated.

[0052] The K value of the polyvinylpyrrolidone resin is preferably from 30 to 120, more preferably from 50 to 120, and even more preferably from 60 to 120. In addition, the K value of the polyvinylpyrrolidone resin is the K value of the polyvinylpyrrolidone resin before crosslinking. Furthermore, the K value is a viscosity characteristic value related to the molecular weight. The K value can be obtained, for example, from the description in the product manual, etc., and can be calculated, for example, using the Fikentscher formula. The K value is calculated, for example, by applying the relative viscosity value at 25 °C measured using a capillary viscometer to the following Fikentscher formula.

[0053] K value = (1.5 log η rel − 1) / (0.15 + 0.003c) + (300c log η rel + (c + 1.5c log η rel ) 2 ) 1 / 2 / (0.15c + 0.003c 2 )

[0054] In the formula, η rel represents the relative viscosity of the aqueous solution of the polyvinylpyrrolidone resin as the object to be measured with respect to water, and c represents the concentration (mass %) of the polyvinylpyrrolidone resin as the object to be measured in the aqueous solution of the polyvinylpyrrolidone resin as the object to be measured.

[0055] If the K value of the polyvinylpyrrolidone resin is too small, even if the polyvinylpyrrolidone resin is crosslinked, it is difficult to remain in the hollow fiber membrane containing the vinylidene fluoride resin, and there is a tendency that it is difficult to suitably maintain the hydrophilicity of the hollow fiber membrane. In addition, if the K value of the polyvinylpyrrolidone resin is too large, the film-forming property deteriorates, and there is a tendency that it is difficult to manufacture a suitable hollow fiber membrane. For these reasons, it is considered that as long as the polyvinylpyrrolidone resin has such a K value, it is likely to remain moderately in the hollow fiber membrane containing the vinylidene fluoride resin, and the hollow fiber membrane can be moderately hydrophilized. Therefore, it is possible to both suppress the decrease in water permeability due to clogging of the pores of the membrane, etc., and improve the hydrophilicity, thereby improving the permeability of the aqueous liquid. Therefore, it is considered that a hollow fiber membrane with excellent classification characteristics, better permeation performance, and excellent fouling resistance is obtained.

[0056] In addition, the hollow fiber membrane preferably has a pure water permeation coefficient K of 1 × 10 -15 m 2 or more and 22 × 10 -15 m 2 or less. Here, the pure water permeation coefficient K is the permeation coefficient when pure water passes through the hollow fiber membrane, and is the permeation coefficient (Darcy's permeation coefficient) calculated according to Darcy's law and using the following formula (1).

[0057] K = (μ·T·Q) / (ΔP·A) (1)

[0058] In formula (1), K represents the permeation coefficient (m 2 ). In addition, μ represents the viscosity (Pa·s), which represents the viscosity of pure water (Pa·s) here. Additionally, T represents the membrane thickness (m), which represents the thickness of the hollow fiber membrane (m) here. Furthermore, Q represents the flow rate (m 3 / s), which represents the water permeation flow rate (m 3 / s) here. Additionally, ΔP represents the transmembrane differential pressure (Pa). Furthermore, A represents the membrane area (m 2 ).

[0059] Next, the method for measuring the pure water permeation coefficient K will be described.

[0060] The method for measuring the pure water permeation coefficient K is not particularly limited as long as it can be calculated based on the above formula (1). Specifically, examples of the method for measuring the pure water permeation coefficient K include the following measurement methods, etc.

[0061] First, perform a wetting treatment of immersing the hollow fiber membrane to be measured in an aqueous solution of 50% by mass ethanol for 15 minutes, and then washing with pure water for 15 minutes. Use a porous hollow fiber membrane module with one end of the hollow fiber membrane that has undergone this wetting treatment sealed, and use pure water as the raw water to perform external pressure filtration under the conditions of a filtration pressure of 100 kPa and a temperature of 25°C, and measure the water permeation amount per hour. Based on the measured water permeation amount, convert it to the water permeation amount per unit membrane area, per unit time, and per unit pressure, and obtain the water permeation amount (L / m 2 / h) at a transmembrane differential pressure of 0.1 MPa for each effective length of 10 cm, 15 cm, 20 cm, 25 cm, and 30 cm. Based on the measured data of the obtained water permeation amount, substitute it into Darcy's formula and calculate the Darcy permeation coefficient K for each effective length.

[0062] Then, plot the effective length on the horizontal axis and the Darcy permeation coefficient K on the vertical axis, and calculate the Darcy permeation coefficient K at an effective length of 0 cm based on the extrapolated values of the obtained plot, and use it as the pure water permeation coefficient K of the present invention.

[0063] Next, the pure water permeation coefficient K will be described.

[0064] The pure water permeability coefficient K is a coefficient of the passing resistance when pure water passes through the hollow fiber membrane. That is, the larger the calculated pure water permeability coefficient K, the smaller the pure water passing resistance of the hollow fiber membrane, indicating a structure through which water easily flows. On the other hand, the smaller the calculated pure water permeability coefficient K, the larger the pure water passing resistance of the hollow fiber membrane, indicating a structure through which water hardly flows. More specifically, when the hollow fiber membrane is a structure in which each pore existing in the membrane is large, the porosity is large, and the pressure loss is small, the pure water permeability coefficient K becomes larger. On the other hand, when the hollow fiber membrane is a dense structure in which each pore existing in the membrane is small and the porosity is small, the pure water permeability coefficient K becomes smaller.

[0065] As long as the structure of the hollow fiber membrane, especially the structure in the membrane thickness direction, is uniform, the pure water permeability coefficient K becomes a constant value regardless of the pressure change during measurement and the length (membrane thickness) of the passing part of the hollow fiber membrane. On the other hand, if the pure water permeability coefficient K varies according to the membrane thickness, it indicates that the structure of the hollow fiber membrane, such as porosity, pore diameter, pore shape, etc., changes in the membrane thickness direction.

[0066] Specifically, for a hollow fiber membrane having an asymmetric structure that changes in the membrane thickness direction, the pure water permeability coefficient K from the region with a small pure water permeability coefficient K to the region with a large pure water permeability coefficient K is as follows. First, let K in the region with a small pure water permeability coefficient K be Ks, and let K in the region with a large pure water permeability coefficient K be Kl. Also, let the thickness of the region with a small pure water permeability coefficient K be Ts, let the thickness of the region with a large pure water permeability coefficient K be Tl, and let the overall thickness (membrane thickness) of the hollow fiber membrane be T. In this case, the pure water permeability coefficient K of the hollow fiber membrane is defined by the following formula (2).

[0067] T / K = Ts / Ks + Tl / Kl (2)

[0068] Thus, the pure water permeability coefficient K of the asymmetric structure hollow fiber membrane is determined according to the ratios of the regions with a small pure water permeability coefficient K and a large pure water permeability coefficient K to the overall membrane thickness of the hollow fiber membrane and the magnitude of the difference in the absolute values of Ks and Kl. That is, the pure water permeability coefficient K of the hollow fiber membrane varies according to the degree of asymmetry of the hollow fiber membrane. Specifically, in the case of a small degree of asymmetry, there is a tendency for the pure water permeability coefficient K of the hollow fiber membrane to become smaller. In addition, in the case of a large degree of asymmetry, there is a tendency for the pure water permeability coefficient K of the hollow fiber membrane to become larger. Thus, by obtaining the pure water permeability coefficient K of the hollow fiber membrane, the pure water permeation performance and the degree of asymmetry of the hollow fiber membrane can be evaluated. Specifically, if the pure water permeability coefficient K of the hollow fiber membrane is large, the pure water permeation performance is high, and if the pure water permeability coefficient K of the hollow fiber membrane varies, it can be regarded as a change in the degree of asymmetry.

[0069] Here, the pure water permeation coefficient K of the hollow fiber membrane according to this embodiment is a value contributing to the membrane structure as described above. The pure water permeation coefficient K contributing to the membrane structure is preferably 1×10 -15 m 2 or more and 22×10 -15 m 2 or less, more preferably 2×10 -15 m 2 or more and 17×10 -15 m 2 or less, and still more preferably 2.3×10 -15 m 2 or more and 10×10 -15 m 2 or less. When the pure water permeation coefficient K is too small, as described above, the resistance to the passage of pure water increases, and there is a tendency that sufficient permeation performance is difficult to exhibit. In addition, when the pure water permeation coefficient K is too large, although excellent permeation performance can be exhibited, there is a tendency that the classification characteristics decrease too much. For these reasons, it is considered that by making the pure water permeation coefficient K within the above range, both the decrease in classification characteristics can be suppressed and the permeation performance for a liquid containing water can be excellent.

[0070] The hollow fiber membrane according to this embodiment preferably has a classification particle size of 0.5 μm or less. The classification particle size refers to the particle size of the smallest particle that can be blocked from passing through the hollow fiber membrane. Specifically, for example, the particle size at which the rejection rate of the hollow fiber membrane becomes 90% can be cited. Such a classification particle size is preferably as small as possible, but in order to maintain excellent permeation performance, it is limited to about 0.001 μm. Therefore, the minimum value of the classification particle size is about 0.001 μm, and preferably about 0.01 μm from the viewpoint of permeation performance. Thus, the classification particle size is preferably 0.5 μm or less, more preferably 0.001 to 0.5 μm, still more preferably 0.01 to 0.5 μm, and particularly preferably 0.02 to 0.1 μm. If the classification particle size of the hollow fiber membrane is too large, even if the permeation performance is improved, the classification characteristics decrease, and there is a tendency that the applicable range of the object to be removed becomes narrow. Therefore, if the classification particle size of the hollow fiber membrane is within the above range, both the decrease in permeation performance can be suppressed and excellent classification characteristics can be exhibited.

[0071] The applicable range of the object to be removed by the hollow fiber membrane varies depending on the classification particle size. Specifically, if the classification particle size is 0.05 to 0.1 μm, it can be used as a microfiltration membrane for removing microorganisms and viruses. In addition, if the classification particle size is 0.001 to 0.01 μm, it can be used as an ultrafiltration membrane for removing minute pathogenic bacteria and proteins. In addition, if the classification particle size is 0.002 μm or less, it can be used as a reverse osmosis membrane for desalination and the like.

[0072] As described above, the hollow fiber membrane according to the present embodiment has excellent fractionation characteristics such that it can be used as a precision filtration membrane for removing microorganisms and viruses by making the fractionation particle size fall within the above range, and can exhibit excellent permeation performance at a membrane thickness that can achieve the required strength.

[0073] In addition, for the hollow fiber membrane according to the present embodiment, the water permeation rate at a transmembrane differential pressure of 0.1 MPa is preferably 1000 to 40000 L / m 2 / hour, more preferably 3000 to 30000 L / m 2 / hour, and even more preferably 3500 to 20000 L / m 2 / hour. If the water permeation rate is too low, there is a tendency for the water permeation performance to deteriorate. If the water permeation rate is too high, there is a tendency for the fractionation characteristics to decline. Thus, if the water permeation rate falls within the above range, a hollow fiber membrane with even better permeation performance and fractionation characteristics can be produced. In addition, the water permeation rate at a transmembrane differential pressure of 0.1 MPa corresponds to the pure water permeation velocity (FW) at a transmembrane differential pressure of 0.1 MPa for the membrane in the wet state described below.

[0074] For the hollow fiber membrane according to the present embodiment, when the thickness of the hollow fiber membrane is L (m), the pure water permeation coefficient is preferably 0.4×10 -11 ×L (m 2 ) or more and 6×10 -11 ×L (m 2 ) or less, more preferably 0.8×10 -11 ×L (m 2 ) or more and 4×10 -11 ×L (m 2 ) or less, and even more preferably 1×10 -11 ×L (m 2 ) or more and 3×10 -11 ×L (m 2 ) or less. That is, in the hollow fiber membrane, when the horizontal axis is the membrane thickness L (m) and the vertical axis is the pure water permeation coefficient K (m [[ID=3']) 2 ), the slope is preferably 0.4×10 -11 to 6×10 -11 or less, more preferably 0.8×10 -11 to 4×10 -11 , and even more preferably 1×10 -11 to 3×10 -11 .

[0075] The pure water permeability coefficient K is a value that depends on the internal structure of the hollow fiber membrane as described above. If the internal structure of the hollow fiber membrane is homogeneous in the membrane thickness direction, its value will not change even if the membrane thickness varies. If the inclination is within the above range, it is considered that the structure of the hollow fiber membrane suitably becomes asymmetric. That is, it is considered that there is a fine layered portion related to the fractionation characteristics near one of its surfaces, etc., and other portions are less likely to contribute to the decrease in permeability, and the pores formed within this portion are relatively large. This fine layered portion functions as a separation layer, and other portions function as a support layer. And, this support layer does not have large pores called macropores in the membrane cross-section, and is a so-called three-dimensional mesh structure in which communication pores exist in any three-dimensional direction. In addition, if the inclination is within the above range, even if the thickness of the entire hollow fiber membrane changes, the thickness of the fine layered portion that functions as the separation layer hardly changes, and the thickness of the portion that functions as the support layer changes. Therefore, even if the thickness of the hollow fiber membrane increases, the fine layered portion related to the fractionation characteristics does not thicken, and a hollow fiber membrane with better permeation performance while maintaining excellent fractionation characteristics can be realized. That is, the inclination is within the above range because there is a tendency that the proportion of the separation layer in the overall thickness of the hollow fiber membrane decreases even if the thickness of the hollow fiber membrane increases. Therefore, there is a tendency that if the inclination is too small, the degree of asymmetry of pores, etc. in the membrane thickness direction is not sufficiently increased, and if the overall thickness of the hollow fiber membrane becomes thick, sufficient permeation performance cannot be exhibited. In addition, if the inclination is too large, the degree of asymmetry is too large, and macropores, etc. occur in the portion that functions as the support layer, and there is a tendency that the portion that should function as the support layer cannot sufficiently function as the support layer. That is, there is a tendency for the strength of the hollow fiber membrane to decrease, and depending on the specific situation, there is a tendency that it is difficult to suitably manufacture the hollow fiber membrane. Therefore, if the inclination is within the above range, a hollow fiber membrane that maintains excellent fractionation characteristics and has better permeation performance can be obtained.

[0076] The hollow fiber membrane according to the present embodiment preferably has a single layer structure. That is, even if the hollow fiber membrane has an asymmetric structure in which the size of pores, etc. is different in the membrane thickness direction as described above, its material is preferably formed of the same layer. More specifically, the hollow fiber membrane preferably has a single layer structure instead of a structure in which the above-described separation layer and support layer are separately formed and laminated. By adopting such a structure, a hollow fiber membrane with better permeation performance and fractionation characteristics and less likely to be damaged by peeling, etc. inside the membrane can be obtained.

[0077] This is considered to be due to the following reasons.

[0078] When the fine layered portion involving hierarchical characteristics as described above is thin as in the case of the hollow fiber membrane according to the present embodiment and has high permeability. At this time, if such a fine layer is to be formed separately, it may not be formed appropriately. In contrast, if the fine layered portion and other portions are formed from the same layer, i.e., a single layer, the fine layered portion can be formed uniformly on the surface. In addition, if the fine layered portion and the other portions are a single layer, peeling and the like occurring at the interface can be sufficiently suppressed.

[0079] Therefore, it is considered that a hollow fiber membrane having better permeability and hierarchical characteristics and being less likely to have damage such as peeling inside the membrane can be obtained.

[0080] The strength of the hollow fiber membrane is not particularly limited as long as it can be used as a hollow fiber membrane. Specifically, the tensile strength of the hollow fiber membrane is preferably 3 to 15 N / mm 2 , more preferably 3 to 10 N / mm 2 , further preferably 3 to 7 N / mm 2 . In addition, specifically, the elongation of the hollow fiber membrane is preferably 30 to 250%, more preferably 50 to 200%, and further preferably 70 to 200%. As the strength of the hollow fiber membrane, if the tensile strength and elongation are within the above ranges, it can be suitably used as a hollow fiber membrane. In addition, the tensile strength is obtained from the load at which the hollow fiber membrane cut into a specified size is stretched at a specified speed and the hollow fiber membrane breaks, and the elongation is the value indicating the elongation of the hollow fiber membrane at the time of its break.

[0081] The hollow fiber membrane according to the present embodiment contains a crosslinked body of a polyvinylpyrrolidone resin as described above, and thus the hollow fiber membrane is hydrophilized. This hollow fiber membrane is preferably manufactured by the manufacturing method described later. That is, in this hollow fiber membrane, it is preferable that the crosslinked body is a crosslinked body obtained by crosslinking the polyvinylpyrrolidone resin contained in the hollow fiber membrane before crosslinking when forming the hollow fiber membrane before crosslinking. As this crosslinked body, a crosslinked body obtained by crosslinking the polyvinylpyrrolidone resin kneaded into the hollow fiber membrane before crosslinking is preferable. Thereby, when forming the hollow fiber membrane before crosslinking, a polyvinylpyrrolidone resin as a hydrophilic resin is kneaded together with a vinylidene fluoride resin in the raw material of the hollow fiber membrane, and thus a softer and more excellent stretchable hollow fiber membrane can be obtained. It is considered that this is because when forming the hollow fiber membrane before crosslinking, the kneaded hydrophilic resin acts as a plasticizer. In contrast, when forming the hollow fiber membrane before crosslinking, if the raw material does not contain a hydrophilic resin, the obtained hollow fiber membrane may sometimes lack flexibility.

[0082] As for the hollow fiber membrane, when it contains a crosslinking body that crosslinks a polyvinylpyrrolidone resin contained in the hollow fiber membrane before crosslinking during the formation of the hollow fiber membrane before crosslinking, the flexibility of the hollow fiber membrane is excellent, and by making the strength within the above range, it is possible to achieve a high practical strength that can sufficiently suppress liquid leakage caused by breakage or the like, that is, so-called fiber leakage, even when the hollow fiber membrane itself is bent or deformed. From this point, it is also preferable to contain a crosslinking body that crosslinks a polyvinylpyrrolidone resin contained in the hollow fiber membrane before crosslinking during the formation of the hollow fiber membrane before crosslinking. Thus, the hollow fiber membrane according to the present embodiment contains the crosslinking body, and thus becomes a hollow fiber membrane having excellent strength with not only high tensile strength but also high elongation as described above, and is suitable for use as a hollow fiber membrane.

[0083] The hollow fiber membrane preferably satisfies the following relationship for the water permeation rate of pure water in a dry state. In addition, as for the hollow fiber membrane, when it contains a crosslinking body that crosslinks a polyvinylpyrrolidone resin contained in the hollow fiber membrane before crosslinking during the formation of the hollow fiber membrane before crosslinking, the water permeation rate of pure water in a dry state often satisfies the following relationship. From this point, it is preferable to contain a crosslinking body that crosslinks a polyvinylpyrrolidone resin contained in the hollow fiber membrane before crosslinking during the formation of the hollow fiber membrane before crosslinking.

[0084] Specifically, first, the ratio (FD / FW) of the water permeation rate of pure water (FD) at a transmembrane pressure difference of 0.1 MPa in a dry state to the water permeation rate of pure water (FW) at a transmembrane pressure difference of 0.1 MPa in a wet state is preferably 40% or more, more preferably 60% or more, and still more preferably 80% or more.

[0085] In addition, the water permeation rate of pure water (FD) at a transmembrane pressure difference of 0.1 MPa in a dry state and the water permeation rate of pure water (FW) at a transmembrane pressure difference of 0.1 MPa in a wet state are different depending on whether the hollow fiber membrane is in a wet state or a dry state, and are the permeation rates measured under the same other conditions.

[0086] As the permeation rate (FD) of pure water under an intermembrane differential pressure of 0.1 MPa in the dry state, for example, the permeation rate measured by the following method can be cited. First, the hollow fiber membrane to be measured is dried. The drying is not particularly limited as long as the hollow fiber membrane can be dried. For example, drying for 24 hours or more in a forced-air constant-temperature dryer at 60°C can be cited. More specifically, the dried hollow fiber membrane can be a hollow fiber membrane in a state where the moisture content of the hollow fiber membrane reaches a state of sufficient equilibrium with the 60°C air in the dryer through such drying. Using this dried hollow fiber membrane, with pure water as the raw water, external pressure filtration is carried out under the conditions of a filtration pressure of 0.1 MPa and a temperature of 25°C, and the water permeation amount per minute is measured. Based on the measured water permeation amount, it is converted into the water permeation amount per unit membrane area, per unit time, and per unit pressure to obtain the permeation rate of pure water (L / m 2 / h: LMH).

[0087] As the permeation rate (FW) of pure water under an intermembrane differential pressure of 0.1 MPa in the wet state, except for using a wet hollow fiber membrane instead of a dry hollow fiber membrane, it is measured by the same method as the measurement method of FD. The wet treatment to make the hollow fiber membrane in a wet state is not particularly limited. For example, a treatment such as immersing the hollow fiber membrane in an aqueous solution of 50% by mass of ethanol for 20 minutes and then washing it with pure water for 20 minutes can be cited.

[0088] The water permeation rate (FD10) of pure water at a transmembrane pressure of 0.1 MPa in the dry state after repeating the wet state and the dry state alternately 10 times each is preferably such that the ratio (FD10 / FW) of FD10 of FW is equal to FD / FW. Specifically, the ratio (FD10 / FW) of FD10 of FW is preferably 40% or more, more preferably 60% or more, and still more preferably 80% or more. In addition, not only the surface of the hollow fiber membrane is hydrophilized, but also the micropores of the hollow fiber membrane are hydrophilized. That is to say, even when the entire hollow fiber membrane is hydrophilized, if the hydrophilicity of the entire hollow fiber membrane can be ensured at a high level during measurement, FD / FW and FD10 / FW are approximately 100%. Additionally, in such cases, FD / FW and FD10 / FW may sometimes exceed 100% due to various reasons such as measurement errors. Moreover, if there are portions where polyvinylpyrrolidone-based resins as hydrophilic resins are peeled off or portions that are not sufficiently hydrophilized, these portions will become water flow resistances. Therefore, depending on the proportion of these portions, FW and the water permeation rate (FW10) of pure water at a transmembrane pressure of 0.1 MPa in the wet state after repeating the wet state and the dry state alternately 10 times each decrease. As a result, FD / FW and FD10 / FW decrease and are less than 100%. Further, the dry state after repeating the wet state and the dry state alternately 10 times each specifically refers to the operation of repeating 10 times the process of changing the wet hollow fiber membrane to the dry state, that is, changing the wet hollow fiber membrane to the dry state and then changing the wet hollow fiber membrane to the dry state again.

[0089] In hollow fiber membranes prepared by conventional hydrophilization methods such as the impregnation method described in Japanese Patent Publication Gazette No. Hei 9-512857, there is generally a tendency for the water permeation rate (FD10) of pure water at a transmembrane pressure of 0.1 MPa in the dry state after repeating the wet state and the dry state alternately 10 times each to decrease. This is considered to be due to the following reasons. First, in conventional hydrophilization methods such as the impregnation method, only a hydrophilic resin is coated on the surface of the hollow fiber membrane and crosslinked. Therefore, it is difficult for the hydrophilic resin to enter the micropores of the hollow fiber membrane, and the anchoring effect of the crosslinked body of the hollow fiber membrane and the hydrophilic resin is likely to decrease. As a result, the crosslinked body of the hydrophilic resin is easily peeled off, and if the wet state and the dry state are repeated about 10 times, the crosslinked body of the hydrophilic resin peels off more. Therefore, it is considered that FD10 is likely to decrease.

[0090] Furthermore, hollow fiber membranes generally tend to have a decreased pure water permeation rate after drying compared to before drying. That is, FD tends to be less than FW. In addition, as described above, in the hollow fiber membranes prepared by conventional hydrophilic treatment methods, FD10 tends to decrease easily. Therefore, in the case of hollow fiber membranes prepared by conventional hydrophilic treatment methods, in order to suppress such a decrease in the permeation rate, it is often the case that the hollow fiber membranes are subjected to moisture retention treatment, protective treatment, etc. before drying. In contrast, in the hollow fiber membranes according to the present embodiment, when the hollow fiber membranes contain a crosslinked product obtained by crosslinking a polyvinylpyrrolidone resin contained in the hollow fiber membrane before crosslinking as the hollow fiber membrane, such treatment is not required to suppress the decrease in FD and FD10.

[0091] The shape of the hollow fiber membrane according to the present embodiment is not particularly limited. The hollow fiber membrane may be in the shape of a hollow fiber, with one side of the length direction open and the other side may be open or closed. As the shape of the hollow fiber membrane, for example, a shape of a hollow fiber with one side of the length direction open and the other side closed can be cited. In addition, as the shape of the open side of the hollow fiber membrane, for example, Figure 1 the case of the shape shown, etc. Additionally, Figure 1 is a partial perspective view of the hollow fiber membrane according to the embodiment of the present invention.

[0092] The outer diameter R1 of the hollow fiber membrane is preferably 0.5 to 7 mm, more preferably 1 to 2.5 mm, and further preferably 1 to 2 mm. If the outer diameter is such, it is a suitable size as the hollow fiber membrane provided in a device for implementing a separation technique using the hollow fiber membrane.

[0093] The inner diameter R2 of the hollow fiber membrane is preferably 0.4 to 3 mm, more preferably 0.6 to 2 mm, and further preferably 0.6 to 1.2 mm. If the inner diameter of the hollow fiber membrane is too small, the resistance of the permeate (pressure loss in the tube) becomes large, and there is a tendency for poor flow. In addition, if the inner diameter of the hollow fiber membrane is too large, the shape of the hollow fiber membrane cannot be maintained, and there is a tendency for the membrane to be easily flattened or skewed, etc.

[0094] The film thickness T of the hollow fiber membrane is preferably 0.2 to 1 mm, more preferably 0.25 to 0.5 mm, and still more preferably 0.25 to 0.4 mm. If the film thickness of the hollow fiber membrane is too thin, the strength is insufficient and there is a tendency to easily deform such as skew. In addition, if the film thickness is too thick, it is difficult to suppress the occurrence of large pores, etc., and there is a tendency to be difficult to obtain a suitable membrane structure. There may be a case where the strength decreases depending on the specific situation. On the other hand, the hollow fiber membrane according to the present embodiment can maintain high water permeability even when the film thickness is changed. Therefore, from the viewpoint of strength, a hollow fiber membrane with a relatively thick film thickness can be used according to the usage environment of components, etc.

[0095] If the outer diameter R1, inner diameter R2, and film thickness T of the hollow fiber membrane are respectively within the above ranges, the hollow fiber membrane provided in the device for implementing the separation technology using the hollow fiber membrane is of a suitable size, and the miniaturization of the device can be achieved.

[0096] In addition, the manufacturing method of the hollow fiber membrane according to the present embodiment is not particularly limited as long as it can manufacture the above-mentioned hollow fiber membrane. As such a manufacturing method, for example, the following manufacturing methods can be cited. As such a manufacturing method, a method including the following steps can be cited, etc., that is: a step of preparing a membrane-forming stock solution containing a vinylidene fluoride resin, a polyvinylpyrrolidone resin, and a solvent (preparation step); a step of extruding the membrane-forming stock solution in a hollow fiber shape (extrusion step); a step of solidifying the membrane-forming stock solution extruded in a hollow fiber shape to form a pre-crosslinked hollow fiber membrane (forming step); and a crosslinking step of crosslinking the polyvinylpyrrolidone resin in the pre-crosslinked hollow fiber membrane. Since such a manufacturing method has a crosslinking step of crosslinking the polyvinylpyrrolidone resin in the hollow fiber membrane, the above-mentioned hollow fiber membrane can be suitably manufactured. That is, a hollow fiber membrane containing a crosslinked body of a polyvinylpyrrolidone resin can be suitably manufactured.

[0097] First, the preparation step in the manufacturing method according to the present embodiment is not particularly limited as long as it can prepare a membrane-forming stock solution containing the vinylidene fluoride resin, the polyvinylpyrrolidone resin, and the poor solvent. As the preparation step, specifically, for example, a method of heating and stirring the raw materials of the membrane-forming stock solution can be cited. In addition, kneading is preferably performed during heating and stirring. That is, a method of mixing the vinylidene fluoride resin, the polyvinylpyrrolidone resin, and the solvent, which are the raw materials of the membrane-forming stock solution, at a specified ratio and kneading them in a heated state is preferred. Thereby, a membrane-forming stock solution in which the components as the raw materials of the membrane-forming stock solution are uniformly dispersed is obtained, and a hollow fiber membrane can be suitably manufactured. In addition, during kneading, for example, a twin-screw kneading device, a kneader, a stirrer, etc. can be used.

[0098] The solvent used herein is preferably a poor solvent for the vinylidene fluoride resin. Examples of the poor solvent for the vinylidene fluoride resin include solvents that are miscible with the vinylidene fluoride resin to form a single-phase state under conditions above a specific temperature and that can cause phase separation due to a decrease in miscibility caused by a temperature drop.

[0099] The preparation process is preferably carried out at a temperature lower than the melting point of the vinylidene fluoride resin. That is, the temperature during the preparation of the membrane-forming stock solution is preferably lower than the melting point of the vinylidene fluoride resin. In addition, when a poor solvent for the vinylidene fluoride resin is used as the solvent, the preparation process is preferably carried out at a temperature lower than the melting point of the vinylidene fluoride resin and higher than the temperature at which phase separation starts due to the temperature drop. That is, it is preferably carried out at a temperature such that the temperature during the preparation of the membrane-forming stock solution is lower than the melting point of the vinylidene fluoride resin and higher than the temperature at which phase separation starts due to the temperature drop. Moreover, as the temperature during the preparation of the membrane-forming stock solution, a temperature of 60°C or higher and lower than the melting point of the vinylidene fluoride resin is preferred, and more preferably 90 to 140°C. If the temperature is too low, the viscosity of the membrane-forming stock solution increases, and there is a tendency that a hollow fiber membrane having a suitable membrane structure cannot be obtained. Specifically, a suitable three-dimensional mesh structure cannot be formed on the layer that functions as the support layer of the hollow fiber membrane, and spherulites or large pores are likely to be formed in this layer, and there is a tendency that the strength of the obtained hollow fiber membrane decreases. In addition, if the temperature is too high, there is a tendency that a hollow fiber membrane having a suitable membrane structure cannot be obtained. Specifically, due to the thermal degradation of the polyvinylpyrrolidone resin, a suitable three-dimensional mesh structure cannot be formed on the layer that functions as the support layer of the hollow fiber membrane, and there is a tendency that spherulites or large pores are likely to be formed in this layer, or conversely, a dense structure is formed. As a result, there is a tendency that it is difficult to obtain a hollow fiber membrane having excellent fractionation characteristics and permeation characteristics. For these reasons, if the temperature during the preparation process is within the above range, a membrane-forming stock solution containing the vinylidene fluoride resin, the poor solvent, and the polyvinylpyrrolidone resin can be suitably obtained while suppressing damage caused by heat, etc., to the polyvinylpyrrolidone resin. Therefore, a suitable membrane-forming stock solution can be obtained, and a hollow fiber membrane having excellent permeation performance, fractionation characteristics, and strength can be manufactured.

[0100] The membrane-forming stock solution thus obtained is used for the manufacture of hollow fiber membranes. At this time, the obtained membrane-forming stock solution is preferably sufficiently degassed. And after being metered using a metering pump such as a gear pump, it is used for the manufacture of the hollow fiber membranes described later.

[0101] The vinylidene fluoride resin and the polyvinylpyrrolidone resin can use the resins described above.

[0102] The solvent is not particularly limited as long as it can be used as the solvent contained in the casting dope used in the production of hollow fiber membranes. In addition, as described above, the solvent is preferably a poor solvent for the vinylidene fluoride resin. Further, the poor solvent is not particularly limited as long as it is soluble in the vinylidene fluoride resin to form a single-phase state under conditions above a specific temperature and can cause phase separation due to a decrease in temperature. In addition, a water-soluble solvent is preferably used as the poor solvent. If it is a water-soluble solvent, water can be used when extracting the solvent from the hollow fiber membrane after film formation, and the extracted solvent can be disposed of by biological treatment or the like. In addition, examples of the poor solvent include γ-butyrolactone, ε-caprolactone, methanol, acetone, and caprolactone. From the viewpoints of environmental load, safety, and cost, γ-butyrolactone is particularly preferred among the solvents exemplified above. In addition, the poor solvent can be used alone or in combination of two or more of the exemplified solvents.

[0103] As the content of each component in the casting dope, the following contents can be cited. First, the content of the vinylidene fluoride resin is 20 to 35 parts by mass, more preferably 20 to 30 parts by mass, based on the total mass of the vinylidene fluoride resin, the poor solvent, and the polyvinylpyrrolidone resin. The content of the poor solvent is 45 to 70 parts by mass, more preferably 50 to 70 parts by mass, further preferably 55 to 65 parts by mass, based on the total mass. The content of the polyvinylpyrrolidone resin is 5 to 20 parts by mass, more preferably 8 to 20 parts by mass, further preferably 10 to 15 parts by mass, based on the total mass. In addition, the content of the vinylidene fluoride resin is preferably 1.54 to 4.38, more preferably 1.6 to 3.91, further preferably 1.67 to 3.13, in terms of mass ratio to the content of the polyvinylpyrrolidone resin. As long as the content of each component in the casting dope is the above content, a hollow fiber membrane with a more suitable content of the cross-linked product of the polyvinylpyrrolidone resin can be produced.

[0104] The casting dope only needs to contain the vinylidene fluoride resin, the polyvinylpyrrolidone resin, and the solvent, and it can also be composed of these. In addition, as the casting dope, other components can also be contained in addition to these three components. Examples of other components include various additives such as surfactants, antioxidants, ultraviolet absorbers, lubricants, anti-caking agents, dyes, and additives that promote the phase separation of the casting dope. In addition, as additives that promote the phase separation of the casting dope, solvents other than the poor solvents such as glycerin, ethylene glycol, tetraethylene glycol, water, ethanol, and methanol, and resins such as polyethylene glycol, polyethylene oxide, polyvinyl alcohol, polymethyl methacrylate, and polyacrylate methyl can be cited. As this resin, it can also be a copolymer of the above resins. In addition, as additives that promote the phase separation of the casting dope, the exemplified compounds can be used alone or in combination of two or more.

[0105] The extrusion process in the manufacturing method according to this embodiment is not particularly limited as long as the casting dope is extruded into a hollow fiber shape. As the extrusion process, Figure 2 The process of extruding the casting dope from a nozzle for forming hollow fibers as shown, etc. In addition, Figure 2 is a schematic diagram showing an example of a nozzle for forming hollow fibers used in the manufacturing method according to the embodiment of the present invention. In addition, Figure 2 (a) shows a cross-sectional view thereof, Figure 2 (b) is a top view showing the outlet side of the nozzle for forming hollow fibers that discharges the casting dope. Specifically, the nozzle 21 for forming hollow fibers here includes an annular outer discharge port 26 and a circular or annular inner discharge port 27 disposed inside the outer discharge port 26. And this nozzle 21 for forming hollow fibers is provided at the end of a flow pipe 24 through which the casting dope flows, and discharges the casting dope flowing in the flow pipe 24 from the outer discharge port 26 through a flow path 22 inside the nozzle. In addition, while discharging the casting dope from the outer discharge port 26, this nozzle 21 for forming hollow fibers circulates an internal coagulating liquid through the flow pipe 25 and discharges it from the inner discharge port 27 through a flow path 23 inside the nozzle. Thereby, the hollow fiber-shaped casting dope extruded from the nozzle 21 for forming hollow fibers is brought into contact with the internal coagulating liquid.

[0106] And, as this internal coagulating liquid, as long as it is a coagulating liquid that can be used in manufacturing a hollow fiber membrane containing a vinylidene fluoride resin, it is not particularly limited. As the internal coagulating liquid, for example, the distance (HSP distance) from the solubility parameter of the casting dope is preferably 5 to 200 (MPa) 1 / 2 , more preferably 50 to 200 (MPa) 1 / 2 , further preferably 100 to 180 (MPa) 1 / 2By using the internal coagulating liquid having such an HSP distance, it is possible to suitably solidify from the inner peripheral surface of the hollow fiber-shaped film-forming stock solution extruded from the hollow fiber forming nozzle. That is, it is considered that the solvent exchange between the inner peripheral surface side of the hollow fiber-shaped film-forming stock solution extruded from the hollow fiber forming nozzle and the internal coagulating liquid proceeds at a suitable speed. Therefore, it is considered that a hollow fiber membrane having a suitable structure near the inner peripheral surface side is obtained, and the hollow fiber membrane having excellent permeation performance and fractionation characteristics can be manufactured more suitably. Therefore, the hollow fiber membrane having excellent permeation performance and fractionation characteristics can be manufactured more suitably.

[0107] Here, the HSP distance is a parameter for evaluating the affinity between a certain substance and another substance, and is defined by the following formula using Hansen's three-dimensional solubility parameters (dD, dP, dH) (specifically, refer to the non-patent literature: Hansen, Charles (2007). Hansen Solubility Parameters: A user’s handbook, Second Edition. BocaRaton, Fla: CRC Press.).

[0108] HSP distance = [4 × (dD stock solution - dD solvent) 2 + (dP stock solution - dP solvent) 2 + (dH stock solution - dH solvent) 2 0.5

[0109] Here, dD is the van der Waals force, dP is the dipole moment force, dH is the hydrogen bond strength. The closer the HSP distance on the three-dimensional coordinate calculated according to the above definition formula is to 0, the higher the compatibility of the two components is judged, the slower the solvent exchange rate in the NIPS method, and the coarser the pore diameter of the contact surface.

[0110] In addition, the solubility parameter used in this specification is the Hansen parameter. For parameters not described in the Hansen parameter, the Hoy parameter can be used. Parameters not described in these two can be estimated by the Hansen parameter formula (refer to Allan F. M. barton, “CRC Handbook of solubility parameters and other cohesion parameters” CRC Corp. 1991). In the case of a mixed solvent, a parameter calculated by the sum rule based on the mass of each solubility parameter can be used.

[0111] In addition, an example of the solubility parameter is shown in Table 1 below.

[0112]

[0113] In the present embodiment, it is preferable to select the solvent, polyvinylpyrrolidone resin, and internal coagulation liquid contained in the casting dope in such a manner as to satisfy the HSP distance. In addition, the internal coagulation liquid may be a single solvent or a combination of two or more solvents. When two or more solvents are used in combination, for example, as the internal coagulation liquid, there may be mentioned a mixed solvent in which a solvent having a large HSP distance from the casting dope and a solvent having a small HSP distance from the casting dope are mixed at an arbitrary ratio to adjust the HSP distance from the casting dope. At this time, the types and amounts of the solvents to be mixed are not particularly limited. In addition, examples of the solvent having a large HSP distance from the casting dope include water, glycerin, etc. Further, examples of the solvent having a small HSP distance from the casting dope include γ-butyrolactone, dimethylacetamide, etc.

[0114] Examples of the mixed solvent used as the internal coagulation liquid include a mixed solvent of dimethylacetamide and glycerin, a mixed solvent of γ-butyrolactone and glycerin, a mixed solvent of γ-butyrolactone and ethylene glycol, a mixed solvent of γ-butyrolactone and water, a mixed solvent of dimethylacetamide and water, a mixed solvent of dimethylacetamide and ethylene glycol, a mixed solvent of dimethylformamide and water, etc. Among them, from the viewpoint of good formability of the hollow fiber membrane, a mixed solvent of γ-butyrolactone and glycerin, and a mixed solvent of dimethylacetamide and water are preferable.

[0115] From the viewpoint of ensuring the uniformity of the internal coagulation liquid, the temperature of the internal coagulation liquid is preferably 40 to 170°C. That is, as the temperature of the internal coagulation liquid, it is preferably adjusted between 40 and 170°C.

[0116] The forming step in the manufacturing method according to the present embodiment is not particularly limited as long as it is a step of solidifying the extruded hollow fiber-shaped casting dope to form a hollow fiber membrane. As this forming step, specifically, for example, there may be mentioned a step of forming a hollow fiber membrane by bringing the extruded hollow fiber-shaped casting dope into contact with an external coagulation liquid. More specifically, this forming step may include a step of immersing the hollow fiber-shaped casting dope extruded in the extrusion step in the external coagulation liquid stored in an external coagulation tank.

[0117] The external coagulating liquid is not particularly limited as long as it can coagulate the extruded hollow fiber-shaped film-forming stock solution by contacting it. Specifically, as the external coagulating liquid, an aqueous solution containing water and salts or solvents can be cited. Here, as the salts, various salts such as sulfates, chlorides, nitrates, and acetates can be cited. Among them, sodium sulfate is preferred. In addition, in the aqueous solution containing salts, the salt concentration is preferably 30 to 300 g / L, more preferably 50 to 300 g / L, and further preferably 100 to 280 g / L. If the concentration is too low or too high, there is a tendency that it is difficult to obtain a hollow fiber membrane with a suitable membrane structure. Specifically, if the concentration is too low, the solvent exchange rate in the forming process becomes fast, and the obtained hollow fiber membrane is over-refined, and there is a tendency for the permeation performance to decrease. In addition, if the concentration is too high, the solvent exchange rate in the forming process becomes slow, and there is a tendency for the fractionation characteristics of the obtained hollow fiber membrane to decrease.

[0118] The temperature of the external coagulating liquid is not particularly limited as long as it is a temperature at which the extruded hollow fiber-shaped film-forming stock solution can be coagulated by contacting it. As the temperature of the external coagulating liquid, when using a poor solvent for the vinylidene fluoride-based resin as the solvent, it is preferably higher than the temperature at which phase separation starts based on the temperature change. If the temperature of the external coagulating liquid is set to such a temperature, a hollow fiber membrane with excellent permeation performance and fractionation characteristics can be suitably manufactured. It is considered that this is for the following reasons. First, when manufacturing the film-forming stock solution, instead of using a good solvent for the vinylidene fluoride-based resin, a poor solvent for the vinylidene fluoride-based resin as described above is used, and the hollow fiber-shaped film-forming stock solution is brought into contact with the external coagulating liquid in a state where phase separation due to the temperature change does not occur. Thereby, solvent exchange between the solvent in the film-forming stock solution and the external coagulating liquid is caused, and the resin in the film-forming stock solution is coagulated. Therefore, the solvent exchange rate becomes a suitable rate compared with the case of using a good solvent, that is, the so-called conventional NIPS method. Therefore, it is considered that a hollow fiber membrane with excellent permeation performance and fractionation characteristics can be suitably manufactured.

[0119] The temperature of the external coagulating liquid is preferably higher than the temperature at which phase separation starts based on the temperature change. Specifically, it is preferably 45°C or higher, more preferably 50°C or higher. In addition, the temperature of the external coagulating liquid is preferably below the boiling point of the external coagulating liquid, more preferably 90°C or lower, and further preferably 85°C or lower. If the temperature of the external coagulating liquid is too low, the obtained hollow fiber membrane becomes denser, and there is a tendency that it is difficult to form an asymmetric structure. In addition, if the temperature of the external coagulating liquid is below the temperature at which phase separation starts based on the temperature change, it becomes the TIPS method, and it is difficult to form a suitable hollow fiber membrane. Moreover, if the temperature of the external coagulating liquid is too high, the viscosity of the membrane-forming stock solution decreases, resulting in a decline in the fractionation characteristics. In addition, there is a tendency that the water permeability is too high. Furthermore, if the temperature of the external coagulating liquid is above its boiling point, the external coagulating liquid boils and vibrates, and there is a tendency that the production of the hollow fiber membrane is unstable.

[0120] The temperature at which phase separation starts is the temperature at which a solution containing the vinylidene fluoride-based resin, the poor solvent, and the polyvinylpyrrolidone-based resin, for example, the membrane-forming solution, starts to phase separate when the temperature is lowered. As the temperature at which phase separation starts, specifically, it is measured as follows (in detail, refer to the non-patent literature: Structure and Physical Property Control of Polymer Alloys and Latest Technologies, Toshiaki Oshima, Norinori Seki, Akio Imai, Information Organization). First, place a glass slide and a coverslip on the stage of an optical microscope equipped with a temperature controller, and heat until the glass slide and the coverslip reach 120°C. Sandwich the membrane-forming stock solution in a homogeneous phase state between the heated glass slide and the coverslip. Then, gradually lower or raise the temperature of the glass slide and the coverslip, for example, lower the temperature by 3°C each time, and visually confirm the turbidity (caused by the difference in refractive index between the two phases) generated during phase separation, and measure the confirmed temperature. Set this temperature as the temperature at which phase separation starts. That is, in this measurement method, if the membrane-forming stock solution is transparent, it is considered to be in a homogeneous phase state, and if it is turbid, it is considered to be in a phase-separated state, and the temperature at which turbidity is locally confirmed is set as the temperature at which phase separation starts (phase separation start temperature) for measurement.

[0121] The forming step may also pass through a gas (usually air) before the extruded hollow fiber-shaped membrane-forming stock solution contacts the external coagulating liquid. That is, the forming step may also make the hollow fiber-shaped membrane-forming stock solution extruded in the extrusion step pass through a gas and then contact the external coagulating liquid. The distance passed through the gas is not particularly limited. For example, it is preferably 5 to 300 mm. Passing through the gas can appropriately perform the solvent exchange between the extruded hollow fiber-shaped membrane-forming stock solution and the internal coagulating liquid, making the hollow fiber shape stable and improving the spinnability. In addition, in the manufacturing method according to the present embodiment, this passing through the gas may not be performed.

[0122] The manufacturing method according to this embodiment can also stretch the hollow fiber membrane formed by the forming step in the longitudinal direction. The stretching method is not particularly limited, and for example, stretching treatment performed in a heated water tank can be cited. In addition, after stretching, if the force applied during stretching is released, it contracts in the longitudinal direction. If such stretching and contraction are performed, the permeation performance of the hollow fiber membrane is improved. It is considered that the independent pores existing in the membrane crack to become communicating pores, the connectivity inside the membrane is improved, and the permeation performance is improved. Moreover, if such stretching and contraction are performed, the direction of the fibers of the hollow fiber membrane is homogenized, and there is also an advantage of improved strength. In addition, the manufacturing method according to this embodiment may not perform such stretching and contraction.

[0123] The manufacturing method according to this embodiment can also clean the hollow fiber membrane formed by the forming step. As the cleaning method, for example, a method of hot water cleaning the hollow fiber membrane in a water tank at 80°C or higher can be cited. By this hot water cleaning, the hydrophilicity inside the hollow fiber membrane is appropriately improved. This is because the polyvinylpyrrolidone resin inside the hollow fiber membrane diffuses in the membrane due to this hot water cleaning.

[0124] The crosslinking step of the manufacturing method according to this embodiment is not particularly limited as long as it can crosslink the polyvinylpyrrolidone resin contained in the hollow fiber membrane. As this crosslinking step, for example, a step of immersing the hollow fiber membrane (hollow fiber membrane before crosslinking) in an aqueous solution containing a radical initiator, a step of immersing the hollow fiber membrane in a strong acid or strong base, a step of performing heat treatment on the hollow fiber membrane, and a step of performing radiation treatment on the hollow fiber membrane can be cited. As the crosslinking step, from the viewpoint of suppressing the deterioration of the vinylidene fluoride resin and being easy to operate, among the above steps, the step of immersing the hollow fiber membrane in an aqueous solution containing a radical initiator is particularly preferred.

[0125] The step of immersing in an aqueous solution containing a radical initiator preferably performs heat treatment during or after the immersion. In addition, as the aqueous solution containing a radical initiator, as long as it is an aqueous solution containing a radical initiator capable of starting the crosslinking reaction of the polyvinylpyrrolidone resin, for example, an aqueous solution of 1% by mass of a radical initiator can be cited. As the radical initiator, for example, sodium persulfate, ammonium persulfate, and hydrogen peroxide can be cited. Among them, hydrogen peroxide is preferred from the viewpoint of easily obtaining a hollow fiber membrane with high permeation performance.

[0126] The heating temperature in the step of performing heat treatment is only required to be a temperature capable of starting the crosslinking reaction of the polyvinylpyrrolidone resin, and for example, about 170 to 200°C is preferred.

[0127] In addition, the hollow fiber membrane according to the present embodiment can be used for membrane filtration. Specifically, for example, the hollow fiber membrane can be modularized as follows, and the modularized structure can be used for membrane filtration. More specifically, a specified number of the hollow fiber membranes according to the present embodiment are bundled and cut into a specified length, and filled into a housing having a specified shape. The ends of the hollow fiber bundle are fixed to the housing by a thermosetting resin such as polyurethane resin or epoxy resin to be modularized. In addition, as the structure of this module, there are various structures such as a structure in which both ends of the hollow fiber membrane are open and fixed, and a structure in which one end of the hollow fiber membrane is open and fixed while the other end is sealed but not fixed. The hollow fiber membrane according to the present embodiment can be used in any module structure.

[0128] The hollow fiber membrane according to the present embodiment is modularized as described above. For example, Figure 3 as shown, it can be assembled into a membrane filtration device. In addition, Figure 3 FIG. is a schematic diagram showing an example of a membrane filtration device including the hollow fiber membrane according to the embodiment of the present invention. The membrane filtration device 31 includes a membrane module 32 in which the hollow fiber membrane is modularized as described above. And, for example, the membrane module 32 may have a structure in which the hollow portion of the upper end portion 33 of the hollow fiber membrane is open and the hollow portion of the lower end portion 34 is sealed with an epoxy resin. In addition, for example, the membrane module 32 may have a structure using 70 hollow fiber membranes having an effective length of 100 cm. And, the membrane filtration device 31 introduces a liquid to be treated from the inlet 35 and discharges the liquid (filtered water) filtered through the membrane module 32 from the outlet 36. Accordingly, filtration using the hollow fiber membrane is performed. In addition, the air introduced into the membrane filtration device 31 is discharged from the air extraction port 37.

[0129] The hollow fiber membrane according to the present embodiment is modularized as described above and used for various applications such as water purification treatment, drinking water production, industrial water production, and wastewater treatment.

[0130] As described above, this specification discloses various technical aspects, and the main technologies are summarized as follows.

[0131] One aspect of the present invention relates to a hollow fiber membrane, which is a porous hollow fiber membrane containing a vinylidene fluoride resin. The hollow fiber membrane has an inclined structure in which the pore diameter of the pores inside the hollow fiber membrane gradually decreases toward the inner peripheral surface side or the outer peripheral surface side. The hollow fiber membrane contains a crosslinked body of a polyvinylpyrrolidone resin, so that the hollow fiber membrane is hydrophilized.

[0132] According to such a structure, a hollow fiber membrane having excellent permeation performance, fractionation characteristics, and strength can be obtained.

[0133] It is considered that this is due to the following reasons.

[0134] First, the hollow fiber membrane has an inclined structure in which the pore diameter of the pores inside the membrane gradually decreases toward the inner peripheral surface side or the outer peripheral surface side. Therefore, a fine layered portion related to the hierarchical property and other portions with relatively large pores (fine holes) are formed. For example, the fine layered portion related to the hierarchical property is formed on the surface or the like, and in other portions, the pores (fine holes) formed therein are relatively large. Thus, a decrease in the permeation performance is suppressed.

[0135] Moreover, since such a hollow fiber membrane contains a vinylidene fluoride-based resin, it tends to have relatively high hydrophobicity. Even such a hollow fiber membrane can improve its hydrophilicity by containing a crosslinked body of a polyvinylpyrrolidone-based resin. In addition, instead of simply containing a polyvinylpyrrolidone-based resin, it contains a crosslinked body of a polyvinylpyrrolidone-based resin, and the detachment of the polyvinylpyrrolidone-based resin is suppressed, and the effect of improving the hydrophilicity can be maintained. By improving the hydrophilicity in this way, the hollow fiber membrane can form suitable pores as described above, and can further improve the permeability to a liquid containing water. In addition, since the hollow fiber membrane contains a vinylidene fluoride-based resin, a hollow fiber membrane with excellent strength is obtained.

[0136] For these reasons, a hollow fiber membrane with excellent permeation performance, hierarchical property, and strength is obtained. In addition, by improving the hydrophilicity, the fouling resistance can be improved.

[0137] In addition, in the above-mentioned hollow fiber membrane, it is preferable that: the content of the crosslinked body is 0.1% by mass or more and less than 15% by mass.

[0138] According to such a structure, a hollow fiber membrane that maintains excellent hierarchical properties, has better permeation performance, and further has excellent fouling resistance is obtained.

[0139] This is because the hollow fiber membrane containing a vinylidene fluoride-based resin can be moderately hydrophilized, and the hydrophilicity can be improved while suppressing a decrease in water permeability due to clogging of the pores of the membrane or the like.

[0140] In addition, in the above-mentioned hollow fiber membrane, it is preferable that: the K value of the polyvinylpyrrolidone-based resin is 30 to 120.

[0141] According to such a structure, a hollow fiber membrane that maintains excellent hierarchical properties, has better permeation performance, and further has excellent fouling resistance is obtained.

[0142] It is considered that this is because of the following reasons. If it is a polyvinylpyrrolidone resin with such a K value, it can remain moderately in the hollow fiber membrane containing a vinylidene fluoride resin, and can moderately hydrophilize the hollow fiber membrane. Therefore, when suppressing the decrease in water permeability due to clogging of the pores of the membrane and the like, the hydrophilicity is improved, and thus the permeability of the liquid containing water can be improved.

[0143] For these reasons, a hollow fiber membrane is obtained which maintains excellent fractionation characteristics, has better permeation performance, and furthermore has excellent fouling resistance.

[0144] In addition, in the hollow fiber membrane, preferably: the water permeation amount at a transmembrane differential pressure of 0.1 MPa is 1000 - 40000 L / m 2 / hour, and the fractionation particle size is 0.001 - 0.5 μm.

[0145] According to such a structure, a hollow fiber membrane with better permeation performance and fractionation characteristics is obtained.

[0146] In addition, in the hollow fiber membrane, preferably: the hollow fiber membrane has a single-layer structure.

[0147] According to such a structure, a hollow fiber membrane with better permeation performance and fractionation characteristics, and difficult to cause damage such as peeling inside the membrane is obtained.

[0148] It is considered that this is based on the following reasons.

[0149] When the fine layered part related to the fractionation characteristics as described above is thin when the permeation performance is high as in the hollow fiber membrane according to one aspect of the present invention. At this time, if trying to form such a fine layer separately, it may not be formed appropriately. In contrast, if the fine layered part and other parts are formed from the same layer, that is, a single layer, the fine layered part can be formed uniformly on the surface. In addition, since the fine layered part and the other parts are a single layer, the occurrence of peeling and the like at the interface can be sufficiently suppressed.

[0150] For these reasons, a hollow fiber membrane with better permeation performance and fractionation characteristics, and difficult to cause damage such as peeling inside the membrane is obtained.

[0151] In addition, in the hollow fiber membrane, preferably: the crosslinked body is a crosslinked body obtained by crosslinking a polyvinylpyrrolidone resin, and the polyvinylpyrrolidone resin is contained in the hollow fiber membrane before crosslinking when forming the hollow fiber membrane before crosslinking.

[0152] According to such a structure, a hollow fiber membrane with better permeation performance and fractionation characteristics, and excellent strength with high tensile strength and elongation rate is obtained.

[0153] In addition, another aspect of the present invention relates to a method for manufacturing a hollow fiber membrane, which is used to manufacture the hollow fiber membrane and includes the following steps: a step of preparing a membrane-forming stock solution containing a vinylidene fluoride resin, a polyvinylpyrrolidone resin, and a solvent; a step of extruding the membrane-forming stock solution in a hollow fiber shape; a step of solidifying the extruded membrane-forming stock solution in a hollow fiber shape to form a pre-crosslinked hollow fiber membrane; and a crosslinking step of crosslinking the polyvinylpyrrolidone resin in the pre-crosslinked hollow fiber membrane.

[0154] According to this method, the hollow fiber membrane can be suitably manufactured.

[0155] In addition, in the method for manufacturing the hollow fiber membrane, it is preferable that: in the membrane-forming stock solution, the content of the vinylidene fluoride resin is 1.54 to 4.38 in terms of mass ratio relative to the content of the polyvinylpyrrolidone resin.

[0156] According to this method, a hollow fiber membrane with a more suitable content of the crosslinked body of the polyvinylpyrrolidone resin can be suitably manufactured.

[0157] In addition, in the method for manufacturing the hollow fiber membrane, it is preferable that: the crosslinking step is a step of immersing the pre-crosslinked hollow fiber membrane in an aqueous solution containing a radical initiator.

[0158] According to this method, the polyvinylpyrrolidone resin contained in the pre-crosslinked hollow fiber membrane can be simply crosslinked.

[0159] Next, the present invention will be described more specifically by way of examples, but the scope of the present invention is not limited to these. Examples

[0160] [Example 1]

[0161] First, polyvinylidene fluoride (hereinafter sometimes simply referred to as PVDF) (Kynar741 manufactured by Arkema Co., Ltd.) was used as the vinylidene fluoride resin, γ-butyrolactone (GBL manufactured by Mitsubishi Chemical Corporation) was used as the solvent, and polyvinylpyrrolidone (SOKALAN K-90 manufactured by BASF Japan Co., Ltd., K value: 90) was used as the polyvinylpyrrolidone resin to prepare a mixture in a mass ratio of 25:62:13. In addition, γ-butyrolactone is a poor solvent for polyvinylidene fluoride. In addition, the content of polyvinylidene fluoride relative to polyvinylpyrrolidone is 25 / 13, about 1.92.

[0162] The mixture was dissolved in a dissolution tank at a constant temperature of 95 °C to obtain a membrane-forming stock solution. After kneading the membrane-forming stock solution, it was taken out from Figure 2Extrusion of a nozzle (hollow fiber membrane forming nozzle) with a double-ring structure having an outer diameter of 1.6 mm and an inner diameter of 0.8 mm is shown. At this time, as the internal coagulation liquid, γ-butyrolactone (GBL manufactured by Mitsubishi Chemical Corporation) and glycerin (refined glycerin manufactured by Kao Corporation) were mixed at a mass ratio of 15:85 at a constant temperature of 65 °C and discharged simultaneously with the membrane-forming stock solution. The HSP distance between this internal coagulation liquid and the membrane-forming stock solution is 163 (MPa). 1 / 2 .

[0163] The membrane-forming stock solution extruded together with this internal coagulation liquid was immersed in an external coagulation liquid at 60 °C formed from a 180 g / L aqueous sodium sulfate solution after a flight distance of 40 mm. Thereby, the membrane-forming stock solution was solidified to obtain a hollow fiber membrane. In addition, this external coagulation liquid is a non-solvent for polyvinylidene fluoride.

[0164] Next, the obtained hollow fiber membrane was stretched and shrunk and then washed with hot water at 90 °C for 2 hours. Thereby, the solvent (γ-butyrolactone) and the polyvinylpyrrolidone-based resin (polyvinylpyrrolidone) were extracted and removed from the hollow fiber membrane. Then, crosslinking treatment (crosslinking insolubilization treatment) was carried out by heating the obtained hollow fiber membrane (hollow fiber membrane before crosslinking) and polyvinylpyrrolidone in a 1% hydrogen peroxide solution. The content of the crosslinked product of polyvinylpyrrolidone at this time was 1.9 mass%.

[0165] The obtained hollow fiber membrane has an outer diameter of 1.3 mm, an inner diameter of 0.8 mm, and a membrane thickness of 0.25 mm.

[0166] In addition, the membrane structure of the hollow fiber membrane according to Example 1 was confirmed using a scanning electron microscope (S-3000N manufactured by Hitachi, Ltd.). The results are shown in Figures 4 to 9 .

[0167] First, Figure 4 is a figure showing a scanning electron microscope photograph of the cross section of the hollow fiber membrane according to Example 1. Next, Figure 5 is a figure showing a scanning electron microscope photograph of the vicinity of the outer peripheral surface in the cross section of the hollow fiber membrane according to Example 1. In addition, Figure 6 is a figure showing a scanning electron microscope photograph of the vicinity of the central portion in the cross section of the hollow fiber membrane according to Example 1. In addition, Figure 7 is a figure showing a scanning electron microscope photograph of the vicinity of the inner peripheral surface in the cross section of the hollow fiber membrane according to Example 1. Specifically, Figure 5 is an enlarged view showing Figure 4 the frame line 61 shown. Figure 6 is an enlarged view showing Figure 4 the frame line 62 shown. Figure 7 is an enlarged view showingFigure 4 The figure of the frame line 63 shown.

[0168] As can be seen from these figures, the hollow fiber membrane involved in Example 1 is a porous hollow fiber membrane, having an inclined structure in which the pore diameter of the air holes of the hollow fiber membrane gradually decreases toward the inner peripheral surface side or the outer peripheral surface side. That is, it can be known that the sizes of the air holes in the hollow fiber membrane are different in sequence in the thickness direction. In addition, it can be known that a fine layered portion is formed near the outer peripheral surface, and a portion other than that is formed into a portion sparser than it. Specifically, using image analysis software (Image-Pro Plus manufactured by PURANETORON Co., Ltd.), Figure 5 The photograph near the outer peripheral surface shown is binarized, and the porosity calculated by determining the threshold value by the Otsu method is 34%, and the porosity calculated with the threshold value of 210 is 67%. Similarly, using image analysis software (Image-Pro Plus manufactured by PURANETORON Co., Ltd.), Figure 7 The photograph near the inner peripheral surface shown is binarized, and the porosity calculated by determining the threshold value by the Otsu method is 50%, and the porosity calculated with the threshold value of 210 is 78%.

[0169] Figure 8 The figure is a scanning electron microscope photograph showing the outer peripheral surface of the hollow fiber membrane involved in Example 1. Figure 9 The figure is a scanning electron microscope photograph showing the inner peripheral surface of the hollow fiber membrane involved in Example 1. From these figures, it can also be known that a fine layered portion is formed near the outer peripheral surface, and a portion other than that is formed into a portion sparser than it.

[0170] Using image analysis software (Image-Pro Plus manufactured by PURANETORON Co., Ltd.), Figure 8 The photograph of the outer peripheral surface shown is binarized, and the arithmetic average value of the diameters of the holes formed on the outer peripheral surface (outer peripheral side fine pore diameter) calculated by determining the threshold value by the Otsu method is 0.13 μm. In addition, using image analysis software (Image-Pro Plus manufactured by PURANETORON Co., Ltd.), Figure 9 The photograph of the inner peripheral surface shown is binarized, and the arithmetic average value of the diameters of the holes formed on the inner peripheral surface (inner peripheral side fine pore diameter) calculated by determining the threshold value by the Otsu method is 5 μm. In addition, the ratio of the inner peripheral side fine pore diameter to the outer peripheral side fine pore diameter (inner peripheral side fine pore diameter / outer peripheral side fine pore diameter) is 38 times.

[0171] The water permeability of the obtained hollow fiber membrane is calculated based on the amount of filtrate per unit time under the following operation using the hollow fiber membrane and the obtained amount and the membrane area.

[0172] Using this hollow fiber membrane, it was fabricated asFigure 3 The membrane filtration device 31 shown in the figure. The membrane module 32 loaded in the membrane filtration device 31 includes 20 hollow fibers with an effective membrane length of 20 cm, and the upper end portion 33 is sealed with an epoxy resin. The hollow portion of the hollow fiber membrane is open at the upper end portion 33, and the hollow portion of the hollow fiber membrane is sealed with an epoxy resin at the lower end portion 34. The membrane filtration device 31 filters pure water from the outer peripheral surface side of the hollow fiber membrane through the inlet 35, and the filtered water is obtained through the outlet 36 on the inner peripheral surface side located at the upper end portion. At this time, the transmembrane pressure difference was adjusted to 0.1 MPa.

[0173] The water permeation rate obtained by this measurement method, that is, the water permeation rate at a transmembrane pressure difference of 0.1 MPa is 5000 L / m 2 / hour. In addition, the hollow fiber membrane used in this measurement is a swollen hollow fiber membrane, and the water permeation rate here corresponds to the pure water permeation rate (FW) at a transmembrane pressure difference of 0.1 MPa in the wet state. In addition, as the hollow fiber membrane used, a dry hollow fiber membrane and a dry hollow fiber membrane after alternately repeating the wet state and the dry state 10 times each were also measured for FD and FD10, respectively.

[0174] In addition, the cut-off diameter of the produced hollow fiber membrane was measured by the following method.

[0175] The rejection rates of at least two kinds of particles with different particle diameters (such as Cataloid SI-550, Cataloid SI-45P, Cataloid SI-80P manufactured by Nichiei Shokubai Kasei Co., Ltd.) were measured. Based on this measured value, in the following approximate formula, the value of S when R is 90 was obtained and used as the cut-off diameter.

[0176] R = 100 / (1 - m × exp(-a × log(S)))

[0177] The constants a and m in the above formula are determined according to the hollow fiber membrane and are calculated based on the measured values of the rejection rates of two or more kinds. In addition, for the ultrafiltration membrane region, the molecular weight (weight average molecular weight) of standard polyethylene oxide (TSKgel manufactured by Tosoh Corporation) that can be removed by more than 90% is described.

[0178] The cut-off diameter obtained by this measurement method is 0.02 μm.

[0179] The pure water permeation coefficient K of the produced hollow fiber membrane was calculated by the above method to be 4 × 10 -15 m 2 .

[0180] In addition to changing the discharge amount of the dope solution, a plurality of hollow fiber membranes with different film thicknesses were similarly manufactured, and the pure water permeability coefficient K of each was calculated. Then, the change in the pure water permeability coefficient K with respect to the change in the film thickness was plotted, and the slope at this time was calculated. The slope was 2.29×10 -11 .

[0181] The strength of the obtained hollow fiber membrane was measured. Specifically, the tensile strength and elongation of the hollow fiber membrane were measured.

[0182] The tensile strength of the hollow fiber membrane was measured as follows.

[0183] First, the obtained hollow fiber membrane was cut into pieces 5 cm long. The cut hollow fiber membrane was used as a test piece for measuring strength.

[0184] Next, a tensile test of the test piece was carried out in water at 25°C at a speed of 100 mm / min using an automatic plotter (AG-Xplus manufactured by Shimadzu Corporation). At this time, the tensile strength was determined from the load at break.

[0185] The tensile strength obtained by this measurement method was 5.2 N / mm 2 .

[0186] In addition, the elongation of the hollow fiber membrane was measured as follows.

[0187] The elongation was determined from the elongation of the test piece at break in the above tensile test.

[0188] The elongation obtained by this measurement method was 180%.

[0189] From these, it can be seen that the hollow fiber membrane according to Example 1 is a hollow fiber membrane with excellent permeation performance, fractionation characteristics, and strength.

[0190] In addition, the following protein adsorption test was carried out to evaluate the hydrophilicity of the hollow fiber membrane.

[0191] The obtained hollow fiber membrane was dried and cut into 2 g based on its weight in the dry state. After wetting treatment of the cut hollow fiber membrane, it was immersed in a 1000 ppm bovine serum albumin (A7906-10G manufactured by Sigma-Aldrich) phosphate buffer solution for 24 hours. The concentration (protein concentration) of bovine serum albumin in the phosphate buffer solution after 24 hours of immersion was measured. Based on this measurement result, the reduction amount of the protein concentration due to the immersion of the hollow fiber membrane was calculated, and based on this reduction amount, the amount of protein attached to the hollow fiber membrane (protein attachment amount: mg / g) was calculated. The hollow fiber membrane in the state of adsorbing bovine serum albumin was immersed in a phosphate buffer solution not containing bovine serum albumin for 24 hours, and the concentration of bovine serum albumin dissolved in this phosphate buffer solution was measured. Based on this measurement result, the amount of dissolved protein (protein dissolution amount: mg / g) was calculated. Then, based on the difference between this protein attachment amount and the protein dissolution amount, the amount of bovine serum albumin adsorbed on the hollow fiber membrane (protein adsorption amount: mg / g) was calculated. The results are shown in Figure 10 In addition, the same evaluation was also performed on the hollow fiber membrane related to Comparative Example 1 described later, and it is also shown together in Figure 10 In.

[0192] In addition, Figure 10 is a graph showing the evaluation results of the hydrophilicity of each hollow fiber membrane related to Example 1 and Comparative Example 1. In addition, the vertical axis represents the protein adsorption amount (mg / g).

[0193] [Example 2]

[0194] Except that polyvinylpyrrolidone (PVPK-120 manufactured by ISP Japan Co., Ltd., K value: 120) was used as the polyvinylpyrrolidone resin, a hollow fiber membrane was prepared in the same manner as in Example 1. The content of the cross-linked product of polyvinylpyrrolidone in the obtained hollow fiber membrane was 4.9% by mass. The outer peripheral side pore diameter, inner peripheral side pore diameter, ratio of the inner peripheral side pore diameter to the outer peripheral side pore diameter, water permeation amount (FW) under a membrane inter-difference pressure of 0.1 MPa, FD, FD10, pure water permeation coefficient K, inclination when arranging the change of the pure water permeation coefficient K with respect to the membrane thickness change, classification particle size, tensile strength, and elongation rate of the obtained hollow fiber membrane were measured by the same method as in Example 1 and are shown in Table 1. It can be seen that the obtained hollow fiber membrane, like Example 1, is a hollow fiber membrane with excellent permeation performance and classification characteristics and excellent strength.

[0195] [Example 3]

[0196] A hollow fiber membrane was prepared in the same manner as in Example 1, except that polyvinylpyrrolidone (PVPK-60 manufactured by ISP Japan Co., Ltd., K value: 60) was used as the polyvinylpyrrolidone resin. The content of the cross-linked product of polyvinylpyrrolidone in the obtained hollow fiber membrane was 0.6% by mass. The outer peripheral side pore diameter, inner peripheral side pore diameter, ratio of the inner peripheral side pore diameter to the outer peripheral side pore diameter, water permeation rate (FW) under a membrane inter-difference pressure of 0.1 MPa, FD, FD10, pure water permeation coefficient K, slope when arranging the change in the pure water permeation coefficient K with respect to the membrane thickness change, classification particle size, tensile strength, and elongation rate of the obtained hollow fiber membrane were measured in the same manner as in Example 1 and are shown in Table 1. It can be seen that the obtained hollow fiber membrane, like that of Example 1, is a hollow fiber membrane with excellent permeation performance, classification characteristics, and strength.

[0197] [Example 4]

[0198] A hollow fiber membrane was prepared in the same manner as in Example 1, except that the washing time of hot water washing performed after stretching and shrinking the hollow fiber membrane was changed to other than 20 minutes. In addition, since the washing time of this Example 4 was shorter than that of Example 1, it was an example in which the remaining amount of the cross-linked product of polyvinylpyrrolidone was intended to be more than that of the hollow fiber membrane obtained in Example 1. The content of the cross-linked product of polyvinylpyrrolidone in the obtained hollow fiber membrane was 9.2% by mass. The outer peripheral side pore diameter, inner peripheral side pore diameter, ratio of the inner peripheral side pore diameter to the outer peripheral side pore diameter, water permeation rate (FW) under a membrane inter-difference pressure of 0.1 MPa, FD, FD10, pure water permeation coefficient K, slope when arranging the change in the pure water permeation coefficient K with respect to the membrane thickness change, classification particle size, tensile strength, and elongation rate of the obtained hollow fiber membrane were measured in the same manner as in Example 1 and are shown in Table 1. It can be seen that the obtained hollow fiber membrane, like that of Example 1, is a hollow fiber membrane with excellent permeation performance, classification characteristics, and strength.

[0199] [Example 5]

[0200] By washing the hollow fiber membrane before crosslinking in Example 1, polyvinylpyrrolidone contained in the membrane was removed until the content of polyvinylpyrrolidone contained in the membrane was less than 0.1% by mass. The hollow fiber membrane from which polyvinylpyrrolidone had been removed was completely dried. Then, the dried hollow fiber membrane was immersed in an aqueous solution containing 50% by mass of ethanol to moisten it. And the moistened hollow fiber membrane was immersed in pure water for 24 hours. Thus, the whole hollow fiber membrane was in a state of containing water. The hollow fiber membrane in this state was immersed in an aqueous solution containing 1% by mass of polyvinylpyrrolidone (SOKALANK-90 manufactured by BASF Japan Co., Ltd., K value: 90). By crosslinking the hollow fiber membrane immersed in polyvinylpyrrolidone in the same manner as in Example 1, a hollow fiber membrane containing a crosslinked product of polyvinylpyrrolidone was obtained. The outer peripheral side pore diameter, inner peripheral side pore diameter, ratio of the inner peripheral side pore diameter to the outer peripheral side pore diameter, water permeation amount (FW) at a membrane inter-differential pressure of 0.1 MPa, FD, FD10, pure water permeation coefficient K, inclination when arranging the change of the pure water permeation coefficient K with respect to the change of the membrane thickness, classification particle size, tensile strength, and elongation rate of the obtained hollow fiber membrane were measured in the same manner as in Example 1 and are shown in Table 1.

[0201] [Comparative Example 1]

[0202] Polyvinylpyrrolidone in the hollow fiber membrane was washed as much as possible, and a hollow fiber membrane was obtained in the same manner as in Example 1 except that the crosslinking-insoluble treatment of polyvinylpyrrolidone was not performed.

[0203] The obtained hollow fiber membrane was not hydrophilized, the permeation resistance became large, and sufficient permeation performance could not be obtained. The content of the crosslinked product of polyvinylpyrrolidone in the obtained hollow fiber membrane was 0% by mass because the crosslinking-insoluble treatment was not performed. In addition, the content of polyvinylpyrrolidone in the obtained hollow fiber membrane was less than 0.1% by mass.

[0204] In addition, as described above, the hydrophilicity of the hollow fiber membrane obtained in Comparative Example 1 was evaluated by the same method as in Example 1. The results are shown in Figure 10 .

[0205] In addition, according to Figure 10 it is known that the amount of protein adsorption of the hollow fiber membrane according to Example 1 in which polyvinylpyrrolidone is contained in the membrane and the crosslinking-insoluble treatment is performed is less than that of Comparative Example 1 in which the crosslinking-insoluble treatment is not performed. From this, it can be seen that the hollow fiber membrane according to Example 1 contains a crosslinked product of a polyvinylpyrrolidone resin, and thus the obtained hollow fiber membrane is hydrophilized.

[0206] [Comparative Example 2]

[0207] Polyvinyl alcohol (PVA-205 manufactured by Kuraray Co., Ltd.) was used instead of polyvinylpyrrolidone resins. When performing the crosslinking insolubilization treatment, a sulfuric acid acidic aqueous solution of 1% glutaraldehyde was used as the crosslinking treatment liquid. Except for this, a hollow fiber membrane was produced in the same manner as in Example 1. The content of the crosslinked product of polyvinyl alcohol in the produced hollow fiber membrane was 3.0% by mass. In addition, this content was calculated by dissolving the produced hollow fiber membrane in N-methylpyrrolidone, which is a good solvent for vinylidene fluoride resins, and measuring the weight of the remaining crosslinked product. The produced hollow fiber membrane did not obtain sufficient permeation performance.

[0208] The conditions and pure water permeation coefficients of the above Examples and Comparative Examples are shown in Table 2 below. In addition, the resin in the table is the resin contained together with the vinylidene fluoride resin, "PVP" is polyvinylpyrrolidone, and "PVA" is polyvinyl alcohol. In addition, the content of the crosslinked product in Comparative Example 2 represents the content of the crosslinked product of PVA, and the content of the crosslinked product other than that represents the content of the crosslinked product of PVP.

[0209]

[0210] It can be seen from Table 2 and the above description that Examples 1 to 5 are excellent in permeation performance and fractionation characteristics, and also excellent in strength, compared with Comparative Examples 1 and 2. In addition, it can be seen that the case of crosslinking polyvinylpyrrolidone kneaded in the hollow fiber membrane before crosslinking (Examples 1 to 4) has a higher elongation rate than the case of crosslinking polyvinylpyrrolidone after impregnating the hollow fiber membrane before crosslinking with an aqueous solution of polyvinylpyrrolidone (Example 5).

[0211] This application is based on Japanese Patent Application No. 2014-063791 filed on March 26, 2014, the content of which is incorporated in this application.

[0212] In order to describe the present invention, the present invention has been appropriately and sufficiently described by the embodiments in the above description. However, it should be recognized that those skilled in the art can easily make changes and / or improvements to the above-described embodiments. Therefore, as long as the changes or improvements implemented by those skilled in the art do not deviate from the scope of the claims described in the claims, such changes or improvements should be interpreted as being included in the scope of the claims.

[0213] Industrial Applicability

[0214] According to the present invention, it is possible to provide a hollow fiber membrane and a method for manufacturing the same, which are excellent in both permeation performance and fractionation characteristics and also excellent in strength.

[0215] Symbol Explanation

[0216] 21 Nozzle for forming hollow fiber

[0217] 22 and 23 flow paths

[0218] 24 and 25 flow-through pipes

[0219] 26 outer discharge port

[0220] 27 inner discharge port

[0221] 31 membrane filtration device

[0222] 32 membrane module

[0223] 33 upper end portion

[0224] 34 lower end portion

[0225] 35 inlet

[0226] 36 outlet

[0227] 37 air extraction port.

Claims

1. A method for manufacturing a hollow fiber membrane, the hollow fiber membrane being a porous hollow fiber membrane containing a vinylidene fluoride resin, the hollow fiber membrane having an inclined structure in which the pore diameter of the pores inside the hollow fiber membrane gradually decreases toward the inner peripheral surface side or the outer peripheral surface side, the hollow fiber membrane contains a cross-linked body of a polyvinylpyrrolidone resin, whereby the hollow fiber membrane is hydrophilized, the ratio (FD10 / FW) of the pure water permeation rate (FD10) at a membrane inter-differential pressure of 0.1 MPa in the dry state after wet and dry states are each repeatedly alternated 10 times to the pure water permeation rate (FW) at a membrane inter-differential pressure of 0.1 MPa in the wet state is 80% or more, The permeation rate (FW) of pure water under a transmembrane differential pressure of 0.1 MPa in the wet state is 3500 to 20000 L / m 2 / hour The pure water permeation coefficient is 1.37×10 when the thickness of the hollow fiber membrane is L (m). -11 ×L (m 2 ) or more and 3.92×10 -11 ×L (m 2 ) or less. the method for manufacturing the hollow fiber membrane includes the following steps: a step of preparing a membrane-forming dope, the membrane-forming dope containing the vinylidene fluoride resin, the polyvinylpyrrolidone resin, and a poor solvent that is miscible with the vinylidene fluoride resin under conditions of a specific temperature or higher to form a single-phase state and causes phase separation due to a temperature drop, and the step of preparing the membrane-forming dope is carried out at a temperature lower than the melting point of the vinylidene fluoride resin and higher than the temperature at which phase separation starts due to the temperature drop, that is, 90°C to 140°C; a step of extruding the membrane-forming dope in a hollow fiber shape and bringing an internal coagulating liquid into contact with the extruded hollow fiber-shaped membrane-forming dope; a step of bringing an external coagulating liquid into contact with the extruded hollow fiber-shaped membrane-forming dope under conditions of a temperature higher than the temperature at which phase separation starts due to the temperature drop and below the boiling point of the external coagulating liquid, that is, 50°C or higher and 90°C or lower, so as to solidify the extruded hollow fiber-shaped membrane-forming dope to form a pre-crosslinked hollow fiber membrane; and a cross-linking step of cross-linking the polyvinylpyrrolidone resin inside the pre-crosslinked hollow fiber membrane.

2. The method for manufacturing a hollow fiber membrane according to claim 1, wherein: the content of the cross-linked body is 0.1% by mass or more and less than 15% by mass.

3. The method for manufacturing a hollow fiber membrane according to claim 1, wherein: the K value of the polyvinylpyrrolidone resin is 30 to 120.

4. The method for manufacturing a hollow fiber membrane according to claim 1, wherein: the classification particle size is 0.001 to 0.5 μm.

5. The method for manufacturing a hollow fiber membrane according to claim 1, wherein: the hollow fiber membrane has a single-layer structure.

6. The method for manufacturing a hollow fiber membrane according to claim 1, wherein: the cross-linked body is a cross-linked body obtained by cross-linking a polyvinylpyrrolidone resin, and the polyvinylpyrrolidone resin is included in the pre-crosslinked hollow fiber membrane when the pre-crosslinked hollow fiber membrane is formed.

7. The method for manufacturing a hollow fiber membrane according to claim 1, wherein: in the membrane-forming dope, the content of the vinylidene fluoride resin relative to the content of the polyvinylpyrrolidone resin is 1.54 to 4.38 in terms of mass ratio.

8. The method for manufacturing a hollow fiber membrane according to claim 1, wherein: The crosslinking step is a step of immersing the hollow fiber membrane before crosslinking in an aqueous solution containing a radical initiator.

9. A hollow fiber membrane, which is produced by the method for producing a hollow fiber membrane according to any one of claims 1 to 8.

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