JP6274642B2 - Porous hollow fiber membrane and method for producing the same - Google Patents

Description

  The present invention relates to a porous hollow fiber membrane that is excellent in water permeability and blocking performance against micropathogens, has high chemical resistance, and can stably maintain blocking performance against micropathogens for a long period of time, and a method for producing the same.

  The filtration process using porous hollow fiber membranes has a higher separation performance and is easier to maintain and manage than conventional chemical treatment processes such as coagulation sedimentation and sand filtration. It is out. In particular, in the production of drinking water, as in the pharmaceutical manufacturing and food industry fields, contamination of the production line as well as consumer infection is caused not only by contamination of the production line, but also by contamination with micropathogens such as viruses that are smaller in size than microorganisms. Due to the danger, ultrafiltration membranes with smaller pore diameters than microfiltration are increasingly being applied. And since the minimum size of a micropathogen is about 22 nm, development of the porous hollow fiber membrane which has a separation layer of 20 nm or less of exclusion capability is calculated | required by the ultrafiltration membrane for drinking water use.

  Conventionally, various methods for producing an ultrafiltration membrane are known, but a phase separation method excellent in relatively wide pore diameter control from a small pore diameter to a large pore diameter is often used. As such a phase separation method, a non-solvent induced phase separation method (hereinafter also referred to as “NIPS method”) and a thermally induced phase separation method (hereinafter also referred to as “TIPS method”) are known. Yes.

  In the NIPS method, a uniform solution (film-forming stock solution) prepared by dissolving a polymer in a solvent is discharged and brought into contact with a coagulating liquid containing a non-solvent, so that the solvent in the film-forming stock solution and the coagulation bath A concentration gradient is generated between the non-solvents, and the phase separation phenomenon proceeds by replacing the non-solvent with the solvent in the film-forming stock solution using this as a driving force. In the NIPS method, the rate of substitution from the solvent to the non-solvent is changed from the contact surface to the porous hollow fiber because the concentration gradient decreases from the surface in contact with the coagulating liquid to the membrane forming stock solution to the inside of the porous hollow fiber membrane. Slow down inside the membrane. The slower the solvent exchange rate, the more the phase separation proceeds and the pores tend to become coarser. In general, porous hollow fiber membranes produced by the NIPS method form dense pores on the contact surface. At the same time, it tends to have an inclined structure in which the pore diameter gradually becomes larger toward the inside of the porous hollow fiber membrane.

  The TIPS method generates heat diffusion by cooling a homogeneous solution (film-forming stock solution) prepared by kneading a polymer with a latent solvent at a high temperature, and proceeds phase separation using this as a driving force. It is. In such a TIPS method, the generated thermal diffusion proceeds at a much faster rate than the concentration diffusion generated in the NIPS method, and therefore the degree of phase separation progress in the cross-sectional direction of the porous hollow fiber membrane is substantially equal. Therefore, the porous hollow fiber membrane produced by the TIPS method tends to have a porous structure in which pores having a relatively uniform pore diameter are formed in the cross-sectional direction of the porous hollow fiber membrane.

  And in order to achieve both the small pore size and water permeability required for virus removal, it is necessary to reduce the thickness of the separation layer from the Hagen Poiseuille's formula showing the relationship between water permeability, pore size and film thickness. In the TIPS method, it is difficult to thin only the separation layer. Therefore, when a porous hollow fiber membrane having a small pore diameter and high water permeability is produced, production by the NIPS method that can easily produce a separation layer having a thickness of several μm by an inclined structure is frequently used.

  On the other hand, as a filter medium used for the filtration process with a porous hollow fiber membrane, for example, a hollow fiber membrane formed of a polymer having excellent processability in a hollow shape, a flat membrane formed of a polymer in a sheet shape, etc. are combined. A membrane module or the like is used. However, when such a membrane module is used for water treatment or the like, there is a problem that the membrane surface is clogged by turbidity separated by filtration, and as a result, the water permeability is lowered and membrane clogging is likely to occur.

  The above-mentioned film clogging is generally caused by physical clogging caused by accumulation of turbidity in pores and on the membrane surface, and chemical clogging caused by adsorption of organic substances etc. on the membrane substrate. are categorized. In many cases, after a certain amount of time or a certain amount of water is filtered, the membrane is washed in order to eliminate clogging of the membrane and restore the amount of permeated water. For example, air scrubbing treatment that vibrates the membrane by continuously or intermittently sending air into the raw water during filtration or backwash operation is used as an effective means for eliminating physical blockage. ing. In addition, if the pressure resistance of the membrane is high, it is possible to perform filtration while applying a larger water pressure, so that the time until cleaning can be maintained longer and the frequency of cleaning can be reduced.

  On the other hand, as a means for eliminating chemical blockage, it is effective to decompose and remove accumulated organic substances using chemicals, for example, an oxidizing agent such as sodium hypochlorite or an alkali such as sodium hydroxide. It is used as. However, such chemicals not only decompose the accumulated organic matter, but can also decompose the polymer that composes the membrane. Therefore, continuous chemical cleaning causes the pores of the membrane to become coarse. Thus, it becomes difficult to maintain the separation performance of the membrane for a long time. Therefore, in order to maintain the separation performance over a long period of time while eliminating the decrease in the amount of permeated water due to membrane clogging, the chemical resistance is such that the separation performance can be sufficiently maintained even after washing with chemicals. Sought in the separation layer.

  In recent years, for the purpose of preventing the deterioration of the film due to such chemicals, a film made of an inorganic material having excellent chemical resistance and a fluorine-based polymer such as polytetrafluoroethylene has been commercialized. However, since these materials are difficult to dissolve in a solvent, it has been difficult to produce a membrane having a membrane shape effective for removing micropathogens and a membrane having high separation performance by the NIPS method.

  On the other hand, among fluoropolymers, polyvinylidene fluoride resin (hereinafter also referred to as “PVDF resin”) is relatively excellent in processability, and is therefore often used as a material for porous hollow fiber membranes. On the other hand, the PVDF resin has a drawback of low resistance to alkali as compared with other fluoropolymers. Therefore, it has been extremely difficult to produce a porous hollow fiber membrane made of PVDF resin that can withstand long-term use that involves washing with alkali.

  A method for producing a porous hollow fiber membrane by PVIPS resin by the NIPS method has been reported in many literatures so far. For example, Patent Document 1 discloses a porous hollow fiber membrane in which physical strength is not reduced by a chemical by producing a porous hollow fiber membrane having a low β crystal ratio, a low crystallinity, and a small specific surface by the NIPS method. A method of manufacturing is disclosed. In Patent Document 2, PVDF resin, latent solvent, and inorganic fine powder are melted and kneaded, molded and cooled into a film shape, and subsequently extracted in the cross-sectional direction of the film by sequentially extracting the latent solvent and inorganic fine powder. There is a disclosure that a porous hollow fiber membrane having a uniform crystal structure can be obtained, and the crystal structure of the porous hollow fiber membrane has a low β crystal ratio and a low crystallinity, so that the chemical strength of the separation layer is strong. It has been confirmed that separation performance can be maintained over a long period of use. Furthermore, Patent Document 3 discloses a PVDF ultrafiltration membrane that removes 4 logs or more of viruses.

International Patent Publication No. 2007/119850 pamphlet JP-A-3-42025 JP 2010-94670 A

  However, the porous ultrafiltration membrane made of PVDF manufactured by the method disclosed in the above patent document is considered to be unsuitable for the filtration application to drinking water requiring high safety for the following reasons. .

  That is, the porous hollow fiber membrane described in Patent Document 1 has a crystal structure that changes in the direction of the membrane cross-section, and has a structure that is strong against chemical degradation near the center of the membrane cross-section, Although the chemical strength of the separation layer in the vicinity of the membrane surface is weak (poor chemical resistance), the deterioration of the strength and elongation of the entire film thickness can be suppressed, but the separation layer can be used with long-term use or by chemical cleaning. It is easily expected that the separation performance will deteriorate and the separation performance will deteriorate. Therefore, the porous hollow fiber membrane produced by the manufacturing method described in Patent Document 1 is not suitable for drinking water applications that require removal of micropathogens such as viruses for a long period of time.

  Further, the porous hollow fiber membrane described in Patent Document 2 is produced by the TIPS method, and has a porous structure in which pores having a relatively uniform pore diameter are formed in the cross-sectional direction of the porous hollow fiber membrane. Therefore, if the pore diameter is reduced in order to ensure the removal performance of micropathogens such as viruses, the water permeation performance is significantly reduced. In addition, when a porous hollow fiber membrane is produced by the production method described in Patent Document 2, pores are formed by dropping the added inorganic fine powder, etc. It tends to be insufficient in the ability to prevent micropathogens. In particular, when there are aggregates of inorganic fine powder at the time of production, coarse pores having a pore size larger than the particle size of the inorganic fine powder are formed, so that the ability to prevent micropathogens tends to be significantly reduced. .

  Furthermore, the ultrafiltration membrane made of PVDF described in Patent Document 3 is for the purpose of removing viruses and is inferior in chemical resistance of the separation layer. Is destroyed, and the separation performance gradually decreases with long-term use.

  The present invention has been made in view of the above problems, and has excellent water permeability and blocking performance against micropathogens, and has only extremely high chemical resistance capable of suppressing the coarsening of pores for chemical cleaning and the like. In addition, it is an object to provide a porous hollow fiber membrane and a method for producing the same that can stably maintain a blocking performance against micropathogens for a long period of time.

  In order to solve the above problems, the present inventors have conducted extensive research and found that having a specific porous structure and compressive strength is indispensable for solving the above problems, A production method for obtaining such a porous hollow fiber membrane with good reproducibility has been found, and the present invention has been completed.

That is, the present invention provides the following (1) to (16).
(1)
A porous hollow fiber membrane made of PVDF resin, wherein one surface of the porous hollow fiber membrane is defined as a surface (A) and the other surface is defined as a surface (B). ), The cross-sectional structure is a three-dimensional network structure, the pore diameter of either surface of the porous hollow fiber membrane is 0.05 μm or less, and the α-type structural crystal (H _α ) And β-type structural crystal (H _β ) (H _α / H _β ) is a porous hollow fiber membrane having a ratio of 0.5 or more.
(2)
The pore diameter constituting the one surface (A) is 0.05 μm or less, the average thickness of the trunk constituting the other surface (B) is 0.5 to 10 μm, and the pore diameter is 0.1 μm or more. The porous hollow fiber membrane according to (1), which is 1 μm or less.
(3)
It has a two-layer structure consisting of an inner layer having a surface (B) in the hollow portion and an outer layer having a surface (A), and the outer layer has a continuous pore size from the outer layer surface toward the boundary with the inner layer surface. The porous hollow fiber membrane according to (1) or (2), which increases to
(4)
The porous hollow fiber membrane according to any one of (1) to (3), wherein the pore size distribution of the inner layer is isotropic.
(5)
The porous hollow fiber membrane according to any one of (1) to (4), wherein the outer layer has a thickness of 1 μm to 100 μm.
(6)
The porous hollow fiber membrane according to any one of (1) to (5), wherein the inner layer has a thickness of 20 μm or more and 1000 μm or less.
(7)
The porous hollow fiber membrane according to any one of (1) to (6), wherein the compressive strength is 0.8 MPa or more.
(8)
Even after 30 days of operation, the porous hollow fiber is air bubbled by introducing air at a flow rate of 1 Nm ³ / h from the through-hole into the hollow part of the porous hollow fiber membrane every 30 minutes. The porous hollow fiber membrane according to any one of (1) to (7), wherein the inner layer and the outer layer in the membrane do not peel off.
(9)
In the method for producing a porous hollow fiber membrane made of PVDF resin, which is composed of two layers of an inner layer having a surface (B) in the hollow portion and an outer layer having the surface (A), the polymer stock solution constituting the inner layer and the A porous hollow fiber membrane in which a polymer stock solution constituting an outer layer is simultaneously discharged from a triple-structure spinning nozzle, the outer layer is phase-separated by a non-solvent induced phase separation method, and the inner layer is phase-separated by a thermally induced phase separation method Manufacturing method.
(10)
The method for producing a porous hollow fiber membrane according to (9), wherein the polymer in the polymer stock solution constituting the inner layer and the polymer in the polymer stock solution constituting the outer layer are thermoplastic resins.
(11)
The method for producing a porous hollow fiber membrane according to (9) or (10), wherein the thermoplastic resin is a PVDF resin.
(12)
The polymer in the polymer stock solution constituting the outer layer is PVDF resin, the polymer in the polymer stock solution constituting the inner layer is PVDF resin, the discharge temperature of the polymer stock solution constituting the outer layer, and the polymer stock solution constituting the inner layer The method for producing a porous hollow fiber membrane according to any one of (9) to (11), wherein the discharge temperature is in the range of 200 to 280 ° C.
(13)
The porous hollow fiber membrane according to any one of (9) to (12), wherein the polymer in the polymer stock solution constituting the outer layer is PVDF resin, and the good solvent for dissolving the PVDF resin is sulfolane. Production method.
(14)
The method for producing a porous hollow fiber membrane according to any one of (9) to (13), wherein the polymer stock solution constituting the inner layer contains a PVDF resin, an organic liquid, and silica.

  According to the porous hollow fiber membrane of the present invention, not only has excellent water permeability and blocking performance against micropathogens, and has extremely high chemical resistance capable of suppressing the coarsening of pores for chemical cleaning, etc., but also micropathogens Can be stably maintained for a long period of time.

The electron micrograph of the surface (A) of the outer layer of Example 1. The electron micrograph of the surface (B) of the inner layer of Example 1. FIG.

  Embodiments of the present invention will be described below. Note that the following embodiment is an example for explaining the present invention, and the present invention is not limited to this embodiment. Moreover, this invention can be implemented with a various form, unless it deviates from the summary.

The porous hollow fiber membrane of the present embodiment is a porous hollow fiber membrane made of PVDF resin, and when one surface thereof is the surface (A) and the other surface is the surface (B), the surface (A ) To the surface (B), the cross-sectional structure is a three-dimensional network structure, the pore diameter of either surface is 0.05 μm or less, and the α-type structure crystal (H _α ) in the crystal part of the PVDF resin beta type structure crystals (H _beta) and a ratio of (H α _/ H _β) is equal to or not less than 0.5.

  From the viewpoint of ensuring the strength of the porous hollow fiber membrane, the cross-sectional structure is a three-dimensional network structure from the surface (A) to the surface (B), and the pore diameter of either surface is 0.05 μm or less. Is needed. Here, the three-dimensional network structure of the porous hollow fiber membrane can be confirmed with a field of view of 500 to 10,000 times using an electron microscope. In the present specification, the pore diameter of the surface of the hollow fiber membrane is measured using a uniform latex sphere having various particle diameters, the latex sphere suspension is filtered, and the blocking rate is measured. From the relationship between the blocking rate and the blocking rate, the particle diameter at which 90% blocking was observed was taken as the pore size, and the average value of 6 points extracted at random. If the pore diameter on either surface exceeds 0.05 μm, micropathogens or the like tend to permeate. From the viewpoint of developing higher performance, the pore diameter on one surface is preferably 18 nm or less, and more preferably 15 nm or less.

Further, in order to improve chemical resistance, particularly resistance to alkali that promotes deterioration of PVDF resin, the ratio of α-type structural crystal (H _α ) and β-type structural crystal (H _β ) in the crystal part of PVDF resin ( H _α / H _β ) is required to be 0.5 or more. Here, as the crystal structure of the PVDF resin, there are known three structures of α-type and β-type, and γ-type having a very small amount. And since the crystal structure of the β-type structure is thermodynamically unstable, if the crystal contains a lot of β-type structure, a portion susceptible to deterioration by chemicals is located near the boundary between the crystal part and the amorphous part. As a result, the chemical resistance of the entire porous hollow fiber membrane tends to decrease. The ratio (H _α / H _β ) between the α-type structure crystal (H _α ) and the β-type structure crystal (H _β ) in the crystal part of the PVDF resin is more preferably 0.7 or more.

  The porous hollow fiber membrane of this embodiment has a pore diameter of 0.05 μm or less constituting one surface (A) from the viewpoint of improving the strength of the porous hollow fiber membrane and eliminating micropathogens with high efficiency. It is more preferable that the average thickness of the trunk constituting the other surface (B) is 0.5 to 10 μm and the pore diameter is 0.1 μm or more and 1 μm or less. More preferably, the average thickness of the trunk constituting the surface (B) is 0.5 to 10 μm, and the pore diameter is 0.1 μm or more and 1 μm or less, and more preferably, the surface (B) is formed. The average thickness of the trunk is 0.5 μm or more and 5 μm or less, and the pore diameter is 0.3 μm or more and 0.7 μm or less.

  Here, the porous hollow fiber membrane of the present embodiment has an inner layer facing the hollow portion and an outer layer facing outward from the viewpoint of balancing water permeability performance, blocking performance against micropathogens, chemical resistance, etc. at a high level. It preferably has a two-layer structure consisting of Hereinafter, the two-layer structure will be described in detail.

  When the porous hollow fiber membrane has the above two-layer structure, the thickness of the outer layer described above is not particularly limited, but is preferably 1 μm or more and 100 μm or less from the viewpoint of complementation when there is a defect, more preferably It is 5 μm or more and 50 μm or less.

  When the porous hollow fiber membrane has the above two-layer structure, the thickness of the inner layer described above is not particularly limited, but is preferably 20 μm or more and 1000 μm or less from the viewpoint of maintaining strength and ensuring water permeability. Is from 100 μm to 500 μm.

  Thus, when the porous hollow fiber membrane has a two-layer structure, it is preferable that the cross-sectional structure has a three-dimensional network structure from the surface (A) of the outer layer to the surface (B) of the inner layer. Further, it is more preferable that the outer layer has a pore diameter that continuously increases from the surface (A) of the outer layer toward the boundary with the surface (B) of the inner layer.

  When the porous hollow fiber membrane has the above two-layer structure, the pore diameter of the outer layer having the surface (B) is preferably 0.05 μm or less in order to eliminate micropathogens.

  In the case where the porous hollow fiber membrane has the above two-layer structure, the average thickness of the trunk constituting the inner layer having the surface (B) is 0. It is preferably 5 to 10 μm and the pore diameter is 0.1 μm or more and 1 μm or less. More preferably, the average thickness of the trunk constituting the inner layer is not less than 0.5 μm and not more than 5 μm, and the pore diameter is not less than 0.3 μm and not more than 0.7 μm.

  The thermoplastic resin constituting the porous hollow fiber membrane includes polyolefins such as polyethylene and polypropylene, polyvinylidene fluoride, ethylene-vinyl alcohol copolymer, polyamide, polyetherimide, polystyrene, polysulfone, polyvinyl alcohol, and polyphenylene. Examples include ether, polyphenylene sulfide, cellulose acetate, polyacrylonitrile, and the like. Among these, PVDF resin, polysulfone, polyethylene and the like are preferable from the viewpoint of water permeability, blocking performance against micropathogens, chemical resistance, etc., and polyvinylidene fluoride is particularly preferable in view of chemical durability such as chemical resistance. preferable.

  When the porous hollow fiber membrane has the above two-layer structure, in order to suppress peeling of the two layers, the outer layer having the inner layer having the surface facing the hollow part (B) and the outer surface facing the surface (A), Each preferably has a three-dimensional network structure. Here, the three-dimensional network structure of the inner layer and the outer layer can be confirmed with a field of view of 500 to 10,000 times using an electron microscope. Thus, when each porous hollow fiber membrane has a three-dimensional network structure, in order to suppress separation of the two layers, the pore diameter of the outer layer may continuously increase from the outer layer surface toward the boundary with the inner layer surface. More preferred. On the other hand, the pore size distribution of the inner layer is more preferably isotropic because the strength is easily maintained.

  As for the porous hollow fiber membrane of this embodiment, that whose compressive strength is 0.8 Mpa or more is more preferable. When the compressive strength is less than 0.8 MPa, the membrane tends to be crushed in the course of repeated operation of filtration backwashing, so that the durability tends to decrease. In addition, in this specification, the compressive strength of a porous hollow fiber membrane means the value measured by the method as described in the Example mentioned later.

And the porous hollow fiber membrane of this embodiment carries out air bubbling by introduce | transducing the air of the flow volume of 1 Nm < ³ > / h from a through-hole at a frequency of 1 minute in 30 minutes to the hollow part of a porous hollow fiber membrane, It is preferable that the inner layer and the outer layer in the porous hollow fiber membrane do not peel after operating for 30 days.

  The porous hollow fiber membrane having the above-mentioned two-layer structure, for example, discharges the polymer stock solution constituting the inner layer and the polymer stock solution constituting the outer layer simultaneously from a triple-structure spinning nozzle, and the outer layer is phase-separated by a non-solvent induced phase separation method. The inner layer can be obtained by phase separation by a thermally induced phase separation method. Hereinafter, the example using PVDF resin as a polymer is explained in full detail.

  The porous hollow fiber membrane of this embodiment can be obtained by discharging a membrane-forming stock solution containing at least a PVDF resin and a solvent and advancing phase separation. Here, the desired phase separation can be advanced by adjusting the discharge temperature of the film-forming stock solution and the temperature of the coagulating liquid brought into contact with the film-forming stock solution.

  Specifically, the discharge temperature of the polymer stock solution constituting the outer layer and the discharge temperature of the polymer stock solution constituting the inner layer are not particularly limited, but are in the range of 200 to 280 ° C. in order to generate α-type structural crystals. Is more preferable, and the range of 200 to 260 ° C. is more preferable.

  In this example, the membrane forming stock solution contains a PVDF resin as a polymer component constituting the porous hollow fiber membrane. Here, the PVDF resin means a homopolymer of vinylidene fluoride or a copolymer containing 50% or more of vinylidene fluoride in a molar ratio. The PVDF resin is preferably a homopolymer from the viewpoint of obtaining a porous hollow fiber membrane having excellent strength. Further, when the PVDF resin is a copolymer, other copolymer monomers copolymerized with the vinylidene fluoride monomer can be appropriately selected from known ones and are not particularly limited. For example, a fluorine-based monomer or a chlorine-based monomer can be preferably used.

  The weight average molecular weight (Mw) of the PVDF resin is not particularly limited, but is preferably 100,000 or more and 1,000,000 or less, and more preferably 200,000 or more and 700,000 or less. By setting the weight average molecular weight to 200,000 or more and 1,000,000 or less, the viscosity of the film-forming stock solution can be controlled within a range suitable for spinning. In addition, by using a PVDF resin having a weight average molecular weight of 700,000 or less, substitution (exchange) between a solvent and a non-solvent proceeds quickly, and a separation layer with higher chemical resistance can be formed.

  The film-forming stock solution contains a solvent. The solvent means a solvent capable of dissolving 1% by weight or more of PVDF resin at 80 ° C. within one week. On the other hand, a solvent that can dissolve the PVDF resin at 80 ° C. within one week is defined as a non-solvent.

Here, Hansen's solubility parameter (hereinafter also referred to as “HSP”) used as a tool for estimating the affinity between the resin and the solvent or non-solvent will be described in detail. HSP is a solubility index indicating how much a certain resin dissolves in a solvent or non-solvent, and the solubility characteristics are expressed in three-dimensional coordinates, and the distance (hereinafter also referred to as “HSP distance”). Those close to each other are judged to have high solubility. Here, the three-dimensional coordinate axis is represented by a dispersion term dD, a polarity term dP, and a hydrogen bond term dH. The dispersion term dD is the van der Waals force, the polar term dP is the dipole moment force, and the hydrogen bond term dH is the hydrogen bond force of water, alcohol, or the like. Specifically, the solvent or non-solvent of the PVDF resin is calculated by calculating the HSP distance from the solubility parameter of the PVDF resin to the solubility parameter of the solvent or non-solvent on the three-dimensional coordinates with dD, dP, and dH as axes. Can be evaluated. The formula for calculating the HSP distance is shown below.
HSP distance = [4 × (dDPVDF-dD solvent) 2+ (dPPVDF-dP solvent) 2 + 2 + (dHPVDF-dH solvent) 2] 0.5

  Thermodynamically, the closer the HSP distance is to 0, the higher the compatibility between the resin and the solvent or non-solvent. Computer software HSPiP developed by Hansen and Abbott includes a database describing the function of calculating the HSP distance and Hansen parameters for various resins and solvents or non-solvents. The solubility parameter (SP value) used in this specification refers to a value obtained by correcting the value described in the Solvent list and Polymer list included in the HSPiP 3rd version to 25 ° C. Here, resins and solvents or non-solvents not described in the Solvent list are calculated by an estimation method using a neural network method called Y-MB. By inputting the molecular structure into Y-MB calculation software attached to HSPiP, it is automatically decomposed into atomic groups, and the HSP value and molecular volume are calculated.

  In the present embodiment, in the production of a porous hollow fiber membrane by the NIPS method, a porous hollow fiber membrane having a separation layer with improved chemical resistance is produced by increasing the phase separation speed. More specifically, a PVDF resin and a solvent having an arbitrary affinity for a non-solvent are selected, and NIPS phase separation is induced in a coagulation liquid containing an appropriate non-solvent, whereby the phase separation rate is increased. The speed is increased, thereby forming a separation layer having high blocking performance.

  By selecting a solvent having a low compatibility with the PVDF resin as the solvent contained in the film-forming stock solution, the phase separation speed can be further increased. From this viewpoint, it is preferable that the HSP distance between the PVDF resin and the solvent is 5.20 or more and 8.00 or less. By selecting a solvent having an HSP distance of 5.20 or more with the PVDF resin, the affinity with the PVDF resin is lowered, and the solvent is easily diffused into the coagulation bath. It is possible to further improve the chemical resistance of the. In addition, by selecting a solvent having an HSP distance with the PVDF resin of 8.00 or less, the formation temperature of the porous structure of the film-forming stock solution can be lowered, and the temperature of the coagulation liquid can be set low. The difference in temperature between the membrane forming stock solution and the low-temperature coagulating liquid accelerates the substitution (exchange) of the solvent and non-solvent, and increases the phase separation speed, so that the chemical resistance of the separation layer can be further improved. It becomes.

Specific examples of the solvent, Glycerol Diacetate, Ethyl Lactate, Diethylene Glycol Monobutyl Ether, gamma-Butyrolactone (GBL), Methylene Chloride, Methyl Acetate, Dimethyl Isosorbide, 1,3-Dioxolane, Dibasic Esters (DBE), Dipropylene Glycol Mono n -Butyl Ether, Sulfolane (Tetramethylene Sulphone), Methyl Ethyl Ketone (MEK), Ethylene Glycol Monomethyl Ether, Cyclohexone, Propylene lycol Monomethyl Ether, Isophorone, Caprolactone (Epsilon), Tetrahydrofuran (THF), Propylene Glycol Phenyl Ether, other single solvent such as Texanol, a mixed solvent obtained by mixing them at an arbitrary ratio can be mentioned. In addition, about the solubility parameter of a mixed solvent, the parameter calculated by the addition law based on the weight ratio (a: b) of a solvent like the following formula is used.
[DD mixed solvent, dP mixed solvent, dH mixed solvent] = [(a × dD solvent 1 + b × dD solvent 2), (a × dP solvent 1 + b × dP solvent 2)) (a × dH solvent 1 + b × dH solvent 2) ] / (A + b)

  In addition to the PVDF resin and the solvent described above, the film-forming stock solution may contain a non-solvent and an additive. By blending the additive, the average pore diameter on the membrane surface can be easily controlled. For example, when a film-forming solution containing an additive is mixed with a coagulating liquid containing a non-solvent to cause NIPS phase separation, or after coagulating PVDF resin, the additive is eluted. A film surface having a desired average pore diameter can be formed according to the type and blending amount of the agent or the film forming conditions. Such additives are not particularly limited, and organic compounds, inorganic compounds, and the like are used. As the organic compound, those that are soluble in both the solvent used when dissolving the PVDF resin and the non-solvent used when causing the NIPS phase separation are preferably used. For example, water-soluble polymers such as polyvinylpyrrolidone, polyethylene glycol, polyethyleneimine, polyacrylic acid, and dextran, surfactants, glycerin, saccharides, proteins, and the like can be given. As the inorganic compound, those that dissolve in both the solvent used when dissolving the PVDF resin and the non-solvent used when causing the NIPS phase separation are preferable. For example, calcium chloride, magnesium chloride, lithium chloride, barium sulfate, etc. Can be mentioned.

  In particular, an embodiment in which the polymer in the polymer stock solution constituting the outer layer is PVDF resin and the good solvent for dissolving PVDF resin is sulfolane is preferable. Moreover, the aspect in which the polymer stock solution which comprises an inner layer contains PVDF resin, an organic liquid, and a silica is preferable.

  The content ratio of each component in the membrane-forming stock solution is not particularly limited, but from the viewpoint of separation performance, water permeability, and strength, the PVDF resin is 5% by weight or more and 35% by weight or less with respect to the total amount of the film-forming stock solution. The solvent is preferably 20 wt% or more and 95 wt% or less, the additive is 0 wt% or more and 40 wt% or less, and the non-solvent is preferably 0 wt% or more and 5 wt% or less. When the content ratio of the PVDF resin is 5% by weight or more, the strength of the separation layer tends to be further increased. Moreover, when the content ratio of the PVDF resin is 35% by weight or less, it becomes easy to adjust the film-forming stock solution to a viscosity suitable for spinning, and the separation layer is suppressed from being densified, and the water permeability is further improved. There is a tendency. Moreover, since there exists a possibility of causing gelatinization of a film forming undiluted solution for a long term, the content rate of a non-solvent has the preferable range of 0 to 5 weight%.

  The method for preparing the film-forming stock solution is not particularly limited, and can be performed by any method. For example, a film-forming stock solution can be prepared by mixing the PVDF resin described above with an additive and a non-solvent in a solvent and stirring. Here, the temperature at which the PVDF resin is dissolved in the solvent is preferably equal to or higher than the melting point (Tm) of the PVDF resin and lower than the boiling point of the solvent. The melting point (Tm) of the PVDF resin is a temperature at which the molecular chain in the crystal region starts to move with the movement of the center of gravity, and is a specific value depending on the chemical structure of the PVDF resin. The melting point (Tm) of the PVDF resin can be determined by differential scanning calorimetry (DSC) as will be described later. Thus, when mixed with the solvent at a melting point (Tm) or higher of the PVDF resin, the polymer crystals are loosened and can be quickly dissolved in a solvent having a low affinity with the polymer. The upper limit of the melting temperature of the PVDF resin is the boiling point of the solvent. However, from the viewpoint of safe treatment by avoiding the danger of flammable explosion of the solvent, the upper limit of the melting temperature of the PVDF resin is preferably 10 ° C lower than the boiling point. Yes. Moreover, it is preferable to deaerate the film-forming stock solution using a vacuum pump during or after dissolution. By passing through the deaeration process, a stable stock solution with few impurities and few bubbles can be obtained.

  The membrane-forming stock solution thus obtained is ejected from a molding nozzle such as a T die or a hollow nozzle and brought into contact with a coagulating liquid containing at least a non-solvent, and a porous hollow fiber membrane is produced by advancing the phase separation phenomenon. The This porous hollow fiber membrane can be formed by a so-called wet film forming method in which a film-forming stock solution is extruded from a molding nozzle and coagulated in a coagulating liquid, or a predetermined empty space after the film-forming stock solution is extruded from a molding nozzle. Any of the so-called dry and wet film forming methods in which a running section is secured and then solidified in a coagulating liquid can be applied. In addition, the discharge shape of the film-forming stock solution is not particularly limited, and can be arbitrarily set to, for example, a flat film shape, a tube shape, a hollow fiber shape, or the like. The film thickness of the porous hollow fiber membrane to be produced is not particularly limited, but is preferably in the range of 25 μm to 500 μm. By setting the film thickness to 25 μm or more, the outflow of micropathogens due to the breakage of the film can be more reliably prevented. In addition, by setting the film thickness to 500 μm or less, it is possible to suppress the formation of a sponge-like structure with poor communication, and thus the water permeability of the film can be significantly improved.

  The coagulation liquid described above contains at least a non-solvent. By selecting a non-solvent contained in the coagulation liquid that has a high affinity for the solvent that dissolves the PVDF resin, the phase separation speed is increased, and the chemical resistance of the membrane can be further enhanced. From this viewpoint, it is preferable that the non-solvent dissolves 100 mg or more of the solvent. In addition, the non-solvent to be used is not limited to one type, and may be a mixture of two or more types of non-solvents.

  Specific examples of the non-solvent include single non-solvents such as water, glycerin, methanol, ethanol, and 2-propanol, and mixed non-solvents obtained by mixing these at an arbitrary ratio.

  In the method for producing a porous hollow fiber membrane of the present embodiment, the temperature of the coagulating liquid is preferably higher than the starting temperature for forming the porous structure of the membrane forming stock solution. When the temperature of the coagulation liquid is equal to or lower than the porous structure formation start temperature of the film-forming stock solution, the heat-induced phase separation tends to progress and a thick and uniformly dense separation layer tends to be generated. Here, in this specification, the porous structure formation start temperature of the film-forming stock solution is the heat-induced phase separation start temperature of the film-forming stock solution, and this porous structure formation temperature is measured using an optical microscope with a temperature controller. The film forming stock solution can be obtained by observing the porous structure formed in the process of heating to the temperature at the time of discharge, holding for 2 minutes to dissolve, and cooling at 100 ° C./min.

  Conventionally, in order to make hollow fiber membranes have high water permeability, if the pore diameter or the openability ratio is increased, the polymer trunk becomes thin, so the compression strength of the hollow fiber membrane could not be increased. It has been found that the compression strength can be increased by thickening the polymer trunk in the longitudinal direction of the thread membrane. Since the thickness of the polymer trunk in the longitudinal direction of the hollow fiber membrane is considered to strongly affect the compression strength of the hollow fiber membrane, it is estimated that the compression strength of the hollow fiber membrane is increased by increasing the thickness of the stem. Is done. The thickness of the trunk constituting the surface (B) of the inner layer is preferably 0.5 μm or more and 10 μm or less. By setting the thickness of the trunk to 0.5 μm or more, the compression resistance can be increased. In addition, by setting the thickness of the trunk to 10 μm or less, water permeability does not deteriorate due to the trunk becoming thicker and the hole becoming smaller. More preferably, they are 1.5 micrometers or more and 8 micrometers or less, More preferably, they are 2 micrometers or more and 6 micrometers or less. On the other hand, the thickness of the trunk constituting the surface (A) of the outer layer is preferably 0.01 to 0.5 μm, more preferably 0.01 μm to 0.3 μm, and still more preferably 0.01 μm to 0 μm. .1 μm or less.

  In this specification, the measurement of the thickness of a trunk means what was computed with the following method. First, a scanning electron microscope is used to photograph the outer surface of the hollow fiber membrane from a direction perpendicular to the outer surface at a magnification that allows the shape of a large number of holes to be clearly confirmed as much as possible. Next, 100 distances between the holes in the direction perpendicular to the longitudinal direction of the hollow fiber membrane are measured, and the arithmetic average is taken as the thickness of the trunk.

  Depending on the solvent used and the molecular weight of the thermoplastic resin, the viscosity of the film-forming solution may be lowered and the desired film shape may not be maintained when discharging the film-forming solution above the melting point of the raw thermoplastic resin. . In such a case, it can be held in an arbitrary shape by discharging the form-retaining fluid simultaneously with the film-forming stock solution. As such a shape-retaining fluid, a well-known fluid commonly used in this type of field can be used as appropriate, and is not particularly limited. For example, in addition to glycerin, glycol ethers such as tetraethylene glycol and ethylene glycol, polyvinyl Examples thereof include a material to which a thickener such as pyrrolidone is added, and a high-viscosity fluid in which a component containing a polymer is dissolved in a solvent. The shape-retaining fluid preferably has a viscosity of 1.0 Pa · s to 5,000 Pa · s. When the viscosity of the shape-retaining fluid is 1 Pa · s or more, mixing of the film-forming stock solution and the shape-retaining fluid can be suppressed, and the shape-retaining fluid can be easily retained in any shape. Further, when the viscosity of the shape-retaining fluid is 5000 Pa · s or less, the spinnability when discharged simultaneously with the film-forming stock solution is improved. The viscosity of the shape-retaining fluid described above is a value measured under the condition of 20 ° C. using a single cylindrical rotational viscometer based on the viscosity measurement method of “JIS K7117-1”. Further, the shape-retaining fluid preferably has a boiling point and a decomposition temperature that are lower than the discharge temperature of the film-forming stock solution. When the boiling point of the shape-retaining fluid is lower than the discharge temperature, the shape-retaining fluid does not boil at the time of discharge, and bubbles are not mixed into the porous hollow fiber membrane, so that pinholes can be prevented from occurring. Moreover, when the decomposition temperature of the shape retention fluid is lower than the discharge temperature, the shape retention fluid does not burn, and the strength of the resulting porous hollow fiber membrane does not decrease. In general, when a thermally induced phase separation method is used with PVDF as a raw material, the proportion of α-type structure crystals is increased, which is convenient for maintaining the strength of the film. Specifically, using a solvent that dissolves PVDF resin at room temperature, when the raw film forming solution is discharged at a temperature equal to or higher than the melting point of the PVDF resin and discharged to a spinning bath below the melting point, the form of phase separation is non-solvent induced. It goes through a solidification process due to the middle of heat induction. In such a phase separation form, it is possible to achieve both α-structure crystals having strong chemical resistance while maintaining the fine pore diameter which is an advantage of the non-solvent induced phase separation method. Further, when the ink is discharged at a temperature exceeding 200 ° C., the tendency is further increased, and sufficient chemical resistance can be imparted. The upper limit of the temperature is the decomposition temperature of the thermoplastic resin.

When it is desired to further increase the membrane strength of the porous hollow fiber membrane, for example, a multi-tubular nozzle such as a hollow fiber molding nozzle having two or more annular discharge ports is used, and one of the two adjacent annular discharge ports is used. From the melt-kneaded material containing at least polyvinylidene fluoride resin, organic liquid and inorganic fine powder, and from the other, the film-forming stock solution is discharged, and is higher than the porous structure formation temperature of the film-forming stock solution, It is preferable that a multilayer porous hollow fiber structure is formed by contact with a coagulation liquid lower than the porous structure formation start temperature, and the organic liquid is extracted and removed from the obtained multilayer porous hollow fiber structure. By extracting and removing the organic liquid from the obtained multilayer porous hollow fiber structure, a porous hollow fiber membrane having extremely high physical strength, high water permeability, and good adhesion at the layer interface can be obtained. . Here, in forming the multilayer kneaded hollow fiber structure with the melt-kneaded product, the solvent contained in the film forming stock solution preferably has an HSP distance of 5.20 or more and 8.00 or less with the PVDF resin. When the solvent contained in the film-forming stock solution has a HSP distance of 5.20 or more with the PVDF resin, the solvent easily dissolves PVDF, so that the above-mentioned film-forming stock solution is directed toward the melt-kneaded product. A structure in which the viscosity of the melt-kneaded product can be prevented from drastically decreasing due to the immersion of the solvent, and as a result, the phase separation rate of the melt-kneaded product does not become too fast, and the structure has good communication even in the vicinity of the joint between the two layers. Is finally maintained, and a porous membrane having high pure water permeation performance and high physical strength can be obtained. In addition, when the difference in HSP distance between the PVDF resin and the solvent is 8.00 or less as the solvent contained in the film-forming stock solution, the solvent easily penetrates into the melt-kneaded material, so that the adhesiveness at the bonding interface As a result, a porous film having a high physical strength such as prevention of peeling of two layers during actual use can be obtained.
Further, as an example of the inner layer, there is a thermally induced phase separation method which is a suitable manufacturing method of a three-dimensional network sponge structure. Thermally induced phase separation is a method in which a thermoplastic polymer is mixed with a latent solvent that is a non-solvent near room temperature but becomes a solvent at a high temperature (higher than the compatible temperature). And then cooled to below the solidification temperature of the thermoplastic polymer, taking advantage of the decrease in the dissolving power of the latent solvent in the thermoplastic polymer during the cooling process. This is a method of separating a dilute phase (solvent rich phase) and then extracting and removing a latent solvent to obtain a porous body composed of a solidified polymer rich phase formed during phase separation (H. Matsuyama, Chemical Engineering, 43 (1998) 453-464, or DR Lloyd, et al., Journal of Membrane Science, 64 (1991). Such as -11). In addition to the thermoplastic polymer and its potential solvent, an inorganic filler such as finely divided silica is added and mixed by heating, and the porous filler is obtained by extracting and removing the inorganic filler together with the potential solvent in the extraction step after cooling and solidification. The method is also included as a kind of thermally induced phase separation method. Examples of potential solvents include when the thermoplastic polymer is polyethylene, polypropylene, polyvinylidene fluoride, such as dibutyl phthalate, dihexyl phthalate, dioctyl phthalate, di (2-ethylhexyl) phthalate, diisodecyl phthalate, etc. Examples thereof include phthalic acid esters and mixtures thereof.
One suitable method for obtaining a porous hollow fiber membrane using a heat-induced phase separation method is to extrude a membrane material polymer that is a thermoplastic polymer and its potential solvent (and inorganic filler if necessary). A hollow fiber molding spout (annular hole for extruding the heated mixture on the extrusion surface, and for discharging the hollow portion forming fluid inside the annular hole. This is a method of extruding the melt from a nozzle provided with a circular hole) into a hollow fiber while injecting a hollow portion forming fluid into the hollow portion to cool and solidify, and then extracting and removing the potential solvent (and inorganic filler). . The hollow portion forming fluid is injected into the hollow portion so that the hollow portion of the hollow fiber-like extrudate is not crushed and closed during cooling and solidification, and is substantially inert to the extruded melt (chemically). Use gas or liquid that does not change. Cooling and solidification after extrusion can be performed by air cooling, liquid cooling, or a combination of both. The gas or liquid that is the cooling medium is required to be substantially inert to the extrudate. The extraction of the latent solvent (or inorganic filler) is carried out using a volatile liquid or aqueous solution that is substantially inert to the cooled solidified product and has excellent dissolving power for the latent solvent (or inorganic filler).
Examples of a preferable method for producing a porous hollow fiber membrane having a high outer surface opening ratio and a three-dimensional network sponge structure include the following method using a thermally induced phase separation method. In addition to the membrane material polymer and its potential solvent, an inorganic filler is added and mixed by heating, and the inorganic filler is extracted and removed together with the potential solvent by extraction after cooling and solidification. As the inorganic filler, finely divided silica having an average primary particle size of 0.005 μm to 0.5 μm and a specific surface area of 30 m ² / g to 500 m ² / g is preferable. Such finely divided silica has good dispersibility during heating and mixing, so that structural defects are hardly generated in the obtained film, and extraction and removal can be easily performed with an alkaline aqueous solution. The amount of the membrane material polymer at the time of heating and mixing is 15% by weight to 25% in the case of a material having a specific gravity of about 1 g / cm ³ , such as polyethylene and polypropylene, from the viewpoint of the balance between the strength of the obtained membrane and the opening property. In the case of a material having a specific gravity of about 1.7 g / cm 3, such as polyvinylidene fluoride, 25% by weight to 45% by weight, which is about 1.7 times that of a material having a specific gravity of 1, is preferable. Further, the weight ratio of the latent solvent / fine silica is preferably 1.0 or more and 2.5 or less, particularly preferably 1.2 or more and 1.8 or less, from the viewpoint of the balance between the strength of the obtained film and the opening property.
The preferred amount of the high molecular weight of the membrane material for heating and mixing is 15% by weight to 35% in the case of a material having a specific gravity of about 1 g / cm ³ , such as polyethylene and polypropylene, from the viewpoint of the balance between strength and openability of the obtained membrane. In the case of a material having a specific gravity of about 1.7 g / cm ³ , such as weight% and polyvinylidene fluoride, the amount is about 1.7 times that of the material having a specific gravity of 1 to 25% by weight to 60% by weight.

  Hereinafter, although an example is given and the present invention is explained in detail, the present invention is not limited to these.

  (1) Weight average molecular weight (Mw) of PVDF resin, (2) Melting point (Tm) of PVDF resin, (3) Water permeability of porous hollow fiber membrane, (4) Porous hollow fiber in Examples and Comparative Examples Dextran removal rate of membrane, (5) Chemical resistance of porous hollow fiber membrane, (6) Defect occurrence rate of porous hollow fiber membrane, (7) Compressive strength, (8) Peelability evaluation are as follows. Measurement and evaluation were performed.

(1) Weight average molecular weight (Mw) of PVDF resin
Using 50 mL of a sample obtained by dissolving PVDF resin in DMF at a concentration of 1.0 mg / mL, GPC measurement was performed under the following conditions, and the weight average molecular weight (PMMA conversion) was obtained.
Apparatus: HPLC-8220GPC (Tosoh Corporation)
Column: Shodex KF-606M, KF-601
Mobile phase: 0.6 mL / min DMF
Detector: Differential refractive index detector

(2) Melting point (Tm) of PVDF resin
15 mg of PVDF resin was sealed in a sealed DSC container and heated at a rate of temperature increase of 10 ° C./min using a DSC-6200 manufactured by Seiko Electronics Co., Ltd. The peak of the melting peak observed during this temperature increase process was the melting point of the PVDF resin. (Tm).

(3) Permeability test of porous hollow fiber membrane (L / (m ² · hr))
Seal one end of a wet hollow fiber membrane of about 10 cm length, put an injection needle into the hollow part at the other end, and inject pure water at 25 ° C. into the hollow part at a pressure of 0.1 MPa from the injection needle. The amount of pure water permeated into the water was measured, and the pure water permeability was determined from the following equation.
Pure water permeability (L / (m ² · hr)) = (60 (min / hr) × water permeability (L)) / (π × membrane inner diameter (m) × membrane effective length (m) × measurement time (min ))

(4) Dextran removal rate of porous hollow fiber membrane (%)
An injection needle is inserted into the hollow part at both ends of the wet hollow fiber membrane, and dextran having a molecular weight of 2 million (Dextran 2000, manufactured by Amersham bioscience) 1 under the conditions of a temperature of 25 ° C., a filtration differential pressure of 50 kPa, and a membrane surface linear velocity of 0.5 m / s. An external pressure cross-flow filtration of a 1,000 ppm aqueous solution was performed for 25 minutes. Next, the concentration of dextran in the initial raw water and the filtrate collected after 25 minutes of filtration was determined using a suggested refractometer. Finally, the dextran removal rate with a molecular weight of 2 million was determined from the following formula.
Removal rate (%) = (1- (Dextran concentration after completion of filtration for 25 minutes) / (Dextran concentration in initial raw water)) × 100

(5) Chemical resistance test of porous hollow fiber membrane (5th day pore diameter retention rate (%))
As a chemical resistance test of the porous hollow fiber membrane, the evaluation was performed based on the 5th day pore size retention rate (%). The 5th day pore size retention rate (%) was determined by measuring the wet porous hollow fiber membrane with water at 40 ° C. Immerse in a mixed aqueous solution of 4% by weight of sodium oxide and 0.5% by weight of sodium hypochlorite at an effective chlorine concentration for 5 days, and remove dextran with a molecular weight of 2 million (5) before and after immersion in the chemical solution. The dextran removal rate (E0) before immersion and the dextran removal rate (E5) after chemical degradation by immersion for 5 days were substituted into the following formula.
Day 5 pore size retention (%) = (E5 / E0) × 100

(6) Number of defects generated in porous hollow fiber membrane One end of a wet hollow fiber membrane having a length of about 1 m is sealed, an injection needle is inserted into the hollow part at the other end, and air is blown from the injection needle at a pressure of 0.2 MPa. The number of defects generated was determined by injecting into the hollow part and measuring the number of locations where bubbles emerge from the outer surface.
(7) Compressive strength (MPa)
One end of a wet hollow fiber membrane having a length of about 5 cm was sealed, the other end was opened to the atmosphere, pure water at 40 ° C. was pressurized from the outer surface, and permeate was discharged from the open end of the atmosphere. At this time, the whole amount was filtered without circulating the membrane feed water, that is, the whole amount filtration method was adopted. The pressurization pressure was increased from 0.1 MPa in increments of 0.01 MPa, the pressure was maintained at each pressure for 15 seconds, and the permeated water coming out from the open end of the atmosphere was sampled during the 15 seconds. While the hollow part of the hollow fiber membrane is not crushed, the absolute value of the permeated water amount (mass) also increases as the pressurized pressure increases. However, when the pressurized pressure exceeds the compressive strength of the hollow fiber membrane, the hollow part is crushed. Since the clogging starts, the absolute value of the permeated water amount decreases even though the pressurizing pressure increases. The compression pressure at which the absolute value of the amount of permeated water was maximized was taken as the compressive strength.
(8) Peelability evaluation 200 wet hollow fiber membranes having a length of about 1 m were bundled and inserted into a 1-inch diameter PVC pipe, and both ends were bonded and fixed. Polyurethane was used as the adhesive, and the adhesive layer thickness was 50 mm. At this time, one end of the hollow fiber membrane was sealed to provide five 10 mm-diameter holes (penetrating ports) penetrating the bonded portion, and the other end was opened to produce a membrane module. The river water was taken out from the opening end by introducing river water from the through hole and applying water pressure in the case. The water pressure at this time was kept constant at 0.2 MPa. Further, air bubbling was performed by introducing air at a flow rate of 1 Nm ³ / h from the through-hole at a frequency of 1 minute every 30 minutes. In order to evaluate the peelability of the (A) layer and the (B) layer after 30 days of operation, the presence or absence of a peeled portion was confirmed by applying an air pressure of 0.1 MPa from the filtered water side. When there was a peeled portion, the peelability was “present”, and when there was no peeled portion, the peelability was “no”.
(9) Measurement of ratio of α-type structure crystal and β-type structure crystal in crystal part of PVDF resin IR spectrum of the film surface was measured by ATR method with a resolution of 8 cm ⁻¹ using Nicoletti S50 / Continuum of Thermo Scientific Co., Ltd. .
From the peak height (H _α ) of the signal of the α-type structure crystal appearing at 763 cm ^{−1 in} the obtained spectrum and the peak height (H _β ) of the signal of the β-type crystal appearing at 840 cm ⁻¹ , Was used to calculate the ratio of α-type structure crystals and β-type structure crystals.
α type / β type structure crystal ratio = H _α / H _β

[Example 1]
A porous hollow fiber membrane of Example 1 having a membrane structure of a two-layer hollow fiber membrane having an outer layer and an inner layer was obtained using a triple-structure spinning nozzle. In the outer layer, vinylidene fluoride homopolymer having a weight average molecular weight of 350,000 as PVDF resin (Arkema, Kynar 741) 24% by weight, polyvinyl pyrrolidone having a K value of 17 (BASF, rubiscol K17) 17% by weight, Sulfolane (manufactured by Wako Pure Chemical Industries, Ltd.) 59% by weight was mixed and dissolved at 180 ° C. to prepare a film forming stock solution. The melting point (Tm) of this vinylidene fluoride homopolymer was 174 ° C., the decomposition temperature was 375 ° C., and the porous structure formation starting temperature of this film-forming stock solution was 42 ° C.
The inner layer is composed of 34% by weight of vinylidene fluoride homopolymer (Solef 6020, manufactured by Solvay Solexis), 33.8% by weight of bis (2-ethylhexyl) phthalate, 6.8% by weight of dibutyl phthalate, and 25.4% fine silica. The melt-kneaded material of the weight%, air was used as the hollow part forming fluid, and these were used as triple ring spinning nozzles (outer layer outermost diameter 2.0 mm, inner layer outermost diameter 1.8 mm, hollow part forming layer outermost diameter 0.9 mm) was simultaneously discharged at 200 ° C. to obtain a hollow fiber molded product.
The extruded hollow fiber-shaped molded article passed through a free running distance of 80 mm, and then proceeded with non-solvent-induced phase separation in water at 60 ° C., and was wound up skein at a speed of 20 m / min. The obtained two-layer hollow fiber extrudate was immersed in isopropyl alcohol to extract and remove bis (2-ethylhexyl) phthalate and dibutyl phthalate. Subsequently, it was immersed in water for 30 minutes to swell the hollow fiber membrane. Subsequently, it was immersed in a 20 wt% NaOH aqueous solution at 70 ° C. for 1 hour, and further washed with water to extract and remove finely divided silica.
Table 1 shows the composition, production conditions, and various performances of the porous hollow fiber membrane of Example 1 obtained.

[ Reference example ]
A porous hollow fiber membrane of Example 2 having a membrane structure of a two-layer hollow fiber membrane having an outer layer and an inner layer was obtained using a triple-structure spinning nozzle. Here, 20% by weight of PVDF homopolymer, 17% by weight of polyvinyl pyrrolidone having a K value of 17 and 63% by weight of Triacetin (manufactured by Aldrich, 99%) are mixed and dissolved at 200 ° C. The membrane forming stock solution was used. The melting point (Tm) of this vinylidene fluoride homopolymer was 174 ° C., the decomposition temperature was 375 ° C., and the porous structure formation starting temperature of this film-forming stock solution was 102 ° C.
The inner layer, spinning conditions, post-treatment conditions, and the like were the same as in Example 1, and a porous hollow fiber membrane (porous hollow fiber membrane) of a reference example was obtained.
Table 1 shows the composition, production conditions, and various performances of the porous hollow fiber membrane of the obtained reference example .

[Comparative Example 1]
A porous hollow fiber membrane of Comparative Example 1 having a membrane structure of a two-layer hollow fiber membrane having an outer layer and an inner layer was obtained using a spinning nozzle having a triple structure. Here, the same PVDF polymer 24% by weight as in Example 1, 17% by weight of K17 polyvinylpyrrolidone and 59% by weight of sulfolane (manufactured by Wako Pure Chemical Industries, Ltd.) are mixed and dissolved at 180 ° C. Used. The melting point (Tm) of this vinylidene fluoride homopolymer was 174 ° C., the decomposition temperature was 375 ° C., and the porous structure formation starting temperature of this stock solution was 42 ° C. Further, as a film forming stock solution for the inner layer, a melt kneaded material of 34% by weight of vinylidene fluoride homopolymer (Solef 6020, manufactured by Solvay Solexis) and 66% by weight of gamma butyrolactone was used. Furthermore, air was used as the hollow portion forming fluid. By discharging these simultaneously from a triple ring spinning nozzle (outer layer outermost diameter 2.0 mm, inner layer outermost diameter 1.8 mm, hollow part forming layer outermost diameter 0.9 mm) at a temperature of 200 ° C., Obtained.
The extruded hollow fiber-shaped molded article passes through a free running distance of 80 mm, and then phase separation proceeds in water at 23 ° C., and the inner layer advances phase separation in the free running part and water, and is skeined at a speed of 20 m / min. Rolled up. The obtained two-layer hollow fiber extrudate was sufficiently washed with water to remove the solvent component.
In Table 1, the compounding composition of the porous hollow fiber membrane of the obtained comparative example 1, manufacturing conditions, and various performance are shown.
[Comparative Example 2]
A porous hollow fiber membrane of Comparative Example 2 having a membrane structure of a two-layer hollow fiber membrane having an outer layer and an inner layer was obtained using a spinning nozzle having a triple structure. Here, as a film forming stock solution for the outer layer, vinylidene fluoride homopolymer having a weight average molecular weight of 350,000 (Kynar 741 manufactured by Arkema Co., Ltd.) 24% by weight, polyvinyl pyrrolidone having a K value of 17 (manufactured by BASF, Rubiscor K17) 17 Weight% and 59% by weight sulfolane (manufactured by Wako Pure Chemical Industries) were mixed and dissolved at 180 ° C. The melting point (Tm) of this vinylidene fluoride homopolymer was 174 ° C., the decomposition temperature was 375 ° C., and the porous structure formation starting temperature of this film-forming stock solution was 42 ° C. In addition, as a film forming stock solution for the inner layer, 34% by weight of vinylidene fluoride homopolymer (Solef 6020 manufactured by Solvay Solexis), 33.8% by weight of bis (2-ethylhexyl) phthalate, 6.8% by weight of dibutyl phthalate A melt kneaded product of 25.4% by weight of finely divided silica was used. Furthermore, air was used as the hollow portion forming fluid. By discharging these from a triple ring spinning nozzle (outer layer outermost diameter 2.0 mm, inner layer outermost diameter 1.8 mm, hollow part forming layer outermost diameter 0.9 mm) at 180 ° C. at the same time, Obtained.
The extruded hollow fiber-shaped molded article passed through a free running distance of 80 mm, and then proceeded with non-solvent-induced phase separation in water at 60 ° C., and was wound up skein at a speed of 20 m / min. The obtained two-layer hollow fiber extrudate was immersed in isopropyl alcohol to extract and remove bis (2-ethylhexyl) phthalate and dibutyl phthalate. Subsequently, it was immersed in water for 30 minutes to swell the hollow fiber membrane. Subsequently, it was immersed in a 20 wt% NaOH aqueous solution at 70 ° C. for 1 hour, and further washed with water to extract and remove finely divided silica.
Table 1 shows the composition, production conditions, and various performances of the porous hollow fiber membrane of Comparative Example 2 obtained. It was found that the 5-day pore diameter retention was lower than that of Example 1.

  As described above, according to the present invention, not only has excellent water permeability and blocking performance against micropathogens, and has extremely high chemical resistance that can suppress pore coarsening for chemical cleaning, etc., but also against micropathogens. A porous hollow fiber membrane capable of maintaining the blocking performance stably for a long period of time can be easily realized with good reproducibility. Therefore, it can be used widely and effectively in filtration applications that require cleaning of turbidity and other contaminants accumulated on the membrane surface using chemicals such as alkaline aqueous solutions, and filtration applications that require stable filtration performance over a long period of time. Can be accumulated on the membrane surface by filtration, for example, water treatment at water purification plants, filtration treatment of river water and lake water, filtration and purification of industrial water and wastewater treatment, pretreatment of seawater desalination, etc. It can be used widely and effectively in a field where both high water permeability and chemical resistance are required for the membrane, such as cleaning dirt such as turbidity with chemicals.

Claims (13)

A porous hollow fiber membrane made of PVDF resin,
When one surface of the porous hollow fiber membrane is the surface (A) and the other surface is the surface (B), the cross-sectional structure from the surface (A) to the surface (B) is a three-dimensional network structure,
The layer containing the surface (A) is a good solvent for at least PVDF resin , and contains a good solvent having a boiling point exceeding 200 ° C.
The α-type structural crystal (Hα) in the crystal part of the PVDF resin in the layer containing the surface (A), the pore diameter of the surface (A) being one surface of the porous hollow fiber membrane is 0.05 μm or less A porous hollow fiber membrane in which the ratio (Hα / Hβ) of β to β-type structural crystal (Hβ) is 0.5 or more.   The pore diameter constituting the one surface (A) is 0.05 μm or less, the average thickness of the trunk constituting the other surface (B) is 0.5 to 10 μm, and the pore diameter is 0.1 μm or more. The porous hollow fiber membrane according to claim 1, which is 1 µm or less.   It has a two-layer structure consisting of an inner layer having a surface (B) in the hollow portion and an outer layer having a surface (A), and the outer layer has a continuous pore size from the outer layer surface toward the boundary with the inner layer surface. The porous hollow fiber membrane according to claim 1 or 2, wherein The porous hollow fiber membrane according to claim 3 , wherein the pore size distribution of the inner layer is isotropic. The porous hollow fiber membrane according to claim 3 or 4 , wherein the outer layer has a thickness of 1 µm to 100 µm. The porous hollow fiber membrane according to any one of claims 3 to 5, wherein the inner layer has a thickness of 20 µm or more and 1000 µm or less. Even after 30 days of operation, the porous hollow fiber is air bubbled by introducing air at a flow rate of 1 Nm ³ / h into the hollow part of the porous hollow fiber membrane every 30 minutes from the through-hole. The porous hollow fiber membrane according to any one of claims 3 to 6 , wherein the inner layer and the outer layer in the membrane do not peel off. The porous hollow fiber membrane according to any one of claims 1 to 7, wherein the compressive strength is 0.8 MPa or more. A method for producing a porous hollow fiber membrane comprising a PVDF resin and having a layer containing the surface (A),
Layer containing the surface (A) is a good solvent for PVDF resin, phase separation by a non-solvent induced phase separation method using a good solvent having a boiling point greater than 200 ° C.,
When one surface of the porous hollow fiber membrane is the surface (A) and the other surface is the surface (B), the cross-sectional structure from the surface (A) to the surface (B) is a three-dimensional network structure,
The α-type structural crystal (Hα) in the crystal part of the PVDF resin in the layer containing the surface (A), the pore diameter of the surface (A) being one surface of the porous hollow fiber membrane is 0.05 μm or less A method for producing a porous hollow fiber membrane, wherein a porous hollow fiber membrane having a ratio (Hα / Hβ) of β to β-type structural crystal (Hβ) of 0.5 or more is produced. In the method for producing a porous hollow fiber membrane, which is made of PVDF resin and is constituted by two layers of an inner layer having a surface (B) in a hollow portion and an outer layer having a surface (A),
The polymer stock solution constituting the inner layer and the polymer stock solution constituting the outer layer are simultaneously discharged from a triple-structure spinning nozzle,
The outer layer is a good solvent for PVDF resin having a boiling point phase-separated by a non-solvent induced phase separation method using a good solvent of greater than 200 ° C., the inner layer is phase separated by thermally induced phase separation method,
The outer layer produces a porous hollow fiber membrane in which the ratio (Hα / Hβ) of α-type structural crystal (Hα) to β-type structural crystal (Hβ) in the crystal part of PVDF resin is 0.5 or more. For producing a conductive hollow fiber membrane. The polymer in the polymer stock solution constituting the outer layer is PVDF resin, the polymer in the polymer stock solution constituting the inner layer is PVDF resin,
The method for producing a porous hollow fiber membrane according to claim 10 , wherein the discharge temperature of the polymer stock solution constituting the outer layer and the discharge temperature of the polymer stock solution constituting the inner layer are in the range of 200 to 280 ° C. The method for producing a porous hollow fiber membrane according to claim 10 or 11 , wherein the polymer in the polymer stock solution constituting the outer layer is PVDF resin, and the good solvent for dissolving the PVDF resin is sulfolane. The method for producing a porous hollow fiber membrane according to any one of claims 10 to 12 , wherein the polymer stock solution constituting the inner layer contains a PVDF resin, an organic liquid, and silica.