JP5856887B2 - Method for producing porous membrane - Google Patents

Description

  The present invention relates to a method for producing a porous 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. .

  The filtration process using a porous membrane is widely used in wastewater treatment because it has higher separation performance and is easier to maintain and manage than conventional chemical treatment processes such as coagulation sedimentation and sand filtration. . 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 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 phase separation methods that are excellent in controlling pore diameters over a relatively wide range from small pore diameters to large pore diameters are used relatively frequently. 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 this NIPS method, the rate of substitution from the solvent to the non-solvent is slow from the contact surface to the inside of the porous membrane because the concentration gradient decreases from the surface in contact with the coagulating solution to the inside of the porous membrane. Become. Since the lower the solvent exchange rate, the phase separation proceeds and the pores tend to become coarser. Therefore, in general, a porous membrane produced by the NIPS method forms 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 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 progress of phase separation in the cross-sectional direction of the porous membrane is substantially equal. Therefore, the porous membrane manufactured 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 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 membrane having a small pore diameter and high water permeability is produced, production by the NIPS method, which 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 in a filtration process using a porous membrane, for example, a membrane module in which a hollow fiber membrane in which a polymer excellent in processability is formed in a hollow shape or a flat membrane in which a polymer is formed in a sheet shape is combined. Etc. are used. However, when such a membrane module is used for water treatment or the like, there is a problem in that the membrane surface is clogged by the 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.

  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 the fluorine-based polymers, polyvinylidene fluoride resin (hereinafter also referred to as “PVDF resin”) is comparatively excellent in processability and is often used as a material for a porous film. 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 membrane made of PVDF resin that can withstand long-term use involving washing with alkali.

  A method for producing a porous membrane by the NIPS method using PVDF resin has been reported in many documents so far. For example, Patent Document 1 discloses a method for producing a porous film in which physical strength is not reduced by a chemical by producing a porous film having a low β crystal ratio, a low crystallinity, and a small specific surface by the NIPS method. 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 membrane having a uniform crystal structure can be obtained. Since the crystal structure of the porous membrane has a low β crystal ratio and a low crystallinity, the chemical strength of the separation layer is strong, It has been confirmed that separation performance can be maintained over 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 Japanese Patent No. 2835365 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 film described in Patent Document 1 has a crystal structure that changes in the film cross-sectional direction, and has a structure that is strong against chemical deterioration near the center of the film cross-section. The chemical strength of the adjacent separation layer is weak (it is inferior in chemical resistance), so the deterioration of strength and elongation of the entire film thickness can be suppressed, but the separation layer deteriorates with long-term use or chemical cleaning However, it is easily expected that the separation performance is lowered. Therefore, the porous membrane produced by the manufacturing method described in Patent Document 1 is unsuitable for drinking water applications that require long-term removal of micropathogens such as viruses.

  Moreover, the porous film described in Patent Document 2 is manufactured 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 film. 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 membrane is produced by the production method described in Patent Document 2, pores are formed by dropping the added inorganic fine powder, etc., so that the obtained porous membrane is capable of blocking micropathogens such as viruses. Performance tends to be insufficient. 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. It aims at providing the manufacturing method of the porous membrane made from PVDF which can hold | maintain the prevention performance with respect to a micropathogen stably for a long period.

  In order to solve the above-mentioned problems, the present inventors have made extensive studies, and as a result, in the NIPS method, the crystal state of the PVDF resin and the rate of substitution (exchange) between the solvent and the non-solvent at the time of discharging the film forming stock solution. Pay attention and adjust the temperature of the membrane-forming stock solution and the coagulating solution so that they are adjusted to the prescribed conditions, thereby producing a porous membrane, compared to the polymer membrane obtained by the conventional manufacturing method. The present invention has been completed by discovering that a porous membrane capable of removing a micropathogen stably and sufficiently for a long period of time can be produced, as well as excellent resistance and blocking performance against micropathogens and extremely high chemical resistance.

That is, the present invention provides the following (1) to (5).
(1) A method for producing a porous membrane, comprising producing a porous membrane by a non-solvent-induced phase separation method by discharging a film-forming stock solution containing at least a polyvinylidene fluoride resin and a solvent, and bringing it into contact with a coagulation solution containing at least a non-solvent Because
The discharge temperature of the film 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 coagulating liquid is higher than the porous structure formation start temperature of the film forming stock solution. Characterized by high,
Method for Producing Porous Membrane (2) The saturated dissolution concentration C _max of the solvent with respect to the non-solvent (saturated weight of the solvent (mg) / non-solvent 1 L) and the solvent concentration C in the non-solvent are represented by the following formula ( 1) and (2);
C _max ≧ 100 (mg / L) (1)
(C _max −C) × 100 / C _max > 80 (%) (2)
Satisfying the relationship of
The manufacturing method of the porous membrane as described in said (1).
(3) The HSP distance between the solvent and the polyvinylidene fluoride resin satisfies 5.20 or more and 8.00 or less,
The manufacturing method of the porous membrane as described in said (1) or (2).
(4) The boiling point of the solvent is a discharge temperature of the film-forming stock solution + 15 ° C. or higher and satisfies a decomposition temperature of the polyvinylidene fluoride resin,
The manufacturing method of the porous membrane as described in any one of said (1)-(3).
(5) In a method for producing a porous film containing a polyvinylidene fluoride resin,
Using a hollow fiber molding nozzle having two or more annular discharge ports, from one of the two adjacent annular discharge ports, a melt-kneaded material containing at least a polyvinylidene fluoride resin, an organic liquid and inorganic fine powder, and from the other A film-forming stock solution containing at least a polyvinylidene fluoride resin and a solvent satisfying an HSP distance of 5.20 or more and 8.00 or less between the polyvinylidene fluoride resin and the polyvinylidene fluoride resin is simultaneously discharged under the conditions satisfying claim 1, and the film-forming stock solution A multilayer porous hollow fiber structure is formed by contacting with a coagulating liquid that is higher than the porous structure formation start temperature of the melt kneaded product and lower than the porous structure formation start temperature of the melt-kneaded product,
The organic liquid is extracted and removed from the obtained multilayer porous hollow fiber structure,
A method for producing a porous membrane.

  According to the present invention, it has excellent water permeation performance and prevention performance against micropathogens, and has extremely high chemical resistance that can suppress pore coarsening for chemical cleaning, etc. A porous membrane that can be stably held can be easily produced with good reproducibility.

  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 membrane manufacturing method of this embodiment is a porous membrane in which phase separation is advanced by the NIPS method by discharging a membrane-forming stock solution containing at least a PVDF resin and a solvent and bringing it into contact with a coagulation solution containing at least a non-solvent. A membrane is produced. In the present embodiment, desired phase separation proceeds 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. Details will be described below.

  The film-forming stock solution contains a PVDF resin as a polymer component constituting the porous 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 film 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 × (dD _PVDF −dD _solvent ) ² + (dP _PVDF −dP _solvent ) ² + (dH _PVDF −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 membrane by the NIPS method, a porous 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 solvents include 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 Sulfone), Methyl Ethyl Ketone (MEK), Ethylene Glycol Monomethyl Ether, Cyclohexanone, Propylene Glycol Monomethyl Ether, Isophorone, Caprolactone (Epsilon), Tetrahydrofuran (THF), Propylene Glycol Phenyl Ether, Texanol, etc. In addition to one solvent, a mixed solvent in which these are mixed at an arbitrary ratio may be used. 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.

  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 discharged 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, thereby proceeding with a phase separation phenomenon, thereby producing a porous membrane. The porous membrane is formed by extruding a film-forming stock solution from a molding nozzle and coagulating it in a coagulating liquid, or a so-called wet film-forming method, or a predetermined idle running section 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 the solidification is performed in a coagulating liquid after ensuring the above, 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 thickness of the porous film 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 membrane of the present embodiment, the temperature of the coagulating liquid is required to be higher than the porous structure formation start temperature of the membrane-forming stock solution. If the temperature of the coagulation liquid is equal to or lower than the starting temperature for forming the porous structure of the film-forming stock solution, the heat-induced phase separation proceeds and a thick, uniform and dense separation layer is formed, so it has the desired small pore diameter and high water permeability. The film structure cannot be obtained. 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.

In addition, the solvent and non-solvent used in the present embodiment are C _max (saturated weight of solvent (mg) / non-solvent 1 L) of the solvent with respect to the non-solvent, and C is the solvent concentration in the non-solvent. Sometimes, it is preferable to satisfy the relationship of the following formulas (1) and (2).
C _max ≧ 100 (mg / L) (1)
(C _max −C) × 100 / C _max > 80 (%) (2)
By using a solvent and a non-solvent that satisfy the relationship of the formulas (1) and (2), the solvent is rapidly replaced (exchanged) with the non-solvent when the film-forming stock solution comes into contact with the coagulation liquid, and phase separation is performed. The speed is increased and the chemical resistance of the separation layer tends to be further enhanced. From this viewpoint, in the above formula (1), the left side is more preferably 500 (mg / L) or more, and in the above formula (2), the left side is more preferably 95% or more. In the measurement of C _max , in the case of a combination of a solvent and a non-solvent that dissolves endlessly, such as ethanol and water, the C _{max is set} to ∞ and used for the calculation of the above formulas (1) and (2). The upper limit value on the left side of the formula (2) is 100%.

  In the method for producing a porous membrane of the present embodiment, the discharge temperature of the raw film forming solution is required to be not lower than the melting point (Tm) of the PVDF resin and lower than the decomposition temperature (Td) of the PVDF resin. By setting the discharge temperature of the film-forming stock solution to Tm or higher, phase separation proceeds from a state where the PVDF resin crystals are loosened, so that the degree of crystallinity is lowered and a porous film having higher chemical resistance is obtained. When the film-forming stock solution is discharged at a temperature lower than Tm, phase separation proceeds from a state in which the PVDF resin is partially crystallized until the discharge, so that the degree of crystallinity increases and the resulting porous membrane has chemical resistance. It drops significantly. On the other hand, when discharged at a Td of PVDF resin or higher, the PVDF resin is oxidized and decomposed, thereby destroying the molecular structure and extremely reducing the physical strength.

  In addition, from the viewpoint of producing a porous film having a uniform and stable quality, the solvent contained in the film forming stock solution should have a boiling point not lower than the discharge temperature + 15 ° C. and lower than the Td of PVDF resin. Is preferred. When the boiling point of the solvent is equal to or higher than the discharge temperature + 15 ° C., the boiling of the solvent after the film-forming stock solution is discharged can be suppressed, and the occurrence of defects such as pinholes is suppressed. In addition, although the upper limit of the boiling point of a solvent is not specifically limited, It is preferable that it is less than the decomposition temperature of PVDF resin from a viewpoint of avoiding oxidation and decomposition | disassembly of PVDF resin.

  When the film-forming stock solution is discharged at a Tm of PVDF resin or more, depending on the solvent used or the molecular weight of the PVDF resin, the viscosity of the film-forming stock solution may be low and an arbitrary film shape may not be maintained. 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, polyvinyl ether or glycol ether such as Tetraethylene glycol or ethylene glycol is used. 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 in the porous film, so that pinholes can be prevented from occurring. In addition, when the decomposition temperature of the shape-retaining fluid is lower than the discharge temperature, the shape-retaining fluid does not burn and the strength of the obtained porous film does not decrease.

  Further, in the case of a hollow fiber-like porous membrane, when it is desired to increase the strength of the membrane, a multi-tubular nozzle such as a hollow fiber molding nozzle having two or more annular discharge ports is used in the manufacturing method of the present embodiment. The melt-kneaded material containing at least the polyvinylidene fluoride resin, the organic liquid and the inorganic fine powder is discharged from one of the two adjacent annular discharge ports, and the above-mentioned film-forming stock solution is discharged from the other, and the film-forming stock solution is porous. A multilayer porous hollow fiber structure is formed by contact with a coagulation liquid that is higher than the structure formation start temperature and lower than the porous structure formation start temperature of the melt-kneaded product, and the organic liquid is extracted from the obtained multilayer porous hollow fiber structure It is preferable to remove. By extracting and removing the organic liquid from the obtained multilayer porous hollow fiber structure, a porous 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 two-layer joint 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.

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

In Examples and Comparative Examples, (1) PVDF resin weight average molecular weight (Mw), (2) PVDF resin melting point (Tm), (3) Saturated solvent concentration C _{max with} respect to non-solvent, (4) Porous The water permeability of the membrane, (5) the dextran removal rate of the porous membrane, (6) the chemical resistance of the porous membrane, and (7) the defect occurrence rate of the porous membrane were measured and evaluated by the following methods, respectively.

(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 is 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 is the melting point of the PVDF resin. (Tm).

(3) Saturated dissolution concentration of solvent with respect to non-solvent (C _max )
Stir 100 mg of non-solvent at 25 ° C. at 100 rpm, titrate the solvent into the non-solvent using a burette, and measure the maximum amount (saturated weight) that the solvent uniformly diffuses and dissolves in the non-solvent within 1 hour. _{Was used} to calculate the saturated dissolution concentration (C _max ).
C _max = amount of solvent titrated (mg) / amount of non-solvent used (0.1 L)

(4) Permeability test of porous 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 ))

(5) Dextran removal rate of porous membrane (%)
The injection needles are inserted into the hollow portions 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 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

(6) Chemical resistance test of porous membrane (5th day pore size retention rate (%))
As the chemical resistance test of the porous membrane, the evaluation was conducted based on the 5th day pore size retention rate (%). The 5th day pore size retention rate (%) was determined by using a wet porous hollow fiber membrane at 40 ° C. sodium hydroxide. Is immersed in a mixed aqueous solution of 4% by weight of sodium hypochlorite and 0.5% by weight of an effective chlorine concentration for 5 days, and the dextran removal rate of 2 million molecular weight of (5) above and before is immersed in the chemical solution. The measurement was performed, and the dextran removal rate (E ₀ ) before immersion and the dextran removal rate (E ₅ ) after chemical deterioration due to 5-day immersion were substituted into the following formulas and calculated.
Day 5 pore diameter retention rate (%) = (E ₅ / E ₀ ) × 100

(7) Number of defects in the porous membrane One end of a wet hollow fiber membrane with a length of about 1 m is sealed, an injection needle is inserted into the hollow portion at the other end, and air is blown into the hollow portion from the injection needle at a pressure of 0.2 MPa. The number of defects was determined by measuring the number of locations where bubbles emerge from the outer surface.

[Example 1]
As PVDF resin, vinylidene fluoride homopolymer having a weight average molecular weight of 350,000 (Arkema, Kynar 741) 24% by weight, polyvinyl pyrrolidone having a K value of 17 (BASF, Rubiscol K17) 17% by weight, sulfolane (sum) 59% by weight (manufactured by Koyo Pure Chemical) 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.
This film-forming stock solution is discharged at 200 ° C. together with glycerin (made by Wako Pure Chemicals, special grade) as an internal liquid from a double ring spinning nozzle (outer diameter 1.3 mm, intermediate diameter 0.7 mm, inner diameter 0.5 mm). The porous membrane of Example 1 having a membrane structure of a hollow fiber membrane after passing through a free running distance of 40 mm, proceeding with non-solvent induced phase separation in water at 60 ° C., and removing the solvent (porous hollow fiber) Membrane).
Table 1 shows the blending composition, production conditions, and various performances of the obtained porous membrane of Example 1.

[Example 2]
The same PVDF homopolymer as in Example 1, 20% by weight of polyvinyl pyrrolidone having a K value of 17 and 63% by weight of Triacetin (manufactured by Aldrich, 99%) were mixed and dissolved at 200 ° C. Was prepared. 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.
This film-forming stock solution was discharged at 250 ° C. together with glycerin (made by Wako Pure Chemicals, special grade) as an internal liquid from the same spinning nozzle as in Example 1, and after passing through a free running distance of 40 mm, Solvent-induced phase separation was advanced to remove the solvent, and a porous membrane (porous hollow fiber membrane) of Example 2 having a hollow fiber membrane structure was obtained.
Table 1 shows the composition, production conditions, and various performances of the porous membrane of Example 2 obtained.

[Example 3]
20% by weight of vinylidene fluoride homopolymer having a weight average molecular weight of 300,000 as a PVDF resin (SOLEF6008 manufactured by Solvay Solexis), 17% by weight of polyvinylpyrrolidone having a K value of 12 (manufactured by BASF, Kollidon 12PF), N -Methyl-2-Pyrrolidone (NMP) (manufactured by Kanto Chemical Co., Ltd., special grade) 63% 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 390 ° C., and the formation start temperature of the porous structure of this film-forming stock solution was 35 ° C.
This film-forming stock solution was discharged from the same spinning nozzle as in Example 1 at 180 ° C. together with glycerin (made by Wako Pure Chemicals, special grade) as an internal solution, passed through a 20 mm free running distance, and then non-solvent in 60 ° C. water. The induced phase separation was advanced and the solvent was removed to obtain a porous membrane (porous hollow fiber membrane) of Example 3 having a hollow fiber membrane structure.
Table 1 shows the composition, production conditions, and various performances of the porous membrane of Example 3 obtained.

[Example 4]
As PVDF resin, vinylidene fluoride homopolymer having a weight average molecular weight of 440,000 (SOLBE 6012, manufactured by Solvay Solexis Co., Ltd.), polyvinylpyrrolidone having a K value of 12 (BASF, Kollidon 12PF), 15% by weight, Triacetin (Aldrich, 99%) was mixed with 63% by weight and dissolved at 220 ° C. to prepare a film forming stock solution. The melting point (Tm) of this vinylidene fluoride homopolymer was 174 ° C., the decomposition temperature was 390 ° C., and the formation start temperature of the porous structure of this film forming stock solution was 102 ° C.
This film-forming stock solution was discharged at 230 ° C. together with glycerin (made by Wako Pure Chemicals, special grade) as an internal liquid from the same spinning nozzle as in Example 1, and after passing through an idle running distance of 60 mm, glycerin 40% at 110 ° C. Non-solvent induced phase separation was allowed to proceed in a 40 wt% Triacetin mixed solution, and the solvent was removed to obtain a porous membrane (porous hollow fiber membrane) of Example 4 having a hollow fiber membrane structure.
Table 1 shows the composition, production conditions, and various performances of the porous membrane of Example 4 obtained.

[Example 5]
A porous membrane of Example 5 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, the same film-forming stock solution as in Example 1 was used for the outer layer, and 34% by weight of vinylidene fluoride homopolymer (SOLEF6020 manufactured by Solvay Solexis, Inc.) and 33.8% by weight of bis (2-ethylhexyl) phthalate were used for the inner layer. %, Dibutyl phthalate 6.8% by weight, finely divided silica 25.4% by weight, and the hollow part forming fluid is air, respectively, and these are used as triple ring spinning nozzles (outer layer outer diameter 2.0 mm) The hollow fiber-shaped molded product was obtained by simultaneously discharging from the inner layer outermost diameter 1.8 mm and the hollow portion forming layer outermost diameter 0.9 mm.
The extruded hollow fiber-shaped molded article passes through an idle running distance of 80 mm, and then the outer layer advances non-solvent induced phase separation in water at 60 ° C., and the inner layer advances thermally induced phase separation in the empty running part and water, The skein was wound up at a speed of 20 m / min. The obtained two-layer hollow fiber extrudate was immersed in methylene chloride to extract and remove bis (2-ethylhexyl) phthalate and dibutyl phthalate, and then dried. Subsequently, after immersing in a 50% by weight aqueous ethanol solution for 30 minutes, 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.
The porous membrane of Example 5 obtained in this way consists of an outer layer having high separation performance and an inner layer having high water permeability and mechanical strength, and the outer layer and inner layer are joined discontinuously, and the interface between the two layers. Also, it had a structure with extremely high communication. Moreover, the porous membrane of Example 5 showed the outstanding intensity | strength and water-permeable performance compared with the porous membrane of Examples 1-4.
Table 1 shows the composition, production conditions, and various performances of the porous membrane of Example 5 obtained.

[Comparative Example 1]
The same PVDF polymer as in Example 1, 24% by weight, 17% by weight of K17 polyvinylpyrrolidone, and 59% by weight of sulfolane (manufactured by Wako Pure Chemical Industries, Ltd.) were 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 stock solution was 42 ° C.
This film-forming stock solution was discharged from the same spinning nozzle as in Example 1 with glycerin (made by Wako Pure Chemicals, special grade) as an internal liquid at 140 ° C., passed through a 60 mm free running distance, and then non-solvent in water at 60 ° C. The induced phase separation was advanced, the solvent was removed, and a porous membrane (porous hollow fiber membrane) of Comparative Example 1 having a hollow fiber membrane structure was obtained.
Table 1 shows the composition, production conditions, and various performances of the obtained porous membrane of Comparative Example 1. The porous membrane of Comparative Example 1 had a very low pore diameter retention rate of 10% on the fifth day, and the chemical resistance of the separation layer was lower than that of the porous membranes of Examples 1 to 5.

[Comparative Example 2]
Using the same film-forming stock solution as in Comparative Example 1, discharged from the same spinning nozzle as in Example 1 together with glycerin (made by Wako Pure Chemicals, special grade) at 200 ° C., passed through a free running distance of 60 mm, and then formed a porous structure of the stock solution Thermally induced phase separation is allowed to proceed in 23 ° C. water below the starting temperature (42 ° C.), and the solvent is removed to obtain a porous membrane (porous hollow fiber membrane) of Comparative Example 2 having a hollow fiber membrane structure. It was.
Table 1 shows the composition, production conditions, and various performances of the porous membrane of Comparative Example 2 obtained. The porous membrane of Comparative Example 2 is different from the porous membranes of Examples 1 to 5 in that a very dense and thick separation layer is formed, the water permeability is extremely low, and the pore diameter retention rate cannot be measured. Met.

[Comparative Example 3]
27% by weight of vinylidene fluoride homopolymer having a weight average molecular weight of 350,000 as PVDF resin (Akema, Kynar 741) and 15% by weight of polyethylene glycol having a weight average molecular weight of 35,000 (Merck, polyethylene glycol 35000), Dissolved in 58% by weight of dimethylacetamide (manufactured by Wako Pure Chemicals, special grade) at 70 ° C. to obtain 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 75 ° C. or higher and 0 ° C. or lower.
This film-forming stock solution was discharged from the same spinning nozzle as in Example 1 at 70 ° C. with water as the internal solution, passed through an idle running distance of 200 mm, and then proceeded with non-solvent-induced phase separation in water at 77 ° C. The solvent was used to obtain a porous membrane (porous hollow fiber membrane) of Comparative Example 3 having a hollow fiber membrane structure.
Table 1 shows the composition, production conditions, and various performances of the obtained porous membrane of Comparative Example 3. Unlike the porous membranes of Examples 1 to 5, the porous membrane of Comparative Example 3 had extremely low chemical resistance in the separation layer.

  As described above, according to the method for producing a porous membrane of the present invention, it has excellent water permeability and blocking performance against micropathogens, and has extremely high chemical resistance that can suppress pore coarsening for chemical cleaning and the like. In addition, it is possible to easily and easily produce a porous membrane that can stably retain the blocking performance against micropathogens for a long period of time. 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 (5)

A method for producing a porous membrane, comprising producing a porous membrane by a non-solvent-induced phase separation method by discharging a film-forming stock solution containing at least a polyvinylidene fluoride resin and a solvent, and contacting a coagulating liquid containing at least a non-solvent. ,
The discharge temperature of the film 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 coagulating liquid is higher than the porous structure formation start temperature of the film forming stock solution. Characterized by high,
A method for producing a porous membrane. The saturated dissolution concentration C _max of the solvent with respect to the non-solvent (saturated weight of the solvent (mg) / non-solvent 1 L) and the solvent concentration C in the non-solvent are represented by the following formulas (1) and (2):
C _max ≧ 100 (mg / L) (1)
(C _max −C) × 100 / C _max > 80 (%) (2)
Satisfying the relationship of
The method for producing a porous membrane according to claim 1. The HSP distance between the solvent and the polyvinylidene fluoride resin satisfies 5.20 or more and 8.00 or less,
The manufacturing method of the porous membrane of Claim 1 or 2. The boiling point of the solvent is not less than the discharge temperature of the film-forming stock solution + 15 ° C. and satisfies the decomposition temperature of the polyvinylidene fluoride resin,
The manufacturing method of the porous membrane in any one of Claims 1-3. In a method for producing a porous membrane comprising a polyvinylidene fluoride resin,
Using a hollow fiber molding nozzle having two or more annular discharge ports, from one of the two adjacent annular discharge ports, a melt-kneaded material containing at least a polyvinylidene fluoride resin, an organic liquid and inorganic fine powder, and from the other A film-forming stock solution containing at least a polyvinylidene fluoride resin and a solvent satisfying an HSP distance of 5.20 or more and 8.00 or less between the polyvinylidene fluoride resin and the polyvinylidene fluoride resin is simultaneously discharged under the conditions satisfying claim 1, and the film-forming stock solution A multilayer porous hollow fiber structure is formed by contacting with a coagulating liquid that is higher than the porous structure formation start temperature of the melt kneaded product and lower than the porous structure formation start temperature of the melt-kneaded product,
The organic liquid is extracted and removed from the obtained multilayer porous hollow fiber structure,
A method for producing a porous membrane.