JP2008062228A - Method for preparing porous membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for preparing porous hollow fiber membrane excellent in permeability, fractionation capability, physical strength, process controllability, cost, and good-pore formability that can be made easily at a low cost and is suitable for water treatment such as water purification, potable water preparation, industrial water preparation and waste water treatment. <P>SOLUTION: The method for preparing porous polyvinylidene fluoride membrane is characterized by comprising a coagulation step wherein an aqueous solution containing at least one kind of salt dissolved therein is used for coagulation. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention is excellent in permeation performance and fractionation performance suitable for water treatment such as water purification treatment, drinking water production, industrial water production, and wastewater treatment, and also has excellent process controllability, cost performance, and good pore formation. The present invention relates to a method for producing a vinylidene resin porous membrane.

  In recent years, the technology of separation means using a separation membrane having selective permeability has been remarkably advanced. Such separation operation technology has been put into practical use as a series of purification systems including separation means, washing means, sterilization means, etc., for example in the manufacturing process of drinking water, ultrapure water and pharmaceuticals, and sterilization and finishing of brewed products. ing. In these application fields, water refinement (advanced treatment), safety improvement, and accuracy improvement are required at a high level, and the use of separation membranes is progressing. In view of the above situation, the characteristics required for the separation membrane are becoming more sophisticated. The most important membrane properties are permeation performance and fractionation performance. The balance is important for both performances, and it is desirable to be able to remove smaller particles at higher permeation rates, and the achievement of this is largely due to membrane pore connectivity and membrane surface structure.

  Under such circumstances, separation membranes made of various membrane materials are manufactured. The physical strength and chemical strength performance of the separation membrane is largely derived from the characteristics of the membrane material. In recent years, porous membranes using vinylidene fluoride resin resins have been developed and used.

  On the other hand, the permeation performance and fractionation performance of the separation membrane depend greatly on the membrane production method. Phase separation is often used as a method for producing a separation membrane having excellent permeation performance and fractionation performance. Manufacturing methods using such phase separation can be broadly divided into non-solvent induced phase separation methods and thermally induced phase separation methods.

  In the non-solvent induced phase separation method, a uniform polymer solution composed of a polymer and a solvent undergoes phase separation due to the concentration change due to the ingress of the non-solvent or evaporation of the solvent to the external atmosphere. As a method for producing a separation membrane using such a non-solvent induced phase separation method, a polymer solution in which a vinylidene fluoride resin resin is dissolved in a good solvent is used at a temperature considerably lower than the melting point of the vinylidene fluoride resin resin. The method of forming an asymmetric porous structure by non-solvent-induced phase separation after being extruded from a die or cast on a glass plate and then brought into contact with a liquid containing a non-solvent of vinylidene fluoride resin resin Known (Patent Document 1). However, in general, a non-solvent induced phase separation method tends to form a dense film, and it is difficult to achieve a sufficient number of surface pores and a sufficiently large surface pore diameter. In addition, it is difficult to control the phase separation in a non-solvent, and since the non-solvent is essential, the production cost is high, and macrovoids (coarse pores) are likely to be generated.

  On the other hand, the thermally induced phase separation method usually comprises the following steps. (1) A mixture of a polymer and a solvent having a high boiling point is melted at a high temperature. (2) Allow the polymer to solidify by cooling at an appropriate rate to induce phase separation. (3) Extract the used solvent.

  In the thermally induced phase separation method, (1) inorganic fine particles and an organic liquid are melt-kneaded into a vinylidene fluoride resin resin and extruded from a die at a temperature equal to or higher than the melting point of the vinylidene fluoride resin resin. There is a method of forming a porous structure by pressing and molding, cooling and solidifying, and then extracting an organic liquid and inorganic fine particles (Patent Document 2). This method is easy to control the porosity and does not form macrovoids and can obtain a relatively homogeneous and high-strength film. However, a solvent such as 1,1,1-trichloroethane is used in the organic liquid extraction process. Since it is necessary to use it, it is a manufacturing method having the disadvantage that the manufacturing cost becomes extremely high.

  (2) A method in which a hydrophilic porogen and a poor solvent of the resin are dissolved in a vinylidene fluoride resin and discharged into a cooling bath containing a poor solvent having a concentration of 60 to 100% by weight for solidification. (Patent Document 3). Although this method does not form macrovoids and a relatively homogeneous and high strength film can be obtained, there is a problem in terms of safety and cost because a liquid containing a high concentration of poor solvent is required for the cooling bath. is doing.

Prior art documents related to the present invention include the following.
JP 56-56202 A JP-A-3-215535 JP 2003-138422 A JP-A-60-97001 JP 2003-138422 A JP-A-2005-146230

  In view of the above-mentioned problems of the prior art, the present invention is suitable for water treatment such as water purification treatment, drinking water production, industrial water production, wastewater treatment, permeation performance, fractionation performance, strength, process controllability, cost performance An object of the present invention is to provide a method for producing a hollow fiber membrane, which makes it possible to inexpensively and easily produce a porous membrane excellent in good pore forming properties.

  As a result of intensive investigations by paying attention to factors that contribute to phase separation, the present inventors have controlled the exchange rate between a non-solvent (water) and a solvent by using a salt aqueous solution in the solidifying step, and the surface of the porous membrane It has been found that the formation of a dense layer can be suppressed.

The above problems are solved by the following (1) to (3).
(1) A method for producing a vinylidene fluoride porous membrane comprising a step of coagulating using an aqueous salt solution in which at least one salt is dissolved.
(2) The method for producing a vinylidene fluoride porous membrane as described above, wherein the salt concentration of the salt aqueous solution is not less than 30 g / L in weight concentration and in the range of 10 to 100% with respect to the saturated aqueous solution concentration.
(3) The method for producing a vinylidene fluoride porous membrane as described above, wherein the porous membrane is a hollow fiber membrane.

  In the present invention, there is provided a method for producing a porous membrane having high water permeability, high separation performance and excellent process controllability and cost by using a vinylidene fluoride resin having high mechanical strength and chemical strength. The

  The manufacturing method of the vinylidene fluoride resin porous membrane of this invention is shown. The manufacturing method other than the coagulation step of the vinylidene fluoride resin solution can apply a known technique, and there are manufacturing, coagulation, and extraction steps of the resin-containing solution, and heat treatment and the like may be appropriately performed. . For example, a vinylidene fluoride resin porous membrane can be produced by the methods described in Patent Documents 4 to 6.

  The solvent used in the present invention is a solvent that can dissolve the vinylidene fluoride resin and is water-soluble. Examples thereof include cyclohexanone, isophorone, γ-butyrolactone, ε-caprolactone, N-methyl-2-pyrrolidone, dimethyl sulfoxide, dimethylacetamide, dimethylformamide, methyl ethyl ketone, acetone, tetrahydrofuran, and trimethyl phosphate.

  The salts used in the present invention are monovalent or polyvalent ones such as halides, sulfates, nitrates, chlorates, lactates and acetates, and it is possible to use anhydrous salts or hydrated salts. it can.

  The salt aqueous solution of the present invention is a solution in which at least one salt is dissolved, and may be a mixed solution composed of a plurality of salts. By using an aqueous salt solution in the solidifying step, the exchange rate of water (non-solvent) and water-soluble solvent can be reduced. That is, it is possible to form a porous film that suppresses formation of a dense layer on the film surface due to rapid structural fixation. The higher the salt concentration, the lower the exchange rate between water and the water-soluble solvent, and the larger the number of surface pores and the surface pore diameter of the resulting membrane.

  The concentration of the aqueous salt solution is preferably at least 30 g / L in weight concentration and in the range of 10 to 100%, more preferably in the range of 10 to 60% with respect to the saturated aqueous solution concentration. If it exceeds 60% with respect to the saturated solution concentration, it is not preferable in terms of handleability.

  The composition of the membrane forming solution, the production method, and the salt concentration in the coagulation step can be appropriately selected according to the pore size of the target porous membrane, and an ultrafiltration membrane, a microfiltration membrane, and a larger large pore membrane are produced. Is possible.

  In the present invention, it is also possible to perform a stretching treatment in order to improve the pure water permeation rate and strength of the porous membrane. As the stretching method, methods such as hot stretching, cold stretching, and heat setting can be appropriately combined according to the intended strength. However, if the degree of stretching is too high, the entire porous membrane obtained is fibrillated and the micropores become slit-like, so that the separation accuracy is lowered and the strength in the circumferential direction is adversely decreased. . In the filtration of the membrane, the strength in the circumferential direction is also important. Therefore, it is necessary to control the stretching ratio within a range in which the membrane surface does not become slit-like micropores but maintains a circular or elliptical shape. Stretching may be performed by an arbitrary method such as performing in a state where a solvent or the like is present after formation, or after extracting the solvent or the like.

  A membrane module is obtained by bundling a predetermined number of porous membranes after drying and storing them in a case having a predetermined shape, and then fixing the ends with urethane resin, epoxy resin or the like. For example, in the case of a hollow fiber membrane, the membrane module is of a type in which both ends of the hollow fiber membrane are fixed open, one end of the hollow fiber membrane is fixed open and the other end is sealed but not fixed Various types are known, such as types.

  Hereinafter, the present invention will be described specifically by way of examples. In addition, this invention does not receive any limitation by this.

  Polyvinylidene fluoride (hereinafter may be abbreviated as PVDF) as a vinylidene fluoride-based resin (SOLEF6010 manufactured by Solvay Advanced Polymers Co., Ltd.) and γ-butyrolactone (hereinafter abbreviated as γ-BL) as a solvent Then, a mixed solution was prepared so that silica as inorganic particles and glycerin as a flocculant were in a ratio of 30: 39: 15: 16 by weight ratio, respectively. The composition of this mixed solution is shown in Table 1.

  The above mixed solution was heated and kneaded (temperature: 150 ° C.) in a twin-screw kneading extruder, and the extruded strand was passed through a pelletizer to form chips. This chip was extruded using an extruder (150 ° C.) equipped with a double ring nozzle having an outer diameter of 1.6 mm and an inner diameter of 0.8 mm. At this time, tetraethylene glycol was injected into the hollow portion of the extrudate.

  The extruded product extruded into the air from the spinning nozzle is placed in a water bath (temperature: 60 ° C.) consisting of a 10% aqueous solution of sodium sulfate by weight over a distance of 3 cm and solidified by passing through an approximately 100 cm water bath. I let you. The aqueous sodium sulfate solution used was 32% with respect to the saturated aqueous solution concentration of 60 ° C. Next, the obtained hollow fiber-like material was stretched in hot water at 80 ° C. so as to be about twice as long as the original length in the fiber direction, wound around a frame, and washed with running water in 80 ° C. warm water for 120 minutes. The solvent (γ-butyrolactone), the flocculant (glycerin), and the injection solution (tetraethylene glycol) were extracted and removed.

  The hollow fiber membrane thus obtained was immersed in an aqueous solution of sodium hydroxide at a weight percent concentration of 5% at 40 ° C. for 120 minutes to extract and remove inorganic particles (silica), followed by a water washing and drying step to obtain a hollow fiber membrane. Got. The manufactured hollow fiber membrane was tested according to the following method. The test results are shown in Table 2. A scanning electron micrograph taken for observing the film surface structure is shown in FIG.

Various Measurement (Analysis) Methods and Apparatuses (1) Fractionated particle diameter The blocking rate of at least two kinds of particles having different particle diameters is measured, and R in the following approximate expression (1) based on the measured values, R Was obtained as a fractional particle size.
R = 100 / (1−m × exp (−a × log (S))) (1)
In the formula (1), a and m are constants determined by the hollow fiber membrane, and are calculated based on measured values of two or more types of rejection. However, the fractional particle size in the case where the rejection rate of 0.1 μm diameter particles is 90% or more is expressed as <0.1 μm.

(2) Pure water permeation rate Using a single-end open type hollow fiber membrane module with an effective length of 3 cm, pure water is used as raw water, the filtration pressure is 50 kPa, and the temperature is 25 ° C., from the outside of the hollow fiber membrane. The amount of permeation per hour was measured by filtering inside (filtering with external pressure), and calculated by a numerical value converted into the amount of permeation per unit membrane area, unit time, and unit pressure.

Comparative Example 1
A hollow fiber membrane was obtained in the same manner as in Example 1 except that the extruded product was cooled and solidified using water at 60 ° C. Table 1 shows the composition of the mixed solution used for the production of the produced hollow fiber membrane, and Table 2 shows the test results. A scanning electron micrograph taken for observing the film surface structure is shown in FIG.

  Polyvinylidene fluoride (Solve Advanced Polymers, SOLEF6010) as a vinylidene fluoride resin, γ-butyrolactone as a solvent, and tetraethylene glycol (hereinafter sometimes abbreviated as TEG) as a flocculant, The mixture was prepared so that the ratio was 35:40:25.

  The above mixed solution was heated and kneaded (temperature: 150 ° C.) in a twin-screw kneading extruder, and the extruded strand was passed through a pelletizer to form chips. This chip was extruded using an extruder (150 ° C.) equipped with a double ring nozzle having an outer diameter of 1.6 mm and an inner diameter of 0.8 mm. At this time, γ-butyrolactone was injected into the hollow portion of the extrudate.

The extruded product extruded into the air from the nozzle is placed in a water bath (temperature: 30 ° C.) consisting of a 10% sodium sulfate aqueous solution by weight over a distance of 3 cm and solidified by passing through a water bath of about 100 cm. I let you. Next, the obtained hollow fiber-like material is stretched in hot water at 80 ° C. so as to be about twice as long as the original length in the fiber direction, wound on a frame, and washed with running water in 80 ° C. warm water for 120 minutes. The solvent (γ-butyrolactone), the flocculant (tetraethylene glycol), and the injection solution (γ-butyrolactone) were extracted and removed. The test results of the manufactured hollow fiber membrane are shown in Table 2.

Comparative Example 2
A hollow fiber membrane was obtained in the same manner as in Example 2 except that the extruded product was cooled and solidified using water at 30 ° C. The test results of the manufactured hollow fiber membrane are shown in Table 2.

  Polyvinylidene fluoride as a vinylidene fluoride-based resin (SOLE 6010, manufactured by Solvay Advanced Polymers Co., Ltd.), ε-caprolactone as a solvent, hydrophobic silica (in Japan Aerosil Co., Ltd., R-972) silica as inorganic particles, and Then, glycerin as an aggregating agent was prepared so that the weight ratio was 35: 47: 10: 8.

  The above mixed solution was heated and kneaded (temperature: 150 ° C.) in a twin-screw kneading extruder, and the extruded strand was passed through a pelletizer to form chips. This chip was extruded using an extruder (150 ° C.) equipped with a double ring nozzle having an outer diameter of 1.6 mm and an inner diameter of 0.8 mm. At this time, tetraethylene glycol was injected into the hollow portion of the extrudate.

  The extruded product extruded into the air from the nozzle is placed in a water bath (temperature: 80 ° C.) consisting of a 10% aqueous solution of sodium sulfate by weight over a distance of 3 cm and solidified by passing through an approximately 100 cm water bath. I let you. Next, the obtained hollow fiber-like material was stretched in hot water at 80 ° C. so as to be about twice as long as the original length in the fiber direction, wound around a frame, and washed with running water in 80 ° C. warm water for 120 minutes. The solvent (γ-butyrolactone), the flocculant (glycerin), and the injection solution (tetraethylene glycol) were extracted and removed. The test results of the manufactured hollow fiber membrane are shown in Table 2.

Comparative Example 3
A hollow fiber membrane was obtained in the same manner as in Example 1 except that the extruded product was cooled and solidified using 80 ° C. water. The test results of the manufactured hollow fiber membrane are shown in Table 2.

  A hollow fiber membrane was obtained in the same manner as in Example 1 except that the extruded product was cooled and solidified using a sodium sulfate aqueous solution (temperature 60 ° C.) having a weight percent concentration of 18%. Table 1 shows the composition of the mixed solution used for the production of the produced hollow fiber membrane, and Table 2 shows the test results.

  A hollow fiber membrane was obtained in the same manner as in Example 1 except that the extruded product was cooled and solidified using a 28% aqueous solution of sodium sulfate (temperature 60 ° C.). Table 1 shows the composition of the mixed solution used for the production of the produced hollow fiber membrane, and Table 2 shows the test results.

2 is a scanning electron micrograph of the outer surface of a vinylidene fluoride resin porous membrane produced by the method of Example 1. FIG. 2 is a scanning electron micrograph of the outer surface of a vinylidene fluoride resin porous membrane produced by the method of Comparative Example 1.

Claims (3)

  A method for producing a vinylidene fluoride porous membrane, comprising a step of coagulating using an aqueous salt solution in which at least one salt is dissolved.   2. The method for producing a vinylidene fluoride porous membrane according to claim 1, wherein the salt concentration of the aqueous salt solution is in the range of 10 to 100% with respect to the saturated aqueous solution concentration and having a weight concentration of 30 g / L or more.   2. The method for producing a vinylidene fluoride porous membrane according to claim 1, wherein the porous membrane is a hollow fiber membrane.

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