JP2018143930A - Method for producing porous membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a porous membrane which suppresses a used amount of an oxidant for a short period of time, and can decompose an aperture auxiliary remained on a porous membrane precursor.SOLUTION: A method for producing a porous membrane includes: a film production step of solidifying a film production stock solution containing 10-30 mass%, preferably, 15-25 mass% of a fluorine resin and 1-20 mass%, preferably, 5-12 mass% of an aperture auxiliary with a solidification liquid to form a porous membrane precursor 30; and a decomposition step of decomposing and removing the aperture auxiliary remained on the porous membrane precursor 30 with a hypochlorite aqueous solution, where pH (measurement temperature: 25°C) of the hypochlorite aqueous solution in the decomposition step is kept to 9.5 or more using at least one of NaOH or Ca(OH). In the method for producing the porous membrane, the fluorine resin is polyvinylidene fluoride, the aperture auxiliary is a water-soluble polymer, preferably, one or more of polyvinyl pyrrolidone, polyvinyl alcohol and polyethylene glycol.SELECTED DRAWING: Figure 1

Description

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

In the fields of food industry, medical care, electronics industry, etc., separation membranes (porous membranes) such as porous hollow fiber membranes were used for the purpose of concentrating and recovering useful components, removing unnecessary components, and producing fresh water. Microfiltration membranes, ultrafiltration membranes, reverse osmosis filtration membranes and the like are frequently used.
For example, the separation membrane forms a porous membrane precursor by coagulating a membrane-forming stock solution in which cellulose acetate, polyacrylonitrile, polysulfone, fluororesin and the like are dissolved in a solvent with a coagulating solution (a membrane-forming step), and the porous membrane It is produced by removing the solvent remaining in the film precursor and drying it.
For example, since a separation membrane made of a fluororesin is soluble in an organic solvent such as dimethylformamide, dry and wet spinning methods and wet spinning methods are often used for the production of the separation membrane.

By the way, in the production of the hollow fiber membrane, in order to obtain a pore size satisfying the target fraction and sufficient water permeability, a section (empty space) in which the membrane-forming raw solution is discharged from the nozzle and contacts the air until reaching the coagulation liquid. It is important how the phase separation state is controlled in the traveling section).
In order to control this phase separation state, it is a well-known technique to contain a pore opening aid in the membrane forming stock solution. As the pore opening aid, a high molecular weight hydrophilic polymer such as polyethylene glycol or polyvinyl pyrrolidone is usually used.

However, in the porous film precursor obtained by the film forming process, the pore opening aid usually remains in a solution state. If the pore opening aid remains in the porous membrane precursor, it becomes difficult for the porous membrane to exhibit high water permeability.
Therefore, in order to express the water permeability of the porous membrane, the pore opening aid remaining in the porous membrane precursor is decomposed and removed using various oxidizing agents.

For example, in Patent Document 1, a chemical solution containing an oxidizing agent such as hypochlorite is cooled to a low temperature (50 ° C. or lower), the porous membrane precursor is immersed in the chemical solution, and the chemical solution is applied to the porous membrane precursor. A method is described in which a porous membrane precursor holding a chemical solution is heated in a gas phase after being held at a low temperature, and further washed with water to decompose the hydrophilic polymer remaining in the porous membrane precursor. .
Patent Document 2 discloses that a porous membrane precursor is brought into contact with a porous membrane precursor in a short period of time by contacting a chemical solution containing an oxidizing agent such as hypochlorite with a temperature of 30 to 120 ° C. Describes a method of decomposing the remaining hydrophilic polymer.
Patent Document 3 describes a method of decomposing a hydrophilic polymer while suppressing decomposition of hypochlorite using a hypochlorite aqueous solution whose initial pH is adjusted to 9 to 12. Yes.

JP-A-2005-220202 International Publication No. 2013/018900 Japanese Patent No. 4724914

However, in the case of the method described in Patent Document 1, in order to sufficiently decompose the hydrophilic polymer remaining in the porous membrane precursor obtained by the film forming step, a step of immersing in a chemical solution at a low temperature, It was necessary to repeat the heating step in the gas phase and the subsequent water washing step for a plurality of cycles, which required time.
Although the hydrophilic polymer can be decomposed in a short time by using the method described in Patent Document 2, the hydrophilic polymer is eluted in the chemical solution containing an oxidizing agent such as hypochlorite during the decomposition, and as a result, under heating and high temperature. As a result, the oxidizing agent is deactivated. For this reason, the oxidizing agent becomes difficult to be effectively used for decomposing the hydrophilic polymer, and it is necessary to use a large amount of the oxidizing agent in order to sufficiently decompose the hydrophilic polymer.
As described in Patent Document 3, even if the pH of the initial hypochlorite aqueous solution is adjusted to 9 to 12, even when the traveling porous membrane precursor is continuously processed, a hydrophilic polymer is used. During the decomposition, the hydrophilic polymer gradually eluted into the hypochlorite aqueous solution and the pH was lowered, the pH became less than 9, and the hypochlorite was decomposed.
Thus, conventionally, no technique has been found that can decompose the pore opening assistant remaining in the porous membrane precursor in a short time and with a reduced amount of oxidizing agent used.

  The present invention has been made in view of the above circumstances, and provides a method for producing a porous membrane capable of decomposing the opening aid remaining in the porous membrane precursor in a short period of time while suppressing the amount of oxidant used. This is the issue.

  As a result of intensive studies, the present inventors have, for example, supplied a base during the decomposition to the hypochlorite aqueous solution used for the decomposition of the pore opening assistant, and the hypochlorite aqueous solution during the decomposition of the pore opening assistant. By maintaining the pH at 9.5 or higher, the deactivation of hypochlorite is suppressed, the decomposition of the pore opening assistant remaining in the porous membrane precursor is reduced in a short time, and the hypochlorite The present inventors have found that it can be decomposed while suppressing the amount used, and have completed the present invention.

That is, this invention has the following aspects.
[1] A film-forming step of forming a porous film precursor by coagulating a film-forming stock solution containing a fluororesin and an opening aid with a coagulating liquid; and forming the porous film precursor with a hypochlorite aqueous solution And a decomposition step of decomposing and removing the remaining pore opening assistant, and maintaining the pH (measurement temperature: 25 ° C.) of the hypochlorite aqueous solution in the decomposition step at 9.5 or higher. Method.
[2] The method for producing a porous membrane according to [1], wherein a base is supplied to the hypochlorite aqueous solution and the pH (measurement temperature: 25 ° C.) is maintained at 9.5 or higher in the decomposition step.
[3] The method for producing a porous membrane according to [2], wherein the base is at least one of sodium hydroxide and calcium hydroxide.
[4] The porous membrane according to [2] or [3], wherein a base is supplied when the pH (measurement temperature: 25 ° C.) of the hypochlorite aqueous solution in the decomposition step is 10.5 or less. Production method.
[5] The method for producing a porous membrane according to any one of [1] to [4], wherein the concentration of the hypochlorite aqueous solution in the decomposition step is 1.0% by mass or more.
[6] The method for producing a porous membrane according to any one of [1] to [5], wherein the fluororesin is polyvinylidene fluoride.
[7] The method for producing a porous membrane according to any one of [1] to [6], wherein the pore opening aid is a hydrophilic polymer.
[8] The method for producing a porous membrane according to [7], wherein the hydrophilic polymer is at least one selected from the group consisting of polyvinyl pyrrolidone, polyvinyl alcohol, and polyethylene glycol.
[9] The porosity according to any one of [1] to [8], wherein the concentration of the pore opening aid in the hypochlorite aqueous solution in the decomposition step is 0.1 to 20.0 mass%. A method for producing a membrane.
[10] The method for producing a porous membrane according to any one of [1] to [9], wherein the temperature of the hypochlorite aqueous solution in the decomposition step is 80 to 100 ° C.

  According to the method for producing a porous membrane of the present invention, it is possible to decompose the pore opening assistant remaining in the porous membrane precursor in a short time and with a reduced amount of the oxidizing agent used.

It is a schematic block diagram which shows an example of the decomposition | disassembly apparatus used for the manufacturing method of the porous membrane of this invention.

Hereinafter, an embodiment of the present invention will be described in detail by taking a hollow fiber membrane as an example of a porous membrane.
In the present specification, after completion of the pore opening aid decomposition, the one that has been washed with water is referred to as a porous membrane, and the portion before the pore opening aid decomposition is referred to as a porous membrane precursor. When the porous membrane is a hollow fiber membrane, the one that has been washed with water after decomposition of the opening aid is called a hollow fiber membrane, and the one before the opening aid decomposition is called a hollow fiber membrane precursor. .
Moreover, since pH changes with temperature, all pH described in this specification is the value measured in 25 degreeC.

  The method for producing a porous membrane according to the present embodiment includes a step of forming a porous membrane precursor by coagulating a film-forming stock solution containing a fluororesin and a pore opening aid with a coagulating liquid (hereinafter referred to as “film-forming step”). And the pore opening aid remaining in the porous membrane precursor by washing the porous membrane precursor obtained in the membrane formation step with a hypochlorite aqueous solution whose pH is maintained at 9.5 or higher. Obtained by decomposing and removing (hereinafter also referred to as “decomposition step”), a step of washing the porous membrane precursor after the decomposition step with water (hereinafter also referred to as “water washing step”), and a water washing step. And a step of drying the obtained porous membrane (hereinafter also referred to as “drying step”).

<Film forming process>
The film forming step is a step of forming a porous film precursor by coagulating a film forming stock solution containing a fluororesin and an opening aid with a coagulating liquid.
Examples of the fluororesin include polyvinylidene fluoride, polyvinyl fluoride, copolymers of vinylidene fluoride and other monomers (for example, hexafluoropropylene, acrylic monomers, and the like). These fluororesins may be used individually by 1 type, and may use 2 or more types together. Among these, polyvinylidene fluoride is preferable because the acid resistance and durability of the porous film are improved.
10-30 mass% is preferable and, as for content of the fluororesin in 100 mass% of film forming stock solutions, 15-25 mass% is more preferable. If the content of the fluororesin is within the above range, the stability during film formation can be maintained well, and a suitable hollow fiber membrane structure tends to be formed.

Examples of the pore opening aid include hydrophilic polymers, and specific examples include polyvinyl pyrrolidone, polyvinyl alcohol, and polyethylene glycol. These opening aids may be used alone or in combination of two or more.
1-20 mass% is preferable and, as for content of the hole opening assistant in 100 mass% of film forming undiluted | stock solution, 5-12 mass% is more preferable. If the content of the pore opening aid is 1% by mass or more, it becomes easy to form a hollow fiber membrane precursor, and if it is 20% by mass or less, an increase in the viscosity of the membrane-forming stock solution can be suppressed. It can be maintained well.

When polyvinylpyrrolidone is used as the pore opening aid, the polyvinyl pyrrolidone preferably has a K value of 30 to 120.
Here, the K value is often used as a parameter corresponding to the molecular weight of polyvinylpyrrolidone, and is calculated from the following equation (1) based on the kinematic viscosity measurement result in the capillary viscometer method of the aqueous solution of polyvinylpyrrolidone. The

In the above formula (1), c is the mass (g) of polyvinylpyrrolidone in 100 ml of the solution, and η _rel is the ratio of the kinematic viscosity of the polyvinylpyrrolidone aqueous solution to the kinematic viscosity of water.

  If the K value of polyvinylpyrrolidone is within the above range, the formability will be good and the desired hollow fiber membrane will tend to be easily obtained. In particular, when polyvinyl pyrrolidone having a high K value, that is, a high molecular weight is used, a hollow fiber membrane having a good membrane structure tends to be easily formed. On the other hand, polyvinyl pyrrolidone having a small K value, that is, a low molecular weight, is preferable in that it is more easily removed from the hollow fiber membrane precursor in the decomposition step described later. Therefore, the same kind of polyvinylpyrrolidone having different K values may be appropriately mixed depending on the purpose.

The film-forming stock solution is prepared by dissolving a fluororesin and a pore opening aid in a solvent.
Examples of the solvent include water-soluble aprotic polar solvents represented by dimethylformamide, dimethylacetamide, N-vinylpyrrolidone and the like.
The temperature of the film-forming stock solution is not particularly limited, but is usually 20 to 60 ° C.

  When spinning into a hollow fiber membrane, the membrane forming raw solution is discharged into a coagulating liquid from a nozzle having an annular discharge port, and solidified in the coagulating liquid to form a hollow fiber membrane precursor. The spinning may be carried out by either a dry-wet spinning method in which the film-forming stock solution is introduced into the coagulating liquid through an idle running portion in contact with air, or a wet spinning method in which the film-forming stock solution is directly introduced into the coagulating liquid.

There is no restriction | limiting in particular in the structure of a hollow fiber membrane precursor, For example, the thing provided with the porous base material may be used, and it may be a multilayer structure and durable with respect to the rubbing at the time of handling.
Although it does not specifically limit as a porous base material, For example, the hollow knitted string, braided string, etc. which were made with various fibers are mentioned, Various materials can be used individually or in combination. Examples of fibers used for the hollow knitted string and braided string include synthetic fibers, semi-synthetic fibers, regenerated fibers, and natural fibers. The form of the fiber may be any of monofilament, multifilament, and spun yarn.

As the coagulation liquid, water, alcohols, glycerin, ethylene glycol or the like can be used alone or in combination. The coagulation liquid may be a mixed liquid of water and a good solvent for the hydrophobic polymer.
The temperature of the coagulation liquid is not particularly limited, but is usually 60 to 90 ° C.

<Disassembly process>
The decomposition step is a step of washing the porous membrane precursor obtained by the film formation step with a hypochlorite aqueous solution and decomposing and removing the pore opening assistant remaining in the porous membrane precursor. At this time, the porous membrane precursor is washed while maintaining the pH of the hypochlorite aqueous solution at 9.5 or higher.
In the porous membrane precursor such as the hollow fiber membrane precursor obtained by the membrane forming process, the pore opening aid such as polyvinylpyrrolidone remains, and therefore there is almost no membrane water permeability at this stage. Therefore, the pore opening aid remaining in the porous membrane precursor is decomposed and removed by washing the porous membrane precursor with a specific hypochlorite aqueous solution.

Examples of hypochlorite include sodium salt, potassium salt, calcium salt of hypochlorous acid. Among these, sodium hypochlorite is preferable.
The concentration of the hypochlorite aqueous solution is preferably 1.0 to 13.0% by mass, and more preferably 6.0 to 12.0% by mass. If the concentration of the hypochlorite aqueous solution is 1.0% by mass or more, a desired cleaning effect can be easily obtained in a short time, and if it is 13.0% by mass or less, the cost can be suppressed.

In order to suppress the decomposition of hypochlorite during the cleaning of the porous membrane precursor, the pH of the hypochlorite aqueous solution during the decomposition step is maintained at 9.5 or higher. Here, “maintaining the pH at 9.5 or higher” means that if the pH of the hypochlorite aqueous solution being washed is measured at 25 ° C., it is always 9.5 or higher.
If the pH of the hypochlorite aqueous solution is not maintained at 9.5 or higher during the decomposition step, the decomposition of hypochlorite becomes significant and a sufficient cleaning effect cannot be obtained.

Moreover, it is preferable that pH of the hypochlorite aqueous solution in a decomposition process is maintained at 13.2 or less. If the pH of the hypochlorite aqueous solution is maintained at 13.2 or less, coloring of the hollow fiber membrane due to the dehydrofluorination reaction of a fluororesin such as polyvinylidene fluoride can be suppressed. Here, “maintaining the pH at 13.2 or lower” means that if the pH of the aqueous hypochlorite solution being washed is measured at 25 ° C., it is always 13.2 or lower.
The hypochlorite aqueous solution is preferably maintained at a pH of 10.0 to 13.0.

In order to maintain the pH of the hypochlorite aqueous solution at 9.5 or higher, it is preferable to supply a base to the hypochlorite aqueous solution during the decomposition step.
Examples of the base include hydroxides, alkali metal salts, alkaline earth metal salts, alkali metal carbonates, alkaline earth metal carbonates, and the like. Among these, a hydroxide is preferable from the viewpoint that the deactivation suppression effect of hypochlorite is high.
Examples of the hydroxide include sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide and the like. These hydroxides may be used individually by 1 type, and may use 2 or more types together.

Moreover, as above-mentioned, when the pH of hypochlorite aqueous solution exceeds 13.2, a hollow fiber membrane will become easy to color. From the viewpoint of preventing coloration, it is preferable to use a poorly soluble strong base as the base. The poorly soluble strong base can suppress the pH of the hypochlorite aqueous solution from becoming too high even when supplied in excess, and has a high effect of suppressing the deactivation of hypochlorite.
Examples of the hardly soluble strong base include calcium hydroxide and magnesium hydroxide, and calcium hydroxide is preferable.
Here, “slightly soluble” refers to those having a solubility at the use temperature of 0.5 [g / 100 g-water] or less.

  On the other hand, when a water-soluble strong base such as sodium hydroxide or potassium hydroxide is used as the base, from the viewpoint of preventing coloring, hypochlorite is monitored while monitoring so that the pH of the hypochlorite aqueous solution does not become too high. It is preferable to supply to the aqueous acid salt solution. In this case, after supplying a water-soluble strong base to the hypochlorite aqueous solution, an aqueous solution of a strong base may be supplied to the hypochlorite aqueous solution in order to rapidly disperse uniformly. In order to prevent the concentration of the hypochlorite aqueous solution from diluting due to the supply of the strong base aqueous solution, the solution in which the strong base is dissolved in the hypochlorite aqueous solution is used as a hypochlorite aqueous solution. Can also be supplied.

  It should be noted that a strong base and a weak acid salt can be used in combination in that the pH of the hypochlorite aqueous solution does not become excessively high even if supplied in excess, but the pH is lowered during the decomposition step. In order to suppress it, it is necessary to input a large amount. If it is a strong base such as sodium hydroxide, potassium hydroxide, or calcium hydroxide described above, the pH of the hypochlorite aqueous solution can be maintained at 9.5 or higher even in a small amount, and the cost is excellent. In particular, sodium hydroxide and calcium hydroxide are preferable in that they can be obtained at low cost.

  In order to maintain the pH of the hypochlorite aqueous solution at 9.5 or higher, hypochlorous acid is considered in consideration of the time from the supply of the base to the reflection of the pH of the hypochlorite aqueous solution. It is preferable to supply the base when the pH of the aqueous salt solution becomes 10.5 or less.

By the way, as described above, even when the pH of the initial hypochlorite aqueous solution is adjusted, in the case where the traveling porous membrane precursor is continuously treated, it is opened during the decomposition of the pore opening aid. There was a problem that the pore assistant gradually eluted into the hypochlorite aqueous solution and the pH was lowered, the pH became less than 9.5, and the hypochlorite was decomposed. In order to maintain the pH of the hypochlorite aqueous solution at 9.5 or higher, it is conceivable to increase the pH of the initial hypochlorite aqueous solution in advance, but if the pH exceeds 13.2, the pH will be hollow. The yarn film tends to be easily colored.
Therefore, in the present invention, it is preferable to maintain the pH of the hypochlorite aqueous solution during the decomposition step at 9.5 to 13.2, thereby preventing the hollow fiber membrane from being colored, Elution into a hypochlorite aqueous solution can suppress the decomposition of hypochlorite. In particular, if a base is supplied during the decomposition step, specifically, if a base is supplied when the pH of the hypochlorite aqueous solution is 10.5 or less, the pH of the hypochlorite aqueous solution is 9 .5 to 13.2 can be easily maintained.
The base is preferably supplied by adjusting the type, concentration, supply amount, etc. so that the pH of the hypochlorite aqueous solution does not exceed 13.2. Further, the pH of the hypochlorite aqueous solution tends to gradually decrease due to elution of the pore opening aid. Therefore, the pH of the hypochlorite aqueous solution is easily maintained at 13.2 or lower. However, when the pH may exceed 13.2 due to supply of a base, an acid such as hydrochloric acid or acetic acid is supplied. The pH is preferably adjusted to be in the range of 9.5 to 13.2 (preferably 10.5 to 13.2).

  As the concentration of the pore opening aid in the hypochlorite aqueous solution increases, the amount of base supplied to maintain the pH increases, so the concentration of the pore opening aid in the hypochlorite aqueous solution is 20. 0 mass% or less is preferable and 10.0 mass% or less is more preferable. When the concentration of the pore opening aid in the hypochlorite aqueous solution exceeds 20.0% by mass, it is preferable to replace the hypochlorite aqueous solution with a new one.

In addition, if a pore opening assistant such as polyvinylpyrrolidone is present in the hypochlorite aqueous solution, it is necessary to adjust the pH of the initial hypochlorite aqueous solution during the decomposition and cleaning of the pore opening assistant. The pH of the aqueous hypochlorite solution is lowered, and hypochlorite is decomposed and deactivated.
However, according to the present invention, since the pH of the hypochlorite aqueous solution during the decomposition step is maintained at 9.5 or higher, even if a pore opening aid is present in the hypochlorite aqueous solution (that is, Further, even if the concentration of the pore opening aid in the hypochlorite aqueous solution is 0.1% by mass or more), it is possible to suppress the deactivation of the hypochlorite and to obtain a sufficient cleaning effect.

  Decomposition and cleaning of the pore opening assistant such as polyvinylpyrrolidone with an aqueous hypochlorite solution is preferably performed at 80 to 100 ° C, more preferably 90 to 100 ° C. If the decomposition cleaning is performed at 80 ° C. or higher, a sufficient decomposition rate of polyvinylpyrrolidone can be obtained, so that the decomposition cleaning can be completed within a desired time. On the other hand, if the decomposition cleaning is performed at 100 ° C. or lower, it is not necessary to keep the heating part at 1 atm or higher, so that it is possible to prevent complication of the apparatus such as providing a seal part during continuous processing. Moreover, cost can be suppressed. Moreover, the deactivation of hypochlorite can be suppressed more.

The decomposition process is performed using, for example, the decomposition apparatus 10 of FIG.
The decomposition apparatus 10 shown in FIG. 1 contacts a chemical solution (hypochlorite aqueous solution) with respect to the decomposition tank 11, traveling rolls 12 a to 12 d for traveling the hollow fiber membrane precursor 30, and the hollow fiber membrane precursor 30. And a base addition means 14 for supplying a base to the chemical tank 13. In addition, the decomposition apparatus 10 includes heating means (not shown) that supplies steam to the decomposition tank 11 and heats the inside of the decomposition tank 11 in order to perform decomposition cleaning of the opening assistant at a desired temperature.

A drain discharge port 15 is provided at the bottom of the decomposition tank 11.
The chemical tank 13 and the base addition means 14 are provided in the decomposition tank 11, and the base is supplied from the base supply means 18 provided above the decomposition tank 11 to the chemical liquid tank 13 through the base addition means 14. It is like that.

  The chemical tank 13 is provided with a partition plate 21 for overflow and a chemical discharge port 22. A pH measuring instrument 20 is installed downstream of the chemical solution outlet 22.

The chemical tank 13 is disposed so that the traveling rolls 12 b and 12 c are immersed in the chemical tank 13. Thereby, the hollow fiber membrane precursor 30 travels in the chemical solution tank 13, and as a result, the hypochlorite aqueous solution comes into contact with the hollow fiber membrane precursor 30.
In the chemical tank 13, the supply of the hypochlorite aqueous solution from the chemical liquid supply means 16 provided outside the decomposition tank 11 to the chemical liquid tank 13 and the discharge of the hypochlorite aqueous solution from the chemical liquid tank 13 are continuous. To be done. The liquid discharged from the chemical liquid tank 13 overflows the partition plate 21 and is then discharged from the chemical liquid discharge port 22.

  The material of the decomposition tank 11, the traveling rolls 12 a to 12 d, the chemical solution tank 13, the base addition means 14, the drain discharge port 15, the partition plate 21, and the chemical solution discharge port 22 may be those having oxidation resistance and heat resistance. Although it does not specifically limit, For example, titanium, polytetrafluoroethylene, polyetheretherketone (PEEK), a ceramic, etc. are mentioned. Moreover, as a special material form, it is the material illustrated above to materials with poor oxidation resistance such as stainless steel and aluminum, and for the decomposition tank 11, the chemical liquid tank 13, the drain outlet 15, and the chemical outlet 22 Examples of the inner surface lining, the traveling rolls 12a to 12d, and the partition plate 21 coated on the outer surface can also be exemplified.

  In the decomposition step using the decomposition apparatus 10 of FIG. 1, first, saturated steam at normal pressure is continuously supplied into the decomposition tank 11, and the decomposition tank 11 is filled with saturated steam and heated. Here, the temperature in the decomposition tank 11 is ideally about 100 ° C., which is a saturated steam temperature at normal pressure, but may be lower than that (about 80 ° C.). On the other hand, the hypochlorite aqueous solution is supplied from the chemical liquid supply means 16 to the chemical liquid tank 13 disposed in the decomposition tank 11 by the supply pump 17. Here, a heater (not shown) is provided between the supply pump 17 and the chemical solution tank 13 outside the decomposition tank 11 so that the chemical solution is heated outside the decomposition tank 11 and then supplied to the chemical solution tank 13. Also good.

  Next, after confirming that the hypochlorite aqueous solution in the chemical solution tank 13 has reached a steady temperature, the hollow fiber membrane precursor 30 is introduced into the decomposition tank 11. The traveling speed of the hollow fiber membrane precursor 30 in the decomposition tank 11 is preferably 4 to 50 m / min, for example.

  The hollow fiber membrane precursor 30 introduced into the decomposition tank 11 passes through the traveling roll 12a and is heated by saturated steam filling the decomposition tank 11. Since the chemical | medical solution tank 13 is arrange | positioned at the traveling rolls 12b and 12c, while the hollow fiber membrane precursor 30 passes the traveling rolls 12b and 12c, the hypochlorite aqueous solution heated by saturated steam is hollow. By contacting with the thread membrane precursor 30, the hollow fiber membrane precursor 30 is washed, and the opening aid contained in the hollow fiber membrane precursor 30 is decomposed and removed.

  Next, the hollow fiber membrane precursor 30 is led out of the decomposition tank 11 through the traveling roll 12d.

In the decomposition tank 11, the hollow fiber membrane precursor 30 is first heated by saturated steam. Thereafter, the heated hollow fiber membrane precursor 30 is brought into contact with the hypochlorite aqueous solution heated in the chemical bath 13. The aqueous hypochlorite solution in contact with the hollow fiber membrane precursor 30 immediately penetrates into the hollow fiber membrane precursor 30. Further, since the hollow fiber membrane precursor 30 contacted and permeated with the hypochlorite aqueous solution is kept warm with saturated water vapor by traveling in the decomposition tank 11, it is opened by the contacted and permeated hypochlorite aqueous solution. The decomposition of the pore assistant also starts and proceeds almost simultaneously with the penetration of the hypochlorite aqueous solution.
The condensed water of saturated water vapor is discharged from the drain discharge port 15.

  Although there is no restriction | limiting in particular in the residence time of the hollow fiber membrane precursor 30 in the chemical | medical solution tank 13, The residence time from which a hypochlorite aqueous solution will fully contact and hold | maintain the hollow fiber membrane precursor 30 is set.

The pH of the hypochlorite aqueous solution discharged from the chemical solution discharge port 22 is measured by the pH measuring instrument 20. At this time, when the pH of the hypochlorite aqueous solution is not within the predetermined range, in order to suppress the deactivation of hypochlorite, the hypochlorite aqueous solution in the chemical tank 13 The base is supplied in the middle of the decomposition step, and the pH of the hypochlorite aqueous solution in the chemical bath 13 is controlled to be maintained at 9.5 or higher (preferably the pH is 9.5 to 13.2). .
Here, if the pH of the hypochlorite aqueous solution in the pH measuring instrument 20 is 9.5 or higher, “the pH of the hypochlorite aqueous solution in the chemical bath 13 can be maintained at 9.5 or higher”. I reckon.

Supply of the base to the hypochlorite aqueous solution in the chemical tank 13 is performed by the base supply pump 19 while monitoring the pH of the hypochlorite aqueous solution in the chemical tank 13 outside the decomposition tank 11. From the base supply means 18 provided in, the base is suitably supplied via the base addition means 14 provided in the decomposition tank 11. At this time, feedback control or the like can be incorporated for automatic control.
A heater (not shown) may be provided between the base supply pump 19 and the chemical solution tank 13 outside the decomposition tank 11 so that the base is heated outside the decomposition tank 11 and then supplied to the chemical solution tank 13. . If the concentration of the supplied base is too low, a large amount of base aqueous solution is added to maintain the pH, and the sodium hypochlorite concentration is lowered and the resolution is lowered. In addition, when the concentration of the supplied base is too high, a portion where the base is concentrated locally is generated before the aqueous solution is mixed, leading to the coloring of the film as described above. Therefore, the base concentration to be supplied is preferably 1 to 10 mol / L, for example.
In addition, pH shown in this specification is the value measured in 25 degreeC. A cooler (not shown) is provided between the chemical solution outlet 22 and the pH measuring instrument 20, and the temperature of the discharged chemical solution is set to 25 ° C. and then measured.

  The concentration and flow rate of hypochlorite in the chemical bath 13 can be controlled so that the pH of the hypochlorite aqueous solution is 9.5 or more (preferably the pH is 9.5 to 13.2). It is preferable to set by. Specifically, it is appropriately set from the viewpoint of the remaining state of the pore opening aid in the hollow fiber membrane precursor 30 and the use efficiency of hypochlorite and base.

  Thus, according to the decomposition process using the decomposition apparatus 10 of the illustrated example, the heated hypochlorite aqueous solution is brought into contact with the hollow fiber membrane precursor 30 to wash the hollow fiber membrane precursor 30. When the hypochlorite aqueous solution comes into contact with the hollow fiber membrane precursor 30, penetration of the hypochlorite aqueous solution into the hollow fiber membrane precursor 30 and the hypochlorite contained in the hypochlorite aqueous solution Decomposition of the pore opening aid proceeds almost simultaneously. Moreover, since the pH of the hypochlorite aqueous solution is maintained at 9.5 or higher, the decomposition and deactivation of hypochlorite can be suppressed. As a result, the opening aid remaining in the hollow fiber membrane precursor 30 can be decomposed in a short time while suppressing the amount of oxidant (hypochlorite) used. Further, if the pH of the hypochlorite aqueous solution is maintained at 13.2 or less, the resulting hollow fiber membrane can be prevented from being colored.

<Washing process>
The water washing step is a step of washing the porous membrane precursor after the decomposition step with water.
The washing method is not particularly limited, and examples include a method of immersing in water. Further, as the water washing step, for example, two steps of a decompression step-cleaning liquid supply step and a decompression step-cleaning liquid supply step-depressurization step described in JP-A-2008-161755 may be performed.
Although the water washing time is not particularly limited, it is usually 10 minutes or more in the case of the method of immersing in water, and it is about 1 minute in the case of the two or three steps described in JP-A-2008-161755.

<Drying process>
A drying process is a process of drying the porous membrane obtained by the water washing process.
The drying method is not particularly limited, and examples thereof include a method of introducing the porous membrane into a drying device such as a hot air dryer.

<Effect>
According to the method for producing a porous membrane of the present invention described above, the porous membrane precursor obtained by the membrane-forming process is washed with a hypochlorite aqueous solution, so that the remaining pores in the porous membrane precursor Auxiliary agents can be decomposed. Moreover, in the present invention, the pH of the hypochlorite aqueous solution is maintained at 9.5 or higher. Therefore, it is possible to prevent the hypochlorite from being decomposed or deactivated during the decomposition process, and in a short time, while reducing the amount of oxidizing agent (hypochlorite) used, the pore opening aid Can be disassembled. In particular, if the pH of the hypochlorite aqueous solution is maintained at 13.2 or less, coloring of the porous membrane can be prevented.
If a base is supplied to the hypochlorite aqueous solution in the decomposition step in the middle of the decomposition step, the pH of the hypochlorite aqueous solution in the decomposition step can be easily maintained at 9.5 or more.

  Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited thereto.

[Example 1]
Polyvinylidene fluoride A (trade name Kyner 761A, mass average molecular weight: 720,000), polyvinylpyrrolidone B (Nippon Shokubai Co., Ltd., trade name K-79, mass average) so as to have the mass ratio shown in Table 1. Molecular weight: 900,000, K value: 79), N, N-dimethylacetamide (manufactured by Samsung Fine Chemical Co., Ltd.) were mixed and dissolved by stirring at 60 ° C. to prepare a film-forming stock solution (1) and a film-forming stock solution (2). .
Separately, a sodium hypochlorite aqueous solution (manufactured by Shintomo Co., Ltd.) was diluted with water so as to have a concentration of 11.5% by mass. The pH of the diluted aqueous sodium hypochlorite solution was 13.0.

Next, a nozzle having a hollow portion formed at the center and an annular discharge port formed in order so that two types of film-forming stock solutions can be applied and laminated on the outside is prepared and kept at 30 ° C. In the state, a polyester multifilament single filament braid (multifilament; 420T / 180F) is introduced into the hollow portion as a porous base material, and a film-forming solution (2) and a film-forming solution (1) are provided on the outer periphery thereof. The hollow fiber membrane precursor was obtained by coating from the inside and coagulating in a coagulating liquid (mixed solution of 8 parts by mass of N, N-dimethylacetamide and 92 parts by mass of water) kept at 75 ° C. (film forming process) ).
With respect to the obtained hollow fiber membrane precursor, the amount of permeated water was measured by allowing pure water having a temperature of 25 ° C. and a gauge pressure of 0.1 MPa to pass from the outside to the inside of the hollow fiber membrane precursor. Moreover, the color of the hollow fiber membrane precursor was confirmed visually. These results are shown in Table 2.

Next, 200 g of an aqueous sodium hypochlorite solution was heated to 100 ° C. Subsequently, 7.6 g of an aqueous solution of polyvinyl pyrrolidone B having a concentration of 20% by mass in a state heated to 100 ° C. was added to prepare an aqueous sodium hypochlorite solution containing 1.4% by mass of polyvinyl pyrrolidone.
As described above, the pore opening assistant is eluted into the hypochlorite aqueous solution during the decomposition of the hole opening assistant. Therefore, in Example 1 and Example 2 below, polyvinyl pyrrolidone is contained in the sodium hypochlorite aqueous solution in advance so that the sodium hypochlorite aqueous solution is in contact with the hollow fiber membrane precursor (ie, The state of the aqueous sodium hypochlorite solution during the decomposition process) was reproduced.

The concentration and pH of the aqueous sodium hypochlorite solution immediately after the addition of polyvinylpyrrolidone (elapsed time 0 minutes) were measured. These results are shown in Table 2.
The concentration of the sodium hypochlorite aqueous solution was measured as follows.
The collected sodium hypochlorite aqueous solution is rapidly cooled in an ice bath at 0 ° C. to stop the decomposition of sodium hypochlorite, and then the temperature of the solution is lowered to 25 ° C., then the handy water quality meter “AQUAB” manufactured by Shibata Kagaku Co., Ltd. The concentration of the sodium hypochlorite aqueous solution was measured using “AQ-101”.
Moreover, the pH of the sodium hypochlorite aqueous solution was measured using a multi-water quality meter “MM-60R” manufactured by Toa DKK Corporation and a pH composite electrode “GST-5731C”.

  The sodium hypochlorite aqueous solution to which polyvinyl pyrrolidone is added has the pH of the liquid lowered with time. The pH of the sodium hypochlorite aqueous solution was measured and monitored, and a sodium hydroxide aqueous solution having a concentration of 1 mol / L was appropriately supplied midway so that the pH of the sodium hypochlorite aqueous solution was maintained at 9.5 or higher.

At the timing when 1 minute has passed since the addition of polyvinylpyrrolidone, 1.0 g of the hollow fiber membrane precursor obtained by the film-forming step was immersed in an aqueous sodium hypochlorite solution, washed at 100 ° C. for 2 minutes, and then hollow. The thread membrane precursor was taken out.
Subsequently, the taken out hollow fiber membrane precursor was washed with pure water at 25 ° C. for 10 minutes to obtain a hollow fiber membrane. About the obtained hollow fiber membrane, the amount of permeated water was measured by allowing pure water having a temperature of 25 ° C. and a gauge pressure of 0.1 MPa to pass from the outside to the inside of the hollow fiber membrane. Moreover, the color of the hollow fiber membrane was confirmed visually. These results are shown in Table 2.
Subsequently, the hollow fiber membrane precursor obtained in the sodium hypochlorite aqueous solution by the film-forming process in the same manner as when 1 minute had passed at each timing of 5 minutes, 10 minutes, and 15 minutes after the addition of polyvinylpyrrolidone. 1.0 g of the body was immersed to obtain a hollow fiber membrane. The amount of permeated water was measured in the same manner for the obtained hollow fiber membrane. Moreover, the color of the hollow fiber membrane was confirmed visually. These results are shown in Table 2.
In parallel with the washing of the hollow fiber membrane precursor, the concentration of the sodium hypochlorite aqueous solution was measured at each timing of 1 minute, 5 minutes, 10 minutes, and 15 minutes after the addition of polyvinylpyrrolidone. did. These results are shown in Table 2.
Table 2 shows the pH measurement results at each timing when 1 minute, 5 minutes, 10 minutes, and 15 minutes had elapsed since the addition of polyvinylpyrrolidone.
The total amount of 1 mol / L aqueous sodium hydroxide solution added dropwise to maintain the pH of the aqueous sodium hypochlorite solution at 10.0 or higher during the 17 minutes after the addition of polyvinylpyrrolidone was 35.5 g. It was.
Note that Example 1 corresponds to an example, and Example 2 corresponds to a comparative example.

"Example 2"
The same procedure as in Example 1 was conducted except that the sodium hypochlorite aqueous solution was not supplied with a 1 mol / L sodium hydroxide aqueous solution. The results are shown in Table 3.

  In Tables 2 and 3, “elapsed time after addition of PVP” is the elapsed time (minutes) from the addition of polyvinylpyrrolidone to the sodium hypochlorite aqueous solution.

  As is apparent from the results in Table 2, in Example 1, the concentration of the aqueous sodium hypochlorite solution was 3.9% by mass or more even after 15 minutes had elapsed since the addition of polyvinylpyrrolidone to the aqueous sodium hypochlorite solution. And decomposition of sodium hypochlorite was suppressed. From this result, by supplying a base such as sodium hydroxide to the sodium hypochlorite aqueous solution on the way, while maintaining the pH of the sodium hypochlorite aqueous solution at 9.5 or higher during the decomposition step, the hollow fiber It was shown that the membrane precursor can be washed with an aqueous sodium hypochlorite solution.

  The hollow fiber membranes obtained by washing the hollow fiber membrane precursor with the aqueous sodium hypochlorite solution after each time had elapsed after adding polyvinyl pyrrolidone to the aqueous sodium hypochlorite solution, The amount of permeated water was high, and the water permeation performance was kept high. Moreover, the obtained hollow fiber membrane maintained white and coloring was suppressed.

In addition, 1.0 g of hollow fiber membrane precursors were immersed in an aqueous sodium hypochlorite solution prepared in the same manner as in Example 1 without adding polyvinylpyrrolidone, washed at 100 ° C. for 2 minutes, and then hollow fiber. The hollow fiber membrane obtained by taking out the membrane precursor and washing it with pure water at 25 ° C. for 10 minutes was measured for the amount of permeated water and found to be 22.3 g / cm ² / min and the membrane color was white. It was.
From this result, Example 1 in which the pH of the sodium hypochlorite aqueous solution in which the hollow fiber membrane precursor was immersed was maintained at 9.5 or higher was obtained when polyvinyl pyrrolidone was not added even though polyvinyl pyrrolidone was added. It was found that a hollow fiber membrane having the same permeated water amount was obtained.

  On the other hand, as is clear from the results in Table 3, in Example 2, the base was not supplied to the sodium hypochlorite aqueous solution halfway, so the initial pH of the initial sodium hypochlorite aqueous solution was as high as 13.0. As time passed, the pH of the sodium hypochlorite aqueous solution fell below 9.5, and the concentration of sodium hypochlorite significantly decreased. Moreover, the amount of permeated water of the hollow fiber membrane decreased with the passage of time, and the water permeation performance could not be maintained.

DESCRIPTION OF SYMBOLS 10 Decomposition | disassembly apparatus 11 Decomposition tank 12a Traveling roll 12b Traveling roll 12c Traveling roll 12d Traveling roll 13 Chemical solution tank 14 Base addition means 15 Drain discharge port 16 Chemical supply means 17 Supply pump 18 Base supply means 19 Base supply pump 20 pH measuring instrument 21 Partition Plate 22 Chemical solution outlet 30 Hollow fiber membrane precursor

Claims (10)

A film forming step of coagulating a film forming stock solution containing a fluororesin and an opening aid with a coagulating liquid to form a porous film precursor;
A decomposition step of decomposing and removing the pore opening aid remaining in the porous membrane precursor with a hypochlorite aqueous solution,
A method for producing a porous membrane, wherein the pH (measurement temperature: 25 ° C.) of the hypochlorite aqueous solution in the decomposition step is maintained at 9.5 or higher.   The method for producing a porous membrane according to claim 1, wherein a base is supplied to the hypochlorite aqueous solution in the decomposition step to maintain the pH (measurement temperature: 25 ° C) at 9.5 or higher.   The method for producing a porous membrane according to claim 2, wherein the base is at least one of sodium hydroxide and calcium hydroxide.   The method for producing a porous membrane according to claim 2 or 3, wherein a base is supplied when the pH (measurement temperature: 25 ° C) of the hypochlorite aqueous solution in the decomposition step becomes 10.5 or less.   The manufacturing method of the porous membrane as described in any one of Claims 1-4 whose density | concentration of the hypochlorite aqueous solution in the said decomposition | disassembly process is 1.0 mass% or more.   The manufacturing method of the porous membrane as described in any one of Claims 1-5 whose said fluororesin is polyvinylidene fluoride.   The method for producing a porous membrane according to any one of claims 1 to 6, wherein the pore opening aid is a hydrophilic polymer.   The method for producing a porous membrane according to claim 7, wherein the hydrophilic polymer is at least one selected from the group consisting of polyvinyl pyrrolidone, polyvinyl alcohol, and polyethylene glycol.   The manufacturing method of the porous membrane as described in any one of Claims 1-8 whose density | concentration of the pore opening aid in the hypochlorite aqueous solution in the said decomposition | disassembly process is 0.1-20.0 mass%. .   The manufacturing method of the porous membrane as described in any one of Claims 1-9 whose temperature of the hypochlorite aqueous solution in the said decomposition | disassembly process is 80-100 degreeC.

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