JP2004168828A - Method for producing porous membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a porous membrane, by which an additive polymer can efficiently be removed from a film comprising a membrane-forming polymer and the additive polymer in a short time. <P>SOLUTION: This method for producing the porous membrane comprises bringing a film comprising the polymer (1) and the polymer (2) having higher oxidative decomposability than that of the polymer (1) into contact with an aqueous solution containing an oxidizing agent and an oxidizing agent decomposition-accelerating agent to decompose away at least one part of the polymer (2). The decomposition of the oxidizing agent is accelerated, and the decomposition of the polymer (2) can thereby be accelerated. Hence, the polymer (2) can efficiently be removed in a short time to achieve a high penetration performance. Accordingly, a process such as a spinning process and a washing process can continuously be carried out, whereby the method for producing the porous membrane is a profitable producing method. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a porous membrane having a high permeability coefficient.
[0002]
[Prior art]
Microfiltration membranes, ultrafiltration membranes made of cellulose acetate, polyacrylonitrile, polysulfone, etc. in the fields of food industry, medical field, electronics industry, etc. Reverse osmosis membranes and the like are used, and many of these membranes use porous hollow fiber membranes produced by wet or dry-wet spinning.
[0003]
Water permeability is one of the most important performances required for porous membranes.However, when trying to improve water permeability, the concentration and coagulation of the spinning solution are generally increased to increase the membrane pore size. Adjust the liquid. When wet or dry-wet spinning is performed, a stock solution obtained by dissolving a polymer forming a film and an additive polymer in a solvent is used.
[0004]
At this time, as the additive polymer, a high-molecular-weight hydrophilic polymer such as polyethylene glycol or polyvinylpyrrolidone is added to improve the viscosity of the stock solution during spinning and stabilize the film formation state. Many.
[0005]
A porous membrane produced by wet or dry-wet spinning has a high water-permeability because a large amount of high-molecular-weight additive polymer remains in the membrane at the stage where the coagulation is completed after passing through a coagulation bath. Function cannot be exhibited. Therefore, after the solidification is completed, it is necessary to remove the high molecular weight additive polymer remaining in the film.
[0006]
As a method for removing the additive polymer, a method for removing the polymer using an oxidizing agent or a hydrolyzing agent is known (for example, see Patent Document 1).
[0007]
In the production of the porous membrane, spinning, coagulation, and washing are continuously performed, so that the production cost can be reduced. Here, when it is necessary to perform the cleaning for a long time, the equipment must be extremely large in order to manufacture continuously, so that it is desired that the water permeation performance is expressed by a decomposition treatment in a time as short as possible. .
[0008]
Increasing the concentration of the oxidizing agent is basically effective for shortening the decomposition treatment time, but increasing the concentration of the oxidizing agent not only increases the cost of the oxidizing agent but also increases the cleaning time. Since the corrosion of the equipment to be used becomes severe, there is a problem that the cost is further increased due to the necessity of a corrosion-resistant material, the frequency of equipment replacement, and the like.
[0009]
Also, when disposing of the oxidant after use, it is not possible to flow high-concentration oxidant as wastewater, so decompose the oxidant to lower its concentration or add a large amount of water to dilute the oxidant. It is necessary to do. Therefore, the higher the concentration of the oxidizing agent, the higher the labor and cost.
[0010]
[Patent Document 1]
JP-A-2-302449
[Problems to be solved by the invention]
An object of the present invention is to provide a method for producing a porous film capable of efficiently removing an additive polymer in a short time from a porous film composed of a polymer forming a film and an additive polymer.
[0012]
[Means for Solving the Problems]
That is, the gist of the present invention is to provide a porous membrane containing a polymer (1) and a polymer (2) having higher oxidative decomposability than the polymer (1), comprising an oxidizing agent and a decomposition accelerator of the oxidizing agent. A method for producing a porous membrane, which is brought into contact with an aqueous solution to decompose and remove at least a part of the polymer (2).
[0013]
When the oxidizing agent is an aqueous hypochlorite solution and the decomposition accelerator is a nickel-based catalyst, the decomposition reaction of the polymer (2) is effectively promoted even when the concentration of hypochlorous acid is low. More preferably, the nickel-based catalyst is a nickel oxide catalyst.
[0014]
Further, when the nickel-based catalyst is in a powder form, the reaction rate can be further improved.
[0015]
When the time for contacting the porous membrane with the aqueous solution is less than 10 minutes, it is preferable to continuously perform the production process.
[0016]
It is preferable that the concentration of hypochlorous acid in the aqueous solution be in the range of 100 to 5000 mg / L, because washing can be performed efficiently.
[0017]
When the temperature of the aqueous solution is 70 ° C. or higher, the reaction rate can be further improved.
[0018]
When the polymer (1) is a hydrophobic polymer and the polymer (2) is a hydrophilic polymer, the production method of the present invention can be suitably applied. In particular, the polymer (1) is polyvinylidene fluoride, and the polymer (2) Is polyvinyl pyrrolidone, the production method of the present invention can be particularly suitably applied.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described. The method for producing a porous membrane according to the present invention is not limited to the form of the membrane, and can be used for producing a flat membrane, a tubular membrane, a hollow fiber membrane, and the like.
[0020]
Hereinafter, an embodiment of the present invention will be described using a method for producing a hollow fiber membrane as an example.
[0021]
In the method for producing a porous membrane according to the present invention, the polymer (1) that can be formed into a hollow fiber membrane is dissolved in a solvent, and this solution is used as a spinning dope and is once spun into the air from a spinneret. The dry-wet spinning method of introducing into a non-solvent and coagulating in the non-solvent is preferably used. In addition to the above steps, a stretching step or the like may be included, and the ink may be directly discharged into a non-solvent without passing through air.
[0022]
As the spinneret, any spinneret that can be formed into a hollow fiber membrane is used. For example, spinning is performed using a spinneret such as a double annular nozzle. At this time, the polymer non-solvent may be sent to the center of the spinneret to solidify the inner surface of the hollow fiber membrane.
[0023]
The polymer (1) used in the present invention can be used as long as it can form a porous hollow fiber membrane by wet or dry-wet spinning, but a hydrophobic polymer is preferable, and specifically, polysulfone or polyethersulfone is used. And polyacrylonitrile, cellulose derivatives, fluorine resins such as polyvinylidene fluoride, polyamide, polyester, polymethacrylate, polyacrylate and the like. Further, copolymers of these resins or those obtained by introducing a substituent into a part thereof may be used. Further, a mixture of two or more resins may be used.
[0024]
In particular, fluororesins, especially polyvinylidene fluoride, have high durability against hypochlorous acid and can be preferably used in the production method of the present invention.
[0025]
The stock solution for spinning in the present invention is obtained by uniformly dissolving the polymer (1) in a solvent. At this time, if the concentration of the polymer (1) is too low or too high, the spinning stability is impaired and a desired porous structure cannot be obtained. Preferably, 15% by mass is more preferred. The upper limit is preferably 30% by mass, more preferably 25% by mass.
[0026]
A polymer (2) having higher oxidative decomposability than the polymer (1) is added to the stock solution as an additive for controlling phase separation. Here, “highly oxidatively decomposable” means that when an oxidation treatment is performed under the same conditions, it is more easily decomposed to lower the molecular weight.
[0027]
As the polymer (2), a hydrophilic polymer is preferable, and specific examples include polyvinylpyrrolidone, polyethylene glycol, polyvinyl acetate, and polyvinyl alcohol. Among them, polyvinylpyrrolidone is more preferable because a good porous structure can be easily formed.
[0028]
The lower limit of the concentration of the polymer (2) is preferably 1% by mass, more preferably 5% by mass. The upper limit is preferably 20% by mass, and more preferably 12% by mass.
[0029]
The solvent used in the present invention is not particularly limited as long as it can dissolve the film-forming resin, but can be uniformly mixed with water since the undiluted solution absorbs moisture in the free running section of the dry-wet spinning. Preferred are N, N-dimethylformamide, N, N-dimethylacetamide, dimethylsulfoxide, N-methyl-2-pyrrolidone, N-methylmorpholine-N-oxide, and the like, alone or in combination, or A solvent obtained by adding a non-solvent to these solvents as long as the solubility is not impaired is preferably used.
[0030]
When mixing a non-solvent, water, alcohols, glycerin, ethylene glycol and the like can be used, and water is most preferred.
[0031]
The stock solution discharged from the spinneret passes through the idle running section and then comes into contact with the non-solvent in the coagulation bath, whereby a porous hollow fiber membrane is formed. As the non-solvent used in the coagulation bath, water, alcohols, glycerin, ethylene glycol and the like can be used alone or in combination. Further, the above-mentioned solvents may be mixed.
[0032]
The porous hollow fiber membrane spun under the above conditions has a large pore diameter and potentially high water permeability, but when coagulation is completed, a large amount of the polymer (2) remains in the membrane. Therefore, the function as a highly water-permeable membrane cannot be exhibited as it is. Therefore, the polymer (2) remaining in the film at the stage of completion of coagulation is decomposed and removed by contact with an oxidizing agent.
[0033]
As the oxidizing agent, ozone, aqueous hydrogen peroxide, and an aqueous solution of hypochlorite can be used. Among them, an aqueous solution of hypochlorite is preferable because it has high oxidizing power, is inexpensive, and has excellent handleability.
[0034]
When hypochlorite is used as an oxidizing agent, decomposition and removal of the polymer (2) are achieved by the active oxygen generated by the decomposition of hypochlorous acid decomposing the polymer (2) having high oxidative decomposability. It is thought to be done.
[0035]
At this time, when a decomposition accelerator for accelerating the decomposition of hypochlorous acid is added to the aqueous solution, the amount of active oxygen generated is increased, so that the decomposition reaction of the polymer (2) can be accelerated. As the decomposition accelerator, a nickel-based catalyst is preferably used, and more preferably, a nickel oxide catalyst such as nickel trioxide (Ni ₂ O ₃ ) is used.
[0036]
As a form of the nickel-based catalyst, if the surface area of the nickel-based catalyst is increased to increase the contact area with the hypochlorite aqueous solution, the effect of promoting the decomposition of hypochlorous acid is enhanced. Therefore, the shape of the nickel-based catalyst is preferably powdery, and more preferably, the state in which the nickel-based catalyst powder is uniformly dispersed in the aqueous hypochlorite solution.
[0037]
In addition, those obtained by solidifying the powder of the nickel-based catalyst into granules or tablets, those obtained by supporting the powder of the nickel-based catalyst on a granular carrier, and the like can also be used.
[0038]
In carrying out the step of decomposing and washing the polymer (2) by bringing the porous membrane into contact with the aqueous hypochlorite solution, a hypochlorite aqueous solution is placed in a tank, and a nickel-based catalyst is dispersed therein. Alternatively, the decomposition cleaning may be performed in a batch process in which the porous membrane is immersed for a predetermined time, but in this case, at the end of the decomposition cleaning process, hypochlorite is decomposed and the concentration is reduced. It is necessary to renew the hypochlorite aqueous solution each time is performed.
[0039]
On the other hand, for example, from one end of a tank filled with a nickel-based catalyst, a high-concentration hypochlorite aqueous solution is continuously introduced, and from the other end, an aqueous solution having a reduced hypochlorous acid concentration is taken out. If the porous membrane is brought into contact with the aqueous solution in the tank while being moved, the step of decomposing and washing the polymer (2) can be performed continuously, so that the productivity is excellent.
[0040]
In addition, since hypochlorous acid is decomposed by the nickel-based catalyst in the aqueous solution taken out from the tank, the concentration of hypochlorous acid can be reduced to several mg / L by appropriately setting concentration conditions, temperature conditions, and the like. Can be reduced. Therefore, it is possible to greatly simplify the degree of wastewater treatment.
[0041]
The temperature of the aqueous solution containing the hypochlorite and the nickel-based catalyst is preferably 70 ° C. or higher because the higher the temperature, the more easily the effect of accelerating the decomposition is exhibited. The upper limit of the temperature is preferably 98 ° C. or lower to prevent boiling, since the contact operation between the aqueous solution and the porous membrane is performed.
[0042]
If the concentration of the hypochlorite aqueous solution is 100 mg / L or more, the decomposition reaction of hypochlorite by the nickel-based catalyst is likely to occur. On the other hand, if the concentration is too high, the corrosiveness becomes high. Therefore, the upper limit of the concentration is preferably 5000 mg / L, and more preferably 1000 mg / L.
[0043]
If the addition amount of the nickel-based catalyst is too large, it is uneconomical and there is a concern that the nickel-based catalyst may be attached to the membrane and taken out. Therefore, the upper limit is 100 g or less per 1 kg of the hypochlorite aqueous solution, and 70 g or less. The following is more preferred. If the amount is too small, the particles will not be uniformly dispersed. Therefore, the lower limit is 0.5 g or more, more preferably 1 g or more, per 1 kg of the aqueous hypochlorite solution.
[0044]
If the contact time between the aqueous hypochlorite solution and the porous membrane is too long, the spinning step and the time required for the coagulation step cannot be balanced, and it is substantially difficult to perform continuous washing. It is preferably less than 10 minutes, more preferably 5 minutes or less. In order to achieve desired water permeability, the lower limit of the contact time is preferably 1 minute or more.
[0045]
【Example】
Hereinafter, the present invention will be described in more detail based on examples.
[0046]
<Example 1>
Polyvinylidene fluoride resin was used as the polymer (1), and polyvinylpyrrolidone was used as the polymer (2).
[0047]
The spinning solution is a polyvinylidene fluoride resin (manufactured by Atofina Japan Co., Ltd., trade name Kynar 301F), a polyvinylidene fluoride resin (manufactured by Atofina Japan Co., trade name Kainer 9000), and polyvinylpyrrolidone (manufactured by ISP, trade name K) -90, molecular weight 1200000) and N, N-dimethylacetamide were heated and dissolved with stirring so that the mass ratio became 12: 8: 10: 80. The prepared spinning dope was spun by a dry-wet spinning method.
[0048]
0.5 g of powdered nickel trioxide (manufactured by Soegawa Rikagaku Co., Ltd.) was added to 600 mL of an aqueous solution of sodium hypochlorite having a temperature of 70 ° C. and a hypochlorous acid concentration of 1000 mg / L, and the powder was mixed well. Then, the porous hollow fiber membrane was immersed, and the polyvinyl pyrrolidone was decomposed and washed for 2 minutes. The water flux of the porous hollow fiber membrane after washing was 158 m ³ / m ² / hr / MPa.
[0049]
<Comparative Example 1>
The porous hollow fiber membrane obtained by spinning under the same conditions as in Example 1 was washed under the same conditions as in Example 1 except that no nickel catalyst was added, and the water flux was measured. As a result, the water flux was 97 m ³ / m ² / hr / MPa.
[0050]
【The invention's effect】
According to the method for producing a porous hollow fiber membrane of the present invention, in decomposing the polymer (2) remaining in the porous hollow fiber membrane with the oxidizing agent aqueous solution, a decomposition accelerator of the oxidizing agent is used in combination. Thus, the decomposition effect is promoted, and the decomposition and removal can be performed in a shorter time, so that the spinning step and the washing step can be continuously performed.
[0051]
In this case, it is more effective to use hypochlorite as an oxidizing agent and use a nickel-based catalyst as a decomposition accelerator.
[0052]
Further, when the polymer (1) forming the film is polyvinylidene fluoride and the polymer (2) is polyvinylpyrrolidone, the production method of the present invention can be more effectively applied.

Claims (9)

A porous membrane containing a polymer (1) and a polymer (2) having higher oxidative decomposability than the polymer (1) is brought into contact with an aqueous solution containing an oxidizing agent and a decomposition accelerator of the oxidizing agent, and the polymer ( 2. A method for producing a porous membrane in which at least a part of 2) is decomposed and removed. The method for producing a porous membrane according to claim 1, wherein the oxidizing agent is hypochlorite, and the decomposition accelerator is a nickel-based catalyst. The method for producing a porous membrane according to claim 2, wherein the nickel-based catalyst is a nickel oxide catalyst. The method for producing a porous membrane according to claim 2, wherein the nickel-based catalyst is in a powder form. The method for producing a porous membrane according to any one of claims 1 to 4, wherein a time for bringing the porous membrane into contact with the aqueous solution is less than 10 minutes. The method for producing a porous membrane according to any one of claims 2 to 5, wherein the concentration of hypochlorous acid in the aqueous solution is in a range of 100 to 5000 mg / L. The method for producing a porous membrane according to claim 1, wherein the temperature of the aqueous solution is 70 ° C. or higher. The method for producing a porous membrane according to any one of claims 1 to 7, wherein the polymer (1) is a hydrophobic polymer, and the polymer (2) is a hydrophilic polymer. The method for producing a porous membrane according to any one of claims 1 to 8, wherein the polymer (1) is polyvinylidene fluoride and the polymer (2) is polyvinylpyrrolidone.