JP2954327B2 - Porous hollow fiber membrane - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a porous hollow fiber membrane, and particularly when used for microfiltration by an external pressure total filtration method, is sharp and has a characteristic structure. The present invention relates to a porous hollow fiber membrane having high filtration accuracy, excellent filtration life, and practically satisfactory mechanical strength.

[Problems to be solved by conventional technology and invention]

Membrane separation technology has attracted attention in terms of energy saving and compactness, and has made remarkable progress. Above all, microfiltration membranes have been in practical use for a long time, and since they have been used in the manufacture of pharmaceuticals and the medical field for the purpose of removing microorganisms,
It is used as an excellent separation and purification technology in many fields such as the food industry, biotechnology industry, electronic industry, and nuclear industry. In addition to the separation membrane, there is also an example of application to a battery diaphragm, a gas-permeable waterproof membrane, and the like. It is not an exaggeration to say that a microfiltration membrane is used in any industry in any form.

In the microfiltration membrane, a very wide variety of materials such as celluloses, polyolefins, fluororesins, polyamides, polyvinyl chloride, polyvinyl alcohol, polycarbonate, polysulfone, and ceramics are used as the membrane material depending on the application. Among them, polysulfone-based resins have attracted attention as having high mechanical strength and excellent heat resistance and chemical resistance.

As a form of the microfiltration membrane, a flat membrane, a tube membrane, a hollow fiber membrane, and the like are known. Among them, the hollow fiber membrane is capable of increasing the membrane packing density per unit volume of the module, and has a simple module structure. For this reason, it has attracted attention in recent years because it is easy to make it modular and aseptic, and it is possible to perform backwashing and cross-flow filtration and prevent a decrease in water permeation rate.

Such a hollow fiber type microfiltration membrane is generally manufactured by any one of a drawing method and a phase separation method. The drawing method is a method in which after a crystalline polymer such as polypropylene or polyethylene is melt-spun, cleavage is caused between crystal lamellas by cold drawing, and the holes are expanded and opened by hot drawing. This method has the advantage of obtaining a hollow fiber membrane that is stronger in strength than the phase-separated cheeks, but on the principle of its opening, the hole diameter that can be opened is limited to a relatively small one. The problem is that only a uniform film having no pore size distribution can be manufactured. In addition, since the porosity of the entire membrane is small, only those having low water permeability can be obtained.

On the other hand, the phase separation method is a method in which a stock solution prepared by dissolving a polymer material in a solvent is extruded into the shape of a hollow fiber, immersed in a coagulation bath mainly composed of a non-solvent, and subjected to sol-gel conversion. This is a method of forming a film. The whole hollow fiber membrane formed by this method has a network structure, and has a larger pore size and a higher porosity than the drawing method. It is also relatively easy to produce a non-uniform membrane having a pore size distribution in the thickness direction of the membrane. For example, a structure in which the pore size is continuously increased from the inner surface side to the outer surface side is known.When such a hollow fiber membrane is used for total filtration at an external pressure, a pull filter is used. It is said that with the same effect as when used, a decrease in filtration speed due to clogging can be reduced and the filtration life is excellent. However, if the rate of change in the film thickness direction is increased, the mechanical strength of the membrane cannot be reduced, and there is a problem in that it is difficult to maintain the strength of a hollow fiber membrane that does not use a support that can withstand practical use.

[Means for solving the problem]

The present inventor has conducted intensive studies to overcome the above-mentioned drawbacks of the known hollow fiber membrane, and as a result, especially when used for microfiltration by an external pressure total filtration method, is sharp and owing to its characteristic structure. The present inventors have found a porous hollow fiber membrane having a combination of highly reliable filtration accuracy, excellent filtration life, and practically satisfactory mechanical strength, and have reached the present invention.

That is, the present invention provides a porous hollow fiber membrane comprising an organic polymer, wherein the membrane wall of the hollow fiber membrane substantially forms a two-layer structure of an inner layer and an outer layer, and the inner layer is constituted by a network structure of the organic polymer, An object of the present invention is to provide a porous hollow fiber membrane characterized in that the outer layer is composed of an organic polymer texture highly oriented in the length direction of the hollow fiber.

The porous hollow fiber membrane of the present invention is characterized by the structure of the membrane wall, and the membrane wall is composed of an inner layer composed of a network structure and a fibrous structure highly oriented in the length direction of the hollow fiber. And a substantially two-layer structure including an outer layer. Such a characteristic structure has never been known before, and is completely different from a conventional porous hollow fiber membrane in which the whole membrane is substantially composed of a network structure.

 Hereinafter, the present invention will be described in more detail.

Typical examples of scanning electron microscope photographs (SEM photographs) of the inner surface, outer surface, and cross section of the porous hollow fiber membrane of the present invention are shown in FIGS. That is, FIG. 1 is an SEM photograph of the inner surface showing an example of the porous hollow fiber membrane of the present invention, FIG. 2 is a SEM photograph of the outer surface of the porous hollow fiber membrane, and FIG. 4 is an SEM photograph of a cross section of the conductive hollow fiber membrane cut in a direction substantially perpendicular to the length direction of the hollow fiber.

As shown in FIGS. 1 to 3, the porous hollow fiber membrane of the present invention has a substantially two-layer structure of an inner layer and an outer layer. The inner layer is substantially composed of a network similar to that found in conventional microfiltration membranes, and the outer layer is substantially composed of a fibrous tissue. The fibrous structure of this outer layer is highly oriented in the length direction of the hollow fiber, and the angle between the central axis in the longitudinal direction of the fibrous structure and the central axis in the longitudinal direction of the hollow fiber is within 10 degrees. Some fibrous tissues make up over 90% of the tissues that make up the outer layer. Such a structure found in the outer layer is a novel structure which is not known at all in the conventional porous hollow fiber membrane.

The porous hollow fiber membrane of the present invention has an inner layer having an average pore size of 0.
It is preferably from 0.1 to 5 μm, more preferably from 0.1 to 5 μm.
１1 μm is desirable for use in microfiltration. Here, the average pore diameter is a value calculated as an average pore diameter of pores by a SEM photograph of a cross section as shown in FIG.

Further porous hollow fiber membrane of the present invention has a pure water permeability of preferably ^{1,000~20,000 / m 2 · hr · atm} , more preferably has a very high value of 3,000~10,000 / m ² · hr · atm Things. The pure water permeability refers to pure water at 25 ° C. from the outer surface side of the porous hollow fiber membrane which is pressurized and permeated, and the amount of permeated water is measured as a unit membrane area (outer surface area), a unit time, and It is converted per unit pressure.

When the porous hollow fiber membrane of the present invention is used for microfiltration by external pressure total filtration, the effect of its characteristic structure is remarkably exhibited. That is, when the porous hollow fiber membrane of the present invention is used for microfiltration by external pressure total filtration, the entire membrane wall is constituted by a network structure, and water permeability is higher than that of a uniform membrane in which the average pore diameter does not substantially change in the thickness direction. It has the characteristics that the decrease in speed is small and the filtration life is long. This is a very great advantage in using a microfiltration membrane. The reason why the decrease in the water permeation rate is small is not known in detail, but the outer layer of the two-layer structure, which is probably the feature of the porous hollow fiber membrane of the present invention, plays the role of a prefilter in the case of external pressure filtration. It seems to be because. An appropriate thickness of the outer layer for exhibiting such an effect is 1 to 20% of the total film thickness. If the thickness of the outer layer is less than 1% of the total film thickness, the effect of the pre-filter is insufficient, and if it exceeds 20% of the total film thickness, the ratio of the inner layer composed of a network structure is reduced, so that the filtration accuracy is reduced. There is a risk of falling.

The porous hollow fiber membrane of the present invention has excellent mechanical strength because the average pore diameter of the entire membrane does not unnecessarily increase.
In addition, since the fibrous structure of the outer layer is highly oriented in the length direction of the hollow fiber, the effect of increasing the tensile strength of the hollow fiber is also considered to contribute to the improvement of the mechanical strength.

The porous hollow fiber membrane of the present invention may be produced by any method. For example, a stock solution containing an organic polymer as a membrane material is extruded together with an internal coagulation solution from a double-tube nozzle. In a method of forming a hollow fiber type separation membrane, in which the whole is immersed in an external coagulating liquid after passing through the air part of a certain distance from the nozzle, the so-called dry-wet method, the amount of water present in the air part is high, and It is manufactured when is set high. Here, the draft ratio referred to herein is defined by the following equation.

Draft ratio = (winding speed) / (discharge linear speed) = v (D ² −d ² ) π / Q (where v is the winding speed, D is the outer diameter of the film forming stock solution discharge slit, and d is the manufacturing speed). The inner diameter of the membrane undiluted liquid discharge slit and Q indicate the discharge amount of the film undiluted solution.) If the water content in the air portion is large, the film undiluted solution absorbs a large amount of water from the outer surface when passing through the air portion. The outer surface and its vicinity are partially gelled. Also, the membrane-forming stock solution is pulled in the length direction of the aerial thread under the gravity in the aerial portion and the tension of winding. Therefore, when the draft ratio is set high, the outer surface of the membrane-forming stock solution and the vicinity thereof are partially gelled and, at the same time, receive a strong tension in the length direction of the hollow fiber. It is believed that an outer layer having a structure substantially composed of fibrous polymer highly oriented in the length direction is formed. The water content and the draft ratio in the aerial part necessary for producing the porous hollow fiber membrane of the present invention vary depending on conditions such as the polymer of the membrane material, the composition and temperature of the membrane-forming stock solution, and the transit time in the aerial part. Although it can not be said, the water content in the aerial part is 100 g / m ³ or more,
Draft ratio is 2 or more, preferably, the water content in the aerial part is 50
0 g / m ³ or more, and draft ratio is 4 or more.

The organic polymer used as the membrane material of the porous hollow membrane of the present invention includes cellulose, polyamide, polyacrylonitrile, polysulfone, and the like, and copolymers of these polymers. It may be. Further, a blend of two or more types of mutually compatible polymers may be used. Among them, a polysulfone-based polymer is preferable because of its excellent heat resistance and chemical resistance. Examples of the polysulfone-based polymer include those having a repeating unit represented by the following general formula (I) or (II).

In the formulas (I) and (II), X _{1 to} X ₆ represent a non-dissociative substituent exemplified by an alkyl group such as a methyl group and an ethyl group, a halogen such as chlorine and bromine, or —COOH, − SO ₃
Represents a dissociable substituent such as H, and m, n, o, p and q are 0
Shows an integer of ~ 4. Generally, polysulfone in which all of m, n, o, p and q are 0 is easily available, and is preferably used in the present invention. However, the polysulfone-based polymer used in the present invention is not limited to the above.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited thereto.

Example 1 16% by weight of polyether sulfone (5200P powder, manufactured by ICI), 9% by weight of polyvinylpyrrolidone (average molecular weight: 360,000, manufactured by Aldrich), N-methyl-2-pyrrolidone 75
The weight percent was dissolved at 60 ° C. to prepare a membrane-forming stock solution. The membrane-forming stock solution was mixed with a core solution composed of tetraethylene glycol.
After being extruded from a double tube type spinning nozzle and passed through an aerial part of 10 cm, it was led to a coagulation bath made of water and solidified to spin a hollow fiber membrane. The temperature of the stock solution, core solution, and coagulation bath
The temperature was set at 50 ° C. The aerial part through which the film forming stock solution passed was covered with a cylindrical material around the film forming stock solution, and steam was supplied from the outside to adjust the water content in the cylindrical material to 500 to 600 g / m ³ . Outer diameter 0.66mm, inner diameter 0.
36 mm.

Under the above film forming conditions, a hollow fiber membrane having an inner diameter of 0.17 mm and an outer diameter of 0.30 mm was spun at a speed of 25 m / min. 80 ° C for the formed hollow fiber
After washing with warm water for 24 hours, the membrane performance was evaluated. As a disqualification, permeability of pure water ^{4,200 / m 2 · hr · atm} , the particle size
100% rejection of 0.22 μm latex. Scanning electron micrographs of the inner surface, outer surface, and cross section of this porous hollow fiber membrane are shown in FIGS. 1, 2, and 3, respectively, and have a two-layer structure. Was.

Example 2 The composition of a film forming stock solution was prepared by mixing 15% by weight of polyether sulfone and polyvinylpyrrolidone (manufactured by Aldrich, average molecular weight of 40,00).
0) A hollow fiber membrane having an inner diameter of 0.20 mm and an outer diameter of 0.32 mm was spun at a speed of 28 m / min in the same manner as in Example 1 except that 15% by weight and 70% by weight of dimethylformamide were used. The membrane performance of this hollow fiber membrane is such that the permeation performance of pure water is 5,900 / m ²
100% of 2 μm latex was blocked. The structure of the hollow fiber was a two-layer structure similar to that of Example 1.

[Brief description of the drawings]

FIG. 1 is a scanning electron micrograph showing the fiber shape of the inner surface of the porous hollow fiber membrane obtained in Example 1, and FIG. 2 is the outer surface of the porous hollow fiber membrane obtained in Example 1. FIG. 3 is a scanning electron micrograph showing the fiber shape of a cross section of the porous hollow fiber membrane obtained in Example 1. FIG.

Claims (5)

(57) [Claims] 1. A porous hollow fiber membrane comprising an organic polymer, wherein the membrane wall of the hollow fiber membrane has a two-layer structure of an inner layer and an outer layer, wherein the inner layer is composed of an organic polymer network, and the outer layer is hollow. A porous hollow fiber membrane comprising a fibrous structure of an organic polymer oriented in the length direction of the yarn. 2. The thickness of the outer layer is 1 to 20 times the thickness of the entire membrane wall.
% Of the porous hollow fiber membrane according to claim 1. 3. The porous hollow fiber membrane according to claim 1, wherein the average pore size of the inner layer is 0.01 to 5 μm. 4. The pure water permeability is 1,000 to 20,000 / m ² · hr.
The porous hollow fiber membrane according to any one of claims 1 to 3, wherein the porous hollow fiber membrane is atm. 5. The porous hollow fiber membrane according to claim 1, comprising a polysulfone-based polymer.