JP7807972B2 - Hollow fiber microfiltration membrane - Google Patents

Description

This disclosure relates to a hollow fiber microfiltration membrane with high water permeability and high membrane strength that can be used as a filtration membrane in the food industry and water treatment.

Patent Document 1 describes an invention for a porous hollow fiber membrane whose membrane wall is composed of a hydrophobic polymer and a hydrophilic polymer, and describes polysulfone as the hydrophobic polymer (claims).

Patent Document 2 describes an invention of a porous hollow fiber membrane in which the pore shape is circular when the inner surface is observed with a scanning electron microscope at 200x magnification, the pore shape is irregular when the outer surface is observed with a scanning electron microscope at 1,000x magnification, the pores near the inner surface are larger than the pores near the outer surface when the membrane cross section is observed with a scanning electron microscope at 200x magnification, and the pure water flux from the inside to the outside of the hollow fiber membrane at 25°C is 10,000 to 30,000 L/m ² /h/bar, and it also describes that the hollow fiber membrane contains a hydrophobic polymer and a hydrophilic polymer, and the hydrophobic polymer is a polysulfone-based polymer (claims).

Patent No. 5431347 International Publication No. 2016/182015

The objective of this disclosure is to provide a hollow fiber microfiltration membrane that has low membrane resistance and a high water permeation rate.

The present disclosure provides a hollow fiber microfiltration membrane made of a hydrophobic polymer,
The hollow fiber microfiltration membrane has, from the outside to the inside, a dense outer surface layer having a surface pore size of 3 μm or less, a porous outer sponge structure layer, a porous intermediate layer containing macrovoids having a minor axis of 20 μm or more, and a porous inner sponge structure layer,
The pore size increases successively in the order of the outer sponge structure layer, the porous intermediate layer containing macrovoids with a minor axis of 20 μm or more, and the inner sponge structure layer.
The hollow fiber microfiltration membrane has a pure water permeation rate of 8,000 to 40,000 L/ ^m2 ·h at 0.1 MPa.

The hollow fiber microfiltration membrane disclosed herein has a high water permeation rate, high breaking strength, and is less susceptible to fiber breakage, making it suitable for use as a filtration membrane for food refining and filtration membrane for brewer's yeast liquid.

Electron microscope (SEM) photographs (60x and 400x) of the cross section of the hollow fiber membrane of Example 1, (b) is an enlarged photograph of (a). 1 is an electron microscope (SEM) photograph (3000x magnification) of the inner and outer surfaces of the hollow fiber membrane of Example 1. 1 is an electron microscope (SEM) photograph (60x magnification) of a cross section of the hollow fiber membrane of Example 2. 1 is an electron microscope (SEM) photograph (60x magnification) of a cross section of the hollow fiber membrane of Example 3. 1 is an electron microscope (SEM) photograph (60x magnification) of a cross section of the hollow fiber membrane of Example 4. 1 is an electron microscope (SEM) photograph (3000x magnification) of the inner surface of the hollow fiber membrane of Example 4. 1 is an electron microscope (SEM) photograph (3000x magnification) of the outer surface of the hollow fiber membrane of Example 4. 1 is an electron microscope (SEM) photograph (60x magnification) of a cross section of the hollow fiber membrane of Comparative Example 1. 1 is an electron microscope (SEM) photograph (60x magnification) of a cross section of the hollow fiber membrane of Comparative Example 2.

(1) Hollow Fiber Microfiltration Membrane The hollow fiber microfiltration membrane of the present disclosure is made of a hydrophobic polymer.
Examples of the hydrophobic polymer include polyethersulfone (PES), sulfonated polyethersulfone, polysulfone, and polyvinylidene fluoride, and it is preferable to include polyethersulfone.

The hollow fiber microfiltration membrane disclosed herein has, from the outside to the inside, a dense outer surface layer with a surface pore size of 3 μm or less, a porous outer sponge structure layer, a porous middle layer containing macrovoids with a short diameter of 20 μm or more, and a porous inner sponge structure layer.

The thickness of the outer surface dense layer (skin layer) having a surface pore size of 3 μm or less is preferably 15 μm or less, more preferably in the range of 1 to 10 μm, in order to reduce water permeation resistance.
The surface pore size is preferably in the range of 0.1 to 3.0 μm. When the surface pore size is 0.1 μm or more, the water permeation resistance does not become too low, and when it is 3.0 μm or less, there is no problem in terms of functionality (foreign matter blocking performance) as a filtration membrane for food purification or a filtration membrane for beer yeast liquid.

The porous outer sponge structure layer is a mesh-like porous layer formed between the outer surface dense layer (skin layer) and the porous intermediate layer containing macrovoids with a minor axis of 20 μm or more, and the pore size of the porous outer sponge structure increases continuously from the outside to the inside of the hollow fiber microfiltration membrane.
The porous outer sponge structure layer accounts for approximately 20% of the total thickness of the film.

The porous intermediate layer containing macrovoids with a minor axis of 20 μm or more preferably has an area occupied by macrovoids with a minor axis of 20 μm or more in the cross section of the hollow fiber microfiltration membrane of 15 to 25%.
The "cross section of a hollow fiber microfiltration membrane" is a cross section of a hollow fiber microfiltration membrane cut perpendicular to the longitudinal direction.
The "minor diameter" is the dimension of the shortest part in a cross section of a hollow fiber microfiltration membrane cut perpendicular to the longitudinal direction.

The thickness of the porous intermediate layer containing macrovoids with a minor axis of 20 μm or more is preferably equal to or slightly greater than the major axis of the macrovoids, and is preferably a layer that accounts for 50 to 70% of the total membrane thickness of the hollow fiber microfiltration membrane.
Macrovoids refer to giant pores with a minor axis of 20 to 200 μm that exist within the membrane thickness of a hollow fiber microfiltration membrane, and are distinguished from the pores in the mesh-like porous layer (sponge layer).
The area occupied by macrovoids in the cross section of a hollow fiber microfiltration membrane is preferably 10 to 35%, more preferably 15 to 25%. When the area occupied by macrovoids is 10% or more, a decrease in water permeability (pure water permeation rate) can be prevented, and when it is 35% or less, a decrease in breaking strength and breaking elongation can be prevented.

The porous internal sponge structure layer has a mesh-like porous (sponge) structure that does not have macrovoids, and the pore size increases continuously from the outside to the inside of the microfiltration membrane.
The porous inner sponge structure layer preferably occupies 10 to 30% of the inner thickness of the hollow fiber microfiltration membrane.

The porous internal sponge structure layer preferably has an inner dense surface layer (skin layer) with a surface pore size of 3 μm or less.
The porous internal sponge structure layer further has an inner dense inner surface layer (skin layer) with a surface pore size of 3 μm or less, and more preferably, nodules are formed on the membrane surface of the inner dense inner surface layer.
It is more preferable that the porous internal sponge structure layer further has an inner surface dense layer (skin layer) with a surface pore size of 3 μm or less, the surface pore size of the inner surface dense layer is 0.1 to 3.0 μm, and nodules are formed on the membrane surface of the inner surface dense layer.
Nodules are irregular surface structures consisting of connected protrusions on the surface of the inner surface dense layer (skin layer). The presence of nodules prevents fouling substances from consolidating on the membrane surface, thereby suppressing a decrease in filtration rate during the filtration operation of the microfiltration membrane.
The thickness of the inner surface dense layer (skin layer) is preferably 15 μm or less in order to reduce water permeation resistance, and more preferably within the range of 1 to 10 μm.

In the hollow fiber microfiltration membrane of the present disclosure, the pore size increases successively in the order of the outer sponge structure layer, the porous intermediate layer containing macrovoids with a minor axis of 20 μm or more, and the inner sponge structure layer.
The hollow fiber microfiltration membrane of the present disclosure has macrovoids formed in the membrane, which increases the porosity of the inner layer and makes it possible to obtain a microfiltration membrane with high water permeability (pure water permeation rate).

The hollow fiber microfiltration membrane disclosed herein can also be preferably used in a filtration process in which a liquid to be treated is sent inside the hollow fiber membrane and the filtrate is removed from the outer surface of the hollow fiber membrane while performing depth filtration. Therefore, a sponge structure in which the pore size is largest on the inside of the hollow fiber membrane and continuously decreases toward the outside is preferred.
Furthermore, as one form of hollow fiber microfiltration membrane disclosed herein, even in a filtration membrane that includes a dense inner surface layer having a surface pore size of 3 μm or less as a constituent layer, a sponge structure in which the pore size is largest on the inside of the hollow fiber membrane and gradually decreases toward the outside is preferred because it can remove foreign matter in multiple stages and therefore provides excellent blocking performance.

The hollow fiber microfiltration membrane disclosed herein preferably has a breaking strength of 500 g/fiber or more, and more preferably 550 g/fiber or more. High breaking strength makes it less likely to break when subjected to external forces and is preferable because it can withstand high filtration pressures.

The hollow fiber microfiltration membrane disclosed herein preferably has a burst pressure of 1 MPa or more.

The hollow fiber microfiltration membrane of the present disclosure preferably has an inner diameter of 1000 to 1700 μm and an outer diameter of 1800 to 2600 μm, more preferably an inner diameter of 1100 to 1600 μm and an outer diameter of 1900 to 2500 μm.
An inner diameter of 1000 μm or more is preferable because it can prevent clogging when filtering food, beer yeast liquid, etc., and an inner diameter of 1700 μm or less is preferable because it can increase the effective membrane area per module.
In order to provide the hollow fiber microfiltration membrane of the present disclosure with a good balance between mechanical strength and water permeability, the membrane thickness is preferably 100 to 500 μm, more preferably 200 to 400 μm.

The hollow fiber microfiltration membrane disclosed herein preferably has a residual polyvinylpyrrolidone mass of 0.20% or less, and more preferably 0.10% or less, relative to the total mass of the hollow fiber microfiltration membrane.

As an example of an application of the hollow fiber microfiltration membrane of the present disclosure, a preferable example is one in which the rejection rate of colloidal silica with a diameter of 0.125 μm is 50% or less.
In the application of water purification filtration, a colloidal silica rejection rate of 0.125 μm diameter is preferably 95% or less.

The hollow fiber microfiltration membrane of the present disclosure has a pure water permeability (PWP) of 8,000 to 40,000 L/m ² ·h.

(2) Membrane-forming solution composition The membrane-forming solution composition for producing the hollow fiber microfiltration membrane of the present disclosure can contain a hydrophobic polymer, a solvent, a non-solvent, a pore-forming agent, and the like.
The membrane-forming solution composition can be produced by first adding and dissolving a pore-forming agent to a mixed solution consisting of a solvent and a non-solvent, and then adding and dissolving a hydrophobic polymer.

The hydrophobic polymer may be polyethersulfone, sulfonated polyethersulfone, polysulfone, polyvinylidene fluoride, or the like, but is preferably a material containing polyethersulfone, and more preferably polyethersulfone, because these hydrophobic polymers have excellent heat resistance and chemical resistance.
As the polyethersulfone, for example, Sumikaexcel 5200P manufactured by Sumika Chemtex Co., Ltd. can be used.

When PES is included as a hydrophobic polymer, it is desirable to reduce the PES content in the membrane-forming solution composition to increase the water permeability of the hollow-fiber microfiltration membrane. However, if the content is too low, pressure resistance will be weakened, so a content of 15% by mass or more is preferred. On the other hand, increasing the PES concentration to improve pressure resistance will decrease water permeability, so the PES component is preferably 20% by mass or less. A content of 16 to 18% by mass is more preferred.

The solvent may be N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), dimethylacetamide, N,N-dimethylformamide, or the like.
When polyethersulfone is used as the hydrophobic polymer and water permeability is to be increased, NMP or DMSO is preferred.
If necessary, a non-solvent for the polymer can be added to the membrane-forming solution composition.
As the non-solvent, ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol (PEG), glycerin (GL), etc. can be used.
PEG and GL are preferred for increasing the pore size of the hollow fiber membrane and thereby increasing water permeability, and GL is more preferred for increasing the viscosity of the membrane-forming solution composition to spin hollow fibers with high circularity and thereby obtain hollow fiber membranes with high pressure resistance.
The concentration of the non-solvent in the membrane-forming solution composition is preferably 5 to 15% by mass, more preferably 8 to 12% by mass.

As the pore-forming agent, polyvinylpyrrolidone (PVP), calcium carbonate, magnesium oxide, etc. can be used.
The water permeability can be improved by adding a pore-forming agent to the membrane-forming solution composition, and after membrane formation, washing with water, sodium hypochlorite, acid, etc. to extract and remove the pore-forming agent.
From the viewpoint of work safety, PVP, which can be removed and washed with low concentration sodium hypochlorite, is preferable to calcium carbonate and magnesium oxide, which require strong acid washing.
The weight-average molecular weight of PVP is preferably 40,000 to 360,000, and in order to increase the pore size of the hollow fiber membrane and thereby increase the water permeability, the weight-average molecular weight is more preferably about 360,000.
The PVP concentration in the membrane-forming solution composition is 3 to 10% by mass, preferably 5 to 7% by mass.

(3) Method for Producing Hollow-Fiber Microfiltration Membrane The hollow-fiber microfiltration membrane of the present disclosure can be produced using the above-described membrane-forming solution composition. An example of the production process will be described below in order.

The above-mentioned membrane-forming solution composition is prepared, degassed, and then spun to obtain a hollow fiber membrane.
The temperature of the membrane-forming solution composition during spinning is preferably 30 to 70°C, more preferably 40 to 60°C, because if the temperature is too low, the pore size of the hollow fiber membrane becomes small and the water permeability decreases, whereas if the temperature of the membrane-forming solution is too high, the viscosity decreases and fiber breakage occurs during spinning.
In spinning, a membrane-forming solution composition is discharged from the outer periphery of a double-tube spinning nozzle, and simultaneously, a non-solvent (internal coagulation liquid) for the membrane-forming components is discharged from the central hole.

The internal coagulation liquid may be a mixed solution of water, diethylene glycol (DEG), and diethylene glycol monomethyl ether (DMM).
The temperature of the internal coagulation liquid is preferably 40 to 80° C., more preferably 50 to 70° C. The content ratio of each component of the internal coagulation liquid is as follows:
The water content is preferably 5 to 20% by mass, more preferably 7 to 17% by mass, and even more preferably 9 to 15% by mass.
The DEG content is preferably 17 to 45 mass %, more preferably 25 to 40 mass %, and even more preferably 27 to 35 mass %.
The content of DMM is preferably 40 to 80% by mass, more preferably 50 to 70% by mass, and even more preferably 55 to 65% by mass.

Thereafter, the spun hollow fiber is guided from the double-tube spinning nozzle through a drying space to a coagulation tank where it is coagulated to obtain a hollow fiber membrane.
The temperature of the drying space is preferably 90°C to 110°C, more preferably 95°C to 105°C, in order to increase the pore size on the outer surface of the hollow fibers and obtain a membrane with high water permeability.
The distance of the drying space is preferably 0 to 100 mm.
The coagulation liquid in the coagulation tank can be water, and the temperature of the coagulation tank (temperature of the coagulation bath) is preferably 50 to 70°C.

Thereafter, the hollow fiber membrane is passed through a washing tank containing water at 50° C. to wash away the solvent, and then wound up and dried.
The washing and removal step is a step of removing the pore-forming agent contained in the hollow fiber membrane.
The washing method is not particularly limited, and washing with running water, washing with circulating running water in which washing water is circulated, immersion washing, immersion and agitation washing, etc. can be applied.
When PVP is removed by immersion washing, the hollow fiber membrane is immersed in an aqueous solution of sodium hypochlorite with a concentration of 1000 ppm for 72 hours.
To perform cleaning in a shorter time, the substrate is immersed in an aqueous solution with a concentration of 2000 ppm for 36 hours.
Thereafter, the hollow fiber membrane is washed with running water to remove the sodium hypochlorite, and then dried.

(1) Pure water permeability coefficient (PWP)
The hollow fiber microfiltration membranes obtained in the examples and comparative examples were cut to a length of 10 cm, and with one end closed, pure water was supplied from the other end at 0.1 MPa, and the volume of pure water permeating through the hollow fiber membrane in a certain period of time was measured.
This volume was divided by the collection time (h) and the membrane area (m ² ) of the inner surface of the hollow fiber membrane to determine the pure water permeability coefficient [L/m ² ·h] at 0.1 MPa.

(2) Breaking strength (g/piece) and breaking elongation (%)
The breaking strength and elongation of the hollow fiber microfiltration membrane were measured using a small benchtop testing machine (Shimadzu EZ Test). The breaking strength and elongation were measured for a wet hollow fiber membrane with an effective length of 5 cm when the crosshead was moved at a rate of 10 mm/min.

(3) Burst test A hollow fiber microfiltration membrane was cut into a length of 1 m and immersed in pure water for 5 minutes or more. After that, with one end closed, dry air was pumped into the other end, and the pressure was increased to 1 MPa at a rate of 0.2 MPa per minute. Hollow fiber membranes that burst were rated as failing, and those that did not burst were rated as passing.

(4) Membrane structure (observation method of hollow fiber cross section, inner surface, outer surface, occupied area of macrovoids)
The hollow fiber microfiltration membranes obtained in the examples and comparative examples were cut, and scanning electron microscope (SEM) photographs of the cross section, inner surface, and outer surface were taken at magnifications of 60 to 400 times for the cross section, and 3000 times for the inner and outer surfaces.
After obtaining a cross-sectional image of the hollow fiber membrane, the macrovoids were painted black using a felt-tip pen (Pentel Co., Ltd.), and the image was imported into a personal computer.
The area occupied by macrovoids in the cross-sectional area of the film was measured using image analysis software (ImageJ Ver. 5.6.0, manufactured by MITANI Corporation).

(5) Colloidal silica rejection rate (%)
The hollow fiber microfiltration membranes obtained in the examples and comparative examples were cut into lengths of 10 cm and immersed in pure water for 30 minutes.
A colloidal silica dispersion (Fuso Chemical Co., Ltd. PL-7) with a secondary particle diameter of 0.125 μm was adjusted to a concentration of 200 mg/L and subjected to internal pressure crossflow filtration under conditions of a membrane inlet supply pressure of 0.100 MPa and an outlet pressure of 0.099 MPa. 50 minutes after the start of filtration, the permeate and concentrate were collected.
The turbidity of the raw solution, permeate, and concentrate was measured using a portable turbidity meter (2100P manufactured by HACH), and the colloidal silica rejection rate was calculated using the following formula.
Blocking rate (%) =
100 - {(permeate turbidity x 100) / [(undiluted solution turbidity) + (concentrated solution turbidity)] / 2}

(6) PVP residual amount (mass%)
The hollow fiber microfiltration membrane was decomposed by the salicylic acid-added Kjeldahl method, and the total nitrogen concentration was measured by indophenol absorptiometry to determine the residual PVP amount (%). Salicylic acid was added during the Kjeldahl decomposition in order to detect and quantify nitrate nitrogen as total nitrogen.
Approximately 2 g of the entire cross section of a hollow fiber microfiltration membrane that had been dried in an oven at 55°C for 6 hours was weighed out as a sample and placed in a Kjeldahl flask. Sulfuric acid and salicylic acid were added and the mixture was heated for oxidative decomposition.
The total nitrogen concentration of the sample solution after decomposition was quantified in accordance with the indophenol absorptiometry method described on pages 1 and 2 of JIS K0102.42. The total nitrogen concentration was converted into PVP weight, and the percentage value obtained by dividing the total nitrogen concentration by the weight of the resin component of the hollow fiber filtration membrane was defined as the residual PVP amount (%).

Example 1
<Film forming solution composition>
Polyvinylpyrrolidone (Tokyo Chemical Industry Co., Ltd. K90) (PVP) was added to a solvent consisting of N-methyl-2-pyrrolidone (NMP) and glycerin (GL) shown in Table 1, and dissolved by heating at 80°C for about 1 hour.
Next, polyethersulfone (Sumika Excel 5200P, manufactured by Sumika Chemtex Co., Ltd.) (PES) was added to the solution and dissolved by heating at 80° C. for about 6 hours to obtain a membrane-forming solution composition.

<Production of hollow fiber semipermeable membrane>
The above membrane-forming solution composition was degassed for 18 hours at 80° C. The degassed membrane-forming solution composition was extruded and spun through a double-tube spinning nozzle heated to 50° C.
The internal coagulation liquid shown in Table 1 was used, and the material was extruded from a double-tube spinning nozzle, then passed through a drying space (99°C) 50 mm in distance to dry, and then passed through a coagulation tank filled with water at 60°C.
Thereafter, the membrane was passed through a washing tank containing water at 50° C. to wash away the solvent, and the hollow fiber microfiltration membrane was wound up.
The wound hollow fiber membrane was immersed in a 1000 ppm aqueous solution of sodium hypochlorite for 72 hours to remove PVP, and then the hollow fiber membrane was washed with running water to remove the sodium hypochlorite.
Thereafter, the film was dried in a dryer at 55°C for 4 hours.

The obtained hollow fiber membrane was subjected to the above-mentioned measurements. The results are shown in Table 1.
The amount of PVP remaining before PVP removal was 0.23%, and the amount of PVP remaining after immersion and washing with sodium hypochlorite was 0.06%.
FIG. 1 shows an SEM photograph of the cross-sectional structure of the hollow fiber microfiltration membrane, and FIG. 2 shows SEM photographs of the inner and outer surface structures.
As can be seen from FIG. 1, the membrane had a sponge structure in which the pore size continuously decreased from the inner surface to the outer surface, and a macrovoid structure in the membrane thickness.
Pores of 3 μm or less were observed on the inner and outer surfaces of the hollow fiber, and the rejection rate of colloidal silica with a secondary particle diameter of 0.125 μm was 46%, indicating that the pore diameter was 0.1 to 3 μm.

Example 2
A hollow fiber microfiltration membrane was obtained in the same manner as in Example 1, except that the discharge rates of the membrane-forming solution composition and the internal coagulation solution extruded from the double-tube spinning nozzle were adjusted to set the inner diameter of the hollow fiber membrane to 1,450 μm and the outer diameter to 2,070 μm. The above-mentioned measurements were carried out on the obtained hollow fiber membrane. The results are shown in Table 1. An SEM photograph of the cross-sectional structure of the hollow fiber microfiltration membrane is shown in Figure 3.

Example 3
A hollow fiber microfiltration membrane was obtained in the same manner as in Example 1, except that the inner diameter of the hollow fiber membrane was 1510 μm and the outer diameter was 2290 μm. The obtained hollow fiber membrane was subjected to the above-mentioned measurements. The results are shown in Table 1. An SEM photograph of the cross-sectional structure of the hollow fiber microfiltration membrane is shown in Figure 4.

Example 4
A hollow fiber microfiltration membrane was obtained in the same manner as in Example 1, except that the composition of the internal coagulation liquid was 11% by mass of water, 30.4% by mass of DEG, and 58.6% by mass of DMM, and the inner diameter of the hollow fiber membrane was 1220 μm and the outer diameter was 1930 μm. The obtained hollow fiber membrane was subjected to the above-mentioned measurements. The results are shown in Table 1.
An SEM photograph of the cross-sectional structure of the hollow fiber microfiltration membrane is shown in FIG. 5, an SEM photograph of the inner surface structure is shown in FIG. 6, and an SEM photograph of the outer surface structure is shown in FIG.
Pores of 2 μm or less were observed on the inner and outer surfaces of the hollow fiber, and the rejection rate of colloidal silica with a secondary particle diameter of 0.125 μm was 91%, indicating that the pore diameter was 0.1 to 2 μm.
After immersion and washing with sodium hypochlorite, the amount of PVP remaining was 0.05%.

Comparative Example 1
A hollow fiber microfiltration membrane was obtained in the same manner as in Example 1, except that the membrane-forming solution composition was 17% by mass of PES, 38% by mass of DMSO, and 45% by mass of PEG, and the hollow fiber membrane had an inner diameter of 800 μm and an outer diameter of 1300 μm. The obtained hollow fiber membrane was subjected to the above-mentioned measurements. The results are shown in Table 1. An SEM photograph of the cross-sectional structure of the hollow fiber microfiltration membrane is shown in Figure 8.
No macrovoids were observed, and the water permeability was significantly inferior to that of the Examples.

Comparative Example 2
A hollow fiber microfiltration membrane was obtained in the same manner as in Example 1, except that the composition of the internal coagulation liquid was 13% by mass of water, 29.7% by mass of DEG, and 57.3% by mass of DMM, and the inner diameter of the hollow fiber membrane was 1340 μm and the outer diameter was 1980 μm. The above-mentioned measurements were carried out on the obtained hollow fiber membrane. The results are shown in Table 1. An SEM photograph of the cross-sectional structure of the hollow fiber microfiltration membrane is shown in Figure 9.

The hollow fiber microfiltration membrane of the present invention has high pure water permeability, high breaking strength and high breaking elongation, and can be used as a filtration membrane in the food industry and for water treatment.

Claims (8)

A hollow fiber microfiltration membrane made of a hydrophobic polymer,
The hollow fiber microfiltration membrane has, from the outside to the inside, a dense outer surface layer having a surface pore size of 3 μm or less, a porous outer sponge structure layer, a porous intermediate layer containing macrovoids having a minor axis of 20 μm or more, and a porous inner sponge structure layer,
The pore size increases successively in the order of the outer sponge structure layer, the porous intermediate layer containing macrovoids with a minor axis of 20 μm or more, and the inner sponge structure layer.
a dense inner surface layer having a surface pore size of 3 μm or less is further formed inside the porous internal sponge structure layer, and nodules are formed on the membrane surface of the dense inner surface layer;
The hollow fiber microfiltration membrane has a pure water permeability coefficient of 8,000 to 40,000 L/ ^m2 ·h at 0.1 MPa. The hollow fiber microfiltration membrane according to claim 1, further comprising an inner surface dense layer having a surface pore diameter of 3 μm or less inside the porous internal sponge structure layer, the surface pore diameter of the inner surface dense layer being 0.1 to 3.0 μm, and nodules being formed on the membrane surface of the inner surface dense layer. 2. The hollow fiber microfiltration membrane according to claim 1, wherein the porous intermediate layer containing the macrovoids having a minor axis of 20 μm or more has an occupied area of the macrovoids having a minor axis of 20 μm or more of 15 to 25% in the cross section. The hollow fiber microfiltration membrane according to any one of claims 1 to 3 , having a breaking strength of 500 g/membrane or more. The hollow fiber microfiltration membrane according to any one of claims 1 to 3 , having a burst pressure of 1 MPa or more. The hollow fiber microfiltration membrane according to any one of claims 1 to 3 , having an inner diameter of 1000 µm or more and 1700 µm or less. The hollow fiber microfiltration membrane according to any one of claims 1 to 3 , wherein the hydrophobic polymer is polyethersulfone. 4. The hollow fiber microfiltration membrane according to claim 1 , wherein the residual weight of polyvinylpyrrolidone in the hollow fiber microfiltration membrane is 0.10% by mass or less based on the total mass of the hollow fiber microfiltration membrane.