JP4869272B2 - Porous multilayer hollow fiber membrane - Google Patents

Description

  The present invention relates to a porous hollow fiber membrane made of a thermoplastic resin, which is suitable for filtration applications such as turbidity and membrane separation activated sludge process, and is less susceptible to abrasion on the outer surface due to air scrubbing or the like and has high cleaning recovery.

  In recent years, filtration methods using porous hollow fiber membranes that have the advantages of improving the safety of treated water and saving the space of equipment are becoming widespread in fields such as decontamination in clean water. Porous hollow fiber membranes have a wide range of operating conditions including high blocking performance that can reliably remove bacteria such as Cryptosporidium and turbid components, high water permeability for treating large amounts of water, chemical cleaning and high operating pressure. High strength that can be used for a long time is required. Furthermore, in the external pressure filtration operation in which the filtration is performed from the outer surface side to the inner surface side of the membrane, the outer surface of the membrane is rubbed by air scrubbing to remove the filtration object accumulated on the outer surface, that is, the pores on the outer surface. The blockage causes deterioration of water permeability performance with time, which is a big problem.

  The idea of obtaining a porous hollow fiber membrane having both high blocking performance and high water permeability by bonding a small pore blocking layer and a large pore strength support layer is disclosed in Patent Document 1, for example. As a specific production method, in Patent Document 1, melt extrusion is performed without adding a solvent to polyethylene, and the lamellar crystal stack is cleaved and opened by performing high-magnification stretching in the longitudinal direction of the hollow fiber to form a porous film. Polyethylene having different MI values are melt-extruded separately from two annular nozzles arranged concentrically using the stretched hole method, which is a method of manufacturing, and resins having different MI values, that is, usually having different molecular weights. Utilizing the property of having different pore diameters when stretched and opened, the hollow fiber membrane is formed such that the outer layer side and the inner layer side have different pore diameter layers.

However, in this production method, (1) the viscosity of the melt is high due to the production method without adding a solvent, and the high molecular weight polymer has low fluidity and is difficult to be molded. However, the rupture strength and compressive strength, which are strengths perpendicular to the stretching axis, are more likely to decrease when filtration is performed. (3) Molecular weight estimated from the MI value of the polyethylene used However, there are disadvantages such as the low outer layer is about 10 to 150,000 and the inner layer is about 40 to 80,000, or the scratch resistance is low, and it is impossible to obtain a film with high scratch resistance and high strength.
Further, Patent Document 2 discloses a film having an asymmetric structure in which the mechanical strength in the longitudinal direction of the film is strong and the pore size distribution is narrow by using PVDF having a high viscosity, that is, a high molecular weight. However, this film has a problem that the entire film is made of a high molecular weight thermoplastic resin, so that the elastic modulus of the film is high, or the thread does not shake greatly during air scrubbing, so that the cleaning recovery is low. .
Therefore, until now, a porous hollow fiber membrane that has both high scuff resistance on the outer surface by air scrubbing and the like and high cleaning recovery has not been obtained.
JP 63-75116 A Japanese Patent Laid-Open No. 07-173323

  The present invention provides a porous hollow fiber membrane made of a thermoplastic resin that is suitable for filtration applications such as turbidity removal and membrane separation activated sludge process, and is less susceptible to abrasion on the outer surface due to air scrubbing or the like, and has high cleaning recovery properties. To the goal.

As a result of intensive studies to solve the above problems, the inventors of the present invention have formed a layer made of a high molecular weight thermoplastic resin having a weight average molecular weight of 500,000 or more on the outermost surface, and a weight average molecular weight on the inner surface side. It was found that laminating a layer having a low molecular weight of less than 400,000 is extremely important in order to achieve both high scratch resistance on the outer surface and high cleaning recovery by air scrubbing, and the present invention has been reached.
That is, the present invention
(1) A porous hollow fiber membrane comprising at least two layers made of a thermoplastic resin, wherein the thermoplastic resin constituting one layer (A) on the inner surface side of the hollow fiber membrane has a weight average molecular weight of less than 400,000 the weight average molecular weight of the thermoplastic resin constituting one layer of the outermost surface (B) is Ri der 500,000, the first layer (a) and the thermoplastic resin is a vinylidene fluoride that constitutes the first layer (B) A porous multilayer hollow fiber membrane characterized in that it is a homopolymer , and (2) the porous multilayer hollow fiber membrane according to (1), wherein the aspect ratio of the outer surface pore is 1/3 or more and 3 or less , and (3) aspect ratio in the surface pores is porous multilayer hollow fiber membrane, according to, characterized in that 1/4 to 4 (1) or (2).

  According to the present invention, it is possible to obtain a porous hollow fiber membrane which is suitable for filtration applications such as turbidity and hardly scratches the outer surface due to air scrubbing or the like and has a high detergency.

Hereinafter, the present invention will be described specifically and in detail.
Thermoplastic resins (thermoplastic polymers) are not easily deformable at room temperature and are elastic and do not exhibit plasticity, but they exhibit plasticity by appropriate heating and become moldable. It is a resin that undergoes reversible changes back to the elastic body and does not produce chemical changes such as molecular structure during that time (edited by the Chemical Dictionary Dictionary, the 6th edition of the Chemical Dictionary, Kyoritsu Shuppan, pages 860 and 867, 1963).

  Examples of thermoplastic resins include resins described in the section of thermoplastics (pages 1069 to 1125) of 14705 chemical products (Chemical Industry Daily, 2005), and Chemical Handbook Application Edition, revised edition 3 (The Chemical Society of Japan) , Maruzen, 1980), pages 809-810. Specific examples include polyethylene, polypropylene, polyvinylidene fluoride, ethylene-vinyl alcohol copolymer, polyamide, polyetherimide, polystyrene, polysulfone, polyvinyl alcohol, polyphenylene ether, polyphenylene sulfide, cellulose acetate, polyacrylonitrile, and the like. . Among these, crystalline polyethylene, polypropylene, polyvinylidene fluoride, ethylene-vinyl alcohol copolymer, polyvinyl alcohol, and the like can be suitably used from the viewpoint of strength development. In addition, among these crystalline thermoplastic resins, hydrophobic crystalline thermoplastic resins such as polyolefins such as polyethylene and polypropylene, polyvinylidene fluoride, etc., which are hydrophobic and have high water resistance, and can be expected to have durability in the filtration of ordinary aqueous liquids. Can be more suitably used. Further, among these hydrophobic crystalline thermoplastic resins, polyvinylidene fluoride, which is excellent in chemical durability such as chemical resistance, can be used particularly suitably. Examples of the polyvinylidene fluoride include a vinylidene fluoride homopolymer and a vinylidene fluoride copolymer having a vinylidene fluoride ratio of 50 mol% or more. Examples of the vinylidene fluoride copolymer include a copolymer of vinylidene fluoride and one or more selected from ethylene tetrafluoride, propylene hexafluoride, ethylene trifluoride chloride, or ethylene. As the polyvinylidene fluoride, a vinylidene fluoride homopolymer is most preferable.

The porous hollow fiber membrane of the present invention, which balances blocking performance, water permeability performance, and strength at a high level, and further achieves high scuff resistance on the outer surface and high cleaning recovery by air scrubbing, is at least two layers inside and outside the present invention. A multilayer film.
Of the two layers, one layer (A) on the inner surface side is a so-called support layer, and has a function to ensure high mechanical strength such as pressure resistance of the porous hollow fiber membrane and to reduce water permeability as much as possible. . Further, this one layer (A) is composed of a thermoplastic resin having a weight average molecular weight of less than 400,000. Since it is made of a thermoplastic resin having a weight average molecular weight of less than 400,000, the hollow fiber membrane is easy to bend and is likely to be shaken during air scrubbing. The use of a thermoplastic resin having a molecular weight of less than 400,000 for the support layer also has an effect of reducing abrasion. The reason is not clear, but it is presumed that the softening of the support layer disperses and lowers the pressing force on the outer surface when the membranes are in contact with each other, thereby reducing abrasion. Moreover, if the weight average molecular weight of one layer (A) is 100,000 or more, it is preferable because high mechanical strength can be expressed. More preferably, it is 200,000 or more.

Of the at least two inner and outer layers of the porous hollow fiber membrane, one outermost layer (B) is a so-called blocking layer, and has a function of blocking the permeation of foreign substances contained in the liquid to be treated by a small surface pore diameter. Demonstrate. Further, this one layer (B) is composed of a thermoplastic resin having a weight average molecular weight of 500,000 or more . By being composed of a thermoplastic resin having a weight average molecular weight of 500,000 or more, it is possible to suppress abrasion on the outer surface of the film due to air scrubbing and to exhibit high water permeability over a long period of time. Moreover, if the weight average molecular weight of 1 layer (B) is less than 2 million, since the whole film | membrane shakes well at the time of air scrubbing and high cleaning recoverability can be exhibited, it is preferable. More preferably, it is less than 1.5 million. Further, as described in Example 1 of the present invention, the outer surface of the hollow fiber membrane of the present invention is extruded and molded as a melt-kneaded product of a high molecular weight thermoplastic resin, a plasticizer, and inorganic fine powder. A film having an outer surface with high surface smoothness and high surface hardness can be obtained, and scratch resistance can be further improved.

  The aspect ratio of the surface of one layer (B), that is, the outer surface hole, is preferably 1/3 or more and 3 or less. If the aspect ratio of the outer surface hole is 1/3 or more, the outer surface which is the blocking layer will not crack when stress is applied in the longitudinal direction of the hollow fiber by air scrubbing etc., and excellent blocking performance will be maintained for a long time In addition, if it is 3 or less, it is preferable to suppress a decrease in water permeability due to rubbing when it is shaken in the circumferential direction of the membrane by air scrubbing or the like. The aspect ratio of the outer surface hole is more preferably ½ or more and 2 or less, and further preferably 2/3 or more and 1.5 or less. The aspect ratio of the outer surface hole here means (average hole diameter in the longitudinal direction of the hollow fiber) / (average hole diameter in the circumferential direction of the hollow fiber) on the outer surface. Since the hollow fiber membrane sways in the circumferential direction of the hollow fiber during air scrubbing, the effect of rubbing is great. Therefore, when the hole diameter in the circumferential direction of the hole on the outer surface is small, the influence of the hole blockage is larger, and the water permeation performance is likely to deteriorate. The aspect ratio of the outer surface hole is measured as follows. First, using a scanning electron microscope, the outer surface is photographed from a direction perpendicular to the outer surface at a magnification that allows the shape of as many holes as possible to be clearly confirmed. Next, on the photograph, five lines are drawn at almost equal intervals in the longitudinal direction of the hollow fiber and in the direction perpendicular thereto, that is, in the circumferential direction of the hollow fiber, and the length of the lines crossing the holes in the photograph. Measure the thickness. And the arithmetic mean value of a measured value is calculated | required in each of a hollow fiber longitudinal direction and a circumferential direction, and it is set as the average hole diameter in each direction. The aspect ratio of the outer surface hole is defined as (average hole diameter in the hollow fiber longitudinal direction) / (average hole diameter in the hollow fiber circumferential direction). In order to increase the accuracy of the hole diameter measurement, it is preferable that the number of hole diameters traversed by the 10 lines in the vertical and horizontal directions is 20 or more. If the pore diameter is about 0.1 μm to 1 μm, it is appropriate to use an electron microscope image with a magnification of about 5000 times.

  Also, the aspect ratio of one layer (A) on the inner surface side is preferably ¼ or more and four or less from the viewpoint of manifesting mechanical strength. If the aspect ratio of the inner surface hole is 1/4 or more, sufficient strength in the longitudinal direction of the hollow fiber membrane, that is, tensile strength can be increased, and if it is 4 or less, the thickness in the film thickness direction of the hollow fiber membrane. It is possible to increase the strength, that is, the compressive strength and burst strength, which are important mechanical strengths during external pressure filtration. The aspect ratio of the inner surface hole is more preferably 1/3 or more and 3 or less, and further preferably 1/2 or more and 2 or less. When the surface of one layer (A) is the innermost surface layer, the aspect ratio of the inner surface hole can be obtained from a scanning electron microscope photograph of the inner surface in the same manner as the aspect ratio of the outer surface hole. When there is a further layer on the inner surface side, it may be obtained from a photograph of a scanning electron microscope of the cross section in the film longitudinal direction and the cross section in the film circumferential direction of one layer (A).

  Moreover, it is preferable that one layer (A) has a homogeneous three-dimensional network structure. Homogeneous here means that the structure has a small change in the pore diameter in the film thickness direction. By adopting such a homogeneous structure, it is difficult to generate a weak portion such as a macrovoid, so that the mechanical strength such as pressure resistance can be increased while maintaining the water permeability of the porous hollow fiber membrane. It becomes possible. In addition, the three-dimensional network structure here refers to a structure in which a resin is innumerable columnar and is joined to both ends to form a three-dimensional structure. In the three-dimensional network structure, almost all of the resin forms a columnar body, and an infinite number of resin lumps that are seen in a so-called spherulite structure are hardly seen. The void portion of the three-dimensional network structure is surrounded by a columnar body of thermoplastic resin, and each portion of the void portion communicates with each other. As described above, since most of the used resin forms a columnar material that can contribute to the strength of the hollow fiber membrane, a high-strength support layer can be formed. In addition, chemical resistance is improved. The reason why chemical resistance is improved is not clear, but because there are a large number of pillars that can contribute to strength, even if part of the pillars are attacked by chemicals, the overall strength of the layer is not significantly affected. It is thought that. On the other hand, in the spherulite structure, since the resin is collected in the lump, the number of columnar objects is relatively small and the strength is low. Therefore, it is considered that when a part of the columnar material is invaded by the chemical, the strength of the entire layer is likely to be affected. A schematic diagram of such an isotropic three-dimensional network structure is shown in FIG. In FIG. 1, it can be seen that the gap b is formed by joining the columnar objects a. For reference, a schematic diagram of the spherulite structure is shown in FIG. In FIG. 2, it can be seen that the spherulites c are partially dense, and the gaps between the dense parts of the spherulites c are voids d.

  Moreover, it is preferable that one layer (B) also has a three-dimensional network structure. The change in the hole diameter in the film thickness direction may be an anisotropic structure in which the diameter of each hole gradually increases from the outer surface toward the inside of the film, and the diameter of each hole is almost independent of the distance from the surface. A uniform structure that is uniform may be used, but a homogeneous structure that can easily maintain the blocking performance when the surface is worn due to abrasion is more preferable. FIG. 3 shows a schematic diagram of the pore diameter change in the cross-sectional direction of the hollow fiber membrane in each case. FIG. 3 (1) is a diagram showing a change in the hole diameter in the film thickness direction when one layer (B) has an isotropic three-dimensional network structure, and FIG. It is a figure which shows a hole diameter change in case of an anisotropic three-dimensional network structure where a hole diameter changes in the thickness direction. 3 (1) and 3 (2), the vertical axis represents the hole diameter ratio based on the cross-sectional central hole diameter, and the horizontal axis represents the film thickness normalized to 1. From the viewpoint of scratch resistance, it is preferable to have a homogeneous three-dimensional network structure because the blocking performance hardly changes even if the surface is scratched. In FIG. 3A, the boundary between the first layer (A) and the first layer (B) is a transition layer in which the diameter of each hole changes rapidly in the film thickness direction. There may be a boundary layer.

The number of layers forming the multilayer and the ratio of the thicknesses of the layers can be appropriately set depending on the purpose. In the case of the purpose of a filtration membrane, for example, in the case of two layers, the combination of a small pore diameter and a thin outer layer and a large pore diameter and a thick inner layer is effective for combining dense pores and high water permeability. .
The thickness of one layer (A) is preferably 10/100 or more and 99/100 or less of the film thickness. If it is 10/100 or more, it can be sufficiently shaken by air scrubbing or the like to exhibit high cleaning recoverability, and if it is 99/100 or less, sufficient blocking performance can be exhibited by the outer blocking layer. More preferably, the film thickness is 30/100 or more and 97/100 or less, and further preferably 60/100 or more and 95/100 or less.

The thickness of one layer (B) is preferably 1/100 or more and 40/100 or less of the film thickness. By making the thickness of the blocking layer relatively thick in this way, it is possible to maintain a high blocking performance even when the outer surface is somewhat worn due to abrasion, while maintaining a good balance between desirable blocking performance and high water permeability. . More preferably, the film thickness is 3/100 or more and 20/100 or less, and further preferably 5/100 or more and 15/100 or less.
In addition, it is preferable that the surface hole diameter of 1 layer (B), ie, an outer surface hole diameter, is 0.01 micrometer or more and less than 5 micrometers. If it is 0.01 μm or more, the filtration resistance of the dense surface is small, and practically sufficient water permeability can be exhibited. In addition, when it is rubbed, a decrease in water permeability due to clogging of the surface holes can be suppressed. Also, if it is 5 μm or less, it is possible to develop turbidity that is an important required function of the filtration membrane, and it is possible to block and deposit most of the material to be filtered on the outer surface of the hollow fiber. High cleaning recovery. More preferably, they are 0.05 micrometer-2 micrometers, More preferably, they are 0.05 micrometer-0.5 micrometer. The surface pore diameter can be obtained by the arithmetic average of the average pore diameter in the membrane longitudinal direction and the average pore diameter in the membrane circumferential direction used in the aspect ratio measurement.

The porosity of the surface of the single layer (B) is not particularly limited as long as it is appropriately determined depending on the purpose, but is preferably 20% or more from the viewpoint of filtration stability of the liquid to be treated containing suspended substances. More preferably, it is 23% or more, and further preferably 25% or more. From the viewpoint of increasing the mechanical strength of the surface portion, the porosity is preferably 80% or less. More preferably, it is 60% or less, More preferably, it is 50% or less. For example, as described in International Publication No. PCT / WO 01/53213 A1, the aperture ratio is obtained by overlaying a transparent sheet on a copy of an electron microscope image, and blacking out the hole using a black magic pen or the like. Then, by copying the transparent sheet onto white paper, the hole portion is clearly distinguished from black and the non-hole portion is clearly distinguished from white, and thereafter, it can be obtained using commercially available image analysis software.
The hollow fiber membrane of the present invention is prepared, for example, by preparing two types of mixtures of thermoplastic resin and organic liquid with different molecular weights, melt and co-extruding from a concentric hollow fiber forming nozzle at an appropriate ratio, and then extracting the organic liquid It can be obtained by removing. An inorganic fine powder may be added together with the organic liquid.

The present invention will be described more specifically based on examples. Hereinafter, methods for measuring various physical properties will be described. All measurements were performed at 25 ° C.
(1) Thread diameter (mm)
Cut the hollow fiber membrane thinly with a razor or the like in the direction perpendicular to the longitudinal direction of the membrane, and measure the major axis and minor axis of the inner diameter and the major axis and minor axis of the outer diameter using a microscope. The outer diameter was determined.

(2) Pure water permeability (l / m ² /hr/0.1 MPa / 25 ° C.)
Seal one end of a wet hollow fiber membrane about 10 cm long, put an injection needle into the hollow part at the other end, inject pure water into the hollow part from the injection needle at a pressure of 0.1 MPa, and permeate to the outer surface The amount of permeated water permeated was measured, and the pure water permeability was determined by the following equation.
Here, the effective membrane length refers to the net membrane length excluding the portion where the injection needle is inserted.

(3) Aspect ratio of outer surface hole and inner surface hole A photograph taken with an arbitrary magnification capable of clearly confirming the shape of a large number of holes on the inner and outer surfaces of the porous hollow fiber membrane as much as possible by a scanning electron microscope was used. On the photograph, five lines were drawn at approximately equal intervals in the longitudinal direction and the circumferential direction of the hollow fiber, and the length of these lines across the hole in the photograph was measured. And the arithmetic average value of the measured value was calculated | required in each of the hollow fiber longitudinal direction and the circumferential direction, it was set as the average hole diameter in each direction, and the aspect ratio of the outer surface and the inner surface hole was calculated by the following formula | equation, respectively.

(4) Scratch resistance (%)
A porous hollow fiber membrane having a wet length of about 10 cm is arranged on a metal plate, and suspended water in which fine sand particles (particle size 130 μm: FujiBrown FRR # 120) are suspended in water at 20% by mass is placed above the membrane. A nozzle set at 70 cm was sprayed onto the outer surface of the membrane at a pressure of 0.07 MPa. After spraying for 15 minutes, the membrane was turned over and sprayed again for 10 minutes. The pure water permeability was measured before and after spraying, and the scratch resistance was determined from the following formula.

(5) Cleaning recovery rate (%)
Using a pressure-type hollow fiber membrane module with an effective membrane length of 2000 mm and a membrane area of 50 m ² , the sand filtration backwash waste water with a turbidity of about 100 degrees and a water temperature of 18-25 ° C. is filtered for 29 minutes and then air scrubbed for 1 minute. The pure water permeability in the module before and after air scrubbing was measured, and the cleaning recovery rate was measured from the following equation. The air flow rate during air scrubbing was 4.6 NL / min / module.

(6) Weight average molecular weight Mw
The weight average molecular weight of the vinylidene fluoride homopolymer used is GPC (HLC-8220 GPC manufactured by Tosoh, column: one KF-606M (6.0 mm ID × 15 cm) manufactured by Shodex + one KF-601 (6.0 mm ID × 15 cm)) Measured with A GPC sample was prepared as follows. First, an eluent (DMF) was added to vinylidene fluoride homopolymer to adjust the concentration to 1.0 mg / ml. Thereafter, the mixture was heated to 40 ° C. (30 min) and allowed to stand overnight to dissolve. Thereafter, the mixture was filtered through a 0.45 micron filter (Chromatodisc 25N manufactured by GL Sciences Inc.), and the filtrate was used as a GPC sample. Moreover, the calibration curve was created using PMMA.

[Example 1]
Vinylidene fluoride homopolymer as a thermoplastic resin, a mixture of bis (2-ethylhexyl) phthalate and dibutyl phthalate as an organic liquid, finely divided silica (trade name: AEROSIL-R972, manufactured by Nippon Aerosil Co., Ltd.) as an inorganic fine powder, The two-layer hollow fiber membrane was melt-extruded by two extruders. As a melt-kneaded product for the outer layer, the composition is a vinylidene fluoride homopolymer (Solef 6020 manufactured by Solvey, weight average molecular weight 780,000): bis (2-ethylhexyl) phthalate: dibutyl phthalate: fine powder silica = 34.0: 33.8 : 6.8: 25.4 (mass ratio) melt-kneaded product was used as a melt-kneaded product for the inner layer, and the composition was vinylidene fluoride homopolymer (Solef 6008 manufactured by Solvey, weight average molecular weight 230,000): Bisphthalate (2 -Ethylhexyl): dibutyl phthalate: fine silica = 36.0: 35.3: 5.0: 23.7 (mass ratio) of melt-kneaded materials, and air as a fluid for forming a hollow part, respectively. From a hollow fiber molding nozzle having an outer diameter of 2.00 mm and an inner diameter of 0.92 mm at a resin temperature of ° C., the theoretical film thickness ratio of the outer layer: inner layer = 10: 90 Was extruded at becomes such an amount ratio.

The extruded hollow fiber-shaped molded product was cooled and solidified by being introduced into a water bath at 40 ° C. after running in the air of 60 cm, and wound up skein at a speed of 40 m / min. The obtained two-layer hollow fiber extrudate was immersed in methylene chloride to extract and remove bis (2-ethylhexyl) phthalate and dibutyl phthalate, and then dried. Next, after immersing in a 50% by mass ethanol aqueous solution for 30 minutes, it was immersed in water for 30 minutes to wet the hollow fiber membrane. Subsequently, it was immersed in 20 mass% NaOH aqueous solution at 70 degreeC for 1 hour, and also water washing was repeated, and the fine powder silica was extracted and removed.
Table 1 shows the outer diameter, inner diameter, outer surface hole aspect ratio, inner surface hole aspect ratio, pure water permeability, scratch resistance, and cleaning recovery rate of the obtained membrane. Both performances showed excellent values.
In addition, when the weight average molecular weight was measured with the outer layer portion from the outer surface side of the obtained hollow fiber membrane as the outer layer portion, and the inner layer portion from 50% thickness of the total film thickness from the inner surface side, It was the same as the weight average molecular weight before extrusion of the polymer used for both layers. In the following Examples and Comparative Examples, the respective weight average molecular weights were measured and found to be the same as the weight average molecular weight before extrusion of the polymers used in both layers.

[Example 2]
A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that a vinylidene fluoride homopolymer (Solef 6010 manufactured by Solvey Co., Ltd., weight average molecular weight 300,000) was used for the melt-kneaded product for the inner layer.
Table 1 shows the outer diameter, inner diameter, outer surface hole aspect ratio, inner surface hole aspect ratio, pure water permeability, scratch resistance, and cleaning recovery rate of the obtained membrane. Both performances showed excellent values.

[Example 3]
The organic hollow liquid obtained in Example 2 and the porous hollow fiber membrane after extraction and removal of the inorganic fine powder were stretched to a length of 200 cm and then naturally relaxed. Thereafter, heat treatment was performed at 140 ° C. for 1.5 hours without fixing both ends, whereby the yarn length was shortened, and the final yarn length was 260 cm (final draw ratio: 1.3 times).
Table 1 shows the outer diameter, inner diameter, outer surface hole aspect ratio, inner surface hole aspect ratio, pure water permeability, scratch resistance, and cleaning recovery rate of the obtained membrane. Both performances showed excellent values.

[Example 4]
A porous hollow fiber membrane was formed in the same manner as in Example 3 except that after stretching twice and heat treatment at 140 ° C. for 1.5 hours was performed with both ends of the hollow fiber membrane fixed to prevent the yarn length from shrinking. Got. The final yarn length was 400 cm (final draw ratio: 2.0 times).
Table 1 shows the outer diameter, inner diameter, outer surface hole aspect ratio, inner surface hole aspect ratio, pure water permeability, scratch resistance, and cleaning recovery rate of the obtained membrane. Both performances showed excellent values.

[Example 5]
Porous hollow in the same manner as in Example 2 except that the yarn length was extended from 100 cm to 300 cm and heat treatment at 140 ° C. for 1.5 hours was carried out with both ends of the hollow fiber membrane fixed to prevent the yarn length from shrinking. A yarn membrane was obtained. The final yarn length was 300 cm (final draw ratio: 3.0 times).
Table 1 shows the outer diameter, inner diameter, outer surface hole aspect ratio, inner surface hole aspect ratio, pure water permeability, scratch resistance, and cleaning recovery rate of the obtained membrane. Both performances showed excellent values.

[Comparative Example 1]
A porous hollow fiber membrane was obtained in the same manner as in Example 2 except that Solef 6010 manufactured by Solvey Co., Ltd. and vinylidene fluoride homopolymer having a weight average molecular weight of 300,000 were used as the vinylidene fluoride homopolymer used for the outer layer melt-kneaded product. It was. The obtained porous hollow fiber membrane was different from the Example membrane, and a membrane having low scratch resistance was obtained.

[Comparative Example 2]
A porous hollow fiber membrane is obtained in the same manner as in Example 2 except that Solef 6020 manufactured by Solvey Co., Ltd. and vinylidene fluoride homopolymer having a weight average molecular weight of 780,000 are used as the vinylidene fluoride homopolymer used for the melt-kneaded product for the inner layer. It was. The obtained porous hollow fiber membrane was different from the Example membrane, and a membrane having a low cleaning recovery rate by air scrubbing was obtained.

  According to the present invention, it is possible to supply a porous multilayer hollow fiber membrane that is suitable for filtration applications such as turbidity and has both scratch resistance and washing recoverability.

It is a schematic diagram of an isotropic three-dimensional network structure. It is a schematic diagram of a spherulite structure. It is the schematic diagram which showed the example of the hole diameter change of the film thickness direction of a porous hollow fiber membrane.

Claims (3)

A porous hollow fiber membrane comprising at least two layers made of a thermoplastic resin, the weight average molecular weight of the thermoplastic resin constituting one layer (A) on the inner surface side of the hollow fiber membrane being less than 400,000, the outermost the weight average molecular weight of the thermoplastic resin constituting one layer of the surface (B) is Ri der 500,000, the first layer (a) and one layer (B) in the thermoplastic resin is a vinylidene fluoride homopolymer which constitutes the porous multilayer hollow fiber membrane, characterized in that.   2. The porous multilayer hollow fiber membrane according to claim 1, wherein the aspect ratio of the outer surface hole is 1/3 or more and 3 or less.   The porous multilayer hollow fiber membrane according to claim 1 or 2, wherein the aspect ratio of the inner surface pore is ¼ or more and 4 or less.

Images