Porous hollow fiber membrane and method for producing porous hollow fiber membrane

Abstract

The present invention makes it possible to provide a porous hollow fiber membrane having a large pore diameter near the surface, a high porosity, and a high compressive strength. This porous hollow fiber membrane contains a polyolefin. The limiting viscosity of the polyolefin or of the porous hollow fiber membrane is 4.5 dL / g or more.

Description

Porous hollow fiber membrane and method for producing porous hollow fiber membrane

[0001] The present invention relates to a porous hollow fiber membrane and a method for producing the porous hollow fiber membrane.

[0002] Solid-liquid separation (clarification) procedures are required to separate and remove suspended matter in various processes, including water supply treatment to obtain drinking water or industrial water from natural water sources such as seawater, river water, lake water, and groundwater, treatment of domestic wastewater such as sewage to produce reclaimed water and clear water suitable for discharge, and removal of yeast and sediment components such as polymers, proteins, polyphenols, and tannins produced by yeast from brewed alcoholic beverages produced by fermenting sugar-containing liquids with yeast. In the production of saccharified liquid from starch liquid, insoluble components not decomposed by enzymes are removed. In water supply treatment, such clarification procedures remove suspended matter (e.g., clay, colloids, and bacteria) from natural water sources, which are suspended matter. In sewage treatment, suspended matter from sewage and suspended matter (e.g., sludge) from treated water following biological treatment (secondary treatment) using activated sludge are removed. Furthermore, clarification of fermented alcoholic beverages such as wine and saccharified solutions has traditionally been carried out by diatomaceous earth filtration, but in recent years membrane filtration has become increasingly popular in place of this method.

[0003] Conventionally, these clarification operations have mainly been carried out by pressure flotation, sedimentation, sand filtration, diatomaceous earth filtration, coagulation-sedimentation sand filtration, etc., but in recent years membrane filtration has become more widespread. The advantages of membrane filtration include the following: (1) The clarification level of the obtained water is high and stable (i.e., the obtained water is highly safe). (2) The installation space for the filtration device is small. (3) Automatic operation is easy.

[0004] For example, in water treatment, membrane filtration is used as an alternative to coagulation-sedimentation sand filtration, or as a means for further improving the quality of treated water after coagulation-sedimentation sand filtration by installing it after coagulation-sedimentation sand filtration. In sewage treatment, the use of membrane filtration is also being considered for separating sludge from secondary sewage effluent.

[0005] These membrane filtration clarification operations mainly use hollow fiber ultrafiltration or microfiltration membranes (pore sizes ranging from several nanometers to several hundred nanometers). Filtration methods using hollow fiber filtration membranes include internal pressure filtration, which filters from the inner surface of the membrane to the outer surface, and external pressure filtration, which filters from the outer surface to the inner surface. Of these, external pressure filtration is advantageous because it allows for a large membrane surface area on the side in contact with the suspended raw water, thereby reducing the suspended solids load per unit membrane surface area. Patent Documents 1 and 2 disclose hollow fibers and methods for producing them.

[0006] Thermally induced phase separation is a well-known method for producing porous membranes. This method uses a thermoplastic resin and an organic liquid. The organic liquid is a solvent that does not dissolve the thermoplastic resin at room temperature but dissolves it at high temperatures, i.e., a latent solvent. Thermally induced phase separation involves kneading a thermoplastic resin and an organic liquid at high temperatures, dissolving the thermoplastic resin in the organic liquid, and then cooling to room temperature to induce phase separation. The organic liquid is then removed to produce a porous body. This method has the following advantages: (a) It enables film production even with polymers such as polyethylene, which do not have suitable solvents that can dissolve them at room temperature. (b) Because the thermoplastic resin is dissolved at high temperatures and then cooled and solidified to form a film, crystallization is promoted during film production, making it easier to obtain a high-strength film, especially when the thermoplastic resin is a crystalline resin.

[0007] Due to the above advantages, it is widely used as a method for producing porous membranes (see, for example, Non-Patent Documents 1 to 4).

[0008] Japanese Patent Publication No. 60-023130 Japanese Patent Publication No. 5-9332

[0009] Hideto Matsuyama, "Preparation of Polymer-Based Porous Membranes by Thermally Induced Phase Separation (TIPS) Method," Chemical Engineering Magazine, Kagaku Kogyosha, June 1998, pp. 45-56; Akira Takizawa, "Membranes," IPC Corporation, January 1992, pp. 404-406; D. R. Lloyd, et al., "Journal of Membrane Science," vol. 64, 1991, pp. 1-11.

[0010] Conventionally, it has been known from Patent Document 1 that a porous hollow fiber membrane is produced using a low molecular weight polyethylene. However, due to the low molecular weight, the strength is insufficient and the membrane is not durable enough for practical use. In addition, Patent Document 2 discloses a method for producing a porous membrane using an ultra-high molecular weight polyethylene. However, the membrane has a flat shape, and the pore size and porosity near the surface are not sufficiently large.

[0011] An object of the present invention is to provide a porous hollow fiber membrane having a large pore size near the surface, a high porosity, and high compressive strength.

[0012] That is, the present invention is as follows. [1] A porous hollow fiber membrane containing a polyolefin, wherein the polyolefin or the porous hollow fiber membrane has an intrinsic viscosity of 4.5 dL / g or more. [2] The porous hollow fiber membrane according to [1], wherein the polyolefin or the porous hollow fiber membrane has an intrinsic viscosity of 6.6 dL / g or more and 18 dL / g or less. [3] The porous hollow fiber membrane according to [1] or [1], wherein a non-reversing heat flow ratio is 35% or more. [4] The porous hollow fiber membrane according to [1] or [2], wherein a non-reversing heat flow ratio is 60% or more. [5] The porous hollow fiber membrane according to [1] or [2], wherein the non-reversing heat flow ratio is 60% or more and 90% or less. [6] The porous hollow fiber membrane according to any one of [1] to [5], wherein the non-reversing heat flow endothermic enthalpy is 60 J / g or more. [7] The porous hollow fiber membrane according to any one of [1] to [6], wherein the non-reversing heat flow endothermic enthalpy is 80 J / g or more and 105 J / g or less. [8] The porous hollow fiber membrane according to any one of [1] to [7], wherein the pore size is 0.15 μm or more in a thickness direction range of 0.5% to 1.5% of the membrane thickness from the outermost surface of one of the inner surface and the outer surface. [9] The porous hollow fiber membrane according to any one of [1] to [8], wherein the porosity is 25% or more in a thickness direction range of 0.5% to 1.5% of the membrane thickness from the outermost surface of one of the inner and outer surfaces.

[10] The porous hollow fiber membrane according to any one of [1] to [9], wherein the pore aspect ratio is 2.0 or less in a thickness direction range of 0.5% to 1.5% of the membrane thickness from the outermost surface of one of the inner and outer surfaces.

[11] The porous hollow fiber membrane according to any one of [1] to

[10] , wherein the pore size is 0.23 μm or more in a thickness direction range of 0.5% to 1.5% of the membrane thickness from the outermost surface of one of the inner and outer surfaces.

[12] The porous hollow fiber membrane according to any one of [1] to

[11] , wherein the ratio of the pore size in a thickness direction range of 0.5% to 1.5% from the outermost surface of one of the inner and outer surfaces to the pore size of the one surface (pore size of one surface / pore size in a thickness direction range of 0.5% to 1.5% from the outermost surface of one surface) is 1.29 or less.

[13] The porous hollow fiber membrane according to any one of [1] to

[12] , wherein the compressive strength is 0.4 MPa or more.

[14] The porous hollow fiber membrane according to any one of [1] to

[13] , wherein the porous hollow fiber membrane has a three-dimensional mesh structure.

[15] The porous hollow fiber membrane according to any one of [1] to

[14] , wherein the polyolefin is polyethylene.

[16] A method for producing a porous hollow fiber membrane, comprising adding an organic liquid and an inorganic fine powder to a polyolefin having an intrinsic viscosity of 4.5 dL / g or more.

[17] The method for producing a porous hollow fiber membrane according to

[16] , wherein the polyolefin is a polyolefin having an intrinsic viscosity of 6.6 dL / g or more and 18 dL / g or less.

[18] The method for producing a porous hollow fiber membrane according to

[16] or

[17] , wherein the inorganic fine powder is silica.

[19] The Hansen solubility parameter distance between the organic liquid and the polyolefin is 8.5 MPa. ０．５

[20] The method for producing a porous hollow fiber membrane according to any one of

[16] to

[19] , wherein the organic liquid comprises at least one selected from the group consisting of phthalate esters, sebacate esters, citrate esters, acetyl citrate esters, adipate esters, trimellitate esters, oleate esters, palmitate esters, stearates, phosphate esters, fatty acids having from 6 to 30 carbon atoms, and epoxidized vegetable oils.

[21] The method for producing a porous hollow fiber membrane according to any one of

[16] to

[20] , further comprising: melt-kneading a polyolefin solution obtained by adding the organic liquid and the inorganic fine powder to the polyolefin; discharging the resulting molten mixture; and cooling the molten mixture only in an air layer after discharging.

[22] The method for producing a porous hollow fiber membrane according to

[21] , wherein the air layer has a wind speed of 2.0 m / sec or less.

[23] The method for producing a porous hollow fiber membrane according to any one of

[16] to

[22] , wherein a hindered phenol-based antioxidant is further added to the polyolefin as an antioxidant.

[24] The method for producing a porous hollow fiber membrane according to any one of

[16] to

[23] , wherein the polyolefin is polyethylene.

[0013] According to the present invention, it is possible to provide a porous hollow fiber membrane having a large pore size near the surface, a high porosity, and high compressive strength.

[0014] FIG. 1 is a schematic diagram of a three-dimensional mesh structure. FIG. 2 is a schematic diagram of an example of an apparatus for producing a porous hollow fiber membrane. FIG. 3 is a schematic diagram of an example of an apparatus for producing a porous hollow fiber membrane. FIG. 4 is an electron microscope photograph of a membrane cross section near the outer surface of the porous hollow fiber membrane produced in Example 2. FIG. 5 is MDSC analysis data of the porous hollow fiber membrane produced in Example 3. FIG. 6 is MDSC analysis data of the porous hollow fiber membrane produced in Example 10. FIG. 7 is MDSC analysis data of the porous hollow fiber membrane produced in Example 13. FIG. 8 is MDSC analysis data of the porous hollow fiber membrane produced in Comparative Example 4.

[0015] The present invention will be described in detail below with reference to the following embodiments, but the present invention is not limited to the following embodiments.

[0016] (Porous hollow fiber membrane) Hereinafter, the porous hollow fiber membrane of this embodiment and the porous hollow fiber membrane obtained by the production method of this embodiment will be described.

[0017] The porous hollow fiber membrane of this embodiment and the porous hollow fiber membrane obtained by the production method of this embodiment contain a polyolefin, and the polyolefin or the porous hollow fiber membrane has an intrinsic viscosity of 4.5 dL / g or more, preferably 6.6 dL / g or more, and preferably 18 dL / g or less. From the viewpoint of strength and abrasion resistance, the intrinsic viscosity is preferably within the above range. The porous hollow fiber membrane may be composed solely of a polyolefin having an intrinsic viscosity of 4.5 dL / g or more, preferably 6.6 dL / g or more, and preferably 18 dL / g or less, or may be composed solely of a polyolefin having an intrinsic viscosity of 4.5 dL / g or more, preferably 6.6 dL / g or more, and preferably 18 dL / g or less, and another thermoplastic resin, or may further contain other components.

[0018] In the present application, the intrinsic viscosity is a value measured in accordance with ISO 1628-3 (2010), and is measured at 25° C. using decahydronaphthalene as a solvent (which may contain a predetermined amount, for example, 1 g / L, of an antioxidant such as 2,6-di-t-butyl-4-methylphenol). More specifically, the intrinsic viscosity can be measured by the method described in the examples.

[0019] In the present application, polyolefin or polyolefin resin refers to a polymer containing an olefin hydrocarbon as a monomer component. Here, polyolefin also includes copolymers of olefin hydrocarbons with monomers other than olefins, and the copolymerization ratio of olefin hydrocarbon units is preferably 95% by mass or more, more preferably 97% by mass or more, and even more preferably 99% by mass or more. In the present application, "polyolefin resin" and "polyolefin" have the same meaning. The polyolefin used in the present application is preferably a crystalline polyolefin. There is no limitation on the type of polyolefin used in the present application, and examples include homopolymers or copolymers containing olefin hydrocarbons as monomers, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Specific examples include polyethylene; polypropylene; polybutene; ethylene-propylene random copolymers; and ethylene-propylene rubber. The polyolefin is preferably polyethylene. Among these, the type of polyethylene is not particularly limited, but examples include polyethylenes such as low-density polyethylene, linear low-density polyethylene, medium-density polyethylene, high-density polyethylene, and ultra-high molecular weight polyethylene. Of these, the ultra-high molecular weight polyethylene may be a polymer polymerized using a known catalyst, but ultra-high molecular weight polyethylene polymerized using a Ziegler catalyst or a metallocene catalyst is preferably used. Furthermore, two or more polyolefins with different structures or molecular weights may be mixed.

[0020] The porous hollow fiber membrane preferably contains a thermoplastic resin as a main component. The thermoplastic resin preferably contains a polyolefin resin as a main component, more preferably a polyolefin having an intrinsic viscosity of 4.5 dL / g or more, preferably 6.6 dL / g or more, and preferably 18 dL / g or less, as a main component, and even more preferably a polyethylene having an intrinsic viscosity of 4.5 dL / g or more, preferably 6.6 dL / g or more, and preferably 18 dL / g or less, as a main component. The resin component contained in the porous hollow fiber membrane is preferably the polyolefin resin alone. Here, "contained as a main component" means that the thermoplastic resin or the porous hollow fiber membrane contains 50% by mass or more (preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably 100% by mass) in terms of solid content.

[0021] The intrinsic viscosity of the polyolefin or the porous hollow fiber membrane is 4.5 dL / g or more, preferably 6.6 dL / g or more, and preferably 18 dL / g or less. The intrinsic viscosity of the thermoplastic resin other than the polyolefin is preferably 4.5 dL / g or more, more preferably 6.6 dL / g or more, more preferably 18 dL / g or less, even more preferably 14 dL / g or less, even more preferably 12 dL / g or less, and even more preferably 10 dL / g or less. Furthermore, the mixture is not limited to a single molecular weight polyolefin and optional other thermoplastic resin, but may also contain a mixture of polyolefins and optional other thermoplastic resins having different molecular weights. Only the intrinsic viscosity of the polyolefin may be within the above range, or only the intrinsic viscosity of the porous hollow fiber membrane may be within the above range, or both the intrinsic viscosities of the polyolefin and the porous hollow fiber membrane may be within the above range.

[0022] The viscosity average molecular weight (Mv) of the polyolefin and other thermoplastic resins is preferably 500,000 or more, and more preferably 900,000 to 4,000,000. In addition, the thermoplastic resins are not limited to those having a single molecular weight, and a mixture of thermoplastic resins having different molecular weights may also be used.

[0023] The hollow fiber membrane in the form of the porous hollow fiber membrane refers to a membrane having a hollow ring shape. The porous membrane has a hollow fiber membrane structure, which makes it possible to increase the membrane area per unit volume of the module compared to a planar membrane. The porous hollow fiber membrane may have pores that communicate with the interior of the porous hollow fiber membrane in the thickness direction from one surface to the other.

[0024] The porous hollow fiber membrane preferably has a three-dimensional mesh structure. In this specification, a three-dimensional mesh structure refers to a structure as schematically shown in FIG. 1. For example, thermoplastic resins a, such as polyolefins, are bonded to form a mesh, and voids b are formed. In the three-dimensional mesh structure, lumps of resin with a so-called spherulite structure are hardly observed. It is preferable that the voids b in the three-dimensional mesh structure are surrounded by thermoplastic resins a, such as polyolefins, and that each portion of the voids b is interconnected. Since most of the thermoplastic resins, such as polyolefins, used form a three-dimensional mesh structure that can contribute to the strength of the porous hollow fiber membrane, it is possible to form a support layer with high strength. Chemical resistance is also improved. The reason for the improved chemical resistance is unclear, but it is thought that this is because, due to the large amount of thermoplastic resin that forms the mesh that contributes to strength, even if part of the mesh is corroded by chemicals, the strength of the layer as a whole is not significantly affected.

[0025] The porous hollow fiber membrane of this embodiment preferably has a non-reversing heat flow ratio of 35% or more. The non-reversing heat flow ratio is the ratio of non-reversing heat flow to reversing heat flow in MDSC measurement. Here, temperature-modulated DSC (MDSC) measurement is a method of DSC measurement in which, unlike normal DSC measurement using a constant rate of temperature increase, the temperature is periodically increased and decreased relative to the average rate of temperature increase. When the MDSC method is used, three melting curves are obtained: the total heat flux component (i.e., the total heat flow measured in a conventional DSC measurement), the heat flux component that can reversibly follow temperature modulation (reversing heat flow), and the irreversible component (non-reversing heat flow) obtained by subtracting the reversing heat flow from the total heat flow. A high ratio of non-reversing heat flow indicates that there are more components that cannot follow the cyclic temperature rise and fall compared to the components that can follow reversibly, i.e., there are many polymer chains that are strongly entangled. If the ratio is 35% or higher, there are many components with strongly entangled polymer chains, and the porous hollow fiber membrane exhibits high compressive strength or burst strength. It is preferably 50% or more, more preferably 60% or more, even more preferably 60% to 100%, and even more preferably 60% to 90%.

[0026] The porous hollow fiber membrane of this embodiment preferably has a non-reversing heat flow endothermic enthalpy of 60 J / g or more. The non-reversing heat flow endothermic enthalpy indicates the amount of enthalpy at the time of melting the non-reversing heat flow component, and a non-reversing heat flow endothermic enthalpy of 60 J / g or more indicates that the polymer chains are more tightly entangled. The non-reversing heat flow endothermic enthalpy is preferably 70 J / g or more and 120 J / g or less, and more preferably 80 J / g or more and 105 J / g or less.

[0027] The porous hollow fiber membrane of this embodiment preferably has a pore size of 0.15 μm or more in a thickness direction range of 0.5% to 1.5% of the membrane thickness from the outermost surface of one of the inner and outer surfaces. A pore size of 0.15 μm or more can exhibit practical water permeability. Because this range is close to the surface of the porous hollow fiber membrane, clogging in this range is presumed to result in rapid accumulation of the substance to be filtered on the surface, resulting in a decrease in water permeability. More preferably, the pore size is 0.17 μm or more, and more preferably 0.45 μm or less. The pore size can be measured, for example, by the method described in the Examples. Preferably, the one surface of the porous hollow fiber membrane of this embodiment is on the side of the liquid to be filtered. Furthermore, in this specification, the "thickness direction range of 0.5% to 1.5% of the membrane thickness from the outermost surface" refers to the range from 0.5% of 100% of the membrane thickness from the outermost surface to 1.5% of 100% of the membrane thickness from the outermost surface.

[0028] Furthermore, the porous hollow fiber membrane of this embodiment preferably has a porosity of 25% or more in a thickness direction range of 0.5% to 1.5% of the membrane thickness from the outermost surface of one of the inner and outer surfaces. If the porosity is 25% or more, the rate of pore blockage due to fouling substances such as suspended matter inside the membrane is low, allowing for stable filtration operation over a long period of time. More preferably, it is 28% or more. The porosity can be measured, for example, by the method described in the Examples. The "one of the inner and outer surfaces" in the above porosity is preferably the same surface as one of the inner and outer surfaces in the pore size preferably 0.15 μm or more described above, and more preferably the surface on the side of the liquid to be filtered.

[0029] Furthermore, the porous hollow fiber membrane of this embodiment preferably has a pore aspect ratio of 2.0 or less in a thickness direction range of 0.5% to 1.5% of the membrane thickness from the outermost surface of one of the inner and outer surfaces. When the aspect ratio is 2.0 or less, buckling on the minor axis side is suppressed when pressure is applied in the minor axis direction of the pore, making it less likely for the pore to become blocked. Similarly, collapse of the entire porous hollow fiber membrane caused by buckling on the minor axis side is suppressed. The pore aspect ratio can be measured, for example, by the method described in the Examples. In the above aspect ratio, "one of the inner and outer surfaces" is preferably the same surface as one of the inner and outer surfaces in the pore diameter preferably equal to or greater than 0.15 μm, and more preferably the surface on the side of the liquid to be filtered.

[0030] The porous hollow fiber membrane of this embodiment preferably has a pore size of 0.23 μm or more in a thickness direction range of 0.5% to 1.5% of the membrane thickness from the outermost surface of one of the inner and outer surfaces. A pore size of 0.23 μm or more can exhibit practical water permeability. It is more preferably 0.23 μm or more and 0.40 μm or less. The pore size can be measured, for example, by the method described in the Examples. The porous hollow fiber membrane of this embodiment preferably has a pore size of 0.15 μm or more (more preferably 0.17 μm or more, and even more preferably 0.45 μm or less) in a thickness direction range of 0.5% to 1.5% of the membrane thickness from the outermost surface of one of the inner and outer surfaces, and a pore size of 0.23 μm or more (more preferably 0.23 μm or more and 0.40 μm or less) in a thickness direction range of 0.5% to 1.5% of the membrane thickness from the outermost surface of the other of the inner and outer surfaces. The pore size in the thickness direction within a range of 0.5% to 1.5% of the film thickness from the outermost surface of the other of the inner and outer surfaces is preferably larger than the pore size in the thickness direction within a range of 0.5% to 1.5% of the film thickness from the outermost surface of one of the inner and outer surfaces. The one surface or the other surface refers to a different surface of the inner and outer surfaces. The other surface is preferably the filtrate side. It is preferable that the one surface is the surface on the side of the liquid to be filtered, and the other surface is the surface on the side of the filtrate. Here, for example, in the case of an external pressure filtration system, the outer surface is the surface on the side of the liquid to be filtered, and the inner surface is the surface on the side of the filtrate. Furthermore, in the case of an internal pressure filtration system, the inner surface is the surface on the side of the liquid to be filtered, and the outer surface is the surface on the side of the filtrate.

[0031] Furthermore, in the porous hollow fiber membrane of this embodiment, the ratio of the pore size in the thickness direction from the outermost surface of one of the inner and outer surfaces to the pore size of the one surface (pore size of one surface / pore size in the thickness direction from the outermost surface of one surface to the pore size of the one surface) is preferably 1.29 or less. When the ratio is 1.29 or less, the difference between the pore size in the thickness direction from the outermost surface of one of the inner and outer surfaces to the pore size of the one surface is small, thereby increasing the capacity of the substance to be filtered captured in the thickness direction and suppressing leakage of the substance to be filtered, thereby exhibiting stable blocking performance during filtration. More preferably, it is 1.26 or less. In the above ratio, the "inner surface and one of the surfaces" is preferably the same surface as one of the inner and outer surfaces at the pore size preferably having a pore size of 0.23 μm or more, and more preferably the surface on the filtrate side.

[0032] The compressive strength of the porous hollow fiber membrane of this embodiment is preferably 0.4 MPa or more. A high compressive strength results in a porous hollow fiber membrane with high strength and high burst strength. A compressive strength of 0.4 MPa or more provides sufficient durability against the pressure applied during filtration, allowing for stable filtration operation over a long period of time. A compressive strength of 0.5 MPa or more is even more preferable.

[0033] (Method for Producing Porous Hollow Fiber Membrane) The method for producing a porous hollow fiber membrane of this embodiment includes adding an organic liquid and an inorganic fine powder to a polyolefin having an intrinsic viscosity of 4.5 dL / g or more. The method for producing a porous hollow fiber membrane of this embodiment is preferably the method for producing the porous hollow fiber membrane of this embodiment described above. The method for producing a porous hollow fiber membrane of this embodiment preferably includes the steps of: extruding a molten mixture containing a polyolefin having an intrinsic viscosity within a predetermined range, and optionally other thermoplastic resins, an organic liquid, and inorganic fine powders, from a spinneret having an annular outlet to form a hollow fiber-shaped molten mixture; and solidifying the hollow fiber-shaped molten mixture and then extracting and removing the organic liquid and inorganic fine powder to produce a porous hollow fiber membrane. The molten mixture may be composed of two components, a thermoplastic resin and a solvent, or may be composed of three components, a thermoplastic resin, an inorganic fine powder, and a solvent. In this specification, the terms "organic liquid" and "organic liquid" are synonymous. Furthermore, inorganic fine powder and inorganic fine powder are synonymous.

[0034] The polyolefin and other optional thermoplastic resins used in the method for producing a porous hollow fiber membrane of this embodiment are resins that are elastic at room temperature and do not exhibit plasticity, but become plastic and moldable when appropriately heated. Furthermore, the polyolefin and other optional thermoplastic resins are resins that return to their original elastic bodies when cooled and cooled, without undergoing chemical changes such as molecular structure change (see, for example, "Chemical Dictionary 6, Condensed Edition, Edited by the Editorial Committee of the Chemical Dictionary, Kyoritsu Shuppan, pp. 860 and 867, 1963").

[0035] Examples of thermoplastic resins other than polyolefins include the resins described in the "Thermoplastics" section (pages 829-882) of "12695 Chemical Products" (The Chemical Daily, 1995) and the resins described in "Chemical Handbook, Applied, Revised, 3rd Edition" (edited by the Chemical Society of Japan, Maruzen, 1980), pages 809-810. Specific examples of thermoplastic resins other than polyolefins include fluororesins such as polyvinylidene fluoride, ethylene-vinyl alcohol copolymers, polyamides, polyetherimides, polystyrenes, polysulfones, polyvinyl alcohols, polyphenylene ethers, polyphenylene sulfide, cellulose acetate, polyacrylonitrile, and the like. Among these, crystalline thermoplastic resins such as fluororesins such as polyvinylidene fluoride, ethylene-vinyl alcohol copolymers, and polyvinyl alcohol are preferably used in terms of strength. More preferably, fluororesins such as polyvinylidene fluoride, which are highly water-resistant due to their hydrophobicity and therefore can be expected to be durable in filtering ordinary aqueous liquids, can be used.

[0036] The total mass proportion of polyolefin and other optional thermoplastic resins in the melt-kneaded product is preferably 10% by mass or more and 50% by mass or less, more preferably 15% by mass or more and 40% by mass or less. If it is 10% by mass or more, mechanical strength is easily ensured, and if it is 50% by mass or less, there is no decrease in water permeability. Furthermore, the mass proportion of polyolefin in the melt-kneaded product is preferably 10% by mass or more and 50% by mass or less, more preferably 15% by mass or more and 40% by mass or less.

[0037] The organic liquid used in this embodiment is a latent solvent for the polyolefin and other optional thermoplastic resins. In this embodiment, the latent solvent refers to a solvent that barely dissolves the polyolefin and other optional thermoplastic resins at room temperature (25°C), but can dissolve the polyolefin and other optional thermoplastic resins at temperatures higher than room temperature. The organic liquid need only be liquid at the melt-kneading temperature with the polyolefin and other optional thermoplastic resins, and does not necessarily have to be liquid at room temperature.

[0038] Examples of organic liquids for polyolefins include phthalate esters such as dibutyl phthalate, diheptyl phthalate, dioctyl phthalate, di(2-ethylhexyl) phthalate, diisodecyl phthalate, and ditridecyl phthalate; sebacate esters such as dibutyl sebacate; citrate esters; acetyl citrate esters; adipate esters such as dioctyl adipate; trimellitate esters such as trioctyl trimellitate; oleate esters; palmitate esters; stearic acid esters; phosphate esters such as tributyl phosphate and trioctyl phosphate; fatty acids having 6 to 30 carbon atoms; epoxidized vegetable oils; glycerin esters such as propylene glycol dicaprate and propylene glycol dioleate; paraffins such as liquid paraffin; and mixtures thereof. Among these, it is preferable to contain at least one selected from the group consisting of phthalate esters, sebacate esters, citrate esters, acetyl citrate esters, adipate esters, trimellitate esters, oleate esters, palmitate esters, stearic acid esters, phosphate esters, fatty acids having from 6 to 30 carbon atoms, and epoxidized vegetable oils, and it is more preferable to contain only at least one selected from the group consisting of phthalate esters, sebacate esters, citrate esters, acetyl citrate esters, adipate esters, trimellitate esters, oleate esters, palmitate esters, stearic acid esters, phosphate esters, fatty acids having from 6 to 30 carbon atoms, and epoxidized vegetable oils. The organic liquid may be of one type, or two or more types may be used.

[0039] The organic liquid has a Hansen solubility parameter distance of 8.5 MPa with respect to the polyolefin. ０．５ The distance between the Hansen solubility parameters for polyolefins is preferably 8.5 MPa or less. ０．５ If the tensile strength is 7.0 MPa or less, the entanglement of the polymer chains will be strong, and high compressive strength or burst strength can be achieved. ０．５ or less, and more preferably 6.0 MPa or less ０．５The reason is as follows: Although the reason is not clear, sebacic acid esters and adipic acid esters are particularly preferred because they can exert high strength.

[0040] The distance (Ra) between the Hansen solubility parameters of polyethylene and an organic liquid can be calculated using the following formula. The following formula is a relational expression between the Hansen (three-dimensional) solubility parameter of polyethylene and the Hansen (three-dimensional) solubility parameter of an organic liquid, and is used to evaluate the solubility of polyethylene and an organic liquid. The right-hand side of the following formula represents the three-dimensional solubility range of the Hansen solubility parameters, and quantitatively represents the distance (Ra) from the Hansen (three-dimensional) solubility parameters of polyethylene (σdp, σpp, σhp) to the Hansen (three-dimensional) solubility parameters of the organic liquid (σdm, σpm, σhm). Ra = (4 x (σdm - σdp) ２ +(σpm-σpp) ２ + (σhm-σhp) ２ ) ０．５ [In the formula, σdm and σdp represent the dispersion force terms of the organic liquid or solvent and polyethylene, respectively, σpm and σpp represent the dipole binding force terms of the organic liquid or solvent and polyethylene, respectively, and σhm and σhp represent the hydrogen bond terms of the organic liquid or solvent and polyethylene, respectively.]

[0041] Generally, when a porous hollow fiber membrane is produced using a polyolefin and an organic liquid whose Hansen solubility parameter distance is outside the above range, a porous hollow fiber membrane cannot be obtained or the porous hollow fiber membrane has a spherulite structure. However, by not only adjusting the Hansen solubility parameter distance between the polyolefin and the organic liquid but also mixing inorganic fine powder and air-cooling as described below, it is possible to achieve a high level of compatibility between the trade-offs of strength, filtration stability, and pore size.

[0042] The mass proportion of the organic liquid in the melt-kneaded product is preferably 10% by mass or more and 90% by mass or less, more preferably 20% by mass or more and 80% by mass or less. When the mass proportion of the organic liquid is 10% by mass or more, the thermoplastic resin can be stably dissolved, and when it is 90% by mass or less, the viscosity is sufficient for spinning a porous hollow fiber membrane, allowing for stable production.

[0043] Examples of inorganic fine powders include silica, alumina, titanium oxide, zirconia oxide, and calcium carbonate, with silica being preferred. The average primary particle size of the inorganic fine powder is preferably 3 nm to 500 nm, more preferably 5 nm to 100 nm. Among these, fine silica having an average primary particle size of 3 nm to 500 nm is preferred. Hydrophobic silica fine powder, which is less likely to aggregate and has good dispersibility, is more preferred, and even more preferred is hydrophobic silica having a MW (methanol wettability) value of 30% by volume or more. The MW value here refers to the volume percent of methanol that completely wets the powder. Specifically, silica is placed in pure water, and while stirring, methanol is added below the liquid surface. When 50% by mass of the silica has settled, the volume percent of methanol in the aqueous solution is calculated. The "average primary particle size of the inorganic fine powder" mentioned above refers to a value determined by analysis of an electron microscope photograph. Specifically, a group of inorganic fine powders is first pretreated according to the method of ASTM D3849. Thereafter, the diameters of 3,000 to 5,000 particles photographed in the transmission electron microscope photograph are measured, and the average primary particle diameter of the inorganic fine powder is calculated by arithmetically averaging these values.

[0044] The mass proportion of the inorganic fine powder in the melt-kneaded product is preferably 5% by mass or more and 50% by mass or less, more preferably 10% by mass or more and 40% by mass or less. If the mass proportion of the inorganic fine powder is 5% by mass or more, the effect of kneading the inorganic fine powder can be fully exhibited, and if it is 40% by mass or less, stable spinning can be achieved. The total mass proportion of the polyolefin, the organic liquid, and the inorganic fine powder in the melt-kneaded product is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and may even be 100% by mass. It is preferable that the total mass proportion is in the above-mentioned numerical range, and that the mass proportions of the polyolefin, the organic liquid, and the inorganic fine powder are each in the above-mentioned numerical range.

[0045] An antioxidant may be used in the melt-kneaded product to inhibit oxidation of the polyolefin and other optional thermoplastic resins. The antioxidant is selected from Irganox 1010, Irganox 1035, Irganox 1076, Irganox 1098, Irganox 1135, Irganox 1141, Irganox 1330, Irganox 1425WL, Irganox 1520L, Irganox 245, Irganox 259, Irganox 3114, Irganox 565, Irgafos 168, Irganox PS800FL, Irganox PS802FL, Irgastab FS042, and dibutylhydroxytoluene. Among the above antioxidants, a hindered phenol-based antioxidant is preferred, and Irganox 1010 is more preferred.

[0046] A mixture of polyolefin, other optional thermoplastic resin, organic liquid, and inorganic fine powder can be obtained by mixing using a Henschel mixer, Banbury mixer, Plosser mixer, etc. When mixing the three components of polyolefin, other optional thermoplastic resin, organic liquid, and inorganic fine powder, the three components may be mixed simultaneously, or the components may be added in any order and mixed at each addition stage.

[0047] The polyolefin, other optional thermoplastic resins, and organic liquid may be directly fed separately to a melt-kneading extrusion device such as a twin-screw extruder without pre-kneading using a Henschel mixer, etc. To improve kneadability, the mixture may be melt-kneaded once after mixing to form pellets, and the pellets may be fed to a melt-kneading extrusion device, extruded into hollow fibers, and cooled and solidified to form hollow fibers.

[0048] The mixture can be melt-kneaded using a conventional melt-kneading means, for example, an extruder. While the use of an extruder will be described below, the melt-kneading means is not limited to an extruder. An example of a manufacturing apparatus used to carry out the manufacturing method of this embodiment is shown in FIGS. 2 and 3.

[0049] The porous hollow fiber membrane manufacturing apparatus shown in Figure 2 includes an extruder 10, a hollow fiber forming nozzle 20, a coagulation bath 30 in which a solution for coagulating the membrane forming solution is stored, and a plurality of rollers 50 for transporting and winding up the porous hollow fiber membrane 40. The space S shown in Figure 2 is a free-running section through which the membrane forming solution discharged from the hollow fiber forming nozzle 20 passes before reaching the solution in the coagulation bath 30. The free-running section S is the section from the discharge outlet of the hollow fiber forming nozzle 20 to the water surface in the coagulation bath 30. A container such as a cylinder may be used as the free-running section S from the discharge outlet, if necessary. After passing through the coagulation bath 30, the membrane is wound up into a hank or the like, if necessary.

[0050] The method for producing a porous hollow fiber membrane of this embodiment preferably further includes melt-kneading a polyolefin solution obtained by adding the organic liquid and the inorganic fine powder to the polyolefin, discharging the resulting melt-kneaded mixture, and cooling the melt-kneaded mixture only in an air layer after discharging. The polyolefin solution and the melt-kneaded mixture obtained by melt-kneading the polyolefin solution may have the same composition. The porous hollow fiber membrane production apparatus shown in Figure 3 is similar to that shown in Figure 2 and includes an extruder 10, a hollow fiber forming nozzle 20, and multiple rollers 50 for transporting and winding the porous hollow fiber membrane 40. Unlike that shown in Figure 2, the production apparatus shown in Figure 3 does not include a coagulation bath and is cooled in an air layer.

[0051] A hollow fiber forming nozzle 20 having one or more concentrically arranged annular discharge ports is attached to the tip of the extruder 10, and the molten mixture is extruded by the extruder 10 and discharged from the hollow fiber forming nozzle 20.

[0052] When the molten mixture is extruded from the annular discharge port, it is preferable to discharge the mixture so that the spinneret discharge parameter R (1 / sec) is 10 or more and 1000 or less, since this results in high productivity, stable spinning, and a membrane with high strength. Here, the spinneret discharge parameter R is the value obtained by dividing the discharge linear velocity V (m / sec) by the slit width d (m) of the discharge port. The discharge linear velocity V (m / sec) is the discharge volume (m) of the molten mixture per hour. ３ / sec) is multiplied by the cross-sectional area of ​​the outlet (m ２) is the value obtained by dividing the R by the original length. If R is 10 or more, there is no problem such as pulsation of the fiber diameter of the hollow extrudate, and stable spinning can be performed with good productivity. Furthermore, if R is 1000 or less, the breaking elongation, which is one of the important strengths of the obtained porous hollow fiber membrane, can be maintained at a sufficiently high level. The breaking elongation is the percentage of elongation relative to the original length when the membrane is pulled in the longitudinal direction.

[0053] The draft ratio when extruding the molten mixture from the annular discharge port is preferably 1.0 or more and 10 or less. The draft ratio is the ratio of the extrusion linear speed (m / sec) to the take-up speed (m / sec) after exiting the coagulation bath. When the draft ratio is 1.0 or more, tension is applied to the molten mixture, allowing for stable production of a hollow molten mixture. When the draft ratio is 10 or less, the aspect ratio of the pores in the resulting porous hollow fiber membrane can be appropriately adjusted.

[0054] The hollow fiber-shaped molten mixture discharged from the discharge port is solidified by passing through a refrigerant such as air or water. In the present method for producing a porous hollow fiber membrane, cooling in an air layer (air cooling) is preferred. Typically, in thermally induced phase separation, the mixture is passed through an air-flow section S, which is composed of an air layer, and then through a coagulation bath 30 containing water or the like. In contrast, the present production method uses air cooling to increase the pore size and porosity near one surface of the porous hollow fiber membrane. This is presumably because air cooling suppresses rapid solidification and allows time for pore growth. In particular, when using a high-molecular-weight thermoplastic resin, the high viscosity of the molten mixture results in extremely slow pore growth during phase separation, making it difficult to enlarge the pores near the surface and increase the porosity. The present production method makes it possible to provide a porous hollow fiber membrane with high porosity and appropriate pore size using a high-molecular-weight thermoplastic resin.

[0055] Furthermore, in the cooling in the air layer (air cooling), the air velocity of the air layer during cooling is preferably 2.0 m / sec or less. In the production method of this embodiment, air is used as the hollow section-forming fluid, but high-temperature air is passed through the hollow fiber forming nozzle. As a result, the inner surface is slowly cooled, solidification is delayed, and the pore size increases. Furthermore, when a polyolefin with an intrinsic viscosity of 4.5 dL / g or more is used, the viscosity of the molten mixture increases, and the pore size in the thickness direction of 0.5 to 1.5% of the membrane thickness from the outermost surface of the porous hollow fiber membrane does not grow and becomes small. Therefore, the ratio of the pore size in the thickness direction of 0.5 to 1.5% of the membrane thickness from the outermost surface of the inner surface to the pore size on the other surface increases. However, this pore size ratio can be reduced by setting the air velocity of the air layer to 2.0 m / sec or less. Methods for controlling the air velocity of the air layer include a local exhaust hood or placing a container in the air layer, but the production method of this embodiment is not particularly limited as long as the air velocity can be controlled.

[0056] When cooling using only an air layer, the material is passed through rollers or directly wound onto a skein, requiring improvements in viscosity and shape retention. Typically, increasing the polymer concentration or using a high-molecular-weight polymer increases viscosity; however, the former reduces the porosity that contributes to filtration, while the latter is prone to problems such as molding defects. Adding inorganic fine powder improves the viscosity and shape retention of the melt-kneaded material without the constraints of polymer molecular weight or concentration. Furthermore, the inorganic fine powder maintains the pore shape, allowing air cooling without pore deformation. Furthermore, when cooling using only an air layer, crystalline polymers such as polyethylene typically form a spherulite structure, making it difficult to form a three-dimensional network structure with good pore interconnectivity. In this case, adding inorganic fine powder inhibits the growth of the spherulite structure, making it possible to form a three-dimensional network structure even when cooling using an air layer.

[0057] The organic liquid and inorganic fine powder can be extracted and removed simultaneously if they can be extracted and removed using the same solvent. Usually, they are extracted and removed separately.

[0058] The organic liquid is extracted and removed using a liquid suitable for extraction that is miscible with the organic liquid without dissolving or denaturing the thermoplastic resin used. Specifically, this can be done by contacting the resin by a technique such as immersion. The liquid is preferably volatile so that it can be easily removed from the porous hollow fiber membrane after extraction. Examples of such liquids include alcohols and methylene chloride. If the organic liquid is water-soluble, water can also be used as the extraction liquid.

[0059] The removal of inorganic fine powder by extraction is usually carried out using an aqueous liquid. For example, when the inorganic fine powder is silica, the powder is first brought into contact with an alkaline solution to convert the silica into silicate, and then brought into contact with water to remove the silicate by extraction.

[0060] It does not matter whether the organic liquid or the inorganic fine powder is extracted and removed first. If the organic liquid is immiscible with water, it is preferable to extract and remove the organic liquid first, and then extract and remove the inorganic fine powder. Since the organic liquid and the inorganic fine powder are usually mixed and coexist in the organic liquid-rich phase, this is advantageous as it allows the inorganic fine powder to be extracted and removed smoothly.

[0061] In this way, a porous hollow fiber membrane can be obtained by extracting and removing the organic liquid and inorganic fine powder from the coagulated porous hollow fiber membrane. The coagulated porous hollow fiber membrane can be stretched longitudinally at a stretching ratio of 3 or less at any of the following stages: (i) before the extraction and removal of the organic liquid and inorganic fine powder; (ii) after the extraction and removal of the organic liquid and before the extraction and removal of the inorganic fine powder; (iii) after the extraction and removal of the inorganic fine powder and before the extraction and removal of the organic liquid; or (iv) after the extraction and removal of the organic liquid and inorganic fine powder. Generally, stretching a porous hollow fiber membrane longitudinally improves its water permeability, but reduces its pressure resistance (e.g., burst strength and compressive strength), and the resulting membrane often does not have sufficient strength for practical use after stretching. However, the porous hollow fiber membrane obtained by the manufacturing method of this embodiment has high mechanical strength. Therefore, stretching at a stretching ratio of 1.1 to 3.0 is possible. Stretching improves the water permeability of the porous hollow fiber membrane. The stretching ratio here refers to the value obtained by dividing the hollow fiber length after stretching by the hollow fiber length before stretching. For example, when a porous hollow fiber membrane with a hollow fiber length of 10 cm is stretched to a hollow fiber length of 20 cm, the stretching ratio is 2 times according to the following formula: 20 cm ÷ 10 cm = 2

[0062] The stretching is preferably carried out at a space temperature of 0° C. to 160° C. If the temperature is higher than 160° C., the stretching unevenness will be large and the elongation at break and water permeability will be reduced, which is not preferred, while if the temperature is lower than 0° C., the possibility of breakage during stretching is high and this is not practical. The space temperature during the stretching step is more preferably 10° C. to 140° C., and even more preferably 20° C. to 100° C.

[0063] In this embodiment, it is preferable to stretch a porous hollow fiber membrane containing an organic liquid. A porous hollow fiber membrane containing an organic liquid is less likely to break during stretching than a porous hollow fiber membrane not containing an organic liquid. Furthermore, a porous hollow fiber membrane containing an organic liquid can shrink more after stretching, which increases the flexibility in setting the shrinkage rate after stretching.

[0064] It is also preferable to stretch a porous hollow fiber membrane containing an inorganic fine powder. A porous hollow fiber membrane containing an inorganic fine powder is less likely to be flattened during stretching due to the hardness of the porous hollow fiber membrane caused by the presence of the inorganic fine powder contained therein. This also prevents the pore size of the finally obtained porous hollow fiber membrane from becoming too small or the fiber diameter from becoming too thin. In this embodiment, it is more preferable to stretch a porous hollow fiber membrane containing both an organic liquid and an inorganic fine powder.

[0065] For the reasons described above, it is preferable to stretch a porous hollow fiber membrane containing either an organic liquid or an inorganic fine powder rather than stretching the porous hollow fiber membrane after the extraction is completed, and furthermore, it is more preferable to stretch a porous hollow fiber membrane containing both an organic liquid and an inorganic fine powder rather than stretching a porous hollow fiber membrane containing either an organic liquid or an inorganic fine powder.

[0066] The method of extracting a stretched porous hollow fiber membrane has the advantage that the extraction solvent can easily penetrate into the porous hollow fiber membrane because the stretching increases the voids on the surface and inside of the porous hollow fiber membrane. The method of performing extraction after the stretching and subsequent shrinking step has the advantage that, since the porous hollow fiber membrane has a low tensile modulus and is easily bent, when extraction is performed in a liquid flow, the porous hollow fiber membrane is likely to sway due to the liquid flow, which enhances the stirring effect and enables efficient extraction in a short time.

[0067] In this embodiment, when a process for stretching and then shrinking a porous hollow fiber membrane is included, a porous hollow fiber membrane with a low tensile modulus can be finally obtained. Here, "low tensile modulus" means that the fibers easily stretch with a small force and return to their original shape when the force is removed. A low tensile modulus means that the porous hollow fiber membrane does not collapse flat, but is easily bent and easily shaken by the water flow during filtration. The uneven bending of the fibers due to the water flow causes the layer of contaminants adhering to and accumulating on the membrane surface to easily peel off without growing, allowing a high volume of filtered water to be maintained. Furthermore, when the fibers are forcibly shaken by flushing or air scrubbing, the shaking is large, enhancing the cleaning recovery effect.

[0068] Regarding the degree of shrinkage of the yarn length when shrinking after drawing, it is desirable that the yarn length shrinkage ratio relative to the increase in yarn length due to drawing be in the range of 0.3 to 0.9. For example, when a 10 cm yarn is drawn to 20 cm and then drawn to 14 cm, the yarn length shrinkage ratio is 0.6 according to the following formula: Yarn length shrinkage ratio = {(maximum yarn length at drawing) - (yarn length after shrinkage)} / [(maximum yarn length at drawing) - (original yarn length)] = (20 - 14) / (20 - 10) = 0.6. A yarn length shrinkage ratio of 0.9 or more is likely to result in low water permeability, while a yarn length shrinkage ratio of less than 0.3 is likely to result in a high tensile modulus, which is undesirable. In this embodiment, a yarn length shrinkage ratio of 0.50 to 0.85 is more preferable.

[0069] Furthermore, by employing a process of stretching the porous hollow fiber membrane to the maximum fiber length at the time of stretching and then shrinking it, the finally obtained porous hollow fiber membrane will not break even when stretched to the maximum fiber length at the time of stretching during use. Here, when the stretching ratio is X and the fiber length shrinkage rate relative to the increment in fiber length due to stretching is Y, the ratio Z, which represents the degree of guaranteed breaking elongation, can be defined by the following formula: Z = (maximum fiber length at stretching - fiber length after shrinkage) / fiber length after shrinkage = (XY - Y) / (X + Y - XY). Z is preferably 0.2 or more and 1.5 or less, more preferably 0.3 or more and 1.0 or less. If Z is too small, the guaranteed breaking elongation is reduced, and if Z is too large, the possibility of breakage during stretching increases but the water permeability decreases.

[0070] Furthermore, in the manufacturing method of this embodiment, when the process includes a step of stretching and then shrinking, the tensile breaking elongation is such that breakage at low elongation is extremely rare, and the distribution of tensile breaking elongation can be narrowed.

[0071] The space temperature in the stretching and subsequent shrinking steps is preferably in the range of 0° C. to 160° C. in terms of shrinkage time and physical properties. If it is lower than 0° C., shrinkage takes too long and is not practical, while if it exceeds 160° C., it may exceed the melting point of the thermoplastic resin, resulting in molding defects, a decrease in breaking elongation, and a decrease in water permeability, which is not preferred.

[0072] In this embodiment, it is preferable to crimp the porous hollow fiber membrane during the shrinking step, which allows a highly crimped porous hollow fiber membrane to be obtained without being crushed or damaged.

[0073] Generally, porous hollow fiber membranes have a straight, unbent tubular shape. Therefore, when they are bundled together to form a filtration module, there is a high possibility that the gaps between the hollow fibers will be small, resulting in a fiber bundle with low porosity. In contrast, when porous hollow fiber membranes with a high degree of crimping are used, the individual fibers are bent, resulting in an averagely wider spacing between the porous hollow fiber membranes, resulting in a fiber bundle with high porosity. Furthermore, filtration modules made of porous hollow fiber membranes with a low degree of crimping have fewer porosity in the fiber bundle, particularly when used under external pressure, which increases flow resistance and prevents effective transmission of filtration pressure to the center of the fiber bundle. Furthermore, the cleaning effect of the interior of the fiber bundle is reduced when removing filtration deposits from the porous hollow fiber membranes by backwashing or flushing. Fiber bundles made of porous hollow fiber membranes with a high degree of crimping have a large porosity, maintaining the gaps between the porous hollow fiber membranes even during external pressure filtration, making them less susceptible to drift.

[0074] The porous hollow fiber membrane obtained by the production method of this embodiment preferably has a crimp degree in the range of 1.5 to 2.5. A crimp degree of 1.5 or more is preferred for the reasons described above, and a crimp degree of 2.5 or less can suppress a decrease in the filtration area per volume.

[0075] Examples of methods for crimping a porous hollow fiber membrane include a method in which, during the process of stretching and then shrinking, the porous hollow fiber membrane is taken up while being sandwiched between a pair of gear rolls having periodically irregularities or a pair of sponge belts having irregularities.

[0076] In the production method of this embodiment, drawing is preferably performed using a take-up machine consisting of a pair of opposing caterpillar belts. In this case, take-up machines are used on the upstream and downstream sides of drawing, and in each take-up machine, the porous hollow fiber membrane is sandwiched between the opposing belts, and the yarn is fed by moving both belts at the same speed in the same direction. In this case, drawing is preferably performed by setting the yarn feeding speed on the downstream side faster than that on the upstream side. By performing drawing in this manner, it becomes possible to draw the yarn without slipping due to the drawing tension during drawing and to prevent the yarn from being flattened.

[0077] Here, the endless track belt preferably has an inner surface that contacts the drive roll made of a highly elastic belt such as a fiber-reinforced belt, and an outer surface that contacts the porous hollow fiber membrane made of an elastic body. It is more preferable that the compressive modulus of elasticity in the thickness direction of the elastic body is 0.1 MPa or more and 2 MPa or less, and that the thickness of the elastic body is 2 mm or more and 20 mm or less. It is particularly preferable that the elastic body on the outer surface is made of silicone rubber from the viewpoint of chemical resistance and heat resistance.

[0078] If necessary, the stretched membrane may be heat-treated to increase its compressive strength. Heat treatment is preferably carried out at a temperature of 80°C or higher and 160°C or lower. A temperature of 160°C or lower can suppress a decrease in breaking elongation and water permeability, while a temperature of 100°C or higher can increase its compressive strength. Heat treatment is preferably carried out on the porous hollow fiber membrane after extraction, as this reduces changes in fiber diameter, porosity, pore size, and water permeability.

[0079] Hereinafter, the present embodiment will be described in more detail with reference to examples and comparative examples, but the present embodiment is not limited to these examples.

[0080] The measurement method used in this embodiment is as follows.

[0081] Unless otherwise specified, all of the following measurements were carried out at 25° C. Below, the evaluation methods will be explained, followed by a description of the manufacturing methods and evaluation results of the examples and comparative examples.

[0082] The composition and manufacturing conditions of the membrane, as well as various performance characteristics, are shown in Table 1.

[0083] (1) Intrinsic Viscosity The intrinsic viscosity of the polyethylene powders obtained in the Examples and Comparative Examples was measured in accordance with ISO 1628-3 (2010). The solvent used was 20 mL of decahydronaphthalene (to which 1 g / L of 2,6-di-t-butyl-4-methylphenol had been added) that had been degassed using a vacuum pump and purged with nitrogen. A Cannon-Fenske viscometer (Shibata Scientific Instruments Co., Ltd.: Product No. -100) was used as the viscosity tube. For example, when measuring a form other than a powder (e.g., a molded product), such as a porous hollow fiber membrane, it is preferable to pulverize the material into a powder form. Pulverization methods include a multi-stage pulverization method in which coarse pulverization is followed by fine pulverization, and a method in which all steps up to micronization are performed in a single stage, but the method is not limited thereto. If the particles obtained after pulverization do not reach the desired particle size even when using a fine pulverizer, they may be pulverized using an ultrafine pulverizer, which is capable of further pulverization. Specific examples of the grinding means include grinding means using a hammer mill, turbo mill, jet mill, pin mill, centrifugal mill, Rotoplex, Pulverizer, wet grinding, chopper mill, ultra rotor, etc., and room temperature or freeze-grinding methods can be used. Freeze-grinding may be performed by freezing with a liquid gas such as liquid nitrogen at a low temperature (for example, -50°C or lower, -100°C or lower, etc.). Grinding may also be performed in the absence of oxygen (for example, oxygen concentration of 5% or lower, 2% or lower, etc.). Grinding methods used in the freeze-grinding process include pin mills, hammer mills, jet mills, etc.

[0084] (2) Outer diameter, inner diameter, and membrane thickness The porous hollow fiber membrane was thinly sliced ​​with a razor or the like in a direction perpendicular to the longitudinal direction of the membrane within an area 15 cm apart in the longitudinal direction of the membrane, and the major diameter (referred to as the "inner major diameter") and minor diameter (referred to as the "inner minor diameter") on the inner diameter side of the cross section of the membrane, and the major diameter (referred to as the "external major diameter") and minor diameter (referred to as the "external minor diameter") on the outer diameter side of the cross section of the membrane were measured using a microscope, and the inner diameter and outer diameter were calculated using the following formulas (2) and (3), respectively. The difference between the calculated outer diameter and the inner diameter was divided by 2 to calculate the membrane thickness. 20 points were measured, and the average values ​​were used as the inner diameter (mm), outer diameter (mm), and membrane thickness (mm) under those conditions.

[0085] (3) Pore size, porosity, aspect ratio, and overall membrane pore structure within a range of 0.5% to 1.5% of the membrane thickness from the outermost surface. A porous hollow fiber membrane sample was immersed in 100% ethanol for 24 hours to wet it, and then immersed in water to replace the ethanol with water. The water replacement procedure was repeated five times to completely replace the ethanol with water. The hollow fiber membrane sample was then frozen in liquid nitrogen and fractured to prepare a membrane cross-section sample. Using a Hitachi SU8000 series electron microscope at an accelerating voltage of 3 kV, the vicinity of one surface of the hollow fiber membrane (in this example, the region near the outer surface of the hollow fiber membrane in a cross section perpendicular to the longitudinal direction of the porous hollow fiber membrane) was observed. Images were taken at a magnification that allowed the shapes of 20 or more pores to be confirmed, and in this example and comparative example, images were taken at 10,000x magnification. Images were also taken in the same manner near the inner surface of the hollow fiber membrane. Using the photographed image, as described in International Publication No. 2001 / 53213, a transparent sheet was placed on top of a copy of the image, and the holes were filled in black using a black pen or the like. The transparent sheet was then copied onto white paper to clearly distinguish the holes as black and the non-holes as white. Then, using commercially available image analysis software Winroof 2018 Ver. 4.23.1, binarization was performed using discriminant analysis. The porosity was determined by calculating the area occupied by the binarized image. The pore size was determined by calculating the circle-equivalent diameter for all holes within a thickness range of 0.5% to 1.5% of the film thickness from the outermost surface in the image, and adding up the hole areas of each hole in order from the largest diameter. The sum was determined as the diameter of the hole where the sum reached 50% of the total hole area of ​​each hole. The aspect ratio was determined by measuring the acicular ratio (= absolute maximum length / diagonal width) of all holes within a thickness direction range of 0.5% to 1.5% of the film thickness from the outermost surface in the image using image analysis software Winroof 2018 Ver. 4.23.1, and taking the average value as the aspect ratio. In the table, one surface refers to the outer surface, and the other surface refers to the inner surface.

[0086] (4) Compression resistance (MPa) One end of a sample of a porous hollow fiber membrane approximately 5 cm long was sealed, and the other end was open to the atmosphere. Pure water at 40 ° C. was applied to the outer surface using the total filtration method, and permeated water was discharged from the end open to the atmosphere. The applied pressure was increased in 0.05 MPa increments from 0.1 MPa, and the pressure was maintained at each pressure for 15 seconds. The permeated water coming out of the end open to the atmosphere during this 15-second period was sampled. As the applied pressure increased, the absolute value of the amount of permeated water (weight) also increased until the hollow portion of the porous hollow fiber membrane was crushed. However, when the applied pressure exceeded the compressive strength of the porous hollow fiber membrane, the hollow portion collapsed and began to clog, so the absolute value of the amount of permeated water decreased despite the increase in applied pressure. The applied pressure at which the absolute value of the amount of permeated water was maximized was defined as the compression resistance.

[0087] (5) MDSC: A TA Instruments Discovery 2500 was used, and an Al Hermetic Pan and Al Hermetic Lid, also manufactured by TA Instruments, were used as sample pans. The membrane was cut to a length equal to the inner diameter (approximately 5 mm) of the Al Hermetic Pan and subjected to DSC measurement. The membrane weighed 1.5 to 2.0 mg. The cut membrane was placed on the inner diameter of the pan, and the lid was placed on top so that it was convex downward, and then clamped using a dedicated jig manufactured by TA Instruments. An empty sample pan was used as a reference, and modulated DSC measurement was performed at an average heating rate of 1.0 °C / min. The modulation period of the modulated DSC was 60 seconds, and the modulation amplitude was ±0.8 seconds. Three melting curves were obtained: the total heat flux component (= normal DSC measurement) (Total Heat Flow), the heat flux component that can reversibly follow temperature modulation (Reversing Heat Flow), and the irreversible component (Non-reversing Heat Flow) obtained by subtracting the Reversing Heat Flow from the Total Heat Flow. From the melting curves, the Total Heat Flow, Reversing Heat Flow, and Non-reversing Heat Flow are calculated as shown in Figures 5 to 8. Apparatus: DSC Discovery 2500 manufactured by TA Instruments Atmosphere: Nitrogen (flow rate 50 ml / min) Weight: Approximately 2 mg Length: Approximately 5 mm Heating rate: 1.0°C / min Measurement mode: Modulated Heat Only Modulation period: 60 s Modulation amplitude: ±0.8°C DSC pan: Hermitic Al pan + Hermitic Al lid

[0088] (6) Filtration Stability A porous hollow fiber membrane sample was immersed in 100% ethanol for 24 hours to wet it, and then immersed in water to replace the ethanol. A mini-module was fabricated using one porous hollow fiber membrane with an effective length of 20 cm. Pure water was deionized and filtered through a UF membrane with a molecular weight cutoff of 6000. A 100 mL aqueous solution of 0.01 wt% 0.10 μm polystyrene uniform latex (manufactured by Magsphere) was prepared as a model solution for the filtration test, and a cross-flow internal pressure filtration test was performed at a filtration pressure of 10 kPa and a module circulation linear velocity of 1.0 m / sec. Filtration was performed for 30 minutes, during which both the filtrate and the concentrated solution were returned to the original solution and circulated. The filtrate was collected 5, 10, 15, 20, 25, and 30 minutes after the start of filtration, and the original solution was also collected at the same time. The absorbance of the collected filtrate and the original solution was measured using a spectrophotometer (Hitachi High-Tech Science Corporation, Ratio Beam Spectrophotometer U-5100), and the rejection rate was calculated using the following formula: Rejection rate [%] = (1 - (absorbance of filtrate / absorbance of original solution)) x 100 If the difference between the minimum and maximum rejection rates at 5, 10, 15, 20, 25, and 30 minutes after the start of filtration was within 20%, the filtration was judged to be stable (◯), and if a difference of more than 20% occurred, the filtration was judged to be unstable (△).

[0089] Example 1: Polyethylene (UH650 manufactured by Asahi Kasei Corporation, intrinsic viscosity 6.6 dL / g) was used as the thermoplastic resin, bis-2-ethylhexyl adipate (DOA manufactured by New Japan Chemical Co., Ltd.) was used as the organic liquid, and finely divided silica (manufactured by Nippon Aerosil Co., Ltd., trade name: AEROSIL-R972) was used as the inorganic fine powder. A hollow fiber membrane was melt-extruded using an extruder with a hollow fiber nozzle. The molten mixture had a composition of polyethylene:bis-2-ethylhexyl adipate:finely divided silica = 19:54:27 (mass ratio). The molten mixture was extruded from a hollow fiber nozzle with an outer diameter of 2.0 mm and an inner diameter of 0.7 mm, using air as the hollow portion-forming fluid, at a discharge temperature of 220°C. Using the method shown in the schematic diagram of Figure 3, a hollow fiber-shaped melt-kneaded material extruded at a discharge temperature of 220°C was taken up at a speed of 5.0 m / min while air-cooling and wound into a hank to obtain a hollow fiber. The obtained hollow fiber was immersed in isopropyl alcohol to extract and remove bis-2-ethylhexyl adipate, and then dried. Next, it was immersed in a 50% by mass aqueous ethanol solution for 30 minutes, then in water for 30 minutes, and then in a 20% by mass aqueous sodium hydroxide solution at 70°C for 1 hour. Further water washing was repeated to extract and remove the finely divided silica, yielding a porous hollow fiber membrane. Table 1 shows the detailed composition and conditions.

[0090] (Example 2) A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that dibutyl sebacate (DBS, manufactured by Toyokuni Oil Mills Co., Ltd.) was used as the organic liquid and the composition of the molten mixture was polyethylene:dibutyl sebacate:finely powdered silica = 19:61:20 (mass ratio).

[0091] (Example 3) A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that polyethylene (UH910 manufactured by Asahi Kasei Corporation, intrinsic viscosity 14.9 dL / g) was used as the thermoplastic resin and the composition of the molten mixture was polyethylene:bis(2-ethylhexyl) adipate:finely powdered silica = 19:61:20 (mass ratio).

[0092] (Example 4) A porous hollow fiber membrane was obtained in the same manner as in Example 2, except that Irganox 1010 (manufactured by BASF) was used as the antioxidant and the composition of the molten mixture was polyethylene: dibutyl sebacate: finely divided silica: Irganox 1010 = 19:61:19.8:0.2 (mass ratio).

[0093] (Example 5) A hollow fiber-shaped molten kneaded material extruded at a discharge temperature of 220°C by the method shown in the schematic diagram of Figure 2 was allowed to travel through the air for 4.0 seconds, then introduced into a coagulation bath at 30°C, taken up at a speed of 5.0 m / min, and wound into a hank to obtain a hollow fiber-shaped material. A porous hollow fiber membrane was obtained in the same manner as in Example 1 except for the above.

[0094] Example 6 A hollow fiber-shaped molten kneaded material extruded at a discharge temperature of 220°C was allowed to travel through the air for 4.0 seconds by the method shown in the schematic diagram of Figure 2, and then introduced into a coagulation bath at 30°C, where it was taken up at a speed of 5.0 m / min and wound into a hank to obtain a hollow fiber-shaped material. A porous hollow fiber membrane was obtained in the same manner as in Example 2 except for the above.

[0095] Example 7 A hollow fiber-shaped molten kneaded material extruded at a discharge temperature of 220°C by the method shown in the schematic diagram of Figure 2 was allowed to travel through the air for 4.0 seconds, then introduced into a coagulation bath at 30°C, taken up at a speed of 5.0 m / min, and wound into a hank to obtain a hollow fiber-shaped material. A porous hollow fiber membrane was obtained in the same manner as in Example 3 except for the above.

[0096] Example 8 A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that dibutyl sebacate was used as the organic liquid and the air velocity during air cooling was set to 2.3 m / sec.

[0097] Example 9 A porous hollow fiber membrane was obtained in the same manner as in Example 8, except that the air velocity during air cooling was set to 2.0 m / sec.

[0098] Example 10 A porous hollow fiber membrane was obtained in the same manner as in Example 8, except that the air velocity during air cooling was set to 1.3 m / sec.

[0099] Example 11 A porous hollow fiber membrane was obtained in the same manner as in Example 8, except that the air velocity during air cooling was set to 0.3 m / sec.

[0100] Comparative Example 1 A porous hollow fiber membrane was obtained in the same manner as in Example 5, except that polyethylene (SH800 manufactured by Asahi Kasei Corporation, intrinsic viscosity 3.2 dL / g) was used as the thermoplastic resin and the composition of the melt-kneaded product was polyethylene:dibutyl sebacate:finely powdered silica = 19:54:27 (mass ratio). The intrinsic viscosity of the porous hollow fiber membrane of Comparative Example 1 was less than 4.5 dL / g.

[0101] Comparative Example 2 A porous hollow fiber membrane was obtained in the same manner as in Comparative Example 1, except that the hollow fiber-shaped molten kneaded material was cooled only by air cooling. The intrinsic viscosity of the porous hollow fiber membrane of Comparative Example 2 was less than 4.5 dL / g.

[0102] Comparative Example 3 An experiment was carried out in the same manner as in Example 1, except that no finely powdered silica was used and the composition of the molten mixture was polyethylene UH650:dibutyl sebacate=20:80 (mass ratio). However, the hollow fiber-shaped molten mixture was broken intermittently, and no sample could be obtained.

[0103] (Example 12) A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that finely powdered silica was not used and the composition of the melt-kneaded product was polyethylene UH910:dibutyl sebacate = 20:80 (mass ratio). When the obtained porous hollow fiber membrane was observed, it was found that the pore structure was non-uniform, such as a spherulite structure, making it impossible to analyze the pore size, etc.

[0104] Example 13 A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that dibutyl phthalate (manufactured by C.G. Ester Co., Ltd.) was used as the organic liquid.

[0105] Comparative Example 4 A porous hollow fiber membrane was obtained in the same manner as in Example 14, except that polyethylene (SH800 manufactured by Asahi Kasei Corporation, intrinsic viscosity 3.2 dL / g) was used as the thermoplastic resin.

[0106] Example 14 A porous hollow fiber membrane was obtained in the same manner as in Example 5, except that dibutyl phthalate (manufactured by C.G. Ester Co., Ltd.) was used as the organic liquid.

[0107]

[0108] The porous hollow fiber membrane of the present invention has large pore diameters near the surface, a high porosity, and high compressive strength, and therefore can be used for membrane filtration such as water and sewage treatment.

[0109] a Thermoplastic resin b Void portion 10 Extruder 20 Hollow fiber forming nozzle 30 Coagulation bath 40 Porous hollow fiber membrane 50 Roller S Idle running portion

Claims

1. A porous hollow fiber membrane comprising a polyolefin, wherein the intrinsic viscosity of the polyolefin or the porous hollow fiber membrane is 4.5 dL / g or more.

2. The porous hollow fiber membrane according to claim 1, wherein the intrinsic viscosity of the polyolefin or the porous hollow fiber membrane is 6.6 dL / g or more and 18 dL / g or less.

3. The porous hollow fiber membrane according to claim 1 or 2, having a non-reversing heat flow ratio of 35% or more.

4. The porous hollow fiber membrane according to claim 1 or 2, having a non-reversing heat flow ratio of 60% or more.

5. The porous hollow fiber membrane according to claim 1 or 2, wherein the non-reversing heat flow ratio is 60% or more and 90% or less.

6. The porous hollow fiber membrane according to claim 1 or 2, having a non-reversing heat flow endothermic enthalpy of 60 J / g or more.

7. The porous hollow fiber membrane according to claim 1 or 2, having a non-reversing heat flow endothermic enthalpy of 80 J / g or more and 105 J / g or less.

8. A porous hollow fiber membrane as described in claim 1 or 2, in which the pore size is 0.15 μm or more in a thickness direction range of 0.5% to 1.5% of the membrane thickness from the outermost surface of one of the inner and outer surfaces.

9. A porous hollow fiber membrane as described in claim 1 or 2, in which the porosity is 25% or more in a thickness direction range of 0.5% to 1.5% of the membrane thickness from the outermost surface of one of the inner and outer surfaces.

10. A porous hollow fiber membrane as described in claim 1 or 2, in which the aspect ratio of the pores is 2.0 or less in a thickness direction range of 0.5% to 1.5% of the membrane thickness from the outermost surface of one of the inner and outer surfaces.

11. A porous hollow fiber membrane as described in claim 1 or 2, in which the pore size is 0.23 μm or more in a thickness direction range of 0.5% to 1.5% of the membrane thickness from the outermost surface of one of the inner and outer surfaces.

12. A porous hollow fiber membrane according to claim 1 or 2, wherein the ratio of the pore size in a thickness direction range of 0.5% to 1.5% of the film thickness from the outermost surface of one of the inner and outer surfaces to the pore size of the one surface (pore size of one surface / pore size in a thickness direction range of 0.5% to 1.5% of the film thickness from the outermost surface of one surface) is 1.29 or less.

13. A porous hollow fiber membrane according to claim 1 or 2, having a compressive strength of 0.4 MPa or more.

14. The porous hollow fiber membrane according to claim 1 or 2, having a three-dimensional mesh structure.

15. The porous hollow fiber membrane according to claim 1 or 2, wherein the polyolefin is polyethylene.

16. A method for producing a porous hollow fiber membrane, comprising adding an organic liquid and an inorganic fine powder to a polyolefin having an intrinsic viscosity of 4.5 dL / g or more.

17. A method for producing a porous hollow fiber membrane according to claim 16, wherein the polyolefin has an intrinsic viscosity of 6.6 dL / g or more and 18 dL / g or less.

18. A method for producing a porous hollow fiber membrane according to claim 16 or 17, wherein the inorganic fine powder is silica.

19. The Hansen solubility parameter distance of the organic liquid to the polyolefin is 8.5 MPa. ０．５ The method for producing a porous hollow fiber membrane according to claim 16 or 17, wherein:

20. A method for producing a porous hollow fiber membrane as described in claim 16 or 17, wherein the organic liquid contains at least one selected from the group consisting of phthalate esters, sebacate esters, citrate esters, acetyl citrate esters, adipate esters, trimellitate esters, oleate esters, palmitate esters, stearic acid esters, phosphate esters, fatty acids having 6 to 30 carbon atoms, and epoxidized vegetable oils.

21. A method for producing a porous hollow fiber membrane according to claim 16 or 17, further comprising: melt-kneading a polyolefin solution obtained by adding the organic liquid and the inorganic fine powder to the polyolefin; discharging the resulting molten and kneaded mixture; and cooling the molten and kneaded mixture only in an air layer after discharging it.

22. A method for producing a porous hollow fiber membrane as described in claim 21, wherein the wind speed of the air layer is 2.0 m / sec or less.

23. A method for producing a porous hollow fiber membrane according to claim 16 or 17, further comprising adding a hindered phenol-based antioxidant to the polyolefin as an antioxidant.

24. A method for producing a porous hollow fiber membrane according to claim 16 or 17, wherein the polyolefin is polyethylene.

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