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

Abstract

The present invention makes it possible to provide a porous hollow fiber membrane having high filtration performance and durability against high temperatures. The present invention provides a porous hollow fiber membrane containing a polyolefin, the porous hollow fiber membrane characterized by having a non-reversing heat flow enthalpy of 30 J / g or less and including 1 or less voids.

Description

Porous hollow fiber membrane and method for manufacturing a porous hollow fiber membrane

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

[0002] Solid-liquid separation (turbidity removal) is necessary to separate and remove suspended solids in various processes, including: water treatment to obtain drinking water and industrial water from natural water sources such as seawater, river water, lake water, and groundwater; wastewater treatment to produce recycled water suitable for discharge; and removal of yeast, high-molecular-weight substances, proteins, polyphenols, tannins, and other sediment components from brewed alcoholic beverages produced by fermenting sugar-containing liquids with yeast. In the production of saccharified liquid from starch liquids, insoluble components that were not broken down by enzymes are removed. In such turbidity removal operations, for water treatment, turbid substances (clay, colloids, bacteria, etc.) originating from natural water sources, which are suspended water, are removed, while for wastewater treatment, suspended solids (sludge, etc.) in treated water that has undergone biological treatment (secondary treatment) with activated sludge, etc., are removed. Furthermore, while diatomaceous earth filtration has traditionally been used to remove turbidity from fermented alcoholic beverages such as wine and saccharified liquids, membrane filtration has recently become more widespread as an alternative to these methods.

[0003] Traditionally, these turbidity removal operations have been carried out mainly by methods such as pressurized flotation, sedimentation, sand filtration, diatomaceous earth filtration, or coagulation-sedimentation-sand filtration, but in recent years membrane filtration has become increasingly popular. The advantages of membrane filtration include the following: (1) The level of turbidity removal in the resulting water is high and stable (i.e., the safety of the resulting water is high). (2) The installation space for the filtration equipment 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, for example, installed after coagulation-sedimentation-sand filtration to further improve the water quality of treated water filtered by coagulation-sedimentation-sand filtration. In wastewater treatment, the use of membrane filtration is also being considered for separating sludge from secondary treated wastewater.

[0005] These membrane filtration operations primarily utilize hollow fiber ultrafiltration membranes or microfiltration membranes (with pore sizes ranging from a few nanometers to several hundred nanometers). There are two filtration methods using hollow fiber filtration membranes: an internal pressure filtration method that filters from the inner surface to the outer surface of the membrane, and an external pressure filtration method that filters from the outer surface to the inner surface. Patent documents 1 and 2 disclose hollow fibers and methods for manufacturing the same.

[0006] When producing cell cultures, it is often necessary to remove waste products from the culture. Currently, advances in biological manufacturing processes have enabled large-scale production of cell cultures, often using processing vessel devices to produce recombinant proteins, virus-like particles (VLPs), gene therapy particles, and vaccines. Cell storage devices that remove metabolic waste and replenish the culture with nutrients are widely available. Generally, this storage is carried out by perfusion filtration of the processing vessel culture using hollow fiber membranes, employing tangential flow filtration. In addition, bioprocess operations are long-term, and continuous culture is utilized. Such operations can last for days, weeks, or even months. It is desirable that many typical components, such as filters, function adequately for such periods without fouling, or at least without the need for maintenance or replacement.

[0007] Hollow fiber membranes are often used for perfusion of cell cultures and separation of products, but such use can lead to membrane fouling by cell debris. This fouling can then cause the desired product to be retained within the membrane rather than passing through it. Hollow fiber membranes are generally made from polyethersulfone (PES), polysulfone, cellulose, polyvinylidene fluoride, polypropylene, polyethylene, and other materials, and there is a continuous demand for membranes that are fouling resistant and capable of filtering solutions with high solids content. Fluoropolymers such as polyethylene (PE) and polyvinylidene fluoride (PVDF) are hydrophobic materials that have been commonly used in industrial filtration, and are known to not adsorb certain molecules very well. Specifically, polyphenols, polysaccharides, and tannins are adsorbed at lower levels. On the other hand, the membrane structure varies greatly depending on the manufacturing method, and even with the same material, the fouling phenomenon on the membrane differs significantly depending on the manufacturing method. Furthermore, in these applications, hollow fiber membranes may be washed before use and between manufacturing batches for repeated use. In the above cleaning process, hot water (for example, 80°C or higher) or steam is used to sterilize the inside of the hollow fiber membrane module, thereby subjecting the hollow fiber membrane to high temperatures.

[0008] Incidentally, thermally induced phase separation is a known method for producing porous films. This method uses a thermoplastic resin and an organic liquid. The organic liquid used 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 is a method in which the thermoplastic resin and the organic liquid are kneaded at high temperature to dissolve the thermoplastic resin in the organic liquid, and then phase separation is induced by cooling to room temperature, and the organic liquid is further removed to produce a porous material. This method has the following advantages: (a) It makes it possible to form films even with polymers such as polyethylene for which there is no suitable solvent that can dissolve at room temperature. (b) Since the film is formed by dissolving at high temperature and then cooling and solidifying, crystallization is promoted during film formation, especially when the thermoplastic resin is a crystalline resin, making it easier to obtain a high-strength film.

[0009] Due to the advantages mentioned above, this method is widely used as a method for manufacturing porous membranes (see, for example, Non-Patent Documents 1-4).

[0010] Japanese Patent Publication No. 60-023130 Patent No. 2835365

[0011] Editorial Committee of the Dictionary of Plastics and Functional Polymer Materials, "Dictionary of Plastics and Functional Polymer Materials," Sangyo Chosakai, February 2004, pp. 672-679. Hideto Matsuyama, "Fabrication of Polymer-Based Porous Membranes by Thermally Induced Phase Separation Method (TIPS Method)," Chemical Engineering Journal, Kagaku Kogyosha, June 1998 issue, pp. 45-56. Akira Takizawa, "Membranes," IPC Co., Ltd., January 1992, pp. 404-406. D. R. Lloyd, et al., "Journal of Membrane Science," 64, 1991, pp. 1-11.

[0012] High filtration performance and high temperature resistance are required for porous hollow fiber membranes. As described in Patent Documents 1 and 2, it was known that porous hollow fiber membranes could be manufactured using polyethylene. However, Patent Documents 1 and 2 did not contain any information on a technology that achieves both high filtration performance through a homogeneous structure and high temperature resistance.

[0013] The present invention aims to provide a porous hollow fiber membrane that has high filtration performance and durability against high temperatures.

[0014] In other words, the present invention is as follows: [1] A porous hollow fiber membrane containing a polyolefin, characterized in that the non-reversing heat flow exothermic enthalpy is 30 J / g or less and the number of voids is 1 or less. [2] The porous hollow fiber membrane according to [1], characterized in that the reversing heat flow endothermic enthalpy is 112 J / g or less. [3] The porous hollow fiber membrane according to [1] or [2], characterized in that the total heat flow endothermic enthalpy is 200.5 J / g or more. [4] The porous hollow fiber membrane according to any one of [1] to [3], characterized in that the intrinsic viscosity of the polyolefin or the porous hollow fiber membrane is 5.0 dL / g or less. [5] A porous hollow fiber membrane according to any one of [1] to [4], characterized in that the non-reversing heat flow exothermic enthalpy is 15 J / g or less. [6] A porous hollow fiber membrane according to any one of [1] to [5], wherein the polyolefin is polyethylene and has a three-dimensional network structure. [7] A method for producing a porous hollow fiber membrane, comprising the steps of melt-kneading a mixture containing a thermoplastic resin and an organic liquid, or a mixture containing particles of a thermoplastic resin, an organic liquid, and an inorganic fine powder to extrude a hollow fiber-like melt-kneaded molded body, and then extracting and removing the organic liquid or the organic liquid and the inorganic fine powder from the hollow fiber-like melt-kneaded molded body to produce a porous hollow fiber membrane, wherein the distance of the Hansen solubility parameter of the porous hollow fiber membrane is 0.5 at 6.0 MPa or less. [8] The method for producing a porous hollow fiber membrane according to [7], wherein the organic liquid body comprises at least one selected from the group consisting of phthalates, sebacates, citric acids, acetyl citrates, adipic acids, trimellitic acids, oleates, palmitates, stearates, phosphates, fatty acids having 6 to 30 carbon atoms, and epoxidized vegetable oils.[9] A method for producing a porous hollow fiber membrane according to [7] or [8], further comprising: melting and 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 with only an air layer after discharging it.

[10] A method for producing a porous hollow fiber membrane according to any one of [7] to [9], characterized in that the inorganic fine powder contains silica.

[0015] The present invention makes it possible to provide a porous hollow fiber membrane that has high filtration performance and durability against high temperatures.

[0016] This is a schematic diagram of a three-dimensional network structure. This is a schematic diagram of an example of an apparatus for manufacturing porous hollow fiber membranes. This is a schematic diagram of an example of an apparatus for manufacturing porous hollow fiber membranes. This is MDSC analysis data of the porous hollow fiber membrane manufactured in Example 3. This is MDSC analysis data of the porous hollow fiber membrane manufactured in Comparative Example 1. This is an example of a void in Comparative Example 2.

[0017] Embodiments of the present invention will be described in detail below. However, the present invention is not limited to the following embodiments.

[0018] (Porous Hollow Fiber Membrane) The porous hollow fiber membrane of this embodiment and the porous hollow fiber membrane obtained by the manufacturing method of this embodiment will be described below.

[0019] The porous hollow fiber membrane of this embodiment and the porous hollow fiber membrane obtained by the manufacturing method of this embodiment preferably contain a polyolefin, and the intrinsic viscosity of the polyolefin or the porous hollow fiber membrane is preferably 5.0 dL / g or less. Preferably, it is more preferable to contain a polyolefin as the main component in a viscosity of 2.0 dL / g or more, more preferably 2.5 dL / g or more, preferably 5.0 dL / g or less, and more preferably 4.0 dL / g or less. Preferably, it is more preferable to contain a polyolefin as the main component in a viscosity of 2.0 dL / g or more and 5.0 dL / g or less, and even more preferably 2.5 dL / g or more and 4.0 dL / g or less. In particular from the viewpoint of strength, it is preferable that the intrinsic viscosity is within the above range. It may consist only of polyolefin and other thermoplastic resins, or it may also contain other components.

[0020] In this application, the intrinsic viscosity is a value measured in accordance with ISO 1628-3 (2010), and is measured at 25°C using decahydronaphthalene (which may contain a predetermined amount, for example, 1 g / L of an antioxidant such as 2,6-di-t-butyl-4-methylphenol) as the solvent. More specifically, the intrinsic viscosity can be measured by the method described in the examples.

[0021] For example, when measuring materials in a form different from powder (e.g., molded bodies), such as porous hollow fiber membranes, it is preferable to grind them into a powder. Grinding methods include multi-stage grinding methods that involve coarse grinding followed by fine grinding, and single-stage methods that achieve fineness in one stage, but the method is not limited to these. If the particles after grinding do not reach the desired particle size even with a fine grinder, further grinding may be performed using an ultrafine grinder capable of further fine grinding. Specific grinding methods include using hammer mills, turbo mills, jet mills, pin mills, centrifugal mills, Rotoplex, Pulverizers, wet grinding, chopper mills, and Ultrarotors, and room temperature or freeze grinding methods can be used. Freeze grinding may be performed by freezing with a low-temperature liquid gas such as liquid nitrogen (e.g., below -50°C, below -100°C, etc.). Grinding may also be performed in an oxygen-free environment (e.g., oxygen concentration of 5% or less, 2% or less, etc.). Grinding methods in freeze grinding treatment include pin mills, hammer mills, jet mills, etc.

[0022] In this application, polyolefin or polyolefin resin refers to a polymer containing olefin hydrocarbons as monomer components. Here, polyolefins also include copolymers of olefin hydrocarbons and monomers other than olefins, but 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 this application, "polyolefin resin" and "polyolefin" are synonymous. The polyolefin used in this application is preferably a crystalline polyolefin. Furthermore, there are no limitations on the type of polyolefin used in this application, and examples include homopolymers or copolymers using olefin hydrocarbons such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene as monomers. Specifically, examples include polyethylene; polypropylene; polybutene; ethylene-propylene random copolymer; ethylene-propylene rubber, etc. The above polyolefin is preferably polyethylene. The type of polyethylene is not particularly limited, but examples include low-density polyethylene, linear low-density polyethylene, medium-density polyethylene, high-density polyethylene, and ultra-high molecular weight polyethylene. Furthermore, two or more polyolefins with different structures and molecular weights may be mixed.

[0023] The porous hollow fiber membrane preferably contains a thermoplastic resin as its main component. The thermoplastic resin preferably contains a polyolefin resin as its main component, and preferably has an intrinsic viscosity of 5.0 dL / g or less. Preferably contains polyethylene as its main component in an amount of 2.0 dL / g or more, more preferably 2.5 dL / g or more, preferably 5.0 dL / g or less, and more preferably 4.0 dL / g or less. Preferably contains polyethylene as its main component in an amount of 2.0 dL / g or more and 5.0 dL / g or less, and even more preferably 2.5 dL / g or more and 4.0 dL / g or less. The resin component contained in the porous hollow fiber membrane preferably consists only of the polyolefin resin. Here, "contains as a main component" means containing 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) on a solid content basis of the thermoplastic resin or porous hollow fiber membrane.

[0024] The intrinsic viscosity of the above polyolefin or the above porous hollow fiber membrane is preferably 5.0 dL / g or less. Preferably, it contains polyethylene as the main component in a viscosity of 2.0 dL / g or more, more preferably 2.5 dL / g or more, preferably 5.0 dL / g or less, and more preferably 4.0 dL / g or less. Preferably, it contains polyethylene as the main component in a viscosity of 2.0 dL / g or more and 5.0 dL / g or less, and even more preferably 2.5 dL / g or more and 4.0 dL / g or less. Furthermore, it is not limited to a single molecular weight polyolefin and other thermoplastic resins as optional components, but a mixture of polyolefins with multiple molecular weights and other thermoplastic resins as optional components may be used. Only the intrinsic viscosity of the above polyolefin may be within the above range, only the intrinsic viscosity of the above porous hollow fiber membrane may be within the above range, or the intrinsic viscosity of the above polyolefin and the above porous hollow fiber membrane may be within the above range.

[0025] The viscosity-average molecular weight (Mv) of polyolefins and other thermoplastic resins is preferably 100,000 or more, more preferably 200,000 or more, preferably 4,000,000 or less, and more preferably between 200,000 and 4,000,000. Furthermore, it is not limited to thermoplastic resins with a single molecular weight, but a mixture of thermoplastic resins with multiple different molecular weights may be used.

[0026] The hollow fiber membrane described above as a porous hollow fiber membrane refers to a membrane having a hollow annular shape. Because the porous membrane has the membrane structure of a hollow fiber membrane, it is possible to increase the membrane area per module unit volume compared to a planar membrane. The porous hollow fiber membrane may have pores that communicate in the thickness direction within the membrane from one surface to the other.

[0027] The porous hollow fiber membrane described above preferably has a three-dimensional network structure. In this specification, a three-dimensional network structure schematically refers to a structure like that shown in Figure 1. For example, thermoplastic resins a, such as polyolefins, are joined together to form a network, and voids b are formed. In the three-dimensional network structure, there are almost no lumps of resin with a so-called spherulite structure. The voids b of the three-dimensional network structure are surrounded by thermoplastic resins a, such as polyolefins, and it is preferable that each part of the voids b is in communication with one another. Since most of the thermoplastic resins such as polyolefins used form a three-dimensional network structure that can contribute to the strength of the porous hollow fiber membrane, it becomes possible to form a support layer with high strength. Chemical resistance is also improved. The reason for the improved chemical resistance is not clear, but it is thought that because there is a large amount of thermoplastic resin that forms a network that can contribute to strength, even if a part of the network is affected by a chemical, the strength of the entire layer is not greatly affected.

[0028] In this embodiment, the porous hollow fiber membrane preferably has a non-reversing heat flow exothermic enthalpy of 30.0 J / g or less. The non-reversing heat flow exothermic enthalpy, reversing heat flow endothermic enthalpy, and total heat flow endothermic enthalpy can be measured by MDSC measurement. Here, temperature-modulated DSC (MDSC) measurement is a method that differs from normal constant-rate heating DSC measurement in that it performs DSC measurement while applying periodic heating and cooling to the average heating rate. Using the MDSC method, three melting curves can be obtained: the total heat flux component (approximately the normal DSC measurement), the heat flux component that can reversibly track 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 low non-reversing heat flow exothermic enthalpy indicates that irreversible crystallization is not observed during periodic heating and cooling; in other words, the porous hollow fiber film is in a stable crystalline state rather than an unstable state during crystallization. This is preferable because it exhibits strength and durability against high temperatures. The non-reversing heat flow exothermic enthalpy is preferably 15.0 J / g or less, and more preferably 10.5 J / g or less. The lower limit of the non-reversing heat flow exothermic enthalpy is not particularly limited, but may be, for example, 0.1 J / g or more.

[0029] In this embodiment, the porous hollow fiber membrane preferably has a reversing heat flow endothermic enthalpy of 112.0 J / g or less. A low reversing heat flow endothermic enthalpy indicates that there are few components that reversibly melt, i.e., a stable crystalline state. Therefore, a low reversing heat flow endothermic enthalpy is preferable because it allows for the development of strength and provides durability against high temperatures. The reversing heat flow endothermic enthalpy is preferably 110.0 J / g or less. Because it may affect the melting point, the reversing heat flow endothermic enthalpy is preferably 70.0 J / g or more, and more preferably 80.0 J / g or more.

[0030] In this embodiment, the porous hollow fiber membrane preferably has a Total Heat Flow endothermic enthalpy of 200.0 J / g or higher. A higher endothermic enthalpy is preferable because it leads to a higher degree of crystallinity, resulting in greater strength and durability against high temperatures. The Total Heat Flow endothermic enthalpy is preferably 200.5 J / g or higher. A high endothermic enthalpy and high degree of crystallinity can lead to a high elastic modulus of the porous hollow fiber membrane, which may affect its flexibility, so a value of 250 J / g or less is preferable, and 230 J / g or higher is more preferable.

[0031] In this embodiment, it is preferable that the porous hollow fiber membrane has one or fewer voids per unit area (130 μm x 130 μm). Here, a void is a cavity defect having an area 50 times or more the average pore area in the observation area (e.g., an area in the membrane cross-section). If there are cavity defects (voids) with an area 50 times or more the average pore area, the localized film thickness resistance decreases, the fluid flow inside the membrane becomes non-uniform, and filtration becomes unstable. Conversely, if the number of voids per unit area (130 μm x 130 μm) is one or less, the structure is homogeneous and stable filtration is possible. The number of voids is preferably 0.8, more preferably 0.5, even more preferably 0.2, and most preferably 0, eliminating localized non-uniformity of fluid flow and enabling more stable filtration. The number of voids can be measured, for example, by the method described in the examples. The above phenomenon is represented by the pure water permeability relative to the bubble point pore diameter. When the pure water permeability relative to the bubble point pore diameter is 9000 LMH / μm or less, the reduction in membrane resistance due to voids, which is undesirable in practical use, is suppressed, meaning a homogeneous structure is achieved, which is preferable. However, since a certain level of filtration performance is necessary, a pure water permeability relative to the bubble point pore diameter of 2000 LMH / μm or more is preferable. Preferably, it is 3000 LMH / μm or more, more preferably 4000 LMH / μm or more, preferably 8000 LMH / μm or less, more preferably 7500 LMH / μm or less, preferably 3000 LMH / μm or more and 8000 LMH / μm or less, and more preferably 4000 LMH / μm or more and 7500 LMH / μm or less.

[0032] The porous hollow fiber membrane of this embodiment preferably has a pure water permeability of 500 LMH or more. If it is 5800 LMH or more, the filtration resistance is low and high filtration performance can be achieved. Preferably it is 700 LMH or more, and more preferably 800 LMH or more.

[0033] The compressive strength of the porous hollow fiber membrane in this embodiment is preferably 0.2 MPa or higher. A higher compressive strength results in a porous hollow fiber membrane with high strength and a high burst strength. A compressive strength of 0.2 MPa or higher provides sufficient durability against the pressure applied during filtration, enabling stable filtration operation over a long period. A compressive strength of 0.25 MPa or higher is even more preferable.

[0034] (Method for manufacturing porous hollow fiber membranes) The method for manufacturing porous hollow fiber membranes according to this embodiment may include, for example, a step of adding an organic liquid and an inorganic fine powder to a polyolefin having an intrinsic viscosity of 5.0 dL / g or more. The method for manufacturing porous hollow fiber membranes according to this embodiment is preferably the method for manufacturing porous hollow fiber membranes according to this embodiment as described above. The method for manufacturing porous hollow fiber membranes according to this embodiment preferably includes a step of extruding a hollow fiber-shaped melt-kneaded molded body by melt-kneading a mixture containing a thermoplastic resin (e.g., polyolefins with an intrinsic viscosity in a predetermined range and optionally other thermoplastic resins) and an organic liquid, or a mixture containing particles of thermoplastic resin, an organic liquid, and an inorganic fine powder, and then extruding the molded body from, for example, a spinneret having an annular discharge port, and thereafter (for example, after the hollow fiber-shaped melt-kneaded material has solidified), a step of extracting and removing the organic liquid or the organic liquid and inorganic fine powder from the hollow fiber-shaped melt-kneaded molded body to produce a porous hollow fiber membrane. The molten compound may consist of two components: a thermoplastic resin and an organic liquid, or it may consist of three components: a thermoplastic resin, inorganic fine powder, and an organic liquid. In this specification, "organic liquid" and "organic liquid body" are synonymous. Similarly, "inorganic fine powder" and "inorganic fine powder body" are synonymous.

[0035] The thermoplastic resin used in the method for producing a porous hollow fiber membrane of the present embodiment is not particularly limited, and for example, may be one that contains polyolefin or consists of polyolefin. When the thermoplastic resin contains polyolefin or consists of polyolefin, preferred examples of the thermoplastic resin are as described above or in other descriptions. The thermoplastic resin (e.g., polyolefin and other optional thermoplastic resin components) used in the method for producing a porous hollow fiber membrane of the present embodiment is elastic and does not exhibit plasticity at normal temperature, but becomes plastic and moldable upon appropriate heating. In addition, the thermoplastic resin (e.g., polyolefin and other optional thermoplastic resin components) returns to its original elastic state after cooling and temperature reduction, and does not undergo chemical changes such as changes in molecular structure during this process (see, for example, "Edited by Chemical Dictionary Editorial Committee, Enlarged Reprint Edition of Kogyo Daijiten Volume 6, Kyoritsu Shuppan, pages 860 and 867, 1963").

[0036] Examples of thermoplastic resins other than polyolefin include resins described in the "Thermoplastics" section (pages 829 to 882) of "Chemical Products of 12695" (Kagaku Kogyo Nipposha, 1995), and resins described on pages 809 to 810 of "Kagaku Binran Oyo-hen Revised 3rd Edition" (edited by The Chemical Society of Japan, Maruzen, 1980). Specific examples of thermoplastic resins other than polyolefin include fluororesins such as 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 thermoplastic resins such as fluororesins (e.g., polyvinylidene fluoride), ethylene-vinyl alcohol copolymer, and polyvinyl alcohol can be suitably used from the viewpoint of strength development. More preferably, fluororesins such as polyvinylidene fluoride, which are hydrophobic and thus have high water resistance and can be expected to have durability in filtration of ordinary aqueous liquids, can be used.

[0037] The thermoplastic resin used in melt mixing (e.g., fed into the extruder 10) is a particle obtained by crushing and / or pulverizing pelletized or granular thermoplastic resin to adjust its particle size. By using a particle obtained by crushing and / or pulverizing pelletized or granular thermoplastic resin as the thermoplastic resin, it is possible to produce a porous hollow fiber membrane with high filtration performance and low variation in membrane performance. The D50 particle size of the thermoplastic resin fed into the extruder 10 may be 30 μm or more, 50 μm or more, 500 μm or less, or 50 to 500 μm.

[0038] Means for crushing and / or pulverizing pelletized and granular thermoplastic resins include multi-stage pulverization methods that coarsely pulverize the pellets and then finely pulverize them, and methods that perform fine pulverization in a single stage, but the method is not limited to these. If the particles after pulverization do not reach a predetermined particle size even with a fine pulverizer, further pulverization may be performed using an ultrafine pulverizer capable of further fine pulverization. Specific pulverization methods include using hammer mills, turbo mills, jet mills, pin mills, centrifugal mills, Rotoplex, Pulverizers, wet milling, chopper mills, and ultrarotors, and room temperature or freeze pulverization methods can be used. For example, freeze pulverization is preferable for vinylidene fluoride resins, which have a low glass transition temperature of approximately -35°C. Freeze pulverization may be performed by freezing the pelletized or granular thermoplastic resin with a low-temperature liquid gas such as liquid nitrogen (e.g., below -50°C, below -100°C, etc.). Pulverization may also be performed in an oxygen-free environment (e.g., oxygen concentration of 5% or less, 2% or less, etc.). Examples of grinding methods used in freeze-milling include pin mills, hammer mills, and jet mills.

[0039] Classification is performed using an appropriate classifier to obtain particles within a specified particle size range. If particles within a specified particle size range are to be obtained from the classified particles, further classification may be performed using another classifier, and then fine powder below the specified particle size may be removed, with the remaining particles (medium powder) being used as the product. Particles larger than the target particle size range after classification can also be crushed again to obtain particles within the specified particle size range. The equipment used for classification is not particularly limited and includes vibrating screens, inertial airflow classifiers, and rotary vane classifiers.

[0040] In addition, when two types of polymers are mixed, each polymer may be pulverized and then mixed using a mixer before use. Classification may be performed after pulverization or after mixing, and is not particularly limited. It is preferable that all polymers to be charged into the mixer have a particle size dispersion degree V described below falling within the suitable range described below.

[0041] The particle size distribution of the pulverized particles can be measured using a laser diffraction or scattering type particle size distribution measuring apparatus.

[0042] The particles whose particle size is adjusted by crushing and / or pulverization preferably have a volume-based median diameter (D50 particle diameter) obtained from the particle size distribution in the range of 50 to 500 µm, more preferably 70 to 400 µm. When the diameter is 50 µm or more, stable feeding can be achieved without causing poor biting into a screw during feeding, for example, when melt-kneading is performed by an extruder or the like. When the diameter is 500 µm or less, porous hollow fiber membranes can be stably produced without occurrence of poor dissolution or the like. In the present specification, D10 particle diameter, D50 particle diameter, and D90 particle diameter refer to values measured by using a laser diffraction particle size measuring apparatus.

[0043] The total mass proportion of polyolefin and any optional other thermoplastic resin in the melt-kneaded product is preferably 10% by mass or more, more preferably 15% by mass or more, and preferably 50% by mass or less, more preferably 40% by mass or less, and 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. When the content is 10% by mass or more, mechanical strength is easily ensured, and when the content is 50% by mass or less, no reduction in water permeability occurs. Further, 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.

[0044] The organic liquid used in this embodiment is a potential solvent for the polyolefin and other optional thermoplastic resins. In this embodiment, a potential solvent is a solvent that hardly dissolves the polyolefin and other optional thermoplastic resins at room temperature (25°C), but can dissolve them at temperatures higher than room temperature. It is sufficient that the solvent is liquid at the melt-mixing temperature with the polyolefin and other optional thermoplastic resins, and it does not necessarily need to be liquid at room temperature.

[0045] 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; adipic esters such as dioctyl adipicate; trimellitic esters such as trioctyl trimelliticate; oleic acid esters; palmitic acid esters; stearic acid esters; phosphate esters such as tributyl phosphate and trioctyl phosphate; fatty acids with 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. In particular, it is preferable that the organic liquid body contains at least one selected from the group consisting of phthalates, sebacates, citrates, acetyl citrates, adipicates, trimelliticates, oleates, palmitates, stearates, phosphates, fatty acids having 6 to 30 carbon atoms, and epoxidized vegetable oils, and it is more preferable that it contains only at least one selected from the group consisting of phthalates, sebacates, citrates, acetyl citrates, adipicates, trimelliticates, oleates, palmitates, stearates, phosphates, fatty acids having 6 to 30 carbon atoms, and epoxidized vegetable oils. The above organic liquid body may be one type, or two or more types of organic liquid bodies may be used.

[0046] The above organic liquid has a Hansen solubility parameter distance of 6.0 MPa relative to the above polyolefin. ０．５ The following is preferable: The distance of the Hansen solubility parameter to the polyolefin is 6.0 MPa. ０．５ The following conditions allow for high solubility and suppress the formation of voids due to undissolved or non-uniform phase separation. Furthermore, the entanglement of polymer chains becomes stronger, resulting in high compressive or bursting strength. Preferably, 5.9 MPa. ０．５ The following, and more preferably 5.8 MPa ０．５ The following is the reason, although it is not entirely clear, sebacate esters, adipic esters, and citric acid are particularly preferred as they can exhibit high strength.

[0047] The distance (Ra) between the Hansen solubility parameters of polyethylene and an organic liquid is calculated using the following formula. The following formula represents the relationship between the Hansen (three-dimensional) solubility parameters of polyethylene and the Hansen (three-dimensional) solubility parameters of the organic liquid, and is used to evaluate the solubility of polyethylene and the organic liquid. The right-hand side of the following formula represents the three-dimensional solubility range of the Hansen solubility parameters and quantitatively expresses 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 × (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 bonding force terms of the organic liquid or solvent and polyethylene, respectively; and σhm and σhp represent the hydrogen bonding terms of the organic liquid or solvent and polyethylene, respectively.] When organic liquids were mixed, the solubility parameters were calculated as a mixed organic liquid by performing ratio calculations according to the mass ratio.

[0048] Generally, when producing porous hollow fiber membranes with polyolefins and organic liquids where the distance between the Hansen solubility parameters is outside the above range, it is not possible to obtain porous hollow fiber membranes, or a porous hollow fiber membrane with a spherulite structure may be obtained. However, by adjusting the distance between the Hansen solubility parameters of the polyolefin and organic liquid, as well as by mixing in inorganic fine powder and using air cooling as described later, it is possible to achieve a high level of balance between strength, filtration stability, and pore size, which are otherwise trade-offs.

[0049] The mass percentage of the organic liquid in the above molten mixture is preferably 10% by mass or more, more preferably 20% by mass or more, preferably 90% by mass or less, more preferably 80% by mass or less, preferably 10% by mass or more and 90% by mass or less, and more preferably 20% by mass or more and 80% by mass or less. If the mass percentage of the organic liquid is 10% by mass or more, the thermoplastic resin can be stably dissolved, and if it is 90% by mass or less, it has sufficient viscosity for spinning porous hollow fiber membranes and can be manufactured stably.

[0050] 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 or more, more preferably 5 nm or more, preferably 500 nm or less, more preferably 100 nm or less, preferably 3 nm to 500 nm, and more preferably 5 nm to 100 nm. Among these, fine silica with an average primary particle size of 3 nm to 500 nm is preferred. Hydrophobic silica fine powder that is less prone to aggregation and has good dispersibility is more preferred, and even more preferably hydrophobic silica with an MW (methanol wettability) value of 30% by volume or more. The MW value here refers to the volume percentage of methanol required to completely wet the powder. Specifically, it is determined by adding silica to pure water, stirring, and then adding methanol below the liquid surface, and finding the volume percentage of methanol in the aqueous solution when 50% by mass of the silica settles. The "average primary particle size of the inorganic fine powder" mentioned above refers to the value obtained from the analysis of electron microscope images. Specifically, a group of inorganic fine powders is first pre-treated according to the ASTM D3849 method. Then, the diameters of 3,000 to 5,000 particles captured by a transmission electron microscope are measured, and the average primary particle diameter of the inorganic fine powder is calculated by taking the arithmetic mean of these values.

[0051] The mass percentage of inorganic fine powder in the above molten mixture is preferably 5% by mass or more, more preferably 10% by mass or more, preferably 50% by mass or less, more preferably 40% by mass or less, preferably 5% by mass or more and 50% by mass or less, and more preferably 10% by mass or more and 40% by mass or less. If the mass percentage of inorganic fine powder is 5% by mass or more, the effect of kneading with inorganic fine powder can be fully expressed, and if it is 40% by mass or less, stable spinning is possible. The total mass percentage of the above polyolefin, the above organic liquid and the above inorganic fine powder in the above molten mixture is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and may be 100% by mass. It is preferable that the total mass percentage is within the above numerical range, and that the mass percentages of the above polyolefin, the above organic liquid, or the above inorganic fine powder are within the above numerical range.

[0052] An antioxidant may be used in the above molten compound for the purpose of suppressing the oxidation of polyolefin and other thermoplastic resins, which are optional components. 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, Irgaphos 168, Irganox PS800FL, Irganox PS802FL, Irgastabu FS042, and dibutylhydroxytoluene. Preferably, it is a hindered phenol antioxidant from among the above antioxidants, and more preferably Irganox 1010.

[0053] A mixture consisting of polyolefin and other optional thermoplastic resins, organic liquids, and inorganic fine powders can be obtained by mixing using a Henschel mixer, Banbury mixer, Proscheer mixer, or the like. When mixing the three components of polyolefin and other optional thermoplastic resins, organic liquids, and inorganic fine powders, the three components may be mixed simultaneously, or each component may be added in any order and mixed at each stage of addition.

[0054] Without pre-mixing using a Henschel mixer or the like, the polyolefin, other thermoplastic resins (optional components), and organic liquids may be directly supplied separately to a melt-mixing extruder such as a twin-screw extruder. To improve mixability, the mixture may be melt-mixed once after mixing to form pellets, which may then be supplied to a melt-mixing extruder, extruded into hollow fibers, and cooled and solidified to form hollow fibers.

[0055] The melt-mixing of the mixture can be carried out using conventional melt-mixing methods, such as an extruder. The following describes the case using an extruder, but the means of melt-mixing are not limited to an extruder. An example of a manufacturing apparatus used to carry out the manufacturing method of this embodiment is shown in Figures 2 and 3.

[0056] The apparatus for producing a porous hollow fiber membrane shown in FIG. 2 includes an extruder 10, a hollow fiber molding nozzle 20, a coagulation bath 30 storing a solution for coagulating a film-forming stock solution, and a plurality of rollers 50 for conveying and winding the porous hollow fiber membrane 40. The space S shown in FIG. 2 is an idle section through which the film-forming stock solution discharged from the hollow fiber molding nozzle 20 passes before reaching the solution in the coagulation bath 30. The idle section S refers to the portion from the discharge port of the hollow fiber molding nozzle 20 to the water surface of the coagulation bath 30. If necessary, a container such as a cylinder may be used for the idle section S from the discharge port. After passing through the coagulation bath 30, the product is wound into a skein or the like as needed.

[0057] The method for producing a porous hollow fiber membrane of the present embodiment preferably further includes: melt-kneading a polyolefin melt obtained by adding the organic liquid and the inorganic fine powder to the polyolefin; discharging the obtained melt-kneaded product; and after discharging the melt-kneaded product, cooling only in an air layer. Note that the polyolefin melt and the melt-kneaded product obtained by melt-kneading the polyolefin melt may have the same composition. The apparatus for producing a porous hollow fiber membrane shown in FIG. 3 includes, as in FIG. 2, an extruder 10, a hollow fiber molding nozzle 20, and a plurality of rollers 50 for conveying and winding the porous hollow fiber membrane 40. Unlike the production apparatus shown in FIG. 2, the production apparatus shown in FIG. 3 is cooled in an air layer without providing a coagulation bath.

[0058] For the melt-kneaded product, a hollow fiber molding nozzle 20 having one or more annular discharge ports arranged concentrically is attached to the tip of the extruder 10, and the melt-kneaded product is extruded by the extruder 10 and discharged from the hollow fiber molding nozzle 20.

[0059] When extruding the melt-kneaded product from the annular discharge port, it is preferable to perform discharge such that the spinneret discharge parameter R (1 / sec) is a value of 10 or more and 1000 or less, because this enables obtaining high productivity, spinning stability, and a membrane with further high strength. Here, the spinneret discharge parameter R is a 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 per unit time of the melt-kneaded product (m ３ / sec) by the cross-sectional area of the discharge port (m ２This is the value obtained by dividing by ). If R is 10 or greater, there are no problems such as pulsation of the yarn diameter of the hollow extruded material, and spinning can be performed stably with good productivity. Also, if R is 1000 or less, the elongation at break, which is one of the important strengths of the resulting porous hollow fiber membrane, can be maintained at a sufficiently high level. Elongation at break is the ratio of elongation to the original length when the membrane is pulled in the longitudinal direction.

[0060] Furthermore, the draft ratio when extruding the molten mixture from the annular discharge port is preferably 1.0 or higher, preferably 10 or lower, and more preferably 1.0 or higher and 10 or lower. The draft ratio is the ratio of the discharge linear velocity (m / sec) to the winding speed (m / sec) after leaving the solidification bath. When the draft ratio is 1.0 or higher, tension is applied to the molten mixture, allowing for the stable production of hollow molten mixtures. When the draft ratio is 10 or lower, the aspect ratio of the pores in the resulting porous hollow fiber membrane can be appropriately adjusted.

[0061] The hollow fiber molten mixture discharged from the discharge port solidifies by passing through a coolant such as air or water. The manufacturing method of this porous hollow fiber membrane may also involve cooling in an air layer (air cooling). Normally, in the heat-induced phase separation method, it is common to pass the mixture through an air-flow section S consisting of an air layer, and then through a solidification bath 30 containing water or the like. On the other hand, in the manufacturing method of the present invention, the pore diameter and porosity near one surface of the porous hollow fiber membrane can be increased by air cooling. This is because air cooling allows for more time to solidify under tension, enabling the growth of strong and stable crystals. As a result, the non-reversing heat flow exothermic enthalpy can be reduced to 30.0 J / g or less. Furthermore, rapid solidification can be suppressed, and more time can be gained for pore growth. In particular, when using high molecular weight thermoplastic resins, the viscosity of the molten mixture is high, so pore growth in phase separation is extremely slow, and it has been difficult to enlarge the pores near the surface and increase the porosity. The present invention provides a porous hollow fiber membrane with high filtration performance and durability against high temperatures.

[0062] Furthermore, in the cooling (air cooling) in the air layer described above, it is preferable that the air velocity of the air layer during cooling is 2.0 m / sec or less. In the manufacturing method of this embodiment, air is used as the fluid for forming the hollow part, but high-temperature air is flowed through it as it passes through the nozzle for forming the hollow fiber. As a result, the inner surface cools slowly and solidification is delayed, so the pore diameter becomes larger. Furthermore, when polyolefins with an intrinsic viscosity of 4.5 dL / g or more are used, the viscosity of the molten mixture becomes high, and the pore diameter in the thickness direction range of 0.5% to 1.5% of the film thickness from the outermost surface of the inner surface of the porous hollow fiber membrane does not grow and becomes small. As a result, the ratio of the pore diameter in the thickness direction range of 0.5% to 1.5% of the film thickness from the outermost surface of the inner surface to the pore diameter of the other surface becomes large, but this ratio of pore diameters 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 installing a local exhaust hood or a container in the air layer, but in the manufacturing method of this embodiment, it is not particularly limited as long as the air velocity can be controlled.

[0063] When cooling with only an air layer, it is necessary to improve viscosity and shape retention because the material is passed through rollers or directly wound onto a skein or similar device within the air layer. Typically, increasing viscosity is achieved by increasing the polymer concentration or using polymers with high molecular weights, but the former reduces the porosity that contributes to filtration, and the latter is prone to problems such as molding defects. By adding inorganic fine powder, the viscosity and shape retention of the molten mixture can be improved without the constraints of polymer molecular weight or concentration, and furthermore, because the inorganic fine powder maintains the pore shape, air cooling can be performed without deformation of the pores. In addition, when cooling with only an air layer is performed, crystalline polymers such as polyethylene usually form a spherulite structure, making it difficult to form a three-dimensional network structure with good pore connectivity. In this case, by adding inorganic fine powder, the growth of the spherulite structure is suppressed, making it possible to form a three-dimensional network structure even when cooling with an air layer.

[0064] The extraction and removal of organic liquids and inorganic fine powders can be performed simultaneously if they can be extracted and removed using the same solvent. However, they are usually extracted and removed separately.

[0065] The extraction and removal of the organic liquid is performed 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 contact, such as by 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.

[0066] The extraction and removal of inorganic fine powders is usually carried out using an aqueous liquid. For example, if the inorganic fine powder is silica, it can be done by first contacting it with an alkaline solution to convert the silica into silicate, and then contacting it with water to extract and remove the silicate.

[0067] The order in which organic liquids and inorganic fine powders are extracted and removed does not matter. However, if the organic liquid is immiscible with water, it is preferable to extract and remove the organic liquid first, followed by the inorganic fine powder. This is advantageous because, normally, the organic liquid and inorganic fine powder coexist in a concentrated subphase of the organic liquid, allowing for smoother extraction and removal of the inorganic fine powder.

[0068] In this way, a porous hollow fiber membrane can be obtained by extracting and removing organic liquids and inorganic fine powders from a solidified porous hollow fiber membrane. The porous hollow fiber membrane after solidification can be stretched in the longitudinal direction within a stretching ratio of up to 3 times at any of the following stages: (i) before extraction and removal of organic liquids and inorganic fine powders, (ii) after extraction and removal of organic liquids and before extraction and removal of inorganic fine powders, (iii) after extraction and removal of inorganic fine powders and before extraction and removal of organic liquids, or (iv) after extraction and removal of organic liquids and inorganic fine powders. Generally, stretching a porous hollow fiber membrane in the longitudinal direction improves its water permeability, but its pressure resistance (e.g., burst strength and compressive strength) decreases, so the stretched membrane often does not have practical strength. However, the porous hollow fiber membrane obtained by the manufacturing method of this embodiment has high mechanical strength. Therefore, stretching with a stretching ratio of 1.1 times to 3.0 times is possible. Stretching improves the water permeability of porous hollow fiber membranes. The stretching ratio, in this context, refers to the value obtained by dividing the length of the hollow fiber after stretching by the length of the hollow fiber before stretching. For example, if a porous hollow fiber membrane with a length of 10 cm is stretched to a length of 20 cm, the stretching ratio is 2 times, according to the following formula: 20 cm ÷ 10 cm = 2

[0069] Stretching is preferably carried out at an ambient temperature of 0°C to 160°C. If the temperature is higher than 160°C, the stretching pattern will be large, the elongation at break will decrease, and the water permeability will be low, which is undesirable. If the temperature is below 0°C, the possibility of stretching breakage will be high and it will not be practical. It is more preferable to keep the ambient temperature during the stretching process at 10°C or higher, even more preferably at 20°C or higher, even more preferably at 140°C or lower, even more preferably at 100°C or lower, even more preferably at 10°C to 140°C, and even more preferably at 20°C to 100°C.

[0070] 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 prone to breakage during stretching than a porous hollow fiber membrane without an organic liquid. Furthermore, a porous hollow fiber membrane containing an organic liquid allows for greater shrinkage after stretching, thus increasing the flexibility in setting the shrinkage rate after stretching.

[0071] Furthermore, it is preferable to stretch a porous hollow fiber membrane containing inorganic fine powder. A porous hollow fiber membrane containing inorganic fine powder is less likely to be flattened during stretching due to the stiffness of the porous hollow fiber membrane caused by the presence of inorganic fine powder. It is also possible to prevent the pore diameter of the final porous hollow fiber membrane from becoming too small or the fiber diameter from becoming too thin. In this embodiment, it is even more desirable to stretch a porous hollow fiber membrane containing both an organic liquid and inorganic fine powder.

[0072] For the reasons stated above, it is preferable to stretch a porous hollow fiber membrane containing either an organic liquid or an inorganic fine powder rather than stretching a porous hollow fiber membrane after extraction is complete, and it is even 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.

[0073] Furthermore, the method of extracting stretched porous hollow fiber membranes has the advantage that, because stretching increases the number of voids on the surface and inside the porous hollow fiber membrane, the extraction solvent can easily penetrate into the interior of the porous hollow fiber membrane. In addition, the method of stretching and then shrinking the membrane before extraction has the advantage that, as described later, the porous hollow fiber membrane becomes flexible and has a low tensile modulus. Therefore, when extraction is performed in a liquid flow, the porous hollow fiber membrane becomes more prone to shaking due to the liquid flow, increasing the stirring effect and enabling highly efficient extraction in a short time.

[0074] In this embodiment, when the porous hollow fiber membrane is stretched and then contracted, a porous hollow fiber membrane with a low tensile modulus can be obtained. Here, "low tensile modulus" means that the fibers are easily stretched with little 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 flatten and collapses easily, is easily bent, and sways easily with the water flow during filtration. Because the bending of the fibers does not remain constant and sways with the water flow, the layer of contaminants adhering to and accumulating on the membrane surface does not grow and peels off easily, and a high filtration water volume can be maintained. Furthermore, when the fibers are forcibly shaken by flushing or air scrubbing, the shaking is greater and the cleaning and recovery effect is enhanced.

[0075] Regarding the degree of yarn length shrinkage when shrinking after stretching, it is desirable that the yarn length shrinkage rate relative to the increase in yarn length due to stretching be in the range of 0.3 to 0.9. For example, when a 10 cm yarn is stretched to 20 cm and then to 14 cm, the yarn length shrinkage rate is 0.6 according to the following formula: Yarn length shrinkage rate = {(Maximum yarn length during stretching) - (Yarn length after shrinkage)} / [(Maximum yarn length during stretching) - (Original yarn length)] = (20 - 14) / (20 - 10) = 0.6 If the yarn length shrinkage rate is 0.9 or higher, the water permeability performance tends to be low, and if it is less than 0.3, the tensile modulus tends to be high, which is undesirable. In this embodiment, it is more preferable that the yarn length shrinkage rate is in the range of 0.50 to 0.85.

[0076] Furthermore, by employing a process of stretching the porous hollow fiber membrane to its maximum length and then shrinking it, the resulting porous hollow fiber membrane will not break even when stretched to its maximum length during use. Here, when the stretching ratio is X and the yarn length shrinkage rate with respect to the increase in yarn length due to stretching is Y, the rate Z, which represents the degree of guarantee of elongation at break, can be defined by the following formula: Z = (maximum yarn length at stretch - yarn length after shrinking) / yarn length after shrinking = (XY - Y) / (X + Y - XY) Z is preferably 0.2 or more, more preferably 0.3 or more, preferably 1.5 or less, more preferably 1.0 or less, preferably 0.2 to 1.5 or less, and more preferably 0.3 to 1.0 or less. If Z is too small, the guarantee of elongation at break will be reduced, and if Z is too large, the possibility of breakage during stretching will increase while the water permeability performance will be low.

[0077] Furthermore, in the manufacturing method of this embodiment, if the process includes stretching followed by shrinking, the tensile elongation at low elongation is significantly reduced, and the distribution of tensile elongation at break can be narrowed.

[0078] In the stretching and subsequent shrinking process, the ambient temperature should preferably be between 0°C and 160°C, considering the shrinkage time and physical properties. Temperatures below 0°C result in a long shrinkage time, making it impractical. Temperatures above 160°C may exceed the melting point of the thermoplastic resin, leading to molding defects, reduced elongation at break, and poor water permeability, which is undesirable.

[0079] In this embodiment, it is preferable to crimp the porous hollow fiber membrane during the shrinkage process. This makes it possible to obtain a porous hollow fiber membrane with a high degree of crimping without crushing or damaging it.

[0080] Generally, porous hollow fiber membranes have a straight, tubular shape, so when bundled together to form a filtration module, there is a high possibility that the gaps between the hollow fibers will not be sufficient, resulting in a bundle with low porosity. In contrast, using porous hollow fiber membranes with a high degree of crimp allows the spacing between the porous hollow fiber membranes to widen on average due to the curvature of the individual fibers, resulting in a bundle with high porosity. Furthermore, filtration modules made of porous hollow fiber membranes with a low degree of crimp have fewer pores in the bundle, especially when used under external pressure, which increases flow resistance and prevents the filtration pressure from being effectively transmitted to the center of the bundle. Moreover, the cleaning effect inside the bundle is reduced when removing filtration deposits from the porous hollow fiber membrane during backwashing or flushing. Bundles made of porous hollow fiber membranes with a high degree of crimp have a large degree of porosity, and the gaps between the porous hollow fiber membranes are maintained even under external pressure filtration, making flow deviation less likely.

[0081] The porous hollow fiber membrane obtained by the manufacturing 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 higher is preferable for the reasons mentioned above, and a crimp degree of 2.5 or lower can suppress a decrease in the filtration area per unit volume.

[0082] Methods for crimping porous hollow fiber membranes include stretching and then shrinking the porous hollow fiber membrane while shrinking it, for example, by clamping it between a pair of gear rolls with periodically irregular grooves or a pair of sponge belts with irregular grooves and then taking it in.

[0083] Furthermore, in the manufacturing method of this embodiment, it is preferable to perform the stretching using a take-up machine consisting of a pair of opposing endless track belts. In this case, the take-up machines are used on the upstream and downstream sides of the stretching process, and in each take-up machine, a porous hollow fiber membrane is sandwiched between the opposing belts, and the yarn is fed by moving both belts at the same speed and in the same direction. In this case, it is preferable to perform the stretching by making the yarn feeding speed on the downstream side faster than the yarn feeding speed on the upstream side. When stretching is performed in this manner, it is possible to stretch the yarn without slipping under the stretching tension and to prevent the yarn from being flattened.

[0084] In this context, the continuous 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 material. Furthermore, it is even more preferable that the compressive modulus of the elastic material in the thickness direction is 0.1 MPa or more and 2 MPa or less, and that the thickness of the elastic material is 2 mm or more and 20 mm or more. In particular, it is preferable to use silicone rubber for the elastic material on the outer surface from the viewpoint of chemical resistance and heat resistance.

[0085] Furthermore, if necessary, the stretched film may be heat-treated to increase its compressive strength. The heat treatment is preferably performed at a temperature between 80°C and 160°C. Below 160°C, a decrease in elongation at break and water permeability can be suppressed, while above 100°C, compressive strength can be increased. In addition, it is preferable to perform the heat treatment on the porous hollow fiber film after extraction is completed, as this minimizes changes in fiber diameter, porosity, pore size, and water permeability.

[0086] The embodiment will be described in more detail below with reference to examples and comparative examples, but this embodiment is not limited to these examples.

[0087] The measurement method used in this embodiment is as follows.

[0088] All measurements below were performed at 25°C unless otherwise specified. The evaluation methods are described below, followed by a description of the manufacturing methods and evaluation results for the examples and comparative examples.

[0089] Table 1 shows the film's composition, manufacturing conditions, and various performance characteristics.

[0090] (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). As the solvent, 20 mL of decahydronaphthalene (with 1 g / L of 2,6-di-t-butyl-4-methylphenol added) that had been degassed with a vacuum pump and replaced with nitrogen was used. As the viscosity tube, a Cannon-Fenske type viscometer (manufactured by Shibata Scientific Instruments Co., Ltd.: product number -100) was used.

[0091] When measuring materials in forms other than powder (e.g., molded bodies), such as porous hollow fiber membranes, the materials were ground into powder for measurement. Grinding methods included multi-stage grinding (coarse grinding followed by fine grinding) and single-stage grinding. If the particles did not reach the desired particle size even with a fine grinder, further grinding was performed using an ultrafine grinder capable of fine grinding. Specific grinding methods included room temperature or freeze grinding using grinding equipment such as hammer mills, turbo mills, jet mills, pin mills, centrifugal mills, Rotoplex, Pulverizers, wet grinding, chopper mills, and Ultrarotors. Freeze grinding was performed by freezing with a low-temperature liquid gas (below -50°C or below -100°C) such as liquid nitrogen. Grinding was also performed in an oxygen-free environment (oxygen concentration below 5% or 2%). Pin mills, hammer mills, or jet mills were used as grinding methods in the freeze grinding process.

[0092] (2) Outer diameter, inner diameter, and film thickness. Thin sections were obtained by cutting the porous hollow fiber membrane in a direction perpendicular to the longitudinal direction of the membrane within a region spaced 15 cm apart in the longitudinal direction of the membrane using a razor or the like. The major axis ("internal major axis") and minor axis ("internal minor axis") on the inner diameter side of the cross-section of the section were measured using a microscope, and the major axis ("external major axis") and minor axis ("external minor axis") on the outer diameter side were measured, and the inner diameter and outer diameter were calculated using the following formulas (2) and (3). The film thickness was calculated by subtracting the inner diameter from the calculated outer diameter and dividing the difference by 2. Twenty measurements were taken, and the average values ​​were taken as the inner diameter (mm), outer diameter (mm), and film thickness (mm) under those conditions.

[0093] (3) Pure water permeability (L / m ２ ( / hr) The hollow fiber membrane was immersed in a 50% by mass ethanol aqueous solution for 30 minutes, and then immersed in water for 30 minutes to wet the hollow fiber membrane. One end of a wet hollow fiber membrane approximately 10 cm long was sealed, and a needle was inserted into the hollow portion of the other end. Pure water at 25°C was injected into the hollow portion from the needle at a pressure of 0.1 MPa, and the amount of pure water that permeated to the outer surface was measured. The pure water permeation flux was determined using the following formula. Here, the effective membrane length refers to the net membrane length excluding the portion in which the needle is inserted. Ten measurements were taken, and the average value was taken as the pure water permeability under each condition.

[0094] (4) Bubble point pressure (MPa), bubble point pore diameter (μm) These were measured in accordance with the method for measuring the maximum pore diameter (also known as the bubble point method) described in ASTM:F316-86. The measurement was performed on a 5 cm long hollow fiber membrane using ethanol as the liquid and compressed air as the pressurizing gas at 25°C and a pressurization rate of 0.05 atm / second. The maximum pore diameter was calculated from the obtained bubble point pressure using the following formula (11). Furthermore, when ethanol is used as the liquid, the surface tension at 25°C is 21.97 dynes / cm. (The Chemical Society of Japan, ed., Basic Chemical Handbook, Revised 3rd Edition, II-82, Maruzen, 1984)

[0095] (5) Compressive Strength (MPa) One end of a porous hollow fiber membrane sample, approximately 5 cm in length, was sealed, and the other end was left open to the atmosphere. Pure water at 40°C was pressurized from the outer surface using a total filtration method, and permeate was discharged from the open end. The pressurized pressure was increased in increments of 0.05 MPa starting from 0.1 MPa, and the pressure was held for 15 seconds at each pressure. Permeate coming out of the open end during these 15 seconds was sampled. As long as the hollow portion of the porous hollow fiber membrane does not collapse, the absolute value of the permeate volume (weight) increases with increasing pressurized pressure. However, when the pressurized pressure exceeds the compressive strength of the porous hollow fiber membrane, the hollow portion collapses and occlusion begins, so the absolute value of the permeate volume decreases despite the increasing pressurized pressure. The pressurized pressure at which the absolute value of the permeate volume is maximum was defined as the compressive strength.

[0096] (6) Thermal shrinkage rate A sample of porous hollow fiber membrane with a length of 5.0 cm was heated in an oven at 120°C for 1 hour, and the length of the fiber was measured before and after heating. The thermal shrinkage rate was calculated from the length of the fiber before and after heating using the following formula: Thermal shrinkage rate = (length of fiber after heating) / (length of fiber before heating) × 100 Three measurements were taken, and the average value of the three measurements was taken as the thermal shrinkage rate for each condition. If the porous hollow fiber membrane sample was in a wet state, it was dried at 50°C for 12 hours, and the test was conducted on the porous hollow fiber membrane sample in a dry state.

[0097] (7) MDSC A Discovery 2500 manufactured by TA Instruments was used, and the sample pans used were Al Tzero Pan and Al Tzero Lid manufactured by TA Instruments. The film was cut to a length of 5 mm and subjected to DSC measurement. The weight of the film was 2.0 to 3.4 mg. 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 ±0.2 °C. Three melting curves were obtained: the total heat flux component (approximately the normal DSC measurement, Total Heat Flow), the heat flux component that can reversibly track 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. Figures 4 and 5 show the MDSC analysis data of the porous hollow fiber membranes produced in Example 3 and Comparative Example 1.

[0098] A baseline was drawn in the melting curve range from 60°C to 150°C, with the upper part of the baseline representing exothermic enthalpy and the lower part representing endothermic enthalpy. The area of ​​the upward-convex portion of the Non-reversing Heat Flow below the endothermic peak temperature was defined as the Non-reversing Heat Flow exothermic enthalpy, the area of ​​the downward-convex portion of the Reversing Heat Flow was defined as the Reversing Heat Flow endothermic enthalpy, and the area of ​​the downward-convex portion of the Total Heat Flow was defined as the Total Heat Flow endothermic enthalpy.

[0099] Equipment: TA Instruments DSC Discovery 2500 Atmosphere: Nitrogen (flow rate 50 ml / min) Weight: Approx. 3 mg Length: Approx. 5 mm Heating rate: 1.0 °C / min Measurement mode: Modulated Heat Only Modulation period: 60 s Modulation amplitude: ±0.2 °C DSC pan: Hermetic Al Pan + Hermetic Al Lid

[0100] (8) Void Number A Hitachi SU8000 series electron microscope was used to photograph the central part of the cross-section of the porous hollow fiber membrane at an acceleration voltage of 3 kV. The images were taken at a magnification such that they did not include the outer or inner surfaces, but only the cross-section of the porous hollow fiber membrane. In this example and comparative example, observation was performed at 1000x magnification. For image analysis, commercially available image analysis software Winroof2018 Ver4.23.1 was used. After calibrating the length per pixel to match the scale displayed when imaging with the electron microscope, binarization was performed using discriminant analysis. An opening process was performed once on the pore information (black area) image obtained after binarization, and the area of ​​each pore or cavity defect (black area) in the observation target area (130 μm vertical × 130 μm horizontal) after processing was calculated. The calculated area value of each black area was set to 0 μm. ２ 0.1 μm or more ２ Less than 0.1 μm ２ 0.2 μm or more ２ Less than 0.1 μm ２ The areas were classified into distinct ranges, and the number of black areas within each range was counted. Then, the upper limit area value (e.g., 0.1–0.2 μm) of the range with the highest count was calculated. ２ Then 0.2 μm ２ The average pore area of ​​the film was defined as ), and black areas (cavity defects) with an area 50 times or more than the average pore area were defined as voids. The number of voids per 130 μm x 130 μm area was defined as the "void count". Figure 6 shows an example of voids in Comparative Example 2. The void count was judged as follows: Many: 5 or more voids Medium: 2 or more but less than 5 voids Few: 1 or fewer voids

[0101] (Example 1) Polyethylene (SH800, manufactured by Asahi Kasei Corporation, intrinsic viscosity 3.2 dL / g) was used as the thermoplastic resin, dibutyl sebacate (DBS, manufactured by Toyokuni Oil Co., Ltd.) as the organic liquid, fine silica powder (manufactured by Nippon Aerosil Co., Ltd., trade name: AEROSIL-R972) as the inorganic fine powder, and pentaerythritol tetrakis[3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate] (Irganox 1010, manufactured by BASF, hereafter referred to as Irganox 1010) as the antioxidant. Melt extrusion of a hollow fiber membrane was performed using an extruder with a hollow fiber molding nozzle. A molten compound with the composition polyethylene:dibutyl sebacate:fine silica powder:irganox 1010 = 20:52:27:1 (mass ratio) was extruded from a hollow fiber forming nozzle with an outer diameter of 3.0 mm and an inner diameter of 1.7 mm, using air as the fluid for forming the hollow section, at a discharge temperature of 180°C. The hollow fiber-like molten compound extruded at a discharge temperature of 175°C was taken up at a speed of 5.0 m / min and wound into a skein to obtain a hollow fiber-like material, as shown in the schematic diagram in Figure 3. The obtained hollow fiber-like material was immersed in isopropyl alcohol to extract and remove dibutyl sebacate and irganox 1010, and then dried. Next, the material was immersed in a 50% by mass ethanol aqueous solution for 30 minutes, then immersed in water for 30 minutes, and then immersed in a 20% by mass sodium hydroxide aqueous solution at 70°C for 1 hour. Further washing with water was repeated to extract and remove the fine silica powder, thereby obtaining a porous hollow fiber membrane. Table 1 shows the detailed composition and conditions.

[0102] (Example 2) A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that bis-2-ethylhexyl adipate (DOA, manufactured by Taoka Chemical Industries, Ltd.) was used as the organic liquid.

[0103] (Example 3) A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that the composition of the molten mixture was polyethylene:dibutyl sebacate:fine silica powder:Irganox 1010 = 20:55:24:1 (mass ratio).

[0104] (Comparative Example 1) A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that a hollow fiber molten mixture extruded at a discharge temperature of 180°C using the method shown in the schematic diagram of Figure 2, was subjected to air travel for 4.0 seconds, then guided into a solidification bath at 30°C, and withdrawn at a speed of 5.0 m / min.

[0105] (Comparative Example 2) A porous hollow fiber membrane was obtained in the same manner as in Comparative Example 1, except that dibutyl phthalate (DBP, manufactured by CG Ester Co., Ltd.) was used as the organic liquid, and the composition of the molten mixture was polyethylene:dibutyl phthalate:fine silica powder:Irganox 1010 = 20:55:24:1 (mass ratio).

[0106] (Comparative Example 3) A porous hollow fiber membrane was obtained in the same manner as in Example 3, except that dibutyl phthalate (DBP, manufactured by CG Ester Co., Ltd.) was used as the organic liquid.

[0107] (Comparative Example 4) A porous hollow fiber membrane was obtained in the same manner as in Comparative Example 1, except that dibutyl phthalate (DBP, manufactured by CG Ester Co., Ltd.) was used as the organic liquid.

[0108] (Comparative Example 5) A porous hollow fiber membrane was obtained in the same manner as in Comparative Example 1, except that di(2-ethylhexyl) phthalate (DEHP) (manufactured by CG Ester Co., Ltd.) was used as the organic liquid.

[0109] (Comparative Example 6) A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that di(2-ethylhexyl) phthalate (DEHP) (manufactured by CG Ester Co., Ltd.) was used as the organic liquid.

[0110]

[0111] a Thermoplastic resin b Hollow section 10 Extruder 20 Hollow fiber molding nozzle 30 Solidification bath 40 Porous hollow fiber membrane 50 Roller S Irrigation section

Claims

1. A porous hollow fiber membrane containing a polyolefin, characterized in that the non-reversing heat flow exothermic enthalpy is 30 J / g or less and the number of voids is 1 or less.

2. The porous hollow fiber membrane according to claim 1, characterized in that the reversing heat flow endothermic enthalpy is 112 J / g or less.

3. The porous hollow fiber membrane according to claim 1 or 2, characterized in that the total heat flow endothermic enthalpy is 200.5 J / g or more.

4. The porous hollow fiber membrane according to claim 1 or 2, characterized in that the intrinsic viscosity of the polyolefin or the porous hollow fiber membrane is 5.0 dL / g or less.

5. The porous hollow fiber membrane according to claim 1 or 2, characterized in that the non-reversing heat flow exothermic enthalpy is 15 J / g or less.

6. The porous hollow fiber membrane according to claim 1 or 2, wherein the polyolefin is polyethylene and has a three-dimensional network structure.

7. A method for producing a porous hollow fiber membrane, comprising the steps of: melt-kneading a mixture containing a thermoplastic resin and an organic liquid, or a mixture containing particles of a thermoplastic resin, an organic liquid, and an inorganic fine powder to extrude a hollow fiber-like melt-kneaded molded body; and subsequently extracting and removing the organic liquid or the organic liquid and the inorganic fine powder from the hollow fiber-like melt-kneaded molded body to produce a porous hollow fiber membrane, wherein the distance of the Hansen solubility parameter of the porous hollow fiber membrane is 0.5 or less at 6.0 MPa.

8. The method for producing a porous hollow fiber membrane according to claim 7, wherein the organic liquid body comprises at least one selected from the group consisting of phthalates, sebacates, citrates, acetyl citrates, adipicates, trimelliticates, oleates, palmitates, stearates, phosphates, fatty acids having 6 to 30 carbon atoms, and epoxidized vegetable oils.

9. A method for producing a porous hollow fiber membrane according to claim 7 or 8, further comprising: melting and 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 with only an air layer after discharging.

10. The method for producing a porous hollow fiber membrane according to claim 7 or 8, characterized in that the inorganic fine powder contains silica.

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