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

Abstract

[Problem] To provide a porous hollow fiber membrane having excellent chemical resistance in addition to blocking performance suitable for filtration applications, water permeability performance, and the like. [Solution] To solve the above-mentioned problem, this porous hollow fiber membrane is a porous hollow fiber membrane which is formed from a thermoplastic resin and which has a crystallization initiation temperature of 140 DEG C or less and a crystal enthalpy of fusion of 10 J / g or less at the crystallization initiation temperature or less.

Description

This application claims the benefit of Japanese patent application No. 2023-197728, filed on November 21, 2023, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

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

[0002] Water treatment using membrane separation technology has been widely used in various industrial fields such as pharmaceuticals, medicine, food, semiconductors, water purification and sewage. In recent years, it has also been actively used in the field of water purification to remove turbidity from river water to produce drinking water and industrial water, as well as in the field of sewage treatment to remove turbidity and purify domestic wastewater and industrial wastewater. As filter materials used for such membrane separation, there are hollow fiber membranes formed into hollow tubes with high molecular weight resin having excellent processability, flat membranes formed into sheets, and membrane modules in which these are assembled are used.

[0003] Among them, the porous hollow fiber membrane used for deturbidity of river water and sea water is particularly important. Since a large amount of water needs to be processed, in addition to the blocking performance, a higher water permeability is also required. Furthermore, from the perspective of increasing the filtration area, since an external pressure filtration method is adopted, the hollow fiber membrane needs to have a compressive strength that will not be destroyed by compression from the outside during the filtration operation. In addition, when the hollow fiber membrane is used continuously for water treatment, the substances separated by filtration will cause blockage on the membrane surface and reduce water permeability. Therefore, after filtering for a period of time or filtering a certain amount of water, the hollow fiber membrane should be cleaned regularly and the organic matter accumulated on the membrane surface should be removed with chemicals.

[0004] However, this chemical cleaning not only degrades the accumulated organic matter, but also degrades the polymer that forms the hollow fiber membrane. Therefore, if the chemical cleaning is repeated, the hollow fiber membrane will gradually deteriorate, making it impossible for the hollow fiber membrane module to operate continuously for a long time. Therefore, the hollow fiber membrane must also have high chemical resistance.

[0005] For example, Patent Documents 1 and 2 propose hollow fiber membranes having both high water permeability and high membrane strength in addition to barrier performance. However, even if the membrane strength is high at the beginning of operation, it is hard to say whether it is strong enough to withstand repeated chemical cleaning.

[0006] On the other hand, the thermally induced phase separation method (TIPS method) is a known method for manufacturing a membrane. 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 the thermoplastic resin at high temperatures, that is, it is a potential solvent. The thermoplastic resin and the organic liquid are mixed at high temperature, the thermoplastic resin is dissolved in the organic liquid, and then cooled to room temperature, phase separation is induced, and the organic liquid is further removed to produce a porous material. Since this method dissolves at high temperature and then rapidly cools and solidifies to form a membrane, especially when the thermoplastic resin is a crystalline resin, crystallization is promoted, and a high-strength membrane is easily obtained, so it is widely used as a method for manufacturing porous membranes. Prior art literature Patent Literature

[0007] [Patent Document 1] Japanese Patent No. 5717987 [Patent Document 2] Japanese Patent No. 6824284 Summary of the invention Problems to be solved by the invention

[0008] An object of the present invention is to provide a porous hollow fiber membrane having a blocking performance and a water permeability suitable for filtration applications and excellent chemical resistance. Solution to the problem

[0009] The present inventors have conducted intensive studies to solve the above-mentioned problems and have completed the present invention. That is, the present invention is as follows. [1] A hollow fiber membrane, which is a porous hollow fiber membrane formed of a thermoplastic resin, having a crystallization starting temperature of 140° C. or lower and a crystal melting enthalpy of 10 J / g or lower at or below the crystallization starting temperature. [2] The porous hollow fiber membrane according to [1], wherein the crystal melting enthalpy of the porous hollow fiber membrane as a whole is lower than 58 J / g. [3] The hollow fiber membrane according to [1] or [2], wherein the crystal melting enthalpy of a peak appearing on the higher temperature side of a main peak of crystal melting of the porous hollow fiber membrane is 0.1 J / g or more. [4] The hollow fiber membrane according to any one of [1] to [3], wherein the peak temperature appearing on the higher temperature side of the main crystal melting peak of the porous hollow fiber membrane is 10° C. or more higher than the main crystal melting peak temperature. [5] The hollow fiber membrane according to any one of [1] to [4], wherein the porous hollow fiber membrane has a degree of crystallinity of less than 60%. [6] The hollow fiber membrane according to any one of [1] to [5], wherein the crystal melting enthalpy at or below the crystallization starting temperature is 15% or less of the crystal melting enthalpy of the entire porous hollow fiber membrane. [7] The hollow fiber membrane according to any one of [2] to [6], wherein the weight average molecular weight (Mw) of the thermoplastic resin is 400 kDa or less. [8] The hollow fiber membrane according to any one of [1] to [7], wherein the thermoplastic resin contains a polyvinylidene fluoride resin. [9] The hollow fiber membrane according to any one of [1] to [8], wherein the inner diameter of the porous hollow membrane is less than 0.75 mm and the compressive strength is 0.3 MPa or more.

[10] The hollow fiber membrane according to any one of [1] to [9], wherein the heterogeneous bonding rate of the porous hollow fiber membrane is 9% or more.

[11] A method for manufacturing a hollow fiber membrane according to any one of [1] to

[10] , wherein the method includes a multi-stage heat treatment process of, after stretching the hollow fiber membrane, heat treating the membrane at a melting point of -50°C to -40°C of a thermoplastic resin containing polyvinylidene fluoride resin, and then heat treating the membrane at a melting point of -35°C to -25°C of the thermoplastic resin.

[12] A method according to

[11] , wherein a mixture of the three components of the thermoplastic resin, the organic liquid and the inorganic fine powder is melt-kneaded and extruded to form hollow fibers, and then the organic liquid and the inorganic fine powder are extracted to produce the porous hollow fiber membrane. Effects of the Invention

[0010] According to the present invention, there can be provided a porous hollow fiber membrane produced by the TIPS method, having barrier performance and high water permeability and having high chemical resistance capable of long-term continuous operation, and a method for producing the porous hollow fiber membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

[0012] Hereinafter, a mode for implementing the present invention (hereinafter referred to as "this embodiment") will be described in detail. In addition, the present invention is not limited to the following this embodiment, and can be implemented with various modifications within the scope of the gist thereof.

[0013] <Porous hollow fiber membrane> The porous hollow fiber membrane of the present invention (hereinafter sometimes simply referred to as "hollow fiber membrane") is formed of a thermoplastic resin. Here, the thermoplastic resin preferably contains a fluororesin, but may be formed of only a fluororesin.

[0014] The fluororesin preferably contains at least one selected from the group consisting of polyvinylidene fluoride resin (PVDF), chlorotrifluoroethylene resin, tetrafluoroethylene resin, ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), hexafluoropropylene resin and a mixture of these resins, or may be composed only of at least one selected from the group consisting of polyvinylidene fluoride resin (PVDF), chlorotrifluoroethylene resin, tetrafluoroethylene resin, ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), hexafluoropropylene resin and a mixture of these resins. Among these, the thermoplastic resin preferably contains at least one of a polyvinylidene fluoride resin and a chlorotrifluoroethylene resin, and more preferably contains at least a polyvinylidene fluoride resin.

[0015] Polyvinylidene fluoride resin refers to homopolymers and / or vinylidene fluoride copolymers containing vinylidene fluoride. Vinylidene fluoride copolymers are polymers having a residual structure of vinylidene fluoride, typically copolymers of vinylidene fluoride monomers and fluorine-based monomers other than vinylidene fluoride, and known substances can be appropriately selected. In addition, multiple vinylidene fluoride copolymers can also be contained.

[0016] The polyvinylidene fluoride resin is preferably a homopolymer from the viewpoint of excellent strength. In the case of a copolymer, it preferably contains 50% or more of vinylidene fluoride by molar ratio from the same viewpoint.

[0017] It should be noted that the thermoplastic resin may be one type or a combination of two or more types.

[0018] The porous hollow fiber membrane of the present invention has a crystallization initiation temperature of 140°C or lower, preferably 100°C or higher and 140°C or lower, and more preferably 120°C or higher and 140°C or lower. Furthermore, the crystal melting enthalpy of the porous hollow fiber membrane of the present invention at or below the crystallization starting temperature is preferably small, and specifically needs to be 10 J / g or less.

[0019] Here, the crystallization starting temperature and the crystal melting enthalpy can be analyzed by temperature modulation DSC (MDSC). The MDSC measurement is different from the conventional DSC measurement of constant temperature increase. It is a method of performing DSC measurement while periodically raising and lowering the temperature relative to the average heating rate. The MDSC measurement can separate the heat flow obtained from the ordinary DSC into a reversing heat flow (Reversing Heat Flow) part that can change with the cyclic increase and decrease of the temperature and an irreversible heat flow (Non-Reversing Heat Flow) part that cannot change with the cyclic increase and decrease of the temperature. The crystal melting heat is observed in the reversible heat flow, while crystallization is only observed in the irreversible heat flow. Therefore, it is one of the preferred embodiments to perform MDSC measurement when evaluating crystal melting and crystallization respectively. The crystallization onset temperature is determined based on the non-reversing heat flow described below.

[0020] The porous hollow fiber membrane of the present invention is a hollow fiber membrane having the characteristics that the amount of crystallites melting at or below the crystallization starting temperature is small and the polymer is easy to recrystallize. The imperfection of the polymer crystal is caused by the crystal size (thickness of the plate-like crystal), and the larger the crystal size, the higher the melting point, and the smaller the crystal size, the lower the melting point. Here, the above-mentioned crystal melting enthalpy at or below the crystallization starting temperature refers to the microcrystal portion that melts at low temperature. By controlling these microcrystals to be as small as possible through heat treatment, a structure that is stable to heat and chemicals can be obtained, and a hollow fiber membrane with excellent durability even if the molecular weight and crystallinity are low can be obtained. Therefore, the crystal melting enthalpy at or below the crystallization starting temperature needs to be 10 J / g or less. Generally speaking, the crystal size distribution of a polymer depends on the manufacturing method such as the polymerization method. If hollow fiber membranes are manufactured and compared under the same conditions, the crystal size is often affected by the properties of the polymer itself. Generally speaking, a membrane with a wide distribution of polymer crystal size will be affected by the smaller crystallites. After being impregnated with chemicals, the strength and other properties of the membrane will decrease and the deviation will increase, and there is a tendency for the membrane performance to deteriorate. The porous hollow fiber membrane of the present invention uses a polymer with a wide crystal size distribution. By heat treating the hollow fiber membrane manufactured using the polymer, the microcrystals with smaller crystal size (i.e., low melting point) are converted (recrystallized) into larger crystals. As a result, it is found that the proportion of microcrystals is reduced, thereby improving chemical resistance. However, it is estimated that excellent chemical resistance cannot be exhibited in the following two cases (1) Even at or below the heat treatment temperature, if the crystal size is large and the melting point is close to the heat treatment temperature, it takes time for the crystal to melt, and the transformation into larger crystals will not occur significantly. (2) The crystals with melting points above the heat treatment temperature are larger in size. In addition, when the crystal size distribution is narrow, the crystals cannot be converted into larger crystals through heat treatment. In addition, in the porous membrane of the present embodiment, there is a crystallization starting temperature at or below the heat treatment temperature (140°C or below in the present application), which proves that small crystals still remain in the porous fiber membrane before heat treatment, and according to the above principle, large crystals can be made to exist through heat treatment. On the other hand, the case where the crystallization temperature is higher than the heat treatment temperature indicates that there are no smaller crystals at or below the heat treatment temperature, and thus larger crystals cannot be produced. Therefore, if the crystallization start temperature of the porous hollow fiber membrane of the present invention is 140°C or lower, recrystallization is more easily promoted, and the melting enthalpy of the crystals at or below the crystallization start temperature can be made less than 10 J / g.

[0021] It should be noted that if the crystallization starting temperature is lower than 100° C., the crystal structure will change under actual use conditions, which is not preferred. Therefore, it is preferably 120° C. or higher.

[0022] The crystal melting enthalpy of the porous hollow fiber membrane of the present invention is preferably less than 60 J / g, more preferably less than 58 J / g. As described above, by controlling the number of microcrystals to be small, the crystal melting enthalpy of this part can be reduced, and even if the crystal ratio is low, a given durability can be ensured.

[0023] Furthermore, the crystal melting enthalpy at or below the crystallization start temperature is preferably smaller than the overall crystal melting enthalpy, preferably 20% or less, more preferably 15% or less of the overall crystal melting enthalpy.

[0024] In addition, the porous hollow fiber membrane of the present invention has the characteristic of being easy to recrystallize, and has the following advantages: if the temperature is raised to above the recrystallization temperature, a crystalline phase having a melting point higher than the main peak of crystal melting will be formed. For structural stabilization, the higher the crystal melting enthalpy of the crystalline phase, the better, and it is preferred to appear on the higher temperature side relative to the main peak of crystal melting. Specifically, it is preferably 0.1 J / g or more and 10°C or more.

[0025] Here, the porous hollow fiber membrane that is easy to recrystallize refers to a membrane with a wide distribution of crystal sizes and a large number of microcrystals with small crystal sizes. It is believed that the heat treatment can recrystallize the microcrystals with low melting points, thereby increasing their crystal sizes, raising their melting points, and stabilizing the membrane structure.

[0026] In addition, the crystallinity of the porous hollow fiber membrane of the present invention is preferably 30% or more and less than 70%. The porous hollow fiber membrane of the present invention has few microcrystals and has a sheet-like thickness, so by making the crystallinity of 30% or more, the decrease in membrane strength caused by chemical erosion can be suppressed. The crystallinity is preferably 35% or more, and more preferably 40% or more. On the other hand, by making the crystallinity lower than 70%, the membrane will not become too brittle and will be less likely to be damaged by deformation due to pressure during filtration. The crystallinity is more preferably lower than 65%, and even more preferably lower than 60%.

[0027] In addition, the weight average molecular weight (Mw) of the porous hollow fiber membrane of the present invention is preferably 100,000 to 500,000, more preferably 200,000 to 400,000. When Mw is less than 100,000, the mechanical strength of the obtained membrane decreases. In addition, for high molecular weight polymers exceeding 500,000, the tensile elongation at break of the membrane decreases, and the viscosity when melted in an organic liquid increases, so phase separation becomes time-consuming, and there is a tendency for water permeability to decrease. In addition, high molecular weight polymers have poor solubility, so it is difficult to melt evenly in an organic liquid, and there is a problem of unstable quality.

[0028] In addition, the porous hollow fiber membrane of the present invention preferably contains heterogeneous sequences in a certain ratio, because a membrane having excellent chemical resistance can be obtained. Specifically, it is preferred 1 The heterologous sequence ratio in the molecule measured by H-NMR was 9.0% or more.

[0029] The inner diameter of the porous hollow fiber membrane of the present invention is preferably 0.4 mm or more and less than 5 mm. When the inner diameter is 0.4 mm or more, the pressure loss of the liquid flowing in the hollow fiber membrane will not become too large, and when the inner diameter is less than 1 mm, it is easy to show sufficient compression strength and burst strength under a relatively thin membrane thickness. More preferably, it is 0.5 mm or more and less than 0.8 mm.

[0030] In addition, the film thickness is preferably 0.1 mm or more and 1.0 mm or less. When the film thickness is 0.1 mm or more, it is easy to show sufficient compression strength and burst strength, and when the film thickness is 1.0 mm or less, the filtration resistance is small and it is easy to show sufficient water permeability in practical use. The film thickness is more preferably 0.15 mm or more and 0.25 mm or less.

[0031] In addition, the hollow fiber membrane of the present invention preferably has a pure water permeation rate of 1000 (L / m 2The pure water used here is distilled water or water filtered through an ultrafiltration membrane or reverse osmosis membrane with a molecular weight cutoff of less than 10,000. When the permeability of pure water is low, the number of membrane modules required to process a specified amount in a certain period of time increases, and the space occupied by the filtration equipment increases. In order to avoid this, the filtration pressure can be set higher to process a specified amount in a certain period of time, but in this case, the membrane module is required to have a higher pressure resistance, and the energy cost required for filtration will also increase, and productivity will deteriorate. From this point of view, it is desirable that the pure water permeability is higher. Specifically, the pure water permeability is preferably 1000 L / m 2 / h or more, more preferably 2000L / m 2 / h or more, more preferably 3000L / m 2 / h or more.

[0032] In addition, the tensile elongation at break of the porous hollow fiber membrane of the present invention is preferably 30% or more and 240% or less. If it is less than 30%, the risk of membrane rupture is high when the membrane is forcibly shaken during washing of the membrane module such as flushing or air washing, and if it is more than 300%, the compressive strength or rupture strength is easily weakened. It is more preferably 40% or more and 220% or less.

[0033] In addition, for chemical resistance, it is not only the absolute value of the tensile elongation at break, but also the retention rate relative to the initial value that is important. The retention rate is preferably 60% or more, more preferably 70% or more, in anticipation of long-term use.

[0034] From the viewpoint of increasing the filtration area, the porous hollow fiber membrane of the present invention is mainly used for external pressure filtration. Therefore, the strength relative to the external pressure direction, i.e., the compressive strength, which is used to prevent the hollow fiber membrane from being damaged during filtration operation, needs to be 0.3MPa or more. If the compressive strength is 0.3MPa or more, it can maintain its shape for a long time in water treatment applications where operating pressure is applied for a long time.

[0035] The porous hollow fiber membrane of the present invention is preferably a porous, three-dimensional network structure. The three-dimensional network structure of the present invention is a pattern such as Figure 1 For example, the thermoplastic resin a is bonded to form a grid, and a void portion b is formed. In the three-dimensional network structure, the so-called spherulite structure of the resin block is hardly observed. The void portion b of the three-dimensional network structure is preferably surrounded by the thermoplastic resin a, and the various parts of the void portion b are interconnected. Since most of the thermoplastic resin used forms a three-dimensional network structure that can contribute to the strength of the hollow fiber membrane, a high-strength support layer can be formed. In addition, chemical resistance is also improved. Although the reason for the improvement in chemical resistance is not clear, it is believed that it may be due to the large amount of thermoplastic resin that forms a grid that contributes to strength, so even if a part of the grid is eroded by the drug, it will not have a significant impact on the strength of the entire layer.

[0036] The hollow fiber membrane may have a single-layer structure or a multi-layer structure of two or more layers. The layer having the surface on the filtrate side is referred to as layer (A), and the layer having the surface on the filtrate side is referred to as layer (B). For example, the functions of layer (A) and layer (B) are shared as follows: layer (A) is set as a so-called blocking layer, which uses its small surface pore size to prevent foreign matter contained in the treated liquid (raw water) from passing through the membrane, and layer (B) is set as a so-called supporting layer, which has the function of minimizing the reduction of water permeability while ensuring high mechanical strength. The sharing of functions between layer (A) and layer (B) is not limited to the above.

[0037] Below, the case where the above-mentioned hollow fiber membrane is a multilayer structure, layer (A) is set as a blocking layer, and layer (B) is set as a two-layer structure of a supporting layer is described. The thickness of layer (A) as the surface of the filtered liquid side is preferably 1 / 100 or more and less than 40 / 100 of the membrane thickness. In this way, by making the thickness of layer (A) thicker, it can be used even if the raw water contains insoluble substances such as sand or coagulants. This is because even if there is some wear, the surface pore size does not change. As long as it is within the range of this thickness, a balance between the desired blocking performance and high water permeability can be achieved. The thickness of layer (A) is more preferably 2 / 100 or more and 30 / 100 or less of the membrane thickness. The thickness of layer (A) is preferably 1μm or more and 100μm or less, more preferably 2μm or more and 80μm or less.

[0038] <Method for producing porous hollow fiber membrane> The method for manufacturing a porous hollow fiber membrane of the present embodiment includes: a step of ejecting a molten mixture containing a thermoplastic resin, an organic liquid and an inorganic fine powder from a spinneret having an annular nozzle to form a hollow fiber-shaped molten mixture; and a step of extracting and removing the organic liquid and the inorganic fine powder after solidifying the hollow fiber-shaped molten mixture to produce a porous membrane (preferably a porous hollow fiber membrane).

[0039] It should be noted that the melt-kneaded product may consist of two components, namely, a thermoplastic resin and a solvent, or may consist of three components, namely, a thermoplastic resin, an inorganic fine powder, and a solvent. The thermoplastic resin used in the method for producing the porous hollow fiber membrane of the present embodiment is the same as the thermoplastic resin used in the porous hollow fiber membrane of the present embodiment described above. In addition, a thermoplastic resin is a resin that returns to its original elastomer when cooled and the temperature drops, and does not undergo chemical changes such as molecular structure during this period (for example, see "Chemical Dictionary 6 Reduced Edition" edited by the Chemical Dictionary Editorial Committee, Kyoritsu Publishing, pages 860 and 867, 1963).

[0040] The concentration of the thermoplastic resin in the melt-kneaded product is preferably 30% by mass or more and 48% by mass or less, and more preferably 32% by mass or more and 45% by mass or less. If the concentration is 30% by mass or more, it is easy to ensure mechanical strength, and if the concentration is 48% by mass or less, the water permeability will not be reduced.

[0041] In addition, when the porous hollow fiber membrane of the present embodiment is a membrane of a two-layer structure, the mass ratio of the thermoplastic resin in the melt-kneaded product of the layer (B) as the layer having the filtrate side surface is preferably 34 mass % or more and 48 mass % or less, more preferably 35 mass % or more and 45 mass % or less. The mass ratio of the thermoplastic resin in the melt-kneaded product of the layer (A) is preferably 10 mass % or more and 35 mass % or less, more preferably 12 mass % or more and less than 35 mass %. If it is 10 mass % or more, the pore size and mechanical strength of the surface can be taken into account; if it is 35 mass % or less, the water permeability will not be reduced.

[0042] As the organic liquid, an organic liquid that is a potential solvent for the thermoplastic resin used in this embodiment is used. In this embodiment, the potential solvent refers to a solvent that hardly dissolves the thermoplastic resin at room temperature (25° C.) but can dissolve the thermoplastic resin at a temperature higher than room temperature. The organic liquid only needs to be liquid at the melt kneading temperature with the thermoplastic resin and does not need to be liquid at room temperature.

[0043] The mass ratio of the organic liquid in the melt-kneaded product is preferably 10 mass % or more and 70 mass % or less, and more preferably 20 mass % or more and 60 mass % or less. If the mass ratio of the organic liquid is 10 mass % or more, the thermoplastic resin can be stably dissolved; if it is 70 mass % or less, the porous film can be stably manufactured because it has sufficient viscosity in spinning.

[0044] Examples of the inorganic fine powder include silicon dioxide, aluminum oxide, titanium oxide, zirconium dioxide, and calcium carbonate, and silicon dioxide is preferred. In addition, the average primary particle size of the inorganic fine powder is preferably 3 nm and above and 500 nm and below, more preferably 5 nm and above and 100 nm and below. Among them, the silica fine powder having an average primary particle size of 3 nm and above and 500 nm and below is preferred. The inorganic fine powder is more preferably a hydrophobic silica fine powder that is not easy to agglomerate and has good dispersibility, and further preferably a hydrophobic silica having a methanol wettability (MW) value of 30% by volume and above. The methanol wettability value here refers to the value of the volume % of methanol that completely wets the powder. Specifically, the methanol wettability value is determined by adding silica to pure water, adding methanol below the liquid surface while stirring, and finding the volume % of methanol in the aqueous solution when 50% by mass of silica is precipitated. The above-mentioned "average primary particle size of inorganic fine powder" means a value obtained by analyzing electron microscope photographs. That is, first, a group of inorganic fine powders are pre-treated using the method of ASTM D3849. Thereafter, the diameters of 3000 to 5000 particles photographed on the transmission electron microscope photographs are measured, and their values ​​are arithmetic averaged to calculate the average primary particle size of the inorganic fine powder.

[0045] In addition, the mass ratio of the inorganic fine powder in the above-mentioned melt-kneaded product is preferably 5 mass % or more and 50 mass % or less, and more preferably 10 mass % or more and 40 mass % or less. If the mass ratio of the inorganic fine powder is 5 mass % or more, the effect of the inorganic fine powder kneading can be fully demonstrated; if it is 40 mass % or less, stable spinning can be achieved.

[0046] A mixture of the aforementioned thermoplastic resin such as polyvinylidene fluoride and an organic liquid, or a mixture of a thermoplastic resin such as polyvinylidene fluoride, an organic liquid and an inorganic fine powder can be obtained by mixing using a Henschel mixer, a Banbury mixer, a Ploughshare mixer or the like.

[0047] As for the order in which the three components, namely, a thermoplastic resin such as polyvinylidene fluoride, an organic liquid and an inorganic fine powder, are mixed, compared with mixing the three components at the same time, first mixing the inorganic fine powder and the organic liquid so that the organic liquid is fully adsorbed on the inorganic fine powder, and then adding and mixing the thermoplastic resin such as polyvinylidene fluoride, it is advantageous in terms of improving the melt moldability, the porosity and the mechanical strength of the obtained porous film.

[0048] Alternatively, the thermoplastic resin such as polyvinylidene fluoride and the organic liquid may be directly supplied to a melt-kneading extruder such as a twin-screw extruder without pre-kneading by a Henschel mixer or the like. In order to improve the kneading property, the mixture may be melt-kneaded once and pelletized, and the pellets may be supplied to a melt-kneading extruder, extruded into a hollow fiber, and cooled and solidified to form a hollow fiber.

[0049] The melt kneading of the mixture can be performed by a common melt kneading method, for example, using an extruder. The following description will be made on the case where an extruder is used, but the melt kneading method is not limited to an extruder. Figure 2 An example of a production apparatus for carrying out the production method of the present embodiment is shown.

[0050] Figure 2 The porous hollow fiber membrane manufacturing apparatus shown includes an extruder 10 , a hollow fiber molding nozzle 20 , a coagulation bath 30 storing a solution for coagulating a membrane-forming stock solution, and a plurality of rollers 50 for conveying and winding a porous hollow fiber membrane 40 . Figure 2 The space S shown is an air moving portion through which the membrane-forming stock solution ejected from the hollow fiber molding nozzle 20 passes before reaching the solution in the coagulation bath 30 .

[0051] A hollow fiber forming nozzle 20 having one or more annular ejection ports arranged in a concentric circle is installed at the front end of the extruder 10, and the molten kneaded product is extruded by the extruder 10 and ejected from the hollow fiber forming nozzle 20. In the case of manufacturing a multilayer structure membrane, there are the following methods: a method of installing a hollow fiber forming nozzle 20 having two or more annular ejection ports at the front end of the extruder 10, and supplying the molten kneaded product to each annular ejection port using a different extruder 10 for extrusion; a method of coating the remaining layers after manufacturing one of the multiple layers. For example, in the former method of manufacturing using different extruders, the molten kneaded products supplied separately are merged and overlapped at the ejection port to obtain a hollow fiber extrudate having a multilayer structure. At this time, by extruding molten kneaded products of different compositions from adjacent annular ejection ports, a multilayer membrane having different pore sizes of adjacent layers can be obtained. Different compositions refer to different constituent materials of the molten kneaded products or different constituent ratios even if the constituent materials are the same. Even if they are the same thermoplastic resin, if the molecular weight and molecular weight distribution are significantly different, they are considered to be different in composition. The place where the melt-kneaded products of different compositions merge may be at the lower end surface of the hollow fiber molding nozzle 20 or may not be at the lower end surface of the hollow fiber molding nozzle 20.

[0052] When the molten kneaded material is extruded from the annular nozzle, it is preferred to expel the material in a manner such that the nozzle extrusion parameter R (1 / sec) is 10 or more and 1000 or less, because high productivity and spinning stability can be obtained, and thus a high-strength film can be obtained. Here, the nozzle extrusion parameter R is the value obtained by dividing the extrusion line speed V (m / sec) by the slit width d (m) of the nozzle. The extrusion line speed V (m / sec) is the extrusion capacity (m) of the molten kneaded material per unit time. 2 / second) divided by the cross-sectional area of ​​the nozzle (m 2 ) obtained. If R is 10 or more, the filament diameter of the hollow extrudate does not have problems such as pulse-like fluctuations, and the spinning can be stably spun with good productivity. In addition, if R is 1000 or less, the elongation at break, which is one of the important strengths of the obtained porous hollow fiber membrane, can be kept sufficiently high. The elongation at break is the elongation relative to the original length when stretched along the length direction of the membrane. In the case of a porous hollow fiber membrane with a multilayer structure, the value obtained by dividing the ejection linear velocity V of the molten mixture formed by the resin confluence and stacking by the slit width d of the ejection port as the spinning port ejection parameter R is more preferably in the range of 50 and above and 1000 and below.

[0053] The hollow fiber-shaped molten kneaded material ejected from the ejection port is solidified by passing through a refrigerant such as air or water. However, depending on the porous hollow fiber membrane used as the target, after passing through the above-mentioned air moving portion S composed of an air layer, it is passed through a coagulation bath 30 containing water or the like. That is, the air moving portion S is a portion from the ejection port of the hollow fiber molding nozzle 20 to the water surface of the coagulation bath 30. As required, a container such as a cylinder may be used in the air moving portion S from the ejection port. After passing through the coagulation bath 30, it is wound up on a reel or the like as required.

[0054] The time for the melt-kneaded material to pass through the air gap portion S is called the air gap time, and the air gap time is preferably 0.05 seconds or more. If the air gap time is 0.05 seconds or more, the polymer molecules are oriented in the air gap portion, thereby further improving the compressive strength. The air gap time is more preferably 0.1 seconds or more and 2.0 seconds or less. As long as it is 2.0 seconds or less, it can be stably manufactured. Preferably, it is 0.12 seconds or more and 1.0 seconds or less.

[0055] In addition, when the difference between the ejection temperature of the ejection outlet of the molten kneaded product and the temperature in the coagulation bath 30 is ΔT and the air gap time is t, the cooling rate ΔT / t is preferably 105°C / s or more and 2100°C / s or less. It is believed that if it is 105°C / s or more, the speed of phase separation becomes faster, so the phase separation time becomes shorter, it is difficult to form an inhomogeneous dry, and the strength coefficient is improved. It is more preferably 210°C / s or more and 1750°C / s or less. In the hollow fiber after solidification, the polymer-rich phase and the organic liquid-rich phase exist finely separated. It should be noted that, for example, when inorganic fine powder is added, when the inorganic fine powder is silica fine powder, the silica fine powder exists unevenly in the organic liquid-rich phase. By extracting and removing the organic liquid and the inorganic fine powder from the hollow fiber, the organic liquid-rich phase becomes pores. Thus, a porous hollow fiber membrane can be obtained.

[0056] The extraction and removal of the organic liquid and the extraction and removal of the inorganic fine powder can be performed simultaneously if the same solvent can be used for extraction and removal. Usually, they are extracted and removed separately. For the extraction and removal of the organic liquid, a liquid is used that does not dissolve or modify the thermoplastic resin used and is suitable for mixing and extraction with the organic liquid. Specifically, it can be carried out by contacting it with a method such as immersion. In order to facilitate removal from the hollow fiber membrane after extraction, the liquid is preferably volatile. Examples of the liquid include alcohols, dichloromethane, etc. If the organic liquid is water-soluble, water can also be used as the extraction liquid.

[0057] The extraction and removal of the inorganic fine powder is usually carried out using an aqueous liquid. For example, when the inorganic fine powder is silicon dioxide, it can be carried out by first contacting it with an alkaline solution to convert the silicon dioxide into silicate, and then contacting it with water to extract and remove the silicate. It does not matter which one is performed first, the extraction and removal of the organic liquid or the extraction and removal of the inorganic fine powder. In the case where the organic liquid and water are immiscible, it is preferred to perform the extraction and removal of the organic liquid first, and then perform the extraction and removal of the inorganic fine powder. Since the organic liquid and the inorganic fine powder are usually mixed and coexist in the organic liquid concentrated phase, the extraction and removal of the inorganic fine powder can be smoothly performed, which is advantageous.

[0058] In this way, by extracting and removing the organic liquid and the inorganic fine powder from the solidified porous hollow fiber membrane, a porous hollow fiber membrane can be obtained.

[0059] Furthermore, the solidified hollow fiber membrane can be stretched along the length direction of the porous hollow fiber membrane within a stretching ratio of up to 3 times at any of the following stages: (i) before extraction and removal of the organic liquid and the inorganic fine powder; (ii) after extraction and removal of the organic liquid and before extraction and removal of the inorganic fine powder; (iii) after extraction and removal of the inorganic fine powder and before extraction and removal of the organic liquid; (iv) after extraction and removal of the organic liquid and the inorganic fine powder. Generally speaking, if the hollow fiber membrane is stretched in the longitudinal direction, the water permeability is improved, but the pressure resistance (such as burst strength and compression strength) is reduced, so it is often not possible to become a membrane with practical strength after stretching. However, the porous membrane (such as a porous hollow fiber membrane) obtained by the manufacturing method of this embodiment has high mechanical strength. Thus, it is possible to implement stretching with a stretching ratio of more than 1.1 times and within 3.0 times. By stretching, the water permeability of the porous membrane (e.g., porous hollow fiber membrane) is improved. The stretching ratio mentioned 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 extension of 20 cm, according to the following formula, the stretching ratio is 2 times. 20cm÷10cm=2

[0060] In addition, the stretching of the hollow fiber membrane is preferably performed at a space temperature of 0°C or more and 160°C or less. When the temperature is higher than 160°C, not only the stretching spot is large, but also the elongation at break is reduced and the water permeability is reduced, so it is not preferred. When the temperature is lower than 0°C, the possibility of stretching breakage is high, so it is not practical. The space temperature in the stretching process is more preferably 10°C or more and 140°C or less, and further preferably 20°C or more and 100°C or less.

[0061] In this embodiment, it is preferred to stretch the hollow fiber membrane containing an organic liquid. Compared with the hollow fiber membrane not containing an organic liquid, the hollow fiber membrane containing an organic liquid breaks less during stretching. Furthermore, in the case of the hollow fiber membrane containing an organic liquid, the shrinkage of the hollow fiber membrane after stretching can be increased, so the degree of freedom in setting the shrinkage rate after stretching is increased. In addition, it is preferred to stretch the hollow fiber membrane containing inorganic fine powder. In the case of the hollow fiber membrane containing inorganic fine powder, the hollow fiber membrane has hardness due to the presence of the inorganic fine powder contained in the hollow fiber membrane, so the hollow fiber membrane becomes difficult to be flattened during stretching. In addition, it is also possible to prevent the pore size of the finally obtained hollow fiber membrane from becoming too small or the wire diameter from becoming too small. In the present embodiment, it is more desirable to stretch a hollow fiber membrane containing both an organic liquid and an inorganic fine powder.

[0062] Based on the above reasons, it is more preferable to stretch a hollow fiber membrane containing either an organic liquid or an inorganic fine powder than to stretch a hollow fiber membrane after the extraction is completed, and it is further preferable to stretch a hollow fiber membrane containing both an organic liquid and an inorganic fine powder than to stretch a hollow fiber membrane containing either an organic liquid or an inorganic fine powder.

[0063] In addition, the method of extracting the stretched hollow fiber membrane has the advantage that the surface and internal voids of the hollow fiber membrane increase due to stretching, so the extraction solvent can easily penetrate into the hollow fiber membrane. In addition, the method of extracting after the stretching and subsequent shrinking steps, as described later, has the advantage that the hollow fiber membrane has a low tensile elastic modulus and is easy to bend, so when the extraction is carried out in a liquid flow, the hollow fiber membrane is easily shaken by the liquid flow, and the stirring effect is increased, so it has the advantage that efficient extraction can be carried out in a short time.

[0064] In the present embodiment, in the case of a process including stretching the hollow fiber membrane and then shrinking it, a hollow fiber membrane with a low tensile modulus of elasticity can be finally obtained. Here, "low tensile modulus of elasticity" means that the fiber is easily stretched under a small force and returns to its original state when the force disappears. If the tensile modulus of elasticity is low, the hollow fiber membrane will not be flattened, it is easy to bend, and it is easy to be shaken by the water flow during filtration. Since the fiber is shaken irregularly with the bending of the water flow, the layer of pollutants attached and deposited on the surface of the membrane cannot grow and is easy to peel off, so a high level of filtered water can be maintained. Further, in the case of forcibly shaking the fiber by flushing or air washing, the shaking becomes larger and the cleaning recovery effect becomes higher.

[0065] Regarding the extent of fiber length contraction when shrinking after stretching, it is desirable to make the fiber length contraction rate relative to the fiber length increment caused by stretching be in the range of 0.3 or more and 0.9 or less. For example, when a 10 cm fiber is stretched to 20 cm and then shrunk back to 14 cm, the fiber length contraction rate is 0.6 according to the following formula. Fiber length shrinkage rate = {(maximum fiber length when stretched) - (fiber length after shrinkage)} / [(maximum fiber length when stretched) - (fiber initial length)] = (20-14) / (20-10) = 0.6 When the fiber length shrinkage is 0.9 or more, the water permeability tends to decrease; when it is less than 0.3, the tensile elastic modulus tends to increase, which is not preferred. In this embodiment, the fiber length shrinkage is more preferably in the range of 0.50 or more and 0.85 or less.

[0066] Furthermore, by adopting the step of stretching the hollow fiber membrane to the maximum fiber length during stretching and then shrinking it, the finally obtained hollow fiber membrane will not break even when stretched to the maximum fiber length during stretching during use. Here, when the stretching ratio is X and the fiber length contraction rate with respect to the increase in fiber length due to the stretching is Y, the rate Z indicating the degree of guarantee of the elongation at break can be defined by the following formula. Z = (maximum fiber length during stretching - fiber length after contraction) / fiber length after contraction = (XY-Y) / (X+Y-XY) Among them, Z is preferably 0.2 or more and 1.5 or less, and more preferably 0.3 or more and 1.0 or less. If Z is too small, the degree of guarantee of elongation at break is reduced, and if Z is too large, the water permeability becomes lower compared with the high possibility of breakage during stretching.

[0067] In addition, the production method of the present embodiment includes the steps of stretching and then shrinking, so that the tensile elongation at break is extremely rare at low elongation, and the distribution of the tensile elongation at break can be narrowed.

[0068] From the viewpoint of shrinkage time and physical properties, the space temperature in the process of stretching and then shrinking is preferably in the range of 0° C. or more and 160° C. or less. If it is lower than 0° C., shrinkage takes time and is not practical, while if it exceeds 160° C., elongation at break decreases and water permeability decreases, which is not preferred. In this embodiment, the hollow fiber membrane is preferably curled in the shrinking step, so that a hollow fiber membrane with a high degree of curling can be obtained without being crushed or damaged.

[0069] Usually, the hollow fiber membrane is in the form of a straight tube without bending. Therefore, when it is bundled into a filtering component, the gap between the hollow fibers is eliminated, and the possibility of forming a fiber bundle with low porosity is high. In contrast, if a hollow fiber membrane with high curl is used, the hollow fiber membrane interval is evenly expanded due to the bending of each fiber, and a fiber bundle with high porosity can be formed. In addition, the filter component composed of hollow fiber membranes with low curl, especially when used under external pressure, the gap of the fiber bundle becomes less, the flow resistance increases, and the filtering pressure cannot be effectively transmitted to the central part of the fiber bundle. Furthermore, when the filtered sediment is peeled off from the hollow fiber membrane by backwashing or flushing, the cleaning effect inside the fiber bundle also deteriorates. The fiber bundle composed of hollow fiber membranes with high curl has a large porosity, and the gap between the hollow fiber membranes can be maintained even if external pressure filtration is performed, and it is not easy to have bias flow. Therefore, the hollow fiber membrane obtained by the manufacturing method of this embodiment preferably has a curling degree in the range of 1.5 or more and 2.5 or less. When it is 1.5 or more, it is preferred for the above reasons, and when it is 2.5 or less, the reduction of the filtration area per unit volume can be suppressed.

[0070] It should be noted that as a method for curling the above-mentioned hollow fiber membrane, there can be cited a process in which, in the process of stretching and then shrinking it, the hollow fiber membrane is placed between, for example, a pair of gear rollers with periodic grooves or a pair of sponge-like belts with grooves, and they are taken out from them during shrinkage.

[0071] In addition, in the manufacturing method of the present embodiment, it is preferred to use a removal device composed of a pair of opposite infinite track type belts for stretching. In this case, a removal device is used on the upstream side and the downstream side of the stretching, and in each removal device, the hollow fiber membrane is placed between the opposite belts, and the fiber is carried by moving the two belts in the same direction at the same speed. In addition, in this case, it is preferred to stretch the fiber carrying speed on the downstream side higher than the fiber carrying speed on the upstream side. If stretching is performed in this way, no tensile tension is generated during stretching, no sliding occurs, and the fiber can be prevented from being flattened.

[0072] Here, the inner side of the endless track type belt in contact with the driving roller is preferably made of a high elastic belt such as a fiber reinforced belt, and the outer surface in contact with the hollow fiber membrane is made of an elastomer. In addition, the compressive elastic modulus of the elastomer along its thickness direction is 0.1MPa or more and 2MPa or less, and more preferably the thickness of the elastomer is 2mm or more and 20mm or less. From the viewpoint of chemical resistance and heat resistance, it is particularly preferred that the elastomer on the outer surface is silicone rubber.

[0073] Furthermore, the manufacturing method of this embodiment includes a multi-stage heat treatment process, which is to heat treat the thermoplastic resin containing polyvinylidene fluoride resin at a melting point of -50°C to -40°C after stretching the hollow fiber membrane, and then heat treat the thermoplastic resin at a melting point of -35°C to -25°C. This is because heat treatment of the hollow fiber membrane after the stretching can change the membrane's sheet structure, thereby improving its crystal structure and mechanical properties. The heat treatment temperature refers to the temperature in the heat treatment device, preferably between -50°C and -10°C, the melting point of the thermoplastic resin. More specifically, the heat treatment temperature is preferably 100° C. or more and 160° C. or less, and more preferably 120° C. or more and 150° C. or less.

[0074] The heat treatment time is preferably 1 hour or longer, more preferably 3 hours or longer. Careful heat treatment for a long time causes the crystallites that melt at low temperatures to disappear, thereby forming a hollow fiber membrane with a thick sheet structure (large crystal size) and a stable crystal structure. As for the temperature profile, a single-stage type with a single temperature condition may be used, but a multi-stage type is preferably used in which heat treatment is first performed at the low temperature side within the above temperature range and then at the high temperature side. The hollow fiber membrane of the present invention has the characteristic of being easy to recrystallize from low temperature, so it is believed that the multi-stage method of gradually increasing the temperature from low temperature has the effect of further promoting the recrystallization of the microcrystalline part. It should be noted that the heat treatment method can be intermittent or continuous. In addition, in the preparation method of the present application, although the inorganic fine powder is extracted and removed as described above, the presence of trace residues promotes the recrystallization of the microcrystalline part. Although the specific mechanism is unclear, it is speculated that the residual inorganic fine powder can serve as a starting point for promoting recrystallization.

[0075] Furthermore, from the viewpoint of reducing changes in fiber diameter, porosity, pore size, and water permeability, it is desirable to heat-treat the hollow fiber membrane after the extraction. Example

[0076] 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.

[0077] In the examples, a molten stock solution was first prepared, and then a porous hollow fiber membrane was produced, and the membrane properties were evaluated. The production conditions are shown below.

[0078] [Example 1] Vinylidene fluoride homopolymer (Kynar 720 manufactured by ARKEMA) was used as the thermoplastic resin. The pelletized Kynar 720 was low-temperature pulverized using a Linrex mill (manufactured by HOSOKAWAMICRON) and then classified using a vibrating sieve machine to remove particles with a sieve opening of 355 μm or more, thereby obtaining particles with a median particle size (d50) of 96 μm. Then, 23.0 mass % of hydrophobic silica (AEROSIL-R972 manufactured by Japan AEROSIL Co., Ltd.), 31.3 mass % of di(2-ethylhexyl) phthalate (DEHP) (manufactured by CGEstar Co., Ltd.), 5.7 mass % of dibutyl phthalate (DBP) (CGEstar Co., Ltd.) and 40.0 mass % of the above-mentioned vinylidene fluoride homopolymer were mixed, and the resulting mixture was melt-kneaded in an extruder, and melt-extruded at 240°C while supplying air as a fluid for forming the hollow portion through a hollow fiber molding nozzle (outer diameter 1.72 mm, inner diameter 0.92 mm) installed at the top of the extruder. The hollow fiber molten mixture extruded at 240°C moves in the air for 0.24 seconds, then is cooled and solidified in a coagulation bath added with 25°C water, pulled by a first belt puller at a speed of 37m / min, passes through a first heating tank (0.8m long) with a space temperature controlled at 40°C, is stretched 2 times by a second belt puller at a speed of 74m / min, passes through a second heating tank (0.8m long) with a space temperature controlled at 140°C, is shrunk 1.5 times by a third belt puller at a speed of 55m / min, and is then wound on a reel. The obtained hollow fiber was immersed in dichloromethane at 30° C. for 1 hour or longer to extract and remove di(2-ethylhexyl) phthalate and dibutyl phthalate, and then dried. Then, the material was immersed in a 50 mass % ethanol aqueous solution for 30 minutes, immersed in water for 30 minutes, immersed in a 20% sodium hydroxide solution at 70° C. for 1 hour, and then repeatedly washed with water to extract and remove the hydrophobic silica. Then, the membrane was placed in a dryer and heat treated at a set temperature of 125°C for 3 hours. The temperature was then raised to 140°C and heat treated for another 5 hours to obtain a porous hollow fiber membrane sample.

[0079] [Example 2] A sample of a porous hollow fiber membrane was produced in the same manner as in Example 1 except that the hollow fiber molding nozzle had an outer diameter of 2.00 mm and an inner diameter of 0.90 mm.

[0080] [Example 3] A sample of a porous hollow fiber membrane was produced in the same manner as in Example 1 except that a vinylidene fluoride homopolymer (Kynar 740 manufactured by ARKEMA) was used as the thermoplastic resin. Kynar 740 was pulverized at low temperature and the median particle size (d50) of the classified particles was 93 μm.

[0081] [Example 4] A sample of a porous hollow fiber membrane was produced in the same manner as in Example 3 except that the hollow fiber molding nozzle had an outer diameter of 2.00 mm and an inner diameter of 0.90 mm.

[0082] [Example 5] A sample of a porous hollow fiber membrane was produced in the same manner as in Example 1 except that the particles pulverized and classified in Examples 1 and 3 were mixed at a ratio of Kynar 720:Kynar 740=1:1 (mass ratio) as the thermoplastic resin.

[0083] [Example 6] A sample of a porous hollow fiber membrane was produced in the same manner as in Example 1 except that the particles pulverized and classified in Examples 1 and 3 were mixed at a ratio of Kynar 720:Kynar 740=1:3 (mass ratio) as the thermoplastic resin.

[0084] [Example 7] A porous hollow fiber membrane sample was produced in the same manner as in Example 1 except that a vinylidene fluoride homopolymer (KynarFlex2850 manufactured by ARKEMA) was used as the thermoplastic resin. KynarFlex2850 was pulverized at low temperature and the median particle size (d50) of the classified particles was 90 μm.

[0085] [Example 8] A sample of a porous hollow fiber membrane was produced in the same manner as in Example 1 except that the particles pulverized and classified in Examples 1 and 7 were mixed at a ratio of Kynar 720: Kynar Flex 2850 = 3:2 (mass ratio) as the thermoplastic resin.

[0086] [Comparative Example 1] As the thermoplastic resin, a vinylidene fluoride homopolymer (SOLEF6010 manufactured by Solvay) was used. A porous hollow fiber membrane sample was produced in the same manner as in Example 1 except that SOLEF6010 was a powder and was not pulverized and classified before use.

[0087] [Comparative Example 2] As the thermoplastic resin, a vinylidene fluoride homopolymer (KF W#1000 manufactured by Kureha Corporation) was used. A porous hollow fiber membrane sample was produced in the same manner as in Example 1 except that KF W#1000 was used without pulverization and classification because it was a powder.

[0088] [Comparative Example 3] As the thermoplastic resin, a vinylidene fluoride homopolymer (Kynar 761 manufactured by ARKEMA) was used. A porous hollow fiber membrane sample was produced in the same manner as in Example 1 except that Kynar 761 was a powder and was not pulverized and classified before use. It should be noted that, since the molecular weight of the obtained polymer was high, the compressive strength of the obtained porous hollow fiber membrane sample was improved, but the pure water permeability and tensile elongation at break suitable for filtration applications were not obtained.

[0089] <Evaluation> The following evaluations were performed on each sample of the obtained porous hollow fiber membrane. Unless otherwise specified, the measurement was performed at 25°C. The measurement results and evaluation results are shown in Table 1.

[0090] (1) Determination of inner diameter, outer diameter and film thickness The hollow fiber membrane was thinly sliced ​​at intervals of 15 cm in a direction perpendicular to the length direction of the membrane using a razor or the like, and the major and minor diameters of the inner diameter and the major and minor diameters of the outer diameter of the cross section were measured using a microscope, and calculated using the following formula. This measurement was performed 10 times, and the average value was used as the inner diameter, outer diameter, and membrane thickness under this condition. Inner diameter (mm) = (inner long diameter + inner short diameter) / 2 ·Outer diameter (mm) = (outer major diameter + outer minor diameter) / 2 ·Film thickness (mm)=(outer diameter-inner diameter) / 2

[0091] (2) Pure water permeability After the hollow fiber membrane is immersed in a 50 mass % ethanol aqueous solution for 30 minutes, it is then wetted by replacing it with pure water. One end of the wet hollow fiber membrane of about 100 mm in length is sealed, and an injection needle is inserted into the hollow part of the other end. Pure water at 25°C is injected into the hollow part from the injection needle at a pressure of 0.1 MPa, and the amount of pure water permeating through the outer surface is measured, and the pure water permeation is calculated by the following formula. The effective length of the membrane refers to the net membrane length excluding the part where the injection needle is inserted. This measurement was performed 10 times, and the average value was taken as the pure water permeation under this condition. ·Pure water permeability (L / m 2 / h) = permeate volume / (π×membrane inner diameter×membrane effective length×measurement time) ※Permeate volume (L), membrane inner diameter (m), membrane effective length (m), measurement time (h)

[0092] (3) Tensile elongation at break The tensile load and the displacement at break were measured under the following conditions. The measurement was carried out in accordance with JIS K7161, and a hollow fiber membrane was used as a sample as it was. Measuring instrument: AGS-X (50N) desktop precision universal testing machine manufactured by Shimadzu Corporation Distance between chucks: 50mm ·Stretching speed: 200mm / min The tensile elongation at break was calculated from the obtained results in accordance with JIS K 7161. This measurement was performed 10 times, and the average value was taken as the tensile elongation at break under the conditions.

[0093] (4) Compression strength One end of a wet hollow fiber membrane of about 5 cm in length is sealed, and the other end is opened to the atmosphere. Pure water at 40°C is pressurized from the outer surface, and permeated water flows out from the end open to the atmosphere. At this time, a method of filtering the entire amount of membrane supply water without circulating it, i.e., a full-volume filtration method, is adopted. The pressurization pressure is increased from 0.1 MPa to 0.05 MPa, and maintained at each pressure for 30 seconds, and the permeated water flowing out from the end open to the atmosphere is collected during this period. When the hollow part of the hollow fiber membrane is not destroyed, the absolute value of the permeated water amount (mass) increases with the increase of the pressurization pressure, but when the pressurization pressure exceeds the compressive strength of the hollow fiber membrane, the hollow part is destroyed and begins to be clogged, so the absolute value of the permeated water amount decreases in contrast to the increase in the pressurization pressure. The pressurization pressure when the absolute value of the permeated water amount reaches the maximum is taken as the compressive strength. This measurement was performed 10 times, and the average value was taken as the compressive strength under this condition.

[0094] (5) Chemical resistance test A hollow fiber membrane having a length of about 100 mm was immersed in a 50% by mass ethanol aqueous solution for 30 minutes and then substituted with pure water to make it wet. Next, an aqueous solution containing 4.0% by mass of sodium hydroxide and 2.0% by mass of sodium hypochlorite in terms of effective chlorine concentration was prepared. The hollow fiber membrane was then immersed in an aqueous solution at 25° C. for 17 days, and then subjected to a tensile test to determine the retention rate (%) of the tensile elongation at break before and after the immersion. It should be noted that the tensile test was conducted with n 10, and the average value was calculated. Retention rate (%) = (after immersion / before immersion) × 100

[0095] (6) Temperature Modulated Differential Scanning Calorimetry (MDSC) 5 mg of the hollow fiber membrane was cut as a sample and subjected to MDSC measurement under the following conditions. The crystal melting temperature, crystal melting enthalpy, crystallinity, etc. were obtained by total heat flow analysis. The results of total heat flow analysis are shown in Figure 3 . The main peak of crystal melting is the peak with the widest heat absorption area during the heating process. After further heating, the heat absorption value returns to the baseline, and then the temperature is raised again, the case where a small peak appears after the main peak (○), and the case where no peak appears is (×). For example, in the case of Example 1, the crystal melting peak temperature of the main peak is 170.0°C, and the temperature of the peak that appears after the main peak is 181.8°C. In addition, the crystallization onset temperature was obtained by irreversible heat flow analysis. The results of irreversible heat flow analysis are shown in Figure 4 During the temperature increase process, the inflection point where the irreversible heat flow significantly rises from the baseline is taken as the crystallization start temperature. For example, in the case of Example 1, 139.6° C. is the crystallization start temperature. ·Device: Discovery DSC2500 (manufactured by TA Instruments) Atmosphere: Nitrogen 50mL / min Sample plate: Tzero Aluminum ·Measurement mode: Modulated Heat Only Temperature range: -20℃→200℃ Heating rate: 1℃ / min Amplitude: 0.2℃ Cycle: 60s It should be noted that the degree of crystallinity = ΔH obs / ΔH ° (ΔH ° =105 J / g), ΔH obs is the observed crystal melting enthalpy under full heat flux, ΔH ° is the equilibrium melting enthalpy.

[0096] (7) Heterogeneous binding rate 0.6 mL of deuterated DMF was added to 30 mg of hollow fiber membrane, and the mixture was heated to 50°C to dissolve. Then, the mixture was subjected to the following conditions: 1 In H-NMR measurement, the heterologous bonding rate was determined by the following formula from the integrated values ​​of the signals derived from the HT (head-tail) and HH (head-head) bonds of PVDF. Device: JEOL ECS400 Pulse width: 45° Waiting time: 3 seconds Number of points: 512 Chemical shift standard: The signal of CHO in DMF was set at 8.02 ppm. *Heterogeneous binding rate (%) = HH / (HH+HT)

[0097] (8) Weight average molecular weight (Mw) The hollow fiber membrane was dissolved in DMF at a concentration of 1.0 mg / mL and subjected to GPC measurement under the following conditions to determine the weight average molecular weight (PMMA equivalent). Instrument: HLC-8420GPC (Tosoh Corporation) ·Column: TSKgel guradcolumn SuperAW-HT SKgel AWM-H (6.0mmID×15cm) 2 pieces ·Column temperature: 40℃ ·Eluent: DMF containing 5mM LiBr Flow rate: 0.6mL / min Injection volume: 30μL

[0098]

Table 1

[0099] According to Table 1 Figure 3 and Figure 4 It can be seen from the results in that the samples of the examples are more excellent in all evaluation items in a balanced manner than the samples of the comparative examples. Each of the samples of the comparative examples showed inferior results in at least one evaluation item compared with the samples of the examples. Industrial Applicability

[0100] According to the present invention, there can be provided a porous hollow fiber membrane produced by the TIPS method, having barrier performance and high water permeability and having high chemical resistance capable of long-term continuous operation, and a method for producing the porous hollow fiber membrane. Explanation of symbols

[0101] 10 Extruder 20 Nozzles for hollow fiber molding 30 coagulation bath 40 Porous hollow fiber membrane More than 50 rollers

Claims

1. A porous hollow fiber membrane formed of a thermoplastic resin, wherein the crystallization starting temperature is 140°C or lower, and the crystal melting enthalpy at or below the crystallization starting temperature is 10 J / g or lower. 2 . The porous hollow fiber membrane according to claim 1 , wherein the crystal melting enthalpy of the porous hollow fiber membrane as a whole is lower than 58 J / g. 3 . The porous hollow fiber membrane according to claim 1 , wherein the crystal melting enthalpy of a peak appearing on the higher temperature side of a main peak of crystal melting of the porous hollow fiber membrane is 0.1 J / g or more.

4. The porous hollow fiber membrane according to claim 1 or 2, wherein a peak temperature appearing on the higher temperature side of the main peak of crystal melting of the porous hollow fiber membrane is 10°C or more higher than the main peak temperature of crystal melting. 5 . The porous hollow fiber membrane according to claim 1 , wherein the porous hollow fiber membrane has a degree of crystallinity of less than 60%. 6 . The porous hollow fiber membrane according to claim 2 , wherein the crystal melting enthalpy at or below the crystallization starting temperature is 15% or less of the crystal melting enthalpy of the entire porous hollow fiber membrane. 7 . The porous hollow fiber membrane according to claim 1 , wherein the weight average molecular weight (Mw) of the thermoplastic resin is 400 kDa or less. 8 . The porous hollow fiber membrane according to claim 1 , wherein the thermoplastic resin contains a polyvinylidene fluoride resin. 9 . The porous hollow fiber membrane according to claim 1 , wherein the inner diameter of the porous hollow fiber membrane is less than 0.75 mm and the compressive strength is 0.3 MPa or more. 10 . The porous hollow fiber membrane according to claim 1 or 2 , wherein the heterogeneous bonding rate of the porous hollow fiber membrane is 9% or more.

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