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

Abstract

To provide a porous hollow fiber membrane that exhibits blocking performance and water permeability performance suitable for filtration applications, as well as having excellent chemical resistance.SOLUTION: In order to provide a solution to the problem to be solved, the porous hollow fiber membrane of the present invention is made of a thermoplastic resin, and characterized in that a crystallization onset temperature is 140°C or lower, and enthalpy of crystal fusion at and below the crystallization onset temperature is 10 J / g or less.SELECTED DRAWING: None

Description

Technical Field

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

Background Art

[0002] Water treatment by membrane separation technology has been widely used in various industrial fields such as pharmaceuticals, medicine, food, semiconductors, and water and sewage. In recent years, applications in the field of water treatment for producing drinking water and industrial water by removing turbidity from river water, etc., and in the field of sewage treatment for removing turbidity and purifying domestic sewage and industrial wastewater, etc. have also become active. As filter media for membrane separation, there are hollow fiber membranes formed by forming a polymer resin with excellent processability into a hollow tubular shape, flat membranes formed into a sheet shape, etc., and membrane modules formed by aggregating these are used.

[0003] Among these, in particular, porous hollow fiber membranes used for removing turbidity from river water and seawater are adopted in an external pressure filtration method from the viewpoints of treating a large amount of water, having high water permeability performance in addition to blocking performance, and further increasing the filtration area. Therefore, a compressive strength is required so that the hollow fiber membrane is not crushed by compression from the outside during the filtration operation. In addition, when the hollow fiber membrane is continuously used for water treatment, the membrane surface is blocked by the substances separated by filtration, and the water permeability decreases. Therefore, after a certain time or after filtering a certain amount of water, etc., the hollow fiber membrane is regularly cleaned, and an operation is performed to remove organic substances and the like accumulated on the membrane surface with chemicals.

[0004] However, such cleaning with chemicals deteriorates not only the accumulated organic substances but also the polymer constituting the hollow fiber membrane. Therefore, if chemical cleaning is repeated, the hollow fiber membrane gradually deteriorates, and the hollow fiber membrane module cannot be continuously operated for a long time. Therefore, the hollow fiber membrane also requires high chemical resistance.

[0005] For example, Patent Document 1 and Patent Document 2 propose hollow fiber membranes that achieve both high water permeability performance and membrane strength in addition to blocking performance. However, even if the film strength at the initial stage of operation is high, it is difficult to say that it can sufficiently withstand repeated chemical cleaning.

[0006] On the other hand, as a method for manufacturing a film, a thermally induced phase separation method (TIPS method) is known. In this manufacturing method, a thermoplastic resin and an organic liquid are used. As the organic liquid, a solvent that does not dissolve the thermoplastic resin at room temperature but dissolves it at high temperature, that is, a potential solvent, is used. The thermoplastic resin and the organic liquid are kneaded at a high temperature to dissolve the thermoplastic resin in the organic liquid, and then cooled to room temperature to induce phase separation, and further the organic liquid is removed to produce a porous body. Since this manufacturing method dissolves at a high temperature and then quickly cools and solidifies to form a film, especially when the thermoplastic resin is a crystalline resin, crystallization is promoted and there is a feature that a high-strength film is easily obtained, and it is frequently used as a method for manufacturing a porous membrane.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

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 and a method for manufacturing the porous hollow fiber membrane, which have excellent blocking performance and water permeability suitable for filtration applications, and excellent chemical resistance.

Means for Solving the Problems

[0009] As a result of intensive studies to solve the above problems, the present inventors have completed the present invention. That is, the present invention is as follows. [1] A porous hollow fiber membrane made of a thermoplastic resin, A hollow fiber membrane characterized in that the crystallization start temperature is 140 ° C or lower and the crystal melting enthalpy at a temperature equal to or lower than the crystallization start temperature is 10 J / g or lower. [2] The hollow fiber membrane according to [1], characterized in that the crystal melting enthalpy of the entire porous hollow fiber membrane is less than 58 J / g. [3] The hollow fiber membrane according to [1] or [2], characterized in that the crystal melting enthalpy of the peak appearing on the higher temperature side than the crystal melting main peak 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], characterized in that the peak temperature appearing on the higher temperature side than the crystal melting main peak of the porous hollow fiber membrane is 10 ° C or more higher than the crystal melting main peak temperature. [5] The hollow fiber membrane according to any one of [1] to [4], characterized in that the crystallinity of the porous hollow fiber membrane is less than 60%. [6] The hollow fiber membrane according to any one of [1] to [5], characterized in that the crystal melting enthalpy at a temperature equal to or lower than the crystallization start 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], characterized in that 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], characterized in that the thermoplastic resin contains a polyvinylidene fluoride-based resin. [9] The hollow fiber membrane according to any one of [1] to [8], characterized in that 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], characterized in that the hetero-bonding ratio of the porous hollow fiber membrane is 9% or more.

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

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

[12] The manufacturing method according to

[11] , wherein the porous hollow fiber membrane is manufactured by melting and kneading a mixture composed of the above-mentioned thermoplastic resin, organic liquid, and inorganic fine powder, extruding and forming it into hollow fibers, and then extracting the organic liquid and inorganic fine powder.

Advantages of the Invention

[0010] According to the present invention, there can be provided a porous hollow fiber membrane and a manufacturing method thereof, which are manufactured by the TIPS method, have high water permeability in addition to blocking performance, and have high chemical resistance enabling continuous operation over a long period.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments for carrying out the present invention will be described in detail. Note that the present invention is not limited to the following embodiments, and various modifications can be made 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 made of a thermoplastic resin. Here, the thermoplastic resin preferably contains a fluororesin and may consist only of a fluororesin.

[0014] As the fluororesin, it is preferably at least one selected from the group consisting of vinylidene fluoride resin (PVDF), chlorotrifluoroethylene resin, tetrafluoroethylene resin, ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-monochlorotrifluoroethylene copolymer (ECTFE), hexafluoropropylene resin, and mixtures of these resins, and it may consist of only at least one selected from the group consisting of vinylidene fluoride resin (PVDF), chlorotrifluoroethylene resin, tetrafluoroethylene resin, ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-monochlorotrifluoroethylene copolymer (ECTFE), hexafluoropropylene resin, and mixtures of these resins. Among these, it is preferable that the thermoplastic resin contains at least one of vinylidene fluoride-based resins and chlorotrifluoroethylene-based resins, and more preferably contains at least a vinylidene fluoride-based resin.

[0015] The vinylidene fluoride-based resin means containing a homopolymer of vinylidene fluoride and / or a vinylidene fluoride copolymer. The vinylidene fluoride copolymer is a polymer having a residue structure of vinylidene fluoride, and is typically a copolymer of a vinylidene fluoride monomer and other fluorine-based monomers, etc., and known ones can be appropriately selected and used. Also, a plurality of vinylidene fluoride copolymers may be contained. From the viewpoint of excellent strength, the vinylidene fluoride-based resin is preferably a homopolymer, and when it is a copolymer, it preferably contains vinylidene fluoride in a molar ratio of 50% or more from the same viewpoint.

[0016] Note that the thermoplastic resin may be only one kind or a combination of a plurality of kinds.

[0017]

[0018] ​In addition, the crystallization onset temperature of the porous hollow fiber membrane of the present invention is 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. In addition, for the porous hollow fiber membrane of the present invention, it is preferable that the crystallization melting enthalpy at a temperature equal to or lower than the crystallization onset temperature is small, and specifically, it is required to be 10 J / g or lower.

[0019] Here, the crystallization onset temperature, crystallization melting enthalpy, etc. can be analyzed by temperature-modulated DSC (MDSC). MDSC measurement is a method of performing DSC measurement while adding periodic temperature increase and decrease to the average temperature increase rate, which is different from the normal DSC measurement at a constant heating rate. When performing MDSC measurement, the heat flow obtained from normal DSC can be separated into a Reversing Heat Flow component that can follow the periodic temperature increase and decrease and a Non-Reversing Heat Flow component that cannot follow the periodic temperature increase and decrease. The crystallization melting heat quantity is observed in the Reversing Heat Flow, and crystallization is observed only in the Non-Reversing Heat Flow. Therefore, when separating and evaluating crystallization melting and crystallization, performing MDSC measurement is one of the preferred embodiments. The crystallization onset temperature is determined from the Non-Reversing Heat Flow described later.

[0020] The porous hollow fiber membrane of the present invention is a hollow fiber membrane in which the amount of microcrystals that melt at a temperature equal to or lower than the crystallization onset temperature is small, and the polymer has the characteristic of being easily recrystallized. The imperfection of polymer crystals is caused by the crystal size (the thickness of lamellar crystals). When the crystal size is large, the melting point rises, and when it is small, the melting point drops. Here, the crystallization melting enthalpy at a temperature equal to or lower than the crystallization onset temperature is a microcrystalline site that melts at a low temperature. By controlling this microcrystal to be as small as possible by heat treatment, a stable structure can be obtained against heat and chemicals. Even if the molecular weight and crystallinity are low, a hollow fiber membrane with excellent durability can be obtained. Therefore, it is required to be 10 J / g or lower. Generally, the crystal size of a polymer has a distribution depending on the manufacturing method such as the polymerization method. When manufacturing hollow fiber membranes under the same conditions and comparing them, they are often affected by the properties of the polymer itself. Usually, those with a wide distribution of polymer crystal sizes are affected by the small crystal sites with small sizes, resulting in a large decrease and variation in strength etc. after chemical immersion, and the membrane performance tends to deteriorate. The porous hollow fiber membrane of the present invention uses a polymer with a wide distribution of crystal sizes, and by performing heat treatment on the hollow fiber membrane manufactured using the polymer, microcrystals (i.e., low melting points) with small crystal sizes are converted (recrystallized) into crystals with large sizes. As a result, it has been found that the chemical resistance is improved by reducing the proportion of microcrystals. However, in the following two cases, it is presumed that excellent chemical resistance cannot be exhibited. 1) Even below the heat treatment temperature, if there are many crystal sizes having a melting point close to the heat treatment temperature, it takes time for the crystals to melt, and the conversion to large-sized crystals is not significantly manifested. 2) If there are many crystal sizes having a melting point above the heat treatment temperature and the distribution of crystal sizes is narrow, they cannot be converted into large-sized crystals by heat treatment. Also, in the porous membrane of the present embodiment, the fact that the crystallization start temperature exists below the heat treatment temperature (in this application, 140°C or lower) is evidence that small-sized crystals remained in the porous hollow fiber membrane before heat treatment. By heat treatment, large-sized crystals can be made to exist based on the above principle. On the other hand, in the case of a crystallization temperature higher than the heat treatment temperature, it means that there are no small-sized crystals below the heat treatment temperature in the first place, and large-sized crystals cannot be generated. Therefore, for the porous hollow fiber membrane of the present invention, if the crystallization start temperature is 140°C or lower, recrystallization is easily promoted, and the crystal melting enthalpy below the crystallization start temperature can be made 10 J / g or lower.

[0021] In addition, since it is not preferable for the crystallization start temperature to remain below 100°C because the crystal structure changes under actual use conditions, it is preferably 120°C or higher.

[0022] The crystallization 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 reduction of microcrystals, the crystallization melting enthalpy of that part decreases, so even if the crystal ratio is low, it is possible to ensure a predetermined durability.

[0023] Also, the crystallization melting enthalpy below the crystallization start temperature is preferably smaller than the overall crystallization melting enthalpy, preferably 20% or less, and more preferably 15% or less.

[0024] Furthermore, the porous hollow fiber membrane of the present invention has the characteristic of being easily recrystallized, and when the temperature is raised above the recrystallization temperature, a crystal phase with a melting point higher than the main crystallization melting peak is formed. The crystallization melting enthalpy of this crystal phase is preferably larger for structural stabilization, and preferably appears on the higher temperature side than the main crystallization melting peak. Specifically, it is preferably 0.1 J / g or more and 10 °C or more.

[0025] Here, the porous hollow fiber membrane that is easily recrystallized refers to a membrane having a wide distribution of crystal sizes and many microcrystalline sites with small crystal sizes. It is considered that by the heat treatment, the microcrystalline sites with a low melting point are recrystallized, the crystal size becomes larger, the melting point increases, and the membrane structure is stabilized.

[0026] Also, the crystallinity of the porous hollow fiber membrane of the present invention is preferably 30% or more and less than 70%. Since the porous hollow fiber membrane of the present invention has few microcrystals and a lamellar thickness, by setting the crystallinity to 30% or more, it is possible to suppress a decrease in membrane strength against chemical erosion. The crystallinity is more preferably 35% or more, and even more preferably 40% or more. On the one hand, by setting the crystallinity to less than 70%, the membrane does not become too brittle and can be made less susceptible to damage from deformation due to pressure during filtration. The crystallinity is more preferably less than 65% and even more preferably less than 60%.

[0027] Also, the weight average molecular weight (Mw) of the porous hollow fiber membrane of the present invention is preferably 100,000 or more and 500,000 or less, and more preferably 200,000 or more and 400,000 or less. If Mw is less than 100,000, the mechanical strength of the resulting membrane becomes small. Also, in the case of a high molecular weight polymer exceeding 500,000, the tensile elongation at break of the membrane becomes low and the viscosity when melted in an organic liquid becomes high, so that phase separation takes time and the water permeation performance tends to be low. Furthermore, high molecular weight polymers have poor solubility, so it becomes difficult to uniformly melt in an organic liquid, and there is a problem that the quality becomes unstable.

[0028] Also, the porous hollow fiber membrane of the present invention preferably contains a heterogeneous sequence at a certain ratio in order to obtain a hollow fiber membrane with high chemical resistance. Specifically, 1 in the 1H-NMR measurement the ratio of the heterogeneous sequence in the molecule is preferably 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. If it is 0.4 mm or more, the pressure loss of the liquid flowing inside the hollow fiber membrane does not become too large, and if it is less than 1 mm, sufficient compressive strength and bursting strength can be easily exhibited even with a relatively thin membrane thickness. More preferably, it is 0.5 mm or more and less than 0.8 mm.

[0030] Also, the membrane thickness is preferably 0.1 mm or more and 1.0 mm or less. If it is 0.1 mm or more, sufficient compressive strength and bursting strength can be easily exhibited, and if it is 1.0 mm or less, the filtration resistance is small, so that sufficient water permeation performance can be obtained in practical use. More preferably, it is 0.15 mm or more and 0.25 mm or less.

[0031] Furthermore, when the porous hollow fiber membrane of the present invention permeates pure water at 25°C under a filtration pressure of 0.1 MPa, the pure water permeation rate per unit membrane area based on the inner surface of the hollow fiber membrane is 1000 L / m 2 / h or more is preferable. The pure water used herein is distilled water or water filtered through an ultrafiltration membrane or a reverse osmosis membrane having a molecular weight cut-off of 10,000 or less. When the pure water permeation rate is low, the number of membrane modules required to treat a predetermined amount within a certain time increases, and the space occupied by the filtration equipment becomes large. To avoid this, it is possible to treat a predetermined amount within a certain time by setting a high filtration pressure. However, in this case, the membrane module is required to have high pressure resistance, and the energy cost required for filtration also increases, resulting in deteriorated productivity. From such a viewpoint, it is preferable that the pure water permeation rate is high. Specifically, the pure water permeation rate is preferably 1000 L / m 2 / h or more, more preferably 2000 L / m 2 / h or more, and even more preferably 3000 L / m2 / 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 lower than 30%, when the membrane is forcibly shaken during membrane module cleaning such as flushing or air scrubbing, the risk of membrane breakage increases. If it is higher than 300%, the compressive strength and burst strength tend to be weak. More preferably, it is 40% or more and 220% or less.

[0033] In terms of chemical resistance, not only the absolute value of the tensile elongation at break but also a high retention rate with respect to the initial value is important. The retention rate is preferably 60% or more, more preferably 70% or more, assuming long-term use.

[0034] ​The porous hollow fiber membrane of the present invention is mainly used in an external pressure filtration method from the viewpoint of increasing the filtration area. Therefore, strength against the external pressure direction is required so that the hollow fiber membrane does not collapse during the filtration operation, and it is preferable that the compressive strength is 0.3 MPa or more. If the compressive strength is 0.3 MPa or more, it is possible to continuously maintain its shape in water treatment applications where the operating pressure is applied for a long time.

[0035] The porous hollow fiber membrane of the present invention is porous and desirably has a three-dimensional network structure. In the present invention, the three-dimensional network structure is schematically the structure shown in FIG. 1. For example, the thermoplastic resin a is joined to form a network, and the void portion b is formed. In the three-dimensional network structure, so-called lumps of resin having a spherulite structure are hardly seen. The void portion b of the three-dimensional network structure is surrounded by the thermoplastic resin a, and it is preferable that each part of the void portion b communicates with each other. Since most of the thermoplastic resin used forms a three-dimensional network structure that can contribute to the strength of the hollow fiber membrane, it is possible to form a support layer with high strength. Also, the chemical resistance is improved. The reason for the improvement in chemical resistance is not clear, but it is thought that it may be because the amount of the thermoplastic resin that forms a network that can contribute to the strength is large, so even if a part of the network is invaded by a chemical, it does not have a great impact on the strength of the entire layer.

[0036] Further, the hollow fiber membrane may have a single-layer structure or a multi-layer structure of two or more layers. Let the layer having the surface on the filtrate side be layer (A), and the layer having the surface on the filtrate side be layer (B). For example, layer (A) may be a so-called blocking layer that exhibits a function of blocking the membrane permeation of foreign substances contained in the liquid to be treated (raw water) due to a small surface pore diameter, and layer (B) may be a so-called support layer that ensures high mechanical strength and has a function of not reducing the water permeability as much as possible. The sharing of functions between layer (A) and layer (B) is not limited to the above.

[0037] The case of a two-layer structure in which the hollow fiber membrane has a multilayer structure, with layer (A) as the barrier layer and layer (B) as the support layer, will be described. The thickness of layer (A), which is the layer having the surface on the filtrate side, is preferably 1 / 100 or more and less than 40 / 100 of the total membrane thickness. By making the thickness of layer (A) relatively thick in this way, it can be used even if the raw water contains insoluble substances such as sand and aggregates. This is because even if it is slightly worn, the surface pore diameter does not change. Within this thickness range, a desirable balance between blocking performance and high water permeability can be achieved. More preferably, it is 2 / 100 or more and 30 / 100 or less of the membrane thickness. The thickness of layer (A) is preferably 1 μm or more and 100 μm or less, and more preferably 2 μm or more and 80 μm or less.

[0038] <Method for manufacturing a porous hollow fiber membrane> The method for manufacturing the porous hollow fiber membrane of the present embodiment includes a step of extruding a melt-kneaded product containing a thermoplastic resin, an organic liquid, and inorganic fine powder from a spinneret having an annular discharge port to form a hollow fiber-shaped melt-kneaded product, and a step of coagulating the hollow fiber-shaped melt-kneaded product and then extracting and removing the organic liquid and the inorganic fine powder to produce a porous membrane (preferably a porous hollow fiber membrane).

[0039] Note that the melt-kneaded product may consist of two components, a thermoplastic resin and a solvent, or may consist of three components, a thermoplastic resin, inorganic fine powder, and a solvent. The thermoplastic resin used in the method for manufacturing the porous hollow fiber membrane of the present embodiment is the same as that used in the porous hollow fiber membrane of the present embodiment described above. Further, the thermoplastic resin is a resin that returns to its original elastomer when cooled and the temperature drops, and no chemical changes such as molecular structure occur during that time (see, for example, "Edited by the Editorial Committee of the Chemical Dictionary, Chemical Dictionary 6th abridged edition, Kyoritsu Shuppan, pages 860 and 867, 1963").

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

[0041] Further, when the porous hollow fiber membrane of the present embodiment is a two-layer structure membrane, the mass ratio of the thermoplastic resin in the melt-kneaded product of the layer (B) which is the layer having the filtrate-side surface is preferably 34% by mass or more and 48% by mass or less, more preferably 35% by mass or more and 45% by mass or less. The mass ratio of the thermoplastic resin in the melt-kneaded product of the layer (A) is preferably 10% by mass or more and 35% by mass or less, more preferably 12% by mass or more and less than 35% by mass. If it is 10% by mass or more, both the pore diameter on the surface and the mechanical strength can be achieved, and if it is 35% by mass or less, the deterioration of water permeability does not occur.

[0042] As the organic liquid, a substance that becomes a latent solvent with respect to the thermoplastic resin used in the present embodiment is used. In the present embodiment, the latent solvent means 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. It only needs to be liquid at the melt-kneading temperature with the thermoplastic resin, and does not necessarily need to be liquid at normal temperature.

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

[0044] Examples of the inorganic fine powder include silica, alumina, titanium oxide, zirconia, calcium carbonate, etc., and silica is preferred. Further, the average primary particle diameter of the inorganic fine powder is preferably 3 nm or more and 500 nm or less, more preferably 5 nm or more and 100 nm or less. Among them, fine silica powder with an average primary particle diameter of 3 nm or more and 500 nm or less is preferred. As the inorganic fine powder, hydrophobic silica fine powder that is difficult to aggregate and has good dispersibility is more preferred, and further preferably hydrophobic silica with an MW (methanol wettability) value of 30% by volume or more. The MW value mentioned here is the value of the volume percentage of methanol in which the powder is completely wetted. Specifically, silica is placed in pure water, and methanol is added under the liquid surface while stirring. When 50% by mass of the silica has settled, the volume percentage of methanol in the aqueous solution is determined and used to determine the MW value. The above-mentioned "average primary particle diameter of the inorganic fine powder" means the value obtained from the analysis of electron micrographs. That is, first, a group of inorganic fine powders is pretreated by the method of ASTM D3849. Then, the diameters of 3000 to 5000 particles shown in the transmission electron micrograph are measured, and the average primary particle diameter of the inorganic fine powder is calculated by taking the arithmetic mean of these values.

[0045] Furthermore, the mass ratio of the inorganic fine powder in the melt-kneaded product is preferably 5% by mass or more and 50% by mass or less, more preferably 10% by mass or more and 40% by mass or less. If the mass ratio of the inorganic fine powder is 5% by mass or more, the effect of kneading the inorganic fine powder can be fully manifested, and if it is 40% by mass or less, spinning can be carried out stably.

[0046] The mixture composed of a thermoplastic resin such as polyvinylidene fluoride and an organic liquid, or the mixture composed 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 Proshear mixer, or the like.

[0047] When mixing three components including a thermoplastic resin such as polyvinylidene fluoride, an organic liquid, and inorganic fine powder, in terms of the order, rather than mixing the three components simultaneously, it is more advantageous in terms of improving melt formability, the porosity of the resulting porous membrane, and mechanical strength to first mix the inorganic fine powder and the organic liquid to allow the inorganic fine powder to sufficiently adsorb the organic liquid, and then blend and mix a thermoplastic resin such as polyvinylidene fluoride.

[0048] Without performing preliminary kneading using a Henschel mixer or the like, the thermoplastic resin such as polyvinylidene fluoride and the organic liquid may be directly supplied to a melt kneading and extruding apparatus such as a twin-screw extruder separately. In order to improve kneadability, after mixing, melt kneading is performed once to pelletize, and these pellets are supplied to the melt kneading and extruding apparatus and extruded and formed into a hollow fiber shape, and then cooled and solidified to obtain hollow fibers.

[0049] The melt kneading of the mixture can be performed using ordinary melt kneading means, for example, an extruder. Although the case of using an extruder will be described below, the means of melt kneading is not limited to an extruder. An example of the manufacturing apparatus used to implement the manufacturing method of the present embodiment is shown in FIG. 2.

[0050] In the manufacturing apparatus for a porous hollow fiber membrane shown in FIG. 2, it includes an extruder 10, a nozzle 20 for forming hollow fibers, a coagulation bath 30 in which a solution for coagulating the membrane-forming stock solution is stored, and a plurality of rollers 50 for transporting and winding up the porous hollow fiber membrane 40. The space S shown in FIG. 2 is a travel-free portion through which the membrane-forming stock solution discharged from the nozzle 20 for forming hollow fibers passes until it reaches the solution in the coagulation bath 30.

[0051] The melt-kneaded material is extruded by an extruder 10 and discharged from a hollow fiber forming nozzle 20 having one or more annular discharge ports arranged concentrically, with the hollow fiber forming nozzle 20 being attached to the tip of the extruder 10. When manufacturing a film with a multilayer structure, a hollow fiber forming nozzle 20 having two or more annular discharge ports is attached to the tip of the extruder 10, and the melt-kneaded material is supplied and extruded from different extruders 10 to each of the annular discharge ports, or after manufacturing one layer in the multilayer, there is a method of applying the remaining layers. For example, in the method of manufacturing using different extruders in the former case, a hollow fiber-like extrudate having a multilayer structure can be obtained by causing the melt-kneaded materials supplied respectively to merge and overlap at the discharge port. At this time, by extruding melt-kneaded materials with different compositions from adjacent annular discharge ports, a multilayer film with different pore diameters in adjacent layers can be obtained. Different compositions from each other refer to the case where the constituent materials of the melt-kneaded material are different, or the case where even if the constituent materials are the same, the composition ratios are different. Even for the same type of thermoplastic resin, when the molecular weight and molecular weight distribution are clearly different, they are regarded as having different constituent materials. The merging position of the melt-kneaded materials with different compositions from each other may be the lower end face of the hollow fiber forming nozzle 20 or may be different from the lower end face of the hollow fiber forming nozzle 20.

[0052] When extruding the melt-kneaded material from the annular discharge port, it is preferable to discharge so that the spinneret discharge parameter R (1 / second) has a value of 10 or more and 1000 or less, because high productivity, spinning stability, and a higher-strength film can be obtained. Here, the spinneret discharge parameter R is a value obtained by dividing the discharge linear velocity V (m / second) by the slit width d (m) of the discharge port. The discharge linear velocity V (m / second) is a value obtained by dividing the discharge capacity per unit time (m 3 / second) of the melt-kneaded material by the cross-sectional area (m 2 ) of the discharge port. If R is 10 or more, there are no problems such as the yarn diameter of the hollow extrudate pulsating, and spinning can be stably performed with good productivity. Also, 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 maintained sufficiently high. The elongation at break is the elongation ratio with respect to the original length when pulled in the longitudinal direction of the membrane. In the case of a porous hollow fiber membrane having a multilayer structure, the range of the spinning port discharge parameter R, which is the value obtained by dividing the discharge linear velocity V of the laminated and melt-kneaded material after the resins merge by the slit width d of the discharge port, is more preferably 50 or more and 1000 or less.

[0053] The hollow fiber-shaped melt-kneaded material discharged from the discharge port is solidified by passing a refrigerant such as air or water. Depending on the target porous hollow fiber membrane, after passing through the above-described air-running portion S composed of an air layer, it passes through a coagulation bath 30 filled with water or the like. That is, the air-running portion S is the portion from the discharge port of the nozzle 20 for forming hollow fibers to the water surface of the coagulation bath 30. A container such as a cylinder may be used for the air-running portion S as needed from the discharge port. After passing through the coagulation bath 30, it is wound around a bobbin or the like as needed.

[0054] The time for the melt-kneaded material to pass through the air-running portion S is called the air-running time, and the air-running time is desirably 0.05 seconds or more. When the air-running time is 0.05 seconds or more, the polymer molecules are oriented in the air-running portion, so that the compression resistance strength can be further increased. The air-running time is more preferably 0.1 seconds or more and 2.0 seconds or less. If it is 2.0 seconds or less, stable production is possible. Desirably, it is 0.12 seconds or more and 1.0 second or less.

[0055] Also, when the difference between the discharge temperature at the discharge port of the melt-kneaded material and the temperature in the coagulation bath 30 is ΔT and the air-running time is t, it is preferable that the temperature drop rate ΔT / t is 105 °C / s or more and 2100 °C / s or less. When it is 105 °C / s or more, the phase separation rate becomes faster, so the phase separation time becomes shorter, it is difficult to form a non-uniform stem, and it is considered that the strength coefficient is improved. More preferably, it is 210 °C / s or more and 1750 °C / s or less. In the hollow fiber after solidification, a polymer-rich partial phase and an organic liquid-rich partial phase are finely separated and present. For example, when an inorganic fine powder is added and the inorganic fine powder is fine silica, the fine silica is unevenly distributed in the organic liquid-rich partial phase. By extracting and removing the organic liquid and the inorganic fine powder from this hollow fiber, the organic liquid-rich partial 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 carried out simultaneously if they can be extracted and removed with the same solvent. Usually, they are extracted and removed separately. For the extraction and removal of the organic liquid, a liquid suitable for extraction is used, which is miscible with the organic liquid without dissolving or denaturing the thermoplastic resin used. Specifically, it can be carried out by bringing them into contact by means such as immersion. The liquid is preferably volatile so that it can be easily removed from the hollow fiber membrane after extraction. Examples of the liquid include alcohols and methylene chloride. 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 silica, it can be carried out 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. It doesn't matter which of the extraction and removal of the organic liquid and the extraction and removal of the inorganic fine powder is carried out first. When the organic liquid is immiscible with water, it is preferable to first carry out the extraction and removal of the organic liquid and then carry out the extraction and removal of the inorganic fine powder. Usually, since the organic liquid and the inorganic fine powder are miscibly coexistent in the organic liquid-rich partial phase, the extraction and removal of the inorganic fine powder can proceed smoothly, which is advantageous.

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

[0059] Then, for the hollow fiber membrane after solidification, stretching in the longitudinal direction of the porous hollow fiber membrane can be carried out within a range of a stretching ratio of 3 times or less at any stage of (i) before the extraction and removal of the organic liquid and the inorganic fine powder, (ii) after the extraction and removal of the organic liquid and before the extraction and removal of the inorganic fine powder, (iii) after the extraction and removal of the inorganic fine powder and before the extraction and removal of the organic liquid, and (iv) after the extraction and removal of the organic liquid and the inorganic fine powder. Generally, when a hollow fiber membrane is stretched in the longitudinal direction, the water permeation performance improves, but the pressure resistance performance (e.g., burst strength and compressive strength resistance) decreases. Therefore, after stretching, it often does not become a membrane with practical strength. However, the porous membrane (e.g., porous hollow fiber membrane) obtained by the manufacturing method of this embodiment has high mechanical strength. Therefore, stretching with a draw ratio of 1.1 times or more and 3.0 times or less is feasible. By stretching, the water permeation performance of the porous membrane (e.g., porous hollow fiber membrane) improves. The draw ratio mentioned here 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, when a porous hollow fiber membrane with a hollow fiber length of 10 cm is stretched to increase the hollow fiber length to 20 cm, from the following formula, the draw ratio is 2 times. 20 cm÷10 cm = 2

[0060] Also, it is desirable to stretch the hollow fiber membrane at a space temperature of 0°C or higher and 160°C or lower. When it is higher than 160°C, the stretching marks are large, and the elongation at break and water permeation performance decrease, which is not preferable. When it is less than 0°C, the possibility of stretching breakage is high and it is not practical. It is more preferable that the space temperature during the stretching process is 10°C or higher and 140°C or lower, and even more preferably 20°C or higher and 100°C or lower.

[0061] In this embodiment, it is preferable to stretch the hollow fiber membrane containing an organic liquid. The hollow fiber membrane containing an organic liquid has fewer breaks during stretching than the hollow fiber membrane not containing an organic liquid. Furthermore, since the hollow fiber membrane containing an organic liquid can cause a greater shrinkage of the hollow fiber membrane after stretching, the degree of freedom in setting the shrinkage rate after stretching increases. Also, it is preferable to stretch the hollow fiber membrane containing inorganic fine powder. Due to the hardness of the hollow fiber membrane due to the presence of the inorganic fine powder contained in the hollow fiber membrane, the hollow fiber membrane is less likely to be flattened and crushed during stretching. It can also prevent the pore diameter of the finally obtained hollow fiber membrane from becoming too small or the fiber diameter from becoming too thin. In this embodiment, it is more desirable to stretch the hollow fiber membrane containing both an organic liquid and inorganic fine powder.

[0062] For the above reasons, it is preferable to stretch the hollow fiber membrane containing either the organic liquid or the inorganic fine powder rather than stretching the hollow fiber membrane after the extraction is completed. Further, it is more preferable to stretch the hollow fiber membrane containing both the organic liquid and the inorganic fine powder rather than stretching the hollow fiber membrane containing either the organic liquid or the inorganic fine powder.

[0063] In addition, the method of extracting the stretched hollow fiber membrane has the advantage that since the voids increase on the surface and inside of the hollow fiber membrane by stretching, the extraction solvent easily penetrates into the hollow fiber membrane. Further, the method of performing the extraction after the steps of stretching and then shrinking has the advantage that, as will be described later, since the hollow fiber membrane has a low tensile modulus of elasticity and is easily bent, when the extraction is performed in a liquid flow, the hollow fiber membrane is easily shaken by the liquid flow, and the stirring effect is increased, so that efficient extraction can be performed in a short time.

[0064] In the present embodiment, when the hollow fiber membrane has the steps of stretching and then shrinking, a hollow fiber membrane having a low tensile modulus of elasticity can be finally obtained. Here, "having a low tensile modulus of elasticity" means that the thread is easily stretched by a small force and returns to its original state when the force is removed. When the tensile modulus of elasticity is low, the hollow fiber membrane does not collapse flat, is easily bent, and is easily shaken by the water flow during filtration. By the thread shaking without a constant bend according to the water flow, the layer of contaminants adhering and depositing on the membrane surface does not grow and is easily peeled off, and the amount of filtered water can be maintained at a high level. Further, when the thread is forcibly shaken by flushing or air scrubbing, the shaking is large and the cleaning recovery effect is high.

[0065] Regarding the degree of thread length contraction when performing contraction after stretching, it is desirable that the thread length contraction rate with respect to the thread length increment by stretching be in the range of 0.3 or more and 0.9 or less. For example, when a 10 cm thread is stretched to 20 cm and then to 14 cm, from the following formula, the thread length contraction rate is 0.6. Thread length contraction rate ={(maximum thread length during stretching)-(thread length after contraction)} / [(maximum thread length during stretching)-(original thread length)]=(20 - 14) / (20 - 10)=0.6 When the yarn length shrinkage rate is 0.9 or more, the water permeability is likely to be low, and when it is less than 0.3, the tensile elastic modulus is likely to be high, which is not preferable. In the present embodiment, it is more preferable that the yarn length shrinkage rate is in the range of 0.50 or more and 0.85 or less.

[0066] Further, by adopting the step of stretching the hollow fiber membrane to the maximum yarn length during stretching and then shrinking it, the finally obtained hollow fiber membrane will not break even when it is stretched to the maximum yarn length during use. Here, when the draw ratio is X and the yarn length shrinkage rate with respect to the yarn length increment due to stretching is Y, the rate Z representing the degree of guarantee of the elongation at break can be defined by the following formula. Z = (maximum yarn length during stretching - yarn length after shrinkage) / yarn length after shrinkage = (XY - Y) / (X + Y - XY) 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 guarantee of the elongation at break will be reduced. If Z is too large, the possibility of breakage during stretching will be high while the water permeability will be low.

[0067] Further, in the production method of the present embodiment, when the step of stretching and then shrinking is included, the tensile elongation at break has extremely few breaks at low elongation, and the distribution of the tensile elongation at break can be narrowed.

[0068] The space temperature in the step of stretching and then shrinking is desirably in the range of 0°C or more and 160°C or less from the viewpoints of the shrinkage time and physical properties. If it is lower than 0°C, the shrinkage takes a long time and is not practical. If it exceeds 160°C, the elongation at break decreases and the water permeability becomes low, which is not preferable. In the present embodiment, it is preferable to wind the hollow fiber membrane during the shrinkage step. Thereby, a hollow fiber membrane with a high winding degree can be obtained without being crushed or damaged.

[0069] Generally, since the hollow fiber membrane has a straight tubular form without bending, when bundled into a filtration module, there is a high possibility that the gaps between the hollow fibers cannot be formed and a yarn bundle with a low porosity is formed. On the other hand, when using a hollow fiber membrane with a high degree of crimp, the average interval between hollow fiber membranes can be widened due to the bending of individual fibers, resulting in a fiber bundle with a high porosity. In addition, a filtration module composed of a hollow fiber membrane with a low degree of crimp has fewer voids in the fiber bundle when used under external pressure, increasing the flow resistance and preventing the effective transmission of filtration pressure to the central part of the fiber bundle. Furthermore, when removing filtration deposits from the hollow fiber membrane by backwashing or flushing, the cleaning effect inside the fiber bundle also becomes smaller. A fiber bundle composed of a hollow fiber membrane with a high degree of crimp has a large porosity, and the gap between the hollow fiber membranes is maintained even under external pressure filtration, making it difficult for uneven flow to occur. Therefore, the hollow fiber membrane obtained by the manufacturing method of the present embodiment preferably has a degree of crimp in the range of 1.5 or more and 2.5 or less. When it is 1.5 or more, it is preferable for the above reasons, and when it is 2.5 or less, a decrease in the filtration area per unit volume can be suppressed.

[0070] As the method for crimping the hollow fiber membrane, during the process of stretching and then shrinking, while shrinking the hollow fiber membrane, for example, a method of pulling it while sandwiching it between a pair of gear rolls with periodic unevenness or a pair of sponge belts with unevenness can be mentioned.

[0071] In addition, in the manufacturing method of the present embodiment, it is preferable to perform stretching using a take-up machine composed of a pair of opposing endless track belts. In this case, the take-up machine is used on the upstream side and the downstream side of stretching. In each take-up machine, the hollow fiber membrane is sandwiched between the opposing belts, and the yarn feeding is performed by moving both belts in the same direction at the same speed. In this case, it is preferable to perform stretching by making the yarn feeding speed on the downstream side faster than the yarn feeding speed on the upstream side. By performing stretching in this way, it becomes possible to stretch without slipping against the stretching tension during stretching and to prevent the yarn from being flattened and crushed.

[0072] Here, the endless track type belt preferably has a high-elasticity belt such as a fiber-reinforced belt on the inner side in contact with the driving roll, and an outer surface in contact with the hollow fiber membrane made of an elastic body. Further, it is more preferable that the compression elastic modulus in the thickness direction of the elastic body is 0.1 MPa or more and 2 MPa or less, and the thickness of the elastic body is 2 mm or more and 20 mm. In particular, it is preferable to use a silicone rubber for the elastic body on the outer surface from the viewpoints of chemical resistance and heat resistance.

[0073] And in the manufacturing method of the present embodiment, after stretching the hollow fiber membrane, it is heat-treated at a melting point of the thermoplastic resin containing polyvinylidene fluoride resin of -50°C to -40°C, and then heat-treated at a melting point of the thermoplastic resin of -35°C to -25°C, and includes a multi-stage heat treatment process. This is because by performing heat treatment on the stretched hollow fiber membrane, the lamellar structure of the membrane can be changed, and the crystal structure and mechanical properties can be improved. The heat treatment temperature is the temperature inside the heat treatment apparatus, and a melting point of the thermoplastic resin of -50°C to -10°C is preferable. 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] Also, the heat treatment time is preferably 1 hour or more, and more preferably 3 hours or more. By performing heat treatment slowly for a long time, the disappearance of microcrystalline sites that melt at low temperatures progresses, and a hollow fiber membrane with a thick lamellar structure (large crystal size) and a stable crystal structure can be obtained. The temperature profile may be a one-stage type with one temperature condition, but in the above temperature range, a multi-stage type in which heat treatment is first performed on the low-temperature side and then on the high-temperature side is preferable. The hollow fiber membrane of the present invention has the characteristic of being easily recrystallized from a low temperature. By adopting a multi-stage method and gradually raising the temperature from a low temperature, it is considered that the recrystallization of microcrystalline sites can be further promoted. Regarding the heat treatment method, either a batch type or a continuous type may be used. In addition, in the manufacturing method of the present application, although the inorganic fine powder is extracted and removed as described above, since there is a very small amount of residue, recrystallization of the microcrystalline part can be promoted. Although the detailed mechanism is unknown, it is presumed that the remaining inorganic fine powder can serve as a starting point and promote recrystallization.

[0075] Furthermore, it is desirable to perform the heat treatment on the hollow fiber membrane after the extraction is completed, in terms of suppressing small changes in the yarn diameter, porosity, pore diameter, and water permeation performance.

Examples

[0076] Hereinafter, examples and comparative examples will be given for detailed explanation, but the present invention is not limited to these descriptions.

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

[0078] [Example 1] As the thermoplastic resin, vinylidene fluoride homopolymer (manufactured by Arkema: Kynar720) was used. Pelletized Kynar720 was cryogenically pulverized with a Ringlex mill (manufactured by Hosokawa Micron) and then classified with a vibrating sieve machine to remove particles with an opening size of 355 μm or more, obtaining particles with a median diameter (d50) of 96 μm. Next, 23.0% by mass of hydrophobic silica (manufactured by Nippon Aerosil Co., Ltd.: AEROSIL-R972), 31.3% by mass of di(2-ethylhexyl) phthalate (DEHP) (manufactured by CJS Ester Co., Ltd.), 5.7% by mass of dibutyl phthalate (DBP) (manufactured by CJS Ester Co., Ltd.), and 40.0% by mass of the vinylidene fluoride homopolymer were mixed, and the resulting mixture was melt-kneaded with an extruder. While supplying air as the fluid for forming the hollow part through a hollow fiber forming nozzle (outer diameter 1.72 mm, inner diameter 0.92 mm) attached to the tip of the extruder, melt extrusion was performed at 240°C. The hollow fiber-like melt-kneaded product extruded at 240 °C traveled in the atmosphere for 0.24 seconds, and then was cooled and solidified in a coagulation bath filled with water at 25 °C, taken up by a first belt take-up machine at a speed of 37 m / min, passed through a first heating bath (0.8 m long) controlled at a space temperature of 40 °C, doubled in length by a second belt take-up machine at a speed of 74 m / min, passed through a second heating bath (0.8 m long) controlled at a space temperature of 140 °C, shrunk by 1.5 times by a third belt take-up machine at a speed of 55 m / min, and then wound up by a winder. The obtained hollow fiber was immersed in methylene chloride at 30 °C for 1 hour or more, and di(2 -ethylhexyl) phthalate and dibutyl phthalate were extracted and removed, and then dried. Next, it was immersed in a 50% by mass aqueous ethanol solution for 30 minutes, then immersed in water for 30 minutes, immersed in a 20% by mass aqueous sodium hydroxide solution at 70 °C for 1 hour, and then washed repeatedly to extract and remove hydrophobic silica. Thereafter, it was put into a dryer, heat-treated at a set temperature of 125 °C for 3 hours, the temperature was raised to 140 °C, and further heat-treated for 5 hours to obtain a sample of a porous hollow fiber membrane.

[0079] [Example 2] A sample of a porous hollow fiber membrane was prepared in the same manner as in Example 1, except that the nozzle for forming the hollow fiber was changed to 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 prepared in the same manner as in Example 1, except that a vinylidene fluoride homopolymer (manufactured by Arkema: Kynar740) was used as the thermoplastic resin. After cryogenic grinding of Kynar740, the median diameter (d50) of the classified particles was 93 μm.

[0081] [Example 4] A sample of a porous hollow fiber membrane was prepared in the same manner as in Example 3, except that the nozzle for forming the hollow fiber was changed to an outer diameter of 2.00 mm and an inner diameter of 0.90 mm.

[0082] [Example 5] The particles pulverized and classified in Example 1 and Example 3 were mixed at a mass ratio of Kynar720:Kynar740 = 1:1, and a sample of the porous hollow fiber membrane was prepared in the same manner as in Example 1, except that it was used as a thermoplastic resin.

[0083] [Example 6] The particles pulverized and classified in Example 1 and Example 3 were mixed at a mass ratio of Kynar720:Kynar740 = 1:3, and a sample of the porous hollow fiber membrane was prepared in the same manner as in Example 1, except that it was used as a thermoplastic resin.

[0084] [Example 7] A sample of the porous hollow fiber membrane was prepared in the same manner as in Example 1, except that a vinylidene fluoride copolymer (manufactured by Arkema: KynarFlex2850) was used as the thermoplastic resin. After cryogenic pulverization of KynarFlex2850, the median diameter (d50) of the classified particles was 90 μm.

[0085] [Example 8] The particles pulverized and classified in Example 1 and Example 7 were mixed at a mass ratio of Kynar720:KynarFlex2850 = 3:2, and a sample of the porous hollow fiber membrane was prepared in the same manner as in Example 1, except that it was used as a thermoplastic resin.

[0086] [Comparative Example 1] A vinylidene fluoride homopolymer (manufactured by Solvay: SOLEF6010) was used as the thermoplastic resin. Since SOLEF6010 is a powder, a sample of the porous hollow fiber membrane was prepared in the same manner as in Example 1, except that it was used without pulverization and classification.

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

[0088] [Comparative Example 3] As the thermoplastic resin, a vinylidene fluoride homopolymer (manufactured by Arkema: Kynar 761) was used. Since Kynar 761 is a powder, a sample of the porous hollow fiber membrane was prepared in the same manner as in Example 1, except that it was used without being pulverized or classified. Note that for the sample of the obtained porous hollow fiber membrane, due to the increase in the molecular weight of the obtained polymer, the compressive strength was improved, but the pure water permeability and the elongation at break in tension suitable for filtration applications could not be obtained.

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

[0090] (1) Measurement of inner diameter, outer diameter, and membrane thickness The hollow fiber membrane was thinly cut with a razor or the like at 15 cm intervals in the membrane longitudinal direction in a perpendicular direction, and the major and minor axes of the inner diameter and the major and minor axes of the outer diameter of the cross section were measured using a microscope and calculated by the following formula. This measurement was performed 10 times, and the average value was taken as the inner diameter, outer diameter, and membrane thickness under those conditions. · Inner diameter (mm) = (inner major axis + inner minor axis) / 2 · Outer diameter (mm) = (outer major axis + outer minor axis) / 2 · Membrane thickness (mm) = (outer diameter - inner diameter) / 2

[0091] (2) Pure water permeability The hollow fiber membrane was immersed in a 50 mass% ethanol aqueous solution for 30 min and then wetted by replacing it with pure water. One end of a wet hollow fiber membrane of about 100 mm in length was sealed, an injection needle was inserted into the hollow part at the other end, and pure water at 25 °C was injected into the hollow part at a pressure of 0.1 MPa from the injection needle, and the permeation amount of the pure water permeating to the outer surface was measured, and the pure water permeability was calculated by the following formula. The effective membrane length is 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 permeability under those conditions. · Pure water permeability (L / m 2 / hr) = permeation amount / (π × membrane inner diameter × effective membrane length × measurement time) ※Water permeation volume (L), inner diameter of the membrane (m), effective length of the membrane (m), measurement time (hr)

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

[0093] (4) Compression resistance strength One end of a wet hollow fiber membrane about 5 cm long was sealed, the other end was open to the atmosphere, and pure water at 40 °C was pressurized from the outer surface to discharge permeated water from the open end to the atmosphere. At this time, a method of filtering the entire amount without circulating the membrane supply water, that is, the total amount filtration method, was adopted. The pressurizing pressure was increased in increments of 0.05 MPa from 0.1 MPa, and held for 30 seconds at each pressure. During this period, the permeated water coming out from the open end to the atmosphere was collected. When the hollow part of the hollow fiber membrane does not collapse, the absolute value of the water permeation volume (mass) increases as the pressurizing pressure increases. However, when the pressurizing pressure exceeds the compression resistance strength of the hollow fiber membrane, the hollow part collapses and blockage begins, so the absolute value of the water permeation volume decreases as the pressurizing pressure increases. The pressurizing pressure at which the absolute value of the water permeation volume becomes maximum was taken as the compression resistance strength. This measurement was carried out 10 times, and the average value was taken as the compression resistance strength under those conditions.

[0094] (5) Chemical resistance test A hollow fiber membrane about 100 mm long was immersed in a 50 mass% ethanol aqueous solution for 30 min, and then wetted by replacing it with pure water. Next, an aqueous solution containing 4.0 mass% sodium hydroxide and 2.0 mass% sodium hypochlorite with an available chlorine concentration was prepared. Next, the hollow fiber membrane was immersed in an aqueous solution at 25°C for 17 days, and then a tensile test was conducted to determine the retention rate (%) of the tensile fracture elongation before and after immersion. The tensile test was performed n10 times, and the average value was calculated. · Retention rate (%) = (after immersion / before immersion) × 100

[0095] (6) Temperature Modulated Differential Scanning Calorimetry (MDSC) A 5 mg sample of the hollow fiber membrane was cut out and subjected to MDSC measurement under the following conditions to determine the crystal melting temperature, crystal melting enthalpy, crystallinity, etc. by Total Heat Flow analysis. The results of the Total Heat Flow analysis are shown in Figure 3. The crystal melting main peak is the peak with the widest endothermic area during heating. Further, after heating and the endothermic value returned to the baseline, if a minute peak appeared after further heating, it was regarded as having a peak after the main peak (〇), and if not, it was regarded as (×). 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. Also, the crystallization onset temperature was determined by Non-Reversing Heat Flow analysis. The results of the Non-Reversing Heat Flow analysis are shown in Figure 4. The inflection point at which the Non-Reversing Heat Flow clearly rises from the baseline during heating was defined as the crystallization onset temperature. For example, in the case of Example 1, 139.6°C is the crystallization onset temperature. · Apparatus: Discovery DSC2500 (manufactured by TA Instruments) · Atmosphere: Nitrogen 50 mL / min · Sample pan: Tzero Aluminum · Measurement mode: Modulated Heat Only · Temperature range: -20°C → 200°C · Heating rate: 1°C / min · Amplitude: 0.2°C · Period: 60 s Note that crystallinity = ΔH obs / ΔH ° (ΔH ° = 105 J / g) ΔH obsis the enthalpy of crystal melting, ΔH, observed by Total Heat Flow ° is the equilibrium melting enthalpy.

[0096] (7) Heterogeneous bond ratio Add 0.6 mL of deuterated DMF to 30 mg of the hollow fiber membrane, dissolve it by heating to 50 °C, and then, under the following conditions, 1 perform 1H-NMR measurement. From the integral values of the signals derived from the HT (head-tail) and HH (head-head) bonds of PVDF, the heterogeneous bond ratio was determined by the following formula. · Apparatus: JEOL ECS400 · Pulse width: 45° · Waiting time: 3 seconds · Number of integrations: 512 times · Chemical shift reference: The signal of CHO in DMF was set to 8.02 ppm. * Heterogeneous bond ratio (%) = HH / (HH + HT)

[0097] (8) Weight average molecular weight (Mw) Dissolve the hollow fiber membrane in DMF at a concentration of 1.0 mg / mL and perform GPC measurement under the following conditions to determine the weight average molecular weight (in terms of PMMA). · Apparatus: HLC-8420GPC (Tosoh Corporation) · Column: TSKgel guradcolumn SuperAW-H Two TSKgel AWM-H (6.0 mm ID × 15 cm) · Column temperature: 40 °C · Eluent: DMF containing 5 mM LiBr · Flow rate: 0.6 mL / min · Injection volume: 30 μL

[0098]

Table 1

[0099] From the results in Table 1, Figures 3 and 4, it was found that the samples of each example were well-balanced and excellent in all evaluation items compared to the samples of the comparative example. It was found that each sample of the comparative example showed inferior results compared to the sample of the example in at least one evaluation item.

Industrial Applicability

[0100] According to the present invention, there can be provided a porous hollow fiber membrane and a method for producing the same, which are produced by the TIPS method, have high water permeability in addition to the blocking performance, and have high chemical resistance that enables continuous operation over a long period.

Explanation of Signs

[0101] 10 Extruder 20 Nozzle for forming hollow fibers 30 Coagulation bath 40 Porous hollow fiber membrane 50 Multiple rollers

Claims

1. A porous hollow fiber membrane made of a thermoplastic resin, A porous hollow fiber membrane, characterized in that the crystallization onset temperature is 140°C or lower, and the crystalline melting enthalpy at temperatures below the crystallization onset temperature is 10 J / g or lower.

2. 2. The porous hollow fiber membrane according to claim 1, wherein the crystalline melting enthalpy of the entire porous hollow fiber membrane is less than 58 J / g.

3. 3. The porous hollow fiber membrane according to claim 1, wherein the crystalline melting enthalpy of a peak appearing at a higher temperature side than the crystalline melting main peak of the porous hollow fiber membrane is 0.1 J / g or more.

4. 3. The porous hollow fiber membrane according to claim 1, wherein a 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. 3. The porous hollow fiber membrane according to claim 1, wherein the porous hollow fiber membrane has a crystallinity of less than 60%.

6. 3. The porous hollow fiber membrane according to claim 2, wherein the crystalline melting enthalpy at a temperature equal to or lower than the crystallization onset temperature is 15% or less of the crystalline melting enthalpy of the entire porous hollow fiber membrane.

7. The porous hollow fiber membrane according to claim 1 or 2, characterized in that the thermoplastic resin has a weight average molecular weight (Mw) of 400 kDa or less.

8. 3. The porous hollow fiber membrane according to claim 1, wherein the thermoplastic resin contains a polyvinylidene fluoride resin.

9. 3. The porous hollow fiber membrane according to claim 1 or 2, characterized in that the porous hollow membrane has an inner diameter of less than 0.75 mm and a compressive strength of 0.3 MPa or more.

10. The porous hollow fiber membrane according to claim 1 or 2, characterized in that the heterogeneous binding rate of the porous hollow fiber membrane is 9% or more.

11. A method for producing a porous hollow fiber membrane, comprising a multi-stage heat treatment step in which a hollow fiber membrane is stretched, then heat-treated at a melting point of a thermoplastic resin containing a polyvinylidene fluoride resin from -50°C to -40°C, and then heat-treated at a melting point of the thermoplastic resin from -35°C to -25°C.

12. The method according to claim 11, wherein the porous hollow fiber membrane is produced by melting and kneading a mixture of three components, namely, the thermoplastic resin, the organic liquid, and the inorganic fine powder, extruding the mixture to form hollow fibers, and then extracting the organic liquid and the inorganic fine powder.

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