Hollow fiber membrane and method for manufacturing the same

Abstract

The present invention relates to a hollow fiber membrane, wherein a thickness L of a spherically structured layer is 60 μm or more and 500 μm or less, the spherically structured layer has a first face and a second face, an average diameter Da1 of the spherically structured in a region Sa1 within 10 μm from the first face and an average diameter Db2 of the spherically structured in a region Sb2 from 10 μm to 20 μm from the second face satisfy a relational expression of Da1 > Db2, and the spherically structured satisfies a specific parameter.

Description

Technical Field

[0001] This invention relates to hollow fiber membranes and their manufacturing methods. Background Technology

[0002] Separation membranes such as fine filtration membranes and ultrafiltration membranes are used in water treatment and food / pharmaceutical applications for clarification, concentration, and separation. However, in recent years, with the expansion of the application range of separation membranes, research has been conducted on their application in applications where the filtered liquid is difficult to filter and high-precision separation is required. The requirements for separation membranes that can balance filtration and separation have been further increased.

[0003] Patent Document 1 discloses a hollow fiber membrane, which is a porous hollow fiber membrane containing a vinylidene fluoride-based resin, and has an inclined structure in which the pore size of the hollow fiber membrane decreases sequentially to at least one side of the inner and outer peripheral surfaces. Furthermore, this hollow fiber membrane is described as having excellent permeability, grading characteristics, and strength.

[0004] Patent Document 2 discloses a hollow fiber membrane having a spherical structure formed from polyvinylidene fluoride resin using a thermally induced phase separation method, and having a larger spherical structure within 10 μm of the outer surface than near the inner surface. It describes that by having a large spherical structure near the outer surface, it is less prone to clogging, maintains high water permeability, and exhibits high strength due to the uniform inner layer.

[0005] Furthermore, Patent Document 2 discloses a method for manufacturing such a membrane by imparting a temperature gradient in the thickness direction to a polyvinylidene fluoride resin solution before solidification. Specifically, it describes heating a tube head formed into a hollow fiber shape, etc., causing the spherical structure on the heated side surface to coarsen.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: International Publication No. 2015 / 146469

[0009] Patent Document 2: International Publication No. 2016 / 006611 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] The hollow fiber membrane disclosed in Patent Document 1 is a three-dimensional network structure in which the pore size gradually decreases to one side. This type of membrane has poor strength and is unsuitable for operation under high loads, such as external pressure cross-flow filtration. Furthermore, the separation layer in this membrane is very thin, making its separation function easily reduced by membrane damage during operation. Therefore, it is difficult to maintain stable separation performance over a long period.

[0012] The membrane in Patent Document 2 has small, uniform spherical structures inside, which can block components in the liquid and thus maintain separation performance for a long time. Furthermore, large spherical structures exist near the outer surface, allowing for the capture of clogging components (components that cause clogging). However, the area where clogging components can be captured is only near the outer surface. Therefore, the following problem exists: depending on the liquid being filtered, especially liquids with high turbidity or organic content, and operating time, the time it can maintain permeability becomes shorter. Therefore, there is a demand to further extend the time it can maintain permeability, even for liquids that are difficult to filter.

[0013] The purpose of this invention is to provide a hollow fiber membrane and a method for manufacturing the same, which can maintain separation and water permeability, for hollow fiber membranes obtained using polyvinylidene fluoride resins.

[0014] Methods for solving problems

[0015] To achieve the above objectives, the present invention includes the following methods 1 to 19.

[0016] 1. Hollow fiber membrane, which has a layer with a spherical resin structure.

[0017] The thickness L of the aforementioned spherical structure layer is 60 μm or more and 500 μm or less.

[0018] The aforementioned spherical structure has a first surface and a second surface.

[0019] Regarding the region Sa, which is 10×(n-1)~10×nμm away from the aforementioned first surface... n The average diameter Da of the spherical structure in n And the region Sb, which is 10×(n-1)~10×nμm away from the aforementioned second surface. n The average diameter Db of the spherical structure in n Da1 > Db2, and

[0020] The minimum value of a natural number i that satisfies the following conditions (1) and (2) min For 3≤i min ≤(L-20) / 10.

[0021] (1)Da1 / Da i ≥1.1

[0022] (2) -0.3μm≤Da i -Db2≤0.3μm

[0023] Where n is a natural number, and in the case of 3≤i minIn the range ≤(L-20) / 10, discard the digits after the decimal point of (L-20) / 10.

[0024] 2. The hollow fiber membrane according to claim 1 above, wherein i min ≤L×0.75 / 10.

[0025] Among them, in the aforementioned i min In the range ≤L×0.75 / 10, discard the digits after the decimal point of L×0.75 / 10.

[0026] 3. The hollow fiber membrane according to 1 or 2 above, wherein 1.10 <Da1 / Da imin <4.00.

[0027] 4. The hollow fiber membrane according to any one of claims 1 to 3 above, wherein 0.50 μm <Db2<2.00μm。

[0028] 5. The hollow fiber membrane according to any one of 1 to 4 above, wherein 1.00 ≤ Da1 / Da2 ≤ 1.10.

[0029] 6. The hollow fiber membrane according to any one of 1 to 5 above, wherein the first surface is the side of the filtered liquid.

[0030] 7. The hollow fiber membrane according to any one of 1 to 6 above, wherein the first surface is the outer surface of the hollow fiber membrane.

[0031] 8. The hollow fiber membrane according to any one of 1 to 7 above, wherein when the throat diameter obtained by analyzing the pore network model of the hollow fiber membrane is used as the throat diameter of the pores in the spherical structure gap,

[0032] Regarding the region Sa, which is 10×(n-1)~10×nμm away from the aforementioned first surface... n The average diameter da of the throat diameter of the spherical structure gap in the middle n And the region Sb, which is 10×(n-1)~10×nμm away from the aforementioned second surface. n The average diameter (db) of the throat diameter of the spherical structure gap in the middle n da1>db2, and

[0033] The minimum value of a natural number j that satisfies the following conditions (1) and (2) min For 3≤j min ≤(L-20) / 10.

[0034] (1)da1 / da j ≥1.15

[0035] (2)da j-db2 ≤ 0.10 μm

[0036] Among them, in the foregoing 3 ≤ j min ≤ (L - 20) / 10, the numbers after the decimal point of (L - 20) / 10 are discarded.

[0037] 9. The hollow fiber membrane according to item 8 above, wherein j min ≤ L × 0.5 / 10.

[0038] Among them, in the foregoing j min ≤ L × 0.5 / 10, the numbers after the decimal point of L × 0.5 / 10 are discarded.

[0039] 10. The hollow fiber membrane according to item 8 or 9 above, wherein, regarding the number of throats Na n in the foregoing region Sa n obtained by the foregoing pore network model analysis n and the number of throats Nb n in the foregoing region Sb

[0040] the minimum value k of the natural number k that satisfies the following conditions (3) and (4) min is 3 ≤ k min ≤ (L - 20) / 10.

[0041] (3) Na1 / Na k ≤ 0.90

[0042] (4) Nb2 - Na k ≤ 400

[0043] Among them, in the foregoing 3 ≤ k min ≤ (L - 20) / 10, the numbers after the decimal point of (L - 20) / 10 are discarded.

[0044] Among them, the analysis region in the foregoing analysis is a cuboid including an arbitrary 50-μm square region of the foregoing first surface and a 50-μm square region of the foregoing second surface opposite to the foregoing first surface.

[0045] 11. The hollow fiber membrane according to any one of items 8 to 10 above, wherein 0.10 μm < db2 < 10.0 μm.

[0046] 12. The hollow fiber membrane according to any one of items 1 to 11 above, wherein

[0047] the foregoing spherical structure contains a polyvinylidene fluoride-based resin,

[0048] there are hydrophilic polymers on the surface and inside of the foregoing spherical structure,

[0049] The aforementioned hydrophilic polymer is present in 1.0 parts by weight or more of 100 parts by weight of the aforementioned polyvinylidene fluoride resin.

[0050] 13. The hollow fiber membrane according to 12 above, wherein the ratio (mass %) of the hydrophilic polymer to the polyvinylidene fluoride resin after the hollow fiber membrane has been impregnated in a 3000ppm sodium hypochlorite aqueous solution (pH 12.5) at 60°C for 30 hours is set as P1, and the ratio (mass %) of the hydrophilic polymer to the polyvinylidene fluoride resin before impregnation is set as P0, wherein the percentage of the ratio P1 / P0 of P1 to P0 is 70% or less.

[0051] 14. A method for manufacturing a hollow fiber membrane, comprising the following steps:

[0052] (a) The process of dissolving a polyvinylidene fluoride resin in a poor solvent to obtain a polyvinylidene fluoride resin solution;

[0053] (b) The process of holding the aforementioned polyvinylidene fluoride resin solution at the temperature at which a primary nucleation is formed;

[0054] (c) Following step (b) above, a process of spraying a polyvinylidene fluoride resin solution in a hollow fiber form through piping and nozzles, and, in at least one of the aforementioned piping or nozzles, applying a temperature gradient along the thickness direction of the polyvinylidene fluoride resin solution; and

[0055] (d) After the aforementioned step (c), a step in which the aforementioned polyvinylidene fluoride resin solution is immersed in a cooling bath, thereby causing the aforementioned polyvinylidene fluoride resin solution to solidify through solid-liquid thermal phase separation.

[0056] The temperature gradient ΔT (°C) applied through the aforementioned process (c) and the time t (seconds) for applying the temperature gradient are 50 ≤ ΔT × t ≤ 300.

[0057] 15. The method for manufacturing a hollow fiber membrane according to 14 above, wherein, in the aforementioned step (c), the thickness Ls of the polyvinylidene fluoride resin imparting the temperature gradient is 5 relative to the thickness L of the aforementioned spherical structure layer. <Ls / L<40。

[0058] 16. The method for manufacturing hollow fiber membrane according to 14 or 15 above, wherein the temperature T2 (°C) in the aforementioned step (b), the temperature T4 (°C) of the cooling bath in the aforementioned step (d), and the crystallization temperature Tc (°C) of the aforementioned polyvinylidene fluoride resin satisfy (Tc-T4) / (T2-T4)<0.50.

[0059] 17. The method for manufacturing a hollow fiber membrane according to the aforementioned 16, which satisfies 15 <Tc-T4<35。

[0060] 18. The method for manufacturing a hollow fiber membrane according to any one of 14 to 17 above, wherein, after the aforementioned step (d), it further comprises the following steps (e) and (f),

[0061] (e) The process of introducing hydrophilic polymers into hollow fiber membranes;

[0062] (f) A heat treatment process at 100°C or higher following the aforementioned process (e).

[0063] 19. The method for manufacturing a hollow fiber membrane according to the aforementioned 18, wherein, in the aforementioned step (e), an aqueous solution containing a hydrophilic polymer is passed through the hollow fiber membrane and then subjected to radiation irradiation.

[0064] Invention Effects

[0065] According to the present invention, a hollow fiber membrane capable of maintaining separation and water permeability and a method thereof can be provided. Attached Figure Description

[0066] Figure 1 These are electron microscope images of the vicinity of the first and second surfaces in the radial cross-section of a hollow fiber membrane.

[0067] Figure 2 This is a schematic diagram illustrating a method for calculating the average diameter of a spherical structure.

[0068] Figure 3 This is a graph showing the measurement results of the average diameter of the spherical structure relative to the distance from the first surface.

[0069] Figure 4 This is a diagram illustrating the pore network model analysis of a hollow fiber membrane.

[0070] Figure 5 This is a top view showing a specific example of the configuration of a tube head used to manufacture hollow fiber membranes.

[0071] Figure 6 yes Figure 5 The image shows an AA section view of the pipe head.

[0072] Figure 7 yes Figure 5 The image shows a bottom view of the pipe head.

[0073] Figure 8 This is a diagram showing an example of the shape of a piping system. Detailed Implementation

[0074] The following describes embodiments of the present invention, but the present invention is not limited to these embodiments and can be implemented in any way without departing from the spirit of the present invention. In this specification, the proportions (percentages, parts, etc.) of the weight basis are the same as the proportions (percentages, parts, etc.) of the mass basis.

[0075] The hollow fiber membrane described in the embodiments of the present invention has a resin spherical structure layer, wherein the thickness L of the aforementioned spherical structure layer is 60 μm or more and 500 μm or less, the aforementioned spherical structure layer has a first surface and a second surface, and a region Sa with a distance of 10×(n-1) to 10×n μm from the aforementioned first surface. n The average diameter Da of the spherical structure in n And the region Sb, which is 10×(n-1)~10×nμm away from the aforementioned second surface. n The average diameter Db of the spherical structure in n Da1 > Db2, and the minimum value of the natural number i that satisfies the following conditions (1) and (2) is i. min For 3≤i min ≤(L-20) / 10.

[0076] (1)Da1 / Da i ≥1.1

[0077] (2) -0.3μm≤Da i -Db2≤0.3μm

[0078] Where n is a natural number, and in the case of 3≤i min In the range ≤(L-20) / 10, discard the digits after the decimal point of (L-20) / 10.

[0079] Hollow fiber membrane

[0080] The hollow fiber membrane described in the embodiments of the present invention has a layer with a spherical resin structure. The hollow fiber membrane may consist only of a layer with a spherical structure, or it may have other layers. It should be noted that the layer with a spherical resin structure refers to a layer with a spherical structure made of resin.

[0081] The resin constituting the spherical structure is preferably a thermoplastic resin containing chain polymers, and polyvinylidene fluoride (PVDF)-based resins are particularly preferred due to their high chemical resistance. Here, PVDF-based resins refer to resins containing at least one of PVDF homopolymers and PVDF copolymers. PVDF-based resins can contain various PVDF copolymers. In other words, the spherical structure preferably contains PVDF-based resins. PVDF-based resins in the spherical structure means that the thermoplastic resin component constituting the spherical structure is substantially composed of PVDF-based resins. When the spherical structure contains PVDF-based resins, in addition to thermoplastic resins, other resins and polyols or surfactants that can be mixed with thermoplastic resins may also be included in a proportion of 50% by weight or less. Furthermore, the spherical structure may inevitably contain impurities in addition to these components, the content of which is preferably, for example, 1% by weight or less.

[0082] Vinylidene fluoride copolymers are polymers having vinylidene fluoride residue structures. Typically, they are copolymers of vinylidene fluoride monomers with other fluorinated monomers. Examples of other fluorinated monomers include one or more monomers selected from vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, and trifluorochloroethylene. Furthermore, without impairing the effects of the present invention, monomers other than the aforementioned fluorinated monomers, such as ethylene, can be copolymerized.

[0083] In addition, the spherical layer may contain, in addition to thermoplastic resin, other resins and polyols or surfactants that can be mixed with thermoplastic resin in a proportion of less than 50% by weight.

[0084] In this specification, "spherical structure" refers to a solid portion with a substantially circular cross-section perpendicular to the length direction of the hollow fiber membrane (in other words, parallel to the radial direction of the hollow fiber membrane), and the structure formed by connecting the solid portions as described below. When the structure is not a whole, especially when the cross-section is substantially circular, it is sometimes referred to as a "solid portion." Substantially circular includes perfect circles and ellipses. The solid portions are connected by sharing a portion with each other in the planar or thickness direction of the separation membrane (hollow fiber membrane). There is no particular limitation on the shape of the cross-section parallel to the length direction of the hollow fiber membrane; it can be any shape, such as substantially circular or columnar.

[0085] The separation membrane (hollow fiber membrane) exhibits high strength and elongation due to its spherical structure, and high water permeability due to the inclusion of voids between the spherical solid parts.

[0086] During filtration, the first surface is preferably positioned upstream in the filtration direction, and the second surface is preferably positioned downstream in the filtration direction. The filtrate preferably flows from the first surface to the second surface. In other words, the first surface is preferably the filtrate side. In other words, in the case of so-called external pressure filtration, the outer surface of the hollow fiber membrane is the first surface and the inner surface is the second surface; in the case of internal pressure filtration, the inner surface of the hollow fiber membrane is the first surface and the outer surface is the second surface.

[0087] Next, use Figure 1 The diameter of the spherical structure relative to the thickness direction of the hollow fiber membrane will be explained.

[0088] For convenience, the outer surface of the hollow fiber membrane will be referred to as the first surface and the inner surface as the second surface in the following description. Figure 1 These are electron microscope images of the radial cross-section of the hollow fiber membrane, (a) showing the vicinity of the first surface and (b) showing the vicinity of the second surface.

[0089] In the cross-section, the regions separated by 10 μm intervals from the first surface are labeled with symbols Sa1, Sa2, ... Sa1, Sa2, ... Sa1, Sa3, Sa4, Sa5, Sa6, Sa7, Sa8, Sa9, Sa1, Sa1, Sa2, Sa3, Sa4, Sa5, Sa6, Sa1, Sa1, Sa2, Sa3, Sa4, Sa5 ... n …In addition, the regions separated by 10 μm from the second surface (0–10 μm, 10 μm–20 μm, … 10 × (1-n)–10 × n μm…) are respectively labeled with symbols Sb1, Sb2, … Sb n That is, let the region 10×(n-1) to 10×nμm from the first surface be Sa. n Let the region 10×(n-1) to 10×nμm from the second surface be Sb. n It should be noted that, here, n is a natural number.

[0090] like Figure 1 As shown, a photograph is taken in a manner that allows identification of the arc forming the first surface of the hollow fiber membrane. At this time, the tangent of the central portion of the arc forming the first surface is aligned with the horizontal direction of the photograph. In the resulting photograph, a straight line Ma1 is drawn passing through the intersection points of the arc forming the first surface and the left and right ends of the photograph. The straight line Ma1 is preferably parallel to the horizontal direction of the photograph, but it can also be inclined.

[0091] Then, draw a line Ma2 parallel to line Ma1, located 10 μm closer to the second surface. Repeat this process to draw lines Ma3, Ma4…Ma… n Region Sa1 is the region between lines Ma1 and Ma2. Line Ma... n It is a straight line parallel to line Ma1, drawn from a point 10 × n μm forward from line Ma1 towards the second surface. Region San The line Ma in the photo is straight. n With line Ma (n+1) The area between.

[0092] Regarding the second side, photographs are taken in a manner that allows for confirmation of its arc. At this time, the tangent to the center portion of the arc forming the second side is aligned with the horizontal direction of the photograph. In the resulting photograph, a straight line Mb1 is drawn passing through the intersection points of the arc forming the first side and the left and right ends of the photograph. The straight line Mb1 is preferably parallel to the horizontal direction of the photograph, but it can also be inclined.

[0093] Furthermore, with the straight line Ma n Similarly, starting from the second surface, draw straight lines at 10μm intervals. In other words, draw a straight line Mb2 parallel to straight line Mb1, located 10μm closer to the first surface than straight line Mb1. By repeating the same operation, it is possible to draw straight lines Mb3…Mb n Region Sb n It is a straight line Mb n withMb (n+1) The area between.

[0094] Region Sa n The average diameter Da of the spherical structure in n Region Sb n The average diameter Db of the spherical structure in n Perform the calculation as follows.

[0095] First, electron microscope images of the radial section are taken. There is no particular limitation on the magnification during imaging, as long as it allows for the measurement of 15 or more spherical structures. For example, if the average diameter of the spherical structures is 1–3 μm, then observation at 1000x to 5000x magnification ensures a sufficient number of spherical structures for calculating the average diameter, and is therefore preferred.

[0096] In shooting direction (and) Figure 1 , 2 On the direction perpendicular to the paper, exclude solid portions from the test object that overlap with other spheres and are located deeper than other spheres. It should be noted that solid portions located deeper than other spheres are... Figure 2 Examples are given for X1 to X4. Additionally, solid parts that break off at the outer edge of the captured image are excluded from the measurement.

[0097] Within the remaining solid portion, for example... Figure 2 The solid part X5 shown is not connected to other solid parts; in other words, it is a solid part whose overall outline can be confirmed. Its major axis (in...) is measured. Figure 2 (Used as a single-dot dash in Chinese).

[0098] On the other hand, in solid parts connected to other solid parts, it is impossible to confirm a portion of the outline. Therefore, when only a portion of the outline of the solid part can be confirmed, the measurement is performed based on the inferred overall image as follows. In spheres X6, X7, etc., which have continuous outlines, the longest line segment with both ends located on its outline is drawn, and its length is taken as the major axis. In addition, in X8 to X10, which have discontinuous outlines, the longest line segment with both ends located on one or two of the outlines is taken as the major axis. In cases (A) or (B) below, the corresponding solid parts are excluded from the measurement object.

[0099] (A) Cases where the confirmed outline is less than 50% of the overall outline of the approximate circle estimated for the solid part (e.g., X11, 12).

[0100] (B) The case where the two ends of the above line segment are located at the two ends of a continuous outline (e.g., X8).

[0101] It should be noted that when the end of the electron microscope image used for tissue evaluation is cut off, the diameter of the spherical structure at that end is not measured. Instead, the diameter of the spherical structure (major axis of the spherical structure) of each solid part being measured is determined, and their arithmetic mean is calculated to obtain the region Sa. n The average diameter Da of the spherical structure in n and region Sb n The average diameter Db of the spherical structure in n .

[0102] This method will be used to target Figure 1 The results obtained by measuring the average diameter of the spherical structure along the thickness direction are shown in Figure 3 The horizontal axis represents the distance along the thickness direction from the first surface, and the vertical axis represents the average diameter of the spherical structures in each region. For ease of drawing, the average diameter Da is... n The plotting is done at a distance of 10×(n-1)μm from the first surface. For example, the average diameter Da3 of region Sa3 is plotted at a distance of 20μm in the thickness direction.

[0103] The average diameter Da1 of the spherical structure in region Sa1 and the average diameter Db2 of region Sb2 satisfy the relationship Da1>Db2. By making the average diameter Da1 of region Sa1 near the first surface greater than the average diameter Db2 of region Sb2 near the second surface, the liquid flow resistance near the first surface decreases and the permeability increases.

[0104] The inventors discovered that by further minimizing the natural number i that satisfies the following conditions (1) and (2), min For 3≤i min≤(L-20) / 10, thus maintaining permeability and separation properties for a long time. Wherein, in the above 3≤i min In the range ≤(L-20) / 10, the decimal part of (L-20) / 10 is discarded. Additionally, as explained later, L represents the thickness of the layer in the spherical structure.

[0105] (1)Da1 / Da i ≥1.1

[0106] (2) -0.3μm≤Da i -Db2≤0.3μm

[0107] valuei min Being within the above range means: a region Sa with an average diameter smaller than the average diameter Da1 of the nearest region of the first face and having an average diameter equal to the average diameter Db2 of the spherical structure of region Sb2. i It is located closer to the second surface than Sa2. Therefore, the blocking components can be more widely dispersed along the thickness direction of the membrane. As a result, the operating time can be extended. Preferably, i min ≥3, preferably i min ≥5, further preferably i min ≥1 / 2×L / 10. Where, in the above i min In the range ≥1 / 2×L / 10, discard the digits after the decimal point of 1 / 2×L / 10.

[0108] exist Figure 3 In the middle, as the spherical structure moves from the first face towards the second face, the average diameter decreases, in i min When = 5, condition (2) is satisfied: -0.30 ≤ Da5 - Db2 ≤ 0.30. At this time, Da1 / Da5 = 1.29, which also satisfies condition (1). Figure 3 In the example, since the film thickness (the thickness L of the spherical structure layer) is 240 μm, it also satisfies 3 ≤ i min ≤(L-20) / 10.

[0109] Considering water permeability and tensile strength, the thickness L of the spherical layer (hollow fiber membrane) is preferably 60 μm or more and 500 μm or less. From the viewpoint of dispersing the clogging sites in the thickness direction based on depth filtration, it is more preferably 100 μm or more and 500 μm or less. From the viewpoint of membrane area, it is even more preferably 150 μm or more and 350 μm or less.

[0110] In addition, the above value i min Preferred i min ≤L×0.75 / 10. In other words, the preferred values ​​satisfying (1) and (2) are the range from the first surface to 3 / 4 of the total thickness of the film. Wherein, in the above imin In ≤ L×0.75 / 10, the numbers after the decimal point of L×0.75 / 10 are discarded.

[0111] Under this condition, for all natural numbers m greater than the numerical value i min and less than or equal to (L - 20) / 10, it is preferable to satisfy the following condition (4).

[0112] (4) -0.30 ≤ Da m -Db2 ≤ 0.30

[0113] The regions satisfying the above (4) have average diameters close to each other. Therefore, within the range of i min < m ≤ (L - 20) / 10, the structure is uniform. In addition, according to the definition of the range of m, the region occupied by the homogeneous structure in the film thickness is more than 1 / 4. Therefore, sufficient separation performance can be obtained. In addition, when the film surface is damaged during film formation or operation, the thickness of the layer with a homogeneous structure is also within this range. Thus, deterioration of separation performance and blocking performance can be suppressed. The layer with a uniform structure is at least preferably 10 μm or more. The layer with a uniform structure is more preferably 20 μm or more, and still more preferably 30 μm or more.

[0114] In addition, it is preferable that 1.10 < Da1 / Da imin < 4.00. By making Da1 / Da imin greater than 1.10, the dispersion of the blocking components in the thickness direction is promoted. In addition, by making Da1 / Da n less than 4.00, high strength can be maintained. The relationship of Da1 / Da i is more preferably 1.15 < Da1 / Da imin < 3.50, and still more preferably 1.20 < Da1 / Da imin < 3.00.

[0115] In addition, it is preferable that 0.50 μm < Db2 < 2.00 μm. By making the average diameter Db2 of the spherical structure near the second surface greater than 0.50 μm, a hollow fiber membrane with high water permeability can be obtained. In addition, by making Db2 less than 2.00 μm, blocking performance suitable for sterilization etc. can be imparted.

[0116] In addition, it is preferable that 1.00 ≤ Da1 / Da2 ≤ 1.10. By making Da1 / Da2 1.00 or more, the average diameter of the spherical structure becomes smaller from the first surface. Therefore, a more suitable structure can be obtained in the present invention. By making Da1 / Da2 1.10 or less, the average diameter of the spherical structure slowly becomes smaller from the first surface. Therefore, i min can be increased, and an improvement in filtration performance can be expected.

[0117] Furthermore, the first surface is preferably the outer surface of the hollow fiber membrane (the outer surface of the hollow fiber membrane is the first surface). By placing the first surface on the outer surface of the hollow fiber membrane, the membrane area can be increased.

[0118] Next, use Figure 4 The throat diameter of the spherical structure gap relative to the thickness direction of the hollow fiber membrane will be explained.

[0119] Figure 4 These are images of the pore network model analysis of hollow fiber membranes. (a) shows the vicinity of the first surface, and (b) shows the vicinity of the second surface. In both images, the upper side is the membrane surface.

[0120] A pore network model refers to a model in which the porous part of a porous body is mechanically divided at the throat of the pore, and spheres (sometimes referred to as pores) with a volume equivalent to the divided area and tubes (sometimes referred to as throats) with an area equivalent to the divided surface are made. The porous part of the porous body is represented as a network structure of pores and throats.

[0121] In the analysis of pore network models, any three-dimensional image obtained by overlaying electron microscope photographs can be used. The size of the analysis area is not particularly limited. When the average diameter of the spherical structure is 1 to 3 μm, if a three-dimensional image obtained by overlaying photographs of a 50 μm square area is used, it can ensure a sufficient number of pore throats in the spherical structure gaps for analysis, and is therefore preferred.

[0122] Generally, the permeability of porous materials is quantitatively analyzed by performing Hagenpoeia fluid analysis on the water flow within the throat. Here, assuming equal porosity within the analysis region, the larger the average throat diameter—in other words, the larger the average pore throat diameter—the higher the permeability.

[0123] The average throat diameter da1 of the spherical structure gaps in region Sa1 and the average throat diameter db2 of region Sb2 preferably satisfy the relationship da1>db2. That is, when the throat diameter obtained by analyzing the pore network model for hollow fiber membranes is set as the throat diameter of the spherical structure gaps, for region Sa1, which is 10×(n-1)~10×nμm from the aforementioned first surface, the throat diameter is set as follows. n The average diameter da of the throat diameter of the spherical structure gap in the middle n And the region Sb, which is 10×(n-1)~10×nμm away from the aforementioned second surface. n The average diameter (db) of the throat diameter of the spherical structure gap in the middle n, preferably da1 > db2. By making the average pore throat diameter da1 of the region Sa1 near the first surface larger than the average pore throat diameter db2 of the region Sb2 near the second surface, the liquid permeability resistance near the first surface becomes smaller, and thus the water permeability is improved.

[0124] The inventors found that: by further making the minimum value j of the natural number j that satisfies the following conditions (1) and (2) min be 3 ≤ j min ≤ (L - 20) / 10, the water permeability and separation performance can be maintained for a long time. Among them, in the above 3 ≤ j min ≤ (L - 20) / 10, the decimal part of (L - 20) / 10 is discarded.

[0125] (1) da1 / da j ≥ 1.15

[0126] (2) da j - db2 ≤ 0.10 μm

[0127] The value j min being within the above range means that: the region Sa that is smaller than the average pore throat diameter da1 in the nearest region of the first surface and has an average pore throat diameter equivalent to that of the region Sb2 j is located closer to the second surface than Sa2. Thus, the clogging components can be more widely dispersed along the thickness direction of the membrane. As a result, the operation time can be extended. Preferably j min ≥ 3, more preferably j min ≥ 5. In addition, it is further preferably j min ≤ L × 0.5 / 10. Among them, in the above j min ≤ L × 0.5 / 10, the decimal part of L × 0.5 / 10 is discarded.

[0128] And, regarding the number of throats Na n in the aforementioned region Sa n manufactured through the foregoing analysis and the number of throats Nb n in the aforementioned region Sb n , preferably Na1 < Nb2. That is, regarding the number of throats Na n in the region Sa n and the number of throats Nb n in the region Sb n , preferably Na1 < Nb2. By increasing the number of throats near the second surface, the branching of the permeation path increases, and the bypass performance during clogging is improved.

[0129] The number of throats Na n in the region San refers to the number of throats whose center of gravity points are located in region Sa n Among them, the throats may not all (entirely) fall within region Sa n .

[0130] The inventors of the present invention found that: by further making the minimum value k of the natural number k satisfying the following conditions (3) and (4) min be 3 ≤ k min ≤ (L - 20) / 10, the water permeability and separation property can be maintained for a long time. Among them, in the above 3 ≤ k min ≤ (L - 20) / 10, the numbers after the decimal point of (L - 20) / 10 are discarded.

[0131] (3) Na1 / Na k ≥ 0.90

[0132] (4) Nb2 - Na k ≤ 400

[0133] Among them, the analysis region in the above analysis is a cuboid including an arbitrary 50-μm square region on the first side and a 50-μm square region on the second side opposite to the first side. Sa n and Sb n are regions of 50 μm × 50 μm × 10 μm.

[0134] The value k min being within the above range means that: a region Sa having more throats than the number of throats Na1 in the nearest region to the first side and having the same number of throats as the number of throats Nb2 in region Sb2 k is located closer to the second side than Sa2. Thus, the clogging components can be more widely dispersed along the thickness direction of the membrane. As a result, the operation time can be extended. Preferably, k min ≥ 3, more preferably k min ≥ 5. In addition, it is further preferably k min ≤ L × 0.5 / 10. Among them, in the above k min ≤ L × 0.5 / 10, the numbers after the decimal point of L × 0.5 / 10 are discarded.

[0135] In addition, it is preferable that 0.10 μm < db2 < 10.0 μm. Or, it is preferable that 0.1 μm < db2 < 10 μm. By making the average pore throat diameter db2 of the spherical structure gaps near the second side greater than 0.10 μm, a hollow fiber membrane with high water permeability performance can be obtained. In addition, by making db2 less than 10.0 μm, a blocking property suitable for sterilization and the like can be imparted.

[0136] Furthermore, in the hollow fiber membrane described in the embodiments of the present invention, hydrophilic polymers are preferably present on the surface and inside the spherical structure. This allows for the maintenance of water permeability and the achievement of excellent fouling resistance.

[0137] Examples of hydrophilic polymers include polymers containing vinyl alcohol, ethylene glycol, vinylpyrrolidone, methacrylic acid, allyl alcohol, cellulose, and vinyl acetate. Furthermore, examples of copolymer polymers containing hydrophilic groups include polyvinyl alcohol with a saponification degree of less than 99%, vinylpyrrolidone-vinyl acetate copolymers, vinylpyrrolidone-vinylcaprolactam copolymers, and vinylpyrrolidone-vinyl alcohol copolymers, preferably including at least one of these.

[0138] The content of the hydrophilic polymer is preferably 1.0 part by mass or more relative to 100 parts by mass of the hydrophobic polymer. More preferably, the content of the hydrophilic polymer is 1.0 part by mass or more and 6.0 parts by mass or less relative to 100 parts by mass of the hydrophobic polymer, and even more preferably, it is 1.0 part by mass or more and 4.0 parts by mass or less.

[0139] When the content of hydrophilic polymers is greater than 6.0 parts by weight, the flow path may be narrowed due to the hydrophilic polymers, and the permeability of the liquid may be reduced. For example, when the spherical structure contains polyvinylidene fluoride resin, the content of hydrophilic polymers is preferably within the above range relative to 100 parts by weight of polyvinylidene fluoride resin.

[0140] Furthermore, regarding the hollow fiber membrane described in the embodiments of the present invention, when the ratio (mass %) of the hydrophilic polymer to the polyvinylidene fluoride resin after the hollow fiber membrane has been immersed in a 3000 ppm sodium hypochlorite aqueous solution (pH 12.5) at 60°C for 30 hours is defined as P1, and the ratio (mass %) of the hydrophilic polymer to the polyvinylidene fluoride resin before immersion is defined as P0, the percentage of P1 / P0 is preferably 70% or less. By immersing the hydrophilic polymer present on the surface of the spherical structure in the sodium hypochlorite aqueous solution, it is removed from the hollow fiber membrane, but the hydrophilic polymer present inside remains. Therefore, P1 / P0 represents the mass ratio of the hydrophilic polymer inside the spherical structure to the total mass of the hydrophilic polymer.

[0141] The content of hydrophilic polymers in hollow fiber membranes can be quantified using X-ray electron spectrometry (XPS), total internal reflection infrared spectroscopy (ATR-IR), and proton nuclear magnetic resonance spectroscopy (1H-NMR). The following will use... 1 This will be illustrated using an example of H-NMR and hollow fiber membranes containing fluoropolymer-based hydrophobic polymers. 1Methods for quantifying the amount of hydrophilic polymer introduced by ¹H-NMR include dissolving a 2 cm hollow fiber membrane in 1 mL of dimethyl sulfoxide and then using… 1 The determination is performed using H-NMR. For example, in the case where the spherical structure contains polyvinylidene fluoride resin, the determination is performed at any two points on the hollow fiber membrane to determine the amount of hydrophilic polymer when the proportion of detected PVDF resin is set to 100.

[0142] <Manufacturing Method of Hollow Fiber Membrane>

[0143] In the method for manufacturing hollow fiber membrane according to the embodiments of the present invention, for example, when the spherical structure comprises a polyvinylidene fluoride resin, the following steps (a) to (d) are included.

[0144] (a) A process of dissolving a polyvinylidene fluoride resin in a poor solvent to obtain a polyvinylidene fluoride resin solution.

[0145] (b) The process of holding the aforementioned polyvinylidene fluoride resin solution under conditions of forming a primary nucleus.

[0146] (c) After the aforementioned step (b), a process of spraying a polyvinylidene fluoride resin solution in the form of hollow fibers through piping and nozzles, and, in at least one of the aforementioned piping or nozzles, applying a temperature gradient along the thickness direction of the polyvinylidene fluoride resin solution.

[0147] (d) After the aforementioned step (c), the aforementioned polyvinylidene fluoride resin solution is immersed in a cooling bath, thereby causing the aforementioned polyvinylidene fluoride resin solution to solidify through solid-liquid thermal phase separation.

[0148] Furthermore, the temperature gradient ΔT (°C) applied through the aforementioned process (c) and the time t (seconds) for applying the temperature gradient are preferably 50 ≤ ΔT × t ≤ 300.

[0149] In step (a), a poor solvent is used to induce solid-liquid thermal phase separation in the subsequent step (d). Examples of solvents that induce solid-liquid thermal phase separation include medium-chain alkyl ketones, esters, and organic carbonates such as cyclohexanone, isophorone, γ-butyrolactone, methyl isopentyl ketone, dimethyl sulfoxide, and propylene carbonate, as well as their mixtures.

[0150] A higher resin concentration in the resin solution facilitates the formation of primary nuclei, resulting in a smaller average diameter of the spherical structure and thus a separation membrane with high strength and elongation. Conversely, a lower resin concentration in the resin solution leads to a higher porosity of the resulting separation membrane, resulting in high water permeability. Based on these considerations, to balance water permeability and strength and elongation, the resin concentration in the resin solution is preferably 30% by weight or more and 60% by weight or less.

[0151] The dissolution temperature T1℃ in step (a) is preferably above the crystallization temperature Tc℃. The crystallization temperature Tc can be measured using a differential scanning calorimetry (DSC) device. The crystallization temperature Tc refers to the temperature at which the crystallization peak rises during the process of sealing a mixture with the same composition as the resin solution used for film formation into a sealed DSC container, heating it to the dissolution temperature at a heating rate of 10℃ / min, maintaining it for 30 minutes to achieve uniform dissolution, and then cooling it at a cooling rate of 10℃ / min.

[0152] Specifically, the temperature T1℃ is more preferably (Tc+20)℃ or higher, and even more preferably (Tc+30)℃ or higher. Furthermore, the temperature T1℃ is preferably (Tc+100)℃ or lower, and more preferably (Tc+90)℃ or lower. More specifically, the temperature T1℃ is preferably 100℃ or higher, and more preferably 110℃ or higher. Additionally, the temperature T1℃ is preferably 200℃ or lower, and more preferably 190℃ or lower.

[0153] In addition, in order to uniformly form a spherical structure through solid-liquid thermal phase separation, it is preferable to have a polyvinylidene fluoride resin solution that can be uniformly dissolved. For this purpose, the dissolution time is preferably 2 hours or more, and more preferably 4 hours or more.

[0154] The resin solution may contain other additives.

[0155] Step (b) is a step in which the polyvinylidene fluoride resin solution is held under conditions that allow for the formation of primary nuclei. Based on X-ray diffraction results, etc., it can be considered that the formation of the spherical structure is a crystallization process. Generally, the crystals that begin to form during the crystallization of crystalline polymers such as polyvinylidene fluoride resins are called primary nuclei. These primary nuclei grow to form a spherical structure.

[0156] The growth of spherical structures continues until they collide with each other, at which point growth ceases. Therefore, the final particle size of the spherical structure depends on the number of primary nuclei initially formed. In other words, it can be considered that to obtain small, micro-spherical structures, it is preferable to form multiple primary nuclei, while to obtain large, giant spherical structures, it is preferable to form a small number of primary nuclei. That is, controlling the formation of primary nuclei is effective for controlling the development of spherical structures.

[0157] The formation of primary nuclei in the resin solution can be controlled by temperature, pressure, and / or time. Specifically, the primary nucleation temperature T2 is preferably above the temperature of the cooling bath in step (d), more preferably above the crystallization temperature Tc, and even more preferably above (Tc+20)°C. Furthermore, it is preferably below the resin dissolution temperature T1°C, more preferably below (Tc+55)°C.

[0158] As the pressure, the resin solution is preferably pressurized to 0.5 MPa or more, more preferably to 0.8 MPa or more. By retaining the resin solution under this pressure, the primary nucleation at the aforementioned temperature is stably advanced. The upper limit of the pressure is preferably 3.0 MPa.

[0159] The holding time under the conditions of primary nucleation is preferably 10 seconds or more, and preferably 20 seconds or more. The primary nucleation of crystallization in thermally induced phase separation is carried out slowly in the region below the melting temperature to above the crystallization temperature Tc (referred to as the "quasi-stable region" in this specification). By setting the resin solution under the aforementioned conditions, the number of primary nuclei formed can be controlled relatively stably.

[0160] Specifically, step (c) refers to: spraying the resin solution after step (b) into a hollow fiber shape through a tubular molding nozzle. At this time, the resin can be supplied to the molding nozzle via piping provided after step (b) and before step (c). An example of the shape of the molding nozzle is shown below. Figure 5 .like Figure 5 As shown, the tube head 1 for manufacturing hollow fiber membranes includes an inner nozzle 11 and an annular nozzle 12 disposed outside the inner nozzle 11 in a manner that surrounds the inner nozzle. Figure 6 , 7 As shown, the inner nozzle 11 has an inner nozzle inlet 111 and an inner nozzle outlet 112, and the annular nozzle 12 has an annular nozzle inlet 121 and an annular nozzle outlet 122. Resin solution is ejected from the annular nozzle of the outer tube, while fluid forming a hollow portion is ejected from the nozzle of the inner tube, thereby shaping (processing) the resin solution into a hollow shape.

[0161] Examples of piping shapes are shown below. Figure 8 The pipe head 1 is connected to the piping 2. Specifically, the piping outlet 132 is connected to the annular nozzle inlet 121. The resin solution is introduced from the piping inlet 131, and the resin solution exiting from the piping outlet 132 is introduced into the annular nozzle 12 through the annular nozzle inlet 121.

[0162] Furthermore, step (c) includes the following step: in at least one of the aforementioned piping or pipe fittings, applying a temperature gradient along the thickness direction to the polyvinylidene fluoride resin solution. "Applying a temperature gradient along the thickness direction" means that in the resin solution flowing in the piping or in the resin solution formed in a hollow fiber shape in the pipe fitting, a portion of the temperature along the thickness direction is different from the temperature of other portions. For example, in the resin solution formed in a hollow fiber shape, the temperature is relatively increased from one side to the other.

[0163] As a specific method for imparting a temperature gradient along the thickness direction, at least one of the following (1) to (3) can be listed.

[0164] (1) Make the temperature of tube head 1 higher or lower than the temperature of the supplied resin solution.

[0165] (2) The temperature of the injected liquid passing through the nozzle 11 inside the tube head 1 is higher or lower than the temperature of the supplied resin solution.

[0166] (3) Ensure that the temperature of the front section of the piping that supplies liquid to pipe head 1 is higher or lower than the temperature of the supplied resin solution.

[0167] By implementing at least one of (1) to (3) above, it is possible to make the spherical structure near either the outer or inner surface of the hollow fiber membrane larger than the spherical structure near the other surface. The temperature of the tube head 1, the temperature of the injection liquid, and the temperature of the piping in (1) to (3) above are respectively denoted as T3 (°C).

[0168] In methods that relatively increase the temperature, the resin temperature can also be lowered except for the portion where a spherical structure with a large average diameter is desired. However, in this case, the effect of reducing the spherical structure on the side where the resin temperature is lowered is less than the effect of increasing the resin temperature to enlarge the spherical structure. This is because, as described above, nucleus formation occurs slowly within the quasi-stable region.

[0169] For example, when a temperature gradient is applied to the resin solution in a short time by cooling at a specific temperature, this becomes an insufficient condition for primary nucleus formation. On the other hand, the number of primary nuclei after formation is greatly affected by the increase in temperature of the resin solution, resulting in a decrease in the number of spherical structures after curing. In other words, the behavior of primary nuclei in the resin solution changes differently with respect to temperature changes of heating and cooling.

[0170] Therefore, when the diameter of the spherical structure changes by "imposing a temperature gradient", it is preferable to use a process that partially heats the resin solution.

[0171] The means of controlling temperature (temperature control means) mentioned in (1) to (3) above are not limited to specific devices or methods. Examples of temperature control means are shown below.

[0172] In (1) above, the tube head can be heated by a heater arranged around the tube head, or the tube head can be heated by a mold temperature controller before use.

[0173] In (2) above, the injection fluid can be heated by a heater or the like before it reaches pipe head 1. The injection fluid can flow in the piping connected to pipe head 1 after heating, or the piping itself can be heated by a heater.

[0174] In (3) above, the piping can be heated simply by using a heater or a mold temperature controller. Regarding the shape of the piping, from the viewpoint of imparting a temperature gradient along the thickness direction of the hollow fiber membrane, a cylindrical pipe is preferred.

[0175] In addition, the “temperature” T3 in (1) to (3) above refers to the set temperature of each temperature control means (e.g., heater or mold temperature controller).

[0176] From the perspective of "imposing a temperature gradient along the thickness direction", the methods of increasing the temperature of the tube head 1 in (1) and increasing the temperature of the injected liquid in (2) are preferred in terms of controlling the unevenness of the temperature gradient in the circumferential direction of the hollow fiber membrane. In the method of increasing the temperature of the front-end piping that supplies liquid to the tube head 1 in (3) above, a piping and tube head design is required such that the resin solution heated near the pipe wall in the piping is also supplied near the wall in the tube head 1. However, compared with the case of heating the tube head 1, it is not necessary to heat the injected liquid. As will be described later, the injected liquid may boil and mix with air bubbles due to heating conditions. Therefore, it is also suitable to use a method (3) that can heat only the resin solution. In this case, the temperature of the tube head 1 in the rear section is preferably controlled to a temperature of T2 to T3 in order to suppress cooling.

[0177] Furthermore, when heating the piping, it is preferable to heat the piping laid in the vertical direction. In piping laid in the horizontal direction, the resin solution mixes within the piping due to its own weight, making it difficult to maintain a temperature gradient along the thickness direction. In piping laid in the vertical direction, mixing of the resin solution in the thickness direction can be suppressed.

[0178] In the process of applying a temperature gradient, if the temperature gradient ΔT (°C) and the time t (seconds) for applying the temperature gradient are set to 50 ≤ ΔT × t ≤ 300, then the minimum value of the natural number i that satisfies the following conditions (1) and (2) can be obtained. min Let 3≤i min ≤(L-20) / 10.

[0179] (1) Da1 / Da i ≥1.1

[0180] (2) -0.3μm ≤ Da i -Db2 ≤ 0.3μm.

[0181] Here, the temperature gradient ΔT is the absolute value of T3 - T2, and the time t is the time during which the resin solution flows through the pipe or the pipe head with the temperature gradient applied (referred to as the residence time). By making ΔT × t ≥ 50, a heat gradient can be applied to the deep part in the thickness direction of the resin solution. Therefore, a gradient can also be generated from the surface of the film to the deep part in the thickness direction with respect to the diameter of the spherical structure. If ΔT × t is too large, the thermal uniformity in the thickness direction of the resin solution is improved, and thus the diameter of the spherical structure in the obtained film becomes uniform in the thickness direction. In contrast, by making ΔT × t ≤ 300, a gradient of the spherical structure as described above can be formed. Therefore, it is preferably 50 ≤ ΔT × t ≤ 300, and more preferably 60 ≤ ΔT × t ≤ 250.

[0182] The time t for applying the temperature gradient is preferably 0.1 second or more and 20 seconds or less, and more preferably more than 5 seconds and 10 seconds or less.

[0183] In the case of applying a temperature gradient to the resin solution by heating the hollow fiber membrane tube head 1 shown in Figures 5-7 , the time t for applying the temperature gradient (heating time when heating the resin solution) is the time taken for the resin solution to pass through the tube head. In addition, in the case of applying a temperature gradient to the resin solution by heating the upstream pipe of the tube head 1, the time t is the time taken for the resin solution to pass through this pipe. For example, as shown in Figure 8 , in the case of supplying the resin solution from the pipe 2 to the tube head 1 and heating the outer peripheral part of the pipe 2, the time from when the resin passes through the pipe inlet 131 to the pipe outlet 132 is the time t. <{

[0184] Furthermore, in step (c), the thickness Ls of the polyvinylidene fluoride - based resin with the temperature gradient applied is preferably 5 < Ls / L < 40 with respect to the thickness L of the layer of the spherical structure. The thickness Ls of the polyvinylidene fluoride - based resin with the temperature gradient applied refers to the flow path thickness of the pipe with the temperature gradient applied. For example, in the case of applying a temperature gradient to the tube head 1 as shown in Figure 6 , since the polyvinylidene fluoride - based resin passes through the annular nozzle 12, the thickness Ls of the polyvinylidene fluoride - based resin is the thickness of the annular nozzle 12. In other words, it is the value obtained by subtracting the inner diameter from the outer diameter of the annular nozzle 12 and dividing by 2.

[0185] In addition, when a temperature gradient is imparted to the upstream piping for supplying liquid to the pipe head 1, if the pipe through which the polyvinylidene fluoride resin passes is a circular pipe, the radius thereof becomes the thickness Ls of the polyvinylidene fluoride resin. When the pipe has a shape other than a circular pipe, the equivalent circular pipe radius of the pipe becomes the thickness Ls of the polyvinylidene fluoride resin. The equivalent circular pipe radius is a value obtained by 2×A / P based on the cross-sectional area A of the pipe and the wetted perimeter P of the pipe. The wetted perimeter P is the perimeter of the liquid contact portion in the pipe cross-section.

[0186] The piping position for calculating the thickness Ls is the position closest to the heat source. When the heat source is a heat source that heats at a point, such as a heat medium for supplying liquid to a mold or a rod heater, in the pipe through which the polyvinylidene fluoride resin passes, it is the flow path thickness at the position closest to the heat source. When the heat source heats the pipe in a surface manner, such as a strip heater or a double pipe, it is the flow path thickness at the center position within the range covered by the heat source in the length direction of the pipe through which the polyvinylidene fluoride resin passes.

[0187] By making Ls / L > 5, it is possible to prevent heat conduction through the entire thickness of the layer of the spherical structure, and a structure is obtained in which the spherical structure on the first surface side is coarse and the spherical structure on the second surface side is dense, such as the hollow fiber membrane described in the embodiments of the present invention. By making Ls / L < 40, it is possible to prevent the spherical structure in only the region Sa1 on the first surface side from becoming coarse, and a structure is obtained in which the size of the spherical structure gradually decreases toward the second surface side. More preferably, 10 < Ls / L < 40, and further preferably, 15 < Ls / L < 35.

[0188] The polyvinylidene fluoride resin solution ejected in a hollow fiber shape passes through the air and is then immersed in a cooling bath. At this time, it is preferably passed through the air for 0.3 seconds or more. The time of passing through the air refers to the time from when the polyvinylidene fluoride resin solution ejected from the pipe head in step (c) reaches the cooling bath, and for convenience, it is hereinafter denoted as "air walking time".

[0189] As the air walking time, if it is 0.3 seconds or more, the time for the heat applied in step (c) to conduct to the inner surface side can be ensured, which is preferable. It is preferably 1 second or more, and further preferably 1.5 seconds or more. In addition, when the air walking time is long, cooling is performed from the outer surface, so it is preferably 5 seconds or less. Further preferably, it is 3 seconds or less.

[0190] The air walking time can be calculated by the following formula.

[0191] Air walking time (seconds) = Air walking distance (m) / Traction speed in the cooling bath (m / second)

[0192] Here, the overhead travel distance refers to the length of a straight line drawn vertically downwards from the lowest point of the tube head to the upper surface of the cooling bath. The traction speed in the cooling bath can be calculated based on the rotational speed (rpm) and diameter (m) of the cooling bath rollers using the following formula. π is the mathematical constant pi. For example, when a roller with a diameter of 0.2m rotates at 10rpm, the traction speed is approximately 0.1m / s.

[0193] Traction speed (m / s) = Roller rotation speed (rpm) / 60 × π × Roller diameter (m)

[0194] In addition, when it passes through the air, the temperature, humidity, solvent vapor concentration, etc. can be adjusted.

[0195] In step (d), after step (c), the aforementioned polyvinylidene fluoride resin solution is cured by immersing it in a cooling bath, thereby utilizing solid-liquid thermal phase separation.

[0196] Thermally induced phase separation refers to a method of curing a resin solution obtained by dissolving it in a poor or good solvent at a temperature above the crystallization temperature Tc by cooling it. Examples of thermally induced phase separation include (A) and (B) below.

[0197] (A) A resin solution that is uniformly dissolved at high temperature separates into a liquid-liquid form with a thick resin phase and a thin resin phase due to the decrease in the solubility of the solution when cooled.

[0198] (B) The resin solution that is uniformly dissolved at high temperature crystallizes when cooled to a temperature below the crystallization temperature Tc, and the phases separate into a solid-liquid type consisting of a polymer solid phase and a polymer dilute solution phase.

[0199] In liquid-liquid separation membranes, a dense phase is used to form a fine three-dimensional network structure; conversely, in solid-liquid separation membranes, a spherical structure is formed. Therefore, a solid-liquid separation membrane is preferably used in the manufacture of the separation membrane according to embodiments of the present invention.

[0200] The preferred cooling bath is a mixture of a poor solvent (50% to 95% by weight) and a good solvent (5% to 50% by weight) and a non-solvent (50% to 50% by weight). By keeping the concentration of the non-solvent below 50% by weight, thermally induced phase separation-based coagulation can be preferentially achieved compared to coagulation based on non-solvent-induced phase separation. It should be noted that a low concentration of the good solvent results in a higher coagulation rate; therefore, by lowering the temperature of the cooling bath, coagulation can be promoted even with a high concentration of the good solvent, thus smoothing the surface of the separation membrane.

[0201] As a poor solvent, it is preferable to use the same poor solvent as the resin solution.

[0202] Examples of good solvents include N-methyl-2-pyrrolidone, dimethyl sulfoxide, dimethylacetamide, dimethylformamide, methyl ethyl ketone, acetone, tetrahydrofuran, tetramethylurea, trimethyl phosphate, and other lower alkyl ketones, esters, amides, and their mixtures.

[0203] In addition, non-solvents include water, hexane, pentane, benzene, toluene, methanol, ethanol, carbon tetrachloride, o-dichlorobenzene, trichloroethylene, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butanediol, pentanediol, hexanediol, low molecular weight polyethylene glycol, and other aliphatic hydrocarbons, aromatic hydrocarbons, aliphatic polyols, aromatic polyols, chlorinated hydrocarbons, or other chlorinated organic liquids and their mixtures.

[0204] It should be noted that, in this specification, "good solvent" refers to a solvent that can dissolve more than 5% by weight of the solute even at low temperatures below 60°C. Conversely, "poor solvent" refers to a solvent that cannot dissolve more than 5% by weight of the solute at low temperatures below 60°C, but can dissolve more than 5% by weight of the solute in a high-temperature region above 60°C and below the melting point of polyvinylidene fluoride resins. "Non-solvent" refers to a solvent that neither dissolves nor swells up to the melting point of the solute or the boiling point of the solvent.

[0205] By lowering the cooling bath temperature T4 below the crystallization temperature Tc of the polyvinylidene fluoride (PVDF) resin, solid-liquid phase separation is induced. The relationship between the cooling bath temperature T4 and the crystallization temperature Tc, as well as the primary nucleation temperature T2, will be explained below.

[0206] In solid-liquid thermally induced phase separation, crystallization (hereinafter also referred to as nucleation) occurs due to cooling, and the growth of the formed crystal nuclei occurs, resulting in phase separation into a polymer solid phase and a polymer dilute solution phase. It can be considered that in solid-liquid phase separation, growth occurs starting from primary nuclei formed in step (b), as well as random nucleation and growth not starting from primary nuclei. In growth starting from primary nuclei, the number of spherical structures generated is related to the number of primary nuclei. The growth of spherical structures continues until they come into contact with surrounding spherical structures; therefore, if the number of primary nuclei is small, growth is promoted, resulting in coarse spherical structures, and if the number of primary nuclei is small, dense spherical structures are generated.

[0207] Growth starting from the primary nuclei and random nucleation and growth not starting from the primary nuclei depend on the cooling rate of the resin solution. That is, it depends on the difference between the primary nucleation temperature T2 and the cooling bath temperature T4. The smaller this difference, the easier the former crystallization is, and the larger this difference, the easier the latter crystallization is. That is, in step (c), when a temperature gradient is imparted in the thickness direction of the hollow fiber membrane and the resin solution in a state where the number of primary nuclei varies in the thickness direction is rapidly cooled, random nucleation occurs regardless of the number of primary nuclei. Therefore, it is difficult to generate a structure in which the average diameter of the spherical structure varies in the thickness direction of the hollow membrane. On the other hand, when the resin in a state where the number of primary nuclei varies in the thickness direction is slowly cooled, growth starting from the primary nuclei occurs. Thus, a hollow fiber membrane in which the average diameter of the spherical structure varies in the thickness direction can be obtained.

[0208] In addition, the crystallization temperature Tc is also affected by the cooling rate of the resin solution. The faster the cooling rate, the higher the crystallization temperature and the faster the phase separation occurs. The cooling rate depends on the difference between the primary nucleation temperature T2 and the cooling bath temperature T4.

[0209] In the method for manufacturing a hollow fiber membrane according to an embodiment of the present invention, through trial and error in view of the above characteristics, it was found that: by manufacturing under the following temperature conditions, the average diameter of the spherical structure varies in the thickness direction of the hollow membrane, and thus the present invention was completed.

[0210] (Tc - T4) / (T2 - T4) < 0.50

[0211] T2 - T4 is a parameter that constitutes the overall cooling rate, and Tc - T4 is a parameter that contributes to growth. It was found that: among the overall cooling rates, by reducing the temperature difference that contributes to growth, growth starting from the primary nuclei occurs. More preferably, (Tc - T4) / (T2 - T4) < 0.45, and further preferably, (Tc - T4) / (T2 - T4) < 0.40. In the method for manufacturing a hollow fiber membrane according to an embodiment of the present invention, it is preferable to satisfy the relationship of 15 < Tc - T4 < 35. By making Tc - T4 greater than 15°C, the solidification time can be prevented from becoming too long, and the manufacturing equipment can be prevented from becoming too large. In addition, it was found that: by making Tc - T4 less than 35°C, growth starting from the above-mentioned primary nuclei occurs.

[0212] In addition, T2 - T4 is preferably less than 100°C, and further preferably less than 80°C.

[0213] The hollow fiber membrane solidified in step (d) can be manufactured through the following steps thereafter.

[0214] After step (d), stretching is preferably performed to increase the permeability and tensile strength by enlarging the gaps between the spherical structures. The temperature around the membrane during stretching is preferably 50°C or higher and 140°C or lower, more preferably 55°C or higher and 120°C or lower, and even more preferably 60°C or higher and 100°C or lower. The stretching ratio is preferably 1.1 times or higher and 4 times or lower, more preferably 1.1 times or higher and 2 times or lower.

[0215] When stretched at temperatures above 50°C, the membrane can be stretched stably and uniformly. When stretched at temperatures below 140°C, the membrane is stretched at a temperature lower than the melting point of polyvinylidene fluoride resin (177°C). Therefore, even when stretched, the membrane will not melt, thus maintaining the membrane structure while increasing the porosity and improving water permeability.

[0216] Furthermore, stretching is preferred because it allows for easy temperature control. Stretching can also be performed in gases such as steam.

[0217] Water is preferred as a liquid due to its simplicity. When stretching is performed at temperatures above approximately 90°C, low molecular weight polyethylene glycol or similar materials can also be used. On the other hand, without such stretching, compared to when stretching is performed, water permeability and tensile strength decrease, but elongation at break and resistance increase. Therefore, the presence or absence of a stretching process and the stretching ratio of the stretching process can be appropriately set according to the intended use of the separation membrane.

[0218] When stretching occurs in a liquid, the above temperature conditions can be applied to the temperature of the liquid; when stretching occurs in a gas, the above temperature conditions can be applied to the temperature of the gas. It should be noted that these manufacturing methods can be applied without particular limitation as long as the thermoplastic resin forms a spherical structure through thermally induced phase separation.

[0219] Alternatively, heat treatment can be performed after step (d). The glass transition temperature of polyvinylidene fluoride (PVDF) resins is around -49°C, and they shrink slowly depending on the operating environment. Therefore, by pre-shrinking them at a temperature higher than the operating temperature, shrinkage during use can be suppressed. Preferably, heat treatment is performed at the operating temperature +10°C, and more preferably at the operating temperature +20°C or higher, until shrinkage disappears.

[0220] In addition, it is preferable to further include the following steps (e) and (f) after step (d).

[0221] (e) The process of introducing hydrophilic polymers into hollow fiber membranes.

[0222] (f) A heat treatment process at 100°C or higher following the aforementioned process (e).

[0223] This describes a method for introducing a hydrophilic polymer into a hollow fiber membrane. Examples of methods include: physically adsorbing the hydrophilic polymer by passing an aqueous solution containing the polymer through the hollow fiber membrane or by impregnating the membrane with the polymer; rendering the polymer insoluble by irradiation; or forming covalent bonds through a chemical reaction with reactive groups present in the hollow fiber membrane. Irradiation is particularly preferred because it allows the hydrophilic polymer to adhere more firmly to the surface of the spherical structure. Specifically, in step (e), irradiation is preferably performed after the aqueous solution containing the hydrophilic polymer is introduced into the hollow fiber membrane.

[0224] Furthermore, by introducing the hydrophilic polymer in the above-mentioned process, compared with methods such as mixing the hydrophilic polymer into the film-forming solution and then forming the film, the hydrophilic polymer can be attached to the surface of the spherical structure, thereby improving the dirt resistance.

[0225] If the concentration of the hydrophilic polymer aqueous solution is too low, a sufficient amount of hydrophilic polymer will not be introduced. Therefore, the concentration of the copolymer (hydrophilic polymer) in the above aqueous solution is preferably 10 ppm or more, more preferably 100 ppm or more, and most preferably 500 ppm or more. However, if the concentration of the hydrophilic polymer in the aqueous solution is too high, there is a concern about an increase in leaching from the module. Therefore, the concentration of the copolymer (hydrophilic polymer) in the above aqueous solution is preferably 100,000 ppm or less, more preferably 10,000 ppm or less.

[0226] It should be noted that, in cases where the aforementioned hydrophilic polymers are poorly soluble or insoluble in water, they can be compatible with organic solvents that do not dissolve hollow fibers or with water, and can dissolve in a mixed solvent of organic solvents that do not dissolve hollow fibers and water. Specific examples of organic solvents that can be used in the aforementioned organic solvents or mixed solvents include, but are not limited to, alcohol-based solvents such as methanol, ethanol, or propanol.

[0227] Furthermore, if the proportion of organic solvent in the above-mentioned mixed solvent increases, there is a possibility that the hollow fibers may swell, and the pore size of the hollow fiber membrane may change. Therefore, the mass fraction of organic solvent in the above-mentioned mixed solvent is preferably 60% or less, more preferably 10% or less, and most preferably 1% or less.

[0228] In the aforementioned radiation irradiation, alpha rays, beta rays, gamma rays, X-rays, ultraviolet rays, or electron beams can be used. From the viewpoints of safety and simplicity, radiation methods using gamma rays or electron beams are preferred. The radiation dose is preferably 15 kGy or more, more preferably 25 kGy or more. By setting it to 15 kGy or more, the hydrophilic polymer can be effectively introduced. Furthermore, the aforementioned radiation dose is preferably 100 kGy or less. This is because if the radiation dose exceeds 100 kGy, the copolymer may easily undergo three-dimensional cross-linking, decomposition of the ester group of the vinyl carboxylate monomer unit, etc.

[0229] To inhibit cross-linking reactions during radiation exposure, antioxidants can be used. Antioxidants are substances that readily donate electrons to other molecules; examples include water-soluble vitamins such as vitamin C, polyphenols, and alcoholic solvents such as methanol, ethanol, or propanol, but are not limited to these. These antioxidants can be used alone or in combination of two or more. Safety must be considered when using antioxidants; therefore, antioxidants with low toxicity, such as ethanol and propanol, are suitable.

[0230] As described above, the amount of the aforementioned hydrophilic polymer introduced into the hollow fiber membrane can be quantified using total internal reflection infrared spectroscopy (ATR-IR). Alternatively, quantification can be performed using X-ray electron spectrometry (XPS) or similar methods, depending on the requirements.

[0231] Furthermore, after introducing the hydrophilic polymer into the hollow fiber membrane, it is preferable to heat-treat the hollow fiber membrane. Examples of heat treatment steps for the hollow fiber membrane include applying heat in a dry state and applying heat under humid conditions such as water vapor. Through heat treatment, a portion of the polyvinylidene fluoride (PVDF) resin shrinks, allowing a portion of the hydrophilic polymer to enter the interior of the spherical structure. If the heat treatment temperature is too low, a sufficient amount of hydrophilic polymer will not be introduced; therefore, the heat treatment temperature is preferably 100°C or higher, more preferably 120°C or higher, and most preferably 150°C or higher. On the other hand, if the heat treatment temperature is too high, it exceeds the melting point of PVDF, causing the hollow fiber membrane to dissolve, resulting in micropore blockage and reduced permeability. Therefore, the heat treatment temperature is preferably below the melting point of the PVDF copolymer.

[0232] Example

[0233] The present invention will now be described in more detail by way of examples, but the present invention is not limited to these examples at all.

[0234] (1) Region Sa n Average diameter Da n The thickness L of the spherical structure layer (film thickness L)

[0235] For the cross-section perpendicular to the length direction of the hollow fiber membrane (the cross-section parallel to the thickness direction), an electron microscope (SU1510) manufactured by HITACHI was used to photograph at 1000x magnification to visualize the arc forming the first surface. In the resulting image, straight lines Ma1, Ma2…Ma are gradually drawn at 10μm intervals from the first surface toward the second surface. n For the straight line Ma n and Ma n+1 The area Sa enclosed by the left and right edges of the photo n For all measurable spherical structures, measure their major diameter. Calculate their arithmetic mean to obtain the average diameter Da. n .

[0236] On the first surface and in area Sa n Without falling into a single captured image, multiple fields of view, staggered along the thickness direction and partially overlapped, are captured at the same magnification. The captured images are then connected by overlapping them with the same structure, thereby obtaining a continuous image in the thickness direction.

[0237] Furthermore, the thickness L (membrane thickness L) of the spherical structure is determined using the following method. The membrane thickness L is calculated based on the outer and inner diameters of the hollow fiber membrane using the formula L = (outer diameter - inner diameter) / 2. Regarding the outer and inner diameters of the hollow fiber membrane, the membrane is cut perpendicular to the axial direction using a single-edged knife or similar tool, and the cross-section is observed using a microscope or similar method to determine the diameter of the circle. In the case of a flattened circle, the lengths of the longest diameter portion (major axis) and the shortest diameter portion (minor axis) are measured, and the average of these two lengths is taken as the diameter.

[0238] (2) The average diameter Db2 of region Sb2

[0239] For a cross-section perpendicular to the length direction of the hollow fiber membrane (a cross-section parallel to the thickness direction), an electron microscope was used to photograph at 1000x magnification. In the obtained images, straight lines Mb1, Mb2, and Mb3 were drawn from the second surface of the separation membrane towards the first surface. The major axis of all measurable spherical structures within the region Sb enclosed by Mb2, Mb3, and the left and right ends of the photograph were measured. The arithmetic mean of the obtained values ​​was calculated to obtain the average diameter Db2.

[0240] (3) Region Sa n Average throat diameter (average diameter of throat) da n And the number of larynx Na n Region Sb n Average throat diameter (average diameter of throat) db n And the number of larynxes Nb n

[0241] After coating the hollow fiber membrane with RuO4, resin was filled into the pores. Using the focused ion beam etching function of an electron microscope (Helios G4 CX) manufactured by FEI, 200 images of cross-sections perpendicular to the length direction of the hollow fiber membrane (parallel to the thickness direction) were taken at 250 μm intervals. Starting from the portion in contact with the arc forming the first surface, cubic images with one side of 50 μm were taken sequentially towards the second surface. For the cuboid formed by connecting the cubes, a pore network model was used for analysis. The centroid position and diameter of each throat were determined, and the region Sa was calculated. n The average diameter of a larynx at the center of gravity is taken as the average orifice diameter da. n Furthermore, calculate the number of larynxes Na. n Furthermore, as the analysis object based on the porous network model, cubes with one side of 50μm are sequentially photographed from the part in contact with the arc forming the second surface toward the first surface. These cubes are then connected, and the resulting cuboid is transformed into a similarly calculated region Sb as described above. n Average throat diameter (average diameter of throat) db n And the number of larynxes Nb n .

[0242] (4) Water permeability of the separation membrane

[0243] A small module, approximately 10 cm in length and composed of 1 to 10 hollow fiber membranes, was fabricated. Distilled water was fed through the first side and filtered using total volume filtration at 25°C and a filtration pressure difference of 18.6 kPa. The permeate flow rate (m³) was measured over a specified time. 3 The values ​​obtained are then converted into units of time (hr) and units of effective membrane area (m²). 2 ), and 50 kPa are used for calculation.

[0244] (5) Crystallization temperature Tc of polyvinylidene fluoride resin solution

[0245] Using a Seiko Electronics DSC-620-0, a mixture of polyvinylidene fluoride resin and a film-forming polymer stock solution with the same composition as the solvent was sealed into a sealed DSC container. The mixture was heated to the dissolution temperature at a rate of 10°C / min and held for 30 minutes to allow for homogeneous dissolution. Subsequently, the mixture was cooled at a rate of 10°C / min, and the temperature at which the crystallization peak was observed during this process was taken as the crystallization temperature Tc.

[0246] (6) Evaluation of actual liquid handling

[0247] The commercially available unfiltered beer "Galaxy Plateau Beer" (trade name, hereinafter referred to as the evaluation beer) containing brewer's yeast was used. 2L of evaluation beer was prepared and kept at 0°C in a container. The circuit was assembled as follows: The evaluation beer was pumped from this container to the outer surface of the hollow fiber membrane and returned to the container. Simultaneously, the filtrate filtered by the hollow fiber membrane was collected using a container different from the container containing the evaluation beer. At this time, the inlet and outlet pressures of the evaluation beer in the module, as well as the pressure on the filtration side, could be measured. The evaluation beer was introduced at a flow rate of 1.5 m / sec at the membrane surface linear velocity. Furthermore, the filtration speed was adjusted to 1 m / d, and the operation was performed using constant flow filtration. In this state, the evaluation beer was pumped to the outer surface of the hollow fiber membrane at 5±3°C, and cross-flow filtration was continuously performed, filtering a portion of the material. As operation continued, the hollow fiber membrane became clogged, and the permeability gradually decreased. However, due to the constant flow filtration, the intermembrane pressure difference (TMP) increased. TMP was calculated using the following formula. Here, Pi is the inlet pressure, Po is the outlet pressure, and Pf is the pressure on the filter side.

[0248] TMP=((Pi+Po) / 2)-Pf

[0249] The time [h] required for TMP to rise to 150 kPa was measured, and the beer throughput was calculated using the following formula. The beer throughput increases when high permeability can be maintained for a longer period. The beer throughput decreases when permeability decreases rapidly.

[0250] Beer processing capacity [L / m] 2 = Filtration speed 100 [L / m 2 / h]×Time until the pressure rises to 150kPa[h].

[0251] (7) Preventive

[0252] For the beer used in the evaluation, after sampling the filtrate and the filtrate, the samples were allowed to stand at room temperature for 30 minutes, and the turbidity was measured using a Nephelometry turbidimeter. The rejection rate was then calculated using the following formula.

[0253] Rejection rate (%) = (Original solution turbidity - Filtrate turbidity) / Original solution turbidity

[0254] (8) Determination of the content of hydrophilic polymers (coated polymers)

[0255] A 2 cm hollow fiber membrane was dissolved in 1 mL of dimethyl sulfoxide using a JNM-ECZ400R dimethyl sulfoxide solution manufactured by Nippon Electron Ltd. 1The determination was performed using ¹H-NMR. This measurement was conducted at any two points on the hollow fiber membrane to determine the amount of hydrophilic polymer when the detected PVDF ratio was set to 100. It should be noted that in the examples and comparative examples, the hydrophilic polymer is referred to by the following abbreviation.

[0256] PVP: Polyvinylpyrrolidone

[0257] PVP / PVAc: Random copolymer of vinylpyrrolidone and vinyl acetate

[0258] PEG: Polyethylene Glycol

[0259] PEGMA: Polyethylene glycol methacrylate

[0260] (9) Durability of chemical washing

[0261] The hollow fiber membrane containing the hydrophilic polymer was immersed in ethanol for 30 minutes, then immersed in pure water for 60 minutes to moisten it, and then cleaned in an oven at 95°C for 5 hours. Afterwards, it was immersed in an aqueous solution containing sodium hypochlorite at a concentration of 3000 ppm (based on available chlorine) and sodium hydroxide at a concentration of 0.04% by mass for 30 hours, and then washed with running water for 2 hours. The hydrophilic polymer content of the hollow fiber membrane was determined using the same procedure as described above. Furthermore, based on the results, the percentage of the ratio (by mass) of the hydrophilic polymer to PVDF obtained by (9) relative to the ratio (by mass) of the hydrophilic polymer to PVDF obtained by (8) was calculated and presented in the table as "Residual Coating Polymer After Immersion in the Solution".

[0262] [Example 1]

[0263] 38 wt% of vinylidene fluoride homopolymer with a weight-average molecular weight of 417,000 and 62 wt% of γ-butyrolactone were dissolved at 150 °C. The resin solution was then held at 100 °C (primary nucleus holding temperature T2) for 20 seconds under a pressure of 1.2 MPa, within the range of (Tc+20) °C to (Tc+55) °C.

[0264] Subsequently, the resin solution is supplied to the pipe head via a pipe whose outer periphery is heated to 108°C (T3). The residence time in the pipe heated to 108°C is 7.05 seconds (the time for imparting the temperature gradient), and the radius of the pipe is 4.0 mm (the thickness Ls of the polyvinylidene fluoride resin imparting the temperature gradient).

[0265] The resin solution was introduced into the outer tube of the tube head, while simultaneously, a 90% by weight aqueous solution of γ-butyrolactone at 100°C was sprayed from the inner tube of the tube head. The sprayed resin solution was cured for 5 minutes in a bath containing a 90% by weight aqueous solution of γ-butyrolactone at 25°C (cooling bath temperature T4). After curing, the resulting membrane was washed with water and stretched to 1.5 times its original length in warm water at 95°C. After washing with water, the stretched hollow fiber membrane was impregnated in 30% glycerol for 2 hours and then air-dried overnight. Subsequently, it was heat-treated in steam at 125°C for 1 hour.

[0266] The cross-section perpendicular to the length direction of the obtained hollow fiber membrane exhibits a spherical structure. The average diameter Da1 of region Sa1 is 2.62 μm, and the average diameter Db2 of region Sb2 is 1.84 μm. The natural number i satisfies the above conditions (1) and (2). min The average diameter Da5 of the spherical structures in region Sa5 is 2.02 μm. The average throat diameter da1 of region Sa1 is 1.40 μm, and the average throat diameter db2 of region Sb2 is 1.14 μm. The natural number j that satisfies the above conditions (1) and (2) is 5. min The average pore throat diameter (da5) of the spherical interstitial structures in region Sa5 is 1.20 μm. The membrane exhibits a pure water permeability of 3.6 m / hr and a beer filtration capacity of 400 L / m³. 2 With a rejection rate of 92%, it can maintain permeability and separation properties for a long time.

[0267] [Example 2]

[0268] 38 wt% of vinylidene fluoride homopolymer with a weight average molecular weight of 417,000 and 62 wt% of γ-butyrolactone were dissolved at 150 °C. The resin solution was then held at 100 °C for 20 seconds under a pressure of 1.2 MPa, within the range of (Tc+20) °C to (Tc+55) °C.

[0269] Subsequently, the resin solution is conveyed to the pipe head via piping that has its outer periphery heated to 125°C. The residence time of the resin solution in the piping heated to 125°C is 7.05 seconds, and the radius of the piping is 4.0 mm.

[0270] The resin solution was introduced into the outer tube of the tube head, while a 90% by weight aqueous solution of γ-butyrolactone at 100°C was sprayed from the inner tube of the double-tube head. The sprayed resin solution was cured for 1 minute in a bath containing a 90% by weight aqueous solution of γ-butyrolactone at 25°C. After curing, the resulting membrane was washed with water and stretched to 1.5 times its original length in warm water at 95°C. The stretched hollow fiber membrane was washed with water, impregnated in 30% glycerol for 2 hours, and then air-dried overnight. Subsequently, it was heat-treated in steam at 125°C for 1 hour.

[0271] The cross-section perpendicular to the length direction of the obtained hollow fiber membrane exhibits a spherical structure. The average diameter Da1 of region Sa1 is 3.40 μm, and the average diameter Db2 of region Sb2 is 2.15 μm. The natural number i satisfies the above conditions (1) and (2). min The average diameter Da5 of the spherical structures in region Sa5 is 2.30 μm. The average throat diameter da1 of region Sa1 is 1.45 μm, and the average throat diameter db2 of region Sb2 is 1.14 μm. The natural number j that satisfies the above conditions (1) and (2) is 5. min The average throat diameter (da5) of the spherical interstitial structures in region Sa5 is 1.24 μm. The membrane exhibits a pure water permeability of 4.4 m / hr and a beer filtration capacity of 700 L / m³. 2 With a rejection rate of 82%, it can maintain permeability and separation properties for a long time.

[0272] [Example 3]

[0273] 36 wt% of vinylidene fluoride homopolymer with a weight average molecular weight of 417,000 and 64 wt% of γ-butyrolactone were dissolved at 150 °C. The resin solution was then held at 99 °C for 20 seconds under a pressure of 1.2 MPa, within the range of (Tc+20) °C to (Tc+55) °C.

[0274] Subsequently, the resin solution is conveyed to the pipe head via piping that has its outer periphery heated to 108°C. The residence time of the resin solution in the piping heated to 108°C is 9.50 seconds, and the radius of the piping is 4.0 mm.

[0275] The resin solution was introduced into the outer tube of the tube head, while simultaneously, a 90% by weight aqueous solution of γ-butyrolactone at 100°C was sprayed from the inner tube of the tube head. The sprayed resin solution was cured for 5 minutes in a bath containing a 90% by weight aqueous solution of γ-butyrolactone at 25°C. After curing, the resulting membrane was washed with water and stretched to 1.5 times its original length in warm water at 95°C. After washing with water, the stretched hollow fiber membrane was impregnated in 30% glycerol for 2 hours and then air-dried overnight. Subsequently, it was heat-treated in steam at 125°C for 1 hour.

[0276] The cross-section perpendicular to the length direction of the obtained hollow fiber membrane exhibits a spherical structure. The average diameter Da1 of region Sa1 is 3.70 μm, and the average diameter Db2 of region Sb2 is 1.86 μm. The natural number i satisfies the above conditions (1) and (2). min The average diameter Da6 of the spherical structures in region Sa6 is 2.10 μm. The average throat diameter da1 of region Sa1 is 1.66 μm, and the average throat diameter db2 of region Sb2 is 1.27 μm. The natural number j satisfies the above conditions (1) and (2).min The average throat diameter (da6) of the spherical interstitial structures in region Sa6 is 1.29 μm. The membrane exhibits a pure water permeability of 4.6 m / hr and a beer filtration capacity of 450 L / m³. 2 With a rejection rate of 91%, it can maintain water permeability and separation properties for a long time.

[0277] [Example 4]

[0278] 38 wt% of vinylidene fluoride homopolymer with a weight average molecular weight of 417,000 and 62 wt% of dimethyl sulfoxide were dissolved at 120 °C. The resin solution was then held at 75 °C for 20 seconds under a pressure of 1.2 MPa, within the range of (Tc+20) °C to (Tc+55) °C.

[0279] Subsequently, the resin solution is fed into a nozzle heated to 85°C and ejected 5.8 seconds after entering the nozzle. The flow path width of the annular nozzle in the nozzle is 1.75 mm.

[0280] The resin solution was introduced into the outer tube of the tube head, while a 90% by weight aqueous solution of dimethyl sulfoxide at 75°C was sprayed from the inner tube of the tube head. The sprayed resin solution was cured for 5 minutes in a bath containing a 90% by weight aqueous solution of dimethyl sulfoxide at 18°C. After curing, the resulting membrane was washed with water and stretched to 1.5 times its original length in warm water at 95°C. After washing with water, the stretched hollow fiber membrane was impregnated in 30% glycerol for 2 hours and then air-dried overnight. Subsequently, it was heat-treated in steam at 125°C for 1 hour.

[0281] The cross-section perpendicular to the length direction of the obtained hollow fiber membrane exhibits a spherical structure. The average diameter Da1 of region Sa1 is 3.52 μm, and the average diameter Db2 of region Sb2 is 1.90 μm. The natural number i satisfies the above conditions (1) and (2). min The average diameter Da7 of the spherical structures in region Sa7 is 1.90 μm. The average throat diameter da1 of region Sa1 is 1.50 μm, and the average throat diameter db2 of region Sb2 is 1.27 μm. The natural number j that satisfies the above conditions (1) and (2) is 7. min The average pore throat diameter (da7) of the spherical interstitial structures in region Sa7 is 1.31 μm. The membrane exhibits a pure water permeability of 4.3 m / hr and a beer filtration capacity of 480 L / m³. 2 With a rejection rate of 90%, it can maintain permeability and separation properties for a long time.

[0282] [Example 5]

[0283] 38 wt% of vinylidene fluoride homopolymer with a weight average molecular weight of 417,000 and 62 wt% of γ-butyrolactone were dissolved at 150 °C. The resin solution was then held at 98 °C for 20 seconds under a pressure of 1.2 MPa, within the range of (Tc+20) °C to (Tc+55) °C.

[0284] Subsequently, the resin solution is conveyed to the pipe head via piping that has its outer periphery heated to 132°C. The residence time of the resin solution in the piping heated to 132°C is 7.05 seconds, and the radius of the piping is 4.0 mm.

[0285] The resin solution was introduced into the outer tube of the tube head, while simultaneously, a 90% by weight aqueous solution of γ-butyrolactone at 100°C was sprayed from the inner tube of the tube head. The sprayed resin solution was cured for 5 minutes in a bath containing a 90% by weight aqueous solution of γ-butyrolactone at 25°C. After curing, the resulting membrane was washed with water and stretched to 1.5 times its original length in warm water at 95°C. After washing with water, the stretched hollow fiber membrane was impregnated in 30% glycerol for 2 hours and then air-dried overnight. Subsequently, it was heat-treated in steam at 125°C for 1 hour.

[0286] The cross-section perpendicular to the length direction of the obtained hollow fiber membrane exhibits a spherical structure. The average diameter Da1 of region Sa1 is 4.30 μm, and the average diameter Db2 of region Sb2 is 2.24 μm. The natural number i satisfies the above conditions (1) and (2). min For 12, region Sa 12 The average diameter Da of the spherical structure in 12 The average pore throat diameter da1 in region Sa1 is 1.55 μm, and the average pore throat diameter db2 in region Sb2 is 1.24 μm. The natural number j that satisfies conditions (1) and (2) above... min For 12, region Sa 12 The average throat diameter da of the spherical structure gap in the middle 12 The membrane has a diameter of 1.34 μm. Its pure water permeability is 5.1 m / hr, and its beer filtration capacity is 720 L / m³. 2 With a rejection rate of 81%, it can maintain permeability and separation properties for a long time.

[0287] [Example 6]

[0288] 38 wt% of vinylidene fluoride homopolymer with a weight average molecular weight of 417,000 and 62 wt% of γ-butyrolactone were dissolved at 150 °C. The resin solution was then held at 97 °C for 20 seconds under a pressure of 1.2 MPa, within the range of (Tc+20) °C to (Tc+55) °C.

[0289] Subsequently, the resin solution is conveyed to the pipe head via piping that has its outer periphery heated to 140°C. The residence time of the resin solution in the piping heated to 140°C is 6.83 seconds, and the radius of the piping is 4.0 mm.

[0290] The resin solution was introduced into the outer tube of the tube head, while simultaneously, a 90% by weight aqueous solution of γ-butyrolactone at 100°C was sprayed from the inner tube of the tube head. The sprayed resin solution was cured for 5 minutes in a bath containing a 90% by weight aqueous solution of γ-butyrolactone at 25°C. After curing, the resulting membrane was washed with water and stretched to 1.5 times its original length in warm water at 95°C. After washing with water, the stretched hollow fiber membrane was impregnated in 30% glycerol for 2 hours and then air-dried overnight. Subsequently, it was heat-treated in steam at 125°C for 1 hour.

[0291] The cross-section perpendicular to the length direction of the obtained hollow fiber membrane exhibits a spherical structure. The average diameter Da1 of region Sa1 is 4.60 μm, and the average diameter Db2 of region Sb2 is 2.29 μm. The natural number i satisfies the above conditions (1) and (2). min For 17, region Sa 12 The average diameter Da of the spherical structure in 12 The average pore throat diameter da1 in region Sa1 is 1.86 μm, and the average pore throat diameter db2 in region Sb2 is 1.32 μm. The natural number j satisfies the above conditions (1) and (2). min For 17, region Sa 17 The average throat diameter da of the spherical structure gap in the middle 17 The membrane has a diameter of 1.60 μm. Its pure water permeability is 5.8 m / hr, and its beer filtration capacity is 800 L / m³. 2 It has a blocking rate of 72% and can maintain permeability for a long time, but shows a slight deterioration in separation.

[0292] [Comparative Example 1]

[0293] The temperature of the outer periphery of the piping was set to 100°C. Otherwise, the hollow fiber membrane was obtained using the same method as described in Example 1.

[0294] The cross section perpendicular to the length direction of the obtained hollow fiber membrane exhibits a spherical structure, but the average diameter Da1 of region Sa1 is 1.83 μm and the average diameter Db2 of region Sb2 is 1.79 μm. There is no natural number i that satisfies the above conditions (1) and (2).

[0295] The average throat diameter da1 of region Sa1 is 1.14 μm, the average throat diameter db2 of region Sb2 is 1.14 μm, and there is no natural number j that satisfies the above conditions (1) and (2).

[0296] The membrane has a pure water permeability of 2.0 m / hr and a beer filtration capacity of 150 L / m³. 2 It has a rejection rate of 96%, and although its separation ability is high, it cannot maintain water permeability for a long time.

[0297] [Comparative Example 2]

[0298] 38 wt% of vinylidene fluoride homopolymer with a weight-average molecular weight of 417,000 and 62 wt% of γ-butyrolactone were dissolved at 150 °C. The resin solution was held at 95 °C for 20 seconds under a pressure of 1.2 MPa, within the range of (Tc+20) °C to (Tc+55) °C, and then fed into a tube head with the outer tube heated to 99 °C. The resin solution was ejected 1.68 seconds after entering the tube head. The resin solution was passed into the outer tube of the tube head, while simultaneously, a 90 wt% aqueous solution of γ-butyrolactone at 95 °C was ejected from the inner tube of the tube head. The ejected resin solution was cured for 5 minutes in a bath containing a 90 wt% aqueous solution of γ-butyrolactone at 5 °C. After curing, the resulting membrane was washed with water and stretched to 1.5 times its original length in warm water at 95 °C. The stretched hollow fiber membrane was washed with water, impregnated in 30% glycerol for 2 hours, and then air-dried overnight. Subsequently, a heat treatment was performed in steam at 125°C for 1 hour.

[0299] The cross-section perpendicular to the length direction of the obtained hollow fiber membrane exhibits a spherical structure, but the average diameter Da1 of region Sa1 is 2.58 μm, and the average diameter Db2 of region Sb2 is 1.86 μm. The natural number i satisfies the above conditions (1) and (2). min The average diameter Da2 of the spherical structure in region Sa2 is 1.96 μm. The average throat diameter da1 of region Sa1 is 1.39 μm, and the average throat diameter db2 of region Sb2 is 1.14 μm. The natural number j satisfies the above conditions (1) and (2). min The average throat diameter da2 of the spherical interstitial structures in region Sa2 is 1.15 μm. The membrane has a pure water permeability of 2.5 m / hr and a beer filtration capacity of 250 L / m³. 2 It has a rejection rate of 96%, and although its separation ability is high, it cannot maintain water permeability for a long time.

[0300] [Comparative Example 3]

[0301] 38 wt% of vinylidene fluoride homopolymer with a weight-average molecular weight of 417,000 and 62 wt% of dimethyl sulfoxide were dissolved at 120 °C. The resin solution was held at 75 °C for 20 seconds under a pressure of 1.2 MPa, within the range of (Tc+20) °C to (Tc+55) °C, and then fed into a tube head with the outer tube heated to 80 °C. The resin solution was ejected 1.68 seconds after entering the tube head. The resin solution was passed into the outer tube of the tube head, while simultaneously, a 90 wt% aqueous solution of dimethyl sulfoxide at 75 °C was ejected from the inner tube of the tube head. The ejected resin solution was cured for 5 minutes in a bath containing a 90 wt% aqueous solution of dimethyl sulfoxide at 5 °C. After curing, the resulting membrane was washed with water and stretched to 1.5 times its original length in warm water at 95 °C. The stretched hollow fiber membrane was washed with water, impregnated in 30% glycerol for 2 hours, and then air-dried overnight. Subsequently, a heat treatment was performed in steam at 125°C for 1 hour.

[0302] The cross-section perpendicular to the length direction of the obtained hollow fiber membrane exhibits a spherical structure. The average diameter Da1 of region Sa1 is 2.54 μm, and the average diameter Db2 of region Sb2 is 1.81 μm. The natural number i satisfies the above conditions (1) and (2). min The average diameter Da2 of the spherical structure in region Sa2 is 1.89 μm. The average throat diameter da1 of region Sa1 is 1.34 μm, and the average throat diameter db2 of region Sb2 is 1.10 μm. The natural number j that satisfies the above conditions (1) and (2) is 2. min The average throat diameter da2 of the spherical interstitial structures in region Sa2 is 1.14 μm. The membrane exhibits a pure water permeability of 2.7 m / hr and a beer filtration capacity of 220 L / m³. 2 It has a rejection rate of 96%, and although its separation ability is high, it cannot maintain water permeability for a long time.

[0303] [Comparative Example 4]

[0304] 38 wt% of vinylidene fluoride homopolymer with a weight average molecular weight of 417,000 and 62 wt% of dimethyl sulfoxide were dissolved at 120 °C. The resin solution was held at 75 °C for 20 seconds under a pressure of 1.2 MPa, within the range of (Tc+20) °C to (Tc+55) °C, and then fed into a tube head that had its outer tube heated to 138 °C. The resin solution was ejected 5.23 seconds after entering the tube head.

[0305] The resin solution was introduced into the outer tube of the tube head, while a 90% by weight aqueous solution of γ-butyrolactone at 75°C was sprayed from the inner tube of the tube head. The sprayed resin solution was cured for 5 minutes in a bath containing a 90% by weight aqueous solution of γ-butyrolactone at 5°C. After curing, the resulting membrane was washed with water and stretched to 1.5 times its original length in warm water at 95°C. After washing with water, the stretched hollow fiber membrane was impregnated in 30% glycerol for 2 hours and then air-dried overnight. Subsequently, it was heat-treated in steam at 125°C for 1 hour.

[0306] The cross-section perpendicular to the length direction of the obtained hollow fiber membrane exhibits a spherical structure. The average diameter Da1 of region Sa1 is 3.35 μm, and the average diameter Db2 of region Sb2 is 3.11 μm. There is no natural number i that satisfies the above conditions (1) and (2). min .

[0307] The membrane has a pure water permeability of 7.8 m / hr and was not used in beer filtration tests.

[0308] The manufacturing conditions for each hollow fiber membrane are shown in Tables 1 and 2, and the evaluation results of the obtained hollow fiber membranes are shown in Tables 3 and 4. It should be noted that, regarding Tables 3 and 4, "Da1 / Da..." i The specific numerical representation of the item, including the value of n = i, is applied using i. min As the value of i. That is, in Tables 3 and 4, in "Da1 / Da i The value specifically shown in the line is actually "Da1 / Da". imin The value of "". The same applies to the case where i is set to j or k.

[0309] [Table 1]

[0310] Table 1

[0311]

[0312] [Table 2]

[0313] Table 2

[0314]

[0315] [Table 3]

[0316] Table 3

[0317]

[0318] [Table 4]

[0319] Table 4

[0320]

[0321] [Example 7]

[0322] As the hydrophilic polymer (coating polymer), PVP / PVAc (Kollidon VA64; manufactured by BASF) was used. The hollow fiber membrane prepared in Example 1 was immersed for 1 hour in a 1.0% by mass aqueous solution of ethanol containing 1000 ppm of the hydrophilic polymer comprising a vinylpyrrolidone / vinyl acetate random copolymer. Then, it was irradiated with 25 kGy of gamma rays to introduce the vinylpyrrolidone / vinyl propionate random copolymer into the hollow fiber membrane, and subjected to heat treatment in an oven at 150°C for 1 hour.

[0323] The obtained hollow fiber membrane was subjected to the above-mentioned process. 1 The content of PVP / PVAc (the mass fraction of hydrophilic polymer when the polyvinylidene fluoride resin is set to 100 parts by mass) was determined by H-NMR, and the water permeability of pure water after the introduction of hydrophilic polymer and after washing was also determined.

[0324] [Example 8]

[0325] PEG was used as the hydrophilic polymer. Otherwise, the hydrophilic polymer was introduced using the same method as in Example 7, and the results are shown in Table 5.

[0326] [Example 9]

[0327] PEGMA was used as the hydrophilic polymer. Otherwise, the hydrophilic polymer was introduced using the same method as in Example 7, and the results are shown in Table 5.

[0328] [Example 10]

[0329] After heat treatment in an oven at 125°C for 1 hour, the hollow fiber membrane prepared in Example 1 was immersed in a 1.0% by mass ethanol aqueous solution containing 10,000 ppm PVP / PVAc for 1 hour, and then irradiated with 25 kGy γ rays to introduce PVP / PVAc into the hollow fiber membrane.

[0330] [Table 5]

[0331] Table 5

[0332]

[0333] The present invention has been described in detail with reference to specific embodiments, but various changes and modifications can be made without departing from the spirit and scope of the invention, which will be apparent to those skilled in the art. This application is based on and incorporates by reference to Japanese patent applications filed on April 28, 2021 (Japanese Patent Application No. 2021-075668), June 30, 2021 (Japanese Patent Application No. 2021-108523), and August 27, 2021 (Japanese Patent Application No. 2021-138656).

[0334] Industrial practicality

[0335] The hollow fiber membrane of the present invention is suitable for use in water treatment applications such as drainage treatment, water purification, and industrial water production, as well as in the manufacture of food and pharmaceuticals. Furthermore, the hollow fiber membrane of the present invention is suitable for use as a fine filtration membrane or ultrafiltration membrane in these applications.

[0336] Explanation of reference numerals in the attached figures

[0337] 1. Pipe head

[0338] 2 piping

[0339] 11 Inner nozzle

[0340] 12 Annular Nozzles

[0341] 111 Inner nozzle inlet

[0342] 112 Inner nozzle outlet

[0343] 121 Annular nozzle inlet

[0344] 122 Annular nozzle outlet

[0345] 131 Piping entrance

[0346] 132 Piping outlet

Claims

1. Hollow fiber membrane, which has a layer with a spherical resin structure. The thickness L of the spherical structure layer is greater than 60 μm and less than 500 μm. The spherical structure has a first surface and a second surface. Regarding the region Sa that is 10×(n-1)~10×nμm away from the first surface n The average diameter Da of the spherical structure in n and the region Sb at a distance of 10×(n-1)~10×nμm from the second surface n The average diameter Db of the spherical structure in n Da1 > Db2, and The minimum value of a natural number i that satisfies the following conditions (1) and (2) min For 3≤i min ≤ (L-20) / 10, (1)Yes1 / Yes i ≥1.1 (2)-0.3μm≤Da i -Db2≤0.3μm in, Where n is a natural number, and in the case 3≤i min In the range ≤ (L-20) / 10, discard the number after the decimal point in (L-20) / 10.

2. The hollow fiber membrane according to claim 1, wherein, i min ≤L×0.75 / 10, Among them, in the i min In the range ≤L×0.75 / 10, discard the digits after the decimal point of L×0.75 / 10.

3. The hollow fiber membrane according to claim 1 or 2, wherein, 1.10<Da1 / Da imin <4.00。 4. The hollow fiber membrane according to claim 1 or 2, wherein, 0.50μm <Db2<2.00μm。 5. The hollow fiber membrane according to claim 1 or 2, wherein, 1.00≤Da1 / Da2≤1.

10.

6. The hollow fiber membrane according to claim 1 or 2, wherein, The first side is the side of the filtered liquid.

7. The hollow fiber membrane according to claim 1 or 2, wherein, The first surface is the outer surface of the hollow fiber membrane.

8. The hollow fiber membrane according to claim 1 or 2, wherein, When the throat diameter, obtained through analysis of the pore network model of the hollow fiber membrane, is used as the throat diameter of the spherical structure gap, Regarding the region Sa that is 10×(n-1)~10×nμm away from the first surface n The average diameter da of the throat diameter of the spherical structure gap in the middle n and the region Sb at a distance of 10×(n-1)~10×nμm from the second surface n The average diameter (db) of the throat diameter of the spherical structure gap in the middle n da1>db2, and The minimum value of a natural number j that satisfies the following conditions (1) and (2) min For 3≤j min ≤ (L-20) / 10, (1)yes1 / yes j ≥1.15 (2)from j -db2≤0.10μm Wherein, in the case of 3≤j min In the range ≤ (L-20) / 10, discard the number after the decimal point in (L-20) / 10.

9. The hollow fiber membrane according to claim 8, wherein, j min ≤L×0.5 / 10, Wherein, in the j min In the range ≤L×0.5 / 10, discard the digits after the decimal point of L×0.5 / 10.

10. The hollow fiber membrane according to claim 8, wherein, Regarding the region Sa obtained through the analysis of the pore network model n the number of throats Na n in the region and Sb n the number of throats Nb n , Na1 < Nb2, and The minimum value of the natural number k that satisfies the following conditions (3) and (4) min For 3≤k min ≤ (L-20) / 10, (3)Na1 / Na k ≤0.90 (4) Nb2-Na k ≤400 Wherein, in the case of 3≤k min In the range ≤ (L-20) / 10, discard the digits after the decimal point of (L-20) / 10. The analysis region in the analysis is a cuboid comprising any 50μm square region of the first face and a 50μm square region of the second face opposite to the first face.

11. The hollow fiber membrane according to claim 8, wherein, 0.10μm <db2<10.0μm。 12. The hollow fiber membrane according to claim 1 or 2, wherein, The spherical structure comprises polyvinylidene fluoride resin. Hydrophilic polymers are present on the surface and inside the spherical structure. The amount of the hydrophilic polymer is 1.0 or more parts by weight relative to 100 parts by weight of the polyvinylidene fluoride resin.

13. The hollow fiber membrane according to claim 12, wherein, When the hollow fiber membrane is immersed in a 3000ppm sodium hypochlorite aqueous solution at 60°C and pH 12.5 for 30 hours, the ratio of the hydrophilic polymer to the polyvinylidene fluoride resin is defined as P1 (by mass%), and the ratio of the hydrophilic polymer to the polyvinylidene fluoride resin before immersion is defined as P0 (by mass%), the percentage of P1 / P0 is less than 70%.

14. A method for manufacturing a hollow fiber membrane, comprising the following steps: (a) The process of dissolving polyvinylidene fluoride resin in a poor solvent to obtain a polyvinylidene fluoride resin solution; (b) a step of holding the polyvinylidene fluoride resin solution at a temperature for forming primary nuclei, wherein the temperature for forming primary nuclei is above the crystallization temperature and below the melting temperature; (c) a step of, after step (b), spraying the polyvinylidene fluoride resin solution in a hollow fiber form through piping and nozzles, and, in at least one of the piping or nozzles, applying a temperature gradient along the thickness direction of the polyvinylidene fluoride resin solution; and (d) After step (c), a step in which the polyvinylidene fluoride resin solution is solidified by immersing it in a cooling bath through solid-liquid thermal phase separation. The temperature gradient ΔT (°C) imparted by the aforementioned process (c) and the time t (seconds) for imparting the temperature gradient satisfy 50 ≤ ΔT × t ≤ 300. The hollow fiber membrane has a layer of resin with a spherical structure, and the thickness L of the spherical structure layer is more than 60 μm and less than 500 μm.

15. The method for manufacturing a hollow fiber membrane according to claim 14, wherein, In step (c), the thickness Ls of the polyvinylidene fluoride resin imparting a temperature gradient is 5 relative to the thickness L of the layer of the spherical structure. <Ls / L<40。 16. The method for manufacturing a hollow fiber membrane according to claim 14 or 15, wherein, The temperature T2 (°C) in step (b), the temperature T4 (°C) of the cooling bath in step (d), and the crystallization temperature Tc (°C) of the polyvinylidene fluoride resin satisfy (Tc-T4) / (T2-T4)<0.

50.

17. The method for manufacturing a hollow fiber membrane according to claim 16, wherein it satisfies 15 <Tc-T4<35。 18. The method for manufacturing a hollow fiber membrane according to claim 14 or 15, wherein, Following step (d), the following steps (e) and (f) are also included. (e) The process of introducing hydrophilic polymers into hollow fiber membranes; (f) A heat treatment process at or above 100°C following the process (e).

19. The method for manufacturing a hollow fiber membrane according to claim 18, wherein, In step (e), the aqueous solution containing the hydrophilic polymer is passed through a hollow fiber membrane and then irradiated with radiation.

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