Hollow fiber membrane and method for manufacturing the same

Abstract

The aim is to improve stain resistance while maintaining water permeability, that is, to suppress clogging in the membrane. [Solution] The hollow fiber membrane of the present invention comprises a spherical structure containing a polyvinylidene fluoride resin, wherein a hydrophilic polymer is present on the surface and inside the spherical structure, and the hydrophilic polymer present inside the spherical structure accounts for 70% by mass or less of the total hydrophilic polymer contained in the entire hollow fiber membrane, with the amount of the hydrophilic polymer being 1.0 part by mass or more per 100 parts by mass of the polyvinylidene fluoride resin.

Description

[Technical Field]

[0001] This invention relates to microfiltration membranes and ultrafiltration membranes used in water treatment applications such as wastewater treatment, water purification, and industrial water production, as well as in applications such as food and pharmaceutical manufacturing. [Background technology]

[0002] Separation membranes such as microfiltration membranes and ultrafiltration membranes are used in water treatment, food, and pharmaceutical applications for purposes such as clarification, concentration, and separation. In recent years, with the expansion of the application range of separation membranes, their use in applications requiring highly difficult-to-filter liquids or high-precision separation is being considered, and the demand for separation membranes that combine both filtration and separation capabilities is increasing.

[0003] In particular, when processing fermented liquids such as beer and wine in the food industry using hollow fiber membrane modules, problems arise such as the formation of a layer on the membrane surface consisting of removed microorganisms and their fragments, which can clog the membrane, leading to increased filtration pressure and a decrease in filtration speed over time.

[0004] A membrane structure has been developed that is less prone to clogging and exhibits good filtering performance. This structure involves a hollow fiber membrane with a surface pore diameter on one side that is larger than that of the substance to be removed, and a minimum pore diameter layer on either the other surface or within the film thickness, allowing for so-called depth filtration by trapping impurities inside the membrane.

[0005] Patent Document 1 discloses a porous hollow fiber membrane containing a vinylidene fluoride-based resin, in which a hydrophilic polymer is attached to the surface of a spherical structure by performing a heat treatment process at 145°C or lower and a hydrophilic polymer introduction process such as radiation irradiation, thereby suppressing clogging in the membrane.

[0006] Patent Document 2 discloses a porous hollow fiber membrane containing a vinylidene fluoride-based resin, having an inclined structure in which the pore diameter of the pores in the hollow fiber membrane gradually decreases toward at least one of the inner and outer peripheral surface sides, and the hollow fiber membrane is hydrophilized by including a crosslinked body of a polyvinylpyrrolidone-based resin.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] The hollow fiber membrane disclosed in Patent Document 1 has a problem that the micropores of the hollow fiber membrane with a microstructure are blocked by heat treatment at 145°C or higher, resulting in a decrease in water permeability. Also, when the hydrophilic polymer is 1.0 part by mass or more with respect to 100 parts by mass of the hydrophobic polymer, the water permeability decreases.

[0009] In Patent Document 2, since PVDF and a hydrophilic polymer are kneaded during film formation, the proportion of the hydrophilic polymer on the surface is small, and therefore it is presumed that the hydrophilic effect is also small.

[0010] An object of the present invention is to improve the antifouling property while maintaining the water permeability, that is, to suppress clogging in the membrane, in a hollow fiber membrane using a polyvinylidene fluoride-based resin.

Means for Solving the Problems

[0011] In order to achieve the above object, the present invention has any one of the following configurations [1] to

[10] .

[0012] [1] A hollow fiber membrane containing a spherical structure containing a polyvinylidene fluoride resin, A hydrophilic polymer is present on the surface and inside of the spherical structure, The hydrophilic polymer present inside the spherical structure is 70% by mass or less of the hydrophilic polymer contained in the entire hollow fiber membrane, A hollow fiber membrane containing 1.0 part by mass or more of the hydrophilic polymer with respect to 100 parts by mass of the polyvinylidene fluoride resin.

[0013] [2] A hollow fiber membrane containing a spherical structure containing a polyvinylidene fluoride resin, The ratio (mass %) P1 of the hydrophilic polymer to the polyvinylidene fluoride resin after immersing the hollow fiber membrane in an aqueous solution of 3000 ppm sodium hypochlorite (pH 12.5) at 60 °C for 30 hours is 70% or less of the ratio P0 before immersion (percentage of P1 / P0), A hollow fiber membrane containing 1.0 part by mass or more of the hydrophilic polymer with respect to 100 parts by mass of the polyvinylidene fluoride resin.

[0014] [3] The hollow fiber membrane has a first surface and a second surface, The average diameter Da of the spherical structure in the region Sa of 10×(n - 1) to 10×n μm from the first surface n and the average diameter Db of the spherical structure in the region Sb of 10(n - 1) to 10×n μm from the second surface n (n is a natural number), Da1 > Db2, [[ID=二十五]] n [[ID=二十六]] The average diameter Db of the spherical structure n For (n is a natural number), Da1 > Db2, The thickness L of the layer having the spherical structure is 60 μm or more and 500 μm or less, The minimum value i of the natural number i that satisfies the following conditions (1) and (2) min is 3 ≤ i min ≤ (L - 20) / 10 (where the decimal part of (L - 20) / 10 is rounded down). The hollow fiber membrane according to the above [1] or [2]. (1) Da1 / Da i ≥ 1.1 (2) -0.3 μm ≤ Da i - Db2 ≤ 0.3 μm [4] i min≤ L × 0.75 / 10 (where L × 0.75 / 10 is truncated). The hollow fiber membrane described in [3] above.

[0015] [5] 1.10 <Da1 / Da imin <4.00 The hollow fiber membrane described in [3] or [4] above.

[0016] [6] 0.50 μm <Db2<2.00μmである A hollow fiber membrane according to any of the above [3] to [5].

[0017] [7] The first surface is the outer surface of the hollow fiber membrane. A hollow fiber membrane according to any of the above [3] to [6].

[0018] [8] The hydrophilic polymer is a polymer containing vinyl alcohol, ethylene glycol, vinylpyrrolidone, methacrylic acid, allyl alcohol, cellulose, and vinyl acetate. A hollow fiber membrane according to any of the above [1] to [7].

[0019] [9](a) A step of forming a hollow fiber by phase separation of a solution containing polyvinylidene fluoride resin, (b) A step of introducing a hydrophilic polymer into the hollow fiber, (c) A step of performing a heat treatment at 150°C or higher after step (b) above. A method for producing a hollow fiber membrane, characterized by having the following features.

[0020]

[10] The method for producing a hollow fiber membrane according to [9], characterized in that step (b) involves passing a hollow fiber through an aqueous solution in which the hydrophilic polymer is dissolved, and then irradiating it with radiation. [Effects of the Invention]

[0021] The hollow fiber membrane of the present invention has excellent stain resistance while maintaining water permeability. Furthermore, the manufacturing method of the present invention is suitable for producing such a hollow fiber membrane. [Brief explanation of the drawing]

[0022] [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. [Figure 2] This is a schematic diagram illustrating a method for calculating the average diameter of a spherical structure. [Figure 3] This figure shows the results of measuring the average diameter of the spherical structure as a function of distance from the first surface. [Figure 4] This is a top view showing a specific example of the configuration of a nozzle used in the manufacture of hollow fiber membranes. [Figure 5] Figure 4 shows a cross-sectional view of the nozzle, AA. [Figure 6] Figure 4 is a bottom view of the nozzle. [Modes for carrying out the invention]

[0023] 1. Structure of hollow fiber membranes (1) Hollow fiber membrane The hollow fiber membrane of the present invention has a layer of spherical resin structures. The hollow fiber membrane may consist only of the spherical structure layer, or it may have other layers.

[0024] As the resin constituting the spherical structure layer, a thermoplastic resin made of a chain polymer is preferred, and polyvinylidene fluoride resin is particularly preferred due to its high chemical resistance. Here, polyvinylidene fluoride resin means a resin containing at least one of a vinylidene fluoride homopolymer and a vinylidene fluoride copolymer. The polyvinylidene fluoride resin may contain multiple types of vinylidene fluoride copolymers.

[0025] A vinylidene fluoride copolymer is a polymer having a vinylidene fluoride residue structure, and is typically a copolymer of a vinylidene fluoride monomer and another fluorinated monomer. Examples of other fluorinated monomers include one or more monomers selected from the group consisting of vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, and trifluoroethylene chloride. Furthermore, monomers other than the above fluorinated monomers, such as ethylene, may be copolymerized to an extent that does not impair the effects of the present invention.

[0026] Furthermore, the spherical structure layer may contain, in addition to the thermoplastic resin, other resins that are miscible with the thermoplastic resin, and polyhydric alcohols or surfactants in a proportion of 50% by mass or less.

[0027] In this specification, "spherical structure" refers to a structure formed by solid portions with a substantially circular cross-section perpendicular to the longitudinal direction of the hollow fiber membrane (i.e., parallel to the radial direction of the hollow fiber membrane), and by the connection of these solid portions as described below. When referring specifically to a portion with a substantially circular cross-section, rather than the entire structure, it may be called a "solid portion." A substantially circular shape includes a perfect circle and an ellipse. Each solid portion is connected to the others by sharing a portion of it in the planar or thickness direction of the separation membrane. The shape of the cross-section parallel to the longitudinal direction of the hollow fiber membrane is not particularly limited and can take any shape, such as a substantially circular or columnar shape.

[0028] The separation membrane exhibits high strength and elongation due to its spherical structure, and also demonstrates high water permeability due to the presence of voids between the spherical solid parts.

[0029] During filtration, it is preferable that the first surface is positioned upstream in the filtration direction and the second surface is positioned downstream in the filtration direction. It is preferable that the filtered liquid flows from the first surface towards the second surface. In other words, when used in 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, and when used in internal pressure filtration, the inner surface of the hollow fiber membrane is the first surface and the outer surface is the second surface.

[0030] Next, using Figure 1, we will describe the diameter of the spherical structure relative to its position in the thickness direction of the hollow fiber membrane. While the diameter of the spherical structure is not limited, having the structure described below creates a so-called depth filtration mechanism that prevents turbidity not only on the surface but throughout the entire layer. This is preferable because it allows for a depth filtration mechanism with low filtration resistance that is less prone to clogging, thus maintaining high water permeability performance over time.

[0031] In the following explanation, 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. Figure 1 is an electron microscope image of the radial cross-section of the hollow fiber membrane, where (a) is the vicinity of the first surface and (b) is the vicinity of the second surface.

[0032] In the cross-section, regions are divided into 10 μm intervals from the first surface, starting from 0 to 10 μm, 10 μm to 20 μm, ...10 × (1-n) μm to 10 × n μm..., with the designations Sa1, Sa2, ...Sa n The symbols are added. Additionally, the regions are divided into 10 μm intervals from the second surface, starting from 0 to 10 μm, 10 μm to 20 μm, and 10 × (1-n) to 10 × n μm, and the symbols Sb1, Sb2, and Sbn are added to each of these regions, respectively.

[0033] As shown in Figure 1, take a photograph so that the arc, which is the first surface of the hollow fiber membrane, can be seen. At this time, align the tangent to the center of the arc forming the first surface with the horizontal direction in the photograph. In the obtained photograph, draw a straight line Ma1 passing through the intersection points of the arc forming the first surface and the left and right edges of the photograph. It is preferable that the straight line Ma1 is parallel to the horizontal direction of the photograph, but it may be tilted.

[0034] Furthermore, a line Ma2 parallel to this line is drawn at a position approximately 10 μm closer to the second plane than line Ma1. Similarly, lines Ma3, Ma4, ... Ma are drawn. n We draw lines. Region Sa1 is the region between lines Ma1 and Ma2. Line Man is a line parallel to line Ma1, drawn at a position 10 × nμm away from line Ma1 in the second plane direction. Region Sa n In the photograph, the straight line Ma nand the straight line Ma (n+1) It is the region between [the two points].

[0035] Similarly, take a photograph of the second surface so that its arc can be seen. At this time, align the tangent to the center of the arc forming the second surface with the horizontal direction in the photograph. In the resulting photograph, draw a straight line Mb1 that passes through the intersection points of the arc forming the first surface and the left and right edges of the photograph. It is preferable that the straight line Mb1 is parallel to the horizontal direction of the photograph, but it may be tilted.

[0036] Furthermore, the straight line Ma n Similarly, draw straight lines from the second surface at 10 μm intervals. That is, draw a straight line Mb2 parallel to this straight line Mb1, at a position 10 μm closer to the first surface than straight line Mb1. Repeat the same operation to draw straight lines Mb3...Mb n It is possible to draw the region Sb. n is the straight line Mb n and Mb (n+1) It is the region between [the two points].

[0037] Average diameter Da of spherical structures in region San n , area Sb n The average diameter Db of the stadium structure in n It is calculated as follows:

[0038] First, electron microscope images are taken of the radial cross-section. The magnification during imaging is not particularly limited as long as it is sufficient to measure the length of 15 or more spherical structures. However, if the average diameter of the spherical structures is 1 to 3 μm, observation at 1000 to 5000x is preferable because it ensures a sufficient number of spherical structures to be observed for calculating the average diameter.

[0039] Solid parts that overlap with other spheres and are located further back than other spheres in the direction of imaging (perpendicular to the plane of the paper in Figures 1 and 2) (shown as X1 to X4 in Figure 2) are excluded from measurement. Solid parts that are interrupted at the outer edge of the image are also excluded from measurement.

[0040] Of the remaining solid parts, those that are not connected to other solid parts, such as solid part X5 shown in Figure 2, that is, those whose entire outline can be seen, have their major axis (shown by a dashed line in Figure 2) measured.

[0041] On the other hand, in solid parts connected to other solid parts, part of the contour line cannot be confirmed. Therefore, when only a part of the contour line of a solid part can be confirmed, the overall shape is estimated and then measured as follows: For spheres X6, X7, etc. with continuous contour lines, the longest line segment whose ends are located on the contour line is drawn, and its length is taken as the major axis. For X8~X10 with discontinuous contour lines, the longest line segment whose ends are located on one or two of the contour lines is taken as the major axis. However, in the following cases: • If the visible contour is less than 50% of the overall roughly circular contour of the solid part (e.g., X11, 12), • When both ends of the above line segment lie on the ends of a continuous contour line (e.g., X8) Furthermore, if the tissue is interrupted at the edge of the electron microscope image used for the determination, the diameter of the spherical structure in the tissue at that edge will not be measured.

[0042] Figure 3 shows the results of measuring the average diameter of the spherical structure in the thickness direction for Figure 1 using this method. The horizontal axis represents the distance in the thickness direction from the first surface, and the vertical axis represents the average diameter of the spherical structure in each region. For the sake of drawing convenience, the average diameter Da n The plots are located at a distance of 10 × (n-1) μm from the first surface, and for example, the average diameter Da3 of region Sa3 is plotted at a distance of 20 μm in the thickness direction.

[0043] The average diameter Da1 of the spherical structure in region Sa1 and the average diameter Db2 of region Sb2 satisfy the relationship Da1 > Db2. Because the average diameter Da1 of region Sa1 near the first surface is larger than the average diameter Db2 of region Sb2 near the second surface, the fluid flow resistance near the first surface is reduced, and thus the water permeability is improved.

[0044] Furthermore, the smallest value of a natural number i that satisfies the following conditions (1) and (2) is imin 3≦i min The fact that ≤(L-20) / 10 (where the decimal part of (L-20) / 10 is truncated) ensures that water permeability and separation are maintained for a long period of time. (1) Da1 / Da i ≥1.1 (2) -0.3μm≦Da i —Db2 ≤ 0.3 μm value i min The fact that it falls within the above range means that region Sa has an average diameter smaller than the average diameter Da1 in the nearest region to the first surface, and an average diameter equivalent to the average diameter Db2 of the spherical structure in region Sb2. i However, it is located closer to the second surface than Sa2. This allows the clogging components to be dispersed more widely in the thickness direction of the film. As a result, it becomes possible to extend the operating period. min ≥3 is preferable, i min ≥5 is preferred, i min A more preferable value is ≥ 1 / 2 × L / 10 (1 / 2 × L / 10 rounded down to the nearest whole number).

[0045] In Figure 3, the average diameter of the spherical structure decreases as we move from the first face to the second face, i min At =5, condition (2) -0.30 ≤ Da5 - Db2 ≤ 0.30 is satisfied. At this time, Da1 / Da5 = 1.29, so condition (1) is also satisfied. Since the film thickness is 240 μm, 3 ≤ i min It also satisfies ≤(L-20) / 10.

[0046] The thickness L of the hollow fiber membrane is preferably 60 μm to 500 μm, considering water permeability and strength and elongation; more preferably 100 μm to 500 μm, from the viewpoint of dispersing blockage locations in the thickness direction by depth filtration; and even more preferably 150 μm to 350 μm, from the viewpoint of membrane area.

[0047] Also, the above value i minIt is preferable that i ≦ 0.75×L / 10 (rounded down to the nearest integer). That is, to satisfy (1) and (2), it is preferable that the range is up to 3 / 4 from the first surface with respect to the thickness of the entire membrane. Under this condition, for all natural numbers k greater than i min and less than or equal to (L - 20) / 10, it is preferable that the following condition (4) is satisfied. (4) - 0.30 ≦ Da k ―Db2 ≦ 0.30 Since the regions satisfying the above (4) have mutually close average diameters, in the range where i min < k ≦ (L - 20) / 10, the structure is homogeneous. Also, from the definition of the range of k, since the region occupied by the homogeneous structure in the membrane thickness is 1 / 4 or more, sufficient separation performance can be imparted. Also, even when the membrane surface is damaged during film formation or operation, the thickness of the homogeneous structure layer being within this range can suppress the deterioration of separation performance and blocking performance. The layer with a uniform structure is preferably at least 10 μm or more. It is preferably 20 μm or more, more preferably 30 μm or more.

[0048] Also, it is preferable that 1.10 < Da1 / Dai min < 4.00. Since Da1 / Da imin is greater than 1.10, the dispersion of the clogging components in the thickness direction is promoted. Also, since Da1 / Da n is less than 4.00, high strength can be maintained. The relationship of Da1 / Da i is preferably 1.15 < Da1 / Da imin < 3.50, more preferably 1.20 < Da1 / Da imin < 3.00.

[0049] Also, it is preferable that 0.50 μm < Db2 < 2.00 μm. Since the average diameter Db2 of the spherical structure near the second surface is greater than 0.50 μm, a hollow fiber membrane with high water permeation performance can be obtained. Also, since Db2 is less than 2.00 μm, blocking performance suitable for sterilization etc. can be imparted.

[0050] (2) Hydrophilic polymer In hollow fiber membranes, hydrophilic polymers are present on the surface and inside the spherical structure. This configuration allows for excellent stain resistance while maintaining water permeability.

[0051] Furthermore, it is preferable that the ratio P1 / P0 of the ratio (mass%) of hydrophilic polymers to polyvinylidene fluoride resin after immersion of the hollow fiber membrane in a 3000 ppm sodium hypochlorite (pH 12.5) aqueous solution at 60°C for 30 hours, relative to the ratio P0 before immersion, is 70% or less. Hydrophilic polymers present on the surface of the spherical structure are removed from the hollow fiber membrane by immersion in sodium hypochlorite, but hydrophilic polymers present inside remain. Therefore, P1 / P0 represents the mass ratio of hydrophilic polymers inside the spherical structure to the total mass of hydrophilic polymers. The hydrophilic polymer content of the hollow fiber membrane can be quantified by X-ray electron spectroscopy (XPS), total internal reflection infrared spectroscopy (ATR-IR), proton nuclear magnetic resonance spectroscopy (1H-NMR), etc. Below, we will explain using the case where the hollow fiber membrane contains a fluororesin-based hydrophobic polymer as an example, using 1H-NMR. The method for quantifying the amount of hydrophilic polymer introduced by 1H-NMR involves dissolving a 2 cm hollow fiber membrane in 1 mL of dimethyl sulfoxide and measuring the amount by 1H-NMR. This measurement was performed at two arbitrary points on the hollow fiber membrane, and the amount of hydrophilic polymer was determined when the proportion of detected PVDF was set to 100.

[0052] The hydrophilic polymer content is preferably 1.0 part by mass or more per 100 parts by mass of hydrophobic polymer. More preferably, it is 1.0 part by mass or more and 6.0 parts by mass or less, and even more preferably 1.0 part by mass or more and 4.0 parts by mass or less.

[0053] If the hydrophilic polymer content is greater than 6.0 parts by mass, the hydrophilic polymer narrows the flow path, reducing the permeability of the liquid.

[0054] (3) Types of hydrophilic polymers 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 degree of saponification of less than 99%, vinylpyrrolidone-vinyl acetate copolymer polymers, vinylpyrrolidone-vinyl caprolactam copolymer polymers, and vinylpyrrolidone-vinyl alcohol copolymer polymers, and it is preferable to include at least one of these.

[0055] 2. Method for manufacturing hollow fiber membranes An example of the manufacturing method for the hollow fiber membrane described above is explained below. The manufacturing method described below is (a) A step of forming a hollow fiber by phase separation of a solution containing vinylidene fluoride copolymer, (b) A step of introducing a hydrophilic polymer into the heat-treated hollow fiber. (c) A step of heat-treating the hollow fiber membrane. It has.

[0056] (1) Hollow fiber formation process The method for producing a hollow fiber membrane according to the present invention comprises the following steps. (a) A step of dissolving a polyvinylidene fluoride resin in a poor solvent to obtain a polyvinylidene fluoride resin solution, (b) A step of holding the polyvinylidene fluoride resin solution under conditions that allow primary nucleus formation to proceed, (c) After step (b) above, a step of discharging a polyvinylidene fluoride resin solution in the form of hollow fibers through the piping and double nozzle, and applying a temperature gradient from the radially inner side to the outer side of at least one of the piping or double nozzle, (d) After (c) above, a step of passing the polyvinylidene fluoride resin solution through air, preferably for 0.3 seconds or more. (e) A step after step (d) in which the polyvinylidene fluoride resin solution is immersed in a cooling bath to solidify the polyvinylidene fluoride resin solution by solid-liquid thermally induced phase separation.

[0057] Step (c) is not mandatory, but it is necessary if you are performing the hole diameter control described above.

[0058] In step (a), a poor solvent is used to induce solid-liquid thermally induced phase separation in the subsequent step (d). Examples of solvents that induce solid-liquid thermally induced phase separation include medium-chain alkyl ketones, esters, and organic carbonates such as cyclohexanone, isophorone, γ-butyrolactone, methyl isoamyl ketone, dimethyl sulfoxide, and propylene carbonate, as well as mixed solvents thereof.

[0059] The higher the resin concentration in the resin solution, the easier it is for primary nuclei to form, resulting in a smaller average diameter for the spherical structure and, consequently, a separation membrane with high elongation strength. However, a lower resin concentration in the resin solution results in a higher porosity of the manufactured separation membrane, leading to higher water permeability. From these viewpoints, in order to achieve both water permeability and elongation strength, the resin concentration in the resin solution is preferably between 30% by mass and 60% by mass.

[0060] The dissolution temperature T1°C in step (a) is preferably equal to or greater than the crystallization temperature Tc°C. The crystallization temperature Tc can be measured using a differential scanning calorimetry (DSC) device. The crystallization temperature Tc is the temperature at which the crystallization peak rises when a mixture of the same composition as the resin solution used for film formation is sealed in a sealed DSC container, heated to the dissolution temperature at a heating rate of 10°C / min, held for 30 minutes to dissolve uniformly, and then cooled at a cooling rate of 10°C / min.

[0061] Specifically, the temperature T1°C is more preferably (Tc+20)°C or higher, and even more preferably (Tc+30)°C or higher. It is also preferably (Tc+100)°C or lower, and more preferably (Tc+90)°C or lower. More specifically, the temperature T1°C is preferably 100°C or higher, more preferably 110°C or higher. It is also preferably 200°C or lower, and more preferably 190°C or lower.

[0062] Furthermore, in order to homogeneously form spherical structures by solid-liquid thermally induced phase separation, it is preferable that the polyvinylidene fluoride resin solution is uniformly dissolved. For this reason, the dissolution time is preferably 2 hours or more, and more preferably 4 hours or more.

[0063] The resin solution may contain other additives.

[0064] Step (b) is a step in which the polyvinylidene fluoride resin solution is held under conditions that promote the formation of primary nuclei. Based on the results of X-ray diffraction, the formation process of the spherical structure is considered to be a crystal formation process. Generally, when crystalline polymers such as polyvinylidene fluoride resins crystallize, the crystals that are initially formed are called primary nuclei. These primary nuclei grow to form a single spherical structure.

[0065] The growth of spherical structures continues until they collide with each other, at which point growth stops. Therefore, the final particle size of the spherical structures depends on the number of primary nuclei initially formed. In other words, it is preferable to form a large number of primary nuclei to obtain small microspherical structures, and a small number of primary nuclei to obtain large giant spherical structures. Thus, controlling the progression of primary nucleus formation is effective in controlling the formation of spherical structures.

[0066] The progression of primary nucleation 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 (e), more preferably above the crystallization temperature Tc°C, and even more preferably above (Tc+20)°C. Furthermore, the temperature at which the resin dissolves is preferably T1°C or lower, and more preferably below (Tc+55)°C.

[0067] Preferably, the resin solution is pressurized to 0.5 MPa or higher, and more preferably to 0.8 MPa or higher. By keeping the resin solution under this pressure, the formation of primary nuclei at the aforementioned temperature proceeds stably. Preferably, the upper limit of the pressure is 3.0 MPa.

[0068] The duration for maintaining the conditions necessary for primary nucleus formation is preferably 10 seconds or more, and more preferably 20 seconds or more. In thermally induced phase separation, the formation of primary nuclei of crystals proceeds gradually in the region from below the melting temperature to above the crystallization temperature Tc (referred to as the "metastable region" in this specification), but by setting the resin solution under the above conditions, the number of primary nuclei that form relatively stably can be controlled.

[0069] Specifically, step (c) involves discharging the resin solution that has gone through step (b) into a hollow fiber shape through a tubular molding die. At this time, the liquid may also be supplied to the molding die via piping provided after step (b) and before step (c). An example of the shape of the molding die is shown in Figure 4. As shown in Figure 4, the die 1 for manufacturing hollow fiber membranes comprises an inner nozzle 11 and an annular nozzle 12 provided on the outside of the inner nozzle 11 so as to surround the inner nozzle. As shown in Figures 5 and 6, 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. The resin solution is discharged from the annular nozzle of the outer tube, and at the same time, a fluid that forms the hollow part is discharged from the nozzle of the inner tube, thereby molding (processing) the resin solution into a hollow shape.

[0070] Furthermore, step (c) includes a step of applying a temperature gradient along the thickness direction to the polyvinylidene fluoride resin solution formed into hollow fibers. "Applying a temperature gradient along the thickness direction" means making the temperature of one part of the resin solution formed into hollow fibers different from the temperature of other parts along its thickness direction. More specifically, it means making the temperature of the resin solution formed into hollow fibers relatively higher from one surface to the other.

[0071] As a specific method for creating a temperature gradient in the thickness direction, (1) Make the temperature T3 of nozzle 1 higher than the temperature of the supplied resin solution. (2) The temperature T3 of the injected liquid passing through the inner nozzle 11 of the nozzle 1 is made higher than the temperature of the supplied resin solution. (3) The temperature T3 of the piping preceding the supply of liquid to nozzle 1 is made higher than the temperature of the supplied resin solution. One example is to perform at least one of the following actions.

[0072] By performing at least one of the above (1) to (3), the spherical structure near either the outer or inner surface of the hollow fiber membrane can be made larger than the spherical structure near the other surface.

[0073] In the method of relatively increasing the temperature, it is also possible to lower the resin temperature in areas other than those where a spherical structure with a large average diameter is desired. However, in this case, the effect of reducing the size of the spherical structure on the side where the resin temperature is lowered is smaller than the effect of increasing the size of the spherical structure by raising the resin temperature. This is because, as mentioned above, the formation of the primary nucleus proceeds gradually in the metastable region.

[0074] For example, when a temperature gradient is applied to the resin solution in a relatively short time by cooling to a specific temperature, the conditions are insufficient for the formation of primary nuclei. On the other hand, the number of primary nuclei after formation is relatively greatly affected by the increase in temperature of the resin solution and decreases, resulting in a decrease in the number of spherical structures after solidification. In other words, the behavior of primary nuclei in a resin solution differs depending on whether the temperature is increased or decreased.

[0075] Therefore, when changing the diameter of the spherical structure by "applying a temperature gradient," a step of partially raising the temperature of the resin solution is preferably used.

[0076] In (1) above, there are no particular limitations on the method of raising the temperature of nozzle 1; any method is possible, such as fixing a heater around the nozzle or loading the nozzle into a mold temperature controller.

[0077] In (2) above, there are no particular limitations on the method of raising the temperature of the injection fluid; any method is possible, such as raising the initial temperature of the injection fluid or heating it in the piping that supplies the fluid to nozzle 1.

[0078] In (3) above, there are no particular limitations on the method for raising the temperature of the piping preceding the supply of liquid to the nozzle 1, and any method such as a heater or mold temperature controller can be used. Regarding the shape of the piping, a cylindrical piping is preferred from the viewpoint of applying a temperature gradient in the thickness direction of the hollow fiber membrane.

[0079] In terms of "applying a temperature gradient in the thickness direction," the method of raising the temperature of the nozzle 1 as described in (1) above, and the method of raising the temperature of the injection liquid as described in (2) above, are preferable for suppressing unevenness in the temperature gradient in the circumferential direction of the hollow fiber membrane. In the method of raising the temperature of the preceding piping that supplies liquid to the nozzle 1 as described in (3) above, it is necessary to design the piping and nozzle so that the resin solution, which is heated near the wall of the piping in the piping, is similarly passed through the vicinity of the wall inside the nozzle 1. However, compared to the case where the nozzle 1 is heated, it is not necessary to heat the injection liquid. As will be described later, depending on the heating conditions, the injection liquid may boil and bubbles may be mixed in, so the method of (3) which can heat only the resin solution is also preferable. In this case, it is preferable to control the temperature of the downstream nozzle 1 to a temperature of T2 to T3 in order to suppress cooling.

[0080] Furthermore, when heating pipes, it is preferable to heat pipes laid vertically. In pipes laid horizontally, the weight of the resin solution causes mixing within the pipe, making it difficult to maintain the temperature gradient applied in the thickness direction. However, in pipes laid vertically, mixing of the resin solution in the thickness direction can be suppressed.

[0081] In the process of applying a temperature gradient, it is preferable that the average rate of temperature increase when applying the temperature gradient, i.e., the average rate of temperature change of the resin solution (rate of temperature increase), be 30°C / min or more and 700°C / min or less, and that the application time (heating time) be 0.1 seconds or more and 20 seconds or less.

[0082] By maintaining an average heating rate of 30°C / min or more, the spherical structure on the separation membrane surface can be enlarged, maintaining high filtration efficiency. By maintaining a temperature change rate of 700°C / min or less, the thickness of the homogeneous structure on the inner surface side of the hollow fiber membrane can be ensured, maintaining the rejection rate. By applying such temperature changes to the resin solution to create a temperature gradient, the number of primary nuclei in parts of the resin solution can be controlled.

[0083] Furthermore, a heating time of 0.1 seconds or more increases the spherical structure on the surface of the separation membrane, resulting in a hollow fiber membrane that maintains high filtration efficiency. On the other hand, a heating time of 20 seconds or less ensures sufficient thickness of the homogeneous structure on the inner surface of the hollow fiber membrane, thus maintaining the rejection rate. For the sake of clarity, the terms "heating time" and "average heating rate" will be used below.

[0084] The average heating rate can be calculated from the following relationship.

[0085] Average heating rate (°C / min) = (T3 - T2) (°C) / heating time (minutes) Furthermore, when a temperature gradient is applied to the resin solution by heating the hollow fiber membrane nozzle 1 shown in Figures 4(A), (B), and (C), the heating time is the time it takes for the resin solution to pass through the nozzle. When heating the piping preceding the nozzle 1, the heating time is the time it takes for the solution to pass through that piping.

[0086] The heating time and average heating rate can be set arbitrarily, but it is preferable to set them within a range where the injection liquid does not boil due to the effects of heating. If the injection liquid boils, air bubbles will be mixed into the hollow part, resulting in unevenness in the diameter of the hollow fiber membrane produced. In addition, the areas that are expanded due to the mixing of air bubbles become areas where stress concentrates when external force is applied, which may cause the fibers to break during the production or operation of the hollow fiber membrane.

[0087] The polyvinylidene fluoride resin solution, extruded in a hollow fiber form, is immersed in a cooling bath after passing through the air. It is preferable to allow it to pass through the air for 0.3 seconds or more. The time spent passing through the air is the time from the extrusion of the polyvinylidene fluoride resin solution from the double nozzle in step (c) until it comes into contact with the cooling bath, and will hereafter be referred to as the "free travel time" on the toilet bowl. A free-running time of 0.3 seconds or more is preferable, as it allows sufficient time for the heat applied in step (c) to transfer to the inner surface. Preferably, it is 1 second or more, and more preferably 1.5 seconds or more. Furthermore, if the free-running time is long, cooling from the outer surface will progress, so 5 seconds or less is preferable, and more preferably 3 seconds or less.

[0088] The free-running time can be calculated using the following formula. Free-running time (seconds) = Free-running distance (m) ÷ Pull-out speed in the cooling bath (m / sec) Here, the free-running distance is the length of a straight line drawn vertically downwards from the bottom of the double-walled pipe fitting to the top surface of the cooling bath. The take-up speed in the cooling bath can be calculated from the following formula using the rotational speed (rpm) and roll diameter (m) of the cooling bath roll. π is the ratio of a circle's circumference to its diameter (pi). For example, if a roll with a diameter of 0.2m is rotating at a speed of 10rpm, the take-up speed will be approximately 0.1m / second. Pulling speed (m / sec) = Roll rotation speed (rpm) / 60 × π × Roll diameter (m) Furthermore, when passing the mixture through the air, the temperature, humidity, and solvent vapor concentration may be adjusted.

[0089] In step (d), after step (c), the polyvinylidene fluoride resin solution is immersed in a cooling bath to solidify the polyvinylidene fluoride resin solution by solid-liquid thermally induced phase separation.

[0090] Thermally induced phase separation is a method of solidifying a resin solution dissolved in a poor or good solvent at a temperature above its crystallization temperature (Tc) by cooling. Examples of thermally induced phase separation include: • A liquid-liquid type resin solution in which a uniformly dissolved resin solution separates into a concentrated phase and a dilute phase of the resin due to a decrease in the solution's solubility when the temperature drops, and One example is the solid-liquid type, in which a resin solution that is uniformly dissolved at high temperatures undergoes crystallization when the temperature drops below the crystallization temperature Tc, resulting in phase separation into a polymer solid phase and a polymer dilute solution phase.

[0091] In the liquid-liquid type, a fine three-dimensional network structure is formed by the concentrated phase, while in the solid-liquid type, a spherical structure is formed. Therefore, the solid-liquid type is preferably used for the production of the separation membrane of the present invention.

[0092] A mixed liquid containing a poor solvent or good solvent with a concentration of 50% to 95% by mass and a non-solvent with a concentration of 5% to 50% by mass is preferred as the cooling bath. By keeping the non-solvent concentration at 50% by mass or less, thermally induced phase separation can be preferentially promoted over non-solvent-induced phase separation. Although a lower concentration of the good solvent results in a higher solidification rate, it is possible to promote solidification and smooth the separation membrane surface even with a high concentration of the good solvent by lowering the temperature of the cooling bath.

[0093] It is preferable to use the same poor solvent as the resin solution as the poor solvent used in the resin solution.

[0094] 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 mixed solvents thereof.

[0095] Examples of non-solvents include water, hexane, pentane, benzene, toluene, methanol, ethanol, carbon tetrachloride, o-dichlorobenzene, trichloroethylene, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butylene glycol, pentanediol, hexanediol, low molecular weight polyethylene glycol, aliphatic hydrocarbons, aromatic hydrocarbons, aliphatic polyhydric alcohols, aromatic polyhydric alcohols, chlorinated hydrocarbons, or other chlorinated organic liquids and their mixed solvents.

[0096] In this specification, "good solvent" means a solvent capable of dissolving 5% by mass or more of a solute even at low temperatures below 60°C. "Poor solvent" means a solvent that cannot dissolve 5% by mass or more of a solute at low temperatures below 60°C, but can dissolve 5% by mass or more of a solute in the high-temperature range of 60°C or above and below the melting point of polyvinylidene fluoride resin. "Non-solvent" means a solvent that neither dissolves nor swells the solute up to the melting point of the solute or the boiling point of the solvent.

[0097] By setting the cooling bath temperature T4 lower than the crystallization temperature Tc of the polyvinylidene fluoride resin, solid-liquid phase separation is induced. The relationship between the cooling bath temperature T4, the crystallization temperature Tc, and the primary nucleation temperature T2 is described below.

[0098] In solid-liquid thermally induced phase separation, crystallization (hereinafter referred to as nucleation) occurs due to cooling, and the growth of the resulting crystal nuclei leads to phase separation into a polymer solid phase and a polymer dilute solution phase. In solid-liquid phase separation, it is thought that growth originates from the primary nuclei formed in step (b), and that random nucleation and growth do not originate from the primary nuclei occur. In growth originating from the primary nuclei, the number of spherical structures generated correlates with the number of primary nuclei. Since the growth of spherical structures proceeds until they come into contact with surrounding spherical structures, a small number of primary nuclei promotes growth and generates coarse spherical structures, while a small number of primary nuclei generates dense spherical structures.

[0099] The growth initiated by these primary nuclei and the random nucleation and growth not initiated by primary nuclei depend on the cooling rate of the resin solution. That is, they depend on the difference between the primary nucleation temperature T2 and the cooling bath temperature T4. The smaller the difference, the more easily the former crystallization progresses, and the larger the difference, the more easily the latter crystallization progresses. In other words, if the resin solution in step (c), where a temperature gradient is applied in the thickness direction of the hollow fiber membrane and the number of primary nuclei changes in the thickness direction, is rapidly cooled, random nucleation occurs regardless of the number of primary nuclei, making it difficult to form a structure in which the average diameter of the spherical structure changes in the thickness direction of the hollow membrane. On the other hand, if the resin solution in which the number of primary nuclei changes in the thickness direction is gradually cooled, growth initiated by the primary nuclei progresses, making it possible to obtain a hollow fiber membrane in which the average diameter of the spherical structure changes in the thickness direction.

[0100] Furthermore, 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 proceeds. The cooling rate depends on the difference between the primary nucleation temperature T2 and the cooling bath temperature T4.

[0101] In the method for manufacturing hollow fiber membranes of the present invention, after trial and error in consideration of the above characteristics, it was discovered that the average diameter of the spherical structure in the thickness direction of the hollow membrane changes when manufactured under the following temperature conditions, leading to the invention.

[0102] (Tc-T4) / (T2-T4)<0.50 T2 - T4 is a parameter representing the overall cooling rate, while Tc - T4 is a parameter contributing to growth. It has been found that by reducing the temperature difference contributing to growth among the overall cooling rates, growth starting from primary nuclei proceeds. More preferably, (Tc - T4) / (T2 - T4) < 0.45, and even more preferably, (Tc - T4) / (T2 - T4) < 0.40. In the method for producing the hollow fiber membrane of the present invention, it is preferable to satisfy the relationship of 15 < Tc - T4 < 35. When Tc - T4 is greater than 15°C, it is possible to suppress the coagulation time from becoming too long and prevent the manufacturing equipment from becoming excessively large. Also, when Tc - T4 is less than 35°C, it has been found that the growth starting from the above-mentioned primary nuclei proceeds.

[0103] Also, T2 - T4 is preferably less than 100°C, and more preferably less than 80°C.

[0104] The hollow fiber membrane coagulated in step (d) may be manufactured through the following steps thereafter. It is also preferable to expand the voids between the spherical structures after step (d) to improve water permeability and perform stretching to enhance the breaking strength. 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 more and 4 times or less, more preferably 1.1 times or more and 2 times or less.

[0105] 4]When stretching under a temperature condition of 50°C or higher, stable and uniform stretching can be achieved. When stretching under a temperature condition of 140°C or lower, since stretching is performed at a temperature lower than the melting point (177°C) of the polyvinylidene fluoride - based resin, the membrane does not melt even when stretched, and the voids can be expanded while maintaining the structure of the membrane, thereby improving the water permeability.

[0106] Also, stretching is preferably performed in a liquid because temperature control is easier. However, stretching may also be performed in a gas such as steam.

[0107] While water is a convenient and preferred liquid, when stretching at temperatures of approximately 90°C or higher, low molecular weight polyethylene glycol can also be used. On the other hand, if stretching is not performed, the water permeability and tensile strength will decrease compared to when stretching is performed, but the degree of elongation at rupture and the inhibitory effect will improve. Therefore, the presence or absence of a stretching step and the stretching ratio of the stretching step can be appropriately set according to the application of the separation membrane.

[0108] The above temperature conditions apply to the liquid temperature when stretching is performed in a liquid, and to the gas temperature when stretching is performed in a gas. These manufacturing methods are not particularly limited and can be applied to any thermoplastic resin that forms a spherical structure by thermally induced phase separation.

[0109] (2) Process for introducing hydrophilic polymer The types of hydrophilic polymers are as described in the section on hollow fiber membranes. Methods for introducing hydrophilic polymers include passing an aqueous solution containing the hydrophilic polymer through the hollow fiber membrane or immersing it in the membrane to physically adsorb the polymer, insolubilizing the hydrophilic polymer by radiation, or forming covalent bonds through chemical reactions with reactive groups present in the hollow fiber membrane. In particular, radiation irradiation is preferred because it allows the hydrophilic polymer to adhere more firmly to the surface of the spherical structure.

[0110] Furthermore, the above-described process of introducing hydrophilic polymers allows for the attachment of hydrophilic polymers to the surface of the spherical structure, compared to, for example, a method in which hydrophilic polymers are kneaded into the film-forming stock solution before film formation, thereby improving stain resistance.

[0111] If the concentration of the hydrophilic polymer aqueous solution is too low, a sufficient amount of hydrophilic polymer will not be introduced. Therefore, the copolymer concentration in the aqueous solution is preferably 10 ppm or higher, more preferably 100 ppm or higher, and most preferably 500 ppm or higher. However, if the concentration of the hydrophilic polymer in the aqueous solution is too high, there is a concern that the amount of leached material from the module will increase, so the copolymer concentration in the aqueous solution is preferably 100,000 ppm or less, and more preferably 10,000 ppm or less.

[0112] If the hydrophilic polymer is poorly soluble or insoluble in water, the hydrophilic polymer may be dissolved in an organic solvent that does not dissolve hollow fibers, or in a mixed solvent of an organic solvent that is compatible with water and does not dissolve hollow fibers, and water. Specific examples of organic solvents that can be used in the above organic solvent or mixed solvent include, but are not limited to, alcohol-based solvents such as methanol, ethanol, or propanol.

[0113] Furthermore, if the proportion of organic solvent in the mixed solvent is high, the hollow fibers may swell, potentially changing the pore size of the hollow fiber membrane. Therefore, the mass fraction of organic solvent in the mixed solvent is preferably 60% or less, more preferably 10% or less, and most preferably 1% or less.

[0114] Alpha rays, beta rays, gamma rays, X-rays, ultraviolet rays, or electron beams can be used for the aforementioned radiation irradiation. Here, from the viewpoint of safety and simplicity, radiation methods using gamma rays or electron beams are preferred. The radiation dose is preferably 15 kGy or more, and more preferably 25 kGy or more. By setting the dose to 15 kGy or more, hydrophilic polymers can be effectively introduced. Furthermore, the above irradiation dose is preferably 100 kGy or less. This is because if the irradiation dose exceeds 100 kGy, copolymers may be prone to three-dimensional crosslinking or decomposition of the ester group portion of the vinyl carboxylate monomer unit.

[0115] Antioxidants may be used to suppress cross-linking reactions during radiation irradiation. Antioxidants are substances that readily donate electrons to other molecules, and examples include, but are not limited to, water-soluble vitamins such as vitamin C, polyphenols, or alcoholic solvents such as methanol, ethanol, or propanol. These antioxidants may be used alone or in mixtures of two or more. When using antioxidants, safety must be considered, so antioxidants with low toxicity, such as ethanol or propanol, are preferably used.

[0116] The amount of the hydrophilic polymer introduced into the hollow fiber membrane can be quantified by total internal reflection infrared spectroscopy (ATR-IR), as described above. Furthermore, if necessary, it can also be quantified by X-ray electron spectroscopy (XPS) or other methods.

[0117] (3) Heat treatment process Heat treatment processes for hollow fiber membranes include methods of applying heat in a dry state and methods of applying heat in a humid state such as under water vapor. Heat treatment causes a portion of the polyvinylidene fluoride resin to shrink, allowing a portion of the hydrophilic polymer to be incorporated into the spherical structure. If the heat treatment temperature is too low, a sufficient amount of hydrophilic polymer will not be introduced. 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 will exceed the melting point of polyvinylidene fluoride, causing the hollow fiber membrane to melt, the micropores to become blocked, and the water permeability to decrease. Therefore, the heat treatment temperature is preferably below the melting point of the polyvinylidene fluoride copolymer. [Examples]

[0118] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way by these examples.

[0119] The following abbreviations are used in the examples and comparative examples.

[0120] PVP: Polyvinylpyrrolidone PVP / PVAc: Vinylpyrrolidone / Vinyl Acetate Random Copolymer PEG: Polyethylene glycol PEGMA: Polyethylene glycol methacrylate (1) Area Sa n Average diameter Da n A cross-section perpendicular to the length direction (parallel to the thickness direction) of the hollow fiber membrane was photographed at 1000x magnification using a HITACHI electron microscope (SU1510) to capture the arc forming the first surface. In the resulting photograph, straight lines Ma1, Ma2...Ma are visible from the first surface toward the second surface.n Draw lines at 10 μm intervals, and draw a straight line Ma n and Ma n+1 The area enclosed by the left and right edges of the photograph Sa n The major axis was measured for all measurable spherical structures. The arithmetic mean was calculated to obtain the average diameter Da n I obtained it.

[0121] One image shows the first plane and region Sa n If the same structure could not be captured, multiple fields of view were shifted in the thickness direction so that parts of them overlapped, and photographed at the same magnification. By stitching the captured photographs together so that identical structures overlapped, a continuous image in the thickness direction was obtained.

[0122] (2) Average diameter Db2 of region Sb2 A cross-section perpendicular to the length direction (parallel to the thickness direction) of the hollow fiber membrane was photographed at 1000x magnification using an electron microscope. In the resulting image, lines Mb1, Mb2, and Mb3 were drawn from the second surface of the separation membrane to the first surface, and the major axes of all measurable spherical structures within the region Sb enclosed by Mb2, Mb3 and the left and right edges of the photograph were measured. The arithmetic mean of the obtained values ​​was calculated to obtain the average diameter Db2.

[0123] (3) Crystallization temperature Tc of polyvinylidene fluoride resin solution Using a Seiko Electronics DSC-620-0, a mixture of polyvinylidene fluoride resin and a solvent with the same composition as the film-forming polymer stock solution was sealed in 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 dissolve uniformly. Subsequently, the temperature at which the crystallization peak rose during the cooling process at a rate of 10°C / min was observed was defined as the crystallization temperature Tc.

[0124] (4) Permeability of the separation membrane A small module approximately 10 cm in length, consisting of 1 to 10 hollow fiber membranes, was fabricated. Distilled water was then introduced through the first surface under conditions of 25°C temperature and a filtration differential pressure of 18.6 kPa, and the entire volume was filtered. The amount of permeate (m³) over a certain period of time was then measured. 3 The values ​​obtained by measuring ) are expressed as unit time (hr) and unit effective film area (m²). 2), calculated by converting to a value per 50 kPa.

[0125] (5) Measurement of hydrophilic polymer content A 2 cm hollow fiber membrane was dissolved in 1 mL of dimethyl sulfoxide and measured by 1H-NMR using a JEOL JNM-ECZ400R. This measurement was performed at two arbitrary points on the hollow fiber membrane, and the amount of hydrophilic polymer was determined when the proportion of detected PVDF was set to 100.

[0126] (6) Distribution of hydrophilic polymers in the depth direction After washing the hollow fiber membrane with distilled water, the surface was exposed using a single-edged blade. Cleaning was performed by Ar gas cluster ion beam (Ar-GCIB) etching, followed by TOF-SIMS measurement using an ION-TOF TOF.SIMS 5. The primary ion was Bi3++, and the secondary ion polarity was set to positive and negative. To correct for intensity distribution due to sample tilt, the relative intensity of the hydrophilic polymer-derived intensity distribution was taken for all ions detected at each pixel, and the content of the hydrophilic polymer in the film thickness direction was determined.

[0127] (6) Chemical wash durability The hollow fiber membrane was immersed in ethanol for 30 minutes, and then immersed in pure water for 60 minutes to wet it. Afterward, the material was washed in a 95°C oven for 5 hours. Then, it was immersed for 30 hours in an aqueous solution containing sodium hypochlorite at an effective chlorine concentration of 3000 ppm, and further containing sodium hydroxide at a concentration of 0.04% by mass, followed by rinsing with running water for 2 hours. The hydrophilic polymer content of this hollow fiber membrane was measured in the same manner as the hydrophilic polymer content measurement described above.

[0128] [Reference example 1] 38% by mass of vinylidene fluoride homopolymer with a weight-average molecular weight of 417,000 and 62% by mass of gamma butyrolactone were dissolved at 150°C. This resin solution was held at a pressure of 1.2 MPa for 20 seconds at 100°C (primary nucleus holding temperature T2), which is within the range of (Tc+20)°C to (Tc+55)°C.

[0129] Subsequently, the outer tube was fed into a double-tube nozzle heated to 108°C (heating temperature T3). The resin solution was discharged 4.7 seconds (heating time) after entering the double-tube nozzle. The average heating rate = (T3-T2)(°C) / heating time (minutes) is shown in the table.

[0130] While the resin solution was passed through the outer tube of the nozzle, a 90% by mass aqueous solution of γ-butyrolactone at 100°C was simultaneously discharged from the inner tube of the double-tube nozzle. The discharged resin solution was solidified for 1 minute in a bath at 25°C (cooling bath temperature T4) consisting of a 90% by mass aqueous solution of γ-butyrolactone. After solidification, the resulting film was washed with water and stretched to 1.5 times its original size in 95°C hot water. After stretching, the hollow fiber film was washed with water, immersed in 30% glycerin for 2 hours, and then air-dried overnight. Subsequently, it was heat-treated under 125°C steam for 1 hour.

[0131] The resulting hollow fiber membrane had a spherical structure in its cross-section perpendicular to its length. The average diameter Da1 of region Sa1 was 2.62 μm, the average diameter Db2 of region Sb2 was 1.84 μm, and the natural number imin satisfying the requirements of this application was 5. The average diameter Da5 of the spherical structure in region Sa5 was 2.02 μm.

[0132] [Example 1] PVP / PVAc (Coridon VA64; manufactured by BASF) was used as the hydrophilic polymer.

[0133] The hollow fiber membrane prepared in Reference Example 1 was immersed for 1 hour in a 1.0% by mass aqueous ethanol solution containing 1000 ppm of a hydrophilic polymer consisting of the prepared vinylpyrrolidone / vinyl acetate random copolymer. Then, 25 kGy of gamma rays were irradiated to introduce the vinylpyrrolidone / vinyl propanoate random copolymer into the hollow fiber membrane, and the membrane was heat-treated in a 150°C oven for 1 hour.

[0134] The PVP / PVAc content of the obtained hollow fiber membrane was determined by the 1H-NMR measurement described above, and the content of hydrophilic polymers in the film thickness direction was determined by TOF-SIMS measurement.

[0135] [Example 2] The hydrophilic polymer was introduced in the same manner as in Example 1, except that PEG was used as the hydrophilic polymer, and the results shown in Table 1 were obtained.

[0136] [Example 3] The hydrophilic polymer was introduced in the same manner as in Example 1, except that PEGMA was used as the hydrophilic polymer, and the results shown in Table 1 were obtained.

[0137] [Comparative Example 1] After heat treatment in an oven at 125°C for 1 hour, the hollow fiber membrane prepared in Reference Example 1 was immersed for 1 hour in a 1.0 mass% ethanol aqueous solution containing 10,000 ppm of PVP / PVAc dissolved in it, and then irradiated with 25 kGy of gamma rays to introduce PVP / PVAc into the hollow fiber membrane.

[0138] [Table 1] [Industrial applicability]

[0139] The hollow fiber membrane of the present invention can be used in water treatment applications such as wastewater treatment, water purification, and industrial water production, as well as in applications such as food and pharmaceutical manufacturing. In particular, it can be suitably used for filtering fermented liquids such as beer and wine in the food industry. [Explanation of Symbols]

[0140] 1. Die head for manufacturing hollow fiber membranes 11 Inner nozzle 12 annular nozzles 111 Inner nozzle inlet 112 Inner nozzle outlet 121 Annular nozzle inlet 122 Annular nozzle outlet

Claims

1. It is a hollow fiber membrane containing a spherical structure that includes a polyvinylidene fluoride resin, After immersing the hollow fiber membrane in a 3000 ppm sodium hypochlorite (pH 12.5) aqueous solution at 60°C for 30 hours, the ratio of the hydrophilic polymer (mass%) P1 to the ratio P0 before immersion (percentage of P1 / P0) to the polyvinylidene fluoride resin is 70% or less. The hydrophilic polymer is at least one selected from the group consisting of polymers containing vinyl alcohol, polymers containing ethylene glycol, polymers containing vinylpyrrolidone, polymers containing methacrylic acid, polymers containing allyl alcohol, polymers containing cellulose, and polymers containing vinyl acetate. A hollow fiber membrane containing 1.0 part by mass or more of the hydrophilic polymer per 100 parts by mass of polyvinylidene fluoride resin.

2. The hollow fiber membrane has a first surface and a second surface, From the first surface, the region Sa is 10 × (n-1) to 10 × n μm. ｎ Average diameter Da of the spherical structure in ｎ And, from the second surface, a region Sb of 10(n-1) to 10 × nμm ｎ Average diameter Db of the spherical structure ｎ For (where n is a natural number), Da １ > Db ２ And, The thickness L of the layer having the spherical structure is 60 μm or more and 500 μm or less. The smallest value of a natural number i that satisfies the following conditions (1) and (2) ｍｉｎ 3 ≤ i ｍｉｎ The hollow fiber membrane according to claim 1, wherein ≤ (L-20) / 10 (where (L-20) / 10 is truncated). (1) Yes １ / Da ｉ ≧1.1 (2)-0.3μm≦Da ｉ ―Db ２ ≦0.3μm

3. i ｍｉｎ The hollow fiber membrane according to claim 2, wherein L × 0.75 / 10 ≤ L × 0.75 / 10 (where L × 0.75 / 10 is rounded down to the nearest whole number).

4. 1.10 < Da １ / Da ｉｍｉｎ The hollow fiber membrane according to claim 2 or 3, wherein the temperature is <4.

00.

5. 0.50 μm < Db ２ The hollow fiber membrane according to claim 2 or 3, wherein the thickness is <2.00 μm.

6. The hollow fiber membrane according to claim 2 or 3, wherein the first surface is the outer surface of the hollow fiber membrane.

7. The hollow fiber membrane according to claim 1 or 2, wherein the content of the hydrophilic polymer is 6.0 parts by mass or less per 100 parts by mass of polyvinylidene fluoride resin.

8. (a) A step of forming hollow fibers by phase separation of a solution containing polyvinylidene fluoride resin, (b) A step of introducing a hydrophilic polymer into the hollow fiber, (c) A step of performing a heat treatment at 150°C or higher after step (b) above. It is characterized by having, A method for producing a hollow fiber membrane, wherein the hydrophilic polymer is at least one selected from the group consisting of a polymer containing vinyl alcohol, a polymer containing ethylene glycol, a polymer containing vinylpyrrolidone, a polymer containing methacrylic acid, a polymer containing allyl alcohol, a polymer containing cellulose, and a polymer containing vinyl acetate.

9. The method for producing a hollow fiber membrane according to claim 8, characterized in that step (b) involves passing an aqueous solution containing the hydrophilic polymer through the hollow fiber membrane and then irradiating it with radiation.

10. The method for producing a hollow fiber membrane according to claim 9, wherein the concentration of the aqueous solution in which the hydrophilic polymer is dissolved is 10,000 ppm or less.

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