Polyamide hollow fiber membrane, hollow fiber membrane module, and method for producing polyamide hollow fiber membrane

Abstract

An object of the present invention is to provide a polyamide hollow fiber membrane whose filtration performance is unlikely to change even when stored in a storage solution for a long period of time, a hollow fiber membrane module, and a method for producing a polyamide hollow fiber membrane. The polyamide hollow fiber membrane of the present invention is made of a polyamide resin and satisfies at least one of the following characteristics (1) to (5): (1) Fe content is 4.30 ppm or less (2) Cr content is less than 1.00 ppm (3) Cu content less than 0.20 ppm (4) Mg content less than 0.60 ppm (5) Zn content less than 0.30 ppm

Description

[Technical Field]

[0001] The present invention relates to a polyamide hollow fiber membrane having excellent storage stability, a hollow fiber membrane module, and a method for producing a polyamide hollow fiber membrane. [Background technology]

[0002] Polyamide hollow fiber membranes are generally used as hollow fiber membrane modules housed in a module case. Hollow fiber membrane modules have excellent liquid filtration properties and are used in a variety of filtration applications, including the filtration of raw materials, intermediates, and chemical solutions used in pharmaceuticals and semiconductors.

[0003] For example, Patent Document 1 discloses a material made of polyamide resin, which, in a bubble point test in which air pressure is applied in a liquid having a surface tension of 12 mN / m at 20°C, has an initial bubble point of 0.40 MPa or more and a burst bubble point of 0.55 MPa or more, and an internal pressure water permeability of 50 L / (m) using pure water at 25°C. 2 Polyamide hollow fiber membranes with a thermal conductivity of 1000 kJ / cm2 or higher have been proposed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-68005 Summary of the Invention [Problem to be solved by the invention]

[0005] A common method for storing hollow fiber membrane modules after use in various applications is to immerse them in a storage solution such as water, an organic solvent, or a mixture thereof. However, the present inventors have newly discovered that long-term storage of hollow fiber membrane modules in a storage solution results in a significant change in the filtration performance of the polyamide hollow fiber membranes.

[0006] The present invention solves the above-mentioned problems of the conventional art, and aims to provide a polyamide hollow fiber membrane, a hollow fiber membrane module, and a method for producing a polyamide hollow fiber membrane, the filtration performance of which is unlikely to change even when stored in a storage solution for a long period of time. [Means for solving the problem]

[0007] As a result of extensive research aimed at solving the above problems, the present inventors have found that the change in filtration performance of polyamide hollow fiber membranes when stored for a long period of time in a storage solution is due to specific metal elements contained in the polyamide hollow fiber membranes. The present invention was completed through further research based on this finding.

[0008] That is, the present invention provides the following aspects of the invention. <1> A polyamide hollow fiber membrane formed from a polyamide resin, A polyamide hollow fiber membrane that satisfies at least one of the following characteristics (1) to (5): (1) Fe content is 4.30 ppm or less (2) Cr content is less than 1.00 ppm (3) Cu content less than 0.20 ppm (4) Mg content less than 0.60 ppm (5) Zn content less than 0.30 ppm <2> The relative viscosity is 2.0 to 6.5. <1> The polyamide hollow fiber membrane according to claim 1. <3> In structural analysis using X-ray diffraction, the ratio of γ crystals to the total amount of α crystals and γ crystals is 0 to 37%. <1> or <2> The polyamide hollow fiber membrane according to claim 1. <4> A polyamide hollow fiber membrane has a dense layer on the lumen surface and / or the outer surface. <1> ~ <3> 10. The polyamide hollow fiber membrane according to claim 9, wherein the polyamide hollow fiber membrane is a polyamide hollow fiber membrane. <5> The rejection rate of particles with a diameter of 50 nm is 90% or more. <1> ~ <4> 10. The polyamide hollow fiber membrane according to claim 9, wherein the polyamide hollow fiber membrane is a polyamide hollow fiber membrane. <6> The retention rate of particles with a particle size of 5 nm calculated using the following formula is 90% or more. <1> ~ <5> 10. The polyamide hollow fiber membrane according to claim 9, wherein the polyamide hollow fiber membrane is a polyamide hollow fiber membrane. Retention rate (%) = (rejection rate of 5 nm particles after storage / rejection rate of 5 nm particles before storage) x 100 Storage conditions: The polyamide hollow fiber membrane is immersed in propylene glycol monomethyl ether acetate and stored at 23°C for 2 years without pressure. <7> External pressure permeability: 50-2000L / (m 2 ·atm·h), <1> ~ <6> 10. The polyamide hollow fiber membrane according to claim 9, wherein the polyamide hollow fiber membrane is a polyamide hollow fiber membrane. <8> The retention rate of external pressure water permeability calculated by the following formula is 90 to 110%. <1> ~ <7> 10. The polyamide hollow fiber membrane according to claim 9, wherein the polyamide hollow fiber membrane is a polyamide hollow fiber membrane. Retention rate (%) = (external pressure water permeability after storage / external pressure water permeability before storage) x 100 Storage conditions: The polyamide hollow fiber membrane is immersed in propylene glycol monomethyl ether acetate and stored at 23°C for 2 years without pressure. <9> In a bubble point test in which air pressure is applied in 2-propanol at 20°C, the initial bubble point is 0.20 MPa or more and the burst bubble point is 0.30 MPa or more. <1> ~ <8> 10. The polyamide hollow fiber membrane according to claim 9, wherein the polyamide hollow fiber membrane is a polyamide hollow fiber membrane. <10> In a bubble point test in which air pressure was applied in 2-propanol at 20°C, The retention rate of the initial bubble point and burst bubble point calculated by the following formula is 90 to 110%. <1> ~ <9> 10. The polyamide hollow fiber membrane according to claim 9, wherein the polyamide hollow fiber membrane is a polyamide hollow fiber membrane. Retention rate (%) = (initial bubble point or burst bubble point after storage / initial bubble point or burst bubble point before storage) x 100 Storage conditions: The polyamide hollow fiber membrane is immersed in propylene glycol monomethyl ether acetate and stored at 23°C for 2 years without pressure. <11> The module case <1> ~ <10> 10. A hollow fiber membrane module containing the polyamide hollow fiber membrane according to any one of claims 1 to 9. <12> A first step of mixing at least a polyamide resin and a sulfone using a multi-screw extruder to prepare a membrane-forming solution; a second step of forming a hollow fiber membrane by using a double-tube nozzle for producing hollow fibers having a double-tube structure, discharging the membrane-forming solution from an outer annular nozzle and discharging an internal solution from an inner nozzle, and immersing the resulting solution in a coagulation bath containing water and / or a polyhydric alcohol; and a third step of removing the organic solvent from the hollow fiber membrane formed in the second step; The method for producing a polyamide hollow fiber membrane, wherein the sulfone has a pH of 5.2 to 6.8 at 25° C. when dissolved in water to form a 5% by mass aqueous solution. <13> The sulfones are dimethyl sulfone and / or sulfolane. <12> A method for producing the polyamide hollow fiber membrane described in claim 1. <14> The method further includes a fourth step of washing the hollow fiber membrane with an organic solvent after the third step to remove at least one metal element selected from the group consisting of Fe, Cr, Cu, Mg, and Zn. <12> or <13> A method for producing the polyamide hollow fiber membrane described in claim 1. [Effects of the Invention]

[0009] The polyamide hollow fiber membrane of the present invention has a reduced content of specific metal elements, and therefore, even when stored in a storage solution for a long period of time, its filtration performance is unlikely to change, and it has excellent storage stability. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram of an apparatus for measuring the external pressure water permeability of a polyamide hollow fiber membrane. [Figure 2] FIG. 1 is a schematic diagram of an apparatus for measuring the bubble point of a polyamide hollow fiber membrane. DETAILED DESCRIPTION OF THE INVENTION

[0011] 1. Polyamide hollow fiber membrane The polyamide hollow fiber membrane of the present invention is formed from a polyamide resin and satisfies at least one of the following characteristics (1) to (5): The polyamide hollow fiber membrane of the present invention will be described in detail below. (1) Fe content is 4.30 ppm or less (2) Cr content is less than 1.00 ppm (3) Cu content less than 0.20 ppm (4) Mg content less than 0.60 ppm (5) Zn content less than 0.30 ppm

[0012] <Polyamide resin> The type of polyamide resin forming the polyamide hollow fiber membrane of the present invention is not particularly limited, and examples thereof include polyamide homopolymers, polyamide copolymers, and mixtures thereof. Specific examples of polyamide homopolymers include polyamide 6, polyamide 66, polyamide 46, polyamide 610, polyamide 612, polyamide 11, polyamide 12, polyamide MXD6, polyamide 4T, polyamide 6T, polyamide 9T, and polyamide 10T. Specific examples of polyamide copolymers include copolymers of polyamide with polyethers such as polytetramethylene glycol or polyethylene glycol. The proportion of the polyamide component in the polyamide copolymer is not particularly limited, and examples thereof include the proportion of the polyamide component of preferably 70 mol% or more, more preferably 80 mol% or more, even more preferably 90 mol% or more, and particularly preferably 95 mol% or more. By ensuring that the proportion of the polyamide component in the polyamide copolymer satisfies the above range, excellent durability can be achieved. The polyamide resins may be used singly or in combination of two or more.

[0013] The polyamide resin used in the present invention may be crosslinked or not, as long as it can be molded into a fiber shape. From the viewpoint of cost reduction, a non-crosslinked polyamide resin is preferred.

[0014] <Polyamide hollow fiber membrane> The polyamide hollow fiber membrane of the present invention has a reduced content of at least one metal element selected from the group consisting of Fe, Cr, Cu, Mg, and Zn (hereinafter, these metal elements are also referred to as "specific metal elements").

[0015] Specific metal elements are substances that cause large changes in filtration performance, such as rejection rate and external pressure water permeability, when polyamide hollow fiber membranes are stored in a storage solution for a long period of time. The reason why specific metal elements cause such large changes in the filtration performance of polyamide hollow fiber membranes when stored in a storage solution for a long period of time is not clear, but it is presumed that the specific metal elements cause degradation by cutting or the like of the polyamide resin that constitutes the polyamide hollow fiber membrane, or that chemical reactions occur between the specific metal elements and other impurities in the polyamide hollow fiber membrane, resulting in changes in the porous structure of the polyamide hollow fiber membrane.

[0016] Among the specific metal elements, the content of Fe is 4.30 ppm or less, preferably 3.50 ppm or less, more preferably 3.00 ppm or less, even more preferably 2.50 ppm or less, still more preferably 2.00 ppm or less, even more preferably 1.50 ppm or less, and particularly preferably 1.20 ppm or less, in order to obtain a polyamide hollow fiber membrane whose filtration performance is unlikely to change even after long-term storage in a storage solution. The Fe content may be, for example, 0.00 to 4.30 ppm, 0.10 to 3.50 ppm, 0.20 to 3.00 ppm, 0.50 to 2.50 ppm, 0.60 to 2.00 ppm, 0.70 to 1.50 ppm, or 0.80 to 1.20 ppm.

[0017] Among the specific metal elements, the Cr content is less than 1.00 ppm, preferably 0.70 ppm or less, more preferably 0.60 ppm or less, even more preferably 0.50 ppm or less, still more preferably 0.40 ppm or less, and particularly preferably 0.30 ppm or less, in order to obtain a polyamide hollow fiber membrane whose filtration performance is unlikely to change even after long-term storage in a storage solution. Examples of the Cr content include 0.00 to 0.99 ppm, 0.00 to 0.70 ppm, 0.05 to 0.60 ppm, 0.05 to 0.50 ppm, 0.10 to 0.40 ppm, and 0.10 to 0.30 ppm.

[0018] Among the specific metal elements, the content of Cu is less than 0.20 ppm, preferably 0.17 ppm or less, more preferably 0.15 ppm or less, even more preferably 0.14 ppm or less, still more preferably 0.13 ppm or less, and particularly preferably 0.10 ppm or less, in order to obtain a polyamide hollow fiber membrane whose filtration performance is unlikely to change even after long-term storage in a storage solution. Examples of the Cu content include 0.00 to 0.19 ppm, 0.00 to 0.17 ppm, 0.02 to 0.15 ppm, 0.02 to 0.14 ppm, 0.05 to 0.13 ppm, and 0.05 to 0.10 ppm.

[0019] Among the specific metal elements, the content of Mg is less than 0.60 ppm, preferably 0.50 ppm or less, more preferably 0.40 ppm or less, even more preferably 0.30 ppm or less, still more preferably 0.20 ppm or less, and particularly preferably 0.10 ppm or less, in order to obtain a polyamide hollow fiber membrane whose filtration performance is unlikely to change even after long-term storage in a storage solution. The content of Mg is, for example, 0.00 to 0.59 ppm, 0.00 to 0.50 ppm, 0.02 to 0.40 ppm, 0.02 to 0.30 ppm, 0.05 to 0.20 ppm, or 0.05 to 0.10 ppm.

[0020] Among the specific metal elements, the content of Zn is less than 0.30 ppm, preferably 0.25 ppm or less, more preferably 0.20 ppm or less, even more preferably 0.17 ppm or less, still more preferably 0.13 ppm or less, and particularly preferably 0.10 ppm or less, in order to obtain a polyamide hollow fiber membrane whose filtration performance is unlikely to change even after long-term storage in a storage solution. The Zn content may be, for example, 0.00 to 0.29 ppm, 0.00 to 0.25 ppm, 0.02 to 0.20 ppm, 0.02 to 0.17 ppm, 0.05 to 0.13 ppm, or 0.05 to 0.10 ppm.

[0021] In order to more effectively suppress changes in filtration performance even when stored in a storage solution for a long period of time, the polyamide hollow fiber membrane of the present invention preferably has the contents of any two of the specific metal elements within the above-mentioned numerical range, more preferably the contents of any three of the specific metal elements within the above-mentioned numerical range, even more preferably the contents of any four of the specific metal elements within the above-mentioned numerical range, and particularly preferably the contents of all (five) of the specific metal elements within the above-mentioned numerical range.

[0022] Specific embodiments of the polyamide hollow fiber membrane of the present invention include those that satisfy features (1) and (2); features (1) and (3); features (1) and (4); features (1) and (5); features (2) and (3); features (2) and (4); features (2) and (5); features (3) and (4); features (3) and (5); or features (4) and (5).

[0023] Other specific embodiments of the polyamide hollow fiber membrane of the present invention include those that satisfy features (1), (2), and (3); features (1), (2), and (4); features (1), (2), and (5); features (1), (3), and (4); features (1), (3), and (5); features (1), (4), and (5); features (2), (3), and (4); features (2), (3), and (5); features (2), (4), and (5); or features (3), (4), and (5).

[0024] Other embodiments of the polyamide hollow fiber membrane of the present invention specifically include embodiments that satisfy features (1), (2), (3), and (4); features (1), (2), (3), and (5); features (1), (2), (4), and (5); features (1), (3), (4), and (5); or features (2), (3), (4), and (5).

[0025] Further, other embodiments of the polyamide hollow fiber membrane of the present invention specifically include embodiments that satisfy the features (1), (2), (3), (4) and (5).

[0026] In the present invention, the content of metal elements in a polyamide hollow fiber membrane is measured by preparing a sample by decomposing and / or dissolving the dried polyamide hollow fiber membrane in nitric acid, and then using the prepared sample by inductively coupled plasma (ICP) atomic emission spectroscopy.

[0027] The relative viscosity of the polyamide hollow fiber membrane of the present invention is, for example, 2.0 to 6.5, and from the viewpoint of improving the external pressure water permeability before long-term storage in a polyamide hollow fiber membrane with a reduced content of a specific metal element, it is preferably 2.5 to 5.5, more preferably 3.0 to 5.0, even more preferably 3.5 to 4.5, and particularly preferably 4.0 to 4.3. The relative viscosity of the polyamide hollow fiber membrane can be adjusted by appropriately selecting the relative viscosity of the polyamide resin constituting the polyamide hollow fiber membrane.

[0028] In the present invention, the relative viscosity of a polyamide hollow fiber membrane is measured by preparing a sample by dissolving the polyamide hollow fiber membrane in 96% by mass sulfuric acid to a concentration of 1 g / dL, and using the prepared sample at 25°C using an Ubbelohbe viscometer.

[0029] In the polyamide hollow fiber membrane of the present invention, the proportion of γ crystals relative to the total amount of α crystals and γ crystals, as determined by crystal structure analysis using X-ray diffraction, is, for example, 0 to 37%. In polyamide hollow fiber membranes with reduced content of specific metal elements, the proportion of γ crystals relative to the total amount of α crystals and γ crystals is preferably 5 to 30%, more preferably 10 to 26%, and even more preferably 15 to 25%, from the viewpoint of improving the external pressure water permeability before long-term storage. The proportion of γ crystals relative to the total amount of α crystals and γ crystals in the polyamide hollow fiber membrane can be adjusted to a desired range by adjusting the relative viscosity of the polyamide hollow fiber membrane. For example, when the polyamide resin forming the polyamide hollow fiber membrane is polyamide 6, the proportion of γ crystals can be adjusted to the above-mentioned preferred range by adjusting the relative viscosity of the polyamide hollow fiber membrane to the above-mentioned preferred range.

[0030] In the present invention, the ratio of γ crystals to the total amount of α crystals and γ crystals is a value determined by determining the peak areas of α crystals and γ crystals by crystal structure analysis using X-ray diffraction, and then calculating the ratio of the peak area of ​​γ crystals to the sum of the peak areas of α crystals and γ crystals. Specifically, the ratio of γ crystals to the total amount of α crystals and γ crystals is a value measured using an X-ray diffractometer under the following conditions and method. Pretreatment: Cut a polyamide hollow fiber membrane perpendicular to its length and fix it to a pole point sample plate with double-sided tape. The pole point sample plate is positioned so that the length of the sample is parallel to the optical axis when 2θ = 0°. Measurement method: WAXD reflection method 2θ / θ method is used. Measurement conditions: X-ray Cu-Kα radiation (1.54 Å), 50 kV 300 mA, thin film, standard multipurpose sample stage / pole point sample plate, parallel beam method, receiving side Soller slit = long slit used, slits: DS / SS / RS = 1.0 mm / 1.0 mm / 1.0 mm, vertical limiting slit = 10 mm, scan range: 2θ = 2° to 60°, scan speed: 2° / min, step 0.02°. Analysis method: Performed by multiple peak separation method (profile fitting using pseudo-Voigt function) in the 2θ = 5° to 36° region. Specific analysis conditions are as follows: 1) The background is the area under the line connecting the corrected intensity at 2θ=8° (calculated as the average intensity from 2θ=7.5° to 8.5°) and the corrected intensity at 2θ=36° (calculated as the average intensity from 2θ=35.5° to 36.5°). 2) The halo pattern due to the amorphous component is assigned as a Gaussian function so that it is tangent to the peak shapes at 2θ = 15° to 17° and 2θ = 27° to 29°. At this time, the center of the halo pattern is set to 2θ = 19° to 21°, and the full width at half maximum is set to about 10. 3) For the diffraction lines from crystalline components, a symmetric pseudo-Voigt function is assigned to match the peak top and waveform. Profile fitting is performed with the halo pattern parameters fixed. The peak 2θ position, half-width, height, and the contribution ratio of the Gaussian function and Lorentzian function are set during fitting. Jade+9.8 is used for fitting. 4) Using the multiple peak separation method, the peak area of ​​the crystalline portion, the peak area of ​​the α crystal, and the peak area of ​​the γ crystal are determined, and the ratio of the γ crystal to the total amount of the α crystal and the γ crystal is calculated using the following formula 1. <Expression 1> The ratio of γ crystals to the total amount of α crystals and γ crystals (%) = {γ crystal peak area / (γ crystal peak area + α crystal peak area)} × 100

[0031] The polyamide hollow fiber membrane of the present invention may have a dense layer on the lumen-side surface and / or outer surface to improve the rejection rate of fine particles. In the present invention, the "dense layer" refers to a region in which, when the cross section of the polyamide hollow fiber membrane is observed, the porous structure of a specific region adjacent to the lumen-side or outer surface of the polyamide hollow fiber membrane is denser and has more micropores than the porous structure of other regions (e.g., a region near the middle between the lumen-side and outer surfaces), and this dense region with more micropores is the part that determines the fractionation characteristics of the polyamide hollow fiber membrane. This also includes cases where the presence of substantially no pores is observed (i.e., when the presence of no pores is observed in a scanning electron microscope (SEM) photograph at a magnification of 10,000 times). The dense layer can be observed in a scanning electron microscope (SEM) photograph. In the polyamide hollow fiber membrane of the present invention, the thickness of the dense layer is not particularly limited, but is, for example, 0.01 to 2.0 μm, preferably 0.1 to 1.5 μm. The polyamide hollow fiber membrane has a dense layer, which improves the blocking ability of fine particles.

[0032] The polyamide hollow fiber membrane of the present invention has, as one of its membrane separation or filtration performances, a particle rejection rate of 50 nm in diameter of preferably 90% or more, more preferably 95% or more, even more preferably 98% or more, and particularly preferably 99% or more. The rejection rate of fine particles varies depending on the application and purpose of use of the module. Suitable examples of filtration performance include a rejection rate of 20 nm in diameter, a rejection rate of 10 nm in diameter, and a rejection rate of 5 nm in diameter of preferably 90% or more, more preferably 95% or more, even more preferably 98% or more, and particularly preferably 99% or more. Thus, the polyamide hollow fiber membrane of the present invention has a pore structure that can separate fine particles with a high rejection rate, and has excellent fine particle removal performance.

[0033] In the present invention, the particle rejection rate for each particle size is calculated from the proportion of gold colloid particles removed when a filtration test is conducted using gold colloid particles having a predetermined average particle size. Gold colloid particles have a very narrow particle size distribution, so a filtration test using gold colloid can accurately reflect the particle rejection rate of a hollow fiber membrane. Specifically, a filtration test using gold colloid particles involves adding 2 mmol / L of tris(hydroxymethyl)aminomethane to an aqueous dispersion containing 10 ppm of gold colloid having a predetermined average particle size, performing constant-pressure dead-end filtration under conditions of a filtration pressure of 0.3 MPa and a filtration temperature of 25°C, and filtering the filtrate at an accumulated filtration volume of 0.005 (m 3 / m 2 The absorbance of the second aliquot at a wavelength of 524 nm is measured, and the rejection rate of particles of each particle size is calculated using the following formula 2. <Expression 2> Particle rejection rate (%) = {(absorbance of unfiltered solution - absorbance of filtrate) / absorbance of unfiltered solution} x 100

[0034] The polyamide hollow fiber membrane of the present invention is characterized by its excellent storage stability, with its filtration performance remaining stable even after long-term storage in a storage solution. Polyamide hollow fiber membranes are generally used as hollow fiber membrane modules housed in a module case. After use in various applications, hollow fiber membrane modules are typically stored by immersing them in a storage solution. In the present invention, "use" of a polyamide hollow fiber membrane or hollow fiber membrane module refers to passing a filtrate (stock solution before filtration) from one side of the inner surface or outer surface of the polyamide hollow fiber membrane to the other, thereby achieving filtration. The type and amount of the filtrate, the type and amount of components to be classified, and the filtration performance are not limited. In the present invention, the type of "storage solution" is not limited, and examples include water and / or an organic solvent. The storage solution may contain additives such as preservatives to enhance storage stability, if necessary. In the present invention, "storage" refers to immersing the membrane in a storage solution until the next use, preferably by static storage without passing a liquid through the membrane. The storage temperature is not particularly limited, but is preferably 20 to 40°C. The storage period is not particularly limited, but is usually one month or more, and from the viewpoint of significantly exhibiting the effects of the present invention, it is preferably six months or more, more preferably one year or more, even more preferably two years or more, and particularly preferably three years or more.

[0035] The polyamide hollow fiber membrane of the present invention has a retention rate of rejection of particles with a particle diameter of 5 nm, calculated by the following formula 3, of preferably 90% or more, more preferably 95% or more, even more preferably 97% or more, still more preferably 98% or more, and particularly preferably 99% or more. <Expression 3> Retention rate of rejection rate (%) = (rejection rate of 5 nm particles after storage / rejection rate of 5 nm particles before storage) × 100 Storage conditions: The polyamide hollow fiber membrane is immersed in propylene glycol monomethyl ether acetate and stored at 23°C for 2 years without pressure.

[0036] The polyamide hollow fiber membrane of the present invention preferably has an external pressure water permeability of 50 to 2000 L / (m 2·atm·h), more preferably 100 to 1500 L / (m 2 ·atm·h), more preferably 150 to 1000 L / (m 2 ·atm·h), and even more preferably 200 to 800 L / (m 2 ·atm·h), more preferably 250 to 800 L / (m 2 ·atm·h), particularly preferably 300 to 800 L / (m 2 However, the preferred external pressure water permeability varies depending on the application and purpose of use of the module, and also on the rejection rate, which is the filtration performance of the polyamide hollow fiber membrane.

[0037] In a polyamide hollow fiber membrane having the ability to block 90% or more of particles with a particle size of 5 nm, the external pressure water permeability is preferably 50 to 1000 L / (m 2 ·atm·h), more preferably 100 to 800L / (m 2 ·atm·h), more preferably 150 to 600 L / (m 2 ·atm·h), and even more preferably 200 to 600 L / (m 2 ·atm·h), particularly preferably 250 to 400 L / (m 2 ·atm·h).

[0038] In a polyamide hollow fiber membrane that allows 10% or more of particles with a particle size of 5 nm to pass through but blocks 90% or more of particles with a particle size of 10 nm, the external pressure water permeability is preferably 100 to 1500 L / (m 2 ·atm·h), more preferably 150 to 1000L / (m 2 ·atm·h), more preferably 200 to 800 L / (m 2 ·atm·h), and even more preferably 250 to 600 L / (m 2 ·atm·h), particularly preferably 300 to 500 L / (m 2 ·atm·h).

[0039] In a polyamide hollow fiber membrane that allows 10% or more of particles with a particle size of 10 nm to pass through but blocks 90% or more of particles with a particle size of 20 nm, the external pressure water permeability is preferably 200 to 2000 L / (m2 ·atm·h), more preferably 250 to 1750 L / (m 2 ·atm·h), more preferably 300 to 1500 L / (m 2 ·atm·h), and even more preferably 500 to 1250 L / (m 2 ·atm·h), particularly preferably 600 to 1000 L / (m 2 ·atm·h).

[0040] In a polyamide hollow fiber membrane that allows 10% or more of particles with a particle size of 20 nm to pass through but blocks 90% or more of particles with a particle size of 50 nm, the external pressure water permeability is preferably 500 to 2000 L / (m 2 ·atm·h), more preferably 600 to 1750L / (m 2 ·atm·h), more preferably 800 to 1500 L / (m 2 ·atm·h), and even more preferably 1000 to 1500 L / (m 2 ·atm·h), particularly preferably 1250 to 1500 L / (m 2 ·atm·h).

[0041] As described above, the polyamide hollow fiber membrane of the present invention has high external pressure water permeability, so that the flow rate of the treatment liquid can be set high, and the filtration efficiency can be increased.

[0042] In the present invention, the external pressure water permeability of the polyamide hollow fiber membrane is a value measured by external pressure filtration, specifically, a value measured by the method described in the examples below.

[0043] The polyamide hollow fiber membrane of the present invention has an external pressure water permeability retention calculated by the following formula 4 of preferably 90 to 110%, more preferably 95 to 105%, even more preferably 97 to 103%, still more preferably 98 to 102%, and particularly preferably 99 to 101%. <Expression 4> Retention rate of external pressure water permeability (%) = (external pressure water permeability after storage / external pressure water permeability before storage) x 100 Storage conditions: The polyamide hollow fiber membrane is immersed in propylene glycol monomethyl ether acetate and stored at 23°C for 2 years without pressure.

[0044] As one of the filtration performance characteristics, the polyamide hollow fiber membrane of the present invention preferably has an initial bubble point of 0.20 MPa or more and a burst bubble point of 0.30 MPa or more in a bubble point test in which air pressure is applied in 2-propanol having a surface tension of 21 mN / m at 20° C. The initial bubble point and burst bubble point vary depending on the application and purpose of use of the module, but preferred examples of filtration performance include an initial bubble point of 0.30 MPa or more and a burst bubble point of 0.40 MPa or more, an initial bubble point of 0.35 MPa or more and a burst bubble point of 0.45 MPa or more, an initial bubble point of 0.40 MPa or more and a burst bubble point of 0.55 MPa or more being even more preferred, and an initial bubble point of 0.45 MPa or more and a burst bubble point of 0.65 MPa or more being even more preferred. The initial bubble point is preferably 0.20 to 1.20 MPa, more preferably 0.30 to 1.10 MPa, even more preferably 0.35 to 1.00 MPa, still more preferably 0.40 to 0.90 MPa, and particularly preferably 0.45 to 0.80 MPa. The burst bubble point is preferably 0.30 to 1.20 MPa, more preferably 0.40 to 1.10 MPa, even more preferably 0.45 to 1.00 MPa, still more preferably 0.55 to 0.90 MPa, and particularly preferably 0.65 to 0.80 MPa. The bubble point indicates that the polyamide hollow fiber membrane has an appropriate pore size for high filtration accuracy.

[0045] In the present invention, the bubble point test is a commonly used measurement method for determining the maximum pore size, and is widely used to estimate pore size because the measurement is simple and rapid. The principle and method of the bubble point test are described in JIS standard K3832. The initial bubble point is the pressure at which air permeates through the membrane surface and bubbles begin to emerge when air pressure is applied to a hollow fiber membrane, and the burst bubble point is the pressure at which bubbles begin to emerge from almost the entire membrane. The initial bubble point and burst bubble point are specifically values ​​measured by the methods described in the Examples below.

[0046] In the polyamide hollow fiber membrane of the present invention, in the bubble point test, the retention of the initial bubble point (hereinafter sometimes referred to as "IBP") and burst bubble point (hereinafter sometimes referred to as "BBP") calculated by the following formula 5 is preferably 90 to 110%, more preferably 95 to 105%, even more preferably 97 to 103%, still more preferably 98 to 102%, and particularly preferably 99 to 101%. <Formula 5> Retention rate of IBP or BBP (%) = (IBP or BBP after storage / IBP or BBP before storage) × 100 Storage conditions: The polyamide hollow fiber membrane is immersed in propylene glycol monomethyl ether acetate and stored at 23°C for 2 years without pressure.

[0047] The inner and outer diameters of the polyamide hollow fiber membrane of the present invention are not particularly limited and are appropriately set depending on the intended use, etc. The inner diameter is, for example, 100 to 3000 μm, preferably 150 to 1500 μm, more preferably 200 to 1000 μm, and even more preferably 350 to 800 μm. The outer diameter is, for example, 250 to 5000 μm, preferably 300 to 3000 μm, more preferably 400 to 2000 μm, and even more preferably 450 to 600 μm.

[0048] The inner and outer diameters of the polyamide hollow fiber membrane can be measured by observing the cross section of the polyamide hollow fiber membrane under an optical microscope at 200 times magnification.

[0049] From the viewpoint of improving processability when the polyamide hollow fiber membrane of the present invention is housed in a module case to form a hollow fiber membrane module, it is preferable that the values ​​of the breaking strength, breaking elongation, and tensile modulus are within the ranges described below. In the present invention, the breaking strength, breaking elongation, and tensile modulus are the average values ​​measured in accordance with JIS L-1013, with a chuck distance of 50 mm, a pulling speed of 50 mm / min, and five measurements.

[0050] The breaking strength of the polyamide hollow fiber membrane is preferably 1.5 to 30 MPa, more preferably 2.5 to 25 MPa, even more preferably 3.5 to 15 MPa, and even more preferably 4 to 10 MPa.

[0051] The breaking elongation of the polyamide hollow fiber membrane is preferably 10 to 500%, more preferably 20 to 350%, even more preferably 40 to 320%, and still more preferably 100 to 300%.

[0052] The tensile modulus of the polyamide hollow fiber membrane is preferably 10 to 100 MPa, more preferably 15 to 80 MPa, even more preferably 20 to 70 MPa, and even more preferably 20 to 60 MPa.

[0053] The polyamide hollow fiber membrane of the present invention is a hollow fiber membrane formed primarily from a polyamide resin, but may contain other resin components, softeners, curing agents, crosslinking agents, antioxidants, stabilizers, dispersants, lubricants, flame retardants, antioxidants, antistatic agents, and other additives as needed, provided that the effects of the present invention are not impaired. These may be used alone or in combination of two or more. The total content of these components is preferably 10% by mass or less of the entire polyamide hollow fiber membrane.

[0054] The polyamide hollow fiber membrane of the present invention may have a coating layer such as an organic coating layer or an inorganic coating layer on the lumen surface and / or the outer surface. The thickness of the coating layer is not particularly limited, but is, for example, 0.001 to 100 μm.

[0055] 2. Polyamide hollow fiber membrane manufacturing method The polyamide hollow fiber membrane of the present invention can be produced by employing specific production conditions using the thermally induced phase separation method (TIPS method).

[0056] Specifically, the method for producing the polyamide hollow fiber membrane of the present invention is carried out through the following first to third steps. First step: A membrane-forming solution is prepared by mixing at least a polyamide resin and a sulfone using a multi-screw extruder. The sulfone has a pH of 5.2 to 6.8 at 25°C when dissolved in water to form a 5% by mass aqueous solution. Step 2: Using a double-tube nozzle for producing hollow fibers with a double-tube structure, the membrane-forming solution is discharged from the outer annular nozzle and the internal solution is discharged from the inner nozzle, and the resulting mixture is immersed in a coagulation bath containing water and / or a polyhydric alcohol to form a hollow fiber membrane. Third step: The organic solvent is removed from the hollow fiber membrane formed in the second step.

[0057] Hereinafter, each step of the method for producing the polyamide hollow fiber membrane of the present invention will be described in detail.

[0058] <1st process> In the first step, a membrane-forming solution is prepared by mixing at least a polyamide resin and a sulfone using a multi-screw extruder.

[0059] A common method for preparing a membrane-forming solution is to charge raw materials such as a polyamide resin and an organic solvent into a tank equipped with a heater and a stirrer, and prepare the membrane-forming solution in a batchwise manner. However, the conventional method for preparing a membrane-forming solution in a batchwise manner cannot reduce the content of a specific metal element in a polyamide hollow fiber membrane to a desired range.

[0060] Therefore, in the present invention, in order to reduce the content of the specific metal element in the polyamide hollow fiber membrane to a desired range, a method is adopted in which at least a polyamide resin and a sulfone are quantitatively charged into a multi-screw extruder to continuously prepare a membrane-forming solution.Furthermore, in order to reduce the content of the specific metal element in the polyamide hollow fiber membrane to a desired range, the sulfone used has a pH of 5.2 to 6.8 at 25°C when dissolved in water to form a 5% by mass aqueous solution.

[0061] The reason why the content of a specific metal element in a polyamide hollow fiber membrane can be reduced to a desired range by adopting a method of continuously preparing a membrane-forming solution using a multi-screw extruder and sulfones that have a pH of 5.2 to 6.8 at 25°C when dissolved in water to form a 5% by mass aqueous solution is not clear, but it is presumed to be due to the following reasons: The first reason is that the continuous method using a multi-screw extruder increases the solubility of polyamide resin in the sulfones compared to the batch method using a tank, thereby shortening the time required for dissolution, and therefore shortening the residence time of the membrane-forming solution in the production equipment (contact time with the metal), which is presumed to prevent the specific metal element from being mixed into the membrane-forming solution from the production equipment. The second reason is that the sulfones have higher compatibility with polyamide resins than conventional organic solvents used in preparing the membrane-forming solution, and therefore the time required for dissolution can be further shortened, which in turn can further shorten the residence time of the membrane-forming solution in the manufacturing equipment (contact time with metals), and therefore it is presumed that this can further suppress the incorporation of specific metal elements originating from the manufacturing equipment into the membrane-forming solution.

[0062] The multi-screw extruder is not particularly limited as long as it is equipped with multiple screws, but from the viewpoint of versatility, a twin-screw extruder is preferred. The outer diameter and L / D of the screw are not particularly limited and may be designed appropriately depending on the target production volume, but the outer diameter is preferably φ30 or more, and the L / D is preferably 25 or more. The screw configuration is also not particularly limited, but the arrangement of full flight and kneading disks, etc. may be designed appropriately to ensure stable production of the membrane-forming solution. The screw rotation speed is also not particularly limited and may be designed appropriately, but is preferably 30 rpm or more. The set temperature of the multi-screw extruder may be designed appropriately, but is preferably 150 to 300°C. In addition, it is preferable that the multi-screw extruder is equipped with a device for quantitatively feeding the polyamide resin and sulfones.

[0063] Examples of sulfones include dimethyl sulfone, sulfolane, diethyl sulfone, diphenyl sulfone, 1,3-propane sulfone, 1,4-butane sulfone, busulfan, sulfolene, ethyl methyl sulfone, and methyl phenyl sulfone. These may be used alone or in combination of two or more. Of these, dimethyl sulfone and / or sulfolane are preferred from the viewpoint of further reducing the content of specific metal elements in the polyamide hollow fiber membrane.

[0064] When dimethyl sulfone is used as the sulfone, the pH of a 5% by mass aqueous solution (5% by mass of dimethyl sulfone, 95% by mass of water) at 25°C is preferably 5.2 to 6.5. When sulfolane is used as the sulfone, the pH of a 5% by mass aqueous solution (5% by mass of sulfolane, 95% by mass of water) at 25°C is preferably 6.3 to 6.8. When dimethyl sulfone and sulfolane are used in combination as the sulfones, it is preferable that the pH of a 5% by mass aqueous solution of either or both of them is within the above-mentioned preferred range. The pH of a 5% by mass aqueous solution of the sulfone can be adjusted to the desired pH, for example, by adjusting the amount of oxidizing agent or reducing agent used in synthesizing the sulfone.

[0065] In the membrane-forming solution, the mass ratio of polyamide resin to sulfones (polyamide resin:sulfones) is preferably 5:95 to 50:50, more preferably 20:80 to 40:60, and even more preferably 26:74 to 30:70. By satisfying this mass ratio, it becomes easier to adjust the particle rejection rate, initial bubble point, burst bubble point, and external pressure water permeability within the aforementioned ranges.

[0066] <Second process> In the second step, a double-tube nozzle for hollow fiber production having a double-tube structure is used, and the membrane-forming solution is discharged from the outer annular nozzle and the internal solution is discharged from the inner nozzle, and the resulting mixture is immersed in a coagulation bath containing water and / or a polyhydric alcohol to form a hollow fiber membrane.

[0067] Here, the double tubular nozzle for producing hollow fibers can be a spinneret with a double annular structure, such as that used in producing core-sheath composite fibers in melt spinning. The diameters of the outer annular nozzle and the inner nozzle of the double tubular nozzle for producing hollow fibers can be appropriately set depending on the inner and outer diameters of the polyamide hollow fiber membrane.

[0068] In the second step, the internal liquid discharged from the inner nozzle of the double-tubular nozzle for hollow fiber production can be either liquid or gas, as long as it is inert to the polyamide resin. Liquids are preferred because they allow for spinning even under conditions where the viscosity of the membrane-forming solution is low and filament formation is difficult. The liquid used as the internal liquid is not particularly limited, as long as it is inert to the polyamide resin. However, a good solvent with a high affinity for the polyamide resin can be used to create relatively large pores on the inner surface of the polyamide hollow fiber, while a poor solvent can be used to create relatively small pores on the inner surface of the polyamide hollow fiber. Specific examples of such good solvents include glycerin, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol 200, γ-butyrolactone, ε-caprolactone, propylene glycol, benzyl alcohol, 1,3-butanediol, and sulfolane. Specific examples of such poor solvents include polyethylene glycol having an average molecular weight of 300 to 1000, polypropylene glycol having an average molecular weight of 400 to 1000, higher fatty acids, and liquid paraffin. These solvents may be used alone or in combination of two or more. When the membrane-forming solution has high viscosity and excellent spinnability, a method of injecting a gas such as an inert gas may be used.

[0069] Among these internal liquids, glycerin, 1,3-butanediol, triethylene glycol, tetraethylene glycol, polyethylene glycol 200, and sulfolane are preferably used.

[0070] In the second step, a coagulation bath containing water and / or a polyhydric alcohol is used. By using such a coagulation bath, a polyamide hollow fiber membrane having the above-described properties can be formed. Specific examples of polyhydric alcohols used in the coagulation bath include glycerin, ethylene glycol, propylene glycol, butylene glycol, diethylene glycol, dipropylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, polyethylene glycol (200-400), and 1,3-butanediol. Among these polyhydric alcohols, glycerin, ethylene glycol, diethylene glycol, propylene glycol, 1,3-butanediol, and polyethylene glycol 200 are preferred. These polyhydric alcohols may be used singly or in combination of two or more.

[0071] Furthermore, when a coagulation bath containing water and a polyhydric alcohol is used as the coagulation bath, the composition ratio thereof is not particularly limited, but the mass ratio of polyhydric alcohol:water is preferably 25-80:75-20, more preferably 40-70:60-30.

[0072] An example of an internal liquid and coagulation bath for obtaining a polyamide hollow fiber membrane having a dense layer formed on the lumen surface and a porous layer with pores of relatively large pore diameters that supports the dense layer is, for example, at least one selected from the group consisting of glycerin, polyethylene glycol having an average molecular weight of 300 to 1000, polypropylene glycol having an average molecular weight of 400 to 1000, and triethylene glycol as the internal liquid, and at least one selected from the group consisting of diethylene glycol, tetraethylene glycol, and propylene glycol, or an aqueous solution containing at least one of these in a proportion of 40 to 80% by mass (preferably 40 to 60% by mass) as the coagulation bath.

[0073] The temperature of the coagulation bath is not particularly limited, but is typically −20 to 100°C, preferably −10 to 80°C, and more preferably 0 to 40°C. Changing the temperature of the coagulation bath can alter the crystallization rate, thereby changing the pore size, water permeability, and strength. Generally, lower coagulation bath temperatures tend to result in smaller pore size, lower water permeability, and improved strength, while higher coagulation bath temperatures tend to result in larger pore size, higher water permeability, and lower strength. However, this can also depend on the solubility of the solvent contained in the membrane-forming solution with the internal liquid and the crystallization rate of the resin itself. To achieve the external pressure water permeability and particle rejection rate of the polyamide hollow fiber membrane within the aforementioned ranges, a low coagulation bath temperature is preferred, but depending on the conditions, a low temperature is not necessarily required. Keeping the coagulation bath temperature within the above ranges increases the membrane strength and reduces the energy required for temperature control.

[0074] The flow rate of the membrane-forming solution when it is discharged from the outer annular nozzle of the double-tubular nozzle for hollow fiber production is not particularly limited, and may be, for example, 2 to 20 g / min, preferably 3 to 15 g / min, and more preferably 4 to 10 g / min. The flow rate of the internal solution is appropriately set taking into consideration the diameter of the inner nozzle of the double-tubular nozzle for hollow fiber production, the type of internal solution used, the flow rate of the membrane-forming solution, etc., and may be, for example, 0.1 to 2 times, preferably 0.2 to 1 time, and more preferably 0.4 to 0.7 times the flow rate of the membrane-forming solution.

[0075] By carrying out the second step in this manner, the membrane forming solution discharged from the double tubular nozzle for hollow fiber production is coagulated in the coagulation bath to form a polyamide hollow fiber membrane.

[0076] <3rd process> In the third step, the organic solvent is removed from the hollow fiber membrane formed in the second step. The method for removing the organic solvent from the hollow fiber membrane is not particularly limited, and examples include immersing the hollow fiber membrane in a cleaning solution consisting of water or an aqueous solution, or winding the hollow fiber membrane on a bobbin, a fence, or a skein winder and exposing the wound hollow fiber membrane to running water consisting of the cleaning solution. These methods can remove organic solvents, such as sulfones contained in the hollow fiber membrane, components of the coagulation bath, and components of the internal solution. A cleaning solution that is inexpensive, has a low boiling point, and can be easily separated after washing due to differences in boiling points is preferred, and water is particularly suitable. If water alone does not provide sufficient cleaning properties, an aqueous solution containing a component that promotes cleaning properties may be used. Examples of components that promote cleaning properties include, but are not limited to, solvents such as methanol, ethanol, isopropanol, acetone, diethyl ether, and petroleum ether, as well as surfactants. The cleaning time is not particularly limited, but is typically 0.2 hours to 2 months, preferably 0.5 hours to 1 month, and more preferably 2 hours to 10 days. In order to effectively remove the organic solvent remaining in the polyamide hollow fiber membrane, the components of the cleaning solution may be changed, the cleaning solution may be stirred, or the flow rate of the cleaning solution may be changed.

[0077] By carrying out the third step in this manner, the polyamide hollow fiber membrane of the present invention is produced.

[0078] <4th process> The polyamide hollow fiber membrane obtained through steps 1 to 3 has a reduced content of specific metal elements, but in order to further reduce the content of specific metal elements, a fourth step of washing the polyamide hollow fiber membrane with an organic solvent may be added after step 3.

[0079] A preferred method for cleaning polyamide hollow fiber membranes is to immerse the lumen side surface and / or outer surface of the polyamide hollow fiber membrane in an organic solvent as a cleaning solvent, and more preferred is to immerse the lumen side surface and outer surface in an organic solvent. Furthermore, during immersion, it is preferred to pass the cleaning solvent from one of the lumen side surface or outer surface of the hollow fiber membrane to the other. In cleaning, the polyamide hollow fiber membranes may be washed directly, or a hollow fiber membrane module may be prepared by housing the polyamide hollow fiber membranes in a module case, and the hollow fiber membrane module may be filled with a cleaning solvent for cleaning. From the viewpoint of cleaning operability, a method in which the hollow fiber membrane module is filled with a cleaning solvent for cleaning is preferred.

[0080] The immersion time is preferably one day or more, more preferably three days or more, even more preferably one week or more, and particularly preferably one month or more. There is no particular upper limit to the immersion time, but it is usually less than one year. The temperature of the washing solvent during the immersion treatment is preferably room temperature (23°C) or higher, more preferably 35°C or higher. There is no particular upper limit to the temperature of the washing solvent, but it is usually below the boiling point of the washing solvent used.

[0081] Furthermore, the polyamide hollow fiber membrane may be subjected to ultrasonic treatment continuously or temporarily during immersion. For example, when the hollow fiber membrane module is filled with a cleaning solvent and washed, the ultrasonic output is preferably 50 kW or more per inch of the module, more preferably 80 kW or more, and even more preferably 100 kW or more. There is no particular upper limit to the ultrasonic output, but it is usually 200 kW. The ultrasonic treatment time is preferably 5 minutes or more, more preferably 15 minutes or more, more preferably 1 hour or more, and particularly preferably 3 hours or more. There is no particular upper limit to the ultrasonic treatment time, but it is usually less than 10 hours.

[0082] During this immersion, fresh cleaning solvent may be added during immersion, or part or all of the cleaning solvent may be replaced with fresh cleaning solvent multiple times in order to improve cleaning efficiency.

[0083] Furthermore, when passing a cleaning solvent from one side of the hollow fiber membrane's lumen surface to the other side of its outer surface during cleaning, a preferred method is to feed the cleaning solvent through a liquid passage port on either the lumen side or the outer storage space of the hollow fiber membrane module, pass it through the inner wall of the hollow fiber membrane, and then discharge it from the other liquid passage port. It is preferable not to reuse a cleaning solvent that has already passed through the hollow fiber membrane for cleaning. The direction of the liquid passage may be changed midway to improve cleaning performance.

[0084] When a cleaning solvent is passed from one side of the lumen surface or the outer surface of the hollow fiber membrane to the other, the amount of the liquid passed is preferably 10 kg or more, more preferably 50 kg or more, even more preferably 300 kg or more, and particularly preferably 500 kg or more per inch of the module when, for example, the hollow fiber membrane module is filled with the cleaning solvent to be washed. The temperature of the cleaning solvent passed is preferably room temperature (23°C) or higher, more preferably 35°C or higher. There is no particular upper limit to the temperature of the cleaning solvent, but it is usually below the boiling point of the cleaning solvent used.

[0085] The flow rate of the solution is not particularly limited, but for example, the volume of the cleaning solvent per hour is in the range of 0.1 to 1000 times the internal volume of the module, and preferably 1 to 100 times. The flow rate may be intentionally changed to enhance the cleaning effect.

[0086] The organic solvent used as the washing solvent is not particularly limited, and known organic solvents can be used. Examples of organic solvents include alkylene glycol monoalkyl ether carboxylates, alkylene glycol monoalkyl ethers, alkyl lactates, alkyl alkoxypropionates, cyclic lactones (preferably having 4 to 10 carbon atoms), monoketone compounds which may contain a ring (preferably having 4 to 10 carbon atoms), alkylene carbonates, alkyl alkoxyacetates, alkyl pyruvates, dialkyl sulfoxides, cyclic sulfones, dialkyl ethers, monohydric alcohols, glycols, alkyl acetates, and N-alkylpyrrolidones. These may be used alone or in combination of two or more.

[0087] The organic solvents used for cleaning are propylene glycol monomethyl ether acetate (hereinafter sometimes referred to as "PGMEA"), isopropanol, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, ethyl lactate, methyl methoxypropionate, cyclopentanone, cyclohexanone, γ-butyrolactone, diisoamyl ether (isoamyl ether), butyl acetate, isoamyl acetate, 4-methyl-2-pentanol, N-methylpyrrolidone, diethylene glycol Preferably, the alkyl ether is at least one selected from the group consisting of ethylene glycol, dipropylene glycol, propylene glycol, ethylene carbonate, propylene carbonate, cycloheptanone, 2-heptanone, butyl butyrate, isobutyl isobutyrate, undecane, pentyl propionate, isopentyl propionate, ethylcyclohexane, mesitylene, decane, 3,7-dimethyl-3-octanol, 2-ethyl-1-hexanol, 1-octanol, 2-octanol, ethyl acetoacetate, dimethyl malonate, methyl pyruvate, and dimethyl oxalate.

[0088] Since cleaning solvents made of these organic solvents have better cleaning properties when the water content is low, it is preferable not to add water other than water unintentionally contained due to moisture absorption by the organic solvent, etc. The water content in the cleaning solvent is preferably 1% by mass or less relative to 100% by mass of the cleaning solvent.

[0089] By carrying out the fourth step in this way, a polyamide hollow fiber membrane having a further reduced content of the specific metal element can be produced. However, since the specific metal element in the polyamide hollow fiber membrane cannot be removed even by thorough washing with water or an organic solvent, the content of the specific metal element cannot be sufficiently reduced by washing treatment using water or an organic solvent alone. Therefore, to obtain the polyamide hollow fiber membrane of the present invention in which the content of the specific metal element is reduced to a specific value or less, it is necessary to produce it by a method including the first to third steps.

[0090] 3. Hollow fiber membrane module The polyamide hollow fiber membranes of the present invention can be housed in a module case and used as a hollow fiber membrane module. The size of the module case is not particularly limited, and can be appropriately designed to suit each application. Known methods can be used to process the hollow fiber membrane module. Specifically, a polyamide hollow fiber membrane bundle is housed in a cylindrical module case, and then the ends of the module case, together with the hollow fiber membrane bundle, are sealed with a potting material. Potting can be performed using either double-end potting or single-end potting. Next, the potting portion that sufficiently seals the hollow fiber membrane bundle is cut to open the space on the lumen side of the hollow fiber membranes. It is necessary that the space on the lumen side of the hollow fiber membranes in the module and the space outside the hollow fiber membranes are securely separated and leak-free. It is preferable to attach a cap with a liquid passage port to the module end where the space on the lumen side of the hollow fiber membranes is open, allowing liquid to pass through the lumen side space. It is also preferable to provide a liquid passage port in the module case to allow liquid to pass through the space outside the hollow fiber membranes in the module case. The shape of the fluid passage port may be selected to suit the intended use.

[0091] The potting material may be a known potting material, and specific examples include polyurethane resin, epoxy resin, polyolefin resin, etc. Among these, from the viewpoint of enhancing resistance to organic solvents, epoxy resin and polyolefin resin are preferred, and among polyolefin resins, polyethylene and polypropylene are preferred, with polyethylene being more preferred.

[0092] The polyamide hollow fiber membrane and hollow fiber membrane module of the present invention have excellent filtration performance in various fields and are characterized by their resistance to change even after long-term storage in storage solutions, making them suitable for a variety of applications. Specifically, they can be used for the filtration or final filtration of raw materials and intermediates used in pharmaceutical manufacturing, the filtration of chemical solutions used for cleaning pharmaceutical manufacturing equipment, and the filtration of chemical solutions used in semiconductor manufacturing, specifically, the filtration of chemical solutions used after each step or before moving on to the next step in semiconductor device manufacturing processes including lithography, etching, ion implantation, and stripping processes. More specifically, they can be used to filter chemical solutions such as developers, rinse solutions, wafer cleaning solutions, line cleaning solutions (e.g., piping cleaning solutions), prewet solutions, wafer rinse solutions, resist solutions, solutions for forming underlayer films, solutions for forming upper layer films, solutions for forming hard coats, aqueous developers, aqueous rinse solutions, strippers, removers, etching solutions, acidic cleaning solutions, phosphoric acid, and phosphoric acid-hydrogen peroxide mixtures. Other applications include, for example, the filtration of chemicals such as developers for polyimide, resists for sensors, resists for lenses, and rinse solutions. [Example]

[0093] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples.

[0094] Various properties were measured or evaluated by the following methods.

[0095] [Relative viscosity of polyamide hollow fiber membrane] Each polyamide hollow fiber membrane obtained in the Examples, Comparative Examples, and Reference Examples was dissolved in 96% by mass sulfuric acid to a polyamide hollow fiber membrane concentration of 1 g / dL, and the viscosities were measured at 25°C using an Ubbelohbe viscometer.

[0096] [Metal element content of polyamide hollow fiber membrane] (1) Metal element content of hollow fiber membrane before organic solvent washing (step 4) 0.5 g of each polyamide hollow fiber membrane obtained in the Examples, Comparative Examples, and Reference Examples was mixed with 5 mL of nitric acid, heated to 100°C over 10 minutes, and held at 100°C for 5 minutes. The mixture was then heated to 140°C over 3 minutes and held at 140°C for 5 minutes. The mixture was then heated to 180°C over 5 minutes and held at 180°C for 10 minutes. The polyamide hollow fiber membrane was decomposed and / or dissolved in the nitric acid, and then the volume was adjusted to 50 mL with ultrapure water. The metal element content (ppm) of this sample was measured using a Thermo Fisher Scientific iCAP6500Duo ICP atomic emission spectrometer. The measured value was the average value obtained by subtracting the average value of a blank run (n=10) conducted simultaneously with the sample run. If the calculated metal element content was within 3σ of the standard deviation obtained from the blank run (n=10), the content was recorded as 0.00 ppm. (2) Metal element content of hollow fiber membrane after organic solvent washing (step 4) Each polyamide hollow fiber membrane obtained in Examples, Comparative Examples, and Reference Examples was housed in a 10-inch module case at a housing ratio of 25% (housing ratio (%) = total cross-sectional area of ​​the housed hollow fiber membrane bundle / cross-sectional area of ​​the module case inner tube × 100), and potting was performed to prepare a hollow fiber membrane module. The module was filled with PGMEA (immersing both the inner and outer sides of the hollow fiber membranes) and maintained at 35°C for 1 week. Subsequently, ultrasonic treatment was performed at an output of 800 kW for 1 hour. After that, 10,000 kg of PGMEA was pumped into the module through a liquid passage port in the hollow fiber membrane lumen side space of the module and passed through the hollow fiber membranes. After all the PGMEA had passed through, the module was disassembled, the housed hollow fiber membranes were removed, and the PGMEA was dried and removed. The metal element content (ppm) of the resulting polyamide hollow fiber membranes was measured using the above-mentioned method (1).

[0097] [Ratio of γ crystals to the total amount of α crystals and γ crystals in polyamide hollow fiber membrane] The ratio of γ crystals to the total amount of α crystals and γ crystals of each polyamide hollow fiber membrane obtained in the examples, comparative examples, and reference examples was measured using the RINT-TTR III (CBO) manufactured by Rigaku Corporation as an X-ray analyzer by the method described above.

[0098] [External pressure water permeability of polyamide hollow fiber membrane] Each polyamide hollow fiber membrane obtained in the Examples, Comparative Examples, and Reference Examples was cut to a length of 9 to 12 cm, and syringe needles of a diameter matching the inner diameter were inserted into the hollow portions at both ends. One syringe needle was sealed with a cap, and the other syringe needle was connected to the outlet, and the membrane was set in the apparatus shown in Figure 1. Subsequently, pure water at 25°C was passed through the membrane with a liquid feed pump 1 for a predetermined time (h) while adjusting the outlet valve 5 to maintain a constant pressure of 0.05 MPa. The volume (L) of water that permeated the membrane and accumulated in the receiver 6 was measured as the permeate volume, and the external pressure permeability was calculated using the following equation 6. The inlet pressure was measured using the inlet pressure gauge 2 shown in Figure 1, and the outlet pressure was measured using the outlet pressure gauge 4 shown in Figure 1. <Formula 6> External pressure permeability (L / (m 2 ·atm·h)) = Permeate volume (L) / [Outer diameter (m) × 3.14 × Length (m) × {(Inlet pressure (atm) + Outlet pressure (atm)) / 2} × Time (h)]

[0099] [Fine particle rejection rate of polyamide hollow fiber membrane] (1) Rejection rate of 50 nm particles The rejection rate of particles with a diameter of 50 nm (Gold colloid-50 nm, manufactured by British Biocell International) was measured by the method described above. (2) Rejection rate of 20 nm particles The rejection rate of particles with a diameter of 20 nm (Gold colloid-20 nm, manufactured by British Biocell International) was measured by the method described above. (3) Rejection rate of 10 nm particles The rejection rate of particles with a diameter of 10 nm (Gold colloid-10 nm, manufactured by British Biocell International) was measured by the method described above. (4) Rejection rate of 5 nm particles The rejection rate of particles with a diameter of 5 nm (Gold colloid-5 nm, manufactured by British Biocell International) was measured by the method described above.

[0100] [Bubble point of polyamide hollow fiber membrane] Using the apparatus shown in FIG. 2, the bubble point of each of the polyamide hollow fiber membranes obtained in the Examples, Comparative Examples, and Reference Examples was measured by the following method. Ten 20 cm long polyamide hollow fiber membranes were prepared and bent into a U-shape. Approximately 1 cm of the end of the hollow fiber membranes near the opening was heat-sealed to block the hollow. Next, a 5 cm long soft nylon tube for air piping (outer diameter 8 mm, inner diameter 6 mm) was prepared, one end was blocked with a silicone plug, and a potting agent (polyurethane resin) was introduced up to approximately 4 cm. Next, the hollow fiber membrane bundle was inserted into the potting agent from the heat-sealed end and left to stand until the potting agent hardened. After the potting agent hardened, the nylon tube and the potting portion were cut above the heat-sealed portion of the hollow fiber membrane to open the hollow fiber membrane lumen side. At this time, it was visually confirmed that the potting agent had not penetrated into the hollow portion and that the potting agent had filled the gaps between the hollow fiber membranes. If the hollow portion was maintained without any problems, it was used as a bubble point measurement sample. Next, in the bubble point test, it is necessary to fill the pores of the hollow fiber membrane with a liquid. Therefore, 2-propanol (surface tension of 21 mN / m at 20°C) was introduced into a glass container 13, the bubble point measurement sample 12 was immersed therein, and the pressure was reduced for several seconds to fill the pores with the liquid. The bubble point measurement sample 12 immersed in 2-propanol was set as shown in Figure 2, and air was pumped into the hollow fiber membrane lumen at 0.4 MPa / min to increase the pressure. The pressure at which bubbles first appeared in the hollow fiber membrane was measured and defined as the initial bubble point (IBP). The pressure was then continued to be increased, and the pressure at which bubbles appeared throughout the entire membrane was measured and defined as the burst bubble point (BBP).

[0101] [Breaking strength, breaking elongation, and tensile modulus of polyamide hollow fiber membrane] The breaking strength, breaking elongation, and tensile modulus of each polyamide hollow fiber membrane obtained in the Examples, Comparative Examples, and Reference Examples were measured in accordance with JIS L-1013 using a tensile testing machine (Autograph AG-H) manufactured by Shimadzu Corporation under conditions of a chuck distance of 50 mm, a tensile speed of 50 mm / min, and five measurements, and the average value of the five measured values ​​was used.

[0102] [Storage test of polyamide hollow fiber membrane] Each polyamide hollow fiber membrane obtained in the Examples, Comparative Examples, and Reference Examples was housed in a 5-inch module case at a housing ratio of 20%, and potting was performed to fabricate two hollow fiber membrane modules for each. The hollow fiber membrane module (polyamide hollow fiber membrane housed in the module) was used for PGMEA filtration by supplying 30 L of PGMEA through the liquid passage port in the hollow fiber membrane outer space of the fabricated hollow fiber membrane module and discharging the filtrate through the liquid passage port in the hollow fiber membrane inner space. The same procedure was performed for each of two hollow fiber membrane modules. One of the two hollow fiber membrane modules was disassembled, and the housed polyamide hollow fiber membrane was removed. The PGMEA was then dried and removed, and the filtration performance (fine particle rejection rate, external pressure permeability, IBP, BBP) of the resulting polyamide hollow fiber membrane was measured. The measured values ​​obtained here were exactly the same as the filtration performance of each polyamide hollow fiber membrane obtained in the Examples, Comparative Examples, and Reference Examples. That is, the filtration performance of the polyamide hollow fiber membrane did not change with use. Next, 10 L of PGMEA was newly introduced as a storage solution into another hollow fiber membrane module after use through the fluid passage port in the hollow fiber membrane lumen side space of the module, immersing the lumen side and outer side of the hollow fiber membranes in the hollow fiber membrane module. At this time, the pressure inside the module was 0 MPa. In this state, all fluid passage ports of the module were sealed, and the module was stored at room temperature (23°C) for two years. After storage, the module was disassembled, the hollow fiber membranes stored therein were removed, and the PGMEA was dried and removed. The filtration performance of the resulting polyamide hollow fiber membranes was measured using the methods described above. Then, each retention rate (%) was calculated according to the following formulas 3 to 5. <Expression 3> Retention rate of rejection rate (%) = (rejection rate of particles of each size after storage / rejection rate of particles of each size before storage) × 100 <Expression 4> Retention rate of external pressure water permeability (%) = (external pressure water permeability after storage / external pressure water permeability before storage) x 100 <Formula 5> Retention rate of IBP or BBP (%) = (IBP or BBP after storage / IBP or BBP before storage) × 100

[0103] The raw materials used in the Examples, Comparative Examples, and Reference Examples are shown below. <Polyamide resin> PA1: Polyamide 6 A1030BRT manufactured by Unitika PA2: Polyamide 6 A1030BRF-BA manufactured by Unitika PA3: Polyamide 6 obtained by solid-state polymerization of A1030BRT at 170°C for 15 hours under a N2 stream. PA4: Polyamide 6 obtained by solid-state polymerization of A1030BRT at 170°C for 50 hours under a N2 stream. PA5: Polyamide 6 obtained by solid-state polymerization of A1030BRT at 170°C for 70 hours under a N2 stream. PA6: Polyamide 66 obtained by solid-state polymerization of Unitika Polyamide 66 A125 at 170°C for 30 hours under a nitrogen gas flow. PA7: Polyamide 610 CM2001 manufactured by Toray Industries, Inc. PA8: Polyamide 6, a homogeneous dry blend of PA3 with 0.05% by mass of sodium hydroxide. PA9: Polyamide 6, which is a homogeneous dry blend of PA3 with 0.05% by mass of potassium hydroxide.

[0104] <Sulfones> Manufacturing Example 1 15.7 parts by mass of dimethyl sulfoxide was added to and dissolved in 24.3 parts by mass of a 27.5% by mass aqueous solution of hydrogen peroxide (aqueous solution consisting of 27.5 parts by mass of hydrogen peroxide and 72.5 parts by mass of water). The resulting solution was gradually heated under a nitrogen atmosphere and maintained at 85°C for 1.5 hours. The solution was then concentrated to a total volume of 1.25 times its original volume and allowed to stand at room temperature for 48 hours to precipitate crystals. The crystals were collected by filtration and dried to obtain dimethyl sulfone. The pH of the resulting 5% by mass aqueous solution of dimethyl sulfone was 4.8. This dimethyl sulfone is referred to as "DMS-1."

[0105] Manufacturing Example 2 Dimethyl sulfone was obtained by the same procedure as in Production Example 1, except that a 25.0% by mass aqueous solution of hydrogen peroxide was used instead of the 27.5% by mass aqueous solution of hydrogen peroxide. The pH of the resulting 5% by mass aqueous solution of dimethyl sulfone was 5.2. This dimethyl sulfone is referred to as "DMS-2."

[0106] Manufacturing Example 3 Dimethyl sulfone was obtained by the same procedure as in Production Example 1, except that a 22.5% by mass aqueous solution of hydrogen peroxide was used instead of the 27.5% by mass aqueous solution of hydrogen peroxide. The pH of the resulting 5% by mass aqueous solution of dimethyl sulfone was 5.8. This dimethyl sulfone is referred to as "DMS-3."

[0107] Production Example 4 Dimethyl sulfone was obtained by the same procedure as in Production Example 1, except that a 20.0% by mass aqueous solution of hydrogen peroxide was used instead of the 27.5% by mass aqueous solution of hydrogen peroxide. The pH of the resulting 5% by mass aqueous solution of dimethyl sulfone was 6.5. This dimethyl sulfone is referred to as "DMS-4."

[0108] Manufacturing Example 5 Dimethyl sulfone was obtained by the same procedure as in Production Example 1, except that a 17.5% by mass aqueous solution of hydrogen peroxide was used instead of a 27.5% by mass aqueous solution of hydrogen peroxide. The pH of the obtained 5% by mass aqueous solution of dimethyl sulfone was 6.9. This dimethyl sulfone is referred to as "DMS-5."

[0109] Manufacturing Example 6 A sealed reactor was charged with 0.45 parts by mass of tert-butylcatechol and 230 parts by mass of sulfur dioxide and heated to 100°C. Then, 162 parts by mass of 1,3-butadiene was added at a flow rate of 0.38 parts by mass / min and stirred at 100°C for 1 hour. After releasing the pressure inside the reactor, 720 parts by mass of water was added, the reactor was cooled to 60°C, and the contents were filtered to obtain an aqueous 3-sulfolene solution. 1,000 g of the resulting aqueous 3-sulfolene solution (2.70 mol of 3-sulfolene) was charged into the sealed reactor along with 4.80 parts by mass of Raney nickel catalyst (50% by mass water content). Next, while maintaining the temperature at 30-40°C, hydrogen was introduced into the sealed reactor, pressurizing it to 1.0 MPa. The mixture was stirred for 3 hours while maintaining the pressure, and then filtered to obtain an aqueous sulfolane solution. The resulting aqueous sulfolane solution was heated to remove water by distillation, yielding crude sulfolane. Next, 100 parts by mass of crude sulfolane and 0.5 parts by mass of a 20.0% by mass aqueous hydrogen peroxide solution (an aqueous solution consisting of 20.0 parts by mass of hydrogen peroxide and 80.0 parts by mass of water) were charged into a nitrogen-purged reactor, and the mixture was stirred at low speed at 60°C for 24 hours. After that, the mixture was heated and decompressed to distill off water and impurities, yielding sulfolane. The pH of the resulting 5% by mass aqueous sulfolane solution was 6.3. This sulfolane is designated "SFL-1."

[0110] Manufacturing Example 7 Sulfolane was obtained by the same procedure as in Production Example 6, except that a 22.5% by mass aqueous solution of hydrogen peroxide was used instead of the 20.0% by mass aqueous solution of hydrogen peroxide. The pH of the resulting 5% by mass aqueous solution of sulfolane was 6.6. This sulfolane is referred to as "SFL-2."

[0111] Manufacturing Example 8 Sulfolane was obtained by the same procedure as in Production Example 6, except that a 25.0% by mass aqueous solution of hydrogen peroxide was used instead of the 20.0% by mass aqueous solution of hydrogen peroxide. The pH of the resulting 5% by mass aqueous solution of sulfolane was 6.8. This sulfolane is referred to as "SFL-3."

[0112] Manufacturing Example 9 Sulfolane was obtained by the same procedure as in Production Example 6, except that a 27.5% by mass aqueous solution of hydrogen peroxide was used instead of the 20.0% by mass aqueous solution of hydrogen peroxide. The pH of the resulting 5% by mass aqueous solution of sulfolane was 7.0. This sulfolane is referred to as "SFL-4."

[0113] The pH of the 5% aqueous solution of dimethyl sulfone or sulfolane was measured at 25°C using a "D-51" manufactured by Horiba Ltd.

[0114] Example 1 A twin-screw extruder (Ikegai Corporation, PCM30) equipped with a polyamide feeder and a powder feeder was used. PA1 was used as the raw polyamide resin and DMS-3 was used as the medium. The twin-screw extruder was operated at a screw rotation speed of 100 rpm and a cylinder temperature of 200°C. PA1 was fed into the twin-screw extruder at a rate of 28 parts by mass / h through the polyamide feeder, and DMS-3 was fed into the twin-screw extruder at a rate of 72 parts by mass / h through the powder feeder (i.e., the composition ratio of the resulting film-forming solution was PA1 / DMS-3 = 28 / 72). A film-forming solution in which the polyamide resin and DMS-3 were uniformly dissolved was discharged from the tip of the twin-screw extruder (Step 1). Once the discharge of the membrane dope was stabilized, a spinning device capable of delivering the solution to a spinneret (a double-tube nozzle for hollow fiber production with a double-tube structure (spinneret hole diameter: outer diameter 1.5 mm, inner diameter 0.6 mm)) via a metering pump was attached to the membrane dope outlet at the tip of the twin-screw extruder, and the membrane dope was extruded from the outer annular nozzle at a rate of 5 g / min. The temperature of the spinning device was 200 °C. Concurrently, an inner solution consisting of glycerin was extruded from the inner nozzle at a rate of 2.0 g / min. The extruded spinning dope and inner solution were introduced through a 10 mm air gap into a coagulation bath containing a 50% by mass propylene glycol aqueous solution at 5 °C and cooled to solidify, forming a hollow fiber membrane. The hollow fiber membrane was then wound onto a bobbin at a take-up speed of 20 m / min (Step 2). The residence time of the raw materials from the introduction of the raw materials into the twin-screw extruder through the introduction device to the extrusion of the membrane dope from the nozzle was a maximum of 15 minutes. The obtained hollow fiber membrane was immersed in water for 24 hours to extract (wash) the solvent, and then dried in a hot air dryer at 50°C for 1 hour to obtain a polyamide hollow fiber membrane (third step). The resulting polyamide hollow fiber membrane had an outer diameter of 550 μm and an inner diameter of 300 μm. SEM observation confirmed the formation of a dense layer on the lumen-side surface of the polyamide hollow fiber membrane.

[0115] Examples 2, 3, 7, 8, 12 to 15, Comparative Examples 1 and 2, Reference Examples 1 and 2 Polyamide hollow fiber membranes were obtained in the same manner as in Example 1, except that the polyamide resin and medium were changed to those shown in Table 1. The obtained polyamide hollow fiber membranes all had an outer diameter of 550 μm and an inner diameter of 300 μm, and SEM observation confirmed the formation of a dense layer on the lumen side surface of the hollow fiber membrane.

[0116] Examples 4 to 6 Polyamide hollow fiber membranes were obtained by the same procedure as in Example 3, except that the rate at which the raw materials were quantitatively fed into the twin-screw extruder was changed and the composition ratio of the membrane-forming solution was changed to that shown in Table 1. The obtained polyamide hollow fiber membranes all had an outer diameter of 550 μm and an inner diameter of 300 μm, and SEM observation confirmed the formation of a dense layer on the lumen-side surface of the hollow fiber membrane.

[0117] Examples 9 to 11, Comparative Example 3 Polyamide hollow fiber membranes were obtained by the same procedure as in Example 3, except that the powder quantitative feeder was replaced with a liquid quantitative feeder as an accessory device for the twin-screw extruder, and the medium was changed from DMS-3 to the one shown in Table 1. The obtained polyamide hollow fiber membranes all had an outer diameter of 550 μm and an inner diameter of 300 μm, and SEM observation confirmed the formation of a dense layer on the lumen side surface of the hollow fiber membrane.

[0118] Comparative Examples 4 to 6 Polyamide hollow fiber membranes were obtained by the same procedure as in Example 9, except that the medium was changed from SFL-2 to γ-butyllactone in Comparative Example 4 (pH of a 5% aqueous solution measured by the above method was 5.5), to ε-caprolactone in Comparative Example 5 (pH of a 5% aqueous solution measured by the above method was 6.5), and to propylene carbonate in Comparative Example 6 (pH of a 5% aqueous solution measured by the above method was 7.0). The obtained polyamide hollow fiber membranes all had an outer diameter of 550 μm and an inner diameter of 300 μm, and SEM observation confirmed the formation of a dense layer on the lumen surface of the hollow fiber membrane.

[0119] Comparative Example 7 A 2L tank equipped with a stirrer, which could be heated and sealed, was used, and 504g of PA3 was used as the polyamide resin and 1296g of DMS-3 was used as the medium. The tank was adjusted to a stirrer speed of 20 rpm and an overall temperature of 200°C. PA3 and DMS-3 were placed in the tank, and stirred at 200°C and 20 rpm for 60 minutes to ensure that the PA3 and DMS-3 were dissolved uniformly. Next, the membrane dope was extruded from the outer annular nozzle at a rate of 5 g / min through a metering pump equipped at the membrane dope outlet of the tank and a spinneret (a double-tube nozzle for hollow fiber production with a double-tube structure (the hole diameter of the spinneret was 1.5 mm outer diameter, 0.6 mm inner diameter)) equipped via the metering pump. The temperatures of the metering pump and the double-tube nozzle for hollow fiber production were set to 200°C. In parallel, an inner solution composed of glycerin was extruded from the inner nozzle at a rate of 2.0 g / min. The extruded spinning dope and inner solution were introduced through an air gap of 10 mm into a coagulation bath composed of a 50% by mass aqueous propylene glycol solution at 5°C and cooled and solidified to form a hollow fiber membrane, which was then wound up around a bobbin at a winding speed of 20 m / min. The process up to this point was designated "Process A." The hollow fiber membrane was wound up around the bobbin for 2.5 hours. In this case, the residence time from when the raw materials were fed into the device until the film-forming solution was extruded from the nozzle was a minimum of 85 minutes and a maximum of 225 minutes. The obtained hollow fiber membrane was immersed in water for 24 hours to extract (wash) the solvent, and then dried in a hot air dryer at 50°C for 1 hour to obtain a polyamide hollow fiber membrane. The obtained polyamide hollow fiber membrane had an outer diameter of 550 μm and an inner diameter of 300 μm, and SEM observation confirmed the formation of a dense layer on the inner surface of the hollow fiber membrane.

[0120] Comparative Example 8 A hollow fiber membrane wound around a bobbin was obtained using the same procedure as in step A of Comparative Example 7. The obtained hollow fiber membrane was then immersed in water for washing. During immersion, the water was stirred and new water was continuously poured in at a flow rate of 0.5 L / min, allowing the water to overflow and extract (wash) the solvent for two months. The membrane was then dried in a hot air dryer at 50°C for one hour to obtain a polyamide hollow fiber membrane. The obtained polyamide hollow fiber membrane had an outer diameter of 550 μm and an inner diameter of 300 μm, and SEM observation confirmed the formation of a dense layer on the lumen surface of the hollow fiber membrane.

[0121] Table 1 shows the production conditions for the polyamide hollow fiber membranes obtained in Examples 1 to 15, Comparative Examples 1 to 8, and Reference Examples 1 and 2, as well as the results of measuring the metal element content before and after organic solvent washing. Table 2 also shows the results of membrane evaluation of the polyamide hollow fiber membranes obtained in Examples 1 to 15, Comparative Examples 1 to 8, and Reference Examples 1 and 2. However, for Comparative Example 1 only, the polyamide hollow fiber membrane was washed with an organic solvent by the method described above in "(2) Metal element content of hollow fiber membrane after organic solvent washing (fourth step)" and was used to evaluate "filtration performance before storage" and "retention rate (%) in storage test." Note that the filtration performance of the polyamide hollow fiber membrane before storage did not change after organic solvent washing and use.

[0122] [Table 1]

[0123] [Table 2]

[0124] From Tables 1 and 2, it was confirmed that the polyamide hollow fiber membranes of Examples 1 to 15 had a low content of specific metal elements, and therefore their filtration performance was unlikely to change even when stored for a long period of time in a storage solution, and they had excellent storage stability. Furthermore, in Examples 1 to 15, when the polyamide hollow fiber membranes were washed with an organic solvent, the specific metal elements were able to be removed to a certain extent, and more hygienic polyamide hollow fiber membranes were obtained. On the other hand, the polyamide hollow fiber membranes of Comparative Examples 1 to 8 had a high content of specific metal elements, and therefore their filtration performance changed significantly when stored in the storage solution for a long period of time. Furthermore, in Comparative Examples 1 to 8, when the polyamide hollow fiber membranes were washed with an organic solvent, the specific metal elements could hardly be removed.

[0125] The polyamide hollow fiber membranes of Examples 3 to 11 had a low content of specific metal elements, and the proportion of γ crystals to the total amount of α crystals and γ crystals was within the range of 15 to 25%, so their external pressure water permeability before storage was superior to that of the other Examples.

[0126] The polyamide hollow fiber membranes of Comparative Examples 1 to 3 used a medium containing a 5% by mass aqueous solution of sulfones with a pH outside the range of 5.2 to 6.8, and therefore contained a large amount of specific metal elements, and the filtration performance changed significantly when stored in the storage solution for a long period of time.

[0127] The polyamide hollow fiber membranes of Comparative Examples 4 to 6 used media other than sulfones, and therefore contained a large amount of specific metal elements, and their filtration performance changed significantly when stored in the storage solution for a long period of time.

[0128] The polyamide hollow fiber membranes of Comparative Examples 7 and 8 contained a large amount of specific metal elements because the membrane-forming solution was prepared batchwise using a tank, and the filtration performance changed significantly when stored in the storage solution for a long period of time.

[0129] In Reference Examples 1 and 2, sodium or potassium components were intentionally added to the raw materials to prepare polyamide hollow fiber membranes containing a large amount of sodium or potassium, and the membranes were evaluated. As a result, it was confirmed that the prepared polyamide hollow fiber membranes were resistant to changes in filtration performance even when stored in a storage solution for a long period of time, and had excellent storage stability. From these results, it was found that not all metal elements affect the storage stability of polyamide hollow fiber membranes, but the content of specific metal elements affects the storage stability of polyamide hollow fiber membranes. [Explanation of symbols]

[0130] 1: Liquid transfer pump 2: Inlet pressure gauge 3: Hollow fiber membrane 4: Outlet pressure gauge 5: Outlet valve 6: Saucer 7: Air inlet 8: Regulator 9: Booster tank 10: Speed ​​controller 11: Pressure sensor 12: Bubble point measurement sample 13: Glass container 14:2-Propanol 15: Digital pressure indicator 16: Two-way valve

Claims

1. A polyamide hollow fiber membrane formed from a polyamide resin, A polyamide hollow fiber membrane that satisfies all of the following characteristics (1) to (5): (1) Fe content is 3.00 ppm or less (2) Cr content is less than 1.00 ppm (3) Cu content less than 0.20 ppm (4) Mg content is less than 0.60 ppm (5) Zn content less than 0.30 ppm

2. A polyamide hollow fiber membrane as described in claim 1, having a Cr content of 0.70 ppm or less.

3. The polyamide hollow fiber membrane according to claim 1, having a relative viscosity of 2.0 to 6.

5.

4. 2. The polyamide hollow fiber membrane according to claim 1, wherein, in structural analysis by X-ray diffraction, the proportion of γ crystals to the total amount of α crystals and γ crystals is 0 to 37%.

5. The polyamide hollow fiber membrane according to claim 1, which has a dense layer on the lumen side surface and / or the outer surface of the polyamide hollow fiber membrane.

6. 2. The polyamide hollow fiber membrane according to claim 1, wherein the rejection rate of particles having a particle diameter of 50 nm is 90% or more.

7. 2. The polyamide hollow fiber membrane according to claim 1, wherein the retention of the rejection rate of particles having a particle diameter of 5 nm calculated by the following formula is 90% or more. Retention rate (%) = (rejection rate of particles with a particle size of 5 nm after storage / rejection rate of particles with a particle size of 5 nm before storage) x 100 Storage conditions: The polyamide hollow fiber membrane is immersed in propylene glycol monomethyl ether acetate and stored at 23° C. for 2 years without applying pressure.

8. External pressure permeability: 50 to 2000 L / (m 2 2. The polyamide hollow fiber membrane according to claim 1, wherein the viscosity is 1 / 2 atm.

9. The polyamide hollow fiber membrane according to claim 1, wherein the retention of external pressure water permeability calculated by the following formula is 90 to 110%. Retention rate (%) = (external pressure water permeability after storage / external pressure water permeability before storage) x 100 Storage conditions: The polyamide hollow fiber membrane is immersed in propylene glycol monomethyl ether acetate and stored at 23° C. for 2 years without applying pressure.

10. 2. The polyamide hollow fiber membrane according to claim 1, which has an initial bubble point of 0.20 MPa or more and a burst bubble point of 0.30 MPa or more in a bubble point test in which air pressure is applied in 2-propanol at 20°C.

11. In a bubble point test in which air pressure was applied in 2-propanol at 20°C, The polyamide hollow fiber membrane according to claim 1, wherein the retention rates of the initial bubble point and the burst bubble point calculated by the following formula are 90 to 110%. Retention rate (%) = (initial bubble point or burst bubble point after storage / initial bubble point or burst bubble point before storage) x 100 Storage conditions: The polyamide hollow fiber membrane is immersed in propylene glycol monomethyl ether acetate and stored at 23° C. for 2 years without applying pressure.

12. A hollow fiber membrane module comprising a module case containing the polyamide hollow fiber membrane according to any one of claims 1 to 11.

13. a first step of mixing at least a polyamide resin and a sulfone using a multi-screw extruder to prepare a membrane-forming solution; a second step of forming a hollow fiber membrane by using a double-tube nozzle for producing hollow fibers having a double-tube structure, discharging the membrane-forming solution from an outer annular nozzle and discharging an internal solution from an inner nozzle, and immersing the resulting mixture in a coagulation bath containing water and / or a polyhydric alcohol; and a third step of removing the organic solvent from the hollow fiber membrane formed in the second step; The method for producing a polyamide hollow fiber membrane according to any one of claims 1 to 11, wherein the sulfone has a pH of 5.2 to 6.8 at 25°C when dissolved in water to form a 5% by mass aqueous solution.

14. The method for producing a polyamide hollow fiber membrane according to claim 13, wherein the sulfone is dimethyl sulfone and / or sulfolane.

15. 14. The method for producing a polyamide hollow fiber membrane according to claim 13, further comprising a fourth step, after the third step, of washing the hollow fiber membrane with an organic solvent to remove at least one metal element selected from the group consisting of Fe, Cr, Cu, Mg, and Zn.

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