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

Abstract

The purpose of the present invention is 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 preservation solution for a long period of time. The polyamide hollow fiber membrane according to the present invention is formed from a polyamide resin, and satisfies at least one of the following characteristics (1)-(5). (1) The Fe content is 4.30 ppm or less. (2) The Cr content is less than1.00 ppm. (3) The Cu content is less than 0.20 ppm. (4) The Mg content is less than 0.60 ppm. (5) The Zn content is less than 0.30 ppm.

Description

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

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

[0002] Polyamide hollow fiber membranes are commonly used as hollow fiber membrane modules housed in module cases. These hollow fiber membrane modules offer excellent liquid filtration capabilities 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, in Patent Document 1, in a bubble point test in a liquid formed from polyamide resin with a surface tension of 12 mN / m at 20°C and air pressure applied, the initial bubble point was 0.40 MPa or higher, the burst bubble point was 0.55 MPa or higher, and the internal pressure permeability using pure water at 25°C was 50 L / (m³). 2 Polyamide hollow fiber membranes with a thermal conductivity of 0.5°H or higher have been proposed.

[0004] Japanese Patent Publication No. 2016-68005

[0005] The 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 inventors have newly discovered that storing hollow fiber membrane modules in a storage solution for a long period of time results in a significant change in the filtration performance of the polyamide hollow fiber membrane.

[0006] The present invention aims to solve the above-mentioned conventional problems and to provide a polyamide hollow fiber membrane, a hollow fiber membrane module, and a method for manufacturing a polyamide hollow fiber membrane, which do not change in filtration performance even when stored for a long period of time in a preservation solution.

[0007] The inventors of this invention conducted thorough research to solve the above problems and found that the change in filtration performance when polyamide hollow fiber membranes are stored in a preservation solution for a long period of time is due to specific metal elements contained in the polyamide hollow fiber membranes. The present invention was completed by further research based on this finding.

[0008] In other words, the present invention provides the invention in the following embodiments: <1> A polyamide hollow fiber membrane formed of a polyamide resin, which satisfies at least one of the following features (1) to (5): (1) Fe content is 4.30 ppm or less (2) Cr content is less than 1.00 ppm (3) Cu content is less than 0.20 ppm (4) Mg content is less than 0.60 ppm (5) Zn content is less than 0.30 ppm <2> The polyamide hollow fiber membrane according to <1>, wherein the relative viscosity is 2.0 to 6.5 <3> The polyamide hollow fiber membrane according to <1> or <2>, wherein, in structural analysis by X-ray diffraction, the ratio of γ crystals to the total amount of α crystals is 0 to 37%. <4> A polyamide hollow fiber membrane according to any one of <1> to <3>, wherein the polyamide hollow fiber membrane has a dense layer on the inner surface and / or outer surface of the polyamide hollow fiber membrane. <5> A polyamide hollow fiber membrane according to any one of <1> to <4>, wherein the rejection rate of particles with a particle size of 50 nm is 90% or more. <6> A polyamide hollow fiber membrane according to any one of <1> to <5>, wherein the retention rate of the rejection rate of particles with a particle size 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) × 100 Storage conditions: Immerse the polyamide hollow fiber membrane in propylene glycol monomethyl ether acetate and store at 23°C for 2 years without pressurization. <7> External pressure permeability is 50 to 2000 L / (m 2A polyamide hollow fiber membrane according to any of <1> to <6>, wherein the pressure is atm·h. <8> A polyamide hollow fiber membrane according to any of <1> to <7>, wherein the retention rate of the external pressure permeability calculated by the following formula is 90 to 110%. Retention rate (%) = (External pressure permeability after storage / External pressure permeability 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 pressurization. <9> A polyamide hollow fiber membrane according to any of <1> to <8>, wherein in a bubble point test in 2-propanol with air pressure applied at 20°C, the initial bubble point is 0.20 MPa or higher and the burst bubble point is 0.30 MPa or higher. <10> A polyamide hollow fiber membrane according to any one of <1> to <9>, wherein the retention rate of the initial bubble point and burst bubble point calculated by the following formula in a bubble point test performed in 2-propanol with air pressure at 20°C is 90 to 110%. Retention rate (%) = (Initial bubble point or burst bubble point after storage / Initial bubble point or burst bubble point 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 pressurization. <11> A hollow fiber membrane module in which the polyamide hollow fiber membrane according to any one of <1> to <10> is housed in a module case. <12> A method for producing a polyamide hollow fiber film, comprising: a first step of preparing a film-forming stock solution by mixing at least a polyamide resin and sulfones using a multi-screw extruder; a second step of forming a hollow fiber film by discharging the film-forming stock solution from the outer annular nozzle and the inner liquid from the inner nozzle using a double-tube nozzle for hollow fiber production, and immersing the film in a solidification bath containing water and / or polyhydric alcohol; and a third step of removing an organic solvent from the hollow fiber film formed in the second step, wherein the sulfones have a pH of 5.2 to 6.8 at 25°C when dissolved in water to make a 5% by mass aqueous solution. <13> The method for producing a polyamide hollow fiber film according to <12>, wherein the sulfones are dimethyl sulfone and / or sulfolane.<14> A method for producing a polyamide hollow fiber membrane according to <12> or <13>, further comprising 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.

[0009] Because the polyamide hollow fiber membrane of the present invention has a reduced content of specific metal elements, its filtration performance does not change easily even when stored for a long period of time in a storage solution, and it has excellent storage stability.

[0010] This is a schematic diagram of an apparatus for measuring the external pressure permeability of a polyamide hollow fiber membrane. This is a schematic diagram of an apparatus for measuring the bubble point of a polyamide hollow fiber membrane.

[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 is less than 0.20 ppm (4) Mg content is less than 0.60 ppm (5) Zn content is less than 0.30 ppm

[0012] <Polyamide Resin> The type of polyamide resin used to form the polyamide hollow fiber membrane of the present invention is not particularly limited, but examples include polyamide homopolymers, polyamide copolymers, or mixtures thereof. Specifically, 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, polyamide 10T, etc. Specifically, examples of polyamide copolymers include copolymers of polyamide and polyethers such as polytetramethylene glycol or polyethylene glycol. Furthermore, the ratio of the polyamide component in the polyamide copolymer is not particularly limited, but for example, the proportion of the polyamide component is 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 satisfying the above range for the ratio of the polyamide component in the polyamide copolymer, excellent durability can be provided. Polyamide resins may be used individually or in combination of two or more types.

[0013] The polyamide resin used in this invention may or may not be crosslinked, as long as it can be molded into a fibrous 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] Certain metal elements are substances that cause significant changes in filtration performance, such as the rejection rate and water permeability under external pressure, when polyamide hollow fiber membranes are stored in a preservation solution for extended periods. The reason why the filtration performance of polyamide hollow fiber membranes changes significantly due to certain metal elements when stored in a preservation solution for extended periods is not clear, but it is presumed that the polyamide resin constituting the polyamide hollow fiber membrane deteriorates due to the cutting or other processes caused by the specific metal elements, or that chemical reactions occur between the specific metal elements and other impurities in the polyamide hollow fiber membrane, resulting in changes to the porous structure of the polyamide hollow fiber membrane.

[0016] In order to obtain a polyamide hollow fiber membrane whose filtration performance does not change easily even when stored for a long period of time in a storage solution, the Fe content among the specific metal elements 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, even more preferably 2.00 ppm or less, even more preferably 1.50 ppm or less, and particularly preferably 1.20 ppm or less. Examples of the Fe content include 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] In order to obtain a polyamide hollow fiber membrane whose filtration performance does not change easily even when stored for a long period of time in a storage solution, the Cr content among the specific metal elements 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, even more preferably 0.40 ppm or less, and particularly preferably 0.30 ppm or less. 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, or 0.10 to 0.30 ppm.

[0018] In order to obtain a polyamide hollow fiber membrane whose filtration performance does not change easily even when stored for a long period of time in a storage solution, the Cu content among the specific metal elements 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, even more preferably 0.13 ppm or less, and particularly preferably 0.10 ppm or less. Examples of 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, or 0.05 to 0.10 ppm.

[0019] In order to obtain a polyamide hollow fiber membrane whose filtration performance does not change easily even when stored for a long period of time in a storage solution, the content of Mg among the specific metal elements 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, even more preferably 0.20 ppm or less, and particularly preferably 0.10 ppm or less. Examples of the Mg content include 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] In order to obtain a polyamide hollow fiber membrane whose filtration performance does not change easily even when stored for a long period of time in a storage solution, the Zn content among the specific metal elements 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, even more preferably 0.13 ppm or less, and particularly preferably 0.10 ppm or less. Examples of the Zn content include 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] From the viewpoint of more effectively suppressing changes in filtration performance even when the polyamide hollow fiber membrane of the present invention is stored in a storage solution for a long period of time, preferably the content of any two of the specified metal elements is within the numerical range, more preferably the content of any three of the specified metal elements is within the numerical range, even more preferably the content of any four of the specified metal elements is within the numerical range, and particularly preferably the content of all (five) of the specified metal elements is within the numerical range.

[0022] Examples of embodiments of the polyamide hollow fiber membrane of the present invention include 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 embodiments satisfying features (4) and (5).

[0023] Furthermore, other embodiments of the polyamide hollow fiber membrane of the present invention include, specifically, 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 embodiments satisfying features (3), (4), and (5).

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

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

[0027] The relative viscosity of the polyamide hollow fiber membrane of the present invention is, for example, 2.0 to 6.5. From the viewpoint of making the external pressure water permeability before long-term storage good 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, still 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 the polyamide hollow fiber membrane is measured at 25°C using an Ubbelohde viscometer with a sample prepared by dissolving the polyamide hollow fiber membrane in 96% by mass sulfuric acid to a concentration of 1 g / dL.

[0029] In the polyamide hollow fiber membrane of the present invention, in the crystal structure analysis by X-ray diffraction method, the ratio of γ-crystal to the total amount of α-crystal and γ-crystal is, for example, 0 to 37%. From the viewpoint of making the external pressure water permeability before long-term storage good in a polyamide hollow fiber membrane with a reduced content of a specific metal element, it is preferably 5 to 30%, more preferably 10 to 26%, and still more preferably 15 to 25%. The ratio of γ-crystal to the total amount of α-crystal and γ-crystal in the polyamide hollow fiber membrane can be adjusted to the 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 ratio of γ-crystal can be adjusted to the above preferred range by adjusting the relative viscosity of the polyamide hollow fiber membrane to the above-mentioned preferred range.

[0030] In this invention, the ratio of γ crystals to the total amount of α crystals and γ crystals is a value obtained 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: A polyamide hollow fiber film is cut perpendicular to its length and fixed to a pole sample plate with double-sided tape. The pole sample plate is positioned parallel to the optical axis when the length of the sample is 2θ = 0°. • Measurement method: The WAXD reflection method 2θ / θ method is employed. - Measurement conditions: X-ray Cu-Kα line (1.54 Å), 50 kV 300 mA, thin film, standard multi-purpose sample stage / pole sample plate, parallel beam method, receiving side solar slit = long slit used, slit: 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 set to 0.02°. - Analysis method: Multiple peak separation method (profile fitting using pseudo-Voigt function) is performed in the 2θ = 5° to 36° region. The specific analysis conditions are as follows. 1) The background is defined as the area under the straight 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 as to be tangent to the peak shapes at 2θ = 15° to 17° and 2θ = 27° to 29°. In this case, the center of the halo pattern should be at 2θ = 19° to 21°, and the full width at half maximum should be around 10. 3) The diffraction lines due to the crystalline component are assigned a Symmetric pseudo-Voigt function to match the peak top and waveform. Profile fitting is performed after fixing the parameters of the halo pattern. The 2θ position of the peak, full width at half maximum, height, and the contribution ratio of the Gaussian function and the Lorentz function are set during fitting. The fitting will use Jade + 9.8.4) By the multi-peak separation method, obtain the peak area of the crystal part, the peak area of α-crystals, and the peak area of γ-crystals, and calculate the ratio of γ-crystals to the total amount of α-crystals and γ-crystals from the following formula 1. <Formula 1> Ratio of γ-crystals to the total amount of α-crystals and γ-crystals (%) = {peak area of γ-crystals / (peak area of γ-crystals + peak area of α-crystals)} × 100.

[0031] The polyamide hollow fiber membrane of the present invention may have a dense layer on the inner cavity side surface and / or the outer surface in order to improve the blocking rate of fine particles. In the present invention, the "dense layer" means that when observing the cross-section of the polyamide hollow fiber membrane, the porous structure of a specific region adjacent to the inner cavity side surface or the outer surface of the polyamide hollow fiber membrane is denser than the porous structure of other regions (for example, the region near the middle of the inner cavity side surface and the outer surface), and fine pores are aggregated. It is the region where the fine pores are aggregated and dense, and it is the part that determines the fractionation characteristics of the polyamide hollow fiber membrane, including the case where the presence of pores is not substantially recognized (when the presence of pores is not recognized 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. When the polyamide hollow fiber membrane has a dense layer, the blocking property of fine particles becomes good.

[0032] As one of the membrane separation performance or filtration performance of the polyamide hollow fiber membrane of the present invention, the blocking rate of particles with a particle size of 50 nm is preferably 90% or more, more preferably 95% or more, still more preferably 98% or more, and particularly preferably 99% or more. The blocking rate of fine particles varies depending on the application and purpose of use when made into a module. As a suitable example of filtration performance, the blocking rate of particles with a particle size of 20 nm, the blocking rate of particles with a particle size of 10 nm, and the blocking rate of particles with a particle size of 5 nm are also preferably 90% or more, more preferably 95% or more, still more preferably 98% or more, and particularly preferably 99% or more, respectively. Thus, the polyamide hollow fiber membrane of the present invention has a pore structure capable of separating fine particles with a high blocking rate and is excellent in the removal performance of fine particles.

[0033] In this invention, the rejection rate of particles of each particle size is calculated from the proportion of gold colloid particles removed when a filtration test is performed using gold colloid particles having a predetermined average particle size. Because gold colloid particles have a very narrow particle size distribution, a filtration test using gold colloid can accurately reflect the particle rejection rate of the hollow fiber membrane. Specifically, the 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 the conditions of a filtration pressure of 0.3 MPa and a filtration temperature of 25°C, and filtration the filtrate with a cumulative filtration volume of 0.005 m³. 3 / m 2 The solution is divided into portions, and the absorbance of the second portion at a wavelength of 524 nm is measured. The rejection rate of each particle size is then calculated using the following formula 2. <Formula 2> Particle rejection rate (%) = {(Absorbance of unfiltered stock solution - Absorbance of filtrate) / Absorbance of unfiltered stock solution} × 100

[0034] The polyamide hollow fiber membrane of the present invention is characterized by its excellent storage stability, as its filtration performance does not change easily even when stored for a long period of time in a storage solution. Polyamide hollow fiber membranes are generally used as hollow fiber membrane modules housed in module cases, and the common method of storage after using hollow fiber membrane modules for various purposes is to immerse them in a storage solution. In the present invention, "use" of the polyamide hollow fiber membrane or hollow fiber membrane module means filtering by passing the filtration stock (stock before filtration) from one side of the inner surface or outer surface of the polyamide hollow fiber membrane to the other, and the type and amount of the filtration stock, the type and amount of the components to be classified, and the filtration performance are not limited. Also, in the present invention, the type of "storage solution" is not limited, and examples include water and / or organic solvents. The storage solution may contain additives such as preservatives as necessary to improve its shelf life. Also, in the present invention, "storage" means keeping it immersed in the storage solution until the next use, and it is preferable to store it standing without passing the solution through it. 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. 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 retention rate of the rejection rate of particles with a particle size of 5 nm calculated by the following formula 3 of the polyamide hollow fiber membrane of the present invention is preferably 90% or more, more preferably 95% or more, still more preferably 97% or more, even more preferably 98% or more, and particularly preferably 99% or more. <Formula 3> Retention rate of rejection 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) × 100 Storage conditions: Immerse the polyamide hollow fiber membrane in propylene glycol monomethyl ether acetate and store it at 23 ° C for 2 years without applying pressure.

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

[0037] In a polyamide hollow fiber membrane having the performance of blocking 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 800 L / (m 2 ·atm·h), still more preferably 150 to 600 L / (m 2 ·atm·h), 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 having the ability to transmit more than 10% of particles with a particle size of 5 nm but block more than 90% of particles with a particle size of 10 nm, the external pressure permeability is preferably 100 to 1500 L / (m²). 2 (atm·h), more preferably 150 to 1000 L / (m 2 (atm·h), more preferably 200 to 800 L / (m 2 (atm·h), more preferably 250 to 600 L / (m 2 (atm·h), particularly preferably 300 to 500 L / (m 2 It is ・atm・h).

[0039] In a polyamide hollow fiber membrane having the ability to transmit more than 10% of particles with a particle size of 10 nm but block more than 90% of particles with a particle size of 20 nm, the external pressure permeability is preferably 200 to 2000 L / (m 2 (atm·h), more preferably 250 to 1750 L / (m 2 (atm·h), more preferably 300 to 1500 L / (m 2 (atm·h), more preferably 500 to 1250 L / (m 2 (atm·h), particularly preferably 600 to 1000 L / (m 2 It is ・atm・h).

[0040] In a polyamide hollow fiber membrane having the ability to transmit more than 10% of particles with a particle size of 20 nm but block more than 90% of particles with a particle size of 50 nm, the external pressure permeability is preferably 500 to 2000 L / (m²). 2 (atm·h), more preferably 600 to 1750 L / (m 2 (atm·h), more preferably 800 to 1500 L / (m 2 (atm·h), more preferably 1000 to 1500 L / (m 2 (atm·h), particularly preferably 1250 to 1500 L / (m 2 It is ・atm・h).

[0041] Thus, because the polyamide hollow fiber membrane of the present invention has high external pressure permeability, the flow rate of the treatment liquid can be set high, and the filtration efficiency can be improved.

[0042] In the present invention, the external pressure 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 a retention rate of the external pressure water permeability calculated by the following formula 4, preferably 90 to 110%, more preferably 95 to 105%, even more preferably 97 to 103%, even more preferably 98 to 102%, and particularly preferably 99 to 101%. <Formula 4> Retention rate of external pressure water permeability (%) = (External pressure water permeability after storage / External pressure water permeability 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 pressurization.

[0044] The polyamide hollow fiber membrane of the present invention preferably exhibits the following filtration performance characteristics: in a bubble point test conducted by applying air pressure in 2-propanol with a surface tension of 21 mN / m at 20°C, the initial bubble point is 0.20 MPa or higher, and the burst bubble point is 0.30 MPa or higher. The initial bubble point and burst bubble point vary depending on the application and purpose of use when the membrane is assembled into a module. However, as a preferred example of filtration performance, it is more preferable that the initial bubble point is 0.30 MPa or higher and the burst bubble point is 0.40 MPa or higher; even more preferable that the initial bubble point is 0.35 MPa or higher and the burst bubble point is 0.45 MPa or higher; even more preferable that the initial bubble point is 0.40 MPa or higher and the burst bubble point is 0.55 MPa or higher; and most preferably that the initial bubble point is 0.45 MPa or higher and the burst bubble point is 0.65 MPa or higher. Furthermore, 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, even 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, even 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 this invention, the bubble point test is a commonly used measurement method for determining the maximum pore diameter. Because the measurement is simple and quick, it is widely used to estimate the pore diameter. 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 from the membrane surface and bubbles begin to form when air pressure is applied to the hollow fiber membrane, and the burst bubble point is the pressure at which bubbles begin to form from approximately the entire membrane. Specifically, the initial bubble point and the burst bubble point are values ​​measured by the method described in the examples below.

[0046] The polyamide hollow fiber membrane of the present invention has a retention rate 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 in the bubble point test, preferably 90 to 110%, more preferably 95 to 105%, even more preferably 97 to 103%, even 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 pressurization.

[0047] The inner and outer diameters of the polyamide hollow fiber membrane of the present invention are not particularly limited and can be set appropriately according to 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 a polyamide hollow fiber membrane can be measured by observing a cross-section of the polyamide hollow fiber membrane under an optical microscope at 200x 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 tensile speed of 50 mm / min, and a number of measurements of 5.

[0050] The tensile 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 elongation at break of the polyamide hollow fiber membrane is preferably 10 to 500%, more preferably 20 to 350%, even more preferably 40 to 320%, and even 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 mainly of polyamide resin, but may contain other resin components, softeners, curing agents, crosslinking agents, antioxidants, stabilizers, dispersants, lubricants, flame retardants, anti-aging agents, antistatic agents, and other additives as needed, as long as they do not impair the effects of the present invention. These may be used individually or in combination of two or more. Preferably, the total content of these components is 10% by mass or less of the total polyamide hollow fiber membrane.

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

[0055] 2. Method for Manufacturing Polyamide Hollow Fiber Membrane The polyamide hollow fiber membrane of the present invention can be manufactured by employing specific manufacturing conditions using the thermally induced phase separation method (TIPS method).

[0056] Specifically, the method for producing a polyamide hollow fiber membrane according to the present invention is carried out through the following three steps. Step 1: A film-forming stock solution is prepared by mixing at least a polyamide resin and sulfones using a multi-screw extruder. The sulfones have a pH of 5.2 to 6.8 at 25°C when dissolved in water to make a 5% by mass aqueous solution. Step 2: Using a double-tube nozzle for hollow fiber production, the film-forming stock solution is discharged from the outer annular nozzle and the internal liquid is discharged from the inner nozzle, and the film is immersed in a coagulation bath containing water and / or polyhydric alcohol to form a hollow fiber membrane. Step 3: The organic solvent is removed from the hollow fiber membrane formed in Step 2.

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

[0058] <Step 1> In Step 1, a film-forming stock solution is prepared by mixing at least a polyamide resin and sulfones using a multi-screw extruder.

[0059] A common method for preparing a film-forming stock solution involves adding raw materials such as polyamide resin and organic solvents to a tank equipped with a heater and stirrer, and preparing the stock solution in a batch process. However, conventional batch-process methods for preparing film-forming stock solutions cannot reduce the content of specific metal elements in the polyamide hollow fiber membrane to the desired range.

[0060] Therefore, in the present invention, in order to reduce the content of specific metal elements in the polyamide hollow fiber membrane to a desired range, a method is employed in which at least polyamide resin and sulfones are quantitatively fed into a multi-screw extruder to prepare a film-forming stock solution in a continuous manner. Furthermore, in the present invention, in order to reduce the content of specific metal elements in the polyamide hollow fiber membrane to a desired range, the sulfones used are those whose pH at 25°C is 5.2 to 6.8 when dissolved in water to make a 5% by mass aqueous solution.

[0061] The reason why the content of specific metal elements in polyamide hollow fiber membranes can be reduced to a desired range by employing a method of continuously preparing a film-forming stock solution using a multi-screw extruder and sulfones whose pH at 25°C is 5.2 to 6.8 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 has a higher solubility of the polyamide resin in the sulfones than the batch method using tanks, so the time required for dissolution can be shortened, and as a result the residence time (contact time with metal) of the film-forming stock solution in the manufacturing equipment can be shortened, and it is presumed that this can suppress the contamination of the film-forming stock solution with specific metal elements originating from the manufacturing equipment. The second reason is that the sulfones have higher compatibility with polyamide resins compared to conventional organic solvents used when preparing the film-forming stock solution. This allows for a further reduction in the time required for dissolution, thereby further shortening the residence time (contact time with metal) of the film-forming stock solution in the manufacturing equipment. Consequently, it is presumed that this will further suppress the contamination of the film-forming stock solution with specific metal elements originating from the manufacturing equipment.

[0062] A multi-screw extruder is not particularly limited as long as it has multiple screws, but a twin-screw extruder is preferred from the viewpoint of versatility. The outer diameter and L / D ratio of the screws are not particularly limited and can be appropriately designed according to the desired production volume, but the outer diameter is preferably φ30 or more, and the L / D ratio is preferably 25 or more. The screw configuration is also not particularly limited, but the arrangement of the full flight and kneading discs should be appropriately designed to ensure stable production of the film-forming stock. The rotation speed of the screws is also not particularly limited and can be appropriately designed, but is preferably 30 rpm or more. The set temperature of the multi-screw extruder can be appropriately designed, but is preferably 150 to 300°C. Furthermore, it is preferable that the multi-screw extruder is equipped with a device for quantitatively feeding 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, ethylmethyl sulfone, and methylphenyl sulfone. These may be used individually 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 at 25°C of a 5% by mass aqueous solution (5% by mass of dimethyl sulfone, 95% by mass of water) is preferably 5.2 to 6.5. When sulfolane is used as the sulfone, the pH at 25°C of a 5% by mass aqueous solution (5% by mass of sulfolane, 95% by mass of water) is preferably 6.3 to 6.8. When dimethyl sulfone and sulfolane are used in combination as the sulfone, it is preferable that the pH of the 5% by mass aqueous solution of either or both is within the above preferred range. The pH of the 5% by mass aqueous solution of sulfones can be adjusted to the desired pH, for example, by adjusting the amount of oxidizing agent or reducing agent used when synthesizing the sulfones.

[0065] In the film-forming stock 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 the above mass ratio, it becomes easier to adjust the particle rejection rate, initial bubble point, burst bubble point, and external pressure permeability within the aforementioned ranges.

[0066] <Second Step> In the second step, a double-tube nozzle for hollow fiber manufacturing is used to discharge the film-forming raw material from the outer annular nozzle and the internal liquid from the inner nozzle, and the resulting material is immersed in a coagulation bath containing water and / or polyhydric alcohol to form a hollow fiber membrane.

[0067] Here, as the double-tubular nozzle for hollow fiber production, a nozzle having a double annular structure, similar to those used in melt spinning to produce core-sheath type composite fibers, can be used. The diameters of the outer annular nozzle and the inner nozzle of the double-tubular nozzle for hollow fiber production can be appropriately set according to the inner and outer diameters of the polyamide hollow fiber membrane.

[0068] Furthermore, in the second step, the internal liquid discharged from the inner nozzle of the double-tubular nozzle for hollow fiber production can be either a liquid or a gas, provided it is inert to the polyamide resin. However, liquids are preferred because they allow spinning even under conditions where the viscosity of the film-forming stock solution is low and filament formation is difficult. The liquid used as the internal liquid is not particularly limited, provided it is inert to the polyamide resin. However, a good solvent with high affinity for the polyamide resin can be used when it is desired to create relatively large pores on the inner surface of the polyamide hollow fiber, while a poor solvent can be used when it is desired 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 with an average molecular weight of 300 to 1000, polypropylene glycol with an average molecular weight of 400 to 1000, higher fatty acids, and liquid paraffin. These solvents may be used individually or in combination of two or more. Furthermore, if the film-forming stock solution has high viscosity and excellent stringability, a method of introducing 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 employing such a coagulation bath, a polyamide hollow fiber membrane with the above-mentioned 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 individually or in combination of two or more.

[0071] Furthermore, when using a coagulation bath containing water and polyhydric alcohol, there are no particular restrictions on the composition ratio of these components, but the mass ratio of polyhydric alcohol to water is preferably 25-80:75-20, and more preferably 40-70:60-30.

[0072] An example of an internal liquid and coagulation bath for obtaining a polyamide hollow fiber membrane comprising a dense layer formed on the luminal surface and a porous layer having relatively large pores that support the dense layer is, for example, an internal liquid which may be at least one selected from the group consisting of glycerin, polyethylene glycol with an average molecular weight of 300 to 1000, polypropylene glycol with an average molecular weight of 400 to 1000, and triethylene glycol, and a coagulation bath which may be 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).

[0073] The temperature of the solidification bath is not particularly limited, but is usually -20 to 100°C, preferably -10 to 80°C, and more preferably 0 to 40°C. By changing the temperature of the solidification bath, the crystallization rate can be changed, thereby changing the pore size, water permeability, and strength. Generally, a lower temperature of the solidification bath tends to result in smaller pore size, lower water permeability, and improved strength, while a higher temperature tends to result in larger pore size, higher water permeability, and decreased strength. However, this can also vary depending on the solubility of the solvent in the film-forming solution and the internal liquid, as well as the crystallization rate of the resin itself. To keep the external pressure water permeability and particle rejection rate of the polyamide hollow fiber membrane within the aforementioned ranges, a low temperature of the solidification bath is preferable, but it is not always necessary to have a low temperature depending on the conditions. If the temperature of the solidification bath is within the above range, the strength of the membrane can be increased while also reducing the energy required for temperature control.

[0074] Furthermore, the flow rate when discharging the film-forming stock solution from the annular nozzle on the outside of the double-tubular nozzle for hollow fiber production is not particularly limited, but for example, it can be 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 liquid is set appropriately considering the diameter of the inner nozzle of the double-tubular nozzle for hollow fiber production, the type of internal liquid used, the flow rate of the film-forming stock solution, etc., but for example, it can be 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 film-forming stock solution.

[0075] Thus, by carrying out the second step, the film-forming raw material discharged from the double-tubular nozzle for hollow fiber production solidifies in the coagulation bath to form a polyamide hollow fiber film.

[0076] <Third Step> In the third step, organic solvents are removed from the hollow fiber membrane formed in the second step. The method for removing organic solvents from the hollow fiber membrane is not particularly limited, and examples include immersing it in a washing solution consisting of water or an aqueous solution, winding the hollow fiber membrane onto a bobbin, fence, or winding machine and exposing the wound hollow fiber membrane to running water consisting of the washing solution. By such methods, organic solvents such as sulfones, components of the coagulation bath, and components of the internal liquid contained in the hollow fiber membrane can be removed. As the washing solution, it is preferable to use one that is inexpensive, has a low boiling point, and can be easily separated after washing by the difference in boiling point, and water is preferred. If the washing effect is insufficient with water alone, an aqueous solution in which a component that promotes washing effect is dissolved in water may be used. The component that promotes washing effect is not particularly limited, but examples include solvents such as methanol, ethanol, isopropanol, acetone, diethyl ether, and petroleum ether, and surfactants. The washing time is not particularly limited, but for example, it is 0.2 hours to 2 months, preferably 0.5 hours to 1 month, and more preferably 2 hours to 10 days. To effectively remove organic solvents remaining on the polyamide hollow fiber membrane, the composition of the washing solution may be changed, the washing solution may be stirred, or the flow rate of the washing solution may be changed.

[0077] Thus, by carrying out the third step, the polyamide hollow fiber membrane of the present invention is manufactured.

[0078] <Fourth Step> Although the polyamide hollow fiber membrane obtained through steps 1 to 3 has a reduced content of specific metal elements, a fourth step may be added after step 3 in which the polyamide hollow fiber membrane is washed with an organic solvent in order to further reduce the content of specific metal elements.

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

[0080] The immersion time is preferably one day or longer, more preferably three days or longer, even more preferably one week or longer, and particularly preferably one month or longer. 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, and 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 cleaning by filling the hollow fiber membrane module with a cleaning solvent, the ultrasonic output is preferably 50 kW or more, more preferably 80 kW or more, and even more preferably 100 kW or more per inch of module. 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, even 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 these immersion processes, to improve cleaning efficiency, new cleaning solvent may be added during immersion, or some or all of the cleaning solvent may be replaced with new cleaning solvent multiple times.

[0083] Furthermore, when passing a cleaning solvent from one of the inner or outer surfaces of a hollow fiber membrane to the other during cleaning, it is preferable to deliver the cleaning solvent from one of the liquid passage ports in the inner or outer storage space of the hollow fiber membrane module, pass it through the inner wall of the hollow fiber membrane, and discharge the cleaning solvent from the other liquid passage port. It is preferable not to reuse the cleaning solvent that has passed through the hollow fiber membrane once for cleaning. The direction of liquid passage may be changed midway to improve cleaning performance.

[0084] When a cleaning solvent is passed from one side of the hollow fiber membrane's inner or outer surface to the other, the amount of solvent passed through 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, for example, when filling a hollow fiber membrane module with the cleaning solvent for cleaning. The temperature of the cleaning solvent is preferably room temperature (23°C) or higher, and 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 liquid is not particularly limited, but for example, the volume of cleaning solvent per hour can be in the range of 0.1 to 1000 times the internal volume of the module, and preferably 1 to 100 times. Furthermore, 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 any known organic solvent can be used. Examples of organic solvents include alkylene glycol monoalkyl ether carboxylate, alkylene glycol monoalkyl ether, alkyl lactate, alkyl alkoxypropionate, cyclic lactone (preferably having 4 to 10 carbon atoms), monoketone compounds which may contain a ring (preferably having 4 to 10 carbon atoms), alkylene carbonate, alkyl alkoxyacetate, alkyl pyruvate, dialkyl sulfoxide, cyclic sulfone, dialkyl ether, monohydric alcohol, glycol, alkyl acetate, and N-alkylpyrrolidone. These may be used individually or in combination of two or more.

[0087] The organic solvents used as washing solvents include 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, and diethylene glycol. Preferably, it is one or more 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 acetoethyl acetate, dimethyl malonate, methyl pyruvate, and dimethyl oxalate.

[0088] Since cleaning solvents composed of these organic solvents exhibit better cleaning performance when they contain less water, it is preferable not to add water other than water unintentionally contained due to moisture absorption of the organic solvent. The water content in the cleaning solvent is preferably 1% by mass or less per 100% by mass of the cleaning solvent.

[0089] Thus, by carrying out the fourth step, a polyamide hollow fiber membrane with a further reduced content of specific metal elements can be produced. However, since specific metal elements in polyamide hollow fiber membranes cannot be removed even by thorough washing with water or organic solvents, washing treatment with water or organic solvents alone is not sufficient to sufficiently reduce the content of specific metal elements. Therefore, in order to obtain the polyamide hollow fiber membrane of the present invention in which the content of specific metal elements is reduced to or below a specific value, it is necessary to manufacture it by a method including the above steps 1 to 3.

[0090] 3. Hollow Fiber Membrane Module The polyamide hollow fiber membrane 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, after housing the polyamide hollow fiber membrane bundle in a cylindrical module case, the end inside the module case is sealed together with the bundle of hollow fiber membranes using a potting material. At this time, either double-ended potting or single-ended potting may be used. Next, the potted portion where the hollow fiber membrane bundle is sufficiently sealed is cut, and the space on the lumen side of the hollow fiber membrane is opened. It is necessary that the space on the lumen side of the hollow fiber membrane inside the module and the space on the outside of the hollow fiber membrane are reliably separated and there is no leakage. 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 membrane is opened, so that liquid can pass through the space on the lumen side. Furthermore, it is preferable to provide liquid passage ports in the module case to allow liquid to pass through the space outside the hollow fiber membrane within the module case. The shape of the liquid passage ports should be appropriate for each application.

[0091] The potting material can be any known potting material, specifically polyurethane resin, epoxy resin, or polyolefin resin. Of these, epoxy resin and polyolefin resin are preferred from the viewpoint of improving resistance to organic solvents, 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 and are characterized by their ability to maintain their filtration performance even after long-term storage in a storage solution, making them suitable for various applications. Specifically, they can be used for filtering raw materials and intermediates used in pharmaceutical manufacturing, for final filtration, for filtering chemical solutions used for cleaning pharmaceutical manufacturing equipment, and for filtering chemical solutions used in semiconductor manufacturing. More specifically, they can be used in semiconductor device manufacturing processes, including lithography, etching, ion implantation, and stripping processes, to filter chemical solutions used after the completion of each process or before moving to the next process. More specifically, they can be used to filter chemical solutions such as developers, rinses, wafer cleaning solutions, line cleaning solutions (e.g., pipe cleaning solutions), pre-wetting solutions, wafer rinses, resists, underlayer forming solutions, upper layer forming solutions, hard coat forming solutions, aqueous developers, aqueous rinses, stripping solutions, removers, etching solutions, acidic cleaning solutions, phosphoric acid, and phosphoric acid-hydrogen peroxide mixtures. Other applications include filtering chemicals such as developers and rinse solutions for polyimides, sensor resists, and lens resists.

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

[0094] Various characteristics were measured or evaluated using the following methods.

[0095] [Relative viscosity of polyamide hollow fiber membranes] Each polyamide hollow fiber membrane obtained in the examples, comparative examples, and reference examples was dissolved in 96% by mass sulfuric acid to a concentration of 1 g / dL, and the viscosity was measured at 25°C using an Ubellobe viscometer.

[0096] [Metal Element Content of Polyamide Hollow Fiber Membranes] (1) Metal Element Content of Hollow Fiber Membranes 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, the temperature was raised to 100°C in 10 minutes and held at 100°C for 5 minutes, then the temperature was raised to 140°C in 3 minutes and held at 140°C for 5 minutes, and then the temperature was raised to 180°C in 5 minutes and held at 180°C for 10 minutes to decompose and / or dissolve the polyamide hollow fiber membrane in nitric acid, and then the volume was finalized to 50 mL with ultrapure water. Using this as a sample, the metal element content (ppm) was measured using a Thermo Fisher Scientific iCAP6500Duo ICP emission spectrometer. The measured value was obtained by subtracting the average value of the operation blank performed simultaneously with the measurement of the above sample with n=10. Furthermore, if the determined value of the metal element content was within the range of 3σ of the standard deviation obtained in the n=10 operational blank, the content was set to 0.00 ppm. (2) Metal element content of hollow fiber membranes after organic solvent washing (fourth step) Each polyamide hollow fiber membrane obtained in the examples, comparative examples, and reference examples was placed in a 10-inch module case at a occupancy rate of 25% (occupancy rate (%) = total cross-sectional area of ​​the hollow fiber membrane bundle to be placed / cross-sectional area of ​​the inner tube of the module case × 100), and potting was performed to produce a hollow fiber membrane module. The inside of this module was filled with PGMEA (immersing either the inner or outer side of the hollow fiber membrane), and held at 35°C for one week. Next, it was sonicated at an output of 800 kW for one hour. After that, 10,000 kg of PGMEA was delivered to the module from the liquid passage port in the inner space of the hollow fiber membrane of the module, and the liquid was passed through the hollow fiber membrane. After passing all of the PGMEA through the module, the module was disassembled, the hollow fiber membrane that was housed inside was removed, and the PGMEA was dried off. The resulting polyamide hollow fiber membrane was then measured for its metal element content (ppm) using the method (1) described above.

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

[0098] [External Pressure Water Permeability of Polyamide Hollow Fiber Membranes] Each polyamide hollow fiber membrane obtained in the examples, comparative examples, and reference examples was cut to 9-12 cm lengths. A needle with a diameter matching the inner diameter was inserted into the hollow portions at both ends. One needle was sealed with a cap, and the other needle was connected to the outlet. The membrane was then set in the apparatus shown in Figure 1. Subsequently, while pure water at 25°C was passed through the membrane using the liquid transfer pump 1 for a predetermined time (h), the valve of the outlet valve 5 was adjusted to maintain a constant pressure of 0.05 MPa. The volume (L) of water that permeated through the membrane and accumulated in the receiving tray 6 was measured as the permeate volume, and the external pressure water permeability was calculated using the following formula 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 water permeability (L / (m) 2 (atm / h) = Permeation rate (L) / [Outer diameter (m) × 3.14 × Length (m) × {(Inlet pressure (atm) + Outlet pressure (atm)) / 2} × Time (h)]

[0099] [Rejection Rate of Fine Particles in Polyamide Hollow Fiber Membranes] (1) Rejection Rate of 50 nm Particles The rejection rate of particles with a particle size of 50 nm (Gold colloid-50 nm, manufactured by British BioCell International) was measured using the method described above. (2) Rejection Rate of 20 nm Particles The rejection rate of particles with a particle size of 20 nm (Gold colloid-20 nm, manufactured by British BioCell International) was measured using the method described above. (3) Rejection Rate of 10 nm Particles The rejection rate of particles with a particle size of 10 nm (Gold colloid-10 nm, manufactured by British BioCell International) was measured using the method described above. (4) Rejection Rate of 5 nm Particles The rejection rate of particles with a particle size of 5 nm (Gold colloid-5 nm, manufactured by British BioCell International) was measured using the method described above.

[0100] [Bubble Points of Polyamide Hollow Fiber Membranes] Using the apparatus shown in Figure 2, the bubble points of each polyamide hollow fiber membrane obtained in the examples, comparative examples, and reference examples were measured by the following method. Ten polyamide hollow fiber membranes, each 20 cm long, were prepared, bent into a U-shape, and the end of the hollow fiber membrane on the opening side was heat-sealed to close the hollow portion. Next, a 5 cm long flexible nylon tube for air piping (outer diameter 8 mm, inner diameter 6 mm) was prepared, one end was sealed with a silicone stopper, and potting agent (polyurethane resin) was introduced up to about 4 cm. Next, the bundle of hollow fiber membranes 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 potting portion were cut above the heat-sealed part of the hollow fiber membrane, opening the inner lumen side of the hollow fiber membrane. At this time, it was visually confirmed whether the potting agent had entered the hollow part and whether the potting agent was filled between the hollow fiber membranes. If the hollow was maintained without any problems, this was used as the bubble point measurement sample. Next, in the bubble point test, it is necessary to fill the pores of the hollow fiber membrane with liquid. Therefore, 2-propanol (surface tension of 21 mN / m at 20°C) was introduced into the glass container 13, and the bubble point measurement sample 12 was immersed in it, and the pressure was reduced for several seconds to fill the pores with liquid. The bubble point measurement sample 12 immersed in 2-propanol was set as shown in Figure 2, and air was sent to the inner lumen of the hollow fiber membrane at 0.4 MPa / min to increase the pressure. The pressure at which bubbles first emerged from the hollow fiber membrane was confirmed and this was designated as the initial bubble point (IBP). The pressure was continued to increase, and the pressure at which bubbles emerged from approximately the entire membrane was confirmed and this was designated as the burst bubble point (BBP).

[0101] [Breaking Strength, Elongation at Breaking, and Tensile Modulus of Polyamide Hollow Fiber Membranes] The breaking strength, elongation at breaking, 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 the conditions of a chuck distance of 50 mm, a tensile speed of 50 mm / min, and a number of measurements of 5. The average value of the 5 measured values ​​was adopted.

[0102] [Storage Test of Polyamide Hollow Fiber Membranes] Each polyamide hollow fiber membrane obtained in the Examples, Comparative Examples, and Reference Examples was placed in a 5-inch module case at a 20% occupancy rate, and potting was performed to produce two hollow fiber membrane modules. 30 L of PGMEA was supplied through the liquid passage port in the outer space of the hollow fiber membrane of the prepared hollow fiber membrane module, and the filtrate was discharged through the liquid passage port in the inner space of the hollow fiber membrane. The hollow fiber membrane module (polyamide hollow fiber membrane housed in the module) was used for filtering PGMEA. The same procedure was performed for each of the two hollow fiber membrane modules. One of the two hollow fiber membrane modules was disassembled, the contained polyamide hollow fiber membrane was removed, and after drying and removing the PGMEA, the filtration performance (fine particle rejection rate, external pressure permeability, IBP, BBP) of the obtained 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. In other words, there was no change in the filtration performance of the polyamide hollow fiber membrane due to use. Next, 10 L of PGMEA was newly passed through the liquid passage port in the inner space of the hollow fiber membrane of another hollow fiber membrane module as a preservation solution, immersing both the inner and outer sides of the hollow fiber membrane inside the module. At this time, the pressure inside the module was 0 MPa. In this state, all liquid passage ports of the module were sealed and stored at room temperature (23°C) for two years. After storage, the module was disassembled, the stored hollow fiber membrane was removed, and the PGMEA was dried off. The filtration performance of each of the obtained polyamide hollow fiber membranes was then measured using the method described above. The retention rate (%) was then calculated using the following formulas 3 to 5. <Equation 3> Retention rate of rejection rate (%) = (Rejection rate of particles of each particle size after storage / Rejection rate of particles of each particle size before storage) × 100 <Equation 4> Retention rate of external pressure permeability (%) = (External pressure permeability after storage / External pressure permeability before storage) × 100 <Equation 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 Resins> ・PA1: Polyamide 6 A1030BRT manufactured by Unitika Corporation ・PA2: Polyamide 6 A1030BRF-BA manufactured by Unitika Corporation ・PA3: Polyamide 6 obtained by solid-phase polymerization of A1030BRT at 170°C for 15 hours under an N2 gas flow ・PA4: Polyamide 6 obtained by solid-phase polymerization of A1030BRT at 170°C for 50 hours under an N2 gas flow ・PA5: Polyamide 6 obtained by solid-phase polymerization of A1030BRT at 170°C for 70 hours under an N2 gas flow ・PA6: Polyamide 66 obtained by solid-phase polymerization of Polyamide 66 A125 manufactured by Unitika Corporation at 170°C for 30 hours under an N2 gas flow ・PA7: Polyamide 610 CM2001 manufactured by Toray Industries, Inc. - PA8: Polyamide 6 obtained by uniformly driving-blending 0.05% by mass of sodium hydroxide with PA3. - PA9: Polyamide 6 obtained by uniformly driving-blending 0.05% by mass of potassium hydroxide with PA3.

[0104] <Sulfones> Production Example 1 15.7 parts by mass of dimethyl sulfoxide was dissolved in 24.3 parts by mass of a 27.5% by mass aqueous solution of hydrogen peroxide (an 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 held at 85°C for 1.5 hours. The entire volume of the solution was then concentrated to 1.25 times its original volume, and the solution was allowed to stand at room temperature for 48 hours to precipitate crystals. These crystals were collected by filtration and dried to obtain dimethyl sulfone. The pH of a 5% by mass aqueous solution of the obtained dimethyl sulfone was 4.8. This dimethyl sulfone is referred to as "DMS-1".

[0105] In the second production example, dimethyl sulfone was obtained using the same procedure as in the first production example, except that a 25.0% by mass aqueous solution of hydrogen peroxide was used instead of a 27.5% by mass aqueous solution of hydrogen peroxide. The pH of a 5% by mass aqueous solution of the obtained dimethyl sulfone was 5.2. This dimethyl sulfone is referred to as "DMS-2".

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

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

[0108] Production Example 5: Dimethyl sulfone was obtained using 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 a 5% by mass aqueous solution of the obtained dimethyl sulfone was 6.9. This dimethyl sulfone is referred to as "DMS-5".

[0109] Production Example 6 0.45 parts by mass of tert-butylcatechol and 230 parts by mass of sulfur dioxide were placed in a sealed reactor and the temperature was raised to 100°C. Then 162 parts by mass of 1,3-butadiene was injected at a flow rate of 0.38 parts by mass / min, and the mixture was stirred at 100°C for 1 hour. After releasing the pressure in the reactor, 720 parts by mass of water were added, and after cooling to 60°C, the contents were filtered to obtain an aqueous solution of 3-sulfolene. 1000 g of the obtained aqueous solution of 3-sulfolene (2.70 mol of 3-sulfolene) was charged into a sealed reactor together with 4.80 parts by mass of Raney nickel catalyst (50% by mass, water-containing). Next, the temperature was maintained at 30-40°C, hydrogen was introduced into the sealed reactor and the pressure was increased to 1.0 MPa. The mixture was stirred for 3 hours while maintaining the pressure, and then filtered to obtain an aqueous solution of sulfolane. The obtained aqueous solution of sulfolane was heated and the water was removed by distillation to obtain crude sulfolane. Next, 100 parts by mass of crude sulfolane and 0.5 parts by mass of a 20.0% by mass aqueous solution of hydrogen peroxide (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. After stirring at low speed at 60°C for 24 hours, water and impurities were removed by heating and reducing pressure to obtain sulfolane. The pH of a 5% by mass aqueous solution of the obtained sulfolane was 6.3. This sulfolane is referred to as "SFL-1".

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

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

[0112] Production Example 9: Sulfolane was obtained using the same procedure as in Production Example 6, except that a 27.5% by mass hydrogen peroxide aqueous solution was used instead of a 20.0% by mass hydrogen peroxide aqueous solution. The pH of a 5% by mass aqueous solution of the obtained 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" measuring device manufactured by Horiba, Ltd.

[0114] Example 1 A twin-screw extruder (PCM30, manufactured by Ikegai Co., Ltd.) equipped with a polyamide quantitative feeding device and a powder quantitative feeding device as auxiliary equipment was used. PA1 was used as the polyamide resin raw material, and DMS-3 was used as the medium. The twin-screw extruder was operated under conditions of screw rotation of 100 rpm and all cylinder temperatures of 200°C. PA1 was quantitatively fed into the twin-screw extruder at a rate of 28 parts by mass / h from the polyamide quantitative feeding device, and DMS-3 was quantitatively fed into the twin-screw extruder at a rate of 72 parts by mass / h (i.e., the composition ratio of the resulting film-forming stock solution was PA1 / DMS-3 = 28 / 72). Discharge of a film-forming stock solution in which the polyamide resin and DMS-3 were uniformly dissolved was confirmed from the tip of the twin-screw extruder (first step). Once the discharge of the film-forming solution stabilized, a spinning device was attached to the discharge port of the film-forming solution at the tip of the twin-screw extruder. This device was connected to a spinning nozzle (a double-tube nozzle for hollow fiber production with a double-tube structure (outer diameter 1.5 mm, inner diameter 0.6 mm)) via a metering pump, and the film-forming solution was extruded at 5 g / min from the outer annular nozzle. The spinning device was set to 200°C. Simultaneously, an internal liquid consisting of glycerin was discharged at 2.0 g / min from the inner nozzle. The extruded spinning solution and internal liquid were immersed in a solidification bath consisting of a 50% by mass aqueous solution of propylene glycol at 5°C via a 10 mm air gap to cool and solidify, forming a hollow fiber film. This film was then wound onto a bobbin at a winding speed of 20 m / min (second step). The residence time of the raw materials, from the time each raw material was fed from the feeding device to the twin-screw extruder until the film-forming solution was extruded 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, etc., and then dried in a hot air dryer at 50°C for 1 hour to obtain a polyamide hollow fiber membrane (third step). The obtained polyamide hollow fiber membrane had an outer diameter of 550 μm and an inner diameter of 300 μm. Furthermore, SEM observation confirmed the formation of a dense layer on the inner surface of the polyamide hollow fiber membrane.

[0115] Examples 2, 3, 7, 8, 12-15, Comparative Examples 1, 2, and Reference Examples 1, 2: Polyamide hollow fiber membranes were obtained using the same procedure 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 the formation of a dense layer on the inner surface of the hollow fiber membrane was confirmed by SEM observation.

[0116] Examples 4-6 Polyamide hollow fiber membranes were obtained using the same procedure as in Example 3, except that the rate at which raw materials were quantitatively fed into the twin-screw extruder was changed to alter the composition ratio of the film-forming stock solution shown in Table 1. All obtained polyamide hollow fiber membranes 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 membranes.

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

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

[0119] Comparative Example 7 A 2L tank equipped with a stirrer and capable of heating and sealing was used, with 504g of PA3 as the polyamide resin and 1296g of DMS-3 as the medium. PA3 and DMS-3 were added to the tank, which was adjusted to a stirrer speed of 20 rpm and an overall temperature of 200°C, and stirred at 200°C x 20 rpm for 60 minutes to ensure uniform dissolution of PA3 and DMS-3. Next, the film-forming stock solution was extruded at a rate of 5g / min from the outer annular nozzle through a metering pump provided at the discharge port of the tank and a spinneret (double-tube hollow fiber manufacturing double-tube nozzle (spinneret hole diameter: outer diameter 1.5 mm, inner diameter 0.6 mm)) provided via the metering pump. The temperature of the metering pump and the double-tube hollow fiber manufacturing double-tube nozzle was set to 200°C. In parallel, an internal liquid consisting of glycerin was discharged at a rate of 2.0g / min from the inner nozzle. The extruded spinning solution and internal liquid were immersed in a solidification bath consisting of a 50% by mass aqueous solution of propylene glycol at 5°C through a 10 mm air gap to cool and solidify, forming a hollow fiber membrane. This membrane was then wound onto a bobbin at a winding speed of 20 m / min. This process was designated as "Process A". The winding of the hollow fiber membrane onto the bobbin took 2.5 hours. In this case, the residence time from the time the raw materials were introduced into the apparatus until the film-forming solution was extruded from the nozzle ranged from a minimum of 85 minutes to a maximum of 225 minutes. The obtained hollow fiber membrane was immersed in water for 24 hours to extract the solvent (wash), 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. 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 was obtained by winding it onto a bobbin using the same procedure as in Step A of Comparative Example 7. Next, the obtained hollow fiber membrane was immersed in water for washing. During immersion, the water was stirred, and fresh water was continuously added at a flow rate of 0.5 L / min, allowing the solvent to be extracted (washed) for two months while the water was overflowing. After that, the membrane was 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 inner surface of the hollow fiber membrane.

[0121] Table 1 shows the manufacturing conditions for the polyamide hollow fiber membranes obtained in Examples 1-15, Comparative Examples 1-8, and Reference Examples 1 and 2, as well as the measurement results of the metal element content before and after organic solvent washing. Table 2 also shows the results of the membrane evaluation of the polyamide hollow fiber membranes obtained in Examples 1-15, Comparative Examples 1-8, and Reference Examples 1 and 2. However, only in Comparative Example 1, the polyamide hollow fiber membrane washed with an organic solvent using the method described in "(2) Metal element content of hollow fiber membrane after organic solvent washing (fourth step)" was used to evaluate the "filtration performance before storage" and the "retention rate (%) in the storage test." There was no change in the filtration performance of the polyamide hollow fiber membrane before storage due to organic solvent washing and use.

[0122]

[0123]

[0124] Tables 1 and 2 show that the polyamide hollow fiber membranes of Examples 1 to 15 have low content of specific metal elements, resulting in less change in filtration performance even after long-term storage in a preservation solution, and thus excellent storage stability. Furthermore, in Examples 1 to 15, the specific metal elements were removed to some extent when the polyamide hollow fiber membranes were washed with an organic solvent, resulting in more hygienic polyamide hollow fiber membranes. On the other hand, the polyamide hollow fiber membranes of Comparative Examples 1 to 8 had high content of specific metal elements, and their filtration performance changed significantly when stored in a preservation solution for long periods. In addition, in Comparative Examples 1 to 8, the specific metal elements could hardly be removed when the polyamide hollow fiber membranes were washed with an organic solvent.

[0125] The polyamide hollow fiber membranes of Examples 3 to 11 had a low content of specific metal elements, and furthermore, the ratio of γ crystals to the total amount of α crystals was in the range of 15 to 25%, resulting in superior water permeability under external pressure before storage compared to the other examples.

[0126] In Comparative Examples 1 to 3, the polyamide hollow fiber membranes used a 5% aqueous solution of sulfones with a pH outside the range of 5.2 to 6.8 as the medium. As a result, they contained a high amount of specific metal elements, and their filtration performance changed significantly when stored for a long period in the storage solution.

[0127] The polyamide hollow fiber membranes of Comparative Examples 4-6 used materials other than sulfones as a medium, resulting in a high content of specific metal elements. Therefore, their filtration performance changed significantly after long-term storage in the storage solution.

[0128] The polyamide hollow fiber membranes of Comparative Examples 7 and 8 were prepared using a batch-type film-forming method with tanks, resulting in high levels of specific metal elements. Consequently, their filtration performance changed significantly after long-term storage in the preservation solution.

[0129] In Reference Examples 1 and 2, sodium or potassium components were intentionally added to the raw materials to produce polyamide hollow fiber membranes containing high levels of sodium or potassium, which were then evaluated. As a result, it was confirmed that the produced polyamide hollow fiber membranes exhibited excellent storage stability, with little change in filtration performance even after long-term storage in a preservation solution. From these results, it was found that not all metal elements affect the storage stability of polyamide hollow fiber membranes; rather, the content of specific metal elements has an impact on the storage stability of polyamide hollow fiber membranes.

[0130] 1: Liquid transfer pump 2: Inlet pressure gauge 3: Hollow fiber membrane 4: Outlet pressure gauge 5: Outlet valve 6: Receiving tray 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 display 16: Two-way valve

Claims

1. A polyamide hollow fiber membrane formed from a polyamide resin, satisfying at least one of the following characteristics (1) to (5): (1) Fe content of 4.30 ppm or less (2) Cr content of less than 1.00 ppm (3) Cu content of less than 0.20 ppm (4) Mg content of less than 0.60 ppm (5) Zn content of less than 0.30 ppm 2. The polyamide hollow fiber membrane according to claim 1, wherein the relative viscosity is 2.0 to 6.

5.

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

4. The polyamide hollow fiber membrane according to claim 1, wherein the polyamide hollow fiber membrane has a dense layer on the luminal surface and / or the outer surface.

5. The polyamide hollow fiber membrane according to claim 1, wherein the rejection rate of particles with a particle size of 50 nm is 90% or more.

6. The polyamide hollow fiber membrane according to claim 1, wherein the retention rate of the rejection rate of particles with a particle size 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) × 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 pressurization.

7. External pressure permeability is 50 to 2000 L / (m). 2 A polyamide hollow fiber membrane according to claim 1, wherein the properties are ・atm・h.

8. The polyamide hollow fiber membrane according to claim 1, wherein the retention rate of the external pressure permeability calculated by the following formula is 90 to 110%. Retention rate (%) = (External pressure permeability after storage / External pressure permeability 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 pressurization.

9. The polyamide hollow fiber membrane according to claim 1, wherein in a bubble point test conducted by applying air pressure in 2-propanol at 20°C, the initial bubble point is 0.20 MPa or higher and the burst bubble point is 0.30 MPa or higher.

10. The polyamide hollow fiber membrane according to claim 1, wherein in a bubble point test conducted in 2-propanol under air pressure at 20°C, the retention rate of the initial bubble point and burst bubble point calculated by the following formula is 90 to 110%. Retention rate (%) = (Initial bubble point or burst bubble point after storage / Initial bubble point or burst bubble point 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 pressurization.

11. A hollow fiber membrane module in which a polyamide hollow fiber membrane according to any one of claims 1 to 10 is housed in a module case.

12. A method for producing a polyamide hollow fiber film, comprising: a first step of preparing a film-forming stock solution by mixing at least a polyamide resin and sulfones using a multi-screw extruder; a second step of forming a hollow fiber film by discharging the film-forming stock solution from the outer annular nozzle and the internal liquid from the inner nozzle using a double-tube nozzle for hollow fiber production, and immersing the film in a solidification bath containing water and / or polyhydric alcohol; and a third step of removing an organic solvent from the hollow fiber film formed in the second step, wherein the sulfones have a pH of 5.2 to 6.8 at 25°C when dissolved in water to make a 5% by mass aqueous solution.

13. The method for producing a polyamide hollow fiber membrane according to claim 12, wherein the sulfones are dimethyl sulfone and / or sulfolane.

14. A method for producing a polyamide hollow fiber membrane according to claim 12 or 13, further comprising 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.

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