Composite hollow fiber membrane

Abstract

To provide a hollow fiber membrane that is hard to be broken or ruptured when a load of torsion and high-pressure gas is applied to the hollow fiber membrane.SOLUTION: A composite hollow fiber membrane includes a polyamide hollow fiber membrane having a porous layer, and a polyimide layer having a dense layer. The polyimide layer is provided in at least one surface of the polyamide hollow fiber membrane. It is preferable that a porosity of the innermost part and the outermost part of the polyamide hollow fiber membrane is lower than a porosity of a center part of the polyamide hollow fiber membrane.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a composite hollow fiber membrane. [Background technology]

[0002] Compared to conventional methods such as distillation, which utilizes phase change and requires thermal energy, membrane separation technology is an energy-saving process because, in theory, it only requires the pressure energy needed to drive a pump, making it an important technology that contributes to reducing greenhouse gas emissions.

[0003] Separation membranes are also used in the field of gas separation, and polyimide resins are often used as the material for polymer-based separation membranes (e.g., Non-Patent Document 1). This is because polyimide resins have a better balance of gas permeability and selectivity than other resins, and also have excellent physical and chemical properties such as high heat resistance and high strength.

[0004] Separation membranes are mainly available in two shapes: flat sheet membranes and straw-shaped hollow fiber membranes. Hollow fiber membranes have the advantage of high membrane area efficiency per volume (e.g., Patent Document 1), and polyimide hollow fiber membrane modules are often used in gas separation membrane modules. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Journal of the Society of Fiber Science and Technology, 1995, Vol. 51, No. 2, p. 21 [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2019-18142 Summary of the Invention [Problem to be solved by the invention]

[0007] When gas separation membranes are used, the gas to be treated is often pressurized. While the aforementioned polyimide hollow fiber membranes are highly strong and can withstand pressure, they generally lack tensile elongation and toughness. Therefore, twisting can occur in the hollow fiber membranes due to changes in the flow rate of treated and non-treated gases, turbulence, and pressure changes during operation of the equipment. The twisting and the load of the high-pressure gas can easily cause the hollow fiber membranes to tear or break.

[0008] Therefore, the main object of the present invention is to solve the above problems and to provide a hollow fiber membrane that is less likely to break or rupture when subjected to torsion and a load from high-pressure gas. [Means for solving the problem]

[0009] The present inventors have conducted extensive research to solve the above problems and have found that a composite hollow fiber membrane comprising a polyamide hollow fiber membrane having a porous layer and a polyimide layer having a dense layer, with the polyimide layer being provided on the surface of the polyamide hollow fiber membrane, makes the composite hollow fiber membrane less susceptible to tearing and fracture when subjected to torsion and loads from high-pressure gas. The present invention was completed through further research based on this finding.

[0010] That is, the present invention provides the following aspects. Item 1. A composite hollow fiber membrane comprising a polyamide hollow fiber membrane having a porous layer and a polyimide layer having a dense layer, the polyimide layer being provided on at least one surface of the polyamide hollow fiber membrane. Item 2. The composite hollow fiber membrane according to Item 1, wherein the porosity of the innermost and outermost parts of the polyamide hollow fiber membrane is lower than the porosity of the central part of the polyamide hollow fiber membrane. Item 3. The composite hollow fiber membrane according to Item 1 or 2, wherein the polyamide hollow fiber membrane has a dense layer. Item 4. The composite hollow fiber membrane according to any one of Items 1 to 3, wherein the porosity of the central portion of the polyamide hollow fiber membrane is 60 to 80%. Item 5. The composite hollow fiber membrane according to any one of Items 1 to 4, wherein the polyamide hollow fiber membrane has an outer diameter of 200 to 600 μm. Item 6. The composite hollow fiber membrane according to any one of Items 1 to 5, wherein the polyimide layer further has a porous layer. Item 7. A hollow fiber membrane module comprising a module case and the composite hollow fiber membrane according to any one of Items 1 to 6 housed in the module case. [Effects of the Invention]

[0011] The composite hollow fiber membrane of the present invention is less likely to break or rupture when subjected to torsion and a load from high-pressure gas, and therefore, by using the composite hollow fiber membrane of the present invention, the durability of the composite hollow fiber membrane is improved, enabling long-term gas separation with low energy and in a small space. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic cross-sectional view of a polyamide hollow fiber membrane cut perpendicular to the longitudinal direction. [Figure 2] FIG. 2 is a partial enlarged view of the area surrounded by the dotted line in FIG. [Figure 3] FIG. 2 is a partial enlarged view of the area surrounded by the dotted line in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0013] 1.Definition In the present invention, the term "polyamide hollow fiber membrane" refers to a filtration membrane in the form of a hollow fiber formed using a polyamide resin and having a porous layer alone or a dense layer and a porous layer.

[0014] In the present invention, the term "polyimide layer" refers to a layer formed on the surface of a polyamide hollow fiber membrane, which is made of a polyimide resin and which comprises a dense layer alone or a dense layer and a porous layer.

[0015] In the present invention, the term "dense layer" refers to a region in the polyamide hollow fiber membrane and polyimide layer where the presence of pores is substantially not observed in a scanning electron microscope (SEM) photograph at 10,000x magnification.

[0016] In the present invention, the term "porous layer" refers to a porous region in a polyamide hollow fiber membrane or a polyimide layer that has a continuous porous structure in which the presence of substantial pores is observed in a scanning electron microscope (SEM) photograph at a magnification of 2000 times.

[0017] 2.Composite hollow fiber membrane The composite hollow fiber membrane of the present invention is characterized by comprising a polyamide hollow fiber membrane having a porous layer and a polyimide layer having a dense layer, and the polyimide layer is provided on the surface of the polyamide hollow fiber membrane. The composite hollow fiber membrane of the present invention will be described in detail below.

[0018] [Polyamide hollow fiber membrane] The polyamide hollow fiber membrane, which is a main component of the composite hollow fiber membrane of the present invention, has a porous layer alone or a dense layer and a porous layer, and is made of a polyamide resin. When the composite hollow fiber membrane of the present invention is used as a gas separation membrane, the polyamide hollow fiber membrane mainly functions as a support for the polyimide layer, imparting pressure resistance and toughness to the composite hollow fiber membrane of the present invention, making the composite hollow fiber membrane less susceptible to tearing and fracture when subjected to torsion and high-pressure gas loads. Furthermore, the polyamide resin has resistance to a wide range of organic solvents, making it possible to form a solvent-soluble polyimide layer using organic solvents.

[0019] The type of polyamide resin is not particularly limited, and examples thereof include polyamide homopolymers, polyamide copolymers, and mixtures thereof. Specific examples of polyamide homopolymers include polyamide 6, polyamide 66, polyamide 46, polyamide 610, polyamide 612, polyamide 11, polyamide 12, polyamide MXD6, polyamide 4T, polyamide 6T, polyamide 9T, and polyamide 10T. Specific examples of polyamide copolymers include copolymers of polyamide with polyethers such as polytetramethylene glycol or polyethylene glycol. The proportion of the polyamide component in the polyamide copolymer is not particularly limited, and examples of the proportion of the polyamide component include preferably 70 mol% or more, more preferably 80 mol% or more, even more preferably 90 mol% or more, and particularly preferably 95 mol% or more. By ensuring that the proportion of the polyamide component in the polyamide copolymer satisfies the above range, the polyamide hollow fiber membrane can be provided with even better organic solvent resistance. The polyamide resins may be used singly or in combination of two or more.

[0020] Among these polyamide resins, polyamide 6 is preferably used as a resin for forming polyamide hollow fiber membranes because it is easy to achieve both favorable pressure resistance and solvent resistance.

[0021] The polyamide resin may be crosslinked or not, but from the viewpoint of reducing production costs, it is preferable that the polyamide resin is not crosslinked.

[0022] The relative viscosity of the polyamide resin is not particularly limited, but may be, for example, 2.0 to 7.0, preferably 2.5 to 6.0, and more preferably 3.0 to 5.0. Having such a relative viscosity improves moldability and controllability of phase separation during the production of polyamide hollow fiber membranes, and makes it possible to provide the polyamide hollow fiber membrane with excellent shape stability. Here, the relative viscosity refers to the value measured with an Ubbelohde viscometer at 25°C using a solution in which 1 g of polyamide resin is dissolved in 100 mL of 96% sulfuric acid.

[0023] In addition to the polyamide resin, the polyamide hollow fiber membrane may contain a filler, if necessary, to the extent that the effects of the present invention are not impaired. The inclusion of a filler can improve the strength, elongation, and elastic modulus of the polyamide hollow fiber membrane. In particular, the inclusion of a filler has the effect of making the polyamide hollow fiber membrane less likely to deform even when high pressure is applied to the polyamide hollow fiber membrane. The type of filler to be added is not particularly limited, and examples thereof include fibrous fillers such as glass fiber, carbon fiber, potassium titanate whisker, zinc oxide whisker, calcium carbonate whisker, wollastonite whisker, aluminum borate whisker, aramid fiber, alumina fiber, silicon carbide fiber, ceramic fiber, asbestos fiber, gypsum fiber, and metal fiber; talc, hydrotalcite, wollastonite, zeolite, sericite, mica, kaolin, clay, pyrophyllite, bentonite, asbestos, and the like. Examples of fillers include silicates such as sintered silicate and alumina silicate; metal compounds such as silicon oxide, magnesium oxide, alumina, zirconium oxide, titanium oxide, and iron oxide; carbonates such as calcium carbonate, magnesium carbonate, and dolomite; sulfates such as calcium sulfate and barium sulfate; metal hydroxides such as calcium hydroxide, magnesium hydroxide, and aluminum hydroxide; and inorganic materials such as non-fibrous fillers such as glass beads, glass flakes, glass powder, ceramic beads, boron nitride, silicon carbide, carbon black, silica, and graphite. These fillers may be used alone or in combination of two or more. Among these fillers, preferred are talc, hydrotalcite, silica, clay, and titanium oxide, and more preferred are talc and clay. The filler content is not particularly limited, but may be, for example, 5 to 100 parts by weight, preferably 10 to 75 parts by weight, and more preferably 25 to 50 parts by weight per 100 parts by weight of polyamide resin. By including the filler in such an amount, the strength, elongation, and elastic modulus of the polyamide hollow fiber membrane can be improved.

[0024] The polyamide hollow fiber membrane may contain additives such as thickeners, antioxidants, surface modifiers, lubricants, and surfactants, as needed, for the purpose of pore size control and improving membrane performance.

[0025] The outer diameter of the polyamide hollow fiber membrane is appropriately set depending on the application of the composite hollow fiber membrane, the thickness of the porous layer or the porous layer and dense layer, the pressure resistance to be provided, etc., but in consideration of the relationship between the effective membrane area when packed into a module, the membrane strength, the pressure loss of the fluid flowing through the hollow portion, and the buckling pressure, the outer diameter of the hollow fiber membrane is 100 μm or more, preferably 200 to 2000 μm, and more preferably 250 to 1500 μm.From the viewpoint of making the composite hollow fiber membrane less susceptible to rupture and breakage when subjected to torsion and loads from high-pressure gas, the outer diameter is preferably 200 to 900 μm, more preferably 250 to 550 μm, and particularly preferably 250 to 380 μm. The inner diameter of the polyamide hollow fiber membrane is appropriately set depending on the use of the composite hollow fiber membrane, the thickness of the porous layer or the porous layer and dense layer, the pressure resistance and gas permeability to be provided, the thickness of the polyimide layer when a polyimide layer is provided on the lumen side surface of the polyamide hollow fiber membrane, etc., and is, for example, 50 to 1500 μm, preferably 75 to 1000 μm, and more preferably 75 to 800 μm. In addition to the above viewpoints, the inner diameter of the polyamide hollow fiber membrane is preferably 50 to 500 μm, more preferably 50 to 230 μm, and particularly preferably 75 to 130 μm, from the viewpoint of making the composite hollow fiber membrane less susceptible to tearing and fracture when subjected to loads from torsion and high-pressure gas. In the present invention, the outer diameter and inner diameter of the polyamide hollow fiber membrane are values ​​that can be determined by observing five composite hollow fiber membranes with an optical microscope at a magnification of 200x, measuring the outer diameter and inner diameter (both at the point where they are maximum diameter) of the polyamide hollow fiber membranes that make up each composite hollow fiber membrane, and calculating the average value of each.

[0026] The thickness of the polyamide hollow fiber membrane is appropriately set depending on the use of the composite hollow fiber membrane, the thickness of the porous layer or the porous layer and the dense layer, the pressure resistance and gas permeability to be provided, the thickness of the polyimide layer when a polyimide layer is provided on the lumen side surface of the polyamide hollow fiber membrane, etc., and is, for example, 25 to 500 μm, preferably 50 to 350 μm. Furthermore, in addition to the above viewpoints, the thickness of the polyamide hollow fiber membrane is preferably 50 to 250 μm, more preferably 70 to 120 μm, from the viewpoint of making the composite hollow fiber membrane less susceptible to tearing and fracture when subjected to loads from torsion and high-pressure gas. In the present invention, the thickness of the polyamide hollow fiber membrane is a value calculated by subtracting the inner diameter from the outer diameter and dividing the value by 2.

[0027] [Porous layer of polyamide hollow fiber membrane] The polyamide hollow fiber membrane constituting the composite hollow fiber membrane of the present invention has a porous layer. As described above, the porous layer is a porous region having a continuous pore structure in which the presence of pores is substantially recognized in a scanning electron microscope (SEM) photograph at 2000x magnification in the polyamide hollow fiber membrane.

[0028] The pore size of the porous layer of the polyamide hollow fiber membrane is not particularly limited as long as it has a strength sufficient to hold the polyimide layer and does not significantly impede fluid permeation.

[0029] [Dense layer of polyamide hollow fiber membrane] The polyamide hollow fiber membrane constituting the composite hollow fiber membrane of the present invention may have a dense layer, if necessary. As described above, the "dense layer" refers to a region in the polyamide hollow fiber membrane where the presence of pores is substantially absent in a scanning electron microscope (SEM) photograph at 10,000x magnification.

[0030] The dense layer of the polyamide hollow fiber membrane may be formed on the lumen surface, the outer surface, or both surfaces of the polyamide hollow fiber membrane. However, from the viewpoint of making the composite hollow fiber membrane less susceptible to tearing and fracture when subjected to torsion and high-pressure gas loads, it is preferable that the dense layer be formed on at least one of the lumen surface or the outer surface of the polyamide hollow fiber membrane, and it is more preferable that the dense layer be formed on both surfaces. When observing the dense layer using a scanning electron microscope (SEM), if the dense layer is present on the outer surface of the polyamide hollow fiber membrane, the polyamide hollow fiber membrane can be cut to an appropriate size, placed on a sample stage, and then vapor-deposited with Pt, Au, Pd, or the like before observation.If the dense layer is present on the lumen surface of the polyamide hollow fiber membrane, the polyamide hollow fiber membrane can be cut longitudinally with a sharp blade such as a scalpel to expose the lumen surface, then cut to an appropriate size, placed on a sample stage, and then vapor-deposited with Pt, Au, Pd, or the like before observation.

[0031] The thickness of the dense layer is not particularly limited and can be adjusted to the desired value by appropriately adjusting the solvent, concentration, and temperature of the membrane-forming solution, the type and temperature of the coagulation solution, etc., but from the viewpoint of improving gas permeability, it is preferably 2.0 μm or less, more preferably 1.8 μm or less, even more preferably 1.5 μm or less, and even more preferably 1.2 μm or less. The lower limit of the dense layer thickness is not particularly limited, but examples include 0.1 μm or more, preferably 0.3 μm or more, and more preferably 0.5 μm or more. Specifically, the thickness of the dense layer of a polyamide hollow fiber membrane can be 0.1 to 2.0 μm, 0.3 to 1.5 μm, or 0.5 to 1.2 μm. In the present invention, the thickness of the dense layer is a value obtained by measuring the distance (thickness) of an area where substantially no pores are found in the polyamide hollow fiber membrane constituting the composite hollow fiber membrane at 10,000x magnification in a scanning electron microscope (SEM) photograph, and calculating the average value.

[0032] The porosity of the polyamide hollow fiber membrane is not particularly limited, and can be adjusted to a desired value by appropriately adjusting the solvent, concentration, and temperature of the membrane-forming solution and the type and temperature of the coagulation solution in the production of the polyamide hollow fiber membrane, but from the viewpoint of ease of membrane production and from the viewpoint of increasing membrane strength so that the permeation amount can be increased by increasing the operating pressure during gas separation, the porosity of the central portion is preferably 60 to 80%, more preferably 63 to 78%, even more preferably 65 to 77%, and even more preferably 66 to 76%. In addition to the above viewpoints, from the viewpoint of making the composite hollow fiber membrane less likely to break or rupture when subjected to torsion and loads from high-pressure gas, the porosity of the central portion of the polyamide hollow fiber membrane is preferably 60 to 80%, more preferably 63 to 69%.

[0033] Furthermore, from the viewpoint of achieving both a smoother polyimide layer and higher gas permeability, the porosity of the innermost and outermost portions of the polyamide hollow fiber membrane is preferably lower than the porosity of the central portion of the polyamide hollow fiber membrane. In this case, from the viewpoint of achieving both a smoother polyimide layer and higher gas permeability, the porosity of the innermost and outermost portions of the polyamide hollow fiber membrane is preferably 30 to 76%, more preferably 35 to 74%, and even more preferably 40 to 72%. Furthermore, when a polyamide hollow fiber membrane is twisted, a greater shear stress acts on the outermost portion than on the innermost portion, so the porosity of the outermost portion is preferably lower than that of the innermost portion. From this viewpoint, the porosity of the outermost portion of the polyamide hollow fiber membrane is preferably 30 to 55%, more preferably 35 to 43%. From the same viewpoint, the porosity of the innermost portion of the polyamide hollow fiber membrane is preferably 58 to 75%, more preferably 58 to 61%. From the same viewpoint, the difference between the porosity of the central portion of the polyamide hollow fiber membrane and the porosity of the outermost portion (porosity of the central portion - porosity of the outermost portion) is preferably 15 to 30%, more preferably 20 to 30%. From the same viewpoint, the difference between the porosity of the central portion of the polyamide hollow fiber membrane and the porosity of the innermost portion (porosity of the central portion - porosity of the innermost portion) is preferably 3 to 14%, more preferably 3 to 12%. From the same viewpoint, the difference between the porosity of the innermost portion of the polyamide hollow fiber membrane and the porosity of the outermost portion (porosity of the innermost portion - porosity of the outermost portion) is preferably 10 to 25%, more preferably 15 to 23%.

[0034] In the present invention, the porosity of a polyamide hollow fiber membrane is calculated by the following method. Scanning electron microscope (SEM) photographs are taken at five locations equally spaced in the thickness direction of the polyamide hollow fiber membrane on a cross section of a composite hollow fiber membrane cut perpendicular to the longitudinal direction. Specifically, the photographs are taken as follows. FIG. 1 is a schematic cross-sectional view of a polyamide hollow fiber membrane cut perpendicular to the longitudinal direction, and FIGS. 2 and 3 are partial enlarged views of the area surrounded by the dotted line in FIG. 1. As shown in FIG. 2, a line is drawn from the center of the lumen of the hollow fiber membrane to the outer surface, and this line is divided into five equal parts from the lumen surface to the outer surface. Then, as shown in FIG. 2, the midpoint of the five equally divided lines is determined. Then, as shown in FIG. 3, photographs are taken at five locations at a uniform magnification such that the midpoint is at the center of the photographed image, only the hollow fiber membrane portion is included in the photographed image, and the number of pores is set to 30 to 300. Image analysis software (ImageJ) was used to analyze the images of three of the five SEM photographs: the outermost (region A in Figure 3), innermost (region C in Figure 3), and central (region B in Figure 3). The pore and polymer regions were distinguished by binarization, and the area ratio (%) of the total pore area to the area of ​​the analyzed region was calculated. This was measured at three different locations on the cross section of the hollow fiber membrane, and the average value of the three locations was calculated to represent the average porosity of the outermost, innermost, and central regions. The specific ImageJ procedures are as follows: The image to be analyzed was imported into ImageJ, the analysis range was specified, and the brightness value at which the histogram peak, obtained by the "Analyze > Histogram" operation, was set as the threshold (i.e., the lower threshold level was set to the brightness value at which the histogram peak was highest, and the upper threshold level was set to 255). The image was then binarized using "Image > Adjust > Threshold."Next, check "Area" in "Analyze>Set measurements" and calculate the total area of ​​the analysis range by operating "Analyze>Measure." Then, check "Area" and "Limit to threshold" in "Analyze>Set measurements" and operate "Analyze>Measure" again to calculate the area of ​​the polymer and void parts, and the area ratio (%) of the void parts to the total area is taken as the porosity.

[0035] [Polyimide layer] In the composite hollow-fiber membrane of the present invention, the polyimide layer has a dense layer and is formed of polyimide. When the composite hollow-fiber membrane of the present invention is used as a gas separation membrane, the polyimide layer mainly functions as a layer that exhibits gas separation performance.

[0036] The polyimide layer may be provided on any surface of the polyamide hollow fiber membrane. In the composite hollow fiber membrane of the present invention, when the inner diameter of the polyamide hollow fiber membrane is relatively small and the composite hollow fiber membrane is less likely to break or rupture when subjected to torsion and a load from high-pressure gas, the polyimide layer is preferably provided on the outer surface of the polyamide hollow fiber membrane from the viewpoint of further improving gas permeability in the lumen of the composite hollow fiber membrane.On the other hand, when a hollow fiber membrane module containing a plurality of composite hollow fiber membranes is used for gas separation, the polyimide layer is preferably provided on the lumen-side surface of the polyamide hollow fiber membrane from the viewpoint of easily preventing peeling of the polyimide layer due to contact between the contained composite hollow fiber membrane modules, etc.

[0037] In the present invention, the polyimide is a polymer obtained by polycondensation of a tetracarboxylic acid monomer having an aromatic ring and a diamine monomer having an aromatic ring via an imide bond. In the present invention, the structure of each monomer is not particularly limited as long as the polyimide has solvent solubility.

[0038] The polyimide used to form the polyimide layer is not particularly limited to a solvent-soluble or calcined polyimide, provided that it can be formed on one surface of the polyamide hollow fiber membrane, but the solvent-soluble polyimide is preferred because the calcination temperature of the calcined polyimide layer generally exceeds the melting point of the polyamide resin forming the polyamide hollow fiber membrane. Furthermore, the solvent-soluble polyimide is also preferred because it can be used to form a thin, dense layer using a non-solvent-induced phase separation method.

[0039] The polyimide layer may contain additives such as a cure retarder, a thickener, an antioxidant, a surface modifier, a lubricant, and a surfactant, as required.

[0040] The thickness of the polyimide layer (the total thickness of the polyimide layer including the dense layer and the porous layer) is not particularly limited, but from the viewpoint of suppressing the occurrence of defects to improve gas separation performance and suppressing a decrease in gas permeability, it is preferably 0.2 to 50 μm, more preferably 0.5 to 45 μm, and even more preferably 30 to 45 μm. In the present invention, the thickness of the polyimide layer is a value determined by measuring the distance (thickness) of the polyimide layer present on the surface of the polyamide hollow fiber membrane at 10 regularly spaced points in a scanning electron microscope (SEM) photograph of the cross section of the composite hollow fiber membrane at a magnification of 1000 to 5000 times, and calculating the average value.

[0041] The polyimide layer may be composed only of a dense layer, but from the viewpoint of further improving the gas permeation rate, it is preferable that the polyimide layer include a dense layer and a porous layer. The thickness of the dense layer in the polyimide layer is not particularly limited, but from the viewpoint of more satisfactorily achieving both improved gas separation performance by suppressing the occurrence of defects and good gas permeability, it is preferably 0.01 to 5 μm, more preferably 0.05 to 1 μm, and even more preferably 0.05 to 0.5 μm. In the present invention, the thickness of the dense layer in the polyimide layer is a value determined by measuring the distance (thickness) of non-porous regions included in the polyimide layer at 10 regularly spaced locations in a scanning electron microscope (SEM) photograph of the cross section of the composite hollow fiber membrane at a magnification of 10,000 times and calculating the average value.

[0042] 3. Manufacturing method of composite hollow fiber membrane The method for producing the composite hollow fiber membrane of the present invention is not particularly limited as long as it can produce a composite hollow fiber membrane having the above structure, but a suitable example is a production method including the following first to fourth steps. First step: A membrane-forming solution is prepared by dissolving polyamide resin at a concentration of 20% by weight or more in an organic solvent that has a boiling point of 150°C or higher and is incompatible with polyamide resin at temperatures below 100°C, at a temperature of 100°C or higher. Second step: A step of solidifying the polyamide resin into a membrane by extruding the membrane-forming solution in a predetermined shape into a coagulation bath at 100°C or less. In this step, a coagulation liquid compatible with the organic solvent used in the membrane-forming solution is brought into contact with at least one surface of the membrane-forming solution extruded in the predetermined shape, thereby forming a polyamide hollow fiber membrane. Third step: The membrane forming solution solvent and the coagulation solution are removed from the polyamide hollow fiber membrane formed in the second step. Fourth step: A polyimide layer is formed on at least one surface of the polyamide hollow fiber membrane obtained by carrying out the third step.

[0043] [Formation of polyamide hollow fiber membrane] Hereinafter, the first to third steps for forming the polyamide hollow fiber membrane will be described in detail for each step.

[0044] [1st step] In the first step, a film-forming solution is prepared by dissolving polyamide resin at a concentration of 20% by weight or more in an organic solvent having a boiling point of 150°C or higher and which is incompatible with polyamide resin at temperatures below 100°C at a temperature of 100°C or higher.

[0045] Examples of organic solvents that have a boiling point of 150°C or higher and are incompatible with polyamide resins at temperatures below 100°C include aprotic polar solvents, glycerin ethers, polyhydric alcohols, organic acids and organic acid esters, and higher alcohols. Specific examples of aprotic polar solvents include sulfolane, dimethyl sulfone, dimethyl sulfoxide, γ-butyrolactone, δ-valerolactone, ε-caprolactone, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, ethylene carbonate, and propylene carbonate. Specific examples of glycerin ethers include diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, diethylene glycol dibutyl ether, and tetraethylene glycol dimethyl ether. Specific examples of polyhydric alcohols include glycerin, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, hexylene glycol, 1,3-butanediol, polyethylene glycol (molecular weight 100 to 10,000), etc. Specific examples of organic acids and organic acid esters include dimethyl phthalate, diethyl phthalate, diisopropyl phthalate, dibutyl phthalate, butyl benzyl phthalate, methyl salicylate, oleic acid, palmitic acid, stearic acid, lauric acid, etc. Among these organic solvents, from the viewpoint of obtaining a polyamide hollow fiber membrane with higher strength, preferred are aprotic polar solvents and polyhydric alcohols; more preferred are sulfolane, dimethyl sulfone, γ-butyrolactone, δ-valerolactone, ε-caprolactone, propylene glycol, hexylene glycol, 1,3-butanediol, and polyethylene glycol (molecular weight 100 to 600); even more preferred are sulfolane, dimethyl sulfone, γ-butyrolactone, δ-valerolactone, and ε-caprolactone; and even more preferred are dimethyl sulfone. These organic solvents may be used alone or in combination of two or more.Although sufficient effects can be obtained by using one of these organic solvents alone, a more effective polyamide hollow fiber membrane can sometimes be produced by mixing two or more of them due to differences in the order and structure of phase separation.

[0046] The concentration of the polyamide resin in the membrane-forming solution may be 20% by weight or more, preferably 23 to 50% by weight, more preferably 25 to 38% by weight, and even more preferably 28 to 35% by weight. By ensuring that the concentration of the polyamide resin in the membrane-forming solution is within this range, the porosity of the central portion of the polyamide hollow fiber membrane is more likely to fall within the above-mentioned specified range, making membrane production easier and increasing the membrane strength.

[0047] Furthermore, in the first step, when dissolving the polyamide resin in the organic solvent, it is necessary to maintain the temperature of the solvent at 100°C or higher. Specifically, it is desirable to dissolve the polyamide resin at a temperature 10 to 50°C higher, preferably 20 to 40°C higher, than the phase separation temperature of the membrane-forming solution to be prepared. The phase separation temperature of the membrane-forming solution refers to the temperature at which liquid-liquid phase separation or solid-liquid phase separation due to crystal precipitation occurs when a mixture of the polyamide resin and the organic solvent at a sufficiently high temperature is gradually cooled. The phase separation temperature can be measured using a microscope equipped with a hot stage, etc.

[0048] In the first step, the temperature conditions for dissolving the polyamide resin in the organic solvent may be appropriately set in the temperature range of 100°C or higher in accordance with the above-mentioned index depending on the type of polyamide resin and the type of organic solvent used, but are preferably 120 to 250°C, more preferably 140 to 220°C, and even more preferably 160 to 200°C.

[0049] Furthermore, the membrane-forming solution may contain, as necessary, fillers, thickeners, antioxidants, surface modifiers, lubricants, surfactants, etc., in order to control the pore size of the polyamide hollow fiber membrane or improve its performance.

[0050] The membrane-forming solution prepared in the first step is supplied to the second step at the same temperature (ie, at 100° C. or higher).

[0051] [Second process] The second step is a step of solidifying the polyamide resin into a membrane by extruding the membrane-forming solution prepared in the first step into a coagulation bath at 100°C or less in a predetermined shape. In this step, a coagulation liquid compatible with the organic solvent used in the membrane-forming solution is brought into contact with at least one surface of the membrane-forming solution extruded in a predetermined shape, thereby forming a polyamide hollow fiber membrane having a porous layer alone or a hollow fiber shape having both a dense layer and a porous layer.

[0052] In the case where the porosity of one of the innermost and outermost portions of a polyamide hollow fiber membrane is lower than the porosity of the central portion of the polyamide hollow fiber membrane and no dense layer is present on that one surface, or where the porosity of one of the innermost and outermost portions is lower than the porosity of the central portion of the polyamide hollow fiber membrane and a dense layer is present on that one surface, in the second step, the one surface is contacted with a coagulation liquid containing a coagulation liquid that is compatible with the organic solvent used in the membrane-forming solution but has a low affinity for the polyamide resin, and the other surface is contacted with a coagulation liquid that is compatible with the organic solvent used in the membrane-forming solution but has a high affinity for the polyamide resin. In addition, in the case of a polyamide hollow fiber membrane in which the porosity of both the innermost and outermost parts is lower than the porosity of the central part of the polyamide hollow fiber membrane and no dense layer is present on the surface, or in the case in which the porosity of both the innermost and outermost parts is lower than the porosity of the central part of the polyamide hollow fiber membrane and a dense layer is present on the surface, in the second step, both surfaces may be contacted with a coagulation liquid containing a coagulation liquid that is compatible with the organic solvent used in the membrane-forming solution but has low affinity for the polyamide resin.

[0053] Specifically, the coagulation liquid that is compatible with the organic solvent used in the membrane-forming solution but has low affinity for the polyamide resin is a solvent that is compatible with the organic solvent used in the membrane-forming solution at temperatures of 25° C. or lower but does not dissolve the polyamide resin at temperatures below the boiling point or 200° C. Specific examples of the coagulation liquid that is compatible with the organic solvent used in the membrane-forming solution but has low affinity for the polyamide resin include aqueous solvents such as water and aqueous solutions with a water content of 80% by weight or higher; monohydric alcohols such as 1-propanol, 2-propanol, and isobutanol; glycol ethers such as polyethylene glycols with an average molecular weight of 300 or higher, polypropylene glycols with an average molecular weight of 400 or higher, diethylene glycol diethyl ether, triethylene glycol monomethyl ether, and propylene glycol monoethyl ether; and glycol acetates such as triacetin and propylene glycol monoethyl ether acetate. Among these, polyethylene glycol, triacetin, and triethylene glycol monomethyl ether, each having an average molecular weight of 300 to 600, are preferred; polyethylene glycol 300, polyethylene glycol 400, and polyethylene glycol 600 are more preferred; and polyethylene glycol 300 and polyethylene glycol 400 are even more preferred. These solvents may be used alone or in combination of two or more. In the present invention, the average molecular weight of polyethylene glycol and polypropylene glycol is a number-average molecular weight calculated based on the hydroxyl value measured in accordance with JIS K 1557-6:2009 "Plastics - Polyurethane raw polyols - Test methods - Part 6: Determination of hydroxyl value by near-infrared (NIR) spectroscopy."

[0054] Furthermore, by adding a solvent used in the coagulation liquid described below that is compatible with the organic solvent used in the membrane-forming solution but has a low affinity for the polyamide resin (a solvent that is compatible with the organic solvent used in the membrane-forming solution at a temperature of 25°C or less and dissolves the polyamide resin at a temperature of the boiling point or less) to the coagulation liquid that is compatible with the organic solvent used in the membrane-forming solution but has a low affinity for the polyamide resin, the porosity of the innermost and / or outermost parts can be made lower than the porosity of the central part of the polyamide hollow fiber membrane, and no dense layer can be formed on the surface of the surface part with the lower porosity. When a solvent (preferably at least one selected from the group consisting of glycerin, diglycerin, 1,3-butanediol, 1,4-butanediol, diethylene glycol, tetraethylene glycol, and polyethylene glycol 200) is contained in the coagulation liquid, the content of the solvent is preferably 10 to 95% by weight, more preferably 20 to 85% by weight, and even more preferably 40 to 80% by weight, from the viewpoint of maintaining the porous layer, although it depends on the combination of solvents.

[0055] The coagulation liquid that is compatible with the organic solvent used in the membrane-forming solution and has a high affinity for the polyamide resin may be any solvent that is compatible with the organic solvent used in the membrane-forming solution at a temperature of 25° C. or lower and dissolves the polyamide resin at a temperature of the boiling point or lower. Specific examples of the coagulation liquid that is compatible with the organic solvent used in the membrane-forming solution and has a high affinity for the polyamide resin include glycerin, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol 200, propylene glycol, 1,3-butanediol, 1,4-butanediol, sulfolane, N-methyl-2-pyrrolidone, γ-butyrolactone, and δ-valerolactone. Among these, from the viewpoint of obtaining a polyamide hollow fiber membrane having the above-mentioned porosity, preferably, at least one selected from the group consisting of 1,4-butanediol, 1,3-butanediol, glycerin, propylene glycol, diethylene glycol, tetraethylene glycol, and polyethylene glycol 200 is used; more preferably, at least one selected from the group consisting of 1,4-butanediol, glycerin, propylene glycol, diethylene glycol, and tetraethylene glycol.

[0056] In the second step, to form a polyamide hollow fiber membrane, a double-tube nozzle for hollow fiber production having a double-tube structure is used, and the membrane-forming solution is discharged from the outer annular nozzle and the internal coagulation solution is discharged from the inner nozzle, followed by immersion in a coagulation bath. In this case, when the porosity of at least one of the innermost and outermost portions of the polyamide hollow fiber membrane is lower than that of the central portion of the polyamide hollow fiber membrane and no dense layer is formed on the one surface, or when the porosity of at least one of the innermost and outermost portions is lower than that of the central portion of the polyamide hollow fiber membrane and a dense layer is formed on the one surface, a coagulation solution that is compatible with the organic solvent used in the membrane-forming solution but has low affinity for the polyamide resin can be used in the one surface side of the internal coagulation solution or the coagulation bath. In the case where the porosity of both the innermost and outermost parts of a polyamide hollow fiber membrane is lower than the porosity of the central part of the polyamide hollow fiber membrane and no dense layer is present on one of the surfaces, or in the case where the porosity of both the innermost and outermost parts is lower than the porosity of the central part of the polyamide hollow fiber membrane and a dense layer is present on one of the surfaces, it is sufficient to use, for both the inner coagulation liquid and the coagulation bath, a coagulation liquid that is compatible with the organic solvent used in the membrane-forming solution but has low affinity with the polyamide resin. Furthermore, when a coagulation liquid compatible with the organic solvent used in the membrane-forming solution but with low affinity for the polyamide resin is used as the internal coagulation liquid, and a coagulation bath using a coagulation liquid compatible with the organic solvent used in the membrane-forming solution but with high affinity for the polyamide resin, a polyamide hollow fiber membrane is obtained in which the porosity of the innermost part is lower than the porosity of the central part of the polyamide hollow fiber membrane and no dense layer is present on the lumen-side surface, or the porosity of the innermost part is lower than the porosity of the central part of the polyamide hollow fiber membrane and a dense layer is present on the lumen-side surface, and the central part and outer surface are porous layers.Furthermore, when a coagulation liquid compatible with the organic solvent used in the membrane-forming solution and having a high affinity for the polyamide resin is used as the internal coagulation liquid, and a coagulation bath compatible with the organic solvent used in the membrane-forming solution and having a low affinity for the polyamide resin is used, a polyamide hollow fiber membrane can be obtained in which the porosity of the outermost part is lower than that of the central part of the polyamide hollow fiber membrane and no dense layer is present on the outer surface, or the porosity of the outermost part is lower than that of the central part of the polyamide hollow fiber membrane and a dense layer is present on the outer surface, and the central part and the lumen-side surface are porous layers. Note that the internal coagulation liquid used in forming the polyamide hollow fiber membrane passes through a double annular nozzle, so it is preferable that it does not contain water whose boiling point is equal to or lower than the temperature of the double annular nozzle.

[0057] The double-tubular nozzle for producing hollow fibers may be a spinneret having a double-tubular structure such as that used for producing core-sheath composite fibers in melt spinning. The diameters of the outer annular nozzle and the inner nozzle of the double-tubular nozzle for producing hollow fibers may be appropriately set depending on the inner and outer diameters of the polyamide hollow fiber membrane.

[0058] The flow rate of the membrane-forming solution discharged from the outer annular nozzle of the hollow fiber production double-tubular nozzle is not particularly limited because it depends on the slit width, but may be, for example, 2 to 30 g / min, preferably 3 to 20 g / min, and more preferably 5 to 15 g / min. The flow rate of the internal coagulation liquid is appropriately set taking into consideration the diameter of the inner nozzle of the hollow fiber production double-tubular nozzle, the type of internal liquid used, the flow rate of the membrane-forming solution, etc., but may 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 membrane-forming solution.

[0059] In the second step, the temperature of the coagulation bath may be 100°C or lower, preferably -20 to 100°C, more preferably 0 to 60°C, even more preferably 2 to 20°C, and particularly preferably 2 to 10°C. The suitable temperature of the coagulation bath varies depending on the organic solvent used in the membrane-forming solution, the composition of the coagulation solution, etc., but generally, lower temperatures tend to favor thermally induced phase separation, while higher temperatures tend to favor non-solvent phase separation. That is, when producing a polyamide hollow fiber membrane having a dense layer formed on the lumen-side surface, it is preferable to set the coagulation bath at a low temperature to increase the pore size of the dense layer on the lumen-side surface, and it is preferable to set the coagulation bath at a high temperature to make the dense layer on the lumen-side surface denser and the internal structure coarser.

[0060] The temperature of the internal coagulation liquid may be about the set temperature of the double tubular nozzle, for example, 120 to 250°C, preferably 160 to 230°C, and more preferably 180 to 220°C.

[0061] By carrying out the second step in this manner, a polyamide hollow fiber membrane having a porous layer is formed.

[0062] [3rd step] In the third step, the membrane forming solution solvent and the coagulation solution are removed from the polyamide hollow fiber membrane formed in the second step.

[0063] The method for removing the membrane-forming solution solvent and the coagulation solution from the polyamide hollow fiber membrane is not particularly limited, but a method in which the polyamide hollow fiber membrane formed in the second step is immersed in an extraction solvent to extract and remove them is preferred.

[0064] The extraction solvent used for extraction and removal is preferably one that is inexpensive, has a low boiling point, and can be easily separated after extraction due to differences in boiling points, such as water, glycerin, methanol, ethanol, isopropanol, acetone, diethyl ether, hexane, petroleum ether, and toluene. Among these, water, methanol, ethanol, isopropanol, and acetone are preferred; water, methanol, and isopropanol are more preferred. In particular, when extracting a water-soluble membrane-forming solution solvent and a coagulation solution, winding the solution while showering it with water allows for simultaneous solvent extraction, which is efficient. Furthermore, isopropyl alcohol, petroleum ether, and the like are suitable for extracting water-insoluble organic solvents such as phthalate esters and fatty acids.

[0065] When the membrane-forming solution solvent and the coagulation solution are extracted and removed by immersing the polyamide hollow fiber membrane in the extraction solvent, the time for immersing the polyamide hollow fiber membrane in the extraction solvent is not particularly limited, but may be, for example, 0.2 hours to 2 months, preferably 0.5 hours to 1 month, and more preferably 2 hours to 10 days. In order to effectively extract and remove the coagulation solution remaining in the polyamide hollow fiber membrane, the extraction solvent may be replaced or stirred.

[0066] By carrying out the third step in this manner, a polyamide hollow fiber membrane having a porous layer is obtained from which the membrane-forming solution solvent and the coagulation solution have been removed.

[0067] After the third step, it is preferable to dry and remove the extraction solvent from the polyamide hollow fiber membrane, which can be done by known drying methods such as natural drying, hot air drying, reduced pressure drying, and vacuum drying.

[0068] In order to increase the strength of the polyamide hollow fiber membrane and provide it with excellent pressure resistance, the polyamide hollow fiber membrane may be stretched in one axial direction (longitudinal direction) simultaneously with or after drying.

[0069] To simultaneously dry and stretch the polyamide hollow fiber membrane in the uniaxial direction, the membrane may be dried while tension for stretching is applied to the membrane. The temperature conditions for simultaneously drying and stretching the membrane in the uniaxial direction are not particularly limited as long as both drying and stretching are possible, and may be, for example, 40°C or higher, preferably 40 to 160°C, more preferably 50 to 140°C, and even more preferably 120 to 140°C.

[0070] Furthermore, when stretching in the uniaxial direction after drying, the temperature conditions during drying are not particularly limited as long as the adhering extraction solvent can be evaporated, and examples include 40° C. or higher, preferably 40 to 160° C., more preferably 50 to 140° C., and even more preferably 120 to 140° C. Furthermore, when stretching in the uniaxial direction after drying, the temperature conditions during stretching are not particularly limited as long as it is −10 to 140° C., preferably 0 to 120° C., but from the viewpoint of further improving the liquid permeability, it is desirable that the temperature be above the glass transition point of the polyamide resin used (more preferably 50 to 120° C., and even more preferably 60 to 100° C.).

[0071] The uniaxial stretching may be carried out by a known method, for example, by continuously winding the membrane from a low-speed roll to a high-speed roll. Alternatively, the membrane may be cut to a predetermined length and stretched by holding both ends of the membrane using a tensile tester or the like, or by manual stretching.

[0072] The stretching ratio is, for example, 1.2 to 5, preferably 1.2 to 3. From the viewpoint of increasing the strength of the polyamide hollow fiber membrane and imparting excellent pressure resistance, the stretching ratio is preferably 1.2 to 2.4, more preferably 1.2 to 2.0.

[0073] [Polyimide layer formation] [4th step] The fourth step for forming a polyimide layer on at least one surface of the polyamide hollow fiber membrane obtained by carrying out the third step will be described in detail below.

[0074] The method for forming the polyimide layer is roughly divided into a method for forming a polyimide layer consisting of only a dense layer and a method for forming a polyimide layer including a dense layer and a porous layer.

[0075] The method for forming a polyimide layer consisting of only a dense layer is not particularly limited as long as it can provide a polyimide layer having the above-described structure and properties. Preferred examples include the following forming methods.

[0076] A method for forming a polyimide layer consisting only of a dense layer on the lumen surface of a polyamide hollow fiber membrane includes, for example, injecting a polyimide coating liquid into the hollow portion of the polyamide hollow fiber membrane to form a polyimide coating film on the lumen surface, and then volatilizing the solvent in the polyimide coating film to form a polyimide layer.

[0077] The polyimide coating solution is prepared by dissolving a solvent-soluble polyimide in an organic solvent. Examples of commercially available solvent-soluble polyimides include Evonik's Polyimide P84 (registered trademark) and Huntsman's Matrimid (registered trademark).

[0078] The solvent for dissolving polyimide is not particularly limited as long as it dissolves the polyimide at the desired concentration. Generally, polyimide has high solubility in solvents having a certain level of polarity or higher. Examples of such solvents include dimethylformamide, N-methyl-2-pyrrolidinone (NMP), dimethylacetamide, methylene chloride, tetrahydrofuran (THF), chloroform, and dioxane.

[0079] The concentration of the coating liquid needs to be adjusted appropriately depending on the viscosity. However, from the viewpoints of maintaining a polyimide layer at a desired thickness on the surface of the polyamide hollow fiber membrane, preventing defects caused by the polyimide layer becoming too thin, preventing unevenness in the thickness of the polyimide layer, and workability, when forming a polyimide layer consisting only of a dense layer, the polyimide concentration in the polyimide coating liquid is preferably 0.5 to 25% by weight, more preferably 1 to 20% by weight, and even more preferably 3 to 8% by weight.

[0080] The method for injecting a polyimide coating liquid into the hollow portion of a polyamide hollow fiber membrane is not particularly limited as long as it can form a polyimide layer of the desired thickness on the lumen-side surface of the polyamide hollow fiber membrane. However, a preferred method involves forming a module using polyamide hollow fiber membranes and injecting the polyimide coating liquid into the hollow portion of the polyamide hollow fiber membranes of the module. Specifically, this injection method is performed as follows: First, a desired number of polyamide hollow fiber membranes of a desired length are bundled together. Next, a hard tube is prepared, and a rubber stopper or the like of an appropriate length is inserted into one end opening of the tube to plug the end opening. Next, a two-component thermosetting resin is injected through the end opening opposite the plugged end of the tube to fill the inner space of the tube with the resin. One end of the bundled polyamide hollow fiber membranes is then heat-sealed to seal the opening, and the membrane is inserted into the tube filled with the thermosetting resin until the tip of the membrane touches the plug. The thermosetting resin is then cured in this state. Next, the area of ​​the hardened resin on the plug side is cut together with the tube to open the hollow part of the polyamide hollow fiber membrane. After that, an appropriate jig is attached to the tube on the open side and a polyimide coating liquid is injected.

[0081] The injection method is not particularly limited, and examples thereof include an injection method using a syringe, a pressure-feeding method using pressurized gas, and an injection method using a pump such as a peristaltic pump, a plunger pump, a diaphragm pump, or a gear pump.

[0082] The speed at which the polyimide coating liquid is injected is not particularly limited, but is, for example, about 0.04 to 2.5 m / sec as a linear velocity, and from the viewpoints of suppressing defects in the polyimide layer, suppressing unevenness in the thickness of the polyimide layer, and improving production efficiency, the linear velocity is preferably 0.1 to 1.0 m / sec, and more preferably 0.1 to 0.5 m / sec.

[0083] In order to form a polyimide layer of uniform thickness on the surface of the polyamide hollow fiber membrane, it is preferable to blow a gas into the hollow portion of the polyamide hollow fiber membrane after injecting the polyimide coating solution. The gas used for blowing is not particularly limited, but dry air or nitrogen is preferred from the viewpoint of preventing reaction with the polyimide coating solution and from the viewpoint of safety.

[0084] The blowing speed is not particularly limited, but is, for example, about 1 to 50 m / sec as a linear speed, and is preferably 2.5 to 20 m / sec, more preferably 5 to 10 m / sec, from the viewpoint of suppressing defects in the polyimide layer, suppressing unevenness in the thickness of the polyimide layer, and from the viewpoint of production efficiency.

[0085] In order to volatilize the solvent from the polyimide layer formed on the surface of the polyamide hollow fiber membrane, it is preferable to heat-dry the polyamide hollow fiber membrane into which the polyimide coating solution has been injected. The heating temperature is not particularly limited, but from the viewpoint of further promoting volatilization and suppressing deterioration of the physical properties of the polyamide hollow fiber membrane due to heat, it is preferably 40 to 160°C, more preferably 60 to 140°C, and even more preferably 80 to 120°C. The heating time is not particularly limited, but from the above viewpoints, it is preferably 10 minutes to 24 hours, more preferably 15 minutes to 16 hours, even more preferably 30 minutes to 8 hours, and even more preferably 1 to 5 hours. Furthermore, it is preferable to promote solvent volatilization under reduced pressure.

[0086] Furthermore, methods for forming a polyimide layer consisting only of a dense layer on the outer surface of a polyamide hollow fiber membrane include, for example, immersing the polyamide hollow fiber membrane in a bath of the polyimide coating liquid described in the method for forming a polyimide layer consisting only of a dense layer on the lumen surface, or applying the polyimide coating liquid to the outer surface of the polyamide hollow fiber membrane to form a polyimide coating film on the surface, and then volatilizing the solvent to form a polyimide layer. In order to form a polyimide coating film with a uniform thickness, the gas may be blown onto the outer surface of the polyamide hollow fiber membrane. Furthermore, in order to volatilize the solvent from the polyimide layer formed on the membrane surface, the polyamide hollow fiber membrane to which the polyimide coating liquid has been applied may be subjected to a heat-drying treatment under the conditions described in the method for forming a polyimide layer consisting only of a dense layer on the lumen surface.

[0087] Next, the method for forming a polyimide layer including a dense layer and a porous layer on the surface of a polyamide hollow fiber membrane is not particularly limited as long as a polyimide layer having the above-described structure and properties can be obtained. A suitable example is the phase separation method described below.

[0088] The polyimide and organic solvent used are the same as those used when forming a polyimide layer consisting of only a dense layer. However, the concentration of the polyimide coating liquid is preferably an amount such that the concentration of polyimide in the polyimide coating liquid is 5 to 25% by weight, more preferably 8 to 22% by weight, and even more preferably 12 to 20% by weight, from the viewpoint of preventing the dense layer of the polyimide layer from becoming too thin after phase separation and causing defects, and from the viewpoint of maintaining the structure of the polyimide layer.

[0089] A polyimide layer including a dense layer and a porous layer can be formed on the outer surface of a polyamide hollow fiber membrane by, for example, immersing the polyamide hollow fiber membrane in a bath of the polyimide coating solution, or by applying the polyimide coating solution to the outer surface of the polyamide hollow fiber membrane and then contacting the membrane with a coagulation bath containing a solvent that is incompatible with polyimide but compatible with the solvent in which the polyimide is dissolved. Water or alcohol is preferred as the coagulation bath, with water being the most preferred. To form a polyimide layer with a uniform thickness, the gas may be blown onto the outer surface of the polyamide hollow fiber membrane.

[0090] When a polyimide layer including a dense layer and a porous layer is formed on the outer surface of a polyamide hollow fiber membrane, the solvent used to prepare the polyimide coating solution is preferably removed by immersion in an extraction solvent. The solvent used in the coagulation bath may be used as the extraction solvent for the extraction removal.

[0091] Furthermore, it is preferable to remove the extraction solvent by drying, which can be performed by known drying methods such as natural drying, hot air drying, reduced pressure drying, and vacuum drying.

[0092] A method for forming a polyimide layer consisting of a dense layer and a porous layer on the inner surface of a polyamide hollow fiber membrane is, for example, similar to the method for forming a polyimide layer consisting of only a dense layer on the lumen surface of a polyamide hollow fiber membrane, by injecting a polyimide coating liquid into the hollow portion of the polyamide hollow fiber membrane, blowing a gas into the hollow portion of the polyamide hollow fiber membrane as needed, sealing both ends of the polyamide hollow fiber membrane by thermocompression or the like, and then contacting the outer surface of the polyamide hollow fiber membrane through the pores of the polyamide hollow fiber membrane with a coagulation bath consisting of a solvent that is incompatible with polyimide but compatible with the solvent in which the polyimide is dissolved, preferably water or alcohol, with water being most preferred.

[0093] By carrying out the fourth step in this manner, the composite hollow fiber membrane of the present invention having a polyimide layer on at least one surface of the polyamide hollow fiber membrane can be obtained.

[0094] 4. Hollow fiber membrane module The composite hollow fiber membrane of the present invention is housed in a module case equipped with an inlet for the gas to be treated, an outlet for the permeated gas, etc., and used as a composite hollow fiber membrane module.

[0095] Specifically, the composite hollow fiber membrane module may have a structure in which the composite hollow fiber membranes of the present invention are bundled and housed in a module case, and one or both ends of the composite hollow fiber membranes are sealed and fixed with a potting agent. The composite hollow fiber membrane module may have an opening connected to a flow path passing through the outer wall surface side of the composite hollow fiber membrane and an opening connected to the hollow portion of the composite hollow fiber membrane, as an inlet for the gas to be treated or an outlet for the permeated gas.

[0096] The shape of the composite hollow fiber membrane module is not particularly limited, and may be a dead-end module or a cross-flow module, but a cross-flow module is preferable for gas separation membranes that separate only specific gases using a pressure difference (concentration difference) as a driving source.Specific examples include cross-flow modules in which a hollow fiber membrane bundle is packed straight, both ends of the hollow fiber membrane bundle are sealed, and the seals at both ends of the hollow fiber membrane bundle are cut to create two flow paths on the side of the filter case.

[0097] The packing rate of the composite hollow fiber membranes inserted into the module case is not particularly limited, but for example, the volume of the composite hollow fiber membranes, including the volume of the hollow portions, relative to the internal volume of the module case is preferably 15 to 75 volume %, more preferably 25 to 65 volume %, and even more preferably 35 to 55 volume %. By satisfying such a packing rate, a sufficient filtration area can be ensured, and the operation of packing the composite hollow fiber membranes into the module case can be facilitated, allowing the potting agent to easily flow between the composite hollow fiber membranes.

[0098] The potting agent used in the manufacture of a composite hollow fiber membrane module is not particularly limited, but when the composite hollow fiber membrane module is used for gas separation, it is preferable to use one that is not too hard so that fluctuations during operation can be absorbed. Examples of such potting agents include polyamide, silicone resin, urethane resin, epoxy resin, melamine resin, polyethylene, polypropylene, phenolic resin, polyimide, and polyurea resin. Among these potting agents, those that shrink or swell little when cured are preferred, and suitable examples include polyamide, silicone resin, epoxy resin, polyethylene, and urethane resin. These potting agents may be used alone or in combination of two or more.

[0099] The material of the module case used in the composite hollow fiber membrane module is not particularly limited, and examples thereof include stainless steel, polyamide, polyester, polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl chloride, polysulfone, polyethersulfone, polycarbonate, polyarylate, polyphenylene sulfide, etc. Among these, preferred are stainless steel, polyamide, polyethylene, polypropylene, polytetrafluoroethylene, polycarbonate, polysulfone, and polyethersulfone, and more preferred are stainless steel, polyamide, polyethylene, polypropylene, and polytetrafluoroethylene. [Example]

[0100] The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0101] 1.Measurement method [Outer and inner diameters of polyamide hollow fiber membrane] The five composite hollow fiber membranes were observed under an optical microscope at a magnification of 200 times, and the outer and inner diameters of the polyamide hollow fiber membranes constituting the composite hollow fiber membranes were measured, and the average values ​​were calculated.

[0102] [Porosity of polyamide hollow fiber membrane] Scanning electron microscope (SEM) photographs were taken at five locations equidistantly spaced in the thickness direction of the polyamide hollow fiber membrane in a cross section obtained by cutting the composite hollow fiber membrane perpendicular to the longitudinal direction. Specifically, the photographs were taken as follows. Fig. 1 is a schematic cross-sectional view of a polyamide hollow fiber membrane cut perpendicular to the longitudinal direction, and Figs. 2 and 3 are partial enlarged views of the area surrounded by the dotted line in Fig. 1. As shown in Fig. 2, a line was drawn from the center of the lumen of the hollow fiber membrane to the outer surface, and this line was divided into five equal parts from the lumen surface to the outer surface. Then, as shown in Fig. 2, the midpoint of the five equally divided lines was determined. Then, as shown in Fig. 3, photographs were taken at five locations at a uniform magnification such that the midpoint was at the center of the photographed image, only the hollow fiber membrane portion was included in the photographed image, and the number of pores was set to 30 to 300. Image analysis software (ImageJ) was used to analyze the images of three of the five SEM photographs: the outermost (region A in Figure 3), innermost (region C in Figure 3), and central (region B in Figure 3). The pore and polymer regions were distinguished by binarization, and the area ratio (%) of the total pore area to the analyzed region was calculated. This was measured at three different locations on the cross section of the hollow fiber membrane, and the average value of the three locations was calculated to represent the average porosity of the outermost, innermost, and central regions. The specific ImageJ procedure was as follows: The image to be analyzed was imported into ImageJ, the analysis range was specified, and the brightness value at which the histogram peak, obtained by the "Analyze > Histogram" operation, was set as the threshold (i.e., the lower threshold level was set to the brightness value at which the histogram peak was highest, and the upper threshold level was set to 255). The image was then binarized using "Image > Adjust > Threshold."Next, check "Area" in "Analyze>Set measurements" and calculate the total area of ​​the analysis range by operating "Analyze>Measure." Then, check "Area" and "Limit to threshold" in "Analyze>Set measurements" and operate "Analyze>Measure" again to calculate the area of ​​the polymer and void parts. The area ratio (%) of the void parts to the total area was defined as the porosity.

[0103] [Polyimide layer thickness] The cross section of the composite hollow fiber membrane cut perpendicular to the longitudinal direction was observed with a scanning electron microscope (SEM) at a magnification of 1000 to 5000. In the obtained SEM photograph, the distance (thickness) of the polyimide layer present on the surface of the polyamide hollow fiber membrane was measured at 10 equally spaced locations, and the average value was calculated.

[0104] [Dense layer thickness] A cross section of the composite hollow fiber membrane cut perpendicular to the longitudinal direction was observed with a scanning electron microscope (SEM) at a magnification of 10,000. In the obtained SEM photograph, the distance (thickness) of the region where the polyamide hollow fiber membrane and the polyimide layer constituting the composite hollow fiber membrane were substantially free of pores was measured at 10 equally spaced locations, and the average value was calculated.

[0105] [Test to confirm whether composite hollow fiber membranes will break when high-pressure gas is supplied after being twisted] Twenty of the composite hollow fiber membranes of Examples 1 to 6 and the hollow fiber membrane of Comparative Example 1 were cut into 12 cm lengths, formed into membrane bundles, and then heat-pressed at one end. The bundles were then housed in a case consisting of 9 mm inner diameter, 4 cm long, and 8 cm long nylon tubes connected by a silicone tube, with the crimped end facing the 4 cm nylon tube. A 10 mL plastic cup was filled with 5 g of urethane resin, and the case containing the membrane bundle was immersed with the crimped end in the urethane resin. The case was then attached to a swing-type centrifuge so that the centrifugal force was directed toward the cup, and centrifuged at 100 rpm for 120 minutes to cure the resin. The plastic cup was then removed, and the crimped portion was cut off along with the nylon tube from the end of the cured urethane resin, exposing the hollow fiber membrane and forming an opening. The 8-cm nylon tube was then removed, and a measurement module was obtained in which the hollow fiber membrane bundle was fixed with urethane resin within the nylon tube over a certain length from the opening, with the hollow fiber membrane bundle exposed from the end of the nylon tube opposite the opening without being cured with urethane resin. Next, the end of the hollow fiber membrane bundle in the longitudinal direction where the hollow fiber membrane bundle was exposed was grasped, and with the measurement module suspended, the nylon tube was rotated once around the circumference of the composite hollow fiber membrane to apply a twist, returned to its original position, and then rotated once more in the opposite direction to apply a twist. The grasped end was then released, thermocompressed, and sealed with urethane resin. An air pressure of 1.0 MPa was then applied to the opening on the nylon tube side for 30 seconds, after which the 20 hollow fiber membranes were visually observed for tears or breaks. The above procedure was performed on 30 measurement modules, and if there was one or more breaks or ruptures in the composite hollow fiber membrane per measurement module, it was deemed to be defective, and the number of measurement modules judged to be defective was counted. A measurement module with five or fewer defective modules was deemed to be pass.

[0106] 2. Test Example [Example 1] 280 g of polyamide 6 chips (Unitika Ltd., A1030BRT, relative viscosity 3.53) and 720 g of dimethyl sulfone (Tokyo Chemical Industry Co., Ltd.) were dissolved by stirring at 190 °C for 1.5 hours, then the stirring speed was reduced and degassed for 1 hour to prepare a membrane-forming solution. A polyamide hollow fiber membrane production apparatus was used, using a spinneret with a double-tube structure that discharged the membrane-forming solution from an outer annular nozzle and an internal coagulation solution from an inner nozzle. The membrane-forming solution was delivered via a metering pump to a spinneret maintained at 200 °C and extruded using a gear pump. The internal coagulation solution was a mixture of 50 wt% polyethylene glycol 300 (PEG300) and 50 wt% glycerin. The extruded membrane-forming solution was poured into a coagulation bath containing 60 wt% 1,4-butylene glycol (1,4BG) aqueous solution at 5 °C and cooled to solidify, forming a polyamide porous membrane. The wound polyamide porous membrane was immersed in water for 24 hours for solvent extraction (washing), and then dried by passing through a hot air dryer (cabinet temperature 130°C) without stretching to obtain a polyamide hollow fiber membrane. The resulting polyamide hollow fiber membrane was cut to a length of 50 cm, and both ends were thermocompression bonded. It was then immersed for 2 seconds in a polyimide coating solution bath containing a commercially available polyimide (Huntsuman, Matrimid 5218) dissolved in THF to a concentration of 5 wt %. It was then removed from the coating solution bath and dried at 80°C for 1 hour while hanging to prevent the coating layer from peeling, yielding a composite hollow fiber membrane. A sufficient number of these composite hollow fiber membranes were produced and subjected to each measurement. Table 1 shows the measurement results for each measurement.

[0107] [Example 2] A membrane bundle of 20 polyamide hollow fiber membranes, 60 cm long, was prepared using the same manufacturing method as in Example 1, except that the extrusion rate of the membrane-forming solution was adjusted to obtain the outer and inner diameters listed in Table 1. A nylon hard tube with an inner diameter of 8 mm and a length of 50 mm was prepared, and a rubber stopper approximately 20 mm long was inserted into one end opening of the tube to plug the end opening. Next, a two-component epoxy resin was injected into the opening opposite the rubber stopper to fill the inner space of the tube. One end of the prepared polyamide hollow fiber membrane was then heat-sealed to seal the opening, and the membrane was inserted into the tube filled with epoxy resin until the tip of the end touched the rubber stopper. The epoxy resin was then cured in this state. Next, the region of the cured epoxy resin on the rubber stopper side of the tube was cut, along with the tube, to open the hollow portion of the polyamide hollow fiber membrane, thereby obtaining a module for injecting a polyimide coating solution. The same polyimide coating solution as in Example 1 was filled into a plastic syringe. The syringe was connected to the nylon hard tube side of the polyimide coating solution injection module with a silicone tube, and the polyimide coating solution was extruded into the hollow portion of the polyamide hollow fiber membrane at a linear velocity of 0.16 m / s. Next, the syringe was removed, and nitrogen was blown at a linear velocity of 6.6 m / s, followed by drying at 80°C for 1 hour. The portion of the composite hollow fiber membrane of the injection module where it was embedded with epoxy resin was then cut off together with the nylon hard tube to obtain a composite hollow fiber membrane. A sufficient number of these composite hollow fiber membranes were produced and subjected to each measurement. Table 1 shows the measurement results for each measurement.

[0108] [Example 3] A composite hollow fiber membrane was obtained in the same manner as in Example 1, except that a mixed solution of 70% by weight of PEG300 and 30% by weight of glycerin was used as the internal coagulation liquid, and the extruded membrane-forming solution was poured into a water coagulation bath at 25°C and cooled to solidify. A sufficient number of these composite hollow fiber membranes were produced to perform each measurement, and were subjected to each measurement. Table 1 shows the measurement results for each measurement.

[0109] [Example 4] 350 g of polyamide 6 chips (Unitika Ltd., A1030BRT, relative viscosity 3.53) and 650 g of dimethyl sulfone (Tokyo Chemical Industry Co., Ltd.) were dissolved by stirring at 190 °C for 1.5 hours, then the stirring speed was reduced and degassed for 1 hour to prepare a membrane-forming solution. A spinneret with a double-tube structure was used as the polyamide hollow fiber membrane production apparatus, which discharged the membrane-forming solution from an outer annular nozzle and an internal coagulation solution from an inner nozzle. The membrane-forming solution was delivered via a metering pump to a spinneret maintained at 200 °C and extruded using a gear pump. A mixture of 70 wt% polyethylene glycol 300 (PEG300) and 30 wt% glycerin was used as the internal coagulation solution. The extruded membrane-forming solution was poured into a water coagulation bath at 20 °C and cooled to solidify, forming a polyamide porous membrane. The wound polyamide porous membrane was immersed in water for 24 hours for solvent extraction (washing), and then dried by passing through a hot air dryer (cabinet temperature 130°C) without stretching to obtain a polyamide hollow fiber membrane. The resulting polyamide hollow fiber membrane was cut to a length of 50 cm, and both ends were thermocompression bonded. It was then immersed for 2 seconds in a polyimide coating solution bath containing a commercially available polyimide (Huntsuman, Matrimid 5218) dissolved in THF to a concentration of 5 wt %. It was then removed from the coating solution bath and dried at 80°C for 1 hour while hanging to prevent the coating layer from peeling, yielding a composite hollow fiber membrane. A sufficient number of these composite hollow fiber membranes were produced and subjected to each measurement. Table 1 shows the measurement results for each measurement.

[0110] [Example 5] A polyamide hollow fiber membrane prepared in the same manner as in Example 3 was cut to a length of 50 cm and thermocompression-bonded at both ends. The membrane was then immersed for 2 seconds in a polyimide coating bath containing a commercially available polyimide (Huntsuman, Matrimid 5218) dissolved in DMAc to a concentration of 15 wt %. The membrane was then removed from the coating bath and immediately immersed in a water bath. The DMAc was subsequently desolvated with water for 24 hours, and the membrane was dried at 50°C for 1 hour to obtain a composite hollow fiber membrane. A sufficient number of these composite hollow fiber membranes were produced and subjected to each measurement. Table 1 shows the results of each measurement.

[0111] [Example 6] A composite hollow fiber membrane was produced in the same manner as in Example 4, except that the extrusion rate of the membrane-forming solution was adjusted so that the outer diameter and inner diameter were as shown in Table 1. The composite hollow fiber membrane was produced in the number sufficient for each measurement and subjected to each measurement. Table 1 shows the measurement results for each measurement.

[0112] [Comparative Example 1] Polyimide (Huntsuman, Matrimid 5218) was dissolved in DMAc to a concentration of 15 wt % with stirring at 30°C for 24 hours. Stirring was stopped and the mixture was degassed for 1 hour to prepare a membrane-forming solution. A spinneret with a double-tube structure was used as the polyamide hollow fiber membrane production apparatus, which discharged the membrane-forming solution from an outer annular nozzle and an internal coagulation solution from an inner nozzle. The membrane-forming solution was delivered to the spinneret at room temperature via a metering pump, and water was used as the internal coagulation solution. The extruded membrane-forming solution was poured into a water coagulation bath and cooled to solidify, forming a polyimide porous membrane. The resulting polyimide porous membrane was dried at 50°C for 1 hour to obtain a polyimide hollow fiber membrane. A sufficient number of these hollow fiber membranes were produced and subjected to each measurement. Table 1 shows the measurement results for each measurement.

[0113] [Table 1]

[0114] The composite hollow fiber membranes of Examples 1 to 6 each comprised a polyamide hollow fiber membrane having a porous layer and a polyimide layer having a dense layer, and the polyimide layer was provided on at least one surface of the polyamide hollow fiber membrane. Therefore, when the composite hollow fiber membrane was subjected to torsion and a load from high-pressure gas, the composite hollow fiber membrane was less likely to break or fracture.

[0115] In particular, comparing Example 1 and Example 3, Example 3 has a dense layer formed on at least one of the inner surface or outer surface of the polyamide hollow fiber membrane, and therefore, compared to Example 1, the composite hollow fiber membrane is less likely to break or fracture when subjected to torsion and high-pressure gas loads.

[0116] Furthermore, comparing Example 3 and Example 4, the porosity of the central part of the polyamide hollow fiber membrane in Example 4 was in the range of 63 to 69%, and therefore, compared to Example 3, the composite hollow fiber membrane was even less likely to break or fracture when subjected to torsion and high-pressure gas loads.

Claims

1. A polyamide hollow fiber membrane having a porous layer and a polyimide layer having a dense layer, A composite hollow fiber membrane, wherein the polyimide layer is provided on at least one surface of the polyamide hollow fiber membrane.

2. 2. The composite hollow fiber membrane according to claim 1, wherein the porosity of the innermost and outermost portions of the polyamide hollow fiber membrane is lower than the porosity of the central portion of the polyamide hollow fiber membrane.

3. The composite hollow fiber membrane according to claim 1 , wherein the polyamide hollow fiber membrane has a dense layer.

4. The composite hollow fiber membrane according to claim 1, wherein the porosity of the central portion of the polyamide hollow fiber membrane is 60 to 80%.

5. The composite hollow fiber membrane according to claim 1, wherein the outer diameter of the polyamide hollow fiber membrane is 200 to 600 μm.

6. The composite hollow fiber membrane according to claim 1 , wherein the polyimide layer further comprises a porous layer.

7. A hollow fiber membrane module comprising a module case containing the composite hollow fiber membrane according to any one of claims 1 to 6.

Images