JP7807773B2 - Composite hollow fiber membrane and its manufacturing method - Google Patents

Description

The present invention relates to a composite hollow fiber membrane having an aliphatic polyamide hollow fiber membrane with a dense layer and a support layer, and a separation function layer containing a crosslinked resin obtained by interfacial polycondensation, and a method for producing the same.

In recent years, the chemical industry has been seeking more energy-efficient processes in order to build a sustainable society and achieve carbon neutrality. Distillation processes in particular account for a large proportion of the energy consumed by the entire chemical industry, so there is a need for a shift to more energy-efficient processes, and a shift to the energy-efficient membrane separation process is being considered.

Membrane separation processes are an industrially established technology used in the fields of water purification and seawater desalination, as well as in the manufacturing processes of various industrial products that use water as a solvent. Polymer membranes are the mainstream material for separation membranes used in membrane separation processes, as they are relatively easy to mold, an inexpensive mass-production process can be easily established, and they are lightweight, flexible, and easy to handle.

In the chemical industry, organic solvents are widely used, and the introduction of membrane separation processes requires separation membrane materials to be sufficiently resistant to organic solvents. High-boiling-point aprotic polar solvents, in particular, have been in high demand in recent years for applications such as solvents for solution reactions in chemical synthesis, extraction media for purification, solvents for polymer materials utilizing their high solubility, and cleaning agents, and their consumption has been increasing. Representative aprotic polar solvents include dimethylacetamide, dimethylformamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, γ-butyrolactone, propylene carbonate, and tetrahydrofuran, and separation membranes are required to be resistant to these solvents. However, these solvents dissolve and corrode separation membrane materials such as polysulfone, polyvinylidene fluoride, and cellulose acetate, which have traditionally been widely used in water treatment. Therefore, these separation membranes developed for water treatment cannot be adapted for organic solvent membrane separation processes. Furthermore, limited polymer materials, such as polyethylene and polyethylene tetrafluoride, have long been used in organic solvent membrane separation processes. However, separation membranes made from these polymeric materials have been put to practical use as microfiltration membranes (with pore sizes ranging from about 0.1 μm to several μm) and as flat membranes of the dead-end filtration (total filtration) type, but cannot be used for the purpose of separating high molecular weight substances or low molecular weight solutes dissolved in organic solvents, and have not been put to practical use as hollow fiber membranes suitable for the cross-flow method, which is an effective operating method for concentrating the solutes to be recovered. Hollow fiber filtration modules have significant advantages in terms of use and manufacturing, such as a simple flow path design and the elimination of parts such as spacer members and O-ring members that are required in flat membrane filtration modules.

Currently, aprotic polar solvents are purified and recovered, or separated from dissolved solutes, by distillation. However, typical aprotic polar solvents have relatively high boiling points of around 150-200°C, and are susceptible to denaturation such as oxidation. Therefore, separating and purifying solutes by distillation requires not only energy consumption but also technology to suppress denaturation. Furthermore, if the solute to be separated and recovered is sensitive to heat, distillation cannot be applied in the first place, which presents a problem.

In response to these challenges, membrane separation processes do not involve phase changes such as solvent evaporation, making it possible to develop energy-saving processes. They are also unaffected by the oxidation or thermal denaturation of solvents, making them useful for separating solutions containing heat-sensitive solutes. Furthermore, if nanofiltration membranes could be provided that enable the efficient separation of low-molecular-weight solutes (molecular weights of approximately 200-1000) and solvents (molecular weights of approximately 100), it is expected that the range of separation targets would expand beyond the limited applications of existing microfiltration membranes. This type of separation membrane field is called OSN (organic solvent nanofiltration).

Development of OSN membranes has progressed in recent years, with several attempts and practical applications underway. One approach is to use membranes made from inorganic materials such as ceramics, which have high solvent resistance, but production costs and scale-up issues have hindered widespread adoption. Regarding the application of polymeric materials, a typical method is to produce polymer membranes using the non-solvent-induced phase separation (NIPS) process used to produce conventional water treatment membranes, followed by a post-processing crosslinking process to impart organic solvent resistance to the polymer membrane and render it insoluble in organic solvents. The NIPS method uses a membrane-forming solution in which a polymer is dissolved in the aforementioned polar solvent, but because the polymer itself has low solvent resistance, a robust post-processing crosslinking process is required.

For example, Non-Patent Document 1 proposes a nanofiltration membrane that is solvent-resistant by crosslinking a polyimide flat membrane produced by the NIPS method. However, this nanofiltration membrane does not have sufficient resistance to aprotic polar solvents.

Non-Patent Document 2 proposes a reverse osmosis membrane for desalination in aqueous systems (seawater desalination) that uses a polysulfone hollow fiber membrane fabricated by the NIPS method as a substrate and forms an interfacially polymerized membrane on the inner surface of the hollow fiber membrane. However, this polysulfone hollow fiber membrane has no resistance to organic solvents, particularly aprotic polar solvents.

Furthermore, to improve the permeability of membranes with extremely small pore sizes, such as OSN membranes, it is necessary to make the separation active layer as thin as possible. In asymmetric membranes fabricated using the NIPS method, the separation active layer is typically as thin as a few micrometers. With more advanced asymmetry (the formation of a dense surface layer), it is possible to thin it down to approximately 0.5 micrometers. This asymmetric structure allows for the formation of membranes with superior permeability. Furthermore, for membranes such as OSNs that are intended for high-pressure operation above 10 bar, mechanical strength in solvents is also important. Because most membranes fabricated using the NIPS method are made using amorphous polymers, their mechanical strength is not necessarily high. Especially when subjected to solvent swelling, such as in solvents, a decrease in elastic modulus occurs. Therefore, to maintain membrane strength, it is necessary to form the membrane on a separate backing carrier that maintains the membrane's strength. The same is true for the polyimide membranes mentioned above, where polyester or polyolefin nonwoven fabrics are used as backing carriers.

Meanwhile, another approach to fabricating OSN membranes, known as thermally induced phase separation (TIPS), is being investigated for obtaining nanofiltration membranes. The TIPS method is a relatively new technique that involves selecting a solvent that dissolves the polymer material at high temperatures but not at low temperatures. The resulting homogeneous polymer solution is then cooled to a temperature below the binodal line, which is the boundary between the one-phase and two-phase regions, inducing phase separation and solidifying the structure through polymer crystallization and glass transition. The TIPS method can be applied to polymers that do not dissolve in solvents at low temperatures, making it suitable for crystalline polymers with high solvent resistance. Furthermore, the TIPS method tends to form a homogeneous, sponge-like structure, resulting in high-strength separation membranes. Therefore, it is a suitable technique for obtaining pressure-resistant OSN membranes, particularly for fabricating hollow-fiber freestanding membranes.

For example, Patent Document 1 proposes a nanofiltration hollow fiber membrane formed using a polyamide resin produced by the TIPS method, having a molecular weight cutoff of 200 to 1000 and a methanol permeation rate of 0.03 L/( ^m2 ·bar·h) or more.

Patent document 2 also proposes a polyamide hollow fiber membrane for use as an ultrafiltration membrane or nanofiltration membrane, manufactured using the TIPS method, having a dense layer formed on at least one side and striated depressions extending in one direction on the surface of the dense layer.

International Publication No. 2022/071123 International Publication No. 2022/071122

Journal of Membrane Science 476(2015)530-553 nature communications (2021)12:2338

In order to use membrane separation processes as an alternative to distillation processes, separation membranes are required that have excellent permeability to organic solvents and excellent blocking performance for solutes in organic solvent-based treated liquids. However, because these two properties are contradictory, it is extremely difficult to achieve high levels of both permeability and blocking performance.

The polyamide hollow fiber membranes in Patent Documents 1 and 2 are suitable for cross-flow filtration and have high resistance to various organic solvents. However, because their mechanical strength in organic solvents is insufficient, it is difficult to increase the operating pressure and thereby increase the amount of organic solvent that permeates. Furthermore, their ability to reject low-molecular-weight solutes in organic solvent-based treated liquids is unsatisfactory, leaving room for improvement in terms of achieving both high levels of permeability and rejection.

The present invention aims to provide a composite hollow fiber membrane and a method for manufacturing the same that achieves both high levels of organic solvent permeability and high levels of solute rejection in organic solvent-based treated liquids.

The inventors conducted extensive research to solve the above-mentioned problems and discovered that by providing a separation function layer containing a crosslinked resin obtained by interfacial polycondensation on the surface of an aliphatic polyamide hollow fiber membrane having a dense layer and a support layer, it is possible to obtain a composite hollow fiber membrane that achieves both high levels of organic solvent permeability and high levels of solute rejection in organic solvent-based treated liquids. The present invention was completed through further research based on this finding.

That is, the present invention provides the following aspects.
Item 1. A composite hollow fiber membrane comprising an aliphatic polyamide hollow fiber membrane having a dense layer and a support layer, and a separation function layer containing a crosslinked resin obtained by interfacial polycondensation, the separation function layer being provided on the surface of the dense layer, and having a burst pressure of 2.25 MPa or more measured under the following conditions:
<Burst pressure>
While N-methyl-2-pyrrolidone is passed through a module prepared using a composite hollow fiber membrane, the pressure is increased by 0.25 MPa at 10-minute intervals, and the pressure (MPa) at which the composite hollow fiber membrane breaks is measured.
Item 2. The composite hollow fiber membrane according to Item 1, wherein the crosslinked resin is a crosslinked aromatic polyamide resin.
Item 3. The composite hollow fiber membrane according to Item 1 or 2, wherein the support layer has a porosity of 60 to 80%.
Item 4. The composite hollow fiber membrane according to any one of Items 1 to 3, wherein the dense layer has a thickness of 0.1 to 5 μm.
Item 5. The composite hollow fiber membrane according to any one of Items 1 to 4, wherein the dense layer is provided on the lumen surface side of the aliphatic polyamide hollow fiber membrane.
Item 6. The composite hollow fiber membrane according to any one of Items 1 to 5, wherein the aliphatic polyamide hollow fiber membrane has a molecular weight cutoff of 10,000 to 300,000.
Item 7. The composite hollow fiber membrane according to any one of Items 1 to 6, wherein the aliphatic polyamide hollow fiber membrane has a burst pressure of 2.1 MPa or more as measured under the following conditions:
<Burst pressure>
A water pressure is applied to a module made using an aliphatic polyamide hollow fiber membrane, and the pressure (MPa) is measured when the aliphatic polyamide hollow fiber membrane breaks.
Item 8. The composite hollow fiber membrane according to any one of Items 1 to 7, wherein when a solution containing N-methyl-2-pyrrolidone as a solvent and diphenyl sulfone as a solute is filtered, the rejection rate of diphenyl sulfone is 30% or more.
Item 9. The composite hollow fiber membrane according to any one of Items 1 to 8, having an N-methyl-2-pyrrolidone permeation rate of 6 L/( ^m2 ·h) or more at 25°C.
Item 10. A filtration method, comprising filtering a liquid to be treated, which contains an organic solvent and a solute, using the composite hollow fiber membrane according to any one of Items 1 to 9.
Item 11. A hollow fiber membrane module comprising a module case and the composite hollow fiber membrane according to any one of Items 1 to 9 housed in the module case.
Item 12. A method for producing a composite hollow fiber membrane, comprising the following first to fourth steps:
a first step of preparing a film-forming solution by dissolving an aliphatic polyamide resin at a concentration of 20% by weight or more in an organic solvent having a boiling point of 150°C or more and being incompatible with the aliphatic polyamide resin at a temperature below 100°C, at a temperature of 100°C or more;
a second step in which the membrane-forming solution is extruded into a coagulation bath at 100°C or less in a predetermined shape to coagulate the aliphatic polyamide resin into a membrane, in which a coagulation liquid that is compatible with the organic solvent used in the membrane-forming solution but has low affinity for the aliphatic polyamide resin is brought into contact with at least one surface of the membrane-forming solution extruded into the predetermined shape, thereby forming an aliphatic polyamide hollow fiber membrane having a dense layer on at least one surface;
a third step of removing the membrane-forming solution solvent and the coagulation solution from the aliphatic polyamide hollow fiber membrane formed in the second step; and a fourth step of forming a separation functional layer containing a crosslinked resin obtained by interfacial polycondensation on the surface of at least one of the dense layers of the aliphatic polyamide hollow fiber membrane obtained by performing the third step.

The composite hollow fiber membrane of the present invention has high mechanical strength in organic solvents (due to its high burst pressure in pressure resistance tests in organic solvents), allowing for increased operating pressure and increased organic solvent permeation rates. Furthermore, the membrane has a separation functional layer containing a crosslinked resin obtained by interfacial polycondensation on the surface of the dense layer of the aliphatic polyamide hollow fiber membrane, enabling it to achieve both high levels of organic solvent permeability and high levels of rejection of solutes (particularly low-molecular-weight solutes with molecular weights of approximately 200 to 1000) in organic solvent-based treated liquids. Furthermore, because the composite hollow fiber membrane of the present invention is formed from a hollow fiber membrane containing an aliphatic polyamide and a separation functional layer containing a crosslinked resin, it has excellent resistance to various types of organic solvents and can stably maintain membrane performance even when in contact with various types of organic solvents used industrially. Therefore, it can be suitably used in membrane separation processes as an alternative to distillation processes.

FIG. 1 is a schematic diagram of an apparatus used to measure the molecular weight cutoff, burst pressure, permeation rate (Flux), and rejection rate of a composite hollow fiber membrane or an aliphatic polyamide hollow fiber membrane. FIG. 1A is a schematic diagram of a module used when measuring the burst pressure of an aliphatic polyamide hollow fiber membrane, and FIG. 1B is a schematic diagram of an apparatus used to measure the burst pressure of an aliphatic polyamide hollow fiber membrane. FIG. 1 is an image analysis diagram after binarization processing for calculating the porosity of the support layer of the aliphatic polyamide hollow fiber membrane of Example 1. 1 is a scanning electron microscope image (magnification 10,000 times) of the lumen side surface of the aliphatic polyamide hollow fiber membrane of Example 1 (before the formation of a separation function layer). 1 is a scanning electron microscope image (magnification 20,000 times) of the lumen surface of the aliphatic polyamide hollow fiber membrane of Example 1 (before the formation of a separation function layer). 1 is a scanning electron microscope image (magnification 10,000 times) of the lumen side cross section of the aliphatic polyamide hollow fiber membrane of Example 1 (before the formation of a separation function layer). 1 is a scanning electron microscope image (magnification: 10,000 times) of the lumen side cross section of the composite hollow fiber membrane of Example 1. 1 is a scanning electron microscope image (magnification: 20,000 times) of the lumen side cross section of the composite hollow fiber membrane of Example 1. 1 is a scanning electron microscope image (magnification: 10,000 times) of the lumen side surface of the composite hollow fiber membrane of Example 1. 1 is a scanning electron microscope image (magnification: 20,000 times) of the lumen side surface of the composite hollow fiber membrane of Example 1. FIG. 1 is a schematic diagram illustrating an example of calculating the porosity of the support layer of a composite hollow fiber membrane, and is a cross-sectional schematic diagram of an aliphatic polyamide hollow fiber membrane cut in a direction perpendicular to the longitudinal direction. FIG. 12 is a partial enlarged view of the area surrounded by the dotted line in FIG. 11 . FIG. 12 is a partial enlarged view of the area surrounded by the dotted line in FIG. 11 .

1. Definitions In the present invention, the terms "ultrafiltration" or "ultrafiltration membrane" refer to a filtration having a molecular weight cutoff set within a range of 1,000 to 1,000,000 or a filtration membrane having a molecular weight cutoff within a range of 1,000 to 1,000,000, and the terms "nanofiltration" or "nanofiltration membrane" refer to a filtration having a molecular weight cutoff set within a range of 200 to 1,000 or a filtration membrane having a molecular weight cutoff within a range of 200 to 1,000.

In the present invention, "aliphatic polyamide hollow fiber membrane" means a filtration membrane in the form of a hollow fiber formed using an aliphatic polyamide resin and having a dense layer and a support layer.

In the present invention, "dense layer" refers to a region in an aliphatic polyamide hollow fiber membrane where dense micropores are concentrated, and where the presence of pores is substantially absent in a scanning electron microscope (SEM) photograph at a magnification of 10,000x or more.

In the present invention, the term "support layer" refers to a region of an aliphatic polyamide hollow fiber membrane other than the dense layer, which is a porous region having a continuous pore structure in which the presence of substantial pores can be observed in a scanning electron microscope (SEM) photograph at 2000x magnification.

In the present invention, the term "separation functional layer" refers to a layer that is provided on the surface of the dense layer of an aliphatic polyamide hollow fiber membrane and contains a crosslinked resin obtained by interfacial polycondensation.

2. Composite hollow fiber membrane The composite hollow fiber membrane of the present invention comprises an aliphatic polyamide hollow fiber membrane having a dense layer and a support layer, and a separation function layer containing a crosslinked resin obtained by interfacial polycondensation, the separation function layer being provided on the surface of the dense layer, and characterized in that the burst pressure measured under the following conditions is 2.25 MPa or more. The composite hollow fiber membrane of the present invention will be described in detail below.
<Burst pressure>
While N-methyl-2-pyrrolidone is passed through a module prepared using a composite hollow fiber membrane, the pressure is increased by 0.25 MPa at 10-minute intervals, and the pressure (MPa) at which the composite hollow fiber membrane breaks is measured.

[Aliphatic polyamide hollow fiber membrane]
The aliphatic polyamide hollow fiber membrane, which is a main component of the composite hollow fiber membrane of the present invention, has a dense layer and a support layer and is formed from an aliphatic polyamide resin. By using an aliphatic polyamide resin as a constituent resin of the hollow fiber membrane, the composite hollow fiber membrane of the present invention can be endowed with resistance to a wide range of organic solvents and also with high mechanical strength in organic solvents (by increasing the burst pressure in a pressure resistance test in an organic solvent), thereby enabling an increase in operating pressure and an increase in the amount of organic solvent permeated.

The type of aliphatic polyamide resin is not particularly limited, but examples include aliphatic polyamide homopolymers, aliphatic polyamide copolymers, and mixtures thereof. Specific examples of aliphatic polyamide homopolymers include polyamide 6, polyamide 66, polyamide 46, polyamide 610, polyamide 612, polyamide 11, and polyamide 12. Specific examples of aliphatic polyamide copolymers include copolymers of aliphatic polyamides with polyethers such as polytetramethylene glycol and polyethylene glycol. The proportion of aliphatic polyamide components in the aliphatic polyamide copolymer is not particularly limited, but examples include a proportion of aliphatic polyamide components of preferably 70 mol% or more, more preferably 80 mol% or more, even more preferably 90 mol% or more, and particularly preferably 95 mol% or more. By ensuring that the proportion of aliphatic polyamide components in the aliphatic polyamide copolymer satisfies the above range, the aliphatic polyamide hollow fiber membrane can be endowed with even better organic solvent resistance and mechanical strength. The aliphatic polyamide resins may be used alone or in combination of two or more.

Of these aliphatic polyamide resins, polyamide 6 is preferred as a resin for forming aliphatic polyamide hollow fiber membranes because it easily achieves both good pressure resistance and solvent resistance and also easily improves the permeability of organic solvents.

Aliphatic polyamide resins may be crosslinked or not, but from the perspective of reducing manufacturing costs, non-crosslinked resins are preferred.

The relative viscosity of the aliphatic polyamide resin is not particularly limited, but examples include 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 aliphatic polyamide hollow fiber membranes, enabling the aliphatic polyamide hollow fiber membranes to have excellent shape stability. Here, relative viscosity refers to the value measured with an Ubbelohde viscometer at 25°C using a solution in which 1 g of aliphatic polyamide resin is dissolved in 100 mL of 96% sulfuric acid.

In addition to the aliphatic polyamide resin, the aliphatic polyamide hollow fiber membrane may optionally contain a filler 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 aliphatic polyamide hollow fiber membrane. In particular, the inclusion of a filler also has the effect of making the aliphatic polyamide hollow fiber membrane less susceptible to deformation, even when subjected to high pressure during filtration. The type of filler to be added is not particularly limited, but examples include fibrous fillers such as glass fiber, carbon fiber, potassium titanate whiskers, zinc oxide whiskers, calcium carbonate whiskers, wollastonite whiskers, aluminum borate whiskers, 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, of the filler per 100 parts by weight of the aliphatic polyamide resin. By including the filler in such an amount, the strength, elongation, and elastic modulus of the aliphatic polyamide hollow fiber membrane can be improved.

Aliphatic polyamide hollow fiber membranes may contain additives such as thickeners, antioxidants, surface modifiers, lubricants, and surfactants as needed to control pore size and improve membrane performance.

The outer diameter of the aliphatic polyamide hollow fiber membrane is set appropriately depending on the application of the aliphatic polyamide hollow fiber membrane, the thickness of the dense layer and support layer, the desired liquid permeability, etc. However, taking into consideration the relationship between the effective membrane area when packed into a module, membrane strength, pressure loss of the fluid flowing through the hollow portion, and buckling pressure, the outer diameter of the hollow fiber membrane is 400 μm or more, preferably 450 to 4000 μm, and more preferably 500 to 3500 μm. Furthermore, the inner diameter of the aliphatic polyamide hollow fiber membrane is not particularly limited, but examples include 100 to 3000 μm, preferably 200 to 2500 μm, more preferably 300 to 2000 μm, and even more preferably 300 to 1500 μm. In the present invention, the outer diameter and inner diameter of the aliphatic polyamide hollow fiber membrane are values that can be determined by observing five hollow fiber membranes with an optical microscope at a magnification of 200 times, measuring the outer diameter and inner diameter (both at the point where they are maximum diameter) of each hollow fiber membrane, and calculating the average value of each.

The thickness of the aliphatic polyamide hollow fiber membrane is set appropriately depending on the application of the aliphatic polyamide hollow fiber membrane, the thickness of the dense layer and support layer, the desired liquid permeability, etc., but is, for example, 50 to 600 μm, preferably 100 to 350 μm. In the present invention, the thickness of the aliphatic polyamide hollow fiber membrane is calculated by subtracting the inner diameter from the outer diameter and dividing the value by 2.

Filtration membranes (separation membranes) are classified in order of the size of the substance they separate into microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) membranes, from largest to smallest. For filtration membranes below UF, molecular weight cutoff is used as an indicator of the size of the substance they capture. The molecular weight cutoff is determined by a permeation test using a standard substance of known molecular weight. The molecular weight cutoff is determined by the lower limit molecular size (Daltons: Da) that is retained by 90% or more in the permeation test. In the present invention, the molecular weight cutoff of ultrafiltration membranes is 1,000 to 1,000,000, and the molecular weight cutoff of nanofiltration membranes is 200 to 1,000.

When a separation layer containing a crosslinked resin obtained by interfacial polycondensation is formed on the surface of the dense layer of an aliphatic polyamide hollow fiber membrane, the fractionation performance of the aliphatic polyamide hollow fiber membrane, which is dominated by the dense layer, reflects the characteristics of the dense layer surface where the interfacial polycondensation reaction takes place. Therefore, the fractionation performance of the aliphatic polyamide hollow fiber membrane significantly affects the formation and characteristics of the separation layer. In order to form a separation layer on the surface of the dense layer that has excellent blocking performance for solutes (particularly low-molecular-weight solutes with molecular weights of approximately 200 to 1,000) in organic solvent-based treated liquids, the molecular weight cutoff of the aliphatic polyamide hollow fiber membrane of the present invention is preferably in the range of that of an ultrafiltration membrane, more preferably 10,000 to 300,000, even more preferably 20,000 to 100,000, even more preferably 30,000 to 80,000, even more preferably 40,000 to 70,000, and particularly preferably 50,000 to 70,000. If the molecular weight cutoff is within the above range, a uniform separation functional layer can be formed on the surface and the surface layer portion (inside the layer near the surface) of the dense layer to form a composite, and the composite hollow fiber membrane of the present invention can be obtained which achieves both higher levels of organic solvent permeability and higher levels of solute rejection in the organic solvent-based treated liquid.The molecular weight cutoff of the aliphatic polyamide hollow fiber membrane can be adjusted to the desired value by appropriately adjusting the thickness of the dense layer and the pore size of the support layer, etc.

In the present invention, when the molecular weight cutoff of an aliphatic polyamide hollow fiber membrane is greater than 3000, it is a value determined using dextran as the standard substance and water as the solvent. Specifically, the molecular weight cutoff of an aliphatic polyamide hollow fiber membrane (when greater than 3000) is a value determined when a crossflow module is prepared using the aliphatic polyamide hollow fiber membrane, and an aqueous solution containing a plurality of dextrans of known molecular weights at predetermined concentrations is used as the circulating liquid (raw solution) in an internal pressure crossflow system. The dextran concentrations in the permeate are measured by high performance liquid chromatography, and the solute rejection at each molecular weight is calculated according to the following formula. The results are then plotted on a graph with molecular weight on the horizontal axis and rejection on the vertical axis, and the molecular weight is the molecular weight at the intersection of the obtained approximation curve and a rejection of 90%.
Solute rejection rate (%) = {(dextran concentration in stock solution - dextran concentration in permeate) / dextran concentration in stock solution} × 100

In the present invention, when the molecular weight cutoff of an aliphatic polyamide hollow fiber membrane is 3000 or less, it is a value determined using monodisperse polystyrene as a standard substance and N-methyl-2-pyrrolidone (hereinafter also referred to as "NMP") as a solvent. Specifically, the molecular weight cutoff of an aliphatic polyamide hollow fiber membrane (when 3000 or less) is a value determined when a crossflow module is prepared using the aliphatic polyamide hollow fiber membrane and an NMP solution containing polystyrenes of known molecular weights at predetermined concentrations is used as a circulating liquid (raw solution) in an internal pressure crossflow system. The polystyrene concentrations in the permeate are measured by high performance liquid chromatography, the solute rejection at each molecular weight is calculated according to the following formula, and the results are plotted on a graph with molecular weight on the horizontal axis and rejection on the vertical axis, and the molecular weight is the molecular weight at the intersection of the obtained approximation curve and a rejection of 90%.
Solute rejection rate (%) = {(polystyrene concentration in raw solution - polystyrene concentration in permeate) / polystyrene concentration in raw solution} x 100

The aliphatic polyamide hollow fiber membrane, which serves as the base material for the composite hollow fiber membrane, is responsible for the membrane's mechanical properties. The radial stress characteristics of the hollow fiber membrane, which are primarily related to its pressure resistance, are important. Stress can be applied as internal pressure to the hollow portion, and the radial breaking stress can be evaluated by the burst pressure of the internal pressure. Furthermore, because the operating pressure during membrane filtration is also related to the permeation rate, it is preferable that the aliphatic polyamide hollow fiber membrane have sufficiently high pressure resistance. The burst pressure of the aliphatic polyamide hollow fiber membrane of the present invention is not particularly limited and can be adjusted to the desired value by appropriately adjusting the material properties of the aliphatic polyamide resin, such as the crystallinity and molecular weight, the ratio of the hollow fiber membrane's inner diameter to its membrane thickness, the thickness and porosity of the dense layer, the thickness and porosity of the support layer, and the pore size of the support layer. However, from the perspective of increasing the operating pressure during membrane filtration to increase the permeation rate, the burst pressure is preferably 2.1 MPa or higher, more preferably 2.5 MPa or higher, even more preferably 2.8 MPa or higher, and even more preferably 3.0 MPa or higher. The upper limit of the burst pressure of an aliphatic polyamide hollow fiber membrane is usually 4.0 MPa or less, preferably 3.8 MPa or less. Specifically, the burst pressure of an aliphatic polyamide hollow fiber membrane is preferably 2.1 to 4.0 MPa, more preferably 2.5 to 3.8 MPa, even more preferably 2.8 to 3.8 MPa, and still more preferably 3.0 to 3.8 MPa. In the present invention, the burst pressure of an aliphatic polyamide hollow fiber membrane refers to the pressure (MPa) at which the aliphatic polyamide hollow fiber membrane ruptures when hydraulic pressure is applied to a module prepared using the aliphatic polyamide hollow fiber membrane.

Specifically, the burst pressure of an aliphatic polyamide hollow fiber membrane is measured by the following method. First, a module 8 shown in FIG. 2A is prepared. Specifically, ten hollow fiber membranes 8a are cut to 30 cm lengths and bundled together. Next, a nylon hard tube 8b with an outer diameter of 8 mm, an inner diameter of 6 mm, and a length of 50 mm is prepared. A rubber stopper approximately 20 mm long is inserted into one end opening of the tube to plug the end opening. Next, a two-component epoxy resin is injected into the opening opposite the rubber stopper to fill the inner space of the tube. The bundle of hollow fiber membranes is then bent into a roughly U-shape, and both ends of the hollow fiber membranes are heat-sealed to prevent the epoxy resin from penetrating the hollow portion. The end tip is then inserted into the tube filled with epoxy resin until it touches the rubber stopper, and the epoxy resin is cured in this state. Next, the region of the hardened epoxy resin on the rubber stopper side is cut off together with the tube, thereby producing a module 8 in which the hollow portions at both ends of the hollow fiber membrane are open.
Next, the module 8 is set in the device shown in FIG. 2b, and water pressure is applied to the module 8 by a hand pump 9, and the pressure (MPa) at which the module 8 breaks is taken as the burst pressure.

The crystallinity of the aliphatic polyamide hollow fiber membrane is not particularly limited and is typically 25% or higher. From the perspective of improving solvent resistance and pressure resistance, it is preferably 30% or higher, more preferably 35% or higher, even more preferably 40% or higher, even more preferably 45% or higher, and particularly preferably 50% or higher. The crystallinity of the aliphatic polyamide hollow fiber membrane can be adjusted to the desired value, for example, by annealing (heat treatment). Annealing can be performed using methods such as dry heat and autoclave.

In the present invention, the crystallinity of an aliphatic polyamide hollow fiber membrane is a value measured by X-ray diffraction (XRD). Specifically, multiple hollow fiber samples are arranged closely together on a measurement stage, and a scattering pattern spectrum of scattering vectors and scattering intensity is obtained by scanning the scattering angle. The obtained spectrum is then spectrally separated into crystalline and amorphous components, and the crystallinity is determined by quantifying them.

[Dense layer]
The dense layer is a region in the aliphatic polyamide hollow fiber membrane where dense micropores are concentrated, and where the presence of pores is substantially not observed in a scanning electron microscope (SEM) photograph at a magnification of 10,000 times or more.

The dense layer may be formed on the lumen-side surface of the aliphatic polyamide hollow fiber membrane, on the outer surface, or on both surfaces. However, from the viewpoint of achieving a higher level of both organic solvent permeability and blocking performance for solutes in the organic solvent-based treated liquid, it is preferable to form it on the lumen-side surface. When observing the dense layer with a scanning electron microscope (SEM), if the dense layer is present on the outer surface of the aliphatic polyamide hollow fiber membrane, the membrane can be cut to an appropriate size, mounted 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-side surface of the aliphatic polyamide hollow fiber membrane, the membrane can be cut longitudinally with a sharp blade such as a scalpel to expose the lumen-side surface, cut to an appropriate size, mounted on a sample stage, and then vapor-deposited with Pt, Au, Pd, or the like before observation.

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. However, from the perspective of improving the blocking performance of solutes in the organic solvent-based treated solution and from the perspective of stabilizing the separation functional layer, it is preferably 0.1 μm or more, more preferably 0.2 μm or more, and even more preferably 0.3 μm or more. Furthermore, from the perspective of improving the liquid permeability of the organic solvent, it is preferably 2.0 μm or less, more preferably 1.8 μm or less, even more preferably 1.5 μm or less, even more preferably 1.2 μm or less, and even more preferably 1.0 μm or less. Specifically, the thickness of the dense layer is preferably 0.1 to 2.0 μm, more preferably 0.2 to 1.8 μm, even more preferably 0.3 to 1.5 μm, even more preferably 0.3 to 1.2 μm, and even more preferably 0.3 to 1.0 μm. In the present invention, the thickness of the dense layer is a value determined by measuring the distance (thickness) of a region where the presence of pores is substantially not observed at 10 points at regular intervals in a scanning electron microscope (SEM) photograph of the cross section of an aliphatic polyamide hollow fiber membrane at a magnification of 10,000 times, and calculating the average value.

[Support layer]
The support layer is a region of the aliphatic polyamide hollow fiber membrane other than the dense layer, and is a porous region having a continuous pore structure in which the presence of substantial pores can be seen in a scanning electron microscope (SEM) photograph at a magnification of 2000 times.

There are no particular restrictions on the pore size of the support layer, as long as it is strong enough to maintain the dense layer and does not significantly impede fluid permeation.

The porosity of the support 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., during the production of aliphatic polyamide hollow fiber membranes. However, from the standpoint of ease of membrane production and increasing membrane strength so that the operating pressure during membrane filtration can be increased to increase the amount of liquid permeated, the porosity is preferably 60 to 80%, more preferably 63 to 78%, even more preferably 65 to 77%, and even more preferably 66 to 76%.

In the present invention, the porosity of the support layer is calculated as follows. Scanning electron microscope (SEM) photographs are taken at five locations evenly spaced along the thickness of the support layer of an aliphatic polyamide hollow fiber membrane in a cross section of an aliphatic polyamide hollow fiber membrane or composite hollow fiber membrane cut perpendicular to the longitudinal direction. Specifically, the photographs are taken as follows. Figure 11 is a schematic cross-sectional view of an aliphatic polyamide hollow fiber membrane cut perpendicular to the longitudinal direction. Figures 12 and 13 are partial enlarged views of the area surrounded by dotted lines in Figure 11, illustrating an example of an aliphatic polyamide hollow fiber membrane 8a having a dense layer 10 and a support layer 11. Note that the separation function layer is omitted in Figures 11 to 13. As shown in Figure 12, a straight line is drawn from the center of the lumen of the aliphatic polyamide hollow fiber membrane 8a toward the outer surface, and this line is divided into five equal parts from the lumen-side surface of the support layer 11 to the outer surface. The midpoints of the five equal parts are then determined as shown in Figure 12. 13, five locations are photographed at a uniform magnification so that the midpoint is at the center of the photographed image, the photographed image includes only the support layer 11 portion, and the number of pores is 30 to 300. For the five SEM photographs taken, image analysis of the regions is performed using image analysis software (ImageJ), and the pore portions and polymer portions are distinguished by binarization processing, and the area ratio (%) of the total pore area to the area of the analyzed region is calculated for each, and the average value is calculated.

[Separation functional layer]
The separation functional layer is a layer provided on the surface of the dense layer of the aliphatic polyamide hollow fiber membrane in order to improve the blocking performance of solutes in the organic solvent-based liquid to be treated. The composite hollow fiber membrane of the present invention, which has a separation functional layer on the surface of the dense layer of the aliphatic polyamide hollow fiber membrane, has significantly improved blocking performance of solutes in the organic solvent-based liquid to be treated (particularly low-molecular-weight solutes with molecular weights of about 200 to 1,000) compared to aliphatic polyamide hollow fiber membranes not having a separation functional layer, and can achieve both high levels of organic solvent permeability and high levels of blocking performance of solutes in the organic solvent-based liquid to be treated.

The separation functional layer may be provided on the surface of the dense layer on the lumen-side surface of the aliphatic polyamide hollow fiber membrane or on the surface of the dense layer on the outer surface of the aliphatic polyamide hollow fiber membrane. However, it is preferably provided on the surface of the dense layer on the lumen-side surface of the aliphatic polyamide hollow fiber membrane. This is because providing a separation functional layer on the surface of the dense layer on the outer surface of the aliphatic polyamide hollow fiber membrane may lead to peeling of the separation functional layer due to contact between the hollow fiber membranes during filtration, or peeling at the interfaces between both ends of the module and the sealant during module fabrication. Furthermore, when an aliphatic polyamide hollow fiber membrane has dense layers on both sides, the separation functional layer may be provided on the surface of only one of the dense layers or on the surfaces of both dense layers. However, for the reasons mentioned above, it is preferable that the separation functional layer be provided only on the surface of the dense layer on the lumen-side surface.

The separation functional layer is a layer primarily composed of a crosslinked resin obtained by interfacial polycondensation. More specifically, it is a layer with a network structure formed from a crosslinked resin obtained by the interfacial polycondensation reaction of two or more polyfunctional monomers, including a trifunctional or higher functional monomer. Because the separation functional layer is formed from a crosslinked resin, it has excellent resistance to various types of organic solvents and can stably maintain membrane performance even when in contact with various types of industrially used organic solvents. The crosslinked resin is not particularly limited as long as it has a crosslinked structure obtained by interfacial polycondensation, and examples include crosslinked polyamide resins and crosslinked polyester resins. One or more types of crosslinked resins may be included. In the case of crosslinked polyamide resins, typically, a bifunctional or higher polyfunctional amine and a bifunctional or higher polyfunctional acyl halide are used as the polyfunctional monomers. In the case of crosslinked polyester resins, typically, a bifunctional or higher polyhydric alcohol and a bifunctional or higher polyfunctional acyl halide are used as the polyfunctional monomers. The use of a highly reactive acyl halide facilitates the interfacial polycondensation reaction to proceed rapidly. In the present invention, from the viewpoint of improving the solvent resistance of the separating functional layer and from the viewpoint of obtaining a composite hollow fiber membrane that can achieve both higher levels of liquid permeability for organic solvents and higher levels of blocking performance for solutes in organic solvent-based treated liquids, the crosslinked resin constituting the separating functional layer is preferably a crosslinked polyamide resin, more preferably a crosslinked aromatic polyamide resin. Hereinafter, as a representative example, a case where the crosslinked resin constituting the separating functional layer is a crosslinked polyamide resin will be described.

Polyfunctional amines, which are raw material monomers for crosslinked polyamide resins, are amines that have at least two amino groups in one molecule. Examples include phenylenediamine, xylylenediamine, 3,5-diaminobenzoic acid, 3-aminobenzylamine, 4-aminobenzylamine, in which two amino groups are bonded to a benzene ring at either the ortho, meta, or para positions; aromatic diamines such as biphenyl and diphenylmethane, in which two hydrogen atoms of an aromatic hydrocarbon bonded directly or via a functional group are replaced with two amino groups; and compounds in which a benzene ring is bonded directly or via a functional group, such as 1,3,5-triaminobenzene, 1,2,4-triaminobenzene, biphenyl, and diphenylmethane, in which two hydrogen atoms of the aromatic hydrocarbon are replaced with two amino groups. Examples of suitable aromatic polyfunctional amines include aromatic triamines, such as compounds in which three hydrogen atoms of an aromatic hydrocarbon bonded directly or via a functional group are substituted with three amino groups; aromatic tetraamines, such as compounds in which four hydrogen atoms of an aromatic hydrocarbon bonded directly or via a functional group to a benzene ring, such as biphenyl or diphenylmethane, are substituted with four amino groups (e.g., 3,3',4,4'-tetraaminobiphenyl); aliphatic polyfunctional amines, such as ethylenediamine and propylenediamine; and alicyclic polyfunctional amines, such as 1,2-diaminocyclohexane, 1,4-diaminocyclohexane, 4-aminopiperidine, piperazine, and 4-aminoethylpiperazine. The polyfunctional amines may be further substituted with substituents such as halogen atoms, sulfo groups, alkyl groups, and fluoroalkyl groups. These polyfunctional amines may be used alone or in combination of two or more. Furthermore, among these polyfunctional amines, from the viewpoints of improving the blocking performance of solutes in the organic solvent-based liquid to be treated, reactivity, handleability, ease of availability, and the like, preferred are aromatic polyfunctional amines, more preferred are aromatic di- to tetrafunctional amines, even more preferred are at least one aromatic diamine selected from the group consisting of m-phenylenediamine, p-phenylenediamine, xylylenediamine, 3,5-diaminobenzoic acid, 3-aminobenzylamine, and 4-aminobenzylamine, and particularly preferred is m-phenylenediamine.

The polyfunctional acyl halides used as raw material monomers for crosslinked polyamide resins are acyl halides containing at least two halogenated carbonyl groups per molecule. Examples include trifunctional acyl halides such as trimesic acid chloride, 1,3,5-cyclohexanetricarboxylic acid trichloride, and 1,2,4-cyclobutanetricarboxylic acid trichloride; aromatic bifunctional acyl halides such as biphenyldicarboxylic acid dichloride, azobenzenedicarboxylic acid dichloride, terephthalic acid chloride, isophthalic acid chloride, and naphthalenedicarboxylic acid chloride; aliphatic bifunctional acyl halides such as adipoyl chloride and sebacoyl chloride; and alicyclic bifunctional acyl halides such as cyclopentanedicarboxylic acid dichloride, cyclohexanedicarboxylic acid dichloride, and tetrahydrofurandicarboxylic acid dichloride. These polyfunctional acyl halides may be used alone or in combination. In order to obtain a crosslinked polyamide resin, it is necessary to use a polyfunctional amine and/or a polyfunctional acyl halide having three or more functionalities. However, from the viewpoints of improving the solvent resistance of the separation functional layer, improving the blocking performance of solutes in the organic solvent-based treated liquid, reactivity, handleability, and ease of availability, it is preferable to use a trifunctional acyl halide, and more preferably trimesic acid chloride.

The separation functional layer may contain a non-crosslinked polyamide resin (a linear polyamide resin obtained by the interfacial polycondensation reaction of a diamine and a bifunctional acyl halide), but from the perspective of improving the solvent resistance of the separation functional layer, it is preferable that it does not contain a non-crosslinked polyamide resin. If the separation functional layer contains a non-crosslinked polyamide resin, the content of the non-crosslinked polyamide resin is preferably 20% by weight or less, more preferably 10% by weight or less, and even more preferably 5% by weight or less.

The separation functional layer may contain additives such as thickeners, antioxidants, surface modifiers, lubricants, surfactants, etc. as needed.

While the thickness of the separation functional layer is not particularly limited, it is extremely thin (less than 0.1 μm), making it extremely difficult to accurately measure its thickness even when observed with a scanning electron microscope (SEM) at magnifications of 100,000 to 200,000 times. Because the separation functional layer of the present invention is observed as a completely integrated part of the dense layer of the aliphatic polyamide hollow fiber membrane, determining its thickness is difficult. However, if the separation functional layer has a significantly uneven morphology, it will be observed as an uneven image distinct from the dense layer, and the presence of the separation functional layer can be determined from the SEM image. In this case, determining the thickness of the separation functional layer is also difficult. Meanwhile, the morphology in SEM images of the surface after the interfacial polycondensation reaction differs from that of the dense layer of an aliphatic polyamide hollow fiber membrane that has not undergone the interfacial polycondensation reaction (e.g., numerous circular protrusions not present on the surface of the dense layer can be seen), and therefore the presence of the separation functional layer can be confirmed by comparing the SEM images.

The presence of a separation functional layer can also be confirmed by an air leak test of a composite hollow fiber membrane. The air leak test involves applying an air pressure of approximately 0.2 MPa to the hollow portion of a dry composite hollow fiber membrane to trap the pressurized air, and evaluating the decrease in trapping pressure over time. When an aliphatic polyamide hollow fiber membrane has a pore size in the range of an ultrafiltration membrane, the aliphatic polyamide hollow fiber membrane is completely unable to maintain the trapping pressure. On the other hand, a composite hollow fiber membrane completely combined with a separation functional layer can almost completely maintain the trapping pressure. Therefore, the presence of a separation functional layer can be confirmed by testing whether the trapping pressure can be maintained. Furthermore, because the quality of the separation functional layer can be determined by quantifying the retention of the trapping pressure, the air leak test is effective in checking for defects in the separation functional layer. In one embodiment of the composite hollow fiber membrane of the present invention, when an air leak test is conducted with a membrane area (S) and a total space volume (V) to be pressurized defined by the following formula 1, the air leak amount (pressure drop) per 5 minutes is preferably 0.05 MPa or less, more preferably 0.02 MPa or less, and even more preferably 0.005 MPa or less.
(Formula 1) V/S=0.0002×0.45/R
R is the inner diameter (mm) of the composite hollow fiber membrane. The error of V/S is within ±10%.
When detecting air leakage through a hollow fiber membrane by pressure drop, it is necessary for quantitative measurement that the membrane area S (the leaking portion) and the total pressurized space volume V be relatively constant. In the case of hollow fiber membranes, the hollow volume of the hollow fiber itself fluctuates with changes in the inner diameter, and the ratio of volume to membrane area is proportional to the inner diameter. Furthermore, the module size and leak tester size also vary depending on factors such as the thickness of the hollow fiber membrane, and therefore these must be corrected as shown in Equation 1 above. In the present invention, the optimal value was found based on an air leak test using a hollow fiber membrane with an inner diameter of 0.45 mm and ^a space of 0.0002 m3 per ^m2 . For hollow fiber membranes with inner diameters of 0.1 to 3 mm, which are suitable for the present invention, the conditions were standardized for hollow fiber membranes with different inner diameters using the relationship described in Equation 1 above.

Specifically, the air leak rate (pressure drop) of a composite hollow fiber membrane is measured by the following method. First, 20 composite hollow fiber membranes are cut into 30 cm lengths and bundled together. Next, a polybutylene terephthalate (PBT) tube with an outer diameter of 12.7 mm, an inner diameter of 9.5 mm, and a length of 53 mm is prepared. A silicone tube with an inner diameter of 12 mm and a length of approximately 50 mm is inserted approximately 15 mm into one end opening of the tube to connect the two. A rubber stopper approximately 20 mm long is inserted into the silicone tube on the side opposite the end inserted into the PBT tube to plug the one end opening. Next, a two-component epoxy resin mixture is injected into the opening on the side opposite the rubber stopper to fill the space inside the tube with the epoxy resin. One end of the bundle of composite hollow fiber membranes prepared above is then heat-sealed to prevent the epoxy resin from penetrating the hollow portion, and the end is inserted into the tube filled with the epoxy resin until the tip of the end touches the rubber stopper, and the epoxy resin is cured in this state. Next, the region of the cured epoxy resin on the rubber stopper side is cut together with the tube to open the hollow portion. The same procedure is performed on the other end to prepare a cross-flow module with open hollow portions at both ends of the composite hollow fiber membrane. The membrane area of the module is calculated based on the internal area of the hollow fibers, and from the internal diameter, number, and effective length of the hollow fibers.
One end of the outlet connected to the hollow part of the fabricated module is sealed, and a valve and a pressure line with a pressure gauge between the valve and the module are attached to the other end. Air pressure of 0.2 MPa is applied to the hollow part, and the valve is closed to contain the pressure. Immediately after that, the pressure is checked over time, and the amount of pressure change after 5 minutes is measured. The containment volume (total pressurized space volume V) including the hollow fibers, module, and pressure line is estimated, and the length of the line, etc. is controlled so that the ratio of the membrane area S to the total pressurized space volume V is within ±10% of the relationship in Equation 1 above.

[Burst pressure of composite hollow fiber membrane]
The composite hollow fiber membrane of the present invention has a burst pressure of 2.25 MPa or more as measured under the conditions described below, and has high mechanical strength in organic solvents (excellent pressure resistance), so that the operating pressure during membrane filtration can be increased to increase the amount of organic solvent that permeates through the membrane.
<Burst pressure>
While N-methyl-2-pyrrolidone is passed through a module prepared using a composite hollow fiber membrane, the pressure is increased by 0.25 MPa at 10-minute intervals, and the pressure (MPa) at which the composite hollow fiber membrane breaks is measured.

From the perspective of increasing the operating pressure during membrane filtration and further increasing the amount of organic solvent permeated, the burst pressure of the composite hollow fiber membrane is preferably 2.5 MPa or more, more preferably 2.75 MPa or more. The upper limit of the burst pressure of the composite hollow fiber membrane is typically 4.0 MPa or less. Specifically, the burst pressure of the composite hollow fiber membrane is preferably 2.5 to 4.0 MPa, more preferably 2.75 to 4.0 MPa. The burst pressure of the composite hollow fiber membrane can be adjusted to the desired value by appropriately adjusting the materials used to form the aliphatic polyamide hollow fiber membrane and the separation function layer, the thickness of the dense layer, the thickness and porosity of the support layer, and the pore size of the pores in the support layer, etc.

Specifically, the burst pressure of the composite hollow fiber membrane is measured by the following method. First, 20 composite hollow fiber membranes are cut into 30 cm lengths and bundled together. Next, a polybutylene terephthalate (PBT) tube with an outer diameter of 12.7 mm, an inner diameter of 9.5 mm, and a length of 53 mm is prepared. A silicone tube with an inner diameter of 12 mm and a length of approximately 50 mm is inserted approximately 15 mm into one end opening of the tube to connect the two. A rubber stopper approximately 20 mm long is inserted into the silicone tube on the side opposite the end inserted into the PBT tube to plug the one end opening. Next, a two-component epoxy resin mixture is injected into the opening on the side opposite the rubber stopper to fill the space inside the tube with the epoxy resin. One end of the bundle of composite hollow fiber membranes prepared above is then heat-sealed to prevent the epoxy resin from penetrating the hollow portion, and the end is inserted into a tube filled with the epoxy resin until the tip of the end touches the rubber stopper, and the epoxy resin is cured in this state. Next, the region of the cured epoxy resin on the rubber stopper side is cut together with the tube to open the hollow portion. The same procedure is performed on the other end to produce a cross-flow module with open hollow portions at both ends of the composite hollow fiber membrane.
The fabricated crossflow module 6 is connected to the internal pressure separation treatment line shown in Figure 1, and NMP is continuously permeated through the crossflow module 6 by the liquid supply circulation pump 2. In the module, the pressures measured by the primary pressure gauge 3 and the secondary pressure gauge 4 are regulated by the regulator 5, and the arithmetic mean value of the pressures measured by the primary pressure gauge 3 and the secondary pressure gauge 4 is defined as the applied pressure, which is increased by 0.25 MPa at 10-minute intervals. The pressure (MPa) at which the composite hollow fiber membrane ruptures, i.e., the pressure at which a sudden drop in pressure occurs, is defined as the burst pressure.

[Permeability of organic solvents and blocking performance of solutes]
The composite hollow fiber membrane of the present invention has the separation functional layer provided on the surface of the dense layer of the aliphatic polyamide hollow fiber membrane, and therefore can achieve both high levels of liquid permeability for organic solvents and high levels of blocking performance for solutes in organic solvent-based treated liquids, and has, for example, the following liquid permeability and blocking performance.

In one embodiment of the composite hollow fiber membrane of the present invention, the NMP permeation rate at 25°C and a pressure 75% of the burst pressure of the composite hollow fiber membrane is preferably 6 L/( ^m2 ·h) or more, more preferably 8 L/( ^m2 ·h) or more, even more preferably 10 L/( ^m2 ·h) or more, still more preferably 11 L/( ^m2 ·h) or more, even more preferably 12 L/( ^m2 ·h) or more, and still more preferably 14 L/( ^m2 ·h) or more. The upper limit of the NMP permeation rate is usually 20 L/( ^m2 ·h) or less. Specifically, the NMP permeation rate at 25°C and a pressure 75% of the burst pressure of the composite hollow fiber membrane is preferably 6 to 20 L/( ^m2 ·h), more preferably 8 to 20 L/( ^m2 ·h), even more preferably 10 to 20 L/( ^m2 ·h), still more preferably 11 to 20 L/( ^m2 ·h), even more preferably 12 to 20 L/( ^m2 ·h), and even more preferably 14 to 20 L/( ^m2 ·h). The NMP permeation rate can be adjusted to a desired value by appropriately adjusting the molecular weight cutoff of the aliphatic polyamide hollow fiber membrane, the thickness of the dense layer, the type of material forming the separation functional layer, etc.

The NMP permeation rate is a value measured by internal pressure filtration and is a value measured by the following procedure. First, 20 composite hollow fiber membranes are cut into 30 cm lengths and bundled together. Next, a polybutylene terephthalate (PBT) tube with an outer diameter of 12.7 mm, an inner diameter of 9.5 mm, and a length of 53 mm is prepared. A silicone tube with an inner diameter of 12 mm and a length of approximately 50 mm is inserted approximately 15 mm into one end opening of the tube to connect them. A rubber stopper approximately 20 mm long is inserted into the silicone tube on the side opposite the end inserted into the PBT tube to plug the one end opening. Next, a two-component epoxy resin mixture is injected through the opening on the side opposite the rubber stopper to fill the space inside the tube with the epoxy resin. One end of the bundle of composite hollow fiber membranes prepared above is then heat-sealed to prevent the epoxy resin from penetrating the hollow portion, and the end is inserted into a tube filled with the epoxy resin until the tip of the end touches the rubber stopper, and the epoxy resin is cured in this state. Next, the region of the cured epoxy resin on the rubber stopper side is cut together with the tube to open the hollow portion. The same procedure is performed on the other end to produce a cross-flow module with open hollow portions at both ends of the composite hollow fiber membrane.
The fabricated crossflow module 6 is connected to the internal pressure separation treatment line shown in Figure 1, and the flowing liquid is continuously passed through the crossflow module 6 by the liquid supply circulation pump 2. NMP is used as the flowing liquid. In the module, the pressures of the primary pressure gauge 3 and the secondary pressure gauge 4 are adjusted by the regulator 5 so that the arithmetic mean value of the pressures of the primary pressure gauge 3 and the secondary pressure gauge 4 is 75% of the burst pressure of the composite hollow fiber membrane. Of the flowing liquid passing through the module, that which has passed through the pores of the composite hollow fiber membrane is recovered as permeated liquid separated from the flowing liquid, and the remainder is circulated back to the separation treatment line. The permeated liquid flowing out of the crossflow module 6 between 2 and 4 hours after the start of circulation is recovered in the receiver 7, and the NMP permeation rate (L/( ^m2 ·h)) is calculated according to the following formula:
NMP permeation amount = volume (L) of NMP permeated to the outside of the composite hollow fiber membrane / [inner diameter (m) of composite hollow fiber membrane × 3.14 × effective filtration length (m) of composite hollow fiber membrane × 20 (membranes) × time (h)]

Effective filtration length of composite hollow fiber membrane: The length of the portion of the outer surface of the composite hollow fiber membrane that is not coated with epoxy resin in the crossflow module.

The composite hollow fiber membrane of the present invention is preferably an ultrafiltration membrane or nanofiltration membrane, and more preferably a nanofiltration membrane, i.e., one with a molecular weight cutoff of 200 to 1,000. However, because measuring the molecular weight cutoff is complicated, a simple method for identifying the classification of filtration membranes was used to evaluate the rejection performance of specific substances. In this invention, the rejection rate of diphenyl sulfone (molecular weight: 218.27), which corresponds to the minimum molecular weight cutoff for OSN, was used. Furthermore, the molecular weight cutoff of less than 1,000 was confirmed using the rejection rate of monodisperse polystyrene (molecular weight: 1,000). The composite hollow fiber membrane of the present invention had a rejection rate of 90% or more for polystyrene with a molecular weight of 1,000 and a rejection rate of 30% or more for diphenyl sulfone, a low molecular weight substance, thereby satisfying the performance requirements of a nanofiltration membrane.

In one embodiment of the composite hollow fiber membrane of the present invention, the rejection rate of diphenyl sulfone when a solution containing NMP as the solvent and diphenyl sulfone as the solute is filtered is preferably 30% or more, more preferably 35% or more, even more preferably 50% or more, even more preferably 60% or more, and particularly preferably 70% or more. The rejection rate of diphenyl sulfone can be adjusted to the desired value by appropriately selecting the type of material forming the separation functional layer, etc.

The rejection of diphenyl sulfone or polystyrene is a value measured by internal pressure filtration, and is a value measured using the same measurement procedure as for the measurement of the NMP permeation amount, except that an NMP solution containing 0.2 wt% of diphenyl sulfone or polystyrene is used. The concentration of diphenyl sulfone or polystyrene can be measured by liquid chromatography. The rejection of diphenyl sulfone or polystyrene is calculated using the following formula.
Rejection rate = 100 - (concentration of permeate/concentration of flow-through) x 100

[Organic solvent resistance]
The composite hollow fiber membrane of the present invention is formed mainly from an aliphatic polyamide resin and a crosslinked resin (preferably a crosslinked aromatic polyamide resin), and therefore has the property of suppressing changes in strength and elongation and stably maintaining the membrane structure even when it comes into contact with various types of organic solvents (organic solvent resistance). More specifically, the composite hollow fiber membrane of the present invention is resistant to organic solvents such as alcohols, aprotic polar solvents, hydrocarbons, higher fatty acids, ketones, esters, and ethers. Specific examples of such organic solvents include the following:
Alcohols: primary alcohols such as methanol, ethanol, n-propanol, n-butanol, and benzyl alcohol; secondary alcohols such as isopropyl alcohol and isobutanol; tertiary alcohols such as tertiary butyl alcohol; and polyhydric alcohols such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, 1,3-butanediol, and glycerin.
Ketones: acetone, methyl ethyl ketone, cyclohexanone, diisopropyl ketone, etc.
Ethers: 3-methoxybutanol, 3-methoxybutyl acetate, tetrahydrofuran, diethyl ether, diisopropyl ether, 1,4-dioxane, ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monomethyl ether acetate, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, and the like.
Aprotic polar solvents: N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, sulfolane, and the like.
Esters: ethyl acetate, isobutyl acetate, ethyl lactate, dimethyl phthalate, diethyl phthalate, ethylene carbonate, propylene carbonate, etc.
Hydrocarbons: petroleum ether, pentane, hexane, heptane, benzene, toluene, xylene, liquid paraffin, gasoline, and mineral oil.
Higher fatty acids: fatty acids having 4 or more carbon atoms (preferably 4 to 30 carbon atoms) other than the carboxyl group, such as oleic acid, linoleic acid, and linolenic acid.

[Application]
The composite hollow fiber membrane of the present invention is suitably used, for example, as an ultrafiltration membrane or nanofiltration membrane in fields such as the semiconductor industry, chemical industry, food industry, pharmaceutical industry, medical product industry, etc. In particular, the composite hollow fiber membrane of the present invention has high resistance to aprotic polar solvents, and is suitably used in industrial fields that use such solvents for filtration of a liquid to be treated that contains the solvent and a solute.

In addition, the composite hollow fiber membrane of the present invention is resistant to various organic solvents, making it suitable for use in membrane separation processes that treat liquids containing organic solvents and solutes.

3. Manufacturing Method of Composite Hollow Fiber Membrane The manufacturing method of 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-described structure. A suitable example includes a manufacturing method including the following first to fourth steps.
First step: A membrane-forming solution is prepared by dissolving an aliphatic 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 being incompatible with the aliphatic polyamide resin at temperatures below 100°C at a temperature of 100°C or higher.
Second step: A step of coagulating the aliphatic 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 that is compatible with the organic solvent used in the membrane-forming solution but has low affinity for the aliphatic polyamide resin is brought into contact with at least one surface of the membrane-forming solution extruded in the predetermined shape, thereby forming an aliphatic polyamide hollow fiber membrane having a dense layer on at least one surface.
Third step: The membrane forming solution solvent and the coagulation solution are removed from the aliphatic polyamide hollow fiber membrane formed in the second step.
Fourth step: A separation functional layer containing a crosslinked resin obtained by interfacial polycondensation is formed on the surface of at least one of the dense layers of the aliphatic polyamide hollow fiber membrane obtained by carrying out the third step.

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

[First step]
In the first step, a film-forming solution is prepared by dissolving an aliphatic 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 being incompatible with the aliphatic polyamide resin at temperatures below 100°C at a temperature of 100°C or higher.

Examples of organic solvents that have a boiling point of 150°C or higher and are incompatible with aliphatic 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 aliphatic polyamide hollow fiber membranes 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. While sufficient effects can be obtained when using one of these organic solvents alone, using a mixture of two or more may result in the production of more effective aliphatic polyamide hollow fiber membranes due to differences in the order and structure of phase separation.

The concentration of the aliphatic polyamide resin in the membrane-forming solution may be 20% by weight or higher, 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 aliphatic polyamide resin in the membrane-forming solution is within this range, a support layer with a porosity of 60 to 80% can be obtained, providing the aliphatic polyamide hollow fiber membrane with excellent pressure resistance. As a result, the composite hollow fiber membrane of the present invention can be provided with excellent pressure resistance and organic solvent permeability.

In addition, in the first step, when dissolving the aliphatic 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 resin at a temperature 10 to 50°C higher, and 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 aliphatic 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.

In the first step, the temperature conditions for dissolving the aliphatic polyamide resin in the organic solvent may be appropriately set within a temperature range of 100°C or higher in accordance with the aforementioned indicators, depending on the type of aliphatic 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.

In addition, fillers, thickeners, antioxidants, surface modifiers, lubricants, surfactants, etc. may be added to the membrane-forming solution as needed to control the pore size of the aliphatic polyamide hollow fiber membrane or improve its performance.

The film-forming solution prepared in the first step is subjected to the second step at the same temperature (i.e., above 100°C).

[Second step]
The second step is a step of coagulating the aliphatic 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, and in this step, a coagulation liquid (hereinafter sometimes referred to as "coagulation liquid for forming a dense layer") that is compatible with the organic solvent used in the membrane-forming solution but has a low affinity for the aliphatic polyamide resin is brought into contact with at least one surface of the membrane-forming solution extruded in the predetermined shape, thereby forming an aliphatic polyamide hollow fiber membrane having a dense layer on at least one surface.

In the second step, the membrane-forming solution is extruded into a predetermined shape into a coagulation bath, and a dense layer is formed on the surface where it comes into contact with the dense-layer-forming coagulation liquid. Near the surface where the membrane-forming solution comes into contact with the dense-layer-forming coagulation liquid, non-solvent phase separation due to solvent exchange occurs more frequently than thermally induced phase separation due to cooling. This results in the formation of a denser structure on the surface than in conventional TIPS methods, resulting in an aliphatic polyamide hollow fiber membrane having the aforementioned molecular weight cutoff.

When a dense layer is to be formed on only one surface of the aliphatic polyamide hollow fiber membrane, in the second step, one surface of the membrane-forming solution extruded into a predetermined shape is contacted with a dense layer-forming coagulating liquid, and the other surface is contacted with a coagulating liquid (hereinafter sometimes referred to as "support layer-forming coagulating liquid") that is compatible with the organic solvent used in the membrane-forming solution and has a high affinity for the aliphatic polyamide resin. When a dense layer is to be formed on both surfaces of the aliphatic polyamide hollow fiber membrane, in the second step, both surfaces of the membrane-forming solution extruded into a predetermined shape are contacted with a dense layer-forming coagulating liquid.

Specifically, the dense layer-forming coagulation liquid is a solvent that is compatible with the organic solvent used in the film-forming solution at temperatures below 25°C, but does not dissolve the aliphatic polyamide resin at temperatures below its boiling point or below 200°C. Specific examples of dense layer-forming coagulation liquids include aqueous solvents such as water and aqueous solutions with a water content of 80% by weight or more; monohydric alcohols such as 1-propanol, 2-propanol, and isobutanol; glycol ethers such as polyethylene glycol with an average molecular weight of 300 or more, polypropylene glycol with an average molecular weight of 400 or more, 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, preferred are polyethylene glycol, triacetin, and triethylene glycol monomethyl ether with an average molecular weight of 300 to 600; more preferred are polyethylene glycol 300, polyethylene glycol 400, and polyethylene glycol 600; and even more preferred are polyethylene glycol 300 and polyethylene glycol 400. These solvents may be used alone or in combination of two or more. In the present invention, the average molecular weights of polyethylene glycol and polypropylene glycol are number average molecular weights 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."

Furthermore, the coagulation liquid for forming the dense layer may contain, to the extent that a dense layer can be formed, a solvent used in the coagulation liquid for forming the support layer, such as glycerin (a solvent that is compatible with the organic solvent used in the membrane-forming solution at temperatures below 25°C and dissolves the aliphatic polyamide resin at temperatures below its boiling point). When the coagulation liquid for forming the dense layer contains a solvent used in the coagulation liquid for forming the support layer (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), the content of the solvent is preferably 5 to 38% by weight, more preferably 10 to 35% by weight, even more preferably 15 to 32% by weight, and even more preferably 15 to 30% by weight, from the viewpoint of forming a dense layer of an appropriate thickness and obtaining an aliphatic polyamide hollow fiber membrane having the aforementioned molecular weight cutoff.

The solidifying liquid for forming the support layer may be any solvent that is compatible with the organic solvent used in the film-forming solution at temperatures below 25°C and that dissolves the aliphatic polyamide resin at temperatures below its boiling point. Specific examples of solidifying liquid for forming the support layer 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, gamma-butyrolactone, delta-valerolactone, and aqueous solutions containing 20% or more by weight of any of these. Among these, from the viewpoint of obtaining a support layer having the above-mentioned porosity, preferred are 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, and an aqueous solution containing at least one of these in a proportion of 25 to 75% by weight; and more preferred are at least one selected from the group consisting of 1,4-butanediol, glycerin, propylene glycol, diethylene glycol, and tetraethylene glycol, and an aqueous solution containing at least one of these in a proportion of 40 to 80% by weight (preferably 40 to 60% by weight).

To form an aliphatic polyamide hollow fiber membrane, the second step involves using a double-tube nozzle for hollow fiber production with a double-tube structure. The membrane-forming solution is discharged from the outer annular nozzle and the internal coagulation liquid is discharged from the inner nozzle, and the membrane is immersed in a coagulation bath. In this case, a dense layer-forming coagulation liquid can be used for at least one of the internal coagulation liquid and the coagulation bath. When a dense layer-forming coagulation liquid is used for both the internal coagulation liquid and the coagulation bath, a dense layer is formed on both the inner and outer surfaces, resulting in an aliphatic polyamide hollow fiber membrane with a support layer on the interior. When a dense layer-forming coagulation liquid is used as the internal coagulation liquid and a support layer-forming coagulation liquid is used as the coagulation bath, a dense layer is formed on the inner surface, and the internal and outer surfaces are support layers. Furthermore, when a coagulating liquid for forming a support layer is used as the internal coagulating liquid and a coagulating liquid for forming a dense layer is used as the coagulating bath, a dense layer is formed on the outer surface, and an aliphatic polyamide hollow fiber membrane in which the lumen side surface and the interior are support layers is obtained. Note that, since the internal coagulating liquid used in forming the aliphatic polyamide hollow fiber membrane passes through a double annular nozzle, it is preferable that the internal coagulating liquid does not contain water whose boiling point is equal to or lower than the temperature of the double annular nozzle.

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

The flow rate at which the membrane forming solution is discharged from the outer annular nozzle of the double-tubular nozzle for hollow fiber production is not particularly limited as 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 set appropriately taking into consideration 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 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.

In the second step, the temperature of the coagulation bath may be 100°C or less, 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 an aliphatic polyamide hollow fiber membrane with 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.

The temperature of the internal coagulation liquid may be approximately 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.

By carrying out the second step in this manner, the membrane-forming solution coagulates in the coagulation bath, and an aliphatic polyamide hollow fiber membrane is formed with a dense layer formed on at least one surface.

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

There are no particular restrictions on the method for removing the membrane forming solution solvent and coagulation liquid from the aliphatic polyamide hollow fiber membrane, but a preferred method is to immerse the aliphatic polyamide hollow fiber membrane formed in the second step in an extraction solvent to extract and remove them.

The extraction solvent used for extraction and removal is preferably 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 water-soluble membrane-forming solution solvents and coagulation solutions, winding the membrane 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.

When the membrane-forming solution solvent and coagulation liquid are extracted and removed by immersing the aliphatic polyamide hollow fiber membrane in an extraction solvent, the time for which the aliphatic polyamide hollow fiber membrane is immersed 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. The extraction solvent may be replaced or stirred to effectively extract and remove the coagulation liquid remaining in the aliphatic polyamide hollow fiber membrane.

By carrying out the third step in this manner, an aliphatic polyamide hollow fiber membrane is obtained from which the membrane-forming solution solvent and coagulation liquid have been removed and which has a dense layer on at least one surface.

After the third step, it is preferable to dry and remove the extraction solvent from the aliphatic polyamide hollow fiber membrane. Drying and removal of the extraction solvent can be carried out by known drying processes such as natural drying, hot air drying, reduced pressure drying, and vacuum drying.

In addition, to increase the strength of the aliphatic polyamide hollow fiber membrane and provide it with excellent pressure resistance, the aliphatic polyamide hollow fiber membrane may be stretched uniaxially (longitudinal) simultaneously with or after drying.

To simultaneously dry and stretch uniaxially, the aliphatic polyamide hollow fiber membrane can be dried while tension for stretching is applied. The temperature conditions for simultaneously drying and stretching uniaxially are not particularly limited, as long as both drying and stretching are possible. Examples include temperatures of 40°C or higher, preferably 40 to 160°C, more preferably 50 to 140°C, and even more preferably 120 to 140°C.

When stretching in the uniaxial direction after drying, the temperature conditions during drying are not particularly limited as long as they allow the adhering extraction solvent to volatilize, but 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. When stretching in the uniaxial direction after drying, the temperature conditions during stretching are not particularly limited as long as they are -10 to 140°C, preferably 0 to 120°C. However, from the perspective of further improving liquid permeability, it is desirable for the temperature to be above the glass transition point of the aliphatic polyamide resin used (more preferably 50 to 120°C, even more preferably 60 to 100°C).

Uniaxial stretching can be performed by known methods, such as continuous winding from a low-speed roll to a high-speed roll. Alternatively, an aliphatic polyamide hollow fiber membrane cut to a specific length can be stretched by holding both ends of the membrane using a tensile tester, or by manual stretching.

The stretching ratio is, for example, 1.2 to 5 times, preferably 1.2 to 3 times. From the perspective of increasing the strength of the aliphatic polyamide hollow fiber membrane and providing it with excellent pressure resistance, the stretching ratio is preferably 1.2 to 2.4 times, more preferably 1.2 to 2.0 times.

[Formation of separation functional layer]
Hereinafter, the fourth step for forming a separation functional layer containing a crosslinked resin obtained by interfacial polycondensation on the surface of at least one of the dense layers of the aliphatic polyamide hollow fiber membrane obtained by carrying out the third step will be described in detail.

The cross-linked resin that constitutes the separation function layer is as described in the [Separation Function Layer] section of "2. Composite Hollow Fiber Membrane" above. Below, we will explain a typical example in which the cross-linked resin is a cross-linked polyamide resin.

Crosslinked polyamide resins are obtained by interfacial polycondensation of polyfunctional amines and polyfunctional acyl halides. To obtain crosslinked polyamide resins, at least one of the polyfunctional amines or polyfunctional acyl halides must contain a trifunctional or higher functional compound.

In the separation functional layer formation process, for example, a solution containing a polyfunctional amine (hereinafter also referred to as a polyfunctional amine solution) and an organic solution containing a polyfunctional acyl halide that is immiscible with the solvent of the polyfunctional amine solution (hereinafter also referred to as a polyfunctional acyl halide solution) are used to perform interfacial polycondensation on the surface of the dense layer, thereby forming a separation functional layer on the surface of the dense layer. Water is typically used as the solvent for the polyfunctional amine solution. However, if the polyfunctional amine is water-insoluble, an organic solvent such as dimethylformamide can be used instead of water, or a mixed solvent of water and an organic solvent such as dimethylformamide can also be used. When m-phenylenediamine is used as the polyfunctional amine, water is preferably used as the solvent. If an organic solvent such as dimethylformamide is used instead of water, the organic solvent must not dissolve the hollow fiber membrane and must be immiscible with the organic solvent of the polyfunctional acyl halide solution. The aliphatic polyamide hollow fiber membrane of the present invention has excellent resistance to a wide variety of organic solvents. Therefore, in the present invention, even an organic solvent such as dimethylformamide, which dissolves conventional hollow fiber membranes, can be used as the solvent for the polyfunctional amine solution.

There are no particular restrictions on the concentration of the polyfunctional amine in the polyfunctional amine aqueous solution, but it is typically approximately 0.01 to 10% by weight, preferably 0.1 to 5% by weight, more preferably 0.5 to 4% by weight, and even more preferably 1 to 3% by weight.

The polyfunctional amine aqueous solution may contain a surfactant and a phase transfer catalyst, etc., to the extent that they do not inhibit the interfacial polycondensation reaction. The surfactant improves the wettability of the dense layer surface and reduces the interfacial tension between the polyfunctional amine solution and the polyfunctional acyl halide solution. Examples of surfactants include sodium dodecyl sulfate and sodium dodecylbenzenesulfonate. The phase transfer catalyst promotes the reaction between the immiscible solutions of the polyfunctional amine solution and the polyfunctional acyl halide solution. Examples of phase transfer catalysts include tertiary amines such as triethylamine and quaternary ammonium salts such as trioctylmethylammonium chloride. If the additives are added, the total content of the additives is preferably 0.5 to 20 wt%, more preferably 1 to 10 wt%. Within this range, the effects of each additive can be effectively achieved.

To perform interfacial polycondensation on the dense layer, it is preferable to first contact the surface of the dense layer with the polyfunctional amine solution. Contact is preferably uniform and without gaps across the dense layer. Examples of contact methods include immersing the aliphatic polyamide hollow fiber membrane in the polyfunctional amine solution and passing the polyfunctional amine solution through the interior of the aliphatic polyamide hollow fiber membrane. The contact time between the dense layer and the polyfunctional amine solution is not particularly limited, but is preferably 0.5 to 10 minutes, more preferably 1 to 5 minutes. This time range allows the polyfunctional amine solution to sufficiently impregnate the dense layer while preventing excessive impregnation of the support layer. The aliphatic polyamide hollow fiber membrane may also be wetted with a solvent such as water before contact with the polyfunctional amine solution. This promotes impregnation of the polyfunctional amine solution.

After contacting the dense layer with the polyfunctional amine solution, it is preferable to remove any excess polyfunctional amine solution. This improves interfacial uniformity and reduces the likelihood of defects in the separation layer. Removal methods include, for example, blowing away excess polyfunctional amine remaining on the dense layer surface with an airflow or ventilating the hollow space of an aliphatic polyamide hollow fiber membrane. The duration of the airflow or ventilating is not particularly limited, but is preferably 5 seconds to 3 minutes, and more preferably 10 seconds to 2 minutes. A shorter duration may result in insufficient drainage, while a longer duration may result in excessive removal of the polyfunctional amine solution or dryness of the contact surface, potentially inhibiting optimal interfacial polycondensation. Alternatively, a liquid immiscible with the solvent for the polyfunctional amine solution may be passed over the dense layer surface. This method also achieves the same effects as airflow. In this case, it is preferable to use the solvent for the polyfunctional acyl halide solution, as described below. When an immiscible liquid is passed through the hollow portion of an aliphatic polyamide hollow fiber membrane, the liquid may be passed either before or after draining with an air stream, and the amount of liquid passed is about 5 to 50 times the internal volume of the hollow fiber.

Next, the dense layer that has been contacted with the polyfunctional amine solution is contacted with a polyfunctional acyl halide solution, and a separation functional layer containing cross-linked polyamide resin is formed on the surface of the dense layer through interfacial polycondensation.

The organic solvent for the polyfunctional acyl halide solution should be immiscible with the solvent for the polyfunctional amine solution, dissolve the polyfunctional acyl halide, not dissolve the aliphatic polyamide hollow fiber membrane, and be inert to the polyfunctional amine and polyfunctional acyl halide. Examples of such organic solvents include hydrocarbon solvents, chlorinated solvents, and fluorinated solvents. These may be used alone or in combination. Of these, hydrocarbon solvents are preferred from the standpoints of ease of handling and environmental impact. Examples of hydrocarbon solvents include aliphatic hydrocarbons and aromatic hydrocarbons. Examples of aliphatic hydrocarbons include linear aliphatic hydrocarbons such as hexane and octane, and branched aliphatic hydrocarbons such as isooctane. Examples of aromatic hydrocarbons include toluene and xylene.

There are no particular restrictions on the concentration of the polyfunctional acyl halide in the polyfunctional acyl halide solution, but it is typically about 0.01 to 5% by weight, preferably 0.01 to 1% by weight, and more preferably 0.05 to 0.5% by weight. Within this range, a separation functional layer with a uniform and sufficient thickness can be formed on the surface of the dense layer.

The polyfunctional acyl halide solution can be brought into contact with the dense layer surface coated with the polyfunctional amine solution in the same manner as described above for contacting the dense layer surface with the polyfunctional amine solution. The contact time is not particularly limited, as long as it is long enough for the interfacial polycondensation reaction between the polyfunctional amine and the polyfunctional acyl halide to proceed sufficiently and for a separation functional layer to form on the surface of the dense layer. It is typically about 5 seconds to 10 minutes, preferably 10 seconds to 5 minutes, and more preferably 30 seconds to 3 minutes.

Next, it is preferable to remove the excess polyfunctional acyl halide solution using a method similar to the method for removing the excess polyfunctional amine solution described above. It is also effective to remove the excess polyfunctional acyl halide solution by washing and replacing only the contact surface with the solvent of the polyfunctional acyl halide solution, followed by draining. When passing the solvent of the polyfunctional acyl halide solution through the hollow portion of the aliphatic polyamide hollow fiber membrane, the solution may be passed either before or after draining with an airflow, and the amount of solution passed is approximately 5 to 50 times the internal volume of the hollow fiber.

After removing excess polyfunctional acyl halide solution, it is preferable to dry the contact surface. This allows the separation functional layer to be firmly fixed onto the dense layer. Drying methods include, for example, placing the composite hollow fiber membrane in a dryer or applying or ventilating hot air only to the surface on which the separation functional layer is formed. The temperature during drying or ventilating is not particularly limited, but is preferably 50 to 120°C, more preferably 80 to 100°C. The drying time can be set appropriately so that the solvent in the polyfunctional acyl halide solution can be sufficiently removed, but is typically approximately 0.5 to 60 minutes, preferably 0.5 to 30 minutes, more preferably 1 to 20 minutes, and even more preferably 1 to 10 minutes.

The dried composite hollow fiber membrane may be used as is, or may be further washed. Unreacted polyfunctional amines and polyfunctional acyl halides may remain in the membrane. If these remain in the membrane, they may leach out during use of the composite hollow fiber membrane, so they are preferably removed by washing. Water is preferably used for washing. Examples of washing methods include immersion in water and passing water through the hollow portions of the composite hollow fiber membrane. The temperature during washing is not particularly limited, but is typically between room temperature and 70°C. The washing time can be adjusted appropriately depending on the water temperature. If immersion at room temperature is used, it is preferable to leave the membrane for 12 hours or more.

The washed composite hollow fiber membrane may be used as is, or may be dried again. A dried composite hollow fiber membrane is more advantageous for subsequent processes such as modularization. Drying should be carried out under conditions that substantially eliminate moisture, and the drying temperature is preferably 25 to 100°C, more preferably 40 to 70°C.

Below, as a suitable example of the fourth step, a method of forming a separation functional layer on the surface of the dense layer on the lumen side of the aliphatic polyamide hollow fiber membrane will be described in detail.

One method for forming a separation functional layer on the surface of the dense layer on the inner surface of an aliphatic polyamide hollow fiber membrane is to inject a polyfunctional amine solution into the hollow portion of the aliphatic polyamide hollow fiber membrane, then inject a polyfunctional acyl halide solution into the hollow portion, and cause interfacial polycondensation between the polyfunctional amine and the polyfunctional acyl halide on the dense layer, thereby forming a separation functional layer on the surface of the dense layer.

The method for injecting the polyfunctional amine solution and polyfunctional acyl halide solution into the hollow spaces of the aliphatic polyamide hollow fiber membranes is not particularly limited as long as it can uniformly coat the surface of the dense layer. However, a preferred method involves forming a module using aliphatic polyamide hollow fiber membranes, injecting the polyfunctional amine solution into the hollow spaces of the aliphatic polyamide hollow fiber membranes of the module, and then injecting the polyfunctional acyl halide solution. Specifically, this injection method involves the following steps: First, prepare a bundle of the desired number of aliphatic polyamide hollow fiber membranes of the desired length. Next, prepare a rigid tube and insert a rubber stopper or similar of an appropriate length into one end opening of the tube to plug the end opening. Next, inject a two-component thermosetting resin into the end opening opposite the plugged end of the tube, filling the interior space of the tube with the resin. One end of the bundled aliphatic polyamide hollow fiber membranes is then heat-sealed to seal the openings, and the membrane is inserted into the tube filled with the thermosetting resin until the tip of the end touches the stopper, allowing the thermosetting resin to harden in this state. The region of the cured resin on the stopper side is then cut along with the tube to open the hollow portion of the aliphatic polyamide hollow fiber membrane. An appropriate jig is then attached to the tube on the open side, and a polyfunctional amine solution is injected, followed by a polyfunctional acyl halide solution. A wetting solution may be injected before the injection of the polyfunctional amine solution.

The injection method is not particularly limited, and examples include injection methods using a syringe, pressure-feeding methods using pressurized gas, and injection methods using pumps such as peristaltic pumps, plunger pumps, diaphragm pumps, and gear pumps.

The speed at which the polyfunctional amine solution and polyfunctional acyl halide solution are injected is not particularly limited, but is, for example, approximately 0.01 to 2 m/sec as a linear velocity, and from the viewpoints of suppressing uneven wetting, achieving uniform coating, and production efficiency, is preferably 0.05 to 1 m/sec. The polyfunctional amine solution and polyfunctional acyl halide solution are preferably injected continuously to ensure the specified contact time. The contact time is as described above.

As described above, in order to remove excess polyfunctional amine solution and polyfunctional acyl halide solution, it is preferable to ventilate a gas into the hollow space of the aliphatic polyamide hollow fiber membrane after injecting the polyfunctional amine solution and the polyfunctional acyl halide solution. There are no particular restrictions on the gas used for ventilating, but dry air or nitrogen is preferred from the standpoints of preventing side reactions and safety.

The gas aeration speed is not particularly limited, but is, for example, approximately 1 to 100 m/s as a linear velocity. From the viewpoints of preventing defects in the separation functional layer due to poor interfacial polycondensation caused by removal of excess reaction liquid, preventing peeling of the formed separation functional layer, and production efficiency, it is preferably 2.5 to 70 m/s, more preferably 10 to 50 m/s. Aeration is preferably carried out continuously for a predetermined period of time, and while there are no particular limitations on the aeration time, it is preferably 5 seconds to 5 minutes, more preferably 20 seconds to 2 minutes.

After injecting the polyfunctional acyl halide solution, it is preferable to heat-treat and dry the membrane surface in order to promote the interfacial polycondensation reaction, fix the separation functional layer to the surface of the dense layer, and remove solvents, etc. The heating temperature and heating time are as described above.

After the interfacial polycondensation reaction is complete, it is preferable to wash the composite hollow fiber membrane to remove unreacted materials and by-products. The washing method and conditions are as described above.

The tubing sections used when injecting the polyfunctional amine solution and polyfunctional acyl halide solution are cut and removed at the appropriate time, for example, before or after heat-treating the composite hollow fiber membrane.

Methods for forming a separation functional layer on the surface of the outer dense layer of an aliphatic polyamide hollow fiber membrane include, for example, sequentially immersing the aliphatic polyamide hollow fiber membrane in baths of a polyfunctional amine solution and a polyfunctional acyl halide solution, or sequentially applying a polyfunctional amine solution and a polyfunctional acyl halide solution to the surface of the outer dense layer of the aliphatic polyamide hollow fiber membrane, and, if necessary, draining the solution and blowing gas to remove excess liquid, repeating the process. These treatments can be performed as a batch process on a hollow fiber bundle, or continuously in a roll-to-roll process. Even when a separation functional layer is formed on the surface of the outer dense layer of an aliphatic polyamide hollow fiber membrane using these methods, it is preferable to perform post-treatments such as heat treatment and washing.

In addition, methods for forming separation functional layers on the surface of the lumen side and outer dense layer of an aliphatic polyamide hollow fiber membrane include, for example, a method of forming a separation functional layer on the surface of the lumen side dense layer of the aliphatic polyamide hollow fiber membrane and a method of forming a separation functional layer on the surface of the outer dense layer of the aliphatic polyamide hollow fiber membrane, in any order or in combination.

It is also preferable that the composite hollow fiber membrane be dried, as this facilitates application to the potting process for fixing the composite hollow fiber membrane to the module. Furthermore, a dried composite hollow fiber membrane is also preferable because it can also be used in the air leak test. The composite hollow fiber membrane of the present invention can exhibit desired performance even when it is a dried membrane. Typically, when ultrafiltration membranes or reverse osmosis membranes are dried, the pores shrink or become clogged due to the surface tension caused by the evaporation of water, often resulting in a loss of permeability. However, the composite hollow fiber membrane of the present invention can exhibit desired properties even when it is a dried membrane before the treatment liquid is passed through it. Drying conditions are not particularly limited, but ventilation drying at a temperature of 25 to 100°C is preferred, and more preferably 40 to 70°C is preferred.

By carrying out the fourth step in this manner, a composite hollow fiber membrane of the present invention is obtained, which has a separation functional layer on the surface of at least one of the dense layers of the aliphatic polyamide hollow fiber membrane.

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 liquid to be treated, an outlet for the permeated liquid, etc., and used as a hollow fiber membrane module.

Specifically, the hollow fiber membrane module may be configured by bundling the composite hollow fiber membranes of the present invention, housing them in a module case, and sealing and fixing one or both ends of the composite hollow fiber membranes with a potting agent. The hollow fiber membrane module may be provided with an opening connected to a flow path passing through the outer wall surface 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 liquid to be treated or an outlet for the filtrate.

The shape of the hollow fiber membrane module is not particularly limited and may be a dead-end module or a cross-flow module. Specific examples include dead-end modules in which hollow fiber membrane bundles are folded into a U-shape and packed, with the ends of the hollow fiber membrane bundle sealed and then cut open; dead-end modules in which hollow fiber membrane bundles with the hollow opening at one end closed by heat sealing or other means are packed straight, with the open end of the hollow fiber membrane bundle sealed and then cut open; dead-end modules in which hollow fiber membrane bundles are packed straight, both ends of the hollow fiber membrane bundle are sealed, and only one end is cut to expose the opening; and cross-flow modules in which hollow fiber membrane bundles are 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.

The filling 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%. Meeting this filling rate ensures a sufficient filtration area, facilitates the loading of the composite hollow fiber membranes into the module case, and allows the potting agent to flow easily between the composite hollow fiber membranes.

There are no particular restrictions on the potting agent used in the manufacture of hollow fiber membrane modules. However, if the hollow fiber membrane module is to be used for processing organic solvents, it is desirable that the potting agent be resistant to organic solvents. Examples of such potting agents include polyamide, silicone 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 and are not too hard are preferred. Suitable examples include polyamide, silicone resin, epoxy resin, and polyethylene. These potting agents may be used alone or in combination of two or more.

The material of the module case used in the hollow fiber membrane module is not particularly limited as long as it is durable against the solvent used. Examples of suitable materials include metals and polymers, such as polyamide, polyester, polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl chloride, polysulfone, polyethersulfone, polycarbonate, polyarylate, and polyphenylene sulfide. Among these, polyamide, polyethylene, polypropylene, polytetrafluoroethylene, polycarbonate, polysulfone, and polyethersulfone are preferred, with polyamide, polyethylene, polypropylene, and polytetrafluoroethylene being even more preferred.

Hollow fiber membrane modules using the composite hollow fiber membrane of the present invention are used in ultrafiltration or nanofiltration applications for removing foreign matter from solvents, concentrating useful components in solvents, recovering solvents, etc. in fields such as the semiconductor, chemical, food, pharmaceutical, and medical product industries.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

1. Measurement method [Outer diameter and inner diameter of aliphatic polyamide hollow fiber membrane]
The five aliphatic polyamide hollow fiber membranes were observed under an optical microscope at a magnification of 200 times, and the outer and inner diameters of each hollow fiber membrane (both at the maximum diameter points) were measured, and the average values were calculated.

[Porosity of Support Layer]
Scanning electron microscope (SEM) photographs were taken at five locations equidistantly spaced in the thickness direction of the support layer of the aliphatic polyamide hollow fiber membrane in a cross section of the composite hollow fiber membrane cut perpendicular to the longitudinal direction. Specifically, the photographs were taken as follows. Fig. 11 is a schematic cross-sectional view of the aliphatic polyamide hollow fiber membrane cut perpendicular to the longitudinal direction. Figs. 12 and 13 are partial enlarged views of the area surrounded by the dotted line in Fig. 11 , showing an example of an aliphatic polyamide hollow fiber membrane 8a having a dense layer 10 and a support layer 11. Note that the separation function layer is omitted in Figs. 11 to 13. As shown in Fig. 11, a straight line was drawn from the center of the lumen of the aliphatic polyamide hollow fiber membrane 8a to the outer surface, and this line was divided into five equal parts from the lumen-side surface of the support layer 11 to the outer surface. The midpoints of the five equal parts were then determined as shown in Fig. 12. 13, five locations were photographed at a uniform magnification such that the midpoint was at the center of the photographed image, the photographed image included only the support layer 11 portion, and the number of pores was 30 to 300. For the five SEM photographs taken, image analysis of the regions was performed using image analysis software (ImageJ), and the pore portions and polymer portions were distinguished by binarization processing, and the area ratio (%) of the total pore area to the area of the analyzed region was calculated, and the average value was calculated.
The specific operations in ImageJ are as follows: The image to be analyzed is imported into ImageJ, the analysis range is specified, and the brightness value at which the peak of the histogram obtained by the "Analyze>Histogram" operation is highest is set as the threshold (i.e., the lower threshold level is set to the brightness value at which the peak of the histogram is highest, and the upper threshold level is set to 255), and the image is binarized using "Image>Adjust>Threshold." Next, "Area" was checked in "Analyze>Set measurements" and the total area of the analysis range was determined by operating "Analyze>Measure." Then, "Analyze>Set measurements" was checked and "Area" and "Limit to threshold" were checked, and the "Analyze>Measure" operation was again performed to calculate the areas of the polymer portion and the pore portion, and the area ratio (%) of the total pore area to the area of the analysis region was calculated.

[Thickness of dense layer]
The cross section of the aliphatic polyamide hollow fiber membrane was cut perpendicular to the longitudinal direction and observed with a scanning electron microscope (SEM) at a magnification of 10,000. In the obtained SEM photograph, the distance (thickness) of a region where the presence of pores was substantially not observed was measured at 10 regularly spaced locations, and the average value was calculated.

[Crystallization degree of aliphatic polyamide hollow fiber membrane]
The crystallinity of the aliphatic polyamide hollow fiber membrane was measured by X-ray diffraction (XRD). Multiple hollow fiber samples were closely spaced on a measurement stage, and a scattering pattern spectrum of scattering vectors and scattering intensity was obtained by scanning the scattering angle. The obtained spectrum was separated into crystalline and amorphous components, and the crystallinity was determined by quantifying the components.

[Molecular weight cutoff of aliphatic polyamide hollow fiber membrane (greater than 3000)]
Ten aliphatic polyamide hollow fiber membranes were cut to 30 cm lengths and bundled together. Next, a nylon hard tube with an outer diameter of 8 mm, an inner diameter of 6 mm, and a length of 50 mm was prepared. 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 bundle of prepared aliphatic polyamide hollow fiber membranes was then heat-sealed to prevent the epoxy resin from penetrating the hollow space. The end was then inserted into the tube filled with the epoxy resin until its tip touched the rubber stopper, and the epoxy resin was allowed to cure 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 space. The same procedure was performed on the other end to produce a cross-flow module 6 with open hollow spaces at both ends of the aliphatic polyamide hollow fiber membranes.
The prepared crossflow module 6 was connected to the internal pressure separation treatment line shown in Figure 1, and the flowing liquid (raw liquid) was continuously passed through the crossflow module 6 by the liquid circulation pump 2. The flowing liquid was an aqueous solution containing five types of dextrans with molecular weights of 5000, 10000, 40000, 70000, or 500000 at 0.5 wt%, 0.5 wt%, 0.2 wt%, 0.2 wt%, or 0.4 wt%, respectively. The pressures of the primary pressure gauge 3 and the secondary pressure gauge 4 were adjusted by regulator 5 so that the arithmetic mean value of the pressures of the primary pressure gauge 3 and the secondary pressure gauge 4 was 1 bar. Of the flowing liquid permeating through the module, that which permeated the pores of the polyamide hollow fiber membrane was recovered as permeate separated from the flowing liquid, and the remainder was circulated back to the separation treatment line. Two hours after the start of circulation, the permeate flowing out of the cross-flow module 6 was collected in a receiver 7, and the dextran concentration in each permeate was measured by high-performance liquid chromatography. The rejection rate for each molecular weight was calculated according to the following formula: Based on the rejection rate results for dextran of each molecular weight, a graph was created with the molecular weight of the dextran used on the horizontal axis and the rejection rate on the vertical axis, and the molecular weight at the intersection of the obtained approximation curve and the 90% rejection rate was taken as the molecular weight cutoff.
Solute rejection rate (%) = {(dextran concentration in stock solution - dextran concentration in permeate) / dextran concentration in stock solution} × 100

[Molecular weight cutoff of aliphatic polyamide hollow fiber membrane (3000 or less)]
The molecular weight cutoff was determined in the same manner as above, except that an NMP solution containing 0.05% by weight of each of four types of polystyrene having molecular weights of 370, 1000, 2000, and 3000 was used as the flowing liquid, the arithmetic mean value of the pressure measured by the primary pressure gauge 3 and the pressure measured by the secondary pressure gauge 4 was set to 15 bar, and the rejection rate was calculated according to the following formula:
Solute rejection rate (%) = {(polystyrene concentration in raw solution - polystyrene concentration in permeate) / polystyrene concentration in raw solution} x 100

[Burst pressure of aliphatic polyamide hollow fiber membrane]
The module 8 shown in Figure 2a was fabricated. First, ten aliphatic polyamide hollow fiber membranes 8a were cut to 30 cm lengths and bundled together. Next, a nylon hard tube 8b with an outer diameter of 8 mm, an inner diameter of 6 mm, and a length of 50 mm was prepared. 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. The bundle of aliphatic polyamide hollow fiber membranes was then bent into a roughly U-shape, and both ends of the aliphatic polyamide hollow fiber membrane were heat-sealed to prevent the epoxy resin from penetrating the hollow space. The end was then inserted into the tube filled with epoxy resin until the tip of the end touched the rubber stopper, and the epoxy resin was allowed to harden in this state. Next, the region of the cured epoxy resin on the rubber stopper side was cut off together with the tube, thereby producing a module 8 in which the hollow portions at both ends of the aliphatic polyamide hollow fiber membrane were open.
Next, the module 8 was set in the device shown in FIG. 2b, and water pressure was applied to the module 8 using a hand pump 9, and the pressure (burst pressure, unit: MPa) at which the module 8 broke was measured.

[Observation of Separation Functional Layer]
In a cross section of a composite hollow fiber membrane cut perpendicular to the longitudinal direction, the composite surface and its vicinity were observed with a scanning electron microscope (SEM) at a magnification of 10,000 or 20,000. The composite surface was also observed from above, either on the inner or outer side, with an SEM at a magnification of 10,000 or 20,000.

[Air leak amount (pressure drop amount) of composite hollow fiber membrane]
Twenty composite hollow fiber membranes were cut into 30 cm lengths and bundled together. Next, a polybutylene terephthalate (PBT) tube with an outer diameter of 12.7 mm, an inner diameter of 9.5 mm, and a length of 53 mm was prepared. A silicone tube with an inner diameter of 12 mm and a length of approximately 50 mm was inserted approximately 15 mm into one end opening of the tube to connect the two. A rubber stopper approximately 20 mm long was inserted into the silicone tube opposite the end inserted into the PBT tube to plug the one end opening. Next, a two-component epoxy resin was injected through the opening opposite the rubber stopper to fill the inner space of the tube. One end of the bundle of composite hollow fiber membranes prepared above was then heat-sealed to prevent the epoxy resin from penetrating the hollow space. The end tip was inserted into the epoxy resin-filled tube until it touched the rubber stopper, and the epoxy resin was allowed to harden in this state. Next, the cured epoxy resin was cut along with the tube at the rubber stopper side to open the hollow portion. The same procedure was performed on the other end to produce a cross-flow module with open hollow portions at both ends of the composite hollow fiber membrane. The membrane area of the module was calculated based on the inner area of the hollow fibers and the inner diameter, number, and effective length of the hollow fibers.
One end of the outlet connected to the hollow part of the fabricated module was sealed, and a valve and a pressure line with a pressure gauge were attached to one end between the valve and the module. Air was pressurized to 0.2 MPa in the hollow part, and the pressure was contained by closing the valve. Immediately after that, the pressure was checked over time, and the change in pressure after 5 minutes was measured. The containment volume (total pressurized space volume V) including the hollow fibers, module, and pressure line was estimated, and the length of the line was adjusted so that the ratio of the membrane area S to the total pressurized space volume V was within ±10% of the relationship in the following formula:
V/S=0.0002×0.45/R
R is the inner diameter (mm) of the composite hollow fiber membrane.

[Burst pressure of composite hollow fiber membrane]
The crossflow module 6 prepared above was connected to the internal pressure separation treatment line shown in Figure 1, and NMP was continuously permeated through the crossflow module 6 by the liquid supply circulation pump 2. In the module, the pressures measured by the primary pressure gauge 3 and the secondary pressure gauge 4 were regulated by the regulator 5, and the arithmetic mean value of the pressures measured by the primary pressure gauge 3 and the secondary pressure gauge 4 was defined as the applied pressure, which was increased by 0.25 MPa at 10-minute intervals. The pressure (MPa) at which the composite hollow fiber membrane ruptured, i.e., the pressure at which a sudden drop in pressure occurred, was defined as the burst pressure.

[NMP permeation amount]
The crossflow module 6 prepared as described above was connected to the internal pressure separation treatment line shown in Figure 1, and the flowing liquid was continuously passed through the crossflow module 6 by the liquid circulation pump 2. NMP was used as the flowing liquid. In the module, the pressures of the primary pressure gauge 3 and the secondary pressure gauge 4 were adjusted by the regulator 5 so that the arithmetic mean value of the pressures of the primary pressure gauge 3 and the secondary pressure gauge 4 was 75% of the burst pressure of the composite hollow fiber membrane. Of the flowing liquid passing through the module, that which permeated the pores of the composite hollow fiber membrane was recovered as permeated liquid separated from the flowing liquid, and the remainder was circulated back to the separation treatment line. The permeated liquid flowing out of the crossflow module 6 between 2 and 4 hours after the start of circulation was collected in the receiver 7, and the NMP permeation rate (L/(m2·h)) was calculated according to the following formula.
NMP permeation amount = volume (L) of NMP permeated to the outside of the composite hollow fiber membrane / [inner diameter (m) of composite hollow fiber membrane × 3.14 × effective filtration length (m) of composite hollow fiber membrane × 20 (membranes) × time (h)]

Effective filtration length of composite hollow fiber membrane: The length of the portion of the outer surface of the composite hollow fiber membrane that is not coated with epoxy resin in the crossflow module.

[Rejection rates of polystyrene and diphenyl sulfone]
The rejection rates of polystyrene and diphenyl sulfone were measured in the same manner as in the measurement of the above-mentioned [NMP permeation amount], except that an NMP solution containing 0.2 wt % of polystyrene or diphenyl sulfone was used. The concentrations of the obtained permeated liquid and eluted liquid were measured using the RI detection values of each component separated by liquid chromatography (SEC mode, mobile phase: DMF), and the rejection rates (%) of polystyrene and diphenyl sulfone were calculated from the following formula.
Rejection %=100−(concentration of permeate/concentration of flow-through)×100

2. Test Example [Example 1]
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 180 ° C for 1.5 hours, and the stirring speed was reduced to degass for 1 hour to prepare a membrane-forming solution. The membrane-forming solution was pumped via a metering pump to a spinneret maintained at 200 ° C, and a mixture of 70 wt % polyethylene glycol 300 (PEG300) and 30 wt % glycerin was passed through as the internal coagulation liquid. The extruded membrane-forming solution was poured into a coagulation bath consisting of a 60 wt % aqueous 1,4-butylene glycol (1,4BG) solution at 5 ° C and cooled to solidify, forming an aliphatic polyamide porous membrane. The wound aliphatic 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 an aliphatic polyamide hollow fiber membrane. The outer diameter, inner diameter, support layer porosity, dense layer thickness, molecular weight cutoff, burst pressure, and crystallinity of the obtained aliphatic polyamide hollow fiber membrane are shown in Table 1. Figure 3 is an image analysis diagram after binarization processing to calculate the porosity of the support layer of the obtained aliphatic polyamide hollow fiber membrane. Figure 4 is a scanning electron microscope image (magnification 10,000x) of the lumen side surface of the obtained aliphatic polyamide hollow fiber membrane. Figure 5 is a scanning electron microscope image (magnification 20,000x) of the lumen side surface of the obtained aliphatic polyamide hollow fiber membrane. Figure 6 is a scanning electron microscope image (magnification 10,000x) of the lumen side cross section of the obtained aliphatic polyamide hollow fiber membrane.
A membrane bundle of 20 aliphatic polyamide hollow fiber membranes, each 40 cm long, was prepared. 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 aliphatic polyamide hollow fiber membrane was then heat-sealed to seal the opening, and the membrane was inserted into the epoxy resin-filled tube until the tip of the end touched the rubber stopper. The epoxy resin was then cured in this state. The region of the cured epoxy resin facing the rubber stopper was then cut along with the tube to open the hollow portion of the aliphatic polyamide hollow fiber membrane, yielding a module for injecting interfacial polymerization solutions.
Water was poured into the resin-fixed open end of the module and immersed in water for 1 hour to wet the entire membrane. The module was then removed from the water and gently shaken to remove any adhering water. Nitrogen was then blown in through the open end to remove any remaining water from the hollow fiber, completing the wetting process. A PP connector with a three-way cock was then attached to the open end of the module, and a circuit device was then connected to inject the interfacial polymerization solution and nitrogen gas for ventilation into the hollow fiber membrane.
An aqueous solution containing 2 wt% m-phenylenediamine (MPD) was passed through the open end of the module at a linear velocity of 0.12 m/s for 1 minute. The flow path was then switched to an airflow, and nitrogen gas was passed through at a linear velocity of 20 m/s for 1 minute. A hexane solution containing 0.1 wt% trimesoyl chloride (TMC) was passed through at a linear velocity of 0.12 m/s for 1 minute. The flow path was then switched to an airflow, and nitrogen gas was passed through at a linear velocity of 20 m/s for 1 minute. The module was then removed from the circuit device and placed in an oven at 100°C for 5 minutes to dry. The dried module was immersed in pure water and washed for 24 hours, with several water changes. The washed module was then placed in a convection dryer at 50°C and dried. The hollow fiber membranes were then cut near the epoxy resin portion of the dried module, yielding a bundle of 20 composite hollow fiber membranes. A module was fabricated using the obtained composite hollow fiber membrane, and an air leak test and a filtration test were performed. Table 1 shows the measurement results for each of the above measurements. FIG. 7 is a scanning electron microscope image (magnification: 10,000x) of the lumen side cross section of the obtained composite hollow fiber membrane. FIG. 8 is a scanning electron microscope image (magnification: 20,000x) of the lumen side cross section of the obtained composite hollow fiber membrane. FIG. 9 is a scanning electron microscope image (magnification: 10,000x) of the lumen side surface of the obtained composite hollow fiber membrane. FIG. 10 is a scanning electron microscope image (magnification: 20,000x) of the lumen side surface of the obtained composite hollow fiber membrane.

[Example 2]
The following interfacial polymerization was carried out using the aliphatic polyamide hollow fiber membrane prepared in Example 1. First, a module into which the interfacial polymerization solution was injected was prepared in the same manner as in Example 1, and the module was subjected to a wetting treatment in the same manner, followed by connection to a circuit device for liquid passage.
A dimethylformamide solution containing 2 wt% 3,3',4,4'-tetraaminobiphenyl (TAB) was passed through the open end of the module at a linear velocity of 0.12 m/s for 1 minute. The flow path was then switched to an airflow, and nitrogen gas was passed through at a linear velocity of 20 m/s for 1 minute. A hexane solution containing 0.1 wt% trimesoyl chloride (TMC) was passed through at a linear velocity of 0.12 m/s for 1 minute. The flow path was then switched to an airflow, and nitrogen gas was passed through at a linear velocity of 20 m/s for 1 minute. The module was then removed from the circuit device and placed in an oven at 100°C for 1 minute to dry. The dried module was immersed in pure water and washed for 24 hours, with several water changes. The washed module was then placed in a convection dryer at 50°C and dried. The hollow fiber membranes of the dried module were then cut near the epoxy resin portion to obtain a bundle of 20 composite hollow fiber membranes. A module was fabricated using the obtained composite hollow fiber membrane, and an air leak test and a filtration test were carried out. Table 1 shows the measurement results for each of the above measurements.

[Comparative Example 1]
A composite hollow fiber membrane was produced in the same manner as in Example 1, except that in preparing the membrane-forming solution, 280 g of polyamide 6 chips (A1030BRT, manufactured by Unitika Ltd., relative viscosity 3.53) and 720 g of dimethyl sulfone (manufactured by Tokyo Chemical Industry Co., Ltd.) were dissolved by stirring at 190°C for 1.5 hours, and the stirring speed was reduced to degas for 1 hour to prepare a membrane-forming solution. Table 1 shows the measurement results for each of the above measurements.

[Comparative Example 2]
A composite hollow fiber membrane was produced in the same manner as in Example 1, except that in preparing the membrane-forming solution, 250 g of polyamide 6 chips (A1030BRT, manufactured by Unitika Ltd., relative viscosity 3.53) and 750 g of dimethyl sulfone (manufactured by Tokyo Chemical Industry Co., Ltd.) were dissolved by stirring at 180°C for 1.5 hours, and the stirring speed was reduced to degas for 1 hour to prepare a membrane-forming solution. Table 1 shows the measurement results for each of the above measurements.

[Comparative Example 3]
A composite hollow fiber membrane was produced in the same manner as in Example 1, except that in preparing the membrane-forming solution, 230 g of polyamide 6 chips (A1030BRT, manufactured by Unitika Ltd., relative viscosity 3.53) and 770 g of dimethyl sulfone (manufactured by Tokyo Chemical Industry Co., Ltd.) were dissolved by stirring at 180°C for 1.5 hours, and the stirring speed was reduced to degas for 1 hour to prepare a membrane-forming solution. Table 1 shows the measurement results for each of the above measurements.

[Comparative Example 4]
A composite hollow fiber membrane was produced in the same manner as in Example 1, except that the membrane-forming solution was prepared by stirring and dissolving 280 g of polyamide 6 chips (Unitika Ltd., A1030BRT, relative viscosity 3.53) and 720 g of dimethyl sulfone (Tokyo Chemical Industry Co., Ltd.) at 180°C for 1.5 hours, then reducing the stirring speed and degassing for 1 hour, and that a mixture of 60 wt% polyethylene glycol 300 (PEG300) and 40 wt% glycerin was used as the internal coagulation liquid. No dense layer was observed in the aliphatic polyamide hollow fiber membrane before interfacial polymerization. The estimated pore size by bubble point method was equivalent to 5 nm. Table 1 shows the measurement results for each of the above measurements.

[Comparative Example 5]
In preparing the membrane forming solution, 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 180°C for 1.5 hours, and the stirring speed was reduced to degass for 1 hour to prepare the membrane forming solution. Polyethylene glycol 400 (PEG400) was used as the internal coagulation liquid. The resulting aliphatic polyamide hollow fiber membrane was stretched 1.3 times by passing it between rolls operating at different speeds. An aliphatic polyamide hollow fiber membrane was produced in the same manner as in Example 1. The produced hollow fiber membrane was subjected to various measurements without undergoing interfacial polymerization. Table 1 shows the measurement results for each of the measurements.

The composite hollow fiber membranes of Examples 1 and 2 had a separation function layer containing a crosslinked aromatic polyamide resin on the surface of the dense layer of the aliphatic polyamide hollow fiber membrane, and had an NMP burst pressure of 2.25 MPa or more, resulting in a very high NMP permeation rate of 6 L/( ^m2 ·h) or more, a diphenyl sulfone rejection rate in the NMP solution of 30% or more, and a polystyrene rejection rate of 1000 molecular weight of 90% or more, thereby providing extremely high rejection performance at the nanofiltration level and achieving both high levels of NMP permeability and diphenyl sulfone rejection. On the other hand, the composite hollow fiber membranes of Comparative Examples 1 to 4 and the aliphatic polyamide hollow fiber membrane of Comparative Example 5 had low NMP burst pressures, resulting in low NMP permeability and poor NMP permeability.

1 Flowing liquid tank 2 Liquid feed pump 3 Primary pressure gauge 4 Secondary pressure gauge 5 Regulator 6 Cross flow module 7 Receptacle 8 Burst pressure evaluation module 8a Aliphatic polyamide hollow fiber membrane 8b Nylon hard tube 9 Hand pump 10 Dense layer 11 Support layer

Claims (12)

A composite hollow fiber membrane comprising: an aliphatic polyamide hollow fiber membrane having a dense layer and a support layer; and a separation function layer containing a crosslinked resin obtained by interfacial polycondensation, wherein the separation function layer is provided on the surface of the dense layer, and the burst pressure measured under the following conditions is 2.25 MPa or more.
<Burst pressure>
While N-methyl-2-pyrrolidone is passed through a module prepared using a composite hollow fiber membrane, the pressure is increased by 0.25 MPa at 10-minute intervals, and the pressure (MPa) at which the composite hollow fiber membrane breaks is measured. The composite hollow fiber membrane described in claim 1, wherein the crosslinked resin is a crosslinked aromatic polyamide resin. A composite hollow fiber membrane as described in claim 1, wherein the support layer has a porosity of 60 to 80%. A composite hollow fiber membrane as described in claim 1, wherein the dense layer has a thickness of 0.1 to 5 μm. A composite hollow fiber membrane as described in claim 1, wherein the dense layer is located on the inner surface side of the aliphatic polyamide hollow fiber membrane. A composite hollow fiber membrane as described in claim 1, wherein the aliphatic polyamide hollow fiber membrane has a molecular weight cutoff of 10,000 to 300,000. 2. The composite hollow fiber membrane according to claim 1, wherein the aliphatic polyamide hollow fiber membrane has a burst pressure of 2.1 MPa or more as measured under the following conditions:
<Burst pressure>
A water pressure is applied to a module made using an aliphatic polyamide hollow fiber membrane, and the pressure (MPa) is measured when the aliphatic polyamide hollow fiber membrane breaks. A composite hollow fiber membrane as described in claim 1, in which the rejection rate of diphenyl sulfone when a solution in which the solvent is N-methyl-2-pyrrolidone and the solute is diphenyl sulfone is filtered is 30% or more. 2. The composite hollow fiber membrane according to claim 1, wherein the permeation rate of N-methyl-2-pyrrolidone at 25°C is 6 L/(m ² ·h) or more. A filtration method using the composite hollow fiber membrane described in any one of claims 1 to 9 to filter a liquid to be treated containing an organic solvent and a solute. A hollow fiber membrane module comprising a module case containing a composite hollow fiber membrane according to any one of claims 1 to 9. A method for producing a composite hollow fiber membrane, comprising the following first to fourth steps:
a first step of preparing a film-forming solution by dissolving an aliphatic polyamide resin at a concentration of 20% by weight or more in an organic solvent having a boiling point of 150°C or more and being incompatible with the aliphatic polyamide resin at a temperature below 100°C, at a temperature of 100°C or more;
a second step in which the membrane-forming solution is extruded into a coagulation bath at 100°C or less in a predetermined shape to coagulate the aliphatic polyamide resin into a membrane, in which a coagulation liquid that is compatible with the organic solvent used in the membrane-forming solution but has low affinity for the aliphatic polyamide resin is brought into contact with at least one surface of the membrane-forming solution extruded into the predetermined shape, thereby forming an aliphatic polyamide hollow fiber membrane having a dense layer on at least one surface;
a third step of removing the membrane-forming solution solvent and the coagulation solution from the aliphatic polyamide hollow fiber membrane formed in the second step; and a fourth step of forming a separation functional layer containing a crosslinked resin obtained by interfacial polycondensation on the surface of at least one of the dense layers of the aliphatic polyamide hollow fiber membrane obtained by performing the third step.