Polyacrylonitrile porous hollow fiber membrane

Abstract

An object of the present invention is to provide a polyacrylonitrile porous hollow fiber membrane having a molecular weight cutoff of 500 kJ or more, which is suitable for internal pressure cross-flow filtration. The polyacrylonitrile porous hollow fiber membrane of the present invention is characterized by having a minimum pore size layer near the inner surface, and the average pore size of the minimum pore size layer being 50 nm or more.

Description

[Technical Field]

[0001] The present invention relates to a polyacrylonitrile porous hollow fiber membrane, and more particularly to a polyacrylonitrile porous hollow fiber membrane that has an excellent balance between pore size and membrane strength and is suitable for internal pressure cross-flow filtration with little leachable matter. [Background technology]

[0002] Microfiltration (MF) and ultrafiltration (UF) membranes are used in the manufacturing process of biopharmaceuticals, vaccines, etc. for the purposes of purifying, concentrating, and buffer exchange of target proteins. Membranes used for these purposes are made from a variety of materials, including cellulose acetate (CA), polyethylene (PE), polysulfone (PS), polyvinylidene fluoride (PVDF), and polyacrylonitrile (PAN). Among these, PAN is characterized by its excellent chemical stability and mechanical properties, as well as its excellent resistance to fouling by proteins and other substances.

[0003] PAN porous membranes are generally produced by non-solvent-induced phase separation (NIPS), but one issue with NIPS-produced PAN porous membranes is that it is difficult to increase the pore size. This means that large molecular weight proteins cannot be efficiently passed through. Furthermore, when concentrating large substances such as viruses and gold particles or performing buffer exchange, membranes with smaller pore sizes than necessary must be used, which makes it difficult to increase the filtration rate.

[0004] Patent Document 1 discloses a PAN porous membrane with a molecular weight cutoff of 360,000. Patent Document 2 discloses a polyacrylonitrile-based filtration membrane having a minimum pore size layer with particle blocking performance of 0.05 μm or less on only the outer surface. Non-Patent Document 1 discloses a composite membrane having a PAN layer formed on a PE fiber layer and finger-like voids throughout the PAN layer, with a molecular weight cutoff of 633,000. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5099930 [Patent Document 2] Japanese Patent Application Publication No. 11-70326 [Non-patent literature]

[0006] [Non-Patent Document 1] Journal of Membrane Science, 2010, 351, 160-167 Summary of the Invention [Problem to be solved by the invention]

[0007] The PAN porous membrane disclosed in Patent Document 1 has a molecular weight cutoff of 360,000, which is only about twice the molecular weight of globulin, a typical pharmaceutical protein, and there is room for improvement in terms of efficient protein permeation. There is also room for improvement in terms of efficient purification of large particles such as viruses.

[0008] Patent Document 2 discloses a technology for a PAN-based filtration membrane having a minimum pore size layer with particle blocking capacity of 0.05 μm or less on only one surface, preferably on the outer surface of the membrane. When using hollow fiber membranes for purification, concentration, buffer exchange, etc. of biological substances such as proteins, peptides, nucleic acids, viruses, and substances bound to them, internal pressure crossflow filtration, in which the source liquid flows through the hollow portion, is often performed because it reduces dead volume compared to flowing the source liquid outside the hollow fiber. In crossflow filtration, by providing a minimum pore size layer near the surface on the side where the source liquid flows, substances larger than the pore size can be prevented from penetrating the membrane interior, and the return of the source liquid can suppress the retention of large substances on the membrane surface, thereby reducing clogging during filtration. When performing internal pressure crossflow filtration using hollow fiber membranes with a minimum pore size layer on the outer surface, large substances can penetrate the membrane from the inner surface and be trapped inside the membrane, resulting in rapid clogging.

[0009] The composite PAN porous membrane disclosed in Non-Patent Document 3 has finger-like voids throughout the PAN layer. In this case, a support layer is necessary because the PAN layer alone weakens the membrane strength, but support layers are generally made of hydrophobic materials, and the presence of a polyethylene (PE) support layer as in Non-Patent Document 3 causes problems such as protein adsorption, resulting in a decrease in recovery rate and clogging.

[0010] The problem to be solved by the present invention is to provide a polyacrylonitrile porous hollow fiber membrane that has a molecular weight cutoff of 500 kJ or more, which is suitable for internal pressure cross-flow filtration, and exhibits high water permeability. Also, the problem to be solved by the present invention is to provide a polyacrylonitrile porous hollow fiber membrane that exhibits high BSA flux retention. [Means for solving the problem]

[0011] As a result of extensive research, the present inventors have found that the above problems can be solved by using a polyacrylonitrile porous hollow fiber membrane with a specific configuration, and have thus completed the present invention.

[0012] That is, the present invention is as follows. [1] A polyacrylonitrile porous hollow fiber membrane having a minimum pore size layer near the inner surface, the minimum pore size layer having an average pore size of 50 nm or more. [2] The polyacrylonitrile porous hollow fiber membrane according to [1], wherein the average pore diameter is 70 nm or less. [3] A polyacrylonitrile porous hollow fiber membrane with a molecular weight cutoff of 500,000 or more. [4] A polyacrylonitrile porous hollow fiber membrane according to any one of [1] to [3], in which the change in water permeation rate due to CIP treatment is 10% or less. [5] The polyacrylonitrile porous hollow fiber membrane according to any one of [1] to [4], which does not have a support. [6] The polyacrylonitrile porous hollow fiber membrane according to any one of [1] to [5], which has a breaking strength of 5 MPa or more. [7] The polyacrylonitrile porous hollow fiber membrane according to any one of [1] to [6], having a burst pressure of 1.5 MPa or more. [8] The polyacrylonitrile porous hollow fiber membrane according to any one of [1] to [7], having a membrane thickness of 0.1 mm or more and 0.5 mm or less. [9] The polyacrylonitrile porous hollow fiber membrane according to any one of [1] to [8], wherein the polyacrylonitrile is a copolymer of acrylonitrile / acrylic acid ester / ionic monomer.

[10] A polyacrylonitrile porous hollow fiber membrane of any of [1] to [9], used for the purification, concentration, and buffer exchange of biological substances.

[11] A membrane-forming solution containing polyacrylonitrile is poured into the outer nozzle of the double annular nozzle, and an internal coagulation solution is poured into the inner nozzle. The method for producing a polyacrylonitrile porous hollow fiber membrane according to any one of [1] to

[10] , characterized in that an inner draw ratio (A / B), which is the ratio of the inner diameter (B) of the polyacrylonitrile porous hollow fiber membrane to the outer diameter (A) of the inner nozzle, is 0.75 or less.

[12] The method for producing a polyacrylonitrile porous hollow fiber membrane according to

[10] , wherein the membrane-forming solution contains an additive having a weight-average molecular weight of less than 6,000. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a polyacrylonitrile porous hollow fiber membrane having a molecular weight cutoff of 500 kJ or more suitable for internal pressure cross-flow filtration and exhibiting high water permeability. Furthermore, it is preferable that the hollow fiber membrane of the present invention can exhibit a high BSA flux retention rate. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is an image of a polyacrylonitrile porous hollow fiber membrane as an example of this embodiment, observed with a scanning electron microscope (SEM). [Figure 2] FIG. 2 is a diagram illustrating the relationship between the spinneret of the double annular nozzle and the polyacrylonitrile porous hollow fiber membrane of the present embodiment. [Figure 3] FIG. 2 is a diagram illustrating a method for measuring the average pore size of the minimum pore size layer. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described. The present invention is not limited to the following embodiment, and various modifications can be made within the scope of the gist of the present invention.

[0016] [Polyacrylonitrile porous hollow fiber membrane] The polyacrylonitrile porous hollow fiber membrane of this embodiment (sometimes referred to as a PAN porous hollow fiber membrane in this specification) has a minimum pore size layer near the inner surface, and the average pore size of the minimum pore size layer is 50 nm or more. In addition, other polyacrylonitrile porous hollow fiber membranes of this embodiment have a molecular weight cutoff of 500,000 or more. The PAN porous hollow fiber membrane of this embodiment preferably consists of only a porous membrane. The porous membrane contained in the PAN porous hollow fiber membrane of this embodiment preferably consists of only a PAN porous membrane. The PAN porous hollow fiber membrane of this embodiment preferably does not consist of only closed pores, and preferably has continuous pores that communicate in the thickness direction.

[0017] The PAN porous hollow fiber membrane of this embodiment has a minimum pore size layer near the inner surface, and preferably on the inner surface. FIG. 1 is a cross-sectional SEM image of an example of a PAN porous hollow fiber membrane according to this embodiment, including the inner and outer surfaces. In the figure, the upper side is the outer surface, and the lower side is the inner surface. FIG. 1(B) is a cross-sectional SEM image of FIG. 1(A) to which a diagram (tie-shaped diagram) schematically showing the change in pore size in the thickness direction has been added. In FIG. 1(B), the tie-shaped image showing the pore size indicates that the wider the tie, the larger the pore size, and the narrower the tie, the smaller the pore size. In FIG. 1, the pore size is smallest on the inner surface, and the pore size changes toward the outside. The layer with the smallest pore size (minimum pore size layer) is present on the inner surface. The vicinity of the inner surface can be determined by the method described in the Examples below. Also, "having on the inner surface" means that at least a part of the minimum pore size layer includes the inner surface.

[0018] The PAN porous hollow fiber membrane contains polyacrylonitrile. The mass ratio of polyacrylonitrile to 100 mass% of the resin components contained in the PAN porous hollow fiber membrane is preferably 80 mass% or more, more preferably 90 mass% or more, and even more preferably 95 mass% or more. In particular, it is preferable that the resin component contained in the PAN porous hollow fiber membrane is polyacrylonitrile alone. Examples of the polyacrylonitrile include the acrylonitrile polymers described below.

[0019] The PAN porous hollow fiber membrane preferably does not substantially contain any compounds other than the polyacrylonitrile (for example, additives described below). Here, "substantially free" may mean 1% by mass or less, preferably 0.1% by mass or less, and more preferably 0.01% by mass or less, based on 100% by mass of the hollow fiber membrane.

[0020] When internal pressure cross-flow filtration, which is often performed in the purification, concentration, and buffer exchange of biological substances, is performed, the presence of a minimum pore size layer near the inner surface prevents substances larger than the minimum pore size from penetrating into the membrane. The return of the filtrate prevents large substances from remaining on the membrane surface, thereby reducing clogging during filtration.

[0021] The average pore size of the minimum pore size layer is 50 nm or more, preferably 51 nm or more, more preferably 52 nm or more, and is preferably 70 nm or less. When the average pore size of the minimum pore size layer is 50 nm or more, the molecular weight cutoff can be 500,000 or more. When the average pore size of the minimum pore size layer is 70 nm or less, the water permeability is further improved (for example, the water permeability described in the examples below can be 200 LMH or more). The average pore size of the minimum pore size layer can be measured by the method described in the Examples below.

[0022] The PAN porous hollow fiber membrane of this embodiment preferably exhibits a change in water permeability due to CIP treatment of 10% or less, more preferably 5% or less, and even more preferably 3% or less. Membranes used for purifying proteins, viruses, etc. are often subjected to alkaline treatment by CIP after being incorporated into the protein production process. CIP treatment is performed for repeated use, such as by storing the membrane after use in purifying proteins, etc., and then reusing it for the next protein purification. Since changes in pore size due to CIP treatment affect the protein recovery rate and impurity removal rate, it is preferable that the pore size does not change due to CIP treatment. The change in water permeability due to CIP treatment is an indicator of the absence of pore size changes due to CIP treatment. The change in water permeability due to CIP treatment can be measured using the method described in the Examples below. CIP treatment may be performed by immersing the membrane in a 0.2% sodium hypochlorite aqueous solution at 50°C for 24 hours.

[0023] The PAN porous hollow fiber membrane of this embodiment preferably does not have a support. The term "support" used herein refers to a material used to improve the strength of the membrane. The support may be a porous or non-porous structure that does not contain PAN. Examples include, but are not limited to, nonwoven fabrics made of hydrophobic polymers (such as homopolymers and copolymers of polyester, polyethylene, polypropylene, polysulfone, and polyvinylidene fluoride). Furthermore, the support is generally used by being combined with the membrane inside or adjacent to the membrane, but the installation location is not limited to these. Because the support must be strong, it is usually made of a hydrophobic material. When proteins are filtered using a membrane with a hydrophobic support, there is a concern that the proteins will adsorb to the hydrophobic support, resulting in a decrease in protein recovery rate and clogging. To prevent protein adsorption, it is preferable to have no support.

[0024] The PAN porous hollow fiber membrane of this embodiment preferably has a breaking strength of 5 MPa or more, more preferably 5.5 MPa or more, even more preferably 6.5 MPa or more, and particularly preferably 6.6 MPa or more. A breaking strength of 5 MPa or more provides excellent membrane strength, making it less likely to suffer from defects such as thread breakage when used in protein purification or the like. The breaking strength can be measured by the method described in the Examples below.

[0025] The PAN porous hollow fiber membrane of this embodiment preferably has a burst pressure of 1.5 MPa or more, more preferably 1.7 MPa or more, even more preferably 2.35 MPa or more, and particularly preferably 2.4 MPa or more. A burst pressure of 1.5 MPa or more provides excellent membrane strength, making the membrane less likely to burst due to pressure increases caused by clogging or sudden pressure fluctuations due to operational errors when using the membrane in protein purification or the like, and reducing the likelihood of failure in purifying the target substance.

[0026] The membrane thickness of the PAN porous hollow fiber membrane of this embodiment is preferably 0.1 mm to 0.5 mm, more preferably 0.2 mm to 0.4 mm. When the membrane thickness is 0.1 mm or more, the membrane strength is excellent and defects such as thread breakage and rupture during use of the membrane are unlikely to occur. Furthermore, when the membrane thickness is 0.5 mm or less, the filtration resistance of the membrane is small and the filtration rate is unlikely to decrease.

[0027] The PAN porous hollow fiber membrane of this embodiment has a smallest pore size layer located near the inner surface and an average pore size of the smallest pore size layer of 50 nm or more, so that the filtration rate does not decrease even after filtering the source liquid containing biological substances using internal pressure crossflow for a long period of time. The BSA flux retention rate of the PAN porous hollow fiber membrane of this embodiment is preferably 70% or more, more preferably 75% or more, and even more preferably 77% or more. The BSA flux retention rate refers to a value measured by the method described in the Examples below.

[0028] (Application) The PAN porous hollow fiber membrane of this embodiment can be suitably used for purifying, concentrating, buffer exchanging, etc. biological substances in the production process of biological substances such as proteins, peptides, nucleic acids, viruses, and substances bound to them.

[0029] (Manufacturing method) A method for producing the PAN porous hollow fiber membrane of this embodiment will be described. The PAN porous hollow fiber membrane of this embodiment is produced, for example, by dissolving an acrylonitrile polymer in a mixed solvent of an organic solvent and an additive in a membrane-forming solution, which is extruded from a double annular nozzle together with an internal coagulation solution, passed through an air gap, a coagulation bath, and a water washing tank to form a hollow fiber, recovered in a reel, cut to a desired length, recovered as a fiber bundle, and then subjected to a hot water treatment.When producing a dry PAN porous hollow fiber membrane, the fiber bundle after the hot water treatment can be immersed in a glycerin aqueous solution, the excess glycerin aqueous solution is removed, and then dried in a hot air dryer. The film-forming solution preferably contains the acrylonitrile polymer, an organic solvent, and additives, and more preferably consists of only the acrylonitrile polymer, an organic solvent, and additives. The film-forming solution preferably does not contain any compound having a molecular weight of 6000 or more other than the acrylonitrile polymer.

[0030] Examples of the acrylonitrile-based polymer include an acrylonitrile homopolymer, and an acrylonitrile-based copolymer containing at least 70% by mass or more (preferably 85 to 100% by mass) of structural units derived from acrylonitrile and 30% by mass or less (preferably 0 to 15% by mass or less) of structural units derived from one or more vinyl compounds copolymerizable with acrylonitrile.

[0031] The vinyl compound copolymerizable with acrylonitrile is not particularly limited as long as it is a known compound copolymerizable with acrylonitrile. Preferred examples of the copolymerization component include acrylic esters such as methyl acrylate, ethyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate; ionic monomers such as acrylic acid, itaconic acid, vinyl acetate, 2-hydroxyethyl acrylate, sodium acrylic sulfonate, sodium methallylsulfonate, sodium p(para)-styrenesulfonate, hydroxyethyl methacrylate, ethyl methacrylate triethylammonium chloride, and ethyl methacrylate trimethylammonium chloride; vinylpyrrolidone; and the like.

[0032] The acrylonitrile-based polymer preferably contains a structural unit derived from acrylonitrile and a structural unit derived from a vinyl compound copolymerizable with two kinds of acrylonitrile. The acrylonitrile-based polymer is more preferably a copolymer containing a structural unit derived from acrylonitrile, a structural unit derived from an acrylic acid ester (preferably methyl acrylate), and a structural unit derived from an ionic monomer (preferably sodium methallylsulfonate), and further preferably a copolymer consisting only of a structural unit derived from acrylonitrile, a structural unit derived from an acrylic acid ester (preferably methyl acrylate), and a structural unit derived from an ionic monomer (preferably sodium methallylsulfonate).

[0033] The mass proportion of the structural units derived from the acrylic acid ester relative to 100% by mass of the acrylonitrile-based polymer is preferably more than 0% by mass and not more than 30% by mass, more preferably 1 to 20% by mass, and even more preferably 2 to 15% by mass. The mass proportion of the constituent units derived from the ionic monomer relative to 100 mass% of the acrylonitrile-based polymer is preferably more than 0 mass% and 30 mass% or less, more preferably 0.05 to 15 mass%, and even more preferably 0.1 to 10 mass%. The mass proportion of the structural units derived from the acrylic acid ester is preferably higher than the mass proportion of the structural units derived from the ionic monomer, and more preferably is at least 5% by mass higher than the mass proportion of the structural units derived from the ionic monomer.

[0034] The mass ratio of the acrylonitrile-based polymer to 100% by mass of the membrane-forming solution is not particularly limited as long as it is within a concentration range that allows membrane formation and ensures the membrane obtained has membrane performance, but 2 to 50% by mass is preferred. If it is less than 2% by mass, the viscosity of the membrane-forming solution is low, making membrane formation difficult, while if it is more than 50% by mass, the viscosity of the membrane-forming solution is too high, making membrane formation difficult. To achieve high water permeability or a large molecular weight cutoff, a lower acrylonitrile-based polymer concentration is better, while a higher acrylonitrile-based polymer concentration is better to ensure sufficient membrane strength. Therefore, the acrylonitrile-based polymer concentration is more preferably in the range of 5 to 35% by mass, and even more preferably in the range of 10 to 25% by mass.

[0035] Examples of the organic solvent for dissolving the acrylonitrile polymer include propylene carbonate, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, γ-butyrolactone, ethylene carbonate, N-methyl-2-pyrrolidone, 2-pyrrolidone, and hexamethylene phosphoamide. These organic solvents may be used alone or in combination of two or more. Because of their good solubility for the acrylonitrile polymer, it is preferable to prepare the stock solution by mixing propylene carbonate and dimethyl sulfoxide.

[0036] The additives may be any that are compatible with the organic solvent and do not dissolve the acrylonitrile polymer. To control the viscosity and dissolution state of the original solution, examples of additives that can be used include water, salts, alcohols such as isopropyl alcohol, methanol, ethanol, propanol, and butanol, ketones such as acetone and methyl ethyl ketone, glycols such as diethylene glycol, triethylene glycol, tetraethylene glycol, and polyethylene glycol (weight-average molecular weight: 200 to 6,000, preferably 200 to less than 6,000, more preferably 200 to less than 4,000, and even more preferably 200 to less than 1,000), and glycerin and polyvinylpyrrolidone (weight-average molecular weight: 1,000 to 6,000, preferably 1,000 to less than 6,000, and more preferably 1,000 to less than 4,000). Multiple additives may be added, and the types and amounts may be adjusted as needed. The additives are removed by water washing and hot water treatment during the film-forming process. Additives with a small weight-average molecular weight are easily removed by washing and are less likely to remain in the PAN porous hollow fiber membrane. When using a PAN porous hollow fiber membrane for protein purification, etc., additives remaining in the PAN porous hollow fiber membrane are preferably small in molecular weight to prevent the additives from leaching out and contaminating proteins, etc., and to prevent the residual additives from being decomposed and leached by CIP treatment, resulting in an increase in pore size and resulting in a change in fractionation performance. The weight-average molecular weight of the additive is preferably less than 6,000, more preferably less than 4,000, and even more preferably less than 1,000. In terms of adjusting the viscosity of the stock solution and ease of removal in water washing and hot water treatment steps, the additive is preferably polyethylene glycol, more preferably polyethylene glycol with a weight-average molecular weight of 200 to 6,000, even more preferably polyethylene glycol with a weight-average molecular weight of 200 or more but less than 6,000, even more preferably polyethylene glycol with a weight-average molecular weight of 200 or more but less than 4,000, even more preferably polyethylene glycol with a weight-average molecular weight of 200 or more but less than 4,000, and particularly preferably polyethylene glycol with a weight-average molecular weight of 200 or more but less than 1,000. The additives with a weight-average molecular weight of 6000 or less are washed out of the hollow fiber membrane during the manufacturing process and do not substantially remain in the hollow fiber membrane. If an additive with a large molecular weight (e.g., a weight-average molecular weight of more than 6000) is used, the additive will remain in the hollow fiber membrane, and the additive remaining in the hollow fiber membrane may be removed during CIP treatment or the like, which may cause changes in the structure or pore size of the hollow fiber membrane.

[0037] The mass ratio of the additives to the film-forming solution (100 mass %) is 1 to 40 mass %, preferably 1 to 30 mass %, but the optimum concentration is determined depending on the type and molecular weight of the additives used.

[0038] The temperature of the membrane-forming solution fed into the outer nozzle may be 10 to 90°C, or 30 to 80°C.

[0039] The internal coagulation liquid is used to form the hollow portion of the hollow fiber membrane. The internal coagulation liquid is an aqueous solution of a good solvent that dissolves acrylonitrile polymers, such as propylene carbonate, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, γ-butyrolactone, ethylene carbonate, N-methyl-2-pyrrolidone, etc. In order to control the viscosity of the internal coagulation liquid, it is also possible to add glycols such as tetraethylene glycol and polyethylene glycol, and non-solvents such as glycerin. The aqueous solution of the good solvent preferably contains 30% by mass or more, more preferably 45% by mass or more, of the good solvent relative to 100% by mass of the aqueous solution, and more preferably 60% by mass or more in order to reduce the filtration resistance in the inner surface layer of the membrane. In order to smoothly promote mass transfer between the organic solvent contained in the membrane-forming solution and the good solvent and water contained in the internal coagulation solution, the good solvent used in the internal coagulation solution is preferably the same as the organic solvent used in the membrane-forming solution. When the organic solvents used in the membrane-forming solution are propylene carbonate and dimethyl sulfoxide, it is preferable to use them alone or in combination as the good solvent for the internal coagulation solution.

[0040] The temperature of the internal coagulation liquid introduced into the internal nozzle may be 10 to 90°C, or may be 30 to 80°C.

[0041] The location of the minimum pore size layer is controlled by the composition and temperature of the membrane-forming solution, the composition and temperature of the internal coagulation solution, the length and temperature of the air gap, the concentration of the solvent and the concentration of the non-solvent, and the composition and temperature of the coagulation bath, among others. The composition of the internal coagulation solution has a particularly large effect. By selecting conditions that facilitate the coagulation of the membrane-forming solution by the internal coagulation solution, the minimum pore size layer is easily formed on the inner surface side (preferably near the inner surface, more preferably on the inner surface), and by selecting conditions that make it difficult for the coagulation of the membrane-forming solution by the internal coagulation solution to proceed, the minimum pore size layer is easily formed on the outer surface side.

[0042] The hollow fiber membrane can be produced using a known tube-in-orifice type double annular nozzle. More specifically, the above-mentioned membrane-forming solution is discharged from the outer nozzle of the double annular nozzle and the internal coagulation solution is discharged from the inner nozzle simultaneously, and the resulting mixture is passed through an air gap and then coagulated in a coagulation bath to obtain a hollow fiber membrane.

[0043] The diameter of the double annular nozzle can be selected arbitrarily. However, when forming pores with a molecular weight cutoff of 500 kJ or more, the inner draw ratio (A / B), which is the ratio of the inner diameter (B) of the obtained hollow fiber to the outer diameter (A) of the inner nozzle, is preferably 0.75 or less. This will be explained using Figure 2. A manufacturing solution 14 is discharged from the outer nozzle of the spinneret 1 of the double annular nozzle, and an internal coagulation solution 15 is discharged from the inner nozzle. The inner diameter of the inner nozzle is 11, the outer diameter of the inner nozzle is 12, and the outer diameter of the outer nozzle is 13. The obtained hollow fiber membrane 2 has an inner diameter 21, an outer diameter 22, and a membrane thickness 23. The present inventors have attempted to increase the pore size of the minimum pore size layer near the inner surface of the hollow fiber membrane by increasing the solvent concentration of the internal coagulation liquid or further promoting phase separation near the inner surface. However, they found that pulsation and drawdown in the air gap and closed pores formed by excessive phase separation reduced the interconnectivity of pores, reducing membrane permeability and making the pore size near the inner surface larger than the pore size near the outer surface. As a result, they were unable to produce a PAN porous hollow fiber membrane that had a minimum pore size layer near the inner surface and an average pore size of 50 nm or more. Therefore, the inventors conducted further studies and found that a completely unconventional idea of ​​setting the dimensions of the spinneret nozzle and the diameter of the hollow fiber membrane within a specific range can produce a PAN porous hollow fiber membrane having a minimum pore size layer with an average pore size within a specific range near the inner surface. More specifically, the inventors conducted extensive studies and found that by setting the internal draw ratio to 0.75 or less, a membrane can be obtained in which the average pore size of the minimum pore size layer near the inner surface of the hollow fiber membrane is 50 nm or more. Although the detailed mechanism is unknown, it is thought that the polymer-dilute phase formed by phase separation, which occurs when the raw material solution comes into contact with the internal liquid near the interface with the internal liquid at the discharge part of the spinneret, is stretched in the circumferential direction of the hollow fiber by reducing the internal draw ratio, leading to an increase in the pore size near the inner surface of the resulting hollow fiber. The present inventors discovered that hollow fibers having an average pore size of 50 nm or more in the minimum pore size layer obtained by membrane production under conditions of an internal draw ratio of 0.75 or less have a molecular weight cutoff of 500,000 or more, leading to the completion of the present invention. An internal draw ratio of 0.75 or less is preferable because the average pore size of the minimum pore size layer is 50 nm or more, and an internal draw ratio of 0.69 or less is more preferable because the average pore size of the minimum pore size layer is 51 nm or more. If the internal draw ratio is too small, the fiber diameter fluctuation at the time of discharge from the spinneret becomes too large, and defects in the hollow fiber shape, such as uneven wall thickness due to a large deviation between the center of the outer diameter and the center of the inner diameter, are likely to occur. Therefore, an internal draw ratio of 0.2 or more is preferable, and 0.3 or more is more preferable. Furthermore, the average pore size of the minimum pore size layer is preferably 70 nm or less. In order to increase the average pore size of the minimum pore size layer beyond 70 nm, it is necessary to promote phase separation excessively, which results in the formation of many closed pores with reduced interconnectivity, reducing the membrane permeability and resulting in a water permeation rate of less than 200 LMH. A water permeation rate of less than 200 LMH is undesirable because it increases the time required for protein purification. By reducing the average pore size of the minimum pore size layer to 70 nm or less, the water permeation rate becomes 200 LMH or more, enabling highly efficient protein purification.

[0044] The air gap refers to the gap between the double annular nozzle and the coagulation bath. To stabilize the temperature, solvent concentration, and non-solvent concentration in the air gap, the air gap may be surrounded by a cylindrical tube or the like, a gas having a constant temperature and humidity may be supplied to the air gap, or the air in the air gap may be exhausted using an exhaust duct or the like.

[0045] The length of the air gap is preferably 5 to 500 mm, more preferably 5 to 250 mm, and even more preferably 5 to 100 mm. The temperature of the air gap may be 10 to 90°C, or may be 30 to 80°C. The time required to pass through the air gap may be 0.01 to 10 seconds.

[0046] The coagulation bath may be a liquid that does not dissolve the polymer (e.g., polyacrylonitrile), such as water, alcohols such as methanol and ethanol, ethers, or aliphatic hydrocarbons such as n-hexane and n-heptane, but water is preferred. The coagulation rate can also be controlled by adding the above-mentioned good solvent to the coagulation bath.

[0047] The temperature of the coagulation bath is −30° C. to 90° C., preferably 0° C. to 90° C., and more preferably 0° C. to 80° C. If the temperature of the coagulation bath is higher than 90° C. or lower than −30° C., the state of the membrane surface in the coagulation bath is unlikely to be stable.

[0048] The membrane removed from the coagulation bath is washed in a water washing tank to remove the solvent and additives. If the membrane is wound up while still containing a large amount of solvent, the residual solvent may cause changes in the membrane structure or adhesion of the hollow fibers to each other. To increase the diffusion rate of the solvent and additives to be removed and to improve the efficiency of the water washing, the water washing temperature is preferably 30 to 90°C. If the water washing temperature is lower than 30°C, the efficiency of washing the solvent and additives decreases, and if it exceeds 90°C, the surface condition of the membrane becomes difficult to stabilize.

[0049] The membrane is taken up from the washing bath and wound into a reel by a winding machine. The reel may be in the form of a flat frame or a round drum. To prevent the membrane from drying and shrinking, it is preferable to spray water on the membrane with a shower or to wind it underwater when winding it in air.

[0050] After cutting a portion of the membrane wound on a skein and recovering it as a fiber bundle, the fiber bundle is immersed in hot water to remove residual solvent and additives and to impart a thermal history to the membrane. The temperature of the hot water is preferably 30 to 90°C. If the hot water temperature is less than 30°C, the efficiency of cleaning the solvent and additives decreases, and if it exceeds 90°C, the surface condition of the membrane becomes unstable.

[0051] When a dried fiber bundle is required, the hollow fiber bundle is dried in a drying process. To prevent pore size shrinkage during drying, the hollow fiber bundle is immersed in a glycerin aqueous solution of a desired concentration, and then the excess glycerin aqueous solution is drained off, followed by drying in a hot air dryer or vacuum dryer. The drying temperature is preferably 30 to 90°C. If the drying temperature is lower than 30°C, the drying speed of the fiber bundle will be slow, resulting in reduced productivity and insufficient drying. Therefore, the drying temperature is preferably 30°C or higher. Furthermore, if the drying temperature exceeds 90°C, the PAN porous hollow fiber membrane will shrink due to heat, resulting in a decrease in pore size, so the drying temperature is preferably 90°C or lower. [Example]

[0052] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples.

[0053] The test methods shown in the examples are as follows.

[0054] (1) Average pore size of the smallest pore size layer (Cross-section preparation) A dry porous hollow fiber membrane was prepared. If the porous hollow fiber membrane was wet, it was dried in a hot air dryer at 40°C to prepare a dry porous hollow fiber membrane. The porous hollow fiber membrane was fixed to a sample stage with carbon paste, and then a cross section perpendicular to the fiber length direction was prepared using an ion milling device E-3500 (manufactured by Hitachi High-Tech Corporation) under the following cross section preparation conditions, for example. However, the resin skeleton was not deformed by heat during cross section preparation, and no sputtered sample pieces were attached. In addition, the cross section was smooth. [Cross-section preparation conditions] Accelerating voltage: 3 kV Irradiation current: 20 μA (Cross-sectional SEM observation) The hollow fiber membrane sample with the cross section prepared in the above (cross section preparation) was osmium coated using an osmium coater (HPC-30W, Vacuum Device Co., Ltd.) under the following conditions: voltage adjustment knob set to 4.5, discharge time 1.5 seconds, and osmium tetroxide gas atmosphere at approximately 7 Pa. The observation sample was then irradiated with an electron beam under the following conditions, and the contrast was adjusted automatically within the field of view to be photographed. Using the above conditions, SEM image 1 (Figure 3(i)) was obtained, in which the hollow fiber membrane thickness direction and the vertical direction of the SEM image are parallel, the image includes the innermost surface of the hollow fiber membrane, the hollow fiber cross section is at the bottom of the field of view, and the background is at the top of the field of view. Note that this SEM image is focused on a smooth cross section and does not contain distortion of the resin skeletal structure due to charge buildup or abnormal contrast. [Observation conditions] SEM: Hitachi High-Technologies Corporation SU7000 scanning electron microscope Accelerating voltage: 1 kV Working distance: 1.5 mm Detector: MD detector (backscattered electron image) Magnification: 30,000x Image size: 960 x 1280 pixels The field of view of SEM image 1 was then shifted parallel to the film thickness direction toward the outer surface, and SEM image 2 was obtained. However, the field of view was slightly overlapped so that the structure in SEM image 2 and the structure in SEM image 1 would smoothly connect. The process of acquiring SEM images while shifting the field of view in the film thickness direction and slightly overlapping it with the previous field of view continued, and imaging was stopped when the outer surface was included in the SEM image. At this time, it is desirable that the outer surface be parallel to the lateral direction of the SEM image. (Image analysis) Labels (text and scale bar) were trimmed from the resulting SEM images. For SEM image 1 and SEM images containing the outer surface, the background, i.e., the area outside the membrane cross section, was trimmed away. The trimming method used binarization using the higher-intensity value of two thresholds calculated during ternarization using the multi-Otsu method. Pixels with a brightness higher than the threshold were labeled white, and pixels with a brightness lower than the threshold were labeled black (Figure 3(ii)). When the resulting binarized image was converted into an intensity matrix, row A, the row closest to the top edge that did not contain white, was extracted. When SEM image 1 was converted into an intensity matrix, row A was defined as the innermost surface, and rows above row A, which represent the background but not the hollow fiber membrane, were trimmed away (Figure 3(iii)). Using a similar method, rows below row A, which represent the background but not the hollow fiber membrane, were trimmed away from the SEM image containing the outer surface. All of the above SEM images, obtained by shifting the field of view in the direction of the hollow fiber membrane thickness, were tiled so that the hollow fiber membrane structure was smoothly connected, resulting in a single cross-sectional image B that did not include the background above the innermost surface or the background below the outermost surface. However, because the cross-sectional image B is rectangular, if there is any lateral misalignment during tiling, it may be removed by trimming after tiling. For the cross-sectional image B, the top edge of the first image on the innermost surface side was defined as 0 μm, and it was divided into regions every 0.5 μm from there (Figure 3(iv)). If the width in the film thickness direction of the region including the outermost surface was less than 0.5 μm, the region from the outermost surface to 0.5 μm above was defined as the outermost surface region. Each divided region, every 0.5 μm, was filtered using a 5 × 5 pixel median filter, and then a threshold was calculated using Otsu's method and binarized (Figure 3(v)). Analysis was performed using, for example, the Python library OpenCV. Regions with a brightness higher than the threshold were binarized so that they were black, and regions below that threshold were white. The binary image was then closed using a 2 × 2 pixel kernel to create the image to be analyzed. The area S of all isolated white areas in each analysis target image was measured, and the circular equivalent diameter R of each pore was calculated using R = (4S / π)^(1 / 2). The number-average pore size of each analysis target image was then determined, and the area with the smallest average pore size was defined as the smallest pore size layer. The region from the innermost surface to 5% of the film thickness in the film thickness direction was defined as the region near the inner surface, and the region from the outermost surface to 5% of the film thickness in the film thickness direction was defined as the region near the outer surface.

[0055] (2) Inner diameter, outer diameter and film thickness The hollow fiber membrane was thinly sliced ​​perpendicular to the fiber length using a razor or similar tool, and the four inner major and minor diameters and outer major and minor diameters of each cross section were measured using a microscope for each hollow fiber membrane condition. The inner diameter was calculated as the average of the four inner major diameters and four inner minor diameters, and the outer diameter was calculated as the average of the four outer major diameters and four outer / inner diameters. The membrane thickness was calculated as ([outer diameter] - [inner diameter]) / 2. The membrane area was calculated as [pi] × [inner diameter] × [effective length of membrane] × [number of hollow fibers].

[0056] (3) Water permeability A porous hollow fiber membrane was prepared with its pores sufficiently moistened with water. If the porous hollow fiber membrane was dry, it was immersed in a solvent with low surface tension, such as ethanol, which does not change the membrane structure, and then replaced with water to moisten it. One end of the moist hollow fiber membrane, with an effective length of approximately 10 cm, was sealed, a syringe needle was inserted into the hollow at the other end, and pure water at 25°C was injected into the hollow through the needle at a pressure of 0.1 MPa. The amount of pure water permeating to the outer surface was measured, and the water permeation rate was calculated using the following formula. Here, the effective length refers to the net membrane length excluding the portion where the syringe needle was inserted. Water permeability [LMH] = (60 [min / hr] x permeable water volume [L]) / (π x inner diameter [m] x effective length [m] x measurement time [min])

[0057] (4) Molecular weight cutoff Using the same method as for measuring water permeability, a wet hollow fiber membrane with an effective length of approximately 20 cm, whose pores had been sufficiently wetted with water, was inserted with syringe needles at both ends and incorporated into an internal pressure cross-flow filtration circuit. 750 ppm aqueous solutions of Poly(ethylene oxide) MW 100,000 to 200,000 (ACROS ORGANICS), Polyethylene oxide MW 300,000 (Thermo Scientific), Polyethylene Glycol MW 500,000 (Fujifilm Wako Pure Chemical Corporation), and Polyethylene oxide MW 1,000,000 (Thermo Scientific) were circulated at a linear circulation speed of 0.3 m / sec and a filtration pressure of 0.05 MPa. The original liquid and filtrate were sampled 3 minutes after the start of filtration, and their refractive indices at 20°C were compared using a digital refractometer (ATAGO, RX-5000i-Plus). The removal rate (%) was calculated as (1 - refractive index of filtrate / refractive index of original liquid) x 100. A graph of the removal rate versus the molecular weight of polyethylene oxide or polyethylene glycol was created, and two points straddling the 90% removal rate were connected with a straight line. The molecular weight at the intersection with the 90% removal rate was taken as the molecular weight cutoff.

[0058] (5) Weight-average molecular weight of additive The molecular weight was measured using a Tosoh HLC-8220 GPC (gel permeation chromatography) column, a TSKgel superAWM-H column, DMF (LiBr 10 mmol / L), a sample volume of 50 μL (1 mg / mL), and an RI detector under the following measurement conditions: molecular weight was measured in terms of a polyethylene glycol standard sample.

[0059] (6) Albumin filtration performance PBS buffer was prepared by dissolving 9.6 g of Dulbecco's PBS(-) powder "Nissui" (Nissui Pharmaceutical Co., Ltd.) in 1 L of water and adjusting the temperature to 25°C. Bovine serum albumin (Sigma-Aldrich, BSA) was dissolved in the PBS buffer to a concentration of 250 ppm and adjusted the temperature to 25°C to prepare a BSA solution. Using the same method as for measuring water permeability, syringe needles were inserted into both ends of a wet hollow fiber membrane with an effective length of approximately 20 cm, the pores of which had been sufficiently wetted with water, and the membrane was then installed in an internal pressure cross-flow filtration circuit. PBS buffer was filtered for 5 minutes at a circulation linear velocity of 0.5 m / sec and a filtration pressure of 0.05 MPa, and the contents of the filtration circuit and hollow fiber membrane were replaced with PBS buffer. Subsequently, a BSA solution was filtered at a circulation linear velocity of 0.5 m / sec and a filtration pressure of 0.05 MPa. The filtrate was collected every 5 minutes from 0 to 60 minutes after the start of filtration, and the filtration rate (L / (m 2 hr 98kPa) and cumulative filtration volume of BSA solution for 60 minutes (L / (m 2 The BSA flux retention rate was calculated using the following formula: BSA flux retention rate (%) = F 55-60 / F 0-5 ×100 F 0-5 : Filtration rate (L / (m) from 0 to 5 minutes after the start of filtration 2 ·hr·98kPa)) F 55-60 : Filtration rate (L / (m) from 55 to 60 minutes after the start of filtration 2 ·hr·98kPa))

[0060] (7) Breaking strength A hollow fiber membrane with its pores sufficiently moistened with water was prepared using the same method as for measuring water permeability. A tensile test was performed on a 30 mm long membrane using an Autograph AGS-X 50N manufactured by Shimadzu Corporation at a temperature of 25°C and a tensile speed of 50 mm / min. The strength at which the membrane broke was measured four times, and the average value was taken as the breaking strength (MPa).

[0061] (8) Burst pressure A dry porous hollow fiber membrane was prepared. If the porous hollow fiber membrane was wet, it was dried in a hot air dryer at 40°C to prepare a dry porous membrane. One end of the dry hollow fiber membrane with an effective length of approximately 3 cm was sealed, and a pressure line was connected to the other end. The hollow fiber was immersed in pure water at 40°C, and air was supplied through the pressure line to pressurize the hollow portion. The pressure at which the hollow fiber ruptured was measured as the burst pressure (MPa).

[0062] (9) CIP treatment A porous hollow fiber membrane with its pores sufficiently moistened with water was prepared using the same method as in the water permeability measurement. The porous hollow fiber membrane was subjected to CIP treatment by immersing it in a 0.2% sodium hypochlorite aqueous solution at 50°C for 24 hours. After removing the porous hollow fiber membrane from the 0.2% sodium hypochlorite aqueous solution, it was immersed in pure water, and the pure water was replaced until the pH of the immersion water became neutral.

[0063] Example 1 A homogeneous membrane-forming solution was prepared by dissolving 19% by weight of a PAN copolymer (91.2% by weight, 8.6% by weight, 0.2% by weight sodium methallylsulfonate) with an intrinsic viscosity [η] = 1.2 (PAN copolymer) and 23% by weight of polyethylene glycol 600 (PEG600, molecular weight 600) in a mixed solvent of 8.7% by weight of propylene carbonate (PC) and 49.3% by weight of dimethyl sulfoxide (DMSO). This solution was maintained at 65°C and extruded from a spinneret (double annular nozzle 0.4mm-0.5mm-1.3mm) together with an internal coagulation solution consisting of a mixture of 50% by weight of DMSO and 50% by weight of water. The solution was then passed through a 100mm air gap and immersed in a coagulation bath of water at 70°C. A cylinder surrounded the spinneret and the coagulation bath, and the temperature of the air gap inside the cylinder was controlled at 70°C. The membrane removed from the coagulation bath was passed through a washing bath containing 50°C water and then wound onto a flat reel at a speed of 20 m / min. The wound membrane was cut at both ends of the reel and immersed in a bundled state in 55°C water for 2 hours for heat treatment. The properties of the obtained membrane are shown in Table 1. More specifically, the hollow fiber membrane of Example 1 had a minimum pore size layer on the inner surface.

[0064] Example 2 A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that the internal coagulation liquid was a mixed solution of 65 mass % DMSO and 35 mass % water. More specifically, the hollow fiber membrane of Example 2 had a minimum pore size layer on the inner surface.

[0065] (Comparative Example 1) A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that the spinneret was a double annular nozzle 0.5 mm-0.7 mm-1.3 mm.

[0066] (Comparative Example 2) A porous hollow fiber membrane was obtained in the same manner as in Comparative Example 1, except that the internal coagulation liquid was a mixed solution of 65 mass % DMSO and 35 mass % water. Although the DMSO concentration in the internal coagulation liquid was increased from the porous membrane of Comparative Example 1, the molecular weight cutoff could not be increased.

[0067] (Comparative Example 3) A porous hollow fiber membrane was obtained in the same manner as in Example 1, except that the spinneret was a double annular nozzle 0.5 mm-0.9 mm-1.7 mm.

[0068] Comparative Example 4 18% by mass of a PAN copolymer (91.2% by mass, 8.6% by mass, 0.2% by mass sodium methallylsulfonate) with an intrinsic viscosity [η] = 1.2 and 16% by mass of polyvinylpyrrolidone (BASF, Luvitec K17, weight-average molecular weight 9,000) were dissolved in a mixed solvent of 33% by mass of PC and 33% by mass of DMSO to prepare a homogeneous membrane-forming solution. This solution was maintained at 60°C and extruded from a spinneret (double annular nozzle 0.5mm-0.7mm-1.3mm) together with an internal coagulation solution consisting of a mixed solution of 90% by mass of DMSO and 10% by mass of water. After passing through a 20mm air gap, the membrane was immersed in a coagulation bath of water at 80°C to complete the coagulation. A cylinder surrounded the spinneret and the coagulation bath, and the temperature of the air gap inside the cylinder was controlled at 60°C. The membrane was then taken up on a flat reel at a spinning speed of 10 m / min. The wound membrane was cut at both ends of the reel and collected in a bundle. The outer diameter, inner diameter, membrane thickness, average pore size of the smallest pore layer, molecular weight cutoff, and water permeation rate before and after CIP treatment were measured, and the results are listed in Table 1. The smallest pore layer of this membrane was located near the outer surface. When measuring the molecular weight cutoff of this membrane, rapid clogging prevented sufficient recovery of the filtrate, making it impossible to measure the molecular weight cutoff. Furthermore, the flux retention rate during BSA filtration was low at 54%. The hollow fiber removed from the coagulation bath was passed through a washing bath containing 50°C water in the same manner as in Example 1, and then wound onto a flat skein at a spinning speed of 10 m / min. The wound membrane was cut at both ends of the skein and immersed in a bundled state in 55°C water for 2 hours for heat treatment. The water permeability of this membrane was 114 LMH.

[0069] (Comparative Example 5) An attempt was made to spin a porous hollow fiber membrane in the same manner as in Example 1, except that the internal coagulation liquid was a mixed solution of 70% by mass of DMSO and 30% by mass of water. However, the hollow fiber broke between the spinneret and the coagulation bath due to drawdown, and the porous membrane could not be recovered.

[0070] [Table 1] [Explanation of symbols]

[0071] 1 spindle 11 Inner diameter of the inner nozzle of the spinneret 12 Outer diameter of inner nozzle of spinneret 13 Outer nozzle diameter of spinneret 14 Film forming stock solution 15 Internal coagulation liquid 2. Hollow fiber membrane 21 Inner diameter of hollow fiber membrane 22 Outer diameter of hollow fiber membrane 23 Hollow fiber membrane thickness

Claims

1. A polyacrylonitrile porous hollow fiber membrane having a minimum pore size layer near the inner surface, the minimum pore size layer having an average pore size of 50 nm or more.

2. The polyacrylonitrile porous hollow fiber membrane according to claim 1, wherein the average pore diameter is 70 nm or less.

3. A polyacrylonitrile porous hollow fiber membrane having a molecular weight cutoff of 500,000 or more.

4. 4. The polyacrylonitrile porous hollow fiber membrane according to claim 1, wherein the change in water permeation rate due to CIP treatment is 10% or less.

5. The polyacrylonitrile porous hollow fiber membrane according to claim 1 or 3, which does not have a support.

6. The polyacrylonitrile porous hollow fiber membrane according to claim 1 or 3, having a breaking strength of 5 MPa or more.

7. The polyacrylonitrile porous hollow fiber membrane according to claim 1 or 3, having a burst pressure of 1.5 MPa or more.

8. The polyacrylonitrile porous hollow fiber membrane according to claim 1 or 3, having a membrane thickness of 0.1 mm or more and 0.5 mm or less.

9. The polyacrylonitrile porous hollow fiber membrane according to claim 1 or 3, wherein the polyacrylonitrile is a copolymer of acrylonitrile / acrylic acid ester / ionic monomer.

10. The polyacrylonitrile porous hollow fiber membrane according to claim 1 or 3, which is used for purifying, concentrating, or exchanging buffers for biological substances.

11. A membrane forming solution containing polyacrylonitrile is poured into the outer nozzle of the double annular nozzle, and an internal coagulation solution is poured into the inner nozzle.

4. The method for producing a polyacrylonitrile porous hollow fiber membrane according to claim 1, wherein an inner draw ratio (A / B), which is the ratio of the inner diameter (B) of the polyacrylonitrile porous hollow fiber membrane to the outer diameter (A) of the inner nozzle, is 0.75 or less.

12. The method for producing a polyacrylonitrile porous hollow fiber membrane according to claim 11, wherein the membrane-forming solution contains an additive having a weight average molecular weight of less than 6,000.

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