Nanofiltration membrane and method for manufacturing the same

Abstract

A purpose of the present invention is to provide a nanofiltration membrane that has a molecular weight cut-off of 200-1000, has a superior permeated liquid quantity with regard to ethanol, and can be suitably used as an organic solvent nanofiltration membrane.　This nanofiltration membrane is formed using a polyamide resin, and has a molecular weight cut-off of 200-1000 and an ethanol permeation quantity of 0.03 L / (m2∙bar∙h) or greater.

Description

[Technical Field]

[0001] The present invention relates to a nanofiltration membrane having a molecular weight cutoff of 200 to 1000 and excellent methanol permeate volume, and is suitable for use as an organic solvent nanofiltration membrane, and to a method for producing the same. The present invention also relates to a nanofiltration method using the said nanofiltration membrane. [Background technology]

[0002] Traditionally, nanofiltration membranes have been used in the water purification field for the removal of pesticides, odor components, and hardness components, and in the industrial field for pretreatment in the production of RO water and ultrapure water, desalting of soy sauce and dairy products, and purification of amino acids and lactic acid. In recent years, due to the social context of energy conservation and reduction of carbon dioxide emissions, attempts have been made to replace energy-intensive distillation separation processes with membrane separation processes in the fields of chemical production, purification, and recycling. In such applications, various organic solvents are used, and the target of separation is often low molecular weight components, so the process is sometimes called organic solvent nanofiltration (OSN), and the membrane used is sometimes called an organic solvent nanofiltration membrane (OSN membrane).

[0003] The definition of nanofiltration varies, and its scope is not always clearly defined. However, the IUPAC definition recommends defining nanofiltration as a porous material with a pore size of 2 nm or less. Furthermore, since nanofiltration targets substances larger than those used in reverse osmosis, it is sometimes defined as a porous material with a pore size of 1-2 nm. On the other hand, the pore size of nanofiltration is difficult to observe and measure even with an electron microscope, and there is variability in pore size. Therefore, the representative pore size of the membrane is not sufficient to represent the separation performance of the membrane, and molecular weight cutoff is mainly used as an indicator of separation performance. It should be noted that the size of target substances that can be separated by nanofiltration membranes is not clearly divided around the molecular weight cutoff, but rather has a certain range. Specifically, materials with a molecular weight cutoff of 200-1000 are generally classified as nanofiltration. As described later, in this specification, when the terms "nanofiltration" or "nanofiltration membrane" are used, it means filtration in which the molecular weight cutoff is set within the range of 200 to 1000, or a filtration membrane in which the molecular weight cutoff is within the range of 200 to 1000.

[0004] Phase separation is a widely used industrial method for fabricating filtration membranes using polymer materials as raw materials. Phase separation methods are broadly classified into non-solvent-induced phase separation (NIPS) and thermal-induced phase separation (TIPS). The NIPS method involves dissolving the polymer material in a good solvent to create a homogeneous polymer solution, and then immersing this polymer solution in a non-solvent. Phase separation occurs due to the infiltration of the non-solvent and the elution of the good solvent into the external atmosphere. The NIPS method results in a finger-like structure formed by macrovoids. Its advantages include simple equipment, ease of forming a dense layer on the surface, ease of increasing flow rate, and a long history of use in fabricating many porous membranes. Disadvantages of the NIPS method include the tendency for the strength of the filtration membrane to be insufficient and the need for a good solvent that dissolves at room temperature. On the other hand, the TIPS method is a relatively new method in which a solvent that does not dissolve in polymer materials at low temperatures but dissolves them at high temperatures is selected, and a homogeneous polymer solution dissolved at high temperatures is cooled to a temperature below the binodal line, which is the boundary between the one-phase and two-phase regions, thereby inducing phase separation and fixing the structure through polymer crystallization or glass transition. The TIPS method tends to produce a sponge-like homogeneous structure, and its general advantages include the ability to achieve high strength and its applicability to polymers for which there are no solvents that dissolve at low temperatures. General disadvantages include the complexity of the equipment and the difficulty in creating dense structures such as nanofiltration membranes.

[0005] On the other hand, organic solvent filtration membranes need to be resistant to a wide range of organic solvents. As an example of a filtration membrane that is resistant to a wide range of organic solvents, polyamide filtration membranes formed using polyamide resin are known.

[0006] Regarding polyamide filtration membranes produced by the NIPS method, it is known that, for example, an asymmetric polyamide hollow fiber consisting of a thin isolation membrane and a hot support membrane can be obtained by extruding a spinning solution containing 15-25% by weight of polyamide and 5-20% by mass of polyethylene glycol, together with formic acid and a coagulating core solution, into a precipitate solution having a pH difference of 3 or more from the coagulating core, stretching it in a wet state, and then drying it (see, for example, Patent Document 1).

[0007] Furthermore, a hollow fiber membrane is known that is a polyamide filtration membrane manufactured by the TIPS method, which has a semipermeable layer on its outer surface with a thickness of 0.1 μm to 10 μm, an outer diameter of 80 to 450 μm, and an inner diameter of 40 to 350 μm, and the hollow fiber membrane contains 70% by weight or more of at least one aliphatic polyamide selected from the group consisting of polyamide 4, polyamide 6, polyamide 11, polyamide 12, polyamide 46, polyamide 66, and polyamide 610 (see, for example, Patent Document 2). This hollow fiber membrane is said to provide a hollow fiber membrane having high water permeability or high moisture permeability and high selectivity.

[0008] Furthermore, as a polyamide filtration membrane manufactured by the TIPS method, an ultrafiltration membrane is known that is formed using polyamide resin and is characterized by having a dense layer formed on at least one surface (see, for example, Patent Document 3). This ultrafiltration membrane is said to have excellent resistance to various organic solvents and can stably maintain its membrane properties even when in contact with various types of organic solvents used industrially. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 58-65009 [Patent Document 2] Japanese Patent Publication No. 2015-198999 [Patent Document 3] Japanese Patent Publication No. 2016-193430 [Overview of the project] [Problems that the invention aims to solve]

[0010] In nanofiltration membranes, the pressure is increased during filtration to ensure a sufficient amount of liquid permeate (permeate volume). The asymmetric polyamide hollow fiber described in Patent Document 1 has been evaluated for permeate volume under a pressure of 0.2 bar (= 0.02 MPa). However, according to the inventor's research, since the asymmetric polyamide hollow fiber described in Patent Document 1 is manufactured by the NIPS method, it was found that if filtration is performed under a pressure of about 0.3 MPa, the membrane of the asymmetric polyamide hollow fiber ruptures, making it impossible to perform organic solvent nanofiltration with a molecular weight cutoff of 200 to 1000.

[0011] Furthermore, although the hollow fiber membrane in Patent Document 2 is manufactured by the TIPS method and therefore has sufficient strength to withstand organic solvent nanofiltration, the inventors' research has shown that when performing organic solvent nanofiltration with a molecular weight cutoff of 200 to 1000, a sufficient amount of permeate is not observed, making efficient organic solvent nanofiltration impossible.

[0012] Furthermore, the ultrafiltration membrane described in Patent Document 3 has not been evaluated for performance in organic solvent nanofiltration with a molecular weight cutoff of 200 to 1000. Therefore, further investigation is needed to ensure a sufficient permeate volume while performing organic solvent nanofiltration using the technology described in Patent Document 3.

[0013] Therefore, the object of the present invention is to provide a nanofiltration membrane that has a molecular weight cutoff of 200 to 1000, has an excellent permeate volume relative to methanol, and can be suitably used as an organic solvent nanofiltration membrane. [Means for solving the problem]

[0014] The inventors of the present invention conducted intensive studies to solve the above problems. As a result, they found that by manufacturing a polyamide filtration membrane under specific manufacturing conditions in the TIPS method, a nanofiltration membrane with a fractional molecular weight of 200 to 1000 and excellent methanol permeation liquid volume can be obtained.

[0015] Specifically, in the production of the polyamide filtration membrane, by satisfying the following steps (1) to (3), a nanofiltration membrane with a fractional molecular weight of 200 to 1000 and a methanol permeation amount of 0.03 L / (m 2 ·bar·h) or more was found. The present invention was completed by further studies based on such findings. (1) A step of preparing a casting solution in which a polyamide resin is dissolved at a concentration of 25% by mass or more at a temperature of 100°C or higher in an organic solvent having a boiling point of 150°C or higher and not compatible with the polyamide resin at a temperature of less than 100°C. (2) A step of coagulating the polyamide resin into a film by extruding the casting solution into a coagulation bath at 100°C or lower in a predetermined shape. In this step, a coagulation liquid containing polyethylene glycol and / or polypropylene glycol having an average molecular weight of 400 to 1000 is brought into contact with at least one surface of the casting solution extruded in the predetermined shape to form a nanofiltration membrane. (3) A step of removing the coagulation liquid from the nanofiltration membrane formed in (2).

[0016] That is, the present invention provides an invention in the following aspects. Item 1. A nanofiltration membrane formed using a polyamide resin, having a fractional molecular weight of 200 to 1000 and a methanol permeation amount of 0.03 L / (m 2 ·bar·h) or more, a nanofiltration membrane. Item 2. The nanofiltration membrane according to Item 1, having a fractional molecular weight of 250 to 990. Item 3. The nanofiltration membrane according to Item 1, which is a hollow fiber membrane having an outer diameter of 450 μm or more. Item 4. The nanofiltration membrane according to any one of items 1 to 3, wherein the polyamide resin consists of only one type of aliphatic polyamide resin having methylene groups and amide groups in a molar ratio of -CH2-:-NHCO-=4:1 to 10:1. Item 5. The nanofiltration membrane according to any one of items 1 to 4, wherein the polyamide resin is polyamide 6. Item 6. A nanofiltration membrane according to any of items 1 to 5, used for organic solvent nanofiltration. Item 7. A nanofiltration method for filtering a liquid to be treated containing a solute or particles using a nanofiltration membrane described in any of Items 1 to 6. Item 8. The nanofiltration method according to Item 7, wherein the solvent contained in the liquid to be treated is an organic solvent. Item 9. A nanofiltration membrane module comprising a module case containing a nanofiltration membrane as described in any of Items 1 to 6. Item 10. Method for producing a nanofiltration membrane, including the following steps 1 to 3: The first step involves preparing a film-forming stock solution by dissolving a polyamide resin at a concentration of 25% by mass or more in an organic solvent having a boiling point of 150°C or higher and being immiscible with the polyamide resin at temperatures below 100°C, at a temperature of 100°C or higher. A step of solidifying a polyamide resin into a film by extruding the aforementioned film-forming stock solution into a solidification bath at 100°C or lower in a predetermined shape, a second step of forming a nanofiltration film by contacting at least one surface of the film-forming stock solution extruded in a predetermined shape with a solidification solution containing polyethylene glycol having an average molecular weight of 400 to 1000 and / or polypropylene glycol having an average molecular weight of 400 to 1000, and A third step involves removing the coagulation solution from the nanofiltration membrane formed in the second step. Item 11. A method for manufacturing a hollow fiber membrane-shaped nanofiltration membrane, The second step is to use a double-tube nozzle for manufacturing hollow fibers, in which the film-forming raw material is discharged from the outer annular nozzle and the internal coagulation solution is discharged from the inner nozzle, and the fibers are immersed in a coagulation bath. The method for producing a nanofiltration membrane according to claim 10, wherein at least one of the internal coagulation solution and the coagulation bath contains a coagulation solution having an average molecular weight of 400 to 1000 polyethylene glycol and / or polypropylene glycol having an average molecular weight of 400 to 1000. Item 12. A method for producing a nanofiltration membrane according to item 10 or 11, comprising a step of stretching the nanofiltration membrane after the third step in one direction simultaneously with or after the drying treatment. [Effects of the Invention]

[0017] The nanofiltration membrane of the present invention is superior in terms of permeate volume and can achieve improved productivity, energy savings, and cost reduction in manufacturing processes across various industries. Furthermore, the nanofiltration membrane of the present invention has excellent resistance to a wide range of organic solvents and can stably maintain its membrane properties even when in contact with various types of organic solvents used industrially. Therefore, it can be suitably used for organic solvent nanofiltration and enables the provision of novel industrial processes, such as an alternative to distillation.

[0018] Furthermore, the nanofiltration membrane of the present invention can also be applied to conventional aqueous filtration processes. Due to its high hydrophilicity, when the substance to be removed is hydrophilic, the removal performance can be improved by the adsorption effect. On the other hand, since the adsorption of hydrophobic substances is suppressed, fouling, which occurs when hydrophobic substances cover the membrane surface and reduce the processing flow rate, can be prevented, thereby achieving efficient filtration. [Brief explanation of the drawing]

[0019] [Figure 1] Figure a is a schematic diagram of the module used for methanol permeation measurement, and figure b is a schematic diagram of the apparatus used for methanol permeation measurement. [Modes for carrying out the invention]

[0020] 1.Definition In this invention, "nanofiltration membrane" refers to a filtration medium having a molecular weight cutoff of 200 to 1000. "Nanofiltration" refers to a filtration process performed using a nanofiltration membrane. Furthermore, in this invention, "organic solvent nanofiltration membrane" refers to a nanofiltration membrane used in a filtration process for a liquid to be treated that contains an organic solvent. "Organic solvent nanofiltration" refers to a nanofiltration process performed on a liquid to be treated that contains an organic solvent.

[0021] 2. Nanofiltration membrane The nanofiltration membrane of the present invention is a nanofiltration membrane formed using a polyamide resin, having a molecular weight cutoff of 200 to 1000 and a methanol permeability of 0.03 L / (m³). 2 It is characterized by having a value of ≥ ∫·bar·h. The nanofiltration membrane of the present invention will be described in detail below.

[0022] [Components] The nanofiltration membrane of the present invention is formed from a polyamide resin. By using a polyamide resin as the constituent resin in the nanofiltration membrane of the present invention, it becomes possible to achieve resistance to a wide range of organic solvents.

[0023] The type of polyamide resin used as the constituent resin is not particularly limited, but examples include polyamide homopolymers, polyamide copolymers, or mixtures thereof. Specific examples of polyamide homopolymers include polyamide 6, polyamide 66, polyamide 46, polyamide 610, polyamide 612, polyamide 11, polyamide 12, polyamide MXD6, polyamide 4T, polyamide 6T, polyamide 9T, and polyamide 10T. Specific examples of polyamide copolymers include copolymers of polyamide with polyethers such as polytetramethylene glycol or polyethylene glycol. The ratio of the polyamide component in the polyamide copolymer is not particularly limited, but for example, the proportion of the polyamide component is preferably 70 mol% or more, more preferably 80 mol% or more, even more preferably 90 mol% or more, and particularly preferably 95 mol% or more. By satisfying the above range for the ratio of the polyamide component in the polyamide copolymer, even better organic solvent resistance can be achieved.

[0024] From the viewpoint of further improving resistance to a wide range of organic solvents, a suitable example of a polyamide resin used as a constituent resin is one consisting of only one type of aliphatic polyamide resin having methylene groups and amide groups in a molar ratio of -CH2-:-NHCO-=4:1 to 10:1.

[0025] The polyamide resin used as the constituent resin can be either crosslinked or not, but from the viewpoint of reducing manufacturing costs, non-crosslinked resin is preferred.

[0026] Furthermore, while there are no particular limitations on the relative viscosity of the polyamide resin, examples include 2.0 to 7.0, preferably 3.0 to 6.0, and more preferably 2.0 to 4.0. By having such a relative viscosity, it is possible to improve moldability and controllability of phase separation during the production of nanofiltration membranes, and to provide the nanofiltration membranes with excellent dimensional stability. Here, relative viscosity refers to the value measured using an Ubbelohde viscometer at 25°C with a solution obtained by dissolving 1 g of polyamide resin in 100 mL of 96% sulfuric acid.

[0027] In the present invention, the polyamide resin used as the constituent resin may be used alone or in combination of two or more types.

[0028] The nanofiltration membrane of the present invention may contain fillers in addition to the polyamide resin, as necessary, to the extent that they do not impair the effects of the present invention. By including fillers, the strength, elongation, and elastic modulus of the nanofiltration membrane can be improved. In particular, by including fillers, the nanofiltration membrane becomes less prone to deformation even when high pressure is applied during filtration. There are no particular limitations on the type of filler to be added, but examples include fibrous fillers such as glass fibers, carbon fibers, potassium titanate whiskers, zinc oxide whiskers, calcium carbonate whiskers, wollastonite whiskers, aluminum borate whiskers, aramid fibers, alumina fibers, silicon carbide fibers, ceramic fibers, asbestos fibers, gypsum fibers, and metal fibers; talc, hydrotalcite, wollastonite, zeolite, sericite, mica, kaolin, clay, pyrophyllite, bentonite, and asbestos. Examples of fillers include silicates such as talc 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 glass beads, glass flakes, glass powder, ceramic beads, boron nitride, silicon carbide, carbon black, silica, and graphite as non-fibrous fillers. These fillers may be used individually or in combination of two or more. Among these fillers, talc, hydrotalcite, silica, clay, and titanium oxide are preferred, and talc and clay are more preferred.

[0029] The filler content is not particularly limited, but for example, 5 to 100 parts by mass, preferably 10 to 75 parts by mass, and more preferably 25 to 50 parts by mass of filler per 100 parts by mass of polyamide resin. By including filler in such a content, the strength, elongation, and elastic modulus of the nanofiltration membrane can be improved.

[0030] Furthermore, the nanofiltration membrane of the present invention may contain additives such as thickeners, antioxidants, surface modifiers, lubricants, and surfactants, as needed, for purposes such as controlling pore size and improving membrane performance.

[0031] [Shape / Structure] The shape of the nanofiltration membrane of the present invention is not particularly limited and can be selected from any shape such as a hollow fiber membrane or a flat membrane. However, a hollow fiber membrane is preferred in the present invention because it has a large filtration area per unit volume of module, enabling efficient filtration.

[0032] In the nanofiltration membrane of the present invention, a dense layer is formed on the surface of at least one side. In the present invention, the "dense layer" refers to a region in which dense micropores are aggregated, and which is a region in which the presence of pores is substantially not observed in a scanning electron microscope (SEM) image at a magnification of 10,000x. In the nanofiltration membrane of the present invention, the filtration performance, such as the molecular weight cutoff and methanol permeability, is almost entirely due to the dense layer. To observe the dense layer with a scanning electron microscope, if the nanofiltration membrane is a flat membrane, it can be cut to an appropriate size, placed on a sample stage, and then observed after vapor deposition treatment with Pt, Au, Pd, etc. Furthermore, if the nanofiltration membrane is a hollow fiber membrane, when observing the dense layer on the outer surface, it is possible to cut it to an appropriate size, place it on a sample stage, and then perform a vapor deposition treatment with Pt, Au, Pd, etc., as with a flat membrane, and then observe it. However, when observing the dense layer on the inner surface, the hollow fiber membrane should be cut along its longitudinal direction with a sharp blade such as a scalpel to expose the inner surface, then cut to an appropriate size, place it on a sample stage, and then perform a vapor deposition treatment with Pt, Au, Pd, etc., and then observe it.

[0033] In the nanofiltration membrane of the present invention, the thickness of the dense layer is not particularly limited, but for example, it can be 10 to 2000 nm, preferably 100 to 1500 nm, more preferably 200 to 1000 nm, even more preferably 400 to 1000 nm, and particularly preferably 440 to 930 nm. In the present invention, the thickness of the dense layer is a value obtained by measuring the distance (thickness) of the region where substantially no pores are observed in an SEM image of the cross-section of the nanofiltration membrane at a magnification of 10,000x at 10 or more locations, and calculating the average value.

[0034] The nanofiltration membrane of the present invention only needs to have a dense layer formed on at least one of its surfaces. For example, if the nanofiltration membrane of the present invention is a hollow fiber membrane, it is sufficient that a dense layer be formed on at least one of the inner surface and the outer surface. Also, for example, if the nanofiltration membrane of the present invention is a flat membrane, it is sufficient that a dense layer be formed on at least one of the outer surface and the back surface. From the viewpoint of suitably providing the fractional molecular weight and methanol permeability described later, a preferred example of the nanofiltration membrane of the present invention is one in which the dense layer is provided on only one surface. Furthermore, a preferred example of the nanofiltration membrane of the present invention being a hollow fiber membrane is one in which a dense layer is provided on the inner surface and no dense layer is provided on the outer surface.

[0035] In the nanofiltration membrane of the present invention, the regions other than the dense layer have a porous structure. Hereinafter, the regions other than the dense layer may also be referred to as the "porous region." Specifically, the porous region refers to the region in which the presence of pores is substantially observed in a scanning electron microscope (SEM) image at a magnification of 2000x. Since the performance of the nanofiltration membrane of the present invention is determined almost entirely by the dense layer, the porous region can be considered a so-called support layer, and the pore size in the porous region is not particularly limited as long as it does not have sufficient strength to hold the dense layer and does not significantly hinder fluid permeability.

[0036] When the nanofiltration membrane of the present invention is a hollow fiber membrane, its outer diameter is appropriately set according to its application, the thickness of the dense layer, the amount of permeate to be contained, etc. However, considering the relationship between the effective membrane area when filled into a module, membrane strength, pressure loss of the fluid flowing through the hollow part, and buckling pressure, the outer diameter of the hollow fiber membrane can be said to be 450 μm or more, preferably 450 to 4000 μm, more preferably 500 to 3500 μm, even more preferably 700 to 3000 μm, and particularly preferably 700 to 2000 μm. Furthermore, when the nanofiltration membrane of the present invention is a hollow fiber membrane, other examples of the range of its outer diameter can be 500 to 2000 μm or 500 to 1980 μm. Furthermore, when the nanofiltration membrane of the present invention is in the form of a hollow fiber, its inner diameter is not particularly limited, but for example, it can be 100 to 3000 μm, preferably 200 to 2500 μm, more preferably 300 to 2000 μm, and even more preferably 300 to 1500 μm. Also, when the nanofiltration membrane of the present invention is a hollow fiber membrane, another example of the range of its inner diameter is 300 to 1260 μm. In the present invention, the outer diameter and inner diameter of the hollow fiber membrane are obtained by observing five hollow fiber membranes with an optical microscope at a magnification of 200x, measuring the outer diameter and inner diameter (both at the point of maximum diameter) of each hollow fiber membrane, and calculating the average value of each.

[0037] The thickness of the nanofiltration membrane of the present invention is appropriately set according to the application and shape of the nanofiltration membrane, the thickness of the dense layer, the amount of permeate to be contained, etc., but in nanofiltration, from the viewpoint of filtration under high pressure operation, a range of 50 to 600 μm, preferably 100 to 350 μm, is mentioned. Other examples of the thickness range of the nanofiltration membrane of the invention include 150 to 750 μm or 200 to 720 μm. When the nanofiltration membrane of the present invention is in the shape of a hollow fiber, the thickness of the nanofiltration membrane is calculated by dividing the value obtained by subtracting the inner diameter from the outer diameter by 2.

[0038] [Fractional molecular weight and methanol permeation] The nanofiltration membrane of the present invention has a molecular weight cutoff of 200 to 1000 and a methanol permeability of 0.03 L / (m³). 2It is above (·bar·h), and while preventing the permeation of low molecules with a molecular weight of 1000 or less, it is possible to have an excellent permeate volume with respect to an organic solvent.

[0039] The range of the molecular weight cut-off of the nanofiltration membrane of the present invention may be 200 to 1000, preferably 250 to 990, more preferably 250 to 950, still more preferably 500 to 900, and particularly preferably 600 to 850. Further, as another example of the range of the molecular weight cut-off of the nanofiltration membrane of the invention, 280 to 990 can be mentioned. The molecular weight cut-off represents the pore size of a membrane that can block 90% or more of a substance having a specific molecular weight, and is represented by the molecular weight of the substance that can be blocked.

[0040] In the present invention, the molecular weight cut-off is a value determined by the following method. A solution prepared by dissolving polyethylene glycol with a known molecular weight in pure water at 0.1% by mass is used as the stock solution, and filtration is carried out at a pressure of 0.3 MPa to recover the liquid that has passed through the membrane. The concentration of polyethylene glycol in the permeate is measured, and the rejection rate is calculated according to the following formula. Using polyethylene glycols of various molecular weights, the rejection rates are calculated respectively, and based on the results, a graph is created with the molecular weight of the polyethylene glycol used on the horizontal axis and the rejection rate of each polyethylene glycol on the vertical axis, and the molecular weight at the intersection of the obtained approximate curve and the 90% rejection rate is determined as the molecular weight cut-off.

Equation

[0041] The methanol permeation rate of the nanofiltration membrane of the present invention may be 0.03 L / (m 2 ·bar·h) or more, but usually 0.03 to 5.00 L / (m 2 ·bar·h), preferably 0.10 to 3.00 L / (m [[ID=2Z]] 2 ·bar·h), more preferably 0.30 to 1.50 L / (m 2 ·bar·h), still more preferably 0.30 to 1.20 L / (m 2 ·bar·h). Further, as another example of the range of the methanol permeation rate of the nanofiltration membrane of the present invention, 0.10 to 1.20 L / (m2 Examples include (bar·h). By having such methanol permeability, it becomes possible to perform filtration in organic solvent nanofiltration with excellent filtration efficiency and a quantity that satisfies practical levels.

[0042] In this invention, the methanol permeate is a value measured by internal pressure filtration when the nanofiltration membrane is a hollow fiber membrane, and is measured by the following procedure. First, 10 hollow fiber membranes are cut to a length of 30 cm and bundled together. Next, a rigid nylon tube with an outer diameter of 8 mm, an inner diameter of 6 mm, and a length of 50 mm is prepared, and a rubber stopper about 20 mm in length is inserted into the opening at one end of the tube to seal the opening at that end. Next, a two-part epoxy resin that cures at room temperature is inserted into the opening of the tube opposite to the one with the rubber stopper, and the space inside the tube is filled with the epoxy resin. Then, the bundle of hollow fiber membranes prepared above is bent into a roughly U-shape, and both ends of the hollow fiber membranes are inserted into the tube filled with the epoxy resin until the tips of the ends touch the rubber stopper, and the epoxy resin is allowed to cure in that state. Next, the region of the hardened epoxy resin part on the rubber stopper side is cut along with the tube to obtain a module in which the hollow portions at both ends of the hollow fiber membrane are open. A schematic diagram of the module is shown in Figure 1a. Next, the module is set in the apparatus shown in Figure 1b, and methanol (100% methanol) at 25°C is flowed through the inside of the hollow fiber membrane of the module under a pressure of approximately 0.3 MPa for a certain period of time. The volume of methanol that permeates to the outside of the hollow fiber membrane is determined, and the methanol permeate amount (L / (m³)) is calculated according to the following formula. 2 Calculate bar(h).

number

[0043] Furthermore, in this invention, the methanol permeate is a value measured by dead-end filtration when the nanofiltration membrane is a flat membrane, and is measured by the following procedure: Using a flat membrane cross-flow tester connected to a high-pressure pump (for example, a Sepa-CF flat membrane test cell manufactured by GE Water Technologies), a flat membrane-shaped nanofiltration membrane of a predetermined size (19.1 cm × 14.0 cm, effective membrane area in the cell: 155 cm²) is measured. 2 Cut the material into pieces and fix it to the cell. Pour methanol at 25°C through it and collect the methanol that permeates at a predetermined pressure. Measure the volume (L) and calculate the methanol permeate amount (L / (m³)) according to the following formula. 2 Calculate bar(h).

[0044]

number

[0045] In the nanofiltration membrane of the present invention, the molecular weight cutoff and methanol permeability should satisfy the ranges described above, but from the viewpoint of suitably balancing the removal capacity of low molecular weight substances and the permeability of organic solvents, it is preferable that the molecular weight cutoff is 300 to 900 and the methanol permeability is 0.30 to 1.50 L / (m³). 2 (bar·h), more preferably a fractional molecular weight of 500-900 and methanol permeability of 0.30-1.50 L / (m³) 2 (bar·h), more preferably a fractional molecular weight of 500-850 and methanol permeability of 0.40-0.95 L / (m³) 2 (bar·h), particularly preferably a fractional molecular weight of 600-850 and a methanol permeability of 0.40-0.95 L / (m³). 2 Examples include (bar·h). Furthermore, in the nanofiltration membrane of the present invention, other examples of the range of molecular weight cutoff and methanol permeability include a molecular weight cutoff of 280 to 990 and a methanol permeability of 0.10 to 1.20 L / (m³). 2 (bar·h), or fractionation molecular weight of 600-990 and methanol permeation rate of 0.37-1.20 L / (m³) 2 (bar·h), or fractionation molecular weight of 600-880 and methanol permeation rate of 0.41-0.92 L / (m³) 2Examples include ·bar·h).

[0046] [Tensile strength and elongation] The nanofiltration membrane of the present invention has sufficient strength to withstand organic solvent nanofiltration, and one example of its specific characteristics is that it has excellent tensile strength and elongation.

[0047] Specifically, when the nanofiltration membrane of the present invention is a hollow fiber membrane, the tensile strength can be, for example, 3 to 40 MPa, preferably 5 to 35 MPa, and more preferably 10 to 30 MPa. Also, when the nanofiltration membrane of the present invention is a hollow fiber membrane, the elongation can be, for example, 50 to 400%, preferably 100 to 300%, and more preferably 100 to 250%.

[0048] In this invention, the tensile strength and elongation of the hollow fiber membrane are values ​​measured by the following procedure. The hollow fiber membrane is cut to a length of 100 mm, and a tensile test is performed in an environment of room temperature (25°C) and humidity (60%) with a gripping distance of 50 mm and a tensile speed of 50 mm / min to measure the load (N) and elongation (mm) at the time of fracture. Separately, the cross-sectional area (mm) of the hollow fiber membrane is measured. 2 To determine the following: Tensile tests and cross-sectional area measurements are performed using five hollow fiber membranes. The average values ​​of load, elongation, and cross-sectional area at fracture are calculated, and these average values ​​are used to calculate the tensile strength and elongation ratio according to the following formula.

number

[0049] [Soil solvent resistance] The nanofiltration membrane of the present invention possesses the property of maintaining a stable membrane structure by suppressing changes in strength and elongation even when in contact with various types of organic solvents (organic solvent resistance). More specifically, the nanofiltration 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 are as follows: Alcoholic beveragesPrimary 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 Examples include acetone, methyl ethyl ketone, cyclohexanone, and diisopropyl ketone. Ethers : Tetrahydrofuran, diethyl ether, diisopropyl ether, 1,4-dioxane, etc., and glycol ethers such as ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, and propylene glycol monomethyl ether. aprotic polar solvent : N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, sulfolane, etc. Esters : Ethyl acetate, isobutyl acetate, ethyl lactate, dimethyl phthalate, diethyl phthalate, ethylene carbonate, propylene carbonate, propylene glycol monomethyl ether acetate, etc. Hydrocarbons : Petroleum ether, pentane, hexane, heptane, benzene, toluene, xylene, liquid paraffin, gasoline, and mineral oil. higher fatty acids : Fatty acids with 4 or more carbon atoms (preferably 4 to 30) other than the carboxyl group, such as oleic acid, linoleic acid, and linolenic acid.

[0050] In particular, a preferred example of the organic solvent resistance possessed by the nanofiltration membrane of the present invention is resistance to at least one, preferably all, of the following organic solvents. Alcoholic beverages Isopropyl alcohol, benzyl alcohol, ethylene glycol, glycerin. Ketones Acetone, methyl ethyl ketone, cyclohexanone. Ethers Tetrahydrofuran, diethyl ether, propylene glycol monomethyl ether. aprotic polar solvent N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone. Esters : Ethyl acetate, isobutyl acetate, dimethyl phthalate. Hydrocarbons Hexane, heptane, benzene, toluene, gasoline, mineral oil. higher fatty acids Oleic acid, linoleic acid.

[0051] Specifically, the organic solvent resistance of the nanofiltration membrane of the present invention is such that, when immersed in the organic solvent at 25°C for 14 hours, the rate of change in the tensile strength and elongation of the ultrafiltration membrane after immersion is ±30% or less, preferably less than ±20%, compared to before immersion. Specifically, the rate of change in tensile strength and elongation is calculated according to the following formula.

number

[0052] The strength and elongation of the nanofiltration membrane are measured under the conditions described in the [Tensile Strength and Elongation] section above if the nanofiltration membrane is a hollow fiber membrane, and under the conditions described in the [Tensile Strength and Elongation] section above if the nanofiltration membrane is a flat membrane, except that a sample of the flat membrane cut into strips with a width of 10 mm and a length of 100 mm is used.

[0053] [Application] The nanofiltration membrane of the present invention is used as a filtration membrane for filtering a liquid to be treated containing solute or particles, and for separating or concentrating nanoscale solute or particles. The solute or particles cut off from the liquid to be treated using the nanofiltration membrane of the present invention are nanoscale and have a molecular weight greater than or equal to the fractionation molecular weight.

[0054] Furthermore, the nanofiltration membrane of the present invention has an excellent permeate volume relative to organic solvents, and is therefore suitable for use in organic solvent nanofiltration. The type of organic solvent used in the liquid to be treated for organic solvent nanofiltration is not particularly limited, but specifically, the organic solvents exemplified in the [Organic Solvent Resistance] section above are examples.

[0055] To perform nanofiltration using the nanofiltration membrane of the present invention, the nanofiltration membrane of the present invention can be incorporated into a nanofiltration membrane module described later.

[0056] 3. Method for manufacturing nanofiltration membranes The method for producing the nanofiltration membrane of the present invention is not particularly limited, as long as it satisfies the aforementioned molecular weight cutoff and methanol permeability. However, a preferred example is a production method comprising the following steps 1 to 3. The nanofiltration membrane of the present invention is difficult to obtain using conventionally known general thermal-induced phase separation methods (TIPS method) or non-solvent-induced phase separation methods (NIPS method) alone. However, the production method comprising the following steps 1 to 3 employs the principles of both the TIPS method and the NIPS method, thereby enabling the efficient production of the nanofiltration membrane of the present invention. 1st step A film-forming stock solution is prepared by dissolving a polyamide resin at a concentration of 25% by mass or more in an organic solvent having a boiling point of 150°C or higher and being immiscible with the polyamide resin at temperatures below 100°C, at a temperature of 100°C or higher. 2nd process A process for solidifying a polyamide resin into a film by extruding the film-forming stock solution in a predetermined shape into a solidification bath at 100°C or lower, wherein in this process, a solidification solution containing polyethylene glycol with an average molecular weight of 400 to 1000 and / or polypropylene glycol with an average molecular weight of 400 to 1000 is brought into contact with at least one surface of the film-forming stock solution extruded in the predetermined shape to form a nanofiltration film. 3rd process The coagulated liquid is removed from the nanofiltration membrane formed in the second step.

[0057] The following describes each of the three processes mentioned above in detail.

[0058] [1st step] In the first step, a film-forming stock solution is prepared by dissolving a polyamide resin at a concentration of 25% by mass or more in an organic solvent having a boiling point of 150°C or higher and being immiscible with the polyamide resin at temperatures below 100°C, at a temperature of 100°C or higher.

[0059] Organic solvents having a boiling point of 150°C or higher and being immiscible with polyamide resin at temperatures below 100°C include, for example, 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. Examples of polyhydric alcohols include glycerin, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, hexylene glycol, 1,3-butanediol, and polyethylene glycol (molecular weight 100 to 10000). 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, and lauric acid. Among these organic solvents, from the viewpoint of obtaining a nanofiltration membrane with higher strength, aprotic polar solvents and polyhydric alcohols are preferred; more preferably sulfolane, dimethyl sulfone, γ-butyrolactone, δ-valerolactone, ε-caprolactone, propylene glycol, hexylene glycol, 1,3-butanediol, and polyethylene glycol (molecular weight 100-600); even more preferably sulfolane, dimethyl sulfone, γ-butyrolactone, δ-valerolactone, and ε-caprolactone; particularly preferred are sulfolane and dimethyl sulfone. These organic solvents may be used individually or in combination of two or more.While sufficient effects can be obtained by using one of these organic solvents individually, mixing two or more can sometimes lead to the creation of even more effective nanofiltration membranes due to differences in the order and structure of phase separation.

[0060] The concentration of the polyamide resin in the film-forming stock solution is not particularly limited, but is limited to 25% by mass or more. Preferably, it is 25-50% by mass, more preferably 25-40% by mass, and even more preferably 25-35% by mass. By satisfying the above range for the concentration of the polyamide resin in the film-forming stock solution, the nanofiltration membrane can be given excellent strength while satisfying the aforementioned molecular weight cutoff and methanol permeability. Furthermore, if the concentration of the polyamide resin in the film-forming stock solution is less than 25% by mass, the molecular weight cutoff will exceed 1000, and there is a tendency for a nanofiltration membrane not to be obtained.

[0061] Furthermore, in the first step, when dissolving the polyamide resin in the organic solvent, it is necessary to keep the solvent temperature at 100°C or higher. Specifically, it is desirable to dissolve the resin at a temperature 10 to 50°C higher, preferably 20 to 40°C higher, than the phase separation temperature of the prepared film-forming stock solution. The phase separation temperature of the film-forming stock solution refers to the temperature at which liquid-liquid phase separation or solid-liquid phase separation by crystal precipitation occurs when a mixture of the polyamide resin and the organic solvent at a sufficiently high temperature is gradually cooled. The phase separation temperature can be measured using a microscope equipped with a hot stage or the like.

[0062] In the first step, the temperature conditions for dissolving the polyamide resin in the organic solvent can be appropriately set in a temperature range of 100°C or higher according to the indicators described above, depending on the type of polyamide resin and organic solvent used, but preferably 120 to 250°C, more preferably 140 to 220°C, and even more preferably 160 to 200°C.

[0063] Furthermore, fillers, thickeners, antioxidants, surface modifiers, lubricants, surfactants, etc., may be added to the film-forming stock solution as needed to control the pore size of the nanofiltration membrane and improve its performance.

[0064] The film-forming stock solution prepared in the first step is subjected to the second step at the same temperature (i.e., above 100°C).

[0065] [Second process] In the second step, the film-forming stock solution prepared in the first step is extruded in a predetermined shape into a solidification bath at 100°C or lower to solidify the polyamide resin into a film, wherein in this step, a solidification solution containing polyethylene glycol with an average molecular weight of 400 to 1000 and / or polypropylene glycol with an average molecular weight of 400 to 1000 (hereinafter sometimes referred to as "solidification solution for forming a dense layer") is brought into contact with at least one surface of the film-forming stock solution extruded in the predetermined shape to form a nanofiltration film.

[0066] In the film-forming stock solution extruded in the second step, near the surface of the surface in contact with the coagulation solution for forming the dense layer, non-solvent phase separation due to solvent exchange proceeds more favorably than thermally induced phase separation due to cooling. This results in the formation of a dense layer with a denser structure than conventional polyamide films, thereby enabling the aforementioned molecular weight cutoff and methanol permeability to be achieved.

[0067] To form a dense layer on only one surface of the nanofiltration membrane, in the second step, one surface of the film-forming stock solution extruded in a predetermined shape is brought into contact with the dense layer-forming coagulation solution, and the other surface is brought into contact with a coagulation solution that is compatible with the organic solvent used in the film-forming stock solution and has high affinity for the polyamide resin (hereinafter sometimes referred to as the "porous structure-forming coagulation solution"). To form a dense layer on both surfaces of the nanofiltration membrane, in the second step, both surfaces of the film-forming stock solution extruded in a predetermined shape are brought into contact with the dense layer-forming coagulation solution.

[0068] The average molecular weight of polyethylene glycol and / or polypropylene glycol used in the coagulation solution for forming a dense layer may be 400 to 1000, but from the viewpoint of suitably providing the aforementioned molecular weight cutoff and methanol permeability, it is preferably 400 to 800, and more preferably 400 to 600. If polyethylene glycol and / or polypropylene glycol with an average molecular weight of less than 400 is used, the molecular weight cutoff will exceed 1000, and there is a tendency that a nanofiltration membrane cannot be obtained. In the present invention, the average molecular weight of polyethylene glycol and / or polypropylene glycol is the number average molecular weight calculated based on the hydroxyl value measured in accordance with JIS K 1557-6:2009 "Plastics - Polyurethane raw material polyols - Test method - Part 6: Method for determining hydroxyl value by near-infrared (NIR) spectroscopy".

[0069] Specific examples of polyethylene glycol and / or polypropylene glycol used in the coagulation solution for forming a dense layer include polyethylene glycol 400, polyethylene glycol 600, polyethylene glycol 800, polyethylene glycol 1000, and polypropylene glycol 400. Among these, polyethylene glycol 400, polyethylene glycol 600, and polypropylene glycol 400 are preferred.

[0070] The coagulation solution for forming a dense layer may use only polyethylene glycol or polypropylene glycol with a predetermined average molecular weight, or a combination of polyethylene glycol and polypropylene glycol with a predetermined average molecular weight may be used.

[0071] Furthermore, the coagulation solution for forming a dense layer is preferably composed of polyethylene glycol and / or polypropylene glycol, but water may be included in addition to polyethylene glycol and / or polypropylene glycol, to the extent that it can suitably provide the aforementioned molecular weight cutoff and methanol permeability. When water is included in the coagulation solution for forming a dense layer, the water content can be, for example, 80% by mass or less, preferably 40% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less.

[0072] The coagulation solution for forming a porous structure can be any solvent that is compatible with the organic solvent used in the film-forming stock solution at a temperature of 25°C or lower, and that dissolves the polyamide resin at a temperature below its boiling point. Specific examples of the coagulation solution for forming a porous structure include glycerin, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol with an average molecular weight of 200 to 800, propylene glycol, 1,3-butanediol, sulfolane, N-methyl-2-pyrrolidone, γ-butyrolactone, δ-valerolactone, and aqueous solutions containing 20% ​​or more by mass of these. Among these, preferably, are aqueous solutions containing at least one selected from the group consisting of glycerin, propylene glycol, diethylene glycol, and polyethylene glycol with an average molecular weight of 200 to 600, and these in a proportion of 20 to 75% by mass (preferably 25 to 75% by mass); more preferably, aqueous solutions containing at least one selected from the group consisting of glycerin, diethylene glycol, tetraethylene glycol, and propylene glycol, and at least one of these in a proportion of 40 to 80% by mass (preferably 40 to 60% by mass), or aqueous solutions containing polyethylene glycol with an average molecular weight of 200 to 600 and polyethylene glycol with an average molecular weight of 200 to 600 in a proportion of 20 to 75% by mass; particularly preferably, aqueous solutions containing propylene glycol and propylene glycol in a proportion of 40 to 80% by mass (preferably 40 to 60% by mass).

[0073] If a hollow fiber membrane is to be formed as a nanofiltration membrane, the second step is to use a double-tube nozzle for hollow fiber production, discharging the film-forming raw material from the outer annular nozzle and the internal coagulation solution from the inner nozzle, and immersing the membrane in a coagulation bath. In this case, a coagulation solution for forming a dense layer should be used in at least one of the internal coagulation solution and the coagulation bath. If a coagulation solution for forming a dense layer is used in both the internal coagulation solution and the coagulation bath, a dense layer will be formed on both the inner and outer surfaces, resulting in a hollow fiber membrane with a porous interior. Furthermore, if a coagulation solution for forming a dense layer is used as the internal coagulation solution and a coagulation solution for forming a porous structure is used as the coagulation bath, a dense layer will be formed on the inner surface, and a hollow fiber membrane with a porous interior and outer surfaces will be obtained. Furthermore, if a coagulation solution for forming a porous structure is used as the internal coagulation solution and a coagulation solution for forming a dense layer is used as the coagulation bath, a dense layer will be formed on the outer surface, and a hollow fiber membrane with a porous interior and inner surface will be obtained. Furthermore, since the internal coagulation solution used when forming the hollow fiber membrane passes through a double annular nozzle, it is preferable that it does not contain water whose boiling point is below the temperature of the double annular nozzle.

[0074] As a double-tubular nozzle for hollow fiber production, a nozzle having a double-tubular structure, similar to those used in melt spinning to produce core-sheath type composite fibers, can be used. The diameters of the outer annular nozzle and the inner nozzle of the double-tubular nozzle for hollow fiber production can be appropriately set according to the inner and outer diameters of the hollow fiber membrane.

[0075] Furthermore, the flow rate when discharging the film-forming stock solution from the annular nozzle on the outside of the double-tubular nozzle for hollow fiber production is not particularly limited as it depends on the slit width, but for example, it can be 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 solution is set appropriately considering the diameter of the inner nozzle of the double-tubular nozzle for hollow fiber production, the type of internal solution used, the flow rate of the film-forming stock solution, etc., but it can be 0.1 to 2 times, preferably 0.2 to 1 time, and more preferably 0.4 to 0.7 times the flow rate of the film-forming stock solution.

[0076] Furthermore, if a flat membrane is to be formed as a nanofiltration membrane, the second step may be to use the dense layer forming coagulation solution as a coagulation bath and extrude the film-forming raw material into the coagulation bath in a predetermined shape and immerse it in the bath.

[0077] In the second step, the temperature of the coagulation bath should be 100°C or lower, preferably -20 to 100°C, more preferably 0 to 60°C, even more preferably 2 to 20°C, and particularly preferably 2 to 10°C. The suitable temperature of the coagulation bath may vary depending on the organic solvent used in the film-forming stock solution, the composition of the coagulation solution, etc. Generally, lower temperatures tend to favor thermally induced phase separation, while higher temperatures tend to favor non-solvent phase separation. That is, when producing a hollow fiber membrane in which a dense layer is formed on the luminal surface, it is preferable to set the coagulation bath to a low temperature in order to increase the pore size of the dense layer on the luminal surface, and it is preferable to set the coagulation bath to a high temperature in order to make the dense layer on the luminal surface more dense and to make the internal structure coarser.

[0078] Furthermore, if a hollow fiber membrane is formed as the nanofiltration membrane, the temperature of the internal coagulation solution should be around the set temperature of the double-tube nozzle, for example, 120 to 250°C, preferably 160 to 230°C, and more preferably 180 to 220°C.

[0079] Thus, by carrying out the second step, the film-forming stock solution solidifies in the solidification bath, and a nanofiltration film is formed with a dense layer on at least one surface.

[0080] [3rd step] In the third step, the coagulated liquid is removed from the nanofiltration membrane formed in the second step. The method for removing the coagulated liquid from the nanofiltration membrane is not particularly limited, but a method of extracting and removing the coagulated liquid, which has undergone phase separation in the nanofiltration membrane by immersion in an extraction solvent, is preferred. As the extraction solvent used for the extraction and removal of organic solvents, it is preferable to use one that is inexpensive, has a low boiling point, and can be easily separated after extraction by the difference in boiling point, etc. Examples include water, glycerin, methanol, ethanol, isopropanol, acetone, diethyl ether, hexane, petroleum ether, toluene, etc. Among these, water, methanol, ethanol, isopropanol, and acetone are preferred; more preferably, water, methanol, and isopropanol are preferred. In particular, when extracting a coagulated liquid that is soluble in water, it is efficient to perform solvent extraction at the same time by winding while showering with water. Furthermore, when extracting organic solvents that are insoluble in water, such as phthalates and fatty acids, isopropyl alcohol, petroleum ether, etc. can be suitably used. Furthermore, when extracting and removing the coagulated liquid by immersing the nanofiltration membrane in the extraction solvent, there are no particular restrictions on the immersion time of the nanofiltration membrane in the extraction solvent, but examples include 0.2 hours to 2 months, preferably 0.5 hours to 1 month, and more preferably 2 hours to 10 days. In order to effectively extract and remove the coagulated liquid remaining on the nanofiltration membrane, the extraction solvent may be replaced or stirred.

[0081] Thus, by carrying out the third step, the nanofiltration membrane of the present invention is obtained.

[0082] [Drying process (4th process)] The nanofiltration membrane after the third step preferably includes a fourth step of drying off the extraction solvent. The drying off of the extraction solvent can be carried out by known drying treatments such as natural drying, hot air drying, reduced pressure drying, or vacuum drying.

[0083] Furthermore, the nanofiltration membrane may be stretched in one direction simultaneously with or after the drying treatment of the extraction solvent. By stretching the nanofiltration membrane in one direction, it becomes possible to improve the tensile strength and elongation while maintaining the aforementioned range of molecular weight cutoffs and increasing the methanol permeability. To stretch the nanofiltration membrane in one direction simultaneously with the drying treatment, the drying treatment should be performed while tension for stretching is applied to the nanofiltration membrane. The mechanism by which unidirectional stretching of the nanofiltration membrane can increase the methanol permeability while maintaining the aforementioned range of molecular weight cutoffs is not entirely clear, but it can be considered as follows, for example. That is, it is thought that the pores in the dense layer expand in an elliptical shape due to the stretching, and at this time, the minor axis of the ellipse maintains the original pore diameter, thereby maintaining the molecular weight cutoffs, and the area of ​​the entire ellipse becomes larger than the area of ​​the original pore, and as a result, the aforementioned effect is produced.

[0084] When stretching a nanofiltration membrane in one direction, the stretching ratio can be, for example, 1.2 to 5 times, preferably 1.2 to 3 times. Stretching can be carried out by known methods, for example, by winding from a low-speed roll to a high-speed roll continuously. Alternatively, the membrane may be cut to a certain length and stretched using a tensile testing machine or the like by gripping both ends, or manual stretching may be used.

[0085] 4. Nanofiltration membrane module The nanofiltration membrane of the present invention is housed in a module case equipped with a liquid to be treated inlet, a permeate outlet, etc., and is used as a nanofiltration membrane module.

[0086] When the nanofiltration membrane of the present invention is in the shape of a hollow fiber, it is used as a hollow fiber membrane module.

[0087] Specifically, the hollow fiber membrane module may have a structure in which the hollow fiber nanofiltration membranes of the present invention are bundled together, housed in a module case, and one or both ends of the bundle of hollow fiber nanofiltration membranes are sealed and fixed with a potting agent. The hollow fiber membrane module may have an opening connected to a flow path passing through the outer wall surface of the hollow fiber nanofiltration membrane, and an opening connected to the hollow portion of the hollow fiber nanofiltration membrane, which serve as an inlet for the liquid to be treated or an outlet for the filtrate.

[0088] The shape of the hollow fiber membrane module is not particularly limited and may be a dead-end module or a cross-flow module. Specifically, examples include: a dead-end module in which a bundle of hollow fiber membranes is bent into a U-shape and packed, and the ends of the bundle of hollow fiber nanofiltration membranes are sealed and then cut to create an opening; a dead-end module in which a bundle of hollow fiber nanofiltration membranes with one end of the hollow opening sealed by heat sealing or the like is packed straight, and the open end of the bundle is sealed and then cut to create an opening; a dead-end module in which a bundle of hollow fiber nanofiltration membranes is packed straight, both ends of the bundle are sealed, and only one end is cut to expose the opening; and a cross-flow module in which a bundle of hollow fiber nanofiltration membranes is packed straight, both ends of the bundle are sealed, the sealed parts at both ends of the bundle are cut, and two flow channels are created on the side of the filter case.

[0089] The packing ratio of the hollow fiber nanofiltration membrane inserted into the module case is not particularly limited, but for example, the volume of the hollow fiber nanofiltration membrane, including the volume of the hollow portion relative to the volume inside the module case, is 30 to 90 volume%, preferably 35 to 75 volume%, and more preferably 45 to 65 volume%. By satisfying such a packing ratio, a sufficient filtration area can be secured, the packing work of the hollow fiber nanofiltration membrane into the module case can be facilitated, and the potting agent can be easily flowed between the hollow fiber nanofiltration membranes.

[0090] The potting agent used in the manufacture of hollow fiber membrane modules is not particularly limited, but if the hollow fiber membrane modules are to be treated with an organic solvent, it is desirable to have an organic solvent. 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 exhibit small shrinkage and swelling upon curing and are not excessively hard are preferred, and suitable examples include polyamide, silicone resin, epoxy resin, and polyethylene. These potting agents may be used individually or in combination of two or more.

[0091] The material of the module case used in the hollow fiber membrane module is not particularly limited, and examples include 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, and more preferably polyamide, polyethylene, polypropylene, and polytetrafluoroethylene.

[0092] Furthermore, when the nanofiltration membrane of the present invention is in a flat membrane shape, it can be used as a plate-and-frame type, stack-type sheet module, spiral type module, rotating flat membrane type module, etc.

[0093] The nanofiltration membrane module utilizing the nanofiltration membrane of the present invention is used in fields such as the semiconductor industry, chemical industry, food industry, pharmaceutical industry, and medical product industry for purposes such as removing foreign substances from solvents, concentrating useful components from solvents, recovering solvents, and purifying water. Furthermore, one embodiment of the nanofiltration membrane module utilizing the nanofiltration membrane of the present invention is suitably used for organic solvent nanofiltration. [Examples]

[0094] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0095] 1.Measurement method [Outer and inner diameters of the hollow fiber membrane, and thickness of the hollow fiber membrane] Five hollow fiber membranes were observed under an optical microscope at a magnification of 200x. The outer and inner diameters (at the point of maximum diameter) of each membrane were measured, and their average values ​​were calculated. The thickness of the hollow fiber membrane was calculated by subtracting the inner diameter from the outer diameter and dividing by 2.

[0096] [Thickness of the dense layer] A cross-section of a hollow fiber membrane [treated with platinum deposition using a deposition apparatus (MSP-1S type magnetron sputtering apparatus, manufactured by Vacuum Device Co., Ltd.) with a discharge voltage of 45 mA and deposition time of 15 seconds] was observed using a scanning electron microscope (SEM) at a magnification of 10,000x. The distance (thickness) of the region where virtually no pores were observed was measured at 10 or more locations, and the average value was calculated.

[0097] [Methanol permeation] First, the module shown in Figure 1a was fabricated. Specifically, ten hollow fiber membranes were first cut to a length of 30 cm and bundled together. Next, a rigid nylon 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 in length was inserted into the opening at one end of the tube to seal the end. Then, a two-part epoxy resin that cures at room temperature was inserted into the opening on the opposite side of the tube from the rubber stopper, filling the space inside the tube with the epoxy resin. After that, the bundle of hollow fiber membranes prepared was bent into a roughly U-shape, and both ends of the hollow fiber membranes were inserted into the tube filled with epoxy resin until the tips of the ends touched the rubber stopper. The epoxy resin was then allowed to cure in this state. Subsequently, the area of ​​the cured epoxy resin on the rubber stopper side was cut along with the tube to create a module with the hollow portions at both ends of the hollow fiber membranes open.

[0098] Next, the module is set in the apparatus shown in Figure 1b, and methanol (100% methanol) at 25°C is flowed through the inside of the hollow fiber membrane of the module under a pressure of approximately 0.3 MPa for a certain period of time. The volume of methanol that permeates to the outside of the hollow fiber membrane is then determined, and the methanol permeate amount (L / (m³)) is calculated according to the following formula. 2 The value of bar (h) was calculated.

number

[0099] [Fractional Molecular Weight] Commercially available GPC standard polyethylene glycols (PEG, Agilent Technologies, molecular weights 194, 238, 282, 420, 600, 1000, 1500, 4000) were dissolved in methanol at a concentration of 0.1% by mass. This stock solution was then passed through a filtration system at a pressure of 0.3 MPa. The permeate was collected, and the polyethylene glycol concentration in the permeate was measured by high-performance liquid chromatography. The rejection rate was calculated according to the following formula. Based on the rejection rates for each molecular weight of polyethylene glycol, a graph was created with the molecular weight of the polyethylene glycol used on the x-axis and the rejection rate on the y-axis. The molecular weight at the intersection of the resulting approximation curve and the point of 90% rejection was determined as the fractional molecular weight.

number

[0100] [Tensile strength and elongation] A hollow fiber membrane cut to approximately 10 cm was used as a sample, and a tensile test was performed in an environment of 25°C (room temperature) and 60% humidity with a gripping distance of 50 mm and a tensile speed of 50 mm / min. The load (N) and elongation (mm) at the time of fracture were measured. Separately, the cross-section of the hollow fiber membrane was observed under an optical microscope at a magnification of 200x, and the outer diameter and inner diameter (both at the point of maximum diameter) of the hollow fiber membrane were measured. From the obtained outer diameter and inner diameter, the cross-sectional area (mm²) of the hollow fiber membrane was calculated. 2 The following was determined. Tensile tests and cross-sectional area measurements were performed using five hollow fiber membranes. The average values ​​of load, elongation, and cross-sectional area at fracture were calculated, and the tensile strength and elongation ratio were determined using these average values ​​according to the following formula.

number

[0101] [Organic solvent resistance] Hollow fiber membranes were immersed in each organic solvent at 25°C for 14 hours. The tensile strength and elongation of the polyamide hollow fiber membranes were measured before and after immersion under the conditions described above, and the rate of change was calculated according to the following formula.

number

[0102] 2. Manufacturing of hollow fiber membranes [Example 1] 260 g of polyamide 6 chips (Unitika Ltd., A1030BRT, relative viscosity 3.53) and 740 g of sulfolane (Tokyo Chemical Industries, Ltd.) were stirred at 180°C for 1.5 hours to dissolve, and the stirring speed was reduced and degassed for 1 hour to prepare a film-forming stock solution. The film-forming stock solution was delivered via a metering pump to a spinneret kept warm at 210°C and extruded at 13.0 g / min. The spinneret used had an outer diameter of 1.5 mm and an inner diameter of 0.6 mm. Polyethylene glycol 400 (PEG400, average molecular weight 400) was flowed at a delivery rate of 5.0 g / min as an internal coagulation solution (coagulation solution for forming a dense layer). The extruded film-forming stock was immersed through a 10 mm air gap into a solidification bath consisting of a 50% by mass aqueous solution of propylene glycol (PG) at 5°C (a solidifying solution for forming porous structures) to cool and solidify, and then taken up at a take-up rate of 20 m / min. The resulting polyamide hollow fibers were immersed in water for 24 hours to extract the solvent, and then dried by passing them through a hot air dryer (internal temperature 130°C) without stretching to obtain a polyamide hollow fiber film.

[0103] [Example 2] A polyamide hollow fiber film was obtained under the same conditions as in Example 1, except that the film-forming stock solution was prepared using 320 g of polyamide 6 chips and 680 g of sulfolane.

[0104] [Example 3] A polyamide hollow fiber membrane was manufactured under the same conditions as in Example 2, except that the internal coagulation solution (coagulation solution for forming a dense layer) was changed to polyethylene glycol 600 (PEG600, average molecular weight 600).

[0105] [Example 4] A polyamide hollow fiber membrane was manufactured under the same conditions as in Example 2, except that the internal coagulation solution (coagulation solution for forming a dense layer) was changed to polypropylene glycol 400 (PPG400, average molecular weight 400).

[0106] [Example 5] A polyamide hollow fiber membrane was produced under the same conditions as in Example 2, except that the film-forming stock solution was prepared using 320 g of polyamide 6 chips, 544 g of dimethyl sulfone, and 136 g of sulfolane.

[0107] [Example 6] A polyamide hollow fiber membrane was obtained under the same conditions as in Example 3, except that the coagulation bath was changed to a 20% by mass polyethylene glycol 600 (PEG600, average molecular weight 600) aqueous solution (coagulation solution for forming porous structures).

[0108] [Example 7] A polyamide hollow fiber film was obtained under the same conditions as in Example 1, except that the film-forming stock solution was prepared using 250 g of polyamide 11 chips (Lilsan BESV0 A FDA, manufactured by Arkema, with a relative viscosity of 2.50) and 750 g of γ-butyrolactone (manufactured by Wako Pure Chemical Industries, Ltd.).

[0109] [Example 8] A polyamide hollow fiber membrane was manufactured under the same conditions as in Example 6, except that the hollow fiber membrane, which had been cooled and solidified, was immersed in water for 24 hours to extract the solvent, and then passed through a supply roller, a hot air dryer (internal temperature 130°C), and a take-up (stretching) roller in that order, to simultaneously perform drying and stretching (stretching ratio 2 times).

[0110] [Comparative Example 1] A polyamide hollow fiber membrane was manufactured under the same conditions as in Example 2, except that the internal coagulation solution (coagulation solution for forming a dense layer) was changed to polyethylene glycol 300 (PEG300, average molecular weight 300).

[0111] [Comparative Example 2] A polyamide hollow fiber film was obtained under the same conditions as in Example 1, except that the film-forming stock solution was prepared using 240 g of polyamide 6 chips and 760 g of sulfolane.

[0112] 3. Results of physical property evaluation of hollow fiber membranes In all of the polyamide hollow fiber membranes of Examples 1-8 and Comparative Examples 1-2, a dense layer was formed on the inner surface. For each polyamide hollow fiber membrane, the outer diameter, inner diameter, thickness of the hollow fiber membrane, thickness of the dense layer, methanol permeation rate, molecular weight cutoff, tensile strength, and elongation were measured and the results are shown in Table 1.

[0113] A film-forming stock solution with a resin concentration of 25% by mass or more is used, and the average molecular weight is 400 or more and 100 0 Polyamide hollow fiber membranes (Examples 1-8) produced using polyethylene glycol or polypropylene glycol as the internal coagulation solution had a molecular weight cutoff of 200-1000, yet exhibited a methanol permeation rate of 0.03 L / (m³). 2 It was possible to achieve a high permeate volume of 100% or more (bar·h). On the other hand, even when using a film-forming stock solution with a resin concentration of 25% by mass or more, the polyamide hollow fiber membrane (Comparative Example 1) produced using polyethylene glycol with an average molecular weight of 300 as the internal coagulation solution had a high molecular weight cutoff of 1800 and could not be used for nanofiltration. Furthermore, the average molecular weight was 400 or more and 100 0 Even when polyethylene glycol was used as the internal coagulation solution, the polyamide hollow fiber membrane (Comparative Example 2) produced using a film-forming stock solution with a resin concentration of 24% by mass had a high molecular weight cutoff of 1300 and could not be used for nanofiltration.

[0114] [Table 1]

[0115] 4. Resistance to organic solvents Table 2 shows the results of measuring the organic solvent resistance of the hollow fiber membranes of Examples 1 and 7. The results confirmed that the hollow fiber membranes of Examples 1 and 7 possessed resistance to a wide range of organic solvents. Furthermore, since the hollow fiber membranes of Examples 2-6 and 8 are formed from polyamide, similar to Examples 1 and 7, it is clear from these results that the hollow fiber membranes of Examples 2-6 and 8 also exhibit excellent organic solvent resistance.

[0116] [Table 2] [Explanation of Symbols]

[0117] 1 module 1a Hollow fiber membrane 1b Tube filled with cured epoxy resin 2. Liquid transfer pump 3. Pressure gauge 4. Pressure relief valve 5. Drip tray 6. Methanol that has permeated to the outside of the hollow fiber membrane

Claims

1. A nanofiltration membrane formed using polyamide resin, having a molecular weight cutoff of 200 to 1000 and a methanol permeability of 0.03 to 5.00 L / (m³). 2 A nanofiltration membrane with bar-h.

2. The nanofiltration membrane according to claim 1, wherein the fractionated molecular weight is 250 to 990.

3. The nanofiltration membrane according to claim 1, wherein the hollow fiber membrane has an outer diameter of 450 μm or more.

4. The aforementioned polyamide resin has methylene groups and amide groups -CH 2 A nanofiltration membrane according to any one of claims 1 to 3, comprising only one type of aliphatic polyamide resin having a molar ratio of -:-NHCO- = 4:1 to 10:

1.

5. The nanofiltration membrane according to any one of claims 1 to 4, wherein the polyamide resin is polyamide 6.

6. A nanofiltration membrane according to any one of claims 1 to 5, used for organic solvent nanofiltration.

7. A nanofiltration method for filtering a liquid to be treated containing a solute or particles using a nanofiltration membrane according to any one of claims 1 to 6.

8. The nanofiltration method according to claim 7, wherein the solvent contained in the liquid to be treated is an organic solvent.

9. A nanofiltration membrane module comprising a module case containing a nanofiltration membrane according to any one of claims 1 to 6.

10. A method for producing a nanofiltration membrane according to any one of claims 1 to 6, comprising the following steps 1 to 3: The first step involves preparing a film-forming stock solution by dissolving a polyamide resin at a concentration of 25% by mass or more in an organic solvent having a boiling point of 150°C or higher and being immiscible with the polyamide resin at temperatures below 100°C, at a temperature of 100°C or higher. A step of solidifying a polyamide resin into a film by extruding the aforementioned film-forming stock solution into a solidification bath at 100°C or lower in a predetermined shape, the second step of forming a nanofiltration film by contacting at least one surface of the film-forming stock solution extruded in a predetermined shape with a solidification solution containing polyethylene glycol having an average molecular weight of 400 to 1000 and / or polypropylene glycol having an average molecular weight of 400 to 1000, and A third step involves removing the coagulation solution from the nanofiltration membrane formed in the second step.

11. This is a method for manufacturing a hollow fiber membrane-shaped nanofiltration membrane. The second step is to use a double-tube nozzle for manufacturing hollow fibers, in which the film-forming raw material is discharged from the outer annular nozzle and the internal coagulation solution is discharged from the inner nozzle, and the fibers are immersed in a coagulation bath, A method for producing a nanofiltration membrane according to claim 10, wherein at least one of the internal coagulation solution and the coagulation bath is a coagulation solution containing polyethylene glycol having an average molecular weight of 400 to 1000 and / or polypropylene glycol having an average molecular weight of 400 to 1000.

12. A method for producing a nanofiltration membrane according to claim 10 or 11, further comprising a step of stretching the nanofiltration membrane after the third step in one direction simultaneously with or after the drying treatment.

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