Porous membrane and method for producing same

Abstract

To provide a porous film that can be produced at low cost and conveniently and has high fouling resistance and a method for producing the same.SOLUTION: Provided is a porous film that has a porous structure and contains a polymer blend comprising a base polymer and poly(2-methoxyethyl acrylate). Also provided is a method for producing a porous film, the method comprising a step for preparing a film-producing solution containing a base polymer, poly(2-methoxyethyl acrylate), and a solvent, and a step for depositing a porous film by phase separation using the film-producing solution.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to porous membranes and methods for making same. [Background technology]

[0002] Membrane treatment technology uses porous membranes to separate solutes, such as polymers dissolved in a liquid, and contaminants contained in the liquid. Examples of water treatment using membrane treatment technology include water purification, sewage treatment, seawater desalination, and industrial water treatment. In addition to water treatment, membrane treatment technology can also be used to treat samples containing biomolecules. Currently, water treatment facilities using membrane treatment technology are rapidly becoming larger, and it is expected that membrane treatment technology will be used on a wider scale in the future. However, membrane fouling occurs when membranes are used for a long period of time, which increases the power costs of membrane treatment, as well as the costs of cleaning and replacement, making it a major problem in membrane treatment technology.

[0003] Therefore, the development of membranes that suppress fouling (low-fouling membranes) has been actively pursued. For example, as techniques for physically or chemically immobilizing polymers with fouling-suppressing properties (low-fouling properties) on the surface of porous membranes, methods such as (a) physical attachment (e.g., Non-Patent Document 1), (b) grafting using ultraviolet light or plasma (e.g., Non-Patent Document 2), and (c) ATRP: atom transfer radical polymerization (e.g., Non-Patent Document 3) have been proposed. Furthermore, (d) phase separation induced membrane formation has been proposed as a membrane formation technique in which a low-fouling polymer is contained in the membrane raw material (for example, Non-Patent Document 4). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] K. Akamatsu et al, Ind. Eng. Chem. Res., 50 (2011) 12281-12284 [Non-patent document 2] K. Akamatsu et al., Sep. Purif. Technol., 204 (2018) 298-303 [Non-patent document 3] YC Chiang et al, J. Membr. Sci., 339 (2009) 151-159 [Non-patent document 4] GV Dizon et al, J. Membr. Sci., 550 (2018) 45-58 Summary of the Invention [Problem to be solved by the invention]

[0005] As a technique for physically and chemically fixing a low-fouling polymer to the surface of a porous membrane, (a) physical attachment techniques are often simple, but the modified polymer is prone to peeling. In addition, (b) grafting using ultraviolet light or plasma, and (c) ATRP methods provide high membrane stability, but require multiple modification steps and are therefore not very practical. In addition, (d) for phase separation-induced membrane formation, blends of various polymers have been investigated, but expensive polymers are often used, which poses a cost issue.

[0006] In view of the above problems, an object of the present disclosure is to provide a porous membrane that can be produced inexpensively and easily and has high fouling resistance, and a method for producing the same. [Means for solving the problem]

[0007] The means for solving the above problems include the following aspects. <1> A porous membrane having a porous structure, comprising a blend polymer including a base polymer and poly(2-methoxyethyl acrylate). <2> The porous structure is an asymmetric porous structure in which the porosity increases as the pore diameter increases from one surface side to the other surface side of the porous membrane. <1> The porous membrane according to claim 1. <3> The base polymer is one or more polymers selected from the group consisting of polyvinylidene fluoride, polysulfone, polyethersulfone, polyvinyl chloride, polyacrylonitrile, cellulose acetate, and polyamide. <1> or <2> The porous membrane according to claim 1. <4> preparing a membrane-forming solution containing a base polymer, poly(2-methoxyethyl acrylate), and a solvent; a step of precipitating a porous membrane by a phase separation method using the membrane-forming solution; A method for producing a porous membrane comprising: <5> The base polymer is one or more polymers selected from the group consisting of polyvinylidene fluoride, polysulfone, polyethersulfone, polyvinyl chloride, polyacrylonitrile, cellulose acetate, and polyamide. <4> A method for producing the porous membrane described in <6> The phase separation method is a non-solvent induced phase separation method. <4> or <5> A method for producing the porous membrane described in [Effects of the Invention]

[0008] According to the present disclosure, a porous membrane that can be produced inexpensively and easily and has high fouling resistance, and a method for producing the same are provided. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic diagram showing an example of a method for producing a porous membrane according to the present disclosure. FIG. [Figure 2] FIG. 1 is a diagram showing FT-IR (ATR) spectra of the membrane surfaces of the porous membranes produced in the examples and comparative examples. [Figure 3] 1 is a FE-SEM image showing the surface of the porous membranes produced in the examples and comparative examples. [Figure 4] 1 is a FE-SEM image showing the cross section of the porous membranes produced in the examples and comparative examples. [Figure 5] 1 is a graph showing the relationship between pure water permeability coefficient and membrane thickness with respect to the PMEA blend ratio. [Figure 6]1 is a graph showing the results of a bovine serum albumin (BSA) permeation test. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. Furthermore, the term "process" in this specification does not only refer to an independent process, but also includes a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved. In this disclosure, poly(2-methoxyethyl acrylate) may be referred to as "MEA polymer" or "PMEA."

[0011] Prior to the porous membrane of the present disclosure, the present inventors focused on MEA polymer, which had been developed as a biomaterial, and discovered that by modifying a porous membrane with MEA polymer by plasma graft polymerization, it is possible to produce a porous membrane with excellent fouling suppression properties. However, plasma graft polymerization has problems such as a complicated modification process and difficulty in scaling up. Therefore, we investigated a simpler method for producing porous membranes with excellent fouling suppression. As a result, we discovered that porous membranes with high fouling resistance can be produced inexpensively and easily by using a membrane-forming solution in which a base polymer and poly(2-methoxyethyl acrylate) are dissolved in a specific good solvent and fabricating the porous membrane using a phase separation-induced membrane formation method.

[0012] <Porous membrane> The porous membrane according to the present disclosure is a porous membrane having a porous structure, which contains a blend polymer including a base polymer and poly(2-methoxyethyl acrylate).

[0013] The base polymer is the polymer that is the most abundant (mass%) in the porous membrane according to the present disclosure and serves as the base of the porous membrane. The base polymer is not particularly limited as long as it can be used to form a porous membrane as a blend polymer with the MEA polymer. Examples of the base polymer include polyvinylidene fluoride, polysulfone, polyethersulfone, polyvinyl chloride, polyacrylonitrile, cellulose acetate, and polyamide. The porous membrane may contain one or more types of base polymer. Polyvinylidene fluoride (PVDF) is preferred as the base polymer from the viewpoints of membrane formability, compatibility with the MEA polymer, membrane strength, availability, and the like.

[0014] PVDF has excellent film-forming properties and excellent mechanical and chemical durability, making it suitable as a material for the porous membrane of the present disclosure. PVDF can be a homopolymer or copolymer, such as a polyvinylidene fluoride-hexafluoropropylene copolymer or a polyvinylidene fluoride-chlorotrifluoroethylene copolymer. The molecular weight of PVDF is not particularly limited, but for example, a polymer having a weight average molecular weight of 10,000 to 10,000,000 can be used.

[0015] The content (mass %) of the base polymer in the porous membrane according to the present disclosure depends on the type of base polymer, but when PVDF is used, for example, it is 55 to 95 mass %, or may be 65 to 85 mass %, from the viewpoints of membrane formability, membrane strength, etc.

[0016] (Poly(2-methoxyethyl acrylate)) Poly(2-methoxyethyl acrylate) (hereinafter referred to as MEA polymer) is a polymer obtained by polymerizing 2-methoxyethyl acrylate. The MEA polymer is an inexpensive material and is commercially available. The MEA polymer may be a homopolymer of 2-methoxyethyl acrylate or a copolymer of 2-methoxyethyl acrylate and other monomers.

[0017] The molecular weight of the MEA polymer is not particularly limited, but for example, an MEA polymer having a weight average molecular weight of 10,000 to 5,000,000 can be used.

[0018] The content (mass %) of the MEA polymer in the porous membrane according to the present disclosure depends on the type of base polymer, but may be, for example, 5 to 45 mass %, or 15 to 35 mass %, from the viewpoints of membrane formability, membrane strength, etc.

[0019] (Other ingredients) The porous membrane according to the present disclosure may contain components other than the base polymer and MEA polymer (other components) to the extent that the fouling resistance is not significantly impaired. The other components include polymers other than the base polymer and the MEA polymer, and additives. Other components include, for example, hydrophilic substances for pore size control, such as ethylene glycol, diethylene glycol, tetraethylene glycol, polyethylene glycol, polyvinylpyrrolidone, and glycerin, and one or more of these may be contained.

[0020] (Porous structure) The shape, size, distribution, and form of the pores in the porous structure of the porous membrane according to the present disclosure are not particularly limited. Depending on the application of the porous membrane, for example, when used as a separation membrane, an asymmetric porous membrane having an asymmetric porous structure in which the porosity increases as the pore diameter (average) increases from one side of the porous membrane to the other side is preferred. An asymmetric porous membrane typically has a sponge-like structure on one side and a dense structure with small pores on the other side. By using the dense layer as the separation surface, it can be used as a microfiltration membrane, ultrafiltration membrane, nanofiltration membrane, or reverse osmosis membrane.

[0021] The pore size is not particularly limited, but is 1 to 1000 μm on one side of the asymmetric porous membrane and 0.001 to 50 μm on the other side. The pore size is measured by observing each side of the porous membrane with a field emission scanning electron microscope (FE-SEM), measuring the maximum diameter of 50 randomly selected pores, and calculating the number average.

[0022] Furthermore, the porosity of the porous membrane according to the present disclosure is not particularly limited. When the porous membrane according to the present disclosure is an asymmetric porous membrane, the porosity varies greatly between the dense layer and the support layer that function as the separation surface. Therefore, the porosity cannot be generalized, but the average porosity of the entire membrane can be, for example, 25 to 85%.

[0023] (film thickness) The thickness of the porous membrane according to the present disclosure is not particularly limited, but if the membrane is too thin, it may be easily damaged during production, installation, or use, while if it is too thick, the solution may not easily pass through, which may increase power costs. The membrane thickness of the porous membrane according to the present disclosure may be selected depending on the application of the membrane, and is, for example, 10 μm to 1.0 mm. The membrane thickness is calculated as the average value of thicknesses measured at five randomly selected points.

[0024] <Method of manufacturing porous membrane> The method for producing a porous membrane according to the present disclosure includes the steps of: preparing a membrane-forming solution containing a base polymer, poly(2-methoxyethyl acrylate), and a solvent; and a step of precipitating a porous membrane by a phase separation method using the membrane-forming solution.

[0025] (Process for preparing a membrane forming solution) First, a membrane-forming solution containing a base polymer, poly(2-methoxyethyl acrylate), and a solvent is prepared. The base polymer and poly(2-methoxyethyl acrylate) that are the membrane materials can be the materials described above.

[0026] The solvent is not particularly limited as long as it can dissolve the base polymer and poly(2-methoxyethyl acrylate). For example, when PVDF is used as the base polymer, the solvent is limited because PVDF has excellent physical and chemical durability, but N-methyl-2-pyrrolidone (NMP) can be suitably used as a good solvent in which both PVDF and PMEA can be dissolved. When a polymer other than PVDF is used as the base polymer, a good solvent can be selected based on, for example, the Hansen solubility parameter. An example of a solvent other than NMP is dimethylacetamide. The solution may contain the above-mentioned "other components" as components other than the base polymer and poly(2-methoxyethyl acrylate).

[0027] The higher the blend ratio of MEA polymer in the membrane-forming solution, the higher the proportion of MEA polymer in the membrane, making it easier to produce a porous membrane with high water permeability. However, a lower proportion of base polymer reduces membrane strength. Furthermore, if the total polymer concentration in the membrane-forming solution is too low, membrane strength decreases, while if it is too high, the resulting porous membrane has low permeability. Therefore, the mass ratio of the base polymer to the MEA polymer (base polymer:MEA polymer) in the membrane-forming solution is preferably 30:1 to 1:1, and more preferably 5:1 to 2:1. The content of the base polymer in the membrane-forming solution is, for example, 10 to 30% by mass, preferably 15 to 25% by mass, and the content of the MEA polymer in the membrane-forming solution is, for example, 1 to 10% by mass, preferably 3 to 8% by mass.

[0028] (Step of precipitating a porous film by phase separation) The membrane-forming solution is used to precipitate a porous membrane by a phase separation method. The phase separation method is a membrane-forming technique for producing an asymmetric membrane, and examples thereof include the following methods. (A)Non-solvent induced phase separation (NIPS) A method to induce phase separation by interdiffusion of solutions in a non-solvent (solvent: soluble, polymer: insoluble) (B) Thermally Induced Phase Separation (TIPS) A method of inducing phase separation by heating or cooling the solution after casting (C) Water vapor-induced phase separation method A method in which the casting liquid is brought into contact with water vapor in a humidity-controllable space to induce phase separation. (Vapor Induced Phase Separation: VIPS)

[0029] In the method for producing a porous membrane according to the present disclosure, any of NIPS, TIPS, and VIPS may be used as the phase separation method. Here, an example of NIPS will be described. Figure 1 shows an example of a method for producing a porous membrane according to the present disclosure by NIPS. First, a membrane-forming solution (casting liquid) consisting of a base polymer (membrane material polymer), MEA polymer, solvent (good solvent), additives, etc. is thinly spread (cast) onto a flat surface such as a glass plate. If left in this state for a certain period of time (several seconds to several minutes), the good solvent will evaporate only from the surface, increasing the polymer concentration on the surface and forming a thin film. From this state, the glass plate is suddenly immersed in a coagulation liquid (poor solvent). The poor solvent penetrates the film surface and mixes with the good solvent, reducing the solubility of the polymer and causing it to solidify (phase separation induction). In addition, the good solvent escapes into the poor solvent, creating holes in the solidified polymer. Because the polymer concentration is higher on the surface side when it is dried and solidification progresses rapidly on the surface side, a dense layer is formed, while the interior solidifies more slowly, creating an asymmetric membrane with a sponge-like porous structure, and the whitened membrane can be recovered after a few minutes.

[0030] In the present disclosure, when a porous membrane is precipitated by NIPS, the method is not limited to the above method. For example, mass production is possible by continuously casting the porous membrane onto a roll of nonwoven fabric, allowing it to pass through a poor solvent, and then winding it up. The poor solvent for depositing the porous membrane according to the present disclosure may be selected depending on the types of base polymer and good solvent. For example, when PVDF is used as the base polymer and NMP as the good solvent, water can be used as the poor solvent.

[0031] By the above method, a porous membrane with excellent low-fouling properties (low-fouling membrane) can be produced inexpensively and simply. The uses of the porous membrane according to the present disclosure are not particularly limited, but for example, when used in water treatment, fouling can be effectively suppressed, which can greatly contribute to reducing not only membrane production costs but also running costs and membrane replacement costs. [Example]

[0032] The porous membrane and the method for producing the same according to the present disclosure will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0033] (reagent) The reagents used in the examples are as follows: ·Poly(vinylidene fluoride)[PVDF](Solef(registered trademark)6010.,SOLVAY.,Mw:300,000-320,000[Da] powder) 1-Methyl-2-pyrrolidone [NMP] (Wako Special Grade, Fujifilm Wako Pure Chemical Industries, Ltd.) Deionized water (DI water) (Elix® Essential 5 (UV), Millipore) 2-Methoxyethyl Acrylate [MEA] (Wako First Grade, Fujifilm Wako Pure Chemical Industries, Ltd.) 2,2'-Azobis(isobutyronitrile) [AIBN] (Wako Special Grade, Fujifilm Wako Pure Chemical Industries, Ltd.) 1,4-Dioxane (special grade reagent, Fujifilm Wako Pure Chemical Industries, Ltd.) Tetrahydrofuran [THF] (special grade reagent, Fujifilm Wako Pure Chemical Industries, Ltd.) Hexane (special grade reagent, Fujifilm Wako Pure Chemical Industries, Ltd.) ·Bovine Serum Albumin[BSA](pH5.2, SIGMA-ALDRICH)

[0034] <Preparing the membrane material> The base polymer used was commercially available polyvinylidene fluoride (PVDF). Poly(2-methoxyethyl acrylate) (PMEA) was synthesized by the following procedure. 20 g of distilled MEA monomer and 100 g of 1,4-dioxane were placed in a round-bottom flask and nitrogen was bubbled through for 30 min. After bubbling, 0.08 g of AIBN and a small stirrer were added, and radical polymerization was carried out at a polymerization temperature of 75°C for 24 h. After the radical polymerization was completed, the polymerized solution was poured into a beaker containing 500 mL of hexane, resulting in the precipitation of a polymer insoluble in hexane. At this point, the entire mixed solution became cloudy, but a white precipitate with a high degree of polymerization was present at the bottom of the beaker, and this precipitate was obtained as the target polymer. The cloudy solution, which contained hexane and 1,4-dioxane other than the precipitate (polymer), was removed, and the polymer was completely dissolved in 10 mL of THF. If the polymer did not dissolve with 10 mL, another 10 mL was added. Hexane, in an amount 15 times the amount of THF used to dissolve the polymer, was added to the solution to precipitate the polymer again. This procedure was repeated a total of three times to obtain MEA polymer (PMEA).

[0035] <Production of porous membrane> (Preparation of membrane-forming solution) As shown in Table 1, the blend ratios of PVDF:PMEA were set to 15:0, 15:1, 15:3, 15:5, and 15:7, and N-methyl-2-pyrrolidone (NMP) was used as the solvent and stirred at 70 °C for 1 to 4 hours to prepare a film-forming solution (cast solution).

[0036]

Table 1

[0037] (Film formation by phase separation method) After the prepared cast solution was naturally cooled to room temperature, it was spread on a glass plate and thinly and uniformly spread with a gap knife with a thickness of 200 μm. After 30 seconds, the glass plate was immersed in a non-solvent (pure water) to precipitate a porous membrane (PMEA0 to PMEA7) by phase separation.

[0038] [Evaluation] <FT-IR analysis> FT-IR (ATR) surface spectrum measurement was performed on the prepared membrane to confirm the presence of PMEA on the membrane surface. The FT-IR (Fourier transform infrared spectroscopy) spectra of the membrane surfaces of each membrane are shown in Figure 2. Peaks derived from C=O specific to PMEA could be confirmed at around 1740 cm-1 in the membranes blended with PMEA (PMEA1, 3, 5, 7). Also, it was confirmed that the ratio of the C=O peak intensity of PMEA to the peak intensity of PVDF increased with the increase in the PMEA / PVDF blend ratio. Therefore, it was verified that the increase in the PMEA / PVDF blend ratio contributed to the increase in the proportion of PMEA present on the membrane surface.

[0039] <FE-SEM observation> The FE-SEM was used to observe the structures of the membrane surface and the membrane cross-section. Figure 3 is a FE-SEM image showing the surface (opposite side to the glass substrate at the time of casting) of each membrane, and Figure 4 is a FE-SEM image showing the cross-section of each membrane. Both films have an asymmetric porous structure, with the porous structure differing between one side (the side opposite the glass substrate during casting) and the other side. Also, as can be seen in Figure 4, the voids in the films blended with PMEA are larger than those in the films not blended with PMEA.

[0040] <Pure water permeation experiment> A pure water permeation test was conducted using the cross-flow method with a flow rate of 2 L / min and a supply temperature of 25°C to determine the pure water permeability coefficient (Lp) and compare the water permeability performance of the membranes. Furthermore, measurements were taken at five random points on the membrane surface using a micrometer, and the average value was evaluated as the membrane thickness. Figure 5 is a graph showing the relationship between the pure water permeability coefficient Lp and membrane thickness as a function of the PMEA blend ratio. The higher the Lp, the lower the membrane resistance, and water permeates at lower pressure, which is advantageous for membrane separation. It can be seen that blending PMEA significantly improves the membrane's water permeability. PMEA3 had the highest pure water permeability coefficient Lp, while PMEA5 and PMEA7 had lower pure water permeability coefficients Lp. This is presumably due to the high total polymer content (blend polymer content) rather than an increase in the PMEA blend ratio. Furthermore, there was no significant difference in membrane thickness, and it was found that the membrane blended with PMEA was able to exhibit high water permeability without reducing the membrane thickness, i.e., without reducing the membrane strength. It is believed that the change in the internal structure of the membrane due to the blending of PMEA (Figure 4) had a significant impact on the increase in the pure water permeability coefficient.

[0041] <Bovine serum albumin (BSA) permeation test> After calculating the pure water permeability coefficient, the flux was set to 4 × 10 -6 [m 3 m -2 s -1 ] was measured at a constant interval of 10 min (the permeate sampling time was approximately 5 min) for a total of 30 min. After the pure water permeation, the entire feed solution was adjusted to 1000 ppm BSA, and the BSA permeation test was carried out for a total of 180 min. During the BSA permeation test, the permeate was sampled for 5 min at 10 min intervals, as in the pure water permeation test.

[0042] To determine the rejection rate, the feed solution was sampled at 30-minute intervals using a vial, and the BSA concentration was calculated using a TOC-V (Shimadzu Corporation) with a previously prepared BSA calibration curve. Permeate samples were collected every 30 minutes, but in the case of low flux, a sufficient amount of permeate could not be collected in the vial. Therefore, the sample was diluted with pure water and the TOC measurement results were multiplied by the dilution factor to correct the concentration. The apparent rejection rate was calculated using the following formula from the feed solution concentration and permeate concentration obtained from the TOC measurement results. R obs =(1-C p / C b ) R obs : Apparent rejection rate C p : Permeate concentration (mol / m 3 ) C b :Feed liquid concentration (mol / m 3 )

[0043] Figure 6 is a graph showing the results of the BSA permeation test. After the start of the test, the pure water was replaced with a 1000 ppm BSA aqueous solution, and the membranes without PMEA blend showed a rapid drop in permeability, indicating poor low-fouling properties. On the other hand, the membranes with PMEA blend, particularly PMEA3, 5, and 7, showed a slight drop in permeability, but remained nearly constant until the end of the test, demonstrating excellent low-fouling properties. It can also be seen that the low-fouling properties improved as the proportion of PMEA blend increased.

[0044] As mentioned above, blending PMEA resulted in changes in the internal structure of the membrane and improved water permeability. Furthermore, it was revealed that increasing the PMEA / PVDF blend ratio contributes to the presence of more PMEA on the membrane surface. Therefore, PMEA-blended PVDF membranes are expected to suppress fouling. [Industrial Applicability]

[0045] The porous membrane according to the present disclosure can suppress fouling in a variety of applications, and may potentially open up new applications beyond water treatment, such as protein fractionation. It is believed to have particularly excellent low-fouling properties against proteinaceous and polysaccharide-like substances.

Claims

1. A porous membrane having a porous structure, which contains a blend polymer including a base polymer and poly(2-methoxyethyl acrylate), which is a homopolymer of 2-methoxyethyl acrylate, wherein the content of the poly(2-methoxyethyl acrylate) relative to the total mass of the porous membrane is 15 to 35 mass %.

2. The porous membrane according to claim 1, wherein the porous structure is an asymmetric porous structure in which the porosity increases as the pore diameter increases from one side of the porous membrane to the other side.

3. 3. The porous membrane according to claim 1, wherein the base polymer is one or more polymers selected from the group consisting of polyvinylidene fluoride, polysulfone, polyethersulfone, polyvinyl chloride, polyacrylonitrile, cellulose acetate, and polyamide.

4. preparing a membrane-forming solution containing a base polymer, poly(2-methoxyethyl acrylate) which is a homopolymer of 2-methoxyethyl acrylate, and a solvent; a step of precipitating a porous membrane by a non-solvent induced phase separation method using the membrane forming solution; A method for producing a porous membrane comprising:

5. The method for producing a porous membrane according to claim 4, wherein the base polymer is one or more polymers selected from the group consisting of polyvinylidene fluoride, polysulfone, polyethersulfone, polyvinyl chloride, polyacrylonitrile, cellulose acetate, and polyamide.

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