Composite semipermeable membrane and method for manufacturing the same

Abstract

The main objective of the present invention is to provide a composite semipermeable membrane that achieves both chemical resistance to alkaline agents and other substances, and water permeability. [Solution] A composite semipermeable membrane is provided, comprising a porous support and a separation functional layer provided on at least one surface of the porous support, wherein the separation functional layer contains polyvinyl alcohol, and the amount of hydroxyl groups relative to the amount of carbon, when the elemental composition of the surface of the separation functional layer is measured by X-ray photoelectron spectroscopy, is 20% or less.

Description

Technical Field

[0001] The present invention is useful as a reverse osmosis membrane (RO membrane) or a nanofiltration membrane (NF membrane) used for water treatment such as the regeneration treatment of industrial wastewater, seawater desalination, brackish water desalination, or ultrapure water production.

Background Art

[0002] The RO membrane is one of the filtration membranes that remove impurities such as ions and salts from an aqueous solution. The RO membrane is mainly used in a filtration process using the reverse osmotic pressure method. The reverse osmotic pressure method refers to a method in which a pressure higher than the osmotic pressure is applied to an aqueous solution containing impurities such as ions and salts through a filtration membrane to filter the impurities. The RO membrane exhibits excellent filtration performance and water permeation performance, and can purify a solution close to pure water. In a water treatment plant using an RO membrane, heat treatment in the purification process is unnecessary, so the energy consumption can be reduced compared to other evaporation methods and the like. The NF membrane has a relatively low rejection rate of ions and salts among RO membranes, and usually, the pore diameter of the NF membrane is about 1 to 2 nm.

[0003] When a water treatment plant using an RO membrane is operated for a long time, impurities contained in the solution may accumulate on the surface of the RO membrane, and the filtration performance may deteriorate (fouling). Countermeasures against fouling include methods for modifying the surface of the RO membrane and methods for cleaning the surface of the RO membrane. In the cleaning method of the RO membrane, there are a chemical cleaning method for removing deposits with an alkaline agent or an acidic agent, etc., and a physical cleaning method (reverse pressure cleaning) for removing deposits by applying pressure from the opposite direction to the direction of the pressure permitted during normal operation. Also, as a method for enhancing the cleaning effect, there is a method of combining a chemical cleaning method and reverse pressure cleaning.

[0004] Patent Document 1 discloses, as an example of an RO membrane with a modified surface, a composite semipermeable membrane in which a hydrophilic polyvinyl alcohol (PVA) is coated on a separation functional layer containing polyamide.

[0005] Patent Document 2 discloses an example of a chemical cleaning method for RO membranes, which involves contacting the surface of an RO membrane with an alkaline aqueous solution containing a surfactant, and then contacting it with an oxalic acid aqueous solution.

[0006] Patent Document 3 discloses a composite semipermeable membrane as an example of an RO membrane capable of reverse pressure cleaning, in which a separation functional layer made of polyvinyl alcohol (C-PVA) containing cationic functional groups is provided on the surface of a porous support made of polyphenylene oxide containing anionic functional groups.

[0007] Non-patent document 1 discloses that the hydroxyl groups (OH groups) contained in PVA are hydrolyzed by alkaline agents. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] International Publication No. 1997 / 034686 [Patent Document 2] Japanese Patent Publication No. 2016-185514 [Patent Document 3] International Publication No. 2021 / 049623 [Non-patent literature]

[0009] [Non-Patent Document 1] Makoto Shiraishi and Shoichi Matsumoto, "On the decrease in viscosity of polyvinyl alcohol in alkaline aqueous solutions," Polymer Chemistry, The Society of Polymer Science, Japan, September 21, 1961, Vol. 19, No. 212, pp. 722-727. [Overview of the project] [Problems that the invention aims to solve]

[0010] In composite semipermeable membranes like the one shown in Patent Document 3, the porous support and the separation functional layer are bonded together by strong ionic bonds. Therefore, even when backwashing is performed, peeling of the separation functional layer is less likely to occur. However, if the crosslinking of C-PVA is insufficient, the OH groups in the C-PVA will be hydrolyzed by alkaline agents. Therefore, when strong cleaning methods such as backwashing with alkaline agents are used, sufficient resistance may not be maintained.

[0011] Furthermore, in composite semipermeable membranes such as those shown in Patent Document 3, the water permeability may decrease due to partial blockage of pores by PVA that has penetrated the support layer.

[0012] There is a demand for composite semipermeable membranes that can achieve both chemical resistance to alkaline agents and other substances, as well as water permeability.

[0013] The main objective of the present invention is to provide a composite semipermeable membrane that achieves both chemical resistance to alkaline agents and other substances, and water permeability. [Means for solving the problem]

[0014] As a result of diligent research, the inventors have discovered that by contacting a porous support containing anionic functional groups with a mixed solution of C-PVA and a crosslinking agent, and simultaneously inducing electrostatic adsorption of C-PVA onto the surface of the porous support and a crosslinking reaction of PVA, a composite semipermeable film that achieves both excellent chemical resistance and water permeability can be obtained, leading to the present invention.

[0015] The composite semipermeable membrane according to the present invention and the method for producing the same have the following configurations [1] to

[12] . [1] A porous support and A separation functional layer provided on at least one surface of the porous support, A composite semipermeable membrane having, The separation functional layer contains polyvinyl alcohol, A composite semipermeable membrane in which the amount of hydroxyl groups relative to the amount of carbon, when measured by elemental composition measurement using X-ray photoelectron spectroscopy on the surface of the separation functional layer, is 20% or less. [2] The composite semipermeable membrane according to [1], wherein the polyvinyl alcohol contains a structure represented by the following formula (1).

Chemical formula

Number

[10] The porous support is In the step of bringing the mixture into contact with the aforementioned mixed solution, A method for producing a composite semipermeable membrane according to [8] or [9], wherein the pH of the mixed solution is less than 5.

[11] The degree of saponification (A) of the polyvinyl alcohol, as shown in formula (3) below, A method for producing a composite semipermeable membrane as described in [8], wherein the ratio is 90% or more and less than 100%.

number

[12] The method for producing a composite semipermeable membrane according to [8], wherein the composite semipermeable membrane is in the shape of a hollow fiber. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a composite semipermeable membrane that achieves both chemical resistance to alkaline agents and other substances, as well as water permeability. [Brief explanation of the drawing]

[0017] [Figure 1]This is a schematic diagram illustrating the process of forming a separation functional layer on the surface of a porous support. [Figure 2] This is a schematic diagram of a performance evaluation system used to evaluate the performance of composite semipermeable membranes. [Modes for carrying out the invention]

[0018] The present invention will be described below. However, the embodiments described below are merely illustrative. The present invention is not limited in any way to the embodiments described below.

[0019] 1.Composite semipermeable membrane The composite semipermeable membrane according to the present invention comprises a porous support and a separation functional layer provided on at least one surface of the porous support. The separation functional layer can impart a separation function to the composite semipermeable membrane, which removes impurities such as ions and salts from an aqueous solution. The porous support can impart properties such as heat resistance, impact resistance, mechanical strength, and chemical resistance to the composite semipermeable membrane.

[0020] From the viewpoint of suppressing the peeling of the separation functional layer during water treatment and washing, it is preferable that the separation functional layer and the porous support are strongly bonded by chemical bonds. Specific examples of bonding include providing anionic functional groups on the porous support and cationic functional groups on the separation functional layer, thereby conferring ionic bonds between the porous support and the separation functional layer. From the viewpoint of chemical resistance, it is preferable that the separation functional layer has a cross-linked structure.

[0021] The composite semipermeable membrane may contain various known additives to optimize its membrane properties. Examples of additives include polymers (such as polystyrene), fillers, surfactants, and hydrophilic agents (such as polyethylene glycol and polyvinylpyrrolidone).

[0022] 2. Separation functional layer The separation functional layer included in the composite semipermeable membrane according to the present invention is preferably a polymer containing a cationic functional group, from the viewpoint of conferring ionic bonds with the porous support. Examples of cationic functional groups include amino groups, imino groups, ammonium groups, sulfonium groups, or phosphonium groups, but a quaternary ammonium group is preferred from the viewpoint of ease of availability and handling. The counterion of the cationic functional group is not particularly limited, but a chloride ion is simple and preferred.

[0023] A polymer containing cationic functional groups is preferably C-PVA. By using C-PVA, a high level of separation performance for monovalent ions, polyvalent ions, or neutral substances can be imparted to the composite semipermeable membrane.

[0024] The specific molecular structure of C-PVA is not particularly limited, but examples include compounds containing the structure represented by the following formula (4). [ka] (In general formula (4), the solid line represents a carbon-carbon single bond. R1 represents an aliphatic compound with 8 to 10 carbon atoms containing a cationic functional group. R2 represents an aliphatic compound with 2 to 4 carbon atoms containing a hydroxyl group. R3 represents an aliphatic compound with 4 to 6 carbon atoms containing an acetate group. m, n, and l each represent an independent natural number.)

[0025] Structural units having cationic functional groups are preferably free of ester bonds, from the viewpoint of suppressing hydrolysis by alkaline agents. C-PVA may be a homopolymer or a copolymer. It may also contain monomers containing two or more cationic functional groups.

[0026] Structural units having hydroxyl groups are preferably 1,3-dioxane ring structures, from the viewpoint of suppressing oxidation of the separation functional layer by chlorine.

[0027] In the above C-PVA, the introduction rate (M) of structural units containing cationic functional groups is preferably 0.3% or more and 3.0% or less. M is represented by the following formula (5).

number

[0028] A rate of introduction of structural units containing cationic functional groups (M) of 0.3% or more is preferable because it increases the amount of ionic bonding of C-PVA to the surface of the porous support containing anionic functional groups, thereby improving the removal rate of NaCl. Furthermore, a rate of M of 3.0% or less is preferable because it prevents the charge density in the separation functional layer from becoming too high, keeps the pore size of the separation functional layer within an appropriate range, and allows for sufficient separation performance of the composite semipermeable membrane. More preferably, M is 0.5% to 2.5%, and even more preferably 0.8% to 2.0%.

[0029] Specific examples of C-PVA include, for instance, PVA copolymers containing a cyclized polymer of diallyldimethylammonium chloride represented by formula (6) below, PVA copolymers containing a polymer of 3-(methacrylamide)propyltrimethylammonium chloride represented by formula (7) below, or PVA copolymers obtained by partially reacting glycidyltrimethylammonium chloride represented by formula (8) below with the hydroxyl groups of PVA, which are preferred from the viewpoint of chemical resistance to alkaline agents and the like. [ka] (In the formula, the solid line represents a carbon-carbon single bond. B) - (where m, n, and l represent the counterion for the ammonium group. m, n, and l are each independent natural numbers.) [ka] (In the formula, solid lines represent carbon-carbon single bonds. Double lines represent carbon-oxygen double bonds. B) - (where m, n, and l represent the counterion for the ammonium group. m, n, and l are each independent natural numbers.) [ka] (In the formula, the solid line represents a carbon-carbon single bond. B) - (where m, n, and l represent the counterion for the ammonium group. m, n, and l are each independent natural numbers.)

[0030] The C-PVA contained in the separation functional layer preferably has a crosslinked structure. Specific examples of the crosslinked structure include a structure in which the C-PVA molecules are crosslinked via multiple cyclic acetals. Multiple cyclic acetals can be obtained by condensing adjacent hydroxyl groups contained in R2 in general formulas (1) and (4). The above crosslinked structure is preferred because it provides excellent chemical resistance to alkaline agents.

[0031] When the elemental composition of the separation functional layer surface is measured by X-ray photoelectron spectroscopy, the amount of residual OH groups relative to the amount of carbon is preferably 20% or less. By keeping the amount of residual OH groups at 20% or less, a denser crosslinking structure can be obtained between the C-PVA molecules, resulting in superior chemical resistance to alkaline agents.

[0032] The thickness of the separation functional layer is preferably 1 nm or more and 100 nm or less. A thickness of 1 nm or more is preferable because it provides sufficient mechanical strength to the separation functional layer. A thickness of 100 nm or less is also preferable because it provides sufficient water permeability to the separation functional layer. More preferably, the thickness of the separation functional layer is 2 nm or more and 50 nm or less, and even more preferably 2 nm or more and 15 nm or less.

[0033] 3. Porous support The porous support included in the composite semipermeable membrane according to the present invention is preferably excellent in chemical resistance. Examples of materials for the porous support include polyphenylene resins such as polyphenylene ether resin, polyphenylene sulfide resin, or polyphenylene oxide (PPO) resin. In the present invention, it is preferable to include PPO resin in that it is excellent in terms of heat resistance, impact resistance, mechanical strength, or chemical resistance.

[0034] The porous support preferably contains anionic functional groups, from the viewpoint of imparting ionic bonds with the separation functional layer described above. A preferred example of anionic functional group is a sulfonic acid group, from the viewpoint of hydrophilizing the hydrophobic polyphenylene oxide resin. By hydrophilizing the porous support with sulfonic acid groups, the water permeability during water treatment can be improved.

[0035] When PPO resin is selected as the material for the porous support, and sulfonic acid groups are introduced into the PPO resin, for example, a copolymer shown in formula (9) below is formed. [ka] (In the formula, solid lines represent carbon-carbon single bonds. Double lines represent carbon-carbon double bonds. M) - (where m and n represent independent natural numbers.)

[0036] 4. Method for fabricating porous supports Next, an example of a method for producing a porous support will be described. While there are no particular limitations on the method for producing a porous support, examples include the dry method, the wet-dry method, and the wet method. In the wet-dry method, first, the resin (polymer material) that will be the material for the porous support is dissolved in an organic solvent to prepare a film-forming stock solution. Next, the obtained film-forming stock solution is evaporated and dried to obtain the porous support. The wet-dry method yields a denser porous support compared to other methods, thus improving the mechanical strength of the porous support.

[0037] The polymer material is, for example, the material for the porous support. Examples of organic solvents for dissolving the polymer material include the aprotic polar solvents N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), or N-methyl-2-pyrrolidone (NMP). From the viewpoint of solubility of the resin material at high temperatures, NMP is particularly preferred.

[0038] The number-average molecular weight of the polymer material is preferably between 5,000 and 500,000. A number-average molecular weight of 5,000 or more is preferable because it results in a sufficiently viscous film-forming solution, allowing for the production of a porous support with sufficient strength. A number-average molecular weight of 500,000 or less is preferable because it allows for dissolution at high temperatures in aprotic polar solvents.

[0039] The polymer material may contain various known additives to optimize the film properties of the resulting composite semipermeable membrane. Examples of additives include polymers (such as polystyrene), fillers, surfactants, and hydrophilic agents (such as polyethylene glycol and polyvinylpyrrolidone).

[0040] The form of the porous support is not particularly limited, but a flat membrane or a hollow fiber membrane is preferred. Furthermore, the hollow fiber membrane form is more preferred because it facilitates the fabrication of the separation functional layer described later. Porous supports having these forms can all be manufactured by methods known to those skilled in the art.

[0041] A method for forming a porous support having a flat film morphology includes, for example, casting a film-forming solution onto a substrate, and optionally allowing a certain drying period, then immersing the resulting molded body in a solidification bath.

[0042] As a method for forming a porous support having the form of a hollow fiber membrane, for example, a film-forming stock solution is discharged from the outer slit of a double-cylindrical spinning nozzle in the form of a hollow cylinder, and a fluid selected from non-solvents, latent solvents, good solvents, or mixtures thereof, or liquids incompatible with the film-forming solvent, or even gases such as nitrogen or air, is extruded together with the film-forming stock solution from the inner nozzle pore, and after a drying period of a certain duration as desired, the resulting molded body is immersed in a solidification bath.

[0043] The non-solvent used during the molding of the porous support is not particularly limited, but, as with known film-forming methods, water, alcohol, and polyhydric alcohols (ethylene glycol, diethylene glycol, triethylene glycol, glycerin, etc.) are preferred, and a mixture of these may also be used. From the viewpoint of simplicity and economy, it is preferable to use water as the main component.

[0044] From the viewpoint of controlling the solvent exchange rate during the coagulation process and achieving a desirable membrane structure, it is preferable to add a film-forming solvent (NMP, DMAc, etc.) to the non-solvent of the coagulation bath. From the viewpoint of controlling the viscosity of the coagulation bath, polysaccharides or water-soluble polymers may be added to the coagulation bath.

[0045] The temperature of the film-forming solution during the molding of the porous support is preferably above the temperature at which the polymer material does not precipitate due to thermally induced phase separation, and below the boiling point of the nonpolar proton solvent. If the polymer material is PPO, the temperature is preferably 40°C or higher, and more preferably 60°C or higher. The upper limit of the temperature is preferably below the boiling point of the nonpolar proton solvent, more preferably 150°C or lower, and even more preferably 100°C or lower. For example, if the temperature of the film-forming solution is within the above range, the viscosity of the film-forming solution will be within an appropriate range, making molding and structural control easier. In addition, the evaporation rate of the good solvent in the film-forming solution and the solvent exchange rate in the solidification bath will be increased, so the density of the polymer on the film surface will not become too dense, making it possible to adjust the structure of the porous support and the water permeability performance to a suitable range.

[0046] It is preferable to allow a certain solvent drying time before immersing the film-forming stock solution in the coagulation bath. The drying time and temperature are not particularly limited, but should be adjusted so that the structure of the porous support obtained in the end is as desired. For example, it is preferable to partially dry the film-forming stock solution for a period of 0.01 seconds to 600 seconds at an ambient temperature of 5°C to 200°C.

[0047] The temperature of the solidification bath is appropriately selected from the viewpoints of controlling the pore size of the porous support, economy, and work safety. Specifically, a temperature of 5°C to 100°C is preferred, and a temperature of 10°C to 80°C is more preferred. When the temperature is within this range, the demixing process proceeds appropriately, resulting in a porous structure for the porous support, and a support with low permeation resistance and excellent strength can be obtained.

[0048] The immersion time in the solidification bath should be adjusted to allow sufficient time for the porous support structure to be formed by phase separation. From the viewpoint of allowing sufficient solidification and efficiency in the manufacturing process, a range of 0.1 seconds to 1000 seconds is preferred, and a range of 1 second to 600 seconds is more preferred.

[0049] The porous support obtained after completing the membrane structure formation in the coagulation bath is preferably washed with water. The washing method is not particularly limited; it may be immersed for a sufficient amount of time, or it may be washed with running water for a certain period of time while being conveyed between rolls. The washed support membrane may be stored in a water-soaked state or in a dry state.

[0050] 5. Introduction of anionic functional groups into porous supports The porous support preferably contains anionic functional groups, from the viewpoint of conferring ionic bonds with the separation functional layer. When the porous support does not contain anionic functional groups, a method for introducing anionic functional groups into the porous support is described below. Note that, in order to obtain the final composite semipermeable film, a polymer material containing anionic functional groups may be prepared in advance, and the porous support may be manufactured from that.

[0051] Preferred examples of anionic functional groups introduced into a porous support include, for example, sulfonic acid groups, from the viewpoint of hydrophilizing a hydrophobic polyphenylene oxide resin. The introduction of sulfonic acid groups into a porous support (sulfonation treatment) can be carried out, for example, by bringing the porous support and a sulfonating agent into contact for a certain period of time.

[0052] Examples of sulfonating agents include chlorosulfuric acid, fuming sulfuric acid, sulfur trioxide, or sulfuric acid. From the viewpoint of low cost, efficiency of the work process, environmental impact, and controllability of the sulfonation treatment, sulfuric acid is preferred. Furthermore, when using sulfuric acid, if the porous support is, for example, PPO, it readily undergoes electrophilic substitution reactions on the benzene ring, which is the main chain of PPO, and since PPO does not contain electron-withdrawing functional groups such as sulfonyl groups in its main chain, the sulfonation treatment can be easily performed.

[0053] The concentration of sulfuric acid used in the sulfonation treatment (sulfuric acid concentration) is preferably 90% by mass or more and 98% by mass or less, and more preferably 92% by mass or more and 96% by mass or less. According to the inventor's findings, sulfonation treatment can also be performed using a relatively low concentration of sulfuric acid, such as 92% by mass or more and 96% by mass or less. The contact time between the porous support and sulfuric acid depends on the sulfuric acid concentration and reaction temperature, but is preferably 10 seconds or more and 6 hours or less, and more preferably 30 seconds or more and 2 hours or less.

[0054] If the sulfuric acid concentration is lower than the range described above, or if the reaction time is shorter, the sulfonation treatment may not be sufficient, the formation of the separation functional layer described later may not proceed sufficiently, and sufficient separation performance may not be obtained. Conversely, if the sulfuric acid concentration is higher than the range described above, or if the reaction time is longer, the sulfonation treatment may proceed excessively, resulting in swelling of the surface of the porous support, an increase in pore size, and potentially resulting in insufficient separation performance.

[0055] One method for bringing a porous support into contact with a sulfonating agent is to immerse the porous support in the sulfonating agent. When the porous support has the form of a flat membrane or a hollow fiber membrane, a batch processing method in which the porous support is immersed in a tank filled with the sulfonating agent, or a continuous immersion processing method using roller conveyance, can be suitably used.

[0056] When immersing a porous support in a sulfonating agent, the porous support may be in either a dry or hydrated state. For example, when the sulfonating agent is sulfuric acid, it is preferable to first immerse the support in a low-concentration sulfuric acid solution (e.g., 80% to 90% by mass) and then perform treatment with a high-concentration sulfuric acid solution (e.g., 90% by mass or more) from the viewpoint of reducing the heat generated by mixing sulfuric acid and water and the concentration change in the sulfuric acid bath. When immersing a dry porous support, it is preferable to directly perform the sulfonation treatment with a high-concentration sulfuric acid solution (e.g., 90% by mass or more).

[0057] The sulfuric acid-treated porous support is preferably washed with pure water. To ensure no sulfuric acid remains, it is preferable to extend the washing time with pure water or to perform a multi-stage washing process. After washing, if necessary, immersion in an aqueous solution of an inorganic salt such as sodium sulfate, sodium chloride, or potassium chloride to replace the counterions of the sulfonic acid groups from protons to inorganic ions is also preferable from the viewpoint of improving the thermal stability of the polymer.

[0058] If the porous support before sulfonation is PPO, the sulfonation treatment yields, for example, the copolymer shown in Chemical Formula 6.

[0059] The degree of sulfonation of the porous support surface is not particularly limited, but should be selected to optimize the performance or mechanical strength of the resulting composite semipermeable film.

[0060] 6. Formation of the separation functional layer Next, an example of a method for forming the separation functional layer will be described. Prepare the porous support prepared in 5. above and a polymer containing hydroxyl groups. The polymer containing hydroxyl groups preferably contains cationic functional groups from the viewpoint of conferring ionic bonds between the porous support and the separation functional layer. Examples of cationic functional groups include amino groups, imino groups, ammonium groups, sulfonium groups, or phosphonium groups, but quaternary ammonium groups are preferred from the viewpoint of ease of availability and ease of handling.

[0061] The polymer containing hydroxyl groups is not particularly limited, but C-PVA is preferred, for example. Referring to Figure 1, in a mixed solution 100 of C-PVA 101 and a solvent 103 containing a crosslinking agent, a crosslinked portion 102 is formed on a portion of the C-PVA 101. When a porous support 104 containing anionic functional groups is brought into contact with the mixed solution 100, C-PVA 101 is electrostatically adsorbed onto the surface of the porous support 104, and then crosslinking of C-PVA 101 proceeds, forming a dense crosslinked portion 105. By selecting the above steps, the molecules of C-PVA 101 electrostatically adsorbed onto the porous support 104 can be prevented from penetrating from the surface of the porous support 104 into the pores, thereby obtaining a composite semipermeable membrane with excellent water permeability.

[0062] The peak-top molecular weight of C-PVA is preferably between 15,000 and 60,000. A peak-top molecular weight of 15,000 or more is preferable because it can further suppress the penetration of C-PVA molecules, which are electrostatically adsorbed onto the porous support, from the surface of the porous support into the pores, thereby obtaining a composite semipermeable membrane with excellent water permeability. Furthermore, a molecular weight of 60,000 or less is preferable because it allows for sufficient electrostatic adsorption of C-PVA molecules onto the porous support, forming a uniform separation functional layer. More preferably, the peak-top molecular weight of C-PVA is between 30,000 and 50,000.

[0063] The peak-top molecular weight mentioned above is determined by the differential molecular weight distribution curve obtained by gel permeation chromatography (GPC). A differential molecular weight distribution curve is a graph obtained by plotting the derivative of the concentration fraction (dw) with respect to the logarithm of the molecular weight (dlog[M]) on the vertical axis and the molecular weight on the horizontal axis. The peak-top molecular weight is the molecular weight value corresponding to the peak-top on the vertical axis of the differential molecular weight distribution curve.

[0064] The specific molecular structure of C-PVA is not particularly limited, but examples include compounds containing the structure represented by the general formula (2) mentioned above.

[0065] Structural units having cationic functional groups are preferably free of ester bonds, from the viewpoint of suppressing hydrolysis by alkaline agents. C-PVA may be a homopolymer or a copolymer. C-PVA may or may not contain monomers containing two or more cationic functional groups.

[0066] The degree of acetylation in the above C-PVA is preferably less than 10%. That is, the degree of saponification (A) of C-PVA is preferably 90% or more and less than 100%. A is represented by the following formula (10).

number

[0067] When A is less than 90%, the crosslinking density of the separation functional layer decreases, and the separation performance of the resulting composite semipermeable membrane tends to decline. The degree of saponification is more preferably 95% or higher, and even more preferably 99% or higher.

[0068] As a crosslinking agent, aldehydes are preferred because they are simple and economical. Examples of aldehydes (aldehyde compounds) that can be used include formaldehyde, acetaldehyde, propionaldehyde, glyoxal, malondialdehyde, succinaldehyde, glutaraldehyde, benzaldehyde, orthophthalaldehyde, isophthalaldehyde, terephthalaldehyde, and 1,3,5-benzenetricarbaldehyde. Of these, formaldehyde and glutaraldehyde are preferred from the viewpoint of having excellent reactivity as crosslinking agents and good acid hydrolysis resistance of the crosslinked product. Glutaraldehyde is particularly preferred.

[0069] The solvent used in the mixed solution containing C-PVA and a crosslinking agent is not particularly limited, but it is preferable to select a solvent capable of dissolving 100 ppm or more of C-PVA, and more preferably a solvent capable of dissolving 5000 ppm or more of C-PVA. Furthermore, it is preferable to select a solvent capable of dissolving 0.1% by mass or more of an aldehyde, and more preferably a solvent capable of dissolving 1% by mass or more of an aldehyde. The solvent is preferably, for example, water, methanol, ethanol, isopropyl alcohol, or a mixture thereof. In order to effectively promote the adsorption of C-PVA onto the porous support by electrostatic adsorption, it is preferable that the solvent contains 50% by mass or more of water as the main component.

[0070] The materials exemplified above can be appropriately selected to form a separation functional layer. For example, by crosslinking (acetalizing) the hydroxyl groups of C-PVA with a crosslinking agent such as glutaraldehyde, the hydrophilicity of the C-PVA separation functional layer can be suppressed and densified. As shown above, a composite semipermeable membrane with excellent permeability selectivity and high desalination performance can be obtained.

[0071] The acetalization crosslinking described above is not particularly limited, but for example, in C-PVA, it refers to the formation of an intramolecular crosslink structure by dehydration condensation between two hydroxyl groups of adjacent vinyl alcohol structural units and an aldehyde group, or the formation of a crosslink structure by linking polymer molecules via multiple cyclic acetals using dialdehydes or trialdehydes.

[0072] The mixed solution of C-PVA and the crosslinking agent is preferably adjusted to a pH of less than 5. More preferably, the pH is between 1 and 4. By appropriately adjusting the pH of the mixed solution, a denser separation functional layer can be formed. As the acid, sulfuric acid, hydrochloric acid, acetic acid, citric acid, etc., can be used, but sulfuric acid is more preferable because it is simple and effective in the crosslinking reaction.

[0073] The preparation time for the mixed solution of C-PVA and aldehyde is preferably 1 second to 60 minutes, and more preferably 1 second to 20 minutes. If the preparation time is too long, the crosslinking reaction will proceed, and the molecular weight of C-PVA will become excessive, which may impair the density during electrostatic adsorption. The porous support may be immersed in the mixed solution of C-PVA and aldehyde after preparation, or the preparation of the mixed solution of C-PVA and aldehyde and the immersion of the porous support in the mixed solution may be performed simultaneously.

[0074] To promote the crosslinking reaction of C-PVA on the surface of the porous support, it is preferable to adjust the temperature when the porous support is immersed to 50°C or higher and 100°C or lower. More preferably, it is 50°C or higher and 80°C or lower, and even more preferably 60°C or higher and 70°C or lower. The immersion time of the porous support is preferably 10 minutes or more, and more preferably 15 hours or more. By appropriately adjusting the above temperature and immersion time, a denser separation functional layer can be formed.

[0075] Factors involved in the cross-linking reaction include pH, immersion temperature, and immersion time. From the standpoint of work efficiency, the pH and immersion temperature conditions may be appropriately changed to reduce the immersion time. [Examples]

[0076] The present invention will be described in more detail below based on specific examples, but the present invention is not limited in any way to the following examples and can be implemented with appropriate modifications without changing the gist of the invention.

[0077] [Measurement and evaluation methods] (Measurement of residual OH groups) The degree of acetalization of the separation functional layer was evaluated by measuring the elemental composition of the separation functional layer, which had been modified with a chemical modification method, using X-ray photoelectron spectroscopy (XPS). More specifically, first, the separation functional layer was subjected to a gas-phase reaction with trifluoroacetic anhydride (standard reagent). The gas-phase reaction was carried out by leaving the separation functional layer in a sealed polytetrafluoroethylene container under an atmosphere of saturated trifluoroacetic anhydride vapor at room temperature for 3 hours. Through the above gas-phase reaction, a trifluoromethyl group was introduced to the OH group in R2 of general formulas (1) and (4) that does not contribute to the cross-linking structure. (See formula (11) below) [ka] (In general formula (11), solid lines represent carbon-carbon single bonds, and double lines represent carbon-oxygen double bonds. n represents an independent natural number.)

[0078] The fluorine and carbon content ratios of the separation functional layer into which trifluoromethyl groups were introduced were analyzed using XPS, and the amount of remaining OH groups was calculated using the following formula (12).

number

[0079] The degree of acetalization can be calculated from the value of the remaining OH group obtained above using the following formula (13).

number

[0080] (Elemental composition measurement by XPS) The conditions for elemental composition measurement using XPS are as follows: [Measurement conditions] Device name: K-Alpha+ (manufactured by Thermo Fisher Scientific) Excitation X-rays: Monochromatized Al Kα rays X-ray output: 12kV, 2.5mA Photoelectron escape angle: 90° Spot size: 200 μmφ Pass energy: 50 eV Step: 0.1eV

[0081] (Fabrication of a performance evaluation module for composite semipermeable membranes) A composite semipermeable membrane having a hollow fiber shape (hereinafter referred to as the hollow fiber membrane) and a sleeve were prepared, and the hollow fiber membrane was bundled in a U-shape and inserted into the plastic sleeve. A thermosetting resin was injected into one end of the sleeve and cured to seal that end. A portion of the sealed end was cut to obtain the open surface of the hollow fiber membrane, and the membrane area based on the outer diameter was 0.1 m². 2 An evaluation module 200 was fabricated (Figure 2).

[0082] (Measurement of permeate volume) The RO performance of the evaluation module 200 was evaluated using a membrane performance testing apparatus equipped with a shell 201, O-ring 202, evaluation fluid tank 300, evaluation fluid 301, supply pump 302, flow rate adjustment valve 303, pressure adjustment valve 304, etc., as shown in Figure 2.

[0083] Specifically, under conditions of 25°C, a pressure of 0.5 MPa was applied to a 1500 ppm sodium chloride (NaCl) aqueous solution, and the NaCl solution was allowed to flow from the inner surface to the outer surface of the hollow fiber membrane. (Standard conditions) After the flow treatment was performed for 1 hour, the permeate water was collected from the opening surface of the hollow fiber membrane and the amount of permeate water was measured.

[0084] Based on the above measurements, the amount of permeate per unit membrane area per day under the above standard conditions (standard condition permeate flux: Flux) was calculated using the following formula (14).

number

[0085] (Measurement of sodium chloride removal rate) The sodium chloride (NaCl) concentration (salt concentration) of the supply aqueous solution with a concentration of 1500 ppm used in the measurement of the permeate volume described above, and the membrane permeate collected in the measurement of the permeate volume described above, were measured using an electrical conductivity meter (Toa DKK CM-25R). Based on the measurement results, the NaCl removal rate was calculated using the following formula (15).

number

[0086] (Performance retention rate after immersion in alkaline solution) A composite semipermeable membrane was immersed in a sodium hydroxide aqueous solution (NaOH) adjusted to pH 12 at room temperature for 800 hours. The NaOH solution was replaced every 24 hours. Under the above standard conditions, permeability tests were performed before and after immersion to measure the removal rate of NaCl ions. From the NaCl rejection rate before and after immersion, the performance retention rate after immersion in the NaCl solution was calculated using the following formula (16).

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[0087] 1. Fabrication of a porous support Polyphenylene oxide PPO646 (hereinafter referred to as PPO) manufactured by SABIC Innovative Plastics Co., Ltd. and N-methyl-2-pyrrolidone (hereinafter abbreviated as NMP) were prepared. NMP and PPO were mixed so that the concentration of NMP was 30% by mass, and the resulting mixed solution was kneaded at 130°C to dissolve the PPO in the mixed solution and obtain a homogeneous film-forming stock solution.

[0088] Next, a double-tube nozzle was prepared. The film-forming stock solution obtained above was prepared as the external liquid supplied to the flow channel on the outer surface side of the double-tube (outer channel), and an NMP aqueous solution containing 20% ​​by mass of NMP was prepared in the hollow section as the internal liquid supplied to the flow channel on the inner surface side of the double-tube (hollow channel). Next, pressure was applied to the double-tube, causing the film-forming stock solution to be discharged from the outer channel, and simultaneously, the NMP aqueous solution to be discharged from the hollow channel. The discharged solutions were dried at room temperature by the air gap method, and then immersed in a solidification bath filled with pure water at 25°C to obtain a PPO hollow fiber membrane. The obtained PPO hollow fiber membrane was thoroughly washed to remove impurities such as residual solvent, and then dried again at room temperature. As a result, a porous support having a hollow fiber shape was obtained. The outer diameter of the obtained PPO hollow fiber membrane was 168 μm, and the film thickness was 101 μm.

[0089] 2. Sulfonation treatment of porous supports 800 dried PPO hollow fiber membranes were formed into 1m long loops, and the openings at both ends were bundled together. Next, these bundles were immersed in sulfuric acid adjusted to 94.5% by mass for 85 minutes to sulfurize the outer surface of the hollow fiber membranes. After the sulfurization treatment, the sulfonated PPO hollow fiber membrane bundles were immersed in a 10°C pure water bath and washed with water until the pH of the washing solution became neutral.

[0090] 3. Synthesis of C-PVA copolymer A round-bottom flask was prepared, and a reflux condenser, stirrer, nitrogen inlet, thermometer, and monomer inlet were attached to it. 125 g of methanol, 460 g of vinyl acetate (VAc), 6.38 g of 60% by mass diallyldimethylammonium chloride (DADMAC) aqueous solution as a cationic monomer, and a catalyst were added to the round-bottom flask, and the temperature was raised to 60°C while introducing nitrogen. 2,2'-azobis(2,4-dimethylvaleronitrile) was used as the reaction initiator. To ensure a uniform copolymer composition distribution, a portion of the highly reactive DADMAC was pre-filled into the reaction vessel, and the remainder was fed in during the reaction. 9.18 g of 60% by mass DADMAC aqueous solution was dissolved in 30 g of methanol and added dropwise to the solution in the flask under strong stirring over 2 hours using a metering pump. The non-volatile content at the end of the reaction was 40% by mass. The reaction was stopped by lowering the temperature to room temperature and blowing oxygen into the round-bottom flask, yielding the polymerization solution.

[0091] While maintaining the temperature of the polymerization solution at 40°C, a methanol solution containing 10% by mass of sodium hydroxide was added in an amount of 50 mmol relative to the vinyl acetate units in the copolymer, and saponification was carried out for 1 hour. The resulting solid was crushed in a mixer, and then thoroughly washed with methanol to remove excess alkaline components and impurities such as monomers. After that, the polymer powder was air-dried and then vacuum-dried at 50°C for 24 hours to obtain the target C-PVA copolymer.

[0092] The degree of saponification of the obtained C-PVA was 99.7%. The peak top molecular weight measured by GPC was 37,500.

[0093] 4. Formation of a separation functional layer on the outer surface of a porous support. (Example 1) An aqueous solution containing 300 ppm of the above-mentioned C-PVA copolymer and a 1.0% by mass aqueous solution of glutaraldehyde (GA) adjusted to pH 3 were prepared. The solutions were mixed using a static mixer and brought into contact with the outer surface of the sulfonated PPO hollow fiber membrane. Subsequently, the sulfonated PPO hollow fiber membrane was immersed in the GA aqueous solution containing the C-PVA copolymer at a temperature of 60°C for 72 hours. After the immersion treatment, it was washed with deionized water for 30 minutes.

[0094] (Example 2) In the immersion treatment, the separation functional layer was formed under the same conditions as in Example 1, except that the temperature was set to 90°C and the immersion time to 15 hours.

[0095] (Example 3) The separation functional layer was formed under the same conditions as in Example 1, except that the GA aqueous solution was adjusted to pH 4.

[0096] (Example 4) The separation functional layer was formed under the same conditions as in Example 1, except that the GA aqueous solution was adjusted to pH 1.

[0097] (Example 5) The separation functional layer was formed under the same conditions as in Example 1, except that the GA aqueous solution was adjusted to pH 1 and the immersion time was set to 15 hours during the immersion treatment.

[0098] (Comparative Example 1) In the immersion treatment, the separation functional layer was formed under the same conditions as in Example 1, except that the immersion time was set to 40 hours.

[0099] (Comparative Example 2) The separation functional layer was formed under the same conditions as in Example 1, except that the GA aqueous solution was adjusted to pH 5.

[0100] Table 1 shows the results of measuring and evaluating the composite semipermeable membranes of Examples 1-5 and Comparative Examples 1 and 2 using the method described above.

[0101] [Table 1]

[0102] The results shown in Table 1 indicate that in the composite films of Examples 1 to 5, the amount of remaining OH groups is 20% or less, and the 1,3-diol structure is increased, which is considered to result in excellent retention of the NaCl removal rate after immersion in alkaline solution.

[0103] In contrast, in Comparative Example 1, the residual OH group content was high at 21.90%, and the retention rate of NaCl removal performance decreased after alkaline immersion. This is thought to be because the immersion temperature was low and the crosslinking reaction did not proceed sufficiently.

[0104] In Comparative Example 2, the residual OH group content was high at 23.95%, and the retention rate of NaCl removal performance decreased after immersion in the alkaline solution. This is thought to be because the pH during immersion was high and the crosslinking reaction did not proceed sufficiently.

[0105] From these results, it became clear that the composite semipermeable membrane obtained in the examples can achieve both chemical resistance to alkaline agents and other substances, as well as water permeability, compared to the composite semipermeable membrane obtained in the comparative examples. [Industrial applicability]

[0106] The composite semipermeable membrane of the present invention can be suitably used as a material for reverse osmosis membranes (RO membranes) used in water treatment such as industrial wastewater regeneration, seawater desalination, or brine desalination, from the viewpoint of its superiority in preventing fouling. [Explanation of symbols]

[0107] 100 Mixed solution, 101 C-PVA, 102 Crosslinked section, 103 Solvent, 104 Porous support, 105 Dense crosslinked section, 200 Evaluation module, 201 Shell, 202 O-ring, 300 Evaluation liquid tank, 301 Evaluation liquid, 302 Supply pump, 303 Flow rate control valve, 304 Pressure control valve.

Claims

1. A porous support, A separation functional layer provided on at least one surface of the porous support, A composite semipermeable membrane having, The separation functional layer contains polyvinyl alcohol, A composite semipermeable membrane in which the amount of hydroxyl groups relative to the amount of carbon, as measured by elemental composition measurement of the surface of the separation functional layer by X-ray photoelectron spectroscopy, is 20% or less.

2. The composite semipermeable membrane according to claim 1, wherein the polyvinyl alcohol comprises a structure represented by the following formula (1). 【Chemistry 1】 (In general formula (1), the solid line represents a carbon-carbon single bond. R １ R represents an aliphatic compound with 8 to 10 carbon atoms that contains a cationic functional group. ２ R represents an aliphatic compound with 2 to 4 carbon atoms that contains a hydroxyl group. ３ (This represents an aliphatic compound with 4 to 6 carbon atoms containing an acetate group. m, n, and l each represent independent natural numbers.)

3. The introduction rate (M) of the cationic functional group contained in the polyvinyl alcohol, as represented by the following formula (2), A composite semipermeable membrane according to claim 2, wherein the concentration is 0.3% or more and 3.0% or less. [Math 1] (In general formula (2), m, n, and l represent independent natural numbers of m, n, and l as shown in general formula (1).)

4. The cationic functional group contained in the polyvinyl alcohol is The composite semipermeable membrane according to claim 2, wherein the group is a quaternary ammonium group.

5. The aforementioned polyvinyl alcohol, A composite semipermeable membrane according to claim 1, comprising a polyvinyl alcohol having a 1,3-dioxane ring structure.

6. The porous support is A composite semipermeable membrane according to claim 1, comprising a polyphenylene oxide containing a sulfonic acid group.

7. The thickness of the separation functional layer is A composite semipermeable film according to claim 1, wherein the wavelength is between 1 nm and 100 nm.

8. A porous support, A separation functional layer is provided on at least one surface of the porous support, A method for producing a composite semipermeable membrane having the following characteristics: (a) A step of dissolving a thermoplastic resin in a solvent to produce the porous support, (b) A step of imparting an anionic functional group to at least one surface of the porous support, (c) A step of forming the separation functional layer on at least one surface of the porous support via the anionic functional group, Equipped with, In step (c) above, The porous support said above, A method for producing a composite semipermeable membrane, characterized by contacting it with a mixed solution containing polyvinyl alcohol containing cationic functional groups and a crosslinking agent.

9. The porous support said above, In the step of bringing the mixture into contact with the aforementioned mixed solution, The temperature of the mixed solution is A method for producing a composite semipermeable membrane according to claim 8, wherein the temperature range is 50°C or more and 100°C or less.

10. The porous support said above, In the step of bringing the mixture into contact with the aforementioned mixed solution, The pH of the aforementioned mixed solution is A method for producing a composite semipermeable membrane according to claim 8 or 9, wherein the value is less than 5.

11. The degree of saponification (A) of the polyvinyl alcohol, as expressed in the following formula (3), A method for producing a composite semipermeable membrane according to claim 8, wherein the percentage is 90% or more and less than 100%. [Math 2] (In general formula (3), n and l represent the same n and l as in general formula (1).)

12. The composite semipermeable membrane, A method for producing a composite semipermeable membrane according to claim 8, wherein the membrane is hollow fiber in shape.

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