Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
  • B01D61/14—Ultrafiltration; Microfiltration
  • B01D61/18—Apparatus therefor
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
  • B01D69/02—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor characterised by their properties
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
  • B01D69/10—Supported membranes; Membrane supports
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
  • B01D69/12—Composite membranes; Ultra-thin membranes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
  • B01D71/06—Organic material
  • B01D71/30—Polyalkenyl halides
  • B01D71/32—Polyalkenyl halides containing fluorine atoms
  • B01D71/34—Polyvinylidene fluoride
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
  • C08J9/28—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof by elimination of a liquid phase from a macromolecular composition or article, e.g. drying of coagulum

Definitions

• separation membranes such as microfiltration membranes and ultrafiltration membranes have been used in a variety of fields, including water treatment (such as water purification and wastewater treatment), the medical field (such as blood purification), and the food industry.
• Patent Document 1 discloses a technique for improving low fouling and separation performance by controlling the pore size distribution of separation membranes made of polymers including polyvinylidene fluoride resins.
• Patent Document 2 discloses a technique for providing fluoropolymer membranes that combine higher porosity, higher permeability, and improved mechanical properties.
• separation membranes made from the above-mentioned polymeric polyvinylidene fluoride resins are excellent in abrasion resistance as well as chemical resistance.
• separation membranes made from the above-mentioned polyvinylidene fluoride resins have a low porosity in the inner layer, making them prone to sludge adhesion, resulting in frequent fouling and reduced water permeability. Therefore, the present invention aims to provide a separation membrane that has low fouling and excellent water permeability while maintaining high chemical resistance and abrasion resistance.
• a separation membrane comprising a porous resin layer containing a polymer mainly composed of a polyvinylidene fluoride resin and another layer, the porous resin layer being disposed on a surface portion thereof, the value of a for the polymer is determined by approximation using the following formula 1 from the radius of gyration ⁇ S 2 > 1/2 measured for the polymer by GPC-MALS (gel permeation chromatography equipped with a multi-angle light scattering detector) and the absolute molecular weight M w of the polymer, and is 0.40 or more and 0.48 or less; the weight-average molecular weight of the polyvinylidene fluoride resin measured by GPC (gel permeation chromatography) is 300,000 or more; the porosity occupied by macrovoids in a region within a depth of 15 ⁇ m from the surface of the porous resin layer is 28% or more and 80% or less;
• a separation membrane where
• a method for producing a separation membrane according to any one of (1) to (3) (i) a polymer solution preparation step of dissolving the polymer containing polyvinylidene fluoride resin as a main component using the polymer containing polyvinylidene fluoride resin as a main component, a first non-solvent containing 20% or more of a substance having a bound water amount of 1.7 g or more and 3.0 g or less per 1 g, a pore-opening agent, and a solvent to obtain a polymer solution; (ii) a porous resin layer forming step of coagulating the polymer solution in a coagulation bath containing a second non-solvent to form the porous resin layer.
• the present invention makes it possible to provide a separation membrane that has excellent low fouling and water permeability while maintaining high chemical resistance and abrasion resistance by including a polymer whose main component is a specific high-molecular-weight polyvinylidene fluoride resin.
• FIG. 1 is an enlarged image of a cross section of a separation membrane, illustrating the "three-dimensional network structure.”
• the separation membrane comprises a porous resin layer containing a polymer (hereinafter sometimes referred to as a "specific polymer”) whose main component is a polyvinylidene fluoride resin, and another layer, and the porous resin layer is disposed on the surface portion.
• a polymer hereinafter sometimes referred to as a "specific polymer”
• main component is a polyvinylidene fluoride resin
• the porous resin layer being disposed on the surface means that the porous resin layer is present on at least one surface of the separation membrane.
• the porous resin layer may be present on only one surface of the separation membrane, or on both surfaces. Furthermore, the porous resin layer may be present on only a portion of the surface of the separation membrane, or may constitute the entire surface.
• Polyvinylidene fluoride resin refers to a vinylidene fluoride homopolymer or vinylidene fluoride copolymer.
• vinylidene fluoride copolymer refers to a polymer having a vinylidene fluoride residue structure, and is typically a copolymer of vinylidene fluoride monomer and other fluorine-based monomers. Examples of such fluorine-based monomers include vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, and trifluorochloroethylene.
• the vinylidene fluoride copolymer may also be copolymerized with ethylene or other fluorine-based monomers, to the extent that the effects of the present invention are not impaired.
• Constant polyvinylidene fluoride resin as the main component means that the proportion of polyvinylidene fluoride resin in the polymer that constitutes the porous resin layer is 50% by mass or more. To ensure high chemical resistance, this proportion is preferably 55% by mass or more, and more preferably 60% by mass or more.
• secondary components other than polyvinylidene fluoride resin include, but are not limited to, acrylic resins that control the hydrophilicity of the porous resin layer. There is no particular upper limit to the proportion, and it may be 100% by mass. In other words, the polymer that constitutes the porous resin layer may consist solely of polyvinylidene fluoride resin.
• the separation membrane according to this embodiment must use a specific polymer having an a value of 0.40 or more and 0.48 or less.
• the a value is determined by approximation using the following formula 1 from the radius of gyration ⁇ S 2 > 1/2 measured for the specific polymer by GPC-MALS (gel permeation chromatography equipped with a multi-angle light scattering detector) and the absolute molecular weight Mw of the specific polymer.
• ⁇ S 2 > 1/2 bM w a ... (Formula 1)
• ⁇ S 2 > 1/2 means the radius of gyration of the specific polymer
• M w means the absolute molecular weight of the specific polymer.
• a When the value of a is 0.40 or more, the ⁇ S 2 > 1/2 becomes moderately large relative to Mw , and the polymer is moderately entangled in the porous resin layer. This is presumably to rigidify the polymer and improve the abrasion resistance of the separation membrane.
• the value of a when the value of a is 0.48 or less, excessive entanglement of the polymer is prevented and the polymer density in the surface layer of the porous resin layer is homogenized. As a result, it is presumed that excellent low-fouling properties are exhibited in the separation membrane.
• a means for setting the value of a to 0.40 or more and 0.48 or less is a method using linear PVDF (polyvinylidene fluoride). The value of a is preferably 0.41 or more and 0.44 or less.
• the value of b is 0.20 or less. It is presumed that a value of b of 0.20 or less allows the polymer to be moderately entangled in the porous resin layer, resulting in excellent low-fouling properties in the separation membrane. There is no particular lower limit for the value of b, but it is usually 0.10 or more.
• a and b above can be determined based on the relationship between the radius of gyration ⁇ S 2 > 1/2 and the absolute molecular weight M w , as measured by GPC-MALS, which is a gel permeation chromatograph (GPC) equipped with a multi-angle light scattering detector (MALS) and a differential refractometer (RI).
• GPC-MALS gel permeation chromatograph
• MALS multi-angle light scattering detector
• RI differential refractometer
• NMP N-methyl-2-pyrrolidone
• a and b can be determined by approximating the relationship between the radius of gyration ⁇ S 2 > 1/2 measured by GPC-MALS and the absolute molecular weight Mw as shown in the following formula 1 using a method commonly used in polymer research called a conformation plot. This method is common, as described in, for example, "Size Exclusion Chromatography” (by Mori Sadao, Kyoritsu Shuppan Co., Ltd., first edition, 1992).
• the conformation plot can be approximated by linear approximation using the least squares method on a double logarithmic graph of formula 1 within the measurement range of the detector.
• ⁇ S 2 > 1/2 bM w a ... (Formula 1)
• the weight-average molecular weight of the polyvinylidene fluoride resin measured by GPC must be 300,000 or more. It is believed that a weight-average molecular weight of 300,000 or more for the polyvinylidene fluoride resin appropriately reduces the amount of terminal functional groups, lowering reactivity with chemicals and improving the chemical resistance of the separation membrane. Furthermore, the weight-average molecular weight of the polyvinylidene fluoride resin is preferably 400,000 or more, and more preferably 500,000 or more. There are no particular restrictions on the upper limit of the weight-average molecular weight of the polyvinylidene fluoride resin, but it is typically 1.5 million or less.
• the weight-average molecular weight of the polyvinylidene fluoride resin can be measured by GPC equipped with a differential refractive index detector. Measurement by GPC is performed by dissolving the polymer that constitutes the porous resin layer in a solvent. A salt may be added to the solvent to improve the solubility of the polymer.
• GPC When measuring polyvinylidene fluoride resin by GPC, it is preferable to use, for example, NMP with 0.1 mol/L of lithium chloride added as the solvent.
• the separation membrane of this embodiment requires that the porosity occupied by macrovoids in the region within 15 ⁇ m depth from the surface of the porous resin layer be 28% or more and 80% or less.
• a porosity of 28% or more narrows the region into which fouling components can penetrate, including the pores, and prevents an increase in filtration resistance.
• a porosity of 30% or more is preferable, and 33% or more is more preferable. This further reduces the flow resistance of permeate when it flows through the porous resin layer, enabling high water permeability to be achieved.
• the porosity occupied by such macrovoids in the region within 15 ⁇ m depth from the surface of the porous resin layer is preferably 70% or less, and more preferably 50% or less.
• Macrovoids and the porosity occupied by macrovoids are defined as follows:
• the separation membrane is stained with a 3.2 mg/mL aqueous solution of a fluorescent substance (3,3,3',3'-tetramethyl-1,1'-bis(4-sulfobutyl)indocarbocyanine sodium) for 24 hours and then washed with distilled water.
• a fluorescent substance (3,3,3',3'-tetramethyl-1,1'-bis(4-sulfobutyl)indocarbocyanine sodium) for 24 hours and then washed with distilled water.
• FV3000 confocal laser microscope
• a 200 ⁇ m x 200 ⁇ m area parallel to the surface is observed at a resolution of 1024 x 1024 pixels and a magnification of 60x, extending from the surface to a depth of 40 ⁇ m, with intervals of 0.2 ⁇ m.
• the unstained areas observed in this manner can be defined as macrovoids.
• the porosity can be calculated by extracting a cross-sectional image perpendicular to the surface of the porous resin layer from the three-dimensional image created by stitching together the obtained images.
• the average pore size on the surface of the porous resin layer must be 10 nm or more and 100 nm or less. In order to exhibit excellent water permeability, the average pore size on the surface of the porous resin layer is more preferably 20 nm or more. Furthermore, in order to exhibit excellent low-fouling properties, the average pore size on the surface of the porous resin layer is preferably 80 nm or less, and more preferably 60 nm or less.
• the average pore size on the surface of the porous resin layer is measured as follows.
• the surface of the porous resin layer is observed at 10,000 times magnification using a scanning electron microscope (hereinafter referred to as SEM), and the diameter of each hole is calculated from the area of each hole, assuming that the hole is circular.
• SEM scanning electron microscope
• the porous resin layer preferably has a three-dimensional mesh structure, as this homogenizes the polymer density in the surface layer due to the entanglement of polymers whose main component is polyvinylidene fluoride resin, further enhancing low-fouling properties.
• three-dimensional mesh structure refers to a structure in which the polymers constituting the porous resin layer of the separation membrane according to this embodiment are spread three-dimensionally in a mesh-like pattern, as shown in Figure 1.
• the three-dimensional mesh structure has pores and voids separated by the polymers that form the mesh.
• the average number density of convex portions having a cross-sectional area of 0.015 ⁇ m 2 or more and 0.10 ⁇ m 2 or less in a plane 50 nm high from the reference surface is 0.16/ ⁇ m 2 or less.
• the average number density of the predetermined convex portions is more preferably 0.10/ ⁇ m 2 or less, and the lower the number, the better. It is most preferable that the average number density of the predetermined convex portions is 0/ ⁇ m 2 .
• the cross-sectional area of the convex portions on the surface of the porous resin layer, at a plane 50 nm high from the reference surface is calculated as follows: The surface of the separation membrane sample is observed in contact mode using an atomic force microscope in air, and a 2.5 ⁇ m square area is randomly selected and measured. For the obtained image, a reference surface is determined, and the cross-sectional area of each convex portion on a cross section parallel to the reference surface at a position 50 nm high from the reference surface is calculated.
• the reference surface is a plane defined based on the international standard ISO 25178 Surface Texture (Surface Roughness Measurement), and is the average height plane of the measured surface in the evaluation area.
• the average number density of convex portions having a cross-sectional area of 0.015 ⁇ m 2 or more and 0.1 ⁇ m 2 or less on a plane 50 nm high from the reference surface is calculated as follows.
• the convex portions having a cross-sectional area of 0.015 ⁇ m 2 or more and 0.10 ⁇ m 2 or less are counted.
• the total number of convex portions obtained is divided by the area of the measurement region to calculate the average number density of the convex portions.
• the average number density of the convex portions is determined by measuring any five visual fields and averaging the average number densities of the convex portions in the five visual fields.
• Methods for keeping the average number density of convex portions within the preferred range include using linear PVDF (polyvinylidene fluoride) and, in the production of separation membranes, limiting the heating time in air to 20 minutes or less at a space temperature of 100°C or higher, as described below.
• linear PVDF polyvinylidene fluoride
• the root mean square roughness Rq per 10 ⁇ m square area on the surface of the porous resin layer is preferably 30 nm or more and 50 nm or less. If the root mean square roughness Rq is 30 nm or more, the size of the particles formed by the polymer in the porous resin layer is large, which increases the interconnectivity of the porous resin layer and enables high water permeability to be achieved. If the root mean square roughness Rq is 50 nm or less, it is possible to prevent sludge from adhering and remaining on the membrane surface and thereby clogging the membrane. It is more preferable that the root mean square roughness Rq is 31 nm or more and 40 nm or less.
• the root mean square roughness Rq is determined by observing the surface of a separation membrane sample in water using an atomic force microscope in contact mode, randomly selecting 10 ⁇ m square areas, taking five images from five different areas, and averaging the Rq values. To calculate Rq for a 10 ⁇ m square area, only images from areas within the 10 ⁇ m square image where the difference between Rq values in any 10 ⁇ m x 5 ⁇ m area is 15 nm or less are used.
• One method for controlling Rq within a preferred range is to set the heating time in air at a space temperature of 100°C or higher, as described below, to 0.5 minutes or more and 7 minutes or less during the production of the separation membrane.
• the other layer is not particularly limited as long as it is a component that can overlap with the porous resin layer to form a layer.
• the other layer is preferably a support.
• the "support” is something that supports the porous resin layer and provides strength to the separation membrane.
• the material of the support is not particularly limited, and may be an organic material, an inorganic material, or the like, but organic fibers are preferred because they are easy to make lightweight. More preferably, the material is a woven or nonwoven fabric made of organic fibers such as cellulose fibers, cellulose triacetate fibers, polyester fibers, polypropylene fibers, or polyethylene fibers. Among these, nonwoven fabrics are preferred because their density is relatively easy to control, they are easy to manufacture, and are inexpensive.
• the thickness of the support is preferably in the range of 50 ⁇ m or more and 1 mm or less. More preferably, it is in the range of 70 ⁇ m or more and 500 ⁇ m or less. A support thickness of 50 ⁇ m or more makes it easier to maintain the strength of the separation membrane, and a thickness of 1 mm or less ensures sufficient water permeability.
• the thickness of the porous resin layer is preferably 50 ⁇ m or more, more preferably 80 ⁇ m or more, and even more preferably 100 ⁇ m or more. If the thickness of the porous resin layer is above the lower limit, the support is not exposed to the membrane surface, which prevents an increase in filtration pressure due to contaminant components adhering to the support, and also avoids the problem of not being able to restore filtration performance after cleaning.
• the thickness of the porous resin layer is preferably 500 ⁇ m or less, more preferably 300 ⁇ m or less, and even more preferably 200 ⁇ m or less. If the thickness of the porous resin layer is below the upper limit, good water permeability can be maintained.
• a portion of the resin forming the porous resin layer penetrates at least the surface layer of the support, forming a composite layer with the support at least in that surface layer.
• the porous resin layer is firmly fixed to the support by a so-called anchor effect, making it possible to prevent the porous resin layer from peeling off from the support.
• the separation membrane according to this embodiment can be applied to various liquid filtration methods and membrane filtration devices in the field of water treatment. In particular, it can be suitably used in the field of sewage treatment.
• the operating conditions are preferably such that the pure water permeability at 25°C and 5 kPa is 0.15 m3 / m2 /hr or more, and more preferably 0.5 m3 / m2 /hr or more.
• the separation membrane according to the present embodiment described above can typically be produced by the method described below.
• the separation membrane according to this embodiment can be produced, for example, by a method including the following steps (i) and (ii).
• a polymer solution preparation step in which the polymer containing a polyvinylidene fluoride resin as a main component is dissolved using a polymer containing a polyvinylidene fluoride resin as a main component, a first non-solvent containing 20% or more of a substance having a bound water amount of 1.7 g or more and 3.0 g or less per 1 g, a pore-opening agent, and a solvent to obtain a polymer solution.
• a manufacturing method can include, for example, the above steps (i) and (ii), and in the (ii) porous resin layer forming step, form a porous resin layer on at least one surface of the support.
• a coating of a polymer solution containing the above-mentioned resin (a polymer primarily composed of polyvinylidene fluoride resin), a first non-solvent containing 20% or more of a substance with a bound water content of 1.7 g to 3.0 g per gram, a pore-opening agent, and a solvent is formed on the surface of the aforementioned support, and the support is then impregnated with the polymer solution.
• the support is then immersed in a coagulation bath containing a second non-solvent to coagulate the resin and form a porous resin layer on the surface of the support. From the perspective of film-forming properties, it is generally preferable to select the temperature of the polymer solution within the range of 15 to 120°C.
• the density of the support is preferably 0.7 g/ cm3 or less, more preferably 0.6 g/ cm3 or less. If the density of the support is within this range, it is suitable for accepting the resin that forms the porous resin layer and forming an appropriate composite layer of the support and the resin. On the other hand, from the viewpoint of the strength as a separation membrane, the density of the support is preferably 0.3 g/ cm3 or more. The density referred to here is the apparent density, which can be determined from the area, thickness, and weight of the support.
• the non-solvent is a liquid that does not dissolve the resin.
• the first non-solvent in step (i) above controls the rate of resin solidification and thereby the size of the pores.
• the first non-solvent in step (i) above can be water, alcohols such as methanol or ethanol, or a substance with high water retention.
• a non-solvent containing a substance with high water retention as the first non-solvent in order to achieve a porosity of 28% or more of the macrovoids in the region within a depth of 15 ⁇ m from the surface of the porous resin layer.
• a substance with a bound water content of 1.7 g or more per gram is preferred.
• examples of such substances include glycerin, proline, and sodium lactate, with glycerin being particularly preferred.
• Substances with a bound water content of 1.7 g or more per gram tend to easily form hydrogen bonds with polymers and other materials in the polymer solution, which increases the viscosity of the polymer solution.
• increasing the viscosity of the polymer solution through hydrogen bonding reduces the tendency of the polymer in the polymer solution, which is primarily composed of a high-molecular-weight polyvinylidene fluoride resin, to aggregate.
• the amount of bound water is preferably 3.0 g or less, and more preferably 2.6 g or less.
• the bound water is water that is bound to a highly water-absorbent substance by hydrogen bonding and has its molecular motion restricted, and is distinguished from free water, which is adsorbed water and can move freely within the substance.
• the free water content is calculated from the integral of the endothermic peak appearing near 0°C using a DSC6200 manufactured by Seiko Instruments Inc. under conditions of a nitrogen atmosphere, a temperature rise of 2°C/min, and a temperature rise of -80 to 25°C, by the following formula 2, by dissolving the vacuum-dried sample in distilled water to a concentration of 10 wt %.
• a first non-solvent containing 20% or more of a substance with a bound water amount of 1.7 g to 3.0 g per gram refers to a non-solvent in which 20% or more of a substance with a bound water amount of 1.7 g to 3.0 g per gram is contained in the non-solvent.
• the proportion of this substance contained in the first non-solvent is preferably 50% to 100% by mass, where the total amount of the first non-solvent is 100% by mass. More preferably, it is 60% to 100% by mass.
• the first non-solvent may further contain another non-solvent as a component separate from the substance having a bound water content of 1.7 g or more and 3.0 g or less per gram.
• another non-solvent include those similar to the second non-solvent described below.
• the other non-solvent and the second non-solvent may be the same or different.
• the pore-opening agent is extracted when the resin layer is immersed in a coagulation bath, making the resin layer porous. It is preferable for the pore-opening agent to be highly soluble in the coagulation bath.
• inorganic salts such as calcium chloride and calcium carbonate can be used.
• polyoxyalkylenes such as polyethylene glycol (PEG) and polypropylene glycol
• water-soluble polymers such as polyvinyl alcohol, polyvinyl butyral, and polyacrylic acid
• surfactants While any pore-opening agent can be selected, polymers containing PEG as the main component or surfactants are preferred.
• polymers containing PEG as the main component with a weight-average molecular weight of 10,000 or more, or surfactants with a polyoxyalkylene structure, a fatty acid ester structure, and a hydroxyl group are particularly preferred.
• Using these pore-opening agents facilitates achieving a porosity of 28% or more, which is the void ratio occupied by macrovoids within a region 15 ⁇ m deep from the surface of the porous resin layer.
• "mainly composed” means that the component is contained in an amount of 50% by mass or more.
• surfactants having a polyoxyalkylene structure, a fatty acid ester structure, and a hydroxyl group include polyethylene glycol monooleate, polyethylene glycol monostearate, polyoxyethylene sorbitan monolaurate (Tween 20), polyoxyethylene sorbitan monostearate (Tween 60), and polyoxyethylene sorbitan monooleate (Tween 80).
• polyoxyethylene sorbitan monolaurate Tween 20
• polyoxyethylene sorbitan monostearate Tween 60
• polyoxyethylene sorbitan monooleate Tween 80
• the solvent dissolves the resin.
• the solvent acts on the resin and pore-opening agent, promoting their formation of a porous resin layer.
• solvents that can be used include NMP, N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetone, and methyl ethyl ketone.
• NMP, DMAc, DMF, and DMSO which have high resin solubility, are preferred.
• a hydrophilic resin is a resin that has a high affinity for water and dissolves in water, or a resin whose contact angle with water is smaller than that of polyvinylidene fluoride resins.
• hydrophilic resins include cellulose esters such as cellulose acetate or cellulose acetate propionate, fatty acid vinyl esters, polyvinyl acetate, polyvinylpyrrolidone, acrylic acid ester or methacrylic acid ester polymers such as ethylene oxide, propylene oxide, or polymethyl methacrylate, or copolymers of these polymers.
• the second non-solvent in step (ii) above acts to control the rate of resin solidification and thereby the size of the pores and macrovoids.
• Water or alcohols such as methanol or ethanol can be used as the second non-solvent. Among these, water is preferred as the second non-solvent from the standpoint of ease of waste liquid treatment and cost.
• the second non-solvent may also be a mixture containing these.
• a polymer primarily composed of polyvinylidene fluoride resin is dissolved in a first non-solvent, a pore-opening agent, and a solvent to obtain a polymer solution.
• the polymer primarily composed of polyvinylidene fluoride resin (specific polymer) is preferably 5 to 30% by mass
• the pore-opening agent is 0.1 to 15% by mass
• the solvent is 40 to 94.9% by mass
• the first non-solvent is 0.5 to 20% by mass.
• the content of the specific polymer in the polymer solution is preferably 5% by mass or more.
• the content is preferably 30% by mass or less. The content is more preferably within the range of 8 to 20% by mass.
• the content of the pore-opening agent in the polymer solution is preferably 0.1% by mass or more.
• the content is preferably 15% by mass or less.
• a more preferred range for the pore-opening agent content in the polymer solution is 0.5 to 10% by mass.
• the solvent content in the polymer solution is preferably 40% by mass or more.
• the above content is preferably 94.9% by mass or less.
• the solvent content in the polymer solution is more preferably in the range of 60 to 90% by mass.
• the solvent be in the range of 40 to 94.8% by mass and the first non-solvent be in the range of 0.5 to 20% by mass in the polymer solution. More preferably, the solvent be in the range of 40 to 94.4% by mass and the first non-solvent be in the range of 0.5 to 15% by mass.
• hydrophilic resin in addition to the polyvinylidene fluoride resin as the polymer dissolved in the polymer solution.
• the amount of hydrophilic resin be 0.1 to 10 parts by mass per 100 parts by mass of the polymer whose main component is polyvinylidene fluoride resin added to the polymer solution.
• the above-mentioned polymer solution is solidified in a coagulation bath containing a second non-solvent to form a porous resin layer.
• the coagulation bath can be a second non-solvent or a mixture containing the second non-solvent and a solvent.
• the second non-solvent is preferably at least 80% by mass, from the viewpoint of controlling the coagulation rate of the resin and the surface pore size, and appropriately generating macrovoids.
• the second non-solvent is more preferably in the range of 85 to 100% by mass.
• the temperature of the coagulation bath is typically preferably selected within the range of 15 to 80°C, from the viewpoint of appropriately controlling the coagulation rate. A more preferred temperature range is 20 to 60°C.
• a porous resin layer on at least one surface of the support in the porous resin layer forming step.
• the polymer solution coating on the support can be formed by applying the polymer solution to the support or by immersing the support in the polymer solution. When applying the polymer solution, it may be applied to one side or both sides of the support. In this case, although it depends on the composition of the polymer solution, it is preferable to use a support having a density of 0.7 g/cm3 or less , because this allows the support to be appropriately impregnated with the polymer solution.
• the washing method can be selected appropriately depending on the type of solvent and pore-opening agent, and is not particularly limited.
• One example is a method of immersing the substrate in hot water at 60 to 100°C for 1 to 10 minutes.
• the porous resin layer formation process it is preferable to carry out a heat drying process in air at a space temperature of 100°C or higher.
• the space temperature is preferably 100°C or higher and 175°C or lower to prevent melting of the polymer, whose main component is polyvinylidene fluoride resin.
• the heating method and conditions can be selected appropriately depending on the membrane and are not particularly limited. Heating the membrane increases the particle size of the polymer that forms the membrane, and can increase the root mean square roughness (Rq).
• the heating time in the heat drying step is preferably 20 minutes or less.
• the average number density of the specific convex portions described above can be set to 0.16/ ⁇ m2 or less .
• the heating time By setting the heating time to 0.5 minutes or more, moisture on the membrane surface is removed, and the polymer changes due to heating occur uniformly.
• the heating time By setting the heating time to 7 minutes or less, it is possible to prevent the polymer from shrinking and forming large pores locally, which would lead to deterioration of filtration properties.
• the shape of the separation membrane produced can be controlled by the manner in which the polymer solution coagulates in the porous resin layer formation process.
• a flat separation membrane for example, a film-like support made of nonwoven fabric, metal oxide, metal, etc. can be coated with the polymer solution and then immersed in a coagulation bath.
• the polymer solution can be discharged from the outer periphery of the double-tube nozzle, and the core liquid can be discharged from the center, simultaneously into a coagulation bath containing a second non-solvent. It is preferable to use a good solvent, such as that used in the polymer solution preparation process, as the core liquid.
• a separation membrane can be formed on the surface of a hollow fiber support made of a polymer, metal oxide, metal, or the like.
• Methods for forming a separation membrane on the surface of a hollow fiber support made of a polymer include, for example, using a triple-tube nozzle to simultaneously discharge the solution that serves as the raw material for the hollow fiber support and the polymer solution, or applying the polymer solution to the outer surface of a hollow fiber support that has already been formed into a membrane, and then passing the coated surface through a second non-solvent in a coagulation bath.
• the separation membrane of this embodiment can be used as either an ultrafiltration membrane or a microfiltration membrane.
• the water to be treated with the separation membrane of this embodiment is not particularly limited, but is preferably used in the separation of activated sludge for the biological treatment of sewage or wastewater containing suspended solids of relatively large particles.
• the resulting polymer solution was injected into a GPC (column: Showa Denko K.K.; Shodex KF-806M ⁇ 8.0 mm ⁇ 30 cm, two columns connected in series; differential refractive index detector: Showa Denko K.K.; RI-71 sensitivity -16) under the following conditions to measure the weight-average molecular weight.
• the injected polymer solution eluted from the column within a range of 28.5 to 43 minutes.
• the weight average molecular weight of the polyvinylidene fluoride resin constituting the porous resin layer is shown in the item "Weight average molecular weight of PVDF" in the table below.
• Injection volume 0.2mL
• Standard sample Monodisperse polystyrene manufactured by Tosoh Corporation
• Solvent NMP with 0.1M lithium chloride added
• the obtained polymer solution was injected into a GPC-MALS (column: Showa Denko K.K.; Shodex KF-806M ⁇ 8.0 mm ⁇ 30 cm, two columns connected in series, differential refractometer: Wyatt Technology; Optilab rEX, MALS: Wyatt Technology; DAWN HeLEOS) under the following conditions and measured.
• the injected polymer solution eluted from the column in the range of 27-43 minutes.
• Solvent 0.1M lithium chloride added NMP Flow rate: 0.5mL/min Injection volume: 0.3mL
• Equation 4 From the polymer concentration c i at elution time t i obtained from RI and the excess Rayleigh ratio R ⁇ i at elution time t i obtained from MALS, sin 2 ( ⁇ /2) and (K ci /R ⁇ i ) 1/2 were plotted (Berry plot or Zimm plot; Equation 5 below), and the absolute molecular weight M wi at each elution time t i was calculated from the value of ⁇ 0 in the approximate equation.
• K is an optical constant calculated from Equation 4 below.
• dn/dc in Equation 4 is the amount of change in the refractive index of the polymer solution with respect to a change in polymer concentration, i.e., the refractive index increment.
• the absolute molecular weight M wi at each elution time t i calculated from the above formula 5 was plotted on the x-axis and the radius of gyration ⁇ S 2 > 1/2 at each elution time t i was plotted on the y-axis, and the value of a for the polymer constituting the porous resin layer was determined by approximating with the above formula 1 within the molecular weight range of 140,000 to 1,000,000 so as to fall within the measurement range of the detector. The approximation was performed by linear approximation using a logarithmic graph of formula 1 and applying the least squares method. The value of a for the polymer constituting the porous resin layer is shown in the table below under the heading "value of a in formula 1."
• (iii) Amount of Bound Water in the First Non-Solvent The amount of bound water was calculated after determining the amount of free water.
• the free water content was calculated from the integrated value of the endothermic peak appearing near 0°C using a DSC6200 manufactured by Seiko Instruments Inc. under conditions of a nitrogen atmosphere, a temperature increase of 2°C/min, and a temperature increase from -80 to 25°C, using the following formula 2, by dissolving the vacuum-dried sample in distilled water to a concentration of 10 wt %.
• the sludge filtration resistance calculated from the permeate volume for the first 5 seconds during filtration was defined as Res Ax .
• the filtration resistance was calculated from the following formula 7.
• Res P ⁇ t ⁇ S/( ⁇ L) (Formula 7)
• L amount of filtered water
• the mixture was stirred for 1 minute at a stirring speed of 450 rpm.
• the filter holder was filled with distilled water, and the mixture was stirred for 1 minute at a stirring speed of 450 rpm.
• Activated sludge filtration and membrane cleaning were repeated for two minutes, and Res A1 to Res A5 were measured.
• Res An+1 - Res An was calculated for n values from 1 to 4, and the average value was taken as the increase in clogging filtration resistance. Note that the smaller the increase in clogging filtration resistance value, the better the separation membrane's anti-fouling properties can be evaluated.
• the separation membrane for which the increase in clogging filtration resistance was measured was immersed in a 12,000 mg/L aqueous sodium hypochlorite solution (100 mL) for 250 hours while maintaining the temperature at 40°C, and then immersed in water for at least 2 hours to replace the chemical solution with water.
• the increase in clogging filtration resistance was measured for the resulting separation membrane, and the increase in clogging filtration resistance after chemical immersion was determined.
• Root mean square roughness (Rq) of the surface of the porous resin layer The separation membrane was cut into 1 cm squares and attached to a sample stand with the surface to be measured facing up to prepare a sample. The surface of this sample separation membrane was observed with an atomic force microscope (Dimension FastScan, manufactured by Bruker AXS), and the root mean square roughness (Rq) of the surface of the porous resin layer was calculated.
• the number of convex portions having a cross-sectional area of 0.015 ⁇ m 2 or more and 0.10 ⁇ m 2 or less was counted, and the average number density was calculated.
• the specific measurement conditions were as follows: Scanning mode: Contact mode Probe: Silicon cantilever (manufactured by Bruker AXS; ScanAsyst-Fluid) Scanning range: 2.5 ⁇ m x 2.5 ⁇ m Scanning speed: 0.5 Hz Scanning resolution: 256 x 256 Measurement temperature: 25°C The measurement was performed for any five visual fields, and the average value of the average number density of the convex portions in the five visual fields was taken as the average number density of the convex portions of the porous film.
• the average number density of convex portions having a cross-sectional area of 0.015 ⁇ m2 or more and 0.10 ⁇ m2 or less in a plane 50 nm high from the reference surface on the surface of the porous resin layer is shown in the item "Average number density of specified convex portions" in the table below.
• Example 1 DMF as a solvent, PEG (weight average molecular weight 20,000) as a pore-opening agent, and glycerin (amount of bound water per 1 g: 1.9 g) as a first non-solvent were added to polyvinylidene fluoride (hereinafter referred to as "PVDF") 1 (Solef 6013, linear PVDF manufactured by Solvay Specialty Chemicals), and the mixture was thoroughly stirred at a temperature of 100°C to prepare a polymer solution having the following composition ratio.
• PVDF polyvinylidene fluoride
• the evaluation results of the obtained separation membrane are shown in Table 1.
• the weight average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 ⁇ m from the surface of the porous resin layer was 31%, and the average pore diameter at the surface of the porous resin layer was 46 nm.
• the pure water permeability which is an indicator of water permeability performance
• the increase in clogging filtration resistance which is an indicator of low fouling performance, both showed excellent values.
• Example 2 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 1, except that the composition of the polymer solution was changed as shown below. PVDF: 11% by mass DMF: 76% by mass PEG: 9% by mass Glycerin: 4% by mass The evaluation results of the obtained separation membrane are shown in Table 1.
• the weight-average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 ⁇ m from the surface of the porous resin layer was 34%, the average pore size at the surface of the porous resin layer was 56 nm, and both the pure water permeability and the increase in clogging filtration resistance showed excellent values.
• Example 3 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 2, except that PVDF2 (Solef6020, linear PVDF manufactured by Solvay Specialty Chemicals) was used instead of PVDF1.
• the evaluation results of the obtained separation membrane are shown in Table 1.
• the weight-average molecular weight of PVDF was 583,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 ⁇ m from the surface of the porous resin layer was 36%, the average pore size at the surface of the porous resin layer was 37 nm, and both the pure water permeability and the increase in clogging filtration resistance showed excellent values.
• Example 4 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 1, except that the composition of the polymer solution was as shown below.
• the amount of bound water per 1 g of the substance contained in the first non-solvent was 0.0 g for water and 1.9 g for glycerin. That is, among the substances contained in the first non-solvent, glycerin corresponds to a substance having an amount of bound water per 1 g of 1.7 g to 3.0 g, and water corresponds to another non-solvent.
• PVDF 14% by mass DMF: 69% by mass
• PEG 9% by mass
• Water 4% by mass
• Glycerin 4% by mass
• Table 1 The evaluation results of the obtained separation membrane are shown in Table 1.
• the weight-average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 ⁇ m from the surface of the porous resin layer was 35%, the average pore size at the surface of the porous resin layer was 39 nm, and both the pure water permeability and the increase in clogging filtration resistance showed excellent values.
• Example 5 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 1, except that the composition of the polymer solution was changed as shown below. PVDF: 14% by mass DMF: 69% by mass PEG: 9% by mass Glycerin: 8% by mass The evaluation results of the obtained separation membrane are shown in Table 1.
• the weight-average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 ⁇ m from the surface of the porous resin layer was 35%, the average pore size at the surface of the porous resin layer was 35 nm, and both the pure water permeability and the increase in clogging filtration resistance showed excellent values.
• Example 6 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 5, except that the first non-solvent of the polymer solution was changed to a sodium lactate solution (approximately 70%). The amount of bound water per gram of sodium lactate was 2.5 g. The evaluation results of the obtained separation membrane are shown in Table 1.
• the weight-average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 ⁇ m from the surface of the porous resin layer was 34%, the average pore size at the surface of the porous resin layer was 38 nm, and both the pure water permeability and the increase in clogging filtration resistance showed excellent values.
• Example 1 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 1, except that water was used as the first non-solvent.
• the evaluation results of the obtained separation membrane are shown in Table 2.
• the weight average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 ⁇ m from the surface of the porous resin layer was 11%, the average pore diameter at the surface of the porous resin layer was 57 nm, and the increase in clogging filtration resistance was inferior to the results of the Examples.
• Example 2 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 2, except that water was used as the first non-solvent.
• the evaluation results of the obtained separation membrane are shown in Table 2.
• the weight average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 ⁇ m from the surface of the porous resin layer was 10%, the average pore diameter at the surface of the porous resin layer was 76 nm, and the increase in clogging filtration resistance was inferior to the results of the Examples.
• Comparative Example 3 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Comparative Example 2, except that PVDF2 was used instead of PVDF1.
• the evaluation results of the obtained separation membrane are shown in Table 2.
• the weight average molecular weight of PVDF was 583,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 ⁇ m from the surface of the porous resin layer was 8%, the average pore diameter at the surface of the porous resin layer was 68 nm, and the increase in clogging filtration resistance was inferior to the results of the Examples.
• Comparative Example 4 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Comparative Example 3, except that the composition of the polymer solution was as shown below. PVDF: 9% by mass DMF: 78% by mass PEG: 9% by mass Water: 4% by mass The evaluation results of the obtained separation membrane are shown in Table 2.
• the weight average molecular weight of PVDF was 583,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 ⁇ m from the surface of the porous resin layer was 26%, the average pore diameter at the surface of the porous resin layer was 84 nm, and the increase in clogging filtration resistance was inferior to the results of the Examples.
• Example 5 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 1, except that the composition of the polymer solution was changed as shown below. PVDF: 8% by mass DMF: 79% by mass PEG: 9% by mass Glycerin: 4% by mass The evaluation results of the obtained separation membrane are shown in Table 2.
• the weight average molecular weight of PVDF was 384,000, the value of a in the above formula 1 was 0.42, the porosity occupied by macrovoids in the region within a depth of 15 ⁇ m from the surface of the porous resin layer was 45%, the average pore diameter at the surface of the porous resin layer was 109 nm, and the increase in clogging filtration resistance was inferior to the results of the Examples.
• Example 6 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 1, except that PVDF3 (Solef 6008, linear PVDF manufactured by Solvay Specialty Chemicals) was used instead of PVDF1.
• the evaluation results of the obtained separation membrane are shown in Table 2.
• the increase in clogging filtration resistance was excellent.
• the increase in clogging filtration resistance after chemical immersion was inferior to that of Example 1 and Example 9 described below.
• Example 7 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 1, except that the composition of the polymer solution was as shown below and PVDF4 (KF1300 manufactured by Kureha) was used instead of PVDF1.
• Cellulose acetate was added as a hydrophilic resin.
• PVDF 15% by mass DMF: 70% by mass
• PEG 9% by mass
• Glycerin 2% by mass
• Water 4% by mass Cellulose acetate: 0.3% by mass
• the resulting separation membrane was heated in air at a space temperature of 140°C for 3.5 minutes.
• the evaluation results for each separation membrane are shown in Table 3.
• the weight-average molecular weight of PVDF was 344,000, the value of a in Equation 1 above was 0.42, and the porosity of macrovoids in the region within a depth of 15 ⁇ m from the surface of the porous resin layer was 45%.
• the average number density of the predetermined convex portions was 0/ ⁇ m 2 in Example 7 and 0.16/ ⁇ m 2 in Example 8, and the average pore diameter on the surface of the porous resin layer was 45 nm in Example 7 and 46 nm in Example 8.
• the root-mean-square roughness Rq of the surface of the porous resin layer was 28 nm in Example 7 and 35 nm in Example 8. Both Examples 7 and 8 showed excellent pure water permeability, but Example 8 showed a better value than Example 7.
• the rejection rate of polystyrene latex after rubbing was excellent in both Examples 7 and 8.
• Example 9 A separation membrane was formed in the same manner as in Example 8, except that the heating time in air at a space temperature of 140° C. was changed to 24 minutes.
• the evaluation results of the obtained separation membrane are shown in Table 3.
• the weight average molecular weight of PVDF was 344,000, the value of a in the above formula 1 was 0.42, and the porosity occupied by macrovoids in the region within a depth of 15 ⁇ m from the surface of the porous resin layer was 45%.
• the average pore diameter on the surface of the porous resin layer was 40 nm, the average number density of the predetermined convex portions was 0.20 / ⁇ m 2 , and the root mean square roughness Rq of the surface of the porous resin layer was 53 nm.
• the increase in clogging filtration resistance showed an excellent value.
• the polystyrene latex rejection rate after rubbing was 14%, which was a lower value than Examples 7 and 8, but was still a sufficiently excellent value.
• the increase in clogging filtration resistance after chemical immersion showed a better value than Comparative Example 6.
• Example 7 A separation membrane having a porous resin layer with a three-dimensional network structure was formed in the same manner as in Example 1, except that the composition of the polymer solution was as shown below and Solef 460 (branched PVDF) manufactured by Solvay Specialty Chemicals was used as the PVDF. Branched PVDF: 17% by mass DMF: 70% by mass PEG: 9% by mass Glycerin: 4% by mass The evaluation results of the obtained separation membrane are shown in Table 4.
• the weight-average molecular weight of PVDF was 730,000, the value of a in the above formula 1 was 0.31, the porosity occupied by macrovoids in the region within a depth of 15 ⁇ m from the surface of the porous resin layer was 19%, the average pore diameter on the surface of the porous resin layer was 56 nm, the average number density of the predetermined convex portions was 0.72/ ⁇ m2 , and the polystyrene latex rejection rate after abrasion was inferior to the result of Example 9.

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