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
• • 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
  • B01D71/08—Polysaccharides
  • B01D71/10—Cellulose; Modified cellulose
• • 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
• • 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
  • B01D71/66—Polymers having sulfur in the main chain, with or without nitrogen, oxygen or carbon only
  • B01D71/68—Polysulfones; Polyethersulfones
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B5/00—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts
  • B32B5/18—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts characterised by features of a layer of foamed material
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/44—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis
• • 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
  • C08J9/36—After-treatment
  • C08J9/40—Impregnation
  • C08J9/42—Impregnation with macromolecular compounds

Definitions

• the present invention relates to porous membranes for use in fields such as water treatment, pharmaceutical manufacturing, food industry, and fermentation.
• porous membranes such as microfiltration membranes and ultrafiltration membranes have been used in a variety of fields, including water treatment (such as water purification or wastewater treatment), medical treatment (such as blood purification), and the food industry.
• water treatment such as water purification or wastewater treatment
• medical treatment such as blood purification
• food industry recent filtration stock solutions (liquids to be filtered) contain organic matter with small molecular weights, such as enzymes, proteins, and polysaccharides. Filtration stock solutions containing organic matter are prone to clogging porous membranes, and have traditionally been considered difficult to filter.
• porous membranes that can efficiently filter such stock solutions.
• porous membranes must be resistant to fouling and chemicals.
• Patent Documents 1 and 2 disclose, for example, porous membranes containing polyvinylidene fluoride resin and polyvinylidene fluoride resin and a cellulose resin.
• Sterilization methods include heat-based treatments such as dry heat sterilization and steam sterilization, electromagnetic wave treatments such as gamma ray sterilization, and chemical sterilization using ethylene oxide gas.
• heat-based treatments such as dry heat sterilization and steam sterilization, electromagnetic wave treatments such as gamma ray sterilization, and chemical sterilization using ethylene oxide gas.
• steam sterilization is particularly preferred due to the simplicity of the sterilization equipment and safety for the human body. Therefore, it is important that porous membranes containing hydrophobic and hydrophilic polymers are heat-resistant so that their structure and performance, particularly their organic matter removal performance, do not change even when subjected to repeated high-temperature steam sterilization.
• Patent Document 1 reports an example of a porous membrane containing polyvinylidene fluoride resin and cellulose resin, which has chemical resistance and stain resistance.
• the porous membrane disclosed in Patent Document 1 experienced changes in the surface pore structure due to high-temperature steam sterilization, resulting in a decrease in its ability to block organic matter. This is thought to be due to the low interaction between the polymer chains that make up the porous membrane and the high mobility of the polymer chains. As the pores shrink during steam heating, the surrounding pores are pulled, causing the pores to expand, resulting in pore diameters of 20 nm or more.
• Patent Document 2 a porous membrane primarily composed of polyvinylidene fluoride resin is brought into contact with steam in advance, thereby suppressing subsequent thermal changes.
• the method described in Patent Document 2 does not have heat resistance, and therefore merely enlarges the pores in advance, but the pores are relatively large and cannot prevent the intrusion of organic matter, and there is an issue in that it is not possible to achieve high organic matter blocking properties even for filtration stock solutions containing organic matter with small molecular weights.
• the present invention aims to provide a porous membrane that is heat-resistant in addition to being stain-resistant and chemical-resistant, and to provide a method for manufacturing a porous membrane that combines a hydrophobic polymer with a cellulose-based resin with a high degree of saponification as a hydrophilic polymer.
• the present invention provides a porous membrane having the following configuration and a method for producing a porous membrane.
• a porous membrane whose main component is a hydrophobic polymer, which contains a cellulose-based resin on its surface, one surface being surface A and the other surface being surface B, wherein the average pore size (hereinafter referred to as the surface pore size) of surface A is smaller than the average pore size of surface B, the surface pore size of surface A is 5 nm or more and 50 nm or less, and the contact angle of surface A with water is 10° or more and 40° or less.
• the hydrophobic polymer comprises at least one thermoplastic resin selected from the group consisting of polyvinylidene fluoride resins, polyethersulfone resins, and polysulfone resins, and the intensity ratio (Is/Ix) of the peak intensity Is derived from the absorption wavelength (1034 cm ⁇ 1 ) of the pyranose ring of the cellulose resin to the following peak intensity Ix derived from the hydrophobic polymer, as measured on the surface A by an ATR-IR method (attenuated total reflection spectroscopy), is 0.6 or more and 1.5 or less: ⁇ Absorption wavelength of hydrophobic polymer> In the case of polyvinylidene fluoride resin, it is 881 cm ⁇ 1 derived from polyvinylidene fluoride, in the case of polyethersulfone resin, it is 1578 cm ⁇ 1 derived from polyethersulfone, and in the case of polysulfone resin, it is 1580 cm ⁇
• ATR-IR method attenuated total reflection infrared (ATR) infrared spectroscopy
• a method for producing a porous membrane comprising: a step (A) of dissolving a polymer resin in a solvent to obtain a polymer solution; and a step (B) of solidifying the polymer solution in a non-solvent to form a porous membrane, wherein the step (A) comprises a hydrophobic polymer and a cellulose resin; the non-solvent in the step (B) comprises 90 to 100% by weight of water, and the temperature of the non-solvent is 6 to 60°C; and a step (C) of contacting at least one surface of the formed porous membrane with an alkaline solution after the steps (A) and (B).
• step (10) The method for producing a porous membrane according to (9), wherein the surface is a surface having a smaller average pore diameter (hereinafter referred to as surface A), and the treatment of step (C) is performed on surface A.
• step (1) The method for producing a porous membrane according to (9) or (10), characterized in that in step (A), the hydrophobic polymer is a polyvinylidene fluoride resin as a main component, and in step (C), the alkaline solution contains a 0.0001N to 0.01N sodium hydroxide aqueous solution, and the formed porous membrane is immersed in the alkaline solution.
• step (B) of solidifying the polymer solution in a non-solvent to form a porous membrane the porous membrane is formed on the surface of a porous structure, and the outermost surface of the formed porous membrane becomes surface A
• step (C) of contacting the porous membrane with the alkaline solution surface A is treated.
• the method for filtering a liquid according to (13) characterized in that the liquid is a sugar solution containing sugar, protein, and their reaction products.
• the present invention makes it possible to provide a porous membrane that is heat-resistant in addition to being stain-resistant and chemical-resistant, and that can ensure high organic substance rejection even when the filtration concentrate contains organic substances with low molecular weights.
• FIG. 1 is a schematic diagram illustrating the concept of the filtration state of a porous membrane of the present invention.
• FIG. 1 is a schematic diagram illustrating the concept of filtration by a conventional porous membrane.
• a filtrate containing organic matter with a small molecular weight is a filtrate containing organic matter with a weight-average molecular weight of 15,000 Da to 40,000 Da.
• organic matter include proteins and polysaccharides. These filtrates have traditionally been difficult to filter.
• the porous membrane of the present invention as a separation membrane, it is possible to highly efficiently filter filtrate containing organic matter with an average molecular weight of 15,000 Da to 40,000 Da, such as enzymes, proteins, and polysaccharides.
• heat resistant means that the structure and properties of the porous membrane do not change even when exposed to high temperatures. In the present invention, this means that there is little change in organic substance blocking properties before and after steam sterilization. Membranes that are not heat resistant show a large change in organic substance blocking properties before and after steam sterilization, and their organic substance blocking properties deteriorate after steam sterilization.
• the porous membrane according to an embodiment of the present invention is a porous membrane comprising a hydrophobic polymer and a cellulose-based resin, with the surface in contact with the liquid to be treated designated surface A and the other surface designated surface B, and the average pore size (hereinafter referred to as the average surface pore size) of surface A being smaller than that of surface B. That is, the average surface pore size of surface A must be smaller than that of surface B, and the contact angle of surface A with water must be 40° or less.
• a surface pore size of 5 to 50 nm prevents contaminants and substances to be removed in the raw filtrate from penetrating into the porous membrane, thereby achieving high fouling resistance.
• the surface pore size will be described later.
• a contact angle of 40° or less on surface A indicates that, as described above, in a porous membrane containing a hydrophobic polymer and a cellulose-based resin, the number of hydroxyl groups on the surface in contact with the liquid to be treated is increased, resulting in the formation of numerous hydrogen bonds.
• a contact angle of 40° or less on surface A with water is an indicator of excellent heat resistance.
• a smaller contact angle on surface A with water is preferable, as it reduces the movement of polymer chains, and 35° or less is more preferable.
• a lower limit on the contact angle is preferably 10° or more, from the perspective of converting the number of hydroxyl groups while the cellulose-based resin is dispersed in the hydrophobic polymer in the porous membrane.
• the contact angle is measured as a static contact angle using a contact angle meter.
• the contact angle in this invention is determined by measuring the static contact angle with the water surface using the air-bubble method, where air bubbles are brought into contact with the porous membrane surface while it is submerged in water, and the static contact angle is measured.
• FIG. 1 shows a schematic diagram of the filtration of a raw liquid, which has been considered difficult to filter using the porous membrane of the present invention.
• Figure 1 is a conceptual schematic diagram showing a portion of the cross section of a porous membrane.
• the filtration direction FL is from surface A to surface B.
• porous membrane 101 has a surface layer 102 and an inner layer 103.
• Surface A of surface layer 102 has many fine pores with a pore size of 5 to 50 nm (shown between the fine mesh lines in Figure 1).
• the pore size of inner layer 103 is larger than that of surface layer 102.
• Organic matter 201 contained in the liquid to be filtered is larger than the pore size of surface A and cannot penetrate the porous membrane.
• the porous membrane of the present invention has strong interactions between polymer chains, which reduces the mobility of polymer chains even when subjected to high-temperature steam sterilization using water vapor. As a result, there is little change in pore size and fine organic matter 201 does not penetrate the pores.
• FIG 2 shows a schematic diagram of the filtration of a difficult-to-filter raw liquid using a previously known porous membrane.
• the porous membrane of Patent Document 1 has weak interactions between polymer chains, which results in high mobility of the polymer chains when steam sterilized. This results in a large change in pore size, causing the fine pores to become finer and the coarse pores to become coarser.
• coarse pores are present on the surface that comes into contact with the liquid being treated (the areas indicated by the partially coarse mesh in Figure 2). Therefore, fine organic matter 201 can easily penetrate the pores, reducing the organic matter blocking ability.
• the hydrophobic polymer contained in the porous membrane may be a polysulfone resin, a polyethersulfone resin, or a polyvinylidene fluoride resin, but preferably includes a polyvinylidene fluoride resin.
• a polyvinylidene fluoride resin refers to a vinylidene fluoride homopolymer or a vinylidene fluoride copolymer.
• a vinylidene fluoride copolymer refers to a polymer having a vinylidene fluoride residue structure.
• a polymer having a vinylidene fluoride residue structure is typically a copolymer of a vinylidene fluoride monomer and another fluorine-based monomer.
• fluorine-based monomers examples include vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, and trifluorochloroethylene.
• the vinylidene fluoride copolymer may also be copolymerized with ethylene or other fluorine-based monomers, as long as the effects of the present invention are not impaired.
• surface A is mainly composed of a hydrophobic polymer and contains a large amount of cellulose-based resin within an appropriate range. It is preferable that Is/Ix is 0.6 to 1.2, more preferably 0.6 to 1.0, and particularly preferably 0.6 to 0.8 .
• the ATR-IR method can analyze a depth of up to 5 ⁇ m from the outermost surface. Therefore, it is possible to observe the composition of the compounds on the surface that comes into contact with the liquid being treated.
• Surface A has good chemical resistance because it is mainly composed of hydrophobic polymers.
• the hydrophobic polymer is a polyvinylidene fluoride resin
• three crystalline structures are known: ⁇ -type, ⁇ -type, and, in very small amounts, ⁇ -type. It is generally known that the ratio of ⁇ -type, ⁇ -type, and ⁇ -type changes when polyvinylidene fluoride resin is exposed to chemicals or stressed by heat or other factors.
• the peak intensities representing polyvinylidene fluoride resins 881 cm -1 is a peak intensity common to all crystals and does not change with chemical treatment.
• the pyranose ring in their chemical structure is not affected by chemical treatment. Therefore, the ratio and state of the hydrophobic polymer and cellulose-based resin on surface A can be evaluated.
• the ratio of the peak intensity Iso (the absorption wavelength of S0 is 1744 cm ⁇ 1 ) derived from the acetyl groups of the cellulose-based resin to the peak intensity (Is) derived from the pyranose rings of the cellulose-based resin is 0.6 or less (Iso/Is), which is preferable because a high proportion of acetyl groups in the cellulose-based resin at the film surface are converted to hydroxyl groups, resulting in excellent heat resistance. Since a higher proportion of hydroxyl groups improves heat resistance, a ratio of 0.5 or less is preferred, more preferably 0.2 or less, and even more preferably 0.1 or less.
• pyranose rings are not affected by the saponification treatment, while acetyl groups are converted to hydroxyl groups by the saponification treatment, resulting in a decrease in their proportion.
• a state in which there are fewer acetyl groups relative to the peak intensity derived from the pyranose rings in the cellulose-based resin indicates a high proportion of hydroxyl groups in the cellulose-based resin.
• the lower limit of Iso/Is can be set arbitrarily. From the viewpoint of production efficiency, Iso/Is is preferably at least 0.01. Within this range, a porous membrane having excellent heat resistance and chemical resistance can be efficiently produced.
• a porous membrane with a molecular weight cutoff of 10,000 to 200,000 is preferable because it prevents the organic matter to be removed and concentrated in the filtrate from penetrating the porous membrane, making it more likely to exhibit high organic matter blocking properties.
• a porous membrane with a molecular weight cutoff of 10,000 to 60,000 is more preferable, with a molecular weight cutoff of 10,000 to 30,000 being particularly preferable.
• the molecular weight cutoff is defined as the molecular weight at which a removal rate of 90% is achieved when several types of dextran aqueous solutions are filtered.
• the dextran aqueous solution was a mixture of 500 ppm each of Fluka dextrans with average molecular weights of 1500 Da, 6000 Da, 15000-25000 Da, and 40000 Da, and Aldrich dextrans with average molecular weights of 60000 Da and 200000 Da.
• the solution to be filtered was subjected to crossflow filtration at a membrane surface linear velocity of 0.9 m/sec, and the filtrate and the water to be filtered at this point were sampled and subjected to GPC measurement to calculate the removal rate.
• a surface pore diameter of 5 to 50 nm on surface A of the porous membrane is preferable, as it prevents contaminants in the filtrate and substances to be removed from penetrating into the porous membrane, making it more likely to exhibit high fouling resistance. It also maintains high organic substance blocking properties and is more likely to exhibit heat resistance.
• a surface pore diameter of 12 nm or less is more preferable, and 8.0 nm or less is even more preferable. The surface pore diameter is the diameter of the pores present in the surface when observing the surface of the porous membrane.
• an image obtained by SEM observing the surface of the porous membrane is binarized using the free software "ImageJ.”
• ImageJ free software
• RenyiEntropy under Threshold binarization threshold
• select Area in Analyze Particles to determine the area of each pore, and the diameter calculated assuming each pore is a circle is used as the surface pore diameter.
• To calculate the average surface pore diameter average the diameters of more than 1,000 pores.
• the standard deviation of the surface pore diameter of the surface A of the porous film be 10.0 nm or less, many pores tend to shrink uniformly during steam treatment, maintaining high organic substance blocking properties and making it easier to exhibit heat resistance.
• the standard deviation is preferably 5.0 nm or less, and more preferably 2.5 nm or less.
• the lower limit of the standard deviation is 1.0 nm or more.
• the porous membrane of the present invention may be composed of a single layer or multiple layers.
• the layer in contact with the liquid to be treated (the layer including surface A) contains a large amount of cellulose-based resin, and that the layer including surface B has an Is/Ix ratio of 0.6 or less. Having a large amount of cellulose-based resin in the layer in contact with the liquid to be treated is preferable, as it tends to exhibit heat resistance in the region that blocks organic matter.
• the layer including surface B contains less cellulose-based resin, and undergoes thermal deformation and expands its pore size, resulting in a fine surface in contact with the liquid to be treated and a coarse structure in the other layers, which is preferable from the perspective of improving the permeability of the permeated liquid.
• the average pore size of surface A of the porous membrane of the present invention is smaller than the average pore size of surface B, and the surface pore size of surface A is 5 to 50 nm.
• the shape of the porous membrane of the present invention may be a flat plate, hollow fiber, tubular, etc., and an appropriate shape can be selected depending on the type of filtration device used and the properties of the raw liquid to be filtered.
• the porous membrane of the present invention can be obtained by a manufacturing method comprising step (A) of dissolving a polymer in a solvent to obtain a polymer solution, followed by step (B) of solidifying the polymer solution in a non-solvent to form a porous membrane, and step (C) of contacting the porous membrane with an alkaline solution, wherein the non-solvent used in step (B) contains 90-100% by weight of water and the temperature of the non-solvent is 6°C to 60°C.
• Step (A): the step of obtaining a polymer solution and step (B): the step of forming a porous membrane are non-solvent-induced phase separation methods, which contain a hydrophobic polymer as the main component and a cellulose-based resin, and can obtain surface pores with a pore size of 5-12 nm.
• the porous membrane manufacturing method of the present invention comprises step (A) of dissolving a polymer resin in a solvent to obtain a polymer solution, and step (B) of solidifying the polymer solution in a non-solvent to form a porous membrane, wherein step (A) contains a hydrophobic polymer and a cellulose resin, and step (B) the non-solvent contains 90-100% by weight of water and the temperature of the non-solvent is 6°C to 60°C.
• step (C) the porous membrane is brought into contact with an alkaline solution in step (C), which converts the acetyl groups in the cellulose resin on surface A of the porous membrane into hydroxyl groups, resulting in the formation of hydrogen bonds in the polymer chains and providing excellent heat resistance.
• the polymers used in step (A) are preferably hydrophobic polymers and cellulose acetate resins, with polyvinylidene fluoride resins and cellulose acetate resins being particularly preferred.
• polyvinylidene fluoride resin refers to a vinylidene fluoride homopolymer or a vinylidene fluoride copolymer.
• vinylidene fluoride copolymer refers to a polymer having a vinylidene fluoride residue structure.
• Polymers having a vinylidene fluoride residue structure are typically copolymers of vinylidene fluoride monomers and other fluorine-based monomers.
• fluorine-based monomers examples include vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, and trifluorochloroethylene.
• vinylidene fluoride copolymer ethylene or other fluorine-based monomers may be copolymerized to the extent that the effects of the present invention are not impaired.
• the weight-average molecular weight of the polyvinylidene fluoride resin may be selected appropriately depending on the required strength and water permeability of the porous membrane. Generally, as the weight-average molecular weight increases, water permeability decreases, and as the weight-average molecular weight decreases, strength decreases. For this reason, a weight-average molecular weight of 50,000 or more and 1,000,000 or less is preferable. In particular, if the raw treatment solution is highly turbid and clogging substances adhering to the porous membrane need to be removed by chemical cleaning and the highly turbid raw treatment solution needs to be repeatedly filtered, a weight-average molecular weight of 100,000 or more and 700,000 or less is preferable. Furthermore, if chemical cleaning is performed multiple times, a weight-average molecular weight of 150,000 or more and 600,000 or less is preferable.
• the cellulose-based resin in the present invention is not particularly limited as long as it has cellulose ester as a molecular unit in the main chain and/or side chain, and other molecular units may also be present.
• molecular units other than cellulose ester include alkenes such as ethylene and propylene, alkynes such as acetylene, vinyl halides, vinylidene halides, methyl methacrylate, and methyl acrylate.
• Ethylene, methyl methacrylate, and methyl acrylate are particularly preferred because they are inexpensive and easy to introduce into the main chain and/or side chain. Introduction can be achieved using known polymerization techniques such as radical polymerization, anionic polymerization, and cationic polymerization.
• homopolymers containing essentially only cellulose ester as a molecular unit are known for their low cost and ease of handling. These homopolymers are preferred because they are inexpensive and easy to handle. Examples of such homopolymers include cellulose acetate, cellulose acetate propionate, and cellulose acetate butyrate.
• the solvent preferably contains a good solvent.
• good solvent refers to a solvent that can dissolve 5% or more by weight of a polymer even at low temperatures below 60°C.
• good solvents include N-methyl-2-pyrrolidone (hereinafter “NMP”), 2-pyrrolidone (hereinafter “2P”), ⁇ -caprolactam (hereinafter “ ⁇ -CL”), dimethyl sulfoxide (hereinafter “DMSO”), dimethylacetamide (hereinafter “DMAc”), dimethylformamide (hereinafter “DMF”), methyl ethyl ketone, acetone, tetrahydrofuran, tetramethylurea or trimethyl phosphate, glycerin, or a mixture thereof.
• NMP N-methyl-2-pyrrolidone
• 2P 2-pyrrolidone
• ⁇ -CL ⁇ -caprolactam
• DMSO dimethyl sulfoxide
• DMAc dimethylacetamide
• the good solvent constitute 40% or more by weight of the solvent, and particularly preferably 60% or more by weight.
• a high content of good solvent is preferred because it allows the polymer chains to expand in the polymer solution and makes it easier to control the viscosity of the solution within an appropriate range.
• non-solvent refers to a solvent that does not dissolve or swell the polymer even when heated to its boiling point.
• non-solvents include aliphatic hydrocarbons such as water, hexane, pentane, benzene, toluene, methanol, ethanol, carbon tetrachloride, o-dichlorobenzene, trichloroethylene, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butylene glycol, pentanediol, hexanediol, and low-molecular-weight polyethylene glycol, aromatic hydrocarbons, aliphatic polyhydric alcohols, aromatic polyhydric alcohols, chlorinated hydrocarbons, other chlorinated organic liquids, and mixtures thereof.
• the polymer concentration (wt %) in the polymer solution is preferably equal to or greater than the entanglement concentration in order to control the viscosity of the solution within an appropriate range. More specifically, 10 to 40 wt % is preferred, 12 to 30 wt % is even more preferred, and 15 to 25 wt % is particularly preferred.
• the porous membrane formation process in step (B), in which a polymer solution is solidified in a non-solvent to form a porous membrane, is a process for forming a porous membrane through non-solvent-induced phase separation.
• the polymer solution comes into contact with the non-solvent, the polymer cannot completely dissolve in the solvent, causing phase separation into a phase rich in polymer and a phase rich in solvent, and each phase coarsens as it coalesces with the surrounding identical phases.
• Step (C) is a step of contacting the porous membrane produced in steps (A) and (B) with an alkaline solution.
• the porous membrane is saponified by immersing it in an alkaline solution at a predetermined temperature for a predetermined period of time.
• the porous membrane produced in steps (A) and (B) is immersed in a 0.0001 to 1.0 N aqueous sodium hydroxide solution at room temperature to 60°C for 10 to 1,440 minutes while stirring.
• Room temperature refers to a range of 15 to 35°C.
• the sodium hydroxide concentration, immersion temperature, and immersion time if the concentration is high, the immersion temperature can be lowered or the immersion time can be shortened.
• the alkaline solution includes an alkaline aqueous solution, and examples of alkaline solutions include, but are not limited to, sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonia (NH 3 ), etc.
• the alkaline solution preferably has a pH of 10 or higher, more preferably a pH of 11 or higher.
• At least surface A has many hydroxyl groups and many hydrogen bonds have been formed, and this can be achieved by contacting only surface A with an alkaline solution of pH 10 or higher for a short period of time.
• the porous membrane of the present invention may further comprise other layers.
• surface A is arranged so as to come into contact with the liquid to be treated, as this makes it difficult for components contained in the raw filtrate to penetrate into the porous membrane, allowing high permeability to be maintained for a long period of time.
• the other layers are not particularly limited as long as they are components that can overlap with the layer containing surface A to form a layered structure, but it is preferable that they have a porous structure with high breaking strength.
• the breaking strength (breaking strength per unit area) is preferably 3 MPa or more, and more preferably 10 MPa or more. If the composite membrane comprising the other layers is in the form of a hollow fiber, the breaking strength of the other layers is preferably 2900 N or more, and more preferably 7800 N or more.
• Liquids can be filtered using a membrane filtration device using the porous membrane of the present invention obtained as described above.
• membrane filtration devices include, but are not limited to, a raw water tank, a booster pump, a module consisting of several thousand to several tens of thousands of porous hollow fiber membranes of the present invention, a filtered water tank, and a backwash pump.
• a method of filtering liquids can be used to remove organic matter and other substances contained in raw water, such as industrial wastewater, by operating the membrane filtration device at an operating pressure of 10 kPa to 1 MPa.
• the liquid filtration method using the porous membrane of the present invention is suitable for filtering liquids containing sugars, proteins, and their reaction products.
• liquids include polysaccharides, gelatin, and Maillard reaction compounds.
• the absorbance at a wavelength of 420 nm be 0.01 to 30 and the Brix be 10 to 75. With the absorbance and Brix at 420 nm in this range, Maillard reaction compounds can be efficiently removed.
• a Brix of 10 or more ensures that the material to be filtered is not too dilute and can be filtered efficiently.
• a Brix of 75 or less is preferred from the perspective of the solubility of the material to be filtered in aqueous solution.
• a Brix of 50 or less is preferred, with 30 or less being particularly preferred.
• An absorbance at 420 nm of 5.0 or less is preferred, particularly from the perspective of the solubility of Maillard reaction compounds. If the absorbance at 420 nm is 0.01 or higher, the Maillard reaction compounds are not too diluted and can be filtered efficiently.
• the absorbance of the sugar solution at 420 nm can be measured using a spectrophotometer.
• the Brix value is calculated by measuring the refractive index using a saccharometer.
• a sugar solution with an absorbance of 0.01 to 30 at a wavelength of 420 nm and a Brix of 10 to 75 can be prepared, for example, by dissolving raw sugar.
• the absorbance and Brix at a wavelength of 420 nm can be adjusted to fall within preferred ranges by diluting or concentrating the solution.
• the steam sterilization assumed in this invention is that described in JIS K 3605 (1992), and the standard steam sterilization conditions are 121°C and 20 minutes. In industrial steam sterilization, the temperature and time may be further increased for safety reasons.
• the static contact angle can be measured using a Drop Master DM500 manufactured by Kyowa Interface Science Co., Ltd.
• the static contact angle was measured by contacting an air bubble (approximately 2 ⁇ L) with the porous membrane surface while it was immersed in pure water.
• the contact angle with the water surface was calculated from the contact angle obtained from the tangent of the air bubble in contact with the surface.
• the measurement water temperature was 20° C., and three air bubbles were measured, and the contact angle was calculated from the average value.
• Ratio of cellulose-based resin/hydrophobic polymer I S /Ix Equation (1)
• (iii) Measurement of the Ratio of Acetyl Groups to Pyranose Rings in Cellulose-Based Resins The measurement sample used was a porous membrane vacuum-dried for 12 hours. Measurements were performed in an atmosphere adjusted to a temperature of 25°C and humidity of 40%.
• An ATR-IR spectrum was obtained by irradiating the surface of the porous membrane with infrared light using a Shimadzu Corporation IRTracer-100, with a Shimadzu Corporation Microm ATR Vision and single-reflection diamond disk as accessories for total reflection. The obtained spectrum was expressed in absorbance, and multipoint baseline correction was performed.
• the saponification ratio of the cellulose-based resin was calculated using the following formula from the peak intensity (Is) derived from the pyranose rings of the cellulose-based resin appearing at 1034 cm ⁇ 1 and the peak intensity (Iso) derived from the acetyl groups of the cellulose-based resin appearing at 1744 cm ⁇ 1 in the obtained spectrum. Measurements were performed at two or more points, and the average value was used.
• a solution was prepared by mixing Fluka dextran having an average molecular weight of 1500 Da, 6000 Da, 15000-25000 Da, and 40000 Da with Aldrich dextran having an average molecular weight of 60000 Da and 200000 Da, each at 500 ppm.
• the obtained dextran solution was subjected to cross-flow filtration at a membrane surface linear velocity of 0.9 m/s, and the filtrate and the water to be filtered at that time were sampled.
• Each sampled solution was subjected to GPC measurement using a Tosoh Technosystems HLC-8320 and an RI detector (double flow system).
• the measurement conditions were pure water as the eluent, a Tosoh Corporation TSKgel G4000PWxl column, a temperature of 40 ° C, and a flow rate of 0.5 mL/min.
• the molecular weight cutoff was calculated by taking the removal rate as the ratio of the value of the dextran solution after filtration to the value of the dextran solution before filtration, and the molecular weight at which the removal rate was 90% was taken as the molecular weight cutoff (Da).
• the amount of gelatin in the permeate was measured, and the gelatin rejection rate was calculated when the amount of gelatin in the unfiltered solution was taken as 100%.
• the amount of gelatin in the permeate was calculated by measuring absorbance at 292 nm.
• the cross-flow linear velocity is the value obtained by dividing the flow rate of the unfiltered solution in a direction perpendicular to the filtration direction by the cross-sectional area of the flow path.
• the transmembrane pressure difference is the difference between the pressure on the raw filtrate side and the pressure on the permeate side across the porous membrane.
• the standard for acceptable change in heat-resistant rejection is a value of 0.5 or more obtained by dividing the organic substance rejection after heat treatment by the organic substance rejection before heat treatment. When the change in organic substance rejection before and after heat treatment is expressed as a percentage, a rejection rate of 50% or more is considered excellent, and a rejection rate of 15% or more is considered sufficient.
• a sugar solution containing Maillard reaction compounds was selected as the solution containing the sugar-protein reaction product.
• the sugar solution (absorbance at 420 nm of 0.01-5.0, Brix: 10-70%) was supplied to a porous membrane at 60°C to achieve a transmembrane pressure difference of 80 kPa and subjected to crossflow filtration at a crossflow linear velocity of 0.8 m/sec.
• the absorbance at 420 nm of the permeate was measured, and the rejection rate of Maillard reaction compounds was calculated when the glycoprotein content of the unfiltered solution was taken as 100%.
• the absorbance at 420 nm of the sugar solution was measured using a spectrophotometer (Shimadzu Corporation: UV-2450).
• the Brix value was calculated by measuring the refractive index using a saccharimeter (Atago Co., Ltd.: Pocket Sugar Meter PAL-1). As above, when the change in organic substance rejection before and after heat treatment is expressed as a percentage, 70% or more is considered excellent rejection, and 50% or more is considered sufficient rejection.
• the discharged support membrane stock solution was solidified in a cooling bath containing an 85% by mass aqueous ⁇ -butyrolactone solution at 20°C and placed 30 mm below the nozzle, to produce a hollow fiber-shaped porous structure having a spherical structure.
• Example 1 In the film production of Example 1, after forming the porous film, step (C) was not performed and the film was not immersed in an alkaline aqueous solution.
• the evaluation results of the porous film are shown in Table 2.
• the static contact angle of the obtained porous film was 44.4 °
• Is/Ix was 0.87
• Iso/Is was 0.63
• the molecular weight cutoff was 40,000.
• the surface pore size was 8.8 nm, with a standard deviation of 3.0 nm.
• the molecular weight cutoff was 50,000, and the structure was not maintained.
• the gelatin rejection rate was 65.7%, while after steam treatment, it was 30.0%, and the rejection rate could not be maintained.
• the value obtained by dividing the organic substance rejection rate after heat treatment by the organic substance rejection rate before heat treatment was low at 46%, and the heat resistance was poor.
• the Maillard reaction compound inhibition rate in the steam-treated sugar solution (absorbance at 420 nm: 1.50, Brix: 10%) was 46.7%, while the Maillard reaction compound inhibition rate in the sugar solution (absorbance at 420 nm: 0.06, Brix: 61%) was 39.0%, both of which were low inhibition rates.
• the results are shown in Table 2.
• the molecular weight cutoff was 17,000, confirming that the structure was maintained.
• the gelatin rejection rate was 80.2%, while after steam treatment, the rejection rate was maintained at 82.5%.
• the organic substance rejection rate after heat treatment divided by the organic substance rejection rate before heat treatment was 102%, indicating acceptable heat resistance.
• the gelatin rejection rate was 90.2%, while after steam treatment, it was 93.5%, maintaining the rejection rate.
• the value obtained by dividing the organic substance rejection rate after heat treatment by the organic substance rejection rate before heat treatment was 104%, indicating acceptable heat resistance.
• Example 4 A porous membrane was obtained in the same manner as in Example 1, except that the NMP in the polymer solution was replaced with DMF and the distilled water in the coagulation bath was set to 40°C.
• the evaluation results of the porous membrane are shown in Table 1.
• the static contact angle of the obtained porous membrane was 32.6°, Is/Ix was 0.99, Iso/Is was 0.28, and the molecular weight cutoff was 55,000. Furthermore, the surface pore size was 11.4 nm, with a standard deviation of 4.1 nm.
• the molecular weight cutoff was 45,000, confirming that the structure was maintained.
• the gelatin rejection rate was 26.2%, while after steam treatment, it was 30.3%, maintaining the rejection rate.
• the value obtained by dividing the organic substance rejection rate after heat treatment by the organic substance rejection rate before heat treatment was 115%, indicating acceptable heat resistance.
• Example 5 A porous membrane was obtained in the same manner as in Example 1, except that the NMP in the polymer solution was replaced with DMF and the distilled water in the coagulation bath was set to 60°C.
• the evaluation results of the porous membrane are shown in Table 1.
• the static contact angle of the obtained porous membrane was 32.7°, Is/Ix was 0.97, Iso/Is was 0.44, and the molecular weight cutoff was 110,000. Furthermore, the surface pore size was 17.1 nm, with a standard deviation of 7.6 nm.
• the molecular weight cutoff was 98,000, confirming that the structure was maintained.
• the gelatin rejection rate was 15.1%, while after steam treatment, it was 16.5%, maintaining the rejection rate.
• the value obtained by dividing the organic substance rejection rate after heat treatment by the organic substance rejection rate before heat treatment was 112%, indicating acceptable heat resistance.

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