JP2024094012A - Ultrafiltration membrane and method for manufacturing ultrafiltration membrane - Google Patents

Abstract

To provide an ultrafiltration membrane which can highly concentrate a gelatin solution.SOLUTION: An ultrafiltration membrane has a gel layer containing gelatin and a support layer containing a thermoplastic resin, and has a pure water permeation coefficient of 0.10 m/h/bar or more and 0.30 m/h/bar or less.SELECTED DRAWING: None

Description

The present invention relates to an ultrafiltration membrane used for concentrating gelatin solutions and a method for producing the ultrafiltration membrane.

In the fields of fermentation, medicine, and food and beverage manufacturing, drying is used as a means of extracting useful substances from solutions. In addition, concentrating the solution before drying can reduce the time and energy required for drying. For example, Patent Document 1 discloses concentrating a gelatin solution using an ultrafiltration membrane with a molecular weight cutoff (MWCO) of 50 kDa or less. Patent Document 2 discloses concentrating a gelatin solution using a cross-flow filtration system.

JP 2014-100110 A Specific Publication No. 2021-516685

Polymers such as gelatin have a wide molecular weight distribution, and an ultrafiltration membrane for concentrating a substance with such a wide molecular weight distribution should ideally have a molecular weight cutoff that coincides with the lower limit of that distribution. However, because there are only a limited number of commercially available ultrafiltration membranes, it is difficult to select an ultrafiltration membrane with a molecular weight cutoff that coincides with the lower limit of the molecular weight distribution. Therefore, ultrafiltration membranes with a molecular weight cutoff smaller than the lower limit of the molecular weight distribution are usually used.

However, there is a trade-off between the water permeability and fractionation performance of a separation membrane, so the smaller the molecular weight fraction of the separation membrane, the lower the water permeability. In other words, to concentrate a substance with a wide molecular weight distribution, a membrane with low water permeability is used, even though the solution also contains substances with large molecular weights. Furthermore, as the concentration progresses, the osmotic pressure on the supply side of the membrane (i.e. the concentrated side) increases, making it difficult for the solution to pass through the separation membrane. As a result, there is a limit to the concentration that can be obtained.

The objective of the present invention is to provide an ultrafiltration membrane capable of concentrating a high concentration.

In an effort to address the above-mentioned issues, the inventors have discovered through extensive research that an ultrafiltration membrane having a gel layer containing gelatin and a support layer containing a thermoplastic resin can achieve both water permeability and blocking performance for gelatin solutions, leading to the invention of the present application.

In order to achieve the above object, the present invention provides the following ultrafiltration membrane coated with a gel layer and a method for producing the same.
[1] An ultrafiltration membrane having a gel layer containing gelatin and a support layer containing a thermoplastic resin, the ultrafiltration membrane having a pure water permeability coefficient of 0.10 m/h/bar or more and 0.30 m/h/bar or less.
[2] The ultrafiltration membrane according to [1], wherein the pure water permeability coefficient of the gel layer is 0.14 m/h/bar or more and 0.38 m/h/bar or less, and the rejection rate R of dextran having a weight average molecular weight of 40,000 of the support layer is 40% or more and 80% or less.
[3] The ultrafiltration membrane according to [1] or [2], wherein the surface free energy ΔGgel of the gel layer is 50 mJ/ ^m2 or more and 90 mJ/ ^m2 or less.
[4] The ultrafiltration membrane according to any one of [1] to [3], wherein the filtration resistivity α of the gel layer is 1×10 ¹³ m/kg or more and 1×10 ¹⁴ m/kg or less.
[5] The ultrafiltration membrane according to any one of [1] to [4], wherein the support layer is made of a thermoplastic resin containing a vinylidene fluoride resin.
[6] The ultrafiltration membrane according to any one of [1] to [5], wherein the value X obtained by dividing the number of surface pores per unit surface area by the average pore size of the surface on the surface of the support layer in contact with the gel layer is 30 to 100 pores/μm ² /nm.
[7] The gel layer is arranged so as to contact one surface of the support layer, and the support layer has openings having a major axis _d1 and a minor axis _d2 on the one surface with which the gel layer contacts, and the average value of the ratio _d1 / _d2 of the major axis _d1 to the minor axis _d2 of the openings is 1.1 to 1.4, the standard deviation of the ratio _d1 / _d2 is 0.3 or less, and the average _d2ave of the minor axes is 5 nm to 15 nm. The ultrafiltration membrane according to any one of [1] to [6].
[8] The ultrafiltration membrane according to any one of [1] to [7], wherein the support layer is a hollow fiber membrane.
[9] The ultrafiltration membrane according to [8], wherein the surface of the hollow fiber membrane in contact with the gel layer is the outer surface.
[10] A method for producing an ultrafiltration membrane according to any one of [1] to [9], comprising a gel layer formation step of filtering a gelatin solution containing gelatin having a weight average molecular weight Mw of 10,000 or more and 360,000 or less through the support layer at a transmembrane pressure difference of at least the critical pressure of the support layer and not more than 4 times the critical pressure, to form the gel layer.
[11] The method for producing an ultrafiltration membrane according to [10], wherein in the gel layer formation step, when the ratio of the viscosity μf of the stock solution to the viscosity μp of the filtrate is μf/μp>1.5, the gelatin solution is subjected to cross-flow filtration such that the flow rate ratio of the flow rate vf of the stock solution to the flow rate vp of the filtrate is 0.02≦vp/vf≦0.3.
[12] The method for producing an ultrafiltration membrane according to [11], wherein in the gel layer formation step, the viscosity _μf of the stock solution is 1.5 mPa·s or more.
[13] The method for producing an ultrafiltration membrane according to [11] or [12], wherein in the gel layer formation step, the gelatin concentration of the raw solution is 1 wt % or more and 10 wt % or less.

The ultrafiltration membrane of the present invention has a support layer and a gel layer containing gelatin, and thus provides an ultrafiltration membrane with water permeability that can highly concentrate a gelatin solution while blocking gelatin.

FIG. 1 is a process schematic diagram showing an example of a method for producing gelatin according to one embodiment of the present invention. FIG. 2 is a schematic cross-sectional view of an ultrafiltration membrane module according to one embodiment of the present invention. FIG. 3 is a schematic flow diagram showing one embodiment of an operating device to which the ultrafiltration membrane module of the present invention is applied. FIG. 4 is a schematic diagram showing a method for measuring the viscosity of a gelatin solution.

The following describes in detail an embodiment of the present invention with reference to the drawings, but the present invention is not limited to these.

<Ultrafiltration membrane>
The ultrafiltration membrane in this embodiment has a support layer and a gel layer covering the support layer, and has a pure water permeability coefficient of 0.10 m/h/bar or more and 0.30 m/h/bar or less.
An ultrafiltration membrane is a membrane that has a certain blocking ability against soluble polymers, and there are various definitions for it. In the ultrafiltration membrane of the present invention, it is preferable that the molecular weight cut-off (MWCO) is 1,000 or more and the pore size is less than 0.1 μm.

<Gel layer>
The gel layer is a gel layer containing gelatin formed to cover the surface of the support layer. The gel layer has water permeability and, unlike the cake layer or dynamic layer made of suspended matter, has a certain degree of blocking ability against soluble polymers. As described below, by forming a gel layer on a support layer having a relatively high degree of permeability, it is possible to achieve both water permeability and blocking rate required for high concentration of gelatin. In addition, by including gelatin in the gel layer, accumulation of gelatin on the membrane surface is suppressed during the process of concentrating the gelatin solution.

The gel layer preferably has a pure water permeability coefficient of 0.14 m/h/bar or more and 0.38 m/h/bar or less. More preferably, it is 0.15 m/h/bar or more and 0.32 m/h/bar or less. A pure water permeability coefficient of 0.14 m/h/bar or more provides a sufficient filtration flux of the gelatin solution. Also, a pure water permeability coefficient of 0.38 m/h/bar or less allows the gelatin gel to form a dense structure, resulting in sufficient gelatin blocking performance.

The weight of the gel layer coated on the support layer is preferably 4.8 g/ ^m2 or more and 48 g/ ^m2 or less per unit membrane area of the support layer. If it is 4.8 g/ ^m2 or more, a suitable amount (i.e., thickness) of the gel layer is present on the support layer to suppress exposure of the support layer. If it is 48 g/ ^m2 or less, the interval between the membranes, i.e., the width of the flow path through which the treated liquid passes, can be secured.

The weight of the gel layer can be measured by decomposing the gelatin contained in the gel layer with a chemical solution, removing it from the membrane by washing, and measuring the change in weight before and after removal. The chemical solution for removing gelatin may be any solution that contains a component that denatures or decomposes protein and does not decompose the support layer. When the support layer is made of polyvinylidene fluoride resin, the chemical solution is preferably an aqueous solution of sodium hypochlorite and sodium hydroxide prepared to a pH of 12.0 and a free chlorine concentration not exceeding 500 ppm.

To confirm the presence of a gel layer containing gelatin, the support layer is immersed in a protein extract, and the resulting protein solution is subjected to polyacrylamide gel electrophoresis (SDS-PAGE) to identify the major bands, which are then excised and subjected to protein analysis using liquid chromatography/mass spectrometry (LC/MS/MS).

The surface free energy ΔGgel of the gel layer is preferably 50 mJ/m ² or more and 90 mJ/m ² or less. When ΔGgel is in the above range, the surface of the gel layer is well wetted with the gelatin solution, so that the amount of solute adsorbed to the gel layer by contact with the gelatin solution can be reduced.
The gelatin gel may be a reversible gel or an irreversible gel, but since the exfoliated gel is recovered as a concentrated liquid together with the liquid to be treated, it is preferable that the gel be a reversible gel that is easily redissolved in the liquid to be treated.

A cake is a layer of particles that are captured and deposited on the surface of a filter medium when a slurry is filtered, and cake filtration is a method of filtration in which the cake acts as a filter medium. Strictly speaking, the gel layer is different from the cake and has a certain degree of blocking ability against soluble polymers, but by applying the cake filtration theory, the filtration resistance of the gel layer can be calculated using the following formula (1).

In the formula (1), W is the amount of solids in the gel layer (kg), A is the filtration area (m ² ), μ is the viscosity of the permeated liquid (Pa·sec), ΔP is the filtration pressure against the gel layer (Pa), and u is the filtration flux (m/sec).

The filtration resistance α of the gel layer is preferably 1×10 ¹³ m/kg or more and 1×10 ¹⁴ m/kg or less, more preferably 2×10 ¹³ m/kg or more and 9×10 ¹³ m/kg or less, and even more preferably 3×10 ¹³ m/kg or more and 7×10 ¹³ m/kg or less.

The filtration resistance α increases as the gel layer is compressed more strongly. When the filtration resistivity α is 1×10 ¹³ m/kg or more, the gel layer is appropriately compressed, so that the membrane spacing, i.e., the flow path width through which the treated liquid passes, can be secured. On the other hand, when the filtration resistivity α is 1×10 ¹⁴ m/kg or less, a gel layer thickness suitable for suppressing exposure of the support layer and a water permeability suitable for concentrating gelatin can be realized.

<Support base>
The support layer contains a thermoplastic resin. Examples of the thermoplastic resin include polyesters such as polytetrafluoroethylene, polyvinylidene fluoride, polylactic acid, polyhydroxyacetic acid, polycaprolactone, and polyethylene adipate, and polymers such as polyurethane, poly(meth)acrylic acid esters, polyvinyl acetal, polyamide, polystyrene, polysulfone, cellulose derivatives, polyphenylene ether, and polycarbonate. The support layer can include one of these polymers, a polymer alloy or blend containing two or more polymers, or a copolymer of monomers that form the above polymers.

Among these, resin components with excellent heat resistance, chemical resistance, etc. include fluorine-based resins such as polytetrafluoroethylene and polyvinylidene fluoride, and sulfone-based resins such as polysulfone and polyethersulfone. Among these, vinylidene fluoride resin is particularly preferred because it has high compatibility with solvents and can easily produce a uniform manufacturing solution. Vinylidene fluoride resin means a resin containing at least one of vinylidene fluoride homopolymer and vinylidene fluoride copolymer. Vinylidene fluoride resin may contain multiple types of vinylidene fluoride copolymer.

A vinylidene fluoride copolymer is 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 copolymers include copolymers of vinylidene fluoride and one or more monomers selected from vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, and trifluorochloroethylene.

For the surface of the support layer in contact with the gel layer, the value X obtained by dividing the number of surface pores per unit surface area by the average surface pore size is preferably 30/μm ² /nm or more and 100/μm ² /nm or less. By being 30/μm ² /nm or more, it is possible to prevent gelatin and other coarse contaminants in the filtrate from penetrating into the support layer. In addition, by being 100/μm ² /nm or less, it is possible to ensure a sufficient number of flow paths through which the filtrate passes through the support layer.

Here, the number of surface pores is the number of pores present in the surface of the support layer when the surface is observed. When the number of surface pores of the support layer is obtained, the image obtained by observing the surface of the support layer with a SEM (scanning electron microscope) is binarized using the free software "ImageJ". When binarizing, after setting 1 pixel in Subtract Background and creating Background, the condition: RenyiEntropy is selected in Threshold (binarization threshold). In the obtained binarized image, the number of pores in the range observed with Analyze Particles is obtained. When the number of surface pores per unit area [pieces/μm ² ] is obtained, 1,000 or more pores are observed and the number of pores is divided by the total area of the observed region to obtain the number of pores.

The surface pore diameter is the diameter of the pores present in the surface of the support layer when the surface is observed. To determine the surface pore diameter of the support layer, the image obtained by observing the surface of the support layer with an SEM is binarized using the free software "ImageJ". When binarizing, set 1 pixel in Subtract Background, select Create Background, and then select the condition: RenyiEntropy in Threshold (binarization threshold). In the obtained binarized image, select Area in Analyze Particles to determine the area of each hole, and the diameter calculated assuming each hole is a circle is used as the surface pore diameter. To determine the average surface pore diameter, the diameters of more than 1,000 holes are averaged.

It is also preferable that the rejection rate R of the dextran with a weight-average molecular weight of 40,000 in the support layer is 40% or more and 80% or less. If it is greater than 80%, the pressure required for filtration becomes too high, and the gelatin solution cannot be concentrated to a high concentration. On the other hand, if it is less than 40%, the gelatin rejection performance in the support layer becomes low, and in order to obtain suitable rejection performance in the entire ultrafiltration membrane, it becomes necessary to densify the above-mentioned gel layer, and as a result, the pressure required for filtration becomes too high, and the gelatin solution cannot be concentrated to a high concentration.

It is preferable that the gel layer is disposed so as to be in contact with one surface of the support layer. The shape of the opening on the surface of the support layer in contact with the gel layer is not particularly limited, but it is preferable that the opening has a substantially elliptical shape. The elliptical shape allows for a larger opening area than a perfect circle with the same blocking hole diameter.

Gelatin dissolved in water is usually in a random coil state, so it can be considered to be approximately spherical. Therefore, if the short diameter of the opening is smaller than the diameter of the sphere, gelatin can be blocked. On the other hand, since the larger the area of the opening, the higher the water permeability, it is preferable that the opening has a shape that is longer in one direction, such as an ellipse, rather than a perfect circle. Therefore, it is preferable that the average value of the ratio d1/d2 of the long diameter d1 and the short diameter d2 of the opening is 1.1 to 1.4, and the standard deviation of the ratio d1/d2 is 0.3 or less. By having the average value of d1/d2 be 1.1 or more, high water permeability can be obtained. By having the average value be 1.4 or less, the entire opening is easily covered with a gel layer, and the gel layer is less likely to deform even when pressure is applied by filtration, so that removability is maintained.

In addition, the average _d2ave of the minor diameter d2 is preferably 5 nm or more and 15 nm or less. By having the size approximately the same as that of the gelatin to be concentrated, a large opening area can be secured while maintaining a certain blocking performance.

The shape of the membrane that is the support layer is not particularly limited, and examples include flat membranes, hollow fiber membranes, and tubular membranes. Hollow fiber membranes are preferred because they allow for a larger membrane area per unit area of the device. In addition, hollow fiber membranes have the advantage that the gel layer covers the support layer in the circumferential direction of its cross section, making it difficult for the gel layer to peel off from the support layer.

The gel layer preferably covers the outer surface of the support layer, which is a hollow fiber membrane. By forming the gel layer on the outer surface of the support layer, the effect of the gel layer can be obtained without narrowing the flow path inside the hollow fiber membrane. The liquid to be treated is preferably supplied to the side where the gel layer is provided. Therefore, a hollow fiber membrane having a gel layer on the outside of the support layer is applied to an external pressure type module.

The support layer may be one layer, or may have two or more layers. For example, the support layer may have a first layer having a spherical structure and a second layer provided on the first layer, having a three-dimensional mesh-like structure, and being denser than the first layer. The gel layer is formed on the layer with the three-dimensional mesh-like structure.

<Method of manufacturing ultrafiltration membrane>
A specific embodiment of the production method will be described below. Note that the ultrafiltration membrane of the present invention is not limited to a specific production method.

<Gel layer forming step>
The gel layer can be formed, for example, by filtering a gelatin solution through a support layer and depositing the gelatin on the support layer.
The gelatin used to form the gel layer has a weight average molecular weight Mw of 10,000 or more and 360,000 or less.

Filtration may be performed at a transmembrane pressure difference that is equal to or greater than the critical pressure of the support layer and equal to or less than four times the critical pressure. Specifically, the transmembrane pressure difference is preferably equal to or greater than 50 kPa and equal to or less than 200 kPa. If the transmembrane pressure difference is equal to or greater than 50 kPa, the critical pressure is often exceeded, and the gelatin can be appropriately compacted to improve the rejection rate. If the transmembrane pressure difference is equal to or less than 200 kPa, appropriate water permeability can be obtained, allowing operation within the allowable pressure range of the ultrafiltration membrane. The transmembrane pressure difference is more preferably equal to or greater than 100 kPa and equal to or less than 200 kPa.

Here, the critical pressure is the pressure at which the rate of increase of the filtration flux relative to the transmembrane pressure begins to decrease. In the present invention, the critical pressure means the pressure indicated by the intersection of line 1 of J=Jmax (maximum value of filtration flux) and line 2, which is the tangent to equation (2) at TMP=0, in a graph with the ordinate representing filtration flux (J) and the abscissa representing transmembrane pressure (TMP).

Here, _Pf is the pressure on the raw liquid side, and _Pp is the pressure on the filtrate side.

Furthermore, the gel layer formation process is preferably carried out by cross-flow filtration, more specifically, by cross-flow filtration using a hollow fiber membrane module incorporating a hollow fiber-shaped support layer. Cross-flow filtration allows the amount of gelatin deposition to be uniform within the module. In the case of cross-flow filtration, the raw liquid side pressure Pf is the average value of the module inlet pressure and the module outlet pressure.

The critical pressure is a value that varies depending on the molecular weight, concentration, temperature, and cross-flow linear velocity of the solute in the raw solution during cross-flow filtration. It is preferable to obtain the critical pressure value by filtering the gelatin solution to be concentrated under the same conditions as those for the actual concentration.

In addition, when the ratio μf/μp of the viscosity μf of the stock solution (i.e., gelatin solution) to the viscosity μp of the filtrate in cross-flow filtration is μf/μp>1.5, it is desirable to set the flow rate ratio vp/vf of the flow rate vf of the stock solution to the flow rate vp of the filtrate in the range of 0.02≦vp/vf≦0.3.

It is preferable to deposit the gelatin solution on the membrane surface while filtering it immediately before carrying out the gelatin concentration process. By depositing it while filtering it, it is possible to coat the gel layer in a reversible gel state, which is preferable since the gel layer can be recovered after the concentration process.

In the gelatin manufacturing process, the membrane may be washed with a chemical solution, and there is a risk that part or all of the coated gel layer may be decomposed or detached by the washing.

The viscosity _μf of the original solution (gelatin solution) is preferably 1.5 mPa·s or more.
The gelatin concentration of the original solution (gelatin solution) is preferably 1 wt % or more and 10 wt % or less.

Other methods for forming the gel layer include immersing the support layer in a gelatin solution to allow the gelatin to adsorb and then drying the support layer, or applying the gelatin solution to the surface of the support layer by nozzle coating, spray coating, etc.

<Support layer forming step>
Methods for producing a support layer from a thermoplastic resin include thermally induced phase separation, non-solvent induced phase separation, melt extraction, and stretching and perforation methods, among which thermally induced phase separation and non-solvent induced phase separation are preferred.

In thermally induced phase separation, the resin solution is solidified by cooling it after dissolving it at high temperatures. In non-solvent induced phase separation, the resin solution is solidified by contacting it with a non-solvent. In either method, the speed of phase separation can be controlled to control the opening size of the support layer to a size suitable for the membrane concentration process of the gelatin solution.

When using the non-solvent induced phase separation method, the solvent for the resin solution is preferably a good solvent for the resin. For example, good solvents for vinylidene fluoride resin include lower alkyl ketones such as N-methyl-2-pyrrolidone, dimethylacetamide, dimethylformamide, methyl ethyl ketone, acetone, and tetrahydrofuran, esters, amides, and mixtures thereof. Here, a good solvent is a solvent that can dissolve 5% or more by weight of vinylidene fluoride resin even at low temperatures below 60°C.

The non-solvent with which the resin solution is brought into contact is defined as a solvent that does not dissolve or swell the vinylidene fluoride resin up to the melting point of the vinylidene fluoride resin or the boiling point of the solvent. Examples of non-solvents for vinylidene fluoride resin include water, hexane, pentane, benzene, toluene, methanol, ethanol, carbon tetrachloride, o-dichlorobenzene, trichloroethylene, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butylene glycol, pentanediol, hexanediol, low molecular weight polyethylene glycol, and other aliphatic hydrocarbons, aromatic hydrocarbons, aliphatic polyhydric alcohols, aromatic polyhydric alcohols, chlorinated hydrocarbons, or other chlorinated organic liquids and mixed solvents thereof.

When the support layer has two or more layers, these layers may be formed simultaneously or sequentially. When forming simultaneously, a multi-tube nozzle may be used to simultaneously inject multiple resin solutions. When forming sequentially, the first layer may be formed by phase separation, and then the resin solution that forms the second layer may be applied, followed by further phase separation. A nozzle, slit coater, or spray coating may be used to apply the resin solution for the second layer. When forming three or more layers, these operations are performed sequentially.

When the support layer has a layer with a spherical structure and a layer with a three-dimensional mesh-like structure, thermally induced phase separation can be used to form the layer with the spherical structure, and a non-solvent induced phase separation method can be used to form the three-dimensional mesh-like structure.

<Module>
An example of a membrane module structure in which the ultrafiltration membrane is a hollow fiber membrane will be described below. The membrane module is preferably an external pressure type membrane module. In an external pressure type membrane module, the liquid to be treated flows on the outside of the hollow fiber membrane, and the permeated liquid flows on the inside of the hollow fiber membrane.

Figure 2 is a schematic diagram showing one embodiment of the ultrafiltration membrane module according to the present invention. In the following, for convenience, the permeate outlet 3 side of Figure 2 is the upper direction, and the raw liquid inlet 2 side is the lower direction. The ultrafiltration membrane 5 is filled in a container 1 having a raw liquid inlet 2, a permeate outlet 3, and a raw liquid outlet 4. The ultrafiltration membrane 5 is embedded in a first potting part 8 and a second potting part 9, and the first potting part 8 and the second potting part 9 are bonded to the container 1. The lower end of the ultrafiltration membrane 5 embedded in the first potting part 8 is sealed. In addition, the first potting part 8 has a plurality of through holes for passing the raw liquid introduced from the raw liquid inlet 2. On the other hand, the upper end of the ultrafiltration membrane 5 embedded in the second potting part 9 is embedded in an open state.

When the viscosity of the gelatin solution increases due to concentration, the cross-flow pressure loss in the treated liquid flow path increases, reducing the pump discharge rate or increasing operating power costs. In an external pressure type hollow fiber membrane module, the cross-sectional area of the cross-flow flow path can be increased by reducing the packing density of the hollow fiber membranes, thereby suppressing the pressure loss.

The filling rate of the hollow fiber membrane is preferably 20% or more and 50% or less. A filling rate of 20% or more provides a sufficient membrane area, and a filling rate of 50% or less provides a sufficient cross-flow channel cross-sectional area.

<Production method of gelatin>
FIG. 1 is a process diagram of a gelatin production method including a membrane concentration step.
As shown in Fig. 1, in order to produce a gelatin solution, the raw material is crushed, degreased, deashed, pretreated (acid treatment or alkali treatment), and extracted. The obtained gelatin solution is subjected to a concentration process including a membrane concentration process and a thermal concentration process in this order to obtain a gelatin concentrated solution. The gelatin concentrated solution is then subjected to, for example, post-treatment (sterilization and molding) and drying to obtain gelatin.

The gelatin solution to be processed in the membrane concentration process can be produced, for example, by degreasing and de-ashing the raw material, rinsing it with water, pretreating it with an acid or alkali, rinsing it again with water, and then extracting the gelatin to obtain the gelatin solution.

Typical raw materials for gelatin include bone raw materials such as cow bones and pig bones, leather raw materials such as cowhide and pig skin, and fish raw materials such as shark, all of which have collagen-containing tissue. In particular, when using bone raw materials such as cow bones and pig bones as raw materials, it is preferable to crush the raw materials.

Gelatin extraction may be carried out batchwise, and the number of extractions is not particularly limited. Specifically, extractions may be carried out 1 to 8 times, but generally 3 to 7 times is preferred.

The gelatin concentration of the gelatin solution is preferably 1% by mass or more and less than 10% by mass at the time of extraction. If it is less than 1% by mass, a large amount of gelatin remains in the raw material after extraction, which is not economical, and if it is 10% by mass or more, the gelatin may decompose more than necessary during extraction or the jelly strength of the extracted gelatin may decrease.

In the production of the gelatin solution, the raw material for gelatin is physically and chemically decomposed to produce a gelatin solution, so the polymer chains of the resulting gelatin are cut in places from the original chains. The weight-average molecular weight (hereafter simply referred to as molecular weight) has a wide distribution, ranging from 10,000 to 360,000, including helical structures consisting of multiple chains. In addition, collagen peptides with a molecular weight of less than 10,000 and low gelling ability are also produced as a by-product.

The present invention will be described below with reference to specific examples, but the present invention is not limited to these examples.

<Method of measuring gel layer>
As a method for confirming the presence of a gel layer covering the support layer, polyacrylamide gel electrophoresis (SDS-PAGE) and liquid chromatography mass spectrometry (LC/MS/MS) will be described.

60 μL of NuPAGE (registered trademark) LDS Sample Buffer (4x) (Life Technologies Japan, Inc.), 5 μL of 0.5 mol/L dithiothreitol (DTT), and 135 μL of water were mixed to make 200 μL, which was used as the extract.

Using scissors that had been thoroughly wiped with methanol, 0.5 g of the ultrafiltration membrane was cut out, and 200 μL of the extract was repeatedly passed through the inside of the membrane. The ultrafiltration membrane was then finely chopped and placed in a 1.5 mL Eppendorf tube. The above extract was added thereto, vigorously vortexed, and ultrasonically treated for 5 minutes using an ultrasonic cleaner, after which the supernatant was heat-treated at 90°C for 10 minutes.
A molecular weight marker (Novex (registered trademark) Sharp pre-stained protein standard (Life Technologies Japan, Inc.)) of 5 μL and the prophase membrane extract of 25 μL were loaded onto an acrylamide gel (NuPAGE (registered trademark) 4-12% Bis-Tris gel, 1.0 mm, 10 well (Life Technologies Japan, Inc.)). Electrophoresis was performed under the conditions shown below, and the gel after electrophoresis was stained with CBB.

[Electrophoresis conditions]
Running buffer: NuPAGE (registered trademark) MES SDS Running Buffer (20x) (Life Technologies Japan, Inc.) was prepared by diluting 20 times with water. Running conditions: 200 V constant voltage, 40 minutes. Electrophoresis apparatus: STC-808 (Tefco Corporation).

The resulting major band (approximately molecular weight 50,000 to 100,000) was excised and washed in water.
The resulting mixture was then reductively carbamidomethylated using dithiothreitol and iodoacetamide in 7 mol/L guanidine, 0.5 mol/L Tris-HCl, and 0.01 mol/L EDTA (pH 8.5), and then digested with lysyl endopeptidase (Lys-C) in 25 mmol/L ammonium bicarbonate at 37° C. for 20 hours.

After collecting the digestion solution, the peptides in the gel were extracted using 30 vol% acetonitrile and 50 vol% acetonitrile to prepare the in-gel digestion sample.

LC/MS/MS measurements were performed under the following conditions. Before measuring the in-gel digestion samples, a standard (ESI-L Low Concentration Tuning Mix, Agilent Technologies) was measured to calibrate the mass spectrometer.

[device]
Mass spectrometer: maXis impact (Bruker Daltonics, Inc.)
Liquid chromatograph: LC-30A system (Shimadzu Corporation)

[LC conditions]
Column: BEH300 C18 (2 mm I.D. x 150 mm, Waters Corporation)
Column temperature: 50 ° C.
Mobile phase flow rate: 0.2 mL per minute
Detection wavelength: 214 nm
Mobile phase A: Water/formic acid mixture (1000:1)
Mobile phase B: Acetonitrile/water/formic acid mixture (900:100:1)
Injection volume: 50 μL
Sample cooler temperature setting: 5℃

The obtained measurement data was analyzed using the analysis software MASCOT 2.4.1 (MASCOT Science Inc.), and it was confirmed that the extracted protein was gelatin.

<Method for measuring the pure water permeability coefficient of ultrafiltration membrane>
A small module with a length of about 10 cm was prepared by adjusting the membrane filling amount so that the membrane area A was 0.001 to 0.002 ^m2 , and distilled water was fed from the gel layer side and filtered in its entirety under conditions of a temperature of 25°C and a filtration differential pressure of 18.6 kPa. The amount of permeated water ( ^m3 ) over 10 min was measured, and the value ( ^m3 / ^Am2 /10 min/18.6 kPa) was converted per unit effective membrane area ( ^m2 ), unit time (h), and unit filtration differential pressure (bar) to calculate the pure water permeability coefficient L (m/h/bar).

<Method of Removing Gel Layer>
A 40°C pH 12.0 aqueous sodium hydroxide solution was fed from the gel layer side of the module filled with ultrafiltration membranes so that the filtration flux was 10 LMH, and the entire amount was filtered for 1 hour. After filtering the aqueous sodium hydroxide solution, the chemical solution was replaced with an aqueous sodium hypochlorite and sodium hydroxide solution with an effective free chlorine concentration of 500 ppm at 40°C pH 12.0, and filtration was continued for another 1 hour.

To confirm that the gel layer had been removed, the ultrafiltration membrane was cut out from the processed module, embedded in resin, and then a thin-film section sample was prepared using a microtome. The composition of the membrane cross section of the sample was analyzed using time-of-flight secondary ion mass spectrometry (TOF-SIMS). No peaks of gelatin-derived components (CN-, CNO-) were detected between the embedding resin and the support layer.

<Method for measuring pure water permeability coefficient of gel layer>
Distilled water was fed into the module after the above treatment from the side covered with the gel layer and filtered in its entirety under conditions of a temperature of 25° C. and a filtration differential pressure of 18.6 kPa. The amount of permeated water (m ³ ) over 10 minutes was measured, and the value (m ³ /Am ² /10 min/18.6 kPa) was converted per unit effective membrane area (m ² ), unit time (h), and unit filtration differential pressure (bar) to calculate the pure water permeability coefficient Lm (m/h/bar).
The pure water permeability coefficient Lg of the gel layer was calculated by the following formula (3).

where L is the pure water permeability coefficient of the ultrafiltration membrane before the gel layer is removed, and Lm is the pure water permeability coefficient of the ultrafiltration membrane after the gel layer is removed.

<Method for measuring surface free energy>
To calculate the surface free energy, the water contact angle, formamide contact angle, and diiodomethane contact angle of the ultrafiltration membrane were measured using analysis software attached to the contact angle meter (manufactured by Kyowa Interface Science Co., Ltd.: FAMAS). The Kaelble-Uy theory was used as the analysis theory. The ultrafiltration membrane, which is a hollow fiber membrane, was cut into a length of 3 cm, immersed in a 25% by mass aqueous solution of ethanol overnight, and then immersed and replaced in water for 2 hours or more, and a smooth surface was prepared using a hand press machine to prepare a sample. The ultrafiltration membrane, which is a flat membrane, was cut into a 3 cm square, immersed in a 25% by mass aqueous solution of ethanol overnight, and then immersed and replaced in water for 2 hours or more to prepare a sample. The sample was fixed to a three-state sample stand (manufactured by Kyowa Interface Science Co., Ltd.) with a three-state magnet (manufactured by Kyowa Interface Science Co., Ltd.) so that the surface of the gel layer was facing down, and placed in a three-state cell (manufactured by Kyowa Interface Science Co., Ltd.) containing distilled water, and bubbles were generated on the surface of the gel layer. A contact angle meter (manufactured by Kyowa Interface Science Co., Ltd.: DropMaster DM500) was used to measure the static contact angle in an atmosphere of 25°C, and the value obtained by dividing this value from 180° was used as the water contact angle. The smooth sample was then dried under reduced pressure, and the sample was placed on a contact angle meter sample stand so that the surface of the gel layer was facing up and fixed with cellophane tape. A droplet of formamide or diiodomethane was dropped on the surface of the gel layer, and the static contact angle was measured in an atmosphere of 25°C using a contact angle meter, and this value was used as the formamide contact angle or diiodomethane contact angle.

<Method for measuring dextran rejection rate>
The measurement method when a hollow fiber membrane was used as the shape of the separation membrane is described below. A housing with a diameter of 5 mm and a length of 17 cm was filled with hollow fiber membranes, and both ends were potted with 0.5 cm of an epoxy resin-based chemical reaction adhesive "Quick Mender" (registered trademark) manufactured by Konishi Co., Ltd., and cut to open, to prepare a module containing three hollow fiber membranes. The module was then washed by running distilled water through it at a flow rate of 100 mL/min for 1 hour.

Next, dextran having an average molecular weight of 40,000 or less (No. 31389) manufactured by Fluka was dissolved in distilled water to a concentration of 0.5 mg/mL to prepare an aqueous dextran solution (stock solution).
The stock solution was passed through the inside of the hollow fiber membrane of the module, and filtration was performed from the inside to the outside. The temperature of the stock solution when passing it was 25° C., and the flow rate was adjusted so that the stock solution flow rate was 20 mL/min and the filtration flow rate was 0.24 mL/min. The module stock solution outlet liquid and filtrate were collected for 15 minutes from 60 minutes to 75 minutes after the stock solution was passed, and the stock solution inlet liquid was collected 75 minutes after the stock solution was passed.

The concentration of the collected liquid was measured using a differential refractometer (Shimadzu RID-20A). A calibration curve for dextran concentration was created using an aqueous solution of dextran dissolved in distilled water to give concentrations of 0.1, 0.3, and 0.5 mg/mL. The sieving coefficient (SC) of the dextran used in the measurement was calculated from the dextran concentration at the module stock solution inlet (Ca), the dextran concentration at the outlet (Cb), and the dextran concentration in the filtrate (Cf) using formula (4).

The SC was taken as the transmittance of dextran with a weight-average molecular weight of 40,000. Furthermore, the value obtained by subtracting the transmittance from 100% was taken as the rejection rate R of dextran with a weight-average molecular weight of 40,000.

<Method for measuring surface pore size of support layer>
The surface pore size of the support layer was calculated using the free software "ImageJ" from the image obtained by observing the surface of the support layer with a scanning electron microscope (SEM). The observation magnification may be appropriately set according to the pore size. For example, when observing a pore with a pore size of 10 nm, it may be set to 100,000 times. First, a binarized image is obtained from the SEM image using ImageJ. After creating background with 1 pixel in Subtract Background, the condition: RenyiEntropy was selected in Threshold (binarization threshold). In the obtained binarized image, the area of each hole was obtained by selecting Area and Fit Ellipse in Analyze Particles, and the major axis and minor axis were calculated assuming that each hole was an ellipse. The average values and standard deviations of the surface pore diameter and major axis/minor axis ratio were calculated using a data set of more than 1,000 pores.

<Method for measuring solution viscosity>
Gelatin (Gelatin Silver, manufactured by Nitta Gelatin Co., Ltd.) was dissolved in distilled water at 60° C. to a predetermined concentration. The temperature was adjusted to 60° C., and viscosity was measured using a capillary viscometer as shown in FIG. 4. The flow rate of the stock solution was adjusted to the flow rate for the rejection test.

<Support Layer Formation Example 1>
A membrane-forming stock solution was prepared by mixing 38% by mass of PVDF1 (KF1300 manufactured by Kureha Corporation) and 62% by mass of γ-butyrolactone and dissolving them at 160° C. This membrane-forming stock solution was discharged from a double-tube nozzle while accompanying an 85% by mass aqueous solution of γ-butyrolactone as a hollow portion-forming liquid, and solidified in a cooling bath containing an 85% by mass aqueous solution of γ-butyrolactone at a temperature of 20° C. and placed 30 mm below the nozzle, to prepare a hollow fiber-shaped carrier having a spherical structure. 12% by mass of PVDF2 (Arkema; Kynar (registered trademark) 710, weight average molecular weight 180,000 Da), 4% by mass of cellulose diacetate (CDA, Eastman; CA-398-3), 4% by mass of cellulose triacetate (CTA, Eastman; CA-436-80S), and 80% by mass of N-methyl-2-pyrrolidone (NMP) were mixed and stirred at 120 ° C. for 4 hours to prepare a polymer solution. Next, the coating polymer solution was uniformly applied to the outer surface of the hollow fiber-shaped carrier at 10 m / min (thickness 50 μm). One second after application, the carrier coated with the polymer solution was immersed in distilled water at 43 ° C. for 10 seconds to solidify and form a support layer. The rejection rate R of dextran with a weight average molecular weight of 40,000 of the obtained support layer was 49%.

<Support Layer Formation Example 2>
A hollow fiber-shaped carrier having a spherical structure was prepared in the same manner as in Membrane Formation Example 1. 12% by mass of PVDF2, 5.1% by mass of CDA, 2.1% by mass of CTA, 68.8% by mass of NMP, and 12% by mass of 2-pyrrolidone (2P) were mixed and stirred at 120°C for 4 hours to prepare a polymer solution. Next, the coating polymer solution was uniformly applied to the outer surface of the hollow fiber-shaped carrier at 10m/min (thickness 50μm). One second after application, the carrier coated with the polymer solution was immersed in distilled water at 37°C for 10 seconds to solidify and form a support layer. The rejection rate R of dextran with a weight average molecular weight of 40,000 of the obtained support layer was 55%.

<Support Layer Formation Example 3>
A hollow fiber-shaped carrier having a spherical structure was prepared in the same manner as in Membrane Formation Example 1. 12% by mass of PVDF2, 4.8% by mass of CDA, 2.4% by mass of CTA, 68.8% by mass of NMP, and 12% by mass of ε-caprolactam (ε-CL) were mixed and stirred at 120°C for 4 hours to prepare a polymer solution. Next, the coating polymer solution was uniformly applied to the outer surface of the hollow fiber-shaped carrier at 10m/min (thickness 50μm). The carrier coated with the polymer solution was immersed in distilled water at 20°C for 10 seconds 1 second after coating to solidify and form a support layer. The rejection rate R of dextran with a weight average molecular weight of 40,000 of the obtained support layer was 71%.

<Support Layer Formation Example 4>
A support layer was formed in the same manner as in Membrane Formation Example 1, except that the carrier coated with the polymer solution was coagulated by immersing it in distilled water at 60° C. for 10 seconds 1 second after coating. The rejection rate R of the obtained support layer for dextran having a weight average molecular weight of 40,000 was 25%.

Example 1
Three of the support layers obtained in Membrane Example 1 were inserted into a polycarbonate module case having a diameter of 5 mm and a length of 17 cm, and both ends were fixed with an epoxy potting agent. The ends were cut to obtain a module with both ends open. The cylindrical module case was provided with ports at two locations near both ends so that the filtrate stock solution could be circulated outside the hollow fiber support layer.

Gelatin (Gelatin Silver manufactured by Nitta Gelatin Co., Ltd.) was dissolved in distilled water at 60°C to a concentration of 5% by weight. The viscosity at this time was 1.5 mPa/s. The prepared gelatin aqueous solution was supplied to the support layer of the module at 60°C so that the transmembrane pressure difference was 160 kPa, and cross-flow filtration was performed for 1 hour at a membrane surface linear velocity of 1.0 m/sec, and the permeate was returned to the filtrate stock tank. The transmembrane pressure difference of 160 kPa is equal to or greater than the critical pressure and equal to or less than four times the critical pressure. Unless otherwise specified in the following examples and comparative examples, a gel layer was formed at a transmembrane pressure difference within the above-mentioned range.
After the cross-flow filtration, the gelatin aqueous solution in the module was discharged and washed with water to obtain an ultrafiltration membrane having a gel layer. The pure water permeability of the obtained ultrafiltration membrane was 0.21 m/h/bar.
A gelatin aqueous solution with a concentration of 10% by weight was prepared and fed to the ultrafiltration membrane of the module at 60° C. so that the transmembrane pressure difference was 100 kPa, and cross-flow filtration was performed at a membrane surface linear velocity of 1.0 m/sec, the permeate flow rate (L/h) was measured, and the filtration flux J (LMH) was calculated. The permeate was then recovered.
The rejection rate T of gelatin was calculated from the following formula (5).

Here, Abs _f is the absorbance of the filtrate at a wavelength of 292 nm, and Abs _p is the absorbance of the permeated liquid at a wavelength of 292 nm.

In addition, a similar module was prepared in addition to the module used to measure the rejection rate, and the gel layer was removed to calculate the pure water permeability coefficient of the gel layer.

The results of the examples are shown in Table 1. Example 1 showed good results with a filtration flux J of 25 LMH and a gelatin rejection rate T of 81%.

Example 2
The procedure was the same as in Example 1, except that the support layer obtained in Membrane Formation Example 2 was used. Example 2 showed good results, with a filtration flux J of 17 LMH and a gelatin rejection rate T of 86%.

Example 3
The procedure was the same as in Example 1, except that the support layer obtained in Membrane Formation Example 3 was used. Example 3 showed good results, with a filtration flux J of 15 LMH and a gelatin rejection rate T of 90%.

Example 4
The procedure was the same as in Example 2, except that an aqueous gelatin solution with a concentration of 5% by weight was prepared and supplied to the ultrafiltration membrane of the module at 60° C. so that the transmembrane pressure difference was 70 kPa, and an ultrafiltration membrane having a gel layer was obtained. Example 4 showed good results with a filtration flux J of 16 LMH and a gelatin rejection rate T of 85%.

Example 5
The procedure was the same as in Example 2, except that an aqueous gelatin solution with a concentration of 5% by weight was prepared and supplied to the ultrafiltration membrane of the module at 60° C. so that the transmembrane pressure difference was 200 kPa, and an ultrafiltration membrane having a gel layer was obtained. Example 5 showed good results with a filtration flux J of 13 LMH and a gelatin rejection rate T of 86%.

Example 6
The hollow fiber obtained in Membrane Production Example 1 was cut to a length of 1.5 m and immersed in compressed hot water at 125° C. for 1 hour to form a support layer, in the same manner as in Example 1. Example 6 showed good results with a filtration flux J of 17 LMH and a gelatin rejection rate T of 86%.

<Comparative Example 1>
The procedure was the same as in Example 1, except that an aqueous gelatin solution with a concentration of 5% by weight was prepared and supplied to the ultrafiltration membrane of the module at 60° C. so that the transmembrane pressure difference was 30 kPa, and an ultrafiltration membrane having a gel layer was obtained. The transmembrane pressure difference of 30 kPa is less than the critical pressure.
The results of the comparative examples are listed in Table 2. Comparative Example 1 showed good results with a filtration flux J=46 LMH, while the gelatin rejection rate T=65%.

<Comparative Example 2>
The same procedure as in Example 2 was followed, except that an aqueous gelatin solution with a concentration of 5% by weight was prepared and fed to the ultrafiltration membrane of the module at 60° C. so that the transmembrane pressure difference was 30 kPa, and an ultrafiltration membrane having a gel layer was obtained. The transmembrane pressure difference of 30 kPa is less than the critical pressure. Comparative Example 2 showed good results with a filtration flux J of 31 LMH, while the gelatin rejection rate T was 78%.

<Comparative Example 3>
The same procedure as in Example 2 was followed except that an aqueous gelatin solution with a concentration of 5% by weight was prepared and fed to the ultrafiltration membrane of the module at 60° C. so that the transmembrane pressure difference was 200 kPa, and an ultrafiltration membrane having a gel layer was obtained. The transmembrane pressure difference of 200 kPa is four times the critical pressure. Comparative Example 3 showed good results with a gelatin rejection rate T of 86%, while the filtration flux J was 9 LMH.

<Comparative Example 4>
The procedure was the same as in Example 1, except that the support layer obtained in Membrane Formation Example 4 was used. Example 3 showed good results with a filtration flux J of 45 LMH, while the gelatin rejection rate T was 68%.

<Comparative Example 5>
The hollow fiber obtained in Membrane Example 2 was cut to a length of 1.5 m and immersed in compressed hot water at 125° C. for 1 hour to form a support layer, in the same manner as in Example 1. Comparative Example 5 showed a good result with a gelatin rejection rate T of 100%, while the filtration flux J was less than 1 LMH.

<Comparative Example 6>
A support layer was prepared in the same manner as in Example 2, and a collagen peptide (Collagen Pro manufactured by Nitta Gelatin Co., Ltd.) aqueous solution with a concentration of 5% by weight was prepared and supplied to the ultrafiltration membrane of the module at 60° C. so that the transmembrane pressure difference was 160 kPa. The transmembrane pressure difference of 160 kPa is less than the critical pressure. No gel layer was formed.

According to Tables 1 and 2, in Examples 1 to 6, which are examples of ultrafiltration membranes with a pure water permeability coefficient of 0.10 m/h/bar or more and 0.30 m/h/bar or less, the filtration flux was high and the gelatin rejection rate was high compared to the comparative example. By simultaneously satisfying both filtration performance and rejection performance, it is possible to economically concentrate the gelatin solution.

These results demonstrate that the ultrafiltration membrane coated with a gel layer according to one embodiment of the present invention is an ultrafiltration membrane capable of highly concentrating a gelatin solution.

The ultrafiltration membrane of the present invention is preferably used for the production of gelatin or concentrated gelatin solutions, as well as for the concentration of gelatin-containing stock solutions in the fermentation field, pharmaceutical field, food and beverage field, etc., which involves the cultivation of microorganisms or cultured cells.

1: Container 2: Stock solution inlet 3: Permeate outlet 4: Stock solution outlet 5: Ultrafiltration membrane 6: Stock solution side space 7: Filtrate side space 8: First potting part 9: Second potting part 10: Membrane module 12: Stock solution tank 13: Filtrate tank 14: Supply pump 15: Constant temperature water tank 16: Capillary tube 21: Concentrated solution valve 22: Filtrate valve 31: Concentrated solution flow meter 32: Filtrate flow meter 41: Stock solution introduction pressure gauge 42: Stock solution outlet pressure gauge 43: Filtrate outlet pressure gauge 44: Filtrate introduction pressure gauge 45: Tube inlet pressure gauge 46: Tube outlet pressure gauge 51: Stock solution thermometer

Claims (13)

An ultrafiltration membrane having a gel layer containing gelatin and a support layer containing a thermoplastic resin,
An ultrafiltration membrane having a pure water permeability coefficient of 0.10 m/h/bar or more and 0.30 m/h/bar or less. The gel layer has a pure water permeability coefficient of 0.14 m/h/bar or more and 0.38 m/h/bar or less;
The rejection rate R of the dextran having a weight average molecular weight of 40,000 in the support layer is 40% or more and 80% or less.
The ultrafiltration membrane of claim 1. The surface free energy ΔGgel of the gel layer is 50 mJ/ ^m2 or more and 90 mJ/ ^m2 or less;
The ultrafiltration membrane according to claim 1 or 2. The filtration resistivity α of the gel layer is 1×10 ¹³ m/kg or more and 1×10 ¹⁴ m/kg or less.
The ultrafiltration membrane according to any one of claims 1 to 3. The support layer is made of a thermoplastic resin containing vinylidene fluoride resin.
The ultrafiltration membrane according to any one of claims 1 to 4. On the surface of the support layer in contact with the gel layer, the value X obtained by dividing the number of surface pores per unit surface area by the average pore size on the surface is 30 to 100 pores/μm ² /nm;
The ultrafiltration membrane according to any one of claims 1 to 5. the gel layer is disposed so as to be in contact with one surface of the support layer;
the support layer has an opening having a major axis _d1 and a minor axis _d2 on the one surface in contact with the gel layer,
the average value of the ratio _d1 / _d2 of the major axis _d1 to the minor axis _d2 of the opening is 1.1 or more and 1.4 or less;
The standard deviation of the ratio _d1 / _d2 is 0.3 or less;
The average _d2ave of the short diameter is 5 nm or more and 15 nm or less.
The ultrafiltration membrane according to any one of claims 1 to 6. The support layer is a hollow fiber membrane.
The ultrafiltration membrane according to any one of claims 1 to 7. The surface of the hollow fiber membrane in contact with the gel layer is the outer surface.
The ultrafiltration membrane of claim 8. A method for producing the ultrafiltration membrane according to any one of claims 1 to 9,
The present invention includes a gel layer forming step of filtering a gelatin solution containing gelatin having a weight average molecular weight Mw of 10,000 or more and 360,000 or less through the support layer at a transmembrane pressure difference of not less than the critical pressure of the support layer and not more than 4 times the critical pressure to form the gel layer.
A method for manufacturing an ultrafiltration membrane. In the gel layer forming step,
When the ratio of the viscosity μf of the stock solution to the viscosity μp of the filtrate is μf/μp>1.5, the gelatin solution is subjected to cross-flow filtration so that the flow rate ratio of the flow rate vf of the stock solution to the flow rate vp of the filtrate is 0.02≦vp/vf≦0.3.
A method for producing the ultrafiltration membrane according to claim 10. The method for producing an ultrafiltration membrane according to claim 11 , wherein in the gel layer forming step, the viscosity _μf of the stock solution is 1.5 mPa·s or more. In the gel layer forming step, the gelatin concentration of the stock solution is 1 wt % or more and 10 wt % or less.
A method for producing the ultrafiltration membrane according to claim 11 or 12.

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