Film fabrication via a combination of evaporation-induced phase separation and precipitate-induced phase separation.

Abstract

The present invention relates to an ultrafiltration membrane and a method for producing the same.

Description

Technical Field

[0001] The present invention relates to an ultrafiltration membrane and a method for manufacturing the same.

Background Art

[0002] Ultrafiltration membranes generally feature very narrow pore sizes between 2 nm and 100 nm (as described in "Terminology for membranes and membrane processes." in Non-Patent Document 1) and are used in separation / purification processes at the molecular level. Regarding the retention of the membrane, it can be similarly defined based on the molecular weight cut-off (MWCO: molecular weight cut-off). The MWCO relates to a dissolved compound (usually dextran) having the minimum molecular weight (in daltons) at which 90% of the dissolved compound is retained by the membrane, or alternatively, to the molecular weight (in daltons) of a molecule at which 90% of the molecules having that molecular weight are retained by the membrane. For example, a membrane with a MWCO of 10 kDa retains at least 90% of dextran with a molecular weight of 10 kDa or more. Common operations performed via ultrafiltration are concentration of the target molecule, buffer exchange, or removal of impurities by size exclusion.

[0003] The production of ultrafiltration membranes is generally realized by the process scheme schematically shown in FIG. 1. The process scheme includes producing a solution of a membrane-forming polymer in a suitable solvent that may contain additional additives (hydrophilic agents or pore-forming agents) as required, casting the solution with a defined thickness onto a suitable carrier (endless steel belt, steel drum), transferring the solution film to a non-solvent bath to induce a phase separation process, and extracting, washing, drying, and winding up the formed membrane film.

[0004] In addition to chemical and physical characteristics such as thickness, fracture pressure, porosity, and resistance to solvents / acids / bases, membranes can be characterized primarily by their permeability P and blocking R profiles. Generally, it is desirable to produce membranes with very high permeability at a given blocking profile. This results in faster processing in almost all unit operations while maintaining the required separation efficiency.

[0005] Permeability and blocking properties are determined by various structural characteristics of the membrane. Blocking properties are determined by the size of the smallest pores in the membrane. In the case of ultrafiltration membranes, the smallest pores are located in the top layer / top portion (see Figure 2). Permeability is determined by the pore size (gradient) throughout the membrane. High permeability with a given blocking profile can be achieved, in particular, by a membrane with high asymmetry, having small pores in the desired range in the top layer / top portion, followed by a steep gradient toward larger pores towards the bottom layer.

[0006] The pore size, as well as the pore size gradient, in the top layer / top portion can be adjusted to some extent by the properties of the polymer solution and the membrane manufacturing process. This allows for an improved blocking-to-permeability ratio, resulting in more desirable membrane properties. Figure 3 illustrates an example of an ultrafiltration membrane exhibiting a similar blocking profile but different permeability due to the difference in pore size gradient. However, using the general membrane formation approach shown in Figure 1, the achievable blocking-to-permeability ratio is limited by the thermodynamic and dynamic aspects of membrane formation itself.

[0007] The pore size at each location within the membrane is determined not only by the polymer content in the cast solution at the start of phase separation, but also by the aggregation time between the start of phase separation and the solidification of the membrane body. Therefore, a feasible approach to achieving larger pores in the membrane body along with smaller pores in the top layer / top portion of the membrane is to use two polymer solutions for membrane formation: a thinner layer of solution containing a large amount of polymer to form the top layer / top portion, and a larger layer of solution containing a smaller amount of polymer to form the larger pores in the membrane body. However, this approach is labor-intensive, as it requires not only precise control of layer thickness but also doubles the amount of mixing, pumping, and layering techniques. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] Journal of Membrane Science, 120, 149-159, 1996 [Overview of the project] [Problems that the invention aims to solve]

[0009] Therefore, the fundamental technical problem of the present invention is to provide an ultrafiltration membrane having an improved inhibitory-to-permeable ratio efficiently and quickly. [Means for solving the problem]

[0010] Solutions to the above technical problems are realized by embodiments characterized in the claims.

[0011] In particular, the present invention relates to a method for manufacturing an ultrafiltration membrane, (a) A step of preparing a polymer solution containing a polymer, a highly volatile solvent, a low-volatile solvent, and a non-solvent, (b) A step of applying a polymer solution to the surface of a carrier or a support layer to form a polymer layer partially or completely on the carrier or on the support layer and / or in the support layer, (c) A step of transferring the polymer layer from step (b) together with an optional support layer through a (controlled) evaporation zone to partially evaporate a highly volatile solvent at a relative humidity of less than 20% RH, (d) A step of introducing the polymer layer from step (c) together with an arbitrary support layer into a precipitation bath containing a precipitant for the polymer in the polymer layer / polymer solution, This includes methods.

[0012] The present invention allows for the production of ultrafiltration membranes achieving an excellent inhibitory-to-permeability ratio by applying a single-layer approach combining precipitate-induced phase separation and evaporation-induced phase separation. This is achieved by using a combination of solvents in the polymer solution. One solvent is highly volatile (i.e., a highly volatile solvent), and the other is almost non-volatile (i.e., a low-volatility solvent). The film formation process includes an additional evaporation zone, where the highly volatile solvent evaporates only from the top portion of the polymer solution film. This process is schematically illustrated in Figure 4. Preferably, the present invention involves only one polymer solution coating step and therefore, apart from step (b), does not involve any further polymer solution coating steps.

[0013] Compared to conventional approaches, preferably, no additional machinery or equipment is required. Preferably, toxic solvents or other substances of high concern are not used. Preferably, manufacturing costs can be reduced through the new process design. Furthermore, preferably, easy control (equilibrium state) of key process parameters is possible.

[0014] When a highly volatile solvent partially evaporates, a higher polymer concentration is achieved near the top of the membrane. If the structure is completed during the precipitation process, this feature is preserved, resulting in a more asymmetric membrane structure. This leads to a higher rejection rate, enabling a faster separation process with higher membrane permeability.

[0015] In this invention, the term "ultrafiltration membrane" refers to a membrane having a MWCO of 1 kDa to 1000 kDa. The determination of the molecular weight cutoff can be carried out according to the US standard ASTM E1343-90 ("Standard test method for molecular weight cutoff evaluation of flat sheet ultrafiltration membranes"). Membrane terminology is summarized, for example, in "Terminology for membranes and membrane processes" (Non-Patent Literature 1).

[0016] For pore sizes of at least 0.1 μm, i.e., for microfiltration membranes with an average pore size of 0.1 μm to 10 μm, the pore size is determined using capillary flow porometry. This is a gas / liquid porometry method in which the differential pressure and flow rate of the gas passing through the membrane sample are first measured in a wet state and then in a dry state. Before measurement, the membrane sample is brought into contact with the wetting liquid so that all pores are filled with this liquid. After the pores are filled and the sample has been introduced, the measurement cell must be closed and the measurement started. After the measurement has started, the gas pressure is automatically and gradually increased, and the pore diameters corresponding to the applied pressure are emptied by the gas pressure. This is continued until the relevant pore range is covered, i.e., until the liquid is removed even from the smallest pores present in the measurement range. The pressure is then reduced again, and the measurement is automatically repeated for the currently dry sample. The pore size distribution is calculated from the difference between two pressure-flow curves using the Young-Laplace equation (see also A. Shrestha, "Characterization of porous membranes via porometry," 2012, Mechanical Engineering Graduate Theses & Dissertations, Paper 38, University of Colorado at Boulder).

[0017] For measuring pore sizes larger than 10 μm and up to 1 mm, the image analysis-based method described in Journal of Membrane Science 372 (2011), pp. 66-74, may be used.

[0018] For pore sizes smaller than 0.1 μm, a liquid-liquid displacement method similar to capillary flow porosimetry is used. However, instead of gas flow rate, the flow rate of the displacement liquid is measured in proportion to the increase in differential pressure (see also R. Daevila, "Characterization of ultra and nanofiltration commercial filters by liquid-liquid displacement porosimetry," 2013). Because this method is complex, the cutoff is determined by filtering model materials and creating sieve curves that show the membrane's cutoff behavior across various molecular weights (see ASTM E1343-90).

[0019] According to the present invention, in step (a), a polymer solution is prepared comprising a polymer, a highly volatile solvent, a low-volatile solvent, and a non-solvent. Any known means can be applied to prepare each polymer solution.

[0020] The polymer(s) used in the manufacture of the ultrafiltration membrane are not particularly limited. Therefore, any one or more polymers suitable for membrane formation can be used. The dissolved polymer (membrane-forming polymer) in the polymer solution may be, for example, polyethersulfone (PES), polysulfone, polyphenylenesulfone, polyetherimide, cellulose acetate (CA) such as cellulose diacetate and cellulose triacetate, cellulose esters, and cellulose ethers. The polymer solution may contain one or more of these membrane-forming polymers, preferably one. The membrane-forming polymer is preferably polyethersulfone and / or cellulose acetate. Most preferably, polyethersulfone is used as the membrane-forming polymer(s).

[0021] The high-volatility solvent and the low-volatility solvent are not particularly limited, except that they have corresponding high volatility and low volatility, respectively. In this regard, the term "high volatility" means that each substance has a boiling point below 100 °C at 1013 hPa and a vapor pressure of at least 50 hPa at 20 °C. The high-volatility solvent has a boiling point (bp) below 100 °C at 1013 hPa and a vapor pressure (vp) of at least 50 hPa at 20 °C. The low-volatility solvent has a boiling point of at least 100 °C at 1013 hPa and a vapor pressure below 50 hPa at 20 °C (see also DIN 53170). Both the high-volatility solvent and the low-volatility solvent are preferably miscible with the liquid of the precipitation bath. In particular, the high-volatility solvent and the low-volatility solvent are preferably miscible with water in the case of an aqueous precipitation bath.

[0022] Examples of high-volatility solvents are acetone (bp 56 °C, vp 246 hPa), formaldehyde (bp -19 °C, vp 4400 hPa), tetrahydrofuran (bp 65 °C, vp 170 hPa), methyl acetate (bp 57 °C, vp 228 hPa), methyl ethyl ketone (bp 80 °C, vp 105 hPa), methyl formate (bp 32 °C, vp 638 hPa), and ethyl formate (bp 54 °C, vp 266 hPa). The boiling point (bp) at 1013 hPa and the vapor pressure (vp) at 20 °C are shown in parentheses. Preferably, the high-volatility solvent is selected from the group consisting of acetone, methyl acetate, methyl formate, and ethyl formate. The solvent can be used alone or in any combination. Preferably, one high-volatility solvent is used. High-volatility solvents such as acetone, methyl acetate, methyl formate, ethyl formate, and tetrahydrofuran are beneficial because their viscosities are low, resulting in a lower-viscosity cast solution / polymer solution, and as a result, improved processability is obtained compared to a cast solution having a higher viscosity (and the same amount of solids).

[0023] Low-volatility solvents include, for example, 2-pyrrolidone (bp 245°C, vp 0.04hPa), N-methyl-2-pyrrolidone (bp 203°C, 0.32hPa), Nn-butyl-2-pyrrolidone / N-butyl-2-pyrrolidone (bp 240.6°C, vp 0.13hPa, Carl Roth SDS), dimethylacetamide (bp 165°C, vp 3.3hPa), 2-hydroxy-N,N-dimethylpropanamide (bp 1031hPa at at least 224°C, vp ≤ 0.12hPa, Sigma Aldrich SDS), dihydrolevoglucocenone (bp 1007hPa at 227°C, vp 0.28hPa at 25°C, Sigma Aldrich SDS), DMSO (bp 189°C, vp The pressure may be 0.556 hPa, and DMF (bp 153°C, vp 3.77 hPa). Preferably, the low-volatility solvent is selected from the group consisting of 2-pyrrolidone, N-butyl-2-pyrrolidone, dihydrolevoglucocenone, and 2-hydroxy-N,N-dimethylpropanamide. The solvent may be used alone or in any combination. Preferably, one low-volatility solvent is used. When using non-REACH solvents such as 2-pyrrolidone, N-butyl-2-pyrrolidone, 2-hydroxy-N,N-dimethylpropanamide, and dihydrolevoglucocenone, it is advantageous to obtain a film that has a REACH substance content below the detection limit or that does not contain any REACH substance.

[0024] REACH is a European regulation aimed at improving the protection of human health and the environment from risks posed by chemical substances. Based on this regulation by ECHA (European Chemicals Agency), the manufacture and / or use of certain substances are prohibited or strictly restricted in the European Union. Some substances tend to be prohibited or are almost certainly prohibited. These "candidates" are also not preferred for use. In this specification, the term "REACH substance" refers to both types of substances, those already prohibited and potential candidates. The term "non-REACH" means a beneficial state where the substance (solvent) is not registered as a substance of concern. Examples of currently beneficial solvents are acetic acid, acetone, N-butyl-2-pyrrolidone, dihydrolevoglucosenone, ethanol, isopropanol, ethyl formate, glycerol, glycerol-1,3-diacetate, glycerol triacetate, methyl acetate, 2-hydroxy-N,N-dimethylpropanamide, 2-pyrrolidone, 2,2-dimethyl-1,3-dioxolan-4-methanol, tetrahydrofuran, and water. The term "detectable content" means, for example, a content of 50 μg / mL or more when extracted with 1 mL of extraction liquid (e.g., water or ethanol) at a temperature of 60 °C to 80 °C for 24 hours on the surface of a 6 cm 2 membrane.

[0025] The polymer solution further contains a non-solvent for the corresponding polymer. In this regard, the term "non-solvent" refers to a liquid that cannot dissolve the membrane-forming polymer. Preferably, the non-solvent for the membrane-forming polymer (or each membrane-forming polymer) has a solubility of at most 1 wt%, very preferably at most 0.1 wt% under standard conditions. Standard conditions can be defined by IUPAC (Quantities, Units and Symbols in Physical Chemistry (iupac.org)): standard temperature (273.15 K or 0 °C) and pressure (10 5Pa) (IUPAC Compendium of Chemical Terminology, 3rd edition, International Union of Pure and Applied Chemistry; 2006. Online version 3.0.1, 2019. "STP" in https: / / doi.org / 10.1351 / goldbook.S06036).

[0026] If the polymer solution contains a non-solvent or other precipitating agent for the corresponding film-forming polymer, the maximum concentration of the non-solvent (or precipitating agent) is insufficient to cause precipitation of the film-forming polymer. Preferably, the polymer solution consists only of a highly volatile solvent and a low volatile solvent for the corresponding film-forming polymer, the corresponding film-forming polymer, and the non-solvent (mixture).

[0027] Suitable non-solvents include, for example, water, glycerol, methanol, ethanol, n-propanol, and isopropanol, as well as mixtures thereof. Unless otherwise specified, the above definitions and considerations regarding non-solvents apply similarly to all aspects of the present invention. Preferably, the non-solvent is water.

[0028] The polymer solution may contain 10% to 35% by weight of a (film-forming) polymer, 5% to 70% by weight of a highly volatile solvent, 20% to 80% by weight of a low-volatile solvent, and 0.1% to 15% by weight of a non-solvent, based on the total mass of the polymer solution. The content of the highly volatile solvent is preferably 5% to 60% by weight, more preferably 10% to 50% by weight, based on the total mass of the polymer solution. The content of the low-volatile solvent is preferably 20% to 70% by weight, based on the total mass of the polymer solution. The content of the non-solvent is preferably 0.5% to 15% by weight, more preferably 1.0% to 10% by weight, based on the total mass of the polymer solution. If the polymer solution contains two or more (film-forming) polymers, a highly volatile solvent, a low-volatile solvent, and / or a non-solvent, the above amounts refer to the sum of each component.

[0029] The polymer solution may further contain one or more additives. Suitable additives include, for example, swelling agents, solubilizers, hydrophilic agents, and / or pore-forming agents (pologens). Each of these additives is known to those skilled in the art and is compatible with the film-forming polymer. For example, polyethylene glycol (PEG), particularly PEG 1000 or PEG 2000, glycerol, or polyvinylpyrrolidone (PVP) may be used as a swelling agent or pore-forming agent. Such additives may be present in the polymer solution in amounts of 0% to 10% by weight. The polymer solution preferably consists only of the corresponding film-forming polymer, a highly volatile solvent, a low-volatile solvent, a non-solvent, and optionally one or more additives, preferably pore-forming agents and / or swelling agents.

[0030] The polymer solution does not require a specified thermal pretreatment; that is, thermal pretreatment for film / structure formation is not necessary. More specifically, it is preferable that the polymer solution does not have a lower or upper critical solution temperature.

[0031] According to the present invention, in step (b), a polymer solution is applied to the surface of a carrier or a support layer to form a polymer layer partially or completely on the carrier, on the support layer, and / or in the support layer. This produces a (single-pass) coated support layer that is transported with the carrier (as schematically shown in Figure 4). In the case of a film without a support layer, the polymer solution can be applied directly to the carrier. In different configurations, the cast solution can be applied directly to a freely floating support layer, a suspended support layer, or a stretched support layer, for example, by using a die or a drop curtain, or by moving it through a compartment containing the polymer solution. Preferably, a carrier optionally having a support layer is used. Methods for applying polymer solutions are known to those skilled in the art from the prior art and can be used without particular limitation in the method of the present invention. Such methods include, for example, transporting the carrier and / or support layer through, for example, a doctor blade system or slot die from which the corresponding polymer solution is dispensed. Step (b) is preferably carried out using a slot die.

[0032] In step (b), the polymer solution may partially or completely penetrate into any support layer. For example, the polymer layer may penetrate the support layer to a degree of at least 25%, preferably at least 50%, and more preferably at least 75%. Preferably, the polymer layer penetrates the support layer to a degree of less than 100%.

[0033] The support layer is not particularly limited. Therefore, all support layers known to those skilled in the art from the prior art can be used. For example, the support layer may be a nonwoven web, a woven fabric, or an open-type microfiltration membrane. Examples of nonwoven webs are polyolefin nonwovens such as PP / PE core-shell nonwovens and polyester nonwovens. The support layer is preferably a polyolefin nonwoven or a polyolefin membrane.

[0034] The thickness of the support layer may be, for example, 30 μm to 300 μm, preferably 50 μm to 250 μm, and more preferably 80 μm to 200 μm.

[0035] Any carrier suitable for conventional film manufacturing methods may be used. The carrier preferably has a flat surface and is inert to the substance used (e.g., polymer solution and its components). Preferably, a moving belt (conveyor belt) or drum (as schematically shown in Figure 4) functions as the carrier, preferably made of steel or plastic foil, which may allow the method to be carried out continuously.

[0036] In one preferred embodiment, the carrier and / or support layer in step (b) moves relative to the polymer solution at a speed of 30 m / h to 500 m / h, more specifically 60 m / h to 400 m / h, and very preferably 100 m / h to 300 m / h, when the polymer solution is applied. That is, the carrier and / or support layer is conveyed at the above speed, for example, through a doctor blade system or slot die for applying the polymer solution.

[0037] The coating temperature in step (b) may be from 4°C to (bp of the highly volatile solvent at its lowest boiling point - 20% of the bp of the highly volatile solvent at its lowest boiling point (in °C)), more preferably from 4°C to (bp of the highly volatile solvent at its lowest boiling point - 25%), and most preferably from 4°C to (bp of the highly volatile solvent at its lowest boiling point - 30%). For example, if the boiling point of the highly volatile solvent at its lowest boiling point is 100°C, the range from 4°C to (bp of the highly volatile solvent at its lowest boiling point - 20%) means a range of 4°C to (100°C - 20°C) = 4°C to 80°C.

[0038] According to the present invention, in step (c), the polymer layer (including an optional support layer) from step (b) is transported through an evaporation zone to partially evaporate the highly volatile solvent at a relative humidity of less than 20% RH. For this purpose, the polymer layer (including an optional support layer) can be transported with the same carrier used in step (b) or together with the support layer by suitable transport means known in the art. In this regard, the term “partially” is not particularly limited. For example, at least 0.01% by weight of the highly volatile solvent is evaporated relative to 100% by weight of the highly volatile solvent in the (initial) polymer solution. Preferably, at least 0.02% by weight, more preferably at least 0.04% by weight, and / or up to 2.0% by weight, more preferably up to 0.5% by weight of the highly volatile solvent is evaporated relative to 100% by weight of the highly volatile solvent in the (initial) polymer solution. Preferably, further components of the (initial) polymer solution (excluding the highly volatile solvent) are not (substantially) evaporated during step (c). Furthermore, it is preferable that no further components, such as water, are (substantially) introduced into the polymer solution during step (c), for example, from the atmosphere.

[0039] To determine the amount of highly volatile solvent from each polymer solution film, the solution can be cast onto a substrate and placed on a laboratory balance under the given conditions used in the casting process. Since the vapor pressures of the additional components are orders of magnitude lower than those of the highly volatile solvent, the total weight loss can be considered approximately the total amount of highly volatile solvent evaporated. The degree of evaporation of the highly volatile solvent can be controlled, for example, by the evaporation time and temperature in step (c).

[0040] For example, the evaporation time in step (c) may be 0.50 seconds to 1.0 minute, preferably 2.0 seconds to 20 seconds, and more preferably 5.0 seconds to 15 seconds. When such evaporation times are applied, sufficient and not excessive evaporation of the highly volatile solvent is achieved while suppressing the evaporation of the low-volatility solvent. This results in a robust skin structure / top portion for high retention, which is thin enough so as not to add further durability to the film.

[0041] The temperature in step (c) may be from 4°C to 20% of the bp of the highly volatile solvent (at its lowest boiling point), more preferably from 4°C to 25% of the bp of the highly volatile solvent (at its lowest boiling point), and most preferably from 4°C to 30% of the bp of the highly volatile solvent (at its lowest boiling point). The coating temperature in step (b) is preferably the same as or lower than the temperature in step (c). Preferably, the coating temperature in step (b) is lower than the temperature in step (c). The above temperatures preferably prevent the formation of bubbles in the film, which could lead to large defects. When such temperatures are applied, sufficient and not excessive evaporation of the highly volatile solvent is achieved while suppressing the evaporation of the low-volatility solvent.

[0042] The evaporation zone allows for control of the atmosphere with respect to humidity, temperature, and optionally, airflow conditions. In particular, the humidity in the evaporation zone is reduced to preferably less than 20% RH, more preferably less than 18% RH, more preferably less than 15% RH, more preferably less than 10% RH, more preferably 0% RH to 5% RH, and most preferably 0% RH, compared to the ambient temperature used in process (c). Typically, this makes it possible to avoid the opposite effect caused by water diffusing into the film (i.e., openings in the top portion of the film) compared to the present invention. Furthermore, lower humidity may result in a narrowing of the pore size in the top portion of the resulting film, as the atmosphere may have a greater ability to accept the (evaporated) highly volatile solvent. The desired relative humidity (RH) can be adjusted, for example, by introducing an inert gas such as a dry gas, preferably nitrogen or carbon dioxide, into the evaporation zone. In the evaporation zone, the gas flow can be carried out as laminar flow, for example, on the surface of the polymer layer from process (b).

[0043] According to the present invention, in step (d), the polymer layer (including an arbitrary support layer) from step (c) is introduced into a precipitation bath containing a precipitant for the polymer in the polymer layer / polymer solution (as schematically shown in Figure 4). The precipitant used in step (d) is not particularly limited. According to the present invention, the same precipitant as in conventional polymer film production methods can be used. The precipitant causes precipitation (phase inversion) of the film-forming polymer in the polymer layer. The precipitant may be one compound or a mixture of two or more compounds. Preferably, the precipitant is in liquid form in the precipitation bath, more preferably the precipitant itself is liquid in the precipitation bath, or the precipitation bath consists only of the precipitant. Preferably, the precipitant is a non-solvent for the film-forming polymer(s), where the precipitation bath preferably consists only of a precipitant selected from the group consisting of water, alcohol, and water with additives. More preferably, the precipitation bath consists only of water as the precipitant. The precipitant may be the same as the non-solvent contained in the polymer solution. Preferably, other components of the polymer layer (excluding the polymer in the polymer layer / polymer solution) (e.g., highly volatile solvents, low volatile solvents, and non-solvents, and optionally one or more additives) are removed from the polymer layer during step (d). Preferably, other components of the polymer layer in the polymer layer / polymer solution (excluding the polymer in the polymer layer / polymer solution) (e.g., highly volatile solvents, low volatile solvents, and non-solvents, and optionally one or more additives) are completely miscible with the precipitant in the precipitation bath.

[0044] The temperature of the deposition bath is not subject to any special restrictions. For example, the temperature of the deposition bath may be 1°C to 60°C, preferably 3°C to 30°C, and more preferably 4°C to 20°C.

[0045] The method of the present invention may further include (e) introducing a polymer layer (with an optional support layer) into one or more rinsing tanks after step (d) (as schematically shown in Figure 4). The rinsing tank used in step (e) is not particularly limited. The same rinsing tanks as those used in conventional polymer film production methods can be used. Rinsing processes are known to those skilled in the art and can be selected according to requirements relating to the film (e.g., relating to residual contaminants, extracts, and leaches (extracts and leaches, respectively)). Suitable rinsing tanks may include, for example, water, monohydric or polyhydric alcohols, or other hydrophilic liquids.

[0046] As described above, the polymer solution may contain a cellulose compound such as cellulose acetate or cellulose ester. To obtain the corresponding hydrolysis product, the method of the present invention (including steps (a) to (d) or (a) to (e)) may further include the step of (f) hydrolyzing the cellulose compound.

[0047] The hydrolysis method used in step (f) is not particularly limited. According to the present invention, the same hydrolysis method as in conventional polymer film manufacturing methods, such as introducing a polymer layer (with any support layer) into an alkaline bath, can be used. Examples of general hydrolysis methods are described, for example, in U.S. Patent No. 7,422,686. A suitable alkaline bath may, for example, contain 50% KOH in water or alcohol, or a mixture thereof. The exposure time may be, for example, 1.0 to 30 minutes.

[0048] The method of the present invention may include, in addition to or instead of step (f), a further step of crosslinking polymer chains in the polymer layer (g).

[0049] The crosslinking method used in step (g) is not subject to any special limitations. According to the present invention, the same crosslinking method as in conventional polymer film manufacturing methods can be used. A suitable crosslinking method is, for example, the crosslinking of regenerated cellulose by a method such as that described in U.S. Patent No. 7,422,686.

[0050] Further aspects of the present invention relate to ultrafiltration membranes obtained by the method of the present invention for manufacturing ultrafiltration membranes. The above-described definitions and embodiments also apply to this aspect of the present invention. Similarly, the following definitions and embodiments also apply to the method of the present invention for manufacturing ultrafiltration membranes. The ultrafiltration membrane of the present invention is preferably a flat membrane.

[0051] As described above, the method of the present invention applies a monolayer approach. Partial evaporation of the highly volatile solvent results in a higher polymer concentration near the top of the membrane. The completion of the structure during the precipitation step preserves this feature and thus results in a more asymmetric membrane structure. This preferably results in a rejection rate that allows for a faster separation process with higher membrane permeability.

[0052] The ultrafiltration membrane of the present invention preferably comprises a top portion and an asymmetrical membrane body portion adjacent to the top portion. The top portion corresponds to the portion of the polymer layer where evaporation of the highly volatile solvent occurred during step (c). The top portion and the membrane body portion are continuous within the membrane (i.e., they do not have a defined interface).

[0053] The top layer of the membrane (also known as the skin layer) has a dense structure with tiny pores that cannot be resolved by SEM. This surface / top layer structure continues into the main membrane body, which is the adjacent layer containing a spongy structure. Due to the significant asymmetry of the membrane at certain points, the pores can be resolved and directly measured by SEM. Very close to the top layer, pores on the underside can be measured directly, as shown in Figure 9. Furthermore, the pore diameter can be measured on the opposite side of the membrane (the part directly facing the inside or surface of the membrane).

[0054] The first portion of the membrane body adjacent to the top portion has a smaller average pore size than the second (opposite) portion of the membrane body (as shown, for example, in Figure 10), and the average pore size increases (continuously or discontinuously) from the first portion to the second portion of the asymmetrical membrane body. The ratio of the average pore sizes between the second and first portions of the membrane body (average pore size of the second portion of the membrane body / average pore size of the first portion of the membrane body) is preferably at least 5:1, more preferably at least 10:1, and most preferably at least 15:1. For example, in the membrane shown in Figure 10, the average pore ratio is about 0.47 μm to about 0.03 μm (below the top portion), which is about 16:1. The average pore size of the top portion is smaller than the average pore size of the first portion of the membrane body. The ratio of the average pore size between the second portion and the top portion of the membrane body (average pore size of the second portion of the membrane body / average pore size of the top portion) is preferably at least 20:1, more preferably at least 30:1, and most preferably at least 50:1. The asymmetrical membrane body is preferably homogeneous, i.e., (substantially) free of macrovoids.

[0055] As used herein, an asymmetric membrane portion is a membrane portion in which a first portion of the asymmetric membrane portion has an average pore size smaller than the average pore size of a second portion (on the opposite side) of the asymmetric membrane portion, and the average pore size increases (continuously or discontinuously) from the first portion to the second portion of the asymmetric membrane portion.

[0056] The top portion may correspond to 5% or less, preferably 4% or less, and more preferably 3% or less (excluding 0.0%) of the total thickness of the membrane. The main membrane portion may correspond to 95% or more, preferably 96% or more, and more preferably 97% or more (excluding 100%) of the total thickness of the membrane. Preferably, the ultrafiltration membrane consists only of the top portion and the main membrane portion. The main membrane portion includes an optional support layer.

[0057] The top portion may have a thickness of 0.5 μm to 10 μm, preferably 0.5 μm to 5.0 μm, more preferably 1.0 μm to 3.0 μm, or 0.5 μm to 1.0 μm.

[0058] The main body of the film may have a thickness of 70 μm to 390 μm, preferably 90 μm to 345 μm, and more preferably 100 μm to 250 μm.

[0059] The film may have a total thickness of 80 μm to 400 μm, preferably 100 μm to 350 μm, and more preferably 120 μm to 280 μm.

[0060] Preferably, the membrane does not contain REACH substances in a detectable amount, and more preferably, it does not contain any REACH substances at all.

[0061] As defined above, the film-forming polymer may be polyethersulfone, polysulfone, polyphenylenesulfone, polyetherimide, cellulose acetate, cellulose ester, and cellulose ether. Furthermore, a cellulose compound such as cellulose acetate or cellulose ester can be subjected to any hydrolysis (any step (f) of the method of the present invention) to obtain regenerated cellulose, and the regenerated cellulose or other polymer can be subjected to any crosslinking method (any step (g) of the method of the present invention). Therefore, in a preferred embodiment of the ultrafiltration membrane of the present invention, the polymer layer contains a polymer selected from the group consisting of polyethersulfone, polysulfone, polyphenylenesulfone, polyetherimide, cellulose acetate, regenerated cellulose, crosslinked regenerated cellulose, or other crosslinked polymers described above (e.g., crosslinked polyethersulfone or crosslinked polysulfone), cellulose ester, and cellulose ether. The polymer layer may contain one or more of these polymers, preferably one. More preferably, the polymer layer comprises polyethersulfone and / or cellulose acetate, more preferably consisting of them, most preferably comprising polyethersulfone, and more preferably consisting of it alone.

[0062] There are no particular limitations on the possible uses of the membrane of the present invention. The membrane of the present invention can be used for filtration, more particularly for filtering viruses, proteins, or macromolecules.

[0063] A further aspect of the present invention relates to a method for enhancing the blocking properties of an ultrafiltration membrane, comprising the steps of exposing a layer of a polymer solution containing a polymer, a highly volatile solvent, a low-volatile solvent, and a non-solvent to an evaporation zone under a dry atmosphere, and then introducing the polymer layer into a precipitation bath containing a precipitant for the polymer in the polymer solution, thereby partially evaporating the highly volatile solvent from the top portion of the polymer layer. The above definitions and embodiments apply similarly to this aspect of the present invention.

[0064] A method for enhancing the blocking properties of an ultrafiltration membrane may include a further step of applying a polymer solution to the surface of a carrier or a support layer before, preferably immediately before, exposing the polymer layer to the evaporation zone, thereby partially or completely forming a polymer layer on the carrier or on and / or within the support layer.

[0065] In a preferred embodiment of a method for enhancing the blocking properties of an ultrafiltration membrane, the evaporation zone under a dry atmosphere may have a relative humidity of less than 20% RH, preferably less than 18% RH, more preferably less than 15% RH or less than 10% RH, more preferably 0% RH to 5% RH, and most preferably 0% RH at the temperature used. In another preferred embodiment, the residence time (evaporation time) in the evaporation zone is 0.50 seconds to 1.0 minute, preferably 2.0 seconds to 20 seconds, and more preferably 5.0 seconds to 15 seconds.

[0066] A further aspect of the present invention relates to the use of an evaporation zone prior to, preferably immediately before, a deposition bath in a method for enhancing the blocking properties of an ultrafiltration membrane, wherein in the evaporation zone, under a dry atmosphere, the highly volatile solvent partially evaporates from the top portion of a layer of polymer solution comprising a polymer, a highly volatile solvent, a low-volatile solvent, and a non-solvent. The above definitions and embodiments apply similarly to this aspect of the present invention.

[0067] The use of an evaporation zone may include a further step of applying a polymer solution to the surface of a carrier or a support layer before, preferably immediately before, the evaporation zone, to partially or completely form a polymer layer on the carrier or on and / or within the support layer.

[0068] In a preferred embodiment of the use of the evaporation zone, the dry atmosphere may be an atmosphere having a relative humidity of less than 20% RH, preferably less than 18% RH, more preferably less than 15% RH or less than 10% RH, more preferably 0% RH to 5% RH, and most preferably 0% RH at the temperature used. In another preferred embodiment, the residence time (evaporation time) in the evaporation zone is 0.50 seconds to 1.0 minute, preferably 2.0 seconds to 20 seconds, and more preferably 5.0 seconds to 15 seconds. [Brief explanation of the drawing]

[0069] [Figure 1] This figure shows a schematic diagram of the manufacturing process for ultrafiltration membranes. [Figure 2] This figure shows a scanning electron microscope image of a cross-section of a PES ultrafiltration membrane, illustrating both the narrow pore size in the top layer / top portion and the pore size gradient in the membrane body (Mag = magnification). [Figure 3] This figure shows scanning electron microscope images of two types of cellulose acetate ultrafiltration membranes with high (left) and low (right) asymmetry in pore size within the membrane body (Mag = magnification). [Figure 4] This figure shows a schematic diagram of the manufacturing process for the ultrafiltration membrane of the present invention. [Figure 5] This figure shows a scanning electron microscope image of film 1 manufactured in the example. [Figure 6] This figure shows a scanning electron microscope image of film 2 manufactured in the example. [Figure 7]This figure shows a comparison between the current state of technology process (square) and the combination of evaporation and precipitation for CA films produced using a mixture of N-butyl-2-pyrrolidone and acetone (circular). [Figure 8] This figure compares the current state-of-the-art process (square) with the combination of evaporation and precipitation for PES films produced using a mixture of N-butyl-2-pyrrolidone and acetone (circular). [Figure 9] This figure shows a scanning electron microscope image of an ultrafiltration membrane having a continuous top portion and a membrane body portion. [Figure 10] This figure shows a scanning electron microscope (SEM) image of an ultrafiltration membrane comprising a top portion that cannot be resolved by SEM, and a membrane body portion having a first portion adjacent to the top portion and a second portion with a wider pore size. The selected pore size was determined and is shown in Figure 10. [Modes for carrying out the invention]

[0070] The present invention will be further illustrated by the following examples, but will not be limited thereto. [Examples]

[0071] Example 1: Visualization of asymmetry in the main body portion of the film The method of the present invention was applied under the following conditions to produce two different ultrafiltration membranes, 1 and 2:

[0072] [Table 1]

[0073] The resulting asymmetry can be visualized using scanning electron microscopy (SEM). The average pore size in 10 different horizontal sections across the entire SEM image (cross-section of the membrane, see Figures 5 and 6) is visually determined by binarizing the image and plotting the location against the observed pore size. The average pore size for one section can be determined from 50 pores. Figures 5 and 6 show the asymmetry of the membrane body portions of membrane 1 and membrane 2.

[0074] Example 2: Determination of the ratio of inhibitory to permeable substances The method of the present invention was applied under the following conditions to produce two further ultrafiltration membranes 3 and 4:

[0075] [Table 2]

[0076] Furthermore, as shown in Figure 1, similar CA and PES films were fabricated using processes based on current technological standards. Films fabricated using only low-volatility solvents or without (controlled) evaporation (Figure 1) exhibited lower performance, which can be visualized by plotting the probe molecule retention against the buffer permeation flux, as shown in Figures 7 and 8.

[0077] When a highly volatile solvent partially evaporates, a higher concentration of the polymer is achieved at the top of the film, directly resulting in a smaller pore size. Furthermore, this leads to a higher degree of asymmetry throughout the film. The higher the degree of asymmetry, the higher the permeability at the same blocking rate compared to a more symmetrical film.

[0078] In the field of ultrafiltration membranes, evaluating asymmetry is difficult. This is because the pore size of the top portion is so small that it cannot be visualized using conventional microscopic methods. Only the larger pores on the other side of the membrane can be evaluated by optical methods.

[0079] The following methods are proposed to accurately determine the degree of asymmetry: The blocking capability against a desired target molecule is determined through cross-flow filtration simulation. The target molecule depends on the blocking range of the membrane and is not limited to a single specific molecule. The average pore size at a point 50 μm away from the top is determined via SEM. The average of at least 10 pore diameters is calculated. The product of the rejection rate and the average pore size reflects the asymmetry of the membrane. A high value corresponds to a high degree of asymmetry.

[0080] The specific characteristics of the membrane regarding inhibitory and permeable properties can be explained by plotting the inhibitory rate of specific target molecules (here, lysozyme (Figure 7) and vitamin B12 (Figure 8), respectively) against membrane permeability. The values ​​for membranes manufactured according to the present invention generally show higher permeation flux at a given retention rate or higher retention at a given permeation flux. As shown in Figures 7 and 8, membranes manufactured by the method of the present invention provide improved performance in terms of inhibitory / permeable ratio compared to corresponding membranes manufactured by conventional processes. This can result in higher permeation flux at a given retention rate, as in the case of CA membranes (Figure 7), or higher retention at a given permeation flux, as in the case of PES membranes (Figure 8), compared to methods of the current state of the art.

Claims

1. A method for manufacturing an ultrafiltration membrane, (a) A step of preparing a polymer solution containing a polymer, a highly volatile solvent, a low-volatile solvent, and a non-solvent, (b) A step of applying the polymer solution to the surface of a carrier or a support layer to form a polymer layer partially or completely on the carrier or on the support layer and / or in the support layer, (c) A step of transferring the polymer layer from step (b) together with any support layer through an evaporation zone to partially evaporate the highly volatile solvent at a relative humidity of less than 20% RH, (d) A step of introducing the polymer layer from step (c) together with any support layer into a precipitation bath containing a precipitant for the polymer in the polymer layer, Methods that include...

2. The method according to claim 1, wherein the polymer solution comprises, with respect to the total mass of the polymer solution, 10% to 35% by weight of the (film-forming) polymer, 5% to 70% by weight of the highly volatile solvent, 20% to 80% by weight of the low volatile solvent, and 0.1% to 15% by weight of a non-solvent.

3. The method according to claim 1 or 2, wherein the polymer is selected from the group consisting of polyethersulfone, polysulfone, polyphenylenesulfone, polyetherimide, cellulose acetate, cellulose ester, and cellulose ether, and combinations thereof.

4. The method according to any one of claims 1 to 3, wherein the highly volatile solvent is selected from the group consisting of acetone, formaldehyde, tetrahydrofuran, methyl acetate, methyl ethyl ketone, methyl formate, and ethyl formate, and combinations thereof.

5. The method according to any one of claims 1 to 4, wherein the low-volatility solvent is selected from the group consisting of 2-pyrrolidone, N-methyl-2-pyrrolidone, N-butyl-2-pyrrolidone, dimethylacetamide, 2-hydroxy-N,N-dimethylpropanamide, dihydrolevoglucocenone, DMSO, and DMF, and combinations thereof.

6. The method according to any one of claims 1 to 5, wherein the non-solvent in the polymer solution is selected from the group consisting of water, glycerol, methanol, ethanol, n-propanol, isopropanol, and combinations thereof.

7. The method according to any one of claims 1 to 6, wherein the evaporation time in step (c) is 0.50 seconds to 1.0 minute.

8. The method according to any one of claims 1 to 7, wherein the temperature in step (b) and step (c) is independently from 4°C to (boiling point of the highly volatile solvent - 20% of the boiling point of the highly volatile solvent (°C)).

9. The method according to any one of claims 1 to 8, further comprising the step of introducing the polymer layer having any support layer into one or more rinse tanks after step (d).

10. An ultrafiltration membrane obtained by a method for producing an ultrafiltration membrane according to any one of claims 1 to 9.

11. The aforementioned membrane comprises a top portion and an asymmetrical membrane body portion adjacent to the top portion. The top portion and the main membrane portion are continuous within the membrane. The first portion of the membrane body adjacent to the top portion has an average pore size smaller than the average pore size of the second portion of the membrane body, and the average pore size increases from the first portion to the second portion of the asymmetric membrane body, The ultrafiltration membrane according to claim 10, wherein the average pore size of the top portion is smaller than the average pore size of the first portion of the membrane body.

12. The ultrafiltration membrane according to claim 11, wherein the top portion corresponds to 5% or less of the membrane thickness, and the membrane body portion corresponds to 95% or more of the membrane thickness.

13. The ultrafiltration membrane according to any one of claims 10 to 12, wherein the membrane does not contain REACH material.

14. The ultrafiltration membrane according to any one of claims 10 to 13, wherein the first portion of the membrane body adjacent to the top portion has an average pore size smaller than the average pore size of the second portion opposite to the membrane body, and the ratio of the average pore sizes between the second portion of the membrane body and the first portion of the membrane body is at least 5:1.

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