METHOD FOR PRODUCING A POROUS SINGLE-LAYER POLYMER MEMBRANE

Description

[0001] The present invention relates to a process for producing a porous single-layer polymer membrane.

[0002] In the production of filtration membranes in phase separation processes, casting solutions (homogeneous mixtures of different substances) are typically first cast in layers or in the form of shaped bodies such as hollow fibers. The casting solution composition or the thermodynamic state of the casting solution is then altered until demixing / phase separation is achieved. Phase separation results in an integral phase with a coherent solid component. This component is retained as a porous membrane or shaped body from the manufacturing process. Such processes are carried out in one or more different process steps. Essentially, four different mechanisms for initiating phase separation are distinguished: (1) non-solvent induced phase separation (NIPS), (2) vapor-induced phase separation (VIPS), (3) evaporation-induced phase separation (EIPS), and (4) thermally induced phase separation (TIPS). and combinations thereof.

[0003] Another known technique is the production of porous membranes from multiple layers of casting solutions that differ in their composition. This process is often referred to as co-casting or multi-coating. In contrast to processes in which only a single casting solution is used, the co-casting process produces multi-layer membranes. This is due to the fact that co-castingseveral layers of casting solution are applied simultaneously to a moving carrier in such a way that they form several membrane-forming polymer layers.

[0004] According to IUPAC recommendations, filtration membranes are classified based on their retentive properties and pore size. Based on pore size, a general distinction is made between microfiltration membranes (average pore size: 0.1 to 10 µm), ultrafiltration membranes (average pore size: 0.01 to less than 0.1 µm), nanofiltration membranes (average pore size: 0.001 to less than 0.01 µm), and reverse osmosis membranes (average pore size: 0.0001 to less than 0.001 µm) (see Shang-Tian Yang, Bioprocessing for Value-Added Products from Renewable Resources, 2007).

[0005] Regarding the retentive properties of the membranes, a similar definition can be made based on the molecular weight cut-off(MWCO). The MWCO refers to the solute with the lowest molecular weight in Daltons at which 90% of the solute is retained by the membrane, or alternatively, to the molecular weight of a molecule in Daltons at which 90% of the molecules with that molecular weight are retained by the membrane. Filtration membranes with an MWCO of test molecules or particles with a hydrodynamic radius of 0.1 to 10 µm are referred to as microfiltration membranes. Corresponding assignments of MWCO ranges are as follows: Ultrafiltration membranes: 0.01 µm to less than 0.1 µm; Nanofiltration membranes: 0.001 µm to less than 0.01 µm; and reverse osmosis membranes: 0.0001 µm to less than 0.001 µm. Method for determining the mean pore size

[0006] Depending on the pore size to be determined, different methods are used. In the field of microfiltration, capillary flow porometry is primarily used. Capillary flow porometry is a gas / liquid porosimetry method in which the differential gas pressures and flow rates through a membrane sample are measured first in the wet state and then in the dry state.

[0007] Before the measurement, the membrane sample is brought into contact with a wetting liquid so that all existing pores are filled with this liquid. After filling the pores and introducing the sample, the measuring cell is closed and the measurement is 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 continues until the relevant pore area has been measured, i.e., until even the smallest pores in the measurement area are free of liquid. The pressure is then reduced again and the measurement is automatically repeated on the now dry sample. The pore size distribution is calculated from the difference between the two pressure-flow rate curves using the Young-Laplace equation. (See Shrestha, Aabhash, "Characterization of porous membranes via porometry" (2012), Mechanical Engineering Graduate Theses & Dissertations, Paper 38.)).

[0008] The characterization of the pore size distribution in ultra- and nanofiltration membranes cannot be carried out using gas / liquid porosimetry because the pores in these membranes have a significantly smaller diameter. According to the Laplace equation, the pressure required to displace the wetting liquid increases to orders of magnitude of up to 50 to 70 bar for a given surface tension. On the one hand, such pressures make handling more difficult and are partly limited by the equipment's boundary conditions. On the other hand, the pressures exerted on the membrane and the resulting forces are to be expected to change the structure and, in extreme cases, a loss of structural integrity. The physical properties obtained here with regard to the pore size distribution would not correspond to those of an intact membrane. In order to be able to carry out characterization despite this, a reduction in the surface orThe interfacial tension of the wetting and displacing media is desired. For this reason, a liquid-liquid displacement of two immiscible liquids is used. The interfacial tension between these two media is significantly lower than the surface tension between a gas and a liquid. Analogous to gas / liquid porosimetry, the Laplace relationship applies to the displacement of one liquid by another, and a similar measurement method can be used, with the difference that flow rates of the displacing liquid are recorded as a function of the differential pressure increase, rather than gas flow rates. (See R. Dávila, Characterization of Ultra and Nanofiltration Commercial Filters by Liquid-Liquid Displacement Porosimetry, 2013.). Procedure for determining the MWCO

[0009] The MWCO of various membranes is determined by recording sieve curves. Samples of a polymer (dextran, polystyrene, pullulan, etc.) with a broad size distribution are filtered through the respective membrane. Both the feed solution and the filtrate are then characterized using size exclusion chromatography. The depletion of certain polymer sizes in the filtrate can be used to determine the retentive properties of the membrane, and a sieve curve can be calculated. The polymer weight at which 90% rejection is achieved is referred to as the MWCO. (See C. Loh, Effects of Additives and Coagulant Temperature on Fabrication of High Performance PVDF / Pluronic F127 Blend Hollow Fiber Membranes via Nonsolvent Induced Phase Separation, Chinese Journal of Chemical Engineering, 20 (1) 71-79 (2012)).

[0010] According to the invention, the mean pore size and the MWCO can be determined as described above under "Method for determining the mean pore size" and "Method for determining the MWCO".

[0011] In addition to the above parameters, filtration membranes can also be classified according to their pore size distribution across the membrane cross-section. Filtration membranes with a (largely) uniform pore size across the membrane cross-section are referred to as symmetric, whereas membranes with pronounced pore size gradients across the membrane cross-section are referred to as asymmetric.

[0012] In the course of this description, an asymmetry factor can be defined. This describes the ratio of the largest to smallest pore size within the membrane cross-section. A factor of 1.5 or higher can be considered an asymmetric membrane. If the asymmetry factor is less than 1.5, the membrane is symmetric.

[0013] Depending on the desired membrane properties, different manufacturing processes are used. Symmetric microfiltration membranes (e.g., made of cellulose acetate (CA), cellulose nitrate (CN), or polyvinylidene fluoride (PVDF)) are primarily manufactured using EIPS processes. These single-step processes allow only limited control of the mass transport processes and the associated membrane formation processes. This method prevents the creation of pore size gradients within the membrane, making asymmetric microfiltration and ultrafiltration membranes inaccessible.

[0014] Asymmetric and symmetric nanofiltration membranes (e.g., made of CA or polyethersulfone (PESU)), however, can be produced using NIPS processes. However, these single-step processes are not capable of producing asymmetric microfiltration and ultrafiltration membranes.

[0015] Asymmetric microfiltration membranes made of polysulfone (PSU) or polyethersulfone (PES) can be produced using VIPS processes. Currently, access to asymmetric ultrafiltration membranes is only possible through a combination of VIPS and NIPS processes in two-stage processes. Since these two-stage processes involve many parameters that must be controlled, there is a need for a simplified process for the production of asymmetric ultrafiltration membranes.

[0016] The above processes for producing asymmetric membranes include a NIPS (non-solvent-induced phase separation) process step. In non-solvent-induced phase inversion processes, the surfaces of the top and bottom sides of the forming membrane exhibit significantly lower porosity at the casting solution / precipitant (air side) and casting solution / support (belt or carrier side) phase interfaces than in the membrane interior. This is essentially due to two mechanisms: By introducing non-solvent and removing solvent from the casting solution film in a NIPS process at the casting solution / precipitant phase interface, gelation of the membrane-forming polymer is induced, resulting in the formation of a dense polymer layer with low porosity (see F. Altena, Phase separation phenomena in cellulose acetate solutions in relation to asymmetric membrane formation, Dissertation, 1982). The resulting gel layer acts as a diffusive barrier to further penetration of precipitant. This reduces the mass transfer between precipitant and solvent and thus the rate of membrane formation directly below the casting solution / precipitant phase boundary. In this way, gel formation inside the membrane is prevented and phase separation primarily occurs, leading to the formation of cellular, sponge-like, or finger-like substructures with higher porosity than in the gel layer.This mechanism can be advantageous in the production of highly asymmetric nanofiltration membranes, but prevents the production of open-pore symmetric and asymmetric microfiltration or ultrafiltration membranes (i.e., microfiltration or ultrafiltration membranes with high surface porosity) in a one-step NIPS process. Furthermore, the nascent structures of the membrane formed during phase separation at interfaces (casting solution / precipitant and casting solution / support) tend to coalesce during the further formation process, assuming no gel formation occurs, to reduce the interfacial tension between the polymer-rich, polymer-poor, and the adjacent phase (atmosphere or support). This leads to a significant reduction in surface porosity on the support and air sides.The resulting membranes exhibit a surface porosity that is significantly lower than the total porosity of the membrane (see A. Deratani et al., Retainment of pore connectivity in membranes prepared with vapor-induced phase separation, Journal of Membrane Science 362 (2010), pages 360-373). The porosities on the air side and belt side of the membrane for commercial microfiltration membranes are typically 15–40%. The total porosity of such membranes is typically more than 70%. This porosity difference is particularly disadvantageous because it leads to faster blockage of the membrane side exposed to the flow and a resulting reduction in permeability during a filtration step. Total porosity is defined as the average porosity across the entire membrane cross-section. This can be easily calculated from geometric and gravimetric data of a membrane sample: ε p = d Mem ⋅ A Mem m Mem ρ Poly The d Mem for the thickness of the membrane sample, A meme for the membrane area, m Mem for the mass of the membrane sample and ρ Poly for the density of the membrane-forming polymer.

[0017] A disadvantage of co-casting The main disadvantage of the multilayer membranes obtained is the often abrupt change in pore sizes between the layers. The abrupt reduction in pore size when moving from one layer to another results in performance losses: a wide range of particle sizes is produced at the interface of the co-casting generated layers are retained.

[0018] This particle loading at the layer boundary leads to a loss of filter service life.

[0019] Often co-castingused to enlarge the pores on the surface of the resulting membrane and increase the flow rate compared to membranes with dense skin layers. However, the enlargement of the pores when using the co-casting process is accompanied by a disadvantageous increase in the overall thickness. Thick layers consisting of several layers, as obtained using the co-casting process, can be packed less densely than single-layer membranes. For example, when pleating filter elements, the overall thickness of the membrane determines the maximum area that can be fitted into a given geometry of a filter element in the form of a pleat. Therefore, the thickness should be kept as low as possible. Furthermore, with co-cast membranes there is a risk of delamination / layer separation during pleating, which is not the case with single-layer membranes.

[0020] In this context, it must be mentioned that an enlargement of the pores due to co-casting is not accompanied by an increase in surface porosity. Even if co-casting inner membrane surfaces enclosed by the bulk material of the membrane are accessible with larger pores, the surface porosity cannot be increased.

[0021] WO 00 / 13768 A1 discloses a method for producing a microporous glass membrane, in which a coating consisting of a suspension of glass particles is applied to an inorganic porous support element, which is converted into a continuous glass phase upon heating. The porous support element is subsequently at least partially removed by treatment with acid. Porous polymer membranes or production methods therefor are not described.

[0022] JP 2011-101837 A, Example 1, describes the preparation of a membrane for separating oil and water. First, a self-supporting polyethylene terephthalate (PET) sheet is coated with a PVA solution, heated, and dried. The dried sheet is subsequently coated with a cellulose acetate casting solution, heated, and dried again. The resulting PET-PVA-cellulose acetate composite structure is then bonded to a fabric support by heat, and the polyvinyl alcohol surrogate layer is removed by treatment with water.

[0023] DE 103 26 741 A1 discloses an asymmetric cellulose membrane with a porous upper layer, a porous lower layer, and a middle layer with two isotropic regions located therebetween. The first isotropic region, adjacent to the upper layer, has smaller pores than the upper layer, but larger pores than the second isotropic region adjacent to the lower layer. The pores of the second isotropic region are smaller than the pores of the lower layer.

[0024] EP 2 134 455 B1 discloses a method for producing microporous asymmetric (cf. paragraph

[0061] ) polymer membranes, in which a casting solution containing a membrane-forming polymer (e.g. cellulose acetate, cf. paragraph

[0026] ) is cast onto a moving support simultaneously with a coagulant solution containing a non-solvent for the membrane-forming polymer (e.g. water or acetone, cf. paragraph

[0036] ) and a coating aid (e.g. polyethylene glycol or polyether), wherein a portion of the coagulant solution (cf. paragraphs

[0032] to

[0034] and

[0038] and claim 8) diffuses through the interface between the two layers into the casting solution and triggers a phase inversion with membrane formation (paragraph

[0052] and Figure 5 ). The process does not involve any final treatment of the multilayer composite with a precipitation bath (see paragraphs

[0009] and

[0045] ).

[0025] EP 1 509 314 B1 discloses a process for producing microporous asymmetric polymer membranes whose dense skin layer is made more open-pored by dissolving the uppermost polymer layer of the membrane (cf. paragraphs

[0009] ,

[0010] ,

[0012] ,

[0015] ,

[0017] ,

[0024] and

[0025] ), but does not contain any information on how the specific embodiments were obtained.

[0026] US 2015 / 0258499 A1 discloses an asymmetric ultrafiltration membrane with a skin layer, which is used for the removal of viruses from protein solutions. The membrane is a composite of a first micro- or ultrafilter layer and a second ultrafilter layer, wherein the composite is formed by the substantially simultaneous pouring ( co-casting ) of two or more polymer casting solutions onto a moving support. The two filter layers differ in their particle retention capacity.

[0027] EP 1 307 280 B1 discloses a co-casting manufacturing process for multilayer membranes, which are produced by applying several casting solution layers, each containing a membrane-forming polymer, to a moving support. Phase inversion results in a membrane whose individual layers each have a different pore size, with the pore size changing abruptly at the interface between adjacent layers—characterized by a "demarcation line." Membranes produced from only a single casting solution layer are also disclosed as comparative examples, although the structure of these membranes is not characterized (cf. paragraphs

[0072] and

[0088] in conjunction with

[0090] . Figure 9 ).

[0028] EP 2 604 329 A1 describes a membrane with localized asymmetries and a method for its production.

[0029] The present invention is therefore based on the object of providing a method for producing a single-layer polymer membrane with high surface porosity and a small difference between surface and total porosity.

[0030] This object is achieved by the embodiments characterized in the claims.

[0031] In particular, the present invention relates to a process for producing a porous single-layer polymer membrane, which comprises the following steps: (A) Providing a membrane-forming casting solution comprising a membrane-forming polymer and a solvent therefor; (B) Providing a non-membrane-forming and non-precipitating protective solution; (C) Providing a support; (D) Applying at least the casting solution and the protective solution to the support to form a film comprising a casting solution layer and an adjacent protective solution layer; (E) Contacting the film with a precipitating agent; and (F) Removing the protective solution layer, wherein in step (D) the film is formed by applying the casting solution to the support, thereby forming the casting solution layer, and applying the protective solution to the casting solution layer, thereby forming the protective solution layer, or applying the protective solution to the support, thereby forming the protective solution layer, and applying the casting solution to the protective solution layer, thereby forming the casting solution layer, and then applying another protective solution to the casting solution layer, thereby forming another protective solution layer, and wherein the protective solution comprises 2-pyrrolidone or a non-membrane-forming polymer (surrogate polymer) selected from the group consisting of polyvinylpyrrolidone, polyethylene glycol, polysaccharides and / or polyvinyl alcohol, and a solvent therefor.

[0032] A porous single-layer polymer membrane is obtainable from the process according to the invention. This differs from polymer membranes obtained by co-casting processes in that the porous polymer membrane obtained by the manufacturing process according to the invention has an integral polymer layer instead of multiple (e.g., two) distinct layers. Furthermore, due to the process, a high surface porosity of 40% or more and a total porosity that is 0.8 to 1.4 times the surface porosity can be achieved.

[0033] The surface porosity of a membrane surface is defined as the ratio of the pore area to the total membrane area directly at the respective membrane surface. According to the invention, the surface porosity can be determined as described below.

[0034] The basis is an SEM image of the respective surface. The resulting image is first binarized using computer-assisted analysis to differentiate between membrane body (bulk material) and pores. The Otsu method is preferably used (Nobuyuki Otsu (1979). "A threshold selection method from gray-level histograms". IEEE Trans. Sys., Man., Cyber. 9 (1): 62-66). The binarized image is then evaluated for its respective surface fractions to obtain a value for the surface porosity.

[0035] The special structural features of the single-layer polymer membrane obtained by the inventive manufacturing process can be achieved through the presence of the protective solution layer. The protective solution layer influences the mass transfer of the casting solution layer with the environment, as well as the interfacial tension between the contacting phases, which has a significant impact on the precipitation of the membrane or the phase inversion / phase separation in step (E). In conventional manufacturing processes, phase separation is influenced solely by the ambient conditions (e.g., composition of the atmosphere or the precipitation bath, pressure, and temperature). The presence of the protective solution layer provides an additional possibility for influencing phase separation, allowing the production of previously inaccessible polymer membranes.

[0036] The membrane-forming casting solution is not subject to any particular restriction. According to the present invention, any solution of a polymer suitable for membrane formation can be used. The casting solution is a homogeneous solution of the membrane-forming polymer which also contains a solvent for the polymer. Optionally, according to the present invention, one or more auxiliaries can be present in the casting solution. Suitable auxiliaries according to the invention are swelling agents, solubilizers, hydrophilizing agents, pore formers (porogens) and / or non-solvents for the membrane-forming polymer. Such auxiliaries are known to the person skilled in the art and are adapted to the membrane-forming polymer. The casting solution preferably consists of the membrane-forming polymer, the solvent and optionally one or more auxiliaries, preferably a non-solvent.

[0037] If the casting solution contains a non-solvent (or other precipitant) for the membrane-forming polymer, the non-solvent (or precipitant) is present at a maximum concentration that is insufficient to lead to precipitation of the membrane-forming polymer. The casting solution preferably consists of the solvent for the membrane-forming polymer, the membrane-forming polymer, and a non-solvent (mixture). Suitable non-solvents include, for example, water, glycerol, isopropanol, ethanol, and mixtures thereof.

[0038] Preferably, the membrane-forming polymer is selected from the group consisting of cellulose acetate, cellulose nitrate, polysulfone, polyvinylidene difluoride (PVDF), polyethersulfone, polyetheretherketone (PEEK), polyacrylonitrile, polymethyl methacrylate (PMMA), and mixtures thereof, with cellulose acetate being preferred. The membrane-forming polymer is particularly preferably a mixture of cellulose diacetate and cellulose triacetate.

[0039] The solvent for the membrane-forming polymer is a liquid capable of dissolving the membrane-forming polymer. It preferably has a solubility of at least 2 wt.%, particularly preferably at least 5 wt.%, for the membrane-forming polymer (or each of the membrane-forming polymers) under standard conditions (25°C, 1013 hPa). Suitable solvents are known to those skilled in the art. Suitable solvent / polymer combinations are, for example, dichloromethane and cellulose acetate, 2-pyrrolidone and polyethersulfone, acetone and cellulose acetate, N-methylpyrrolidone and polyethersulfone, dioxane and cellulose acetate, γ-butyrolactone and polyethersulfone, and mixtures thereof. Preferred solvents for cellulose acetate are dioxane and acetone, with a mixture of these being particularly preferred. A suitable solvent for polyethersulfone is N-methylpyrrolidone.

[0040] Based on his technical knowledge, the person skilled in the art is able to prepare a casting solution from the membrane-forming polymer and the solvent.

[0041] Step (A) preferably comprises the steps (A1) Initially introducing the solvent for the membrane-forming polymer; (A2) Adding the membrane-forming polymer to the solvent; (A3) Optionally adding the auxiliary agent(s), preferably a non-solvent for the membrane-forming polymer; and (A4) Stirring the mixture of the solvent, the polymer, and optionally the auxiliary agent(s) to form the homogeneous casting solution.

[0042] The protective solution provided in step (B) is non-precipitating. This means that it is formulated in such a way that, upon contact with the casting solution in step (D), it does not trigger precipitation (phase inversion or phase separation) of the casting solution layer. Furthermore, the protective solution is non-membrane-forming. This means that it is formulated in such a way that no membrane or membrane layer emerges from it.

[0043] According to the invention, a solution of a polymer incapable of membrane formation (surrogate polymer) and a solvent can be used. However, it is also possible for the protective solution to contain no surrogate polymer and / or smaller molecules to modify the viscosity or solution properties. A protective solution that does not contain a surrogate polymer comprises or consists of 2-pyrrolidone.

[0044] Preferably, the protective solution has a viscosity of at least 0.8 cP under normal conditions (25°C, 1013 hPa). It is also preferred that the viscosity of the protective solution under normal conditions is not more than 100,000 cP.

[0045] According to the invention, the protective solution may contain a membrane-forming polymer, but only in a concentration that is insufficient for the protective solution to form a membrane (layer). Preferably, a membrane-forming polymer is present in a proportion of no more than 5% by weight, preferably no more than 1% by weight. It is preferred that the protective solution not contain any membrane-forming polymer (i.e., polymer capable of membrane formation).

[0046] In step (B), a protective solution is preferably provided in a manner similar to step (A), with the proviso that the protective solution contains a polymer incapable of membrane formation (surrogate polymer). This means that, according to the invention, the protective solution can be prepared like a conventional casting solution for membrane formation, using a surrogate polymer incapable of membrane formation instead of a membrane-forming polymer.

[0047] The surrogate polymer is one or more polymers selected from the group consisting of polyvinylpyrrolidone, polyethylene glycol, polysaccharides, and / or polyvinyl alcohol. The content of the surrogate polymer in the protective solution is preferably at least 1 wt.%, more preferably at least 5 wt.%, and especially preferably at least 20 wt.%. It is preferred that the content of the surrogate polymer is not more than 80 wt.%, more preferably not more than 70 wt.%, and especially preferably not more than 60 wt.%.

[0048] According to the invention, suitable solvents for preparing the preferred protective solution containing the surrogate polymer are all solvents capable of dissolving the surrogate polymer and which do not act as precipitants for the membrane-forming polymer. Solvent mixtures can also be used as solvents for the surrogate polymer. Suitable pairings of surrogate polymer and solvent for this purpose according to the invention include, for example, polyethylene glycol (PEG) and N-methylpyrrolidone (NMP), polyvinylpyrrolidone (PVP), dioxane, PVP, and acetone.

[0049] The protective solution may contain the same excipients as the casting solution, including a non-solvent for the membrane-forming polymer, provided that the concentration does not lead to precipitation at the interface between the casting solution and the protective solution.

[0050] According to the invention, the protective solution may contain a precipitant (especially a non-solvent) or a precipitant mixture for the membrane-forming polymer, but only in a concentration that is insufficient to lead to precipitation of the membrane-forming polymer upon contact of the protective solution with the casting solution. Suitable non-solvents for the protective solution include, for example, water, glycerol, isopropanol, or ethanol.

[0051] According to a preferred embodiment of the present invention, the protective solution does not contain a precipitant for the membrane-forming polymer.

[0052] A solvent or solvent mixture different from the solvent of the casting solution can be used. Preferably, the same solvent (mixture) is used for the protective solution as for the casting solution. Preferred solvents for the protective layer are dioxane and acetone, with a mixture of these being particularly preferred.

[0053] Particularly preferably, the protective solution and the protective solution layer formed therefrom consist of the surrogate polymer, the solvent therefor and a non-solvent.

[0054] Step (B) preferably comprises the steps (B1) introducing the solvent for the surrogate polymer; (B2) adding the surrogate polymer to the solvent for the surrogate polymer; (B3) optionally adding the excipient(s), and (B4) stirring the mixture of the solvent for the surrogate polymer and the surrogate polymer to form the protective solution.

[0055] The auxiliary agent optionally used in step (B3) may, for example, be a non-solvent for the membrane-forming polymer.

[0056] The support provided in step (C) is not subject to any particular restriction. Any support suitable for prior art membrane production processes can be used in accordance with the invention. The support preferably has a flat surface and is inert to the substances used (casting solution, protective solution, their components, etc.). A moving belt (conveyor belt) or a drum preferably serves as the support, which allows for continuous operation.

[0057] In step (D), at least the casting solution and the protective solution are applied to the carrier. This forms a film comprising a casting solution layer and a protective solution layer, with the casting solution layer adjacent to the protective solution layer. According to the invention, the casting solution layer directly adjoins the protective solution layer, i.e., there are no further layers in between. The casting solution layer and the protective solution layer have a common interface.

[0058] According to the invention, the time period between the application of the casting solution layer and the application of the protective solution layer is not limited. However, to prevent undesirable premature precipitation / phase inversion, it is advantageous to keep this time period as short as possible. This time period is preferably no more than 5 minutes, more preferably no more than 1 minute, and most preferably no more than 30 seconds.

[0059] Furthermore, it is advantageous if step (E) follows directly after step (D), i.e., no further step is performed between steps (E) and (D). In particular, it is advantageous not to perform a drying step between steps (E) and (D).

[0060] Apart from the casting solution provided in step (A), no other casting solution suitable for forming a polymer membrane is used in the process according to the invention. This means that the casting solution provided in step (A) is the only casting solution for membrane formation. The membrane is formed solely from this single casting solution. The film formed in step (D) thus contains only one casting solution.

[0061] However, it is possible to apply one or more additional protective solutions in addition to the casting solution and the protective solution, thereby obtaining a film with a casting solution layer, a protective solution layer, and further protective solution layer(s). Preferably, the film consists of the casting solution layer and the protective solution layer, so that no further layers or films are present on the support. This means that the support is preferably coated only with the film comprising the casting solution layer and the protective solution layer.

[0062] According to a preferred embodiment, in step (D), the film is formed by applying the casting solution to the support, thereby forming the casting solution layer, and applying the protective solution to the casting solution layer, thereby forming the protective solution layer. According to an alternative embodiment, in step (D), the protective solution is applied to the support, thereby forming the protective solution layer, and the casting solution is applied to the protective solution layer, thereby forming the casting solution layer, and then another protective solution is applied to the casting solution layer, thereby forming another protective solution layer. That is, it is possible for the casting solution layer to be located between the protective solution layer and the support. In this case, the protective solution layer is present as an overlayer.Alternatively, the protective solution layer can be located between the casting solution layer and the support. In this case, a protective solution layer is present as an overlayer and another protective solution layer is present as a underlayer. Preferably, the film formed in step (D) consists of the casting solution layer and the protective solution layer, with the casting solution layer located between the support and the protective solution layer (the protective solution is present as an overlayer).

[0063] The application of the casting and protective solution(s) in step (D) is not subject to any particular restriction and can be carried out, for example, by doctor blade or by means of a nozzle.

[0064] The application of the casting and protective solution(s) in step (D) can be carried out successively (sequentially, as described for example in EP 0 689 863 B1) or substantially simultaneously, as described for example in US 2015 / 0258499 A1.

[0065] As already mentioned above, the protective solution layer influences phase separation. In particular, surprisingly, regardless of whether the protective solution layer is present as an underlayer and / or overlayer, a significantly increased surface porosity is obtained on that main surface of the membrane that shares an interface with a protective solution layer compared to conventional processes. Furthermore, the membranes (obtained from the process according to the invention) have a total porosity that approximately corresponds to the surface porosity of that main surface(s) of the membrane that forms a common interface with the protective solution layer(s) during the process.

[0066] Presumably, the above surprising effects of the protective solution layer can be explained by two mechanisms.

[0067] Firstly, if the protective solution layer is present as an overlayer in the casting solution layer, no phase separation initially occurs. Unlike with conventional processes, there is no direct contact with a layer containing a precipitant in a concentration that would trigger phase separation. In the case of the overlayer according to the invention, the phase separation of the casting solution layer is diffusion-limited: The precipitant must first diffuse through the protective layer and accumulate in the casting solution layer before precipitation is triggered. As a result, it is surprisingly observed that the gel formation on the precipitant side observed above for conventional processes does not occur. Thus, the production process according to the invention does not produce a skin layer ( "skin layer" ) with small pores and low surface porosity, but rather an open-pore structure with high surface porosity is formed.

[0068] The second mechanism explains in particular the remarkable fact that, compared to a membrane produced with direct contact to the support, a higher support-side surface porosity is observed when the protective layer solution is present as an underlayer and the casting solution layer comes into direct contact with the precipitant in step (E) (not according to the invention). (However, this effect also comes into play, at least in part, when the protective solution layer is present as an overlayer (according to the invention)) If the casting solution is applied directly to the support, a precisely defined phase boundary forms between the solid support and the casting solution layer. In contrast, the various components of the underlayered protective solution layer, on the one hand, and the casting solution layer, on the other hand, can partially diffuse into the other layer.According to this explanatory model, the casting solution layer is at least partially seeded or impregnated with components of the protective solution layer through diffusive mass transfer. The temporary impregnation influences phase inversion for the duration of membrane production in such a way that, in the case of the underlayer, comparable effects are observed as in the case of the overlayer. Furthermore, the underlayer influences the interfacial tension of the membrane on the support side, which reduces coalescence during membrane formation. In this way, nascently formed structures are more strongly preserved, which also leads to higher surface porosity. The structure of the membrane is not disturbed or altered, since the protective solution layer is completely removed in step (F).However, overlaying the casting solution with the protective layer is preferred over underlaying it, because in the case of overlaying the above effects (in particular high surface porosity and suppression of the formation of a skin layer) are more pronounced.

[0069] The precipitant used in step (E) is not subject to any particular restriction. The same precipitants as in conventional manufacturing processes for polymer membranes can be used in the invention. The precipitant leads to precipitation (phase inversion) of the membrane-forming polymer in the casting solution layer. The precipitant can be gaseous or liquid, or present as a dissolved solid. The precipitant can be a single compound or a mixture of several compounds. Preferably, the precipitant is contained in a liquid; more preferably, the precipitant itself is a liquid. Step (E) is preferably carried out by immersing the film in a liquid containing or consisting of the precipitant.

[0070] The precipitant is preferably a non-solvent for the membrane-forming polymer or each of the membrane-forming polymers. The non-solvent is a liquid that is not capable of dissolving the membrane-forming polymer. The non-solvent for the membrane-forming polymer (or each of the membrane-forming polymers) preferably has a solubility of (in each case) at most 1 wt. %, particularly preferably at most 0.1 wt. % under standard conditions. A preferred precipitant is also a solvent for the surrogate polymer, with water being particularly preferred. If the precipitant is also a solvent for the surrogate polymer, the protective solution layer can be removed in a one-step process.

[0071] The removal of the protective solution layer(s) in step (F) is not subject to any particular restriction. According to the invention, it is possible to remove the protective solution layer(s) mechanically. Preferably, the protective layer is removed by applying a solvent for the surrogate polymer, wherein the solvent for the surrogate polymer is a non-solvent for the membrane-forming polymer. Step (F) is preferably carried out by immersing the film in a liquid containing or consisting of the solvent for the surrogate polymer. Preferably, water is used as the solvent for the surrogate polymer.

[0072] Alternatively, the surrogate polymer can be chemically degraded by one or more agents. The agent(s) selectively degrade the surrogate polymer into low-molecular-weight soluble components, while the membrane-forming polymer is inert to chemical degradation by the agent(s).

[0073] Subsequently, the single-layer membrane obtained from the casting solution layer can be subjected to one or more washing steps and / or drying steps.

[0074] In another aspect, a porous single-layer polymer membrane without a skin layer is described, wherein at least one of the main surfaces of the polymer membrane has a surface porosity of at least 40% and the total porosity of the polymer membrane is 0.8 to 1.4 times the at least 40% surface porosity. Such a membrane can be obtained by the manufacturing process according to the invention.

[0075] "Principal surfaces" are the two membrane surfaces with the largest surface area.

[0076] As already mentioned above, the single-layer membrane differs from polymer membranes obtained by co-casting processes in that the porous polymer membrane obtained by the manufacturing process according to the invention has an integral polymer layer instead of several (e.g., two) distinct layers. According to the prior art, the individual layers are separated from one another by a sudden or discontinuous change in pore size or in other properties (density, porosity, membrane polymer) at the interface between the individual layers, e.g., in the form of the demarcation line between adjacent polymer layers known from EP 1 307 280 B1.

[0077] At least one main surface of the membrane obtained by the manufacturing process according to the invention has a surface porosity of at least 40%, preferably at least 50%, more preferably at least 60%, especially preferably at least 70%, most preferably at least 80%. Preferably, both main surfaces independently have the above surface porosities.

[0078] It is sufficient if one of the main surfaces of the membrane has a surface porosity of at least 40%. This means that one of the main surfaces of the membrane can have a surface porosity of less than 40%. It is preferred that at least the main surface facing the precipitant has a surface porosity of at least 40%. Preferably, both main surfaces of the membrane have a surface porosity of at least 40%.

[0079] The membrane has no skin layer (" skin layer"), which is associated with a high flow rate. The presence of a skin layer on a main surface can be excluded in particular if its average pore size is at least 10 nm, preferably at least 15 nm. With such an average pore size, porous structures can still be seen in a scanning electron micrograph of the main surface in question at 20,000x magnification. To determine the average pore size of the main surface, the above-described method of liquid-liquid displacement porosimetry based on two immiscible liquids can be used.

[0080] The total porosity of the membrane obtained by the production process according to the invention lies in the range of the surface porosity of the main surface with higher surface porosity (more porous main surface) and is 0.8 times to 1.4 times the at least 40% surface porosity. If both main surfaces of the membrane have a surface porosity of at least 40%, the total porosity is 0.8 times to 1.4 times the higher surface porosity. Preferably, the total porosity is at least 0.9 times, preferably at least 0.95 times, particularly preferably at least 0.99 times the surface porosity of the more porous main surface. Preferably, the total porosity is at most 1.3 times, preferably at most 1.2 times, particularly preferably at most 1.1 times the surface porosity of the more porous main surface.The membrane obtained by the production process according to the invention has a total porosity of at least 32%, preferably at least 40%, particularly preferably at least 50%, even more preferably at least 60%, even more preferably at least 70% and most preferably at least 80%.

[0081] High porosity means that a high proportion of the membrane volume is occupied by pores. Therefore, higher porosity is associated with higher efficiency (separation or filtration performance) for the same membrane volume and pore size.

[0082] The thickness of the membrane obtained by the manufacturing process according to the invention is fundamentally unrestricted. The thickness is preferably at most 400 µm, more preferably at most 300 µm, and particularly preferably at most 250 µm.

[0083] The single-layer porous polymer membrane obtained by the manufacturing method according to the invention can be either symmetrical or asymmetrical. According to a preferred embodiment, the membrane is asymmetrical, i.e., it has an asymmetry factor of at least 1.5. The asymmetry factor is preferably at least 2.5. The upper limit of the asymmetry factor of the membrane is not limited and is preferably 10. That is, the polymer membrane preferably has an asymmetry factor of 1.5 to 10.

[0084] The asymmetry factor of an asymmetric membrane or membrane layer is the ratio of the maximum pore size to the minimum pore size of the membrane or membrane layer, whereby the maximum or minimum pore size of a membrane (layer) is based on the entire body of the membrane (layer). The integral pore size distribution can be determined analogously to the determination of surface porosity. An SEM image of the membrane cross-section (perpendicular to the main membrane surfaces) is taken with sufficient resolution. The resulting image is first binarized using computer-assisted analysis to differentiate between the membrane body and the pore. The Otsu method mentioned above is preferably used (Nobuyuki Otsu (1979), "A threshold selection method from gray-level histograms", IEEE Trans. Sys., Man., Cyber. 9 (1), 62-66).In the next step, the binarized image is evaluated for the respective distances between the membrane body and the pore across the entire membrane body to obtain an integral pore size distribution. The asymmetry factor can be calculated from the respective minimum and maximum values.

[0085] The average pore size of the polymer membrane obtained by the production process according to the invention is not subject to any particular restriction. For example, the membrane can be a micro-, ultra-, nano-, or reverse osmosis membrane. The membrane is preferably a microfiltration membrane or ultrafiltration membrane, particularly preferably an ultrafiltration membrane.

[0086] According to a preferred embodiment, the membrane obtained by the manufacturing process according to the invention is composed of a membrane-forming polymer selected from the group consisting of cellulose acetate, cellulose nitrate, polysulfone, polyvinylidene difluoride (PVDF), polyethersulfone, polyetheretherketone (PEEK), polyacrylonitrile, polymethyl methacrylate (PMMA), and mixtures thereof, with cellulose acetate being preferred. A mixture of cellulose diacetate and cellulose triacetate is particularly preferred.

[0087] There are no particular restrictions regarding the possible use of the membrane obtained by the production process according to the invention. It can be used for filtration, in particular for the filtration of viruses, proteins, or macromolecules. Figure 1shows an SEM image of the side (main surface) of the membrane from Example 1, formed from a casting solution layer adjacent to an overlay solution (air side) during fabrication. A porous structure extending into the depth of the membrane is clearly visible. Figure 2 shows an SEM image of the support-side (glass plate-side) main surface of the membrane from Example 1 and shows a typical pore structure. The structure is largely unaffected by the overlayer (in contrast to the overlayer-side main surface) and shows a very similar structure to the support-side main surface of the membrane from Comparative Example 1. Figure 3 shows an SEM image of the air-side (the air side is the side opposite the support side) main surface of the membrane from Comparative Example 1. No pores are visible at 10,000 times magnification. Figure 4shows an SEM image of the carrier-side main surface of the membrane from Comparative Example 1. No significant differences can be seen compared to the tape-side surface structure of the membrane from Example 1. Figure 5 shows an SEM image of the carrier-side main surface of the membrane from Comparative Example 2. A pronounced pore structure can be seen. Figure 6 shows an SEM image of the overlayer-side main surface of the membrane from Comparative Example 2. A skin layer without any recognizable pores on the surface is clearly visible. Figure 7 shows an SEM image of the membrane from Example 2. The membrane side that was in contact with the protective solution (surrogate solution 1) (support side) is visible. It is clearly visible that the support-side main surface of the membrane exhibits high surface porosity. Figure 8shows an SEM image of the membrane from Comparative Example 3. The membrane side that was in contact with the glass surface (support side) can be seen and has a low surface porosity compared to Example 2. Figure 9 shows an SEM image of the air side of the membrane from Example 3. The globular structure, high surface porosity and the absence of a skin layer are clearly visible. Figure 10 shows an SEM image of the air side of the membrane from Comparative Example 4. The absence of porous structures and the presence of a skin layer are clearly visible. Figure 11 shows an SEM image of the membrane from Example 2. The membrane side that was in contact with protective solution 2 (surrogate solution 2) (air side) is visible. The globular structure, high surface porosity, and the absence of a skin layer are clearly visible. Figure 12shows an SEM image of the membrane from comparative example 3. The membrane side that was not in contact with the glass surface (air side) can be seen.

[0088] The present invention is further illustrated by the following non-limiting examples. Examples

[0089] Example 1: Production of an asymmetric microfiltration membrane from cellulose acetate using the precipitation bath process Casting solution

[0090] The casting solution was prepared from the components listed in Table 1 in a stirred reactor at 40 °C. Acetone and dioxane were added as initial components, and the solids were added in powder form. Once the polymer solution was no longer turbid (after approximately 3 hours), formamide was added. The casting solution in this composition is stable for approximately 48 hours at room temperature. Table 1: Composition of the casting solution Components Mass fraction [%] Cellulose triacetate (Eastman, CA 398) (12.8%) 12,8 Cellulose diacetate (Acetati, Aceplast PC / FG) (3.2%), 3,2 acetone 28 Dioxane 28 Formamide 28 Protection solution

[0091] The protective solution (overlay) was prepared in the same way as the casting solution. The composition of the protective solution is given in Table 2. Table 2: Composition of the protective solution Components Mass fraction [%] Polyvinylpyrrolidone (PVP, M w =360,000 g / mol) of type “K90” from BASF 10 acetone 30 Dioxane 30 Formamide 30

[0092] Unless otherwise stated, the protective and casting solutions of the following examples were prepared as described in Example 1. Membrane production

[0093] The coating solution with the composition shown in Table 1 was evenly spread onto the glass plate with a doctor blade to a thickness of 300 µm in a dry (anhydrous) nitrogen atmosphere at a temperature of 22 °C on a glass plate used as a support. The overcoat was then doctored over the coating solution film to a thickness of 200 µm. The application process was completed in less than 30 seconds.

[0094] A water bath at 22°C was set up in an air atmosphere at 22°C and <40% relative humidity, which could completely cover the glass plate. The coated layers on the glass plate were completely immersed in the water bath from one side of the glass plate in less than 20 seconds and shaken for 30 minutes. The water was replaced after these 30 minutes, and the glass plate with the now-precipitated membrane was shaken again for 30 minutes. After complete removal of the overlayer, the membrane was lifted from the glass plate and dried on a nonwoven support at 22°C. The following characterizations were performed on the dry membrane: scanning electron microscopy (SEM) and flow-through.

[0095] SEM images of the main surfaces of the membrane from Example 1 are shown in Figures 1 and 2 shown. From Figure 1 An open-pored structure with high porosity can clearly be seen.

[0096] The permeability was 0.89±0.1 mL / (min·cm 2< ·bar) Comparison example 1

[0097] For Comparative Example 1, a casting solution identical to the casting solution of Example 1 was used. The protective solution was omitted. Membrane production

[0098] The coating solution with the composition shown in Table 1 was evenly spread onto the glass plate with a doctor blade to a thickness of 300 µm in a dry (anhydrous) nitrogen atmosphere at a temperature of 22 °C on a glass plate used as a support. No protective solution was applied. The coating process was completed in less than 30 seconds.

[0099] A water bath at 22°C was set up in an air atmosphere at 22°C and <40% relative humidity, completely covering the glass plate. The coated layer on the glass plate was completely immersed in the water bath from one side of the glass plate in less than 20 seconds and shaken for 30 minutes. The water was replaced after 30 minutes, and the glass plate, with the now-precipitated membrane, was shaken again for 30 minutes. The membrane was then lifted from the glass plate and dried on a nonwoven support at 22°C. The following characterizations were performed on the dry membrane: scanning electron microscopy (SEM) and flow-through.

[0100] SEM images of the main surfaces of the membrane from Comparative Example 1 are shown in Figures 3 and 4 shown. From Figure 3 a dense structure with low porosity (skin layer) is clearly visible. The permeability was less than 0.1 mL / (min·cm 2< ·bar) Comparative example 2: Example 3 of EP 2 134 455 B1

[0101] Comparative Example 2 is based on Example 3 of EP 2 134 455 B1. A protective solution containing a precipitant at a concentration that leads to precipitation of the membrane-forming polymer upon contact with the casting solution was used for the overcoating. Otherwise, Comparative Example 2 was carried out as in Example 1. Table 3: Composition of the casting solution Components Mass fraction [%] Polyethersulfone (PESU) of the type "Ultrason E6020" from BASF 13 PEG 400 70 N-Methylpyrrolidone (NMP) 30 Table 4: Composition of the overlay solution Components Mass fraction [%] Polyethylene glycol 400 (PEG 400) 80 Water 20

[0102] SEM images of the main surfaces of the membrane from Comparative Example 1 are shown in Figures 5 and 6 shown. From Figure 6 a dense structure with low porosity (skin layer) is clearly visible. Example 2 : Underclass and overclass

[0103] The compositions of the casting solution and protective solutions used in this example are given in Tables 5, 6 and 7. Table 5: Composition of the casting solution Components Mass fraction [%] Polyethersulfone of the type "Ultrason E6020" from BASF 12,00 % 2-Pyrrolidone 77,00 % PVP-VA copolymer of the type "PVP S630" from Ashland 3,00 % Glycerin 5,00 % Water 3,00 % Table 6: Composition of protective solution 1 (undercoating solution) Components Mass fraction [%] 2-Pyrrolidone 70 % Polyethylene glycol (Mw = 30000 g / mol) 30 % Table 7: Composition of protective solution 2 (overcoating solution) Components Mass fraction [%] 2-Pyrrolidone 100 Membrane production

[0104] Protective solution 1 (surrogate solution 1) was heated to 60 °C and evenly spread onto a glass plate with a doctor blade to a thickness of 100 µm under an air atmosphere at 22 °C and <40% relative humidity. The applied protective solution and the glass plate were then cooled to room temperature.

[0105] The coating solution was then evenly applied to surrogate solution 1 using a squeegee with a thickness of 200 µm. Protective solution 2 (surrogate solution 2) was then applied with a squeegee with a thickness of 100 µm.

[0106] A water bath was set up at 22°C, capable of completely covering the glass plate. The glass plate with the film, consisting of spread-out casting solution and surrogate solutions, was completely immersed into the water bath from one side of the glass plate in less than 20 seconds and shaken for 30 minutes. After these 30 minutes, the water was replaced, and the glass plate with the now-precipitated membrane was shaken again for 30 minutes, during which the protective solution layers dissolved, leaving only the membrane on the glass plate. The membrane was then lifted from the glass plate and dried on a nonwoven support at 22°C. Characterizations were performed on the dry membrane.

[0107] The result is in Figures 7 and 11 shown. The Figures 7 and 11The SEM images shown were obtained under the following conditions: imaging device: FEI Quants 200 F; accelerating voltage: 19 kV; magnification: 4000x. Comparison example 3

[0108] The composition of the casting solution used in this example is given in Table 8. Table 8: Casting solution Components Mass fraction [%] Polyethersulfone of the type "Ultrason E6020" from BASF 12,00 2-Pyrrolidone 77,00 PVP-VA copolymer of the type "PVP S630" from Ashland 3,00 Glycerin 5,00 Water 3,00 Membrane production

[0109] The casting solution was evenly applied to a glass plate using a 250 µm thick doctor blade and exposed to air for 3 minutes. A water bath was set up at 22 °C, which could completely cover the glass plate. The coated casting solution on the glass plate was completely immersed in the water bath from one side of the glass plate in less than 20 seconds and shaken for 30 minutes. The water was replaced after these 30 minutes, and the glass plate, with the now-precipitated membrane, was shaken again for 30 minutes. The membrane was then lifted from the glass plate and dried on a nonwoven support at 22 °C. Characterizations were performed on the dry membrane.

[0110] The result is in Figure 8 and Figure 12 and shown in Table 9. Table 9: Comparison of Example 2 with Comparative Example 3 Example 2 Comparison example 3 Surface porosity (air side) 40% 0% Surface porosity (belt or carrier side) 63% 22% Example 3

[0111] The compositions of the casting solution and protective solution used in this example are given in Tables 10 and 11. Table 10: Composition of the casting solution Components Mass fraction [%] Polyethersulfone of the type "Ultrason E6020" from BASF 12,00 2-Pyrrolidone 77,00 PVP-VA copolymer of the type "PVP S630" from Ashland 3,00 Glycerin 5,00 Water 3,00 Table 11: Composition of the protective solution Components Mass fraction [%] 2-Pyrrolidone 100 Membrane production

[0112] A 250 µm thick coating solution was evenly applied to a glass plate using a doctor blade, and immediately followed by a 50 µm thick protective solution coating using a doctor blade. A water bath was set up at 22 °C to completely cover the glass plate. The glass plate with the coated coating solution and protective solution was completely immersed in the water bath from one side of the glass plate in less than 20 seconds and shaken for 30 minutes. The water was replaced after these 30 minutes, and the glass plate with the now-precipitated membrane was shaken again for 30 minutes. After complete removal of the overcoat / protective solution layer, the membrane was lifted from the glass plate and dried on a nonwoven support at 22 °C. Characterizations were performed on the dry membrane.

[0113] The results are in Figure 9 and shown in Tables 13 and 14. Comparison example 4

[0114] The composition of the casting solution used in this example is given in Table 12. Table 12 Composition of the casting solution Components Mass fraction [%] Polyethersulfone of the type "Ultrason E6020" from BASF 12,00 2-Pyrrolidone 77,00 PVP-VA copolymer of the type "PVP S630" from Ashland 3,00 Glycerin 5,00 Water 3,00 Membrane production

[0115] A 250 µm thick coating solution was evenly applied to a glass plate using a doctor blade. A water bath was set up at 22 °C, sufficient to completely cover the glass plate. The coating solution spread on the glass plate was completely immersed in the water bath from one side of the glass plate in less than 20 seconds and shaken for 30 minutes. After these 30 minutes, the water was replaced, and the glass plate, with the now-precipitated membrane, was shaken again for 30 minutes. The membrane was then removed from the glass plate and dried on a nonwoven support at 22 °C. Characterizations were performed on the dry membrane.

[0116] The results are in Figure 10 and shown in Tables 13 and 14. Table 13 Example 3 Comparison example 4 Surface porosity (air side) 42% 0% Surface porosity (belt side) 21% 20% Table 14 Example 3 Comparison example 4 Permeability [mL / (min·cm 2 < ·bar)] 9 ± 3 < 1

[0117] As can be seen from Table 13, Example 3 has a significantly higher permeability, and thus a higher flow rate, than Comparative Example 4, mainly due to the higher surface porosity of the air side.

[0118] To determine the permeability of the prepared membranes, they were wetted in NaCl solution (0.9 wt%) and then transferred to a permeability measuring cell. This consisted of eight identical metal cylinders (steel pressure vessel, 200 mL capacity, Sartorius Stedim Biotech GmbH). The measuring cells could be filled with a defined volume of saline solution. A pressure of 0.1 to 1 bar (depending on permeability) was then applied, and the bottom valve was opened. The permeate was collected and weighed with time resolution. The calculation of the specific permeability is as follows: χ spez = V p t ⋅ A a ⋅ p

[0119] This is V p the permeate volume, tthe time required to convert this permeate volume V p to catch, A a the membrane flow area and p the applied pressure.

[0120] The process according to the invention makes it possible to produce single-layer porous polymer membranes with high surface and total porosity, thus resulting in thin membranes with high filtration performance. The process according to the invention opens up a novel control option for influencing phase inversion during membrane formation from a casting solution, making the process less sensitive to fluctuations in parameters such as pressure, temperature, and precipitant concentration during phase inversion.

Claims

1. Method for producing a porous monolayer polymer membrane, which comprises the following steps: (A) providing a membrane-forming casting solution which comprises a membrane-forming polymer and a solvent therefor; (B) providing a non-membrane-forming and nonprecipitating protective solution; (C) providing a support; (D) applying at least the casting solution and the protective solution to the support to form a film comprising a casting solution layer and a protective solution layer bordering thereon; (E) contacting the film with a precipitant; and (F) removing the protective solution layer, wherein in step (D) the film is formed by applying the casting solution to the support to form the casting solution layer, and applying the protective solution to the casting solution layer to form the protective solution layer, or applying the protective solution to the support to form the protective solution layer, and applying the casting solution to the protective solution layer to form the casting solution layer, and subsequently applying a further protective solution to the casting solution layer to form a further protective solution layer, and wherein the protective solution comprises 2-pyrrolidone or a polymer not capable of forming a membrane (surrogate polymer), selected from the group consisting of polyvinylpyrrolidone, polyethylene glycol, polysaccharides and / or polyvinyl alcohol, and a solvent therefor.

2. Method according to claim 1, wherein the protective solution comprises the polymer not capable of forming a membrane (surrogate polymer) and the solvent therefor.

3. Method according to claim 1 or 2, wherein the protective solution contains no precipitant for the membrane-forming polymer.