Solution of polyarylsulfone polymer in N-(2 '-ethoxyl)-2-pyrrolidone for membrane preparation and application of polyarylsulfone polymer in N-(2'-ethoxyl)-2-pyrrolidone

Abstract

The invention relates to a solution comprising a polyarylsulfone polymer selected from the group of polyethersulfones and polysulfones and N-(2 '-hydroxyethyl)-2-pyrrolidone, to a method for producing a membrane thereof and to the use of the membrane for a separation process.

Description

[0001] The present invention relates to a solution comprising a polyarylsulfone polymer selected from the group of polyethersulfone and polysulfone and N-(2'-hydroxyethyl)-2-pyrrolidone, a method for producing a membrane thereof and the use of the membrane in separation processes.

[0002] Polyarylsulfone polymers, such as polysulfone (sold particularly under the trade names: S. ) and polyethersulfone (especially those sold under the trade names: E. ) are high-performance polymers which are used in a variety of technical applications due to their mechanical properties as well as their chemical and thermal stability. One major technical application of polyarylsulfone polymers is their use as a raw material for the production of polymer membranes, such as dialysis membranes or ultrafiltration membranes. Polymer membranes are used in separation processes and are widely used, for example, in medical applications, food technology, biotechnology, the pharmaceutical industry and water treatment.

[0003] In JG Wijmans et al., Eur. Polym. J., 1983, Vol. 19, No. 12, pp. 1143-1146, it is pointed out that polyarylsulfone polymers have limited solubility in many common solvents. In particular, concentrated solutions of polysulfone in various solvents tend to become turbid and precipitate.

[0004] WO 2019042749 A1 discloses a method for producing a membrane by contacting a polymer solution comprising a polymer, a first solvent and a cosolvent with a coagulant. As the first solvent, N-methylpyrrolidone and N-ethylpyrrolidone are claimed. Polyvinylpyrrolidone is used as a water-soluble polymer. In addition, a membrane obtained by this method is disclosed. The post-treatment of the membrane includes an oxidation step with sodium hypochlorite. In addition, the production of a membrane from a polymer solution is described in WO 2015056145 A1.

[0005] WO 2017220386 A1 discloses the use of a solution of polysulfone in N-acyl-morpholine for making an ultrafiltration membrane. Polyvinyl pyrrolidone is used as a water-soluble polymer. In the absence of an added water-soluble polymer, the obtained membrane is not water permeable. The post-treatment of the membrane includes an oxidation step using sodium hypochlorite.

[0006] WO 2021191043 describes the use of N-n-butyl-2-pyrrolidone as an alternative solvent for different polymer classes, including polyarylsulfone polymers. The use of such solutions in combination with water-soluble polymers in membrane making is also described. Different polyvinylpyrrolidone grades are used as water-soluble polymers. Post-treatment of the membrane includes an oxidation step using sodium hypochlorite.

[0007] PCT / EP2021 / 082449 describes a solution comprising at least one sulfone polymer and N-tert-butyl-2-pyrrolidone as an alternative solvent. The use of such a solution in combination with a water-soluble polymer in membrane production is also described. A combination of different grades of polyvinylpyrrolidone is used as the water-soluble polymer. Post-treatment of the membrane includes an oxidation step using sodium hypochlorite.

[0008] In the above-mentioned prior art, an additional oxidation step using, for example, sodium hypochlorite is required during post-treatment of the membrane.

[0009] In S.Munari et al., Desalination, 1988, Vol. 70, pp. 265-275, a microporous polysulfone membrane is prepared from a casting solution of polysulfone, a solvent and polyvinyl pyrrolidone as an additive. N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, and N-methyl-2-pyrrolidone are mentioned as common solvents. It is pointed out that due to the low molecular weight of the polymer, the polysulfone solution is characterized by low viscosity and is therefore difficult to cast. In order to overcome this problem, it has become common practice to add high molecular weight polyvinyl pyrrolidone to the polysulfone solution. In the experiment, N,N-dimethylacetamide was used as a solvent, which is classified as a chemical of very high concern in the European Union. The post-treatment of the membrane consists of washing with water.

[0010] US 2005 / 0170183 A1 describes molded bodies, at the surface of which substituents characterized by different formulae are incorporated. These molded bodies contain polyarylethers, which can be polyarylsulfone polymers (e.g. polyethersulfone), among others. The production of molded bodies in the form of films or thin films, for example from solutions of polyethersulfone in N,N-dimethylacetamide or dimethylsulfoxide, is described. For the production of films, the use of poly(ethylene glycol) in solution is also mentioned.

[0011] The functionalization of the easily produced molded bodies (films or membranes) is carried out by treating the molded bodies with aqueous H2SO4 containing reagents and carbonyl compounds in a heterogeneous reaction. Among others, N-(2'-hydroxyethyl)-2-pyrrolidone is mentioned as an example of a reagent, i.e. N-(2'-hydroxyethyl)-2-pyrrolidone is not used as a solvent but as a reagent for modifying the undissolved molded body.

[0012] EP 3756753 A1 discloses a combination of pyrrolidone-based solvents as an alternative to conventionally used solvents for producing films. The combination comprises 2-pyrrolidone and N-alkyl-2-pyrrolidone, in particular N-n-butyl-2-pyrrolidone. As film-forming polymers, polyarylsulfone polymers are described, which are used in combination with a water-soluble polymer, the latter being a mixture of polyvinylpyrrolidone and poly(ethylene glycol).

[0013] In C. Kahrs et al., Polymer [polymer] 2020, Vol. 186, 122071, the formation of films by non-solvent-induced phase separation using different solvents is described. N-methylpyrrolidone and N,N-dimethylacetamide as conventional solvents are compared with 2-pyrrolidone and N,N-dimethyllactamide as alternative solvents. In all four solvent systems, polyvinylpyrrolidone and poly (ethylene glycol) of different concentrations and molecular weights are applied. The viscosity of the polymer solution and the film characteristics are discussed.

[0014] Generally, there is a continuous demand for solvents suitable for replacing the currently used solvents in specific applications. One demand is a toxicological profile (profile) that is less problematic than conventionally used solvents such as N-methyl-2-pyrrolidone. With regard to sulfone polymer solutions, these solvents should be able to obtain solutions with high sulfone polymer content without turbidity in addition. In addition, the solution further containing a water-soluble polymer should be stable and clear, because these factors affect the pore structure and therefore affect the quality of the film obtained by these solutions. Since the solvents conventionally used for film production produce low-viscosity polymer solutions that are difficult to cast, it is also necessary to produce a solvent system with a polymer solution of higher viscosity. In addition, it is required that the solvent used for the film production of the phase inversion induced by the non-solvent shows complete miscibility with the water used as a coagulant. With regard to the film obtained by using these polymer solutions, it is important to achieve at least the same film quality standards and possibly even better film quality. In particular, the water permeability of such films should be as high as possible, together with the few defects or large voids visible in the cross section of the film. Another requirement is acceptable mechanical stability and therefore the long life of the film.

[0015] An object of the present invention is to find a readily usable and obtainable solvent of low toxicological concern, which dissolves the polyarylsulfone polymers, produces a solution of low turbidity and high viscosity, and thus allows the direct preparation of membranes. Another object of the present invention is to provide a method for preparing membranes based on these solutions, which is as simple as possible, for example by avoiding lengthy and / or hazardous post-processing of the membranes. Another object of the present invention is to provide membranes that show improved membrane properties.

[0016] We have surprisingly found that the above objects can be achieved by a solution comprising:

[0017] (a) a polyarylsulfone polymer selected from polyethersulfones comprising repeating units of formula I

[0018]

[0019] and a polysulfone comprising a repeating unit of formula II

[0020]

[0021] or a mixture thereof, wherein the weight average molecular weight M of the polyarylsulfone polymer is w in the range of 40000 g / mol to 105000 g / mol, and the polyarylsulfone polymer comprises at least 95 wt.-% of repeating units of formulae I and II, based on the total weight of the polyarylsulfone polymer,

[0022] as well as

[0023] (b) N-(2'-Hydroxyethyl)-2-pyrrolidone.

[0024] Polyarylsulfone polymer

[0025] The polyarylsulfone polymer comprises at least 95 wt.-% (% / weight), preferably at least 97 wt.-%, more preferably at least 98 wt.-% and particularly preferably at least 99 wt.-% of repeating units of the formulae I and II, based on the total weight of the polyarylsulfone polymer.

[0026] The polyarylsulfone polymer is a polymer containing -S(=O)2- units in the polymer. The -S(=O)2- units are also called sulfonyl units.

[0027] The polyarylsulfone polymers contain aromatic groups. In addition to 1,4-phenylene units, the polyarylsulfone polymers may also contain units based on minor isomers, like 1,2-phenylene or 1,3-phenylene units.

[0028] The polyarylsulfone polymers contain terminal groups at the ends of the polymer chains, such as Cl-, OH-, MeO-, tert-butyl-, or aryl groups. Different chain ends may contain different terminal groups. The polyarylsulfone polymers typically contain 0.02 wt.-% to 1 wt.-% (% by weight), preferably 0.03 wt.-% to 0.8 wt.-% of terminal groups, based on the total weight of the polyarylsulfone polymer.

[0029] The polyarylsulfone polymer contains <5 wt.-%, preferably <3 wt.-%, more preferably <2 wt.-%, and particularly preferably <1 wt.-% of additional chemical groups based on the total weight of the polyarylsulfone polymer, which are not represented by the repeating units of formula I and II. These additional chemical groups are typically end groups or minor aromatic isomers as described above.

[0030] As examples of polyethersulfone polymers comprising repeating units of formula I, mention is made of ETHYLENE, available from BASF SE. E level, like E 2020P, E 3010, E 6020P or E-7020P.

[0031] As an example of a polysulfone polymer comprising repeating units of formula II, mention is made of the polysulfone polymer available from BASF S level, like S 3010 or S 6010.

[0032] The weight average molecular weight of the polymer is M w The summed mass based on molecules with a certain molecular mass. Larger (i.e., higher mass) molecules have a greater contribution to this value than smaller (i.e., lower mass) molecules. M w and their determination are well known in the art. w It can be determined, for example, by gel permeation chromatography (GPC) using a suitable column (stationary phase), solvent (liquid phase) and detector (e.g., via refractive index or UV). These chromatographic units must be calibrated using standard polymers of known molecular weight (e.g., polystyrene). Taking into account the detector signal and the calibration curve, an attached computer system calculates a chromatogram that represents the molecular weight distribution of the corresponding sample. The molecular weight distribution describes the number of molecules present in a sample versus their molar mass. M w and number average molecular weight M n It is obtained through computational analysis. n is based on the number of molecules with a specific mass. Using these two values, the polydispersity index (PDI = M w / M n ), the polydispersity index is a measure of the breadth of the molecular weight distribution.

[0033] The polyarylsulfone polymer exhibits a weight average molecular weight value M of 40000 to 105000 g / mol, preferably 45000 to 100000 g / mol, and more preferably 50000 to 95000 g / mol as determined by gel permeation chromatography (GPC) in tetrahydrofuran (THF) and polystyrene (PS) as a standard. w Preferred polyethersulfone polymers comprising repeating units of formula I exhibit weight average molecular weight values ​​of 40,000 to 100,000 g / mol, preferably 48,000 to 90,000 g / mol, and preferred polysulfone polymers comprising repeating units of formula II exhibit weight average molecular weight values ​​of 45,000 to 70,000 g / mol, preferably 53,000 to 60,000 g / mol, as determined by GPC in THF and PS as a standard.

[0034] Preferred polyarylsulfone polymers exhibit a polydispersity index of 2 to 5, preferably 2.5 to 4.7. Preferred polyethersulfone polymers comprising repeating units of formula I exhibit PDI values ​​of 2.5 to 3.6, preferably 2.7 to 3.4, and preferred polysulfone polymers comprising repeating units of formula II exhibit PDI values ​​of 3.8 to 4.7, preferably 4 to 4.5.

[0035] Glass transition temperature T g It is the temperature region when the polymer segments become mobile and the sample undergoes a reversible transition from a solid amorphous region to a softer state with higher flexibility. The glass transition temperature and its determination are well known in the art. The glass transition temperature can be determined, for example, by differential scanning calorimetry (DSC), which measures heat capacity versus temperature, wherein the temperature is changed at a constant rate (e.g., 10K / min). Typically, the sample is first cooled and then heated at the same rate. The glass transition temperature can be obtained from the resulting figure by geometric analysis or computational analysis.

[0036] Preferred polyarylsulfone polymers exhibit a glass transition temperature of 160°C to 250°C, preferably 170°C to 240°C, and more preferably 180°C to 230°C, measured by differential scanning calorimetry (DSC) according to ISO 11357-1 (2017) and 11357-2 (2020) at a heating rate of 10 K / min.

[0037] The viscosity value (reduced viscosity, VN) is related to the molecular weight of the polymer and can be measured in a 1 wt.-% polymer solution in N-methylpyrrolidone based on ISO 1628-5 (1998). Thus, the elution time (t) of a defined volume of polymer solution in an Ubbelohde 1C-capillary is related to the run time (t0) of the pure solvent and is then normalized with the polymer concentration (c in g / ml) according to equation 1:

[0038]

[0039] The viscosity values ​​are given in ml / g. Preferred polyarylsulfone polymers exhibit viscosity values ​​of 40 ml / g to 130 ml / g, preferably 50 ml / g to 120 ml / g, and most preferably 60 ml / g to 110 ml / g in a 1 wt.-% polymer solution in N-methylpyrrolidone based on ISO 1628-5 (1998).

[0040] Solvents

[0041] The solution of the present invention contains N-(2'-hydroxyethyl)-2-pyrrolidone (CAS No. 3445-11-2) as a solvent, which is commercially available on an industrial scale. This solvent is a protic solvent and has low toxicological concerns compared to various frequently used aprotic pyrrolidone derivatives (such as N-methyl-2-pyrrolidone), which is classified as "May damage fertility. May damage the unborn child" according to the GHS hazard statement (Globally Harmonized System of Classification and Labelling of Chemicals). Polyarylsulfone polymers and mixtures of polyarylsulfone polymers with water-soluble polymers can be well dissolved by N-(2'-hydroxyethyl)-2-pyrrolidone, resulting in clear solutions showing unexpectedly high viscosity. Both clarity and minimum viscosity are requirements for preparing high-quality films from such solutions.

[0042] Solution

[0043] The solution according to the invention comprises preferably 50 wt.-% to 99 wt.-%, more preferably 55 wt.-% to 95 wt.-%, particularly preferably 60 wt.-% to 90 wt.-%, very particularly preferably 65 wt.-% to 85 wt.-% and most preferably 70 wt.-% to 80 wt.-% of N-(2′-hydroxyethyl)-2-pyrrolidone, based on the total weight of the solution.

[0044] The solutions according to the invention comprise 1 to 50 wt.-%, preferably 5 to 45 wt.-%, more preferably 7 to 40 wt.-%, particularly preferably 10 to 35 wt.-% and very particularly preferably 12 to 25 wt.-% of polyarylsulfone polymer, based on the total weight of the solution.

[0045] Water soluble polymer

[0046] The solution of the present invention may further comprise a water-soluble polymer, i.e. a polymer that can be easily dissolved in water at 21°C to give a clear solution at a concentration of at least 10 g polymer / 100 g water. These water-soluble polymers affect the solution viscosity and thus act as viscosity modifiers. In addition, these water-soluble polymers act as pore formers in the membrane preparation process and thus strongly affect the membrane characteristics. Typically, the water-soluble polymers that fill the formed pores are removed in a post-treatment step of the membrane preparation. This step may include different washing and / or oxidation steps to remove the water-soluble polymers that leave empty pores after removal. Among other parameters, the polymer type and molecular weight strongly affect the effort necessary to remove the water-soluble polymers during post-treatment of the membrane.

[0047] Preferably, the water-soluble polymer is selected from the group of polyvinylpyrrolidone and poly(alkylene oxide) or mixtures thereof, typically exhibiting a number average molar mass M of at least 250 g / mol. n More preferably, the water-soluble polymer is selected from the group consisting of polyvinylpyrrolidone, poly(ethylene oxide), poly(propylene oxide) and poly(ethylene oxide) / poly(propylene oxide)-block-copolymers or mixtures thereof, exhibiting an M of at least 250 g / mol. n Particularly preferably, the water-soluble polymer is selected from the group consisting of polyvinylpyrrolidone and poly(ethylene oxide) or mixtures thereof, wherein M n The water-soluble polymer is preferably polyvinylpyrrolidone, wherein the solution viscosity is characterized by a K value of 12 or more determined according to the Fikentscher method described by Fikentscher in Cellulosechemie 13, 1932 (58). The K value is determined by viscosity measurements of polymer solutions as described by Fikentscher and is known in the art. A very particularly preferred water-soluble polymer is polyvinylpyrrolidone, which has an M of at least 250 g / mol. n and a solution viscosity characterized by a K value of 12 or more determined according to the Fikentscher method described by Fikentscher in Cellulosechemie 13, 1932 (58).

[0048] Preferably, the solution comprises 0.1 to 15 wt.-%, more preferably 2 to 10 wt.-% and particularly preferably 3 to 7 wt.-% of water-soluble polymer, based on the total weight of the solution.

[0049] More preferably, the solution comprises 0.1 wt.-% to 15 wt.-%, preferably 2 wt.-% to 10 wt.-%, and more preferably 3 wt.-% to 7 wt.-% of a water-soluble polymer selected from the group of water-soluble polyvinylpyrrolidones and water-soluble poly(alkylene oxides) or mixtures thereof, based on the total weight of the solution.

[0050] Particularly preferably, the solution comprises 0.1 wt.-% to 15 wt.-%, preferably 2 wt.-% to 10 wt.-% and more preferably 3 wt.-% to 7 wt.-%, based on the total weight of the solution, of a water-soluble polymer selected from the group of water-soluble polyvinylpyrrolidone, water-soluble poly(ethylene oxide), water-soluble poly(propylene oxide) and water-soluble poly(ethylene oxide) / poly(propylene oxide)-block-copolymers or mixtures thereof.

[0051] Very particularly preferably, the solution comprises 0.1 wt.-% to 15 wt.-%, preferably 2 wt.-% to 10 wt.-% and more preferably 3 wt.-% to 7 wt.-% of a water-soluble polymer selected from the group of water-soluble polyvinylpyrrolidones and water-soluble poly(ethylene oxide) or mixtures thereof, based on the total weight of the solution.

[0052] Most preferably, the solution comprises 0.1 to 15 wt.-%, preferably 2 to 10 wt.-%, and more preferably 3 to 7 wt.-% of water-soluble polyvinylpyrrolidone, based on the total weight of the solution.

[0053] additive

[0054] The solution of the present invention may further comprise an additive, i.e., one or more additives or a mixture thereof. The additive is a compound that dissolves a polyarylsulfone polymer selected from the group consisting of polyethersulfones comprising repeating units of formula I and polysulfone polymers comprising repeating units of formula II or a mixture thereof at only a low concentration of <1 g polyarylsulfone polymer / 100 g additive at 21°C. In the method of membrane preparation, the additive affects the speed of solvent exchange and thus the speed of the precipitation process and thus controls membrane properties, like pore size and number.

[0055] Preferably, the additive is selected from water, C1-C4 alkanols, C2-C8 alkanediols, oligo(alkylene glycols) and C3-C 12More preferred additives are methanol, ethanol, propanol, isopropanol, n-butanol, isobutanol, sec-butanol, tert-butanol, ethylene glycol, di(ethylene glycol), tri(ethylene glycol), tetra(ethylene glycol), propane-1,2-diol, di(propylene glycol), tri(propylene glycol), tetra(propylene glycol), propane-1,3-diol, butane-1,2-diol, butane-1,3-diol, butane-1,4-diol, butane-2,3-diol, pentane-1, 2-diol, pentane-1,3-diol, pentane-1,4-diol, pentane-1,5-diol, pentane-2,3-diol, pentane-2,4-diol, hexane-1,2-diol, hexane-1,3-diol, hexane-1,4-diol, hexane-1,5-diol, hexane-1,6-diol, hexane-2,5-diol, heptane-1,2-diol, heptane-1,7-diol, octane-1,8-diol, octane-1 ,2-diol, hexane-1,5-diene-3,4-diol, neopentyl glycol, 2-methylpentane-2,4-diol, 2,4-dimethylpentane-2,4-diol, 2-ethylhexane-1,3-diol, 2,5-dimethylhexane-2,5-diol, 2,2,4-trimethylpentane-1,3-diol, pinacol, glycerol, butane-1,2,3-triol, butane-1,2,4-triol, pentane-1,2,3- triol, pentane-1,2,4-triol, pentane-1,2,5-triol, hexane-1,2,3-triol, hexane-1,2,4-triol, hexane-1,2,6-triol, hexane-1,3,4-triol, hexane-1,3,5-triol, hexane-1,3,6-triol, hexane-1,4,5-triol, trimethylolmethane, 1,1,1-trimethylolethane, 1,1,1-trimethylolpropane or a mixture thereof.

[0056] Preferably, the solution comprises 0.1 to 20 wt.-%, more preferably 1 to 15 wt.-%, particularly preferably 2 to 13 wt.-% and very particularly preferably 5 to 12 wt.-% of additives, based on the total weight of the solution.

[0057] Co-solvent

[0058] In addition to N-(2'-hydroxyethyl)-2-pyrrolidone, the solutions of the invention may also contain further solvents designated herein as co-solvents. In the process of membrane preparation, co-solvents influence the speed of solvent exchange and thus the speed of the precipitation process and thus control membrane properties, like pore size and number.

[0059] Preferred co-solvents are solvents that are easily miscible with N-(2'-hydroxyethyl)-2-pyrrolidone in any proportion. More preferred suitable co-solvents are, for example, selected from high-boiling ethers, esters, ketones, asymmetric halogenated hydrocarbons, anisole, γ-valerolactone, N,N-dimethylformamide, dimethyl sulfoxide, dihydro-levulose ketone, methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate, sulfolane, N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, N-n-butyl-2-pyrrolidone, N-tert-butyl-2-pyrrolidone, N,N-dimethyl-2-hydroxypropionamide, and N,N-diethyl-2-hydroxypropionamide. Particularly preferred co-solvents are γ-valerolactone, N-tert-butyl-2-pyrrolidone and N-n-butyl-2-pyrrolidone.

[0060] In a preferred embodiment, at least 10 wt.-%, more preferably at least 50 wt.-%, particularly preferably at least 80 wt.-% and very particularly preferably at least 90 wt.-% of the total weight of all solvents of the solution is N-(2'-hydroxyethyl)-2-pyrrolidone.

[0061] In a more preferred embodiment, no co-solvent is used in the solution and N-(2'-hydroxyethyl)-2-pyrrolidone is the only solvent used.

[0062] Solution preparation

[0063] The solutions of the present invention can be easily prepared by combining the different components of the solution. Usually, there is no specific order in which the different components are added. Therefore, the solutions of the present invention can be prepared by combining all components, i.e., the polyarylsulfone polymer, N-(2'-hydroxyethyl)-2-pyrrolidone, and (if necessary) other components (such as water-soluble polymers, additives, or cosolvents), and dissolving all compounds according to any method known in the art.

[0064] Solution processes may, for example, advantageously be supported or accelerated by increasing the temperature of the mixture and / or by mechanical manipulations like stirring, shaking, subjecting to ultrasound (eg, by using a sonication bath), and the like.

[0065] In a preferred embodiment, the polyarylsulfone is first dissolved in N-(2'-hydroxyethyl)-2-pyrrolidone at 60° C. under stirring, and subsequently, the water-soluble polymer and additives are added and stirred until a clear solution is obtained.

[0066] The solutions of the present invention generally show a viscosity of 0.1 Pa·s to 150 Pa·s, preferably 0.5 Pa·s to 75 Pa·s, particularly preferably 1 Pa·s to 50 Pa·s, and most preferably 2 Pa·s to 30 Pa·s as determined with a Brookfield viscometer DV-IPrime from Brookfield Engineering Laboratories, Inc. Middleboro, USA, with a RV 6 rotor at 60° C. with 5-100 rpm (revolutions per minute). The shear rate, i.e., the revolutions per minute, is generally selected according to the viscosity of the sample. Typically, low viscosity samples are measured at higher shear rates, while higher viscosity samples are measured at lower shear rates.

[0067] High solution viscosity, as shown by the solutions of the present invention, benefits the casting process during film preparation by producing better film quality. In addition, in the case of high solution viscosity, during film preparation, it is generally not necessary to use high molecular weight water-soluble polymers to (further) increase viscosity.

[0068] The measurement of turbidity is the nephelometric turbidity unit (NTU) determined with a calibrated nephelometer or turbidimeter, which represents the amount of light reaching the detector on the light beam side after being scattered by the sample. Preferably, the solution of the invention exhibits a turbidity of 0 to 2 NTU, more preferably 0 to 1 NTU, particularly preferably 0 to 0.9 NTU as determined at 60° C. using a turbidimeter 2100AN (Hach Lange GmbH, Düsseldorf, Germany) with an 860 nm filter.

[0069] Low solution turbidity is a prerequisite for the production of high-quality thin films and thus high-performance membranes, since large voids and defects in the membrane (which would affect the balance between pure water permeability and molecular weight cut-off) are thereby avoided.

[0070] Membrane preparation

[0071] We have further discovered a method for preparing a membrane using a solution comprising:

[0072] (a) a polyarylsulfone polymer selected from the group consisting of polyethersulfones comprising repeating units of formula I and polysulfones comprising repeating units of formula II or mixtures thereof, wherein the weight average molecular weight M of the polyarylsulfone polymer is w in the range of 40000 g / mol to 105000 g / mol, and the polyarylsulfone polymer comprises at least 95 wt.-% of repeating units of formulae I and II, based on the total weight of the polyarylsulfone polymer,

[0073] (b) N-(2'-hydroxyethyl)-2-pyrrolidone, and

[0074] (c) Water-soluble polymers.

[0075] In the context of the present application, membrane is understood to be a semi-permeable structure capable of separating two fluids or separating molecules and / or ionic components or particles from a liquid. The membrane acts as a selective barrier, allowing some particles, substances or chemicals to pass through while retaining others. The membrane can have various geometric shapes, such as flat sheets, spirally wound, pillow-shaped, tubular, single-hole hollow fibers or porous hollow fibers.

[0076] Separation by using a membrane can be operated in different ways, for example driven by pressure, by a concentration gradient or by a gradient like an electric potential or a temperature gradient. Examples of pressure-driven operations are microfiltration, ultrafiltration or nanofiltration or reverse osmosis operations. Examples of concentration-based operations are dialysis or forward osmosis. A good overview is given in WO 2017 / 045985 or M.Ulbricht, Polymer [Polymer] 47 (2006), pp. 2217-2262 (which are hereby incorporated by reference herein). A preferred membrane is an ultrafiltration membrane.

[0077] Membranes can be prepared by different methods such as phase separation (phase inversion) of polymers, sol-gel method, interfacial reaction, stretching, extrusion, track etching, microfabrication, etc.

[0078] Preferably, the membrane according to the present invention is prepared by liquid non-solvent induced phase separation (NIPS), which comprises the following steps:

[0079] a) preparing a solution comprising a polyarylsulfone polymer, N-(2'-hydroxyethyl)-2-pyrrolidone and a water-soluble polymer, wherein the polyarylsulfone polymer is selected from the group consisting of polyethersulfone comprising repeating units of formula I and polysulfone comprising repeating units of formula II or a mixture thereof, wherein the weight average molecular weight M of the polyarylsulfone polymer is w In the range of 40000 g / mol to 105000 g / mol, the polyarylsulfone polymer comprises at least 95 wt.-% of repeating units of formulae I and II, based on the total weight of the polyarylsulfone polymer.

[0080] b1) Shaping the polymer solution into a certain geometry, such as, for example, a fiber, a tube or a flat sheet geometry, by methods such as, for example, extrusion of the polymer solution or preparation of a film from the polymer solution or casting the polymer solution.

[0081] b2) Solidifying the geometric shape formed in step b1) by exposing the polymer solution to a coagulant.

[0082] Step a)

[0083] The solution of step a) corresponds to the polymer solution as described above. Preferably, the solution further contains additives and / or cosolvents as described above. The preparation of the solution of step a) can be carried out as described above.

[0084] In a preferred embodiment, the solution of step a) comprises 5 to 45 wt.-% of polyarylsulfone polymer, 55 to 95 wt.-% of N-(2'-hydroxyethyl)-2-pyrrolidone, and 1 to 15 wt.-% of water-soluble polymer, based on the total weight of the solution.

[0085] In a more preferred embodiment, the solution of step a) comprises 10 to 35 wt.-% of polyarylsulfone polymer, 60 to 90 wt.-% of N-(2'-hydroxyethyl)-2-pyrrolidone and 2 to 10 wt.-% of water-soluble polymer, based on the total weight of the solution.

[0086] In one embodiment, the solution may be degassed and / or heated before proceeding to the next step.

[0087] Steps b1) and b2)

[0088] Steps b1) and b2) can be carried out continuously, i.e. not as separate steps, for example in the case of continuous extrusion of the polymer solution into the coagulation bath, or in separate steps, for example first forming the polymer film and then transferring it to the coagulation bath after a certain drying time.

[0089] The formed polymer solution still contains polyarylsulfone polymer, solvent, water-soluble polymer, etc. The phase separation of polymer and solvent has not yet started or completed, that is, the polymer has not yet completely solidified. In the case of extrusion molding, the polymer solution is made into a fiber or tubular geometry. In the case of film production or casting molding, the polymer solution is made into a flat sheet geometry.

[0090] Exposing the shaped polymer solution to the coagulant (step b2) can be carried out, for example, in a coagulation bath containing the coagulant. The polyarylsulfone polymer should have a low solubility in the coagulant, ie <1 g polyarylsulfone polymer / 100 g coagulant at 21°C.

[0091] The contact with the coagulant causes a non-solvent-induced demixing of the homogeneous polymer solution, which leads to the solidification of the polymer and thus to the formation of a film in the corresponding geometry of the polymer solution. The film structure and morphology depend strongly on the process parameters used here, as well as on the nature and presence of the different compounds present in the solution. The water-soluble polymers and additives have essentially two functions: on the one hand, they adjust the solution viscosity to a high level, which makes it easier to form a film, for example by casting, and on the other hand, they act as pore formers, which strongly determine the performance characteristics of the film.

[0092] Suitable coagulating agents are, for example, liquid water, water vapor and mixtures of water with alcohols, cosolvents, and / or the solvent of the solution of the invention (i.e., N-(2'-hydroxyethyl)-2-pyrrolidone). Suitable alcohols are, for example, selected from C1-C4 alkanols, C2-C8 alkanediols, oligo(alkylene glycols), C3-C 12 A monool, diol or triol of the group of alkantriols (as they can be used as additives in the solutions of the invention (see above)), or n The poly(ethylene oxide) is 100 g / mol to 1000 g / mol.

[0093] Suitable co-solvents are, for example, selected from high boiling ethers, esters, ketones, asymmetric halogenated hydrocarbons, anisole, γ-valerolactone, N,N-dimethylformamide, dimethyl sulfoxide, sulfolane, dihydro-levulose ketone, methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate, N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, N-n-butyl-2-pyrrolidone, N-tert-butyl-2-pyrrolidone, N,N-dimethyl-2-hydroxypropionamide, and N,N-diethyl-2-hydroxypropionamide. Preferred co-solvents are γ-valerolactone, N-tert-butyl-2-pyrrolidone and N-n-butyl-2-pyrrolidone.

[0094] Preferably, the coagulant is a mixture comprising liquid water and a solvent N-(2'-hydroxyethyl)-2-pyrrolidone or a mixture comprising liquid water and an alcohol (eg, M n The coagulant is preferably a mixture of 100 g / mol to 1000 g / mol of poly(ethylene oxide)) and / or a mixture comprising liquid water and a cosolvent. More preferably, the coagulant is a mixture comprising liquid water and the solvent N-(2'-hydroxyethyl)-2-pyrrolidone. Based on the total weight of the coagulant, the coagulant may comprise 10 wt.-% to 90 wt.-% of water and 90 wt.-% to 10 wt.-% of alcohol and / or a cosolvent or solvent N-(2'-hydroxyethyl)-2-pyrrolidone, preferably 30 wt.-% to 70 wt.-% of water and 70 wt.-% to 30 wt.-% of alcohol and / or a cosolvent or solvent N-(2'-hydroxyethyl)-2-pyrrolidone. Typically, the amounts of all components of the coagulant add up to 100%.

[0095] Further details of process steps a) and b) depend on the desired membrane geometry and the scale of production, which includes laboratory scale or commercial / industrial scale.

[0096] In a preferred method, a flat sheet membrane is prepared. The method comprises:

[0097] a) preparing a solution comprising a polyarylsulfone polymer, N-(2'-hydroxyethyl)-2-pyrrolidone and a water-soluble polymer, wherein the polyarylsulfone polymer is selected from the group consisting of polyethersulfone comprising repeating units of formula I and polysulfone comprising repeating units of formula II or a mixture thereof, wherein the weight average molecular weight M of the polyarylsulfone polymer is w In the range of 40000 g / mol to 105000 g / mol, the polyarylsulfone polymer comprises at least 95 wt.-% of repeating units of formulae I and II, based on the total weight of the polyarylsulfone polymer.

[0098] b1) preparing a polymer film from the solution of step a) by applying the solution of step a) to a substrate

[0099] b2) exposing the polymer film to a coagulant.

[0100] Step a)

[0101] The solution of step a) corresponds to the polymer solution as described above. Preferably, the solution further contains additives and / or cosolvents as described above. The preparation of the solution of step a) can be carried out as described above.

[0102] In a preferred embodiment, the solution of step a) comprises 5 to 45 wt.-% of polyarylsulfone polymer, 55 to 95 wt.-% of N-(2'-hydroxyethyl)-2-pyrrolidone, and 1 to 15 wt.-% of water-soluble polymer, based on the total weight of the solution.

[0103] In a more preferred embodiment, the solution of step a) comprises 10 wt.-% to 35 wt.-% of polyarylsulfone polymer, 60 to 90 wt.-% of N-(2'-hydroxyethyl)-2-pyrrolidone and 2 to 10 wt.-% of water-soluble polymer, based on the total weight of the solution.

[0104] Preferably, the solution is prepared under stirring at a temperature of 20°C to 100°C, preferably 40°C to 90°C, more preferably 50°C to 70°C. Typically, the prepared solution is degassed for 2h to 24h, preferably 6h to 20h, more preferably 10h to 14h. Subsequently, the solution is preferably heated again at a temperature of 20°C to 100°C, preferably 40°C to 90°C, more preferably 50°C to 70°C for 1h to 4h, preferably 1.5h to 3h.

[0105] Step b1)

[0106] The preparation of polymer films from the solutions of the invention can be carried out, for example, by casting or other methods like rolling, spraying, etc., preferably by casting at a temperature of 40° C. to 90° C., preferably 50° C. to 70° C., using, for example, a casting knife and optionally an automatic coater onto a substrate, preferably a polymer substrate (like, for example, a polymer film from biaxially oriented poly(ethylene terephthalate), for example, available under the trade name ), a glass substrate, or a metal substrate (e.g., a metal conveyor belt).

[0107] In a preferred embodiment, the polymer film is produced by casting using a casting knife having openings of 100 μm to 500 μm, more preferably 200 μm to 400 μm.

[0108] Step b2)

[0109] Typically, the polymer film is allowed to stand for 5s to 150s, preferably 10s to 100s, more preferably 20s to 60s and then subjected to a coagulation bath containing a coagulant as described above. The immersion is carried out at 10°C to 50°C, preferably 15°C to 40°C, more preferably 20°C to 30°C for 3min to 20min, preferably 5min to 15min. Subsequently, the film is detached from the substrate and is usually subjected to post-treatment.

[0110] In another preferred method, a non-flat sheet membrane is prepared. The method comprises:

[0111] a) preparing a solution comprising a polyarylsulfone polymer, N-(2'-hydroxyethyl)-2-pyrrolidone and a water-soluble polymer, wherein the polyarylsulfone polymer is selected from the group consisting of polyethersulfone comprising repeating units of formula I and polysulfone comprising repeating units of formula II or a mixture thereof, wherein the weight average molecular weight M of the polyarylsulfone polymer is w In the range of 40000 g / mol to 105000 g / mol, the polyarylsulfone polymer comprises at least 95 wt.-% of repeating units of formulae I and II, based on the total weight of the polyarylsulfone polymer.

[0112] b) continuously shaping the solution of step a) and stratifying the solution into non-planar geometric shapes by directly exposing the solution to a coagulant that causes continued solidification of the shaped solution.

[0113] Step a)

[0114] The solution of step a) corresponds to the polymer solution as described above. Preferably, the solution further contains additives and / or cosolvents as described above. The preparation of the solution of step a) can be carried out as described above.

[0115] In a preferred embodiment, the solution of step a) comprises 5 to 45 wt.-% of polyarylsulfone polymer, 55 to 95 wt.-% of N-(2'-hydroxyethyl)-2-pyrrolidone, and 1 to 15 wt.-% of water-soluble polymer, based on the total weight of the solution.

[0116] In a more preferred embodiment, the solution of step a) comprises 10 wt.-% to 35 wt.-% of polyarylsulfone polymer, 60 to 90 wt.-% of N-(2'-hydroxyethyl)-2-pyrrolidone and 2 to 10 wt.-% of water-soluble polymer, based on the total weight of the solution.

[0117] Preferably, the solution is prepared under stirring at a temperature of 20°C to 100°C, preferably 40°C to 90°C, more preferably 50°C to 70°C. Typically, the prepared solution is degassed for 2h to 24h, preferably 6h to 20h, more preferably 10h to 14h. Subsequently, the solution is preferably heated again at a temperature of 20°C to 100°C, preferably 40°C to 90°C, more preferably 50°C to 70°C for 1h to 4h, preferably 1.5h to 3h.

[0118] Step b)

[0119] The shaping of the solution of step a) into a non-flat geometry can be carried out, for example, by extruding the solution through an extrusion nozzle to obtain, for example, a fiber-like geometry. In order to maintain the desired shape, the polymer solution is usually extruded directly into a coagulation bath containing a coagulant as described above, which causes a demixing of polymer and solvent and thus causes the polymer to solidify in the desired shape. For example, if one or more additional hollow needles are present in the extrusion nozzle, through which a coagulant solution is additionally injected into the interior of the fiber being formed, a single-tube or multi-tube geometry is obtained as described below.

[0120] The preferred membrane showing non-flat geometry is a hollow fiber membrane (single hole hollow fiber or porous hollow fiber). They can be prepared by different spinning techniques, for example, by melt spinning, dry spinning or wet spinning. Thus, the solution obtained in the above-mentioned step a) is extruded through an extrusion nozzle (also referred to as a spinneret) containing a desired number of hollow needles (step b). During extrusion, the condensed liquid is injected into the extruded polymer through the hollow needle. By this arrangement, the extruded polymer film obtains a hollow cylindrical geometry, and forms parallel continuous channels extending in the extrusion direction in the extruded polymer. Preferably, the porous structure on the outer surface of the extruded membrane is formed in the following manner: the outer surface of the extruded hollow polymer fiber is contacted with a mild coagulant (such as water vapor) so that the shape is fixed on the outer surface without an active layer, i.e., a highly porous filter layer. Subsequently, the membrane is contacted with a coagulant. Parameters of the process like extrusion speed, temperature, nozzle geometry, type of coagulant, concentration have an influence on and can therefore be used to control the pore size and distribution as well as the shape and thickness of the membrane and hence the properties of the membrane. The hollow fiber membranes can optionally be wound on rolls and / or bundled into hollow fiber bundles.

[0121] The membranes prepared by the above procedure are usually post-treated by washing and / or an optional oxidation step.

[0122] In one embodiment, the membrane prepared by the method described above (steps ab) is exposed to a solution containing an oxidatively active component after an optional washing step. In this case, the solution containing the oxidatively active component is preferably an aqueous solution. Washing is performed after the oxidation step in order to remove the oxidant. Preferred are water-soluble oxidants, such as, for example, sodium hypochlorite or halogens, especially chlorine, in a concentration range of 500 wt.-ppm to 5000 wt.-ppm, more preferably 1000 wt.-ppm to 4000 wt.-ppm and particularly preferably 1500 wt.-ppm to 3500 wt.-ppm, based on the total weight of the aqueous oxidizing solution.

[0123] Oxidation and washing are performed to remove water-soluble polymers and thus form pores. Oxidation may be followed by washing, or vice versa. Oxidation and washing may also be performed simultaneously in one step. Preferably, the membrane is oxidized with a hypochlorite solution or gaseous chlorine at a pH of 20°C to 90°C, preferably 35°C to 80°C, more preferably 50°C to 70°C and 9 to 10, preferably 9.3 to 9.7 for 0.5h to 4h, preferably 1h to 3h, more preferably 1.5h to 2.5h, and then washed with water. Washing with water is typically performed in a water bath for 2h to 24h, preferably 4h to 20h, more preferably 8h to 16h, and then usually washed with water at 20°C to 90°C, preferably 35°C to 80°C and more preferably 50°C to 70°C for one to five times, preferably three times. In a further step, the membrane is typically washed with a sodium bisulfite solution, preferably using 0.1 wt.-% to 1 wt.-%, preferably 0.3 wt.-% to 0.7 wt.-%, more preferably 0.4 wt.-% to 0.6 wt.-% aqueous sodium bisulfite solution.

[0124] In the case where the solutions of the invention based on N-(2'-hydroxyethyl)-2-pyrrolidone are used for film preparation, preferably no oxidative post-treatment is performed. As mentioned above, due to the high viscosity of the solutions of the invention, it is not necessary to use high molecular weight water-soluble polymers to increase the viscosity. Instead, low molecular weight water-soluble polymers can be used, which do not require oxidative post-treatment to remove.

[0125] Furthermore, a membrane is found, which is prepared using the solution of the present invention, the solution comprising a polyarylsulfone polymer selected from the group consisting of a polyethersulfone comprising a repeating unit of formula I and a polysulfone comprising a repeating unit of formula II or a mixture thereof, N-(2'-hydroxyethyl)-2-pyrrolidone and a water-soluble polymer, wherein the weight average molecular weight M of the polyarylsulfone polymer is w In the range of 40000 g / mol to 105000 g / mol, the polyarylsulfone polymer comprises at least 95 wt.-% of repeating units of formulae I and II, based on the total weight of the polyarylsulfone polymer.

[0126] As mentioned above, the membrane preparation preferably comprises the following steps:

[0127] a) preparing a solution comprising a polyarylsulfone polymer, N-(2'-hydroxyethyl)-2-pyrrolidone and a water-soluble polymer, wherein the polyarylsulfone polymer is selected from the group consisting of polyethersulfone comprising repeating units of formula I and polysulfone comprising repeating units of formula II or a mixture thereof, wherein the weight average molecular weight M of the polyarylsulfone polymer is w In the range of 40000 g / mol to 105000 g / mol, the polyarylsulfone polymer comprises at least 95 wt.-% of repeating units of formulae I and II, based on the total weight of the polyarylsulfone polymer.

[0128] b1) Shaping the polymer solution into a certain geometry, such as, for example, a fiber, a tube or a flat sheet geometry, by methods such as, for example, extrusion of the polymer solution or preparation of a film from the polymer solution or casting the polymer solution.

[0129] b2) Solidifying the geometric shape formed in step b1) by exposing the polymer solution to a coagulant.

[0130] Preferably, the solution used for membrane production further contains additives and / or co-solvents as described above. The preparation of the solution of step a) can be carried out as described above. Typically, the membrane is post-treated by different washing and / or oxidation steps as described above.

[0131] The performance of a membrane can be specified, among other things, by its pure water permeability (PWP) and its molecular weight cut-off. PWP reflects the permeability of the membrane to pure water according to the following formula (

[0132] Equation 2), depends on the membrane area, the applied pressure, and the duration of the permeation experiment:

[0133]

[0134] PWP: Pure water permeability [kg / barhm 2 ]

[0135] m: Mass of permeate water [kg]

[0136] A: membrane area [m 2 ]

[0137] P: Pressure [bar]

[0138] t: time of penetration experiment [h].

[0139] In order to achieve high flow rates, the membranes according to the present invention preferably include a flow rate greater than 100 kg / h·m for ultrafiltration membranes. 2 bar, more preferably greater than 125 kg / h·m 2 bar, and particularly preferably greater than 150 kg / h·m 2 bar and preferably greater than 25 kg / h·m for nanofiltration membranes 2 bar, more preferably greater than 50 kg / h·m 2 bar, and particularly preferably greater than 100 kg / h·m 2 · High PWP of Ba.

[0140] The pure water permeability of the membrane is determined prior to determining its molecular weight cut-off to avoid limited permeation values ​​due to clogging / fouling of the pores by the polymer used in the molecular weight cut-off determination.

[0141] The weight average molecular weight cut-off (MWCO) of the membrane is the minimum weight average molecular weight (M w ) of a polyethylene oxide) standard that is retained by the membrane to at least 90%. It is usually given in kilodaltons (kDa). For example, a MWCO of 18.4 kDa means that MWCOs of 18.4 kDa and higher are w The poly(ethylene oxide) of the present invention is retained to at least 90%. For ultrafiltration membranes, the MWCO of the membrane according to the present invention is typically 2 kDa-200 kDa, preferably 10 kDa-200 kDa, and more preferably 10 kDa-100 kDa, and for nanofiltration membranes, the upper MWCO limit is typically <10 kDa and the lower MWCO limit is preferably at least 5 kDa, more preferably at least 3 kDa and particularly preferably at least 2 kDa.

[0142] The membranes of the present invention can be used in any kind of separation process of gaseous or liquid mixtures, for example water treatment applications like drinking water purification, treatment of industrial or municipal wastewater, desalination of seawater or brackish water, dialysis, purification of pharmaceuticals, plasmolysis and food processing.

[0143] In a general embodiment of the process of the invention for preparing a flat sheet membrane, a polyethersulfone having a weight average molecular weight of 48000 g / mol to 90000 g / mol comprising repeating units of formula I is mixed with N-(2'-hydroxyethyl)-2-pyrrolidone and polyvinylpyrrolidone showing a K value of at least 12. The different polymers are used in such an amount that the resulting solution contains 10 wt.-% to 35 wt.-% of polyethersulfone and 2 wt.-% to 10 wt.-% of polyvinylpyrrolidone. In order to accelerate the dissolution of these polymers, the mixture is heated at a temperature of 50° C. to 70° C. with stirring until a homogeneous clear viscous solution is obtained. The solution is then degassed for 10 h to 14 h and heated at 50° C. to 70° C. for another 1.5 h to 3 h. Subsequently, the solution is cast onto a glass plate at a temperature of 50° C. to 70° C. using a casting knife with an opening of 200 μm to 400 μm. The resulting film is allowed to stand for 20 to 60 seconds and then subjected to a water-based coagulation bath containing 30 to 70 wt.% water and 70 to 30 wt.% N-(2'-hydroxyethyl)-2-pyrrolidone for 5 to 15 minutes at 20 to 30° C. After detachment from the glass substrate, the film is transferred to a water bath at room temperature, where it is allowed to stand for 8 to 16 hours, then washed three times with water at 50 to 70° C. and stored in a wet state.

[0144] In a general embodiment of the method of the present invention for preparing a hollow fiber membrane, a polysulfone having a weight average molecular weight of 53000 g / mol to 60000 g / mol and comprising repeating units of formula II is mixed with N-(2'-hydroxyethyl)-2-pyrrolidone and a polysulfone having a number average molecular weight M of at least 250 g / mol. n The different polymers are used in an amount such that the resulting solution contains 10 wt.-% to 35 wt.-% of polyethersulfone and 2 wt.-% to 10 wt.-% of poly(ethylene oxide). In order to accelerate the dissolution of these polymers, the mixture is heated at a temperature of 40° C. to 90° C. with stirring until a uniform, clear, viscous solution is obtained. The solution is then degassed for 10 h to 14 h and then reheated at 40° C. to 90° C. for 1.5 h to 3 h. The solution is then subjected to a wet spinning process at 50° C. to 70° C., wherein the solution is extruded through an extrusion nozzle (spinneret). The pore fluid, i.e., the above-mentioned condensation liquid injected into the extruded polymer through a hollow needle, contains 20 vol.-% to 30 vol.-% of water and 70 vol.-% to 80 vol.-% of the solvent N-(2'-hydroxyethyl)-2-pyrrolidone. The resulting fiber is immersed in a coagulation bath containing water, wound on a winder, and subsequently exposed to an aqueous solution containing 1500 wt.-ppm to 3500 wt.-ppm NaOCl at 50° C. to 70° C. and a pH of 9.3 to 9.7 for 1 h to 3 h. The membrane is then washed with water at 50° C. to 70° C. and once with a 0.4 wt.-% to 0.6 wt.-% aqueous sodium bisulfite solution. The membrane is stored in a wet state.

[0145] The solvent N-(2'-hydroxyethyl)-2-pyrrolidone was found to readily dissolve the polyarylsulfone polymer, producing a solution with low turbidity.

[0146] Compared with other solvents, especially other pyrrolidone derivatives, the use of N-(2'-hydroxyethyl)-2-pyrrolidone shows different advantages:

[0147] First, N-(2'-hydroxyethyl)-2-pyrrolidone is readily available and has low toxicological concerns compared to, for example, the frequently used N-methylpyrrolidone.

[0148] Secondly, the resulting solutions of the present invention show high viscosity, which benefits the casting process during film preparation by producing better film quality.

[0149] Third, due to the high viscosity of the solution, during the film preparation, it is not necessary to use high molecular weight water-soluble polymers to increase the viscosity. Instead of this, the high viscosity of the solution allows the use of water-soluble polymers with relatively low K values ​​or molecular weight values. These types of polymers can be removed from the resulting film by simple washing with water without the need for an oxidation step. Therefore, advantageously, a process step can be avoided and the use of potentially hazardous chemicals (and required additional washing steps) can therefore be avoided.

[0150] Fourthly, it has furthermore been found that the use of solutions of the invention containing low molecular weight water-soluble polymers, with or without oxidative post-treatment, results in membranes having a favorable balance of pure water permeability and molecular weight cut-off and thus in improved membrane performance compared to membranes known from the art.

[0151] Examples

[0152] Abbreviations and compounds used in the examples:

[0153] DMAc N,N-Dimethylacetamide

[0154] DMSO Dimethyl sulfoxide

[0155] DSC Differential Scanning Calorimetry

[0156] GPC Gel Permeation Chromatography

[0157] HEP N-(2'-Hydroxyethyl)-2-pyrrolidone

[0158] MWCO Molecular weight cut-off

[0159] NMP N-Methyl-2-pyrrolidone

[0160] NTU Nephelometric Turbidity Unit

[0161] PEO Poly(ethylene oxide)

[0162] PWP Pure Water Permeability

[0163] TBP N-tert-butyl-2-pyrrolidone

[0164] 2P 2-Pyrrolidone

[0165] AMD 3L N,N-dimethyl-2-hydroxypropionamide (N,N-dimethyl lactamide)

[0166] This article abbreviates it as "AMD3L"

[0167] E 3010 polyethersulfone having a viscosity value of 66 ml / g (measured in a 1 wt.-% polymer solution in N-methylpyrrolidone based on ISO 1628-5 (1998); a glass transition temperature of 225° C. (DSC, 10 K / min; according to ISO 11357-1 (2017) and 11357-2 (2020)); a molecular weight Mw of 58000 g / mol (GPC, in THF, PS standards), and Mw / Mn=3.3, abbreviated as “E3010”

[0168] E 6020P polyethersulfone having a viscosity value of 81 ml / g (measured in a 1 wt.-% polymer solution in N-methylpyrrolidone based on ISO 1628-5 (1998); a glass transition temperature of 225° C. (DSC, 10 K / min; according to ISO 11357-1 (2017) and 11357-2 (2020)); a molecular weight Mw of 75000 g / mol (GPC, in THF, PS standards), and Mw / Mn=3, abbreviated as “E6020P”

[0169] E 7020P polyethersulfone having a viscosity value of 100 ml / g (measured in a 1 wt.-% polymer solution in N-methylpyrrolidone based on ISO 1628-5 (1998); a glass transition temperature of 225° C. (DSC, 10 K / min; according to ISO 11357-1 (2017) and 11357-2 (2020)); a molecular weight Mw of 92000 g / mol (GPC, in THF, PS standards), and Mw / Mn=3.0, abbreviated as “E7020P”

[0170] K90 has a molecular weight M of 1000000 g / mol to 1500000 g / mol w and polyvinylpyrrolidone characterized by a solution viscosity of 90 determined according to the Fikentscher method (Fikentscher, Cellulosechemie 13, 1932 (58)), which is abbreviated as "K90"

[0171] K85 has a molecular weight of 1100000 g / mol w and a polyvinylpyrrolidone characterized by a solution viscosity of 85 determined according to the Fikentscher method (Fikentscher, Cellulosechemie 13, 1932 (58)), which is abbreviated as "K85"

[0172] K30 has a molecular weight M of 44000 g / mol to 540000 g / mol w and polyvinylpyrrolidone characterized by a solution viscosity of 30 determined according to the Fikentscher method (Fikentscher, Cellulosechemie 13, 1932 (58)), which is abbreviated as "K30"

[0173] K25 has a molecular weight M of 28000 g / mol to 340000 g / mol w and a polyvinylpyrrolidone characterized by a solution viscosity of 25 determined according to the Fikentscher method (Fikentscher, Cellulosechemie 13, 1932 (58)), which is abbreviated as "K25"

[0174] K17 has a molecular weight M of 7000 g / mol to 11000 g / mol w and a polyvinylpyrrolidone characterized by a solution viscosity of 17 determined according to the Fikentscher method (Fikentscher, Cellulosechemie 13, 1932 (58)), which is abbreviated as "K17"

[0175] K12 has a molecular weight M of 2000 g / mol to 3000 g / mol w and a polyvinylpyrrolidone characterized by a solution viscosity of 12 determined according to the Fikentscher method (Fikentscher, Cellulosechemie 13, 1932 (58)), which is abbreviated as "K12"

[0176] E 400 has a number average molecular weight M of 400 g / mol, calculated from the OH number according to DIN 53240 n Poly(ethylene oxide), abbreviated as "PEO 400"

[0177] Determination of solution turbidity

[0178] The polymer solution turbidity was measured at 60°C with a turbidimeter 2100AN (Hach, Düsseldorf, Germany) using a 860 nm filter and expressed in nephelometric turbidity units (NTU).

[0179] Determination of solution viscosity

[0180] Polymer solution viscosity was measured using a Brookfield viscometer DV-I Prime (Brookfield Engineering Laboratories, Inc., Middleboro, USA) with a RV 6 spindle at 5-100 rpm at 60° C. The shear rate utilized depended on the solution viscosity and is given in the table below.

[0181] Determination of membrane water permeability

[0182] The pure water permeability (PWP) of the membrane was tested using a pressure cell with a diameter of 74 mm using ultrapure water (saline-free, filtered through a Millipore UF system) at 23° C. and 1 bar water pressure. The pure water permeability (PWP) was calculated as follows (Equation 2):

[0183]

[0184] PWP: Pure water permeability [kg / barhm 2 ]

[0185] m: Mass of permeate water [kg]

[0186] A: membrane area [m 2 ]

[0187] P: Pressure [bar]

[0188] t: time of penetration experiment [h].

[0189] Determination of membrane MWCO

[0190] In subsequent tests, solutions of poly(ethylene oxide)-standards with increasing molecular weight were used as feed to be filtered through the membrane at a pressure of 0.15 bar. The molecular weight of the permeate was determined for each poly(ethylene oxide)-standard used by GPC measurements of the feed and the permeate.

[0191] Examples 1-17: Viscosity and turbidity of polyarylsulfone polymer solutions in different solvents

[0192] use DAC 600.1Vac-P (Hauschild & Co. KG, Hamm, Germany) prepared 20 wt.-% of the given concentration of the polymer solution of the present invention using different polyarylsulfone polymers within 30 minutes of mixing at a speed of 200 rpm, 800 rpm and 1200 rpm. For comparison, the corresponding solutions were prepared using other solvents according to the same procedure. The solution viscosity and turbidity were measured at 60 ° C according to the above procedure. The results are summarized in Table 1.

[0193] Table 1: Viscosity and turbidity of 20 wt.-% polymer solutions at 60 °C

[0194]

[0195] The data summarized in Table 1 show that, at a given concentration of polyarylsulfone polymer, the solution of the present invention using N-(2'-hydroxyethyl)-2-pyrrolidone as solvent shows significantly higher viscosity levels compared to corresponding solutions based on other solvents (comparative examples). This is still effective if the type of polyarylsulfone polymer is changed. Higher solution viscosity is advantageous because it contributes to the method steps of film casting, thereby producing better films. In addition, the solution turbidity of the solution of the present invention is at a low level required for obtaining high-quality films. Higher turbidity levels may cause defects in the film and therefore reduce their separation performance. Compared to solutions based on other solvents (comparative examples), turbidity is at a similar level.

[0196] Membrane preparation - general procedure

[0197] The amounts given in this general procedure are general ranges, the exact amounts for the corresponding experiments can be found in Table 2 and Tables 4-7. To a three-necked flask equipped with a magnetic stirrer were added 65 g or 81 g of solvent, 15 g to 19 g of Polymer, 4g to 8g Polyvinylpyrrolidone or poly(alkylene oxide) (e.g., E 400), as given in Table 2 and Tables 4-7. The mixture was heated at 60°C with gentle stirring until a homogeneous clear viscous solution was obtained, generally referred to as a solution. The solution was degassed at room temperature overnight.

[0198] Afterwards, the membrane solution was heated at 60° C. for another 2 hours and cast onto a glass plate at 60° C. using an Erichsen coater (Coatmaster 510, Erichsen GmbH & Co KG, Hemer, Germany) operating at a speed of 5 mm / s with a casting knife (300 microns). The membrane film was allowed to stand for 30 seconds and then immersed in a water-based coagulation bath at 25° C. for 10 minutes, the coagulation bath consisting of a mixture of the same solvent used to prepare the above polymer solution and water in a ratio of 40:60 based on weight. After the membrane had been detached from the glass plate, the membrane was exposed to post-treatment A or post-treatment B.

[0199] Post-treatment A (no oxidation step, only washing)

[0200] The membrane was washed three times with 60°C water.

[0201] Optional post-treatment B (oxidation and washing steps)

[0202] The membrane was transferred to a water bath containing 2000 ppm NaOCl solution at 60° C. and a pH of 9.5 for 2 h. The membrane was then washed with water at 60° C. and once with a 0.5 wt.-% sodium bisulfite solution to remove active chlorine.

[0203] After work-up, the membranes were stored in a wet state.

[0204] Examples 18-20: Solutions with adjusted viscosity and their films with and without oxidative post-treatment

[0205] The inventive polymer solutions as well as the comparative polymer solutions were prepared according to the above procedure using the polymer types and polymer amounts as given in Table 2. The amount of water-soluble polymer was kept constant. The type of water-soluble polymer was selected so that the prepared solution showed a target viscosity of 16 Pa·s-17 Pa·s.

[0206] Viscosity and turbidity were measured according to the above methods. According to the procedure described above, two membranes were prepared from each of these solutions in each case, including washing with water alone as post-treatment in one case (post-treatment A), and post-treatment with NaOCl oxidation and washing with sodium bisulfite solution in the other case (post-treatment B). For each membrane, pure water permeability and molecular weight cut-off were determined according to the above methods. The results are summarized in Table 3.

[0207] Table 2: Solutions of E 6020P in different solvents and in combination with different water-soluble polymers. All solutions were adjusted to a viscosity of 16 to 17 Pa·s at 20 rpm.

[0208] E6020P K17 K25 K30 K85 K90 HEP AMD3L 2P Example 18 15g 5g --- --- --- --- 80g --- --- Comparative Example 19 15g --- --- --- 5g --- --- 80g --- Comparative Example 20 15g --- --- 2.5g --- 2.5g --- --- 80g

[0209] Table 3: Solution-solution properties of E 6020P in different solvents and in combination with different water-soluble polymers and the properties of the resulting membranes without or with oxidative post-treatment.

[0210]

[0211] These experiments show that, for a given amount of water-soluble polymer, In the case of the solutions of the invention of E 6020P and N-(2'-hydroxyethyl)-2-pyrrolidone, water-soluble polymers with lower K values ​​(according to Fikentscher, see above) lead to the target solution viscosity, whereas in the case of the other solvents, water-soluble polymers with higher K values ​​are required. This is due to the higher viscosity contribution of the solvent of the invention, N-(2'-hydroxyethyl)-2-pyrrolidone. Since the K value is a measure of the molar mass, this means that solvents other than N-(2'-hydroxyethyl)-2-pyrrolidone require water-soluble polymers of higher molar mass to obtain the same target viscosity. A certain viscosity level is required to obtain high-quality films by casting.

[0212] Another advantage in the case of N-(2'-hydroxyethyl)-2-pyrrolidone is due to the fact that lower molecular weight water-soluble polymers can be removed from the resulting membrane by simple washing with water. In this case, an oxidative post-treatment is not necessary, whereas in the case of other solvents, only higher molecular weight water-soluble polymers can be significantly removed if an oxidative post-treatment is performed. In the case of N-(2'-hydroxyethyl)-2-pyrrolidone, even without an oxidative post-treatment, the resulting membranes show a combination of a higher pure water permeability and a lower molecular weight cut-off, whereas the membranes produced from the other solutions show a lower PWP and a higher MWCO. In the case of the other solvents, an oxidative post-treatment results in a higher PWP but at the same time a significantly higher MWCO.

[0213] Examples 21-48: Solutions with different water-soluble polymers and their films without oxidative post-treatment

[0214] Different films were prepared according to the above procedure. In doing so, different water-soluble polymers, i.e. different polyvinyl pyrrolidone grades or poly (ethylene oxide) (PEO 400) were used. The viscosity and turbidity of the polymer solution before the casting step were determined according to the above procedure. The post-treatment of the film was completed only by washing with water (post-treatment A). The PWP and MWCO of the obtained film were analyzed according to the above method. The results are summarized in Tables 4-7.

[0215] Table 4: Preparation of K12 and PEO 400 Composition and characteristics of E 3010 membrane

[0216]

[0217] Table 5: Prepared with K12 Composition and characteristics of E 3010 membrane

[0218]

[0219] Table 6: Prepared with K25 Composition and characteristics of E 3010 membrane

[0220]

[0221] Table 7: Prepared with K30 Composition and characteristics of E 3010 membrane

[0222]

[0223] Tables 4-7 show that the solutions according to the invention based on N-(2'-hydroxyethyl)-2-pyrrolidone show a higher solution viscosity than the corresponding NMP solutions. Therefore, the solutions according to the invention are better suited for cast films. If the resulting membrane is not post-treated oxidatively but only post-treated by washing with water, the resulting membrane shows a favorable balance of PWP and MWCO in the case of N-(2'-hydroxyethyl)-2-pyrrolidone, while in the case of NMP, extremely low PWP values ​​occur, which are not suitable for efficient separation procedures.

Claims

1. A solution comprising (a) a polyarylsulfone polymer selected from polyethersulfones comprising repeating units of formula I and a polysulfone comprising a repeating unit of formula II or a mixture thereof, wherein the weight average molecular weight M of the polyarylsulfone polymer is w in the range of 40000 g / mol to 105000 g / mol, and the polyarylsulfone polymer comprises at least 95 wt.-% of repeating units of the formulae I and II, based on the total weight of the polyarylsulfone polymer, as well as (b) N-(2'-Hydroxyethyl)-2-pyrrolidone.

2. The solution according to claim 1, wherein The solution comprises 50 wt.-% to 99 wt.-% N-(2'-hydroxyethyl)-2-pyrrolidone, based on the total weight of the solution.

3. The solution according to any one of claims 1 to 2, wherein The solution comprises 1 wt.-% to 50 wt.-% of the polyarylsulfone polymer, based on the total weight of the solution.

4. The solution according to any one of claims 1 to 3, comprising a water-soluble polymer showing a solubility in water at 21°C of at least 10 g / 100 g water.

5. The solution according to claim 4, wherein The solution comprises 0.1 wt.-% to 15 wt.-% of a water-soluble polymer, based on the total weight of the solution.

6. The solution according to any one of claims 4 to 5, wherein The water-soluble polymer is selected from polyvinyl pyrrolidone, poly(alkylene glycol) or a mixture thereof.

7. The solution according to any one of claims 1 to 6, comprising an additive, the polyarylsulfone polymer being soluble in the additive at a concentration of <1 g polyarylsulfone polymer / 100 g additive at 21°C.

8. The solution according to claim 7, wherein The solution comprises 0.1 wt.-% to 20 wt.-% of the additive, based on the total weight of the solution.

9. The solution according to any one of claims 7 to 8, wherein The additive is selected from the group consisting of: Water, C1-C4 alkanol, C2-C8 alkanediol, oligo(alkylene glycol) and C3-C 12 alkantriol or a mixture thereof.

10. A solution according to any one of claims 1 to 9, which exhibits a viscosity of 4 Pa ​​as determined with a Brookfield viscometer DV-IPrime with RV 6 spindle at 60°C and 5-100 rpm (revolutions per minute). . s to 150Pa . s solution viscosity.

11. The solution according to any one of claims 1 to 10, which exhibits a solution turbidity of 0 NTU (nephelometric turbidity unit) to 1 NTU as determined with a turbidimeter at 60°C using an 860 nm filter.

12. A method for preparing a membrane using the solution according to any one of claims 4 to 11.

13. The method according to claim 12, comprising the steps of: a) preparing a solution according to any one of claims 4 to 11, b1) preparing a polymer film from the solution of step a) by applying the solution of step a) to a substrate, and b2) exposing the polymer film to a coagulant.

14. The method according to claim 12, comprising the steps of: a) preparing a solution according to any one of claims 4 to 11, and b) continuously shaping the solution of step a) and stratifying the solution into non-planar geometric shapes by directly exposing the solution to a coagulant that causes continued solidification of the shaped solution.

15. Use of the membrane according to any one of claims 12 to 14 for purification of drinking water, treatment of industrial or municipal wastewater, desalination of seawater or brackish water, dialysis, purification of pharmaceuticals, plasmolysis and food processing.

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