Solution of sulfone polymers in 2-(2-oxopyrrolidin-1-yl)ethyl acetate for the use of membranes

Abstract

The present application relates to a solution comprising at least one polyarylsulfone selected from the group of polyethersulfone and polysulfone as polymer and 2-(2-oxopyrrolidin-1yl)ethyl acetate (HEPA), the process of making a membrane and the use of this membrane for separation processes.

Description

[0001] Solution of sulfone polymers in 2-(2-oxopyrrolidin-1-yl)ethyl acetate for the use of membranes

[0002] The present application relates to a solution comprising at least one polyarylsulfone selected from the group of polyethersulfone and polysulfone as polymer and 2-(2-oxopyrrolidin-1yl)ethyl acetate (HEPA), the process of making a membrane and the use of this membrane for separation processes.

[0003] Polyarylsulfone polymers, such as polysulfones (i.a. merchandized under the trade names: III- trason® S, UJU Paryls®) and polyethersulfones (i.a. merchandized under the trade names: III- trason® E, Veradel®, Sumicaexel®) are high performance polymers which are used in a variety of technical applications because of their mechanical properties and their chemical and thermal stability. One major technical application of polyarylsulfone polymers is their usage as raw materials for the production of polymer membranes, for example, dialysis or ultrafiltration membranes. Polymer membranes are used in separation processes and are widely applied, e.g., in medical applications, the food technology, biotechnology, pharmaceutical industries, and water treatment.

[0004] In J. G. Wijmans et. al., Eur. Polym. J., 1983, Vol.19, No.12, pp. 1143-1146 it is pointed out that polyarylsulfone polymers have a limited solubility in many common solvents. As disclosed by M. Munari in Desalination 1988, 70, 265-275 polyarylsulfones are used as materials for porous membranes. Due to the low molecular weight of poyarylsulfones their solutions are characterized by low viscosities and are therefore difficult to cast.

[0005] WO 2019042749 A1 discloses a process for making a membrane by bringing a polymer solution comprising a polymer, a first solvent, and a co-solvent in contact with a coagulating agent. As first solvent N-methylpyrrolidone and N-ethylpyrrolidone, both polar aprotic solvents, are claimed. Polyvinylpyrrolidone which have high molecular weights is used as a water-soluble polymer which acts as pore forming agent and solution viscosity modifier at the same time. Furthermore, a membrane obtained by this process is disclosed. Unfortunately N-methylpyrrolidone as well as N-ethylpyrrolidone exhibits numerous toxicological disadvantages as they are classified as CMR (cancerogenic, mutagenic or toxic for reproduction) agents.

[0006] WO 2021191043 as well as PCT / EP2021 / 082449 describe the use of N-n-butyl-2-pyrrolidone and N-tert.butyl-2-pyrrolidone as alternative aprotic solvents for N-methylpyrrolidon in different polymer classes, such as polyarylsulfone polymers. The use of such solvents for polyarylsulfone polymers and solution of these together with water-soluble polyvinylpolymers for the membrane fabrication is also disclosed herein. But both alternative solvents are not biodegradable. Thus, washing water incurred during the membrane production process including these solvents has to be purified before it will be combined with the wastewater that is normally purified in the wastewater treatment. Furthermore, nanofiltration membranes with pore sizes below 3 nm are not received with such solvents.

[0007] 2-(2-Oxopyrrolidin-1-yl)ethyl acetate (HEPA) (CAS-No. 52580-83-3) with formula (la) is prepared by acetylation of N-(2’-hydroxyethyl)-2-pyrrolidone. N-(2’-hydroxyethyl)- 2-pyrrolidone (CAS-No. 3445-11-2) is commercially available in industrial scale. 2-(2- Oxopyrrolidin-1yl)ethyl acetate (HEPA) is a polar aprotic solvent and of low toxicological concern in contrast 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 statements (Globally Harmonized System of Classification and Labelling of Chemicals). Furthermore 2-(2-oxopyrrolidin-1yl)ethyl acetate (HEPA) is readily biodegradable according to OECD 301 B with International Standard ISO 9439. This means that washing water containing HEPA that incurred during the membrane production process can be combined with the wastewater which is purified in the wastewater treatment plant without any further pretreatment.

[0008] There is a great ongoing demand of solvents which are suitable to replace the currently used solvents in specific applications. These solvents must fulfill different demands like showing less problematic toxicological profile and should show a good biodegradation degree which allows to combine them with the wastewater stream without any pretreatment compared to the conventionally used solvents, like N-methyl-2-pyrrolidone (NMP), 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). Therefore, it is an object of the present invention to provide solutions with high sulfone polymer content without turbidity. The solution containing water-soluble polymers should be stable and clear, since this influences the pore structure and thus the quality of the membranes obtained from these solutions. Since the solvents conventionally used for membrane fabrication result in low viscous polymer solutions, which are difficult to cast, there is also the demand for solvent systems leading to polymer solu- tions with higher viscosity. Furthermore, it is required, that the solvents utilized for membrane production by non-solvent induced phase inversion show complete miscibility with water which is used as coagulant. Regarding membranes obtained by using these polymer solutions, it is important, that at least the same standard of membrane quality and possibly an even better membrane quality is achieved. In particular, the water permeability of such membranes should be 30kg / h m2bar or higher combined with a molecular weight cutoff which can be used for high flux nanofiltration membranes with a pore size below 3 nm.

[0009] Surprisingly, these problems will be solved by a solution comprising a) a polyarylsulfone polymer selected from the group of polyethersulfone polymers comprising repeating unit of formula (I) and polysulfone polymer comprising repeating unit of formula (II) or mixtures thereof, wherein the weight average molecular weight Mwof the polyarylsulfone polymer is in the range from 40000 to 105000 g / mol and the polyarylsulfone polymer comprises at least 95 wt.-% of the repeating units of formula (I) and / or (II) based on the total weight of the polyarylsulfone polymer, and b) 2-(2-Oxopyrrolidin-1yl)ethyl acetate.

[0010] The inventive solution will be advantageous, if the polyarylsulfone polymer is a polyethersulfone polymer comprising repeating unit of formula (I) wherein the weight average molecular weight Mwof the polyethersulfone polymer is in the range from 40000 to 105000 g / mol and the polyethersulfone polymer comprises at least 95 wt.-% of the repeating units of formula (I) based on the total weight of the polyethersulfone polymer.

[0011] The inventive solution will be advantageous, if the solution comprises 10 to 25 wt.-% of polyarylsulfone polymer based on the total weight of the solution.

[0012] The inventive solution will be advantageous, if the solution comprises 60 to 89 wt.-% of 2-(2- oxopyrrolidin-1-yl)ethyl acetate based on the total weight of the solution.

[0013] The inventive solution will be advantageous, if the solution comprising 1 to 15 wt.-% of a water - soluble polymer showing a solubility in water at 21°C of at least 10 g per 100 g water based on the total weight of the solution.

[0014] The inventive solution will be advantageous, if the water-soluble polymer is selected from polyvinylpyrrolidone, poly(alkylene glycol) with an Mw of 200 to 2,000,000 g / mol or mixtures thereof.

[0015] The inventive solution will be advantageous, if the solution comprises 60 to 88 wt.-% of 2-(2- oxopyrrolidin-1yl)ethyl acetate and 1 to 15 wt.-% of an additive, in which the polyarylsulfone polymer is soluble at 21°C at concentration of < 1g polyarylsulfone polymer per 100 g additive both based on the total weight of the solution.

[0016] The inventive solution will be advantageous, if the additive is selected from the group of C1-C4 alkanols, C2-C8 alkanediols, oligo(alkylene glycol)s, and C3-C12 alkanetriols or mixtures thereof.

[0017] The inventive solution will be advantageous, if it shows a solution viscosity of 2 to 30 Pa s as determined with a Brookfield Viscometer DV-I Prime with RV 6 spindle at 60°C and 20- 100 rpm.

[0018] The inventive solution will be advantageous, if it shows a turbidity of 0 to 1 ,5 NTU as determined with a turbidimeter employing a filter of 860 nm at 60°C.

[0019] Another embodiment of the invention is a process for the preparation of a membrane using the inventive solution.

[0020] The inventive process comprising the following steps: a) preparing a solution according to any of claims 1 to 9, b) preparing a polymer film from the solution of step a) by applying the solution of step a) to a substrate and c) exposing the polymer film to a coagulant.

[0021] Another embodiment of the invention is the use of inventive membranes received by the inventive process for drinking water purification, treatment of industrial or municipal wastewater, desalination of sea or brackish water, dialysis, purification of pharmaceutical products, plasmol- ysis and food processing.

[0022] Polyarylsulfone polymer

[0023] The polyarylsulfone polymer comprises at least 95wt.-% (% per weight), preferably at least 97wt.-%, more preferably at least 98wt.-%, and particularly preferably at least 99wt.-% of the repeating units of formula (I) and / or (II) based on the total weight of the polyarylsulfone polymer. The polyarylsulfone polymer is a polymer comprising -S(=O)2- units in the polymer. -S(=O)2- units are also designated as sulfonyl units.

[0024] The polyarylsulfone polymer comprises aromatic groups. Beside 1 ,4-phenylene units, the polyarylsulfone polymer can also comprise units based on minor isomers, like 1 ,2-phenylene or 1 ,3- phenylene units.

[0025] The polyarylsulfone polymer comprises endgroups at the polymer chain ends, like CI-, OH-, MeO-, tert. butyl-, or arylgroups. The different chain ends may comprise different endgroups.

[0026] The polyarylsulfone polymer usually comprises 0.02 to 1wt.-% (% by weight) endgroups, preferably 0.03 to 0.8wt.-% based on the total weight of the polyarylsulfone polymer.

[0027] The polyarylsulfone polymer comprises < 5wt.-% further chemical groups, which are not represented by the repeating units of formula (I) and / or (II), preferably < 3wt.-%, more preferably < 2wt.-%, and particularly preferably < 1wt.-% based on the total weight of the polyarylsulfone polymer. These further chemical groups typically are endgroups or minor aromatic isomers as described above.

[0028] As examples for polyethersulfone polymers comprising the repeating unit of formula (I) Ultra- son® E grades available from BASF SE, like Ultrason® E 3010, Ultrason® E 6020 P or Ultra- son® E 7020 P are mentioned.

[0029] As examples for polysulfone polymers comprising the repeating unit of formula (II) Ultrason® S grades available from BASF SE, like Ultrason® S 3010, Ultrason® S 6010 or UJU Paryls®, which is a polysulfone polymer comprising the repeating unit of formula (II) available from UJU New Materials, are mentioned. The weight average molecular weight Mwof a polymer is based on the summarized mass of molecules having a certain molecular mass. To this value, larger, i.e. higher mass molecules have a larger contribution than smaller, i.e. lower mass molecules. Mwand its determination are well known in the art. Mwcan be, for example, determined by gel permeation chromatography (GPC), using a suitable column (stationary phase), solvent (liquid phase), and detector (e.g., via refractive index or UV). These chromatography units have to be calibrated using standard polymers of known molecular weight (e.g., polystyrene). Considering the detector’s signal and the calibration curve, attached computer systems calculate the chromatogram which represents the molecular weight distribution of the respective sample. The molecular weight distribution describes the number of molecules present in a sample versus their molar mass. Mwand the number average molecular weight Mnare received via computational analysis. Mnis based on the number of molecules bearing a specific mass. Using these two values, also the polydispersity index (PDI = Mw / Mn) can be calculated which is a measure for the broadness of the molecular weight distribution.

[0030] The polyarylsulfone polymers show weight-average molecular weight values Mwof 40000 to 105000 g / mol, preferably 45000 to 100000 g / mol, and more preferably 50000 to 95000 g / mol as determined via gel permeation chromatography (GPC) in tetrahydrofuran (THF) and polystyrene (PS) as standard. The preferred polyethersulfone polymers comprising the repeating unit of formula (I) show weight-average molecular weight values of 40000 to 100000 g / mol, preferably 48000 to 92000 g / mol as determined via GPC in THF and PS as standard.

[0031] The preferred polyarylsulfone polymers show polydispersity indices of 2 to 5, preferably 2.5 to 4.7. The preferred polyethersulfone polymers comprising the repeating unit of formula (I) show PDI-values of 2.7 to 3.8, preferably 2.8 to 3.6.

[0032] The glass transition temperature Tgis the temperature region at which polymer chain segments become mobile and the sample undergoes a reversible transition from solid, amorphous regions to a softer state of higher flexibility. The glass transition temperature and its determination is well known in the art. It can be determined for example by differential scanning calorimetry (DSC), measuring the heat capacity versus the temperature, whereby the temperature is changed at constant speed (e.g., 10 K / min). Usually, the sample is first cooled and then heated with the same speed. The glass transition temperature can be received from the resulting diagram by geometrical or computational analysis.

[0033] The preferred polyarylsulfone polymers, especially the polyethersulfone polymers, show glass transition temperatures of 160 to 250 °C, preferably 170 to 240 °C, and more preferably from 180 to 230 °C, measured via differential scanning calorimetry (DSC) according to ISO 11357-1 (2017) and11357-2 (2020) at a heating rate of 10 K / min. The viscosity number (reduced viscosity, VN) correlates with the molecular weight of the polymer and can be measured based on ISO 1628-5 (1998) in a 1wt.-% polymer solution in N-methylpyrrolidone. Thereby, the elution time (t) of a defined volume of the polymer solution in an Ubbelohde 1C-capillary is related to the running time of the pure solvent (to) and normalized afterwards with the polymer concentration (c in g / ml) according to equation 1 :

[0034] The viscosity number is given in ml / g. The preferred polyarylsulfone polymers, especially the polyethersulfone polymers show viscosity numbers of 40 to 130 ml / g, preferably 50 to 120 ml / g, and most preferably of 60 to 110 ml / g, based on ISO 1628-5 (1998) in a 1wt.-% polymer solution in N-methylpyrrolidone.

[0035] Solvent

[0036] The inventive solution comprises 2-(2-oxopyrrolidin-1yl)ethyl acetate (HEPA) [CAS 52580-83-3] as solvent, which is available by acetylation of N-(2’-hydroxyethyl)-2-pyrrolidone which is commercially available in industrial scale. 2-(2-Oxoxpyrrolidin-1yl)ethyl acetate is a polar-aprotic solvent, biodegradable according to OECD 301 B with International Standard ISO 9439 and of low toxicological concern in contrast to various frequently used aprotic pyrrolidone derivates, such as N-methyl-2-pyrrolidone which is classifies as “May damage fertility. May damage unborn child.” According to the GHS (Globally Harmonized System of Classification and Labelling of Chemicals). Polyarylsulfone polymers and mixtures of polyarylsulfone polymers, preferably polyethersulfone polymers or mixtures of different polyethersulfone polymers, with water-soluble polymers can be well dissolved by 2-(2-oxopyrrolidin-1yl)ethyl acetate leading to clear solutions showing surprisingly high viscosities. Both, clarity and a minimum viscosity value are requirements for the preparation of high-quality membranes from such solutions.

[0037] Water-soluble polymers

[0038] The inventive solution may further comprise a water-soluble polymer, i.e., a polymer that can be readily dissolved in water to give a clear solution in concentrations of at least 10 g polymer per 100 g water at 21 °C. These water-soluble polymers influence the solution viscosity and therefore act as viscosity modifiers. Furthermore, these water-soluble polymers act as pore forming agent in the process of membrane preparation and therefore strongly influence the membrane characteristics. Usually, the water-soluble polymers filling the formed pores are removed in a work-up step of membrane preparation. This step can comprise different washing and / or oxidiz- ing steps to remove the water-soluble polymers, which, after removal, leave the empty pores. Beside other parameters, polymer type and molecular weight strongly influence the effort necessary for removal of the water-soluble polymers during the work-up of the membrane.

[0039] Preferably, the water-soluble polymer is selected from the group of polyvinylpyrrolidone and poly(alkylene glycol)s or mixtures thereof usually showing a weight-average molecular weight values Mwbetween 200 to 2,000,000 g / mol. More preferably, the water-soluble polymer is selected from the group of polyvinylpyrrolidone, poly(ethylene oxide), polypropylene oxide), and poly(ethylene oxide) / poly(propylene oxide)-block-copolymers or mixtures thereof showing a Mw between 200 and 2,000,000 g / mol. Particularly preferably, the water-soluble polymer is selected from the group of polyvinylpyrrolidone and poly(ethylene oxide) or mixtures thereof with Mw between 200 and 2,000,000 g / mol and, in the case of polyvinylpyrrolidone with a solution viscosity characterized by the K-value of 12 or higher determined according to the method of Fikentscher 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 a polyvinylpyrrolidone with Mwof at least 2000 g / mol and a solution viscosity characterized by the K-value of 12 or higher determined according to the method of Fikentscher described by Fikentscher in Cellulosechemie 13, 1932 (58).

[0040] Preferably, the solution comprises 1 to 15wt.-% of water-soluble polymer, more preferably 2 to 10wt.-%, and particularly preferably 3 to 7wt.-% based on the total weight of the solution. More preferably, the solution comprises 1 to 15wt.-% of water-soluble polymer selected from the group of water-soluble polyvinylpyrrolidone and water-soluble poly(alkylene glycol)s or mixtures thereof, preferably 2 to 10wt.-%, and more preferably 3 to 7wt.-% based on the total weight of the solution.

[0041] Particularly preferably, the solution comprises 1 to 15wt.-% of water-soluble polymer selected from the group of water-soluble polyvinylpyrrolidone, water-soluble poly(ethylene oxide), water- soluble polypropylene oxide), and water-soluble poly(ethylene oxide) / poly(propylene oxide)- block-copolymers or mixtures thereof, preferably 2 to 10wt.-%, and more preferably 3 to 7wt.-% based on the total weight of the solution.

[0042] Very particularly preferably, the solution comprises 1 to 15wt.-% of water-soluble polymer selected from the group of water-soluble polyvinylpyrrolidone and water-soluble poly(ethylene oxide) or mixtures thereof, preferably 2 to 10wt.-%, and more preferably 3 to 7wt.-% based on the total weight of the solution. Most preferably, the solution comprises 1 to 15wt.-% of water-soluble polyvinylpyrrolidone, preferably 2 to 10wt.-%, and more preferably 3 to 7wt.-% based on the total weight of the solution.

[0043] Additives

[0044] The inventive solution may further comprise an additive, i.e. , one or more additives or mixtures thereof. Additives are compounds, which dissolve the polyarylsulfone polymer selected from the group of polyethersulfone comprising repeating units of formula (I) and polysulfone polymer comprising repeating units of formula (II) or mixtures thereof only at low concentrations of < 1 g polyarylsulfone polymer per 100 g additive at 21 °C, preferably polyethersulfones comprising repeating units of formula (I) were dissolved alone. In the process of membrane preparation, additives influence the velocity of solvent exchange and thus of the precipitation process and therefore control membrane properties like pore size and number.

[0045] Preferably, the additive is selected from the group of water, C1-C4 alkanols, C2-C8 alkanediols, oligo(alkylene glycol)s, and C3-C12 alkanetriols or mixtures thereof. More preferred additives are methanol, ethanol, propanol, isopropanol, n-butanol, iso-butanol, 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-

[0046] 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-

[0047] 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, hexa-1 ,5-diene-3, 4-diol, neopentyl glycol, 2- methyl pentane-2, 4-diol, 2, 4-dimethylpentane-2, 4-diol, 2-ethylhexane-1 , 3-diol,

[0048] 2, 5-dimethylhexane-2, 5-diol, 2,2,4-trimethylpentane-1 ,3-diol, pinacol, glycerol, butane-

[0049] 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-

[0050] 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 mixtures thereof. Much more preferred additives are 1 ,2-propandiol,

[0051] 1.3-propandiol, propanol, isopropanol and glycerol.

[0052] Preferably, the solution comprises 1 to 15 wt.-%, more preferably 1 to 14 wt.-%, particularly preferably 2 to 13 wt.-% and very particularly preferably 5 to 11 wt.-% of the additive based on the total weight of the solution.

[0053] Cosolvents

[0054] Beside 2-(2-oxopyrrolidin-1yl)ethyl acetate, the inventive solution may comprise further solvents herein designated as cosolvents. In the process of membrane preparation, cosolvents influence the velocity of solvent exchange and thus of the precipitation process and therefore control membrane properties like pore size and number.

[0055] Preferred cosolvents are solvents that are readily miscible with 2-(2-oxopyrrolidin- 1 yl)ethyl acetate at any ratio. More preferred suitable cosolvents are, for example, selected from high-boiling ethers, esters, ketones, asymmetrically halogenated hydrocarbons, anisole, gammavalerolactone, N,N-dimethylformamide, dimethylsulfoxide, dihydrolevoglucosenone, 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, methyl 1-methyl-5- oxopyrrolidine-3-carboxylate, N,N-dimethyl-2-hydroxypropanoic amide, and N,N-diethyl- 2-hydroxypropanoic amide. Particularly preferred cosolvents are gamma-valerolactone, N-tert- butyl-2-pyrrolidone, methyl 1-methyl-5-oxopyrrolidine-3-carboxylate, N-(2’-hydroxyethyl)-2- pyrrolidone and N-n-butyl-2-pyrrolidone.

[0056] In a preferred embodiment, at least 10wt.-%, more preferably at least 50wt.-%, particularly preferably at least 80wt.-%, and very particularly preferably at least 90wt.-% of the total weight amount of all solvents of the solution is 2-(2-oxopyrrolidin-1yl)ethyl acetate.

[0057] In a more preferred embodiment, no cosolvent is used in the solution and 2-(2-oxopyrrolidin- 1 yl)ethyl acetate is the only solvent used.

[0058] Preparation of the solution

[0059] The solution of the invention can be easily prepared by combining the different components of the solution. In general, there is no specific order in which the different components are to be added. Therefore, the solution of the invention can be prepared by combining all the components, i.e. polyarylsulfone polymer, especially polyethersulfone , 2-(2-oxopyrrolidin-1yl)ethyl acetate, and if need be, further components like water-soluble polymer, additives, or cosolvents, and dissolving all compounds according to any process known in the art

[0060] The solution process may, for example, advantageously be supported or accelerated by increasing the temperature of the mixture and / or by mechanical operations like stirring, shaking, subjection to ultrasound (e.g. by using an ultrasonification bath) etc.

[0061] In a preferred embodiment under stirring at 60 °C, first the polyarylsulfone is dissolved in 2-(2-oxopyrrolidin-1yl)ethyl acetate and subsequently, the water-soluble polymer and additives are added and stirred until a clear solution is received.

[0062] The inventive solution usually shows a viscosity of 0.1 to 80 Pa s, preferably 0.5 to 60 Pa s, particularly preferably 1 to 50 Pa s, and most preferably of 2 to 30 Pa s as determined with a Brookfield Viscometer DV-I Prime from Brookfield Engineering Laboratories, Inc. Middleboro, USA) with RV 6 spindle at 60 °C with 5-100 rpm (rounds per minute). The shear rate, i.e., rounds per minute, is usually selected depending on the viscosity of the sample. Typically, low viscous samples are measured at higher shear rates, whereas higher viscous samples are measured at lower shear rates.

[0063] High solution viscosities, as shown by the inventive solutions, are beneficial for the casting process during membrane preparation by leading to better film quality. Furthermore, in case of high solution viscosities, during membrane preparation there is generally no need to use high molecular weight water-soluble polymers to (further) increase the viscosity.

[0064] A measure for turbidity is the nephelometric turbidity unit (NTU) determined with a calibrated nephelometer or turbidimeter, expressing the amount of light reaching a detector at the side of a light beam after being scattered of the sample. Preferably, the inventive solution shows a turbidity of 0 to 2 NTU, more preferably of 0 to 1.5 NTU, particularly preferably of 0 to 1.4 NTU as determined with a turbidimeter 2100 AN (Hach Lange GmbH, Dusseldorf, Germany) employing a filter of 860 nm at 60 °C.

[0065] A low solution turbidity is a prerequisite for the preparation of high-quality films and thus, high performance membranes, because hereby macrovoids and defects in the membranes are avoided which would affect the balance between pure water permeability and molecular weight cut-off.

[0066] Preparation of the membrane

[0067] We have further found a process for preparing a membrane using a solution comprising

[0068] (a) a polyarylsulfone polymer selected from the group of polyethersulfone comprising the repeating unit of formula (I) and polysulfone comprising the repeating unit of formula (II) or mixtures thereof, wherein the weight average molecular weight Mwof the polyarylsulfone polymer is in the range from 40000 to 105000 g / mol and the polyarylsulfone polymer comprises at least 95wt.-% of the repeating units of formula (I) and (II) based on the total weight of the polyarylsulfone polymer,

[0069] (b) 2-(2-oxopyrrolidin-1yl)ethyl acetate, and

[0070] (c) a water-soluble polymer.

[0071] In the context of this application, a membrane shall be understood to be a semipermeable structure capable of separating two fluids or separating molecular and / or ionic components or particles from a liquid. A membrane acts as a selective barrier, allowing some particles, substances or chemicals to pass through, while retaining others. The membrane may have various geometries such as flat sheet, spiral wound, pillows, tubular, single bore hollow fiber or multiple bore hollow fiber. Separation by using membranes can be operated in different ways, e.g., driven by pressure, by concentration gradients or by gradients like electric potential or temperature gradients. Examples for pressure driven operations are micro-, ultra- or nanofiltration or reverse osmosis operations. Examples for concentration-based operations are dialysis or forward osmosis. A good overview is given in WO 2017 / 045985 or in M. Ulbricht, Polymer 47 (2006), pp.2217-2262 which are herewith incorporated herein by reference. A preferred membrane is the ultrafiltration membrane.

[0072] Membranes can be prepared by different methods, like phase separation (phase inversion) of polymers, sol-gel process, interface reaction, stretching, extrusion, track-etching, microfabrication, etc.

[0073] Preferably, the membranes according to the invention are prepared by liquid nonsolvent- induced phase separation (NIPS), which comprises the following steps: a) Preparing a solution comprising a polyarylsulfone polymer selected from the group of polyethersulfone comprising the repeating unit of formula (I) and polysulfone comprising the repeating unit of formula (II) or mixtures thereof, wherein the weight average molecular weight Mwof the polyarylsulfone polymer is in the range from 40000 to 105000 g / mol and the polyarylsulfone polymer comprises at least 95wt.-% of the repeating units of formula (I) and (II) based on the total weight of the polyarylsulfone polymer, 2-(2-oxopyrrolidin- 1 yl)ethyl acetate, and a water-soluble polymer. b) Shaping the polymer solution into a certain geometry, like, for example, fiber, tubular or flat sheet geometries, by methods like, for example, extrusion of the polymer solution or preparation of a film from or casting the polymer solution. c) Solidifying the geometry shaped in Step b) by exposing the polymer solution to a coagulant.

[0074] Step a)

[0075] The solution of step a) corresponds to a 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 performed as described above.

[0076] In a preferred embodiment, the solution of step a) comprises 10 to 25wt.-% of the polyarylsulfone polymer, 60 to 89wt.-% of 2-(2-oxopyrrolidin-1yl)ethyl acetate, and 1 to 15wt.-% of water-soluble polymer based on the total weight of the solution. In a more preferred embodiment, the solution of step a) comprises 14 to 20wt.-% of the polyarylsulfone polymer, 60 to 84wt.-% of 2-(2-oxopyrrolidin-1yl)ethyl acetate, and 2 to 10wt.-% of water-soluble polymer based on the total weight of the solution.

[0077] In one embodiment, the solution may be degassed and / or heated before proceeding to the next step.

[0078] Step b) and c)

[0079] Steps b) and c) can either be performed continuously, i.e. not as separated steps, for example, in case of the continuous extrusion of the polymer solution into a coagulation bath, or in separated steps, for example first forming a polymer film, which is then transferred to a coagulation bath after a certain drying time.

[0080] The shaped polymer solution still contains the polyarylsulfone polymer, the solvent, the water- soluble polymers etc. The phase separation of polymer and solvent has not started or is not yet completed, i.e. the polymer is not yet completely solidified. In case of form shaping by extrusion, the polymer solution is brought into a fiber- or tubular-shaped geometry. In case of form shaping by film generation or casting, the polymer solution is brought into a flat sheet geometry. The exposition of the shaped polymer solution to the coagulant (step c) can, for example, take place in a coagulation bath containing a coagulant. The polyarylsulfone polymer should have a low solubility in the coagulant, i.e. < 1 g polyarylsulfone polymer per 100 g of coagulant at 21 °C.

[0081] The contact to the coagulant induces the nonsolvent-induced de-mixing of the homogeneous polymer solution, which results in the solidification of the polymer and thus the membrane formation in the respective geometry of the shape of the polymer solution. The membrane structure and morphology is strongly dependent on the process parameters used hereby, as well as on the nature and presence of the different chemical compounds present in the solution. The water-soluble polymer and the additives have mainly two functions: on the one hand side they adjust the solution viscosity on a high level which makes film formation, e.g., by casting easier, on the other side they function as pore formation agents, strongly determining the membrane’s performance properties.

[0082] Suitable coagulants are, for example, liquid water, water vapor and mixtures of water with alcohols, cosolvents, and / or the solvent of the inventive solution, i.e., 2-(2-oxopyrrolidin-1yl)ethyl acetate. Suitable alcohols are, for example, mono-, di- or trialcohols selected from the group of C1-C4 alkanols, C2-C8 alkanediols, oligo(alkylene glycol)s, C3-C12 alkanetriols as they can be used as additives in the inventive solution (see above), or poly(ethylene oxide) with Mwof 100 to 1000 g / mol. Suitable cosolvents are, for example, selected from high-boiling ethers, esters, ketones, asymmetrically halogenated hydrocarbons, anisole, gamma-valerolactone, N,N-dimethylformamide, dimethylsulfoxide, sulfolane, dihydrolevoglucosenone, 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, methyl 1-methyl-5-oxopyrrolidine-3-carboxylate, N,N-dimethyl- 2-hydroxypropanoic amide and N,N-diethyl-2-hydroxypropanoic amide. Preferred cosolvents are gamma-valerolactone, N-tert-butyl-2-pyrrolidone, methyl 1-methyl-5-oxopyrrolidine-3- carboxylate, N-(2’-hydroxyethyl)-2-pyrrolidone and N-n-butyl-2-pyrrolidone.

[0083] Preferably, coagulants are mixtures comprising liquid water and the solvent 2-(2’-pyrrolidin- 1 yl)ethyl acetate or mixtures comprising liquid water and alcohols, e.g., poly(ethylene oxide) with Mw of 100 to 1000 g / mol and / or mixtures comprising liquid water and cosolvents. More preferably, coagulants are mixtures comprising liquid water and the solvent 2-(2-oxopyrrolidin- 1yl)ethyl acetate. Said coagulants may comprise from 10 to 90wt.-% water and 90 to 10wt.-% alcohol and / or co-solvent(s) or the solvent 2-(2-oxopyrrolidin-1yl)ethyl acetate, preferably 30 to 70wt.-% water and 70 to 30wt.-% alcohol and / or co-solvent(s) or the solvent 2-(2-oxopyrrolidin- 1 yl)ethyl acetate, based on the total weight of the coagulant. Generally, the amount of all components of the coagulant adds up to 100%.

[0084] Further details of process steps a) and b) depend on the desired geometrical structure of the membrane and the scale of production, which includes lab scale or commercial / industrial scale.

[0085] In a preferred process, a flat sheet membrane is prepared. This process comprises: a) Preparing a solution comprising a polyarylsulfone polymer selected from the group of polyethersulfone comprising the repeating unit of formula (I) and polysulfone comprising the repeating unit of formula (II) or mixtures thereof, wherein the weight average molecular weight Mwof the polyarylsulfone polymer is in the range from 40000 to 105000 g / mol and the polyarylsulfone polymer comprises at least 95wt.-% of the repeating units of formula (I) and (II) based on the total weight of the polyarylsulfone polymer, 2-(2-oxopyrrolidin- 1 yl)ethyl acetate, and a water-soluble polymer. b) Preparing a polymer film from the solution of step a) by applying the solution of step a) to a substrate c) Exposing the polymer film to a coagulant. Step a)

[0086] The solution of step a) corresponds to a 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 performed as described above.

[0087] In a preferred embodiment, the solution of step a) comprises 10 to 25wt.-% of the polyarylsulfone polymer, 60 to 89wt.-% of 2-(2-oxopyrrolidin-1yl)ethyl acetate, and 1 to 15wt.-% of water-soluble polymer based on the total weight of the solution.

[0088] In a more preferred embodiment, the solution of step a) comprises 14-20wt.-% of the polyarylsulfone polymer, 60 to 84wt.-% of 2-(2-oxopyrrolidin-1yl)ethyl acetate and 2 to 10wt.-% of water- soluble polymer based on the total weight of the solution.

[0089] Preferably, the solution is be prepared under stirring at a temperature of 20 to 100 °C, preferably 40 to 90 °C, more preferably 50 to 70 °C. The ready solution typically is degassed for 2 to 24 h, preferably 6 to 20 h, more preferably 10 to 14 h. Subsequently, the solution is preferably re-heated at a temperature of 20 to 100 °C, preferably 40 to 90 °C, more preferably 50 to 70 °C for 1 to 4 h, preferably 1.5 to 3 h.

[0090] Step b)

[0091] The preparation of a polymer film from the inventive solution can be performed, for example, by casting or other methods like rolling, spraying etc., preferably by casting onto a substrate, preferably, a polymeric substrate (like, e.g., polymer films from biaxially oriented poly(ethylene terephthalate), e.g., under trade name Hostaphan®), a glass substrate, or metal substrates (e.g., metal conveyor belt) using, for example, a casting knife and, optionally, an automated coating machine at a temperature of 40 to 90 °C, preferably 50 to 70 °C.

[0092] In a preferred embodiment, the polymer film is generated by casting, using a casting knife with an aperture of 100 to 500 pm, more preferably of 200 to 400 pm.

[0093] Step c)

[0094] Typically, the polymer film is allowed to rest for 5 to 150 s, preferably 10 to 100 s, more preferably 20 to 60 s and is then subjected to the coagulation bath containing a coagulant as described above. The immersion takes place at 10 to 50 °C, preferably 15 to 40 °C, more preferably at 20 to 30 °C for 3 to 20 min, preferably 5 to 15 min. Subsequently, the membrane is detached from the substrate and usually subjected to work-up.

[0095] In a further preferred process, a non-flat sheet membrane is prepared. This process comprises: a) Preparing a solution comprising a polyarylsulfone polymer selected from the group of polyethersulfone comprising the repeating unit of formula (I) and polysulfone comprising the re- peating unit of formula (II) or mixtures thereof, wherein the weight average molecular weight Mwof the polyarylsulfone polymer is in the range from 40000 to 105000 g / mol and the polyarylsulfone polymer comprises at least 95wt.-% of the repeating units of formula (I) and (II) based on the total weight of the polyarylsulfone polymer, 2-(2-oxopyrrolidin-1yl)ethyl acetate, and a water-soluble polymer. b) Continuous shaping and de-mixing of the solution of step a) to a non-flat geometry by direct exposition of the solution to a coagulant inducing constant solidification of the shaped solution.

[0096] Step a)

[0097] The solution of step a) corresponds to a 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 performed as described above.

[0098] In a preferred embodiment, the solution of step a) comprises 10 to 25wt.-% of the polyarylsulfone polymer, 60 to 89wt.-% of 2-(2-oxopyrrolidin-1yl)ethyl acetate, and 1 to 15wt.-% of water-soluble polymer based on the total weight of the solution.

[0099] In a more preferred embodiment, the solution of step a) comprises 14-20wt.-% of the polyarylsulfone polymer, 60 to 84wt.-% of 2-(2-oxopyrrolidin-1yl)ethyl acetate, and 2 to 10wt.-% of water-soluble polymer based on the total weight of the solution.

[0100] Preferably, the solution is be prepared under stirring at a temperature of 20 to 100 °C, preferably 40 to 90 °C, more preferably 50 to 70 °C. The ready solution typically is degassed for 2 to 24 h, preferably 6 to 20 h, more preferably 10 to 14 h. Subsequently, the solution is preferably re-heated at a temperature of 20 to 100 °C, preferably 40 to 90 °C, more preferably 50 to 70 °C for 1 to 4 h, preferably 1.5 to 3 h.

[0101] Step b)

[0102] Shaping of the solution of step a) to a non-flat geometry can, for example, be performed via extrusion of the solution through an extrusion nozzle, to get, for example, a fiber-like geometry. To maintain the desired shape, the polymer solution is usually extruded directly into a coagulation bath containing a coagulant as described above, which induces de-mixing of polymer and solvent and thus polymer solidification in the desired shape. If, for example, one or more additional hollow needles are present in the extrusion nozzle, through which additionally a solution of a coagulant is injected to the inside of the developing fibers, single- or multi-tubular geometries are obtained as described in the following.

[0103] Preferred membranes showing a non-flat geometry are hollow fiber membranes (single bore hollow fibers or multiple bore hollow fibers). They can be prepared by different spinning tech- nologies, for example by melt spinning, dry spinning or wet spinning. Hereby, the solution obtained in above mentioned step a) is extruded through an extrusion nozzle (also called spinneret) containing the required number of hollow needles (step b). The coagulation liquid is injected through the hollow needles into the extruded polymer solution during extrusion. By this setup, the extruded polymer membrane gets a hollow cylindrical geometry and parallel continuous channels extending in extrusion direction are formed in the extruded polymer solution. Preferably, the porous structure on the outer surface of the extruded membrane is formed by bringing the outer surface of the extruded hollow polymer solution in contact with a mild coagulation agent as for example water vapor such that the shape is fixed without an active layer, i.e., a highly porous filtration layer, on the outer surface. Subsequently, the membrane precursor is brought in contact with a coagulation agent. The parameters of the process, like extrusion speed, temperature, nozzle geometry, type of coagulant, concentrations have an effect on and thus can be used to control pore size and distribution as well as the membrane’s shape and thickness and thus the performance of the membrane. Hollow fiber membranes can be optionally wound up onto rolls and / or bundled to bundles of hollow fibers.

[0104] The membranes prepared by the procedures described above are usually worked up by washing and / or optional oxidizing steps.

[0105] If high molecular weight water-soluble polymers are used, the membrane prepared by the processes described above (steps a-b) will be exposed, after an optional washing step, to a solution containing an oxidatively active component. In this case, the solution containing an oxidatively active component preferably is an aqueous solution. To remove the oxidant, washing is performed after the oxidation step. Preferred is a water-soluble oxidant, such as, e.g., sodium hypochlorite or halogens, especially chlorine in a concentration range of 500 to 5000wt.-ppm, more preferably from 1000 to 4000wt.-ppm and particularly preferably from 1500 to 3500wt.- ppm based on the total weight of the aqueous oxidation solution.

[0106] Oxidation as well as washing is performed in order to remove the water-soluble polymer(s) and thus, to form the pores. Oxidation may be followed by washing or vice versa. Oxidation and washing may as well be performed simultaneously in one step. Preferably, the membrane is oxidized with hypochlorite solution or gaseous chlorine at 20 to 90 °C, preferably 35 to 80 °C, more preferably 50 to 70 °C and at a pH of 9 to 10, preferably 9.3 to 9.7 for 0.5 to 4 h, preferably 1 to 3 h, more preferably 1 .5 to 2.5 h, and subsequently washed with water. Washing with water is typically performed in a water bath for 2 to 24 h, preferably for 4 to 20 h, more preferably for 8 to 16 h, and subsequently usually one to five times, preferably three times with water at 20 to 90 °C, preferably 35 to 80 °C and more preferably 50 to 70 °C. It is an advantage that for the inventive solution based on 2-(2-oxopyrrolidin-1yl)ethyl acetate and water-soluble polymers for membrane preparation no oxidative workup is needed. As described above, due to the high viscosities of the inventive solutions, there is no need to use high molecular weight water-soluble polymers to increase the viscosity. Instead of this, low molecular weight water-soluble polymers can be used, which do not require oxidative work-up to be removed.

[0107] Moreover, a membrane was found, which is prepared using the inventive solution comprising a polyarylsulfone polymer selected from the group of polyethersulfone comprising the repeating unit of formula (I) and polysulfone comprising the repeating unit of formula (II) or mixtures thereof, wherein the weight average molecular weight Mwof the polyarylsulfone polymer is in the range from 40000 to 105000 g / mol and the polyarylsulfone polymer comprises at least 95wt.-% of the repeating units of formula (I) and (II) based on the total weight of the polyarylsulfone polymer, 2-(2-oxopyrrolidin-1yl)ethyl acetate and a water-soluble polymer.

[0108] As described above, the membrane preparation preferably comprises the following steps a) Preparing a solution comprising a polyarylsulfone polymer selected from the group of polyethersulfone comprising the repeating unit of formula (I) and polysulfone comprising the repeating unit of formula (II) or mixtures thereof, wherein the weight average molecular weight Mwof the polyarylsulfone polymer is in the range from 40000 to 105000 g / mol and the polyarylsulfone polymer comprises at least 95wt.-% of the repeating units of formula (I) and (II) based on the total weight of the polyarylsulfone polymer, 2-(2-oxopyrrolidin- 1 yl)ethyl acetate and a water-soluble polymer. b) Shaping the polymer solution into a certain geometry, like, for example, fiber, tubular or flat sheet geometries, by methods like, for example, extrusion of the polymer solution or preparation of a film from or casting the polymer solution. c) Solidifying the geometry shaped in step b) by exposing the polymer solution to a coagulant.

[0109] Preferably, the solution used for the membrane production further contains additives and / or cosolvents as described above. The preparation of the solution of step a) can be performed as described above. Typically, the membrane is worked up by different washing and / or oxidizing steps as described above.

[0110] Beside others, the performance of a membrane can be specified by its pure water permeability (PWP) and its molecular weight cut-off. The PWP reflects the permeability of the membrane towards pure water in dependance on the membrane area, the applied pressure, and time of the permeation experiment according to the following formula (equation 2):

[0111] PWP: pure water permeability [kg I bar h m2] m: mass of permeated water [kg]

[0112] A: membrane area [m2]

[0113] P: pressure [bar] t: time of the permeation experiment [h].

[0114] To achieve high flow rates, the membranes according to the invention preferably comprise a high PWP of more than 100 kg / h m2bar, more preferably more than 125 kg / h m2bar, and particularly preferably more than 150 kg / h m2bar for ultrafiltration membranes and preferably more than 25 kg / h m2bar, more preferably more than 50 kg / h m2bar, and particularly preferably more than 100 kg / h m2bar for nanofiltration membranes.

[0115] The pure water permeability of the membrane is determined before the determination of 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.

[0116] The rejection of the molecular weight cut-off (MWCO) was calculated by equation (2) measured by filtering a solution of polyethylene oxide (Pluriol® 8000 E, 0.1 wt% in ultra-pure water) by the membrane at a pressure of 0.2 bar, in which CF and CP represent the concentration in initial feed and in permeate, respectively. For polyethylene oxide - standard (MWCO = 1) the concentrations of the feed and permeate are determined by GPC-measurement (refractive index detector). According to Matsuyama et al. in Ind. Eng. Chem. Res. 2017, 56, 11302 PEO 8000 has a calculated stokes radius of 2,5nm. High flux nano-filtration membranes combine high flux of 30 kg / h m2bar or more with a high sieving coefficient for PEO 8000 of more than 90%.

[0117] The inventive membranes can be used for any kind of separation process of gaseous or liquid mixtures, for example water treatment applications like drinking water purification, treatment of industrial or municipal waste water, desalination of sea or brackish water, dialysis, purification of pharmaceutical products, plasmolysis, and food processing. In a general embodiment of the inventive process to prepare a flat sheet membrane, polyethersulfone comprising the repeating unit of formula (I) with a weight average molecular weight of 48000 to 90000 g / mol is mixed with 2-(2-oxopyrrolidin-1yl)ethyl acetate, polyvinylpyrrolidone showing a K-value of at least 12 and a C2-C8 alkanediol as additive. The different polymers are used in an amount such that the resulting solution comprises 10 to 25wt.-% of the polyethersulfone and 2 to 10wt.-% of polyvinylpyrrolidone. To accelerate the dissolution of the polymers, the mixture is heated under stirring at a temperature of 50 to 70 °C until a homogeneous clear viscous solution is obtained. The solution is then degassed for 10 to 14 h and re-heated at 50 to 70 °C for 1.5 to 3 h. Subsequently, the solution is casted onto a glass plate with a casting knife with an aperture of 200 to 400 pm at a temperature of 50 to 70 °C. The resulting film is allowed to rest for 20 to 60 s and then subjected to a water-based coagulation bath comprising 30 to 70wt.-% water and 70 to 30wt.-% of glycerol at 20 to 30 °C for 5 to 15 min. After detachment of the glass substrate, the membrane is transferred into a water bath at room temperature, left there for 8 to 16 h, subsequently washed three times with water at 50 to 70 °C, and stored in a wet state.

[0118] In a general embodiment of the inventive process to prepare a flat sheet membrane, polysulfone comprising the repeating unit of formula (II) with a weight average molecular weight of 53000 to 60000 g / mol is mixed with 2-(2-oxopyrrolidin-1yl)ethyl acetate and poly(ethylene oxide) showing a number average molecular weight Mn of at least 250 g / mol. The different polymers (and solvents) are used in an amount such that the resulting solution comprises 10 to 35 wt.-% of the polysulfone and 2 to 10 wt.-% of poly(ethylene oxide). To accelerate the dissolution of the polysulfone, the mixture is heated under stirring at a temperature of 40 to 90 °C until a homogeneous clear viscous solution is obtained. The solution is then degassed for 10 to 14 h, and subsequently re-heated at 40 to 90 °C for 1.5 to 3 h. Subsequently, the solution is casted onto a glass plate with a casting knife with an aperture of 200 to 400 pm at a temperature of 50 to 70 °C. The resulting film is allowed to rest for 20 to 60 s and then subjected to a water coagulation bath at 20 to 30 °C for 5 to 15 min. After detachment of the glass substrate, the membrane is transferred into a water bath at room temperature, left there for 8 to 16 h, subsequently washed three times with water at 50 to 70 °C, and stored in a wet state.

[0119] It was found that the solvent 2-(2-oxopyrrolidin-1yl)ethyl acetate readily dissolves polyarylsulfone polymers leading to solutions of low turbidity.

[0120] Compared to other aprotic polar solvents, particularly other pyrrolidone derivatives, the utilization of 2-(2-oxopyrrolidin-1yl)ethyl acetate shows different advantages:

[0121] Firstly, 2-(2-oxopyrrolidn-1yl)ethyl acetate is easily accessible and of low toxicological concern compared, for example, with the frequently used aprotic polar solvent N-methylpyrrolidone. Secondly, the resulting inventive solutions show high viscosities, which are beneficial for the casting process during membrane preparation by leading to better film quality.

[0122] Thirdly, due to the high viscosities of the solution, during membrane preparation there is no need to use high molecular weight water-soluble polymers to increase the viscosity. Instead of this, the high viscosities of the solutions allow for the usage of water-soluble polymers of relatively low K-values or molecular weight values. These kinds of polymers can be removed from the resulting membranes by simple washing with water, without requiring an oxidation step. Thus, advantageously, one process step and thus, the usage of potentially hazardous chemicals (and required further washing steps) can be avoided. Fourthly, it was found furthermore, that the usage of the inventive solutions containing low molecular weight water-soluble polymers leads to membranes with an advantageous balance of pure water permeability and high flux with a high sieving coefficient for PEO 8000 of more than 90% so that these membranes can be used has high flux nano-filtration membranes compared to membranes known from the art. Additionally the use of the inventive solutions containing 2- (2-oxopyrrolidin-1yl)ethyl acetate in the process of making membranes causes defect-free cross sections in the filtration layer as well as in the bottom layer.

[0123] Examples

[0124] Abbreviations and compounds used in the examples:

[0125] PWP pure water permeation

[0126] MWCO molecular weight cutoff

[0127] NTU nephelometric turbidity unit

[0128] HEPA 2-(2-Oxopyrrolidin-1-yl)ethyl acetate (synonym: 1-(acetoxyethyl-2-pyrrolidone

[0129] [CAS 52580-83-3]

[0130] NMP N-methyl-2-pyrrolidone [CAS 872-50-4]

[0131] Ultrason® E 3010 Polyethersulfone with a viscosity number (measured based on ISO 1628-5 (1998) in a 1wt.-% polymer solution in N-methylpyrrolidone) of 66 ml / g; a glass transition temperature (DSC, 10 K / min; according to ISO 11357-1 (2017) and 11357-2 (2020)) of 225 °C; a molecular weight Mw(GPC in THF, PS standard) of 52000 g / mol, and Mw / Mn= 3.0, which is abbreviated as “E3010”

[0132] Ultrason® E 6020 P Polyethersulfone with a viscosity number (measured based on ISO 1628-5 (1998) in a 1wt.-% polymer solution in N-methylpyrrolidone) of 81 ml / g; a glass transition temperature (DSC, 10 K / min; according to ISO 11357-1 (2017) and 11357-2 (2020)) of 225 °C; a molecular weight Mw(GPC in THF, PS standard) of 72000 g / mol, and Mw / Mn= 3.4, which is abbreviated as “E6020P”

[0133] Ultrason® E 7020 P Polyethersulfone with a viscosity number (measured based on ISO 1628-5 (1998) in a 1wt.-% polymer solution in N-methylpyrrolidone) of 105 ml / g; a glass transition temperature (DSC, 10 K / min; according to ISO 11357-1 (2017) and 11357-2 (2020)) of 225 °C; a molecular weight Mw(GPC in THF, PS standard) of 90000 g / mol, and Mw / Mn= 3.4, which is abbreviated as “E7020P”

[0134] Kollidon® K12 Polyvinylpyrrolidone with a molecular weight Mwof 3000 to 4000 g / mol and a solution viscosity characterized by the K-value of 12, determined according to the method of Fikentscher (Fikentscher, Cellulosechemie 13, 1932 (58)), which is abbreviated as “K12”

[0135] Pluriol® 8000 E poly(ethylene oxide) PE08000; BASF SE; Average molecular weight MW: 7 510 g / mol (calculated from GPC in water with 0.01 mol phosphate buffer pH 7.4, TSKgel GMPWXL column, Tosoh Bioscience with poly(ethylene oxide) standard 106 - 1 522 000 g / mol); Polydispersity MW / MN: 1.1 ; solution viscosity characterised by the K-value of 28.8 (de- termined according to the method of Fikentscher (Fikentscher, Cellu- losechemie 13, 1932 (58)))

[0136] The polymer solution turbidity was measured with a turbidimeter 2100AN (Hach Lange GmbH, Dusseldorf, Germany) employing a filter of 860 nm at 60 °C and expressed in nephelometric turbidity units (NTU). NTU values below 1 are preferred.

[0137] The pure water permeance (PWP) of the membranes was tested using a pressure cell with a diameter of 74 mm using ultrapure water (salt-free water, filtered by a Millipore UF-system) at 23 °C and 1 bar water pressure. The pure water permeation (PWP) is calculated as follows (equation

[0138] PWP: pure water permeance [kg I bar h m2] m: mass of permeated water [kg]

[0139] A: membrane area [m2]

[0140] P: pressure [bar] t: time of the permeation experiment [h].

[0141] In a subsequent test, a solution of poly(ethylene oxide) (Pluriol® 8000 E, 0.1 wt% in ultra-pure water) was used as feed to be filtered by the membrane at a pressure of 0.2 bar and rejection (MWCO %) is calculated by equation (2) in which OF and OP represent the concentrations in initial feed and in permeate, respectively. For poly(ethylene oxide)-standard (MWCO 1) the concentrations of the feed and permeate are determined by GPC-measurement (refractive index detector) 100 r%] (2)

[0142] MWCO 1 : feed PEO 8000

[0143] According to Matsuyama et. al (Ind. Eng. Chem. Res. 2017, 56, 11302) PEO 8000 has a calculated stokes radius of 2.5 nm. High flux nano-filtration membranes combine high flux of more than 30 kg / h m2bar with a high sieving coefficient for PE08000 of more than 90 %.

[0144] Example 1-3: Viscosity and turbidity of polyarylsulfone polymer solutions in different solvents

[0145] Into a three-neck flask equipped with a magnetic stirrer there were added 80 g of solvent S1 and 20 g polymer as given in table 1. The mixture was heated under gentle stirring at 60°C until a homogeneous clear viscous solution, usually referred to as solution was obtained. The solution was degassed overnight at room temperature.

[0146] For comparison, corresponding solutions were prepared using other solvents according the same procedure. The solution viscosity and turbidity was measured at 60 °C according to the procedures described above. The results are summarized in table 1.

[0147] Table 1. Viscosity and turbidity of polymer solutions at 60 °C

[0148] The data summarized in table 1 shows that at a given concentration of polyarylsulfone polymer, the inventive solutions using HEPA as solvent show significantly higher viscosity levels compared to the corresponding solutions based on NMP (comparative examples). This remains valid also if the type of polyarylsulfone polymer is changed. A higher solution viscosity is advantageous, because it facilitates the process step of film casting resulting in better films. Furthermore, the solution turbidities of the inventive solutions are on a low level which is required to get high quality membranes. Higher turbidity levels could cause defects in the membranes and thus lower their separation performance. The turbidities are on similar levels compared to solutions based on other solvents (comparative examples).

[0149] Preparation of membranes using HEPA as polymer solvent

[0150] General procedure

[0151] Into a three-neck flask equipped with a magnetic stirrer there were added 65 g of solvent S1 , 19 g Ultrason® polymer with 6 g polyvinylpyrrolidone and 10 g 1 ,2-propandiol as given in table 2. The mixture was heated under gentle stirring at 60°C until a homogeneous clear viscous solution, usually referred to as solution was obtained. The solution was degassed overnight at room temperature.

[0152] After that the membrane solution was reheated at 60°C for 2 hours and casted onto a glass plate with a casting knife (300 microns) at 60°C using an Erichsen Coating machine (Coatmas- ter 510, Erichsen GmbH & Co KG, Hemer, Germany) operating at a speed of 5 mm / s. The membrane film was allowed to rest for 30 seconds before immersion in a water-based coagulation bath at 25°C for 10 minutes. After the membrane had detached from the glass plate, the membrane was carefully transferred into a water bath for 12 h. Afterwards the membrane was washed with water at 60°C three times and stored wet.

[0153] Table 3: Compositions and properties of Ultrason® E 3010 membranes prepared with PVP K12;

[0154] Posttreatment: water. Coagulation: water-glycerol (50 / 50 wt / wt)

[0155] Polymer solutions produced with HEPA according to the invention show higher solution viscosity and membranes fabricated thereof showed higher water permeability values of higher than 10 kg / h m2bar in combination with PE08000 rejection values higher than 90 %. compared to membranes known from the art these are high flux nano-filtration membranes.

[0156] Also, the PESLI nano filtration membrane from HEPA according to example 4 is showing defect- free cross-sections which are mandatory for high flux nanofiltration membranes. Thus Figure 1 which is divided into three scanning electron micrograph of the membrane of example 4 showing in Figure 1 CS according to the invention the cross-section with a well-established nano porous filtration layer on the top supported by sponge-type substructure with increasing pore sizes from top to bottom. Figure 1 TL shows the well-established nano porous filtration layer which is homogenous regarding the pore sizes and Figure 1 BL shows the bottom layer as a spongetype substructure with well established pore-size gradient on which the filtration layer is supported. Also, here no macrovoids are visible in this bottom layer.

[0157] In contrast to this the scanning electron micrograph of the comparative example 4 in Figure 2 which is also divided into Figure 2 CL, Figure 2 TL and Figure 2 BT showing the cross-section, the filtration layer and the bottom layer. As seen in Figure 2 CL macrovoids are inside the membrane cross-section visible indicating a less controlled phase inversion process which leads to the PWP and MWCO values which are not in the range of high-flux nanofiltration membranes as seen in table 4.

Claims

Claims:

1. Solution comprising a) a polyarylsulfone polymer selected from the group of polyethersulfone polymers com-and polysulfone polymer comprising repeating unit of formula (II)or mixtures thereof, wherein the weight average molecular weight Mwof the polyarylsulfone polymer is in the range from 40000 to 105000 g / mol and the polyarylsulfone polymer comprises at least 95 wt.-% of the repeating units of formula (I) and / or (II) based on the total weight of the polyarylsulfone polymer, and b) 2-(2-Oxopyrrolidin-1yl)ethyl acetate.

2. The solution according to claim 1 wherein the polyarylsulfone polymer is a polyethersulfone polymer comprising repeating unit of formula (I)wherein the weight average molecular weight Mw of the polyethersulfone polymer is in the range from 40000 to 105000 g / mol and the polyethersulfone polymer comprises at least 95 wt.-% of the repeating units of formula (I) based on the total weight of the polyethersulfone polymer.

3. The solution according to any of claim 1 to 2 wherein the solution comprises 10 to 25 wt.- % of polyarylsulfone polymer based on the total weight of the solution.

4. The solution according to any of claim 1 to 3 wherein the solution comprises 60 to 89 wt.- % of 2-(2-oxopyrrolidin-1-yl)ethyl acetate based on the total weight of the solution.

5. The solution according to any of claim 1 to 4 wherein the solution comprising 1 to 15 wt- % of a water -soluble polymer showing a solubility in water at 21°C of at least 10 g per 100 g water based on the total weight of the solution.

6. The solution according to claim 5 wherein the water-soluble polymer is selected from polyvinylpyrrolidone, poly(alkylene glycol) with an Mwof 200 to 2,000,000 g / mol or mixtures thereof.

7. The solution according to any of claim 1 to 6 wherein the solution comprises 60 to 88 wt.- % of 2-(2-oxopyrrolidin-1yl)ethyl acetate and 1 to 15 wt.-% of an additive, in which the polyarylsulfone polymer is soluble at 21 °C at concentration of < 1g polyarylsulfone polymer per 100 g additive both based on the total weight of the solution.

8. The solution according to claim to 7 wherein the additive is selected from the group of C1-C4 alkanols, C2-C8 alkanediols, oligo(alkylene glycol)s, and C3-C12 alkanetriols or mixtures thereof.

9. The solution according to any of claim 1 to 8 showing a solution viscosity of 2 to 30 Pa s as determined with a Brookfield Viscometer DV-I Prime with RV 6 spindle at 60°C and 20- 100 rpm.

10. The solution to any of claim 1 to 9 showing a turbidity of 0 to 1 ,5 NTU as determined with a turbidimeter employing a filter of 860 nm at 60°C.

11. A process for the preparation of a membrane using the solution according to any of claim 1 to 10.

12. The process according to claim 11 comprising the following steps: d) preparing a solution according to any of claims 1 to 9, e) preparing a polymer film from the solution of step a) by applying the solution of step a) to a substrate and f) exposing the polymer film to a coagulant.

Images