The present invention relates to a solution comprising at least one polymer P, at least one water soluble polymer and a N-n-butyl-2-pyrrolidone, the process of making a membrane and the use of this membrane for water treatment.

Polyvinylidene fluoride (PVDF), ethylene chlorotrifluoroethylene and sulfone polymers such as polysulfone, polyethersulfone and polyphenylsulfone are high performance polymers which are used in a variety of technical applications because of their mechanical properties and their chemical and thermal stability. Sulfone polymers, however, have limited solubility in many com mon solvents. In particular low molecular weight fractions of sulfone polymers cause turbidity of solutions of sulfone polymers, as described in J.G Wijmans and C.A. Smolders, Eur. Polym. J. 19, No. 12, pp 1143 to 1146 (1983).

US 5885456 discloses N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAC), dime- thylacrylamide (DMAD) or dimethylsulfoxide (DMSO) as suitable solvent for sulfone polymers.

One major technical application is the use of sulfone polymers as raw materials for the produc tion of membranes, for example ultrafiltration membranes (UF membranes), as described in US 4207182 and US 5885456. The process of producing membranes of sulfone polymers includes dissolving sulfone polymers in a solvent, coagulating the sulfone polymer from such solvent and further post-treatment steps. The selection of the solvent is essential to the process and has impact on the properties of the obtained membrane, including but not limited to the membranes’ mechanical stability, water permeability and size of pores.

EP 2804940 describes the use of N-n-butyl-2-pyrrolidone as non-reprotoxic solvent for the pol ymer production of different kind of polymers such as polysulfons, polyethersulfons and polyvi- nylpyrrolidons. The use of N-n-butyl-2-pyrrolidone (NBP) as solvent in a solution comprising a polymer P and a water soluble polymer for making a membrane with better pure water permea tion combined with a molecular weight cutoff in the ultrafiltration range (10 - 100 kDa) is not disclosed.

In the field of solvents there is an ongoing demand for alternative solvents which may replace presently used solvents in specific applications. In case of polyvinylidene fluoride (PVDF), eth ylene chlorotrifluoroethylene (ECTFE) and sulfone polymers alternative solvents should be able to prepare solutions that allow a high content of PVDF, ECTFE and sulfone polymer without turbidity. Regarding membranes made there from 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 as high as possible combined with a mo lecular weight cutoff in the ultrafiltration range of 10 to 100 kDa.

It was an object of the present invention to provide an alternative solvent for poly vinylidene flu oride (PVDF), ethylene chlorotrifluoroethylene (ECTFE) and sulfone polymers and for the pro cess of making membranes. The alternative solvent should fulfill the requirements listed above.

Accordingly, the solution as defined above and a process for the making of membranes have been found.

To the sulfone polymer

The solution comprises a polymer P selected from the group of poly vinylidene fluoride, ethylene chlorotrifluoroethylene and sulfone polymer. The term “sulfone polymer” shall include a mixture of different sulfone polymers.

A sulfone polymer comprises -SO2- units in the polymer, preferably in the main chain of the polymer.

Preferably, the sulfone polymer comprises at least 0.02 mol -SO2- units, in particular at least 0.05 mol -SO2- units per 100 grams (g) of polymer. More preferred is a sulfone polymer comprising at least 0.1 mol -SO2- units per 100 g of polymer. Most preferred is a sulfone polymer comprising at least 0.15 mol -SO2- units, in particular at least 0.2 mol -SO2- units per 100 g of polymer.

Usually a sulfone polymer does comprise at maximum 2 mols -SO2- units, in particular at maximum 1.5 mols of -SC>2- units per 100 grams (g) of polymer. More preferred is a sulfone polymer comprising at maximum 1 mol and of -SC>2- units per 100 grams of polymer. Most preferred is a sulfone polymer comprising at maximum 0.5 of mol-SC>2- units per 100 grams of polymer.

Preferably, the sulfone polymer comprises aromatic groups, shortly referred to as an aromatic sulfone polymer. In a preferred embodiment, the sulfone polymer is an aromatic sulfone polymer, which consists to at least 20% by weight, in particular to at least 30% by weight of aromatic carbon atoms. An aromatic carbon atom is a carbon atom, which is part of an aromatic ring system.

More preferred is an aromatic sulfone polymer, which consists to at least 40 % by weight, in particular to at least 45 % by weight of aromatic carbon atoms.

Most preferred is an aromatic sulfone polymer, which consists to at least 50 % by weight, in particular to at least 55 % by weight of aromatic carbon atoms.

Preferably, the sulfone polymer may comprise aromatic groups that are selected from 1 ,4- phenylene, 1,3-phenylene, 1,2-phenylene, 4,4’-biphenylene, 1,4-naphthylene, or 3-chloro-1,4- phenylene.

The aromatic groups may be linked by, for example, units selected from -SO2- _, -SO-,

-S-, -0-, -CHr, -C (CH ₃ ) ₂ .

In a preferred embodiment, the sulfone polymer consists to at least 80 % by weight, more preferably to at least about 90 % by weight and most preferably to at least 95, respectively at least 98 % by weight of groups selected from the above aromatic groups and linking groups.

Examples of most preferred sulfone polymers are: polyethersulfone of formula I which is, for example, available from BASF under the trade name Ultrason® E, polysulfone of formula II which is, for example, available from BASF under the trade name Ultrason® S and polyphenylsulfone of formula III which is, for example, available from BASF under the trade name Ultrason® P.

To the polyvinylidene fluoride (PVDF) and ethylene chlorotrifluoroethylene (ECTFE)

Globally 60 % of all membranes for water filtration are based on partially fluorinated polymers such as polyvinylidene fluoride (PVDF) and ethylene chlorotrifluoroethylene (ECTFE). The poly vinylidene fluoride which are usable for the invention can be used in different forms. Preferable are PVDF grades in powder and pellet form. These PVDF grades are applicable in the invention as linear or gel-free products with weight average molecular weights Mw in the range from 300 - 320 kDa (Solef® 6010), 380 - 400 kDa (Solef® 6012), 570-600 kDa (Solef® 1015) and 670 - 700 kDa (Solef® 6020) available from Solvay Speciality Polymers. ECTFE is available with a melt flow index of 1.0 (tested at 2.16 kg and 5.0 kg as Halar® 901 and 902 from Solvay Special ity Polymers.

To the water soluble polymers

The water soluble polymer helps to adjust the viscosity of the solution. The main purpose of the water solution polymer is to support the formation of the pores. In the coagulation step during the process of making the membrane the water soluble polymer becomes distributed in the co agulated membrane and thus becomes the place holder for pores. The water soluble polymer may be any known water soluble polymer selected from the group of polyvinyl pyrrolidone and polyalkylene oxides with a molar mass of 8000 g/mol or higher. Pre ferred water soluble polymers are selected from the group of polyvinyl pyrrolidone, polyethylene oxide, polypropylene oxide, polyethylene oxide / polypropylene oxide block copolymers and mixtures thereof. A very preferred water soluble polymer is polyvinyl pyrrolidone.

To the solution

The solution may comprise further additives. These additives are selected from the group of C2- C4 alkanol, C2-C4 alkanediol, C3-C4 alkanetriol, polyethylene glycol with a molar mass in the range of 100 to 1000 g/mol and mixtures of those. Preferred additives are ethanol, n-propanol, iso-propanol, n-butanol, iso-butanol, tert-butanol, ethylene glycol, 1 , 1 -ethandiol, 1,2-propandiol, 1,3-propandiol, 2,2-propandiol, 1,2,3- propantriol, 1 ,1 , 1-propantriol, 1 , 1 ,2-propantriol, 1,2,2- propantriol, 1,1,3- propantriol, 1 , 1 , 1-butantriol, 1 ,1 ,2-butantriol, 1 , 1 ,3-butantriol, 1 , 1 ,4-butantriol, 1,2,2,-butantriol, 2,2,3-butantriol, 2-methyl-1,1,1-triolpropan, 2-methyl- 1 , 1 ,2-triolpropan, 2methyl-1,2,3-triolpropan, 2-methyl- 1,1,3-triol-propan.

In a preferred embodiment up to 25 wt.-%, in particular up to 15 wt.%, based on the solution is an additive.

In a more preferred embodiment the amount of additive is in the range of 0.1 to 10 wt.%, in par ticular 5 to 10 wt.-% based on the solution.

The solution may comprise further solvents besides the N-n-butyl-2-pyrrolidone, hereinafter re ferred to as co-solvents.

Preferred are co-solvents that are miscible with the N-n-butyl-2-pyrrolidone in any ratio. Suitable co-solvents are, for example, selected from high-boiling ethers, esters, ketones, asymmetrically halogenated hydrocarbons, anisole, dimethylformamide, dimethyl sulfoxide, sulfolane, N-me- thyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, N,N-dimethyl-2-hydroxypropanoic amide and N,N- diethyl-2-hydroxypropanoic amide.

In a preferred embodiment at least 10 % by weight, in particular at least 90 % by weight by weight of the total amount of all solvents of the solution is N-n-butyl-2-pyrrolidone.

In a most preferred embodiment no co-solvent is used in the solution and N-n-butyl-2-pyrrolidine is the only solvent used. Preferably, the solution comprises 5 to 50 parts by weight, in particular 10 to 40 parts by weight, more preferably 20 to 30 parts by weight of sulfone polymer per 100 parts by weight of the total amount of all solvents.

In a most preferred embodiment the solution comprises 5 to 50 parts by weight, in particular 10 to 40 parts by weight, more preferably 20 to 30 parts by weight of polymer P per 100 parts by weight of the total amount of N-n-butyl-pyrrolidone.

Preferably, the solution comprises 1 to 40 wt.-%, in particular 10 to 30 wt.-%, more preferably 15 to 20 wt.-% of polymer P according to the solution.

In a most preferred embodiment the solution comprises 0.1 to 15 wt.-%, in particular 1 to 10 wt- %, more preferably 5 to 10 wt.-% of water soluble polymers according to the solution.

The solution may be prepared by adding the polymer P and the water soluble polymer to the N- n-butyl-2-pyrrolidone and dissolving the polymer P according to any process known in the art. The dissolution process may be supported by increasing the temperature of the solution and/or by mechanical operations like stirring. In an alternative embodiment the polymer P may be al ready synthesized in N-n-butyl-2-pyrrolidine or a solvent mixture comprising N-n-butyl-2- pyrrolidine.

To the process of making a membrane

In the context of this application a membrane shall be understood to be a semipermeable struc ture capable of separating two fluids or separating molecular and/or ionic components or parti cles 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 geome tries such as flat sheet, spiral wound, pillows, tubular, single bore hollow fiber or multiple bore hollow fiber.

For example, membranes can be reverse osmosis (RO) membranes, forward osmosis (FO) membranes, nanofiltration (NF) membranes, ultrafiltration (UF) membranes or microfiltration (MF) membranes. These membrane types are generally known in the art and are in detail de scribed in literature. A good overview is found also in earlier European patent application No. 15185604.4 (PF 78652) which is here with incorporated herein by reference. A preferred mem brane is the ultrafiltration (UF) membrane. Membranes may be produced according to a process comprising the following steps: a) providing a solution comprising a polymer P, N-n-butyl-2-pyrrolidine and fur ther comprising a water soluble polymer, b) contacting the solution with a coagulant c) optionally oxidizing and washing the obtained membrane

The solution in step a) corresponds to the solution described above. The water soluble polymer helps to adjust the viscosity of the solution. The main purpose of the water solution polymer is to support the formation of the pores. In the following coagulation step b) the water soluble poly mer becomes distributed in the coagulated membrane and thus becomes the place holder for pores.

The water soluble polymer may be any known water soluble polymer. Preferred water soluble polymers are selected from the group of polyvinyl pyrrolidone and polyalkylene oxide with a mo lar mass of 8000 g/mol or higher like polyethylene oxide, polypropylene oxide, polyethyleneox- ide / polypropylene oxide block copolymers and mixtures thereof. A very preferred water soluble polymer is polyvinyl pyrrolidone.

In a preferred embodiment, the solution in step a) comprises 75 to 90 wt.-% of the polymer P and 10 to 25 wt.-% of the water soluble polymer, based on the total weight of the polymer P and water soluble polymer.

Preferably, the solution comprises 65 to 75 wt.-% of the polymer P and 25 to 35 wt.-% of the water soluble polymer based on the total weight of the polymer P and water soluble polymer.

The solution may optionally be degassed before proceeding to the next step.

In step b) the solution is contacted with a coagulant. In this step coagulation of the polymer P occurs and the membrane structure is formed.

The polymer P should have low solubility in the coagulant. Suitable coagulants are, for example, liquid water, water vapor, alcohols or mixtures thereof.

Suitable alcohols are, for example, mono-, di- or trialkanols selected from the group of the group of C2-C4 alkanol, C2-C4 alkanediol, C3-C4 alkanetriol, polyethylene oxide with a molar mass of 100 to 1000 g/mol as they can be used as additives in the inventive solution. Preferred mixtures of the coagulants are mixtures comprising liquid water and alcohols, more preferably are mix- tures comprising liquid water and the alcohols that were optionally used as additive in the in ventive solution. A preferred coagulant is liquid water.

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 scale.

For a flat sheet membrane detailed process steps a) and b) could be as follows: a1) adding the water soluble polymer to the solution comprising a polymer P and N-n- butyl-2-pyrrolidine a2) heating the solution until a viscous solution is obtained; typically the solution is kept at a temperature of 20 to 100 °C, preferably 40 to 80°C, more preferably 50 to 60°C. a3) further stirring of the solution until a homogenous mixture is formed; typically ho mogenization is finalized within 5 to 10 h, preferably within 1 to 2 hours b1) Casting the solution obtained in a3) on a support and thereafter transferring the casted film into a coagulation bath, which is preferably water.

For the production of single bore hollow fiber or multiple bore hollow fibers step b1) may per formed by extruding the solution obtained in a3) through an extrusion nozzle with the required number of hollow needles. The coagulating liquid is injected through the hollow needles into the extruded polymer during extrusion, so that parallel continuous channels extending in extrusion direction are formed in the extruded polymer. Preferably the pore size on an outer surface of the extruded membrane is controlled by bringing the outer surface after leaving the extrusion nozzle in contact with a mild coagulation agent such that the shape is fixed without active layer on the outer surface and subsequently the membrane is brought into contact with a strong coagulation agent.

Further process step c) is optional. In a preferred embodiment process step c) is performed. Oxidation as well as washing is performed in order to remove the water soluble polymer and 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.

For oxidation any oxidant may be used. Preferred is a water soluble oxidant such as in particu lar sodium hypochlorite.

According to the invention solutions of polymer P are obtained that show no or at least less tur bidity. The solutions are suitably for the manufacturing of membranes. Membranes obtained have high mechanical stability and have excellent separation characteristics. In particular, membranes have good molecular weight cutoffs (MWCO) in the range of 10 to 100 kDa com bined with better values for the water permeability (PWP) as those mentioned in the art.

The membranes obtained by the process of the invention may be used for any separation pur pose, for example water treatment applications, treatment of industrial or municipal waste water, desalination of sea or brackish water, dialysis, plasmolysis, food processing.

Examples

Abbreviations and compounds used in the examples:

PWP pure water permeation

MWCO molecular weight cutoff

NTU nephelometric turbidity unit

NBP N-n-butyl-2-pyrrolidone

NMP N-methyl-2-pyrrolidone

DMAc N,N-dimethylacetamide 12PD 1,2-propandiol

Ultrason ^® E 3010 Polyethersulfone with a viscosity number (ISO 307, 1157, 1628; in 0.01 g/mol phenol/1,2 orthodichlorobenzene 1 :1 solution) of 66; a glass tran sition temperature (DSC, 10°C/min; according to ISO 11357-1/-2) of 225 °C; a molecular weight Mw (GPC in THF, PS standard): 58000 g/mol, Mw/Mn = 3.3

Ultrason ^® P 3010 Polyphenylenesulfone with a viscosity number (ISO 307, 1157, 1628; in 0.01 g/mol phenol/1,2 orthodichlorobenzene 1:1 solution) of 71 ; a glass transition temperature (DSC, 10°C/min; according to ISO 11357-1/-2) of 220 °C; a molecular weight Mw (GPC in THF, PS standard): 48000 g/mol, Mw/Mn = 2.7

Polysulfon S 6020 Polysulfone with a viscosity number (ISO 307, 1157, 1628; in 0.01 g/mol phenol/1,2 orthodichlorobenzene 1:1 solution) of 81; a glass transition temperature (DSC, 10°C/min; according to ISO 11357-1/-2) of 187 °C; a molecular weight Mw (GPC in THF, PS standard): 60000 g/mol, Mw/Mn = 3.7

In a 4 I vessel equipped a with stirrer, Dean-Stark-trap, nitrogen inlet and temperature control 430.62 g of dichlorodiphenyl sulfone, 342.08 g of Bisphenol A and 222.86 g of anhydrous potassium carbonate with particle size of 32.4 pm were mixed under nitrogen in 641 ml of NMP. The reaction mixture was firstly heated to 90° C within 1 h. Subsequently, the reaction water and NMP were being continuously distilled off at a pressure of 300 mbar. The NMP content in the reac tion vessel was continuously refilled and kept constant. The mixture was reacted for 8 h at 190° C. After adding 1609 ml of cold NMP, methylchloride was passed ( 01/h) through the reaction mixture for 45 minutes at 140 °C. Then the mixture was cooled down and the inor ganic constituents were filtered off. The polymer was then isolated as pearls by precipitation of the resulting polymer solution in water. After extraction with water for 20 h at 85 °C, the product was dried under reduced pressure (< 100 mbar) at 150 ° C for 24 hours, giving a white powder polysulfon S 6020.

Luvitec ^® K30 Polyvinylpyrrolidone with a solution viscosity characterised by the K- value of 30, determined according to the method of Fikentscher (Fikentscher, Cellulosechemie 13, 1932 (58))

Luvitec ^® K90 Polyvinylpyrrolidone with a solution viscosity characterised by the K- value of 90, determined according to the method of Fikentscher (Fikentscher, Cellulosechemie 13, 1932 (58))

Pluriol ^® 400E Polyethylene oxide with an average molecular weight of 400 g/mol cal culated from the OH numbers according to DIN 53240.

The polymer solution turbidity was measured with a turbidimeter 2100AN (Hach Lange GmbH, Dusseldorf, Germany) employing a filter of 860 nm and expressed in nephelometric turbidity units (NTU). Low NTU values are preferred. The pure water permeation (PWP) of the membranes was tested using a pressure cell with a diameter of 60 mm using ultrapure water (salt-free water, filtered by a Millipore UF-system). A high PWP allows a high flow rate and is desired.

In a subsequent test, solutions of polyethylene oxide-standards with increasing molecular weight were used as feed to be filtered by the membrane at a pressure of 0.15 bar. By GPC- measurement of the feed and permeate, the molecular weight of the permeate of each polyeth ylene oxide-standard used was determined. The weight average molecular weight (MW) cut-off of the membranes (MWCO) is the molecular weight of the first polyethylene oxide standard which is withhold to at least 90% by the membrane. For example, a MWCO of 18400 means that PEG of molecular weight of 18400 and higher are withhold to at least 90 %. It is desired to have a MWCO in the range from 10 to 100 kDa.

Preparation of membranes using NBP as polymer solvent General procedure

Into a three-neck flask equipped with a magnetic stirrer there were added 65 or 75 ml of Solvent S1, 19 g Ultrason® polymer, 6 g Luvitec® polyvinylpyrrolidone with optional second additives (1,2-propandiol, Pluriol® 400) as given in tables 1-3. The mixture was heated under gentle stir ring at 60°C until a homogeneous clear viscous solution, usually referred to as solution was obtained. The solution was degassed overnight at room temperature.

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 operating at a speed of 5 mm/min. The membrane film was allowed to rest for 30 seconds before immersion in a water-based coagulation bath at 25°C for 10 minutes (Table 4). 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 transferred into a bath containing 2500 ppm NaOCI at 50°C for 4.5 h. The membrane was then washed with water at 60°C and one time with a 0.5 wt.-% solution of sodium bisulfite to remove active chlorine. After several washing steps with water the membrane was stored wet until characterization regarding pure water permeability (PWP) and minimum pore size (MWCO) started. Table 2 summarizes the membrane properties.

Membranes produced with NBP according to the invention show improved separation charac teristics over membranes known from the art. Membranes produced with NBP show higher wa ter permeability values in combination with MWCO values in the ultrafiltration range (10-100 kDa) compared to membranes known from the art. Table 1: Compositions and properties of Ultrason ^® E 3010 membranes prepared; MWCO in [Da], PWP in [kg/h m ² bar]. Table 2: Compositions and properties of Ultrason ^® P 3010 membranes prepared; MWCO in [Da], PWP in [kg/h m ² bar].

Table 3: Compositions and properties of Ultrason ^® S 6020 P membranes prepared; MWCO in [Da], PWP in [kg/h m ² bar] and turbidity in [NTU].

Insoluble crystalline cyclic polysulfone dimers pose in solutions for membrane manufacturing problems either by filter clogging or can cause imperfections on the membrane surface (S. Sa- varier et. al, Desalination 2002, 144, 15-20). Polymer solutions of S6020 P in NBP are clear and transparent compared to solutions in DMAc. The content of cyclic dimers is better dissolved by NBP compared to DMAc as shown by solution turbidity. In addition, PSU membranes obtained from NBP solutions according to the invention show improved separation have better separation performance e.g. significant higher permeability combined with MWCO in the ultrafiltration range. Table 4: Compositions of the coagulation bath employed for membrane preparation