Membranes, such as ultrafiltration and nanofiltration membranes, need constant improvements to meet the future challenges of industry. Objects are for example a small mean pore diameter, a low molecular weight cut-off, a high pure water permeability, a high rejection rate, a long-term stability in continuous operation, a high feed recovery, an easy regeneration, and a simple fabrication process.

The object was solved by an integral asymmetric membrane comprising a polyamide-imide and a sulfonated polyarylether sulfone.

The object was also solved by a method for producing an integral asymmetric membrane comprising a) providing a dope solution comprising a polyamide-imide, a sulfonated polyarylether sulfone, and a solvent, and

b) adding a coagulant to the dope solution to coagulate the polyamide-imide and the

sulfonated polyarylether from the dope solution to obtain the membrane. Typically, a membrane is a discrete, thin interface that moderates the permeation of

chemical species in contact with it. Some species may be readily transported through the membrane, while other species may be retained or slowed down. This typically leads to permselectivity, that is, selective permeation of species. The membrane is an integral asymmetric membrane, which usually means that the membrane may have a skin layer and a porous sublayer and that the membrane is obtainable of the same material (e.g. the same polyamide-imide and the same sulfonated polyarylether sulfone) in a single process, such as the method according to the invention. Typically, the porous sublayer usually serves as support, and the skin layer serves as selective barrier.

For comparison, thin film composite (TFC) membranes are not integral asymmetric membranes. TFC membranes are usually obtainable by depositing a thin barrier layer on a porous substructure in a two-step process, and the two layers are usually made of different materials. Thus, it is usually not possible to produce hollow fiber membranes in form of TFC membranes.

The membrane has usually a molecular weight cut-off (MWCO) from 500 to 5,000 Dalton, preferably from 600 to 3,000 Da, and in particular from 700 to 2,000 Da. The membrane has usually a pure water permeability (PWP) from 30 to 300 LMH/bar, preferably from 40 to 200 LMH/bar, and in particular from 50 to 100 LMH/bar.

The membrane comprises a polyamide-imide, which are usually commercially available.

The polyamide-imide preferably comprises a repeating unit of the formula (II)

where R comprises an aromatic group and n is at least 3. In a preferred form R is a moiety that includes at least one C6-C16 aryl or heteroaryl, optionally substituted with one or more substituents selected from C1-C12 alkyl, (C6-Ci8)(hetero)aryl(Ci- Ci ₂ )alkyl, (C ₆ -Ci8)(hetero)aryl(C2-Ci2)alkene, C3-C12 cycloalkyl, (C ₆ -Ci8)(hetero)aryl(C ₃ -Ci2) cycloalkyl, C1-C12 haloalkyl, (C6-Ci8)(hetero)aryl(Ci-Ci2)haloalkyl, C1-C12 alkoxy, (C6-Cis)(he- tero)aryl(Ci-Ci2)alkoxy, C6-C18 (hetero)aryloxy, C6-C18 (hetero)arylamino or a C6-C18 (hetero)aryl group, wherein any of the aryl or heteroaryl group is further optionally substituted with a halogen or hydroxyl.

In a more preferred form R is a moiety that includes at least one C6-C16 aryl, optionally substituted with one or more substituents selected from C1-C12 alkyl, (C6-Ci8)aryl(Ci-Ci2)alkyl, (C ₆ -Ci8)aryl(C2-Ci2)alkene, C3-C12 cycloalkyl, (C ₆ -Ci ₈ )aryl(C ₃ -Ci2)cycloalkyl, C1-C12 alkoxy,

(C6-Ci8)aryl(Ci-Ci2)alkoxy, C6-C18 aryloxy, C6-C18 arylamino or a C6-C18 aryl group, wherein any of the aryl or heteroaryl group is further optionally substituted with a halogen or hydroxyl.

In particular, R is a moiety selected from C6-C12 aryl, C6-C12 aryl substituted with C6-C18 aryloxy, C6-C12 aryl substituted with (C6-Ci8)aryl(Ci-Ci2)alkyl, such as the following formulae:

The index n is preferably at least 5, more preferably at least 10.

The molecular weight (M _n ) of the polyamide-imide is usually from 3000 to 100,000 Da, preferably from 5000 to 50,000 Da.

The membrane may comprise 70 to 99 wt%, preferably 80 to 98 wt%, and in particular 85 to 97 wt% of the polyamide-imide. The polyamide-imide may have a solubility in N-methyl pyrrolidone (NMP), N,N-dimethyl- acetamide, dimethylformamide, sulfolane, dimethyllactamide, and/or caprolactam (preferably in NMP) of at least 5 wt%, preferably at least 10 wt%, at 20 °C. The membrane comprises a sulfonated polyarylether sulfone.

The term "sulfonated" means that a certain proportion of the repeating units of the polyarylether sulfone is sulfonated and carry at least one (e.g. one or two, preferably two) sulfonate group bound to an aryl group. The sulfonate group may be present in anionic form -S03 ^_ or as acid form -SO3H, wherein the anionic form may be present for as alkali metal salt (e.g. Na, K or Li).

Typically, 0.1 to 20 mol%, preferably 0.5 to 10 mol%, and in particular 1 to 5 mol% of the repeating units of the polyarylether sulfone carry at least one (e.g. one or two, preferably two) sulfonate group bound to an aryl group. The amount of the repeating units of the polyarylether sulfone which carry at least one sulfonate group bound to an aryl group can be determined using ¹ H-NMR spectroscopy or potentiometric titration or IR-spectroscopy.

Suitable sulfonated polyarylether sulfones are based on polyarylether sulfones which comprise at least one (e.g. one, two or three) repeating unit selected from the formlae la to lo, preferably from la, Ig or Ik, and in particular from Ig:

In the repeating units la to lo one or more 1 ,4-dihydroxyphenyl units may be replaced by resorcinol or dihydroxynaphthalene units.

In a preferred form the sulfonated polyarylether sulfones are based on polyarylether sulfones which comprise two repeating units selected from the formulae la to lo, preferably selected from Ik and Ig or from Ik and lb.

In another preferred form the sulfonated polyarylether sulfone is based on a polyarylether sulfone which comprises essentially only one kind of the repeating unit selected from the formulae la to lo, preferably from la (also known as polysulfone PSU), Ig (also known as polyphenylsulfone PPSU), and in particular from Ig.

The sulfonated polyarylether sulfone is preferably a sulfonated polysulfone (sPSU), a sulfonated polyethersulfone (sPESU), a sulfonated polyphenylenesulfone (sPPSU), or a mixture thereof.

The sulfonated polyarylether sulfone is in particular a sulfonated polyphenylenesulfone.

The sulfonated polyarylether sulfone may be prepared by sulfonation of a polyarylether sulfone, for example by sulfonation with concentrated H2SO4.

In another form sulfonated polyarylether sulfone is obtainable by reacting

— at least one aromatic dihalide (M1 a),

— a dialkali metal salt of at least one aromatic diol (M2a), and

— at least one sulfonated monomer selected from sulfonated aromatic dihalide (M1 b) and/or sulfonated aromatic diol (M2b).

Examples of aromatic dihalides (M1 a) include: bis(4-chlorophenyl)sulfone, bis(4-fluorophenyl) sulfone, bis(4-bromophenyl) sulfone, bis(4-iodophenyl) sulfone, bis(2-chlorophenyl) sulfone, bis(2-fluorophenyl) sulfone, bis (2-methyl-4-chlorophenyl) sulfone, bis(2-methyl-4-fluorophenyl) sulfone, bis(3,5-dimethyl-4-chlorophenyl) sulfone, bis(3,5-dimethyl-4-flurophenyl) sulfone and corresponding lower alkyl substituted analogs there-of. They may be used either individually or as a combination of two or more monomeric constituents thereof. Particular examples of dihalides are bis(4-chlorophenyl) sulfone (also designated (4,4'-dichlorophenyl) sulfone;

DCDPS) and bis(4-fluorophenyl) sulfone.

Examples of aromatic diols (M2a) are: hydroquinone, resorcinol, 1 ,5-dihydroxynaphthalene, 1 ,6-dihydroxynaphthalene, 1 ,7-dihydroxynaphthalene, 2,7-dihydroxynaphthalene, 4,4'-bis- phenol, 2,2'-bisphenol, bis(4-hydroxyphenyl) ether, bis(2-hydroxyphenyl) ether, 2,2-bis(4-hy- droxyphenyl)propane, 2,2-bis(3-methyl-4-hydroxy-phenyl)propane, 2,2-bis(3,5-dimethyl-4-hy- droyphenyl)propane, bis(4-hydroxyphenyl)methane, and 2,2-bis(3,5-dimethyl-4-hydroxypenyl)- hexafluoropropane. Preferred of them are hydroquinone, resor-cinol, 1 ,5-dihydroxynaphthalene, 1 ,6-dihydroxynaphthalene, 1 ,7-dihydroxynaphthalene, 2,7-dihydroxynaphthalene, 4,4'-biphenol, bis(4-hydroxyphenyl) ether, and bis(2-hydroxyphenyl) ether. They may be used either individually or as a combination of two or more monomeric constituents M2a. Particular examples of such aromatic diols are 4,4'-bisphenol and 2,2'-bisphenol.

Suitable compounds M1 b and M2b are the mono- or poly-sulfonated equivalents of the above described non-sulfonated monomeric constituents M1 a and M2a.

The degree of polymerization of the thus obtained polymer may be in the range of 40 to 120, in particular 50 to 80 or 55 to 75. In another form the sulfonated polyarylether sulfone is obtainable by polymerizing a) non-sulfonated monomers of the general formulae M1 a and M2a

HO— Ar— OH (M2a)

and b) at least one sulphonated monomer of the general formulae M1 b and M2b

,S0 ₃ H

HO— Ar— OH (M2b) where Ar is divalent aromatic residue,

Hal is F, CI, Br or I,

n and m independently are 0, 1 or 2, provided that n and m are not simultaneously 0, and the aryl groups of M1 a and M2a may carry at least one C1-C4 alkyl group.

where Hal is F, CI, Br or I.

An example for the monomer M2a is

^HO ^ T^ ^OH

An example for the sulfonated monomer M1 b is

where Hal is F, CI, Br or I. An example for the sulfonated monomer M2b is

The molar ratio of (M1 a +M1 b) : (M2a +M2b) is usually 0,95 to 1 .05, in particular 0,97 to 1 ,03.

The molar proportion of sulfonated monomers M1 b and/or M2b is usually in the range of 0.1 to 20 mol%, preferably 0.5 to 10 mol%, and in particular 1 to 5 mol%, based on the total mol number of the momoners M1 a, M1 b, M2a and M2b. The molecular weight (e.g. Mw) of the sulfonated polyarylether sulfone may be 10,000 to

200,000 g/mol, preferably 20,000 to 150,000 g/mol, and in particular 40,000 to 90,000 g/mol.

The membrane may comprise 1 to 20 wt%, preferably 2 to 12 wt%, and in particular 2 to 8 wt% of the sulfonated polyarylether sulfone.

The weight ratio of the polyamide-imide to the sulfonated polyarylether sulfone may be in the range from 70:30 to 99:1 preferably from 80:20 to 98:2, and in particular from 85:15 to 95:5.

Preferably, the membrane is a hollow fiber membrane, such as a single bore hollow fiber membrane or a multibore hollow fiber membrane.

Suitable multibore membranes comprise more than one longitudinal channels, such as 2 to 100. The channels may have circular, ellipsoid or rectangular form. The diameter of the channels may be from 0.05 to 3 mm. The wall thickness in such multiple channel membranes is normally from 0.02 to 1 mm at the thinnest position, preferably 30 to 500 μηη, more preferably 100 to

300 μηη. Normally, the membranes and carrier membranes have an essentially circular, ellipsoid or rectangular diameter, which may be from 2 to 10 mm. Preferably, membranes are essentially circular. In one embodiment the rejection layer is located on the inside of each channel of said multiple channel membrane.

The invention further relates to a method for producing the integral asymmetric membrane comprising the polyamide-imide and the sulfonated polyarylether sulfone

containing the steps of

a) providing a dope solution comprising a polyamide-imide, a sulfonated polyarylether sulfone, and a solvent, and

b) adding a coagulant to the dope solution to coagulate the polyamide-imide and the

sulfonated polyarylether from the dope solution to obtain the membrane.

The method for producing the integral asymmetric membrane preferably includes non-solvent induced phase separation (NIPS). In the NIPS process, the polymers used as starting materials are dissolved in the solvent optionally together with any further additive(s) used. In a next step, a porous polymeric membrane may be formed under controlled conditions in a coagulation bath. In most cases, the coagulation bath contains water as coagulant, or the coagulation bath is an aqueous medium, wherein the matrix forming polymer is not soluble.

The dope solution may comprise 5 to 35 wt%, preferably 8 to 28 wt%, and in particular 12 to 22 wt% of the polyamide-imide.

The dope solution may comprise 0.1 to 10 wt%, preferably 0.5 to 5.0 wt%, and in particular 1.0 to 3.0 wt% of the sulfonated polyarylether sulfone.

The polyamide-imide and the the sulfonated polyarylether sulfone are usually present in dissolved form in the dope solution. The dope solution may optionally be heated until a solution is obtained; typically temperature of the dope solution is 5-250 °C, preferably 25-150 °C, more preferably 50-90 °C.

Suitable solvents for the dope solution are N-methylpyrrolidone, DMF, dimethylacetamide, dimethyllactatmide, N-formylmorpholin, sulfolane. Suitable coagulants are water, methanol, ethanol, n-propanol, iso-propanol, n-butanol, sec- butanol, iso-butanol, n-pentanol, sec.-pentanol, iso-pentanol, 1 ,2-ethanediol, ethylene glycol, diethylene glycol, triethylene glycol, propyleneglycol, dipropyleneglycol, glycerol,

neopentylglycol, 1 ,4-butanediol, 1 ,5-pentanediol, pentaerythritol, and mixtures thereof. The dope solution may comprise an additive, such as a polyethylene glycol, which may have a molecular weight from 100 to 5000 g/mo, preferably from 200 to 1000 g/mol. Preferably, the additive is present in dissolved form. The dope solution may comprise 1 to 30 wt%, preferably 5 to 20 wt%, and in particular 10 to 16 wt% of the additive. The hollow fiber membranes may be manufactured by extruding the polymers, which form a semi-permeable membrane after coagulation through an extrusion nozzle with several hollow needles. A 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. The invention further relates to a use of the membrane for filtering an aqueous liquid through the membrane. The filtration may be made by conventional filtration processes and parameters, which are known to experts. The liquid may contain at least 80 wt%, preferably at least 90 wt%, and in particular at least 95 wt% water. Usually, the liquid is industrial waste water, sea water, surface water, ground water, process water, drinking water, liquid food (e.g. a beverage, such as beer, wine, juices, dairy products, or soft drinks). Preferably, the liquid is industrial waste water, such as from textile industry which contains a dye. In a preferred form the membrane is used for filtering an aqueous liquid which contains a dye and optionally an inorganic salt.

Polyamide-imide of Formula A, based on the monomers trimellitic anhydride chloride, 4,4'-oxydianiline and meta-phenylenediamine (molar ratio about 1 / 0.7 / 0.3), intrinsic

Formula A sPPSU-1 : sulfonated polyphenylenesulfone of Formula B; molecular weight (Mw, determind by

GPC in DM Ac using PMMA standards for calibration) of 64.2 kDa, 2.5 mol% units based on 3,3 ^' -Di-sodiumdisulfate-4,4 ^' -dichlorodiphenylsulfone (determined by IR- spectroscopy).

Formula B sPESU-1 : sulfonated polyethersulfone according to Formula C, based on a copolyethersulfone having 5 mol% of hydrochinone-based units, which was sulfonated in cone. H2SO4 at room temperature (10 g copolymer in 100 ml cone. H2SO4 stirring for 4 h), precipitated in water, washed until the pH of the washing water was >6. The viscosity number was 76,5 ml/g (determined at 25°C in NMP, 1 wt%-solution), the content of sulfonated units was 4,6 mol% (H-NMR).

Formula C

The pure water permeability (PWP) was determined with a lab-scale cross-flow ultrafiltration system. Deionized water was pumped through a membrane for 30 min with a trans-membrane pressure (TMP) of 0.4 bar and the pure water permeability (PWP) was taken by measuring the mass of the permeate within a fixed time and given as LMH/bar, which equals L nr ² r ¹ bar ¹ .

The effective mean pore size, pore size distribution and molecular weight cut-off (MWCO) of the hollow fibers were characterized by the solute transport method (Van der Bruggen, Water Res. 36 (2002) 1360-1368; Singh, J. Membr. Sci. 142 (1998) 1 1 1-127). PEG with various molecular weights was used as the model solutes. Aqueous PEG solutions with a concentration of 200 ppm were run through the fibers at 1 bar. The PEG concentrations of the feed solution (Cf) and the permeate (Cp) were determined by using a Total Carbon Analyzer

The water contact angles of the membrane surfaces were measured using a Contact Angle Geniometer (Rame Hart, USA) with deionized water as the probe liquid at 23±0.5 °C. The membrane samples were freeze dried overnight just prior to tests. The measurements were carried out in air with sessile drops. A drop of deionized water was introduced on the membrane surface and the contact angle was taken and calculated by the software. At least ten readings were taken at random locations for each sample and the averaged value was reported.

Example 1 - Preparation of membrane An optimized non-solvent induced phase separation (NIPS) dry-jet wet spinning process was adopted to fabricate the hollow fiber membranes.

Before preparing the spinning dope solutions, PAI-1 and sPPSU-1 and sPESU-1 polymer flakes were first dried in a vacuum oven at 70 °C overnight to remove moisture.

Then, the dope solution was prepared by gradually adding the solid into a liquid mixture of N- methyl-2-pyrrolidinone (NMP) and polyethylene glycol with a molecular weight of 400 Da (PEG 400) under stirring. The dope solution contained 18 wt% PAI-1 , 2 wt% sPPSU-1 or sPESU-1 , 67 wt% NMP and 13 wt% PEG 400. Each mixture was heated to 60 °C and stirred for 2 days to form a homogeneous solution.

Prior to hollow fiber spinning, the dope solution was degassed overnight to remove air bubbles that might be trapped in the solution. During spinning, the dope and bore fluid solution (90 wt% water and 10 wt% NMP) were pumped through the spinneret (inner diameter 0.48 mm, outer diameter 1 .2 mm) with dope flow rate of 2 ml/min, a bore fluid flow rate of 1.5 ml/min, air gap length 6 cm, take up speed 3.5 m/min, and water as external coagulant.

After spinning, the as-spun hollow fibers were rinsed in a clean water bath for 2 days to remove the residual solvent and additive and for complete phase inversion. After that, the membranes were submerged in a 50 wt% glycerol aqueous solution for another 2 days in order to prevent pore collapse during the subsequent air-drying process. The characteristics of the resulting membrane ("PAI-1 + sPPSU-1 " and "PAI-1 + sPESU-1 ") were summarized in Table 1 .

Comparative membrane without sPPSU-1 and without sPESU-1 : The above procedure was repeated, but without the sPPSU-1 or sPESU-1 . The dope solution contained 20 wt% PAI-1 , 67 wt% NMP and 13 wt% PEG 400. The characteristics of the resulting comparative membrane ("PAI-1 ") were summarized in Table 1 .

The data in Table 1 demonstrated that the inventive membrane had a lower pure water permeability PWP and lower molecular weight cutt-off MWCO.

Table 1

Example 2 - Rejections to electrolytes

The effective rejections of the hollow fiber membranes prepared in Example 1 to inorganic salts were obtained by using various electrolytes as the feeds at 1 bar. The initial salt concentration of each electrolyte solution was 1000 ppm (NaCI, MgC , Na2S0 ₄ and MgS0 ₄ neutral solutions), and the feed and permeate salt concentrations were determined by conductivity measurements. The salt rejection R (%) was calculated as follows: R = (1 - Cp/Cf) ^* 100%, where Cp and Cf are the salt or dye concentrations in the permeate and feed solutions, respectively. In general, the both membranes of Example 1 have relatively low rejections of less than 10% to the four inorganic salts. In the textile industry, the inorganic salts are usually used to enhance the dye uptake by the fabric and to maximize the exhaustion of dye molecules. More than 90% of the inorganic salts permeate through the membranes, offering great potential to reuse these salts in the next dying processes. For the treatment of high-salinity wastewater, this ultralow salt rejection would also help increase the permeation flux since the effects of osmotic pressure and concentration polarization effects are minimized.

Example 3 - Rejections to dye solutes

The feasibilities of the membranes prepared in Example 1 for dye removal were evaluated by conducting the UF tests using different dye solutions as feeds. Organic textile dyes of Rose Bengal (an acid dye, mol weight 1018 g/mol), Alcian Blue 8GX (an basic dye, mol weight 1299 g/mol) and Procion Yellow H-E3G (a reactive dye, mol weight 1632 g/mol) were obtained from Sigma-Aldrich.

Rose Bengal Alcian Blue 8GX

Procion Yellow H-E3G

The initial dye concentration of each feed was kept at 200 ppm. The dye concentrations in both the feed and permeate were measured using a UV-vis spectrophotometer. The dye rejection R was calculated as in Example 2.

The results are summarized in Tables 2 to 4 and showed that the inventive membranes had a low PWP and a high rejection. Table 2: Dye Rose Bengal (Molar weight 1018 g/mol) at solution pH 4.5

Table 3: Dye Alcian Blue 8GX (Molar weight 1299 g/mol) at solution pH 6

Table 4: Dye Procion Yellow H-E3G (PY-H) (Molar weight 1632 g/mol) at solution pH 6

PWP [LMH/bar] Rejection

PAI 90 99.9

PAI+sPPSU-1 80 99.9 Example 4 - Long-term performance

For the long-term performance a 1 .0 L dye solution was used as the feed and was continuously run through the membrane of Example 1 until the predetermined testing duration or recovery rate was reached. During the total recirculation operation mode, the permeate of the module was bypassed into the feed tank to maintain the dye concentration of 200 ppm in the feed. The permeate sample was collected at certain time intervals to monitor the membrane performance. In order to ensure the repeatability of the experiments, at least three membrane samples were tested for each testing condition and the averaged results were reported.

The evolution of the dye rejection was tested as a function of time in a continuous test of 170 h at 1 bar using 200 ppm RB (pH=4.5) and PY-H (pH=6.0) as the feed solutions, separately.

The membrane exhibited an excellent dye rejection of higher than 99% over the entire testing period of 170 h.