Method for the production of a filtration membrane having a mean molecular weight cut-off of < 1000 g/mol

Abstract

The invention relates to a process for preparing a filtration membrane having an average molecular out-off of <1000 g/mol.

Claims

1. A process for preparing a filtration membrane having an average molecular cut-off less than 1000 g/mol, the process comprising: a) contacting at least one filtration membrane having an average molecular cut-off between 3,000 g/mol and 15,000 g/mol, and comprising at least one bonded textile fabric coated with at least one of: a polyether sulphone and a hydrophilicized polyether sulphone, with an aqueous medium having a water content of not less than 99%, b) contacting the filtration membrane from step a) with at least one polyethylene glycol in the presence of at least one different aliphatic, cyclic or non-cyclic, branched or unbranched, saturated or unsaturated, mono-, di- or trihydric alcohol, wherein the polyethylene glycol is used in an amount of 5 wt % to 50 wt %, based on the combined amount of polyethylene glycol and alcohol, and the sum total of polyethylene glycol and alcohol used in process step b) is greater than 95 wt %, based on the overall amount of the mixture used, and c) drying the filtration membrane from step b).

2. The process according to claim 1, wherein the filtration membrane used in process step a) has a salt retention between 5% and 15%, wherein this salt retention is measured at 25 C. with an aqueous 2000 ppm MgSO.sub.4 solution in a crossflow filtration unit at a feed volume flow of 4 l/h and a feed pressure of 10 bar.

3. The process according to claim 1, wherein, in process step a), the filtration membrane is bonded textile fabric coated with the hydrophilicized polyether sulphone.

4. The process according to claim 1, wherein, in process step a), the aqueous medium has a conductivity less than or equal to 10 S/cm at 25 C.

5. The process according to claim 1, wherein, in process step a), the filtration membrane is contacted for 10 s to 600 s.

6. The process according to claim 1, wherein the alcohol comprises monohydric aliphatic saturated alcohols.

7. The process according to claim 1, wherein the polyethylene glycols have an average molar mass of 106 g/mol to 2000 g/mol.

8. The process according to claim 1, wherein the polyethylene glycols are polyethylene glycols of formula (I) ##STR00002## where n is from 2 to 10.

9. The process according to claim 1, the filtration membrane is treated in process step b) for 2 min to 6 min.

10. The process according to claim 1, wherein the amount of polyethylene glycol used in process step b) is between 10 wt % and 40 wt %, based on the combined amount of polyethylene glycol and alcohol.

11. The process according to claim 1, wherein the filtration membrane is dried at a temperature of 40 C. to 70 C.

12. The process according to claim 1, wherein the filtration membrane is dried for 1 to 15 min.

13. The process according to claim 1, wherein the filtration membrane obtained has an average molecular cut-off between 200 g/mol and 1000 g/mol and a salt retention between 80% and 99% coupled with a flux of 6 to 30 l/m.sup.2/h.

14. The process according to claim 1, wherein the filtration membrane obtained has a salt retention which decreases by less than 2% within a period of 1 week at pH less than 1 measured at 25 C.

15. The process according to claim 1, wherein: the bonded textile fabric comprises fibrous nonwoven webs laid from polyethylene or polypropylene or from mixtures thereof, and have a thickness of 50 to 250 m; the bonded textile fabric comprises a coating membrane comprising greater than 98 wt % of polyether sulphone having an average molecular weight between 4,000 and 100,000 g/mol based on the total amount of the membrane, or greater than 98 wt % of hydrophilicized polyether sulphone having an average molecular weight between 40,000 and 100,000 g/mol based on the overall amount of the membrane; the aqueous medium has a water content of 99.9% and a conductivity less than or equal to 1 S/cm at 25 C.; in process step a), the filtration membrane s contacted with the aqueous medium for 10 s to 350 s; the alcohol comprises methanol, ethanol, 1-propanol, 2-propanol, n-butanol, i-butanol, t-butanol, 1-pentanol, 2-pentanol, or 3-pentanol or mixtures thereof; the polyethylene glycols have a molar mass of 106 to 600 g/mol and are polyethylene glycols of formula (I) ##STR00003## where n is from 2 to 10; in process step b), the polyethylene glycol is used in an amount of 10 wt % to 40 wt % based on the combined amount of polyethylene glycol and alcohol, and the total amount of polyethylene glycol and alcohol used is greater than 98 wt %, based on the overall amount of the mixture used; and the filtration membrane is dried at a temperature of 50 C. to 65 C.

Description

EXAMPLES

Example 1

(1) A filtration membrane comprising a bonded textile fabric coated with a hydrophilicized polyether sulphone (PESH), the UH004 product from Microdyn Nadir, was subjected to a treatment comprising 4 steps. 1st The surface of the membrane was rinsed with deionized water for 30 s by means of a spray bottle. 2nd Thereafter the rinsed membrane was fully immersed in a solution of 20 wt % polyethylene glycol (200 g/mol) in 2-propanol for 3 minutes. During this impregnating step, the solution was kept covered in order to avoid any evaporation of the solvent. 3rd The membrane was then removed from the 2-propanol/polyethylene glycol bath and laid aside for 10 minutes under atmospheric conditions. Ail the while a lid was used to keep the membrane covered up. 4th The membrane was finally dried for 10 minutes in a circulating air drying cabinet at 50 C.

(2) Before and after this treatment, the membranes were tested with an aqueous solution of 2000 ppm of MgSO.sub.4 in a crossflow filtration cell at a pressure of 10 bar and a feed volume flow of 4 l/h to determine their retention and flux. Table 1 shows the result of the measurement after a test period of about 4 hours, when an equilibrium state was safely established.

(3) TABLE-US-00001 TABLE 1 Membrane Before treatment After treatment No. Flux/l/m.sup.2h Retention/% Flux/l/m.sup.2h Retention/% 1 55.0 11.7 7.9 98.9

Example 2

(4) A filtration membrane comprising a bonded textile fabric coated with a hydrophilicized polyether sulphone (PESH), the UH004 product from Microdyn Nadir, was subjected to a treatment comprising 3 steps. 1st The top and bottom sides of the membrane were rinsed with deionized water for 30 s at a time by means of a water hose. 2nd Thereafter the rinsed membrane was fully immersed in a solution of 20 wt % polyethylene glycol (200 g/mol) in ethanol for 3 minutes. During this impregnating step, the solution was kept covered in order to avoid any evaporation of the solvent. 3rd The membrane was finally dried for 5 minutes in a circulating air drying cabinet at 60 C.

(5) Before and after this treatment, the membrane was tested with an aqueous solution of 2000 ppm of MgSO.sub.4 in a crossflow filtration cell at a pressure of 10 bar and a feed volume flow of 4 l/h to determine their retention and flux. Table 2 shows the result of the measurement after a test period of about 4 hours, when an equilibrium state was safely established.

(6) TABLE-US-00002 TABLE 2 Membrane Before treatment After treatment No. Flux/l/m.sup.2h Retention/% Flux/l/m.sup.2h Retention/% 2 53.6 13.3 4.0 99.1

Example 3

(7) A filtration membrane comprising a bonded textile fabric coated with a polyether sulphone (PES), the UP005 product from Microdyn Nadir, was subjected to a treatment comprising 3 steps. 1st The top and bottom sides of the membrane were rinsed with deionized water for 30 s at a time by means of a water hose. 2nd Thereafter the rinsed membrane was fully immersed in a solution of 20 wt % polyethylene glycol (200 g/mol) in ethanol for 3 minutes. During this impregnating step, the solution was kept covered in order to avoid any evaporation of the solvent, 3rd The membrane was finally dried for 10 minutes in a circulating air drying cabinet at 50 C.

(8) Before and after this treatment, the membrane was tested with an aqueous solution of 2000 ppm of MgSO.sub.4 in a crossflow filtration cell at a pressure of 10 bar and a feed volume flow of 4 l/h to determine their retention and flux. Table 3 shows the result of the measurement after a test period of about 4 hours, when an equilibrium state was safely established.

(9) TABLE-US-00003 TABLE 3 Membrane Before treatment After treatment No. Flux/l/m.sup.2h Retention/% Flux/l/m.sup.2h Retention/% 3 235.0 9.3 8.5 98.3

Example 4

(10) A filtration membrane comprising a bonded textile fabric coated with a hydrophilicized polyether sulphone (PESH), the UH004 product from Microdyn Nadir, was subjected to a treatment comprising 3 steps, in order to test it subsequently for its stability in highly concentrated add. 1st The top and bottom sides of the initial membrane were placed for 5 minutes in a water bath filled with deionized water. 2nd Thereafter the rinsed membrane was fully immersed in a solution of 20 wt % polyethylene glycol (200 g/mol) in ethanol for 5 minutes. During this impregnating step, the solution was kept covered in order to avoid any evaporation of the solvent. 3rd The membrane was finally dried for 5 minutes in a circulating air drying cabinet at 60 C.

(11) The membrane thus obtained was tested with an aqueous solution of 2000 ppm of MgSO.sub.4 in a crossflow filtration cell at a pressure of 10 bar and a feed volume flow of 4 l/h to determine its retention and flux. The result is shown in the first column of Table 4.

(12) A second membrane obtained by the process described above was placed for 7 days at room temperature in an aqueous solution of 20 wt % H.sub.2SO.sub.4. In order to avoid any evaporation of the solution and also external influences, the vessel in which the membrane was placed was tightly sealed.

(13) After this treatment, the membrane was rinsed with deionized water and tested with an aqueous solution of 2000 ppm of MgSO.sub.4 in a crossflow filtration cell at a pressure of 10 bar and a feed volume flow of 4 l/h to determine its retention and flux. The result is shown in the second column of Table 4.

(14) TABLE-US-00004 TABLE 4 Without immersion After immersion in Membrane in acidic solution 20 wt % H.sub.2SO.sub.4 No. Flux/l/m.sup.2h Retention/% Flux/l/m.sup.2h Retention/% 4 4.1 99.3 4.8 97.4

(15) The result reveals that there is no significant decrease in the flux and retention of a nanofiltration membrane following immersion in a strongly acidic solution. The membrane accordingly exhibits extraordinarily good stability in extremely acidic media.

Example 5

(16) Two filtration membranes (membrane A and membrane B), comprising a bonded textile fabric coated with a hydrophilicized polyether sulphone (PESH), the UH004 product from Microdyn Nadir, were subjected to two different treatments comprising 3 steps. 1st The top and bottom sides of the membranes were rinsed with deionized water for 30 s by means of a spray bottle. 2nd Thereafter the rinsed membranes were fully immersed in a solution of 22 wt % polyethylene glycol (200 g/mol) in deionized water (membrane A) or 22 wt % polyethylene glycol (200 g/mol) in ethanol (membrane B) for 3 minutes. During this impregnating step, the solutions were kept covered in order to avoid any evaporation of the solvent. 3rd The membranes were finally dried for 10 minutes in a circulating air drying cabinet at 50 C.

(17) After this treatment, the membranes were tested with an aqueous solution of 2000 ppm of MgSO.sub.4 in a crossflow filtration cell at a pressure of 10 bar and a feed volume flow of 4 l/h to determine their retention and flux. Table 5 shows the result of the measurement after a test period of about 4 hours, when an equilibrium state was safely established.

(18) TABLE-US-00005 TABLE 5 Membrane A- Membrane B- Membrane PEG in water PEG in ethanol No. Flux/l/m.sup.2h Retention/% Flux/l/m.sup.2h Retention/% 5 128.7 10.8 5.6 97.0