MECHANICALLY STABLE ULTRAFILTRATION MEMBRANE, AND METHOD FOR PRODUCING SAME

Abstract

The invention relates to a mechanically stable ultrafiltration membrane and to a method for producing such an ultrafiltration membrane.

Claims

1. A method for producing an ultrafiltration membrane, comprising: (a) applying a first polymer solution to a support layer, to form a first polymer layer on and/or partially or completely in the support layer, (b) contacting the coated support layer from step (a) with a gas, which comprises a nonsolvent, relative to the polymer in the first polymer solution, (c) applying a second polymer solution to the first polymer layer, to form a second polymer layer on the first polymer layer, and (d) introducing the multiply coated support layer into a precipitating bath, which comprises a precipitant, relative to the polymer in the first and/or second polymer solution.

2. The method of claim 1, wherein the first and the second polymer solutions independently of one another have the same or different solids contents of 2 to 40 wt %.

3. The method of claim 2, wherein the first polymer solution has a higher solids content than the second polymer solution.

4. The method of claim 1, wherein the first and the second polymer solutions independently of one another have viscosities of 800 to 40000 mPa*s.

5. The method of claim 1, wherein the application temperatures in (a) and (c) independently of one another are from 4° C. to 40° C.

6. The method of claim 1, wherein the support layer in (a) and the coated support layer in (c) move at a speed of 30 to 500 m/h relative to the respective first and second polymer solution during application thereof.

7. The method of claim 1, wherein the gas comprises from 1.0 to 20 g/m.sup.3 of nonsolvent.

8. The method of claim 1, wherein the gas which comprises a nonsolvent, relative to the polymer in the first polymer solution, comprises nitrogen as carrier gas and a nonsolvent (mixture) consisting of 50 to 100 vol % of water and 0 to 50 vol % of ethanol, based on the total amount of nonsolvent.

9. The method of claim 1, wherein the exposure time in (b) is from 500 ms to 20 s.

10. The method of claim 1, wherein the exposure temperature in (b) is at least 10° C. below the boiling point of the lowest-boiling constituent of the first polymer solution, said boiling point being above 50° C.

11. The method of claim 1, wherein (b) is carried out using a channel in which the flow of nonsolvent-containing gas impinges on the coated support layer at an angle of between 0 and 45°.

12. The method of claim 1, wherein the depth of penetration of the nonsolvent from the atmosphere formed by the nonsolvent-containing gas into the applied first polymer solution in (b) is less than 80%, based on the total depth of the applied first polymer solution.

13. The method of claim 1, further comprising: (c1) contacting the multiply coated support layer of (c) with a gas which comprises a nonsolvent, relative to the polymer in the second polymer solution, between (c) and (d).

14. The method of claim 1, further comprising: (e) introducing the multiply coated support layer into one or more rinsing tanks after (d).

15. The method of claim 1, wherein the first and/or second polymer solution comprises a cellulose ester.

16. The method of claim 15, further comprising (f) hydrolyzing the cellulose ester(s).

17. The method of claim 1, further comprising (g) crosslinking the polymer chains in the polymer layers.

18. The method of claim 1, wherein (c) is performed one or more times before (d), using different or identical polymer solutions.

19. An ultrafiltration membrane obtained by the method for producing an ultrafiltration membrane as claimed in claim 1.

20. The ultrafiltration membrane of claim 19, wherein a convective pressure rise, as determined by the test method below, takes place at an impact count of above 600, the test method being characterized in that a test element (K) having a diameter of 20 mm lands repeatedly with a force of at least 1 N but not more than 250 N on a water-wetted ultrafiltration membrane under test, which is located in a housing and sealed off above and below, the pressure above the membrane being P.sub.1 and the pressure below the membrane being P.sub.2, with P.sub.1>P.sub.2, and a sensor (S) detects the pressure rise below the membrane, with the sensor (S) for the pressure rise detecting, further to a diffusive component, a convective component as soon as the membrane is damaged by an impact of the test element (K).

21. The ultrafiltration membrane of claim 19, wherein the ultrafiltration membrane comprises two or more polymer layers disposed on one another on a support layer, and the first polymer layer, which is disposed directly on and/or partially or completely in the support layer has a damping region on the side bordering the second polymer layer, which is disposed on the first polymer layer.

22. The ultrafiltration membrane of claim 21, wherein the polymer layers having a sponge structure, and the outermost polymer layer has an outer, retentive skin layer.

23. The ultrafiltration membrane of claim 22, wherein the outer, retentive skin layer has a thickness of 0.01 to 2.0 μm.

24. The ultrafiltration membrane of claim 21, wherein the first polymer layer has a microporous sponge structure having a mean pore size of 0.05 to 30 μm, and the sponge structure of the first polymer layer penetrates at least 25% into the support layer.

25. The ultrafiltration membrane of claim 21, wherein the support layer has a thickness of 30 to 300 μm, the first polymer layer has a thickness of 10 to 100 μm, with the first polymer layer penetrating at least 25% into the support layer, the second polymer layer has a thickness of 20 to 100 μm, the thickness of the second polymer layer being less than or equal to the thickness of the combination of the first polymer layer and support layer, and the damping region of the first polymer layer accounts for up to 20% of the thickness of the first polymer layer.

26. The ultrafiltration membrane of claim 21, wherein the ultrafiltration membrane has a total thickness of 50 to 400 μm.

27. The ultrafiltration membrane of claim 21, wherein the polymer layers independently of one another have an asymmetric or a symmetrical sponge structure.

28. The ultrafiltration membrane of claim 27, wherein the first and second polymer layers have an asymmetric sponge structure.

29. The ultrafiltration membrane of claim 28, wherein the first and second polymer layers, taken together, have a complex structure wherein the pores initially widen from a first main surface within the second polymer layer, the pores in the interior of the membrane then taper on entry into the first polymer layer, and the pores in the course of the structure widen again toward a second main surface.

30. The ultrafiltration membrane of claim 21, wherein the polymer layers comprise cellulose derivates comprising cellulose ester, cellulose nitrate, regenerated cellulose and/or crosslinked regenerated cellulose.

31. The ultrafiltration membrane of claim 21, wherein the ultrafiltration membrane retains bovine serum albumin to an extent of not more than 50%.

32. The method of claim 1, wherein the gas comprises from 12 to 17 g/m.sup.3 of nonsolvent.

Description

[0074] The figures are as follows:

[0075] FIG. 1: Illustrative apparatus for implementing the method of the invention

[0076] FIG. 2: Illustrative apparatus for implementing the method for investigating the impact resistance of the membrane (11)

[0077] FIG. 3: SEM image of the membrane from Example 1

[0078] FIG. 4: Enlargement of the SEM image from FIG. 3 to show the damping region (23, 33) and the sponge structures (22, 24) located above/below said region in the second and first polymer layers

[0079] FIG. 5: Results of the method for investigating the impact resistance of different ultrafiltration membranes (M-1 to M-3)

[0080] FIG. 6: Illustrative structure of the membrane of the invention for different depths of diffusive penetration (50%, 33%, 25%) of the nonsolvent-containing gas in step (b)

[0081] The present invention is elucidated in more detail by means of the following, nonlimiting examples.

Determination of the (Mean) Pore Sizes To determine the (mean) pore sizes of the membrane, a method was employed that is based on membrane cross sections analyzed by scanning electron microscopy (SEM) (described in P. Poelt et. al., Journal of Membrane Science 2012, vol. 399-400, pp. 86-94).

EXAMPLE 1

Production of an Ultrafiltration Membrane with Damping Region and Enhanced Impact Resistance

[0082] A first polymer solution consisting of 75 wt % of dimethylacetamide (DMAc), 15 wt % of cellulose acetate (Acetati, type Aceplast PC/FG) and 5 wt % of PEG 1000, 5 wt % of water is applied via slot die to a PP/PE core-shell nonwoven (OMB-60; Mitsubishi) moving at 100 m/h orthogonally to the outlet opening. The polymer film then passes through a chamber 10 cm in length, containing an atmosphere with 35% relative humidity (rh) at a temperature of 20° C. On departure from the chamber, the polymer film is moved orthogonally to the outlet opening of a second application medium (carriage with doctor blade), and second polymer solution consisting of 78 wt % of DMAc, 11 wt % of cellulose diacetate (Acetati, type Aceplast PC/FG), 4 wt % of PEG 2000, 4 wt % of glycerol and 3 wt % of water is applied, and is then transferred for phase separation into a water precipitation bath at a temperature of 6° C. The rest of the process, described above, takes place at room temperature.

[0083] The membrane obtained is then passed through a number of rinsing tanks into a tank with 50% KOH, in which the acetyl groups are hydrolyzed. Relative to comparable single-layer or multilayer membranes, the regenerated cellulose membrane has a particular stability in the z-direction. By crosslinking, in accordance for example with the methods described in DE 102004053787 A1, the membrane can be converted into a crosslinked regenerated cellulose membrane (cellulose hydrate membrane) which likewise has the special mechanical stability. SEM images of the membrane obtained are shown in FIGS. 3 and 4.

EXAMPLE 2

Investigation of the Impact Resistance of Different Ultrafiltration Membranes

[0084] Two conventional ultrafiltration membranes (M-1 and M-2) of regenerated cellulose (RC) (a single-layer membrane with a cut-off limit of 100 kDa (M-1) and a multilayer membrane with a cut-off limit of 300 kDa (M-2), produced by a conventional Cocast method (double coating method) without step (b)) and an ultrafiltration membrane according to the invention (M-3) of regenerated cellulose with a damping region and a cut-off limit of 300 kDa, produced by means of the method of the invention, were subjected to the aforementioned testing method for determining the impact resistance or the convective pressure rise. In this case a pressure P.sub.1=3 bar and a pressure P.sub.2=1 bar were applied, and a cylindrical test element having a diameter of 20 mm which landed repeatedly on the membranes with a force of 85 N. The results obtained are depicted in FIG. 5 and show that with the ultrafiltration membrane (M-3) with damping region, according to the invention, the achievable impact resistance is much higher by comparison with the conventional ultrafiltration membranes (M-1 and M-2).

EXAMPLE 3

Production of an Ultrafiltration Membrane with Damping Region and Enhanced Impact Resistance in a Chamber

[0085] A first polymer solution consisting of 50 wt % of acetone, 28 wt % of dioxane, 2 wt % of water, 12 wt % of cellulose acetate (Acetati, type Aceplast PC/FG) and 8 wt % of glycerol is applied via doctor blade and carriage to a PP/PE core-shell nonwoven (OMB-60; Mitsubishi) moving at 70 m/h orthogonally to the outlet opening. Directly bordering the carriage is a chamber 30 cm in length, containing an atmosphere of nitrogen as carrier gas and also 19 g/m.sup.3 of a 1:19 ethanol:water mixture and maintained at a constant temperature of 20° C. In the upper region of the chamber, a stream of a preconditioned atmosphere flows through, this stream being controlled in such a way as not to exceed a flow of 0.2 m/s, so that in the lower region of the chamber there are no eddies where the polymer film runs through the chamber. The chamber is operated in such a way that there is always an overpressure of 1 mbar relative to the surrounding space. The ambient environment undergoes suction withdrawal for correct removal of the small amounts of ethanol. On departure from the chamber, the polymer film is moved orthogonally to the outlet opening of a second application medium (carriage with doctor blade), and second polymer solution consisting of 50 wt % of acetone, 28 wt % of dioxane, 2 wt % of water, 12 wt % of cellulose acetate (Acetati, type Aceplast PC/FG) and 8 wt % of glycerol, and is then transferred for phase separation into a water precipitation bath at a temperature of 15° C.

[0086] The membrane obtained is then passed through a number of rinsing tanks into a tank with 50% KOH, in which the acetyl groups are hydrolyzed. The membrane is rinsed and excess alkali is neutralized with diluted acetic acid. Relative to comparable single-layer or multilayer membranes, the regenerated cellulose membrane has a particular stability in the z-direction. By crosslinking, in accordance for example with the methods described in DE 102004053787 A1, the membrane can be converted into a crosslinked regenerated cellulose membrane (cellulose hydrate membrane) which likewise has the special mechanical stability.

LIST OF REFERENCE SYMBOLS

[0087] 1 Support layer [0088] 2 First polymer solution [0089] 3 Nonsolvent-containing gas (relative to first polymer solution) [0090] 4 Second polymer solution [0091] 5 Precipitation bath [0092] 6 Rinsing tank [0093] 11 Wetted membrane on support [0094] 21 Outer, retentive skin layer [0095] 22 Sponge structure (second polymer layer) [0096] 23, 33 Damping region [0097] 24 Sponge structure (first polymer layer) [0098] 25 Textile support layer [0099] 46 Second polymer layer [0100] 47 First polymer layer [0101] K Test element [0102] M-1 Conventional ultrafiltration membrane (single-layer membrane, RC, 100 kDa) [0103] M-2 Conventional ultrafiltration membrane (multi-layer membrane, RC, 300 kDa) [0104] M-3 Ultrafiltration membrane of the invention (multi-layer membrane, RC, 300 kDa) [0105] P.sub.1 Pressure obtaining above the membrane (11) [0106] P.sub.2 Pressure obtaining below the membrane (11) [0107] S Sensor