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 which penetrates at least 25% into the support layer, thereby forming a coated support layer; (b) contacting the coated support layer from (a) with a gas, which comprises a nonsolvent, relative to the polymer in the first polymer solution, leading to a depth of penetration of the nonsolvent from an atmosphere formed by the nonsolvent-containing gas into the applied first polymer solution of at least 5.0% but less than 80%, based on a total depth of the applied first polymer solution, wherein the gas comprises from 1.0 g/m.sup.3 to 20 g/m.sup.3 of the nonsolvent; (c) applying a second polymer solution to the first polymer layer, to form a second polymer layer on the first polymer layer of the coated support layer, thereby forming a multiply coated support 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, wherein the first and second polymer layers comprise cellulose derivates, wherein the support layer is a nonwoven web, a woven fabric or an open microfilter membrane, and wherein after (d) 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.

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 wt % 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 mPa*s to 40000 mPa*s.

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

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

9. The method of claim 1, wherein an 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.

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

11. 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).

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

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

14. The method of claim 13, further comprising (f) hydrolyzing the cellulose ester(s).

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

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

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

18. The method of claim 1, wherein in (b) the coated support layer of (a) is guided through a chamber which possesses a corresponding gas atmosphere, or the contacting is accomplished by the supply of a corresponding gas stream, whereby the volume flow of gas is 10 m.sup.3/h to 600 m.sup.3/h at velocities of 0.3 m/s to 8 m/s (v(gas)).

19. The method of claim 1, wherein the first and second polymer layers independently comprise cellulose derivates selected from the group consisting of cellulose ester, cellulose nitrate, regenerated cellulose, and crosslinked regenerated cellulose.

20. The method of claim 1, wherein the support layer is made of polyolefin, polyester, polyamide, polyethersulfone, or regenerated cellulose.

Description

(1) The figures are as follows:

(2) FIG. 1: Illustrative apparatus for implementing the method of the invention

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

(4) FIG. 3: SEM image of the membrane from Example 1

(5) 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

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

(7) 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)

(8) The present invention is elucidated in more detail by means of the following, nonlimiting examples.

DETERMINATION OF THE (MEAN) PORE SIZES

(9) 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

(10) 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.

(11) 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

(12) 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

(13) 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.

(14) 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

(15) 1 Support layer 2 First polymer solution 3 Nonsolvent-containing gas (relative to first polymer solution) 4 Second polymer solution 5 Precipitation bath 6 Rinsing tank 11 Wetted membrane on support 21 Outer, retentive skin layer 22 Sponge structure (second polymer layer) 23, 33 Damping region 24 Sponge structure (first polymer layer) 25 Textile support layer 46 Second polymer layer 47 First polymer layer K Test element M-1 Conventional ultrafiltration membrane (single-layer membrane, RC, 100 kDa) M-2 Conventional ultrafiltration membrane (multi-layer membrane, RC, 300 kDa) M-3 Ultrafiltration membrane of the invention (multi-layer membrane, RC, 300 kDa) P.sub.1 Pressure obtaining above the membrane (11) P.sub.2 Pressure obtaining below the membrane (11) S Sensor