Method for producing a porous monolayer polymer membrane, porous monolayer polymer membrane, and use thereof for filtration

Abstract

The present invention relates to a method for producing a porous monolayer polymer membrane, to a porous monolayer polymer membrane, and to the use of the polymer membrane for filtration.

Claims

1. A method for producing a porous monolayer polymer membrane, which comprises the following steps: (A) providing a membrane-forming casting solution which comprises a membrane-forming polymer and a solvent therefor; (B) providing a non-membrane-forming and nonprecipitating protective solution; (C) providing a support; (D) applying at least the casting solution and the protective solution to the support to form a film comprising a casting solution layer and a protective solution layer bordering thereon, wherein the film is formed by applying the casting solution to the support to form the casting solution layer, and applying the protective solution layer to the casting solution layer to form the protective solution layer; (E) contacting the film with a precipitant; and (F) removing the protective solution layer by dissolving or chemically decomposing the protective layer solution.

2. The method as claimed in claim 1, wherein the protective solution comprises a polymer not capable of forming a membrane (surrogate polymer) and a solvent therefor.

3. The method of claim 1, wherein the protective solution contains no precipitant for the membrane-forming polymer.

4. The method of claim 2, wherein the protective solution contains no precipitant for the membrane-forming polymer.

5. The method of claim 1, wherein the porous monolayer polymer membrane is asymmetric.

6. The method of claim 5, wherein the porous monolayer polymer membrane has an asymmetry factor of 1.5 to 10.

7. The method of claim 1, wherein the membrane-forming polymer is selected from cellulose acetate, cellulose nitrate, polysulfone, polyvinylidene difluoride, polyethersulfone, polyetheretherketone, polyacrylonitrile, or polymethyl methacrylate.

Description

(1) FIG. 1 shows an SEM micrograph of the side (major surface) of the membrane from Inventive Example 1 that is formed from a casting solution layer which during production bordered an upper coating solution (air side). Clearly in evidence is a porous structure which extends into the depth of the membrane.

(2) FIG. 2 shows an SEM micrograph of the support-side (glass plate-side) major surface of the membrane from Inventive Example 1, and shows a typical pore structure. The structure (in contrast to the upper coating-side major surface) is largely unaffected by the upper coating and shows a very similar structure to the support-side major surface of the membrane from Comparative Example 1.

(3) FIG. 3 shows an SEM micrograph of the air-side (the air side is the side opposite the support side) major surface of the membrane from Comparative Example 1. At ten thousand times magnification there are no pores in evidence.

(4) FIG. 4 shows an SEM micrograph of the support-side major surface of the membrane of Comparative Example 1. There are no significant differences in evidence relative to the belt-side surface structure of the membrane from Inventive Example 1.

(5) FIG. 5 shows an SEM micrograph of the support side major surface of the membrane from Comparative Example 2. A pronounced pore structure is in evidence.

(6) FIG. 6 shows an SEM micrograph of the upper coating-side major surface of the membrane from Comparative Example 2. Clearly in evidence is a skin layer without perceptible pores on the surface.

(7) FIG. 7 shows an SEM micrograph of the membrane from Inventive Example 2. In evidence is the membrane side which was in contact with the protective solution (surrogate solution 1) (support side). It is clearly evident that the support-side major surface of the membrane has a high surface porosity.

(8) FIG. 8 shows an SEM micrograph of the membrane from Comparative Example 3. In evidence is the membrane side which was in contact with the glass surface (support side) and which has a low surface porosity in comparison to Inventive Example 2.

(9) FIG. 9 shows an SEM micrograph of the air side of the membrane from Inventive Example 3. Clearly in evidence are the globular structure, a high surface porosity, and the absence of a skin layer.

(10) FIG. 10 shows an SEM micrograph of the air side of the membrane from Comparative Example 4. Clearly in evidence are the absence of porous structures and the presence of a skin layer.

(11) FIG. 11 shows an SEM micrograph of the membrane from Inventive Example 2. In evidence is the membrane side which was in contact with the protective solution 2 (surrogate solution 2) (air side). Clearly in evidence are the globular structure, a high surface porosity, and the absence of a skin layer.

(12) FIG. 12 shows an SEM micrograph of the membrane from Comparative Example 3. In evidence is the membrane side, which was not in contact with the glass surface (air side).

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

EXAMPLES

Inventive Example 1: Production of an Asymmetric Microfiltration Membrane from Cellulose Acetate in a Precipitation Bath Process

(14) Casting Solution

(15) The casting solution was prepared from the constituents from Table 1 in a stirred reactor at 40° C. The acetone and dioxane components were introduced initially and the solids were added in powder form. As soon as the polymer solution was no longer turbid (after about 3 hours), formamide was added. In this composition the casting solution can be kept for about 48 hours at room temperature.

(16) TABLE-US-00001 TABLE 1 Composition of the casting solution Constituents Mass fraction [%] Cellulose triacetate 12.8 (Eastman, CA 398) (12.8%) Cellulose diacetate 3.2 (Acetati, Aceplast PC/FG) (3.2%), Acetone 28 Dioxane 28 Formamide 28

(17) Protective Solution

(18) The protective solution (upper coating) was prepared as for the preparation of the casting solution. The composition of the protective solution is reported in Table 2.

(19) TABLE-US-00002 TABLE 2 Composition of the protective solution Constituents Mass fraction [%] Polyvinylpyrrolidone 10 (PVP, M.sub.w = 360 000 g/mol) of “K90” type from BASF Acetone 30 Dioxane 30 Formamide 30

(20) Unless otherwise indicated, the protective and casting solutions in the following examples were prepared as described in Inventive Example 1.

(21) Membrane Production

(22) The casting solution with the composition from Table 1 was coated out evenly to a thickness of 300 μm, using a knife coater, onto a glass plate used as support, in a dry (anhydrous) nitrogen atmosphere at a glass plate and atmosphere temperature of 22° C. Thereafter the upper coating was knife-coated over the casting solution film in a thickness of 200 μm. The application process was over after less than 30 seconds.

(23) Set up in an air atmosphere with 22° C. and <40% relative humidity was a water bath at 22° C. which was able to fully cover the glass plate. The coated layers on the glass plate were immersed fully into the water bath in less than 20 seconds, starting from one side of the glass plate, and shaken for 30 minutes. After these 30 minutes, the water was replaced and the glass plate, with the now precipitated membrane, was shaken again for 30 minutes. After complete removal of the upper coating, the membrane was lifted from the glass plate and dried on a fleece underlay at 22° C. The dry membrane was subjected to the following characterizations: scanning electron microscopy (SEM) and throughflow.

(24) SEM micrographs of the major surfaces of the membrane from Inventive Example 1 are shown in FIGS. 1 and 2. Clearly apparent from FIG. 1 is an open-pore structure with high porosity.

(25) The permeability was 0.89±0.1 mL/(min.Math.cm.sup.2.Math.bar)

Comparative Example 1

(26) For Comparative Example 1, the casting solution used was identical to the casting solution from Inventive Example 1. The protective solution was omitted.

(27) Membrane Production

(28) The casting solution with the composition from Table 1 was coated out evenly to a thickness of 300 μm, using a knife coater, onto a glass plate used as support, in a dry (anhydrous) nitrogen atmosphere at a glass plate and atmosphere temperature of 22° C. No protective solution was applied. The application process was over after less than 30 seconds.

(29) Set up in an air atmosphere with 22° C. and <40% relative humidity was a water bath at 22° C. which was able to fully cover the glass plate. The coated layer on the glass plate was immersed fully into the water bath in less than 20 seconds, starting from one side of the glass plate, and shaken for 30 minutes. After these 30 minutes, the water was replaced and the glass plate, with the now precipitated membrane, was shaken again for 30 minutes. After that, the membrane was lifted from the glass plate and dried on a fleece underlay at 22° C. The dry membrane was subjected to the following characterizations: scanning electron microscopy (SEM) and throughflow.

(30) SEM micrographs of the major surfaces of the membrane from Comparative Example 1 are shown in FIGS. 3 and 4. Clearly apparent from FIG. 3 is a dense structure with low porosity (skin layer).

(31) The permeability was less than 0.1 mL/(min.Math.cm.sup.2.Math.bar)

Comparative Example 2: Example 3 from EP 2 134 455 B1

(32) Comparative Example 2 is based on Example 3 of EP 2 134 455 B1. In this case, for the upper coating, a protective solution was used which comprises a precipitant in a concentration which on contact with the casting solution leads to the precipitation of the membrane-forming polymer. Comparative Example 2 was otherwise carried out like Inventive Example 1.

(33) TABLE-US-00003 TABLE 3 Composition of the casting solution Constituents Mass fraction [%] Polyethersulfone (PESU) of 13 “Ultrason E6020” type from BASF PEG 400 70 N-Methylpyrrolidone (NMP) 30

(34) TABLE-US-00004 TABLE 4 Composition of the upper coating solution Constituents Mass fraction [%] Polyethylene glycol 400 (PEG 400) 80 Water 20

(35) SEM micrographs of the major surfaces of the membrane from Comparative Example 1 are shown in FIGS. 5 and 6. Clearly apparent from FIG. 6 is a dense structure with a low porosity (skin layer).

Inventive Example 2: Lower Coating and Upper Coating

(36) The compositions of the casting solution and protective solutions used in this example are reported in tables 5, 6 and 7.

(37) TABLE-US-00005 TABLE 5 Composition of the casting solution Constituents Mass fraction [%] Polyethersulfone of “Ultrason 12.00% E6020” type from BASF 2-Pyrrolidone 77.00% PVP-VA copolymer of “PVP 3.00% S630” type from Ashland Glycerol 5.00% Water 3.00%

(38) TABLE-US-00006 TABLE 6 Composition of protective solution 1 (lower coating solution) Constituents Mass fraction [%] 2-Pyrrolidone 70% Polyethylene glycol (M.sub.w = 30 000 g/mol) 30%

(39) TABLE-US-00007 TABLE 7 Composition of protective solution 2 (upper coating solution) Constituents Mass fraction [%] 2-Pyrrolidone 100

(40) Membrane Production

(41) Protective solution 1 (surrogate solution 1) was heated to 60° C. and coated out uniformly to a thickness of 100 μm using a knife coater on a glass plate under an air atmosphere at 22° C. and <40% relative humidity. The protective solution applied and also the glass plate were subsequently cooled to room temperature.

(42) The casting solution was subsequently applied uniformly to the surrogate solution 1 with a thickness of 200 μm using a knife coater. Then protective solution 2 (surrogate solution 2) was coated over it in a thickness of 100 μm by means of a knife coater.

(43) A water bath with 22° C. was set up which was able to fully cover the glass plate. The glass plate with the film consisting of coated-out casting solution and surrogate solutions was immersed completely into the water bath in less than 20 seconds, starting from one side of the glass plate, and shaken for 30 minutes. After these 30 minutes, the water was replaced and the glass plate with the now precipitated membrane was shaken again for 30 minutes, during which the protective solution layers dissolved and only the membrane remained on the glass plate. After that, the membrane was lifted from the glass plate and dried on a fleece underlay at 22° C. The dry membrane was subjected to the characterizations.

(44) The result is shown in FIGS. 7 and 11. The SEM micrographs represented in FIGS. 7 and 11 were obtained under the following conditions: recording instrument: FEI Quants 200 F; accelerating voltage: 19 kV; magnification: 4000 times.

Comparative Example 3

(45) The composition of the casting solution used in this example is reported in table 8.

(46) TABLE-US-00008 TABLE 8 Casting solution Constituents Mass fraction [%] Polyethersulfone of “Ultrason 12.00 E6020” type from BASF 2-Pyrrolidone 77.00 PVP-VA copolymer of “PVP 3.00 S630” type from Ashland Glycerol 5.00 Water 3.00

(47) Membrane Production

(48) The casting solution was applied uniformly with a thickness of 250 μm, using a knife coater, to a glass plate, and was exposed to the air atmosphere for 3 minutes. A water bath with 22° C. was set up, which was able to fully cover the glass plate. The coated-out casting solution on the glass plate was immersed fully into the water bath in less than 20 seconds, starting from one side of the glass plate, and shaken for 30 minutes. After these 30 minutes the water was replaced and the glass plate, with the now precipitated membrane, was shaken again for 30 minutes. After that the membrane was lifted from the glass plate and dried on a fleece underlay at 22° C. The dry membrane was subjected to the characterizations.

(49) The result in shown in FIG. 8 and FIG. 12 and in table 9.

(50) TABLE-US-00009 TABLE 9 Comparison of Inventive Example 2 with Comparative Example 3 Inventive Comparative Example 2 Example 3 Surface porosity (air side) 40%  0% Surface porosity (belt or support side) 63% 22%

Inventive Example 3

(51) The compositions of the casting solution and protective solution used in this example are reported in tables 10 and 11.

(52) TABLE-US-00010 TABLE 10 Composition of the casting solution Constituents Mass fraction [%] Polyethersulfone of “Ultrason 12.00 E6020” type from BASF 2-Pyrrolidone 77.00 PVP-VA copolymer of “PVP 3.00 S630” type from Ashland Glycerol 5.00 Water 3.00

(53) TABLE-US-00011 TABLE 11 Composition of the protective solution Constituents Mass fraction [%] 2-Pyrrolidone 100

(54) Membrane Production

(55) A casting solution was applied evenly with a thickness of 250 μm, using a knife coater, to a glass plate, and directly thereafter was coated with the protective solution as upper coating in a thickness of 50 μm by means of a knife coater. A water bath with 22° C. was set up, which was able to fully cover the glass plate. The glass plate with the coated-out casting solution and protective solution was immersed completely into the water bath in less than 20 seconds, starting from one side of the glass plate, and shaken for 30 minutes. After these 30 minutes the water was replaced and the glass plate, with the now precipitated membrane, was again shaken for 30 minutes. After complete removal of the upper coating/protective solution layer, the membrane was lifted from the glass plate and dried on a fleece underlay at 22° C. The dry membrane was subjected to the characterizations.

(56) The results are shown in FIG. 9 and also in tables 13 and 14.

Comparative Example 4

(57) The composition of the casting solution used in this example is reported in table 12.

(58) TABLE-US-00012 TABLE 12 Composition of the casting solution Constituents Mass fraction [%] Polyethersulfone of “Ultrason 12.00 E6020” type from BASF 2-Pyrrolidone 77.00 PVP-VA copolymer of “PVP 3.00 S630” type from Ashland Glycerol 5.00 Water 3.00

(59) Membrane Production

(60) A casting solution was applied evenly with a thickness of 250 μm, using a knife coater, to a glass plate. A water bath with 22° C. was set up, which was able to fully cover the glass plate. The coated-out casting solution on the glass plate was immersed completely into the water bath in less than 20 seconds, starting from one side of the glass plate, and shaken for 30 minutes. After these 30 minutes the water was replaced and the glass plate, with the now precipitated membrane, was again shaken for 30 minutes. After that, the membrane was lifted from the glass plate and dried on a fleece underlay at 22° C. The dry membrane was subjected to the characterizations.

(61) The results are shown in FIG. 10 and also in tables 13 and 14.

(62) TABLE-US-00013 TABLE 13 Inventive Comparative Example 3 Example 4 Surface porosity (air side) 42%  0% Surface porosity (belt side) 21% 20%

(63) TABLE-US-00014 TABLE 14 Inventive Comparative Example 3 Example 4 Permeability [mL/(min .Math. cm.sup.2 .Math. bar)] 9 ± 3 <1

(64) As can be seen from table 13, Inventive Example 3, owing primarily to the higher surface porosity of the air side, has a substantially higher permeability and therefore a higher throughflow than Comparative Example 4.

(65) To determine the permeability of the membranes produced, they were wetted in NaCl solution (0.9 wt %) and then transferred into a permeability measurement cell. This cell consisted of eight identical metal cylinders (pressure-resistant steel container, 200 mL capacity, Sartorius Stedim Biotech GmbH). The measurement cells could be filled with a defined volume of salt solution. Then a pressure of 0.1 to 1 bar (according to permeability) was applied and the bottom valve was opened. The permeate was collected and weighed in a time-resolved manner. The specific permeability is calculated as follows:

(66) ${\chi }_{\mathrm{spec}}=\frac{V_p}{t.Math.A_a.Math.p}$

(67) Here, V.sub.p is the permeate volume, t the time required to collect this permeate volume V.sub.p, A.sub.a the membrane inflow area, and p the applied pressure.

(68) Through the method of the invention it is possible to provide monolayer porous polymer membranes with high surface and total porosity, hence allowing thin membranes with high filtration performance to be obtained. Through the method of the invention an innovative control option is gained for influencing the phase inversion during formation of membranes from a casting solution, and consequently the method is less sensitive to fluctuations in parameters such as pressure, temperature, and precipitant concentration during the phase inversion.