SERIAL ARRANGEMENT HAVING MULTIPLE PLIES OF ASYMMETRIC FILTER MEDIA, PRODUCTION METHOD, FILTRATION UNIT, USE OF THE ARRANGEMENT, AND CHARACTERIZATION METHOD

Abstract

The present invention relates to a serial arrangement comprising n plies of asymmetric filter media, wherein n is at least two and the pore size of the n plies substantially continuously decreases in the thickness direction of the serial arrangement, to a production method for the serial arrangement, to a filtration unit comprising the serial arrangement, to the use of the serial arrangement, and to a method for characterizing the pores of a filter medium.

Claims

1. A serial arrangement of filter media that comprises n plies, wherein n is at least two, each of the n plies is an asymmetric filter medium having an asymmetry factor of at least 1.5, the n plies have an overall asymmetry factor of at least 10, and the pore size of the n plies substantially continuously decreases in the thickness direction of the serial arrangement.

2. The serial arrangement as claimed in claim 1, wherein the filter medium is a microporous membrane.

3. The serial arrangement as claimed in claim 1, wherein the arrangement is an assembly composed of n plies of asymmetric filter media which are loosely stacked on top of one another and are, in the edge regions of the plies, embedded in a filter housing.

4. The serial arrangement as claimed in claim 1, comprising 2 to 10.

5. The serial arrangement as claimed in claim 1, having a thickness of 100 to 1000 μm.

6. The serial arrangement as claimed in claim 1, wherein the pore size of the n plies decreases linearly, concavely or convexly in the thickness direction.

7. The serial arrangement as claimed in claim 1, wherein the thickness of the plies is, independently of one another, 50 to 250 μm.

8. The serial arrangement as claimed in claim 1, wherein the plies are constructed from PVDF, PTFE, cellulose ester, cellulose hydrate, polyamide, polysulfone, polyarylsulfone, polyacrylic acid, polymethacrylic acid, acrylic acid-methacrylic acid copolymer, polyethersulfone or a mixture thereof.

9. A method for producing a serial arrangement as claimed in claim 1, comprising, for the production of each of the n plies, in each case the steps of (A) providing a casting solution containing a membrane-forming polymer and a solvent, (B) forming a film by application of the casting solution to a level support, (C) conditioning the film by introduction of the film into a gaseous atmosphere containing a first precipitant and (D) precipitating the ply by introduction of the conditioned film into a precipitation bath containing a second precipitant, in order to obtain the n plies; and (E) laying the n plies on top of one another.

10. A filtration unit comprising a serial arrangement as claimed in claim 1, wherein the filtration unit is a cassette, a plate module, a hollow-fiber module, a pleated cartridge or a capsule.

11. The serial arrangement as claimed in claim 1, configured for filtration of viruses and/or for filtration of mycoplasmas.

12. A method for characterizing the pores of a filter medium, comprising the steps of (1) fixing the filter medium; (2) producing a cross-sectional preparation for scanning electron microscopy by cutting, grinding and polishing of the fixed filter medium in the thickness direction; (3) generating a gray-scale image of the cross-section of the filter medium by scanning electron microscopy analysis of the preparation; (4) generating a binarized image by binarization of the gray-scale image, the binarized image being made up of first and second pixels and the first pixels representing a pore wall and the second pixels representing a pore interior; (5) subdividing the binarized image into four sections, with the result that each section depicts an equally thick region of the cross-section of the filter medium and has multiple rows of first and second pixels, the rows comprising series of second pixels; (6) determining section-by-section the number of second pixels of each series of second pixels and the number of series of second pixels; and (7) calculating section-by-section the average pore size from the arithmetically averaged number of second pixels per series of second pixels.

13. The method as claimed in claim 12, wherein the filter medium is a microporous membrane.

14. The serial arrangement as claimed in claim 1, comprising 2 or 3, plies.

15. The serial arrangement as claimed in claim 1, having a thickness of 150 to 600 μm.

16. The serial arrangement as claimed in claim 1, wherein the thickness of the plies is, independently of one another, 100 to 200 μm.

17. The serial arrangement as claimed in claim 1, wherein the plies are constructed from polyethersulfone.

18. The serial arrangement as claimed in claim 10 configured for sterile-filtration.

Description

[0118] FIG. 1 shows schematically a serial arrangement according to the invention having two (n=2) plies F1 and F2. Each ply is subdivided into 4 sections F1-1 to F1-4 or F2-1 to F2-4. The filtration direction is indicated by an arrow pointing downward. Two V-shaped lines converging in the filtration direction depict, in each case, decreasing of the pore size of the n plies in the thickness direction or filtration direction. The different gradients of the lines show that, in the preferred embodiment depicted in FIG. 1, the two plies have different asymmetry factors. The two plies are depicted at a distance from one another, but can in reality directly adjoin one another. In this preferred embodiment, the pore sizes of the adjacent plies are approximately identical in the boundary region, and this is depicted by the approximately identical horizontal distance of the respective converging lines in the region of the plies F1-4 and F2-1.

[0119] FIG. 2 shows schematically the determination of the wastage of a serial arrangement.

[0120] FIG. 3 shows, on the left-hand side, exemplary SEM images obtained by use of a SEEM detector. The SEM images (cross-sectional images) are arranged according to the order of the serial arrangement according to the invention. On the right-hand side, the pore-size profile is shown across the total thickness of the serial arrangement (subdivided into four sections per ply, obtained by the characterization method according to the invention), the average pore size of each ply being depicted by a disk in each case.

[0121] FIG. 4 shows schematically a linear (A), a convex (B) and a (C) concave pore-size profile.

[0122] FIG. 5 shows schematically comparative examples (A to C, E and F) and a serial arrangement according to the invention having two plies (D). [0123] A: The serial arrangement has a ply having an asymmetry factor <1.5. [0124] B: The serial arrangement has a ply having an asymmetry factor <1.5. [0125] C: The pore size does not substantially continuously fall in the thickness direction. The wastage is more than 30%. [0126] D: Example according to the invention. [0127] E: The serial arrangement has a ply having an asymmetry factor <1.5. [0128] F: The pore size does not substantially continuously fail in the thickness direction. The wastage is more than 30%.

[0129] FIG. 6 shows the pore-size profiles of the serial arrangements examined in Example 1 and also the wastage.

[0130] FIG. 7 shows schematically the pore-size profiles of the serial arrangements examined in Example 1 and also experimentally determined service lives of the arrangements (Caro/Ovomaltine).

[0131] FIG. 8 shows the pore-size profiles of the serial arrangements examined in Example 2 and also the wastage.

[0132] FIG. 9 shows the service lives SL (Caro/Ovomaltine) in kg/m.sup.2 for three-ply serial arrangements according to the invention in comparison with two-ply comparative examples which lack either the prefilter, i.e., the first filter ply, or the middle filter, i.e., the second filter ply.

[0133] The following examples serve to further elucidate the present invention without being limited thereto.

EXAMPLES

[0134] Characterization of Pores

[0135] For the characterization of the pores of the examined membranes using a MATLAB® routine, the following set of parameters was taken as a basis. [0136] Loading of the image (dimensions of, for example, 3584×2671 pixels) [0137] Specification of the image width on the basis of the resolution/magnification: this was adjusted such that clear transitions between pore wall and pore space are identifiable. [0138] Cropping of the image, so that the membrane completely fills the image (manually in MATLAB®) [0139] Input of the physical porosity (measured)=80% [0140] Specification of minimum pore-interior size and minimum pore-wall diameter=0.05 μm (resolution limit due to the penetration depth of electrons) [0141] Definition of the number of sections=4 Median filters before binarization=3 or median filters after binarization=3 [0142] Binarization with threshold-value criterion “Porosity” [0143] Median filters before binarization=3 [0144] Hole closing/filling filter of variable intensity, depending on particular average pore size (in segments of 500 image rows; intensity of the hole closing/filling filter: average pore size of the respective segment of 500 image rows/10) [0145] Output of the average pore sizes per section.

[0146] Method for Determining Service Life

[0147] Caro/Ovomaltine Service Life

[0148] Service life (soil absorption capacity; SL) was ascertained by filtration with a test filtration composition based on an aqueous suspension of a mixture consisting of Ovomaltine® powder and Caro® powder (0.0463% by weight of Caro® powder; 0.0145% by weight of Ovomaltine® powder, the remainder being water) (Caro/Ovomaltine SL). The measurement was carried out using a 47 mm filter housing at a pressure of 1 bar. Filtration was carried out until the transmembrane flow rate was only 5% of the initial transmembrane flow rate. The filtrate was collected and weighed. The amount of filtrate in gram per filter surface area corresponds to the service life.

[0149] DMEM/PP3 Service Life

[0150] In an alternative method, service life (DMEM/PP3) was carried out by filtration with a test filtration composition based on an aqueous suspension containing 13.4 g/L Dulbecco's Modified Eagle's Medium (DMEM) and 7.5 g/L proteose peptone No. 3 (PP3). The measurement was carried out using a 25 mm polypropylene filter housing at a pressure of 2 bar. Filtration was carried out until the transmembrane flow rate was only 10% of the initial transmembrane flow rate. The filtrate was collected and weighed. The amount of filtrate in gram per filter surface area corresponds to the service life.

Example 1—n=2

[0151] What were examined were a serial arrangement according to the invention (“SSB 20 Prototype”, “1650043.7/1553743.16” “2-ply example”) and also the commercially available Domnick Hunter® HC, Millipore® SHC, Pall® ECV and Life Assure from 3M Innovative Properties. The serial arrangements were examined as specified above. The porosity profiles obtained therefrom are shown in FIG. 6. Simplified diagrams of the porosity profiles are shown in FIG. 7.

[0152] The service lives of the five different serial arrangements having two plies (n=2) were determined as specified above. The results are shown in FIG. 7 and Table 1.

[0153] In the case of the exemplary embodiment according to the invention (“1650043.71553743.16” in FIG. 7), the service life (Caro/Ovomaltine) was 508 kg/m.sup.2, whereas in the case of the comparative examples from the prior art, the service lives were only between 132 and 341 kg/m.sup.2, the filters in all the examples being sterile filters having the same separation rate (nominal pore size 0.2 μm). Corresponding results were obtained for the DMEM/PP3 service life: the example according to the invention achieved a service life of 2546 kg/m.sup.2, whereas the comparative examples merely achieved service lives of 2063 kg/m.sup.2 (Domnick Hunter® HC), 2060 kg/m.sup.2 (Millipore® SHC), 1423 kg/m.sup.2 (Pall® ECV) and 1560 kg/m.sup.2 (Life Assure).

Example 2—n=2 or 3

[0154] Serial arrangements according to the invention and comparative arrangements were produced and examined as described above (characterization of pores and determination of service life).

[0155] The results are shown in Table 1 and FIG. 8.

TABLE-US-00001 TABLE 1 Three plies Two plies 3-ply Comparative Comparative 2-ply Domnick MP Pall 3M Life example Example 1 Example 2 example Hunter HC SHC ECV assure Prefilter Largest section 12.4 15.2 6.2 15.9 12.5 14.6 2.6 10.4 membrane Smallest section 8.1 10.1 2.4 6 2.1 1.7 1.1 2.8 Asymmetry 1.5 1.5 2.6 2.7 6.0 8.6 2.4 3.7 Thickness (μm) 155 130 110 160 130 140 200 130 Wastage (μm) 0 95 50 0 0 0 0 50 Middle Largest section 5.1 6.6 8.7 No middle filter membrane Smallest section 2.5 2.1 2.1 Asymmetry 2.0 3.1 4.1 Thickness (μm) 135 110 135 Wastage (μm) 0 35 135 End-filter Largest section 1.6 1.3 1.3 3.2 2.15 4.1 1.6 1.8 membrane Smallest section 0.8 0.7 0.7 0.8 1.45 1.3 1.4 1.3 Asymmetry 2.0 1.9 1.9 4.0 1.5 3.2 1.1 1.4 Thickness (μm) 140 160 160 130 120 140 125 110 Wastage (μm) 0 0 0 0 65 100 70 0 Overall Thickness (μm) 430 400 400 290 250 280 325 240 Wastage (μm) 0 130 185 0 65 100 140 35 Wastage (%) 0 33 46 0 26 36 43 15 Overall asymm. 16 22 9 20 9 11 2 8 Caro SL (kg/m.sup.2) 451 44 45 508 341 282 179 132 DMEM/PP3 SL (kg/m.sup.2) 2526 1823 1700 2546 2063 2060 1423 1560 Flow rate (L/m.sup.2) 3300 1900 1700 12 800 15 000 10 400 11 600 14 200

[0156] The entries indicated by “Largest section” or “Smallest section” in Table 1 specify the largest or smallest average pore size from the four section-based pore sizes of the respective ply in micrometers.

[0157] As can be seen from Table 1, none of the two-ply serial arrangements Domnick Hunter HV®, Pall® ECV and 3M Innovative Properties Life Assure and the three-ply Comparative Example 2 had an overall asymmetry of 10 or more. Furthermore, the serial arrangements Pall® ECV and 3M Innovative Properties Life Assure both had a ply having an asymmetry factor of less than 1.5. Moreover, the serial arrangements Millipore® SHC and Pall® ECV did not have a substantially continuously decreasing pore size of the two plies in the thickness direction. The wastage was 36% and 43%, respectively. The aforementioned two-ply and three-ply serial arrangements of the comparative examples had a lower service life than the serial arrangements according to the invention having an identical ply number.

[0158] Comparative Examples 1 and 2 of Example 2 did not have a substantially continuously decreasing pore size of the three plies in the thickness direction. The wastage was 33% and 46%, respectively. As already mentioned, the aforementioned had a lower service life than the three-ply serial arrangement according to the invention of Example 2.

[0159] FIG. 9 depicts the service lives Caro/SL of the three-ply example and of the three-ply comparative examples/Counter Examples 1 and 2 (see Table 1 above). Moreover, FIG. 9 shows that, in the case of a particularly preferred embodiment of the serial arrangement according to the invention having three plies and a wastage of at most 30%, it was possible to achieve a particularly high service life of 451 kg/m.sup.2 whereas in the case of a modification of the three-ply example to form a two-ply serial arrangement by removal of the first ply, i.e., the prefilter PF, the service life dropped to 159 kg/m.sup.2. The same tendency applies to a modification of the three-ply serial arrangement in which the second ply, i.e., the middle filter EF1, is missing. In said modification, the service life dropped to only 65 kg/m.sup.2. By contrast, in the case of the three-ply comparative examples “Counter Example 1” and “Counter Example 2”, which had a distinctly higher wastage of 33% (Counter Example 1) and 46% (Counter Example 2), the removal of either the first (PF) or second (EF1) ply from the three-ply serial arrangement hardly had an influence on the anyway distinctly lower service life, which was not more than 124 kg/m.sup.2 in the case of Counter Example 1 and its two-ply modifications and not more than 113 kg/m.sup.2 in the case of Counter Example 2 and its two-ply modifications.

[0160] The present invention provides a serial arrangement having n plies of asymmetric microporous filter media, especially membranes, that efficiently achieves a high service life. Dead volumes and premature blocking can be avoided. Moreover, with the aid of the characterization method according to the invention, it is possible to characterize filter media exactly, meaning that, in the production method for the serial arrangement, the pore-size profile of each ply can be set in a controlled manner.