MICROPOROUS POLYVINYL FLUORIDE PLANAR MEMBRANE AND PRODUCTION THEREOF

Abstract

Hydrophobic flat membrane made from a vinylidene fluoride polymer with a wall, a first surface, and a second surface. The membrane has on its first surface a network structure with open pores and on its second surface a continuous skin in which pores are formed, and adjacent to the skin of the second surface a supporting layer with an isotropic pore structure across the wall thickness, wherein the supporting layer extends over at least 80% of the wall thickness and wherein the pores of the supporting layer have an average diameter of less than 1 μm. The weight average of the molecular weight M.sub.W of the vinylidene fluoride polymer lies in the range from 300 000 to 500 000 daltons, and the polydispersivity M.sub.W/M.sub.N is greater than 5.5.

The pores in the skin of the second surface have a closed perimeter in the plane of the skin and an average ratio of the extension in the direction of the longest axis thereof to the extension in the direction of the shortest axis thereof of at most 5. The pores in the first surface and second surface have an essentially isotropic distribution of their orientation. The porosity of the membrane lies in the range from 50 to 90 vol. % and the wall thickness in the range from 50 to 300 μm. The membrane has a maximum separating pore diameter d.sub.max in the range from 0.05 to 1.5 μm.

Claims

1. Hydrophobic membrane comprising a flat membrane made from a vinylidene fluoride polymer, wherein the membrane has a wall with a wall thickness, a first surface, and a second surface, wherein the membrane has a network structure with open pores on the first surface thereof and a continuous skin on the second surface thereof, in which pores are formed, wherein the membrane, adjacent to the skin of the second surface, has a supporting layer having an open-pored, microporous, and sponge-like pore structure that is substantially isotropic across the wall thickness, the supporting layer extending over at least 80% of the wall thickness and comprising pores having an average diameter of less than 1 μm, and wherein the vinylidene fluoride polymer forming the membrane has a weight-average molecular weight MW in the range from 300 000 to 500 000 daltons and a polydispersivity MW/MN, given by the ratio of the weight-average molecular weight MW and the number average MN of the molecular weight, that is greater than 5.5, the pores in the skin of the second surface have a closed perimeter in the plane of the skin, the pores in the skin of the second surface having an average ratio of the extension in the direction of the longest axis thereof to the extension in the direction of the shortest axis thereof of at most 5, and the pores in the first surface and second surface having an essentially isotropic distribution of their orientation when viewed perpendicular to the surface, the porosity of the membrane lies in the range from 50 to 90 vol. % and the wall thickness in the range from 50 to 300 μm, and the membrane has a maximum separating pore diameter d.sub.max in the range from 0.05 to 1.5 μm determined according to the bubble point method.

2. The membrane according to claim 1, wherein the membrane has a transmembrane flow for isopropyl alcohol in the range from 3 to 15 ml/(cm.sup.2.Math.min.Math.bar), measured at 25° C.

3. The membrane according to claim 1, wherein the membrane has a transmembrane flow for water vapor of at least 35 l/(m.sup.2.Math.h), determined by means of a flat membrane module with a membrane area of 40 cm.sup.2 at a salt water circuit temperature of 80° C. and a distillate circuit temperature of 30° C., a volume flow in the circuits of 200 l/h, a pressure level in the circuits of 500 mbar at the inlet to the flat membrane module, and a salt concentration in the salt circuit of 36 g/l.

4. The membrane according to claim 1, wherein the membrane has an elongation at break of at least 50% at room temperature.

5. The membrane according to claim 1, wherein the membrane has a breaking strength of at least 200 cN/mm.sup.2 at room temperature.

6. The membrane according to claim 1, wherein the membrane has a volume porosity in the range from 70 to 85 vol. %.

7. The membrane according to claim 1, wherein the membrane has a wall thickness in the range from 60 to 150 μm.

8. The membrane according to claim 1, wherein the membrane has a maximum separating pore diameter d.sub.max in the range from 0.1 to 1.0 μm.

9. A method for producing the membrane according to claim 1 from a vinylidene fluoride homopolymer or copolymer, comprising at least the following steps: a) preparing a homogeneous casting solution of 20-30 wt. % of a polymer component made from at least one vinylidene fluoride polymer in 80-70 wt. % of a solvent system, wherein the casting solution of the polymer component and solvent system has on cooling a critical demixing temperature and a solidification temperature, and a miscibility gap below the critical demixing temperature in the liquid state of aggregation, and wherein the solvent system contains a compound A and a compound B, which are liquid and can be mixed homogeneously with each other at the dissolving temperature, and wherein a solvent for the polymer component is selected for compound A and compound B is a non-solvent for the polymer component, b) forming of the casting solution into a film with a first surface and a second surface in a forming tool, which has a tool temperature above the critical demixing temperature, and c) placing the first side of the film onto a conditionable carrier, which is conditioned to a cooling temperature below the solidification temperature, resulting in cooling of the film via the conditionable carrier at such a rate that a thermodynamic non-equilibrium liquid-liquid phase separation into a polymer-rich and a polymer-poor phase takes place, and subsequently, on passing below the solidification temperature, solidification of the polymer-rich phase takes place, forming a membrane structure; at the same time d) bringing the second surface of the film into contact with a gaseous atmosphere, e) drawing the film with the formed membrane structure from the carrier, f) removing at least part of the solvent system from the film to obtain the flat membrane, and wherein the polymer component has a weight-average molecular weight MW in the range from 300 000 to 500 000 daltons and a polydispersivity MW/MN, given by the ratio of the weight-average molecular weight MW and the number average MN of the molecular weight, that is greater than 5.5.

10. The method according to claim 9, wherein glyceryl triacetate, glyceryl diacetate, 2-(2-butoxyethoxy-)ethyl acetate, dibutyl phthalate, adipic acid diethyl ester, adipic acid dibutyl ether, butyl diglycol acetate, butyl glycol acetate, glycol diacetate, propylene carbonate, butyrolactone, or ε-caprolactam, or a mixture of the compounds mentioned, is used as compound A.

11. The method according to claim 9, wherein dioctyl adipate, glyceryl monoacetate, glycerol, glycol, diglycol, or castor oil, or a mixture thereof, is used as compound B.

12. The method according to claim 9, wherein the carrier has a temperature in the range from 30 to 80° C.

13. The method according to claim 9, wherein the gaseous atmosphere has a temperature in the range from 20 to 25° C.

14. The method according to claim 9, wherein the conditionable carrier is a conditionable and rotating casting roller, which with a part of its perimeter on the bottom is immersed in a bath filled with a liquid cooling medium.

15. The method according to claim 14, wherein the liquid cooling medium comprises a solvent and a non-solvent for the polymer component, wherein the cooling medium acts as a non-solvent for the polymer component at the cooling temperature.

Description

[0078] The invention is to be explained in more detail on the basis of the following examples and figures. The content of the figures is as follows:

[0079] FIG. 1: Scanning electron microscopic (SEM) image of the first side (roller side) of the membrane according to Example 3 at 2000× magnification.

[0080] FIG. 2: SEM image of the first side (roller side) of the membrane according to Example 3 at 8000× magnification.

[0081] FIG. 3: SEM image of the second side (air side) of the membrane according to Example 3 at 2000× magnification.

[0082] FIG. 4: SEM image of the second side (air side) of the membrane according to Example 3 at 8000× magnification.

[0083] FIG. 5: SEM image of a cross-section across the wall of the membrane according to Example 3 at 2000× magnification.

[0084] FIG. 6: SEM image of the first side (roller side) of the membrane according to Example 5 at 8000× magnification.

[0085] FIG. 7: SEM image of the second side (air side) of the membrane according to Example 5 at 8000× magnification.

[0086] FIG. 8: Scanning electron microscopic (SEM) image of a cross-section across the wall of the membrane according to Example 5 at 2000× magnification.

[0087] FIG. 9: SEM image of the first side (roller side) of the membrane according to Comparison example 1 at 2000× magnification.

[0088] FIG. 10: SEM image of the first side (roller side) of the membrane according to Comparison example 1 at 8000× magnification.

[0089] FIG. 11: SEM image of the second side (air side) of the membrane according to Comparison example 1 at 2000× magnification.

[0090] FIG. 12: SEM image of the second side (air side) of the membrane according to Comparison example 1 at 8000× magnification.

[0091] To determine the properties of the flat membrane according to the invention, the following methods were used:

[0092] Maximum Separating Pore:

[0093] The diameter of the maximum separating pore is determined by means of the bubble point method (ASTM No. 128-99 and F 316-03), for which the method described in DE-A-36 17 724 is suitable. Thereby, d.sub.max results from the vapor pressure P.sub.B associated with the bubble point according to the equation

d.sub.max=σ.sub.B/P.sub.B

where σ.sub.B is a constant that is primarily dependent on the wetting liquid used during the measurement. For IPA, σ.sub.B is 0.61 μm.Math.bar at 25° C.

Transmembrane Flow for Isopropyl Alcohol (Permeability for IPA):

[0094] Disc-shaped membrane samples are stamped out of the membrane to be tested and then clamped fluid-tight at the perimeter in a suitable sample holder such that a free measuring area of 17.35 cm.sup.2 results. The sample holder is located in a test cell through which isopropyl alcohol (IPA) can flow under pressure.

[0095] The test cell is incorporated in a testing apparatus and subjected to a flow of isopropyl alcohol (IPA) ultrapure, conditioned to 25° C. at a defined test pressure (approx. 0.2 bar). The filtered IPA volume obtained during a measuring time of 2 minutes, i.e. the permeate generated during measurement, is determined gravimetrically or volumetrically. Before measurement is begun, the system must be purged of air. In order to determine the TMF, the input and output pressure is measured on the test cell in the testing apparatus. The measurement is performed at 25° C.

[0096] The transmembrane flow, TMF, is determined according to formula (III)

[00001]$\begin{array}{cc}\mathrm{TMF}=\frac{\mathrm{Vw}}{\Delta .Math..Math.t.Math.A_M.Math.\Delta .Math..Math.p}.Math.\left[\frac{\mathrm{ml}}{{\mathrm{cm}}^2.Math.\min .Math.\mathrm{bar}}\right]& \left(\mathrm{III}\right)\end{array}$

where:
Vw=volume of IPA [ml] flowing through the membrane sample during the measuring period
Δt=measuring time [min]
A.sub.M=area of the membrane sample penetrated (17.35 cm.sup.2)
Δp=pressure set during the measurement [bar]

Transmembrane Flow for Water Vapor:

[0097] The measurement of the transmembrane flow for water vapor is performed on a flat membrane module with a test surface of 40 cm.sup.2. Two liquid circuits are connected to this flat membrane module, wherein the feed stream (salt water circuit) is connected to the flat membrane module in such a way that it flows along one side of the flat membrane to be tested. The distillate circuit absorbing the permeate flows through the flat membrane module on the other side of the flat membrane. The starting volumes of the circuits were each 1.6 l. During measurement, the distillate circuit continually increases as a result of the added permeate. The salt water circuit has a salt content of 36±0.5 g/l, which is kept constant by conductivity measurement while adding deionized water.

[0098] The volume flow in both circuits is set to 200 l/h±5%, wherein the circuits are guided in counter-current flow with respect to each other. The distillate circuit is conditioned to a temperature of 30° C. and the salt water circuit to a temperature of 80° C. Both circuits are kept at the same pressure level, with a pressure of 500 mbar set at the respective inlet to the flat membrane module.

[0099] In order to determine the transmembrane flow for water vapor, the increase in weight in the distillate circuit is determined gravimetrically over time. The minimum measurement time is 15 minutes.

[0100] The transmembrane flow for water vapor in l/(m.sup.2.Math.h) is determined as the increase in weight, or the resulting increase in volume per time unit, referred to the current membrane surface (inner surface) of the flat membrane module used.

Force and Elongation at Break:

[0101] Measuring the force at break of the membrane takes place using a standard, universal testing machine from Zwick (Ulm, Germany). For this purpose, samples are cut from the flat membrane to be tested, the edges thereof oriented in the production direction and transverse to the production direction. The samples have a width of 15 mm and are clamped in the testing machine such that a free length of 25 cm results.

[0102] The membrane samples are stretched at constant speed in the longitudinal direction or in the transverse direction of the samples until break. The force required for this is measured as a function of the change in length and retained in a force/elongation curve. The measurement takes place as multiple determinations on five membrane samples at 100 mm clamping length and at a traction speed of 500 mm/min. The pretension weight is 2.5 cN. The measurement is performed at room temperature.

[0103] The force required for breaking, BK, is determined as the average numeric value in cN and the elongation at rupture achieved thereby as a % of the original length. The breaking strength σ.sub.B of the membrane sample in cN/mm.sup.2 is obtained by standardizing the breaking force BK to the cross-sectional area A.sub.Q of the membrane wall, which results from the sample width and the membrane thickness.

Molecular Weight, Polydispersivity:

[0104] The determination of the molecular weight and the mole mass distribution (polydispersivity) takes place using gel permeation chromatography (GPC; columns: PSS GRAM: 10 μm, G, 30, 100, 3000 Å) on polystyrene standards with N-methyl-2-pyrrolidone (NMP)/0.01M LiCl as the eluent, and at a flow rate of 1 ml/min. The sample concentration is 3 mg/ml, and the injection volume is 100 μl (injection system TSP AS 3000). The oven temperature is set to 70° C., and the detection takes place with the Shodex RI 71 differential refractometer. The number average M.sub.N and the weighted average M.sub.W of the molar mass distribution are determined from the molar mass distribution using conventional methods. The dispersivity results from the ratio of the weight average M.sub.W to the number average M.sub.N, thus M.sub.W/M.sub.N.

Pore Size in the Surfaces:

[0105] The determination of the average diameter of the pores in the surfaces takes place using image analysis methods based on scanning electron microscope images of the surfaces at 500× magnification (outer surface) or 2000× magnification (inner surface). The scanning electron microscope images of the surfaces were also used to assess the ratio of the longitudinal extension of the pores to the transverse extension thereof.

Volume Porosity:

[0106] A sample of at least 0.5 g of the membrane to be examined is dry weighed. The membrane sample is subsequently placed in a liquid that moistens the membrane material, however without causing swelling, for 24 hours such that the liquid penetrates into all pores. For the present PVDF membranes, a silicone oil with a viscosity of 200 m Pa s at 25° C. (Merck) is used. The permeation of liquid into the membrane pores is visually discernable in that the membrane sample changes from an opaque to a glassy, transparent state. The membrane sample is subsequently removed from the liquid, liquid adhering to the membrane sample is removed by centrifuging at approx. 1800 g, and the mass of the thus pretreated wet, i.e. liquid-filled, membrane sample is determined by weighing.

[0107] The volume porosity c is determined according to the following formula:

[00002]$\mathrm{Volume}.Math..Math.\mathrm{porosity}.Math..Math.\mathrm{.Math.}=\frac{\left(m_{\mathrm{wet}}-m_{\mathrm{dry}}\right)/\rho .Math..Math.\mathrm{liquid}}{\left(m_{\mathrm{wet}}-m_{\mathrm{dry}}\right)/\rho .Math..Math.\mathrm{liquid}+m_{\mathrm{dry}}/\rho .Math..Math.\mathrm{polymer}}$

where: [0108] m.sub.dry=weight of the dry membrane sample after wetting and drying [g] [0109] m.sub.wet=weight of the wet, liquid-filled membrane sample [g] [0110] ρ.sub.liquid=density of the liquid used [g/cm.sup.3] [0111] ρ.sub.polymer=density of the membrane polymer [g/cm.sup.3]

EXAMPLES 1-4

[0112] A mixture of PVDF powders of type Hylar 461 and Solef 6020 from Solvay Solexis (mixture ratio 50:50) was melted in an extruder at 235-245° C. The polymer melt was mixed in a mixer with a solvent system consisting of 40 wt. % glyceryl triacetate (component A) and 60 wt. % dioctyl adipate (component B) at 230-245° C. and subsequently processed to form a homogeneous solution. The polymer proportion was set at 25.5 wt. %. The PVDF mixture used in the casting solution had a weighted-average molecular weight M.sub.W of 457 000 daltons and a polydispersivity M.sub.W/M.sub.N of 6.92.

[0113] The finished casting solution was poured by means of a sheeting die conditioned to 210° C. onto a conditioned metal casting roller to form a film with a thickness of approximately 100 μm. The temperature of the casting roller was varied between 40 and 70° C. The film located on the casting roller was fed through a climate-controlled zone with a climate of approximately 23° C. and 55% relative humidity and drawn off the casting roller after solidification. The residence time of the film on the casting roller was approximately 10 s.

[0114] The film thus obtained was extracted with isopropyl alcohol at a temperature of approximately 60° C. to remove the solvent system and then dried in a convection oven at a temperature of 80° C.

[0115] The properties of the flat membranes thus obtained are summarized in Table 1.

TABLE-US-00001 TABLE 1 Temperature TMF of casting ml/(cm.sup.2 .Math. d.sub.max Thickness roller min .Math. bar) μm μm Example 1 40° C. 3.08 0.28 92 Example 2 50° C. 3.51 0.36 95 Example 3 60° C. 4.41 0.46 93 Example 4 70° C. 5.04 0.51 92

[0116] The weight-average molecular weight M.sub.W and polydispersivity M.sub.W/M.sub.N of the PVDF component contained in the flat membranes thus produced substantially corresponded to those of the starting material.

[0117] For the membrane according to Example 3, a transmembrane flow for water vapor of 55.6 l/(m.sup.2.Math.h) as well as a breaking strength of 350 cN/mm.sup.2 in the longitudinal direction and 385 cN/mm.sup.2 in the transverse direction were determined. The elongation at break of this membrane was 74.1% in the longitudinal direction and 119.7% in the transverse direction. The volume porosity was in the range 75-80 vol. %.

[0118] FIGS. 1 to 4 show scanning electronic microscopic (SEM) images of the surfaces of the membrane according to Example 3 at 2000× and 8000× magnification.

[0119] The SEM images of the surface of the membrane which was against the casting roller during production of the membrane (roller side, first side of the membrane) exhibit a pronounced network structure with open pores at 2000× and 5000× magnification, in which spaces between in part filament-like webs form a pore system (FIG. 1, 2). The SEM images at 2000× and 5000× magnification of the surface of the membrane that was exposed to the air during production of the membrane (air side, second side of the membrane) show a uniform and relatively even structure of the surface, which has island-shaped pores (FIG. 3, 4). The pores have an irregular shape but do not exhibit a preferred orientation.

[0120] A comparison of FIG. 2 with FIG. 4 shows that the pores of the flat membrane according to Example 3 in the first surface are larger than the pores in the second surface.

[0121] FIG. 5 shows an SEM image of a cross-section across the wall of the flat membrane according to Example 3 at 2000× magnification. In the cross-section adjacent to the second surface (top left of image, air side), a fine-pored supporting layer is evident, which essentially extends over the entire cross-section and has an isotropic pore structure without a gradient across the wall thickness with regard to the pore size. The average diameter of the pores in the supporting layer lies below 1 μm.

EXAMPLES 5-6

[0122] The procedure was the same as in Examples 1 and 2. Unlike Examples 1 and 2, however, a mixture of PVDF types Hylar 461 and Solef 6020 was used in a mixture ratio of 30:70. The resulting polymer component had a weight-average molecular weight M.sub.W of 355 000 daltons and a polydispersivity M.sub.W/M.sub.N of 7.84.

[0123] The properties of the flat membranes according to Examples 5 and 6 are summarized in Table 2.

TABLE-US-00002 TABLE 2 TMF Temperature TMF Water of casting ml/(cm.sup.2 .Math. d.sub.max Thickness vapor roller min .Math. bar) μm μm l/(m.sup.2 .Math. h) Example 5 40° C. 6.73 0.45 89 64.4 Example 6 50° C. 6.80 0.44 84 61.7

[0124] The flat membrane according to Example 5 had a breaking strength of 298 cN/mm.sup.2 in the longitudinal direction and 396 cN/mm.sup.2 in the transverse direction. The elongation at break of this membrane was 74.9% in the longitudinal direction and 77.4% in the transverse direction. For the flat membrane according to Example 6, a breaking strength of 365 cN/mm.sup.2 in the longitudinal direction and 487 cN/mm.sup.2 in the transverse direction were determined. The elongation at break of this membrane was 96.5% in the longitudinal direction and 139.7% in the transverse direction.

[0125] FIGS. 6 and 7 show scanning electronic microscopic (SEM) images of the surfaces of the membrane according to Example 5 at 8000× magnification. The SEM image of the surface of the membrane according to Example 5 which was against the casting roller during production of the membrane (roller side, first side of the membrane) has, as in the previous Example 3, a pronounced network structure with open pores, in which spaces between in part filament-like webs form a pore system (FIG. 6). The SEM image of the surface of the membrane that exposed to the air during production of the membrane (air side, second side of the membrane) shows a uniform and relatively even structure of the surface, which has island-shaped pores (FIG. 7). The pores have an irregular shape but do not exhibit a preferred orientation. A comparison of FIG. 6 with FIG. 7 shows that the pores of the flat membrane according to Example 5 in the first surface are larger than the pores in the second surface.

[0126] FIG. 8 shows an SEM image of a cross-section across the wall of the flat membrane according to Example 5 at 2000× magnification. Here again, in the cross-section adjacent to the second surface (top left of image, air side), a fine-pored supporting layer is evident, which essentially extends over the entire cross-section and has an isotropic pore structure without a gradient across the wall thickness with regard to the pore size. The average diameter of the pores in the supporting layer lies below 1 μm.

[0127] For the flat membrane according to Example 6, a similar image results with regard to the surfaces and the cross-section, dispensing with the need for a separate presentation.

EXAMPLE 7

[0128] The procedure was the same as in Example 1. Unlike in Example 1, however, a solvent system consisting of 35 wt. % glyceryl triacetate (component A) and 65 wt. % dioctyl adipate (component B) was used. The casting roller temperature was 40° C. as in Example 1.

[0129] The properties of the flat membranes according to Example 7 are summarized in Table 3.

TABLE-US-00003 TABLE 3 TMF Temperature TMF Water of casting ml/(cm.sup.2 .Math. d.sub.max Thickness vapor roller min .Math. bar) μm μm l/(m.sup.2 .Math. h) Example 7 40° C. 5.69 0.39 100 55.7

[0130] The flat membrane according to Example 7 had a breaking strength of 320 cN/mm.sup.2 in the longitudinal direction and 355 cN/mm.sup.2 in the transverse direction. The elongation at break of this membrane was 69.7% in the longitudinal direction and 87.3% in the transverse direction.

COMPARISON EXAMPLE 1

[0131] The procedure was the same as in Example 1. Deviating from Example 1, a solvent system consisting of 60 wt. % glyceryl triacetate (component A) and 40 wt. % dioctyl adipate (component B) was used. The casting roller temperature was also 40° C. as in Example 1.

[0132] The properties of the flat membranes according to Comparison example 1 are shown in Table 4.

TABLE-US-00004 TABLE 4 TMF Temperature TMF Water of casting ml/(cm.sup.2 .Math. d.sub.max Thickness vapor roller min .Math. bar) μm μm l/(m.sup.2 .Math. h) Comparison 40° C. 1.32 0.90 96 31.8 example 1

[0133] The flat membrane according to Comparison example 1 had a breaking strength of 437 cN/mm.sup.2 in the longitudinal direction and 413 cN/mm.sup.2 in the transverse direction. The elongation at break of this membrane was 119.1% in the longitudinal direction and 111.2% in the transverse direction.

[0134] As proven by the scanning electronic microscopic (SEM) images of the surfaces of the membrane according to Comparison example 1 at 2000× and 8000× magnification, the membrane according to comparison example 1 has a first and a second surface (roller side and air side) with a pronounced spherulitic structure (FIG. 9-12). In particular in the first surface, a structure clearly differing from the inventive network-like surface structure is evident, in which the particulate or spherulitic segments are in part connected to each other by fibrils. The second side of the membrane according to this comparison example has an increased roughness due to its spherulitic structure.

COMPARISON EXAMPLES 2-5

[0135] The procedure was the same as in Examples 1 to 4. Deviating from Examples 1 to 4, however, the PVDF type Solef 6020 was used in a proportion of 100% as the polymer component. For the PVDF type Solef 6020, a weight-average molecular weight M.sub.W of 552 000 daltons and a polydispersivity M.sub.W/M.sub.N of 5.1 were determined.

[0136] The properties of the flat membranes according to Comparison examples 2 to 5 are summarized in Table 5.

TABLE-US-00005 TABLE 5 Temperature TMF of casting ml/(cm.sup.2 .Math. d.sub.max Thickness roller min .Math. bar) μm μm Comparison 40° C. 1.29 0.90 94 example 2 Comparison 50° C. 1.81 0.90 89 example 3 Comparison 60° C. 1.81 0.43 96 example 4 Comparison 70° C. 2.62 0.39 95 example 5

[0137] For the membrane according to Comparison example 3, a transmembrane flow for water vapor of 51.1 l/(m.sup.2.Math.h) as well as a breaking strength of 381 cN/mm.sup.2 in the longitudinal direction and of 662 cN/mm.sup.2 in the transverse direction were determined. The elongation at break of this membrane was 93.3% in the longitudinal direction and 232.4% in the transverse direction. The volume porosity for the flat membranes according to Comparison examples 2 to 5 was also in the range from 75 to 80 vol. %.