Macroporous filtration membrane

Abstract

Hydrophilic flat-sheet membrane based on a hydrophobic first polymer from the group consisting of aromatic sulfone polymers and a hydrophilic second polymer, wherein the membrane has a thickness in the range between 30 and 200 μm, a first and a second surface and a supporting layer having a three-dimensional sponge-like network structure, wherein the supporting layer has a first cover layer on the side thereof facing the first surface and a second cover layer on the side thereof facing the second surface, which cover layers are formed integrally with the supporting layer, and wherein the first and second surfaces have approximately oval or circular openings which penetrate the first and second cover layers, respectively, and are connected to the supporting layer, wherein the average diameter of the openings in the surfaces differ by a factor of less than 2, wherein the three-dimensional network structure of the supporting layer is made up of thick branches and a continuous pore system, and the predominant proportion of the branches have a diameter of at least 0.5 μm at the thinnest point thereof and wherein the pores in the supporting layer are larger than the openings in the surfaces.

Claims

1. A hydrophilic, macroporous filtration membrane comprising: a flat-sheet membrane based on a film-forming hydrophobic first polymer from the group consisting of aromatic sulfone polymers, and a hydrophilic second polymer, the membrane consisting of a first surface, a second surface, and a supporting layer extending between the first and second surfaces and having a three-dimensional sponge-like network structure, the supporting layer has a first cover layer on the side thereof facing the first surface and a second cover layer on the side thereof facing the second surface, which cover layers are formed integrally with the supporting layer, and the first and second surfaces have approximately oval or circular openings which penetrate the first and second cover layers, respectively, and are connected to the supporting layer, wherein an average diameter of the openings in the first surface and an average diameter of the openings in the second surface differ by a factor of less than 2, the three-dimensional sponge-like network structure of the supporting layer is made up of thick branches and a continuous pore system, and a predominant proportion of the branches have a diameter of at least 0.5 μm at the thinnest point thereof, the pores in the supporting layer are larger than the openings in the surfaces, a diameter of the pores in the supporting layer, starting from the first surface cover layer in a direction of the second cover layer, is essentially constant in a first area of the supporting layer over at least 50% of the thickness of the supporting layer, and the filtration membrane has a thickness in the range between 30 and 200 μm.

2. The macroporous filtration membrane according to claim 1, wherein a porous proportion of the area of the first surface is at least 20%.

3. The macroporous filtration membrane according to claim 2, wherein the porous proportion of the area of the first surface is less than 60%.

4. The macroporous filtration membrane according to claim 1, wherein the first or second cover layer, respectively, has a thickness in the range from 3 to 10 μm.

5. The macroporous filtration membrane according to claim 1, wherein the average diameter of the openings in the second surface is 0.5 to 1.5 times as large as the average diameter of the openings in the first surface.

6. The macroporous filtration membrane according to claim 1, wherein the average diameter of the openings in the second surface lies in the range from 2 to 20 μm.

7. The macroporous filtration membrane according to claim 1, wherein the filtration membrane thickness is in the range from 50 to 150 μm.

8. The macroporous filtration membrane according to claim 1, wherein the filtration membrane has a volume porosity in the range from 65 to 85 vol. %.

9. The macroporous filtration membrane according to claim 1, wherein the filtration membrane has a diameter, d.sub.max, of a maximum separating pore in the range from 5 to 10 μm as determined by means of the bubble point method.

10. The macroporous filtration membrane according to claim 1, wherein the supporting layer has pores with a diameter of at least 1/10 of the membrane thickness.

11. The macroporous filtration membrane according to claim 1, wherein the filtration membrane has a transmembrane flow, TMF, for water in the range from 700 to 4000 ml/(cm.sup.2.Math.min.Math.bar).

12. The macroporous filtration membrane according to claim 1, wherein the filtration membrane has a tensile strength of at least 500 cN/mm.sup.2 relative to the cross-sectional area thereof.

13. The macroporous filtration membrane according to claim 1, wherein the aromatic sulfone polymer is a polysulfone or a polyethersulfone.

14. The macroporous filtration membrane according to claim 1, wherein the hydrophilic polymer is selected from the group consisting of a polyvinylpyrrolidone, a polyethylene glycol, a polyvinyl alcohol, a polyglycol monoester, a polysorbate, a polyacrylate, a carboxyl methylcellulose, a polyacrylic acid, or a modification or copolymer of these polymers.

15. An infusion set for administering infusion solutions, comprising a drip chamber and an infusion tube connected to the drip chamber, wherein a membrane according to claim 1 is inserted in the infusion tube in the area of the outlet of the drip chamber.

Description

(1) The content of the figures is as follows:

(2) FIG. 1: Scanning electron microscopic (SEM) image of a cross section of the membrane according to Example 1 at 250 times magnification.

(3) FIG. 2: SEM image of a portion of the cross section of the membrane according to Example 1 in the area of the membrane side which was facing the casting roller during production (roller side), at 2000 times magnification.

(4) FIG. 3: SEM image of the surface of the membrane according to Example 1, which surface was facing the casting roller during production (roller side), at 100 times magnification.

(5) FIG. 4: SEM image of the surface of the membrane according to Example 1, which surface was facing away from the casting roller during production (air side), at 100 times magnification.

(6) FIG. 5: SEM image of the surface of the membrane according to Example 1, which surface was facing the casting roller during production (roller side), at 500 times magnification.

(7) FIG. 6: SEM image of the surface of the membrane according to Example 1, which surface was facing away from the casting roller during production (air side), at 500 times magnification.

(8) FIG. 7: SEM image of the entire cross section of the membrane according to Example 1 at approximately 1600 times magnification.

(9) FIG. 8: SEM image of a cross section of the membrane according to Example 2 at 250 times magnification.

(10) FIG. 9: SEM image of the cross section of the membrane according to Example 2 in the area of the membrane side at 500 times magnification.

(11) FIG. 10: SEM image of the surface of the membrane according to Example 2, which surface was facing the casting roller during production (roller side), at 100 times magnification.

(12) FIG. 11: SEM image of the surface of the membrane according to Example 2, which surface was facing away from the casting roller during production (air side), at 100 times magnification.

(13) FIG. 12: SEM image of the entire cross section of the membrane according to Example 2 at approximately 1600 times magnification.

(14) FIG. 13: SEM image of a cross section of the membrane according to Example 3 at 250 times magnification.

(15) FIG. 14: SEM image of the cross section of the membrane according to Example 3 in the area of the membrane side, at 500 times magnification.

(16) FIG. 15: SEM image of the surface of the membrane according to Example 3, which surface was facing the casting roller during production (roller side), at 100 times magnification.

(17) FIG. 16: SEM image of the surface of the membrane according to Example 3, which surface was facing away from the casting roller during production (air side), at 100 times magnification.

(18) FIG. 17: SEM image of a cross section of the membrane according to Comparative Example 1.

(19) FIG. 18: SEM image of the surface of the membrane according to Comparative Example 1, which surface was facing the roller during production (roller side).

(20) FIG. 19: SEM image of the surface of the membrane according to Comparative Example 1, which surface was facing away from the roller during production (air side).

(21) FIG. 20: SEM image of a cross section of the membrane according to Comparative Example 2.

(22) FIG. 21: SEM image of the surface of the membrane according to Comparative Example 2, which surface was facing the roller during production (roller side).

(23) FIG. 22: SEM image of the surface of the membrane according to Comparative Example 2, which surface was facing away from the roller during production (air side).

(24) FIG. 23: SEM image of a cross section of the membrane according to Comparative Example 3b at 100 times magnification.

(25) FIG. 24: SEM image of the cross section of the membrane according to Comparative Example 3b at 250 times magnification.

(26) FIG. 25: SEM image of the surface of the membrane according to Comparative Example 3b, which surface was facing the roller during production (roller side), at 500 times magnification.

(27) FIG. 26: SEM image of the surface of the membrane according to Comparative Example 3b, which surface was facing away from the roller during production (air side), at 500 times magnification.

(28) FIG. 27: SEM image of a cross section of the membrane according to Comparative Example 4.

(29) FIG. 28: SEM image of the surface of the membrane according to Comparative Example 4, which surface was facing the roller during production (roller side).

(30) FIG. 29: SEM image of the surface of the membrane according to Comparative Example 4, which surface was facing away from the roller during production (air side).

(31) FIG. 30: SEM image of a cross section of the membrane according to Comparative Example 6.

(32) The following methods are used for characterizing the membranes:

(33) Determination of the Viscosity of the Casting Solution:

(34) The viscosity of the casting solution is determined at a temperature of 60° C. by means of a rotational rheometer (RheoStress 1, Haake) using a Z20 DIN cylinder sensor device. The measurement takes place at a shear rate of 10 s.sup.−1.

(35) Determination of Volume Porosity

(36) Four samples of approximately 15 cm.sup.2 of the membrane to be tested are weighed out and placed in approximately 50 ml water for 16 h. The samples are subsequently removed from the water and the excess water is removed by means of blotting paper. The samples thus pretreated are weighed to determine the wet weight and afterwards dried at 50° C. for 16 h. After cooling, the weight of the dry samples is determined (dry weight).

(37) The volume porosity is determined from the average value of the water uptake (wet weight minus dry weight) relative to the average value of the dry weight of the samples, using the densities for water and for the polymer (hydrophobic first polymer) forming the membrane.

(38) Determination of the Transmembrane Flow (Water Permeability):

(39) 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 housing that can be penetrated under pressure by water. The clamped membrane sample is then penetrated, from the side on which the surface of the membrane with the smaller pores is located, by deionized water conditioned to 25° C. at a defined pressure between 0.1 and 0.2 bar. The water volume that flows through the membrane sample during a measuring period of 60 s is determined gravimetrically or volumetrically.

(40) The transmembrane flow, TMF, is determined according to formula (III)

(41) $\begin{array}{cc}\mathrm{TMF}\left[\frac{1}{m^2.Math.h.Math.\mathrm{bar}}\right]=\frac{V_w}{\Delta t.Math.A_M.Math.\Delta p}.Math.600& \mathrm{III}\end{array}$
where: Vw=volume of water [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]
Determination of the Maximum Separating Pore:

(42) The diameter of the maximum separating pore is determined by means of the bubble point method (ASTM No. 128-61 and F 316-86), where for example 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 water, σ.sub.B is 2.07 μm.Math.bar at 25° C.
Determination of Breaking Force and Tensile Strength:

(43) Measuring the breaking force 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.

(44) The membrane samples are stretched at constant speed in the longitudinal direction or in the transverse direction of the samples until rupture. The force required for this is measured as a function of the change in length and retained in a force/elongation diagram. 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 force required to break, BK, is determined as the average numeric value in cN and the elongation at break achieved thereby as a % of the original length.

(45) The tensile 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.

(46) Examination of the Fluid Column Maintained by the Membrane:

(47) A sample with a diameter of approximately 1.8 cm is stamped out of the membrane to be tested. The sample is embedded in a sample holder fluid-tight at the outer edge thereof such that a free area with a diameter of 1.2 cm (1.13 cm.sup.2 measurement area) results in the center. The sample holder is provided with an inlet port at the upper end thereof and with an outlet port at the lower end thereof. The upper end of a flexible tube having an inside diameter of 3 mm and a length of approximately 1.75 m is connected to the outlet port. The lower end of the tube is positioned at a height of 1 m below the membrane sample and discharges openly into a collection container. The inlet port is connected to a water container that contains approximately 1 l.

(48) The flexible tube is clamped off by means of a roller clamp and the water container above the sample holder is filled with water. At the beginning of the test, the roller clamp is opened and the water flow through the membrane sample and the tube is started. After passage of the entire amount of water provided, it is tested whether a fluid column of 1 m in height is maintained by the membrane sample in the tube beneath the membrane.

EXAMPLE 1

(49) 33 kg γ-butyrolactone, 33 kg ε-caprolactam, 4.2 kg glycerine, and 0.8 kg water were placed as a solvent system in a heatable vessel and mixed into a homogeneous liquid. 1.3 kg sulfonated polyethersulfone (SPES) with a degree of sulfonation of 5% was dissolved within 1 h in this solvent system while stirring, and then 15.22 kg polyethersulfone (PES, Ultrason E6020, BASF) was sprinkled in while stirring and dissolved over 4 h. Afterwards, 12.50 kg polyvinylpyrrolidone (PVP, K30, ISP), was stirred in, finely distributed, and homogenized. By applying vacuum and purging with nitrogen, the oxygen was largely removed from the vessel. Afterwards, the vessel was heated to 80° C. and a homogeneous casting solution was produced within 36 h under intensive stirring. Afterwards, the casting solution was cooled to 70° C. and degassed by means of vacuum. The solution obtained had a viscosity of 11.6 Pa s at 40° C.

(50) The finished casting solution was poured by means of a casting mold conditioned to 80° C. onto a metal casting roller conditioned to 65° C. to form a film with a thickness of approximately 130 μm. The film, located on the casting roller, was fed through a climate-controlled zone and for approximately 26 s impinged with a climate of 50° C. and 63% relative humidity (dew point approximately 40° C.) before said film was fed into a coagulation bath filled with water conditioned to 65° C. After a residence time of 26 s to form the membrane structure, the film was drawn off from the casting roller by means of a drawing off roller. In subsequent wash baths, the membrane was fixed in water increasing in temperature step-wise to 90° C. and the solvent together with a large part of the PVP was extracted. The drying of the membrane took place by means of a drum dryer.

(51) The membrane thus produced was permanently hydrophilic, spontaneously wettable with water, and had a thickness of approximately 95 μm and a maximum separating pore of 6.9 μm as determined by means of the bubble point method. It had a transmembrane flow of approximately 1320 ml/(cm.sup.2.Math.min.Math.bar) and a porosity of approximately 81 vol. %. An average tensile strength of 770 cN/mm.sup.2 in the longitudinal direction (production direction) and 760 cN/mm.sup.2 in the transverse direction was measured for the membrane. The average elongation at break was 28% in the longitudinal direction and 47% in the transverse direction. The membrane maintained a fluid column of at least one meter without any problems in the test.

(52) FIG. 1 shows a cross section of the membrane from Example 1, in which on the surfaces of the membrane on the roller side (right side in the SEM image shown in FIG. 1) and also on the air side (left side in the SEM image shown in FIG. 1), a cover layer having pores can be recognized. A coarse-pored supporting layer extends between the cover layers, wherein the cover layer facing the roller side adjoins a first area of the supporting layer, in which first area the pore size is essentially constant and which first area, starting from the roller side, extends over approximately 70% of the supporting layer. In a second area of the supporting layer, which second area extends to the cover layer corresponding to the air side, the pore structure appears more coarsely pored than in the first area of the supporting layer.

(53) In FIG. 2, which shows a portion of the membrane cross section in the area of the membrane side that was facing away from the casting roller during production of the membrane (air side) at 2000 times magnification, it can be recognized that the membrane structure of the cover layer and also the branches of the supporting structure have a skin on the outer side and a porous structure in the interior, wherein the skin of the cover layers and also of the branches in the supporting layer appears smooth, uniform, and even, as determined by means of scanning electron microscopy. The latter is illustrated by means of sections of the membrane surface on the roller side and on the air side at 500 times magnification in FIG. 5 and FIG. 6.

(54) The membrane of Example 1 had a structure across the thickness thereof, as is depicted in the SEM image of FIG. 7. At the magnification of approximately 1600 times shown here, it is clearly recognizable that the predominant majority of the branches forming the structure have a diameter of more than 0.5 μm at their respectively thinnest point. From FIGS. 3 to 6, it is also clear that the openings or pores in the surfaces of the membrane from Example 1 have an island-sea structure, in which on average approximately oval or circular openings are surrounded by the cover layer in the surfaces, the cover layers thus respectively forming the continuous phase in the form of webs surrounding the openings or pores, which phase surrounds the openings.

EXAMPLE 2

(55) The procedure was the same as in Example 1, except that a coagulation bath temperature of 20° C. was set.

(56) The membrane had a thickness of approximately 100 μm and was likewise permanently hydrophilic and spontaneously wettable with water. It had a maximum separating pore of 8.3 μm, determined by means of the bubble point method, a transmembrane flow of approximately 2080 ml/(cm.sup.2.Math.min.Math.bar) and a porosity of approximately 83 vol. %. The membrane had an average tensile strength of 740 cN/mm.sup.2 in the longitudinal direction (production direction) and 730 cN/mm.sup.2 in the transverse direction. The average elongation at break was 25% in the longitudinal direction and 43% in the transverse direction. The average diameter of the pores in the surface that was facing the casting roller during casting of the film (roller side) was 10.5 μm, and that of the pores in the surface that was initially facing the air during casting of the film (air side) was 8.2 μm. The proportion of pores in the surface on the roller side was 36.2% and on the air side was 6.8%, determined using image analysis methods. The membrane maintained a fluid column of at least one meter without any problems in the test.

(57) The membrane structure resembles that of the membrane from Example 1. By means of FIGS. 8 and 9, which show a cross section of the membrane from Example 2 in 250 times and 500 times magnification, respectively, cover layers can be recognized on both surfaces of the membrane. The supporting layer lying therebetween has a first area extending from the roller side (right side in the SEM image shown in FIGS. 8 and 9) with an essentially isotropic pore structure, which area extends across approximately 70% of the supporting layer. The pore structure in a second area of the supporting layer, which second area extends from the first area of the supporting layer in the direction of the air side of the membrane, appears more coarsely pored than in the first area.

(58) It can also be clearly recognized by means of FIGS. 8 and 9 that the cover layers and the branches have an internal porous structure and a smooth, even skin on the outer side thereof. FIGS. 10 and 11 depict scanning electron microscopic images of the surfaces of the membrane from Example 2 (FIG. 10: roller side; FIG. 11: air side). These surfaces also have a clear island-sea structure, in which the approximately oval or circular openings in the surfaces are surrounded by the cover layer. FIG. 12 shows an SEM image, in approximately 1600 times magnification, of the cross section of the membrane from Example 2. It can be clearly recognized that practically all of the branches or webs have a diameter of at least 0.5 μm at the thinnest point thereof.

EXAMPLE 3

(59) The casting solution from Example 1 was used. The casting solution was poured by means of a casting mold conditioned to 67° C. onto a metal casting roller conditioned to 66° C. to form a film with a thickness of approximately 140 μm. The film located on the casting roller was fed through a climate-controlled zone and for approximately 16 s impinged with a climate of 59° C. and 72% relative humidity (dew point approximately 45° C.) before said film was fed into a coagulation bath filled with water conditioned to 20° C. After a residence time of 16 s to form the membrane structure, the film was drawn off from the casting roller by means of a drawing off roller. In subsequent wash baths, the membrane was fixed in water increasing in temperature step-wise to 90° C. and the solvent, together with a large part of the PVP, was extracted. The drying of the membrane took place by means of a drum dryer.

(60) The membrane thus produced was permanently hydrophilic, spontaneously wettable with water, had a thickness of approximately 110 μm and a maximum separating pore of 9.0 μm, determined by means of the bubble point method. It had a transmembrane flow of approximately 2000 ml/(cm.sup.2.Math.min.Math.bar) and a porosity of approximately 81 vol. %. A tensile strength of 720 cN/mm.sup.2 in the longitudinal direction (production direction) and 700 cN/mm.sup.2 in the transverse direction was measured for the membrane. The average elongation at break was 29% in the longitudinal direction and 44% in the transverse direction. The membrane maintained a fluid column of at least one meter without any problems in the test.

(61) With regard to the structure thereof, i.e. also with regard to the branches forming the structure, the membrane from Example 3 resembled the membrane from Example 1 or Example 2. In the cross section, the membrane from Example 3 had recognizable cover layers on both surfaces based on SEM images. The supporting layer lying therebetween has, according to FIG. 13 and FIG. 14, a first area extending from the roller side (right side in the SEM images shown in FIGS. 13 and 14) with an essentially isotropic pore structure, which first area extends across approximately 60% of the supporting layer. The pore structure in a second area of the supporting layer, which second area extends from this first area of the supporting layer in the direction of the air side of the membrane, appears more coarsely pored than in the first isotropic area. FIGS. 15 and 16 depict scanning electron microscopic images of the surfaces of the membrane from Example 3 (FIG. 15: roller side; FIG. 16: air side). These surfaces also clearly have an island-sea structure, in which the approximately oval or circular openings in the surfaces are surrounded by the cover layer.

COMPARATIVE EXAMPLE 1

(62) The procedure was the same as in Example 1. Departing from Example 1, the casting roller temperature was 60° C. and the coagulation bath temperature 55° C. In addition, the temperature in the climate-controlled zone was 36.5° C. and the relative humidity was approximately 95% (dew point approximately 33° C.). In addition, the method was carried out such that there was a residence time of approximately 18.5 s in the climate-controlled zone and in the coagulation bath, respectively.

(63) The approximately 95 μm thick membrane thus produced had a comparably fine-pored, sponge-like pore structure with fine branches. The maximum separating pore was 2.4 μm, determined by means of the bubble point method. The membrane had a transmembrane flow of approximately 474 ml/(cm.sup.2.Math.min.Math.bar) and thus a transmembrane flow that was too low.

(64) The membrane from Comparative Example 1 shows an asymmetric structure across the wall thickness as seen in the cross section depicted in FIG. 17. In the wall interior, close to the middle of the wall, there is an area with minimal pore size. Starting from this region with minimal pore size, the size of the pores increases towards both surfaces, i.e. an asymmetric area is located on both sides of the area with minimal pore size. FIGS. 18 and 19 depict the surfaces of the membrane from Comparative Example 1 (FIG. 18: roller side; FIG. 19: air side).

COMPARATIVE EXAMPLE 2

(65) The procedure was the same as in Example 1. Departing from Example 1, the casting roller temperature and the coagulation bath temperature was 27° C. In addition, the temperature in the climate-controlled zone was 40° C. and the relative humidity was approximately 78% (dew point approximately 35° C.). In addition, the method was carried out such that there was a residence time of approximately 12.5 s in the climate-controlled zone and in the coagulation bath, respectively.

(66) The membrane thus produced likewise had a fine-pored, sponge-like pore structure. The maximum separating pore was 0.5 μm, determined by means of the bubble point method. The membrane had a transmembrane flow of only approximately 30 ml/(cm.sup.2.Math.min.Math.bar), which was thus significantly too low.

(67) The cross section of the membrane from Comparative Example 2 depicted in FIG. 20 shows an asymmetric pore structure across the wall thickness with a region with minimal pore size located in the wall interior near the middle of the wall. Starting from this area with minimal pore size, the size of the pores increases towards both surfaces, i.e. an asymmetric area is located on both sides of the area with minimal pore size. FIGS. 21 and 22 depict scanning electron microscopic images of the surfaces of the membrane from Comparative Example 2 (FIG. 21: roller side; FIG. 22: air side).

COMPARATIVE EXAMPLES 3A AND 3B

(68) The procedure was the same as in Example 1. Departing from Example 1, the casting roller temperature and the coagulation bath temperature were 79° C. and 75° C., respectively. The temperature in the climate-controlled zone was set to 50° C. and the relative humidity to approximately 75% (dew point approximately 42° C.). In addition, the method was carried out such that there was a residence time of approximately 26 s in the climate-controlled zone and in the coagulation bath, respectively. In addition, departing from Example 1, a film with a thickness of approximately 370 μm (Comparative Example 3a) and approximately 420 μm (Comparative Example 3b) was poured onto the casting roller, from which membranes with a thickness of 300 μm and 350 μm, respectively, were obtained.

(69) The properties of the membranes from Comparative Examples 3a and 3b are listed in Table 1.

(70) TABLE-US-00001 TABLE 1 Thickness TMF Maximum separating Membrane of μm ml/(cm.sup.2 .Math. min .Math. bar) pore μm Comparative 300 870 4.8 Example 3a Comparative 350 270 4.8 Example 3b

(71) The cross section of the membrane from Comparative Example 3b depicted in FIGS. 23 and 24 shows across the wall thickness a clearly asymmetric pore structure with an fine-pored structure on the roller side (right side in the SEM images shown in FIG. 23 and FIG. 24), which pore structure transitions into a very open-pored structure in the wall interior. Towards the air side (left side in the SEM images shown in FIG. 23 and FIG. 24), the structure transitions into a fine-pored area extending across approximately 25% of the wall thickness.

(72) FIGS. 25 and 26 depict scanning electron microscopic images of the surfaces of the membrane from Comparative Example 3b (FIG. 25: roller side; FIG. 26: air side). On the roller side, the membrane from Comparative Example 3b shows a partially network-like and fibrillar structure with predominantly thin, fibril-like delimitations between irregularly shaped pores.

COMPARATIVE EXAMPLE 4

(73) According to the method of Example 1, a homogeneous solution was produced of 13.95 wt. % polyethersulfone (PES, Ultrason E6020, BASF), 1.04 wt. % sulfonated polyethersulfone (SPES) with a degree of sulfonation of 5%, and 11.25 wt. % polyvinylpyrrolidone (PVP, K30, ISP) in a solvent system having 41.48 wt. % γ-butyrolactone, 13.83 wt. % ε-caprolactam, and 18.45 wt. % polyethylene glycol PEG 200, each relative to the weight of the total solution. The final solution had a viscosity of 3.7 Pa s at a measuring temperature of 40° C.

(74) The finished casting solution was poured by means of a casting mold conditioned to 45° C. onto a metal casting roller conditioned to 75° C. to form a film. The film located on the casting roller was fed through a climate-controlled zone and for approximately 26 s impinged with a climate of 50° C. and 74% relative humidity (dew point approximately 47° C.) before said film was fed into a coagulation bath filled with water conditioned to 69.5° C. After a residence time of 26 s to form the membrane structure, the film was drawn off from the casting roller by means of a drawing off roller. In subsequent wash baths, the membrane was fixed in water and the solvent together with a large part of the PVP was extracted. The drying of the membrane took place by means of a drum dryer.

(75) A membrane with a thickness of approximately 280 μm was produced. The membrane had a maximum separating pore of 3.9 μm, determined by means of the bubble point method, and a transmembrane flow of approximately 1250 ml/(cm.sup.2.Math.min.Math.bar).

(76) As proven by the SEM images of the membrane cross section and the surfaces according to FIGS. 27 to 29, the membrane from this Comparative Example 4 had a pronounced asymmetric structure extending across the entire membrane wall, wherein the diameters of the pores in the surfaces of the membrane differed on average by a factor of more than 2.

COMPARATIVE EXAMPLE 5

(77) The procedure was the same as in Comparative Example 4. Departing from Comparative Example 4, a film was poured onto a casting roller conditioned to 85° C., which film, after passing through the climate-controlled zone, was fed into a coagulation bath conditioned to 73° C. to form the membrane structure.

(78) A membrane with a thickness of approximately 340 μm was produced. The membrane had a maximum separating pore of 2.5 μm and a transmembrane flow of approximately 630 ml/(cm.sup.2.Math.min.Math.bar). The structure of the membrane from Comparative Example 5 resembled that of the membrane from Comparative Example 4.

COMPARATIVE EXAMPLE 6

(79) According to the method of Example 1, a homogeneous solution was produced of 13.84 wt. % polyethersulfone (PES, Ultrason E6020, BASF), 1.04 wt. % sulfonated polyethersulfone (SPES) with a degree of sulfonation of 5%, and 11.16 wt. % polyvinylpyrrolidone (PVP, K30, ISP) in a solvent system having 41.15 wt. % γ-butyrolactone, 13.72 wt. % ε-caprolactam, 18.29 wt. % polyethylene glycol PEG 200, and 0.80 wt. % water, each relative to the weight of the total solution. The final solution had a viscosity of 3.7 Pa s at a measuring temperature of 40° C.

(80) The finished casting solution was poured by means of a casting mold conditioned to 45° C. onto a metal casting roller conditioned to 64° C. to form a film with a thickness of approximately 170 μm. The film, located on the casting roller, was fed through a climate-controlled zone and for approximately 10 s impinged with a climate of 46° C. and 42% relative humidity (dew point approximately 29° C.) before said film was fed into a coagulation bath filled with water conditioned to 64° C. After a residence time of 10 s to form the membrane structure, the film was drawn off from the casting roller by means of a drawing off roller. In subsequent wash baths, the membrane was fixed and extrated in water. The drying of the membrane took place by means of a drum dryer.

(81) The membrane had a thickness of approximately 140 μm, a maximum separating pore of 0.8 μm, determined by means of the bubble point method, and a transmembrane flow of approximately 220 ml/(cm.sup.2.Math.min.Math.bar). The membrane had an asymmetric, comparatively fine-pored, sponge-like pore structure across the entire membrane wall with predominantly fine, thin branches. By means of the SEM images, it can be recognized that a layer with very small pores is located in the wall interior, and the pore size increases starting from this layer in the direction of the surfaces (FIG. 30).