U.S. patent application number 11/914768 was filed with the patent office on 2008-08-21 for microfiltration membrane with improved filtration properties.
Invention is credited to Wolfgang Ansorge, Klaus Dombrowski, Oliver Schuster, Friedbert Wechs.
Application Number | 20080197072 11/914768 |
Document ID | / |
Family ID | 36660786 |
Filed Date | 2008-08-21 |
United States Patent
Application |
20080197072 |
Kind Code |
A1 |
Ansorge; Wolfgang ; et
al. |
August 21, 2008 |
Microfiltration Membrane With Improved Filtration Properties
Abstract
Integrally asymmetric flat membrane for microfiltration
comprising at least 40 wt. % of a hydrophobic first sulfone polymer
and a hydrophilic second polymer with a pore size distribution over
the membrane wall, having a separating layer in the wall's
interior, and also having, in the direction from this separating
layer, pore sizes increasing towards the surfaces, the second
surface having pores with a mean diameter of at least 1 .mu.m. The
membrane comprises the hydrophilic second polymer in a
concentration of 0.1-10 wt. %. The separating layer is located in
an region facing the first surface, and the pore size passes
through a maximum in the direction from the asymmetrical region
towards the second surface. Method for producing this membrane from
a casting solution comprising the hydrophobic first sulfone polymer
and the hydrophilic second polymer in a solvent system, the method
comprising the steps of pouring the casting solution, conditioned
to a molding temperature, onto a carrier to form a film, which
carrier has a temperature that is higher in comparison to the
molding temperature, conveying the film through a
climate-controlled zone, initiating the coagulation in a
coagulation bath for the formation of a membrane structure,
withdrawing the membrane structure from the carrier with a speed
that is increased in comparison to the carrier speed, stabilizing,
extracting, and subsequently drying the membrane.
Inventors: |
Ansorge; Wolfgang; (Essen,
DE) ; Schuster; Oliver; (Gevelsberg, DE) ;
Wechs; Friedbert; (Worth, DE) ; Dombrowski;
Klaus; (Koln, DE) |
Correspondence
Address: |
HAMMER & ASSOCIATES, P.C.
3125 SPRINGBANK LANE, SUITE G
CHARLOTTE
NC
28226
US
|
Family ID: |
36660786 |
Appl. No.: |
11/914768 |
Filed: |
June 3, 2006 |
PCT Filed: |
June 3, 2006 |
PCT NO: |
PCT/EP2006/005346 |
371 Date: |
November 19, 2007 |
Current U.S.
Class: |
210/500.41 ;
210/500.27; 264/138 |
Current CPC
Class: |
B01D 2325/38 20130101;
B01D 63/081 20130101; B01D 2325/36 20130101; B01D 67/0016 20130101;
B01D 67/0013 20130101; B01D 71/68 20130101; B01D 67/0027 20130101;
B01D 2325/24 20130101; B01D 69/02 20130101; B01D 69/06 20130101;
B01D 2325/022 20130101; B01D 67/0011 20130101; B01D 2325/023
20130101 |
Class at
Publication: |
210/500.41 ;
210/500.27; 264/138 |
International
Class: |
B01D 39/14 20060101
B01D039/14; B29C 37/00 20060101 B29C037/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 9, 2005 |
DE |
10 2005 026 804.8 |
Claims
1. Integrally asymmetric membrane in the form of a flat sheet, in
particular for microfiltration, based on a film-forming hydrophobic
first polymer from the group of aromatic sulfone polymers, the
membrane having a membrane wall with a first and a second porous
surface and an interior situated between the surfaces, possessing a
porous structure with a pore size distribution over the membrane
wall, and having a separating layer in the wall's interior with a
minimal pore size, and also having, in the direction from this
separating layer towards the first surface, a first asymmetrical
region, and, towards the second surface, a second asymmetrical
region, with pore sizes increasing towards the surfaces, and the
second surface having pores with a mean diameter of at least 1
.mu.m, characterized in that the membrane comprises at least 40 wt.
% of the film-forming hydrophobic first polymer and also comprises
a hydrophilic second polymer, the concentration of the hydrophilic
second polymer being 0.1-10 wt. % relative to the weight of the
membrane, the separating layer with minimal pore size is located in
an region of the membrane wall facing the first surface, and the
pore size passes through a maximum in the direction from the second
asymmetrical region towards the second surface.
2. Membrane according to claim 1, characterized in that the maximum
of the pore size is located in an essentially isotropic region
adjacent to the second asymmetrical region, in which isotropic
region the pore size is essentially constant, whereby the isotropic
region extends over 15 to 70% of the membrane wall.
3. Membrane according to claim 1, characterized in that the
aromatic sulfone polymer is a polysulfone or a
polyethersulfone.
4. Membrane according to claim 1, characterized in that the
hydrophilic second polymer has an average molecular weight MW of
more than 10 000 daltons.
5. Membrane according to claim 4, characterized in that the
hydrophilic second polymer is polyvinylpyrrolidone, polyethylene
glycol, polyvinyl alcohol, polyglycol monoester, polysorbitate,
carboxymethylcellulose, polyacrylic acid, polyacrylate, or a
modification or a copolymer of these polymers.
6. Membrane according to claim 1, characterized in that it further
contains a hydrophilic third polymer, which is different from the
hydrophilic second polymer, whereby the hydrophilic third polymer
is a hydrophilically modified aromatic sulfone polymer.
7. Membrane according to claim 6, characterized in that the
hydrophilically modified aromatic sulfone polymer is present in a
concentration of 1 to 50 wt. % relative to the weight of the
membrane.
8. Membrane according to claim 6, characterized in that the
hydrophilically modified aromatic sulfone polymer is based on the
hydrophobic first aromatic sulfone polymer.
9. Membrane according to claim 6, characterized in that the
hydrophilically modified aromatic sulfone polymer is a sulphonated
sulphone polymer.
10. Membrane according to claim 1, characterized in that the ratio
of the average size of the pores in the first surface to the
average size of the pores in the second surface is at least 5.
11. Membrane according to claim 1, characterized in that it has a
transmembrane flow TMF of at least 10 000 l/(m.sup.2hbar) and the
transmembrane flow at the same time fulfills the condition:
TMF.gtoreq.85 000d.sub.max.sup.2 whereby d.sub.max is the diameter
of the maximum separating pore determined by means of the bubble
point method.
12. Membrane according to claim 11, characterized in that the
transmembrane flow TMF is at least 15 000 l/(m.sup.2hbar).
13. Membrane according to claim 1, characterized in that it has a
volume porosity of at least 75 vol. %.
14. Membrane according to claim 1, characterized in that it has a
filtrate flow rate for an aqueous BSA solution of at least 750
l/hm.sup.2, whereby the filtrate flow rate is determined 15 minutes
after the initiation of a filtration of an aqueous BSA solution
with a BSA concentration of 2 g/l and a pH value of 5 at a
transmembrane pressure of 0.4 bar.
15. Membrane according to claim 14, characterized in that the
filtrate flow rate is at least 1 000 l/hm.sup.2.
16. Membrane according to claim 1, characterized in that it has a
residual filtrate flow rate of at least 35%, whereby the residual
filtrate flow rate is defined as the ratio of the filtrate flow
rate after 120 minutes to the filtrate flow rate after 5 minutes
during a filtration of an aqueous BSA solution with a BSA
concentration of 2 g/l and a pH value of 5 at a transmembrane
pressure of 0.4 bar.
17. Membrane according to claim 16, characterized in that the
filtrate flow rate constant is at least 45%.
18. Membrane according to claim 14, characterized in that it has a
nominal pore of 0.2 .mu.m.
19. A method for producing an integrally asymmetric membrane in the
form of a flat sheet, whereby the method comprises the following
steps: a. producing a homogeneous casting solution from a polymer
component and a solvent system, the polymer component consisting of
10-25 wt. %, relative to the weight of the solution, of a
hydrophobic first polymer from the group of aromatic sulfone
polymers and 2-20 wt. %, relative to the weight of the solution, of
a hydrophilic second polymer, and the solvent system consisting of
5-80 wt. %, relative to the weight of the solvent system, of a
solvent for the polymer component, 0-80 wt. %, relative to the
weight of the solvent system, of a latent solvent for the polymer
component, as well as 0-70 wt. %, relative to the weight of the
solvent system, of a non-solvent for the polymer component, b.
conditioning the homogeneous casting solution to a molding
temperature. c. pouring the homogeneous casting solution onto a
carrier to form a film, which carrier can be temperature controlled
and has a temperature that is at least 15.degree. C. higher than
the molding temperature of the casting solution, and which carrier
has a speed v1, d. conveying the film located on the carrier
through a climate-controlled zone having a temperature in the range
between 35 and 55.degree. C. and a relative humidity in the range
from 40 to 75%, e. introducing the film located on the carrier into
a coagulation medium and initiating the coagulation of the film for
the formation of a membrane structure, f. withdrawing the membrane
structure from the carrier within the coagulation medium by means
of withdrawal device moving with a speed of v2, the speed v2 being
greater than the speed v1 of the carrier, by which means the
membrane structure is drawn, g. stabilizing the membrane structure
in the coagulation medium, h. extracting the resulting membrane and
subsequently drying the membrane.
20. Method according to claim 19, characterized in that the
aromatic sulfone polymer is a polysulfone or a
polyethersulfone.
21. Method according to claim 20, characterized in that the
hydrophilic second polymer has an average molecular weight MW of
more than 10 000 daltons.
22. Method according to claim 21, characterized in that
polyvinylpyrrolidone, polyethylene glycol, polyvinyl alcohol,
polyglycol monoester, polysorbitate, carboxymethylcellulose,
polyacrylic acid, polyacrylate, or a modification or a copolymer of
these polymers is used as the hydrophilic second polymer.
23. Method according to claim 19, characterized in that the
viscosity of the homogeneous casting solution is set to a viscosity
below 10 Pa s, determined at 40.degree. C.
24. Method according to claim 19, characterized in that the casting
solution further contains 0.2-20 wt. %, relative to the weight of
the casting solution, of a hydrophilic third polymer, which is
different from the hydrophilic second polymer, the hydrophilic
third polymer being a hydrophilically modified aromatic sulfone
polymer.
25. Method according to claim 24, characterized in that the
hydrophilically modified aromatic sulfone polymer is a sulphonated
sulphone polymer.
26. Method according to claim 19, characterized in that the ratio
of the speed v2 of the withdrawal device to the speed v1 of the
carrier lies in the range between 1.05:1 and 1.2:1.
27. Method according to claim 19, characterized in that a polar,
aprotic solvent or a protic solvent is used as the solvent.
28. (canceled)
Description
[0001] The invention relates to an integrally asymmetric membrane
for microfiltration, based on a film-forming hydrophobic first
polymer from the group of sulfone polymers, the membrane having a
porous structure with a pore size distribution over the membrane
wall, and having a separating layer in the wall's interior with a
minimal pore size, and also having, in the direction from this
separating layer towards the first surface, a first asymmetrical
region, and, towards the second surface, a second asymmetrical
region, with pore sizes increasing towards the surfaces. The
invention further relates to a method for producing such
membranes.
[0002] Microporous polymer membranes are used in a wide range of
industrial, pharmaceutical, or medical applications for
high-precision filtration. In these applications, membrane
separation processes are gaining increasingly in importance, as
these processes offer the advantage that the substances to be
separated are not thermally stressed or affected. Microfiltration
membranes enable, for example, the removal of fine particles or
micro-organisms in sizes down to the submicron range and are
therefore suitable for producing purified water for use in
laboratories or for the semi-conductor industry. Numerous further
applications of membrane separation processes are known from the
beverage industry, biotechnology sector, or wastewater
technology.
[0003] In these cases, membranes with a high degree of asymmetry
are preferably used, which membranes have a separating layer and an
adjoining microporous supporting structure with coarser pores in
comparison to the separating layer. The pores in the separating
layer thereby determine the actual separation characteristic of the
membrane, i.e. the size of the particles or molecules that are
retained by the membrane is controlled by the size of the pores in
the separating layer. In the application, membranes of this type
are often used in such a way, that the liquid flows into them from
the more open-pored side and so the microporous supporting layer
functions as a prefilter for the separating layer. By this means,
the dirt-loading capacity of the membrane is increased. A fluid
that flows through the membrane in such a way enters first in the
larger pores and finally in the smaller pores of the separating
layer. By this means, particles that are contained in the fluid are
retained in the coarse-pored supporting layer before they reach the
separating layer and can block it.
[0004] In these cases, sulfone polymers like, e.g. polysulfone or
polyethersulfone represent a widely used membrane material, not
least because of their high chemical stability, their temperature
stability or the sterilizability of the membranes manufactured from
them. Admittedly, these polymers are hydrophobic polymers, which
limits their use for the filtration of aqueous media. In addition
it is known that membranes made of hydrophobic material have a
strong, non-specific ability to adsorb, due to which in use often
results rapid covering of the membrane surface by predominantly
higher-molecular components of the liquid to be filtered and
subsequently a decrease in permeability.
[0005] U.S. Pat. No. 5,866,059 discloses a polyethersulfone
membrane for microfiltration, whereby the membrane has a pronounced
asymmetric structure with a skin that has relatively small pores on
the one, first side of the membrane and the pore size increases
from this side over the membrane wall to the other, second side of
the membrane, whereby the pores on the second side are larger than
the pores in the skin on the first side by a factor of 50 to 10
000. Membranes with this type of structure are on the one hand
susceptible to mechanical damages with regard to the layer with the
smallest pores, i.e. the separating layer that is located on the
surface of the membrane. On the other hand, the membranes have only
a moderate mechanical stability due to the specific asymmetric
structure.
[0006] The membranes produced according to U.S. Pat. No. 5,866,059
are hydrophobic and can at most be hydrophilized by an
aftertreatment. In contrast, the integrally asymmetric membranes
based on a sulfone polymer disclosed in U.S. Pat. No. 6,045,899
have hydrophilic characteristics, as during the production of these
membranes a hydrophilic polymer like e.g. polyvinylpyrrolidone is
added to the polymer solution.
[0007] Likewise, U.S. Pat. No. 5,906,742 discloses hydrophilic
integrally asymmetric polymer membranes based on a sulfone polymer.
These membranes have a microporous skin and an adjoining porous
supporting structure, whereby the porous supporting structure has
an isotropic region with essentially constant pore size adjoining
the skin, and further has, adjoining this isotropic region, an
asymmetrical region with pore sizes increasing, starting from the
isotropic region. The isotropic region extends over approximately
15-25% of the wall, whereby the size of the pores in the isotropic
region is somewhat larger than the size of the pores in the
microporous skin. For these membranes as well, the skin, which
forms the separating layer, is susceptible to mechanical damages.
Due to the relatively broad isotropic region with small pore sizes,
relatively high pressure drops are to be expected for these
membranes during the passage of a fluid.
[0008] U.S. Pat. No. 4,933,081 discloses microporous polysulfone
membranes, whose separating layer is located within the membrane
wall for the prevention of the susceptibility of the separating
layer to mechanical damage. These membranes have a pore size
distribution over the membrane wall and have a layer with minimal
pore sizes at a distance of preferably 1-30 .mu.m from one of the
membrane surfaces. The pore size increases in the direction from
this layer with minimal pore size towards both surfaces of the
membrane. The diameter of the pores in the surface facing the
separating layer is smaller by a factor of 10-100 than that of the
pores in the surface facing away from the separating layer, whereby
the membranes in U.S. Pat. No. 4,933,081 have a high asymmetry. The
membranes from U.S. Pat. No. 4,933,081 are, however, hydrophobic,
and must be subjected to a further treatment in order to make them
hydrophilic.
[0009] EP-A-361 085 discloses integrally asymmetric membranes made
of polyethersulfone. In the examples, hollow-fiber membranes are
described that have in the outer region an approx. 50-100 .mu.m
thick fine-pored structure open towards the outside, which
structure at the membrane center passes into an increasingly
coarser-pored structure. Towards the lumen side, the structure
compacts again. The inner surface of this membrane is open-pored.
The transmembrane flows of the membranes revealed in EP-A-361 085
are relatively low.
[0010] Object of the present invention is therefore to provide a
membrane based on a sulfone polymer, in particular for
microfiltration, that has hydrophilic characteristics, a high
permeability as well as a high dirt-loading capacity, and is not
susceptible to mechanical attacks. Object of the present invention
is further to provide a method for producing such a membrane.
[0011] The object according to the invention is achieved through an
integrally asymmetric membrane in the form of a flat sheet, in
particular for microfiltration, based on a film-forming hydrophobic
first polymer from the group of aromatic sulfone polymers, the
membrane having a membrane wall with a first and a second porous
surface and an interior situated between the surfaces, possessing a
porous structure with a pore size distribution over the membrane
wall, and having a separating layer in the wall's interior with a
minimal pore size, and also having, in the direction from this
separating layer towards the first surface, a first asymmetrical
region, and, towards the second surface, a second asymmetrical
region, with pore sizes increasing towards the surfaces, and the
second surface having pores with a mean diameter of at least 1
.mu.m, the membrane being characterized in that it comprises at
least 40 wt. % of the film-forming hydrophobic first polymer, and
also comprises a hydrophilic second polymer, the concentration of
the hydrophilic second polymer being 0.1-10 wt. % relative to the
weight of the membrane, and the separating layer with minimal pore
size is located in an region of the membrane wall facing the first
surface, and the pore size passes through a maximum in the
direction from the second asymmetrical region towards the second
surface.
[0012] Since the separating layer with minimal pore size for the
membrane according to the invention is located within the membrane
wall, it is protected from mechanical damages caused by, for
example, the processing of the membrane for embedding in a housing,
in particular e.g. the pleating of the flat membrane, or cleaning
cycles during use. Preferably the separating layer with minimal
pore size is located at a distance of 3-30% of the thickness of the
membrane wall from the first surface. In addition to using suitable
scanning or transmission electron microscope images, the existence
of an internal separating layer can also be demonstrated by means
of simple staining techniques. By means of staining technique, the
different pore sizes over the membrane cross-section can be made
visible with a light microscope at suitable magnification.
Depending on the size of the pores, the intensity of the staining
of the membrane structure varies, whereby the staining is more
intense the more finely pored the structure is.
[0013] According to the invention, the pore size passes through a
maximum in the direction from the second asymmetrical region
towards the second surface, whereby the maximum has a distance from
the second surface of preferably 3-75% of the thickness of the
membrane wall. In an advantageous embodiment of the membrane
according to the invention, the maximum of the pore size is located
in an essentially isotropic region adjoining the second
asymmetrical region, or is part of this isotropic region, whereby
the isotropic region preferably extends over 15-70% of the membrane
wall. Within the context of the present invention, an essentially
isotropic region is understood to be a region of the membrane wall
with an essentially constant pore size, whereby an assessment is
carried out by means of scanning or transmission electron
microscope images. The isotropic region can also be regarded as a
region in which the flow channels extending through the membrane
wall have an essentially constant average diameter. As is true for
every membrane, the actual pore size also varies somewhat in the
membrane according to the invention, i.e. it has a certain pore
size distribution, even when the pore size distribution appears
visually isotropic. Therefore, the invention comprises also
embodiments with the essentially isotropic region, in which the
pore size changes by a maximum of approx. 15-20%. Due to the
preferred existence of an isotropic region, in which the pore size
does not increase further, an improvement of the mechanical
stability is achieved while simultaneously retaining a high
dirt-loading capacity.
[0014] The pore size of the membrane according to the invention,
after passing through the maximum or passing the isotropic region,
decreases in the direction towards the second surface. In order to
realize a high permeability and a high dirt-loading capacity of the
membrane, a highly open-pored structure and, in particular in the
case of the presence of an isotropic region, large pore diameters
in the second surface are aimed for, whereby the pores in the
surface are smaller than the pores in the layer in which the
maximum of the pore size is present. According to the invention,
the average diameter of the pores in the second surface is at least
1 .mu.m and preferably at least 2 .mu.m and more preferably at
least 5 .mu.m.
[0015] In a preferred embodiment of the membrane according to the
invention, the ratio of the average size of the pores in the second
surface to the average size of the pores in the first surface is at
least 5 and more preferably at least 10. In membranes of this type,
a high dirt-loading capacity is achieved due to the pronounced
asymmetry in connection with the high openness of the second
surface.
[0016] Due to its specific structure, the membrane according to the
invention possesses, simultaneously with the high dirt-loading
capacity, a high stability or mechanical strength, whereby this
stability is also retained at high volume porosities of the
membrane. Such strengths are not achieved in comparable membranes,
in which the asymmetrical region with increasing pore size extends
to the second surface of the membrane wall. The membrane according
to the invention can have, therefore, at the same time a high
volume porosity that is advantageous for a high permeability and
thus for a high transmembrane flow as well as for a high
dirt-loading capacity. Preferably, the membrane according to the
invention has a volume porosity of at least 75 vol. % and more
preferably at least 80 vol. %, whereby it has proved particularly
advantageous if the porosity lies between 80 and 90 vol. %.
[0017] Within the context of the present invention, the
dirt-loading capacity in regards to the pore blocking behavior of
the membrane during the fluid flow through the membrane is
determined by means of a test medium based on an aqueous solution
of soluble instant coffee powder. From the change of the
transmembrane flow TMF.sub.PM of this test solution through the
membrane over time, a statement about the pore blocking behavior of
the membrane and thereby about the dirt-loading capacity can be
derived. The membrane shows thereby a high dirt-loading capacity,
if the transmembrane flow TMF.sub.PM of the test medium over time
changes only slightly, which can be ascribed to the fact that the
membrane is not appreciably blocked. The membrane according to the
invention has preferably a residual of the transmembrane flow
TMF.sub.PM of a test medium, consisting of an aqueous solution of
0.04 g soluble instant coffee powder per liter of water, of at
least 0.5, preferably at least 0.65, whereby the residual of the
transmembrane flow TMF.sub.PM is defined as the ratio of the
TMF.sub.PM after a testing time of 10 minutes to the TMF.sub.PM at
the beginning of the measurement.
[0018] The object according to the invention is further achieved by
a method for producing the membrane according to the invention, the
method comprising the following steps: [0019] a. producing a
homogeneous casting solution from a polymer component and a solvent
system, the polymer component consisting of 10-25 wt. %, relative
to the weight of the solution, of a hydrophobic first polymer from
the group of aromatic sulfone polymers, and 2-20 wt. %, relative to
the weight of the solution, of a hydrophilic second polymer, and
the solvent system consisting of 5-80 wt. %, relative to the weight
of the solvent system, of a solvent for the polymer component, 0-80
wt. %, relative to the weight of the solvent system, of a latent
solvent for the polymer component, as well as 0-70 wt. %, relative
to the weight of the solvent system, of a non-solvent for the
polymer component, [0020] b. conditioning the homogeneous casting
solution to a molding temperature, [0021] c. pouring the
homogeneous casting solution onto a carrier to form a film, which
carrier can be temperature controlled and has a temperature that is
higher than the molding temperature of the casting solution, and
which carrier has a speed v.sub.1, [0022] d. conveying the film
located on the carrier through a climate-controlled zone, [0023] e.
introducing the film located on the carrier into a coagulation
medium and initiating the coagulation of the film for the formation
of a membrane structure, [0024] f. withdrawing the membrane
structure from the carrier within the coagulation medium by means
of withdrawal device moving with a speed of v.sub.2, the speed
v.sub.2 being greater than the speed v.sub.1 of the carrier, by
which means the membrane structure is drawn, [0025] g. stabilizing
the membrane structure in the coagulation medium, [0026] h.
extracting the resulting membrane and subsequently drying the
membrane.
[0027] The solvent system used for the preparation of the casting
solution is to be adapted to the membrane-forming sulfone polymer.
Preferably, the solvent system comprises polar, aprotic solvents
like dimethylformamide, dimethylacetamide, dimethyl sulfoxide,
N-methylpyrrolidone, or a mixture of these, or protic solvents like
.epsilon.-caprolactam. Additionally, the solvent system can contain
up to 80 wt. % of latent solvent, whereby in the context of the
present invention a latent solvent is understood as a solvent that
dissolves the sulfone polymer poorly or only at increased
temperature. In the case of using .epsilon.-caprolactam as the
solvent, for example .gamma.-butyrolactone, propylene carbonate,
polyalkylene glycol can be used. In addition to this, the solvent
system can contain non-solvents for the membrane-forming polymer,
like, e.g., water, glycerin, low-molecular polyethylene glycols
with a weight average of the molecular weight of less than 1 000
daltons or low-molecular alcohols, such as ethanol or
isopropanol.
[0028] For the realization of the method according to the invention
and for the formation of the characteristic structure of the
membrane according to the invention, it is advantageous if the
viscosity of the casting solution is set to a value below 10 Pa s,
and more advantageous if it is set to a value below 5 Pa s, whereby
the viscosity is determined at 40.degree. C. The setting of the
viscosity can occur in particular through the selection and
concentration of the hydrophilic second polymer used in the method
according to the invention.
[0029] The pouring of the casting solution to form a film can take
place according to methods known per se, for example by means of
conventional forming tools like sheeting dies, casting molds, or
doctor blades. At the latest, the casting solution is set to the
molding temperature in the forming tool. The pouring of the casting
solution takes place on a carrier that can be temperature
controlled; here also, one can resort to the conventional carriers,
from which the coagulated membrane can be withdrawn later. For
example, coated papers or steel tapes can be used. Preferably, the
temperature-controllable carrier is a heating roll that can be
temperature controlled, i.e. a casting roller, onto which the film
is poured.
[0030] It is essential to the invention that the temperature of the
carrier is higher than the molding temperature of the casting
solution. By this means, a viscosity gradient develops in the
casting solution over the thickness of the poured film. Due to the
increased carrier temperature, the poured film has a lower
viscosity in the region of the carrier, by which means
coarser-pored structures are formed during later contact with the
coagulation medium. The carrier temperature is preferably at least
15.degree. C. and more preferably at least 20.degree. C. higher
than the molding temperature.
[0031] In order to create the asymmetric structure with interior
separating layer it is furthermore required that the film located
on the carrier be conveyed through a climate-controlled zone, in
which a defined temperature and a defined relative humidity are
set. Preferably, the temperature in the climate-controlled zone
lies in the range from 35-55.degree. C., the relative humidity is
set preferably to values in the range from 40-75%. The retention
time of the film in the climate-controlled zone as well as the
overflow speed of the air over the poured film in the
climate-controlled zone is to be determined such that a
pre-coagulation is induced by pickup of the air humidity acting as
a non-solvent and a separating layer with minimal pore size is
obtained within the membrane wall. The conditions in the
climate-controlled zone have at the same time an influence on the
size of the pores in the separating layer.
[0032] After passing through the climate-controlled zone, the film
located on the carrier is introduced into a coagulation medium and
a coagulation for the formation of the membrane structure is
initiated. Preferably, the coagulation medium is conditioned to a
temperature above room temperature and has more preferably a
temperature above 40.degree. C. In a preferred embodiment of the
method according to the invention, the coagulation medium is water
or a water bath.
[0033] In the coagulation medium, the film is initially
precipitated to form the membrane structure to the extent that the
membrane structure already has a sufficient stability and can be
withdrawn from the carrier, i.e. preferably from the casting
roller. The withdrawal from the casting roller occurs by means of a
withdrawal device, for example by means of a drawing-off roller,
whereby according to the invention the withdrawal speed v.sub.2 is
greater than the speed v.sub.1 of the carrier and the membrane
structure is drawn. Preferably, the ratio of the speed v.sub.2 of
the withdrawal device to the speed v.sub.1 of the carrier lies in
the range between 1.05:1 and 1.2:1. By this means, a high surface
porosity is achieved on the side of the resulting membrane that
faced towards the carrier.
[0034] Following the withdrawal device, the coagulation is
completed in the subsequent coagulation baths and the membrane is
stabilized. These coagulation baths can have a higher temperature
in comparison to the first, previously described coagulation bath.
The temperature can also be increased stepwise from bath to bath.
In the coagulation baths thereby simultaneously occurs an
extraction of the solvent system and, normally, of parts of the
hydrophilic second polymer from the membrane structure, so that the
coagulation baths function simultaneously as wash or extraction
baths. As a coagulation or wash medium in these coagulation or wash
baths, water is preferably used.
[0035] After the extraction, the resulting membrane is dried, for
example, by means of a drum dryer, and the dried membrane is
thereafter wound up. During the extraction and drying of the
membrane, a minor drawing is likewise advantageous, in order to set
well-defined membrane characteristics, such as, e.g. the surface
porosity and the separation characteristics.
[0036] According to the invention, the membranes are based on a
hydrophobic first polymer from the group of aromatic sulfone
polymers and contain in addition a hydrophilic second polymer. As
the aromatic sulfone polymer in the context of the present
invention, e.g. polysulfones, polyethersulfones, polyphenylene
sulfones, polyarylethersulfones or copolymers or modifications of
these polymers or mixtures of these polymers can be used. In a
preferred embodiment, the hydrophobic first polymer is a
polysulfone or a polyethersulfone with the repeating molecular
units shown in formulas (I) and (II) as follows:
##STR00001##
[0037] More preferably, a polyethersulfone according to formula
(II) is used as the hydrophobic first polymer, because this has
lower hydrophobicity than, for example, the polysulfone.
[0038] Long-chain polymers are used advantageously as the
hydrophilic second polymer that have a good compatibility with the
hydrophobic first polymer and have repeating polymer units that are
in themselves hydrophilic. Those hydrophilic polymers are preferred
that have an average molecular weight M.sub.w of more than 10 000
daltons. In the method according to the invention, the polymers
used as the hydrophilic second polymers have at the same time the
function of increasing the viscosity of the homogeneous spinning
solution, i.e. of functioning as a thickener, for which reason
these polymers are also often called thickeners. In addition to
this, these polymers function also as pore-forming agents or
nucleating agents during the formation of the membrane structure.
Preferably, the hydrophilic second polymer is polyvinylpyrrolidone,
polyethylene glycol, polyvinyl alcohol, polyglycol monoester,
polysorbitate, such as, e.g., polyoxyethylene sorbitan monooleate,
carboxymethylcellulose, or a modification or a copolymer of these
polymers. Polyvinylpyrrolidone is especially preferred. In a
further preferred embodiment it is also possible to use mixtures of
different hydrophilic polymers and in particular mixtures of
hydrophilic polymers with different molecular weights, e.g.,
mixtures of polymers whose molecular weights differ by a factor of
5 or more. Preferably, the concentration of the hydrophilic second
polymer in the membrane according to the invention is 0.5-7 wt. %
relative to the weight of the membrane. These polymers furthermore
can, if necessary, be modified chemically or physically in the
membrane. For instance, polyvinylpyrrolidone can be subsequently
cross-linked, e.g. by irradiation with high-energy radiation, and
made water-insoluble thereby.
[0039] For the modification of the surface characteristics of the
membranes according to the invention, additives can be used that
influence the stability of the membrane, the color, the ability to
adsorb or absorb. There are also additives possible that control
the charge of the membrane, e.g., that impart anionic or cationic
character to the membrane. Preferably, the membrane according to
the invention further contains a hydrophilic third polymer that is
different from the hydrophilic second polymer and is a
hydrophilically modified aromatic sulfone polymer. Due to the
presence of such a polymer, the permeability of the membrane as
well as its adsorption characteristics are in particular favorably
influenced and the membrane has permanent hydrophilic properties,
which manifest themselves in the fact that, among other things, the
membrane can be repeatedly steam sterilized and its hydrophilic
characteristics remain preserved, essentially unchanged, even after
for example 30 sterilization cycles. In an especially preferred
embodiment, the hydrophilically modified aromatic sulfone polymer
is present in the membrane according to the invention at a
concentration of 1-50 wt. % relative to the weight of the membrane,
whereby the sum of the polymers yields 100%. Thereby, in the method
for producing the preferred membranes according to the invention,
the polymer component further comprises a hydrophilic third polymer
that is different from the hydrophilic second polymer and is a
hydrophilically modified aromatic sulfone polymer. Preferably, the
casting solution contains the hydrophilically modified aromatic
sulfone polymer homogeneously dissolved at a concentration of
0.2-20 wt. % relative to the weight of the casting solution
[0040] The hydrophilically modified aromatic sulfone polymer can be
of a type, in which hydrophilic functional groups are covalently
bound to the sulfone polymer. It can also be a copolymer based on a
sulfone polymer, in which hydrophilic segments are contained, for
example a copolymer made from a sulfone polymer with a hydrophilic
polymer like, e.g., polyvinylpyrrolidone or polyethylene glycol.
For reasons of compatibility, it is of particular advantage, if the
hydrophilically modified aromatic sulfone polymer is based on the
hydrophobic first aromatic sulfone polymer, i.e., the membrane
structure contains a mixture of a hydrophobic first aromatic
sulfone polymer and a hydrophilic modification of this polymer.
Very good results are achieved when the hydrophilically modified
aromatic sulfone polymer is a sulfonated sulfone polymer, whereby
this sulfonated sulfone polymer has preferably a degree of
sulfonation in the range of 3-10%. Membranes according to the
invention that contain a combination of polyethersulfone and
sulfonated polyethersulfone have particularly high permeabilities
for water and proteins as well as a low tendency for adsorption,
e.g. of proteins, and therefore a low tendency for fouling.
[0041] Not least due to their particular structure and surface
characteristics, the membranes according to the invention are
distinguished by a high permeability and thereby by a high
transmembrane flow for water. The membranes according to the
invention have preferably a transmembrane flow TMF of at least 10
000 I/(m.sup.2hbar), whereby the transmembrane flow subject to the
diameter d.sub.max of the maximum separating pore satisfies at the
same time the condition (III), which mirrors the dependence of the
transmembrane flow on the size of the pores in the separating
layer:
TMF.gtoreq.85 000d.sub.max.sup.2, (III)
whereby d.sub.max is the diameter of the maximum separating pore in
.mu.m and represents the diameter of the maximum pore in the
separating layer. In a preferred embodiment of the invention, the
transmembrane flow satisfies the condition (IV):
TMF.gtoreq.105 000d.sub.max.sup.2. (IV)
[0042] The diameter of the maximum separating pore is determined by
means of the bubble point method (ASTM nos. 128-61 and F 316-86),
for which the method described in DE-A-36 17 724, for example, is
suitable. From this, d.sub.max results from the gas space pressure
P.sub.B associated with the bubble point according to the equation
(V):
d.sub.max=.sigma..sub.B/P.sub.B (V)
where .sigma..sub.B is a constant that is mainly dependent on the
wetting liquid used for measurement. For water, .sigma..sub.B is
2.07 .mu.mbar at 25.degree. C. In a more preferred embodiment, the
transmembrane flow of the membrane according to the invention is at
least 15 000 l/(m.sup.2hbar).
[0043] Surprisingly, the membranes according to the invention show
an excellent permeability for aqueous protein solutions.
Preferably, the membranes according to the invention have a
filtrate flow rate for an aqueous BSA (Bovine Serum Albumin)
solution of at least 750 l/hm.sup.2, whereby the filtrate flow rate
is determined 15 minutes after the beginning of a filtration of an
aqueous BSA solution with a BSA concentration of 2 g/l and a pH
value of 5 at a transmembrane pressure of 0.4 bar. More preferably,
the filtrate flow rate for the aqueous BSA solution is at least 1
000 l/hm.sup.2, excellent membranes according to the invention have
a filtrate flow rate for the aqueous BSA solution of at least 2 000
l/hm.sup.2.
[0044] Aside from having a high filtrate flow rate for aqueous
protein solutions, the membranes according to the invention are
distinguished in that the filtrate flow rate for this type of
protein solutions shows a high stability over the filtration
period, i.e., only a relatively low reduction of the filtrate flow
rate can be determined during the filtration period. Preferred
membranes have a residual filtrate flow rate of at least 35%,
whereby the residual filtrate flow rate is defined as the ratio of
the filtrate flow rate after 120 minutes to the filtrate flow rate
after 5 minutes during a filtration of an aqueous BSA solution with
a BSA concentration of 2 g/l and a pH value of 5 at a transmembrane
pressure of 0.4 bar. More preferably, the residual filtrate flow
rate of the membranes according to the invention is at least 45%
and most preferred at least 50%. Membranes with such favorable flow
rate characteristics in the filtration of protein solutions are
unknown in the prior art. In particular, the flow rate
characteristics for BSA solutions mentioned are also already found
for membranes according to the invention with relatively small
pores in the separating layer, i.e., preferably for membranes that
have a nominal pore of 0.2 .mu.m. Thereby, the nominal pore is
defined via the retention properties of the membrane as regards
specific microorganisms. For instance, a membrane with a nominal
pore of 0.2 .mu.m, for example, retains bacteria of the genus
Brevundimonas diminuta, a membrane with a nominal pore of 0.45
.mu.m retains bacteria of the genus Serratia marcescens, etc. Other
common nominal pore sizes are 0.1 .mu.m, 0.6 .mu.m and 1.2 .mu.m.
The testing or the determination of the nominal pore sizes is
described, for example, in the HIMA Regulation, No. 3, Vol. 4, 1982
(Health Industry Manufacturers Association).
[0045] The membranes according to the invention in the form of flat
sheets, i.e. the flat membranes according to the invention are
suitable in particular for microfiltration. Membranes of this type
have, as a general rule, diameters of the maximum separating pores
of 0.01-10 .mu.m, preferably of 0.1-5 .mu.m and more preferably of
0.2-2 .mu.m. Preferably, the flat membrane according to the
invention has a thickness of 10-300 .mu.m, more preferably of
30-150 .mu.m.
[0046] The invention will now be described in more detail by way of
the following examples and figures, whereby the scope of the
invention is not limited by the examples.
[0047] The drawings show in:
[0048] FIG. 1: a scanning electron microscope (SEM) image of the
cross-section of the membrane according to Example 1, magnified 600
times.
[0049] FIG. 2: a SEM image of a section of the cross-section of the
membrane according to Example 1 in the region of the membrane side
that during production faced towards the casting roller (roller
side) with the layer of minimal pore size, magnified 2700
times.
[0050] FIG. 3: a SEM image of the surface of the membrane according
to Example 1 that during production faced towards the casting
roller (roller side), magnified 500 times.
[0051] FIG. 4: a SEM image of the surface of the membrane according
to Example 1 that during production faced away from the casting
roller (air side), magnified 2000 times.
[0052] FIG. 5: a SEM image of the cross-section of the membrane
according to Example 2, magnified 600 times.
[0053] FIG. 6: a SEM image of a section of the cross-section of the
membrane according to Example 2 in the region of the membrane side
that during production faced towards the casting roller (roller
side) with the layer of minimal pore size, magnified 2700
times.
[0054] FIG. 7: a SEM image of the surface of the membrane according
to Example 2 that during production faced towards the casting
roller (roller side), magnified 500 times.
[0055] FIG. 8: a SEM image of the surface of the membrane according
to Example 2 that during production faced away from the casting
roller (air side), magnified 2000 times.
[0056] FIG. 9: a SEM image of the cross-section of the membrane
according to Example 3, magnified 600 times.
[0057] FIG. 10: a SEM image of a section of the cross-section of
the membrane according to Example 3 in the region of the membrane
side that during production faced towards the casting roller
(roller side) with the layer of minimal pore size, magnified 2700
times.
[0058] FIG. 11: a SEM image of the surface of the membrane
according to Example 3 that during production faced towards the
casting roller (roller side), magnified 500 times.
[0059] FIG. 12: a SEM image of the surface of the membrane
according to Example 3 that during production faced away from the
casting roller (air side), magnified 2000 times.
[0060] In the examples, the following methods for characterizing
the membranes were applied:
Determination of the Volume Porosity:
[0061] Four samples of approx. 15 cm.sup.2 of the membrane to be
examined are weighed out and kept in approx. 50 ml of water for 16
hours. Subsequently, the samples are removed from the water and
excess water is removed by means of blotting paper. The samples
pre-treated in this way are weighed to determine the wet weight and
then dried for 16 hours at 50.degree. C. After cooling, the weight
of the dried samples (dry weight) is determined.
[0062] The volume porosity is determined from the average value of
the uptake of water (wet weight minus dry weight), with respect to
the average value of the dry weight of the samples, using the
densities for water and for the polymer forming the membrane
structure (hydrophobic first polymer).
Transmembrane Flow (Water Permeability):
[0063] From the membrane to be tested, disc-shaped membrane samples
with a diameter of 15 cm are cut out and clamped in a suitable
specimen holder fluid-tight at the perimeter, so that a free
measuring area of 43.20 cm.sup.2 results. The specimen holder is
located in a housing that can be passed through by water under
pressure. Deionized water maintained at 25.degree. C. and at a
defined pressure from 0.4-1.0 bar is then passed through the
clamped membrane sample from the side on which the membrane
separating layer is located. During a measuring period of 60
seconds, the water volume passed through the membrane sample is
gravimetrically or volumetrically determined.
[0064] The transmembrane flow TMF is determined according to the
formula (VI)
T M F [ I m 2 h bar ] = V W .DELTA. t A M .DELTA. p 600 ( VI )
##EQU00001##
where
[0065] V.sub.W=the water volume [ml] passed through the membrane
sample during the measuring period
[0066] .DELTA.t=the measuring period [min]
[0067] A.sub.M=the area of the membrane sample (43.20 cm.sup.2)
that was passed through
[0068] .DELTA.p=the pressure set during the measurement [bar]
Determination of the Dirt-Loading Capacity:
[0069] The dirt-loading capacity is determined via the pore
blocking behavior of the membrane during the liquor passage of the
membrane by a test medium based on soluble instant coffee
powder.
[0070] As the test medium, a solution of 200 mg soluble instant
coffee powder in 5 l deionized water is prepared in a pressure
vessel equipped with a stirrer and by means of the stirrer is
maintained homogeneously during the measurement. A membrane sample
with a diameter of 50 mm, cut out of the membrane to be examined,
is clamped in a filter holder so that the test medium during
testing flows against the side facing away from the separating
layer, i.e. the more open-pored side, which represents the inflow
side. The effective filter area is 9.6 cm.sup.2. The test medium is
fed at a constant pressure of 0.4 bar from the pressure vessel
through the membrane for a period of 10 minutes. The volume of the
test medium passed through is recorded over time and from the data,
the transmembrane flow TMF.sub.PM of the test medium is determined
analogous to determining the transmembrane flow for water over
time. From the change of the transmembrane flow TMF.sub.PM over
time, i.e. from the ratio of the TMF.sub.PM after the measuring
period of 10 minutes to the TMF.sub.PM at the beginning of the
measurement, a statement about the pore blocking behavior of the
membrane and thereby about its dirt-loading capacity can be
derived. The membrane shows thereby a high dirt-loading capacity,
if the transmembrane flow TMF.sub.PM of the test medium over time
changes only slightly, which can be ascribed to the fact that the
membrane is not appreciably blocked.
Determination of the Filtrate Flow Rate of an Aqueous Bovine Serum
Albumin (BSA) Solution
[0071] The filtrate flow rate of an aqueous BSA solution is
determined using a cross-flow laboratory apparatus with
recirculating feed stream. From the membrane to be examined, two
samples of approx. 5.3 cm.sup.2 each are tested in parallel. First
the membrane samples are equilibrated in a phosphate buffer
solution (pH 5, 67 mM) and then placed in the test cell. Initially,
the flow rate for the phosphate buffer solution is measured at 0.4
bar for 60 minutes or until stable values are obtained. Afterwards,
the reservoir for the feed stream is filled with a BSA solution (2
g/l, in phosphate buffer, pH 5) and the filtrate flow rate J.sub.MF
[l/(hm.sup.2)] through the membrane samples is continuously
measured at 0.4 bar for 120 minutes.
EXAMPLE 1
[0072] In a heatable boiler, 55.31 kg of a mixture of 75 wt. % of
.gamma.-butyrolactone and 25 wt. % of c-caprolactam, conditioned to
40.degree. C., was provided and, with stirring, 1.05 kg of
sulfonated polyethersulfone (SPES) with a degree of sulfonation of
5%, was dissolved within 1 hour. Subsequently, 13.95 kg
polyethersulfone (PES, Ultrason E6020, manufactured by BASF) was
sprinkled in with stirring and dissolved over 4 hours. Afterwards,
11.25 kg polyvinylpyrrolidone (PVP, K30, manufactured by ISP) was
finely dispersed, stirred in and homogenized. The oxygen was
largely removed from the boiler by creation of a vacuum and the
application of nitrogen. Following this, the boiler was heated to
95.degree. C. and a homogeneous solution was produced over 8 hours
with intensive stirring. After cooling the solution to 80.degree.
C., 18.44 kg polyethylene glycol PEG 200 was slowly added,
intensively stirred in and homogenized for 3 hours. Afterwards, the
casting solution was cooled to 40.degree. C. and degassed by means
of a vacuum. The resulting homogeneous solution had a viscosity of
3.6 Pa s at 40.degree. C.
[0073] The finished casting solution was poured out by means of a
casting mold conditioned to 40.degree. C. onto a metal casting
roller conditioned to 62.degree. C. to form a film with a thickness
of approx. 160 .mu.m. The film located on the casting roller was
conveyed through a climate-controlled zone and during approx. 11
seconds it was exposed to a climate of 44.degree. C. and 48%
relative humidity before it was introduced into a coagulation bath
of water conditioned to 62.degree. C. After a retention time of 11
seconds for the formation of the membrane structure, the film was
withdrawn by means of a drawing-off roller at a speed increased by
9% in comparison to the casting roller speed, whereby the film or
the membrane structure was drawn in order to open the surface
pores. In the subsequent wash baths, the membrane was fixed in
water at temperatures increasing stepwise to 90.degree. C. and the
solvent together with the greater portion of the PVP was extracted.
The drying of the membrane occurred by means of a drum dryer.
Within the wash and drying areas, there was a further speed
increase of approx. 5%.
[0074] The membrane thus produced was permanently hydrophilic and
spontaneously wettable with water and had a maximum separating pore
of 0.48 .mu.m determined by means of the bubble point method as
well as a nominal pore of 0.2 .mu.m. It had a transmembrane flow of
approx. 27 000 l/(m.sup.2hbar) as well as a porosity of 83 vol. %.
The filtrate flow rate for an aqueous BSA solution was determined
15 minutes after the beginning of the filtration to be 7 400 l/h
m.sup.2. The filtrate flow rates for the BSA solution after 5
minutes and after 120 minutes were 8 200 l/hm.sup.2 and 4 500
l/hm.sup.2 respectively, so that a residual filtrate flow rate of
55% resulted. The membrane had a high dirt-loading capacity. In the
test, it demonstrated a relatively constant transmembrane flow
TMF.sub.PM for the test medium used over time; after 10 minutes the
TMF.sub.PM had dropped by only 20%.
[0075] Over its cross-section, the membrane had a structure with an
interior separating layer, i.e. the layer with the minimal pore
size lay within the membrane wall at a distance of approx. 10 .mu.m
from the side of the membrane, which side during production
initially faced towards the air (FIG. 1, 2). According to the SEM
image shown in FIG. 1, the pore size initially increased, in the
direction from the layer with the minimal pore size to the roller
side, in an asymmetrical region, and remained then practically
unchanged over an region of approx. 1/4 of the wall thickness in an
essentially isotropic region. Shortly prior to reaching the
adjoining membrane surface, the pore size decreased towards the
roller surface. The roller side (FIG. 3) as well as the air side of
the membrane (FIG. 4) had an open-pored structure. The average pore
size of the pores in the roller side of the membrane was
significantly above 5 .mu.m.
EXAMPLE 2
[0076] The procedure was the same as in Example 1, except that, in
the climate-controlled zone a temperature of 44.degree. C. and a
relative humidity of 62% were set.
[0077] The membrane thus obtained had a maximum separating pore of
0.65 .mu.m, determined by means of the bubble point method, and a
nominal pore of 0.45 .mu.m. It had a transmembrane flow of approx.
54 000 l/(m.sup.2hbar) and showed a high permeability to BSA
solutions with, at the same time, a minor drop in permeability over
time. The membrane had likewise a high dirt-loading capacity. It
showed in the test over time a relatively constant transmembrane
flow TMF.sub.PM; after 10 minutes, the TMF.sub.PM had only dropped
to approx. 75% of the initial value.
[0078] Likewise, the membrane had, over its cross-section, the
structure according to the invention with an interior separating
layer (FIG. 5). The pore size initially increased, in the direction
from the layer with the minimal pore size towards the side of the
membrane that faced towards the casting roller during the membrane
production, i.e. towards the roller side in an asymmetrical region,
and remained then essentially unchanged over an region of approx.
1/3 of the wall thickness. Shortly prior to reaching the adjoining
membrane surface, the pore size decreased towards the surface. The
layer with the minimal pore size, i.e. the separating layer was
located within the membrane wall at a distance of approx. 10-15
.mu.m from the adjoining surface, i.e. from the air side of the
membrane (FIG. 6). Roller side (FIG. 7) and air side of the
membrane (FIG. 8) had a pronounced open-pored structure.
EXAMPLE 3
[0079] The procedure was the same as in Example 1. Unlike Example
1, the casting roller and the coagulation bath were set to a
temperature of 70.degree. C. In the climate-controlled zone, a
temperature of 44.degree. C. and a relative humidity of 69%
prevailed. The drawing-off roller in the coagulation bath had a
speed increased by 10% in comparison to the casting roller
speed.
[0080] The membrane thus obtained had a maximum separating pore of
0.87 .mu.m, determined by means of the bubble point method, and a
nominal pore of 0.60 .mu.m. It had a transmembrane flow of approx.
102 000 l/(m.sup.2hbar) and showed a high permeability in regards
to BSA solutions with, at the same time, a minor drop in
permeability over time. The membrane had a high dirt-loading
capacity. In the test, it showed a relatively constant
transmembrane flow TMF.sub.PM for the test medium used over time;
the TMF.sub.PM was still at a high level after one hour.
[0081] The structure of the membrane is apparent from the SEM
images shown in FIG. 9-12. According to this, the membrane likewise
had, over its cross-section, an integrally asymmetrical pore
structure according to the invention with an interior separating
layer (FIG. 9). Similarly, for this membrane, the pore size
initially increased, in the direction from the separating layer
towards the roller side, then remained essentially constant from
approximately the center of the wall, and finally decreased towards
the roller side of the membrane. The layer with the minimal pore
size, i.e. the separating layer was located within the membrane
wall at a distance of approx. 15 .mu.m from the adjoining surface,
i.e. from the air side of the membrane (FIG. 10). The roller side
(FIG. 11) was open-pored with large pores in the surface. The air
side of the membrane (FIG. 12) likewise showed an open-pored
structure with a more uniform size of the pores.
EXAMPLE 4
[0082] In a heatable boiler, 55.31 kg of a mixture of 75 wt. % of
.gamma.-butyrolactone and 25 wt. % of .epsilon.-caprolactam,
conditioned to 40.degree. C., was provided and, with stirring, 15
kg of polyethersulfone (PES, Ultrason E6020, manufactured by BASF)
was sprinkled in with stirring and dissolved over 4 hours.
Subsequently, 11.25 kg polyvinylpyrrolidone (PVP, K30, manufactured
by ISP) was finely dispersed, stirred in and homogenized. The
oxygen was largely removed from the boiler by creation of a vacuum
and the application of nitrogen. Following this, the boiler was
heated to 95.degree. C. and a homogeneous solution was produced
over 8 hours with intensive stirring. After cooling the solution to
80.degree. C., 18.44 kg polyethylene glycol PEG 200 was slowly
added, intensively stirred in and homogenized for 3 hours. Finally,
the casting solution was cooled to 40.degree. C. and degassed by
means of a vacuum.
[0083] The finished casting solution was poured out by means of a
casting mold conditioned to 30.degree. C. onto a metal casting
roller conditioned to 66.degree. C. to form a film with a thickness
of approx. 160 .mu.m. The film located on the casting roller was
conveyed through a climate-controlled zone and during approx. 11
seconds it was exposed to a climate of 42.degree. C. and 51%
relative humidity before it was introduced into a coagulation bath
of water conditioned to 66.degree. C. After formation of the
membrane structure, the film was withdrawn by means of a
drawing-off roller at a speed increased by 6% in comparison to the
casting roller speed, whereby the film or the membrane structure
was drawn in order to open the surface pores. In the subsequent
wash baths, the membrane was fixed in water at temperatures
increasing incrementally to 90.degree. C. and the solvent together
with the greater portion of the PVP was extracted. The drying of
the membrane occurred by means of a drum dryer at approx.
60-80.degree. C. Within the wash and drying areas, there was a
further speed increase of approx. 9%.
[0084] The maximum separating pore of the membrane thus obtained
had a size of 0.47 .mu.m, determined by means of the bubble point
method. The nominal pore was 0.20 .mu.m in size. The membrane had a
transmembrane flow of approx. 27 400 l/(m.sup.2hbar).
[0085] The membrane had, as evidenced by the SEM images, an
asymmetric pore structure with a separating layer situated within
the wall. The structure passed, in the direction from the
separating layer towards the surface that had been facing towards
the casting roller during the production of the membrane (roller
side), into a pronounced coarse-pored structure. In immediate
proximity to the surface, the pore structure re-compacted somewhat,
whereby a good mechanical stability of the surface layer as well as
of the coarse-pored layer lying beneath it was guaranteed. The
structure became likewise coarse-pored towards the other surface
(air side); however to a clearly lesser degree than towards the
roller side.
EXAMPLE 5
[0086] In a heatable boiler, a mixture of 46.45 kg of
.gamma.-butyrolactone and 15.49 kg of .epsilon.-caprolactam,
conditioned to 40.degree. C., was provided and, with stirring,
initially 1.05 kg of sulfonated polyethersulfone (SPES) with a
degree of sulfonation of 5%, and subsequently, 13.95 kg
polyethersulfone (PES, Ultrason E6020, manufactured by BASF) was
sprinkled in with stirring and dissolved over 4 hours. Afterwards,
3.5 kg of the high-molecular PVP-K90 (manufactured by ISP) was
finely dispersed, stirred in and homogenized. The oxygen was
largely removed from the boiler by creation of a vacuum and the
application of nitrogen. Following this, the boiler was heated to
95.degree. C. and a homogeneous solution was produced over 8 hours
with intensive stirring. After cooling the solution to 80.degree.
C., 19.56 kg polyethylene glycol PEG 200 was slowly added,
intensively stirred in and homogenized for 3 hours. Finally, the
casting solution was cooled to 40.degree. C. and degassed by means
of vacuum.
[0087] The finished casting solution was poured out by means of a
casting mold conditioned to 30.degree. C. onto a metal casting
roller conditioned to 66.degree. C. and having a speed of 3.0 m/min
to form a film with a thickness of approx. 160 .mu.m. The film
located on the casting roller was conveyed through a
climate-controlled zone and during approx. 11 seconds it was
exposed to a climate of 41.degree. C. and 47% relative humidity
before it was introduced into a coagulation bath of water
conditioned to 66.degree. C. After formation of the membrane
structure, the film was withdrawn by means of a drawing-off roller
at a speed increased by 6% in comparison to the casting roller
speed, whereby the film or the membrane structure was drawn in
order to open the surface pores. In the subsequent wash baths, the
membrane was fixed in water at temperatures increasing
incrementally to 90.degree. C. and the solvent together with the
greater portion of the PVP was extracted. The drying of the
membrane occurred by means of a drum dryer at approx. 60-80.degree.
C. Within the wash and drying areas, there was a further speed
increase of approx. 9%.
[0088] The maximum separating pore of the membrane thus obtained
had a size of 0.55 .mu.m, determined by means of the bubble point
method. The membrane had a transmembrane flow of approx. 45 000
l/(m.sup.2hbar) and showed a high permeability to BSA
solutions.
EXAMPLE 6
[0089] In a heatable boiler, 22.645 kg N-methylpyrrolidone (NMP)
was provided at 40.degree. C. and, with stirring, first 7.5 kg of
polyethersulfone (PES, Ultrason E6020, manufactured by BASF) and
then 2.063 kg of the high-molecular PVP-K90 (manufactured by ISP)
were finely dispersed, stirred in and homogenized. The oxygen was
largely removed from the boiler by creation of a vacuum and the
application of nitrogen. Following this, the boiler was heated to
90.degree. C. and a homogeneous solution was produced over 6 hours
with intensive stirring. After cooling the solution to 60.degree.
C., a mixture of 1.618 kg water and 16.175 kg polyethylene glycol
PEG 200 was slowly added, intensively stirred in and homogenized
for 3 hours. Finally, the casting solution was cooled to 40.degree.
C. and degassed by means of vacuum.
[0090] The finished casting solution was poured out by means of a
casting mold conditioned to 40.degree. C. at a production speed of
6.0 m/min onto a metal casting roller conditioned to 60.degree. C.
to form a film with a thickness of approx. 160 .mu.m. The film
located on the casting roller was conveyed through a
climate-controlled zone and for approx. 11 seconds it was exposed
to a climate of 43.degree. C. and 57% relative humidity before it
was introduced into a coagulation bath of water conditioned to
60.degree. C. In the subsequent wash baths, the membrane was fixed
in water at temperatures increasing incrementally to 90.degree. C.
and the solvent together with the greater portion of the PVP was
extracted. The drying of the membrane occurred by means of a drum
dryer at approx. 60-80.degree. C. Within the wash and drying areas,
there was a further speed increase of approx. 25%.
[0091] The maximum separating pore of the membrane thus obtained
had a size of 0.56 .mu.m, determined by means of the bubble point
method. The transmembrane flow was approx. 36 000
l/(m.sup.2hbar).
* * * * *