Note: Descriptions are shown in the official language in which they were submitted.
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Method for Producing an Integral-Asymmetric Hollow-Fibre Polymer Mem-
brane Consisting of an Amphiphilic Block Copolmyer, The Hollow-Fibre
Membrane obtained and the Use thereof
Description
The invention relates to a method for producing a self-supporting integral-
asymmetric hollow-fibre polymer membrane having an isoporous outer
skin, a porous inner skin and a sponge-like inner structure in a dry/wet
spinning method. The invention further relates to an integral-asymmetric
hollow-fibre polymer membrane, a filtration module and a use.
The invention thus relates to the field of membrane-support microfiltration,
ultrafiltration and nanofiltration. Such membranes have a porous separa-
tion layer, wherein the size and regularity of the pores determine both the
separation limit as well as the selectivity. The most isoporous possible
membrane is hereby desired, i.e. a membrane with a most regular possi-
ble structure of pores, the diameter of which is as uniform as possible.
Corresponding membranes are producible in different manners. Mem-
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' branes produced according to a so-called phase inversion process are
predominantly used. These membranes have a more or less large statisti-
cal variance during the distribution of the pore size. In addition to a low
selectivity, such membranes also tend to so-called "fouling". This is a fast
blocking of the large pores, since a larger portion of the liquid passing
through the membrane first passes through these large pores. The fouling
is also considerably reduced for isoporous membranes.
In German patent no. 10 2006 045 282 by the applicant, which has been
published as DE 10 2006 045 282 Al, a method is disclosed by means of
which polymer membranes can be produced with isoporous separation-
active surfaces. For this purpose, an amphiphilic block copolymer is dis-
solved in a casting solution with one or more solvents, spread into a film,
and the film is immersed in a precipitation bath.
This method exploits the fact that the polymer blocks of the amphiphilic
block copolymer are not miscible with each other. The block copolymers
therefore form microphases in the casting solution such as a known mi-
celle structure with spherical or cylindrical micelles. Within a short evapo-
ration time, part of the liquid solvent close to the surface evaporates such
that the microphase morphology hardens in a layer of the film close to the
surface that has formed due to the self-organization of the polymer blocks
of the block copolymers, whereas the block copolymers remain dissolved
within the bulk of the casting solution.
By dipping this film in a precipitation bath, the rest of the solvent is dis-
placed, and a known phase inversion process occurs which results in a
known sponge-like structure. In some cases, the previously assumed mi-
crophase-separated isoporous structure of the layer close to the surface is
retained despite being dipped in the precipitation bath. This then transi-
tions directly into the sponge-like structure. Additional descriptions are
contained in DE 10 2006 045 282 Al, the entire disclosed content of which
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is incorporated in the present application.
An overview of other different methods for producing isoporous mem-
branes is also included in DE 10 2006 045 282 Al of the applicant.
However, up until now, this concept of flat membranes has not been suc-
cessfully transferred to hollow-fibre membranes. Due to its considerably
larger ratio of separation surface to packing volume in one module, hollow-
fibre membranes are very interesting for the economical application, in
particular for applications that require large membrane surfaces, such as
drinking water treatment or dialysis.
Up until now, hollow-fibre membranes for the ultrafiltration of aqueous sys-
tems are mainly produced based on polysulfone or respectively polyether-
sulfones or polyether imides, polyvinylidene fluorides (PVDF) or poly-
acrylnitrile (PAN) through phase inversion of corresponding polymer solu-
tions, which are spun as hollow fibres and precipitated in water. These
membranes show no isoporous structure and for this reason have a great-
er tendency for fouling.
An overview of new developments in the production of polymeric hollow
fibres is provided in the article Na Peng et al., "Evolution of polymeric hol-
low fibers as sustainable technologies: Past, present, and future", Pro-
gress in Polymer Science 37 (2012), 1401 ¨ 1424. It describes the tech-
nique of spinning hollow fibres, in which a spinning solution with at least
one polymer is directed through an annular spinneret, wherein another
liquid is directed concentrically on the inside through the spinneret and
penetrates the hollow space of the hollow fibre formed in this manner. De-
pending on the selection of the polymers and of the solutions, which act
from outside or respectively inside on the hollow fibres, mainly impermea-
ble surfaces are achieved.
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Based on this state of the art, the object of the present invention is to
make available hollow-fibre membranes with an isoporous pore structure
in a separation-active surface, which are usable for microfiltration, ultrafil-
tration or nanofiltration.
The object is solved by a method for producing a self-supporting integral-
asymmetric hollow-fibre polymer membrane having an isoporous outer
skin, a porous inner skin and a sponge-like inner structure in a dry/wet
spinning method with the following method steps:
¨ Production of a polymer solution having at least one solvent, in
which
at least one amphiphilic block copolymer having at least two different
polymer blocks is dissolved,
- Pressing the polymer solution through a spinneret formed as a hol-
low-core nozzle or multiple hollow-core nozzle to form a hollow fibre,
in the centre of which a liquid column is spun, which consists of a
precipitant having reduced precipitation activity,
- After passing through a fall section in an atmosphere, immersing the
spun hollow fibre in a precipitation bath to form the hollow-fibre poly-
mer membrane.
The method according to the invention corresponds with a dry/wet spin-
fling method insofar, as after the extruding the hollow fibre passes through
a fall section without external contacting of a liquid precipitation bath
first
("dry") and is then immersed into a bath of a liquid precipitant ("wet").
The execution of the method according to the invention leads to the pro-
duction of polymer hollow-fibre membranes with an isoporous outer sur-
face, which is based on the self-organization of the tailored block copoly-
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mers in microphases of the so-called microphase separation. This isopo-
rous separation-active outer surface transitions into a typical sponge-like
structure of a membrane produced through precipitant-induced phase in-
version. This sponge-like structures gives the hollow-fibre membrane its
stability and permeability so that the hollow-fibre membrane is self-
supporting or respectively self-carrying. The passing of a fall section in an
atmosphere, i.e. a dry part of the process at first, thereby leads to part of
the solvent of the polymer solution evaporating and a microphase-
separated structure being able to solidify at the outer surface, which sur-
vives the immersion into the precipitation bath after passing through the
fall section.
According to the invention, a precipitant with reduced precipitation activity
is used for the inner surface of the hollow-fibre membrane. Tests have
shown that a conventional precipitant such as water leads to the formation
of a mainly impermeable or respectively almost impermeable inner skin.
The inner skin formed in this manner is then so impermeable that it still
holds onto particles, which can pass through the separation-active outer
skin. With such a liquid-impermeable inner skin, the hollow-fibre mem-
brane designed in this manner is generally unsuitable for filtration. The use
of a precipitant with reduced precipitation activity causes the surface for-
mation to progress with lower kinetics than for a normally used precipitant.
The reaction thereby has enough time for a partial separation on the inner
surface so that pores can form inside the surface so that use for filtration
is
enabled.
The liquid column preferably consists of a mixture of at least one precipi-
tant for the at least one block copolymer and at least one solvent mixable
with the precipitant for the at least one block copolymer. Due to the fact
that the solvent or the solvents is or respectively are mixable with the pre-
cipitant in the liquid column, the precipitation activity of the precipitant
is
reduced considerably. The separation of the copolymer from the polymer
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solution thereby leads to the desired pore formation on the inner boundary
surface. The liquid column can preferably also contain at least one pore
forming material, in particular polyethylene glycol (PEG). This can support
the pore formation on the inner surface of the hollow fibre.
Additionally, at least one metal salt is preferably dissolved in the polymer
solution, wherein one of the polymer blocks forms complexes with the
metal, wherein the metal is preferably a main group element of the second
main group in particular magnesium, calcium or strontium, wherein the salt
is in particular advantageously magnesium acetate or another organic salt
of magnesium, calcium or strontium. The metals of the second main group
are more biocompatible than transition metals so that they are preferred
for hollow-fibre membranes with biological applications. Magnesium and
calcium in particular occur in comparatively large amounts in the human
body. Strontium also occurs in small amounts in the body and is not toxic.
The supported effect of the salt in the phase separation is attributed to the
fact that the added salt leads to the formation of partially charged polyelec-
trolytic micelle cores, which positively impact the non-solvent-induced
phase separation.
Alternatively or additional, it is advantageously provided that at least one
carbohydrate is additionally dissolved in the polymer solution, in particular
saccharose, D(+)-glucose, D(-)-fructose and/or cyclodextrin, in particular
a-cyclodextrin. These substances are more biocompatible than the transi-
tion metals and their salts. For use in the method according to the inven-
tion, the carbohydrates show a clear stabilization of the isoporous separa-
tion-active surface during the phase inversion through immersion in a pre-
cipitation bath.
The supporting effect of the carbohydrates in the phase separation is at-
tributed to the fact that the carbohydrates can form hydrogen bonds to the
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hydrophilic block of the block copolymers. Through the hydrogen bonds,
the viscosity of the polymer solution is greatly increased so that a lower
concentration of the block copolymers in the solution is sufficient to form
the structure according to the invention with the isoporous separation-
active layer.
In the case of both types of additives, i.e. metal salts or carbohydrates, a
moderate, thermo-reversible linking of a block type in the polymer solution,
in particular of the pore-forming component, thus takes place for example
through hydrogen bonds or complex formation.
The problem of a permanently belated release of toxic metal ions through
the membrane during its use is thus no longer relevant through the use of
carbohydrates for improving the membrane structure. Since carbohydrates
are non-toxic, the use of the membrane for medically or respectively bio-
logically relevant processes is thus harmless.
Due to the increased viscosity, it is also hereby possible to work with lower
block copolymer concentrations in the polymer solution, which leads to
material savings for the relatively expensive block copolymers. The clean-
ing of the membrane produced with carbohydrates is unproblematic.
Both the membranes produced with metal salts and those produced with
carbohydrates show adjustable pore sizes in a few cases. Thus, the water
flow through the membrane can be adjusted in a broad range via a change
in the pH value of a solution flowing through the pores. The control over
the pH value works when the pore-forming polymer block reacts to chang-
es in the pH value, for example expands or contracts and thus narrows or
widens the pores.
The at least one block copolymer preferably comprises two or more differ-
ent polymer blocks A, B or A, B and C or A, B, C and D of the configura-
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tion A-B, A-B-A, A-B-C, A-B-C-B-A, A-B-C-D, A-B-C-D-C-B-A or multiblock
copolymers based on the aforementioned configurations, wherein the pol-
ymer blocks are respectively selected from the group of polystyrene, poly-
4-vinylpyridine, poly-2-vinylpyridine,
polybutadiene, polyisoprene,
poly(ethylene-stat-butylene), poly(ethylene-alt-propylene), polysiloxane,
polyalkylene oxide, poly-e-caprolactone, polylactic acid, poly alkyl methac-
rylate, polymethacrylic acid, polyalkyl acrylate, polyacrylic acid, polyhy-
droxyethyl methacrylate, polyacrylamide, poly-N-alkylacrylamide, polysul-
fone, polyaniline, polypyrrole, polytriazole, polyvinylimidazole, polyte-
trazole, polyethylene diamine, polyvinyl alcohol, polyvinylpyrrolidone, poly-
oxadiazole, polyvinylsulfonic acid, polyvinyl phosphonic acid or polymers.
Multiblock copolymers comprise structures of the base configurations that
repeat multiple times. Thus, a multiblock copolymer based on the configu-
ration A-B has a structure A-B-A-B etc. or a multiblock copolymer based
on the configuration A-B-C the structure A-B-C-A-B-C etc. The multiblock
copolymers also form the microphases underlying the invention.
The block copolymers and the polymer blocks preferably have a low poly-
dispersity, in particular less than 1.5, in particular less than 1.2. The self-
organization of the block copolymers and the microphase formation is thus
supported.
The polymer lengths of the at least two polymer blocks of the amphiphilic
block copolymers are preferably selected relative with respect to each oth-
er so that a self-organization in the solvent leads to the formation of a
spherical, cylindrical or co-continuous, in particular gyroidal, micelle struc-
ture or microphase structure in the solvent, in particular a length ratio be-
tween approximately 2:1 and approximately 10:1, in particular between
approx. 3:1 and 6:1. These length ratios of the majority components to the
minority components of the block copolymers lead to the desired micelle
structure, i.e. to the inclusion of individual spherical micelles of the
minority
components in the bulk of the majority components or to cylindrical or co-
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continuous, for example gyroidal, micelle structures, in which the minority
components form the cylinders or respectively gyroidal filaments or re-
spectively branchings in the bulk of the majority components.
The block copolymer preferably has a molecular weight between 100 kDa
and 600 kDa, in particular between 170 kDa and 320 kDa. In this range,
the pore size can be adjusted in a particular fine manner through selection
of the molecular weight.
The polymer preferably makes up a percentage by weight between 10 wt.-
% and 40 wt.-%, in particular between 15 M.-% and 25 wt.-%, of the solu-
tion.
In the dry/wet spinning process, the polymer solution is particularly prefer-
ably pressed through the spinneret with an excess press of 0.01 to 0.5
bar, in particular between 0.05 and 0.25 bar. A very regular hollow-fibre
membrane can thus be produced, wherein the required excess pressure
depends among other things on the viscosity of the polymer solution. The
geometry, i.e. in particular radius, gap width and shape, of the spinneret
also plays a role, so that for a selected system the excess pressure or re-
spectively the throughput can be determined securely and simply through
series of tests.
The hollow-core nozzle or multiple hollow-core nozzle to be used accord-
ing to the invention has in the simplest case an annular-slit-shaped, outer
nozzle opening, which is arranged in particular concentrically around a
central nozzle opening. Other hollow-fibre cross-sections can also be cre-
ated by suitable geometries of the hollow-core nozzle or multiple hollow-
core nozzle, for example elliptical, polygonal, star-shaped with 3 or more
rays among other things. With triple- or multi-layered multiple hollow-core
nozzles, it is also possible to co-extrude for example carrier and mem-
brane and, if applicable, even other layers. Such carrier layers are to be
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arranged on the outside and/or inside of the layer according to the inven-
tion.
The fall section preferably has a length of between 1 cm and 100 cm, in
5 particular between 10 cm and 100 cm. Depending on the set excess pres-
sure or respectively throughput and of the set spinning rate, this means
that there is advantageously an evaporation time of between 10 seconds
and 60 seconds before immersion in the precipitation bath. The fall section
can also be determined securely and optimal through a series of tests.
Several solvents are preferably used, wherein the polymer blocks of the
block copolymers in the different solvents are soluble to different degrees
and the solvents are volatile to different degrees. The use of different sol-
vents, which represent solvents of differing quality in particular for the dif-
ferent blocks of the block copolymers, supports the solidification of the
self-organization and microphase formation on the outer surface of the
hollow fibre before immersion in the precipitation bath.
Respectively a polar or non-polar precipitant or precipitant mixture is ad-
vantageously used as precipitant in the liquid column and in the precipita-
tion bath after the fall section independently of each other, in particular
water and/or methanol and/or ethanol and/or acetone and/or a mixture of
two or more of the precipitants or diethyl ether, and respectively a polar or
non-polar solvent, in particular dimethylformamide and/or dimethylacetam-
ide and/or N-methylpyrrolidone and/or dimethylsulfoxide and/or tetrahydro-
furane or a mixture of two or more of the solvents, are used as solvent in
the polymer solution and in the liquid column independently of each other,
wherein the solvent(s) in the liquid column is or respectively are mixable
with the precipitant(s) in the liquid column.
The precipitants or respectively solvents used in the respective precipitant
or precipitant mixture and the respective solvent or solvent mixture are
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also respectively intermixable so that the respectively used precipitant mix-
ture or respectively solvent mixture is single-phase, i.e. homogeneous.
Diethyl ether is an example of a water-insoluble precipitant. For some pol-
ymers, non-aqueous solvents and precipitants are more suitable than
aqueous solvents and precipitants.
The object underlying the invention is also solved by an integral-
asymmetric hollow-fibre polymer membrane, in particular micro-, ultra- or
nanofiltration membrane, produced or producible in an, in particular as
described above according to the invention, dry/wet spinning process,
wherein the hollow-fibre polymer membrane has an isoporous, separation-
active outer surface, which is based on a microphase formation through
self-organization of an amphiphilic copolymer, which transitions into a non-
solvent-induced, phase-inversion-formed, sponge-like inner structure,
wherein an inner surface of the hollow-fibre polymer membrane is formed
porously based on a precipitation with an inner liquid column from a pre-
cipitant with reduced precipitation activity.
The pores of the separation-active outer surface preferably have a ratio of
the maximum pore diameter to the minimum pore diameter of less than 3.
The object underlying the invention is also solved by a filtration module, in
particular microfiltration module, ultrafiltration module or nanofiltration
module, with at least one of the hollow-fibre polymer membranes de-
scribed above according to the invention, as well as by a use of a corre-
sponding integral-asymmetric hollow-fibre polymer membrane according to
the invention for microfiltration, ultrafiltration or nanofiltration, in
particular
for filtering out proteins or other nanoparticulate substances.
The advantages, properties and characteristics named for the individual
invention objects, i.e. the method, the integral-asymmetric hollow-fibre pol-
ymer membrane, the filtration module and the use, also apply without re-
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striction to the respective other invention objects, which relate to each oth-
er.
Further characteristics of the invention will become apparent from the de-
scription of embodiments according to the invention together with the
claims and the included drawings.
The invention is described below in an exemplary manner, without restrict-
ing the general intent of the invention, based on exemplary embodiments
in reference to drawings and figures, wherein we expressly refer to the
drawings with regard to the disclosure of all details according to the inven-
tion that are not explained in greater detail in the text. The figures shown
in:
Fig. 1 a schematic representation of a hollow-fibre-membrane
spinning system,
Fig. 2a), b) SEM images of cross-sections through a hollow-fibre poly-
mer membrane produced according to the invention in two
different enlargement stages,
Fig. 3a), b) SEM images of the outer surface of a hollow-fibre polymer
membrane produced according to the invention in two differ-
ent enlargement stages,
Fig. 4a), b) SEM images of the inner surface of a hollow-fibre polymer
membrane produced according to the invention in two differ-
ent enlargement stages and
Fig. 5a)-c) SEM images of the cross-section, outer surface and inner
surface of a further hollow-fibre polymer membrane pro-
duced according to the invention.
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It will be illustrated below using examples how the hollow-fibre membranes
according to the invention are produced as completely self-supporting in-
tegral-asymmetric membranes.
For this, Fig. 1 shows a spinning system with a spinneret 2 designed as a
hollow-core nozzle, which has a central opening and an annular-slit-
shaped nozzle or respectively annular nozzle arranged around it concen-
trically. Through the annular nozzle, a polymer solution with at least one
amphiphilic block copolymer exits, which is kept ready in a polymer solu-
tion container 4. The polymer solution is pressed into the spinneret 2 by
means of a gas under excess pressure P, in this case nitrogene (N2),
which is supplied to a pressure connection 6 on the container 4. Instead of
an overpressurized gas, a pump can alternatively be used, which controls
the throughput of the polymer solution through the spinneret 2.
Through the central opening or respectively the spinneret 2, a liquid pre-
cipitant solution exits a precipitant container 8. A central liquid column is
thereby spun. The precipitant introduced in the central opening of the spun
hollow fibre 12 is introduced through a pump 10.
A hollow fibre 12 thus exits the spinneret 2, the inner opening or respec-
tively lumen of which is filled with a liquid column of the precipitant from
the container 8, which brings about a precipitation of the polymer from in-
side. The hollow fibre 12 immerses after a fall section 13 into a precipita-
tion bath 14 with a further precipitant, for example water. A liquid-induced
phase-inversion process takes place there and the hollow-fibre polymer
membrane 16 according to the invention is formed from the hollow fibre
12.
Process parameters herein are among other things the selection of the
spinneret 2 or respectively annular-slit nozzle, the polymer solution, the
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liquid column, the excess pressure P or respectively the throughput of pol-
ymer solution, the fall section 13, the precipitant in the precipitation bath
14, the different concentrations or the local shear rate in the spinneret gap.
The parameters are selected such that a mechanically stable hollow fibre
12 is created as soon as it leaves the spinneret 2 in the direction of the
precipitation bath 14 and a formation of a impermeable skin on the inside
of the hollow fibre 12 is avoided.
Several processes take place thermodynamically in quick succession. A
slowed liquid-induced phase inversion process without a dry phase first
takes place on the inside. Irregular and large pores hereby occur due to
the reduced precipitation activity of the inner precipitant. The formation of
the cylindrical pore structures on the outside takes place between the
spinneret 2 and the precipitation bath 14 through evaporation of a solvent.
Since the hollow fibre 12 is spun downwards, gravity also impacts the -
structure formation. After the immersion of the hollow fibre 12 into the pre-
cipitation bath 14, a second liquid-induced phase inversion process occurs
on the outside surface of the hollow fibre 12, which leads to the final for-
mation of the hollow-fibre polymer membrane 16.
The scanning electron microscope (SEM) images in Figures 2 to 5 show a
relatively narrow pore size distribution on the outer skin, which is caused
by the microphase separation and the solvent-precipitant exchange. In
contrast, the inner skin shows elliptical pore structures, which were creat-
ed due to the slowed phase inversion on the inside of the hollow fibre 12
and aligned parallel to the spinning direction due to the shearing in the
gap. In certain cases, considerable hollow spaces occur here, which cre-
ate a porosity, which is needed for the filtration.
It was found that pure water is not suitable as a core liquid. In contrast,
water mixtures with the solvents of the block copolymer solution are suita-
ble for a corresponding coarsely porous inner skin.
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Examples according to the invention are described below.
Production of a Polymer Solution
In order to produce a polymer solution with different concentrations and
solvent mixtures, a polystyrene-block-poly(4-vinylpyridine) block copoly-
mer (PS-b-P4VP) was solved in a mixture of N,N-Dimethylformamide
(DMF) and tetrahydrofuran (THF) at room temperature for one day under
constant stirring with a magnetic stirrer, until the polymer solution became
homogeneous.
Before the spinning of the hollow fibre, the polymer solution was stored at
room temperature without stirring for 12 hours in order to remove the
shearing effect of the stirring and fine air bubbles, which were trapped in
the solution.
A copolymer PS-b-P4VP with a total molecular weight of 154 kg/mol (kDa)
was used as the polymer, whereby 82.7 wt.-% was polystyrol and 17.3 wt.-
Yo were P4VP. The solution contained 27 wt.-% polymer and 73 wt.-% sol-
vent DMF and THF in a ratio of 60:40 wt.-% DMF:THF.
Spinning of the Integral-Asymmetric Hollow-Fibre Membrane
PS-b-P4VP hollow-fibre membranes were spun in a dry/wet spinning
method at different fall heights. The fall heights were 30, 50, 70 and 90
cm. The experimental system is shown in Fig. 1.
The spinneret has a structure with an individual opening in the opening in
a concentric arrangement with diameter of 0.212 mm for the inner liquid
column. The inner diameter of the annular nozzle is 0.353 mm and the
outer diameter 0.702 mm; the nozzle length is 1 mm.
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The polymer solution was pressed out through the opening of the spinner-
et or respectively annular nozzle using nitrogen under excess pressure.
The extrusion rate of the polymer solution was set by the pressure of the
nitrogen gas between 0.05 and 1 bar. The polymer solution flowed through
the annular nozzle or respectively hollow-core nozzle while, as liquid col-
umn, a mixture of water and different water-soluble solvents as internal
precipitant was directed through the inner opening of the spinneret under
controlled flow rates.
The created hollow fibres came from the tip of the spinning nozzle and
passed through a fall section of a certain length before they immersed into
a precipitation bath with pure water.
The variation in the fall section between the spinneret and the precipitation
bath served to let the solvent from the polymer solution evaporate in the
outer area of the hollow-fibre membrane. The hollow-fibre membranes
then reached the precipitation bath with their fall speed, without being
supplied externally with a tractive force.
Both the internal and the external precipitants were held at room tempera-
ture.
The hollow-fibre polymer membranes were then held in the water contain-
er for 24 hours at room temperature in order to remove residual solvent
(DMF) and then dried in a vacuum oven at 60 Celsius for more than 24
hours.
Parameters of some test arrangements are shown in the following table:
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Fall section 90 cm
Flow of the col-
0.5 ml/min
umns
Pressure P
0.2 0.1 0.05 0.2 0.1
(bar)
Liquid Water/ Water/ Water/ Water/mix Water/mix
column DMF THF THE (DMF/THF) (DMF/THF)
Mix 80/20 60/40
80/20 80/20 80/20
ratio (60/40) (60/40)
In these cases, a hollow-fibre membrane according to the invention with
isoporous outer skin and porous inner skin was obtained. Examples of this
are shown in the following.
SEM Images
The following figures show electron-microscope images of results from two
experiments. They concern the two experiments in which the liquid col-
umns had a mixture of water and THF at a weight ratio of 80 wt-% to 20
wt.-%. The fall section was 90 cm, the flow rate of the liquid column 0.5
ml/min.
It is thereby shown that a lower shear rate in the spinneret leads to a more
regular structure of the pores on the outer surface of the hollow-fibre
membrane.
Fig. 2a) and 2b) show the cross-section through the hollow-fibre mem-
brane for an excess pressure of 0.1 bar in two different enlargements. In
particular, the sponge-like structure inside the hollow-fibre membrane can
be clearly seen.
Fig. 3a) and 3b) show the outer surface of the same membrane as in Fig.
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2 in two different enlargement stages. A pore structure with pores having a
very uniform size results in particular for the large enlargement. This struc-
ture is come across over a large area of the outer surface.
Fig. 4a) and 4b) show in different enlargement stages images of the inner
surface of the hollow-fibre membrane from Fig. 2 and 3, wherein the irreg-
ular pore structure from the shearing in the spinneret is shown elongated
as in the spinning direction. This irregular pore structure has no separa-
tion-active effect, but is necessary so that the corresponding membrane
can be used at all for filtration.
Fig. 5 shows the cross-section, the outer surface and the inner surface in
respectively larger scales finally for a case, in which a hollow-fibre mem-
brane was extruded with an excess pressure of 0.05 bar, wherein the
structures of a mainly isoporous outer surface and a porous inner surface
according to the invention as well as a sponge-like structure occur in be-
tween.
All named characteristics, including those taken from the drawings alone
and also individual characteristics, which are disclosed in combination with
other characteristics, are considered alone and in combination as essential
for the invention. Embodiments according to the invention can be realized
by individual characteristics, or a combination of several characteristics.
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Reference Numbers
2 Spinneret
4 Container for polymer solution
6 Pressure connection
8 Container for inner precipitant solution
10 Pump
12 Hollow fibre
13 Fall section
14 Precipitation bath
16 Hollow-fibre polymer membrane
