Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to a process for producing dense
membranes and to (supported) dense membranes so produced.
It is known to produce dense membranes by means of solvent
casting which involves forming a solution of a polymer
comprising a surface active agent and casting it onto a liquid
support to produce a thin layer which is subsequently dried ~by
evaporation of the solvent present in the po:Lymer solution) to
forn a solid, dense membrane. The applied solvent is, however,
generally substantially i~miscible with the liquid support in
order to avoid a reduction of the surface tension of the liquid
support which could lead to instability OI the developing
membrane and possible generation of holes therein.
It would be advantageous to be able to use a solvent which
is substantially soluble in the liquid support without simul-
taneous generation oE undesired holes despite cubstantially
reduced interfacial tension, thus shortening the membrane
solidification time substantially because not all solvent would
have to be removed from the membrane for~ing layer by means of
evaporatlon.
Surprisingly, it has now been found that dense (that is
non-porous) membranes can be produced, starcing from a solution
comprising a polar polymer and/or a polar prepolymer in a polar
solvent which is substantially soluble in the polar liquid used
as support.
Accordingly, the present invention provides a process for
producing dense membranes, wherein a solution comprising a polar
polymer and/or a polar prepolymer in a polar solvent which is
substantially soluble in a polar liquid is allowed to ~pread out
over the surface of the polar liquid! and polar solvent in the
spread out solution is alla~ed to dissolve in the ~olar lialuid.
In ~rticular, the polar liquld in the sp~e~ out solution is
allo~e~ to desolvate.
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It appears that a very 10W - or even completely absent -
inter~acial tension between the polar liquid support and the
polar (pre)polymer solution allows the solution to spread out
spontaneously over the surface of the liquid suppor~ without ~he
ne~d ~or a surface active agent, such as a dispersant, in ~he
solution. Thus, lt ls now possible to produce membranes com-
prising said polymer in the absence of a surface active agent
and e~en to use a liquid support in which a small amount of the
sol~ent used is already present before the solution which
comprises the solvent is spread out. A previous requirement for
continuous refreshing of tpart of) the liquid support (resulting
in a disturbed surface of the suppor~) i5 thereby eliminated.
Advantageously a solution of a polar prepolymer ls used in
the present process, pre~erably in combination with a liquid
support which effects cross-linking of the spread-out pre-
polymer, which makes i~ possible to produce selective, ultra-
thin den~e membranes for application ln molecular separation
processes. Suitable prepolymers comprise, in addition to carbon-
and hydrogen atoms, nitrogen- and/or oxygen atoms, in particular
~0 in the form of ether~bridges. Such prepolymers can be obtained
by reaction of a polyol, such as a polyether polyol and/or a
polyamine, and/or a polyether amine with an Isocyanate
comprising at 1 ast two functional groups such as diphenyl
methane diisocyanate or toluene diisocyanate. Preferred
prepol~mers are obtained by rea~tion of a polyether glycol with
diphenyl methane diisocyanate; ~he polyether glycol suitably has
a molecular weight of from 150-6000, preferably of from
40~-2000.
Instead of a prepolymer it is also possible ~o use a
homopolymer or copolymer which is reasonably soluble in a
suitable polar solvent; in the present process a solution of
a linear polyurethane can suitably be used.
The polar solvent may suitably be selected from organlc
co~pounds with from 1-10 carbon atoms and one or more hetero
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atoms which have at least a good solubility in the polar liquid
support. Suitable organic compounds include ketones, of which
methyl ethyl ketone is preferred because it possesses excellent
desolva~ion (i.e. dissolution and evaporation) properties, in
particular when used in combination with water as support
liquid. It is also possible to use a polar solvent which
additionally comprises a non polar or less polar compound in
order to match the degree of polarity of the (pre)polymer which
is to be dissolved therein.
Preferred polar liquids which are used as support in the
process according to the present invention are water and dilute
aqueous solutions of salts which are most preferably substanti~lly
free of particulates which might adversely affect the formation
of dense membranes. However, other polar liquids, s~ch as
glycerine may also be used.
The process according to the invention is suitably carried
out at room temperature. Ele~ated temperatures (e.g. of from
30-80 ~C) are sometimes preferred in order to decrease the
membrane solidification time; in other cases temperatures below
room temperature are preferred; this can be attained by
maintaining the liquid support at the des~rad temperature.
~ he (pre)polymer solution may be deposited continuously or
batch~wise on the surface of the polar liquid support by known
means, such as a pipette which is held close to the support
surface in order not to disturb this surface. Once the (pre)-
polymer solution has spread out spontaneously over the support
surface and a sufficiently thin liquid film has been formed,
this film is allowed to solldify and to form a solid dense
membrane. Before, during or, preferably, after the desolvation
of the membrane film, the film is recovered from the liquid
support surface by any suitable means. Preferably, the thus
formed dense membrane according to the present invention is
taken up on a pe~meable support which may comprise a layer of
any suitable material, such as porous polypropylene, cloth and
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wire net. Porous polypropylene is pre~erred in view of the
high porosity of this material. Alternatively, at least one
layer can be applled between a selective, dense membrane film
and the permeable support; this intermediate layer may itself be
a dense~ preferably hlghly permeable, film prepared according to
the invention.
With the process according to the invention thin, hole-free
membranes can be obtained with a high selectivity and an
acceptable throughput (permeability) in molecular separation
processes, such as gas purification. The thic~ness of such a
m~mbrane should preferably be less than about 0.1 ~m in order to
attain sufficient permeability, which is required for commercial
application in processes such as the separatlon of carbon
dioxide from methane or the separation of oxygen from nitrogen.
In some cases it is possible to increase the permeability and/or
the selectivity of dense membranes prepared according to the
present invention by coating one surface of such membranes with
a layer of a polar compound. Preferably such a layer comprises a
polyether glycol and/or a polyether amlne. In particular, dense
polyu~ethane membranes obtained from prepolymers of polyether
glycol and multiisocyanate (a mixture of isocyanates comprlsing
two and more reactive groups), coated with a layer of a
polyether glycol with a lower molPcular weight than the
polyether glycol used in the preparation of the prepolymer, show
an increased permeability, compared with similar polyurethane
membranes which ars not coated with a layer of a polar compound.
The invention is further illustrated by the following
Examples.
EE~PLE I
~0 Preparation of dense (supported) membranes.
A dense polyureehane membrane A with a thickness of 0.05 ~m
was prepared by allowing a prepolymer solution in methyl ethyl
ke~one obtained by reaction of polyether glycol with a molecular
wsight of 40Q with diphsnyl methane diisocyanate to spread out
o~er water spontaneously in the absencs of a spreading a8ent.
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After spreading and desolvation the dense polyurethane membrane
A chus obtained was transferred on to a highly permeable poly-
dimethyl siloxane layer (supported by porous polypropylene)
prepared ac ording to C~Yx~anFab~ ~ppliG~inn S.N. 4~,701, A.van
~er Sdr~etal, f~cd ~ y l2, 19~4. ~he n~t~nkt~ Æ~d ~y~o~d
d~EenE~bK~e B w~s b~das ~ xd.un ~ple 3.
Supported dense membrane C was prepared in substantially
the same manner as membrane B, except that polyether glycol with
a molecular weight of 2000 was used to prepare the polyurethane
prepolymer.
EXAMPLE 2
Supported dense membrane D was prepared by coating the free
side of the polyurethane layer of membrane C with polyether
glycol having a molecular weight of 400.
EXAMPLE 3
Permeability and selectivity measurements.
The supported dense membranes B, C and 1) were tested at a
gas pressure of 500 kPa (= 5 bar abs.) on one side of the
membrane and atmospheric permeate pressure on the other side ot
the membrane. The permeability tor C0~ and CH4 of the membranes
was measured; the results of these measurements are given in the
normal form of P/l-values (Nm3.m .day .bar 1) in the following
Table, in which also the selectivity for a gas mixture based on
equal volumes of C0~ and ~4, i.e. the ratio of the permeability
for C0~ and the permeabllity for CH4, is given.
TABLE
Experiment Membrane ~/1 for C02 P/l for CH4 Selectivity
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1 B 0.3 0.01 30
C 4 U.15 ~ ~7
3 D ~ _ 0.22 27
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~'rom the results given ln the Table it is clear that
membranes with an excellent selectivity for the separation of
C2 from a gas mixture of CO~ and CH4 can be prepared with the
process according to the present in~ention.
l~ was found that the use o~ polyether glycol with a higher
molecular weight (~()OU for membrane C, compared with 400 for
membrane B) in preparing the polyurethane prepolymer starting
material prcvided a membrane (C) which showed a substantially
increased permeability for CO2, wlth only slightly lower
selectivity than was measured for supported membrane B.
By coating ~he free side of the polyurethane layer of
supported membrane C wlth polyether glycol with a molecular
weight of 400 the P/l-value ~or C02 was further increased
without loss of selectivity (see the results o~ Experiment 3 for
supported membrane D).
EXAMPLE 4
The selectivity of supported dense membrane B was further-
more measured for a gas mixture of 2 and N2 in a si~ilar manner
as in Experiments 1-3 described in Example 3. The selectivity,
expressed as the ratio of tne permeability for 2 and the
: permeabllity for N2, was found to be 9,8 which is an excellent
value for supported polymeric membranes.
