CA1239260A - Process for producing dense membranes and (supported) dense membranes so produced - Google Patents
Process for producing dense membranes and (supported) dense membranes so producedInfo
- Publication number
- CA1239260A CA1239260A CA000465809A CA465809A CA1239260A CA 1239260 A CA1239260 A CA 1239260A CA 000465809 A CA000465809 A CA 000465809A CA 465809 A CA465809 A CA 465809A CA 1239260 A CA1239260 A CA 1239260A
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- CA
- Canada
- Prior art keywords
- process according
- polar
- solution
- membranes
- dense
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- Separation Using Semi-Permeable Membranes (AREA)
Abstract
A B S T R A C T
PROCESS FOR PRODUCING DENSE MEMBRANES AND
(SUPPORTED) DENSE MEMBRANES SO PRODUCED
Process for producing dense membranes by allowing a solution comprising a polar polymer and/or a polar prepolymer in a polar solvent which is substantially soluble in a polar liquid to spread out over the surface of the polar liquid and allowing the spread out solution to desolvate. The dense membranes can be applied m supported membranes for use in gas separation processes.
PROCESS FOR PRODUCING DENSE MEMBRANES AND
(SUPPORTED) DENSE MEMBRANES SO PRODUCED
Process for producing dense membranes by allowing a solution comprising a polar polymer and/or a polar prepolymer in a polar solvent which is substantially soluble in a polar liquid to spread out over the surface of the polar liquid and allowing the spread out solution to desolvate. The dense membranes can be applied m supported membranes for use in gas separation processes.
Description
3Z66~
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.
~;
1~
- ~2~9~
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 ' ~ .!
~235~
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 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.
:: ~
' ~23~2~
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.
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.
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 _ . . _. _ 1 B 0.3 0.01 30 C 4 U.15 ~ ~7 3 D ~ _ 0.22 27 : :: ___ :~
.~ , ~;~3~
~'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).
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.
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.
~;
1~
- ~2~9~
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 ' ~ .!
~235~
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 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.
:: ~
' ~23~2~
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.
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.
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 _ . . _. _ 1 B 0.3 0.01 30 C 4 U.15 ~ ~7 3 D ~ _ 0.22 27 : :: ___ :~
.~ , ~;~3~
~'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).
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.
Claims (16)
1. 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 spread out over the surface of the polar liquid, polar solvent present in the spread out solution is allowed to dissolve in the polar liquid.
2. A process according to claim 1, wherein said solution is a solution of said prepolymer.
3. A process according to claim 2, wherein the pre-polymer is obtained by reaction of a polyol with an iso-cyanate comprising at least two functional groups.
4. A process according to claim 3, wherein the iso-cyanate is diphenyl methane diisocyanate.
5. A process according to claim 3, wherein the polyol is a polyether polyol.
6. A process according to claim 5, wherein said polyol is a polyether glycol.
7. A process according to claim 6, wherein the poly-ether glycol has a molecular weight of from 150-6000.
8. A process according to claim 7, wherein said molecular weight is from 400-2000.
9. A process according to claim 1, wherein said solution is a solution of a linear polyurethane.
10. A process according to claim 1, wherein the polar solvent comprises an organic compound with 1-10 carbon atoms and one or more hetero atoms.
11. A process according to claim 10, wherein the organic compound is a ketone.
12. A process according to claim 11, wherein the ketone is methyl ethyl ketone.
13. A process according to claim 1, 2 or 9, wherein the polar liquid is water.
14. A process according to claim 1, 2 or 9, wherein the polar liquid is an aqueous solution of a salt.
15. A process according to claim 1, wherein one surface of the membrane formed is coated with a layer of a polar compound.
16. A process according to claim 15, wherein the layer comprises a polyether glycol and/or a polyether amine.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000465809A CA1239260A (en) | 1984-10-18 | 1984-10-18 | Process for producing dense membranes and (supported) dense membranes so produced |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000465809A CA1239260A (en) | 1984-10-18 | 1984-10-18 | Process for producing dense membranes and (supported) dense membranes so produced |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1239260A true CA1239260A (en) | 1988-07-19 |
Family
ID=4128949
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000465809A Expired CA1239260A (en) | 1984-10-18 | 1984-10-18 | Process for producing dense membranes and (supported) dense membranes so produced |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1239260A (en) |
-
1984
- 1984-10-18 CA CA000465809A patent/CA1239260A/en not_active Expired
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