U.S. patent number RE37,656 [Application Number 09/325,135] was granted by the patent office on 2002-04-16 for electrode apparatus containing an integral composite membrane.
This patent grant is currently assigned to W.L. Gore & Associates, Inc.. Invention is credited to Bamdad Bahar, Alex R. Hobson, Jeffrey A. Kolde, Robert S. Mallouk.
United States Patent |
RE37,656 |
Bahar , et al. |
April 16, 2002 |
**Please see images for:
( Certificate of Correction ) ** |
Electrode apparatus containing an integral composite membrane
Abstract
A composite membrane is provided which includes a base material
and an ion exchange resin. The base material has a microstructure
characterized by nodes interconnected by fibrils, or a
microstructure characterized by fibrils with no nodes present. The
ion exchange resin substantially impregnates the membrane such that
the membrane is essentially air impermeable.
Inventors: |
Bahar; Bamdad (Elkton, MD),
Mallouk; Robert S. (Chadds Ford, PA), Hobson; Alex R.
(Elkton, MD), Kolde; Jeffrey A. (Baltimore, MD) |
Assignee: |
W.L. Gore & Associates,
Inc. (Newark, DE)
|
Family
ID: |
23601323 |
Appl.
No.: |
09/325,135 |
Filed: |
June 3, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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404853 |
Mar 15, 1995 |
5547551 |
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Reissue of: |
567466 |
Dec 5, 1995 |
05635041 |
Jun 3, 1997 |
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Current U.S.
Class: |
204/282; 204/296;
429/494; 429/516; 429/530 |
Current CPC
Class: |
H01M
8/106 (20130101); C08J 5/225 (20130101); H01M
8/1039 (20130101); B01D 69/141 (20130101); H01M
8/1044 (20130101); H01M 8/1062 (20130101); H01M
8/1023 (20130101); C25B 13/08 (20130101); B01D
67/0088 (20130101); Y02E 60/50 (20130101); C08J
2327/18 (20130101) |
Current International
Class: |
C08J
5/20 (20060101); C08J 5/22 (20060101); B01D
67/00 (20060101); C25B 13/00 (20060101); B01D
69/14 (20060101); B01D 69/00 (20060101); C25B
13/08 (20060101); H01M 8/10 (20060101); C25B
013/00 () |
Field of
Search: |
;204/296,282,252
;429/33,41,42 ;427/115
;428/220,308.4,422,305.5,306.6,315.5,311.51,315.7,315.9,316.6 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2091166 |
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Jan 1982 |
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GB |
|
51-71888 |
|
Jun 1976 |
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JP |
|
62-240627 |
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Sep 1987 |
|
JP |
|
1-194927 |
|
Aug 1989 |
|
JP |
|
6-29032 |
|
Feb 1994 |
|
JP |
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WO 91/14021 |
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Sep 1991 |
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WO |
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95/16730 |
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Jun 1995 |
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WO |
|
Other References
"Ion Transporting Composite Membranes," Liu, et al., J.
Electrochem. Soc., vol. 137, No. 2, Feb. 1990 The Electrochemical
Society, Inc., pp. 510-515. .
"Ion Transporting Composite Membranes", Penner, et al., Journal
Electrochem Soc., vol. 132, No. 2, Feb. 1985, pp. 514-515. .
"Composite Membranes for Fuel-Cell Applications," Verbrugge, et
al., AIChE Journal, Jan. 1992, vol. 38, No. 1, pp. 93-100..
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Primary Examiner: Bell; Bruce F.
Attorney, Agent or Firm: Morgan & Finnegan, L.L.P.
Parent Case Text
This is a divisional of application Ser. No. 08/404,853, filed on
Mar. 15, 1995, now U.S. Pat. No. 5,547,551.
Claims
Having described the invention, what is claimed is:
1. A.Iadd.n .Iaddend.electrode apparatus adapted for use in an
electrochemical system, the electrode apparatus comprising
.Iadd.composite membrane comprising.Iaddend.:
(a) .[.an.]. .Iadd.at least one .Iaddend.expanded
polytetrafluoroethylene membrane having a porous microstructure of
polymeric fibrils .Iadd.and having a thickness of 80 microns or
less.Iaddend.; and
(b) .[.an.]. .Iadd.at least one .Iaddend.ion exchange material
impregnated throughout the .Iadd.porous microstructure of the
expanded polytetrafluoroethylene .Iaddend.membrane .Iadd.so as to
render an interior volume of the expanded polytetrafluoroethylene
membrane substantially occlusive.Iaddend., the impregnated expanded
polytetrafluoroethylene membrane having a Gurley number of greater
than 10,000 seconds.[., wherein the ion exchange material
substantially impregnates the membrane so as to render an interior
volume of the membrane substantially occlusive.]. .
2. The electrode apparatus of claim 1, wherein the .Iadd.expanded
polytetrafluoroethylene .Iaddend.membrane .[.is expanded
polytetrafluoroethylene having.]. .Iadd.has .Iaddend.a
microstructure defined by nodes interconnected by fibrils.
3. The electrode apparatus of claim 1, wherein the total thickness
of the .Iadd.expanded polytetrafluoroethylene .Iaddend.membrane is
less than 13 .[..mu.m.]. .Iadd.microns.Iaddend..
4. The electrode apparatus of claim 1, wherein the electrochemical
system is a fuel cell.
5. The electrode apparatus of claim 1, wherein the electrochemical
system is an electrodialysis system..Iadd.
6. The electrode apparatus of claim 1, wherein the thickness of the
expanded polytetrafluoroethylene membrane is 60 microns or
less..Iaddend..Iadd.
7. The electrode apparatus of claim 1, wherein the thickness of the
expanded polytetrafluoroethylene membrane is 40 microns or
less..Iaddend..Iadd.
8. The electrode apparatus of claim 1, wherein the thickness of the
expanded polytetrafluoroethylene membrane is 20 microns or
less..Iaddend..Iadd.
9. The electrode apparatus of claim 1, wherein the thickness of the
expanded polytetrafluoroethylene membrane is at least 1.5
microns..Iaddend..Iadd.
10. The electrode apparatus of claim 1, wherein the at least one
ion exchange material is complimented by powder, non-ionic polymer,
or a combination thereof..Iaddend..Iadd.
11. The electrode apparatus of claim 10, wherein the at least one
ion exchange material is complimented by silica..Iaddend..Iadd.
12. The electrode apparatus of claim 10, wherein the at least one
ion exchange material is complimented by
platinum..Iaddend..Iadd.
13. An electrode apparatus adapted for use in an electrochemical
system, the electrode apparatus comprising a composite membrane
consisting essentially of:
(a) at least one expanded polytetrafluoroethylene membrane having a
porous microstructure of polymeric fibrils and having a thickness
of 80 microns or less; and
(b) at least one ion exchange material impregnated throughout the
porous microstructure of the expanded polytetrafluoroethylene
membrane so as to render an interior volume of the expanded
polytetrafluoroethylene membrane substantially occlusive, the
impregnated expanded polytetrafluoroethylene membrane having a
Gurley number of greater than 10,000 seconds..Iaddend..Iadd.
14. The electrode apparatus of claim 13, wherein the expanded
polytetrafluoroethylene membrane has a microstructure defined by
nodes interconnected by fibrils..Iaddend..Iadd.
15. The electrode apparatus of claim 13, wherein the thickness of
the expanded polytetrafluoroethylene membrane is less than 25
microns..Iaddend..Iadd.
16. The electrode apparatus of claim 13, wherein the
electrochemical system is a fuel cell..Iaddend..Iadd.
17. The electrode apparatus of claim 13, wherein the
electrochemical system is an electrodialysis
system..Iaddend..Iadd.
18. The electrode apparatus of claim 13, wherein the at least one
ion exchange material is complimented by silica..Iaddend..Iadd.
19. The electrode apparatus of claim 13, wherein the at least one
ion exchange material is complimented by
platinum..Iaddend..Iadd.
20. An electrode apparatus comprising:
(a) at least one porous polymeric membrane having a microstructure
of micropores with a porosity of greater than 35% and a thickness
of at most 25 microns; and
(b) at least one perfluoro ion exchange material impregnated within
the micropores of the polymeric membrane so as to render them
substantially occlusive..Iaddend..Iadd.
21. The electrode apparatus according to claim 20, wherein the
thickness is at most 20 microns..Iaddend..Iadd.
22. The electrode apparatus according to claim 20, wherein the
porosity is greater than 70%..Iaddend..Iadd.
23. An electrode apparatus according to claim 20, wherein the
perfluoro ion exchange material is locked inside the membrane which
prevents the material from migrating to the surface during drying
of the impregnated perfluoro ion exchange
material..Iaddend..Iadd.
24. An electrode apparatus according to claim 20, wherein the
thickness is at most 6 microns..Iaddend..Iadd.
25. An electrode apparatus comprising:
an ultra-thin composite membrane comprising:
(a) at least oneporous polymeric membrane having a microstructure
of micropores with a porosity of greater than 35% and a thickness
of at most 25 microns; and
(b) at least one perfluoro ion exchange material fully impregnated
within the micropores of the polymeric membrane so as to render
them fully occlusive, wherein the composite membrane is air
impermeable with a Gurley number of greater than 10,000
seconds..Iaddend..Iadd.
26. An electrode apparatus according to claim 25, wherein the
thickness is at most 20 microns..Iaddend..Iadd.
27. An electrode apparatus according to claim 25, wherein the
thickness is at most 6 microns..Iaddend..Iadd.
28. An electrode apparatus according to claim 25, wherein the
porosity is greater than 70%..Iaddend..Iadd.
29. An electrode apparatus according to claim 25, wherein the
composite membrane is heated at 60.degree. C. to 200.degree.
C..Iaddend..Iadd.
30. A fuel cell comprising a composite membrane located between an
anode and a cathode, the composite membrane comprising:
(a) at least one porous polymeric membrane having a microstructure
of micropores with a porosity of greater than 35% and a thickness
of at most 25 microns; and
(b) at least one perfluoro ion exchange material impregnated within
the micropores of the polymeric membrane so as to render them
substantially occlusive..Iaddend..Iadd.
31. A fuel cell according to claim 30, wherein the porosity is
greater than 70%..Iaddend..Iadd.
32. A fuel cell according to claim 30, wherein the composite
membrane is heated at 60.degree. C. to 200.degree.
C..Iaddend..Iadd.
33. A fuel cell according to claim 30, wherein the composite
membrane is heated so as to lock the perfluoro ion exchange
material inside the membrane..Iaddend..Iadd.
34. A fuel cell according to claim 30, wherein the thickness is at
most 20 microns..Iaddend..Iadd.
35. A fuel cell according to claim 30, wherein the thickness is at
most 13 microns..Iaddend..Iadd.
36. A fuel cell comprising a composite membrane located between an
anode and a cathode, the composite membrane comprising:
(a) at least oneporous polymeric membrane having a microstructure
of micropores with a porosity of greater than 35% and a thickness
of at most 25 microns; and
(b) at least one perfluoro ion exchange material fully impregnated
within the micropores of the polymeric membrane, wherein the
composite membrane is air impermeable with a Gurley number of
greater than 10,000 seconds..Iaddend..Iadd.
37. A fuel cell according to claim 36, wherein the porosity is
greater than 70%..Iaddend..Iadd.
38. A fuel cell according to claim 36, wherein the ion exchange
material is locked within the composite
membrane..Iaddend..Iadd.
39. A fuel cell according to claim 37, wherein the ion exchange
material is locked within the composite
membrane..Iaddend..Iadd.
40. A fuel cell according to claim 36, wherein the thickness is at
most 20 microns..Iaddend..Iadd.
41. An electrode apparatus comprising:
(a) at least one expanded polytetrafluoroethylene membrane having a
porous microstructure of polymeric fibrils and a total thickness of
less than 20 microns; and
(b) at least one ion exchange material impregnated throughout the
membrane, the impregnated expanded polytetrafluoroethylene membrane
having a Gurley number of greater than 10,000 seconds, wherein the
ion exchange material substantially impregnates the membrane to
render an interior volume of the membrane substantially
occlusive..Iaddend..Iadd.
42. A electrode apparatus according to claim 41, wherein the
porosity of the expanded polytetrafluoroethylene membrane is
greater than 70%..Iaddend..Iadd.
43. A electrode apparatus according to claim 41, wherein the
perfluoro ion exchange material is locked inside the
membrane..Iaddend..Iadd.
44. A electrode apparatus according to claim 41, wherein the
impregnated membrane is heated at 120.degree. C. to 160.degree.
C..Iaddend..Iadd.
45. A electrode apparatus according to claim 41, wherein the total
thickness is at most 13 microns..Iaddend..Iadd.
46. A fuel cell comprising a composite membrane located between an
anode and a cathode, the composite membrane comprising:
(a) at least one expanded polytetrafluoroethylene membrane having a
porous microstructure of polymeric fibrils and a total thickness of
less than 20 microns; and
(b) at least one ion exchange material impregnated throughout the
membrane, the impregnated expanded polytetrafluoroethylene membrane
having a Gurley number of greater than 10,000 seconds, wherein the
ion exchange material substantially impregnates the membrane to
render an interior volume of the membrane substantially
occlusive..Iaddend..Iadd.
47. A fuel cell according to claim 46, wherein the porosity of the
expanded polytetrafluoroethylene membrane is greater than
70%..Iaddend..Iadd.
48. A fuel cell according to claim 46, wherein the composite
membrane is heated at 60.degree. C. to 200.degree.
C..Iaddend..Iadd.
49. A fuel cell according to claim 46, wherein the composite
membrane is heated at 120.degree. C. to 160.degree.
C..Iaddend..Iadd.
50. A fuel cell according to claim 46, wherein the total thickness
is at most 13 microns..Iaddend..Iadd.
51. An electrode apparatus comprising a laminate of composite
membranes consisting essentially of at least two composite
membranes laminated to each other, wherein the at least two
composite membranes each consist essentially of:
(a) at least one expanded polytetrafluoroethylene membrane having a
porous microstructure of polymeric fibrils and having a thickness
of 80 microns or less; and
(b) at least one ion exchange material impregnated throughout the
porous microstructure of the membrane so as to render an interior
volume of the expanded polytetrafluoroethylene membrane
substantially occlusive, the impregnated membrane having a Gurley
number of greater than 10,000 seconds..Iaddend..Iadd.
52. An electrode apparatus according to claim 51, wherein the
thickness of the laminate is 40 microns or less..Iaddend..Iadd.
53. An electrode apparatus according to claim 51, wherein at least
two of the impregnated membranes are impregnated with ion exchange
material before lamination to form the laminate..Iaddend..Iadd.
54. An electrode apparatus according to claim 51, wherein the
laminate is prepared by the combination of steps comprising:
(i) impregnation of at least one first unimpregnated expanded
polytetrafluoroethylene membrane with ion exchange material to form
a first impregnated membrane,
(ii) lamination of the first impregnated membrane with a second
unimpregnated expanded polytetrafluoroethylene membrane, and
(iii) impregnation of the second unimpregnated expanded
polytetrafluoroethylene to form a second impregnated membrane which
is laminated to the first impregnated membrane..Iaddend..Iadd.
55. An electrode apparatus according to claim 51, wherein
lamination is carried out by heat..Iaddend..Iadd.
56. An electrode apparatus according to claim 51, wherein each of
the impregnated polytetrafluoroethylene membranes have been heated
to a temperature of at least 60.degree. C. and each of the
impregnated membranes have an ionic conductance of at least 22.7
mhos/cm.sup.2 ; wherein the thickness of each of the impregnated
membranes is 20 microns or less; wherein the ion exchange material
is perfluorinated sulfonic acid resin; wherein each of the
impregnated membranes are prepared by multiple impregnations of two
sides of the expanded polytetrafluoroethylene membrane with ion
exchange material..Iaddend..Iadd.
57. An electrode apparatus according to claim 56, wherein the
thickness of the laminate is 40 microns or less..Iaddend..Iadd.
58. A fuel cell comprising a laminate membrane consisting
essentially of at least two composite membranes laminated to each
other, wherein each of the composite membranes are prepared by a
combination of steps consisting essentially of:
providing at least one microporous expanded polytetrafluoroethylene
membrane having a thickness of 80 microns or less;
impregnating the microporous membrane with ion exchange material so
the impregnated membrane has a Gurley number of at least 10,000
seconds;
heating the impregnated membrane to between 60.degree. C. and
200.degree. C..Iaddend..Iadd.
59. A fuel cell according to claim 58, wherein the thickness of the
laminate membrane is 60 microns or less..Iaddend..Iadd.
60. A fuel cell according to claim 58, wherein the thickness of the
laminate membrane is 40 microns or less..Iaddend..Iadd.
61. A fuel cell according to claim 58, wherein the thickness of the
laminate membrane is 20 microns or less..Iaddend..Iadd.
62. A fuel cell according to claim 58, wherein the laminate
membrane has an ionic conductance of at least 8.5
mhos/cm.sup.2..Iaddend..Iadd.
63. A fuel cell according to claim 58, wherein the laminate
membrane has an ionic conductance of at least 22.7
mhos/cm.sup.2..Iaddend..Iadd.
64. A method for preparing an electrode apparatus comprising the
step of:
locating at least one ion exchange membrane between an anode and a
cathode, wherein the ion exchange membrane comprises
(a) at least one porous polymeric membrane having a microstructure
of micropores with a porosity of greater than 35% and a thickness
of at most 25 microns; and
(b) at least one perfluoro ion exchange material impregnated within
the micropores of the polymeric membrane so as to render them
substantially occlusive..Iaddend..Iadd.
65. A method according to claim 64, wherein the thickness is at
most 20 microns..Iaddend..Iadd.
66. A method according to claim 64, wherein the thickness is at
most 13 microns..Iaddend..Iadd.
67. A method according to claim 64, wherein the thickness is at
most 6 microns..Iaddend..Iadd.
68. A method according to claim 64, wherein the electrode apparatus
is a fuel cell..Iaddend..Iadd.
69. A method according to claim 64, wherein the porosity is greater
than 70%..Iaddend..Iadd.
70. A method according to claim 69, wherein the thickness is at
most 6 microns..Iaddend.
Description
FIELD OF THE INVENTION
An electrode apparatus containing an integral composite membrane is
provided which is useful in electrolytic processes and other
chemical separations.
BACKGROUND OF THE INVENTION
Ion exchange membranes (IEM) are used in polymer electrolyte fuel
cells as solid electrolytes. A membrane located between a cathode
and an anode of such a fuel cell, transpods protons formed near the
catalyst at the hydrogen electrode to the oxygen electrode, thereby
allowing a current to be drawn from the fuel cell. These polymer
electrolyte fuel cells are particularly advantageous because they
operate at lower temperatures than other fuel cells. Also, these
polymer electrolyte fuel cells do not contain any corrosive acids
which are found in phosphoric acid fuel cells. In these type fuel
cells, there is a need to eliminate the bulk transfer of reactants
from one electrode to the other, i.e. fluid percolation.
Ion exchange membranes are also used in chloralkali applications to
separate brine mixtures to form chlorine gas and sodium hydroxide.
For best performance, it is preferred that the membrane selectively
transport the sodium ions across the membrane while rejecting the
chloride ions. Also, the ion exchange membrane must eliminate bulk
transfer of electrolytic solution across the membrane, i.e. fluid
percolation.
Additionally, IEMs are useful in the areas of diffusion dialysis,
electrodialysis and in pervaporation and vapor permeation
separations. IEMs may also be used for selective transpod of polar
compounds from mixtures containing both polar and non-polar
compounds.
IEMs must have sufficient strength to be useful in their various
applications. Often, this need for increased strength requires that
an IEM be made relatively thick in cross section, or that the IEM
be reinforced with woven fabrics (macro-reinforcements), both of
which decreases the ionic conductance of the IEM. Additionally,
conventional IEMs exhibit inherent dimensional instability due to
the absorbance of solvents, such as water, for example. Such
dimensional instability renders conventional IEMs substantially
ineffective for many commercial applications.
U.S. Pat. No. 3,692,569 relates to the use of a coating of a
copolymer of fluorinated ethylene and a sulfonyl-containing
fluorinated vinyl monomer on a fluorocarbon polymer that was
previously non-wettable. The fluorocarbon polymer may include
tetrafluoroethylene polymers. This coating provides a topical
treatment to the surface so as to decrease the surface tension of
the fluorocarbon polymer. U.S. Pat. No. 3,692,569 provides for a
fluid percolating structure.
U.S. Pat. No. 4,453,991 relates to a process for making articles
coated with a liquid composition of a perfluorinated polymer,
having sulfonic acid or sulfonate groups in a liquid medium, by
contacting the polymer with a mixture of 25 to 100% by weight of
water and 0 to 75% by weight of a second liquid component, such as
a low molecular weight alcohol, in a closed system. Such a process
provides for a multi-layered structure.
U.S. Pat. No. 4,902,308 relates to a film of porous expanded
polytetrafluoroethylene (PTFE) having its surfaces, both exterior
and internal, coated with a metal salt of perfluoro-cation exchange
polymer. Such a composite product is permeable to air. The air flow
of such a structure, as measured by the Gurley densometer ATSM
D726-58, is about 12 to 22 seconds. Therefore, this structure
provides for fluid percolation.
U.S. Pat. No. 5,082,472 relates to a composite material of a
microporous membrane, such as porous expanded PTFE, in laminar
contact with a continuous ion exchange resin layer, wherein both
layers have similar area dimensions. Surfaces of internal nodes and
fibrils of the expanded PTFE may be coated, at least in part, with
an ion exchange resin coating. The expanded PTFE layer of this
composite membrane imparts mechanical strength to the composite
structure. However, the interior of the expanded PTFE membrane is
untilled so as to not block the flow of fluids. Therefore, U.S.
Pat. No. 5,082,472 provides for fluid percolation.
U.S. Pat. Nos. 5,094,895 and 5,183,545 relate to a composite porous
liquid-permeable article having multiple layers of porous expanded
PTFE, which are bonded together, and which have interior and
exterior surfaces coated with an ion exchange polymer. Such a
composite article is particularly useful as a diaphragm in
electrolytic cells. However, diaphragms are inherently percolating
structures. Japanese Patent Application No. 62-240627 relates to a
coated or an impregnated membrane formed with a perfluoro type ion
exchange resin and a porous PTFE film to form an integral
structure. The resulting composite is not fully occlusive.
Furthermore, the teachings of this application do not provide for
permanent adhesion of the ion exchange resin to the inside surface
of the PTFE film.
There remains a need for a strong, integral composite ion exchange
membrane, having long term chemical and mechanical stability.
SUMMARY OF THE INVENTION
The present invention is an advancement over presently known ion
exchange membranes. In one embodiment of the present invention,
this is accomplished by providing a composite membrane comprising
an expanded polytetrafluoroethylene (PTFE) membrane having a porous
microstructure of polymeric fibrils. The composite membrane is
impregnated with an ion exchange material throughout the membrane.
The impregnated expanded polytetrafluoroethylene membrane has a
Gurley number of greater than 10,000 seconds. The ion exchange
material substantially impregnates the membrane so as to render an
interior volume of the membrane substantially occlusive.
The expanded PTFE membrane may comprise a microstructure of nodes
interconnected by fibrils.
The ion exchange material may be selected from a group consisting
of perfluorinated sulfonic acid resin, perfluorinated carboxylic
acid resin, polyvinyl alcohol, divinyl benzene, styrene-based
polymers, and metal salts with or without a polymer. The ion
exchange material may also be comprised of at least in part a
powder, such as but not limited to, carbon black, graphite, nickel,
silica, titanium dioxide, and platinum black.
A purpose of the present invention is to provide an improved
alternative to the macro-reinforcement of ionomer materials.
Another purpose of the present invention is to provide an ion
exchange membrane having a single integral structure that does not
allow for fluid percolation.
The foregoing and other aspects will become apparent from the
following detailed description of the invention when considered in
conjunction with the accompanying drawing figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross-section of a composite membrane of the
present invention that is fully impregnated with an ion exchange
material.
FIG. 2 is a schematic cross-section of the composite membrane of
the present invention that is fully impregnated with an ion
exchange material and which includes a backing material attached
thereto.
FIG. 3 is a photomicrograph, at a magnification of 2.5 kX, of a
cross-section of an expanded PTFE membrane that has not been
treated with an ion exchange material.
FIG. 4 is a photomicrograph, at a magnification of 5.1 kX, of a
cross-section of an expanded PTFE membrane impregnated with an ion
exchange material, such that the interior volume of the membrane is
substantially occluded.
FIG. 5 is a photomicrograph, at a magnification of 20.0 kx, of a
cross-section of an expanded PTFE membrane, comprised substantially
of fibrils with no nodes present, which has not been treated with
an ion exchange material.
DETAILED DESCRIPTION OF THE INVENTION
As best illustrated by FIG. 1, a composite membrane is provided
which includes a base material 4 and an ion exchange material or
ion exchange resin 2. The base material 4 is a membrane which is
defined by a porous microstructure characterized by nodes
interconnected by fibrils (FIG. 3), or a porous microstructure
characterized substantially by fibrils (FIG. 5). The ion exchange
resin substantially impregnates the membrane so as to render the
interior volume substantially occlusive. The ion exchange resin is
securely adhered to both the external and internal membrane
surfaces, i.e. the fibrils and/or nodes of the base material.
The composite membrane of the present invention may be employed in
various applications, including but not limited to, polarity-based
chemical separations; electrolysis; fuel cells and batteries;
pervaporation; gas separation; dialysis separation; industrial
electrochemistry, such as chloralkali production and other
electrochemical applications; use as a super acid catalyst; or use
as a medium in enzyme immobilization, for example.
The composite membrane of the present invention is uniform and
mechanically strong. As used herein, the term "uniform" is defined
as continuous impregnation with the ion exchange material such that
no pin holes or other discontinuities exist within the composite
structure. The membrane should be "occlusive", meaning that the
interior volume of the porous membrane is impregnated such that the
interior volume is filled with the ion exchange material and the
final membrane is essentially air impermeable having a Gurley
number of greater than 10,000 seconds. A fill of 90% or more of the
interior volume of the membrane should provide adequate occlusion
for purposes of the present invention.
A preferred base material 4 is an expanded polytetrafluoroethylene
(ePTFE) which may be made in accordance with the teachings of U.S.
Pat. No. 3,593,566, incorporated herein by reference. Such a base
material has a porosity of greater than 35%. Preferably, the
porosity is between 70-95%. Preferably the thickness is between
0.06 mils (0.19 .mu.m) and 0.8 mils (0.02 mm), and most preferably
the thickness is between 0.50 mils (0.013 mm) and 0.75 mils (0.019
mm). This material is commercially available in a variety of forms
from W. L. Gore & Associates, Inc., of Elkton, Md., under the
trademark GORE-TEX.RTM., FIG. 3 shows a photomicrograph of the
internal porous microstructure of an embodiment of such an expanded
PTFE membrane. As seen therein, the porous microstructure comprises
nodes interconnected by fibrils which define an interior volume of
the base material 4. Alternatively, the base material 4 may
comprise an ePTFE material having a porous microstructure defined
substantially of fibrils with no nodes present.
To manufacture an ePTFE membrane having a porous microstructure
defined substantially of fibrils with no nodes present, a PTFE that
has a low amorphous content and a degree of crystallization of at
least 98% is used as the raw material. More particularly, a
coagulated dispersion or fine powder PTFE may be employed, such as
but not limited to FLUON.RTM. CD-123 and FLUON.RTM. CD-1 available
from ICI Americas, Inc., or TEFLON.RTM. fine powders available from
E. I. DuPont de Nemours and Co., Inc. (TEFLON is a registered
trademark of E. I. DuPont de Nemours and Co., Inc.) These
coagulated dispersion powders are lubricated with a hydrocarbon
extrusion aid, preferably an odorless mineral spirit, such as
ISOPAR K (made by Exxon Corp.) (ISOPAR is a registered trademark of
the Exxon Corporation). The lubricated powder is compressed into
cylinders and extruded in a ram extruder to form a tape. The tape
is compressed between rolls to an appropriate thickness, usually 5
to 10 mils. The wet tape is stretched traversely to 1.5 to 5 times
its original width. The extrusion aid is driven off with heat. The
dried tape is then expanded longitudinally between banks of rolls
in a space heated to a temperature that is below the polymer
melting point (approximately 327.degree. C.). The longitudinal
expansion is such that the ratio of speed of the second bank of
rolls to the first bank is from about 10-100 to 1. The longitudinal
expansion is repeated at about 1-1.5 to 1 ratio. After the
longitudinal expansion, the tape is expanded traversely, at a
temperature that is less than about 327.degree. C., to at least 1.5
times, and preferably to 6 to 15 times, the width of the original
extrudate, while restraining the membrane from longitudinal
contraction. While still under constraint, the membrane is
preferably heated to above the polymer melting point (approximately
342.degree. C.) and then cooled. This ePTFE membrane is
characterized by the following properties:
(a) average pore size between 0.05 and 0.4 micrometers, and
preferably less than 0.2;
(b) a bubble point between 10 and 60 psi;
(c) a pore size distribution value between 1.05 and 1.20;
(d) a ball burst strength between 0.9 and 17 pounds/force;
(e) an air flow of between 20 Frazier and 10 Gurley seconds;
(f) a thickness between 1.32 .mu.m and 25.4 .mu.m; and
(g) a fiber diameter of between 5 and 20 Nm.
Suitable ion exchange materials 2 include, but are not limited to,
perfluorinated sulfonic acid resin, perfluorinated carboxylic acid
resin, polyvinyl alcohol, divinyl benzene, styrene-based polymers
and metal salts with or without a polymer. A mixture of these ion
exchange materials may also be employed in treating the membrane 4.
Solvents that are suitable for use with the ion exchange material,
include for example, alcohols, carbonates, THF (tetrahydrofuran),
water, and combinations thereof. Optionally, ion exchange materials
may be complemented by finely divided powders or other (non-ionic)
polymers to provide final composites. Such a finely divided powder
may be selected from a wide range of organic and inorganic
compounds such as, but not limited to, carbon black, graphite,
nickel, silica, titanium dioxide, platinum black, for example, to
provide specific added effects such as different aesthetic
appearance (color), electrical conductivity, thermal conductivity,
catalytic effects, or enhanced or reduced reactant transport
properties. Examples of non-ionic polymers include, but are not
limited to, polyolefins, other fluoropolymers such as
polyvinylidene (PVDF), or other thermoplastics and thermoset
resins. Such non-ionic polymers may be added to aid occlusion of
the substrate matrix, or to enhance or reduce reactant transport
properties.
A surfactant having a molecular weight of greater than 100 is
preferably employed with the ion exchange material 2 to ensure
impregnation of the interior volume of the base material 4.
Surfactants or surface active agents having a hydrophobic portion
and a hydrophilic portion may be utilized.
A most preferred surfactant is a nonionic material, octylphenoxy
polyethoxyethanol having a chemical structure: ##STR1##
where x=10 (average),
and is known as Triton X-100, which is commercially available from
Rohm & Haas of Philadelphia, Pa.
As best seen by reference to FIG. 4, the final composite membrane
of the present invention has a uniform thickness free of any
discontinuities or pinholes on the surface. The interior volume of
the membrane is occluded such that the composite membrane is
impermeable to non-polar gases and to bulk flow of liquids.
Optionally, and as shown schematically in FIG. 2, the composite
membrane may be reinforced with a woven or non-woven material 6
bonded to one side of the base material 4. Suitable woven materials
may include, for example, scrims made of woven fibers of expanded
porous polytetrafluoroethylene; webs made of extruded or oriented
polypropylene or polypropylene netting, commercially available from
Conwed, Inc. of Minneapolis, Minn.; and woven materials of
polypropylene and polyester, from Tetko Inc., of Briarcliff Manor,
N.Y. Suitable non-woven materials may include, for example, a
spun-bonded polypropylene from Reemay Inc. of Old Hickory,
Tenn.
The treated membrane may be further processed to remove any
surfactant which may have been employed in processing the base
material as described in detail herein. This is accomplished by
soaking or submerging the membrane in a solution of, for example,
water, isopropyl alcohol, hydrogen peroxide, methanol, and/or
glycerin. During this step, the surfactant, which was originally
mixed in solution with the ion exchange material, is removed. This
soaking or submerging causes a slight swelling of the membrane,
however the ion exchange material remains within the interior
volume of the base material 4.
The membrane is further treated by boiling in a suitable swelling
agent, preferably water, causing the membrane to slightly swell in
the x and y direction. Additional swelling occurs in the
z-direction. A composite membrane results having a higher ion
transport rate that is also strong. The swollen membrane retains
its mechanical integrity and dimensional stability, unlike the
membranes consisting only of the ion exchange material. Also, the
membrane maintains desired ionic transport characteristics. A
correlation exists between the content of the swelling agent within
the membrane structure and transport properties of the membrane. A
swollen membrane will transport chemical species faster than an
unswollen membrane.
Although the membrane has excellent long term chemical stability,
it can be susceptible to poisoning by organics. Accordingly, it is
often desirable to remove such organics from the membrane. For
example, organics can be removed by regeneration in which the
membrane is boiled in a strong acid, such as nitric or chromic
acid.
To prepare the integral composite membrane of the present
invention, a support structure, such as a polypropylene woven
fabric, may first be laminated to the untreated base material 4 by
any conventional technique, such as, hot roll lamination,
ultrasonic lamination, adhesive lamination, or forced hot air
lamination so long as the technique does not damage the integrity
of the base material. A solution is prepared containing an ion
exchange material in solvent mixed with one or more surfactants.
The solution may be applied to the base material 4 by any
conventional coating technique including forwarding roll coating,
reverse roll coating, gravure coating, doctor coating, kiss
coating, as well as dipping, brushing, painting, and spraying so
long as the liquid solution is able to penetrate the interstices
and interior volume of the base material. Excess solution from the
surface of the membrane may be removed. The treated membrane is
then immediately placed into an oven to dry. Oven temperatures may
range from 60.degree.-200.degree. C., but preferably
120.degree.-160.degree. C. Drying the treated membrane in the oven
causes the ion exchange resin to become securely adhered to both
the external and internal membrane surfaces, i.e., the fibrils
and/or nodes of the base material. Additional solution application
steps, and subsequent drying, may be repeated until the membrane
becomes completely transparent. Typically, between 2 to 8
treatments are required, but the actual number of treatments is
dependent on the surfactant concentration and thickness of the
membrane. If the membrane is prepared without a support structure,
both sides of the membrane may be treated simultaneously thereby
reducing the number of treatments required.
The oven treated membrane is then soaked in a solvent, such as the
type described hereinabove, to remove the surfactant. Thereafter
the membrane is boiled in a swelling agent and under a pressure
ranging from about 1 to about 20 atmospheres absolute thereby
increasing the amount of swelling agent the treated membrane is
capable of holding.
Alternatively, the ion exchange material may be applied to the
membrane without the use of a surfactant. This procedure requires
additional treatment with the ion exchange resin. However, this
procedure does not required that the oven treated membrane be
soaked in a solvent, thereby reducing the total number of process
steps. A vacuum may also be used to draw the ion exchange material
into the membrane. Treatment without surfactant is made easier if
the water content of the solution is lowered. Partial solution
dewatering is accomplished by slow partial evaporation of the ion
exchange material solution at room temperature followed by the
addition of a nonaqueous solvent. Ideally, a fully dewatered
solution can be used. This is accomplished in several steps. First,
the ion exchange material is completely dried at room temperature.
The resulting resin is ground to a fine powder. Finally, this
powder is redissolved in a solvent, preferably a combination of
methanol and isopropanol.
Because the composite membrane of the present invention can be made
thinner than a fabric or non-woven reinforced structure, it is
possible to transport ions at a faster rate than previously has
been achieved, with only a slight lowering of the selectivity
characteristics of the membrane.
The following testing procedures were employed on samples which
were prepared in accordance with the teachings of the present
invention.
TEST PROCEDURES
TENSILE TEST
Tensile tests were carried out on an Instron Model 1122 tensile
strength tester, in accordance with ASTM D 638-91. Machine
parameters were set as follows:
Cross head speed: 0.423 cm/sec.
Full Scale load range: 222.4N
Humidity (%): 50
Temperature: 22.8.degree. C.
Grip Distance: 6.35 cm
Specimens were stamped out to conform with Type (II) of ASTM D638.
The specimens had a width of 0.635 cm, and a gauge length of 2.54
cm.
THICKNESS
Thickness of the base material was determined with the use of a
snap gauge (Johannes Kafer Co. Model No. F1000/302). Measurements
were taken in at least four areas of each specimen. Thickness of
the dried composite membrane was also obtained with the use of the
snap gauge. Thicknesses of swollen samples were not measurable with
the snap gauge due to the compression or residual water on the
surface of the swollen membrane. Thickness measurements of the
swollen membranes were also not able to be obtained with the use of
scanning electron microscopy due to interferences with the swelling
agents.
MOISTURE VAPOR TRANSMISSION RATE (MVTR)
A potassium acetate solution, having a paste like consistency, was
prepared from potassium acetate and distilled water. (Such a paste
may be obtained by combining 230 g potassium acetate with 100 g of
water, for example.) This solution was placed into a 133 ml.
polypropylene cup, having an inside diameter of 6.5 cm, at its
mouth. An expanded polytetrafluoroethylene (ePTFE) membrane was
provided having a minimum MVTR of approximately 85,000 g/m.sup.2-
24 hr. as tested by the method described in Crosby U.S. Pat. No.
4,862,730. The ePTFE was heat sealed to the lip of the cup to
create a taut, leakproof, microporous barrier containing the
solution.
A similar ePTFE membrane was mounted to the surface of a water
bath. The water bath assembly was controlled at 23.degree.
C..+-.plus or minus 0.2.degree. C., utilizing a temperature
controlled room and a water circulating bath.
Prior to performing the MVTR test procedure, a sample to be tested
was allowed to condition at a temperature of 23.degree. C. and a
relative humidity of 50%. The sample to be tested was placed
directly on the ePTFE membrane mounted to the surface of the water
bath and allowed to equilibrate for 15 minutes prior to the
introduction of the cup assembly.
The cup assembly was weighed to the nearest 1/1000 g. and was
placed in an inverted manner onto the center of the test
sample.
Water transport was provided by a driving force defined by the
difference in relative humidity existing between the water in the
water bath and the saturated salt solution of the inverted cup
assembly. The sample was tested for 10 minutes and the cup assembly
was then removed and weighed again within 1/1000 g.
The MVTR of the sample was calculated from the weight gain of the
cup assembly and was expressed in grams of water per square meter
of sample surface area per 24 hours.
PEEL STRENGTH
Peel strength or membrane adhesion strength tests were conducted on
membrane samples prepared with reinforced backings. Test samples
were prepared having dimensions of 3 inches by 3.5 inches (7.62
cm.times.8.89 cm). Double coated vinyl tape (type--#419 available
from the 3M Company of Saint Paul, Minn.) having a width of 1 inch
(2.54 cm) was placed over the edges of a 4 inch by 4 inch (10.2
cm.times.10.2 cm.) chrome steel plate so that tape covered all
edges of the plate. The membrane sample was then mounted on top of
the adhesive exposed side of the tape and pressure was applied so
that sample was adhesively secured to the chrome plate.
The plate and sample were then installed, in a horizontal position,
within an Instron tensile test machine Model No. 1000. An upper
crosshead of the tensile test machine was lowered so that the jaws
of the test machine closed flat and tightly upon the sample. The
upper crosshead was then slowly raised pulling the membrane sample
from the reinforced backing. When the membrane detached from the
reinforced backing, the test was complete. Adhesion strength was
estimated from the average strength needed to pull the membrane
from the reinforced backing.
IONIC CONDUCTANCE
The ionic conductance of the membrane was tested using a Palico
9100-2 type test system. This test system consisted of a bath of 1
molar sulfuric acid maintained at a constant temperature of
25.degree. C. Submerged in the bath were four probes used for
imposing current and measuring voltage by a standard "Kelvin"
four-terminal measurement technique. A device capable of holding a
separator, such as the sample membrane to be tested, was located
between the probes. First, a square wave current signal was
introduced into the bath, without a separator in place, and the
resulting square wave voltage was measured. This provided an
indication of the resistance of the acid bath. The sample membrane
was then placed in the membrane-holding device, and a second square
wave current signal was introduced into the bath. The resulting
square wave voltage was measured between the probes. This was a
measurement of the resistance due to the membrane and the bath. By
subtracting this number from the first, the resistance due to the
membrane alone was found.
DIMENSIONAL STABILITY
Reverse expansion in the x and y direction upon dehydration was
measured using a type Thermomechanical Analyzer 2940, made by TA
Instruments, Inc., of New Castle, Del. This instrument was used to
apply a predetermined force to a sample that had been boiled in
water for 30 minutes. A quartz probe placed in contact with the
sample measured any linear changes in the sample as it dried. A
sample was placed in a holder and then dried at 75.degree. C. for
greater than 10 min. The change in dimension (i.e., the shrinkage)
was recorded as a percentage of the original weight.
WEIGHT LOSS WITH TEMPERATURE
A high resolution TGA 2950, Thermogravimetric Analyzer, made by TA
Instruments (Newcastle, Del.) was used to determine the weight loss
of samples with respect to temperature. This weight loss is an
indication of the water content of the ionomer sample.
SELECTIVITY
Two solutions of KCI, having concentrations of 1 molar and 0.5
molar, respectively, were separated using the membranes of the
present invention. Two calomel reference electrodes (available from
Fischer Scientific, Pittsburgh Pa., catalog number 13-620-52) were
placed in each solution, and the potential difference across the
membranes was recorded using a digital multimeter (available from
Hewlett Packard, Englewood Calif., catalog number HP34401A). The
values obtained correspond to the difference of chloride ion
activity across the membrane and are reduced by the rate of anion
migration across the membranes. Therefore the obtained values
provide an indication of the membrane selectivity. The higher the
measured voltage, the better the membrane selectivity.
BUBBLE POINT TEST
Liquids with surface free energies less than that of stretched
porous PTFE can be forced out of the structure with the application
of a differential pressure. This clearing will occur from the
largest passageways first. A passageway is then created through
which bulk air flow can take place. The air flow appears as a
steady stream of small bubbles through the liquid layer on top of
the sample. The pressure at which the first bulk air flow takes
place is called the bubble point and is dependent on the surface
tension of the test fluid and the size of the largest opening. The
bubble point can be used as a relative measure of the structure of
a membrane and is often correlated with some other type of
performance criteria, such as filtration efficiency.
The Bubble Point was measured according to the procedures of ASTM
F316-86. Isopropyl alcohol was used as the wetting fluid to fill
the pores of the test specimen.
The Bubble Point is the pressure of air required to displace the
isopropyl alcohol from the largest pores of the test specimen and
create the first continuous stream of bubbles detectable by their
rise through a layer of isopropyl alcohol covering the porous
media. This measurement provides an estimation of maximum pore
size.
PORE SIZE AND PORE SIZE DISTRIBUTION
Pore size measurements are made by the Coulter Porometer.TM.,
manufactured by Coulter Electronics, Inc., Hialeah, Fla. The
Coulter Poremeter is an instrument that provides automated
measurement of pore size distributions in porous media using the
liquid displacement method (described in ASTM Standard E1298-89).
The Poremeter determines the pore size distribution of a sample by
increasing air pressure on the sample and measuring the resulting
flow. This distribution is a measure of the degree of uniformity of
the membrane (i.e., a narrow distribution means there is little
difference between the smallest and largest pore size). The
Porometer also calculates the mean flow pore size. By definition,
half of the fluid flow through the filter occurs through pores that
are above or below this size. It is the mean flow pore size which
is most often linked to other filter properties, such as retention
of particulates in a liquid stream. The maximum pore size is often
linked to the Bubble Point because bulk air flow is first seen
through the largest pore.
BALL BURST TEST
This text measures the relative strength of a sample by determining
the maximum load at break. The sample is challenged with a 1 inch
diameter ball while being clamped between two plates. The material
is placed taut in the measuring device and pressure applied with
the ball burst probe. Pressure at break is recorded.
AIR FLOW DATA
The Gurley air flow test measures the time in seconds for 100 cc of
air to flow through a one square inch sample at 4.88 inches of
water pressure. The sample is measured in a Gurley Densometer (ASTM
0726-58). The sample is placed between the clamp plates. The
cylinder is then dropped gently. The automatic timer (or stopwatch)
is used to record the time (seconds) required for a specific volume
recited above to be displaced by the cylinder. This time is the
Gurley number.
The Frazier air flow test is similar but is mostly used for much
thinner or open membranes. The test reports flow in cubic feet per
minute per square foot of material at 0.5 inches water pressure.
Air flow can also be measured with the Coulter Porometer. In this
test, the operator can select any pressure over a wide range. The
Porometer can also perform a pressure hold test that measures air
flow during a decreasing pressure curve.
BACKGROUND OF EXAMPLES
As may be appreciated by one skilled in the art, the present
invention provides for an integral composite membrane. No porous
surfaces are exposed in the present invention.
The integral composite membrane of the present invention can be
advantageously employed in electrolytic processes and chemical
separations. In a plate-and-frame type electrodialysis unit, the
membrane of the present invention would take the place of existing
cation exchange membranes. This membrane could be of the type which
is laminated to a spacer screen in accordance with a specific
application. Due to the higher conductance of this membrane
feasible with thinner membranes, an electrodialysis unit could
employ less membrane to achieve a given flux rate, thereby saving
space and cost. If equipment is retrofitted with this membrane, the
voltage requirements would be reduced at a given current, or higher
current could be run at a given voltage. Also, in a diffusion
dialysis system, a given unit employing the membrane of the present
invention would provide a higher flux.
A fuel cell, utilizing the membrane of the present invention,
operates at a higher voltage for a given current density due to the
improved ionic conductance of thinner versions of the membrane of
this invention.
Due to improved water transport across the membrane of the present
invention, high limiting current may be achieved with less fuel gas
humidification, as compared to membranes which have been employed
heretofore. For example, the membrane of the present invention has
a resistance of 0.044 ohm-sq cm. At a current density of 1
A/cm.sup.2, this causes a voltage drop of about 44 mV, or about a
99 mV improvement in cell voltage compared to NAFION 117 membranes
which have a resistance of 0.143 .OMEGA.-cm.sup.3. (NAFION is a
registered trademark of E. I. DuPont de Nemours and Co., Inc.). As
used herein, NAFION 117 means a membrane having a thickness of 7
mils made from perfluorosulfonic acid/tetrafluoroethylene
(TFE)/copolymer. This may reduce losses by about 99 mW/sq cm at
this operating condition for resistance. If the cell operating
voltage increased from 500 mV to 599 mV, the cell voltage
efficiency would increase from 41% to 49% of the theoretical 1.23
V. The decrease in the internal resistance of the cell allows the
design of smaller or more efficient cells.
Without intending to limit the scope of the present invention, the
apparatus and method of production of the present invention may be
better understood by referring to the following examples. All
samples of ePTFE provided in the following examples were made in
accordance with the teachings of U.S. Pat. No. 3,593,566. More
particularly, the ePTFE had the following material properties:
TYPE 1 TYPE 2 Gurley (sec.) 3.3 0.9 Bubble Point (psi) 28.3 32.6
Mass/Area (g/m.sup.2) 6.1 4.4 Density (g/cc) 0.65 0.77 Longitudinal
Maximum Load (lbs.) 1.76 2.18 Transverse Maximum Load (lbs.) 2.33
1.31
As may be appreciated by one skilled in the art, ePTFE membranes
can be made with a wide range of physical property values, with
ranges far exceeding the two examples given above.
Example 1
A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils
(0.02 mm), was mounted on a 6 inch diameter wooden embroidery hoop.
An ion exchange material/surfactant solution was prepared
comprising 95% by volume of a perfluorosulfonic
acid/tetrafluoroethylene copolymer resin solution (in H+ form,
which itself is comprised of 5% perfluorosulfonic
acid/tetrafluoroethylene copolymer resin, 45% water, and 50% a
mixture of low molecular weight alcohols, commercially available
from E.I. DuPont de Nemours, Inc. under the registered trademark
NAFION.RTM. type NR-50 (1100 EW) hereinafter "NR-50") and 5% of a
nonionic surfactant of octylphenoxy polyethoxyethanol (Triton
X-100, commercially available from Rohm & Haas of Philadelphia,
Pa.). This solution was brushed on both sides of the membrane to
impregnate and substantially occlude the interior volume of the
membrane. The sample was then dried in the oven at 140.degree. C.
for 30 seconds. The procedure was repeated two more times to fully
occlude the interior volume. The sample was then soaked in
isopropanol for 5 minutes to remove the surfactant. After rinsing
with distilled water and allowing the sample to dry at room
temperature, a final coat of the ion exchange material/surfactant
solution was applied. The wet membrane was again dried in the oven
at 140.degree. C. for 30 seconds and soaked in isopropanol for 2
minutes. The membrane was finally boiled in distilled water for 30
minutes under atmospheric pressure to swell the treated membrane.
Gurley numbers for this material are summarized in Table 3. Ionic
conductive rates are summarized in Table 4. The tensile strength
may be found in Table 2. Percent weight change of this sample may
be found in Table 6. The swollen membrane was later dried to a
dehydrated state in an oven at 140.degree. C. for 30 seconds. The
thickness of the dried composite membrane was measured and found to
be approximately the same thickness as the base material.
Example 2
A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils
(0.02 mm) and a Gurley Densometer air flow of 2-4 seconds, was
placed on top of a netting of polypropylene obtained from Conwed
Plastics Corp. of Minneapolis, Minn. The two materials were bonded
together on a laminator with 10 psig pressure, a speed of 15 feet
per minute and a temperature of 200.degree. C. No adhesives were
used. The reinforced membrane sample was then placed on a 6 inch
wooden embroidery hoop. A solution was prepared of 96% by volume of
a perfluorosulfonic acid/TFE copolymer resin in alcohol, and 4% of
the nonionic surfactant Triton X-100. This solution was brushed
only on the membrane side to substantially occlude the interior
volume of the membrane. The sample was dried in an oven at
130.degree. C. This procedure was repeated three more times to
fully occlude the interior volume of the membrane. The sample was
then baked in an oven at 140.degree. C. for 5 minutes. The sample
was soaked in isopropanol for 5 minutes to remove the surfactant.
The membrane was then boiled in distilled water for 30 minutes
under atmospheric pressure causing the treated membrane to swell.
Gurley numbers for this material are summarized in Table 3.
This sample was tested for its peel strength in accordance with the
method described above. The linear bond strength was found to be
2.06 lb./sq. in. (1450 kg/m.sup.2).
Example 3
A TYPE 2 ePTFE membrane, having a thickness of 0.5 mils (0.01 mm),
was mounted on a 6 inch diameter wooden embroidery hoop. A solution
of 100% by volume of NR-50 was brushed onto both sides of the
membrane, without the addition of any surfactants, to substantially
occlude the interior volume of the membrane. The sample was then
placed in an oven at 140.degree. C. to dry. This procedure was
repeated four more times until the membrane was completely
transparent and the interior volume of the membrane was fully
occluded. The sample was then boiled in distilled water for 30
minutes at atmospheric pressure causing the membrane to swell.
Gurley numbers for this material are summarized in Table 3.
Example 4
A TYPE 2 ePTFE membrane, having a thickness of 0.5 mils (0.01 mm),
was mounted onto a 6 inch diameter wooden embroidery hoop. A
solution was prepared of 95% by volume NR-50 and 5% of the nonionic
surfactant, Triton X-100. The solution was brushed on both sides of
the membrane with a foam brush and the excess was wiped off. The
wet membrane was dried in an oven at 140.degree. C. for 30 seconds.
Three additional coats of solution were applied to the membrane in
the same manner to fully occlude the interior volume of the
membrane. The membrane was then soaked in isopropanol for 2 minutes
to remove the surfactant. The membrane was rinsed with distilled
water and allowed to dry at room temperature. A final treatment of
the solution was applied. The wet membrane was dried in the oven at
140.degree. C. for 30 seconds, and then soaked in isopropanol for 2
minutes. Finally, the membrane was boiled in distilled water for 5
minutes. Moisture vapor transmission rates for the treated membrane
were measured and are summarized in Table 1.
Example 5
A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils
(0.02 mm), was mounted onto a 6 inch diameter wooden embroidery
hoop. The Gurley Densometer air flow for this membrane was 2-4
seconds. A solution was prepared of 95% by volume NR-50 and 5%
Triton X-100. The solution was brushed on both sides of the
membrane with a foam brush and the excess was wiped off. The wet
membrane was dried in the oven at 140.degree. C. for 30 seconds.
Three additional coats of solution were applied in the same manner.
The membrane was then soaked in isopropanol for 2 minutes. After
rinsing with distilled water and allowing to dry at room
temperature, a final coat of the solution was applied. The wet
membrane was dried in the oven at 140.degree. C. for 30 seconds,
then soaked in isopropanol for 2 minutes. This material was not
boiled. No swelling other than the minor swelling during the
surfactant removal occurred. The ionic conduction rate for this
material is presented in Table 4.
Example 6
A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils
(0.02 mm), was mounted onto a 5 inch diameter plastic embroidery
hoop. The Gurley Densometer air flow for this membrane was 2-4
seconds. A solution was prepared of 95% NR-50 and 5% Triton X-100.
The solution was brushed on both sides of the membrane with a foam
brush and the excess was wiped off. The wet membrane was dried in
the oven at 140.degree. C. for 30 seconds. Two additional coats of
solution were applied in the same manner to fully occlude the
interior volume of the membrane. The membrane was then soaked in
isopropanol for 2 minutes. After rinsing with distilled water and
allowing to dry at room temperature, a final coat of the solution
was applied. The wet membrane was dried in the oven at 140.degree.
C. for 30 seconds, and then soaked in isopropanol for 2 minutes to
remove the surfactant. The sample was rinsed and dried at room
temperature.
This sample was weighed before it was mounted on the 5 inch plastic
hoop. Following treatment, it was removed from the hoop and weighed
again. The ion exchange polymer content was directly calculated by
determining the weight change before and after treatment. The ion
exchange content for this sample was found to be 98.4 mg or 7.77
grams per square meter of membrane.
Example 7
A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils
(0.02 mm) and a Gurley Densometer air flow of 2-4 seconds, was
placed on top of a netting of polypropylene which was obtained from
Applied Extrusion Technologies, Inc. of Middletown, Del. The two
materials were bonded together on a laminator with 10 psig
pressure, a speed of 15 feet per minute and a temperature of
200.degree. C. The reinforced sample was then mounted on a 6 inch
diameter wooden embroidery hoop. A solution was prepared consisting
of the following: 95% by volume NR-50, containing 5% by weight
perfluorosulfonic acid/TFE copolymer resin in a solvent mixture of
less than 25% water, preferably 16-18% water, and the remainder a
mixture of isopropanol and normal propanol; and 5% of Triton X-100
non-ionic surfactant. The solution was brushed on both sides of the
membrane with a foam brush and the excess was wiped off. The wet
membrane was dried in an oven at 140.degree. C. for 30 seconds.
Three additional coats of solution were applied to the membrane in
the same manner to fully occlude the interior volume of the
membrane. The membrane was then soaked in isopropanol for 2 minutes
to remove the surfactant. The membrane was rinsed with distilled
water and allowed to dry at room temperature. A final treatment of
the ion exchange material/surfactant solution was applied. The wet
membrane was dried in the oven at 140 C. for 30 seconds, then
soaked in isopropanol for 2 minutes. Finally, the membrane was
boiled in distilled water for 5 minutes.
Example 8
A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils
(0.02 mm) and a Gurley Densometer air flow of 2-4 seconds, was
mounted on a 6 inch diameter wooden embroidery hoop. A solution was
prepared consisting of the following: 95% NR-50, containing 5% by
weight perfluorosulfonic acid/TFE copolymer resin in a solvent
mixture of less than 25% water, preferably 16-18% water, and the
remainder a mixture being isopropanol and normal propanol; and 5%
of Triton X-100 non-ionic surfactant. The solution was brushed on
both sides of the membrane with a foam brush and the excess was
wiped off. The wet membrane was dried in an oven at 140.degree. C.
for 30 seconds. Three additional coats of solution were applied to
the membrane in the same manner. The membrane was then soaked in
isopropanol for 2 minutes to remove the surfactant. The membrane
was rinsed with distilled water and allowed to dry at room
temperature. A final treatment of the solution was applied. The wet
membrane was dried in the oven at 140.degree. C. for 30 seconds,
then soaked in isopropanol for 2 minutes. Finally, the membrane was
boiled in distilled water for 5 minutes.
Example 9
A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils
(0.02 mm) and a Gurley Densometer air flow of 2-4 seconds, was
mounted on a 6 inch diameter wooden embroidery hoop. The membrane
was first submerged in a solution consisting of 25% Triton X-100
non-ionic surfactant, 25% water, and 50% isopropyl alcohol. Next, a
solution of NR-50 was brushed on both sides of the membrane with a
foam brush and the excess was wiped off. The wet membrane was dried
in an oven at 140.degree. C. for 30 seconds. Three additional coats
of surfactant solution followed by a coat of NR-50 solution were
applied to the membrane in the same manner to fully occlude the
interior volume of the membrane. The membrane was then soaked in
isopropanol for 2 minutes to remove the surfactant. The membrane
was rinsed with distilled water and allowed to dry at room
temperature. A final treatment of the ion exchange
material/surfactant was applied to the membrane. The wet membrane
was dried in the oven at 140.degree. C. for 30 seconds, then soaked
in isopropanol for 2 minutes. Finally, the membrane was boiled in
distilled water for 5 minutes.
Example 10
A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils
(0.02 mm) and a Gurley Densometer air flow of 2-4 seconds, was
mounted on a 6 inch diameter wooden embroidery hoop. The membrane
was first submerged in a solution consisting of 25% Triton X-100
non-ionic surfactant, 25% water, and 50% isopropyl alcohol. Next, a
95% by weight NR-50 solution, containing 5% by weight
perfluorosulfonic acid/TFE copolymer resin in a solvent mixture of
less than 25% water, preferably 16-18% water, and the remainder a
mixture of isopropanol and normal propanol, was brushed on both
sides of the membrane with a foam brush and the excess was wiped
off. The wet membrane was dried in an oven at 140.degree. C. for 30
seconds. Three additional coats of surfactant solution followed by
the NR-50 solution were applied to the membrane in the same manner
to fully occlude the interior volume of the membrane. The membrane
was then soaked in isopropanol for 2 minutes to remove the
surfactant. The membrane was rinsed with distilled water and
allowed to dry at room temperature. A final treatment of the NR-50
solution was applied. The wet membrane was dried in the oven at
140.degree. C. for 30 seconds, then soaked in isopropanol for 2
minutes. Finally, the membrane was boiled in distilled water for 5
minutes.
Example 11
A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils
(0.02 mm) and a Gurley Densometer air flow of 2-4 seconds, was
placed on top of a netting of polypropylene. The two materials were
bonded together on a laminator with 10 psig pressure, a speed of 15
feet per minute and a temperature of 200.degree. C. The reinforced
sample was then mounted on a 6 inch diameter wooden embroidery
hoop. The membrane was first submerged in a solution consisting of
25% Triton X-100 non-ionic surfactant, 25% water, and 50% isopropyl
alcohol. Next, a solution of NR-50 was brushed on both sides of the
membrane with a foam brush and the excess was wiped off. The wet
membrane was dried in an oven at 140.degree. C. for 30 seconds.
Three additional coats of the surfactant solution followed by the
NR-50 solution were applied to the membrane in the same manner to
fully occlude the interior volume of the membrane. The membrane was
then soaked in isopropanol for 2 minutes to remove the surfactant.
The membrane was rinsed with distilled water and allowed to dry at
room temperature. A final treatment of the NR-50 solution was
applied. The wet membrane was dried in the oven at 140.degree. C.
for 30 seconds, then soaked in isopropanol for 2 minutes. Finally,
the membrane was boiled in distilled water for 5 minutes.
Example 12
A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils
(0.02 mm) and a Gurley Densometer air flow of 2-4 seconds, was
mounted on a 6 inch diameter wooden embroidery hoop. A solution
consisting of 5% by weight of perfluorosulfonic acid/TFE copolymer
resin in a solvent mixture of less than 25% water, preferably
16-18% water, and the remainder a mixture of isopropanol and normal
propanol was allowed to evaporate slowly at room temperature. The
resulting resin was ground to a powder with a mortar and pestle.
This resin was then dissolved in methanol under low heat (less than
70.degree. C.). The final solution contained the original resin
content in a base solvent of methanol such that the resin content
of the solution was 5% by weight. The solution was brushed on both
sides of the membrane with a foam brush and the excess was wiped
off. The wet membrane was dried in an oven at 140.degree. C. for 30
seconds. Three additional coats of solution were applied to the
membrane in the same manner to fully occlude the interior volume of
the membrane. The membrane was boiled in distilled water for 5
minutes.
Example 13
A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils
(0.02 mm) and a Gurley Densometer air flow of 2-4 seconds, was
mounted on a 6 inch diameter wooden embroidery hoop. A solution
consisting of 5% by weight of perfluorosulfonic acid/TFE copolymer
resin, in a solvent mixture of less than 25% water, preferably
16-18% water, and the remainder a mixture of isopropanol and normal
propanol, was allowed to evaporate slowly at room temperature. The
resulting resin was ground to a powder with a mortar and pestle.
This resin was then dissolved in methanol under low heat (less than
70.degree. C.). The final solution contained the original resin
content in a base solvent of methanol such that the resin content
of the solution was 5% by weight. This solution was used to prepare
a new solution comprised of a 95% dewatered resin solution, and 5%
Triton X-100 non-ionic surfactant. The solution was brushed on both
sides of the membrane with a foam brush and the excess was wiped
off. The wet membrane was dried in an oven at 140.degree. C. for 30
seconds. Two additional coats of solution were applied to the
membrane in the same manner to fully occlude the interior volume of
the membrane. The membrane was then soaked in isopropanol for 2
minutes to remove the surfactant. The membrane was rinsed with
distilled water and allowed to dry at room temperature. A final
treatment of the resin/Triton X-100 non-ionic surfactant solution
was applied. The wet membrane was dried in the oven at 140.degree.
C. for 30 seconds, then soaked in isopropanol for 2 minutes.
Finally, the membrane was boiled in distilled water for 5
minutes.
Example 14
A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils
(0.02 mm) and a Gurley Densometer air flow of 2-4 seconds, was
mounted on a 6 inch diameter wooden embroidery hoop. A solution
consisting of 5% by weight of perfluorosulfonic acid/TFE copolymer
resin in a solvent mixture of less than 25% water, preferably
16-18% water, and the remainder a mixture of isopropanol and normal
propanol, was allowed to partially evaporate slowly at room
temperature. Before all the solvent evaporated, the viscous liquid
was mixed with methanol. The water content of the resulting
solution was estimated at 5%. The resin content of the solution was
5%. The solution was brushed on both sides of the membrane with a
foam brush and the excess was wiped off. The wet membrane was dried
in an oven at 140.degree. C. for 30 seconds. Three additional coats
of solution were applied to the membrane in the same manner to
fully occlude the interior volume of the membrane. The membrane was
boiled in distilled water for 5 minutes.
Example 15
A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils
(0.02 mm) and a Gurley Densometer air flow of 2-4 seconds, was
placed on top of a netting of polypropylene. The two materials were
bonded together on a laminator with 10 psig pressure, a speed of 15
feet per minute and a temperature of 200.degree. C. The reinforced
sample was then mounted on a 6 inch diameter wooden embroidery
hoop. A solution consisting of 5% by weight of perfluorosulfonic
acid/TFE copolymer resin in a solvent mixture of less than 25%
water, preferably 16-18% water and the remainder a mixture of
isopropanol and normal propanol, was allowed to partially evaporate
slowly at room temperature. Before all the solvent evaporated, the
viscous liquid was mixed with methanol. The water content of the
resulting solution was estimated at 5%. The resin content of the
solution was 5%. The solution was brushed on both sides of the
membrane with a foam brush and the excess was wiped off. The wet
membrane was dried in an oven at 140.degree. C. for 30 seconds.
Three additional coats of solution were applied to the membrane in
the same manner to fully occlude the interior volume of the
membrane. The membrane was boiled in distilled water for 5
minutes.
Example 16
A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils
(0.02 mm) and a Gurley Densometer air flow of 2-4 seconds, was
mounted on a 6 inch diameter wooden embroidery hoop. A solution
consisting of 5% by weight of perfluorosulfonic acid/TFE copolymer
resin in a solvent mixture of less than 25% water, preferably
16-18% water and the remainder a mixture of isopropanol and normal
propanol, was allowed to partially evaporate slowly at room
temperature. Before all the solvent evaporated, the viscous liquid
was mixed with methanol. The water content of the resulting
solution was estimated at 5% The resin content of the solution was
5%. This solution was used to prepare a new solution comprised of
95% of the low-water resin solution, and 5% of the nonionic
surfactant, Triton X-100. The new solution was brushed on both
sides of the membrane with a foam brush and the excess was wiped
off. The wet membrane was dried in an oven at 140.degree. C. for 30
seconds. Two additional coats of solution were applied to the
membrane in the same manner to fully occlude the interior volume of
the membrane. The membrane was then soaked in isopropanol for 2
minutes to remove the surfactant. The membrane was rinsed with
distilled water and allowed to dry at room temperature. A final
treatment of the new solution was applied. The wet membrane was
dried in the oven at 140.degree. C. for 30 seconds, then soaked in
isopropanol for 2 minutes. Finally, the membrane was boiled in
distilled water for 5 minutes.
Example 17
A thermoplastic frame was cut and a membrane of ePTFE was placed at
a center location of the frame. The ePTFE membrane was heat sealed
to the frame. The membrane was then treated in accordance with
Example 1. Alternatively, a fluoroionomer membrane made in
accordance with Example 1 was secured mechanically within a
frame.
This "framed" fluoroionomer composite has utility, by providing a
unitary construction which can be placed in a device, which beyond
serving as an ion exchange medium, can also serve as a sealant
between various components of a cell assembly.
Example 18
TEFLON.RTM. fine powder was blended with ISOPAR K mineral spirit at
a rate of 115 cc per pound of fine powder. The lubricated powder
was compressed into a cylinder and was ram extruded at 70.degree.
C. to provide a tape. The tape was split into two rolls, layered
together and compressed between rolls to a thickness of 0.030 inch.
Next, the tape was stretched transversely to 2.6 times its original
width. The ISOPAR K was driven off by heating to 210.degree. C. The
dry tape was expanded longitudinally between banks of rolls in a
heat zone heated to 300.degree. C. The ratio of speed of the second
bank of rolls to the first bank of rolls was 35:1 and the third
bank of rolls to the second bank of rolls was 1.5:1, for a total of
52:1 longitudinal expansion producing a tape having a width of 3.5
inches. This tape was heated to 295.degree. C. and transversely
expanded 13.7 times in width, while being constrained from
shrinkage and then heated to 365.degree. C. while still
constrained. This process produced a web-like membrane having a
porous microstructure composed substantially of fibrils in which no
nodes were present
Example 19
An ePTFE membrane, having a nominal thickness of 2.2 mils (0.6 mm)
and a Gurley Densometer air flow of 6-9 seconds, was mounted on a 6
inch diameter wooden embroidery hoop. A solution consisting of 5%
by weight of ionomer, such as perfluorosulfonic acid/TFE copolymer
resin in a solvent such as methanol, was brushed on both sides of
the membrane with a foam brush and the excess was wiped off. The
wet membrane was dried in an oven at 140.degree. C. for 30 seconds.
Three additional coats of solution were applied to the membrane in
the same manner to fully occlude the interior volume of the
membrane.
Example 20
An ePTFE membrane, having a nominal thickness of 3 mils (0.8 mm)
and a Gurley Densometer air flow of 6-9 seconds, was mounted on a 6
inch diameter wooden embroidery hoop. A solution consisting of 5%
by weight of ionomer, such as perfluorosulfonic acid/TFE copolymer
resin in a solvent such as methanol, was brushed on both sides of
the membrane with a foam brush and the excess was wiped off. The
wet membrane was dried in an oven at 140.degree. C. for 30 seconds.
Three additional coats of solution were applied to the membrane in
the same manner to fully occlude the interior volume of the
membrane.
Example 21
An ePTFE membrane, having a nominal thickness of 0.75 mils (0.02
mm) and a Gurley Densometer air flow of 2-4 seconds, was mounted on
a 6 inch diameter wooden embroidery hoop. A solution consisting of
5% by weight of ionomer, such as perfluorosulfonic acid/TFE
copolymer resin of 1100 EW in a solvent such as methanol, was
brushed on both sides of the membrane with a foam brush and the
excess was wiped off. The wet membrane was dried in an oven at
140.degree. C. for 30 seconds. Three additional coats of solution
were applied to the membrane in the same manner to fully occlude
the interior volume of the membrane. A second composite membrane
prepared in the same manner, however using a 950 EW
perfluorosulfonic acid/TFE copolymer in a solvent such as ethanol.
The two membranes were then combined (laminated) by use of heat and
pressure. For example, at 190.degree. C. (375.degree. F.) @100 psi
for 1 minute in a heated press or a comparable arrangement in a
heated roll.
Example 22
An ePTFE membrane, having a nominal thickness of 0.75 mils (0.002
mm) and a Gurley Densometer air flow of 2-4 seconds, was mounted on
a 6 inch diameter wooden embroidery hoop. An alcohol solution
consisting of 5% by weight of ionomer, and a finely divided powder,
such as carbon black (10%), was brushed on both sides of the
membrane with a foam brush and the excess was wiped off. The wet
membrane was dried in an oven at 140.degree. C. for 30 seconds.
Three additional coats of solution were applied to the membrane in
the same manner to fully occlude the interior volume of the
membrane. The final composite had a dark appearance.
Example 23
An ePTFE membrane, having a nominal thickness of 0.75 mils (0.002
mm) and a Gurley Densometer air flow of 2-4 seconds, was mounted on
a 6 inch diameter wooden embroidery hoop. A solution consisting of
5% by weight of ionomer, was brushed on both sides of the membrane
with a foam brush and the excess was wiped off. The wet membrane
was dried in an oven at 140.degree. C. for 30 seconds. Three
additional coats of solution were applied to the membrane in the
same manner to fully occlude the interior volume of the membrane.
This composite membrane was then combined (laminated) to another
ePTFE membrane having a nominal thickness of 0.75 (0.002) mm and a
Gurley Densometer air flow of 2-4 second, by use of heat and
pressure (for example 190 C. [375 F.] @100 psi) using a heated
press or a comparable arrangement.
A solution consisting of 5% by weight of ionomer, such as
perfluorosulfonic acid/TFE copolymer resin in a solvent such as
methanol, was brushed on the ePTFE membrane side of the membrane
with a foam brush and the excess was wiped off. The wet membrane
was dried in an oven at 140.degree. C. for 30 seconds. Three
additional coats of solution were applied to the membrane in the
same manner to fully occlude the interior volume of the ePTFE
membrane. A thicker integral composite membrane was thus
formed.
Comparative Samples
NAFION 117, a perfluorosulfonic acid cation exchange membrane,
unreinforced film of 1100 equivalent weight commercially available
from E. I. DuPont de Nemours Co., Inc., having a quoted nominal
thickness of 7 mils (0.18 mm) was obtained. The samples, originally
in the hydrated swollen state, were measured in the x- and
y-directions and weighed.
Without intending to limit the scope of the present invention, data
collected from testing the ion exchange membranes made in
accordance with the procedures of the foregoing examples are
summarized in the following tables. As may be appreciated by one
skilled in the art, these tables reveal that the ion exchange
membrane of this invention has superior ionic conductance and
exceptional dimensional stability compared to known ion exchange
membranes. Furthermore, this inventive membrane has good mechanical
strength in the unswollen state and retains much of its mechanical
strength in the swollen state, whereas conventional membranes are
substantially weakened upon hydration.
TABLE 1 Moisture Vapor Transmission Rates (MVTR) Sample ID* MVTR
(grams/m.sup.2 -24 hrs.) 4 25,040 NAFION 117 23,608 *Measurements
were obtained on samples in their swollen state.
TABLE 1 Moisture Vapor Transmission Rates (MVTR) Sample ID* MVTR
(grams/m.sup.2 -24 hrs.) 4 25,040 NAFION 117 23,608 *Measurements
were obtained on samples in their swollen state.
TABLE 1 Moisture Vapor Transmission Rates (MVTR) Sample ID* MVTR
(grams/m.sup.2 -24 hrs.) 4 25,040 NAFION 117 23,608 *Measurements
were obtained on samples in their swollen state.
TABLE 4 Ionic Conductance Ionic Conductance Sample ID (mhos/sq. cm)
Example 1 22.7 NAFION 117* 7.0 Example 5 8.5 NAFION 117** 4.7
*sample was boiled in distilled water for 30 minutes. **sample was
tested as received from E. I. DuPont de Nemours, Inc.
TABLE 4 Ionic Conductance Ionic Conductance Sample ID (mhos/sq. cm)
Example 1 22.7 NAFION 117* 7.0 Example 5 8.5 NAFION 117** 4.7
*sample was boiled in distilled water for 30 minutes. **sample was
tested as received from E. I. DuPont de Nemours, Inc.
TABLE 4 Ionic Conductance Ionic Conductance Sample ID (mhos/sq. cm)
Example 1 22.7 NAFION 117* 7.0 Example 5 8.5 NAFION 117** 4.7
*sample was boiled in distilled water for 30 minutes. **sample was
tested as received from E. I. DuPont de Nemours, Inc.
TABLE 4 Ionic Conductance Ionic Conductance Sample ID (mhos/sq. cm)
Example 1 22.7 NAFION 117* 7.0 Example 5 8.5 NAFION 117** 4.7
*sample was boiled in distilled water for 30 minutes. **sample was
tested as received from E. I. DuPont de Nemours, Inc.
Although a few exemplary embodiments of the present invention have
been described in detail above, those skilled in the art readily
appreciate that many modifications are possible without materially
departing from the novel teachings and advantages which are
described herein. Accordingly, all such modifications are intended
to be included within the scope of the present invention, as
defined by the following claims.
* * * * *