U.S. patent application number 09/841253 was filed with the patent office on 2002-02-21 for fuel cell comprising a composite membrane.
Invention is credited to Bahar, Bamdad, Hobson, Alex R., Kolde, Jeffry A., Mallouk, Robert S..
Application Number | 20020022123 09/841253 |
Document ID | / |
Family ID | 26991620 |
Filed Date | 2002-02-21 |
United States Patent
Application |
20020022123 |
Kind Code |
A1 |
Bahar, Bamdad ; et
al. |
February 21, 2002 |
Fuel cell comprising a composite membrane
Abstract
A fuel cell comprising at least one composite membrane which
includes a porous support impregnated with ion exchange resin to
make the pores of the support occlusive, wherein the thickness of
the composite membrane is less than 0.025 mm. The membrane is
strong and has good ionic conductivity. Methods of making the
composite membrane are also disclosed.
Inventors: |
Bahar, Bamdad; (Chester,
MD) ; Hobson, Alex R.; (Elkton, MD) ; Kolde,
Jeffry A.; (Avondale, PA) ; Mallouk, Robert S.;
(Chadds Ford, PA) |
Correspondence
Address: |
MORGAN & FINNEGAN, L.L.P.
345 Park Avenue
New York
NY
10154
US
|
Family ID: |
26991620 |
Appl. No.: |
09/841253 |
Filed: |
April 25, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09841253 |
Apr 25, 2001 |
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09209932 |
Jul 8, 1998 |
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6254978 |
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09209932 |
Jul 8, 1998 |
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08903844 |
Jul 31, 1997 |
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08903844 |
Jul 31, 1997 |
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08339425 |
Nov 14, 1994 |
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Current U.S.
Class: |
428/306.6 ;
428/308.4; 428/311.51; 428/315.5 |
Current CPC
Class: |
Y02E 60/10 20130101;
H01M 8/1062 20130101; B32B 7/12 20130101; B32B 2377/00 20130101;
H01M 50/491 20210101; B32B 5/022 20130101; B32B 5/024 20130101;
C08J 2327/18 20130101; H01M 8/1081 20130101; H01M 50/497 20210101;
C25B 13/08 20130101; Y10T 428/249955 20150401; Y10T 428/31544
20150401; H01M 8/1067 20130101; B32B 2329/04 20130101; Y10T
428/249964 20150401; Y10T 428/249954 20150401; H01M 8/106 20130101;
H01M 50/426 20210101; Y02P 70/50 20151101; H01M 8/1023 20130101;
H01M 50/411 20210101; H01M 50/489 20210101; Y02E 60/50 20130101;
Y10T 428/249978 20150401; B32B 2327/18 20130101; H01M 8/1039
20130101; Y10T 428/249958 20150401; H01M 8/1044 20130101; B32B
2369/00 20130101; B32B 5/18 20130101; C08J 5/2275 20130101 |
Class at
Publication: |
428/306.6 ;
428/315.5; 428/308.4; 428/311.51 |
International
Class: |
B32B 003/26 |
Claims
1. A fuel cell comprising at least one integral air impermeable
composite membrane comprising: at least one polymeric support
having a microstructure of micropores, said microstructure defining
a porosity in the range of about 70% to 98% within said polymeric
support, at least one ion exchange resin filling said
microstructure such that said composite membrane is air
impermeable, said composite membrane having a thickness of at most
0.8 mils and an ionic conduction rate of at least 5.1
.mu.mhos/min.
2. The fuel cell according to claim 1, wherein the polymeric
support is expanded polytetrafluoroethylene.
3. The fuel cell according to claim 1, wherein the thickness of the
composite membrane is 0.06 mils to 0.8 mils.
4. The fuel cell according to claim 1, wherein the thickness of the
composite membrane is 0.5 mils to 0.8 mils.
5. The fuel cell according to claim 1, wherein the thickness of the
composite membrane is at most 0.5 mils.
6. The fuel cell according to claim 1, wherein the ion exchange
resin comprises perfluorinated sulfonic acid resin.
7. The fuel cell of claim 1, further comprising a reinforcement
backing bonded to a side thereof.
8. The fuel cell of claim 1, wherein the thickness of the composite
membrane is at most 0.5 mils, wherein the polymeric support is
expanded polytetrafluoroethylene, and wherein the ion exchange
resin comprises perfluorinated sulfonic acid resin.
9. The fuel cell of claim 1, wherein the microstructure includes
nodes interconnected with fibrils.
10. A fuel cell comprising a substantially air occlusive integral
composite membrane having a polymeric support with a microstructure
of pores, said microstructure filled with an ion exchange resin,
said composite membrane has an ionic conduction rate of at least
5.1 .mu.mhos/min, said composite membrane prepared by, (a)
providing a polymeric support having a microstructure of
micropores; (b) applying an ion exchange resin solution to said
polymeric support; and (c) repeating step (b) until said micropores
are sufficiently filled with ion exchange resin to form an air
occlusive integral composite membrane.
11. The fuel cell of claim 10, wherein said step (b) further
includes, (b1) drying said support after each application of ion
exchange resin solution to remove solvent from said solution.
12. The fuel cell according to claim 10, wherein said support
comprises expanded polytetrafluoroethylene.
13. The fuel cell according to claim 10, wherein the composite
membrane has a thickness of 0.06 to 0.8 mils.
14. The fuel cell according to claim 10, wherein the composite
membrane has a thickness of 0.5 to 0.8 mils.
15. The fuel cell according to claim 10, wherein the composite
membrane has a thickness of at most 0.8 mils.
16. The fuel cell according to claim 10, wherein the composite
membrane has a thickness of at most 0.5 mils.
17. The fuel cell according to claim 10, wherein the ion exchange
resin is a perfluorinated sulfonic acid resin.
18. The fuel cell according to claim 17, wherein the polymeric
support is expanded polytetrafluoroethylene.
19. The fuel cell according to claim 18, wherein the composite
membrane has a thickness of at most 0.8 mils.
20. A fuel cell comprising an integral air impermeable composite
membrane comprising: a polymeric support having a microstructure of
micropores, said microstructure defining a porosity in the range of
about 70% to 98% within said support, at least one ion exchange
resin filling said microstructure such that said composite membrane
is air impermeable, said composite membrane having a thickness of
at most 0.8 mils.
21. The fuel cell of claim 20, wherein the thickness of said
composite membrane is in the range of between 0.06 and 0.8
mils.
22. The fuel cell of claim 20, wherein the thickness of said
composite membrane is in the range of between about 0.5 and 0.8
mils.
23. The fuel cell of claim 20, wherein the thickness of said
composite membrane is at most 0.5 mils.
24. The fuel cell of claim 20, wherein the polymeric support is
expanded polytetrafluoroethylene.
25. The fuel cell of claim 20, wherein the polymeric support is
expanded polytetrafluoroethylene and the ion exchange resin is a
perfluorinated sulfonic acid resin.
26. The fuel cell of claim 20, wherein said composite membrane is
prepared by, (a) providing a polymeric support having a
microstructure of micropores; (b) applying an ion exchange resin
solution to said polymeric support; and (c) repeating step (b)
until said micropores are sufficiently filled with ion exchange
resin to form an air impermeable composite membrane.
27. The fuel cell of claim 20, wherein the composite membrane is
heated at 60.degree. C. to 200.degree. C.
28. The fuel cell of claim 20, wherein the composite membrane is
heated at 120.degree. C. to 160.degree. C.
29. The fuel cell of claim 28, wherein the thickness of said
composite membrane is in the range of between about 0.5 and 0.8
mils, wherein the polymeric support is expanded
polytetrafluoroethylene, wherein the ion exchange resin is a
perfluorinated sulfonic acid resin, and wherein said composite
membrane is prepared by, (a) providing a polymeric support having a
microstructure of micropores; (b) applying an ion exchange resin
solution to said polymeric support; and (c) repeating step (b)
until said micropores are sufficiently filled with ion exchange
resin to form an air impermeable composite membrane.
30. A fuel cell consisting essentially of a composite membrane
consisting essentially of: a support having a microstructure of
micropores, said microstructure defining a porosity in the range of
about 20% to 98% within said support, at least one ion exchange
resin filling said microstructure such that said composite membrane
is air impermeable, said composite membrane having a thickness of
at most 0.8 mils, said composite membrane being heated to
60.degree. C. to 200.degree. C., and said ion exchange resin being
a perfluorinated sulfonic acid resin.
31. The fuel cell of claim 30, wherein said support has a pore
diameter of less than 10 microns.
32. The fuel cell of claim 30, wherein the thickness is at most 0.5
mils.
33. The fuel cell of claim 30, wherein the support is a polymeric
support.
34. The fuel cell of claim 35, wherein the membrane is heated at
120.degree. C. to 160.degree. C.
35. A fuel cell comprising a composite membrane consisting
essentially of: a support of expanded polytetrafluoroethylene
having a microstructure of micropores, said microstructure defining
a porosity in the range of about 70% to 98% within said support, at
least one perfluorinated sulfonic acid ion exchange resin filling
at least 90% of said microstructure, said composite membrane having
a thickness of at most 0.8 mils, said composite membrane being
heated to 60.degree. C. to 200.degree. C.
36. A fuel cell according to claim 35, wherein the pore diameter is
less than 10 microns.
37. A fuel cell according to claim 35, wherein the pore diameter is
between 0.05 microns and 5 microns.
38. A fuel cell according to claim 35, wherein the composite
membrane is heated to 120.degree. to 160.degree. C.
39. A fuel cell according to claim 35, wherein the micropores are
fully occluded.
40. A fuel cell according to claim 35, wherein a reinforcement
backing is bonded to the membrane.
Description
FIELD OF THE INVENTION
[0001] An integral composite membrane having a thickness of less
than about 1 mil (0.025 mm) is provided which is useful in
electrolytic processes and other chemical separations.
BACKGROUND OF THE INVENTION
[0002] Ion exchange membranes (IEM) are used in fuel cells as solid
electrolytes. A membrane is located between the cathode and anode
and transports protons formed near the catalyst at the hydrogen
electrode to the oxygen electrode thereby allowing a current to be
drawn from the cell. These membranes are particularly advantageous
as they replace heated acidic liquid electrolytes such as
phosphoric acid fuel cells which are very hazardous.
[0003] Ion exchange membranes are used in chloralkali applications
to separate brine mixtures and form chlorine gas and sodium
hydroxide. The membrane selectively transports the sodium ions
across the membrane while rejecting the chloride ions. IEM's are
also useful in the area of diffusion dialysis where for example,
caustic solutions are stripped of their impurities. The membranes
are also useful for pervaporation and vapor permeation separations
due to their ability to transfer polar species at a faster rate
than their ability to transfer non-polar species.
[0004] These membranes must have sufficient strength to be useful
in their various applications. Often this need for increased
strength requires the membranes to be made thicker which decreases
their ionic conductivity. For example, ion exchange membranes that
are not reinforced such as those commercially available from E.I.
DuPont de Nemours, Inc. and sold under the trademark Nafion are
inherently weak at small thicknesses (e.g., less than 0.050 mm) and
must be reinforced with additional materials causing the final
product to have increased thickness. Moreover, these materials
cannot be reliably manufactured pinhole free.
[0005] U.S. Pat. No. 3,692,569 to Grot 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 (not porous expanded
PTFE). This coating provides a topical treatment to the surface so
as to decrease the surface tension of the fluorocarbon polymer.
U.S. Pat. No. 4,453,991 to Grot relates to a process for making a
liquid composition of a perfluorinated polymer having sulfonic acid
or sulfonate groups in a liquid medium that is contacted with a
mixture of water and a second liquid such as a lower alcohol. The
liquid made by the process may be used as a coating, a cast film,
and as a repair for perfluorinated ion exchange films and
membranes. Cast or coated products made with the liquid composition
had thicknesses on the order of 5 mils (0.125 mm).
[0006] U.S. Pat. No. 4,902,308 to Mallouk, et al. relates to a film
of porous expanded PTFE having surfaces, both exterior and internal
surfaces adjacent to pores, coated with a metal salt of
perfluoro-cation exchange polymer. The base film of porous,
expanded PTFE had a thickness of between 1 mil and 6 mils
(0.025-0.150 mm). The final composite product had a thickness of at
least 1 mil (0.025 mm) and preferably had a thickness of between
1.7 and 3 mils (0.043-0.075 mm). The composite product was
permeable to air and the air flow as measured by the Gurley
densometer ASTM D72658 was found to be between 12 and 22
seconds.
[0007] U.S. Pat. No. 4,954,388 to Mallouk, et al. relates to an
abrasion-resistant, tear resistant, multi-layer composite membrane
having a film of continuous perfluoro ion exchange polymer attached
to a reinforcing fabric by means of an interlayer of porous
expanded PTFE. A coating of a perfluoro ion exchange resin was
present on at least a portion of the internal and external surfaces
of the fabric and porous ePTFE. The composite membrane made in
accordance with the teachings of this patent resulted in
thicknesses of greater than 1 mil (0.025 mm) even when the
interlayer of porous ePTFE had a thickness of less than 1 mil
(0.025 mm).
[0008] U.S. Pat. No. 5,082,472 to Mallouk, et al. relates to a
composite membrane of microporous film in laminar contact with a
continuous perfluoro ion exchange resin layer wherein both layers
have similar area dimensions. Surfaces of internal pores of ePTFE
may be coated at least in part with perfluoro ion exchange resin
coating. The membrane of ePTFE had a thickness of about 2 mils
(0.050 mm) or less and the perfluoro ion exchange layer in its
original state had a thickness of about 1 mil (0.025 mm). The ePTFE
layer of this composite membrane imparted mechanical strength to
the composite structure and the pores of the ePTFE were preferably
essentially unfilled so as to not block the flow of fluids.
[0009] U.S. Pat. Nos. 5,094,895 and 5,183,545 to Branca, et al.
relate to a composite porous liquid-permeable article having
multiple layers of porous ePTFE bonded together and having interior
and exterior surfaces coated with a perfluoro ion exchange polymer.
This composite porous article is particularly useful as a diaphragm
in electrolytic cells. The composite articles are described to be
relatively thick, preferably between from 0.76 and 5 mm.
[0010] U.S. Pat. No. 4,341,615 to Bachot, et al. relates to a
fluorinated resin base material treated with a copolymer of an
unsaturated carboxylic acid and a non-ionic unsaturated monomer for
use as a porous diaphragm in the electrolysis of alkaline metal
chlorides. The fluorinated resin base material may be reinforced
with fibers such as asbestos, glass, quartz, zirconia, carbon,
polypropylene, polyethylene, and fluorinated polyhalovinylidene
(col. 2, lines 13-17). Only 0.1 to 6 percent of the total pore
volume of the support sheet is occupied by the carboxylic
copolymer.
[0011] U.S. Pat. No. 4,604,170 to Miyake et al. relates to a
multi-layered diaphragm for electrolysis comprising a porous layer
of a fluorine-containing polymer having a thickness of from 0.03 to
0.4 mm with its interior and anode-side surface being hydrophilic
and an ion exchange layer on its cathode surface with the ion
exchange layer being thinner than the porous layer but of at least
0.005 mm and the total thickness of the diaphragm being from 0.035
to 0.50 mm.
[0012] U.S. Pat. No. 4,865,925 to Ludwig, et al. relates to a gas
permeable electrode for electrochemical systems. The electrode
includes a membrane located between and in contact with an anode
and a cathode. The membrane, which may be made of expanded
polytetrafluoroethylene, may be treated with an ion exchange
membrane material with the resulting membrane maintaining its
permeability to gas. Membrane thicknesses are described to be
between 1 and 10 mils, (0.025-0.25 mm), with thicknesses of less
than 5 mils (0.125) to be desirable. Examples show that membrane
thicknesses range from 15 to 21 mils.
[0013] Japanese Patent Application No. 62-240627 relates to a
coated or an impregnated membrane formed with a perfluoro type ion
exchange resin and porous PTFE film to form an integral unit. No
water or surfactant were used in the manufacture of this membrane.
The combination is accomplished by fusion or by coating and does
not provide for permanent adhesion of the ion exchange resin to the
inside surface of the PTFE film. The weight ratio of the perfluoro
ion exchange resin to PTFE is described to be in the range of 3 to
90% with a preferable weight ratio of 10 to 30%.
[0014] Japanese Application No. 62-280230 and 62-280231 relate to a
composite structure in which a scrim or open fabric is heat
laminated and encapsulated between a continuous perfluoro ion
exchange membrane and an ePTFE sheet thus imparting tear strength
to the structure. The composite membrane was not used for ionic
conduction.
[0015] Additional research has also been conducted on the use of
perfluorosulfonic acid polymers with membranes of expanded porous
polytetrafluoroethylene such as that described in Journal
Electrochem. Soc., Vol. 132, No. 2, February 1985, p. 514-515. The
perfluoro ion exchange material was in an ethanol based solvent
without the presence of water or surfactant. Moreover, ultrasonic
energy in the treatment of this membrane.
[0016] Heretofore and as represented by the references discussed
above, there is a need for an integral ultra-thin strong ion
exchange composite membrane, with long term chemical and mechanical
stability that has a thickness before swelling of at most 1 mil
(0.025 mm), with more than 90% of the pore volume of the membrane
filled with a perfluoro ionomer to render it at least substantially
occlusive and that is capable of swelling without deterioration of
mechanical properties.
SUMMARY OF THE INVENTION
[0017] An ultra-thin integral composite membrane is provided
including a porous polymeric membrane having a structure of
micropores of polymer with a porosity of greater than 35%, an
average pore diameter of less than 10 microns and a thickness of at
most 0.025 mm and a perfluoro ion exchange polymer impregnated
within the micropores so as to render the micropores substantially
occlusive, wherein the composite membrane is impermeable to gases
and liquids and is substantially free of pinholes. Porous polymeric
membranes suitable for this invention include membranes made of
perfluoroalkyloxy resin, fluorinated ethylene propylene,
polyolefins, polyamides, cellulosics, polycarbonates, fluorinated
and chlorinated polymers, and polysulfones. Perfluoro ion exchange
materials suitable for use with this invention include
perfluorinated sulfonic acid resin, perfluorinated carboxylic acid
resin, polyvinyl alcohol, divinyl benzene, and styrene based
polymers. A reinforcement backing may also be provided.
[0018] Methods for making the ultra-thin integral composite
membranes are also provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic cross-section of the composite
membrane that is fully impregnated with the ion exchange
material.
[0020] FIG. 2 is a schematic cross-section of the composite
membrane that is fully impregnated with the ion exchange material
and has a backing material attached thereto.
[0021] FIG. 3 is a photomicrograph of a cross-section of expanded
PTFE that has not been treated with any ion exchange material at a
magnification of 2.5 kX.
[0022] FIG. 4 is a photomicrograph of a cross-section of expanded
PTFE fully impregnated with an ion exchange material at a
magnification of 5.1 kX.
DETAILED DESCRIPTION OF THE INVENTION
[0023] An ultra-thin composite membrane is provided and includes a
base material of microporous membrane with a thickness less than 1
mil (0.025 mm) having a microstructure of micropores and perfluoro
ion exchange resin that substantially impregnates the microporous
membrane so as to occlude the micropores. The ultra-thin composite
membrane may be employed in many different types of applications
including for example, chemical separation, electrolysis in fuel
cells and batteries, pervaporation, gas separation, dialysis
separation, industrial electrochemistry such as chlor-alkali, and
other electrochemical devices, catalysis as a super acid catalyst
and use as a catalyst support in enzyme immobilization.
[0024] The ultra-thin composite membrane is mechanically strong and
is substantially and uniformly pore occlusive so that it is
particularly useful as an ion exchange material. Ultra-thin is
hereby defined as 1 mil (0.025 mm) or less. Uniform is hereby
defined as continuous impregnation with the ion exchange material
so that no pin holes or other discontinuities exist within the
composite structure. In addition, pore occlusive is hereby defined
as pores being substantially impregnated (i.e., at least 90%) with
the perfluoro ion exchange material rendering the final material
air impermeable with a Gurley number of infinity.
[0025] The microporous membrane which serves as the base material
for the composite has a porosity of greater than 35% and preferably
has a porosity of between 70-95%. The pores of the microstructure
have a diameter less than 10 .mu.m, are preferably between 0.05 and
5 .mu.m, and are most preferably about 0.2 .mu.m. The thickness of
the membrane is at most 1 mil (0.025 mm) preferably between 0.06
mils (0.19 .mu.m) and 0.8 mils (0.02 mm), and most preferably
between 0.50 mils (0.013 mm) and 0.75 mils (0.019 mm). Materials
from which this microporous membrane can be made include for
example, perfluoroalkyloxy (PFA), fluorinated ethylene propylene
(FEP), polyolefins, polyamides, cellulosics, polycarbonates,
fluorinated and/or chlorinated polymers, and polysulfones. A most
preferred material is expanded porous polytetrafluoroethylene
(PTFE) made in accordance with the teachings of U.S. Pat. No.
3,593,566 herein incorporated by reference. This material is
commercially available in a variety of forms from W.L. Gore &
Associates, In. of Elkton, Md., under the trademark GORE-TEX.RTM..
The expanded PTFE membrane can be made in a number of thickness
ranging from 0.00025 inches to 0.125 inches (6 .mu.m to 3 mm) with
the preferred thickness for the present invention being at most 1
mil (0.025 mm) and most preferably between 0.50 mils (0.013 mm) and
0.75 mils (0.019 mm). The expanded PTFE membrane can be made with
porosities ranging from 20% to 98%, with the preferred porosity for
the present invention being 70-95%. FIG. 3 shows a photomicrograph
of the internal microstructure of expanded PTFE used as the base
material.
[0026] An ion exchange material dissolved in a solvent and mixed
with a surfactant is uniformly applied so as to impregnate and
occlude the micropores of the base material. Suitable ion exchange
materials include for example, 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 a membrane. Solvents that are suitable for use
with the ion exchange material include for example, alcohols,
carbonates, THF (tetrahydrofuran), water, and combinations
thereof.
[0027] A surfactant having a molecular weight of greater than 100
must be employed with the ion exchange material to ensure
impregnation of the pores. Surfactants or surface active agents
having a hydrophobic portion and a hydrophilic portion may be
utilized. Preferable surfactants are those having a molecular
weight of greater than 100 and may be classified as anionic,
nonionic, or amphoteric which may be hydrocarbon or
fluorocarbon-based and include for example, Merpol.RTM., a
hydrocarbon based surfactant or Zonyl.RTM., a fluorocarbon based
surfactant, both commercially available from E.I. DuPont de
Nemours, Inc. of Wilmington, Del.
[0028] A most preferred surfactant is a nonionic material,
octylphenoxy polyethoxyethanol having a chemical structure: 1
[0029] where x=10 (average)
[0030] known as Triton.RTM. X100, commercially available from Rohm
& Haas of Philadelphia, Pa.
[0031] FIG. 1 shows a schematic view of the composite membrane with
ion exchange material 2 and the base material 4 so that the
micropores of the interior of the base material 4 are fully
impregnated. The final composite membrane has a uniform thickness
free of any discontinuities or pinholes on the surface. The
micropores of the membrane are 100% occluded thus causing the
composite membrane to be impermeable to liquids and gases. FIG. 4
shows a scanning electron photomicrograph of this composite
membrane.
[0032] Alternatively, the ion exchange material and surfactant
mixture 2 may be applied to the membrane 4 so that most of the
pores are uniformly treated rendering the membrane substantially
impregnated with the ion exchange material. The composite membrane
is still free of any discontinuities or pinholes.
[0033] 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 membrane. Suitable woven
materials include for example, scrims made of woven fibers of
expanded porous polytetrafluoroethylene, commercially available
from W.L. Gore & Associates, Inc., of Elkton, Md.; webs made of
extruded or oriented polypropylene netting commercially available
from Conwed, Inc. of Minneapolis, Minn.; and woven materials of
polypropylene and polyester of Tetko Inc., of Briarcliff Manor,
N.Y. Suitable non-woven materials include for example, a
spun-bonded polypropylene commercially available from Reemay Inc.
of Old Hickory, Tenn.
[0034] The treated membrane may then be further processed to remove
the surfactant with the use of various low molecular weight
alcohols. This is accomplished by soaking or submerging the
membrane in a solution of, for example, water, isopropyl alcohol,
methanol and/or glycerin. During this step, the surfactant which
was originally mixed in solution with the perfluoro ion exchange
material is removed. Slight swelling of the membrane occurs. The
perfluoro ion exchange material remains within the pores of the
base material as it is not soluble in the lower molecular weight
alcohol.
[0035] The membrane is further treated by boiling in a suitable
swelling agent, preferably water causing it to then 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 (at least 20 times higher than in its unswollen
state) that is also strong. The swollen membrane retains its
mechanical integrity unlike the membranes consisting only of the
perfluoro ion exchange material and simultaneously 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.
[0036] Although the membrane has excellent long term chemical
stability, it can be susceptible to poison by organics. The
organics can be removed by regeneration in which the membrane is
boiled in a strong acid such as nitric or chromic acid.
[0037] To prepare the inventive membrane, a support structure such
as a polypropylene woven fabric may first be laminated to the
untreated base membrane by any conventional technique including hot
roll lamination. ultrasonic lamination, adhesive lamination, forced
hot air lamination so long as the technique does not damage the
integrity of the membrane. A solution is prepared containing a
perfluorosulfonic acid resin in solvent mixed with one or more
surfactants. The solution may be applied to the membrane 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 micropores 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 60-200.degree. C., preferably 120-160.degree. C. so as
to lock the perfluoro ion polymer inside the membrane and prevent
it from migrating to the surface during drying This step 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.
[0038] The oven treated membrane is then soaked in a low molecular
weight alcohol, as described above to remove the surfactant. The
membrane is then boiled as described above in a swelling agent
under pressure ranging from 0 to 20 atmospheres absolute thereby
increasing the amount of swelling agent the treated membrane is
capable of holding.
[0039] Alternatively, the ion exchange material may be applied to
the membrane without the use of a surfactant. This procedure
requires additional exposure to the perfluoro ion exchange resin
but does not then need to be soaked in alcohol. A vacuum may also
be used to draw the ion exchange material into the membrane.
[0040] Another alternative to the process of preparing the
inventive membrane involves the selection and use of a surfactant
having low water solubility with the perfluoro ion solution.
Surfactants with low water solubility include Zonyl.RTM. FSO. a
fluorocarbon based surfactant commercially available from E.I.
DuPont de Nemours, Inc. By using this type of surfactant, the heat
treatment step may be eliminated. The resulting ion exchange
treated membrane made by this process may be used for different
aqueous applications and other chemical environments without any
effect due to the surfactant.
[0041] Because the base membrane is exceptionally thin (at most 1
mil) (0.025 mm) with the resulting composite membrane being very
thin and only marginally distorted in the x and y directions, it is
able to selectively transport ions at a faster rate than heretofore
has been achieved with only a slight lowering of the selectivity
characteristics of the membrane.
[0042] The following testing procedures were employed on the
samples prepared as described in the examples described below.
TEST PROCEDURES
AIR PERMEABILITY--Gurley Number Method
[0043] The resistance of samples to air flow was measured by a
Gurley densometer (ASTM D726-58) manufactured by W. & L.E.
Gurley & Sons. The results were reported in terms of Gurley
Number defined as the time in seconds for 100 cubic centimeters of
air to pass through 1 square inch (6.45 sq. cm.) of a test sample
at a pressure drop of 4.88 inches (12.4 cm.) of water.
STRENGTH MODULUS
[0044] Strength testing was carried out on an Instron Model 1122.
Samples were one inch wide. Gauge length (distance between clamps)
was two inches (5.08 cm.). Samples were pulled at a rate of 500%
per minute. The cross head speed was 20 inches per minute.
THICKNESS
[0045] 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 were also obtained with
the use of the snap gauge. Thicknesses of swollen samples were not
measurable due to the compression or residual water on the surface
of the swollen membrane with the snap gauge. Thickness measurements
of the swollen membrane 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)
Potassium Acetate Method
[0046] Moisture vapor transmission rates were determined by the
following procedure. Approximately 70 ml. of a solution consisting
of 35 parts by weight of potassium acetate and 15 parts by weight
of distilled water was placed into a 133 ml. polypropylene cup,
having an inside diameter of 6.5 cm. at its mouth. An expanded
polytetrafluoroethylene (PTFE) membrane having a minimum MVTR of
approximately 85,000 g/m.sup.2-24 hr as tested by the method
described in U.S. Pat. No. 4,862,730 to Crosby and available from
W.L. Gore & Associates, Inc. of Newark, Del., was heat sealed
to the lip of the cup to create a taut, leakproof, microporous
barrier containing the solution.
[0047] A similar expanded PTFE 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.
[0048] The sample to be tested was allowed to condition at a
temperature of 23.degree. C. and a relative humidity of 50% prior
to performing the test procedure. Samples were placed so the
microporous polymeric membrane was in contact with the expanded
polytetrafluoroethylene membrane mounted to the surface of the
water bath and allowed to equilibrate for at least 15 minutes prior
to the introduction of the cup assembly.
[0049] The cup assembly was weighed to the nearest {fraction
(1/1000)} g. and was placed in an inverted manner onto the center
of the test sample.
[0050] Water transport was provided by the driving force between
the water in the water bath and the saturated salt solution
providing water flux by diffusion in that direction. The sample was
tested for 10 minutes and the cup assembly was then removed,
weighed again within {fraction (1/1000)} g.
[0051] 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
[0052] Peel strength or membrane adhesion strength tests were
conducted on samples prepared with reinforced backings. Test
samples were prepared having dimensions of 3 inch by 3.5 inch (7.62
cm.times.8.89 cm). A 4 inch by 4 inch (10.2 cm.times.10.2 cm)
chrome steel plate with an Instron tensile test machine Model No.
1000 were also used. Double coated vinyl tape (3M -#419) 1 inch
(2.54 cm) wide was placed over the edges of the chrome plate so
that tape covered all edges of the plate.
[0053] The sample material was then mounted on top of the adhesive
exposed side of the tape and pressure was applied so that sample
was secured.
[0054] The plate and sample were then installed in the Instron in a
horizontal position. The upper crosshead was lowered so that the
jaws of the machine closed flat and tightened upon the sample. The
upper crosshead was then slowly raised pulling the layer of
perfluoro ion material from the reinforced backing. When the
composite membrane had been detached from the reinforced backing,
the test was complete. Adhesion strength was estimated from the
average strength needed to pull the composite membrane from the
reinforced backing.
ELECTRICAL CONDUCTANCE
[0055] The electrical conductance in the Z-direction (otherwise
known as through conductance) was measured. A sample of swollen
composite membrane (cut to a 1 inch diameter circle) was placed
between two 0.680 inch (1.73 cm) diameter copper contacts. A 5 lb.
(2268 g) weight was placed above the top contact. The contacts were
connected to a Hewlett Packard Model 3478A multimeter. The
resistance was then read. Prior to this measurement, the thickness
of the dried preswollen composite membrane was determined as
described above. Conductance was calculated according to the
formula:
C=1/R
[0056] wherein
[0057] R=resistance measured in ohms
[0058] C=conductance measured in mhos
IONIC CONDUCTION RATE
[0059] The conductivity of the composite membrane was tested to
measure the ionic conduction rate in terms of micromhos per minute.
This test was performed with two 900 ml. compartments between which
the treated membrane was placed. The exposed surface area of
membrane was 7.07 sq in. (45.65 sq cm). One compartment (the
retentate side) was filled with 1 M NaCl solution. The other side
(the permeate side) was filled with pure distilled water. Both
compartments were stirred continuously and at the same speed with
two electric mixers using polypropylene impellers. The conductivity
of the permeate side was recorded every 5 minutes for an hour with
a hand-held conductivity meter, Omega Model No. PHH80. The total
ionic conduction rate was determined by taking the average slope of
a graph of conductivity over time.
LINEAR EXPANSION TESTS
[0060] The percentage swelling in the x- and y-directions were
determined. The length and width of the composite membranes were
first measured with a Mitutoyo Model # 505-627-50 caliper prior to
boiling and swelling. Final measurements were taken after the
samples were boiled and swelled. Percent linear expansion were then
calculated for both the x- and y-directions.
WEIGHT CHANGE
[0061] The percent weight change on samples was also prepared. Here
composite membranes were weighed prior to boiling and swelling and
then after swelling. All weight measurements were taken with
Mettler Balance, Model No. AT400. The percent weight change was
then calculated.
EXAMPLE 1
[0062] A sample of expanded polytetrafluoroethylene membrane made
in accordance with the teachings of U.S. Pat. No. 3,593,566, herein
incorporated by reference. The membrane, with a nominal thickness
of 0.75 mils (0.02 mm) and a 0.2 micrometer pore size, was mounted
on a 6 inch wooden embroidery hoop. A solution was prepared
comprising 95% by volume of a perfluorosulfonic
acid/tetra-fluoroethylene copolymer resin (in H+form) in a solution
of low molecular weight alcohols comprising propanol, butanol, and
methanol known as Nafion NR-50 (1100 EW) commercially available
from E.I. DuPont de Nomours, Inc. and 5% of a nonionic surfactant
of octyl phenoxy poly ethoxyethanol known as Triton X100
commercially available from Rohm & Haas of Philadelphia, Pa.
This solution was brushed on both sides of the membrane so as to
impregnate and substantially occlude the micropore structure. 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
micropores. The sample was then soaked in isopropanol for 5 minutes
to remove the surfactant. After rinsing with distilled water and
allowing to dry at room temperature, a final coat of the
Nafion-surfactant solution as described above 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 10 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 strength modulus may be found in Table
5; percent linear expansion may be found in Table 6; and percent
weight change of this sample may be found in Table 7. The swollen
membrane was later dried to a dehydrolyzed 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
[0063] A sample of expanded porous PTFE membrane made in accordance
with the teachings of U.S. Pat. No. 3,593,566 having a pore size of
0.2 micrometers and 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 of 96% by volume of a perfluorosulfonic
acid TFE copolymer resin in alcohol Nafion NR-50 (1100 EW)
commercially available from E.I. DuPont de Nemours, Inc. and 4% of
the nonionic surfactant Triton X-100 obtained from Rohm & Haas
was prepared. This solution was brushed on the membrane side only
to substantially occlude the micropores and the sample was dried in
an oven at 130.degree. C. This procedure was repeated three more
times to fully occlude the micropores. 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.
[0064] 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
[0065] A sample of expanded porous polytetrafluoroethylene membrane
made in accordance with the teachings of U.S. Pat. No. 3,593,566,
having a thickness of 0.5 mils (0.01 mm) with a pore size of 0.2
micrometer was mounted on a 6 inch wooden embroidery hoop. A
solution of 100% Nafion resin solution, perfluorosulfonic acid/TFE
copolymer resin in a solvent mixture of propanol, butanol, and
methanol known commercially from E.I. DuPont de Nemours, Inc. as
Nafion.RTM. solution NR-50 (1100 EW) without the addition of any
surfactants was brushed onto both sides of the membrane to
substantially occlude the micropores. The sample was then placed in
an oven at 160.degree. C. to dry. This procedure was repeated four
more times until the membrane was completely transparent and the
micropores were 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. The electrical conductivity was measured and summarized
in Table 2.
EXAMPLE 4
[0066] A sample of expanded porous polytetrafluoroethylene membrane
made in accordance with the teachings of U.S. Pat. No. 3,593,566
having a thickness of 0.5 mils (0.01 mm) and a pore size 0.2
micrometers was mounted on a 6 inch wooden embroidery hoop. A
solution of 99% by volume Nafion NR-50 commercially available from
E.I. DuPont de Nemours, Inc. and 1% surfactant mixture was
prepared. The surfactant mixture consisted of 50% of a nonionic
surfactant, Triton X-100 commercially available from Rohm &
Haas Corp. and 50% Zonyl FSO commercially available from E.I.
DuPont de Nemours, Inc. This solution was brushed on both sides of
the membrane and was allowed to dry at room temperature. This
procedure was repeated 4 more times until the sample was completely
transparent and to fully occlude the micropores. The sample was not
treated so as to remove the surfactant. The composite membrane was
boiled in distilled water for 5 minutes causing the membrane to
swell. The Gurley number for this material is summarized in Table
3.
EXAMPLE 5
[0067] A sample of expanded porous polytetrafluoroethylene membrane
made in accordance with the teachings of U.S. Pat. No. 3,593,566,
having a thickness of 0.5 mils (0.01 mm) with a pore size of 0.2
micrometer was mounted onto a 6 inch wooden embroidery hoop. A
solution of 95% by volume Nafion N R-50 (1100 EW) commercially
available from E.I. DuPont de Nemours, Inc. and 5% of a nonionic
surfactant, Triton X-100 commercially available from Rohm &
Haas was prepared. 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 micropores. 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 Nafion-Triton 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.
Moisture vapor transmission rates for the treated membrane were
measured and are summarized in Table 1. The Gurley number of the
treated membrane are summarized in Table 3.
EXAMPLE 6
[0068] A sample of expanded porous polytetrafluoroethylene membrane
made in accordance with the teachings of U.S. Pat. No. 53,593,566,
having a nominal thickness of 0.75 mils (0.02 mm) and a pore size
of 0.2 micrometers was mounted onto a 6 inch wooden embroidery
hoop. The Gurley Densometer air flow on this membrane was 2-4
seconds. A solution of 95% by volume Nafion NR-50 (1100 EW)
commercially available from E.I. DuPont de Nemours, Inc. and 5%
Triton X-100 non-ionic surfactant from Rohm & Haas was
prepared. 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 Nafion 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. 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 7
[0069] A sample of expanded porous polytetrafluoroethylene membrane
made in accordance with the teachings of U.S. Pat. No. 3,593,566,
having a nominal thickness of 0.75 mils (0.02 mm) and a pore size
of 0.2 micrometers was mounted onto a 5 inch plastic embroidery
hoop. The Gurley Densometer air flow on this membrane was 2-4
seconds. A solution of 95% by volume Nafion NR-50 (1100 EW)
commercially available from E.I. DuPont de Nemours, Inc. and 5%
Triton X-100 non-ionic surfactant from Rohm & Haas was
prepared. 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 so as
to fully occlude the micropores. 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 same
Nafion NR-50 Triton X-100 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. No boiling
occurred.
[0070] 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 9.81 grams per square meter of membrane. A sample of
Nafion 115 (5 mils, 0.13 mm) commercially available from E.I.
DuPont de Nemours, Inc. was cut to a 1 inch (25.4 mm) by 1 inch
(25.4 mm) sample, weighed and found to be 216 grams per square
meter.
Nafion Comparative Samples
[0071] 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. The samples were then dried at room
temperature to an unswollen state and then remeasured from which
expansion and weight change measurements found in Tables 6 and 7
were calculated. Naflon 115, a perfluorosulfonic acid cation
exchange membrane, unreinforced film of 1100 equivalent weight also
commercially available from E.I. DuPont de Nemours, Inc., having a
nominal thickness of 5 mils (0.1 mm) was obtained. This sample was
also obtained commercially in the hydrated swollen state.
1TABLE 1 Moisture Vapor Transmission Rates (MVTR) Sample ID* MVTR
(grams/m.sup.2-24 hrs. 5 25,040 Nafion 117 23,608 *Measurements
were obtained on samples in their swollen state.
[0072]
2TABLE 2 Electrical Conductivity Sample ID* Conductivity
(micromhos) 3 1,277 Nafion 117 1,214 *Measurements were obtained on
samples in their swollen state.
[0073]
3TABLE 3 Gurley Numbers Final Swollen Thickness Base Material
Membrane Sample ID (mm)* Gurley No. (sec) Gurley Number (sec) 1
0.02 2-4 Total occlusion 2 0.02 2-4 Total occlusion 3 0.01 2-4
Total occlusion 4 0.01 2-4 Total occlusion 5 0.01 2-4 Total
occlusion *Thickness measurements were obtained on samples prior to
swelling in dried state.
[0074]
4TABLE 4 Ionic Conduction Rate Ionic Conduction Rate Sample ID
(micromhos/minute) 1 119 (swollen) 6 5.1 (unswollen) Nafion 115
15.9 (swollen)
[0075]
5TABLE 5 Strength Modulus Thickness Strength Modulus Sample ID (mm)
(lb per dry sq. in.)** 1 0.02* 15150 Nafion 115 0.13 12750
*Thickness measurements were obtained prior to swelling in dried
state. **Strength modulus measurements were obtained in the swollen
state.
[0076]
6TABLE 6 Percent Linear Expansion Average Average Un- % Un- %
swollen Swollen Expansion swollen Swollen Exapnsion Sample (x) (x)
in (y) (y) in ID (mm) (mm) x-direction (mm) (mm) y-direction 1
124.4 124.4 -- 123.3 123.3 -- Nafion 125.5 137.7 +9.7 127.3 149.7
+17 117
[0077]
7TABLE 7 Percent Weight Change Unswollen Swollen % weight Sample ID
wt (g) wt (wet) (g) change (wet) 1 0.2515 1.0273 +308% Nafion 117
5.5700 7.5106 +35%
* * * * *