U.S. patent application number 11/050194 was filed with the patent office on 2005-06-16 for process for dissolution of highly fluorinated ion-exchange polymers.
Invention is credited to Sun, Qun.
Application Number | 20050131116 11/050194 |
Document ID | / |
Family ID | 34652013 |
Filed Date | 2005-06-16 |
United States Patent
Application |
20050131116 |
Kind Code |
A1 |
Sun, Qun |
June 16, 2005 |
Process for dissolution of highly fluorinated ion-exchange
polymers
Abstract
Highly fluorinated ion-exchange polymers achieve dissolution in
aqueous tetrahydrofuran at lower pressures and temperatures than in
other solvents, with few or no side products being formed.
Inventors: |
Sun, Qun; (Wilmington,
DE) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY
LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
34652013 |
Appl. No.: |
11/050194 |
Filed: |
February 3, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11050194 |
Feb 3, 2005 |
|
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10194491 |
Jul 12, 2002 |
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Current U.S.
Class: |
524/113 ;
524/111; 524/544; 524/545; 524/546 |
Current CPC
Class: |
Y02E 60/50 20130101;
H01M 8/1081 20130101; Y02P 70/50 20151101; C08J 3/095 20130101;
H01M 4/8668 20130101; C08J 5/20 20130101; H01M 2300/0082 20130101;
H01M 8/1039 20130101; H01M 8/1023 20130101 |
Class at
Publication: |
524/113 ;
524/111; 524/544; 524/545; 524/546 |
International
Class: |
C08J 003/00 |
Claims
What is claimed is:
1. A process for making a liquid composition of highly fluorinated
ion-exchange polymer comprising a) contacting the polymer with
aqueous tetrahydrofuran, and b) heating said polymer in contact
with aqueous tetrahydrofuran to form the liquid composition.
2. The process of claim 1 wherein said aqueous tetrahydrofuran is
comprised of water and tetrahydrofuran in a ratio of about 10:90 to
about 90:10 weight %.
3. The process of claim 1 wherein said aqueous tetrahydrofuran is
comprised of water and tetrahydrofuran in a ratio of about 45:55 to
about 55:45 weight %.
4. The process of claim 1 wherein said aqueous tetrahydrofuran is
comprised of less than about 50 weight % tetrahydrofuran.
5. The process of claim 1 wherein the polymer comprises about 1 to
about 15 weight % of the combined weight of polymer and water and
tetrahydrofuran.
6. The process of claim 1 wherein said polymer in contact with
aqueous tetrahydrofuran is heated to a temperature of about
150.degree. C. to about 300.degree. C.
7. The process of claim 1 wherein said polymer in contact with
aqueous tetrahydrofuran is heated in a autoclave.
8. The process of claim 1 wherein said highly fluorinated
ion-exchange polymer is in the acid form, sodium ion form or
potassium ion form, or a combination thereof.
9. The process of claim 1 wherein said highly fluorinated
ion-exchange polymer is in the acid form.
10. The process of claim 1 wherein said ion-exchange polymer is
perfluorinated.
11. The process of claim 1 wherein said ion-exchange polymer has an
ion-exchange ratio of about 3 to about 33.
12. The process of claim 1 wherein said ion-exchange polymer has an
ion-exchange ratio of about 8 to about 23.
13. The process of claim 1 wherein said composition is
substantially free of products from reactions of the solvent during
preparation of said composition.
14. A liquid composition consisting essentially of ion-exchange
polymer in aqueous tetrahydrofuran, at least about 50% of the total
number of halogen and hydrogen atoms in said ion-exchange polymer
being fluorine atoms.
15. The liquid composition of claim 14 wherein the aqueous
tetrahydrofuran is from about 10:90 weight % water:tetrahydrofuran
to about 90:10 weight % water:tetrahydrofuran.
16. The liquid composition of claim 14 wherein the aqueous
tetrahydrofuran is from about 45:55 weight % water:tetrahydrofuran
to about 55:45 weight % water:tetrahydrofuran.
17. The liquid composition of claim 14 wherein said ion-exchange
polymer is in the acid form.
18. The liquid composition of claim 14 wherein said ion-exchange
polymer is about 1 to about 15 weight % of the combined weights of
the polymer and aqueous tetrahydrofuran.
19. The liquid composition of claim 14 wherein said ion-exchange
polymer is perfluorinated.
20. The liquid composition of claim 14 wherein said ion-exchange
polymer has an ion-exchange ratio of about 3 to about 33.
21. The liquid composition of claim 14 wherein said ion-exchange
polymer has an ion-exchange ratio of about 8 to about 23.
22. The liquid composition of claim 14 wherein said composition
contains less than about 3 weight % organic materials other than
said tetrahydrofuran and those associated with said ion exchange
polymer.
Description
FIELD OF THE INVENTION
[0001] This invention is in the field of liquid compositions of
highly fluorinated ion-exchange polymer.
BACKGROUND OF THE INVENTION
[0002] Highly fluorinated ion-exchange polymers, such as the
sulfonyl type disclosed in U.S. Pat. No. 3,282,875, are used in
membrane form as separators in electrochemical cells. The polymers
are also useful as acid catalysts. These applications first used
melt-fabricated, i.e. melt-processed, shapes such as films and
pellets. Because the ion-exchange polymers are difficult to
melt-process in the ionic form, fabrication is carried out on
polymer in a melt-processible precursor form, and the fabricated
article is then hydrolyzed to convert the polymer to the ionic
(also referred to as the ion-exchange) form. Later, methods were
discovered for making liquid compositions of highly fluorinated
ion-exchange polymers: U.S. Pat. Nos. 4,433,082 and 6,150,426. From
such liquid compositions, ion-exchange membranes can be made by
film-casting techniques. Catalyst can be made by coating liquid
compositions on inert substrates. Liquid compositions have also
found use in making electrodes for fuel cells.
[0003] Dissolution of the above polymers in their ionic forms
requires solvents and temperatures sufficient to overcome the
forces that hold the polymer together in the solid state. These
forces include the polar attractions of the ionic groups for one
another. Polar solvents such as water and alcohol can solvate the
ionic groups of the polymer, weakening their interaction, and
promoting dissolution. Heat further weakens intermolecular
attractions.
[0004] Liquid compositions are typically made by putting highly
fluorinated ion-exchange polymer that is in the sulfonic acid form,
in alcohol, in water, or in aqueous alcohol, and heating the
combination to achieve the dissolution of the polymer. The
temperatures necessary are generally 220.degree. C. or higher.
These temperatures are above the boiling point of the solvent at
atmospheric pressure and therefore the dissolution is conducted in
an autoclave. Higher temperatures are necessary with water. Alcohol
is a better solvent, and lower temperatures can be used. However,
the formation of side products such as ether and olefin through the
reaction of the alcohol with the strongly acid polymer contributes
to the development of pressure during dissolution. Water-alcohol,
i.e. aqueous alcohol, is effective at lower temperatures than are
necessary with water alone, and also does not develop pressures so
high as occur with alcohol alone. Nevertheless, though reduced in
quantity, side products still form, adding to reaction pressure,
requiring separation from the liquid composition and disposal, and
resulting in loss of solvent. New solvents are needed that are
effective at lower temperatures and that produce less side
product.
SUMMARY OF THE INVENTION
[0005] A process for making a liquid composition of highly
fluorinated ion-exchange polymer comprising
[0006] a) contacting the polymer with aqueous tetrahydrofuran,
and
[0007] b) heating said polymer in contact with aqueous
tetrahydrofuran to form the liquid composition.
[0008] The invention further provides a liquid composition of
highly fluorinated ion-exchange polymer in aqueous
tetrahydrofuran.
DETAILED DESCRIPTION
[0009] Polymers for use in accordance with the present invention
are highly fluorinated ion-exchange polymers having sulfonate
functional groups. "Highly fluorinated" means that at least about
50% of the total number of halogen and hydrogen atoms in the
polymer are fluorine atoms, preferably at least about 75%, and more
preferably at least about 90%. Most preferably, the polymer is
perfluorinated. The term "sulfonate functional group" refers to
either to sulfonic acid groups or salts of sulfonic acid groups,
preferably alkali metal or ammonium salts. The functional group is
represented by the formula --SO.sub.3X where X is a cation, also
known as a "counterion". X may be H, Li, Na, K or
N(R.sub.1)(R.sub.2)(R.sub.3)(R.sub.4), and R.sub.1, R.sub.2,
R.sub.3, and R.sub.4 are the same or different and are preferably
H, CH.sub.3 or C.sub.2H.sub.5. More preferably, X is H, in which
case the polymer is said to be in the "acid form". X may also be
multivalent, as represented by such ions as Ca.sup.++, and
Al.sup.+++. It is clear to the skilled artisan that in the case of
multivalent counterions, represented generally as M.sup.n+, the
number of sulfonate functional groups per counterion will be equal
to the valence "n".
[0010] Preferably, the polymer comprises a polymer backbone with
recurring side chains attached to the backbone, the side chains
carrying cation exchange groups. Polymers include homopolymers or
copolymers of two or more monomers. Copolymers are typically formed
from a nonfunctional monomer and a second monomer carrying the
cation exchange group or its precursor, e.g., a sulfonyl fluoride
group (--SO.sub.2F), which can be subsequently hydrolyzed to a
sulfonate functional group. For example, copolymers of a first
fluorinated vinyl monomer together with a second fluorinated vinyl
monomer having a sulfonyl fluoride group (--SO.sub.2F) can be used.
Possible first monomers include tetrafluoroethylene (TFE),
hexafluoropropylene, vinyl fluoride, vinylidine fluoride,
trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkyl vinyl
ether), and combinations thereof. TFE is a preferred first
monomer.
[0011] Possible second monomers include fluorinated vinyl ethers
with sulfonate functional groups or precursor groups which can
provide the desired side chain in the polymer. Additional monomers,
including ethylene, propylene, and R--CH.dbd.CH.sub.2 where R is a
perfluorinated alkyl group of 1 to 10 carbon atoms, can be
incorporated into these polymers if desired. The polymers may be of
the type referred to herein as random copolymers, that is
copolymers made by polymerization in which the relative
concentrations of the comonomers are kept as constant as possible,
so that the distribution of the monomer units along the polymer
chain is in accordance with their relative concentrations and
relative reactivities. Less random copolymers, made by varying
relative concentrations of monomers in the course of the
polymerization, may also be used. Polymers of the type called block
copolymers, such as that disclosed in European Patent Application
No. 1,026,152 A1, may also be used.
[0012] Preferred polymers for use in the present invention include
a highly fluorinated, most preferably perfluorinated, carbon
backbone and side chains represented by the formula
--(O--CF.sub.2CFR.sub.f).sub.a--O--CF.sub.2CFR'.sub.fSO.sub.3X
[0013] wherein Rf and R'f are independently selected from F, Cl or
a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0, 1 or
2, and X is H, Li, Na, K or N(R1)(R2)(R3)(R4) and R1, R2, R3, and
R4 are the same or different and are preferably H, CH.sub.3 or
C.sub.2H.sub.5. More preferably X is H. As stated above, X may also
be multivalent.
[0014] The preferred polymers include, for example, polymers
disclosed in U.S. Pat. No. 3,282,875 and in U.S. Pat. Nos.
4,358,545 and 4,940,525. An example of preferred polymer comprises
a perfluorocarbon backbone and the side chain represented by the
formula
--O--CF.sub.2CF(CF.sub.3)--O--CF.sub.2CF.sub.2SO.sub.3X
[0015] where X is as defined above. Polymers of this type are
disclosed in U.S. Pat. No. 3,282,875 and can be made by
copolymerization of tetrafluoroethylene (TFE) and the
perfluorinated vinyl ether
CF.sub.2.dbd.CF--O--CF.sub.2CF(CF.sub.3)--O--CF.sub.2CF.sub.2SO.sub.2F,
perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) (PDMOF),
followed by conversion to sulfonate groups by hydrolysis of the
sulfonyl fluoride groups and ion exchanged as necessary to convert
them to the desired ionic form. An example of a preferred polymer
of the type disclosed in U.S. Pat. Nos. 4,358,545 and 4,940,525 has
the side chain --O--CF.sub.2CF.sub.2SO.sub.3X, wherein X is as
defined above. This polymer can be made by copolymerization of
tetrafluoroethylene (TFE) and the perfluorinated vinyl ether
CF.sub.2.dbd.CF--O--CF.sub.2CF.sub.2SO.sub- .2F,
perfluoro(3-oxa-4-pentenesulfonyl fluoride) (POPF), followed by
hydrolysis and further ion exchange as necessary.
[0016] The polymers of this invention preferably have an ion
exchange ratio of less than about 33. In this application, "ion
exchange ratio" or "IXR" is defined as number of carbon atoms in
the polymer backbone in relation to the cation exchange groups.
Within the range of less than about 33, IXR can be varied as
desired for the particular application. With most polymers, the IXR
is preferably about 3 to about 33, more preferably about 8 to about
23.
[0017] The cation exchange capacity of a polymer is often expressed
in terms of equivalent weight (EW). For the purposes of this
application, equivalent weight (EW) is defined to be the weight of
the polymer in acid form required to neutralize one equivalent of
sodium hydroxide. In the case of a sulfonate polymer where the
polymer has a perfluorocarbon backbone and the side chain is
--O--CF.sub.2--CF(CF.sub.3)--O--CF.sub.2--- CF.sub.2--SO.sub.3H (or
a salt thereof), the equivalent weight range which corresponds to
an IXR of about 8 to about 23 is about 750 EW to about 1500 EW. IXR
for this polymer can be related to equivalent weight using the
formula: 50 IXR+344=EW. While the same IXR range is used for
sulfonate polymers disclosed in U.S. Pat. Nos. 4,358,545 and
4,940,525, e.g., the polymer having the side chain
--O--CF.sub.2CF.sub.2SO.sub.3H (or a salt thereof), the equivalent
weight is somewhat lower because of the lower molecular weight of
the monomer unit containing a cation exchange group. For the
preferred IXR range of about 8 to about 23, the corresponding
equivalent weight range is about 575 EW to about 1325 EW. IXR for
this polymer can be related to equivalent weight using the formula:
50 IXR+178=EW.
[0018] Temperatures for use in the process can be within the range
of about 150.degree. C. to about 300.degree. C. It has been
discovered that with aqueous tetrahydrofuran, the dissolution of
highly fluorinated ion-exchange polymer can be achieved at lower
temperature and pressure than with known solvents. It will be
recognized by those skilled in the art of making such liquid
compositions, that the IXR of the polymer affects the temperature
required. The higher the IXR, the lower the concentration of the
ion-exchange groups in polymer, and the greater the crystallinity
due to the greater tetrafluoroethylene content. Other things being
equal, higher temperatures are necessary to achieve the dissolution
of higher IXR polymers. In the case of polymer of IXR=14.3 in the
acid form using aqueous THF solvent, temperatures of 160.degree. C.
to 260.degree. C. are effective, temperatures of 180.degree. C. to
240.degree. C. are preferred, and temperatures of 190.degree. C. to
220.degree. C. are more preferred.
[0019] In the water plus THF combinations referred to herein under
the general term "aqueous THF" or "aqueous tetrahydrofuran" the
water:tetrahydrofuran ratio, on a weight basis, is about 1:99 to
about 99:1. For rapid dissolution at minimum temperature and
pressure, the water:tetrahydrofuran ratio is preferably about 10:90
to about 90:10, more preferably about 20:80 to about 80:20, still
more preferably about 40:60 to about 60:40, and most preferably
about 45:55 to about 55:45. However, it is found that higher solids
are attainable while maintaining the liquid composition in a
pourable state if tetrahydrofuran (THF) is <50 wt % of the
solvent. If the desire for higher solids liquid compositions takes
precedence over lower dissolution temperatures and pressures, the
preferred water:THF ratio is about 50:50 to about 99:1, more
preferably about 50:50 to about 90:10, still more preferably about
50:50 to about 75:25, and most preferably about 50:50 to about
60:40.
[0020] The acid form and the sodium ion and potassium ion forms and
combination thereof, are preferred forms of the highly fluorinated
ion-exchange polymer for making solution. The sodium and potassium
forms and other ionic forms can be made according to the teachings
of this invention, following substantially the same procedure as
used for the acid form. The acid form is the most preferred form of
the polymer for use in making solution since it is desirable for
most applications that the polymer in the resulting solution be in
acid form. Starting with the acid form avoids a subsequent acid
exchange process step. Somewhat higher temperatures are necessary
when the sodium and potassium forms are used instead of the acid
form.
[0021] A further surprising aspect of this invention is that after
heating to achieve the dissolution of the polymer in aqueous THF,
little or no ether, olefin, or other organic product of the
reaction of the THF with the acid polymer is found. Because of the
absence of these volatile side products, the pressures developed
during dissolution are much lower, which reduces the cost of the
equipment used in making the polymer liquid composition and makes
for a more easily run reaction. Because little or no side products
form, the resulting polymer liquid composition is preferably
substantially free of side products. By "substantially free" is
meant that the polymer liquid composition contains less than about
3 weight %, preferably 1 weight % organic materials other than THF
and those associated with the polymer. The resulting polymer liquid
composition need not be further treated to remove the side
products, which are impurities. As a result, substantially all the
solvent may be recovered and used again. THF recovery is
facilitated by the fact that it forms an azeotrope with water
(boiling point 64.degree. C., 95:5 wt:wt THF:water at atmospheric
pressure).
[0022] The solids concentration in the liquid compositions of this
invention are preferably about 1 to about 15 wt %, more preferably
about 5 to about 12 wt %, and most preferably about 6 to about 10
wt %. As solids concentration increases, viscosity rises until the
liquid composition is not pourable and takes on the character of a
gel. The practical solids limit is determined by the viscosity that
can be tolerated. Temperature enters into this because if the
liquid composition can be kept at higher temperature, or if it is
used as soon as it is made, higher solids liquid compositions are
acceptable.
[0023] The liquid compositions of this invention can be made in any
vessel rated for the pressures encountered at the temperatures used
to achieve dissolution. The material of construction of the vessel
should have corrosion resistance, such as is provided by nickel
alloys such as Hastelloy-C. Dissolution will occur if the vessel
containing the polymer and aqueous THF is simply heated for a
sufficient time. Agitation however, is preferred to reduce the time
needed for dissolution of the polymer in aqueous THF. Agitation may
be accomplished by imparting motion to the vessel itself,
preferably by shaking or rocking. Alternatively and preferably, the
contents alone may be agitated through use of a vessel having an
agitator to stir or mix the polymer and aqueous THF.
EXAMPLES
[0024] The perfluorinated ion-exchange polymer used is Nafion.RTM.
perfluorinated sulfonic acid polymer made by the DuPont Company,
Wilmington Del. USA. The equivalent weight of the polymer is 1060
(IXR=14.3). The polymer is available from Aldrich Chemical Co.
Milwaukee Wis. USA. The solvents, THF and methanol, are reagent
grade. Distilled water is used.
[0025] The reactor used for dissolving the Nafion.RTM. resin in
THF/water solution is a 300 ml vertical stirred autoclave from
Autoclave Engineers (Erie Pa. USA). The reactor is made of
Hastelloy-C. It can be operated up to 3000 psi (20 MPa). Typically,
it takes about 30 minutes to heat the autoclave from room
temperature to 180.degree. C.
Example 1
[0026] The 300 ml autoclave is charged with 16 g perfluorinated
ion-exchange polymer pellets (approximately cylindrical, 1
mm.times.1 mm), 92 g distilled water and 92 g reagent grade THF.
The charge consists of 8 wt % polymer. The autoclave is heated to
190.degree. C. in 30 minutes with agitation speed set at 1000 rpm.
The pressure in the autoclave is 310 psi (2.1 MPa). After 4 hours
heating and agitation the autoclave is cooled to room temperature.
The autoclave pressure is zero (gauge), indicating no formation of
side products such as ethers or olefins that exert vapor pressure
over and above that of the THF:water solution at room temperature.
On draining, the autoclave is found to contain only liquid, a
homogenous liquid composition of the perfluorinated ion-exchange
polymer. No second layer of liquid is seen, further indication that
no side product ethers are formed. The absence of solids shows that
dissolution of the polymer pellets is complete.
Example 2
[0027] The 300 ml autoclave is charged with 16 g perfluorinated
ion-exchange polymer, 92 g distilled water and 92 g THF. The
mixture is heated to 200.degree. C. after 35 minutes at the
agitation speed of 1500 rpm. The autoclave pressure is 370 psi (2.6
MPa) at 200.degree. C. Heating and agitation is continued for an
additional 4 hours. After cooling to room temperature, the
autoclave is drained and found to contain a homogenous liquid
composition of the perfluorinated ion-exchange polymer.
Comparative Example
[0028] For comparison, perfluorinated ion-exchange polymer liquid
composition is made according to the general teaching of U.S. Pat.
No. 4,433,082, Example 11. The autoclave is charged with 17 g
perfluorinated ion-exchange polymer, 23 g methanol, 50 g
n-propanol, and 100 g distilled water. It is heated to 230.degree.
C. and with mechanical agitation for 3 hours. The autoclave
pressure is 1000-1100 psi (6.9-7.6 MPa). After cooling to room
temperature, some pressure remains and is vented. The liquid
obtained separates into two layers. The upper layer, composed
largely of ethers, is about 10% by volume of the total amount of
liquid. This example shows that, compared to the process of this
invention, the use of aqueous alcohol to achieve dissolution of
perfluorinated ion-exchange polymer generates 3-4 fold higher
pressures and significant volumes of solvent side products.
* * * * *