U.S. patent application number 13/045463 was filed with the patent office on 2011-06-30 for stabilized fluoropolymer and method for producing same.
This patent application is currently assigned to DAIKIN INDUSTRIES, LTD.. Invention is credited to Yasuhiro HASHIMOTO, Eiji HONDA, Hideki IIJIMA, Tadashi INO, Tadaharu ISAKA, Masahiro KONDO.
Application Number | 20110159402 13/045463 |
Document ID | / |
Family ID | 34380295 |
Filed Date | 2011-06-30 |
United States Patent
Application |
20110159402 |
Kind Code |
A1 |
HONDA; Eiji ; et
al. |
June 30, 2011 |
STABILIZED FLUOROPOLYMER AND METHOD FOR PRODUCING SAME
Abstract
The present invention provides a method for producing a
stabilized fluoropolymer which comprises producing the stabilized
fluoropolymer by subjecting a treatment target substance containing
a sulfonic-acid-derived-group-containing fluoropolymer to a
fluorination treatment, wherein the
sulfonic-acid-derived-group-containing fluoropolymer is a
fluoropolymer containing --SO.sub.3M (in which M represents H,
NR.sup.1R.sup.2R.sup.3R.sup.4 or M.sup.1.sub.1/L; R.sup.1, R.sup.2,
R.sup.3 and R.sup.4 are the same or different and each represents H
or an alkyl group containing 1 to 4 carbon atoms; and M.sup.1
represents an L-valent metal), and the treatment target substance
has a moisture content of not higher than 500 ppm by mass.
Inventors: |
HONDA; Eiji; (Yokohama-shi,
JP) ; IIJIMA; Hideki; (Fuji-shi, JP) ;
HASHIMOTO; Yasuhiro; (Fuji-shi, JP) ; INO;
Tadashi; (Osaka, JP) ; ISAKA; Tadaharu;
(Osaka, JP) ; KONDO; Masahiro; (Osaka,
JP) |
Assignee: |
DAIKIN INDUSTRIES, LTD.
Osaka
JP
ASAHI KASEI E-MATERIALS CORPORATION
Tokyo
JP
|
Family ID: |
34380295 |
Appl. No.: |
13/045463 |
Filed: |
March 10, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10571317 |
Mar 9, 2006 |
|
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PCT/JP2004/013241 |
Sep 10, 2004 |
|
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13045463 |
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Current U.S.
Class: |
429/483 ; 521/27;
525/326.4; 525/355; 525/356 |
Current CPC
Class: |
H01M 8/1093 20130101;
H01M 4/8605 20130101; H01M 8/1039 20130101; C08F 8/20 20130101;
H01M 8/1004 20130101; Y02E 60/50 20130101; H01M 8/1023 20130101;
Y02P 70/50 20151101; C08J 5/2237 20130101; C08J 2327/18
20130101 |
Class at
Publication: |
429/483 ;
525/326.4; 521/27; 525/355; 525/356 |
International
Class: |
H01M 8/10 20060101
H01M008/10; C08F 8/18 20060101 C08F008/18; C08J 5/22 20060101
C08J005/22 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 10, 2003 |
JP |
2003-318243 |
Aug 3, 2004 |
JP |
2004-226891 |
Claims
1. A method for producing a stabilized fluoropolymer which
comprises producing said stabilized fluoropolymer by subjecting a
treatment target substance containing a
sulfonic-acid-derived-group-containing fluoropolymer to a
fluorination treatment, wherein said
sulfonic-acid-derived-group-containing fluoropolymer is a
fluoropolymer containing --SO.sub.3M (in which M represents H,
NR.sup.1R.sup.2R.sup.3R.sup.4 or M.sup.1.sub.1/L; R.sup.1, R.sup.2,
R.sup.3 and R.sup.4 are the same or different and each represents H
or an alkyl group containing 1 to 4 carbon atoms; and M.sup.1
represents an L-valent metal), and said treatment target substance
has a moisture content of not higher than 500 ppm by mass.
2. The method for producing a stabilized fluoropolymer according to
claim 1, wherein the sulfonic-acid-derived-group-containing
fluoropolymer further contains --SO.sub.2X and/or --COZ (wherein X
represents F, Cl, Br, I or --NR.sup.5R.sup.6 and Z represents
--NR.sup.7R.sup.8 or --OR.sup.9; R.sup.5, R.sup.6, R.sup.7 and
R.sup.8 are the same or different and each represents H, an alkali
metal element, an alkyl group or a sulfonyl-containing group and
R.sup.9 represents an alkyl group containing 1 to 4 carbon
atoms).
3. The method for producing a stabilized fluoropolymer according to
claim 1, wherein the sulfonic-acid-derived-group-containing
fluoropolymer further contains --COOH at the polymer chain terminus
or termini.
4. The method for producing a stabilized fluoropolymer according to
claim 1, wherein the fluorination treatment is carried out using a
gaseous fluorinating agent comprising a fluorine source, said
fluorine source is at least one species selected from the group
consisting of F.sub.2, SF.sub.4, IF.sub.5, NF.sub.3, PF.sub.5, ClF
and ClF.sub.3 and said fluorine source amounts to not less than 1%
by volume of said gaseous fluorinating agent.
5. The method for producing a stabilized fluoropolymer according to
claim 4, wherein the fluorine source is F.sub.2.
6. The method for producing a stabilized fluoropolymer according to
claim 1, wherein the sulfonic-acid-derived-group-containing
fluoropolymer is a copolymer which is at least binary comprising an
acid-derived group-containing perhalovinyl ether represented by the
general formula (I):
CF.sub.2.dbd.CF--O--(CF.sub.2CFY.sup.1--O).sub.n--(CFY.sup.2).sub.m-
-A (I) (wherein Y.sup.1 represents F, Cl, Br, I or a perfluoroalkyl
group, n represents an integer of 0 to 3; n atoms/groups of Y.sup.1
are the same or different; Y.sup.2 represents F, Cl, Br or I; m
represents an integer of 1 to 5; when m is an integer of 2 to 5, m
atoms of Y.sup.2 are the same or different; A represents
--SO.sub.2X or --COZ; X represents F, Cl, Br, I or
--NR.sup.5R.sup.6 and Z represents --NR.sup.7R.sup.8 or --OR.sup.9;
R.sup.5, R.sup.6, R.sup.7 and R.sup.8 are the same or different and
each represents H, an alkali metal element, an alkyl group or a
sulfonyl-containing group and R.sup.9 represents an alkyl group
containing 1 to 4 carbon atoms) and a copolymerizable monomer with
said acid-derived group-containing perhalovinyl ether, said
copolymerizable monomer is an "other vinyl ether" other than said
acid-derived group-containing perhalovinyl ether and an ethylenic
monomer, said copolymer comprises 5 to 40 mole percent of an
acid-derived group-containing perhalovinyl ether unit derived from
said acid-derived group-containing perhalovinyl ether, 60 to 95
mole percent of an ethylenic monomer unit derived from said
ethylenic monomer and 0 to 5 mole percent of an "other vinyl ether
unit" derived from said "other vinyl ether".
7. The method for producing a stabilized fluoropolymer according to
claim 6, wherein n is 0 (zero).
8. The method for producing a stabilized fluoropolymer according to
claim 6, wherein Y.sup.2 is F and m is 2.
9. (canceled)
10. (canceled)
11. (canceled)
12. (canceled)
13. (canceled)
14. A stabilized fluoropolymer, which is obtained by the method for
producing a stabilized fluoropolymer according to claim 1.
15. The stabilized fluoropolymer according to claim 14, which has a
melt index of 0.1 to 20 g/10 minutes as measured under the
conditions of 270.degree. C. and a load of 2.16 kg according to JIS
K 7210.
16. A polymer electrolyte membrane, which contains a hydrolyzate of
the stabilized fluoropolymer according to claim 14.
17. The polymer electrolyte membrane according to claim 16, wherein
the amount of fluoride ion eluted by Fenton treatment comprising
immersing b grams of said polymer electrolyte membrane in a liters
of an aqueous hydrogen peroxide solution having an initial iron(II)
cation concentration of 2 ppm and an initial hydrogen peroxide
concentration of 1% by mass at a membrane/bath ratio [b/a] of 3.2
and maintaining the whole at 80.degree. C. for 2 hours is not
greater than 11.times.10.sup.-4 parts by mass per 100 parts by mass
of said polymer electrolyte membrane.
18. An active substance-immobilized material which comprises a
hydrolyzate of the stabilized fluoropolymer according to claim 14
and an active substance.
19. The active substance-immobilized material according to claim
18, wherein the active substance is a catalyst.
20. The active substance-immobilized material according to claim
19, wherein the catalyst is a platinum-containing metal.
21. A membrane/electrode assembly comprising a polymer electrolyte
membrane and an electrode, wherein said membrane/electrode assembly
satisfies at least one condition selected from the group consisting
of the conditions (1) and (2) given below: (1) said polymer
electrolyte membrane is the polymer electrolyte membrane according
to claim 16, and (2) said electrode is the active
substance-immobilized material according to claim 18.
22. A solid polymer electrolyte fuel cell which comprises the
membrane/electrode assembly according to claim 21.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S.
application Ser. No. 10/571,317 filed Mar. 9, 2006, which is a 371
of PCT International Application No. PCT/JP2004/013241 filed Sep.
10, 2004, and which claims benefit of Japanese Patent Application
No. 2004-226891 filed Aug. 3, 2004 and Japanese Patent Application
No. 2003-318243 filed Sep. 10, 2003. The above-noted applications
are incorporated herein by reference in their entirety.
TECHNICAL FIELD
[0002] The present invention relates to a method for producing a
stabilized fluoropolymer, a stabilized fluoropolymer obtained by
such production method, and a polymer electrolyte membrane
containing a hydrolyzate of such stabilized fluoropolymer.
BACKGROUND ART
[0003] Sulfonic-acid-derived-group-containing fluoropolymers
obtained by copolymerizing tetrafluoroethylene and a
--SO.sub.2F-containing perfluorovinyl ether are known to be useful,
in the form resulting from hydrolysis of --SO.sub.2F, as
electrolyte membranes in fuel cells, chemical sensors and so
forth.
[0004] The hydrolyzates of those
sulfonic-acid-derived-group-containing fluoropolymers, when used,
for example, as fuel cell electrolyte membranes for a long period
of time, reportedly produce, as a result of deterioration thereof,
such a problem as contamination of the wastewater discharged from
the fuel cells with HF.
[0005] Reportedly, an improvement can be produced in this respect
by a certain kind of stabilization treatment, namely when such
sulfonic-acid-derived-group-containing fluoropolymers in solid
state are brought into contact with a fluorine atom
radical-generating compound, such as gaseous fluorine, at 20 to
300.degree. C. to thereby convert at least 40% of unstable groups
at polymer chain termini to stable groups (cf. e.g. Patent Document
1: Japanese Patent Publication S46-23245).
[0006] However, such prior art stabilization treatment has a
problem in that in particular when the
sulfonic-acid-derived-group-containing fluoropolymer to be treated
is one obtained by emulsion polymerization, the rate of conversion
of unstable groups to stable groups becomes insufficient and,
therefore, discoloration and frothing, among others, occur in the
step of melt molding.
DISCLOSURE OF INVENTION
Problems which the Invention is to Solve
[0007] In view of the above-discussed state of the art, it is an
object of the present invention to provide a method for stabilizing
a sulfonic-acid-derived-group-containing fluoropolymer to a
satisfactory extent, a stabilized fluoropolymer obtained by such
method, and a highly durable fuel cell membrane comprising a
hydrolyzate of such stabilized fluoropolymer.
Means for Solving the Problems
[0008] The present invention provides a method for producing a
stabilized fluoropolymer which comprises producing the stabilized
fluoropolymer by subjecting a treatment target substance containing
a sulfonic-acid-derived-group-containing fluoropolymer to a
fluorination treatment, wherein the
sulfonic-acid-derived-group-containing fluoropolymer is a
fluoropolymer containing --SO.sub.3M (in which M represents H,
NR.sup.1R.sup.2R.sup.3R.sup.4 or M.sup.1.sub.1/L; R.sup.1, R.sup.2,
R.sup.3 and R.sup.4 are the same or different and each represents H
or an alkyl group containing 1 to 4 carbon atoms; and M.sup.1
represents an L-valent metal), and the treatment target substance
has a moisture content of not higher than 500 ppm by mass.
[0009] The invention also provides a stabilized fluoropolymer,
which is obtained by the method for producing a stabilized
fluoropolymer.
[0010] The invention further provides a stabilized fluoropolymer
obtained via polymerization of an acid-derived group-containing
perhalovinyl ether represented by the general formula (II):
CF.sub.2.dbd.CF--O--(CFY.sup.2).sub.m-A (II)
(wherein Y.sup.2 represents F, Cl, Br or I, m represents an integer
of 1 to 5; when m is an integer of 2 to 5, m atoms of Y.sup.2 are
the same or different; and A represents --SO.sub.2X or --COZ; X
represents F, Cl, Br, I or --NR.sup.5R.sup.6 and Z represents
--NR.sup.7R.sup.8 or --OR.sup.9; R.sup.5, R.sup.6, R.sup.7 and
R.sup.8 are the same or different and each represents H, an alkali
metal element, an alkyl group or a sulfonyl-containing group and
R.sup.9 represents an alkyl group containing 1 to 4 carbon atoms),
and tetrafluoroethylene, wherein the stabilized fluoropolymer shows
an intensity ratio [x/y] between carboxyl group-due peak [x] and
--CF.sub.2-- due peak [y] of not higher than 0.05 in IR
measurement. The invention further provides a stabilized
fluoropolymer obtained via polymerization of an acid-derived
group-containing perhalovinyl ether represented by the general
formula (II):
CF.sub.2.dbd.CF--O--(CFY.sup.2).sub.m-A (II)
(wherein Y.sup.2 represents F, Cl, Br or I, m represents an integer
of 1 to 5; when m is an integer of 2 to 5, m atoms of Y.sup.2 are
the same or different; and A represents --SO.sub.2X or --COZ; X
represents F, Cl, Br, I or --NR.sup.5R.sup.6 and Z represents
--NR.sup.7R.sup.8 or --OR.sup.9; R.sup.5, R.sup.6, R.sup.7 and
R.sup.8 are the same or different and each represents H, an alkali
metal element, an alkyl group or a sulfonyl-containing group and
R.sup.9 represents an alkyl group containing 1 to 4 carbon atoms)
and tetrafluoroethylene, wherein, in a hydrolyzate of the
stabilized fluoropolymer, the number [X] of main chain terminal
--CF.sub.3 groups per 1.times.10.sup.5 main chain carbon atoms of
the hydrolyzate is not smaller than 10 as calculated using an
integrated intensity due to main chain terminal --CF.sub.3 groups
and an integrated intensity due to --CF.sub.2-- adjacent to an
ether bond in side chains branched from the main chain in the
hydrolyzate, each determined by solid state .sup.19F nuclear
magnetic resonance spectrometry of the hydrolyzate in a state
swollen in an oxygen-containing hydrocarbon compound having a
dielectric constant of not lower than 5.0 and further using an ion
exchange equivalent weight Ew value determined by titrimetric
method.
[0011] The invention further provides a polymer electrolyte
membrane, which contains a hydrolyzate of the stabilized
fluoropolymer.
[0012] The invention further provides an active
substance-immobilized material which comprises a hydrolyzate of the
stabilized fluoropolymer.
[0013] The invention further provides a membrane/electrode assembly
comprising a polymer electrolyte membrane and an electrode, wherein
the membrane/electrode assembly satisfies at least one condition
selected from the group consisting of the conditions (1) and (2)
given below:
(1) the polymer electrolyte membrane is the polymer electrolyte
membrane, and (2) the electrode is the active substance-immobilized
material.
[0014] The invention still further provides a solid polymer
electrolyte fuel cell which comprises the membrane/electrode
assembly.
[0015] In the following, the present invention is described in
detail.
[0016] The method for producing a stabilized fluoropolymer
according to the invention comprises producing the stabilized
fluoropolymer by subjecting a treatment target substance comprising
a sulfonic-acid-derived-group-containing fluoropolymer to a
fluorination treatment.
[0017] The sulfonic-acid-derived-group-containing fluoropolymer is
a fluoropolymer containing --SO.sub.3M (wherein M represents H,
NR.sup.1R.sup.2R.sup.3R.sup.4 or M.sup.1.sub.1/L).
[0018] R.sup.1, R.sup.2, R.sup.3 and R.sup.4 in the above-mentioned
NR.sup.1R.sup.2R.sup.3R.sup.4 are the same or different and each
represents H or an alkyl group containing 1 to 4 carbon atoms.
[0019] The alkyl group containing 1 to 4 carbon atoms is not
particularly restricted but preferably is a straight alkyl group,
more preferably a methyl group.
[0020] The symbol M.sup.1 represents an L-valent metal. The
L-valent metal is a metal belonging to the group 1, 2, 4, 8, 11, 12
or 13 of the periodic table.
[0021] The L-valent metal is not particularly restricted but
includes such metals of the group 1 of the periodic table as Li,
Na, K and Cs, such metals of the group 2 of the periodic table as
Mg and Ca, such metals of the group 4 of the periodic table as Al
etc., such metals of the group 8 of the periodic table as Fe etc.,
such metals of the group 11 of the periodic table as Cu and Ag,
such metals of the group 12 of the periodic table as Zn etc., and
such metals of the group 13 of the periodic table as Zr etc.
[0022] The sulfonic-acid-derived-group-containing fluoropolymer may
further contain, in addition to the above-mentioned group
--SO.sub.3M, --SO.sub.2X and/or --COZ (wherein X represents F, Cl,
Br, I or --NR.sup.5R.sup.6 and Z represents --NR.sup.7R.sup.8 or
--OR.sup.9; R.sup.5, R.sup.6, R.sup.7 and R.sup.8 are the same or
different and each represents H, an alkali metal element, an alkyl
group or a sulfonyl-containing group and R.sup.9 represents an
alkyl group containing 1 to 4 carbon atoms).
[0023] X in the above --SO.sub.2X is preferably F, Cl or Br, more
preferably F.
[0024] The --OR.sup.9 is preferred as Z in the above --COZ.
[0025] The alkali metal element is not particularly restricted but
includes Li, Na, K and Cs, among others.
[0026] The alkyl group is not particularly restricted but includes
alkyl groups containing 1 to 4 carbon atoms such as methyl and
ethyl. The alkyl group may be substituted by a halogen atom or
atoms.
[0027] The sulfonyl-containing group is a fluorine-containing alkyl
group which contains sulfonyl group and may be, for example a
fluorine-containing alkylsulfonyl group, which may optionally have
a terminal substituent. As the fluorine-containing alkylsulfonyl
group, there may be mentioned, for example,
--SO.sub.2R.sub.f.sup.1Z.sup.1 (in which R.sub.f.sup.1 represents a
fluorine-containing alkylene group and Z.sup.1 represents an
organic group) and so forth.
[0028] The organic group may be, for example, --SO.sub.2F, or may
contain such indefinite repetition as
--SO.sub.2(NR.sup.5SO.sub.2R.sub.f.sup.1SO.sub.2).sub.kNR.sup.5SO.sub.2--
(in which k represents an integer of not smaller than 1 and
R.sub.f.sup.1 represents a fluorine-containing alkylene group), for
example
--SO.sub.2(NR.sup.5SO.sub.2R.sub.f.sup.1SO.sub.2).sub.kNR.sup.5SO.sub.2F
(in which k represents an integer of not smaller than 1 but not
larger than 100 and R.sup.5 and R.sub.f.sup.1 are as defined
above). For use in fuel cells, the organic group is preferably free
of --COZ since the hydrolysis product --COOH may cause a stability
problem.
[0029] Further, the sulfonic-acid-derived-group-containing
fluoropolymer may contain --COOH at its polymer chain terminus or
termini.
[0030] For example, The above --COOH group(s) is(are) introduced
into the main chain terminus(termini) of the
sulfonic-acid-derived-group-containing fluoropolymer from the
molecular structure of the polymerization initiator.
[0031] For example, when a peroxydicarbonate or the like is used as
the polymerization initiator, such --COOH groups are formed at the
main chain termini of the sulfonic-acid-derived-group-containing
fluoropolymer. When produced by emulsion polymerization, the
sulfonic-acid-derived-group-containing fluoropolymer generally
contains --COOH at its polymer chain terminus or termini.
[0032] Further, when a perfluoroalkyldicarboxylic acid is used as
the polymerization initiator and the polymerization is carried out
in a nonaqueous system, the polymer chain termini partly have the
corresponding perfluoroalkyl group but, generally, --COOH and --COF
are formed there. This is due to .beta.-scission of the
perhalovinyl ether.
[0033] The sulfonic-acid-derived-group-containing fluoropolymer is
preferably a copolymer which is at least binary comprising an
acid-derived group-containing perhalovinyl ether and a
copolymerizable monomer with the acid-derived group-containing
perhalovinyl ether.
[0034] The acid-derived group-containing perhalovinyl ether is
preferably a compound represented by the general formula (I):
CF.sub.2.dbd.CF--O--(CF.sub.2CFY.sup.1--O).sub.n--(CFY.sup.2).sub.m-A
(I).
[0035] In the above general formula (I), Y.sup.1 represents F, Cl,
Br, I or a perfluoroalkyl group; it is preferably a perfluoroalkyl
group, more preferably a perfluoroalkyl group containing 1 to 3
carbon atoms, still more preferably --CF.sub.3.
[0036] In the above general formula (I), n represents an integer of
0 to 3; the n atoms/groups Y.sup.1 are the same or different. The
integer n is preferably 0 (zero) or 1, more preferably 0.
[0037] In the above general formula (I), Y.sup.2 represents F, Cl,
Br or I; however, F is preferred among others.
[0038] In the above general formula (I), m represents an integer of
1 to 5. When m is an integer of 2 to 5, the m atoms of Y.sup.2 are
the same or different. The integer m is preferably 2.
[0039] The compound of general formula (I) is preferably one in
which Y.sup.2 is F and m is 2, more preferably one in which Y.sup.2
is F, m is 2 and n is 0.
[0040] In the above general formula (I), A represents the
acid-derived group --SO.sub.2X or --COZ (X and Z being as defined
above).
[0041] The --SO.sub.2X and/or --COZ, which the
sulfonic-acid-derived-group-containing fluoropolymer may optionally
have, may also be introduced into the fluoropolymer by
polymerization of the acid-derived group-containing perhalovinyl
ether represented by the general formula (I).
[0042] The acid-derived group-containing perhalovinyl ether is more
preferably a compound represented by the general formula (II):
CF.sub.2.dbd.CF--O--(CFY.sup.2).sub.m-A (II)
(wherein Y.sup.2, m and A are as defined above referring to the
general formula (I)).
[0043] Either one single species or a combination of two or more
species of the acid-derived group-containing perhalovinyl ether may
be used.
[0044] Preferred as the copolymerizable monomer with the
acid-derived group-containing perhalovinyl ether is an "other vinyl
ether" other than the above-mentioned acid-derived group-containing
perhalovinyl ethers and/or an ethylenic monomer. At least one
monomer selected from the group consisting of the "other vinyl
ethers other than the acid-derived group-containing perhalovinyl
ethers and ethylenic monomers" can be selected as the
copolymerizable monomer with the acid-derived group-containing
perhalovinyl ether according to the intended purpose.
[0045] The ethylenic monomer may be a vinyl group-containing
monomer having no ether oxygen atom, and the hydrogen atoms of the
vinyl group may be partly or wholly substituted by a fluorine atom
or atoms.
[0046] As such ethylenic monomer, there may be mentioned, for
example, haloethylenic monomers represented by the general
formula:
CF.sub.2.dbd.CF--Rf.sup.2
(wherein Rf.sup.2 represents F, Cl or a straight or branched
fluoroalkyl group containing 1 to 9 carbon atoms), and
hydrogen-containing fluoroethylenic monomers represented by the
general formula:
CHY.sup.3.dbd.CFY.sup.4
(wherein Y.sup.3 represents H or F and Y.sup.4 represents H, F, Cl
or a straight or branched fluoroalkyl group containing 1 to 9
carbon atoms).
[0047] The ethylenic monomer is preferably at least one monomer
selected from the group consisting of fluorovinyl ethers
represented by CF.sub.2.dbd.CF.sub.2, CH.sub.2.dbd.CF.sub.2,
CF.sub.2.dbd.CFCl, CF.sub.2.dbd.CFH, CH.sub.2.dbd.CFH and
CF.sub.2.dbd.CFCF.sub.3. Among them, perhaloethylenic monomers are
more preferred, perfluoroethylenic monomers are still more
preferred, and tetrafluoroethylene is particularly preferred.
[0048] Among such ethylenic monomers, one single species or two or
more species can be used.
[0049] The "other vinyl ethers" other than the acid-derived
group-containing perhalovinyl ethers are not particularly
restricted but include those vinyl ethers which contain no
acid-derived group, for example perfluorovinyl ethers represented
by the general formula:
CF.sub.2.dbd.CF--O--Rf.sup.3
(wherein Rf.sup.3 represents a fluoroalkyl group containing 1 to 9
carbon atoms or a fluoropolyether group containing 1 to 9 carbon
atoms), and hydrogen-containing vinyl ethers represented by the
general formula:
CHY.sup.5.dbd.CF--O--Rf.sup.4
(wherein Y.sup.5 represents H or F and Rf.sup.4 represents a
straight or branched fluoroalkyl group containing 1 to 9 carbon
atoms, which may optionally contain an ether oxygen atom or
atoms).
[0050] Either one single species or a combination of two or more
species of the "other vinyl ether" may be used.
[0051] Preferred as the sulfonic-acid-derived-group-containing
fluoropolymer is a copolymer which is at least binary obtained by
copolymerization of at least one of the acid-derived
group-containing perhalovinyl ethers and at least one of the
ethylenic monomers. A copolymer which is at least binary obtained
by copolymerization of one acid-derived group-containing
perhalovinyl ether and one ethylenic monomer is more preferred. If
desired, however, use may also be made of copolymers obtained by
copolymerization of an "other vinyl ether" other than the
acid-derived group-containing perhalovinyl ether together with the
acid-derived group-containing perhalovinyl ether and ethylenic
monomer.
[0052] In the practice of the invention, the
sulfonic-acid-derived-group-containing fluoropolymer is preferably
one comprising 5 to 40 mole percent of an acid-derived
group-containing perhalovinyl ether unit derived from the
acid-derived group-containing perhalovinyl ether, 60 to 95 mole
percent of an ethylenic monomer unit derived from the ethylenic
monomer and 0 to 5 mole percent of an "other vinyl ether" unit
derived from the "other vinyl ether".
[0053] A more preferred lower limit to the acid-derived
group-containing perhalovinyl ether unit content is 7 mole percent,
a still more preferred lower limit thereto is 10 mole percent, a
more preferred upper limit thereto is 35 mole percent, and a still
more preferred upper limit is 30 mole percent.
[0054] A more preferred lower limit to the ethylenic monomer unit
content is 65 mole percent, a still more preferred lower limit
thereto is 70 mole percent, a more preferred upper limit thereto is
90 mole percent, and a still more preferred upper limit is 87 mole
percent.
[0055] A more preferred upper limit to the "other vinyl ether" unit
content is 4 mole percent, and a still more preferred upper limit
thereto is 3 mole percent.
[0056] The term "ethylenic monomer unit" means that moiety which is
derived from the molecular structure of the ethylenic monomer and
constitutes a part of the molecular structure of the
sulfonic-acid-derived-group-containing fluoropolymer. Thus, the
tetrafluoroethylene unit means the section [--CF.sub.2--CF.sub.2--]
derived from tetrafluoroethylene [CF.sub.2.dbd.CF.sub.2].
[0057] The term "acid-derived group-containing perhalovinyl ether
unit" means that moiety which is derived from the molecular
structure of the acid-derived group-containing perhalovinyl ether
and constitutes a part of the molecular structure of the
sulfonic-acid-derived-group-containing fluoropolymer.
[0058] The term "other vinyl ether unit" means that moiety which is
derived from the molecular structure of the "other vinyl ether" and
constitutes a part of the molecular structure of the
sulfonic-acid-derived-group-containing fluoropolymer.
[0059] The acid-derived group-containing perhalovinyl ether unit,
ethylenic monomer unit and other vinyl ether unit contents so
referred to herein are the values respectively calculated with the
whole sum of all the monomer units being taken as 100 mole
percent.
[0060] The term "all the monomer units" means the total amount of
all the monomer-derived units constituting the molecular structure
of the sulfonic-acid-derived-group-containing fluoropolymer.
Therefore, the monomers from which "all the monomers units" are
derived constitute the total quantity of all the monomers
participating in the formation of the
sulfonic-acid-derived-group-containing fluoropolymer.
[0061] The "acid-derived group-containing perhalovinyl ether unit"
content (in mole percent) so referred to herein is the percentage
of the number of moles of the acid-derived group-containing
perhalovinyl ether from which the acid-derived group-containing
perhalovinyl ether unit is derived to the total number of moles of
the monomers from which all the monomer units in the
sulfonic-acid-derived-group-containing fluoropolymer are derived.
Similarly, the "ethylenic monomer unit" content (in mole percent)
and "other vinyl ether unit" content (in mole percent) are
respectively the percentages of the number of moles of the
corresponding monomers. These respective unit contents are the
values obtained by carrying out NMR measurement at 300.degree. C.
using a high-temperature .sup.19F nuclear magnetic resonance
spectrometer (model JNM-FX100, product of Nippon Denshi (JEOL),
Japan) without using any solvent. Hereinafter, this measurement is
referred to as high-temperature NMR for short.
[0062] The method of producing the above
sulfonic-acid-derived-group-containing fluoropolymer by
polymerization may be any of the method known in the art, for
example solution polymerization, suspension polymerization and
emulsion polymerization. Emulsion polymerization is preferred,
however, since this method is most effective in producing
stabilized fluoropolymers in accordance with the present
invention.
[0063] When the sulfonic-acid-derived-group-containing
fluoropolymer is produced by emulsion polymerization of a
--SO.sub.2F-containing monomer, for instance, only a small
proportion of this --SO.sub.2F group is converted to --SO.sub.3H in
the process of polymerization. This --SO.sub.3H can be readily
converted to --SO.sub.3NR.sup.1R.sup.2R.sup.3R.sup.4 or
--SO.sub.3M.sup.1.sub.1/L in the presence of
.sup.+NR.sup.1R.sup.2R.sup.3R.sup.4 or M.sup.1L+ (R.sup.1, R.sup.2,
R.sup.3, R.sup.4 and M.sup.1 being as defined above).
[0064] The --SO.sub.2F-containing monomer is not particularly
restricted but may be, for example, an acid-derived
group-containing perhalovinyl ether of the general formula (I) in
which A is --SO.sub.2F. The group --SO.sub.3M (M being as defined
above) contained in the sulfonic-acid-derived-group-containing
fluoropolymer of the invention is not limited to the one derived
from --SO.sub.2F contained in the monomer subjected to emulsion
polymerization but may be, for example, the one introduced by any
of the methods known in the art.
[0065] The method for producing stabilized fluoropolymers according
to the invention comprises subjecting a treatment target substance
containing such a sulfonic-acid-derived-group-containing
fluoropolymer as mentioned above to a fluorination treatment.
[0066] The "treatment target substance" so referred to herein is
the target substance to be subjected to the fluorination
treatment.
[0067] The treatment target substance may be in the form of a resin
powder, pellets, or a molded membrane. From the viewpoint that the
fluorination treatment to be described later herein is successful,
the treatment target substance is desirably in the form of a resin
powder whereas, from the industrial handleability viewpoint, it is
desirably in the form of pellets.
[0068] The conventional method of fluorination treatment is
disadvantageous in that the fluorination of the
sulfonic-acid-derived-group-containing fluoropolymer becomes
insufficient. The reason for the insufficient fluorination is
presumably as follows. Thus, even when the
sulfonic-acid-derived-group-containing fluoropolymer is prepared as
such a solid form as a powder, pellets or moldings via drying
treatment, --SO.sub.3M is generally highly hygroscopic and
therefore that fluoropolymer absorbs moisture in the air. The group
--SO.sub.3M is much higher in hygroscopicity than other functional
groups such as --COOH, salts thereof, --COZ, --SO.sub.2X (Z and X
being as defined above). Due to this high hygroscopicity of
--SO.sub.3M, solids whose substantial main component is the
sulfonic-acid-derived-group-containing fluoropolymer generally have
a moisture content exceeding 500 ppm by mass depending on the
humidity of the atmosphere in which they occur. When a solid, which
contains the sulfonic-acid-derived-group-containing fluoropolymer
and has such a high moisture content, is subjected to the
fluorination treatment in the conventional manner, the fluorine
source (F) is consumed by the reaction (A) represented by
2H.sub.2O+4(F).fwdarw.4HF+O.sub.2 (A)
and, as a result, the fluorination of the
sulfonic-acid-derived-group-containing fluoropolymer is
inhibited.
[0069] In carrying out the method for producing a stabilized
fluoropolymer according to the invention, the treatment target
substance has a moisture content of 500 ppm or below. When the
moisture content exceeds 500 ppm, the fluorination of the
sulfonic-acid-derived-group-containing fluoropolymer is unfavorably
inhibited. A preferred upper limit is 450 ppm, and a more preferred
upper limit is 350 ppm. Provided that the moisture content in the
treatment target substance is within the above range, the lower
limit thereto may beset at 0.01 ppm, for instance, from the economy
and productivity viewpoint.
[0070] The moisture content in the treatment target substance is
the value obtained by measurement using the Karl Fischer titration
method.
[0071] The method for producing a stabilized fluoropolymer
according to the invention makes it possible to prevent such an
inhibitory reaction in the fluorination treatment as the reaction
(A) mentioned above and fluorinate the
sulfonic-acid-derived-group-containing fluoropolymer to a
sufficient extent by carrying out the fluorination treatment under
conditions such that the moisture content of the treatment target
substance is within the above range.
[0072] The method for reducing the moisture content in the
treatment target substance to a level within the above range is not
particularly restricted but may be any of the drying methods known
in the art, for example the method comprising heating at 80 to
130.degree. C. for 2 to 50 hours, if desirable after dehydration by
centrifugation or the like, if desirable while varying the
temperature stepwise, if desirable under reduced pressure; or the
method comprising melting the treatment target substance in a
vented extruder and allowing the water vapor to escape through the
vent hole. The use of the latter method may possibly result in
partial decomposition of --SO.sub.3M, hence is preferred.
[0073] Since the sulfonic-acid-derived-group-containing
fluoropolymer has highly hygroscopic functional groups, the steps
of the above drying and the succeeding fluorination treatment to be
described later herein is preferably carried out in a closed system
or as quickly as possible.
[0074] The fluorination treatment in the method for producing a
stabilized fluoropolymer according to the invention is carried out
using a fluorine source.
[0075] The fluorine source is preferably at least one species
selected from the group consisting of F.sub.2, SF.sub.4, IF.sub.5,
NF.sub.3, PF.sub.5, ClF and ClF.sub.3, and F.sub.2 is more
preferred.
[0076] Preferably, the fluorination treatment is carried out using
a gaseous fluorinating agent comprising such fluorine source as
mentioned above. In this case, the fluorine source preferably
amounts to not less than 1% by volume in the gaseous fluorinating
agent. A more preferred lower limit is 10% by volume.
[0077] The gaseous fluorinating agent comprises the above-mentioned
fluorine source and a gas inert to fluorination.
[0078] The gas inert to fluorination is not particularly restricted
but may be, for example, nitrogen gas or argon gas.
[0079] The fluorination treatment is preferably carried out at a
temperature lower than the melting point of the fluoropolymer,
generally at 250.degree. C. or below, more preferably at room
temperature to 150.degree. C.
[0080] The fluorination treatment can be carried out either
continuously or batchwise.
[0081] The apparatus to be used in the fluorination treatment is
properly selected from among tray type reactors, can type reactors
and like stationary reactors; reactors equipped with a stirring
impeller; rotary kilns, W cone type reactors, V type blenders and
like rotating (reversing) vessel reactors; vibrating reactors;
agitated fluidized bed and other various fluidized bed reactors;
and so forth.
[0082] In the fluorination treatment, a solvent inert to such
fluorine source as a fluorocarbon can be used to maintain the
reaction temperature uniformity. When the treatment target
substance is in the form of a resin powder or pellets, the
fluorination treatment is preferably carried out in a rotating
vessel reactor or a vibrating reactor since the reaction
temperature can be maintained uniformly with ease in such
reactor.
[0083] The above fluorination treatment is a treatment for
converting those unstable groups susceptible to thermal
decomposition which the sulfonic-acid-derived-group-containing
fluoropolymer before the fluorination treatment has to stable
groups hardly susceptible to thermal decomposition.
[0084] Presumably, the fluorination treatment preferably converts
the --CF.sub.2SO.sub.3M (M being as defined above) which the
sulfonic-acid-derived-group-containing fluoropolymer has to
--CF.sub.2H, --CF.sub.3 and/or the like and, further, the --COOH
and/or --SO.sub.2NH.sub.2 which the
sulfonic-acid-derived-group-containing fluoropolymer optionally has
at its chain terminus or termini to --CF.sub.3 and/or --SO.sub.2F,
respectively.
[0085] As a result of these conversions resulting from the above
fluorination treatment, it becomes possible to avoid the
discoloration due to thermal decomposition of such unstable groups
as --SO.sub.3M and the foaming due to decomposition of such
unstable groups as --COOH in the step of melt molding using the
above-mentioned sulfonic-acid-derived-group-containing
fluoropolymer.
[0086] The fluorination treatment can further eliminate such
impurities contained in the treatment target substance as oligomers
and other low-molecular-weight substances, unreacted monomers and
byproducts.
[0087] The --SO.sub.3M groups (M being as defined above) which the
sulfonic-acid-derived-group-containing fluoropolymer has are
converted to --CF.sub.2H, --CF.sub.3 or/and the like, which have no
ion exchange capacity, by the fluorination treatment. When the
--SO.sub.3M groups are the results of conversion of --SO.sub.2F,
which the above-mentioned monomer has, on the occasion of emulsion
polymerization, the conversion of --SO.sub.2F to --SO.sub.3M is
very slight and, therefore, the ion exchange equivalent weight [Ew]
can be maintained without a marked increase thereof even when the
membranes and the like molded from the polymer after the above
fluorination treatment are used for the ion exchange purposes.
[0088] The method for producing a stabilized fluoropolymer of the
invention comprises producing a stabilized fluoropolymer by
carrying out the above fluorination treatment.
[0089] The "stabilized fluoropolymer" as used herein is a
fluoropolymer obtained from a
sulfonic-acid-derived-group-containing fluoropolymer by the
above-mentioned fluorination treatment and now having such stable
groups hardly susceptible to thermal decomposition as --CF.sub.2H,
--CF.sub.3 and --SO.sub.2F in lieu of such unstable groups
susceptible to thermal decomposition as --COOH, --CF.sub.2SO.sub.3M
and --SO.sub.2NH.sub.2 in the original
sulfonic-acid-derived-group-containing fluoropolymer.
[0090] After the above fluorination treatment, the stabilized
fluoropolymer may contain volatile components, for example HF, and
it is desirable that such accompanying components be
eliminated.
[0091] The volatile components are preferably removed using an
extruder having a volatile matter eliminating mechanism, more
preferably using a vented extruder having at least one vent
hole.
[0092] As mentioned above, the treatment target substance to be
subjected to the fluorination treatment is preferably in the form
of a powder and, when a resin powder is used as the treatment
target substance, the treatment target substance after the
fluorination treatment is preferably subjected to melt-kneading in
a vented extruder to eliminate the volatile components and then
pelletized. More preferably, the fluorination treatment is carried
out in such a vented extruder as mentioned above, followed by
volatile matter elimination and pelletization in the same
extruder.
[0093] Even when the treatment target substance is membranous, the
fluorination treatment can stabilize the same in the same
manner.
[0094] The fluorination treatment of membranous bodies is preferred
since the membranous bodies, when they are intended to be used as
electrolyte membranes, will not incur any severe thermal damage
after stabilization treatment, hence such unstable groups as
otherwise resulting from polymer chain cleavage will not be
formed.
[0095] When the treatment target substance is membranous, the
fluorination treatment is preferably carried out, for example, by
using pellets molded by the method comprising melting in such a
vented extruder as mentioned above for degassing through the vent
hole, followed by melt extrusion and using, after membrane molding,
a can type reactor or a reaction apparatus equipped with a winder
for fluorination treatment.
[0096] When drying treatment is carried out prior to the
fluorination treatment using any of the various reactors mentioned
above, the drying is preferably carried out in the manner of vacuum
evacuation or by passing a dry gas through the drier.
[0097] When the sulfonic-acid-derived-group-containing
fluoropolymer in the treatment target substance contains --COF
groups as unstable groups, a relatively high temperature is
required for stabilizing those groups. In this case, the
fluorination treatment can be carried out after converting such
unstable groups to --COOH groups in advance by hydrolysis, for
instance, and adjusting the moisture content to a level not higher
than 500 ppm.
[0098] In accordance with the present invention, the stabilized
fluoropolymer is one resulting from conversion of the polymer chain
terminal --COOH groups generally occurring before fluorination
treatment to such stable groups as --CF.sub.2H and --CF.sub.3
groups by the fluorination treatment, as mentioned above. The rate
of this conversion is very high in the method for producing a
stabilized fluoropolymer according to the invention, and the
intensity ratio [x/y] between carboxyl group-due peak [x] and
--CF.sub.2-- due peak [y] can be reduced to not higher than 0.05 in
infrared spectroscopy [IR] measurement. A preferred upper limit to
the intensity ratio [x/y] is 0.04, and a more preferred upper limit
thereto is 0.03.
[0099] A stabilized fluoropolymer (hereinafter sometimes referred
to as "stabilized fluoropolymer (A)") obtained by the method for
producing a stabilized fluoropolymer according to the invention
also constitutes an aspect of the present invention.
[0100] In the practice of the invention, the stabilized
fluoropolymer is preferably one having the characteristic features
of the stabilized fluoropolymers (A) and also having the
characteristic features of the stabilized fluoropolymers (B)
described later herein, one having the characteristic features of
the stabilized fluoropolymers (A) and also having the
characteristic features of the stabilized fluoropolymers (C)
described later herein, or one having the characteristic features
of the stabilized fluoropolymers (A) and also having the
characteristic features of the stabilized fluoropolymers (B) and
further having the characteristic features of the stabilized
fluoropolymers (C).
[0101] The stabilized fluoropolymer (hereinafter sometimes referred
to as "stabilized fluoropolymer (B)") of the invention is a
stabilized fluoropolymer obtained via polymerization of an
acid-derived group-containing perhalovinyl ether represented by the
general formula (II) given hereinabove (Y.sup.2, m and A being as
defined above) and tetrafluoroethylene, wherein the stabilized
fluoropolymer shows an intensity ratio [x/y] between the carboxyl
group-due peak [x] and the --CF.sub.2-- group-due peak [y] of not
higher than 0.05 in IR measurement.
[0102] The polymerization of the acid-derived group-containing
perhalovinyl ether and tetrafluoroethylene is preferably carried
out in the manner of emulsion polymerization.
[0103] In the above stabilized fluoropolymer (B), the carboxyl
groups [--COOH] are formed mainly as polymer chain terminal groups,
and the --CF.sub.2-- groups occur mainly in the polymer main
chain.
[0104] In the stabilized fluoropolymer (B), a preferred upper limit
to the intensity ratio [x/y] is 0.04, and a more preferred upper
limit thereto is 0.03.
[0105] The method for producing the stabilized fluoropolymers (B)
is not particularly restricted provided that they have an intensity
ratio [x/y] within the above range. They can be obtained with ease
by using the method for producing a stabilized fluoropolymer of the
invention.
[0106] While it can be obtained with high efficiency by the
above-mentioned method for producing a stabilized fluoropolymer of
the invention, the stabilized fluoropolymer (B) is not always
restricted to one obtained by the method for producing a stabilized
fluoropolymer of the invention and, in this respect, conceptually
differ from the above-mentioned stabilized fluoropolymers (A).
[0107] By saying herein simply "stabilized fluoropolymer" without
adding (A), (B) or (C) (to be mentioned later), a superordinate
concept is meant that can include the stabilized fluoropolymer (A),
stabilized fluoropolymer (B) and/or stabilized fluoropolymer (C)
without making any distinction among the stabilized fluoropolymer
(A), the stabilized fluoropolymer (B) and the stabilized
fluoropolymer (C) described later herein.
[0108] The above-mentioned stabilized fluoropolymer shows an
intensity ratio [x/y] within the above range in infrared
spectroscopy [IR] measurement and, therefore, can be the one hardly
causing foaming in the step of melt molding.
[0109] In the practice of the invention, the intensity ratio [x/y]
is calculated from the respective peak intensities obtained by
measurement using an infrared spectrophotometer.
[0110] The above-mentioned carboxyl group-due peak intensity [x] is
the sum of the associated carboxyl group-due absorption peak
intensity observed at around 1776 cm.sup.-1 and the non-associated
carboxyl group-due absorption peak intensity observed at around
1807 cm.sup.-1.
[0111] The above-mentioned --CF.sub.2-- due peak [y] is the
absorption peak due to the overtone of --CF.sub.2--.
[0112] The above stabilized fluoropolymer preferably has a sulfonyl
group content of not higher than 200 ppm. A more preferred upper
limit is 50 ppm.
[0113] The stabilized fluoropolymer preferably has a carboxyl group
content of not higher than 100 ppm. A more preferred upper limit is
30 ppm.
[0114] The sulfonyl group content and carboxyl group content
reported herein are the values obtained by preparing a 150- to
200-.mu.m-thick membrane for measurement by heat-pressing each
stabilized fluoropolymer at 270.degree. C. and 10 MPa for 20
minutes, carrying out spectrum measurement using a FT-IR
spectrometer, and following the procedure described below.
[0115] First, a standard reference sample is separately prepared by
carrying out the fluorination treatment at 150.degree. C. for 20
hours for complete stabilization of unstable groups, and the
difference spectrum is derived from an IR spectrum thereof and an
IR spectrum of the membrane for measurement with normalization
based on the C--F overtone absorption peak, and the intensities of
the sulfonic acid group-due absorption peak observable at around
1056 cm.sup.-1, the associated carboxyl group-due absorption peak
observable at around 1776 cm.sup.-1 and the non-associated carboxyl
group-due absorption peak observable at around 1807 cm.sup.-1 are
read from the difference spectrum obtained. For each absorption,
the absorption peak intensity Abs is obtained with C--F overtone
peak intensity-based normalization.
[0116] The content of each functional group is calculated from the
extinction coefficient .di-elect cons. (cm.sup.3/molcm) of the
absorption peak of each functional group, the specific gravity d
(g/cm.sup.3) of the sample and the sample membrane thickness 1 (cm)
when the C--F overtone intensity is 1, using the equation:
Functional group content(ppm)={Abs.times.(molecular weight of each
functional group)}.times.10.sup.11/.di-elect cons.dl
according to Lambert-Beer's law (Abs=.di-elect cons.cl; c being the
concentration).
[0117] The carboxyl group content so referred to herein is the sum
of the associated and non-associated carboxyl group contents.
[0118] A stabilized fluoropolymer obtained by polymerization of an
acid-derived group-containing perhalovinyl ether represented by the
general formula (II) given above and tetrafluoroethylene, wherein,
in a hydrolyzate of the stabilized fluoropolymer, the number (X) of
main chain terminal --CF.sub.3 groups per 1.times.10.sup.5 main
chain carbon atoms of the hydrolyzate is not smaller than 10 as
calculated using an integrated intensity due to main chain terminal
--CF.sub.3 groups and an integrated intensity due to --CF.sub.2--
adjacent to an ether bond in side chains branched from the main
chain in the hydrolyzate, each determined by solid state .sup.19F
nuclear magnetic resonance spectrometry of the hydrolyzate in a
state swollen in an oxygen-containing hydrocarbon compound having a
dielectric constant of not lower than 5.0 and further using an ion
exchange equivalent weight Ew value by titrimetric method (such
fluoropolymer is hereinafter sometimes referred to as "stabilized
fluoropolymer (C)") also constitutes an aspect of the present
invention.
[0119] In the above stabilized fluoropolymer (C), in which the
number (X) of such main chain terminal --CF.sub.3 groups as
mentioned above is not smaller than 10, the main chain terminal
groups have been stabilized to a sufficient extent.
[0120] The lower limit to the number (X) of main chain terminal
--CF.sub.3 groups is more preferably not lower than 15.
[0121] The number (X) of main chain terminal --CF.sub.3 groups can
be calculated from an integrated intensity of main chain terminal
--CF.sub.3 groups and an integrated intensity of --CF.sub.2--
adjacent to an ether bond in side chains branched from the main
chain in the hydrolyzate, each determined by solid state .sup.19F
nuclear magnetic resonance spectrometry and the an exchange
equivalent weight Ew value of the sample by titrimetric method.
[0122] In the solid state .sup.19F nuclear magnetic resonance
spectrometry, an oxygen-containing hydrocarbon compound having a
dielectric constant of not lower than 5.0 can be used as a swelling
solvent. The oxygen-containing hydrocarbon compound having a
dielectric constant of not lower than 5.0 is not particularly
restricted but there may be mentioned, for example,
N-methylacetamide and the like.
[0123] In the practice of the invention, the sample fluoropolymer
swollen in N-methylacetamide is subjected to solid state .sup.19F
nuclear magnetic resonance spectrometry using a model DSX400
measuring apparatus (product of Bruker Biospin, Germany) at a
number of MAS (Magic Angle Spinning) rotations of 4.8 kHz, a
measurement frequency of 376.5 MHz, a chemical shift standard of
CF.sub.3COOH (-77 ppm) and a measurement temperature of 473K.
[0124] In the above-mentioned solid state .sup.19F nuclear magnetic
resonance spectrometry, the integrated intensity (A) of main chain
terminal --CF.sub.3 groups can be measured from the signal showing
a peak at -79.7 ppm, and the integrated intensity (B) of
--CF.sub.2-- adjacent to an ether bond in side chains branched from
the main chain can be measured from the signal showing a peak at
-76.4 ppm.
[0125] The number (X) of main chain terminal --CF.sub.3 groups can
be calculated from the above-mentioned integrated intensity (A) and
integrated intensity (B) and the ion exchange equivalent weight Ew
determined of the sample by titrimetric method according to the
following formula (III):
X=100000/[{(B/A).times.3/2}.times.{2.times.(Ew-178-50.times.m)/100}+2]
(III)
wherein m is the value of m in the general formula (II) given
hereinabove.
[0126] In the above-mentioned IR measurement, the stabilized
fluoropolymer (C) preferably shows an intensity ratio [x/y] between
carboxyl group-due peak [x] and --CF.sub.2-- due peak [y] of not
higher than 0.05.
[0127] The preferred range of the intensity ratio [x/y] in the case
of the stabilized fluoropolymer (C) is the same as the range
explained hereinabove referring to the stabilized fluoropolymer
(B).
[0128] The method for producing the stabilized fluoropolymer (C) is
not particularly restricted provided that they are produced via
polymerization of an acid-derived group-containing perhalovinyl
ether represented by the general formula (II) give hereinabove and
tetrafluoroethylene. However, the polymerization is preferably
carried out in the manner of emulsion polymerization.
[0129] The stabilized fluoropolymer (C) can be efficiently obtained
by the above-mentioned method for producing a stabilized
fluoropolymer according to the invention. The stabilized
fluoropolymer (C) is not always limited to one obtained by the
method for producing a stabilized fluoropolymer of the invention
and, in this respect, are conceptually different from the
above-mentioned stabilized fluoropolymer (A).
[0130] In the stabilized fluoropolymer (B) and stabilized
fluoropolymer (C), the side chains are short, namely the value of n
is equal to 0 (zero) in the general formula (I) given hereinabove,
as is evident from the general formula (II) given above.
[0131] The present inventors found that those polymers whose side
chains are short are superior in chemical stability, as is evident
from the results of Fenton treatment 1, which are to be mentioned
later herein, and have completed the present invention relating to
the stabilized fluoropolymer (B) and stabilized fluoropolymer
(C).
[0132] The stabilized fluoropolymer of the invention preferably has
a melt index of 0.1 to 20 g/10 minutes as measured under the
conditions of 270.degree. C. and a load of 2.16 kg according to JIS
K 7210. The melt index [MI] so referred to herein is also expressed
by the term melt mass flow rate [MFR].
[0133] In the case of the stabilized fluoropolymer of the
invention, a melt index lower than 0.1 (g/10 minutes) tends to make
melt molding difficult, while a melt index exceeding 20 (g/10
minutes) may readily cause deterioration in durability when the
polymer electrolyte membranes are used in fuel cells.
[0134] A more preferred lower limit to the MI is 0.5 (g/10
minutes), and a more preferred upper limit is 10 (g/10
minutes).
[0135] The MI so referred to herein is expressed in terms of the
weight, in grams, of the test polymer extruded through an orifice
having a specific shape and size at 270.degree. C. under a load of
2.16 kg during 10 minutes according to JIS K 7210.
[0136] The stabilized fluoropolymer of the invention is more
resistant to foaming even in the step of melt molding than the
corresponding unstabilized one.
[0137] The polymer electrolyte membrane of the invention contains a
hydrolyzate of the stabilized fluoropolymer of the invention as
mentioned above.
[0138] The "hydrolyzate of the stabilized fluoropolymer" is the
fluoropolymer obtained by hydrolysis of the stabilized
fluoropolymer.
[0139] As the hydrolyzate of the stabilized fluoropolymer, there
may be mentioned, for example, sulfonic acid type fluoropolymers
resulting from conversion of acid-derived --SO.sub.2F groups in the
stabilized fluoropolymers of the invention to such salt groups as
--SO.sub.3Na or --SO.sub.3K groups by hydrolysis, followed by
conversion of these to --SO.sub.3H groups by reaction with an acid,
and carboxylic acid type fluoropolymers resulting from conversion
of --COOCH.sub.3 groups to such salt type groups as --COONa or
--COOK groups by hydrolysis, followed by conversion thereof to
--COOH groups by reaction with an acid.
[0140] The polymer electrolyte membrane of the invention which
contains the hydrolyzate of the stabilized fluoropolymer, when used
as electrolyte membrane materials in fuel cells, chemical sensors
and the like, will not deteriorate during a long period of use,
hence the situation that fuel cell wastewater contains HF can be
avoided.
[0141] The polymer electrolyte membrane of the invention may
contain one single hydrolyzate species or two or more such species
differing, for example, in m an/or A in the general formula (II)
given hereinabove or differing in copolymerization ratio between
(II) and TFE as the above-mentioned hydrolyzate of the stabilized
fluoropolymer.
[0142] In the polymer electrolyte membrane of the invention, the
amount of fluoride ion eluted by Fenton treatment comprising
immersing b grams of the polymer electrolyte membranes in a liters
of an aqueous hydrogen peroxide solution having an initial iron(II)
cation concentration of 2 ppm and an initial hydrogen peroxide
concentration of 1% by mass at a membrane/bath ratio [b/a] of 3.2
and maintaining the whole at 80.degree. C. for 2 hours is
preferably not greater than 11.times.10.sup.-4 parts by mass per
100 parts by mass of the polymer electrolyte membrane.
[0143] When the polymer electrolyte membrane of the invention shows
the amount of fluoride ion within the above range, the degree of
stabilization of the stabilized fluoropolymers is sufficiently
high.
[0144] The amount of fluoride ion eluted by the above Fenton
treatment is more preferably not greater than 8.0.times.10.sup.-4
parts by mass, still more preferably not greater than
5.0.times.10.sup.-4 parts by mass, per 100 parts by mass of the
polymer electrolyte membrane. Provided that the amount of fluoride
ion eluted by the above Fenton treatment is within the above range,
the amount of fluoride ion of not lower than 1.0.times.10.sup.-4
parts by mass per 100 parts by mass of the polymer electrolyte
membrane are still acceptable from the industrial viewpoint.
[0145] The amount of fluoride ion so referred to herein is measured
by "Fenton treatment 2" to be described later herein and by ion
chromatography (apparatus used: IC-2001, product of Tosoh, Japan;
anion analyzing column used: TSKgel Super IC-Anion, product of
Tosoh, Japan).
[0146] The polymer electrolyte membrane of the invention preferably
has a membrane thickness of 5 to 100 .mu.m. When the thickness is
less than 5 .mu.m, the membranes, when used in fuel cells, will
easily wane in mechanical strength in the process of operation of
the fuel cells, leading to breakage thereof. When the thickness
exceeds 100 .mu.m, the membranes, when used in fuel cells, show
high levels of membrane resistance and cannot manifest sufficient
initial characteristics.
[0147] The term "initial characteristics" as used herein refers to
high numerical values of voltage, power generating capacity in a
wide current density range and other performance characteristics as
observed in current density-voltage curve measurement during fuel
cell operation using the polymer electrolyte membranes of the
invention.
[0148] A more preferred lower limit to the polymer electrolyte
membrane thickness is 10 .mu.m, and a more preferred upper limit
thereto is 75 .mu.m.
[0149] The polymer electrolyte membranes of the invention can be
obtained by carrying out a molding and a hydrolysis.
[0150] The molding may comprise (1) molding the stabilized
fluoropolymer according to the invention into membranous
(film-like) forms or (2) molding a
sulfonic-acid-derived-group-containing fluoropolymer not yet
subjected to the above-mentioned fluorination treatment into
membranous (film-like) forms. In the case of the molding (2)
mentioned above, the fluorination treatment can be carried out
after molding.
[0151] The molding can be carried out, for example, by such melt
molding methods as the T-die molding method, inflation molding
method and calendering-based molding method.
[0152] In the above molding, a third component may be admixed
according to need with the stabilized fluoropolymer in the above
molding (1) or the sulfonic-acid-derived-group-containing
fluoropolymer in the above molding (2).
[0153] The molding conditions can be adequately selected depending
on the molding method employed. In the case of melt molding using a
T die, for instance, the molten resin temperature is preferably 100
to 400.degree. C., more preferably 200 to 300.degree. C.
[0154] If it follows the above-mentioned fluorination treatment,
the hydrolysis may be carried out either before molding or after
molding.
[0155] The hydrolysis is carried out by bringing the stabilized
fluoropolymer of the invention into contact with a strong base such
as an aqueous solution of NaOH or KOH, whereby --SO.sub.2F, for
instance, is converted, by saponification, to a metal salt such as
--SO.sub.3Na or --SO.sub.3K, and --COOCH.sub.3 to a metal salt such
as --COONa or --COOK. After washing with water, the metal salt is
further reacted with an acidic solution such as nitric acid,
sulfuric acid or hydrochloric acid (whereby --SO.sub.3Na, for
instance, is converted to --SO.sub.3H, and --COONa to --COOH),
followed by further washing with water. In this manner, the
acid-derived group represented by A in the general formula (I) or
general formula (II), in particular --SO.sub.2X, can be converted
to a sulfonic acid group, whereby the polymer electrolyte membranes
can be obtained.
[0156] As the method (1) for molding the stabilized fluoropolymer
of the invention into membranous (or film-like) form, there may
also be mentioned the method (casting method) comprising casting a
membrane or film casting liquid onto a supporting member to form a
liquid coating film on the supporting member and then removing a
liquid medium from the liquid coating film.
[0157] The membrane or film casting solution is obtained by the
steps of bringing the sulfonic-acid-derived-group-containing
fluoropolymer after the above-mentioned fluorination treatment into
contact with a strong base such as an aqueous solution of NaOH or
KOH for effecting saponification and, after washing with water,
further reacting the hydrolyzate with an acidic liquid such as
nitric acid, sulfuric acid or hydrochloric acid, followed by
further washing with water, for converting side chain terminal
acid-derived groups to sulfonic acid groups, and by the step of
dispersing or dissolving the polymer thus obtained in an
appropriate solvent comprising water, an alcohol and/or an organic
solvent, for instance, at 80 to 300.degree. C., if necessary using
an autoclave or the like. On the occasion of dispersing or
dissolving the polymer, a third component other than the above
polymer may be admixed with the polymer according to need.
[0158] Usable as the method for casting onto the supporting member
are the conventional coating methods using a gravure roll coater,
natural roll coater, reverse roll coater, knife coater, dip coater,
pipe doctor coater, etc.
[0159] The supporting member to be used in casting is not
restricted but ordinary polymer films, metal foils, alumina
substrates, Si substrates and like substrates can be adequately
used. In forming membrane/electrode assemblies (to be described
later herein), such supporting members can be removed from the
polymer electrolyte membranes, if desired.
[0160] By impregnating porous membranes prepared from the
polytetrafluoroethylene [PTFE] membrane described in Japanese
Patent Publication (Kokoku) H05-75835 with the membrane/film
casting liquid and then removing the liquid medium, it is also
possible to produce polymer electrolyte membranes containing a
reinforcement (the porous membrane). It is also possible to produce
fibrillated fiber-reinforced polymer electrolyte membranes, such as
those shown in Japanese Kokai Publication S53-149881 and Japanese
Patent Publication S63-61337, by adding fibrillated fibers made of
PTFE or the like to the membrane/film casting liquid and, after
casting, removing the liquid medium.
[0161] The polymer electrolyte membrane of the invention may be the
one obtained by subjecting to heat treatment (annealing) at 40 to
300.degree. C., preferably 80 to 220.degree., if desired.
Furthermore, they may be subjected to acid treatment with
hydrochloric acid or nitric acid, for instance, to sufficiently
manifest their intrinsic ion exchange capacity, if desired.
Further, they may be oriented in the direction(s) of expanding
using a transverse uniaxial tenter or a sequential or simultaneous
biaxial tenter.
[0162] The hydrolyzate of the above-mentioned stabilized
fluoropolymer can be used as membranes and/or electrodes for
constituting membrane/electrode assemblies in solid polymer
electrolyte fuel cells, which are to be described later herein.
[0163] The hydrolyzate of the stabilized fluoropolymer may be one
constituting membranes but not constituting electrodes or those not
constituting membranes but constituting electrodes or those
constituting membranes and electrodes in membrane/electrode
assembly constituting the solid polymer electrolyte fuel cell
described later herein.
[0164] The active substance-immobilized material of the invention
comprises a hydrolyzate of the stabilized fluoropolymer as
mentioned above and an active substance.
[0165] Generally, the active substance-immobilized material is the
one obtained by coating a substrate material with a liquid
composition prepared by dispersing the hydrolyzate of the
stabilized fluoropolymer and the active substance in a liquid
medium. When the substrate material is coated with a liquid
composition comprising the above-mentioned stabilized fluoropolymer
and the active substance, that member can be obtained by
hydrolyzing the stabilized fluoropolymer after coating.
[0166] The active substance is not particularly restricted provided
that it can be active in the active substance-immobilized material.
While the active substance is to be properly selected according to
the intended purpose of the active substance-immobilized material
of the invention, a catalyst may be adequately used in some
instances. The active substance-immobilized material of the
invention in which a catalyst is used as the active substance can
be suitably used as an electrode constituting a membrane/electrode
assembly in a fuel cell.
[0167] The catalyst is preferably an electrode catalyst, more
preferably a platinum-containing metal.
[0168] The catalyst is not particularly restricted but may be any
of those generally used as electrode catalysts, including, among
others, metals containing platinum, ruthenium and/or the like; and
organometallic complexes generally containing, as a central
metal(s), one or more metals at least one of which is platinum or
ruthenium.
[0169] The metal containing platinum, ruthenium and/or the like may
be a ruthenium-containing metal, for example simple substance
ruthenium, but preferably is a platinum-containing metal. The
platinum-containing metal is not particularly restricted but may
be, for example, simple substance platinum (platinum black); and
platinum-ruthenium alloys.
[0170] The above catalyst is generally used in a form supported on
a carrier such as silica, alumina or carbon.
[0171] In cases where the particles or solution of the stabilized
fluoropolymer hydrolyzate are or is desired to show good
dispersibility, the liquid medium may include, in addition to
water, alcohols such as methanol; nitrogen-containing solvents such
as N-methylpyrrolidone [NMP]; ketones such as acetone; esters such
as ethyl acetate; polar ethers such as diglyme and tetrahydrofuran
[THF]; carbonate esters such as diethylene carbonate; and other
polar organic solvents. One of these may be used singly or two or
more of these may be used in admixture.
[0172] The above-mentioned liquid composition comprises at least
the stabilized fluoropolymer or the hydrolyzate thereof and the
active substance mentioned above and may further contain another or
other components according to need.
[0173] As the other components for the purpose of membrane molding
by casting or impregnation, for instance, there may be mentioned
alcohols for improving the leveling properties and polyoxyethylenes
for improving the membrane/film-forming properties, among
others.
[0174] The substrate material is not particularly restricted but
includes, among others, porous supports, resin moldings and metal
sheets. Preferred are those electrolyte membranes, porous carbon
electrodes and like materials used in fuel cells and so forth.
[0175] The above-mentioned electrolyte membranes may preferably
comprise a fluororesin commonly so referred to and may comprise the
hydrolyzate of the stabilized fluoropolymer of the invention. The
electrolyte membranes may contain a substance(s) other than the
fluororesin generally so referred to and than the hydrolyzate of
the stabilized fluoropolymer, so long as the substance(s) will not
diminish the properties of the active substance-immobilized
materials.
[0176] The "coating a substrate material with a liquid composition"
consists in applying the above-mentioned liquid composition to the
substrate, drying if necessary, further converting the coating to a
hydrolyzate if necessary, and, generally, further heating the
coating at a temperature not lower than the melting point of the
hydrolyzate of the stabilized fluoropolymer.
[0177] The conditions of the heating mentioned above are not
particularly restricted provided that the hydrolyzate of the
stabilized fluoropolymer and active substance can be immobilized on
the substrate material. For example, the heating is preferably
carried out at 200 to 350.degree. C., for instance, for several
minutes, for example 2 to 30 minutes.
[0178] When it is to be used in a solid polymer electrolyte fuel
cell, the active substance-immobilized material of the invention is
preferably an electrode (also referred to as "catalyst layer")
comprising the hydrolyzate of the stabilized fluoropolymer, carbon
and a catalyst (e.g. Pt).
[0179] The membrane/electrode assembly of the invention is a
membrane/electrode assembly comprising a polymer electrolyte
membrane and an electrode and satisfies at least one condition
selected from the group consisting of the following conditions (1)
and (2):
(1) the polymer electrolyte membrane is the above-mentioned polymer
electrolyte membrane of the invention; (2) the electrode is the
above-mentioned active substance-immobilized material of the
invention.
[0180] The membrane/electrode assembly of the invention can be used
in a solid polymer electrolyte fuel cell, for instance.
[0181] When the polymer electrolyte membrane of the invention is
used in a solid polymer electrolyte fuel cell, the polymer
electrolyte membrane of the invention can be used in a
membrane/electrode assembly (hereinafter sometimes referred to as
"MEA") in which the membrane is sandwiched between an anode and a
cathode and tightly adhered to both. Here, the anode comprises an
anode catalyst layer and has protonic conductivity, and the cathode
comprises a cathode catalyst layer and has protonic conductivity.
The assembly further comprising a gas diffusion layer (to be
mentioned later herein) joined to the outside surface of each of
the anode catalyst layer and cathode catalyst layer is called
"MEA".
[0182] The anode catalyst layer contains a catalyst for fuel (e.g.
hydrogen) oxidation and ready proton formation, and the cathode
catalyst layer contains a catalyst for water formation by reaction
of protons and electrons with an oxidizing agent (e.g. oxygen or
air). For both the anode and cathode, platinum or an alloy
comprising platinum and ruthenium, for instance, is suitably used
as the catalyst, preferably in the form of catalyst particles not
greater than 10-100 angstroms in diameter. Such catalyst particles
are preferably supported on such electrically conductive particles
about 0.01 to 10 .mu.m in size as particles of furnace black,
channel black, acetylene black, carbon black, active carbon or
graphite. The amount of the supported catalyst particles relative
to the projected area of the catalyst layer is preferably not
smaller than 0.001 mg/cm.sup.2 but not greater than 10
mg/cm.sup.2.
[0183] Further, the anode catalyst layer and cathode catalyst layer
preferably contain the hydrolyzate of a fluoropolymer obtained via
polymerization of an acid-derived group-containing perhalovinyl
ether represented by the general formula (II) given hereinabove and
tetrafluoroethylene. The amount of the hydrolyzate of the
perfluorocarbonsulfonic acid polymer as supported relative to the
projected catalyst layer area is preferably from 0.001 mg/cm.sup.2
to not greater than 10 mg/cm.sup.2.
[0184] As the method for making MEAs, there may be mentioned, for
example, the following method. First, the hydrolyzate of the
stabilized fluoropolymer is dissolved in a mixed solvent composed
of an alcohol and water, and a commercial grade of
platinum-supporting carbon (e.g. Tanaka Kikinzoku's TEC10E40E) is
dispersed in the solution to give a paste. A predetermined amount
of this is applied to one side of each of two PTFE sheets and then
dried to form a catalyst layer. Then, the polymer electrolyte
membrane of the invention is sandwiched between the PTFE sheets in
a manner such that the respective coated surfaces face each other,
and the whole is subjected to heat pressing at 100 to 200.degree.
C. for transfer joining or coating. Upon removal of the PTFE
sheets, a MEA can be obtained. Such and other methods for
manufacturing MEAs are well known to those skilled in the art. The
methods for manufacturing MEAs are described in detail in Journal
of Applied Electrochemistry, 22 (1992), pages 1 to 7, for
instance.
[0185] Usable as the gas diffusion layer are commercial grades of
carbon cloth or carbon paper. A typical example of the former is
the carbon cloth E-tek, B-1, which is a product of De Nora North
America, U.S.A. As typical examples of the latter, there may be
mentioned Carbel (registered trademark, Japan Gore-Tex, Japan),
TGP-H, which is a product of Toray, Japan, and the carbon paper
2050, which is a product of Spectracorp, U.S.A., among others.
[0186] The structural unit resulting from integration of an
electrode catalyst layer with a gas diffusion layer is called "gas
diffusion electrode". A MEA can also be obtained by joining such
gas diffusion electrodes to the polymer electrolyte membrane of the
invention. As a typical example of the commercially available gas
diffusion electrode, there may be mentioned the gas diffusion
electrode ELAT (registered trademark; product of De Nora North
America, U.S.A.; carbon cloth being used as the gas diffusion
layer).
[0187] The solid polymer electrolyte fuel cell of the invention
contains the above-mentioned membrane/electrode assembly.
[0188] The solid polymer electrolyte fuel cell is not particularly
restricted provided that it contains the above-mentioned
membrane/electrode assembly. Generally, it may contain electrodes,
a gas and other constituent elements constituting a solid polymer
electrolyte fuel cell.
[0189] The stabilized fluoropolymer and the hydrolyzate thereof
according to the invention are excellent in chemical stability, as
described hereinabove, and, therefore, can be suitably used over a
long period of time as the electrolyte membrane in a fuel cell,
such as a solid polymer electrolyte fuel cell, which is generally
used under severe conditions, and as a material thereof.
[0190] The membrane obtained by cast film formation, the membrane
formed on a porous supporting member, the active
substance-immobilized material, the polymer electrolyte membrane
and the solid polymer electrolyte fuel cell, each mentioned
hereinabove, are obtained by using the hydrolyzate of the
stabilized fluoropolymer. The liquid composition mentioned above is
preferably the one comprising the hydrolyzate of the stabilized
fluoropolymer.
[0191] The polymer electrolyte membranes of the invention are
useful not only as the membrane materials of the solid polymer
electrolyte fuel cells but also as membrane materials for
manufacturing electrolyte membranes for use in lithium cell
membranes, membranes for use in electric soda processes, membranes
for use in electrolysis of water, membranes for use in hydrofluoric
acid electrolysis, membranes for oxygen concentration apparatus,
humidity sensor membranes, gas sensor membranes and separation
membranes, or ion exchange membranes.
EFFECTS OF THE INVENTION
[0192] The method for producing a stabilized fluoropolymer of the
invention makes it possible to fluorinate unstable groups of a
sulfonic-acid-derived-group-containing fluoropolymer to a
satisfactory extent.
[0193] The stabilized fluoropolymer of the invention is excellent
in chemical stability, as mentioned above, and, therefore, the
hydrolyzate thereof can be suitably used as electrolyte membranes
or like membrane materials or electrodes for use in fuel cells,
such as solid polymer electrolyte fuel cells, which are used under
very severe conditions and make it possible to obtain highly
durable fuel cell membranes or electrodes causing very low levels
of the fluoride ion concentration in wastewater.
BEST MODES FOR CARRYING OUT THE INVENTION
[0194] The following examples illustrate the present invention in
further detail. These examples are, however, by no means limitative
of the scope of the invention.
Measurement Methods
1. Moisture Content Measurement
[0195] Using a moisture evaporator (trademark: ADP-351, product of
Kyoto Electronics Manufacturing Co.), the sample was heated, with
dry nitrogen as a carrier gas, and the moisture evaporated was
collected in a Karl Fischer moisture titrator and determined by
coulometric titration.
[0196] The sample size was 1 g, and the measurement temperature was
150.degree. C.
2. Functional Group Determination by IR
[0197] The polymer sample was heat-pressed at 270.degree. C. and 10
MPa for 20 minutes to give a 150- to 200-.mu.m-thick membrane,
which was subjected to spectrometry using a FT-IR spectrograph.
3. Stability Testing with Fenton's Reagent
(1) Fenton Treatment 1
[0198] The polymer sample was heat-pressed at 270.degree. C. and 10
MPa for 20 minutes, and the polymer side chain terminal groups were
then converted to sulfonic acid groups to give a membrane for
stability testing.
[0199] A solution of 1 mg of FeSO.sub.4.7H.sub.2O in 20 ml of a 30%
(by mass) aqueous solution of hydrogen peroxide was placed in a
tetrafluoroethylene/perfluoro (alkyl vinyl ether) copolymer-made
bottle, 3 g of the membrane for stability testing was immersed
therein and maintained at 85.degree. C. for 20 hours. Thereafter,
the whole was cooled to room temperature, the membrane for
stability testing was taken out, and the fluoride ion concentration
in the liquid phase was determined using a fluoride ion meter
(Orion EA940 expandable ion analyzer).
(2) Fenton Treatment 2
[0200] The polymer electrolyte membrane (b grams) was immersed in
an aqueous hydrogen peroxide solution (a liters) with an initial
iron(II) cation concentration of 2 ppm and an initial hydrogen
peroxide concentration of 1% by mass at a membrane/liquid ratio
[b/a] of 3.2 and, after 2 hours of maintenance at 80.degree. C.,
the sample polymer (polymer electrolyte membrane) was removed. The
liquid amount was measured and then appropriately diluted with
distilled water for ion chromatography, and the amount of fluoride
ion F.sup.- was determined by ion chromatography. The measurement
apparatus used was IC-2001, product of Tosoh, Japan, and the anion
analyzing column used was TSKgel Super IC-Anion, product of Tosoh,
Japan. The amount of fluoride ion eluted was expressed in terms of
the mass of fluoride ion eluted per 100 parts by mass of the sample
polymer mass.
4. Ion Exchange Equivalent Weight Ew Measurement
[0201] A 0.1-g section cut out of the sample polymer electrolyte
membrane was immersed in 30 ml of a saturated aqueous solution of
NaCl at a temperature of 25.degree. C. and, after 30 minutes of
standing with stirring, the system was subjected to neutralization
titration with a 0.01 N aqueous solution of sodium hydroxide, with
phenolphthalein as an indicator. The point at which the pH meter
used (TPX-90, product of Toko Kagaku Kenkyusho, Japan) indicated a
value within the range of 6.95-7.05 was taken as the equivalence
point. After neutralization, the Na salt form electrolyte membrane
was rinsed with pure water, then dried under vacuum and weighed.
The ion exchange equivalent weight Ew (g/eq) was calculated from
the equivalent amount M (mmol) of sodium hydroxide required for
neutralization and the weight W (mg) of the Na salt form
electrolyte membrane according to the following equation:
Ew=(W/M)-22
5. Solid State .sup.19F Nuclear Magnetic Resonance Spectrometry
[0202] The solid state .sup.19F nuclear magnetic resonance
spectrometry was carried out under the following conditions. The
sample was subjected to NMR measurement while being immersed in a
swelling solvent in a test tube for NMR spectrometry.
Apparatus: DSX400, product of Bruker Biospin, Germany Number of MAS
rotations: 4.8 kHz Observation frequency: 376.5 MHz Chemical shift
standard: CF.sub.3COOH (-77 ppm) Swelling solvent:
N-Methylacetamide Measurement temperature: 473K
[0203] The method of calculation is described below. The main chain
terminal --CF.sub.3 group (in FIG. 1, a) is detected at -79.7 ppm.
The integrated intensity of this signal is expressed as A. On the
other hand, a signal of the --CF.sub.2-- group adjacent to a side
chain ether (in FIG. 2, b) is observed at -76.4 ppm. The integrated
intensity of this signal is expressed as B. The number (X) of main
chain terminal CF.sub.3 groups per 10.sup.5 main chain-constituting
carbon atoms can be calculated from the values of A and B and the
Ew of the sample as determined by the ion exchange equivalent
weight (Ew) measurement method described above under 4, using the
following equation (III):
X=100000/[{(B/A).times.3/2}.times.{2.times.(Ew-178-50.times.m)/100}+2]
(III)
wherein m is the value of m defined hereinabove referring to the
general formula (II).
6. Fuel Cell Evaluation
[0204] The polymer electrolyte membrane was evaluated in a fuel
cell in the following manner. First, electrode catalyst layers were
produced. Thus, a 5% (by mass) fluoropolymer solution in a solvent
composition composed of ethanol and water at a ratio of 50/50 (by
mass) was prepared and further concentrated to 11% by mass. A
3.31-g portion of the polymer solution was added to 1.00 g of
Pt-carrying carbon (TEC10E40E, Pt content 36.4% by weight, product
of Tanaka Kikinzoku, Japan). Further addition of 3.24 g of ethanol
and thorough mixing up using a homogenizer gave an electrode ink.
This electrode ink was applied onto PTFE sheets by screen printing.
Two coating weight levels were employed. In one case, the amount of
the Pt supported and that of the polymer supported were both 0.15
mg/cm.sup.2 and, in the other, the amount of the Pt supported and
that of the polymer supported were both 0.30 mg/cm.sup.2. After
application, the coat layers were dried at room temperature for 1
hour and then in air at 120.degree. C. for 1 hour to give about
10-.mu.m-thick electrode catalyst layers. Among these electrode
catalyst layers, the one carrying 0.15 mg/cm.sup.2 of Pt and of the
polymer was used as the anode catalyst layer, and the one carrying
0.30 mg/cm.sup.2 of Pt and of the polymer was used as the cathode
catalyst layer.
[0205] The polymer electrolyte membrane was sandwiched between
thus-obtained anode catalyst layer and cathode catalyst layer with
the layers facing each other, and the whole was hot pressed at
160.degree. C. and a contact area pressure of 0.1 MPa, whereby the
anode catalyst layer and cathode catalyst layer were transferred
and joined to the polymer electrolyte membrane to give a MEA.
[0206] Carbon cloths (ELAT (registered trademark) B-1, product of
De Nora North America, U.S.A.) were set, as gas diffusion layers,
on both sides of this MEA (on the outside surfaces of the anode
catalyst layer and cathode catalyst layer), and the whole was
integrated into a cell for evaluation. This cell for evaluation was
set on a cell evaluation apparatus (Toyo Tekunika (Toyo Corp.)
model 890CL fuel cell evaluation system) and, after raising the
temperature to 80.degree. C., hydrogen gas was caused to flow on
the anode side at a rate of 150 cc/min, and air gas was caused to
flow on the cathode side at a rate of 400 cc/min. For gas
humidification, the water bubbling method was used, and hydrogen
gas and air gas were humidified at 80.degree. C. and 50.degree. C.,
respectively. While feeding the humidified hydrogen gas and air gas
to the cell, the current density-voltage curve was measured for
initial characteristics evaluation.
[0207] After the above initial characteristics evaluation,
durability testing was carried out at a cell temperature of
100.degree. C. In each case, the gas humidifying temperature was
60.degree. C. for both the anode and cathode. When the cell
temperature was 100.degree. C., hydrogen gas was fed to the anode
side at 74 cc/min and air gas to the cathode side at 102 cc/min
and, while pressurizing the anode side at 0.30 MPa (absolute
pressure) and the cathode side at 0.15 MPa (absolute pressure),
electricity was generated at a current density of 0.3 A/cm.sup.2.
If the polymers in the membrane and/or electrodes are deteriorated
on such occasion, the fluoride ion concentrations in exhaust water
on the anode side and cathode side will increase. Therefore, the
fluoride ion concentrations in exhaust water were measured at timed
intervals using a model 9609B Nionplus fluoride ion selective
multiple electrode of a model 920Aplus benchtop pH/ion meter
(product of Meditorial, Japan). If a pinhole is formed in the
polymer electrolyte membrane during such durability test, the
so-called crossleak phenomenon, namely leakage of a large amount of
hydrogen gas to the cathode side. For examining the degree of this
crossleak, the hydrogen concentration in the exhaust gas on the
cathode side was measured using a micro GC (CP4900, product of
Varian, Netherlands) and, at the time when this measured value
arrived at a level 10 times the initial level, the test was
finished.
7. Melt Index [MI] Measurement
[0208] The melt index of each fluoropolymer was measured according
to JIS K 7210 under the conditions of 270.degree. C. and a load of
2.16 kg using a type C-5059D melt indexer, product of Toyo Seiki
Seisakusho, Japan. The weight of the polymer extruded was expressed
in terms of grams per 10 minutes.
Example 1
(1) Polymer Synthesis
[0209] A 3000-ml stainless steel stirring autoclave was charged
with 300 g of a 10% aqueous solution of C.sub.7F.sub.15COONH.sub.4
and 1170 g of pure water, followed by thorough evacuation and
nitrogen substitution. After sufficiently evacuating the autoclave,
tetrafluoroethylene [TFE] gas was fed to the autoclave until a gage
pressure of 0.2 MPa, and the temperature was raised to 50.degree.
C. Then, 100 g of CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F was
injected into the autoclave, and the gage pressure was raised to
0.7 MPa by introduction of TFE gas. Thereafter, an aqueous solution
of 0.5 g of ammonium persulfate [APS] in 60 g of pure water was
injected to start the polymerization. For supplementing the TFE
consumed by polymerization, TFE was continuously fed to the
autoclave so that the autoclave inside pressure might be maintained
at 0.7 MPa. The polymerization was continued while continuously
feeding CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F in an amount of
53% by mass relative to the TFE further fed.
[0210] After completion of feeding of 522 g of TFE, the autoclave
inside pressure was released and the polymerization was thereby
terminated. After cooling to room temperature, there was obtained
2450 g of a somewhat cloudy aqueous dispersion containing about 33%
by mass of an SO.sub.2F-containing perfluoropolymer.
[0211] The aqueous dispersion was coagulated with nitric acid, and
the solid was washed with water and dried at 90.degree. C. for 24
hours and further dried at 120.degree. C. for 12 hours to give 800
g of a fluoropolymer (A).
[0212] Then, 1 g of the thus-obtained fluoropolymer A was
immediately put into a tubular oven heated at 150.degree. C. for
evaporating the moisture, which was introduced into a Karl Fischer
moisture measuring apparatus using dry nitrogen as a carrier gas.
The moisture content thus determined was 200 ppm by mass.
[0213] Further, a high-temperature. NMR measurement at 300.degree.
C. revealed that the CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F unit
content in the fluoropolymer A was 19 mole percent.
[0214] The above fluoropolymer A was heat-pressed at 270.degree. C.
and 10 MPa for 20 minutes to give a 170-.mu.m-thick transparent
membrane.
[0215] Upon IR measurement, any sulfonic acid-due peak was not
observed. The intensity ratio [x/y] between the carboxyl group-due
peak [x] and the --CF.sub.2-- group-due peak [y] was 0.23.
(2) Fluorination
[0216] A 200-g portion of the above fluoropolymer A with a moisture
content of 200 ppm was placed in a 1000-ml autoclave (made of
Hastelloy), and the temperature was raised to 120.degree. C. under
evacuation. The procedure comprising evacuation and nitrogen
substitution was repeated three times and, thereafter, nitrogen was
introduced to a gage pressure of 0 MPa. Then, a gaseous
fluorinating agent prepared by diluting fluorine gas to 20% by
volume with nitrogen gas was introduced into the autoclave until
arrival of the gage pressure at 0.1 MPa, and the resulting system
was maintained for 30 minutes.
[0217] Then, the fluorine in the autoclave was discharged and,
after evacuation, a gaseous fluorinating agent prepared by diluting
fluorine gas to 20% by volume with nitrogen gas was introduced
until arrival of the gage pressure at 0.1 MPa and the system was
maintained for 3 hours.
[0218] Thereafter, the autoclave was cooled to room temperature,
the fluorine was discharged therefrom and, after three repetitions
of evacuation and nitrogen substitution, the autoclave was opened.
A stabilized fluoropolymer (B) was thus obtained.
[0219] The above stabilized fluoropolymer B was heat-pressed at
270.degree. C. and 10 MPa for 20 minutes to give a 170-.mu.m-thick
transparent membrane.
[0220] Upon IR measurement, any sulfonic acid-due peak was not
observed. The intensity ratio [x/y] between the carboxyl group-due
peak [x] and the --CF.sub.2-- group-due peak [y] was 0.03.
[0221] The stabilized fluoropolymer obtained was extruded through a
melt indexer [MI meter] at 270.degree. C. under a load of 5 kg to
give an extrudate strand. The strand obtained was again extruded
through the MI meter. After two repetitions of this procedure, the
strand obtained showed almost no discoloration.
Comparative Example 1
[0222] The fluoropolymer A obtained in Example 1 was allowed to
stand in the air for 2 days and the moisture content was then
measured and found to be 700 ppm.
[0223] This polymer was fluorinated in the same manner as in
Example 1 to give a fluoropolymer (C). The upper part of the
autoclave used for producing the fluoropolymer C was found to have
been colored green, indicating the progress of corrosion.
[0224] The fluoropolymer C was heat-pressed at 270.degree. C. and
10 MPa for 20 minutes to give a 160-.mu.m-thick transparent
membrane.
[0225] As a result of IR measurement, the intensity ratio [z/y]
between the sulfonic acid-due peak [z] and the --CF.sub.2--
group-due peak [y] was found to be 0.03. The intensity ratio [x/y]
between the carboxyl group-due peak [x] and the --CF.sub.2--
group-due peak [y] was 0.14.
[0226] The fluoropolymer C obtained was extruded through a MI meter
at 270.degree. C. under a load of 5 kg to give an extrudate strand.
The strand obtained was again extruded through the MI meter. After
two repetitions of this procedure, the strand obtained was found
colored dark brown.
[0227] In view of the above results, the fluorination of the
fluoropolymer A having a moisture content lower than 500 ppm as
carried out in Example 1 could stabilize terminals as compared with
Comparative Example 1 in which the moisture content was 700 ppm. It
was also found that the fluoropolymer in Example 1 was inhibited
from being discolored.
Example 2
[0228] The membrane obtained in Example 1 (2) was treated in a 20%
aqueous solution of sodium hydroxide at 90.degree. C. for 24 hours
and then washed with water. It was then treated in 6 N sulfuric
acid at 60.degree. C. for 24 hours and then washed with water until
the washings showed neutrality, to give a sulfonic acid form
membrane.
[0229] This membrane was thoroughly dried at 110.degree. C., and 3
g of the dried membrane was taken and subjected to stability
testing by Fenton treatment 1. The fluoride ion concentration was 5
ppm.
Comparative Example 2
[0230] A polymer membrane (trade mark: Nafion 117, product of
DuPont) produced by polymerization of
CF.sub.2.dbd.CFOCF.sub.2CF(CF.sub.3)OCF.sub.2CF.sub.2SO.sub.2F was
used as a sulfonic-acid-derived-group-containing fluoropolymer and
subjected to stability testing by Fenton treatment 1 in the same
manner as in Example 2. The fluoride ion concentration was 20
ppm.
[0231] The results of the above stability testing indicated that
the polymer obtained by copolymerization of
CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F and TFE and the
subsequent terminal stabilization of the copolymer is superior in
stability to the polymer produced by using
CF.sub.2.dbd.CFOCF.sub.2CF(CF.sub.3)
OCF.sub.2CF.sub.2SO.sub.2F.
Example 3
(1) Polymer Synthesis
[0232] A 3000-ml stainless steel stirring autoclave was thoroughly
evacuated, followed by nitrogen substitution. The autoclave was
again thoroughly evacuated and, then, charged with 1530 g of
perfluorohexane and 990 g of
CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F, and the temperature was
adjusted to 25.degree. C. Then, tetrafluoroethylene [TFE] gas was
introduced until arrival of the gage pressure at 0.30 MPa, followed
by introduction under pressure of 13.14 g of a 10% (by mass)
solution of the polymerization initiator (C.sub.3F.sub.7COO).sub.2
in perfluorohexane to initiate the polymerization reaction. For
supplementing the TFE consumed by polymerization, TFE was
continuously fed to the autoclave so that the autoclave inside
pressure might be maintained at 0.30 MPa. The polymerization was
continued while further feeding a total of 47 g of
CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F intermittently.
[0233] After completion of feeding of 73 g of TFE, the autoclave
inside pressure was released and the polymerization was thereby
terminated.
[0234] After completion of the polymerization reaction, 1500 ml of
chloroform was added, and the resulting mixture was stirred for 10
minutes. The mixture was then subjected to solid-liquid separation
using a centrifuge, 1500 ml of chloroform was added to the solid
matter, and the mixture was stirred for 10 minutes. This procedure
was repeated three times to wash the polymer. This washed polymer
was then deprived of the residual chloroform at 120.degree. C.
under vacuum to give 128 g of a fluoropolymer (D).
[0235] Then, 1 g of the thus-obtained fluoropolymer D was
immediately put into a tubular oven heated at 150.degree. C. for
evaporating the moisture, which was introduced into a Karl Fischer
moisture measuring apparatus using dry nitrogen as a carrier gas.
The moisture content thus determined was 50 ppm by mass.
[0236] Further, a high-temperature NMR measurement at 300.degree.
C. revealed that the CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F unit
content in the fluoropolymer D was 18 mole percent.
[0237] The above fluoropolymer D was heat-pressed at 270.degree. C.
and 10 MPa for 20 minutes to give a 160-.mu.m-thick transparent
membrane.
[0238] Upon IR measurement, any sulfonic acid-due peak was not
observed. The intensity ratio [x/y] between the carboxyl group-due
peak [x] and the --CF.sub.2-- group-due peak [y] was 0.08.
(2) Fluorination
[0239] A 100-g portion of the fluoropolymer D was treated in the
same manner as in Example 1 to give a stabilized fluoropolymer
(E).
[0240] A thin membrane was prepared by heat pressing in the same
manner as in Example 1 and subjected to IR measurement. Neither
sulfonic acid-due peak nor carboxyl group-due peak was
observed.
Example 4
[0241] The thin membrane of the stabilized fluoropolymer E was
treated in the same manner as in Example 2 to give a sulfonic acid
form membrane.
[0242] This membrane was thoroughly dried at 110.degree. C., and 3
g of the dried membrane was taken and subjected to the
above-mentioned Fenton treatment 1. The fluoride ion concentration
was 5 ppm.
Example 5
(1) Polymer Synthesis
[0243] A 6-liter stainless steel stirring autoclave was charged
with 150 g of a 10% aqueous solution of C.sub.7F.sub.15COONH.sub.4
and 2840 g of pure water, followed by thorough evacuation and
nitrogen substitution. After sufficiently evacuating the autoclave,
tetrafluoroethylene [TFE] gas was fed to the autoclave until a gage
pressure of 0.2 MPa, and the temperature was raised to 50.degree.
C. Then, 180 g of CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F was
injected into the autoclave, and the gage pressure was raised to
0.7 MPa by introduction of TFE gas. Thereafter, an aqueous solution
of 1.5 g of ammonium persulfate [APS] in 20 g of pure water was
injected to start the polymerization.
[0244] For supplementing the TFE consumed by polymerization, TFE
was continuously fed to the autoclave so that the autoclave inside
pressure might be maintained at 0.7 MPa. The polymerization was
continued while continuously feeding
CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F in an amount of 70% by
mass relative to the TFE further fed.
[0245] After completion of feeding of 920 g of TFE, the autoclave
inside pressure was released and the polymerization was thereby
terminated. After cooling to room temperature, there was obtained
4650 g of a somewhat cloudy aqueous dispersion containing about 32%
by mass of a SO.sub.2F-containing perfluoropolymer.
[0246] The aqueous dispersion was coagulated with nitric acid, and
the solid was washed with water and dried at 90.degree. C. for 24
hours and further dried at 120.degree. C. for 12 hours to give 1500
g of a fluoropolymer (F).
[0247] Then, 1 g of the thus-obtained fluoropolymer F was
immediately put into a tubular oven heated at 150.degree. C. for
evaporating the moisture, which was introduced into a Karl Fischer
moisture measuring apparatus using dry nitrogen as a carrier gas.
The moisture content thus determined was 150 ppm by mass.
[0248] Further, a high-temperature NMR measurement at 300.degree.
C. revealed that the CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F unit
content in the fluoropolymer F was 18.6 mole percent.
[0249] The above fluoropolymer F was heat-pressed at 270.degree. C.
and 10 MPa for 20 minutes to give a 170-.mu.m-thick transparent
membrane. As a result of IR measurement, the intensity ratio [z/y]
between the sulfonic acid-due peak [z] and the --CF.sub.2--
group-due peak [y] was found to be 0.05. The intensity ratio [x/y]
between the carboxyl group-due peak [x] and the --CF.sub.2--
group-due peak [y] was 0.20.
(2) Fluorination
[0250] A 700-g portion of the above fluoropolymer F was then placed
in a 3-liter autoclave made of Hastelloy, and the temperature was
raised to 120.degree. C. under evacuation. The procedure comprising
evacuation and nitrogen substitution was repeated three times and,
thereafter, nitrogen was introduced to a gage pressure of -0.5 MPa.
Then, a gaseous fluorinating agent prepared by diluting fluorine
gas to 20% by volume with nitrogen gas was introduced into the
autoclave until arrival of the gage pressure at 0 MPa, and the
resulting system was maintained for 30 minutes.
[0251] Then, the fluorine in the autoclave was discharged and,
after evacuation, a gaseous fluorinating agent prepared by diluting
fluorine gas to 20% by volume with nitrogen gas was introduced
until arrival of the gage pressure at 0 MPa and the system was
maintained for 3 hours.
[0252] Thereafter, the autoclave was cooled to room temperature,
the fluorine was discharged therefrom and, after three repetitions
of evacuation and nitrogen substitution, the autoclave was opened.
A stabilized fluoropolymer (G) was thus obtained.
[0253] The above stabilized fluoropolymer G was heat-pressed at
270.degree. C. and 10 MPa for 20 minutes to give a 170-.mu.m-thick
transparent membrane.
[0254] Upon IR measurement, neither sulfonic acid-due peak nor
carboxyl group-due peak was observed.
[0255] The MI of this stabilized polymer G was 3.5 (g/10 min). The
stabilized fluoropolymer G was molded into a 50-.mu.m-thick thin
membrane by extrusion melt molding at 280.degree. C. using a T die.
This thin membrane was treated in the same manner as in Example 2
to give a sulfonic acid form membrane. The Ew of this sulfonic acid
form membrane was determined by the ion exchange equivalent weight
(Ew) measurement method described hereinabove and found to be 720
(g/eq). Measurements made according to the above-mentioned solid
sate .sup.19F nuclear magnetic resonance measurement method gave
the following results: B/A=302.1, X=20.4 (m=2 in the general
formula (II)).
[0256] Further, this sulfonic acid form membrane was subjected to
the above-mentioned Fenton treatment 2. Fluoride ion was found to
amount to 3.1.times.10.sup.-4 parts by mass per 100 parts by mass
of the membrane.
[0257] Then, using this electrolyte membrane, a membrane/electrode
assembly (MEA) was produced according to the method described
hereinabove. The fluoropolymer used on that occasion in producing
the electrodes was the same polymer as used in the electrolyte
membrane, namely the sulfonic acid form polymer obtained by
conversion treatment of the stabilized fluoropolymer G.
[0258] This MEA was integrated into the cell for evaluation, and
the initial characteristics were measured at a cell temperature of
80.degree. C. by the method described hereinabove. The relationship
between the voltage (V) and current density (A/cm.sup.2) indicated
very good cell performance characteristics: 0.77 V at 0.5
A/cm.sup.2, 0.68 V at 1.0 A/cm.sup.2 and 0.55 V at 1.5
A/cm.sup.2.
[0259] In the durability test at 100.degree. C., the cell could be
operated for 550 hours and thus a high level of durability could be
attained. The fluoride ion concentration in the wastewater after
the lapse of 50 hours was 0.16 ppm on the cathode side and, on the
anode side, it was 0.23 ppm. After 400 hours, the fluoride ion
concentration in the wastewater was 0.22 ppm on the cathode side
and, on the anode side, it was 0.48 ppm.
Comparative Example 3
[0260] A polymer prepared in quite the same manner as in the case
of the fluoropolymer F except that it was not subjected to
fluorination treatment was made into a 50-.mu.m-thick thin membrane
by extrusion melt molding at 280.degree. C. using a T die in the
same manner as in Example 5. The MI of this fluoropolymer F was 3.5
(g/10 min). This thin membrane was treated in the same manner as in
Example 2 to give a sulfonic acid form membrane.
[0261] The Ew of this sulfonic acid form membrane was determined by
the ion exchange equivalent weight (Ew) measurement method
described hereinabove and found to be 720 (g/eq). Measurements made
according to the above-mentioned solid state .sup.19F nuclear
magnetic resonance measurement method gave the following results:
B/A=.infin., X=0 (m=2 in the general formula (II)). Further, this
sulfonic acid form membrane was subjected to the above-mentioned
Fenton treatment 2. Fluoride ion was found to amount to
13.3.times.10.sup.-4 parts by mass per 100 parts by mass of the
membrane.
[0262] Then, using this electrolyte membrane, a membrane/electrode
assembly (MEA) was produced according to the method described
hereinabove. The fluoropolymer used on that occasion in producing
the electrodes was the same polymer as used in the electrolyte
membrane, namely the sulfonic acid form polymer derived from the
fluoropolymer F not subjected to fluorination.
[0263] This MEA was integrated into the cell for evaluation, and
the initial characteristics were measured at a cell temperature of
80.degree. C. by the method described hereinabove. The relationship
between the voltage (V) and current density (A/cm.sup.2) indicated
the following: 0.72 V at 0.5 A/cm.sup.2, 0.44 V at 1.0 A/cm.sup.2
and unworkable at 1.5 A/cm.sup.2.
[0264] In the durability test at 100.degree. C., the cell stopped
working after 85 hours of operation due to crossleak. The fluoride
ion concentration in the wastewater after the lapse of 50 hours was
2.8 ppm on the cathode side and, on the anode side, it was 6.1
ppm.
Comparative Example 4
[0265] The fluorinated fluoropolymer C obtained in Comparative
Example 1 was made into a 50-.mu.m-thick thin membrane by extrusion
melt molding at 280.degree. C. using a T die in the same manner as
in Example 5. The MI of this fluoropolymer C was 3.2 (g/10 min).
This thin membrane was treated in the same manner as in Example 2
to give a sulfonic acid form membrane. The Ew of this sulfonic acid
form membrane was determined by the ion exchange equivalent weight
(Ew) measurement method described hereinabove and found to be 720
(g/eq). Measurements made according to the above-mentioned solid
state .sup.19F nuclear magnetic resonance measurement method gave
the following results: B/A=921, X=6.7 (m=2 in the general formula
(II)). Further, this sulfonic acid form membrane was subjected to
the above-mentioned Fenton treatment 2. Fluoride ion was found to
amount to 12.8.times.10.sup.-4 parts by mass per 100 parts by mass
of the membrane.
[0266] Then, using this electrolyte membrane, a membrane/electrode
assembly (MEA) was produced according to the method described
hereinabove. The fluoropolymer used on that occasion in producing
the electrodes was the same polymer as used in the electrolyte
membrane, namely the sulfonic acid form polymer derived from the
fluoropolymer C.
[0267] This MEA was integrated into the cell for evaluation, and
the initial characteristics were measured at a cell temperature of
80.degree. C. by the method described hereinabove. The relationship
between the voltage (V) and current density (A/cm.sup.2) indicated
the following: 0.75 V at 0.5 A/cm.sup.2, 0.52 V at 1.0 A/cm.sup.2
and 0.18 V at 1.5 A/cm.sup.2.
[0268] In the durability test at 100.degree. C., the cell stopped
working after 120 hours of operation due to crossleak. The fluoride
ion concentration in the wastewater after the lapse of 50 hours was
1.18 ppm on the cathode side and, on the anode side, it was 2.5
ppm.
INDUSTRIAL APPLICABILITY
[0269] The method for a producing a stabilized fluoropolymer of the
invention can be used in preparing materials suited for producing
membrane materials, such as fuel cell electrolyte membranes, which
are to be used under sever conditions.
[0270] The stabilized fluoropolymer of the invention is excellent
in chemical stability, as mentioned above, and, therefore, the
hydrolyzate thereof can be suitably used as electrolyte membranes
or like membrane materials or electrodes for use in fuel cells,
such as solid polymer electrolyte fuel cells, which are used under
very severe conditions and make it possible to obtain highly
durable fuel cell membranes or electrodes causing very low levels
of fluoride ion concentration in wastewater.
BRIEF DESCRIPTION OF THE DRAWINGS
[0271] FIG. 1 is a NMR chart obtained by subjecting the stabilized
fluoropolymer G obtained in Example 5 to solid state .sup.19F
nuclear magnetic resonance spectrometry (the ordinate being
magnified 13 times as compared with FIG. 2).
[0272] FIG. 2 is a NMR chart obtained by subjecting the stabilized
fluoropolymer G obtained in Example 5 to solid state .sup.19F
nuclear magnetic resonance spectrometry.
* * * * *