Note: Descriptions are shown in the official language in which they were submitted.
os~s5s
BACKGROUND
Chlorine is produced almost entirely by electrolytic methods,
primarily from aqueous solutions of alkali metal chlorides. In the
electrolysis of brines, chlorine is produced at the anode, and hydrogen,
together with an alkali metal hydroxide, such as sodium or potassium
hydroxide, at the cathode. As the anode and cathode products must be
kept separate, many cell designs have been developed. These designs
have generally utilized either a diaphragm, or a mercury intermediate
electrode to separate the anolyte and catholyte compartments.
In the diaphragm process, brine is fed continuously into the
electrolytic cell and flows from the anode compartment through an
asbestos diaphragm into the catholyte compartment which contains, for
example, an iron cathode. To minimize back-diffusion and migration,
the flow rate is always such that only part of the salt is converted.
The hydrogen ions are discharged from the solution at the cathode,
forming hydrogen gas and leaving hydroxyl ions. This catholyte solution,
which contains sodium hydroxide and unchanged sodium chloride is evapor-
ated to obtain the sodium hydroxide. In the course of the evaporation
the sodium chloride precipitates, is separated, redissolved, and sent
back into the electrolytic cell. The function of the diaphragm is to
maintain the level of concentration of alkali, to minimize the diffusion-
al migration of hydroxyl ions into the anolyte and to maintain separation
of hydrogen and chlorine. The diaphragm should also have minimal elec-
trical resistance.
In the mercury electrode process, the cation, after discharge,
forms an alloy or amalgam with mercury. The amalgam flows or is pumped
to a separate chamber in whch it is allowed to undergo galvanic reaction,
most often with water, to form hydrogen and a comparatively strong
- 2 - ~
~ 05Z8S8
sodium hydroxide solution containing almost no sodium chloride.
The diaphragm process is inherently cheaper than the mercury
process, but as the former process does not provide chloride-free
alkali, additional processing steps are necessary to purify and concen-
trate the alkali.
Substitution of an ion-exchange membrane material for the
diaphragm has been proposed. Numerous membrane materials have been
suggested. For example, membranes are described in United States Patent
Nos. 2,636,851; 2,967,807; 3,017,338; and British Patent Nos. 1,184,321
and 1,199,952.
Such membranes are substanttally lmpervious to hydraulic flow.
During operation, brine is introduced into the anolyte compartment where-
in chlorine is liberated. Then, in the case of a cation permselective
membrane, sodium ions are transported across the membrane into the
catholyte compartment. The concentration of the relatively pure caustic
produced in the catholyte compartment is determined by the amount of water
added to this compartment from an external source, and by migration of
water, in the cell, i.e. osmosis and/or electro-osmosis. While operation
of a membrane cell has many theoretical advantages, its commercial appli-
cation to the production of chlorine and caustic has been hindering
owing to the often erratic operating characteristics of the cells. A
number of disadvantages have been present when using these membranes,
including a relatively high electrical resistance, poor permselectivity
and oxidative degeneration~
In membrane-type fuel cells, a fuel, such as hydrogen or a
material which decomposes to hydrogen, is oxidized in an o~idation zone
or chamber giving up electrons to an anode. The hydrogen ions formed
migrate by means of an ion-exchange resin to the reduction zone or
~05Z8S8
chamber where they combine with oxygen gas from an oxidant reduced at
the cathode. Water, which contains a minor amount of a peroxide by-
product, is discharged from the cell. Thus, both material and electri-
cal charge balances are maintained as electrons flow from the anode to
the cathode. This electron flow can be utilized to provide useful
electrical energy. Numerous types of membranes have been proposed for
use in fuel cells including polymers of ~ , ~ , ~ -trifluorostyrene (U.S.
Patent 3,341,366) and copolymers of trifluorovinyl sulfonic acid (British
Patent 1,184,321).
It is an object of this invention to provide an improved cation
permselective membrane for use in chlor-alkali cells and fuel cells which
has a low electrical resistance and a high resistance to hydroxyl migra-
tion. It is a further object to provide a membrane which is particularly
resistant to oxidative degradation, particularly in chlorine and peroxide
environments.
It is a still further object to provide chlor-alkali cells of
the diaphragm type with improved electrical properties and which prevent
chloride contamination of the catholyte material. And it is an object
to proyide improved fuel cells of the ion-exchange membrane type.
It is also an object of this invention to provide a novel
method for the preparation of membranes and the use of such membranes
in electrochemical cells. Further objects will become apparent to one
skilled in the art from the following detailed specjfication and the
appended claims.
1052858
THE INVENTION
This invention relates to novel polymeric membranes, the
method of making the membranes, and their use in electrochemical cells
such as a diaphragm type chlor-alkali cell and an ion-exchange type
fuel cell. Radiation techniques are used for the polymerization of
perfluoro-olefinic monomers, and for the grafting of such monomers
onto base films for use in electrochemical cells.
The electrochemical cells in which the membrane described
herein is to be used, are similar to previously used cells, subject to
the modifications described herein. Generally, an enclosure is provided
which is d;vided into a catholyte and anolyte compartment by the membrane
material. In a chlor-alkali diaphragm cell, the catholyte compartment
contains an appropriate cathode, generally a metallic material, such as
iron. The anolyte compartment contains a conductive electrolytically-
active anode, such as graphite or more desirably, a metallic anodehaving a valve metal substrate, such as a titanium, bearing a coating
which contains a precious metal, precious metal oxide or other electro-
catalytically active corrosion-resistant material or mixtures thereof.
The anolyte compartment is provided with an outlet for generated chlorine
gas, an ;nlet for brine solution and an outlet for depleted brine. Sim-
ilarly, the catholyte compartment will have outlets for liquid (caustic)
and gaseous (hydrogen) products and generally, an inlet through which
water and/or sodium hydroxide solution may be added initially. Multi-
compartment cells may also be used, which have one or more buffer com-
partments, porous sheets and cationic membranes.
In operation, a direct current is passed between the elec-
trodes causing the generation of chlorine at the anode and the selective
transport of hydrated sodium ions across the membrane into the catholyte
~OS;~858
compartment wherein they combine with hydroxide ions formed at the
cathode by the electrolysis of water.
The improved membrane of this invention is manufactured by
the process of radiation grafting a fluorinated vinyl compound, such as
~ trifluorostyrene (TFS), onto an inert polymeric film, and
then sulfonating the membrane. The preparation of membranes by radia-
tion grafting techniques has the advantage of permitting the use of a
wide variety of film bases which may be tailored for the desired end-
use, and the flexibility of grafting to the base a predetermined quantity
of functional groups. Moreover the membranes made by this technique do
not require the use of fillers and plasticizers which are used when
fabricating membranes from conventionally made polymers, such as poly
~ -trifluorostyrene. The membranes made by this invention are
therefore homogeneous, integral films which are directly usable in
electrochemical cells.
The choice of base film material to which the fluorinated
vinyl compounds of this invention can be grafted should be chosen from
polymeric film materials which are resistant to oxidative degradation.
For example, grafting can be carried out on films of polymers of
ethylene, propylene, tetrafluoroethylene (TFE), trifluorochloroethylene,
and other halogenated olefinically unsaturated monomers preferably
having 2-3 carbon atoms and films of copolymers of these monomers such
as copolymers of tetrafluoroethylene-hexafluoropropylene (FEP) and TFE-
ethylene. It has been found that it is preferable to utilize fluoro-
carbon base films rather than hydrocarbon base films. Such films aresomewhat more inert, particularly under long-term and extreme conditions.
In many instances, the desired end use of the membrane will dictate the
choice of base film. For example, when the membrane is to be used in a
1~:)5Z8~8
chlor-alkali cell, the fluorocarbon base films should be used, as the
other films will be degraded by the chlorine which is produced during
operation of the cell.
At the present, FEP film is convenient for commercial use;
it is readily available as a staple article in commerce in 48 inch width
rolls, while TFE is not so available. A laminate of FEP films, TFE
films, or a combination may be used. A three layer laminate having a
layer of TFE cloth sandwiched between two layers of FEP is an example
of a useful membrane material. It may also be desirable to utilize a
supported film. For example, the film material may be supported and
laminated to a polymeric cloth, woven mesh, perforated sheet or foram-
inous support member to provide greater mechanical strength. The follow-
ing description of the invention will be illustrated using FEP film as
the base material.
The thickness of the base film for grafting purposes is not
critical. However, generally a greater percentage of graft is obtained
when using a thinner film under similar conditions. As with the choice
of film material, the choice of film thickness will depend on the end
use of the membrane. The electrical resistance of the finished membrane
is generally lower when starting with a thinner film, due to the greater
percentage of grafting when utilizing the thinner films. If similar
graft percentages are obtained for thick and thin films, then the elec-
trical resistances of each will be approximately equal. FEP films
ranging in thickness from 1/2 to 20 mils, preferably about 2 mils to
about 5 mils, provide satisfactory base films for use in chlor-alkali
cells. There is substantially no hydroxyl migration from the catholyte
to the anolyte compartments when using a S mil film. Generally, films
having a thickness of up to 10 mils are useful in fuel cells where
greater mechanical strength or resistance to back-pressure is required.
~OSZ858
The monomeric mater;a1s to be grafted onto the base film
generally comprise one or more fluorinated vinyl compounds such as
perfluoroalkenyl aromatic monomers. These monomers may be depicted
by the general f~rmula:
¢~(CF2)X - CF CF2
wherein x is an integer from zero to eight and the expression (CF2)X
includes linear and branched-chain perfluoroalkyl groups. These com-
pounds which have had all of the non-aromatic carbon-hydrogen bonds
replaced by carbon-fluorine bonds. It has been found that under ex-
10 treme oxidative conditions such as are found in chlor-alkali and fuel
cells, primary degradation of the material occurs at the C-H bonds.
Membranes made from these fluorinated compounds will have a much
greater lifetime than those containing C-H bonds. As C-H bonds on an
aromatic ring are resistant to oxidation, the presence of non-fluorinated
15 aromatic moities in the monomer are not detrimental. The foregoing
monomers, or mixtures thereof, may be directly grafted onto the base
film and then sulfonated, or the monomers may first be sulfonated and
then grafted onto the base film. The aromatic groups provide ready
sites for sulfonation by procedures described below. Alternatively
20 sulfonated fluoroalkenyl monomers may be used. These monomers are
preferably prepared in the sulfonated form and then grafted onto the
base film. For example, one or more monomers of the general formula:
F(CF2)X CF = CF S02F
wherein (CF2~X is as defined above, may be grafted onto a base film and
25 then hydrolyzed to a suitable form (i.e. an acid, alkali metal salt,
ammonium or amine form), see U.S. Patents 3,041,317, 3,560,568 and
3,624,053
~OSZ858
The foregoing perfluorinated monomers may also be co-grafted
with other monomeric materials such as those monomers known to be
susceptible to radiation grafting. For example, ~ -trifluoro-
styrene can be co-grafted with ~ -methylstyrene onto a base film and
then sulfonated to form a useful membrane.
A preferred starting material is ~ -trifluoro-
styrene (TFS). This material may be made in accordance with several
well known methods, such as those described in U.S. Patent Nos.
3,489,807, 3,449,449 and 2,651,627. This invention will be further
illustrated utilizing this compound as the monomer.
It should be noted that pure TFS is not required for this
process. As the grafting will generally be done from solution due to
economic considerations, commercial TFS containing other materials may
be used. It is necessary, however, that the diluent be inert and a
non-solvent for the base film. This means that the diluent must not
either homopolymerize, nor copolymerize with the TFS under the radiation
conditions of this process, nor adversely affect the radiation grafting.
The general procedure for the graft polymerization of TFS
onto the base film, used in each of the following embodiments, is as
follows. The film base, covered with an interlayer of paper and wound
into a roll, is placed in a chamber. A solution of trifluorostyrene
in a suitable solvent is then added to completely i~merse the film base
roll. A vacuum and nitrogen flush is applied and re-applied several
times to remove any dissolved oxygen from the solution and chamber. The
chamber, under a nitrogen atmosphere, is sealed. The temperature is
kept sufficiently low during this procedure to prevent vaporization of
the solvent. The film and trifluorostyrene solution, in the sealed
chamber, are irradiated i.e. for about two weeks at a suitable dose
_ g
lOSZ858
rate, as discussed below. The grafted film is then removed from the
chamber and washed and dried.
The grafting may be performed either using TFS in bulk or
in solution. Generally, the solvent must have similar properties as
described for the TFS diluent discussed above. Table I illustrates
the grafting of TFS onto FEP film at different concentrations of TFS.
The percentage of graft is determined by the difference between the
weight of the final grafted film less the weight of the original base
film, divided by the weight of the original base film.
TABLE I
GRAFTING OF TRIFLUOROSTYRENE AT DIFFERENT CONCENTRATIONS
TFS Conc. Benzene Dose RateTotal Dose Graft
10% 90% 7236 r/hr2.59 Mrad 1.4%
30% 70% 7236 r/hr2.59 Mrad 8.2%
50% 50% 7236 r/hr2.59 Mrad 14.3%
70% 30% 7236 r/hr2.59 Mrad 24.0%
As can be seen from Table I the amount of graft increases with
the concentration of TFS. The amount of graft for membranes useful in
chlor-alkali cells may vary widely, such as from 3 to 40%, and preferably
from about 10 to 30X of TFS graft. Hence, the TFS concentration in the
grafting procedure should be varied accordingly. Membranes designed
for use in fuel cells should have a lower resistance than those used in
chlor-alkali cells. This may be achieved by utilizing a greater per-
centage graft. Preferably such membranes will have about 20% to 50% of
TFS graft. This may be achieved with a higher TFS concentration or by
varying the dose rates.
The solvents for the grafting procedure may be aromatic com-
pounds or halogenated compounds. Preferred solvents for the TFS are
-- 10 --
105Z858
benzene and methylene chloride. Both solvents are inert and inexpen-
sive. Methylene chloride has the further advantage of being non-
flammable. The results of grafting in this solvent are shown in Table
II .
TABLE II
GRAFTING OF TRIFLUOROSTYRENE IN METHYLENE CHLORIDE
TFS Conc. MeC12 Dose Rate Total Dose% Graft
10% 90% 7163 r/hr 2.49 Mrad2.5
30% 70% 7163 r/hr 2.49 Mrad9.7
50% 50% 7163 r/hr 2.49 Mrad17.6
70% 30% 7163 r/hr 2.49 Mrad27.1
A comparison of the results shown in Tables I and II indicates
that the use of methylene chloride as a solvent leads to a greater degree
of grafting than the use of benzene. Additives may be employed to in-
crease the rate of grafting, such as the use of a minor amount of
methanol with benzene.
As shown in Tables III and IV, the grafting of TFS onto the
base film is dose and temperature dependent.
TABLE III
GRAFTING OF TRIFLUOROSTYRENE TO FEP FILM AT ROOM
TEMPERATURE AND 60% TRIFLUOROSTYRENE IN BENZENE SOLUTION
Dose Rate Radiation Time Total Dose% Graft
12,606 r/hr 51.5 hrs. 0.649 Mrad3.3%
12,606 r/hr 121.8 hrs. 1.535 Mrad7.2%
12,606 r/hr 233.0 hrs. 2.937 Mrad15.5%
12,606 r/hr 344 hrs. 4.33 Mrad26.1%
12,606 r/hr 402 hrs. 5.06 Mrad 29.9%
~05Z8S8
TABLE IV
GRAFTING OF TRIFLUOROSTYRENE AT DIFFERENT TEMPERATURES
Temperature TFS Conc.
of Grafting In Benzene Dose Rate Total Dose% Graft
.
-78C 25% 7236 r/hr 3.11 Mrad 0.7%
23C 25% 7236 r/hr 2.96 Mrad 8.3%
Generally the dose rate and temperatures are not critical. It is
possible to utilize dose rates in the range of about 5,000 r/hr up to
about 300,000 r/hr. Preferably the dose rate is kept below about
100,000 r/hr. Too great a total dose may cause degradation of the base
film. Generally a total dose within the range of about .1 to 10 Mrad
is useful, while a range of about .7 to 5 Mrad is preferable. Higher
dose rates also y;eld lower percentage ~rafts for the same total dose.
The temperature may also be widely varied within the range of -78C to
60C; room temperature is preferred for convenience of operations.
As noted above TFS can be grafted onto many base film materials,
in addition to the FEP base film of the previous examples. Results
obtained with several base films are shown in Table V.
TABLE V
GRAFTING OF TRIFLUOROSTYRENE IN OTHER BASE FILMS
Polymeric TFS Conc.
F;lm In Ben~ene Dose Rate Total Dose% Graft
Teflon~D 25% 7236 r/hr 2.24 Mrad 6.8%
(tetrafluoro-
ethylene)
High Density PE 25% 7236 r/hr 2.24 Mrad 5.7%
Polypropylene 25% 7236 r/hr 2.24 Mrad 2.1%
FEP Laminate* 25~ 7236 r/hr 2.24 Mrad 6.0%
Sulfonation of TFS resin has been carried out previously by dis-
solving the resin in suitable solvent prior to sulfonation, see U.S.
Patent 3,341,366. The present method allows sulfonation to be
performed in s u on the grafted membrane with suitable
* 2 mil FEP/2 mil glass/2 mil FEP
~05'~858
sulfonating agents, such as chlorosulfonic acid and fluorosu1fonic
acid.
The grafted membrane is first worked in a suitable solvent
for TFS, such as methanol, benzene or methlene chloride, and then dried
at about 50 C to 60 C for about l/2 to l hour.
A sulfonating bath is prepared by dissolving from 5% to 50%
and preferably about 30% chlorosulfonic acid in a halogenated solvent
such as carbon tetrachloride. The solvent should not be volatile at
the sulfonation temperature and should not be sulfonatable. Generally
the lower concentration chlorosulfonic acid baths are used when a sul-
fonated linear product is desired and the higher concentration baths are
used when a cross-linked product is desired. A higher sulfonation tem-
perature also promotes cross-linking.
The membrane is then introduced into and allowed to dwell in
the bath. The grafted film is left in the chlorosulfonic acid solution
for a suitable period of time at an elevated temperature, i.e. for about
20 to 60 minutes, or longer, at about 130 to 155 C. The amount of
sulfonation may be in the range of about 5% to about 25% as determined
by the difference between the final weight of the sulfonated grafted
film less the weight of the grafted film, divided by the weight of the
grafted film. The film is then removed and boiled in water to convert
the sulfonate groups to the acid form. The film is then added to an
alkali metal hydroxide solution, such as 4-5% solution of potassium
hydroxide to obtain the salt form. It has also been found that a final
treatment in an anionic emulsifier is desirable to lower the resistance
of the film.
The results for various dwell times of the grafted film in the
chlorosulfonic acid bath for a 17% grafted membrane are shown in Table ~I.
1052858
TABLE VI
SULFONATION OF TRIFLUOROSTYRENE GRAFTED 5 MIL FEP FILM
Residence Electrical
Temperature Time % Weight Gain Resistance 40% KOH
130 C10 minutes 8.3% 0.97 ohm-in2
140 C10 minutes 8.5% 0.89 ohm-in2
7.1% 0.56
8.5% 0.54
7.8% O. 87
150 7.5% 1.10
300 5.6% 1.00
155 C10 minutes 8.5% 0.64
7.9% 0.49
7.8% 0.53
150 7.1% 0.80
300 6.9% 1.80
Instead of realizing a continuous weight gain with increasing
time and temperature, it was found that generally sulfonation gradually
5 decreases with time as shown in Table VI. The electrical resistance of
the film, as measured in 40~ KOH, goes through a minimum, so that the
electrical resistance of the membrane (such as from .2 to 2 ohms per
square inch) can be controlled by modification of residence time during
sulfonation. This reduction in weight and change in electrical resis-
10 tance may be due to the formation of sulfones.
The hydroxyl ion diffusion across the membrane is another impor-
tant property of the membrane, since the membrane should retard the dif-
fusion of hydroxyl ions across the membrane to the anode. In the follow-
ing tests the diffusion of hydroxyl ions is measured in a dialyser cell in
15 which one side is filled with 2.5 M NaOH and the other side with distilled
water. The concentration of OH- ion in the distilled water side is plotted
-- 14 --
~05'~858
against time. The flux of OH is calculated from the slope of the plot
as:
J = C V
where J is the OH flux in moles/in2-min
Ct is the slope of the pH time plot
V is the volume of half cell in liters
A is the membrane area exposed in in2.
J can be converted to units of gram/in -hr by multiplying by a factor
of 2.4 x 10 . The diffusion of OH ions across various membranes is
given in Table VII. The data are presented for TFS grafted membranes
made in accordance with the present invention and conventionally grafted
styrene membranesi the base film used was 2 mil FEP.
TABLE VII
HYDROXYL ION DIFFUSION IN GRAFTED MEMBRANES
Resista~ce OH Flux
15 Sample % Graft % Sulfonation (ohm-in ) (Mole/in2-min)
TFS 26.0 8.0 0.500 1.6 x 10 5
TFS 32.9 8.8 0.446 1.8 x 10 5
TFS 17.4 7.5 5.30 2.1 x 10
Styrene 32.0 23.0 0.225 4.7 x 10 5
Styrene 20.0 14.0 1.86 5.4 x 10-6
Styrene 16.0 11.0 2.50 3.2 x 10-6
From a purely back diffusion point of view, the smaller the J values,
the better the membrane. However, the smaller J value follows from a
high electrical resistance. Therefore, a suitable balance between
resistance and flux should be chosen for the membrane in accordance
with the desired end-use of the membrane.
The resistance to oxidation of a membrane is a very important
property since it is related to the life of the cell. The
- 15 -
~05Z858
screening test used to determine relative life involves ferrous ion
catalyzed hydrogen peroxide degradation as described by Hodgdon (R B.
Hodgdon, J. Polymer Science, Part A-l, 171 (1968)). The membrane to be
tested is added to 50 ml of a 3% hydrogen peroxide solution containing
1 ppm of ferrous ion. The mixture is kept at 70 C for 24 hours. The
electrical resistance in 40% KOH is measured before, "Rol', and after,
"Rf", the oxidation test. The weight loss under the conditions of these
tests was also measured. The data in Table VIII is for TFS grafted
membranes made in accordance with the present invention and conven-
0 tionally grafted styrene membranes; the base film used was 2 mil FEP.
TABLE VIII
OXIDATION STABILITIES OF GRAFTED MEMBRANES
Ro Rf 2
SAMPLE % GRAFT % SULFONATION (ohm-in ) (ohm-in ) % WT. LOSS
TFS 25.6 8.8 0.23 3.9 17.4
TFS 25.6 12.0 0.24 2.8 18.3
TFS 32.9 8.8 0.50 1.67 9.9
TFS 32.9 - 8.8 0.44 0.90 ---
TFS 32.9 8.8 0.45 1.28 ---
Styrene 20.2 14.1 0.20 4.0 24.6
Styrene 20.2 14.1 0.23 4.4 ---
As can be seen, the final resistance of the styrene-S03H membranes was
higher than the TFS-S03H membrane. In addition, the weight loss of
styrene-S03H was also much larger under this very severe test. Most of
the TFS-S03H membranes still have a resistance below 3 ohm-in2 and are
considered to be operational but the styrene-S02H membranes would require
too high a Yoltage in operation.
The electrolytic cells containing the ion exchange membranes
of the present invention may be used for effecting the electrochemical
- 16 -
105'~858
decomposition of a number of ionizable compounds. Exemplary of the
various solutions of ionizable compounds which may be electrolyzed
and the products produced are aqueous solutions of alkali metal halides
to produce alkali metal hydroxides and halogens, agueous solutions of
HCl to produce hydrogen and chlorine, aqueous solutions of ammonium
sulfate to produce persulfates; aqueous solutions of borax to produce
perborates; aqueous solutions of alkali metal hydroxides, when using an
oxygen cathode, to produce peroxides; etc. Of these, the most preferred
are aqueous solutions of alkali metal halides, particularly sodium
chloride, and aqueous solutions of HCl. Typically, the solutions of
these ionizable compounds are introduced into the anode compartment of
the cell in which the electrolysis takes place.
In a typical process, utilizing a sodium chloride brine as the
feed to the anode compartment, the feed solution may contain from about
200 to 325 grams per liter sodium chloride. Preferably, the anolyte
liquor has a pH in the range of about 1 to 5, with a pH within the range
of about 2.5 to 4.0 being particularly preferred. This pH of the anolyte
solution may be attained by the addition of hydrochloric acid to the feed
brine solution, typically in amounts of about 2 to 10% by weight. In
general, the cells may be operated over a wide temperature range, e.g.
from room temperature up to the boiling point of the electrolyte al-
though temperatures of from about 65 to 90 C are preferred. Similarly,
a wide variation in the electrical operating conditions are also possible,
with cell voltages of from about 0.5 to 4 amps per square inch being
suitable. The concentration of the sodium hydroxide concentrations of
from about 24 to 33% by weight being preferred. Typically, the sodium
hydroxide recovered from the cathode compartment contains less than about
1~ by weight of sodium chloride while chlorine is produced in the anode
- 17 -
lO5Z858
compartment at a current efficiency in excess of about 95%.
In accordance with the foregoing procedure, six samples of
TFS grafted FEP film were tested in a chlorine cell. All samples were
preboiled for 15-20 minutes before the test. The cell used was a stan-
dard two compartment glass minicell operated between 85-95 C. The
anolyte was circulated with acidified brine at a pH of 2Ø The catho-
lyte was 100 gm/liter sodium hydroxide.
The resistance of each membrane was determined using an AC
bridge. The initial voltage for the cell was determined at 2 ASI and
100 gm/liter caustic strength 85-95 C. The cell was operated and the
cell voltaged determined at various times, and corrected to the initial
conditions of 100 gm/liter sodium hydroxide and the rate of voltage in-
crease calculated. The results are summarized in Table IX.
- 18 -
~052858
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lOS;~858
The "Operation Time" in the table for samples 1-5 was the
lifetime during which the cell voltage was less than 4.5 volts.
A cell voltage of 4.5 volts at 2 ASI does not necessar;ly mean
the membrane had to be removed from the test. Depending on power costs,
higher cell voltages might be economical. Also lower cell voltage could
be obtained by decreasing the current density. The selection of 2 ASI
as an operating current density was used as a method of screening. At
lower current densities the voltage-time rise would be lower and corres-
pondingly longer time would be projected for the membrane to reach a
given cell voltage.
The "Rate Of Increase" in cell voltage was determined from the
linear portion of the variation in cell voltage which occurred after an
initially higher rate of increase and prior to a sharp rise prior to
termination of the test. For example, for samples 3 and 4, the respec-
tive initial rates of increase were 0.60 mv/hr and 1.70 mv/hr each after
about 135 hours. Figure 1 is a plot of the voltage characteristics as
a function of operating time for sample 4.
The use of the membrane of this invention in fuel cells maybe illustrated as follows. Place the membrane in abutting relationship
between two fuel cell electrodes in a conventional fuel cell configura-
tion. The faces of the electrodes away from the membrane respectively
form the sides of compartments for reactants and products of the fuel
cell process, i.e. see U. S. Patent 3,341,366. The electrodes may be
made of platinum black and a binding agent, by techniques well known
in the art. The electrodes are bonded to the membrane by the appli-
cation of an elevated temperature and pressure, i.e. 250 F and 450 psi
. ,
lOS;~85B
Hydrogen gas is supplied at the anode and oxygen gas at the cathode
and current is drawn from the cell by leads affixed to the electrodes
The membranes of this invention can also be used to prepare
anionic exchange membranes. The procedures for this are analogous to
those used for conventional styrene based anionic membranes as reported
in the literature. This can be achieved, for example, with a TFS
grafted membrane, by the standard chemical sequence of halomethylation,
amination and quaternization, or by a nitration and reduction route.
In addition to the radiation technique for grafting the per-
fluoro monomers described above, it has been found that these monomersmay be polymerized by radiation either in bulk or in solution. The
technique of radiation polymerization has the advantage over conventional
catalyzed polymerization (with peroxides or persulfates) in producing
a purer product, i.e. free of catalyst residues both physically admixed
and chemically combined. Generally the conditions set forth above for
grafting may be followed for polymerization and also for sulfonation.
Using a Cobalt-60 source at room temperature, the following results
were obtained.
TABLE X
RADIATION POLYMERIZATION OF TRIFLUOROSTYRENE
TFS Conc. Benzene Conc. Dose Rate Total Dose Polymer Yield
5050% 7236 r/hr 1.98 Mrad 0.6%
1000~ 7236 r/hr 1.98 Mrad 1.4%
This invention has been described in terms of specific embodi-
ments set forth in detail. Alternative embodiments will be apparent to
those skilled in the art in view of this disclosure and accordingly such
2~
lOSZ858
modifications are to be contemplated within the spirit of the invention
as disclosed and claimed herein.
22
