Note: Descriptions are shown in the official language in which they were submitted.
1203S09
The present invention resides in a fluorinated composite
membrane containing ion exchange groups and its use as an ion exchange
membrane in an electrolytic cell.
This application is divided from applicant's copending
Canadian Application Serial No. 379,454 filed on June lO, 1981 which
is directed to a polymeric composite film of the type having two
layers which differ in equivalent weight, each of said layers having
a substantially completely fluorinated polymeric backbone with a
plurality of pendant groups attached thereto, at least a portion of
said pendant groups being a chain of carbon atoms which may be inter-
rupted with one or more oxygen atoms and which terminates with an ion
exchange group; wherein the improvement comprises the two layers
differing in equivalent weight from each other by less than 250 and
the carbon chain which connects the ion exchange group to the polymeric
backbone having from 1 to 3 carbon atoms.
The electrolytic production of chlorine and caustic by the
electrolysis of brine has been well known for many years. Historically,
diaphragm cells using a hydraulically-permeable asbestos diaphragm,
vacuum-deposited onto foraminous steel cathodes, have been widely
commericalized. Such diaphragm cellsJ employing permeable diaphragms,
produce NaCl-containing NaO~I catholytes because NaCl passes through
the diaphragm ~rom the anolyte to the catholyte. Such NaCl-containing
caustic is generally of low caustic concentration and requires a
de-salting process and extensive evaporation of water to obtain a low-
salt, high concentration caustic for industrial purposes.
In recent years, the chlor-alkali industry has focused much
of its attention on developing membrane cells to produce low-salt or salt-
free, high concentration caustic in order to improve quality and avoid
'A 28,985-F
12Q3509
;~
the costly de-salting and evaporation processes.
Membranes have been developed for that purpose which
are substantially hydraulically-impermeable, but which
will permit hydrated Na+ ions to be transported from
the anolyte portion to the catholyte portions, while
substantially preventing transport of Cl ions. Such
cells are operated by flowing a brine solution into the
anolyte portion and by providing salt-free water to the
catholyte portion to serve as the caustic medium. The
anodic reactions and cathodic reactions are not affected
by the use of a membrane cell as opposed to the use of
a diaphragm cell.
In addition to the caustic strength being
important, two other criteria of the operating cell
must also be considered for a complete energy view of
the overall process. One is current efficiency, which
is the ability of the membrane to prevent migration of
the caustic produced at the cathode into the anode
compartment; and the second is the voltage at which the
cell operates, which is partly determined by the elec-
trical resistance of the membrane. Power efficiency is
often used as one term that considers both the current
efficiency and cell voltage. It is defined as the
product of the theoretical voltage, divided by the
actual voltage, multiplied by the actual amount of
caustic produced divided by the theoretical amount of
caustic that could have been produced at a ~iven current.
Thus, it is apparent that power efficiency is reduced
by higher cell voltage or by lower current efficiency.
The membrane has a direct effect on both. The most
common method of comparing cells is to express the
operation as kilowatt hours (KWH) of power consumed per
metric ton (mt) of product produced. This expression
28,985-F -2-
; :
12Q3509 ~''
also considers both voltage ~higher voltage increase
the quantity of KWH consumed), and current efficiency
(lower efficiency decreases the quantity of product
produced). Thus, the lower the value KWH/mt, the
better the performance of the cell. It is apparent that
optimization of a membrane for use in electrolytic
chlor-alkali cells is a trade off between cell volt~ge
which is reflected in membrane electrical resistance,
current efficiency and caustic concentration.
It is well known (G.E. Munn, Nafion~ Men~ranes
- Factors Controlling Performance in the Electrolysic
of Salt Solutions, The Electrochemical Society Meeting,
October, 1977, Atlanta, Georgia) that the current
efficiency of a chlor-alkali cell containing a membrane
is determined primarily by the surface of the membrane
contacting the catholyte. The current efficiency is
dependent on the equivalent weight of the membrane in
contact with the cathol~te and the voltage is dependent
on both the thickeness of the membrane and the equivalent
weight of the membrane. The equivalent weight is the
measure of the concentration of ion exchange functional
groups in the polymer membrane and is simply the weight
of the polymer in the acid form required to neutralize
one equivalent of base. The above publication discloses
that iower equivalent weights (eq. wts.) have lower
electrical resistance (and thus lead to lower cell
voltage), but that higher eq. wts. are required to
obtain sufficient negative ion rejection and thus
acceptable current efficiency. It is well known and
discussed in the publication that voltage drop across
the membrane is directly dependent on thickness; a thin
film being desirable for minimum voltage drop. It thus
follows that ideal membranes would be very thin films
28,985-F -3-
lZ03509
--4--
having higher eq. wts. (1500-2000 for sulfonic acids
membranes of the prior art).
U.S. Patent 3,909,378 teaches a method to
S take advantage of the increased current efficiency
associated with high eq. wts. without absorbing the
full voltage penalty associate-d with these materials.
This patent teaches a composite membrane formed by
laminating a thin, high eq. wt. film to a thicker,
lower eq. wt. film. The thin, higher eq. wt. side of
the film faces the catho~yte in the cell thus resulting
in current efficiency associated with the higher eq.
wt. and voltage associated with the thin layer plus the
minimal voltage of the lower eq. wt. layer. The patent
further teaches that the eq. wts. of the polymers fall
within the range of 1000-2000 or even greater and that
the eq. wt. difference between the low and high eq. wt.
portions of the composite film should be at least 250
and preferably 400. The patent teaches polymers having
sulfonyl type ion exchange groups and that the structure
linking these groups to the main polymer chain are not
critical. The sulfonyl ion exchange groups, according
to the patent may be the sulfonamide form or in the
sulfonic acid form.
U.S. Patents 3,784,399 and 4,085,071 teach
formation of a barrier layer, facing the catholyte, on
a single polymer film by reacting ammonia or N-substituted
amines with one face of a sulfonyl functional polymer
to form sulfonamide ion exchange sites. The main
distinguishing feature of these patents is that the
barrier layer facing the catholyte is introduced by
chemical modification on a single eq. wt. film rather
than by lamination of a barrier film to a support film.
28,985-F -4-
. ~ . ,
` ~ 1203509 ` ~^-
u.s. Patent 4,151,053 also teaches having
barrier layers on the catholyte face of membranes to
achieve enhanced current efficiency without substantial
voltage penalties. The main distinguishing feature of
this patent is that the barrier layer has carboxylic
acid ion exchange groups of the general structure
~OCF2COOM where M is hydrogeni ammonium; quaternary
ammonium, particularly quaternary ammonium having a
molecular weight of 500 or less; and metallic atoms,
particularly alkali or alkaline earth metals. The
patent teaches that each film of the composite membrane
should have eq. wts. in the range of 1000 to 2000 and
that the first film, the high eq. wt. film, should have
an eq. wt. at least 150 higher than the second film.
All of the aforementioned patents use as
starting materials sulfonyl containing fluoropolymers
wherein the sulfonyl is generally contained on a pendant
chain. The useful polymers and monomer precursors for
these type materials are described in U.S.Patent 3,282,875.
fn each patent the preferred sulfonyl containing fluoro- .
polymer is described as derived, by polymerization,
from the monomer
FSO2CF2CF20CFCF2OCF = CF2
CF3
disclosed in U.S. Patent No. 3,282,875. The polymers
are generally copolymers of the above monomer and
tetrafluoroethylene. These copolymers are sold under
the tradename of Nafion~ by E. I. duPont Company and
are well known and have been widely evaluated as membranes
in chlor-alkali cells where the properties of these
copolymers, such as useful eq. wt. ranges, water absorp-
tion and the like, have become accepted as the proper~ies
28,985-F -5-
1203509
of sulfonic acid containing fluorocarbon polymers. In general,
useful eq. wts. for these copolymers when used as membranes
in chlor-alkali cells is not below about 1000 to llO0. Below
these values water absorption increases dramatically and physical
integrity falls sharply. For eq. wts. above about 1800-2000,
electrical resistance becomes so great as to render the copolymers
impractical in chlor-alkali cell use. Preferred eq. wt. ranges
are from about 1100 to about 1500.
United States Patent 4,065,366 teaches the use
of single layer carboxylic acid membranes in chlor-alkali cells.
This patent teaches useful equivalent weight ranges that VarJ
from about 500 to about 2000; the lower range being significantly
lower than that claimed for sulfonic acid membranes. The usefulness
of these membranes in chlor-alkali cells is taught as being
associated with the concentration of the functional groups in
the membrane ~eq. wt.), water absorption of the membrane and
glass transition temperature of the polymer. The most preferable
range for the concentration of the carboxylic acid group in
the polymer is given as 1.1 to 1.7 meq./g of dry polymer (about
600 to about 900 eq. wt.). Excellent current efficiencies are
obtained with these relatively low eq. wt. carboxylic acid polymers
at high caustic concentrations (30-40%), but the voltages reported
in the examples are relatively high for the thicknesses reported
(200 microns) and the current density of the cells (20A/dm2).
According to the present invention there is now
provided in a polymeric composite film of the type having two
28,985-F
12Q3S09
--7--
layers which differ from each other in equivalent weights by
at least 250, each of said layers having a substantially
completely fluorinated polymeric backbone with a plurality of
pendant groups attached thereto, at least a portion of said
pendant groups being a chain of carbon atoms which may be
interrupted with one or more oxygen atoms and which terminates
with a sulfonyl ion exchange group; wherein the improvement
comprises at least one layer having an equivalent weight of
less than 1000 and the chain of carbon atoms having from
1 to 3 carbon atoms.
The ion exchange membranes of the invention are
made by combining at least wo different films of substantially
fluorinated polymers containing ion exchange functional groups.
The present invention, together with those of the
aore-mentioned parent Canadian Application Serial Numlier
379,4S4 filed June 10, 1981 by The Dow Chemical Company and
its divisional Canadian Application Serial Number 443,988
filed December 21, 1983 by The Dow Chemical Company will now
be further described.
Several criteria, aside from the criteria of cell
performance, must be considered as to whether polymers
qualify as membranes in electrolytic cells. When the polymers
are used as films, which are conveniently made by melt
extrusion, or the like, on a commercial scale, the physical
and chemical properties of the film must withstand the en-
vironment of the cell. This severely
~'~? 28,985-F (Div. C) -7-
~ lZQ3509
-- 8 --
restricts the materials usef~l in the harsh environment
of a chlor-alkali cell. The cell is divided by the
membrane into two compartments, an anolyte compartment,
wherein chlorine gas is produced and evolved from an
anode; and a catholyte compartment wherein caustic is
produced at a cathode. These cells normally operate at
temperatures-of from about 70C up to temperatures of
about 100C and are expected to continuously operate at
these conditions for many months and even years. This
chemical environment of strong, hot caustic on one side
and a highly oxidative environment on the other virtually
eliminates the use of most organic polymers as membranes.
The constant churning of gas being evolved through the
liquid electrolyte solutions in the cell severely
limits the physical properties that a film must have in
order to meet the lifetime requirements of the cell.
It is known to physically support polymer films on such
materials as polytetrafluoroethylene scrim to aid in
meeting the life requirements, but even then, the film
must be physically sound to a large degree. Any holes
or tears that develop in the film lead to contamination
of the caustic product in the catholyte with salt from
the anolyte and even worse, can lead to explosive
mixtures of hydrogen in chlorine when cathodes are used
that produce hydrogen along with attendant production
of chlorine on the anode.
It is known in the art that fluoropolymers,
in general, meet the chemical requirements of the
chlor-alkali cell. These fluoropolymers can be sub-
stituted with other halogen atoms such as chlorine orbromine that are not reactive in the cell environment,
but, although contrary to some teachings, these poly-
mers should not contain hydrogen atoms on carbons that
28,985-F
1203509 ~-
make up the main polymer backbone. Carbon-hydrogen
bonds are chemically attacked by both oxidation from
the anolyte components and caustic in the catholyte.
Chemical attack on the polymer backbone can lead to
reduced molecular weight by carbon-carbon bond cleavage
and thus lead to severe damage to the physical properties
of the membrane.
Physical properties of a polymer are de-
lo pendent on polymer structure. A highly crystalline
fluoropolymer made from simple, unsubstituted monomers
.. ... ~ .
such as tetrafluoroethylene is tough, but has extremely
high melting or softening temperatures. Fabr.cation is
difficult or nearly impossible by simple techniques
such as melt extrusion. Homopolymers of long chain,
terminal fluorocarbon olefins which result in polymers
having many pendant qroups are difficult to prepare
because they have a relatively unreactive olefin site
and when formed are often low molecular weight, waxy,
amorphous solids having little, if any, plastic quality.
Materials of this nature are useless as membranes.
Copolymers of the two type monomers described above
often have properties, better than the homopolymers.
Copolymers of tetrafluoroethylene and perfluoroalkyl
2S vinyl ethers (US Patent 3,896,179) have excellent
physical properties and can be conveniently melt fabricated
into films. Thus, polymers with a limited number of
pendant groups can maintain most of the favorable
physical charactexistics of the parent (no long pendant
groups) polymer and also lend themselves to simple
fabrication. The physical strength of a polymer depends
on both the number of pendant groups and the size or
number of atoms and arrangement of atoms (generally
carbon and oxygen in the chain) that make up the pendant
28,985-F
~ 1203S09 ---
- 10 -
group. Thus, the commercial, composite membranes of
the prior art are based on sulfonyl containing copolymers
of tetrafluoroethylene and
FSO2CF2CF2OCFCF2OCF = CF2
CF3
The membranes are made by laminating a thin layer of
1500 eq. wt. polymer onto a thicker layer of 1100 eq.
lo wt. polymer which lends mechanical strength while
adding little electrical resistance (see G. E. ~ull).
Decreasing the equivalent weight of the thicker support
layer would result in somewhat lower electrical resis-
tance, but, because of the added number of pendant
groups, would decrease the structural support needed
for the thin, higher eq. wt. layer. Sulfonyl containing
polymers having shorter pendant groups than those of
the prior art have excellent physical properties and
cell performance characteristics at eq. wts. considera~ly
lower than those of the prior art.
The eq. wt and the hydration per functional
group of a polymer used as a membrane in a chlor-alkali
cell have a direct influence on both of the quantities,
2S voltage and current efficiency, that determine the
overall efficiency at which a cell operates. The water
of hydration per functional group, in effect, determines
the nature and the size of the paths through which ions
must travel to pass through the membrane. Excessive
hydration allows more ions to penetrate into the membrane.
Penetration of the membrane by hydroxide ion leads to
loss in current efficiency. Excessive hydration leads
to transport of hydroxide from catholyte to anolyte and
thus a loss in current efficiency. Equivalent weight
28,985-F
Q3509 ` ~`
determines the number of sites available to transport
the sodium ions from the anolyte to the catholyte. At
a given applied current to the cell, a specific number
of ions must be transported for cell operation. Lower
eq. wt. means a larger number of sites for transport
and thus a lower electrical potential is re~uired to
drive the ions.
Sulfonic acid membranes of the prior art
which have long pendant chains separating,the polymer
backbone from the functional group, hydrate to such a
.. . . ..
large degree that equivalent weights of as low as 1100
to 1200 are not practically useable as barrier layers
in chlor-alkali cells. Sulfonic acid polymers having
shorter pendant groups hydrate less per functional
group at given eg. wt. than do the polymers of the
prior art. Exemplary, composite sulfonic acid membranes
in the present invention are copolymers of tetrafluoro-
ethylene and the monomer FSO2CF2CF20CF = CF2 as well as
terpolymers of the above two monomers and of the general
structure ROCF = CF2 where R is a straight or branched
substantially fluorinated alkyl chain which may be
interruptea by oxygen atoms. Polymers formed from
combinations of the above monomers hydrate less at a
given equivalent weight and perform superior to the
sulfonic acid polymers of the prior art in chlor-alkali
cells. Thus a 1240 equivalent weight, short pendant
chain polymer of the invention operates at equal or
better current efficiency than a 1500 equivalent weight
polymer of the prior art and has lower ele,ctrical
resistance per unit thickness. A laminate of the above
1240 equivalent weight polymer onto a 1100 equivalent
weight polymer of the prior art surprisingly operates
in a chlor-alkali cell superior to a laminate of a 1500
28,985-F ,
1203509
- 12 -
eq. wt. polymer of the prior art onto the same 1100 equivalent weight film
even though the equivalent weight difference is only 140 as opposed to the
minimum difference of 250 and the preferred difference of 400 taught in
United States Patent 3,909,378.
In another example of the present invention, a composite
membrane formed by laminating a film of the same 1240 equivalent weight
material as above onto an 860 eq. wt. copolymer of tetrafluoroethylene
and FSO2CF2CF2qCF=CF2~and then hydrolyzing to obtain the sulfonic acid
salt was shown to be superior to the composite 1500 eq. wt. onto 1100
eq. wt. membrane of the prior art. The material had excellent physical
strength and gave equal or better current efficiency and better cell vol-
tage on a unit thickness basis than the composite membrane of the prior
art. This was surprising since United States Patent 3,gO9,378 teaches
that the low eq. wt. layer should have an eq. wt. of at least 1000.
The main feature of this composite, sulfonic acid membrane is the fact
that one layer of the membrane has an equivalent weight of less than
1000 .
The composite sulfonic acid membranes of the present inven-
tion have (1) a barrier layer, the layer facing the catholyte, that has
a lower water of hydration per functional group than the second layer,
(2) should not have an eq. wt. exceeding about 1300, (3) the eq. wt.
difference between the two layers cannot be less than about 250 and the
eq. wt. of the second layer can be less than 1000 but preferably not
less than about 750. A preferred embodiment is where the second layer
has an eq. wt. of not more than 1300 and does not exceed one-third of
28,985-F
1203S09
-13-
the total thickness of the composite membrane. A more preferred
embodiment is where the minimum possible equivalent weight is
used for both layers while still preserving sufficient
mechanical properties and cell performance. In this embodiment,
the second layer has an equivalent weight in the range of 800
to about 1000 and the first layer, the barrier layer, has an
equivalent weight of from about 1100 to about 1300. It is
entirely within the scope of the present invention to add
mechanical support to the membrane by introducing a third
material in the form of a ibrous mat or a woven fabric or
scrim. When support is added it is preferred that the support
material be incorporated in the second film or layer of the
composite membrane.
From the standpoint of manufacture, it is parti-
cularly convenient to make composite membranes as opposed to
single film membranes wherein one face of the membrane is
chemically modified to produce a barrier stratum such as in
U.S. Patent Nos. 3,784,399, 4,085,071 and 4,151,053. Chemical
reactions on polymers are difficult especially when careful
control of the depth and extent of reaction is necessary on a
polymer film. In addition to the normal kinetic characteristics
of the particular reaction involved, diffusion rates of the
reactants into the polymer structure must also be considered
and in many cases is the controlling factor. Production
of reproducible membranes by this technique requires careful
control and is subject to errors that can result in irre-
t~ievable loss of expensive polymer ma~erials. Production
. of films from polymers that already have the desired functional
groups can be done by standard and well known methods such as
melt extrusion. Composite membranes can be made by either
forming two films and laminating these together or can be
formed by co-extrusion of the two layers.
28,985-F (Div. C) -13-
12(}3S09
-14-
Included in the scope of the present invention
is combining two films, one of which has had one surface
chemically converted from sulfonyl to carboxylic acid
or derivative. The side opposite the carboxylic acid
fun~tion, which still contains sulfonyl function, is
laminated to the second film containing sulfonyl func-
tionality. Also included in the scope is combining two
sulfonyl functional films and then chemically converting
all or part of the sulfonyl functional groups in the
first film to carboxylic acid functional groups. The ;
carbo~ylic acid surface of the composite faces the
catholyte in the operating cell. In these embodiments
the equivalent weight of the first film is less than,
equal to, or no more than 150 higher than the equivalent
weight of the second film. While these techniques do
have the disadvantage of reguiring careful control to
accomplish the chemical conversion reproducibly, the
~ first does not suffer the full disadvantage since only
a limited amount of material, the material for the thin
first layer, is subject to loss. These techniques can
be advantageous when polymers containing the two different,
sulfonic acid and carboxylic acid, unctional groups
are not readily available. Otherwise, the technique of
combininq the two, separate films (the carboxylic acid
functional polymer and the sulfonic acid functional
polymer films) to form the composite membrane is the
preferable method.
In the composite membranes of the present
invention, the b?rrier.laYer or stratum preferably has
a lower water of hydration per functional group than
does the second layer. Water of hydration per functional
group is determined by boiling a dry polymer film in
water or thirty minutes and measuring, by weighing,
28,985-F -14-
"
~'
lZ03509
the "Standard Water Absorption" and from this value
calculating the moles of water absorbed per equivalent
weight of polymer (W. G. F. Grot, et al, Perfluorinated
Ion Exchange Membranes, 141st National Meeting, The
Electrochemical Society, Houston, Texas, May, 1972).
In each embodiment of the membranes of the present
invention, the maximum limit in eguivalent weight for
the barrier layer is lower than the maximum limits set
out in the prior art. Only when the eq. wt. of at
least one o the layers has a value less than 1000 can
the eq. wt. difference exceed 150
ExamPle 1
A terpolymer film having an eguivalent weight
of 1240 and a thickness of 8 mil was prepared by poly-
merizing tetrafluoroethylene, FSO2CF2CF20CF=CF2 and
ClCF2CF2CF=CF2 and then hydrolyzing to the sodium
sulfonate form using caustic in alcohol. The ratio of
the latter two monomers was 8:1. The membrane was
converted to the acid form by soaking in dilute hydro-
chloric acid, dried and then soaked for 30 minutes at
25C in a 30 weight perecent solution of triethanol-
amine in water. The membrane was then air dried and
tested in a small electrolytic cell. The cell had an
anode and a cathode with the ion exchange membrane
sandwiched therebetween, thus separating the cell into
an anode chambor and a cathode chamber. Each electrode
had a sguare shape and had an area of 8.63 sguare
inches (56 cm2). Each electrode had a solid, metal
stud welded to it. Each stud passed through a wall of
the cell and was provided with leak proof seals. Both
studs were connected to a power supply. The stud
connected to the anode was constructed of titanium,
while the stud connected to the cathode was constructed
28,985-F -15- -
1203509
-16-
of steel. The anode, itself, was an expanded titanium
mesh screen coated with a Ruo2-Tio2 mixture, while the
cathode was constructed from woven steel wires.
s
The anode chamber was illed with a saturated
NaC1 brine solution (approximately 25 weight percent
NaCl) and the catholyte chamber was filled with a
, caustic solution having approximately the same NaO~
concentration as the intended cell operation produced.
The cell was energized by applying a constant current
of approximately 8.63 amps, to give a current density
of 1.0 amps per square inch of electrode area. A
saturated brine solution (appoximately 25 weigh~ per-
cent NaCl) was flowed into the anode chamber at a rate
sufficient to maintain the concentration of the anolyte
leaving the cell at approximately 17-20 weight percent
NaCl. Deionized water was flowed into the catholyte
chamber, in a similar manner, at a rate sufficient to
maintain the catholyte leaving the cell at a desired
NaO~ concentration. During the evaluation of each
membrane, the NaOH concentration was varied in order to
determine the cell operaticn over a range of caustic
concentrations.
The temperature of the cell was controlle~d
throughout each evaluation at about 80C by means of an
immersion heater connected to a thermocouple inserted
into the anolyte chamber. During the evaluation of
each membrane the cell voltage was constantly monitored 30 by measuring the difference in vol~age potential
between the anode stud and the cathode stud. For each
evaluation, the cell was operated for several days to
reach equilibrium. Then current efficiency was
determined by collecting the catholyte leaving the cell
28,985-F -16-
lZ03509
-17-
for a given period of time, usually 16 hours, and
determining the amount of NaO~ actually produced, as
compared to the amount theoretically produced at the
applied current. The membrane operated in the above
manner at 3.31 volts at 12% caustic at a current
efficiency of 91.3%. The voltage at 20% caustic was
3.25 and the current efficiency 82.6% and at 32%
~ caustic the voltage was 3.30 and the current efficiency
73.7%.
The water absorption was determined for the
membrane by first drying the membrane film in the S03H
form for 16 hours at 110C, weighing the sample
boiling the sample for 30 minutes in water, blotting
the surface dry with towels and then reweighing the
film. The difference in weight represented the amount
of water absorbed by the film and is commonly referred
to as the "Standard Water Absorption". The water
absorption per functional group was then determined by
calculating the moles of water that one eguivalent of
the polymer absorbed. In this manner the hydration of
the membrane was determined to be 13.8 moles of water
per sulfonate equivalent.
Exam~le 2
A 3.5 mil thick film of the polymer of Example 1
in the sulfonyl fluoride orm (-502 F~ was thermally lami-
nated onto a second film having a thickness of 7 mils,
an equivalent weight of 860 and prepared by copoly-
merizing tetrafluoroethylene and FS02CF2CF20CF=CF2.
The composite ilm was then converted to the acid form
by hydrolysis in base and neutralization with acid.
The film was then evaluated as described in Example 1
with the 1240 equivalent weight layer facing the
28,985-F -17-
12Q3509
-18-
catholyte. The cell operated from 3.07 to 3.09 volts
over a caustic strength range of from 12~o to about 20%
caustic. The current efficiency was essentially the
same as in Example l. The 860 equivalent weight second
film, in the acid form, was determined to have a hydration
of 23.9 moles of water per equivalent of functional
group.
ComParative Example 2
.. 10 . A composite membrane of the prior art^com-
posed of a first film 1.0 mil thick and having an
equivalent weight of 1500 and a hydration o about 15
moles of water per sulfonic acid unctional grollp and a
second film 5.0 mil thick and having an eguivalent
weight of 1100 and a hydration of about 22 moles of
water per sulfonic acid equivalent^was evaluated as in
Example l. The cell voltage was about 3.1 volts over a
range of i2 to 20% caustic and the current efficiency
varied from 89.5% at 12X NaOH to 80% at 20% NaO~. This
membrane was about equal in voltage to the membrane of
Example 2 even though the barrier layer thickness was
only 28% as great~ Clearly the membrane of Example 2
is superior in voltage at comparable thicknesses, and
in current eficiency at comparable caustic concen-
2S tration. .
ExamPle 3
A composite membrane iS prepared by lami-
nating the sulfonyl fluoride form of a 3.5 mil thick
fi~m of the polymer of ~xample 1 to a 4 mil film having
an llOO eguivalent weight and being the same polymer as
the second layer of the composite membrane described in
Comparative Example 2. The membrane operated in the
cell of Example 1 at a voltage essentially the same as
28,985-F -18-
1203509
-19-
that of the cell in comparative Example 2 even though
the thickness was greater and at a current efficiency
better than comparative Example 2 and egual to that of
Example 2.
Exam~le 4
A composite film is prepared by laminating a
2 mil film of an 820 equivalent weigh~ copolymer of
tetrafluoroethylene and C~300C(CF2)30CF=CF2 onto a
-- support layer the same as the second film of Example 2.
The composite fil~ is then con~erted to the salt form
.
by hydrolysis in aqueous alcoholic base. Evaluation of
the film in a cell, with the carboxylic acid face
towards the cathode, demonstrates that the membrane
operates at about the same efficiency as a film made of
the carboxylic acld polymer alone, but at a substan-
tially lower voltage than an equal thickness of the
carboxylic acid polymer. The current efficiency is
about 90% when the catholyte contains 35% caustic. The
composite film has excellent ~echanical properties.
Exam~Ie S
A composite membrane was prepared by thermally
laminating a 3 mil film of a 770 equivalent weight
polymer made from the monomers in Example 1 to a 6.5
mil film of a 1000 equivalent weight polymer made from
the monomers in Example 2. The composite film was then
hydrolyzed from the S02F form to the S03Na form using
caustic in a boiling water-alcohol mixture. The film
was then converted to the acid form by soaking in
dilution of HCl, washed with water and then dried
overnight at 110C in a vacuum oven. The film was then
converted to the 502Cl form by boiling, at reflux, for
20 hours in a 1:1 mixture of phosphorus pentachloride
28, 985-F -19-
` 12Q3509
-20-
and phosphorus oxychloride. The slde of the membrane
having the low equivalent weight (770) was then con-
verted to carboxylic acid functionality using 57%
hydroiodic acid at 80C as described in U.S. patent
4,151,053. The film was then hyd~olyzed using caustic
in a èthanol-water mixture, converted to the acid form,
dried and evaluated, with the carboxylic acid surface
facing the cathode, in the cell described in Example 1.
The cell operated at a voltage from 3.06 to 3.35 at
caustic strengths-varying from 2S to 35% NaO~. The
current efficiency was 82% at 3S% NaOH and the caustic
solution contained 55 ppm sodium chloride.
Com~arative Example S
The 770 equivalent weight film of Example S
was hydrolyzed to.the S02~a form usinq caustic in water
and alcohol, then converted to the acid form, dried and
evaluated as described in Example 1. The current
efficiency was 79% at 9.5Z NaOH and the caustic
solution contained 4000 ppm sodium chloride.
28,985-F -20-
