Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
8~
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 applicants
copending Canadian application Serial No~ 379,45~ filed on
June 10, 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
~luorinated polymeric backbone with a plurality of pendant
groups attached thereto, at least a portion of said pendant
groups ~eing a chain of carbon atoms which may be interrupted
with one or more oxygen atoms and which terminates with a
sulfonyl ion exchange group and wherein the two layers
differ 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
hydraulical~y-permeable asbestos diaphragm, vacuum-deposited
onto foraminous steel cathodes, have been widely commer-
cialized. Such diaphragm cells, employing permeable dia-
phrams, produce NaCl-containing NaOH catholytes because
NaCl passes through the diaphragm from 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 caust~c for industrial purposes.
In recent years, the chlor-alkali industr~ 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
~"',..
28,985-F (Div. B) -l-
~2~
the costly de-salting and evaporation processes.
Membranes have been developed for that purpose which
are substantially hydraulically-imperme~ble, 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 membra~e cell as opposed to the use of
a diaphragm cell.
In addition to the caustic strength belng
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 given current.
Thus, it is apparent that power efficiency is reduced
by higher cell voltage or by lower current efficien~y.
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
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
bet~er 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 voltage
which is reflected in membrane electrical resistance,
current efficiency and caustic concentration.
It is well known (G.E. Munn, Nafion~ Membranes
- Factors Contxolling Performance in the Electrolysis
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 ~urface of the membrane
contacting the catholyte. The current efficiency is
dependent o~ the e~uivalent weight of the membxane in
contact with the catholyte 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 lower 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 reiection 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
28,9a5-F -3- -
~2~
having higher eq. wts. ~1500-2000 for sulfonic acids
membranes of the prior art).
U.S. Patent 3,909,378 teaches a method to
take advantage of the increased current efficiency
associated with high eq. wts. without absorbing the
full voltage penalty associated with these materials.
This patent teaches a composite membrane formed by
laminating a thin, high eg. 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 e~.
wt. and voltage associated with the thin layer plus the
minimal voltage of the lower eq. wt. layer. The paten-t
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 pre~erably 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-subs-tituted
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 e~. wt. ilm rather
than by lamination of a baxrier film to a support film.
28,985-F -4-
~2~L7~
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 hydrogen; ammonium; guaternary
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.
In each patent the preferred sulfonyl containing fluoro-.
polymer is described as derived, by polymerization,
from the monomer
FSO2CF2cF2OcFcF2OcF ~ CF2
CF3
disclosPd 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 properties
28,985-F -5-
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 1100. 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. w~. 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 cellsO This patent teaches useful
equivalent weight ranges that vary 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 taugh~ as being
associated with the concentration of the functional groups in the membrane ~eq.
wt.), water absorption of the membr~ne and glass transition tempera~ure of
th0 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. w~. carboxylic acid polymers a~ high
caustic concentrations ~30-40%), but the voltages reported in the examples
. 20 are relatively high for the thicknesses reported ~200 microns) and the current
density of ~he cells ~20A/dm ).
According to the present invention there is now provided a
substantially completely fluorinated polymeric composite film having a first
layer containing carboxylic acid ion exchange groups and a second layer
containing sulfonyl ion exchange groups, wherein the improvement comprises at
least one layer having an equivalent weight o less than 1000.
28,985-F
., , ~
7~
--7--
The ion exchange membranes of the invention are
made by combining at least two different films of substantially
fluorinated polymers containing ion exchange functional
groups. It is within the scope of the invention and in
fact in some cases preferable, that the e~. wt. of the first
film, the film facing the catholyte in chlor-alkali
electrolytic cells, can be e~ual to or even less than the
eq. wt. of the second film.
The present invention, together with those of
the aforementioned parent application Serial No. 379,454
filed June 10, 1981 ~y The Dow Chemical Company and its
~ivisional application Serial No. 443,987 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
~ualify as membranes in electrolytic cells. Wh~n 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 ev
vironment of the cell. This severelv restricts the materials
useful 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 strony, hot caustic on one
side and a highly oxidative environment on;-the other
virtually eliminates the use of most organic polymers as
membranes. The ~onstant churning of gas being evolved
~,985-F (Div. B) -7-
~L2~7~
-7a-
throu~h the liquid electrolyte solutions in the cell severly
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.
28,985-F (Div. B~ -7a-
~2~
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 shlorine 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
substituted with other halogen atoms such as chlorine or bromine that are not
reactive in the cell anvironment, but, although contrary to some teachings,
these polymers should not contain hydrogen atoms on carbons that 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 tnus lead to severe damage to the physical properties
of the membrane.
Physical properties of a polymer are dependent on polymer structure.
A highly crystalline ~luoropolymer made from simple, unsubstituted monomers such
as tetrafluoroethylene is tough, but has extremely high melting or softening
temperatures. Fabrication 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 groups 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 o~ 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 vinyl
ethers (United States Patent 3,896,179) have excellellt physical properties and
28,985-F
~8~e73L
can be conveniently melt fabricated into films. Thus, polymers with a limited
number of pendant groups can maintain most of the favorable physical
characteristics 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 oE pendant groups and the size or number of atoms and
arrangement o~ atoms (generally carbon and oxygen in the chain) that make up
the pendant group. Thus, the commercial, composite membranes of the prior art
are based on sulfonyl containing copolymers of tetrafluoroethylene and
Fso2cF2cF2ocFcF2ocF = CF2
F3
The membranes are made by laminating a thin layer of 150Q eq. wt. polymer onto
a thicker layer of 1100 eq. wt. polymer which lends mechanical strength while
adding little electrical resistance (see G. E. ~lull). Decreasing the equivalent
weight of the thicker support layer would result in somewhat lower electrical
resistance, 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. considerably 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, 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
28,985-F
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 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 required 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 hydra~e less per functional group at
given eq. w~. 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 FSO2CF2CF2OCF = 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
interrupted 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 electrical resistance per ~mit 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
- 10 -
28,985-F
eq. wt. polymer ofthe prior art onto the same 1100 equiva-
lent weight film even though the equivalent wei~ht difference
is only 140 as opposed to the minimum difference of 250
and the preferred difference of 400 taught in U.S. Patent
3,909,378.
A composite membrane may be formed by laminating
a film of the same 1240 equivalent weight material as
above onto an 860 eq. wt. copolymer of tetrafluoroethylene
and FSO2CF2CF2OCF=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 curren~ efficiency
and better cell voltage on a unit thickness basis than
1~ the composite membrane of the prior art. This was surprising
since U.S. Patent 3,909,378 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 barrier layer of the composite membrane of
the invention may be made from fluoropolymers having
carboxylic acid functional groups. This type of membrane
is particularly desirable when the caustic-product from
the cell is to be evaporated and thus higher concentrations
from the cell result in less energy required for evaporation.
It is particularly advantageous that teh second layer o:E
the composite membrane be sulfonic acid functional fluoro-
. polymers. The eq. wt. range for -the barrier layer is from
500 to 1100 and more preferably from 550 to 1000. The
eq~ wt. range for the second layer is from 750 to 1100
and more preferably from 800 to 1100 and most preferably
from 800 to 1000. It 's preferable to minimize the
28,~85-F ~Div. B) -11-
-12- ~2~
thickness of the barrier layer slnce this is the layer
having the higher e~lectrical resistance. This layer can
be as thin as lOOA, but is preferably from 2.5 to 12.5
micrometer and preferably should not exceed about one-
third to about one-half of the total membrane thic}cness.
Use of lower equivalent weight barrier layers in the
present invention results in improved composite membranes
compared to the sulfonic acid-carboxylic acid composite
membranes of the prior art.
From the standpoint of manufacture, it is
particularly 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.SO Patents 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 irretrievable loss of expensive polymer
materials. 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 togehter or can be fored by
3~ co-extrusion of the two layers.
28,985-F (Div. B) -12-
7' ~ ~ '
Included in the scope of the present invention
is com'oining -two films, one of which has had one surface
chemically converted from sulfonyl to carboxylic acid
or derivativeO The side opposite the carboxylic acid
function, 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
carboxylic acid surfaae of the composite faces the
catholyte in the operating cell. In these embodiments
the equivalent weight of the first film is less thàn,
equal to, or no more than 150 higher than the equivalent
weight of the second film. While these techni~ues do
have the disadvantage of requiring 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, functional groups
are not readily available. Otherwise, the-technique of
combining 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 barrier 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 for thirty minutes and measuring, by weighing,
~3
28,985-F
8~
the "Standard Water Absorption" and from this value
calculating the moles of water absorbed per eguivalent
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 equivalent 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 of the layers has a value less than 1000 can
the eq. wt. difference ex~eed 150.
Example l
A terpolymer film having an equivalent weight
of 1240 and a thickness of 8 mil was prepared by poly-
merizing tetrafluoroethylene, FSOzCF2CF2OCF=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 an~ 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 wi-th the ion exchange membrane
sandwiched therebetween, thus separating the cell into
an anode chamber and a cathode chamber. Each electrode
had a square shape and had an area of 8.63 square
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. rrhe stud
connected to the anode was constructed of titanlum,
while the stud connected to the cathode was constxucted
~J
28,985-F
~z~
of steel. The anode, itself, was an expanded titanium
mesh screen coated with a RuO2-TlO2 mixture, whlle the
cathocle was constructed from woven steel wires.
The anode chamber was filled with a sa~urated
NaCl brine solution (approximately 25 weight percent
NaCl) and the catholyte chamber was filled with a
caustic solution having approximately the same ~aOH
concentration as the intended cell operation produced.
The cell was energized by applying a constant curxent
oE 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 weight 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
NaOH concentration. During the evaluation of each
membrane, the NaOH concentration was varied in order to
determine the cell operation over a range of caustic
concentrations.
The temperature of the cell was controlled
throughout each evaluation at about 80~C by means o~ an
immersion heater connected to a thermocouple inserted
into the anolyte chamber. During the evaluation of
each membrane the cell voltage was constantly monitored
by measuring the difference in voltage potential
between the anode stud ancl 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
,.~.,~
1; . / .~
r
28,985-F
for a given period of time, usually 16 hours, and
determinlng the amount of NaOH 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 3Z%
caustic the voltage was 3.30 and the current efficiency
73.7%.
Thé water absorption was determined for the
membrane by first drying the membrane film in the SO3H
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 equivalent of
the polymer absorbe~. In this manner the hydration of
the membrane was determined to be 13.8 moles of water
per sulfonate equivalent.
~xam~le 2
A 3.5 mil thick film of the polymer of ~xample 1
in the sulfonyl fluoride form (-SO2F) was thermally lami-
nated onto a second film having a thickness o 7 mils,
an equivalent weight of 860 and prepared by copoly-
merizing tetrafluoroethylene and FSO2CF2CF2OCF=CE'2.The composite film was then converted to the acid form
by hydrolysis in hase and neutralization with acid.
The film was then evaluated as described in Example 1
with the 1240 equiva.lent weight layer facing the
28,985-F
~2~
catholyte. The cell operated from 3.07 to 3.09 volts
over a caustic strength range of from 12% to about 20%
caustic. Th~ current efficiency was essentially the
same as in Example 1. 'The 860 equivalent welght second
film, in the acid form, was determined to have a hydration
of 23.9 moles of water per equivalent of functional
group.
Comparatlve ExamPle 2
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 of about 15
moles of water per sulfonic acid functional group and a
second film S.O mil thick and having an equivalent
weight of 1100 and a hydration of about 22 moles of
water per sulfonic acid equivalent was evaluated as in
Example 1. The cell voltage was about 3.1 volts,over a
range of 12 to 20% caustic and the current efficiency
varied from 89.5% at 12% NaOH to 80% at 20% NaOH. This
membrane was about equal in voltage to the membrane o
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 efficiency at comparable caustic concen-
,tration.
Example 3
A composite membrane is prepared by lami-
, nating the sulfonyl fluoride form of a 3.5 mil thick
film of the polymer of Example 1 to a 4 mil film havlng
an 1100 equivalent 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 Examp~e 1 at a voltage essentially the same as
:.
28,985-F
~2~
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 e~ual to that of
Example 2.
Example 4
A composite film is prepared by laminatlng a
2 mil film of an 820 equivalent weight copolymer of
tetrafluoroethylene and CH300C(CF2)30CF=CF2 onto a
support layer the same as the second film of Example 2.
The composite film is'then converted to the salt form
by hydrolysis in aqueous alcoholic base. Evaluation of
the film in a cell, with the carboxylic afid face
towards the cathode, demonstrates that the membrane
operates at about the same efficiency as a film made of
the carboxylic acid polymer alone, but at a substan-
tially lower voltage than an equal khickness of the
carboxylic acid polymer. The current efficiency ~s
about 90% when the catholyte contains 35% caustic. The
composite film has excellent mechanical properties.
Example 5
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 c~mposite film was then
hydrolyzed from the SO?F 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 ~Cl, washed with water and then dried
overnight at 110C in a vacuum oven. The film was then
converted to the S02Cl form by boiling, at reflux, for
20 hours in a 1:1 mixture of phosphorus pentachloride
~ ' / ~
L~ --,2'fr_
28,985-F
7~
and phosphorus oxychloride. The side of the membrane
having the low equivale~t weight (770) was then con-
verted to carboxylic acld functionality uslng 57%
hydroiodic acid at 80C as described in U.S. patent
4,151,053. The film was then hydrolyzed using caustic
in a ethanol-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 25 to 35% NaOH. The
current efficiency was 82% at 35% NaOH and the caustic
solution contained 55 ppm sodium chloride.
Comparàtive ExamPle 5
The 770 equivalent weight film of Example 5
was hydrolyzed to the SOzNa form using 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 9.5% NaOH and the caustic
solution contained 4000 ppm sodium chloride.
2~,985-F
. .
