Note: Descriptions are shown in the official language in which they were submitted.
lZ03508
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QM 32193
INSTALLATION OF ION-EXCHANGE MEMBRANE IN ELECTROLYTIC
CELL
This invention relates to a method of installing
an ion-exchange membrane in an electrolytic cell.
Electrolytic cells are known comprising a
plurality of anodes and cathodes with each anode being
separated from the adjacent cathode by an ion-exchange
membrane which dLvides the electrolytic cell into a
plurality of anode and cathode compartments. The anode
compartments of such a cell are provided with means for
feeding electrolyte to the cell, suitably from a common
header, and with means for removing products of
electrolysis from the cell. Similarly, the cathode
compartments of the cell are provided with means for
removing products of electrolysis from the cell, and
optionally with means for feeding water or other fluid
j~.
~203508
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to the cell. The electrolytic cells may be of the
monopolar or bipolar type.
For example electrolytic cells of the filter
press type may comprise a large number of alternating
anodes and cathodes, for example, fifty anodes
alternating with fifty cathodes, although the cell may
comprise even more anodes and cathodes, for example up
to one hundred and fifty alternating anodes and
cathodes.
In such an electrolytic cell the membranes are
essentially hydraulically impermeable and in use ionic
species, e.g. hydrated ionic species, are transported
across the membrane between the anode and cathode
compartments of the cell. Thus, when an aqueous alkali
lS metal chloride solution is electrolysed in a cell
equipped with cation-exchange membranes the solution is
fed to the anode compartments of the cell and chlorine
produced in the electrolysis and depleted alkali metal
chloride solution are removed from the anode
compartments, alkali metal ions are transported across
the membranes to the cathode compartments of the cell to
which water or dilute alkali metal hydroxide solution
may be fed, and hydrogen and alkali metal hydroxide
solution produced by the reaction of alkali metal ions
with hydroxyl ions are removed from the cathode
compartments of the cell.
Electrolytic cells of the type described may be
used particularly in the production of chlorine and
sodium hydroxide by the electrolysis of aqueous sodium
chloride solution.
In such an electrolytic cell the membrane is
secured to the cell, for example, by clamping between
gaskets. It is desirable that the membrane be installed
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in the cell in a taut state and that the membrane remain
in a substantially taut state when electrolyte is
charged to the cell and the cell is operated. However,
if a membrane is installed in an electrolytic cell in a
dry state and is fixed tautly therein it is found that
in use when electrolyte is contacted with the membrane
in the cell the membrane swells and expands and becomes
slack and may even become wrinkled. As a result there
may be uneven release of gas and an increase in the
voltage of the cell. This is a particular disadvantage
where the cell is designed to operate at low, or zero,
anode-cathode gap.
In order to alleviate this problem of swelling of
the membrane in use it has been proposed to pre-swell
the membrane before installing the membrane in an
electrolytic cell, or example by soaking the membrane
in water, in an aqueous sodium chloride solution, or in
an aqueous sodium hydroxide solution. Ideally, the
membrane should be pre-swelled to an extent
approximately the same as that by which a dry membrane
would be swelled by contact with the electrolyte in the
electrolytic cell.
In US Patent No. 4000057 there is described the
pre-swelling of a membrane before installation of the
membrane in an electrolytic cell the method comprising
contacting the membrane with a liquid medium in which
the membrane exhibits a substantially flat expansion
versus time curve for at least four hours after
contacting the membrane with the liquid medium. Suitable
liquid media include, for example, aqueous solutions of
ethylene glycol, glycerine and higher fatty alcohols.
Although the aforementioned methods do assist in
overcoming the problem of swelling of a membrane when
the membrane is contacted with electrolyte in an
lZ03508
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electrolytic cell they do themselves suffer from
s-~bstantial disadvantages. Thus, the pre-swelled
membranes are wet and remain wet during installation in
the electrolyte cell and are thus difficult to handle.
Special handling precautions may need to be taken, for
example where the membrane has been pre-swelled by
contact with a corrosive liquid, e.g. a caustic soda
solution. Also difficulty may also be experienced in
securing the wet membrane in the electrolytic cell in a
leak-tight manner, for example between a pair of
gaskets.
The present invention relates to a method of
installing an ion-exchange membrane in an electrolytic
cell which does not suffer from the aforementioned
disadvantages.
According to the present invention there is
provided a method of installing an ion-exchange membrane
comprising an organic polymer containing ion-exchange
groups or derivatives thereof convertible to ion-
exchange groups in an electrolytic cell in which method
the ion-exchange membrane is expanded and the expanded
membrane is secured to the electrolytic cell or to a
part thereof, characterised in that the membrane is
expanded by stretching the membrane in order to increase
the surface area per unit weight of the membrane.
In the method of the invention the ion-exchange
membrane in the form of a sheet or film is expanded by
stretching so as to increase the surface area Oe the
membrane per unit weight of membrane.
This expansion of the membrane does not depend on
the use of a liquid medium to swell and thus expand the
membrane. Indeed, the expansion by streching will
generally be effected, and is preferably effected, on a
1l2~350~
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dry membrane, thus avoiding the substantial
disadvantages associated with use of a liquid medium
Furthermore, the expansion is not effected merely by
pressing the membrane at elevated pressure and
temperature.
The stretching of the membrane should be effected
with care in order not to tear the membrane. The use of
elevated temperature during the stretching of the
membrane greatly assists in avoiding tearing of the
membrane.
According to a further embodiment of the present
invention there is provided a method of installing an
ion-exchange membrane in an electrolytic cell which
method comprises heating the ion-exchange membrane to an
elevated temperature, stretching the membrane at the
elevated temperature, and securing the expanded
stretched membrane into the electrolytic cell, or to a
part of the electrolytic cell.
It is further preferred to stretch the membrane
at elevated temperature and to cool the membrane to a
lower temperature, e.g. to a temperature at or near
ambient, whilst restraining the membrane in the expanded
stretched state, and thereafter to secure the expanded
stretched membrane into the electrolytic cell, or to a
part thereof.
The stretching may be effected, for example, by
passing the membrane around and between rollers
operating at different periphe~al speeds, and the
expanded, stretched membrane may be cooled to a lower
temperature. Alternatively, the membrane may be
stretched by applying a stretching force to opposed
qdges of the membrane, and the expanded, stretched
membrane may be cooled to a lower temperature. The
stretching of the membrane may be effected in a
stretching frame or machine.
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The membrane may be stretched uniaxially or
biaxially. Biaxial stretching may be effected in two
directions simultaneously or sequentially.
When the membrane is stretched uniaxially strips
of a relatively stiff material may be attached to
opposed edges of the membrane to prevent contraction of
the membrane in a direction transverse to that in which
the membrane is stretched.
Where the membrane is stretched, e.g. at elevated
temperature, and particularly where the membrane is
subsequently cooled to a lower temperature, for example,
at or near ambient temperature, whilst the membrane is
restrained in the expanded, stretched state, at least
some of the expansion of the membrane effected by
stretching is "locked" into the membrane. When the
expanded, stretched membrane is installed in an
electrolytic cell and secured therein and the membrane
is contacted with an electrolyte, particularly at an
elevated temperature, for example with aqueous alkali
metal chloride solution at a temperature which may be as
high as 95C in a chlor-alkali cell, the expansion which
is "locked" into the membrane is released, or partially
released, and the membrane tends to contract towards its
original state, although the membrane is of course
restrained in the electrolytic cell. This tendancy to
contract is counteracted by the expansion of the
membrane cuased by swelling brought about by contacting
the membrane with the electrolyte, with the result that
the membrane installed in the electrolytic cell remains
taut and does not become wrinkled during use.
It is preferred that the amount of expansion
effected by stretching should be approximately the same
as or greater than the expansion caused by swelling of
1~03508
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the membrane on contact with the electrolyte in the
electrolytic cell so that the membrane, when contacted
with the electrolyte remains taut in the cell. Some
benefit wilL however be obtained even if the amount of
expansion effected by stretching is somewhat less than
expansion caused by swelling of the membrane on contact
with the electrolyte. A suitable amount of expansion to
be effected by stretching may be determined by simple
test.
In general, the expansion of the membrane
effected by stretching should produce an increase oE at
least 2% in the surface area of the membrane per unit
weight of membrane, preferably at least 5~. A large
expansion of the membrane may be effected by stretching
for example, at least 50~, or at least 100~, or even a
10-fold increase or greater in the surface area per unit
weight of the membrane. Where a substantial amount of
stretching is effected additional benefits will be
obtained. Thus, where a large expansion of the membrane
has been effected by stretching use of the membrane in
an electrolytic cell will result in a lower voltage of
operation, with consequent savings in power costs.
Additionally, the products o~ electrolysis may be
produced at a higher current efficiency.
In order that the bulk of the expansion of the
membrane effected by stretching may be "locked" into the
membrane the membrane may be cooled from an elevated
temperature to a lower temperature whilst the membrane
is restrained in the expanded, stretched state. However,
when such a membrane is used in an electrolytic cell the
contraction of the membrane which occurs when the
membrane is contacted with electrolyte at elevated
temperature may be much greater than the expansion
caused by swelling of the membrane by contact with
12~3S08
electrolyte, and the membrane may tend to tear. Whether
or not there is any tendency to tear will of course
depend on the extent of the expansion of the membrane
effected by stretching.
It is preferred, where the extent of expansion of
the membrane which is effected by stretching is substantial,
in order for example to produce a membrane which has a
much increased surface area per unit per weight and which
thus is capable of operating at a substantially reduced
voltage in an electrolytic cell, to anneal the expanded
stretched membrane by heating the membrane at elevated
temperature and subsequently by cooling the membrane to a
lower temperature. In this way sufficient expansion may
be "locked" into the membrane for the membrane to remain
taut and unwrinkled during use in an electrolytic cell
and also any tendency for the membrane to tear during
use may be overcome.
The membrane which is sub~ected to expansion by
stretching will generally be in the form of a film and
may, for example, have a thickness in the range 0.2 to
2 mm.
Although extremely thin membranes may be produced
by stretching the expanded, stretched membrane should
not be so thin that it is highly susceptible to damage
when used in an electrolytic cell. In general the
expanded, stretched membrane will have a thickness of at
least 0.02 mm, preferably at least 0.1 mm.
The elevated temperature at which stretching of
the membrane is effected will depend on the nature of
the membrane. It will in general, however, be in excess
of 40C, preferably in excess of 55C. A suitable
temperature for use with a particular membrane may be
selected by simple experiment. The temperature should
12(~3508
g
not be so high that the organic polymer of the membrane
melts or is degraded to a significant extent. In general
the elevated temperature at which stretching is effected
will not be above 150C.
Where the expanded, stretched membrane is
annealed the annealing temperature may be the same as or
similar to the elevated temperature at which the
membrane is stretched. The annealing temperature may be
higher than the temperature at which stretching is
effected. The time for which the expanded, stretched
membrane is annealed will determine the extent of the
expansion of the membrane which is "locked" into the
membrane when the membrane is subsequently cooled to a
lower temperature, the longer is this annealing time the
less will be the extent of the expansion which remains
"locked" into the membranes. In general, the annealing
time will be at least 1 minute, but in general it will
not be more than 5 hours.
The lower the temperture to which the membrane
may be cooled will be a temperature at which the
membrane does not relax rapidly when the restraining
force, if any is removed from the membrane. It is most
convenient to cool the membrane to a temperature which
is at or near ambient temperature.
In a eurther pre~erred embodiment, particularly
useful where the membrane is to be expanded to a
substantial extent by stretching, the membrane is
stretched at elevated temperature, the membrane is
cooled to a lower temperature, e.g. to a temperature at
or near ambient, whilst restraining the membrane in the
expanded stretched state, and the steps of expansion by
stretching at elevated temperature and cooling are
repeated at least once. In this way the desired amount
. .
lZQ3508
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of expansion of the membrane may be effected by
stretching in a plurality of stages and there is a
decreased possibility of the membrane being damaged,
e.g. by tearing, during the stretching.
The ion-exchange membrane is preferably a cation-
exchange membrane containing acidic groups or
derivatives thereof convertible to acidic groups. In
order to provide resistance to the corrosive environment
encountered in many electrolytic cells, particularly in
chlor-alkali cells, the membrane is preferably a
~luoropolymer, and more preferably a perfluoropolymer,
containing such acidic groups or derivatives thereof.
5uitable acidic groups include sulphonic acid,
carboxylic acid or phosphonic acid. The membrane may
l; contain two or more different acidic groups. Suitable
derivatives of the acidic groups include salts of such
groups, for example metal salts, such groups,
particularly alkali metal salts. Suitable derivatives
include in particular derivatives convertible to acidic
groups by hydrolysis, for example acidic halide groups,
e.g. - SO2F and -COF, nitrile groups - CN, acid amide
groups - CONR2, where R is H or alkyl, and acid ester
groups, e.g. - COOR, where R is an alkyl group.
Suitable cation-exchange membranes are those
described, for example, in the GB Patents Nos. 1184321,
1402920, 1406673, 1455070, 1497748, 1497749, 1518387 and
1531068.
It is preferred to use membranes containing
derivatives of acidic groups which are convertible to
ion-exchange groups by hydrolysis as membranes
containing such groups are generally more susceptible to
stretching. ~or example, where the membrane is a
fluoropolymer containing carboxylic acid groups as ion-
1203508
exchange groups it is preferred to stretch the membrane
in a form in which the carboxylic groups are in the
ester form, e.g. in the form of a methyl ester.
Where the membrane c~ntains groups convertible to
ion-exchange groups by hydr~lysis the hydr~lysis may be
effected, f~r example, by contacting the membrane with
aque~us alkali metal hydroxide solution, e.g. with
aqueous sodium hydroxide solution. As the membrane may
tend to swell on hydrolysis it is preferred to effect
such hydrolysis after the expanded, stretched membrane
has been secured to the electrolytic cell or to a part
there~f.
The membrane may be reinf~rced, for example with
a net ~f a fluor~p~lymer, alth~ugh such reinferced
membranes are n~t preferred as difficulty may be
experienced in stretching the reinf~rcing net. The
membrane may be in the form of a laminate, ~r it may be
c~ated with electrode ~r non-electr~de materials.
The expanded, stretched ion-exchange membrane is
secured in the electr~lytic cell, or to a part of the
electrolytic cell. Where the membrane has been expanded
by stretching at elevated temperature it may be secured
in the electrolytic cell ~r to a part there~f whilst at
the elevated temperature. H~wever, as the expanded
stretched membrane will tend ~o cool t~wards ambient
temperature and thus contract during this securing
procedure it is preeerred to lock the expansi~n into the
membrane prior to securing the membrane int~ the
electr~lytic cell ~r to a part thereof. Thus, it is
pre~erred t~ expand the ion-exchange membrane by
stretching at elevated temperature and to restrain the
membrane in the expanded, stretched state whilst cooling
the membrane to a lower temperature, and preferably to
lZ~3508
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ambient temperature, at which temperature the membrane
retains a substantial proportion of its expanded,
s~retched state when the restraining force is removed.
The expanded stretched membrane may be secured
into the electrolytic cell or to a part thereof by any
convenient means. For example, the membrane may be
securely clamped between a pair of gaskets in the
electrolytic cell, or the membrane may be secured to a
frame which is subsequently installed in the
electrolytic cell, or the membrane may be secured to an
electrode.
The method of the present invention is
particularly suitable ~or application to an ion-exchan~e
membrane which is to be installed in an electrolytic
lS cell of the filter press type. Electrolytic cells of the
filter press type may comprise a large number of
alternating anodes and cathodes with an ion-exchange
membrane positioned between each anode and adjacent
cathode. Such cells may comprise, .or example, fifty
anodes alternating with fifty cathodes, although the cell
may comprise even more anodes and cathodes, for example
up to one hundred and ~ifty alternating anodes and
cathodes.
In the electrolytic cell the electrodes will
3enerally be made of a metal or alloy. ~he nature of the
metal or alloy will depend on-whether the electrode is
to be used as an anode or cathode and on the nature o~
the electrolyte which is to be electrolysed in the
electrolytic cell.
31 Where aqueous alkali metal chloride solution is
to ~e electrolysed and the eiectrode is to be used as an
anode the electrode is suit~bly made o~ a film-~orming
metal o~ an allo~ there~E, e,,c r~ample oE zirconium,
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niobium, tungsten or tantalum, but preferably of
titanium, and the surface of the anode suitably carries
a coating of an electro-conducting electrocatalytically
active material. The coating may comprise one or more
platinum group metals, that is platinum, rhodium,
iridium, ruthenium, osmium or palladium, and/or an oxide
of one or more of these metals. The coating of platinum
group metal and/or oxide may be present in admixture
with one or more non-noble metal oxides, particularly
one or more film-forming metal oxides, e.g. titanium
dioxide.
Electro-conducting electrocatalytically active
materials for use as anode coatings in an electrolytic
cell for the electrolysis of aqueous alkali metal
chloride solution, and methods of application of such
coatings, are well known in the art.
Where aqueous alkali metal chloride solution is
to be electrolysed and the electrode is to be used as a
cathode the electrode is suitably made of iron or steel,
or of other suitable metal, for example nickel. The
cathode may be coated with a material designed to reduce
the hydrogen overpotential of the electrolysis.
Any suitable construction oE electrode may be
used in the electrolytic cell. For example the electrode
may comprise a plurality of elongated members, e.g. rods
or strips, or it may comprise a Eoraminate surface, e.g.
a perforated plate, a mesh, or an expanded metal.
The invention is illustrated by the following
examples.
Example 1
A rectangular section 35 cm x 30 cm was cut from
a 280 micron thick sheet of a cation-exchange membrane
of a copolymer of tetrafluoroethylene and a
~..
12Q3S08
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perfluorovinyl ether containing carboxylic acid groups,
the ion-exchange capacity of the membrane being 1.3
milli equivalents per gram.
Strips of PVC elastic tape were attached to the
sheet at each of the 35 cm long edges of the sheet and
strips of aluminium were attached to t'ne sheet at each
of the 30 cm long edges of the sheet. The sheet was then
mounted in a Bruckner Karo 11 orienter and the
temperature of the sheet was raised to 67C in an oven
associated with the orienter.
The aluminium strips were pulled apart at a rate
of 1 metre per minute until the spacing of the aluminium
strips attached to the sheet had increased by a factor
of 1.5, the PVC elastic strips assisting in the
prevention of "waisting" of the sheet. The sheet, whilst
mounted on the orienter, was then removed from the oven
and cooled to ambient temperature in a stream of air.
The above procedure of stretching the sheet at a
temperature of 67C and cooling of the sheet to ambient
temperature was repeated twice, in the first repeat of
the procedure the spacing of the aluminium strips being
increased by a factor of 2.5 over the original spacing
and in the second repeat of the procedure the spacing of
the aluminium strips being increased by a factor of 4.2
over the original spacing.
The resultant cation-exchange membrane film was
then removed from the orienter. The film relaxed
slightly towards the original dimensions of the sheet.
The thickness of the film after t`nis slight relaxation
was 80 microns.
The film of cation-exchange membrane produced as
described above was securely and tautly clamped between
a pair of gaskets of EPDM rubber and mo~nted in an
electrolytic cell equipped with a 7.5 cm diameter nickel
lZ03S08
-15-
mesh cathode and a 7.S cm diameter titanium mesh anode
coated with a coating of a mixture of RuO2 and Tio2
in a proportion of 3~ RuO2 : 65 TiO2 by weight.
310 g/l aqueous NaCl solution at a pH of 8.0 was
charged to the anode compartment of the cell and water
was charged to the cathode compartment of the cell and
the NaCl was electrolysed therein at a temperature of
90C, the concentration of NaCl in the anode compartment
during electrolysis being 200 g/l.
Chlorine and depleted NaCl solution were removed
from the anode compartment and hydrogen and aqueous NaOH
(35% by weight) were removed from the cathode
compartment.
The electrolysis was effected at a current
density of lkA/m2 and the cell voltage was 3.01 volts.
After a total of 20 days electrolysis the cell
was opened and the cation-exchange membrane examined.
The membrane was found to be taut and not wrinkled.
By way of comparison the above electrolysis
procedure was repeated except that a 280 micron thick
sheet of cation-exchange membrane was installed in the
electrolytic cell, that is a membrane which had not been
subjected to the stretching process.
At a current density of lkA/m2 the voltage was
3.1 volts and the membrane removed from the cell was
found to be wrinkled and no longer taut.
Example 2
The electrolysis procedure of Example 1 was
repeated at a current density of 2kA/m2. In this case
the voltage was 3.24 volts and, as in the case Oe
Example 1 the membrane, when removed from the cell, was
~ound to be taut and unwrinkled.
.,
:IZ~3~
-16-
By way of comparison the above electrolysis
~rocedure was repeated except that a 280 micron thick
sheet of cation-exchange membrane was installed in the
electrolytic cell, that is a membrane which nad not been
subjected to the stretching process.
At a current density of 2 kA/m2 the voltage was
3.4 volts and the membrane removed from the cell was
found to be wrinkled and no longer taut.
Example 3
The electrolysis procedure of Example l was
repeated at a current density of 3 kA/m2. In this case
the voltage was 3.52 volts and, as in the case of
Example 1, the membrane, when removed from the cell, was
found to be taut and unwrinkled.
ay way oE comparison the above electrolysis
procedure was repeated except that a 280 micron thick
sheet of cation-exchange membrane was installed in the
electrolytic cell, that is a membrane which had not been
subjected to the stretching process.
At a current density of 3 kA/m2 the voltage was
3.7 volts and the membrane removed from the cell was
found to be wrinkled and no longer taut.
Example 4
A sample of a cation-exchange membrane of a
copolymer of tetra-fluoroethylene and a perfluorovinyl
ether containing sulphonic acid groups in the form of
the potassium salt of dimensions 11.5 cm x 11.5 cm was
taped at its edges with PVC tape and the thus taped
membrane was clamped in a stentor frame. The membrane
was heated to a temperature of 180C and was drawn
uniaxially at a draw speed Oe 0.8;m/min until the
membrane had been drawn by a factor o~ 2Ø The membrane
was then cooled to ambient temperature and removed from
the stentor frame.
12(:~3S08
-17-
The membrane was installed in an electrolytic
cell as described in Example 1 and tne electrolysi~s
procedure of Example 2 was followed, that is aqueous
NaCl solution was electrolysed at a current density of
2kA/m2. Na~H solution at a concentration of 25~ by
weight was produced at a current efficiency of 50%. The
cell voltage was 2.95 volts.
When the electrolytic cell was opened the
membrane was found to be taut and unwrink-led.
By way of comparison the electrolysis procedure
was repeated except that there was used a membrane as
described above which had not been subjected to the
stretching procedure. The cell operated at a voltage Oe
3.1 volts and NaOH was produced at a current efficiency
of 57~.
When the cell was opened the membrane was found
to be wrinkled and no longer taut.
Example S
The stretching procedure of Example 4 was
repeated except that the membrane which was used was a
copolymer of tetrafluoroethylene and a perfluorovinyl
ether containing carboxylic acid methyl ester groups,
and the temperature to which the membrane was heated
during the stretching was 80C.
The membrar.e was installed in an electrolytic
cell as described in Example 1, hydrolysed by contact
with NaOH solution, and the electrolysis procedure of
Example 3 was followed, that is aqueous NaCl solution
was electrolysed at a current density of 3kA/m2. NaOH
3~ solution at a concentration Oe 35~ by weight was
produced at a current efficiency o~ 94~. The cell
voltage was 3.32 volts.
12()3S08
-18-
When the electrolytic cell was opened the
membrane was found to be taut and unwrinkled.
By way o~ co~arison t~e electrolysis procedure
was repeated except that there was used a membrane as
described above which had not been subjected to the
stretching procedure. The cell operated at a voltage of
3.4 volts and NaOH was produced at a current efficiency
of 94~.
When the cell was opened the membrane was found
to be wrinkled and no longer taut.
Example 6
A sample of an ion-exchange membrane of a
copolymer of tetrafluoroethylene and a perfluorovinyl
ether containing carboxylic acid methyl ester groups as
used in Example S was heated at a temperature of 67-C
and was stretched uniaxially on a stentor frame
following the procedure described in Example 4, except
that the draw rate was 1 m/min and the membrane was
stretched by a actor of 4.3, that is it was stretched
to 430~ of its original length in the direction of
stretch. After completion of the stretching the membrane
was cooled rapidly to ambient temperature in a stream of
air and removed from the frame.
After standing for 15 minutes the membrane was
found to have shrunk by 15% in the direction o~
stretching, so that in this direction it was 365% of its
ori~inal length in this direction.
The electrolysis procedure of Example 1 was
repeated using the above described membrane. After
effecting electrolysis for 20 days the membrane was
found to be taut and unwrinkled.
lZ~3S08
-19-
Example 7 to 9
The procedure of Example 6 was repeated on three
separate samples of membrane except that, prior to
cooling and removal from the stentor frame, the samples
were annealed after completion of the stretching by
S heating at 67C for respectively 1 minute (Example 7), 2
minutes (Example 8) and 3 minutes (Example 9).
After standing for 15 minutes after re~oval from
the frame the membranes were found to have shrunk, in
the direction of stretching, respectively by 11%
(Example 7), 10% (Example 8), and 9% (Example 3), that
is in this direction the membranes were 383% (Example 7),
387% (Example 8), 391% (Example 9) of their orignial
length.
The electrolysis procedure of Example 1 was
repeated using each of the above described membranes.
After effecting electrolysis for 20 days each of the
membranes was found to be taut and unwrinkled.
