Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
" ~56~6Z
The present invention relates to an electrolytic cation
exchange membrane. More particularly, the present invention
relates to a cation exchange membrane suitable for use in elec-
trolysis of water or an aqueous solution such as an aqueous
acidic or alkaline solution, an aqueous alkali metal halide solu-
tion or an aqueous alkali metal carbonate solution, and an elec-
trolytic cell and electrolytic process wherein such a membrane is
employed.
As a process for producing an alkali metal hydroxide
and chlorine by the electrolysis of the above-mentioned aqueous
solution, particularly an aqueous solution of an alkali metal
chloride, a diaphragm method has now been used in p~ace of a mer-
cury method with a vlew to preventing environmental pollution.
Further, in order to efficiently obtain an alkali metal hydroxide
having a high purity in a high concentration, it has been pro-
posed and put into practical use to employ an ion exchange mem-
brane.
However, from the standpoint of energy saving, it is
desired to develop an ion exchange membrane which is capable of
providing a high current efficiency
1- g~e
~Z56(~62
-- 2
and a low cell voltage and which has adequate mechanical
strength. For this purpose, various membranes have been
proposed. However, this object has not yet adequately
been attained.
The present inventors have conducted extensive
researches with an aim to provide an ion exchange
membrane whereby the electrolysis of an aqueous solution
can be conducted with high efficiency and which has
adequate mechanical strength, and as a result, have
succeeded in the development of an ion exchange membrane
which is capable of ~dcquatcly attaining such an object.
Namely, the present invention provides an
electrolytic cation exchange membrane which comprises a
first film made of a fluorinated polymer having cation
exchange groups and containing a fibrillated
tetrafluoroethylene polymer and a second film laminated
thereon, made of a fluorinated polymer having carboxylic
acid groups as its ion exchange groups, said first film
having a greater thickness and smaller specific electric
resistance than said second film.
Such an ion exchange membrane of the present
invention provides superior characteristics, i.e. a high
current efficiency and a low cell voltage, in the
electrolysis, and it has adequate mechanical strength.
In order to obtain a high current efficiency in the
electrolysis of e.g. an alkali metal chloride, the
electrolytic cation exchange membrane is usually required
to have a relatively low ion exchange capacity, although
12S6~62
the desired ion exchange capacity may vary depending upon the
concentration of the alkali metal hydroxide to be produced. Such
a relatively low ion exchange capacity means that the specific
electric resistance is high, whereby the cell voltage will be
high. Accordingly, it is not advisable to prepare the ion
exchange membrane entirely from a fluorinated polymer having a
low ion exchange capacity. For this reason, it has been proposed
to prepare an ion exchange membrane by employing a combination of
a fluorinated polymer having a low ion exchange capacity with a
fluorinated polymer having a high ion exchange capacity and thus
a small specific electric resistance.
However, when a plurality of thin films of a fluori-
nated polymer are laminated to form a membrane, it is usually
difficult to obtain adequate mechanical strength without increas-
ing the total thickness. It is very difficult to reinforce thin
films of a fluorinated polymer by conventional methods. Further
a film of a fluorinated polymer having a high ion exchange capa-
city has usually poor mechanical strength. According to the pre-
sent invention, a fibrillated tetrafluoroethylene polymer isincorporated for the enhancement of the mechanical strength into
the first film of a fluorinated polymer having a high ion
exchange capacity a~d thus a low specific electric resistance and
the second film having a low ion exchange capacity and thus a
high specific electric resistance is made as thin as possible
1256(~62
and laminated on the first film, whereby adequate
mechanical strength can be imparted to the membrane
without increasing the total thickness or without taking
any special measure Eor the improvement of mechanical
strength of the second film. Further, it has been found
that when the above~m2ntioned fibrillated
tetrafluoroethylene polymer is used, uniform and adequate
reinforcement is obtainable and accordingly it is
possible to increase the ion exchange capacity of the
fluorinated polymer having a high ion exchange capacity
to minimize the specific electric resistance, whereby it
is possible to obtain an ion exchange membrane which is
capable of minimizing the cell voltage.
In the present invention, the first film of the
fluorinated polymer containing the fibrillated
tetrafluoroethylene polymer should preferably be made to
have an ion exchange capacity as high as possible so as
to minimize the specific electric resistance. The cation
exchange groups of the first film may be any groups such
as sulfonic acid groups, carboxylic acid groups,
phosphonic acid groups or hydroxyl groups. But sulfonic
acid groups or carboxylic acid groups are preferred.
- The content of the cation exchange groups in the
first film is selected to have an ion exchange capacity
of from 0.7 to 3.0 meq/g dry resin, preferably from 0.9
to 2.0 meq/g dry resin so that the specific electric
'.7~' resistance becomes ~o ~ lower than that of the second
-6, _J
film which will be described hereinafter. In a case
3LZ56~62
-- 5 --
where weak acid groups such as carboxylic acid groups or
phosphoric acid groups are used as the cation exchange
groups, an ion exchange capacity higher than that of the
second film is employed.
Various kinds of fluorinated polymers may be used for
the preparation of the first film. Among them, polymers
having the following repeating units (a) and (b) are
preferably used.
(a)-~-CF2-CXX'-t-
(b) ( CF2-CX ~----
Y-A
where X and ~' is -F, -Cl, -H or -CF3, A is -SO3M, -COOM
or -PO3M2 (where M is hydrogen or an alkali metal or a
grou~ hydrolyzable to such a group), and Y is selected
from the group consisting of-~ CF2~ x ~ ~-~ CF2 ~-x
-~ O-CF -CF-~ and -~ O-ICF CF2-~x I Y
Z Z Z
where Z is -F or a perfluoroalkyl group having from l to
10, preferably l to 5, carbon atoms, and each of x and y
is an integer of from l to 10, preferably l to 5.
The molar ratio of units (a)/units (b) is selected to
provide a fluorinated polymer having an ion exchange
capacity within the above-mentioned range.
The fluorinated polymers are preferably perfluoro
polymers. Preferred perfluoro polymers include a
copolymer of CF2=CF2 and CF2=CFOCF2CF(CF ~OCF2CF2SO2F, a
copolymer of CF2=CF2 and CF2=CFO(CF2)l_5 SO2F, a
copolymer of CF2=CF2 and CF2=CFO(CF2)l_s 3
1256062
-- 6 --
copolymer of CF2=CF2 and CF2=CFOCF2CF(CF3)OCF2CF2COOCH3
As mentioned above, a fibrillated tetrafluoro-
ethylene polymer is incorporated in the fluorinated
polymer for the first film. The fibrillated tetra-
fluoroethylene polymer may be a tetrafluoroethylenepolymer including any modified tetrafluoroethylene which
is capable of forming a branched or web-like
three-dimensional net-work structure when subjected to a
shearing force to Eibrillate. Such a tetrafluoroethylene
polymer may be a so-called fine powder obtained by
emulsion polymerization or a so-called molding powder
obtained by suspension polymerization. Among them, it is
preferred to employ a powder having a surface area of
from 3 to 30 m2/g, more preferably from 6 to 20 m2/g.
The tetrafluoroethylene polymer may be a homopolymer of
tetrafluoroethylene or a copolymer of tetrafluoroethylene
containing preferably at most
5 molar ~ of a monomer exemplified by the formula:
CF2=CF, CF2=CF or CF2=CFCl
Rf ORf
where Rf is a perfluoroalkyl group having from l to 10
carbon atoms, as the comonomer unit.
The molecular weight of the tetrafluoroethylene
polymer used in the present invention is not critical.
However, from the viewpoint of the reinforcing effect,
the molecular weight is preferably at least 2,000,000.
The amount of the fibrillated tetrafluoroethylene
polymer to be incorporated in the first film of the
-~- 12S6()6Z
-- 7 _
fluorinated polymer may vary depending upon the
mechanical strength desired which differs depending upon
the ion exchange capacity of the fluorinated polymer.
But it is usually preferred to incorporate the
fibrillated fluoroethylene polymer in an amount of from
0.5 to 15~ by weight, more preferably from 0.8 to 8~ by
weight, based on the total amount of the first film. If
the amount is too small, no adequate improvement of the
mechanical properties will be obtained. On the other
hand, if the amount exceeds the above range, there will
be drawbacks such as the formation of bubbles in the
membrane during the electrolysis or deterioration in the
fabricability into a film.
- Various methods may be employed for the incorporation
of the fibrillated tetrafluoroethylene polymer into the
film for the cation exchange membrane. However, from the
viewpoints of the operability, the economy and the
properties of the cation exchange membrane obtained, it
is preferred to employ a method wherein the
non-fibrillated polymer is mixed with the film-forming
fluorinated polymer and a shearing stress is applied
during or after the mixing operation to fibrillate the
polymer. The fibrillation is conducted by exerting a
shearing stress to the tetrafluoroethylene polymer, e.g.
by subjecting the mixture containing the
tetrafluoroethylene polymer to roll kneading. For the
fibrillation, it is possible to employ a mixing or
kneading method commonly used for mixing or kneading
12S6~62
plastics, such as kneading between a pair of rolls, kneading by
means of a Banbury* mixer or kneading by means of a monoaxial or
biaxial extruder. The shearing stress is preferably at least 105
dyn/cm, the shear rate is preferably at least 0.1 sec~l, and the
temperature is preferably at least 70C, more preferably at least
100C and at most the melting point of the tetrafluoroethylene
polymer. The time for the mixing or kneading is not critical,
but is usually from 1 minute to 1 hour.
The mixing of the tetrafluoroethylene polymer with the
film-forming fluorinated polymer may be conducted at the same
time as the fibrillation of the tetrafluoroethylene polymer, or
the two polymers may initially be mixed by dry mixing or wet mix-
ing and the mixture thereby obtained is kneaded between a pair of
rolls for fibrillation of the tetrafluoroethylene polymer. If
necessary, after the fibrillation, the film may be subjected to
heat treatment at a temperature of not higher than the melting
point of the tetrafluoroethylene polymer to remove the internal
strain.
By the incorporation of the fibrillated tetrafluo-
roethylene polymer into the first film, the tensile strength, the
bending strength, the abrasion resistance and the anti-edge tear
strength of the membrane thereby obtained will be remarkably
improved. Further, according to the present invention, the tear
strength and stiffness of the membrane can further be
*Trademark
125~Qt;2
g
improved by incorporating a woven or non-woven fabric,
such as a cloth or a net, of a fluorinated polymer in
addition to the fibrils. Such a woven or non-woven
fabric may be made of a fluorinated polymer which is the
same or similar to the one constituting the fibrils. The
thickness and the mesh size of the woven or non-woven
fabric are preferably from 50 to 500 ~m and from 15 to 50
mesh respectively. The woven or non-woven fabric may be
incorporated into the first film under pressure and
heating prior to or subse~uent to the lamination of the
first film with the second film.
In the present invention, various fluorinated
polymers may be used as the fluorinated polymer for the
second film which has carboxylic acid groups as the ion
exchange groups. Among them, it is preferred to use
polymers having the following repeating units (A) and
(B).
(A) ~ CF2--CXX')
(B) ( CF2-CX--~----
Y-COOM
Where X, X', Y and M are as defined above. Among
them, perfluoro copolymers are preferred. For instance,
- there may be mentioned a copolymer of CF2=CF2 and
CF2=CFO(CF2)1 5 COOCH3 or a copolymer of CF2=CF2 and
25 CF2=cFocF2cF(cF3)o(cF2)l-5 3
In order to maximize the performance of the ion
exchange membrane by the combination of the first and
second films of fluorinated polymers according to the
125f~2
present invention, the first film is preferably made to have a
thickness of from 50 to 500,~m and the second film is preferably
made to have a thickness of from 5 to 50,~m, and the ratio of
the thickness of the first film to the thickness of the second
film is preferably selected to be from 2 to 50, more preferably
from 5 to 20.
In the present invention, it is preferred that the sec-
ond film be very thin, as mentioned above. Accordingly, the
reinforcing material is usually introduced to the first film
only. However, in some cases, it is also possible to incorporate
a fibril or a woven or non-woven fabric of a fluorinated polymer
into the second film.
For the lamination of the first and second films, an
optional method may be employed, and in any case, the two films
must be integrated by the lamination. For instance, the lamina-
tion is carried out by pressing or coextruding them preferably at
a temperature of from 100 to 350C under pressure of from 0.5 to
100 kg/cm~. In the present invention, in some cases, the first
film or the second film or both films may be a laminated film
composed of two or more different kinds of films. when these
films are laminated, the respective cation exchange groups should
take a suitable form not subject to decomposition. For instance,
in the case of carboxylic acid groups, they should preferably
take the form of an acid or an ester at the time of lamination,
and in the case of sulfonic acid groups, they should preferably
take the form of -S02F at the time of lamination. The thickness
of the cation exchange membrane obtained by the lamination is
preferably from 80 to 500/~m, more preferably from 100 to 300
~m.
The cation exchange membrane of the present invention
thus obtained by the lamination of the first and second films,
exhibits superior performance by itself. However, if desired, a
gas and liquid permeable porous layer comprising catalytically
-- 10 --
125~62
active particles (see, U.S. Patent No. 4,22~,121, etc.) or a gas
and liquid permeable porous layer comprising catalytically inac-
tive particles (see U.K. Published Patent Application No.
2,064,586, published June 17, 1981) may be provided on one side
or both sides of the membrane to further improve its performance.
The cation exchange membrane of the present invention
is useful for the electrolysis of various aqueous solutions, par-
ticularly an aqueous alkali metal chloride solution as mentioned
above. For instance, when used for the electrolysls of an aque-
ous alkali metal chloride solution, the cation exchange membrane
of the present invention is disposed so that the first film faces
the anode side and the second film faces the cathode side,
whereby the cation exchange membrane of the present invention
exhibits the maximum performance.
The electrolysis of an aqueous alkali metal chloride
~S6(~62
- 12 -
solution with use of the cation exchange membrane of the
present invention, may be conducted under such known
conditions as disclosed in the above-mentioned U.S.
Patent No. 4,367,126 and No. 4,319,969, etc. For
instance, while supplying an aqueous alkali metal
hydroxide solution of preferably from 2.5 to 5.0 N to the
anode compartment and water or a diluted alkali metal
hydroxide to the cathode compartment, the electrolysis is
conducted preferably at a temperature of from 80 to 120C
at a current density of from 10 to 100 A/dm2. In such
electrolysis, it is advisable to minimize heavy metal
ions such as calcium or magnesium in the aqueous alkali
metal chloride solution, as such heavy metal ions tend to
lead to degradation of the cation exchange membrane.
Further, in order to minimize the generation of oxygen at
the anode, an acid such as hydrochloric acid may be added
to the aqueous alkali metal chloride solution.
The electrolytic cell used in the present invention
~s
~,~ may be a monopolar or bipolar type ~e long as it has the
above-mentioned structure. The electrolytic cell used in
the electrolysis of an aqueous solution of an alkali
metal chloride, is made of a material bcing resistant to
the aqueous solution of the alkali metal chloride and
chlorine such as valve metal like titanium in the anode
compartment and is made of a material be~ resistant to
an alkali metal hydroxide and hydrogen such as iron,
stainless steel or nickel in the cathode compartment.
lZ56062
-- 13 --
When the electrodes are placed in the electrolytic
cell of the present invention, they may be disposed to
contact the ion exchange membrane, or they may be placed
with an appropriate space from the ion exchange membrane.
In the foregoing, the use of the membrane of the
present invention has been described primarily with
respect to the electrolysis of an aqueous alkali metal
chloride solution. However, it should be understood that
the membrane of the present invention is likewise
applicable to the electrolysis of water, a halogen acid
(hydrochloric acid or hydrobromic acid) or an alkali
metal carbonate.
Now, the present invention will be described with
reference to Examples which are provided for the purpose
1~ of illustration and are not intended to limit the present
invention.
EXAMPLE 1:
Into a 0.2 stainless steel pressure reactor, 100 g
of deionized water, 0.2 g of C8F17COONH4, 0.50 g of
Na2HPO4 12H2O, 0.29 g of NaH2PO4 2H2O, 0.079 g of
~NH4)2S2O8 and 0.04 g of NaHSO4 were fed, and then 30 g
of CF2=CFO(CF2)3COOCH3 was fed. After thoroughly
deaerating with liquid nitrogen, the temperature was
raised to 40C, and 5.1 kg/cm2 of tetrafluoroethylene was
introduced and reacted. During the reaction,
tetrafluoroethylene was continuously introduced into the
system to maintain the pressure at 5.1 kg/cm2. 7.5 Hours
later, the reaction was terminated and the obtained latex
lZSt;(~6Z
was coagulated by means of concentrated sulfuric acid. The poly-
mer thereby obtained was thoroughly washed with water, then
treated in methanol at 65C for 16 hours and dried to obtain 23.4
g of a copolymer having an ion exchange capacity of 1.80 meq/g.
To the copolymer, 5.5% by weight of PTFE particles (supplied
under the trademark Teflon 6J) were added, and the mixture was
kneaded at 130C by kneading rolls to fibrillate PTFE and then
pressed at 230C to form a film having a thickness of 200~ m.
Into 0.2 ~ stainless steel pressure reactor, 100 g of
deionized water, o.2 g of C8F17COONH4, o.49 g of NaH2PO4 2H2O,
O.052 g of (MH4)2S208 and 0.017 g of isopropanol were fed, and
then 20 g of CF2=CFO(CF2)3COOCH3 was fed. After deaerating with
liquid nitrogen, the temperature was raised to 60C, and 14.5
kg/cm2 of tetrafluoroethylene was introduced and reacted. During
the reaction, tetrafluoroethylene was introduced into the system
to maintain the pressure at 14.5 kg/cm2. 6 hours later, the
reaction was terminated, and the obtained latex was treated in
the same manner as above, whereby 19.6 g of a polymer having an
ion exchange capacity of 1.25 meq/g was obtained. The polymer
was pressed at 230C to form a thin film having a thickness of 30
m.
Then, the two types of films were laminated at 190C by
means of rolls to obtain a double-layered membrane. The membrane
was hydrolyzed at 90C for 16 hours in an aqueous solution con-
taining 25% by weight of sodium hydroxide.
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~.~
lZS6Q62
- 15 -
Then, a two compartment electrolytic cell was formed
by partitioning an anode and a cathode with such a cation
exchange membrane so that the layer having a higher ion
exchange capacity faces the anode side and the layer
having a lower ion exchange capacity faces the cathode
side. Electrolysis of sodium chloride was conducted
under the following conditions by using a RuO2/Ti
expanded metal as the anode and an active nickel/Fe
expanded metal as the cathode and with an electrode
distance of 3 mm. Namely, the electrolysis was carried
out at 90C under a current aensity of 20 A/dm2 at a pH
of the anolyte of 3 while supplying a 4N NaCl aqueous
solution to the anode compartment. As the results, the
cell voltage was 3.07 V and the current efficiency was
95.0% when the concentration of the formed sodium
hydroxide was 35~ by weight.
EXAMPLE 2:
Into a 2 ~ stainless steel pressure reactor, 1000 g
of CF2-CFO(CF2)3COOCH3 and 0-4 g of diisopropyl
peroxydicarbonate were fed. After thoroughly deaerating
with liquid nitrogen, the temperature was raised to 40C,
and 6.7 kg/cm2 of tetrafluoroethylene was introduced and
reacted. 10.5 ~ours later, the reaction was terminated,
and the obtained slurry was diluted with
trichlorotrifluoroethane. Then, the polymer was
precipitated in carbon tetrachloride. After the
reprecipitation, the precipitate was dried at 65C for 16
hours to obtain 146 g of a copolymer having an ion
~Z56~6Z
exchange capacity of 1.91 meq/g. To the copolymer, 9% by weight
of PTFE particles ~supplied under the trademark Teflon 6J) were
added, and the mixture was kneaded at 130C by kneading rolls to
fibrillate PTFE and then pressed at 230C to form a film having a 5 thickness of 200 ~m. Onto this film, the polymer film obtained
in Example 1, having an ion exchange capacity of 1.25 meq/g and a
thickness of 30/~ m was laminated.
Then, a paste was prepared by kneading a mixture com-
prising 10 parts by weight of zirconium oxide having an average
particle size of 7~m, 1 part by weight of PTFE particles, 0.3
part by weight of methyl cellulose ~a 2% aqueous solution), 14
parts by weight of water, 2 parts by weight of cyclohexanol and 1
part by weight of cyclohexanone. The paste was applied by screen
printing to one side of the above laminated membrane ~i.e. on the
polymer layers having an ion exchange capacity of 1.91 meq/g),
then dried and solidified to obtain a deposition of 1.0 mg/cm2.
Then, a paste prepared in the same manner as above except that
silicon carbide having an average particle size of 2 ,~m was
used, was applied on the other side of the laminated membrane
~i.e. on the polymer layer having an ion exchange capacity of
1.25 meq/g), then dried and solidified to obtain a deposition of
0.98 mg/cm2. The membrane was hydrolyzed in an aqueous solution
containing 25% by weight of sodium hydroxide, and then used for
the electrolysis in the same manner as in Example 1. Sodium
hydroxide having a concentration of
- 16 -
lZ56~)62
- 17 -
35~ by weight was obtained at a current efficiency of 94%
and at a cell voltage of 2.8 V.
EXAMPLE 3:
Into a 0.2 Q stainless steel pressure reactor, 70 g
f
CF3
and 0.11 g of diisopropylperoxydicarbonate were fed.
After thoroughly deaerating with liquid nitrogen, the
temperature was raised to 40C, and 9. 3 kg/cm2 of
tetrafluoroethylene was introduced and reacted. 4 Hours
later, 11.5 g of a copolymer having an ion exchange
capacity of 1.14 meq/g was obtained. To this copolymer,
6% by weight of PTFE particles were added and fibrillated
by kneading rolls, and the mixture was pressed at 230C
to form a film having a thickness of 150 ~m. Onto this
film, the film made of a copolymer of tetrafluoroethylene
with CF2=CFO(CF2) 3COOCH3 obtained in Example 1, having an
an ion exchange capacity of 1.25 meq/g and a thickness of
30 ~m, was laminated at 190C by means of rolls. The
laminated membrane was hydrolyzed at 90C for 16 hours in
an aqueous solution containing 25~ by weight of sodium
hydroxide, to obtain an ion exchange membrane. The
electrolysis was conducted in the same manner as in
example 1. Sodium hydroxide having a concentration of
35~ by weight was obtained at a current efficiency of 95%
and at a cell voltage of 3.08V.
~S6~)6Z
- 18 -
EXAMPLE 4:
PTFE cloth (39 mesh x 34 mesh woven cloth by 300
denier PTFE filament) was incorporated at 190C by means
of rolls in the 1.80 meq/g layer of the same laminated
membrane as in Example 1. The membrane was hydrolyzed at
90C for 16 hours in an aqueous solution containing 25
weight % by weight of sodium hydroxide. Then, the
electrolysis was conducted in the same manner as in
Example 1. Sodium hydroxide having a concentration of
35% by weight was obtained at a current efficiency of 95%
and at a cell voltage of 3.13V.
EXAMPLE 5:
Both surfaces of the same laminated membrane as in
Example 1 were roughened with a sand paper so as to have
coase regions of 1 to 5 microns depth. Then, the
membrane was hydrolyzed at 90C for 16 hours in a 25
weight % caustic solution. The electrolysis was
conducted in the same manner as in Example 1. Sodium
hydroxide having a concentration of 35% by weight was
obtained at a current efficiency of 94% and at a cell
voltage of 2.98V.
