U.S. patent application number 16/091918 was filed with the patent office on 2019-04-25 for bipolar membrane.
This patent application is currently assigned to ASTOM Corporation. The applicant listed for this patent is ASTOM Corporation. Invention is credited to Kenji FUKUTA, Masayuki KISHINO, Kouta YUZUKI.
Application Number | 20190118144 16/091918 |
Document ID | / |
Family ID | 59351435 |
Filed Date | 2019-04-25 |
United States Patent
Application |
20190118144 |
Kind Code |
A1 |
KISHINO; Masayuki ; et
al. |
April 25, 2019 |
BIPOLAR MEMBRANE
Abstract
A bipolar membrane in which a cation-exchange membrane and an
anion-exchange membrane are joined to each other, wherein a leakage
ratio of gluconic acid at 60.degree. C. is not more than 1.0%, and
the cation-exchange membrane is supported by a polyolefin
reinforcing member and, further, contains a polyvinyl chloride.
Inventors: |
KISHINO; Masayuki;
(Shunan-shi, JP) ; YUZUKI; Kouta; (Shunan-shi,
JP) ; FUKUTA; Kenji; (Shunan-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ASTOM Corporation |
Tokyo |
|
JP |
|
|
Assignee: |
ASTOM Corporation
Tokyo
JP
|
Family ID: |
59351435 |
Appl. No.: |
16/091918 |
Filed: |
April 13, 2017 |
PCT Filed: |
April 13, 2017 |
PCT NO: |
PCT/JP2017/015180 |
371 Date: |
October 5, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 71/82 20130101;
C08J 5/22 20130101; C08J 2325/14 20130101; B01D 71/26 20130101;
C08J 5/2275 20130101; C08J 2427/06 20130101; B01D 69/125 20130101;
B01D 61/46 20130101; B01D 2323/46 20130101; B01J 47/12 20130101;
B01J 41/04 20130101; B01D 69/02 20130101; C08J 2325/08 20130101;
B01D 61/445 20130101; C08J 5/2287 20130101; C08J 2325/06 20130101;
B01D 67/0006 20130101; B01D 69/12 20130101; C08J 2423/06 20130101;
C08J 2325/18 20130101; B01D 2325/42 20130101; B01J 39/20 20130101;
B01J 41/14 20130101; B01J 39/04 20130101; C08J 5/2243 20130101;
C08J 5/2231 20130101; C08J 2423/12 20130101; B01D 69/10 20130101;
B01D 71/28 20130101; C08J 2353/02 20130101; B01D 71/30
20130101 |
International
Class: |
B01D 61/44 20060101
B01D061/44; B01J 47/12 20060101 B01J047/12; B01J 39/04 20060101
B01J039/04; B01J 39/20 20060101 B01J039/20; B01J 41/04 20060101
B01J041/04; B01J 41/14 20060101 B01J041/14; C08J 5/22 20060101
C08J005/22; B01D 61/46 20060101 B01D061/46; B01D 69/02 20060101
B01D069/02; B01D 69/12 20060101 B01D069/12; B01D 71/28 20060101
B01D071/28 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 14, 2016 |
JP |
2016-081450 |
Claims
1. A bipolar membrane in which a cation-exchange membrane and an
anion-exchange membrane are joined to each other, wherein a leakage
ratio of gluconic acid at 60.degree. C. is not more than 1.0%, and
said cation-exchange membrane is supported by a polyolefin
reinforcing member and, further, contains a polyvinyl chloride.
2. The bipolar membrane according to claim 1, wherein an area ratio
of a portion where the cation-exchange membrane and the
anion-exchange membrane are peeled off is not more than 20% after
the bipolar membrane is dipped in a 6N sodium hydroxide aqueous
solution of 25.degree. C. for one hour and then in pure water of
25.degree. C. for another one hour.
3. The bipolar membrane according to claim 1, wherein the
cation-exchange membrane contains said polyvinyl chloride in an
amount of 10 to 45% by mass.
4. A method of producing a bipolar membrane including a step of
forming a cation-exchange membrane, and a step of forming an
anion-exchange membrane on the surface of said cation-exchange
membrane; wherein the step of forming said cation-exchange membrane
includes: a step of impregnating a polyolefin reinforcing member
with a polymerizable composition obtained by mixing a polyvinyl
chloride (A) and a polymerization-curable component (B) that
contains a monomer (b1) having a cation-exchange group or a monomer
(b2) having a reaction group capable of introducing a
cation-exchange group; and a step of forming a membrane of a
cation-exchange resin that contains the polyvinyl chloride or of a
cation-exchange resin precursor resin by polymerization-curing said
polymerizable composition at a temperature of not lower than
100.degree. C.; and, further, as required, a step of introducing a
cation-exchange group into the cation-exchange resin precursor
resin.
5. The method of producing the bipolar membrane according to claim
4, further including: a step of impregnating a polyolefin
reinforcing member with the polymerizable composition obtained by
mixing the polyvinyl chloride (A) and the monomer (b1) having a
cation-exchange group; and a step of forming a membrane of the
cation-exchange resin that contains the polyvinyl chloride by the
polymerization curing at a temperature of not lower than
100.degree. C.
6. The method of producing the bipolar membrane according to claim
4, further including: a step of impregnating a polyolefin
reinforcing member with the polymerizable composition obtained by
mixing the polyvinyl chloride (A) and the monomer (b2) having a
reaction group capable of introducing a cation-exchange group; a
step of forming the membrane of said cation-exchange resin
precursor resin by the polymerization curing at a temperature of
not lower than 100.degree. C.; and a step of introducing a
cation-exchange group by acting a cation-exchange group introducing
agent upon said cation-exchange resin precursor resin.
7. The method of producing the bipolar membrane according to claim
4, wherein the step of forming the anion-exchange membrane on the
surface of said cation-exchange membrane includes: a step of
applying a polar organic solvent solution of the anion-exchange
resin on the surface of said cation-exchange membrane; and a step
of removing the polar organic solvent.
8. The method of producing the bipolar membrane according to claim
4, wherein the step of forming the anion-exchange membrane on the
surface of said cation-exchange membrane includes: a step of
applying, on the surface of said cation-exchange membrane, a polar
organic solvent solution of an anion-exchange resin precursor resin
having a reaction group capable of introducing the anion-exchange
group; a step of forming a membrane of said anion-exchange resin
precursor resin on the surface of said cation-exchange membrane by
removing the polar organic solvent; and a step of introducing the
anion-exchange group into said anion-exchange resin precursor
resin.
9. The method of producing the bipolar membrane according to claim
4, wherein the step of forming the anion-exchange membrane on the
surface of said cation-exchange membrane includes: a step of
applying, on the surface of said cation-exchange membrane, a polar
organic solvent solution that contains an anion-exchange resin
precursor resin having a reaction group capable of introducing the
anion-exchange group and an anion-exchange group introducing agent;
and a step of removing the polar organic solvent.
Description
TECHNICAL FIELD
[0001] This invention relates to a bipolar membrane obtained by
sticking a cation-exchange membrane and an anion-exchange membrane
together, and to a method of producing the same. More specifically,
the invention relates to a bipolar membrane having improved
adhesiveness between the cation-exchange membrane and the
anion-exchange membrane, and having an improved current efficiency,
and to a method of producing the same.
BACKGROUND ART
[0002] The bipolar membrane is a composite membrane obtained by
sticking a cation-exchange membrane and an anion-exchange membrane
together, and has a function of dissociating water into protons and
hydroxide ions. To utilize this special function, the bipolar
membrane is set in an electrodialyzer together with a
cation-exchange membrane and/or an anion-exchange membrane, and the
electrodialysis is executed to produce an acid and an alkali from a
neutral salt.
[0003] The above bipolar membrane requires a high degree of
adhesiveness, specifically, between the cation-exchange membrane
and the anion-exchange membrane. When put to use, for example, for
the electrodialysis for extended periods of time or under
high-temperature conditions, it is desired that the electrodialysis
can be executed while effectively preventing the membranes from
swelling, without permitting the membranes to be peeled off and
maintaining stability.
[0004] As the bipolar membrane that satisfies the above
requirements, for example, a patent document 1 discloses a bipolar
membrane whose at least either the cation-exchange membrane or the
anion-exchange membrane contains a chlorinated polyolefin. The
above bipolar membrane has a high adhesiveness between the
cation-exchange membrane and the anion-exchange membrane, and
features excellent stability at the time of electrodialysis. From
the standpoint of current efficiency, however, there still remains
a room for improvement.
PRIOR ART DOCUMENT
Patent Document
[0005] Patent document 1: JP-A-2010-132829
OUTLINE OF THE INVENTION
Problems that the Invention is to Solve
[0006] It is, therefore, an object of the present invention to
provide a bipolar membrane in which a cation-exchange membrane and
an anion-exchange membrane are firmly adhered together, and
exhibiting an excellent current efficiency even under
high-temperature conditions, and a method of producing the
same.
Means for Solving the Problems
[0007] According to the present invention, there is provided a
bipolar membrane in which a cation-exchange membrane and an
anion-exchange membrane are joined to each other,
[0008] wherein a leakage ratio of gluconic acid at 60.degree. C. is
not more than 1.0%, and the cation-exchange membrane is supported
by a polyolefin reinforcing member and, further, contains a
polyvinyl chloride.
[0009] In the bipolar membrane of the present invention, it is
desired that:
(1) An area ratio of a portion where the cation-exchange membrane
and the anion-exchange membrane are peeled off is not more than 20%
after the bipolar membrane is dipped in a 6N sodium hydroxide
aqueous solution of 25.degree. C. for one hour and then in pure
water of 25.degree. C. for another one hour; and (2) The
cation-exchange membrane contains the polyvinyl chloride in an
amount of 10 to 45% by mass.
[0010] According to the present invention, further, there is also
provided a method of producing a bipolar membrane including a step
of forming a cation-exchange membrane, and
[0011] a step of forming an anion-exchange membrane on the surface
of the cation-exchange membrane; wherein
[0012] the step of forming the cation-exchange membrane
includes:
[0013] a step of impregnating a polyolefin reinforcing member with
a polymerizable composition obtained by mixing a polyvinyl chloride
(A) and a polymerization-curable component (B) that contains a
monomer (b1) having a cation-exchange group or a monomer (b2)
having a reaction group capable of introducing a cation-exchange
group; and
[0014] a step of forming a membrane of a cation-exchange resin that
contains the polyvinyl chloride or of a cation-exchange resin
precursor resin by polymerization-curing the polymerizable
composition at a temperature of not lower than 100.degree. C.; and,
further, as required,
[0015] a step of introducing a cation-exchange group into the
cation-exchange resin precursor resin.
[0016] In the method of producing the bipolar membrane of the
present invention, it is desired that:
(1) The method of production includes a step of impregnating a
polyolefin reinforcing member with the polymerizable composition
obtained by mixing the polyvinyl chloride (A) and a monomer (b1)
having a cation-exchange group, and the step of forming a membrane
of the cation-exchange resin that contains the polyvinyl chloride
by the polymerization curing at a temperature of not lower than
100.degree. C.; and (2) The method of production includes a step of
impregnating a polyolefin reinforcing member with the polymerizable
composition obtained by mixing the polyvinyl chloride (A) and the
monomer (b2) having a reaction group capable of introducing a
cation-exchange group, a step of forming a membrane of the
cation-exchange resin precursor resin by the polymerization curing
at a temperature of not lower than 100.degree. C., and a step of
introducing a cation-exchange group by acting a cation-exchange
group introducing agent upon the cation-exchange resin precursor
resin.
[0017] In the production method of the invention, further, it is
desired that:
(3) The step of forming the anion-exchange membrane on the surface
of the cation-exchange membrane, further, includes a step of
applying a polar organic solvent solution of the anion-exchange
resin on the surface of the cation-exchange membrane, and a step of
removing the polar organic solvent; (4) The step of forming the
anion-exchange membrane on the surface of the cation-exchange
membrane, further, includes a step of applying, on the surface of
the cation-exchange membrane, a polar organic solvent solution of
an anion-exchange resin precursor resin having a reaction group
capable of introducing the anion-exchange group, a step of forming
a membrane of the anion-exchange resin precursor resin on the
surface of the cation-exchange membrane by removing the polar
organic solvent, and a step of introducing the anion-exchange group
into the anion-exchange resin precursor resin; or (5) The step of
forming the anion-exchange membrane on the surface of the
cation-exchange membrane, further, includes a step of applying, on
the surface of the cation-exchange membrane, a polar organic
solvent solution that contains an anion-exchange resin precursor
resin having a reaction group capable of introducing the
anion-exchange group and an anion-exchange group introducing agent,
and a step of removing the polar organic solvent.
Effects of the Invention
[0018] The present invention is concerned to a bipolar membrane
which uses a cation-exchange membrane as the exchange base membrane
(hereinafter also referred to as "cation-exchange base membrane").
A solution for forming an anion-exchange resin is applied onto the
surface of the cation-exchange base membrane, and the solvent is
removed from the layer that is applied in order to form an
anion-exchange membrane. The cation-exchange base membrane is the
one that is obtained by impregnating a polyolefin reinforcing
member with a polymerizable composition that contains a
polymerization-curable component for forming a cation-exchange
resin and a polyvinyl chloride, followed by curing by
polymerization. In the invention, an important feature resides in
that the cation-exchange base membrane contains the polyvinyl
chloride. This makes it possible to attain both a strong adhesion
between the anion-exchange membrane and the cation-exchange
membrane and an excellent electric current efficiency under a high
temperature condition (60.degree. C.). The reasons for this will
now be described in detail.
[0019] The polyvinyl chloride works as a resin for adhering the
cation-exchange base membrane and the anion-exchange membrane
together. Namely, the polyvinyl chloride exhibits a high degree of
affinity to the polymerization-curable component (B) (e.g.,
styrene, divinylbenzene, etc.) that is used for forming the
cation-exchange base membrane, to the cation-exchange resin (e.g.,
a resin having a specific skeleton that will be described later) or
to the precursor resin thereof (e.g., a precursor resin having a
specific skeleton that will be described later) and, besides, is
highly compatible with various polar organic solvents. It will,
therefore, be understood that the polyvinyl chloride has its high
molecular chain (specifically, amorphous portion) entangled with a
high molecular chain of the cation-exchange resin in the
cation-exchange base membrane, and forms a structure that cannot be
easily separated from the cation-exchange membrane. Besides, in
forming the anion-exchange membrane, the polyvinyl chloride present
in the cation-exchange base membrane migrates partly into the
solvent that forms the anion-exchange resin due to the polar
solvent contained in the solvent for forming the anion-exchange
resin. Therefore, the polyvinyl chloride is made present in the
interface between the cation-exchange base membrane and the
anion-exchange membrane in a form being engaged with both
membranes. As a result, the polyvinyl chloride exhibits a high
degree of anchoring effect, and the adhesiveness is markedly
improved between the two membranes. Here, if the polyvinyl chloride
is added to the anion-exchange membrane, then the polyvinyl
chloride does not migrate insufficiently into the cation-exchange
base membrane that has been polymerized and cured, and a strong
adhesiveness is not obtained.
[0020] Attempts had so far been made even with the conventional
bipolar membranes to improve the adhesiveness based on the
above-mentioned principle. Referring, for instance, to the bipolar
membrane of the patent document 1 obtained by containing the
chlorinated polyolefin in the cation-exchange base membrane,
adhesiveness had also been improved between the anion-exchange
membrane and the cation-exchange membrane relying on the anchoring
effect produced by the chlorinated polyolefin. However the bipolar
membrane of the patent document 1 was not capable of improving the
electric current efficiency.
[0021] Through the study of the conventional bipolar membranes, the
present inventors have discovered that with the cation-exchange
base membrane provided with a reinforcing member such as polyolefin
woven fabric, the junction was poor between the reinforcing member
and the cation-exchange resin, and gaps developed in the interface
between the reinforcing member and the cation-exchange resin
permitting, therefore, the anions such as hydroxide ions to pass
through the gaps and through the cation-exchange membrane during
the electrodialysis. Such unwanted motion of anions means that an
electric current flows without contributing to the hydrolysis
reaction at the time of electrodialysis and, therefore, forming the
acid and alkali in relatively decreased amounts despite of the
amount of electricity that is used causing, therefore, a decrease
in the electric current efficiency.
[0022] In order not to form the gap in the interface between the
reinforcing member and the cation-exchange resin, it can be
contrived to form a cation-exchange base membrane based on the
polymerization and curing under a condition of a temperature which
is so high that the reinforcing member partly melts so that the
reinforcing member and the cation-exchange resin are reliably
melt-adhered together. However, if this method is applied to the
bipolar membrane of the patent document 1, then the chlorinated
polyolefin and the cation-exchange resin undergo the separation in
phase in the cation-exchange base membrane, causing the chlorinated
polyolefin to agglomerate or, in other words, causing the
chlorinated polyolefin to be no longer dispersed homogeneously. As
a result, the chlorinated polyolefin is present in decreased
amounts in the interface between the anion-exchange membrane and
the cation-exchange membrane, the adhesiveness is lost, and the
strength of the membrane decreases.
[0023] In this regard, the present invention does not use the
chlorinated polyolefin but, instead, uses the polyvinyl chloride.
Even when the cation-exchange resin is formed by the polymerization
under such a high temperature condition that the polyolefin
reinforcing member partly melts, the polyvinyl chloride does not
undergo the phase separation and disperses homogeneously in the
cation-exchange resin. This is because the polyvinyl chloride
exhibits very higher affinity to the monomer component used for
forming the cation-exchange base membrane and to the
cation-exchange resin in the membrane and very higher compatibility
to various polar solvents than those exhibited by the chlorinated
polyolefin. This accounts for close adhesion between the polyolefin
reinforcing member and the cation-exchange resin without forming
gaps and for the presence of the polyvinyl chloride in sufficient
amounts in the interface between the anion-exchange membrane and
the cation-exchange membrane. As a result, the present invention
realizes both strong adhesion between the anion-exchange membrane
and the cation-exchange membrane and excellent electric current
efficiency. As for the electric current efficiency, specifically,
the Examples appearing later are demonstrating that when the
electrodialysis is executed under a high temperature condition
(60.degree. C.), the leakage ratio of gluconic acid is suppressed
to be not more than 1.0%, proving that anions such as gluconic acid
ions are effectively suppressed from passing through the
cation-exchange membrane.
MODES FOR CARRYING OUT THE INVENTION
[0024] As described earlier, the bipolar membrane of the present
invention uses a cation-exchange membrane as the exchange base
membrane, and has the anion-exchange membrane formed on one surface
of the cation-exchange base membrane. The cation-exchange base
membrane is supported by the polyolefin reinforcing member and
contains the polyvinyl chloride.
<Polyvinyl Chloride (A)>
[0025] As the polyvinyl chloride (A), there can be used any known
ones without limitation. For instance, there can be used not only a
homopolymer of vinyl chloride monomer but also copolymers
copolymerized with other monomers so far as they do not impair the
properties of the polyvinyl chloride or the object of the
invention. As other monomers that are copolymerizable, there can be
exemplified .alpha.-olefins such as ethylene, propylene, etc. and
vinyl esters such as vinyl acetate, etc. These polyvinyl chlorides
may be used alone or in a combination of two or more kinds, as a
matter of course.
[0026] The chlorine content of the polyvinyl chloride (A) is in a
range of, desirably, 30 to 80% by mass and, specifically, 55 to 70%
by mass. The polyvinyl chloride (A) having a chlorine content in
this range has a high degree of affinity to the polar solvent, and
works advantageously in the mechanism of adhesion.
[0027] From the standpoint of heat resistance, furthermore, it is
desired that the polyvinyl chloride has a high softening point,
e.g., has a Crash Berg flexible temperature (JIS K6734) of not
lower than 60.degree. C. and, more preferably, not lower than
65.degree. C. The polyvinyl chloride that lies within the above
range is capable of holding a high degree of adhesiveness even
under high temperature conditions. Besides, the polyvinyl chloride
has a low bipolar voltage, and does not cause the membrane to peel
off even during the electrodialysis under high temperature
conditions enabling, therefore, the electrodialysis to be executed
maintaining stability. The polyvinyl chlorides, in general, have
Crash Berg flexible temperatures of not higher than 70.degree.
C.
[0028] Though not specifically limited, the polyvinyl chloride (A)
has an average polymerization degree which usually lies in a range
of 500 to 3,000 and, specifically, 800 to 2,000. The longer the
molecular chain of the polyvinyl chloride, the larger the degree of
entanglement with the molecules of the cation-exchange resin and
the higher the degree of adhesiveness. However, too long molecular
chains cause a decrease in the solubility thereof in a solvent, and
migration into the anion-exchange resin decreases. As a result, the
entanglement becomes loose in the interface between the two
membranes, and the adhesiveness decreases between the two
membranes. If the average polymerization degree lies within the
above-mentioned range, an improved adhesiveness is achieved and,
therefore, a bipolar membrane is obtained having a low bipolar
voltage.
[0029] The polyvinyl chloride (A) can be put to use in a known form
such as powder, pellets or the like. Among them, however, the
powdery form is preferred and, more preferably, the powdery form
having an average particle size of 0.1 to 30 .mu.m as measured by
the laser diffraction light scattering method. The powdery
polyvinyl chloride has good affinity to the cation-exchange resin
(e.g., resin having a specific skeleton that will be described
later) or to a precursor resin thereof (e.g., precursor resin
having a specific skeleton that will be described later), and can
be homogeneously dispersed. The powdery polyvinyl chloride can be
obtained by a known suspension polymerization method.
[0030] In the present invention, the polyvinyl chloride (A) may be
contained in the cation-exchange base membrane. This is because, as
described above, the cation-exchange base membrane that is blended
with the polyvinyl chloride exhibits the effect for improving
adhesiveness more than the anion-exchange membrane. It is
allowable, of course, for the anion-exchange membrane to contain
the polyvinyl chloride in a range in which it does not impair the
properties of the bipolar membrane of the present invention.
[0031] Further, the polyvinyl chloride (A) is polymerized in a
state of being made compatible with the monomer (b1) having a
cation-exchange group, with the monomer (b2) having a reaction
group capable of introducing the cation-exchange group, or with a
crosslinking monomer. The polyvinyl chloride (A), therefore, is
present in a state of being entangled with the molecular chain of
the cation-exchange resin. As a result, the polyvinyl chloride (A)
is effectively prevented from separating, and a particularly
improved adhesiveness is obtained. It is, therefore, desired that
the polyvinyl chloride (A) is contained in the cation-exchange
membrane that contains the cation-exchange resin that is obtained,
specifically, by sulfonating a styrene-divinylbenzene copolymer.
This is because the styrene-divinylbenzene copolymer is obtained by
the polymerization of a monomer such as styrene having a very high
degree of affinity to the polyvinyl chloride or by the
polymerization of a crosslinking component such as
divinylbenzene.
[0032] The polyvinyl chloride (A) is contained in the
cation-exchange base membrane in an amount of, preferably, 10 to
45% by mass, more preferably, 15 to 35% by mass and, particularly
preferably, 20 to 30% by mass (on the dry weight basis). If the
amount of the polyvinyl chloride is too small, the effect is not
sufficient for improving the adhesiveness between the
cation-exchange base membrane and the anion-exchange membrane. If
the amount thereof is too large, on the other hand, the resistance
of the membrane so increases as to cause such an inconvenience as a
rise in the bipolar voltage.
<Production of the Cation-Exchange Base Membrane>
[0033] The invention can use a cation-exchange membrane known per
se. as the cation-exchange base membrane. In the invention, the
cation-exchange base membrane has a polyolefin reinforcing member
which imparts strength and heat resistance to the bipolar
membrane.
[0034] As the polyolefin, there can be exemplified homopolymers of
.alpha.-olefins, such as ethylene, propylene, 1-butene and
4-methyl-1-pentene, or random or block copolymers thereof.
Concretely, there can be exemplified low-density polyethylene,
high-density polyethylene, polypropylene, poly (1-butene) and poly
(4-methyl-1-pentene). Among them, low-density polyethylene,
high-density polyethylene and polypropylene are preferred. Further,
from the standpoint of easy availability and affinity to the
cation-exchange resin, the polyethylene type polymers are most
desirably used, such as low-density polyethylene and high-density
polyethylene.
[0035] The polyolefin reinforcing member may assume any form such
as woven fabric, nonwoven fabric or porous film but, preferably
assumes the form of the woven fabric from the standpoint of
strength. The filament of the woven fabric may be in the form of
either a multi-filament or a mono-filament. The mono-filament,
however, is preferred from the standpoint of strength. Moreover,
though it may be suitably selected depending on the use, the
polyolefin woven fabric has a thickness which is, usually, 50 to
500 .mu.m and, preferably, 100 to 300 .mu.m, and the filament has a
thickness of, specifically, 10 to 250 deniers (20 to 200 .mu.m)
from the standpoint of taking a balance between the strength and
the membrane resistance.
[0036] As the cation-exchange resin for forming the cation-exchange
membrane, there can be used any one that has been known per se. For
example, there can be used a resin having a specific skeleton or a
precursor resin that forms the specific skeleton and in which a
cation-exchange group has been introduced. As the precursor resin
for forming the specific skeleton, there can be exemplified
polymers obtained by polymerizing a monomer having an ethylenically
unsaturated double bond such as of vinyl type, styrene type or
acryl type, and copolymers thereof; and hydrocarbon type resins
having an aromatic ring on the main chain thereof, such as
polysulfone, polyphenylene sulfide, polyether ketone, polyether
ether ketone, polyether imide, polyphenylene oxide, polyether
sulfone, and polybenzimidazole.
[0037] There is no particular limitation on the cation-exchange
group if it is a reaction group that is capable of turning into a
negative charge in an aqueous solution. There can be exemplified
sulfonic acid group, carboxylic acid group and phosphonic acid
group. Usually, there is preferably used the sulfonic acid group
that is a strong acid group.
[0038] The cation-exchange base membrane may be produced according
to a method known per se. Representative methods include a method
of impregnating the polyolefin reinforcing member with a
polymerizable composition which contains the polyvinyl chloride (A)
and the polymerization-curable component (B) that contains the
monomer (b1) having a cation-exchange group, and forming a membrane
of the cation-exchange resin by polymerizing and curing the
polymerizable composition (hereinafter often called "method I"),
and a method of impregnating the polyolefin reinforcing member with
a polymerizable composition which contains the polyvinyl chloride
(A) and the polymerization-curable component (B) that contains the
monomer (b2) having a reaction group capable of introducing a
cation-exchange group, forming a membrane of the cation-exchange
resin precursor resin by polymerizing and curing the polymerizable
composition, and introducing a cation-exchange group into the
cation-exchange resin precursor resin (hereinafter often called
"method II").
[0039] The method I will be described, first. This method is
capable of forming a membrane of the cation-exchange resin which is
the same as the resin obtained by introducing a cation-exchange
group into the precursor resin that has the specific skeleton by
simply polymerizing and curing the polymerizable composition
without requiring the step of separately introducing the
cation-exchange group. Concretely, the polymerizable composition is
prepared by mixing together the polyvinyl chloride (A) and the
polymerization-curable components (B) for forming the
cation-exchange resin, such as the monomer (b1) having a
cation-exchange group, a crosslinking monomer and a polymerization
initiator. The polymerizable composition is filled in the gaps in
the polyolefin reinforcing member which is in the form of a woven
fabric and is, thereafter, polymerized and cured to form the
cation-exchange resin. There is thus obtained the desired
cation-exchange base membrane.
[0040] The polymerization-curing temperature is nearly so set that
the polyolefin reinforcing member melts. Though dependent upon the
kinds and the polymerization-curing times of the olefin and the
polymerization-curable component (B), the lower limit of the
polymerization-curing temperature is, usually, a melting point
minus 40.degree. C. and, preferably, a melting point minus
20.degree. C. of the polyolefin that constitutes the reinforcing
member, whereas the upper limit of the polymerization-curing
temperature is a melting point plus 20.degree. C. and, preferably,
a melting point plus 5.degree. C. thereof. Concretely, the lower
limit of the polymerization-curing temperature is 100.degree. C.
and, preferably, 110.degree. C. whereas the upper limit thereof is,
preferably, 160.degree. C. If the polymerization is executed at an
excessively low temperature, then gaps are made present in the
interface between the polyolefin reinforcing member and the
cation-exchange resin, and the current efficiency may decrease. If
the temperature is too high, on the other hand, the polyolefin
reinforcing member may once melt completely, and the strength of
the obtained cation-exchange base membrane may decrease
strikingly.
[0041] The monomer (b1) having the cation-exchange group contained
in the polymerization-curable component (B) may be the one that has
heretofore been used for producing the cation-exchange resin. For
example, there can be used sulfonic acid type monomers, such as
.alpha.-halogenated vinyl sulfonate, styrene sulfonate and vinyl
sulfonate; carboxylic acid type monomers, such as methacrylic acid,
acrylic acid and maleic anhydride; phosphonic acid type monomers,
such as vinyl phosphonate and the like; and salts and esters of the
above monomers.
[0042] The crosslinking monomer is used for densifying the
cation-exchange resin, for suppressing the swelling and for
increasing the membrane strength, and no specific limitation is
imposed thereon. There can be used, for example, divinylbenzene,
divinylsulfone, butadiene, chloroprene, divinylbiphenyl,
trivinylbenzenes, divinylnaphthalene, diallylamine and
divinylpyridine. The crosslinking monomer is used in an amount of,
preferably, 0.1 to 50 parts by mass and, more preferably, 1 to 40
parts by mass per 100 parts by mass of the monomer (b1) that has
the cation-exchange group.
[0043] In addition to the above-mentioned monomer (b1) having the
cation-exchange group and the crosslinking monomer, there may be
added, as required, other monomers capable of copolymerizing with
the above monomers. As the other monomers, there can be used
styrene, chloromethylstyrene, acrylonitrile, methylstyrene,
ethylvinylbenzene, acrolein, methyl vinyl ketone and vinylbiphenyl.
The amounts of the other monomers may differ depending on the
object of addition. Usually, however, the other monomers are added
in a total amount of 0 to 100 parts by mass per 100 parts by mass
of the monomer (b1) that has the cation-exchange group. When the
flexibility is to be imparted, in particular, the other monomers
are added in amounts of 1 to 80 parts by mass and, specifically, 5
to 70 parts by mass.
[0044] As the polymerization initiator, there can be used those
known per se. without any limitation. Concretely, there are used
organic peroxides such as octanoyl peroxide, lauroyl peroxide,
t-butylperoxy-2-ethyl hexanoate, benzoyl peroxide,
t-butylperoxyisobutylate, t-butylperoxylaurate,
t-hexylperoxybenzoate, di-t-butylperoxide and
1,3,3-tetramethylbutylhydroperoxide. The polymerization initiator
is used in an amount of, preferably, 0.1 to 20 parts by mass and,
more preferably, 0.5 to 10 parts by mass per 100 parts by mass of
the monomer (b1) that has the cation-exchange group.
[0045] The polymerizable composition is prepared by blending the
above polymerization-curable component (B) with the polyvinyl
chloride (A) so that the cation-exchange membrane that is finally
obtained will possess the above-mentioned composition. There is no
particular limitation on the method of adding the polyvinyl
chloride. The polyvinyl chloride may be stirred together with the
polymerization-curable component (B) at room temperature such that
a homogeneously mixed polymerizable composition is obtained. Or the
polyvinyl chloride may be stirred and mixed together with the
polymerization-curable component (B) at a temperature at which the
polymerization-curable component (B) does not undergo the
polymerization or, concretely, at a temperature of not higher than
50.degree. C.
[0046] The polymerizable composition may, as required, contain a
chlorinated polyolefin, a thickener and known additives.
[0047] As the chlorinated polyolefin, there can be exemplified a
chlorinated polyolefin that is closely described in, for example,
the patent document 1. Though dependent upon the kinds of the
cation-exchange base membrane and the anion-exchange membrane,
addition of the chlorinated polyolefin further increases the
adhesiveness. The amount of addition thereof is 0 to 10 parts by
mass per 100 parts by mass of the polyvinyl chloride (A).
[0048] As the thickener, there can be used saturated aliphatic
hydrocarbon type polymers such as ethylene-propylene copolymer and
polybutylene; styrene type polymers such as styrene-butadiene
copolymer and the like; and polyolefin powder having an average
particle size of not more than 10 .mu.m. By using the thickener,
the viscosity can be so adjusted as to effectively prevent the
dripping during the membrane-forming work.
[0049] Further, as the additives, there can be used plasticizers
such as dioctyl phthalate, dibutyl phthalate, tributyl phosphate,
styrene oxide, tributyl acetylcitrate, or alcohol esters of fatty
acid or aromatic acid; and hydrochloric acid-trapping agents such
as ethylene glycol diglycidyl ether and the like.
[0050] There is no specific limitation on the method of
impregnating the voids in the polyolefin reinforcing member with
the polymerizable composition. For instance, the polyolefin
reinforcing member is dipped in a vessel filled with the
polymerizable composition. Impregnation with the polymerizable
composition can also be executed by such a method as spray coating
or application by using a doctor blade in addition to the
dipping.
[0051] The polymerizable composition with which the polyolefin
reinforcing member is impregnated as described above is heated in a
polymerization apparatus such as heating oven, and is polymerized
and cured.
[0052] In the step of polymerization, in general, a method is
employed in which the polyolefin reinforcing member impregnated
with the polymerizable composition is held by a polyester film, and
the temperature thereof is elevated starting from the normal
temperature under a pressurized condition. The pressure is,
usually, about 0.1 to 1.0 MPa, and is produced by using an inert
gas such as nitrogen or by using a roll. Due to the application of
pressure, the polymerization is carried out in a state where an
excess of the polymerizable composition present on the outer
interface of the polyolefin reinforcing member is pushed into voids
in the polyolefin reinforcing member, effectively preventing the
occurrence of resin reservoirs.
[0053] Other polymerization conditions vary depending on the kind
of the polymerization-curable component (B), and should be suitably
selected from the known conditions and determined. The
polymerization temperature is set, as described above, to be such
that the polyolefin reinforcing member partly melts. Further, the
polymerization time is, usually, about 3 to about 20 hours though
dependent upon the polymerization temperature and the like. After
the completion of the polymerization and curing, there is obtained
a cation-exchange membrane, i.e., a cation-exchange base membrane
supported by the polyolefin reinforcing member.
[0054] Next, the method II will be described. According to this
method, the cation-exchange base membrane is formed by using a
polymerization-curable component for forming the cation-exchange
resin precursor resin instead of using the polymerization-curable
component for forming the cation-exchange resin used in the method
I. Concretely speaking, the cation-exchange base membrane is
prepared by blending the polymerizable composition with the monomer
(b2) having a reaction group capable of introducing the
cation-exchange group instead of the monomer (b1) having the
cation-exchange group. In this case, too, the cation-exchange base
membrane may be prepared in the same manner as in the method I that
uses the monomer (b1) having the cation-exchange group but
necessitating the step of introducing the cation-exchange group
that will be described later.
[0055] The monomer (b2) having the reaction group capable of
introducing the cation-exchange group may be the one that has
heretofore been used for producing the cation-exchange resin.
Examples thereof include styrene, vinyltoluene, vinylxylene,
ethylvinylbenzene, .alpha.-methylstyrene, vinylnaphthalene and
.alpha.-halogenated styrene.
[0056] In addition to the monomer (b2) having the reaction group
capable of introducing the cation-exchange group and the
crosslinking monomer, it is also allowable to use any other
monomers. As the other monomers, there can be used
chloromethylstyrene, acrylonitrile, acrolein and methyl vinyl
ketone.
[0057] The step of introducing the cation-exchange group is carried
out after the membrane of the cation-exchange resin precursor resin
is formed by polymerizing and curing the polymerizable composition.
In this step, the concentrated sulfuric acid, chlorosulfonic acid
or phosphoric acid is acted, as the cation-exchange group
introducing agent, upon the precursor resin that is obtained so as
to be sulfonated, chlorosulfonated or phosphoniated, or the
precursor resin that is obtained is hydrolyzed to introduce the
cation-exchange group into it. There is thus obtained the desired
cation-exchange base membrane.
[0058] In the present invention, it is desired to employ the method
II that uses the monomer (b2) having the reaction group capable of
introducing the cation-exchange group. This is because in the
present invention, an important feature resides in the formation of
the cation-exchange base membrane from the polymerizable
composition that comprises the polymerization-curable component (B)
and the polyvinyl chloride (A). Here, however, the polyvinyl
chloride (A) dissolves more in the monomer to which no cation group
has been introduced than in the monomer (b1) that has the
cation-exchange group.
[0059] In the invention, further, it is also possible to fill the
gaps of the polyolefin reinforcing member with a solution of a
cation exchange group-containing high molecular compound obtained
by dissolving a cation exchange group-containing high molecular
compound in a solvent instead of relying on the method that uses
the polymerizable composition for forming the above-mentioned
cation-exchange resin or the cation-exchange resin precursor resin.
However, the method that uses the polymerizable composition is
preferred since the polymerization and curing are executed at a
temperature near the melting point of the polyolefin reinforcing
member contributing to improving the adhesiveness between the
reinforcing member and the cation-exchange resin.
[0060] It is desired that the cation-exchange base membrane
prepared as described above has a thickness in a range of 10 to 500
.mu.m and, more preferably, 100 to 300 .mu.m. If the thickness is
too small, the strength of the exchange base membrane may greatly
decrease. If the thickness is too large, inconvenience may occur
such as an increase in the bipolar voltage.
[0061] The cation-exchange base membrane has a burst strength of,
usually, 0.1 to 3.0 MPa though dependent upon the thickness
thereof. Desirably, the thickness of the polyolefin reinforcing
member and the amount of the crosslinking monomer in the whole
monomers are so set that the burst strength of the cation-exchange
base membrane is 0.2 to 1.8 MPa.
[0062] It is desired that the cation-exchange base membrane has an
ion-exchange capacity of, usually, in a range of 0.1 to 4 meq/g
and, specifically, 0.5 to 2.5 meq/g from the standpoint of bipolar
membrane properties, such as voltage drop and current efficiency.
The membrane resistance, too, is, desirably, not larger than 10
.OMEGA.cm.sup.2 and, specifically, in a range of 1 to 5
.OMEGA.cm.sup.2.
<Forming the Anion-Exchange Membrane>
[0063] Next, an anion-exchange membrane is formed on the surface of
the cation-exchange base membrane formed as described above. Though
there is no specific limitation on the method of forming the
anion-exchange membrane, the following three methods can be
preferably employed.
(Precursor Resin Type Two-Step Method)
[0064] On the surface of the cation-exchange membrane, there is
applied a polar organic solvent solution of the anion-exchange
resin precursor resin having the reaction group capable of
introducing the anion-exchange group. The polar organic solvent is
then removed to form a membrane of the anion-exchange resin
precursor resin on the surface of the cation-exchange membrane.
Next, the anion-exchange group is introduced into the
anion-exchange resin precursor resin to thereby form the
anion-exchange membrane on the cation-exchange base membrane.
(Non-Precursor Resin Type One-Step Method)
[0065] A polar organic solvent solution of the anion-exchange resin
is applied onto the surface of the cation-exchange membrane. The
polar organic solvent is then removed, and an anion-exchange
membrane is formed through one step.
(Precursor Resin Type One-Step Method)
[0066] A polar organic solvent solution is applied onto the surface
of the cation-exchange membrane, the polar organic solvent solution
containing the anion-exchange resin precursor resin having the
reaction group capable of introducing the anion-exchange group and,
further, containing the anion-exchange resin introducing agent. The
polar organic solvent is then removed, and an anion-exchange
membrane is formed through one step.
[0067] If the polyvinyl chloride is to be contained in the
anion-exchange membrane, too, then the polyvinyl chloride should be
contained in the polar organic solvent solution that is used in the
above-mentioned three methods.
[0068] In the present invention, prior to forming the
anion-exchange membrane on the cation-exchange base membrane, it is
desired to subject the surface of the cation-exchange base membrane
(surface on the side on where the anion-exchange membrane is to be
formed) to the surface-roughening treatment. For instance, the
arithmetic mean surface roughness Ra is adjusted to be in a range
of 0.1 to 2.0 .mu.m and, specifically, 0.5 to 1.8 .mu.m. Upon
forming the anion-exchange membrane on the rough surface, the
density of the membrane can be increased and, therefore, the
anchoring effect can be increased. As a result, there is obtained a
bipolar membrane having improved adhesiveness between the
cation-exchange base membrane and the anion-exchange resin. The
arithmetic mean surface roughness Ra can be calculated by
processing the image on the surface that is photographed by using
an ultradeep profile microscope.
[0069] The surface can be roughened by a known method. For example,
the surface (to be joined) of the cation-exchange base membrane is
directly rubbed with a sand-paper, or a hard granular material such
as sand is blown thereto. Or, the cation-exchange base membrane may
be formed by impregnating the polyolefin reinforcing member with
the polymerizable composition as described above followed by the
polymerization and curing. In this case, at the time of producing
the cation-exchange membrane, the polymerizable composition is
polymerized and cured by being held by a rough surface-forming film
such as of a polyethylene terephthalate that is forming rough
surface as described above. Thereafter, the film is removed to
thereby form the rough surface.
[0070] As the organic solvent for forming the polar organic solvent
solution used in the above-mentioned three methods, there can be
used the one that does not affect the properties of the
cation-exchange base membrane forming the lower layer but that
accelerates the polyvinyl chloride (A) in the cation-exchange base
membrane to be partly migrated into the anion-exchange membrane.
Concretely, there can be used alcohol, ethylene chloride,
tetrahydrofuran, dimethylformamide and N-methylpyrrolidone. Among
them, the tetrahydrofurane or the dimethylforamide is particularly
preferred from the standpoint of accelerating the migration of the
polyvinyl chloride.
[0071] In the non-precursor resin type one-step method among the
above-mentioned three methods of forming the anion-exchange
membrane, the anion-exchange resin is the known one, e.g., is a
resin having a specific skeleton or a resin obtained by introducing
the anion-exchange group into a precursor resin that has a specific
skeleton. As the precursor resin having the specific skeleton,
there can be exemplified the same resins as those of the case of
the cation-exchange resin. There is no specific limitation on the
anion-exchange group if it is a reaction group capable of serving
as a positive electric charge in an aqueous solution. Examples
thereof include primary to tertiary amino groups, quaternary
ammonium salt group, pyridyl group, imidazole group and quaternary
pyridinium salt group. Usually, the quaternary ammonium salt group
and the quaternary pyridinium salt group which are strongly basic
groups are preferred.
[0072] In the precursor resin type two-step method and the
precursor resin type one-step method among the above-mentioned
three methods of forming the anion-exchange membrane, as the
anion-exchange resin precursor resin, there can be used a high
molecular compound having a monomer unit capable of introducing an
anion-exchange group, such as chloromethylstyrene, vinylpyridine
and vinylimidazole; and high molecular compounds into which has
been introduced a reaction group capable of introducing an
anion-exchange group such as chloromethyl group or bromobutyl group
into a styrene type elastomer, like
polystyrene-poly(ethylene-butylene)-polystyrene triblock copolymer,
polystyrene-poly(ethylene-propylene)-polystyrene triblock
copolymer, polystyrene-polyisoprene block copolymer, and
hydrogenated products thereof.
[0073] The concentration of the precursor resin in the solution may
be suitably set by taking the coating property into consideration.
Though there is no limitation, the concentration thereof is,
usually, 5 to 40% by mass.
[0074] As required, furthermore, the polar organic solvent solution
that contains the above-mentioned precursor resin can be blended
with a high molecular compound to adjust the properties of the
anion-exchange membrane. Specifically, there can be used a high
molecular compound to which no anion-exchange group has been
introduced in order to adjust the anion-exchange capacity and water
content of the anion-exchange membrane, to increase the water-proof
property of the exchange membrane and to suppress the swelling
thereof. As the high molecular compound, there can be exemplified
polystyrene, polystyrene-poly(ethylene-butylene)-polystyrene
triblock copolymer,
polystyrene-poly(ethylene-propylene)-polystyrene triblock
copolymer, polystyrene-polyisoprene block copolymer and styrene
type elastomers such as hydrogenated products thereof.
[0075] Described below are concrete examples of the anion-exchange
group introducing agent.
Aminating Agents:
[0076] Tertiary amines such as trimethylamine and trimethylamine;
and diamine compounds such as
N,N,N',N'-tetramethyl-1,6-hexanediamine,
N,N,N',N'-tetramethyl-1,3-propanediamine, and
N,N,N',N'-tetramethyl-1,2-ethanediamine.
Alkylating Agents:
[0077] Halogenated alkanes such as methyl iodide, ethyl iodide and
methyl bromide; and dibromoalkanes such as dibromobutane and
dibromohexane.
[0078] In the case of the precursor resin type two-step method, the
polar organic solvent solution containing the anion-exchange resin
precursor resin is applied onto the surface of the cation-exchange
membrane followed, as required, by drying. Thereafter, the
anion-exchange groups are introduced therein. The anion-exchange
groups can be introduced by acting the anion-exchange group
introducing agent on the anion-exchange resin precursor resin.
[0079] In the invention, the anion-exchange membrane is formed,
more preferably, by the non-precursor resin type one-step method.
This is because the process of production can be shortened and the
cost of production can be lowered. That is, when the anion-exchange
membrane is formed by the non-precursor resin type one-step method,
no step is required for introducing the anion-exchange groups.
Therefore, the productivity can be increased as compared to the
case where the two steps are required for forming the
anion-exchange resin precursor resin and for introducing the
anion-exchange groups.
[0080] As the method of forming the anion-exchange membrane, the
precursor resin type one-step method, too, is preferred. The
precursor resin type one-step method comprises preparing a polar
organic solvent solution that contains the anion-exchange resin
precursor resin and the anion-exchange group introducing agent, and
applying the polar organic solvent solution followed by drying.
Here, introduction of the anion-exchange groups into the
anion-exchange resin precursor resin takes place during the period
of from when the solution is prepared till when it is dried. Since
the anion-exchange groups are introduced while the anion-exchange
membrane is being formed as described above, there is easily formed
the anion-exchange membrane into which the anion-exchange groups
have been introduced homogeneously.
[0081] In forming the anion-exchange membrane in the invention, a
crosslinked structure can be introduced into the anion-exchange
membrane. Concretely speaking, it is desired to use a diamine
compound such as N,N,N',N'-tetramethyl-1,6-hexanediamine or to use
the dibromoalkane such as dibromobutane or dibromohexane, which is
effective in improving water-proof property of the anion-exchange
membrane and in suppressing the swelling thereof. Specifically
preferably, however, the diamine compound such as
N,N,N',N'-tetramethyl-1,6-hexanediamine is used from the viewpoint
of easy handling.
[0082] The anion-exchange membrane produced as described above has
a thickness that lies in a range of, preferably, 1 to 200 .mu.m.
Like the cation-exchange base membrane, the anion-exchange membrane
has an ion-exchange capacity, usually, in a range of 0.1 to 4 meq/g
and, specifically, 0.5 to 2.5 meq/g from the standpoint of bipolar
membrane properties. Therefore, the amount of applying the polar
organic solvent solution containing the anion-exchange resin or the
precursor resin thereof, the composition of the precursor resin
(ratio of content of monomer units having the reaction groups) and
the amount of the anion-exchange group introducing agent, are so
set that the above-mentioned thickness and the anion-exchange
capacity can be realized.
[0083] In the invention, it is allowable, as required, to suitably
employ a known method of lowering the bipolar voltage by
introducing ions of a heavy metal (e.g., ions of iron, tin,
chromium or ruthenium) having a catalytic action for hydrolysis, an
oxide of the heavy metal or a tertiary amine into the surface of
the cation-exchange base membrane (surface on the side where the
anion-exchange membrane is to be formed) prior to forming the
anion-exchange membrane. From the standpoint of catalytic action,
it is desired that the content of the heavy metal in the bipolar
membrane is in a range of 1 to 5,000 mg/m.sup.2 and, preferably, 5
to 1,000 mg/m.sup.2. Specifically, preferably, there are used tin
ions, ruthenium ions or an oxide of tin or ruthenium from the
standpoint of not being dissolved in acid or alkali and low
toxicity.
[0084] After the anion-exchange membrane has been formed, a heat
treatment may be suitably executed. This enables the anion-exchange
membrane to bite into the rough surface of the cation-exchange base
membrane. As a result, adhesiveness or the strength of junction is
greatly improved between the cation-exchange base membrane and the
anion-exchange membrane. It is desired that the heat treatment is
executed at a temperature higher than, for example, the softening
point of the polyolefin reinforcing member in the exchange base
membrane. In order to improve the anchoring effect due to the rough
surface, furthermore, it is desired that the heat treatment is
executed under a pressurized condition while, for example, holding
the membranes between the steel plates heated in the
above-mentioned temperature range or passing the membranes through
the rollers.
<Bipolar Membrane>
[0085] In the bipolar membrane of the invention produced as
described above, at least the cation-exchange base membrane is
blended with the polyvinyl chloride (A). Therefore, the
cation-exchange base membrane and the anion-exchange membrane are
joined together maintaining a high degree of adhesiveness.
[0086] Concretely speaking, as measured in Examples appearing
later, even after the bipolar membrane of the invention is dipped
in the 6N sodium hydroxide aqueous solution (25.degree. C.) for one
hour and, thereafter, in pure water (25.degree. C.) for another one
hour, the area ratio a portion where the cation-exchange membrane
and the anion-exchange membrane are separated away from each other
has been suppressed to be, generally, not more than 20% and,
preferably, not more than 10%.
[0087] With the bipolar membrane of the invention having such a
high degree of adhesiveness, the membranes are not peeled even when
put to use in the electrodialysis which, therefore, can be
continued for extended periods of time maintaining stability. In
the production of acids and alkalis, in particular, the bipolar
membrane can be employed under a wide range of production
conditions such as of temperatures and the like.
[0088] In the invention, furthermore, there is no need of providing
any particular adhesive layer between the cation-exchange base
membrane and the anion-exchange membrane. Therefore, there is no
need, either, of increasing the membrane voltage of the bipolar
membrane. As measured in Examples appearing later, therefore, the
bipolar voltage is, usually, suppressed to be not higher than 2.0 V
and, preferably, not higher than 1.5 V.
[0089] In the cation-exchange base membrane of the bipolar membrane
of the invention, further, no gap is present between the polyolefin
reinforcing member and the cation-exchange resin. Therefore, an
improved current efficiency is observed when the bipolar membrane
is put to the electrodialysis. This effect is not impaired in the
electrodialysis of even under high temperature conditions. In fact,
when the gluconic acid is to be produced by the electrodialysis
under high temperature conditions by using the bipolar membrane of
the present invention, the gluconic acid leakage ratio (60.degree.
C.) has been suppressed to be, usually, not more than 1.0% and,
preferably, not more than 0.7% as demonstrated in Examples
appearing later. On account of the same reasons, furthermore, the
hydrolysis efficiency is, usually, not less than 98% as measured in
a sodium hydroxide aqueous solution of 60.degree. C. and in a
hydrochloric acid aqueous solution.
EXAMPLES
[0090] Excellent effects of the invention will now be described by
using the following Examples.
[0091] In Examples and in Comparative Examples, properties of the
bipolar membranes were measured as described below.
1) Bipolar Voltage
[0092] There were formed 4-compartment cells of the following
constitution by using bipolar membranes (sample bipolar membranes)
prepared in Examples and in Comparative Examples and by also using
the Neosepta BP-1E (produced by ASTOM Co.) as a control bipolar
membrane. Bipolar voltages were measured by using these cells.
[0093] Positive electrode (Pt plate) (1.0 mol/L of NaOH)/control
bipolar membrane/(1.0 mol/L of NaOH)/sample bipolar membrane/(1.0
mol/L of HCl)/control bipolar membrane/(1.0 mol/L of HCl) negative
electrode (Pt plate)
[0094] The bipolar voltages were measured under the conditions of a
liquid temperature of 25.degree. C. and a current density of 10
A/dm.sup.2 via platinum wire electrodes placed holding the bipolar
membrane therebetween.
2) Hydrolyzing Efficiency
[0095] Platinum electrodes were provided in two compartments of a
glass cell having two compartments separated by a bipolar membrane
having an effective current-carrying area of 4.5 cm.sup.2. An
aqueous solution containing 0.8 mol/L of sodium hydroxide was fed
in an amount of 60 ml into the positive electrode compartment, and
an aqueous solution containing 0.8 mol/L of hydrochloric acid was
fed in an amount of 60 ml into the negative electrode compartment.
After having flown a direct current of 0.45 A at 60.degree. C. for
20 hours, the quantities of the acid and the base in the two
compartments were determined. The current efficiencies for forming
the acid and the base were calculated from the quantities of the
acid and the base that were formed, and an average value of the two
was regarded as a hydrolyzing efficiency of the bipolar
membrane.
3) Gluconic Acid Leakage Rate.
[0096] Platinum electrodes were provided in two compartments of a
glass cell having two compartments separated by a bipolar membrane
having an effective current-carrying area of 4.5 cm.sup.2. An
aqueous solution containing 2.0 mol/L of sodium gluconate was fed
in an amount of 50 ml into the positive electrode compartment, and
an aqueous solution containing 2.0 mol/L of sodium hydroxide was
fed in an amount of 50 ml into the negative electrode compartment.
After having flown a direct current of 0.45 A at 60.degree. C. for
one hour, the quantity of the gluconic acid in the negative
electrode compartment was determined. From the quantity of the
gluconic acid that was obtained, the current efficiency of the
gluconic acid that has permeated through was calculated and was
regarded to be a gluconic acid leakage rate.
4) Adhesiveness of the Bipolar Membrane
[0097] The bipolar membrane was dipped in an aqueous solution
containing 6.0 mol/L of sodium hydroxide of 25.degree. C. for one
hour, taken out therefrom, and was dipped again in pure water of
25.degree. C. for one hour. After taken out from the pure water,
the membrane was analyzed by using an image processing system
(IP-1000 PC manufactured by Asahi Engineering Co.), and the ratio
of an abnormal portion (that was blistered) in 1 cm.sup.2 of the
membrane was calculated as a peeled area (%).
5) Ion-Exchange Capacity of the Ion-Exchange Membrane
[0098] The ion-exchange membrane was dipped in an aqueous solution
containing 1 mol/L of HCl for not less than 10 hours.
[0099] Thereafter, in the case of the cation-exchange membrane,
counter ions of the ion-exchange groups were substituted for the
sodium ions from the hydrogen ions by using an aqueous solution
containing 1 mol/L of NaCl, and the quantity of free hydrogen ions
was determined by the potentiometric titration by using a sodium
hydroxide aqueous solution (A mol=A eq). In the case of the
anion-exchange membrane, on the other hand, counter ions were
substituted for the nitric acid ions from the chloride ions by
using an aqueous solution containing 1 mol/L of NaNO.sub.3, and the
quantity of free chloride ions was determined by the potentiometric
titration by using a silver nitrate aqueous solution (A mol=A eq).
The potentiometric titration was executed by using a potentiometric
titrator (COMT ITE-900 manufactured by Hiranuma Sangyo Co.).
[0100] Next, the same ion-exchange membrane was dipped in an
aqueous solution containing 1 mol/L of NaCl for not less than 4
hours. Thereafter, the ion-exchange membrane was dried at
60.degree. C. under a reduced pressure for 5 hours, and its dry
weight (Dg) was measured. From the above measured values, the
ion-exchange capacity of the ion-exchange membrane was found
according to the following formula,
Ion-exchange capacity [meq/g]=A[eq].times.1000/D[g]
6) Burst Strength of the Cation-Exchange Membrane
[0101] The cation-exchange membrane was dipped in an aqueous
solution containing 0.5 mol/L of NaCl for not less than 4 hours,
and was then washed with the ion-exchanged water to a sufficient
degree. Next, without drying, the membrane was measured for its
burst strength by using the Meullen burst strength tester
(manufactured by Toyo Seiki Co.) in compliance with the
JIS-P8112.
7) Membrane Resistance of the Cation-Exchange Membrane
[0102] The cation-exchange membranes prepared in Examples and in
Comparative Examples were each held in a 2-compartment cell having
platinum black electrode plates. The cell was filled with an
aqueous solution containing 0.5 mol/L of sodium chloride on both
sides of the cation-exchange membrane, and the resistance across
the electrodes of an AC bridge circuit (frequency of 1,000 cycles
per sec.) was measured at 25.degree. C. A difference between the
resistance across the electrodes in this case and the resistance
across the electrodes measured without installing the
cation-exchange membrane was recorded as a membrane resistance.
[0103] The cation-exchange membrane used for the above measurement
was the one that had been rendered, in advance, to be in an
equilibrium state in an aqueous solution containing 0.5 mol/L of
sodium chloride.
8) Wet Thickness of the Cation-Exchange Membrane
[0104] The cation-exchange membrane was dipped in an aqueous
solution containing 0.5 mol/L of NaCl for not less than 4 hours.
Thereafter, the water on the surfaces of the membrane was wiped
away with a tissue paper, and the membrane was measured for its
thickness by using a micrometer MED-25PJ (manufactured by Mitsutoyo
Co.).
[0105] 9) Arithmetic Mean Roughness (Ra) of the Surface of the
Cation-Exchange Membrane
[0106] By using an ultradeep profile microscope VK-8700
(manufactured by Keyence Corp.), the surfaces of the sample
membranes were observed through a 100-power objective lens. Images
were synthesized based on the light quantity data and the color
data of a CCD camera. At the same time, surface ruggedness data
were also obtained. A suitable place of a length of about 100 .mu.m
free of impurity was selected. A roughness curve f(x) was found
from the ruggedness data of the above place, and a mean roughness
Ra along the center line was found from the following formula (1).
The operation was repeated several times, and it was confirmed that
the error was within .+-.5%.
Ra = 1 f .intg. 0 l f ( x ) dx ##EQU00001##
10) Amount of Catalyst
[0107] The cation-exchange membrane that has been treated with a
catalyst was subjected to the X-ray fluorometric analysis to find a
molar ratio of sulfur element and catalyst element. The amount of
the catalyst was calculated from a ratio of the sulfur element
relative to the ion-exchange capacity.
Example 1
1. Preparation of the Cation-Exchange Base Membrane
[0108] The following polymerizable composition was prepared.
TABLE-US-00001 Styrene 58 parts by mass, Chloromethylstyrene 4
parts by mass, Divinylbenzene (purity: 57%, the remainder being 9
parts by mass, ethyl vinyl benzene) Acrylonitrile 29 parts by mass,
Di-t-butyl peroxide 1 part by mass, Tributyl acetylcitrate 17 parts
by mass, Ethylene glycol diglyciyl ether 1 part by weight, and 65
parts by mass. Polyvinyl chloride powder (Crash Berg flexible
temperature: 68.degree. C., chlorine content: 57%, average
polymerization degree: 1,060, average particle size: 1 .mu.m)
[0109] In the above polymerizable composition, a polyethylene woven
fabric (50 denier, mesh vertical:lateral 156:100/inch,
monofilament, filament diameter 86 .mu.m, melting point 125.degree.
C.) was dipped under the atmospheric pressure at 25.degree. C. for
10 minutes. Thereafter, the woven fabric was taken out from the
polymerizable composition and was coated on both sides thereof with
the Teijin Tetron Film (type S, polyethylene terephthalate)
manufactured by Teijin-Du Pont Film Co., 188 .mu.m in thickness as
a peeling material. The woven fabric was then polymerized by being
heated at 120.degree. C. for 5 hours under a nitrogen pressure of
0.3 MPa. One surface of the obtained membrane was treated with a
sand-paper; i.e., there was obtained the membrane having Ra=1.2
.mu.m. Next, the thus obtained membrane was dipped in a mixture of
sulfuric acid of a concentration of 98% and chlorosulfonic acid of
a purity of not less than 90% at a weight ratio of 1:1 maintaining
a temperature of 40.degree. C. for 60 minutes to thereby obtain a
sulfonic acid type cation-exchange membrane. The obtained
cation-exchange membrane possessed an ion-exchange capacity of 1.9
meq/g, a burst strength of 0.4 MPa and a membrane resistance of 3.0
.OMEGA.cm.sup.2. Further, the mass of the woven fabric was
subtracted from the dry mass of the cation-exchange membrane to
find the content of the cation-exchange resin, and the amount of
the polyvinyl chloride in the exchange base resin was found from
the composition ratio of the above amount of the cation-exchange
resin and the polymerizable composition to be 24%. The constitution
of the cation-exchange base membrane and properties thereof were as
shown in Table 1.
2. Imparting the Hydrolyzing Catalyst
[0110] The obtained cation-exchange membrane was dipped in an
aqueous solution containing 2.0 wt % of ruthenium chloride for 60
minutes. The cation-exchange membrane was then taken out therefrom
and was dried at 60.degree. C.
3. Preparation of a Solution for Forming the Anion-Exchange
Membrane
[0111] 100 Grams of a styrene type block copolymer comprising a
polystyrene segment (65% by mass) and a polyisoprene segment (35%
by mass) that has been hydrogenated was dissolved in 1,000 g of a
chloroform and to which were added 100 g of a chloromethylmethyl
ether and 10 g of tin chloride to prepare a reaction solution. The
reaction solution was stirred at 40.degree. C. for 15 hour. After
the stirring, methanol was added to the reaction solution, and the
precipitated solid material was picked up by filtering. The
obtained solid material was washed and was then dried. As a result,
there was obtained a chloromethylated styrene type block copolymer.
Next, the chloromethylated polystyrene of a molecular weight of
5,000 was mixed with the above chloromethylated styrene type block
copolymer to obtain a mixture containing the chloromethylated
polystyrene at a ratio of 40% by mass. The mixture was dissolved in
a tetrahydrofuran to obtain a solution containing 25% by mass of
the chloromethylated polymer. To the solution was added 8% by mass
of an N,N,N',N'-tetramethyl-1,6-hexanediamine to prepare a solution
for forming the anion-exchange membrane that contains the
anion-exchange resin precursor resin and the anion-exchange group
introducing agent.
4. Preparation of the Bipolar Membrane
[0112] The solution for forming the anion-exchange membrane
prepared as described above was applied onto the roughened surface
of the cation-exchange membrane, and was dried at 50.degree. C. for
30 minutes. As a result, there was obtained a bipolar membrane
having the anion-exchange membrane of a thickness of 70 .mu.m.
[0113] Separately, further, the solution for forming the
anion-exchange membrane that was the same as the one used for
forming the bipolar membrane, was applied onto a polyethylene
terephthalate film and was dried. The membrane formed on the
polyethylene terephthalate film was peeled off the film to thereby
obtain an anion-exchange membrane for measuring the ion-exchange
capacity. The anion-exchange membrane was measured for its
ion-exchange capacity to be 1.4 meq/g.
[0114] The obtained bipolar membrane was evaluated for its
adhesiveness, amount of catalyst, bipolar voltage, hydrolytic
efficiency and gluconic acid leakage rate (60.degree. C.). The
constitution of the bipolar membrane and the evaluated properties
thereof were as shown in Table 2.
Examples 2 to 5
[0115] Bipolar membranes were prepared according to the same
procedure as that of Example 1 but changing the kind and amount of
the polyvinyl chloride which was the polymerizable monomer used for
preparing the cation-exchange base membrane as shown in Table 1.
The constitutions and properties of the cation-exchange base
membranes were as shown in Table 1 while the constitutions and
properties of the bipolar membranes were as shown in Table 2.
Example 6
[0116] A bipolar membrane was obtained according to the same
procedure as that of Example 1 but treating the cation-exchange
membrane obtained in Example 1 with an aqueous solution containing
2.0 wt % of tin chloride (II) instead of treating the
cation-exchange membrane with the ruthenium chloride aqueous
solution. The constitution and properties of the bipolar membrane
were as shown in Table 2.
Example 7
[0117] Onto the roughened surface of the cation-exchange membrane
treated with the ruthenium chloride aqueous solution obtained in
Example 1, there was applied a solution obtained by dissolving, in
a tetrahydrofuran, 15 wt % of a partly aminated polystyrene having
a quaternary ammonium base-exchange capacity of 0.9 meq/g followed
by drying at room temperature. As a result, there was obtained a
bipolar membrane having a partly aminated polystyrene layer of a
thickness of 70 .mu.m. The constitution and properties of the
bipolar membrane were as shown in Table 2.
[0118] The partly aminated polystyrene was synthesized as described
below. First, a monomer mixture of styrene and chloromethylstyrene
at a molar ratio of 10:1 was copolymerized in toluene in the
presence of a benzoyl peroxide that was a polymerization initiator
at 70.degree. C. for 10 hours. The obtained reaction solution was
poured into methanol, and the precipitated
styrene-chloromethylstyrene copolymer was recovered. An
N,N,N',N'-tetramethyl-1,2-ethanediamine was acted on the
chloromethyl group of the copolymer to introduce quaternary
ammonium base, and there was obtained a partly aminated
polystyrene.
Example 8
[0119] The chloromethylated polymer solution obtained in Example 1
was applied onto the roughened surface of the cation-exchange
membrane treated with the ruthenium chloride aqueous solution
obtained in Example 1 followed by drying to thereby forma
chloromethylated polymer film of a thickness of 60 .mu.m.
Thereafter, the cation-exchange membrane having the
chloromethylated polymer film was dipped in a methanol solution of
an N,N,N',N'-tetramethyl-1,3-propanediamine (10% by mass) at
30.degree. C. for 50 hours. The cation-exchange membrane was,
thereafter, washed with water to a sufficient degree, and there was
obtained a bipolar membrane. The constitution and properties of the
bipolar membrane were as shown in Table 2.
Example 9
[0120] A bipolar membrane was prepared according to the same
procedure as that of Example 1 but changing the polyolefin
reinforcing member and the polymerization temperature for preparing
the cation-exchange base membrane into those shown in Table 1 and
using a 1,1,3,3-tetramethylbutylhydroperoxide as the polymerization
initiator. The constitution and properties of the cation-exchange
base membranes were as shown in Table 1 while the constitution and
properties of the bipolar membrane were as shown in Table 2.
Example 10
[0121] A bipolar membrane was prepared according to the same
procedure as that of Example 1 but changing the polyolefin
reinforcing member used for preparing the cation-exchange base
membrane into the one shown in Table 1. The constitution and
properties of the cation-exchange base membranes were as shown in
Table 1 while the constitution and properties of the bipolar
membrane were as shown in Table 2.
Comparative Example 1
[0122] A bipolar membrane was prepared according to the same
procedure as that of Example 1 but changing the polymerization
temperature to 80.degree. C. at the time of preparing the
cation-exchange base membrane and using a
t-butyl-2-ethylperoxyhexanoate as the polymerization initiator. The
constitution and properties of the cation-exchange base membranes
were as shown in Table 1 while the constitution and properties of
the bipolar membrane were as shown in Table 2.
Comparative Example 2
[0123] A bipolar membrane was prepared according to the same
procedure as that of Example 1 but preparing the cation-exchange
base membrane without adding the polyvinyl chloride that was the
polymerizable monomer. The constitution and properties of the
cation-exchange base membranes were as shown in Table 1 while the
constitution and properties of the bipolar membrane were as shown
in Table 2.
Comparative Example 3
[0124] A bipolar membrane was prepared according to the same
procedure as that of Example 1 but preparing the cation-exchange
base membrane by using a chlorinated polyethylene (average
molecular weight of 20,000, chlorine content of 66%) in an amount
as shown in Table 1 instead of using the polyvinyl chloride,
executing the polymerization at 80.degree. C. and using the
t-butyl-2-ethylperoxyhexanoate as the polymerization initiator. The
constitution and properties of the cation-exchange base membranes
were as shown in Table 1 while the constitution and properties of
the bipolar membrane were as shown in Table 2.
Comparative Example 4
[0125] A bipolar membrane was prepared according to the same
procedure as that of Example 1 but preparing the cation-exchange
base membrane by using the chlorinated polyethylene (average
molecular weight of 20,000, chlorine content of 66%) in an amount
as shown in Table 1 instead of using the polyvinyl chloride.
The constitution and properties of the cation-exchange base
membranes were as shown in Table 1 while the constitution and
properties of the bipolar membrane were as shown in Table 2.
TABLE-US-00002 TABLE 1 Properties of the cation-exchange
Cation-exchange membrane composition membrane *1 Ion- (parts
Polymerization exchange Burst Membrane *3 Adhesive by Reinforcing
temp. capacity strength resistance *2 Ra (% by resin mass) member
(.degree. C.) (meq/g) (MPa) (.OMEGA. cm.sup.2) (.mu.m) (.mu.m)
mass) Ex. 1 PVC1 65 PE1 120 1.9 0.4 3.0 210 1.2 24 Ex. 2 PVC2 120
PE1 120 1.5 0.5 4.9 198 1.2 34 Ex. 3 PVC2 200 PE1 120 0.7 0.5 8.8
190 1.2 42 Ex. 4 PVC1 45 PE1 120 2.2 0.4 2.8 215 1.2 18 Ex. 5 PVC1
25 PE1 120 2.4 0.4 2.5 224 1.2 12 Ex. 9 PVC1 65 PP 150 1.9 0.6 3.0
213 1.0 24 Ex. 10 PVC1 65 PE2 120 2.1 0.3 2.0 145 1.1 30 Comp. PVC1
65 PE1 80 1.9 1.3 2.7 215 1.2 24 Ex. 1 Comp. -- 0 PE1 120 2.6 0.4
2.0 236 1.2 -- Ex. 2 Comp. CPE 25 PE1 80 2.4 1.3 2.5 218 1.2 12 Ex.
3 Comp. CPE 25 PE1 120 2.4 0.4 2.5 218 1.2 12 Ex. 4 PVC1: Polyvinyl
chloride, chlorine content 57%, average polymerization degree 1060
PVC2: Polyvinyl chloride, chlorine content 57%, average
polymerization degree 1200 CPE: Chlorinated polyethylene, chlorine
content 66%, average polymerization degree 20,000 PE1: Polyethylene
woven fabric (monofilament, filament diameter 50 denier = 86 .mu.m,
mesh vertical:lateral 156/100/inch, melting point: 125.degree. C.)
PP: Polypropylene woven fabric (monofilament, filament diameter 50
denier = 87 .mu.m, mesh vertical:lateral 156/100/inch, melting
point: 165.degree. C.) PE2: Polyethylene woven fabric
(monofilament, filament diameter 30 denier = 67 .mu.m, mesh
vertical:lateral 100/100/inch, melting point: 125.degree. C.) *1:
Amount of the adhesive resin in the polymerizable composition used
for forming the cation-exchange base membrane *2: Thickness of wet
membrane *3: Content of the adhesive resin in the cation-exchange
membrane
TABLE-US-00003 TABLE 2 Anion-exchange membrane composition Catalyst
layer Ion- composition Properties of the bipolar exchange Amount of
membrane capacity Catalyst catalyst *2 *3 *4 *5 Resin species *1
(meq/g) species (mg/m.sup.2) (%) (V) (%) (%) Ex. 1 CMPS/CMSEPS
TMHDA 1.4 Ru 500 0 1.1 99.3 0.3 Ex. 2 CMPS/CMSEPS TMHDA 1.4 Ru 300
0 1.3 99.5 0.3 Ex. 3 CMPS/CMSEPS TMHDA 1.4 Ru 100 0 1.5 99.5 0.2
Ex. 4 CMPS/CMSEPS TMHDA 1.4 Ru 600 9 1.1 99.1 0.6 Ex. 5 CMPS/CMSEPS
TMHDA 1.4 Ru 650 19 1.0 99.0 0.9 Ex. 6 CMPS/CMSEPS TMHDA 1.4 Sn 550
0 1.1 99.3 0.3 Ex. 7 SCMS TMEDA 0.9 Ru 500 0 1.3 99.4 0.3 Ex. 8
CMPS/CMSEPS TMPDA 1.4 Ru 500 12 1.2 99.3 0.3 Ex. 9 CMPS/CMSEPS
TMHDA 1.4 Ru 400 0 1.2 99.2 0.7 Ex. 10 CMPS/CMSEPS TMHDA 1.4 Ru 450
0 1.0 99.0 0.6 Comp. CMPS/CMSEPS TMHDA 1.4 Ru 500 0 1.0 97.9 1.9
Ex. 1 Comp. CMPS/CMSEPS TMHDA 1.4 Ru 1000 100 5.1 97.8 1.7 Ex. 2
Comp. CMPS/CMSEPS TMHDA 1.4 Ru 700 0 1.2 97.2 3.0 Ex. 3 Comp.
CMPS/CMSEPS TMHDA 1.4 Ru 700 100 5.2 99.1 0.8 Ex. 4 CMPS:
Chloromethylated polystyrene CMSEPS: Chloromethylated styrene type
block copolymer SCMS: Styrene-chloromethylstyrene copolymer TMHDA:
N,N,N',N'-tetramethyl-1.6-hexanediamine TMEDA:
N,N,N',N'-tetramethyl-1.2-ethanediamine TMPDA:
N,N,N',N'-tetramethyl-1.3-propanediamine *1: Anion-exchange group
introducing agent *2: Peeled area *3: Bipolar voltage *4:
Hydrolyzing efficiency *5: Gluconic acid leakage ratio
* * * * *