Production of bipolar membranes

A bipolar membrane is produced by fastening an anion exchange membrane to a cation exchange membrane with the aid of an ion-permeable adhesive, by a process in which an aqueous solution of a polyvinylamine is used as the adhesive, and the bipolar membrane, which consists of two ion exchange membranes and the film-like adhesive, is subjected to electrodialysis.

The present invention relates to a process for the production of bipolar 
membranes by fastening an anion exchange membrane to a cation exchange 
membrane with the aid of an adhesive consisting of polyvinylamine. 
Bipolar membranes which are used, for example, for isolating acids or bases 
from their salts by electrodialysis are ion exchange membranes having 
fixed cations on one side and corresponding anions on the other side. 
Bipolar membranes are produced, for example, by firmly anchoring cationic 
or anionic groups to both sides of a neutral membrane by means of a 
chemical treatment (U.S. Pat. No. 4,057,481) or bringing an anion exchange 
membrane into close contact with a cation exchange membrane, for example 
by pressing the membranes on top of one another in the presence of heat 
(British Pat. No. 1,038,777). Attempts have been made to increase the 
stability of such bipolar membranes, obtained by combining anion exchange 
membranes with cation exchange membranes, by applying an ion-perm able 
adhesive between the two membranes. A polymerizable mixture of 
polyethyleneimine and epichlorohydrin (U.S. Pat. No. 2,829,095) or 
polyvinyl chloride and polyvinyl alcohol (Israel Journal of Chemistry, 9 
(1971), 485) has been proposed as an adhesive. 
Bipolar membranes are difficult to produce by the conventional methods. For 
example, in a chemical treatment of the surface, the two layers have to be 
of uniform thickness and must be in contact with one another over the 
entire surface in order to ensure the current flow. On the other hand, the 
layers must not penetrate one another since the membrane would then lose 
its bipolar selectivity. Although combining two monopolar membranes gives 
bipolar membranes possessing defined anionic and cationic layers, this 
method gives rise to difficulties at the contact surface. If the membranes 
are not completely in contact, the resistance increases. The same applies 
where the adhesive is not sufficiently conductive. Moreover, very 
undesirable tears or bubbles may form at the points of contact in bipolar 
membranes of the stated type under the operating conditions. 
Bipolar membranes which consist of the two individual membranes and 
polyvinyl alcohol as an adhesive are prepared according to Ber. Bunsenges. 
Phys. Chem. 68 (1964), 536, by a method in which the cation exchange films 
and the anion exchange films are coated with a polyvinyl alcohol solution, 
the films are laid one on top of the other and heated for one hour at 
60.degree. C., and the bipolar membrane is then dried, and compressed for 
30 minutes at 100.degree. C. Although the resulting bipolar membranes 
exhibit firm adhesion, their swellability in aqueous salt solutions is 
irreversibly restricted, and these bipolar membranes, which possess 
rectifying properties, are therefore unsuitable for electrodialysis. 
We have found that bipolar membranes suitable for electrodialysis are 
obtained from an anion exchange membrane, a cation exchange membrane and 
an ion-permeable adhesive if an aqueous solution of a polyvinylamine is 
used as the adhesive, and the bipolar membrane consisting of the two ion 
exchange membranes and the film-like adhesive is subjected to 
electrodialysis. 
The bipolar membranes can be produced using conventional ion exhcnage 
membranes, as described in, for example, K. S. Spiegler, Principles of 
Desalination, Acad. Press. New York, 1980, page 269. Membranes of this 
type are produced by, for example, copolymerization of styrene and 
divinylbenzene or butadiene, or of acrylonitrile and butadiene, the 
cations being attached firmly to the membrane by, for example, 
sulfochlorination, and the anions by chloromethylation and reaction with 
tertiary amines. The membranes are, for example, from 0.1 to 1 mm thick. 
The adhesive used is an aqueous polyvinylamine solution, an example of a 
suitable polyvinylamine being one in which the amino group may be 
substituted by alkyl of 1 to 4 carbon atoms and which has a molecular 
weight of from 10.sup.4 to 10.sup.6. For example, from 0.5 to 70, 
preferably from 3 to 15%, strength aqueous solutions of polyvinylamine are 
used. Solutions of the stated type are obtained, for example, by a 
conventional method, by acidic or alkaline hydrolysis of 
polyvinylformamide or of polyvinylacetamide with sodium hydroxide solution 
or hydrochloric acid. Aqueous solutions which are obtained by hydrolyzing 
polyvinylformamide with hydrochloric acid by a method in which, for 
example, a 1-50, preferably 5-20%, strength aqueous polyvinylformamide 
solution is treated with hydrochloric acid at from 60.degree. to 
100.degree. C. are particularly useful. The polyvinylamine solutions are 
still liquid and can be easily applied onto the membranes. 
The polyvinylamine solution is applied onto the membranes by brushing on or 
roller-coating, for example at from 10.degree. to 50.degree. C., adequate 
adhesion being achieved, for example, even when the adhesive is applied to 
only one of the two membranes. It is also possible to impregnate the 
membranes on both sides with the solution. The outer membrane surface is 
washed free of the adhesive during finishing of the bipolar membrane. The 
adhesive layer is, for example, from 0.001 to 0.05 mm thick. 
The bipolar membrane produced in this manner and consisting of the two ion 
exchange membranes and the film-like adhesive is strengthened by an 
electrodialysis treatment. The electrodialysis is carried out, for 
example, using a current of from 0.1 to 10, preferably from 0.5 to 5, 
A/dm.sup.2 for from 5 to 50, preferablyfrom 10 to 40, hours at from 
10.degree. to 80.degree. C., preferably from 20.degree. to 40.degree. C. 
The specific procedure is as follows: the preprepared pair of membranes is 
incorporated into the electrodialysis cell so that the cation exchange 
membrane faces the cathode, and the anion exchanger faces the anode. For 
example, an electrodialysis cell as required when the bipolar membrane is 
subsequently used and as illustrated in FIG. 3 can be employed. However, 
the membranes may also be subjected to the electrodialysis after-treatment 
in a separate unit, as illustrated in, for example, FIG. 1. A suitable 
electrolyte for the treatment of the membranes is an aqueous salt solution 
having a salt content of from 0.1 to 5, preferably from 0.5 to 2, moles/l. 
Examples of suitable salt solutions are sodium chloride, sodium acetate 
and sodium hydroxide solutions, but other water-soluble salts, bases or 
acids may also be used. The electrodialysis causes the membranes to adhere 
firmly to one another. After this treatment, they can no longer be 
separated from one another without being destroyed. 
The bipolar membranes obtainable by the novel process are resilient. 
Electrodialysis converts them to a form which would have been adopted by 
the individual membranes under the same conditions. Frequently the 
membranes have a slightly wavy shape; the anion exchange membrane and 
cation exchange membrane have matched up with one another in shape, 
without either of the two membranes exhibiting tears. The bipolar 
membranes have just as long a shelf life as their individual components 
and are generally stored in aqueous sodium chloride solution. 
The bipolar membranes according to the invention are particularly useful 
for electrodialysis processes. It is surprising that the membranes 
connected to one another by the process of the inention are adhesively 
bonded to one another uniformly and irreversibly by the electrodialysis 
after-treatment. Moreover, it was expected that the adhesive strengthened 
in this manner would hinder the current transport and water transport 
within the double membrane. Surprisingly, the bipolar membranes have 
proven resilient and completely usable even after prolonged use. 
Because of these advantageous properties, the bipolar membranes are very 
useful, for example, for cleaving salts back into acids and bases by 
electrodialysis, as described in, for example, J. Membrane Science, 2 
(1977), 109-124. In particular, they are very useful for converting 
aqueous solutions of salts of organic compounds to aqueous solutions of 
the free bases or acids from which the salts of the organic compounds are 
derived. Examples of starting material sfor this electrodialysis are salts 
of organic acids, such as carboxylic acids, hydroxycarboxylic acids or 
amino acids, phenols or salts of organic bases, such as amines or 
heterocycles, and betaines and quaternary ammonium compounds. The 
permeability to the organic ions of this type is very low, so that the 
electrodialysis takes place with high yields of materials and high 
selectivities. For example, from 0.1 to 10 molar aqueous solutions of the 
bases or acids are obtained in this electrodialysis.

EXAMPLE 1 
(a) The adhesive used was 5% strength by weight aqeuous polyvinylamine 
solution (K value 139) which is obtained by hydrolyzing polyvinylformamide 
with hydrochloric acid for 6 hours at 70.degree. C., using 1.22 moles of 
HCL pre kg of polyvinylformamide. 
(b) The adhesive was applied onto five cation exchange membranes available 
commercially under the name Neosepta.RTM. CH-45t and five anion exchange 
membranes available commercially under the name Neosepta.RTM. ACH-45T, a 
thin coat being applied on one side of each membrane by roller coating. 
The membranes were laid one on top of the other with the adhesive-coated 
side and were scraped smooth. 5 bipolar membranes consisting of 5 cation 
exchange membranes and 5 anion exchange membranes available commercially 
under the anmes Selemion.RTM. CMV and Selemion.RTM. AMV were produced in 
the same manner. 
(c) The prepared bipolar membranes obtained as described in paragraph b) 
were incorporated, without further treatment, in an electrodialysis 
apparatus as shown schematically in FIG. 1. The apparatus contained two 
electrolyte compartments (1) with two platinum electrodes (2) and two 
cation exchange membranes (3) available commercially under the name 
Nafion.RTM.. In the other compartments (4), which are separated from the 
electrolyte compartments (1) by the cation exchange membranes (3), the 10 
bipolar membranes (5) produced according to the invention were mounted, 
the polyamine solution being present between the ion exchange membranes (6 
and 7). In the bipolar membranes, the anion exchange membrane (6) faced 
the anode and the cation exchange membrane (7) faced the cathode. The 
individual bipolar membranes (5) were arranged separately from one another 
and fixed by PVC frames. The frames and membranes formed the electrolyte 
compartments, and the colution flowed through inlets in the frames. The 
same solution was passed through all inner compartments (4). The unit has 
the structure of an electrodialysis unit, except that the diluate and 
concentrate were identical. 
(d) 1500 parts of a 5% strength aqueous sodium sulfate solution were passed 
through the electrolyte compartments (1) to bathe the electrodes (2). A 5% 
strength aqueous sodium chloride solution was passed through the 
compartments (4) to bathe the bipolar membranes (5) to be adhesively 
bonded. Both solutions were circulated. The electrodialysis was carried 
out at room temperature and at a current density of 0.75 A/dm.sup.2 and 
was terminated after 38 hours. After this electrodialysis treatment, the 
bipolar membranes were found to be firmly bonded to one another and were 
resilient and had a slightly wavy shape. 
(e) Two pairs of membranes obtained as described in paragraph (b) were 
stored for 38 hours in a 5% strength aqueous sodium chloride solution but 
not subjected to electrodialysis. After this treatment, the membranes 
could readily be separated from one another. No adhesive bonding had taken 
place. 
EXAMPLE 2 
A bipolar membrane produced from the Selemion.RTM. membranes as described 
in Example 1, paragraph (b), was incorporated in an electrodialysis 
apparatus which is shown schematically in FIG. 2. This apparatus differed 
fromthe electrodialysis apparatus described in FIG. 1 in that it contained 
only one bipolar membrane (5) and two compartments (8 and 9) separated 
from one another. 
The electrolyte compartments (1) were flushed with 1500 parts of 5% 
strength aqueous sodium sulfate solution as described in Example 1, 
paragraph (d). Compartment (8) as well as compartment (9) was flushed 
separately with 1500 parts of a 10% strength aqueous sodium acetate 
solution. 
Electrodialysis was carried out for 20 hours at room temperature and at a 
current density of 3 A/dm.sup.2. The result was that 1395 parts of an 
aqueous acetic acid (0.86 mole/kg), corresponding to a current efficiency 
of 54%, were obtained in compartment (9), and 1694 parts of an aqeuous 
solution containing 0.60 mole of NaOH per kg (corresponding to a current 
efficiency of 45%) and 1.06 moles of sodium acetate per kg, were obtained 
in compartment (8). 
EXAMPLE 3 
The procedure described in Example 2 was followd, compartment (9) being 
charged with 100 parts of a 1 molar aqueous solution of sodium sarcosine 
and comaprtment (8) with 1000 parts of an aqueous solution containing 0.5 
mole of sodium acetate and 0.5 mole of acetic acid. The current density 
decreased from 3 A/dm.sup.2 to 0.7 A/dm.sup.2 after an experimental time 
of 12 hours. The result was that 900 parts of an aqueous sarcosine 
solution (1.04 moles/kg, corresponding to a current efficiency of 93% and 
a material yield of 95%) were obtained in compartment (9), and 1207 parts 
of an aqeuous sodium acetate solution (1.57 moles/kg) were obtained in 
compartment (8). 
EXAMPLE 4 
Experiment 2 was repeated in order to test the barrier action of the 
bipolar membranes against Na.sup..sym.. The experiment lasted for 11 hours 
and the current efficiency was 95%. The sodium content in compartment (9) 
decreased from 2.30% to 0.02%, and that in compartment (8) increased from 
2.30% to 3.70%. Accordingly, the membrane had a barrier action of &gt;95% up 
to a concentration difference corresponding to a factor &gt;150. 
EXAMPLE 5 
Electrodialysis was carried out similarly to Example 2, using 1000 parts of 
a one-molar aqueous trisodium citrate solution in compartment (9) and 1000 
parts of 0.1 molar sodium hydroxide solution in compartment (8). 940 parts 
of an aqeuous solution of citric acid (1.0 mole/kg, corresponding to a 
material yield of 94% and a current efficiency of 92%) were obtained in 
compartment (9), and 1264 parts of sodium hydroxide solution (1.72 moles 
of NaOH per kg, corresponding to a material yield of 69%) were obtained in 
compartment (8). 
The bipolar membrane was in use for a total of about 100 hours, in general 
a pH of &lt;7 prevailing on the cation exchange side and a pH of &gt;12 
prevailing on the anion exchange side. The abovementioned yields were also 
obtained at the end of this period. Although the membrane had assumed a 
slightly dark color, it showed no other change compared with its initial 
state. 
EXAMPLE 6 
The apparatus described in Example 2 was modified as shown in FIG. 3. A 
cation exchange membrane (3) was repalced with a Neosepta.RTM. ACH-45T 
anion exchange membrane (10). The bipolar membrane (5) was produced from a 
Neosepta.RTM. CH-45T cation exchange membrane and a Neosepta.RTM. ACH-45T 
anion exchange membrane, as described in Example 1, paragraph (b). The 
electrolyte compartments (1) were flushed with 1500 parts of a 5% strength 
sodium sulfate soltuion. Compartment (9) was charged with 1000 parts of a 
1 molar aqueous solution of .gamma.-aminobutyric acid hydrochloride 
(HCl.H.sub.2 N--(CH.sub.2).sub.3 --COOH), and compartment (8) was charged 
with 1000 parts of a 5% strength aqueous sodium chloride solution. 
Electrodialysis was carried out for 12 hours at room temperature using an 
initial current density of 3 A/dm.sup.2. 
An aqeuous solution of .gamma.-aminobutyric acid (0.95 mole/kg, 
corresponding to a material yield of about 90% and a current efficiency of 
about 75%) was obtained in compartment (9). The CL.sup..crclbar. content 
of this solution was less than 0.1 mole/kg. Compartment (8) contained 
hydrochloride acid having an HCL content of 0.43 mole/kg. 
EXAMPLE 7 
Electrodialysis was carried out for 20 hours as described in Example 6, 
1000 parts of a 1 molar aqueous tetrabutylammonium bisulfate soltuion 
being employed in compartment (9), and 1000 parts of a 5% strength aqueous 
solution of Na.sub.3 PO.sub.4.12 H.sub.2 O) in compartment (8). 
The result was that 994 parts of an aqueous soltuion of tetrabutylammonium 
hydroxide (0.88 mole/kg) and of bistetrabutylammonium sulfate (0.06 
mole/kg) corresponding to a material yield of 88% and a current efficiency 
of 88%, were obtained in compartment (9). After 10 hours, the solution in 
compartment (9) consisted virtually entirely of bistetrabutylammonium 
sulfate. 
EXAMPLE 8 
Electrodialysis was carried out for 20 hours as described in Example 6, 
1000 parts of a 1 molar aqueous solution of triethylammonium acetate being 
employed in compartment (9), and 1000 parts of a 5% strength aqueous 
sodium acetate solution being used in compartment (8). 
The result was that 807 parts of a 2-phase mixture of triethylamine and 
water were obtained in compartment (9). The total amount of triethylamine 
was about 1.1 mole/kg (material yield 89%, current efficiency 62%). 
Compartment (8) contained 1196 parts of an aqeuous acetic acid solution 
(0.70 mole/kg, material yield 84%) 
The bipolar membrane was tested for a total of 50 hours. After it has been 
removed, it was found to be uniformly and intimately bonded. If, on the 
other hand, the prepared membrane (without electrodialysis) has been 
stored in 5% strength NaCL, adhesive bonding of the membranes would not 
have resultled. 
EXAMPLE 9 
A bipolar membrane was produced as described in Example 1, paragraphs (a) 
and (b), using the cation exchange membrane available commercially 
(Ionics) under the name Type 61 CZL 386, and the anion exchange membrane 
available commercially under the name Type 103 QZL 386. 
The bipolar membrane prepared in this manner was used for the 
electrodialysis as described in Example 2, the apparatus being shown in 
FIG. 2. Electrodialysis was carried out for 10 hours at room temperature 
and at a current density of 2.9 A/cm.sup.2. 200 parts of an aqueous 0.25 
molar solution of disodium naphthalene-1,5-disulfonate were used in 
compartment (9), and 100 parts of 0.5% strength sodium hydroxide solution 
were employed in compartment (8). 1937 parts of a 0.24 molar solution of 
monosodium naphthalene-1,5-disulfonate (0.02 mole/kg of the disodium salt, 
current efficiency 50%) were obtained in compartment (9), and about 1000 
parts of 2.2% strength Sodium hydroxide solution were obtained in 
compartment (8). 
When the experiment was complete, the membranes were investigated. It was 
found that a smooth, bubble-free, strongly bonded bipolar membrane has 
formed, which could be separated into its individual components.