Bipolar membrane and method for its production

Abstract: A bipolar membrane comprising a cation-exchange membrane closely adhered to an anion exchange membrane in which cation-exchange groups present at least at the adhered surface of the cation exchange membrane have been ion-exchanged with a heavy metal ion, has a water splitting current efficiency of not less then 80% and a water splitting membrane potential of not more than 2.0 V.

Description: The present invention relates to a novel bipolar membrane and a method for producing it. Bipolar membranes are widely used as diaphragms in electrolysis of water (water splitting) or as separating membranes in reclaiming acids and alkalis from aqueous solutions of salts that are products formed by neutralization between acids and alkalis, and as ion-exchange membranes known in the art. They have a basic structure comprised of a lamination of a cation-exchange membrane and an anion-exchange membrane, and various methods for their production have been proposed. The methods can be exemplified by a method in which a cation-exchange membrane and an anion-exchange membrane are laminated using a mixture of polyethyleneimine and epichlorohydrin and then bonded to each other by curing (Japanese Patent Publication No. 32-3962); a method in which a cation-exchange membrane is bonded to an anion-exchange membrane using an adhesive having the properties of exchanging ions (Japanese Patent Publication No. 34-3961); a method in which a paste material comprising vinyl pyridine and an epoxy compound is coated on the surface of a cation-exchange membrane, followed by exposure to radiation to obtain the product (Japanese Patent Publication No. 38-16633); a method in which a sulfonic acid polymeric electrolyte and an allylamine are adhered to the surface of an anion-exchange membrane, followed by exposure to ionizing radiation (Japanese Patent Publication No. 51-4113); (Japanese Laid-Open Patent Application No. 53-37190); and a method in which a polyethylene film is impregnated with styrene and divinylbenzene followed by polymerization to give a sheet-like material, which is then nipped between frames made of stainless steel, where one side thereof is sulfonated and thereafter the sheet is detached and the remaining side is chloromethylated followed by treatment for amination (U.S. Patent No. 3,562,139). The bipolar membranes obtained by these methods, however, have the problem that a membrane potential much higher (e.g., 2.5 V to 3.0 V, or higher) than the theoretical water-splitting membrane potential (0.83 V) is applied when attempting to split water, resulting in a high-power consumption. They can also never be satisfactory from the point of view of current efficiency. Electrochemica Acta, Vol. 31, No. 9, pp.1175-1176 (1986) reports a production method in which cation- and anion-exchange membranes whose surfaces have been coated with a solution comprising at least one kind of inorganic electrolyte selected from sodium tungstate, chromium nitrate, sodium metasilicate, ruthenium trichloride are put together and then pressed to give a bipolar membrane having a low water-splitting membrane potential. This bipolar membrane, compared with the above numerous bipolar membranes, is characterised by a low water-splitting membrane potential, but, only at the initial stage of use. Thus, this membrane is disadvantageous in that the water-splitting membrane potential increases in a relatively short time, bubbles or blisters may occur at the interface between the cation-exchange membrane and the anion-exchange membrane rendering it of no use, and also, depending on conditions of use, the bipolar membrane may separate into the cation-exchange membrane and the anion-exchange membrane thereby destroying utility. Also, it does not exhibit good currency efficiency and can not be said to be a bipolar membrane satisfactory for use on an industrial scale. WO-A-89/01059 discloses a bipolar membrane formed by conjoining treated anion and cation exchange membranes. The treatment comprises contacting the membranes with an aqueous solution of at least one cation prior to conjoining and contacting at least one face of the membranes with an aqueous alkaline solution. Bipolar membranes are also disclosed in EP-A-0,251,511 and FR-A-2,361,927. An object of the present invention is to provide a bipolar membrane that can be used on an industrial scale. Another object of the present invention is to provide a bipolar membrane that has a water-splitting current efficiency of not less than 80%, and preferably not less than 90%, has a water-splitting membrane potential of not more than 2.0 V, and preferably not more than 1.8 V, and also can be used for a long period of time. Still another object of the present invention is to provide a bipolar membrane comprised of a cation-exchange membrane on the surface of which a film comprising a polymer having anion-exchange groups is formed, thereby giving a peel strength of not less than 0.2 kg.f/25 mm. According to one aspect of the invention, there is provided a process for producing a bipolar membrane, which process comprises bringing a solution of a polymer having an anion-exchange group or a functional group capable of an conversion to an anion-exchange group in a solvent into contact with a surface of a cation-exchange membrane which has been ion-exchanged with a heavy metal ion, removing the solvent so as to form a film of the polymer on the surface of the cation-exchange membrane and, when necessary, converting the functional group into an anion-exchange group. According to a further aspect of the invention, there is provided a bipolar membrane comprising a cation-exchange membrane and an anion-exchange membrane closely adhered to each other at a peel strength of not less than 0.2 kg·f/25mm; the cation-exchange membrane having cation-exchange groups present at least at its adhered surface which have been ion-exchanged with a heavy metal ion and said adhered surface bearing a film comprising a polymer having anion-exchange groups; the bipolar membrane having a water-splitting current efficiency of not less than 80%, and a water-splitting membrane potential of not more than 2.0 V, and being obtainable by the process of the invention. There are no particular limitations on the cation-exchange membrane in a bipolar membrane of the present invention. Any conventional cation-exchange membrane can be used. For example, a cation-exchange membrane having an ion-exchange group such as a sulfonic acid group or a carboxylic acid can be used. In particular, taking account of the purpose of bipolar membranes, most preferred is a cation-exchange membrane having a sulfonic acid group with an exchange group dissociated even under acidic conditions. There are no particular limitations on the cation-exchange capacity of the cation-exchange membrane, and those conventionally known can be used. In general, it may be in the range of 0.5 meq/g to 3 meq/g, and preferably 1 meq/g to 2.5 meq/g. The cation-exchange membrane may be of a polymerized type, a homogeneous type or a nonhomogeneous type. Optionally, it may be provided with a reinforcing material (centre layer), and may be of any kind or form depending on the method by with it is prepared. Ion-exchange membranes comprising a cation-exchange membrane having therein a small proportion of an anion-exchange group may also be satisfactorily used as the cation-exchange membrane for the present invention so long as they have cation transport numbers of not less than 0.9. In the cation-exchange membrane used in the present invention, a counter ion of an ion-exchange group present at least at its adhered surface (i.e., the surface to which the anion-exchange membrane is adhered) is required to have been ion-exchanged with a heavy metal ion to give a heavy metal type membrane. Such a heavy metal (ion) referred to in the present invention may include elements (ions) of the atomic number of from 20 to 90, preferably from 22 to 50, and more preferably from 26 to 50, except for halogens and inert gas elements. In general, those preferably used may specifically include ions of iron (II, III), titanium (IV), tin (II, IV), zirconium (IV), palladium (II), and ruthenium (III). In particular, ions of iron, ruthenium and tin are effective. The cation-exchange membrane of a heavy metal type can be obtained using conventionally known ion-exchange methods. It can be commonly obtained by, e.g., a method in which a cation-exchange membrane is immersed in a solution of a heavy metal salt, and a method in which a solution of a heavy metal salt is coated or sprayed onto a cation-exchange membrane. The heavy metal ions present in such a heavy metal type cation-exchange membrane need not be distributed uniformly in the thickness direction (i.e., to the whole extent) of the membrane, but will be present at least on the side to which the anion-exchange membrane is put and adhered. The ion-exchanged heavy metal ions may be present usually in such a proportion that they comprise 0.001% to 100%, and particularly preferably 0.01% to 50%, of the total ion-exchange capacity. For the preparation of a bipolar membrane of the present invention, a cation-exchange membrane ion-exchanged with heavy metal ions will be bonded to an anion-exchange membrane so that the cation-exchange membrane and the anion-exchange membrane are closely adhered to each other at a peel strength of not less than 0.2 kg·f/25mm in a wet state. A peel strength smaller than the above value is not acceptable because it tends to allow separation of the constituent membranes when the bipolar membrane is used in a wet state as in water splitting, making its long-term use impossible. It enables bubbles or blisters to form at the interface between the constituent membranes resulting in a lowering of current efficiency per unit membrane surface area, and bringing about a gradual increase in membrane potential with use of the biopolar membrane for an increasing period of time, making it impossible to continue its industrial use. A bipolar membrane of the present invention prevents formation of blisters or bubbles at the interface between the cation-exchange membrane and anion-exchange membrane regardless of conditions of use. Although the mechanism by which blisters or bubbles occur in prior art bipolar membranes is not absolutely clear, the inventors for the present invention have confirmed that the causes differ depending on the production method. For example, in order to introduce heavy metal ions to the ion-exchange groups present at the adhered surface of the cation-exchange membrane, a method is often employed in which the cation-exchange membrane is immersed in a solution of a heavy metal salt or a solution of a heavy metal salt is coated or sprayed onto the cation-exchange membrane. In such a method, the resulting bipolar membrane may form blisters or bubbles at its interface when immersed in water, if the anion-exchange membrane is bonded in the state that unreacted heavy metal salt solution remains on the surface of the cation-exchange membrane. Hence, it is a very important factor in the above bipolar membrane that neither unreacted heavy metal salt solution nor water is present at the interface of the cation-exchange membrane and anion-exchange membrane. In the method described above, it is preferred to thoroughly wash and then well dry the adhered surface of the cation-exchange membrane following ion-exchange with the heavy metal ion, and thereafter bond it to the anion-exchange membrane. If blisters or bubbles occur at the interface, they not only cause separation at the interface between the cation-exchange membrane and anion-exchange membrane, but also cause an increase in the membrane potential. Hence, they must be prevented from occurring. A bipolar membrane of the present invention is also required to have a water-splitting current efficiency of not less than 80%, and preferably not less than 90%, and also have a water-splitting membrane potential of not more than 2.0 V, and preferably not more than 1.8 V. With regard to current efficiency, known bipolar membranes can also satisfy an efficiency of 80%. However, bipolar membranes capable of keeping a membrane potential of not more than 2.0 V for a long period of time are almost unknown. In respect of the membrane potential, some conventional bipolar membrane are known to have a membrane potential of about 1.0 V at the initial stage of their use. In almost all of them, however, this membrane potential gradually increases with the lapse of electrodialysis time to exceed 2.0 V in a very short time. The properties of the above bipolar membranes can not be achieved by controlling only one step in their production process. They must be achieved by control while correlating the various requirements with each other. For example, a bipolar membrane having a peel strength of not less than 1.0 kg·f/25mm can be obtained in the form of a laminated bipolar membrane by bonding a cation-exchange membrane to an anion-exchange membrane, or in the form of a single-film bipolar membrane comprising a single film to both surfaces of which different ion-exchange groups have been imparted. The bipolar membranes obtained by such methods tend to have a high water-splitting membrane potential and a low current efficiency. A bipolar membrane of the present invention can not be obtained by application of conventional methods. Rather, the bipolar membrane of the present inventions is obtainable by a process according to the invention, which process comprises bringing a solution of a polymer having an anion exchange group or a functional group capable of conversion to an anion-exchange group in a solvent into contact with a surface of a cation-exchange membrane which has been ion-exchanged with a heavy metal ion, removing the solvent so as to form a film of the polymer on the surface of the cation-exchange membrane and, when necessary, converting the functional group into an anion-exchange group. This method is hereinafter simply referred to as the polymer film method. The peel strength may be affected by the conditions under which the cation-exchange membrane is treated when the cation-exchange membrane is bonded to the anion-exchange membrane. For example, the peel strength is lower and also the resulting bipolar membrane has less favourable water-splitting performance when the polymer film is applied in the state that any unreacted heavy metal salts arising from the introduction of heavy metal ions remain on the cation-exchange membrane. On the other hand, when the polymer film is applied after the surface of the cation-exchange membrane has been washed and then dried, the peel strength becomes 0.3 to 0.5 kg·f/25mm or higher. Such a membrane is well suited for practical use. Moreover, as will be detailed later, a bipolar membrane making use of a cation-exchange membrane whose surface has been roughened and on which the polymer film is formed can give a peel strength of 1 kg·f/25mm or more, which is a highly satisfactory strength. Thus this is a particularly preferred embodiment. In order to keep the water-splitting membrane potential as low as not more than 2.0 V, and preferably not more than 1.8 V, it is a very important factor that the cation-exchange groups present at the adhered surface of the cation-exchange membrane to which the anion-exchange membrane is adhered have been ion-exchanged with the heavy metal ions. The heavy metal ions introduced as a result of such ion exchange are presumed to permeate into the cathode side with the lapse of time and disappear, when water is split using the bipolar membrane. On the basis of research by the inventors for the present application, it is presumed, however, that the heavy metal ions are released in part from the bipolar membrane with the lapse of time but a very small quantity of heavy metal ions remain at the interface, and the remaining heavy metal ions play a role in preventing the membrane potential from increasing. It is still unclear by what mechanism the heavy metal ions remain at the surface and how the heavy metal ions remaining in a very small quantity act so as to prevent the membrane potential increasing. It is presumed that water is split into OH - and H + in the vicinity of the heavy metal ions, which have been ion-exchanged to the ion-exchange groups of the cation-exchange membranp adhered to the anion-exchange membrane, and that the heavy metal ions can move with difficulty compared with H + so that H + preferentially moves in the cathode direction while the heavy metal ions present at the surface adhered to the anion-exchange membrane can remain for a long period of time. On the other hand, heavy metal ions present in the cation-exchange membrane at a region distant from the adhered surface are presumed to move little by little in the cathode direction by the influence of continually flowing H + ions until they are released from the cation-exchange membrane. The water in the vicinity of the heavy metal ions is affected by the heavy metal ions and the water bonds are weakened, so that the water can be separated into H + and OH - with ease by an application of potential. The heavy metal ions are required to be present only at the surface adhered to the anion-exchange membrane and hence it is presumed satisfactory for the heavy metal ions to be in a very small quantity. As will be detailed in Examples given later, the content of heavy metal ions at the interface of a bipolar membrane before use in water splitting can be confirmed by fluorescent X-ray measurement. After use in water splitting for 60 days, the heavy metal ions no longer remain in such a quantity that their presence can be confirmed by fluorescent X-ray measurement. Their presence can be confirmed however at this time by electron spectroscopy chemical analysis (ESCA). The heavy metal ions remain in such a state that their density distribution decreases towards the surface of the cation-exchange membrane opposite to the surface to which the anion-exchange membrane is adhered. Such a phenomenon has been revealed for the first time by the present invention. It can be said to be á surprising phenomenon that the heavy metal ions ion-exchanged to the ion-exchange groups at the interface remain in such a state they are preferentially distributed without release from the bipolar membrane even after use in water splitting. Moreover, it can be also said to be a surprising phenomenon that the heavy metal ions remaining in the state that they are preferentially distributed act to prevent the membrane potential increasing so that water splitting can be carried out at a low membrane potential over a long period of time. On the basis of such action, the bipolar membrane of the present invention enables water-splitting while keeping the current efficiency at not less than 80%, and usually not less than 90%. The polymer film method for preparation of a bipolar membrane according to the present invention will now be described below in detail. This method generally comprises coating the surface of the cation-exchange membrane with a solution prepared by dissolving in a solvent a polymer having an anion-exchange group or a functional group capable of ready conversion to an anion-exchange group, removing the solvent to form a film and if necessary converting the functional group into an anion exchange group. Such a polymer having an anion-exchange group or a functional group capable of ready conversion to an anion-exchange group may preferably be a thermoplastic resin soluble in a solvent suitable for preparation of a solution for forming the film. In order to maintain a high water-splitting efficiency and alkali resistance, the film may preferably be formed as a strongly basic polymer whose anion-exchange group is mainly comprised of a quaternary ammonium group. As for the thickness of such a film, an excessively large thickness results in an increase in the water-splitting membrane potential and an excessively small thickness results in a lowering of the current efficiency. Hence, it is preferable for the thickness to be 5 µm to 300 µm, and particularly 10 µm to 200 µm. There are no particular limitations on the polymer having an anion-exchange group for use in the present invention so long as it is capable of being dissolved in a solvent. Particularly preferred polymers can be exemplified by linear polymers including copolymers of trialkylvinylbenzyl ammonium with a vinyl compound such as styrene, vinyltoluene or acrylonitrile; polymers obtained by chloromethylation and then quaternization of polymers such as polysulfone, polystyrene, polyphenylene oxide, polyether ether ketone, an ethylene-styrene copolymer and a propylene-styrene copolymer; and blend polymers of polysulfone with a polyalkylvinylbenzyl ammonium salt. These linear polymers having an anion-exchange group should preferably have an ion-exchange capacity of 0.4 meq/g to 2.5 meq/g, and particularly 0.6 meq/g to 1.5 meq/g, because those having a large ion-exchange capacity make the film formed liable to be dissolved in water or swell, resulting in a lowering of the water-splitting current efficiency in the bipolar membrane, and those having a small ion-exchange capacity tend to bring about an increase in the water-splitting membrane potential because of an increase in electrical resistance. As for polymers having a functional group capable of ready conversion to an anion-exchange group for use in the present invention, those particularly suitable can be exemplified by polymers having a chloromethyl group, such as chloromethylated polystyrene, polychloromethylstyrene, chloromethylated polyphenylene oxide, chloromethylated polyether ether ketone, and a copolymer of chloromethylated ethylene or propylene with styrene. These polymers having a chloromethyl group may be used optionally together with an inert polymer that does not react with amino compounds, such as polysulfone, polystyrene or polyacrylonitrile, to form a polymer solution. Using this polymer solution, a film serving as a precursor of the anion-exchange membrane is formed on the surface of the heavy metal type cation-exchange membrane, which is then reacted with, e.g., a trialkylamine, a dialkylamine, N,N,N',N'-tetramethyl-1,6-hexamethylenediamine, N,N-dimethylpropanediamine or N,N-dimethylbutanediamine. Thus a film comprised of an anion-exchanger into which a quaternary ammonium salt group and a crosslink have been introduced can be formed. When preparing a bipolar membrane of the present invention, it is also possible to form the polymer film by reacting a polymer having an anion-exchange group or a functional group capable of being converted to an anion-exchange group and a polymer having two or more reactive groups, and if necessary introducing an anion-exchange group, so as to form a crosslinked structure. such a combination of polymers may preferably include, for example, a combination of a polymer having two or more tertiary amino groups with a polymer having two or more epoxy groups, and a combination of a polymer similarly having two or more tertiary amino groups and a polymer having two or more halomethyl group. Examples of suitable polymers having two or more tertiary amino groups are polydialkylvinylbenzylamines, polydialkylaminoethylstyrenes, polydimethyallylamines, N,N,N',N'-tetramethyl-1,6-hexamethylenediamine, N,N,N',N',N"-pentamethyldipropylenetriamine and N,N,N' N'-tetramethyl-1,3-trimethylenediamine. Examples of suitable polymers having two or more epoxy groups are bisphenol-A type diglycidyl ether, resorsin type diglycidyl ether, glycerol triglycidyl ether, novolac type polyglycidyl ether, vinylcyclohexane dioxide, and dicyclopentadiene dioxide. Examples of suitable polymers having two or more halomethyl group are polychloromethylstyrene and chloromethylated polyether ether ketone. A film comprised of a crosslinked ion-exchanger should preferably have an ion-exchange capacity of 0.3 meq/g to 2.0 meq/g, and particularly 0.5 meq/g to 1.5 meq/g, because those having a large ion-exchange capacity tend to become brittle making use in a bipolar membrane difficult, and those having a small ion-exchange capacity tend to bring about an increase in electrical resistance and also bring about an increase in the water-splitting membrane potential. The above polymer solution is prepared by dissolving the polymer as described above in an organic solvent exemplified by ethylene chloride, chloroform, tetrahydrofuran, dimethylformamide, N-methylpyrolidone and methylalcohol, with adjustment of its concentration and viscosity so as to be suitable for its coating on the surface of the heavy metal type cation-exchange membrane to form the desired film. As a specific method of film formation, the film can be obtained usually by a method comprising applying the polymer solution to the surface of the heavy metal type cation-exchange membrane by coating or spraying, followed by drying to remove the solvent, or a method comprising immersing in the polymer solution a heavy metal type cation-exchange membrane one side of which has been covered with an opaque film sheet, glass plate or the like, and drawing up the membrane, followed by a similar drying. In order to increase the peel strength of the bipolar membrane of the present invention and also achieve long-term electrolysis with the bipolar membrane the surface of the cation-exchange membrane to which the anion-exchange membrane is adhered may preferably be roughened.There are no particular limitations on the method of roughing the surface so long as the performance of the cation-exchange membrane is not impaired. In general, it is preferred to use a means of finely roughing the surface of the cation-exchange membrane by brushing it using sandpaper, fine sands or the like, or roughing the membrane surface by bringing it into contact with a guide roll having a rough surface, used when the membrane is rolled up in the process of manufacture. The degree of roughness on the surface of the cation-exchange membrane may be on the basis of a depth of usually 0,1 µ to 100 µ, and preferably 1 µ to 60 µ. The bipolar membrane of the present invention enables long-term water splitting to be carried out at a current efficiency of not less than 80%, usually not less than 90%, and at a membrane potential of not more than 2.0 V, and usually 0.9 V to 1.8 V. Hence, the power consumption for water splitting can be greatly decreased. In particular, under circumstances in which products from neutralization between acids and alkalis must not be discarded as environmental pollution sources, a bipolar membrane of the present invention may be usefully employed to reclaim acids and alkalis from the salts by electrolysis. To describe the present invention in greater detail, the present invention will be described below by giving Examples and Comparative Examples and with reference to Figure 1 which shows the proportion of Fe atoms and S atoms (Fe/S) measured by ESCA, present in a bipolar membrane according to the present invention. The present invention is by no means limited to these examples. In the Examples and Comparative Examples, the characteristics of the bipolar membranes were determined as follows: Using platinum plates as electrodes, a given bipolar membrane with an effective area of 10 cm 2 was provided between them. On the side of its cation-exchange membrane, 100 ml of an aqueous solution of 1.00N hydrochloric acid was fed, and on the side of its anion-exchange membrane 100 ml of an aqueous solution of 1.00N sodium hydroxide was fed. At a current density of 10 A/dm 2 , electric currents were supplied for 4 hours to 16 hours, and thereafter the quantities of acids, bases and salts in the solutions on either side of the membrane were measured. Thus the hydroxyl ion current efficiency η(OH), hydrogen ion current efficiency η(H), chloride ion current efficiency η(Cl) and sodium ion current efficiency η(Na) were determined as water-splitting current efficiency of the bipolar membrane. Wire platinum electrodes were also provided cross the bipolar membrane to measure the drop of membrane potential of the bipolar membrane. The peel strength was measured according to JIS K-6854. A tensile shear tester used therefor was TENSILON UTM-1-5000B (manufactured by TOYO BALDWIN CO., LTD.). Using this tester, the strength was measured on test pieces of 25 mm wide bipolar membranes in a wet state, under conditions of a temperature of 23°C, a relative humidity of 52% and a grip movement rate of 200 mm/min. Example 1 A cation-exchange membrane having sulfonic acid groups as ion-exchange groups NEOSEPTA CM-1 (trademark: available from TOKUYAMA SODA CO., LTD.) was previously rubbed with sandpaper to rough one side surface thereof. Thereafter the membrane was immersed in an aqueous solutions of 2% by weight of ferrous chloride at 25°C for 1 hour, followed by thorough cleaning with ionexchanged water and then air drying at room temperature. The cation-exchange membrane thus treated has an iron content of 98% of the total exchange capacity. It was possible to confirm the presence of iron also by fluorescent X-ray measurement. An aminated polysulfone formed by chloromethlation of polysulfone followed by quaternization using trimethylamine (ion-exchange capacity of the quaternary ammonium group: 0.92 meq/g) was dissolved in a mixed solvent of methanol and chloroform (volume ratio: 1:1) to prepare solution with a concentration of 15% by weight. Next, on the roughed surface of the above cation-exchange membrane thus treated, the solution of aminated polysulfone was coated, followed by drying to produce a bipolar membrane having a layer of the aminated sulfone with a thickness of 90 µm. With regard to the bipolar membrane thus obtained, its performances were measured to reveal that it had a peel strength of 5.1 kg . f/25mm, a water-splitting membrane potential of 1.2 V and current efficiencies of η(H) = 99.2%, η(OH) = 99.2%, η(Cl) = 0.3% and η(Na) = 0.5%. These performances of the bipolar membrane did not change even after the lapse of 2 months, and neither bubbles nor blisters were seen to occur in the membrane. With regard to the bipolar membrane after the lapse of 2 months, its peel strength was measured to reveal that it was 5.2 kg . f/25mm. With regard to the cation-exchange membrane having been separated, the presence of Fe ions on its adhered surface was measured by fluorescent X-ray measurement. The Fe ions, However, did not remain in such a quantity detectable by the fluorescent X-ray measurement. Now with regard to the same cation-exchange membrane, the presence of Fe ions was measured by ESCA. As a result, it was possible to confirm the presence of Fe ions remaining in the density distribution as shown in Fig. 1. Fig. 1 shows a proportion of Fe atoms and S atoms (Fe/S) measured by ESCA, the former being those remaining in the separated cation-exchange membrane from its surface to its interior in the thickness direction and the latter being those of the sulfonic acid groups. This Fig. 1 shows that the Fe ions are present with a density gradient from the surface (adhered surface) of the cation-exchange membrane toward its opposite surface. Comparative Example 1 A bipolar membrane was produced in entirely the same procedure as in Example 1 except that the cation-exchange membrane was not treated with the aqueous ferrous chloride solution. The bipolar membrane thus obtained had performances of a peel strength of 5.3 kg . f/25mm, a water-splitting membrane potential of 3.2 V and current efficiencies of η(H) = 99.3%, η(OH) = 99.3%, η(Cl) = 0.3% and η(Na) = 0.4%. These performances of the bipolar membrane did not change even after the lapse of 2 months, and neither bubbles nor blisters were seen to occur in the membrane. Comparative Example 2 The same cation-exchange membrane as used in Example 1 was immersed in an aqueous 2% ferrous chloride solution and then taken out. Next, an anion-exchange membrane NEOSEPTA AM-1 (trademark: available from TOKUYAMA SODA CO., LTD.) was superposed on the above cation-exchange membrane. The membranes thus put together were pressed in a temperature environment of 130°C under a pressure of 2 kg/cm 2 for 10 minutes. The bipolar membrane thus obtained caused blistering at the interface when immersed in water, resulting in separation into two membranes to have come to be of no use. Comparative Example 3 Comparative Example 2 was repeated except that the cation-exchange membrane as used therein was washed with water to remove the ferrous chloride present on the surface of the cation-exchange membrane, having not been ion-exchanged. Performances were measured on the resulting bipolar membrane to reveal that it had a peel strength of 0.01 kg . f/25mm, a water-splitting membrane potential of 1.2 V at the initial stage, which increased to 2.6 V after the lapse of 24 hours, and current efficiencies of η(H) = 99.0%, η(OH) = 99.0%, η(Cl) = 0.3% and η(Na) = 0.7%. This bipolar membrane caused bubbles at the interface of membrane after the lapse of 16 hours. This bipolar membranes had also so small a peel strength in a wet state that the bubbles occurred became larger after the lapse of 72 hours, so that it became substantially impossible to continue the experiment of electrolysis. Comparative Example 4 The same cation-exchange membrane and anion-exchange membrane as used in Comparative Example 2 were dried. These cation-exchange membrane and anion-exchange membrane were then bonded to each other using an adhesive comprised of a mixed solution of 1 part of bisphenol-A type diglycidyl ether, 1 part of triethylenetetramine and 1 part of methanol. The membranes thus bonded was pressed, and in that state left to stand in a temperature environment of 40°C for 24 hours to give a bipolar membrane. The bipolar membrane thus obtained had a sufficient peel strength, but had a water-splitting membrane potential of as large as 4.3 V. Example 2 Example 1 was repeated except that the surface of the cation-exchange membrane as used therein was not roughed with sandpaper. As a result, the bipolar membrane obtained had performances of a peel strength of 0.28 kg . f/25mm, a water-splitting membrane potential of 1.2 V and current efficiencies of η(H) = 99.2%, η(OH) = 99.2%, η(Cl) = 0.3% and η(Na) = 0.5%. These performances of the bipolar membrane showed a little increase in membrane potential to 1.4 V after the lapse of 2 months, but neither bubbles nor blisters were seen to occur in the membrane. Example 3 Polydimethylvinylbenzylamine (molecular weight: about 10,000) was dissolved in chloroform. Subsequently bisphenol-A diglycidyl ether was dissolved therein. Thus a solution comprising polydimethylvinylbenzylamine, bisphenol-A diglycidyl ether and chloroform in 1:1:10 (weight ratio) was prepared. This solution was coated on a cation-exchange membrane containing Fe, having been treated in the same manner as in Example 1, followed by drying at room temperature for 10 hours to effect curing of the epoxy resin. At the same time, an anion-exchange membrane of a quaternary ammonium group type was formed. A bipolar membrane was thus produced. The bipolar membrane thus obtained had performances of a peel strength of 5.0 kg . f/25mm, a water-splitting membrane potential of 1.3 V and current efficiencies of η(H) = 99.4%, η(OH) = 99.4%, η(Cl) = 0.3% and η(Na) = 0.3%. These performances did not change even after the lapse of 3 months, and none of bubbles or the like were seen to occur in the membrane. Example 4 The same cation-exchange membrane (NEOSEPTA CM-1) as used in Example 1, with its surface having been roughed, was immersed in an aqueous tin chloride (SnCl 2 ) solution of 1% by weight, and thereafter thoroughly washed with ion-exchanged water, followed by air drying at room temperature. The cation-exchange membrane thus treated had a tin content of 3.2% of the total exchange capacity. An aminated polyphenylene oxide with an ion-exchange capacity of 1.02 meq/g, formed by chloromethylation of polyphenylene oxide followed by quaternization using trimethylamine, was dissolved in a mixed solvent of methanol and chloroform (volume ratio: 1:1) to prepare solution with a concentration of 15% by weight. Next, on the above cation-exchange membrane, the solution of aminated polyphenylene oxide was coated, followed by drying to form an anion-exchange membrane with a thickness of 95 µm. A bipolar membrane was thus produced. The bipolar membrane thus obtained had performances of a peel strength of 4.3 kg . f/25mm, a water-splitting membrane potential of 1.3 V and current efficiencies of η(H) = 99.3%, η(OH) = 99.3%, η(Cl) = 0.3% and η(Na) = 0.4%. These performances did not change even after the lapse of 2 months, and neither bubbles nor blisters were seen to occur in the membrane. Example 5 The same cation-exchange membrane (NEOSEPTA CM-1) as used in Example 1, with its surface having been roughed, was immersed in an aqueous ruthenium chloride (RuCl 3 ) solution of 0.1% by weight at 25°C for a day, and thereafter thoroughly washed with ion-exchanged water, followed by air drying at room temperature. This cation-exchange membrane had a ruthenium content of 0.02% of the total exchange capacity. An aminated polystyrene formed by quaternization of chloromethyl groups of a styrene-chloromethylstyrene copolymer (molar ratio of monomers: 10:1) using trimethylamine was dissolved in a mixed solvent of methanol and chloroform (Volume ratio: 1:5) to prepare solution with a concentration of 15% by weight. Next, on the above cation-exchange membrane, the solution of aminated polystyrene was coated, followed by drying to form an anion-exchange membrane with a thickness of 85 µm. A bipolar membrane was thus produced. The bipolar membrane thus obtained had performances of a peel strength of 4.7 kg . f/25mm, a water-splitting membrane potential of 1.2 V and current efficiencies of η(H) = 99.2%, η(OH) = 99.2%, η(Cl) = 0.3% and η(Na) = 0.5%. These performances did not change even after the lapse of 6 months, and neither bubbles nor blisters were seen to occur in the membrane. Example 6 Example 1 was repeated except that the aqueous solution used therein for ion exchange to the cation-exchange membrane was replaced with each of aqueous solutions of the compounds shown in Table 1. As a result, the bipolar membranes obtained had the performances shown in Table 1. In all the bipolar membranes, neither bubbles nor blisters were seen to occur in the membrane. Example 7 On one side of the cation-exchange membrane obtained in Example 5, containing ruthenium ions, a 15% by weight solution prepared by dissolving in organic solvent a chloromethylated polymer having a chlorine content of 1.5 meq/g was coated, and thereafter the solvent was evaporated at room temperature, followed by drying to give a film with a thickness 90 µm. Next, the above cation-exchange membrane was immersed in an aqueous solution of 3% by weight of N,N-dimethylpropanediamine at 25°C for 10 days to effect quaternary amination of chloromethyl groups and crosslinking. A bipolar membrane was thus obtained. The aminated polystyrene film had a thickness of 100 um. The performances of the bipolar membrane thus obtained were measured to reveal that it had a peel strength of 2.1 kg·f/25mm, a water-splitting membrane potential of 1.4 V and current efficiencies of η(H) = 99.0%, η(OH) = 99.0%, η(Cl) = 0.3% and η(Na) = 0.8%. These performances did not change even after the lapse of 3 months. During this period, neither bubbles nor blisters were seen at all to occur in the bipolar membrane.

Claims: A process for producing a bipolar membrane, which process comprises bringing a solution of a polymer having an anion-exchange group or a functional group capable of conversion to an anion-exchange group in a solvent into contact with a surface of a cation-exchange membrane which has been ion-exchanged with a heavy metal ion, removing the solvent so as to form a film of the polymer on the surface of the cation-exchange membrane and, when necessary, converting the functional group into an anion-exchange group. A process according to claim 1, wherein prior to contact with the polymer solution, the surface of the cation-exchange membrane is washed with water to remove any free metal salt compound and the surface is dried. A process according to claim 2, wherein the cation-exchange membrane is ion-exchanged with a heavy metal by immersing the membrane in an aqueous solution of a heavy metal salt or by coating an aqueous solution of a heavy metal salt on the surface of a cation-exchange membrane, prior to washing and drying. A process according to any one of claims 1 to 3, comprising roughening the surface of the cation-exchange membrane. A bipolar membrane comprising a cation-exchange membrane and an anion-exchange membrane closely adhered to each other at a peel strength of not less than 0.2 kg.f/25 mm; the cation-exchange membrane having cation-exchange groups present at least at its adhered surface which have been ion-exchanged with a heavy metal ion and said adhered surface bearing a film comprising a polymer having anion-exchange groups; the bipolar membrane having a water-splitting current efficiency of not less than 80%, and a water-splitting membrane potential of not more than 2.0 V, and being obtainable by a process as defined in any one of the preceding claims. A membrane according to claim 5, wherein there are no free heavy metal salt compounds present at the interface between the cation-exchange membrane and the anion-exchange membrane. A membrane according to claim 5 or claim 6, wherein the anion-exchange membrane is 5µm to 300µm thick, A membrane according to any one of claims 5 to 7, wherein the heavy metal ion is selected from titanium, iron, zirconium, ruthenium, palladium and tin. A membrane according to any one of claims 5 to 8, wherein the anion-exchange membrane has an anion-exchange capacity of 0.4 meq/g to 2.5 meq/g and the cation-exchange membrane has a cation-exchange capacity of 0.5 meq/g to 3 meq/g. A membrane according to any one of claims 5 to 9, wherein the cation-exchange membrane has a rough surface.

Inventors: HANADA FUMIO, HIRAYA KOKI, OHMURA NOBUHIKO, TANAKA SHINSUKE, HANADA, FUMIO, HIRAYA, KOKI, OHMURA, NOBUHIKO, TANAKA, SHINSUKE
Applicants: TOKUYAMA CORP, TOKUYAMA CORPORATION
Application year: 1991
Main classification: C08J5
IPC codes: C08J5, B01D61