JPS6223773B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a novel mechanically superior fluorine-containing cation exchange membrane suitable for use in an oxidizing atmosphere. Specifically, a resin layer having a crosslinked cation exchange group on one side of a cation exchange resin layer without a crosslinked structure has weakly acidic cations than a layer having a crosslinked cation exchange group on the other side. The present invention relates to a fluorine-containing cation exchange resin membrane in which each resin layer has an exchange group. Ion exchange membranes are used in various fields today and have a wide range of applications. Generally, hydrocarbon-based ion exchange resin membranes are extremely stable in a reducing atmosphere, but deteriorate in a short period of time in an oxidizing atmosphere. Particularly in recent years, ion exchange membranes have been increasingly used in oxidizing atmospheres. For example, it is often used for electrolysis of aqueous alkali metal salt solutions, diaphragms for organic electrolytic reactions, electrolytic treatment of waste water, oxidation treatment and reduction treatment of metal ions, and diaphragms for fuel cells. Furthermore, in ordinary dialysis, electrodialysis, etc., when giant organic ions adhere to the membrane surface or inside the membrane, the resistance of the ions when they permeate through the membrane increases and the amount of permeation decreases. In such cases, in order to restore the performance of the membrane, it is effective to remove organic substances that are harmful to the membrane by treatment with an oxidizing agent or heat treatment, and the ion exchange membrane itself must have oxidation resistance and heat resistance. is desirable. On the other hand, so-called ion exchange membrane electrolysis of alkali metal salts, which produces alkali metal hydroxides, halogen gas, oxygen gas, hydrogen, etc. by electrolysis of aqueous solutions of alkali metal salts, is becoming an industrially established technology.
The ion exchange membrane plays a central role in these processes, and it can be said that the quality of the process is determined by the performance of the ion exchange membrane. This has required a membrane with as low electrical resistance as possible and with a high ability to prevent hydroxide ions from leaking through the membrane. To this end, numerous studies and attempts have been made. In order to reduce the permeation leakage of hydroxide ions through the membrane, it is necessary to increase the fixed ion concentration in the membrane.
Increasing the fixed ion concentration in the membrane inevitably increases the electrical resistance of the membrane. Furthermore, the thickness of the film also affects the electrical resistance almost proportionally. To this end, it is currently practiced to provide a thin layer with a high concentration of fixed ions on the membrane surface. However, in general, it is possible to prevent the leakage of hydroxide ions by providing a weakly acidic cation exchange group on the membrane surface to increase the fixed ion concentration, but at the same time, cations, especially polyvalent cations, can be prevented from leaking. Movement within the layer becomes extremely difficult, and multivalent ions such as calcium, magnesium, and strontium inevitably accumulate within the layer, and further react with hydroxide ions of alkali metal hydroxides that come into contact with the membrane. Hydroxide precipitates are formed within the membrane, making it difficult to maintain high performance of the membrane over a long period of time. Therefore, in order to maintain high performance over a long period of time using the above membrane, a highly purified aqueous alkali metal salt solution must be used. Furthermore, these membranes generally do not have a covalent cross-linked structure, which causes problems in mechanical strength, and the membrane expands and contracts significantly due to external liquid concentration, temperature, etc. When using this method, wrinkles occur, which in turn results in an increase in the electrolytic voltage. Furthermore, during electrolysis and handling, the membrane may exhibit fatal defects such as the tendency to generate pinholes in the membrane. The present inventors have already developed a method for manufacturing a cation exchange membrane having a cross-linked structure throughout the membrane (Japanese Patent Application No. 55510/1982).
However, since the cation exchange membrane obtained by this method has a crosslinked structure throughout the membrane, the electrical resistance of the membrane is slightly high. Furthermore, JP-A No. 53-92394 discloses a membrane having three or more different ion exchange resin layers, that is, a membrane consisting of three or more layers parallel to the surface, with sulfonic acid groups present in one end layer and the other. A cation exchange membrane without a crosslinked structure in which a carboxylic acid group is present in at least one of the layers has been proposed. When electrolysis is performed using an aqueous alkali metal salt solution containing 1 ppm or more of calcium ions as the anolyte, this membrane undergoes a rapid increase in cell voltage due to precipitation of calcium within the membrane due to continuous electrolysis for several weeks to several months. This results in an increase in current efficiency and a significant decrease in current efficiency. Another requirement in the industrial alkali metal salt electrolysis process is to increase the salt decomposition rate by ion exchange membrane electrolysis and to obtain highly concentrated caustic alkali with high efficiency. However, many of the various cation exchange membranes that have been developed so far are non-crosslinked, which causes the membrane to swell and contract, resulting in a decrease in current efficiency. In some cases, the design of the membrane does not match the environment in which it is used, such as showing a tendency to shrink on the one hand, and a tendency on the other hand to shrink, resulting in stress within the film and the formation of wrinkles. In order to solve the various problems mentioned above, the present inventors have conducted extensive research and found that the present invention has durability against ions such as calcium that precipitate in a basic atmosphere, and also has resistance to the decomposition of alkali metal salts. We have developed a cation exchange membrane that has a high current efficiency when obtaining high concentration caustic alkali, and is less likely to cause deformation such as wrinkling. That is, the cation exchange membrane of the present invention has a thin layer of a fluorine-containing cation exchange resin having a crosslinked structure on the surface of the membrane that comes into contact with an aqueous alkali metal salt solution, and also has a thin layer of a fluorine-containing cation exchange resin that comes into contact with an alkali metal hydroxide. A resin layer () having a weakly acidic cation exchange group is present on the surface, and a non-crosslinking ion exchange resin is present in the middle between the two, thereby reducing the internal membrane resistance caused by differences in the environments on both surfaces. Try to buffer stress. In this way, the alkali metal salt aqueous solution is highly decomposed (also called desalination), and the swelling of the membrane due to contact with the aqueous solution in the low concentration range is partially suppressed, and the membrane on the surface in contact with the high concentration alkali metal hydroxide is Partially suppresses the contraction of
By maintaining a relatively homogeneous fixed ion concentration,
This prevents a decrease in current efficiency during electrolysis, and can always maintain a predetermined membrane resistance value. Furthermore, regardless of the concentration of the alkali metal salt aqueous solution, the present invention has a dense membrane surface that prevents the entry of multivalent ions such as calcium, and these ions can cause deterioration such as an increase in membrane resistance. It is characterized by extremely high durability. Hereinafter, the embodiments of the present invention will be explained in more detail. The fluorine-containing cation exchange membrane of the present invention includes a cation exchange layer () having a covalent crosslinked structure from one side of the membrane, a cation exchange layer () having no covalent crosslinked structure, and a cation exchange layer () having no covalent crosslinked structure. The cation exchange group is present in a layered manner in the order of layers () having a cation exchange group that is more weakly acidic than the cation exchange group. As a constituent requirement of each layer, the layer () has a covalent crosslinked structure, and the degree of crosslinking is determined by the total monomer mixture of fluorine-containing polyvinyl monomers contained in the polymer copolymer constituting the layer (). It is preferably 2 to 50% by weight.
If the value is smaller than this value, the effect of the film of the present invention is lost, and if it is higher than this value, the electrical resistance of the film increases significantly. The thickness of the layer () is desirably within a range that does not substantially increase the electrical resistance of the membrane, and may be within the range of 1/3 or more of 3000 Å or more with respect to the total thickness of the cation exchange membrane of the present invention, and if it is less than 3000 Å, the cross-linking The effect of the structure is weak, and if the thickness exceeds 1/3 of the total thickness, the electrical resistance of the film increases. Yet another layer ()
The cation exchange group present in the cation exchange group is preferably one whose membrane properties have as little dependence as possible on the pH of the solution, and is generally a cation exchange group with a relatively high degree of dissociation such as a sulfonic acid group, a phosphoric acid group, or a carboxylic acid group. In particular, the sulfonic acid group is a perfluorosulfonic acid group (−
_{CF2SO2 -} ^or

[Formula]) is desirable. Next, the main role of layer () is to support layers () and () present on both sides, and in addition, it has a certain effect of preventing multivalent ions from entering layer (). Therefore, although the thickness of layer () depends on the thickness of layers () and (), it is within the range of 1/10 or more and less than 1/1 of the total thickness of the cation exchange membrane of the present invention. The thickness may be such that the electrical resistance of the cation exchange membrane of the present invention does not increase significantly and the mechanical strength of the cation exchange membrane of the present invention is sufficiently maintained even when the layers () and () are made thin. In addition, the ion exchange group in the layer () may be one that is relatively more acidic than the ion exchange group in the layer ().
Generally, sulfonic acid groups, perfluorosulfonic acid groups, and carboxylic acid groups are preferred. Furthermore, the exchange capacity of the layer () is preferably high in order to reduce the electrical resistance of the membrane, and is generally in the range of 0.5 to 5.0 milliequivalents/gram dry membrane. Next, layer () is not particularly limited as long as it has a cation exchange group that is weaker acidic than the cation exchange group present in layer (). For example, when the cation exchange group present in the layer () is a sulfonic acid, the cation exchange group present in the layer () is a carboxylic acid, an acid amide group having a dissociable hydrogen atom, a phenolic hydroxyl group,
phosphoric acid group, phosphorous acid group,

[Formula] etc., and when −CF ₂ COOH is present in the layer (), the cation exchange group present in the layer () is −
_CH2COOH ,

【formula】

It is necessary to use [expression] etc. Generally, the acid strength is determined by the ion exchange group bonded to the polymer, so it is not possible to specifically define and order the PKa, but the PKa of monomer compounds with similar structures bonded to the ion exchange group is not possible. Large Organic Chemistry, International Critical Table,
It can be determined by comparing with chemical handbooks, etc. The thickness of the layer () must be within the range of 1/3 or more or more than 200 Å of the total thickness of the film of the present invention, so as not to cause an increase in electrical resistance as much as possible and to have a high fixed ion concentration. . The effective exchange capacity of this layer () may be the same as layer (), but it is generally desirable to be between 0.3 and 2.5 meq/g dry membrane. Further, it is desirable that the layer () has no covalent crosslinked structure, and even if there is a crosslinked structure, it is desirable that the amount of crosslinked structure is as low as possible. In the membrane of the present invention, a membrane made only of resin components may be used, but when a membrane with a large area is used industrially, it is desirable to use a reinforcing material. A porous base material is used as a reinforcing material, but from the viewpoint of a cation exchange membrane that has oxidation resistance and heat resistance, a porous base material that also has oxidation resistance and heat resistance is necessary. Emulsion-spun products such as polytetrafluoroethylene fibers,
Fibers made of melt-moldable polymers such as copolymers of polytetrafluoroethylene and ethylene, microporous membranes such as ultrafiltration membranes made of fluorine-containing polymers, and fibrillated fibers. Cloths made of fluorine-based polymers, carbon fibers, and knitted fabrics such as plain woven fabrics and stockinette woven fabrics are used depending on the purpose. Preferably, it is a plain woven fabric made of long fibers made of polytetrafluoroethylene, with a yarn size of 40 to 400 deniers, which has as little effect on the increase in electrical resistance of the membrane as possible. This porous base material may be unevenly distributed in one part of the cation exchange membrane, or may be located somewhere in between. Of course, particularly good results are obtained when the porous base material is subjected to etching or graft polymerization treatment with a compound that has good affinity with the partially polymerized product in order to improve the adhesion between the porous base material and the ion exchange resin component. It is something. Further, the thickness of the film is preferably 0.05 mm to 2 mm. The method for manufacturing the cation exchange membrane of the present invention includes: (A) fusing three polymer membranes that satisfy the structural requirements of layer (), layer (), and layer () under heating and pressure; Method. (B) Layer () or layer () (A )
Methods other than methods of changing the ion exchange groups and/or ion exchange capacity of the layer (), for example using known chemical reactions, to suit the constituent requirements of the layer (); ) In the presence of one or more types of fluorine-containing vinyl monomers, fluorine-containing polyvinyl monomers, and/or polymers of these parts, energy such as heating, ionizing radiation, and ultraviolet rays is applied. A method of polymerization or graft polymerization using (C) Layer () and a fluorine-containing vinyl monomer or fluorine-containing polyvinyl monomer that satisfies the constituent requirements of layer () between two polymer membranes that meet the constituent requirements of layer (). A method comprising producing the layer by polymerization or graft polymerization in the presence of one or more types of polymers and/or partial polymers thereof. (D) Layer (), which satisfies the constituent requirements of layer (), is formed on both sides of a single polymeric film-like material that satisfies the constituent requirements of layer (), using a known chemical reaction.
Polymerization or grafting by changing the ion exchange group and/or ion exchange capacity, or by adding one or more types of fluorine-containing vinyl monomers, fluorine-containing polyvinyl monomers (described below), and/or partial polymers thereof. A method consisting of polymerization. However, among these methods, method (A) involves laminating three polymer membranes, so in some cases, the fusion may not proceed sufficiently. For this reason, the preferred method for carrying out the present invention is
(B), (C), and (D). When producing the cation exchange membrane of the present invention, the following fluorine-containing vinyl monomers, fluorine-containing polyvinyl monomers, and fluorine-containing vinyl monomers that are raw materials for polymer membranes are preferred. (1) In the general formula, CFX=CYZ [where X and Y are -
H, −F, −Cl, −CnF _8o+1 (n=1 to 5), Z is −
A group of fluorine-containing vinyl monomers represented by H, -F, -Cl, -O-CnF _2o+1 (n = 1 to 5)], (2) in combination with the fluorine-containing vinyl monomer (1); As a fluorine-containing vinyl monomer that is polymerizable and has a cation exchange group, a functional group that can be easily introduced with a cation exchange group, or that can be easily converted into a cation exchange group, A fluorine-containing vinyl monomer in which at least a fluorine atom is bonded to the carbon at the α-position relative to the group, preferably a perfluoro vinyl monomer. That is, as the vinyl monomer in (2) above,

[Formula] O=N-A-B,

[Formula] etc. Here, D represents one of _-CF3 , _{-C2F5 ,} etc. A generally represents -X-Y-Z-, -X-Z-Y-, -Z-X-Y
-, -Z-Y-X-, -Y-X-Z-, -Y-Z-
Represented by X-. In this formula, X is (-OCFX) _-l , (-CFX) _-l , [-O(-CFX) _-l ] _-n , Y is (-CFX) _-o , (-O) _-p , (- S) − _p , Z is

【formula】

[Formula] (may be in positions other than para), where l, m, and n are 0 or 1
~10 positive integer, p is 0 or 1, X is F,
One _or more of Cl, H, _-CF3 , _-C2F5 , etc., and X' represents one or more of F, _{-CF3 , C2F5} . B is

[Formula] −PE ₂ , − E, −COE, −CN,

【formula】

[Formula] E is OM, OR ₁ , halogen, OH, NR ₁ R ₂ (M is one or more of metal ions and organic cations; R ₁ and R ₂ are hydrogen, metal ions, organic cations, Saturation with carbon numbers 1 to 20,
(unsaturated alkyl group, alicyclic group, aromatic group, heterocyclic group). Further specific examples of this are as follows.

【formula】

【formula】

【formula】

【formula】

【formula】 【formula】

[Formula] O=N(-CF ₂ )- _o COE, O=C(-CF ₂ )- _o COE, O=N(-CF ₂ )- _o SO ₂ E, O=C(-CF ₂ )- _o
SO ₂ E,

【formula】 【formula】

【formula】

【formula】 【formula】

【formula】

【formula】 【formula】

【formula】

【formula】 【formula】

[Formula] (The position of the substituent on the benzene ring is not limited to the para position.)

【formula】 【formula】

【formula】

[Formula] Also, in order to form a crosslinked structure in the layer (), a fluorine-containing polyvinyl compound (3) such as CFX=CX'-O
-A-O-CX'=CFX is used. As a specific example, CF ₂ = CFO (-CF ₂ ) - _o OCF = CF ₂ (n is a positive integer from 2 to 24) is generally used, but in addition, CFX = CX' - A -X'C=CFX, specifically CF ₂ = CF (-CF ₂
) _-o A fluorine-containing compound such as CF= _CF2 , preferably a perfluoro compound, is used. Here X, X′,
A is as described above. Some typical examples of the method for producing a cation exchange membrane of the present invention will be described. Tetrafluoroethylene and perfluoro(3,6-dioxa-4
- a 0.8 mm film having an exchange capacity of 0.91 meq/g dry film (H ⁺ form) when the sulfonyl fluoride group is hydrolyzed with a copolymer with methyl-7-octensulfonyl fluoride), and A 0.4 mm film containing sulfonyl fluoride groups, whose exchange capacity when hydrolyzed with a copolymer consisting of the same components as 0.87 meq/g dry film (H ⁺ type), is further reacted with ethylenediamine to form a sulfonyl fluoride group. Fluoride groups have been converted to sulfonic acid amide groups. Between these two films, 60 polytetrafluoroethylene threads are inserted vertically and horizontally (100 denier thread).
A cation exchange membrane precursor having a two-layer structure, that is, one layer has a sulfonyl fluoride group and the other layer has a sulfonic acid amide group, which is weaker acid than sulfonic acid, was obtained by sandwiching and heat-sealing the plain woven fabric of Ta. Next, perfluoro{pentamethylene di-(vinyl ether)} and perfluoro(3,6-dioxa-
The two-layer structure described above is a viscous partial polymer obtained by partially polymerizing a mixture of 25, 47, and 28% (by weight) of 4-methyl-7-octensulfonyl fluoride) and perfluoro(pyropyru vinyl ether), respectively. was applied uniformly to the side of the cation exchange membrane containing the sulfonyl fluoride groups. Furthermore, the coated surface was irradiated with ultraviolet rays to complete polymerization.
Separately, the same partial polymer was polymerized with ultraviolet rays and hydrolyzed, and the exchange capacity was measured, and it was found to be 1.06 meq/g dry film (H ⁺ type). The thickness of the coating layer was determined from the difference in thickness before and after coating and was approximately 0.4 mm. The thus obtained cation exchange membrane precursor having a three-layer structure was hydrolyzed to obtain the membrane of the present invention. Another method is to use tetrafluoroethylene and perfluoro(3,6-dioxa-4-methyl-7
-octensulfonyl fluoride) and has an exchange capacity of 0.83 when hydrolyzed.
Milliequivalent/gram dry film (H ⁺ type) is 0.1mm
The above-mentioned polytetrafluoroethylene plain woven fabric was sandwiched between 0.05 mm films, heat-fused and integrated, and then hydrolyzed to obtain a cation exchange membrane having a single layer structure. Next, one side of the obtained membrane was reacted with phosphorus pentachloride to convert the sulfonic acid groups present in a layer of 10 microns from one side to sulfonyl chloride groups. Furthermore, the sulfonyl chloride group was converted into a carboxylic acid group by air oxidation of the sulfonyl chloride group while heating in n-butanol. The above-mentioned partial polymer was applied to the sulfonic acid group side of the cation exchange membrane with a two-layer structure obtained in this way, and the polymerization was completed by irradiating it with ultraviolet rays. It was used as an ion exchange membrane precursor. Further hydrolysis yielded a cation exchange membrane having a three-layer structure according to the present invention. Although the cation exchange membrane of the present invention is thin, the mechanical strength of the membrane is significantly improved because a covalent crosslinked structure is introduced into the membrane. In general, if the exchange capacity of a membrane is increased in order to keep the electrical resistance of the membrane low, the mechanical strength of the membrane becomes weaker. This tendency is particularly strong in membranes without a covalent crosslinked structure, but this can be extremely significantly prevented in the cation exchange membrane of the present invention. Furthermore, since the membrane of the present invention has a covalent crosslinked structure, it has good dimensional stability. At the same time, since there is a weakly acidic cation exchange base layer, when used as a diaphragm in the electrolysis of alkali metal salts, it is possible to obtain a concentrated alkali metal hydroxide with a large ability to prevent membrane leakage of hydroxide ions. This results in high current efficiency. As mentioned above, in the case of ordinary ion-exchange membrane salt electrolysis, etc., the polyvalent cations in the salt water, especially calcium ions, have a significant effect on the deterioration of membrane performance, so the salt water must be purified to a high degree. Must be. Furthermore, in a membrane having a weakly acidic cation exchange group on the cathode side of the membrane, as disclosed in JP-A-53-137888, etc., the movement speed of calcium ions within this weakly acidic cation exchange base layer is slow; There is a problem in that precipitates such as calcium hydroxide are formed in the weakly acidic cation exchange base layer due to adsorption of hydroxide ions into the layer, resulting in a decrease in membrane performance. In contrast, in the cation exchange membrane of the present invention, calcium ions are prevented from entering the membrane due to the presence of a covalent crosslinked structure, and furthermore, the movement speed of calcium ions within the layer is slow. , calcium ions do not fully reach the layer with weakly acidic cation exchange groups, and even if they do reach the membrane, the weakly acidic cations that act as a hydroxide ion leakage prevention layer pass through the membrane even in an alkaline atmosphere. Calcium ions do not accumulate in the layer containing exchange groups, and high performance can be maintained for a long time. Furthermore, the desalination rate of the aqueous alkali metal salt solution can be increased, and for example, in the case of electrolysis of common salt, the concentration can be reduced to about 2 to 3.5 normal. The cation exchange membrane of the present invention is particularly effectively used as a diaphragm in the electrolysis of alkali metal salts, and in this case, it exhibits a particularly effective effect by directing the layer of covalent crosslinked structure toward the anode. This is the case when using it. For this reason, it is desirable that the side of the membrane facing the anode is a perfluorinated layer. There are still many points that are unclear as to why the cation exchange membrane of the present invention exhibits particularly excellent properties, but the reason why it is mechanically superior and has excellent dimensional stability is that the cross-linked structure formed in a thin layer on the membrane surface is highly This seems to be because it is based on a molecular thin film layer. In addition, the reason why high performance is maintained for a long time even when ion species that are inherently harmful to alkali metal salt electrolytic systems, such as relatively high concentrations of calcium, are contained in the salt water is that the surface layer of the membrane Calcium ions move slower than sodium ions within the three-dimensional crosslinked structure layer of the membrane, and are trapped in the anode side surface layer of the membrane. Calcium ions do not reach the weakly acidic cation exchange base layer existing on the surface, and even if they do reach the weakly acidic cation exchange base layer, since there is virtually no crosslinking, the calcium ions pass through the membrane and enter the hydroxide ion leakage prevention layer. This seems to be because no accumulation occurs. The cation exchange membrane obtained in the present invention can be used without any limitation in systems using conventionally known membranes in addition to those described above, and is particularly useful in systems requiring oxidation resistance. For example, diaphragms for electrode reactions, electrolysis of acids and bases, electrolysis of alkali metal salts, diaphragms for electrolysis of plating waste liquid, and other acidic solutions containing heavy metal ions, and diaphragms for fuel cells and other high-temperature or room-temperature batteries. When diaphragms, ion exchange membranes used in electrodialysis, and other charged membranes are used in an oxidizing atmosphere at high temperatures, they can be used almost semi-permanently. Next, when using the membrane of the present invention, conventionally known devices can be used without any restrictions, such as a clamp type battery cell for multi-chamber electrodialysis, a water bath type battery tank, and a clamp type electrolyzer for electrolytic reaction. Conventionally known electrolytic cells such as electrolytic cells, finger electrolytic cells, bipolar electrolytic cells, single-polar electrolytic cells, horizontal electrolytic cells, ultrafiltration, fuel cells, etc. can be used without any restriction. The content of the present invention will be specifically explained in the following Examples, but the present invention is not limited to the Examples below. In the following examples, the electrical resistance of the membranes was measured by alternating current for 1000 cycles at 80°C, and the solutions were measured between 3.5N NaCl and 6.0N NaOH unless otherwise specified. The exchange capacity was determined by immersing the H-type membrane in a predetermined amount of 0.2N-NaOH for 3 hours and back-optimizing the amount of OH lost by the ion exchange reaction using 0.1N-HCl. The exchange capacity for 1 g of H-type membrane was determined. The thickness of the film was determined using a micrometer. The electrolysis experiments were carried out using a two-chamber electrolytic cell with an effective current-carrying area of 1 dm ^2. An insoluble anode made of a titanium wire mesh coated with ruthenium dioxide and titanium dioxide was used as the anode, and a soft iron wire mesh was used as the cathode. Normally electrolysis is carried out at 30A/ ^dm2 , saturated saline is supplied as the anolyte unless otherwise specified, and the decomposition rate is approximately 3.0.
%, and the temperature inside the electrolytic cell was kept at 80-90℃. Example 1 An exchange capacity of 0.91 meq/g dry membrane (H
A sheet of 0.05 mm thick corresponding to the mold) is layered with another sheet of the same 0.05 mm thickness, and between them is a plain woven fabric made by weaving 400 denier tetrafluoroethylene threads at a rate of 50 threads per inch both vertically and horizontally. A single piece made by sandwiching and fusing together was used as a layer (). This film-like polymer was immersed in a hydrolysis bath consisting of 600 parts of water, 400 parts of dimethyl sulfoxide, and 120 parts of caustic potassium at 60°C for 4 hours to form a potassium sulfonate salt, and then immersed in 3N nitric acid for 6 hours. It was soaked for a period of time to form an acid form. After drying this under reduced pressure, one side of the membrane was brought into contact with phosphorus pentachloride vapor at 150°C for 30 minutes to convert the sulfonic acid groups to sulfonyl chloride groups.
Next, this was immersed in n-butyl alcohol to sufficiently swell it, and then heated to 110°C by blowing air into it.
The membrane was heated for 8 hours, and then immersed in a hydrolysis bath again to obtain a cation exchange membrane. An infrared spectrum measurement revealed that carboxylic acid groups were present on the side of this membrane that came into contact with phosphorus pentachloride. A portion of this membrane was placed in a solution of 1N hydrochloric acid and methanol (1:1 volume ratio) containing 1% crystal violet for 20 minutes at room temperature.
When the cross-section of the membrane was time-stained and observed under an optical microscope, the layer containing sulfonic acid groups was stained dark green at a depth of about 0.08 mm from one side of the membrane, and the remaining approximately 0.02 mm was not stained at all. It was found that carboxylic acid groups were present. After this film was heated and dried at 80°C under reduced pressure again, the surface layer on the back side of the film where carboxylic acid groups exist (where sulfonic acid groups are present) was rubbed with sandpaper to make the surface loose, and then the next treatment was performed. did. That is, perfluoro(3,6-dioxa-4-
Methyl-7-octensulfonyl fluoride)
A solution of 60 parts of perfluoroalkyl vinyl ether sulfonyl fluoride, 20 parts of perfluoro(methyl vinyl ether), and 20 parts of perfluoro{pentamethylene-bis(vinyl ether)} in 100 parts of perfluorodimethylcyclobutane. was placed in a stainless steel autoclave, cooled to -80°C, thoroughly purged with nitrogen and degassed, then raised to -40°C and injected with 3 mol% N ₂ based on the total monomers. After introducing F ₂ initiator and keeping at 75° C. for 2 hours, a viscous solution of the polymer subpolymer was obtained. When this was analyzed by infrared rays, a perfluorinated double bond attached to an ether bond was observed at 1840 cm ^-1 . When the viscosity of this was measured, it was 2.4 poise. After uniformly and thinly applying this viscous monomer and partial polymer solution to the surface layer of one side of the polymer membrane with a sparse surface, a mercury vapor ultraviolet lamp (SHL-100UV manufactured by Toshiba) was applied to the coated surface. Irradiated uniformly. After about 150 hours of irradiation, the viscous partially polymerized product was completely polymerized, yielding a polymer membrane having a three-dimensional crosslinked structure only on one membrane surface. Further, a portion was cut out and stained in the crystal violet solution described above at room temperature for 20 hours, and the cross section of the membrane was observed using an optical microscope.
It was dyed dark green with a thickness of mm, and on both sides about
Completely undyed layers were observed at thicknesses of 0.02 mm and 0.001 mm. It was found that the former was a layer containing carboxylic acid groups, and the latter was a layer containing sulfonyl fluoride groups, and had a three-layer structure.
This was then immersed in the water-dimethylsulfoxide-caustic potassium hydrolysis bath described above to obtain the cation exchange membrane of the present invention. Incidentally, the weight increase due to the formation of a three-dimensional crosslinked structure in the polymer membrane was about 1%. Electrolysis of saturated saline water was carried out using this membrane. The calcium concentration in the saturated saline solution was 2.1 ppm. 12N caustic soda was obtained from the cathode chamber with the surface having a three-dimensional crosslinked structure facing the anode and the layer having a weakly acidic cation exchange group facing the cathode. The current effect of caustic soda acquisition is 95%, 6
Electrolysis was continued under the same conditions for several months, but there was no change. Also, the battery voltage started from 3.75V for 3 days.
It rose to 3.82V, but remained constant thereafter. On the other hand, for comparison, 60 parts of perfluoro(alkyl vinyl ether sulfonyl fluoride) containing perfluoro(3,6-dioxa-4-methyl-7-octensulfonyl fluoride) as a main component and 40 parts of perfluoro(methyl vinyl ether) were used. A partially polymerized product was obtained from a mixture consisting of the following by the method described above. Using this partial polymer, a cation exchange membrane having a three-layer structure was prepared by the method described above. When electrolysis of saturated salt water was carried out in the same manner using this membrane, 12N caustic soda was obtained, the electrical efficiency was 93%, and the cell voltage was 3.70V, but after 3 months, the cell voltage is 4.1V, and the current efficiency is 83
%. Furthermore, in order to compare the mechanical strength of the membrane of the present invention and the membrane used for comparison, the membrane that had been electrolyzed for 3 months was taken out from the electrolytic bath, and after being air-dried, a bending test was repeated over 90 degrees Celsius, resulting in a slight cloudiness. We investigated the possibility of pinholes forming in energized parts. As a result, the film used for comparison was found to have pinholes after being folded 15 times, but the film of the present invention did not show any pinholes even after being folded 50 times or more. Also, when the membrane immersed in 1N caustic soda was immersed in 6.0N caustic soda,
The comparative membrane shrank 6%, while the inventive membrane shrank only 3%. Example 2 A 0.15 mm thick film made of copolymer and tetrafluoroethylene was used as the raw material for the layer (). The exchange capacity when this is hydrolyzed is
It was 1.52 meq/g dry film (H type).
On the other hand, CF ₂ = CFO (−CF ₂ ) − ₅ OCF = 20 parts of CF ₂ and

[Formula] was mixed and put into a stainless steel autoclave, and after repeating freezing and vacuum degassing several times, it was kept at 50℃ and irradiated with 40 Mrad gamma rays at a dose rate of 5000 rad/hr from a Co ⁶⁰ radiation source. After that, when the autoclave was opened, an extremely viscous partial polymer had been formed. At this time, the viscosity of this was measured and was 55 poise. So, add the following compound to this The mixture was stirred with a homogenizer to a viscosity of 5 poise to obtain a monomer suspension of a partially polymerized product. This was applied to one side of the above-mentioned film and left to permeate the film to some extent, then covered with a quartz plate and irradiated with ultraviolet rays from a mercury ultraviolet lamp to polymerize. On the other hand,

[Formula] A thin film made of a terpolymer of tetrafluoroethylene and hexafluoropropylene and having an ion exchange capacity of 1.85 milliequivalents/g in an alkaline atmosphere and a thickness of 0.05 mm on a dry membrane was prepared by adding the above-mentioned carboxylic acid ester groups. by heating and fusing onto the surface of the
The cation exchange membrane of the present invention is formed into a single polymer membrane and then immersed in a 10% caustic soda methanol solution at 60°C for 24 hours to hydrolyze the sulfonyl fluoride and carboxylic acid ester contained therein. And so. When a part of this membrane was subjected to the same staining test as in Example 1 in the staining solution used in Example 1, the side to which the partially polymerized product was applied was about 0.01mm from the surface.
The area was stained dark green, and the remaining area was not stained at all. Using this membrane, salt electrolysis was carried out in the same manner as in Example 1 using 3.5N saline containing 2 ppm of calcium ions. However, the pH of the anolyte
When electrolysis was performed with the solution adjusted to 7, the current efficiency was 93% when the caustic soda concentration obtained from the cathode chamber was adjusted to 14, and the cell voltage was 3.80V. On the other hand,
The film with carboxylic acid groups obtained 14N caustic soda and had a current efficiency of 93%, but the cell voltage was 4.12V. When electrolysis was continued under the same conditions for 6 months, the membrane of the present invention did not change the current efficiency due to the caustic soda in the cathode chamber, and the cell voltage increased.
3.92V, whereas the film used for comparison had a current efficiency of 82V with a 14-normal caustic soda rating.
%, and the battery voltage was 4.80V. In addition, the amount of oxygen in the chlorine gas generated at the anode was 2.5% in all cases at the beginning of electrolysis. Therefore, the pH of the anolyte
When the cell voltage was lowered to 0.5 at the inlet of the cell, the amount of oxygen in the chlorine gas generated at the anode decreased to less than 0.2%, but the cell voltage decreased when using the membrane of the present invention.
While it was 3.85V, the film used for comparison soared to 5.7V. Example 3 The partial polymer obtained in Example 1 was uniformly cast into a polytetrafluoroethylene mold, a quartz plate was placed over the mold, and ultraviolet rays were irradiated. 20
After irradiation for a period of time, the quartz plate was removed to obtain a sheet with a thickness of approximately 0.2 mm. When we measured the reflection infrared spectrum of this film, we found that it was present in the partially polymerized product.
It was found that the absorption band attributable to vinyl ether groups near 1850 cm ^-1 almost disappeared, indicating that the polymerization was complete. On the other hand, a 0.05 mm thick sheet made of the copolymer of tetrafluoroethylene and perfluoro(alkyl vinyl ether sulfonyl fluoride) used in Example 1 was immersed in ethylenediamine at room temperature for 24 hours to remove the sulfonyl fluoride groups. was converted into a sulfonic acid amide group. The membrane treated with ethylenediamine was thoroughly washed with water and then dried under reduced pressure. Between the two sheets thus obtained, tetrafluoroethylene and perfluoro(3,6-dioxa-4-methyl-7-octensulfonyl fluoride) obtained in the same manner as in Example 1 are placed. A partial polymer having a weight ratio of 1:1 was uniformly applied. This was further pressed slightly and 20 Mrad of Co ⁶⁰ γ-rays were applied at a dose rate of 1000 rad/hr.
Irradiated. The temperature during irradiation was 60°C. After irradiation, the film-like material was taken out and observed, and it was found that the partially polymerized material had completely polymerized, and the two sheets could not be separated. Also, the total thickness of the film-like material is approximately 0.3
mm, and calculation from this revealed that the thickness of the layer in which the partially polymerized product was polymerized was approximately 0.05 mm. The obtained membrane-like material was further immersed in the hydrolysis bath used in Example 1 to obtain a cation exchange membrane of the present invention. This membrane was subjected to an electrolytic test in the same manner as in Example 1. The calcium concentration in the saturated saline solution used was
It was 2.5ppm. As a result, the current efficiency at the beginning of energization was 94% with 10 standard caustic soda, and 6
Even if electrolysis was continued under the same conditions for several months, there was no change. In addition, the cell voltage also increased from 3.83V at the time of energization for about 2 days.
The voltage rose to 3.90V, but remained constant after that, and no deterioration in membrane performance due to calcium ions was observed.

Claims (1)

[Scope of Claims] 1. A layer () having a cation exchange group having a covalent crosslinked structure, a layer () having a cation exchange group not having a covalent crosslinked structure, and a layer () having a cation exchange group having a covalent crosslinked structure. A fluorine-containing cation exchange membrane consisting of a layer ( ) having a cation exchange group that is weaker acid than the ion exchange group. 2. The cation exchange membrane according to claim 1, wherein the cation exchange groups present in the layer () are sulfonic acid groups. 3. The cation exchange membrane according to claim 1, wherein the cation exchange group present in the layer (2) is either a carboxylic acid group or an acid amide group bonded to a dissociable hydrogen atom. 4. The cation exchange membrane according to claim 1, wherein the cation exchange groups present in the layer () are sulfonic acid groups or carboxylic acid groups. 5. The cation exchange membrane according to claim 1, which is composed of a perfluorocarbon-based polymer compound. 6. The cation exchange membrane according to claim 1, wherein the thickness of the layer () is 200 Å or more and 1/3 or less of the total thickness of the membrane. 7. The cation exchange membrane according to claim 1, wherein the layer () has a thickness of 200 Å or more and 1/3 or less of the total thickness of the membrane. 8. The cation exchange membrane according to claim 1, which is used as a diaphragm for electrolysis of alkali metal salts. 9. The cation exchange membrane according to claim 8, wherein the layer ( ) is used facing the anode.