JPH0149174B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a perfluorocarbon ion exchange membrane obtained by extrusion molding or a perfluorocarbon polymer membrane having a group that can be easily converted into an ion exchange group or into which an ion exchange group can be easily introduced (hereinafter simply a precursor). At least 1 micron of the surface layer of the ion-exchange membrane (referred to as the membrane) is removed and the newly formed surface is exposed to the unique ion-exchange groups that the ion-exchange membrane has, or the ion-exchange groups that the precursor ultimately has, through a chemical reaction. The present invention relates to an improved method for producing a perfluorocarbon-based ion-exchange membrane, which comprises introducing a different type of ion-exchange group or a functional group that can be easily converted thereto, and then carrying out a conversion reaction into an ion-exchange group, if necessary. Ion exchange membranes are used industrially in various fields such as electrodialysis, diffusion dialysis, electrode reaction diaphragms, reverse osmosis, and ultrafiltration. In particular, the ion-exchange membrane method and salt-containing electrolysis technology, which has been actively researched and industrialized in recent years, is currently being used to obtain high-concentration caustic soda and improve current efficiency.
It has reached over 90%, and research is underway to reduce the electrolysis voltage as much as possible in order to achieve further energy savings. Of this electrolytic voltage, apart from the theoretical decomposition voltage, other factors that account for large values in the electrolytic voltage are cathode overvoltage, voltage drop due to electrical resistance of the membrane, and voltage drop due to current interruption due to bubbles. Cathodic overvoltage has been significantly reduced by the development of active cathodes. Furthermore, the electrical resistance of ion exchange membranes has been reduced by changing the membrane composition, membrane thickness, and selection of reinforcing materials to maintain membrane strength. but,
If the thickness of the membrane is made too thin to reduce its electrical resistance, the alkali metal salts contained in the caustic soda produced when the membrane is used for alkali metal salt electrolysis will increase, resulting in the production of impure caustic soda. . In JP-A No. 58-93728, we reported that the membrane structure of an ion exchange membrane made by extrusion molding was non-uniform in terms of membrane cross section, and the surface layer of this ion exchange membrane was removed to form a thin film. By doing so,
We discovered that there is a significant reduction in the electrical resistance of the film other than that due to the reduction in its thickness, and made a proposal. Another possible means of lowering the electrical resistance of the ion exchange membrane is to increase the water content of the membrane. Specifically, it is considered desirable to use a membrane having ion exchange groups that are easily hydrated, such as sulfonic acid groups. However, membranes with a high water content also have a large permeation of salts due to diffusion and a large transfer number of reactive ions, resulting in a decrease in current efficiency and an increase in the rate of salt contamination when obtaining highly concentrated caustic soda in the electrolysis of alkali metal salts. When was it? produce defects. Now, in the electrolysis of alkali metal salts, in order to obtain highly concentrated caustic soda from the cathode chamber and obtain a current efficiency of 90% or more, it is necessary to install a weakly acidic cation exchange group on the surface of the membrane that comes into contact with the caustic soda. It is valid that there exists. Specifically, these include carboxylic acid groups, perfluoro tertiary alcohols, phenolic hydroxyl groups, and sulfonic acid amide groups having a dissociable hydrogen atom, which dissociate in an alkaline aqueous solution and generate a negative charge. However, when using a cation exchange membrane consisting only of these weakly acidic cation exchange groups, there is a drawback that the electrical resistance of the membrane becomes high. Therefore, one of the most ideal ion exchange membrane structures is a layered structure parallel to the membrane surface, consisting of a layer having sulfonic acid groups and a layer having weak acid groups. Furthermore, a relatively thin layer having weak acid groups, for example 100 Å to 10 microns, is sufficient. In order to obtain an ion exchange membrane having such a structure, for example, a cation exchange membrane consisting of two layers of sulfonic acid groups and carboxylic acid groups, a thin film having a sulfonic acid group and a thin film having a carboxylic acid group, or such ion exchange groups are used. Adhering or fusing a thin film that can be easily introduced has been considered and has been carried out in some industries.
However, it is difficult to overlay and integrate membranes having weakly acidic cation exchange groups, such as those with a thickness of 10 microns or less, by adhesion or fusion, and even if it is possible to bond them, it is difficult to combine them with polymers with sulfonic acid groups and carboxylic acid groups. Due to the poor compatibility of polymers with acid groups, when used in the electrolysis of alkali metal salts, they often peel off due to changes in operating conditions such as a partial decrease in the concentration of salt water or a decrease in the pH of the salt water. From this point of view, instead of using a bonded membrane, it is preferable to use a weakly acidic cation exchange group or a membrane that has a group that can easily introduce or convert it into a strongly acidic cation exchange group. A method of introducing groups that can be introduced or converted into a thin layer, or a membrane that has a weakly acidic cation exchange group bonded to it, or a single membrane that has a group that can be introduced or converted to a weakly acidic cation exchange group. A method in which a strongly acidic cation exchange group, a group capable of introducing a strongly acidic cation exchange group, or a group capable of being converted into a strongly acidic cation exchange group, is introduced in a layered manner is desirable. Of course, each ion exchange group is introduced and converted into a group into which each ion exchange group can be introduced or converted to form an ion exchange membrane. In the description of this specification and claims,
As mentioned above, a membrane-like material made of a polymer compound having a group into which an ion exchange group can be easily introduced or which can be exchanged with an ion exchange group is referred to as an ion exchange membrane precursor (or simply a precursor). As is well known, in the technical field dealing with ion exchange membranes, various chemical treatments are applied to the ion exchange membrane itself or to its precursor, and then ion exchange groups are introduced or processed. Whether or not to perform conversion processing is almost arbitrary, and technically they are considered the same. The present inventors, based on the various findings mentioned above,
As a result of various studies with the aim of developing an excellent cation exchange membrane particularly suitable for electrolysis of alkali metal salts, etc., we basically formed a thin film-like ion exchange membrane using an extrusion molding method. At least one side of the exchange membrane is coated with at least one
We have arrived at a method in which ion exchange groups different from the ion exchange groups already present in the membrane are introduced into one surface newly formed by micron scraping. The extrusion molding used in the present invention usually involves melting a perfluorocarbon polymer compound (molecular weight 10,000 or more) having acid halide groups, acid anhydride groups, acid ester groups, etc., which is a precursor of a cation exchange membrane.
It is molded into a film using a molding machine using either an inflation molding method or a T-die molding method. The thickness of the film for use as an ion exchange membrane is usually 10 microns to 800 microns, preferably 50 microns to 250 microns. Here, the perfluorocarbon ion exchange membrane made by homogeneous extrusion molding of the present invention or the precursor that can become the ion exchange membrane is a mixture of fine powder of an infusible ion exchange resin and an inert thermoplastic polymer. The electrochemical performance and membrane structure are essentially different from the so-called heterogeneous membrane formed by melting and forming a membrane using the membrane as a binder. In other words, in such a heterogeneous membrane, inert polymers may cover the ion exchange surface layer during membrane formation, and in such cases, removing the surface layer of the ion exchange membrane reduces electrical resistance. Although it has been reported that this may occur, this is technically completely different from the removal of the surface layer of the homogeneous membrane in the present invention. The homogeneous perfluorocarbon ion exchange membrane of the present invention or its precursor is a group into which a cation exchange group can be introduced, a group convertible to a cation exchange group, or a linear or partially branched membrane with a cation exchange group bonded thereto. It is a perfluorocarbon polymer that can be melt-extruded. In this case, even if an inert polymer such as polytetrafluoroethylene is present in a fibrillated state in order to maintain strong mechanical strength in the polymer membrane, it may also be used as a corrosion-resistant base of the woven or nonwoven fabric. Even if the material is present in a state that does not significantly degrade membrane performance, it is still within the scope of the homogeneous ion exchange membrane of the present invention. Among the so-called homogeneous ion-exchange membranes, there are some that form cluster structures and have non-uniformity in the distribution of ion-exchange groups when looking at the microscopic membrane structure. Needless to say, it is included in the term homogeneous in the invention. A specific example of an ion exchange membrane made of a perfluorocarbon polymer compound or its precursor that can be used in the present invention is a copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether sulfonyl fluoride (by extrusion molding (can be made into a perfluorocarbon polymer film and then hydrolyzed to form a perfluorocarbon sulfonic acid type cation exchange membrane), perfluorocarbon carboxylic acid type membrane or perfluoro tertiary alcohol, and dissociable hydrogen atoms. Perfluorocarbons with functional groups, such as membranes with sulfonic acid amide groups or phosphoric acid groups, those with functional groups that dissociate in aqueous solution to become negatively charged, those that can introduce them, and those that can be converted into them. All polymeric materials of the series are preferably used. Preferred examples of the cation exchange group include a sulfonic acid group, a carboxylic acid group, a phosphoric acid group, a phosphorous acid group, a perfluoro tertiary alcohol group, and a sulfonic acid amide group having a dissociable hydrogen atom. Examples of constituent elements of perfluorocarbon polymers that have such ion exchange groups or groups that can be converted into or introduced into ion exchange groups include ion exchange groups or groups that can introduce or convert ion exchange groups. As a monomer having, CF ₂ = CFO (CF ₂ ) _o −A CF ₂ = CFOCF ₂ (−OCFXOCF _2o ) (—CFX′)
− _n (−CF ₂ OCFX″) − _l A CF ₂ = CFCF ₂ O (−CFX) − _o (−CFX′) − _n (
−O) _−r )−CFX″(− _p A CF ₂ =CF(CF ₂ )− _o A, CF ₂ =CF(−CF ₂ )− _o
(-O)- _r (CFX)- _n A CF ₂ = CF(CFX)- _o (-CFX')- _n (O) _r-
(CFX") - (O) _r - ( _CFX ) - _l A (X, X', X" each represents -F, _-CF3 , _-C2F5.n , m, l, p is 0 or a positive integer up to 12, r is 0 or 1, A is -CN, -
COOH, −COX, −COOM, −COOR, Indicates OH, -OR _1, etc. However, X and X′′′′ are halogens, M is alkali metals, alkaline earth metals,
R ₁ represents a C ₁ -C ₁₂ alkyl group, R ₂ represents a C ₁ -C ₁₂ alkyl group, a hydrogen atom, or the like. ) or a vinyl monomer having a functional group that can be easily converted into a cation exchange group. In addition, examples of monomers copolymerizable with the above monomers include perfluoroolefins, such as CF ₂ =
_CF2 , CF ₂ = CF - CF = CF ₂ , CF ₂ = CFO (CFX) _o OCF =
CF ₂ (X is F, CF ₃ ), etc. Now, these ion exchange groups used in the present invention,
Alternatively, a polymer obtained by extrusion molding a copolymer of a perfluorocarbon monomer having a group capable of introducing an ion exchange group or convertible into an ion exchange group, and a perfluorocarbon monomer copolymerizable with the perfluorocarbon monomer. As a natural result, a molecular film has a skin layer on its surface that has physical properties different from those inside the film. Not only does this skin layer have an effect on increasing the electrical resistance of the membrane, but also ion exchange groups that are different from the ion exchange groups inherent in the ion exchange membrane are formed in one membrane through a chemical reaction. It was unexpectedly discovered that when introduced, it has the effect of delaying the infiltration of reagents into the membrane and lowering the reaction rate. In other words, a chemical reaction in a polymer matrix generally proceeds with the rate-determining step being the diffusion of a reagent into the polymer. On the other hand, if at least 1 micron of the surface layer of the polymer film obtained by extrusion is removed and then a chemical reaction treatment is performed, the reaction proceeds extremely quickly and efficiently, and the introduction reaction of different ion exchange groups etc. It will be done. This fact, together with the effects of the invention proposed in Japanese Patent Application No. 191822/1987, which we had proposed earlier, enabled us to successfully obtain a membrane with low electrical resistance and higher current efficiency. It is not clear why the chemical reactivity is significantly improved by removing a very thin layer from the surface layer of the ion exchange membrane, but in reverse osmosis membranes, a skin layer is formed on the surface layer of the membrane. In other words, so that a dense thin layer exists on the surface of the perfluorocarbon cation exchange membrane during extrusion molding,
When extruded through the slit of the die, there are differences in the polymer structure based on the affinity between the slit material and the polymer, or there are differences in the orientation of the polymer at the part that comes into contact with the slit material. It is assumed that it causes Also,
When the polymer is extruded as a film from the slit, the way the polymer is cooled differs between the surface layer and the inside, and it is speculated that this is manifested as a difference in the crystallinity of the polymer. In any case, it has been confirmed through model experiments that the area that is strongly affected by these effects is approximately 1 micron below the film surface. This surface layer is thought to act as a barrier against reagents penetrating into the membrane during chemical reactions. As a method for removing at least 1 micron from at least one surface of the surface layer of such a homogeneous ion exchange membrane before carrying out a chemical reaction, any known method that is industrially practicable can be used without any restriction in the present invention. . That is, it may be removed by either a chemical reaction or a physical method. Perfluorocarbon compounds, which generally have excellent chemical resistance, are chemically stable and it is easier to physically scrape them off, but chemical methods include sodium naphthalene, n-butyllithium, alkali metals, and alkali metal amalgams. It is relatively easily defluorinated and decomposed by methods such as Alternatively, it is known that perfluorinated compounds having ether bonds are easily decomposed by cleavage of the ether bonds and decomposition by aluminum chloride, aluminum bromide, and the like. Therefore, the surface layer of the cation exchange membrane or its precursor is treated with an organic/inorganic reagent that decomposes such a perfluorocarbon polymer compound to remove at least 1 micron of the surface layer. That is, a method of performing a defluorination reaction and oxidative decomposition, or cutting the main chain or side chain of the surface layer of the perfluorocarbon polymer,
A method of elution and decomposition in concentrated caustic alkali, and other known methods of decomposing and removing the surface layer of a perfluorocarbon polymer, a cation exchange membrane, or a precursor thereof by chemical reaction may be used without any restriction. A physical method is to cut the surface of the perfluorocarbon cation exchange membrane into a thin film with a sharp blade. For example, while the membrane is running on a rotating roll, a sharp blade is cut at a certain distance from the rotating roll. This method continuously removes the surface layer of a polymer membrane to a constant thickness by fixing a knife and moving the membrane on a roll to a constant thickness. It is an effective method. In addition, when performing on a relatively small area,
Methods such as applying sandplast and removing by polishing with sandpaper are also effective. Other treatments such as sputtering and flame treatment can be carried out as appropriate. When removing the surface layer of the membrane by physical or chemical means, the thickness to be removed must be at least 1 micron. Depending on the manufacturing conditions and composition of the perfluorocarbon polymer film, the composition of the polymer material, etc., the thickness and density of adhesion only to the film surface vary significantly, and in some cases, even removal of about 100 Å is not effective. However, generally it is effective if at least 1 micron is removed, and there is no harm in removing 1 micron or more. Particularly effective is when 1 to 10 microns are removed. The thickness to be removed must be selected depending on the thickness of the ion exchange membrane. For example, in the case of a cation exchange membrane with a thickness of 150 microns, removing 10 microns will have little effect on the membrane thickness, and therefore the increase in the salt content in the produced caustic soda will be negligible;
If 50 microns of the micron-thick membrane were removed, the salt content in the resulting caustic soda would increase to a point where it could no longer be ignored. Therefore, the thickness of the surface layer to be removed must be selected depending on the object to which it is applied, but if at least 1 micron or more is removed, introduction of different ion exchange groups by chemical reaction becomes extremely easy. Moreover, removing more than 1/3 of the film thickness is not only meaningless, but also causes a decrease in film strength and an increase in the amount of salt mixed in, which is not preferable. The reaction of introducing a different ion exchange group into the ion exchange membrane whose surface has been removed in this way or the precursor of the ion exchange membrane is as follows:
Specifically, when introducing a sulfonyl chloride group by treating a membrane having a sulfonic acid group with a reaction solution containing phosphorus pentachloride as one component, the sulfonyl chloride group is further treated with oxidation, reduction treatment, amines, phenol compounds, etc. When forming a carboxylic acid group or a sulfonic acid amide group having a dissociable hydrogen atom, the reaction is further carried out as CF ₂ I by reacting with a compound containing a sulfonyl chloride group and a hydrogen atom to form a carboxylic acid group. In the case of conversion, there is a method of directly converting a sulfonic acid group to a carboxylic acid group, a method of desulfonating a sulfonyl chloride group and adding a vinyl compound having a weakly acidic cation exchange group, and a method of adding a vinyl compound having a weakly acidic cation exchange group to a vinyl compound having a weakly acidic ion exchange group. Conventionally known methods such as graft polymerization of compounds proceed extremely efficiently, and the poor reactivity peculiar to polymer reactions is significantly improved. Conversely, sulfonic acid groups are added to membranes that have weakly acidic cation exchange groups such as carboxylic acid groups, perfluoro tertiary alcohols, phenolic hydroxyl groups, phosphoric acid groups, and sulfonic acid amide groups bonded to dissociable hydrogen atoms. It is also an extremely effective method especially when introducing strong acid type ion exchange groups such as sulfonic acid groups of perfluorocarbons. For this purpose, conventionally known chemical reactions can be used without any restrictions.
The introduction of such different ion exchange groups is not limited to those having ion exchange groups, but also those having functional groups that can be easily exchanged with ion exchange groups or groups that can be introduced, such as acid halide groups, acid anhydride groups, and esters. A different type of ion exchange group, or a type that can be converted into a different type of ion exchange group, or a type that can introduce such a group, such as a sulfonyl chloride group,
A sulfinic acid group, _-CF2I , etc. may be introduced and then converted into or introduced into an ion exchange group, which may be selected depending on the purpose, membrane type, ion exchange group, etc. as appropriate. EXAMPLES The present invention will be specifically explained below with reference to Examples, but the present invention is not restricted to these specific examples in any way. Example 1 Tetrafluoroethylene and perfluoro(3,
A T-die extruded sheet of a copolymer consisting of 6-dioxa-4-methyl-7-octensulfonyl fluoride was hydrolyzed to yield an exchange capacity of 0.91 meq/g and a dry membrane (H type) thickness of 175 microns. A film was obtained. This was shaved by about 3 microns on each side by applying a sharp blade perpendicular to the film surface. The thickness of the film was measured at 10 locations with a micrometer and averaged.
It was 169 microns. Separately, a film was also made in which only one side was shaved down by about 6 microns. Next, the above two types of membranes were converted into carboxylic acid groups by converting the sulfonic acid into sulfonyl chloride using phosphorus pentachloride and by air oxidation of the sulfonyl chloride group in alcohol using the method described in JP-A-53-58493. Then, the remaining sulfonyl chloride groups were converted into sodium sulfonate by hydrolysis, and an ion exchange membrane with a double structure consisting of carboxyl groups on the surface layer as ion exchange groups and sulfonic acid groups on the inside was obtained. For membranes with 6 microns removed from only one side,
The deleted faces were converted to carboxylic acid groups. These membranes were used to electrolyze a saline solution. In other words, with an effective current carrying area of 1 dm ² , a current density of 30 A /
dm ² and 80° C., the salt concentration in the anode chamber was set to 3.5N, and 9N of caustic soda was constantly taken out from the cathode chamber.
Table 1 shows the electrical resistance, electrolytic voltage, and other electrolytic performance of the membrane. The electrolysis was carried out with the membrane surface into which carboxylic acid groups were introduced facing the cathode.

[Table] Example 2 The surface layer of the sulfonic acid type cation exchange membrane used in Example 1 was removed to various thicknesses using fine sandpaper (Sankyo Rikagaku, CC1500cw), and the removed membrane surface was was chemically treated in the same manner as in Example 1 to introduce a carboxylic acid group. Next, the infrared absorption spectra of the films of each year were measured and −COOH
The distribution of groups and _-SO3H groups was measured. In addition, for comparison, the same test was carried out for the case where the above-mentioned film surface was not removed. In other words, for the film in which carboxylic acid groups were introduced without surface removal and the film in which carboxylic acid groups were introduced by surface removal at each thickness, first, the chemically treated surface was shaved to various thicknesses, and then the surface was removed by permeation. The membrane surface where sulfonic acid groups were present was removed so that infrared absorption spectra could be measured. As a result, the degree of conversion to carboxylic acid groups and the distribution and ratio of sulfonic acid groups to carboxylic acid groups were determined. The results were as shown in Tables 2 to 4. That is, Table 2 shows the results of the abundance ratio of each ion exchange group in the thickness of the membrane when chemical treatment was carried out without removing the membrane surface, which was carried out for comparison. Furthermore, Table 3 shows the results of the abundance ratio of each ion exchange group in the thickness of the membrane when 10 microns of the membrane surface was removed and then chemically treated. Furthermore, Table 4 shows the thickness after removing the membrane surface and the ratio of both ion exchange groups (-SO ₃ H/-
This result shows that COOH). In order to make the results of Tables 2 to 4 more clear, they are graphed in FIGS. 1 to 3, respectively. In other words, Figure 1 is Table 2
Figure 2 is a graph of the results of Table 3.
FIG. 3 is a graph of the results of Table 4.

【table】

【table】

[Table] Example 3 Tetrafluoroethylene and CF ₂ =CF-O-
A copolymer of (CF ₂ ) ₃ -COOCH ₃ was formed into a 170 micron thick film by extrusion molding. After removing 10 microns from the surface of this film with sandpaper (Sankyo Rikagaku Co., Ltd., CC1500) on one side, it was immersed in a mixed solvent of caustic soda, water, and methanol in a conventional manner to remove carboxylic acid ester groups. Convert it to a sodium carboxylate group, then remove the membrane surface, leave the carboxylic acid group as a sodium carboxylate group, change the back side to a carboxylic acid group, and then heat the sodium carboxylate side to 250℃ to convert the sodium carboxylate group. Disassembled. This membrane with the ion-exchange groups on one side decomposed was converted into a sodium carboxylate type solution of crystal violet in a 1% water-methanol mixture (1:
When dyed in step 1), about 20 microns of the heated surface remained undyed. That is, the ion exchange group is removed only in this portion. To introduce sulfonic acid groups into this membrane
Sulfonic acid groups were introduced by immersing it in a mixed solution of H ₂ SO ₃ ClCsF and heating it at 150° C. for 24 hours. The membrane was then again immersed in caustic soda-water-methanol to convert the ion exchange groups to the sodium form. Similar carboxylic acid group decomposition reactions and sulfonic acid group introduction reactions were also carried out on membranes whose surfaces had not been removed. Next, electrolysis of saturated saline solution was carried out in the same manner as in Example 1. The results are shown in Table 5.

[Table] Example 4 A copolymer of 1 and tetrafluoroethylene was extruded using a T-die to obtain a sheet with a thickness of 0.15 mm. A mesh made of PFA monofilament was heat-softened and pressed into one side for reinforcement, and then a 25 micron thick inflation film made of the same polymer as above was made to cover the PFA monofilament. The films were stacked together and pressed into a single sheet. Next, sandblasting was applied to the surface behind the surface into which the PFA reinforcing material had been pressed, and the surface layer was removed along the uneven surface of the mesh. The thickness removed by sandblasting was measured by a micrometer at several locations, and an average of 7 microns was removed. This membrane was treated with dimethyl sulfoxide,
Hydrolysis with a mixed solution of water and potassium hydroxide yielded a cation exchange membrane having an exchange capacity of 0.87 milliequivalents/g dry membrane with sulfonic acid groups as ion exchange groups. The above-mentioned film bound to sulfonyl fluoride before hydrolysis was reacted with hydrazine to convert it into a sulfinic acid group. In this case, the membrane was placed in a reactor capable of reacting only on one side, and only the side with the surface layer removed was brought into contact with 85% hydrazine hydrate for 30 minutes. After thoroughly washing this membrane with water, add formic acid and 37
The sulfinic acid groups were converted to carboxylic acid groups by immersion in a 5:1 solution of % hydrochloric acid in the presence of oxygen, followed by immersion in a sulfuric acid and potassium dichromate solution and heating. Next, the remaining sulfonyl fluoride groups were converted to sulfonic acid sodium salt by hydrolysis treatment with a mixed solution of dimethyl sulfoxide, water, and caustic soda. On the other hand, with the same membrane having the same sulfonyl fluoride group as above, the same reaction as above was carried out without sandblasting on the same side of the surface that had been sandblasted. These two types of membranes were subjected to electrolysis of saturated saline in the same manner as in Example 1. The results are shown in Table 6.

[Table] Example 5 Using the PFA-reinforced perfluorocarbon sheet bonded with sulfonyl fluoride groups used in Example 4, one membrane surface was sandblasted in the same manner as in Example 4, and then Using a device capable of reacting only on one side, the surface of the membrane whose surface was removed was brought into contact with gaseous methylamine for 6 hours to form sulfonic acid amide groups, and then immersed in a mixed solution of dimethyl sulfoxide, water, and potassium hydroxide. The remaining sulfonyl fluoride groups were converted to potassium sulfonate, and a sulfonamide group was formed as a weakly acidic cation exchange group. On the other hand, for the membrane whose surface had not been removed by sandblasting, methylamine was reacted on the same membrane surface under the same conditions as above, and then the sulfonyl fluoride group was hydrolyzed. Electrolysis of a saline solution was carried out in the same manner as in Example 1. The results are shown in Table 7.

【table】 [Brief explanation of drawings]

The attached drawings, Figure 1, are graphs showing the distribution of sulfonic acid groups and carboxylic acid groups distributed in the membrane when chemically treated without surface removal, and Figure 2 is a graph showing the distribution of sulfonic acid groups and carboxylic acid groups distributed in the membrane when chemically treated without surface removal.
It is a graph showing the distribution state of sulfonic acid groups and carboxylic acid groups distributed in the film when chemically treated after removing 10 microns. Further, FIG. 3 is a graph showing the relationship between the thickness of the surface removed and the ratio of -SO ₃ H to -COOH.

Claims (1)

[Claims] 1. Removal of at least 1 micron of the surface layer of a homogeneous perfluorocarbon ion exchange membrane obtained by extrusion molding, or a precursor having a group capable of converting into an ion exchange group or introducing an ion exchange group. After introducing an ion exchange group different from the specific ion exchange group ultimately possessed by the ion exchange membrane or the precursor into the newly formed surface by a chemical reaction, if necessary, the precursor A method for producing a perfluorocarbon-based ion exchange membrane, which comprises carrying out a reaction of converting or introducing an ion exchange group into a perfluorocarbon-based ion exchange membrane. 2. The manufacturing method according to claim 1, wherein the inherent ion exchange group or the ion exchange group converted from the precursor is a sulfonic acid group. 3. The manufacturing method according to claim 1, wherein the inherent ion exchange group or the ion exchange group converted from the precursor is a carboxylic acid group. 4. The manufacturing method according to claim 1, wherein the ion exchange group converted or introduced by a chemical reaction is a carboxylic acid group. 5. The manufacturing method according to claim 1, wherein the ion exchange group converted or introduced by the chemical reaction is a sulfonic acid amide group having a dissociable hydrogen atom. 6 The ion exchange group converted from the inherent ion exchange group or a variant is a sulfonic acid group, and the ion exchange group introduced onto the surface by a chemical reaction is a carboxylic acid group. Production method. 7. The manufacturing method according to claim 1, wherein a carboxylic acid group is introduced by chemical reaction into a sulfonic acid type cation exchange membrane having an exchange capacity of 0.5 to 1.5 meq/g dry membrane (H type).