JPS64977B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an ion exchange membrane for alkali metal salt electrolysis, in which both surfaces of a homogeneous perfluorocarbon cation exchange membrane made by extrusion molding are removed by at least 1 micron. Ion exchange membranes are used industrially in various fields such as electrodialysis, diffusion dialysis, diaphragm reverse osmosis for electrolytic reactions, and ultrafiltration. In particular, the ion-exchange membrane method salt electrolysis, which has been industrialized in recent years, has made remarkable technological progress as an energy-saving process.
Initially, the aim was to obtain highly concentrated caustic soda with high current efficiency using a perfluorocarbon-based ion exchange membrane, but now the current efficiency is over 90%, and it is now possible to maintain this for a long period of time. Ta. Therefore, the biggest challenge at present is to lower the electrolysis voltage. When electrolytic voltage is analyzed, theoretical decomposition voltage, cathode overvoltage, voltage drop due to membrane electrical resistance, etc. account for a large proportion. Although the theoretical decomposition voltage is difficult to control, active cathodes have been developed to reduce the cathode overvoltage, which contributes to reducing the electrolysis voltage. In addition, attempts have been made to reduce the electrical resistance of the membrane by changing the composition of the ion exchange membrane, selecting reinforcing nets, cloth, etc. Further, reducing the thickness of the membrane is an effective method, but there is a limit due to the correlation with the salt content in the produced caustic soda, and the mechanical strength of the membrane must also be taken into consideration. In addition to the various attempts mentioned above, we found that there is heterogeneity in the cross section of the membrane structure of extrusion-molded ion-exchange membranes, and we investigated the surface layer of perfluorocarbon-based ion-exchange membranes made by extrusion-molding. It has been discovered that by removing the material in the form of a thin film, an ion exchange membrane for electrolysis with unexpectedly low electrical resistance can be obtained.
Conventionally, this is a so-called heterogeneous ion exchange membrane, which is made by mixing a hydrocarbon-based fine powder ion exchange resin and an inert polymer to form a film. is known to have a high electrical resistance, and if its surface layer is removed with sandpaper or the like, fine particles of ion exchange resin will be exposed and the electrical resistance will drop significantly. It is also known that reverse osmosis membranes have a dense thin layer called a skin layer on the membrane surface. It is generally known that in polymer films, particularly extrusion-molded films, there are structural differences between the inside and the surface layer of the film. However, until now, no study has been conducted on the effect of the surface layer of a so-called homogeneous ion exchange membrane on the electrical resistance and other electrochemical properties of such a membrane. We conducted experiments assuming that the nonuniformity of the membrane structure of extruded films also exists in ion exchange membranes, and that this has some effect on the electrical resistance of the membrane, and we were able to complete the present invention. Ivy. The homogeneous cation exchange membrane referred to in the present invention is a membrane composed of an inert thermoplastic polymer in order to retain an insoluble and infusible cation exchange resin in the form of a membrane, which is known as a conventional heterogeneous membrane. This refers to cation exchange membranes. An infusible inert polymer for reinforcing, such as fibrillated polytetrafluoroethylene, coexists with a perfluorocarbon polymer having extrudable cation exchange groups or into which cation exchange groups can be introduced. It also belongs to the class of homogeneous cation exchange membranes of the present invention. In other words, a homogeneous ion-exchange membrane is one in which there is non-uniformity in the distribution of ion-exchange groups in any ion-exchange membrane when viewed from the microstructure.
A perfluorocarbon polymer compound that has an ion exchange group or can easily introduce an ion exchange group can be used as a cation exchange membrane after undergoing the ion exchange group introduction process. All are included. The perfluorocarbon-based cation exchange membrane made by extrusion molding of the present invention includes tetrafluoroethylene and perfluoro(3,6-dioxa-4-methyl-7), which are well known.
There are perfluorosulfonic acid type membranes that are obtained by hydrolyzing a perfluorocarbon-based cation exchange membrane formed by extruding a copolymer of octensulfonyl fluoride (-octensulfonyl fluoride). Generally, there are no particular limitations as long as it is a perfluorocarbon-based cation exchange membrane represented by the following formula. As the cation exchange group, a sulfonic acid group, a carboxylic acid group, a phosphoric acid group, a phosphorous acid group, a tertiary alcohol group, a sulfonic acid amide, etc. are used as appropriate. To illustrate these (x, x', x'' represent -F, -CF ₃ , -C ₂ F _5, respectively. n, m, l, p are positive integers up to 0 or 12, A is -CN, -COOH, -COX, -COOM, -
COOR,

【formula】

【formula】

[Formula] -SO ₃ H, -SO ₃ M, -SO ₂ M, - SO ₂ H, -SO ₂ X, -SO ₂ R, -SO ₃ R, -SH, -P
(OH) ₂

[Formula]-P(OM) ₂ ,

【formula】

【formula】

[Formula] represents −PX ₂ , OH, −OR ₁ , etc. However, x and x' are halogen, M is an alkali metal or alkaline earth metal, R ₁ is a C ₁ to C ₁₂ alkyl group, R ₂
represents a _C1 to _C12 alkyl group or a hydrogen atom. ) or a functional group that can be easily converted into a cation exchange group and a fluorinated olefin, such as CF ₂ =
_CF2 ,

【formula】

[Formula] CF ₂ = CFCl,

[Formula] (Rf is a saturated perfluoroalkyl group) CF ₂ = CF−CF=CF ₂ , CF ₂ = CFO (CFX) _o OCF
= CF ₂ (X is F-CF ₃ ), etc., are copolymerized and formed into a film shape. These membranes are cation exchange membranes that can obtain highly concentrated alkali metal hydroxides with high current efficiency by introducing ion exchange groups as necessary after film formation. In order to improve efficiency, it is necessary to form a two-layer membrane in which different types of layers are present, for example, different types of ion exchange groups or layers with different exchange group densities are present. In particular, to maintain high current efficiency and lower membrane electrical resistance,
For example, there is a membrane that has a sulfonic acid group on the anode side and a carboxylic acid group on the cathode side. Alternatively, there is a membrane with a larger exchange capacity on the membrane surface facing the anode and a smaller one on the cathode side, or an ion exchange membrane with a sulfonic acid group on the anode side, and a phosphoric acid group, tertiary alcohol group, or dissociated membrane on the cathode side. It is preferable to use a thin film bonded with a sulfonic acid amide group having a hydrogen atom that can absorb moisture. Other chemical treatments for cation exchange membranes include hydrolysis reaction, conversion reaction to sulfonic acid groups, conversion reaction of sulfonic acid groups to carboxylic acid groups, conversion reaction of carboxylic acid groups to sulfonic acid groups, sulfonic acid amide. There is also known a means for improving the current efficiency of a cation exchange membrane for electrolysis by carrying out a group introduction reaction or the like. In any case, the present invention applies to any membrane, as long as it is a perfluorocarbon-based ion exchange membrane obtained by extrusion molding at any membrane forming stage, and not only when used as is.
It can be effectively applied even if it goes through a process such as lamination or cloth backing. Now, extrusion molding is usually done by 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, and then using an inflation molding method or T-die molding. It is molded into a film using one of the following methods: 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. Further, as a method for removing at least 1 micron of the surface layer from the surface of such a homogeneous ion exchange membrane, any known industrially applicable method may be used without any restriction. 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 such as sodium, naphthalene,
It is relatively easily defluorinated and decomposed by n-butyllithium, alkali metals, alkali metal amalgams, etc. 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. In other words, a method of performing a defluorination reaction and oxidative decomposition, a method of cutting the main chain or side chain of the surface layer of a perfluorocarbon polymer, and eluting and decomposing it in concentrated caustic alkali, and other chemical reactions. Therefore, other known methods for decomposing and removing the surface layer of a perfluorocarbon polymer compound, a cation exchange membrane, or a precursor thereof may be used without any restriction. A physical method is to cut the surface of the perfluorocarbon cation exchange membrane into a thin layer 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. A method that 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 effective as such. In addition, when performing on a relatively small area,
Effective methods include sandblasting and polishing with sandpaper. Other treatments such as sputtering and flame treatment can also 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. The thickness and density of the dense layer on the membrane surface vary significantly depending on the manufacturing conditions and composition of the perfluorocarbon polymer membrane, the composition of the polymer substance, etc., and in some cases even removal of about 100A° Although it may be effective in some cases, generally it is effective if at least 1 micron is removed, and there is no harm in removing 1 micron or more. The thickness to be removed may 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 has little effect on the membrane thickness, and therefore the increase in salt content in the produced caustic soda is negligible; If 50 microns of 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 at which the surface layer is removed must be selected depending on the object to which it is applied, but if at least 1 micron or more is removed, the electrical resistance of the ion exchange membrane will be significantly reduced. 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. Now, it is not clear why the electrical resistance of the membrane is reduced by just removing a very small amount of the surface layer of the cation exchange membrane, but in reverse osmosis membranes, there is a thin layer called a skin layer on the membrane surface. During extrusion molding, on the surface of the perfluorocarbon-based cation exchange membrane, 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 due to the part that comes into contact with the slit material. It is presumed that there is a difference in the orientation of the polymer, which causes a difference between the surface layer of the film and the inside of the film. It is also speculated that when the polymer is extruded as a film from the slit, the way it is cooled differs between the surface layer and the inside, which is manifested as a difference in the crystallinity of the polymer. In any case, the parts that are strongly affected by these are
It was confirmed through model experiments that the area is 1 micron or less below the film surface. This membrane surface layer seems to act as a kind of barrier when ions enter the membrane or exit from the membrane into the solution. Removal of such a barrier layer would therefore significantly reduce the electrical resistance of the membrane. Furthermore, since it is preferable to remove both surfaces of a cation exchange membrane rather than removing one membrane surface, the present invention is particularly directed to a cation exchange membrane in which both surfaces of the membrane have been removed. When removing only one side, the effect is greater if the side where ions enter the membrane is removed. In addition, when removing the membrane surface layer, 1) a method in which a cation exchange group is introduced by deleting a precursor in which a cation exchange group has not been introduced, 2) a method in which a cation exchange group is introduced and then deleted, 3. ) A method of removing a weakly acidic cation exchange group to improve current efficiency before treatment such as introduction, 4) A method of removing a weakly acidic cation exchange group etc. after introduction to improve current efficiency, etc. Although 2) and 3) are also effective, the most preferred are 2) and 3). Particularly in alkali metal salt electrolysis, in order to maintain high current efficiency when obtaining a high level of caustic soda, a thin film of weakly acidic cation exchange groups is often formed on one side of the membrane, and this is several microns thick. Provides sufficient functionality with its thickness. A layer formed by forming such a thin film through an oxidation/reduction reaction or other reaction, or by fusing a thin film with a weakly acidic cation exchange group onto a strongly acidic cation exchange membrane. may be destroyed. Therefore, when a thin layer of such weakly acidic cation exchange groups is desirably formed by a chemical reaction, it is preferable to remove the surface layer of the membrane before the chemical reaction. The content of the present invention will be specifically explained with reference to the following examples, but the present invention is not limited to the following examples. 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 178 microns. A film was obtained (referred to as film A). About 5 times each side
A sharp blade was applied perpendicularly to the surface of the film to scrape it micron by micron. The thickness of the film was measured with a micrometer and was found to be 168 microns (referred to as film B). Membrane A
In the same way, only one side was shaved by approximately 10 microns (referred to as film C). In addition, membrane A was converted into a carboxylic acid group by converting the sulfonic acid into a sulfonyl chloride using phosphorus pentachloride and oxidizing the sulfonyl chloride group in the air in an alcohol using the method described in JP-A-53-58493. The sulfonyl chloride group of is converted into sodium sulfonate by hydrolysis, and an ion exchange membrane with a double structure consisting of a carboxyl group on the surface layer as an ion exchange group and a sulfonic acid group on the inside is made (referred to as membrane D). Separately, the above treatment was performed using membrane B to obtain an ion exchange membrane (referred to as membrane E). Further, the removed surface of membrane C was subjected to the above treatment to obtain another ion exchange membrane (referred to as membrane F). Similarly, a film obtained by performing the above treatment on the smooth surface of film C is referred to as film G. These membranes were used to electrolyze a saline solution. That is, the effective current carrying area is 1 dm ² , the current density is 30 A/dm ² , and the current density is 80°C using an electrolytic cell consisting of a ruthenium oxide coating on a titanium lath as an anode and a soft iron lath as a cathode.
The salt concentration in the anode chamber is 3.5N, and the salt concentration in the cathode chamber is 9N.
of caustic soda was taken out regularly. In both films, the current efficiency was approximately 95%, and the amount of common salt in the caustic soda was approximately 60 ppm (50% caustic soda equivalent). The electrical resistance and electrolytic voltage of the membrane are shown in Table 1.

[Table] Example 2 Tetrafluoroethylene and CF ₂ =CF-O(-CF ₂
-) ₃ A copolymer of COOCH ₃ was extruded into a film with a thickness of 170 microns. The exchange capacity of this film was 1.45 meq/gram dry film. This was immersed in a 1:1 mixed solution of 10% water and methanol at 50° C. for 16 hours to obtain a cation exchange membrane having carboxylic acid groups. For this film, sandpaper (AA400 manufactured by Nagatsuka Industries) approximately 5 microns each on the anode side, cathode side, and both sides.
, and three types of membranes were obtained. The thickness of the film was measured using a micrometer. We investigated the electrochemical properties of these membranes and used them to conduct salt electrolysis. The results are shown in Table 2. The procedure was the same as in Example 1 except that 35% caustic soda was obtained from the cathode chamber.

[Table] Example 3 Tetrafluoroethylene and CF ₂ =CFO(-CF ₂ -)
A polymer was obtained by copolymerizing ₄ SO ₂ F using azoisobutyronitrile as a catalyst. This was then extruded into a film with a thickness of 150 microns.
This was immersed in a 600:400:15 hydrolysis bath of dimethyl sulfoxide, water, and potassium hydroxide to convert the sulfonyl fluoride groups to sulfonic acid groups.
The conversion capacity was 1.01 meq/dry membrane (H type). One side of this cation exchange membrane is brought into contact with phosphorus pentachloride vapor to convert the sulfonic acid groups into sulfonyl chloride groups, and then air oxidized in n-butyl alcohol to convert them into sulfonic acid groups.
Immerse it in the above hydrolysis bath again and apply approximately
The membrane had a carboxylic acid layer of 10 microns. Sand the surface layer of the membrane obtained here with sandpaper (manufactured by Sankyo Rikagaku Co., Ltd.) on both the anode side and cathode side.
CC600CW) was used to remove approximately 5 microns of the surface layer. Table 3 shows the results of measuring the properties of these membranes and their performance during salt electrolysis. From the cathode chamber
35% caustic soda was obtained regularly.

[Table] Example 4 Tetrafluoroethylene and perfluoro(3,6-dioxa-4-methyl-7) used in Example 1
- octensulfonyl fluoride) copolymer (before hydrolysis treatment), one and both sides of each sheet were shaved by 20 microns by the method of Example 1. Then the sheet is 10%
The sheet was immersed in methanol containing NaOH at room temperature for 10 hours to hydrolyze the sulfonyl fluoride groups in the sheet to form sodium sulfonate groups. In order to further convert it into the H acid form, it was immersed in 1N hydrochloric acid at room temperature for 15 hours. Thereafter, it was washed with water and dried under reduced pressure to obtain an acid type sheet. Furthermore, in order to convert sulfonic acid groups into carboxylic acid groups, we placed an acid-type sheet on the inner wall of a cylindrical glass reactor (inner diameter 10 cm) with the shaved side (for sheets with both sides scraped, any surface) on the outside. Attachment: A Toshiba germicidal lamp (GL-15) was set in the center of the reactor so that it was approximately equidistant from the surface of the sheet. The reactor was immersed in an oil bath and heated to 150°C.
After the inside of the reactor was sufficiently purged with nitrogen, it was evacuated using a vacuum pump. After that, take 1 nitric oxide from the cylinder.
Kg/cm ² was introduced, the reactor was sealed, a germicidal lamp was turned on, and irradiation was performed for 30 minutes. The lamp was then extinguished and nitrogen was introduced into the reactor to purge the nitric oxide. On the other hand, for comparison, a sheet whose one side was not shaved off was also processed under the same conditions. When the infrared spectrum (ATR method) of the treated sheet was measured, the absorption band of carboxylic acid groups was observed at 1780 cm ^-1 in all areas irradiated with light, and the absorption band of sulfonic acid groups was observed at 1060 cm ^-1 . was hardly recognized. On the other hand, on the opposite side, no absorption band at 1780 cm ^-1 was observed, but a strong absorption band at 1060 cm ^-1 was observed. In order to subject these sheets to a cation exchange membrane electrolysis test, the treated sheets were immersed in methanol containing 10% NaOH at room temperature for 2 hours. After immersion, the sample was washed with water and subjected to a saline electrolysis test according to the method of Example 1. The results obtained are shown in Table 4.
When assembling it into an electrolytic cell, the surface on which carboxylic acid groups were present (the surface irradiated with light) was directed toward the cathode side.

【table】

Claims (1)

[Scope of Claims] 1. A cation exchange membrane in which at least 1 micron is removed from both surfaces of a homogeneous perfluorocarbon cation exchange membrane produced by extrusion molding. 2. The ion exchange membrane according to claim 1 is
A cation exchange membrane for alkali metal salt electrolysis having an exchange capacity of 0.5 to 2.0 milliequivalents and 1 gram dry membrane (H type). 3. A cation exchange membrane according to claim 1, wherein the exchange group of the ion exchange membrane is either or both of a sulfonic acid group and a carboxylic acid group. 4. A cation exchange membrane according to claim 3, in which sulfonic acid groups and carboxylic acid groups exist in a layered manner.