JPS6410543B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD OF THE INVENTION The present invention relates to stacked fluorocarbon cation exchange membranes. BACKGROUND ART In recent years, there has been a growing movement to develop new chemical processes using fluorocarbon cation exchange membranes and cation exchange resins that have excellent chemical resistance and heat resistance. A typical example of this trend is in industries that produce caustic soda and chlorine through the electrolysis of table salt, which has advantages in terms of pollution prevention and energy conservation compared to the conventional mercury method and diaphragm method. It is possible to produce caustic soda with the same quality as the mercury method.
The ion exchange membrane method is attracting a lot of attention. The most important factor governing the economic efficiency of the ion exchange membrane method is the characteristics of the cation exchange membrane used, which must satisfy the following conditions. (1) High current efficiency and low electrical resistance. For high current efficiency, the membrane needs to have a sufficiently large ion exchange capacity and low water content, resulting in a high fixed ion concentration within the membrane. On the other hand, in order to have a low electrical resistance, it is advantageous for the water content to be high. Since the water content varies depending on the type of ion exchange group, ion exchange capacity, and concentration of the external liquid, an optimal combination thereof is required. (2) Can withstand high temperature chlorine and alkali for long periods of time. Cation exchange membranes made of fluorocarbon polymers generally withstand the above-mentioned atmospheres well, but depending on the type of ion exchange group, some may not have sufficient chemical stability, so it is important to select an appropriate ion exchange group. is necessary. (3) Must have sufficient physical strength in high temperature and highly concentrated aqueous alkaline solutions. In general, the physical strength of a membrane is determined by the physical structure of the membrane,
Since it varies depending on the polymer composition, ion exchange capacity, type of ion exchange group, etc., it is necessary to realize an optimal combination of these. (4) The manufacturing method is easy and the cost is low. Conventionally, several fluorocarbon cation exchange membranes have been proposed for use in electrolysis of aqueous solutions of alkali metal halides. For example, a copolymer of tetrafluoroethylene and perfluoro-3,6-dioxa-4-methyl-7-octensulfonyl fluoride is hydrolyzed to produce a fluorocarbon cation exchange membrane having sulfonic acid groups in the side chains. What has been done is known. Problems to be Solved by the Invention However, fluorocarbon cation exchange membranes having only sulfonic acid groups exhibit only low current efficiency because of their high water content and, as a result, the low fixed ion concentration within the membrane. Even if the ion exchange capacity is lowered to increase the concentration of fixed ions in the membrane, it is impossible to achieve a current efficiency of over 90%, and on the contrary, the electrical resistance increases, making it impossible to perform electrolysis economically. It was possible. On the other hand, for the same ion exchange capacity compared to membranes with sulfonic acid groups, various fluorocarbon cation exchange membranes with carboxylic acid groups have lower water content in the membrane and higher concentration of fixed ions in the membrane. It is presented for the purpose of improving efficiency. However, although fluorocarbon cation exchange membranes consisting only of carboxylic acid groups can achieve current efficiency of 90% or more, they have the drawback of high electrical resistance, resulting in high electrolysis voltage and, as a result, high power consumption. Furthermore, as mentioned above, in order to combine the characteristics of a sulfonic acid group, which has a low electrical resistance due to a high water content, and the characteristics of a carboxylic acid group, which has a high current efficiency due to a low water content, a carboxylic acid group or A cation exchange membrane with a two-layer structure in which a fluorocarbon polymer having only a group that can be derived into the group and a fluorocarbon polymer having only a sulfonic acid group or a group that can be derived into the group are laminated as membranes. is disclosed. These two-layered cation exchange membranes had a preferable structure in that they took advantage of both the characteristics of the carboxylic acid group and the characteristics of the sulfonic acid group. However, in a two-layered cation exchange membrane, when current is applied for a long period of time under conditions of high current density, the laminated interface may peel off, resulting in a decrease in current efficiency. It is understood that these phenomena are caused by an excessively large difference in water content between the layers at the laminated interface. That is, current efficiency is generally controlled by the concentration of ions fixed in the surface layer of the cation exchange membrane facing the cathode side. The higher the fixed ion concentration in the membrane, and therefore the lower the water content, the higher the current efficiency, and the higher the membrane water content, and the thinner the membrane thickness, the lower the electrolytic voltage tends to be. In addition, in the above-mentioned two-layered cation exchange membrane, it is said that it is preferable to apply current to the layer having the carboxylic acid group toward the cathode side. In order to make the above-mentioned current efficiency as high as possible and the electrolytic voltage as low as possible, it is preferable that the layer having carboxylic acid groups has a low water content and the layer having a sulfonic acid group has a high water content. A problem is encountered in that the difference in water content at the interface becomes large and peeling at the laminated interface is likely to occur. The cause of peeling is not theoretically clear, but during energization, such as when electrolyzing a saline solution, sodium ions move within the membrane from the anode side to the cathode side, accompanied by hydration water, which causes rapid delamination at the lamination interface. A large resistance difference occurs. It is thought that the greater the difference in water content at the laminated interface, the greater the resistance, and the more likely peeling occurs at the laminated interface. Means for Solving the Problems As a result of intensive research in order to overcome the drawbacks of the above-mentioned two-layered cation exchange membrane, the present inventors found that a three-layer membrane consisting of a layer containing a carboxylic acid group and a layer containing a sulfonic acid group has been found. With a laminated cation exchange membrane with a layered structure, by setting the difference in equivalent weight between each layer and the thickness of each layer within a specific range, problems such as peeling at the laminated interface can be solved at once, making it possible to conduct electricity for a long period of time. The present invention has been completed by developing a new cation exchange membrane that stably exhibits an extremely high current efficiency and low electrolytic voltage. That is, the present invention provides a layer () made of a fluorocarbon polymer containing an atomic group -O(CF ₂ ) ₂ COOM (M is H, metal, NH ₄ group) in the side chain or a part of the side chain; It consists of three layers: layer () and layer () made of a fluorocarbon polymer containing an atomic group -O(CF ₂ ) ₃ SO ₃ M (M is H, metal, NH ₄ group) in a part of the side chain. Equivalent weight (EW ₁ ) and thickness (l ₁ ) of (),
A laminated fluorocarbon cation exchange membrane in which the equivalent weight (EW ₂ ) and thickness (l ₂ ) of the layer () and the equivalent weight (EW ₃ ) and thickness (l ₃ ) of the layer () satisfy the following formulas, respectively. It provides: 50≦EW ₂ −EW ₁ ≦200 150≦EW ₂ −EW ₃ ≦500 l ₁ +l ₂ /l ₁ +l ₂ +l ₃ ≦1/2, 1/20≦l ₁ /l ₁ +l ₂ ≦3/4 Here The equivalent weight is the number of grams of polymer containing one equivalent of ion exchange capacity. The present invention will be explained in detail below. The laminated fluorocarbon cation exchange membrane of the present invention is composed of three layers, with an inner layer () made of a fluorocarbon polymer containing sulfonic acid groups, and a layer () made of a fluorocarbon polymer containing carboxylic acid groups on both sides of the inner layer. layer() and layer()
It has a three-layer structure in which layers ( ) made of a fluorocarbon polymer containing sulfonic acid groups whose equivalent weight is smaller than that of the fluorocarbon polymer are laminated. Layer () serves as a kind of buffer layer to reduce the difference in water content between layer () and layer (). The equivalent weight of layer () is
800-2000 preferably 850-1650. If the equivalent weight exceeds 2000, the electrical resistance is too high to be practical. On the other hand, the lower limit of the equivalent weight is constrained by the lower limit of the equivalent weight of the layer (). The equivalent weight of the layer () is smaller than the equivalent weight of the layer (), and the difference is between 50 and 200, preferably
It is 70 or more and 150 or less. Usually, the equivalent weight of the layer () is between 750 and 1950, preferably between 800 and 1600. If the difference in equivalent weight is less than 50, the difference in moisture content between the layers () is too large and peeling phenomenon is likely to occur at the laminated interface. If the difference exceeds 200, no significant improvement in current efficiency can be expected. The equivalent weight of the layer () is also smaller than the equivalent weight of the layer (), and the difference is between 150 and 500, preferably
200 or more and 450 or less. The equivalent weight of the layer () is usually 650-1850, preferably 700-1500. If the equivalent weight of the layer () is less than 650, the membrane will swell significantly and its mechanical strength will decrease. On the other hand, if it exceeds 1850, the electrical resistance of the film becomes too high, which is not preferable. Furthermore, if the difference in equivalent weight between layer () and layer () is less than 150, the difference in water content between the two layers will be small and the effect of lowering the electrolytic voltage will be weakened. If it exceeds 500, the difference in moisture content is too large and a peeling phenomenon occurs at the laminated interface. The proportion of carboxylic acid groups contained in the layer () to all exchange groups in the layer depends on the equivalent weight of the layer, but is generally 20% or more and 100% or less, preferably 40% or more and 100% or less. be. The remaining exchange groups are preferably sulfonic acid groups. When the proportion of carboxylic acid groups present is less than 20%, it is generally difficult to achieve sufficiently high current efficiency. The form in which the carboxylic acid groups exist in the layer ( ) may be uniformly distributed in the layer, or the density of the groups may gradually decrease from the surface layer toward the lamination interface with the layer ( ). It's okay. Uniform distribution here means that the proportion of carboxylic acid groups to the total number of ion exchange groups at any point in the layer is constant, and density refers to the surface or a cross section substantially parallel to this. It refers to the ratio of carboxylic acid groups present between layers of unit thickness along the total number of ion exchange groups in the layer. The thickness of each layer is important as it affects the electrolytic voltage. In particular, the thickness of a layer with a low water content greatly contributes to an increase in electrolytic voltage. Therefore, the sum of the thicknesses of the layer () with a low moisture content and the layer () is less than 1/2 of the total film thickness,
Preferably it is 1/3 or less, and usually 1/4 to 1/8 is most preferable. The sum of the thicknesses of layer () and layer () is preferably as thin as 50μ or less, preferably 30μ or less. The thickness of the layer with the lowest moisture content () is
It accounts for 1/20 or more and 3/4 or less of the sum of the thicknesses of layer () and layer (), preferably 1/20 or more and 2/3 or less. The thickness of the layer () is less than 12μ, preferably 10μ
Below, it is usually 9μ to 1μ. The thickness of the entire film in which Layer (), Layer (), and Layer () are laminated is appropriately determined depending on the equivalent weight of each layer, the film strength of each layer, electrolytic conditions, etc., but the lower limit is 50μ, preferably 75μ. Yes, there is no upper limit other than practical constraints. Laminated cation exchange membrane layers of the present invention ()
has an atomic group -O in the side chain or part of the side chain
(CF ₂ ) ₂ COOM (M is the same as above), and the layer () and layer () have an atomic group -O(CF ₂ ) ₃ SO ₃ M in the side chain or a part of the side chain.
(M is the same as above). The fluorocarbon polymer constituting the layer () is structured as follows. That is, at least one monomer selected from the group of olefins represented by the following general formula, CA ₁ A ₂ = CA ₃ A ₄ −(1) (A ₁ and A ₂ are F or H, A ₃ is F, Cl or H,
A ₄ is F, Cl, CF ₃ , -OR _F , H or CH ₃ , R _F is
C ₁ -C ₅ perfluoroalkyl group) Preferably, at least one monomer selected from the group of fluorinated olefins represented by the following general formula, particularly preferably tetrafluoroethylene and CF ₂ =CFL −( 2) (L=F, Cl, CF ₃ , -OR _F or H, _RF is C ₁ ~
( _C3 perfluoroalkyl group) It can be obtained by copolymerizing a fluorocarbon vinyl monomer having a carboxylic acid group represented by the following general formula or a group derivable into this group. (However, θ is 0 or 1Z is

【formula】

[Formula] - CN, -COOR ₁ ,

[Formula]) (R ₁ is an alkyl group having 1 to 5 carbon atoms or H, R ₂ is H or an alkyl group having 1 to 5 carbon atoms, R ₃ is H or an alkyl group having 1 to 5 carbon atoms) In addition, the layer ( The fluorocarbon polymer constituting ) is produced by ternary copolymerization of the fluorocarbon vinyl monomer of formula (4) described later with monomers represented by formula (1) and formula (3), or by copolymerization of the monomer represented by formula (1) with ( It can be obtained by blending a copolymer of a monomer represented by formula (4) and a copolymer of a monomer represented by formula (1) and a monomer represented by formula (3) in a fixed ratio. The layer () or the fluorocarbon polymer constituting the layer () contains at least one monomer selected from the group of fluorinated olefins represented by formula (1), preferably formula (2), particularly preferably tetrafluoroethylene;
It can be obtained by copolymerizing with a fluorocarbon vinyl monomer represented by the following formula (4). (However, r is 0 or 1Y is halogen) In the compound represented by formula (4), r is preferably 0 or 1, and Y is particularly preferably F. Further, the polymer constituting the layer () may further contain a carboxyl group or a group convertible to the carboxyl group. Describing preferred specific examples of formulas (3) and (4),
In the compound of formula (3), CF ₂ = CFO (CF ₂ ) ₂ COOCH ₃ CF ₂ = CFO (CF ₂ ) ₂ COF etc., and in the compound of formula (4), CF ₂ = CFO(CF ₂ ) ₃ SO ₂ F etc. The ratio of fluorinated olefin and fluorocarbon vinyl monomer contained in the copolymer constituting each layer can be adjusted by appropriately selecting the charging ratio of each monomer and polymerization conditions, and the In coalescence, the amount of carboxylic acid groups present can be adjusted by adjusting the blend ratio. Usually, the fluorocarbon polymer constituting the layer () has an equivalent weight of 750 to 1950, preferably 800 to 1600.
In addition, the proportion of carboxylic acid groups or groups that can be induced into these groups relative to all exchange groups or groups that can be induced into these groups is 20% or more and 100% or less, preferably
Adjusted to be between 40% and 100%. In addition, the fluorocarbon polymer constituting the layer () has an equivalent weight of 800 to 2000, preferably 850 to 1650,
The equivalent weight of the fluorocarbon polymer constituting the layer (2) is adjusted to 650 to 1850, preferably 700 to 1500. The copolymers of the present invention are made by conventional polymerization methods known for use in the homopolymerization or copolymerization of fluorinated ethylene. Methods for producing the copolymer of the present invention include a non-aqueous polymerization method and an aqueous polymerization method, and the polymerization temperature is generally 0 to 200°C, preferably 20 to 100°C. The pressure is 0-200Kg/cm ² , preferably 1-50Kg/
^cm2 . Non-aqueous polymerizations are often carried out in fluorinated solvents. A suitable non-aqueous solvent is inert 1,1,2-trichlor-1,2,
2-trifluoroethane or perfluorohydrocarbons such as perfluoromethylcyclohexane,
These include perfluorodimethylcyclobutane, perfluorooctane, and perfluorobenzene. Aqueous polymerization methods for producing copolymers include emulsion polymerization methods, in which the monomers are contacted with an aqueous medium containing a free radical initiator and an emulsifier to obtain a slurry of polymer particles; There is a suspension polymerization method in which a dispersion of polymer particles is prepared by contacting the polymer particles with an aqueous medium containing both a dispersion stabilizer that is inactive against meridization, and this dispersion is precipitated. Free radical initiators used in the present invention include oxidation-reduction catalysts such as ammonium persulfate and sodium bisulfite; organic peroxides such as t-butyl peroxide and benzoyl peroxide; and azobis-based initiators such as azobisisobutyronitrile. Compounds include fluorine radical generators such as N ₂ F ₂ . The polymer thus obtained is heated and melted to form a film by a commonly known technique. For example, T-die film molding using an extruder, inflation molding, calendar molding, compression molding, etc. are applicable. The cation exchange membrane having a stacked three-layer structure of the present invention can be obtained by the following method. Layer (), layer () formed by the method described above
and fluorocarbon polymer films constituting the layer (). For lamination, generally known methods are used in which the laminated interfaces are heated and melted and bonded, such as compression lamination,
Calendar lamination, etc. are used.
Layer (), layer () and layer () may be laminated at the same time, or layer () and layer () or layer ()
After the layer () is laminated, the layer () or the layer () may be laminated on the other surface of the layer (). The temperature during lamination must be higher than the melting temperature of the fluorocarbon polymer constituting each layer and lower than the decomposition temperature of the polymer,
Although it varies depending on the equivalent weight, molecular weight, type of side chain, etc. of the polymer, it is generally 150°C to 400°C, preferably
Temperatures of 200°C to 350°C are applied. The thickness of each layer is such that the ratio of the sum of the thicknesses of layer () and layer () to the total film thickness is 1/2 or less, preferably 1/3 or less, and the ratio of the sum of the thicknesses of layer () and layer () The layers are laminated so that the ratio of the thickness of each layer to the sum is 1/20 or more and 3/4 or less, preferably 1/10 or more and 2/3 or less. The obtained laminated film-like material having a three-layer structure is usually hydrolyzed with an alkaline aqueous solution to form a layer containing the atomic group -O(CF ₂ ) ₂ COOM and a layer containing the atomic group -O(CF ₂ ). ) It becomes a laminated cation exchange membrane consisting of a layer () containing ₃ SO ₃ M and a layer (). In order to put the cation exchange membrane into practical use, it usually requires
To increase the strength of the membrane, it may be lined with a mechanical reinforcing material such as a mesh, most preferably a mesh made from polytetrafluoroethylene fibers. Reinforced cation exchange membranes are usually used with a reinforcing material embedded in the layer. The laminated cation exchange membrane having a three-layer structure of the present invention can adjust the difference in water content between each layer by setting the difference in equivalent weight between each layer to a specific range, compared to the conventional two-layer structure. It is possible to prevent the peeling phenomenon at the laminated interface observed in laminated cation exchange membranes having a structure. Furthermore, the cation exchange membrane has a carboxylic acid group on one surface, and has extremely excellent effects in that high current efficiency and low electrolytic voltage can be achieved by setting the thickness of each layer within a specific range. When the laminated cation exchange membrane of the present invention is used as a diaphragm for electrolysis of common salt, electricity is applied with the layer ( ) facing the cathode side. When current is applied with the layer ( ) facing the anode side, only low current efficiency is exhibited. EXAMPLES The present invention will be illustrated by the following examples. Example 1 290g in a 1000c.c. stainless steel autoclave
of

[Formula] 1-chloro-2,2-dichloro-trifluoroethane
Add 580g of perfluoropropionyl peroxide and 0.04g of perfluoropropionyl peroxide, and after replacing with nitrogen, raise the temperature to 25℃.
The pressure of tetrafluoroethylene is kept at 2.8
Polymerization was carried out while adjusting the pressure of tetrafluoroethylene so that the polymerization rate was set at Kg/cm ² and constant. After the polymerization was completed, 120 g of polymer was recovered (referred to as Polymer A). A portion of the obtained polymer was 260
A press film was formed at ℃, hydrolyzed with an alkali, and the ion exchange capacity was measured. The ion exchange capacity was determined to be 0.91 meq/g (equivalent weight) by acid-base titration method.
1100). Next, using the same method as for producing Polymer A, the pressure of tetrafluoroethylene was set at 3.4 Kg/cm ² and the temperature was set at 35.
Polymerization was carried out at ℃. After the polymerization was completed, 140 g of polymer was recovered (referred to as Polymer B). The ion exchange capacity was measured in the same manner as Polymer A and found to be 0.71 meq/g (equivalent weight 1410). Also,

[Formula] instead

A polymer having an exchange capacity of 0.77 meq/g (equivalent weight 1300) was obtained (referred to as Polymer C) in the same manner as in the production of Polymer A using the following formula. Using a small extruder, a film A having a thickness of 60 μm was made from polymer A, a film B having a thickness of 20 μm from polymer B, and a film C having a thickness of 8 μm from polymer C. Film B [layer ()] is the inner layer, film A [layer ()], film B, film C [layer ()]
were stacked one on top of the other in order and laminated at 260°C. The laminated film was hydrolyzed in a 1:1 (volume ratio) mixed solution of 6N caustic potash aqueous solution and methanol at 70°C for 16 hours, and then equilibrated with a 0.1N caustic soda aqueous solution. When the cross section of the laminated film was stained, about 8μ from the surface layer of Film C was stained pale blue, about 20μ inside was stained pale yellow, and the remaining about 60μ was stained deep yellow. Electrolysis was performed in the following manner with the surface of the laminated film having the film C facing the cathode side. Using an electrolytic cell consisting of an anode chamber and a cathode chamber with a current-carrying area of 0.06 dm ² (2 cm x 3 cm) through the membrane,
The membrane is assembled with the surface having film C facing the cathode side. A dimensionally stable metal electrode is used as the anode, and an iron plate is used as the cathode. A saturated saline solution is poured into the anode chamber, and the pH is maintained at 3 while adding hydrochloric acid. Water is added to the cathode chamber to keep the concentration constant while circulating a 6.5N caustic soda aqueous solution. The anode chamber and cathode chamber were each kept at 95℃ for 40
Current was applied at a current density of ampere/dm ² , and the current efficiency was calculated by dividing the amount of caustic soda produced in the cathode chamber per hour by the theoretical amount calculated from the amount of current applied. Changes in current efficiency and cell voltage over time were as follows.

[Table] Upon cross-sectional observation of the membrane after energization, no abnormality was observed at the laminated interface. Example 2 Films with thicknesses of 60μ, 20μ, and 20μ, respectively, were produced using polymers A, B, and C used in Example 1 (referred to as film A', film B', and film C', respectively). Film A' (layer ()), film B', and film C' (layer ()) were laminated in order using film B' (layer ()) as an inner layer at 260°C. The laminated film was subjected to hydrolysis treatment and equilibration treatment under the same conditions as in Example 1. Electrolysis was carried out in the same manner as in Example 1, with the surface of the laminated film having the film C' facing the cathode side. Changes in current efficiency and cell voltage over time were as follows.

[Table] Upon cross-sectional observation of the membrane after energization, no abnormality was observed at the laminated interface. Example 3 Polymerization by the same method as Example 1

[Formula] and tetrafluoroethylene copolymer with exchange capacity 0.95 meq/g
(equivalent weight 1050) and 0.78meq/g (equivalent weight 1280)
(referred to as polymers G and F, respectively) and

[Formula] and tetrafluoroethylene copolymer exchange capacity 0.87me
q/g (equivalent weight 1150) of polymer (Polymer E
) was obtained. Using polymers E, F, and G, films with thicknesses of 10μ, 25μ, and 60μ, respectively, were produced using a small extruder (referred to as films E, F, and G, respectively). Films E [layer ()], F, and G [layer ()] were laminated in this order at 260° C. with film F [layer ()] serving as an inner layer. The laminated film was subjected to hydrolysis treatment and equilibration treatment under the same conditions as in Example 1. Electrolysis was carried out in the same manner as in Example 1, with the surface of the laminated film having the film E facing the cathode side, and the concentration of the caustic soda aqueous solution being adjusted to 10N. Changes in current efficiency and cell voltage over time were as follows.

[Table] Comparative Example 1 Same as Example 2, exchange capacity 0.71meq/g
(equivalent weight 1410) of tetrafluoroethylene and

A copolymer of the formula was obtained (referred to as Polymer D). In the same manner as in Example 2, the thickness was increased from Polymer A.
Film A is 60μ, film B is 20μ thicker than polymer B, and film D is 8μ thicker than polymer D.
, and layered film A, film B, and film D in order with film B as the inner layer.
Lamination was carried out at 270°C. After the laminated film was subjected to alkali hydrolysis treatment and equilibration treatment in the same manner as in Example 1, electrolysis was performed in the same manner as in Example 1 with the surface having the film D facing the cathode side. Changes in current efficiency and cell voltage over time were as follows.

[Table] When the cross section of the film was observed after energization, there were many parts where the laminated interface between Film B and Film D was peeled off. Comparative Example 2 After laminating 60μ film A and 8μ film C at 260°C, they were subjected to alkali hydrolysis treatment and equilibration treatment in the same manner as in Example 1, and then carried out with the side with film C facing the cathode side. Electrolysis was carried out in the same manner as in Example 1. Changes in current efficiency and cell voltage over time were as follows.

Claims (1)

[Claims] 1. An atomic group -O in the side chain or a part of the side chain
A layer () consisting of a fluorocarbon polymer containing (CF ₂ ) ₂ COOM (M is H, metal, NH ₄ group) and an atomic group -O (CF ₂ ) ₃ SO ₃ M ( ) in the side chain or a part of the side chain. It consists of three layers, layers () and (), made of a fluorocarbon polymer containing M (M is the same as above), and the equivalent weight (EW ₁ ) and thickness (l ₁ ) of the layer (), and the equivalent weight ( A laminated fluorocarbon cation exchange membrane in _{which the equivalent weight (EW 3 )} and thickness (l ₃ ) of the layer (EW ₂ ) and thickness (l 2 ) satisfy the following formulas. 50≦EW ₂ −EW ₁ ≦200 150≦EW ₂ −EW ₃ ≦500 l ₁ +l ₂ /l ₁ +l ₂ +l ₃ ≦1/2, 1/20≦l ₁ /l ₁ +l ₂ ≦3/4 2 Atomic group -O in the side chain or part of the side chain
A layer () consisting of a fluorocarbon polymer containing (CF ₂ ) ₂ COOM (M is H, metal, NH ₄ group) and an atomic group -O (CF ₂ ) ₃ SO ₃ M ( ) in the side chain or a part of the side chain. It consists of three layers, layers () and (), made of a fluorocarbon polymer containing M (M is the same as above), and the equivalent weight (EW ₁ ) and thickness (l ₁ ) of the layer (), and the equivalent weight ( A laminated fluorocarbon cation exchange membrane whose thickness (EW ₂ ) and thickness (l ₂ ) and the equivalent weight (EW ₃ ) and thickness (l ₃ ) of the layer () satisfy the following formulas is treated with an aqueous alkali metal halide solution. A method of conducting electrolysis with the surface of the exchange membrane facing the cathode when used as a diaphragm for electrolysis. 50≦EW ₂ −EW ₁ ≦200 150≦EW ₂ −EW ₃ ≦500 l ₁ +l ₂ /l ₁ +l ₂ +l ₃ ≦1/2, 1/20≦l ₁ /l ₁ +l ₂ ≦3/4