CA1152452A - Cation exchange membrane preparation and use thereof - Google Patents
Cation exchange membrane preparation and use thereofInfo
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- CA1152452A CA1152452A CA000410363A CA410363A CA1152452A CA 1152452 A CA1152452 A CA 1152452A CA 000410363 A CA000410363 A CA 000410363A CA 410363 A CA410363 A CA 410363A CA 1152452 A CA1152452 A CA 1152452A
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- membrane
- cation exchange
- polymer
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- exchange membrane
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Abstract
ABSTRACT OF THE DISCLOSURE
Cation exchange membranes having good current efficiency and durability comprise a fluorocarbon polymer charac-terized by the presence both of pendant carboxylic acid groups of the formula -- OCF2COOH -- and derivatives thereof and of pendant sulfonic acid groups of the formu-la -- OCF2CF2SO3M -- and derivatives thereof.
Cation exchange membranes having good current efficiency and durability comprise a fluorocarbon polymer charac-terized by the presence both of pendant carboxylic acid groups of the formula -- OCF2COOH -- and derivatives thereof and of pendant sulfonic acid groups of the formu-la -- OCF2CF2SO3M -- and derivatives thereof.
Description
~5~45Z
ACKGROUND OF THE INVENTION
This invention relates to improved cation exchange mem-branes and to methods for their production.
It has been known to the art to obtain a cation exchange membrane of a perfluorocarbon polymer containing pendant sulfonic acid groups by ~aponification of a membrane pre-pared from a copolymer of tetrafluoroethylene and per-fluoro-3,6-dioxa-4-methyl-7-octene-sulfonyl fluoride.
This known perfluorocarbon type cation exchange membrane containing only sulfonic acid groups, however, has the disadvantage that the membrane, when u~ed in the electro-lysis of an aqueous solution of an alkali metal halide, tends to permit penetration therethrough of hydroxyl ion~
back migrating from the cathode compartment because of th,e high hydrophilicity of the sulfonic acid group. As a result, the current efficiency during electrolysis is low. This is a special problem when the electrolysis is used for the production of aqueous solution of caustic soda at concentrations of more than 20 percent. In this .
~Z45~
reaction, the current efficiency is so low that the pro-cess is economically disadvantageous compared with electrolysis of aqueous solutions of sodium chloride by conventional mercury process or diaphragm process.
S The disadvantage of such low current efficiency can bé
alleviated by lowering the exchange capacity of the sul-fonic acid group to less than 0.7 milliequivalent per gram of the H form dry resin. Such lowering, however, results in a serious decrease in the electroconductivity of the membrane and a proportional increase in the power consumption. This solution, therefore, is not without its economic difficulties.
U. S. Patent No. 3,909,378 discloses composite cation ex-change membranes containing sulfonic acid moieties as the ion exchange group and comprising two polymers with dif-ferent e~uivalent weight (EW), that is number of grams of polymer containing one equivalent weight of ion exchange functional group. When such membrane~ are utilized in the electrolysis of aqueous solutions of sodium chloride, high current efficiencies are obtained by effecting the electrolysi~ with the higher EW polymer side of the com-posite membrane facing the cathode. For high current efficiency coupled with low power consumption, the value of EW of the higher EW polymer must be increased and the thicknes~ decreased as much as possible. It is, however, extremely difficult to produce a composite cation ex-change membrane having current efficiency of not less than 90 percent by use of membranes containing only sul-fonic acid groups.
In U. S. Patent No. 3,784,399, Canadian Patent No~.
1,033,097 and 1,033,098 there are suggested cation ex-change membranes wherein the cathode side surface layers of fluorocarbon cation exchange membranes contain ~Z45~ :
sulfonamide group, salts thereof or N-mono-substituted sulfonamide group. These membranes, however, are defi-cient in electrochemical and chemical stabilities.
An object of this invention is to provide fluorocarbon cation exchange membranes which, even in electrolysis for production of caustic soda at high concentration, more effectively inhibit the back migration of hydroxyl ions and enable the electrolysis to proceed constantly with higher current efficiency than the conventional fluorocarbon cation exchange membranes and to provide a method for the manufacture of said membranes.
THE INVENTION
Novel cationic membrane3 characterized by the presence of polymer~ containing pendant carboxylic and sulfonic acid groups have now been discovered. These membranes when used in electrolysis cells, particularly when used in such cells for the electrolysis of sodium chloride to produce aqueous sodium hydroxide, have many advantages compared to conventional cation exchange membranes. A
particular advantage is the durability of the membranes, it having been found that the current efficiency of the membranes remains ~table at well above 90%, even after many months o f operation.
The present invention provide a cation exchange membrane comprising a fluorocarbon polymer containing pendant carboxylic acid groups represented by the formula:
wherein M is hydrogen, ammonium, quaternary ammonium, particularly quaternary ammonium having a molecular weight of 500 or less, or metallic atoms, particularly alkali or alkaline earth metals, the polymer also con-taining pendant sulfonic acid groups of the formula ~5245~
where M has the same meaning as above.
In its simplest form, a cation exchange membrane of this invention is a film forming perfluorocarbon polymer. The thickncss of the film can be varied widely depending on purposes~ There is no particular limit to the thickness, but usually a thickness from 0.5 to 20 mils is suitable for many purposes.
The preferred embodiments of membrane are characterized by the presence of at least one surface or internal layer at least about 100 A in thickness in which the polymer is substituted with pendant carboxylic and sulfonic acid groups represented by the formula as set forth above.
The membranes of this invention can be classified into two major groups. One is a uni-layer film wherein the equivalent weight ~EW) of the cation exchange groups is uniform throughout the membrane. The other is a two-ply film in which a first film having higher EW value and a second film having lower EW value are combined. In each case, the specific fluorocarbon polymer as defined above can be present either homogeneously throughout the film or as stratum on one surface or in internal portion of the membrane. The membrane, however, can sometimes have strata both on opposed surfaces of the membrane in each o the aforesaid groups. As mentioned above, according to preferred embodiment of the two-ply layer, the first film having higher EW value is provided with the specific fluorocarbon polymer, preferably as surface stratum of at least 100 A in thickness on the surface opposite to the side laminated with the second film. For practical purpose, the membrane is usually reinforced with rein-forcing materials selected from the group consisting of woven fabrics of inert fibres and porous films of inert 45~
polymers, preferably polytetrafluoroethylene fibres. When the membrane has a surface stratum of the specific fluoro-carbon polymer of the invention, the reinforcing material is desirably embedded in the membrane on the side opposite to the side having the surface stratum of the specific fluorocarbon polymer of the invention. In the two-ply layer, the reinforcing material is desirably embedded in the second film having lower EW value.
For convenience, detailed explanation is given in the following principally with reference to the uni-layer film having a surface stratum of the specified acid groups.
The structure of the first film in a two-ply layer mem-brane with regard to functional groups is substantially the same as the uni-layer film. When the membrane is used in electrolysis, the side having the stratum is placed in the electrolytic cell to face the cathode side in order to obtain the remarkable benefits of the inven-tion.
Since the cationic membranes of thi~ invention are derived from sulfonic acid group substituted fluorocarbon copoly-mers, they may contain any predetermined proportion of sulfonic acid groups, or derivatives thereof.
It ha~ been observed that the presence of carboxylic acid groups on the surface of the cation exchange membrane, particularly on the surface facing the cathode, remarkab-ly impedes the back migration of hydroxyl ionfi from the cathode compartment during the electrolysi~ of aqueous solutions of alkali metal halide~ such as sodium chloride.
These effects are realized while operating at high cur-rent efficiencies, normally well over 90~. Moreover, themembranes of the invention are especially durable even in the presence of aqueous solutions of sodium hydroxide at a concentration of 20~, or even higher, and also ~5Z452 highly resistant to chlorine gas generated from the anode.
Preferably the polymer forming the surface layer of the mem-brane contains at least20 mol percent of carboxylic acid groups, and the best combinations of economy and effici-ency are normally realized if the content is at least 40mol percent; all based on the total number of all func-tional groups in the surface layer.
The depth of the surface layer can be ascertained by staining techniques. For example, a section of a prepa-red membrane can be immersed for several minutes in anaqueous solution of crystal violet containing 5 to 10%
ethanol as a solubility aid. This dye will stain only the treated sections, and a cross section of the membrane can be examined microscopically.
Alternatively, the thickness of the layer and the content of carboyxlic acid groups can be ascertained by x-ray microprobe analysis.
The membrane~ of this invention may take any of several forms, as i8 particularly illustrated in the examples.
As mentioned above, it may be a simple unilayer film with one or both major surface~ embodying strata with carboxyl$c acid sub5titutents. Alternatively, the mem-brane~ may be composite membranes formed from two appro-priately prepared and substituted perfluorocarbon films bonded together in each of which the EW is from about 1000 to 2000, preferably from 1000 to 1500.
If a two film membrane ic employed, the first film should have an EW which is at least 150 higher than the second film, and its thickness should be up to one half of the total thickness. In fact, the thin film should be as thin as possible to minimize total electrical resistance.
Due to the difficult manufacturing techniques involved, the thin film will generally occupy from about 10 to 45%
of the total thickness.
In other forms of the invention, the membranes may be laminated to reinforcing materials to improve mechanical ~trength. For this purpose fabrics made of polytetrafluoro-ethylene fibres are most suitable, although other mate-rials which are inert to the chemical environment in which the membrane~ are employed may also be used. Par-ticularly, polytetrafluoroethylene films may preferablybe employed as reinforcing materials. If reinforcing materials are utilized, it is particularly advantageous to embed them in the polymer membrane. This can be readily accomplished, for example, at elevated tempera-ture and under reduced pressure as illustrated in theexamples.
In all of these various constructions, the most prefer-red membranes will be constructed with carboxylic groupQ
predominating on one surface, and sulfonic groups predo-minating on the other. In composite membrane~ the filmwith the higher EW will preferably carry the carboxyl groups .
The 8tarting fluorocarbon polymer having sulfonic acid groups in the side chain~ thereof is produced by copoly-merizing a fluorinated ethylene and a vinyl fluorocarbonmonomer having a sulfonyl fluoride group of the generic formula (I) given below:
FSO2CF2CF2O~CFYCF2O)nCF=CF2 ~I) ~wherein, Y represents F or a fluoroalkyl group having 1 to 5 carbon atoms and n an integer having the value of 0 - 3), if necessary, in conjunction with a monomer selected from the class consisting of hexafluoropropylene, CF3CF=CF2 and compounds of the generic formula (II) given ~24S~
below:
F ( CF 2 ) ,~2. ( CFCF 2 ) pCF=CF ( I I ) (wherein, ~ represents an integer having the value of l -3 and p an integer having the value of 0 - 2), thereby giving rise to a polymer possessing a side chain of -OC~CF2SO2F shaping the resultant polymer in the form of a membrane and thereafter converting the side chain -OCF2CF2SO2F of said polymer into the group -OCF2CF2SO3M
through saponification.
Typical examples of fluorinated ethylene include vinyl-idene fluoride, tetrafluoroethylene and chlorotrifluoro-ethylene. Among them, tetrafluoroethylene is most preferred.
Typical example~ of the vinyl fluorocarbon monomer having the sulfonyl fluoride group of the aforementioned generic formula include those enumerated below:
FS02CF2CF20CFzCF2
ACKGROUND OF THE INVENTION
This invention relates to improved cation exchange mem-branes and to methods for their production.
It has been known to the art to obtain a cation exchange membrane of a perfluorocarbon polymer containing pendant sulfonic acid groups by ~aponification of a membrane pre-pared from a copolymer of tetrafluoroethylene and per-fluoro-3,6-dioxa-4-methyl-7-octene-sulfonyl fluoride.
This known perfluorocarbon type cation exchange membrane containing only sulfonic acid groups, however, has the disadvantage that the membrane, when u~ed in the electro-lysis of an aqueous solution of an alkali metal halide, tends to permit penetration therethrough of hydroxyl ion~
back migrating from the cathode compartment because of th,e high hydrophilicity of the sulfonic acid group. As a result, the current efficiency during electrolysis is low. This is a special problem when the electrolysis is used for the production of aqueous solution of caustic soda at concentrations of more than 20 percent. In this .
~Z45~
reaction, the current efficiency is so low that the pro-cess is economically disadvantageous compared with electrolysis of aqueous solutions of sodium chloride by conventional mercury process or diaphragm process.
S The disadvantage of such low current efficiency can bé
alleviated by lowering the exchange capacity of the sul-fonic acid group to less than 0.7 milliequivalent per gram of the H form dry resin. Such lowering, however, results in a serious decrease in the electroconductivity of the membrane and a proportional increase in the power consumption. This solution, therefore, is not without its economic difficulties.
U. S. Patent No. 3,909,378 discloses composite cation ex-change membranes containing sulfonic acid moieties as the ion exchange group and comprising two polymers with dif-ferent e~uivalent weight (EW), that is number of grams of polymer containing one equivalent weight of ion exchange functional group. When such membrane~ are utilized in the electrolysis of aqueous solutions of sodium chloride, high current efficiencies are obtained by effecting the electrolysi~ with the higher EW polymer side of the com-posite membrane facing the cathode. For high current efficiency coupled with low power consumption, the value of EW of the higher EW polymer must be increased and the thicknes~ decreased as much as possible. It is, however, extremely difficult to produce a composite cation ex-change membrane having current efficiency of not less than 90 percent by use of membranes containing only sul-fonic acid groups.
In U. S. Patent No. 3,784,399, Canadian Patent No~.
1,033,097 and 1,033,098 there are suggested cation ex-change membranes wherein the cathode side surface layers of fluorocarbon cation exchange membranes contain ~Z45~ :
sulfonamide group, salts thereof or N-mono-substituted sulfonamide group. These membranes, however, are defi-cient in electrochemical and chemical stabilities.
An object of this invention is to provide fluorocarbon cation exchange membranes which, even in electrolysis for production of caustic soda at high concentration, more effectively inhibit the back migration of hydroxyl ions and enable the electrolysis to proceed constantly with higher current efficiency than the conventional fluorocarbon cation exchange membranes and to provide a method for the manufacture of said membranes.
THE INVENTION
Novel cationic membrane3 characterized by the presence of polymer~ containing pendant carboxylic and sulfonic acid groups have now been discovered. These membranes when used in electrolysis cells, particularly when used in such cells for the electrolysis of sodium chloride to produce aqueous sodium hydroxide, have many advantages compared to conventional cation exchange membranes. A
particular advantage is the durability of the membranes, it having been found that the current efficiency of the membranes remains ~table at well above 90%, even after many months o f operation.
The present invention provide a cation exchange membrane comprising a fluorocarbon polymer containing pendant carboxylic acid groups represented by the formula:
wherein M is hydrogen, ammonium, quaternary ammonium, particularly quaternary ammonium having a molecular weight of 500 or less, or metallic atoms, particularly alkali or alkaline earth metals, the polymer also con-taining pendant sulfonic acid groups of the formula ~5245~
where M has the same meaning as above.
In its simplest form, a cation exchange membrane of this invention is a film forming perfluorocarbon polymer. The thickncss of the film can be varied widely depending on purposes~ There is no particular limit to the thickness, but usually a thickness from 0.5 to 20 mils is suitable for many purposes.
The preferred embodiments of membrane are characterized by the presence of at least one surface or internal layer at least about 100 A in thickness in which the polymer is substituted with pendant carboxylic and sulfonic acid groups represented by the formula as set forth above.
The membranes of this invention can be classified into two major groups. One is a uni-layer film wherein the equivalent weight ~EW) of the cation exchange groups is uniform throughout the membrane. The other is a two-ply film in which a first film having higher EW value and a second film having lower EW value are combined. In each case, the specific fluorocarbon polymer as defined above can be present either homogeneously throughout the film or as stratum on one surface or in internal portion of the membrane. The membrane, however, can sometimes have strata both on opposed surfaces of the membrane in each o the aforesaid groups. As mentioned above, according to preferred embodiment of the two-ply layer, the first film having higher EW value is provided with the specific fluorocarbon polymer, preferably as surface stratum of at least 100 A in thickness on the surface opposite to the side laminated with the second film. For practical purpose, the membrane is usually reinforced with rein-forcing materials selected from the group consisting of woven fabrics of inert fibres and porous films of inert 45~
polymers, preferably polytetrafluoroethylene fibres. When the membrane has a surface stratum of the specific fluoro-carbon polymer of the invention, the reinforcing material is desirably embedded in the membrane on the side opposite to the side having the surface stratum of the specific fluorocarbon polymer of the invention. In the two-ply layer, the reinforcing material is desirably embedded in the second film having lower EW value.
For convenience, detailed explanation is given in the following principally with reference to the uni-layer film having a surface stratum of the specified acid groups.
The structure of the first film in a two-ply layer mem-brane with regard to functional groups is substantially the same as the uni-layer film. When the membrane is used in electrolysis, the side having the stratum is placed in the electrolytic cell to face the cathode side in order to obtain the remarkable benefits of the inven-tion.
Since the cationic membranes of thi~ invention are derived from sulfonic acid group substituted fluorocarbon copoly-mers, they may contain any predetermined proportion of sulfonic acid groups, or derivatives thereof.
It ha~ been observed that the presence of carboxylic acid groups on the surface of the cation exchange membrane, particularly on the surface facing the cathode, remarkab-ly impedes the back migration of hydroxyl ionfi from the cathode compartment during the electrolysi~ of aqueous solutions of alkali metal halide~ such as sodium chloride.
These effects are realized while operating at high cur-rent efficiencies, normally well over 90~. Moreover, themembranes of the invention are especially durable even in the presence of aqueous solutions of sodium hydroxide at a concentration of 20~, or even higher, and also ~5Z452 highly resistant to chlorine gas generated from the anode.
Preferably the polymer forming the surface layer of the mem-brane contains at least20 mol percent of carboxylic acid groups, and the best combinations of economy and effici-ency are normally realized if the content is at least 40mol percent; all based on the total number of all func-tional groups in the surface layer.
The depth of the surface layer can be ascertained by staining techniques. For example, a section of a prepa-red membrane can be immersed for several minutes in anaqueous solution of crystal violet containing 5 to 10%
ethanol as a solubility aid. This dye will stain only the treated sections, and a cross section of the membrane can be examined microscopically.
Alternatively, the thickness of the layer and the content of carboyxlic acid groups can be ascertained by x-ray microprobe analysis.
The membrane~ of this invention may take any of several forms, as i8 particularly illustrated in the examples.
As mentioned above, it may be a simple unilayer film with one or both major surface~ embodying strata with carboxyl$c acid sub5titutents. Alternatively, the mem-brane~ may be composite membranes formed from two appro-priately prepared and substituted perfluorocarbon films bonded together in each of which the EW is from about 1000 to 2000, preferably from 1000 to 1500.
If a two film membrane ic employed, the first film should have an EW which is at least 150 higher than the second film, and its thickness should be up to one half of the total thickness. In fact, the thin film should be as thin as possible to minimize total electrical resistance.
Due to the difficult manufacturing techniques involved, the thin film will generally occupy from about 10 to 45%
of the total thickness.
In other forms of the invention, the membranes may be laminated to reinforcing materials to improve mechanical ~trength. For this purpose fabrics made of polytetrafluoro-ethylene fibres are most suitable, although other mate-rials which are inert to the chemical environment in which the membrane~ are employed may also be used. Par-ticularly, polytetrafluoroethylene films may preferablybe employed as reinforcing materials. If reinforcing materials are utilized, it is particularly advantageous to embed them in the polymer membrane. This can be readily accomplished, for example, at elevated tempera-ture and under reduced pressure as illustrated in theexamples.
In all of these various constructions, the most prefer-red membranes will be constructed with carboxylic groupQ
predominating on one surface, and sulfonic groups predo-minating on the other. In composite membrane~ the filmwith the higher EW will preferably carry the carboxyl groups .
The 8tarting fluorocarbon polymer having sulfonic acid groups in the side chain~ thereof is produced by copoly-merizing a fluorinated ethylene and a vinyl fluorocarbonmonomer having a sulfonyl fluoride group of the generic formula (I) given below:
FSO2CF2CF2O~CFYCF2O)nCF=CF2 ~I) ~wherein, Y represents F or a fluoroalkyl group having 1 to 5 carbon atoms and n an integer having the value of 0 - 3), if necessary, in conjunction with a monomer selected from the class consisting of hexafluoropropylene, CF3CF=CF2 and compounds of the generic formula (II) given ~24S~
below:
F ( CF 2 ) ,~2. ( CFCF 2 ) pCF=CF ( I I ) (wherein, ~ represents an integer having the value of l -3 and p an integer having the value of 0 - 2), thereby giving rise to a polymer possessing a side chain of -OC~CF2SO2F shaping the resultant polymer in the form of a membrane and thereafter converting the side chain -OCF2CF2SO2F of said polymer into the group -OCF2CF2SO3M
through saponification.
Typical examples of fluorinated ethylene include vinyl-idene fluoride, tetrafluoroethylene and chlorotrifluoro-ethylene. Among them, tetrafluoroethylene is most preferred.
Typical example~ of the vinyl fluorocarbon monomer having the sulfonyl fluoride group of the aforementioned generic formula include those enumerated below:
FS02CF2CF20CFzCF2
2 2 2 , 2 2CF2CF2 ,CFCF20CFCF20CF=CF
FSO2CF2CF2CF=CF2 Fso2cF2cF2ocFcF2ocF~cF2 Of the vinyl fluorocarbon monomers having the ~ulfonyl fluoride group available at all, the most desirable is perfluoro~3,6-dioxa-4-methyl-7-octene sulfonyl fluoride), 2 2 2 , 2 2 ~24S~
g A typical example of the fluorovinyl ether of the generic formula (II) which takes part, where necessary, in said copolymerization is perfluoromethyl perfluorovinyl ether.
The membranes of the invention may be prepared starting rom ~he sulfonyl substituted polymers described in United States Patent No. 3,909,378.
Preferably, the membrane is first formed with the sulfonyl substituted polymer which is then converted by reactions described more fully hereinafter to a membrane of the invention.
The preferred copolymer composition for a starting material is such that the fluorinated ethylene monomer content is from 30 to 90 percent by weight, preferably from 40 to 75 percent by weight, and the content of the perfluorovinyl monomer possessing the sulfonyl fluoride group i8 from 70 to 10 percent by weight, preferably from 60 to 25 percent by weight. The material~ are pro-duced by procedures well ~nown in the art for the homo-polymerization or copolymerization of a fluorinated ethylene.
Polymerization may be effected in either aqueouU or non-aqueou~ systems. Generally, the polymerization is per-formed at temperatures of from 0 to 200C under pressure of from 1 to 200 kg~cm2. Frequently, the polymerization in the nonaqueous system is carried out in a fluorinated solvent. Examples of such nonaqueous ~olvent~ include 1,1,2-trichloro-1,2,2-trifluoroethane and perfluorocar-bons such as perfluoromethylcyclobutane, perfluorooctane and perfluorobenzene.
The aqueous system polymerization is accomplished by brin-ging the monomers into contact with an aqueous solvent containing a free radical initiator and a dispersant to ~5i245~2 `--10--produce a slurry of polymer particles, or by other well known procedures.
After the polymerization, the resultant polymer is shaped to form a membrane using any of a variety of well known techniques.
The copolymer is desired to have an EW in the range of rom 1000 to 2000. The membrane having a low EW is desi-rable in the sense that the electric resistance is pro-portionally low. A membrane of a copolymer having a notably low EW is not desirable since the mechanical strength is not sufficient. A copolymer having a notably high EW cannot easily be shaped in the form of membrane.
Thus, the most desirable range of EW is from 1000 to 1500.
The copolymer, after being shaped into a membrane, can be lS laminated with a reinforcing material such as a fabric for improvement of mechanical strength. As the reinfor-cing material, fabrics made of polytetrafluoroethylene fibres are most suitable. The aforesaid layer should preferably be allowed to be present on the surface oppo-site to the side on which the reinforcing material islined.
In the case of the cation exchange membrane having two bonded films which is the preferred embodiment of the invention as mentioned above, two kinds of copolymers having different EW are prepared according to the poly-merization methods as described above, followed by shaping, and fabricated into a composite film. ~he first film is required to have an EW of at least lS0 greater than the EW of the second film and also to have a thickness of not more than one half of the entire thickness. The thickness of the first film is preferab-ly as thin as possible, since electric resistance is greatly increased as the increase in EW. Thus, the ~5i2~S;~
thickness of the first film which depends on EW thereof is required to be 50% or less of the entire thickness, preferably from 45 to 10~.
It is important that the first film of a higher EW is present in the form of a continuous film formed parallel ~o the surface of the membrane.
The overall thickness of said composite cation exchange membrane, though variable with the kind of particular ion exchange group used, the strength required of the copolymer as the ion exchange membrane, the type of electrolytic cell and the conditions of operation, gene-rally has a lower limit of 4 mils and no upper limit.
The upper limit i9 usually fixed in consideration of eco-nomy and other practical purposes.
Said composite membrane may be laminated with fabrics or some other suitable reinforcing material with a view to improvement of the mechanical strength thereof. The re-inforcing material is preferably embedded in the second film. As the reinforcing material, fabrics made of polytetrafluoroethylene filaments are most suitable.
For the preparation of the products of this invention, pendant sulfonyl group~ in the form represented by the formulas:
-OCF2CF2SO2X ~A) and/or -OCF2CF2SO2 = (B) wherein X is halogen, especially fluorine or chlorine;
hydroxyl; alkyl containing up to four carbon atoms; aryl or OZ where Z is a metallic atom, especially an atom of an alkali metal, alkyl containing up to four carbon atoms or aryl; are converted to:
115245~
by treatment with a reducing agent.
Since the conversion to a carboxylic acid group is effec-ted chemically, it can be controlled so as to producq products with substantially any degree of carboxylation which may be desired.
The starting polymers are usually formed from sulfonyl fluoride substituted compounds which remain intact during polymerization. The sulfonyl fluoride groups can direct-ly be treated with a reducing agent to be converted tcthe carboyxlic acid groups. Alternatively, they may be first converted to any of the other derivatives of sul-fonic acid as defined in the above formulas tA) and (B) by known reactions, followed by conversion into the car-boxyllc acid groups. The sulfonyl chloride groups areespecially preferred due to higher reactivity. Therefore, it is more,desirable to convert the ~ulfonyl fluoride to any of the other derivatives of sulfonic acid defined above in connection with the definition of X. Such re-actions can be readily carried out by procedures wellknown to the art.
The formstion of the carboxylic group may follow any of several pathways.
~t may be formed by reduction to a sulfinic acid with a relatively weak reducing agent followed by a heat treat-ment as indicated below:
-OCF2CF2SO2X red. ) OCF2CF2SO2M
~ ~ OCF2COOM.
Conversion into carboyxlic acid groups can more readily be effected when M in the above formulas ifi hydrogen.
Alternatively the treatment may be stepwise in which initially a sulfinic acid is produced, and this is -converted to a carboxylic group by the use of a strong reducing agent. This may take place as indicated below:
-OCF2CF2S02X rea. ~, OCF2CF2S02M
OCF2COOM . ', S With some reducing agents, the treatment may be directly from the sulfonic group to the carboxyl group, as indi-cated by, OCF2CF2S03M ) OCF2COOM.
It is preferred that the concentration of sulfinic acid groups in the final product be relatively low. Accor-dingly, it may be desirable, but not necessary, to oxi-dize sulfinic acid groups to sulfonic acid groups by the following ~equence:
-OCF2CF2S02M , OCF2CF2S03M
This may be accomplished by known procedures utilizing aqueou~ mixturesof sodium hydroxide and hypochlorite.
The reducing agenta which can be used in the present invention are exemplified as shown below. Those skilled in the art are completely familiar with these reducing agent~ and many other similar reducing agent~ a~ well a~
procedure~ by which they are employed. However, some of the reduclng agent~ such as hydrazine having amino groups which are capable of forming sulfonamide groups as dis-closed in Canadian Patent No. 1,033,097 are not suitable for the purpose of the invention, and therefore they are excluded from the scope of the invention.
The reducing agents of the first group are metal hydridea of the generic formula MeLH4, wherein Me representc an alkali metal atom and L an aluminum or boron atom, or Me'Hx, wherein Me' represents an alkali metal atom or alkaline earth metal atom and x is an integer with a value of 1 to 2. These include, for example, lithium .~
..... . . .
- :' - - - -.
.. ,-. , ~l~Z45~
aluminum hydride, lithium boron hydride, potassium boron hydride, sodium boron hydride, sodium hydride and calcium hydride.
The reducing agents of the second group are inorganic acids possessing reducing activity such as, for example, hydroiodic acid, hydrobromic acid, hypophosphorous acid, hydrogen sulfide and arsenious acid.
The reducing agents of the third group are mixtures of metals and acids. Examples of these mixtures include tin, iron, zinc and zinc amalgam and those of acids in-clude hydrochloric acid, sulfuric acid and acetic acid.
The reducing agents of the fourth group are compound~ of low-valency metal~. Exampleq of these compound~ include tannous chloride, ferrou~ sulfate and titanium trichlo-ride. They may be used in con~unction with such acids as hydrochloric acid and sulfuric acid.
The reducing agents of the fifth group are organic metal compounda. Examples of the~e reducing agents include butyl lithium, Grignard reagent, triethyl aluminium and triisobutyl aluminum.
The reducing agent~ of the ~ixth group are inorganic acid salts po~e~ing reducing activity and similar compounds.
Example~ of these reducing agent~ include potassium io-dide, qodium iodide, potas~ium sulfide, sodium sulfide, ammonium ~ulfide, soaium ~ulfite, ~odium dithionite, aodium phosphite, ~odium arsenite, sodium polysulfide and phosphoru~ trisulfide.
The reducing agents of the seventh group are mixture~ of metals with water, steam, alcohols or alkali~. Examples of metals usable in the mixtures include sodium, lithium, aluminum, magnesium, zinc, iron and amalgams thereof.
, ~;z4~2 Examples of alkalis include alkali hydroxides and alco-holic alkalis.
The reaucing agents of the eighth groups are organic compounds possessing a reducing activity such as, for example, triethanol amine and acetaldehyde.
Among the groups as enumerated above, those belonging to the second, third, fourth and sixth groups are found to be preerable.
The optimum conditions for treatment with a reducing agent will be selected depending on the selected reducing agent to be used and on the kind of the substituent X in the SO2X group. Generally, the reaction temperature is in the range of from -50C to 250C, preferably from 0C to 150C, and the reducing agent is used in the form of a gas, liquid or solution. As the solvent for the reaction, there can be used water; polar organic solvents such as methanol, tetrahydrofuran, diglyme, acetonitrile, propi-onitrile or benzonitrile; or nonpolar organic solvent~
such as n-hexane, benzene or cyclohexane or mixtures of such solvents.
The amount of the reducing agent is not less than the equlvalent weight of the sulfonyl group present in the surface. Generally, the reducing agent will be used in large excess. The pH value of the reaction system will be selected on the basis of the particular reducing agent employed.
The reaction can be carried out under reduced, normal or increased pressure. In the reaction involving the use of a gaseous reducing agent, the increased pressure can improve the velocity of the reaction.
~5Z452 The reaction time generally ranges from one minute to 100 hours.
In case of a cation exchange membrane reinforced with a re-inforciny material~ treatment with a reducing agent is pre-ferably applied onto the side opposite to the reinforced side.
The course of the reaction may be followed by analysis of the infrared absorption spectrum of the membrane, as is particularly illustrated in the examples. Key bands in following the reaction are as follows:
sulfonyl chloride -------- 1420 cm 1 sulfinic acid salt ------- 940 cm 1 sulfinic acid salt ------- 1010 cm 1 carboxylic acid ---------- 1780 cm 1 carboxylic acid salt ----~ 1690 cm 1 The specific functional groups of the invention are found to be unitary species having a neutralization point at approximately pKa=2.5 from measurement of electric resis-tance and infrared spectrum by varying pH. Said func-tional groups exhibit characteristic absorptions at 1780 cm 1 ~H form) and at 1690 cm 1 (Na form). Furthermore, when converted into chlorides by treatment with PC15/POC13, they are found to exhibit characteristic absorption at 1810 cm 1 From these measurements, they are identified to be carboxylic acid groups. By elemental analysis by using the combustion method, sulfur is found to be de-crea8ed by one atom per one exchange group. Fluorine atom removal is observed, two atoms per one exchange group, by the alizarin-complexion method. From these results of analysis and also from the fact that carboxy-lic acids are formed by use of a reducing agent containingno carbon atom under an atmosphere in the absence of car-bon atom, the above functional groups are confirmed to be -OCF2COOM. This structure is also evidenced by measurement of NMR spectrum of C13 and Fl9 of the product obtained by the réaction, corresponding to the ab,ove polymer reaction, conducted for the monomer having the functional group 2C 2S02X.
The products of the treatment with a reducing agent may take three typical forms. These are:
1) A~ many -COOM groups as are required may be formed.
2) Not all the -COOM groups required may be formed and excess -SO2M groups may be present.
FSO2CF2CF2CF=CF2 Fso2cF2cF2ocFcF2ocF~cF2 Of the vinyl fluorocarbon monomers having the ~ulfonyl fluoride group available at all, the most desirable is perfluoro~3,6-dioxa-4-methyl-7-octene sulfonyl fluoride), 2 2 2 , 2 2 ~24S~
g A typical example of the fluorovinyl ether of the generic formula (II) which takes part, where necessary, in said copolymerization is perfluoromethyl perfluorovinyl ether.
The membranes of the invention may be prepared starting rom ~he sulfonyl substituted polymers described in United States Patent No. 3,909,378.
Preferably, the membrane is first formed with the sulfonyl substituted polymer which is then converted by reactions described more fully hereinafter to a membrane of the invention.
The preferred copolymer composition for a starting material is such that the fluorinated ethylene monomer content is from 30 to 90 percent by weight, preferably from 40 to 75 percent by weight, and the content of the perfluorovinyl monomer possessing the sulfonyl fluoride group i8 from 70 to 10 percent by weight, preferably from 60 to 25 percent by weight. The material~ are pro-duced by procedures well ~nown in the art for the homo-polymerization or copolymerization of a fluorinated ethylene.
Polymerization may be effected in either aqueouU or non-aqueou~ systems. Generally, the polymerization is per-formed at temperatures of from 0 to 200C under pressure of from 1 to 200 kg~cm2. Frequently, the polymerization in the nonaqueous system is carried out in a fluorinated solvent. Examples of such nonaqueous ~olvent~ include 1,1,2-trichloro-1,2,2-trifluoroethane and perfluorocar-bons such as perfluoromethylcyclobutane, perfluorooctane and perfluorobenzene.
The aqueous system polymerization is accomplished by brin-ging the monomers into contact with an aqueous solvent containing a free radical initiator and a dispersant to ~5i245~2 `--10--produce a slurry of polymer particles, or by other well known procedures.
After the polymerization, the resultant polymer is shaped to form a membrane using any of a variety of well known techniques.
The copolymer is desired to have an EW in the range of rom 1000 to 2000. The membrane having a low EW is desi-rable in the sense that the electric resistance is pro-portionally low. A membrane of a copolymer having a notably low EW is not desirable since the mechanical strength is not sufficient. A copolymer having a notably high EW cannot easily be shaped in the form of membrane.
Thus, the most desirable range of EW is from 1000 to 1500.
The copolymer, after being shaped into a membrane, can be lS laminated with a reinforcing material such as a fabric for improvement of mechanical strength. As the reinfor-cing material, fabrics made of polytetrafluoroethylene fibres are most suitable. The aforesaid layer should preferably be allowed to be present on the surface oppo-site to the side on which the reinforcing material islined.
In the case of the cation exchange membrane having two bonded films which is the preferred embodiment of the invention as mentioned above, two kinds of copolymers having different EW are prepared according to the poly-merization methods as described above, followed by shaping, and fabricated into a composite film. ~he first film is required to have an EW of at least lS0 greater than the EW of the second film and also to have a thickness of not more than one half of the entire thickness. The thickness of the first film is preferab-ly as thin as possible, since electric resistance is greatly increased as the increase in EW. Thus, the ~5i2~S;~
thickness of the first film which depends on EW thereof is required to be 50% or less of the entire thickness, preferably from 45 to 10~.
It is important that the first film of a higher EW is present in the form of a continuous film formed parallel ~o the surface of the membrane.
The overall thickness of said composite cation exchange membrane, though variable with the kind of particular ion exchange group used, the strength required of the copolymer as the ion exchange membrane, the type of electrolytic cell and the conditions of operation, gene-rally has a lower limit of 4 mils and no upper limit.
The upper limit i9 usually fixed in consideration of eco-nomy and other practical purposes.
Said composite membrane may be laminated with fabrics or some other suitable reinforcing material with a view to improvement of the mechanical strength thereof. The re-inforcing material is preferably embedded in the second film. As the reinforcing material, fabrics made of polytetrafluoroethylene filaments are most suitable.
For the preparation of the products of this invention, pendant sulfonyl group~ in the form represented by the formulas:
-OCF2CF2SO2X ~A) and/or -OCF2CF2SO2 = (B) wherein X is halogen, especially fluorine or chlorine;
hydroxyl; alkyl containing up to four carbon atoms; aryl or OZ where Z is a metallic atom, especially an atom of an alkali metal, alkyl containing up to four carbon atoms or aryl; are converted to:
115245~
by treatment with a reducing agent.
Since the conversion to a carboxylic acid group is effec-ted chemically, it can be controlled so as to producq products with substantially any degree of carboxylation which may be desired.
The starting polymers are usually formed from sulfonyl fluoride substituted compounds which remain intact during polymerization. The sulfonyl fluoride groups can direct-ly be treated with a reducing agent to be converted tcthe carboyxlic acid groups. Alternatively, they may be first converted to any of the other derivatives of sul-fonic acid as defined in the above formulas tA) and (B) by known reactions, followed by conversion into the car-boxyllc acid groups. The sulfonyl chloride groups areespecially preferred due to higher reactivity. Therefore, it is more,desirable to convert the ~ulfonyl fluoride to any of the other derivatives of sulfonic acid defined above in connection with the definition of X. Such re-actions can be readily carried out by procedures wellknown to the art.
The formstion of the carboxylic group may follow any of several pathways.
~t may be formed by reduction to a sulfinic acid with a relatively weak reducing agent followed by a heat treat-ment as indicated below:
-OCF2CF2SO2X red. ) OCF2CF2SO2M
~ ~ OCF2COOM.
Conversion into carboyxlic acid groups can more readily be effected when M in the above formulas ifi hydrogen.
Alternatively the treatment may be stepwise in which initially a sulfinic acid is produced, and this is -converted to a carboxylic group by the use of a strong reducing agent. This may take place as indicated below:
-OCF2CF2S02X rea. ~, OCF2CF2S02M
OCF2COOM . ', S With some reducing agents, the treatment may be directly from the sulfonic group to the carboxyl group, as indi-cated by, OCF2CF2S03M ) OCF2COOM.
It is preferred that the concentration of sulfinic acid groups in the final product be relatively low. Accor-dingly, it may be desirable, but not necessary, to oxi-dize sulfinic acid groups to sulfonic acid groups by the following ~equence:
-OCF2CF2S02M , OCF2CF2S03M
This may be accomplished by known procedures utilizing aqueou~ mixturesof sodium hydroxide and hypochlorite.
The reducing agenta which can be used in the present invention are exemplified as shown below. Those skilled in the art are completely familiar with these reducing agent~ and many other similar reducing agent~ a~ well a~
procedure~ by which they are employed. However, some of the reduclng agent~ such as hydrazine having amino groups which are capable of forming sulfonamide groups as dis-closed in Canadian Patent No. 1,033,097 are not suitable for the purpose of the invention, and therefore they are excluded from the scope of the invention.
The reducing agents of the first group are metal hydridea of the generic formula MeLH4, wherein Me representc an alkali metal atom and L an aluminum or boron atom, or Me'Hx, wherein Me' represents an alkali metal atom or alkaline earth metal atom and x is an integer with a value of 1 to 2. These include, for example, lithium .~
..... . . .
- :' - - - -.
.. ,-. , ~l~Z45~
aluminum hydride, lithium boron hydride, potassium boron hydride, sodium boron hydride, sodium hydride and calcium hydride.
The reducing agents of the second group are inorganic acids possessing reducing activity such as, for example, hydroiodic acid, hydrobromic acid, hypophosphorous acid, hydrogen sulfide and arsenious acid.
The reducing agents of the third group are mixtures of metals and acids. Examples of these mixtures include tin, iron, zinc and zinc amalgam and those of acids in-clude hydrochloric acid, sulfuric acid and acetic acid.
The reducing agents of the fourth group are compound~ of low-valency metal~. Exampleq of these compound~ include tannous chloride, ferrou~ sulfate and titanium trichlo-ride. They may be used in con~unction with such acids as hydrochloric acid and sulfuric acid.
The reducing agents of the fifth group are organic metal compounda. Examples of the~e reducing agents include butyl lithium, Grignard reagent, triethyl aluminium and triisobutyl aluminum.
The reducing agent~ of the ~ixth group are inorganic acid salts po~e~ing reducing activity and similar compounds.
Example~ of these reducing agent~ include potassium io-dide, qodium iodide, potas~ium sulfide, sodium sulfide, ammonium ~ulfide, soaium ~ulfite, ~odium dithionite, aodium phosphite, ~odium arsenite, sodium polysulfide and phosphoru~ trisulfide.
The reducing agents of the seventh group are mixture~ of metals with water, steam, alcohols or alkali~. Examples of metals usable in the mixtures include sodium, lithium, aluminum, magnesium, zinc, iron and amalgams thereof.
, ~;z4~2 Examples of alkalis include alkali hydroxides and alco-holic alkalis.
The reaucing agents of the eighth groups are organic compounds possessing a reducing activity such as, for example, triethanol amine and acetaldehyde.
Among the groups as enumerated above, those belonging to the second, third, fourth and sixth groups are found to be preerable.
The optimum conditions for treatment with a reducing agent will be selected depending on the selected reducing agent to be used and on the kind of the substituent X in the SO2X group. Generally, the reaction temperature is in the range of from -50C to 250C, preferably from 0C to 150C, and the reducing agent is used in the form of a gas, liquid or solution. As the solvent for the reaction, there can be used water; polar organic solvents such as methanol, tetrahydrofuran, diglyme, acetonitrile, propi-onitrile or benzonitrile; or nonpolar organic solvent~
such as n-hexane, benzene or cyclohexane or mixtures of such solvents.
The amount of the reducing agent is not less than the equlvalent weight of the sulfonyl group present in the surface. Generally, the reducing agent will be used in large excess. The pH value of the reaction system will be selected on the basis of the particular reducing agent employed.
The reaction can be carried out under reduced, normal or increased pressure. In the reaction involving the use of a gaseous reducing agent, the increased pressure can improve the velocity of the reaction.
~5Z452 The reaction time generally ranges from one minute to 100 hours.
In case of a cation exchange membrane reinforced with a re-inforciny material~ treatment with a reducing agent is pre-ferably applied onto the side opposite to the reinforced side.
The course of the reaction may be followed by analysis of the infrared absorption spectrum of the membrane, as is particularly illustrated in the examples. Key bands in following the reaction are as follows:
sulfonyl chloride -------- 1420 cm 1 sulfinic acid salt ------- 940 cm 1 sulfinic acid salt ------- 1010 cm 1 carboxylic acid ---------- 1780 cm 1 carboxylic acid salt ----~ 1690 cm 1 The specific functional groups of the invention are found to be unitary species having a neutralization point at approximately pKa=2.5 from measurement of electric resis-tance and infrared spectrum by varying pH. Said func-tional groups exhibit characteristic absorptions at 1780 cm 1 ~H form) and at 1690 cm 1 (Na form). Furthermore, when converted into chlorides by treatment with PC15/POC13, they are found to exhibit characteristic absorption at 1810 cm 1 From these measurements, they are identified to be carboxylic acid groups. By elemental analysis by using the combustion method, sulfur is found to be de-crea8ed by one atom per one exchange group. Fluorine atom removal is observed, two atoms per one exchange group, by the alizarin-complexion method. From these results of analysis and also from the fact that carboxy-lic acids are formed by use of a reducing agent containingno carbon atom under an atmosphere in the absence of car-bon atom, the above functional groups are confirmed to be -OCF2COOM. This structure is also evidenced by measurement of NMR spectrum of C13 and Fl9 of the product obtained by the réaction, corresponding to the ab,ove polymer reaction, conducted for the monomer having the functional group 2C 2S02X.
The products of the treatment with a reducing agent may take three typical forms. These are:
1) A~ many -COOM groups as are required may be formed.
2) Not all the -COOM groups required may be formed and excess -SO2M groups may be present.
3) Substantially all -SO2M groups may be present.
In the first instance, no further treatment will be re-quired. In the second and third case, there are two alternative3. A more powerful reducing agent may be em-ployed, or excess -SO2M groups may be converted to car-boxylic acid groups by heat treatment, which is advan-tageously carried out when M is hydrogen. The heating may take place at any selected practical pre~sure at a temperature of from 60C to 400C for a period of from 15 to 120 minutes. The preferred conditions for effi-ciency and economy are atmospheric pressure, 100C to 200C, and 30 to 60 minutes.
Any remaining sulfinic acid groups may be converted into sulfonic acid groups, if desired. Thi~ conversion of the sulfinic acid group to the culfonic acid group can ea~ily be accomplished ~uch as by subjecting the former group to oxidation in an aqueous 801ution of 1 to 5 per-cent NaC10 or an aqueous solution of 1 to 30 percent H2O2 at 40C to 90C for 2 to 20 hours.
The reducing agent to be used for the purpose of this invention is selected, as in ordinary organic reactions, with due consideration to numerous factors such as the ~, ~;Z452 kind of the substitutent X in the SO2X group, the kind of the reducing agent, the kind of the solvent to be used, the temperature of the reaction, the concentration, the pH value, the reaction time and the reaction pressure.
The reducing agents usable for this invention are broadly divided by their reactions as follows.
The reducing agents of the first group can be applied to virtually all SO2X groups. Occasionally the reaction proceeds to an advanced extent to produce a product which appears to be an alcohol.
The reducing ag~nts of the second, third and fourth groups are particularly effective when applied to sulfonyl halide groups of relatively high reactivity.
The reducing agents of the fifth, sixth, ~eventh and lS eighth groups are also effective for application to sul-fonyl halide groups, although use of these reducing agents frequently produces the sulfinic acid aloné. Use of the -SO2F group demands cpecially careful selection of the reaction cond~tions, for it may possibly induce hydrolysi~ in the presence of a reducing agent from the sixth, seventh and eighth groups.
It is possible to convert the -SO2Cl group directly into the carboxylic acid group without going through the inter-mediate of sulfinic acid For example, the conversion can be accomplished by subjecting the membrane of the fluorocarbon polymer possessing the -SO2Cl group to ele-vated temperature and/or to ultra~iolet rays and/or to an organic or inorganic peroxide.
As a matter of course, the reaction of the present in~en-tio~ can be applied to other monomers possessing similar side chains. Thus, fluorocarbon monomers possescing a . .
.. . .
~Z452 sulfinic acid group or carboxylic acid group can readily be synthesized by said reaction.
It will be noted that the ultimate effect of the treat-ment with a reducing agent can be represented by the following reaction:
2 2S03M > 2C OM.
The membranes of this invention have many advantages, some of which have already been mentioned above.
In the course of the electrolysis of a~ueous solution of an alkali metal halide, the portion of the fluorocarbon polymer possessing the carboyxlic acid groups in the cation exchange membrane iB preferably on the side of the catholyte~ Even in the electrolysis performed to produce caustic soda at a high concentration, therefore, the membrane provides effective prevention of the back migra-tion of hydroxyl ions and permits the electrolysis to proceed at high current efficiency.
Particularly when the cation exchange membrane of two-layer construction of this invention is used as the dia-phragm in the electrolysi~ of aqueous solution of sodiumchloride, the prevention of the back migration of hydroxyl ion~ can be obtained quite effectively and the current efficiency maintained at a high level by allowing the high EW layer resulting from the treatment with the re-ducing agent to face the cathode side of the electrolyticcell. AQ a consequence, thi~ cation exchange membrane enables the unit power consumption to be lowered and the prime cost of the product proportionally lowered and, hence, proves to be highly advantageous from the commer-cial point of view.
Generally, the anode compartment in the electrolysis ofaqueous solution of sodium chloride is operated in an acidic state. In consideration of the fact that the apparent pKa value of the carboxylic acid is of the order of 2 to 3, the presence of the thin layer of the carboxy-lic acid group on the anode compartment side brings about an effect of increasing the potential and therefore proves disadvantageouc.
Compared with the conventional membranes, the membranes of this invention have the following advantages in the process of manufacture., Manufacture of the cation ex-change membrane of fluorocarbon polymer substituted bycarboxylic acid groups has heretofore proved to be ex-tremely difficult. This is because the properly substi-tuted fluorocarbon compound monomers are extremely diffi-cult to synthesize. Additionally, copolymers from such monomers with perfluorovlnyl monomers are highly suscep-tible to thermal decomposition and cannot be thermally moulded in the form of a membrane by conventional extru-sion techniques.
Thic invention alleviates the aforesaid difficulties by causing part of the sulfonic acid groups of the fluoro-carbon polymer to be converted into the carboxylic acid group.
The following examples illustrate,the techniques used in the preparation of membrane~ in accordance with the in-vention, although it will'be understood that the membra-nes of certain examples, in which polymers containing only pendant carboxylic acid groups are prepared, and the comparison examples in which the membraneq contain no pendant carboxylic groups, are outside the scope of the present invention as set forth in the claims. Such examples are included for the better understanding of those examples which do relate to the membranes of the invention, incorporating a layer of polymer having both carboxylic acid groups and sulfonic acid groups.
~5Z45~
Tetrafluoroethylene and perfluoro(3,6-dioxa-4-methyl-7-octene sulfonyl fluoride) were copolymerized in 1,1,2-trichloro-1,2,2-trifluoroethane in the presence of per-fluoropropionyl peroxide as the initiator. The polymeri-zation temperature wa~ held at 45C and the pressure maintained at 5 atmospheres during the copolymerization.
The exchange capacity of the resultant polymer, when measured after saponification, was 0.95 milli-equivalent/gram of dry resin.
This copolymer was moulded with heating into film 0.3 mmin thickness. It was then saponified in a mixture of 2.5N caustic soda/50 percent methanol at 60C for 16 hours, converted to the H form in lN hydrochloric acid, and hea-ted at 120C under reflux for 20 hour~ in a 1:1 mixture of phosphorus pentachloride and phosphorus oxychloride to be converted into the sulfonyl chloride form. At the end of the reaction, the copolymer membrane was washed with carbon tetrachloride and then subjected to mea~ure-meant of attenuated total reflection ~pectrum (hereinafter referred to as A.T.R.), which showed a strong ab~orption band at 1420 cm 1 characteristic of sulfonyl chloride.
In a crystal violet solution, the membrane was not stai-ned. Between rame~ made of acrylic resin, two sheets of th~s membrane were fastened in position by meanq of packings made of polytetrafluoroethylene. The frames were immersed in an aqueous 57 percent hydroiodic acid ~olution so that one surface of each membrane would un-dergo reaction at 80C for 24 hours. The A.T.R. of the membrane was then measured. In the spectrum, the absorp-tion band at 1420 cm 1 characteristic of sulfonyl chlo-ride group vanished and an absorption band at 1780 cm 1 characteristic of carboxylic acid group appeared instead.
In the crystal violet solution, a layer of a thickness of about 15 microns on one surface of the membrane was ~, .
- - -' , - , ' .
- : , .
~2~5;~
stained. The cation exchange groups existing on the surface were found to be carboxylic acid groups (100%) by measurement of A . T . R .
By saponifying this membrane in an aqueous solution of 2.5N caustic soda/50 percent methanol at 60C for 16 hours, there was obtained a homogeneous and strong cation exchange membrane.
In an aqueous 0.lN caustic soda solution, this membrane showed a specific conductivity of 10.0 x 10 3 mho/cm.
The specific conductivity of the membrane was determined by initial conversion to a complete Na form, keeping the membrane in a constantly renewed bath of an aqueous 0.lN
caustic soda solution at about 25C for ten hours until equilibrium and subjecting it to an alternating current of 1000 cycles while under an aqueou~ O.lN caustic soda solution at 25C for measurement of the electric resis-tance of the membrane.
The aforementioned Na form electrolytic diaphragm wa~
equilibrated in an aqueous 2.5N caustic soda solution at 90C for 16 hours, incorporated in an electrolytic cell in such way that the treated surface fell on the cathode ~de. It was utilized as the membrane in the electroly-si~ of sodium chloride and its current efficient measured.
The result was 95%.
The electrolytic cell had a service area of 15 cm2 (5 cm x 3cm) and comprised an anode compartment and a cathode compartment separated by the cationic membrane. A metal-lic, dimensionally stable DSA anode was u~ed, and an iron plate was used as the cathode. ~n aqueous 3N sodium chloride solution at pH 3 was circulated through the anode compartment and an aqueous 35 percent caustic soda ~1~;2452 solution through the cathode compartment at 90C. Under these conditions, an electric current was passed between the electrodes at a current density of 50 amperes/dm2.
The current efficiency was calculated by dividing the amount of caustic soda produced in the cathode compart-ment per hour by the theoretical value calculated from the amount of electricity passed.
A sulfonyl fluoride form membrane having an exchange capacity of 0.65 milli-equivalent/gram of dry resin was obtained by a procedure similar to that of Example 1.
The membrane was placed in a flask and tetrahydrofuran was added, Lithium boron hydride was added in a large excess, and the resultant reaction system heated under reflux for 50 hours. At the end of the reaction, the membrane was subjected to measurement of A.T.R. In the spectrum, the absorption band at 1470 cm 1 characteristic of sulfonyl fluoride substantially vanished and large absorption band at 1690 cm 1 characteristic of -COOLi small absorption bands at 940 cm 1 and 1010 cm 1 charac-teristic of sulfinic acid salt appeared instead. Thi~
membrane wa~ allowed to stand in an aqueous 0.1 percent cry~tal violet ~olution (containinq 10 percent ethanol) for three minute~ and, thereafter, the cross section of the membrane was observed under a microscope. The micro-~copic observation revealed a strongly stained layer having a thickness of about 10 micron~ on each ~urface of the membrane.
The carboxylic acid group content in the surface layers, as determined from A.T.R. was about 60 percent.
ExAMæLE 3 The sulfonyl chloride form of the membrane obtained in Example l was reduced in an aqueous 35 percent hypophos-phorous acid solution at 80C for ten hours. sy A.T.R., the adsorption band at 1420 cm 1 characteristic of sul-fonyl chloride group vanished, but the absorption band at 1780 cm 1 characteristic of carboxylic acid group was not very strong. When the membrane was washed with water and treated in 47~ hydrobromic acid at 80C for 20 hours, the absorption band at 1780 cm 1 characteristic of car-boxylic acid group increased in intensity. The cation exchange groups on the surface were measured by A.T.R.
to be 90%.
When the membrane was saponified in an aqueous solution of 2.5N caustic soda/50 percent methanol and then subjec-ted to measurement of A.T.R. the absorption by the car-boxylic acid group at 1780 cm 1 was noted to have been shifted to an absorption band at 1690 cm 1 char'acteristic of carboxylic acid salt. Low intensity absorption due to the sulfinic acid salt group appeared at 940 and 1010 cm 1.
The specific conductivity and current efficiency of the membrane, when measured under the conditions of Example1, were found to be 9.5 x 10 3 mho/cm and 92 percent respectively.
Two sheetfi of the sulfonyl chloride form of the membrane obtained in Example 1 were incorporated in frames simi-lar to those used in Example 1. The frames were immer-sed in triethanol amine so that one surface o each membrane would undergo a reaction at 80C for 20 hours.
The treated surfaces of each membrane were subjected to measurement of A.T.R. In the spectrum, the absorption band at 1420 cm 1 characteristic of the sulfonyl chloride group vanished and strong absorption bands at 940 cm 1 and 1010 cm 1 characteristic of the sulfinic acid salt ~1~;245~
appeared. The membrane was saponified in an aqueous solu-tion of 2.5N caustic soda/50 percent methanol, then heated in 12N hydrochloric acid at 90~C for 30 hours, again con-verted into the Na form by means of 2.5N caustic soda and S subjected to measurement of A.T.R. A sharp absorption band characteristic of a carboxylic acid salt appeared at 1690 cm 1. The absorption bands at 940 cm 1 and 1010 cm practically disappeared. The percentage of carboxy-lic acid 7roups present on the surface as cation exchange groups was 85% by measurement of A.T.R.:
The specific conductivity of this membrane, when measured under the same conditions in Example 1, was found to be 11.0 x 10 3 mho/cm. The current efficiency of the mem-brane, when measured with the treated surface acing the cathode compartment side, was 89 percent.
EYAMPLE S
A ~ulfonyl fluoride orm membrane 0.20 mm in thickness wa~ prepared by a procedure similar to that of Example 1.
One surface of the membrane was saponified with an aque-ou8 solution of 2.SN caustic soda/50 percent methanol.The membrane wa~ spread out with the nonsaponified sur-~sce held downwardly on a plain-weave fabric of polytet-rafluoroethylene 0.15 mm in thickne~s with warp and fil-ling yarns, both of 400-denier multifilaments, repeat each at a rate of 40 yarns per inch. The membrane and fabric were heated to 270C with the membrane simultane-ously drawn against the fabric by mean~ of vacuum so as to embed the fabric in the membrane as a reinforcing material.
This membrane was converted to the sulfonyl chloride form by the same method as used in Example 1. With frames made of acrylic resin, two such membranes were held fast against each other with the fabric-reinforced ~15;;~45~
surface on the inside. The frames containing the two ad-joining membranes were immersed in an aqueous 47 percent hydrobromic acid solution and caused to undergo a reaction at 80~C for 20 hours. After the reaction, the membranes were removed, saponified in an aqueous solution of 2.5N
caustic soda/50 percent methanol and further oxidized in a solution of 2.5N caustic soda/2 5 percent sodium hypochlo-rite at 90C for 16 hours to reconvert unreacted sulfinic acid groups to sulfonate groups. The specific conductivity and current efficiency of the membrane, when measured under the conditions of Example 1 with the treated sur-face held in the direction of the cathode, were found to be 5.0 x 10 3 mho/cm and 95 percent respectively.
The reinforced membrane obtained in the first stage of Example 5 wac caponified. The specific conductivity and current efficiency of the saponified membrane, when mea-sured under the same conditions as in Example 1, were found to be 6.0 x 10 3 mho/cm and 58 percent respectively.
In 1,1,2-trichloro-1,2,2-trifluoroethane, tetrafluoro-ethylene and perfluoro-3,6-dioxa-4-methyl-7-octene sul-fonyl fluoride were copolymerized in the presence of perfluoropropionyl peroxide a~ the polymerization initi-ator, with the polymerization temperature fixed at 45Cand the pressure maintained at 5 kg/cm2G during the polymerization. The polymer obtained is identified as Polymer 1.
The copolymerization was repeated by the same procedure, except the pressure was maintained at 3 kg/cm2 through-'out the polymerization to produce Polymer 2.
~ ~.52452 A portion of each of the polymers thus produced was sub-jected to hydrolysis in a mixture of aqueous 5N caustic soda solution and methanol (volume ratio of 1:1) at 90C
for 16 hours to be converted into the sodium sulfonate form. The exchange capacity of the sodium sulfonate form polymer was found to be 0.74 milliequivalent/g of dry resin in the case of Polymer 1 and 0.91 milliequivalent/g of dry resin in the case of Polymer 2.
Polymer 1 and Polymer 2 were heat moulded to produce separate membranes 2 mils and4mils in thickness. The two membranes were joined face to face and moulded under heating into a composite membrane. The composite mem-brane was treated in the aforementioned hydrolyzing sys-tem to be converted into a sodium sulfonate form compo-lS site membrane.
The composite membrane was converted into the H form bytreatment in an aqueous lN hydrochloric acid ~olution and sub~equently converted into the sulfonyl chloride form by reaction with a mixture of phosphorus pentachlo-ride and phosphoru~ oxychloride ~gravimetric ratio 1:1)at 120C for 40 hour~. At the end of the reaction, the composite membrane was washed for four hours under reflux in carbon tetrachloride and dried under vacuum at 40C.
2S The dried membrane was subjected to measurement of A.T.R.
to reveal that in both the membranes of Polymer 1 and Polymer 2, the absorption band by sulfonyl chloride appeared at 1420 cm 1 and absorption due to the sulfonic acid group at 1060 cm 1 completely disappeared.
Two sheets of the composite membrane were held against each other with the Polymer 2 membrane sides facing in-wardly and, in that state, set in position in frames ~Z45~
made of acrylic resin and fastened up by use of packings made of polytetrafluoroethylene. The frames were immer-sed in an aqueous 57 percent hydroiodic acid solution so that only the exposed surfaces (Polymer 1 membrane side) S would undergo reaction at 80C for 30 hours. The mem-branes were washed with water at 60C for 30 minutes.
The infrared spectrum of each treated surface was mea-sured. In the spectrum, the àbsorption band at 1420 cm 1 characteristic of sulfonyl chloride completely vanished, and an absorption band at 1780 cm 1 characteristic of carboxylic acid group appeared. In crystal violet solu-tion, a stained layer of a width of about 0.3 mil was observed on the Polymer 1 side of the membrane. The ~ation exchange groups on the surface were found to be lS carboxylic acid groups (100%) by A.T.R.
The membrane was saponified in an aqueous solution of ~.SN
caustic soda/50 percent methanol at 60C for 16 hours and then subjected to measurement of A.T.R. In the spectrum, the absorption band of the carboxylic acid group was shifted to 1690 cm 1 in the Polymer 1 side of the mem-brane. On the Polymer 2 side of the membrane, an absorp-tion band at 1055 cm 1 characteristic of sodium sulfonate appeared. The membrane was immer8ed in an agueous 2.5 percent sodium hypochlorite solution and oxidized at 90C
for 16 hour~.
The specific conductivity of the resultant membrane, when measured in an aqueous O.lN caustic soda solution, was found to be 5.2 x 10 3 mho/cm.
The specific conductivity of the membrane was determined after complete conversion into the Na form, keeping the membrane in a constantly renewed bath of an aqueous 0.lN
caustic soda solution at normal room temperature for ten hours until equilibrium and subjecting it to an alterna-ting current of 1000 cycles while under an aqueous 0.lN
~15245~
soda solution at 25C for measurement of the electric resistance of the membrane.
The Na form cation exchange membrane was equilibrated by immersion in an aqueous 2N caustic soda solution at 90C
S for 16 hours, then incorporated in an elec~rolytic céll with the reacted surface, namely the Polymer 1 side, fa-cing the cathode. It was tested for current efficiency as the membrane in the electrolysis of sodium chloride.
The value thus found was 94 percent.
The service area of the electrolytic cell was 15 cm2 (5 cm x 3 cm). It comprised an anode compartment and a cathode compartment separated by the electrolytic membrane.
A metallic anode coated with a noble metal was used as the anode and an iron plate as the cathode. An aqueous 3N
sodium chloride solution at pH 3 wa~ circulated through the anode compartment and an aqueous 30 percent caustic soda solution was circulated through the cathode compart-ment at 90C, Under these conditions, an electric cur-rent wa~ pa~sed between the electrodes at a current den-~ity of 50 ampere~/dm . The current efficiency wa~ cal-culated by divldlng the amount of caustic ~oda produced in the cathode compartment per hour by the theoretical value calculated from the amount of electricity passed.
The passage of the electric current was continued for 2000 hours. Thereafter, the current efficiency wa~ mea-sured and found to be 93.8 percent.
A copolymer (Polymer 3) was prepared by repeating the procedure of Example 6, except that the pressure was maintained at 7 kg/cm2G during the polymerization.
The e~change capacity of Polymer 3, when measured by the ~245Z
same procedure as that of Example 6, was found to be 0.68 milliequivalent/g of dry resin.
By following the procedure of Example 6, a composite membrane comprising a Polymer 2 membrane, 4 mils ln khickness, and a Polymer 3 membrane, 2 mils in thickness, was produced.
The composite membrane was subjected to hydrolysis in a mixture of an aqueous 5N caustic soda solution and metha-nol (volume ratio of 1:1) at 60C for 40 hours, converted into the H form by treatment in an aqueous lN hydrochloric acid solution and thereafter converted again into the ammonium sulfonate form by treatment in an aqueous lN
ammonia solution.
It was then allowed to react in a mixture of phosphorus pentachloride and phosphorus oxychloride (gravimetric ratio 1:1) at 120C for 36 hours to be converted into the sulfonyl chloride form.
The composite membrane was incorporated in a flow-ga~
reaction system and ~o that the Polymer 3 side of the compo~ite membrane was allowed to undergo a contact re-action with 20.0 percent hydrogen iodide gas ~with nitro-gen as the diluting gas) at 100C for 12 hours. The treated surface of the membrane was subjected to measure-ment of A.T.R. In the spectrum, an absorption band due 2S to carboyxlic acid appeared at 1780 cm 1 and the absorp-tion band due to sulfonyl chloride at 1420 cm 1 dis-appeared. In crystal violet solution, a layer of 0.4 mil in thickness was stained. By A.T.R. measurement, about 100% of the cation exchange groups were found to be carboxylic acid groups.
The composite membrane was then hydrolyzed, oxidized, and incorporated in an electrolytic cell with the Polymer ~;2452 3-side facing the cathode compartment. In this electro-lytic system, electrolysis of sodium chloride was carried out under the same conditions as in Example 6, with the concentration of caustic soda circulated to the cathode compartment fixed at 20 percent. The current efficiency was 97 percent, and the specific conductivity was 4.3 x mho/cm.
The current efficiency, when measured after 1700 hours of continued passage of electric current, was 97.2 percent.
Two sheets of the sulfonyl chloride form of the membrane obtained in Example 7 were held against each other with the Polymer 2 sides facing inwardly and, in that state, set in frames of acrylic resin, immersed in an aqueous 20 percent sodium sulfide solution and allowed to react under a continuous flow of nitrogen gas at 70C for two hours. At the end of the reaction, the treated surface of the membrane was subjected to measurement of A.T.R. In the spectrum, the absorption band at 1420 cm 1 characte-ristic of the sulfonyl chloride group disappeared, and absorption bands at 1010 cm 1 and 940 cm 1 characteristic of sulfinic acid salts were observed.
The membrane was immersed in an aqueous 2 5 percent 40-dium hypochlorite solution at 70C for 16 hours and was again subjected to measurement of A.T.R. In th~ spectrum, the adsorptionbands at 1010 cm 1 and 940 cm 1 vanished and an absorption band at 1055 cm 1 characteristic of sodium sulfonate appeared.
30 Two sheets of the membrane treated with sodium sulfide as described above were washed with water and set again ~1~245~2 in the frames so that the Polymer 3 side reacted in a solution of 47 percent hydrogen bromide at 80~C for 20 hoursO In A.T.R. obtained from the membrane, a sharp absorption band due to the carboxylic acid group appeared at 1780 cm 1.
The membrane was saponified in an aqueous solution of 2.5N
caustic soda/50 percent methanol and subjected again to measurement of A.T.R. In the spectrum, the absorption band at 1780 cm 1 vanished, an absorption band due to sodium carboxylate appeared at 1690 cm 1, and minor ab-sorption bands due to sulfinic acid salts appeared at 940 cm 1 and 1010 cm 1. By A.T.R. measurement, about 90%
of the cation exchanye groups on the surface were found to be carboxylic acid group~.
The membrane was then treated in an aqueous 2.5 percent sodium hypochlorite solution at 90C for 16 hours. The specific conductivity of the treated membrane was found to be 4.6 x 10 3 mho/cm. The current efficiency, when measured under the same electrolytic condition~ as those o Example 7, wa~ found to be 92 percent. Substantially the same current efficiency was shown after 1700 hours of continued passage of electric current.
The membrane treated with sodium culfide in Example 8 was saponified in an aqueous 2.SN caustic soda/SO percent methanol at 60C for 16 hours. The membrane wa~ then treated in an aqueous lN hydrochloric acid solution at 60C for 16 hours and thereafter heated in the air at 150C for one hour. The Polymer 3 side of the membrane was subjected to measurement of A.T.R. spectrum. In the spectrum, an absorption band due to the carboxylic acid group appeared at 1780 cm 1 When this membrane was con-verted to the salt form, an absorption band due to ~;245~
carboxylate appeared strongly at 1690 cm 1, and weak absorption bands by sulfinic acid salts appeared at lOlO
cm 1 and 940 cm 1. The cation exchange groups on the surface were found to be carboxylic acid groups (about 90%) by A.T.R. The membrane was oxidized in an aqueous 2.5 percent sodium hypochlorite solution at 90C for 16 hours. The specific conductivity of the oxidized mem-brane was found to be 4.4 x 10 3 mho/cm. The current efficiency of the membrane, when measured under the same conditions as those of Example 7, was found to be 93 per-cent.
.
Tetrafluoroethylene and perfluoro-3,6-dioxa-4-methyl-7-octene sulfonyl fluoride were emulsion polymerized at 70C
under 4.5 atmospheres of tetrafluoroethylene pressure, with ammonium persulfate used as the initiator and the ammonium salt of perfluoro-octanoic acid as the emulsifier.
The polymer consequently obtained was washed with water, then hydrolyzed and thereafter subjected to measurement of exchange capacity by a titrimetric method. The ex-change capacity was found to be 0.80 milli-equivalent/
gram dry resin. This polymer is identified as Polymer 4.
By a procedure similar to that of Example 6, Polymer 2 used in Example 6 and Polymer 4 were combined to produce a composite membrane comprising a Polymer 2 membrane which had a thicknes~ of 4 mil~, and a Polymer 4 membrane which had a thickness of 3 mil~. This composite membrane was spread out with the Polymer 2 side held downwardly on a woven fabric of polytetrafluoroethylene about 0.15 mm in thickness having filling yarns of 400-denier multi-filaments and warp yarns of 200-denier multifilaments x 2 repeat each at a rate of 25 yarns per inch. The mem=
brane and fabric were heated to 270C with the membrane ~`Z452 simultaneously drawn against the membrane by means of vacuum so as to embed the fabric in the membrane as a reinforcing material.
This membrane was converted into the sulfonyl chloride form by the same procedure as used in Example 6. Two sheets of the membrane were held against each other with the Polymer 2 sides (the sides having the fabric embed-ded) acing inwardly by means of frames made of acrylic resin. The Polymer 4 sides of the membrane were allowed to react with hydrogen sulfide gas introduced in a conti-nuous flow at l20C for 20 hours.
The membrane was removed, saponified in an aqueous solu-tion of 2.5N caustic soda/50 percent methanol and there-after oxidized in a solution of 2.5N caustic soda/2.5 percent sodium hypochlorite at 90C for 16 hours. The specific conductivity, when measured by the same method as used in Example 6, was found to be 3.2 x 10 3 mho/cm, The current efficiency was 94 percent. Even after 1000 hours of continued passage of electric current, the cur-rent efficiency remained unchanged.
The reinforced composite membrane obtain in Example 9 wa~saponified. The specific conductivity and current effi-ciency of the resultant membrane, when measured under the same conditions as tho~e of Example 6, were found to be 3.9 x 10 3 mho/cm and 62.1 percent respectively.
, ' ~
In the first instance, no further treatment will be re-quired. In the second and third case, there are two alternative3. A more powerful reducing agent may be em-ployed, or excess -SO2M groups may be converted to car-boxylic acid groups by heat treatment, which is advan-tageously carried out when M is hydrogen. The heating may take place at any selected practical pre~sure at a temperature of from 60C to 400C for a period of from 15 to 120 minutes. The preferred conditions for effi-ciency and economy are atmospheric pressure, 100C to 200C, and 30 to 60 minutes.
Any remaining sulfinic acid groups may be converted into sulfonic acid groups, if desired. Thi~ conversion of the sulfinic acid group to the culfonic acid group can ea~ily be accomplished ~uch as by subjecting the former group to oxidation in an aqueous 801ution of 1 to 5 per-cent NaC10 or an aqueous solution of 1 to 30 percent H2O2 at 40C to 90C for 2 to 20 hours.
The reducing agent to be used for the purpose of this invention is selected, as in ordinary organic reactions, with due consideration to numerous factors such as the ~, ~;Z452 kind of the substitutent X in the SO2X group, the kind of the reducing agent, the kind of the solvent to be used, the temperature of the reaction, the concentration, the pH value, the reaction time and the reaction pressure.
The reducing agents usable for this invention are broadly divided by their reactions as follows.
The reducing agents of the first group can be applied to virtually all SO2X groups. Occasionally the reaction proceeds to an advanced extent to produce a product which appears to be an alcohol.
The reducing ag~nts of the second, third and fourth groups are particularly effective when applied to sulfonyl halide groups of relatively high reactivity.
The reducing agents of the fifth, sixth, ~eventh and lS eighth groups are also effective for application to sul-fonyl halide groups, although use of these reducing agents frequently produces the sulfinic acid aloné. Use of the -SO2F group demands cpecially careful selection of the reaction cond~tions, for it may possibly induce hydrolysi~ in the presence of a reducing agent from the sixth, seventh and eighth groups.
It is possible to convert the -SO2Cl group directly into the carboxylic acid group without going through the inter-mediate of sulfinic acid For example, the conversion can be accomplished by subjecting the membrane of the fluorocarbon polymer possessing the -SO2Cl group to ele-vated temperature and/or to ultra~iolet rays and/or to an organic or inorganic peroxide.
As a matter of course, the reaction of the present in~en-tio~ can be applied to other monomers possessing similar side chains. Thus, fluorocarbon monomers possescing a . .
.. . .
~Z452 sulfinic acid group or carboxylic acid group can readily be synthesized by said reaction.
It will be noted that the ultimate effect of the treat-ment with a reducing agent can be represented by the following reaction:
2 2S03M > 2C OM.
The membranes of this invention have many advantages, some of which have already been mentioned above.
In the course of the electrolysis of a~ueous solution of an alkali metal halide, the portion of the fluorocarbon polymer possessing the carboyxlic acid groups in the cation exchange membrane iB preferably on the side of the catholyte~ Even in the electrolysis performed to produce caustic soda at a high concentration, therefore, the membrane provides effective prevention of the back migra-tion of hydroxyl ions and permits the electrolysis to proceed at high current efficiency.
Particularly when the cation exchange membrane of two-layer construction of this invention is used as the dia-phragm in the electrolysi~ of aqueous solution of sodiumchloride, the prevention of the back migration of hydroxyl ion~ can be obtained quite effectively and the current efficiency maintained at a high level by allowing the high EW layer resulting from the treatment with the re-ducing agent to face the cathode side of the electrolyticcell. AQ a consequence, thi~ cation exchange membrane enables the unit power consumption to be lowered and the prime cost of the product proportionally lowered and, hence, proves to be highly advantageous from the commer-cial point of view.
Generally, the anode compartment in the electrolysis ofaqueous solution of sodium chloride is operated in an acidic state. In consideration of the fact that the apparent pKa value of the carboxylic acid is of the order of 2 to 3, the presence of the thin layer of the carboxy-lic acid group on the anode compartment side brings about an effect of increasing the potential and therefore proves disadvantageouc.
Compared with the conventional membranes, the membranes of this invention have the following advantages in the process of manufacture., Manufacture of the cation ex-change membrane of fluorocarbon polymer substituted bycarboxylic acid groups has heretofore proved to be ex-tremely difficult. This is because the properly substi-tuted fluorocarbon compound monomers are extremely diffi-cult to synthesize. Additionally, copolymers from such monomers with perfluorovlnyl monomers are highly suscep-tible to thermal decomposition and cannot be thermally moulded in the form of a membrane by conventional extru-sion techniques.
Thic invention alleviates the aforesaid difficulties by causing part of the sulfonic acid groups of the fluoro-carbon polymer to be converted into the carboxylic acid group.
The following examples illustrate,the techniques used in the preparation of membrane~ in accordance with the in-vention, although it will'be understood that the membra-nes of certain examples, in which polymers containing only pendant carboxylic acid groups are prepared, and the comparison examples in which the membraneq contain no pendant carboxylic groups, are outside the scope of the present invention as set forth in the claims. Such examples are included for the better understanding of those examples which do relate to the membranes of the invention, incorporating a layer of polymer having both carboxylic acid groups and sulfonic acid groups.
~5Z45~
Tetrafluoroethylene and perfluoro(3,6-dioxa-4-methyl-7-octene sulfonyl fluoride) were copolymerized in 1,1,2-trichloro-1,2,2-trifluoroethane in the presence of per-fluoropropionyl peroxide as the initiator. The polymeri-zation temperature wa~ held at 45C and the pressure maintained at 5 atmospheres during the copolymerization.
The exchange capacity of the resultant polymer, when measured after saponification, was 0.95 milli-equivalent/gram of dry resin.
This copolymer was moulded with heating into film 0.3 mmin thickness. It was then saponified in a mixture of 2.5N caustic soda/50 percent methanol at 60C for 16 hours, converted to the H form in lN hydrochloric acid, and hea-ted at 120C under reflux for 20 hour~ in a 1:1 mixture of phosphorus pentachloride and phosphorus oxychloride to be converted into the sulfonyl chloride form. At the end of the reaction, the copolymer membrane was washed with carbon tetrachloride and then subjected to mea~ure-meant of attenuated total reflection ~pectrum (hereinafter referred to as A.T.R.), which showed a strong ab~orption band at 1420 cm 1 characteristic of sulfonyl chloride.
In a crystal violet solution, the membrane was not stai-ned. Between rame~ made of acrylic resin, two sheets of th~s membrane were fastened in position by meanq of packings made of polytetrafluoroethylene. The frames were immersed in an aqueous 57 percent hydroiodic acid ~olution so that one surface of each membrane would un-dergo reaction at 80C for 24 hours. The A.T.R. of the membrane was then measured. In the spectrum, the absorp-tion band at 1420 cm 1 characteristic of sulfonyl chlo-ride group vanished and an absorption band at 1780 cm 1 characteristic of carboxylic acid group appeared instead.
In the crystal violet solution, a layer of a thickness of about 15 microns on one surface of the membrane was ~, .
- - -' , - , ' .
- : , .
~2~5;~
stained. The cation exchange groups existing on the surface were found to be carboxylic acid groups (100%) by measurement of A . T . R .
By saponifying this membrane in an aqueous solution of 2.5N caustic soda/50 percent methanol at 60C for 16 hours, there was obtained a homogeneous and strong cation exchange membrane.
In an aqueous 0.lN caustic soda solution, this membrane showed a specific conductivity of 10.0 x 10 3 mho/cm.
The specific conductivity of the membrane was determined by initial conversion to a complete Na form, keeping the membrane in a constantly renewed bath of an aqueous 0.lN
caustic soda solution at about 25C for ten hours until equilibrium and subjecting it to an alternating current of 1000 cycles while under an aqueou~ O.lN caustic soda solution at 25C for measurement of the electric resis-tance of the membrane.
The aforementioned Na form electrolytic diaphragm wa~
equilibrated in an aqueous 2.5N caustic soda solution at 90C for 16 hours, incorporated in an electrolytic cell in such way that the treated surface fell on the cathode ~de. It was utilized as the membrane in the electroly-si~ of sodium chloride and its current efficient measured.
The result was 95%.
The electrolytic cell had a service area of 15 cm2 (5 cm x 3cm) and comprised an anode compartment and a cathode compartment separated by the cationic membrane. A metal-lic, dimensionally stable DSA anode was u~ed, and an iron plate was used as the cathode. ~n aqueous 3N sodium chloride solution at pH 3 was circulated through the anode compartment and an aqueous 35 percent caustic soda ~1~;2452 solution through the cathode compartment at 90C. Under these conditions, an electric current was passed between the electrodes at a current density of 50 amperes/dm2.
The current efficiency was calculated by dividing the amount of caustic soda produced in the cathode compart-ment per hour by the theoretical value calculated from the amount of electricity passed.
A sulfonyl fluoride form membrane having an exchange capacity of 0.65 milli-equivalent/gram of dry resin was obtained by a procedure similar to that of Example 1.
The membrane was placed in a flask and tetrahydrofuran was added, Lithium boron hydride was added in a large excess, and the resultant reaction system heated under reflux for 50 hours. At the end of the reaction, the membrane was subjected to measurement of A.T.R. In the spectrum, the absorption band at 1470 cm 1 characteristic of sulfonyl fluoride substantially vanished and large absorption band at 1690 cm 1 characteristic of -COOLi small absorption bands at 940 cm 1 and 1010 cm 1 charac-teristic of sulfinic acid salt appeared instead. Thi~
membrane wa~ allowed to stand in an aqueous 0.1 percent cry~tal violet ~olution (containinq 10 percent ethanol) for three minute~ and, thereafter, the cross section of the membrane was observed under a microscope. The micro-~copic observation revealed a strongly stained layer having a thickness of about 10 micron~ on each ~urface of the membrane.
The carboxylic acid group content in the surface layers, as determined from A.T.R. was about 60 percent.
ExAMæLE 3 The sulfonyl chloride form of the membrane obtained in Example l was reduced in an aqueous 35 percent hypophos-phorous acid solution at 80C for ten hours. sy A.T.R., the adsorption band at 1420 cm 1 characteristic of sul-fonyl chloride group vanished, but the absorption band at 1780 cm 1 characteristic of carboxylic acid group was not very strong. When the membrane was washed with water and treated in 47~ hydrobromic acid at 80C for 20 hours, the absorption band at 1780 cm 1 characteristic of car-boxylic acid group increased in intensity. The cation exchange groups on the surface were measured by A.T.R.
to be 90%.
When the membrane was saponified in an aqueous solution of 2.5N caustic soda/50 percent methanol and then subjec-ted to measurement of A.T.R. the absorption by the car-boxylic acid group at 1780 cm 1 was noted to have been shifted to an absorption band at 1690 cm 1 char'acteristic of carboxylic acid salt. Low intensity absorption due to the sulfinic acid salt group appeared at 940 and 1010 cm 1.
The specific conductivity and current efficiency of the membrane, when measured under the conditions of Example1, were found to be 9.5 x 10 3 mho/cm and 92 percent respectively.
Two sheetfi of the sulfonyl chloride form of the membrane obtained in Example 1 were incorporated in frames simi-lar to those used in Example 1. The frames were immer-sed in triethanol amine so that one surface o each membrane would undergo a reaction at 80C for 20 hours.
The treated surfaces of each membrane were subjected to measurement of A.T.R. In the spectrum, the absorption band at 1420 cm 1 characteristic of the sulfonyl chloride group vanished and strong absorption bands at 940 cm 1 and 1010 cm 1 characteristic of the sulfinic acid salt ~1~;245~
appeared. The membrane was saponified in an aqueous solu-tion of 2.5N caustic soda/50 percent methanol, then heated in 12N hydrochloric acid at 90~C for 30 hours, again con-verted into the Na form by means of 2.5N caustic soda and S subjected to measurement of A.T.R. A sharp absorption band characteristic of a carboxylic acid salt appeared at 1690 cm 1. The absorption bands at 940 cm 1 and 1010 cm practically disappeared. The percentage of carboxy-lic acid 7roups present on the surface as cation exchange groups was 85% by measurement of A.T.R.:
The specific conductivity of this membrane, when measured under the same conditions in Example 1, was found to be 11.0 x 10 3 mho/cm. The current efficiency of the mem-brane, when measured with the treated surface acing the cathode compartment side, was 89 percent.
EYAMPLE S
A ~ulfonyl fluoride orm membrane 0.20 mm in thickness wa~ prepared by a procedure similar to that of Example 1.
One surface of the membrane was saponified with an aque-ou8 solution of 2.SN caustic soda/50 percent methanol.The membrane wa~ spread out with the nonsaponified sur-~sce held downwardly on a plain-weave fabric of polytet-rafluoroethylene 0.15 mm in thickne~s with warp and fil-ling yarns, both of 400-denier multifilaments, repeat each at a rate of 40 yarns per inch. The membrane and fabric were heated to 270C with the membrane simultane-ously drawn against the fabric by mean~ of vacuum so as to embed the fabric in the membrane as a reinforcing material.
This membrane was converted to the sulfonyl chloride form by the same method as used in Example 1. With frames made of acrylic resin, two such membranes were held fast against each other with the fabric-reinforced ~15;;~45~
surface on the inside. The frames containing the two ad-joining membranes were immersed in an aqueous 47 percent hydrobromic acid solution and caused to undergo a reaction at 80~C for 20 hours. After the reaction, the membranes were removed, saponified in an aqueous solution of 2.5N
caustic soda/50 percent methanol and further oxidized in a solution of 2.5N caustic soda/2 5 percent sodium hypochlo-rite at 90C for 16 hours to reconvert unreacted sulfinic acid groups to sulfonate groups. The specific conductivity and current efficiency of the membrane, when measured under the conditions of Example 1 with the treated sur-face held in the direction of the cathode, were found to be 5.0 x 10 3 mho/cm and 95 percent respectively.
The reinforced membrane obtained in the first stage of Example 5 wac caponified. The specific conductivity and current efficiency of the saponified membrane, when mea-sured under the same conditions as in Example 1, were found to be 6.0 x 10 3 mho/cm and 58 percent respectively.
In 1,1,2-trichloro-1,2,2-trifluoroethane, tetrafluoro-ethylene and perfluoro-3,6-dioxa-4-methyl-7-octene sul-fonyl fluoride were copolymerized in the presence of perfluoropropionyl peroxide a~ the polymerization initi-ator, with the polymerization temperature fixed at 45Cand the pressure maintained at 5 kg/cm2G during the polymerization. The polymer obtained is identified as Polymer 1.
The copolymerization was repeated by the same procedure, except the pressure was maintained at 3 kg/cm2 through-'out the polymerization to produce Polymer 2.
~ ~.52452 A portion of each of the polymers thus produced was sub-jected to hydrolysis in a mixture of aqueous 5N caustic soda solution and methanol (volume ratio of 1:1) at 90C
for 16 hours to be converted into the sodium sulfonate form. The exchange capacity of the sodium sulfonate form polymer was found to be 0.74 milliequivalent/g of dry resin in the case of Polymer 1 and 0.91 milliequivalent/g of dry resin in the case of Polymer 2.
Polymer 1 and Polymer 2 were heat moulded to produce separate membranes 2 mils and4mils in thickness. The two membranes were joined face to face and moulded under heating into a composite membrane. The composite mem-brane was treated in the aforementioned hydrolyzing sys-tem to be converted into a sodium sulfonate form compo-lS site membrane.
The composite membrane was converted into the H form bytreatment in an aqueous lN hydrochloric acid ~olution and sub~equently converted into the sulfonyl chloride form by reaction with a mixture of phosphorus pentachlo-ride and phosphoru~ oxychloride ~gravimetric ratio 1:1)at 120C for 40 hour~. At the end of the reaction, the composite membrane was washed for four hours under reflux in carbon tetrachloride and dried under vacuum at 40C.
2S The dried membrane was subjected to measurement of A.T.R.
to reveal that in both the membranes of Polymer 1 and Polymer 2, the absorption band by sulfonyl chloride appeared at 1420 cm 1 and absorption due to the sulfonic acid group at 1060 cm 1 completely disappeared.
Two sheets of the composite membrane were held against each other with the Polymer 2 membrane sides facing in-wardly and, in that state, set in position in frames ~Z45~
made of acrylic resin and fastened up by use of packings made of polytetrafluoroethylene. The frames were immer-sed in an aqueous 57 percent hydroiodic acid solution so that only the exposed surfaces (Polymer 1 membrane side) S would undergo reaction at 80C for 30 hours. The mem-branes were washed with water at 60C for 30 minutes.
The infrared spectrum of each treated surface was mea-sured. In the spectrum, the àbsorption band at 1420 cm 1 characteristic of sulfonyl chloride completely vanished, and an absorption band at 1780 cm 1 characteristic of carboxylic acid group appeared. In crystal violet solu-tion, a stained layer of a width of about 0.3 mil was observed on the Polymer 1 side of the membrane. The ~ation exchange groups on the surface were found to be lS carboxylic acid groups (100%) by A.T.R.
The membrane was saponified in an aqueous solution of ~.SN
caustic soda/50 percent methanol at 60C for 16 hours and then subjected to measurement of A.T.R. In the spectrum, the absorption band of the carboxylic acid group was shifted to 1690 cm 1 in the Polymer 1 side of the mem-brane. On the Polymer 2 side of the membrane, an absorp-tion band at 1055 cm 1 characteristic of sodium sulfonate appeared. The membrane was immer8ed in an agueous 2.5 percent sodium hypochlorite solution and oxidized at 90C
for 16 hour~.
The specific conductivity of the resultant membrane, when measured in an aqueous O.lN caustic soda solution, was found to be 5.2 x 10 3 mho/cm.
The specific conductivity of the membrane was determined after complete conversion into the Na form, keeping the membrane in a constantly renewed bath of an aqueous 0.lN
caustic soda solution at normal room temperature for ten hours until equilibrium and subjecting it to an alterna-ting current of 1000 cycles while under an aqueous 0.lN
~15245~
soda solution at 25C for measurement of the electric resistance of the membrane.
The Na form cation exchange membrane was equilibrated by immersion in an aqueous 2N caustic soda solution at 90C
S for 16 hours, then incorporated in an elec~rolytic céll with the reacted surface, namely the Polymer 1 side, fa-cing the cathode. It was tested for current efficiency as the membrane in the electrolysis of sodium chloride.
The value thus found was 94 percent.
The service area of the electrolytic cell was 15 cm2 (5 cm x 3 cm). It comprised an anode compartment and a cathode compartment separated by the electrolytic membrane.
A metallic anode coated with a noble metal was used as the anode and an iron plate as the cathode. An aqueous 3N
sodium chloride solution at pH 3 wa~ circulated through the anode compartment and an aqueous 30 percent caustic soda solution was circulated through the cathode compart-ment at 90C, Under these conditions, an electric cur-rent wa~ pa~sed between the electrodes at a current den-~ity of 50 ampere~/dm . The current efficiency wa~ cal-culated by divldlng the amount of caustic ~oda produced in the cathode compartment per hour by the theoretical value calculated from the amount of electricity passed.
The passage of the electric current was continued for 2000 hours. Thereafter, the current efficiency wa~ mea-sured and found to be 93.8 percent.
A copolymer (Polymer 3) was prepared by repeating the procedure of Example 6, except that the pressure was maintained at 7 kg/cm2G during the polymerization.
The e~change capacity of Polymer 3, when measured by the ~245Z
same procedure as that of Example 6, was found to be 0.68 milliequivalent/g of dry resin.
By following the procedure of Example 6, a composite membrane comprising a Polymer 2 membrane, 4 mils ln khickness, and a Polymer 3 membrane, 2 mils in thickness, was produced.
The composite membrane was subjected to hydrolysis in a mixture of an aqueous 5N caustic soda solution and metha-nol (volume ratio of 1:1) at 60C for 40 hours, converted into the H form by treatment in an aqueous lN hydrochloric acid solution and thereafter converted again into the ammonium sulfonate form by treatment in an aqueous lN
ammonia solution.
It was then allowed to react in a mixture of phosphorus pentachloride and phosphorus oxychloride (gravimetric ratio 1:1) at 120C for 36 hours to be converted into the sulfonyl chloride form.
The composite membrane was incorporated in a flow-ga~
reaction system and ~o that the Polymer 3 side of the compo~ite membrane was allowed to undergo a contact re-action with 20.0 percent hydrogen iodide gas ~with nitro-gen as the diluting gas) at 100C for 12 hours. The treated surface of the membrane was subjected to measure-ment of A.T.R. In the spectrum, an absorption band due 2S to carboyxlic acid appeared at 1780 cm 1 and the absorp-tion band due to sulfonyl chloride at 1420 cm 1 dis-appeared. In crystal violet solution, a layer of 0.4 mil in thickness was stained. By A.T.R. measurement, about 100% of the cation exchange groups were found to be carboxylic acid groups.
The composite membrane was then hydrolyzed, oxidized, and incorporated in an electrolytic cell with the Polymer ~;2452 3-side facing the cathode compartment. In this electro-lytic system, electrolysis of sodium chloride was carried out under the same conditions as in Example 6, with the concentration of caustic soda circulated to the cathode compartment fixed at 20 percent. The current efficiency was 97 percent, and the specific conductivity was 4.3 x mho/cm.
The current efficiency, when measured after 1700 hours of continued passage of electric current, was 97.2 percent.
Two sheets of the sulfonyl chloride form of the membrane obtained in Example 7 were held against each other with the Polymer 2 sides facing inwardly and, in that state, set in frames of acrylic resin, immersed in an aqueous 20 percent sodium sulfide solution and allowed to react under a continuous flow of nitrogen gas at 70C for two hours. At the end of the reaction, the treated surface of the membrane was subjected to measurement of A.T.R. In the spectrum, the absorption band at 1420 cm 1 characte-ristic of the sulfonyl chloride group disappeared, and absorption bands at 1010 cm 1 and 940 cm 1 characteristic of sulfinic acid salts were observed.
The membrane was immersed in an aqueous 2 5 percent 40-dium hypochlorite solution at 70C for 16 hours and was again subjected to measurement of A.T.R. In th~ spectrum, the adsorptionbands at 1010 cm 1 and 940 cm 1 vanished and an absorption band at 1055 cm 1 characteristic of sodium sulfonate appeared.
30 Two sheets of the membrane treated with sodium sulfide as described above were washed with water and set again ~1~245~2 in the frames so that the Polymer 3 side reacted in a solution of 47 percent hydrogen bromide at 80~C for 20 hoursO In A.T.R. obtained from the membrane, a sharp absorption band due to the carboxylic acid group appeared at 1780 cm 1.
The membrane was saponified in an aqueous solution of 2.5N
caustic soda/50 percent methanol and subjected again to measurement of A.T.R. In the spectrum, the absorption band at 1780 cm 1 vanished, an absorption band due to sodium carboxylate appeared at 1690 cm 1, and minor ab-sorption bands due to sulfinic acid salts appeared at 940 cm 1 and 1010 cm 1. By A.T.R. measurement, about 90%
of the cation exchanye groups on the surface were found to be carboxylic acid group~.
The membrane was then treated in an aqueous 2.5 percent sodium hypochlorite solution at 90C for 16 hours. The specific conductivity of the treated membrane was found to be 4.6 x 10 3 mho/cm. The current efficiency, when measured under the same electrolytic condition~ as those o Example 7, wa~ found to be 92 percent. Substantially the same current efficiency was shown after 1700 hours of continued passage of electric current.
The membrane treated with sodium culfide in Example 8 was saponified in an aqueous 2.SN caustic soda/SO percent methanol at 60C for 16 hours. The membrane wa~ then treated in an aqueous lN hydrochloric acid solution at 60C for 16 hours and thereafter heated in the air at 150C for one hour. The Polymer 3 side of the membrane was subjected to measurement of A.T.R. spectrum. In the spectrum, an absorption band due to the carboxylic acid group appeared at 1780 cm 1 When this membrane was con-verted to the salt form, an absorption band due to ~;245~
carboxylate appeared strongly at 1690 cm 1, and weak absorption bands by sulfinic acid salts appeared at lOlO
cm 1 and 940 cm 1. The cation exchange groups on the surface were found to be carboxylic acid groups (about 90%) by A.T.R. The membrane was oxidized in an aqueous 2.5 percent sodium hypochlorite solution at 90C for 16 hours. The specific conductivity of the oxidized mem-brane was found to be 4.4 x 10 3 mho/cm. The current efficiency of the membrane, when measured under the same conditions as those of Example 7, was found to be 93 per-cent.
.
Tetrafluoroethylene and perfluoro-3,6-dioxa-4-methyl-7-octene sulfonyl fluoride were emulsion polymerized at 70C
under 4.5 atmospheres of tetrafluoroethylene pressure, with ammonium persulfate used as the initiator and the ammonium salt of perfluoro-octanoic acid as the emulsifier.
The polymer consequently obtained was washed with water, then hydrolyzed and thereafter subjected to measurement of exchange capacity by a titrimetric method. The ex-change capacity was found to be 0.80 milli-equivalent/
gram dry resin. This polymer is identified as Polymer 4.
By a procedure similar to that of Example 6, Polymer 2 used in Example 6 and Polymer 4 were combined to produce a composite membrane comprising a Polymer 2 membrane which had a thicknes~ of 4 mil~, and a Polymer 4 membrane which had a thickness of 3 mil~. This composite membrane was spread out with the Polymer 2 side held downwardly on a woven fabric of polytetrafluoroethylene about 0.15 mm in thickness having filling yarns of 400-denier multi-filaments and warp yarns of 200-denier multifilaments x 2 repeat each at a rate of 25 yarns per inch. The mem=
brane and fabric were heated to 270C with the membrane ~`Z452 simultaneously drawn against the membrane by means of vacuum so as to embed the fabric in the membrane as a reinforcing material.
This membrane was converted into the sulfonyl chloride form by the same procedure as used in Example 6. Two sheets of the membrane were held against each other with the Polymer 2 sides (the sides having the fabric embed-ded) acing inwardly by means of frames made of acrylic resin. The Polymer 4 sides of the membrane were allowed to react with hydrogen sulfide gas introduced in a conti-nuous flow at l20C for 20 hours.
The membrane was removed, saponified in an aqueous solu-tion of 2.5N caustic soda/50 percent methanol and there-after oxidized in a solution of 2.5N caustic soda/2.5 percent sodium hypochlorite at 90C for 16 hours. The specific conductivity, when measured by the same method as used in Example 6, was found to be 3.2 x 10 3 mho/cm, The current efficiency was 94 percent. Even after 1000 hours of continued passage of electric current, the cur-rent efficiency remained unchanged.
The reinforced composite membrane obtain in Example 9 wa~saponified. The specific conductivity and current effi-ciency of the resultant membrane, when measured under the same conditions as tho~e of Example 6, were found to be 3.9 x 10 3 mho/cm and 62.1 percent respectively.
, ' ~
Claims (13)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A cation exchange membrane comprising a fluorocarbon polymer containing pendant carboxylic acid groups repre-sented by -OCF2COOM wherein M is selected from the group consisting of hydrogen, ammonium, quaternary ammonium, and metallic atoms, and wherein the fluorocarbon polymer further contains pendant sulfonic acid groups represented by the formula:
-OCF2CF2SO3M.
-OCF2CF2SO3M.
2. A cation exchange membrane as in Claim 1, comprising (a) the said fluorocarbon polymer and (b) a further fluorocarbon polymer having cation exchange groups sub-stantially consisting of sulfonic acid groups represented by -OCF2CF2SO3M, said fluorocarbon polymer (a) existing as at least one layer in a thickness each of at least 100 .ANG. on the surface or in an internal portion of the membrane.
3. A cation exchange membrane as in Claim 2, wherein the fluorocarbon polymer (a) is present as surface stratum at least about 100 .ANG. in thickness on one surface of the membrane.
4. A cation exchange membrane as in Claim 1, wherein at least 20 mol percent of the ion exchange groups contained as pendant groups of the fluorocarbon polymer are -OCF2COOM.
5. A cation exchange membrane of Claim 1, reinforced with a material selected from the group consisting of woven fabrics of inert fibres.
6. A cation exchange membrane of Claim 5, wherein the inert fibres are polytetrafluoroethylene.
7. A cation exchange membrane comprising two bonded polymer films, a first film comprising a fluorocarbon polymer containing pendant carboxylic acid groups repre-sented by the formula:
wherein M is selected from the group consisting of hydro-gen, ammonium, quaternary ammonium, and metallic atoms, and a second film comprising a fluorocarbon polymer con-taining pendant sulfonic acid groups represented by the formula:
wherein M is selected from the group consisting of hydro-gen, ammonium, quaternary ammonium, and metallic atoms, the equivalent weight of the polymer in each film being from 1000 to 2000; the equivalent weight of the polymer in the first film being at least 150 higher than the polymer in the second film; the thickness of the first film being up to 50% of the total thickness; and where-in the fluorocarbon polymer in the first film further contains pendant sulfonic acid groups represented by the formula:
-OCF2CF2SO3M.
wherein M is selected from the group consisting of hydro-gen, ammonium, quaternary ammonium, and metallic atoms, and a second film comprising a fluorocarbon polymer con-taining pendant sulfonic acid groups represented by the formula:
wherein M is selected from the group consisting of hydro-gen, ammonium, quaternary ammonium, and metallic atoms, the equivalent weight of the polymer in each film being from 1000 to 2000; the equivalent weight of the polymer in the first film being at least 150 higher than the polymer in the second film; the thickness of the first film being up to 50% of the total thickness; and where-in the fluorocarbon polymer in the first film further contains pendant sulfonic acid groups represented by the formula:
-OCF2CF2SO3M.
8. A cation exchange membrane as in Claim 7, wherein the first film comprises (a) a fluorocarbon polymer containing pendant carboxylic acid groups represented by -OCF2COOM and (b) a fluorocarbon polymer having cation exchange groups substantially consisting of sulfonic acid groups represented by -OCF2CF2SO3M, said fluorocar-bon polymer (a) existing as at least one layer in a thickness each of at least 100 .ANG. on the surface or in an internal portion of the first film, and further contai-ning pendant sulfonic acid groups represented by -OCF2CF2SO3M.
9. A cation exchange membrane as in Claim 8, wherein the fluorocarbon polymer (a) is present as surface stra-tum at least about 100 .ANG. in thickness on one surface of the membrane.
10. A cation exchange membrane as in Claim 7, wherein at least 20 mol percent of the ion exchange groups con-tained as pendant groups of the fluorocarbon polymer in the first film are -OCF2COOM.
11. A cation exchange membrane of Claim 7, reinforced with a material consisting of woven fabric of inert fibres.
12. A cation exchange membrane of Claim 11, wherein the inert fibres are polytetrafluoroethylene.
13. A cation exchange membrane of Claim 11, wherein the reinforcing material is embedded in the second film.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000410363A CA1152452A (en) | 1975-07-09 | 1982-08-27 | Cation exchange membrane preparation and use thereof |
Applications Claiming Priority (8)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP50084112A JPS5224177A (en) | 1975-07-09 | 1975-07-09 | Manufacturing method of fluorocarbon cathion exchange membrane |
| JP84111/75 | 1975-07-09 | ||
| JP84112/75 | 1975-07-09 | ||
| JP50084111A JPS5224176A (en) | 1975-07-09 | 1975-07-09 | Cathion exchange membrane |
| JP3559376A JPS52119486A (en) | 1976-03-31 | 1976-03-31 | Fluorocarbon composite membraneous product and production thereof |
| JP35593/76 | 1976-03-31 | ||
| CA000256437A CA1219399A (en) | 1975-07-09 | 1976-07-06 | Fluorocarbon polymer membrane with pendant carboxylic acid groups |
| CA000410363A CA1152452A (en) | 1975-07-09 | 1982-08-27 | Cation exchange membrane preparation and use thereof |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1152452A true CA1152452A (en) | 1983-08-23 |
Family
ID=27508069
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000410363A Expired CA1152452A (en) | 1975-07-09 | 1982-08-27 | Cation exchange membrane preparation and use thereof |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1152452A (en) |
-
1982
- 1982-08-27 CA CA000410363A patent/CA1152452A/en not_active Expired
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