CA1219399A - Fluorocarbon polymer membrane with pendant carboxylic acid groups - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A fluorocarbon polymer and a cation exchange membrane formed of fluorocarbon polymer are characterized by the presence in at least one surface layer of the polymer of pendant carboxylic acid groups of the formula -- OCF2COOM --, where M
stands for hydrogen, ammonium, quaternary ammonium or a metal atom.

Description

12~9æ~9 BACKGROUND OF INVENTION
This inventlon relates to improved cation exchange membranes and to methods for their production. The invention is further directed to methods for the electrolysis of an aqueous solution of an alkali metal halide by use of these cation exchange membrane3, and to the electrolytic cells in which the electrolysis takes place.
It has been known to the art to obtain a cation exchange membrane of a perfluorocarbon polymer containing pendant sulfonic acid ~roups by saponification of a membrane prepared from a copol,yrner of tetrafluoroethylene and per-fluoro-3,6-dioxa-4-methyl-7--octene-sulfonyl fluoride. This known perfluorocarbon type cation exchan~e membrane contain-ln~ only sulfonic acid ~roups, however, has the disadvantage that the membrane~ when used in the electrolysis of an aqueous solution of an alkali metal halide, tends to permit penetra-tion therethrough of hydroxyl ions back mi~ratin~ from the cathode compartment because of the high hydrophilicity of the sulfonic acid group. As a result, the current efficiency durin~ electrolysis is low. This is a special problem when the electrolysis is used for the production of a~lueous sol~tion o~ caustic soda at concentrat:lons o,~ more than 20 percent.
In this reaction, the curren~ e~icienc,~ ls so low that the process is econornically disadvantageous compared with electrol,vsls of aaueous solutions of sodium chloride by conventional mercur;y process or diaphra~m process.
The disadvanta~e of such low current ef~iciency can be alleviated b,y lowerin~ the exchange capacity o~ the sulfonic acid group to less than 0.7 milliequivalent per gram of the H form dr,y resin. Such lowerin~, however, results in a serious ,,~

.

121939~

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 exchange membranes containing sulfonic acid moieties as the ion exchange group and comprising two polymers with different equivalent weight (EW), that is number of grams of polymer containing one equivalent weight of ion exchange functional group. When such membranes are utilized in the electrolysis of aqueous solutions of sodium chloride, high current efficiencies are obtained by effecting the electroly-sis with the higher EW polymer side of the composite 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 thickness decreased as much as possible. It is, however, extremely difficult to produce a composite cation exchange membrane having current efficiency of not less than 90 percent by use of membranes containing only sulfonic acid groups.
In U. S. Patent No. 3,784,399, Canadian Patent Nos. 1,033,097 and 1,033,098, there are suggested cation exchange membranes wherein the cathode sidc surface layers of fluorcarbon cation exchan~e membranes contain sulfonamide group, salts thereof or N-mono-substituted sulfonamide group.
These membranes, however, are deficient in electrochemical and chemical stabilities.
An object of this invention is to provide fluoro-carbon cation exchange membranes which, even in electrolyis 1~19~9~

for production of caustic soda at hi~h concentration, more effectively inhibit the back migratiorl 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.
Another ob,~ect of this invention is to provide a method for the electrolysis of the aqueous solution of an alkali metal halide b,y use of such cation exchange membranes.
THE INVENTION
Novel cationic membranes characterized by the presence of carboxylic acids 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 exchan~e membranes. A particular advanta~e is the durability of the membranes, it having been found that the current efficiency of the membranes remains stable at well above 90%, even after many months of operation.
The present invention provides a cation exchan~e membrane comprisin~ a fluorocarbon polymer containlrl~ pend~nt carbox~lic acld ~roups r~pre~ented b~ ~he Pormula:

wherein M is hydro~en: ammoniùm; auaternar,v ammonium, parti-cularly quaternar~ ammonium having a molecular weight of 500 or less; and metallic atoms, particularly alkali or alkaline earth metals.
In its simplest form, a cation exchan~e membrane of this invention is a film formin~ perfluorocarbon polymer.
The thickness of the film can be varied widely dependin~ on purposes. There is no particular limit to the thiclmess, bUt usually a thickness from 0.5 to 20 mils is suitable for many ~9~99 purposes.
The preferred embodiments of membranes 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 acid groups represented by the fo~mula as set forth above.
The membranes of this invention can be classified into two major groups. One is a uni-layer film wherein 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 of 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 2 in thickness on the surface opposite to the side laminated with the second film. For practical purpose, the membrane is usually reinforced with reinforcing materials selected from the group consisting of woven fabrics of inert fibers and porous films of inert polymers, preferably poly-tetrafluoroethylene fibers. When the membrane has a surfacestratum of the specific fluorocarbon 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.

~9;~9 For convenience, detailed explanation is given inthe following principally with reference to the uni-layer film having a surface stratum of the specified carboxylic acid groups. The structure of the first ilm in a two-ply layer membrane 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 carboxylic acid group stratum is placed in the eléctrolytic cell to face the cathode side in order to obtain the remarkable benefits of the invention.
The stratum may also contain sulfonic acid groups which may be represented by the formula:

-ocF2cF2so3M
wherein M has the same meaning as abo~e.
In fact, since the cationic membranes of this invention are derived from sulfonic acid group substituted fluorocarbon copolymers, they may contain any predetermined proportion of sulfonic acid groups, or derivatives thereof.
The specific fluorocarbon polymer constituting the stratum of this invention may contain from 5 to 100 mol percent of carboxylic acid substituents based on total cation exchange groups. Usually, as the distance ~rom the surface increases the relative percent of carboxylic acid substituents decreases, and the relative percent of sulfonic acid substituents increases. In preferred embodiments of the invention, only one surface is predominantly carboxylic acid substituents, and the quantity of sulfonic acid groups will increase with distance from that surface until the opposite surface is predominantly sulfonic acid groups. According to one preferred embodiment, there is a cation exchange membrane comprising (a) a fluorocarbon polymer containing pendant carboxyli,c acid groups of -OCF2COOM and (b) a fluorocarbon 1~9399 polymer having cation exchan~e groups substantially consisting of sulfonic acid groups of -OCF2CF2S03M. According to the most preferred embodiment, the membrane consists of a film of fluorocarbon polymer having -OCF2CF2SO3M cation exchange groups and which is fiber reinforced on one side and has a surface stratu~ of fluorocarbon polymer having -OCF2COOM
cation exchange groups on the other side. It is-also possible by special procedures to prepare embodiments of this invention in which carboxylic acid groups are uniformly distributed thro~l ~h o~t th~gout the membrane.
It has been observed that the presence of carboxylic acid groups on the surface of the cation exchange membrane, particularly on the surface facing the cathode, remarkably impedes the back migration of hydroxyl ions from ~he cathode compartment during the electrolysis of aqueous solutions of alkali metal halides such as sodium chloride. These effects are realized while operating at high current efficiencies, normally well over 90%. Moreover, the membranes 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 highly resistant to chlorine ~ ' gas generated fromZl1anode.
As indicated above, the surfAcc denHiey of cnrboxyJic acid groups mny vary Erom 5 eo 100 mol percent~ The preferred range is from 20 to 100 mol percent, and the best combinations of economy and efficiency are normally realized if the density is from 40 to 100 mol percent; all based on the total number of all functional groups in ~he surface strata, The depth oE the surEace strata can be ascertained by staining techniques. For example, a section of a prepared membrane can be immersed for several minutes in an aqueous bm~

.~,.,,,'Z

~i93~9 solution of crystal violet cont~ining 5 to 1~ 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 density of the carboxylic acld groups can be ascertained by x-ray microprobe analysis.
The membranes of this invention may take any of several forms, as is particularly illustrated in the examples.
As mentioned above, it may be a simple unilayer film with one or both major surfaces embodying strata with carboxylic acid substituents. Alternatively, the membranes may be composite membranes formed from two appropriately prepared and sub-stituted perfluorocarbon films bonded together in each of which the EW is from about 1000 to 2000, preferably 1000 to 1500.
If a two film membrane is 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 strength. For this purpose fabrics made of polytetrafluoro-ethylene fibers are most suitable, although other materials which are inert to the chemical environment in which the membranes are employed may also be used. Particularly, 1219;~9~

polytetrafluoroethylene films may preferably be employed as reinforcing materials. If reinforcing materials are utilized, it is particularly advantageous to em~ed them in the polymer membrane. This can be readily accomplished, for example, at elevated temperature and under reduced pressure as illuestrated in the examples.
In all of hese various constructions, the most preferred membranes will be constructed with carboxylic groups predominating on one surface, and sulfonic groups predominating on the other. In composite membranes the film with the higher EW will preferably carry the carboxyl groups.
The starting fluorocarbon polymer having the sulfonic acid groups as the side chain thereof is produced by copolymer-izing a fluorinated ethylene and a vinyl fluorocarbon monomer 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 form~la (II) ~iven below:
F(CF2)~0(CFC~20)pC~'~CF2 (II) twherein, ~ represents an integer having the value of 1 - 3 v~r~c r~Sc 7~;
~,'i!, 25 and p an integer having the value of 0 - 2), thereby dcri-~ing a polymer possessing a side chain of -OCF2CF2S02F shaping the resultant polymer in the form of a membrane and thereafter converting the side chain -OCF2CF2S02F of said polymer into the group -OCF2CF2S03M through saponification.

~z~9;~

Typical examples of fluorinated ethylene include vinylidene fluoride, tetrafluoroethylene and chlorotrlfluoro-ethylene. Among them, tetrafluoroethylene is most preferred.
Typical examples of the vinyl fluorocarbon monomer having the sul~onyl fluoride group o~ the a~orementioned generic formula include those enumerated below:

2 2 2OCF CF2 Fso2cF2ocFcF2ocF=cF2 ~F3 2CF2CF201 FCF20CFCF20CF=CF
CF3 CF~
Fso2cF2cF2cF=cF2 Fso2cF2cF2ocFcF2ocF=cF2 ~ f the vinyl fluorocarbon monomers having the sulfonyl fluoride group available at all, the most desirable is per-fluoro(3,6-dioxa-4-methyl-7-octene sulfonyl fluoride), Fso2cF2cF2ocFcF2ocF=

A typical example of the fluorovinyl ether of the generlc formula (II) which takes part, where necessary, in said copolymerizat.ion is per~luoromethyl vinyl ether.
The process of the invention i~ applicable to all Or the sulfonyl substituted polymers described in Unlted States Patent 3,909,378.
Advanta~eously, the membrane is first formed with the sulfonyl substituted polymer which is then converted by reactions described more fully hereina~ter to a membrane of the invention.
The preferred copolymer composition for starting 1219;~99 materials is such that the fluorinated ethylene monomer content is from 30 to 90 percent by wei~ht, preferably from 40 to 75 percent by weight, and the content of the perfluorovinyl monomer possessing~ the sulfonyl fluoride group is ~rom 70 to 10 percent by weight, preferably from 50 t~ 25 percent by weight. The materials are produced by procedures ~ell known in the art for the homopolymerization or copolymerization of a fluorinated ethylene.
Polymerization may be effected in either aqueous or nonaqueous systems. Generally, the polymerization is performed at temperatures of from 0 to 200C under pressure of from 1 to 200 kg/cm . Frequently, the polymerization in the nona~ueous system is carried out in a fluorinated solvent. Examples of such nona~ueous solvents include 1,1,2-trichloro-1,2,2-tri-fluoroethane and perfluorocarbons such as perfluoromethyl-cyclobutane, perfluorooctane and perfluorobenzene.
The aqueous system polymerization is accomplished by brin~lng the monomers into contact with an aqueous solvent containing a freè radical initiator and a dispersant to produce a slurry of polymer particles, or by other well known procedures.
After the polymerization, the re~ultant ~olymer is shaped to form a membrane usin~ any o~ a variet;y o~ well known techni~ues.
The copolymer is desired to have an EW ln the ran~e of from 1000 to 2000. The membrane havin~ a low EW is desirable in the sense that; the electric resistance is proportionally 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 ~orm of membrane. Thus, the most desirable ranKe o~

~9~99 El~ is from 1000 to 1500.

The copolymer, after being shaped into a membrane, a ~ ~ r;~
can be laminated with a reinforcing material such as fabric_ -for improvement of mechanical strength. As the reinforcing material, fabrics made of polytetra~luoroethylene fibers are most suitable. The aforesald stratum should preferably be allowed to be present on the surface opposite to the side on which the reinforcing material is lined.
In 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 l;o the polymerization methods as described above, followed by shaping, and fabricated into a composite film. The first film is required to have an EW
f at least 150 ~rreater 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 preferably as thin as possible, since electric resistance is greatly increased as the increase in EW. Thus, the thickness of the first film which depends on EW thereof is required to be 50% or less o;~ th~ en~.lre thlcknes~, pre~er-ably from ll5 t~ 10%.
It ls lmportank that the flrst film of a hig,her EW
ls present in the form of a continllou.s film formed parallel to the surface of the membralle.
The overall thickness o~ said composite cation exchan~e membrane, thou~h variable with the klnd 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, generally has a lower limit of ~21g~99 4 mils and no upper limit. The upper limit is usua]ly fixed in consideration of economy 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 reinforc-ing 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, 10 the pendant sulfonyl groups in the form represented by the formulas:
-OCF2CF2SO2X (A) and/or -OCF CF' SO ~
2 2 2 ~ O (B) wherein X is halogen, especially fluorine or chlorine;
hydroxyl 7 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:
-OC~2COOM
by treatment with a re~lcln~ a~r~ent.
Since the conversion to a carboxylic acid ~roup is effected chemically, it can be controlled so as to produce products with substant~ally any de~ree of carboxylation which may be desired.
The starting polymers are usually formed from sulfonyl fluoride substituted compounds which remain intact durin~ polymerization. l'he sulfonyl fluoride groups can directly be treated with a reducing agent to be converted to 1;~19;~99 t~l~e carboxylic acid groups. Alternatively, they may be fi~st converted to any of the other derivatives of sulfonic acid a~ defined in the above formulas (A) and (B) by known reac-tions, followed by conversion into the carboxylic acid groups.
d~e T 5 ~he sulfonyl chloride groups are especially preferred-~r to higher reactivity. Therefore, it is more desirable to convert the sulfonyl fluoride to any of the other derivatives of' sul-fonic acid defined above in connection with the definition of X. Such reactions can be readily carried out by procedures well known to the art.
The formation of the carboxylic group may follo~ any of several pathways.
It may be formed by reduction to a sulfinic acid with a relatively weak reducing agent followed by a heat treatment as indicated below:
-OCF2CF2S02X - red. ~ OCF2CF2S02M

~ ~ OCF2COOM.
Conversion into carboxylic acid groups can more readily be effected when M in the above formulas is hydrogen. Alter-natively the treatment may be stepwise in which initially asulfinic acid is produced, and this is converted to a carboxy-lic group by the use of a str~ng reducin~ a~ent. This may take place as indicated below:
-OCF2C~2$02X _ed. __~ 0cF2c~2so2M

- ~ OCF2COOM.
With some reducing agents, the treatment may be directly from the sulfonlc ~roup to the carboxyl ~roup, as indicated by, 2 2 3 ~ > OCF2Coo,~q It is preferred that the concentration of sulfinic acid groups in the final product be relatively low.

- ]4 -- 1219;~99 Accordingly, it may be desirable, but not necessary, to oxidize sulfinic acid groups to sulfonic acid groups by the following sequence:

-CF2cF2s2M _ ~ OCF2cF2so3M
This may ~e accomplished by known procedures utilizing aqueous mixtures of sodium hydroxide and hypochlorite.
The reducing agents which can be used in the present invention are exemplified as shown below. Those skilled in the art are completely familiar with these reducing agents and many other similar reducing agents as well as procedures by which they are employed. However, some of the reducing agents such as hydrazine having amino groups which are capable of forming sulfonamide groups as disclosed in German Patent OLS No. 2,437,395 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 hydrides of the generic formula MeLH4, wherein Me represents 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, ~or example, lithium 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, hypo-phosphorous acid, hydrogen sulfide and arsenious acid.
The reducing agents of the third group are mixtures ~21~;~99 of metals and acids. Examples of these mixtures include tin~
iron, zinc and zinc amalgam and those of acids include hydro-chloric acid, sulfuric acid and acetic acid.
The reducing a~ents o~ the fourth group are compounds of low-valency metals. Examples of these compounds include stannous chloride, ferrous sulfate and titanium trichloride.
They may be used in conjunction with cuch acids as hydrochloric acid and sulfuric acid.
The reducing agents of the fifth group are organic metal compounds. Examples of these reducing agents include butyl lithium, Grignard reagent, triethyl aluminium and tri-isobutyl aluminum.
The reducing agents of the sixth group are inorganic acid salts possessing reducing activity and similar compounds.
~xamples of these reducing agents incluae potassium iodide, sodium iodide, potassium sulfide, sodium sulfide, ammonium sulfide, sodium sulfite, sodium d.~thionite, sodium pho~phite, sodium arsenite, sodium poly~ulfide and phosphorl2s trisul~ide.
The reducing agents of the seventh group are mixtures of metals with water, steam, alcohols or alkalis. Examples of metals usable in the mixtures include sodium, llthlum, aluminum, magneslum, zinc, iron and amal~ams thereof. Examples of alkalls include alkal.l hydrox~des and alcoholic alkalis.
The reducln~ agents of the ei~hth groups are organic compounds possesslng a reducing activity such as, for example, triethanol amine and acetaldehyde.
Amon~ the groups as enumerated above, those belonging to the second, thlrd, fourth and sixth ~roups are found to be preferable.
The optimum conditions for treatment with a reducin~

lZ1~399 a~ent will be selected dependin~ 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 ~rom 0C to 150C, and the reducing a~ent 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, di~lyme, acetonitrile, propionitrile or benzonitrile: or nonpolar organic solvents such as n-hexane, benzene or cyclohexane or mixtures of such solvents.
The amount of the reducin~ agent is not less than the equivalent wei~ht of the sulfonyl group present in the surface.
Generally~ the reducing agent will be used in lar~e excess.
The pH value of the reaction system will be selected on the basis of the particular reducin~ agent employed.
The reaction can be carried out under reduced, normal or increased pressure. In the reaction involving the use of a gaseous reducing a~,ent, the increased pressure can improve the velocity of the reaction.
The reaction time generally ranges from one minute to 100 hours.
In case of a cation exchan~e membrane rein;~orced with a r~lnforcin~ materlal, treatment with a reduclng a~,ent is preferably applled onto the side opposlte to the reinforced side.
The course of the reactlon may be followed by analysis of the infrared absorption spectrum of the membrane, as is particularly illustrated in the examples. Key bands in followin~ the reaction are as follows:

12~9;~99 sulfonyl chloride ~ 1420 cm 1 sulfinic acid salt ------------- g40 cm 1 sulfinic acid salt -------------- 1010 cm 1 carboxylic acid ~ ------------ 178Q cm 1 carboxylic acid sal~ -~--------- 1690 cm 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 resistance and infrared spectrum by varying pH. Said functional 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 combustion method, sulfur ~4m is found to be decreased by one atom per one exchange group.
Fluorine atom is observed to be removed by two atoms per one exchange group by alizarin-complexion method. From these results of analysis and also from the fact that carboxylic acids are formed by use of a reducing agent containing no carbon atom under an atmosphere in the absence of carbon atom, the above functional ~roups are con~lrmed to be -OCF2COOM.
This structure ls also evidenced by mca~urem~nt o~ NM~ spectrum of C13 of the product obtained by the reactiGn~ correspondln~
to the above polymer reaction~ conducted for the monomer having ~unctional group ~ OCF2CF2S02X.
The products of the treatment with a reduclng agent may take three typical forms. These are:
; 1) All o~ the -COOM groups required may be formed.
2) Not all -COOM groups required may be formed ~2~9;~99 and -SO2M groups may be present.

3) Substantially all -SO2M groups may be present.
In the first instanceg no further treatment will be required. In the second and third case~ there are two alterna-tives. A more powerful reducing agent may be employed, orthe -SOzM groups may be converted to carboxylic acid groups by heat treatment, which is advantageously carried out when M is nydrogen. The heating may take place at any selected practical pressure at a temperature of from 60C to 400C for a period of from 15 to 120 minutes. The preferred conditions for efficiency and economy are atmospheric pressure, 100~C to 200C, and 30 to 60 minutes.
Any remaining sulfinic acid group may be converted into the sulfonic acid group, if desired. This conversion o~
the sulfinic acid group to the sulfonic acid group can easily be accomplished such as by subJecting the former group to oxidation in an aqueous solution of 1 to 5 percent NaClO or an aq.leous solution of 1 t;o 30 percent ~22 at 40C to 90C
for 2 to 20 hours.
The re~ucing agent to be used for the purpose of this invention is selected, as in ordinary or~anic reactions, with due consideration to numerous ractors such a~ the Icind Or the substltuenl; X in the .~O2X f~rroup, the k:Lnd o~l the reducln~ ag~rent, the kind of the solvent to be used, the temperature oi the reaction, the concentration, the pH value~ thc reactiorl time and the reaction pressure.
The reducin~ a~ents usable for this inv~ntion are broadl~ divided by their reactions as follow~.
The reducing agents of the first group can be applied to virtually all SO2X groups. Occasionally the reaction 1219;~99 proceeds to an advanced extent to produce a product which appears to be an alcohol.
The reducin~ agents of the second~ third and fourth Kroups are particularly e~fective when applied to sulfonyl halide groups of relatively hi~h reactivity.
The reducing agents of the fifth, sixth, seventh and eighth groups are also effective for application to sulfonyl halide groups, although use ol these reducing agents frequently produces the sulfinic acid alone. Use of the -S02F group demands specially careful selection of the reaction conditions, for it may possibly induce hydrolysis in the presence of a reducing a~ent from the sixth, seventh and eighth groups.
It is possible to convert the -S02Cl group directly into the carboxylic acld group without going through the intermediate of sulfinic acid. For example, the conversion can be accomplished by sub~ecting the membrane of the fluoro-carbon polymer possessin~ the -S02Cl group to elevated temperature and/or to ultraviolet rays and/or to an organic or inorganic pero~ide.
As a matter of course, the reaction of the present invention can be applied to other monomers possesslng simllar side chains. Thus, fluorocarbon monomers possesqln~, a ~ulfinlc acid ~roup or carboxyllc aold ~Jroup can readily be syntheslzed by sald reaction.
It will be noted that the ultimate effect of the treatment with a reducin~ agent can be represented by the followlng reaction:
-OCF2cI?2~o3M _ ~ -OCF2COOM .

~21g3g9 The membranes of this invention have many advantages, some of which have already been mentioned above.
In the course of the electrolysis of aqueous solution of an alkali metal halide, the portion of the fluorocarbon polymer possessin~ the carboxylic acid group in the cation exchange membrane is 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 migration 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 diaphragm in the electrolysis of aqueous solution of sodium chloride, the prevention of the back migration of hydroxyl ions can be obtained quite effectively and the current effi-ciency maintained at a high level by allowing the high EW layer resultinF~ frorn the treat~ent with the reducing a~ent to face the cathode side of the electrolytic cell. As a consequence, this cation exchange membrane ena~les the unit power consump~ion to be lowered and the prime cost o~ the product proportionally lowered an~, hence, proves ko be hi~hly ~dvanta~eous from the commercial point oY view.
Generally, the anode compartment in the electrolysis of aqueous solution of sodium chloride is operated in an acidic state. In consideration of the fact that the apparent pKa I value of the carboxylic acid is ~ the order of 2 to 3, the presence of the thin layer of the carboxylic acîd group on the r C C~s~ n,~
anode compartment side brings about an effect of height~ing the potential and therefore proves disadvantageous.

~2~9;~99 Compared with the conventional membranes, the membranes of this invention have the following advanta~es in the process of manufacture. Manufacture of the cation exchange rnembrane of the fluorocarbon polymer substituted with the carboxylic acid group has heretofore proved to be extremely difficult. This is because the properly substituted fluoro-carbon compound monomers are extremely difficult to synthesize.
Additionally, copol~mers from such monomers with perfluorovinyl monomers are highly susceptible to thermal decomposition and cannot be thermally molded in the form of a membrane by conven-tional extrusion techniques.
This invention alleviates the aforesaid difficulties by causing the sulfonic acid group of the fluorocarbon polymer to be converted into the carboxylic acid group.
The following examples are given by way of illustra-tlon only, and are not to be considered limitations of this invention, many apparent variations of whlch are possible without department from the spirit or scope thereof.

Tetrafluoroethylene and perfluoro(3,6-dioxa-4-methyl-7-octene sulfonyl fluoride) were copolymerized in 1,1,2-trichloro-1,2,2-trifluoroethane ln the pr~sence o~ per~luoro-propionyl peroxide as tho inltla~or. The polymerlzation temperature was held at 45C and the pressure maintalned at 5 atmospheres durin~ the copolymerization. The exchan~e capacit,y of the resultant polymer, when measured after saponi~ication, was 0.95 milligram equivalent/gram of dry resin.
This copolymer was molded with heating into film 0.3 mm in thickness. It was then saponified in a rnixture of , 12~939~

2.5N caustic soda/50 percent methanol at 60C for 16 hours, converted to the H form in lN hydrochloric acid, and heated at 120C under reflux for 20 hours 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 tetra-chloride and then subjec~ed to measurement of attenuated total reflection spectrum (hereinafter referred to as A.T.R.), which showed a strong absorption band at 1420 cm 1 characteristic of sulfonyl chloride. In a crystal violet solution, the membrane was not stained. Betw~en frames made of acrylic resin, two sheets of this membrane were fastened in position by means of packings made of polytetrafluoroethylene. The frames were immersed in an aqueous 57 percent hydroiodic acid solution so that one surface of each rnembrane would undergo reaction at 80C for 24 hours. The A.T.R. of the membrane was then measured. In the spectrum, the absorption band at 1420 cm 1 characteristic of sulfonyl chloride group vanished and an absorption band at 1780 cm 1 characterlstic of carboxylic acld group appeared instead. In the crystal violet solution, a layer of a thickness of about 15 microns on one surface of the membrane was stained. The cation exchan~e ~roups ~xl8tln~
on the surrace were found to be carboxylic acld ~¢roups (100~) by measurement o~ A . T ~ R .
3y saponifyin~ 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 catlon exchange membrane.
In an aqueous O.lN caustic soda solution, this membrane showed a specific conductivity of 10.0 x 10 3 mho/cm.

. . .

~ig399 The specific conductivity of the membrane was determined by initial conversion to a co~plete Na form, keeping the membrane in a constantly renewed bath of an aqueous O.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 aqueous O.lN caustic soda solution at 25C for measurement of the electric resistance of the membrane.
The aforementioned Na ~orm electrolytic diaphragm was 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 side.
It was utilized as the membrane ih the electrolysis of sodium chloride and its current efflciency measured. The result was 95%.
The electrolytic cell had a service area of 15 cm2 (5 cm x 3 cm) and comprised an anode compartment and a cathode compartment separated by the cationic membrane. A metallic, dimensionally stable DSA anode was used, and an iron plate was used as the cathode. An aqueous 3N sodium chloride solution at p~l 3 was circulated through the anode compartment and an a~ueous 35 percent caustlc soda solution ~hrou~h the cathode compartment at 90C. Under these conditlons, an electric current was passed between the electrodes at a current density of 50 amperes/dm2. The current efficlency was calculated by divldlng the amount of caustic soda produced in the cathode compartment per hour by the theoretical value calculated from the amount of electricity passed.

The sulfonyl chloride form Or the membrane obtained i2~,9;~99 in Example 1 was saponified in a mixture of 2.5N caustic soda/50 percent methanol. The resultant sulfonic acid form ion exchange membrane was tested for speciric conductivity and current efficiency under the conditions as used in Example 1.
The values found were 13 0 x 10 3 mho/cm and 55% respectively The polymerization of Example 1 was repeated by the same procedure, except that the pressure was maintained at 6 atmospheres during the polymerization. The exchange capacity of the resultant polymer was 0.79 milligram equivalent/gram of dry resin.
The copolymer was molded with heating into a film 0.3 mm in thickness. The membrane was then converted into the sulfonyl chloride form under the same conditions as in Example 1. One surface of the membrane was caused to react with an aqueous 57 percent hydroiodic acid solution at 80C
for 30 hours.
Thereafter, the treated surface of the membrane was 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 an absorptlon band at 1780 cm 1 characteristic Or carboxylic ac:Ld ~roup appeared ~nstead.
In crystal vlolet solu~,ion, a layer havlng a thlckness o~
about 15 microns on one surface of the membrane was stained.
The cation exchange ~roups on the surface were found to be carboxylic acid groups (100%) by A.T.R.
This membrane was saponified in a solution of 2.ON
caustic soda~50 percent methanol at 60C for 40 hours and then treated in an aqueous 2.5 percent sodium hypochlorite at 90C
for 16 hours. The resultant membrane was treated in a solution ~9æ~9 of 2.ON caustic soda/50 percent methanol at 90C for 16 hours.
The A.T.R. of this membrane showed the characteristic absorption of a salt of a carboxylic acid group at 1690 cm 1 on the surface exposed to the reacticn with hydroiodic acid, and an absorption characteristic of a salt of sulfonic acid was observed at 1055 cm 1 on the opposite surface.
The specific conductivity of this membrane was 6.5 x 10 3 mho~cm. The current efficiency measured under the same conditions as those of Example 1, with the surface treated with hydroiodic acid facing the cathode compartment side, was found to be 94%. After continuous passage of current for 1500 hours, while maintainin~ the concentration of alkali in cathode compartment at 30%, the current efficiency was found to be as high as 93.2%.

_ . . .
The sulfonyl chloride formof the membrane obtained in Example 2 was saponified in a solutlon of 2.5N caustlc sodai50 percent methanol. The resultant sulfonic acid type ion exchange membrane was tested for specific conductivity and current efficiency under the same conditions as in Example 1. The values thus obtained were 7.0 x 10 3 mho/cm and 65~ respectively.

Terpolymerization was conducted utilizin~ the mono-mers of Example 1 plus perfluoropropyl vinyl ether following the procedure of Example 1. When the membrane obtained was subjected to the same procedure as tllat of Example 1, it manifested a current efficiency as high as the membrane of Example 1.

:

1~19399 Terpolymerization was carried out using the monomers of Example 1 plus perfluoro-3,6-dioxa-5-methyl nonene-lg CF3CF2CF2OCF(CF3)CF2OC~=CF2. When the membrane obtained was subjected to the same procedure as in Example 1, there were obtained similar results.

A sulfonyl fluoride form membrane having an exchange capacity of o.65 milli~ram equivalent/gram of dry resin was obtained by a procedure similar to that of Example 1. The membrane was placed in a flask and t~trahydrofuran was added.
Lithium boron hydride was added in a large excess, and the resultant reaction system heated under reflux for 50 hours.
~t the end of the react:ion, the membrane was sub~ected 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 16~0 cm 1 characteristic of -COOLi small absorption bands at 940 cm 1 and 1010 cm 1 characteristic of sulfinic acid salt appeared instead. This membrane was allowed to stand in an aqueous 0.1 percent crystal violet solution (containinp 10 percent ethanol) ~r ~hree minutes and, therear~er, the cro~s sec~lon o~ thc membrane was observed under a microscope. rrhe microscopic observation revealed a stron~ly stained layer havin~ a thickness of about 10 microns on each sur~ace of the membrane.
The carbox~lic acid group content ln the surface layers, as determined from ~.T.R. was about 60 percent.

Terpolymerization was carried out using the monomers of Example 1 plus hexafluoropropylene following the procedure of Example 1. When the membrane obtained was subjected to procedures similar to those of Example 1, similar results were obtained.

The sulfonyl chloride form of the membrane obtained in Example 1 was reduced in an aqueous 35 percent hypophosphorus acid solution at 80C for ten hours. By A.T.R., the adsorp-tion band at 1420 cm 1 characteristic of sulfonyl chloride group vanished, but the absorption band at 1780 cm 1 character-istic 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 carboxylic 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 subjected to measurement of A.T.R. the absorption by the carboxylic acid group at 1780 cm 1 was noted to have been 20 shifted to an absorption band at 1690 cm 1 characteristic 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 Example 1, 25 were found to be 9.5 x 10 3 mho/cm and 92 percent respectively.

.
The sulfonyl chloride form of the membrane ~0.65 meg/g) obtained in a manner similar to Example 1 was refluxed in an aqueous solution of tin and hydrochloric acid for 50 hours and then subjected to measurement of A.T.R.
In the spectrum, the absorption band at 1420 cm 1 ~ 28 -1219æ~9 characteristic of sulfonyl chloride group vanished and a sharp absorption band at 1780 cm 1 characteristic of carboxylic acid group appeared. The carboxylic acid groups were present on the surface (100~) by measurement of A~T.R. This membrane was saponi~ied in an aqueous solution of 2.5N caustic soda/50 percent methanol. The specific conductivity and current efficiency of the saponified membrane~ when measured under conditions of Example 1 and maintainin~ the alkali concentration in cathode compartment at 20%, were found to be 2.0 x 10 3 mho/cm and 96 percent respectively.

Example 8 was repeatedg except that an aqueous solution of stannous chloride and hydrochloric acid was used in place of the aqueous solution of tin and hydrochloric acid, ~o give a similar result.

The sulfonyl chloride form of the membrane obtained in Example 1 was submerged in an aqueous 15 percent sodium sulfide solution under a co~tinuous flow o~ nitroge~ g~s at 60C for one hour. At the end of the reactlon, the membrane was sub~ected to measurement of A.T.R. In the spectrum, the absorption barld at 1420 cm 1 charac~erist1c o~ ~ul~onyl ch:loride group completely van1~hed and sharp at)sorptlon bands ~lt 940 cm 1 and 1010 cm 1 characteristic o~ a sulrlnic acid salt appeared.
The membrane was converted into the H form by immerslon in lN
h~ydrochloric acid solution at 60C for 10 hours, and heated under nitrogen gas at l50~C for 120 minutes. It was then converted into the Na ~orm by lmmersicn in a 2.5N caustic soda solution and sub,~ected to measurement of A.T~R. In the spectrum, the absorption bands at 940 cm 1 and 1010 cm 1 1~19399 characteristic of the sulfinic acid salt completely disappeared and a strong absorption band at 1690 cm 1 characteristic of carboxylic acid salt appeared~ The cation exchange groups on the surface were found b~ A.T.R. to be carboxylic acid groups (100%). The membrane was immersed in an aqueous solution of 2.5N caustic soda/50 percent methanol to effect saponification of those sulfonyl chloride groups still remairing in the membrane interior. The specific conductivity and current efficiency of the saponified membrane~ when measured under the same conditions as in Example 1, were found to be 9.2 x 10 3 mho/cm and 95 percent respectively. When the current efficiency was measured under the same conditions as those of Example 1~ except that the alkali concentration in the cathode compartment fixed at 40 percent, there was obtained a va]ue of 97 percent.

The sulfonyl chloride form of the membrane obtalned in Exa~ple 1 was saponified in an aqueous solution of 2.5N
caustic soda/50 percent methanol. The current efficiency of the saponified membrane, when measured under the same condi-tions in Example 1, except that the alkali concentrilt;ioll in the cathode compartment I`ixed at l~o percerlt, wal~ 52 pe~rcel1t.

The procedure of Example 10 was repeated, except an aqueous 5 percent potassium iodide solution was used in place of the aqueous 15 percent sodium sulflde solution. The results were similar to those obtained in Example 10.

Two sheets of the sulfonyl chloride form of the membrane obtained in Example 1 were incorporated in frames 1~19~

similar to those used in Example 1. The frames were immersed in triethanol amine so that one surface of each membrane would undergo a reaction at 80C for 20 hours. The treated sur~aces of each membrane were sub,lected to measurement of A.T.R. In the spectrum, the absorption band at 1420 cm 1 characteristic of the sulfonyl chloride group vanished and stron~ absorption bands at 940 cm 1 and 1010 cm 1 characteristic of the sulfinic acid salt appeared. The membrane was saponified in an aqueous solution of 2.5N caustic soda/50 percent methanol~ then heated in 12N hydrochloric acid at 90C for 30 hours, again converted into the Na form by means of 2.5~ caustic soda and sub~ected to measurement of A.T.R. A sharp absorption band character-istic of a carboxylic acid salt appeared at 1690 cm 1. The absorption bands at 940 cm 1 and 1010 cm 1 practically dis-appeared. The percentage of carboxylic acid groups present on the surface as cation exchan~e ~roups was 85~ by measurement of A.T.R.
The specific conductivity of thiæ 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 membrane, when measured with the treated sur~ace ~aclrl~ the cathode compartment side, was 89 percent.
~XAMPLE 13 The sulfonyl chloride form of the membrane obtairled in Example 1 was allowed to react ln 5 percent hexane-tetra-hydrofuran solution of butyl llthium at 60C for eight hours and then sub~ected to measurement of A.T.R. In the spectrum, the absorption band of the sulfonyl chloride ~roup at 1420 cm 1 dwindled by about 60 percent and absorption bands at 940 and 1010 cm 1 characteristic or' the sulfinic acid salt appeared.

.
, .

EXAMPLE_14 The sulfonyl chloride form of the membrane obtained in Example 1 was saponified, converted into the H form with hydro-chloric acid~ thoroughly dried~ and reacted in a tetrahydro~uran solution of lithium aluminum hydride at 40C for 20 hours. In A.T.R., an absorption band at 1780 cm 1 characteristic of the carboxylic acid ~roup was slightly visible.

A sulfonyl fluoride form membrane 0.20 mm in thickness was prepared by a procedure similar to th~t of Example 1. One surface of the membrane was saponified with an aqueous solution of 2.5N caustic soda/50 percent methanol. The membrane was spread out with the nonsaponified surface held downwardly on a plain-weave fabric of polytetrafluoroethylene 0.15 mm in thick-ness with warp and filling yarns, both of 400-denier multifila-ments, repeat each at a rate of 40 yarns per inch. The membrane and fabric were heated to 270C with the membrane simultaneously drawn against the fabric by means of vacuum so as to embed the fabric in the membrane as a reinforcin~ material.
This membrane was converted to the sulfonyl chloride form by the same method as used in ~xample 1. Wlth frames made of acryli.c resin, two such membralle~3 wer~ held ~ast a~alnst each other wlth the fabric-reinPorc~d surface on the inside. I'he frames containin~ the two ad~oinin~ membranes were immersed in an aqueous 47 percent hydrobromic acld solutlon and caused to under~o a reaction at 80C for 20 hours. After the reaction~
the membranes were removed, saponlfied in an aqueous solution of 2.5N caustic soda/50 percent methanol and further oxidized ir. a solution of 2.5N caustic soda/2.5 percent sodium hypochlorite at 90C for 16 hours. The specific conductivity and current ~Zl9~99 efficiency of the membrane, when measured under the conditions of Example 1 with the treated surface held in the diréction of the cathode, were found to be 5.0 x 10 3 mho/cm and 95 percent respectively.

The reinforced membrane obtained in Example 15 was saponified. The specific conductivity and current efficiency of the saponified membrane, when measured under the same conditions as in Example 1, were found to be 6.0 x 10 3 0 mho/cm and 58 percent respectively.

The treated membrane obtained in Example 1 after oxidation (with 2.5N NaOH/2.5% NaClO) was subjected to separate durability tests by immersion in 45% caustic soda or in 5% sodium hypochlorite at 90C for 300 hours. There-after, the treated surface of the membrane was subjected to measurement of A.T.R. In the spectrum, an absorption band at 1690 cm 1 characteristic of carboxylic acid group was observed. When the membrane was converted into the H form in IN hydrochloric acid, the absorption band shifted to 1780 cm 1, indicating that the membrane had undergone substantially no change during the durability test. The specific conduc-tivity and current efficiency of th0 membrane, when measured after the durability test under the same condit.ions in Example 1, were found to be 9.9 x 10 3 mho/cm and 94 percent in the case of the sample which had been subjected to the durability test in the 45~ caustic soda and 10.2 x 10 3 mho/cm and 96 percent with the sample tested in 5% sodium hypochlorite. The results indicate that the membrane had undergone absolutely 0 no change during the durability tests.

_ .
The sulfonyl chloride form of the membrane obtained SP

12~9;~99 in Example 1 was saponified, then converted into the H form with lN hydrochloric acid, thoroughly dried and treated with phosphorus pentoxide suspended in phosphorus oxychloride at 110C ~or 24 hours. After this treatment by ~.T.R. measurement, absorption bands at 1460 and 1470 cm 1 characteristic of sulfo-nic anhydride were observed. The membrane was allowed to react with lithium aluminum hydride under the same conditions as in Example 14, whereby a similar result was obtained.
E~XAMPLE 18 The procedure of Example 2 were repeated except that the steps of saponification and oxidation were reversed. The results were similar to those obtained in Example 2.

The procedure of Example 1 was repeatedg except the reaction in the hydroiodic acid solution was carried out at 40C for four hours and, after saponification, the reaction in hydroiodic acid solution was repeated at 80C for 25 hours.
The resultant membrane was oxidized ln an aqueous solution of 2.5N caustic soda/2.5 percent sodium hypochlorite and then measured for specific conductivity and current efriciency under the same conditions as in Example 1. The vfll~eq obtained were 11.0 x 10 mho/cm an~ per-c~nt respectlvely.
~XAMPI.~ 20 Polytetrafluoroethylene powder and ~lass fibers were blended. The blend was compression molded under pressure of 300 k~/cm2 to produce a polytetrafluoroethylene sheet with a thickness of 1 millimeter. The sheet was heated in an electric furnace at 320C for one hour to fuse the polytetra-fluoroethylene powder, and thereafter treated with hydrofluoric acid to dissolve the ~lass phase. Consequently, there was l~9~g~

obtained a neutral membrane with a porous structure.
This neutral membrane was coated three times with a 1~1,2-trichloro-1,2,2-trifluoroethane solution of a copolymer of tetrafluroethylene and perfluoro~336-dioxa-4-methyl-7-octenesulfonyl fluoride) having an exchange capacity of 1.2milligr-am equivalents/gram of dried resin, and obtained by a po]ymerization procedure similar to that of Example 1. The solvent was evaporated from the coated membrane. Thereafter,.
the coat was pressed on the membrane at 270C ~or 10 minutes, to produce a laminate with a thickness of 50 ~.
Under the same conditions as those of Example 1, this membrane was converted into the sulfonyl chloride form, and reacted with hydrogen iodide gas at 100C for 40 hours. The A.T.R. indicated complete disappearance of the absorption band at 1420 cm 1 characteristic of the sulfonyl chloride group, and showed sharp absorption at 1780 cm 1 characteristic of carboxylic acid.

In 1,1,2-trichloro-1,2,2-trifluoroethane 3 tetra-fluoroethylene and perfluoro-3,6 dioxa-4-methyl-7-octene sulfonyl fluorlde were copolymerized in the preserce of perfluoropropionyl peroxlde a8 the pol~ymerization inltlator, with the polym~rlzation temperature ~lxed at 45C and the pressure maintained at 5 Ic~/cm2G durin~ the polymerizat:Lon.
The polymer obtained is identifled as Polymer 1.
The copolymerization was repeated by the same pro-cedure, except the pressure was maintained at 3 kg/cm2 throughout the polymerization to produce Polymer 2.
A portion of each o~ the polymers thus produced was sub~ected to hydrolysis in a mixture of aqueous 5N caustic soda 12~9399 solution and methanol (volume ratio of 1:1) at 90C I'or 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 molded to produce separate membranes 2 mils and 4 mils in thickness. The two membranes were joined face to face and molded under heating into a composite membrane. The composite membrane was treated in the aforementioned hydrolyzing system to be con~erted into a sodium sulfonate form composite membrane.
The composite membrane was con~erted into the H form by treatment in an aqueous lN hydrochloric acid solution and subsequently converted lnto the sulfonyl chloride form by-reactlon with a mixture of phosphorus pentachloride and phos-phorus ox~chloride (gravimetric ratio 1:1) at 120~C for 40 hours. At the end of the reaction) the composlte membrane was washed for four hours under reflux in carbon tetrachloride and dried under vacuum at 40C.
The dried membrane was subjected to measurement of A.T.R. to reveal that in both the membrane~ o~ Polymer 1 and Polymer 2, the absorption band by sulfonyl chlorlde 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 a~ain3t each other with the Polymer 2 membrane sides facing inwardly and, in that state, set in position in frames made of acrylic resin and fastened up by use of packings made of polytetrafluoroethylene. The frames were immersed in an aqueous 57 percent hydroiodic acid solution so that only the exposed surfaces (Polymer 1 membrane side) l~rould undergo reaction at 80C for 30 hours. The membranes were washed with water at 60C
for 30 minutes. The infrared spectrum of each treated surface was measured. In the spectrum, the absorption band at 1420 cm 1 characteristic of sulfonyl chloride completely vanished~ and an absorption band at 1780 cm 1 characteristic of carboxylic acid ~roup appeared~ In crystal violet solution, a stained layer of a width of about 0.3 mil was observed on the Polymer 1 side of the membrane. The cation exchange groups on the surface were found to be carboxylic acid groups (100%) by A.T.R.
The membrane was saponified in an aqueous solution of 2.5N 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 membrane. On the Polymer 2 side of the membrane, an absorption band at 1055 cm 1 characteristic of sodlum sulfonate appeared. The membrane was lmmersed in an aqueous 2.5 percent sodium hypochlorite solution and oxidized at 90C for 16 hours.
The speclf~c conductivity o~ the resultant membrane, when measured in an aqueous 0.lN cau~tlc ~30da solut:lon, was round to be 5.2 x 10 3 ~ho/cm.
~ he speclfic conductivity of the membrane wa~
determlned after complete conversion into the Na form, keepin~
the membrane in a constantly renewed bath of an aqueous 0.lN
caustic soda solutlon at normal room temperature for ten hours until equilibrium and subjecting it to an alternating current of 1000 cycles whiie under an aqueous 0.lN soda solution at 25C for measurement of the electric resistance of the membrane.

1219;~99 The Na form cation exchange membrane was equilibrated by immersion in an aqueous 2N caustic soda solution at 90C for lG hours~ then incorporated in an electrolytic cell with the reacted surface~ namely the Polymer 1 side, facing 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 e~lectrolytic 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 solu-tion at pH 3 was circulated through the anode compartment and an aqueous 30 percent caustic soda solution was circulated 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 compartment per hour by the theoretical value calculated from the amount of electricity passed, The passa~e of the electric curre~t was cont:ln~led for 2000 hours, Thereafter, the currenk efriclency was measured and found to be 93,8 percent.

The sulfon,vl chloride form of the composite membrane obtalned in Rxample 21 was saponified in a solution of 2.5N
caustic soda/50 percent methanol, The specific conductivity and current ef~icienc,v of the saponified membrane, when measured under the conditions of Example 21, were found to be 7,5 x lC 3 mho/cm and 7~ percent.

1219;~9~

The procedure of Example 21 was repeated, except that the s~eps of saponiri~ation and oxidation were reversed.
rrhe specific conductivity and current efficiency of the membrane were similar to those obtained in Example 21.

The procedure of Example 21 was repeated, except that the membrane was allowed to react in an aqueous 20 percent potassium io~ide solution at 60C for 30 hours in place Or the treatment ln hydrogen iodide. At the end of the reaction, the treated surface was subjected to measurement of A.T.R. In the 3pectrum, absorption bands due to potassium sulfinate were observed at 1010 cm 1 and g40 cm 1 In crystal violet solution, a surrace layer of a thickness of 0.2 mil on the Polymer 1 side was observed to be stained.
The membrane was saponified under the same condi~ions a3 those of Example 21 and then allowed to react in 57 percent h~drolodic acid at 80C for 30 hourfi. At the end of the reaction, Ithe Polymer 1 slde Or the membrane was sub~ected to measurement o~ A.T.R. In the spectrum~ the absorption by potassium sul~inate completely disappeared and an absorption by carboxylic acld appeared at 1780 cm 1 The cation exchan~.e ~roups on the surface were found to be carboxylic acld groups (about 100%~ by A.T.R.
On the Polymer 2 side of the membrane, an absorption by sulfonic acld appeared at 1060 cm 1. The membrane was further oxidized under the same condltions as those of Example 21. The specific conductiv~ty and current efflciency Or the membrane were foun~
to be 5.3 x 10 3 mho~cm a~d g4 percent respectively.
EXAMP~E 24 A copolymer (Polymer 3~ was prepared by repeating - 3g -~z~9~99 the procedure of Example 21, except that the pressure was maintained at 7 kg/cm G during the polymerization.
The exchange capacity of Polymer ~ when measured by the same procedure as that of Example 219 was found to be 0.~ milliequivalent/g of dr~ resin.
By followin~ the procedure o~ Example 21, a compo-site membrane comprising a Polymer 2 membrane, 4 mils in thickness, and a Polymer 3 membrane3 2 mils in thickness, was produced.
The composite membrane was sub;ected to hydrolysis in a mixture of an a~ueous 5N caustic soda solution and methanol (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 o~ phos-phorus pentachloride and phosphorus oxychloride (gravimetric ratio 1:1) at 120C for 36 hours to be converced lnto the sulfonyl chloride form.
The composite membrane was incorporated in a flow-gas reaction system and so that the Polymer 3 slde of the composite membrane was allowed to under~o a cont~ct reactlon with 20.0 percent hydrogen iodide gas (with nitro~en as the dilutin~ gas) at 100C for 12 hours. The treated surface of the membrane was sub~ected to measurement of A.T.R. In the speatrum, an absorption band due to carboxylic acid appeared at 1780 cm 1 and the absorption band due to sulfonyl chloride at 1420 cm disappeared. In crystal violet solution, a layer of 0.4 mil in thickness was stained. By A.T.R. measurement, 3 about lOQ% o~ the cation exchange groups were found to be ~219399 carboxylic acid groups.
The composite membrane was then hydrolyzed, oxidized, and incorporated in an electrolytic cell with the Polymer 3 side facing the cathode compartment. In this electr~lytic system, electrolysis of sodium chloride was carried out under the same conditions as in Example 21, with the concentration of caustic soda circulated to the cathode compartment fixed at 20 percent.
The current efficiency was 97 percent, and the specific conduc-tivity was 4.3 x 10 3 mho/cm.
l'he current efficiency, when measured after 1700 hours of continued pa~sage of electric current, was 97.2 percent.
CO~PARISON EXAMPLE 6 . .
The sulfonyl chloride form of the composite membrane obtained in Example 24 was hydrolyzed in a solution of 2.5N
caustic soda/50 percent methanol. The specific conductivity of the resultant membrane was 5.2 x 10 3 mho/cm. The membrane was sub~ected to electrolysis under the same conditions as those of Example 24, with the Polymer 3 side f'acing the cathode compartment. The current efflciency was 80.2 percent.

Two sheets of the sul~onyl chlorld~ form Or the membrane obtained in ~xamF~le 24 wer-e held a~a:lnst each other with the Polymer Z sides facin~ inwardly and~ ln that state, set in frames of acrylic resin, immersed ln an aqueous 20 percent sodium sul~lde solutlon and allowed to react under a continuous f`low of nltrogen gas at 70C for two hours. At the end of the reaction, the treated surface of the membrane was sub~ected to measurement of A.T.R. In the spectrum, the absorption band at 1420 cm 1 characteristic of the sulfonyl ~939g chloride group disappeared~ and absorption bands at 1010 cm 1 and 940 cm characteristic of sulfinic acid salts were observed.
The membrane was immersed in an aqueous 2.5 percent sodium hypochlorite solution at 70C for 16 hours and was again sub~ected to measurement of A.T.R. In the spectrumg the adsorp-tion bands at 1010 cm 1 and 940 cm 1 vanished and an absorption band at 1055 cm 1 characteristic of sodium sulfonate appeared.
Two sheets of the membrane treated with sodium sul~ide as described above were washed with water and set again in the frames so that the Polymer 3 side reacted in a solution of 47 percent hydrogen bromide at 80C for 20 hours. 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 f 2.5N caustic scda/50 percent rnethanol and sub~ected again to measurement o:~ A.T.R. In the spectrum, the absorption band at 1780 cm vanished, an absorption band due to sodium carboxy-late appeared at 1690 cm 1, and mlnor absorptlon bands due to sulfinic acid salts appeared at 940 cm 1 and 1010 cm 1. By A.T.R. measurement, about 90% Or the cation exchange groups on the sur~ace were found to be carboxyllc acld ~roups.
The membrane was th~n treate~ ln an aqu~ous 2.5 percent sodium h~ypochloI-lte solut:lon 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 efflctency, when measured u~der the same electrolytic conditions as those of Example 24, was found to be 92 percent. Substantially the same current efficiency was shown a~ter 1700 hours of continued passage of electric current.

1219;~99 EXAMPLE_?6 The rnembrane treated with sodium sulfide in Example 25 was saponified in an aaueous 2.5N caustic soda/50 percent methanol at 60C for 16 ho~rs. The membrane was then treated in an aqueous lN hydrochloric acid solution at 60~C 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 converted to the salt form, an absorption band due to carboxylate appeared strongly at 1690 cm 1, and weak absorp-tion bands by sulfinic acid salts appeared at 1010 cm 1 and 940 cm . 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 hypo-chlorite solution at 90C for 16 hours. The specific conduc-tivity of the oxidized membrane 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 Or Example 24~ was found to be 93 percent.

Tetrarluoroet~yl.ene~ and perr~ oro-3,6 dioxa-ll-methyl-7-octene sulforlyl fluorlde were emulsion polymerlzed at 70C under 4.5 atmospheres of tetrafluoroethylene pressure, with ammonium persulfate used as the initiator and the ammonium salt of perfluoro-octanolc aci~ as the emulsifier.
The polymer consequently obtained was washed with water, t~en hydroly~ed and thereafter sub~ected to measurement of exchange capacity by a titrimetric method. The exchange capacity was found to be 0.80 milligram equivalent/gram dry ~F

~19~99 resin. This polymer is identifled as Polymer 4.
By a procedure similar to that of Example 21, Polymer 2 used in Example 21 and Polymer 4 were combined to produce a composite membrane comprising a Polymer 2 membrane which h~d a thickness of 4 mil~, and a Polymer 4 membrane which had a thlckness of 3 mils. ~his composite membrane was spread out with the Polymer 2 side held downwardly on a woven fabric of polytetrafluo~oeth,vlene about 0.15 mm in thickness having filling yarns of 400-denier multifilaments and warp yarns of 200-denier multifilaments x 2 repeat each at a rate of 25 yarns per inch. The membrane and fabric were heated to 270C with the membrane 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 21. Two sheets of the membrane were held against each other with the Polymer 2 sides (the sides having the fabric embedded) facing lnwardly by means of frames made of acrylic resin. The Polymer 4 sides 2G of the membrane were allowed to react with hydrogen sulfide gas introduced in a contlnuous flow at 120C for 20 hours.
The membrane was removed, ~aponirled ln an aqueous solution of 2.5N caustlc soda/50 percent methanol and there-after oxldized in a solution of 2,5N caustlc soda/2.5 percent sodlum hypochlorite at 90C for 16 hour~, The specific conductivity, when measured by the same method as used in ~xample 21, has found t,o be 3,2 x 10 3 mho~cm, The current efficiency was 94 percent. ~ven after 1000 hours of continued pas~age of electric current, the current. ef~iciency remained unchanged.

1219;~99 The reinforced composite membrane obtained in Example 26 was saponified. The specific conductivity and current efficiency of the resultant membraneg when measured under the same conditions as those of Example 219 were found to be 3.9 x 10 3 mho/cm and 62.1 percent respectively.

The fluorocarbon cation exchange membrane, I'Nafion #315," made by E. I. DuPont de ~iemours & Company was treated by immersion in an aqueous lN hydrochloric acid solution at 60C for 16 hours and then converted into the ammonium sulfonate form with an aqueous lN ammonia solution. The membrane was dried under vacuum at 50C ~or 16 hours and then allowed to .. ~7........... w;fl react ~n a mixture of ~hosphorus pentachloride and phosphorus oxychlorlde (gravimetrlc ratio 1:1) at 120C for 40 hours.

The surfaces, having respective equivalent weights of 1500 and 1100~ were subjected to measurement of A.T.R. In both of the spectra, the absorption band at 1060 cm 1 characteristlc of ~.~d sul~onic acid groupldisappeared and an absorption band at 1420 cm 1 characteristic of sulfonyl chloride was observed.
Two sheets of the membrane were held against each other with the sides havin~ the equlvalent wel~ht 1100 ~acin~ inwardly, inserted in frame~ m~de Or acrylic resin, lmmersed ln an aqueous 57 percent hydrolodiC acid solution and allowed to react at 80C for 24 hours. At the end of the reactlon, the membrane was washed with water and the side of the membrane having the equivalent wei~ht 1500 was subjected to measurement of A.T.R. In the spectrum, the absorption band at 1420 cm 1 ~ R
characteristic of sulfonyl chlorideldisappeared and an absorption band at 1780 cm 1 characteristic of the carboxylic acid group appeared. In the case of the A~T.R. obtained of the side of the membrane having the e~uivalent weight of 110n, the absorption at 1420 cm 1 characteristic of sulfonyl chloride remained intact.
This membrane was saponified in a solution of 2M caustic soda/50 percent methanol at 60C for 4Q hours. Then, it was oxidized in an aqueous 2.5 percent sodium hypochlorite solution at 90C
for 16 hours. Thereafterg the membrane was placed in an electrolytic cell with the side having the equivalent weight 1500 facing the cathode and electrolysis of sodium chloride was carried out under the same conditions as those of Example 24. The c~lrrent efficiency was 96 percent. The specific conductivity was 2.0 x lQ 3 mho/cm.
The current efficiency of the membrane~ when measured after 1500 hours of continued passage of the same 5 electric current 5 was found to be essentially unchanged.
COMPARISON EXAMPLE_ The sul~onyl fluoride form membrane as prepared in Example 1 was fastened in position between frames made of acrylic resin. Only the one surrace was allowed to react with a mixture of ammonia gas with air (about one vol. %) at room temperature for 15 hours. After the reaction, the cross section of the membrane was stained with rqethyl Red, whereby stained layer caused by sulfonamide groups appeared to the depth of 0.02 mm only at the reacted surface. This membrane was subjected to saponification and equilibration under the same conditions as in Example 1, followed by measure-ment of specific conductivity to give the result of 9.8 x 10 3 mho/cm.
Current efficiency was measured under the same conditions as in Example 1, with the surface having the _ 46 -~Z19399 sulfonamide groups facing the cathode side~ to be 87%.
Furthermore~ after continuous passage of current for 1000 hours, the current efficiency measured was as low as 80%.
COMPARI~SON EXAMPLE 9 The sulfonyl fluoride form mem~rane as prepared in Example 1 was treated at onl~ one surface with ethylene diamine at room temperature for 20 hours. A~i,er the reaction, the membrane was washed with diglymeg followed by washing with benzene~ and finally washed with water warmed at about 40C.
When the cross section of this membrane was stained with Methyl Red, only the treated surface was stained to the depth of 0.01 mm to find out that N-substituted sulfonamide was formed. This membrane was subjected to saponification and equilibration under the same conditions as in Example 1. The specific conductivity was measured to be 10.2 x 10 3 mho/cm.
Current efficiency was measured, with the surface ha~-ing the N-substituted sulfonamide groups facing the cathode side, under the same conditions as in Example 1 to be 91%.
After continuous current passage prolonged for 1000 hours, the current efficiency was found to be lowered to 74%.
COMPARIS~N ~XAMPLE 10 The cation exchatl~e mem~rane as prep~red in Compari~on Exanlple 9 was subLIected ko oxldatlve treatment with 2.5% aqueous sodlum hypochlorite solution at 90C f`or 16 hours. ~ith the surface havin~ the N-substituted sulfonamide groups f'acing the cathode side, current ef'ficienc,y was measured under the same condltions as in Example 1 to be 76%. Formation of sulfonic acid groups was confirmed by measurement of A,T.R.
after the oxidative treatment.

Claims (56)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A cation exchange membrane comprising a fluorocarbon polymer containing pendant carboxylic acid groups represented by -OCF2COOM wherein M is selected from the group consisting of hydrogen; ammonium; quaternary ammonium; and metallic atoms.

2. A cation exchange membrane as in Claim 1, comprising (a) a fluorocarbon polymer containing pendant carboxylic acid groups represented by -OCF2COOM and (b) a fluorocarbon polymer having cation exchange groups substan-tially 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, substantially consisting of the fluorocarbon polymer con-taining pendant carboxylic acid groups represented by -OCF2COOM.

5. 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.

6. A cation exchange membrane of Claim 1, rein-forced with a material selected from the group consisting of woven fabrics of inert fibres.

7. A cation exchange membrane of Claim 6, wherein the inert fibres are polytetrafluoroethylene.

8. A cation exchange membrane comprising two bonded polymer films, a first film comprising a fluorocarbon polymer containing pendant carboxylic acid groups represented by the formula -OCF2COOM, wherein M is selected from the group consisting of hydrogen; ammonium; quaternary ammonium;
and metallic atoms; and a second film comprising a fluoro-carbon polymer containing pendant sulfonic acid groups rep-resented by the formula -OCF2CF2SO3M, wherein M is selected from the group consisting of hydrogen; 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.

9. A cation exchange membrane as in Claim 8, wherein the first film comprises (a) a fluorocarbon polymer containing pendant carboxylic acid groups represented by -OCF2COOM and (b) a fluorocarbon polymer having cation ex-change groups substantially consisting of sulfonic acid groups represented by -OCF2CF2SO3M, said fluorocarbon poly-mer (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.

10. A cation exchange membrane as in Claim 9, wherein the fluorocarbon polymer (a) is present as surface stratum at least about 100 .ANG. in thickness on one surface of the membrane.

11. A cation exchange membrane as in Claim 8, wherein the first film substantially consists of the fluorocarbon polymer containing pendant carboxylic acid groups represented by -OCF2COOM.

12. A cation exchange membrane as in Claim 8, wherein at least 20 mol percent of the ion exchange groups contained as pendant groups of the fluorocarbon polymer in the first film are -OCF2COOM.

13. A cation exchange membrane of Claim 8, rein-forced with a material consisting of woven fabric of inert fibres.

14. A cation exchange membrane of Claim 13, wherein the inert fibres are polytetrafluoroethylene.

15. A cation exchange membrane of Claim 13, wherein the reinforcing material is embedded in the second film.

16. A polymer having the repeating units wherein m is 0, 1 or 2, p is 1 to 10, q is 3 to 15, the X's taken together are four fluorines or three fluorines and one chlorine, Y is F or CF3, R1 is H, or M (?), M is alkali metal, alkaline earth metal, ammonium or quater-nary ammonium, and t is the valence of M.

17. The polymer of Claim 16, wherein m is 1

18. The polymer of Claim 17, wherein the X's taken together are four fluorines.

19. The polymer of Claim 18, wherein Y is CF3.

20. A film or membrane of a fluorinated ion exchange polymer having the repeating units wherein m is 0, 1 or 2, p is 1 to 10, q is 3 to 15, the X's taken together are four fluorines or three fluorines and one chlorine, Y is F or CF3, R1 is H, or M(?), M is alkali metal, alkaline earth metal, ammonium or quater-nary ammonium, and t is the valence of M, said polymer having an equivalent weight no greater than about 2000.

21. The film or membrane of Claim 20, wherein m is 1.

22. The film or membrane of Claim 21, wherein the X's taken together are four fluorines.

23. The film or membrane of Claim 22, wherein Y
is CF3.

24. The film or membrane of Claim 20, wherein the equivalent weight is no greater than 1500.

25. The film or membrane of Claim 20, reinforced with fabric.

26. A laminar structure having (a) a base layer of a fluorinated ion exchange polymer, and having on at least one surface thereof (b) a layer of the polymer defined in Claim 16, the polymer of each layer having an equivalent weight no greater than about 2000 wherein the base layer (a) is a fluorinated ion exchange polymer which has pendant side chains which contain -OCF2CF2SO3R2 groups wherein R2 is F, or M(?);M is alkali metal, alkaline earth metal, ammonium or quaternary ammonium; and t is the valence of M.

27. A laminar structure having (a) a base layer of a fluorinated ion exchange polymer, and having on at least one surface thereof (b) a layer of the polymer defined in Claim 19, the polymer of each layer having an equivalent weight no greater than about 2000 wherein the base layer (a) is a fluorinated ion exchange polymer which has pendant side chains which contain -OCF2CF2SO3R2 groups wherein R2 is H, or M?; M

is alkali metal, alkaline earth metal, ammonium or quaternary ammonium; and t is the valence of M.

28. The laminar structure of Claim 26 or 27, wherein the layer (b) is present as surface stratum at least about 100 .ANG. in thickness on one surface thereof.

29. The laminar structure of Claim 26 or 27, wherein the thickness of the layer (b) is up to 50% of the total thickness.

30. A fluorocarbon polymer containing pendant carboxylic acid groups of -OCF2COOM wherein M is selected from the group consisting of hydrogen; ammonium; quaternary ammonium; and metallic atoms, said polymer having the equivalent weight not greater than about 2000.

31. A cation exchange membrane comprising a fluorocarbon polymer according to Claim 30.

32. A cation exchange membrane as in Claim 31, comprising (a) a fluorocarbon polymer containing pendant carboxylic acid groups of -OCF2COOM and (b) a fluorocarbon polymer having cation exchange groups substantially consisting of sulfonic acid groups of -OCF2CF2SO3, said fluorocarbon polymer (a) existing as at least one layer in a thickness of each at least 100 .ANG. on a surface or internally of the membrane.

33. A cation exchange membrane as in Claim 32, wherein the fluorocarbon polymer (a) is present as surface stratum at least about 100 .ANG. in thickness of one surface of the membrane.

34. A cation exchange membrane as in Claim 31, substantially consisting of the fluorocarbon polymer con-taining pendant carboxylic acid groups of -OCF2COOM.

35. A cation exchange membrane as in Claim 31, wherein at least 20 mol percent of the ion exchange groups contained as pendant groups of the fluorocarbon polymer is -OCF2COOM.

36. A cation exchange membrane as in Claim 31, reinforced with a woven fabric of inert fibres.

37. A cation exchange membrane as in Calim 36, wherein the inert fibres are polytetrafluoroethylene.

38. A cation exchange membrane as in Claim 31, comprising two bonded polymer films, a first film comprising a fluorocarbon polymer containing pendant carboxylic acid groups represented by the formula -OCF2COOM, wherein M is selected from the group consisting of hydrogen; ammonium;
quaternary ammonium; and metallic atoms; and a second film comprising a fluorocarbon polymer containing pendant sulfonic acid groups represented by the formula -OCF2CF2SO3M, wherein M is selected from the group consisting of hydrogen; 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 percent of the total thickness.

39. A cation exchange membrane as in Claim 38, wherein the first film comprises (a) a fluorocarbon polymer containing pendant carboxylic acid groups of -OCF2COOM, and (b) a fluorocarbon polymer having cation exchange groups substantially consisting of sulfonic acid groups of -OCF2CF2SO3M, said fluorocarbon polymer (a) existing as at least one layer in a thickness of each at least 100 .ANG. on a surface or internally of the first film.

40. A cation exchange membrane as in Claim 39, wherein the fluorocarbon polymer (a) is present as a surface stratum at least about 100 .ANG. in thickness on one surface of the membrane.

41. A cation exchange membrane as in Claim 38, wherein the first film substantially consists of the fluoro-carbon polymer containing pendant carboxylic acid groups of -OCF2COOM.

42. A cation exchange membrane as in Claim 38, wherein at least 20 mol percent of the ion exchange groups contained as pendant groups of the fluorocarbon polymer in the first film is -OCF2COOM.

43. A cation exchange membrane of Claim 38, reinforced with a material selected from the group consist-ing of woven fabrics of inert fibres.

44. A cation exchange membrane of Claim 43, wherein the inert fibres are polytetrafluoroethylene.

45. A cation exchange membrane of Claim 43, wherein the reinforcing material is imbedded in the second film.

46. A cation exchange membrane comprising at least one continuous layer extending to at least one sur-face of the membrane and formed by a fluorocarbon polymer according to Claim 35.

47. A cation exchange membrane as claimed in Claim 46, having only a first surface layer formed by said polymer.

48. A cation exchange membrane according to Claim 47, wherein the second surface layer of said membrane is formed by a second fluorocarbon polymer containing pen-dant sulfonic acid groups represented by -OCF2CF2SO3M, said second polymer having an equivalent weight not greater than about 2000.

49. An electrolytic cell comprising a housing with separate anode and cathode compartments, separated by a membrane according to Claim 47 or 48, wherein said first surface layer faces the cathode compartment.

50. An electrolytic cell comprising an anode portion and a cathode portion separated by a membrane com-prising (a) a fluorocarbon polymer containing pendant car-boxylic acid groups of -OCF2COOM, and (b) a fluorocarbon polymer having cation exchange groups substantially consist-ing of sulfonic acid groups of -OCF2CF2SO3M, said fluorocar-bon polymer (a) existing as a surface stratum at least about 100 .ANG. in thickness on that surface of the membrane which faces the cathode side of the cell.

51. An electrolytic cell comprising an anode portion and a cathode portion separated by a membrane com-prising two bonded polymer films, a first film comprising a fluorocarbon polymer containing pendant carboxylic acid groups represented by the formula -OCF2COOM, wherein M is selected from the group consisting of hydrogen; ammonium;
quaternary ammonium; and metallic atoms; and a second film comprising a fluorocarbon polymer containing pendant sulfonic acid groups represented by the formula -OCF2CF2SO3M, wherein M is selected from the group consisting of hydrogen; 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 percent of the total thickness, the side of said first film facing the cathode side of the cell.

52. A cation exchange membrane as claimed in claim 46 or 47, wherein the layer of fluorocarbon polymer containing pendant carboxylic acid groups is at least about 100 .ANG. thick.

53. A cation exchange membrane of claim 46 or 47, wherein the layer of fluorocarbon polymer containing pendant carboxylic acid groups has a thickness up to 50 percent of the total thickness.

54. A cation exchange membrane comprising at least a layer of a fluorocarbon polymer containing pendant carboxylic acid groups represented by -OCF2COOM wherein M
is selected from the group consisting of hydrogen, ammonium, quaternary ammonium and metallic atoms and prepared by treat-ing with a reducing agent a starting membrane comprising a corresponding fluorocarbon polymer characterized by the presence of sulfonyl groups represented by the formula:

and/or wherein X is selected from the group consisting of halogen;
hydroxyl; alkyl containing up to four carbon atoms; aryl;
and OZ wherein Z is selected from the group consisting of metallic atoms, alkyl containing up to four carbon atoms and aryl.

55. A cation exchange membrane according to claim 54, wherein at least 20 mol percent of the functional groups in said layer are carboxylic acid groups.

56. A cation exchange membrane according to claim 54 or 55, wherein the thickness of said layer is at least 100 Angstroms.