GB1604173A

GB1604173A - Fluorinated ion excahnge polymers

Info

Publication number: GB1604173A
Application number: GB15539/78A
Authority: GB
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1977-04-20
Filing date: 1978-04-19
Publication date: 1981-12-02
Also published as: IL54539A; CA1126443A; FR2485024B1; ZA782224B; FR2388013B1; IL54539A0; JPS53132094A; IT7822495A0; FR2485024A1; NL7804220A; GB1604174A; DE2817315A1; JPS6147843B2; GB1604176A; GB1604175A; NZ187022A; SU784786A3; AU525395B2; AU3516878A; FR2388013A1

Abstract

Fluorinated ion exchange polymers which have both pendant side chains containing carboxylic groups and pendant side chains which contain sulfonyl groups, when used in the form of membranes to separate the anode and cathode compartments of an electrolysis cell, permit operation at high current efficiency. They are made by oxidation of flourinated polymers which have pendant side chains containing sulfinic groups, or both sufinic and sulfonyl groups. The fluorinated polymers which have pendant side chains containing sulfinic groups, or both sufinic and sulfonyl groups, are in turn made from fluorinated polymers which have pendant side chains containing sulfonyl halide groups by reduction with, for example, hydrazine. Fluorinated ion exchange polymers which have pendant side chains containing -OCF2COOR groups and also pendant side chains which contain sulfonyl groups can also be made by copolymerization of a mixture of monomers, one of which is a vinyl monomer which contains the indicated carboxylic group; and also by treatment of a polymer which contains -OCF2CF2SO3H or salts thereof with a combination of fluorine and oxygen.

Description

(54) FLUORINATED ION EXCHANGE POLYMERS (71) We, E. I. DU PONT DE NEMOURS AND COMPANY, a Corporation organised and existing under the laws of the State of Delaware, United States of America, located at Wilmington, State of Delaware, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention concerns improvements in and relating to fluorinated ion exchange polymers, and particularly to such polymers used in the form of films and membranes used in chloralkali electrolysis cells.

Fluorinated ion exchange membranes are known in the art. The fluorinated ion exchange polymer in such membranes can be derived from a fluorinated precursor polymer which contains pendant side chains in sulfonyl fluoride form.

The sulfonyl fluoride functional groups have been converted to ionic form in various ways, for example, to sulfonate salts by hydrolysis with an alkaline material, to the sulfonic acid by acidification of the salts, and to the sulfonamide by treatment with ammonia. Examples of such teachings in the art can be found in U.S. 3,282,875, U.S. 3,784,399, and U.S. 3,849,243.

Although such polymers and membranes have many desirable properties which make them attractive for use in the harsh chemical environment of a chloralkali cell, such as good long-term chemical stability, their current efficiencies are not as high as is desired, especially when the caustic is produced at high concentration. As transport of hydroxyl ion in a chloralkali cell from the catholyte through the membrane to the anolyte increases, current efficiency drops. Larger amounts of oxygen impurity in the chlorine are thereby produced, and there is a greater buildup of chlorate and hypochlorite contaminants in the brine, which contaminants must be removed and discarded to maintain acceptable cell operation. Current efficiencies of at least 90,0 are highly desirable.

Accordingly, there is a need for polymers and membranes which will permit cell operation at high current efficiencies, and especially for those which will permit operation at high efficiencies over long periods of time. Additionally, it was desired to find a method for modifying the known polymers and membranes which have pendant side chains in sulfonyl fluoride form in such a way to obtain polymers and membranes which will have the high current efficiencies desired.

It has now been found that fluorinated ion exchange polymers and membranes which contain pendant side chains in ionic carboxylic form and pendant side chains in ionic sulfonyl form have high current efficiencies.

There is provided according to the present invention a process which comprises contacting a first fluorinated polymer which contains pendant side chains containing

(or sulfinic) groups, wherein Rf is F, Cl or a C1 to C,O perfluoroalkyl radical, M is H, an alkali metal or an alkaline earth metal, ammonium, substituted ammonium including quaternary ammonium, or hydrazinium including substituted hydrazinium, and n is the valence of M with an oxidising agent, and separating therefrom a second fluorinated polymer which contains pendant side chains containing

groups. More specifically, the first fluorinated polymer is a random polymer which contains pendant side chains which contain sulfinic groups is

wherein m is O, 1 or 2, p is I to 10, q is 3 to 15, s is 0, 1 or 2, t is 0 to 10, each X, which may be the same or different, is H, F, CLEF3 or perfluoroalkoxy, subject to the proviso that at least one X is F (preferably the X's taken together are four fluorines or three fluorines and one chlorine), Y is F or CF3, Z is F or CF3, Ref is F, Cl or a C1 to C10 perfluoroalkyl radical, RZ is F, Cl or OM

M is H, alkali metal, alkaline earth metal, ammonium, substituted ammonium including quaternary ammonium, hydrazinium including substituted hydrazinium, and n is the valence of M. When reduction of sulfonic to sulfinic groups is essentially complete, t in this polymer will be 0. More often, both p and t will be at least l.

By "separating" is meant that the second fluorinated polymer is separated from any excess oxidising agent and by-products of consumed oxidising agent.

A preferred second fluorinated polymer prepared in this way contains pendant side chains, 10 to 95% of which side chains contain

groups and 5 to 90 /O of which side chains contain

groups, wherein R, is F, Cl or C1 to C10 perfluoroalkyl, M is H, an alkali metal, an alkaline earth metal, ammonium, substituted ammonium including quaternary ammonium, or hydrazinium including substituted hydrazinium, and n is the valence of M.

More specifically, such a second fluorinated polymer is a random polymer containing +he units:

wherein m is O, 1 or 2, p is I to 10, q is 3 to 15, r is 1 to 10, s is 0, 1, 2 or 3, each X, which may be the same or different, is H, F, Cl, CF3 or perfluoroalkoxy subject to the proviso that at least one X is F (preferably the X's taken together are four fluorines or three fluorines and one chlorine), Y is F or CF3, Z is F or CF3, Rf is F, Cl or a C, to C,0 perfluoroalkyl radical, R2 is F, Cl or

OM #1# n M is H, alkali metal, alkaline earth metal, ammonium, substituted ammonium including quaternary ammonium, hydrazinium including substituted hydrazinium, and n is the valence of M.

Such a random copolymer can also be produced, as described and claimed in Application No. 8041258, (Serial No. 1,604,175) by a process which comprises the steps of contacting a fluorinated polymer having the repeating units:

where R2 is where M is H, alkali or alkaline earth metal and n is the valence of M, with a mixture of fluorine and oxygen, followed if desired by subjecting the polymeric product therefrom to hydrolysis.

With reference to part of the definition of M, "ammonium, substituted ammonium including quaternary ammonium, or hydrazinium including substituted hydrazinium" includes groups defined more specifically as

wherein R4 is H, lower alkyl such as C, to Ca, or NH2; and R5, R6 and R7 are each independently H or lower alkyl such as C, to Cs, with the understanding that any two of R4, R5, R6 and R7 may join to form a hetero ring, such as a piperidine or morpholine ring. There is also provided according to the invention certain films and membranes of the polymers; and laminar structures containing the polymers.

The ion exchange membranes of the present invention which contain both ionizable carboxylic and ionizable sulfonyl groups as active ion exchange sites are highly desirable in comparison with prior art ion exchange membranes for several distinct reasons. Most importantly, outstanding efficiencies in a chlor-alkali cell have been obtained in comparison with membranes which contain only sulfonic acid ion exchange groups obtained by hydrolysis of pendant sulfonyl groups. For example, treatment of a membrane having pendant sulfonyl groups to modify a surface layer to incorporate carboxylic groups according to the present invention results in a dramatic increase in current efficiency in a chlor-alkali cell. This improvement is considered to be of predominant importance in commercial applicability in reducing the cost of producing a unit of chlorine and caustic.

Illustratively, in a chlor-alkali plant producing, for example, 1000 tons per day of chlorine, the direct savings in electrical power for only a 1% increase in efficiency are very significant.

A need has developed in the chlor-alkali industry for improved ion exchange materials which can be used to replace existing cell compartment separators which have been used for decades without substantial improvement in design.

For use in the environment of a chlor-alkali cell, the membrane must be fabricated from a material which is capable of withstanding exposure to a hostile environment, such as chlorine and solutions which are highly alkaline. Generally, hydrocarbon ion exchange membranes are totally unsatisfactory for this kind of use because such membranes cannot withstand this environment.

For commercial use in the chlor-alkali industry, a film must go beyond the ability to be operable for prolonged time periods in the production of chlorine and caustic. A most important criterion is the current efficiency for conversion of brine in the electrolytic cell to the desired products. Improvement in current efficiency can translate into pronounced savings in the cost of production of each unit of chlorine and caustic. Additionally, from a commercial standpoint the cost of production of each unit of products will be determinative of the commerical suitability of an ion exchange membrane.

The ion exchange polymers of the present invention possess pendant side chains which contain carboxylic groups and pendant side chains which contain sulfonyl groups attached to carbon atoms having at least one fluorine atom connected thereto. The ion exchange polymers of the invention can be made from polymers which contain pendant side chains containing

groups wherein Rf is F, Cl or a C to C10 perfluoroalkyl radical, M is as defined above, and n is the valence of M, by sublecting them to an oxidation. A variety of oxidizing agents are found effective for oxidizing pendant side chains containing

groups to side chains containing

groups.

Among suitable oxidizing agents are oxygen, chromic acid, permanganate salts, vanadate salts in acid solution, nitrous acid, and hypochlorite salts. The term "oxygen" is intended to encompass mixtures of gases which contain oxygen, such as air. The preferred oxidizing agents are oxygen, chromic acid, permanganate salts and vanadate salts because they are more effective.

Oxygen can be used to oxidize the pendant groups defined above when M is H, that is, when the functional group is the free sulfinic acid. With this oxidizing agent, it is preferred to carry out the oxidation in the presence of a metal catalyst. It is also preferred to employ an elevated temperature. At or near room temperature without a catalyst, although oxygen has little or no observable effect even over a period of a few days, significant conversion to carboxyl groups is observed after three to four weeks. At higher temperatures the oxidation is faster; for example, at 50--60"C, without a catalyst, there is a significant amount of oxidation to carboxyl groups by air after only a few days. When oxygen is the oxidant, the polymer, film or membrane to be so treated can simply be exposed to the gas, or it can be contacted with oxygen in a liquid medium such as water. The use of a catalyst is preferred as it speeds the reaction. Metals which can exist in more than one valence state can be used as catalyst. (For present purposes, zero is not counted as a valence state.) For example, salts of iron, vanadium, uranium, cobalt, nickel, copper and manganese have been found effective.

Other effective oxidizing agents for the pendant groups, when the functional group is either in the free sulfinic acid form or in the form of an alkali metal or alkaline earth metal salt thereof, include permanganate in either acidic or basic media, chromic acid, vanadate salts in acidic media, nitrous acid, and hypochlorite salts in basic media. It should be understood that the polymer can be in either free acid form, or salt thereof, when introduced to such oxidizing agents, and that the acid or salt forms may interconvert depending on the pH of the oxidizing medium used. The oxidations are ordinarily carried out at temperatures above room temperature. These oxidations can be carried out in media such as water, or in the presence of inorganic or organic acids such as sulfuric acid, hydrochloric acid, acetic acid, etc.

It has also been observed that pendant

groups can be converted to

groups by placing them in boiling water or a boiling organic or inorganic acid such as formic acid for a period of at least several hours. It is believed that air oxidation may be occurring under such conditions. Some oxidizing agents, such as hydrogen peroxide and nitric acid are ineffective for present purposes. It is a simple matter to distinguish oxidizing agents effective for present purposes from those that are ineffective, merely by determining the presence or absence of characteristic absorption bands in the infrared spectrum of the product corresponding to carboxylic acid groups at about 1785 cell or to salts thereof at about 1680 cm~'.

The ion exchange polymers of the present invention possess pendant side chains which contain carboxylic groups attached to carbon atoms having at least one fluorine atom connected thereto, and pendant side chains which contain sulfonyl groups attached to carbon atoms having at least one fluorine atom connected thereto, as set forth above.

The ion exchange polymers of the present invention which possess pendant side chains which contain carboxylic groups and pendant side chains which contain sulfonyl groups possess general utility as ion exchange resins. When used in a film or membrane to separate the anode and cathode compartments of an electrolysis cell, such as a chloralkali cell, the polymer should have a total ion exchange capacity of 0.5 to 1.6 meq/g (milliequivalents/gram), preferably from 0.8 to 1.2 meq/g. Below an ion exchange capacity of 0.5 meq/g, the electrical resistivity becomes too high, and above 1.6 meq/g the mechanical properties are poor because of excessive swelling of the polymer. The values of p, q and r in the above formulas of the copolymer should be adjusted or chosen such that the polymer has an equivalent weight no greater than about 2000, preferably no greater than about 1500, for use as an ion exchange barrier in an electrolytic cell. The equivalent weight above which the resistance of a film or membrane becomes too high for practical use in an electrolytic cell varies somewhat with the thickness of the film or membrane. For thinner films and membranes, equivalent weights up to about 2000 can be tolerated. For most purposes, however, and for films of ordinary thickness, a value no greater than about 1500 is preferred.

The polymers which contain pendant side chains containing

groups, wherein RS, M and n are as defined hereinabove, are in turn made from known precursor fluorinated polymers which contain pendant side chains containing

groups wherein Rf is as defined above, and A is F or Cl, preferably F. Ordinarily, the functional group in the side chains of the precursor polymer will be present in terminal

groups. When this is the case, the intermediate polymer will contain

(sulfinic) groups, and the polymers prepared therefrom will contain both

groups. The precursor fluorinated polymers employed can be of the type disclosed in U.S.P. 3,282,875, U.S.P. 3,560,568 and U.S.P. 3,718,627. More specifically, the precursor polymers can be prepared from monomers which are fluorinated or fluorine substituted vinyl compounds. The precursor polymers are in general made from at least two monomers, with at least one of the monomers coming from each of the two groups described below.

The first group is of fluorinated vinyl monomers of the formula CX2=CX2, wherein each X, which may be the same or different, is H, Cl, F, CF3 or perfluoroalkoxy subject to the proviso that at least one X is F, such as vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkyl vinyl ether), tetrafluoroethylene and mixtures thereof. In the case of copolymers which will be used in electrolysis of brine, the precursor vinyl monomer desirably will not contain hydrogen, and preferably the X's taken together are four fluorine or three fluorines and one iodine.

The second group is of the sulfonyl-containing monomers containing the precursor group

wherein Rf and A are as defined above. Additional examples can be represented by the general formula CF2=CF-Tk-CF2SO2F wherein T is a bifunctional perfluorinated radical comprising I to 8 carbon atoms, and K is 0 or 1. The particular chemical content or structure of the radical T is not critical, but it must have a fluorine atom attached to the carbon atom to which the -CF2SO2F group is attached. Other atoms connected to this carbon can include fluorine, chlorine, or hydrogen although generally hydrogen will be excluded in use of the copolymer for ion exchange in a chlor-alkali cell. The T radical of the formula above can be either branched or unbranched, i.e., straight-chain and can have one or more ether linkages. It is preferred that the vinyl radical in this group of sulfonyl fluoride containing comonomers be joined to the T group through an ether linkage, i.e., that the comonomer be of the formula CF2=CF-TC F2-SO2 F Illustrative of such sulfonyl fluoride containing comonomers are

The most preferred sulfonyl fluoride containing comonomer is perfluoro(3,6 dioxa - 4 - methyl - 7 - octenesulfonyl fluoride),

The sulfonyl-containing monomers are disclosed in such references as U.S.P.

3,282,875, U.S.P. 3,041,317, U.S.P. 3,718,627 and U.S.P. 3,560,568.

The preferred copolymers utilized as the precursor are perfluorocarbon although others can be utilized as long as the T groups has a fluorine atom attached to the carbon atom which is attached to the -CF2SO2F group. The most preferred copolymer is a copolymer of tetrafluoroethylene and perfluoro(3,6 - dioxa - 4 methyl - 7 - octenesulfonyl fluoride) which comprises 20 to 65 percent, preferably, 25 to 50 percent by weight of the latter.

The precursor copolymer used in the present invention is prepared by general polymerization techniques developed for homo- and copolymerizations of fluorinated ethylenes, particularly those employed for tetrafluoroethylene which are described in the literature. Nonaqueous techniques for preparing the copolymers of the present invention include that of U.S.P. 3,041,317, that is, by the polymerization of a mixture of the major monomer therein, such as tetrafluoroethylene, and a fluorinated ethylene containing a sulfonyl fluoride group in the presence of a free radical initiator, preferably a perfluorocarbon peroxide or azo compound, at a temperature in the range (W200 C. and at pressures in the range 1--200, or more atmospheres. The nonaqueous polymerization may, if desired, be carried out in the presence of a fluorinated solvent. Suitable fluorinated solvents are inert, liquid, perfluorinated hydrocarbons, such as periluoromethylcyclohexane, perfluorodimethlcyclobutane, perfluorooctane, perfluorobenzene and the like, and inert, liquid c lorofluorocarbons such as 1,1,2 trichloro - 1,2,2 - trifluoroethane, and the like.

Aqueous techniques for preparing the precursor copolymer include contacting the monomers with an aqueous medium containing a free-radical initiator to obtain a slurry of polymer particles in non-water-wet or granular form, as disclosed in U.S.

Patent 2,393,967, or contacting the monomers with an aqueous medium containing both a free-radical initiator and a telogenically inactive dispersing agent, to obtain an aqueous colloidal dispersion of polymer particles, and coagulating the dispersion, as disclosed, for example, in U.S.P. 2,559,752 and U.S.P. 2,593,583 The polymers which contain pendant side chains containing

groups are made from the known precursor fluorinated polymers which contain pendant side chains containing

groups by reduction with a compound having the formula

wherein R8 is H or C1 to C8 alkyl radical, and R9 is H or a C1 to C8 alkyl radical, preferably H. The preferred reducing agent is hydrazine in view of its ready availability. Another effective compound is methylhydrazine. Accordingly, the preferred reducing agents are those which have the formula H2NNHR8.

A variety of reaction conditions can be used for the reduction. For example hydrazine has been found effective when used anhydrous, as the known hydrazine hydrate, as a 50 / by weight solution in water, or in solution in other solvents such as dimethylsulfoxide. The reduction with a hydrazine can advantageously be carried out in the presence of an acid acceptor. The acid acceptor can be a tertiary amine such N - methylmorpholine, N,N,N',N - tetramethylethylenediamine, pyridine or triethylamine, or a metal hydroxide such as KOH or NaOH. Use of a hydroxide or tertiary amine is preferred because the sulfinic acid product and by-product hydrogen fluoride can form salts with the hydrazine reagent, and the hydroxide or tertiary amine forms a salt with the sulfinic acid and frees the hydrazine to be available for reducing other sulfonyl halide groups.

Reduction with a hydrazine is ordinarily done at temperatures from about room temperature to about 40"C, although higher temperatures can also be used.

The reaction rate increases as the amount of water in the reaction medium is decreased. Also the reaction rate increases in dimethylsulfoxide as a medium, and in the presence of a hydroxide or tertiary amine.

The sulfinic groups in the reduced polymer will be in the form of sulfinic acid groups, or an alkali metal or alkaline earth metal salt thereof. Similarly, any sulfonyl halide groups that have been hydrolyzed will be in the form of sulfonic acid groups, or an alkali or alkaline earth metal salt thereof. In both cases, the form will depend on the nature of the last medium with which the polymer was treated, and will ordinarily be the salt of the strongest base in medium (or the last medium) to which it is (or was) exposed. Interconversion between acid and salt forms can be accomplished by treatment with solutions of acids or bases, as desired. Treatment times must, of course, be increased as the thickness of the layer to be treated is increased. Following reduction by a hydrazine, it is best to wash the polymer to free it of excess hydrazine before proceeding to the oxidation step. At the same time, an acid or alkaline wash can be carried out, if desired, to put the polymer in free acid or salt form if a specific form is desired for the particular oxidizing agent to be used.

Although the precursor fluorinated polymer can be in the form of powder or granules when subjected to the reduction and oxidation reactions described hereinabove, more often it will be in the form of a film or membrane when subjected to these reactions.

The polymers of the present invention can be in the form of films and membranes.

When the polymers of the invention are in the form of a film, desirable thicknesses of the order of 0.002 to .02 inch are ordinarily used. Excessive film thicknesses will aid in obtaining higher strength, but with the resulting deficiency of increased electrical resistance.

The term "membrane" refers to non-porous structures for separating compartments of an electrolysis cell and which may have layers of different materials, formed, for example, by surface modification of films or by lamination, and to structures having as one layer a support, such as a fabric imbedded therein.

It is possible according to the present invention to make films and membranes wherein the pendant side chains are essentially wholly (i.e., 90 or more) or wholly (i.e., up to about 99%) in the carboxylic form throughout the structure, and also wherein the pendant side chains throughout the structure are in carboxylic and sulfonyl form, for example 10 to 90% of each. Control in this respect is exercised during the reduction of the precursor polymer which contains sulfonyl halide groups to the intermediate polymer which contains sulfinic groups, by using or not using competing reactions which produce different products. For example, when 100% hydrazine is used as the reagent in the reduction reaction, conversion to sulfinic functional group, and eventually to carboxylic functional group can be quite high, of the order of 9W95%, and it is believed that in the ultimate surface layers conversion to sulfinic or carboxyl groups can be as high at 98 or 99%. Use of a combination of hydrazine and a hydroxide in a solvent like water or dimethylsulfoxide will result in reduction of some groups to sulfinic form and hydrolysis of others to sulfonic acid salt form; because the sulfonic salt form is not affected during the oxidation step, the ultimate result is a combination of carboxylic and sulfonyl groups in relative amount which varies with the relative amounts of hydrazine, water and hydroxide used. Control of the relative amounts of carboxylic and sulfonyl functional groups can also be exercized to some extent during the oxidation step. Some oxidizing agents such as chromic and vanadate salts will produce relatively larger amounts of carboxylic and small amounts of the original sulfonic groups, while other oxidizing agents such as hypochlorite will produce relatively smaller amounts of carboxylic and larger amounts of the original sulfonic groups.

In similar fashion, it is possible to make products where various other functional groups are present in pendant side chains, in combination with carboxylic groups in other pendant side chains. For example, the precursor polymer which contains sulfonyl halide groups can be treated with hydrazine in combination with ammonia or a primary amine, whereby not only will some sulfonyl groups be reduced to sulfinic form, but others will be converted to sulfonamide or N-substituted sulfonamide groups. The technique whereby groups of the precursor polymer can be converted to the form ASO2NH)mQ, wherein Q is H, NH4, cation of an alkali metal and/or cation of an alkaline earth metal and m is the valence of Q, are set forth in U.S.P. 3,784,399. Preferred definitions of Q include NH4 and/or cation of an alkali metal particularly sodium or potassium. The technique whereby sulfonyl groups of the precursor polymer can be converted to N-monosubstituted sulfonamide groups and salts thereof are as set forth in U.S.P.

4,085,071.

So that the final film or membrane will have as low an electrical resistivity as possible, it is desirable that essentially all of the sulfonyl halide groups in the precursor polymer be converted to active cation exchange groups, i.e. to either carboxylic or sulfonyl groups of a type which will ionize or form metal salts. In this respect, it is highly undesirable that a film or membrane to be used for ion exchange purposes in an electrolytic cell to have a neutral layer, or that a film or membrane to be used in a chloralkali cell have either a neutral layer or an anion exchange layer. The film and membrane of the present invention do not have neutral or anion exchange layers. In this context, fiber or fabric reinforcing is not considered as a neutral layer, inasmuch as such reinforcing has openings, i.e., its effective area is not coextensive with the area of the film or membrane.

In the case of films and membranes to be used as separators in a chloralkali cell, polymers which contain 40-95% pendant side chains containing carboxylic groups and 540 /O pendant side chains containing sulfonyl groups provide excellent current efficiency. An equally important criterion in a chlor-alkali cell, however, is the amount of power required for each unit of chlorine and caustic. It is considered that the polymers of the type disclosed herein permit a proper combination of operating conditions to realize an excellent and unexpected reduction in power. Since the power requirement (which may be expressed in watthours) is a function of both cell voltage and current efficiency, low cell voltages are desirable and necessary. However, a polymer without a high current efficiency cannot operate effectively from a commercial standpoint even with extremely low cell voltages. Additionally, a polymer with an inherent high current efficiency allows a proper combination of parameters as in fabrication into the fi cell. While the description of said German and Dutch publications is directed to use in a chloralkali cell, it is within the scope of the present disclosure to produce alkali or alkaline earth metal hydroxides and halogen such as chlorine from a solution of the alkali or alkali earth metal salt. While efficiencies in current and power consumption differ, the operating conditions of the cell are similar to those disclosed for sodium chloride.

An outstanding advantage has been found in terms of current efficiency in a chloralkali cell with the fluorinated polymers of the type disclosed herein with pendant groups which contain carboxylic groups and pendant groups which contain sulfonyl groups.

To further illustrate the innovative aspects of the present invention, the following examples are provided.

Example 1 A 4-mil film of a copolymer of tetrafluoroethylene and perfluoro(3,6 - dioxa 4 - methyl - 7 - octenesulfonyl fluoride) having an equivalent weight of 1100 was immersed for 16 hours at room temperature in 85% hydrazine hydrate, washed with water and immersed for 30 minutes at room temperature in a 5% solution of potassium hydroxide. The film was washed with water and immersed at room temperature in a mixture of 25 ml formic acid and 5 ml 37% hydrochloric acid in an atmosphere of oxygen. Increasing conversion to the carboxylic acid form was observed by IR analysis after 5 and 60 hours at room temperature and after an additional 2 hours at 7080 C in this medium.

The degree of conversion to the carboxylic acid form was further increased by heating the film (after a water wash) for 2 hours at 500C in a mixture of 75% acetic acid, 3% concentrated sulfuric acid, 2% chromium trioxide and 20% water. The film was washed with water, and conditioned for cell testing by heating for 2 hours to 70"C in a 10% solution of sodium hydroxide. Aqueous sodium chloride was electrolyzed at a current density of 2.0 asi (amps/in2) to give 35% NaOH at a current efficiency of 91% at a cell voltage of 4.9 volts.

Example 2 A 4-mil film of a copolymer of tetrafluoroethylene and perfluoro(3,6 - dioxa 4 - methyl - 7 - octenesulfonyl fluoride) having an equivalent weight of 1100 was immersed for 48 hours at room temperature in 85% hydrazine hydrate. IR analysis at this point indicates essentially complete conversion to the sulfinic salt form through the entire thickness of the film. The film was then heated for 20 minutes to 90"C in a solution of potassium hydroxide (13%) in aqueous dimethyl sulfoxide (30), rinsed with water and immersed for 20 minutes at room temperature in a mixture of 20 ml aqueous hydrochloric acid and 100 ml glacial acetic acid. The film was again rainsed with water and then heated for 16 hours to 1300C in an atmosphere of oxygen. IR analysis at this point indicated the formation of cF2CO2H functional groups, the presence of XF2SO3H groups, and the complete absence of O2F of the starting material as well as O2R groups of the sulfinate intermediate.

The film was conditioned for cell testing by heating for 2 hours in a 10% solution of sodium hydroxide. Aqueous sodium chloride was electrolyzed at a current density of 2.0 asi to give 32% NaOH at a current efficiency of 91% at a cell voltage of 4.7 volts.

Example 3 A 4-mil film of a copolymer of tetrafluoroethylene and perfluoro(3,6 - dioxa 4 - methyl - 7 - octenesulfonyl fluoride) having an equivalent weight of 1100 was immersed for 20 hours at room temperature in a mixture of 300 ml dimethylsulfoxide, 100 ml N-methylmorpholine and 80 ml 85% hydrazine hydrate.

The film was then washed with dilute potassium hydroxide solution and water and immersed for 10 minutes at room temperature in a mixture of 200 ml acetic acid, 10 ml concentrated sulfuric acid, 2 gm ammonium vanadate, 1 gm vanadyl sulfate and 300 ml water. After that the film was washed with water and exposed to air at room temperature for 6 days.

The film was then converted to the potassium salt form by heating for 30 minutes to 900C in a solution containing 13% potassium hydroxide and 30 ZO dimethylsulfoxide and washing with water.

Analysis by X-ray fluorescence at this point showed a potassium content of 0.965 gram atoms per kg and 0.075 gm atoms of sulfur per kg. This indicates a content of 0.89 meq/gm of carboxylate groups and 0.075 meq/gm of sulfonate groups.

Aqueous sodium chloride was electrolyzed at a current density of 2.0 asi in a cell with this film as the membrane to produce 35% NaOH as a current efficiency of 92% at a cell voltage of 4.9 volts.

Example 4 A 4-mil film of a copolymer of tetrafluoroethylene and perfluoro(3,6 - dioxa 4 - methyl - 7 - octenesulfonyl fluoride) having an equivalent weight of 1100 was immersed for 30 hours at room temperature in 85% hydrazine hydrate (54"d hydrazine on an anhydrous basis). The film was then washed with water and a dilute solution of potassium hydroxide. Infrared analysis indicated that this treatment caused substantially complete reduction of sulfonyl groups to sulfinic groups through the entire thickness of the film.

The film was then heated for 17 hours to 700C in a solution containing 5 Ó chromium trioxide and 50 /O acetic acid in water, rinsed with water, and conditioned for cell testing by heating for 2 hours to 70"C in a 10% solution of sodium hydroxide. Aqueous sodium chloride was electrolyzed at a current density of 2.0 asi to produce 33% sodium hydroxide at a current efficiency of 90 /O and a cell voltage of 4.1 volts.

Example 5 A 7-mil film of a copolymer of tetrafluoroethylene and perfluoro(3,6 - dioxa 4 - methyl - 7 - octenesulfonyl fluoride) having an equivalent weight of 1100 was placed as a liner in a dish and anhydrous hydrazine (95%) was poured into the liner so as to contact only the upper surface of the film. After 2 minutes at room temperature, the hydrazine was removed and the film was washed with water.

The film was then immersed in 50 ml glacial acetic acid, and 25 ml of a 20% solution of sodium nitrite was added in portions during a period of I hour. The film was again washed with water. Staining of a cross-section with Sevron) Brilliant Red 4G indicated a depth of reaction of 0.5 mils.

The unreacted SO2 groups were hydrolyzed by heating for 30 minutes to 90"C in a solution of potassium hydroxide (13%) in aqueous dimethylsulfoxide (30 2O) The film was installed in a chloralkali cell with the hydrazine treated and oxidized side toward the catholyte. Aqueous sodium chloride was electrolyzed at a current density of 2.0 asi to give 25% NaOH at a current efficiency of 81% at a cell voltage of 4.3 volts.

Example 6 A 4-mil film of a copolymer of tetrafluoroethylene and perfluoro(3,6 - dioxa 4 - methyl - 7 - octenesulfonyl fluoride) having an equivalent weight of 1100 was immersed for 16 hours at room temperature in 85% hydrazine hydrate. The film was rinsed with water, then with a hot 2% solution of sodium hydroxide, and again with water. The film was then heated for 1 hour to 5060 C in a solution of 1% potassium bisulfate in 80% acetic acid (balance water), wiped off and exposed to room temperature air for 3 days. The oxidation was completed by 2 hours immersion in a solution of 2% CrOs and 3% H7SO in 75% acetic acid (balance water) at 50"C.

The film was washed with water, then heated to 700C for I hour in a 10% solution of NaOII, and evaluated in a chloralkali cell. Aqueous sodium chloride was electrolyzed at a current density of 2.0 asi (2.0 amps/in2) to give 30 NO sodium hydroxide at a current efficiency of 91% at a cell voltage of 4.5 volts. After 130 days of operation sodium hydroxide was being produced at a concentration of about 32.5% at a current efficiency of 89% and a cell voltage of 4.3 volts.

Example 7 A 7-mil film of a copolymer of tetrafluoroethylene and perfluoro(3,6 - dloxa 4 - methyl - 7 - octenesulfonyl fluoride) having an equivalent weight of 1100, having a T-24 "Teflon" fabric embedded therein, and having one surface layer of about 1 mil depth with had been hydrolyzed to the corresponding potassium salt of the sulfonic acid, was exposed on the sulfonyl fluoride side to a solution of 18 ml hydrazine hydrate and 45 ml N - methylmorpholine in 35 ml dimethylsulfoxide for 15 minutes at room temperature. The sheet was washed with water, dilute potassium hydroxide and again with water. Cutting the sheet through its thickness and staining with Sevron Brilliant Red 4G at this point indicated that reaction with hydrazine had occurred to a depth of 0.6 mils.

The sample was then washed with 5% sulfuric acid and exposed to air for 20 hours at room temperature, followed by chromic acid oxidation like that described in Example 6. The sheet was evaluated in a chloralkali cell at a current density of 2.0 asi, and sodium hydroxide was produced at a concentration of 37 Ó at a current efficiency of 88% and a cell voltage of 4.6 volts after 20 days of operation.

Example 8 A 7-mil film of a copolymer of tetrafluoroethylene and perfluoro(3,6 - dioxa 4 - methyl - 7 - octenesulfonyl fluoride) having an equivalent weight of 1100 was folded and sealed to a bag, except for a small opening to permit the introduction of reagents. A mixture of 250 ml dimethylsulfoxide, 100 ml N - methylmorpholine and 90 ml hydrazine hydrate was poured into the bag and permitted to react for 7 minutes at room temperature. The bag was then emptied, rinsed with water and exposed for I hour on the inside with a solution containing 10% KOH and 10% dimethylsulfoxide, rinsed again with water, and then with dilute acetic acid. The bag was then opened, the film was cut through its thickness, and staining with SevronB Brilliant Red 4G indicated a depth of reaction of 0.6 mils. The film was then treated for 30 minutes with a solution of 5 gm VOSO4, 5 gm NH4VO3 and 5 ml concentrated H2SO4 in 3 liters of water, washed with water and hung up for drying.

After 3 days of air exposure at room temperature, the film was treated for 1 hour with a solution containing 5% acetic acid, 2% K2SO4 and 2% KHSO4 in water.

The film was then washed with water and vacuum laminated to a T-25 "Teflon" fabric, followed by total hydrolysis for 20 minutes in a solution of KOH in aqueous dimethylsulfoxide.

The resulting membrane was evaluated in a chloralkali cell with the hydrazine treated and oxidized surface facing the catholyte. Sodium hydroxide was produced at a concentration of 31% by weight at a current efficiency of 87% and a cell voltage of 5.8 volts at a current density of 2 asi.

In Application No. 80-41258 (Serial No. 1,604,175) we describe and claim a process for making a fluorinated polymer which contain pendant side chains containing

groups wherein Rf is F, Cl or C, to C,0 perfluoroalkyl which comprises contacting a fluorinated polymer which contains pendant side chains containing groups wherein R2 is

M is H, alkali metal or alkaline earth metal, and n is the valence of M, with a mixture of fluorine and oxygen, and, if desired, treating the resulting polymeric product with water or a solution of an alkali metal base to provide a fluorinated ion-exchange polymer which contains pendant side chains containing

groups wherein X is H or alkali metal.

WHAT WE CLAIM IS: 1. A process which comprises contacting a first fluorinated polymer which contains pendant side chains containing

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The sample was then washed with 5% sulfuric acid and exposed to air for 20 hours at room temperature, followed by chromic acid oxidation like that described in Example 6. The sheet was evaluated in a chloralkali cell at a current density of 2.0 asi, and sodium hydroxide was produced at a concentration of 37 Ó at a current efficiency of 88% and a cell voltage of 4.6 volts after 20 days of operation. Example 8 A 7-mil film of a copolymer of tetrafluoroethylene and perfluoro(3,6 - dioxa 4 - methyl - 7 - octenesulfonyl fluoride) having an equivalent weight of 1100 was folded and sealed to a bag, except for a small opening to permit the introduction of reagents. A mixture of 250 ml dimethylsulfoxide, 100 ml N - methylmorpholine and 90 ml hydrazine hydrate was poured into the bag and permitted to react for 7 minutes at room temperature. The bag was then emptied, rinsed with water and exposed for I hour on the inside with a solution containing 10% KOH and 10% dimethylsulfoxide, rinsed again with water, and then with dilute acetic acid. The bag was then opened, the film was cut through its thickness, and staining with SevronB Brilliant Red 4G indicated a depth of reaction of 0.6 mils. The film was then treated for 30 minutes with a solution of 5 gm VOSO4, 5 gm NH4VO3 and 5 ml concentrated H2SO4 in 3 liters of water, washed with water and hung up for drying. After 3 days of air exposure at room temperature, the film was treated for 1 hour with a solution containing 5% acetic acid, 2% K2SO4 and 2% KHSO4 in water. The film was then washed with water and vacuum laminated to a T-25 "Teflon" fabric, followed by total hydrolysis for 20 minutes in a solution of KOH in aqueous dimethylsulfoxide. The resulting membrane was evaluated in a chloralkali cell with the hydrazine treated and oxidized surface facing the catholyte. Sodium hydroxide was produced at a concentration of 31% by weight at a current efficiency of 87% and a cell voltage of 5.8 volts at a current density of 2 asi. In Application No. 80-41258 (Serial No. 1,604,175) we describe and claim a process for making a fluorinated polymer which contain pendant side chains containing groups wherein Rf is F, Cl or C, to C,0 perfluoroalkyl which comprises contacting a fluorinated polymer which contains pendant side chains containing groups wherein R2 is M is H, alkali metal or alkaline earth metal, and n is the valence of M, with a mixture of fluorine and oxygen, and, if desired, treating the resulting polymeric product with water or a solution of an alkali metal base to provide a fluorinated ion-exchange polymer which contains pendant side chains containing groups wherein X is H or alkali metal. WHAT WE CLAIM IS:

1. A process which comprises contacting a first fluorinated polymer which contains pendant side chains containing

groups, wherein Rfl is F, Cl or a C, to C,O perfluoroalkyl radical, M is H, an alkali metal or an alkaline earth metal, ammonium, substituted ammonium including quaternary ammonium, or hydrazinium including substituted hydrazinium, and n is tevalence of M, with an oxidising agent, and separating therefrom a second fluorinated polymer which contains pendant side chains containing

groups.

2. A process according to Claim I wherein said first fluorinated polymer contains pendant side chains containing

groups and pendant side chains containing

groups, wherein Rf, M and n are as defined in Claim 1.

3. A process according to Claim 1 wherein said first fluorinated polymer is a random polymer having the units

wherein mis O, 1 or 2, p is 1 to 10, q is 3 to 15, s is O, 1 or 2, t is O to 10, each X, which may be the same or different, is H, F, Cl, CF3 or perfluoroalkoxy subject to the proviso that at least one X is F, Y is F or CF3, Z is F or CF3, Rf is F, Cl or C1 to C,O perfluoroalkyl, R2 is F, Cl or

OM #l# n M is H, alkali metal, alkaline earth metal, ammonium, substituted ammonium including quaternary ammonium, or hydrazinium including substituted hydrazinium, and n is the valence of M.

4. A process according to Claim 3 wherein the X's taken together are four fluorines or three fluorines and one chlorine.

5. A process according to Claim 3 or 4 wherein t is 0.

6. A process according to Claim 3 or 4 wherein ,, and t are each at least 1.

7. A process according to any one of the preceding claims wherein the first fluorinated polymer is in the form of a film or membrane, and has an ion exchange capacity of 0.5 to 1.6 meq/g.

8. A process according to Claim 7 wherein the film or membrane is contacted with the oxidising agent on at least one surface thereof and to a depth of at least 200 angstroms.

9. A process according to Claim 8 wherein the depth is less than half the thickness of the film or membrane.

10. A process according to any one of the preceding claims wherein 10% to 90 /O of the

groups are converted to

groups.

II. A process according to any one of Claims 1 to 9 wherein at least 90% of the

groups are converted to

groups.

12. A process according to Claim 11 wherein up to 95% of the

groups are converted to

groups.

13. A process according to any one of contacting the preceding claims wherein the first fluorinated polymer is the product of contacting a precursor fluorinated polymer which contains pendant side chains containing

groups wherein X is F or Cl with a compound of the formula

wherein R8 is H or C1 to Ca alkyl and R9 is H or C1 to Ca alkyl, and separating therefrom the first fluorinated polymer.

14. A process according to Claim 13 wherein a film or membrane of the precursor polymer is contacted with

on at least one surface thereof and to a depth of at least 200 angstroms.

15. A process according to Claim 13 or 14 wherein R8 is H and R9 is H.

16. A process according to Claim 15 wherein the contacting with H2N-NH2 is carried out in the presence of an alkali metal hydroxide, an alkaline earth metal hydroxide, or a tertiary amine.

17. A process according to any one of the preceding claims wherein the first fluorinated polymer contains pendant side chains containing

groups wherein M is H and n is 1 and the oxidising agent is oxygen.

18. A process according to Claim 17 wherein contacting with oxygen is carried out in the presence of a catalytic amount of a metal which can exist in more than one valence state.

19. A process according to Claim 18 wherein contacting with oxygen is carried out in the presence of a catalytic amount of a vanadium, iron, uranium, cobalt, nickel, copper or manganese salt.

20. A process according to any one of Claims 1 to 17 wherein the oxidising agent is a permanganate salt, chromic acid, a vanadate salt, nitrous acid or a hypochlorite salt.

21. A process according to Claim I substantially as described in any one of Examples 1 to 8.

22. A fluorinated polymer which contains pendant side chains containing

groups when prepared by a process as claimed in any one of the preceding claims.

23. A fluorinated polymer according to Claim 22 which contains pendant side chains 10 to 95% of which side chains contain

groups and 5 to 90 /O of which side chains contain

groups, wherein Rf is F, Cl or C1 to C10 perfluoroalkyl, M is H, an alkali metal or an alkaline earth metal, and n is the valence of M.

24. A fluorinated polymer according to Claim 23 in the form of a film or membrane, and having an ion exchange capacity of 0.5 to 1.6 meq/g.

25. A fluorinated polymer according to Claim 22 having the units

whereinmisO, 1 or2,pis Ito l0,qis3tol5,risltol0,sis0, l,2or3,eachX, which may be the same or different, is H, F, Cl, CF3 or perfluoroalkoxy subject to the proviso that at least one X is F, Y is F or CF3, Z is F or CF3, Rf is F, Cl or C, to C,0 perfluoroalkyl and R2 is F, Cl or OM

26. A fluorinated polymer according to Claim 25 wherein the X's taken together are four fluorines or three fluorines and one chlorine.

27. A polymer according to Claim 26, wherein B, is F, R2 is

m) and M is H, alkali metal or alkaline earth metal.

28. A film or membrane of a fluorinated ion exchange polymer as claimed in Claim 26 or 27, the polymer having an equivalent weight no greater than 2000.

29. A film or membrane according to Claim 28, wherein the equivalent weight is no greater than 1500.

30. A film or membrane according to Claim 28 or 29, wherein the X's taken together are all fluorines, m is 1 and s is 1.

31. A laminar structure having a base layer of a fluorinated polymer which contains pendant side chains containing

groups wherein B, is F, Cl or C, to C,0 perfluoroalkyl, M is H, an alkali metal or an alkaline earth metal, and n is the valence of M, and having on at least one surface thereof a layer of a fluorinated polymer as claimed in Claim 23 each layer having an ion exchange capacity of 0.5 to 1.6 meq/g.

32. A laminar structure having a base layer of a fluorinated ion exchange polymer, and having on at least one surface thereof a layer of a polymer as claimed in Claim 26 or 27 each layer having an equivalent weight no greater than 2000.

33. A laminar structure according to Claim 32 wherein the base layer is a fluorinated ion exchange polymer which has pendant side chains which contain

groups wherein R2 is F, Cl or

OM #l# n M is H, alkali metal or alkaline earth metal, n is the valence of M, and Ref is F, Cl or C, to C,O perfluoroalkyl.

34. An electrolytic cell comprising a housing with separate anode and cathode sections, said cell being separated by a film or membrane as claimed in Claim 28, 29 or 30.

35. An electrolytic cell comprising a housing with separate anode and cathode sections, said cell being separated by a laminar structure as claimed in Claim 33 or 34.

36. A process for producing halogen and metal hydroxide of an alkali or alkaline earth metal, or combinations thereof, by electrolysis of a halide of said metal employing separate anode and cathode sections in an electrolytic cell, wherein ions of said metal are passed through a film or membrane as claimed in Claim 28, 29 or 30.

37. A process for producing halogen and metal hydroxide of an alkali or alkaline earth metal, or combinations thereof, by electrolysis of a halide of said metal employing separate anode and cathode sections in an electrolytic cell, wherein ions of said metal are passed through a laminar structure as claimed in Claim 33 or 34, the base layer of which laminar structure has sulfonyl groups at least a majority of which are present as ion exchange sites in ionic form, and the surface layer of which faces the cathode section of the cell.