CA1203509A - Composite ion exchange membranes - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A composite, substantially completely fluorinated film containing ion exchange groups and its use as an ion exchange membrane in an electrolytic cell is described. The membranes are comprised of at least two layers wherein each layer contains sulfonyl ion exchange groups the equivalent weight of the two layers differing by more than 250, at least one layer having an equivalent weight of less than 1000. The higher equivalent weight layer should face the cathode in an electrolytic cell.

Description

The present invention resides in a fluorinated composite membrane containing ion exchange groups and its use as an ion exchange membrane in an electrolytic cell.
This application is divided from applicant's copending Canadian Application Serial No. 379,454 filed on June lO, 1981 which is directed to a polymeric composite film of the type having two layers which differ in equivalent weight, each of said layers having a substantially completely fluorinated polymeric backbone with a plurality of pendant groups attached thereto, at least a portion of said pendant groups being a chain of carbon atoms which may be inter-rupted with one or more oxygen atoms and which terminates with an ion exchange group; wherein the improvement comprises the two layers differing in equivalent weight from each other by less than 250 and the carbon chain which connects the ion exchange group to the polymeric backbone having from 1 to 3 carbon atoms.
The electrolytic production of chlorine and caustic by the electrolysis of brine has been well known for many years. Historically, diaphragm cells using a hydraulically-permeable asbestos diaphragm, vacuum-deposited onto foraminous steel cathodes, have been widely commericalized. Such diaphragm cellsJ employing permeable diaphragms, produce NaCl-containing NaO~I catholytes because NaCl passes through the diaphragm ~rom the anolyte to the catholyte. Such NaCl-containing caustic is generally of low caustic concentration and requires a de-salting process and extensive evaporation of water to obtain a low-salt, high concentration caustic for industrial purposes.
In recent years, the chlor-alkali industry has focused much of its attention on developing membrane cells to produce low-salt or salt-free, high concentration caustic in order to improve quality and avoid 'A 28,985-F

;~

the costly de-salting and evaporation processes.
Membranes have been developed for that purpose which are substantially hydraulically-impermeable, but which will permit hydrated Na+ ions to be transported from the anolyte portion to the catholyte portions, while substantially preventing transport of Cl ions. Such cells are operated by flowing a brine solution into the anolyte portion and by providing salt-free water to the catholyte portion to serve as the caustic medium. The anodic reactions and cathodic reactions are not affected by the use of a membrane cell as opposed to the use of a diaphragm cell.

In addition to the caustic strength being important, two other criteria of the operating cell must also be considered for a complete energy view of the overall process. One is current efficiency, which is the ability of the membrane to prevent migration of the caustic produced at the cathode into the anode compartment; and the second is the voltage at which the cell operates, which is partly determined by the elec-trical resistance of the membrane. Power efficiency is often used as one term that considers both the current efficiency and cell voltage. It is defined as the product of the theoretical voltage, divided by the actual voltage, multiplied by the actual amount of caustic produced divided by the theoretical amount of caustic that could have been produced at a ~iven current.
Thus, it is apparent that power efficiency is reduced by higher cell voltage or by lower current efficiency.
The membrane has a direct effect on both. The most common method of comparing cells is to express the operation as kilowatt hours (KWH) of power consumed per metric ton (mt) of product produced. This expression 28,985-F -2-; :

12Q3509 ~'' also considers both voltage ~higher voltage increase the quantity of KWH consumed), and current efficiency (lower efficiency decreases the quantity of product produced). Thus, the lower the value KWH/mt, the better the performance of the cell. It is apparent that optimization of a membrane for use in electrolytic chlor-alkali cells is a trade off between cell volt~ge which is reflected in membrane electrical resistance, current efficiency and caustic concentration.
It is well known (G.E. Munn, Nafion~ Men~ranes - Factors Controlling Performance in the Electrolysic of Salt Solutions, The Electrochemical Society Meeting, October, 1977, Atlanta, Georgia) that the current efficiency of a chlor-alkali cell containing a membrane is determined primarily by the surface of the membrane contacting the catholyte. The current efficiency is dependent on the equivalent weight of the membrane in contact with the cathol~te and the voltage is dependent on both the thickeness of the membrane and the equivalent weight of the membrane. The equivalent weight is the measure of the concentration of ion exchange functional groups in the polymer membrane and is simply the weight of the polymer in the acid form required to neutralize one equivalent of base. The above publication discloses that iower equivalent weights (eq. wts.) have lower electrical resistance (and thus lead to lower cell voltage), but that higher eq. wts. are required to obtain sufficient negative ion rejection and thus acceptable current efficiency. It is well known and discussed in the publication that voltage drop across the membrane is directly dependent on thickness; a thin film being desirable for minimum voltage drop. It thus follows that ideal membranes would be very thin films 28,985-F -3-lZ03509 having higher eq. wts. (1500-2000 for sulfonic acids membranes of the prior art).

U.S. Patent 3,909,378 teaches a method to S take advantage of the increased current efficiency associated with high eq. wts. without absorbing the full voltage penalty associate-d with these materials.
This patent teaches a composite membrane formed by laminating a thin, high eq. wt. film to a thicker, lower eq. wt. film. The thin, higher eq. wt. side of the film faces the catho~yte in the cell thus resulting in current efficiency associated with the higher eq.
wt. and voltage associated with the thin layer plus the minimal voltage of the lower eq. wt. layer. The patent further teaches that the eq. wts. of the polymers fall within the range of 1000-2000 or even greater and that the eq. wt. difference between the low and high eq. wt.
portions of the composite film should be at least 250 and preferably 400. The patent teaches polymers having sulfonyl type ion exchange groups and that the structure linking these groups to the main polymer chain are not critical. The sulfonyl ion exchange groups, according to the patent may be the sulfonamide form or in the sulfonic acid form.
U.S. Patents 3,784,399 and 4,085,071 teach formation of a barrier layer, facing the catholyte, on a single polymer film by reacting ammonia or N-substituted amines with one face of a sulfonyl functional polymer to form sulfonamide ion exchange sites. The main distinguishing feature of these patents is that the barrier layer facing the catholyte is introduced by chemical modification on a single eq. wt. film rather than by lamination of a barrier film to a support film.

28,985-F -4-. ~ . , ` ~ 1203509 ` ~^-u.s. Patent 4,151,053 also teaches having barrier layers on the catholyte face of membranes to achieve enhanced current efficiency without substantial voltage penalties. The main distinguishing feature of this patent is that the barrier layer has carboxylic acid ion exchange groups of the general structure ~OCF2COOM where M is hydrogeni ammonium; quaternary ammonium, particularly quaternary ammonium having a molecular weight of 500 or less; and metallic atoms, particularly alkali or alkaline earth metals. The patent teaches that each film of the composite membrane should have eq. wts. in the range of 1000 to 2000 and that the first film, the high eq. wt. film, should have an eq. wt. at least 150 higher than the second film.
All of the aforementioned patents use as starting materials sulfonyl containing fluoropolymers wherein the sulfonyl is generally contained on a pendant chain. The useful polymers and monomer precursors for these type materials are described in U.S.Patent 3,282,875.
fn each patent the preferred sulfonyl containing fluoro- .
polymer is described as derived, by polymerization, from the monomer FSO2CF2CF20CFCF2OCF = CF2 disclosed in U.S. Patent No. 3,282,875. The polymers are generally copolymers of the above monomer and tetrafluoroethylene. These copolymers are sold under the tradename of Nafion~ by E. I. duPont Company and are well known and have been widely evaluated as membranes in chlor-alkali cells where the properties of these copolymers, such as useful eq. wt. ranges, water absorp-tion and the like, have become accepted as the proper~ies 28,985-F -5-of sulfonic acid containing fluorocarbon polymers. In general, useful eq. wts. for these copolymers when used as membranes in chlor-alkali cells is not below about 1000 to llO0. Below these values water absorption increases dramatically and physical integrity falls sharply. For eq. wts. above about 1800-2000, electrical resistance becomes so great as to render the copolymers impractical in chlor-alkali cell use. Preferred eq. wt. ranges are from about 1100 to about 1500.
United States Patent 4,065,366 teaches the use of single layer carboxylic acid membranes in chlor-alkali cells.

This patent teaches useful equivalent weight ranges that VarJ
from about 500 to about 2000; the lower range being significantly lower than that claimed for sulfonic acid membranes. The usefulness of these membranes in chlor-alkali cells is taught as being associated with the concentration of the functional groups in the membrane ~eq. wt.), water absorption of the membrane and glass transition temperature of the polymer. The most preferable range for the concentration of the carboxylic acid group in the polymer is given as 1.1 to 1.7 meq./g of dry polymer (about 600 to about 900 eq. wt.). Excellent current efficiencies are obtained with these relatively low eq. wt. carboxylic acid polymers at high caustic concentrations (30-40%), but the voltages reported in the examples are relatively high for the thicknesses reported (200 microns) and the current density of the cells (20A/dm2).
According to the present invention there is now provided in a polymeric composite film of the type having two 28,985-F

layers which differ from each other in equivalent weights by at least 250, each of said layers having a substantially completely fluorinated polymeric backbone with a plurality of pendant groups attached thereto, at least a portion of said pendant groups being a chain of carbon atoms which may be interrupted with one or more oxygen atoms and which terminates with a sulfonyl ion exchange group; wherein the improvement comprises at least one layer having an equivalent weight of less than 1000 and the chain of carbon atoms having from 1 to 3 carbon atoms.
The ion exchange membranes of the invention are made by combining at least wo different films of substantially fluorinated polymers containing ion exchange functional groups.
The present invention, together with those of the aore-mentioned parent Canadian Application Serial Numlier 379,4S4 filed June 10, 1981 by The Dow Chemical Company and its divisional Canadian Application Serial Number 443,988 filed December 21, 1983 by The Dow Chemical Company will now be further described.
Several criteria, aside from the criteria of cell performance, must be considered as to whether polymers qualify as membranes in electrolytic cells. When the polymers are used as films, which are conveniently made by melt extrusion, or the like, on a commercial scale, the physical and chemical properties of the film must withstand the en-vironment of the cell. This severely ~'~? 28,985-F (Div. C) -7-~ lZQ3509 restricts the materials usef~l in the harsh environment of a chlor-alkali cell. The cell is divided by the membrane into two compartments, an anolyte compartment, wherein chlorine gas is produced and evolved from an anode; and a catholyte compartment wherein caustic is produced at a cathode. These cells normally operate at temperatures-of from about 70C up to temperatures of about 100C and are expected to continuously operate at these conditions for many months and even years. This chemical environment of strong, hot caustic on one side and a highly oxidative environment on the other virtually eliminates the use of most organic polymers as membranes.
The constant churning of gas being evolved through the liquid electrolyte solutions in the cell severely limits the physical properties that a film must have in order to meet the lifetime requirements of the cell.
It is known to physically support polymer films on such materials as polytetrafluoroethylene scrim to aid in meeting the life requirements, but even then, the film must be physically sound to a large degree. Any holes or tears that develop in the film lead to contamination of the caustic product in the catholyte with salt from the anolyte and even worse, can lead to explosive mixtures of hydrogen in chlorine when cathodes are used that produce hydrogen along with attendant production of chlorine on the anode.

It is known in the art that fluoropolymers, in general, meet the chemical requirements of the chlor-alkali cell. These fluoropolymers can be sub-stituted with other halogen atoms such as chlorine orbromine that are not reactive in the cell environment, but, although contrary to some teachings, these poly-mers should not contain hydrogen atoms on carbons that 28,985-F

1203509 ~-make up the main polymer backbone. Carbon-hydrogen bonds are chemically attacked by both oxidation from the anolyte components and caustic in the catholyte.
Chemical attack on the polymer backbone can lead to reduced molecular weight by carbon-carbon bond cleavage and thus lead to severe damage to the physical properties of the membrane.

Physical properties of a polymer are de-lo pendent on polymer structure. A highly crystalline fluoropolymer made from simple, unsubstituted monomers .. ... ~ .
such as tetrafluoroethylene is tough, but has extremely high melting or softening temperatures. Fabr.cation is difficult or nearly impossible by simple techniques such as melt extrusion. Homopolymers of long chain, terminal fluorocarbon olefins which result in polymers having many pendant qroups are difficult to prepare because they have a relatively unreactive olefin site and when formed are often low molecular weight, waxy, amorphous solids having little, if any, plastic quality.
Materials of this nature are useless as membranes.
Copolymers of the two type monomers described above often have properties, better than the homopolymers.
Copolymers of tetrafluoroethylene and perfluoroalkyl 2S vinyl ethers (US Patent 3,896,179) have excellent physical properties and can be conveniently melt fabricated into films. Thus, polymers with a limited number of pendant groups can maintain most of the favorable physical charactexistics of the parent (no long pendant groups) polymer and also lend themselves to simple fabrication. The physical strength of a polymer depends on both the number of pendant groups and the size or number of atoms and arrangement of atoms (generally carbon and oxygen in the chain) that make up the pendant 28,985-F

~ 1203S09 ---group. Thus, the commercial, composite membranes of the prior art are based on sulfonyl containing copolymers of tetrafluoroethylene and FSO2CF2CF2OCFCF2OCF = CF2 The membranes are made by laminating a thin layer of 1500 eq. wt. polymer onto a thicker layer of 1100 eq.
lo wt. polymer which lends mechanical strength while adding little electrical resistance (see G. E. ~ull).
Decreasing the equivalent weight of the thicker support layer would result in somewhat lower electrical resis-tance, but, because of the added number of pendant groups, would decrease the structural support needed for the thin, higher eq. wt. layer. Sulfonyl containing polymers having shorter pendant groups than those of the prior art have excellent physical properties and cell performance characteristics at eq. wts. considera~ly lower than those of the prior art.

The eq. wt and the hydration per functional group of a polymer used as a membrane in a chlor-alkali cell have a direct influence on both of the quantities, 2S voltage and current efficiency, that determine the overall efficiency at which a cell operates. The water of hydration per functional group, in effect, determines the nature and the size of the paths through which ions must travel to pass through the membrane. Excessive hydration allows more ions to penetrate into the membrane.
Penetration of the membrane by hydroxide ion leads to loss in current efficiency. Excessive hydration leads to transport of hydroxide from catholyte to anolyte and thus a loss in current efficiency. Equivalent weight 28,985-F

Q3509 ` ~`

determines the number of sites available to transport the sodium ions from the anolyte to the catholyte. At a given applied current to the cell, a specific number of ions must be transported for cell operation. Lower eq. wt. means a larger number of sites for transport and thus a lower electrical potential is re~uired to drive the ions.

Sulfonic acid membranes of the prior art which have long pendant chains separating,the polymer backbone from the functional group, hydrate to such a .. . . ..
large degree that equivalent weights of as low as 1100 to 1200 are not practically useable as barrier layers in chlor-alkali cells. Sulfonic acid polymers having shorter pendant groups hydrate less per functional group at given eg. wt. than do the polymers of the prior art. Exemplary, composite sulfonic acid membranes in the present invention are copolymers of tetrafluoro-ethylene and the monomer FSO2CF2CF20CF = CF2 as well as terpolymers of the above two monomers and of the general structure ROCF = CF2 where R is a straight or branched substantially fluorinated alkyl chain which may be interruptea by oxygen atoms. Polymers formed from combinations of the above monomers hydrate less at a given equivalent weight and perform superior to the sulfonic acid polymers of the prior art in chlor-alkali cells. Thus a 1240 equivalent weight, short pendant chain polymer of the invention operates at equal or better current efficiency than a 1500 equivalent weight polymer of the prior art and has lower ele,ctrical resistance per unit thickness. A laminate of the above 1240 equivalent weight polymer onto a 1100 equivalent weight polymer of the prior art surprisingly operates in a chlor-alkali cell superior to a laminate of a 1500 28,985-F , eq. wt. polymer of the prior art onto the same 1100 equivalent weight film even though the equivalent weight difference is only 140 as opposed to the minimum difference of 250 and the preferred difference of 400 taught in United States Patent 3,909,378.
In another example of the present invention, a composite membrane formed by laminating a film of the same 1240 equivalent weight material as above onto an 860 eq. wt. copolymer of tetrafluoroethylene and FSO2CF2CF2qCF=CF2~and then hydrolyzing to obtain the sulfonic acid salt was shown to be superior to the composite 1500 eq. wt. onto 1100 eq. wt. membrane of the prior art. The material had excellent physical strength and gave equal or better current efficiency and better cell vol-tage on a unit thickness basis than the composite membrane of the prior art. This was surprising since United States Patent 3,gO9,378 teaches that the low eq. wt. layer should have an eq. wt. of at least 1000.
The main feature of this composite, sulfonic acid membrane is the fact that one layer of the membrane has an equivalent weight of less than 1000 .
The composite sulfonic acid membranes of the present inven-tion have (1) a barrier layer, the layer facing the catholyte, that has a lower water of hydration per functional group than the second layer,

(2) should not have an eq. wt. exceeding about 1300, (3) the eq. wt.
difference between the two layers cannot be less than about 250 and the eq. wt. of the second layer can be less than 1000 but preferably not less than about 750. A preferred embodiment is where the second layer has an eq. wt. of not more than 1300 and does not exceed one-third of 28,985-F

the total thickness of the composite membrane. A more preferred embodiment is where the minimum possible equivalent weight is used for both layers while still preserving sufficient mechanical properties and cell performance. In this embodiment, the second layer has an equivalent weight in the range of 800 to about 1000 and the first layer, the barrier layer, has an equivalent weight of from about 1100 to about 1300. It is entirely within the scope of the present invention to add mechanical support to the membrane by introducing a third material in the form of a ibrous mat or a woven fabric or scrim. When support is added it is preferred that the support material be incorporated in the second film or layer of the composite membrane.
From the standpoint of manufacture, it is parti-cularly convenient to make composite membranes as opposed to single film membranes wherein one face of the membrane is chemically modified to produce a barrier stratum such as in U.S. Patent Nos. 3,784,399, 4,085,071 and 4,151,053. Chemical reactions on polymers are difficult especially when careful control of the depth and extent of reaction is necessary on a polymer film. In addition to the normal kinetic characteristics of the particular reaction involved, diffusion rates of the reactants into the polymer structure must also be considered and in many cases is the controlling factor. Production of reproducible membranes by this technique requires careful control and is subject to errors that can result in irre-t~ievable loss of expensive polymer ma~erials. Production . of films from polymers that already have the desired functional groups can be done by standard and well known methods such as melt extrusion. Composite membranes can be made by either forming two films and laminating these together or can be formed by co-extrusion of the two layers.

28,985-F (Div. C) -13-12(}3S09 Included in the scope of the present invention is combining two films, one of which has had one surface chemically converted from sulfonyl to carboxylic acid or derivative. The side opposite the carboxylic acid fun~tion, which still contains sulfonyl function, is laminated to the second film containing sulfonyl func-tionality. Also included in the scope is combining two sulfonyl functional films and then chemically converting all or part of the sulfonyl functional groups in the first film to carboxylic acid functional groups. The ;
carbo~ylic acid surface of the composite faces the catholyte in the operating cell. In these embodiments the equivalent weight of the first film is less than, equal to, or no more than 150 higher than the equivalent weight of the second film. While these techniques do have the disadvantage of reguiring careful control to accomplish the chemical conversion reproducibly, the ~ first does not suffer the full disadvantage since only a limited amount of material, the material for the thin first layer, is subject to loss. These techniques can be advantageous when polymers containing the two different, sulfonic acid and carboxylic acid, unctional groups are not readily available. Otherwise, the technique of combininq the two, separate films (the carboxylic acid functional polymer and the sulfonic acid functional polymer films) to form the composite membrane is the preferable method.

In the composite membranes of the present invention, the b?rrier.laYer or stratum preferably has a lower water of hydration per functional group than does the second layer. Water of hydration per functional group is determined by boiling a dry polymer film in water or thirty minutes and measuring, by weighing, 28,985-F -14-"
~' lZ03509 the "Standard Water Absorption" and from this value calculating the moles of water absorbed per equivalent weight of polymer (W. G. F. Grot, et al, Perfluorinated Ion Exchange Membranes, 141st National Meeting, The Electrochemical Society, Houston, Texas, May, 1972).
In each embodiment of the membranes of the present invention, the maximum limit in eguivalent weight for the barrier layer is lower than the maximum limits set out in the prior art. Only when the eq. wt. of at least one o the layers has a value less than 1000 can the eq. wt. difference exceed 150 ExamPle 1 A terpolymer film having an eguivalent weight of 1240 and a thickness of 8 mil was prepared by poly-merizing tetrafluoroethylene, FSO2CF2CF20CF=CF2 and ClCF2CF2CF=CF2 and then hydrolyzing to the sodium sulfonate form using caustic in alcohol. The ratio of the latter two monomers was 8:1. The membrane was converted to the acid form by soaking in dilute hydro-chloric acid, dried and then soaked for 30 minutes at 25C in a 30 weight perecent solution of triethanol-amine in water. The membrane was then air dried and tested in a small electrolytic cell. The cell had an anode and a cathode with the ion exchange membrane sandwiched therebetween, thus separating the cell into an anode chambor and a cathode chamber. Each electrode had a sguare shape and had an area of 8.63 sguare inches (56 cm2). Each electrode had a solid, metal stud welded to it. Each stud passed through a wall of the cell and was provided with leak proof seals. Both studs were connected to a power supply. The stud connected to the anode was constructed of titanium, while the stud connected to the cathode was constructed 28,985-F -15- -of steel. The anode, itself, was an expanded titanium mesh screen coated with a Ruo2-Tio2 mixture, while the cathode was constructed from woven steel wires.
s The anode chamber was illed with a saturated NaC1 brine solution (approximately 25 weight percent NaCl) and the catholyte chamber was filled with a , caustic solution having approximately the same NaO~
concentration as the intended cell operation produced.
The cell was energized by applying a constant current of approximately 8.63 amps, to give a current density of 1.0 amps per square inch of electrode area. A
saturated brine solution (appoximately 25 weigh~ per-cent NaCl) was flowed into the anode chamber at a rate sufficient to maintain the concentration of the anolyte leaving the cell at approximately 17-20 weight percent NaCl. Deionized water was flowed into the catholyte chamber, in a similar manner, at a rate sufficient to maintain the catholyte leaving the cell at a desired NaO~ concentration. During the evaluation of each membrane, the NaOH concentration was varied in order to determine the cell operaticn over a range of caustic concentrations.
The temperature of the cell was controlle~d throughout each evaluation at about 80C by means of an immersion heater connected to a thermocouple inserted into the anolyte chamber. During the evaluation of each membrane the cell voltage was constantly monitored 30 by measuring the difference in vol~age potential between the anode stud and the cathode stud. For each evaluation, the cell was operated for several days to reach equilibrium. Then current efficiency was determined by collecting the catholyte leaving the cell 28,985-F -16-lZ03509 for a given period of time, usually 16 hours, and determining the amount of NaO~ actually produced, as compared to the amount theoretically produced at the applied current. The membrane operated in the above manner at 3.31 volts at 12% caustic at a current efficiency of 91.3%. The voltage at 20% caustic was

3.25 and the current efficiency 82.6% and at 32%
~ caustic the voltage was 3.30 and the current efficiency 73.7%.
The water absorption was determined for the membrane by first drying the membrane film in the S03H
form for 16 hours at 110C, weighing the sample boiling the sample for 30 minutes in water, blotting the surface dry with towels and then reweighing the film. The difference in weight represented the amount of water absorbed by the film and is commonly referred to as the "Standard Water Absorption". The water absorption per functional group was then determined by calculating the moles of water that one eguivalent of the polymer absorbed. In this manner the hydration of the membrane was determined to be 13.8 moles of water per sulfonate equivalent.

Exam~le 2 A 3.5 mil thick film of the polymer of Example 1 in the sulfonyl fluoride orm (-502 F~ was thermally lami-nated onto a second film having a thickness of 7 mils, an equivalent weight of 860 and prepared by copoly-merizing tetrafluoroethylene and FS02CF2CF20CF=CF2.
The composite ilm was then converted to the acid form by hydrolysis in base and neutralization with acid.
The film was then evaluated as described in Example 1 with the 1240 equivalent weight layer facing the 28,985-F -17-catholyte. The cell operated from 3.07 to 3.09 volts over a caustic strength range of from 12~o to about 20%
caustic. The current efficiency was essentially the same as in Example l. The 860 equivalent weight second film, in the acid form, was determined to have a hydration of 23.9 moles of water per equivalent of functional group.

ComParative Example 2 .. 10 . A composite membrane of the prior art^com-posed of a first film 1.0 mil thick and having an equivalent weight of 1500 and a hydration o about 15 moles of water per sulfonic acid unctional grollp and a second film 5.0 mil thick and having an eguivalent weight of 1100 and a hydration of about 22 moles of water per sulfonic acid equivalent^was evaluated as in Example l. The cell voltage was about 3.1 volts over a range of i2 to 20% caustic and the current efficiency varied from 89.5% at 12X NaOH to 80% at 20% NaO~. This membrane was about equal in voltage to the membrane of Example 2 even though the barrier layer thickness was only 28% as great~ Clearly the membrane of Example 2 is superior in voltage at comparable thicknesses, and in current eficiency at comparable caustic concen-2S tration. .

ExamPle 3 A composite membrane iS prepared by lami-nating the sulfonyl fluoride form of a 3.5 mil thick fi~m of the polymer of ~xample 1 to a 4 mil film having an llOO eguivalent weight and being the same polymer as the second layer of the composite membrane described in Comparative Example 2. The membrane operated in the cell of Example 1 at a voltage essentially the same as 28,985-F -18-that of the cell in comparative Example 2 even though the thickness was greater and at a current efficiency better than comparative Example 2 and egual to that of Example 2.

Exam~le 4 A composite film is prepared by laminating a 2 mil film of an 820 equivalent weigh~ copolymer of tetrafluoroethylene and C~300C(CF2)30CF=CF2 onto a -- support layer the same as the second film of Example 2.
The composite fil~ is then con~erted to the salt form .
by hydrolysis in aqueous alcoholic base. Evaluation of the film in a cell, with the carboxylic acid face towards the cathode, demonstrates that the membrane operates at about the same efficiency as a film made of the carboxylic acld polymer alone, but at a substan-tially lower voltage than an equal thickness of the carboxylic acid polymer. The current efficiency is about 90% when the catholyte contains 35% caustic. The composite film has excellent ~echanical properties.

Exam~Ie S
A composite membrane was prepared by thermally laminating a 3 mil film of a 770 equivalent weight polymer made from the monomers in Example 1 to a 6.5 mil film of a 1000 equivalent weight polymer made from the monomers in Example 2. The composite film was then hydrolyzed from the S02F form to the S03Na form using caustic in a boiling water-alcohol mixture. The film was then converted to the acid form by soaking in dilution of HCl, washed with water and then dried overnight at 110C in a vacuum oven. The film was then converted to the 502Cl form by boiling, at reflux, for 20 hours in a 1:1 mixture of phosphorus pentachloride 28, 985-F -19-` 12Q3509 and phosphorus oxychloride. The slde of the membrane having the low equivalent weight (770) was then con-verted to carboxylic acid functionality using 57%
hydroiodic acid at 80C as described in U.S. patent

4,151,053. The film was then hyd~olyzed using caustic in a èthanol-water mixture, converted to the acid form, dried and evaluated, with the carboxylic acid surface facing the cathode, in the cell described in Example 1.
The cell operated at a voltage from 3.06 to 3.35 at caustic strengths-varying from 2S to 35% NaO~. The current efficiency was 82% at 3S% NaOH and the caustic solution contained 55 ppm sodium chloride.

Com~arative Example S
The 770 equivalent weight film of Example S
was hydrolyzed to.the S02~a form usinq caustic in water and alcohol, then converted to the acid form, dried and evaluated as described in Example 1. The current efficiency was 79% at 9.5Z NaOH and the caustic solution contained 4000 ppm sodium chloride.

28,985-F -20-

Claims (3)

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

1. A polymeric composite film of the type having two layers which differ from each other in equivalent weights by at least 250, each of said layers substantially completely fluorinated polymeric backbone with a plurality of pendant groups attached thereto, at least a portion of said pendant groups being a chain of carbon atoms which may be interrupted with one or more oxygen atoms and which terminates with a sulfonyl ion exchange group; wherein the improvement comprises at least one layer having an equivalent weight of less than 1000 and the chain of carbon atoms having from 1 to 3 carbon atoms.

2. The film of Claim 1 wherein an oxygen atom connects the chain of carbon atoms to the backbone.

3. An electrolytic cell comprising an anode in an anode compartment and a cathode in a cathode compartment separated by an ion exchange membrane comprised of the film of Claim 1 wherein the first layer of the film has the higher equivalent weight and faces the cathode compartment.