Method for forming polymer composite films using removable substrates

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A METHO~ FOR FORMING POLYMER COMPOSITE FILMS USING REMOVABLE SUBSTRATES

The invention resides in a method for forming polymer composite films using a removable substrate and particularly for forming ion exchange active composite membranes using a removable substrate.

Ion exchange active fluoropolymer films have been widely used in industry, particularly as ion exchange membranes in chlor-alkali cells. Such mem- branes are made from fluorinated polymers having sites convertible to ion exchange active groups on pendant groups on the polymeric backbone.

Such po~ymers are usually thermoplastic and may be fabricated into films or sheets while in their molten form using mechanical extrusion equipment.

However, such equipment~is operated in the temperature regio~n near the crystalline meIting point of th-e polymer, which is commonly near the decomposition temperature of some of the polymiers. Thus, decomposition may be a problem when some polymers are formed into films by conventional methods. Likewise, it is difficult to 20 make~such polymers into films thinner than about 10 . microns using such techniques. In addition, it is

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difficult to make films of consistent thickness. It would therefore be highly desirable to be able to make thin films having a consistent thickness.

Forming membrane structures and supp~rt structures into multiple layers is the subject of several patents and applications including U.S. Patent

Nos. 3,925,135; 3,909,378; 3,770,567; and 4,341,605.

However, these methods use complicated procedures and equipment including such things as vacuum manifolds, rolls and release media.

Prior art methods for fabricating films from perfluorinated polymers have been limited by the sol- ubility of the polymers and the temperature-dependent viscosity_shear rate behavior of the polymers. To overcome these characteristics of perfluorinated carboxylic ester polymers, workers have tried to swell the polymers using various types of swelling agents and to reduce the fabrication temperatures of the polymers to practical ranges by extraction. Extractions methods -20 have been taught in, for example, U.S. Patent No. 4,360,601. There, low molecular weight oligomers were removed from carboxylic ester polymers. Polymer "fluff" was extracted in a Soxhlet device at atmos- pheric pressure for 24 hours (see Examples 1 and 3 of

U.S. Patent No. 4,360,601). Such treatments has been found to make some fluorinated carboxylic-ester copoly- mers more processible and operate more efficiently in a chlor-alkali cell when in a hydrolyzed form. Such extractions modify the fabricated polymer axticle, for example, by forminy a grease of the polymer as shown in

Example 3 of U.S. Patent No. 4,360,601.

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In addition, such extractions seern to lower processing temperatures of carboxylic ester polymers after isolation. Isolation means s~paration from the polymerization latex by conventional methods of deacti~ vating the surfactant such as freezing, heating, shear- ing, salting out or pF~ adjustment.

British Patent No. 1,286,859 teaches tha-t highly pol~ar organic "solvents" dissolve small amounts of fluorinated vinyl ether/tetrafluoroethylene copolymer in its thermoplastic form. Thermoplastic form means the polymer is in a form which can be molded or pro- cessed above some transition ternperature (such as the glass transition temperature or the melting point) without altering its chemical structure or composition.

The patent teaches the use of "solvents" including butanoI, ethanol, N,N-dimethylacetamide, and

N,N-dimethylaniline.

Similar approaches have been used to swell membranes in their ionic forms. Ionic forms of-mem- branes are membranes which have been converted fromtheir thermoplastic form (-SO2F or -COOCH3) to their ionic forms (-SO3M or -COOM) where M is H , K , Na , or

NH4 or other metal ion.

Prior art workers have used highly polar solvents or mixtures of solvents on substan-tially perflùorinated polymers and less polar solvents on fluorinated polymers containing hydrocarbon components as co--monomers, ter-monomers or crosslinking agents.

However, each of the prior art me-thads for swel-ling, dispersing or extracting the polyme~s has

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certain shortcomings which are known to those prac- ticing the art. Polar solvents have the potential for water absorption or reactivity with the functional groups during subsequent fabrication operations, thus making poor coatings; films, etc. High boiling sol- vents are difficult to remove and frequently exhibit toxic or flammability properties. Functional form (ionic forms) of the polymers can react with solvents. (See Analytical ~hem., 1982, Volume 54, pages 1639-1641).

The more polar of the solvents such as methanol, butanol esters, and ketones as used in U.S. Patent No. 3,740,369; British Patent No. 1,286,859; and Chemical

Abstracts 7906856 have high vapor pressures at ambient conditions, which is desirable for solvent removal; however, they tend to absorb water. Their water content is undesirable because it causes problems in producing continuous coatings and films of hydrophobic polymers. In addition, polar solvents frequently leave residues which are incompatible with the polymers.

Also, they frequently leave residues which are reactive during subsequent chemical or thermal operations if they are not subsequently removed.

Another approach taken by the prior art workers to form films from fluoropolymers include the use of high molecular weight "solven-ts" which have been produced by halo~enating vinyl ether monomers. (See

British Patent No. 2,066,824).

:` : The swelling of the functional (ionic) forms of the fluoropolymers by polar or hydrophilic agents has been known for some time. In addition, the solvent:

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solubility parameters were compared to the swelling ~- effect of 1200 equivalent weight Nafion ion exch2nge membrane (available from E. I. DuPont Company) by Yeo at Brookhaven Laboratory (see PolYmer, 1980, Volume 21, page 432)~

The swelling was found to be proportional to two different ranges of the solubility parameter and a calculation was developed for optimizing ratios~of solvent mixtures. Ionic forms of functional fluoro- polymers may be treated in such a manner, however, thesubsequent physical forming or manipulation of the polymers into usable configurations by any thermal operation is limited when the polymers are in the functional forms. In addition, non-ionic forms of polymers treated in this manner are also limited in the thermoplastic processing range by the stability of the functional group bonds.

Other solva-tion methods have used temperatures near the crystalline melting points of the polymers being solvated, thus requiring either high boiling point "solvents" or high pressure vessels to maintain the system in a solid/li~uid state. See Analytical Chem., 1982, Volume 54, pages 1639-1641.

Burrell states the theory of Bagley [J. Paint . 25 Tech., Volume 41, page 49~ (1969)] predicts a.aon-crystal- line polymer will dissolve in a solvent of similar solubility parameter without chemical similarity, association, or any intermolecular force. However, he fails to mention anything about the solubility of polymers demonstrating crystallinity. ~rc~J~ ~ rk

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The invention is a me-thod for forming pol~ner composite films using removable substrates cornprising: (a) forming a first disper~ion of a first perfluorinated polymer containing sites conver'cible to ion exchange groups and a first dispersant having: a boiling point less than 110C; a density of from 1.55 to 2.97 grams per cubic centimeter; and ~ solu- bility parameter of from greater than 7.1 to 8.2 hilde- brands; (b) depositing the first dispersion onto a first removable substrate; (c) removing the first dispersion from the first dispersion, thereby forming a first film; (d) forming a second dispersion of a second perfluorinated polymer containing sites convertible to ion exchange groups and a second dispersant having: a boiling point less than 110C; a density of from 1.55 to 2.97 grams per cubic centimeter; and a solubility parameter of from greater than 7.1 to 8.2 hildebrands; (e) depositing the second dispersion onto a second removable substrate; (f) removing the second dispersant from the second dispersion, thereby forming a second film; (g) bonding the first film to the second film; and (h) removing the first and the second sub strate. . ` Particularly preferred as a first and as a second dispersant is a compound represented by the general formula:

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wherein: X is selected from F, Cl, Br, and I;

X' is selected from Cl, Br, and I;

Y and Z are independently selected frorn H, F,

Cl, Br, I and R'; R' is selected from perfluoroalkyl radicals and chloroperfluoroalkyl radicals having from l to 6 carbon atoms.

The most preferred first and second dis- persant is 1,2-dibromotetrafluoroethane.

Dispersion, as used herein, means a com- position containing a treating agent and a perfluori- nated polymer containing sites convertible to ion exchange groups. The polymer is at least partially dissolved in the dispersant and is dispersed into the dispersant.

The present invention can be used to make ion - exchange composite films suitable for use in electro- lytic cells, fuel cells and gas or liquid per~eationunits.

Non-ionic forms of perfluorinated polymers described in the following U.S. Patents are suitable for use in the present invention: 3,282,875; 3,909,378; 4,025,405; 4,065,366; 4,116,888; 4,123,336; 4,126,588; 4,151,052; 4,~176,215, 4,178,218; 4,192,725; 4,209,635; 4,212,713; 4,251,333; 4,270,996; ~,329,~35; 4,330,654; 4,~337,137; 4,337,211; 4,340,680; 4,357,218; ~,358,412; , :

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4,358,545; 4,417,969; 4,462,877; 4,470,889; and 4,478,695; and 4,320,205. These polymers usually have equivalent weights of from 500 to 2000.

Particularly preferred for the formation of each layer of the composite films of the present inven- tion are copolymers of monomer I with monomer II (as defined below). Optionally, a third type of monomer may be copolymerized with I and II.

The first type of monomer is represented by the general formula:

CF2=CZZ' (I) where: Z and Z' are independently selected from -H, -

Cl, -F, and CF.

The second monomer consists of one or more monomers selected from compounds represented by the general formula:

Y-(CF)a-(CFR~)b-(CFR'f)c-O-[CF(CFX)-CF-O]n-CF=CF2 (II)

where:

Y is selected from -SO2Z, -CN, -COZ and

C~R3f)(R4f)OH;

Z is selected from I, Br, Cl, F, OR, and NRlR2; R is selected from a branched or linear alkyl radical having from 1 to 10 carbon atoms or an aryl radical;

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R3f and R4f are independently selec-ted from perfluoroalkyl radicals having from 1 to 10 carbon atoms;

Rl and R2 are independently selected from H, a`branched or linear alkyl radical having from 1 to 10 carbon atoms and an aryl radical; a is 0-6; b is 0-6; c is 0 or 1; provided a-~b~c is not equal to 0; X is selec-ted from Cl, Br, F and mixtures thereof when n>l; n is 0 to 6; and

Rf and R'f are independently selected from F,

Cl, perfluoroalkyl radicals having from 1 to 10 carbon atoms and fluorochloroalkyl radicals having from 1 to 10 carbon atoms.

Particularly preferred is when Y is -SO2F or -COOCH3; n is 0 or 1; Rf and R'f are F; X is Cl or F; and a+b~c is 2 or 3.

Although the polymers of each layer can have the same or different radicals for Y, the most pre- ferred composite polymer is one where the polymer of one layer has Y as -SO2F and the polymer of the other layer has Y as -COOCH3.

` ~ By composite films we mean film composed of two or more different polymers. These polymers may differ by type or concentration of sites convertible to ion exchange group. These different polymers are dlsposed in layers parallel to the film surface.

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The third and optional monomer sui-table is one or more monomers selected from the compounds repre- sented by the general formula:

Y ~(CF2)al-(cFRf)b~-(cFR'f)cl~o-[cF(c~2xl)-cF2 Oln,-CF-CF2 (III)

where: Y' is selected from F, Cl and Br; a' and b' are independently 0-3; c' is 0 or 1; provided a'+b'+c' is not equal to 0; n' is 0-6;

Rf and R'f are independently selected from

Br, Cl, F, perfluoroalkyl radicals having from 1 to 10 carbon atoms, and chloroperfluoroalkyl radicals having from 1 to 10 carbon atoms; and X' is selected from F, Cl, Br, and mixtures thereof when n'>1.

Conversion of Y to ion exchange groups is well known in the art and consists of reaction with an alkaline solution.

The monomer FSO2CF2CF2OCF=CF2 has a density of about 1.65 grams per cubic centimeter and polytetra- fluoroethylene has a density of about 2.2 grams per cubic centimeter. A copoly~er of this monomer with tetrafluoroethylene would, thus, have a density between the two values.

It has been discovered that certain perhalo- genated dispersant have a surprising effec-t of dispers- ing the polymers, especially when the polymers are in a finely divided state.

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Dispersants sui~cable for use in the present invention should have the following characteristics: a boiling point less than 110C; a density of from 1.55 to 2.97 grams per cubic centimeter; a solubility parameter of from greater thhn 7.1 to 8.2 hildebrands. - -

- It is desirabl~ that the dispersant ha-s a boiling point of from 30C to 110C. The ease of removal of the dispersant and the degree of dispersant removal is important in the producing of various films, coatings and the like, without residual dispersant, hence a reasonable boiling point at atrnospheric pres- sure allows convenient handling at room conditions yet effective dispersant remova] by atmospheric drying or mild warming.

It is desirable that the dispersant has a density of from 1.55 to 2.97 grams per cubic centi- meter. The polymers of the present invention have densities on the order of from 1.55 to 2.2 grams per cubic centimeter. Primarlly, the polymers have densities in the range of from 1.6 to 2.2 grams per cubic centimeter. Dispersants of the present invention will therefore swell dissolve and disperse small par- ticles of this polymer, aided by the suspending effects of t~è similarity in densities. : The prior art did not balance density. They were interested in forming solutions and solutions do not separate.

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Solubility parameters are related to the cohesive energy density of compounds. Calculating solubility parameters is discussed in U.S. Patent No. ~,3~8,310. It is desirable that the dispersant have a solubility parameter of from greater that 7.1 to 8.2 hildebrands. The similarity in cohesive energy densities between the dispersant and the polymer de-termines the likelihood of dissolving, swelling and dispersing the polymer in the dispersant. It is preferable that the dispersant have a Vapor pressure of up to 760 mm Hg at the specified temperature limits at the point of dispersant removal. The dispersant should be conveniently removed without the necessity of higher temperatures or reduced pressures involving extended heating such as would be necessary in cases similar to U.S. Patent No. 3,692,569 or the examples in British Patent No. 2,066,824 in which low pressures (300 mm) had to be employed as well as non-solvents to compensate ~or the higher~boiling points and low vapor pressures of the complex solvents. It has been found that dispersants represented by the following general -formula are particularly desirable when they - also meet the characteristics discussed above (boiling point, density, and solubility parameter):

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wherein: X is selected from F, Cl, Br, and I;

X' is selected from Cl, Br, and I; Y and Z are independently selected from H,

F, Cl, Br, I and R'; R' is selected from perfluoroalkyl radicals and chloroperfluoroalkyl radicals having from 1 to 6 carbon atoms.

The most preferred dispersants are 1,2-dibromo- tetrafluoroethane (commonly known as Freo ~114 B 2)

BrCF2 -CF2Br and 1,2,3 trichlorotrifluoro'ethane (commonly known as

Freon~113): ;

ClF2C-ccl 2F - , Of these two dispersants, 1,2-dibromotetra1uoroethane is the most preferred dispersant. It has a boiling poin~ of about 47.3C, a density of about 2.156 grams per cubic centimeter, and a solubi'lity parameter of about 7.2 hildebrands.

1,2-dibromotetrafluoroethane is though-t to work particularly well because, though not directly `po~lar, it is highly polarizable. Thus, when 1,2-dibro- m~otetrafluoroethane is a'ssociated with a polar molecule, its electron density shifts and causes it to behave as a polar molecule. Yet, when 1,2~dibromotetrafluoroethane is around a non-polar molecule, it behaves as a non~polar dlspersant. Thus, 1,2-dibromotetrafluoroethane tends

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to dissolve the non-polar backbone of polytetra~luoro- ethylene and also the polar, ion-exchange-containing pendant groups. Its solubility parameter is calculated to be from 7.13 to 7.28 hildebrands.

It is surprising that an off-the-shelf, readily-available compound such as 1,2-dibromotetra- . fluoroethane would act as a solvent for the fluoro- polymers described above. It is even more surprising that 1,2-dibromotetrafluoroethane happens to have a boiling point, a density and a solubility parameter such that it is particularly suitable for use as a solvent/dispersant in the present invention.

In practiciny the present invention, the ` polymer may be in any physical form. However, it is preferably in the form of fine particles to speed dissolution and dispersion of the particles into the dispersant. Preferably, the particle size of the polymers is from 0.01 microns to 840 microns. Most preferably, the particle size is less than 250 microns.

To dissolve and disperse the polymer particles into the dispersant, the polymer particles are placed in contact with the dispersant of choice and intimately mixed. The polymer and the dispersant may be mixed by any of several means including; but not limited to, shaking, stirring, milling or ultra sonic means.

Thorough, intimate contact between the polymer and the dispersant are needed for optimum dissolution and dispersion.

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The polymers of -the present invention are dissolved and dispersed into the dispersants at concen- trations ranging from 0.1 to 50 weight percent of polymer to dispersant. At concentrations below 0.1 weight percent, there is insufficient polymer dissolved and dispersed to be effective as a medium for coating of articles or forming films wikhin a reasonable nurnber of repetitive operations. Conversely, at concentra- tions above 50 weight percent there is sufficient polymer present as a separate phase such that viable,coherent films and coatings of uniform structure cannot be formed without particulate agglomerates, etc.

Preferably, the concentration of the polymer in the dispersant is from 0.1 to 20 weight percent.

More preferably, the concentration of the polymer in the dispersant is from 0.3 to lO weight percent. Most preferably, the concentration is from 5 to 15 weight percent.

The dispersion of the polymer into the dis pe~sant can be conducted at room temperature conditions: However, the optimum dispersion effects are best achieved at temperatures from 10C to 50C. At temperatures above 50C the measures for dissolving and dispersing the polymer have to include pressure confinement for the preferred dispersants or method of condensing the dispersants. Conversely, at temperatures below 1-0C many of ~he polymers of the present invention are below their glass transition temperatures thus causing their dispersions to be difficult to form at reasonable conditlons of mixing, stirring, or grinding.

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The dispersion of the polymers of the present invention into the dispersant are best conducted at atmospheric pressure. However, dispersion effects can be achieved at pressures from 760 to 15,000 mm Hg or greater. At pressures below 760 mm Hg, the opera-tion of the apparatus presents no advantage in dissolving - ~ and dispersing po-lymers, rather hindering permeation into the polymers and thus preventing forminy of the dispersions. ~ -

Conversely, pressures above 760 mm Hg aid in ~dissolviny and dispersing polymers very little compared to the difficulty and complexity of the operation.

Experiments have shown that at about 20 a-tmospheres the amount of polymer dissolved and dispersed in the disper- sant is not appreciably greater.

After the polymer dispersions of the presentinvention have been formed, they are fixed to other polymer films or substrates by sintering or compression to fix the polymer from the dispersion to the substrate.

After coating the first dispersion onto the first substrate and coating the second dispersion onto the second substrate, it is possible to contact the dispersions and form them into a fused composite film while maintaining them in co~tact. The two dispersions, when processed in this manner/ will tend to mingle to some extent.

The following methods are suitable for fixing the dispersion of the present invention -to a substrate.

Dlpping the substrate into the dispersion, followed by

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air drying and sintering at the desired temperature with sufficient repetition to build the desired thick- ness. Sprayi.ng the dispersion on-to the substrate is used to advantage for covering large or irregular shapes. Pouring the dispersion onto the substrate is sometimes used. Painting the dispersion ~7ith brush or roller has been successfully employed. In addition, coatings may be easily applied with metering bars, knoves, or rods. Usually, the coatings or films are built up to the thickness desired by repetitive drying and sintering.

The type of substrate upon which the disper sion of the present invention may be applied can include such things as glass, metal sheets or foils such as aluminum foil, polytetrafluoroethylene tape or sheets, or other polymer films or sheets.

The substrate upon which the dispersion is to be deposited is cleaned or treated in such a way as to assure uniform contact with the dispersion. The sub- strate can be cleansed by washing with a degreaser orsimilar solvent followed by drying to remove any dust or oils from objects to be used as substrates. Metals should usually be acid etched, then washed wi~h a solvent to promote adhesion, if desired, unless the metal is new in which case degreasing is sufficient. . After being cleaned, the substrates may be pre-conditioned by heating or vacuum drying prior to contact with the dispersions and the coating operation.

Temperatures and pressures in the following ranges are preferably used: 20 mm Hg at a temperature of about

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110C is sufficient in all cases; however, mild heat is usually adequate, on the order of about 50C at atrnos- pheric pressure.

After preparation, -the substrates are coated with the dispersion by any of several means including, - but not limited to, dipping, spraying, brushing, pouring. Then the dispersion may be evened out using scraping ~ knives, rods, or other suitable means. The dispersion - - can be applied in a single step or in several s-teps depending on the concentration of the polymer in the dispersion and the desired thickness of the coating or film.

Following the application of the dispersion, the dispersan-t is removed by any of several methods including, but not limited to, evaporation or extraction.

Extraction is the use of some agent which selectively dissolves or mi~es with the dispersant but not the polymer.

These removal means should be employed until a uniform deposition of polymer is obtained and a continuous film is formed.

The dispersant removal is typically carried out by maintaining the coated substrate at temperatures ranging from 10C to 110C, with the preferred heating range being from 20C to 100C. The heating tempera ture seIected depends upon the boiling point of the disper~ant.

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Heating temperatures are customarily in -the range of from 20C to 50C for 1,2-dibromotetrafluoro- ethane.

The pressures employed for the removal of the dispersant from the coa-ted substrate can range from 20 mm Hg to 760 mm Hg depending on the nature of the dispersant, although pressures are typically in the ~ range of from 300 mm Hg to 760 mm Hg f~r 1,2-dlbromo- tetrafluoroethane.

The forming of the coating or film can be carried out as part of the polymer deposition and dispersant removal process or as a separate step by adjusting the thermal and pressure conditions associ- ated with the separation of the polymer from the dis- persant. If the dispersion is laid down in successive steps, a continuous film or coating free from pinholes ~ can be formed without any subsequent heating above ambient temperature b~ control of the rate of evapor- ation. This can be done by vapor/liquid equilibrium in a container or an enclosure; theréfore, the dispersant removal step can be mereiy a drying step or a control- led process for forming a coating or film. If the dispersant is removed as by flash evaporation, a film will not form without a separate heating step.

After the dispersant has been removed, the ~ residual palymer and substrate,:as a separate step, is preferably subjected to a heat source of from 150C to 380C for times ranging from 10 seconds to 120 minutes, depending upon the thermoplastic properties of the polymers. The polymers having melt viscosities on the : , ,

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order of 5 x 105 poise at about 300aC at a shear rate of 1 sec. 1 as measured by a typical capillary rheorneter would re~uire the longer times and higher temperatures within the limits of the chemical group stability.

Polymers with viscosities on the order of 1 poise at ambient temperatures would require no further treat- ment.

The most preferred treatment temperatures are from 270C to 350C and a time of from 0.2 to 45 minutes for the most preferred polymers for use in the present invention. Such polymers- form thin continuous films under the conditions described above.

After two polymers have been applied to their respective substrate, they are contacted with each other at a temperature, at a pressure and for a time sufficient ~o bond the two polymers together. Such temperatures are usually from 150C to 380~C. The pressures suitable pressures up to 2000 psi (13,780 kPa). The times are from 10 seconds to 120 minutes.

Thereafter, the removable substrates are removed. A variety of means can be used to remove the substrate including chemically etching the substrate away, vaporizing the substrate, dissolving the sub- strate, peeling the substrate from the film, peeling the film f*om the substrate, and other physical or chemical means.

Composite films of varying layer thicknesses can be easily produced by the methods and means described above. Such films are suitable as membranes, when in ~' ' .. . .

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their ionic forms, for use in electrochemical cells.

They are particularly useful for the electrolysis of sodium chloride brine solutions to produce chlorine gas and sodium hydroxide solutions. Membranes prepared according to the present invention have surprisingly good current efficiencies ~7hen used in chlor-alkali cells. ~ ~

- ~ EX~MPLES

Example 1

A first polymer having an equivalent weight of 850 was prepared according to the following pro- cedure:

784 grams of CF2=CFOCF2CF2SO2F was added to 4700 grams of deoxygenated water containing 25 grams

NH~02CC7Fl5, 18.9 grams o Na2HPO4 7H20, 15.6 grams of NaH2PO4-H2O and 4 grams of (NH4 )2S208 under a positive pressure of 192 psig (1323 kPa) of tetrafluoroethylene at a temperature of 60C for 88 minutes. The reactor was vented under heat and vacuum to remove residual monomers. The reactor contents was frozen, thawed, and vigorously washed to remove residual salts and soap.

30 grams of the first polymer was made into a dispersion using 270 grams of 1,2-dibromotetrafluoro- ethanè. The dispersion was coated onto an aluminum foil and heated to a temperature of 300C for 1 minute.

The coating and heating steps were repeated until a coating having a thickness of 4 mils (102 microns) was achieved.

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A second copolymer of CF2=CE'2 and

CF2=CFOCF2CF2SO2F having an equivalent weight of 1144 was prepared according to the following procedure. 784 grams of CF2=CFOCF2CF'2SO2F was added to a700 grams of deoxygena~ted water containing 25 grams ~rH4o2Cc7Fl5, 18.9 grams of Na2HPO~ 7H2O, 15.6 grams of ~aH2PO~ H20 - and 4 grams of (~4 )2S2~ under a positi-ve pressure of 250 psig (1723 kPa) of tetrafluoroethylene at a -tem- - perature of 60C for 58 minutes. The reactor was vented under heat and vacuum to remove residual monomers. The reactor contents was frozen, thawed, and vigorously washed to remove residual salts and soap

After vacuum drying, a dispersion was prepared by placing 56 grams of polymér prepared above in a laboratory-size single tier 290 revolutions per minute roller Norton Jar Mill with 168 grams of 1,2-dibromo- te-trafluoroethane. The mixture was mixed in the ball mill overnight at ambient temperature and at atmos- pheric pressure.

To the resulting soft paste 300 grams of 1,2-dibromotetrafluoroethane was added and the mill was rolled an additional 3 hours. The resulting dispersion was found to contain 12.5 weight percent polymer. The mixture was coated onto a sheet of alu- minum foil approximately 38 microns thick by dipping the foil which had been formed into an envelope or pocket shape into the dlspersion. The coated aluminum foil was allowed to air dry. Thus, the dispersant evaporated from the dispersion at ambient temperature.

The aluminum foil coated with the second copolymer was then heated at a temperature of 300C in a muffle furnace for 1 minute to fuse the polymer into a more uniorm film form.

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The resulting thin film was found to be a continuous film and had a thickness of 0.5 mils (12.7 microns).

- The dipping and heating process was repeated 5 5 times until a 2.5 mil ~63.5 microns) thick second polymer film was built up.

The two coated foils above were then pressed together, coated side to coated side, at a temperature of 300C and at a pressure of 400 psig (2756 kPa) for 3 minutes. The resulting two layer composite membrane was hydrolyzed in a 25 weight percent aqueous sodium hydroxide solution. It was then run satisfactorily in a chlor-alkali test cell.

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