CA1267759A - Method for forming polymer composite films using a removable substrate - Google Patents

Abstract

ABSTRACT
The invention is a method for forming polymer composite films using a removable substrate by:
(a) forming a first dispersion of a first perfluorinated polymer containing sites convertible to ion exchange groups dispersed in a first dispersant;
(b) depositing the first dispersion onto a removable substrate;
(c) heating the first dispersion at a tem-perature sufficient to form and fuse a first polymer film;
(d) forming a second dispersion of a second perfluorinated polymer containing sites convertible to ion exchange groups and a second dispersant;
(e) depositing the second dispersion onto the first film;

34,261-F

(f) heating the second dispersion for a time and at a temperature sufficient to form and fuse a second polymer film;
(g) bonding the first film to the second film, thereby forming a composite film; and (h) removing the first substrate.

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

34,261-F

Description

A METHOD FOR FORMING POLYMER COMPOSITE -FILMS
USING A REMOVABLE SUBSTRATE

The invention resides in a method for forming polymer composite films using a removable substrate and partic-ularly 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 mem~ranes in chlor-alkali cells. Such mem-branes are made from fluorinated polymers having sites convertible to ion exchange active groups attached to pendant groups on the polymeric backbone.

Such polymers 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 region near the crystalline melting point o the polyme;r, which is commonly near the decomposition temperature of some of the polymers. Thus, decomposition may be a problem when some polymers are formed into films by conventional methods. Likewise, it is difficult to make such polymers into films thinner than about 10 microns using such techniques. In addition, it is 34,261-F -1-~2~i77~

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

Forming membrane structures and support structures into multiple layers is the su~ject of several pate~ts and applications including U.S. Patent Nos. 3,925,135; 3,909,378; 3,770,567; and 4,341,605.
However, these me~hods use-complicated pr~cedures and eguipment including such things as vacuum manifolds, rolls and release media.

Prior art methods for fabricating films from perfluorinated polymers have been limited by the solubility of the polymers.and the temperature--dependent ~iscosity-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 fabricat~on tem-peratures of the polymers to practical ranges by 20; extraction. Extractions methods have been taught in, for example, U.S. Patent No. 4,360,601. There, low mol-ecular weight oligomers were removed from carboxylic ester polymers. Polymer "fluff" was extracted in a Soxhlet device at atmospheric 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 ~opolymers more proces~ible and operate more efficiently in a chlor-alkali cell when in a hydrolyzed form. Such extractions modiy the abri-cated polymer article, for example, by forming a grease of the polymer as shown in Example 3 of U.S. Patent No.
. 4,360,601.

34,261-F -2-~67~i9 In addition, such extractions seem to lower processing _emperatures of carboxylic ester polymers after isolation. Isolation means separation from the polymerization latex by conventional methods of deacti-vating the surfactant such as freezing, heating, shear-ing, salting out or pH adjustment.
,.
British Patent No. 1,286,859 teaches that highly`polar organic "solvents" di~ssolve 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 temperature (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 butanol, 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 from - their 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

2~ solvents or mixtures of solvents on substantially perfluorinated poiymers and less po.LaL soLvents on fluorinatè~ polymers containing hydrocarbon components as co-monomers, ter-monomers or crosslinking agents.

However, each of the prior art methods for swelling, dispersing or extracting the polymers has 34,261-F -3-~2~77~

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 mak-ng 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 Analytlcal Chem., 1982, Volume 54, pages 1639-1641).

The more polar of the solvents such as meth-anol, butanol esters, and ketones as disclosed 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 hydro-phobic polymers. In addition, polar solvents fre-quently lea~e xesidues 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 "solvents" which have been produced by halogenating vinyl ether monomer6. (See British Patent No. 2,(~66,824).

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 34,261-F -4-solubility parameters were compared to the swelling effect of 1200 equivalent weight Nafion ion exchange 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-poIymers may be treated in such a manner, however, the subsequent 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 solvation methods have used temper-atures 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/liquid state. See Analytical Chem., 1982, Volume 54, pages 1639-1641.

Burrell states the theory of Bagley [J. Pa.int 25 Tech., Volume 41, page 495 (1969)1 predicts a norl- .
crystalline polymer will dissolve in a solven~ of similar solubility parameter without chemical simil-arity, association, or any intermolecular force.
However, he fails to mention anything about the solubility of polymers demonstrating crystallinity.

34,261-F -5-The invention is a method for forming polymer composite films using a removable substrate comprising:
(a) ~orming a first dispersion o~ a first perfluorinated polymer containing sites convertible to ion exchange groups and a dispersant, said 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;
(b) depositing the fir~t diqpersion onto a removable substrate;
(c) heating the ~irst dispersion at a tem-perature sufficient to form and fuse a first polymer film;
(d) forming a second dispersion of a second perfluorinated polymer containing sites convertible to ion exchange groups and a second dispersant, said second dispersant having a boiling point less than 110C a denslty o~ ~rom 1.55 to 2.97 ~ram~ per cubic centimeter; and a solubility parameter of from greater than 7.1 to 8.2 hildebrands;
(e) depositing the second dispersion onto the first film;
2~ (f) heating the second dispersion for a time and at a temperature sufficient to form and fuse a second polymer film;
(g) bonding the first film to the second film, thereby forming a oomposite ~ilm; and ~h) r~movlng the substrate.
Particularly preferred as a ~irst and as a second disper~ant is a compound represented by the general formula:

34,261-F -6-XCF2-CYZX ' 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 chloroperfluor~alkyl ~ad~cals-having from 1 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 per-fluorinated polymer containing sites convertible to ion exchange groups. The polymer is at least partially dissol~ed 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 permeationunits.

Non-ionic forms of perfluorinated polymers .. described in the following U.S. Patents ~e sui.tab.Le for use in the present invention: 3,282,875; 3,~0~,378;
4,025,405; 4,065,366; 4,116,8~; 4,123,336; g,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; 4,329,435; ~,330,65~;
4,337,137; 4,337,211; 4,340,680; 4,357,218; 4,358,412;

34,261-F . -7-4,358,545; 4,417,969; 4,462,877; 4,470,889; and 4,478,695; and European Patent Publication 0,027,009.
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 CF3~ .

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

Y-(cF2)a~(cFRf)b-(cFR~f)c-o-[cF(cF2x)-cF2-o]n-cF=cF2 (II) where:
Y is selected from -SO2Z, -CN,. -COZ and C(R3~)(R4E)oH;
Z is selected ~rorn I, Br, C'.L~ E', OR, and NRIR2;
R is selected from a branched or linear alkyl radical having from 1 to 10 carbon atoms and an aryl radical;

34,261-F -8-lX67~5~

R3f and R~f are independently selected 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 selected from Cl, Br, F and mixtures thereof when n>1; .
n is 0 to 6; and Rf and R'f are independently selected from F, 15 C1, 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 dlfferent 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 l~yer has Y as -COOCH3.

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

34,261-F -9_ ~2677~;9 The third and optional monomer suitable is one or more monomers selected from the compounds repre-sented by the general ~ormula:

Y ~(CF2)al-(cFRf)b,-(cFRlf)cl-o-[cF(cF2xl )-CF2-O~n,-CF=CE'2 (III) , where:
- Y' is selected from F, Cl and Br; - -a' and b' are lndependently 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 ~, 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 a polymer of tetrafluoroethylene has a density of about 2.2 grams ,per cubic,centimeter. Alcopo:Lymer o this monomer Wi't}'l tetrafluoroethylene would, thus, have a density between the two vaLues.

It has been discovered that certain perhalo-genated dispersants have a surprising effect of dis-. persing the polymers, especially when the polymers are in a finely divided state.

34,261-F -10-~2677~;9 Dlspersants suitable for use in the present invention should have the following characteristics:
a boiling point less than llO~C:
a density of from 1.55 to 2.97 grams per cubic centimeter;
a solubility parameter of from greater than 7.1 to 8.2 hildebrands.

It is desira~le that the dispersant has 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 atmospheric pressure allows convenient handling at room conditions yet effective dispersant removal by atmospheric drying or mild warming.

It is desirable that the dispersant has a density of from 1.5~ to 2.97 grams per cubic centi-meter. The polymers of the present invention have 2~ densities on the order of from 1.55 to 2.2 grams per cubic centimeter. Primarily, the polymers have densities in the rang.e 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 effectsof the similarity in dens:ities.
, The prior art did not balance density. They were interested in forming solutions and solutions do not separate.

34,261-F

~L2~7759 Solubility paramèters are related to the cohesive energy density of compounds. Calculating solubility parameters is discussed in U.S. Patent No.
4,348,310.

It is desirable that the dispersant has a solubility paramete.r of from greater than 7.1 to 8.2 hildebrands. The similarity in cohesive energy den-sities between the -dispersant and the polymer determine the likelihood of dissolving, swelling and dispersing the polvmer in the dispersant.

It is preferable that the dispersant has 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 pres-sures involving extended heating such as would be ~ecessary in cases similar to U.S. Patent ~o. 3,692,569 or the examples in British Patent No. 2,066,82g in which low pressures ~300 mm) had to be employed as well as non-solvents to compensate for the higher boiling points and low vapor pressures of the complex solvents.

It has been found that dispersants repre-sented by the following general formula are par-ticularly preferred provided they also meet the charac~eristics discussed above (boi:Lirlg po;int, density, and sol ubi1ity parclme ker):

XCF2-CYZ-X ' 34,261-F -12-~2677~;~

wherein:
X is selected from F, C1, 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 Freon 114 B 2) BrCF 2 - CF 2 Br and 1,2,3-trichlorotrifluoroethane (commonly known as Freon 113):

ClF2C-CCl 2F

Of these two dispersants, 1,2-dibromotetrafluoroethane is the most preferred dispersant. ~t has a boiling point of about 47.3C, a density of about 2.156 grams per cubic centimeter, and a solubility parameter of about 7.2 hildebrands.

1,2-dibromotetrafluoroethane is thought to work particularly well because, though not directly polar, it is highly polarizable. Thus, ~hen 1,2-dibro-motetrafluoroethane is associated with a polar molecule, itæ electron dens.ity shifts and cause~ lt to behave as a polar molecule. Yet, when 1,2-dibromotetrafluoro-ethane is around a non-polar molecule, it behaves as a non-polar dispersant. Thus, 1,2-dibromotetrafluoro-ethane tends to dissolve the non-polar backbone of 34,261-F -13-~7~

polytetrafluoroethylene and also the polar, ion--exchange-containing pendant groups. Its solubility parameter is calculated to ~e from 7.13 to 7.28 hilde-brands.

It is surprising that an off-the-shelf, readily-available compound such as 1,2-dibromote-tra-fluoroethane would act as a solvent for the fluoro-- polymers described above. It is even more surpr~si~g 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 practicing the present invention, the polymer may be in any physical form. However, it is preferably in the form of fine particles to speed dissolution a~d 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 par-ticles 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, mil.Ling OE llltra son:ic means. Thorough, i.ntimate con-tact betweell the polymer and the dispersant are needed for optimum dissolution and dispersion.

34,261-F -14--15~

The polymers of the present invention are dissolved and dispersed into the dispersants at concen-trati~s ranging from O.l to 50 weight percent of polymer to dispersant. At c~ncentrations below 0.1 weight percent, there is insufficient polymer dissolved and dispersed to be effective as a medium for coating of articles or forming filrns within a reasonable number of repetitive operations. Conversely, at concentra-tions a~ove 50 weight percent there is sufficient polymer present as a separate phase such that viable, coherent films and coatings of uniform structure cannot be ormed without particulate agglomerates, etc.

Preferably, the concentration of the polymer in the dispersant is from 0.1 to 20 weight percent.
More preferably, t~e concentration of the polymer i~
the dispersant is from 0.3 to 10 weight percent. Most preera~1y, the concentration is from 5 to 15 weight percent.

The dispersion of the polymer into the dis-persant can be conducted at room temperature con~itions.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 o~ condensing the dispersants. Conversely, at ~temper~tures below l0C
many of the polymers of t~e presenk invention are below their glass transition temperatures thus causing their dispersions to be difficult to form at reasonable conditions of mixing, stirring, or grinding.

34,261-F -15-~267759 The dispersion of the polymers of the present invention into the dispersant are ~est conducted at atmospheric pressure~ However, dispers1on efects ca~
be achieved at pressures from 760 to 15, 000 mm Hg or greater. At presslres below 760 mm Hg, the operation of the apparatus presents no advantage in dissolving and dispersing polymers, rather hindering permeation~into the polymers and thus preventing forming of the disper-slons .

Conversely, pressures above 760 mm Hg aid in dissolving and disp.ersing polymers very little compared to the difficulty and complexity of the operation.
Experiments have shown that at about 20 atmospheres the amount of polymer dissolved and di~persed in the dis-persant is not apprecia~ly greater.

After the polymer first dispersion of ~he present invention has been formed, it is fixed to a substrate by sintering or compression to fix the polymer from the dispersion to the substrate.

2~ The following methods are suitable for fixing the dispersion of the present invention to a substrate.
Dipping the substrate into the dispersion, followed by air drying and sintering at the desired temperature with sufficient repetition to build the desired thick-nesa. Spraying the dispersion onto the substrate is used to advantage for co~ering large or irregular shapes. Pouring the dispersion onto the substrate is sometimes used. Painting the dispersion with brush or roller has been successfully employed. In addition, coatings may be easily applied with metering bars, knives, or rods. Usually, the coatings or fil~s. are 34,261-F -16-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 ~e applied carl include such things as glass, metal sheets or foil such as al~mi~num foil, polytetrafluoroethylene tape, or sheets, or other polymer films or sheets.

The subs,trate upon which the dispersion is to be deposited is cleaned or treated in such ~ way as to assure uniform contact with the dispersion. The sub-strate can be cleansed by washing with a degreaser or similar 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 with a solvent to promote adhesion, if desired, unless the metal is new in which case degreasing is sufficier~t.

After being cleaned, the substrates may be pre-conditioned by heating or vacuum drying prior to contact with the dispersions and the coating oper~ation.
Temperatures and pressures in the following ranges are preferably used: 20 mm Hg at a temperature of 110C or is sufficient in all cases; however, mild heat is usually adequate, on the order of 50~C at atmospheric pressure.

.
~ter 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 usiny scraping knives, rods, or other suitable means. The dispersion can be applied in a single step or in several steps 34,261-F -17-~2~77~i9 ..
dependlng on the concentration of the polymer in the dispersion and the desired thickness of the coating or film.

Following the applicatlon of the dispersion, the dispersant is removed by any of se~eral methods includ~ng,-but not limited to, evaporatio~ or extrac- -tion. Extraction is the use of some agent which selec-tiv-ely dissolves or mixes with the dispersant but not -the polymer.

These removal means should be employed until a uniform depositi~n 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 temper-ature selected depends upon the boiling point of the dispersant.

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 coated subs.trate 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 for 1,2-dibromo-tetrafluoroethane.

34,261-F -18-:

~X6~7~;!3 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 adiusting the thermal and pressure conditions associ-ated with the separation oE the polymer from the dis-pèrsant. rf 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 by control of the rate of evaporation. This can be done by vapor/liquid equilibrium in a container or an enclosure; therefore, the dispersant removal step can be merely a drying step or a controlled 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 polymer and substrate, as a separate step, i~ preferably subjected to a heat source of from 150C to 380C for times ran~ing from 10 seconds to 120 minutes, depending upon the thermoplastic properties of the polymer.
Polymers having a melt viscosity on the order of 5 x 105 poise at a temperature of 300C at a shear rate of l sec.~l as measured by a typical capillary rheometer would require the longer times and higher temperatures within the limits of the chemical group stability.
Polymers with viscosities on the order of l poise at ambient temperatures would require no further treatment.
The mo~t preferred treatment temperatures are from 270CC 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.

34,261-F -l9-, ~;, .~.~.~

~L26,77~9 64693-3834 JEP

After the polymer from the dispersion has been fixed to its substrate, the polymer layer is contacted with a second polymer dispersion formed in the same manner used to form the first dispersion. Thereafter, the second dispersion is fused to form the second film and to bond the second film to the first film. The second film is formed and the two films are fused together by heating the two films at a temperature, at a pressure and for a time su ficient to bond the two polymers together. Such temperatures are usually from 1~0 to 380~C. Pressures up to 2000 psi (13,780 kPa) are usually employed. The time period is generally from 10 seconds to 120 minutes.
Thereafter, the removable substrate should be removed. A variety of means can be used to remove the substrate including chemically etching the substrate away, vapori2ing the s~bstrate, dissolving the sub-strate, peeling the substrate from the film, peeling thefilm from the substrate, and other physical or chemical means.
Composite films of varying thicknesses can be easily produced by the methods and means described above. Such films are suitable as membranes, when in their ionic forms, for use in electrochemical cells.
They are particularly useful for the electrolysis of sodium chloride brine solutions to produce chlorine ga~
and sodium hydroxide solutions. Membranes prepared according to the present invention have surprisingly good current eficiencies when used in chlor-alkali cells.

34,261-F -20-. ~
. .

~2677~i9 64693-3834 JEP

Steps f and g in the Summary of the Invention can be alternately accomplished in one c~ordinated operation, rather than separately. That is, the second film may be formed and fused at the same time it is being fused to the first film.
EXAMPLES
Example 1 A copolymer of C~=CF and CF=CFOCFCFCOOCH was prepared as followed:
50 9 of CF=CFOCFCFCOOCH was added to 250 g of deoxygenated water containing 3 grams of NH4OCC7Fl5, 1.5 grams of NaHPO4"7HO and 1.0 gram of NaHPO4:HO in a glass reactor with stirring at 800 rpm. Next, 50 ml of deoxygenated water containing 0O05 (NH4)SO8 was injected into the reactor and the reactor was kept under a positive pressure of 220 psig (1516 kPaJ using a tetrafluoroethylene gas at a temperature of 50C for 180 minutes. The reactor was vented and the contents was acidified with 50 ml 6N HCl to coagulate the latex and cause the polymer to separate from the emulsion. The polymer was filtered, vigorously washed to remove inorganics, soap and residual monomers and then vacuum dried for 16 hours at a temperature of 85C. The dried polymer weighed 99.2 grams and upon titration wa~ found to be 856 equivalent weight.
A di~persion of the 856 equivalent weight carboxylic ester polymer was made by mixing 49 grams of the polymer with 304 grams of 1,2-dibromotetrafluoro-ethane.

34,261-F -21-1~6775~

A poly~.er having an equivalent weight of 850 was prepared according to the following procedure:

784 grams of CF2=CFOCF2CF2S02F was added to 4700 grams of deoxygenated water-containing 25 grams NH402CC7F1 5, 18.9 grams of Na2HPO~ 7H20, 15.6 grams of NaH2P04-H20 and 4 grams of (NH4 )2S208 under a positive pressure of 192 psig (1323 kPa) of tetrafluoroethylene ~ at a temperature of 60~C 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 polymer was made into a dispersion using 270 grams of 1,2-dibromotetrafluoro-ethane. The dispersion was coated onto an aluminumfoiL 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.

A piece of aluminum foil was coated with a dispersion of the 856 EW carboxylic ester copolymer.
The dispersant was allowed to air dry and the coated foil was fused for 1 minute at a temperature of 250C
between polytetrafluoroethylene coated glass cloth 25 sh~eets. The process was ~epeated to bu:i:ld a 1 mil thick (25.4 ~I) film. 'rhe caLbo~ylic ester copolymer film on the foil was then coated in a like manner with an 850 equivalent weight fluorosulfonyl copolymer dispersion to build a fluorosulfonyl copolymer film until the total film thickness of the two combined films was 5 mils (12.7 I~) The aoated foll was placed 34,261-F -22-., lZ6~77~;~

polymer side down on top of a sized polytetrafluoro-ethylene fa~ric (Prodesco Inc. 12 x 12 leno weave cloth~ which was in turn placed on a vacuum table. The vacuum was applied and the tahle was placed under a heated platen for 4 minutes at a temperature of 250C.
The polytetrafluoroethylene fa~ric was firmly bonded to the support layer polymer.

34,261-F -23-

Claims (17)

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

1. A method for forming polymer composite films using a removable substrate comprising the steps of:
(a) forming a first dispersion of a first perfluorinated polymer containing sites convertible to ion exchange groups and a dispersant, said dispersant having a boiling point less than 110°C; 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;
(b) depositing the first dispersion onto a removable substrate;
(c) heating the first dispersion at a temper-ature sufficient to form and fuse a first polymer film;
(d) forming a second dispersion of a second perfluorinated polymer containing sites convertible to ion exchange groups and a second dispersant, said second dispersant having a boiling point less than 110°C; 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 the first film;
(f) heating the second dispersion for a time and at a temperature sufficient to form and fuse a second polymer film;

34,261-F -24-(g) bonding the first film to the second film; thereby forming a composite film; and (h) removing the substrate.

2. The method of Claim 1 wherein the first and the second perfluorinated polymers are indepen-dently selected from copolymers formed from a first monomer and a second monomer:
wherein the monomer is represented by the general formula:
CF2=CZZ' (I) where:
Z and Z' are independently selected from -H, -C1, -F, and CF3; and the second monomer is represented by the general formula:

Y-(CF2)a-(CFRf)b-(CFR'f)c-O-[CF(CF2X)-CF2-O]n-CF=CF2 (II) where:
Y is selected from -SO2Z, -CN, -COZ and C(R3f)(R4f)OH;
Z is selected from I, Br, C1, F, OR, and NR1R2;
R is a branched or linear alkyl radical having from 1 to 10 carbon atoms or an aryl radlcal;
R3f and R4f are independently selected from perfluoroalkyl radicals having from 1 to 10 carbon atoms;
R1 and R2 are independently selected from H, a branched or linear alkyl radical having from 1 to 10 carbon atoms-and an aryl radical;

34,261-F -25-a is 0-6;
b is 0-6;
c is 0 or 1;
provided a+b+c is not equal to 0;
x is selected from C1, Br, F and mixtures thereof when n>1;
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.

3. The method of Claim 2 including a third monomer represented by the general formula:

Y' -(CF2)a,-(CFRf)b,-(CFR'f)C,-o-[CF(CF2X')-CF2-O]n,-CF=CF2 (III) where:
Y' is selected from F, C1 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, C1, 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, C1, Br, and mixtures thereof when n'>1.

34,261-F -26-

4. The method of Claim, 1 wherein the boil-ing point of the first and the second dispersant is from 30°C to 110°C.

5. The method of Claim 1 wherein the den-sity of the first and the second dispersant is from 1.55 to 2.2 grams per cubic centimeter.

6. The method of Claim 1 wherein the solubil-ity parameter of the first and second dispersant is from greater than 7.1 to 7.5 hildebrands.

7. The method of Claim 1 wherein the den-sity of the first and second dispersant and the density of the first and second polymer are both from 1.55 to 2.2 grams per cubic centimeter.

8. The method of Claim 1 wherein the first and the second dispersants are independently represented by the general formula:

XCF2-CYZX ' wherein:
X is selected from F, C1, Br, and I;
X' is selected from C1, Br, and I;
Y and Z are independently selected from H, F, C1, Br, I and R'; and R' is selected from perfluoroalkyl radicals and chloroperfluoroalkyl radicals having from 1 to 6 carbon atoms.

34,261-F -27-

9. The method of Claim 8 wherein X and X' are C1 or Br.

10. The method of Claim 1 wherein the first and the second polymers are present in the first and the second dispersions at a concentration of from 0.1 to 50 weight percent.

11. The method of Claim 1 wherein the first and the second polymers are present in the first and the second dispersions at a concentration of from 0.3 to 30 weight percent.

12. The method of Claim 1 wherein the remov-able substrate is aluminum.

13. The method of Claim 1 wherein the sub-strate is removed by dissolving with a solvent for the substrate.

14. The method of Claim 1 wherein the sub-strate is removed by an alkaline solution.

15. The method of Claim 1 including heating the coated substrate to a temperature of up to about 300°C to fuse the first film to the second film.

16. A composite film produced by the method of Claim 1.

17. An electrolytic cell comprising an anode and a cathode separated by a composite film, wherein the film is the film of Claim 16.

34,261-F -28-