CA1287712C

CA1287712C - Method for forming polymer films using a removable substrate

Info

Publication number: CA1287712C
Application number: CA000510623A
Authority: CA
Inventors: Jeffrey D. Birdwell
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1985-05-31
Filing date: 1986-06-02
Publication date: 1991-08-20
Anticipated expiration: 2008-08-20
Also published as: US4610762A

Abstract

ABSTRACT

The invention is a method for forming a polymer film using a removable substrate comprising the steps of:
(a) forming a dispersion of a perfluori-nated polymer containing sites convertible to ion exchange groups dispersed in a dispersant having: a boiling point less than 110°C; a density of from 1.55 to 2.2; and a solubility parameter of from greater than 7.1 to 8.2 hildebrands;
(b) depositing the dispersion onto the removable substrate;
(c) removing the dispersant from the disper-sion; and (d) removing the substrate.

Optionally, the removable substrate is provided with a roughened surface.

Particularly preferred as a dispersant is a compound represented by the general formula:

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 chloroperfluoroalkyl radicals having from 1 to 6 carbon atoms.

The most preferred dispersant is 1,2-dibromo-tetrafluoroethane.

Description

~LZ~7'~

A METHOD FOR FORMING POLYMER FILMS `
USING A REMOVABLE SUBSTRATE

The invention resides in a method for forming a polymer film using a removable substrate and parti-cularly for forming an ion exchange active membrane using a removable substrate.

Ion exchange active fluoropolymer films or sheets have been widely used in industry, particularly as ion exchange membranes in chlor-alkali cells. Such membranes are made from fluorinated polymers having 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. How ever; such e~uipment is operated in the ~emperature region near the crystalline melting point of the polymer, 34,250A-F -1-.;.
..~, which is commonly near the decomposition temperature of some of the polvmers. Thus, decomposition may be a prob~
lem when some polymers are formed in-to films by conven-tional methods. Likewise, lt is difficult to make such polymers into films thinner than about 10 microns using such techni~ues. In addition, it is difficult to make films of consistent thickness. Accordingly, i-t would be highly desirable ~o be able to make thin films having a consistent~thickness.

Forming membrane structures and support 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 includiny 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 viscosity-shear rate behavior of the polymers. To overcome these characteristics of per-fluorinated carboxylic ester polymers, workers have tried -to swell the polymers usiny various types of swelling agents and to reduce the fabrication temper-atures of the polymers to practical ranges by extraction.Extrac-tions methods have been ta~g~t in, for example;
U.S. Pa-tent No. 4,360,601. There, low molecular 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 have 34,250A-F -2-;,~

been found to make some fluorinated carboxylic es-ter copolymers more processible and operate more effi-ciently in a chlor-alkali cell when in a hy~rolyzed form. Such extractions modify the fabricated polymer article, for example, by forming grease of che polymer as shown in Example 3 of U.S. Pa-tent No. 4,360,601.

In addition, such extractions seem to lower processing temperatures of carboxylic ester polymers after isolation. Isolation means separation from the polymerization latex by conventional methods of deactivating the surfactant such as freezing, heating, shearing, salting out or pH adjustment.

British Patent No. 1,286,859 teaches that highly polar organic "solvents" dissoive small amounts of a fluorinated vinyl ether/tetrafluoroethylene copolymer in its thermoplastic form. Thermoplastic form means the polymer is in a form which can be molded or processed above some transition temperature (such as the glass transition temperature or the melting point) without altering its chemical s-tructure 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 25. membranes in their ionic forms. Ionic forms of membranes are membranes which have been converted from their thermoplas-tic form (-SO2F or -COOCH3) to their ionic forms (-SO3M or -COOM) where M is H , K , Na , or N~I4 or o-ther metal ion.

34,250A-F -3-~287~l2 Prior art workers have used highly polar solvents or mixtures of solvents on substantially perfluorina-ted polymers and less polar solvents on fluorinated polymers con-taining hydrocarbon com-ponent as co-monomers, ter-monomers or crosslinking agents.
~ , . . .
However, each of the prior art methods for swelling,` dispersing or`extracting the polymers has certain ~hortcomings which are known to those practicing 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 solvents are difficult to remove and frequently exhibit toxic or flammability properties. Functional form (ionic forms) of the polymers can react with solvents. ~See Analytical Chem., 1982, Volume 54, pages 1639-1641).

The more polar of the solvents such as methanol, 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 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.

34,250A-F -4-~nother approach taken by the prior art workers to form films from ~luoropolymers include the use o~
hlgh molecular weight "solvents" which have been produced by halogenating vinyl ether monomers. (See British Patent No. 2,066.824).
The swelling of the functional (icnic) forms of the fluoropolymers by polar or hydrophilic agents has been known for some time. In addition, the solvent 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 fluoropolymers 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 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/liquid state. See ~nalytical Chem., 1982, Volume 54, pages 1639-1641.

*Trade-mark 34,250A-F -5-7~:

Burrell states the theory of Bagley [J. P _ t Tech., Volume 41, page ~95 (1969)] predicts a non -crystalline 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.
It has been found that ion exchange membranes frequently operate more efficiently if the surface is roughened. This is especially true when such membranes are used in processes where a gas is generated adjacent to the membrane. Roughened membranes release the gas from its surface and do not cause gas blinding of the membrane. However, the preparation of such roughened membrane surfaces is difficult. A simple method for producing roughened membranes would be highly desirable.
It has also been found that ion exchange membranes frequently operate more efficiently if the surface has electrically inactive particles embedded into its surface. Such particles cause gas bubbles to be released from the surface of the membrane. However, the prior art method for producing such membranes is not entirely satisfactory because such membranes require specialized materials and additional process steps to the finished ~embrane.

U.S. Patent No. 4,457,815 discloses a permionic membrane having a porous film or surface and electro-catalyst on at least one surface thereof. Also disclosed is an electrolytic cell with the permionic 34,250A-F -6-~ 2 membrane, an electrolytic process utilizing the permionic membrane, and an electrolytic process utilizing electrolytic cell.
.

U.S. Patent No. 4,349,422 discloses a process and cell for electrolysis of alkali metal halides, especially sodium chloride, wherein the anolyte and the catholyte compartments are separated by a fluorinated ion exchange membrane whose surface facing the catholyte compartment is of a polymer having carboxylic functionality and which has a roughened surface which does not exceed 1.5 microns. Such a cell and process operate at high current efficiency, low voltage and low power consumption.
U.S. Patent No~ ~,468,301 discloses a process for the electrolysis of an aqueous alkali metal chloride in a cell which comprises an anode and a cathode which are partitioned by an ion exchange membrane having at least one roughened surface. The electrodes are spaced apart in non-contacting relationship from the membrane surface up to distance of no more than 2 millimeter~s.

U.S. Patent No. 4,539,084 discloses an unreinforced ion exchange membrane which comprises fluorinated polymer which has carboxylic functional groups, which has a hydrogen bubble release layer on the cathode side thereof, and which has a hydrogen bubble release layer on the cathode side thereof, and which has channels open to the outer surface of the anode-facing side thereof. Precursor membrane 7 which may contain partially embedded sacrificial members, and from which the unreinforced ion exchange membrane is made, is also disclosed. The 34,250A-F -7-~2~773~2 unreinforced ion exchange membrane can be used to separa-te the compartments of a chloro-alkali cell, and such a cell operates at low voltage, high curren-t efficiency, and low power consumption.

A method for quickly and easily manufactur-ing ion exchange membranes having a bubble release layer would be highly desirable. It is also a pur-pose of this inventl'~n to prov'ide such a method.

More particularly, the invention resides in a method for forming a polymer film using a removable substrate comprising -the steps of:
(a) forming a dlspersion of a perfluori-nated polymer containing sites convertible to ion exchange groups and a dispersant having: a boiling point of 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 dispersion onto a removable substrate;
(c) removing the dispersant from'the dis-perslon;
(d) removing the substrate.

Particularly preferred is a dispersant having the general formula:
.. ' . .
XCF2-CYZX' wherein:
X is selected from F, Cl, Br, and I;
X' is selected from Cl, Br, and I;

34,250A-F -8-~2~7~
g and Z are independently selec-ted from ~, F, Cl, Cr, I and R';
P' is selected from perfluoroalkvl radicals and chloroperfluoroalkyl radicals having from 1 to 6 carbon atoms.

The most preerred dispersant is 1,2-dibromo-tetrafluoroethane.
.
D.ispersion, as used herein, means a compo-sition containing a treating agent and a perfluori-nated polymer containing sites convertible to ionexchange groups. The polymer is at least par-tially dissolved in the treating agent and is dispersed in the treating agent.

The present invention can be used to make ion exchange media, films and articles for use in electroly-tic cells, fuel cells and gas or liquid permeation uni-ts.

The method of the present invention also forms an ion exchange membrane having a surface of a controlled roughness by a unique process which is easier, more direct, and more easily controlled than prior art methods. The present process and the article are useful in making novel membranes for ion exchange, osmosis and o~ther such applications.
The membranes are particularly useful for ion exchange membranes for chlor-alkali electrolytic cells.

34,250A-F -9-7~

The present invention may also be used to form superior solid polymer electrolyte membranes or fuel cell membranes with enhanced transport phenomena and enhanced bonding to the conductive and catalytic layers, i.e. the replicated surface of the film having the ~ine orders of roughness allowing better adhesion to the catalyst and conductive particles. This allows more int;mate contact than the smooth surface Prom ordinary ~abrication.
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;
15 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; 4,330,654;
4,337,137; 4,337,211; 4,340,680; 4,357,218; 4,358,412;
4,358,545; 4,417,969; 4,462,877; 4,470,889; and 20 4,478,695; European Patent Publication No. 0,027,009.
These polymers usually have equivalent weights of from 500 to 2000.
Particularly preferred are copolymers of 25 monomer I with monomer II (as defined below).
Optionally, a third monomer may be copolymerized with monomers I and II.
The first monomer is represented by the general formula:
CF2=CZZ' (I) 34,250A-F -10-'7:~2 where:
Z and Z' are independen-tly selected frorn -H, -Cl, -F, and CF3.

The second monomer consists of one or more monomers sel~cted 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(R f) (R f)OH;
Z is selected from I, Br, Cl, F, OR, and NRlR2 i R is selected from a branched or linear alkyl radical having from 1 to 10 carbon atoms and an aryl radical;
R3f and R4f 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 Erom Cl, Br, F and mix-tures thereof when n>l;

34,250A-F -11-~.
.

~Zfi~

n is 0 to 6; and R~ and R'f are independently selected ~xom F, Cl, perfluoroalkyl radicals having from 1 to 10 carbon atoms and fluorochloroalkyl radicals having from 1 to 10 carbon a-toms.

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

The third and optional monomer suitable is one or more monomers selected from -the compounds represented by the general formula (CF ) (CFR ) -(CFR' f) ,-O-[CF(CF2X ) CF2 ]n 2 whe.re:
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 radi.cals having from 1 to 10 carbon a-toms, and chloroperfluoro-alkyl radicals having from':L to 10 carbon atoms; and X' is selected from F, Cl, Br, and mixtures thereof when n'~l.

Conversion of Y -to ion exchange groups is well known ln the art and consists of reaction with an alkaline solu-tion.

34,250A-F -12-~:87~2 The monomer FS02CF2CF20CF=CF2 has a density of about 1.65 grams per cubic centimeter and poly-tetrafluoroethylene has a densit~ of abou-t 2.2 grams per cubic centimeter. A copolymer of this monomer with tetrafluoroethylene would -thus, have a densi~ty between the two values.
.
It has been discovered -that certain per-halogenated dispersants have a surprising effect of dispersing the polymers, especially when the polymers are in a finely divided state.

Dispersants suitable for use in -the present invention should have the following characteristics: -a boiling point less than about 110C;
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.

I-t is desirable 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 i~ the producing of various films, coatings and the like, without residual dispersant; hence a reasonable boiling point at atmospheric pressure allows convenient handling a-t ambient temperature yet effective dispersant removal 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 centimeter. The polymers of the present invention 34,250A-F -13-, . .i, 7~
1~-have densities on the order of from 1.55 to 2.2 grams per cubic centime-ter. Primarily, the polymers have densities in the range of fxom 1.6 to 2.2 grams per cubic centimeter. Dispersants of the present invention will therefore swell, dissolve and disperse small particles of this polymer, ai~ed by the suspending effects of the similarity in densi-ties.
..
In the prior art, there was no recognition and thus no attempt made to balance density. The prior art was only interested in forming solutions, and solu-tions do not separate.

Solubility parameters are related to the cohesive energy density of compounds. Calcu-lating solubility parameters is discussed in U.S~Patent No. 4,348,310.

It is impor-tant that the dispersant has a solubility parameter of from greater than 7.1 to 8.2 hildebrands. The similarity in cohesive energy densities between the dispersant and the polymer determine the likelihood of dissolving, swelling and dispersing the polymer in the dispersant.

It is preferable that the dispersan-t has a vapor pressure of up to about 760 millimeters of mercury at the specified temperature limits at the point of dispersant removal. The dispersan-t should be conveniently removed wi-thout the necessity of higher temperatures of reduced pressures involving extended heating such as would be necessary in cases similar 30 to U.S Paten-t No. 3,692,569 or the examples in -34,250A-F -14-,. ,.i British Patent No. 2,066,824 in which low pressures (300 millimeters) had to be employed as well as non-solvents to compensate for the higher boiling points and low vapor pressures of the comple~ solvents.

It has been found that dispersants represented by the following general formula are particularly preferred provided they also meet the characteristics discussed above (boiling point, density, and solubility parameter):

XCF2-CYZ-X ' 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 FreonTM 114 B 2) BrCF2-CF2Br and 1,2,3-trichlorotrifluoroethane (commonly known as FreonT~ 113):
ClF2C CC12F

34,250A-F -15-Z

Of these two dispersants, 1,2-dibromotetrafluoroethane is the most preferred dispersant. It 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, when 1,2-dibromotetra-fluoroethane is associated with a polar molecule, itselectron density shifts and causes it to behave as a polar molecule. Yet, when 1,2-dibromotetrafluoroethane is in contact with a non-polar molecule, it behaves as a non-polar dispersant. Thus, 1,2-dibromotetrafluoro-ethane tends to dissolve the non-polar backbone of polytetrafluoroethylene 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 dispersant for the fluoropolymers 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 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 and dispersion of the particles into the 34,250A-F -16-~17-dispersant. Preferably, the par-ticle size of -the polymers is from 0.01 microns -to 840 mircrons. Most preferably, -the particle size is less than 250 microns.

To dissolve and di~perse the polymer par-ticles into the dispersant, the polymer particles are placed in contact~wih the dispersant of choice and intimately mixed. The polymer and the dis-persant may be mixed by any of several mea~s including-ing, but not limited to, shaking, stirring, milling or ultrasonic means.

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

The polymers of the presen-t invention are dissolved and dispersed into the dispersants at concentrations ranging from 0.1 -to 50 weight percent of polymer to disperant. At concentrations below 0.1 weight percent, there is insufficient polymer dissolved and dispersed to be eEfective as a medium for coating of articles or forming films within a reasonable number of repetitive operations. Conversely, at concentrations above 50 ~eight 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 34,250A-F - -17-,' -18~

the dispersant is from 0.3 to 10 weight percent. Most preferably, the concentration is from 5 to ~5 weight percent.

The dispersion of the polymer into the dispersant can be conducted a-t room tempera-ture conditions. However, the optimum dispersion effe~ts are best achieved at temperatures from 10C to 50C.
At temperatures above 50C the measures for dissolv-ing and dispersing the polymer have to include pres-sure confinement for the preferred dispersants ormethod of condensing.the dispersants. Conversely, at temperatures below 10C many of the polymers of the present invention are below their glass transi-tion temperatures thus causing their dispersions to be difficult to form at reasonable conditions of mixing, stirring, or grinding.

The dispersion of the polymers of -the present invention into the dispersant are best conducted at atmospheric pressure. However, dis-persion effects can be achieved at pressures from - 760 to 15,000 mm Hg or greater. At pressures 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 dispersions.
.
Conversely, pressures above 760 ~m Hg aid is dissolving and dispersing polymers very little compared to the difficulty and complexity of the operation. Experiments have shown that at a pressure of about 20 atmospheres the amount of polymer .

34,250A-F -18-~;. , ~L~8~12 dissolved and dispersed in the dlspersant is not appreciably greater.

After the polymer dispersions of the present invention have been formed, they may be fixed to other polymer Eilms or substrates by sintering or compre~sion to fix the polymer from the aispersion to the substrate.

The follo~ing methods are suitable for fixing the dispersion of the present invention to a subs-trate. Dipping the substrate into the dispersion, followed by air drying and sintering at the desired temperature with sufficlent repetition to build the desired thickness. Spraying the dispersion onto the substrate is used to advantage for covering 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 films are built up to the thickness desired by repetitive drying and sintering.

The type of substra-te upon which the dis-persion of the present inven-tion may be applied can include such things as glass, polytetrafluoroethylene tapes or sheets, me-tal sheets, or other polymer films or objects.

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.

34,250A-F -19-' ~Z8~712 -20~

The substrate 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 subs-trates.
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 sufficient.-Aft'er~ bein'g cleaned, the 'substrates maybe pre-conditioned ~y heating or vacuum drying prior to contact with the dispersions and the coating operation. Temperatures and pressures in the following ranges are preferably used: about 20 mm Hg at about 110C is sufficient in all cases; however, mild heat is usually adequate when applied at a temperature of 50C at atmospheric pressure.

After preparation, the substrates are coated with the dispersion by any of several means including, but not limited to, dipping, spraying, brushing, or 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 steps depending on the concen-tra-tion of the polymer in the dispersion and the desired thickness of the coating or film.

Following the applica-tion of the dispersion, the dispersant is removed by any of several methods including, but not limi-ted -to, evaporation or extraction.
Extraction is the use of some agent which selectively dissolves or mixes with the dispersant but no-t the poly-mer.

34,250A-F -20-.., ;

~2~

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

The dispersant removal is typically carried out by maintaining the coated subs-trate at temperatures ranging from 10C to 110C,~with the preferred heating range being from 20C to 100C.
The heating temperature selected depends upon the -boiling point of the dispersant.

10Heating temperatures are customarily~in the range of from 20C to 50C for 1,2-dibromotetra-fluoroethane.

The pressures employed for the removal of the dispersant from the coated substrate can range 15from 20 mm Hy 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-dibromotetrafluoroethane.

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 associated with the separation of the polymer from the dispersant. 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 by control of the rate of evaporation. This can be done by vapor/
liquid equilibrium in a container or an enclosure;

34,250A F -21-t ~2~ 12 therefore, the dispersan-t 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 s-tep.

After the dispersant has been removed, the residual polymer 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 proper-ties of the polymers. The polymers having melt vis-cosities on the order of 5 x 105 poise at a temper-ature of 300C at a shear rate of 1 sec. 1 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 vis-cosities on the order of 1 poise at ambient temper-atures would require no further treatment.

- The most preferred treatment temperatures 20 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 con-tinuous films under the conditions described above.

After the dispersan-t has been removed, the -substrate should~be removed. A variety of means can be used to remove the substrate including chemically etching the substrate away, vaporizing the substrate, dissolving the substra-te, peeling the substrate from the film, peeling the film from -the substrate, and other physical or chemical means.

34,250A-F -22-, a 7'7~

Films of varying -thickness 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 gas and sodium hydroxide solutions. Membranes prep~ared accord- -ing to the present invention have surprisingly good curr~nt efficiencies when used in chlor-alkali cells. -EXAMPLES

Example 1 A copolymer of CF2=CF2 and CF2=CFOCF2CF2S02F
having equivalent weight of about 1144 was prepared.
The polymer was prepared according to the following procedure. About 784 grams of CF2=CFOCF2S02F was added to about 4700 grams of deoxygenated water containing 25 grams NH4O2CC7F15, 18.9 grams of Na2HPO4"7H20, 15.6 grams of Na2HPO4"H2O, and 4 grams of (NH4)2S2O8 llnder a positive pressure of 250 psig (1722 kPa) of tetra-fluoroethylene at a temperature 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 pre-25 pared by placing 56 grams of polymer prepared above ina laboratory-size single tier 290 revolutions per minute roller Norton Jar Mill wi-th 168 grams of 1,2~dibromotetrafluoroethane. The mix-ture was mixed in -the ball mill overnight at ambient -temperature and at atmospheric pressure.

34,250A-F -23-, ...

To the resulting soft paste about 300 additional grams of 1,2-dibromotetrafluoroethane was added and the mill was rolled an additionàl

3 hours. The resulting dispersion was found to contain 12.5 weight percent polymer. The mix-ture was coated onto a sheet of aluminum foil having a thickening of 38 microns by dipping~the foil into the dispersion. The coated aluminum foil was allowed - to air dry. Thus, the dispersant evaporated from -the dispersion at ambient temperature.

The coated aluminum foil was then heated to a temperature of 300C in a muffle furnace for 1 minutefsinter the polymer into a more uniform film form.

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

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

Two pieces of aluminum foil which had been coated in the above described manner were pressed together, coated side to coa-ted side, under a pressure of 800 psi (5512 kPa) at a temperature of 595F
(313C) for 4 minutes. The resulting film, with aluminum foil on both sides, was hydrolyzed for 16 hours in a 25 weight percent sodium hydroxide aqueous solution at a tempera-ture of 90C. This trea-tment dissolved the aluminum foil and lef-t only -the two 34,250A-F -24-.
.~ .
'4r" .

7~;~

layer film. The two layer film was tested in a chlor-alkali membrane cell. The cell was operated at a temperature of 89C at a current density o~ 2 amps/in2 (0.31 amps/cm2) of electrode surface areas with a 3 millimeter (3000 micron) gap between the anode and cathode. A cathode having an electrocatalyst on its surface was used. Cell voltage was 3.11 volts at 12.9 weight percen-t sodium hydroxide concentration being produ~ed in the cathode chamber. The caustic current efficiency was found to be 92.2 percent. The caustic produced in the catholyte chamber was analy~ed and found to contain 1030 parts per million sodium chlor-ide. The total energy consumed for the production of one metric ton of sodium hydroxide was calculated to be 2259 kilowatt hours.

Example 2 A copolymer of CF2+CF2 and CF2=CFOCF2CF2CO3CH3 was prepared having an equivalent weight of about 847.
About 50 grams of CF2=CFOcF2cF2cO3cH3 was added to 20 about 300 grams of deoxygenated wa-ter con-taining 3.0 grams of NH402CC7F15, 1.5 grams of Na2HPO4 7H20, 1.0 gram NaFi2P04~H20, and 0.20 gram (NH4)2S208 under a positive pressure of tetrafluoroethylene of 250 psig pressure at 50C for 180 minutes in a glass reactor.
The reactor was vented and the reac-tor contents was acidified with abou-t 6 normal HCl to coagulate the polymer. The coagulam was filtered out, vigorously washed and vacuum dried.

About 35 grams of the polymer was ground and mixed overnight in about 315 grams of 1,2-dibromo-tetrafluoroethane in the laboratory jar mill described in Example 1.

34,250A-F -25-,", ~Z~ii7~7~2 The dispersant was analyzed and found to contain about 10 weight percent solids. The dispersion was used to coat a shee-t of aluminum foil having a thickness of about 38 microns b~ dipping the foil, allowing the coating to air dry and sintering the coating of 250F (482C) for about 1 minute in the muffle furnace described in Exampl~e 1.

~ This coating procedure was repeated until a series of coated foils had been made in which the coating thickness varied. Two dips on the various films resulted in film thicknesses of the sintered coating of from 0.8 to 1.8 mils (17.8 to 48.6 microns).

The coated foils were then pressed onto an 850 equivalent weight fluorosulfonyl copolymer films which was 4 mils (101.6 microns) thick. The 850 equivalent weight polymer was prepared as the previous fluorosulfonyl copolymer example except for using a pressure of 192 psig and a run time of 88 minutes. The dried polymer was extruded at 500F (260C) to 550F
(288C) using a Haake Rheomex 254 three-fourths inch vented 25:1 length/diameter 316 stainless steel screw extruder and a six inch die. With a 20 mil (508 microns) die gap, the film was drawn down to about ~-5 mils (101 to 127 microns) thickness and ~uenched on an unheated 316 stainless s-teel roll. The cast film samples were cleaned by deg~easing with acetone and air dried. The coa-ted side of -the foil was placed against the cast film and the two were placed between two sheets of polytetrafluoroethylene coated glass cloth. These were then placed between two photographic plates. The entire sandwich was compressed at 250C in a hydraulic hot press using 20 tons force for five minutes.

34,250A-F -26-", j, ,~

7~2 The combinations were hydrolyzed in a 25 welght percent sodium hydroxide aqueous solution at 90C for 16 hours. This treatment dissolved the aluminum foil from each combination. The resulting two layer films were tested in a chlor-alkali test cell. The cell has an exposed electrode surface of about 8.65 square inch with a titanium anode compartment and a PlexiglassTM cathode compartment. The anode was a ruthenium oxide coated expanded metal electrode. A cathode having an electrocatalyst on its surface was used. Brine containing about 20 weight percent sodium chloride was introduced into the anode compartment and water was added to the cathode compartment as the direct current was passed through the electrodes at about 2 amps per square inch of electrode surface area. The membrane was disposed between the electrodes and bolted between the two cell halves with gas exits and overflows from each half.
The data on these films is in Table I.
TABLE I
Sample No. 1 2 ~ of Dippings 2 5 25 Thickness of coatingso.8 1.8 (mils) Thickness of Pressed 0.20.4-0.6 Loading (mils) Caustic Current 95.6 96.7 30 Efficiency (%) Voltage 3.22 3.33 % NaOH 34.7 33.4 Energy (kwh/metric 2256 2307 ton NaOH

34,250A-F -27-The caustic current efficiency is determinecl ~s the moles of caustic per Faraday~ of current -times 100. That is, the number of moles of caus-tic which were produced in a test period, divided by the -time in seconds times the current over the test period, all dlvided by 96,520 coulombs per ec~uivalent IFaraday).
The resultan-t decimal fraction represents the propor--tion of electrons that produced NaOH. This fraction t.imes 100 gives the caustic current efficiency.

The above data was taken after about 13 days of oeration and essentially unchanged after about 90 days operation.

Example 3 A copolymer of CF2=CF2 and CF2=CFOCF2CF2SO2F
having an equivalent weight of about 1160. The polymerwas prepared according to the following procedure.
About 50 grams of CF2=CFOCF2CF2SO2F was added to about 300 milliliters of deoxygena-ted water containing 3 gram5-N~I4C02C7F15, 1-5 ~rams of Na2HP04-7H20, 1 gram of NaH2PO4-H2O, and 0.1 grams of (NH4)2S2O8 under a posi-tive pressure of 245 pounds per square inch guage (psig) of tetrafluoroethylene at 60C for 75 minutes in~
a glass reactor. The reac-tor was vented under heat and acidified to coagulate the latex. The coagulated polymer was washed repeatedly to remove inorganics clnd soap. The polymer was vacuum dried for 16 hours at 110C.

About 30 grams of the fluorosulfonyl copolymer was ground with about 270 grams of 1,2-dibromotetra-fluoroethane in a lab motar and pestle un-til a viscous dispersion was produced.

34,250A-F -28-- ~.i,f 3Q~Y ~
~;.~ ~ 71~

This dispersion was used to coat a sheet of aluminum foil having a thickness o~ about 38.1 microns.
The coatings was allowed to air dry. Then the coated foil was pressed between glass reinforced polytetra-~luoroethylene backing sheets which were held betweenphotographic plates in a heated press. The pressure was 2000 pounds per square inch gauge and the temperature was 595F (313C). The time was 4 minutes and 20 seconds. Thereafter, the backing sheet was removed from the press and a thin polymeric film remained on the foil.
A second copolymer was prepared. It was a copolymer of CF2=CF2 and CF2=CFOCF2CF2S02F having an equivalent weight of about 974. The polymer was prepared according to the following procedure. About 784 grams of CF2=CFOCF2CF2S02F was added to 470 grams of deoxygenated water containing 25 grams NH402C7F15, about 18.9 grams of Na2HP04 7H20, 15.6 grams of NaH2P04 H20 and 4 grams of (NH4)2S20g under a positive pressure of 220 pounds per square inch guage (psig) of tetrafluoro-ethylene at 60C for 30 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. The film was vacuum dried for 16 hours at a temperature of 85C.
The second film was extruded on a commercially available one inch Killion laboratory extruder with a regular Xaloy"' barrel and screw. The screw was a standard type commonly used to extrude polyethylene.
Blown film was made with a 1-1/4 inch die with a 20 m;l 34,250A-F -29-~21~

(508 micron) gap heated to 550~C using no cooliny ring.
The extruder was operated at 450-550F (232 to 288C) and 20-40 revolutions per minute. The hauloff (a mechanical device to roll up -the film) opera-ted at 1-2 feet per minute. Various thic~nesses of blown film were produced as desired by varying speeds and blowing of the bubble.

A 5 miL (127 microns) extruded polymeric film of fluorosulfonyl copolymer was placed against the coated side of the foil and pressed as above except only 670 pounds per square inch gauge pressure was used. This two layer film was hydrolyzed in 25 weight percent sodium hydroxide aqueous solution for 16 hours at 90C. The foil was etched away in this process.
The resulting two layer film was placed in a test cell (the same cell described in Example 3) with the 83P019 polymer facing the cathode compartment. After 2 days of operating the following results were obtained.

Cell vol-tage was found to be 3.02 volts and the caustic current efficiency was 91.5 percent at a caustic concentration of about 12.56 weight percent.
The sodium chloride concentration in the caustic was analyzed and found -to be about 940 parts per million.
The cell energy was calculated to be abou-t 2211 kilo-watt hours per metric ton of caustic.

Example 4 A copolymer of CF2=CF2 and CF2=CFOCF2CF2C03CH3 was prepared having an equivalent weight of 755. About 50 grams of CF2=CFOCF2CF2C03CEI3 was added to 300 grams 30 of deoxygenated water containing 3.0 grams of NH~02CC7Fl 5, 34,250A-F -30-1.5 grams of Na2HPO~ 7~2O, 1.0 gram NaH2PO~ ~I2O, and 0.10 gram (NH~)2S2O8 under a positive pressure of tetrafluoroethylene of 235 psig (1619 kPa) pressure at a -temperature of 50C for 5 hours in a glass reac-tor.
The reactor was ven-ted and the reactor contents was acidified with 6 normal HCl to coacJulate the latex.
- The coagulam was filtered and washed vigorously to remove inorganics and soap. The polymer was vacuum ~ - dried for 16 hours at a temperature of 85C. -About 15 grams of the polymer prepared above was ground in a lab mortar and pestle with 135 grams of 1,2-dibromotetrafluoroethane to produce a viscous dispersion. The dispersion was used to coat a sheet of aluminum foil which was 38.1 microns thick. The coated foil was pressed in a heated hydraulic press at a pressure of 2000 pounds per square inch pressure at a temperature of 540F (282C) for 4 minutes and 20 seconds between glass reinforced polytetrafluoroethyl-ene backing sheets.

The backing shee-ts were removed from the first polymer film and the coated side of the foil placed against a 5 mil (127 micron) thick film of the second polymer film of Example 3. The pressing opera-tion was repeated using 670 pounds per square inch gauge to attach the first film to the second film. The resul~ing two-layer film was hydroly.zed in a 25 weight percent sodium hydroxide aqueous solution for 16 hours at a -temperature of 90C. The aluminum foil dissolved in this process. The two-layer film was mounted in a test cell with the 755 equivalent weigh-t polymer facing the cathode compartment.

34,250A-F -31-:' ;..

. . .

~2~

After 190 days o~ operation in a chlor-alkali test cell as described i~ E~ample 2 to produce chlorine gas and NaOH ~rom the elec-trolysis of a NaCl brine, produced a 33 weight percent NaOH in an aqueous solu-tion at a 95.6 percent caustic curren-t efficiency and 3.38 volts.

In the preparation of a surface roughened membrane, the surface roughness can be formed by almost any surface treating means including air blasting such things as alumina, sand, zirconium oxide or the like;
belt sanders; oscillating wire brush; chemical e-tching or other well known techniques.

The roughening process for the substrate is carried out by either spraying a surface or chemically etching a surface to the desired degree or roughness.
The roughness of the polymer film Imembrane surface) is determined by the conformation of the polymer to the roughened surface.

The roughening process may embed particles into the roughened substrate. These particles may be left in the substrate and, when later used to form polymer films, may become a part of -the film. This provides a simple means for forming membranes having bubble release particles in their surface.

25; This can be carried ou-t by the following procedure: The roughened surface is prepared. A
dispersion is applied to the roughened surface. The dispersant is removed leaving the polvmer conforming to the surface. If desired, the polymer and subs-trate can 34,250A-F -32-;;, ~L2~ 7~

be further treated by fusing, sintering or pressing to aid in the forming process. The film is then separated from the removable substrate by well known physical or chemical means.

Example 5 A sheet of aluminum foil having a thickness of 2 mils (50.8 microns) soft drawn is etched with 300 ~ grit alumina to impart an irregular, sharp~y featured surface to the aluminum foil. This left some of the particulates embedded in the foil.

The copolymer of Example 4 was prepared having an equivalent weight of about 755. About 15 grams of the polymer prepared above is ground in a lab mortar and pestle with about 135 grams of 1,2-dibromo-tetrafluoroethane to produce a viscous dispersion. The dispersion is used to coat the sheet of aluminum foil prepared above.

The coating is fused for five minutes at 250C and the coating procedure is repeated a suf-ficient number of -times to accumulate a 0.8 (23.2 micronsj thick coating on the foil.

The coated side of the foil is placed against a 4 mil (101.6 microns) thick film of an 850 equivalent weight fluorosulfonyl vinyl ether/ te-trafluoroethylene copolymer. The 850 equivalent weight fluorosulfonyl vinyl ether tetrafluoroethylene copolymer is prepared according to the following procedure: 784 grams of CF2=CFOCF2CF2SO2F is added to 4700 grams of deoxy-genated water containing 25 grams NH402CC7F15, 18.9 34,250A-F -33-7.~2 -3~-grams of Na2HP04"7H20, 15.~ grams of NaH2PO4H20 and 4 grams of (NH4)2S2O8 under a positive pressure of 192 psig of tetrafluoro~thylene at a temperature of 60C
for 88 minu-tes. The reactor is vented under heat and vacuum to remove residual monomers. The reactor con-tents is frozen, thawed, and vigorously washed to remove residual salts and soap.

The dried polymer'is extrudëd at a temper-ature of 500F -to 550F (260C to 288C) using a Haake Rheomex 254 1.9 cm vented 25:1 length/diameter sta~n-less steel screw extruder and a 15 cm die. With a 20 mil (508 microns) die gap, -the film is drawn down to a thickness of 4 to 5 mils (101 to 127 microns) and ~uenched on an unheated stainless steel roll. The cast film samples were cleaned by degreasing with acetone and air dried.

The so-prepared coated aluminum foil and the film are placed between two sheets of polytetrafluoro-ethylene covered glass cloth and this all in turn is placed between two photographic plates. The composite sandwich is pressed for 5 minutes at a pressure of 800 psi (5512 kPa) and at a temperature of 250C on a hydraulic hot press. The composite film and foil was then removed from the press and placed in a 25 weight percent aqueous sodium hydroxide bath at a -temperature of 70C for ~ hours to remove -the foil and hydrol.y~e the film. The membrane film was now in the sodium form and was suitable for use in a minimum brine gap chlor--alkali cell.

3~,250A-F -3~-'~''

Claims

1. A method for forming a polymer film using a removable substrate comprising the steps of:
(a) forming a dispersion of a perfluorinated polymer containing sites convertible to ion exchange groups and a dispersant, said dispersant having a boiling point of 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 dispersion onto the removable substrate;
(c) removing the dispersant from the disper-sion; and (d) removing the substrate.

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

34,250A-F 35 Y-(CF2)a-(CFRf)b-(CFR'f)c-0-[CF(CF2X)-CH2-O]µ-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 NR1R2;
R is selected from a branched or linear alkyl radical having from 1 to 10 carbon atoms and an aryl radical;
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;
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, Cl, perfluoroalkyl radicals having from 1 to 10 carbon atoms and fluorochloroalkyl radicals having from l 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) 34,250A-F 36 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'>l.

4. The method of Claim 1, wherein the boiling point of the dispersant is from 30°C to 110°C.

5. The method of Claim 1, wherein the density of the dispersant is from 1.55 to 2.2 grams per cubic centimeter.

6. The method of Claim 1, wherein the solubility parameter of the dispersant is from greater than 7.1 to 7.5 hildebrands.

7. The method of Claim 1, wherein the density of the dispersant and the density of the polymer are both from 1.55 to 2.2 grams per cubic centimeter.

8. The method of Claim 1, wherein the dispersant is represented by the general formula:

XCF2-CYZX ' wherein:
X is selected Erom F, Cl, Br, and I;
X' is selected from Cl, Br, and I;

34,250A-F 34 Y and 2 are independently selected from H, F, Cl, Br, I and R'; and R' is selected from perfluoroalkyl radicals and chloroperfluoroalkyl radicals having from 1 to 6 carbon atoms.

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

10. The method of Claim 8, wherein the polymer is present in the dispersion at a concentration of from 0.1 to 50 weight percent.

11. the method of Claim 8, wherein the polymer is present in the dispersion at a concentration of from 0.3 to 30 weight percent.

12. The method of Claim 1, wherein the removable substrate is aluminum.

13. The method of Claim 1, wherein the substrate is removed by dissolving with a solvent from the substrate.

14. The method of Claim 1, wherein the substrate is removed by an alkaline solution.

15. The method of Claim 1, including heating the coated substrate to fuse the polymer into a film prior to removing the substrate.

16. The method of Claim 1, including the step of providing the removable substrate with a roughened surface.

34,250A-F 38

17. The method of Claim 16, wherein the substrate is roughened by blasting.

18. The method of Claim 16, wherein the substrate is roughened by chemical etching.

19. A film produced by the method of Claim 1.

20. An electrochemical cell having an anode and a cathode separated by an ion exchange membrane film, wherein the film is the film produced by the method of Claim 1.

34,250A-F 39