CA1209528A

CA1209528A - Fluorpolymer membrane with microporous stretched sheet reinforcement

Info

Publication number: CA1209528A
Application number: CA000394172A
Authority: CA
Inventors: Raimund H. Silva
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1981-01-16
Filing date: 1982-01-14
Publication date: 1986-08-12
Also published as: IT1149559B; JPS57137490A; GB2091166A; FR2498197A1; DE3201092A1; GB2091166B; NL8200153A; IT8219139A0; FR2498197B1; BE891794A

Abstract

ABSTRACT
An ion exchange membrane which comprises at least a layer of fluorinated polymer which contains -COONa or -COOK groups, and optionally a layer of fluorinated polymer which contains -S03Na or -S03K groups, which membrane contains therein a microporous polytetrafluoroethylene sheet which has been stretched in at least one direction, and which membrane has been fabricated by melt lamination, is described. The membrane may optionally have an external reinforcing fabric adhered to one surface thereof. Such membrane can be used as the separator between the compartments of a chloralkali cell, and such a cell operates at low voltage, high current efficiency, and low power consumption.

Description

5z~ -TITLE
Fluorpolymer Membrane With Microporous Stxetched Sheet Reinforcement BACKGROUND OF THE INVENTICN
Fluorinated ion exchange polymers having carboxylic acid and/or sulfonic acid functional groups or salts thereof are known in the art. One principal use of such polymers is as a component of a !;
membrane used to separate the anode and cathode compartments of a chloralkali electrolysis cell.
Such membrane can be in the form of a reinforced or unreinforced film or laminar structure.
It is desirable for use in a chloralkali cell that a membrane provides for operation at 10W
voltage and high current efficiency, and thereby at low power consumption, so as to provide products of high purity at low cost, especially in view of todayls steadily increasing cost of energy. I~ is also desirable that the membrane be tough, so as to 2n resist damage during fabrication and installation in such a cell. As films of the bes~ available ion exchange polymers have low tear strength, it has been found necessary to streng~hen them by fabricating membranes with reinforcement therein, such as a reinforcing fabric.
However, use o~ reinforcement within the membrane is not totally beneficial. A deleterious effect is that use of reinforcement such as fabric results in a thicker membrane, which in turn leads to 30 operation a~ higher voltage because the greater thickness has a higher electrical resistance.
Efforts to lower the resistance by using thinner films in fabricating reinforced membranes are often unsuccessful because the film ruptures in some of the ~D-51~5 35 windows of the fabric during membrane fabrication, 5Z~

resulting in a membrane with leaks. (By "windows"
is meant the open areas of a fabric between adjacent threads of the fabric.) A membrane which leaks is undesirable as it permits anolyte and catholyte to flow into the opposite cell compartment, thereby lowering the current efficiency and contaminating the products made. Additionally, thick layers of polymer at the junctions of threads in a reinforcing fabric also constitute regions of high resistance. (By "junctions" is meant the crossover points where threads in the warp meet threads in the weft.) It is a principal object of this invention to provide an ion exchange membrane which operates at low voltage and high current efficiency, and thereby at low power consumption, and yet has good tear resistance. Another object is to provide a thin, tough ion exchange membrane. Other objects will be apparent hereinbelow.
SUMMARY OF THE INVENTION
Briefly, according to the invention, there is provided a melt fabricated ion exchange membrane which contains at least one layer of fluorinated polymer which contains -COONa or -COOK functional groups, and which membrane contains therein a sheet of microporous polytetrafluoroethylene which has been stretched in at least one direction.
More specifically, in one aspect of the inven~ion there is provided an ion-exchange membrane which comprises a layer of fluorinated polymer which has -COOM functional groups, where M is Na or K, and, completely embedded therein, a microporous polytetrafluoroethylene sheet which has been stretched in at least one direction, said membrane having been fabricated by melt lamination.

.~
,~

52~
. - 3 -In another aspect of the invention there is provided an ion-exchange membrane which comprises a first layer of a first fluorinated polymer which has -COOM functional groups, a second layer of a second fluorinated polymer which has -SO3~ ~unctional groups, where ~ is Na or X, and, completely embedded therein, a microporous poly~etrafluoroethylene sheet which has been s~retched in at least one direction, said membrane having been fabricated by melt 10 lamination.
There are also provided according to the invention an electrochemical cell having such ion exchange membrane as a component part thereof, and an electrolysis process in which such ion exchange 15 membrane is used.
DETAILED DESCRIPTION OF T~ NT ION
The membranes of the present invention are typically prepared from one or more layers of fluorinated polymer which have -COOR and/or -S02W
20 func~ional groups, where R is lower alkyl and W is F
or Cl, and a web of support material.
The first layer of polymer with which the present invention is concerned is typically a carboxylic pol~mer having a fluorinated hydrocarbon 25 back~one chain to which are attached the ~unctional groups or pendant side chains which in turn carry the functional groups. The pendant side chains can contain, for example ~ C~ ~ V

~ ~Jt groups wherein ~ is F or CF3, t is 1 ~o 12, and V
is -COOR or -CN, where R is lower alkyl. Ordinarily, the functional group in the side chains of the polymer will be present in terminal -O t CF t V
~Jt groups. Examples of fluorinated polymers of this ~l2~S~
, ~

kind are disclosed in Bri~ish Patent No~ 1,145,445, U.S. 4,116,888 and U.S. 3,506,635. More ~peciLically, the polymers can be prepared ~rom monomers which are fluorinated or fluorine-substituted vinyl compounds. The pol~mers are usually made from at least two monomers. At least one monamer is a fluorina~ed vinyl ccmpound such as vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, 10 chlorotrifluoroethylene, perfluoro(alkyl vinyl ether), tetrafluoroethylene and mixtures thereof~ In the case of copolymers which will ~e used in electrolysis of brine, the precursor vinyl monGmer desirably will not contain hydrogen. Additionally, 15 at least one moncmer is a fluorinated monGmer which contains a group which can be hydrolyzed to a car~oxylic acid group, e.g., a carboalkoxy or nitrile group, in a side chain as set forth above.
By "fluorinated polymer" is meant a polymer 20 in which, after loss of the R group by hydrolysis to ion exchange form, the number of F atoms is a~ least 90% of the number of F atoms and H atoms.
The monGmers, with the exception of the R
group in the ~COOR, will pre ferably not contain 25 hydrogen, especially if the polymer will be used in the elec~rolysis of brine, and for greatest stability in harsh environments, most preferably will be free of both hydrogen and chlorine, i.e., will be perfluorinated; the R group need not be fluorinated 30 as it is lost during hydrolysis when the functional groups are converted to ion exchange groups.
one exemplary suitable type o carboxyl-containing moncmer is represen~ed by the fonmula "

~2~15~

CF2~ OCF2CF~SOCF2-COOR
y wherein R is lower alkyl, Y is F or CF3, and s is 0, 1 or 2.
Those monomers wherein s is 1 are preferred because : their preparation and isolation in good yield is more : easily accomplished than when s is 0 or 2. The :~ 10 compound CF2 =Q'0~2CFOCF2COOCH3 is an especially useful monomer. Such monomers can 15 be prepared, for example, from compounds having the formula CF2=CF(OCF2CF)sOCF2CF2SO2F

Y
wherein s and Y are as defined atove, by (1) 20 saturating the terminal vinyl group with chlorine to protec~ it in subsequen~ ~teps by converting it to a CF2Cl CFCl- group; (2) oxidation with nitrogen dioxide to convert the -OCF2CF2S02F group to an -OCF2COF group; (3) esterification with an alcohol 25 such as methanol to form an -0CF2COOCH3 group;
and (4~ dechlorination wi~h zinc dust to regenerate ~he terminal CF2=CF- group. It is also possible to replace steps t2) and (3) of this sequence by the steps (a) reduction of the -OCF2CF2SO2F group 30 to a sulf i ni c ac i d, -O CF2CF2 SO2H, or alkali metal or alkaline earth metal salt thereof by trea~ment with a sulfite sal~ or hydrazine; (b) oxida~ion of the sulfinic acid or salt thereof with oxygen or chromic acid, whereby -OCF2COOH groups or 35metal salts thereof are ~ormed; and (c)

2~ 1 ~S~3 -esterification to -OCF2COOCH3 by known methods; this sequence is more fully described in South African Patent 78/2224 of Du Pont (W. G. Grot et al), published 1979 April 25. Preparation of copolymers thereof is described in South African Patent No. 7~/2221 of Du Pont (W. G. Grot et al), published 1979 May 30.
Another exemplary suitable type of carboxyl-containing monomer is represented by the formula CF2=cF~ocF2~FtsocF2-cF - V
Y
wherein V is -COOR or -CN, R is lower alkyl, Y is F or CF3, is F or CF3, and S iS t 1 or 2.
The most preferred monomers are those wherein V is -COOR wherein R is lower alkyl, generally Cl to C5, because o ease in polymerization and conversion to ionic form. Those monomers wherein s is l are also preferred because their preparation and isolation in good yield is more easily accomplished than when s is 0 or 2. Preparation of those monomers wherein V is -COOR where R is lower alkyl, and copolymers thereof, is described in U.S. Patent No. 4,131,740.
The compounds CF2=CFOCF2CFOCF2CF2COOCH3, and CF2=CFO (CF2CF(~) 2cF2cF2cot)(~H3 whose preparation is described therein, are especially useful monomers. Preparation of monomers wherein V is -CN is described in U. S. Patent No.

3,852,326.

'\~1 ~ ~l2~P1 . 5,X~

Yet another suitable ~ype of carboxyl-containing monomer is that having a terminal -O(CF~)~COOCH3 group where v is from 2 to 12, such as 2 ( 2)3CCCC~3 and CF2=CFOCF2CF(CF3) 0(CF2~3cOOcH
Preparation of such monomers and copolymers thereof is described in Japanese Patent Publications 38486/77 and 28586/77, both of Asahi Glass and published for opposition as 44427/78 on 1978 November 29 and as 4133/78 on 1978 February 1~, respectively, and in British Patent No. 1,145,445 of DU Pont (D. G. Anderson et al) published 1969 March 12.
Another class of carbo~yl-containing polymers is represented by polymers having the repeating units _ ~ ~Cx2-CX2~

o s ~_ l OOR
wherein q is 3 to 15, r is 1 to 10, s is 0, 1 or 2, t is 1 to 12, the X's taken together are four flourines or three fluorines and one chlorine, Y is F or CF3, ~ is F or CF3, and R is lower alkyl.
A preferred group of copolymers are those of tetrafluoroethylene and a compound having the formula CF2=CFO(CF2CFO)n(CF2)mCOOR, Y

.~ 5~
..

where n is 0, 1 or 2, m is 1, 2, 3 or 4, Y is F or CF3, and 3' 2 5 3 7 Such copolymers with which the present invention is concerned can be prepared by techniques known in the art, e.g., U.S. Patent No. 3,528,954, U.S. Patent No. 4, 131,740, and South African Patent No. 78/225 of Du Pont ~(D. C. England et al), published 1978 April 19.
When a layer of sulfonyl polymer is present, it is typically a polymer having a fluorinated hydrocarbon backbone chain to which are attached the functional groups or pendant side chains which in turn carry the functional groups. The pendant side chains can contain, for example, CF-CF2-SO2W groups wherein ~f is F C1, or a Rf Cl to C10 perfluoroalkyl radical, and W is F or Cl, preferably F. Ordinarily, the functional group in the side chains of the polymer will be present in terminal -O-CF-CF2SO2F
Rf groups. Examples of fluorinated polymers of this kind are disclosed in U. S. Patent No. 3,282,875, U.S.
Patent No. 3,560,568 and U.S. Patent No. 3,718,627.
More specifically, the polymers can be prepared from monomers which are fluorinated or fluorine substituted vinyl compounds. The polymers are made from at least two monomers, with at least one of the monomers coming from each o the two groups described below.
At least one monomer is a fluorinated viny1.
compound such as vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro (alkyl vinyl ~,.

-~ ~L2~

g ether), tetrafluoroethylene and mixtures thereof. In the case of copolymers which will be use~ in electrolysis of brine, the precursor vinyl monomer desirably will not contain hydrogen.
The second layer may contain groups derived from sulfonyl-containing monomers containing the precursor group -CF-CF2-SO2F, wherein Rf is as Rf defined above. Additional examples can be represented by the general formula CF2=CF-Tk-CF2SO2F wherein T is a bifunctional fluorinated radical comprising 1 to 8 carbon atoms, and k is 0 or 1. Substituent atoms in T include fluorine, chlorine, or hydrogen, although generally hydrogen will be excluded in use of the copolymer for ion exchange in a chloralkali cell.
The most preferred polymers are free of both hydrogen and chlorine attached to carbon, i.e., they are perfluorinated, for greatest stability in harsh environments. The T radical of the formula above can be either branched or unbranched, i.e., straight-chain, and can have one or more ether linkages. It is preferred that the vinyl radical in this group of sulfonyl fluoride containing comonomers be joined to the T group through an ether linkage, i.e., that the comonomer be of the formula CF2=CF-O-T-CF2-SO2F.
Illustrative of such sulfonyl fluoride containing comonomers are CF2=CFOCF2CF2S02F, CF2=CFOCF2CFOCF2CF2S02F, CF2=CFOCF2CFOCF2CFOCF2CF2S02F, CF2=CFCF2CF2SO2F, and 5~
. ~, CF2=cFocF2cFocF2cF2so2F

o CF3.

The most ~referred sulfonyl fluoride containing comonomer is perfluoro(3,6-dioxa-4-methyl-7-octenesulf onyl fluoride), CF2=cFocF2cFocF2cF2so2F.

The sulf onyl-contai ni ng monomers are disclosed in such references as U.S. Patent No.

3,282,875, U.S. Patent No. 3,041,317, U.S. Patent No.

3,718,627 and U.S. Patent No. 3,560,568.

A preferred class of such polymers is 15 represen~ed by poly~ers having the repeating units ~ CX~-CX2 ~~"~ ~ _ CF

O
CF-Rf ~S

wherein h is 3 to 15, j is 1 to 10, p iS 09 1 or 2, the X's taken tosether are four fluorines or three fluorines and one chlorine, Y is F or CF3, and R~ is F, Cl or a Cl to C10 perfluoroalkyl radical.

A most preferred copolymer is a copolymer of 35 tetrafluoroethylene and perfluoro(3,6-dioxa-4-methyl-~ U~52~
,.~

7-octenesulEonyl fluoride) which ccmprises 20 to 65 percent, prefera~ly, 25 to 50 percent by weight of the latter.
Such coFol~mers used in the present invention can be prepared by general polymerization techniques developed for homo- and copolymerizations of fluorinated ethylenes, particularly those employed for tetrafluoroethylene which are described in the literature. Nonaqueous techniques for preparing the 10 copolymers inc~.ude that o~ U.S. Patent No. 3,041,317, that is, by the polymerization of a mixture of the major monomer therein, such as tetrafluoroethylene, and a fluorinated ethylene containing a sulfonyl fluoride group in the presence of a free radical 15 initiator, preferably a perfluorocarbon peroxide or azo compound, at a temperature in the range 0-200C
and at pres~ures in the range of 10 to ~10 pascals (1-200 Atm.) or higher. The nonaqueous polymerization may, if desired, be carried out in the 20 presence of a fluorinated solvent. Suitable fluorina~ed solvents are inert, liquid, per~luorinated hydrocarbons, such as perfluoromethylcyclohexane, perfluorodimethyl-cyclobutane, perfluorooctane, perfluorobenzene and 25 the like, and inert, liquid chloro~luorocar~ns such as 1,1,2-trichloro-1,2-2-trifluoroethane, and the like.
Aqueous techniques for preparing the copolymer include contacting the moncmers with an 30 aqueous medium containing a free-radical initiator to obtain a slurry of polymer particles in non-water-wet or granular form, as disclosed in U.S. Patent ~o.
2,393,967, or contacting the monomers with an aqueous medium containing both a free-radical initiator and a 35 telogenically inactive dispersing agent, to obtain an :~z~ s~

- 12 _ aqueous colloidal dispersion of pol~mer particles, and coagulating ~he disFersion, as disclosed, for example, in U.S. Patent No. 2,559,752 and U.S. Pa~ent No. 2,593,583.
A copolymer which contains different types of functional groups can also be used as a component film in making the membrane of the invention. For example, a terpolymer prepared from a monomer chosen from the group of nonfunctional monomers described 10 above, a monomer from the group of car~oxylic monomers descri~ed above, and additionally a monomer from the group of sulfonyl moncmers descri~ed above, can be prepared and used as one of the film components in making the membrane.
It is further possible to use as a component film of the membrane a film which is a blend of two or mor~ polymers. For example, a blend of a polymer having sulfonyl groups in melt-fabrica~le form with a pol~mer havins carboxyl groups in melt-fabricable 20 form can be prepared and used as one of the component films of ~he membrane of ~his invention~
It is additionally possible to use a laminar film as one of the component films in making the membrane. For example, a film having a layer of a 25 copolymer having sulfonyl groups in melt-fabricable f orm and a layer o a copol~mer having car boxyl groups in melt-fabricable fonm, can also be used as one of the component films in making the membrane of the invention.
An essential oomponent of the membrane o~
the invention is a layer of a first fluorinated polymer which has -COONa or -COOK f unc~i onal gro ups, which has an equivalent weight in the range o~ 400 to 2000, most prefera~ly 1000 to 1100, and which has a ~21 ~5~3 ,~. ~

thickness in the range o~ 13 to 250 microns (0.5 to 10 mils), preferably 25 to 75 microns tl to 3 mils).
~ he membrane of the invention may or may not have, in adherent contact with said layer of first S fluorinated poly~er, an optional component which is a layer of a second fluorinated polymer which has -SO3Na or -SO3K functional groups, which has an equivalent weight in the range of 800 to 2000, most preferably 1100 to 1200, and which has a thickness in 10 the range of 13 to 150 microns (0.5 to 6 mils), prefe~ably 13 to 75 microns (0.5 to 3 mils). when this second layer is present, the thickness of the first layer of first fluorinated polymer should be 13 ~o 150 microns, preferably 13 to 75 microns, and the 15 thickness of the first and second layers taken toge~her should be in the range of 26 to 250 microns (1 to 10 mils), preferably 2~ to 150 microns (1 to 6 mils).
Concerning both the polymer with carboxyl 20 unc~ionality and the polymer with sulfonyl functionality, above an equivalent weigh~ of 2000, the electrical resis~ivity becomes too high, and below the indicated lower equivalent weight limits, the mechanical properties are poor because of 25 excessive swelling of the polymer. The relative amounts of the comoncmers which make up the polymer can be adjusted or chosen such that ~he polymer has a desired equivalent weight. The equivalent weight above which the resistance of a film or membrane 30 becomes too high for practical use in an electrolytic cell varies scmewhat with the thickness of the film or membrane. For thinner films and membranes, equivalent weights up to about 2Q00 can be tolerated. For most purposes, however, and for films of ordinary thickness, a value no greater ~han about 1400 is preferred.
A second essential componen~ of the membrane is a microporous polytetrafluoroe~hylene sheet. This sheet can be a film or extrudate made, or treated by any means to make it, microporous. This sheet should have a thickness in the range of 2.5 to 250 microns (0.1 to 10 mils), preferably 13 to 75 microns (0.5 to 3 mils), and has open-cell porosity, with a pore size 10 in the range of 0.01 to 20 microns, pre~erably 3 to 15 microns. Ey "pore size" is meant an average size of the pores present.
This micro,~orous sheet is one which has been stre~ched in at least one direction so that it ~
15 be tough, and thus impart toughness to ~he membrane of the invention. Stretching results in orientation of the polymer in the sheet. The ability of the microporous sheet to provide toughness in the thin membranes of the invention is an important aspect of 20 the invention. Use of an oriented sheet also provides a more dimensionally s~able membrane. The sheet can be one which has ~een stretched in two mutually perpendicular directions; such sheet has both greater orientation and tou~hness, and provides 25 a membrane which resists tearing in all directions.
A typical such microporous sheet is one of polytetrafluoroethylene having a microstructure charac~erized by nodes interconnected by fibrils, made by high-rate stretching at an elevated 3~ temperature of an unsintered, dried paste extrudate of pol~tetrafluoroethylene, as descri~ed in U.S.
3,962,153, and commercially available from r~. L. Gore Associates, Inc., under ~he trademark Gore-Tex.
I~ the membrane has only the first layer of 35 first fluorinated polymer, the microporous s,heet will ~l2~5~8 , ~

be disposed in said first layer, and should be completely embedded therein. As employed herein, the term "completely embedded" means that the pores of the microporous sheet are filled with said first and/or second fluorinated pol~mer which will subsequently be converted to ion exchange polymer, but that fibrids of polytetrafluoroeth~lene which are part of the microporous sheet may protrude from the surface of the membrane.
If the membrane has both a first layer of first fluorinated polymer and a second layer of second fluorinated polymer, the microporous sheet can be disposed in either layer, or at the ~oundary of the layers, thus extending into both layers. For a 15 membrane in~ended for use in a chloralkali electrolysis process, the microporous sheet will preferably be predominantly in the second layer, and most preferably entirely in the second layer, and the membrane will be employed with the the second layer 20 facing the anode of the cell. In any case, the microporous sheet is completely embedded in the resulting composite structure.
The membranes of the invention may also include a further optional componen~, which is a 25 woven or knitted reinforcement fabric, disposed externally on the membrane, adherent to one of the surfaces, preferably to the exposed surface of the second layer described abo~e.
In the case of woven fabric, weaves such as 30 ordinary basketweave and leno weave are suitable.
The threads of the fabric can be either monofilament or multistranded.
The threads are perhalocarbon polymer threads. As employed herein, the term "perhalocarbon 35 polymer" is employed to refer to a polymer which has -- 15 _ .~

L.2r~$5;Z8 a carbon chain which may or may not contain ether oxygen linkages therein and which is totally substitu~ed by fluorine or by fluorine and chlorine atoms. Preferably the perhalocarbon polymer is a perfluorocarbon polymer~ as it has greater chemical inertness. Typical such polymers include homopolymexs made from tetrafluoroethylene and copolymers of tetrafluoroethylene with hexafluoropropylene and/or perfluoro(alkyl vinyl 10 ethers) with alkyl being 1 to lO car~on atoms such as perfluoro(propyl vinyl ether). An example of a most preferred thread material is polytetrafluoroethylene.
Threads made from chlorotrifluoroethylene polymers are also useful.
So as to have adequate strength in the fa~ric before lamination, and in the mem~rane after 12mination, the threads should be of 50 ta 600 denier, preferably 200 to 400 denier (denier is g/9000 m of thread).
The fabric will ~ypically have a thread count in the range of l.6 to 16 threads/cm (4 to 40 threads (inch~) in each of the warp and weft~
preferably 3 to 10 threads/cm.
The membrane can be made from the component 25 layers of film and the rnicroporous sheet with the aid of heat and pressure. Temperatures of abou~ 200C to 300C are ordinarily required to fuse the polymer films employed and enable the microporous sheet to become completely embedded in ~he film, and, when two 30 films are used, to make the films fuse together; the temperature required may be even above or below this range, however, and will depend on the sEecific polymer or polymers used. The choice of a suitable temperature in any specific case will be clear J
35 inasmuch as too low a temperature will fail to bring . , .

-~ ~2~1$~

about embedment of the microporous sheet as evidenced by a high opacity, and will fail to effect an adequate degree of adherence of the films to each other where there are two ~ilms, and too high a S temperature will cause leaks to form. Pressures of as little as about 2X104 pascals, to pressures exceeding 10 pascals can be used. A hydraulic press is a suitable apparatus for making the membrane, in which case typical pressures are in the range of 2X105 to 107 pascals.
Another apparatus, suitable for continuous preparation of membrane, ~omprises a hollow roll with an internal heater and an internal vacuum source.
The hollow roll contains a series of circumferential 15 slots on its surface which allow the internal vacuum source to draw component materials in the direction of the hollow roll. The vacuum draws the component ma~erials of the membrane onto the hollow roll, such that typical air pressures against the component 20 materials is in the range of sX104 to 105 pascals. A curved stationary plate with a radiant heater faces the top surface of the hollow roll with a spacing of about 6 mm (1/4 inch) between their two surfaces.
During a lamination run, porous release paper is us~d in contacting the hollow roll as a support material to prevent adherence of any component material to the roll surfa~e and to allow vacuum to pull component materials in the direction 30 of the hollow roll. Feed and takeoff means are provided for the component materials and product. In the feed means one idler roll of smalLer diameter than the hollow roll is provided ~or release paper and ccmponent materials. The feed and takeoff means ., 5;~3 , are positioned to allow component materials to pass around the hollow roll over a length of about 5/6 of its circumference. A f~urther idler xoll is provided for the release paper allowing its separation from the other materials. Takeoff means are provided for the release paper and the product membrane~
For use in ion exchange applications and in cells, for example a chloralkali cell for electrolysis of brine, the membrane should have all o~ the ~ ~ctional groups converted to ionizable functional groups. These groups are -COOM groups, and, when present, -SO3~ groups, where M is Na or K. Such conversion is ordinarily and conveniently accomplished by hydrolysis with acid or base, such 15 that th& various function~ groups described above in relation to the melt-fabricable polymers are converted respectively to the free acids or the alkali metal salts thereof. ~ch hydrolysis can be : ~ ~ carried out with an aqueous solution of a ~ neral 20 acid or an alkali metal hydroxide. Base hydrolysis is preferred as it is faster and more complete. Use of hot solutions, such as near the boiling point of the solution, is preferred for rapid hydrolysis. The time required for hydrolysis increases with the 25 thickness of the structure. It is also of advan~age to include a water~miscible organic compound such as dimethylsuloxide in the hydrolysis bath. The free carboxylic and sulfonic acids are convertible to salts with NaOH or KOH.
The membrane of the invention is impermeabLe to hydraulic ~low of liquid. ~A diaphragm~ which is porous, permits hydraulic flow of liquid therethrough with no change in composition, while an ion exchange membrane permits selec~ive permeation by ions and 35 permeation of liquid by diffusion, such tha,t the ~ ~l Z ~ .5z~

material which penetrates the membran~ differs in composition from the liquid in contact with the membrane.) It is an easy matter to determine whether thexe is or is not ~ydraulic flow of liquid by a leak test with gas or liquid.
A principal use of the ion exc'nange membrane of the invention is in electrochemical cells. Such a cell comprises an anode, a compartment for the anode, a cathode, a compartment for the ca~hode, and a 10 membrane which is situated to sepzrate the two said compartments. One example is a chloralkali cell.
The copolymers used in the layers decri~ed herein should be of high enough molecular weight to produce films which are at least moderately s~rong in 15 both the melt-fabricable precursor form and in the hydrolyzed ion exchange form.
To further illustrate the innovative aspects of the present invention, the following examples are provided.
;20 EXAMPLES
Example 1 In a hydraulic press having 20 cm x 20 cm (8 x 8 inch) heatable platens were placed a piece of film of a copolymer of tetrafluoroethylene and methyl 25 perfluoro(4,7-dioxa-5-methyl-8-nonenoate) having an equivalent weight of 1050, said piece of film having a thickness of 36 to 43 microns tl-4-1-7 mils) and being circular 12.5 cm (5 inches) in diameter, and a piece of "Gore Tex" sheet as described hereinakove, 30 said piece of sheet having a ~hickness of 25 microns (1 mil), being circular 12.5 cm in diameter, and having a pore size of 0.5 micron. The assembly of film and microporous sheet was heated at 220C under 3.23x106 pascals (30,000 lb force) for 1 minute, 35 after preheating for 1 minute. The resuLting ,..

.52~3 composite structure had a thickness o' akout 64 microns (2.5 mils), and was almost transparent, which indicated complete emkedment of the microporous sheet in the layer of oopolymer film. The composi te structure was placed in a mixture of 74 w~. % water, 15 wt. ~ dimethylsulfoxide and 11 wt. ~ potassium hydroxide at 80C for 1 hour to hydrolyze the -COOCH3 groups, thus providing an ion exchange membrane having -COOK groups. The membrane was 10 thoroughly washed with water and mounted in a small chloralkali cell. The cell was operated for 8 days at 80C, 31 amps/~m , and 20 wt. % exi ~ brine concentration to produce caustic at 32 wt. % at a current efficiency of 93.7-96.3~, voltage of 15 3.59-3.73 volts, and a power consumption of 2523-2658 kwh/metric ton. During the electrolysis, the functional groups o~ the membranebecame -COONa grouEs.
By way of comparison a copolymer film like ~hat described above, except that it was 51 microns 20 (2 mils) thick, and having no microporous sheet therein, was hydrolyzed as above, soaked in 30 wt.
aqueous NaOH for 2 hours, moun~ed in a small chloralkali cell and operated for 8 days under the same oonditions as above to produce caustic a~ a 25 current efficiency of 94.9-99.9%, voltage of

4.02-4.18 volts, and a power consumption of 2790-2863 Icw h/me t ri c ton .
Example 2 In the hydraulic press described in 30 Example 1 were placed a piece of film of a copolymer like that of Example 1, 12,5 cm in diameter, 25 microns thick, a piece of fabric, 12 5 cm in diameter, and between the film and the fabric, a piece of '7Gore-Tex" sheet as described hereinabove, 3512.5 cm in diameter, 2S microns thick, pore size of 3 ~ 20 -~Z~5~
. ~ ~

to 5 ~ crons. ((The fabric had 10 threads/c~ (25 thread/inch) of 200 denier polytetrafluoroethylene yarn in the warp, and 10 threads/c~ (25 threads/inch) of 400 denier polytetrafluoroethylene yarn in the weft, in a leno weave, and was 175 microns (7 mils) thick)). The assembly of microporous sheet, fabric and film was heated at 270C under 1.0~xl06 pascals (10,000 lb force) for 1 minute, after preheating for l minute. The resulting composite structure had 10 excellent mechanical strength, and was free of leaks when tested under a vacuum of 1.65x104 pascals (25 inches of mercury). The composi~e str ucture was placed in an aqueous solution containing 32 wt. %
NaOH and 1~ wt. ~ methanol at room temperature for 2 15 hours to hydrolyze the -cooca3 groups, thus providing an ion exchange membrane having -COOMa groups ~ The membra~ is useful as a separation between ~he ccmpartments of electrochemical cells.
I NDUS TR IAL AP P L I C~B IL IT Y
The ion exchange membrane of the present invention is technically advanced over membranes of the prior art. It exhibits improved performance characteristics when used as m~mbrane in a chloro~kali cell, including operation at low voltage 25 and high current efficiency, and thus at low power consumption. There is accordingly a substantial saving in operating costs resulting from the lowered consumption of power. The membrane of the invention ; also provides for less gas blinding by chlorine on 30 the anolyte side of the membrane during brine el ectrolys is .
The ion exchange membrane of the inven~ion can also be used in the electrolysis of water to hydrogen and oxygen, and in Donnan dialysis and 35 electrodi ~ ysis processes.

, .

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An ion-exchange membrane which comprises a layer of fluorinated polymer which has - COOM
functional groups, where M is Na or K, a thickness in the range of 13 to 250 microns, and an equivalent weight in the range of 400 to 1400; and, embedded therein, a microporous polytetrafluoroethylene sheet which has been stretched in at least one direction, is from 2.5 to 250 microns in thickness, and has a pore size of 0.01 to 20 microns, said membrane having been fabricated by melt lamination.

2. The membrane of Claim 1 wherein said layer has a thickness in the range of 13 to 75 microns, said perfluorinated polymer has an equivalent weight in the range of 1000 to 1100, and said sheet has a thick-ness in the range of 13 to 75 microns and a pore size in the range of 3 to 15 microns.

3. The membrane of Claim 2 wherein said sheet has been stretched in two mutually perpendicular directions.

4. The membrane of Claim 2 wherein said perfluorinated polymer is a copolymer of tetrafluoroethylene and

5. The membrane of Claim 4 wherein said membrane further comprises an external support fabric adhered thereto, said fabric being woven or knitted, and consisting of filaments of perfluorocarbon polymer.

6. The membrane of Claim 2 wherein said perfluorinated polymer is a copolymer of tetrafluoroethylene and CF2 = CFO(CF2)3 COOM,

7. The membrane of Claim 6 wherein said membrane further comprises an external support fabric adhered thereto, said fabric being woven or knitted, and consisting of filaments of perfluorocarbon polymer.

8. An ion-exchange membrane which comprises a first layer of a first fluorinated polymer which has -COOM functional groups, a thickness of 13 to 150 microns and an equivalent weight of from 400 to 1400;
a second layer of a second fluorinated polymer which has -SO3M functional groups, where M is Na or K, a thickness of 13 to 150 microns and an equivalent weight in the range of 800 to 1400; said two layers having a total thickness of no more than 250 microns;
and, completely embedded in the membrane a microporous polytetrafluoroethylene sheet which has been stretched in at least one direction, is from 2.5 to 250 microns in thickness, and has a pore size of 0.01 to 20 microns, said membrane having been fabricated by melt lamination.

9. The membrane of Claim 8 wherein each of said first and second layers has a thickness in the range of 13 to 75 microns and said two layers have a thickness in the range of 26 to 150 microns, said first perfluorinated polymer has an equivalent weight in the range of 100 to 1100, said second perfluorinated polymer has an equivalent weight in the range of 1100 to 1200, and said sheet has a thickness in the range of 13 to 75 microns and a pore size in the range of 3 to 15 microns.

10. The membrane of Claim 9 wherein said sheet has been stretched in two mutually perpendicular directions.

11. The membrane of Claim 9 wherein said sheet lies at least predominantly in said second layer.

12. The membrane of Claim 11 wherein said second perfluorinated polymer is a copolymer of tetrafluoroethylene and

13. The membrane of Claim 12 wherein said first perfluorinated polymer is a copolymer of tetrafluoroethylene and

14. The membrane of Claim 13 wherein said membrane further comprises an external support fabric adhered thereto, said fabric being woven or knitted, and consisting of filaments of perfluorocarbon polymer.

15. The membrane of Claim 14 wherein said support fabric is adhered to said second layer.

16. The membrane of Claim 12 wherein said first perfluorinated polymer is a copolymer of tetrafluoroethylene and CF2 = CFO(CF2)3COOM,

17. The membrane of Claim 16 wherein said membrane further comprises an external support fabric adhered thereto, said fabric being woven or knitted, and consisting of filaments of perfluorocarbon polymer.
18. The membrane of Claim 17 wherein said support fabric is adhered to said second layer.
19. An electrochemical cell which comprises an anode compartment, an anode situated within the anode compartment, a cathode compartment, a cathode situated within the cathode compartment, and, between said compartments, the membrane of any one of Claim 1, Claim 4 and Claim 5.

20. An electrochemical cell which comprises an anode compartment, an anode situated within the anode compartment, a cathode compartment, a cathode situated within the cathode compartment, and between said compartments, the membrane of any one of Claim 6, Claim 7 and Claim 8.
21. An electrochemical cell which comprises an anode compartment, an anode situated within the anode compartment, a cathode compartment, a cathode situated within the cathode compartment, and between said compartments, the membrane of Claim 13 or Claim 15.
22. An electrochemical cell which comprises an anode compartment, an anode situated within the anode compartment, a cathode compartment, a cathode situated within the cathode compartment, and, between said compartments, the membrane of Claim 16 or

Claim 18.