Note: Descriptions are shown in the official language in which they were submitted.
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~
-- 2
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~
, ~
-- 4 --
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
- 4 -
"
~2~15~
- 5 -
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
CF3
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
-
-- 6 --
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
CF3
CF2=CFO (CF2CF(~) 2cF2cF2cot)(~H3
CF3
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.
-- 6 --
'\~1
~ ~l2~P1 . 5,X~
-- 7 --
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~
ICF2
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
-- 7 --
.~ 5~
..
-- 8 --
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,
CF3
CF2=CFOCF2CFOCF2CFOCF2CF2S02F,
CF3 CF3
CF2=CFCF2CF2SO2F, and
5~
. ~,
-- 10 --
CF2=cFocF2cFocF2cF2so2F
CF2
o
CF3.
The most ~referred sulfonyl fluoride
containing comonomer is perfluoro(3,6-dioxa-4-
methyl-7-octenesulf onyl fluoride),
CF2=cFocF2cFocF2cF2so2F.
CF3
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
CF2
S 2 F
~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-
-- 10 --
~ 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
11 --
:~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
- 12
~21 ~5~3
,~. ~
- 13 -
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
- 13 -
- 14 -
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
- 14 -
~l2~5~8
, ~
-- 15 --
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~$5;Z8
- 16 -
a carbon chain which may or may not contain ether
oxygen linkages therein and which is totally
substitu~ed by fluorine or by fluorine and chlorine
atoms. Preferably the perhalocarbon polymer is a
perfluorocarbon polymer~ as it has greater chemical
inertness. Typical such polymers include
homopolymexs made from tetrafluoroethylene and
copolymers of tetrafluoroethylene with
hexafluoropropylene and/or perfluoro(alkyl vinyl
10 ethers) with alkyl being 1 to lO car~on atoms such as
perfluoro(propyl vinyl ether). An example of a most
preferred thread material is polytetrafluoroethylene.
Threads made from chlorotrifluoroethylene polymers
are also useful.
So as to have adequate strength in the
fa~ric before lamination, and in the mem~rane after
12mination, the threads should be of 50 ta 600
denier, preferably 200 to 400 denier (denier is
g/9000 m of thread).
The fabric will ~ypically have a thread
count in the range of l.6 to 16 threads/cm (4 to 40
threads (inch~) in each of the warp and weft~
preferably 3 to 10 threads/cm.
The membrane can be made from the component
25 layers of film and the rnicroporous sheet with the aid
of heat and pressure. Temperatures of abou~ 200C to
300C are ordinarily required to fuse the polymer
films employed and enable the microporous sheet to
become completely embedded in ~he film, and, when two
30 films are used, to make the films fuse together; the
temperature required may be even above or below this
range, however, and will depend on the sEecific
polymer or polymers used. The choice of a suitable
temperature in any specific case will be clear J
35 inasmuch as too low a temperature will fail to bring
- 16 -
. , .
-~ ~2~1$~
- 17 -
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
- 17 -
.,
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
- 18 -
~ ~l Z ~ .5z~
- 19 -
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
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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
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~Z~5~
. ~ ~
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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.
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