Note: Descriptions are shown in the official language in which they were submitted.
TITLE
Coextruded Multilayer Cation Exchange Membranes
BACKGROUND OF THE INVENTION
Fluorinated polymers containing pendant side
10 chains are often used to prepare ion exchange
membranes. Such membranes have outstanding
properties as barrier membranes to separate the anode
and cathode compartments of electrolytic cells used
for the production of chlorine and caustic.
Fluorinated polymers containing carboxyl
side chains provide excellent current efficiency in
such electrolytic cells, but result in high operating
voltages. By contrast, fluorinated polymers
containing sulfonyl ~roups in the side chains permit
20 operation a~ low voltages, but result in poor current
efficiency in the electrolytic cell
It has previously been recognized that
combinations of polymers having sulfonyl and carboxyl
groups or combinations of polymers having different
25 equivalenk weights are desirable ~o provide an
optimum balance of performance characteristics in an
electrolytic cell using membranes prepared from such
polymers. Under normal cell operation, the water
permeation characteristics of the layers are
sufficiently close that the layers remain in intimate
contact with each other. However, abnormal cell
conditions, such as 1055 of electrical power, failure
of water feed ~o the cathode chamber and particularly
interruption or loss of brine feed to the cell, can
35 produce a large difference in water transport. For
, ,
............. ... ................ ~ . .. .. . .. . . .,, . ~
example, iE the anolyte is depleted below 50 y/l
NaCl, a large quanti-ty of water will be transported
through a Eluorinated polymer having sulfonyl side
chains. However, the water is substantially less
able to permeate a polymer havi.ng carboxyl-containing
side chalns. Thus, during an upset of cell
operation, a composite of two such polymers may
experience a buildup of water at the interface,
resulting in delamination of the composite.
SUMMARY OF THE INVENTION
The instant invention provides composite
cation exchange membranes exhibiting improved
resistance to delamination in an electrolytic cell
and a process for the production of such composite
membranes by coextrusion.
Specifically, the instant invention provides
a process for the preparation of a multilayer cation
exchange membrane by heating at least two
substantially incompatible melt-fabricable
fluorinated polymers having pendant side chains
containing functional groups selected from sulfonyl
and carboxyl to a temperature above the melting point
of the polymers, bringing the molten polymers into
contact while each is at a temperature above the
melting point of the polymer, coextruding the
polymers into a composite film, cooling the resulting
composite to a temperature below the melting points
of each of the films, and subsequently converting the
functional groups to ionizable functional groups.
DETAILED DESCRIPTION OF THE INVENTION
_
The melt-fabricable polymers used in the
ins-tant invention are of the types generally used
previously in the preparation of films or reinforced
laminates for use in electrochemical cells, and
especially chloralkali cells. These include
~.~,4~3~
fluorinated polymers with pendant side chains
containing sulfonyl groups, carboxyl groups, or both.
The melt-fabricable polymer having sulfonyl
functional groups has a fluorinated hydrocarbon
bac]cbone chain to wh:ich are attached functional
groups or pendant side ehains bearing the funetional
groups. The pendant side ehains ean contain, for
example,
CF2CFS02F
Rf
groups wherein Rf is F, Cl, or a Cl to C10
perfluoralkyl radical. Fluorinated polymers of this
type and their preparation are deseribed in detail in
United States Patents 3,282,875, 3,560,568,
3,718,627, and 3,041,317. For ehlor-alkali cells,
perfluorinated polymers are preferred.
The melt-fabricable polymer having
carboxylie funetional groups has a fluorinated
hydroearbon backbone ehain to which are attaehed the
funetional groups or pendant side ehains whieh in
turn carry the funetional groups. The pendant side
chains can eontain, for example,
( CF ) W
Z t
groups wherein Z is F or CF3/ t is 1 to 12 and
preferably 1 to 3, and W is -COOR or -CN, wherein R
is lower alkyl. Preferably, the funetional group in
the side ehains of the polymer will be present in
terminal
--O~ CF ) W
Z t
groups. Sueh fluorinated polymers containing
earboxylie functional groups and their preparation
~3~3~
are disclosed in British Patent 1,145,445 and
United States Patents 3,506,635 and 4,116,888.
Preferred monomers for use in the preparation
o such polymers are found in United States Patents
4,131,740 and 3,852,326. For chlor-alkali cells,
perfluorinated polymers are preferred.
~ he particular polymers used in the present
invention are substantially incompatible.
lO Substantially incompatible polymers are those which, -
after blending above the melting point of each
polymer, separate into distinct phases on cooling ~o ---
room temperature~ Thus, the polymers used to form
the component layers in the present invention can
include one membrane having carboxylic functional
groups and another membrane having sulfonyl
functional groups. Alternatively, two polymers
having similar ohemical composition, for example,
both polymers being characterized by sulfonyl groups,
~0 but substantially incompatible due to variation in
equivalent weight, can also be used in accordance
- with the present invention.
In general, polymers used in the present
invention having sulfonyl groups exhibit an
25 equivalent weight of about from 700 to 1600, and
preferably about from lO00 to 1200. In the event
that two fluorinated polymers having sulfonyl
- functional groups are used~ the equivalent weight of .:-
the polymers ~hould differ by at leas about 200 to ---
30 realize the required degree of incompatibility. --
In generalt the polymers having carboxylic
functional groups exhibit an equivalent weight of -
abo~t from 700 to 1200, and preferably about from
l,050 to 1,150.
4 .
B~
Each of the melt fabricable resins used
should be uniform or well-blended. The carboxylate
resin should be protected from atmospheric moisture
prior to coextrusion to prevent hydrolysis.
In accordance with the present invention,
`the final product contains at least two layers, and
preferably two or three layers. The total thickness
of the final product of the present invention can be
about from 25 to 250 microns. Particularly good
10 balance of cell voltage and durability in a
chloralkali cell is realized with a total thickness
of 75 ~o 150 microns~ and this composite thickness is
accordingly preferred. Each component of a laminate
of the present invention should comprise at least
15 about 5% of the total thickness of a composite. In
general, in a two component laminate prepared from
polymers having carboxylic and sulfonic functional
groups, respectively, the carboxyl containing polymer
preferably comprises about from 15 to 33~ of the
20 total thickness of the composite.
The present laminar structures are prepared
by coextruding the polymers above the melting point
of each polymer. A wide variety of mechanical
variations can be used, including commercially
25 available coextrusion equipment such as that
manufactured by Johnson Plastics Machinery Company of
Chippewa Falls, Wisconsin. For example, the
laminates can be prepared using multiple extruders
feeding into separate zones of a common chamber,
30 following which the layered stream is fed to a single
die. Another embodiment of the present invention
involves the use of a single die known in the film
processing industry as a "coat hanger'l die. In the
use of such a die, multiple extruders feed separate
35 streams into the specialized die. The die expands
31~
the width of the streams and joins ~he two streams
before they leave a sinyle exit orifice of the die.
Within the requirement that the polymers be
above the melting point, the particular processing
5 tempera~ure will depend on known operational
requirements for extrusion of films. The temperature
will be adjusted to obtain continuous smooth films of
uniform caliper without die drips or other defects.
Using the sulfonyl~containing and carboxyl-containing
lO polymers of the present invention, an extrusion
temperature range o about from 260 ~o 330C is
preferred.
After coextrusion of the laminates, they are
cooled to a temperature below the melting point of
15 each polymer and treated according to known
techniques to convert the functional groups to forms
more suitable for use in electrolytic cell
applications. Such conversion also renders the
polymers substantially less melt-fabricable.
A reinforcing layer is generally also added
to the composite structure. A wide variety of
support materials can be used, including woven fabric
or nonwoven material~ In the case of woven fabric,
weaves such as ordinary basket weave and leno weave
25 can be used. The reinforcement threads can be either
monofilament or multistrandedO
Particularly preferred reinforcement
materials are perhalocarbon polymer threads
optionally in conjunction with sacrificial fibers
30 which are dissolved by caustic or destroyed by
oxidizing agents. As used herein, the term
"perhalocarbon polymer" means a polymer which has a
carbon chain optionally containing ether linkages and
which is totally substituted by fluorine or by
35 fluorine and chlorine atoms~ Varticularly preferred
.~ 3~ 3
because of its inert: character is a perfluorocarbon
polymer. Typical of such polymers are those made
from tetrafluoroethylene and copolymers of
tetrafluoroethylene with hexafluoropropylene and/or
perfluorotalkyl vinyl ethers) wherein alkyl is from 1
to 10 carbon atoms.A preferred ether of this type is
perfluoro~propyl vinyl ether).
The reinforcing material is conveniently
applied to the sulfonyl-containing polymer in
conjunction with another sulfonyl-containing polymer
of the same equivalent weight. These two
sulfonyl-con~aining polymers are then laminated, with
the reinforcing material embedded in the composite
layer of sulfonyl-containing polymer.
The laminar structures of the present
invention are particularly useful as membranes for
chloralkali electrolytic cells. Among the cells in
which these laminar structures can be used are
narrow-gap cells, in which the gap between anode and
cathode is no greater than about 3 mm and the laminar
structure is in contact with at least one of the
anode and cathode. Such arrangements minimize the
resistance con~ributed by the anolyte and catholyte,
thus providing for operation at low voltage. The
25 membranes of this invention can also be used in a
solid polymer electrolyte or composite
electrode/membrane arran~ement, in which a thin
porous anode and/or porous cathode are attached
directly to the membrane surface, and rigid current
30 collectors can also be used in contact with these
electrodes.
In any of the above arrangements, either or
both of the electrodes can have a catalytically
active surface layer of the type known in the art for
35 lowering the overvoltage of an electrode. Such
electrocatalyst can be of a type known in the art,
such as those described in U.S. Patents 4,224,121 and
3,134,697, and published UK Patent Application GB
2,009,788A. Preferred cathodic electrocatalysts
include platinum black, Raney nickel and ruthenium
black. Preferred anodic electrocatalysts include
platinum black and mixed ruthenium and iridium oxides.
The membranes described herein can also be
modified on either surface or both surfaces thereof
so as to have enhanced gas release properties, for
example by providing optimum surface roughness or
smoothness, or, preferably, by providing thereon a
gas- and liquid~permeable porous non-electrode
layer~ Such non-electrode layer can be in the form
of a thin hydrophilic coa~ing or spacer and is
ordinarily of an inert electroinactive or
non-electrocatalytic substance. Such non-electrode
layer should have a porosity of 10 to 99~, preferably
30 to 70%, and an average pore diameter of 0~01 to
2000 ~icrons, preferably 0.1 to 1000 microns, and a
thickness generally in the range of 0.1 to 500
microns, preferably 1 to 300 microns. A
non-electrode layer ordinarily comprises an inorganic
component and a binder; the inorganic component can
25 be of a type as set forth in published UK Patent
Application G~ 2~064,586A, preferably tin oxide,
titanium oxide, zirconium oxide, or an iron oxide
such as Fe2O3 or Fe3O4. Other information
regarding non-electrode layers on ion-exchange
30 membranes is found in published European Patent
Application 0,031,660, and in Japanese Published
Patent Applications 56-108888 and 56-112487.
The binder component in a non-electrode
layer, and in an electrocatalyst composition layer,
35 can be, for example, polytetrafluoroethylene, a
3~
fluorocarbon polymer at least the surface of which is
hydrophilic by virtue of treatment with ionizing
radiation in air or a modifying agent to introduce
functional groups such as -COOH or -SO3H (as
described in published UK Patent Application G~
2,0~0,703A) or treatment with an agent such as sodium
in liquid ammonia, a functionally substituted
fluorocarbon polymer or copolymer which has
carboxylate or sulfonate functional groups, or
polytetrafluoroethylene particles modified on their
surfaces with fluorinated copolymer having acid type
functional groups (GB 2,064,586A~. Such binder can
be used in an amount of about from 10 to 50~ by wt.
of the non-electrode layer or of ~he electrocatalyst
composition layer.
Composite structures having non-electrode
layers and/or electrocatalyst composition layers
thereon can be made by various techniques known in
the art, which include preparation of a decal which
is then pressed onto the membrane surface,
application of a slurry in a liquid composition
(e.g., dispersion or solution) of the binder follo~ed
by drying, screen or gravure printing of compositions
in paste form, hot pressing of powders distributed on
the membrane surface, and other methods as set forth
in GB 2,064,586A. Such structures can be made by
applying the indicated layers onto membranes in
melt-fabricable form, and by some of the methods onto
membranes in ion-exchange Eorm; the polymeric
component of the resulting structures when in
melt-fabricable form can be hydrolyzed in known
manner to the ion-exchange form.
Non-electrode layers and electrocatalyst
composition layers can be used in combination in
various ways on a membrane. For example, a surface
of a membrane can be modified with a non-electrode
layer, and an electrocatalyst composition layer
disposed over the latter. It is also possible to
place on a membrane a layer containing both an
electrocatalyst and a conductive non-electrode
material, e.g. a metal powder which has a higher
overvoltage than the electrocatalyst, ~ombined into a
single layer with a binder. One preferred type of
membrane is that which carries a cathodic
electrocatalyst composition on one surface ~hereof,
and a non-electrode layer on the opposite surface
thereof.
Membranes which carry thereon one or more
electrocatalyst layersr or one or more non-electrode
layers, or combinations thereof, can be employed in
an electrochemical cell in a narrow-gap or zero-gap
configuration as described above.
In chloralkali electrolytic cells, the
present membranes show outstanding resistance to
damage or delamina4ion, particularly in non-standard
cell operating conditions. Moreover, the laminates
show particular resistance to delamination or
separation of the layers when the salt in the anolyte
is depleted below 50 gpl. The present laminar
structures also show increased storage life and
permit the efficient preparation of laminates with a
low frequency of defects.
The present invention is further illustrated0 in the following specific examples.
EXAMPLE 1
A two-component laminar structure was
prepared using an apparatus consisting of three
single barrel screw extruders, two of which were used
in this preparation, coupled with heated transfer
lines to a common chamber where the polymer streams
met. The combined polymer stream was transferred
from the cor~on chamber through another heated
transfer line to a six-inch slit die.
One extruder o~ this apparatus, having a one
inch diameter barrel, was charged with cubes of a
copolymer of tetrafluoroethylene (TFE) and methyl
perfluoro(4,7-dioxa-5-methyl-8-noneate) (EVE) having
an equivalent weight of 1,037 and melt flow of 25.5
10 at 270C. The extruder was operated at 20 rpm with a
barrel discharge temperature of 278C.
The hopper to a second extruder, which had a
1 1/4" diameter barrel, was charged with cubes of a
copolymer of tetrafluoroethylene (TFE) and
perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl
fluoride) (PSEPVE) having an equivalent weight of
1,090 and a melt 1Ow of 31.4 at 2S0C. This
extruder was operated at 11 rpm with a barrel
discharge ~emperature of 288C. The common chamber
and die temperatures were maintained at 277C.
A coextruded two-layered film was produced
at the rate of 2.5 cm/sec t5 ft/min~. The total
thickness of the coextruded film varied from 127 to
178 microns.
A piece of this film was hydrolyzed in a
bath containing 30% dimethylsulfoxide (DMSO; and 11%
potassium hydroxide for 20 minutes at 90C. The film
was rinsed and dried. Microtomed cross-sections were
prepared across the width of the film, stained with
Malachite green, and the relative thickness of the
layers measured with a microscope.
The layer containing the carboxylic
copolymer was continuous across the width of the
extruded film and its thickness was 18~ of the total.
12
A sample of the hydroly2ed dou~le layered
film was mounted in a chloralkali cell having an
active area of 45 cm between a dimensionally
stable anode and a mild steel expanded metal
cathode. The cell was operated at 80C with current
of 3.1 KA/m . The anolyte salt content was held at
200 gpl. Water was added to the anolyte to maintain
the concentration of caustic produced at 32 + 1%.
After 5 days of operation, the cell was operating at
95.1~ current efficiency and 3.68 volts. On the
fifth day the membrane was subjected to a simulated
failure of the brine feed. The brine was shut off to
the cell while electrical power, water to the
catholyte and temperature control were maintained.
After 5 hours of continued operation, the anode
chamber was half empty and the residual anolyte
contained only 1-2~ NaCl and the cell voltage had
increased to 5.0 volts. The cell was then shut down,
dissembled, and the membrane was inspected. The
membrane was in perfect condition with no evidence of
separation of the layers.
EXAMPLE 2
The extrusion conditions of Example 1 were
repeated except that the speed of the first extruder
25 was ~educed to 10 rpm. A coextruded two-layered film
was produced at the rate of 2.5 cm/sec (5 ft/min).
The total thickness of the coextruded film varied
from 100 to 152 microns.
A sample of this film was hydrolyzed and
30 examined microscopically as in Example 1. The layer
containing the carboxylic polymer was continuous
across the width of the extruded film and its
thickness averaged 9~ of the total thickness of the
film.
A sample of the hydrolyzed film was mounted
in a laboratory cell with the side containing the
carboxylic polymer toward the cathode. After 5 days
of operation, it was performing at 94.9% current
efficiency and 3.65 volts. The membrane was subject
to a simulated failure of the brine feed as in
Example 1. Upon inspection and examination, the
membrane was in perfect condition with no evidence of
blistering.
COMPARATIVE EXAMPLE A
-
A film was extruded from the TFE/PSEPVE
copolymer used in Example 1, except that the polymer
had an equivalent weight of 1100 and gave a film with
a thickness of 100 microns. A quantity of TFE/EVE
copolymer of 1080 e~uivalent weight was extruded
separately to yield a film of 50 micron thickness. A
sample of each of the above films was chosen and
these samples were then pressed together to exclude
air between the layers and thermally bonded by passing
through a thermal laminator supported on a continuous
web of a porous release paper through which vacuum
was app]ied. The temperature of the laminator was
controlled so that the temperature of the laminate
reached 230-235C at the end of the heated zone.
This thermally bonded double layer film was
then hydrolyzed and tested in a laboratory cell as in
Example 1. After a similar period of 5 days, the
membrane was performing at 97.5% current efficiency
and 3.61 volts, producing 32 + 1% Na~H. The cell was
operated for a total of ~2 days, at which time the
membrane was subjected to a simulated failure of the
brine feed system as in Example 1. Upon cell
disassembly and inspection of the membrane, it was
found to be extensively blistered. Approximately 75%
of the area in the active area of the cell was
13
14
covered with liquid-filled pockets or blisters. I'he
separations had occurred at the interface of the
TFE/PSEPVE and the TFE/EVE layers.
EXAMPLE 3
A portion of the double layered film from
Example 1 in the unhydrolyzed state was laid on top
of an open weave fabric prepared from monofilaments
of a mel~-extrudable perfluorocarbon resin tPFA).
Below the cloth was placed a 50 micron film of
TFE/PSEPVE copolymer of 1100 equivalent weight. This
sandwich was then passed through a thermal laminator
supported on a continouus web of a porous release
paper with vacuum, The temperature of the laminator
was adjusted so that the temperature of the laminate
reached 230-235C at the end of the heated zone. The
perfluorocarbon resin fabric was embedded or
encapsulated in the TFE/PSEPVE copolymer~ The
laminate was hydrolyzed as in Example 1 and found to
have greatly improved tear resistance over the
unreinforced double layer films of Examples 1-2 and
Comparati~e Example A.
A portion of this laminate was evaluated in
a laboratory cell as in Example 1. After two days
the performance was 95.2~ current efficiency and 3.81
volts, operated at 80C, 3.1 KA/m , 200 gpl NaCl in
the anolyte and 32 + 1~ NaOH. The anolyte
concentration was then reduced to 115 gpl salt in the
anolyte to simulate the partial stoppage of brine
flow to the anolyte chamber. The cell was operated
for an additional 26 days at this high salt depletion
and then shut down and the membrane examined. ~o
separation or blisters in the membrane wexe found~
EXAMPLE 4
A coextruded double layer film was prepared
using the general procedure of Example 3, but with a
1~
different type of reinforcing cloth. The cloth
consisted of a warp and fill of 20 threads per inch
of a 200 denier polytetrafluoroethylene fiber and 40
threads per inch of a 50 denier rayon. In the
lamination procedure, a 25 micron film of the 1100
equivalent weight TFE/PSEPVE copolymer was used below
the cloth instead of the 50 micron film used in
Example 3.
After hydrolysis the laminate was evaluated
in a cell under the same conditions as Example 1
After 9 days its performance was 95.3% current
efficiency and 3~69 volts. After 11 days the
membrane was subjected to a stoppage of the brine
feed as in Example 1. The cell was restarted and the
laminate continued to perform at 95-96% current
efficiency and 3.70 volts. After 30 days the cell
was shut down and disassembled. Upon inspection, the
membrane appeared to be in perfect condition with no
blisterin~ apparent.
COMPARATIVE EXAMPLE B
Separate films of 50 micron TFE/EVE and 100
micron TFE/PSEPVE wre pressed together as in
Comparative Example Ao This composite was then used
to prepare a fabric-reinforced laminate as in Example
~5 3. After hydrolysis the performance of the membrane
was evaluated in a cell where after 5 days it was
performing at 95~ current efficiency and 3.9 vol~s
under the conditions of Example 1. This membrane was
sub]ected to a simulated falure of the brine feed
system as in Example 1. Upon removal from the cell
and inspection, the membrane was found to be
extensively blistered with approximately 50% of the
area in the active area of the cell covered with
liquid-filled pockets or blisters.
