Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to diaphragms for electrolytic
cells.
In copending Canadian application No. 244,710, filed
on January 29, 1976, there is disclosed an electrolytic cell
assembly comprising an anode, a cathode and a diaphragm there-
between, wherein the diaphragm comprises discrete fibers of a
self-bGnding thermoplastic material, the fibers having a
diameter of from 0.05 to 40 microns, preferably from 0.05 to
10 microns.
As is known to those skilled in the art, conventionally
asbestos is employed as the diaphragm material. Asbestos has
many advantages as a diaphragm material. These advantages have
been reported and detailed a plurality of times. Yet, there
exists a plurality of disadvantages attendant the use of asbestos
diaphragms, such as swelling of the diaphragm.
Although the invention of the copending application
provides more than adequate substitutive materials for asbestos,
there still exists a need for improving asbestos diaphragms. ~ -
Contemporaneously, it would be advantageous to improve the fiber
diaphragms of the copending application. -
According to the invention, there is provided a
diaphragm for an electrolytic cell, which diaphragm comprises
a body of fibers cemented one to another by a cementing agent
consisting of a heat-fusible hydrophobic material and containing
a hereinafter-defined wetting agent to counteract the hydro-
phobic character of said heat-fusible hydrophobic material, a
wetting agent also being present in the fiber material when the
latter, in the absence of a wetting agent, is hydrophobic in
nature.
The cementing agent preferably is a fluorohydrocarbon
or contains a fluorohydrocarbon.
The fibers of the diaphragm may be made of thermo~-
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plastic or thermosetting resin or may be made of an inorganic
fibrous material. Examples of fibrous materials are, inter alia,
polyolefins, polycarbonates, polyesters and polyamides, specific
examples being polyethylene, polypropylene, hexamethylene adip-
amide and other nylons, polyethylene terephthalate, poly - 4 -
methylpentene -1, polyltetramethylene) terephthalate, poly-
styrene - polyvinylidene chloride copolymers, chlorinated
polyvinyl chloride, polycarbonates of 2 - (4 - hydroxymethyl)
propane (Bisphenol A) and polyphenylene oxide.
Preferably, the diaphragm material is asbestos,
The fiber material may be a mixture of two or more
different materials.
Because of the hydrophobic nature of the thermoplastic
fibers, which is true also for at least part of the surfaces
of the fiber body when made of other fibers, upon cementation,
it is necessary to include within the internal structure or
matrix of the fibers, per se, a hydrophilic material to ensure
the wetting ability of the diaphragm. The term "wetting agent"
used herein means a perfluorinated organic material or an
inorganic material other than asbestos e.g., are the inorganic
materials specified below: -
mica, titanates, such as barium titanate and potassiumtitanate, talc, vermiculite, titanium dioxide, boron nitride,
kaolinite, diatomaceous earth and clays, as well as mixtures
thereof.
Preferred wetting agents are the perfluorinated fatty
acids, alcohols or sulfonate-based surfactants.
Although any heat-fusible cementing agent that is
capable of resisting chemical attack in an electrolytic cell
can be utilized, the fluorohydrocarbons have been found to
be eminently useful. Usually, the cementing agents are utilized
where the diaphragm material is non-fluorinated, or where the
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fibers,per se, are not self-bonding, but the use of these agents
is applicable with all fibers. The cementing agent is in -
general employed in an amount ranging from ten to thirty per-
cent, by weight thereof, based on the weight of the fiber.
Preferably, the cementing is employed in an amount ranging
from fifteen to twenty-five percent, by weight thereof, based ~-
on the weight of the fiber.
The cementing agent may be employed as a dry powder
or as a solution thereof in a non-reactive solvent. When`employed
as a powder, the agent can conveniently be admixed with a fiber
slurry from which to deposit, for example on a cathode screen,
the fibers. Where deployed as a solution, one useful method ~;
, for applying the cementing agent is achieved by first depositing
the fiber slurry on a cathode screen and thereafter, contacting
the deposited fibrous diaphragm with the solution of the cement-
ing agent.
Afrer the cementing agent has been applied, the
diaphragm is then heat treated, conveniently at a temperature
ranging from 100C to 400C for a period of from one-half to
one hour, to fuse the cementing agent and so effect cementation
of the fibers. The temperature and duration of treatment, of
( course, vary with the selected fiber used for the diaphragm
; and with the cementing agent.
Useful fluorohydrocarbon cementing agents contemplated
for use in the present invention include, for example, fluori-
nated ethylene propylene, polyethylene tetrafluoroethylene,
polyethylene chlorotrifl~loLoethylerle, polychlorotrifluoro-
ethylene and polyvinylidene fluoride, as well as mixtures
~; thereof.
Suitable solvents for deploying the fluorohydrocarbon
as a solution include nitrobenzene and hexachlorobenzene.
Generally, a one to ten percent, by weight, solution is utilized.
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In practising the present invention, it is preferred
that where used, the thermoplastic fibers have a fiber diameter
of from 0.05 to forty microns.
The following Examples are intended to illustrate the
invention. In the Examples, all parts are by weight.
EXAMPLE I
To a conventional two percent asbestos slurry contain-
ing 0.5% , (based on the weight of the asbestos) of Fluorad
FC-126 fluorohydrocarbon surfactant ("Fluorad" is a Registered
Trade Mark), was added twenty percent (based on the weight of
the asbestos) of polyethylene chlorotrifluoroethylene powder.
The powder was mixed with the slurry to render the slurry
uniform.
The slurry was then deposited onto a cathode screen
by vacuum deposition substantially as described in Example I
of our copending Application No. 244,710.
The diaphragm was then dried in an oven at 100C
for eight hours. Thereafter, the diaphragm was heat treated to
bond the asbestos fibers to each other as well as to the
cathode, at 260C for one hour.
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The diaphragm was then mounted in a test chlor -
alkali cell and brine electrolysis was carried out therewithin.
The cell having the diaphragm mounted therewithin produced 130
grams per liter of caustic at 95~ current efficiency.
EXAMPLE II
This Example illustrates the use of a cementing agent
with a self-bonding fiber.
Following the procedure of Example I of Application
No. 244,710, a polyethylene chlorotrifluoroethylene fiber
slurry containing the same surfactant as defined therein was
deposited on a cathode screen. After deposition, the fiber-
screen assembly was dried in a vacuum oven at 100C. for a
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period of about eight hours. The dried fibers were then
impregnated with a one percent solution of polyethylene chloro-
trifluoroethylene in nitrobenzene at 70C as a cementing agent.
This was achieved by immersing the assembly within the solution. -
Thereafter, the impregnated fiber-screen assembly was heated at
250C for about one-half hour to bond the fibers.
The cathode was then deployed in a brine electrolysis
test cell. The cell produced a caustic concentration of 130
grams per liter with a 90% current efficiency.
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