Note: Descriptions are shown in the official language in which they were submitted.
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MD27101
This invention relates to porous diaphragms for
electrolytic cells.
More particularly, the invention relates to porous
diaphragms based on tetrafluoroethylene polymers. Such
diaphragms are especially suitable for use in cells
electrolysing alkali metal chloride solutions.
In the specification of our UK Patent 1,081,046 there
is described a method of manufacturing porous diaphragms
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which comprises forming an aqueous dispersion of
polytetrafluoroethylene and a solid particulate additive
such as starch, adding an organic coagulating agent such as
acetone to said dispersion and then drying the coagulated
dispersion. An organic lubricant such as petroleum ether
is then added to the dried coagulated material to serve as
a processing aid when the material is being rolled into a
sheet. On completion of the rollin~ operation the starch
is removed to give the desired porous diaphragm. The
lubricant can also be removed if required.
An improved method of manufacturing porous diaphragms
in which the organic lubricant is replaced by water as the
lubricant is described in the specification of our Canadian
Patent No. 1,004,819 issued on 8th February, 1977. This
method comprises preparing an aqueous dispersion comprising
polytetrafluoroethylene and a removable solid particulate
additive, thickening said aqueous dispersion to effect
agglomeration of the solid particles therein, forming from
the thickened dispersion a dough-like material containing
sufficient water to serve as lubricant in a subsequent sheet
forming operation, forming a sheet of desired thickness from
said dough, for example by calendering and removing solid
particulate additive from the sheet.
Suitable removable solid particulate additives include
starch, for example maize starch and/or potato starch, or
a water-soluble inorganic base or carbonate~ for example
calcium carbonate. If desired, these solid particulate ;
additives may be removed from the diaphragm prior to
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introducing the diaphragm into the cell, for example, by
soaking the diaphragm in an acid, preferably a mineral acid
e g. hydrochloric acid. The diaphragm is then washed with
water to remove the acid and assembled, whilst wet, into
a cell Alternatively, the solid particulate additives may
be removed from the diaphragm in situ in the cell, for example
as described in our copending Canadian application Serial No
205,247 filed on 9th August, 1974, in which there is described
the removal of such additives by treating unextracted diaph-
ragm with an acid, for example hydrochloric acid, containinga corrosion inhibitor, for example propargyl alcohol, or
by filling the cell with a working electrolyte e.g. with an
alkali metal chloride brine and electrolysing the said electro-
lyte
The aforesaid porous diaphragms based on polytetra-
fluoroethylene may be used without any support, but it is
preferred to strengthen the diaphragm by¦incorporating a
sheet of a suitable strengthening material, for example a
polymer gauze such as a polypropylene or fluorinated ethylene/
propylene copolymer gauæe The extra strength of the supported
polytetrafluoroethylene increases the ease of handleability of
the porous diaphragm during assembly into the cell, and the
support substantially prevents any extrusion of polytetra-
fluoroethylene through the coarse net of the metal cathode
during cell operation as a result of the hydrostatic head
applied. Polypropylene is especially suitable as the polymer
gauze because it is flexible, relatively cheap as compared with
fluorinated ethylene/ .
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propylene copolymers for example, and is chemically resistant
to the catholyte liquors in the cell The polypropylene
support may be attached to the polytetrafluoroethylene
diaphragm by calendering the two together through a narrow
setting on the calender rolls, but in practice, this has
not proved entirely satisfactory because of the poor adhesion
of polypropylene to polytetrafluoroethylene After intensive
calendering, the support can separate easily from the
diaphragm with consequent difficulties in handleability of
the diaphragm.
We have now found that the aforesaid disadvantages
associated with the preparation of porous polytetrafluoro-
ethylene diaphragm with polypropylene supports may be obviated
or mitigated by the use of a particular adhesive in the course
of the preparation
According to one aspect of the present invention
we provide a method of manufacturing a porous diaphragm based
on polytetrafluoroethylene which comprises the step of coating
with an aqueous dispersion of polytetrafluoroethylene a
surface of a polypropylene gauze and/or a surface of a sheet
of polytetrafluoroethylene which is already porous or which
contains a removable solid particulate material, and
contacting the said surfaces under pressure thereby providing :
a strengthening support for the diaphragm.
According to another aspect of the present invention :
we provide a method of manufacturing a porous diaphragm
based on polytetrafluoroethylene which comprises forming a ~
sheet of polytetrafluoroethylene in admixture with a solid :~ -
particulate additive to be removed therefrom, coating a
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surface of a polypropylene gauze and/or a surface of the sheet
of polytetrafluoroethylene with an aqueous emulsion of
polytetrafluoroethylene, bonding the polytetrafluoroethylene
sheet to the polypropylene gauze by contacting the said
surfaces under pressure, and subsequently removing the
solid particulate additive from the polytetrafluoroethylene
sheet. A.
It is preferred to coat the polypropylene gauze rather
than the polytetrafluoroethylene sheet, with the aqueous
dispersion of polytetrafluoroethylene since uniform
covering of the gauze can be ensured by using excess of the
aqueous dispersion and by allowing the excess to pass through
the pores in the gauze.
The aqueous dispersion of polytetrafluoroethylene is
conveniently the same aqueous dispersion used in the
preparation of the unextracted polytetrafluoroethylene
diaphragms as described below. Suitably, the aqueous dispersion
contains 20% to 8Q% by weight of polytetrafluoroethylene,
for example 60%. The preferred particle size of the
polytetrafluoroethylene in the aqueous dispersion, whether
used for coating the polypropylene gauze or for preparing the
polytetrafluoroethylene diaphragm, is in the range of 0.05
to 1 micron, for example 0.2 micron.
The polypropylene gauze may be coated with an aqueous
dispersion of polytetrafluoroethylene using any of the
conventional methods of spraying, brushing or dipping.
The coated polypropylene gauze is then placed on top of the
porous or the unextracted polytetrafluoroethylene diaphragm
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and the contacting surfaces are bonded by applying pressure,
most suitably b~lcalendering.
The une~tracted diaphragms may conveniently be prepared
from aqueous dispersions of polytetrafluoroethylene and
the solid particulate additive by the methods described in
our UK Patent No 1, 081, 046 and in our Canadian Patent
-No. 1,004,819, both of said methods being referred to
a~ove.
The solid particulate additives may be removed from
the supported polytetrafluoroethylene diaphragms, either
before or after assembly in the cell, by the methods
described hereinbefore.
Generally, the solid particulate additive has a particle
size substantially all of which are within the range of 5
to 100 microns. The amount of additive will depend on the
permeability desired in the final diaphragm. Thus, the
weight ratio of additive to polytetrafluoroethylene may be,
for example, from 10:1 to 1:10 preferably from 5:1 to 1:1.
In the preparation of the unextracted polytetrafluoro-
ethylene diaphragms, in many cases it is desirable to
incorporate other components in the aqueous dispersion which
are not removed when the sheet is subjected to the treatment
to remove the particulate additive. Examples of such
components include particulate filler generally inorganic
fillers, for example, titanium dioxide which is particularly
preferred, barium sulphate, asbestos, (for example amphibole
or serpentine asbestos), graphi~e and alumina~ Suitably
the non-removable filler has a particle size of, for example,
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less than 10 microns and preferably less than 1 micron. The
weight ratio of filler to the polytetrafluoroethylene may
be for example from 10:1 to 1:10, preferably from 2:1 to
1:2
The diaphragms thus produced are particularly suitable
for use in electrolytic cells for the electrolysis of
alkali metal halides, for the production of chlorine and
caustic alkalies
The invention is illustrated but notlimited in the
following Example in which all parts and percentages are
by weight
EXAMæLE
To 100 parts of an aqueous dispersion of polytetra-
fluoroethylene containing 60% of the polymer in the form of
particles approximately all in the size range 0 15 to 0 2
micron were added 101 parts of water, 60 parts of titanium
dioxide of particle size approximately 0 2 micron, 60 parts
of maize starch of particle size approximately 13 microns ~
and 120 parts of potato starch of particle size less than ;~ -
75 microns. The mixture was then stirred with a paddle
mixer for 30 minutes to form a substantially uniform paste.
This paste was spread on trays and dried at 24 for 48 hours
to a water content 5,7% by weight. 100 parts of the
resultant crumb were mixed with 52 parts of water to form
a dough having a viscosity of 4 x 10 poise The dough was
then spread along the shortest edge of a rectangular piece
of card, and calendered on the card between dual, even-speed
calender rolls, set 3 mm apart, into an oblong sheet
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~fter calendering, the oblong sheet was cut, in the
direction of calendering, into four equal pieces These
were laid congruently over each other to obtain a four-
layered laminate The card was picked up~ rotated 90 in
the horizontal plane, and calendered (directed 90 to the
original direction of calendering) again through the 3 mm
roll separation This process, the successive cutting into
four, stacking, rotating and calendering was repeated until
the composition had been rolled a total of five times The
lQ resultant laminate was cut into four, in the direction of
calendering, stacked, removed from the card, and calendered,
without rotation through 90, theinter-roll space being .
reduced by the thickness of the card After calendering,
the laminate was cut, at right angles to the direction of
calendering, into four equal pieces, stacked, rotated through
90 and calendered again. This process, cutting right angles
to the direction of calendering, stacking, rotating and
calendering was repeated until the composition had been
rolled a total of nine times The resultant essentially
rectangular laminate was then passed through the rolls with
its largest side directed at 90 to the direction of calendering
and with the inter-roll space slightly reduced, no cutting,
stacking or rotating through 90 being involved. This process
was repeated through a gradually reduced inter-roll space,
the same edge of the laminate was 1.5 mm
A square of 22 x 26 mesh gauze woven of 0.011 inch
diameter monofilament polypropylene yarn was sprayed with
an aqueous dispersion of polytetrafluoroethylene containing -
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~060Z86
60% of the polymer in the form of particles approximately
all in the size range 0 15 to 0.2 micron The coated
polypropylene was gauze was allowed to dry for about 5
minutes, placed on top of the laminate, and rolled into the
laminate by calendering through a slightly reduced inter-roll
space
The resultant reinforced sheet was removed from the
rolls and subsequently assembled into an electrolytic cell.
It was observed during storage and before assembly into the
cell that the polypropylene gauze remained firmly attached
to the unextracted polytetrafluoroethylene sheet
The cell was filled with sodium chloride brine at 60C
and allowed to stand for 1 hour After 1 hour the current
was switched on to commence electrolysis of the brine. Initial
voltage was 4.1 volts at 2 kA/m . At this stage there was
no flow through the sheet. After 2 hours on load, cell
voltage had dropped to its usual value of 3.0 volts at 2 kA/m2.
Flow through the diaphragm commenced after 10 hours, and after
18 hours had reached its design valua. Removal of starch
from the sheet could be followed by analysis of carbon
dioxide in the gaseous chlorine From an initial level of
7% carbon dioxide concentration decreased steadily until
after 18 hours it was constant at 0.5%, the levelattributable
to excess carbonate is feed brine, this indicating that ;~
oxidation of starch was complete. After 24 hours, satisfactory
cell operation at a current efficiency of 96 5% at 50% ~;
conversion was achieved ~
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