Note: Descriptions are shown in the official language in which they were submitted.
~0~ 17
This invention relates to improvements in the manu-
facture of porous diaphragms~
More particularly, the invention relates to improve-
ments in the manufacture of porous diaphragms suitable for use
in electrochemical cells.
Porous diaphragms based on tetrafluoroethylene
polymers are especially suitable for use in cells for the
electrolysis of alkali metal chloride solutions. In our U.K.
Patent Specification No. 1,081,046 there is described a method
of manufacturing such diaphragms which comprises forming a filled
polytetrafluoroethylene sheet from an aqueous dispersion of
polytetrafluoroethylene and a removable solid particulate
additive, e.g. starch, by adding an organic coagulating agent
such as acetone to said dispersion and then drying the co-
agulated 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 rolling operation the starch is
removed from the sheet to give the desired porous diaphragm.
The lubricant can also be removed if required.
Another method of manufacturing such porous diaphragms
in which the organLclubricant is replaced by water as lubricant
is described in our Canadian Patent No. 1004819 issued on 8th
February, 1977. This method comprises forming a filled poly-
tetrafluoroethylene sheet from an aqueous dispersion comprising
polytetrafluoroethylene and a removable solid particulate ad-
ditive by thickening said aqueous dispersion to effect agglo-
meration of the solid particles therein, forming from the
thickened dispersion a sheet-formable material containing suf-
ficient water to serve as lubricant in a subsequent sheet forming
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operation, forming a sheet of desired thickness from said
material, for example by passing the material through calender
rolls and removing solid particulate additive from the sheet.
As indica-ted above, suitable removable solid particulate
~, ~ _ , _ _ _ . ,, , _ , , , , , , , . . , . . . , _ _ _ _ _
. _ _ .. _ . _ . .. _ . , . . , ... ... .. .. .. .., . , .., .. _ _, ... _ _ _ _ _ _
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104~217
additives include starch, e.g maize and/or potato starch~
Other removable additives are water-insoluble inorganic bases
or carbonates, e.g calcium carbonate. Cellulose also is a
suitable additive If desired, these solid particulate ad-
ditives, which constitute the means of imparting porosity
to the diaphragm, may be removed from the diaphragm prior to
introducing the diaphragm into the electrolytic cell. Alter-
natively, the solid particulate additives may be removed from
the diaphragms in situ in the cell
Hereafter in this specification and claims the
term filled polytetrafluoroethylene sheet shall mean poly-
tetrafluoroethylene sheet containing removable solid parti- -
culate additive as described above
Unfortunately, however, there are problems asso-
ciated with the development of the use of such diaphragms in
electrolytic cells For example, there is generally a limit
on the dimensions of the diaphragm sheets that can be produced
in practice Of necessity the width of the diaphragm sheet
is governed by the size of the rolls employed in producing the
sheet. The cost of increasing the size of the manufacturing
equipment is exponential with the result that there is an
optimum size of roll which is dependent upon purely commercial
factors, Moreover, diaphragms of simple rectangular sheet form
are extremely difficult to fit onto the complicated cathode
designs of modern diaphragm cells because of the numerous
recesses and protuberances presented by the cathode. The
aforesaid
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104~217
problems are accentuated in the case of diaphragms made
of non-melt-processable materials such as PTFE. The main
reason for this is that it is extremely difficult to join
together small sheets of polytetrafluoroethylene in order
to produce a diaphragm of the desired complex shape and size.
It is an object of the present invention to provide a method
of manufacturing polytetrafluoroethylene diaphragms which
obviates or mitigates the aforesaid disadvantages.
According to the present invention there is pro-
vided a method of manufacturing a porous diaphragm for anelectrolytic cell from a plurality of sheets of filled poly-
tetrafluoroethylene which method comprises fusing a melt-
processable fluorine-containing polymer into said sheets at
or near juxtaposed edges of said sheets at a temperature which
will not substantially decompose the filler in said sheets,
solidifying the melt-processable polymer 54 as to effect
joining of the sheets and thereafter removing filler from
the thus joined sheets.
In one embodiment of the invention two or more
sheets of filled polytetrafluoroethylene are joined along
juxtaposed edges by overlapping said edges with one or more
strips of melt-processable fluorine-containing polymer and
fusing said strip or strips into the areas of the sheets
adjacent to said juxtaposed edges.
However, in a preferred embodiment of the invention,
one or more strips of melt-processable fluorine-containing
polymer can be made to partially overlap one or more edges of
a sheet of filled polytetrafluoroethyethylene and protruding
portions of said strip or strips can be utilised as desired
to bond said polytetrafluoroethylene sheet to other polytetra-
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. ,. . -
104~217
fluoroethylene sheets which have not had melt-processable
strips of fluorine-containing polymer fused thereto. Con-
veniently all four sides of a rectangular sheet of filled
polytetrafluoroethylene can be pro~lided with overlapping
strips of melt-processable fluorine-containing polymers
to give a window-frame effect and such unit window-frames
of melt-processable polymer can be joined to other filled
polytetrafluoroethylene sheets by conventional plastics
fabrication techniques.
The last mentioned procedure can be modified by
replacing the strips of melt-processable fluorine-containing
polymer with tabs of melt-processable polymer at intervals
along one or more edges of a filled polytetrafluoroethylene
sheet.
As aforesaid, the melt-processable fluorine-contain-
ing polymer must be such that it fuses into the filled poly-
tetrafluoroethylene sheet below the temperature at which the
filler substantially decomposes. For example, in the case
where the filler is starch, the upper temperature for the
~0 fusion process must not exceed 300C. Furthermore, it is
important that the melt processable polymer be impermeable
or of porosity not greater than that of the eventual poly-
tetrafluoroethylene diaphragm. Consequently the temperature
at which fusion takes place must not be so high as to allow
decomposition products to blow holes in the melt-processable
polymer.
When the diaphra-~m manufactured according to the
present invention is intended for use in electrolytic cells
then the melt-processable fluorine-containing polymer must
be resistant to conditions in the cell.
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The melt-processable fluorine-containing polymer
used in the presen-t invention is preferably one which sub-
stantially returns to its original form on the removal of
heat and also retains its original properties. By comparison,
polytetrafluoroethylene, which is considered as a non-melt-
processable material in the context of the present invention,
fuses when heat is applied but also decom~oses within a few
degrees of its melting point. In other words the melt vis-
cosity of polytetrafluoroethylene is too high for the ap-
plication of conventional plastics fabrication techniques.
Preferably, the melt-processable fluorine-con-
taining polymer is selected from polychlorotrifluoroethylene,
polyvinylidene fluoride, FEP (a fluorinated ethylene/propylene
copolymer) or a copolymer of ethylene and chlorotrifluoroethy-
lene.
Conveniently the melt-processable fluorine-containing
polymer is fused into the filled polytetrafluoroethylene
sheet by the application of heat and pressure. The temperature
at which the melt-processable polymer is fused into the
polytetrafluoroethylene sheet preferably is lower than the
melting point of polytetrafluoroethylene. The temperatures
and pressures employed depend upon the specific melt-process-
able polymer involved. We have found, however, that in
most cases it is advantageous to operate at a constant
pressure of around 10 psi and to apply heat over a varying
period of time which does not result in deformation of the
diaphragm.
The present invention is also a porous polytetra-
fluoroethylene diaphragm whenever manufactured by a process
as hereinbefore described.
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The diaphragms of the invention may contain a
non-removable filler such as titanium dioxide in order
to render the diaphragm wettable when installed in an
electrolytic cell.
Embodiments of the invention will now be described
with reference to the following Examples in which all parts
and percentages are by weight.
EXAMPLE 1
.
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 ho-irs to a water content of 5.7%. 100 parts of the
resultant crumb we~e mixed with 52 parts of water to form
a sheet-formable material akin to a dough having a viscosity
of 4 x 106 poise. The sheet-farmable material 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.
After 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 lami-
nate. The card was picked up, rotated 90 in the horizontal
plane, and calendered (directed 90 to the original direction
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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 resultant lami-
nate was cut into four, in the direction of calendering,
stacked, removed from the card, and calendered, without
rotation through 90, the inter-roll space being reduced by
the thickness of the card~ After calendering, the laminate
was cut, at xight angles to the direction of calendering,
into four equal pieces, stacked, rotated through 90 and
calendered again. This process, cutting at right angles
to the direction of calendering, stacking, rotating and
calendering was repeated until the composition had been rGlled
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 being fed to the rolls on each
occasion, until the thickness 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 placed on top of the
laminate, and rolled into the laminate by calendering
through a slightly reduced inter-roll space.
The edges of two sheets of filled polytetrafluoroethylene
prepared as aboJe were brought together in a butt-joint and
a strip of FEP of thickness 0.02 inch was laid along the
length of the butt-joint. This composite join was then
heated by means of heated plattens under a pressure of 10 psi
_ g
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1046217
for a period of 10 minutes until a temperature of 275 C
was attained. By this means an adequate joining of the
two PTFE sheets was effected, charring of the starch being
minimised.
The aforesaid sample of joined PTFE sheets was then
immersed in 5N HCl in order to remove the starch filler.
A porous PTFE diaphragm suitable for use in diaphragm cells
was produced. After removal of the starch, the PTFE/FEP
joint was still effective.
EXAMPLE 2
Two starch filled PTFE sheets prepared according to
Example 1 but with an FEP backing sheet instead of poly-
propylene were joined together at their edges by means of
a strip of FEP of thickness 0.01 inch, The welding took
place between parallel flat heating elements 5/16 inch wide
and under a pressure of 24 psi. The elements were lightly
greased with silicone grease to prevent sticking. Pressure
was maintained on the weld for 30 seconds after turning
off the welding cur~ent. The maximum temperature attained
was 280 C and was measured between the interfaces of the
materials with a dwell time of between one and two seconds.
The sample of joined PTFE sheets was immersed in
5N HCl in order to remove the starch filler. A porous PTFE
diaphragm suitable for use in diaphragm cells was produced.
EXAMPLE 3
Two PTFE sheets prepared according to Example 1 but
with cellulose as filler and with FEP backing sheet were
joined together by the technique of Example 2 with the
exception that current was only supplied to the element in
contact with the 0.01 inch thick FEP strip to prevent
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104~Z17
undue degradation of the cellulose. An adequate join was
obtained and the cellulose filler was removea as before to
give a porous PTFE diaphragm.
EXAMPLE 4
The juxtaposed edges of two PTFE sheets prepared
according to Example 1 with starch as filler and each
having an FEP backing sheet were joined together by the
technique of Example 2 except that a strip of Halar (a
copolymer of ethylene and chlorotrifluoro ethylene) (Halar ,
is a Registered Trade Mark) of thickness 0.005 inch was
placed over the butt joint in place of the FEP, and a maxi-
mum temperature of 260C was used. An adequate joining
of the two PTFE sheets was effected and the starch was
removed as before to give a porous diaphragm.
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