U.S. patent number 3,890,417 [Application Number 05/326,475] was granted by the patent office on 1975-06-17 for porous diaphragms.
This patent grant is currently assigned to Imperial Chemical Industries Limited. Invention is credited to Christopher Vallance.
United States Patent |
3,890,417 |
Vallance |
June 17, 1975 |
Porous diaphragms
Abstract
Porous diaphragms suitable inter alia for electrolytic cells are
formed from an aqueous slurry or dispersion of
polytetrafluoroethylene and a solid particular additive utilising
water as lubricant in the sheet forming operation.
Inventors: |
Vallance; Christopher (Runcorn,
EN) |
Assignee: |
Imperial Chemical Industries
Limited (London, EN)
|
Family
ID: |
9794471 |
Appl.
No.: |
05/326,475 |
Filed: |
January 24, 1973 |
Foreign Application Priority Data
Current U.S.
Class: |
264/49;
210/500.25; 210/500.27; 264/127; 156/77 |
Current CPC
Class: |
H01M
50/411 (20210101); B32B 27/20 (20130101); C25B
13/08 (20130101); C08J 9/26 (20130101); Y02E
60/10 (20130101); C08J 2327/18 (20130101) |
Current International
Class: |
C25B
13/00 (20060101); C25B 13/08 (20060101); C08J
9/26 (20060101); C08J 9/00 (20060101); H01M
2/16 (20060101); C08f 047/08 () |
Field of
Search: |
;264/49,127 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
241,695 |
|
Nov 1962 |
|
AU |
|
639,568 |
|
Apr 1962 |
|
CA |
|
961,494 |
|
Jun 1964 |
|
GB |
|
1,081,046 |
|
Aug 1967 |
|
GB |
|
Primary Examiner: Griffin; Ronald W.
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
What we claim is:
1. A method of manufacturing a porous diaphragm suitable for use in
electrolytic cells comprising preparing an aqueous dispersion of
polytetrafluoroethylene and a solid particulate removable filler,
thickening said aqueous dispersion to effect agglomeration of the
solid particules therein, forming from the thickened dispersion a
dough-like material containing water as lubricant for a subsequent
sheet forming operation, calendering said dough-like material to
form a biaxially fibrillated sheet and removing the solid
particulate filler from the sheet to render it porous.
2. A method as claimed in claim 1 wherein the thickening of the
aqueous dispersion is effected by reducing the water content
thereof and water is then added to the thus thickened material to
form the dough.
3. A method as claimed in claim 2 wherein the desired degree of
lubrication for the sheet forming operation is obtained by mixing
water with the thickened material so that a dough having a
viscosity of at least 300 poises at 20.degree.C is obtained.
4. A method as claimed in claim 2 wherein the drying of the aqueous
dispersion is carried out at a temperature of 10.degree. to
100.degree.C.
5. A method as claimed in claim 1 wherein said aqueous dispersion
of polytetrafluoroethylene and a solid particulate removable filler
is dried to a water content of no more than 10% of the total weight
of the dried dispersion prior to the addition of further water for
the formation of the dough.
6. A method as claimed in claim 1 wherein water is added to the
dried dispersion until a dough is attained which has a water
content comprising 20 to 45% of the total weight of the dough.
7. A method as claimed in claim 1 wherein the aqueous dispersion is
thickened by subjecting it to high shearing action and the high
shear action is continued so that a dough having a viscosity of at
least 300 poises at 20.degree.C is obtained.
8. A method as claimed in claim 7 wherein the dispersion has a
water content comprising 2 to 50% of the total weight of the
dispersion.
9. A method as claimed in claim 7 wherein the dispersion is
subjected to high shear conditions by mixing it in a Z-blade
mixer.
10. A method as claimed in claim 1 wherein the aqueous dispersion
is thickened by first subjecting it to mixing action and then
adding a thickening agent to achieve the desired consistency for
the sheet forming operation.
11. A method as claimed in claim 10 wherein the thickening agent is
a copolymer of maleic anhydride and an alkyl vinyl ether.
12. A method as claimed in claim 1 wherein the particle size of the
polytetrafluoroethylene in the aqueous slurry or dispersion is in
the range of 0.05 to 1 micron.
13. A method as claimed in claim 1 wherein the solid particulate
removable filler is substantially insoluble in water and can be
removed by chemical or physical means which does not cause damage
to the polytetrafluoroethylene.
14. A method as claimed in claim 13 wherein the solid particulate
additive is starch or a water-insoluble inorganic base or
carbonate.
15. A method as claimed in claim 13 wherein the solid particulate
filler is removed by soaking the sheet in acid.
16. A method as claimed in claim 1 wherein the solid particulate
filler is an organic polymer removable by dissolving the sheet in
an organic solvent or by hydrolysis or by vaporisation.
17. A method as claimed in claim 1 wherein the solid particulate
removable filler has a particle size substantially within the range
5 to 100 microns.
18. A method as claimed in claim 1 wherein the ratio of solid
particulate filler to polytetrafluoroethylene is 10:1 to 1:10.
19. A method as claimed in claim 1 wherein the aqueous dispersion
contains a non-removable filler.
20. A method as claimed in claim 19 wherein the nonremovable filler
is titanium dioxide, barium sulphate, asbestos, graphite or
alumina.
21. A method as claimed in claim 20 wherein said nonremovable
filler has a particle size less than 10 microns.
22. A method as claimed in claim 19 wherein the weight ratio of
said non-removable filler to polytetrafluoroethylene is 10:1 to
1:10.
23. A method as claimed in claim 1 wherein a coagulant is added to
the aqueous dispersion to assist in the formation of the dough.
24. A method as claimed in claim 23 wherein the coagulant is
brine.
25. A method as claimed in claim 1 wherein the calendering is
carried out by passing the dough rolls a number of times.
26. A method as claimed in claim 25 wherein after some or after
every pass through the rolls, the sheets are rotated through about
90.degree. so that the calendering is carried out biaxially.
27. A method as claimed in claim 1 wherein the diaphragm is
provided wtih a strengthening support.
28. A method as claimed in claim 27 wherein the support is a
polypropylene gauze.
Description
This invention relates to the manufacture of porous diaphragms.
More particularly, the invention relates to the manufacture of
porous diaphragms based on polytetrafluoroethylene.
One method of manufacturing such porous diaphragms comprises
forming an aqueous slurry or dispersion of polytetrafluoroethylene
and a solid particular 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 rolling operation the starch is removed
to give the desired porous diaphragm. The lubricant can also be
removed if required.
A major disadvantage of the above method of manufacturing porous
diaphragms is that the use of an organic lubricant gives rise to
irreproducibility of the diaphragms and this is extremely
undesirable particularly when the diaphragms are intended for use
in multi-modular electrolytic cells where reproducibility is
essential for efficient operation. We have now discovered that the
above disadvantage is obviated or mitigated by the method of the
present invention in which water is used as lubricant which enables
diaphragms of desired reproducibility and permeability to be
produced.
According to the present invention a method of manufacturing a
porous diaphragm comprises preparing an aqueous slurry or
dispersion comprising polytetrafluoroethylene and a solid
particulate additive, thickening said aqueous slurry or dispersion
to effect agglomeration of the solid particles therein, forming
from the thickened slurry or 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 and removing solid particulate additive from the
sheet.
In one embodiment of the invention the thickening of the aqueous
slurry or dispersion is effected by reducing the water content
thereof and water is then added to the thus thickened material to
form the dough.
The desired degree of lubrication for the sheet forming operation
is obtained by mixing water with the thickened material so that a
dough having a viscosity of at least 300 poises is obtained.
Preferably, water is added to the thickened material so that a
dough having a viscosity of between 1.times.10.sup.6 and
7.times.10.sup.6 poises is obtained.
Depending on the end use of the diaphragm, the desired degree of
lubrication can be achieved either by drying the aqueous slurry or
dispersion to a low water content and then adding a considerable
amount of water to form the dough or conversely drying the slurry
or dispersion only slightly and adding comparatively less water to
form the dough.
When the diaphragms are intended for use in electrolytic cells the
aqueous slurry or dispersion is preferably dried to a water content
of no more than 10% of the total weight of the dried
dispersion.
Further preferably, water is added to the dried dispersion until a
dough is attained which has a water content comprising 2 to 50%,
preferably 20 to 45%, of the total weight of the dough.
The drying of the slurry or dispersion can be carried out in any
suitable manner which will not cause damage to the constituents
thereof. Preferably the drying is carried out at a temperature of
10.degree. to 100.degree.C for example 15.degree. to 50.degree.C.
The time for drying will depend, inter alia, on the temperature but
is generally from 10 to 100 hours, for example, 20 to 50 hours.
In a further embodiment of the invention the aqueous slurry or
dispersion is thickened by subjecting it to high shearing action
and the high shear action is continued so that a dough having a
viscosity of at least 300 poises, preferably between
1.times.10.sup.6 and 7.times.10.sup.6 poises, is obtained.
In this embodiment the slurry or dispersion advantageously has a
water content comprising 2 to 50%, preferably 20 to 45% of the
total weight of the dispersion.
A particularly suitable way of subjecting the slurry or dispersion
to high shear conditions is to mix the slurry or dispersion in a
Z-blade mixer.
When the slurry or dispersion is subjected to high shear conditions
the viscosity increases and high shear action is preferably
maintained for a time at least sufficient for the viscosity to
reach the preferred range.
In yet a further embodiment of the invention the aqueous slurry or
dispersion is thickened by first subjecting it to mixing action and
then adding a thickening agent to achieve desired consistency for
the sheet forming operation.
Preferably the thickening agent is a copolymer of maleic anhydride
and an alkyl vinyl ether.
The particles size of the polytetrafluoroethylene in the aqueous
slurry or dispersion is preferably in the range of 0.05 to 1
micron, e.g. 0.1 to 0.2 micron.
The solid particulate additive can be any which is substantially
insoluble in water but which can be removed by a suitable chemical
or physical means which will not cause damage to the
polytetrafluoroethylene. The additive may be starch, for example
maize starch and/or potato starch, or a water-insoluble inorganic
base or carbonate, for example calcium carbonate.
These additives may be removed, for example, by soaking the sheet
in an acid, preferably a mineral acid e.g. hydrochloric acid. Other
additives which may be used include organic polymers which
depending on the properties of the polymer may be removed from the
sheet by dissolving with an organic solvent, by hydrolysis or by
vaporisation. Mixtures of additives may be used and if necessary
various treatments may be given to the sheet to remove the
additive.
Generally, the 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 many cases it is desirable to incorporate other components in
the aqueous slurry or dispersion which are not removed when the
sheet is subjected to the treatment to remove the particulate
additive. Examples of such components include particulate fillers
generally inorganic fillers, for example, titanium dioxide which is
particularly preferred, barium sulphate, asbestos, (e.g. amphibole
or serpentine asbestos), graphite and alumina. Suitably the filler
has a particle size of, for example, less than 10 microns and
preferably less than 1 micron. The weight ratio of filler to
polytetrafluoroethylene may be for example from 10:1 to 1:10,
preferably from 2:1 to 1:2.
In some cases it may be advantageous to add a coagulating agent,
e.g. brine, to the dispersion to assist in the formation of the
dough.
The sheet is generally formed from the dough by calendering.
Preferably, the calendering is carried out by passing the dough
through the rolls a number of times.
Generally, after some, or even after every pass through the rolls,
the sheets are rotated through about 90.degree. so that the
calendering is carried out biaxially.
The diaphragms produced by the process of the present invention
have a wide range of uses but are particularly suitable for use in
electrolytic cells for electrolysis of alkali-metal halides, for
the production of chlorine and caustic alkalis.
They are generally strong enough to be used without any support but
for extra strength it may be desirable to incorporate a sheet of a
suitable strengthening material, for example, a polymer gauze such
as polypropylene gauze.
The process of the present invention enables the production without
difficulty of a successive number of diaphragms over a period of
time, each one having similar permeabilities. This is very
necessary when using the diaphragms in electrolytic cells.
The invention is illustrated in the following Examples in which all
parts and percentages are by weight.
EXAMPLE 1
To 100 parts of an aqueous dispersion of polytetrafluoroethylene
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 particles 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 wtih a
paddlemixer for 30 minutes to form a substantially uniform paste.
This paste was spread on trays and dried at 24.degree. 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.times.10.sup.6 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. 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 laminate. The card was picked up, rotated 90.degree.
in the horizontal plane, and calendered (directed 90.degree. 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 resultant
laminate was cut into four, in the direction of calendering,
stacked, removed from the card, and calendered, without rotation
through 90.degree., the inter-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.degree. 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 rolled a total of nine times. The
resultant essentially rectangular laminate was then passed through
the rolls with its largest side directed at 90.degree. to the
direction of calendering, and with the inter-roll space slightly
reduced, no cutting, stacking or rotating through 90.degree. 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.times.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 resultant reinforced sheet was
removed from the rolls and soaked in cold aqueous 18% hydrochloric
acid for 24 hours. The starch additive was thereby removed leaving
a multi-porous sheet suitable for use as a diaphragm material in
electrolytic cells electrolysing aqueous solutions.
EXAMPLE 2
To 100 parts of an aqueous dispersion of polytetrafluoroethylene
containing 60% of the polymer in the form of particles
approximately all in the size range 0.15 to 0.2 micron were added
70 parts of water, 60 parts of titanium oxide of particle size
approximately 0.2 micron, 60 parts of maize 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
paddlemixer for 3 minutes to form a paste which was then mixed in a
Z-blade mixer for 22 minutes to form a dough having a viscosity of
4.times.10.sup.6 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. 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 laminate.
The card was picked up, rotated 90.degree. in the horizontal plane,
and calendered (directed 90.degree. 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 15 times. The resultant laminate was cut into four, in the
direction of calendering, stacked, removed from the card, and
calendered, without rotation through 90.degree., to the inter-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.degree. and calendering again. This process, cutting at 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.degree. to the direction of calendering, and with the inter-roll
space slightly reduced, no cutting, stacking or rotating through
90.degree. 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.times.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 resultant
reinforced sheet was removed from the rolls and soaked in cold
aqueous 18% hydrochloric acid for 24 hours. The starch additive was
thereby removed leaving a multi-porous sheet suitable for use as a
diaphragm material in electrolytic cells electrolysing aqueous
solution.
EXAMPLE 3
To 100 parts of an aqueous dispersion of polytetrafluoroethylene
containing 60% of the polymer in the form of particles
approximately all in the size range 0.15 to 0.2 micron were added
200 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
paddlemixer for 2 minutes to form a substantially uniform paste
having a viscosity of 3 poise. To this paste was added 27 parts of
a thickening agent known at Viscofas L100 (a copolymer of maleic
anhydride and methyl vinyl ether) and 27 parts of M NaOH, and the
mixture stirred with a paddlemixer to yield a dough having a
viscosity of 1.times.10.sup.6 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. 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
fourlayered laminate. The card was picked up, rotated 90.degree. in
the horizontal plane, and calendered (directed 90.degree. 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 one hundred and ten times.
For the first ninety of these passes through the rolls, accurate
stacking into laminates was not possible due to the nature of the
material. The resultant laminate was cut into four, at right angles
to the direction of calendering, stacked, removed from the card,
rotated through 90.degree., and calendered, the inter-roll space
being reduced by the thickness of the card. This process, cutting
at right angles to the direction of calendering, stacking, rotating
and calendering was repeated until the composition had been rolled
a total of one hundred and fifteen times.
The resultant essentially rectangular laminate was then passed
through the rolls with its largest side directed at 90.degree. to
the direction of calendering, and with the interroll space slightly
reduced, no cutting, stacking or rotating through 90.degree. 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.times.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 resultant reinforced sheet was
removed from the rolls and soaked in cold aqueous 18% hydrochloric
acid for 24 hours. The starch additive was thereby removed leaving
a multi-porous sheet suitable for use as a diaphragm material in
electrolytic cells electrolysing aqueous solutions.
In the process of the invention, high viscosity measurements were
preferably made with a Weissenberg Rheogoniometer, Model R16
(Sangamo Controls Limited). This instrument is equipped with a cone
and plate sample assembly of 2.5 cm diameter with a cone angle of
4.degree.. The cone and plate are manufactured of stainless steel
and the surfaces are polished. For viscosities below
5.times.10.sup.3 poise, the torsion bar torque measuring
accessories were used and the cone was driven in the continuous
mode. For viscosities above 5.times.10.sup.3 poise the cone was
driven in the oscillatory mode and the torque was measured by means
of two type 9203 piezoelectric force transducers (Kistler
Instruments Limited). set at 4 cm apart. The resulting output
signal was compared with the input signal by means of analysers
JM1606 and JX1600A (Solatron Limited). Viscosity measurements
detailed are initial readings at a shear rate of .gamma.=1
sec.sup..sup.-1.
For viscosity measurements below 600 poise however, for example as
in the first viscosity measurement of Example 3, a Brookfield
Viscometer, Model LVF was used. This instrument was equipped with a
spindle number 2 and measurements quoted were initial measurements
taken at speed 30.
We have also conducted tests to show that diaphragms prepared
according to the methods of the present invention can be obtained
with a greater degree of reproducibility than diaphragms obtained
by means of an organic lubricant.
TEST SERIES A
A polytetrafluoroethylene diaphragm was prepared according to the
method of British Pat. No. 1,081,046 using acetone as a coagulant
and petroleum ether as lubricant. Permeability tests were conducted
on these diaphragms, the results being given in Table I below.
Measurements were made with diaphragms installed in an electrolytic
cell.
Table I ______________________________________ Diaphragm No. K perm
______________________________________ 1 8350 2 840
______________________________________ K perm, (i.e. permeability
measured in hrs.sup..sup.-1) ##EQU1##
In Test Series A the diaphragm was installed in the electrolytic
cell and the inorganic filler removed by operation of the cell. The
permeability of the diaphragm is zero at first, increases as starch
is removed and then reduces as the diaphragm becomes blocked during
extended operation of the cell. The coefficient of permeability (K
perm) is based on measurements taken when the permeability of the
diaphragm is at a maximum. Comparison of the permeability factors
indicated in Table I for diaphragms produced by identical
techniques showed a wide deviation as regards reproducibility.
TEST SERIES B
In this test series a number of diaphragms was prepared according
to the process described in Example 1 except that petroleum ether
was used as lubricant instead of water. In this case the inorganic
filler was extracted outside the electrolytic cell by soaking in
16% hydrochloric acid and permeability measurements were made when
the diaphragms had been installed in the electrolytic cell. The
results are shown in Table II below.
Table II ______________________________________ Diaphragm No. K
perm ______________________________________ 3 446 4 765 5 492 6 446
7 490 8 253 9 612 10 321 11 262 12 204 13 292 14 959 Means K perm =
462 Standard Deviation = 226
______________________________________
These results indicate use of an organic lubricant gives rise to
considerable irreproducibility of the diaphragms.
TEST SERIES C
In this series of tests the diaphragms were prepared according to
Example 1 with water being used as lubricant. This time a slightly
different technique was adopted in measuring permeability in that a
special rig was set up for removal of the inorganic filler by acid
extraction and the permeability measurements were not made with the
diaphragms installed in an electrolytic cell but in said rig. The
tests results are indicated in Table III below which also indicates
results of tensile strength tests carried out on the diaphragms on
a standard Instron tensile strength measuring device. We have
established that the tensile strength of a diaphragm is correlated
to its permeability and we have found that the tensile strength
tests are easier to conduct and the results can be used to
accurately predict the permeability of the diaphragm.
Table III ______________________________________ Diaphragm No. K
perm .times. 10.sup.3 Ultimate Tensile Strength (gms
mm.sup..sup.-2) ______________________________________ 15 32 37.2
16 30 40.7 17 20 43.6 18 30 40.0 19 32 39.3 20 32 42.9 21 36 46.0
22 30 37.0 23 28 44.0 24 24 40.0 25 34 40.0 26 34 39.4 27 26 39.2
28 32 49.0 29 20 42.0 30 20 46.0 31 20 45.8 32 20 45.7 33 20 46.0
34 21 47.8 Mean = 27.1 Mean = 42.6 Standard Deviation = 5.8
Standard Deviation = 3.6 ______________________________________
The results listed in Table III indicate that by using water as
lubricant diaphragms of desired reproducibility and permeability
can be obtained.
* * * * *