Note: Descriptions are shown in the official language in which they were submitted.
il4469Z
ffYDROPHILIC FLUCROPOLYMERS
BACKGROUND OF THE INVENTION
In commercial chlor-alkali cells used for the
production of chlorine, hydrogen and sodium hydroxide
from brine, asbestos diaphragms are ordinarily used as
anion exchange membranes between the anolyte and
catholyte compartments.
Diaphragms of this sort are generally satis-
factory, but their electrical resistance is high and
chlor-alkali cells which use such diaphragms therefore
require more electric current for operation than is
desirable.
SUMMARY OF THE INVENTION
It has now been found that by chemically
modifying certain fluoropolymers with sulfur or
phosphorus containing compounds, the fluoropolymers
are made hydrophilic. This ma~es them especially
suited for use in ma~ing diaphragms for electrolytic
cells.
'F-6058-A
~ f
, . .~ . , ,
,
i~4~692
Such diaphragms have less electrical resis-
tance than those made solely of asbestos. There is,
accordingly, less power loss in a chlor-alkali cell
which uses this new diaphragm and the cell is there-
fore more efficient. Indeed, it has been observed thatthe chlorine, hydrogen and caustic production of a cell
using this diaphragm can generally be held at the same
level as one using an asbestos diaphragm while operating
at 10-15~ lower voltage.
The hydrophilic fluoropolymers also have the
inertness of fluoropolymers generally, and this makes
them resistant to attack by the contents of conventional
electrolytic cells.
"Hydrophilic", as used in this context, means
that the modified fluoropolymers do not repel water as
conventional fluoropolymers do, but instead absorb it.
The term more specifically means that the modified
fluoropolymers have water contact angles of 0 to about
50, as measured by the method and apparatus-described
on page 137 of "Contact Anqle, Wettability and Adhesion",
American Chemical Society, 1964.
Specifically, the hydrophilic fluoropolymers
of the invention are chemically modified homopolymers,
or copolymers (meaning they contain two or more dif-
ferent monomer units), derived at least in part fromolefinic monomers completely substituted with fluorine
atoms or completely subst-tuted with a combination of
fluorine atoms and no more than one chlorine, bromine
or iodine atom per monomer. These fluoropolymers can
3~ also contain up to about 75 mol percent of units derived
from other ethylenically unsaturated monomers.
These hydrophilic fluoropolymers contain
non-terminal units represented by the structure
11~4692
F
(1) -- C --
where ~ is
-S02Br
-SCl
-SOC1
-S02Cl
1 0 -SXM
-SxBr
-SxCl
-S-C-OR
S
-S-C-SR
S
SM
--S--P=S
SM
or
OR
-P=O
OR
where
M is hydrogen, sodium, potassium, lithium,
calcium or magnesium;
R is an alkyl radical of 1-12 carbon atoms
or a cycloalkyl radical of 3-12 carbon
atoms; and
x is 1, 2, 3, 4 or 5.
Hydrophilic fluoropolymers containing non-
terminal units of structure (1~ where ~ is
:
;~
~14469Z
OR
--P=O
OR
are preferred for use as diaphragm materials in electro-
lytic cells because of their low electrical resistance.
The hydrophilic fluoropolymers can contain
more than one type of unit of structure (1).
The hydrophilic fluoropolymer will contain
enough units of structure (1) to give it a sulfur or
phosphorus content of about 0.1~-10~, by weight, but
not so many that more than about 1%, by weight, of the
polymer will dissolve in water at 20C.
Sulfur and phosphorus content is determined
gravimetrically by
(1) converting the sulfur present to a
sulfate in an oxygen flask, then
adding BaC12 and weighing the BaSO4
precipitate, as described by J. H.
2Q Karchmer on pages 112-114 of "The
Analytical Chemistry on Sulfu~ and
its Compounds, Part I", Wiley-Inter-
science, 1970; or
(2) converting the phosphorus present to
a phosphate by the Schoniger oxygen
flask method, then adding ammonium
molybdate and weighing the ammonium
molybdophosphate precipitate, as
described by M. Halmann on pages 15,
16, 22 and 23 of "Analytical Chemistry
or Phosphorus Compounds", Wiley-Inter-
science, 1972.
Water solubility is determined by running a
sample of polymer in a Soxhlet extractor for 24 hours,
using deionized water as a solvent.
, . .
11446~2
When a hydrophilic fluoropoly~er of the in-
vention is to be used as a diaphragm material in an
electrolytic cell, it preferably contains enough units
of structure (1) t~ give it an electrical resistance of
about 0.1-100 ohms per square centimeter of surface
area. Resistance is determined using conventional in-
struments to measure the ~oltage and amperage of a
current flowing through the material under actual use
conditions, computing the resistance using Ohmls law,
and then dividing the resistance by the area, in square
centimeters, of the material.
The preparative methods to be described will
give hydrophilic fluoropolymers with the foregoing
chemical and electrical characteristics.
The hydrophilic fluoropolymers of the inven-
tion containing units of structure (1) which are ionic
are useful as materials for ion-exchange membranes.
The hydrophilic fluoropolymers which contain units of
structure (1) which are nonionic can be used as
materials for semi-permeable membranes in osmotic pro-
cedures and in dialysis.
DETAILED D~SCRIPTION OF THE INVENTION
How the Hydro~hilic Fluoropolymers Are Made
The hydrophilic fluoropolymers of the inven-
tion can be made by two methods. Each method requiresa fluoropolymer starting material and a modifying com-
pound.
"Fluoropolymer starting material", as used
here, means a homopolymer or copolymer (meaning the
copolymer contains two or more different monomer units)
derived at least in part from olefinic monomers com-
pletely substituted with fluorine atoms or completely
substituted with a combination of fluorine atoms and
no more than one chlorine, bromine or iodine atom per
monomer.
114469Z
Representative of such fluoropolymer starting
materials are homopolymers ~nd copolymers (in all mono-
mer unit weight ratios) of
(1) tetrafluoroethylene (TFE)
(2) hexafluoropropylene (HFP)
(3) chlorotrifluoroethylene (CTFE)
and
(4) bromotrifluoroethylene (BTFE).
These fluoropolymer starting materials can
also contain up to about 75 mol percent of units of
other ethylenically unsaturated monomers which contain
at least as many fluorine atoms as carbon atoms, for
example, vinylidene fluoride. Optionally, minor amounts
of olefins containing 2-4 carbon atoms can also be
lS present. Illustrative of the fluoropolymer starting
materials which result from use of these other ethyl-
enically unsaturated monomers are those described in
U.S. Patent 4,035,565 - Apotheker and Krusic, granted
July 12, 1977.
When these other ethylenically unsaturated
monomer units are present in a fluoropolymer starting
material which is to be modified and made into a
diaphragm for an electrolytic cell, it is preferred
that the units of structure (1) be present in the form
F
CF2 - C - CF2
to avoid possible alpha,beta elimination of hydrogen
fluoride under extreme conditions of heat and alkalinity
found in some electrolytic cells.
The f1uoropolymers pre~erred as starting
materials because of the low electrical resistance of
the hydrophilic fluoropolymers which result when they
are used are
1~4~692
(1) polytetrafluoroethylene (PTFE)
(2) polychlorotrifluoroethylene ~PCTF~)
(3) polybromotrifluoroethylene (PBTFE)
and
(4) copolymers (in all monomer unit
weight ratios) of
(a) TFE and HFP
(b) TFE and ~TFE
(c) TFE and BTFE
(d) CTF~ and BTFE
and
(e) TFE and perfluoroalkyl vinyl
ether, (especially perfluoro-
methyl vinyl ether).
PCTFE, the TFE-BTFE copolymers and the TFE-
CTFE copolymers are especially preferred because their
reactivity makes it easier to prepare the hydrophilic
fluoropolymers of the invention from them.
Mixtures of fluoropolymer starting materials
can also be used.
The molecular weight of the fluoropolymer
starting material, and of the hydrophilic fluoropolymer
product, is unimportant. A low molecular weight fluoro-
polymer which itself may lack the physical strength
needed for the use intended can be coated on a fabric
substrate made of such things as glass fibers, asbes-
tos or PTFE fibers and then modified, or can be cross-
linked after modification, to provide the strength.
The fluoropolymer starting materials are
either commercially available or can be made by well-
known methods. See for example, "Fluoropolymers",
~eo A. Wall, Wiley-Interscience, 1972, Chapter 1,
; "Polymerizat-ion of Fluoroolefins", pages 1-29.
In both methods of preparing the hydrophilic
fluoropolymer, the fluoropolymer starting material can
~ .....
,
114469Z
be in the form of a powder, a porous or nonporous un-
supported film, a porous reinforced structure, a coating
on an inert fabric, or in the form of fibers.
The fluoropolymer starting materials are
available commercially, or can be conventionally made,
as powders or as dispersions from which powders can be
extracted. These powders ordinarily contain particles
having an average longest dimension of about 0.1-lO
microns, with no particles having a longest dimension
greater than about 50 microns. These dimensions are
measured optically, against a standard.
A porous unsupported film can be made from
powders of fluoropolymer starting material according
to conventional cellulose spongemaking procedures.
Sodium chloride or calcium chloride (about 50~-60~, by
weight of the fluoropolymer starting material) is
first dissolved in an a~ueous dispersion of fluoro-
polymer starting material (about 20~-25% by weight of
solids). This solution-dispersion is then cast on a
plate to a thickness of about 0.1-5 millimeters, air-
dried and then heated to fuse the fluoropolymer starting
material. The resulting film is cooled, stripped from
the plate and then immersed in water for 2-3 days at
20C to extract the chloride and give a porous unsup-
ported film of fluoropolymer starting material.
A nonporous unsupported film of fluoropolymerstarting material can be made according to the fore-
going procedure by simply omitting the addition of
sodium chloride or calcium chloride to the polymer
dispersion, and omitting the subsequent immersion in
water.
A porous reinforced structure of fluoro-
polymer starting material can be made by coating a
fabric of glass fibers, asbestos, PTFE fibers, or the
like, with a dispersion containing about 5%-25% by
1144~92
weight of a fluoropolymer starting material and about
10~-25~ by weight of a polymer soluble in the carrier
and stable at the fluoropolymer fusion temperature,
such as polyvinyl acetate or an acrylic polymer such
as polymethylmethacrylate. The carrier for the ~isper-
sion is a solvent for the polymethylmethacrylate, such
as toluene or methylisobutyl ketone. If the flu
polymer starting material is other than a TFE-HFP co-
polymer, adhesion of that polymer to the fabric can be
improved by adding about 2~-5%, by weight of the
dispersion, of TFE-HFP copolymer (85/15 weight ratio)
to the dispersion. The dispersion is applied to both
sides of the fabric to give a final structure thickness
of about 0.1-5 millimeters.
The coated fabric is heated to fuse the
fluoropolymer starting material and give a continuous
coating. The fabric is then soaked at 50-130C for
2-4 hours, with agitation, in a solvent for the poly-
methylmethacrylate, such as dimethylacetamide or
acetone. This leaches out the polymethylmethacrylate,
giving a porous reinforced structure of fluoropolymer
starting material.
A nonporous coated fabric can be made by
coating a fabric of glass fibers, PTFE fibers, asbes-
tos, or the like, with a dispersion of fluoropolymer
starting material, drying it and then heating it tofuse the fluoropolymer. Enough of the dispersion should
be used to give a final coating weight of about 0.001-
0.5 gram of polymer per square centimeter of fabric and
a final coated fabric thickness of about 0.1-S milli-
meters, as measured with a micrometer. A porous coatedfabric can be made in the same manner by limiting the
final coating weight of the fluoropolymer starting
material to about 0.01-0.10 grams of polymer per
square centimeter of fabric.
114469Z
Fibers of fluoropolymer starting material can
be made by mixing fluoropolymer particles with cellulose
xanthate, forcing the mixture through a suitable spin-
neret and then baking the resulting fibers at the
fluoropolymer's fusion temperature to drive off the
xanthate. This procedure is described in Burrows and
Jordan, U.S. Patent 2,772,444, granted December 4, 1956.
These fibers ordinarily have an average diametér of
about 1-2~ microns, with no fibers having a diameter
greater than about 50 microns. These diameters are
measured optically, against a standard.
In an alternative and preferred procedure, a
porous reinforced structure can be made from a disper-
sion which comprises
(a) a fluoropolymer starting material;
(b) a fibrous material which will act
as a base for the structure;
(c) optionally, a fluoropolymer binder
material; and
(d) a liquid carrier.
This composition can also contain conventional
adjuncts such as wetting agents, surfactants, defoamers
and the like, in the usual amounts.
A porous reinforced structure can be made
from such a composition by first deagglomerating the
fibers of (b) and then forming a mat of the fibers by
removing the carrier, preferably by a papermaking tech-
nique. In a highly preferred embodiment of the inven-
tion, this porous reinforced structure is formed
directly on the cathode screen of an electrolytic
cell.
Any fibrous material can be used in (b) which
can withstand the baking temperature to be used and
which resists attack by the environment in which the
structure is to be used. Illustrative of such
materials are
1144692
11
asbestos
glass fibers
fibers of such fluoropolymers as PTFE
or TFE-HFP copolymers
and
potassium titanate fibers.
Mixtures of such fibrous materials can also be used.
Asbestos is the preferred fibrous material for use in
electrolytic cell diaphragms. Especially preferred is
a chrysotile asbestos whose fibers have an average
diameter of about 200A~ (as measured by electron micro-
scopy) and an average length of about 70 mm.
Similarly, the fluoropolymer to be used as
a binder material in (c) can be any which resists
attack by the environment in which it is to be used.
Illustrative are
PTFE
TFE-HFP copolymers (all monomer
ratios)
polyvinyl fluoride
polyvinylidene fluoride
and
vinylidene fluoride/hexafluoro-
propylene copolymers (all monomer
ratios~.
Mixtures of binder materials can also be used.
In electrolytic cell diaphragms, the TFE-
HFP copolymers are preferred as binder materials
because of their inert nature.
The carrier in ~d) can be any liquid which
will not significantly affect the chemical or physical
characteristics of the structure. Illustrative of
such liquids are
~ater
chlorinated hydrocarbons
11
4692
12
methanol
hexane
and
brine.
When the composition is to be used to make an electro-
lytic cell diaphragm, a 15% by weight brine solution
is preferred as a carrier because it helps keep the
fibrous material in suspension.
The components of the composition are
preferably present in the following concentrations:
(a) fluoropolymer starting material --
10-90% by weight of the total of
~a) and (b), even more preferably
40-60%;
(b) binder -- 10-90% by weight of the
total of (a) and (b), even more
preferably 40-60%;
~a) plus (b) cons~ituting 10-90%, by weight of the
total of (a), tb), and (c), preferably 20-25%;
(c) fibrous material -- 10-90%, by
weight of the total of (a), (b~,
(c), even more preferably 75-80%;
and
(d) carrier -- the remainder.
The composition will contain 0.01-3%, by
weight of solids, preferably about 1~.
Preparative Method One
In the first method of preparation, the
fluoropolymer starting material, in whatever form, is
exposed to radiation while it is in contact with a
suitable modifying compound, or the fluoropolymer and
modifying compound are brought together in the presence
of a peroxy compound such as benzoyl peroxide or t.
butyl peroxide.
Any compound which contains a sulfur or
phosphorus atom which can react with the fluoropolymer
~'
,.~
~L
1144692
starting material can serve as a modifying compound.
Representative of these are
s 2C12
SOC12
M2 SX
MS -C-OR
S
MS -C -SR
lQ S
SM
MS - P - SM
S
OR
H-P=O
OR
O O
.. ..
RO - P - P - 03~(
OR OR
RO-P-O-P-OR
OR OR
and
OR
RO-P
OR
where
3Q M iS hydrogen, sodium, potassium, lithium,
calcium or magnesium;
R is an alkyl radical of 1-12 carbon atoms
or a cycloalkyl radical of 3-12 carbon
atoms; and
x is 1, 2, 3, 4 or 5.
~44~;92
14
Mixtures of modifying compounds can also be
used.
The irradiation is accomplished by first
placing a fluoropolymer starting material, in whatever
form, in a saturated solution of modifying compound in
water or other inert solvent such as benzene. If the
modifying compound is itself a liquid, no other liquid
need be used. The amount of solution or liquid in which
the fluoropolymer starting material is placed should be
such that the starting material is thoroughly wetted,
but not so much that the passage of radiation through
the liquid is impeded. About 0.1-1%, by weight, of a
surfactant such as perfluorooctanoic acid may be added
to aid wetting.
The liquid containing the fluoropolymer start-
ing material and modifying compound is then exposed to
about 0.1-20 megarads of electron radiation or to about
0.1-50 watts per square centimeter of liquid surface of
ultraviolet light, at 20C.
The irradiation is done in conventional equip-
ment. If desired, it can be performed continuously,
with fluoropolymer starting material and modifying com-
pound being fed into the equipment and hydrophilic
fluoropolymer of the invention being removed.
After the irradiation step is comple~ed, the
product is separated from the liquid and washed with
water. Drying is optional and can be done convention-
ally.
The irradiation also crosslinks the hydro-
philic fluoropolymers of the invention based on PCTFE
and TFE-CTFE and TFE-BTFE copolymer starting materials,
thereby physically strengthening them.
When the fluoropolymer starting material and
modifying compound are brought together in the presence
3~ of a peroxy compound; the procedure is the same except
14
` ~14469Z
that irradiation is omitted. Peroxy compound, about
10-100 mol percent of the fluoropolymer starting mate-
rial, is added to the liquid in which the fluoropolymer
has been placed. This liquid, with the fluoropolymer in
it, is then held at 80-150C for 5-36 hours. The poly-
mer is then separated from the liquid, washed with
water and, optionally, dried.
Preparative Method Two
In this method, the fluoropolymer starting
material and modifying compound are brought into con-
tact with each other, under conditions suitable for
reaction, and kept together until the reaction is com-
plete.
This method can only be used with homopolymers
and copolymers of CTFE and BTFE as starting materials.
The modifying compounds used are
Sulfur
SM
MS-P=S
S~
S
MS-C-OR
MS-C-SR
S
or
M2 Sx
where
M is hydrogen, sodium, potassium, lithium,
calcium or magnesium;
R is an alkyl radical of 1-12 carbon atoms
or a cycloalkyl radical of 3-12 carbon
atoms; and
x is 1, 2, 3, 4 or 5.
, .
114469Z
16
~ixtures of these compounds can also be used.
The fluoropolymer starting material, pref-
erably in the form of a porous unsupported film or a
porous reinforced structure, is placed in a 2%-10~, by
weight solution of modifying compound in dimethyl-
formamide or dimethylacetamide. As in Preparative
Method One, only enough of the solution is used to
thoroughly wet the fluorocarbon polymer starting mate-
rial. If desired, a small amount of cesium fluoride
or potassium fluoride can be added to the solution to
suppress side reactions.
The solution, with the fluoropolymer starting
material in it, is then brought to a temperature within
the range of about 20C to about the boiling point of
the solvent used, to bring about the reaction. The
solution is held at that temperature for about 4-200
hours, the reaction at lower temperatures requiring
more time for completion than one carried out at higher
temperatures. When the initially colorless fluoro-
polymer starting material turns black, the reactionis complete.
In both methods of preparation, the modifying
compounds used will provide radicals which will be
attached directly to the fluoropolymer chain. These
compounds, and the radicals they provide, are shown in
the following table:
Attached
Compound Radical (~ Group)
Sulfur -SxBr or -SxC
S2C12 -SCl
SOC12 -SOCl
so2C12 -SO2Cl
M2Sx -SxM
MS-C-OR -S-C-OR
S S
. ~ , ~
-`` 1144692
17
Table (Continued)
Attached
Compound Radical (~ Group)
MS-C-SR -S-C-SR
S S
OR OR
H-P=O -P=O
OR OR
OR 10R
RO-P -P=O
OR OR
SM SM
MS-P=S -S-P=S
SM SM
O O OR
RO-P-P-OR -P=O
,
RO OR OR
RO-P-O-P-OR OR
RO OR -P=O
OR
where
M is hydrogen, sodium, potassium, lithium,
calcium or magnesium;
R is an alkyl radical of 1-12 carbon atoms
or a cycloalkyl radical of 3-12 carbon
atoms; and
x is 1, 2, 3, 4 or 5.
The radicals which result when the phosphorus
compounds are used can be con~erted into
OM
-P=O
OM
, . ,
1~44692
radicals by treating the hydrophilic fluoropolymers
bearing these radicals with a 5~ solution of NaOH in
methanol.
It should be noted that the radicals which
result when the sulfur compounds in the following list
are used as modifiers can be converted into -SO3H
radicals by treating the hydrophilic fluoropolymers
bearing these radicals with saturated chlorine water
or 25%-50% (by weight) aqueous hydrogen peroxide for
2-20 hours at 20C.
sulfur
S2C12
SOC12
S02C12
M2 Sx
MS-C-OR
S
MS-C-SR
S
SM
MS-P-SM
S
H
H-P=~
OM
and
PCl3
where
M is hydrogen, sodium, potassium, lithium,
calcium or magnesium;
R is an alkyl radical of 1-12 carbon atoms
or a cycloalkyl radical of 3-12 carbon
atoms; and
x is 1, 2, 3, 4 or 5.
1144692
13
How the Hydrophilic ~luoropolymer .~re Put Into Usable
Form
Unmodified porous and nonporous unsupported
films and unmodified porous reinforced structures of
fluoropolymer starting material, and unmodified non-
porous fabrics coated with fluoropolymer starting
material, prepared by the methods previously described
and then modified by Preparative Method One or Prepara-
tive Method Two, can be used directly without further
fabrication.
Felt can be prepared from fibers of hydro-
philic fluoropolymer using well-known felting methods.
A porous unsupported film can be made from
a powder of hydrophilic fluoropolymer according to the
lS conventional spongemaking techniques already described.
When these porous products are to be used as
diaphragms in electrolytic cells, they preferably
should be able to pass about 0.01-S cubic centimeters
of liquid per square centimeter of surface area per
minute. This flow rate is measured by conducting a
predetermined amount of liquid through the product,
collecting the liquid and measuring the amount col-
lected, measuring the time required to collect the
liquid, dividing the amount collected by the time
required to collect it, and then dividing that quotient
by the area of the product (in square centimeters).
The flow rate requirement will vary with the
use. When used in chlor-alkali cells, for example,
porous unsupported films or porous reinforced struc-
tures should be able to pass about 0.02-1 cubic centi-
meters of brine per square centimeter of surface area
per minute.
In a porous unsupported film or a porous
reinforced structure, the proper flow rate can be
obtained by varying the thickness of the film or
19
114469Z
~o
structure, or by varying the porosity through changing
the ratio of chloride or polymethylmethacrylate to
fluoropolymer starting material or hydrophilic fluoro-
polymer in the fabrication step. Varying the chloride
ratio is done routinely in the cellulose sponge art
according to well-known principles, and the desired
porosity can be obtained in a film or a reinforced
structure by applying those principles to this tech-
nology.
Ordinarily, the proper flow rate can be
obtained with pores having an average longest trans-
verse (to the direction of flow) dimension of about
0.1-10 microns, with no transverse dimension larger
than about 15 microns. Pore size is measured optically,
against a standard. The porous unsupported film or
porous reinforced structure will have a pore density
of about 10,000-1,000,000 pores per square centimeter
of surface area, as counted with the aid of a micro-
scope.
If the porous unsupported film or porous
reinforced structure is to be used as a diaphragm
material for a chlor-alkali cell, its pores ordinarily
have an average longest transverse (to the direction
of flow) dimension of about 0~1-2 microns, with no
transverse dimension larger than about 10 microns, and
a pore density about 10,000-1,000,000 pores per square
centimeter.
If these porosities and pore densities do
not give precisely the flow rate desired, a few simple
measurements followed by appropriate adjustments in
the amount of chlorides or polymethylmethacrylate used
should give the proper flow rate. In the case of felt,
the proper flow rate can be obtained by coordinating
its thickness with the number of fibers per unit
volume, as is well known.
~14~692
The tensile strengths of the various products
just described need only be high enough to enable the
forms to withstand the stresses encountered in use.
These products have the requisite strength inherently.
After a porous or nonporous unsupported film,
porous reinforced structure, nonporous coated fabric or
porous felt of hydrophilic fluoropolymer has been made,
it can be used directly for whatever use is intended by
simply trimming it to the correct dimensions and placing
it in position in the apparatus used.
The preferred method of preparing a diaphragm
for a chlor-alkali cell is the same as that previously
described for making a preferred porous reinforced
structure starting material, except that a finished
hydrophilic fluoropolymer is used instead of a fluoro-
polymer starting material. As in the previously-
described method, a diaphragm can be made from a dis-
persion which comprises
(a) a hydrophilic fluoropolymer,
preferably in the form of a
powder;
~b) a fibrous material which will
act as a base for the diaphragm;
(c) optionally, a fluoropolymer
binder material; and
(d) a liquid carrier.
This composition, as the one previously
described, can also contain conventional adjuncts such
as wetting agents, surfactants, defoamers and the like,
in the usual amounts.
A chlor-alkali~cell diaphragm can be made
from such a composition by first deagglomerating the
fibers of (b) and then forming a mat of fibers by
removing the carrier, preferably by a papermaking
technique. Even more preferably, the diaphragm is
i 21
~ .. .. .
" --` 114469Z
~2
thus formed in situ on the cathode screen of the cell.
However formed, the diaphragm is then heated to fuse
the fluoropolymer binder, if it is present. The dia-
phragm is then ready for use.
The fibrous materials, the fluoropolymer
binders, the carriers, and the pr~ferred embodiments
and concentrations of these are as previously set forth
in the description of preparina a porous reinforced
structure for modification according to the invention.
Diaphragms for electrolytic cells, made in
situ in this way and as previously described, must meet
the operator's specifications regarding permeability,
current efficiency and dimensional stability. These
specifications vary with the operator, the type of cell
being used, electrical current demands of the cell, and
like factors. One skilled in the diaphragm making art
will use the same skills in preparing these in situ
diaphragms that he does in preparing conventional
asbestos diaphragms, and by applying those skills
will, without difficulty, be able to prepare diaphragms
that will meet all of any manufacturer's requirements.
While the hydrophilic fluoropolymers of the
invention are most useful as diaphragm materials for
electrolytic cells, especially chlor-alkali cells, they
can also be used as materials for membranes to be
used in other ion-exchange procedures, as in desalini-
zation of sea water, and as materials for semi-
permeable membranes to be used in osmotic procedures
and in dialysis.
When a hydrophilic fluoropolymer is to be
used as a membrane for osmosis or dialy~is, an appro-
priate modifying compound must be used to make the
polymer properly ionic or nonionic, as is well known
in the art.
If contaminants which are present in most
~1~469Z
23
electrolytic cells, ion-exchange cells or dialysis
chambers clog a diaphragm or membrane of hydrophilic
fluoropolymer to the point where its efficiency is
diminished, the diaphragm of membrane can be rejuve-
nated by back-flushing it with water, rinsing it with
concentrated nitric acid and then with water.
The following examples illustrate the inven-
tion. In these examples, all parts and percentages
are by weight, unless otherwise indicated.
EXAMPLE 1
A piece of Teflon* fluorocarbon resin cloth
(T-162-42, sold by Stern & Stern Textiles, Inc.) was
preshrunk by baking it for 10 minutes at 270C.
A 15% solids aqueous dispersion of tetra-
fluoroethylene/bromotrifluoroethylene 86/14 copolymer
was sprayed on both sides of the clath to an overall
thickness of 100 microns (dry) and the cloth was then
baked for 20 minutes at 270C.
The resulting coated fabric was then placed
in a vessel containing a solution of
Potassium sulfide 11 parts
Sulfur 6.4 parts
Cesium fluoride 3.0 parts
in 300 parts of dimeihylacetamide~
The vessel was heated in a steam bath for
four hours, while the solution containing the fabric
was stirred.
The coated fabri~c was then removed from the
vessel, washed with distilled water, submerged in
saturated chlorine water and kept there for 16 houxs
at 20C,
The fabric was then rinsed with distilled
water, dried and placed ~n the diaphragm position of
a laboratory chlor-aLkali cell containing saturated
brine! where, in operation, it required a voltage
*denotes trade mark
~,
1144~i9z
24
of 3.0-3.1 to achieve a current density of 0.204 amperes
per square centimeter of diaphragm area.
The flow rate through the diaphragm was found
to have been 1.87 cubic centimeters of brine per square
centimeter per minute.
The fabric coating had a water solubility of
less than 1~.
EXAMPLE 2
(A) The following were added to a vessel:
Dispersion of polychlorotri-200 parts
fluoroethylene sold by 3M Co.
as Kel-F* (30% in methyl-
isobutyl ketone)
Dispersion of tetrafluoro-50 parts
ethylene/hexafluoropropylene
85/15 copolymer resin (30% in
methylisobutyl ketone)
Solution of polymethylmethacry- 188 parts
late resin MW 65,000-100,000
(40~ in aceto~e/toluene 2/1 by
volume)
These components were well mixed and a piece of Teflon
cloth (as per Example 1) dipped into the mixture. The
coated cloth was removed, excess mixture was removed by
drawing it down with wire-wound rods, and the cloth
baked for 30 minutes at 270C.
The coated cloth was cooled to room tempera-
ture and placed in a vessel containing a mixture of
xylene and dimethylacetamide (1/1 by volume). The
temperature of the mixture was raised to the boiling
point of the liquid over a two-hour period and held
there for two hours. The resulting porous reinforced
structure was then removed.
(B) The porous reinforced structure made
in (A) was rolled into a cylindrical shape and placed
in a vessel containing a solution of
Potassium sulfide 45 parts
Sulfur 5 parts
Cesium fluoride 2 parts
* denotes trade mark
24
'~'
1144692
in 1500 parts of dimethylacetamide. The vessel was
sealed and rolled for 88 hours. The porous structure
was then removed, washed with distilled water, submerged
in saturated chlorine water and kept there for three
hours at 20C, with stirring.
The porous reinforced structure was then dried
and placed in the diaphragm position of a laboratory
chlor-alkali cell containing saturated brine, where, in
operation, it required an average voltage of 3.06 to
achieve a current density of 0.204 amperes per square
centimeter of diaphragm area.
The flow rate through the diaphragm was found
to have been 1.04 cubic centimeters of brine per square
centimeter per minute.
The cloth coating had a water solubility of
less than 1%.
EXAMPLE 3
A porous reinforced structure was prepared as
in Example 2(A).
This was air-dried, thoroughly soaked with
dimethylphosphite and then given one megarad of electron
radiation with an electron beam resonant transformer.
The irradiated porous structure was placed in
a 5% solution of sodium hydroxide in methanol, which
was then heated to reflux temperature and held there
for one hour, with stirring. The structure was removed,
washed in distilled water for 15 minutes, dried and
placed in the diaphragm position of a laboratory
chlor-al~ali cell containing saturated brine.
Direct current was applied. The current
flow initially was one ampere at 8.8 volts; after two
hours, the flow was one ampere at 8.1 volts. The flow
rate was found to have been 0.074 cubic centimeters
of brine per square centimeter per minute.
The cloth coating had a water solubility of
less than 1%.
1144692
26
EXAMPLE 4
(1) A mixture of
Tetrafluoroethylene/bromo- 22 parts
trifluoroethylene 84/16
copolymer powder
Methylisobutyl ketone 66 parts
was ball-milled for 30 minutes.
(2) The dispersion of (1) was added to a
mixture of
Tetrafluoroethylene/hexa- 18.3 parts
fluoropropylene 85/15
copolymer resin dispersion
(30% in methylisobutyl ketone)
Solution of polymethylmeth- 68.8 parts
acrylate resin MWn 65,000-
100,000 (40~ in acetone-
toluene 2/1 by volume)
Methylisobutyl ketone 60 parts
Aminopropyltrimethoxysilane 1 part
(3) A piece of asbestos paper of the type
used in chlor-alkali cells was thoroughly soaked in the
product of (2) and then baked for 30 minutes at 315C.
~ 4) The porous reinforced structure of (3)
was placed in a vessel containing a solution of
Dimethylacetamide 850 parts
Potassium polysulfide 33 parts
Potassium fluoride 13 parts
The vessel was filled with nitrogen, sealed and held
for 8 days at room temperature. The coated paper was
then removed.
(5) The p~oduct of (4) was soaked in
saturated chlorine water for 8 hours and then air-dried.
The resulting material was suita~le for use
as a diaphragm material in a chlor-alkali cell.
EXAMPLE 5
(1) A mixture of
~ 3~ Tetrafluoroeth~lene/chloro- 16.5 parts
- trifluoroethylene 87.5/
12.5 copolymer powder
26
1144692
27
Methylisobutyl ketone 50 parts
was ball-milled for 30 minutes.
(2) To the dispersion of (1) was added a
mixture of
Tetrafluoroethylene/hexa- 13.7 parts
fluoropropylene 85/15
copolymer resin dispersion
(30% in methylisobutyl
ketone~
Solution of polymethylmeth- 51.6 parts
acrylate resin MWn 65,000-
100,000 (40% in acetone/
toluene 2/1 by volume)
Aminopropyltrimethoxysilane 0.75 part
(3) The ball mill used in (1) was rinsed
with 15 parts of methylisobutyl ketone, which was
added to the dispersion of (2). An additional 22.5
parts of methylisobutylketone was then added to the
dispersion of (2).
(4) A piece of asbestos paper of the type
used in chlor-alkali cells was thoroughly soaked in
the product of (3) and air-dried. Twenty parts of
methylisobutyl ketone was added to (3) and the paper
again thoroughly soaked in it. The paper was then
air-dried and baked for 30 minutes at 315C.
(5) The following were placed in a glass
jar:
Dimethylacetamide 400 parts
(dried over molecular
sieves)
Potassium sulfide 2.2 parts
Sulfur 6.4 parts
The jar was sealed and rolled for two hours at room
temperature.
(6) The liquid product of (5) was placed
in a vessel. The porous reinforced structure of t4)
was su~merged in the liquid and the vessel placed in
an enclosure. The enclosure was purged with nitrogen,
sealed and allowed to stand undisturbed for two weeks
at room temperature.
27
,: . .
114402
28
The resulting material was suitable for use
as a diaphragm material in a chlor-alkali cell.
EXAMPLE 6
A hydrophilic fluoropolymer of the present
invention was prepared by
(a) providing a fluoroelastomer which is
a copolymer containing (on a percent
by weight basis) 58.0% vinylidene
fluoride units, 39.1% hexafluoro-
propylene units and 2.9% bromotri-
fluoroethylene units, said fluoro-
elastomer having a Mooney viscosity
of 38 at 100C when measured on a
Mooney viscometer using the large
rotor and a ten-minute shearing time;
(b) grinding said fluoroelastomer into
small particles by using a freeze mill
in which the fluoroelastomer was
frozen with liquid nitrogen and then
hammer-shattered into small particles;
(c) mixing 20 grams of the resulting particu-
late fluoroelastomer with 20 grams of
dimethylphosphite;
(d) mixing one gram of benzoyl peroxide
with the resulting mixture;
(e) heating the resulting mixture of reflux
(172C) for 3 hours;
(f) mixing one gram more of benzoyl peroxide
with the resulting mixture;
(g) heating the resulting mixture at reflux
for 2 hours, and then cooling it to
about 25C and pouring it into one
liter of water at 25C;
~h) washing the modified fluoroelastomer
present in the resulting mixture five
28
.
1~4469;2
29
times with fresh distilled water, each
washing cycle including depositing the
fluoroelastomer on a filter;
(i) mixing the fluoroelastomer with one
liter of acetone at 22C;
(j) slowly adding the resulting mixture to
water at 22C with stirring to precipitate
the fluoroelastomer, followed by filtra-
tion; and
(k) drying the resulting modified fluoro-
elastomer in a vacuum oven for two days
at 60C.
The fluoroelastomer described in step (a)
was made by
(1) continuously feeding (per hour) 56 parts
of vinylidene fluoride, 44 parts of hexafluoropropylene
and 2.8 parts of bromotrifluoroethylene to a two-liter
stàinless steel pressure vessel reactor which had been
flushed with nitrogen while operating the stirrer of
the reactor at 500 rpm for thorough mixing of the
reactor contents, and while the contents of the reactor
were heated at 105C under a pressure of 63 kg./cm2
so that the reaction mixture formed in operation (2)
below underwent an emulsion polymerization reaction as
it passed through the reactor, the reactor residence
time being about 20 minutes;
(2) during operation 1, constantly feeding
to the reactor during each hour (for each 100 parts of
monomer) 400 parts of water containing 0.6 part of
ammonium persulfate and 0.12 part sodium hydroxide
and maintaining the reaction mixture at pH of 3.7:
(3) continuously removing from the reactor
the resulting copolymer latex which was continuously
formed during operations 1 and 2;
(4) after discarding the latex obtained
29
Al
1144~92
3Q
during the first four residence times, collecting the
desired quantity of latex and mixing it for uniformity,
the latex having a pH of about 3.7 and a copolymer solids
content of 19.2%; and
(5) isolating the resulting copolymer from
the latex by the gradual addition of a 4% aqueous
solution of potassium alu~inum sulfate until the
copolymer is coagulated, washing the copolymer particles
with distilled water, removing the water by means of
a filter apparatus, and then drying the copolymer in
a circulating air-oven at 100C to a moisture content
of less than 1~.
EXAMPLE 7
The product fluoroelastomer provided in step
(a) Example 6 was vulcanized by mixing 100 parts of
it with 20 parts of magnesium silicate powder, 15 parts
of magnesium oxide, 4 parts of triallyl isocyanurate
and 3 parts of 2,5-dimethyl-2, 5-ditertiarybutylperoxy
hexane, and then heating it in a press for 1~2 hour
at 166C, followed by 24 hours in an oven at 204C.
Two samples of this vulcanized polymer were
immersed in a solution of 11 parts of K2S and 6.4
parts of sulfur in 300 parts of dimethylacetamide for
1/2 and 4 hours respectively at 85C. During this
treatment, the samples swelled and the surface turned
black.
The treated samples were washed with distilled
water and then immersed in chlorine water at 20C for
16 hours. During this treatment, the surface color
changed to light yellow.
Finally, the samples were dried in a vacuum
oven at 50C for 48 hours.
Examination of the wetability and surface
resistivity of the samples gave the results listed in
Table I.
'.~
1144692
TABLE I
Time in Surface Wettability
K2S Sol. Resistivity
(h) (ohm cm)
0 1.85 x 1013 Not Wettable
0.5 3.85 x 108 --
4 4.35 x 109 Wettable
Examination of the surface by ESCA showed increasing
amounts of sulfur on the surface of the samples as
treatment time increased.
TABLE II
Time in K2S Sol. (h) Sulfur Intensity (cps)
0 74
0.5 171
4 410
EXAMPLE 8
(A) A reactor was charged with
TFE/BTFE 87/13 copolymer 20 parts
powder
Xylene 150 parts
Triisopropyl phosphite 13.5 parts
Di-t. butyl peroxide 2.0 parts
The charge was heated to 142C and held
at that temperature for 7 hours while the
vapors which formed were removed by con-
densation.
The resulting product was removed from
the liquid by filtration, washed with xylene
and then methanol, and dried at 100C. It
was found to have a phosphorus content
of 1.16% and a water solubility of less
than 1%.
(B) The product of (A), 7.88 parts dis-
persed in 40 parts of methanol, was
mixed with 14.32 parts of a 55% a~ueous
dispersion of TFE/HFP 85/15 copolymer.
;
114'i69Z
32
(C) To a sparging flask were added
NaCl 630 parts
Asbestos fibers63 parts
(Chlorbestos SP-25*
Johns-Manville Co.)
Distilled water3087 parts
This charge was then sparged with air for
1 1/2 hours.
(D) The product of (B) was added to the
product of (C) and the mixture was
sparged with air for 30 minutes.
(E) A diaphragm was formed from the slurry
of (D) directly on the cathode of a
laboratory chlor-alkali cell by
immersing the cathode in the slurry and
drawing the slurry on the cathode with
vacuum according to the following schedule:
Start 10.2 mm of vacuum
After
1 minute 20.3 mm
2 minutes 33 mm
3 minutes 43.2 mm
4 minutes 53.3 mm
5 minutes 66 mm
6 minutes 78.7 mm
7 minutes 99.1 mm
8 minutes 139.7 mm
9 minutes 180.3 mm
10 minutes 203.2 mm
The cathode was then removed from theslurry,
placed in a horizontal position and dried
by drawing a vacuum of 203.2 mm of Hg on the
manifold for 10 minutes, followed by a
vacuum of 508 mm for 20 minutes. The coated
cathode was then held briefly at 100C under
a vacuum of 508 mm and was then baked for
30 minutes at 275C.
* denotes trade mark
---~ 114~692
(F) The diaphragm-cathode produced in (E)
was placed into a position in a labora-
tory chlor-alkali cell. Direct current
was applied to the electrodes. The cell
required an average voltage of 3.189 to
achieve a current efficiency of 95% as
compared with a conventional asbestos
diaphragm, which in the same application
required an average voltage of 3.4-3.5.
Tetraethyl pyrophosphite and tetraethyl
hypophosphate can be used in the foregoing procedure
in place of triisopropyl phosphite, with substantially
the same results.
.
