Sealant for electrochemical cells

A sealant composition comprises a dispersion of a fluorocarbon polymer with particles no larger than 25 micrometers in a low volatility liquid and up to 50 percent of a higher volatility liquid. Also covered is an applicator for applying the sealant to the gasket of an electrochemical cell.

FIELD OF THE INVENTION 
This invention relates to a sealant composition and an applicator for 
applying it to a gasket of an electrochemical cell. The sealant comprises 
a dispersion of a fluorocarbon polymer having a small particle size in a 
medium containing a liquid of low volatility which is compatible with the 
medium. 
BACKGROUND 
Membrane cells are the state-of-the-art equipment for electrochemical 
reactions such as the electrolysis of sodium chloride to make chlorine and 
sodium hydroxide. Some of the cells have very large membranes. 
Plate-and-frame cells can have an active membrane area of up to 
1.5.times.3.7 meters, which means that the membrane area sealed by the 
gaskets is even larger. Sealing is a particularly difficult task for the 
larger membranes, and these cells frequently use ribbed gaskets, with a 
width of up to 5 cm, between the plate and the membrane. 
It is necessary to use a sealant between the gaskets and the membranes at 
the time the cells are assembled under compressive force, for three 
reasons: 
1. The sealant prevents leakage of the electrolyte during operation. This 
is particularly important when using state-of-the-art membranes which 
contain small channels parallel to the dimensions of the planar surface, 
as taught in U.S. Pat. No. 4,437,951. The sealant must block the end of 
these channels, and must block capillary leakage down the length of 
multifilament membrane reinforcement. 
2. The sealant lubricates the gasket/membrane interface so the force of 
closing the cell does not tear the membranes. The use of reinforced 
gaskets reduces the deformation of the gaskets but does not eliminate the 
need for lubrication. 
3. The sealant provides a release function so the membranes are not damaged 
during disassembly. 
Among the sealants now in use is silicone rubber, which is applied in 
solution and crosslinks on contact with air. The sealant generally must be 
allowed to cure for about five hours or more (until it loses its 
tackiness) without contacting any other surface. This makes application of 
sealant to vertical gasket assemblies difficult, because there is only a 
limited amount of free space available when the clamps are fully 
separated. The silicone sealant is flammable, does not seal membranes with 
channels from sacrificial fibers, and does not provide adequate release 
properties. Spilled sealant cannot be removed with water. 
Du Pont Krytox.RTM. fluorinated grease has advantages over silicone rubber. 
It is effective as a release agent, can seal membranes with small channels 
made by removal of sacrificial fibers in the reinforcement of the membrane 
when using ribbed gaskets, and does not require air drying. This means 
that in a multicell electrolyzer, the Krytox.RTM. can be applied to each 
gasket and the coated gaskets may touch each other overnight prior to 
insertion of membranes. This is particularly important with very large 
cells, where vertical application of gasket seal ant is required because 
the cell frames are too large and heavy to permit horizontal assembly. 
Further, the Krytox.RTM. grease is not flammable. 
However, Krytox.RTM. grease must be applied with care because spilled 
sealant cannot be removed with water. Also, achieving leak-free 
performance is difficult when membranes are reinforced with fabrics made 
from multifilament yarns and when membranes are installed with flat 
gaskets. 
An improved sealant with the following attibutes is desirable: 
1. a low enough viscosity to ease application, but high enough to avoid 
running, particularly on vertical surfaces, after application; 
2. ability to flow into voids, penetrating and plugging channels remaining 
after removal of sacrificial fibers in the membrane reinforcement and 
sealing against capillary leakage along the length of multifilament 
reinforcement fibers, and/or ability to plasticize to a degree a cation 
exchange membrane so that, upon cell closure, the pressure will urge the 
cation exchange polymer itself against and even into any voids. 
3. effectiveness on flat as well as ribbed gaskets; 
4. ease of spill cleanup, preferably with water; 
5. compatablity with other sealants, such as Krytox.RTM. fluorinated 
grease, so that the two sealants can touch each other without detriment if 
the two are being used in the same cell; 
6. chemical compatibility with the reactants, materials of construction of 
the cell, the products and the electrolytic process in general; and 
7. cure time low or not required so that adjacent gaskets with sealant 
applied can touch promptly after application, so an entire large cell can 
be fitted with gaskets and sealant in one day and assembled with wet 
membrane the next day. 
The sealing composition of the present invention has the desired attributes 
and is fully suitable for application on gaskets in the vertical as well 
as horizontal mode. It is particularly effective when applied with the 
applicator of the present invention. 
SUMMARY OF THE INVENTION 
The present invention comprises a sealant composition for use in an 
electrochemical cell comprising a dispersion of a fluorocarbon polymer in 
which the polymer particles are at most 25 micrometers in diameter in a 
liquid medium, the liquid medium comprising a liquid of low volatility 
which is compatible with more volatile liquids in which the fluorocarbon 
polymers are generally commercially available. The medium should be 
essentially inert to cell components and to the desired electrochemical 
reaction. The volatility of the liquid medium should be low enough that 
the sealant composition does not harden or crack prior to closing the 
electrolyzer. 
Preferably, the sealant composition contains a thickener to make the room 
temperature viscosity of the sealant such that it can easily be applied to 
the gasket but will not flow as a result of gravity and, most preferably, 
contains a thickener that facilitates increases in viscosity with time. 
The viscosity should be intermediate between that of a water-based paint 
and that of a paste (such as tooth paste), that is, about 10 to 5000 
poises. 
More preferably, the sealant composition contains emulsions or dispersions 
of polyacrylic acids and their homologs and sufficient solid sodium 
bicarbonate (NaHCO.sub.3), to raise the pH from the desired range for 
mixing the fluoropolymer dispersion and the preferred thickener (a pH 
below 6) to the preferred pH of application (about 6.5 to 7). It has been 
found that, in a short time after application, carbon dioxide (CO.sub.2) 
diffuses out of the sealant, raising the pH to 7.5 to 8, thereby causing 
the sealant to become more viscous. By including a bromthymol blue 
indicator, pH and, hence, viscosity change can easily be noted by the 
changes in color of the sealant. In this most preferred embodiment, the 
viscosity is low during storage and application and increases after 
application. 
It is believed that the smaller particle size polymers are more effective 
in blocking channels and that the low volatility liquid medium softens the 
cation exchange membrane so the pressure of cell closure effectively 
presses the cation exchange polymer against the multifilament fibers and 
perhaps into the voids of the multifilament fibers. 
This sealant can be used on both the anolyte side and the catholyte side or 
it can be used on one side and Krytox.RTM. fluorinated grease can be used 
on the other side. The present sealant and Krytox.RTM. fluorinated grease 
can, without damage, touch each other during the time when other gaskets 
are being sealed but before membranes are installed. 
Other embodiments of the invention are an applicator and process for 
applying the sealant to a ribbed or flat gaskets which comprises (a) a 
means of providing a controlled flow of the sealant to the applicator head 
and (b) a head which has a grooved section on each side which fits the 
ribs of the gasket and has a deeper section in the center into which the 
sealant is urged.

DETAILS OF THE INVENTION 
The composition of the sealant of the present invention, on a weight 
percent (wt. %) basis, is suitably: 
20-50% fluoropolymer on a dry basis, preferably 30-50% and 0-5% thickener, 
preferably 1-2% dispersed in an amount of liquid medium chosen to add to 
100%, the liquid medium being suitably: 
50-100% low-volatility liquid, preferably 75-95%, and 0-50% more volatile 
liquid, preferably 5-25%. 
The ratio of wt. % low-volatility liquid to wt. % fluoropolymer should be 
at least 1:2. 
The fluoropolymer used in making the sealant contains at least 90% fluorine 
(F) atoms attached to carbon atoms, but may contain small amounts of other 
atoms normally present in fluoropolymers, such as hydrogen (H) and 
chlorine (Cl). Preferably, a perfluoropolymer is used, with the proviso 
that it is satisfactory to have ether linkages (--O--) in the polymer. 
Polytetrafluoroethylene is preferred due to its commercial availability and 
cost for use in the sealant of the present invention. Also suitable are 
copolymers of tetrafluoroethylene with perfluoroolefins of 3-10 carbon 
atoms or with perfluorovinyl perfluoroalkyl ethers with 3-10 carbon atoms. 
Further, within the equivalents envisioned would be 
non-fluorine-containing polymers that are hydrophobic and chemically 
resistant or inert to the reactants, products, equipment and operating 
conditions. 
The fluoropolymer fed to the preparation of the sealant may be a largely 
aqueous dispersion or an organosol; the former are more readily available. 
For the purposes of this application, both will be referred to as starting 
dispersions and references to aqueous dispersions should be construed to 
include organosols. The amount of liquid in the starting dispersion of 
fluoropolymer is not critical, but it is preferred to keep the content of 
liquid low to provide more flexibility in producing the sealant of the 
present invention. Dispersions of about 60 wt. % fluoropolymer in water 
are commercially available and are quite suitable. They may contain small 
amounts of nonionic surfactants and may contain very small amounts of 
perfluorinated ionic surfactants. 
Substantially all the polymer particles in the dispersion should be no 
larger than 25 micrometers. Preferably the average particle should be no 
more than 10 micrometers, more preferably no more than 1 micrometer. The 
most preferred and most readily available dispersions have average 
particle sizes of 0.1-0.3 micrometers. 
A purpose of the low-volatility liquid of the medium is to prevent 
hardening and cracking of sealant between application to the gasket and 
installation of the membrane. After the sealant is applied to the gasket 
it frequently must be exposed to air overnight before membranes are 
installed. This is particularly true with very large membranes, which must 
be installed vertically because the cells are too large to move from a 
horizontal to a vertical position after assembly. 
The low-volatility liquid is to prevent evaporation of more than 25 wt. % 
of the total liquid content of a 0.5 mm coating of the sealant overnight, 
even in warm, dry weather. 
Some low-volatility liquids also dissolve in the cation exchange membrane 
to some degree during and after assembly. This is believed to soften or 
plasticize the cation exchange polymer of the membrane, helping it to 
press close to the surface and perhaps into the fissures of any 
multifilament reinforcement fibers which may be present. The plasticizing 
effect is also believed to be helpful in urging the channels remaining 
from the removal of sacrificial fibers in the membrane reinforcement to 
close during pressure assembly of the plate-and-frame cell. Thus, the 
sealing function is facilitated. 
In order for the low-volatility liquid to be sufficiently low in 
volatility, it should have a boiling point at 5 mm Hg of at least 
50.degree. C. 
The low-volatility liquid of the medium must be soluble in and compatible 
with other liquids so as to form a single liquid phase in which the 
fluoropolymer and any thickener is dispersed. That is to say, the 
low-volatility liquid must be soluble in and compatible with the liquids 
in which the fluoropolymer and the optional thickener are normally 
commercially available. It must not interfere with the desired 
electrochemical reaction. For use in the preferred thickened composition, 
it should not be acidic or basic to the extent that it would interfere 
with the performance of the thickener. It is not necessary for the 
low-volatility liquid to be inert to the cell electrolytes since only a 
small amount will be present during assembly of the cell and it may be 
dissolved out during early minutes of cell operation. 
Many low-volatility liquids meet the above requirements. Among them are 
polyethylene glycols and their alkyl or monoaryl ethers; ethylene glycol 
and glycerol; dimethyl sulfoxide; dimethyl formamide; and tetramethylene 
sulfone. In an electrolysis experiment in which various low-volatility 
liquids were added to the catholyte during electrolysis, tetramethylene 
sulfone caused very little foaming, which is desirable. 
Optionally, a thickener may be used in the sealant. This is not necessary 
for horizontal assembly of plate-and-frame cells, because the milk-like 
viscosity of the sealant without thickener would not cause it to run off a 
horizontal gasket. However, larger cells cannot be assembled horizontally 
because they are too large and heavy to turn into the operating position 
in which the membranes are substantially vertical. A much higher viscosity 
of the sealant is needed to make it suitable for application to a vertical 
gasket. This viscosity is at least as high as that of water-based paint, 
approximately 10 poises, and no higher than that of a paste such as 
toothpaste, approximately 5000 poises. Preferably, the viscosity should be 
about 20 to 1000 poises. 
In addition to whether the assembly is horizontal or vertical, the method 
of application will be considered by one skilled in the art in selecting 
the preferred viscosity. The preferred viscosity can be thinner if its 
application is to be with a brush and thicker if the application is by 
putty knife or by using the applicator of the present invention. 
Any thickener known in the art that is compatible with the liquid medium 
can be used. For example, gum arabic may be used. 
The preferred thickeners are emulsions or dispersions of polyacrylic acids 
or their homologs. For example Rohm and Haas Acrysol.RTM. ASE thickeners 
or Acrysol.RTM. ICS-1 thickener may be used. For the purpose of this 
patent, both thickener emulsions and thickener dispersions will be 
referred to as emulsions. These emulsions are quite fluid, suitable for 
blending with the other components if they are on the acid side of pH 6. 
Upon increasing the pH of the sealant blend to about 6.5, some of the 
--COOH groups are converted to --COO.sup.- groups, and the polymer 
dissolves, causing the viscosity to increase to a paste-like level. 
The preferred thickened compositions may be prepared by mixing the 
fluoropolymer starting dispersion and the thickener emulsion with 
low-volatility liquid, and, preferably, a trace of bromthymol blue 
indicator, all at a pH below 6 (with the indicator present the color of 
the sealant at or below pH 6 will be a yellow color). To this fluid is 
added solid NaHCO.sub.3 until the pH is about 6.5-7 (green color). The 
polyacrylic acid dissolves and the mixture thickens to a paste, ready to 
store or apply to gasketing. 
When it is applied to gasketing, the CO.sub.2 diffuses out, the pH 
increases to 7.5-8 (blue color) and the sealant becomes still more viscous 
within 1 to 3 hours, depending upon the thickness of the sealant. 
The sealant may be applied with a brush, preferably covering the middle 
third of the gasket and leaving the outer edges free of sealant to 
minimize contamination of the membrane and electrolyte. If excess sealant 
is applied, it may be scraped off with, in the case of a ribbed gasket, a 
comb designed to leave a thick layer at the desired place and to fit into 
the ribs of the gasket and remove sealant from the areas where little or 
no sealant is desired. It may be desirable to leave a little sealant even 
in the outer edges of the gasket, to minimize leakage and assist in 
eventual release. 
A preferred way to apply the sealant to ribbed gasketing is to use an 
applicator of this invention which is depicted in the figures. While 
depicted as a round device, the applicator may have any other geometry so 
long as the face is essentially flat and sized to fit the gasket upon 
which the sealant is to be placed. 
FIG. 1 depicts the applicator (1) approaching the position to apply sealant 
to a vertical gasket (2), glued to a plate (3) of a plate-and-frame press 
(not shown). The applicator is equipped with a feeding device (A) attached 
to and in fluid communication through an orifice in the head (B) of the 
applicator. The orifice (C) can best be seen in FIG. 3 which shows the 
face of the applicator depicted in FIG. 1 and FIG. 2 which shows a 
cross-section of FIG. 3. As can be seen, particularly in FIG. 3, there are 
parallel grooves (D) that are molded or machined into the face (E) of the 
applicator so as to fit closely the ribs (F) of the gasket. Also shown in 
FIG. 3 are preferred raised lips (G) on opposing edges of the applicator 
to assist in maintaining the applicator's position with respect to the 
gasket when in use (the spacing between the lips should be slightly 
greater than the width of the gasket). 
An application slot (H), or area where the peaks (I) have been removed can 
be seen in FIGS. 2 and 3 in fluid communication with the orifice (C). The 
slot may be of any depth desired to provide a sealant bead of the desired 
thickness. The slot may extend fully across the face of the applicator as 
shown or it may only extend to the trailing edge (J) of the applicator. 
Preferably, the slot is slightly off center as shown so that the sealant 
bead will be closer to the outside edge of the gasket thereby minimizing 
the chance of sealant getting onto the working area of the membrane that 
will be installed. Thin channels (K) can be cut through the grooves in the 
applicator to allow a small amount of sealant to be present across the 
entire width of the applicator. Preferably, the thin channels are slightly 
angled toward the trailing edge as shown. Then, when the applicator is in 
use, it will leave a thin layer of sealant across the entire width of the 
gasket. 
In use, the applicator should be held so all its face area is in complete 
and close contact with the gasket in a manner that the peaks in the face 
of the applicator align with the valleys (L) in the gasket and, if 
present, the lips of the applicator are slightly outside the edges of the 
gasket as shown in FIG. 2. The applicator is slid along the gasket and 
sealant is fed into the applicator. The feeding device (A) may be a 
syringe or calking gun or a similar mechanism. Preferably, it should be 
small enough to be hand-held and fit easily between plates while keeping 
the face of the applicator flat against the gasket. Sealant in the feeding 
device is fed through the orifice (C) and into the slot (H) by applying 
some pressure. If the viscosity of the sealant is precisely adjusted and 
the application slot only extends to the trailing edge, the motion of the 
applicator will tend to create a vacuum at the point where the sealant 
enters the application slot through the orifice and the metering of the 
sealant will become almost automatic. 
As stated above, it may be desirable, particularly for release purposes, to 
have a thin layer of sealant across the entire face of the gasket. If the 
preferred thin channels (K) are omitted, this can be done by placing a 
small bead of sealant across the entire leading edge (M) of the 
applicator, the edge that first touches the gasket as the applicator is 
moved along the gasket. This will also result in a very thin layer across 
the entire gasket. 
EXAMPLES 
1. An aqueous polytetrafluoroethylene dispersion (65.08 g) with 60% solids, 
obtained as Teflon.RTM. 30 dispersion made by E. I. du Pont de Nemours and 
Company, was mixed with 15.33 g of glycerol and 11.08 g of Rohm and Haas 
Acrysol.RTM. ICS-1 thickener with magnetic stirring. Sodium bicarbonate 
(350 mg) was added slowly with magnetic stirring until the mixture was too 
thick for magnetic stirring. Then it was stirred with a spatula. 
2. Teflon.RTM. 30 (235 g) dispersion was mixed with 24.1 g Rohm and Haas 
Acrysol.RTM. ICS-1 thickener and 122 g diethylene glycol and 160 mg 
bromthymol blue. Then 700 mg sodium bicarbonate was added with stirring 
and the mixture became yellow-green and thick like a paste. On prolonged 
exposure to air it turned blue-green. It did not solidify on drying 
overnight. 
The composition was applied to both gaskets of a commercial cell to seal a 
DuPont Nafion.RTM. N-90209 perfluorinated membrane, which contained 
channels from removal of sacrificial fibers. The cell was used to make 
chlorine and sodium hydroxide under typical commercial conditions for 
seven (7) days with no leakage. On shutdown, the cell was disassembled and 
the membrane was released without difficulty. 
3. A similar composition was applied to a large commercial cell 
(1.5.times.3.7 m), which is still operated under typical commercial 
conditions after five (5) months with no leakage. 
4. The applicator depicted in the Figures without the thin channels was 
used to apply a similar sealant composition to a 5 cm wide ribbed gasket 
of a type used commercially. It gave a smooth bead of about 0.5 millimeter 
(mm) thickness in the central area of the gasket. When a small bead of the 
sealant was placed across the leading edge of the applicator, the central 
bead was obtained along with a very thin film of sealant across the entire 
gasket.