Electrode manufacturing process and flow-through capacitor produced therefrom

An electrode is made by depositing a mixture of particulate carbon and very fine thermoplastic binder on the surface of a fluid permeable dielectric web. The coated web is then passed between a pair of rollers that apply heat to at least the Vicat softening point of the binder and simultaneously apply pressure to fuse the carbon and binder to the web. Two of such electrodes are employed to manufacture a flow-through capacitor suitable for use in water treatment.

TECHNICAL FIELD 
This invention relates to a novel method for the continuous production of 
an electrode in the form of a dielectric web coated with a high surface 
area layer of an active material in the form of a particulate, 
electrically conductive substance. The electrically conductive substance 
is caused to adhere to the web by means of a thermoplastic binder present 
in a sufficiently small volume that it does not interfere with the 
electrical conductivity characteristics of the active material. There is 
further disclosed the manufacture of a novel flow-through capacitor 
employing the coated web as the capacitor dielectric and plates. 
BACKGROUND ART 
The closest known approaches to the web coating process of this invention 
are described in Koslow U.S. Pat. Nos. 5,019,311; 5,147,722; 5,189,092; 
5,249,948; and 5,331,037, their parent applications, their corresponding 
foreign patent applications and patents, and the references cited therein. 
See also co-pending patent application Ser. No. 08/813,055, filed 
concurrently herewith by the present inventors and entitled "Continuous 
Solid State Web Coating Apparatus and Webs Produced Thereby," now U.S. 
Pat. No. 5,792,513. The closest known prior art to the flow-through 
capacitors of this invention are U.S. Pat. Nos. 5,192,432; 5,360,540; 
5,415,768 inter alia of Marc D. Andelman. 
The above-mentioned Koslow patents disclose processes for the production of 
composite materials which are characterized by primary particles 
interconnected by a binder material. Some of these processes require high 
pressure and shear or extrusion through a die with carefully controlled 
back pressure. These prior art processes are extremely useful in producing 
a wide variety of articles including extruded solid forms such as 
activated carbon filters. 
The above-mentioned Andelman patents disclose capacitors designed to 
receive fluids therethrough for various applications such as 
chromatography and fluid purification. These capacitors operate by 
removing ionized particles and solutes by electrical attraction to the 
charged plates. In one embodiment, Andelman proposes a wound cylindrical 
capacitor having radial flow between the inside and the outside of the 
cylinder. Such an arrangement has the advantage of efficiency. However, in 
order to obtain such a construction, six or eight separate and individual 
layers must be wound around a central core. In accordance with the present 
invention, a cylindrical capacitor of somewhat similar configuration is 
obtained by use of a layered composite electrode produced by the method of 
this invention. 
It would be desirable to impregnate, cover, or otherwise treat, a 
relatively fragile, fluid permeable, dielectric base material web with an 
electrically conductive material that has a high surface area and is also 
porous to fluid flow. One example would be a nonwoven or plastic web 
coated with carbon particles and binder particles on at least one side. 
Two such coated webs could be wound together as, for example, in the known 
method of manufacturing a metallized film capacitor. This would produce a 
capacitor capable of functioning in essentially the same manner as the 
prior art flow-through capacitors but would be much easier to manufacture. 
However, the fragile nature of the underlying base material would make it 
impractical to employ the known prior art techniques which require high 
pressure and shear. Furthermore, the most desirable binder materials are 
in the form of extremely finely divided particulate material which is 
difficult to employ because it is non-flowable due to the high innate 
cohesion between the particles created by electrostatic and van der Waal 
forces. 
In the prior art referred to above, the powdered active material is caused 
to bind to the substrate by means of a thermoplastic material with which 
it is intimately mixed. However, the pressures and temperatures involved 
would not permit their application to fragile substrates such as the webs 
described herein. Accordingly, it is a primary object of the present 
invention to provide a method for continuously coating a relatively 
fragile web with a particulate conductive material and a very finely 
divided particulate thermoplastic binder. Another object is to produce a 
flow-through capacitor from such a web. Other objects, features, and 
advantages will become apparent from the following description and 
appended claims. 
DISCLOSURE OF THE INVENTION 
In accordance with the present invention a loose, dry composite mixture is 
formed which comprises particles of an electrically conductive material 
and particles of a thermoplastic binder. The binder particles are quite 
small in size, preferably on the order of 20 microns, and no greater than 
approximately 40 microns on average. The particle size of the electrically 
conductive material may be over a large range, for example 5-1500 microns. 
The small size of the thermoplastic binder particles causes them to adhere 
to the particles of the electrically conductive material by electrostatic 
and van der Waal forces. In addition to their tendency to stick to the 
particles of the electrically conductive material, the binder particles 
also have a high innate cohesion. 
The composite powder may be uniformly applied to the surface of a moving 
dielectric web by means of a knurled roll dispenser or by other means well 
known in the prior art. The coated web can be preheated and then passed 
through the nip of a pair of laminating rollers, at least one of which is 
heated, that apply heat and elevated pressure to fuse the powder particles 
to each other and to the dielectric web. During this process, it is also 
possible to incorporate an additional dielectric web on the upper surface 
of the powder layer. This second web is generally not preheated and 
generally passes in contact with the unheated laminator roll. The 
electrically conductive powder now emerges between two layers of 
dielectric medium that provide strength and protection to the composite 
sheet. 
If a second dielectric sheet has been used, it is thereafter peeled away 
with ease because its adhesion is relatively low because of its greater 
distance from the heated roll and lack of preheating. Next the remaining 
web is folded and combined with a third, electrically conductive, web such 
as graphite foil, expanded metal, etc. One edge of this third web is 
allowed to extend beyond the margin of the folded dielectric composite 
sheet. This combination is then passed through a heated laminator that 
applies heat and elevated pressure to cause all of the layers to fuse into 
a final composite electrode medium. 
Two lengths of this final composite medium may be laid upon each other and 
rolled to form a cylindrical capacitor. Alternatively, two or more of 
these composite electrodes may be stacked upon each other to form a 
capacitor stack. The two edges of the electrode core extending beyond the 
composite edge are used to make electrical connections to end caps or 
other appropriate structures.

BEST MODE FOR CARRYING OUT THE INVENTION 
Any of a large number of electrically conductive particulate agents may be 
applied to an underlying web in accordance with this invention. The 
critical features of this invention, however, reside in the thermoplastic 
binder which is employed to coalesce the conductive particles and adhere 
them to the underlying web. For this purpose, the thermoplastic binder 
must be in the form of very small particles and must be present in a small 
enough volume that they do not interfere with the consolidation of the 
conductive powder into a continuous electrically conductive medium. 
Preferably, the binder particles will have an effective diameter of not 
more than 40 microns on average with an average optimum size of 20 microns 
or less. A binder which is suitable for the process of this invention may 
be produced from normally solid, synthetic organic polymeric thermoplastic 
resins by the method disclosed in U.S. Pat. No. 3,432,483 of Peoples, et 
al. Examples of suitable binders are Microthene.RTM. F, microfine 
polyolefin powders produced by Quantum Chemical Company such as, for 
example, their low density polyethylene designated FN-510 and their 
ethylene-vinyl acetate copolymer designated FE-532. FIG. 2 illustrates the 
typical particle size distribution of Microthene FN-510 powder. Similar 
suitable binders can be high density polyethylene, linear low density 
polyethylene, nylon, polypropylene, or other thermoplastic resins or some 
thermoset resins, such as dry thermoset resins sold by Plenco, Inc. 
FIG. 1 illustrates an exemplary apparatus for the manufacture of an 
electrode in accordance with this invention. A supply roll 10 provides a 
web 12 of the substrate dielectric to be treated, such as a spun bonded 
nonwoven or other porous dielectric medium. Downstream from supply roll 10 
is a knurled roller 13 positioned to receive the composite powder 14 of 
this invention from a hopper 16 and apply the powder to the upper surface 
of the web 12. The surface of the knurled roller 13 may be designed to 
provide a substantially continuous coating or, alternatively, a coating of 
a specific design such as, for example, stripes on the web surface. A 
brush 18 may be employed to aid in removing the composite powder from the 
knurled roller 13. Such apparatus is conventional. Thereafter, the web 12 
is passed through the nip 20 between a heated idler roller 22 and a drive 
roller 24. A pneumatic cylinder 26 is connected via a rod 28 to the axle 
of the heated idler roller 22 to maintain a desired pressure on the web 
within the nip 20. In passing over the surface of the heated idler roller 
22, the binder is heated to a temperature greater than its Vicat softening 
temperature as it enters the nip 20, the temperature being below the 
melting temperature of the binder. The temperature is also below the 
melting temperature of the electrically conductive material. Within this 
nip the binder material fuses under both heat and pressure with the 
conductive material and with the dielectric web, while the electrically 
conductive powder particles are pressed and consolidated into intimate 
electrical contact. 
Note: The Vicat softening temperature is defined by Quantum Chemical 
Company, Cincinnati, Ohio, as " . . . the temperature at which the 
finished [thermoplastic] article becomes too soft to withstand stresses 
and keep its shape. It is the temperature at which a flat-ended needle of 
1 mm cross section under a load of 1 kg penetrates 1 mm into a . . . 
specimen. In the Vicat test, the temperature of the specimen is increased 
at a uniform rate." 
In the illustrated apparatus there is provided a second supply roll 30 of a 
web 32 which may be of the same or a different material from that of base 
web 12. In one configuration, this web is selected to be removable from 
the composite web being manufactured, its main function being to secure 
the particles through the fusion operation. (In another configuration, 
this web 32 could be an electrically conductive foil which would form an 
electrode capable of stacking in a linear arrangement or suitable for 
winding into the flow-through capacitor described below.) This web is also 
passed between the nip 20 of the rollers 22, 24 and on the top of the 
particulate material which is being fused. Accordingly, the web 34 which 
leaves the roller 24 is a composite sandwich web. Upon leaving the nip 20, 
the binder cools and hardens, thereby forming the desired composite. The 
composite web 34 passes onto a take-up roll 36. 
The process can be arranged to carry out the application of a second 
capacitive layer of electrically conductive medium on the opposite side of 
the dielectric layer or the conductive foil layer by arranging two 
systems, such as shown in FIG. 1, in tandem and allowing a second layer of 
composite powder to be applied and fused. This would result in a composite 
structure similar in arrangement to the structure illustrated in FIG. 5. 
EXAMPLE 1 
Carbon Electrode 
The substrate used was a 23 cm wide web 12 of dielectric material selected 
to be sufficiently "tight" to prevent passage of the particulate 
electrically conductive material, such as carbon, but porous to the water 
to be treated. It was a calendared meltblown non-woven polypropylene with 
5.mu. pore size, part number WD 902 supplied by Web Dynamics of East 
Stroudsburg, Pa. The composite powder mixture 14 was 17% by weight 
ethylene-vinyl acetate copolymer, (FE532 of U.S.I. Chemicals of New York, 
N.Y.), 5% by weight flake graphite (20-30.mu.), 10% graphite powder 
(80/325 mesh), and 68% activated carbon (200-325 mesh). 
The 23 cm wide upper web 32 was 0.8 ounce/sq. yard polyester. The webs 
moved at the rate of 0.6 meter/min and the composite powder was laid down 
in the amount of 310 g/m.sup.2. The heated roller 22 was 25 cm in diameter 
and heated by hot oil to a temperature of 135.degree. C. The binder 
reached its Vicat softening temperature of 75-80.degree. C. in the nip. 
Pressure in the nip was maintained at approximately 72 kg/cm.sup.2. This 
process produced a sandwich web 34. 
A suitably long piece of the sandwich web 34 was cut and the polyester 
layer of web 32 was peeled therefrom, leaving the exposed electrically 
conductive layer of web 12 having a coating of fused activated carbon, 
flake and powder graphite, and binder. Laid upon this fused coating was a 
0.025 cm thick graphite foil 38 which was perforated to render it water 
permeable. As illustrated in FIG. 3, one edge 40 of the foil extended 
substantially along the centerline of the web 12, while the other edge 42 
extended slightly beyond the edge 44 of the web. Although only a portion 
is shown, the foil extended substantially the length of the web 12. The 
coated web 12 was then folded along its centerline to enclose the inner 
edge 40 of the foil, while leaving its outer edge 42 exposed. The folded 
web was then passed between the nip 20 of the rollers 22, 24 to fuse the 
foil and web together. The result was the electrode 46 illustrated in FIG. 
4 wherein the inner edge of the foil is protected and insulated and the 
outer edge remains extended beyond the composite web 12. It will be 
understood that the thickness of the web 12 is greatly exaggerated in 
order to show the construction. It will also be understood that folding 
over of the web 12 around the foil is a preferred construction. The foil 
could also extend the full width of the web and still function to form the 
flow-through capacitor to be described. However, in this preferred 
embodiment the non-extending edge is better protected. It will also be 
apparent that structures other than a foil could be employed. For example, 
an expanded nickel metal foil could serve the same function. Furthermore, 
other non-structural elements can be substituted for the foil, such as an 
additional layer of powdered electrically conductive material, which could 
be cast into and bonded within web 12. 
EXAMPLE 2 
Flow-through Capacitor 
Two pieces 46a, 46b of the electrode were cut to the desired length. One 
was reversed relative to the other and they were laid together as 
illustrated in FIG. 5 such that the extending edges 42a, 42b of the foils 
were on opposite sides. The two electrodes were then wrapped around a 
perforated cylindrical plastic core 48, as shown in FIG. 6. The number of 
turns depends upon the desired capacitance and can be readily ascertained 
by one skilled in the art. Upon completion, there was a cylindrical 
capacitor section 50. Extending from one end of the Upon completion, there 
was a cylindrical capacitor section 50. Extending from one end of the 
section was the foil edge 42a of one electrode and extending from the 
opposite end was the foil edge 42b of the other electrode. 
An electrically conductive circular plastic end cap 52 having an encircling 
rim 54 was secured to one end of the capacitor section 50--making contact 
with foil edge 42a and secured by means of an electrically conductive 
potting compound 56. The end cap 52 provided an electrically conductive 
contact surface for uniform electrical supply along foil edge 42a and 
sealed electrode 46a of the flow-through capacitor which included a 
central opening 58 therethrough. 
In like fashion, a second electrically conductive plastic end cap 60 having 
a similar encircling rim 62 was secured to the other end of the capacitor 
section 50--making contact with foil edge 42b and was similarly secured by 
means of an electrically conductive potting compound 64. This second end 
cap 60 similarly provided an electrically conductive contact surface for 
uniform electrical supply along foil edge 42b and sealed the other 
electrode 46b of the flow-through capacitor while providing a solid, 
fluid-impervious, wall. 
The flow-through capacitor which results from this invention has 
water-permeable electrodes wrapped around a perforated hollow core 48. 
This permits radial flow through the cylindrical section--generally 
inwardly rather than outwardly. Similarly, fluid enters or leaves the 
capacitor through the end cap opening 58. Electrical connections to the 
end caps 52, 60 permit its connection into an electrical circuit. 
A flow-through capacitor can also be constructed by stacking electrodes 46 
in a fashion substantially similar to the structure illustrated in FIG. 5. 
This stack would be arranged within a suitable housing which would seal 
the non-foiled edges of the electrodes and provide electrically conductive 
contact surfaces or bus bars along each of foil edge 42a and 42b. Water or 
other fluid would be directed through the stack in an axial direction, 
exiting out the other end of the stack. 
In both the wound cylinder and stack flow-through capacitors, an electrical 
current can be caused to flow between adjacent electrodes. When water or 
other fluids are forced through the capacitor, each electrode can attract 
oppositely charged ions and temporarily hold them within the composite 
medium. It will be apparent to one skilled in the art that the electrical 
current can be adjusted to attract certain ions, leaving other elements 
unaffected and hold such ions in a manner to serve either as an electrical 
storage device or a means for fluid purification. Furthermore, the 
polarity of each electrode can be reversed, releasing such captive ions 
and thereby allowing them to be washed out of the electrodes through a 
reverse, or other suitable, flow path. 
It is believed that the many advantages of this invention will now be 
apparent to those skilled in the art. It will also be apparent that a 
number of variations and modifications may be made therein without 
departing from its spirit and scope. For example, the foregoing is 
directed to an electrode and capacitor primarily for water treatment. 
However, the invention is equally applicable to use with any liquid or 
gas, hence the use of the term "fluid" in the following claims. 
Accordingly, the foregoing description is to be construed as illustrative 
only, rather than limiting. This invention is limited only by the scope of 
the following claims.