Conductive polymer electrical devices

A method of attaching power leads to a mesh or similar electrode embedded in the surface of a conductive polymer element. A conductor, preferably also mesh, is bonded to the electrode using a conductive adhesive and a polymer layer is applied over the surface of at least the conductor, preferably also over the electrode. The polymer of the coating interpenetrates the openings of the mesh conductor and mesh electrode and bonds to the conductive polymer matrix. This mechanically holds the conductor, electrode, and conductive element in contact with each other.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to electrical devices comprising conductive polymer 
compositions and to a method or attaching power leads to conductive 
polymer elements. 
2. Discussion of the Prior Art 
Electrical devices, such as for example, heaters and current limiting 
devices, comprising conductive polymer compositions are described in the 
literature and are commercially available. 
In such devices a conductive polymer composition is attached in some manner 
to a source of electrical power. This is generally provided by what is 
referred to in the art as an electrode which is in contact with the 
conductive polymer composition and which is connected to a source of 
electrical power. One type of electrode that can be used with conductive 
polymer compositions is a wire mesh or grid electrode at least partially 
embedded in the conductive composition. The grid or mesh must then be 
attached to a power lead in some fashion. Several techniques for attaching 
such leads have been suggested. One method comprises soldering or welding 
power leads to a portion of the mesh which extends beyond the edge of the 
conductive polymer element. Because of this extension, production of 
heaters by a continuous extrusion process is limited to substantially 
rectangular heaters. Also, protruding edges of the mesh electrode are 
difficult to cut to the desired size. Attachment of a power lead to the 
center of the mesh by welding or soldering creates a protrusion on the 
planar surface of the heater making direct contact between the entire 
heater surface and the surface of the object to be heated difficult or 
impossible to maintain. 
A discussion of attaching electrodes to conductive polymer compositions can 
be found in U.S. Pat. No. 3,351,882 (Kohler et al.) and U.K. Pat. No. 
1,167,551 (Texas Instruments). In the U.K. patent, perforated electrodes 
are maintained in good electrical contact with a PTC element of a heater 
by an insulating sleeve. The material of the sleeve and PTC element 
coalesce in the perforations to maintain this contact. Electrical leads 
can be connected to the electrodes by any suitable manner, as by peeling 
away a portion of the outer jacket and soldering the leads to the 
perforated electrode strips. As mentioned above, soldering leads to 
electrodes of this type is frequently undesirable. 
SUMMARY OF THE INVENTION 
This invention provides an improved method of attaching electrical power 
leads to an electrical device comprising a conductive polymer element. One 
aspect of this invention provides an electrical device comprising: 
(a) a conductive polymer element comprising conductive particles dispersed 
in a polymer matrix; 
(b) an electrode having a plurality of openings therein secured to the 
surface of said element; 
(c) a conductor having a plurality of openings superimposed over at least a 
portion of said electrodes and conductive element and bonded thereto with 
an electrically conductive adhesive; and 
(d) a layer of polymeric material covering said conductor and 
interpenetrating the openings of said conductor and electrode, said 
polymeric material bonding to said conductive element, electrode and 
conductor, thereby retaining said conductor in electrical contact with 
said electrode and conductive element. 
Another aspect of this invention comprises a method of attaching electrical 
power leads to an electrical device comprising a conductive element 
composed of a conductive polymer composition comprising conductive 
particles dispersed in a polymer matrix which comprises: 
(a) securing an electrode having a plurality of openings to the surface of 
said element; 
(b) superimposing a conductor having a number of openings therein over at 
least a portion of said electrode, said conductor being coated with an 
electrically conductive adhesive on at least the surface thereof which 
contacts said electrodes; 
(c) applying a layer of polymeric material over said conductor so that said 
polymeric material interpenetrates the openings of said conductor and 
electrode and bonds to the polymeric matrix of said conductive element, 
thereby retaining said conductor in good electrical contact with said 
electrode and conductive element; and 
(d) attaching a power lead to said conductor.

DETAILED DESCRIPTION OF THE INVENTION 
Conductive polymer compositions and their use in electrical devices are 
well known in the art. For example, see U.S. Pat. Nos. 2,978,665 (Vernet 
et al), 3,243,753 (Kohler), 3,311,862 (Rees), 3,351,882 (Kohler et al), 
4,017,715 (Whitney et al) and 4,177,376 (Horsma et al) and copending and 
commonly assigned Applications Ser. Nos. 750,149 (Kamath et al), 751,095 
(Toy et al) 798,154 (Horsma) 943,659 (van Konynenburg), 965,343 (van 
Konynenburg et al) now U. S. Pat. No. 4,237,441, 965,344 (Middleman et al) 
now U.S. Pat. No. 4,238,812 and 365,345 (Middleman et al), and 
applications filed concurrently herewith, now Ser. Nos. 141,984 and 
141,988 respectively, the disclosures of which are incorporated herein by 
reference. In general, these compositions comprise conductive particles 
of, for example, carbon black, graphite or particulate metal, dispersed in 
an polymer matrix. 
The conductive element used in the practice of this invention can comprise 
one or more layers of conductive polymer composition. When more than one 
layer is included in the element, the conductive polymer composition of 
each layer can be different, if desired. For example, conductive devices 
comprising layers of different conductive polymer compositions are 
disclosed in U.S. Pat. No. 4,177,376 (Horsma et al). As described in this 
patent, conductive polymer compositions can remain of relatively constant 
wattage, or resistance, with increasing temperature or can exhibit a 
positive temperature coefficient of resistance (PTC) and undergo a sharp 
increase in resistance at a given temperature or temperature range with a 
corresponding decrease in power. In the electrical devices used in 
accordance with this invention, the conductive element can comprise one or 
more layers of constant wattage material, i.e. material which exhibits a 
zero temperature coefficient of resistance (ZTC), PTC material or NTC 
material, i.e. material which exhibits a negative temperature coefficient 
of resistance. The conductive element can be a shaped article other than a 
layer or layered structure, if desired. 
The electrode having a number of openings therein is of a highly conductive 
material. Preferably the electrode is a metal, for example, nickle or 
nickel coated copper. The electrode is preferably a mesh of metal wire or 
filaments. Other structures having openings therein such as grids, 
expanded metal, stranded wire, wire rovings, silver coated nylon fabric, 
graphite fabric or mats, and the like can be used. The electrode is 
embedded in or otherwise attached to the conductive element over at least 
a portion of the surface thereof. Generally, the conductive element is a 
layered structure and the electrode is embedded over substantially all of 
at least one surface of the layered structure. In a preferred embodiment, 
the device comprises two electrodes embedded in opposing surfaces of a 
layered conductive element. 
Preferably, the electrode is embedded in the surface such that conductive 
polymer composition substantially fills the openings in the electrode. To 
ensure good electrical contact with the conductor and electrode, the outer 
surface of the embedded electrode should be free of conductive polymer 
composition. If necessary, conductive polymer can be removed from the 
outer surface of the electrode by scraping, sanding or otherwise abrading 
the surface over the portion of the surface of the electrode which will be 
in contact with the conductor. 
The conductor is also of a highly conductive material and has a plurality 
of openings therein. The conductor can be a mesh, grid, stranded wire, 
wire rovings, expanded metal, graphite fabric and mats, and the like. The 
conductor contacts at least a portion of the electrode. To ensure and 
maintain good electrical contact between the conductor and electrode, 
these two components are bonded together with an electrically conductive 
adhesive. The electrically conductive adhesive is preferably resilient and 
provides a mechanical buffer between the conductor and electrode, enabling 
good electrical contact to be maintained over numerous thermal cycles with 
repeated expansion and contraction of the conductive polymer matrix. The 
adhesive also provides greater area of contact between the conductor and 
electrode at each point of intersection between these components, thus 
preventing burn out which generally occurs at such point contacts. It is 
believed that openings in the conductor as well as the electrode enable 
them to remain in electrical contact during repeated expansion and 
contraction through successive heating cycles. Use of a mechanically 
resilient electrically conductive adhesive is thought to further improve 
maintenance of good electrical contact between these two elements. 
Electrically conductive adhesives typically contain conductive particles 
such as carbon black, graphite or powdered metals, for example silver or 
other high conductive metal dispersed in an adhesive such as, an epoxy, 
silicone, or preferably, a fluoroelastomer based adhesive. The conductive 
adhesive provides an interface between the electrode and conductor at the 
points of intersection between them. However, the adhesive should not fill 
a substantial proportion of the openings of the conductor or electrode. 
One convenient method of applying the adhesive to the interface is to 
apply the adhesive to one side of the conductor such that the majority of 
the openings of the conductor are not filled with adhesive and then 
placing the conductor over the electrode. The adhesive can be applied to 
the conductor by, for example, spraying, brushing, roll-coating, dipping, 
etc. 
The conductor is attached to lead wires which can be connected to a source 
of electrical power. This can be accomplished, for example, by positioning 
the conductor in a manner such that a portion thereof extends beyond the 
edge of the conductive element and securing a connecting wire to the 
extended portion. This can be done by soldering, welding or otherwise 
physically and electrically connecting the lead wire to the conductor. 
When the lead wire is connected to a source of power, current flows 
through the conductor, electrode and conductive element. 
An outer layer of polymeric material covers at least the conductor and 
interpenetrates the openings of the conductor and electrode and bonds to 
the polymer matrix of the conductive element. This mechanically holds the 
conductor, electrode, and conductive element in contact with each other. 
The polymer used should be capable of bonding to the polymer of the 
conductive polymer matrix. The outer polymer layer preferably covers the 
entire surface of the device. For many uses the polymer layer should be an 
insulating layer. However, in some cases it is desirable that the outer 
layer also be conductive. For example, in manufacturing a multilayered 
conductive element with mesh electrodes interspersed between the 
conductive layers. One to these layers can be considered as an outer 
polymer layer. In this case, the polymeric material can be of the same of 
a different conductive polymer composition, as long as the polymer 
matrices of the layers are compatible. The entire device does not need to 
be covered by the polymer layer, for example, when the device is to be 
later incorporated into an apparatus or appliance, the insulation can be 
placed only over the conductor where it makes contact with the electrode. 
The polymeric material can be, for example, a polyolefin such as 
polyethylene or polypropylene, polyvinylidene chloride, polyvinylidene 
fluoride, polytetrafluoroethylene, polychlorotrifluoroethylene, 
tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene 
fluoride-hexafluoropropylene copolymer, epoxy resin, polyurethane, 
silicone rubber or the like. The polymeric layer can be applied over the 
conductor or entire surface of the device by conventional techniques such 
as compression molding, extrusion, lamination, adhesive bonding, etc. 
Turning now to the drawings, FIG. 1 illustrates an embodiment of the 
invention. FIG. 1 shows a sheet heater composed of a conductive polymer 
layer, 1, having embedded in the surface thereof, a mesh electrode, 2. A 
mesh conductor, 3, in the form of a strip is secured to the embedded mesh 
electrode, 2, with a conductive adhesive. Power lead, 4, is soldered to 
the mesh conductor, 3. In this embodiment a second electrode (not shown) 
identical to the first is attached to the opposite side of the heater and 
power lead, 5, extends from a conductor (not shown) identical to 
conductor, 3, but secured to the second electrode. 
FIG. 2 shows an enlarged view of a cross-section of the heater of FIG. 1 
along the line 6-6'. In FIG. 2, the conductive polymer layer, 7, has mesh 
electrodes, 8 and 9, embedded on opposing surfaces thereof. Mesh 
conductors, 10 and 11, are bonded to electrodes, 8 and 9, respectively 
with electrically conductive adhesive, 12, which coats the solid segments 
of the mesh conductors, 10 and 11. Polymeric insulating layers, 13 and 14, 
cover both opposing surfaces of the heater and interpenetrate the openings 
of the mesh conductors, making contact with and bonding to the conductive 
polymer matrix. 
EXAMPLE 
This example further illustrates the invention. In this example a planar 
heater having mesh electrodes was prepared and power leads were connected 
thereto in accordance with the invention. The planar heater comprises 
planar mesh electrodes having between them a conductive polymer layer 
which exhibits a positive temperature coefficient of resistance (PTC) and 
a contiguous layer which exhibits essentially no change of resistance with 
changing temperatures (ZTC or zero temperature coefficient of resistance). 
The conductive composition used to prepare the ZTC layer is the subject of 
copending commonly assigned patent application, Ser. No. 141,984, filed 
concurrently herewith. 
PREATION OF ZTC SHEET MATERIAL 
Master Batch 1 was prepared from the ingredients shown in the Table. The 
ingredients were introduced into a 25 lb. Banbury mixer whose rotor had 
been preheated by steam and was turning at high gear. When the torque had 
increased considerably, the steam to the rotor was turned off and water 
was passed through the rotor to cool it. Mixing was continued at fourth 
gear for 2.5 mins. after the water had been turned on and for a further 2 
mins. at third gear. The mixture was dumped, held on a steam-heated mill, 
extruded into a water bath through a 3.5 inch extruder fitted with a 
pelletizing die, and chopped into pellets. The pellets were dried under 
vacuum at 60.degree. C. for at least 18 hours. 
Master Batch 2 was prepared from the ingredients shown in the Table. The 
ingredients were introduced into a 25 lb. Banbury mixer whose rotor was 
water-cooled and was turning at high gear; mixing was carried out at 
fourth gear for 2 mins. and at third gear for 1.75 mins. The mixture was 
dumped, cooled and granulated. The granules were dried under vacuum at 
60.degree. C. for at least 18 hours. 
The final mix, containing the ingredients shown in the Table, was prepared 
by introducing 11,523 g. of Master Batch 1, 3,127 g. of Master Batch 2, 
3,480 g. of high density polyethylene (Marlex 6003) and 77.7 g. of 
antioxidant into a 25 lb. Banbury mixer whose rotor was water-cooled and 
was turning at high hear; mixing was carried out at high gear for 4 mins. 
and at low gear for 1 min. The mixture was dumped, held on a steam-heated 
mill, extruded into a water bath through a 3.5 inch extruder fitted with a 
pelletizing die, and chopped into pellets. The pellets were dried under 
vacuum at 70.degree. C. for 24 hours, and then extruded into sheet 12 
inches wide and 0.021 inch thick, using a two and one half inch 
Davis-Standard Extruder fitted with a 15 inch sheet die and operating at 
20 RPM with a throughput of 4 feet/minute. The sheet was stored under 
argon. 
PREATION OF PTC SHEET MATERIAL 
The ingredients shown in the Table for the PTC material were introduced 
into a 25 lb Banbury mixer. The mixture was dumped from the Banbury and 
converted into sheet by the same procedure as the Final Mix. The sheet was 
stored under argon. 
PREATION OF HEATER 
Rectangles 8.75.times.9 inch were cut from the ZTC sheet material and from 
the PTC sheet material, and dried under vacuum at 60.degree. C. for 9 
hours. Two rectangles 8.times.9 inch were cut from a sheet of fully 
annealed nickel mesh that had been thoroughly cleaned. The rectangles were 
sprayed until the nickel was completely covered, but the mesh apertures 
were not filled, with a composition containing 60 parts by weight of 
methyl ethyl ketone and 40 parts of Electrodag 502 which is an adhesive 
composition comprising graphite particles dispersed in a fluoroelastomer, 
specifically a copolymer of vinylidene fluoride and hexafluoropropylene. 
The coated mesh rectangles were dried under vacuum for 2 hours at 
100.degree. C. 
The PTC, ZTC and mesh rectangles were laminated to each other by layering a 
fluoroglass sheet (a release sheet of a glass-fiber reinforced fluorinated 
polymer), a mesh electrode, a PTC layer, a ZTC layer, another mesh 
electrode, and another fluoroglass sheet in a mold and pressing with a 12 
inch press with plate temperatures of (224.degree. C.) (top) and 
218.degree. C. (bottom) for 3.5 minutes at 14 tons ram pressure. The mold 
was then cooled in an 18 inch cold press with air cooling at 14 tons ram 
pressure for 5 minutes. 
ATTACHMENT OF THE POWER LEADS 
The resulting heater blank was masked, leaving 1.5 inch at each end 
unmasked. A razor was used to scrape away PTC or ZTC material (which had 
been pressed through the coated mesh) from the mesh on opposite sides of 
the heater in the unmasked area. The scraped area on each side of the 
heater blank was then further abraded with a grit blaster using 320 mesh 
grit and 40 pounds per square inch pressure. 
Strips 1.0.times.10.0 inch were cut from flat and fully annealed Cu mesh 
which had been thoroughly cleaned. One side of the strips was coated with 
a silver/silicone contact elastomer and strips were then dried in vacuum 
at room temperature for a minimum of 4 hours. One end of each of the 
strips was then bent back at a 45.degree. angle and 
0.008.times.0.187.times.6.0 inch flat Copper wire was soldered onto the 
bent end. One of these strips was applied to each of the abraded areas of 
the heater blank with the silver side toward the heater and then each of 
the conductors was covered with a 8.75.times.1.5.times.0.011 inch 
polyethylene sheet. The assembly was placed between two 0.5 inch aluminum 
plates and compression molded at 200.degree. C. for 3 min. at 5000 lbs. 
pressure, and then placed in the cold press for 10 minutes at 5000 lbs. 
pressure. 
TABLE 
__________________________________________________________________________ 
MASTER BATCH 1 
MASTER BATCH 2 
FINAL MIX PTC 
g. % wt. 
% Vol. 
g. % wt. 
% Vol. 
g. % wt. 
% Vol. 
g. % 
% 
__________________________________________________________________________ 
Vol. 
Carbon Black 1 (Raven 8000) 
-- -- -- 6628 
42.1 
28 1317 
7.2 
7.6 
-- -- -- 
Carbon Black 2 (Furnex N765) 
-- -- -- -- -- -- -- -- -- 7001 
44 29.6 
Polyethylene 1 (Marlex 6003) 
-- -- -- -- -- -- 3480 
19.1 
10.7 
8592 
54 68.1 
Polyethylene 2 (Alathon 7050) 
6186 
27.3 
49 8837 
56.1 
70 4900 
26.9 
15.0 
-- -- -- 
Inert Filler (Glass Beads) 
16306 
72.1 
50 -- -- -- 8308 
45.6 
65.9 
-- -- -- 
Antioxidant 138 0.6 1 276 
1.8 2 203 
1.2 0.8 318 
2 2.3 
__________________________________________________________________________ 
NOTES: 
Raven 8000 (Available from City Services Co.) has a particle size (D) of 
13 millimicrons and a surface area (s) of 935 m.sup.2 /g. 
Furnex N765 (Available from City Services Co.) has a particle size (D) of 
60 millimicrons and a surface area (s) of 32 m.sup.2 /g. 
Marlex 6003 is a high density polyethylene with a melt index of 0.3 which 
is available from Phillips Petroleum Co. 
Alathon 7050 is a high density polyethylene with a melt index of 18.0 
which is available from E.I. DuPont de Nemours & Co. 
The glass beads are available from Potters Industries as Potters #3000 
with CP01 coating. They are spherical glass beads with a diameter of 444 
microns and having a surface coating of a wetting or coupling agent 
thereon. 
The antioxidant used was an oligomer of 4,4thio bis (3methyl-6-t-butyl 
phenol) with an average degree of polymerization of 34, as described in 
U.S. Pat. No. 3,986,981.