US 3344385 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
p 1967 D. M. BARTOS ETAL 3,34 ,385
FLEXIBLE RESISTANCE ELEMENT WITH FLEXIBLE AND STRETCHABLE TERMINAL ELECTRODES Filed Jan. 4, 1965 'IIIIIII'II/IIII' Q in INVENTORS. Dona/0M4 fiar/os Y Raymondd Price 3 H TTORNEV United States Patent 3,344,385 FLEXIBLE RESISTANCE ELEMENT WITH FLEX- IBLE AND STRETCHABLE TERMINAL ELEC- TRODES Donald M. Bartos, Midland, and Raymond J. Price, Bay City, Mich, assignors to Dow Corning Corporation, Midland, Mich, a corporation of Michigan Filed Jan. 4, 1965, Ser. No. 423,228 2 Claims. (Cl. 338212) ABSTRACT OF THE DISCLOSURE Flexible and stretchable elas-tomeric resistance element, having flexible and stretchable braided, open weave metallic terminal electrodes, which have the capacity to maintain intimate contact with the elastomeric, resist ance element during flexing, bending, and stretching of the element while it is in use. Such intimate contact between an elastomeric resistance element and its terminal electrodes is necessary if current is to flow uniformly through the resistance element.
The present invention relates to electrically conductive flexible or stretchable materials, and more particularly, to the provision of terminal connections for such materials.
Various types of electrically conductive plastic and elastomeric materials have been proposed. Generally, such materials are formed of electrically conductive particles suspended in a-natural or synthetic rubber, or plastic, flexible or stretchable carrier. By proper selection of materials it is now possible to provide flexible and/or stretchable materials having any balance of electrical and physical properties. Such devices have potential application in fields such as resistance elements, heating elements, tapes, pads, blankets, clothing and many others.
A major problem heretofore, with such materials, has been the difliculty in attaching suitable electrodes to the material for connection to an electrical power source. Electrodes must have intimate contact with a resistance element throughout its entire length. The ability to maintain intimate elecrode contact during flexing, bending, and stretching of the element in use is considered by many design engineers to be the most important criterion in selecting such devices for a particular use. This has been particularly true in heating applications wherein a relatively large area is to be uniformly heated. If electrodes are attached only at points on the electrically conductive resistance element, electrical current is distributed nonuniformly through the area resulting in high temperatures (or hot spots) in areas of high current concentration and low temperatures in areas remote from the electrodes. It is therefore desirable to provide an electrode arrangement wherein current is caused to flow uniformly through the resistance element. It is toward this object that the present invention is directed.
Another object is the provision of an electrode, which is capable of stretching and yielding with the flexible resistive element for relatively wide stretchable electrical elements. A further object is to provide a method of making a wide flexible conductor having yieldable electrodes made integrally therewith.
In accordance with these and other objects, there is provided by the present invention a yieldable, stretchable, low resistance electrode, which is bonded to a flexible and/or elastomeric electrical resistance element. Resistance elements may be either supported or unsupported. The electrode may, for example, be a stretchable metallic conductor which is processed into the resistance element before the element has been vulcanized or cured. If desired, flexible electrical insulation can be bonded over both the resistance element and the electrodes.
Other objects and many of the attendant advantages of the present invention will become apparent to those skilled in the art by a consideration of the following detailed description When read in conjunction with the acccompanying drawings wherein:
FIG. 1 is a view in perspective of a heating pad made in accordance with the present invention;
FIG. 2 is a cross-sectional view of the heating pad of FIG. 1 taken on the line 2--2 of F IG. 1;
FIG. 3 is a view in perspective of the heating element ofFIG. 1; and
FIG. 4 is a diagrammatic view illustrating a method of making insulated elements in accordance with the present invention.
Referring now to the drawings wherein like reference numerals designate like or corresponding parts throughout the several views, there is shown in FIG. 1 a heating pad shown generally as 10. The heating pad is covered by a layer of insulating material 11, and has a pair of electrodes 12 and 13 embedded therein, and projecting from the pad for connection to an electrical power source. It will be realized that in practice the leads emerging from suitable locations on the pad will also be insulated to prevent short circuiting and danger to the user.
As may be seen from FIGS. 2 and 3, the electrodes are embedded in a layer of flexible conducting material 14 which may be of any known type which is in turn covered by a layer 11 of insulation. Numerous examples of suitable conductive materials are described in copending US. patent application, Ser. No. 408,263, filed Nov. 2, 1964. The material may merely be flexible, or it may be elastomeric in nature. It is preferable that the material be capable of flowing or being made to flow at some time in the manufacturing process and be capable of bonding or adhering to the electrodes.
It is desirable that the electrodes be capable of flexing with the conducting material without loss of bond or intimate contact. For example, if the conducting material is elastomeric in nature, the electrode must be capable of stretching or yielding with that material during flexing or stretching. A suitable type electrode is an open weave or braided metallic conductor such as the type of braid commonly used as a shield in audio equipment such as microphone cables. In a specific embodiment, an electrode consisting of 50 strands of tinned copper wire braided in tubular form was utilized. Before embedding it, the electrode was expanded to render it more stretchable and flattened to conform to the flat pad. The open weave braided electrode is particularly desirable due to the large surface area of the conductors, high current capability, and assurance of excellent bonding since the material of the resistance element can be flowed through the open weave. While it is also possible to use conductive elastomeric materials, the resistance of most such materials is too high due to a voltage drop along the electrode to obtain uniform current distribution.
For uniform heating in a resistance element having substantially uniform resistance per unit of volume, it is desirable to provide current paths of equal length throughout the material. The heat generated by the conducting substantially equal current distribution and substantially equal resistance will thus result in uniform temperature.
Uniform current distribution is accomplished in accordance with the present invention by extending the electrodes 12 and 13 through substantially the entire length of the heating element in parallel directions as shown in FIG. 3. Since the distance between electrodes is substantially uniform, the heat produced will also be substantially uniform.
Patented Sept. 26, 1967- As shown in FIG. 4, insulated heating elements made in accordance with the present invention may easily be mass produced. Assuming that the conductive material used as a resistance element is an elastomeric such as a conductive dispersion in a silicone rubber carrier, a web 14 of uncured or tacky conducting material, which may be supported, if desired, is fed between a first pair of pressure rollers 16 and 17. Electrode material 13, held, for example, on a reel 18 is also fed into the space between pressure rollers 16 and 17 in the positions at which it is to be embedded into the conducting material 14. The pressure rollers force the electrode material into the uncured conductor which, because of its uncured state, is caused to flow into tight adherence with the electrodes. A pair of webs 11 and 11a of insulating material, also preferably, but not necessarily, in uncured form are fed with the conductive web into the space between a second pair of pressure rollers 19 and 20. The insulating jacket could be silicone rubber, for example. The insulation will adhere well to either cured or uncured conductive material. The composite web is then fed through a curing oven 21, where it is cured as necessary, depending on the type of material used, and is then wound on a takeup reel 22, where it may be stored until ready for use. It is to be understood that, while not as desirable in many applications, the materials may be cured before assembly and conductive adhesives may be flowed through the electrodes to secure them to the webs.
Obviously, the insulated heating elements need not be produced in rolls as shown in FIG. 4. However, the same basic steps are involved in making single small elements. The braid electrodes are forced into an uncured sheet of conductive material. The conductive material may, if desired, be provided with fabric reinforcement. Insulation, if desired, is placed over the conducting material and cured as necessary.
Many other modifications and variations of the present invention will become obvious to those skilled in the art. Therefore, it is to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
That which is claimed is: 1. A flexible resistance element comprising an elastomeric body of flexible electrically conductive material, and a system of flexible and stretchable low resistance electrodes embedded therein, said embedded electrodes each having a length in said material corresponding substantially to the length of said material in the direction in which said electrodes are embedded, and said embedded electrodes being each formed as a braided open weave metallic conductor. 2. A resistance element as defined in claim 1, but including further a layer of insulating material surrounding said elastomeric body of flexible electrically conductive material and said low resistance electrodes.
References Cited UNITED STATES PATENTS 2,406,367 8/1946 Griffith et al. 244l34 2,473,183 6/1949 Watson 219549 X 2,559,077 7/1951 Johnson et al. 219-549X 2,885,461 5/1959 Cafiero 3159-222X 2,952,001 9/1960 Morey 3382l0 3,022,412 2/ 1962 Waters 219-549 3,060,303 10/1962 Skoglund et al. 219-549 3,281,579 10/1966 Glicksman 219-535 FOREIGN PATENTS 975,264 11/ 1964 Great Britain.
RICHARD M. WOOD, Primary Examiner.
V. Y. MAYEWSKY, Assistant Examiner.