1. Field of the Invention
This invention relates to elongate conductors containing conductive polymers, especially ignition cables for automobiles.
2. Introduction to the Invention
The term "conductive polymer" is used in this specification to denote a composition which comprises a polymeric component and, dispersed or otherwise distributed in the polymeric component, a particulate conductive filler. Conductive polymers are well known. When the polymeric component is crystalline, many conductive polymers exhibit positive temperature coefficient (PTC) behavior in a temperature range which starts somewhat below the crystalline melting point. In general, the less crystalline the polymeric component, the less likely it is that the conductive polymer will exhibit sharp PTC behavior. When carbon black is the conductive filler, the nature of the carbon black is also an important factor in determining how resistivity varies with temperature (resistivities referred to in this specification are volume resistivities). Conductive polymers can be shaped in any appropriate way, but are preferably melt extruded, and can be crosslinked, e.g. by radiation, after they have been shaped. Conductive polymers have been widely used, for example, in antistatic flooring, as shielding in high voltage cables, and in devices in which current passes through the conductive polymer between metal electrodes, particularly self-regulating heaters and circuit protection devices which use PTC conductive polymers. For further information about conductive polymers, reference may be made for example to U.S. Pat. Nos. 4,237,441, 4,304,987, 4,388,607, 4,545,926, 4,560,498, 4,591,700, 4,724,417, 4,774,024, 4,935,156, 5,049,850, 5,250,228, and 5,378,407, the disclosures of which are incorporated herein by reference.
Ignition cables in automobiles must meet very stringent requirements. Thus, they must have satisfactory resistance and capacitance characteristics over a wide temperature range, and must retain those characteristics over a period of years in a very hostile physical environment. Many attempts have been made to provide such cables--see for example U.S. Pat. Nos. 2,790,053, 3,991,397, 4,330,493, 4,363,019, 4,375,632, 4,677,418, 4,704,596, 4,748,436, 4,780,700, 4,894,490, 4,970,488, 5,034,719, and 5,057,812, the disclosures of which are incorporated herein by reference. However the cable constructions which have proved commercially acceptable are complicated and expensive to make, and often require, in order to provide the desired combination of resistance, capacitance and size, two or more conductive components which must be applied in separate operations.