1. Field of the Invention
This invention relates to conductive polymer compositions and devices and assemblies comprising such compositions.
2. Introduction to the Invention
Conductive polymer compositions and electrical devices comprising them are well-known. Such compositions comprise a polymeric component, and dispersed therein, a particulate conductive filler such as carbon black or metal. Conductive polymer compositions are described in U.S. Pat. Nos. 4,237,441 (van Konynenburg et al), 4,388,607 (Toy et al), 4,534,889 (van Konynenburg et al), 4,545,926 (Fours et al), 4,560,498 (Horsma et al), 4,591,700 (Sopory), 4,724,417 (Au et al), 4,774,024 (Deep et al), 4,935,156 (van Konynenburg et al), 5,049,850 (Evans et al), and 5,250,228 (Baigrie et al), and in pending U.S. application Ser. Nos. 07/894,119 (Chandler et al, filed Jun. 5, 1992), now U.S. Pat. No. 5,378,407, issued Jan. 3, 1995, Ser. No. 08/085,859 (Chu et al, filed Jun. 29, 1993), now U.S. Pat. No. 5,451,919, and Ser. No. 08/173,444 (Chandler et al, filed Dec. 23, 1993). The disclosure of each of these patents and applications is incorporated herein by reference.
Such compositions often exhibit positive temperature coefficient (PTC) behavior, i.e. they increase in resistivity in response to an increase in temperature, generally over a relatively small temperature range. The temperature at which this increase occurs is the switching temperature T.sub.s and may be defined as the temperature at the intersection point of extensions of the substantially straight portions of a plot of the log of the resistance of a PTC element against temperature which lie on either side of the portion of the curve showing a sharp change in slope. The increase from the resistivity at 25.degree. C. (.rho..sub.25) to a peak resistivity (.rho..sub.peak, i.e. the maximum resistivity which the composition exhibits above T.sub.s) is the PTC anomaly height.
PTC conductive polymer compositions are particularly suitable for use in electrical devices such as circuit protection devices, heaters, and sensors which respond to changes in ambient temperature, current, and/or voltage conditions. For many applications it is desirable that the composition have as low a resistivity and as high a PTC anomaly height as possible. A low resistivity allows preparation of small devices which have low resistance. Such devices need little space on a printed circuit board or other substrate and contribute little resistance to an electrical circuit during normal operation. A high PTC anomaly height allows the device to withstand the necessary applied voltage. Although the resistivity of a conductive polymer composition can be decreased by adding more conductive filler, it is generally the case that such an addition reduces the PTC anomaly, possibly by decreasing the amount of crystalline polymer which contributes to the PTC anomaly or by physically reinforcing the polymeric component and decreasing the expansion at the melting temperature.
In addition to a low resistivity and high PTC anomaly, the location of the switching temperature of the conductive polymer composition is also important. Some applications, e.g. automotive applications in which an electrical device is located under the hood, require that T.sub.s be sufficiently high that the polymeric component is not adversely affected by a high ambient temperature. For other applications, e.g. for battery protection, it is necessary that T.sub.s be low enough that there is no heat damage to the substrate or surrounding components when the device switches to the high temperature/high resistance state.