Electrical devices comprising conductive polymer compositions

Claim 1

Self-regulating strip heater comprising (1) an elongate core of a melt-extruded electrically conductive polymer composition which (a) has a resistivity at 70° F. of 100 to 50,000 ohm.cm, (b) comprises an organic thermoplastic polymer and conductive carbon black dispersed therein, and (c) exhibits PTC characteristics; and (2) two longitudinally extending electrodes which are embedded in and surrounded by said elongate core parallel to each other, and which are in direct physical and electrical contact with the conductive polymer composition; the average linearity ratio between the electrodes being at most 1.2; and the heater having been prepared by a process which comprises (i) melt-extruding a molten thermoplastic electrically conductive polymer composition over and into direct physical and electrical contact with the electrodes, thus forming an elongate core of the melt-extruded conductive polymer composition having two longitudinally extending electrodes embedded therein parallel to each other; the conductive polymer composition comprising an organic thermoplastic polymer and conductive carbon black dispersed therein, and being such that when it is melt-extruded in this way, it does not need a subsequent annealing treatment at a temperature above the crystalline melting point of the polymer in order to have a resistivity at 70° F. of less than 50,000 ohm.cm; and (ii) cooling the whole of the melt-extruded conductive polymer composition to a temperature below its melting point, the cooled composition having a resistivity at 70° F. of 100 to 50,000 ohm.cm and exhibiting PTC characteristics; without subjecting the heater, at any stage after the whole of the melt-extruded conductive polymer composition has cooled to a temperature below its melting point, to a heat treatment in which substantially all of the cooled conductive polymer composition is reheated above the crystalline melting point of the organic polymer.