Abstract: A strip heater (10) having at least one elongate control element (12a), at least one elongate heating element (12b) and a plurality of elongate conductors (14). An electrical insulator (18) partially encloses the elements (12) and the conductors (14). The elements (12) are physically separated and electrically connected by one of the conductors (14) such that both of the elements (12a and 12b) can be simultaneously placed in relative thermal contact with an object to be heated (19). An equally preferred alternate embodiment (310) has a dividing strip separating further separating the heating element (12b) from the control element (12a) and at a fixed distance such that the heating element (12b) is maintained generally parallel to the control element (12a).

Abstract: An assembly comprising a flexible corrugated metal tube and an elongate resistive heating element that is within the tube and which can be connected to a power supply to provide an elongate electrical heater. The tube has a wall thickness less than 0.012 inch, a corrugation depth greater than 0.030 inches and a distance between corresponding points in adjacent corrugations from 0.02 inch to 0.75 inch. The corrugations extend around the entire periphery of the tube.

Abstract: Novel PTC conductive polymer compositions contain a mixture of two crystalline polymers of different melting points, the higher melting of the polymers having a melting point which is at least 160.degree. C. and at least 25.degree. C. higher than the melting point of the other polymer. The compositions do not increase in resistivity by a factor more than 2 when maintained at 150.degree. C. for 1000 hours, and are therefore particularly suitable for self-limiting heaters which can be used on apparatus which is periodically subjected to high temperatures, e.g. during steam-cleaning thereof.

Abstract: The flex life of devices comprising a relatively inflexible PTC conductive polymer element is improved by surrounding the PTC element with a layer of a relatively flexible polymeric composition which is melt-fused thereto. Particularly useful devices are self-limiting strip heaters in which the polymeric component of the PTC element comprises a mixture of two crystalline polymers having substantially different melting points, the higher melting polymer having a melting point of at least 160.degree. C., preferably at least 200.degree. C., for example a mixture of polyvinylidene fluoride and a copolymer of ethylene and tetrafluoroethylene. Such devices can be made by melt-extruding a layer of the relatively flexible polymeric composition around the PTC element, and heating to cause melt-fusion of the polymers at the interface of the layer and the PTC element.

Abstract: The flex life of devices comprising a relatively inflexible PTC conductive polymer element is improved by surrounding the PTC element with a layer of a relatively flexible polymeric composition which is melt-fused thereto. Particularly useful devices are self-limiting strip heaters in which the polymeric component of the PTC element comprises a mixture of two crystalline polymers having substantially different melting points, the higher melting polymer having a melting point of at least 160.degree. C., preferably at least 200.degree. C., for example a mixture of polyvinylidene fluoride and a copolymer of ethylene and tetrafluoroethylene. Such devices can be made by melt-extruding a layer of the relatively flexible polymeric composition around the PTC element, and heating to cause melt-fusion of the polymers at the interface of the layer and the PTC element.

Abstract: A method of annealing a PTC conductive polymer composition comprising a mixture of two crystalline polymers. Compositions having improved electrical characteristics are obtained by annealing at a temperature between the melting points of the two polymers, preferably closer to the melting point of the lower melting polymer. Particularly useful results are obtained when the annealing method is applied to a self-limiting heater in which the PTC core comprises carbon black dispersed in a mixture of polymers, one of which has a melting point of at least 160.degree. C., preferably at least 200.degree. C., e.g. a mixture of polyvinylidene fluoride and an ethylene/tetrafluoroethylene copolymer.