H - Electricity – 01 – B
Patent
H - Electricity
01
B
31/158, 309/85
H01B 1/24 (2006.01) B29C 47/02 (2006.01) H01C 7/02 (2006.01) H05B 3/14 (2006.01)
Patent
CA 1168433
MP0728/730/732 ABRIDGEMENT The invention relates to PTC conductive polymer compositions and devices containing them. The novel compositions contain a conductive filler, e.g. carbon black, which is dispersed a mixture of two crystalline polymers of different melting points. The higher melting of the tWD polymers has a melting point which is at least 160°C and at least 25°C higher than the melting point of the other polymer. A preferred mixture of polymers is poly(vinylidene fluoride) and an ethylene/tetrafluoroethylene copolymer. The compositions do not increase in resistivity by a factor more than 2 when maintained at 150°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 tempersture, e.g. during steam-cleaning thereof. The compositions are preferably annealed at a temperature between the melting points of the two polymers. The novel compositions are relatively brittle and preferred devices are rendered more flex-resistant by the presence of a layer of a relatively flexible composition which is melt-fused to a PTC element composed of the novel composition. A preferred heater is illustrated in Figure 1. -2-
377699
Fetherstonhaugh & Co.
Raychem Corporation
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