| abstract |
Compositions which exhibit PTC behavior with a switching temperature (Ts) above 0 DEG C., which have a resistivity at a temperature below Ts of less than 7 ohm. cm, and which comprise a crystalline polymer and a particulate filler component which comprises carbon black having a particle size, D, which is from 20 to 150 millimicrons and a surface area S in m2/gram such that S/D is not more than 10. The composition preferably has a peak resistivity of at least 1000 ohm. cm and is electrically stable when aged at elevated temperature. The quantity <IMAGE> is preferably less than 1, especially less than 0.5. Shaped articles of such compositions are prepared by a process in which the carbon black is dispersed in the molten polymer and the dispersion is then melt-shaped, the total energy used in preparing and melt-shaping the dispersion being 1 to 300 hp. hr. ft-3, preferably 1 to 100 hp. hr. ft-3, especially 1 to 50 hp. hr. ft-3. The compositions are particularly useful in circuit protection devices. |