Ptc element and process for producing the same

Abstract

A PTC element comprising a conductive sheet of a crystalline polyolefin matrix and a conductive filler, having provided on both sides thereof an electrode of a metallic foil, the PTC element having a resistivity at 20.degree. C. (.rho..sub.20) of not more than 1.8 .OMEGA..multidot.cm, a peak resistivity (.rho..sub.p) of not less than 2.0.times.10.sup.6 .OMEGA..multidot.cm, and a temperature difference �T.sub.a (0.degree.C.)-T.sub.b (.degree.C.)! of not greater than 10.degree. C., wherein T.sub.a (.degree.C.) is a temperature at which the resistivity is 10.sup.6 times the resistivity at 20.degree. C. (.rho..sub.20) and T.sub.b (.degree.C.) is a temperature at which the resistivity is 10 times the resistivity at 20.degree. C. (.rho..sub.20). The PTC element is produced by a process comprising mixing a crystalline polyolefin and a conductive filler, molding the mixture into a conductive sheet, press bonding under heat a metallic foil to both sides of the conductive sheet, and repeatedly subjecting the conductive sheet to a heating and cooling cycle consisting of heating at a temperature of not lower than (the melting point of the crystalline polyolefin minus 5.degree. C.) and then cooling to a temperature lower than (the melting point of the crystalline polyolefin minus 5.degree. C.). The PTC elements exhibit an extremely low resistivity at 20.degree. C. and a high resistivity at a peak temperature, showing a very steep increase in resistance in a narrow temperature range.