| 摘要 |
<p>This invention relates to conductive polymer compositions, especially PTC compositions, which comprise a conductive carbon black dispersed in a copolymer of an olefin and a polar comonomer. It has been discovered that conductive polymer compositions having improved voltage stability are obtained by use of a carbon black having a particle size greater than 18 millimicrons, a d-spacing greater than 360 and a surface area which is less than where S is the DBP absorption of the carbon black, the carbon black being dispersed in a crystalline copolymer of an olefin and at least 10% of a polar comonomer. The melt index of the copolymer is preferably less than 20. The composition preferably also comprises another crystalline polymer which serves as a matrix for the copolymer. The composition is preferably cross-linked, e.g., to a gel fraction of at least 0.6. The invention includes the novel compositions, methods of preparing them, and electrical devices containing them. The novel compositions are particularly useful in electrical devices, especially self-limiting heaters, which comprise at least two electrodes which are connectable to a source of electrical power to cause current to pass through the composition.</p> |
