Radiation cross-linking of ptc conductive polymers

Abstract: The higher the voltage applied to an electrical device comprising a PTC conductive polymer, the more likely it is that intermittent application of the voltage will cause the device to fail. According to the invention, the likelihood of such failure is substantially reduced by irradiatng the PTC conductive polymer (1) so that it is very highly cross-linked, for example to a dosage of at least 50 Mrads, preferably at least 80 Mrads, especially at least 120 Mrads. In this way, for example, it is possible to make a circuit protection devce which will continue to provide effective protection even after repeated exposure to a voltage of 200 volts.

Description: Conductive polymer compositions exhibiting PTC behavior, and electrical devices comprising them, have been described in published documents and in our earlier specificatons. Reference may be made, for example, to U. S. Patents Nos. 2,952,761, 2,978,665, 3,243,753, 3,351,882, 3,571,777, 3,757,086, 3,793,716, 3,823,217, 3,858,144, 3,861,029, 4,017,715, 4,072,848, 4,085,286, 4,117,312, 4,177,376, 4,177,446, 4,188,276, 4,237,441, 4,242,573, 4,246,468, 4,250,400, 4,255,698, 4,272,471, 4,276,466 and 4,314,230; J. Applied Polymer Science 19 , 813-815 (1975), Klason and Kubat; Polymer Engineering and Science 18 , 649-653 (1978), Narkis et al; German OLS 2,634,999, 2,746,602, 2,755,076, 2,755,077, 2,821,799 and 3,030,799; European Published Applications Nos. 0028142, 0030479, 0038713, 0038714, 0038715 and 0038718; pending European Applications No. 81,301,767.0, 81,301,768.8 and 81,302,201.9; and pending U.S. Applications Nos. 176,300, 184,647, 254,352, 272,854 and 300,709. The disclosures of these patents, publications and applications are incorporated herein by reference. It is known to cross-link PTC conductive polymers by radiation, and in practice the dosages employed have been relatively low, e.g. 100,000 to 200,000 Gy (10-20 Mrads). Higher dosages have, however, been proposed for some purposes. Thus OLS 2,634,999 recommends a dose of 200,000 to 450,000 Gy (20-45 Mrads); U.K. Specification No. 1,071,032 describes irradiated compositions comprising a copolymer of ethylene and a vinyl ester or an acrylate monomer and 50-400% by weight of a filler, e.g. carbon black, the radiation dose being about 20,000 to about 1,000,000 Gy (about 2 to about 100 Mrads), preferably about 20,000 to about 200,000 Gy (about 2 to about 20 Mrads), and the use of such compositions as tapes for grading the insulation on cables; and US-A-3 351 882 discloses the preparation of electrical devices by embedding electrodes having a considerable area and an irregular surface, e.g. a metal mesh, in a resistor composed of a PTC conductive polymer, followed by cross-linking of the conductive polymer. The stated purpose of using electrodes of considerable area is to avoid excessive current concentrations and consequent damage to the conductive polymer. The stated purposes of the radiation are (a) to cross-link the conductive polymer adjacent the electrodes, so that the electrodes are firmly gripped, and (b) to cross-link the bulk of the conductive polymer so that it will resist softening. The cross-linking can be effected by radiation, and the patent discloses subjecting the entire resistor to a dose of 500,000 to 1,000,000 Gy (50 to 100 megarads) of radiation of one or two million electron volt electrons. As shown by US-A-3 858 144 and 3 861 029, a dose of 20,000 to 150,000 Gy (2-15 megarads) is sufficient to prevent softening of PTC conductive polymers, and higher doses are regarded as disadvantageous because they reduce crystallinity. The higher the voltage applied to an electrical device comprising a PTC conductive polymer, the more likely it is that intermittent application of the voltage will cause the device to fail. This has been a serious problem, for example, in the use of circuit protection devices where the voltage dropped over the device in the "tripped" (i.e. high resistance) condition is more than about 200 volts. [Voltages given herein are DC values or RMS values for AC power sources.] We have now discovered that the likelihood of such failure can be substantially reduced by irradiating the conductive polymer so that it is very highly cross-linked. Our experiments indicate that the higher the radiation dose, the greater the number of "trips" (i.e. conversions to the tripped state) a device will withstand without failure. The radiation dose is, therefore at least 600,000 Gy (60 Mrads), particularly at least 800,000 Gy (80 Mrads), with yet higher dosages, e.g. at least 1,200,000 Gy 120 Mrads or at least 1,600,000 Gy (160 Mrads), being preferred when satisfactory PTC characteristics are maintained and the desire for improved performance outweighs the cost of radiation. The present invention provides a process for the preparation of an electrical device comprising (1) a cross-linked PTC conductive polymer element comprising a polymeric component consisting essentially of one or more polymers, and (2) two substantially planar, parallel, metal electrodes which can be connected to a power source to cause current to flow through the PTC element, which process comprises cross-linking the PTC element by irradiating it after the electrodes have been secured thereto, characterized in that the electrodes are metal foil electrodes and the essential parts of the PTC element are irradiated to a dosage of at least 600,000 Gy (60 Mrads). When reference is made herein to an electrode being "substantially planar", we mean an electrode whose shape and position in the device are such that substantially all the current enters (or leaves) the electrode through a surface which is substantially planar. The present invention is particularly useful for circuit protection devices, but is also applicable to heaters, particularly laminar heaters. In this invention, each of the electrodes is substantially planar and is a metal foil. Since the metal foil electrodes are applied to the PTC element before it is irradiated, there is a danger that gases evolved during irradiation will be trapped. PTC conductive polymers suitable for use in this invention are disclosed in the patents and applications referenced above. Their resistivity at 23°C is preferably less than 1250 ohm.cm, eg. less than 750 ohm.cm, particularly less than 500 ohm.cm, with values less than 50 ohm.cm being preferred for circuit protection devices. The polymeric component should be one which is cross-linked and not significantly degraded by radiation. The polymeric component is preferably free of thermosetting polymers and often consists essentially of one or more crystalline polymers. Suitable polymers include polyolefins, eg. polyethylene, and copolymers of at least one olefin and at least one olefinically unsaturated monomer containing a polar group. The conductive filler is preferably carbon black. The composition may also contain a non-conductive filler, eg. alumina trihydrate. The composition can, but preferably does not, contain a radiation cross-linking aid. The presence of a cross-linking aid can substantially reduce the radiaton dose required to produce a particular degree of cross-linking, but its residue generally has an adverse effect on electrical characteristics. Shaping of the conductive polymer will generally be effected by a melt-shaping technique, eg. by melt-extrusion or molding.

Claims: A process for the preparation of an electrical device comprising (1) a cross-linked PTC conductive polymer element comprising a polymeric component consisting essentially of one or more polymers, and (2) two substantially planar, parallel, metal electrodes which can be connected to a power source to cause current to flow through the PTC element, which process comprises cross-linking the PTC element by irradiating it after the electrodes have been secured thereto, characterized in that the electrodes are metal foil electrodes and the essential parts of the PTC element are irradiated to a dosage of at least 600,000 Gy (60 Mrads). A process according to claim 1 characterized in that the essential parts of the device are irradiated to a dosage of at least 800,000 Gy (80 Mrads).

Inventors: DOLJACK FRANK ANTHONY, JACOBS STEPHEN M, MCTAVISH MARY SANDRA, TAYLOR JAMES MICHAEL, DOLJACK, FRANK ANTHONY, JACOBS, STEPHEN M., MCTAVISH, MARY SANDRA, TAYLOR, JAMES MICHAEL
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION
Application year: 1982
Main classification: H01C7
IPC codes: H01C7, H01B1