Claims
- 1. An electrical device which comprises
- (1) a resistive element which comprises first and second elongate wire electrodes which are embedded in a continuous strip of conductive polymer;
- (2) an insulating jacket;
- (3) an auxiliary member which contains interstices and which is separated from the resistive element by the insulating jacket; and
- (4) blocking material which (i) fills interstices in the auxiliary member and (ii) contacts the insulating jacket but is not bonded to the insulating jacket,
- wherein at least one of the following conditions is present
- (a) the blocking material has been applied by a pressure extrusion,
- (b) the blocking material has been applied in the form of a liquid, and
- (c) the device has a thermal efficiency which is at least 1.05 times that of an identical heater which does not comprise the blocking material.
- 2. A device according to claim 1 wherein the blocking material comprises a polymeric compound.
- 3. A device according to claim 1 wherein the blocking material is electrically insulating.
- 4. A device according to claim 1 wherein the blocking material is electrically conductive.
- 5. A device according to claim 1 wherein the auxiliary member is a braid.
- 6. A device according to claim 5 wherein the braid is a metallic grounding braid.
- 7. A device according to claim 1 wherein the blocking material fills at least 20% of the interstices of the auxiliary member.
- 8. A device according to claim 7 wherein the blocking material fills at least 30% of the interstices of the auxiliary member.
- 9. A device according to claim 1 wherein the blocking material comprises the same material as the insulating jacket.
- 10. A device according to claim 1 wherein the blocking material comprises a thermally conductive particulate filler selected from the group consisting of ZnO, Al.sub.2 O.sub.3, graphite and carbon black.
- 11. A device according to claim 1 wherein the interstices of the auxiliary member comprise at least 30% of the surface area of the auxiliary member.
- 12. A device according to claim 1 which is surrounded by concrete.
- 13. A device according to claim 1 wherein the blocking material completely fills the interstices in the auxiliary member.
- 14. A flexible elongate electrical heater which comprises
- (1) an elongate resistive heating element which comprises first and second elongate wire electrodes which are embedded in a continuous strip of conductive polymer;
- (2) a first elongate jacket which is composed of an insulating polymeric material, and which surrounds the heating element;
- (3) a metallic braid which surrounds and contacts the first insulating jacket; and
- (4) a second elongate jacket which is composed of a polymeric material, which surrounds and contacts the metallic braid, and a part of which passes through apertures in the metallic braid to fill at least 20% of the apertures and to contact but not bond to the first jacket.
- 15. A method making an electrical device which comprises
- (A) providing a device which comprises
- (1) a resistive element which comprises first and second elongate wire electrodes which are embedded in a continuous strip of conductive polymer,
- (2) an insulating jacket, and
- (3) an auxiliary member which contains interstices and which is separated from the resistive element by the insulating jacket; and
- (B) filling interstices in the auxiliary member with a blocking material which (i) passes through the interstices and (ii) contacts the insulating jacket but does not bond to the insulating jacket.
- 16. A method according to claim 15 wherein the blocking material comprises a polymeric compound.
- 17. A method according to claim 15 wherein the interstices are filled by extruding the blocking material over the auxiliary member.
- 18. A method according to claim 15 wherein the blocking material is in the form of a liquid.
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation application of copending, commonly assigned application No. 07/322,969 (Batliwalla et al), filed Mar. 13, 1989, now U.S. Pat. No. 5,111,032, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
This invention relates to electrical devices comprising an insulating jacket.
2. Introduction to the Invention
Electrical devices such as electrical heaters, heat-sensing devices and other devices whose performance depends on thermal transfer characteristics are well-known. Such devices generally comprise a resistive element and an insulating jacket. Many devices comprise an auxiliary member which is separated from the resistive element by the insulating jacket. The auxiliary member is most commonly a metallic braid which is present to act as a ground, but which also provides physical reinforcement. Particularly useful devices are heaters which comprise resistive heating elements which are composed of conductive polymers (i.e. compositions which comprise an organic polymer and, dispersed or otherwise distributed therein, a particulate conductive filler), particularly PTC (positive temperature coefficient of resistance) conductive polymers, which render the heater self-regulating. Self-regulating strip heaters are commonly used as heaters for substrates such as pipes.
The effectiveness of a heater depends on its ability to transfer heat to the substrate to be heated. This is particularly important with self-regulating heaters for which the power output depends upon the temperature of the heating element. Consequently, much effort has been devoted to improving the heat transfer from heater to substrate, including the use of a heat-transfer material, e.g. a heat-transfer cement, slurry or adhesive, between the heater and the substrate, and the use of clamps or a rigid insulating layer to force the heater into contact with the pipe. However, these solutions are not free from disadvantages. Heat-transfer materials are often messy to apply and, if "cured", may restrict removal or repositioning of the heater. Clamps or other rigid materials may restrict the expansion of a PTC conductive polymer in the heater, thus limiting its ability to self-regulate.
We have now realized in accordance with the present invention, that the presence of air gaps (or other zones of low thermal conductivity) within an electrical device, particularly a self-regulating heater, has an adverse effect on the performance of the device and that by taking measures to increase the thermal conductivity of such zones, substantial improvements in efficiency can be obtained. The invention is particularly valuable for improving the efficiency of devices which comprise an auxiliary member, e.g. a metallic grounding braid, having interstices therein, since conventional manufacturing techniques result in air being trapped in such interstices. The preferred method of increasing the thermal conductivity of the zones of low thermal conductivity is to fill them with a liquid (including molten) material which thereafter solidifies in place.
In one aspect, this invention provides an electrical device which comprises
In a second aspect, this invention provides a flexible elongate electrical heater which comprises
In a third aspect, this invention provides a method of making a device of the first aspect of the invention.
US Referenced Citations (39)
Foreign Referenced Citations (5)
| Number |
Date |
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| 0136795 |
Apr 1985 |
EPX |
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Feb 1989 |
EPX |
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Continuations (1)
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Number |
Date |
Country |
| Parent |
322969 |
Mar 1989 |
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Patent Grant
5300760
