Electric heating – Heating devices – With heating unit structure
Patent
1992-11-06
2000-08-29
Paik, Sang
Electric heating
Heating devices
With heating unit structure
219544, 338214, H05B 334, H01C 700
Patent
active
061112348
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electrical devices comprising resistive heating elements, in particular self-regulating strip heaters which comprise resistive heating elements composed of a conductive polymer composition which exhibits PTC behavior.
2. Introduction to the Invention
For many applications, it is desirable to heat a substrate, e.g. a pipe or a tank, by means of an elongate heater comprising a resistive heating element. Often it is necessary to provide an electrically insulating jacket around the resistive heating element in order to prevent electrical shorting between the resistive element and an electrically conductive substrate. While such insulating jackets provide electrical insulation and environmental protection, they may not have adequate abrasion resistance. As a result, braids are sometimes provided over the insulating jacket for toughness and abrasion resistance. When the braid is metallic, it can also act as a grounding braid.
SUMMARY OF THE INVENTION
In recent years, there has been an increasing emphasis on the desirability of reducing the flammability of elongate heaters having polymeric insulating jackets, particularly self-regulating conductive polymer heaters. A standard way of assessing the flammability of an elongate heater is the Underwriter's Laboratory VW-1 flame test, published in Reference Standard for Electrical Wires, Cables, and Flexible Cords, UL 1581, No. 1080, Aug. 15, 1983. Heaters which contain polyolefin jackets, and/or resistive elements comprising conductive polymers based on polyolefins are less likely to pass the VW-1 test than heaters which contain fluoropolymer jackets and/or resistive elements comprising fluoropolymers. A heater which comprises a metallic grounding braid is generally more flammable than the corresponding non-braided heater. The flammability of a heater can be reduced by using (in the insulating jacket and/or in the resistive element if it is composed of a conductive polymer) a polymer which has low flammability, for example by using a fluorinated polymer instead of a polyolefin. Flammability can also be reduced by incorporating flame retardants, e.g. antimony trioxide and/or halogen-containing additives, into the polymer. However, these expedients suffer from disadvantages such as added cost and weight, processing difficulties, and inferior physical properties such as flexibility. In addition, there are circumstances where the use of halogen-containing materials is forbidden or discouraged.
We have discovered that the flammability of an elongate heater can be reduced by providing it with an additional insulating jacket, or by replacing a single insulating jacket (including one of two insulating jackets) by two or more jackets. In this way, a heater which fails the VW-1 test can be converted into one which passes the VW-1 test. When a further insulating jacket is added to an existing heater, on top of, or underneath, the conventional jacket(s), the reduction in flammability is not determined by (though it may be influenced by) the flammability of the material of the further insulating jacket. Even jackets which are made of materials which would normally be regarded as flammable can be effective. For example, we have obtained remarkable reductions in flammability by wrapping a thin film of polyethylene terephthalate around the conventional insulating jackets of known heaters. Similarly, when a single insulating jacket is replaced by a combination of two insulating jackets, the combination may be one which, for properties other than flammability, is substantially equivalent to the single jacket. For example, we have found that by replacing a single polyolefin-based insulating jacket by two insulating jackets made of the same material and having the same total thickness, a reduction in flammability is achieved.
Elongate heaters having two (or even more) insulating jackets have been used, or proposed for use, in the past, but only for purposes which do not, so far as we know, have any connec
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Batliwalla Neville S.
Thompson James C.
Gerstner Marguerite E.
Paik Sang
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