Electric heating – Heating devices – With heating unit structure
Patent
1993-03-18
1995-04-04
Evans, Geoffrey S.
Electric heating
Heating devices
With heating unit structure
219543, 338212, H05B 334, B29C 6534
Patent
active
054039932
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention relates to an article comprising a conductive material, preferably a conductive polymeric material, which may be used, for example, for modifying, e.g. joining or repairing one or more substrates. In particular the invention relates to such articles, preferably in the shape of tapes, in which the flow of electrical current, and also the heat output of various parts of the article are specifically controlled.
INTRODUCTION TO THE INVENTION
Conductive polymers are well known. They comprise a polymeric component and, dispersed or otherwise distributed therein, a particulate conductive filler, e.g. carbon black. Conductive polymers have been widely used as electrical heaters. The heaters may be in the form of heat-recoverable articles per se or may be in the form of heat-recoverable articles so that, by powering the heater, the articles can be caused to recover, for example to join, to repair, or to reinforce one or more substrates around or against which the article recovers. Recently, it has been shown that conductive polymers which retain substantial strength above their melting point, especially sintered polymers such as ultra high molecular weight polyethylene (UHMWPE), are particularly useful for modifying plastic pipes, and EP-A-0157640 (MP0922 EPC) describes a heat recoverable coupler comprising carbon black containing UHMWPE. Also WO88/06517 and WO89/05230 (MP1180COM PCT1 and 2) describe conductive polymeric heaters in the form of tapes preferably made from UHMWPE. These tapes may be heat stable and can be arranged in a number of configurations for a number of applications. For example, as exemplified by FIG. 8 of WO80/06517, the conductive tape may be placed over a butt joint between two pipes and an elastomeric band used to press the tape against the joint. As the tape heats, with the elastomeric band pressing it against the pipes, it fuses to the pipes and joins them.
The tapes described in WO88/06577 and WO89/05230 typically comprise a conductive polymeric strip with two electrodes extending along the long edges of the tape so that current flows across the strip.
SUMMARY OF THE INVENTION
We have found that for some applications it is desirable for there to be non-uniform current flow, and hence non-uniform heating of such a tape. Thus, for example, where a conductive polymeric tape is wrapped around a butt joint it is desirable to prevent overheating in the butt area This may be caused by the presence of a gap, which may be up to 1 cm, between the two pipe ends to be joined. We have found that overheating can be avoided by reducing the resistance of the part of the tape overlapping the butt area.
Thus, the present invention provides an article comprising: having a different electrical resistance from that of another part of the tape; and electrical contact with the conductive element, which can be connected to a source of power to cause electrical current to flow across the tape; strip extending part way only along the length of the tape, between the electrodes, and separated from the electrodes by conductive material of different electrical resistance.
The different resistance values may be achieved, for example, by using materials of different resistivity, or by varying the thickness, or both. Where materials of different resistivity are used, these may be, for example, conductive polymeric materials of different resistivity, or completely different materials, e.g. a coating of a metal, e.g a metal spray, such as a flame sprayed metal, or an additional conductive braid. Where the thickness is varied, this may be done, for example by adding additional layers in certain regions.
Preferably the part of lower resistance is in the shape of the strip extending part way along the length of the tape between the electrodes, and separated from the electrodes by conductive material of higher resistance. Thus the lower resistance central strip is heated to a lesser amount than the higher resistance borders. However, where the conduct
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Cordia Johannes M.
Rogers Bruce
Burkard Herbert G.
Evans Geoffrey S.
Gerstner Marguerite E.
N.V. Raychem S.A.
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