DE29824578U1 - Electric heating element - Google Patents

Description

Electric heating element

The invention relates to an electric heating element according to the preamble of claim 1.
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DE 42 21 454 A1 discloses a heating element made from electrically conductive fiber fabrics. To produce such a heating element, it is disadvantageously necessary to press the fiber fabric at the intersection points. In practice, the structure formed sometimes does not have a uniform volume resistance over the surface. This leads to undesirable local overheating, which can lead to the destruction of the heating element.

Heated exterior rear-view mirrors for motor vehicles are also known from the state of the art. A heating layer applied to the glass pane behind a mirror layer is made from a film that has metallic conductor tracks 0 printed on it as electrical resistance heating elements.

For example, DE 37 11 035 C2 discloses electric surface heaters in which a circuit board carrying metallic conductor tracks is glued to a heat storage device, e.g. a stone slab. The metallic conductor tracks are usually made of copper.

If the metallic conductor breaks, the heating effect is at least partially lost. The carrier that holds the heating layer is then only heated in sections.

The object of the invention is to eliminate the disadvantages of the prior art. In particular, it is intended to provide a simple

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and cost-effective heating element that is robustly constructed.

This object is achieved by the features of claim 1. Advantageous embodiments of the invention result from the features of claims 2 to 13.

According to the invention, it is provided that the electrically conductive non-metallic fibers are designed as a fleece and wherein the fleece is impregnated in sections with a metallic conductor.

A fleece is a layered structure made up of randomly arranged fibers.

It has surprisingly been shown that embedding such a fleece in a plastic layer does not impair the electrical properties of the fleece. It was expected that the fine fibers, which are not specifically electrically connected to one another, would be electrically insulated by the plastic. Nevertheless, good heating performance is easily achieved with the structure according to the invention. The fleece is impregnated in sections with a metallic conductor. This makes it easy to provide reliable contact with the fleece. The impregnation can be carried out by means of galvanization. However, impregnation by introducing molten metallic material into the fleece is also conceivable.

A further advantage of the heating layer according to the invention is that the fibers are non-metallic in nature and therefore the risk of corrosion is eliminated.

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In addition, the fibers in the fleece are undirected. The production of special conductor tracks is not necessary. There is no risk of the heating effect being lost due to a conductor track breaking. A homogeneous heating effect is achieved over the entire surface of the fleece. Local overheating is not observed. Such a heating layer can be produced easily and inexpensively.

According to a design feature of the invention, the non-metallic fibers are carbon fibers. Carbon advantageously has a negative characteristic curve. This means that the electrical resistance decreases with increasing temperature.

The carbon fibers can conveniently be made from a polyacrylonitrile precursor. Such fibers have a diameter of about 2 to 8, preferably 6 μιη. They are characterized by high strength, high elastic modulus and low density.

The metallic conductor is preferably made of copper. The impregnation is preferably carried out galvanically by dipping the fleece section by section into an acidic electrolyte bath, preferably containing copper ions.

The plastic layer can be made of a resin, especially an epoxy resin. Such a resin is particularly easy to process. It ensures reliable protection of the fleece against environmental influences.

Another design feature is that the heating layer is a prefabricated assembly part. This makes mass production easier.

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Any element to be heated can be used as a carrier. It is considered particularly advantageous if the carrier is an external rear-view mirror for a motor vehicle with a mirror layer attached to the back of a glass pane. The carrier can also be a stone slab, preferably a marble slab. However, the use of other carriers is also conceivable, such as aircraft wings, helicopter rotor blades and the like.

It has proven to be useful to attach the heating layer with one side to the back of the carrier by means of a first rubber-elastic mass. By using the first rubber-elastic mass, different expansion coefficients of the carrier and the heating layer can be compensated.

In particular, when manufacturing a room heater, it is advantageous to have a thermal insulation layer, in particular a mica layer, attached to the other side of the heating layer using a second elastic mass. This causes a targeted flow of heat onto the carrier. The carrier is thus heated particularly well.

A cable connected to the metallic conductor can be routed through an opening provided in the thermal insulation layer. This enables a particularly compact design of the electrical heating element.

To manufacture an electric heating element, a method comprising the following steps is provided:

a) Cutting a fleece made of electrically conductive, non-metallic fibres,

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b) Embedding the fleece in a layer made of plastic and

c) Attaching the fleece to a carrier.

The process enables the production of an electric heating element in a simple and cost-effective manner. The electric heating element is particularly robust. Due to the special design of the heating layer, there is no need to fear a loss of the heating effect over the heating surface, at least in sections.

According to a variant of the process, the fleece can be impregnated in sections with a metallic conductor, preferably copper, before step b. The impregnation is advantageously carried out galvanically by dipping the fleece in sections into an acidic electrolyte bath, preferably containing copper ions. Instead of copper, a copper alloy or another suitable metal alloy can also be used. The production of such an impregnation layer is simple and inexpensive. Following the impregnation, it is expedient to embed the fleece in a plastic layer. This provides simple and inexpensive protection against the effects of environmental influences.

It is expedient to use a fleece in which the non-metallic fibers are carbon fibers, the fleece being partially impregnated with an electrically conductive metallic conductor, preferably containing copper. The fleece is advantageously embedded in a layer made of plastic.

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The invention is explained in more detail below using exemplary embodiments. They show:

Fig. 1 is a schematic cross-sectional view of an exterior rearview mirror,

Fig. 2 is a schematic cross-sectional view of an electric heater and

Fig. 3 is a sectional view along the line A-A' in Fig. 2.

In Fig. 1, an exterior rear-view mirror for motor vehicles has a mirror pane 1, on the back of which a mirror layer 2 is provided. On the side of the mirror layer 2 facing away from the mirror pane 1, a heating layer 3 for heating the exterior rear-view mirror is provided.

The heating layer 3 contains a fleece layer made of electrically conductive, non-metallic fibers. In the present embodiment, the fibers are carbon fibers 4, which are made from a polyacrylonitrile precursor. Such a fiber material is commercially known, for example, under the trade name Tenax®. The fleece layer formed from the carbon fibers 4 is embedded in a layer made from plastic 5.

In this embodiment, the connections to a power supply are made by a self-adhesive copper foil (not shown here), which is glued to the heating layer 3 at a suitable location. Since the fiber ends of the carbon fibers 4 penetrate the adhesive layer of the copper foil, a reliable electrical connection can be made.

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The plastic layer 5 can also be designed in such a way that it covers the side of the heating layer 4 facing away from the mirror layer 2, so that the carbon fibers 4 are protected against external influences. The material of the fleece layer can be adapted to the respective application in terms of thickness, fiber strength and the area used. In particular, the following materials known under the trade name Tenax® can be used:

[mm] BP-1030A-ES BP-1040A-ES BP-IlOOA-ES Width [mm] 1000 1000 1000 length [mm] 100 100 100 thickness [g/ ^m2 ] 0.42 0.55 1.70 Area weight 30 40 100 Binder type [%] modif. Poly
ester modif. Poly
ester modified epoxy Binder content 16 20 12 tensile strenght [N/50mm
] vertical [N/50mm
] 59 67 479 horizontal 34 43 289 Elongation at break [%] vertical [%] 0.3 0.1 0.5 horizontal 0.4 0.1 0.4 Tear-resistant
ity [G] vertical [G] 115 133 308 horizontal [Ω] 140 156 432 Electric
Resistance 1.82 0.63 0.32

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Since the fleece layer is deformable before being embedded in the plastic layer, it can be adapted to the curvature of the exterior rear-view mirror. Holes can also be provided for the connections of the adjustment elements of the exterior rear-view mirror.

Fig. 2 shows a schematic cross-sectional view of an electric radiator. A stone plate 6 is covered on one side with a first silicone rubber 7.

Above this is the heating layer 3. It is made of carbon fibers 4 which are embedded in a layer made of plastic 5. The electrically conductive fibers 4 are impregnated with a metallic conductor at the edges. The metallic contact zones produced in this way are indicated by hatching. The contact zones are electrically connected to lines 9 which are connected to an overheating protection 10. The overheating protection 10 is connected to a power supply (not shown here).

The overheating protection 10 is mounted here on a mica plate 11. The mica plate 11 is glued by means of a second silicone rubber layer 12 onto a first silicone rubber layer 7 which embeds the heating layer 3.

Fig. 3 shows a section along the line A-A' in Fig. 2, from which the arrangement of the contact zones impregnated in sections with a metallic conductor 8 can be seen again.

0 Of course, the heating layer 3 can also be present as a separate assembly element. In this case, it is advantageous that both the carbon fibers 4 of the fleece and the contact zones impregnated with the metallic conductor 8 are covered with the plastic 5.

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To produce the heating element according to the second embodiment, a fleece made of carbon fibers 4 is placed in a clamping device with a cover frame and electrically conductively contacted. The fleece is then dipped together with the clamping device into an acidic electrolyte bath containing copper ions. The fleece then remains in the electrolyte for about 5 minutes without current. The electrolyte completely soaks the fleece.

Then the material is copper-plated at a current density of 1 A/qdm for 5 minutes. The current density can then be increased to 2 to 4 A/qdm and a top layer can be electroplated. This process takes about 10 to 20 minutes. After the electroplating process, the fleece is rinsed in the usual way, removed from the clamping device and dried at 60 to 70° in a circulating air drying oven for 20 minutes.

Cables are then soldered to the contact zones impregnated with metallic copper. The contacted fleece is then embedded in liquid plastic. With the exception of the contact zones, the fleece is penetrated by the plastic. The contact zones 5 can also be surrounded with plastic for electrical insulation.

The plastic layer can also be made of a different plastic material than the epoxy resin used here. However, this material should be able to embed the 0 fibers and protect them from external influences.

Furthermore, any other non-metallic fibers can be used instead of the carbon fibers, provided they are electrically conductive. The nonwoven fabric according to the invention can

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consist of undirected fibers or directed layers. The use of filament yarn, short fiber cuts and fiber paper is also possible. Woven or other flat fiber materials can also be used. 5

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11 List of reference symbols

1 glass pane

2 Mirror layer 3 Heating layer

4 Carbon fiber

5 Plastic

6 Stone slab

7 first silicone rubber layer 10 8 metallic conductor

9 Connection cables

10 Overtemperature protection&zgr;

11 Mica layer

12 second silicone rubber layer 15

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Claims (13)

1. Electric heating element with a heating layer ( 3 ) applied to a carrier ( 1 , 6 ) in the form of an electrical resistance heater, wherein the heating layer is formed from electrically conductive non-metallic fibers ( 4 ) which are embedded in a layer made of plastic ( 5 ), characterized in that the electrically conductive non-metallic fibers ( 4 ) are designed as a fleece and wherein the fleece is impregnated in sections with a metallic conductor ( 8 ). 2. Electric heating element according to claim 1, wherein the non-metallic fibers are carbon fibers ( 4 ). 3. Electric heating element according to one of the preceding claims, wherein the metallic conductor ( 8 ) is made of copper. 4. Electric heating element according to one of the preceding claims, wherein the plastic ( 5 ) is made of a resin, in particular an epoxy resin. 5. Electric heating element according to one of the preceding claims, wherein the heating layer ( 3 ) is a prefabricated assembly part. 6. Electric heating element according to one of the preceding claims, wherein the carrier ( 1 , 6 ) is an outside rear-view mirror for a motor vehicle with a mirror layer ( 2 ) attached to a rear side of a glass pane ( 1 ). 7. Electric heating element according to one of the preceding claims, wherein the carrier ( 1 , 6 ) is a stone slab ( 6 ), preferably a marble slab. 8. Electric heating element according to one of the preceding claims, wherein the heating layer ( 3 ) is attached with its one side to a rear side of the carrier ( 1 , 6 ) by means of a first rubber-elastic mass ( 7 ). 9. Electric heating element according to claim 8, wherein a thermal insulation layer, in particular a mica layer ( 11 ), is provided on the other side of the heating layer ( 3 ) and is fastened by means of a second rubber-elastic mass ( 12 ). 10. Electric heating element according to one of the preceding claims, wherein a line ( 9 ) connected to the metallic conductor ( 8 ) is guided through an opening provided in the thermal insulation layer ( 11 ). 11. Electric heating element according to one of the preceding claims, wherein the impregnation is produced galvanically by dipping the fleece in sections into an acidic electrolyte bath, preferably containing copper ions. 12. Electric heating element according to one of the preceding claims, wherein the fleece is impregnated by introducing molten metallic material. 13. Electric heating element according to one of the preceding claims, wherein the heating layer ( 3 ) is designed as a prefabricated assembly part.