EP0926925A1 - Carbon fibre heating panel and methode of making the same - Google Patents

Abstract

The resistive heating element (10) is a layer of cellulosic-precursor, carbon fiber fabric containing 92%-96% carbon. Fabric emissivity exceeds 0.7. An Independent claim is included for the method of manufacture, essentially by assembly and impregnation with thermoplastic and polymerization. The carbon fibers are obtained from cellulosic fabric by heating ultimately to 700-1000 degrees C.

Description

Field of the invention

The present invention relates to an electric heating panel comprising a resistive heating element made of carbon fibers.

A more particularly targeted field of application is that of room heating panels for domestic, industrial or tertiary use (communities, hospitals, ...).

Invention background

Various electric heating devices have been proposed using a resistive element of essentially two-dimensional shape made of carbon fibers.

The resistive element may be formed from a sheet of short fibers obtained by papermaking, a tablecloth or unidirectional ribbon made of fibers carbon, or a fabric comprising chain carbon fibers and fibers insulating weft. Reference may be made in particular to documents US-A-5 459 327, FR-A-2 263 658, FR-A-2 578 377 and FR-A-2 744 872. The use of such resistive elements is advantageous in the case where the heating device must conform to the shape of the parts to be heated, for example molds or wing structures to be defrosted. It was also proposed in document DE-A-42 21,455 to use a heating element in the form of a carbon fabric. The resistive element made of carbon fibers, provided with connection electrodes, is generally placed between two layers of insulating material, for example in glass fibers, the assembly being embedded in a plastic material such as polymerized thermosetting resin.

Known heating devices using resistive elements carbon fibers generally work by conduction or even by convection.

However, an object of the present invention is to exploit the possibility use carbon fiber heating elements to make thin radiant heating panels, therefore thin size.

It has been noted that the so-called radiant heating panels currently commercially available, in addition to being relatively thick, dissipate a significant part of the calories produced by convection.

Another object of the present invention is to provide a panel for heater with which the heater is produced with a radiation rate important for a relatively low surface temperature, comparison with known radiant panels.

In addition, the use of carbon fibers, generally good electrically conductive, for effect heating elements Joule, poses the problem of making an element having an electrical resistance high enough. One solution may be to lengthen the resistive path by imposing a curvy course on the current by leaving openings or cuts in the carbon fiber element, as shown for example in document DE-A-42 21 455 already cited. However, such a solution presents disadvantages. In addition to requiring an additional step at the manufacturing, it presents the risk of establishing short circuits between parallel branches, on either side of an opening or cut, except to widen this, and therefore increase the size and / or waste of matter.

Yet another object of the invention is therefore to provide a panel for heating in which the heating element consists of a fiber texture of carbon having a relatively high resistivity so that it is not necessary to give it a special geometry aimed at increasing the electrical resistance.

Brief description of the invention

According to a first aspect of the invention, these aims are achieved thanks to an electric heating panel comprising a shaped resistive element essentially two-dimensional carbon fiber, connected electrodes electrically to the electrical element, and at least one layer of material electrically insulating located on each side of the resistive element, the layers of insulating material, the resistive element and the electrodes being embedded in a plastic material, panel in which the resistive element consists of a layer of carbon fiber fabric with a cellulose precursor, the rate of carbon in the fibers being between 92% and 96% and the emissivity of the fabric being greater than 0.7.

Thus, a characteristic of the panel according to the invention resides in the use of a heating element having an emissivity such that it behaves almost like a black body with emissivity ε = 1.

Another characteristic of the panel according to the invention is due to the presence of carbon fibers with a cellulose precursor which make it possible to have sufficient electrical resistance with a high carbon content, then than with other carbon precursors, in particular with polyacrylonitrile (PAN), sufficient electrical resistance can only be obtained in the event of much lower carbonization, as indicated in document US-A-4,816,242.

According to a feature of the panel according to the invention, it has a radiation rate higher than 10% for a surface temperature of 80 ° C or even, preferably, greater than 25% and up to at least 30 %. By way of comparison, a radiation rate of 30% is only obtained for a surface temperature of 200 ° C or more with radiant panels of the prior art.

According to a second aspect of the invention, it relates to a method of manufacture of a radiant electrical panel, comprising the stages of production of a resistive element in carbon fibers of essentially shaped two-dimensional, installation of connection electrodes in contact with the resistive element, arrangement of at least one layer of material electrically insulating on each side of the resistive element, impregnation of the resistive element provided with electrodes and layers of electrically insulating material by a thermosetting resin, and polymerization of the resin, a process in which the realization of the resistive element includes the provision of a fiber fabric cellulosics and carbonization of the fabric by heat treatment carried out, in final, at a maximum temperature between 700 ° C and 1000 ° C, so that the carbon content in the fibers is between 92% and 96%.

Thus, features of the invention arise from the use of a cellulosic precursor and carrying out a heat treatment allowing to produce a carbon fiber fabric having electrical resistance sufficient and very good emissivity. It is then possible to carry out thin and efficient radiant panels.

Preferably, the cellulosic precursor is a rayon and the heat treatment is carried out at a final temperature of between 780 ° C and 820 ° C for a period of between 0.7 min and 1.3 min.

The layers of insulating material can be formed by textures, for example glass fiber fabrics.

The plastic coating the resistive element, the electrodes and the insulating layers is a polymerized thermosetting resin, advantageously added with a black dye in order to optimize the appearance and the emissivity of the panel, therefore its radiative character.

The electrodes are advantageously constituted by strips metallic bonded to the fabric, for example copper strips, perforated, the perforations allowing the passage of the resin for a better bond between the electrodes and the carbon tissue. Alternatively, the electrodes can also be formed by local deposition of a conductive resin on the fabric, or by dipping the ends of the fabric in molten copper.

Other features and advantages of the panel and the process according to the present invention will emerge on reading the detailed description given below for information but not limitation, with reference to the drawings.

Brief description of the drawings

• Figure 1 is an exploded perspective view showing the different components of a particular embodiment of a panel heating according to the invention, before their assembly;
• Figure 2 is a partial sectional view illustrating the assembly by vacuum molding of the components of a heating panel, according to Figure 1;
• Figure 3 shows the variation over time of the temperature measured on the front face of a panel produced in accordance with the invention, during successive power-up and power-down cycles;
• Figure 4 shows the variation over time of the temperature measured at the front of the panel and the ambient temperature during a period of operation of a panel produced in accordance with the invention;
• Figure 5 very schematically shows an assembly of emissivity measurement; and
• Figures 6 and 7 illustrate the variations in electrical resistance in function of the final carbonization temperature and the carbon content for a carbon fiber fabric from a rayon precursor.

Description of preferred embodiments

The components of a radiant heating panel according to a embodiment of the invention are illustrated in Figure 1, before their assembly.

The heating element 10 consists of a layer, or fold, rectangular carbon fiber fabric. It is provided, along each little side, an electrode 12 connected to a supply conductor 14.

On the side of the front face of the panel, intended to constitute the face radiant, the face of the heating element 10 is coated with a layer of material electrically insulating, in this case one or two plies 20a, 20b of fiber fabric of glass. More than two plies of electrically insulating material could be planned, but without penalizing the transfer to the front of the panel of calories produced by the heating element.

At the rear, the face of the heating element is coated with a material electrically and thermally insulating, formed by one, or preferably several layers 22a, ..., 22d for example of glass fiber fabric.

A reflective layer 24, such as a metallic foil, for aluminum example can optionally be inserted between two layers rear insulators to reflect the heat flow of the heating element. It is also possible to add a layer to the back of the heating element of honeycomb material in order to limit the temperature on the rear face.

The carbon fibers in the heating element come from a carbon precursor, after carbonization. According to a characteristic of the invention, the precursor is of the cellulosic type and the carbonization is not absolutely complete, the carbon content in the fibers, after carbonization, being between 92% and 96%, therefore relatively high. Note that the terms "carbon fibers" here refer to fibers in which the rate of carbon is not necessarily equal to 100%.

Carbonization is produced by heat treatment carried out, in final, at a maximum temperature between 700 ° C and 1000 ° C. Under 700 ° C, carbonization is insufficient, the fibers retain a character essentially insulating, while above 1000 ° C, the carbon content is so high that the electrical resistance becomes insufficient.

Preferably, the precursor is a rayon. The final temperature heat treatment is then preferably between 780 ° C and 820 ° C approximately and the maintenance of the fabric at this temperature is carried out for a period preferably between 0.7 min and 1.3 min. The heat treatment may advantageously include a pre-carbonization phase at a temperature between 350 ° C and 420 ° C approximately, for a longer period long.

The use of a cellulosic precursor, associated with a treatment relatively moderate thermal, leads to a carbon fiber fabric having a satisfactory resistivity to constitute a good heating element by Joule effect. In addition, we end up with a fabric having a very good emissivity, greater than 0.7, or even greater than 0.9, therefore close to that of the body black.

The electrodes 12 are advantageously constituted by strips conductive metal, for example copper, provided with perforations 12a. They can be fixed to the fabric 10 by gluing using an adhesive able to withstand the temperatures encountered during assembly of the sign. Such an adhesive is for example formed by a strip of glass fabric sticker applied to the electrodes and to the fabric 10.

As a variant, each electrode can be formed by a local deposit, on the fabric, a layer of conductive resin, for example epoxy resin loaded with carbon powder. Another possibility is to train electrodes by immersing each end of the fabric 10 in copper in fusion.

The assembly of the panel can be carried out by vacuum molding (Figure 2) after impregnation with a composition comprising a resin thermosetting.

The fiberglass and carbon fiber fabric layers are impregnated separately, preferably. It is possible to use layers of glass fiber fabric prepreg.

The impregnating composition, in addition to the resin, can comprise a hardener, a hardening accelerator and a black coloring agent. The agent dye is intended at least for the impregnation of the front insulating layer and the heating element, in order to optimize the emissivity of the panel. The resin can be an epoxy resin. The black dye, not electrically conductive, is by example of carbon black diluted to 2% by weight in an epoxy resin itself mixed with 3% by weight maximum in the impregnation composition basic.

Insulating layers and carbon fabric with electrodes connected to the supply wires are placed between a mold 30 and a waterproof film 32. The space between the mold 30 and the film 32 is closed at its periphery by a seal seal 34, for example a sealant, is connected to a vacuum source by a pipeline 36.

The components of the panel can be overcome by a drainage layer 40 having the function of collecting the excess of impregnation composition. The layer 40 is for example a felt of polyester. Between the felt 40 and the panel can be disposed a separating film perforated 42. The latter, for example made of polyamide, makes it possible to limit drainage of the composition for impregnation with the felt 40 and facilitates the separation panel. The mold 30 is constituted by a metal plate, by example a stainless steel plate whose surface is coated with an agent release agent, for example the product sold under the name "MOLD WIZ F57 NC "by the company of the United States of America Axel.

As is well known for all molding processes vacuum polymerization, the vacuum is established gradually, by steps, as the temperature increases necessary for the polymerization of the resin.

After demolding and trimming, the panel is fitted with a plug electric and can be directly used. The front of the panel can be with a decorative motif and the panel can be framed. He can too present a particular color or mixture of colors and be highlighted shape on demand, for example with curvatures.

Techniques other than vacuum molding can be used for panel assembly. So the different layers prepregs can be press molded. Another technique is placing the different non-impregnated layers in a mold in which the impregnating composition is injected (process known as RTM for "Resin Transfer Molding ").

According to an alternative embodiment, the insulating layer 20 at the front of the panel can be in the form of a glass plate for example vitroceramic.

In all cases, the front face can be frosted, for example by shot blasting, in order to create a relief increasing the radiating surface.

From the above, it appears that a radiant panel can be obtained relatively simply and quickly, with a limited total thickness, less than 4 mm, or even typically less than 2 mm. Such a panel, possibly after decoration, can be easily integrated into a room residential. It can also be designed as a heating panel removable or mobile.

Another advantage of a radiant panel is its rate of radiation relatively higher than 10%, even 25% and can even reach at least 30% with a surface temperature of 80 ° C, so relatively little high. The radiation rate is the ratio between the radiated power and the power absorbed by the panel.

A particular example of embodiment will now be described.

Example

The heating element is a carbon fiber fabric obtained by carbonization of a satin rayon fabric with a surface mass equal to 270 g / m ² . The carbonization heat treatment comprises a pre-carbonization at a temperature of approximately 375 ° C followed by a final carbonization at a temperature of 790 ° C for approximately 1 min. A rectangular fold of dimensions 530 x 330 mm is cut from the carbon fiber fabric, the mass of the fold being 46 g.

Electrodes made up of two copper strips of 1/10 ^e mm thickness perforated over their entire length, width equal to 10 mm and length equal to 330 mm, are placed along the two short sides of the ply of fiber fabric. carbon and bonded to the latter by an adhesive strip based on a glass cloth, having a temperature resistance up to 180 ° C. The electrodes are equipped with electrical connection wires soldered in tin on one of their ends. The measured resistance of the ply of carbon fiber fabric is 210 W.

A ply of twill glass fabric with dimensions equal to 700 x 500 mm and with a mass equal to 110 g is placed on the front face, while four folds of taffeta glass fabric of dimensions equal to 700 x 500 mm and of equal mass at 330 g are placed on the rear face.

The folds of carbon fabric and glass are impregnated with a composition comprising an epoxy resin added with a hardener, a accelerator and a black dye consisting of a mixture at 2% by weight of carbon black in an epoxy paste. The perforations of the electrodes allow the passage of the impregnation composition to ensure good contact between the electrodes and the carbon tissue, even when the electrodes are not glued to the fabric.

The whole is polymerized under vacuum as described with reference to the figure 2.

The panel obtained is equipped with an electrical outlet and tested under an alternating supply voltage equal to 230 V.

Figure 3 shows the evolution of the temperature at the front surface of the panel during successive cycles including a power-up period 15 min, followed by a 15 min power off period. The speed of temperature rise and fall, on and off, and the reproducibility of the cycles are remarkable.

Figure 4 shows the evolution of the temperature on the front surface (curve A) and the ambient temperature (curve B) in the center of a 60 m ^{3 room} on a wall of which the panel is installed, and is maintained under a voltage of 230 V. The consistency of the surface temperature value (which changes in parallel with the ambient temperature) is remarkable.

The emissivity of the heating element of the panel, i.e. the fabric carbon fiber coated with epoxy resin was measured as follows (figure 5).

A sample 30 of fabric coated with epoxy resin is provided electrodes and energized to obtain a surface temperature 100 ° C. The heat flux produced, passing successively through a modulator 32, a diaphragm 34 and a lens 36 is focused on a detector 38 of photoelectric type which delivers a voltage proportional to the luminance of the sample material. The modulator 32 is a rotary disc comprising alternately opaque and transparent areas to thermal radiation. he collects the radiative flux emitted, freeing itself from the surrounding noise.

The voltage delivered by the detector is 7.5 µV, whatever the face of the sample presented opposite the detector.

For reference, the sample 30 is replaced by a black body whose surface is brought to 100 ° C. The radiative power delivered by the body black is the maximum reference value, corresponding to an emissivity ε = 1. The voltage collected at the detector output is 8.0 µV.

We deduce, for sample 30, an emissivity ε equal to 0.937.

The radiation level of the panel is measured by wearing the panel radiant at a surface temperature of 80 ° C in a room at 19 ° C in which the atmosphere is stable and controlled. A receiving plate placed 1 meter from distance facing the radiant panel, and over which are distributed uniformly 150 thermocouples, collects the radiated temperature beam by the system. From this data, the radiation rate is calculated. The measured value of the radiation rate is approximately 30%.

Comparative essay

From a rayon fabric such as that used in the example previous, different carbonization cycles are carried out which distinguish the each other by the final heat treatment temperature of carbonization.

Figure 6 shows the variation of the electrical resistance measured in function of the final carbonization temperature for fabric samples dimensions 10 cm x 10 cm, the measurements being made directly on the fabric not equipped with electrode in the warp direction and in the direction respectively frame. We note that the variations in both directions, warp and weft, are practically identical.

Figure 7 shows the variation of the electrical resistance as a function the carbon level measured for the same samples.

It is found that the resistance becomes relatively low for final carbonization temperatures above 1000 ° C and corresponding carbon higher than 96%, while below a temperature final 700 ° C corresponding approximately to a carbon content of 92%, the resistance becomes relatively high.

Claims (19)

Electric heating panel comprising a resistive element of essentially two-dimensional shape made of carbon fibers, electrodes electrically connected to the electric element, and at least one layer of electrically insulating material situated on each face of the resistive element, the layers of material insulating, the resistive element and the electrodes being embedded in a plastic material,
characterized in that the resistive element consists of a layer of carbon fiber fabric with a cellulose precursor, the carbon content in the fibers being between 92% and 96% and the emissivity of the fabric being greater than 0.7 . Panel according to claim 1, characterized in that the emissivity of the tissue is greater than 0.9. Panel according to any one of claims 1 and 2, characterized in that the radiation rate is greater than 10% for a surface temperature of 80 ° C. Panel according to claim 3, characterized in that the rate of radiation is greater than 25% for a surface temperature of 80 ° C. Panel according to any one of claims 1 to 4, characterized in that the resistive element consists of a layer of fabric Rayon precursor carbon fibers. Panel according to any one of claims 1 to 5, characterized in that the layers of insulating material are at least partly consisting of layers of glass fiber fabric. Panel according to any one of claims 1 to 6, characterized in that the plastic is a thermosetting resin polymerized with a black dye. Panel according to any one of claims 1 to 7, characterized in that the electrodes consist of strips conductive metallic perforated. Panel according to any one of claims 1 to 8, comprising a radiating front face and a rear face opposite the face front, characterized in that a reflective sheet is inserted between the face back of the panel and the resistive element. Panel according to any one of claims 1 to 9, comprising a radiating front face and a rear face opposite the face front, characterized in that a layer of honeycomb material is disposed at the rear of the resistive element. Panel according to any one of claims 1 to 10, comprising a radiating front face, characterized in that it comprises a glass plate on the front. Panel according to any one of claims 1 to 11, comprising a radiating front face, characterized in that the front face is frosted. Method of manufacturing a radiant electrical panel, comprising the steps of producing a resistive element in carbon fibers of essentially two-dimensional shape, placing connection electrodes in contact with the resistive element, arrangement of at least a layer of electrically insulating material on each face of the resistive element, impregnation of the resistive element provided with the electrodes and layers of electrically insulating material with a thermosetting resin, and polymerization of the resin,
characterized in that the production of the resistive element comprises the supply of a fabric of cellulosic fibers and the carbonization of the fabric by heat treatment carried out, ultimately, at a maximum temperature of between 700 ° C and 1000 ° C, so that the carbon content in the fibers is between 92% and 96%. Method according to claim 13, characterized in that the final heat treatment temperature is between 780 ° C and 820 ° C. Method according to any one of claims 13 and 14, characterized in that it includes a pre-carbonization phase at a temperature between 350 ° C and 420 ° C. Process according to any one of Claims 13 to 15, characterized in that the fabric is kept at the final temperature for one duration between 0.7 min and 1.3 min. Process according to any one of Claims 13 to 16, characterized in that the impregnation is carried out by an epoxy resin added with a black dye. Process according to any one of Claims 13 to 17, characterized in that the polymerization of the resin is carried out during a phase for shaping the panel by vacuum molding. Process according to any one of Claims 13 to 18, characterized in that the electrodes are put in place by bonding strips metal perforated along two opposite edges of the resistive element.

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