Electric heating mat

Abstract

The invention relates to an electric surface heating system or heating mat based on an electrically conductive polymer film or conductive polymer foam, which heats only locally where people, animals, or objects are located on the mat. This allows for energy savings compared to heating the entire surface. Ideally, this localized heat generation functions without any external electronic control or regulation.

Description

[0001] The invention relates to an electric surface heating system or heating mat based on an electrically conductive polymer film or conductive polymer foam that heats only where people, animals, or objects are located on the mat. This allows energy savings compared to full-surface heating. Ideally, this local heat generation works without any external electronic control or regulation.

[0002] Electric surface heaters have a wide range of applications, including wall heating, underfloor heating, mirror heating, terrarium heating, waterbed heating, heated doormats, and many more. While large-area heat distribution is desired for heating rooms, heated doormats or heated blankets for pets, such as dogs, require heat only where there is direct contact.

[0003] Common electric surface heating systems generate heat by converting electrical energy (Joulian heating). They consist, for example, of conductive plastics that are fully or partially contacted with electrodes, which can also be designed as conductor tracks. Alternatively, metallic conductor tracks on the heating surface, created on an insulating substrate by etching or printing, can themselves be used as resistance heating.

[0004] DE 10 2004 058 721 A1 discloses a multilayer heating element that serves as a pressure-dependent electrical resistor in a composite structure. Surfaces can thus be heated according to their coverage. WO 2017 / 098842 A1 relates to a heating device, specifically a seat heater for motor vehicles. It comprises a sheet-like electrical heating element with two electrically insulating layers with a plurality of round, convex-shaped sections. A sheet-like heating element is arranged on the inside of the upper electrically insulating layer and is connected to an electrode in each of the convex-shaped sections. A corresponding second electrode is arranged on the inside of the lower electrically insulating layer. The two electrodes are spaced apart from one another.If pressure is applied to the outside of the upper insulating layer, the two electrodes come into contact and a current flows which is in turn converted into heat. US 6 906 293 B2 discloses a seat heater for motor vehicles. It comprises a pressure-sensitive sensor mat with several sections that are connected to one another by flexible strips. The flexible strips each comprise two carrier films that are connected to one another by spacers. Electrodes are arranged at a distance from one another on the inside of the carrier films between the spacers. If pressure is applied to the flexible strips, the two electrodes touch one another and current flows. Another seat heater is described in DE 11 2012 005 448 T5. It comprises a sensor that detects the occupancy state with the help of a thermistor and a control circuit connected to it.

[0005] What all these electric surface heating systems have in common is that the position and fixation of the electrodes definitively determines the local current flow and thus the local heat generation. Locally selective control is then only possible if individual sectors of the heating surface are actively regulated.

[0006] An alternative is disclosed in patent JPH0624768, which describes a partial and selective current supply using a pressure-sensitive resistor. A disadvantage of this invention is that, depending on the desired spatial resolution, multiple pressure sensors must be implemented. In patent JPH09245937, this disadvantage is overcome by making the electrically conductive heating layer itself pressure-sensitive, so that electrical heat is only generated where a force or pressure is applied. A disadvantage of this solution, however, is that even in the unloaded case, a residual current flows due to the still existing finite resistance, as a result of which a small amount of energy is permanently consumed. The present invention also overcomes this disadvantage, since no quiescent current flows in the unloaded case. No quiescent current, as defined by this invention, means that the current intensity is less than 1 mA.

[0007] The object of the invention is to provide a technical solution for an electric surface heating system or heating mat based on an electrically conductive polymer film or an electrically conductive polymer foam that heats only where people, animals, or objects are located on the surface / mat and that does not generate any current when unloaded. This allows heating energy to be reduced. The technical solution requires no sensors or electrical controls.

[0008] Specifically, the solution to the problem is achieved by contacting a material made of electrically conductive plastic (1) with electrodes (2), (3) on the top and bottom. Additionally, spacers (4) made of an electrically non-conductive material (see [figure omitted]) are located on the top and / or bottom of the conductive plastic. Fig. 1 and Fig. 2). This means there is no material or frictional contact between the electrodes. Only when pressure increases due to a local load on the surface does close contact between the electrodes and the heating element occur, allowing current to flow in this area and generating heat.

[0009] The electrically conductive plastic can be either an intrinsically conductive plastic or a plastic that has been made conductive by the addition of additives.

[0010] Doped poly-3,4-ethylenedioxythiophene, polyaniline, polypyrrole or polythiophene can be used as intrinsically conductive polymers.

[0011] Non-intrinsically electrically conductive plastics can be made conductive by adding electrically conductive additives. Suitable additives include, for example, carbon black, graphite, graphene, metal particles, and / or carbon nanotubes. These plastics include polymers with a main chain consisting exclusively of carbon, such as polyethylene and polypropylene, as well as polyamides, polyurethanes, polyesters, and silicones.

[0012] The electrically conductive plastic can be solid, porous, or foamed. Depending on the underlying polymer, it can be rigid or flexible.

[0013] Particularly preferred are conductive plastics with a positive temperature coefficient (PTC) of electrical resistance, which realize an independent reduction of the current and thus of the heat generation with increasing temperature.

[0014] The electrical conductivity of the plastic is between 10 2< and 10 5< S / m, preferably between 10 2< and 10 4< S / m.

[0015] Advantageously, the flat electrodes possess a certain degree of mechanical flexibility, which allows for reversible pressing and detaching of the contact from the heating element under pressure. Suitable flat electrodes can be, for example, metal foils, metal-coated polymer foils, metallic wire mesh, metallized fabrics, or conductive foams that ensure a sufficiently low electrical supply resistance. The material of the flat heating element (1) is preferably a conductive plastic in the form of a foil or plate, or a conductive foam.

[0016] To prevent the flow of current in the unloaded state, it is necessary to attach electrically non-conductive spacers (4) at specific intervals, either point-like or linearly, between the electrodes (2), (3) and the electrically conductive surface heating element (1). These spacers prevent the electrodes from accidentally coming into localized contact with the surface heating element. By attaching the spacers according to the invention, the current intensity is reduced entirely to zero. The spacers (4) can be thin, flexible foam films or thin textile fibers. Care must be taken to ensure that the area covered by the spacers (4) is very small, ideally less than 10%, compared to the total area of the heating mat. Preferred embodiments

[0017] In a first embodiment, the electrically conductive plastic (1) consists of a conductive foam plate, the electrodes (2), (3) of a metallic wire mesh and the spacers (4) of thin polyester fibers, which are laid at a distance of several centimeters between the foam plate and the electrodes.

[0018] In a second embodiment, thin foam pads with a lateral extension of a few millimeters are glued to the foam plate at a distance of several centimeters as spacers (4).

[0019] In a third embodiment, the electrodes (2) and (3) are realized using a metallized mesh. A significant advantage of these electrodes over the metallic wire mesh of the first and second embodiments is their greater flexibility and lower weight. Examples Example 1

[0020] This example demonstrates the operating principle of the invention. A conductive PE foam (ELS-M) measuring 470 x 320 mm and 6 mm thick is covered on both sides with stainless steel mesh electrodes. The mesh electrodes consist of stainless steel wires with a mesh size of 1.4 mm and are loosely attached to the foam at the edge. PET plastic filaments with a diameter of 0.5 mm were woven into the wire mesh between the lower mesh electrode and the conductive foam at a distance of approximately 6 cm. Between the upper mesh electrode and the conductive foam, 28 foam plates (2 mm thick) were glued at a distance of approximately 8 cm as spacers. In principle, other materials and molded bodies can also be used as spacers, as long as they do not prevent the surface contact of the electrodes with the conductive foam under load.

[0021] If a voltage of 60 volts is applied to the electrodes, no measurable current flows through the heating mat when the mat is not under load. If the mat is locally loaded, a significantly higher current begins to flow at that point. In one example, the load was applied, dictated by the geometry of the support, in a ring-shaped area with an inner diameter of 3.5 cm and an outer diameter of 6.6 cm. This corresponds to a contact area of 24.6 cm². If the area is loaded with a mass of 9.4 kg, a current of 140 mA flows. This corresponds to a local current density of 5.7 mA / cm². If the load is increased to 13.3 kg, the current increases to 160 mA or 6.5 mA / cm². This results in a temperature increase of 30 - 35 K compared to the unloaded part of the mat.

[0022] In a second variant, a weight of 80 kg is applied to the central part of the mat, in a rectangle measuring 31 x 20 cm. The current is now 1.3 A and the local current density is 2.1 mA / cm².

[0023] If a person weighing approximately 75 kg steps onto the mat, a current of 1.34 A flows. Assuming a sole area of approximately 500 cm², this results in a current density of 2.7 mA / cm². The electrical power of 80 watts generated in this way leads to a rapid heating of the mat beneath the feet, whereby after approximately 10 seconds a temperature increase of between 15 and 25 degrees Celsius, depending on foot contact, can be detected by thermography ( Fig. 3 ). Example 2

[0024] This example demonstrates the importance of spacers for reducing the quiescent current in the unloaded case by not using any spacers. A conductive foam measuring 21 x 21 cm and 7 mm thick is covered on both sides with stainless steel mesh electrodes. The mesh electrodes consist of stainless steel wires with a mesh size of 1.4 mm and are loosely attached to the foam at the edge. Spacers are missing. If a voltage of 60 volts is applied to the electrodes, a small but easily measurable current of 10 mA flows through the heating mat without any load. This current is caused by random point contacts. If the mat is locally loaded, a higher current begins to flow at that point, comparable to that in Example 1. List of reference symbols

[0025] 1 conductive plastic 2 upper electrode 3 lower electrode 4 spacer 5 heating mat

Claims

1. Electric surface heater (5) comprising or consisting of an electrically conductive plastic body (1) and an upper (2) and a lower electrode (3) to which an electric voltage is applied, characterized in that at least one of the two electrodes is flexible, and that between the upper electrode and the plastic body (1) and / or the lower electrode (3) and the plastic body (1), thin spacers (4) of very limited size are attached at defined distances from one another, so that no current flows in the unloaded state, but that under load the flexible electrode sags, and a material-bonded contact to the electrically conductive plastic body (1) develops, and that a local flow of current and heating is thereby realized.

2. Electric surface heater according to Claim 1, characterized in that the electrically conductive plastic body (1) is intrinsically conductive, and preferably contains or consists of doped poly-3,4-ethylenedioxythiophene, polyaniline, polypyrrole or polythiophene.

3. Electric surface heater according to Claim 1, characterized in that the electrically conductive plastic body (1) is made conductive through the inclusion of additives, preferably through the inclusion of carbon black, graphite, graphene, metal particles and / or carbon nanotubes.

4. Electric surface heater according to Claim 3, characterized in that the plastic of the electrically conductive plastic body (1) is selected from the group of polymers with a primary chain consisting exclusively of carbon, preferably polyethylene vinyl acetate copolymer, or from the group of polyethylene, polypropylene, polyvinyl fluoride, polyvinylidene fluoride, polyvinyl chloride, polyvinylidene chloride or of polyamides, polyurethanes, polyesters or silicones.

5. Electric surface heater according to Claim 1, characterized in that the conductive plastic body (1) is present either in solid form or in porous or foamed form.

6. Electric surface heater according to Claim 1, characterized in that the conductive plastic body has a positive temperature coefficient (PTC) of electrical resistance.

7. Electric surface heater according to Claim 1, characterized in that the electrical conductivity of the plastic of the electrically conductive plastic body (1) is between 102 and 105 S / m, preferably between 102 and 104 S / m.

8. Electric surface heater according to Claim 1, characterized in that the planar electrodes are metal foils, metal-coated polymer foils, metal wire mesh, metallized mesh or conductive foams.

9. Electric surface heater according to Claim 1, characterized in that the spacers (4) are electrically non-conductive and are attached with a certain spacing from one another at points or in a linear arrangement on the upper and / or lower side of the electrically conductive plastic body (1).

10. Electric surface heater according to Claim 9, characterized in that the spacers (4) are foam foils or textile fibres and the surface area covered by the spacers is less than 10% of the total area of the electrically conductive plastic body (1).

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