WO2008133562A1

WO2008133562A1 - Heating device

Info

Publication number: WO2008133562A1
Application number: PCT/SE2007/050290
Authority: WO
Original assignee: Intelliohm Ab
Priority date: 2007-04-30
Filing date: 2007-04-30
Publication date: 2008-11-06
Also published as: EP2160925A4; EP2160925A1

Abstract

Heating device comprising two elongated electrodes arranged at a distance and being inter-connected by a semiconducting heat generating member of a polymer based material having positive temperature coefficient regarding resistivity (PTC-material), wherein the heat generating member comprises electrode interconnection sections of a low resistivity PTC material compared with the PTC material of intermediate section. There is also provided a method of manufacturing a heating device.

Description

HEATING DEVICE

The present invention concerns an improved heating device comprising a wide bendable, electrically conductive, polymeric mat, adapted for division into lengths and mounting of these lengths solely or side by side in contact with the object to be heated and provided with electrodes arranged along each side edge of the mat, to which electrodes a current is con- nectable, whereby the current is conducted through the device, which heats up and emits heat and whereby the mat partly comprises a material composition whose volume resistivity increases when the temperature of the material composition increases.

TECHNICAL BACKGROUND

Publications, US 6,737,611 and WO 0156333 describe a floor heating device consisting of a wide bendable heat mat of an electrically conductive material wherein the volume resistivity increases with temperature (T). Such a material is in the art defined as a Positive Temperature Coefficient (PTC) material. Since the material conducts electricity, although much less than a metal it is in the art also called semiconductive. The mat comprises two electrodes each extending along the each edge. A fixed voltage (U) is connected over the said electrodes resulting in a current (I) flowing, to and fro the electrodes through the semiconductive material in the heat mat and causes a heating of the mat. When the volume resistivity of the material increases with increased temperature (T), the resistance (R) of the heat mat increases. The heat generated can be expressed as P = U²/R(T). That implies that the heat generating power decreases even though the voltage (U) is kept constant. This is the basis for so called self limiting PTC-devices. The mat is electrically insulated by the use of a co- extruded outer layer(s) of a non-conducting polymer. The heat mat is cut into preferred lengths, connected electrically to each other and mounted side by side underneath at least a surface layer of a floor.

Such a mat can be manufactured using coextrusion, lamination techniques or combinations thereof. In manufacturing by coextrusion a so called semiconductive compound is melted and pressurised in an extruder and thereafter fed into a so called coat-hanger die, shaping the melt of semiconductive material into a thin sheet or film like shape. The electrodes are at the same time drawn through channels in the die and becomes surrounded and in fact partly impregnated with the melted semiconductive material and thereafter in the same tool covered by an insulating layer of a non-conducting polymer delivered by a second extruder through corresponding coat-hanger shaped distribution channels. Sheet coextrusion using so called coat-hanger dies is thoroughly described by the book: "Extrusion dies for plastics and rubber: design and engineering computations / Walter Michaeli; with contributions by Ulrich Dombrowski et al. Hanser 2003". The sheet like extrudate is thereafter conveyed onto chill rollers where it solidifies and is cooled down to a temperature where it can be printed with information and finally coiled and cut into required lengths.

However, heating mats of this type are shown to have poor long time stability. Occasionally a hot zone is developed at the very contact between the electrode and the PTC-material. In other circumstances a hot zone develops at a distance within approximately 15 - 20 mm from the electrode. In both cases, the appearing hot zones together with the PTC- characteristic of the material gives a localised increase in the volume resistivity, rendering the mat a total lower heat generating power. This localized heating will spread along the length of the mat and the floor heating mat will not work as intended.

There are several causes of this problem. Firstly there will always be a contact resistance appearing between the electrode and the semiconductive layer. If no counter active measures are taken this contact resistance will be high enough to cause a local over-temperature at the interface of the conductor. This will lead to a creation of a hot zone at the very contact between the electrode and the semiconductive layer. Another cause of localised overheating that may occur in co-extruded heat mats is due to thickness variations of the semiconducting layer, and may be explained as follows. Figs. 1 and 2 show cross sections of the heat mat of the regions near the electrodes. The distance between the electrodes is about 380 mm. As can be seen from the figures are that the mat is not exactly of the same thickness over the width. In particular the conducting layer, the PTC-material, has a varying thickness near the electrode. The electrode has a certain thickness and has to be surrounded by conductive material. The coextrusion process together with the shaping and cooling of the of the mat imposes also a localised thinning of the mat in a region extending approximately 15 mm inside of the electrodes. Furthermore the same region inside of the electrodes are subjected to shear deformation during the time from it leaves the die and until it contacts the chill roller(s). The localised shear will to some extent disrupt the formation of the electrical connections between the conducting particles and thereby rendering that particular part a higher volume resistivity. The publications US 3,858,144 and US 4,426,339 discloses methods of reducing the contact problem. These two publications refer to self limiting heater cables. In publication US 3,858,144 the contact problem is claimed to be solved using an increased proportion of carbon black at the electrode interface relative to that of the remainder of the semiconductive material in the heating cable. The method described in US 4,426,339 claims that the electrode should have a temperature above the melting point of the polymer melt, when it becomes covered by the melt in order to solve the contact problem. However trials have shown that none of these methods fully solve the problem with local overheating at the electrode interface. The tendency of overheating in the zone reaching to a distance of 15 - 20 mm inside of the electrodes remained unaffected.

Since the mat is comprised of thermoplastic materials no attempts were made to solve the problem using cross-linkable elastomers (rubbers) as described in publications US 4,348,584 and US 4,444,708.

Another problem associated with floor heating mats of this type is caused by inherent wavi- ness of the mat. In coextrusion, during solidification of the polymer materials, they contracts much more than the electrodes. That will force the electrodes to buckle or deform with the result of the appearance of a waviness of the mat. This waviness can to some extent be counteracted in the shaping process, but not completely. As the temperature goes down the waviness will increase. During installation, this waviness will to a large extent be suppressed as the mat becomes squeezed between the top floor and the base floor. However a closer study of the resulting deformation pattern reveals that the inherent long waves of the mat is transformed into shorter waves. That waviness may impose deformation of the semiconductive material in the vicinity of the electrode. Such a deformation will also increase the material volume resistivity. As mentioned earlier, where the mat can not transfer heat by direct contact to the floor surfaces a temperature increase will result and in case of the original design this will also lead to localized overheating in the vicinity of the electrode.

However a more serious problem, associated with the inherent waviness of the floor heating mat, became obvious after several years of operation. Normally if the installation was car- ried out properly the inherent waviness could be accommodated and the floor heating mat showed no signs of damage. However during installation in somewhat colder ambient temperatures or if the floor heating mat had a too low temperature the waviness of the mat became to large to be accommodated. Sometimes the laying procedures of the parquet or laminate floor caused some displacement of the floor heating mat actually causing extra waviness to the mat. In such cases, the electrodes, at the contact points with the surrounding floor surfaces, will be subjected to extra compressive stresses that can cause the electrode to be subjected to a large plastic deformation and causing a wrinkle of the said electrode. When the floor is subjected to repetitive loads from people moving on top of the floor, the heavily deformed wrinkle will be subjected to fatigue loads, since the top floor rides on the deformed mat. This phenomenon can in some cases lead to fatigue failure of the electrode. The breakage of a live electrode can lead to the formation of an electric arc appearing between the ends of the failed electrode. Such an electric arc can in worst case at least theoretically create a fire. If one inserts a flexible polymer foamed mat, commonly used underneath parquet or laminate floor, between the floor heating mat and the sub floor the tendency to form wrinkles becomes less, but not to the extent that a fire hazard is avoided. Other kinds of foamed products like extruded foamed polystyrene (XPS) sheets will give further improvements, but wrinkles in the electrodes may still appear.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an improved heating mat compared to the prior art, e.g. as disclosed in US 6,737,611 and WO 0156333. However it should be considered to applicable to any design of any wide self limiting heating mat. Objectives of the present invention are:

To counteract the tendency of the appearance of localized overheating at the vicinity of the electrodes. This is achieved by placing a PTC-material around and inside of the electrode that has a lower volume resistivity and also a shallower PTC-curve than the PTC material in the main part of the heating mat.

To secure that there will not appear any sharp wrinkles in the electrodes. This is achieved by the introduction of a free space between the two objects that clamp the heat mat allowing the outer portion and including the electrodes of the mat to flex more or less freely and thereby avoiding any tendency of forming sharp wrinkles of the electrodes. To render the insulating material of the heating mat an increased fire resistance by adding fillers such as aluminium- tri- hydrate (ATH) and/or magnesium hydroxide (MgOH).

To render any material in contact with the heating mat vastly improved fire resistance by for example using additives such as fillers of ATH and/or MgOH.

To use standard earth potential protection devices in order to detect the appearance of an electric arc formed as a result of an electrode breakage and within a split second turn off the power supply of the heating mat. This can be accommodated by the use of a metal foil attached underneath to the cellular foamed sheet.

To provide self limiting devices for floor heating and wall mounted panel heaters, which quickly and easily may be installed.

These and other objects are met by the device as defined by the independent claims.

SHORT DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates a cross section of the device according to the present invention. Fig. 2 illustrates in cross section the difference between the prior art design and the improved design.

Fig. 3 shows in graphical form the PTC-charcteristic of PTC-material 1.

Fig. 4 illustrates the addition of a soft flexible cellular plastic sheet placed under the heat mat.

Fig. 5 illustrates one example of the way the heat mats are installed for a floor heating application.

Fig. 6 illustrates an exploded view of one example of how the heat mats are installed for wall mounted heating panels.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to one embodiment, disclosed in cross-section in fig. 1, there is provided a heating device 20 comprising two elongated electrodes 4 arranged at a distance and being inter- connected by a semi conducting heat generating member 1,2 of a polymer based material having positive temperature coefficient regarding resistivity (PTC-material), wherein the heat generating member comprises electrode interconnection sections 2 of a low resistivity PTC material compared with the PTC material of intermediate section 1. Thus, one solution to the local over-heating for a co-extruded heating device 20 in the form of a heat-mat is achieved by widening the channel in the coextrusion die containing the electrode and increasing the feeding rate of material from the third extruder (PTC-material 2) by a substantial amount, giving a volume of higher conductivity around the electrode and occupying a region reaching as long as up to 20 mm inside of the electrodes. This solution also avoided the problem of a localized heating zone appearing in the part where the mat is thinner. Figure 2 is illustrative. The left hand side shows the original design and showing the regions of risk of localised overheating. The right hand side shows the improved solution. As is shown in figs. 1 and 2, the electrode interconnection sections 2 may be arranged to surround the electrodes 4. However, the electrode interconnection sections may not completely surround the electrodes 4, as long as a sufficient electrical contact area is established. In one embodiment, the electrodes are laminated to one or both sides of the electrode interconnection sections 2 of a flat heating member 1, 2. As is indicated above, the problem of localized overheating a short distance from the electrodes can be avoided by letting the electrode interconnection sections extend an inward distance relative to the electrodes 4. In order to achieve a flexible heating mat, the electrodes 4 may be flexible.

The low resistivity PTC-material of the electrode interconnection sections preferably has a shallower PTC-curve compared with the PTC material of intermediate section. In most applications the heating device must be electrically insulated and therefore it comprising one or more electrically insulating outer layers 5 enclosing the electrodes and the heat generating member. In order to achieve increased fire resistance one or more fillers may be added into the insulating layer or layers 5. According to one embodiment, the PTC-material of the intermediate section 1 and said low resistivity PTC-material 2 consists of a semicrystalline polymer and electrically conductive fillers, and the insulating layers or layers may be comprised of a semicrystalline polymer compatible with said two PTC-materials. The electrically conductive filler material in the PTC materials may be carbon black, and the insulating layers 5 may be filled with flame and smoke suppressing additives like aluminium-tri- hydrate (ATH) and magnesium- hydroxide (MgOH). Exfoliated nanosized clay particles may also be added in order to improve fire resistance of the insulating layers 5. The heat generating member may be manufactured by a thermoplastic processing method such as extrusion and coextrusion and the insulating layers may be co-extruded in the same extrusion die or separately laminated to the heat generating member.

According to one embodiment, a free space is introduced between two objects, adapted to clamp said insulation outer layers 5 allowing the outer portions, including the electrode means 4 to flex more or less freely.

One or more heating devices of this type may be comprised in a heating arrangement wherein the electrodes in the heating devices are connected to electrical current feeder means. As is schematically shown in fig. 4, the heating arrangement may comprise a foamed sheet 6 of a cellular polymer placed on one side of each heating device 20, the sheet having a width that is less than the distance between the electrodes in the heating device and are arranged to not overlap the electrodes. In order to improve the electrical safety of the heating arrangement, a metal foil may be arranged on a major side of the cellular foamed sheet and is electrically connected to earth potential. Furthermore, an earth fault breaker may be connected to and arranged to break the power supply to the electrical feeder means upon detection of a fault. In order to reduce the fire hazard, the foamed sheet 6 comprises flame and smoke suppressing additives, as described above.

There is also provided a method of manufacturing a heating device comprising the steps:

• providing an elongated semiconducting heat generating member of a polymer based material having positive temperature coefficient regarding resistivity (PTC-material), wherein the heat generating member comprises electrode interconnection sections of a low resistivity PTC material compared with the PTC material of intermediate section, and

• attaching two elongated electrodes to the elongated semiconducting heat generating member , one to each electrode interconnection section.

The step of providing the elongated semiconducting heat generating member may be performed by co extrusion of two PTC materials of different characteristics. Moreover, the steps of providing the elongated semiconducting heat generating member and attaching the electrodes may be performed in one single coextrusion step. In one embodiment, the thermoplastic materials used are cross-linked after a forming operation. Alternatively, the step of attaching the electrodes may be performed by lamination of the electrodes to the elongated semiconducting heat-generating member. The semicrystalline material may be any flexible semicrystalline polymer or a copolymer for example an ethylene-ethyl acrylate polymer (EEA), ethylene-butyl acrylate polymer (EBA), ethylene-methyl acrylate polymer (EMA), ethylene-vinyl acetate polymer (EVA) or a so called plastomer. Plastomers is a family name for a family of homogeneous, ethylene alpha- olefin polymers prepared with metallocene catalysts. In particular for an application like foor heating, the crystalline melting point should be in the range of 75-99 ⁰C or rather in the range of 80-96 ⁰C and the electrically conductive filler material should be carbon black. For an application like wall mounted panel heaters a semicrystalline polymer or co-polymer with a melting point in the range of 96 - 130 ⁰C is appropriate. Also here the preferred electrically conductive filler is carbon black.

The mat may be provided with fully covering electrically insulating layers so that the device is ready to be used without further insulation or any risks of electric flash-over. Preferably the electrically insulating layer is co-extruded with the electrically conductive mat, but may also be provided in other ways, such as foliating. The electrically insulating layer may, for example, be a low density polyethylene (LDPE), linear low density polyethylene (LLDPE), polypropylene (PP), polyester or any of the previously mentioned copolymers, which preferably should have a higher melting point than the semicrystalline PTC-materials of the mat. Also some plastomers are conceivable. The electrically insulating layer may comprise a plurality of layers of different materials, and an outer layer for better scratch resistance, for example, firmly attached to each other. Preferably, electrically insulating filler material and flame retarding materials, such as aluminium tri-hydrate (ATH) and/or magnesium hydroxide (MgOH) and/or exfoliated nanoclay, may be added.

In fig. 1 the heat mat according to a first embodiment of the present invention is illustrated in cross section, which comprises an electrically conducting core of a semicrystalline polymer with an electrically conducting filler material, such as carbon black. Such material is called a semiconductive compound. Semiconductive compounds based on semicrystalline polymers exhibite a Positive Temperature Coefficient (PTC) regarding volume resistivity as a function of temperature and are therefore denoteted PTC-materials. Fig. 2 shows the difference between the prior art design using only one kind of semiconductive material exhibiting a positive temperature coefficient, here denoted PTC-material 1 and the improved design with electrode interconnection sections of a second PTC-material 2. PTC-material 2 has a much lower volume resistivity compared to that of PTC-material 1. This difference in volume resistivity can easily be conceived by using more carbon black in PTC-material 2. In that case also the lower volume resistivity renders PTC-material 2 a shallower PTC characteristic. Another possibility is to use a so called conductive carbon black. The semicrystal- line polymer is any of the previously mentioned. The electrodes 4 are placed along the edges 3 of the mat and enclosed by PTC-material 2. PTC material 2 occupies a region containing the said electrodes and stretches inwards at a distance of typically 10 - 20 mm.

There are a large number of different types of carbon blacks that may be used. Depending on the type of carbon black used, different percentages is required to achieve the right volume resistivity at 20 ⁰C and so called PTC-characteristic, i. e. how the volume resistivity of the electrically conducting mat material increases with increasing temperature, which causes the resistance in the floor heating device to increase, which in turn brings the power down and therefore also limits the temperature in the floor heating device preventing over- temperatures. Preferably the electrode 4 is a tin coated copper thread or copper wire.

Exterior of the electrically conducting entity comprised of the heat generating member of- core materials 1, 2 and electrodes 4 fully covering electrically insulating layers 5 are provided. Different countries may have different requirements of the minimum thickness and number of insulating layers around electrically conducting products. From the graph in fig. 3 it is shown that the volume resistivity in PTC-material 1 for 60W/m² at a temperature of 20 ⁰C is about 700 Ohmcm but as high as about 1200 Ohmcm at a temperature of 30 ⁰C. PTC- material 2 can be formulated in such a way that it preferably has a volume resistivity of only 10 Ohmcm at 20 ⁰C. The thickness of the core ought to be within the range 0,3-1 mm.

There are several means for avoiding that fire hazard. Firstly one can insert a soft foamed plastic sheet between the mat and the base floor and so arranged that the soft foam occupies only a certain width of the mat inside of the electrodes as depicted in fig. 4. This foamed sheet can be laid separately before the installation of the floor heating mat or preferably attached to the floor heating mat during fabrication. The foamed plastic sheet has preferably a thickness of 2 - 4 mm. The distance between the electrodes and the edges of foamed mat is preferably in the range of 5 - 30 mm. Now the inherent waviness of the mat will be superseded by shorter waves. The mat can still move a bit in the horizontal direction. However since the electrodes are not any more squeezed between the floor surfaces, there will not appear any sharp wrinkles in the electrodes. Fatigue failure will not appear and consequently no electric arc will be formed and the fire hazard is nullified. In another embodiment of the invention harder sheets can be inserted between the floor heating mat and the base floor. One such preferred sheet is made of extruded and foamed polystyrene (XPS). Such XPS sheets are among other makes sold under the trade names Depron and Kapron. The dimensions referred to for the soft foamed layer applies also for this embodiment. The said improvements can be generalised and used in many other applications apart from floor heating. Even if the said improvements will minimise the risk of a fire hazard one can add flame re- tardants in both the insulating layers and foam and XPS in order to further lessen any risk of fire hazard.

In order to prevent the electrodes 4 to be subjected to mechanical damage as described earlier, the embodiment incorporates, as shown in fig. 4, a flexible foamed plastic sheet 6 which is symmetrically placed under the heat mat. If the distance between the electrodes 4 is denoted s, the width w of the sheet 6 should preferably be in the interval [s - 10, w, s - 60]. The thickness of the sheet 6 should preferably be in range of 2.0 to 4,0 mm. Most conveniently for installation purposes the sheet 6 is adhered to the heat mat at the time of manufacture. In this embodiment suitable for floor heating, this sheet is made of closed cell PE- foam.

During compounding of the semicrystalline polymer and the electrically conductive filler material, preferably also small amounts of antioxidants are added, which gives good longtime ageing stability to the prepared compound, and therefore the device according to the present invention. In order to install a heated floor by means of the present device the sub- floor 11 is measured and a number of lengths of the mat 10 is cut so that the sub floor will be substantially covered when the lengths of mat are mounted side by side. See figs. 1 and 5. The cut end edges 7 are insulated by means of insulating tape 8 or the like over the cut end surfaces 7. At one of the end edges 7, preferably at the same end of the floor for all of the mat lengths, the ends 9 of the electrodes are laid open, whereafter the mat lengths are mounted side by side, not overlapping, on top of the preferably insulated subfloor. Each mat lengths is connected in parallel to mains voltage, whereby each connection of the ends (9) of the electrodes of course must be insulated.

As a conclusion two actual examples will be illustrated, which shall not be seen as a limitation of the scope, but only as possible embodiments of many conceivable embodiments within the scope of claim 1. A conceivable floor heating device according to the present invention comprises an electrically conductive core of semicrystalline polymer, ethylene- butyl- aery late (EBA) with 17 % (by weight) butyl acrylate (BA) as a matrix, which polymer has a crystalline melting point at about 90 ⁰C. Into this polymer a carbon black of the type N774, about 36 % by weight, is mixed to achieve proper electrically conductive properties in PTC- material 1 and a carbon black of the type N550 with about 35 % per weight for obtaining the proper PTC-characteristics for PTC-material 2.

The electrically conductive core has a thickness of approximately 0,4 mm and a width of 380 mm. Two exterior electrically insulating layers are coextruded around the electrically conductive core to a total thickness exceeding 0,4 mm each side. The electrically insulating layer comprises an inner layer of ethylene-butyl acrylate polymer (EBA) with a butyl acry- late percentage of 14 -29 %.

The insulating layers will in this embodiment be filled with smoke and flame retardants such as aluminium trihydrate (ATH) and/or magnesium hydroxide (MgOH) in order to achieve a limiting oxygen index (LOI) higher than 40 %. Two threadlike electrodes, one along each side edge of the mat, are embedded in the electrically conductive core. Depending on the applied mains voltage the cross sectional area of the electrodes will be in the range of 0,5 to 1,5 mm². Antioxidants are also added.

The heat mat can also be used in wall mounted electric heating panels. Traditionally such heating panels use electric resistance threads for heating. The heating thread is connected to the mains voltage in series with a thermostat or a thermo-regulator. The heating panels are provided with a visible warning text explaining the risk of overheating and fire hazard if the heating panel is covered with a towel or similar. The present invention, properly used, will remove the risk of overheating. Since one wants to minimise the total area of the wall occupied by heating panels, the temperatures need to be higher than those for the large area floor heating application. However the temperatures should not in any circumstance be too high creating a risk of overheating even if the panel heater is covered with a blanket or similar. For that application a semicrystalline polymer or co-polymer with a melting point in the range of 95 to 130 ⁰C is appropriate. In order to ensure long time stability of the heating mat one needs to use two PTC-materials as described earlier.

A second embodiment is depicted in fig. 6 and concerns the use of heating elements for use as wall mounted panel heater. The heat mats 10 are placed side by side in the vertical direction inside an encasement made of sheet metal. The electrical connections are in principle the same as for the floor heating application, however with tape 8 having a higher tempera- ture resistance. The figure is more or less self explanatory. The mat 10 with included sheets of cellular foam is firmly mounted between the back 12 and front 13 plates. Due to the higher temperatures the cellular foam may in this case be comprised of a closed cell PP- foam. This application requires much higher power and therefore a higher operating temperature than the floor heating application. That will in turn require the use of a polymer with higher crystalline melting point. A conceivable wall mounted panel heater according to the present innovation comprises a core of semicrystalline ethylene-butyl-acrylate-polymer (EBA) with 7 % butyl acrylate, which polymer has a crystalline melting point of about 107 ⁰C . Into this polymer a carbon black of the type N550, about 34 % by weight is mixed to achieve the proper conductivity in PTC-material 1 and about 37 % by weight of a more con- tuctive carbon black as N 472 is mixed to achieve the proper conductivity levels of PTC- material 2. The insulting layers can be made from a LDPE and will also in this embodiment be filled with sufficient amounts of ATH and/or MgOH to achieve a limiting oxygen index higher than 35 %. The higher specific power and temperatures involved in this application can cause irreversible changes in the conductivity of the heat mat. Therefore it will be advantageous to use standard procedures such as electrone beam (EB) or silane-crosslinking after manufacturing of the heat mats in order to render the heat mat improved temperature stability.

Claims

1. Heating device (20) comprising two elongated electrodes (4) arranged at a distance and being interconnected by a semiconducting heat generating member (1,2) of a polymer based material having positive temperature coefficient regarding resistivity (PTC-material), wherein the heat generating member comprises electrode interconnection sections (2) of a low resistivity PTC material compared with the PTC material of intermediate section (1).

2. Heating device as claimed in claim 1, wherein the electrode interconnection sections (2) are arranged to surround the electrodes (4).

3. Heating device as claimed in claim 1, wherein the electrode interconnection sections (2) extends an inward distance relative to the electrodes (4).

4. Heating device as claimed in claim 1, wherein the electrodes (4) are flexible.

5. Heating device as claimed in claim 1, wherein said low resistivity PTC- material of the electrode interconnection sections has a shallower PTC-curve compared with the PTC material of intermediate section.

6. Heating device as claimed in claim 1, comprising one or more electrically insulating outer layers (5) enclosing the electrodes and the heat generating member.

7. Heating device as claimed in claim 1, wherein it is formed as a flat wide heat mat.

8. Heating device as claimed in claim 1, wherein an increased fire resistance is achieved by adding one or more fillers into the insulating layer or layers (5).

9. Heating device as claimed in claim 1, wherein said PTC-material of the intermediate section (1) and said low resistivity PTC-material (2) consists of a semicrystalline polymer and electrically conductive fillers.

10. Heating device as claimed in claim 8, wherein the insulating layer or layers (5) are comprised of a semicrystalline polymer compatible with said two PTC- materials.

11. Heating device as claimed in claim 1, wherein the heat generating member (1,2) is manufactured by a thermoplastic processing method such as extrusion and coextrusion.

12. Heating device as claimed in claim 8, wherein insulating layers (5) are co- extruded together with the heat generating member.

13. Heating device as claimed in claim 8, wherein insulating layers are laminated to the heat generating member.

14. Heating device as claimed in claim 1, wherein the electrically conductive filler material in the PTC materials is carbon black.

15. Heating device as claimed in claim 8, wherein the insulating layers (5) are filled with flame and smoke suppressing additives like aluminium-tri-hydrate (ATH) and magnesium- hydroxide (MgOH) and exfoliated nanosized clay particles.

16. Heating device as claimed in claim 8, wherein a free space is introduced between two objects, adapted to clamp said insulation outer layers (5) allowing the outer portions, including the electrode means (4) to flex more or less freely.

17. Heating arrangement comprising one or more heating devices according to anyone of the preceding claims and electrical current feeder means connected to the electrodes in the heating devices.

18. Heating arrangement according to the preceding claim, comprising a foamed sheet (6) of a cellular polymer placed on one side of each heating device, the sheet having a width that is less than the distance between the electrodes in the heating device and are arranged to not overlap the electrodes.

19. Heating arrangement according to claim 18, wherein a metal foil is arranged on a major side of the cellular foamed sheet and is electrically connected to earth potential.

20. Heating arrangement according to the preceding claim, wherein an earth fault breaker is arranged to break the power supply to the electrical feeder means upon detection of a fault.

21. Heating arrangement according to claim 18, wherein the foamed sheet (6) comprises flame and smoke suppressing additives, like aluminium-tri-hydrate (ATH) and magnesium- hydroxide (MgOH) and exfoliated nanosized clay particles.

22. Method of manufacturing a heating device comprising the steps:

providing an elongated semi conducting heat generating member of a polymer based material having positive temperature coefficient regarding resistivity (PTC-material), wherein the heat generating member comprises electrode interconnection sections of a low resistivity PTC material compared with the PTC material of intermediate section, and

attaching two elongated electrodes to the elongated semiconducting heat generating member , one to each electrode interconnection section.

23. Method according to claim 22 wherein the step of providing the elongated semiconducting heat generating member is performed by coextrusion of two PTC materials of different characteristics.

24. Method according to claim 23 wherein the steps of providing the elongated semiconducting heat generating member and attaching the electrodes are performed in one single coextrusion step.

25. Method according to claim 22, wherein used thermoplastic materials are cross- linked after a forming operation.

26. Method according to claim 22 wherein the step of attaching the electrodes is performed by lamination of the electrodes to the elongated semiconducting heat generating member.