WO2013045254A1

WO2013045254A1 - Vehicle seat heating element comprising a heating cable with metallic filaments

Info

Publication number: WO2013045254A1
Application number: PCT/EP2012/067524
Authority: WO
Inventors: Filip Lanckmans; Steve Verstraeten
Original assignee: Nv Bekaert Sa
Priority date: 2011-09-29
Filing date: 2012-09-07
Publication date: 2013-04-04
Also published as: RS57550B1; EP2761977B1; CN103765984A; EP2761977A1; PL2761977T3

Abstract

A vehicle seat heating element is described comprising a heating cable and means for feeding electrical current to the heating cable. The heating cable comprises metallic filaments. A predetermined number of the metallic filaments in the heating cable are each individually wrapped with one or more non-electrically conductive filaments or individually wrapped with non-electrically conductive fibers or individually wrapped with one or more non-electrically conductive tapes. The benefit of such vehicle seat heating element is that a longer reliably functioning lifefime is obtained.

Description

Vehicle seat heating element comprising a heating cable with metallic filaments

Description

Technical Field

[0001] The invention relates to the field of vehicle (e.g. car) seat heating elements that are

comprising electrical heating cables.

Background Art

[0002] Electrical heating cables comprising metallic filaments (e.g. 15 - 150 metallic filaments) are known and are used in seat heating in cars. Each of the metallic filaments may have a diameter that is of a magnitude of about 50 μιτι. Car seat heating can be achieved by installing the electrical heating cables in the seats, e.g. in the form of one or more loops, to form a car seat heating element. In the car seat heating element, such a heating cable is connected to a power feeding unit that delivers current, whereby the element can be heated to a suitable temperature.

[0003] An important requirement among the requirements for vehicle seat heating systems is a long lifetime during which the car seat heating system is operating correctly and reliably.

[0004] In car seat heating elements or systems, electrical heating cables are subject to dynamic bending forces. Hence, the flex life (resistance to dynamic bending) is an important parameter for the durability and lifetime of the heating cable and hence of the car seat heating element or system. A way to increase the flex life or flex endurance of a heating cable and consequently of the car seat heating element up to the required level, is the use in the heating cable of metallic filaments with lower diameter. However, decreasing the diameter of the metallic filaments exponentially increases the production costs of the heating cable and of the car seat heating element.

[0005] In order to protect the metallic filaments from corrosion (and especially from galvanic corrosion) and to increase the flex life of heating cables to the required levels for car seat heating applications, the heating cable of the car seat heating element can be provided with a polymer sheath. For the best values of flex life, high grade polymer coatings are required. These high grade polymer coatings (e.g. perfluoroalkoxy polymer, PFA) have the drawback that they are expensive and difficult to apply.

[0006] When individual metallic filaments of the heating cable of the car seat heating element are damaged or broken, it can lead to a local difference in electrical properties over the length of the heating cable. A so-called hot-spot can occur: at the position of the broken filament(s) the heat generation is higher than along the rest of the length of the heating cable. Hot-spots are to be avoided as they create a safety hazard. WO 01/058315 - relating to a device for heating a component in a vehicle environment - describes a way to resolve the hot-spot formation which occurs at the interruption (breakage) of part of the metallic filaments of the heating cable. The solution provides a heating cable constructed from a number of strands of which a predetermined number of strands are individually electrically insulated with an insulating lacquer layer. [0007] Although individual insulation of the strands or of the metallic filaments by means of lacquering is an effective way to solve hot-spot formation, the individual lacquering or individual coating of the strands or of the metallic filaments of the heating cable has a serious drawback in that it is a complex and very expensive process. Furthermore, it is technically very difficult to apply a uniform and effective lacquer on fine metallic filaments or strands. If the lacquer layer is not applied uniformly nor dried and cured correctly, bending of the heating cable during its use can damage the lacquer layer, resulting in lower lifetime of the heating cable or in insufficient prevention of hot-spots. Furthermore, there is little or no contribution of the lacquering of the filaments or the strands to the flex life of the heating cable.

Disclosure of the Invention

[0008] It is an objective of the invention to provide a vehicle (e.g. car) seat heating element that has a long lifetime during which it is functioning correctly and reliably (including that it has excellent flex life and effective hot-spot prevention) and which is easy to manufacture.

[0009] According to the invention is provided a vehicle seat heating element (e.g. a car seat heating element) comprising a heating cable and means for feeding electrical current to said heating cable. The heating cable comprises metallic filaments, preferably twisted or cabled together. A predetermined number of the metallic filaments of the heating cable are each individually wrapped with one or more non-electrically conductive filaments or individually wrapped with non-electrically conductive fibers or individually wrapped with one or more non-electrically conductive tapes.

[0010] The non-electrically conductive fibers can e.g. be provided in the form of spun yarns which are wrapped around metallic filaments.

[001 1] The long lifetime of the vehicle car seat heating element of the invention during which it is functioning correctly and reliably is obtained by the synergetic effects of preventing the occurrence of hot spots and by an increase in the flex fatigue resistance of the heating cable of the vehicle seat heating element. The formation of hot-spots is effectively prevented by the insulation of metallic filaments with the non-conductive fibers or non- conductive filaments or non-conductive tapes that are wrapped around individual metallic filaments. Surprisingly, the wrapping results in an important increase of the flex fatigue resistance as well, resulting in a longer lifetime during which the vehicle seat heating element is functioning correctly.

[0012] In an embodiment of the invention, the wrapping is performed in Z- and in S-direction around the axis of the metallic filament. A way of wrapping in Z- and in S-direction around the axis of the metallic filaments is by wrapping part of the fibers or part of the filaments or part of the tapes in S-direction and part of the fibers or filaments or tapes in Z-direction around the axis of the metallic filament. The advantage is that a more stable heating cable is obtained with such metallic filaments. Preferably, the metallic filament contains a same amount of fibers or filaments or tapes wrapping in S-direction as in Z-direction, as the result is the best stability of the heating cable and it enhances the coverage.

[0013] For instance, the metallic filament is wrapped by an even number of non-electrically

conductive filaments (or fibers or tapes), wherein half of the filaments (or fibers or tapes) is wrapped around the metallic filament in S-direction and the other half in Z-direction. The benefit is a stabilization of the heating cable. For instance two non-electrically conductive filaments (or fibers or tapes) are used in wrapping, one is wrapped in Z- direction, one in S-direction.

[0014] In another embodiment of the invention, the wrapping is performed in only one direction around the axis of the metallic filament. This can be in S- or in Z- direction. A benefit of this embodiment compared to wrapping in both directions is that a heating cable with a lower diameter is achieved.

[0015] Although any non-electrically conductive filaments, fibers or tapes can in principle be used to wrap the metallic filaments, examples of preferred filaments are polyester, polyurethane, polyamide, fiberglass, polybenzobisoxazole (PBO), aramid, polypropylene, polyethylene, melt yarn, bicomponent fibers or bicomponent filaments (preferably of the type with a sheath with a lower melting temperature). High tenacity polyester filaments are more preferred as their higher tensile strength results in an even more pronounced increase in flex life of the heating cable. Filaments for wrapping are preferably having a diameter between 12 and 70 micrometer.

Fibers of discrete length can also be used to wrap the metallic filaments, examples are natural fibers (e.g. cotton) or synthetic fibers (polyester, polyamide, polypropylene, polyethylene...).

[0016] A particularly preferred type of filaments that can be used for wrapping is tapes. A tape is a particular type of filament: a tape has a cross section that is substantially flat, showing a thickness and a width. For the invention, tapes are used that preferably have a width over thickness ratio of the cross section of at least 10, preferably at least 15. Preferably, the width over thickness ratio of the tapes is lower than 50, more preferably lower than 35. Tapes offer the benefit that effective results are obtained in terms of flex fatigue and hot spot prevention, while the diameter of the heating cable can be limited. Preferred is where the windings of the tape are not overlapping, but touching each other in subsequent turns of wrapping, which results in a smoother wrapped filament and cable construction.

Such tapes in polyester, polyamide, polyolefin (e.g. polyethylene or polypropylene) can be used. Polyester tapes are preferred however, thanks to their interesting combination of properties. More preferred are flame retardant polyester tapes.

Preferred tapes are having a cross section with a thickness between 10 and 40 micrometer, more preferably between 10 and 25 micrometer, even more preferably between 12 and 25 micrometer. The latter range is providing a combination of a sufficiently thin heating cable while the wrapping is providing an important increase in flex life of the heating cable in the vehicle seat heating element. Preferably the width of the cross section of the tape is at least 100 micrometer, more preferably at least 200 micrometer, even more preferably at least 300 micrometer.

Preferably the width of the tape is less than 500 micrometer.

Specific examples of cross sections of tapes that can be used in the invention are e.g. 250 micrometer by 12 micrometer, 350 micrometer by 12 micrometer, 370 micrometer by 12 micrometer and 250 micrometer by 23 micrometer, e.g. in polyester.

[0017] Preferably, all metallic filaments of the heating cable are each individually wrapped with one or more non-electrically conductive filaments or by non-electrically conductive fibers (e.g. in which the non-electrically conductive fibers are combined in a yarn to wrap around the metallic filament) or by one or more non-electrically conductive tapes. However, the invention also works if a minimum number of metallic filaments are individually wrapped. This minimum number is suitably chosen depending on how many non-insulated metallic filaments that would be able to lead to hot-spot formation at breakage or damage. The position of the metallic filaments of the heating cable that are individually insulated may also be chosen with respect to the risk of the occurrence of hot-spots in the vehicle seat heating element. If some metallic filaments in the heating cable run more risk of breaking, these should preferably be wrapped by non-electrically conductive filaments or by non- electrically conductive fibers, or by non-electrically conductive tapes. If the situation occurs that these most vulnerable metallic filaments (which are individually wrapped) are damaged and/or broken, there is still effective prevention of the occurrence of hot-spots, while the wrapping fibers, filaments or tapes offer strength to avoid full failure of the heating cable due to dynamic bending load.

[0018] Preferably, the wrapping is covering at least 50% of the surface of the metallic filament being wrapped. Fifty percent coverage gives a good amount of protection against hot spots. Preferably, at least 70% of the surface is covered, which results in better hot spot prevention. More preferably, at least 90% of the surface of the metallic filament is covered. Even more preferably, at least 95% is covered of the surface of the metallic filament being wrapped. Even more preferred, at least 99% is covered of the surface of the metallic filament being wrapped. 95% and 99% coverage give a high safety factor against hot spots. When cables are made with these coverage percentages, the process of wrapping is set for total coverage, but due to variation in the production process, it is possible that some spots on the metallic filament is not covered. Most preferably, total coverage is achieved of the surface of the metallic filament that is being wrapped. With total coverage, the avoidance of hot-spots is maximum, total coverage requires higher care in the production process however.

[0019] A preferred way of production of the heating cable used in the vehicle seat heating

element according to the invention is by means of twisting and/or cabling operations with the metallic filaments (some or all of which are wrapped). Preferably the metallic filaments are twisted or cabled into strands and these strands are twisted or cabled together into a heating cable, after which the heating cable can be cut to length and mounted in the vehicle seat element including connecting the heating cable to means for feeding electrical current to the heating cable.

[0020] In a specific embodiment, a heating cable is provided wherein the non-electrically

conductive fibers or non-electrically conductive filaments or non-electrically conductive tapes comprise polymeric filaments or polymeric fibers or polymeric tapes or wherein the non-electrically conductive fibers or non-electrically conductive filaments or non- electrically conductive tapes are comprising fibers or filaments or tapes with a polymeric sheath and wherein at least part of the non-electrically conductive fibers or non- electrically conductive filaments or non-electrically conductive tapes are molten or softened and than solidified to form a polymeric layer around said metallic filament. To this end, low melting or core-sheath non-electrically conductive monofilaments, preferably as tape, with a low melting sheath (meaning that the sheath has a lower melting point than the body) are preferred. The benefit is that an even more effective insulation of the metallic filament is created. Full closure of the cross section of the heating cable can be obtained this way, which results in avoidance of capillarity.

[0021] In another specific embodiment, the heating cable of the vehicle seat heating element is provided with a polymeric sheath. The polymeric sheath can be provided by coating the heating cable with an appropriate polymer, e.g. via extrusion coating. This way, the heating cable is embedded in a plastic material. Examples of polymers that can be used are polytetrafluorethylene (PTFE), copolymers of tetrafluoromethylene and

hexafluoropropylene (FEP), perfluoroalkoxy polymer (PFA). The polymeric coating sheath is providing an efficient resistance of the heating cable against corrosion, including against galvanic corrosion. Thanks to the presence of individually wrapped metallic filaments, the polymeric coating can be a lower grade or cheaper coating (e.g. polyamide 12 or TPE): high grade coatings contribute to the flex life of the heating cable, contribution which is less or not required in heating cables according to the invention as the wrapping of metallic filaments is in itself creating flex life of the heating cable. PFA coatings for instance, exist in different grades, the grades with higher temperature stability result in higher flex life contribution, but are more expensive in material cost and in applying the coating. A PFA grade with temperature stability of 260°C is much more expensive than a PFA grade with 225°C temperature stability and needs higher temperature during the application process.

[0022] Preferably a heating cable provided with a polymeric sheath is having a polymeric sheath in which the polymer is devoid of fluorine. Examples of such polymers are polyurethane, polyamide (e.g. polyamide 12), thermoplastic polyester, TPE (thermoplastic elastomer) - e.g. a co-polyester thermoplastic elastomer or PVC. Use of a polymer for the polymeric sheath that is devoid of fluorine has the benefit that cheaper and more environmentally friendly coatings can be used. [0023] In a specific embodiment a polymeric sheath of the heating cable is formed by melting or softening followed by solidification of at least part of the non-electrically conductive fibers or non-electrically conductive filaments. The benefit is that no additional coating material is required to form a polymeric sheath around the heating cable.

[0024] In a preferred embodiment of the invention, a vehicle seat heating element is provided wherein a heating cable is provided that is having an electrical resistance below 3 Ohm/meter (measured at 20°C). The invention is of particular interest for vehicle seat heating element that comprise heating cables with a resistance below 3 Ohm/meter. For heating cables with higher resistance, heating cables made out of bundles of bundle drawn stainless steel filaments are available at an interesting cost and with an intrinsically good flex life and durability. Below 3 Ohm/meter however, heating cables consisting of bundle drawn stainless steel filaments are expensive, because of the large section of the cable (high amount of material). Hence another solution than bundle drawn stainless steel filaments is preferably required for heating cables with resistance below 3 Ohm/meter.

[0025] The invention is of specific interest for vehicle seat heating elements with heating cables with resistance below 1 Ohm/meter (measured at 20°C) and still even more for heating cables with resistance below 0.75 Ohm/meter (measured at 20°C).

[0026] Such vehicle seat heating elements according to the invention with heating cable with resistance below 3 Ohm/meter, or below 1 Ohm/meter or below 0.75 Ohm/meter can beneficially be provided with stainless steel monofilaments (preferably single end drawn) or using metallic filaments that are comprising a copper or copper alloy layer and a steel layer. Such products have the benefit that they have excellent functional lifetime and can be produced in an economical way.

[0027] Examples of metallic filaments that can be used in the invention are stainless steel

filaments. Other examples are metallic filaments that are comprising a copper or copper alloy layer and a steel layer.

[0028] The metallic filaments that can be used in the invention are electrical conductors made from suitable metals or can be multilayered metallic filaments. Examples of suitable metallic filaments are copper, stainless steel, or some other suitable metal or metal alloy with good electrical conductivity properties. Examples of multilayer metallic filaments that can be used are metallic filaments that have a copper or copper alloy layer and a steel layer, preferably a concentric steel layer. For instance the copper or copper alloy layer can be the core layer surrounded by a stainless steel layer. An alternative is a steel core layer surrounded by a copper or copper alloy layer (so called copper clad steel).

[0029] Preferably, the metallic filaments are having a substantially round cross section. With substantially round cross section is meant that the cross section is circular, or oval. If the cross section is oval, the difference between the largest and smallest diameter of the cross section is less than 10%, preferably less than 5%, more preferably less than 2% of the largest diameter of the cross section.

[0030] In a specific embodiment of the invention, the metallic filament is made from a low carbon steel grade or at least comprising a layer made from a low carbon steel grade. A low carbon steel grade is a steel grade where - possibly with exception for silicon and manganese - all the elements have a content of less than 0.50 % by weight, e.g. less than 0.20 % by weight, e.g. less than 0.10 % by weight. E.g. silicon is present in amounts of maximum 1.0 % by weight, e.g. maximum 0.50 % by weight, e.g. 0.30 % by weight or 0.15 % by weight. E.g. manganese is present in amount of maximum 2.0 % by weight, e.g. maximum 1.0 % by weight, e.g. 0.50 % by weight or 0.30 % by weight. Preferably for the invention, the carbon content ranges up to 0.20 % by weight, e.g. ranging up to 0.06 % by weight. The minimum carbon content can be about 0.02 % by weight. In a more preferred embodiment, the minimum carbon content can be about 0.01 % by weight. The low carbon steel composition has mainly a ferrite or pearlite matrix and is mainly single phase. There are no martensite phases, bainite phases or cementite phases in the ferrite or pearlite matrix.

[0031] The use of a low carbon steel grade for the metallic filaments has a number of benefits. A heating cable with high flexibility and good flex life is obtained. The high flexibility is of interest when using the heating cable in a vehicle seat heating element where the heating cable needs to be given a complex arrangement in the vehicle seat heating element.

[0032] In another specific embodiment of the invention, the metallic filament is made from a high carbon steel grade or at least comprising a layer made from a high carbon steel grade. With high carbon steel is meant a steel grade having a carbon content between 0.30 and 1.70% by weight. For the invention, preferably high carbon steel grades with carbon content between 0.40 and 0.95% by weight are used, even more preferably high carbon steel grades with carbon content between 0.55% and 0.85% by weight. The high carbon steel grades can contain alloy elements. The use of a high carbon steel grade has a number of additional benefits. The strength of the metallic filaments comprising the high carbon steel layer is higher. The heating cable made thereof has been shown to give a higher flex life when compared to alternative heating cables with similar diameter of metallic filaments; e.g. compared to stainless steel monofilament heating cables or compared to heating cables comprising stainless steel layers in the filaments.

[0033] In a specific embodiment of the invention the metallic filaments are single end drawn, i.e. one single metallic filament is drawn through drawing means, in contrast to bundle drawing.

[0034] In another specific embodiment of the invention, the metallic filaments have been end drawn, i.e. the process of drawing is the final process in making the metallic filaments, meaning that no heat treatments follow. A heating cable in which the metallic filaments are end drawn is having an improved flex life. [0035] In another specific embodiment of the invention, the metallic filaments have been end annealed resulting in an annealed microstructure of the metallic filaments in the heating cable of the vehicle seat heating element. It is of interest that the heating cable made with metallic filaments that have been end annealed is having higher flexibility. Higher flexibility of the heating cable is a benefit when the heating cable has to be bent into a specific shape in the vehicle seat heat element.

[0036] When using metallic filaments made from a low carbon or with a high carbon steel grade, or when using metallic filaments with a layer made from a low carbon or from a high carbon steel grade, it is possible to have a corrosion resistant metal as cover layer (e.g. as a layer more towards or at the sheath than the layer made from a low carbon or from a high carbon steel grade) or as outer metallic layer on the metallic filament. Preferably, the corrosion resistant metal coating or layer on the metallic filament is between 1 and 10 % by weight of the metallic filament, more preferably between 2 and 6 % by weight, even more preferably between 3 and 5 % by weight. Examples of corrosion resistant metal coatings are zinc and nickel.

[0037] High carbon and low carbon filaments with a metallic coating or cover layer (and

especially with a zinc coating or with a nickel coating) exist and are used for a number of different applications. When using these type of metallic filaments, the production of a heating cable - and consequently also of the vehicle seat heating element according to the invention - is facilitated and made more cost effective by the use of raw material for the metallic filaments that is already in use for metallic wires for other applications. For use for to the invention, the metallic filaments will need to be drawn further (preferably in single end drawing), to finer diameters, compared to the diameters required for other, existing applications. Where a metallic coating or cover layer is used, the metallic coating or cover layer can be provided on a wire of larger diameter, which is being drawn further to the required final diameter for the metallic filament.

[0038] Preferably, the diameter of the metallic filaments used to manufacture the heating cable for the vehicle seat heating element is in the range of 30 to 150 micrometer. More preferably 50 to 100 micrometer, even more preferably 75 to 100 micrometer. The contribution to the increase of the flex life of the heating cable by the non-electrically conductive wrapping fibers or filaments results in it that coarser metallic filaments can be used. It is known that finer metallic filaments result in heating cables with higher flex life, but the finer metallic filaments have a higher cost and result in a more expensive heating cable. Consequently, the invention allows to use higher diameter metallic filaments, e.g. in the range of 75 to 100 micrometer.

[0039] Where diameter is mentioned, it is meant as equivalent diameter, which is for a non- round cross section the diameter of a circle having the same surface as the non-round cross section. Brief Description of Figures in the Drawings

[0040] Figure 1 shows a vehicle seat heating element according to the invention.

[0041] Figure 2 shows a core-sheath metallic filament that is wrapped with two non-electrically conductive monofilaments as can be used in the invention.

[0042] Figure 3 shows an example of a heating cable that can be used in the invention.

Mode(s) for Carrying Out the Invention

[0043] Figure 1 shows an example of a vehicle seat heating element 10 according to the

invention. The element comprises a power supply 1 1 (e.g. via a connection to the battery of the vehicle), a control circuit 13 and a heating cable 15 connected to the control circuit 13 via means 17 for feeding electrical current to said cable. The means 17 for connecting the heating cable can be of any type known in the art, e.g. via crimped connecters. The vehicle seat heating element 10 of figure 1 comprises a heating cable 15 that comprises metallic filaments, wherein a predetermined number of the metallic filaments in the heating cable are each individually wrapped with one or more non-electrically conductive filaments or individually wrapped with non-electrically conductive fibers or individually wrapped with non-electrically conductive tapes. The wrapped metallic filaments are electrically connected in parallel in the means 17 for feeding electrical current so that each of the wrapped metallic filaments is fed with electrical current.

Vehicle seat heating elements according to the invention can comprise more than one heating cable, wherein the heating cables are mounted in serial or in parallel connection or in a combination of serial and parallel connections.

[0044] Figure 2 shows a wrapped metallic filament 20 that can be used in the invention. The metallic filament is of the core-sheath type, with a core 22 of a first metal or metal alloy and a sheath 24 of a second metal or metal alloy. In one example, the core 22 can be steel and the sheath 24 copper or copper alloy. In another example, the core 22 can be copper or copper alloy and the sheath 24 can be a steel layer, e.g. a stainless steel layer. In yet another example, the core 22 can be low carbon steel wire and the sheath 24 can be zinc. The metallic filament is wrapped by two non-electrically conductive filaments or by two non-electrically conductive tapes 26 and 28. The first non-electrically conductive filament or tape 26 is wrapped in Z-direction and the second non-electrically conductive filament or non-electrically conductive tape 28 is wrapped in S-direction around the metallic filament.

[0045] Figure 3 shows a heating cable 300 that can be used in the vehicle seat heating element of the invention (e.g. as shown in figure 1 ). Metallic filaments 310 are wrapped by non- electrically conductive filaments or non-electrically conductive tapes 312. The wrapped metallic filaments are twisted together to form yarns or strands 314. The yarns or strands 314 are twisted together to form a heating cable which is coated afterwards to have a polymeric sheath 316. The polymeric sheath can e.g. be in polyamide 12 or in TPE. [0046] An example of a heating cable that can be used in the invention (e.g. as shown in figure 1 ) is a heating cable of the construction 4*7, with copper clad steel filaments (and specifically CCS40, which is a metallic filament with a steel core and a copper sheath and having 40% of the electrical conductivity of a copper filament of the same diameter) of 63 μιη diameter wrapped with two 87 dtex polyester monofilaments. Each of the two 87 dtex polyester filaments is wrapped with 750 turns per inch (27755 turns per meter) around the metallic filament, one in S-direction, the other in Z-direction. Seven of these wrapped metallic filaments are twisted together with 40 turns per meter (in Z-direction) to form a strand, four of these strand are twisted together with 60 turns per meter (in S-direction) to form the heating cable. In a comparative folding endurance test the heating cable had a four times longer flex life than the same heating cable without the wrapping with the polyester monofilaments. A 30% higher flex life was obtained than the same heating cable without the wrapping with the polyester monofilaments but with a PFA coating around the heating cable.

[0047] In another example, the wrapping is performed with four polyester monofilaments, two in S-direction around the metallic filaments and two in Z-direction.

[0048] Another example of heating cable that can be used in the invention (e.g. as in figure 1 ) is a heating cable of the construction 6*7, with copper clad steel filaments (and specifically CCS40, which is a metallic filament with a steel core and a copper sheath and having 40% of the electrical conductivity of a copper filament of the same diameter) of 63 μιτι diameter wrapped with two 87 dtex polyester monofilaments, each of the two 87 dtex polyester monofilaments is wrapped with 750 turns per inch (27755 turns per meter) around the metallic filament, one in S-direction, the other in Z-direction. Seven of these wrapped metallic filaments are twisted together with 40 turns per meter (in Z-direction) to form a strand, six of these strands are twisted together with 65 turns per meter (in S- direction) to form the heating cable. The heating cable showed to have an excellent flex life.

[0049] Tables 1 to 4 provide comparative flex endurance results (in number of flexing cycles to failure, 10000 cycles being considered the acceptance criteria in this test) of heating cables and hence of vehicle seat heating elements.

Table 1 shows test results for heating cables made from 35 copper clad steel filaments of 63 μιτι each. The electrical resistance of the heating cables is 0.39 Ohm/meter.

Table 2 shows test results for heating cables made from 21 metallic filaments with a copper core and a steel sheath. The metallic filaments have a diameter of 88 μιτι. The electrical resistance of the heating cable is 0.39 Ohm/meter.

Table 3 shows test results for heating cables made with 19 copper clad steel filaments of 84 μιη each. The electrical resistance of the heating cables is 0.40 Ohm/meter. The test results indicate that it is possible to make heating cables according to the invention with metallic filaments of 84 μητι and that satisfy the flex endurance requirements.

Table 4 shows test results for heating cables made with 33 low carbon steel filaments with a zinc layer; these filaments have a diameter of 60 μιτι. The heating cable has an electrical resistance of 1.2 Ohm/meter.

Table 1 : Flex endurance results for heating cables made from 35 copper clad steel filaments of 63 μιτι each

Table 2: Flex endurance results for heating cables made from 21 metallic filaments with a copper core and a steel sheath, the metallic filaments have a diameter of 88 μιτι.

Heating cable particulars Number of cycles to failure

Unwrapped metallic filaments 1787

Each metallic filament is individually 17483

wrapped with 87 dtex polyester filaments

Unwrapped metallic filaments, the heating 5497

cable is extrusion coated with PA12

Each metallic filament is individually 33058

wrapped with 87 dtex polyester filaments

and the heating cable is extrusion coated

with PA12

Table 3: Flex endurance results for heating cables made with 19 copper clad steel filaments of 84 μητι each

Table 4: Flex endurance results for heating cables made from with 33 low carbon steel filaments with a zinc layer, filament diameter 60 μιτι

[0050] Another example of heating cable that can be used in the invention (e.g. as in figure 1 ) is a heating cable of the construction 7*7, with copper clad steel filaments (and specifically CCS40) of 63 μιτι diameter wrapped with a 12 μητι by 250 μιτι polyester tape. Seven of these wrapped metallic filaments are twisted together (in Z-direction) to form a strand, seven of these strands are twisted together (in S-direction) to form the heating cable. The heating cable was provided with an extruded PA12 coating. The diameter of the so- formed heating cable (after extrusion coating) is 1.07 mm. The heating cable showed to have an excellent flex life.

[0051] Another example of heating cable that can be used in the invention (e.g. as in figure 1 ) is a heating cable of the construction 5*7, with copper clad steel filaments (and specifically CCS40) of 80 μιτι diameter wrapped with a 12 μητι by 370 μιτι polyester tape. Seven of these wrapped metallic filaments are twisted together (in Z-direction) to form a strand, five of these strands are twisted together (in S-direction) to form the heating cable. The heating cable was provided with an extruded PA12 coating. The diameter of the so- formed heating cable (after extrusion coating) is 1.16 mm. The heating cable showed to have an excellent flex life.

[0052] The experiments have shown that the vehicle seat heating elements that include the cited examples of heating cables have efficient hot-spot prevention. When the heating cable of the vehicle seat heating element is dynamically flexed up to failure of individual filaments, the electrical resistance of the heating cable increased, which clearly indicates that the broken metallic filament(s) are not making contact with unbroken metallic filaments, which if it would occur, would lead to hot-spots.

[0053] Elements and features of the different embodiments and examples can be combined while staying within the content and scope of the invention.

Claims

1. Vehicle seat heating element comprising a heating cable and means for feeding electrical current to said heating cable, wherein said heating cable comprises metallic filaments, wherein a predetermined number of said metallic filaments in said heating cable are each individually wrapped with one or more non-electrically conductive filaments or individually wrapped with non-electrically conductive fibers or individually wrapped with one or more non-electrically conductive tapes.

2. A vehicle seat heating element as in claim 1 , wherein all metallic filaments in said heating cable are each individually wrapped with one or more non-electrically conductive filaments or by non-electrically conductive fibers or by one or more non-electrically conductive tapes.

3. A vehicle seat heating element as in any of the preceding claims, wherein said wrapping is performed in Z- and in S-direction around the axis of said metallic filament.

4. A vehicle seat heating element as in any of the preceding claims, wherein said wrapping is covering at least 50% of the surface of the metallic filament being wrapped.

5. A vehicle seat heating element as in any of the preceding claims, wherein said heating cable comprises metallic filaments in twisted or cabled form.

6. A vehicle seat heating element as in any of the preceding claims, wherein said non-electrically conductive fibers or non-electrically conductive filaments or non-electrically conductive tapes comprise polymeric filaments or polymeric fibers or polymeric tapes or wherein said non- electrically conductive fibers or non-electrically conductive filaments or non-conductive tapes comprise fibers or filaments or tapes with a polymeric sheath and wherein at least part of said non-electrically conductive fibers or non-electrically conductive filaments or non-electrically conductive tapes are molten or softened and then solidified to form a polymeric layer around said metallic filament.

7. A vehicle seat heating element as in any of the preceding claims, wherein said heating cable has a polymeric sheath.

8. A vehicle seat heating element as in claim 7, wherein said polymeric sheath is devoid of fluorine in the polymer.

9. A vehicle seat heating element as in claim 8, wherein said polymer sheath is polyamide 12 or TPE.

10. A vehicle seat heating element as in claims 7, 8 or 9, wherein the polymeric sheath of said heating cable is formed by melting or softening followed by solidification of at least part of said non-electrically conductive fibers or non-electrically conductive filaments or non-electrically conductive tapes.

1 1. A vehicle seat heating element according to any of the preceding claims, wherein the

resistance of said heating cable is below 3 Ohm/meter.

12. A vehicle seat heating element according to any of the preceding claims, wherein at least part of the metallic filaments are stainless steel filaments or wherein at least part of the metallic filaments are metallic filaments comprising a copper or copper alloy layer and a steel layer.

13. A vehicle seat heating element as in any of the preceding claims, wherein the diameter of the metallic filaments is in the range of 30 to 150 micrometer.