EP1484945B1 - Electrical heating cable or heating band - Google Patents

Abstract

Electrical heating cable or tape with insulating polytetrafluoro-ethylene (PTFE) sheaths in a layer structure, in which at least one of the PTFE layers is protected from shock by at least one adjacent insulating layer of melt-processable fluoropolymer (I).

Description

The present invention relates to an electrical heating cable or an electrical heating tape having arranged in the layer structure insulating sheaths made of polytetrafluoroethylene.

For a variety of applications, for example, for the heating of aggressive media, heating cables in a coaxial arrangement are known (DE-A 28 50 722), in which the heating element is enclosed by a fluoropolymer as insulating material. This insulation is covered by a copper wire mesh, where the individual wires are additionally nickel-plated to avoid corrosion. This braid of the copper wires is the electrical protective conductor of the line, which is provided to avoid accident hazards, for example, by short circuit in the electrically conductive part, within the line. The protective conductor is covered by an outer plastic sheath which, for protection against aggressive media of the environment, e.g. made of a fluoropolymer. Advantage of a coaxial arrangement thus constructed, in addition to the wide range of application of this line due to the high temperature resistant and also against aggressive media insensitive materials that such lines can be produced in virtually any length with high flexibility.

DE-U-200 06 222 discloses an electrical heating cable or an electrical heating tape having layered structure of insulating sheaths made of polytetrafluoroethylene, wherein the insulation is shock-proofed by a PTFE tape in combination with a glass fiber fabric.

The same applies to known electric heating tapes (GB 2 092 420 A, GB 2 130 459 A), which are e.g. be used for so-called Rohrbegleitheizungen or on steam-purged pipes for temperature maintenance or to increase the temperature. Finally, so-called self-limiting heating tapes with a semiconductor heating element are also in use. Since the heat dissipation is controlled automatically here as a function of the ambient temperature, such heating bands are particularly suitable for use in potentially explosive areas.

But it happens again and again that due to external operating conditions when using heating cables, such as coaxial design, with external pressure load of the outer jacket squeezed so strong and the insulation is pushed away from the heating element, that protective conductor and heating conductor touch each other or the insulating distance between protective and heating conductors has become so low that it comes to glowing or spark discharges. Also, at the Damage Broken wires of the protective conductor get into the insulation and thus lead to failure of the entire heating cable. These are criteria that must be taken into account, in particular, in the case of heating cables which are used in explosion-proof systems and to which special safety requirements are therefore made as preventive explosion protection. However, these criteria must also be taken into account with regard to the applicable standards (DIN VDE 0170/0171, EN 50014 and EN 50019), which require, for example, a protective conductor which ensures sufficient coverage of the surface of the conductor insulation, as well as a separate crimp test with a following Testing the insulating capacity of the conductor insulation. An increase in the wall thickness of both the insulation and the outer shell to avoid these difficulties does not help here, apart from the fact that these measures lead to a significant increase in diameter of the entire line and at an increased cost due to the larger amount of fluoroplastic.

A resistant to external mechanical stress electrical heating cable with coaxial layer structure is already known (DE-ES 101 07 429). A glass ceramic tape layer in the layer structure above the conductor insulation of this cable should provide protection against mechanical damage in conjunction with a likewise air-permeable reinforcement layer. On both sides of these two layers air-impermeable layers of an extrudable fluoropolymer are provided so that an air cushion can form between them. Apart from this complex and the cable diameter-increasing layer construction leads the deliberately induced air cushion in Kabelinnem to a significant impairment of the heat conduction away from the heating element to the cable surface and thus to a deterioration of the efficiency of the heating cable itself.

In order to avoid this, but nevertheless to meet the requirements of the applicable standards for sufficient resistance to external shock or pressure stresses, it has already been proposed (EP 0 609 771 B1), below and / or above the protective conductor in a generic electrical heating line a or several layers of a strip of high mechanical strength plastics, such as polyimide provide. Such a winding is able to withstand high compressive stresses external shocks are absorbed damped, damage to the conductor insulation is avoided.

Based on this prior art, the invention is another way to protect located in the layer structure of a heating cable or a heating tape sheaths (conductor insulation, sheath, outer protective sheath) made of polytetrafluoroethylene even at maximum mechanical force by impact or crushing stresses.

According to the invention, at least one of the polytetrafluoroethylene sheaths is shock-proofed by at least one adjacent insulating layer of a melt-processable fluoropolymer. The invention is based on the recognition that sufficient protection against external mechanical stresses can be achieved by the juxtaposition of polymer layers of the same polymer family, but different polymer structure. Thus, according to the proposal of the invention, the polytetrafluoroethylene with its fibrous, so-called fibrils, polymer structure protected by the adjacent thermoplastic polymer with its amorphous structure. This is due to the fact that in shock or impact stresses in contrast to the fiber structure, the amorphous polymer structure impact absorbing.

An advantageous embodiment of the invention results in an electrical heating cable in a coaxial arrangement with a central conductor, an insulation made of polytetrafluoroethylene and a protective conductor in the form of stranded or -geflochtener wires and an outer protective sheath when the polytetrafluoroethylene - insulation, one or more layers, is shock-proofed by at least one adjacent insulating layer of a melt processable fluoropolymer.
A particularly advantageous embodiment of a heating line in a coaxial arrangement results according to the invention when, below the conductor enclosing polytetrafluoroethylene - insulation, and thus directly on the conductor itself, the shock-absorbing insulating layer is arranged from a processable from the melt fluoropolymer. The use of similar materials for mechanical protection compared to the known heating cables or lines, the continuous temperature resistance, a necessary property for heating cables, significantly increased. The heating line according to the invention has no air cushion in the layer structure, the heat generated by the conductor heating thus reaches without much heat accumulation to the cable / line surface, ie where it is needed. The line construction is production-technically problem-free, by the extruded polymeric protective layers the line diameters can be kept small.

Since, as a rule, the polytetrafluoroethylene insulation is subjected to a temperature treatment for the purpose of sintering the polymer material, the shrinkage of the polytetrafluoroethylene caused thereby results in compaction of the layer structure. The line is therefore, in contrast to the known heating cable with air cushion, also longitudinally watertight, while known glass fiber braids, mica tapes or inorganic films still have an undesirable wicking and therefore provide an ideal moisture transport.

As stated above, in addition to the described heating cables in a coaxial design and electrical heating bands of various embodiments in use. If such a heating tape consists, for example, of parallel-guided feed cores and a conductor contacting these conductors at intervals heating coil and an intermediate sheath and / or an outer sheath made of polytetrafluoroethylene, then in carrying out the invention at least one sheath layer by at least one adjacent insulating layer of a processable from the melt Fluoropolymer shock resistant.
In another variant of a heating strip with parallel, uninsulated feeders and a guided parallel to these and contacted with the feeders at intervals heating wire and a common envelope made of polytetrafluoroethylene, the sheath is shock-protected by at least one adjacent insulating layer of a processable from the melt fluoropolymer according to the invention ,
Self-limiting heating tapes have proved to be advantageous for special applications, for example in explosion protection, in these heating tapes with parallel, uninsulated feeders and a surrounding semiconductive sheath and a common insulation and / or an outer protective sheath made of polytetrafluoroethylene in turn, the common insulation and / or Protective jacket shockproof according to the proposal of the invention.

The purpose of the longitudinal water-tightness and compactness of the heating lines or heating strips according to the invention, in continuation of the invention, is also to weld or bond the shock-absorbing insulating layers with the polytetrafluoroethylene sheathings. At the same time the bending fatigue strength of such arrangements is substantially increased.
The thickness of the impact absorbing layer is 0.1 to 0.8 mm, preferably 0.2 to 0.5 mm. The thickness to be selected in the case of heating cables in coaxial arrangement and on the conductor immediately located shock absorbing insulating layer depends essentially on the respective conductor diameter. For example, with a conductor diameter of 1.5 mm, the impact absorbing layer is 0.2 mm.

The invention offers particular advantages even if the conductor insulation consists of a polytetrafluoroethylene tape wound with overlap of the edges, for example with a rectangular cross section. In this case, according to the invention, the gussets formed by the winding of the band are filled with the fluoropolymer of the impact absorbing layer. The adhesion of the adjoining layers is improved, the further compactness gained thereby ensures a high stability of the line against kinking and bending.

The shock-absorbing layer according to the invention consists of a melt processable fluoropolymer. Since, in the case of a generic heating cable or a heating band, the task also depends on a high long-term temperature resistance, if appropriate also under the influence of aggressive media, it is advantageous to produce the impact-absorbing layer of a tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (TFA / PFA). However, the tetrafluoroethylene / hexafluoropropylene copolymer (FEP) or the polytetrafluoroethylene-perfluoromethylvinyl ether copolymer, also known under the trade name HYFLON MFA, are, depending on the field of application, advantageous polymers for carrying out the invention.

Other known melt-processable fluoropolymers, such as polyvinylidene fluoride (PVDF) or ethylene-tetrafluoroethylene (ETFE), may sometimes find advantageous use.

A particularly advantageous embodiment of the invention results in a polytetrafluoroethylene envelope from a wound polytetrafluoroethylene tape if this has a plano-convex cross-sectional shape. Compared to common bands having a rectangular cross-sectional shape, the plano-convex shape after winding and sintering of the polytetrafluoroethylene band results in a compact envelope with a closed smooth outer surface. This is of particular advantage when the outer surface is exposed to aggressive media in the environment.

Another advantageous possibility to bring about an improvement in the insulating quality compared to that of rectangular strips is to form the strip of polytetrafluoroethylene in cross-section as a flat profile, with tapering from the center to both sides edge regions and at the edges regular band. After winding the strip with overlapping edges and sintering of the strip material (PTFE) results in the leakage of the band edges in the overlap area in a particularly smooth, self-contained insulating surface. Advantageously, the edges of the band made of polytetrafluoroethylene are drawn wide, wherein the edge width on both sides of the band thickness determining middle region is at least 45%, preferably 50 to 80%, of the entire width of the band.

The thickness of the polytetrafluoroethylene tape advantageously used according to the invention is from 20 to 200 μm, preferably from 40 to 160 μm. The tape thickness decreases to the edges (edge) down to 5 microns and less. The bandwidth is expediently 5 to 50 mm, preferably 10 to 30 mm.

The same band dimensions apply with particular advantage also for the case that in addition to the insulation and the outer protective sheath consists of a wound tape made of polytetrafluoroethylene.

In this case, it may sometimes be advantageous to arrange an impact-absorbing insulating layer of a melt processable fluoropolymer below the polytetrafluoroethylene wrap layer (s). A further advantageous embodiment of the invention would be the one or both sides of the protective conductor an impact absorbing insulating layer of a processable from the melt fluoropolymer adjacent to enclose the protective conductor so with these insulating layers.

The invention will be explained in more detail with reference to the in Figures 1 and 2 as particularly advantageous embodiments of electrical heating cables in a coaxial arrangement.

To increase the flexibility of the heating cable according to the invention, the conductor 1, as shown in Figure 1, for example, a number of individual resistance wires. The conductor insulation is designated 2, it consists of a high temperature resistant polytetrafluoroethylene, the term "polytetrafluoroethylene", as above, includes cole tetrafluoroethylene polymers provided with modifying additives, but in such an amount that the polymer, such as PTFE itself, from the melt is not processable.

In the preferred embodiment of the invention, the polytetrafluoroethylene used consists of a first unsintered tape or film material wound on the heating conductor in the unsintered state, preferably with overlap, for example up to 50%, and sintered in the wound state by a corresponding temperature treatment. The individual tape layers are fused or welded into a compact insulation.

The protective conductor 3 consists of individual metallic wires, such as nickel-plated copper wires, which are stranded on the insulation 2 or woven to achieve the widest, covering over the circumference covering.

Outwardly, the heating line is completed by the jacket 4, which is useful, since such lines are also used in the sphere of influence of aggressive media, such as in the chemical industry, is made of a suitably suitable plastic material. Fluorinated polymers which are applied in extruded form or in which a winding of initially unsintered and wound wound PTFE tapes forms the outer termination of the heating cable have also proved advantageous as cladding materials.

In order to prevent now that at outer pressure load (shock) squeezed the jacket 4 and / or pushed away from the protective conductor 3, so it comes to the damage to the heating cable and possibly cable failure, is below the shell 4, the shock absorbing layer 5 is provided. This layer of extrudable fluoropolymer, amorphous in accordance with the invention, dampens the impact energy applied from the outside and thus prevents conduction damage or destruction.

A particularly advantageous embodiment of the invention is shown in FIG 2. The heating cable, again in a coaxial design, consists of the heating element 6, for example, a plurality of individual stranded together or braided resistance wires. The conductor insulation is denoted by 7, it consists in the present embodiment of one or more layers of a band of polytetrafluoroethylene (PTFE). After sintering the PTFE material, this tape wound up in the unsintered state and sintered in the wound state forms a compact, longitudinally watertight casing which is also insensitive to aggressive media, but due to the material structure is not sufficiently suitable for absorbing shock or impact stress without damage , In order to make this heating line suitable for use under extreme mechanical external loads, for example, to be able to use it in explosion-proof (endangered) systems, the invention provides the shock absorbing layer 8 of a melt processable fluoropolymer. This layer covers the conductor 6 directly, because of the smaller in relation to the diameter of the line conductor diameter, the wall thickness of the layer 8 can be kept extremely thin. Compared to the solution according to FIG. 1, this substantially saves polymer material, moreover, this embodiment leads to a reduced overall diameter compared with the above exemplary embodiment, but above all with respect to the prior art.

Due to the layer 8, which acts due to the material structure as it were as a resilient buffer layer upon impact on the line, the adjacent conductor insulation 7 is mechanically protected. The insulation is not squeezed or pushed away from the conductor 6, its insulating effect is maintained. An external shock is absorbed damped, damage to the conductor insulation 7 is not to be feared. This line construction according to the invention substantially enhances the material-specific properties of PTFE and PFA (TFA, MFA). The higher hardness of PTFE coupled with the higher elasticity of PFA, for example, lead in this composite construction to a significant increase in pressure and shock resistance or stability.

Since the substructure remains undamaged in shock / impact, there is also no risk of wire breakage within the protective conductor 9 or a failure of the heating cable by broken wires that could penetrate through a damaged insulation 7. The heating cable according to the invention therefore meets all safety requirements, in particular those for explosion protection. In addition, this heating line according to the invention is inexpensive to produce, partly because of the simplified compared to the prior art steps, partly because of the lower amounts of material, which are also still assigned to the same polymer family. A particular advantage when a high continuous temperature resistance is required, for example in hot steam cleaning plants with operating temperatures between 300 ° and 320 ° C.

The outer jacket 10 is in this embodiment also made of a winding of PTFE - bands, subjected in the wound state of a temperature treatment and thus welded or fused to form a compact enclosure. The particular cross-sectional shape of the PTFE tape provided according to the invention results in a particularly smooth, self-contained surface. A rupture of the individual tape layers under impact or impact stress is avoided by the inventive solution to arrange a shock-absorbing polymer layer of the same polymer family in the layer structure of the heating line.

The heating line according to the invention according to Figure 2 is also characterized by particularly favorable external dimensions. For a total diameter of z. B. 4.8 mm, the diameter of the conductor 6 is 1.4 mm, the wall thickness of the shock absorbing layer 8 is 0.2 mm, the insulation 7 has a wall thickness of 0.6 mm, the thickness of the braid 9 is 0, 4 mm and the jacket 10 has a wall thickness of 0.5 mm.
Notwithstanding the preferred embodiment of FIG. 2, other variants are possible. Thus, insulating layers of PTFE and PFA, for example, in the layer structure of the heating line alternate, such as PTFE / PFA / PTFE or PFA / PTFE / PFA, the prerequisite is as in the embodiments that these insulating layers adjacent to each other.

The effect of the invention can also be achieved, if notwithstanding the illustrated embodiments, known heating cable or heating cables, too in deviating from the coaxial design embodiments are resistant to shock and pressure stresses and adjacent to the PTFE cladding used there according to the invention insulating layers of melt processable fluoropolymers.

Claims (22)

1. Electric heating cable or electric heating band comprising insulating sheaths (2, 4, 7, 10) made of polytetrafluoroethylene arranged in a layered structure, characterised in that at least one of the polytetrafluoroethylene sheaths (2, 4, 7, 10) is protected against shock by at least one adjacent insulating layer (5, 8) made of a melt-processable fluoropolymer.
2. Electric heating cable according to claim 1 in coaxial arrangement with a central conductor (1, 6), an insulation (2, 7) made of polytetrafluoroethylene and a protective conductor (3, 9) in the form of twisted or woven metallic wires, and an outer protective jacket (4, 10), characterised in that the polytetrafluoroethylene insulation (2, 7), in one or more layers, is protected against shock by at least one adjacent insulating layer (5, 8) made of a melt-processable fluoropolymer.
3. Electric heating cable according to claim 2 in coaxial arrangement with a single-layer insulation (7) made of polytetrafluoroethylene surrounding the central conductor (6) and a protective conductor (9) covering it and an outer protective jacket (10), characterised in that a shock-absorbing insulating layer (8) made of a melt-processable fluoropolymer is arranged below the polytetrafluoroethylene insulation (7), and therefore directly on the conductor (6).
4. Electric heating band according to claim 1 with parallel feed strands consisting of insulated electric conductors and a heater coil contacting the conductors of these feed strands at intervals and an intermediate jacket and/or an outer jacket made of polytetrafluoroethylene, characterised in that at least one jacket layer is protected against shock by at least one adjacent insulating layer made of a melt-processable fluoropolymer.
5. Electric heating band according to claim 1 with parallel, uninsulated feed conductors and a heater wire guided parallel thereto and contacting the feed conductors at intervals and a common sheath made of a polytetrafluoroethylene, characterised in that the sheath is protected against shock by at least one adjacent insulating layer made of a melt-processable fluoropolymer.
6. Electric heating band according to claim 1 with parallel, uninsulated feed conductors and a semi-conducting sheath surrounding them and a common insulation and/or an outer protective jacket made of polytetrafluoroethylene, characterised in that the common insulation and/or the protective jacket is protected against shock by at least one adjacent insulating layer made of a melt-processable fluoropolymer.
7. Heating cable according to claim 2 or 3, characterised in that the thickness of the shock-absorbing insulating layer (5, 8) is 0.1 to 0.8 mm, preferably 0.2 to 0.5 mm, depending on the respective conductor diameter (1, 6).
8. Heating cable or heating band according to claim 1, or any one of the following claims, characterised in that the shock-absorbing insulating layer (5, 8) is welded or glued to the polytetrafluoroethylene sheath (insulation, jacket) (2, 4, 7, 10).
9. Heating cable or heating band according to claim 1 or any one of the following claims, with a sheath (2, 4, 7, 10) made of a polytetrafluoroethylene band wound with overlapping of the edges, characterised in that the interspaces formed by the winding of the band are filled with the fluoropolymer of the shock-absorbing insulating layer (5, 8).
10. Heating cable or heating band according to claim 1 or any one of the following claims, characterised in that the shock-absorbing insulating layer (5, 8) consists of a tetrafluoroethylene/perfluoroalkylvinylether copolymer (TFA/PFA).
11. Heating cable or heating band according to claim 1 or any one of the following claims, characterised in that the shock-absorbing insulating layer (5, 8) consists of a tetrafluoroethylene/hexafluoropropylene copolymer (FEP).
12. Heating cable or heating band according to claim 1 or any one of the following claims, characterised in that the shock-absorbing insulating layer (5, 8) consists of a polytetrafluoroethylene/perfluoromethylvinylether (MFA).
13. Heating cable or heating band according to claim 1 or any one of the following claims, characterised in that the polytetrafluoroethylene of the sheath (2, 4, 7, 10) is sintered.
14. Heating cable or heating band according to claim 9, characterised in that the band made of polytetrafluoroethylene has a rectangular cross-sectional shape.
15. Heating cable or heating band according to claim 9, characterised in that the band made of polytetrafluoroethylene has a plano-convex cross-sectional shape.
16. Heating cable or heating band according to claim 9 characterised in that the band made of polytetrafluoroethylene is configured with a flat cross-sectional profile, with edge regions tapering from the centre to either side and a regular band course at the edges.
17. Heating cable or heating band according to claim 16, characterised in that the edges of the band made of polytetrafluoroethylene are drawn-out widthways, the edge width on either side of the central region determining the band thickness being at least 45%, preferably 50 to 80% of the total width of the band.
18. Heating cable or heating band according to claim 15 or any one of the following claims, characterised in that the thickness of the band made of polytetrafluoroethylene is 20 to 200 µm, preferably 40 to 160 µm, which decreases towards the margins (edge) to 5 µm and less.
19. Heating cable or heating band according to claim 18, characterised in that the width of the band made of polytetrafluoroethylene is 5 to 50 mm, preferably 10 to 30 mm.
20. Heating cable or heating band according to claim 1 or any one of the following claims, characterised in that the outer protective jacket consists of a wound band made of polytetrafluoroethylene.
21. Heating cable or heating band according to claim 20, characterised in that a shock-absorbing insulating layer made of a melt-processable fluoropolymer is arranged below the winding layer(s) made of polytetrafluoroethylene.
22. Heating cable or heating band according to claim 2 or any one of the following claims, characterised in that, on one or both sides, a shock-absorbing insulating layer made of a melt-processable fluoropolymer adjoins the protective conductor.

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