GB2304510A

GB2304510A - Flexible heating elements

Info

Publication number: GB2304510A
Application number: GB9617358A
Authority: GB
Inventors: Thomas Gilmour Graham
Original assignee: FLEXEL INTERNATIONAL Ltd
Current assignee: FLEXEL INTERNATIONAL Ltd
Priority date: 1995-08-17
Filing date: 1996-08-19
Publication date: 1997-03-19
Also published as: GB9617358D0

Abstract

A generally planar, flexible heating element, and methods of manufacturing same, comprising a generally planar electrically resistive heating layer (10) sandwiched between generally planar electrically insulating layers (12,14) and having electrically conductive contact strips extending along opposite edges of the resistive heating layer, in electrical contact therewith. The element may include an earth screening layer (20) of electrically conductive material applied to at least one of said electrically insulating layers. The laminated product may include additional composite layers (16,18) The element may also include apertures (24) formed in gaps in the resistive layer in such a manner as to improve the bonding of the various layers of the laminated product.

Description

"Improvements in or relating to Flexible Heating Elements" The present invention relates to improvements in planar, flexible heating elements.

Various types of planar, flexible heating elements of this general type are known, employing different types and arrangements of resistive layers, insulating layers and contact strips. The improvements provided by the present invention are applicable to most, if not all, heating elements of this general type, but are intended particularly to be applied to elements of the type manufactured by the present Applicants and marketed under the trade mark FLEXEL. Most particularly, the improvements are intended to be applied to FLEXEL-type heating elements in which the resistive heating layer is printed onto a substrate layer which forms one of the insulating layers sandwiching the resistive heating element.

In accordance with a first aspect of the invention, there is provided a generally planar, flexible heating element of the type comprising a generally planar electrically resistive heating layer sandwiched between generally planar electrically insulating layers and having electrically conductive contact strips extending along opposite edges of the resistive heating layer, in electrical contact therewith.

Preferably, an earth screening layer of electrically conductive material is applied to an outer surface of at least one of said electrically insulating layers.

Preferably, said earth screening layer comprises a substantially continuous layer of said conductive material. Preferably also, said conductive material is aluminium or other earth bonding conductor. Most preferably, said aluminium comprises aluminium foil.

In a preferred embodiment, said resistive heating layer is sandwiched between first and second layers of insulating material, and said earth screening layer is embedded in a third insulating layer applied to an outer surface of said first insulating layer. Most preferably, a fourth insulating layer is applied to an outer surface of said second insulating layer.

In a particularly preferred embodiment, said first insulating layer is formed from polyester, and said second, third and fourth layers are composite layers each comprising an outer layer of polyester and an inner layer of polyethylene.

In accordance with a second aspect of the invention, there is provided a method of forming a generally planar, flexible heating element of the type comprising a generally planar electrically resistive heating layer sandwiched between generally planar electrically insulating layers and having electrically conductive contact strips extending along opposite edges of the resistive heating layer, in electrical contact therewith; said method comprising the steps of laminating together first and second electrically insulating layers with said resistive heating layer and said contact strips sandwiched therebetween.

Preferably, said earth screening layer comprises a substantially continuous layer of said conductive material. Preferably also, said conductive material is aluminium or other earth bonding conductor. Most preferably, said aluminium is comprises aluminium foil.

In a preferred embodiment, said earth screening layer is embedded in a third insulating layer, and said method further comprises the step of laminating said third insulating layer to the outer surface of said first insulating layer. Most preferably, a fourth insulating layer is laminated to the outer surface of said second insulating layer simultaneously with the lamination of said third insulating layer to said first insulating layer.

In a particularly preferred embodiment, said first insulating layer is formed from polyester, said second, third and fourth layers are composite layers each comprising an outer layer of polyester and an inner layer of polyethylene, and the lamination of the respective layers includes the application of heat such that said layers are bonded together by said polyethylene.

In accordance with a third aspect of the invention, there is provided a generally planar, flexible heating element of the type comprising a generally planar electrically resistive heating layer sandwiched between generally planar electrically insulating layers and having electrically conductive contact strips extending along opposite edges of the resistive heating layer, in electrical contact therewith; wherein a plurality of apertures are formed in said heating element.

Preferably, said resistive heating layer includes a plurality of gaps therein, and said apertures are formed in said gaps. Most preferably, said resistive heating layer comprises a plurality of substantially parallel strips extending between and substantially at right angles to said contact strips, said gaps being formed between said substantially parallel strips.

Preferably also, said apertures are substantially circular.

In a preferred embodiment, said resistive heating layer is sandwiched between first and second layers of insulating material, and third and fourth insulating layers are applied to outer surfaces of said first and second insulating layers. Most preferably, said apertures each comprise a first aperture formed in said first and second insulating layers, and a second, smaller aperture, of similar shape to said first aperture and concentric therewith, formed in said third and fourth insulating layers such that the periphery of said second aperture sealingly encloses the periphery of said first aperture.

Preferably also, the heating element includes electrically insulating edge portions formed by said insulating layers and extending along the outer edges of said resistive heating layer, at least some of said apertures being formed in said edge portions.

The heating element may further include an earth screening layer.

In accordance with a fourth aspect of the invention, there is provided a method of forming a generally planar, flexible heating element of the type comprising a generally planar electrically resistive heating layer sandwiched between generally planar electrically insulating layers and having electrically conductive contact strips extending along opposite edges of the resistive heating layer, in electrical contact therewith; said method comprising the steps of laminating together first and second electrically insulating layers with said resistive heating layer and said contact strips sandwiched therebetween, and forming a plurality of apertures in said heating element.

Preferably, said resistive heating layer is formed with a plurality of gaps therein, and said apertures are formed in said gaps. Most preferably, said resistive heating layer comprises a plurality of substantially parallel strips extending between and substantially at right angles to said contact strips, said gaps being formed between said substantially parallel strips.

Preferably also, said apertures are substantially circular.

In a preferred embodiment, third and fourth insulating layers are applied to outer surfaces of said first and second insulating layers. Most preferably, said apertures each comprise a first aperture formed in said first and second insulating layers, and a second, smaller aperture, of similar shape to said first aperture and concentric therewith, formed in said third and fourth insulating layers such that the periphery of said second aperture sealingly encloses the periphery of said first aperture.

The method may further include the application of an earth screening layer.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a schematic, exploded side view illustrating the construction of a heating element in accordance with the present invention; Fig. 2 is a schematic plan view of half of the width of a heating element having the construction illustrated in Fig. 1; and Fig. 3 is schematic, sectional side view of one of a plurality of apertures formed in the heating element of Figs. 1 and 2; and Fig. 4 is a schematic, exploded side view illustrating an alternative embodiment of a heating element without an earth screening layer.

Referring firstly to Fig. 1, a heating element in accordance with a preferred embodiment of the invention comprises a flexible, electrically resistive heating layer 10 sandwiched between first and second layers 12 and 14 of electrically insulating material, and third and fourth outer layers 16 and 18 of insulating material. An earth screening layer 20 of electrically conductive material is embedded within at least the third insulating layer 16.

In this embodiment, the resistive heating layer 10 is printed onto the inner surface of the first insulating layer 12. The second, third and fourth insulating layers 14, 16 and 18 are composite layers, each comprising an outer layer of polyester 14a, 16a and 18a, and an inner layer of polyethylene 14b, 16b and 18b.

The earth screening layer 20 preferably comprises a layer of aluminium foil or other earth bonding conductor which is bonded to the inner surface of polyester layer 16a prior to the lamination together of the various layers 12, 14, 16 and 18. The earth screening layer might alternatively be provided by other means, such as vacuum deposition. However, this would provide a layer which is thinner and less robust than that provided by a layer of foil.

The heating element is formed by firstly printing the resistive heating layer 10 onto the inner surface of the first insulating layer 12. Electrically conductive contact strips 22 (Fig. 2) are then applied along opposite edges of the resistive heating layer in electrical contact therewith. The first insulating layer 12 is then laminated together with the second insulating layer 14. The third insulating layer 16, including the earth screening layer 20 (when such a layer is included), and the fourth insulating layer 18, are then laminated simultaneously to the outer surfaces of the first and second insulating layers respectively.

Heat is applied during lamination, such that the various layers are bonded together by the polyethylene layers 14b, 16b, 18b.

The bond between the aluminium layer 20 and the third insulating layer 16 is of relatively low strength, to enable electrical contacts to be made with the contact strips, for connecting the heating element to a power supply. Such contacts may be established using metallic crimps which penetrate the insulating layers to make contact with the contact strips 22, as is known in the art.

The heating element is adapted to be manufactured as a continuous strip from rolls of the insulating materials, using well known techniques, with the contact strips 22 extending parallel to the lateral edges of the strip. The strip can then be cut to form panels of the required length. The outer insulating layers 18a and 16a provide a "double-insulated" product. An earth screening layer might also be applied to a "single-insulated" product, such as that provided by the basic sandwich of the first and second insulating layers 12 and 14.

The contact strips 22 may be applied by any of a number of known techniques, such as are disclosed in GB-A1511874 or WO-A-90/09086.

In an alternative embodiment, a plurality of apertures 24 are formed in the element, as shown in Fig. 2. The resistive heating layer 10 is applied in a pattern having gaps 26 therein, and the apertures 24 are formed in the gaps 26 in the resistive heating layer 10. In this example, the resistive heating layer is formed as a plurality of parallel strips 28, extending between and substantially at right angles to the contact strips 22. The gaps 26 lie between the parallel strips 28, and the apertures 24 are formed along the length of the gaps 26. In this example the heating element also includes an insulating portion 30 formed by the laminated insulating layers between the contact strips 22 and the lateral edges of the element. Apertures 24 are formed in this portion also.

The apertures provide a means of securely bonding the heating element to a structure within which the element is to be incorporated, such as in the situation where the element is used to provide underfloor heating and is to be located between a concrete surface and flooring tiles laid upon the concrete surface. The cement which would normally be used to bond the tiles to the underlying surface penetrates through the apertures 24, allowing the tiles to be bonded securely to the concrete surface and retaining the element securely in position between the tiles and concrete surface.

The apertures 24 may be formed simply by punching through the heating element following lamination of the various layers. In the present example, however, the apertures are formed by punching a set of first apertures following lamination of the first and second insulating layers 12, 14, and punching a set of second apertures, similarly shaped to the first apertures but smaller and concentric therewith. This is illustrated in Fig. 3, wherein the layers of the element are designated by the same numerals as in Fig. 1, and wherein the first and second apertures are designated 24a and 24b. It will be appreciated that the thickness of the element is exaggerated in comparison with the diameter of the aperture, for clarity of illustration.

Punching the first apertures 24a following lamination of the first and second insulating layers 12 and 14 results in the third and fourth insulating layers 16 and 18 bonding directly to one another through the first apertures 24a during their subsequent lamination.

When the second, smaller aperture 24b is punched in the directly-bonded region of the third and fourth insulating layers 16 and 18, the periphery of the second aperture 24b effectively encapsulates the periphery of the first aperture 24a. This arrangement provides improved protection for the resistive heating layer 10 against, for example, water penetration, than would be the case if the apertures 24 were simply punched through the completed heating element after all of the layers had been laminated.

The preferred embodiment of the invention, including the double insulating layers previously referred to, provides a heating element which is relatively tough and resistant to damage during installation and subsequent use.

In the embodiment of Fig. 4, the earth screening layer 20 is omitted, for use in applications where such a layer is not required or is undesirable.

Improvements and modifications may be incorporated without departing from the scope of the invention.

Claims

1. A generally planar, flexible heating element of the type comprising a generally planar electrically resistive heating layer sandwiched between generally planar electrically insulating layers and having electrically conductive contact strips extending along opposite edges of the resistive heating layer, in electrical contact therewith.

2. A heating element as claimed in Claim 1, wherein an earth screening layer of electrically conductive material is applied to at least one of said electrically insulating layers.

3. A heating element as claimed in Claim 2, wherein said earth screening layer comprises a substantially continuous layer of said conductive material.

4. A heating element as claimed in Claim 2 or Claim 3, wherein said conductive material is aluminium or other earth bonding conductor.

5. A heating element as claimed in Claim 4, wherein said aluminium comprises aluminium foil.

6. A heating element as claimed in any one of Claims 2 to 5, wherein said resistive heating layer is sandwiched between first and second layers of insulating material, and said earth screening layer is embedded in a third insulating layer applied to an outer surface of said first insulating layer.

7. A heating element as claimed in Claim 6, wherein a fourth insulating layer is applied to an outer surface of said second insulating layer.

8. A heating element as claimed in Claim 6 or Claim 7, wherein said first insulating layer is formed from polyester, and said second, third and fourth layers (if present) are composite layers each comprising an outer layer of polyester and an inner layer of polyethylene.

9. A method of forming a generally planar, flexible heating element of the type comprising a generally planar electrically resistive heating layer sandwiched between generally planar electrically insulating layers and having electrically conductive contact strips extending along opposite edges of the resistive heating layer, in electrical contact therewith; said method comprising the steps of laminating together first and second electrically insulating layers with said resistive heating layer and said contact strips sandwiched therebetween.

10. A method as claimed in Claim 9, and including the step of applying an earth screening layer of electrically conductive material to an outer surface of at least one of said electrically insulating layers.

11. A method as claimed in Claim 10, wherein said earth screening layer is embedded in a third insulating layer, and said method further comprises the step of laminating said third insulating layer to the outer surface of said first insulating layer.

12. A method as claimed in Claim 11, wherein a fourth insulating layer is laminated to the outer surface of said second insulating layer simultaneously with the lamination of said third insulating layer to said first insulating layer.

13. A method as claimed in any one of Claims 9 to 12, wherein said first insulating layer is formed from polyester, said second, third and fourth layers, where present, are composite layers each comprising an outer layer of polyester and an inner layer of polyethylene, and the lamination of the respective layers includes the application of heat such that said layers are bonded together by said polyethylene.

14. A generally planar, flexible heating element of the type comprising a generally planar electrically resistive heating layer sandwiched between generally planar electrically insulating layers and having electrically conductive contact strips extending along opposite edges of the resistive heating layer, in electrical contact therewith; wherein a plurality of apertures are formed in said heating element.

15. A heating element as claimed in Claim 14, wherein said resistive heating layer includes a plurality of gaps therein, and said apertures are formed in said gaps.

16. A heating element as claimed in Claim 15, wherein said resistive heating layer comprises a plurality of substantially parallel strips extending between and substantially at right angles to said contact strips, said gaps being formed between said substantially parallel strips.

17. A heating element as claimed in any one of Claims 14 to 16, wherein said apertures are substantially circular.

18. A heating element as claimed in any one of Claims 14 to 17, wherein said resistive heating layer is sandwiched between first and second layers of insulating material, and third and fourth insulating layers are applied to outer surfaces of said first and second insulating layers.

19. A heating element as claimed in Claim 18, wherein said apertures each comprise a first aperture formed in said first and second insulating layers, and a second, smaller aperture, of similar shape to said first aperture and concentric therewith, formed in said third and fourth insulating layers such that the periphery of said second aperture sealingly encloses the periphery of said first aperture.

20. A heating element as claimed in any one of Claims 14 to 19, wherein the heating element includes electrically insulating edge portions formed by said insulating layers and extending along the outer edges of said resistive heating layer, at least some of said apertures being formed in said edge portions.

21. A heating element as claimed in any one of Claims 14 to 20, wherein there is included an earth screening layer.

22. A method of forming a generally planar, flexible heating element of the type comprising a generally planar electrically resistive heating layer sandwiched between generally planar electrically insulating layers and having electrically conductive contact strips extending along opposite edges of the resistive heating layer, in electrical contact therewith; said method comprising the steps of laminating together first and second electrically insulating layers with said resistive heating layer and said contact strips sandwiched therebetween, and forming a plurality of apertures in said heating element.

23. A method as claimed in Claim 22, wherein said resistive heating layer is formed with a plurality of gaps therein, and said apertures are formed in said gaps.

24. A method as claimed in Claim 23, wherein said resistive heating layer comprises a plurality of substantially parallel strips extending between and substantially at right angles to said contact strips, said gaps being formed between said substantially parallel strips.

25. A method as claimed in any one of Claims 22 to 24, wherein said apertures are substantially circular.

26. A method as claimed in any one of Claims 22 to 25, wherein third and fourth insulating layers are applied to outer surfaces of said first and second insulating layers.

27. A method as claimed in Claim 26, wherein said apertures each comprise a first aperture formed in said first and second insulating layers, and a second, smaller aperture, of similar shape to said first aperture and concentric therewith, formed in said third and fourth insulating layers such that the periphery of said second aperture sealingly encloses the periphery of said first aperture.

28. A method as claimed in any one of Claims 22 to 27, further including the application of an earth screening layer.

29. A generally planar, flexible heating element substantially as hereinbefore described with reference to the accompanying drawings.

30. A method of forming a generally planar, flexible heating element substantially as hereinbefore described with reference to the accompanying drawings.