GB2263556A - Heatable mirrors - Google Patents

Abstract

A heatable mirror comprises a substrate (1) bearing a reflective coating (2) and incorporating a heating element. At least two current collectors (3, 4) are formed on or bonded to the substrate at the rear thereof, separated from the reflective coating by a dielectric layer (7) and a copper layer (6). Conductive material (5) is deposited on the substrate over the current collectors thereby to form in situ an electric resistance heating element extending between the current collectors. <IMAGE>

Description

HEATABLE MIRRORS This invention relates to heatable mirrors comprising a substrate bearing a reflective coating and incorporating a heating element.

It is known to provide mirrors with a conductive heating element on their rear faces and to glue current collectors in electrical contact with that conductive heating element for the supply and distribution of heating current. In such known heatable mirrors it is of course necessary to use an electrically conductive glue. The composition of such glues is a compromise as between conductivity, which is afforded by a conductive filler, usually silver, and adhesive properties, which latter are diminished as the conductive filler content increases. Clearly, increasing the proportion of a filler such as silver will have a marked effect on the cost of such glues.

Such glues are not easy to apply in such a manner that their conductivity in situ is uniform unless they contain a rather large quantity of conductive filler which makes them expensive and tends to give insufficient adhesive bond strength.

This is unsatisfactory, and it is an object of this invention to provide a heatable mirror which does not suffer from this disadvantage.

According to the present invention, there is provided a heatable mirror comprising a substrate bearing a reflective coating and incorporating a heating element, characterised in that at least two current collectors are formed on or bonded to said substrate, separated from said reflective coating by a dielectric layer, and in that conductive material is deposited on the substrate over said current collectors thereby to form in situ an electric resistance heating element which extends between said current collectors and is electrically isolated from said reflective coating.

The invention avoids any necessity for a compromise as to the properties of any adhesive which may be used for securing the current collectors in place. The collectors, if not formed in situ on the rear of the substrate, may be bonded thereto using a glue which may be selected solely according to its adhesive properties. There is no need to use, and indeed no point in using, a conductive glue for gluing the collectors to a dielectric layer, so the expense of any such glue is avoided. We have found to our surprise that a conductive material may be deposited on the substrate over said current collectors with a good electrical contact and a good bond between the conductive material and the collectors, and without giving rise to any problems due to differential thermal expansion which would tend to disrupt that contact.

In some preferred embodiments of the invention, said reflective coating and said current collectors are applied to opposite faces of a dielectric substrate. In such embodiments, the substrate itself serves as said dielectric layer, and the mirror thus provided is of the "front- silvered" type. In such embodiments, the current collectors may be constituted by metallised strips1 for example of copper, which are formed in situ on the substrate.

In other preferred embodiments of the invention, a dielectric layer is applied on top of said reflective layer and said current collectors are bonded to that dielectric layer. Thus the invention also provides a "rearsilvered" mirror in which the reflective layer lies behind, and is therefore protected by, the substrate. Of course such substrate must be transparent. In such embodiments, the current collectors may suitably be constituted by strips of metallic or metallised tape which are glued to the dielectric layer.

In embodiments of the invention constituted as "rear-silvered" mirrors, it is preferred that said substrate bears a polymeric coating overlying said reflective coating and extending at least part way over side edge faces of the substrate. Such polymeric coating has the advantage of affording a measure of protection to the edges of the reflective layer against corrosion due to atmospheric pollutants.

In some such preferred embodiments of the invention, said dielectric layer is constituted by said polymeric coating. This is one very suitable way of providing such edge protection without increasing the number of layers of material required to be applied to the substrate.

Preferably, said conductive material is applied to form a conductive film extending between said collectors. A conductive film may be applied easily, and it allows uniform dissipation of heat when in use. Uniform dissipation of heat over the area of the heating element is beneficial for the avoidance of "hot spots". The uniform dissipation of heat is easier to achieve when using a conductive film than when the heating element is of some other form, such as a ladder-like array of parallel conductive trips extending between the current collectors, and furthermore, the uniformity of such dissipation is also less susceptible to disruption with a film than with a ladder-like array should the heating element become damaged.

Advantageously, said conductive material is applied as a paint.

Such paint will naturally adhere to subjacent current collectors and to the area of the substrate between such collectors to form a conductive film without any special additional measures being taken. Such a coating may readily be applied by any conventional painting technique, for example by brushing, spraying or rolling, but in order to promote uniformity of coating thickness, it may sometimes be thought preferable to use a curtain coating technique.

In some embodiments of the invention wherein said substrate bears a polymeric coating overlying said reflective coating and extending at least part way over side edge faces of the substrate, and said conductive material is applied to form a conductive film extending between said collectors, it is preferred that said conductive film is constituted by said polymeric coating. This presents useful economies in manufacture by saving on the number of layers which must be applied to the substrate for achieving the same result. Such embodiments are especially useful in low-voltage systems, for example 12 volt systems, where it is not essential for safety or other reasons that the heating element be insulated.

In other preferred embodiments, said conductive material is covered with an electrically insulating overcoating. This affords electrical insulation to the heating element which is particularly desirable in systems which are to operate at other than low voltages, and it also helps to avoid any operating problems due to condensation or icing on the heating element when the heater is first switched on.

In such embodiments, a saving in the number of layers to be applied to the mirror may be achieved if, as is preferred, said electrically insulating overcoating is constituted by a said polymeric coating.

Preferred embodiments of the invention will now be described by way of Example only, and with reference to the accompanying diagrammatic drawings, in which: Figures 1 to 3 are cross-sectional views of three embodiments of heatable mirror in accordance with this invention.

EXAMPLE 1 In Figure 1, a sheet of glass 1 is cut to size and then has a reflective layer 2 applied to a face thereof to form a "front-silvered" mirror. Current collectors 3, 4 are applied to the rear face of the glass sheet 1. Such collectors may be constituted by metallised strips for example of copper or by conductive enamel strips deposited directly onto the glass, or they may be formed by strips of a metal such as copper which are bonded to the mirror using an adhesive. Such metal strips msy for example be in the form of a metallised self-adhesive foil which is applied to the glass.The rear face of the glass sheet 1 is then coated with a layer of conductive paint which is allowed to dry to form a conductive film constituting an electric resistance heating element 5 extending between the collectors 3, 4, which heating element is separated from the reflective layer 2 by the glass sheet 1. A suitable conductive paint for forming the resistance heating element 5 is that marketed by Yamaichi International (Europe) Limited of London, under their trade designation "Exothermic Paint MRX-OOl".

In a variant of this Example, the reflective layer 2, the current collectors 3, 4 and the paint layer 5 are all applied to a large glass sheet before that sheet is cut to size to form mirrors as illustrated.

In another variant, the glass from which the sheet 1 is cut is in the form of a spherical bowl, so as to form a curved mirror.

EXAMPLE 2 In Figure 2, a sheet of glass 1 has applied to its rear face a reflective layer 2 of silver which is overcoated by a protective layer of copper 6. The mirror thus formed is an unpainted conventional "rear-silvered" mirror which is ready for the application of a dielectric protective paint layer and heating means in accordance with this invention. It may be convenient to form such a mirror on a large sheet of glass and then cut that sheet to size.

A dielectric protective paint layer 7 is applied on top of the copper layer in such a way that it extends also to cover the edges of the previously deposited reflective silver layer 2. Current collectors 3, 4 are applied to the rear face of the dielectric paint layer 7. Such collectors are suitably formed by strips of metallised self-adhesive foil which is bonded to the dielectric paint layer 7.The rear face of the glass sheet 1 is then coated with a layer of conductive polymeric paint, for example that marketed by Yamaichi International (Europe) Limited of London, under their trade designation "Exothermic Paint MRX-001", which is allowed to dry to form a conductive polymer film constituting an electric resistance heating element 5 extending between the collectors 3, 4, which heating element is separated 'irom the reflective layer 2 by the dielectric paint layer 7. As illustrated, the pclymeric coating 5 overlies the reflective coating 2 and the protective layers of copper 6 and paint 7 applied thereto, and also extends at least part way over side edge faces of the substrate constituted by the glass sheet 1, thus affording a measure of protection to the edges of the mirror.

In a variant, the glass from which the sheet 1 is cut is in the form of a bowl, so as to form a curved mirror.

EXAMPLE 3 In Figure 3, a sheet of glass 1 has applied to its rear face a reflective layer 2 of silver which is overcoated by protective layers of copper 6 and dielectric paint 7. The mirror thus formed is a conventional "rear-silvered" mirror which is ready for the application of heating means in accordance with this invention. Current collectors 3, 4 are applied to the rear face of the dielectric paint layer 7. Such collectors are suitably formed by strips of metallised self-adhesive foil which is bonded to the dielectric paint layer 7.

The rear face of the glass sheet 1 is then coated with a layer of conductive polymeric paint, such as that marketed by Yamaichi International (Europe) Limited of London, under their trade designation "Exothermic Paint MRX 00111, and that paint is allowed to dry to form a conductive polymer film constituting an electric resistance heating element 5 extending between the collectors 3, 4, which heating element is separated from the reflective layer 2 by the dielectric paint layer 7. It may be convenient to form such a mirror and heating element on a large sheet of glass and then cut that sheet to size. The conductive paint layer 5 is then overcoated with an electrically insulating overcoating 8 which extends at least part way over side edge faces of the substrate constituted by the glass sheet 1, thus affording a measure of protection to the edges of the reflective coating layer 2. A suitable material for forming the insulating overcoating 8 is also available from Yamaichi International (Europe) Limited of London, under their trade designation "Insulation Paint MRX-003"

Claims (10)

1. A heatable mirror comprising a substrate bearing a reflective coating and incorporating a heating element, characterised in that at least two current collectors are formed on or bonded to said substrate, separated from said reflective coating by a dielectric layer, and in that conductive material is deposited on the substrate over said current collectors thereby to form in situ an electric resistance heating element which extends between said current collectors and is electrically isolated from said reflective coating.

2. A heatable mirror according to claim 1, wherein said reflective coating and said current collectors are applied to opposite faces of a dielectric substrate.

3. A heatable mirror according to claim 1, wherein a dielectric layer is applied on top of said reflective layer and said current collectors are bonded to that dielectric layer.

4. A heatable mirror according to claim 3, wherein said substrate bears a polymeric coating overlying said reflective coating and extending at least part way over side edge faces of the substrate.

5. A heatable mirror according to claims 3 and 4, wherein said dielectric layer is constituted by said polymeric coating.

6. A heatable mirror according to any preceding claim1 wherein said conductive material is applied to form a conductive film extending between said collectors.

7. A heatable mirror according to claim 6, wherein said conductive material is applied as a paint.

8. A heatable mirror according to claim 4 and claim 6 or 7, wherein said conductive film is constituted by said polymeric coating.

9. A heatable mirror according to any preceding claim1 wherein said conductive material is covered with an electrically insulating overcoating.

10. A heatable mirror according to claims 4 and 9, wherein said electrically insulating overcoating is constituted by a said polymeric coating.