Composite material, especially for reflectors

Details Composite material, especially for reflectors Composite material, especially for reflectors

2000-07-11

2003-09-30

Thibodeau, Paul (Department: 1773)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Physical dimension specified

C428S336000, C428S413000, C428S414000, C428S416000, C428S457000, C428S458000, C428S469000, C428S912200, C428S913000

Reexamination Certificate

active

06627307

ABSTRACT:

The present invention relates to a composite material, in particular for reflectors, having a metallic support in strip form, an intermediate layer and having a multilayer optical system which is applied to the intermediate layer.
To characterize the reflective properties of a composite material of this nature, the spectral coefficient of the total reflection and that of the diffuse reflection are measured (DIN 5036). Different demands are imposed on the optical properties of reflectors. In many cases, the aim is to have a composite material with a high total reflectivity, i.e. with low reflection losses. If, in addition, a mirror nature, i.e. a low scattering of the light at the reflector surface or a low level of diffuse reflection, is required, the production of such a composite material involves a very high level of outlay in production engineering terms. In some cases, a higher level of diffuse reflection, and in certain circumstances even an absorptive behaviour with regard to the total reflectivity, is also desired. In some special applications, the electromagnetic radiation wavelength which is to be reflected may lie in the UV or in the IR range.
The base material used for reflectors with a high total reflectivity and a low diffuse component, is generally rolled aluminium of a minimum purity of 99.8%, for which, since the crude aluminium has a sensitive surface, an intermediate layer has to be applied in order to protect against mechanical and chemical influences and thus maintain its suitability for use. Such a protective intermediate layer is formed in a wet-chemical process which is referred to overall as anodizing and comprises an electrobrightening treatment and an anodic oxidation. By changing the purity and/or the roughness depth, it is possible to influence the level of total reflection, while the level of diffuse reflection can be influenced by making controlled changes to the rolled structure. In the wavelength range of visible light (380 to 780 nm), the total reflection of composite materials of this nature is from 83% to 87%. Considerable process engineering outlay, primarily a high outlay on rolling technology, as well as a high level of purity of the material used, are required in order to achieve a low level of diffuse reflection, in particular of below 4%, resulting in high costs. Despite these drawbacks, this type of semi-finished product in strip form has become established throughout the world as a standard product for the lighting industry.
Furthermore, it is known, when using less high-grade aluminium support material in strip form, to apply to the support a layer of eloxal which consists of Al
2
O
3
in a thickness of approx. 1 to 3 &mgr;m and is then applied to a multilayer optical system. The reduced thickness eloxal layer allows the surface to be sufficiently rough and hard and to be free from defects. A highly reflective high-purity aluminium layer is deposited on this eloxal layer. This high-purity aluminium layer is optically dense, about 50 to 80 nm thick and has a total reflection of more than 91% in the visible light range. Yet more layers which increase the reflection can be applied to this reflective layer. For example, a subsequent continuous strip coating which is described in the journal “Metalloberfläche” 50 (1996), 10, improves the optical efficiency of such aluminium strips up to a total light reflectivity of approx. 95%. However, the diffuse light reflectivity of material with a mirror nature is improved only to an insignificant extent. The optically active surface has a good mechanical load-bearing capacity and sufficient chemical resistance for use as a reflector material. However, the intermediate eloxal layer which serves as a smoothing layer is relatively brittle, so that there is a risk of cracks being formed in the event of extreme mechanical deformation processes.
A significant drawback of the two known optically active composite materials described above is that they are limited to aluminium as the support material. This entails complex, sometimes environmentally disadvantageous process steps during production, such as wet-chemical brightening, anodizing and, if appropriate, the vacuum coating or the need to use high-purity, specially rolled and therefore expensive starting material.
U.S. Pat. No. 5,527,562 has disclosed, as the intermediate layer, a levelling application to an aluminium support which serves as a substrate for a subsequent vacuum coating with an optically reflective layer system. This application is of an organic silicon sol which is applied to suitably pretreated aluminium. The special sol is intended to bring about good levelling of the aluminium-strip surface and high total reflectivities after the coating in vacuo by sputtering of Al, SiO
2
, TiO
2
. A drawback of this arrangement is that the use of a very specific formulation for the sol layer necessitates a high process engineering outlay.
Furthermore, it is known that curved, rigid reflector bodies (e.g. for headlights) are provided with a levelling coating layer, to which a single-layer reflective application of aluminium is then applied in vacuo, and this in turn is provided with a protective layer. A drawback of this is that the total reflectivity does not exceed that of the aluminium layer, i.e. approx. 90%, and that the resultant surface does not exhibit sufficient mechanical strength. In particular, the surface is so unable to withstand loads imposed by forming operations that under standard industrial conditions further processing would be impossible.
The present invention is based on the object of providing a composite material, in particular for reflectors, of the type described in the introduction, with which it is possible, in a less complex and therefore less expensive manner, as required, to achieve any desired total reflectivity, in particular even values of over 95%, and any desired level of diffuse reflection, in particular even values of below 4% (according to DIN 5036) and which has a high mechanical strength and chemical resistance. The composite material should not be limited to a support made from aluminium and should be highly deformable.
According to the invention, this is achieved by the fact that a non-metallic protective layer, which consists of a low-absorption material and has a thickness of from 5 to 20 nm, preferably from 5 to 10 nm, is applied to the multilayer optical system.
The protective layer may preferably be a sputtered layer, in particular a layer produced by reactive sputtering, a CVD or PECVD layer or a layer which is produced by vaporization, in particular by electron bombardment or from thermal sources. The protective layer may consist predominantly of a material with a low refractive index, such as for example SiO
2
.
The invention opens up entirely new technical and economic possibilities relating to the production of composite reflector materials. In particular, depending on requirements, it is possible, without reducing the mechanical strength of the composite material, to apply an eloxal layer or, in particular, a coating layer with a thickness which is adapted to the roughness of the support, which may characteristically lie within the range from about 2 to 20 &mgr;m, for the intermediate layer. The coating material may be a polycondensate produced on the basis of one or more monomers, a polyadduct or a polymer produced in particular by free-radical polymerization. Irrespective of the basis on which the coating-material mixture is produced, it is possible, by suitably selecting the mixture and/or the application process and/or the curing parameters, to set slight or, as far as possible, complete levelling of the support surface, resulting in a planar surface with a minimal roughness depth or a desired undulating or rough structure. Thus, with regard to its influence on the reflectivity, the coating material has the same importance as the rolled surfaces of the known aluminium strips.
In this case, the protective layer plays the major role. According to the current state of scientific knowledge in t

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