Optical: systems and elements – Mirror – With support
Reexamination Certificate
1999-02-11
2002-07-09
Shafer, Ricky D. (Department: 2872)
Optical: systems and elements
Mirror
With support
C359S359000, C359S360000, C359S584000, C359S585000
Reexamination Certificate
active
06416194
ABSTRACT:
The present invention is directed to thermostable back-surface mirrors with controllable reflectance, comprising a substantially transparent glass substrate which is coated with a thermostable mirror coating. The coated glass mirrors can be tempered and bent to a desired shape while remaining reflective. The present invention also is directed to methods for the production of such thermostable back-surface mirrors.
BACKGROUND
Reflective coatings for transparent glass substrates, such as automotive, architectural or decorative glazing, industrial mirrors, components of scientific or navigational instruments, and optical lenses, are known to those skilled in the art. It would be desirable to have a back-surface mirror which adequately retains its reflective properties under thermal stresses encountered during tempering or bending the glass substrate; however, known mirror coatings are functionally diminished in reflectance or are otherwise degraded to an unacceptable degree when subjected to such high temperatures.
According to prior known methods, the glass substrate is first tempered and shaped prior to coating. Thus, the resulting shaped articles must be coated individually, rather than in bulk as part of a single, large glass sheet. This has the substantial disadvantage of increasing production time, cost and complexity. Also, in order to coat an article which is not flat, e.g, a curvo-planar article, specialized coating apparatus is required, which results in further expense. A reflective glass coating is disclosed by Chesworth et al in European Patent Application 0 301 755. This prior art coating comprises a layer of at least one metal having an atomic number of 22 to 29 and a thin layer of aluminum applied over the metal layer, the latter for thermal protection during tempering or bending. Heat processable, metallic coatings, and vacuum coatings, are disclosed in U.S. Pat. No. 5,705,278 to Gillery et al.
Also known are front-surface mirrors. Front-surface mirrors suffer the disadvantage that it is difficult to achieve adequate long-term adhesion between the substrate and the metal mirror coating. The front-surface mirror coating is applied in a cold coating process as the last step after the glass has been formed and bent to shape. Also known are transparent mirrors, that is, half-mirrors, such as those employing alternating layers of TiO2/SiO2. The deposition of a large number of alternating layers for this type of mirror structure is costly and complex. Some known mirrors are provided with a condensation or ice control mechanism employing electrical heating. A typical structure would include a silver metal film applied on a surface of the glass by silkscreen or other technique, to act as an electrical resistance heater. A black paint layer is then applied over the silver metal film. Both the black paint layer and the silver metal film then must be dried simultaneously at about 600° C., during which the glass is bent into the desired shape.
It is an object of this invention to provide thermostable, optionally colored, back-surface mirrors comprising a substantially transparent glass substrate, wherein the mirror coating is thermostable at the tempering or bending temperature of the glass substrate.
It is an object of at least certain preferred embodiments of the invention to provide back-surface mirrors with the desired reflectivity values ranging from above 53% to 85%, having color or no color, which are thermostable at the bending temperature of the glass substrate.
It is a further object of the invention to provide methods for the manufacture of the above-described Thermostable, reflectivity-controllable and color-controllable, back-surface mirrors.
Additional objects and advantages of the present invention will be readily understood by those skilled in the art given the benefit of the following disclosure of the invention and detailed description of certain preferred embodiments.
SUMMARY
The present invention provides a highly reflective, thermostable, chromium-based back-surface mirror, optionally being optically colored, comprising a transparent glass substrate and a thermostable mirror coating on a surface of the glass substrate. It should be understood that, as used here, a “back surface” mirror is reflective through the glass substrate, that is, when viewed through the glass substrate from the surface opposite that surface which caries the thermostable mirror coating. In accordance with one aspect, the novel thermostable back-surface mirrors have a reflective chromium-based mono-layer. As used here, “chromium-based” means chromium or any suitable Ni—Cr alloy and “reflective chromium-based mono-layer” means a single layer of Cr or Ni—Cr which is the only reflective material in the mirror coating. In certain preferred embodiments, these mirrors have been thermally processed, i.e., tempered and/or heated for bending. The reflective chromium-based mono-layer in such mirrors has a thickness exceeding that of chromium layers employed in prior known mirrors. More specifically, the reflective chromium-based mono-layer is sufficiently thick to maintain mirror-quality, back-surface reflectivity after such heat treatment, even without an overlying protective layer. Exemplary chromium-based mirrors disclosed here employ a chromium or Ni—Cr layer 140 nm thick. Embodiments of the present invention employing a soda-lime-silica glass substrate with a 140 nm thick chromium-based mono-layer and no protective material overlying the mono-layer, still have coating capacity and mirror-quality back- surface reflectance, e.g., greater than 53% back-surface reflectance, after tempering and bending. The front surface of the chromium-based mono-layer, which is exposed to atmosphere during the heating, oxidizes to form a self-protective CrOx surface imparting a substantially non-reflective, blue-colored appearance. In contrast, mirrors of the above-mentioned EP application 0 301 755 employ a chromium reflective layer only 60 nm thick and require a protective aluminum layer overlying the front surface to maintain back-surface reflectivity after heat treatment.
In accordance with another aspect of the invention, thermostable back-surface mirrors have a reflective chromium-based layer, i.e., Cr or Ni—Cr, together with a reflectivity enhancing metal layer of silver or copper between the glass substrate and the chromium-based layer. In certain preferred embodiments, the reflectivity enhancing metal layer is sandwiched between chromium-based layers. The reflectivity enhancing layer, especially in the case of silver, should be protected against oxidation, both from the glass side and from the outside. The overlying chromium-based layer protects the outside. A buffer layer is used to protect the glass side. The second (i.e., glass side) chromium-based layer serves as such required buffer in the preferred embodiments mentioned above, wherein the reflectivity enhancing silver or copper metal layer is sandwiched between chromium-based layers. Alternative buffer layers between the reflectivity enhancing layer and the glass include a layer of Si. The buffer should be sufficiently thin to be substantially transparent to visible light, so as to maintain back-surface reflectance. It should be sufficiently thick to provide adequate protection against oxidation of the reflectivity enhancing metal layer. A buffer layer also can be used in preferred embodiments of the invention employing a chromium-based mono-layer for reflectivity.
In accordance with another aspect of the invention, the thermostable mirror coating includes an oxide layer between the above-mentioned layers and the glass substrate. In certain preferred embodiments, the oxide layer is a color-forming layer of copper oxide (CuO
x
) or tin oxide (SnO
2
). In other preferred embodiments, the oxide layer is a colorless oxide, e.g., WO
3
. The thermostable mirror coating is reflective through the glass substrate, that is, the mirror is reflective when viewed from the glass substrate surface opposite that which carries the thermostable mirror coating
Banner & Witcoff LTD
Shafer Ricky D.
Turkiye Sise ve Cam Fabrikalari A.S.
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