Heat-bendable mirrors

IS 2 172 244 T3

DESCRIPTION

Thermo-bending mirrors.

The present invention relates to mirrors and more particularly to mirrors that are formed using flat substrates such as glass that are subsequently heat bent into a desired curved configuration.

Curved mirrors are commonly used as rear view mirrors for motor vehicles, as reflective surfaces for telescopes, and the like. They can be formed by first curving a glass sheet into a desired curved configuration, and subsequently applying a reflective coating to one or the other face of the substrate. For example, curved mirrors of the type used in carnivals can be made to give fun, warped reflections of an observer by first forming a sheet of glass to the desired shape, and then coating a surface of the glass with metallic silver and a top layer of protective paint. .

Mirrors can also be made by applying a reflective coating to a glass substrate using a magnetroan spraying technique of the type described in US Patent to Chapin, No. 4,166,018. Chrome or silver can be used as a reflective layer. When making curved mirrors using a magnetroan sputtering process, the glass substrates for the mirrors are first bent as desired typically in a size that would produce two or more mirrors. After washing the curved glass pieces, they are placed on a suitable support and coated using magnetron sputtering. Due to the curvature of the substrates, the reflective coatings thus produced have not been precisely uniform. The manufacturing process itself is cumbersome and time consuming as it requires multiple small glass substrates to be manually placed on a support that passes through a magnetron spraying apparatus, and requires that each of the individual pieces of the resulting mirror be removed. by hand from the carrier sheet once the spraying operation is finished.

To avoid these problems, it will be desirable to first spray-deposit a reflective coating onto a sheet of flat glass to form a mirror, and then heat-bend and cut the mirror to size. US Patent 4,826,525 (Chesworth et al.) Shows an example for the preparation of a mirror by using consecutive chromium and aluminum coatings on glass. In general, when flat glass sheets are coated with a reflective layer using chromium, for example, as reflective metal, and then heat-bent at the softening temperature of the glass, the coatings can produce defects that may be called pits. Pitting appears as small, visually detectable circular defects that have poor reflectance. The phenomenon of pitting (sometimes called “pin holes” is not fully understood, but it is estimated that it is a function of the stresses developed during the bending operation of the reflective, spray-deposited films that form the reflective layer.

The present invention relates to a thermoformable mirror that is capable of being configured at elevated temperatures to obtain a curved mirror without pitting and without significant changes in reflectance. The thermoformable mirror comprises a substrate, preferably glass, leading from the surface of the substrate outward, a transparent amorphous dielectric layer and a reflective layer. The latter comprises a first and a second contiguous but different metal film. The first metal film is a reflective film that is positioned closer to the substrate than the second metal film, and the second metal film comprises a protective film that is less reflective than the first metal film. The second metal film can be made of niobium, tungsten, tantalum, iron or nickel, with niobium being by far preferred. The reflective metal film, which is preferably aluminum, has a thickness sufficient to give the mirror a reflectivity of at least 50%, and the second metal film is present in a sufficient thickness to protect the first metal film and avoid a significant reduction in reflectivity during thermoforming of the mirror. The product described herein may also have a protective film located further from the substrate than the reflective layer, the protective film preferably comprising a dielectric oxide or nitride such as sputter deposited silicon nitride, sputter deposited aluminum oxide or oxide of silicon deposited by cathodic pulverization. Of these, silicon nitride is preferred.

When a thermoformable mirror of the invention is heat formed at a temperature higher than the temperature at which the reflective coating layers are deposited, atomic and / or structural diffusion rearrangements can occur between the various deposited films, changing the reflective properties of the film. curved mirror produced. The thermoformable mirrors of the invention, however, largely and preferably fully retain their important mirror-oaptic properties (low transmissivity, high reflectance) when heated and bent in this way, and are furthermore free of the pitting phenomenon. .

In another embodiment, the invention provides a curved mirror that is produced by forming a thermoformable mirror of the type described above, and subjecting the mirror to bending forces at a temperature at which the substrate is capable of plastic deformation (for example, the temperature of water transition in the case of glass substrates). The flat mirror is curved at that temperature into a desired curved configuration to produce the curved mirror, the latter then cooling while maintaining its curved configuration. The resulting curved mirror desirably retains at least about 100% of the reflectance and no more than about 150% of the transmissivity of the thermoformable flat mirror from which it was manufactured, and is sensitive2

ES 2 172 244 T3 free of observable pitting.

The curved mirrors of the invention desirably show a hemispherical reflectance (measured using a reflectometer and integrating sphere over the wavelength range of 200 to 2600 nm) of at least 50% and a transmissivity of no more than 4, 0% approximately. Here "reflectance" is measured using a reflectometer using a tungsten lamp at a filament temperature of 2854 ° K at an angle of incidence of 25 ° ± 5 ° using a detector cell that approximately duplicates human sight (CIE standard photoepic curve ) and an integrating sphere. In addition to showing good optical properties for a mirror product, the film stack should be physically and chemically durable in both its flat and curved states.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic sectional view, in cross section, of a thermoformable mirror of the invention; and Figure 2 is a schematic view showing the use of a heat curlable apparatus.

With reference to Figure 1, a thermoformable mirror (10) of the invention is shown in which the flat, sheet-like substrate is glass. Deposited by sputtering on the flat surface (20) of the substrate, in sequence, there is an amorphous layer (14), a reflective layer (16) comprising a first and second metallic films (20, 22), and a protective layer ( 18).

In the embodiment of Figure 1, the amorphous layer (14) is sprayed directly onto the flat surface (20) of the glass substrate, using well-known magnetron spraying techniques of the type described in US Patent to Chapin 4,166. 018. "Spray deposited", "spray deposited" and analogous terms are used herein to refer to coatings of the type produced by magnetron spray.

The film (14) is amorphous, that is, it does not show an observable crystal pattern or crystal arrangement when viewed using cross-sectional transmission electron microscope ("XTEM"). In general, it has been found that pinhole or pitting can be eliminated by using this amorphous layer between the glass substrate and the reflective top layer. The amorphous layer is a dielectric material, preferably an oxide or a nitride, examples of which are silicon nitride, titanium nitride and tin oxide. Of these, silicon nitride is preferred. The amorphous layer is desirably used at a thickness of at least 200A, preferably at least 300A, and preferably on the order of 300-400A. Sufficient thickness of the amorphous materials is used in order to avoid pitting. .

Referring again to Figure 1, the reflective layer (16) is composed, as mentioned above, of two contiguous metal films (20 and 22), the film (20) being a reflective metal film of sufficient thickness to give the resulting mirror a reflectance (hemispheric reflectance, measured using a reflectoometer and integrating sphere over a wavelength range of 200 to 2600 nm) of at least 50% and a transmissivity of no more than about 4%, and the metal film (22) serves to protect reflective metal film (20). It is desirable to select the reflective metal film from the group consisting of aluminum, titanium, zirconium, molybdenum, copper, hacnium, gold, and stainless steel, and combinations thereof. Aluminum is the preferred reflective metal. Chrome, nickel-chrome alloys, and silver should be avoided as reflective metal film, as these reflective materials promote pinholes or pitting. However, it is believed that small amounts of these materials can be used in combination with aluminum or other reflective metals such as those listed above.

The protective metal coating (22) is made of a different metal from the metal used for the reflective layer (20). The reflective metal film (22) is less reflective than the reflective layer (20) and preferably the metal used for the film (22) is less reflective than the metal used for the reflective metal film (20) when comparing metallic films of the same thickness. The film (22) is preferably formed by a metal selected from the group consisting of niobium, tungsten, teanthal, iron and nickel or their combinations. Niobium is the preferred metal for the protective layer (22).

The protective metallic film (22) is used at a thickness that provides good protection against the oxidation of aluminum or other reflective metal film located below it, but the thickness of this film should be so great that reflectance is considerably reduced after bending. . Within these parameters, the thickness of the metal film (22) can vary as desired and offers some control of the reflected color of the mirror. Niobium, as noted above, is the preferred metal film and is desirably used at a thickness of between 50 ^TOA and 400A^TO approximately, being 100-150 ^TOAt the preferred range. In the preferred embodiment, the reflective metal film (20) is aluminum and the protective metal film (22) is niobium, the aluminum being present with a thickness on the order of about 200 to 1000, preferably 530^TOA, and niobium being present with a thickness of the order of about 50-400 A^TO and preferably 100-150 A^TO. These metallic films are contiguous, that is, they are formed one on top of the other.

In Figure 1 an optional protective layer is shown as (18), and this layer is desirably deposited by cathode spray directly on the reflective layer (16), that is, directly on the surface of the protective metallic layer (22). The outer protective layer (18) additionally provides some control over the reflected color of the mirror, and serves, with the protective metallic film (22), to prevent oxygen from reaching the reflective metallic film.

ES 2 172 244 T3 tante (20). Silicon nitride and zinc oxide are suitable materials for the protective layer (18), with silicon nitride being preferred, and the protective layer (18) may have a thickness of the order of about 50 to 200 A.

In a more preferred embodiment, the coating on the glass substrate comprises the amorphous film (14), the reflective metallic film (20), and the protective metallic film (22) being formed one on top of the other, ie, as adjoining coatings. The optional protective layer (18), when used, desirably but not necessarily, is formed on the outer surface of the protective metal film (22). Also, it is desired that the amorphous layer (14) be formed directly on the surface (20) of the glass substrate.

It will be understood that one or more layers of sputtering materials may be positioned between / or on either side of the amorphous layer (14), the reflective layer (16), or the protective layer (18), provided that such additional layers do not contribute to pitting, discoloration, or other objectionable coating failure when the mirror is thermoformed. For example, stainless steel can be applied to one side of the amorphous layer (14); When applied to the amorphous layer, the stainless steel acts to improve the reflectivity of the reflective metallic film (20).

Figure 2 schematically shows a portable heated mold in heat-forming bent glass sheets. Molds of this type are commonly used in the formation of, for example, curved automobile windshields and curved glass sheets which are subsequently to be provided with a mirrored surface for use as rear-view mirrors of motor vehicles and the like. The mold consists of a female part (40) having a concave upper surface (42), and a male part (44) having a downwardly directed convex surface (46). During use, the portions of the mold are heated to the softening temperature of glass, and a thermo-bending mirror as described in relation to Figure 1 is placed on the surface of the female member with its cathodic spray coating directed downward. . As the flat glass sheet is heated to its softening point, it sags downward in accordance with the upper surface (42) of the mold. The male mold portion is then pushed down against the other surface of the glass sheet and serves to ensure a smooth shaping of the glass sheet against the surface (42). Once the bending is finished, the molds are cooled below the glass substrate transition point, the parts of the mold are separated and the bent mirror is removed. Mold temperatures of this type are commonly on the order of 1110-1130 '^: F.

The reflective coatings of the mirrors of the invention, before and after bending, should demonstrate substantial durability. That is, the coatings should show resistance to abrasion, extreme heat and cold, moisture, and solvents such as alcohols and salt spray. The abrasion resistance can be measured by sliding a standard pencil eraser (Blaisdell® # 536T or equivalent), counterbalanced with a load of 1 kg, alternatively on a coated surface, cleaned with methanol for 100 cycles. Coatings of acceptable durability shall not show significant film loss or loss of reflectivity. The adherence of the sprayed film stack to the substrate can be checked by trying to peel off the coating with a pressure sensitive adhesive tape, as described in MIL-C-48497A. Alcohol resistance can be checked by rubbing an area of ​​the coating with a clean cloth dipped in isopropanol using hand pressure. A salt spray test is described in ASTM B117, and is continued for 240 hours. To check moisture resistance, a coated specimen is exposed for 500 hours in a humidity chamber maintained at 45 ° C and 98% -100% relative humidity. After each of the tests described above, the tested coatings are visually examined for any defects.

The invention can be more easily understood with reference to the following non-limiting example.

Using a commercial magnetroan spray line manufactured by Airco, Inc., the upper, clean surfaces of the flat glass sheets were exposed to spray from various targets in a series of successive zones, the rate of travel of the sheets being of glass and the electrical power supplied to the various magnetron spraying units such as to provide the desired thicknesses of the sprayed films. Initially, silicon will be sprayed from a silicon target into an atmosphere containing nitrogen to produce the deposition of silicon nitride at a thickness of 400^TOTo approximately. Aluminum was then sprayed from an aluminum niobium target from a niobium target in an argon atmosphere at a thickness of about 530^TOAy 110^TOA, respectively. Finally, silicon was sprayed from a silicon target in an atmosphere containing nitrogen to give a final silicon nitride layer of about 100^TOTO.

The resulting thermoformable mirror will be measured for transmissivity, reflectance and color properties and then subjected to the bending procedure described above at a temperature of approximately 1130 F. After the resulting curved mirror is removed from the mold, the mirror will be examined. to see coating defects and will also be tested for reflectance, transmissivity and color. No cloudiness or other phasic defects were observed. The reflectance before and after bending was 59%. The transmissivity will increase from 2.2% to 2.6% during bending, and the initial and final color coordinates of the bent film (Hunter System L, a, b, Illuminant D 65) were L = 73.3 , a = 0.65 and b = 3.4. The curved mirror was subjected to the saline spray, humidity, abrasion, adhesion and rubbing tests with alcohol described above, without appearing

ES 2 172 244 T3 notable defects.

Although a preferred embodiment of the present invention has been described, it should be understood that various changes, adaptations and modifications may be made therein without departing from the spirit of the invention or the scope of the claims that follow.