MXPA96002186A - Mirrors and your producc - Google Patents

Abstract

The present invention relates to a rear surface mirror comprising a glass substrate having a thickness greater than 2mm having a reflective coating comprising a reflective layer and at least two layers that improve reflection, thickness and refractive indexes The layers of said reflector coating are selected to provide a visible light reflection of at least 65% with a reflection color having a value a * greater than -6, wherein the reflection improving layer, which is closest to said glass substrate is a layer of silicon having an optical thickness of less than 125 nm. A method for producing rear surface mirrors, comprising depositing on a hot glass strip, with a thickness greater than 2mm, during the glass production process, an inner silicon layer that improves the reflection having an optical thickness less than 125nm , and a second layer that improves the reflection followed by a reflecting layer, the resulting mirrors have a visible light reflection of at least 65% with a reflection color having a value a * greater than

Description

MIRRORS AND THEIR PRODUCTION This invention relates to mirrors and to a method for producing them. The European patent specification EP 0 583 871 A1 describes a method for producing mirrors by the application of a coating, which comprises a reflective layer and at least two layers that intensify reflection, to a hot glass ribbon, for example a ribbon of floating glass, during the glass production process. The layer furthest from the light source to be reflected, during its use, is considered as the reflecting layer, with the layers that intensify the reflection being between the light source and the reflective layer. The reflective layer can be a high refractive index layer, for example of silicon, of a silicon oxide having a refractive index of at least 1.9, of tantalum oxide, of tin oxide or of titanium oxide, with the layers that increase the reflection being alternatively of a relatively low refractive index and a relatively high refractive index. A reflection enhancing layer, which has a high refractive index, can be, for example, any of the high refractive index materials, listed above for the reflective layer. However, since silicon has a greater absorption for visible light than that of the listed metal oxides, it is generally preferred, in accordance with the teachings of European patent EP 0 583 871 A1, in its use for a mirror of back surface, a layer of metal oxide as the inner layer, which increases the reflection, high refractive index, in order to achieve the required high reflection of visible light. Thus, in a preferred structure for a rear surface mirror, the arrangement of layers is: glass inner layer (which increases the reflection) of metal oxide intermediate layer (which increases the reflection) of refractive index relatively low outer layer ( reflective) of silicon. The low refractive index reflecting layer may be silicon oxide having a refractive index lower than that of the reflective layer and that of the refractive enhancing layer, of high refractive index, and in Any case less than two. At least the layers that increase the reflection are formed with a thickness very close to n? / 4, where n is a whole number non (preferably 1) and? is the wavelength of light in the visible region of the spectrum, so that the layers act to increase reflection by an interference effect. The mirrors, according to the invention of the patent EP 0 583 871 A1, have a number of advantageous advantages over the conventional mirrors. They are not only produced by the application of a reflection coating on the glass, during the glass production process (thus avoiding the need for a separate off-line process, for the application of the reflection coating), but can also be applied the coating layers by pyrolytic processes (for example the chemical vapor deposition), using the heat of the glass to produce these pyrolytic coatings of high durability. However, there is a tendency for these mirrors to exhibit a color in the reflection, which is significantly greener than that of a conventional mirror. This tendency in the reflection of a green color generally increases with the increase of the reflection (according to the thicknesses of the layers, especially the layers that increase the reflection, they approximate more closely an? / 4, where? Is the length of the wave of light, towards the middle of the visible region of the spectrum). Also, it is especially significant in the rear surface mirrors, where most of the reflected light passes twice through the thickness of the glass, which can impart a green tint to the transmitted light, as a result of the presence of ferrous iron in the Glass. The present inventor has found that, through the careful selection of the thicknesses and the refractive indices of the individual layers of the mirror coating, a rear surface mirror can be produced, having a visible light reflection of at least 65%, and, in the preferred embodiments, at least 70%, and a reflection color having a value a * greater than -6. In accordance with the present invention, a rear surface mirror is provided which comprises a glass substrate carrying a reflection coating, which includes a reflective layer and at least two layers that increase reflection, thickness and refractive index of the coating layers are selected to provide a visible light reflection of at least 65% and a reflection color having a value a * greater than -6. The term "visible light reflection", as used in the present specification and in the claims, refers to the percentage of light reflected under the Observation Conditions of Source 1931 of Illuminant D65. The reflection colors mentioned in the present specification and in the claims and defined in terms of the values of a * and / ob *, are colors defined in accordance with the CIÉ system (ASTM Designation: E 308-85) and , as mentioned, they are measured using the Observation Conditions of Source 1931 of Illuminant D65. The rear surface mirrors, according to the invention, preferably have a visible light reflection of at least 73%. The rear surface mirrors, according to the invention, preferably have a reflection color with a value of a * greater than -5. The glass substrate can be of a float glass and normal, but not necessarily, it will have a thickness greater than about 2 mm and less than about 12 mm. For most applications that require significant areas of the mirror, the glass will have a nominal thickness of 3 mm or more. As in the invention of the patent EP 0 583 871A1, the reflective layer is a layer with a high refractive index, while the layers that increase the reflection are alternatively of relatively low and high refractive indexes. The reflective layer is the layer that, in use, is farthest away from the light source to be reflected, and the layers that increase reflection are between the light source and the reflective layer. It will be appreciated that the layers that increase reflection may reflect more light than the reflective layer.

The thicknesses of the reflection-enhancing layers can be selected, in a generally known manner, such that reflections from the interfacial areas, between the reflection-increasing layer, adjacent to the glass (i.e., the inner layer which increases the reflection, remote from the reflective layer) and the glass and between the two layers that increase the reflection, reinforce the reflections from the face of the reflective layer, adjacent to the intermediate layer that increases the reflection. This will happen when the layers, internal and intermediate, that increase the reflection, each have an optical thickness of around n? / 4, where, in each case,? is the wavelength of light in the visible region of the spectrum, ie from about 400 nm to 750 nm, and n is a non-integer; n may be the same or different for each of the layers, but is preferably 1 in each case. The thickness of the reflective layer can be similarly selected so that the reflections from the face of the reflection layer, adjacent to the intermediate layer that increases the reflection (ie, the interfacial area between those two layers) is reinforced by the reflections from the outer face of the reflective layer. Unless the outer face of the reflective layer is in contact with an even higher refractive index layer, this will occur when the optical thickness of the reflection layer is around n? / 4, where? is the wavelength of light in the visible region of the spectrum and n is an integer non, normally 1. The desired high reflection of visible light is more easily achieved, while avoiding a strong unwanted green tint in the color of the reflection, using silicon for both the reflective layer (that is, the outer layer of the reflection coating) and for the internal layer that increases reflection (closer to the glass). To control the tendency to a strongly green reflection color, while, at the same time, a high reflection of the light is achieved, an internal layer can be used, which increases the reflection, of optical thickness less than 125 nm, and is preferred use an internal layer, which increases reflection, with optical thickness less than 100 nm and especially less than 90 nm (but greater than 50 nm). A particularly preferred reflection-enhancing inner layer is a silicon layer having a thickness in the range of 14 nm to 19 nm. The optical thickness of the reflective layer is generally less critical than the thickness of the reflecting layer, but will normally be greater than 100 nm and, in order to control the loss of light by absorption (especially when it is of silicon), will be less than 150 nm. In practice, it was found that increasing the optical thickness of the silicon reflecting layer, while maintaining the thickness of the layers that increase reflection, tends to reduce the green dye of the reflection color, ie, there is an increase from a *, and a less green reflection can be achieved with the use of a silicon reflective layer with an optical thickness greater than 125 nm. A particularly preferred reflection layer is a layer of silicon having a thickness greater than 20 nm and especially a thickness in the range of 25 to 35 nm. The refractive index of silicon can be as large as about 5 (see P J Martin, R P Netherfield, W G Sainty and D R McKenzie in Thin Solid FilmsllOO (1983), pages 141-147) although smaller values are often found. It is known in the art that the refractive index varies with the wavelength. In this specification and in the claims, references to the "refractive index" are intended to mean, in a conventional manner, the refractive index for light of wavelength of 550 nm (Y »to avoid doubt, it is confirmed in this that the optical thicknesses mentioned herein are calculated optical thicknesses of the refractive index for light of wavelength of 550 nm). It is believed that, in practice, the refractive index of silicon varies, depending on the precise physical form of the silicon and the presence of any impurity, for example oxygen, nitrogen or carbon. For the purposes of the present invention, the presence of these impurities can be tolerated (and, indeed, it is difficult in practice to produce silicon coatings online, without the significant incorporation of oxygen and / or carbon), provided that the refractive index is not reduced below 2.8. Thus, the term "silicon", as used herein with reference to the relatively high refractive index layers, refers to the material which is predominantly silicon, but which may contain minor proportions of impurities, provided that the refractive index is at least 2.8; preferably, the refractive index of the silicon used is at least 3.0. In order to achieve the desired reflection and color, with the aforementioned reflection enhancing inner layer, the reflection enhancing intermediate layer adjacent to the reflective layer may have an optical thickness greater than 125 nm and it is preferred to use an intermediate layer that it increases the reflection of an optical thickness greater than 140 nm (but usually less than 200 nm). The reflection enhancing layer, adjacent to the reflective layer, must be of a relatively low refractive index, generally a refractive index of less than 1.8 and preferably a refractive index of less than 1.6. For a high reflection of visible light, a material should be used which is not substantially absorbed in the visible region of the spectrum. A suitable and convenient layer material is silicon oxide, which, however, may contain minor proportions of additional elements, such as carbon or nitrogen, and the term "silicon oxide" is used herein. encompassing the silicon oxides that contain these impurities. However, in order to obtain a high transmission of light, it is preferred to use a silicon oxide with a silicon: oxygen ratio of approximately 1: 2, and a low level of impurities, so that its refractive index is find below 1.6 and preferably below 1.5. A particularly preferred reflection-enhancing intermediate layer is a layer of silicon oxide having a thickness in the range of 95 to 130 nm, although other materials with sufficiently low refractive index, for example the aluminum. As in EP 0 583 871 A1, the layers of the required refractive index can be applied to a hot glass ribbon during the glass production process by means of pyrolytic processes. It is generally convenient to use a chemical vapor deposition process to apply any silicon or silicon oxide layer that may be required. Thus, for example, a layer of silicon can be deposited (directly or indirectly) on the hot glass substrate by the deposit of chemical vapor from a silane gas, conveniently in a gaseous diluent, for example nitrogen. It is generally more convenient to use monosilane, although other silanes, such as dichlorosilane, can also be used. A suitable process for depositing such a silicon layer is described in British patent GB 1 507 996. If it is desired, for example, to improve the alkali resistance of the silicon coating, the reactive gas may contain a proportion of a compound gaseous electron donor, especially an ethylenically unsaturated hydrocarbon compound, for example ethylene, as an additive (although the use of a high proportion of such compounds will normally be avoided, since their presence tends to produce the incorporation of oxygen, which is believed to it is derived from glass, in the silicon coating, with the consequent reduction in the refractive index). A silicon oxide layer for use as the low refractive index reflecting layer (ie, an intermediate layer) can similarly be deposited by the chemical vapor deposition technique from a silane gas, conveniently in a gaseous diluent, mixed with oxygen or a source of oxygen. A mixture of silane and an ethylenically unsaturated hydrocarbon, together with carbon dioxide or an alternative oxygen compound which serves as a source of oxygen, such as a ketone, for example acetone, can also be used. The relative concentrations of the silane and the source of the oxygen used will depend on the refractive index required; in general, the lower the refractive index required, the higher the used ratio of the oxygen-containing compound to the silane will be. Again, the silane used is preferably a monosilane. When a coating layer is applied to a floating glass ribbon, chemical vapor deposition techniques can conveniently be carried out within the flotation bath, i.e. where the glass is supported on a bath of molten metal, under a protective atmosphere (but preferably after the glass has been subjected to stretching of finishing, ie at a glass temperature below 7502C), or after the tape has emerged from the float bath. When a gas containing monosilane is used to deposit the layers of silicon or silicon oxide, it is preferred to carry out the deposition of those layers in the flotation bath, where the glass is at a temperature in the range of 600 to 750 ° C. , in order to achieve a satisfactory deposit regime. Preferred layers of silicon and silicon oxide, used in the practice of the present invention, while reflecting in the visible region of the spectrum, are substantially transparent in the infrared region, so that their presence (as opposed to silver layers) traditionally used for the mirrors) on the surface of the glass, during annealing, will not have any substantial detrimental effect on the annealing of the coated glass. This means that the mirrors can be easily produced online in a flotation glass process, with the mirrors being annealed in a known manner. According to a further aspect of the present invention, a method for producing rear surface mirrors is provided, which comprises depositing, on a hot glass ribbon, during the production process of the glass, two layers that increase reflection, followed by by a reflecting layer, and thus the resulting mirrors will have a reflection of visible light of at least 65%, with a reflection color having a value of a * greater than -6. The preferred layers of silicon and silicon oxide, used in the mirrors of the present invention, have a high degree of chemical durability, and, in contrast to conventional silver mirrors, the mirrors will not require chemical protection by a paint of back. However, the silicon has a limited resistance to scrapes and, if desired, an additional protective layer, for example metal oxide, especially tin oxide, can be supplied on the reflective coating. This can be conveniently done by an applied pyrolytic coating technique, after depositing the reflector coating, during the glass production process. However, care is required to prevent the required conditions from detrimentally affecting the properties of the silicon reflecting layer and it may be appropriate to delay the application of the tin oxide protective layer until after a surface layer of silicon oxide is on the surface. silicon, for example, as described in U.S. Patent No. 4,661,381. The mirrors of the present invention are useful for many purposes, including domestic use as mirrors in bathrooms and bedrooms. For many applications, the mirrors will be provided with a darkening layer, preferably a substantially opaque layer, on the reflective coating. Thus, according to a preferred aspect of the present invention, a rear surface mirror of the invention additionally comprises an opacity layer. This opacity layer may be a layer of paint or a previously formed element assembled against the coated glass. When an opacity paint is used to form the paint layer, it may be a paint based on an alkyd resin, optionally containing an amino resin, for example melamine, and may have an organosilane sizing incorporated. The paint will normally contain an opacifying agent, for example carbon black, preferably in an amount of at least 1.4% by weight, based on the weight of the dry paint. Because the reflective coatings of the present invention are chemically durable, the paint may be lead free. The mirror coatings of the present invention are preferably deposited on a hot glass ribbon, in line, during the glass production process. The coated tape is cut in line to form individual mirrors and will usually be then cut off line to supply Separate mirrors of the required size. Opacity paint, which may be solvent based or water based, may be applied off-line, conveniently by a curtain coating process or roller coating process, and preferably before further cutting the mirrors off. line Alternatively, the opacity paint can be applied inline by a spraying or roller process. Because the paint does not require chemical durability, thin layers of paint will suffice, so thinner layers of 50 microns and preferably thinner (per economy) of 25 microns can be used (the specified thickness is a dry thickness), a typical thickness in dry or as curing is 18 to 30 microns for the paintings covered by curtain and 15 ± 5 microns for paintings applied by roller or spray. A sizing layer can be applied to the mirror coating before applying the paint, or the paint can have a sizing incorporated in its composition. A suitable sizing is an organosilane and a sizing particularly suitable for use with the alkyd-based paints is an organosilane having amino end groups, such as an aminopropyltrimethoxysilane. This sizing is stable in water and moistens the underlying surface to be painted. It can be applied as a 1-2% aqueous solution in deionized water. Alternatively, when the organosilane size is incorporated into the paint, the size is preferably present in an amount of 1 to 6% by weight, based on the weight of the paint, with a typical solids content of about 62%, more preferably about 1% by weight. The application of paints to mirror coatings of the kind generally described in EP 0 583 871 A1 is discussed in more detail in WO 95/18774. Alternatively, the opacity layer may be in the form of a previously formed element, assembled against the coated glass. Such a preformed element may be a plastic film adhered to the side coated with the glass, or it may be a separate fold assembled against the side coated with the glass, for example a board having a dark side, for example a face painted black, against the coated glass. In the production of mirrors, according to the invention, a coated tape can be cut in line, to form individual mirrors, and will usually be then cut off line to provide separate mirrors of the required size. The opacity element can be assembled on the off-line mirrors, preferably before cutting the mirrors off-line. The opacity element preferably comprises a self-adhesive plastic film (which makes it possible to achieve a securely backed product). The plastic film is translucent to opaque and carries a translucent pressure sensitive adhesive or (when the film is translucent) opaque. The carbon black is preferably present in the adhesive as an opacity element. The adhesive is preferably an acrylic based adhesive. The plastic film is preferably a polyolefin film, such as polyethylene or polypropylene, and is preferably oriented axially. Such biaxial orientation can increase the impact performance of the mirror backed with safety. Alternatively, the plastic film may comprise a polyester film. The films can be clear and transparent or colored. A more preferred film is a polypropylene film bearing an acrylic adhesive which, in its pressure sensitive form, it is known for use with polyester films used for use in glass for protection against bomb explosion or for solar control, this adhesive is known to be compatible with glass. A typical self-adhesive film has a total thickness of about 90 microns, the plastic film and the adhesive have respective approximate thicknesses of 60 and 30 microns. The adhesion between the rear surface of the mirror and the self-adhesive plastic film can be increased by the use of a size, which is applied to the reflector coating before the application of the self-adhesive plastic film thereon. The size is preferably an organosilane, preferably having amino or epoxy end groups, and in particular the size can be an aminopropyltrimethoxysilane in aqueous solution. This sizing is stable in deionized water and moistens the underlying surface that is to be covered by the self-adhesive film. It can be applied as a 2% aqueous solution in deionized water. In an alternative embodiment of the present invention, the opacity element comprises a separate sheet having a face assembled in contact with the rear surface of the mirror. Preferably, the opacity element of the mirror assembly comprises a board having a darkened surface, more preferably a matt surface, which is assembled, for example, by the use of frame elements or other mechanical additives to the rear surface of the mirror. The board can, for example, comprise a hard board painted with a black matte paint, with the painted surface assembled, for a rear surface mirror, adjacent to the reflective coating of the mirror. Alternatively, the opacity element may comprise black paper. In each of those specified embodiments, the opacity element is assembled together with the glass substrate carrying the reflective coating, to form a unitary mirror assembly as a composite assembly. However, in a further alternative embodiment, the opacity element may comprise a wall of a house, preferably having a darkened surface adjacent to the mirror, and the mirror assembly of the present invention comprises the glass substrate carrying the reflective coating. assembled on the wall. The use of opacity elements in the mirror assemblies in which the reflective coatings are, may be of the kind generally described in EP 0 583 871 A1, which is discussed more fully in WO 95/18773.

Returning to the structure of the reflector coating, one skilled in the art will appreciate that additional layers of low and high refraction indices of a quarter wave can be added (n? / 4, where n is an integer non, preferably 1) to the stack of layers that form the reflector coating, to further intensify the reflection. It may also be possible to incorporate additional layers not of a quarter wave, between the inner and outer layers, although, in the case of such layers, they are generally considered better as forming part of a composite intermediate layer, which, considered as a single composite layer, it must have a thickness so that the phase differences of the light reflected from the interface of the composite intermediate layer and the internal layer that increases the reflection, reinforces the reflected light from the interface between the layer intermediate composite and the reflective layer. Similarly, an additional layer can be included between the inner layer and the glass, although it will usually be of an intermediate refractive index between the refractive index of the inner layer and the glass. The invention is illustrated, but not limited, by the following schematic drawings, its description and the following Examples. In the drawings: Figure 1 is a section through a mirror, according to the invention; Figure 2 is a section through a mirror, as illustrated in Figure 1, with the addition of a protective layer or an opacity layer in the form of a plastic paint or film, on the reflective coating; Figure 3 is a section through a mirror, as illustrated in Figure 1, with the addition of an opacity layer in the form of a separate fold on the reflector coating; Figure 4 is a representation of the arrangement of the coating stations on a floating glass production line, for the production of mirrors, according to the method of the present invention; Figure 5 is a section through a gas distributor, suitable for use in any of the coating stations 15, 16 and 17, indicated in Figure 4, for depositing a layer of the reflective coating used in the present invention. , by the chemical vapor deposit. With reference to Figure 1, a rear surface mirror comprises a floating glass substrate 1, which carries a reflective coating 2, which comprises an internal layer 3, which increases the reflection, of silicon, an intermediate layer 4, which increases the reflection, of silicon oxide and an outer layer 5 of reflection of silicon. The layer thicknesses can be as discussed below. The production of these mirrors, which have a visible light reflection of at least 65% and a reflection color with a value of a * greater than -6, is described in the following Example. In Figures 2 and 3, the same numbers are used to designate the same substrate and layers, as in Figure 1. Figure 2 further shows an additional layer 6, which may be a protective layer (e.g., a thin layer). tin oxide) or an opacity layer, in the form of a paint layer or a self-adhesive plastic film (when layer 6 is an opacity layer, it will normally be thicker than the other layers shown). Figure 3 shows an opacity layer 7 in the shape of a board, which preferably has an obscuring surface adjacent to the glass, assembled by means of a frame 8 against the coated surface of the glass. Figure 4 schematically illustrates a floating glass production line, which comprises a glass melting section 11, a section 12 of the float bath, to form the molten glass in a continuous belt, a hardening section 13, for annealing the glass ribbon, and a cellar section 14, for cutting glass pieces from the belt, for storage and / or distribution and use. For the production of mirrors, according to the invention, each of the three coating stations, to respectively apply the inner, intermediate and outer layers, will normally be placed in or between the section 12 of the flotation bath and the section 13 of tempered glass; in the illustrated embodiment of the invention, the three coating stations, 15, 16, 17 are arranged in section 12 of the flotation bath, as shown in Figure 4. The location of each coating station is selected to being in a position where the glass ribbon has substantially reached its final thickness (usually below a glass temperature of about 750SC), so that it does not undergo further stretching, which could form cracks in any applied coating layer , but (at least for the inner and intermediate layers) where the temperature remains high enough for the formation of one more pyrolytic layer. Referring to Figure 5, a double flow coating gas distributor beam, generally designated 20, useful for the practice of the present invention, comprises a frame 21, formed by internally and externally spaced walls, 22 and 24 , which define enclosed cavities, 26 and 28, through which a suitable heat exchanger means is circulated, to maintain the distributor beam at a desired temperature. Gaseous precursors, supplied through the supply conduit 30 of the cooled fluid, which extend along the distributor beam, are admitted through the downlines 32 spaced along the supply conduit to a delivery chamber 34. , inside a manifold 36 carried by the frame 20. The precursor gases admitted through the descending lines 32 are discharged from the delivery chamber 34 through the passage 38 towards and along the glass surface 18 (shown in FIG. shape of a ribbon floating on the molten tin bath 19) both upstream and downstream (with respect to the direction of movement of the ribbon) in the direction of the arrows in Figure 5. The offset plates 40 can be supplied inside the delivery chamber to equalize the flow of the precursor materials through the distributor beam, to ensure that the materials are discharged against the glass in a flow jo laminate smooth, uniform, completely through the beam. Spent precursor materials, like a certain amount of the surrounding atmosphere around the bundles, are collected and removed through the exhaust chambers 42, along the sides of the distributor beam. Various types of suitable dispensing devices for chemical vapor deposition are generally known in the prior art and are described, for example, in U.A.A., Nos. 4,469,045, 4,504,526 and 5,065,696. The following Examples illustrate the present invention without limiting it. In the Examples, in-line mirrors were produced using a float glass production line, having coating gas distributors disposed in coating stations 15, 16 (2 distributors) and 17, shown in Figure 4. In the Examples , gas flows are indicated in volume, measured at room temperature and at a pressure of 1 bar, and all gas flows are mentioned by width in meters of the coated tape.

Example 1 Glass mirrors, for use as rear surface mirrors, were produced using a laminar vapor coating process. Four separate, equally spaced coating beams, as illustrated in Figure 5, were used to apply successive layers of silicon (1 beam), silicon oxide (2 beams) and silicon (1 beam) to a clear floating glass ribbon, which has a thickness of 4 mm and which advances in the glass tempering furnace at a speed of 555 meters / hour Each of the coating beams is located in the floating bath, where the glass ribbon is supported on a bath of molten metal, with the beam upstream (with reference to the direction of advance of the glass) located in a position where the Glass temperature is approximately 710SC.

The four coating beams were fed with the gas mixtures shown below: No modifications were required in the conditions of the glass tempering furnace to anneal the resulting coated tape, which had a highly reflective appearance. The individual mirrors were cut from the tape in a conventional manner and the optical properties were measured using the Source Observation Conditions 1931 of Illuminant D65, on the uncoated side of the glass, with the following results. The reflection of visible light was found to be 73.5% with reflected light having a value of a * of -5.1 and a value of b of +0.6. The value of a * was somewhat lower than the preferred minimum value of -5, however (in common with the other measured optical properties) it was in accordance with the calculated value for a coated glass having the observed combination of layer thickness and refractive indexes (the inner layer, which increases reflection, has a thickness of 18 nm and a refractive index of 4.4, the intermediate layer, which increases reflection, has a thickness of 105 nm and a refractive index of 1.46 and the outer reflection layer has a thickness of 19 nm and a refractive index of 4.3). The same theory predicts that, for a coating comprising an internal layer, which increases the reflection, of silicon with a thickness of 18 nm, an intermediate layer, which increases the reflection, of silicon oxide with a thickness of 110 nm and an outer layer of silicon reflection of thickness of 25 nm and with refractive indexes of the layers as those specified above, the corresponding values would be: Reflection of visible light 74% to * -3.8 b +2.3 showing that the preferred coatings of the invention can be produced similarly by minor modification of the thicknesses of the layers. A mirror, produced substantially in accordance with the previous example (but having a visible light reflection of 74% with a * of -5.0 and b * of +0.9, was backed with a black polyethylene film, of 200 microns, which It has a solvent-based pressure-sensitive acrylic adhesive applied to the coating.The application of the backing resulted in a small change in the optical properties and the measured reflection of visible light of 72.5%, with a * of -5.6 , b * of +0.1 Thus, it can be seen that the present invention allows the production in line of rear surface mirrors of high durability and having a reflection color close to neutral and close to that of a conventional silver mirror ( a * = -2.5, b * = +1.5, when formed on the basic floating glass, used in the previous example).

EXAMPLE 2 The procedure of Example 1 was repeated, except that the clear 4 mm floating glass ribbon was advanced in the glass tempering furnace at a speed of 690 meters / hour, the double flow beams 1 (upstream) ) and 4 (downstream) were replaced by single-flux coating laminar bundles, of the kind described in EP 0 305 102B, and the double-flux beam 2 was replaced by a modified version of such a single laminar coating bundle., in which the water cooling was replaced by the oil cooling, the graphite blocks by metal blocks and the gas flow restriction element by a sequence of deflection plates, corresponding to those used in the double flow beams. The gas flows are as shown below (given in liters per minute for beams 1, 2 and 3 and in kilograms per hour for beam 4, except for acetone in beam 2, which is given in cubic centimeters (cc ) of liquid acetone per minute): The resulting coated glass had a visible light reflection of 70% with the reflected light having a value of a * of -5.8 and a value of b * of 0.7. The thicknesses and refractive indices of the individual layers were 19 nm, 4.4 (internal layer that increases reflection), 80 nm, 1.46 (intermediate layer that increases reflection) and 25 nm, 4.3 (reflective layer).

Example 3 Example 1 was repeated, except that the 4 mm clear glass float tape was advanced in the glass tempering furnace at a speed of 750 meters / hour, the beams of double flow (upstream) and (downstream) were replaced by simple lamellar coating beams of the kind described in EP 0 305 102B, and the gas flows are as shown below (given in liters per minute for beams 1, 2 and 3, and in kilograms per hour for beam 4): The resulting coated glass had a visible light reflection of 73%, with reflected light having a value of a * of -2.6 and a value of b * of +3.2. The thicknesses and refractive indices of the individual layers were 28 nm, 4.5 (inner layer that increases reflection), 101 nm, 1.45 (intermediate layer that increases reflection), 30 nm, approximately 4 (reflective layer). Increasing the thickness of the silicon reflecting layer (compare Examples 1, 2 and 3) not only increases the value of a * (to thereby become less negative), but also increases the value of b *. Market research showed that increased b * values are readily acceptable to the consumer, and that preferred mirrors according to the invention have values of a * in the range of -4 to -2, and values of b * in the range from 2.5 to 4.5, preferably accompanied by a light reflection of at least 72%.

Claims (19)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following is claimed as property: CLAIMS 1. A rear surface mirror, which comprises a glass substrate that bears a reflection coating, which includes a reflective layer and at least two layers that increase reflection, characterized in that the thicknesses and the refractive indexes of the layers of the coating provide a visible light reflection of at least 65%, with a color of reflection that has a value of a * greater than -6.
2. 2. A rear surface mirror, according to claim 1, characterized in that the thicknesses and refractive indices of the coating layers provide a reflection color having a value of a * greater than -5.
3. 3. A rear surface mirror, according to claim 1 or claim 2, characterized in that the inner layer, which increases the reflection, is of an optical thickness of less than 125 nm.
4. 4. A rear surface mirror, according to claim 3, characterized in that the inner layer, which increases the reflection, is of an optical thickness of less than 100 nm.
5. 5. A rear surface mirror, according to any of the previous claims, characterized in that the internal layer that increases the reflection is silicon.
6. 6. A rear surface mirror, according to claim 5, characterized in that the inner layer, which increases the reflection, is a layer of silicon having a thickness in the range of 14 nm to 19 nm.
7. 7. A rear surface mirror, according to any of the preceding claims, characterized in that the intermediate layer, which increases the reflection, has an optical thickness greater than 125 nm.
8. 8. A rear surface mirror, according to claim 7, characterized in that the intermediate layer, which increases the reflection, has an optical thickness greater than 140 nm.
9. 9. A rear surface mirror, according to any of the previous claims, characterized in that the intermediate layer, which increases the reflection, is a layer of silicon oxide.
10. 10. A rear surface mirror, according to claim 9, characterized in that the inter-media layer, which increases the reflection, is a layer of silicon oxide, having a thickness in the range of 95 nm to 130 nm.
11. 11. A rear surface mirror, according to any of the preceding claims, characterized in that the reflective layer is a layer of silicon.
12. 12. A rear surface mirror, which comprises a glass substrate, characterized in that the substrate has an inner layer, which improves reflection, silicon, optical thickness less than 100 nm, an intermediate layer, which increases reflection, of silicon oxide, optical thickness greater than 140 nm and an external reflective layer of silicon.
13. 13. A method for producing rear surface mirrors, characterized in that two layers are deposited on a hot glass ribbon during the glass production process, which increase the reflection, followed by a reflection layer, the resulting mirrors have a reflection of visible light of at least 65%, with a reflection color having a value of a * greater than -6.
14. 14. A method, according to claim 13, characterized in that the layers are pyrolytically deposited.
15. 15. A method, according to claim 14, characterized in that the layers are deposited by chemical vapor deposition.
16. 16. A rear surface mirror, characterized in that it is produced by a method according to any of claims 13 to 15.
17. 17. A rear surface mirror, according to any of claims 1 to 12 and 16, characterized by an opacity layer on the reflecting layer.
18. 18. A rear surface mirror, according to claim 17, characterized in that the opacity layer is a layer of paint.
19. 19. A rear surface mirror, according to claim 17, characterized in that the opacity layer is an element that provides opacity, in the form of a plastic film, adhered to the rear side of the mirror.