Stock material or miscellaneous articles – Composite – Of quartz or glass
Reexamination Certificate
1999-12-21
2002-05-14
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of quartz or glass
C428S216000, C428S428000, C428S432000, C428S433000, C428S434000, C428S336000, C428S697000, C428S698000, C428S699000
Reexamination Certificate
active
06387515
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a transparent substrate, especially made of glass, intended to be incorporated into glazing and provided, on at least one of its faces, with an antireflection coating.
An antireflection coating usually consists of a stack of interferential thin layers, generally an alternation of layers based on a dielectric material having high and low refractive indices. The purpose of such a coating, deposited on a transparent substrate, is to reduce its light reflection, and therefore to increase its light transmission. A substrate coated in this way therefore has its transmitted light/reflected light ratio increased, thereby improving the visibility of objects placed behind it. When it is desired to achieve the maximum antireflection effect it is then preferable to provide both faces of the substrate with this type of coating.
One useful application is in the equipping of buildings. Glazing with an antireflection effect is, according to this application, used, for example, as shop windows so as to better distinguish objects placed behind the window, even in configurations in which the interior lighting is low compared with that outside. It may also be used as counter glass.
Recently, another application has been envisaged, especially at the request of motor-vehicle manufacturers. This is to give a windscreen an antireflection effect while still meeting the requirements in force which insist on high levels of light transmission, generally greater than 75%, and a very low residual haze (less than 1% of the transmitted light). The desired antireflection effect may, for example, consist in increasing the light transmission T
L
, which amounts to improving the visual comfort of the driver and his passengers. This also reduces the undesirable reflections which may hinder the driver, particularly the reflection of equipment inside the vehicle.
Hitherto, this application has not been developed to any great extent because of the fact that the level of mechanical and chemical durability required is difficult to achieve.
This is because the antireflection coating in the windscreen is placed at least on the 1 face, that is to say that face of the windscreen facing out of the passenger compartment.
However, that face of the windscreen is exposed to a number of stresses. Thus, the to-and-fro movement of the windscreen wipers when they are operating results in significant abrasion, and the impingement of various particles—dust or chippings—damages the coating in places, both from a mechanical standpoint and a chemical (corrosion) standpoint.
To solve these problems and therefore to develop glazing with an antireflection coating that has sufficient mechanical and chemical durability, for the purpose of being used as a windscreen, the Applicant has already proposed in document WO 97/43224 that at least some of the layers forming part of the antireflection stack of layers of materials having alternately high and low refractive indices be layers deposited by a pyrolysis technique.
This stack is not completely satisfactory since, because of the very fact that there is a large number of interfaces, the risk of the stack being fragile cannot be entirely avoided.
SUMMARY OF THE INVENTION
The object of the invention is therefore to propose an antireflection coating having improved mechanical performance, especially for the purpose of using the glazing which is provided with it as a motor-vehicle windscreen.
To do this, the subject of the invention is a transparent substrate, especially made of glass, provided on at least one of its faces with an antireflection coating A, characterized in that said coating consists of a single thin layer formed from at least two materials of different refractive indices, the composition of which thin layer varies continuously through its thickness. Preferably, the thin layer defined above has a geometrical thickness of between 100 and 400 nm.
Advantageously, this layer has at least one first region in its thickness (that region closest to the substrate) in which the refractive index increases and at least one second region (especially that closest to the atmosphere) in which the refractive index decreases. Between these two regions, there may also be an intermediate region of a relatively constant refractive index. Thus, it is possible to have a first region in which the index varies from the value I(0) of approximately 1.45 to 1.65 to the value I(1) of at least 2, especially 2.1 to 2.4, then an optional second region which is very thin compared with the first, in which the index remains approximately at the value I(2) and then a final region in which the index decreases again towards a value I(3), which may be closer to I(1) or may be slightly less than it. The particular feature of this layer is that the index changes gradually, so that this really acts as a monolayer. More complex index variations are also possible (more than two alternations of increasing index region and of decreasing index region).
Because of its actual construction, the antireflection coating according to the invention makes the glazing perfectly suited to use as a motor-vehicle windscreen.
Moreover, the mechanical integrity of the continuous layer allows it to undergo the curving operation without any difficulty.
Furthermore, the antireflection coating according to the invention has a very effective ultra-violet filtration effect, particularly in the wavelength range lying between 400 and 500 nanometers. This effect is advantageously combined with that of the thermoplastic, such as PVB, used as interlayer in the laminated glazing.
This thus avoids, even more effectively, the risk of the plastics and paints present inside the passenger compartment of the motor vehicle fading.
Advantageously, the layer is formed only from two materials of different refractive index.
According to this embodiment, the thin layer is preferably based on SiO
x
N
y
where x and y vary continuously through its thickness from 0 to 2 and from 0 to 1.33, respectively.
Still according to this embodiment, it is also preferable in that region of the layer which lies closest to the interface with the ambient environment, such as the air, for x to vary in a strictly increasing manner from 0 to 2 and for y to vary in a strictly decreasing manner from 1.33 to 0.
According to another embodiment, the thin layer according the invention is based on Si
z
Ti
1-z
O
2
where z varies in a strictly continuous manner through its thickness from 0 to 1.
Preferably, in that reiogion of the layer which lies closest to the interface with the ambient environment, z varies in a strictly increasing manner from 0 to 1.
In order to optimize the antireflection effect of the graded-index “monolayer” according to the invention, the reflective index of the thin layer preferably lies between 1.35 and 1.75, advantageously between 1.38 and 1.70, in that region which lies at a distance of between 0 and 10 nanometers from the interface with the ambient environment, such as the air.
The characteristics defined above allow the glazing provided with the substrate according to the invention to fully meet the necessary requirements of the windscreen application, namely the combination of an R
L
value of less than 7%, and even less than 6%, at normal incidence and less than 10% at an angle of incidence of 60°, and a T
L
value of at least 75% at normal incidence.
It is also possible, according to the invention, to choose one of the materials used in the composition of the graded-index “monolayer” so that it provides a hydrophobic function on the external side of the substrate.
The substrate according to the invention is advantageously provided on that face not having the antireflection coating with a stack of thin layers which includes at least one functional layer, for example made a metallic functional layer of the silver type.
As to the type of stack, this may be a stack with the following sequence:
dielectric/silver/dielectric or dielectric/silver/dielectric/silver/dielectric.
For more details concerning these types
Joret Laurent
Lebellac David
Maurer Marc
Pennie & Edmonds LLP
Piziali A.
Saint-Gobain Vitrage
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