Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
1999-08-18
2001-01-30
Pianalto, Bernard (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S162000, C427S299000, C427S407100, C427S419300, C427S508000, C427S558000, C427S559000, C427S595000
Reexamination Certificate
active
06180188
ABSTRACT:
DESCRIPTION
The present invention relates to a method for preparing an optic material by depositing on a substrate at least two layers of inorganic polymeric material, each of these layers containing at least one metal oxide or metalloid oxide, the deposited layers being densified cross-linked by exposure to ultraviolet rays.
The invention also concerns the optic material which may be prepared using this method.
The optic materials which may be prepared using said method are in particular multilayer materials such as antireflective materials and reflective materials.
The antireflective and reflective materials are made up of an organic or inorganic substrate, coated with several layers of which some have desired specific optic properties.
More precisely, interference dielectric mirrors comprise a substrate, coated with a dielectric film which reflects one or more desired wavelengths, while nonetheless showing relatively low intrinsic absorption in comparison with metals conventionally used to make mirrors.
Antireflective or reflective materials offer a host of applications.
For example, organic or inorganic substrates, namely plastics and glass substrates in particular, coated with an antireflective film are of special interest in the following areas: ophthalmic products and video, or architectural applications such as glass panels placed outside or inside buildings.
In addition, antireflective materials and interference dielectric mirrors may also be used in high-energy lasers, solar, heat and photovoltaic applications or even in integrated optic systems.
Methods are already known in the prior art with which these antireflective materials or interference dielectric mirrors can be produced. These methods are cited below.
Also, if plastics such as polycarbonates, polyacrylates, polyallylcarbonates and others are of particular interest in the ophthalmic sector, glass substrates are also of interest especially in the area of general optics and in the area of screens, such as visualisation screens.
It is easy to understand that with a loss in reflection rate of approximately 4% for each air-glass interface encountered, the average glass index being 1.5, the overall loss for a complex optic system is sometimes consequential.
Therefore, opticians have long sought to create coatings having optic properties, in particular antireflective films using physical methods for vacuum depositing, grouped under the technological term PVD (Physical Vapor Deposition).
These methods include simple or reactive spraying, simple or reactive evaporation by electronic or ionic heating either aided or unaided, etc . . .
Despite the excellent optic, chemical and mechanical quality of the deposits, these techniques require heavy sophisticated equipment which is costly and the methods are mostly time-consuming. This is especially true when the surface of the components to be treated is extensive. The consequence is that such methods are generally ill-adapted to the production of cheaper series.
For example, only cathode-ray tube screens for the most up-market television sets are currently equipped with antireflective coatings applied using the PVD technique.
This is why gentle chemical depositing methods, in particular sol-gel depositing methods, appear to offer an alternative of interest to physical methods of vacuum depositing.
With the sol-gel depositing method it is possible to prepare films deposited on substrates having various optic properties. Said method, compared with conventional methods of vacuum depositing, offers a certain number of advantages among which particular mention may be given to deposition generally conducted at ambient temperature and at atmospheric pressure without the need for a heat stage at very high temperatures, a reduced equipment capital layout, and easy, quick implementation of the method providing great flexibility of use.
The deposition of metal or non-metal oxides having optic properties using the sol-gel method has been given extensive research. It would appear that the sol-gel systems or processes can be grouped into two categories: polymeric processes or systems and colloidal processes or systems.
Each system requires different preparations and operating conditions which are related to the properties of the desired treatment solutions, and to the type of oxide concerned.
The polymeric system consists of using monomer, oligomer or low molecular weight precursors, in solution form and having good molecular homogeneity, which are subsequently converted into an oxide by baking after application on the substrate. The deposited liquid finally changes viscosity as the solvent gradually evaporates until it forms a gel on the substrate. The solid network obtained, still saturated with solvent, is then converted into an oxide by heating the system generally to high temperatures up to 500° C. A dense, hard layer is obtained which adheres strongly to the substrate. The conversion into an oxide is generally accompanied by a heavy loss in mass consisting of water and organic matter, leading to a substantial reduction in the thickness of the layer. This induces strong internal, tensile or compressive stresses within the deposit which may cause glazing of the coating in thick films whether with single or multiple components, that is to say whose thickness is greater than a few &mgr;m.
German patents DE A 736 411 and DE A 937 913 for example, mention the use of hydrolytic compounds to prepare various interference films. The major drawback of these methods lies in the compulsory heat treatment at between 500 and 600° C. to convert the polymeric intermediates into final dense ceramics. These high temperatures restrict the choice of type of substrate to be coated and complicate implementation at industrial level.
Patent U.S. Pat. No. 2,466,119 describes a process for preparing reflective and/or antireflective multilayer films, by hydrolysis and condensation of halide mixtures of titanium and/or silicon alkoxides. Control over the porosity of these layers is made by varying the temperature. However, obtaining layers with good mechanical resistance requires heating to temperatures far greater than the temperature that usual plastics can withstand, whose thermal stability is generally 150° C. at the most.
Patent U.S. Pat. No. 2,584,905 describes the preparation of thin reflective layers from alcohol solutions of TiCl
4
and a silicon alkoxide. Here again, it is necessary to have recourse to a high temperature heat treatment stage in order to achieve proper densification of the oxides. In this method, the problems of glazing and flaking related to material densifying considerably reduce the preparation of highly reflective multilayer constructions.
Patent U.S. Pat. No. 3,460,956 describes the preparation of reflective films in TiO2 from hydrolysates of tetralkyl titanates in an alcohol medium. However, for efficient conversion of the polymer film into a dense oxide, the film needs to undergo heating to a high temperature, in the region of 500° C., which is detrimental and penalising for all organic substrates.
Patents U.S. Pat. Nos. 2,768,909 and 2,710,267 describe the production of reflective films in TiO
2
from alcohol sols of a titanium alkoxide, these sols able to be hydrolysed by atmospheric humidity. This approach also requires high temperature baking of the condensed intermediates, and the layers obtained are not abrasive resistant.
Patent U.S. Pat. No. 4,272,588 concerns the possibility of increasing the reflectivity of mirrors in noble metals and the possibility of making the latter chemically passive, through the deposition of TiO
2
and Ta
2
O
5
dielectric layers derived from molecular precursors.
Such coatings are obtained by compulsory heating to approximately 400° C.
Therefore the polymeric material generally used for thin optic layers with a high refractive index (lying for example between 1.9 and 2.1) is titanium oxide (TiO
2
). However, in order to obtain layers with mechanical resistance to abrasion, densification needs to be conducted at a high temperature, close to 4
Belleville Philippe
Prene Philippe
Burns Doane Swecker & Mathis L.L.P.
Commissariat A l'Energie Atomique
Pianalto Bernard
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