Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2002-01-25
2004-02-24
Dawson, Robert (Department: 1712)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S372200, C427S384000, C427S402000, C427S407100, C427S419200
Reexamination Certificate
active
06696104
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a light-transmissive substrate having a light-transmissive, low-ohmic coating and in particular to a cathode ray tube comprising a display screen having an electroconductive coating.
The invention also relates to a method of manufacturing an electroconductive coating on a substrate.
Electroconductive coatings are used inter alia, as anti-static layers on display screens of display devices, in particular cathode ray tubes (CRTs). Said layers have a sheet resistance of, for example, 10
6
to 10
10
&OHgr;/□ and are thus sufficiently electroconductive to ensure that a high electrostatic voltage present on the outside surface of the display screen is removed within a few seconds. Thus, the user does not experience an unpleasant shock if he touches the screen. Besides, the attraction of atmospheric dust is reduced.
Since it may be hazardous to health, shielding from electromagnetic radiation is becoming ever more important. Devices provided with cathode ray tubes, such as display tubes for TVs and monitor tubes, comprise a number of radiation sources which may be hazardous to the user's health if he is exposed to said sources for a long period of time. A substantial part of the electromagnetic radiation generated can be screened off with metal in a simple manner via the housing of the cathode ray tube. However, radiation emitted via the display screen may substantially add to the amount of radiation to which the user is exposed.
This problem is solved by applying a well-conducting coating on the surface of the display screen. Said coating must also be sufficiently transparent in the wavelength range from 400 to 700 nm, i.e., the transmission should be at least 60%. A well-known material which can be used for a transparent and well-conducting coating which meets said requirements is indium-doped tin oxide (ITO). Such a layer can be provided by means of vacuum evaporation or sputtering. Said method requires, however, expensive vacuum equipment. ITO layers can also be manufactured by firing spin-coated or sprayed layers of solutions of indium-tin salts. Said firing operation should be carried out at a temperature of at least 300° C. This temperature is much too high to be used with a complete display tube which, in order to preclude damage to parts of the display tube, can withstand temperatures of typically 160° C.
In German Patent Application DE-A-4229192, a description is given of the manufacture of an anti-static coating for, inter alia, a display screen, said coating being made from poly-3,4-ethylene dioxythiophene and a trialkoxysilane to improve the adhesion. By way of example, a coating is manufactured by providing a desalinated aqueous solution of poly-3,4-ethylene dioxythiophene, polystyrene sulphonic acid and 3-glycidoxypropyl trimethoxysilane on a glass plate, whereafter said glass plate is dried. Said poly-3,4-ethylene dioxythiophene is previously prepared by oxidatively polymerizing the monomer 3,4-ethylene dioxythiophene by means of an Fe(III) salt in water in the presence of polystyrene sulphonic acid to preclude precipitation. The anti-static layer thus obtained has a thickness of 0.6 &mgr;m (600 nm) and a sheet resistance of 50 k&OHgr;/□. This sheet resistance is sufficient to bring about an antistatic effect.
A disadvantage of said known layer is that such layers have relatively poor mechanical properties, especially a low scratch resistance, and they are not well suited to be used in anti-reflection coatings because of the relatively low index of refraction of said layer.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to provide, inter alia, a substrate, like a display screen of a cathode ray tube, having a coating, said coating providing an effective shield against electromagnetic radiation and exhibiting good optical and mechanical properties. A further object of the invention is to provide a simple method of manufacturing such light-transmissive well-conducting optical coatings, while it is preferably possible, in particular, to carry out said method at relatively low temperatures (typically up to 170° C.) at which no damage is caused to parts of a cathode ray tube.
A coated substrate as described in the opening paragraph is characterized in that the coating is provided on a substrate containing SiO
2
, as a major component, the coating comprising a first layer and a second layer adjacent to each other, each of the first and second layers comprising SiO
2
as a major component, wherein the first layer comprises a high index material having an intrinsic index of refraction which is higher than the index of refraction of the substrate, and the second layer has an index of refraction which is lower than the index of refraction of the substrate and comprises a conductive polymer, the coating having a sheet resistance below 10 k&OHgr;/□.
In accordance with the above-mentioned requirements, such a layer provides an excellent shield against electromagnetic fields. The conductivity of the coating is primarily due to the conductive polymer. A major component of the substrate, the first and second layer is SiO
2
(in this respect it is remarked that, within the framework of the invention SiO
2
is to be broadly interpreted and includes glasses comprising SiO
2
in whatever form, such as float glass and screen glass used for and in cathode ray tubes). This allows a good adhesion of the layers to each other and to the substrate. The thermal coefficients of expansion of the layers and the substrate are comparable because a major component is the same. As regards layers of pure ITO deposited on glass, this increases the adhersion between the glass and the layer and reduces the occurrence of thermal tensions during or after manufacturing. Coatings of pure conductive polymer lack sufficient scratch resistance. However, the conductive coating in accordance with the invention, comprises a layer of mainly SiO
2
and is much more scratch-resistant. In the coating in accordance with the invention, the first layer has an index of refraction which is higher than the index of refraction of the substrate, due to the material having an intrinsic (i.e. as a bulk material) index of refraction which is higher than the index of refraction of the substrate, whereas, due to the conductive polymer, the index of refraction of the second layer is lower than that of the substrate. The combination of a first layer having an index of refraction which is higher than the substrate, and a second layer having an index of refraction which is lower than the substrate allows good anti-reflective properties to be attained. Preferably one of the layers comprises a light-absorbing material. In this manner, it is possible to control the light absorbing properties of the coating. Preferably both the first and the second layer comprise an alkoxy compound. The alkoxy-compound provides a strong mechanical coupling between the coating and the substrate and between the layers. Coatings according to the invention can be applied by means of a wet-coating method, not requiring high temperatures. Metal oxide particles, and in particular ATO, ITO and TiO
2
particles are suited for use in the first layer. Particularly useful are particles with a very high index of refraction, i.e. higher than 2.0. The index of refraction of the first layer is thereby substantially increased, which increases the possible use of such layers in anti-reflection coatings. Preferably, each layer is a ¼&lgr; layer, i.e. for each layer the product of the index of refraction of the layer and the thickness of the layer is ¼&lgr; (plus or minus 25%) of the wavelength of visible light (380-780 nm). A suitable thickness range for each of the first and second layers between 50 nm and 150 nm.
ITO and ATO are preferred in those circumstances where a charge build-up of the outer layer of the coating is to be prevented. The at least partly conductive nature of ITO and ATO particles prevents electrical charge from accumulating.
An electrocon
Aben Gerardus V. A.
Compen Johannes M. A. A.
Willems Henricus X.
Zwerver Bauke H.
Dawson Robert
Feely Michael J
LandOfFree
Light-transmissive substrate having a light-transmissive,... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Light-transmissive substrate having a light-transmissive,..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Light-transmissive substrate having a light-transmissive,... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3323277