Television – Video display – Cathode-ray tube
Reexamination Certificate
1994-10-19
2001-03-27
Nguyen, Madeleine (Department: 2622)
Television
Video display
Cathode-ray tube
C345S060000, C313S479000, C358S305000
Reexamination Certificate
active
06208389
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a display device, in particular a cathode ray tube, comprising a display screen having an antistatic, light-absorbing coating which contains latex particles of electroconductive polypyrrole.
The invention also relates to a method of manufacturing an antistatic, light-absorbing coating on a display screen.
2. Description of the Related Art
Antistatic coatings are used on display screens of display devices, in particular cathode ray tubes (CRTs). Such coatings are 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.
Light-absorbing layers which reduce the light transmission are used on display screens of display devices, for example cathode ray tubes and liquid crystal display devices (LCD and LC-TV), to improve the contrast of the image reproduced. By virtue thereof, the necessity of changing the glass composition of the display screen is avoided and the possibilities of bringing the light transmission to a desired value in a simple manner are increased. Such filtering layers reduce the transmission of incident ambient light as well as light originating from the internal light source, for example the CRT phosphors. The incident ambient light passes through the light-absorbing coating and the glass display screen, subsequently, reflects off the rough phosphor layer on the inside of the display screen and again passes through the display screen and the light-absorbing coating. If the transmission of the light-absorbing coating amounts to T, then the intensity of the reflected ambient light is reduced by a factor of T
2
. Light originating from the CRT phosphors passes only once through the light-absorbing coating, so that the intensity of this light is reduced only by a factor of T. The combination of these effects causes an increase of the contrast by a factor of T.
In published European Patent Application EP-A-533256, assigned to the present assignee, a description is given of the manufacture of an antistatic, light-absorbing coating which comprises a layer of a matrix of silicon dioxide and latex particles consisting of an electroconductive polypyrrole compound. In the known method, an aqueous dispersion of polypyrrole latex particles is added to an aqueous solution of a hydrolysed alkoxy silane compound, whereafter the dispersion is provided on the display screen and the hydrolysed alkoxy silane compound, for example tetraethyl orthosilicate, is converted to silicon dioxide by means of a treatment at temperatures between 150° C. and 170° C. for at least 30 minutes. The polypyrrole dispersion is prepared in the presence of an excess of water-soluble polymer as the steric stabilizer, for example polyvinyl alcohol. Since conductive polypyrrole is black, the coating obtained also has light-absorbing properties and a neutral grey exterior.
Since polypyrrole is an electroconductive material, a coating on the basis of a polypyrrole dispersion would result in an electroconductive layer. The presence of an excess of a steric stabilizer, however, prevents the layer from being electroconductive. In the known method, the coating is rendered electroconductive (antistatic) by removing the excess stabilizer by a centrifuging treatment for 1 hour at 11000 r.p.m., whereafter the polypyrrole particles are again dispersed in water. This method is time consuming due to the additional process steps.
If desired, the known coating can be supplemented with layers having an antireflective or antiglare effect or with layers which improve the scratch resistance. These additional layers are usually provided by spinning or spraying of an alcoholic alkoxy silane compound. Therefore, it is often necessary to subject the first coating formed from the aqueous mixture of the polypyrrole dispersion and the alkoxy silane compound to a thermal drying treatment to preclude that this layer is washed away when the additional layer is provided.
SUMMARY AND DESCRIPTION OF THE INVENTION
It is an object of the invention to provide, inter alia, a display device comprising a display screen having an antistatic, light-absorbing coating which adheres firmly to the glass display screen, is homogeneous and has satisfactory optical properties. The layer must have a simple composition and also be suitable to bring the light transmission to the desired value. It must also be possible to combine the layer with additional layers in order to obtain an antireflective and antiglare effect. The invention also aims at providing a simple method of manufacturing such coatings, which method must be practicable, in particular, at relatively low temperatures at which no damage is done to components of a cathode ray tube. In addition, the method must be suitable for optionally providing additional layers without a previous temperature treatment of the antistatic and light-absorbing coating.
These objects are achieved by a display device, in particular a cathode ray tube, as described in the opening paragraph, which display device is characterized in accordance with the invention in that the coating consists predominantly of a mixture of latex particles of polypyrrole and antimony-doped tin oxide particles.
For the polypyrrole compound use can be made of polypyrrole, N-substituted polypyrrole and &bgr;-substituted polypyrrole. For the substituents use can be made of: alkyl groups having, for example, up to 5 carbon atoms, aryl groups, alkoxy groups, nitro groups and halogen atoms. Such materials and the preparation of latex particles, inter alia, for use as an antistatic coating, are described in U.S. Pat. No. 4,959,162. Also this American patent describes a centrifuging step and a re-dispersion step to purify the latex which has been prepared. Preferably, the latex particles consist of unsubstituted polypyrrole.
In view of the optical properties and the uniformity and homogeneity of the layer, it is desirable for the latex particles of polypyrrole to have uniform dimensions. The latex particles are spherical and the average diameter preferably ranges between 30 and 80 nm.
In addition to polypyrrole particles, the coating in accordance with the invention comprises particles of antimony-doped tin oxide (ATO or Sb:SnO
2
). The ATO particles used are very small, their dimensions preferably being smaller than 30 nm. By virtue of such small dimensions of both the polypyrrole particles and the ATO particles no visible light scattering occurs and the coating formed is transparent. The particles are sufficiently electroconductive if the molar ratio Sb/Sn is less than 0.3. By virtue of the small dimensions of the ATO particles, Van der Waals' forces occur which cause a sufficiently firm adhesion of the ATO particles to a glass substrate. The ATO particles are provided on the display screen from an aqueous, stable ATO suspension (hydro-sol) which also comprises the polypyrrole particles. In the suspension, the ATO particles are not sterically stabilized but charge stabilized. Such a suspension enables very thin conductive layers having a surface resistance of approximately 1 M&OHgr;/square to be manufactured. The preparation of such an ATO suspension, inter alia, for use as an antistatic coating is described in U.S. Pat. No. 4,775,412.
The coating in accordance with the invention does not comprise a matrix of SiO
2
. The layer is built up of only two essential components, namely ATO particles and polypyrrole particles, the ATO particles providing the antistatic properties and the polypyrrole particles providing the light-absorbing properties of the layer. Both properties are combined in a single layer. As a result of the method described below, the coating consists of a homogeneous mixture of both types of particles. The layer thickness of the coating ranges between 50 and 400 nm. The colour of the coating is neutral grey, i.e.
Aben Gerardus V. A.
Krings Leo H. M.
Nguyen Madeleine
Spain Norman N.
U.S. Philips Corporation
LandOfFree
Display device comprising a display screen having an... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Display device comprising a display screen having an..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Display device comprising a display screen having an... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2521455