Coating processes – Electrical product produced – Fluorescent or phosphorescent base coating
Reexamination Certificate
1995-05-04
2001-09-04
Talbot, Brian K. (Department: 1762)
Coating processes
Electrical product produced
Fluorescent or phosphorescent base coating
C427S106000, C427S108000, C427S126300
Reexamination Certificate
active
06284306
ABSTRACT:
The invention relates to a display device comprising a display screen which is provided with a filtering layer of silicon dioxide and a dye.
The invention also relates to a method of manufacturing a filtering layer of silicon dioxide on a display screen of a display device.
Filtering layers serving to reduce the light transmission are used on the faceplates of display devices, such as cathode ray tubes (CRTs) and liquid crystal display devices (LCD and LC-TV), to improve the contrast of the reproduced image. By virtue thereof, there is no necessity to change the glass composition of the display screen 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 both incident ambient light and light originating from CRT phosphors. As the filtering layers can be provided in a uniform manner, the transmission of the filtering layers is also uniform. The incident ambient light passes through the filtering layer and the glass display screen, after which it reflects off the rough phosphor layer on the inside of the display screen and again passes through the display screen and the filtering layer. If the transmissivity of the filtering layer is designated as T, the intensity of the reflected ambient light is reduced by a factor of T
2
. Light originating from CRT phosphors passes through the filtering layer only once, so that the intensity of this light is reduced by only a factor of T, resulting in an increase of the contrast by a factor of T.
In U.S. Pat No. 4,987,338 a description is given of a cathode ray tube having a display screen provided with a filtering layer consisting of silicon dioxide and a dye having a selective light absorption which is maximal in the wavelength range of 575±20 nm. Said Patent Specification also describes a method of manufacturing such a layer. In said method an alcoholic solution of an alkoxysilane compound, such as tetraethyl orthosilicate Si(OC
2
H
5
)
4
(TEOS), acidified with hydrochloric acid and to which a dye is added, is spin coaled onto a display screen. In this case, for example Rhodamine B is used as the dye. Silicon dioxide starts forming in said solution. By a treatment at an increased temperature, the formation of silicon dioxide is completed and a layer of silicon dioxide which also contains the dye is formed.
A disadvantage of the known filtering layer is its purplish-violet appearance which in the case of much ambient light leads to an undesired purplish background of the reproduced image. A further disadvantage of the known dyes used is that they are not resistant to light, so that when the layer is exposed to ambient light the transmission of the layer increases with passage of time, and hence the contrast-increasing effect decreases.
It is an object of the invention to provide, inter alia, a display device having a display screen which is provided with a filtering layer which has a non-coloured appearance, which is resistant to prolonged exposure to ambient light and to customary cleaning liquids such as ethanol and ammonium hydroxide and which additionally has the desired scratch resistance. The invention also aims at providing a simple method of manufacturing such a filtering layer on a display screen, which filtering layer may have, if desired, a high-gloss appearance.
This object is achieved by a display device as described in the opening paragraph, which is characterized according to the invention in that the dye is black and in that the layer also contains an oxide of a metal selected from the group formed by Ge, Zr, Al and Ti. By incorporating a black dye in the oxidic filtering layer, the display screen obtains a neutral appearance which varies from light grey to black, depending on the concentration of said black dye. Many known black dyes are unsuitable because they are insoluble in the alcoholic solution of the alkoxysilane compounds used. Other so-called black dyes exhibit a blue or brown colour, instead of a black colour, in the filtering layer. Black dyes which are suitable for use in accordance with the invention are Orasol Black CN™ (Colour Index: Solvent Black 28) and Orasol Black RL™ (Colour Index: Solvent Black 29) available from Ciba Geigy; Zapon Black X51™ (Colour Index: Solvent Black 27) available from BASF and Lampronol Black™ (Colour Index: Solvent Black 35) available from ICI. Said dyes enable high-gloss black filtering layers to be manufactured. A very suitable dye is Orasol Black CN™ (Colour Index: Solvent Black 28) because it has a high resistance to light. The chemical structural formula of the latter dye is unknown; according to the information provided by the supplier it is a mono-azo chromium complex. Dependent upon the desired transmission, the filtering layer comprises dye stuff in a quantity of 1-15% by weight. In the wavelength range between 410 and 680 nm the transmission of the filtering layer comprising said dye is substantially constant and hence spectrally neutral. It has been found that these and other dyes can readily be leached when the filtering layer is in contact with customary cleaning liquids such as ethanol, acetone, diluted acetic acid, ammonium hydroxide, soap and salt water. By incorporating an oxide of Ge, Zr, Al or Ti or a mixture of one or more than one of said metal oxides in the silicon dioxide, a filtering layer is obtained which is better resistant to leaching of the dye. The above metal oxides can be incorporated in the filtering layer on the basis of a mixture of an alkoxysilane and a corresponding alkoxy compounds of said metals, such as tetraethyl orthogermanate Ge(OC
2
H
5
)
4
(TEOG), tetrabutyl orthozirconate Zr(OC
4
H
9
)
4
(TBOZ), tetrapropyl orthozirconate Zr(OC
3
H
7
)
4
(TPOZ), tripropyl orthoaluminate Al(OC
3
H
7
)
3
(TPOAl) and tetraethyl orthotitanate Ti(OC
2
H
5
)
4
(TEOTi) in an alcoholic solution. The filtering layer is formed in the hereinafter described manner.
In a favourable embodiment the filtering layer comprises 2-15 mole %, preferably 5-12 mole %, of germanium oxide or zirconium oxide in relation to silicon dioxide. Below 5 mole % the filtering layer is less resistant to cleaning liquids and above 15 mole % no further improvement of the resistance to leaching is obtained and the filtering layer becomes unnecessarily expensive. In addition, above 15 mole % of zirconium oxide, there is a greater risk of gellification of the alcoholic solution after a few hours at room temperature, which would render the solution unsuitable for further processing.
According to the invention, the object of providing a method of manufacturing a filtering layer of silicon dioxide on a display screen of a display device is achieved in that the filtering layer is manufactured by providing, on the display screen, an alcoholic solution of an alkoxysilane compound, an alkoxy compound of at least one metal selected from the group formed by Ge, Zr, Al and Ti, acidified water and a black dye, followed by a treatment at an increased temperature, thereby forming the filtering layer comprising silicon dioxide, an oxide of the metal and the dye.
A suitable alkoxysilane compound for use in the method in accordance with the invention is tetraethyl orthosilicate (TEOS). Other alkoxysilane compounds of the type Si(OR)
4
, which are known per se, and oligomers thereof can alternatively be used, wherein R represents an alkyl group, preferably a C
1
-C
5
alkyl group. Preferably, the alcoholic solution is applied to the display screen by spin coating. After drying and heating to, for example, 160° C. for 30 minutes a grey to black, smooth and high-gloss filtering layer is obtained in this manner. To obtain a homogeneous, smooth layer it may be beneficial to add a surface-active substance, for example in quantities of from 0.001 to 5 % by weight, to the solution. If desired, the alcoholic solution can be applied by spraying, thereby forming a mat filtering layer having anti-glare properties. For the alcohol, use can be made of ethanol, propanol, butanol, diacetone alcohol or a mi
Oomen Emmanuel W. J. L.
Van Lierop Joseph G.
Bartlett Ernestine C.
Talbot Brian K.
U.S. Philips Corporation
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