A cathode ray tube

Abstract: A cathode ray tube (1) comprises an envelope (2) including a faceplate (5) with inner and outer surface and an antistatic layer (10) covering the outer surface of the faceplate for discharging static charges accumulating in the faceplate. The antistatic layer (10) is formed by using a solution containing an alcoholate of silicon as main constituent. The antistatic layer (10) contains a stabilising substance for maintaining antistatic characteristics. The antistatic layer (10) also has light filtering characteristics when the layer contains xanthene dye as filtering substance.

Description: This invention relates to a cathode ray tube and more particularly, to an antistatic layer provided in front of a faceplate of the cathode ray tube. It is known that a cathode ray tube can reproduce letters and pictures by electron beam bombardment of phosphor screen formed on an inner surface of a faceplate of glass. The electron beam is emitted from an electron gun assembly placed inside a neck of an envelope including the faceplate. The phosphor screen includes dot-shaped or stripe-shaped red, green and blue phosphors which are distributed regularly on the inner surface of the faceplate. The cathode ray tube has a problem due to the glass faceplate. Since the surface resistance of the faceplate is high, static charges due to the electron beam accumulate on the faceplate during tube operation. Because of the accumulation of the static charges, dust and fluff in the atmosphere are absorbed on to the outer surface of the faceplate. Also, when someone touches the faceplate during tube operation, they receive an electrical shock. In order to solve the problems due to the accumulation of the static charges, it has been proposed that the outer surface of the faceplate is covered with an antistatic layer which can discharge static charges accumulated on the faceplate during tube operation. For example, it is disclosed in U.S. patent No.4,563,612 issued on January 7, 1986 that a cathode ray tube has an antistatic, glare-reducing, image-transmitting coating on an external viewing surface of a glass viewing window. The coating has a rough surface for imparting the glare-reducing characteristics and is composed essentially of a silicate material and a metallic compound in proportions to impart the desired antistatic characteristics without substantially degrading the image-transmitting capability of the coating. Further, it is also disclosed that the formulation may contain pigment particles and/or dyes to reduce the brightness up to about 50 percent of its initial value and/or to modify the spectral distribution of the transmitted image. However, the coating can not exhibit a satisfactory antistatic effect in practical use. Since the silicate material composing the coating substantially has no conductivity, the resistance value of the coating is not sufficiently reduced even if the small amount of metal compounds are contained in the coating. Further, when the amount of the compound added is increased to reduce the resistance value, strength and optical characteristics of the coating deteriorate. Another cathode ray tube for solving the accumulation of static charges is disclosed in Japanese Patent Disclosure No.61-118946. An outer surface of a faceplate is covered with double layers, which consists of an antireflection layer and an antistatic layer formed on the antireflection layer. The antireflection layer consists of transparent SiO 2 and has rough surface for improving the contrast of the reproduced images. The antistatic layer is formed on the outer surface of the faceplate by spraying a solution which contains an alcoholate of silicon as its main constituent and contains silanole radical. Since the antistatic layer can absorb moisture in the atmosphere due to the silanole radical, the resistance value of the layer can be effectively reduced. However, when using the antistatic layer, the silanol radical is reduced with the passage of time through the progressive glassification of the silicon forming the basis of the layer. Because of reduction of the silanol radical, the resistance value of the layer increases in accordance with reduction of the moisture absorption capability. As a result, the antistatic effect deteriorates. Accordingly, the antistatic layer lacks stability of antistatic characteristics. An object of this invention is to provide a cathode ray tube with a thin layer provided in front of a faceplate for improving reproduced images. Therefore, the invention may provide a cathode ray tube comprising an envelope including a faceplate with inner and outer surfaces and a sidewall portion; a neck, and a cone connecting the faceplate to the neck; an electron gun provided inside the neck for emitting at least one electron beam; a phosphor screen provided on the inner surface of the faceplate for emitting a visible light by bombardment of the electron beam; and a thin layer provided on the outer surface of the faceplate for preventing accumulation of static charges on the faceplate. The thin layer is formed by a solution which contains an alcoholate of silicon as main constituent and a stabilizing substance present in an operative concentration for maintaining antistatic characteristics of the layer, as defined in the characterising clause of claim 1. According to the invention, since the thin layer for preventing accumulation of static charges contains a stabilizing substance, the resistance value of the antistatic layer may not increase with the passage of time. Accordingly, a stable antistatic layer can be obtained. A non-limiting theoretical explanation can be considered for illustration only. The antistatic layer, which is formed by using a solution of an alcoholate of silicon, is composed of a SiO 2 film partially having a silanol radical. In the conventional antistatic layer, the silanole radical will cause a dehydrating condensation reacting with passage of time, and thus, moisture absorption capability due to the silanole radical will disappear through the glassification of the layer. On the contrary, since the antistatic layer of the invention contains stabilizing substance, the glassification mentioned above can be effectively prevented. It is assumed that the stabilizing substance is present in such a way that it separates neighbouring silanol radicals and thus prevents the reaction of the silanol radicals in the layer. As a result, the dehydrating condensation reaction can be prevented and thus the increase in the resistance value of the layer with the passage of time can be prevented. The stabilizing substance is preferably an organic substance, which is solid at normal temperature, can be dissolved in water or an organic solvent such as alcohol, and has a molecular weight of 100 to 5000. For example, one or more dyes, such as anthraquinone group dyes composed of anthraquinone and its derivatives, azo group dyes and carbonium dyes, can be used. Other dyes, such as xanthene dyes and phthalein dyes including Sulpho Rhodamine B (colour Index 45100) and Rhodamine B (colour Index 45170), Kayanol Milling Red 6BW(Acid Violet 97),and Kayaset Blue K-FL (Solvent Blue 70), can be used as the stabilizing substance. These dyes of Sulpho Rhodamine B, Rhodamine B, Kayanol Milling Red 6BW,and Kayaset Blue K-FL are marketed by Nippon Kayaku Co., Ltd. The amount of the stabilizing substance in the antistatic layer can be adjusted depending on the molecular weight and specific gravity of the substance. The amount of the substance is preferably between 0.01 wt% and 75 wt%. If the amount is less, prevention of deterioration of the antistatic layer can not be expected. Also, if the amount is more, transmissivity and adhesion of the layer is reduced for practical use. The antistatic layer of this invention can contain metal salts, such as salt of Li, Na, Ba, Sr and Ca, as moisture absorbent. In order that the invention may be more readily understood embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a side view of a cathode ray tube in accordance with one embodiment of the invention, and Figure 2 is an enlarged diagram showing part of the molecular structure of an antistatic layer shown in Figure 1. Prefered embodiments of this invention will be explained with reference to the drawings. In Figure 1, a cathode ray tube 1 includes an envelope 2 which is hermetic and is made of glass. The envelope 2 has a neck 3 and a cone 4 as a continuation of the neck 3. The envelope 2 also has a faceplate 5 sealed with the cone 4 by frit glass. A metal tension band 6 for preventing explosion is wound around the outer periphery of a sidewall portion 7 of the faceplate 5. An electron gun 8, which emits three electron beams, is provided in the neck 3. On the inner surface of the faceplate 5, there is provided a phosphor screen 9 which consists of a plurality of phosphor stripes for emitting red, green and blue lights and light absorbing stripes between the phosphor stripes. A shadow mask (not shown), which has a plurality of apertures for bombarding the phosphor stripes by the electron beams, is placed adjacent to the phosphor screen 9. A deflection yoke (not shown) is attached to the outside of the cone 4 for deflecting the electron beams to scan the phosphor screen 9. The outer surface of the faceplate 5 is covered with an antistatic layer 10 to reduce the surface resistance of the faceplate 5. As shown in Figure 2, the antislatic layer 10 contains stabilizing substances 11, which is composed of methyl violet and separates the silanol radicals. Although the antistatic layer 10 is shown as a two-dimensional structure in Figure 6, the actual antistatic layer is three dimensional. Since the antistatic layer 10 contained stabilizing substances 11 separating the silanol radicals, the resistance value of the antistatic layer 10 did not increase with the passage of time and the antistatic layer 10 could maintain stable antistatic characteristics. Also, since the antistatic layer 10 contained methyl violet as the stabilizing substances, the external light reflectivity was reduced by 20 % and the contrast was also improved. The antistatic layer 10, of course, was electrically connected to the metal band 6 to effectively discharge the static charges which would be accumulated on the faceplate 5. The antistatic layer was formed as follows. Embodiment 1 A coating solution having the following composition was prepared. Ethyl silicate 7 wt% Hydrochloric acid 3 wt% Methyl violet 0.2 wt% Water 2 wt% Isopropyl alcohol Remainder The solution was coated on the outer surface of the faceplate of the assembled cathode ray tube by spin coating After coating, the antistatic layer was formed by drying. The resistance value of the layer was 5x10 9 Ωcm, by measurement. A heat-resistance test was carried out by leaving the cathode ray tube with the antistatic layer for 500 hours at a temperature of 80°C to evaluate the the stability of the antistatic layer with the passage of time. As the result of the test, the resistance value did not increase to more than 5x10 10 Ωcm, and the antistatic layer maintained satisfactory antistatic characteristics. On the contrary, after the heat-resistance test mentioned above, an antistatic layer which did not contain the stabilizing substance deteriorated and was accompanied by an increase in resistance from 5x10 9 Ωcm to 1x10 13 Ωcm. Embodiment 2 An antistatic layer according to another embodiment contained lithium chloride as a moisture absorbent in addition to violet dye as the stabilizing substance. A coating solution having the following composition was prepared. Ethyl silicate 7 wt% Hydrochloric acid 3 wt% Lithium chloride 1 wt% Violet dye 0.2 wt% Water 2 wt% Isopropyl alcohol Remainder The solution was coated on the outer surface of the faceplate of the assembled cathode ray tube by spin coating. After coating, the antistatic layer was formed by drying. The resistance value of the layer was 1×10 8 Ωcm, by measurement. As mentioned above, a heat-resistance test was carried out under the same conditions. after the test, the resistance value did not increase to more than 1×10 9 Ωcm, and this result indicating the antistatic layer maintained satisfactory antistatic characteristics. Embodiment 3 An antistatic layer according to a further embodiment contained saccharin with a molecular weight of 183 as the stabilizing substance. A coating solution having the following composition was prepared. Ethyl silicate 7 wt% Hydrochloric acid 3 wt% Saccharin 0.2 wt% Water 2 wt% Isopropyl alcohol Remainder The solution was coated on the outer surface of the faceplate of the assembled cathode ray tube by spin coating . After coating, the antistatic layer containing the stabilizing substance saccharin was formed by drying. The resistance value of the layer was 5×10 9 Ωcm, by measurement. A heat-resistance test was carried out under the same condition mentioned above. After the test, the resistance value did not increase to more than 5x10 10 Ωcm. This result meant that the antistatic layer had an excellent stability. According to further embodiments of the invention, an antistatic layer with not only antistatic characteristics but also light filtering characteristics is explained. In other words, the antistatic layer is a light filtering with antistatic characteristics by containing a filtering substance of particular organic dye(s) which can act as the stabilizing substance for maintaining antistatic characteristics. Embodiment 4 A coating solution having the following composition was prepared. The solution was coated on the outer surface of the faceplate with a size of 25 inches by a spin coating method after assembling the cathode ray tube. After coating, a light filtering layer, which contained the light filtering substance acting as the stabilizing substance for maintaining antistatic characteristics, was formed by drying. In the case of the embodiment, the amount of Sulpho Rhodamine B contained in the filtering layer was 4.0g, 2.0g, 1.5g, 1.0g, 0.5g, 0.3g, 0.1g, 0.05g, and 0.02g. In Table 1, evaluations of reproduced images obtained from the cathode ray tubes with the light filtering layers and results of the heat-resistance test carried out under the same conditions mentioned above are shown. As a comparison, a 25-inch-size cathode ray tube, which has a glass plate containg Nd 2 O 3 as the light filter, was evaluated. In Table 1, the body colour was evaluated whether, when black images were reproduced by these colour cathode ray tubes, the images were recognised by human sight as natural black without the black being tinged with any other colour. In practice, a black pattern of 50mm x 50mm was reproduced in the centre of the phosphor screen, and the periphery of the pattern was made white. The shade of the black pattern (reddish, bluish, green, etc.) was evaluated while illuminating the faceplate with an incandescent lamp from an angle of 45° with respect to the outer surface of the faceplate so that the illumination on the outer surface of the faceplate was 500 lux. Evaluation standards are specified thus: Recognition as natural black without being tinged by any colour was indicated as o ○ , slight colouration noticed but hardly any problem was indicated as ○, colouration being rather strong and tending to cause problems was indicated as ▵, and colouration being so strong that the pattern was not as black was indicated as x. As seen from Table 1, if the amount of the dye was increased, the BCP increased and the contrast was improved. However, the body colour gradually became more strongly tinged. As also seen from Table 1, if the amount of the dye was between 0.3g and 4.0g, the contrast was improved, and if the amount of the dye was between 0.02g and 1.5g, antistatic characteristics of the filtering layer were stabilized. Further, if the amount was between 0.3g and 1.5g, a filtering layer which had no problem in respect of body colour, improved contrast, and stable antistatic characteristics was obtained. Embodiment 5 The filtering layer of this embodiment further contained 1 wt% of LiCl as moisture absorbent for improving antistatic characteristics, compared to the filtering layer of Embodiment 4. Table 2 shows heat-resistance test results carried out under the same conditions mentioned above. As seen from Table 2, the filtering layer had stabilized antistatic characteristics.

Claims: A method of covering the outer surface of the faceplate (5) of a cathode ray tube (1) with an antistatic layer (10), which tube comprises an envelope (2) including the faceplate (5) with inner and outer surfaces and a sidewall portion (7), a neck (3), and, a cone (4) connecting the faceplate to the neck, an electron gun (8) provided inside the neck for emitting at least one electron beam, a phosphor screen (9) provided on the inner surface of the faceplate for emitting a visible light by bombardment of the electron beam, characterised in that the antistatic layer (10) is formed on the outer surface of the faceplate from a solution containing an alcoholate of silicon as main constituent and a stabilising substance present in an operative concentration for maintaining antistatic characteristics of the antistatic layer, and wherein the stabilising substance is organic material which is soluble in water, has molecular weight in the range from 100 to 5000 and is at least one selected from the group consisting of pigment, dye, anthraquinone group dyes composed of anthraquinone and/or its derivatives, azo group dyes, carbonium dyestuffs, xanthene dyes, phthalein dyes and saccharin and dried. A cathode ray tube having a faceplate covered with an antistatic coating by the method according to claim 1 wherein the antistatic coating contains 0.01wt% to 75wt% of stabilising material. A cathode ray tube having a faceplate covered with an antistatic coating by the method according to claim 1 or a cathode ray tube according to claim 2 wherein the stabilising substance is at least one selected from the group consisting of anthraquinone group dyestuffs composed of anthraquinone and its derivatives, azo group dyes and carbonium dyes. A cathode ray tube having a faceplate covered with an antistatic coating by the method according to claim 1 or a cathod ray tube according to one of the claims 2 or 3 wherein the antistatic coating further contains moisture absorbent in an operative concentration for maintaining antistatic characteristics of the antistatic coating. A cathode ray tube according to claim 4 wherein the moisture absorbent is at least one compound of the following elements: Li, Ba, Sr and Ca, optionally lithium chloride. A cathode ray tube having a faceplate covered with an antistatic coating by the method according to claim 1 or a cathode ray tube according to claims 2 - 5 wherein the stabilising substance comprises one or more of the following light-filtering substances: Rhodamine B, sulpho-Rhodamine B, Kayanol milling red, acid violet, methyl violet, violet dye, and Kayaset Blue K-FL. A cathode ray tube as claimed in claim 6 wherein said stabilising substance comprises both sulpho-Rhodamine B and Kayaset Blue K-FL. A cathode ray tube as claimed in any one of claims 2 to 6, wherein the stabilising substance comprises saccharin. A cathode ray tube as claimed in claims 2 - 8 wherein the antistatic layer is applied to a transparent substrate, such as a plate, which is itself applied to the outer surface of the faceplate.

Inventors: ITOU TAKEO, MATSUDA HIDEMI, TANAKA HAJIME, ITOU, TAKEO, MATSUDA, HIDEMI, TANAKA, HAJIME, Itou, Takeo, c/o Patent Division, Matsuda, Hidemi, c/o Patent Division, Tanaka, Hajime, c/o Patent Division
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: H01J29
IPC codes: H01J29