Method of manufacturing cathode-ray tube

Abstract: According to the invention, an antistatic/­ anti-reflecting film (3) of high adhesive strength can be formed easily by forming an SiO₂ film on a cathode-ray tube (1) faceplate (2) by means of a condensation reaction of polyalkyl siloxane consisting essentially of condensed alkyl silicates. As a result, the sintering conditions for forming an antistatic/anti-reflecting film (3) can be set adequately. The antistatic effect can be further enhanced, reflection of the external light can be decreased, and workability can be greatly improved.

Description: The present invention relates to a method of manufacturing a cathode-ray tube and, more particularly, to a method of forming a film having anti-reflecting and antistatic properties on the outer surface of a cathode-ray tube faceplate. Various non-glare treatments are commonly used to decrease reflection of external light on the outer surface of a cathode-ray tube faceplate, thereby to diminish the adverse effects of the reflected light. In one of these non-glare treatments, an alcohol solution consisting of alcoholate of Si, Si(OR) 4 is spray-coated on the outer surface of the faceplate, thereby forming numerous fine projections thereon. Japanese Patent Disclosure (Kokai) No. 61-118932 discloses a practical non-glare treatment in which the film formed by the spray-coating of an alcohol solution of Si(OR) 4 on the outer surface of a faceplate is sintered at 150°C or less, thereby to endow the film with antistatic properties. Since the sintering temperature is relatively low, the adherence of the film to the faceplate may be reduced. To prevent this reduction of adherence, HNO 3 is added to the alcohol solution. The above non-glare treatment, in which the film is formed of an alcohol solution of Si(OR) 4 , takes place in the manner shown below. (1) Hydrolysis (generation of a silanol group) ≡ Si - OR + H 2 O → ≡ Si - OH + ROH (2) Condensation of a silanol group (generation of a siloxane bond) ≡ Si - OH + RO - Si ≡ → ≡ Si - O - Si ≡ + ROH ≡ Si - OH + HO - Si ≡ → ≡ Si - O - Si ≡ + H 2 O In the above reaction, the silanol group gives an antistatic effect to the film, and siloxane bond serves to increase the adhesion of the film to the faceplate. Reaction (2) is promoted when the film is heated. As long as the film is moderately heated, the silanol group remains in the film, whereby the film is sufficiently antistatic. In this case, however, the adherence of the film to the faceplate is insufficient since the siloxane bonds in the film is small in number. On the other hand, when the film is overheated, it cannot be adequately antistatic. Although acid such as HNO 3 can promote reaction (1), thereby reducing the time required for the aging of the coated film, it cannot serve to increase the adherence of the film sufficiently. US-A-4 596 745 discloses a coating for reducing specular reflection on optical glass screens which comprises a partially hydrolized metal alkoxide polymer. These alkoxides have the general formula M(OR) 4 where M is selected from the group consisting of silicon, titanium and zirconium where R is alkyl with 1 to 6 carbon atoms. The equivalent titanium and/or zirconium oxides is about 15% of total solids by weight. A presently preferred coating mixture is prepared by dissolving tetraethyl orthosilicate in alcohol, at an elevated tempera-ture; gradually adding a mixture of nitric acid and water; gradually adding titanium butoxide and/or zirconium n-propoxide; and, adding and mixing additional water and alcohol. The coating is applied by a method comprising the steps of cleaning the surface of the optical glass screen; preheating the glass screen; coating the solution onto the glass screen; and, baking the glass screen and solution, at a temperature of 500°C or more which is high enough to drive off the solvent and bond the coating mixture to the glass surface. BE-A-681 941 discloses a composition comprising a polyalkyl siloxane of the formula R-O-[(OR) 2 Si-O] n -O-R wherein n is an integer of 2 to 10, at least one of the groups R is a -B(OH) 2 group and the remaining groups R represent a group -C 2 H 5 , provided that the boron/silicon ratio is in a range of from 1/40 to 3/1. JP-A-61 290 622 & Patent Abstracts of Japan, Vol. 11, no. 154 (E-508) [2601], 19th May 1987 disclose a film having minute unevenness of transparent SiO 2 which is formed on the outer surface of the face plate of a cathode-ray tube. Said film is formed so as to cover an effective face of the fluorescent surface formed inside the face plate. Said forming method is as follows: Firstly, 1% of HNO 3 is added to an ethanol solution of Si(OC 2 H 5 ) 4 as a catalyzer for being applied to the outer surface of the face plate preheated up to about 40°C by spraying with a spray nozzle in order to make the outer surface coarse. Next, said cathode-ray tube is given the heat treatment at about 150°C to firmly bond the SiO 2 film thereon. Thereafter, the whole surface of said film is evenly rubbed by a felt made of stainless steel. US-A-4 535 026 discloses an antireflective silica coating for vitreous material which is substantially non-reflecting over a wide band of radiations. This is achieved by providing the coating with a graded degree of porosity which grades the index of refraction between that of air and the vitreous material of the substrate. To prepare the coating, there is first prepared a silicon-alkoxide-based coating solution of particular polymer structure produced by a controlled proportion of water to alkoxide and a controlled concentration of alkoxide to solution, along with a small amount of catalyst. The primary solvent is alcohol and the solution is polymerized and hydrolized under controlled conditions prior to use. The prepared solution is applied as a film to the vitreous substrate and rapidly dried. It is thereafter heated under controlled conditions to volatilize the hydroxyl radicals and organics therefrom and then to produce a suitable pore morphology in the residual porous silica layer. The silica layer is then etched in order to enlarge the pores in a graded fashion, with the largest of the pores remaining being sufficiently small that radiations to be passed through the substrate are not significantly scattered. For use with quartz substrates, extremely durable coatings which display only 0.1% reflectivity have been prepared. DE-A-3 735 817 discloses a coating solution for forming a SiO 2 coating being free of halide ions and having excellent storage stability, which can be produced without adding a doping agent, the amount of solvent being unlimited, and an easy solvent separating taking place during production thereof, wherein a coating solution is obtained by reacting an alkoxysilane and/or an oligomer thereof with water in presence of a solid acid catalyst and a solvent. Patent Abstracts of Japan, Vol. 10, no. 235, 14th August 1986, page 74 C 366 & JP-A-61-68350 disclose a process wherein a metallic alcoholate (e.g.; ethyl silicate) and a slight amount of hydrolysis adjusting agent (e.g.; hydrochloric acid) are added to an alcohol solvent (e.g.; ethanol, propanol). The resulted solution is stirred under the conditions of about 15∼25°C, about 30∼60% relative humidity and about 200∼600 revolutions/min rotating speed to hydrolyze part of the metallic alcoholate by the moisture in the air and to gel adequately the solution. The above-mentioned solution is coated on the inorganic base material (e.g.; glass plate) and is dried; therafter the coating is baked for about 10-60 min at about 280-350°C by which the transparent inorganic film (antireflecting film) having many fine uplifts of 0.1∼2µ height is formed on the inorganic base material. Patent Abstracts of Japan, Vol. 10, no. 306, (E-446) [2362], 17th October 1986 & JP-A-61-118932 disclose a process for manufacturing a Braun tube wherein the outside surface of a front panel is coated by spraying with spraying liquid, which is prepared by adding HNO 3 as a catalyst to alcohol solution consisting of, for example, solute Si(OC 2 H 5 ) 4 , solvent C 2 H 5 OH. And, it is performed the firing at low temperature, for example, at 80°C, in 30 minutes. Thereby, sufficient nonglaring effect may be obtained, as well as, generating the antistatic effect. The forming of SiO 2 coat without getting dirty due to the adhesion of dust and refuse, even if it is used for long times, may be obtained. It is an object of the present invention to provide a method of manufacturing a cathode-ray tube, wherein an anti-reflecting film having a sufficient antistatic effect is formed on, and strongly adhered to, a faceplate. According to the present invention, there is provided a method of manufacturing a cathode ray tube having a faceplate coated with an antistatic film, comprising the steps of: coating on the faceplate of the cathode ray tube a solution containing polyalkyl siloxane and silanol groups, said solution being obtained by condensing alkyl silicate in an average range of a dimer to a hexamer; and sintering the coated solution at a sufficient temperature below about 200°C, between 300C° and 400°C and in a sufficient time to partially condense the polyalkyl siloxane and the silanol groups and to leave a part of said silanol groups in order to enable the film to produce the antistatic effect, thereby forming a SiO 2 antistatic film containing the left part of the silanol groups on said faceplate. A polyalkyl siloxane is a condensate of two or more alkyl silicate monomers represented by the following formula: wherein R is an alkyl group (methyl, ethyl propyl, and butyl) and n = 0, 1, 2, 3,... Polyalkyl siloxane which is obtained by condensing alkyl silicate in an average range of a dimer to a hexamer is used for the following reasons. When alkyl silicate is condensed to a certain degree, e.g., in the range of a dimer to a hexamer, a film has a higher strength than that of polyalkyl siloxane containing noncondensed alkyl silicate monomers, as can be apparent from Figs. 1 and 2 to be described later. When alkyl silicate is condensed into a hexamer or more, the resultant product tends to be easily gelled and is thus not practical. A low condensate cannot contain only the same type of oligomer, as in the case of a polymer. The low condensate usually contains alkyl silicates having different molecular weights. Even if alkyl silicates having different molecular weights in the range of a dimer to a hexamer are mixed, the effect of the present invention can be achieved. As a major composition of the solution containing polyalkyl siloxane, an alcohol solution added with an acid or alkali and water, such as a normal alcoholate solution, is used in order to promote hydrolysis. A methyl, ethyl, propyl, or butyl group can be used as an alkyl group in polyalkyl siloxane. However, a methyl or ethyl group is preferable since hydrolysis is facilitated. The polyalkyl siloxane solution is coated on the surface of the faceplate of a cathode-ray tube by spraying, dispensing, or dipping. Sintering varies depending on the sintering time and temperature. At a temperature of about 100°C, the sintering time may be 10 to 15 minutes; about 200°C, 5 to 10 minutes; and 300 to 400°C, 5 minutes or less. In some cases, sintering is substantially unnecessary if an ageing period of about a week is allowed (namely, if the coated faceplate can be exposed in air for about a week). The silanol group formed by condensation according to the method of the present invention is obtained when a -OR group is hydrolyzed in the same manner as alkyl silicate of the conventional method described in Japanese Patent Disclosure (Kokai) No. 61-118932. The silanol group is partially condensed to form a siloxane bond. The condensation of the present invention is characterized in that a certain number of siloxane bonds and silanol groups are already contained in an alkyl siloxane solution which is to be coated on the faceplate and to be sintered or dried. Therefore, a film having a high adhesive force can be obtained even at an early stage of condensation of the silanol group. As a result, the present invention has the following two effects. According to the first effect, the sintering conditions of the solution containing alkyl siloxane can be set adequately, such as a sintering temperature or sintering time as defined above, and a film having a sufficient adhesive force can be formed. As a result, labor and manufacturing facility can be decreased, thus providing an inexpensive cathode-ray tube easily. In the following the terms "conventional" and "prior art" are used with respect to faceplates and or methods of manufacturing a cathode ray tube having a faceplate wherein a polyalkyl siloxane containing noncondensed alkyl silicate monomers are used. For example, assume that a conventional faceplate having a film formed by spraying and a faceplate of the present invention having a film of the same thickness as the conventional one are compared. The relationship between the sintering time and the strength of the film is as shown in Fig. 1. More specifically, in Fig. 1, the axis of ordinate represents the strength of the film and the axis of abscissa represents the time of sintering the film. The temperature is 115°C and is constant. Note that the strength of the film is expressed by means of a maximum number of rubbing times with which the film is not damaged or removed by a rubbing test using an eraser with a load of 0.5 kg/cm 2 . As is apparent from Fig. 1, in accordance with a conventional method, when sintering is performed for 30 minutes, the film is removed after rubbing was repeated about 15 to 20 times. In contrast to this, in accordance with the present invention, a film having a strength to endure rubbing of about 150 times can be obtained by sintering within 30 minutes. When sintering is performed for 1 hour, a film strength capable of enduring rubbing of 200 times or more can be obtained in accordance with the present invention, whereas a film strength capable of enduring rubbing as low as about 80 times can be obtained in accordance with the conventional method. Fig. 2 shows a relationship between the sintering temperature and the strength of the film under the same experimental conditions as in the case of Fig. 1. In Fig. 2, the axis of ordinate represents the strength of the film and the axis of abscissa represents the sintering temperature. The sintering time is 10 minutes and is constant. As is apparent from Fig. 2, when the sintering temperature is 115°C, a strength capable of enduring rubbing of about 60 times can be obtained according to the present invention, whereas a strength capable of enduring rubbing of about 15 times can be obtained according to the conventional method. In fine, according to the present invention, a film strength equal to or higher than the conventional film strength can be obtained with a sintering time of about 1/5 the conventional case provided that the temperature is constant. In other words, a film strength of equal to or higher than the conventional film strength can be obtained with less strict sintering conditions. The second effect of the present invention is to provide a sufficient antistatic effect. The antistatic effect is obtained by the silanol group. The parameters that influence the antistatic effect are: (1) the thickness of the film; and (2) the sintering conditions. The larger the film thickness and the weaker the sintering, the higher the antistatic effect. However, the adhering strength is inversely proportional to these parameters. In the present invention, since sufficient adhering strength can be maintained with less strict sintering conditions, i.e., the sintering time of about 1/5 the conventional case, the antistatic effect can be further enhanced. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a graph showing a relationship between the strength of the film and the time of sintering the film; Fig. 2 is a graph showing a relationship between the strength of the film and the temperature during sintering; and Fig. 3 is a view for explaining the structure of a color cathode-ray tube used in Example 1 of the present invention. The present invention will be described by way of its examples. Example 1 A coating solution having the following composition was prepared. Composition polyalklyl siloxane (average degree of polymerization: tetramer) 5 wt% nitric acid 3 wt% water 2 wt% isopropyl alcohol 90 wt% The solution was coated on outer surface 2 of the faceplate of color cathode-ray tube 1 shown in Fig. 3 by spraying. Cathode-ray tube 1 was sintered in a sintering furnace at a temperature of 115°C for 10 minutes to form an antistatic/anti-reflecting film 3 having projections of average thickness of 0.7 µm on an outer surface 2 of the faceplate. Note that reference numeral 4 in Fig. 3 denotes an explosion-proof band. Subsequently, resultant tube 1 was mounted in a television receiver in a room at a temperature of 20°C and a humidity of 40%. The surface of the faceplate was not charged and the antistatic effect was thus confirmed. When tube 1 was subjected to a rubbing test using an eraser, it was confirmed that the film had a strength capable of enduring rubbing of 60 times with a load of 0.5 kg/cm 2 . As a comparative example, a conventional solution of Si(OR) 4 disclosed in Japanese Patent Disclosure (Kokai) No. 61-118932 was coated on the faceplate by spraying and sintered at a temperature of 115°C for ten minutes, thereby forming a film on the faceplate. The film on the outer surface of the faceplate which was obtained in this manner by the conventional method was resistant to rubbing of only 15 times when rubbing was performed with a load of 0.5 kg/cm 2 . In order to obtain a film having the same strength as in Example 1 with the conventional method, sintering must be performed at a temperature of 210°C for ten minutes. In this case, however, the surface of the faceplate was charged, and a sufficient antistatic effect could not be obtained. Example 2 A coating solution as in Example 1 was coated on the outer surface of the faceplate of a color cathode-ray tube as in Example 1 by a conventional dispensing method. The resultant tube was sintered at a temperature of 115°C for five minutes, thus forming an antistatic/anti-reflecting film having projections of average thickness of 0.1 m. A sufficient antistatic effect was confirmed in Example 2 as well. A film strength capable of enduring rubbing of 300 times or more using an eraser with a load of 1 kg/cm 2 was obtained. As is apparent from Examples 1 and 2 described above, according to the present invention, an antistatic/anti-reflecting film having a sufficient adhering strength can be formed within a short period of time. As a result, the sintering conditions can be set less strict, the antistatic effect can be further enhanced, reflection of external light can be decreased, and workability can be greatly improved.

Claims: A method of manufacturing a cathode ray tube having a faceplate coated with an antistatic film, comprising the steps of: coating on the faceplate of the cathode ray tube a solution containing polyalkyl siloxane and silanol groups, said solution being obtained by condensing alkyl silicate in an average range of a dimer to a hexamer; and sintering the coated solution at a sufficient temperature below about 200°C, or between 300°C and 400°C and in a sufficient time to partially condense the polyalkyl siloxane and the silanol groups and to leave a part of said silanol groups in order to enable the film to produce the antistatic effect, thereby forming a SiO 2 antistatic film containing the left part of the silanol groups on said faceplate.

Inventors: ITOU TAKEO, MATSUDA HIDEMI, YAGI OSAMU, YOSHIZAKO MAMORU, ITOU, TAKEO, MATSUDA, HIDEMI, YAGI, OSAMU, YOSHIZAKO, MAMORU, ITOU, TAKEO C/O PATENT DIVISION, MATSUDA, HIDEMI C/O PATENT DIVISION
Applicants: TAMA CHEMICALS CO LTD, TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA, TAMA CHEMICALS CO., LTD.
Application year: 1987
Main classification: H01J9
IPC codes: C09K3, H01J29, H01J9, H05F1