METHOD FOR FORMING A PHOSPHOR SCREEN OF A CATHODE RAY TUBE

Abstract: Method for a phosphor screen of a cathode ray tube in­ cludes the steps of forming a photo-tacky layer on a panel of a cathode ray tube, exposing the photo-tacky layer to form an adhesive pattern on the layer, and adhering phosphor powder on the adhesive pattern, in which these exposing and adhering steps are repeated. Before the next exposing step, the photo-­ tacky layer is heated and then exposed after cooling. The heat­ ing step prevents the sensitivity of the layer from deterioration caused by the preceding adhering step.

Description: The present invention relates to a method for forming a phosphor screen of a cathode ray tube, more particularly to an improvement of a phosphor powder coating method. In a colour cathode ray tube, three different phosphors that respectively emit blue, green and red light are coated in a regular array (in a set pattern) in the form of stripes or dots on the inner surface of a panel which displays an image. Conventionally known methods for forming such phosphor coating layers include a phosphor slurry method and a phosphor powder coating method. The phosphor slurry method uses phosphor in a slurry mixed with photoresist material. In forming a phosphor screen, such a method inconveniently uses a lot of water for the development. The phosphor powder coating method recently developed however has the advantage of being free from such water development. In the powder coating method, a thin layer of an aromatic diazonium salt, or a photo-tacky composition which contains an aromatic diazonium as a photosensitive component and exhibits adhesivity on exposure to light is formed on the inner surface of a panel. Phosphor particles are brought into contact with and held in a thin layer on a sticky surface which is formed on the exposed portion, and then remaining excess phosphor particles are removed from the thin layer, whereby a patterned phosphor layer is formed. In this method, first, a photo-sticky or phototacky material, i.e. a photosensitive composition containing a diazonium salt, is coated on the inner surface of the panel in the form of an aqueous solution, heated and dried to give a solid thin layer. The usual procedure in this case is that the panel is coated with an aqueous solution of the photosensitive composition while rotating at low speed and subsequently the speed of rotation is increased, so as to throw the photosensitive composition aqueous solution off the panel. Then the coated solution is heated and dried to form a solid thin layer on the panel by means of an infrared heater, facing the panel, which raises the panel temperature to about 50°C. Next, ultraviolet ray irradiation (exposure) is effected through a shadow mask assembled with the panel. In this case, the panel is cooled beforehand to 30°-40°C, this being done to prevent the heat of the panel causing heating and thermal expansion of the shadow mask and consequent shift of the positions at which exposure is to be effected. The ultraviolet ray irradiation positions in this process correspond to locations that will be impinged upon by electron beams in order to cause the phosphor to emit light, i.e., they correspond to locations where the phosphor is to be coated. In a portion of the thin layer which is irradiated by ultraviolet rays, there is produced a particle acceptable adhesive surface by diazonium salt photolysis reaction. Next, after the shadow mask is removed, a phosphor powder of a first colour is brought into contact with the layer, thereby causing adhesion of phosphor to an amount corresponding to the positions of the particle acceptable adhesive surface thus obtained. In order to contact such phosphor powder on the adhesive surface a phosphor powder sliding method may be employed. Next, excess phosphor powder is removed from the thin layer by air blowing or similar means. In this manner, a first colour phosphor layer is formed only on the portions that were exposed. Next, the positions where a phosphor powder of a second colour is to be fixed are exposed via a shadow mask and the second colour phosphor powder is adhered only to the exposed portions in the same manner as was used in forming the first colour phosphor. Then a phosphor powder of a third colour is adhered to the inner surface of the panel by a similar procedure. The above operation results in a panel on which phosphors of three colour triads constituting a phosphor screen are respectively formed at locations which will be struck by electron beams for causing emission of light. In this method, however, there is the problem that the amount of ultraviolet irradiation energy needed for producing a powder acceptable adhesion region in order to effect adhesion of the second and third colours is 1.5-2 times greater than the corresponding ultraviolet irradiation needed for the first colour. In other words, sensitivity is lower with the second colour and third colour applied. Even then the second colour and third colour adhesion patterns are inferior to the first colour pattern and the quality of the second colour and third colour adhesion patterns is lower. For this reason, in the case that the method applies to coating the phosphor in a minute and precise dot- or stripe-shape on the entire surface of a panel of the colour cathode ray tube, a part of the phosphor dots may peel off or fail to thorougly adhere, as a result, causing the quality of the phosphor screen to deteriorate. One object of the present invention is to provide an improved phosphor screen forming method in which deterioration of sensitivity and of phosphor adhesion pattern quality from the second colour on are prevented. According to the invention this object is achieved by a method according to claim 1. In the case of coating three kinds of phosphors respectively emitting blue, green and red for forming a colour cathode ray tube phosphor screen, the exposing steps are repeated three times. The photo-tacky layer is heated before every exposing step after the adhering step for a first phosphor. In such a heating before exposure step, the photo-tacky layer is heated in the range of 40°C to 110°C. The layer fails to be activated thoroughly if the layer is at a lower temperature than 40°C and is decomposed if the temperature of the layer is over 110°C. The typical material of the photo-tacky layer contains diazonium salt as a main component. It is desirable to use infrared irradiation and or hot air blowing for the heating before exposure step. Consequently in every exposing step after adhering a first phosphor, the photo-tacky layer is activated and the sensitivity recovers about equal to the conditions in a first exposing step. In this manner, the exposure times in the second and the following exposing steps can be shortened so as to be as short as that of the first exposing step. In addition, the adhesive amount of phosphors is thoroughly secured over the whole of the phosphor screen surface, and the screen quality is enhanced. Though the heating before exposure step may be applied to all exposing steps, one heating before exposure step may be applied through the entire steps. In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a flow-chart showing the steps of one embodiment of the invention, and Figs. 2 to 7 are schematic views illustrating the steps in fig. 1. An embodiment will now be described with reference to Figs. 1-7. (Step A) First, a material which produces the adhesion to adhere powder on being irradiated with ultraviolet rays was used. This is a photosensitive aqueous solution containing per 100 parts by weight The material is coated to a thickness of about 1 µm on the inner surface of the panel of a 20-inch colour cathode ray tube, so forming a photo-tacky layer. In this process, first, the whole of the inner surface of the pannel 1, which is rotating at low speed, is wetted with the aqueous solution, and then the speed of rotation is increased in order to throw excess aqueous solution off the panel and form a photo-tacky layer 2 (Fig. 2). (Step B) Next, the panel inner surface was brought facing an infrared heater 3 and the temperature of panel 1 was raised to about 50°C to effect heating and drying (Fig. 3) and so form dried photo-tacky layer 2 on the panel inner surface. (Step C) Next, a shadow mask was assembled with the panel and when panel 1 reached a temperature of about 35°C or less by cooling, it was set on a lampr housing 4 and locations where blue phosphor was to be formed were exposed for about 2 minutes through shadow mask 6 by means of a 1 kW ultrahigh pressure mercury vapor lamp located below the panel inner surface and about 300 mm away from it. Hereupon, a pattern of adhesive surfaces capable of accepting powder was formed at the exposed sites, i.e., the locations where blue phosphor was to be fixed (Fig. 4). (Step D) Next, the shadow mask was removed and about 50 g of ZnS: Ag blue phosphor 7 with an average particle diameter of 5 µm poured onto the panel inner surface and caused to slide over the whole of the panel inner surface (Fig. 5), whereby the blue phosphor adhered to those locations of the panel inner surface to which blue phosphor was to be fixed. (Step E) Then, the excess phosphor on unexposed portions was blown off by dry air at about 8.5 m/sec from a spray gun which had seven 0.5 mm diameter nozzle holes at 50 mm intervals and was located about 200 mm from the panel inner surface, thereby effecting so-called air development and forming a set blue phosphor pattern 8 (Fig. 6). (step F) Next, the panel inner surface was brought facing an infrared heater 3 and the panel was heated again to about 50°C (Fig. 7). (steps G, H and I) The panel was assembled with a shadow mask and cooled to about 35°C or less and, as with the blue phosphor, approximately 2 minute exposure was effected and adhesion of Zns: CuAl phosphor with an average particle diameter of about 5 µm and air development were effected to form a set Zns: CuAl green phosphor pattern. (step J) Further, as with the step F, heating was effected. (step K, L and M) After heating, cooling, approximately 2 minute exposure, adhesion of phosphor and air development were effected to form a pattern of Y₂O₂S: Eu red phosphor with an average particle diameter of about 5 µm. In this manner, a panel on which phosphors of three colour triads, blue, green and red, were adhered was produced. (step N) pure water was supplied onto the inner surface of this panel at a pressure of about 1.5 kg/cm² by a 0.5 mm diameter spray nozzle, to wash excess photo-tacky material and phosphors off the panel inner surface. Examination of the phosphor screen after washing showed that no phosphor had become detached and that there was good balance of the three colours, the amounts of blue, green and red phosphor adhering being 3.2 mg/cm², 3.15 mg/cm² and 3.8 mg/cm² respectively. Next, the following test was conducted for the purpose of comparison with the method of the invention. The abovedescribed photosensitive aqueous solution was used and approximately 2 minute exposure was effected for blue, green and red and phosphor layers, but without heating the photo-tacky layer prior to exposure for the second colour and third colour. When water washing as in the exemplified method of the invention was effected, the amounts of blue, green and red phosphor adhering were low, at 1.6 mg/cm², 1.0 mg/cm² and 0.8 mg/cm² respectively and there was also found to be detachment of phosphor. The time for achieving the same amount of phosphor adhesion as in the exemplified method of the invention was 2 minutes for blue, 3 minutes 10 seconds for green and 3 minutes 45 seconds for red, and also a fixing step using ammonia vapour was necessary. When the temperature to which the photo-tacky layer was heated prior to exposure for adhesion of the phosphor of the second colour was varied, it was found that the amount of phosphor adhering increased at 40°C and over and reached a peak at 50°C or more. Heating the photo-tacky layer is limited by its decomposition. The diazonium salts used are capable of being heated up to about 110°C. Air blowing also may be used for heating separately or together with the infrared irradiation. Since air blowing elevates the temperature of the thin photo-tacky layer discretely from the panel if the panel is at a room temperature, the photo-tacky layer in the next step is cooled fast and uniformly over the entire surface due to the panel. It is believed that the layer may recover the photo-sensitivity in each of the exposing steps since these heating and cooling steps before exposure step adjust the water content of the photo-tacky layer. Therefore, it is desirable that throughout the phosphor screen forming process, the air environment may comprise non-dry air with about 20% to about 80% of relative humidity at a room temperature. Dry air however may be used both for the air blowing for developing step and the hot air blowing for heating before exposure step. Further, although the developing and heating steps are separated as the steps (E) and (F) in the embodiment above described, these steps may be combined in a single step by using hot air in the developing step. The photo-tacky material, besides the material described in the embodiment, may be used with p-Diazo-methoxybenzene chloride-zinc chloride, o-Diazo-methoxybenzene chloride-zinc chloride and so on. As described above, the adoption of the invention makes it possible to prevent the sensitivity of the photo-tacky material from the deterioration on the exposing step for coating a plurality of phosphor on a panel, thereby, a high quality phosphor screen being formed. The invention can be applied to a phosphor screen of a cathode ray tube such as a colour cathode ray tube, a cathode ray oscilloscope and the other type cathode ray tubes.

Claims: 1. A method for forming a phosphor screen of a cathode ray tube comprising the steps of: coating a photo-tacky layer on a panel of the cathode ray tube; exposing a surface of the photo-tacky layer to form a pattern of adhesive surfaces on the layer; and adhering phosphor powder to the pattern, in which method the exposing and adhering steps are repeated, two times or more, characterized in that, at least a heating before exposure step for heating the photo-tacky layer to a temperature of 40°C to 110°C before each exposing step is contained in the steps after the adhering step for a first phosphor, and the photo-tacky layer is exposed at a lower temperature than that of said heating before exposure step. 2. The method of claim 1, wherein the heating before exposure step is interposed between a preceding step for adhering a phosphor and a following step for exposing, another phosphor. 3. The method of claim 1 or claim 2, wherein the phosphors are three kinds of phosphors emitting blue, green and red respectively, and the exposing and adhering steps are repeated for every phosphor. 4. The method of any one of claims 1 to 3, wherein the photo-tacky layer contains diazonium salt as a main component. 5. The method of any one of claims 1 to 4, wherein the photo-tacky layer is heated at 50°C or more in the heating before exposure step and the photo-tacky layer is maintained at less than 40.C in the exposing step. 6. The method of any one of claims 1 to 5, wherein the photo-tacky layer is heated by hot air in the heating before exposure step. 7. The method of any one of claims 1 to 5, wherein the photo-tacky layer is heated by infrared irradiation in the heating before exposure step. 8. The method of any one of claims 1 to 7, wherein all the steps are performed in an atmosphere of non dry air. 9. The method of any one of claims 1 to 8, wherein the method comprises the steps of: coating a photo-tacky layer on a panel of the cathode ray tube; drying the photo-tacky layer; exposing the photo-tacky layer to form a pattern of adhesive surfaces on the layer; adhering phosphor powder onto the layer; and removing excess phosphor powder from the panel; wherein the steps except for the coating and drying are repeated for every phosphor. 10. The method of claim 9, wherein the method comprises the steps of: a first exposing step for exposing selectively the photo-tacky layer to form a first predetermined pattern of adhesive surfaces on the layer; a first adhering step for adhering a first phosphor powder on the adhesive surfaces; a first removing step for removing excess first phosphor powder; a heating before exposure step for heating the photo-tacky layer; a second exposing step for exposing selectively the photo-tacky layer to form a second predetermined pattern of adhesive surfaces on the layer; and a second adhering step for adhering a second phosphor powder on the adhesive surfaces; a second removing step for removing excess second phosphor powder.

Inventors: ITOU, TAKEO PATENT DIV. TOSHIBA CORP. PRINC. OFF., SAGOU, SEIJI PATENT DIV. TOSHIBA CORP. PRINC. OFF.
Applicants: KABUSHIKI KAISHA TOSHIBA, TOSHIBA CORPORATION
Application year: 1986
Main classification: H01J9
IPC codes: H01J9