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Process for Producing a Phosphor

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Process for Producing a Phosphor Europa,schesP_ MM M II M MM IN M I M M MM II J European Patent Office _ _ _ © Publication number: 0 372 560 B1 Office_„. europeen des brevets EUROPEAN PATENT SPECIFICATION © Date of publication of patent specification: 08.03.95 © Int. CI.6: C09K 11/02 © Application number: 89122598.9 @ Date of filing: 07.12.89 © Process for producing a phosphor. ® Priority: 08.12.88 JP 310648/88 © Proprietor: KASEI OPTONIX, LTD. 12-7, Shibadaimon 2-chome @ Date of publication of application: Mlnato-ku 13.06.90 Bulletin 90/24 Tokyo 105 (JP) © Publication of the grant of the patent: @ Inventor: Tono, Hideo 08.03.95 Bulletin 95/10 Kasel Optonlx Ltd., 1060 Naruda, Odawara-shl © Designated Contracting States: Kanagawa-ken (JP) DE GB IT NL Inventor: Mlyazakl, Tomohlro Kasel Optonlx Ltd., 1060 Naruda, © References cited: Odawara-shl GB-A- 2 034 910 Kanagawa-ken (JP) US-A- 4 287 257 PATENT ABSTRACTS OF JAPAN, AP-No. © Representative: Wachtershauser, Gunter, Prof. JP860312537, Tokyo, JP; & JP-A-63 284 290 Dr. Patentanwalt Tal 29 D-80331 Munchen (DE) 00 o CO m CM oo Note: Within nine months from the publication of the mention of the grant of the European patent, any person ® may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition CL shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee LU has been paid (Art. 99(1) European patent convention). Rank Xerox (UK) Business Services (3. 10/3.09/3.3.3) EP 0 372 560 B1 Description The present invention relates to a process for producing a phosphor, notably for a cathode ray tube. As is well known, phosphor screens for color television cathode ray (picture) tubes are composed of 5 blue, green, and red emitting phosphor dots or stripes of blue, green, and red emitting phosphor components disposed regularly on a face plate. The phosphor screens for color television picture tubes are prepared by a photo-printing method. That is, a first light emitting phosphor component is dispersed in a solution of a light sensitive resin such as, for example, an aqueous solution of polyvinyl alcohol activated with a dichromate, to provide a phosphor slurry. The phosphor slurry obtained is applied over the whole io surface of a face plate by an appropriate coating method such as by rotary coating, slurry coating, and then the coated layer is irradiated by energy rays such as ultraviolet rays, in conformity with a desired pattern, whereby the resin is hardened and insolubilized at the portions irradiated by the energy rays (light exposure). Thereafter, the resin at the non-irradiated portions (unhardened resin portions) is dissolved away by a solvent (development) to form dots or stripes composed of the first light emitting phosphor component. 75 Then, by successively repeating slurry coating, exposure and development in the same manner as described above using second and third light emitting phosphor components, dots or stripes composed of the second and third light emitting phosphor components are formed on the face plate. In this case, as matter of course, the energy ray irradiation must be so controlled that the dots or stripes composed of each of the first, second, and third light emitting phosphor components are repeatedly and regularly disposed on 20 the face plate without being overlapped with each other. Then, the fluorescent screen containing the resin component thus prepared is fired at a proper temperature to decompose and volatilize the resin component, whereby a desired phosphor screen is obtained. For making phosphor screens for color television picture tubes by a photo-printing method using the above-mentioned phosphor slurries, the following requirements must be met. 25 1 . The phosphor screen must be dense and have little pin holes or the like; 2. One light emitting phosphor component must not intermix with other light emitting component phosphors, i.e. color mixing must not occur; 3. The phosphor slurry must have high light exposure sensitivity and must be easy to work with; 4. Formation of the light emitting elements (dots or stripes) must be excellent, i.e. the edges must be 30 sharp, the dots must be circular, and the stripes must be linear; 5. The exposed phosphor screen must not fall off the face plate by high pressure aqueous development treatment during the development process, i.e. it must have a high adhesive strength. Heretofore, various studies and improvements have been made with respect to the surface treatment of phosphors, the compositions and the method for the preparation of phosphor slurries with an aim to satisfy 35 the above requirements. For example, Japanese Examined Patent Publication No. 21675/1985 discloses a phosphor having its surface coated with zinc hydroxide for the purpose of improving items 2 and 3 among the above requirements. With respect to the color television picture tubes, there has been an increasing demand for improving the image quality in recent years. For this purpose, it is necessary to form finer picture elements on a face 40 plate so that a highly fine image can be formed. To fulfill the above demand, it is firstly required to satisfy the requirement that the adhesive strength of the screen portion hardened by the exposure is high (above requirement 5) and the requirement that color mixing must not occur (above requirement 2), simultaneously. However, in order to increase the adhesive strength of phosphor screens by conventional methods for forming phosphor screens, a method is usually 45 employed wherein the drying is strengthened at the time of coating, whereby there will be a difficulty such that the higher the adhesive strength of the phosphor screen, the more the unnecessary phosphors tend to remain on other position, thus leading to color mixing. With the above-mentioned phosphor coated with zinc hydroxide, it is hardly possible to satisfy such mutually opposing requirements at the same time. It is conceivable to increase the amount of zinc hydroxide coated in order to adequately prevent color mixing in 50 the above-mentioned phosphor coated with zinc hydroxide. However, if the amount of the coated zinc hydroxide increases, the action for aggregation of zinc hydroxide colloid increases in the phosphor slurry, whereby the dispersibility of the phosphor in the slurry deteriorates, which in turn tends to lead to a decrease of the adhesive strength, formation of pin holes or a deterioration in the sharpness of the edge. US-A-4 287 257 describes a process for adhering a zinc hydroxide onto the surface of a phosphor in a 55 slurry which is colloidal and tends to decrease the dispersibility of the powder. The present invention has been made in view of the above problems, and it is an object of the present invention to provide a process for producing phosphor which is capable of preventing the occurrence of color mixing of the phosphor screen and increasing the adhesive strength of the phosphor screen at the 2 EP 0 372 560 B1 same time and which is excellent in the dispersibility in the slurry, and a method for treating the surface thereof. The present inventors have conducted extensive researches to accomplish the above object and as a result, have found that among phosphors having zinc oxide attached in their surface, those obtained by 5 forming and attaching zinc oxide on the phosphor surface in a suspension of a phosphor, have the above- mentioned desired properties for forming phosphor screens. Namely, the phosphor as prepared by the process of the present invention comprises a phosphor and zinc oxide formed in a suspension of the phosphor and attached to the surface of the phosphor. Here, "zinc oxide is formed in a suspension" means that zinc oxide is formed by reacting zinc ions and an alkaline io solution in the suspension. Now, the present invention will be described in detailed with reference to the preferred embodiments. In the companying drawings: Figures 1(a) and (b) are microscopic enlarged photographs of phosphor particles having zinc oxide formed in a suspension attached on their surface. is Figures 2(a) and (b) are microscopic enlarged photographs of phosphor particles having zinc oxide attached merely by mixing phosphor and ready made zinc oxide on their surface. Figures 3(a) and (b) are microscopic enlarged photographs of phosphor particles having zinc hydroxide attached on their surface. Figures 4(a), (b) and (c) are diagrammatical views of the above-mentioned phosphor particle having zinc 20 oxide attached on its surface, the phosphor particle having zinc oxide deposited on its surface and the phosphor particle having zinc hydroxide attached on its surface. Figure 5 is a graph showing the temperature and pH ranges in which zinc oxide is formed. Figures 6 and 7 are graphs showing cross-contamination of the phosphor having zinc oxide attached on its surface, the phosphor having zinc oxide deposited on its surface and the phosphor having zinc 25 hydroxide attached on its surface. Figures 1(a) and (b) show photographs of the phosphor produces by the process of the present invention enlarged 5,000 and 20,000 magnifications, respectively, by an electron microscope. Figures 2(a) and (b) show, for the purpose of comparison with the phosphor produces by the process of the present invention, electroscopic photographs with 5,000 and 20,000 magnifications, respectively, of a phosphor 30 having zinc oxide deposited on its surface by adding a dispersion of commercially available zinc oxide to a suspension of the phosphor, followed by stirring and mixing. Further, Figures 3(a) and (b) are electron microscopic photographs with 5,000 and 20,000 magnifications, respectively, of a phosphor having zinc hydroxide formed in a suspension of the phosphor attached on its surface.
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