1. Field of the Invention
This invention relates to a phosphor having a coating made of pigment particles (which acts as a color filter), and is called a pigment coated phosphor, and more particularly to pigment coated phosphors employed in cathode ray tubes for color televisions and to the manufacturing process thereof.
2. Description of the Prior Art
As is well known in the art, adhesion of blue pigment particles or red pigment particles to the respective surfaces of blue emitting phosphor particles or red emitting phosphor particles used in a cathode ray tube for color television markedly enhances the contrast of an image formed on the cathode ray tube, because a part of the visible region of the emitted spectra inherent to the uncoated phosphor is cut due to the filter effect of pigment particles adhering thereto resulting in a clearer emission color and further, the pigment particles can absorb a part of the external light to reduce the reflection of light therefrom (as disclosed in Japanese Laid-Open Patent Application No. 50-56146). The following conditions are required in order for pigment particles to exert the above-described effects with high efficiency when coated on phosphors of cathode ray tubes for color televisions;
1. Pigment particles should adhere uniformly to the surface of the phosphor,
2. Adhesion power of the pigment particles to the phosphor must be strong enough to prevent the particles from detaching from the surface of the phosphor during the process of phosphor-coating,
3. Resinous binders employed for allowing pigment particles to adhere to the phosphor must be such that they decompose and vaporize by heat application at the time of fluorescent screen-formation, and so on.
Until now pigment coated phosphors have been manufactured according to the following steps of: (1) dispersing pigment particles into an aqueous solution of a suitable resin such as polyvinyl pyrrolidone (PVP), (2) dispersing phosphor particles into a gelatin solution, (3) mixing the two dispersions and stirring them throughly, and (4) drying the resulting precipitate, as disclosed in, for example, U.S. Pat. No. 3,275,466.
However, such a conventional method is too time-consuming in practical use; for instance, 10 days are required for preparation of the dispersion of pigment particles, several hours are required for formation of the phosphor dispersion and further, several hours are required for drying the precipitate produced and furthermore, the precipitate produced after sufficient mixing must be carefully washed with water. In addition, there are other disadvantages. For example, since in pigment coated phosphors manufactured by such a conventional method as described above the surface of each of the phosphor particles is coated with clusters formed by pigment particles which tend to flocculate, a large quantity of pigment is necessary for obtaining the prescribed rate of coverage of the phosphor surface and therefore, there results a significant decrease in luminance. Further, pigment clusters once adhered to the phosphor surface are easily separated from the phosphor surface by rubbing with a small physical force as, for instance, when treated in a ball mill or the like.