Stir-in fluorescent lamp phosphor and method of making same

#This invention relates to a method for coating particles of a fluorescent lamp phosphor with a polymer to prevent agglomeration and improve dispersability of the phosphor in aqueous and organic binder media. More particularly, the invention relates to a method of coating a magnesium tungstate fluorescent lamp phosphor.

#Fluorescent lamp phosphors which emit white light may be comprised of phosphors which emit in the red, blue and green spectral regions. Magnesium tungstate is a blue-emitting fluorescent lamp phosphor which may be blended with manganese-activated zinc silicate, a green-emitting fluorescent lamp phosphor, and tin-activated strontium orthophosphate, a red-emitting fluorescent lamp phosphor, to produce a white-emitting fluorescent lamp phosphor.

#The interior surface of a fluorescent lamp envelope is coated with a phosphor which has been suspended in an organic or aqueous binder. Many fluorescent lamp phosphors, including manganese-activated zinc silicate and tin-activated strontium orthophosphate, are readily dispersed in such binders and are said to have "stir-in" capability. However, some fluorescent lamp phosphors, such as magnesium tungstate, have a very fine particle size and tend to agglomerate when mixed into the binder. Because of this tendency to agglomerate, the binder suspension containing such phosphors must be milled to provide a smooth coating on the inside walls of a fluorescent lamp envelope. Such milling adds time and expense to the phosphor manufacturing and blending process.

#Some fluorescent lamp phosphors have very fine and/or irregularly shaped particles. For example, the particles of magnesium tungstate fluorescent lamp phosphor are very fine and have a shape which resembles a kernel of popcorn, with many surfaces and cavities. Magnesium tungstate phosphor has a low bulk density because its irregularly shaped particles cannot be closely packed. It is believed that magnesium tungstate phosphor particles agglomerate either because of hydrogen bonding between particles or the presence of surface electrostatic charges which tend to attract particles to one another. This tendency of magnesium tungstate phosphor to agglomerate results in poor dispersion of the phosphor in liquid binder vehicles.

#The method of the invention involves the formation of a polymer coating on the particle surfaces of a fluorescent lamp phosphor to prevent agglomeration of the phosphor in organic and aqueous binder media. A latex polymer and fumed silica are added to an aqueous slurry of the phosphor so that there is a 1:1 ratio of polymer solids to silica. The slurry is stirred until a uniform dispersion is obtained. The liquid is then separated from the solids by decantation. The resulting solids are dried and screened to produce polymer-coated fluorescent lamp phosphor particles consisting of about 0.5 percent by weight polymer and about 0.5 percent by weight silica. Dispersability of the polymer-coated fluorescent lamp phosphor in both aqueous and organic binder media is vastly improved, resulting in a smooth, even phosphor coating on the inside walls of a fluorescent lamp envelope. Costly and time-consuming milling of the phosphor in the liquid binder is no longer necessary when the method of this invention is followed.

#A latex is an aqueous colloidal suspension of at least one organic polymer or copolymer. The amount and type of latex polymer is not critical to the method of the invention. It has been determined that from about 0.2 to 2.0 percent polymer by weight of phosphor is sufficient to impart good dispersion properties. Anionic latex polymers are preferred. Some latex polymers which give acceptable results include: 1) a carboxylated acrylate acrylonitrile copolymer manufactured by Reichold under the name of Synthemul DL-0685; 2) a styrene/acrylic copolymer manufactured by Union Carbide Corp. under the name of UNOCAL RES1018; and 3) an acrylic polymer manufactured by Rohm & Haas Co. under the name of Rhoplex B-85.