UNITED STATES PATENT OFFICE 2,475,437 PROSPHORESCENT PIGMENT Ferdinand Stern, New York, N

Patented July 5, 1949 2,475,437 UNITED STATES PATENT OFFICE 2,475,437 PROSPHORESCENT PIGMENT Ferdinand Stern, New York, N. Y. No Drawing. Application March 9, 1948, Serial No. 13,939 7 Cairns. (C. 252-301.4) 1 2 This invention relates to phosphorescent pig to 1300° C. to the mixture. At the lower end of ments, especially of the alkaline earth metal Sul this muffling temperature range a relatively long fide type, and has for its object the provision of treating period is required, and, conversely, at an improved alkaline earth metal sulfide phos the upper end of the temperature range the heat phorescent pigment and particularly an improved ing period must be relatively short and must be calcium sulfide-strontium sulfide phosphorescent carried out With care to prevent damage to the pigment. pigment. In general, muffling temperatures with The phosphorescent properties of pigments of in the range of 1000 to 1200° C. give particularly the alkaline earth metals, particularly When ap effective results. The heat treatment is con propriately activated, have long been recognized. 0 tinued for 1 to 2 hours, which is generally suffi For example, phosphors of calcium sulfide and cient to effect the necessary diffusion of the acti strontium sulfide, and mixtures thereof, have vator throughout the pigment, and the resulting heretofore been commonly activated by bismuth. mixture is allowed to cool under non-oxidizing I have discovered that the luse of copper as an conditions, and is then carefully ground. The auxiliary activator With bismuth produces sig 5 grinding should be relatively gentle in order to nificant increases in the phosphorescent bright avoid destroying the phosphorescence by crush ness of alkaline earth metal Sulfide phosphors. ing. The heat treatment may advantageously be These increases in phosphorescent brightness carried out in two stages of 40 to 60 minutes each generally range between about 100 and 200% for with intermediate cooling and grinding. the period of 30 minutes to 8 hours after excita 20 Although the phosphorescent pigment of the tion of the phosphor. The present invention, invention may comprise a single alkaline earth based on that discovery, involves an improved metal Sulfide, Such as calcium Sulfide or Stron phosphorescent pigment of the alkaline earth tium sulfide, it is preferably composed for the metal sulfide type containing as activators mi most part of calcium Sulfide and strontium Sul nute amounts of both bismuth and copper. 25 fide in ratios varying from 80:20 to 20:80. Sig The activation of the alkaline earth metal Sul nificant increases in phosphorescence are ob fide is commonly effected by muffling or other tained from the copper activator, used in con Wise suitably heat-treating at elevated tempera junction with the bismuth activator, even when ture a mixture of the sulfide, or a plurality of the the formulation of the phosphors is widely varied. sulfides, and a minute amount of the activator in Thus the ratios of crude calcium sulfide to crude the form of a salt, such for example as bismuth Strontium sulfide may be varied from 100:0 to nitrate. While it is customary to refer to the ac 0:100 and significant improvement in the phos tivator in the resulting phosphorescent pigment phorescent brightness Will be obtained as a result as "bismuth,' it Will be understood that the bis of the inclusion of the copper activator along muth is not present in the elemental form but with the bismuth activator. Formulations based as a compound of bismuth resulting from the de On the carbonates and Oxides of Calcium and composition at elevated temperature of the bis strontium together With elemental sulfur also muth Salt in the presence of the alkaline earth show improvement when copper is used as an ac metal sulfide and any flux, sulfur or reducing tivator in accordance with the invention. agent that may be mixed there with. 40 Only a minute amount of each activator is in The improved phosphorescent pigment of the cluded in the pigment, usually a few hundredths invention is similarly made by heat treating at or less of one per cent of bismuth, and a few elevated temperature a mixture comprising an thousandths or less of one per cent of copper by alkaline earth metal sulfide either as a single Sul weight being sufficient, the percentages being fide or as a plurality of such sulfides, and con 45 based on the weight of the alkaline earth metal taining as activators minute amounts of both sulfide or sulfides. For practical reasons it is a bismuth salt and a copper salt. In addition to the activators, small amounts of fluxes, such, for more convenient and more accurate to express example, as one or more alkali metal salts, Sul the minute amounts of the two activators in fur and a reducing agent may be included in the 50 cluded in the finished pigment in terms of the mixture. The heat treatment is carried out in amounts admixed with the raw alkaline earth a non-oxidizing and preferably inert or slightly metal sulfide or sulfides prior to heat treatment. reducing atmosphere such, for example, as sul Expressed thus in Weight per cent based on the fur vapor from the added sulfur or carbon mon total weight of alkaline earth metal sulfide or Sul Oxide from the added reducing agent, and may be 55 fides in the mixture prepared for heat treatment, advantageously conducted in a retort or muffle the optimum and useful ranges of the two acti externally heated to impart a temperature of 900 vators are as follows: - . 2,475,437 4. about 1-2% by weight based on the weight of Optimum the mixture. range Useful range Elemental Sulfur, say about 5% by weight, and Copper (calculated as Cu).----------- 0.0031-0, 0055 0.0005-0.007 magnesium oxide, also around 5% by weight, Bismuth (calculated as Bi).----------- 0.015–0.07 0.005-0.028 S may also be advantageously included in the mix ture. Sulfur stabilizes the sulfide pigments and also protects the mixture from oxidation during In Straight bismuth-activated phosphors, phos the heat treatment, and magnesium oxide in phorescence is at a maximum when the bismuth hibits any tendency of the mixture to fuse or concentration is close to 0.03% for high calcium 0 Sinter. In addition, the mixture may advanta phosphors and 0.017% for high strontium phos geously include a reducing agent readily decom phors. As hereinbefore explained, these figures posable by heat, such as starch. are based on the Weight of crude alkaline earth Warious SaltS or oxides of the activating metals metal Sulfide (e. g. combined calcium sulfide and (copper and bismuth) may be added separately Strontium Sulfide) and are equivalent to one aton 5 or together to different ingredients of the crude of bismuth for each 12,000 molecules of the sul mix Without appreciably affecting the phospho fide or Sulfides. When copper is included with rescent properties of the finished pigment. The bismuth, phosphorescence is at a maximum preferred method of mixing is to grind the acti throughout the period of 1 minute to 8 hours after vators into the flux with a little alcohol, add excitation with copper in the range of 0.0031 to 20 the crude calcium sulfide and strontium sulfide 0.0055% and bismuth in the range of 0.015 to and grind to a uniform mix, and finally magnesia, 0.017%. Thus, in these phosphors the weight is ground in. The alcohol is allowed to evapo of copper is about one-third to one-fifth the rate, then Sulfur and starch are ground in. In Weight of bismuth. The wider useful ranges of large Scale practice for the manufacture of cop copper and bismuth hereinbefore recited yield per-bismuth-activated pigments, all of the in phosphors Superior to the best straight bismuth gredients, except the calcium Sulfide and stron activated phosphors. For example, throughout tium sulfide, may be advantageously sprayed with the entire range of 0.005-0.028% added bismuth, a water solution of the activators, then dried and as little as 0.0005% added copper is effective in mixed With the calcium sulfide and strontium Synergistically decreasing the rate of phosphores. 30 Sulfide. cent decay in accordance with the invention. In The compositions of a typical Series of phos creasing amounts of copper up to 0.007% are also phors of the invention are given below, the war effective throughout the aforementioned range ious constituents being expressed in terms of the of bismuth content. Copper contents materially amount included in the crude mixture prepared above 0.007% tend to lose their effectiveness and 3. for heat treatment: even increase the rate of phosphorescent decay Gians of the bismuth-activated pigment. Copper nitrate .3H2O ------------------ 0.0012 Because of the difficulty of uniformly distrib Calcium sulfide ------------------------ X. uting the minute amounts of bismuth and cop Sulfur -------------------------------- 0.2 per salts throughout the pigment mixture, these Bismuth Submitrate .H2O --------------- 0.0024 salts are advantageously those capable of being Strontium Sulfide ---------------------- y dissolved in a volatile Solvent which has no ad Sodium carbonate ---------- - - - - - - - - - - 0.36 verse effect upon the composition of the mix Lithium sulfate ----------------------- 0.18 ture. Thus, bismuth nitrate, bismuth Subnitrate Magnesia ----------------------------- 0.40 (to which bismuth nitrate largely hydrolyzes in Sulfur -------------------------------- 0.89 water), bismuth chloride and bismuth citrate Starch (rice or corn) ------------------ 0.44. may be used as the bismuth compound, and cop per nitrate, copper chloride, copper sulfate and In this series of phosphors the ratio of calcium copper acetate are examples of Suitable copper sulfide to strontium sulfide (Cas: SrS) was varied compounds.

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