Patent GB-678165-A (GB)
Title: Improvements in or relating to the deposition of powdered materials on surfaces by sedimentation
IPC: H01J9/22
Filed: 1949-04-12
Abstract: 678,165. Luminescent materials. GENERAL ELECTRIC CO., Ltd., and WELCH, E. E. July 12, 1950 [April 12, 1949], No. 9933/49. Class 39 (i). [Also in Group XV] In a method of forming upon a surface an adherent layer of powdered material by sedimentation from an aqueous settling liquid in which the powder is insoluble or only partly soluble a water-soluble alginate is added to the settling liquid. The alginate acts as a dispersing and wetting agent and as a binder. The method is particularly applicable to depositing luminescent powders on the envelopes of discharge tubes such as cathode-ray tubes, whether or not interiorly coated with metal, e.g. silver, as the soluble alginate partly reacts with the glass or silver to form a gelatinous precipitate of calcium or silver alginate respectively. The method may also be applied to similarly reacting vitreous materials, metals or to mica. The alginate may be ammonium alginate, or an alginate of sodium, potassium, lithium or magnesium, or triethanolamine alginate of the trade name " Manucol T." The ammonium alginate may be a highly polymerized grade in a concentration between 30 milligrams and 2.5 grams, e.g. 500 milligrams, per litre of the suspending liquid. To ensure a uniform layer thickness a small proportion of one or more electrolytes not reacting with the alginate may be added, e.g. magnesium chloride, nitrate or sulphate or ammonium chloride, in a concentration of 5-20 millimols per litre of the suspending liquid. The layer may be given an overbinder of tetraethyl orthosilicate dissolved in alcohol at a concentration of 1-10 per cent and/or baked in air at a temperature of 350- 450‹ C. When baked, the alginate concentration may be as low as 30-50 milligrams per litre. The method permits the use of relatively coarse powder particles of 5-30 Á. equivalent diameter. The surface to be coated is cleared and covered to a depth with a settling liquid containing water-soluble alginate, then a concentrated suspension of powder in an aqueous liquid containing the samewater-soluble alginate or other dispersing agent, or pure water added, then a settling period of 1-2 hours allowed before removing the supernatant liquid by decanting slowly, syphoning or draining, finally the coating being dried by warm air. The ammonium alginate used may be clarified by settling for a long period, or by centrifuging of filtering. The powder is wetted with alginate solution or other dispersing agent, e.g. blood serum, or the phosphate preparation sold under the Registered Trade Mark " Calgon," or with pure water, in a ball mill, i.e. alginate may be used only in the settling liquid. Ammonium chloride electrolyte at a concentration of 5-10 millimols. per litre is preferably employed in the settling liquid in the absence of " Calgon " or other dispersing agent. When Calgon " is used magnesium chloride or nitrate at 10 millimols per litre is preferably used, or at 2-5 millimols per litre concentration if pure water or alginate solution is employed. With some powders the suspending liquid in the concentrated suspension may periodically be replaced to remove any injurious soluble material. The dried powder layer may be washed with an aqueous solution of a calcium salt, e.g. calcium chloride or nitrate, or an acid, e.g. very dilute hydrochloric acid, to liberate insoluble alginic acid, to prevent the alginate film or deposited soluble alginate from redissolving during any further process involving soaking in water. Residual electrolyte preferably is removed by washing with warm distilled water. Double or triple layers may be produced. Thymol may be added to the solution to prevent attack by bacteria. A powder concentration of 30 gms. per litre in the concentrated suspension liquid may be used. Various specific examples are described. Specification 669,848 is referred to.