Patent GB-802910-A (GB)
Title: Improvements in or relating to electric incandescent lamps
IPC: H01K1/32
Filed: 1955-07-14
Abstract: 802,910. Coating electric lamp bulbs internally. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. July 14, 1955 [July 19, 1954], No. 20374/55. Class 39(2) [Also in Group XL(a)] Bulbs for electric incandescent lamps are provided with an internal surface coating or finely divided, porous, amorphous silica which has the moisture gettering ability or property of accumulating at least 0.4 per cent by weight of moisture, at the normal operation minimum temperature of the lamp. Preferably the applied silica coating itself possesses, before sealing, not less than 1.55 per cent and not more than 4.8 per cent by weight of moisture. The requisite moisture gettering property of the silica coating is stated to be attained by a preliminary baking of finely divided silica powder at a temperature not exceeding 625‹C. followed by a cooling to room temperature, during which cooling it is maintained under substantially moisture- 'free conditions. A series of curves are furnished showing moisture loss against firing temperatures for silica powders which originally possessed 12 per cent by weight of moisture, and illustrating how the silica loses its moisture gettering characteristics at temperatures above 625‹C. The finely divided silica powder possessing the requisite properties is preferably applied to the inside surface of a lamp bulb by an electrostatic process, the bulb 22, Fig. 8, being mounted on an insulating lava chuck 36 which is rotated either manually or automatically while subjected to heating at a temperature of about 100‹C. by gas-air burners 42. Fig. 7 shows a " smoke " generator unit comprising a silica powder and silica smoke reservoir 46 with a bottom outlet 48 through which silica is admitted into a mixing venturi 50 connected rearwardly through a regulating valve 52 and dryer 68 to an air compressor 66. Silica powder is thus fed through a conduit 54 and impinges on a target 56, thereby forming a smoke of finely divided silica particles suspended in air. The silica within the reservoir 46 is maintained in a finely divided state by a power-driven agitator 70 near the base of the reservoir. A conduit 60 leading from the top of the reservoir and embodying a manually controlled butterfly valve 64 connects the reservoir to a smoke - injector nozzle-assembly 62, Fig. 8, located within the insulating lava chuck 36 carrying a lamp bulb 22. The nozzle assembly 62 is disposed circumferentially about a probe 76 connected to a source of high voltage pulsating direct current; and a collar 80 which supports the nozzle assembly within the lava chuck provides a return conduit or outlet passage for the air that is present within the bulb and for the silica smoke which does not deposit on the bulb wall during the coating operation. The coated bulb is baked or lehred while being rotated, to drive off any moisture that may have accumulated during the coating operation, e.g. a lehring temperature of 350‹C. for a period of 10 to 20 seconds will be employed for a silica coating comprising silica powder fired originally at a temperature of about 500‹C. Clear glass bulbs and commercially available silica are employed in carrying out the invention. A small amount of a known moisture getter consisting of 20 per cent aluminium and 80 per cent zirconium suspended in a nitrocellulose, amyl-acetate, binder vehicle may be painted on the lamp stem. Specification 653,323 and U.S.A. Specification 2,661,438 are referred to.