Method of improving the output and maintenance of a fluorescent lamp
This is a continuation of application Ser. No. 634,959, filed Nov. 24, 1975, now abandoned. 
    
    
     THE INVENTION 
     This invention concerns fluorescent lamps. Such lamps are low pressure mercury arc discharge devices which have electrodes at each end of an elongated glass envelope and which contain a phosphor coating on the inner surface of the glass envelope. The invention is particularly concerned with lamps utilizing lead activated barium mesosilicate phosphor. This phosphor is an ultraviolet emitting phosphor peaking at about 351 nanometers. 
     I have found that in such lamps the initial output and the maintenance can be significantly improved by adding a heat decomposable or vaporizable chloride, for example, ammonium chloride, to the phosphor coating suspension. I believe that the chloride improves lamp performance by eliminating any lead oxide that may occur during lamp processing, particularly during the lehring operation. The lead activator in the phosphor has a tendency to become oxidized during the lehring operation and to form a film of yellow PbO on the surface of the phosphor crystals. Although the amount of PbO formed is very small, it is nevertheless sufficient to cause a shift of approximately 3 nanometers in the peak of the emission due to the filtering effect. The NH 4  Cl addition according to the invention reacts readily with the PbO to form PbCl 2  which is sufficiently volatile at the lehring temperature of approximately 650° C. to be eliminated from the surface of the phosphor film. 
     The barium mesosilicate phosphor can be prepared by conventional methods in which typical raw mix constituents, such as BaCO 3 , H 2  SiO 3 , BaCl 2  and Pb acetate, are mixed together in proportions to satisfy the phosphor formulation corresponding to BaSiO 5  :Pb.sub..03 F.sub..09 and fired in silica crucibles at temperatures ranging from 1000 to 1100° C. for 4 to 8 hours. After cooling, the phosphor is ball-milled in water for 2 hours and filtered to remove the excess water. The wet cake is resuspended in a conventional water-base vehicle to which is added NH 4  Cl in amounts ranging between 0.5 and 5%, and preferably 2% by weight of the dry phosphor. The resulting suspension may also include the usual wetting and defoaming agents known in the art and is used to coat the fluorescent bulbs in the conventional manner. 
     After drying, the bulbs are lehred for approximately 3 minutes at a temperature ranging between about 600° and 650° C. and the lamps are finished in the conventional manner. 
     As a specific but non-limitative example according to the invention, 400 grams of lead activated barium mesosilicate phosphor were milled for 2 hours in a pebble mill containing 600 ml of deionized water. 
     After milling, the water suspension was filtered under vacuum and the cake was resuspended in 1000 ml of 5.4% polyoxyethylene vehicle diluted with 300 ml deionized water and containing 2.4 grams of fumed aluminum oxide as adherence promoter. 
     The resulting suspension was divided into three equal portions, the first one being used as control. To the other two portions were added respectively 1.35 and 2.7 grams NH 4  Cl corresponding to 1 and 2% by weight of dry phosphor. 
     These suspensions were used to coat fluorescent 4CT12 lamps which gave the following results in relative units of ultraviolet emission: 
     
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            0 Hour  100 Hours  1000 Hours                                 
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Control (no addition)                                                     
              14.352    11,248      9,213                                 
1% NH.sub.4 Cl Addition                                                   
              17,052    14,128     10,941                                 
2% NH.sub.4 Cl Addition                                                   
              17,136    14,660     11,885                                 
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     It can be seen that the chloride additions increased the initial output from 14,352 to over 17,000, an increase of more than 18%. 
     A similar test was conducted in 48T12-VHO lamps giving the following values: 
     
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                                     1000                                 
                                     Hours                                
                                     Main-                                
            0      5000     1000     ten-                                 
            Hour   Hours    Hours    ance                                 
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Control (no addition)                                                     
              28,125   17,428   13,499 47.8%                              
2% NH.sub.4 Cl Addition                                                   
              32,353   24,905   19,705 60.9%                              
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     In this test the chloride addition increased initial output by 15% and also increased the 100 hour maintenance by 13%. 
     When the suspension used to coat the phosphor is organic solvent based, instead of water-base, the heat decomposable or vaporizable chloride can comprise an organic chloride which is soluble in the solvent.