Fluorescent lamp with a low reflectivity protective film of aluminum oxide

The light output and lumen maintenance of fluorescent lamps with or without a conductive film, are improved by coating the glass envelope or the conductive glass envelope with a thin transparent film of Al.sub.2 O.sub.3. The alumina film is obtained from a coating of a binderless suspension of submicron size Al.sub.2 O.sub.3 particles in lightly acidified water and has a thickness of 20 to 80 nanometers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention concerns fluorescent lamps, which are electric lamps 
comprising a glass envelope having a coating of phosphor on its inner 
surface, which have electrodes at each end and which contain a fill 
including low pressure mercury vapor. 
This invention particularly relates to protective coatings applied to 
conventional fluorescent lamp glass envelopes. 
2. Description of the Prior Art 
It is well known in the art that the light output and the lumen maintenance 
of fluorescent lamps are affected by a progressive darkening of the bulb 
during the useful life of the lamp. The darkening is commonly attributed 
to disclorations resulting from the amalgamation of mercury with sodium at 
the inner surface of the glass under the influence of impinging 
ultraviolet radiations. Mercury is present in the lamp fill and sodium is 
present in the glass. 
In a special type of fluorescent lamp the inner surface of the glass is 
coated with a transparent electroconductive layer of tin oxide or indium 
oxide in order to achieve satisfactory ignition characteristics. In this 
particular case, the darkening is compounded by additional discolorations 
resulting from the conductive coating. This is particularly true of the 
white conductive SnO.sub.2 which can be reduced to the black SnO. 
Many types of protective coatings in fluorescent lamps have been disclosed 
most of which are relatively thick and porous due to the method of 
application by dispersion in an organic binder followed by the 
conventional coating, drying and baking process. 
Such is the case for a coating of zinc oxide, titanium oxide or cerium 
oxide disclosed in U.S. Pat. No. 2,774,903, issued to L. Burns, Dec. 18, 
1956. 
In U.S. Pat. No. 3,141,990, issued to J. G. Ray, July 12, 1964, the 
TiO.sub.2 is 12 to 25 microns thick. 
Another thick layer of TiO.sub.2 is disclosed in U.S. Pat. No. 3,379,917, 
issued Apr. 23, 1968 to R. Menelly and an alumina layer of 1 to 10 microns 
thick with a thin layer of titania is disclosed in U.S. Pat. No. 
3,599,029, issued Aug. 10, 1971 to Martyny. 
While the thick protective layers achieve some improvement in maintenance, 
they also introduce the disadvantage of reducing the initial light output. 
In order to reduce the initial light loss, much thinner coatings of 
TiO.sub.2 and ZrO.sub.2 have been disclosed, as in U.S. Pat. No. 3,377,494 
to R. W. Repsher on Apr. 9, 1968. 
However, thin films of TiO.sub.2 have been noted to cause starting problems 
in fluorescent lamps, hence the disclosure of Sb.sub.2 O.sub.3 additions 
to such films in U.S. Pat. No. 3,541,376, issued Nov. 17, 1970 to Sadoski 
and Schreurs. 
In U.S. Pat. No. 3,748,518, issued June 14, 1972 to D. Lewis, it is stated 
that a titania film 10 to 20 nanometers thick reflects the ultra violet 
radiation back into the phosphor. U.S. Pat. No. 3,624,444, issued to F. 
Berthold on Nov. 30, 1971 discloses the necessity of protective layers 
over tin oxide conductive films to prevent the formation of black stains 
already occurring after 50 operating hours. In this case, the protective 
layers have a thickness of 50 to 150 nanometers and consist of oxides of 
titanium, zirconium, hafnium, niobium and tantalum. 
Very thin films with a thickness less than 200 nanometers have so far only 
been produced by vapor deposition or from hydrolyzed solutions of 
relatively expensive metal organic compounds such as tetraisopropyl 
titanate or titrabutyl titanate and require elaborate controls to produce 
the required thickness with reproducible accuracy. 
SUMMARY OF THE INVENTION 
The object of the invention is to provide a protective coating within a 
fluorescent lamp which will improve the maintenance while increasing the 
initial light output. A further object is to provide a film which is most 
economical and easily applicable to high speed production. 
In accordance with the invention, the increase in inital light output is 
achieved by a compact film of aluminum oxide between 20 and 80 nanometers 
thick acting as an anti-reflective layer for the visible light. In 
addition, the film of Al.sub.2 O.sub.3 acts as a protective barrier 
between the glass and the phosphor or between the conductive layer and the 
phosphor in the electroconductive fluorescent lamps. According to my 
preferred process, the film of Al.sub.2 O.sub.3 is applied by flushing 
down the bulb an aqueous dispersion of fumed alumina having a surface area 
of 100 m.sup.2 /gram minimum and drying the film with hot air or 
preferably by zone drying, as disclosed in my U.S. Pat. No. 3,676,176. The 
phosphor coating is then applied in the usual manner using an organic 
binder. A single lehring is sufficient and the fluorescent lamp is 
processed in the manner known in the art.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Fumed alumina with a surface area equal or greater than 100 m.sup.2 per 
gram is commercially available and can readily be suspended in water with 
homogenizers or colloid mills. The fluidity of such suspensions depends on 
the concentration and the pH and the latter can be adjusted with mineral 
or organic acids. I found it very convenient to prepare fumed alumina 
suspensions containing 30% solids by weight. The pH of the suspension is 
adjusted to a value between 2 and 4 with a slight addition of HCL. Such 
dispersions remain stable for months and can be diluted with water to the 
desired concentration level. In preparing the Al.sub.2 O.sub.3 coating 
according to the invention, advantage is taken of the exceptional positive 
surface charge developed by the fumed alumina particles in aqueous or 
other highly polar suspensions. This positive charge renders the resulting 
films highly substantive to glass and, consequently, very strongly 
adherent to the glass envelope of the fluorescent lamp. This strong 
adherence is still maintained if the glass has been previously coated with 
an electroconductive tin oxide or indium oxide film. Furthermore, the 
subsequent phosphor layer which normally has a slight negative charge is 
now substantive to the positively charged Al.sub.2 O.sub.3 layer. In 
consequence, the interpostion of my Al.sub.2 O.sub.3 layer between the 
glass surface and the phosphor film results in increased overall adherence 
and reduces the manufacturing losses due to knocking or shaping the 
fluorescent lamp envelope. 
As a specific example of protective coatings in fluorescent lamps, a series 
of 40T12 fluorescent lamp bulbs were coated at different concentrations of 
Al.sub.2 O.sub.3 in order to determine the optimum thickness of Al.sub.2 
O.sub.3 film. The coatings were obtained by diluting a 30% solids by 
weight fumed alumina suspension with deionized water and adding a suitable 
wetting agent to insure a complete coverage of the glass envelope during 
the down-flushing. A most satisfactory wetting was obtained by adding an 
ampholytic surfactant at a concentration of 0.5% in the final coating. 
After flushing, the glass bulbs were dried by the zone drying method using 
radiant heaters. Such drying is most efficient since it requires about 60 
seconds for a 4 foot bulb. After drying the bulbs were coated with a cool 
white halophosphate phosphor dispersed in an organic vehicle and processed 
into lamps in the manner well known in the art. 
The light output of these lamps is given in the following table expressed 
in percent relative to the control. 
______________________________________ 
RELATIVE LIGHT OUTPUT 
VERSUS Al.sub.2 O.sub.3 CONCENTRATION 
100 HOURS 
500 HOURS 1750 HOURS 
______________________________________ 
1. 12 mg Al.sub.2 O.sub.3 /ml 
100.1 99.7 101.0 
2. 18 mg Al.sub.2 O.sub.3 /ml 
100.6 101.5 103.0 
3. 24 mg Al.sub.2 O.sub.3 /ml 
101.4 101.5 103.0 
4. 30 mg Al.sub.2 O.sub.3 /ml 
101.3 101.5 102.4 
5. 36 mg Al.sub.2 O.sub.3 /ml 
100.5 99.4 100.0 
6. 60 mg Al.sub.2 O.sub.3 /ml 
100.0 98.5 -- 
Control - no precoat 
100 % 100 % 100 % 
______________________________________ 
It is quite apparent from these results that the most efficient film was 
obtained under the operating conditions, at a concentration of 24 mg/ml of 
Al.sub.2 O.sub.3 in the coating. Electron microgaphs of this particular 
film revealed a very thin compact layer of Al.sub.2 O.sub.3 approximately 
50 nanometers thick. 
In another example of Al.sub.2 O.sub.3 coatings according to the invention, 
a direct comparison was established between regular fluorescent lamps at 
various gas compositions and similar lamps with a conductive film of 
indium oxide overcoated with the Al.sub.2 O.sub.3 film. Conductive films 
whether of indium or stannic oxide, are known to cause a brightness loss. 
The conductive film in this test was overcoated with an aluminum oxide 
coating at 24 mg Al.sub.2 O.sub.3 per ml prepared and laid on in the 
manner described in the previous example. The results given in the 
followng tables show that the Al.sub.2 O.sub.3 film according to the 
invention more than compensates for the light loss that would result from 
a conductive film. 
______________________________________ 
CONTROL LAMPS, 
NO CONDUCTIVE FILM, NO Al.sub.2 O.sub.3 FILM 
GAS FILL O HOURS 100 HOURS 500 HOURS 
______________________________________ 
100% Argon 3199 lumens 
3064 lumens 
3014 lumens 
65% Argon 3004 lumens 
2874 lumens 
2844 lumens 
50% Argon 2941 lumens 
2830 lumens 
2806 lumens 
Average Brightness 
3048 lumens 
2923 lumens 
2888 lumens 
______________________________________ 
______________________________________ 
TEST LAMPS WITH INDIUM OXIDE FILM, PLUS Al.sub.2 O.sub.3 FILM 
GAS FILL O HOURS 100 HOURS 500 HOURS 
______________________________________ 
100% Argon 3194 lumens 
3098 lumens 
3042 lumens 
65% Argon 3039 lumens 
2899 lumens 
2870 lumens 
50% Argon 2979 lumens 
2832 lumens 
2798 lumens 
Average Brightness 
3071 lumens 
2943 lumens 
2903 lumens 
______________________________________ 
In another test a direct comparison was established between a group of 
regular lamps, another group with a tin oxide conductive coating and 
finally a group which contained the aluminum oxide protective coating 
according to the invention, over the conductive tin oxide film. This 
coating had again been obtained in a manner similar to that described in 
the first example and at the optimum 24 mg Al.sub.2 O.sub.3 per ml 
concentration. 
______________________________________ 
MAIN- 
O HOUR 100 HOURS TENANCE 
______________________________________ 
Control lamp- 
phosphor alone 
2796 lumens 
2664 lumens 
95.3% 
SnO.sub.2 film 
and phosphor 
2727 lumens 
2559 lumens 
93.8% 
SnO.sub.2 film - 
Al.sub.2 O.sub.3 
film and phosphor 
2755 lumens 
2662 lumens 
96.6% 
______________________________________ 
PG,6