U.S. patent number 3,909,649 [Application Number 05/426,494] was granted by the patent office on 1975-09-30 for electric lamp with light-diffusing coating.
This patent grant is currently assigned to General Electric Company. Invention is credited to Vito J. Arsena.
United States Patent |
3,909,649 |
Arsena |
September 30, 1975 |
Electric lamp with light-diffusing coating
Abstract
A glass electric lamp bulb having on the inside surface thereof
a light-diffusing coating of self-adherent silica particles having
an average particle size, as measured by a scanning electron
microscope, within the range from about 0.5 to 1.2 microns.
Inventors: |
Arsena; Vito J. (Highland
Heights, OH) |
Assignee: |
General Electric Company
(Schenectady, NY)
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Family
ID: |
26995660 |
Appl.
No.: |
05/426,494 |
Filed: |
December 20, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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348347 |
Apr 5, 1973 |
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Current U.S.
Class: |
313/116; 313/112;
427/107; 501/54 |
Current CPC
Class: |
H01K
1/32 (20130101) |
Current International
Class: |
H01K
1/28 (20060101); H01K 1/32 (20060101); H01K
001/26 () |
Field of
Search: |
;117/97 ;313/116,112,110
;220/2.1R ;106/54,52 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Brody; Alfred L.
Attorney, Agent or Firm: Sos, Jr.; Emil F. Kempton; Lawrence
R. Neuhauser; Frank L.
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
This application is a continuation-in-part of application Ser. No.
348,347, filed Apr. 5, 1973, now abandoned.
Ser. No. 426,493 Vito J. Arsena, "Method of Manufacturing
Light-Diffusing Articles", filed concurrently herewith and assigned
the same as this invention.
Claims
What I claim as new and desire to secure by Letters Patent of the
United States is:
1. A glass electric lamp bulb having on the inside surface thereof
a light-diffusing coating of silica particles adhering to said
surface, randomly spaced from one another, and having a diameter
within the range from about 0.5 to 1.2 microns, said coating
characterized by a density of approximately 0.030 to 0.535
milligrams per square centimeter.
2. The glass electric lamp bulb claimed in claim 1 wherein said
silica particles are crystalline silica.
3. The glass electric lamp bulb claimed in claim 1 wherein said
silica particles are amorphous silica.
4. The glass electric lamp bulb claimed in claim 1 wherein the
numerical preponderance by weight of said silica particles have a
diameter larger than the longest wavelength of visible light.
5. The glass electric lamp bulb claimed in claim 1 wherein the
luminous efficiency in lumens per watt of an ordinary incandescent
lamp comprising such a coated bulb is of the order of 1.5 to 6%
less than that of the same lamp with a clear bulb.
6. The glass electric lamp bulb claimed in claim 1 wherein said
particles of silica have a diameter within the range from 0.6 to
0.7 microns.
7. The glass electric lamp bulb claimed in claim 1 wherein said
particles of silica have a diameter within the range from 0.5 to
1.0 microns.
8. The glass electric lamp bulb claimed in claim 1 wherein a
portion of said particles of silica are characterized as being
nonspherical.
9. The glass electric lamp bulb as claimed in claim 1 wherein said
coating density is approximately 0.070 .+-.0.041 mg of silica per
cm.sup.2.
10. The glass electric lamp bulb as claimed in claim 1 wherein said
coating density is approximately 0.315 .+-.0.070 mg of silica per
cm.sup.2.
11. The glass electric lamp bulb as claimed in claim 1 wherein said
coating density is approximately 0.462 .+-.0.072 mg of silica per
cm.sup.2.
12. An incandescent lamp comprising a glass bulb having on the
inside surface thereof a light-diffusing coating of silica
particles adhering to said surface, randomly spaced from one
another, and having a diameter of about 0.5 to 1.2 microns, said
coating characterized by a density of approximately 0.030 to 0.535
milligrams per square centimeter, and a filament mounted inside
said bulb, said coating serving a dual function of a
light-diffusing means and as a means for preventing the dislodgment
of any impurities from the inside surface of the glass bulb which
would tend to lower the breakdown voltage across said filament.
13. The lamp claimed in claim 12 wherein said bulb contains a fill
gas and said tightly packed coating of silica prevents the
dislodgment of any impurities from the bulb surface which would
combine with the fill gas.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to light-diffusing articles such as
incandescent lamps and discharge devices. More particularly, the
invention relates to silica-coated bulbs used in the manufacture of
electric lamps.
2. Description of the Prior Art
Light-diffusing coatings located on the inside surface of a lamp
bulb have been desirable in the lighting industry for some time.
One of the first such effective coatings, known as inside frost,
was developed by Pipkin, U.S. Pat. 1,687,510, Oct. 16, 1928, and
assigned to the assignee of the present invention. The inside frost
diffusion coating is manufactured by etching the inside surface of
a bulb with hydrofluoric acid. Although Pipkin's invention was a
great contribution to the lamp art, etching created an
environmental problem of acid disposal.
Another diffusion coating widely used in the lamp industry and
known as Q-coat was also invented by Pipkin, U.S. Pat. No.
2,545,896. Q-coat is a coating of silica particles predominantly
smaller than one micron deposited on the acid etched inside surface
of the bulb. Although the silica particles are generally deposited
on etched or inside frosted lamps, the Q-coat of silica may be used
on smooth or unetched bulbs Q-coat is formed by particle deposition
from combustion of an organosilicon compound, such as ethyl
orthosilicate. Generally, organosilicon compounds are relatively
expensive compared to naturally occurring silica. It is believed
that Pipkin's particle range, as recited in the patent, of 0.2 to
0.6 microns is a measurement of numereous agglomerates and that in
fact many of the individual particles are smaller than the given
range. This apparent discrepancy may be the result of measurements
made in the 1940's with less sophisticated measuring techniques
that are available at the present time.
Materials other than silica, such as titanium dioxide, have been
used as diffusion coatings, and methods, such as electrostatic
precipitation, Meister et al., U.S. Pat. No. 2,922,065, are known.
In Patent U.S. Pat. No. 3,175,117 -- Kardos, titanium dioxide
particles are interspersed in an organic binder which is dissolved
in an organic solvent to form a slurry which is applied to the
inside of the bulb. This slurry is then dried and burned on the
inside surface. According to the patent, one type of TiO.sub.2
coating consists of two sizes of particles, one having a diameter
below one micron and the other having a diameter between 2 and 4
microns. Inasmuch as the particles are TiO.sub.2 and are tightly
packed to one another, presumably because they are dissolved in an
organic solvent, the light absorption of this coating is believed
to be greater than what is commercially acceptable, namely, 6% or
less.
Other slurry mixtures have tried, on etched as well as unetched
lamp bulbs, in an attempt to reproduce the frosted and Q-coated
bulbs. U.S. Pat. No. 2,661,438 -- Shand claims the use of colloidal
silica particles having a diameter of 0.04 to 0.8 microns mixed
with solid silica particles having a diameter between 1 and 15
microns. The mixture is applied to a preheated bulb in an effort to
drive off the water of the solution. This coating and method have
the disadvantages of poor coating uniformity and short lamp life
caused by the coating containing water. Colloidal silica is
hydrophilic and absorbs water at room temperature while the bulbs
are waiting to be made into lamps.
Various other slurry coatings containing silica of a 2-micron
diameter in an organic solution or 2-micron diameter silica mixed
with 2-micron diameter titanium dioxide also in an organic solution
have been made. However, either light transmittance or filament
hiding power is decreased to a point where the lamp is commercially
unacceptable. It is believed that these disadvantages are caused by
the particles being tightly packed to one another which may be the
result of using an organic solvent in the slurry.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a lamp bulb
with a light-diffusing coating which has relatively low light
absorption, that is, less than approximately 6%. Another object of
the invention is to provide a lamp bulb with an effective diffusion
coating without etching the clear glass bulb through the use of
hydrofluoric acid. Still another object of the invention is to use
a diffusion coating material which costs less than organosilicon
compounds.
Yet another object of the invention is to apply the diffusion
coating to a bulb by a slurry coating method using a water soluble
organic binder which will loosely space the silica particles one
from another to give effective diffusion, filament hiding, and good
adherence of the particles to the glass bulb. A further object of
the invention is to disperse the diffusion coating particles in a
media which is nontoxic and explosion proof.
The objects of the invention are accomplished by coating a lamp
bulb with a slurry containing silica particles, having an average
particle size, measured by a scanning electron microscope, between
0.5 and 1.2 microns, suspended in a polyacrylic acid binder in
ammoniacal water solution. This mixture of silica particles,
polyacrylic acid, ammonium hydroxide and water is relatively
nontoxic, nonflammable, and because it is aqueous, it does not
create a silica inhalation hazard. Furthermore, the silica
particles are obtained through the inexpensive method of ball
milling natural silica.
Coating thickness may be varied by changing the viscosity and the
solid content of the slurry. Variations of viscosity and solid
content will change the silica thickness which, in turn, will give
three different types of diffusion coatings: inside frost, Q-coat
and enamel. Coating densities which correspond to the three types
of bulbs are approximately 0.070 .+-.0.041, 0.315 .+-.0.070, and
0.462 .+-.0.072 mg/cm.sup.2.
It has been found that the particles used may be either amorphous
or crystalline silica. In the preferred embodiment,
microcrystalline alpha quartz from the Pfizer Company is ball
milled down to a particle size of between 1.9 to 2.1 microns as
measured by a Coulter Counter. The mill charge is made of silica,
water, polyacrylic acid and a basic hydroxide such as ammonium
hydroxide. More water and ammonium hydroxide are added after
milling until the desired viscosity and a pH of about 10 is
obtained.
This slurry solution is then squirted into the inside of a clear,
unetched glass bulb. After the bulb drains, there is a preliminary
hot air drying which sets the coating on the bulb. The coating is
then lehred to remove the polyacrylic acid, ammonium hydroxide and
water, which are presumably discharged from the inside of the bulb
as CO.sub.2, N.sub.2 and H.sub.2 O vapor.
It is believed that because the silica particles are surrounded by
polyacrylic acid molecules and because the polymer molecules are
dissolved and ionized in water, an inorganic solvent, as opposed to
prior art organic solutions, the silica particles are loosely
spaced from one another but tightly adhered to the bulb wall. It is
also believed that loosely spaced particles make good diffusion
coatings while still maintaining a satisfactory level transmission
and good filament hiding power.
Other objects and advantages will become apparent from the
following specification and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a fragmented perspective view of an incandescent lamp of
the invention;
FIG. 2 is a fragmented perspective view of another type of lamp of
the invention;
FIG. 3 is a front elevation in section of a part of the apparatus
used in coating bulbs of the invention;
FIG. 4 is a photograph, magnified 2000 times, of a coating used to
give the diffusion properties of an inside frosted lamp;
FIG. 5 is a photograph, magnified 2000 times, of a coating which
has diffusion properties similar to a soft white or Q-coat lamp
bulb;
FIG. 6 is a photograph, also magnified 2000 times, of a coating
which can be used as a substitute for enamel-type lamps;
FIG. 7 is a graph showing light absorption of a soft white lamp of
the prior art;
FIG. 8 is a similar graph showing light absorption of the lamps of
the invention; and
FIG. 9 is a Coulter Counter particle distribution of silica
particles in the coating slurry.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1 of the drawings, an incandescent lamp 20 with a
silica coating 21 is therein illustrated. Lamp 20 is comprised of
bulb or envelope 22, mount 23 and base 24. The mount 23 has a
tungsten filament 25 connected to lead wires 26 and 27 which are
sealed in stem press 28. Envelope 22 and stem press 28 are
hermetically sealed, and exhausted through tube 29. After
exhausting, the envelope may be filled with an inert gas or gas
mixture such as nitrogen and argon (95% argon, 5% nitrogen).
One of the requisites of light-diffusing coating 21 is that it must
adhere to the bulb wall throughout the various lamp processing
operations. The coating must also be an effective, efficient light
diffuser and transmitter. Two basic measurements of diffusion
coatings used to evaluate their effectiveness are light absorption
(arbitrary units) of a coated lamp versus a clear lamp or standard
light source and attenuated beam reading by a photocell versus
horizontal scan along the bulb's major diameter.
Through the use of silica particles of a certain size carried by
polyacrylic acid such as Acrysol A-5, molecular weight less than
300,000, in an H.sub.2 O solvent containing a basic hydroxide, such
as hydroxides of ammonium, potassium and sodium, the problems
inherent in former slurry coating processes have been eliminated. A
particle size range from 0.5 to 1.2 microns, as measured by a
scanning electron microscope, has been found to give effective
light-diffusing coatings. This is true for both crystalline and
amorphous silica. It should be pointed out that the particle size
range of 0.5 to 1.2 microns is the size of the particles on the
envelope wall, also referred to as final particle size. The other
particle size range referred to in the application is the size of
particles in the slurry prior to application to the envelope wall.
The preferred range of the slurry particles is 1.9 to 2.1 microns,
as measured by a Coulter Counter. An overall range of slurry
particle size is 1.3 to 4.4 microns as measured by the Coulter
Counter. This corresponds to the final particle size range
previously recited 0.5 to 1.2 microns.
The coating of the invention gives good diffusion while absorbing
approximately less than 6% of the light as compared to a clear
bulb. Prior art slurry coatings having final particle size of 2.0
microns gave between 8 and 15% absorption. These prior art slurries
use an organic solvent which has a tendency to agglomerate silica
particles to a greater degree than the slurry coating of the
invention.
In the coating of the invention, the silica particles are well
dispersed in a strongly alkaline water medium. With the addition of
ammonium hydroxide, the polyacrylic acid molecules become ionized
and therefore enhance dispersion and limit agglomeration. Although
polyacrylic acid is a preferred binder, the following may also be
used: carboxy-methylcellulose and polyethylene oxides such as
Polyox 750 and Polyox 3000 produced by the Union Carbide Company.
Other coating materials such as calcium pyrophosphate, soda-lime
glass, aluminum silicate and alumina have been applied to glass
lamp bulbs as substitutes for the preferred Williams "Super White"
silica.
Coatings having a final particle size below 0.5 microns selectively
scatter blue and purple light energy to a point where the filament
has an undesirable red color. On the other hand, coatings having
final particle sizes greater than 1.2 microns lack continuity and
have a blotchy appearance. As coating final particle size increases
near 1.2 microns, a slightly aesthetically inferior lamp is
obtained although its diffusion and illuminating properties are
good up to and including 1.2 microns. A lamp with this type of
coating could be used in locations where the aesthetics of the lamp
are not important.
FIG. 9 is a Coulter Counter log-log graph of particle size versus
percent by weight of silica particles used in a slurry of a
preferred embodiment which has a slurry particle size of 1.9 to 2.1
microns. By way of explanation, the Coulter Counter senses the
volume of a particle and then converts the volume of the particle
into a hypothetical sphere of the same volume having a diameter
equal to the particle diameter. These Coulter Counter measurements
are referred to as slurry particle size or slurry particle
diameter. As can be seen from the graph, 90% by weight of the
particles in the slurry are of a size greater than one micron, and
less than 1% by weight of the particles are greater than 6 microns.
Furthermore, the numerical preponderance by weight percent of the
particles are greater than one micron and greater than 0.8 microns.
Coulter Counter measurements, unlike scanning electron microscope
measurements taken at 10-15,000X magnification, are volumetric
measures of agglomerates of more than one particle. In other words,
three final particles of 0.5 microns as measured by the SEM may be
measured as one slurry particle of 1.5 or less microns in diameter
as measured by the Coulter Counter when the three particles are
attached to each other.
The particular slurry illustrated in the graph was obtained by ball
milling for 27 hours at 62 rpm the following ingredients: distilled
or deionized water (3800 gm), a wetting agent such as Antarox
BL-225, GAF Corporation (0.0960 gm), Acrysol A-5, Rohm and Haas
(88.5 gm), Williams Super White silica, Pfizer Chemical (680 gm),
and a quantity of a basic hydroxide such as ammonium hydroxide
sufficient to make the pH equal to 10 .+-.0.5. To this slurry is
added 240 gm distilled water and an additional amount of ammonium
hydroxide sufficient to maintain the above pH value. More or less
water can be added until the reflectance, at the major diameter of
an A-line bulb, is 34 .+-.1. This corresponds to a liquid viscosity
of 55 .+-.8 cps at 25.degree.C, as measured by a Brookfield
Viscosimeter using a No. 1 spindle at 100 rpm. The percent solids
in the above mixture is approximately 12.3 .+-.0.4%. This mixture
would give a diffusion coating similar to an enamelled lamp, and
the coating density would be approximately 0.462 .+-.0.072
mg/cm.sup.2.
A coating which can be used as a substitute for inside frost is
prepared in a similar manner. This coating has a reflectance of 10
.+-.2, a viscosity of 36 .+-.8 cps at 25.degree.C and 3.0 .+-.0.1%
solids. With the preceding suspension, a coating density of 0.070
.+-.0.041 mg/cm.sup.2 should result. To obtain a coating which can
be used as a Q-coat, the solution parameters should be reflectance
of 25 .+-.2, viscosity of 36 .+-.8 cps at 25.degree.C and between
6.3 to 6.7 percent solids in the solution. This generally will
result in a coating density of 0.315 .+-.0.070 mg/cm.sup.2.
The diffusion coating of the invention may be used for lamps other
than the exposed filament type shown in FIG. 1. It can be used, for
instance, in lamp 30 shown in FIG. 2. Lamp 30, like lamp 20 of FIG.
1, has a silica coating 31 of the invention on the inside wall of
bulb 32. Instead of a filament, however, lamp 30 contains an inner
lamp 33 which may be a tungsten-halogen lamp, as illustrated, or a
discharge lamp. Actually, the diffusion coating of the invention
may be used on any light-transmitting substrate which is associated
with a light source of some kind.
As can be seen from graphs 7 and 8, the diffusion coating of the
invention differs from the standard Q-coat bulb with silica smoke
deposited from the burning ethyl orthosilicate. All test readings
used in compiling these curves were taken from 100-watt CC8
filament incandescent lamps. Curve A of FIG. 7 indicates the amount
of red light transmitted by a standard Q-coated lamp, the readings
being taken by a red sensitive photocell. Curve B which is a
compilation of readings from a blue sensitive photocell shows a
smaller amount of blue transmission. This disparity between red and
blue transmissions will make the filament and a portion of the
light it gives off appear red.
By way of contrast, curves A and B of FIG. 8 which represent the
photocell response of the lamp of the invention show an almost
equal amount of red and blue transmissions. The data for curves A
and B of FIG. 8, like the data for the curves of FIG. 7, represent
the respective responses of blue and red sensitive photocells.
Since curves A and B of FIG. 8 are almost identical, this means the
light transmitted by the lamp will be white and not biased by an
excess of red or blue. In many lighting applications, this balanced
white light is a very desirable property.
Another advantage of using silica particles of a size within the
range of the invention is the fact that they will absorb less
moisture. This is desirable because bulbs can be manufactured and
stored, whereas, bulbs of the prior art had to be made into lamps
shortly after they were coated.
One of the objects of the invention is to find a process which is
sufficiently versatile to be able to make the three basic types of
diffusion coatings, namely: inside frost, soft white or Q-coat, and
enamel. Inside frost is obtained by etching the inside surface of a
clear bulb with hydrofluoric acid. Soft white or Q-coat is a
two-step process. First, the bulbs are etched as in inside
frosting, and then the etched bulbs receive a deposit of silica
smoke from the combustion of an organosilicon compound. This
coating requires the use of two steps and two separate pieces of
equipment. Outside enamel is painting the exterior of the bulb, a
process which requires yet another and different piece of
equipment. Accordingly, it would be quite desirable from an
economic standpoint to be able to make the three types of coatings
using the same facilities and machinery.
This objective is accomplished by varying the density (mg/cm.sup.2)
of the coating of the invention by changing the slurry reflectance,
viscosity and percent solid as previously described. Referring now
to FIGS. 4, 5 and 6, the photographs shown therein represent
varying desities of silica on glass bulbs corresponding to the
three types of diffusion coatings. In FIG. 4, the silica particles
are loosely packed with respect to each other and their density on
the bulb wall is comparatively small. The approximate density of
these particles is 0.070 .+-.0.041 mg/cm.sup.2. This coating would
serve as a substitute for inside frost, acid etched bulbs. Inside
frost bulbs have less diffusion than other coatings which is why
the particle density shown in FIG. 4 is smaller than that of FIGS.
5 and 6.
A distinguishing characteristic of the coating of the invention is
the irregular shape of some of the silica particles. In other
words, the particles are not totally spherical. This characteristic
of not all particles being spherical can be observed in all three
figures.
FIG. 5 shows a coating with particle density greater than that
shown in FIG. 4 but less than that shown in FIG. 6. This density is
approximately 0.315 .+-.0.070 gm/cm.sup.2 and corresponds to the
diffusion of a soft white or Q-coat lamp, acid etched plus silica
smoke. It can be observed in FIG. 5 that there are agglomerates or
large particles, and small particles, presumably single particles.
It is believed that this combination of agglomerate and small
particles performs the same function as etched glass and small
silica particles do in Q-coat.
In FIG. 6, there is shown a silica coating with a density of 0.462
.+-.0.072 gm/cm.sup.2 which corresponds to an outside enamel
coating. As can be observed, this coating has the greatest density
of all three specimens. All photographs in the figures are
magnified 2000 times.
Part of the apparatus and method used in depositing the coating is
illustrated in FIG. 3. The coating apparatus 34 is comprised of a
slurry holding tank 35 which has an air inlet 36, slurry inlet 37
and applicator nozzle 38. Holding tank 35 is sealed from the
atmosphere by plate 39, the top part of which forms spillway 40.
Bulb 41 can be held by numerous types of holding means such as the
one shown at 42.
Once bulb 41 is positioned in holding means 42 over applicator
nozzle 38, air pressure is applied to the slurry through opening
36. The pressure increases in the space above the slurry forces
some of the slurry up applicator nozzle 38 onto the inside of the
bulb wall. Any excess slurry falls into spillway 40 and can be
recylced and reused. Slurry replenishment comes through the nozzle
and opening shown at 37.
After the bulb 42 has been coated, an air jet is inserted to give
the coating a quick dry. Following this, the bulbs are lehred in an
oven according to the following oven temperature schedule: heat up
from room temperature to 780.degree.C, bake at 780.degree.C for 10
seconds, bake at 680.degree.C for 15 seconds, cool to upper strain
point of approximately 525.degree.C in 5 seconds, cool to lower
strain point of approximately 475.degree.C in 60 seconds and cool
to room temperature in approximately 60 seconds.
* * * * *