Note: Descriptions are shown in the official language in which they were submitted.
o
~ 1 - LD 9325
METHOD ~ID APPARATUS
FOR COATING FLUO~ESCENT LAMP TUBES
__ _
BACKGROU~D OF THE I~7E~TIO~
_ _ _
1. Field of the invention.
This invention relates to the manufacture of
fluorescent lamps, and, more particularly, to a method
and apparatus for applying a layer of particulate
material to the in~ide of a fluorescent lamp bulb by
electrostatic deposition.
2. Description of the prior art.
In the prior art techniques for manufacturing
fluorescent lamps, phosphor coatings are typically
applied a~ a suspension of particulate material in a
slurry including an organic binder. The organic binder
serve6 the function of holding the phosphor particle~
to the glass bulb ~urface during the manufacturing of
the bulb. A~ter application of the phosphor coating,
the bulb~ are lehred at a high temperature to vaporize
the oryanic binder and bond the phosphor particle~ to
the gla~ bulb ~urfaee and to other phosphor partiele~
to form a uniform, well-bonded eoating on the
~luoreseent lamp bulb. ~hie technique requires heating
of the lamp bulb to a temperature which would eause the
lamp gla~s to ~often. To prevent distortion of the
pluoreseent lamp bulb, straight line fluoreseent lamps
are eonventionally rotated during the lehring process
~o that the gravitational effects are averaged and the
lamp maintains a straight shape.
~9~5~
- 2 LD 9325
U-shaped fluorescent lamps having both ~ets of
lamp terminals at the same end of the lamp raise a
difficulty with respect to lamp coating and lehring
which is not experienced in manufacturing straight
S fluorescent lamps. In prior art techniques of
manufacturing U~haped fluorescent lamps, the phosphor
coatings are typically applied as water suspensions
containing organic polymer binders which act a~
di6per~ing agent~ to provide ~mooth coating
appearance. After the coatings have been applied, the
binders must be removed prior to sealing of the lamp
and filling with the typical fluorescent lamp
atmospheres, because the organic materials of the
binder are incompatible with the fluorescent lamp
atmosphere and tend to cause darkening and 10~6 of lamp
efficacy in lumens per watt over the life of the lamp.
The binders typically are removed by baking at elevated
temperatures, i.e. lehring, for a sufficient time to
vaporize the binders. When folded fluorescent lamp
tubes are subjected to lehring temperatures typically
u~ed for lehring lamps coated with water-based organic
binder coatings (600-630C), the glass can soften
resulting in distortion of the glass tube due to
gravity. It is impractical to roll the folded tube
during the lehring proce6s to average gravitational
effects, and, therefore, lehring must occur at lower
temperature~. However, lower temperature lehriny
significantly lowers lamp efficacy and maintenance due
to the incomplete removal of the organic binder
material~. In one prior art technique for
manufacturing U-shaped fluorescent lamp~ a tin oxide
~tarting ~trip is applied to an interior ~urface of the
fluorescent lamp extending generally from one electrode
around the bend of the lamp to the opposite electrode
~2~
_ 3 _ LD 9325
in order to assi~t in starting of the lamp. If this
coating is applied prior to lamp bending, difficulties
are e~perienced in maintaining electrical continuity of
the starter strip following bending of the gla~s tube
due to the strain on the glass and therefore on the
~tarting strip during bending. Thexefore, the starting
strip is typically applied after the glass tube has
been bent into the desired U-shape. A difficulty
experienced when using tin oxide as the starting strip
results from the use of an in~ulating barrier coating
on the tin oxide coating to overcome the poor adherence
of phosphors to tin oxide and the tendency of the tin
oxide to darken with exposure to the atmosphere inside
the fluorescent lamp. To improve adherence of pho~phor
materials to the tin oxide coating, certain types of
borates, e.g. calcium borate, are included within the
binder material. Removal of the binders from the lamp
following deposition of the phosphors requires a ~till
higher lehring temperature when additional borate
additives are used, which increases the risk of sag in
the U-shaped lamps. To overcome these limitations in
the manufacturing of U-~haped fluore~cent lamps, a
technique of applying phoRphor coatings and bonding the
coatings to the lamp glass without requiring high
temperature lehring iB required.
SUMMARY OF T~IE INVENTION
~ n ob~ect o~ the present invention iB to provide
a method and apparatus or applying phosphor coatings
to the interior surfaces of fluorescent lamp tubes
without requiring the use of binder materials who~e
removal from the lamp requires high temperature
lehring. A more specific object of the
presentinvention i~ to provide an electrostatic coating
- 4 - LD 9325
technique for applying pho~phor layers to the interior
surfaces of a U-~haped fluorescent lamp.
Accordingly, the present invention includes an
electro~tatic coating apparatus having one electrode
positioned outside the glass tube and at a
predetermined position relative to a pair of ~econd
electrodes placed in~ide the glass tube during the
coating process, each of the ~econd electrodes having a
nozzle attached thereto with passages therethrough for
the aelivery of phosphor coating material to the
interior of the glass envelope and a tip for forming a
corona; and connections to a high voltage d-c
electrical power supply for applying ~oltage of a first
polarity to the first electrode and voltage of a second
polarity to the second electrodes; such that a field is
created between the electrode tips which causes the
glass tube to become electrically charged with one
polarity and the particles of phosphor material to
become oppositely charged, ~o that the phosphor
particles are attracted to the interior surface of the
glass tube and adhere thereto.
BRIEF DESCRIPTION OF THE DRAWINGS
Further objects and advantage3 of the present
invention together with its organization, method of
operation, and be~t mode contemplated may be~t be
understood by re~erence to the following de~cription
taken in conjunction with the accompanying drawings in
which:
E'IG~ 1 is a ~chematic elevation view of a
fluore~cent lamp coating apparatus according to the
present invention;
FIG. 2 i~ a ~chematic elevation view, partly in
section, of an apparatu~ according to the preRent
5~
~ 5 - LD 9325
invention for applying coatings to the interior surface
of fluore~cent lamp tubes;
FIG. 3 is a ~chematic partial cross-sectional view
of a nozzle for the coating apparatus of the present
invention, anlarged to illustrate details thereof;
FIG. 4 is an elevational view, partly in section,
of an alternative embodiment of the coating apparatus
of the present invention;
FIG. 5 is a greatly enlarged schematic view
illuetrating the fluorescent particles
electrostatically bonded to the surface of a
fluorescent lamp tube;
FIG. 6 is a greatly enlarged schematic view of a
glass tube wall illustrating the completed bonding of
phosphor particles to a lamp glass surface according to
the present invention; and
FIG. 7 is a block diagram illustrating the
phosphor depo~ition method of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIME~TS
A~ shown schematically in FIG. l, the apparatus
of the pre~ent invention comprises equipment for
coating the interior surface of a ~luorescent lamp
glass tube while the tube iB suspended in a suitable
holding device (not shown). ~he coating apparatus
includes a high voltage electrode lO and a pair of
phosphor supply tubes 26, 28 secured to a movement
mechanism 25 for moving the electrode lO and supply
tubes 26, 28 relative to the glass tube 20 at a
controllable, con~tant rate. ~he movement mechani~m
may include hydraulic, compre~sed air or electric motor
means 27 to provide the controlled movement. The
electrode lO and ~upply tubes 26, 28 are secured to
suitable holders 23 for movement together during
pho~phor deposition. As shown in more detail in
5~
- 6 - LD 9325
FIG. 2~ a high voltage electrode 10 co~pri6ing
conductive rod 12 and conductive tip member 14 i8
disposed 60 that the points 16, 18 at the respective
ends of the conductive member 14 are in close proximity
to the glass tube 20 but are not in contact therewith.
The rod 12 is connected via a ~uitable conductor shown
6chematically as 22 to a high voltage d-c power supply
24. The present apparatus further includes supply
tubes 25 and 28 for receiving via tubee ~5, 27,
respectively, a mixture of dry air and powder from a
powder supply hopper 29 of conventional design, such as
a fluidized bed, and conveying the mi~ture of dry air
and powder to the interior 30 of the glass tube 20 for
coating the interior surface 32 thereof. The nozzles
34 are both of a similar co~struction, one of which is
shown enlarged in FIG. 3. The tube 26 is connected to
a nozzle 34 for example by threads 38. The nozzle 34
includes a plurality of paesages 39 cut through the
closed end wall 47 at a predetermined angle with
respect to the nozzle centerline 40 to concentrate the
phosphor powder in the corona region 42 surrounding tip
44. The angle ~ is determined experimentally to
provide optimum powder flow from the nozzle past the
tip 44 and through the corona region 42 into the
interior of the glass tube for deposition upon the
surface 32, 6hown in FIG. 2. The angle ~ determines
the distance of travel of the phosphor particles before
deposition on the glas~ surface. The pa8sages 39 are
typically 50-100 mil~ in diameter, sub~tantially larger
than the particle size of the phosphor being
deposited. The passages 39 may be cut with a slight
spiral to cause the phosphor particles to swirl as they
pass over the tip. The tube 26 may be a copper tube
having a plastic coating to prevent erosion of the
tubing by fluorescent phosphor particles supplied to
_ 7 _ LD 9325
the interior of the lamp, or alternatively the tube 26
may be a stainless steel tube requiring no inner
lining. The tip 34 is preferably of ~tainle6s ~teel.
The rod 12 and tip member 14 are preferably of copper
or other ~uitable conductive material. Although the
conductive member 14 is shown to be in the plane of the
U-shaped glass tube 20, the rod 12 and tip member 14
may be offset, e.g., above the plane of the paper as
shown in FIG. 2 but in a plane generally parallel to
the plane including the re3pective centerlines 40 of
the tips 44, so that it may be positioned nearer the
top 21 of the bend in the glas~ tube 20. By using the
offset position, the tip member 14 may be positioned to
ensure deposition of phosphor powder over the entire
suxface of the curve in the lamp if required for
particular phosphors.
An alternative embodiment is illustrated
schematically in FIG. 4 for the application of phosphor
coatinys to a different type of U-shaped fluorescent
lamp. The lamp tube 50 is a glass fluorescent tube
used in twin-tube lamps of the type sold by the General
Electric Company under the trademark MOD-U~LINE~ having
a sharp U-bend and smaller diameter, typically T-5,
than the lamp shown in FIG. 2. For coating a lamp of
this configuration the central rod 52 has a tip 54
attached thereto generally aligned with the axi~ 56 of
the rod 52. The pair of ~upply tubes 58 and 60 are
configured to have bend~ 62, 64 and 66, 68,
respectively, to po~ition the supply tubes properly for
ingertion into the legs 74, 76 of the U-shaped lamp
tube 50. The supply tubes have nozzles 70, 72 of a
con~truction similar to nozzle 34, described above, but
of smaller diameter connected respectively thereto.
The materials of the rod 52 and the supply tubes 58 and
60 and the nozzle 70 and 72 are as described above with
5~
- 8 - LD 9325
respect to the ~mbodiment of FIG. 2~ The rod 52 and
tip 54 could be offset from the plane of the glass tube
50, rather than being in ~he plane of the tube 50, so
~hat the tip 54 could be positioned adjacent the
U-6haped bend rather than within the ~-shaped bend.
The tip 54 could be provided by turning the tip member
14 perpendicular to the plane of the glass tube and
positioning one of the tips 16 or 18 in close pro~imity
to the bend of the tube 50 of FIG. 2.
The present invention provides a method of
phosphor deposition as shown in the block diagram of
FIG. 7, as follows: the glass fluore6cent tube is bent
into the U-shaped configuration while heated. While
the glass tube is still hot, it i8 loaded into a
suitable lamp holding mechanism for deposition of the
phosphor coating6. Alternatively, the bulbs may be
allowed to cool and then be reheated. The heating
removes moisture from the surface of the glass tubes
and thereby reduces 6urface conductivity, which would
interfere with the application of charge to the glass
surface. The supply tubes 26 and 28 are inserted into
the legs of the U-~haped lamp, and the electrode tips
16 and 18 are positioned adjacent the glass tube wall
and slightly above and generally adjacent the position
of the ~ips of the nozzles. The 8upply ~ubes 26 and 28
are connected to electrical ground. The power ~upply
24 connected to the rod 12 ~upplies a D.C. voltage in
the range of 20 to 50 kv. The exact settirtg for a
particular deposition i8 establiehed by raising the
voltage to a level at w~lich breakdown occur~ in air and
then reducing the voltage level slightly to avoid
arcing. q~tis ~pacing i~ typically in the range of
about 0.50 inch to about 2.00 inches. A supply of dry
air or other suitable gas i8 provided to the pho~phor
feed hopper to entrain particulate matter in a ~tream
_ g _ LD 9325
flowing vertically upward through the tubes 26, 28 into
the bight of the glass tube. The phosphor particles
are charged as they pass through the corona region 42
The phosphor particle 6ize is typically 3.0 to 15.0
microns plus or minus 15 percent, which is standard for
fluor~scent phosphor particle size. The passages 39
are thus much larger in diameter than the particles and
do not significantly affect particle velocity through
the nozzles. Typically the phosphor particles travel
about four to six inches beyond the openings 45 before
contacting the glass tube wall. The powder supply
nozzles and the electrode member 14 are moved
vertically downward at a rate determined by the desired
thickness of deposition upon the interior surface of
the glass wall, e.g., at about 5 inches per second for
coating the T-12 or approximately 1.5 inch diameter
tube ~hown in the FIG. 2 embodiment. Alternatively,
the glass tube could be moved while the powder supply
tubes and the electrode are kept fixed. If it is
desired to deposit a second layer of pho~phor coating
onto the interior surface of the lamp glass, a 6econd
step of electrostatic deposition may be employed by
moving the nozzels and electrode 10 bacX to their
beginning positions and repeating the procedure
de8cribed above. If a different phosphor is to be used
for the ~econd deposition, the appropriate supply
hopper would be connected to tube~ 25, 27 prior to
insertion of the tubes 25, 27 into the lamp tube.
Following electro~tatic deposition of phosphors the
coated bulb is cooled in air. Whether one layer or two
or ~ore layer~ have been depo~ited, the phoephor
coating i~ humidified by blowing satuxated air into the
interior of the tube 80 that moisture is picked up on
the surface~ of the particulate phosphor material.
- 10 - LD 9325
Following humidification the lamp is lehxed to reMove
the water introduced into the lamp by humidification
and to bond the phosphor particles to the gla~s ~urface
and to other particles in the phosphor coatings, so
that the phosph~r layers will be securely bonded to the
lamp interior surface after manufacture.
During phosphor deposition the glass tube is
maintained at a temperature r~nge from about 150C to
about 500C at which it i5 electrically conductive,
la so that a current flow of approximately 2.5
milliamperes flows through the rod 12 and from the tips
16 and 18 through the glass of the lamp tube and the
phosphor particles in the interior of the glas~ to the
respective tips 44 of the nozzles 34. The powder being
blown through the respective supply tubes into the
glass bulb picks up a negative charge as it passes the
corona point. The current flowing through the glass
wall causes the glass to accumulate a positive charge.
As shown greatly enlarged in FIG. 5, the glass wall 20
accumulates a positive charge, shown at 80, and the
phosphor particles 82 exhibit a negative charge.
Because the glass tube is isolated from the electrical
system and from electrical ground, the positive charge
is retained, and therefore the particulate phosphor is
caused to adhere to the glass sur~ace. This retained
charge will dissipate over time, but if properly
isolated will retain adequate charge or a period of
approximately 12 hours, 80 that the particulate
phosphor can be bonded to the gla~s surface while it is
still being held in place by the electrical
attraction. The charge on the powder in the coating is
retained because of the low conductivity of the
powder. This allows sufficient time for the
humidification and lehring of the coated lamp. A6
shown grea~ly enlarged in FIG. 6, a second layer 83 of
~ LD 9325
phosphor particles 84 of substantially different size
than the particles 82 is deposited over the first layer
31. Humidification c~uses the layers 81, 83 of
phosphor particles to be densified due to the fact that
moisture on the surfaces of the individual particles
causes the phosphor particles to shift slightly
relati~e to each other to reduce spaces between
particles and become more closely packed to the surface
of the glass by the mutual attraction of the
electrostatic charge. This improves the uniforrnity of
the phosphor coatings on the lamp glass. The
particulate layers will be maintained generally
separate along a line shown at 86 at a position
generally corresponding to the thickness of the first
particulate layer 81 from the surface of the glass
wall. Upon lehring the particles of phosphor are bound
together to the glass surface to form uniform, bonded
layers as shown in FIG. 6.
A lower lehring temperature may be employed
following the electrostatic depoæition according to the
present invention than is employed in prior art slurry
deposition, because no organic binder containing carbon
materials is used to initially bond the phosphor
coatings to the glass. The lower lehring temperature,
475C to 600C, which would be inadequate to burn out
organic binder materials, i8 adequate to cause phosphor
bonding and removal of water but is not high enough to
cause softening of the glass. Therefore, the sag which
i~ experienced at high temperature lehring is avoided
for U-shaped lamps rnade according to the present
inVelltiOII, 80 that no di~tortion of lamp shape is
caused by the lehring ~tep. An additional advantage of
the pxesent invention i8 that only a limited amount of
moisture is used in the humidifying of the lamps,
thereby reducing the quantity of water which must be
- 12 - LD 9325
removed by lehring, so that the time required for
lehring iB less than that required by prior art
techniques even though the lehring temperature i6
lower.
The electrostatic deposition process of the
present invention is not adversely affected by the
pre~ence of a ~tarting strip on the interior surface of
the glass tube. For example, on lamps with the tin
oxide starting stripe described above deposited on the
interior of the U-shaped glass tube, the present
invention performs phosphor deposition with no
reduction of adherence of the phosphor to the starting
stripe or insulating barrier layer. Further, the
present invention facilitates deposition of phosphor
mixtures which may include several particle sizes,
because no gravitational separation would occur and the
electrostatic bonding of phosphor particles to the
glass surface would not be affected by particle size~
Phosphors which are difficult to keep in suspension or
are incompatible with an organic binder are readily
applied by the electrostatic deposition process of the
present invention because of the elimination of the
binder.
It will be appreciated by those skilled in the
art that the pre~ent system of electrostatic deposition
of phosphors for fluorescent lamps eliminates the need
for organic binders in phosphor deposition with the
resultant savings of material and energy consumption
while fluorescent lamp production can be completed in
less time.
