Resistive, adhesive-primer coating for a display apparatus and method of making same

A display apparatus comprises a CRT having an evacuated envelope including a faceplate with a luminescent screen on the interior surface thereof. An electron gun is disposed within a neck of the envelope for generating and directing at least one electron beam toward the screen. A funnel connects the neck of the envelope with the faceplate. A deflection yoke is disposed around the envelope at the junction of the neck and the funnel. The yoke is attached to the envelope with an adhesive. The envelope further includes a first conductive coating on the interior surface of the funnel, a second conductive coating on at least a portion of the exterior surface of the envelope, and a third coating on the exterior surface of the envelope underlying the yoke and connected to the second conductive coating. The third coating provides a resistive, adhesive-primer coating that acts as a barrier between the yoke adhesive and the envelope and has a composition consisting essentially of a polychloroprene latex, a conductive filler material selected from the group consisting of carbon black and graphite, and water.

The present invention relates to a display apparatus, such as a cathode-ray 
tube (CRT) with a deflection yoke attached thereto, and, more 
particularly, to a resistive, adhesive-primer coating which forms a 
barrier disposed between the tube and the yoke. 
BACKGROUND OF THE INVENTION 
The display apparatus, such as a television picture tube or a display tube, 
comprises an evacuated envelope which includes a faceplate and a 
rectangular funnel. A tri-color luminescent screen is disposed on the 
interior surface of the faceplate. An electron gun for generating and 
directing at least one, and preferably three electron beams toward the 
screen is disposed within and closes the neck of the funnel. The panel, 
bearing the screen, is sealed to and closes the opposite end of the 
funnel. The interior surface of the funnel is coated with a first 
conductive layer and the exterior surface is partly covered with a second 
conductive layer. The conductive layer on the interior surface of the 
funnel forms the connection between the last electrode of the electron gun 
and the screen on which the electron beams impinge. The conductive layer 
on the interior surface of the funnel thus acts as a second anode and also 
creates a field-free space in the region of the tube in which the electron 
beams are not deflected. The conductive layer on the exterior of the 
funnel is grounded and, together with the interior conductive layer, forms 
a capacitor which serves to smooth the applied high voltage. The apparatus 
further includes a system of deflection coils, called a deflection yoke, 
disposed around the neck-funnel transition region of the envelope. 
It is known in the art to provide a high resistance coating or layer on the 
exterior portion of the tube envelope underneath the deflection yoke, in 
order to reduce interference generated by the deflection system. 
Typically, the high resistance layer has a resistance of between 10.sup.6 
and 10.sup.10 ohms per square. The high resistance layer is electrically 
connected to the second conductive layer disposed on the exterior surface 
of the funnel. Apparatuses made with a low resistance layer, or having no 
coating at all underneath the yoke, develop an inductive charge which 
"flashes-over", or electrically discharges, to the deflection yoke or to 
the second conductive layer on the exterior surface of the funnel. The 
discharge is accompanied with an annoying crackling or sputtering sound. 
The problem is exacerbated in the current generation of high resolution 
display apparatuses in which the horizontal scan rate is twice that of 
prior tubes, thus requiring the deflection yoke to operate at a higher 
current, thereby generating additional heat underneath the yoke and 
increasing the magnitude of the induced charge. The high resistance 
coating underneath the deflection yoke also must be compatible with other 
steps in the manufacturing process, such as alignment and attachment of 
the deflection yoke. In the present manufacturing process, the yoke, after 
alignment, is rapidly secured by an adhesive to the tube. Conventional 
high resistance coatings, such as those known in the art and containing 
polyvinyl acetate and suitable quantities of metal oxides and soots, or 
water-based urethane resins and conductive materials, are unsatisfactory, 
because they are either incompatible with, or slow, the curing process of 
the adhesive. Thus, a need exists for a high resistance coating which is 
compatible with the above-described yoke attachment process, and which 
provides the protection needed to reduce electrical interference from the 
yoke and to prevent the build-up and flash-over of induced charge. 
SUMMARY OF THE INVENTION 
A display apparatus comprises a CRT having an evacuated envelope with an 
interior surface and an exterior surface. The envelope includes a 
faceplate having a luminescent screen on the interior surface thereof. An 
electron gun is disposed within a neck of the envelope for generating and 
directing at least one electron beam toward the screen. A funnel connects 
the neck of the envelope with the faceplate. A deflection yoke is disposed 
around the envelope at the junction of the neck and the funnel. The yoke 
is attached to the envelope with an adhesive. 
The envelope further includes a first conductive coating on the interior 
surface of the funnel, a second conductive coating on at least a portion 
of the exterior surface of the envelope, and a third coating on the 
exterior surface of the envelope underlying the yoke and connected to the 
second conductive coating. The third coating provides a resistive, 
adhesive-primer coating which forms a barrier between the yoke adhesive 
and the envelope. The third coating has a composition consisting 
essentially of a polychloroprene latex, a conductive filler material 
selected from the group consisting of carbon black and graphite, and 
water. A method of making the resistive, adhesive-primer coating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIGS. 1 and 2 show a color display apparatus including a CRT 10, having a 
glass envelope 11 comprising a substantially rectangular faceplate panel 
12 and a tubular neck 14 connected by a rectangular funnel 15. The funnel 
15 has an interior conductive coating 15a that contacts an anode button 16 
and extends into the neck 14. An exterior conductive coating 15b overlies 
at least a portion of the interior coating 15a. An external insulative 
coating 16a surrounds the outer portion of the anode button 16. The panel 
12 comprises a viewing faceplate 18 and a peripheral flange or sidewall 20 
which is sealed to the funnel 15 by a glass frit 21. 
A three color phosphor screen 22 is carried on the inner surface of the 
faceplate 18. The screen 22 preferably is a line screen which includes a 
multiplicity of screen elements comprised of red-emitting, green-emitting 
and blue-emitting phosphor stripes R, G and B, respectively, arranged in 
color groups or picture elements of three stripes or triads in a cyclic 
order (not shown) and extending in a direction which is generally normal 
to the plane in which the electron beams are generated. In the normal 
viewing position of the embodiment, the phosphor stripes extend in the 
vertical direction. Preferably, the phosphor stripes are separated from 
each other by a light-absorbing matrix material (also not shown), as is 
known in the art. Alternatively the screen can be a dot screen. A thin 
conductive layer 24, preferably of aluminum, overlies the screen 22 and 
provides a means for applying a uniform potential to the screen as well as 
for reflecting light, emitted from the phosphor elements, through the 
faceplate 18. The screen 22 and the overlying aluminum layer 24 comprise a 
screen assembly. 
A multi-apertured color selection electrode or shadow mask 25 is removably 
mounted in predetermined spaced relation to the screen assembly, by 
conventional means. An electron gun 26, shown schematically by the dashed 
lines in FIG. 1, is centrally mounted within the neck 14, to generate and 
direct three electron beams 28 along convergent paths, through the 
apertures (not shown) in the mask 25, to the screen 22. The gun 26 may be 
any type of CRT electron gun known in the art. 
The tube 10 is designed to be used with an external magnetic deflection 
yoke 30, located in the region of the funnel-to-neck junction. When 
activated, the yoke 30 subjects the three beams 28 to magnetic fields 
which cause the beams to scan horizontally and vertically in a rectangular 
raster over the screen 22. The initial plane of deflection (at zero 
deflection) is shown by the line P-P in FIG. 1, at about the middle of the 
yoke 30. For simplicity, the actual curvatures of the deflection beam 
paths in the deflection zone are not shown. The yoke 30 is attached to the 
neck 14 by a clamp (not shown) and contacts the envelope in the 
funnel-to-neck junction by means of a plurality of adjustment screws 32, 
shown in FIG. 2. The ends of the screws in contact with the envelope are 
cemented thereto by an adhesive 34. As so far described, the display 
apparatus is conventional. 
A novel high resistance coating 36 having a resistance within the range of 
about 10.sup.6 to 10.sup.10 ohms/square, and preferably about 10.sup.7 to 
5.times.10.sup.7 ohms/square, is provided on the exterior surface of the 
funnel 15, underlying the yoke 30, and extending along the neck of the 
tube to be coextensive with the internal coating 15a. As shown in FIG. 2, 
the high resistance coating 36 extends forward of the yoke attachment 
screws 32 and has a pair of oppositely disposed projections 38 (at the 3 
and 9 o'clock positions) which contact the conductive coating 15b. The 
novel high resistance coating 36 is a water-based, latex material that 
provides an adhesive-primer which acts as a barrier between the yoke 
adhesive and the glass envelope to facilitate accurate attachment of the 
yoke, after yoke alignment. The primer is compatible with both urethane 
and hot-melt adhesives which are used to cement the contacting portions of 
the yoke screws 32 to the envelope and thus permits rapid and accurate 
yoke attachment which speeds the manufacturing process. 
The novel high resistance coating comprises, in weight percent: 
______________________________________ 
polychloroprene latex 88 .+-. 2%, 
(44 wt.% solids) 
a conductive filler material 
2.5 .+-. 0.5%, 
selected from the group consist- 
0.3 .+-. 0.1% 
ing of carbon black and graphite, 
a dispersant sold under 
the trademark MARASPERSE 
and the balance being water 
______________________________________ 
The novel high resistance, adhesive-primer coating is made as follows: 
a) pour 750 g. of hot deionized water (65.degree.-51.degree. C.) into a 
stainless steel beaker; 
b) slowly add 30 g. the dispersant, sold under the trademark MARASPERSE 
available from Reed Lignin Co., Rothchild, Wis., to the hot water and stir 
continuously to form a solution; 
c) gradually stir into the solution 213 g. of carbon black to form a 
conductive slurry; 
d) mix the slurry in a blender at a high speed for two minutes, and then 
transfer the slurry to a a 1 liter plastic bottle; 
e) pour 750 g. of polychloroprene latex adhesive primer, manufactured by 
National Starch and Chemical Co., Bridgewater, N.J., into an empty 1 liter 
plastic bottle, and add 102 g. of the slurry, to form a high resistance 
adhesive-primer solution; 
f) roll the plastic bottle containing the coating materials on a ball mill 
for a minimum of 24 hours; and 
g) test the solution for linear resistance by applying a quantity of the 
solution to a clean glass plate using a flat paint brush with a 50 mm 
width. Allow the solution to air dry and form a coating. Measure the 
linear resistance of the coating with an ohm meter. The resistance must be 
in the range of 10.sup.6 -10.sup.9 ohms/square (a 25.4 mm square pad is 
used for the resistance check). Additional primer may be added to the 
solution to increase the resistance and additional carbon black may be 
added to decrease it. 
When the high resistance adhesive-primer is applied to the tube 10, using a 
brush or other suitable applicator, it must be allowed to air dry for at 
least 15 minutes to form the coating 36 before the yoke 30 is attached to 
the tube. The drying time can be shortened by preheating the tube or 
drying the primer with heated air. It has been determined that the 
resistance can be increased by an order of magnitude (10.times.) by drying 
the coating 36 with heated air. The primer is applied to the exterior 
surface of the envelope, in the region of the funnel-to-neck junction. The 
coating 36 extends along the neck 14 for a distance that is coextensive 
with the interior conductive coating 15a. The coating 36 extends forward 
of the yoke 30 so that the yoke attachment screws 32 contact the coating 
36. The screws are secured to the coating 36 with a hot-melt adhesive, 
sold under the trademark MACROMELT #6238, available from Henkel Adhesives, 
Co., La Grange, Ill. Electrical connection to the grounded conductive 
coating 15b is provided by the projections 38. 
A test was initially conducted using one hundred and twenty-five (125) 35 
V110-degree tubes with conventional yokes and with the novel coating 36. 
Electrical tests of operating television sets with these apparatuses 
showed a slight reduction in discharge-induced set shutdowns. An 
additional 200-tube test, using 31 V and 35 V110-degree tubes, was 
conducted with 2 H yokes mounted on the tubes. The 2 H yoke, which has a 
horizontal scan rate double that of the conventional yoke, generates more 
heat than a conventional yoke and causes an increase in the inductive 
charge buildup. The novel high resistance adhesive-primer coating 36 was 
effective in preventing the flashover or discharge of this induced charge 
by permitting the charge to bleed-off to the grounded conductive coating 
15b. Additionally, the novel coating 36 proved to be compatible with the 
hot melt adhesive used to attach the yoke adjustment screws 32 to the 
coated envelope of very large size tubes having a diagonal screen 
dimension of 79 cm, or larger. While carbon black is the conductive 
additive of choice in the novel coating 36, graphite in the same 
proportion, also can be used, with only a slight decrease in the strength 
of the attachment of the hot melt adhesive to the coating.