Method for improving the adherence of a phosphor-photobinder layer to a glass support

A layer of an aqueous solution containing polyvinyl alcohol and a zirconyl compound is applied to a clean glass surface and dried to produce a precoating thereon. A subsequently-applied phosphor-photobinder coating thereon is exposed to a light image and then developed with a turbulent aqueous liquid. The phosphor-photobinder coating exhibits improved adherence to the surface during the developing step due to the presence of the precoating.

BACKGROUND OF THE INVENTION 
This invention relates to a method for preparing a luminescent screen, as 
for a cathode-ray tube, which screen exhibits markedly improved adherence 
to its supporting surface during the processing thereof. 
In preparing a luminescent screen by the slurry-direct photographic 
process, as described, for example, in U.S. Pat. No. 3,406,068 to H.B. 
Law, a glass surface, such as the inner surface of the glass faceplate for 
a color television picture tube, is coated with an aqueous slurry 
comprising a photosensitizable binder (photobinder), a photosensitizer 
therefor, and particles of phosphor material. The phosphor-photobinder 
coating is dried and then exposed to a light pattern, as by exposure 
through an apartment mask, to produce regions of greater and regions of 
lesser solubility in the coating. The exposed coating is developed by 
removing the more-soluble regions of the coating, as by spraying and/or 
flushing the coating with water or aqueous solution under pressure. The 
adherence of the retained less-soluble regions of the coating to the glass 
surface is important and is particularly critical during the development 
step. The loss of even a small part of the less-soluble regions, which 
should constitute portions of the screen, requires the screen to be 
scrapped. 
It is known that the adherence of the coating to a clean glass surface can 
be improved by applying to the glass surface a very thin precoating of a 
water-soluble polymeric material prior to applying the coating. See, for 
example, Canadian Pat. No. 602,838 to W. W. Slobbe and U.S. Pat. No. 
3,481,733 to L. W. Evans. In a typical process, the surface of a glass 
panel is washed with an aqueous ammonium bifluoride composition, rinsed 
with deionized water, then rinsed with a dilute solution of PVA (polyvinyl 
alcohol) of about 0.2 to 0.5 weight percent concentration and then dried. 
It is theorized that a very thin, perhaps monomolecular, precoating of PVA 
remains on the glass surface, which precoating improves the adherence of a 
subsequently-applied phosphor-photobinder coating. It is not apparent why 
such a thin precoating improves the adherence of the subsequently-applied 
coating. However, it has been observed that aging the PVA precoating in 
air at room temperature, or acidifying the PVA precoating with a mineral 
acid, further improves the adherence of the subsequently-applied phosphor 
coating. When heavier screen weights in the range of 4 to 6 milligrams 
phosphor per square centimeter (mg/cm.sup.2) are desired, improved 
adherence can be achieved by employing a heavy precoating of light-exposed 
dichromate-sensitized PVA or other photosensitive organic colloid with or 
without phosphor particles present. Another method, disclosed in U.S. Pat. 
No. 3,966,474 to S. A. Harper, employs a thick adherent precoating 
consisting essentially of water-insoluble, organic, polymeric particles, 
preferably deposited from an aqueous emulsion thereof. 
It is the practice, when the phosphor-photobinder coating is being dried on 
automatic or semiautomatic machines, to heat the coating and the glass 
support to about 50.degree. to 55.degree. C. When the dry coating is being 
exposed to the light pattern, the temperature of the glass support has 
dropped somewhat to about 40.degree. to 45.degree. C. This practice will 
be referred to herein as the "hot-application" process. It is often 
desirable to maintain lower temperatures in the glass support while the 
coating is being dried and is being exposed in order to reduce energy 
consumption and/or to improve the placement of the retained, less-soluble 
regions of the coating. A "cool-applicatio" process, in which the coating 
is dried at temperatures of about 30.degree. to 38.degree. C. and is 
exposed at temperatures of about 25.degree. to 33.degree. C., has been 
found to be practical. However, prior precoatings have not been as 
effective in improving the adherence of the coating in the 
cool-application process as they are in the hot-application process. 
SUMMARY OF THE INVENTION 
The novel method comprises depositing on a clean glass surface and then 
drying a layer of an aqueous solution containing polyvinyl alcohol and a 
water-soluble zirconyl compound, such as zirconyl nitrate, to produce a 
precoating. Preferably, the aqueous solution contains about 0.01 to 0.50 
weight percent of polyvinyl alcohol and about 0.001 to 0.10 weight percent 
of zirconyl compound. After the precoating has been deposited, a 
phosphor-photobinder coating is deposited on the precoating, and the 
coating is exposed and developed by the slurry direct-photographic process 
as in the prior art. 
By employing the precoating to the glass surface according to the novel 
method, the subsequently-deposited coating exhibits improved adherence to 
the surface. Heavier and thicker phosphor coatings can be applied, 
exposed, and developed without loss of any parts of the screen during the 
development. Either the cool-application or the hot-application process 
may be used. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The novel method is an improvement to the slurry-direct photographic 
process for making a luminescent screen on a glass support. In the novel 
method as in prior methods, a precoating is applied to the surface of the 
support, and then a layer of an aqueous slurry including phosphor 
particles, a binder such as PVA (polyvinyl alcohol), and a photosensitizer 
therefor is applied to the support and dried. 
In the novel method, the precoating is produced by depositing on the glass 
surface and then drying a solution containing PVA and a zirconyl compound. 
The zirconyl compound may be, for example, zirconyl chloride [ZrOCl.sub.2 
], zirconyl bromide [ZrOBr.sub.2 ], or zirconyl iodide [ZrOI.sub.2 ]. The 
preferred compound is zirconyl nitrate [ZrO(No.sub.3).sub.2 ]. 
Combinations of two or more zirconyl compounds may be used. The zirconyl 
compound may be provided in any purity grade provided the impurities 
present do not interfere with the fabrication of the screen. The PVA may 
be any PVA that was previously used or disclosed to be useful for 
precoating glass supports. A PVA useful for precoating generally has a 
degree of hydrolysis in the range of 80 to 95 percent. One useful PVA is 
Vinol No. 540 marketed by Air Products and Chemicals, Inc., Allentown, PA. 
The aqueous precoating solution contains about 0.01 to 0.50 weight percent 
of PVA and about 0.001 to 0.10 weight percent of zirconyl compound with 
respect to the weight of the solution. It is preferred that the dry weight 
of zirconyl compound is between 1 and 100% of the dry weight of the PVA 
present. The precoating solution may be applied by any convenient method 
such as by spraying or flowing the solution on or over the surface. A 
preferred method is to face the glass surface downward and then to apply a 
fountain of the precoating solution to the surface. The pH of the 
precoating solution is generally in the range of about 2.5 to 4.0

EXAMPLE 
A specific example of a preferred precoating solution and the practice of 
the novel method is as follows: Prepare a precoating solution comprising 
in weight percent of dry solids: 
______________________________________ 
PVA (87% hydrolyzed) 0.10% 
Zirconyl nitrate 0.02% 
Water balance 
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Separate aqueous solutions of PVA and zirconyl nitrate are prepared. Then, 
the zirconyl solution and water are added one at a time to the PVA 
solution while slowly stirring the mixture. 
The inner surface of a glass 25V faceplate panel for a color television 
picture tube is thoroughly cleaned. Then, the precoating solution is 
applied to the clean glass surface by facing the surface to be precoated 
downwardly and then applying the solution as a fountain, after which the 
surface is permitted to drain. Preferably, the panel rotates at about 30 
rpm about an axis that is substantially normal to the surface and that is 
inclined from vertical no more than about 10.degree.. After the precoating 
solution is applied, the rotation of the panel is briefly increased to 
about 100 rpm to remove excess solution. Then, infrared heat and flowing 
air are applied to dry the precoating. The precoating and panel may reach 
a temperature up to about 35.degree. C. during the drying step. 
Next, the coating composition is applied to the precoated surface. The 
coating composition comprises a mixture of phosphor particles, a 
photobinder, a photosensitizer for the binder and water, as is known in 
the art. One suitable coating composition contains, in about the following 
proportions, 292 grams of green-emitting, copper-activated zinc-cadmium 
sulfide particles, 233 grams of a ten-weight-percent aqueous solution of 
polyvinyl alcohol (Vinol No. 540), 13 grams of a 45-weight-percent aqueous 
solution of an acrylic copolymer, 14 grams of a ten-weight-percent aqueous 
solution of sodium dichromate and about 402 grams of deionized water. The 
coating composition is thoroughly mixed, and the viscosity of the mixture 
is adjusted to be in the range of about 20 to 50 centipoises. The coating 
composition is then flow coated upon the precoated surface and dried to 
produce a dry coating containing about 4.0 mg/cm.sup.2 of phosphor 
particles. The coating and the panel may reach a temperature up to about 
35.degree. C. during the drying of the coating. 
At this point in the novel method, the panel carries a precoating on the 
inner surface thereof and phosphor-photo-binder coating on the precoating. 
The apertured mask for the panel is then inserted on the studs provided 
therefor and the panel assembly positioned upon a lighthouse platform. 
Ultraviolet light from a small area light source in the lighthouse is 
projected through the mask, which permits a pattern of light to fall 
incident upon and expose the coating. The panel assembly is then removed 
from the lighthouse and the mask is removed from the panel. The exposed 
coating is developed by subjecting the coating to a turbulent quantity of 
aqueous liquid, as by spraying and flushing with water (or an aqueous 
developing solution) under pressure to remove the unexposed and 
substantially unexposed more-soluble regions of the coating while 
retaining the exposed and less-soluble regions of the coating in place. 
The novel method may be used to print any particle pattern (pattern of 
particulate material) upon any glass supporting surface. The particles of 
the coating may be luminescent or nonluminescent, may be light absorbing 
and may be any body color in reflected light. The novel method may be used 
particularly to print viewing-screen structures for cathode-ray tubes. Dot 
and line viewing-screen structures are examples. 
The example illustrates the use of the novel method to produce a screen 
structure (the green-emitting field) for a color television picture tube 
by the slurry-direct photographic process. Many suitable coating 
compositions for the novel method applied to the slurry process are 
described in U.S. Pat. No. 3,269,838 to T. A. Saulnier, Jr. Preferably, 
the coating composition is comprised of a water-soluble binder which is 
cross-linkable into insoluble form by actinic radiation in the presence of 
hexavalent chromium ions, a soluble dichromate photosensitizer for the 
binder, and particles of the pattern material. The binder may be any 
organic colloid, such as gelatin or fish glue, but is preferably a PVA. 
The photosensitizer may be any soluble dichromate; for example, sodium 
dichromate, potassium dichromate, or ammonium dichromate. The 
photosensitizer may be present in amounts of about 2 to 20 weight percent 
of the weight of the binder present. 
The coating is exposed to a pattern of actinic radiation. Any actinic 
radiation such as visible light rays, ultraviolet light rays, or cathode 
rays may be used. A significant quality of the exposure is that, although 
coating portions may be insolubilized by actinic radiation, nevertheless 
the insoluble coating portions may not adhere sufficiently to the 
supporting surface to remain in place through the development step. This 
adherence is improved by the novel method. 
Adherence of the exposed coating may be evaluated in a quantitative manner 
by using the developing-time method. By this method, each of the exposed 
coatings to be tested is developed with a spray of water with the same 
spray nozzle using the same pressure to produce the spray. The developing 
time required to produce the first loss of a less-soluble region of the 
exposed coating is a measure of the adherence of the coating to the glass 
support. 
TABLE I presents some comparative data giving representative developing 
times for various samples prepared by the cool-application process in 
which a glass support received a precoating with a precoating solution 
containing 0.10 weight percent PVA, then received a coating of 
green-emitting phosphor which was dried at temperatures of about 
35.degree. C., then the dried coating was exposed to a pattern of 
ultraviolet light at temperatures of about 131.degree. C. and then 
developed with a water spray. 
TABLES II and III present some comparative data giving representative 
developing times for various samples prepared by the cool-application and 
the hot-application processes respectively in which a glass support 
received a precoating with the indicated precoating solution containing 
about 0.10 weight percent PVA, then received a first coating comprising 
green-emitting phosphor which was dried (at temperatures of about 
49.degree. C. for the hot-application process and 35.degree. C. for the 
cool-application process), then the dried first coating was exposed to a 
pattern of ultraviolet light (at temperatures of about 40.degree. C. for 
the hot-application process and 31.degree. C. for the cool-application 
process), and then developed with a water spray. Then, the developed first 
coating received a second coating comprising blue-emitting phosphor which 
was dried (at temperatures of about 50.degree. C. for the hot-application 
process and 35.degree. C. for the cool-application process). Thereafter 
the dried second coating was exposed to a pattern of ultraviolet light (at 
temperatures of about 40.degree. C. for the hot-application process and 
31.degree. C. for the cool-application process) and then developed with a 
water spray. The indicated developing times in the TABLES are the 
developing times of the second coating required to produce the first loss 
of less-soluble regions from the second coating. 
Each of TABLES I, II and III shows that the novel method produces 
substantially more adherent coatings as compared with similar methods 
using prior precoatings. TABLE I illustrates that the improved adherence 
may be realized with a first coating directly on the precoating. TABLES II 
and III illustrate that the improved adherence may be realized with a 
second coating that has been applied after a first coating. In the tests 
listed in TABLES II and III, the retained less-soluble regions of the 
first and second coatings are each on the precoating and do not overlap 
one another. 
TABLE I 
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COOL-APPLICATION PROCESS - 
FIRST COATING ON PRECOATING 
Developing Time to 
Initial Losses 
Test No. 
Precoating (Seconds) 
______________________________________ 
(1) 0.1% PVA - pH about 5.5 
18 
(2) 0.1% PVA - acidified to 
32 
pH 4.1 
(3) 0.1% PVA with 0.002% 
48 
zirconyl nitrate 
(pH about 4.1) 
(4) 0.1% PVA acidified to 
28 
pH 3.65 with nitric acid 
(5) 0.1% PVA with 0.005% 
73 
zirconyl nitrate 
(pH about 3.65) 
(6) 0.1% PVA acidified to 
137 
pH about 3.15 
(7) 0.1% PVA with 0.02% 
205 
zirconyl nitrate 
(pH about 3.15) 
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TABLE II 
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COOL-APPLICATION PROCESS - 
SECOND COATING ON PRECOATING 
Developing Time to 
Initial Losses 
Test No. Precoating (Seconds) 
______________________________________ 
(8) 0.1% PVA acidified to 
46 
pH 3.15 
(9) 0.1% PVA with 0.02% 
75 
zirconyl nitrate (pH 
about 3.15) 
______________________________________ 
TABLE III 
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HOT-APPLICATION PROCESS - 
SECOND COATING ON PRECOATING 
Developing Time to 
Initial losses 
Test No. Precoating (Seconds) 
______________________________________ 
(10) 0.1% PVA 45 
(11) 0.1% PVA acidified to 
65 
about pH 3.45 
(12) 0.1% PVA with 0.01% 
120 
zirconyl nitrate 
(pH about 3.45) 
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