Pigment coated phosphor

A pigment coated phosphor used as a green emitting phosphor for a high contrast color television cathode ray tube. A green emitting phosphor is coated with TiO.sub.2 --ZnO--CoO--NiO system pigment particles.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a phosphor coated with pigment particles 
(hereinafter referred to as "pigment coated phosphor"), and more 
particularly to a pigment coated phosphor employed in a high contrast 
color television cathode ray tube. 
2. Description of the Prior Art 
As is well known in the art, pigment particles adhering to the surfaces of 
phosphor particles such as blue emitting phosphor particles, green 
emitting phosphor particles and red emitting phosphor particles employed 
in a color television cathode ray tube markedly enhance the contrast of 
the image formed on the cathode ray tube. This is because a part of the 
visible region of the emission spectra inherent in the phosphor is cut due 
to the filter effect of the pigment particles adhering thereto, which 
results in a clearer emission color, and further because the pigment 
particles can absorb a part of the external light to reduce the reflection 
of light thereby. This is disclosed, for instance in U.S. Pat. No. 
3,886,394. 
It is required for the pigment coated phosphor employed in a high contrast 
color television cathode ray tube mentioned above that the reflectance be 
low and the luminance be sufficiently high. That is, in order to lower the 
reflectance of the external light by the pigment coated phosphor, it is 
necessary to increase the amount of the pigment particles adhering to the 
surface of the phosphor and obtain a higher surface coating rate. In such 
a pigment coated phosphor having a high surface coating rate, the 
luminance thereof is inevitably lowered. It is required that the lowering 
of the luminance be controlled to be as small as possible. In other words, 
for a given specific reflectance, the luminance is desired to be as high 
as possible. 
Heretofore, chromium oxide (Cr.sub.2 O.sub.3) have been recommended as 
green pigment particles for a green pigment coated green emitting phosphor 
employed in a high contrast color television cathode ray tube. The pigment 
coated phosphor using the chromium oxide pigment particles satisfies the 
aforesaid requirement to some extent, but not sufficiently. Therefore, it 
has not been put into practical use. That is, at present, blue and red 
pigment coated phosphors are used respectively as the blue and red 
emitting phosphors of a high contrast color television cathode ray tube, 
but a green pigment coated phosphor is not used as the green emitting 
phosphor thereof. 
Under the aforesaid circumstances, there is demanded a green pigment coated 
green emitting phosphor available for practical use whose luminance is 
higher, when comparing phosphors of the same specific reflectance, than 
that of the conventional chromium oxide pigment coated green emitting 
phosphor. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a green pigment coated 
green emitting phosphor whose luminance is higher, when comparing 
phosphors of the same specific reflectance, than that of the conventional 
chromium oxide pigment coated green emitting phosphor. 
As the result of various investigations of the green pigment particles 
adhering to the surface of a green emitting phosphor, the inventors have 
discovered that the aforesaid object can be achieved when TiO.sub.2 
--ZnO--CoO--NiO system pigment particles are used as green pigment 
particles. 
The pigment coated phosphor in accordance with the present invention is 
characterized in that the TiO.sub.2 --ZnO--CoO--NiO system pigment 
particles adhere to the surface of a green emitting phosphor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As TiO.sub.2 --ZnO--CoO--NiO system pigment particles used in the pigment 
coated phosphor of the present invention, there are employed commercially 
available products. The TiO.sub.2 --ZnO--CoO--NiO system pigment contains 
titanium dioxide (TiO.sub.2), zinc oxide (ZnO), cobalt oxide (CoO) and 
nickel oxide (NiO) as main ingredients. The body color of the TiO.sub.2 
--ZnO--CoO--NiO system pigment varies depending upon the method of 
production, particle size, contents of the aforesaid main ingredients, and 
the like. The reflectances at 400 nm, 450 nm, 500 nm, 550 nm, 600 nm, 650 
nm and 700 nm of the TiO.sub.2 --ZnO--CoO--NiO system pigment particles 
employed in the pigment coated phosphor of the present invention are 
within the ranges indicated in Table I below wherein the reflectance is 
shown by a relative value with reference to that of a magnesium oxide 
diffusion plate defined to be 100%. 
Table I 
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Wavelength 
400 450 500 550 600 650 700 
(nm) 
not not not not not 
Reflectance 
more more 15-40 20-45 more more more 
(%) than than than than than 
15 20 25 20 20 
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FIG. 1 represents the reflection spectrum of the TiO.sub.2 --ZnO--CoO--NiO 
system pigment particles employed in the pigment coated phosphor of the 
present invention (curve-a) in comparison with that of the chromium oxide 
pigment particles employed in the conventional pigment coated phosphor 
(curve-b) wherein the reflectance represented by the ordinate is also 
shown by a relative value with reference to that of a magnesium oxide 
diffusion plate defined to be 100%. 
The TiO.sub.2 --ZnO--CoO--NiO system pigment particles employed in the 
pigment coated phosphor of the present invention should preferably have a 
mean particle size of less than 3.0.mu.. When the TiO.sub.2 
--ZnO--CoO--NiO system pigment particles having a mean particle size 
larger than 3.0.mu. are employed, the pigment particles do not firmly 
adhere to the surface of the phosphor since the pigment particles are too 
large. Therefore, when using large particles a larger amount of the 
pigment particles is needed to effect a desired coloration of the 
phosphor, which marked lowers the luminance of the pigment coated phosphor 
obtained. More preferably, the mean particle size is within a range of 
0.1.mu. to 1.5.mu.. 
On the other hand, the green emitting phosphors employed in the pigment 
coated phosphor of the present invention are, for example, a copper 
activated zinc sulfide phosphor (ZnS:Cu), a copper and aluminium activated 
zinc sulfide phosphor (ZnS:Cu,Al), a copper activated zinc cadmium sulfide 
phosphor [(Zn,Cd)S:Cu], a copper and aluminum activated zinc cadmium 
sulfide phosphor [(Zn,Cd)S:Cu,Al], a silver activated zinc cadmium sulfide 
phosphor [(Zn,Cd)S:Ag], a silver and aluminium activated zinc cadmium 
sulfide phosphor [(Zn,Cd)S:Ag,Al], a gold and aluminium activated zinc 
sulfide phosphor (ZnS:Au,Al), a copper, gold and aluminium activated zinc 
sulfide phosphor (ZnS:Cu,Au,Al), a silver activated zinc sulfoselenide 
phosphor [Zn(S,Se):Ag], a silver and aluminium activated zinc 
sulfoselenide phosphor [Zn(S,Se):Ag,Al], a copper activated zinc 
sulfoselenide phosphor [Zn(S,Se):Cu], a copper and aluminum activated zinc 
sulfoselenide phosphor [Zn(S,Se):Cu,Al], a self-activated zinc oxide 
phosphor (ZnO:Zn), and a manganese activated zinc silicate phosphor 
(Zn.sub.2 SiO.sub.4 :Mn). The sulfide phosphors included in the above 
mentioned green emitting phosphors may contain a very small amount of 
halogen, bismuth, antimony, and the like. Among the green emitting 
phosphors mentioned above, the ZnS:Cu,Al phosphor and (Zn,Cd)S:Cu,Al 
phosphor which are practically used as the green emitting phosphors for a 
color television cathode ray tube are especially recommended. These green 
emitting phosphors employed in the pigment coated phosphor of the present 
invention should preferably have a mean particle size within a range of 
3.mu. to 15.mu., more preferably of 4.mu. to 12.mu.. 
As the process for preparing the pigment coated phosphor of the present 
invention by coating the surface of a green emitting phosphor with 
TiO.sub.2 --ZnO--CoO--NiO system pigment particles, the conventional 
process for preparing a pigment coated phosphor as disclosed in the 
aforesaid U.S. Pat. No. 3,886,394 may be adopted. In U.S. Pat. No. 
3,886,394, pigment particles dispersed in an appropriate solution of a 
water soluble resin such as polyvinyl pyrrolidone are mixed with a 
phosphor dispersed in an aqueous solution of gelatin, and then the mixture 
is stirred and the resulting precipitate is dried to obtain a pigment 
coated phosphor. However, in the preparation of a pigment coated phosphor, 
it is important that the pigment particles adhere uniformly and firmly to 
the surface of the phosphor. As the process for having the pigment 
particles adhere to the surface of the phosphor uniformly and firmly are 
recommended, for instance, a process utilizing a suspension polymerization 
method, a process utilizing a copolymerization method and a process using 
a gelatin-gum arabic mixture as a binder. 
In the pigment coated phosphor of the present invention, in general, the 
amount of the TiO.sub.2 --ZnO--CoO--NiO system pigment particles adhering 
to the surface of the green emitting phosphor is preferably within a range 
of 0.02 to 10 parts by weight per 100 parts by weight of the green 
emitting phosphor, though it depends upon various factors such as the kind 
of the green emitting phosphor employed, the particle size thereof, the 
kind of the TiO.sub.2 --ZnO--CoO--NiO system pigment particles employed 
and the particle size thereof. When the amount of the pigment particles is 
not more then 0.02 parts, it is impossible to obtain a sufficiently high 
contrast, while when it is not less than 10 parts, the luminance is 
lowered. 
FIG. 2 represents the relationship between the specific reflectance and the 
luminance of the pigment coated phosphor of the present invention wherein 
TiO.sub.2 --ZnO--CoO--NiO system pigment particles adhere to the surface 
of the ZnS:Cu,Al phosphor (curve-a) in comparison with that of the 
conventional pigment coated phosphor wherein chromium oxide pigment 
particles adhere to the surface of the same ZnS:Cu,Al phosphor (curve-b). 
In FIG. 2, the luminance represented by the ordinate is shown by a 
relative value with reference to that of the uncoated ZnS:Cu,Al phosphor 
defined to be 100%. 
As is clear from FIG. 2, within a range of practical specific reflectance, 
the luminance of the pigment coated phosphor of the present invention is 
always higher, when comparing phosphors of the same specific reflectance, 
than that of the conventional pigment coated phosphor. Though the 
relationship between the specific reflectance and the luminance of the 
pigment coated phosphor of the present invention using a green emitting 
phosphor varies depending upon the kind of the TiO.sub.2 --ZnO--CoO--NiO 
system pigment particles employed and the particle size thereof, the 
result of the experiments indicates that, within the range of the 
practical specific reflectance, the pigment coated phosphor of the present 
invention exhibits higher luminance, when comparing phosphors of the same 
specific reflectance, than that of the conventional pigment coated 
phosphor using chromium oxide pigment particles. 
As is described above, in accordance with the present invention, there is 
provided a pigment coated phosphor whose luminance is higher, when 
comparing phosphors of the same specific reflectance, than that of the 
conventional pigment coated phosphor. 
The pigment coated phosphor of the present invention is used mainly as a 
green emitting phosphor for a high contrast color television cathode ray 
tube. However, it should be noted that the use thereof is not limited only 
to the green emitting phosphor for a high contrast color television 
cathode ray tube. 
The present invention will hereinbelow be described with reference to 
several examples thereof. It should, however, be understood that the 
invention is not limited to these examples. 
EXAMPLE 1 
0.6 parts by weight of gelatin were dissolved in water of 40.degree. C. to 
prepare a 0.3% aqueous solution of gelatin. To the resulting 0.3% aqueous 
solution of gelatin were added 100 parts by weight of copper and aluminium 
activated zinc sulfide green emitting phosphor (ZnS:Cu,Al) having a mean 
particle size of 10.mu., and the resulting mixture was then well stirred 
with a stirrer to disperse the phosphor particles as primary particles. A 
phosphor dispersion was thus obtained. 
On the other hand, 0.4 parts by weight of gum arabic were dissolved in 
water to prepare a 0.3% aqueous solution thereof. 0.2 parts by weight of 
TiO.sub.2 --ZnO--CoO--NiO system pigment particles having a mean particle 
size of about 0.5.mu. (No. 9320, made by Dainichiseika Co., Ltd.) were 
then added to the 0.3% aqueous solution of gum arabic and well stirred 
with a stirrer till the pigment particles became primary particles. A 
pigment dispersion was thus obtained. 
Then, the phosphor dispersion and the pigment dispersion were mixed by 
stirring. The pH of the resulting mixture was adjusted to 4.0 and the 
temperature thereof was lowered to below 10.degree. C. To the mixture was 
then added gradually 1 part by weight of formaline while the mixture was 
continuously stirred. After the mixture was allowed to stand, a 
precipitate was obtained. The supernatant solution was removed and the 
precipitate was washed with water. The precipitate was then separated from 
water and concentrated using a decanter to yield a pigment coated phosphor 
cake containing about 85% of solid content. The cake was dried in a stream 
of air to obtain a pigment coated phosphor of the present invention. 
The conventional pigment coated phosphor using chromium oxide pigment 
particles was prepared in the same manner as that mentioned above except 
0.15 parts by weight of chromium oxide pigment particles having a mean 
particle size of about 0.5.mu. (X-1134, made by Hercules Co., Ltd.) were 
used instead of the aforesaid 0.2 parts by weight of TiO.sub.2 
--ZnO--CoO--NiO system pigment particles. 
As is shown in Table II below, the specific reflectance of the pigment 
coated phosphor of the present invention obtained was almost the same as 
that of the conventional pigment coated phosphor. However, the luminance 
thereof was higher than that of the conventional one. 
EXAMPLE 2 
0.6 parts by weight of gelatin were dissolved in water of 40.degree. C. to 
prepare a 0.3% aqueous solution of gelatin. To the resulting 0.3% aqueous 
solution of gelatin were added 100 parts by weight of copper and aluminium 
activated zinc sulfide green emitting phosphor (ZnS:Cu,Al) having a mean 
particle size of 10.mu., and the resulting mixture was then well stirred 
with a stirrer to disperse the phosphor particles as primary particles. A 
phosphor dispersion was thus obtained. 
On the other hand, 0.4 parts by weight of gum arabic were dissolved in 
water to prepare a 0.3% aqueous solution thereof. 0.3 parts by weight of 
TiO.sub.2 --ZnO--CoO--NiO system pigment particles having a mean particle 
size of about 0.5.mu. (No. 9310, made by Dainichiseika Co., Ltd.) were 
then added to the 0.3% aqueous solution of gum arabic and well stirred 
with a stirrer till the pigment particles became primary particles. A 
pigment dispersion was thus obtained. 
Then, the phosphor dispersion and the pigment dispersion were mixed by 
stirring. The pH of the resulting mixture was adjusted to 4.0 and the 
temperature thereof was lowered to below 10.degree. C. To the mixture was 
then added gradually 1 part by weight of formaline while the mixture was 
continuously stirred. After the mixture was allowed to stand, a 
precipitate was obtained. The supernatant solution was removed and the 
precipitate was washed with water. The precipitate was then separated from 
water and concentrated using a decanter to yield a pigment coated phosphor 
cake containing about 85% of solid content. The cake was dried in a stream 
of air to obtain a pigment coated phosphor of the present invention. 
The conventional pigment coated phosphor using chromium oxide pigment 
particles was prepared in the same manner as that mentioned above except 
0.25 parts by weight of chromium oxide pigment particles having a mean 
particle size of about 0.5.mu. (X-1134, made by Hercules Co., Ltd.) were 
used instead of the aforesaid 0.3 parts by weight of TiO.sub.2 
--ZnO--CoO--NiO system pigment particles. 
As is shown in Table II below, the specific reflectance of the pigment 
coated phosphor of the present invention obtained was almost the same as 
that of the conventional pigment coated phosphor. However, the luminance 
thereof was higher than that of the conventional one. 
EXAMPLE 3 
0.6 parts by weight of gelatin were dissolved in water of 40.degree. C. to 
prepare a 0.3% aqueous solution of gelatin. To the resulting 0.3% aqueous 
solution of gelatin were added 100 parts by weight of copper and aluminium 
activated zinc sulfide green emitting phosphor (ZnS:Cu,Al) having a mean 
particle size of 10.mu., and the resulting mixture was then well stirred 
with a stirrer to disperse the phosphor particles as primary particles. A 
phosphor dispersion was thus obtained. 
On the other hand, 0.4 parts by weight of gum arabic were dissolved in 
water to prepare a 0.3% aqueous solution thereof. 0.5 parts by weight of 
TiO.sub.2 --ZnO--CoO--NiO system pigment particles having a mean particle 
size of about 0.5.mu. (No. 9320, made by Dainichiseika Co., Ltd.) were 
then added to the 0.3% aqueous solution of gum arabic and well stirred 
with a stirrer till the pigment particles became primary particles. A 
pigment dispersion was thus obtained. 
Then, the phosphor dispersion and the pigment dispersion were mixed by 
stirring. The pH of the resulting mixture was adjusted to 4.0 and the 
temperature thereof was lowered to below 10.degree. C. To the mixture was 
then added gradually 1 part by weight of formaline while the mixture was 
continuously stirred. After the mixture was allowed to stand, a 
precipitate was obtained. The supernatant solution was removed and the 
precipitate was washed with water. The precipitate was then separated from 
water and concentrated using a decanter to yield a pigment coated phosphor 
cake containing about 85% of solid content. The cake was dried in a stream 
of air to obtain a pigment coated phosphor of the present invention. 
The conventional pigment coated phosphor using chromium oxide pigment 
particles was prepared in the same manner as that mentioned above except 
0.35 parts by weight of chromium oxide pigment particles having a mean 
particle size of about 0.5.mu. (X-1134, made by Hercules Co., Ltd.) were 
used instead of the aforesaid 0.5 parts by weight of TiO.sub.2 
--ZnO--CoO--NiO system pigment particles. 
As is shown in Table II below, the specific reflectance of the pigment 
coated phosphor of the present invention obtained was almost the same as 
that of the conventional pigment coated phosphor. However, the luminance 
thereof was higher than that of the conventional one. 
EXAMPLE 4 
10 parts by weight of polyvinylbutyral resin were dissolved in 85 parts by 
weight of ethanol. To the resulting solution were added 100 parts by 
weight of copper and aluminium activated zinc sulfide green emitting 
phosphor (ZnS:Cu,Al) having a mean particle size of 10.mu. and 0.8 parts 
by weight of TiO.sub.2 --ZnO--CoO--NiO system pigment particles having a 
mean particle size of about 0.5.mu. (No. 9320, made by Dainichiseika Co., 
Ltd.). Then the mixture was ballmilled to disperse the phosphor and the 
pigment particles uniformly in the solution. A phosphor-pigment dispersion 
was thus obtained. 
Then, the phosphor-pigment dispersion was continuously mixed with water in 
a mixing weight ratio of 1 part of the dispersion: 10 parts of water using 
a linemixer. By the mixing, the solvent (ethanol) was separated from the 
phosphor and the pigment particles which were bound together by the 
polyvinylbutyral and there was obtained a precipitate. The precipitate was 
then taken out and concentrated using a decanter to yield a pigment coated 
phosphor cake containing about 85% of solid content. The cake was dried in 
a stream of air to obtain a pigment coated phosphor of the present 
invention. 
The conventional pigment coated phosphor was prepared in the same manner as 
that mentioned above except 0.65 parts by weight of chromium oxide pigment 
particles having a mean particle size of about 0.5.mu. (X-1134, made by 
Hercules Co., Ltd.) were used instead of the aforesaid 0.8 parts by weight 
of TiO.sub.2 --ZnO--CoO--NiO system pigment particles. 
As is shown in Table II below, the specific reflectance of the pigment 
coated phosphor of the present invention obtained was almost the same as 
that of the conventional pigment coated phosphor. However, the luminance 
thereof was higher than that of the conventional one. 
EXAMPLE 5 
A copper and aluminium activated zinc sulfide phosphor (ZnS:Cu,Al) having a 
mean particle size of 10.mu. was prepared as a green emitting phosphor. 
TiO.sub.2 --ZnO--CoO--NiO system pigment particles having a mean particle 
size of about 0.5 (No. 9310, made by Dainichiseika Co., Ltd.) were 
dispersed uniformly in a nitrocellulosealkyd resin solution (DNT clear 
lacquer, made by Dai Nippon Toryo Co., Ltd.) by a ball mill. A pigment 
particle dispersed adhesive solution was thus obtained. The ingredients of 
the resulting pigment particle dispersed adhesive solution and the amount 
of the ingredients were as follows. 
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DNT clear lacquer 10 parts by weight 
TiO.sub.2 -ZnO-CoO-NiO system 
0.8 parts by weight 
pigment particles 
Diluent (triol/ethyl acetate = 1/1) 
25 parts by weight 
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Then, the aforesaid green emitting phosphor was put into a tank of an 
electrostatic coating machine (made by Ransburg Co., Ltd.), and the 
machine was prepared to charge the green emitting phosphor particles 
sprayed into positive polarity. On the other hand, the aforesaid pigment 
particle dispersed adhesive solution was put into a tank of another 
electrostatic coating machine (White Dragon, made by Nippon Kogei Co., 
Ltd.), and the machine was prepared to charge the microdrops of the 
solution sprayed into negative polarity. The voltage of the electrostatic 
generators of the coating machines was adjusted to 7 kilovolts, and the 
pressure of the air spouted from the compressors of the coating machines 
was adjusted to 4 Kg/cm. Spray guns for the green emitting phosphor and 
the pigment particle dispersed adhesive solution were located so that the 
distance between the tops of the guns was 20 cm and the angle between the 
directions in which the phosphor particles and the microdrops of solution 
were sprayed respectively by the guns was 60.degree.. The green emitting 
phosphor and the pigment particle dispersed adhesive solution were sprayed 
into a cyclone maintained at a temperature ranging from 20 to 30.degree. 
C. at the rate of 100 g/minute and 18 g/minute, respectively. By an 
electrostatic attractive force, the microdrops of the pigment particle 
dispersed adhesive solution was caused to adhere to the surface of the 
green emitting phosphor, and a pigment coated phosphor having a uniform 
coating of pigment particles dropped down on the bottom of the cyclone. 
Thus, the pigment coated phosphor of the present invention was obtained. 
The conventional pigment coated phosphor using chromium oxide pigment 
particles was prepared in the same manner as that mentioned above except 
0.6 parts by weight of chromium oxide pigment particles having a mean 
particle size of about 0.5.mu. (X-1134, made by Hercules Co., Ltd.) were 
used instead of the aforesaid 0.8 parts by weight of TiO.sub.2 
--ZnO--CoO--NiO system pigment particles. 
As is shown in Table II below, the specific reflectance of the pigment 
coated phosphor of the present invention was almost the same as that of 
the conventional pigment coated phosphor. However, the luminance thereof 
was higher than that of the conventional one. 
Table II 
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Example 
Green Pigment 
Specific 
No. Particles Reflectance (%) 
Luminance (%) 
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1 a 78.7 97.8 
b 78.9 96.0 
2 a 74.8 97.0 
b 75.0 95.0 
3 a 69.0 94.2 
b 68.9 92.0 
4 a 60.0 86.4 
b 60.2 78.5 
5 a 71.2 95.7 
b 71.0 93.0 
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a TiO.sub.2 -ZnO-CoO-NiO system pigment particles 
b chromium oxide pigment particles 
* Luminance is shown by a relative value with reference to that of the 
uncoated green emitting phosphor defined to be 100%. 
*The light source usd for measuring the specific reflectance was a 
tungsten lamp.