Method of lacquering the luminophore layer of a color picture tube

The following steps are performed in a method of lacquering the luminophore layer of a colour picture tube: PA1 the luminophore layer is first prewetted, PA1 a first aqueous dispersion of a lacquer is applied to the prewetted luminophore layer, PA1 this first dispersion is fully dried and heated to above the film formation temperature of the lacquer, thereby producing a first lacquer film, PA1 a second aqueous dispersion is applied directly to the first lacquer film, where the said second aqueous dispersion has exactly the same components as the first dispersion and the concentration of the components in the second dispersion corresponds at least to a large extent to the concentration of these components in the first dispersion, and PA1 this second dispersion is fully dried and heated to above the film formation temperature of the lacquer, thereby producing a second lacquer film. This two-stage lacquering process makes it possible obtain screen brightnesses that are about 10% greater than can be obtained by means of conventional single-stage lacquering methods.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of lacquering the luminophore 
layer on the screen trough of a colour picture tube. 
State of the Art 
When lacquering the luminophore layer on the screen trough of a colour 
picture tube, it is customary to make use of an aqueous dispersion of a 
lacquer. Examples of suitable lacquers are given, among others, in U.S. 
Pat. No. 3,317,337 and DE-A-30 46 373 (U.S. Pat. No. 4,409,352). In 
practice there are essentially two methods for applying the dispersion. 
One of these, described--for example--in U.S. Pat. No. 3,317,337, consists 
of arranging the screen trough so that its luminophore surface points 
upwards, filling dispersion into it, and then causing it to revolve at a 
high speed so as to distribute the dispersion, eventually tipping the 
trough sideways to permit excess dispersion to drain off. A second known 
application method consists of spraying the dispersion upwards and against 
the screen, which for this purpose is set either vertically or pointing 
obliquely downwards. Following the spraying, the screen trough is set to 
rotate at speed so as to distribute the dispersion evenly over the 
luminophore layer and, at one and the same time, to throw off the excess 
dispersion. 
With a view to making possible good wetting of the luminophore layer by the 
dispersion, it is common practice to precede the application of the 
dispersion to the luminophore by a prewetting step. Materials and methods 
suitable for this purpose are described, for example, in EP-B-149 838. 
Following the even distribution of the dispersion over the luminophore 
layer, the dispersion is fully dried and then heated to above the film 
formation temperature of the lacquer in order to produce a lacquer film. 
Subsequently--although this no longer forms part of the method of 
lacquering the luminophore layer--vaporized aluminium is deposited on the 
lacquer, so that the latter volatilizes and only an aluminium film is left 
as a reflection layer behind the luminophore layer. Conventional methods 
of lacquering the luminophore layer on the screen trough of a colour 
picture tube therefore comprise the following steps: 
the luminophore layer is prewetted, 
an aqueous lacquer dispersion is distributed evenly over the luminophore 
layer, and 
the dispersion is fully dried and heated to above the film formation 
temperature of the lacquer, thereby producing a lacquer film. 
Apart from these traditional methods with a single lacquering step, methods 
involving two lacquering steps are also known in literature. The 
previously mentioned U.S. Pat. No. 3,317,337, for example, describes a 
method according to which a 6% dispersion of a polyvinyl acetate in water 
is first applied to the luminophore layer. This first dispersion is then 
fully dried and heated to above the film formation temperature of the 
lacquer in order to produce a first lacquer film. The next step consists 
of applying a 10% dispersion of an acrylic resin in water, again followed 
by drying and heating to above the film formation temperature. Neither of 
the two lacquering steps is preceded by prewetting. Both layers are 
applied by spraying a puddle into the centre of the screen trough, which 
is then made to rotate. 
An identical procedure, though using different aqueous dispersions, is 
described in DE-A-27 10 308. According to the examples given in the 
specification, the first dispersion contains either about 3% or about 6% 
of a copolymer of n-butyl acrylate and metacrylic acid, while the second 
dispersion contains about 15% of an acrylic resin. 
Procedures using two layers of lacquer have not proved successful in 
practice largely in view of the fact that the first lacquer film was often 
damaged in the process of applying the second dispersion. No clear 
advantage ensued which could offset this drawback. 
Notwithstanding the many known lacquering methods, all different from each 
other, there still remained the problem of so configuring a method of this 
type as to obtain better lacquering results, which ultimately means 
greater screen brightness. 
SUMMARY OF THE INVENTION 
The method of lacquering the luminophore layer on the screen trough of a 
colour picture tube according to the present invention has the following 
characteristics: 
the luminophore layer is first prewetted, 
a first aqueous dispersion is then applied to the prewetted luminophore 
layer, 
this first dispersion is fully dried and heated to above the film formation 
temperature of the lacquer, thereby producing a first lacquer film, 
a second aqueous dispersion is applied directly onto the first, the said 
second aqueous dispersion having exactly the same components as the first, 
where the concentration of the components in the second dispersion 
corresponds at least to a large extent to the concentration of the 
components in the first dispersion, and 
the second dispersion is fully dried and heated to above the film formation 
temperature of the lacquer, thereby producing a second lacquer film. 
It has been found that if the first lacquering is preceded by prewetting, 
though not the second, lacquering results are obtained that lead to an 
extraordinary increase in the brightness hitherto obtained. Whereas 
conventional further developments of lacquering technique in such matters 
as material selection and procedure brought only very small improvements 
in screen brightness, the method according to the present invention 
produces a brightness increase of the order of 10%. This brightness 
increase was confirmed by several test series, i.e. the lacquering method 
functions in a reliable manner. In other words, the application of the 
second dispersion no loner deforms the first film to such an extent as to 
produce a loss of brightness in many tubes. The situation seems to be that 
the prewetting causes the first lacquer film to adhere so well that the 
application of the second dispersion does not attack it to any appreciable 
extent. In this connection it is important that the application of the 
second dispersion should not be preceded by prewetting. It has been found 
that if the application of the second dispersion is preceded by 
prewetting, the first lacquer layer wall often suffer appreciable changes 
with negative effects on the overall lacquering result. 
The more quickly the application of the second dispersion to the first 
lacquer film is followed by drying, the better the lacquering results, 
i.e. the greater will be the gain in brightness. In this connection it is 
advantageous to apply the second dispersion by means of the per se known 
method of spraying from below against the rotating luminophore layer while 
it is either in a vertical position or points obliquely downwards and then 
setting the screen trough to rotate at a high speed in order to distribute 
the dispersion in a thin layer over the first film and throw off any 
excess dispersion. If another method is used, a case in point being the 
method of spraying a dispersion puddle into the centre of the screen 
trough and throwing off the excess by rotation, the resulting covering is 
unevenly distributed over the surface of the screen and the worst picture 
brightness is obtained where the dispersion remained in contact for the 
longest period of time, i.e. in the centre of the screen when using a 
method in which the dispersion is applied in the form of a puddle at the 
centre. 
For technical reasons, it will be particularly advantageous to produce both 
lacquer films with the same dispersion. But slightly better lacquering 
results will be obtained when the concentration of the components in the 
second dispersion is slightly greater than their concentration in the 
first. The concentration of the components in the second dispersion should 
therefore correspond at least to a large extent to the component 
concentration in the first dispersion.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Tests were carried out with dispersions of different lacquer-forming 
materials--especially acetates and acrylates--in aqueous solution. 
Particularly good results were obtained with a dispersion prepared in 
accordance with the example of DE-A-30 46 373 (U.S. Pat. No. 4,409,352). 
The example there given concerns a dispersion with about 17% of solids. 
The concentration of this dispersion was reduced to about half by the 
addition of demineralized water. 
Following the application of a luminophore layer to the screen trough, the 
first step of the lacquering method, as shown in FIG. 1, consisted of 
prewetting the said luminophore layer. This was done as described in 
EP-B-149 838. The lacquer dispersion produced as per DE-A-30 46 373, i.e. 
a dispersion of 8.5% by weight of polyacrylate in aqueous solution, was 
then applied. As schematically shown in FIG. 2, the application took the 
form of spraying the dispersion through a nozzle 10 onto the luminophore 
layer 11 on a screen trough 12 pointing obliquely downwards. The screen 
trough was slowly rotated during the spraying process. The speed of 
revolution was then increased to distribute the sprayed-on dispersion as a 
fine film over the entire luminophore layer and to eliminate the excess 
dispersion. This was followed by drying and heating to above the film 
formation temperature at just over 40 degrees Celsius. This drying and 
heating to above the film formation temperature was obtained by means of 
infrared radiation within the space of a few minutes. The last two steps, 
i.e. application and distribution of the dispersion and the subsequent 
drying and heating to above the film formation temperature, were then 
repeated. But repetition of steps 2 and 3 is not preceded by the first 
step, i.e. prewetting. 
FIG. 3 illustrates the brightness improvement obtained as compared with the 
conventional lacquering method of DE-A-30 46 373. The dotted line shows 
the relationship between the percentage of solids in the dispersion and 
the beam current needed in a colour picture tube to obtain a given 
brightness in the case of single-layer lacquering. The proportion of 
solids was varied between 8 and 18%. As can be seen, the beam current 
needed to obtain a given brightness hardly changes when the proportion of 
solids lies between 12 and 18%. Percentagewise, this current is designated 
as 100%. If the proportion of solids is diminished, a greater beam current 
will be needed to obtain the given brightness. This is due to the fact 
that in these cases the aluminium layer applied on the lacquer layer will 
be disturbed by the penetrant luminophore layer. If the proportion of 
solids is increased above 18%, the aluminium film will again suffer 
disturbance, though this time not on account of penetration of the 
luminophore layer, but rather because bubbles will be formed as the 
lacquer film evaporates through the aluminium layer. 
The full line in FIG. 3 illustrates the corresponding relationship in the 
case of the method set out in FIGS. 1 and 2. In this case the proportion 
of solids is the sum of the percentages of solids in the two dispersions. 
Since in the embodiment here considered both dispersions contain 8.5% of 
solids, the total for the purposes of FIG. 3 is 17%. It can be seen that 
considerable smaller beam currents are needed to obtain the given 
brightness when this total lies between about 16 and 20%, namely only 
about 90% of the previous optimal value here designated as 100%. With a 
given beam current in actual operation, therefore, the obtainable 
brightness will be about 10% greater than what could previously be 
obtained. 
It was found that for any given total proportion of solids, the lacquering 
result will be slightly improved if the concentration of the components in 
the second dispersion is slightly higher than the concentration of the 
components in the first dispersion. However, the improvement hardly makes 
itself felt in the brightness, but rather in the optical impression of the 
edge area of the screen trough when the tube is not in operation. When 
dispersions of different concentration are used, the concentration of the 
first dispersion should not be excessively low. Thus, the proportion of 
solid lacquer particles in the first solution should not be less than 2%. 
Good results were obtained with 6.5% polyacrylate in the first dispersion 
and 9.5% polyacrylate in the second dispersion. These dispersions were 
once again produced as described in DE-A-30 46 373, increasing only the 
percentage of demineralized water. To this end the second dispersion was 
produced as the mother stock, from which the first dispersion was then 
obtained by further dilution. 
Attention should here be drawn to the fact that further lacquer films can 
be applied in analogy with the second lacquer layer, i.e. by applying a 
lacquer dispersion without prior wetting and then drying and heating to 
above the film formation temperature. The total of the percentages of 
solids in all the layers must then lie within the optimal range 
illustrated by FIG. 3. It has however been found that this procedure will 
not produce a sufficient brightness increase to justify its extra cost. 
The possible repetition of the last two steps described in FIG. 1 is there 
indicated by a dotted line that leads from the end of the last step back 
to the beginning of the penultimate step.