Antistatic coating for, in particular, a cathode ray tube comprising latex particles of a polypyrole compound in a silicon dioxide matrix

An antistatic coating was provided on a substrate 10 by dispersing latex particles 12 from a polypyrrole compound in an aqueous solution of a hydrolyzed alkoxysilane compound, after which the dispersion was applied to the substrate. The hydrolyzed alkoxysilane compound was converted to silicon dioxide by means of a treatment at an increased temperature, thereby forming a matrix 14 in which the latex particles 12 are dispersed and by means of which the latex particles are adhesively secured to the substrate 10. The antistatic coating can particularly suitably be used on the display screen of a cathode ray tube, and, if desired, said coating can simultaneously be used to reduce the light transmission of the display screen.

FIELD OF THE INVENTION 
The invention relates to an antistatic coating on a substrate, in 
particular a display screen of a cathode ray tube, which coating comprises 
an electroconductive polypyrrole compound. 
The invention also relates to a method of manufacturing such an antistatic 
coating. 
Antistatic coatings are applied to, for example, display screens of display 
devices. Such coatings are sufficiently electroconductive to ensure that a 
high electrostatic voltage present on the outside surface of the device is 
removed within a few seconds. By virtue thereof, a user touching a screen 
is not shocked and, additionally the attraction of atmospheric dust is 
reduced. 
BACKGROUND OF THE INVENTION 
In German Patent Application DE 3834526, a description is given of the 
manufacture of transparent, antistatic layers which may consist of, for 
example, polypyrrole which is provided on a substrate from a colloidal 
dispersion, after which a thermal treatment is carried out to improve the 
adhesion. 
A disadvantage of this known method is that the layers manufactured are 
mechanically weak and, for certain applications such as for example 
display screens, insufficiently resistant to solvents. The colloidal 
dispersions used have limited stability and must be processed shortly 
after they have been prepared. 
SUMMARY OF THE INVENTION 
An object of the invention to provide, inter alia, antistatic coatings 
comprising polypyrrole compounds, which coatings adhere firmly to the 
underlying substrate. The coatings must exhibit substantial mechanical 
durability and scratch resistance and must be resistant to solvents and 
cleaning agents. In addition, the coatings must be homogeneous, exhibit 
suitable optical properties and, in particular, be suitable for use on the 
display screen of display devices. The coatings must be suitable for 
bringing the light transmission to a desired value. A further object of 
the invention is to provide a simple method of manufacturing such 
antistatic coatings, which method must be practicable, in particular, at 
temperatures at which no damage is done to components of a cathode ray 
tube. The materials used in the method have to be stable and exhibit long 
keeping qualities. 
These objects are achieved by an antistatic coating on a substrate, in 
particular the display screen of a cathode ray tube as described in the 
opening paragraph, which coating is characterized according to the 
invention in that it comprises latex particles in a matrix of silicon 
dioxide, which latex particles consist of the polypyrrole compound. If 
desired, the coating can be supplemented with layers, which are known per 
se, having an antireflective or antiglare effect or with layers which 
further increase the scratch resistance. 
Polypyrrole, N-substituted polypyrrole and .beta.-substituted polypyrrole 
can be used as the polypyrrole compound. If desired, polyaniline can 
alternatively be used. Suitable substituents are: alkyl groups having, for 
example, up to 5 carbon atoms, aryl groups, alkoxy groups, nitro groups 
and halogen atoms. Such materials are described in U.S. Pat. No. 
4,959,162, inter alia, for use as an antistatic coating. According to a 
preferred embodiment of the invention, the latex particles consist of 
unsubstituted polypyrrole. 
In view of the optical properties and the uniformity and homogeneity of the 
layer, it is desirable for the latex particles to have uniform dimensions. 
The average size of the latex particles preferably ranges from 50 to 150 
nm. 
To suppress the reflection of light on the rear side of the display screen 
of a cathode ray tube, it is efficacious to reduce the light transmission 
of the display screen. Unlike the light originating from the luminescent 
material of the display tube, the reflected light has to pass twice 
through the display screen and the layers present thereon. By virtue 
thereof, a relative improvement can be attained, in particular, if the 
light output of the luminescent material can be increased by other 
independent measures. The antistatic coating according to the invention 
can be efficaciously used to reduce the light transmission of a display 
screen of a cathode ray tube. By virtue thereof, the necessity of changing 
the glass composition of the display screen is avoided and the 
possibilities of bringing the light transmission to substantially any 
desired value in a simple manner are increased. 
The object of providing a method of manufacturing an antistatic coating on 
a substrate is achieved according to the invention in that latex particles 
of a polypyrrole compound are dispersed in an aqueous solution of a 
hydrolyzsed alkoxysilane compound, after which the dispersion is provided 
on the substrate and the hydrolyzed alkoxysilane compound is converted to 
silicon dioxide by means of a treatment at an increased temperature. In a 
preferred embodiment of the method according to the invention, the 
conversion to silicon dioxide takes place by means of a treatment at a 
temperature between 150.degree. C. and 170.degree. C. for at least 30 
minutes. 
An alkoxysilane compound which can suitably be used in the method according 
to the invention is tetraethyl orthosilicate. Alternatively, other 
alkoxysilane compounds of the type Si(OR).sub.4, which are known per se, 
can be used, where R is an alkyl group having, preferably, 1 to 5 carbon 
atoms. 
In Japanese Patent Application (Kokai) JP 1-167254, a description is given 
of a composite material comprising a matrix of silicon dioxide in which 
polypyrrole is dispersed. This material is manufactured by impregnating a 
porous silicon dioxide material, manufactured according to the sol-gel 
method, with the monomer pyrrole and polymerizing it in situ. However, in 
this material no latex particles of polypyrrole are used. The composite 
material is inhomogeneous and, due to its porosity, mechanically weak and 
it has a rough surface. The porous material is unstable, sensitive to 
contamination and gives rise to problems during cleaning of the surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
EXEMPLARY EMBODIMENT 1 
Polypyrrole latex particles were prepared by stirring a mixture of 0.014 
tool of pyrrole, 0.033 tool of iron (III) nitrate and 0.5 g of polyvinyl 
alcohol (as the stabilizer) in 100 ml of water for 24 hours. The formation 
of the polypyrrole particles became apparent from the blackening of the 
mixture. The particles were separated from the mixture by centrifuging for 
1 hour at a speed of rotation of 11000 revolutions per minute, after which 
the particles were again dispersed in water in a quantity of 1% by weight. 
Electron microscopy showed that the latex particles have a regular convex 
shape and that they are monodisperse with an average particle size of 80 
nm. Other suitable methods of preparing polypyrrole compounds are 
described in U.S. Pat. No. 4959162. 
A hydrolyzed alkoxysilane compound was prepared by thoroughly stirring for 
five minutes a mixture of equal quantities by weight of tetraethyl 
orthosilicate and a 1N solution of hydrochloric acid in water. Ethanol was 
formed in said hydrolysis, so that the alkoxysilane compound could 
dissolve. 
A quantity of 4% by weight of the solution of the hydrolysed alkoxysilane 
compound was added to the dispersion with the polypyrrole latex particles. 
One or more than one layer of said dispersion was spin coated on to a 
substrate of glass, after which this substrate was maintained at a 
temperature of 160.degree. C. for 30 minutes, thereby forming a suitably 
adhering smooth layer of silicon dioxide in which the polypyrrole latex 
particles were dispersed. 
FIG. 1 diagrammatically shows a substrate 10 of glass carrying a coating 
which consists of polypyrrole latex particles 12 in a matrix 14 of silicon 
dioxide. At layer thicknesses of 100 to 200 nm, the surface resistance of 
the layer is approximately 4 M.OMEGA./.quadrature.. This is amply 
sufficient for the desired antistatic effect (a surface resistance between 
10.sup.4 and 10.sup.10.OMEGA. is desirable) and makes it possible to bring 
the light-transmission properties to the desired value by changing the 
layer thickness and the concentration of the dispersion, while preserving 
the required antistatic effect. The absorption coefficient of the coating 
was approximately 0.3.times.10.sup.5 /cm, resulting in a light 
transmission between 0.7 and 0.3 at layer thicknesses of 100 to 200 nm. 
EXEMPLARY EMBODIMENT 2 
FIG. 2 diagrammatically shows a cathode ray tube, which is known per se, 
comprising a glass envelope 31 having a display window 32, a cone 33 and a 
neck 34. In the neck there is provided an electron gun 35 for generating 
an electron beam 36. Said electron beam 36 is focused on a display screen 
37 to form a target spot 38. The electron beam 36 is deflected across the 
display screen 37 in two mutually perpendicular directions x-y by means of 
the deflection coil system 39. A layer of a luminescent material 
(phosphor) is present on the display screen 37. The display window 32 is 
provided on the outside with an antistatic layer 40 which is manufactured 
as described in exemplary embodiment 1. 
By means of the invention, effective antistatic coatings were manufactured 
and provided on a display screen of a cathode ray tube in a simple manner, 
said coatings enabling the light-transmission properties to be adjusted 
according to the requirements.