Note: Descriptions are shown in the official language in which they were submitted.
"3~
RCA 6g,534
.
This invention relates to a novel photographic
method for preparing a screen structure for a cathode-ray
tube (CRT) and particularly, but not exclusively, to a novel
method for preparing a light-absorbing matrix for a color
television picture tube.
Color television picture tubes which include a
light-absorbing matrix as a s-tructural part of the lumines-
cent screen have been described previously; for example, in
U. S. patents No. 2,842,697 to F. J. Bingley, and No.
3,146,368 to J. ~. Fiore et al. These patents describe color
television picture tubes of the aperture-mask type (also
called shadow-mask type) in which a light-absorbing matrix
is located on the inner surface of the faceplate of the tube.
lS In this structure, the matrix has openings therein which are
fiLled with phosphor material.
A reverse-printing method for preparing a light-
absorbing matrix for a CRT is described in U. S. patent No.
3,558,310 to E. E. Mayaud. In a preferred embodiment of
that method, the inner surface of the faceplate of a CRT is
coated wi~h a film of water-based photopolymeric material,
typically a dichromate-sensitized polyvinyl alcohol. A light
image 1.5 pro~ected on the film to insolubilize selected
regiorls of the film. The film is developed by flushing the
film with`water to remove the still-soluble regions of the
fi~lm while retaining the insolubilized regions in place.
~:hen, the developed film or stencil and the bare areas of
the surface are overcoated with a layer containing particles
of light-absorbing material, such as graphite. Finall~, the
overcoating is developed by removi~g the stencil together
` '
'~
' "'-
~ ~.1.6~
RCA 68,534
1 with the overlying overcoating, while retaining the over~
coating on the bare areas of the surface not covered by the
stencil.
Such a process produces satisfac-tory light-absorb-
ing matrices in automated and semiautomated factory produc-
tion. However, frequently, the overall appearances of
matrixes are marred by s~reaks and swirlsO Such streaks and
swirls are believed to result from the leaching of soluble
and/or pcrtially-soluble material from the stencil, which
leached material is then redistributed in a random manner by
gravitational and centrifugal forces during and after devel-
opment of the film. Careful examination has shown that these
streaks and swirls are merely cosmetic, resulting from very
slight nonuniformities in the sizes of the openings in the
matrix. Although they may have a trivial effect on the
performance of the tube, nevertheless streaks and swirls
adversely affect the salability of picture tubes which have
them.
Polyvinyl alcohol as used by the CRT maker is a
product of polyvinyl-acetate manufacturing whose main uses
are in the paint and adhesive industry. Attempts have been
made to supply a special electronic-grade of polyvinyl
~alcohol, but the cost is prohibitive because the total
amount used is small. Therefore, the CRT maker uses materi-
als made for the textile, paper and binder industries. The
chemical character of those polyvinyl alcohols is dictated
by the uses in the aforementioned industries.
The principal factors governing t~e properties of
polyviny1 alcohol are the degree of polymerization and the
3 percent hydrolysis. While proper blending of batch-produced
.
6~3~
RCA 68,534
1 polyvinyl alcohol can provide products within a given saponi-
fication range, such materials will contain much larger
amounts of high and low molecular weight fractions and of
portions varying widely in degrees of hydrolysis. This type
of manufacturing variation produces products with varying
de~rees of cold-water solubility. The CRT maker, to make
matrix stencils, dissolves away that part of the polyvinyl
alcohol not insolubilized by the actinic radiation, while
retaining the insolubilized part as a stencil. ~11 regions
of the stencil have some partially-soluble material therein.
This partial solubility of the polyvinyl alcohol varies with
the lots supplied by the manufacturer.
Prior to coating the photopolymeric film on the
faceplate surface, the dry, powdered polyvlnyl alcohol is
15 dissolved in water. This step also may introduce variations
in the cold-water solubility of the stencil-making material.
The method according to the invention is similar to
the previous method disclosed in the above-cited Mayaud patent
~0 except that, after the exposed film is flushed with water or
other àqueous medium to remove the more-soluble reqions of the
film, the retained and sti]l wet fllm reqions are rin~s~ wifh
an aqueous solution containinq borate ions. Solution~ of
boric acid or borax are preferred, althou~h solutions of a
?5 soluble borate, perborate, or other boron com~ound sinql~ or
:in combination may be used.
Rinsing the developed film or stencil with an
aqueous solutlon containing borate ions essentially elimi-
nates the cosmetic defects described above. It is believed
that this rlnsing step has the effect of preventing soluble
; _ 4 _
-- .
~3B
RCA 6~,534
i and partially-soluble material in the film from being leache~
out by the aqueous me~lum and t11en being distributed in a
random manner. Also, the variable cold-water solubility
of the material of the stencil is suppressed by the
method, according to the invention resulting in a more uni-
form pro~uct.
A preferred embodiment for preparing a llght-
absorbing matrix on the inner surface of the faceplate of an
aperture-mask-type color television picture tube will now be
described. First, the inner surface of the faceplate is
cleaned in the usual way, as with water, ammonium bifluoride,
hy~rofluoric acid, detergent, caustic, etc., to remove any
foreign matter. Then the surface is coa-ted with a film of
dichromate-sen~itized polyvinyl alcohol. The film may be
produced by depositing on the surface of the faceplate a
quantity of an aqueous solution containing about:
Weight Percent
Polyvinyl alcohol 3.42
Ammonium dichromate O.34
Water Balance to lO0
The faceplate is rotated and tilted so that the quan-tity of
solution spreads evenly over the surface. During the latter
steps of rotation, infrared heat is applied so that the
` water in the solution evaporates and a dry photopolymeric
2S film is formed on the surface.
An aperture mask for the faceplate is positioned
above and spaced from the film and the assembly is placed in
a lighthouse, which is a known apparatus for ex~osing the
film on the faceplate by projection of light through the
apertures in the mask. In this example, the mask has
.
RCA 6g,534
1 circular apertures with a diameter of about 0.33 mm. and a
center-to-center spacing between apertures of about 0.71 mm.
near the center of the mask. The film is exposed for about
8 minutes to light from a l,000-watt quartz lamp positioned
about 36 cm. from the aperture rask. During the exposure,
light from the lamp is passed through a light pipe or colli-
mator and projec-ted through the mask causing beamlets of
light to pass through the apertures of the mask incident
upon the film. The irradiated regions of the film harden;
that is, become insoluble in water. There is a slight en-
largement of the exposed areas above the size of its associ-
ated aperture (to about 0. 41 n~. ) and a araded hardening at
the margins of the exposed areas. The exposure through the
mask is repea-ted three times, each time with the light
incident at a slightly different angle so that the beamlets
harden the film in groups of three, as in the usual aperture-
mask screen manufacture.
- Following exposure, the assembly is removed from
the lighthouse and the mask is separated from the faceplate.
The exposed coating is subjected -to flushing with a forced
spray of water, which may con-tain a detergent, for about 30
seconds, after which the faceplate is rinsed with water.
Then, the still-wet faceplate and retained fiim regions are
rinsed for about 30 seconds with a solution containing borate
ions. Such borate solutions are described in more detail
below. In this example, the faceplate is rinsed with a 0.5
welght-percent solution of boric acid. Then the retained
film regions and faceplate are dried with or without the
assistance of applied heat. ~t this point in the process,
the lacepla=e surface carries an adhcrcnt stenoil comprised
. ' ' ' .
.
B
RCA 68,534
of open surface areas and of dots of hardened polymeric
film coated on the surface.
The stencil is now overcoated with a composition
comprised of light-absorbing particles. In this example,
the overcoating is produced by applying to the said stencil
a slurry containing about 5.0 weight percent of colloidal
graphite in water and then drying the overcoating. It is de-
sirable to include a trace of wettincj agent in the graphite
slurry in order to facilitate the spreading of the slurry
over the stencil. The overcoating is dried tho,oughly for
about l.5 minutes with the aid of infrared heat. After
coolin~, the overcoating is well adhered both to the stencil
and to the bare faceplate surface not covered by the stencil.
Next, a chemically digestive agent for the poly-
meric dots is applied to develop the overcoating. In this
example, the digestive agent is an a~ueous solution contain-
ing about 4.0 weight-percent hydrogen peroxide. This 501u-
tion may be applied to the overcoating as a wash or as a
spray under pressure. The hydrogen peroxide solu-tion pene-
trates the overcoating and the stencil, causing the light~
hardened polyvinyl alcohol of the stencil to swell and soften.
Subse~uent flushing with water removes the softened stencil
togethex with the ovexlying port1ons of the overcoating,
but leaves behind that portion, or portions, of the overcoat-
25 ing which is adhexed directly to the surface of the faceplate
`not covered'by the stencil. At this point, the faceplate
carries a black light-absorbing matrix having a multiplicity
of circular holes therethrough about 0.41 mm in diameter.
The edges of the pattern are relatively smooth.
The black, l1ght-absorbing matrix is now rinsed
.
38
RCA 68,534
1 with water and dried for about 4 minutes with the aid of
infrared heat. ~hen, the faceplate is processed in the usual
way to deposit red-emitting phosphor dots, green-emit-ting
phosphor dots and blue-emitting phosphor dots about 0.a3 mm
in diameter in the holes of the matrix. The slight enlarye-
ment of the phosphor dots over the holes in the matrix is
achieved by the spreading of light during projection, which
may be enhanced by increasing the exposure time of the har-
dened areas. The luminescent screen may now be processed
in the usual way to apply a reflective metal layer on top
of the phosphor dots and -the black matri~. The screen is
baked and assembled with the aperture mask into a CRT in the
usual way.
.
lS The particular steps described in the e~ample above
may be varied within limits and still fall within the scope
of the method according to the invention. Obviously, the
- method maY be used to Produce screen structures of different
~aterials on other support surfaces, and for prepaxing other
screen structures than that described above. By "scxeen
structure" is meant a structure, luminescent or nonlumi-
nescentj which is a part of the screen or -tarqet of a CRT.
Some variations in the -~ethod are described below.
The photopolymeric film may be produced by coating
a support surfacle,las by dipping, spraying, flow coating or
-spin coating, with a solution of a photopolymeric material.
The preferred photopolymeric material is a polyvinyl alcohol
which is sensiti~ed with a small amount of ammonium dichromate,
sodium dichromate, potassium dichromate, or a soluble salt of
à metal such as iron or mercury; or with an organic
RCA 68,534
1 photosensitizer for a water-based photosensitive material,
such as a diazo compound.
sesides polyvinyl alcohol, other water-ba~ed
photosensitizable polymeric materials may be used. Some
suitable water~~ased materials which can be made photosensi-
tive are proteins such as gelatin, albumen and fish glue;
carbohydrates such as ~um arabic and starch; and synthetic
materials such as polyvinyl pyrollidone. The photopolymeric
film may also contain a small amount of a less water-soluble
polymer for the purpose of increasing the resistance of the
stencil to the erosive action of the chemically-digestive
agent. The photopolyme-ric material is of the type which is
insolubilized when i~t is exposed to a light image. Such
photopolymeric films are referred to as negative acting.
Any pattern form may be used as-a photographic
master for exposing the photosensitive film. Thus~ conven-
tional silver halide images may be used either by projection
or ~ontact printing. In preparing screen structures for
color television picture tubes of the aperture-mask type,
it is prèferred to use the apert~re mask of the tube as a
photographic master for exposin~ the photopolymeric film. In
that case, the mask is closely spaced from the film and the
li~ht source is placed at three separate locations in order
to produce three separate exposures on the film, each at a
different location on the film. The aperture mask may have
apertures that are round or slit shaped or any other shape
or size since the shape and size of the apertures are not
critical. Also, the exposure may be used to produce dot or
slit or line openings in the matrix.
The photosensitive material is exposed to a pattern
~ 9 _
.
R(A 6~,53
1 of ener~y rays in the range and of the type to which the
photosensitive material is sensitive. Where dichromated
- polyvinyl alcohol is the photosensitive material, radiant
energy in the form of electron beams or as light in the blue
and ultraviolet range of the spectrum may be used. Where a
contact master is used, one may use a flood exposure. Where
the exposure is by projection of an image, a small diameter
source is preferred. A small circular light source is
preferred for mak1ng dot screens, and a line light source is
preferred for making slit and line screens.
Where a photoresist technique has been used for
producing the stencil, the exposed photosensitive film is
developed in the manner of the use for that material. In the,
case of dichromated polyvinyl alcohol, the development is
carried out by flushing the surface of the film with water
or with other suitable solvent for the unexposed, still-
soluble photosensitive material. With other films, the same
or other solvents may be used. The development should leave
the minimum residue on the bare support surface so as not
to interfere w1th the subsequent overcoating step.
The solution of borate ions may be applied to the
stencil in any convenient way as by spraying or with a limp
stream while the panel is rotating. The solution is aqueous
and may contain one,or more boron compounds. Some suitable
water-soluble boron compounds are boric acid, borax, sodium
perborate, potassium perborate, isobutyl boric acid,
trimethyl borate and boric anhydr1de. The solutions may
contain 0.2 to 5 weight percent of dissolved boron compound.
Washiilg or rinsing the wet stencil with a solution
of a boron compound as described above has the e~fect o~
-- 10 --
.
.
-
- RCA 68,534
1 reducing or eliminating the physica~ appearance of streaks
and swirls in the finished matrix when viewed in reflected
ambient light. It is believed that the wash or rinse re-
duces the amount of leaching of polymer from the stencil.
This reduces the deposition of the leached polymer. The
variability in the solubility of the stencil is suppressed.
The overcoating may be of any material which is
adherent to the support surface. The overcoating may incl~de
a pigment or phosphor. Where it is desired to produce a
light-absorbing matrix for a cathode-ray tube, it is pre-
ferred to include in the overcoating a relatively high
loading of a dark pigment. The pigment is preferably ele-
mental carbon in the form of carbon black, acetylene black,
or graphite. Other dark pigments that may be used are
silver sulfide, iron oxide, lead sulfide, ferrites, and
manganese dioxide. The pigment may be black, white or
colored. Where it is desired to produce a lumine~cent
structure, it is preferred to include a relative high loading
of phosphor particles in thc overcoating. A process of this
type for preparing patterns of phosphor particles is dis-
closed in U. S. patent No. 2,840,470 to A. K. Levine.
The overcoating must make a bond to the supportsurface that will endure the subse~uent processing, such as
removing the'image stencil and depositing the phosphor dots.
With some materials, such as some commercially-available
dispersions of graphite in water, the graphite upon drying
makes a bond to a glass faceplate which is ade~uate. With
other materials, it may be necessary to include a small
amount of a binder in the overcoating such that the dry
overcoating develops a bond to the support surface through
.
a~
RCA 68,53~
1 the use of the binder. Colloidal silica is a satisfactory
binder for lamp black and acetylene black. For example,
about 10% of a colloidal silica with respec-t to the percent
pigment present produces a strong bond to the glass face-
plate, especially where a small amount of ammonium dichromateis also present. sesides colloidal silica, alkali silicates
may also be used as the binder.
Where a pigment is used for the purpose of making
a light-absorbing matrix for a picture tube of the shadow-
mask type, the pigment must be deposited in sufficient
density to develop the necessary opacity for this purpose.In the case of acetylene black and lamp black, the pigment
should be deposited in a weight of about 0.2 to 2.0 mg/cm2
of surface area and, preferably, about 1.0 mg/cm2 or more
IS in order that sufficient thickness remains after tube pro-
cessing. Where graphite or other pi.gments are used, slightlylower weights are required for achieving the same opacity
in the ~inal graphic image.
The overcoating should also be permeable to and
substantially unaffected by the graphic image developer,
which must sweli or erode or dissolve at least a part of the
image stencil. Where the overcoating is entirely particles,
it is necessarily permeable. Where the overcoating contains
a binder, the overcoating may be permeable by nature or may
be made permeable by cra%ing the overcoating. The bond
between the .supp,orting surface and the overcoating is prefer-
ably not substantially attacked by the graphlc i.mage devel-
oper. When the overcoc,ting-support sur~ace bond i5 both
inert to the attack of the graphic image developer, and is
adherent to the surface, it is possible to develop the graphlc
- 12 ~
.
-\
9~3
RCA 68,534
Canada
LAIMS
1. In a method for producing a screen structure
for a cathode-ray tube including
(a) exposing a film of wa-ter-soluble photopolymeric
material on a support surface to a light image until the
solubilities of the irradiated regions of said film are
selectively xeduced,
~ b) flushing said exposed film with an aqueous medium
to remove only the regions of greater solubilities, thereby
producing a stencil on said surface,
(c) overcoating said stencil and the surface not
covered by said stencil with screen-structure material,
(d) and then removing only said stencil and the over-
coating thereon,
the improvement comprising, subsequent to step (b) and
prior to step (c), rinsing said stencil and support surface
with an aqueous solution containing less than 5 weight percent
of dissolved boron compound.
2. The method defined in claim 1 wherein said
aqueous solution contains dissolved boric acid.
3. The method defined in claim 1 wherein said
aqueous solution contains dissolved borax.
j,~;
