US3700444A - Method of forming a cathode ray tube screen - Google Patents

Abstract

A METHOD OF PATTERN FORMING FOR COLOR TELEVISION PICTURE TUBES WHICH COMPRISES COATING THE FACE PANEL OF A TUBE WITH A SLURRY OF A PHOTOSENSITIZED MATERIAL AND PHOSPHOR BY UTILIZING THE FORCE OF GRAVITY AND THEN EXPOSING THE COATED PANEL TO A SUITABLE SOURCE OF RADIATION THROUGH A NEGATIVE AND DEVELOPING TO FORM A PATTERN.

Description

Oct. 24, 1972 ETAL 3,700,444

METHOD OF FORMING A CATHODE RAY TUBE SCREEN Original Filed April 5, 1967 2 Sheets-Sheet 1- v INVENTORS JOHN J'. MIL 45/? BY 7mz pzus 1/. RYcmm sK/ Oct. 24, 1972 J LL R HAL METHOD OF FORMING A CATHODE RAY Original Filed April 5. 1967 TUBE SCREEN 2 Sheets-Sheet 2 v INIVENTORS Jam J M/LLER BY TWIDMz/s 1/. FYCWMWSK/ ATTOR/Vf V United States Patent O 3,700,444 METHOD OF FORMING A CATHODE RAY TUBE SCREEN John J. Miller and Thaddeus V. Rychlewski, Seneca Falls, N.Y., assignors to Sylvania Electric Products Inc. Continuation of application Ser. No. 627,745, Apr. 3, 1967. This application Feb. 10, 1970, Ser. No. 9,119 Int. Cl. H013 9/20, 31/20 US. Cl. 96-361 1 Claim ABSTRACT OF THE DISCLOSURE A method of pattern forming for color television picture tubes which comprises coating the face panel of a tube with a slurry of a photosensitized material and phosphor by utilizing the force of gravity and then exposng the coated panel to a suitable source of radiation through a negative and developing to form a pattern.

This application is a continuation of application Ser. No. 627,745 filed Apr. 3, 1967 and now abandoned.

BACKGROUND .OF THE INVENTION This invention relates to methods of forming cathode ray tube screens and more particularly to methods of forming mosaic patterns on screens suitable for use in color television picture tubes. It is partcularly useful when used in conjunction with the so-called direct photographic method of screen deposition.

In forming patterned screens in this manner", a prior method utilizes an aqueous medium contaning, in suspension, phosphor particles, an organic polymerizable binder such as polyvinyl alcohol, and a photosensitizer such as ammonium dichromate. A quantity of this mixture, or slurry, is deposited near the center of a cathode ray tube face panel, which has a generally concavo-convex configuration and is positioned with the concave side uppermost, and is then spread thereover, by the action of a constantly varying force generated by tilting and spinning the panel according to a selected and rather complicated schedule. After the slurry has been spread over the panel to provide a layer of substantially uniform area density, it is dried. The phosphor coating on the panel is then exposed through a suitable negative providing a desired pattern, to a source of radiaton which is capable of polymerizing the slurry. Usually this radiation is actinic. The exposed layer is then developed by washing it to remove the unexposed slurry, thus leaving the desired pattern of elemental phosphor areas (dots or stripes) adhered to the panel. The process is repeated for each of a plurality of desired phosphors, such as red-emitting, greenemitting, and blue-emitting, to produce the finished screen.

This method of pattern application works well under some conditions but requires strict controls and faithful adherence to the aforementioned schedule. Also, the rotational spreading of the slurry material becomes more critical and more diflicult after the first pattern of elemental phosphor areas has been applied. Under these conditions the previously applied elemental areas act as barriers to the flow of the secondarily applied slurry layer which is being caused to spread by the constantly changing centripetal force imparted thereto by the rotation of the panel. These barriers cause a condition known as spoking; i.e., bare areas which occur behind the previously applied dots because of the manner in which the secondarily applied slurry flows therearound. Also, it is extremely difficult to achieve a uniform area density of slurry material under these conditions because of the vast difference in the velocity of the slurry at the center of the panel at the start of the spinning schedule and at the edge.

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OBJECTS AND SUMMARY OF THE INVENTION It is an object of the invention to enhance the screening techniques utilized for the manufacture of patterned screen cathode ray tubes.'

It is another object of the invention to provide a method of achieving a substantially uniform area density of slurry material prior to exposure.

It is yet another object of the invention to provide a method of screen deposition that is simple and economical to operate.

These objects are achieved in one aspect of the invention by the provision of a method of forming a pattern on a prepared given area of the interior non-planar surface of a cathode ray tube face panel which comprises the steps of wetting at least the given area with a phosphor carrying, radiation sensitized slurry without regard to the area density thereof and then positioning the panel in a manner to achieve levigation and uniformity of area density of the slurry through the action of a constant force. After the levigation the slurry coated panel is exposed through a patterned negative to a suitable source of radiation and is then subsequently developed to remove the unexposed areas. For a three color cathode ray tube screen the above procedure is repeated twice more with the radiation source offset to a new position relative to the screen.

As used in this application, levigation shall mean the smoothing or evening out of a surface of a fluid or semifiuid material.

Utilization of this method provides enhanced cathode ray tube screens that do not exhibit the degree of spoking found in screens made by prior art practices. It is simple and economical to operate and eliminates the complex tilting and spinning movements of prior methods. Also, it provides an extremely uniform and easily reproducible area density for the slurry, a condition of vast importance as a prelude to forming elemental pattern areas of uniform consistency and thus uniform light output.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1-8 are diagrammatic representations of various stages in the method;

FIG. 9 is a diagrammatic representation of another embodiment; and

FIG. 10 is a diagrammatic view of an exposure device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In referring now to the drawings with greater particularity, in FIG. 1 there is shown a cathode ray tube face panel 10 of a type that might be used for a color television tube. The panel 10 has a non-planar interior surface 12, which in this instance is concave, a skirt 14, and a given area 16 which is to have the pattern formed thereon.

The panel 10 has given area 16 prepared for deposition by having it thoroughly cleaned and dried before commencement of the operation. The given area 16 may also be pre-coated; e.g., with a layer of unsensitized polyvinyl alcohol.

Whether the panel is pre-coated or not the given area must be dry before proceeding to the next step. With the panel thus prepared it is placed in a substantially horizontal position with the concave surface uppermost and a quantity of a photosensitized slurry material 18 is poured or otherwise dispensed thereinto substantially at the center thereof.

A suitable slurry may comprise an aqueous suspension containing about 6 weight percent of an organic, polymerizable binder such as polyvinyl alcohol and about 8 weight percent (of the organic binder) of a photosensitizer such as ammonium dichromate. To this suspension is added a suitable quantity of phosphor having a relatively fine particle size; i.e., 2 to 3 microns. The quantity of phosphor will vary with the material, but the amount for phosphors currently being used will be between about :1 to :1 by weight of organic binder. After sufiicient mixing to assure a substantially homogeneous dispersion the viscosity of the slurry is adjusted by the addition of ethyl alcohol to within the range of 14 to 25 centistockes.

After the slurry has been dispensed into the face panel the panel is tilted as is shown in FIG. 2 to allow the slurry to flow from the center to at least the edge of the given area to accomplish the initial wetting action. After the slurry has flowed to one edge of the given area the panel is tilted in another direction to allow the slurry to flow to another edge of the given area, as is shown in FIG. 3. This action is repeated until all of the given area has been initially wetted. Depending upon the manner in which the slurry is dispensed into the panel, the initial wetting may be accomplished by tilting the panel in three or four directions; i.e., in relation to the points of a compass, the panel may be first tilted toward the north, thence toward the east south east, and thence toward the west south west. If four directions are necessary they may be north, east, south, and west; although not necessarily in that order. The angle of tilt will be determined by the viscosity of the slurry and its flow characteristics and, of course, the time limitations demanded by production quotas. For viscosities within the range of 14 to 25 centistokes an angle of tilt of 60 is satisfactory.

Alternatively, the panel may be rotatively mounted at a substantially 60 angle and have the slurry dispensed thereinto. The panel is rotated at a relatively slow speed, say 3 to 4 r.p.m., until the given area is initially wetted.

Whichever of the above methods of initially wetting the given area is utilized, after the wetting has been achieved the slurry is allowed to substantially repuddle near the center of the panel as shown in FIGS. 4 and 5. The repuddling need not be complete and generally the perimeter of the puddle will be quite uneven, as may be seen in FIG. 5 where the crosshatched section represents the wetted area and the stipped section represents the repuddled slurry.

After the repuddling has occurred the panel is inverted to the position shown in FIG. 6 and allowed to remain substantially immobile until the levigation of the slurry occurs. The levigation occurs in stages as the slurry begins to flow outwardly from the center of the panel toward the edges under the action of the constant force of gravity. The slurry undergoes flowing movement because of the concave interior surfaces of the panel.

Immediately subsequent to inversion the surface of the slurry may have a rough and uneven texture such as is shown in FIG. 7; however, after a predetermined period of time the slurry levigates and assumes the surface shown in FIG. 8. The area density of the slurry layer at this stage will have substantial uniformity between any two points thereon; i.e., there will be on the average less than .5% variation between any two points. This kind of consistency was not possible by prior art methods.

To provide this type of consistent performance, control of several other parameters are necessary in addition to the viscosity of the slurry and the time for levigation; namely, the temperature and the humidity of the atmosphere surrounding the panel during the slurry dispensing, wetting, and levigating.

For a slurry having a viscosity in the range of about 14 to 25 centistokes and a phosphor-PVA ratio within the range set forth above, and a levigating time in the range of about 7 to 13 minutes, a temperature in the range of about 70 to 80 F. and a relative humidity in the range of about 30% to 50% provide excellent results.

During the levigation stage it is desirable that all movement of air or drafts be avoided since this might speed up the evaporation of the solvents within the slurry mixture and cause it to set up with an uneven surface. This condition may be avoided by placing the panel within a substantially confining enclosure during levigation. Some additional control may be had over the levigating of the slurry within the enclosure by providing a suitable hole or holes therein to control convection currents.

The amount of slurry dispensed into the panel is not particularly critical within very broad limits. Obviously there must be an amount of slurry sufircient to provide the desired area density over the given area, but any excess of this amount will not cause any unacceptable results. This is so because the area density is dependent upon the viscosity of the slurry, the temperature, the humidity, and the levigation time-all easily controlled parametersand, of course, the levigation forcegravity which, within the relatively small vertical difference between the panel center and edge, may be assumed to be constant. Thus, any excess of the slurry will flow beyond the perimeter of the given area and may be wiped off after levigation has occurred. 4

Still another embodiment of the invention is shown in FIG. 9 wherein the slurry is hosed upwardly into an already inverted panel. One or more nozzles 20 may be utilized depending upon the size of the panel, and they may be pivotally mounted as at 22 so that all of the given area may be hosed on one pass of the nozzle. This procedure eliminates the steps of wetting, repuddling, and inverting and provides the same results.

It is to be noted that, as used herein, hosing is meant to mean the fluid ejection of material by application of a fluid force or pressure and is to be differentiated from spraying which provides an aerated or atomized particle form of deposition.

Whichever of'the above methods of slurry application is utilized, after the levigation is complete the panel is mated with a suitable negative 24 to provide a desired pattern and is positioned with an exposure device 26, as is shown in FIG. 10. A radiation source 28 is remotely spaced from the panel and upon energization provides the radiation necessary to polymerize the desired areas. After the exposure the panel is developed by washing it with a solvent for the unexposed slurry. For slurries of the type described herein the washing solution may be water.

When the developing is finished the panel is dried and the cycle is repeated. In the case of a three color tube the process is utilized three times.

This invention provides a substantial advance in the art of screening cathode ray tubes. It eliminates the need for highly skilled operators and provides excellent uniformity of area density, a prime prerequisite to manufacturing acceptable color picture tubes. Also, it lends itself readily to automatic procedures and, by elimination of the high speed spinning cycles, virtually eliminates the condition called spoking on the second and subsequent slurry depositions. (The more slowly moving slurry has a better opportunity to flow around previously formed patterns).

While there have been shown and described what are at present considered to be the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claim.

We claim:

1. In the method of forming a pattern on the concave lnterior surface of a cathode ray tube face panel the steps comprising: wetting at least said concave surface with a phosphor particle containing, organic, radiation sensitized, viscous slurry without regard to the area density thereof by depositing a quantity of said slurry material substantially in the center of said concave surface when said surface is upwardly oriented and in a substantially horizontal plane; angularly tilting said surface in at least three directions, one at a time, to substantially wet at least said concave surface; returning said surface to said horizontal plane for a sufficient time to allow substantial repuddling of said slurry material near the center of said surface, positioning said face panel with said concave surface downwardly oriented to achieve levigation and uniformity of area density of said material over said concave surface through the action of gravity; positioning a suitable negative with respect to said phosphor coated surface; exposing said slurry coated concave surface through said negative to a suitable source of radiation whereby portions of said slurry coated surface become selectively unremovable by a given solvent; and forming said pattern by utilizing said given solvent to remove that part of said slurry coated surface which has not become unremovable.

References Cited UNITED STATES PATENTS ALFRED L. LEAVITT, Primary Examiner J. R. BA'ITEN, JR., Assistant Examiner US. Cl. X.R.

11733.5 C, 33.5 CM, 33.5 CF

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