US3653939A

US3653939A - Screening of black-surround color picture tubes

Info

Publication number: US3653939A
Application number: US580A
Authority: US
Inventors: Charles J Prazak
Original assignee: Zenith Radio Corp
Current assignee: Zenith Electronics LLC
Priority date: 1970-01-05
Filing date: 1970-01-05
Publication date: 1972-04-04
Anticipated expiration: 1989-04-04
Also published as: CA919522A; GB1279350A

Abstract

The screen of a black-surround color picture tube is provided with a discontinuous coating of light-absorbing material, discontinuous in that the light-absorbing material is applied to surround those elemental areas of the screen which are to receive deposits of phosphor. The screen is then heated so that the exposed elemental screen areas and the light-absorbing coating attain a uniform temperature. A slurry, including a chosen one of the three phosphor materials, is then dispensed onto the screen and distributed to establish a layer of slurry of a desired thickness over the entire screen surface. The excess slurry is removed by tilting the screen to effect dumping. The slurry layer is then set, without the application of heat, and after the layer has been set, heat is applied to dry the layer. Thereafter, screening continues in conventional fashion.

Description

United States Patent Prazak, III

[451 Apr. 4, 1972 SCREENING OF BLACK-SURROUND COLOR PICTURE TUBES Inventor:

U.S. Cl. ..ll7/33.5 CM, 117/33.5 CP, 117/101 Int. Cl ..C03c 3/28, C03c 17/00, F21v 9/16 Field of Search ..1l7/33.5 CF, 33.5 CM, 101

References Cited UNITED STATES PATENTS Batchelor v.117/101 X Korner et al ..117/33.5 CM X Emmens ..117/101X' Primary Examiner-Alfred L. Leavitt Assistant Examiner-Kenneth P. Glynn Attorney-John J. Pederson [57] ABSTRACT The screen of a black-surround color picture tube is provided with a discontinuous coating of light-absorbing material, discontinuous in that the light-absorbing material is applied to surround those elemental areas of the screen which are to receive deposits of phosphor. The screen is then heated so that the exposed elemental screen areas and the light-absorbing coating attain a uniform temperature. A slurry, including a chosen one of the three phosphor materials, is then dispensed onto the screen and distributed to establish a layer of slurry of a desired thickness over the entire screen surface. The excess slurry is removed by tilting the screen to effect dumping. The slurry layer is then set, without the application of heat, and after the layer has been set, heat is applied to dry the layer. Thereafter, screening continues in conventional fashion.

3 Claims, 6 Drawing Figures Patented A ril 4, 1972 3,653,939

y//////////) V///////Z7l q llb ll llr Ilb llg Ilr H61 a llr Ilb n Hr Attorne Aperhlre Siz Mus) FIG. 6 SE B .g J gg lnvenrcor i Charles J. Prazok,IlI

scR E INc F BLACK-SURROUND COLOR PICTURE TUBES BACKGROUND OF THE INVENTION The present invention is addressed to an improved screening process for use in the manufacture of black-surround color picture tubes as described and claimed in U.S. Pat. No. 3,146,3b8, issued on Aug. 25, 1964 to Joseph P. Fiore et al. Such a tube has light-absorbing material covering those portions of the screen which are interposed between its various phosphor deposits. Advantages of such a structure are very real and include a substantial enhancement of both brightness and contrast.

In preparing the screen of such a tube, there is a choice of applying the green, blue and red phosphors and filling the spaces between the phosphor deposits with light-absorbing material or, alternatively, a grille of the light-absorbing material, having holes into which the phosphors may be deposited, may be formed on the screen initially. There is a preference to forming the grille of light-absorbing material first and then filling its.holes with the various phosphors arranged, of courseQin the necessary interspersed relation to result in a repeating pattern of the three different color phosphor deposits distributed over the entire screen surface. This, in fact, is the procedure currently employed in commercial practice but it has manifested an undesired loss of brightness which is corrected by the present invention. In particular, it has been found that the phosphor deposits are not as well as packed as desired and may even be characterized as porous in comparison with the phosphor deposits of a standard tri-color picture tube that does not have a grille of lightabsorbing material. Applicant has discovered the failure of current practices to achieve the theoretical brightness capability of a black-surround tube and teaches herein an improved method for overcoming the deficiency of past screening processes used in making black-surround color picture tubes.

Accordingly, it is an object of the invention to provide an improved process for screening a black-surround color cathode-ray tube.

It is a specific object of the invention to improvethe screening of such a color picture tube to attain the brightness output of which the screen of such a tube is theoretically capable.

In particular, it is an object of the invention to improve the screening of a black-surround color tube to achieve phosphor deposits that provide at least the same brightness as attained from similar screening processes used in the manufacture of a conventional color picture tube that does not have the blacksurround feature. i

SUMMARY OF THE INVENTION In screening a color cathode-ray tube of the black-surround variety, a light-absorbing material is first applied to the screen in circumscribing relation to those elemental areas of the screen that are to receive deposits of phosphor material. Thereafter, in accordance with the invention a liquid coating composition is dispensed onto the screen in an amount ex-' ceeding that required to coat the screen with a coating layer of a preselected thickness. That liquid coating composition comprises at least a volatilizable constituent which is usually water. The coating composition is distributed, as by rotating and tilting the screen, to form over the elemental screen areas and also over the previously coated circumscribing screen portions a layer of coating composition having a substantially uniform temperature and of the desired thickness and then the excess of the coating composition is removed. Thereafter, a sufficient amount of the volatilizable constituent of the coating composition is evaporated, while maintaining the elemental screen areas and the previously coated screen portions at a substantially uniform temperature. This evaporation increases the viscosity of the remainder of the coating composition so as to fix the layer of coating composition to all surface portions of the screen. After having fixed that layer in position, the screen is heated to dry the layer thoroughly and from this point forward the screening is conducted in conventional manner. i I

In one aspect of the invention, the screen with its previously developed light-absorbing grille is preheated to the extent required to bring the grille and the exposed elemental screen areas that are to receive phosphor material to a uniform high temperature and then a phosphor-containing light-sensitive slurry at room temperature or at an elevated temperature is dispensed onto the screen to form a desired slurry layer. Thereafter, the screen is tilted to such an angle that the excess slurry is dumped out and this occurs with the screen rotating at sufficient speed to retain the slurry layer in situ. The rotation is continued without further heating of the screen until, through evaporation, the slurrylayer attains a set after which heat is applied to dry the layer preparatory to its exposure and further processing.

BRIEF DESCRIPTION OF THE DRAWING The features of the present invention which-are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identify like elements, and in which:

FIGS. 1 to 5 are fragmentary cross sectional views, on an enlarged scale, of a color cathode-ray tube showing sequential process steps in screening a black-surround tube; and

FIG. 6 is a curve relating brightness to the size of the aperturesof the light-absorbing grille of such a tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT The envelope of a shadow mask color tube has a face section that is initially separated from a conically shaped envelope portion which is a convenience in screening. A small or fragmentary portion 10 of such a faceplate is represented in FIG. 1 and its specific dimension and configuration are of no special consequence. The invention may be practiced in screening large or small sized tubes of round or rectangular form. It is equally applicable to the manufacture of tubes in which the phosphor deposits are stripes or the more conventional dot triads. For the purpose of a specific disclosure of the invention, it will be assumed that faceplate IQ is part of a 25 inch rectangular color tube having a mosaic-type of screen, that is to say, having a multiplicity of dot triads distributed over the internal screen surface with each triad comprising a dot of green, a dot of blue and a dot of red phosphor. It should also be noted that the invention is applicable to any ofthe various screening processes which utilize photographic printing in which the inner surface of the screen is provided with a layer of a light-sensitive material for exposure and developing in order to lay down any of the three color phosphors. The phosphor may be included as a component of the light-sensitive coating material or it may be added subsequently, as in the case of dusting. Again, for purposes of the disclosure it will be assumed that screening is accomplished through the use of a water soluble photosensitive slurry comprised of water, polyvinyl alcohol, sensitized with ammoniumdichromate and bearing phosphor particles in suspension. Before this slurry is utilized, however, screen 10 is provided with a grille of lightabsorbing material and its application will be discussed initially.

After the screen has been made chemically clean, it is coated with a removable layer 11 of clear polyvinyl alcohol sensitized with ammonium dichrornate. After layer 11 has been dried, selected portions thereof are exposed to ultraviolet light to establish in the layer interleaved sets of images of those elemental areas of the screen that are to receive assigned ones of the phosphor materials This is accomplished in a step analogous to that conventionally taken in photoresist screening to define the elemental areas of the screen which are to receive a particular phosphor and to distinguish them from the other portions of the screen. This discrimination is easily attained by exposure to ultraviolet light through round holes or apertures of the shadow mask 12 of the tube in process. For that purpose, such a mask is installed within the faceplate portion of the tube envelope after which this subassembly is placed in an exposure chamber so that ultraviolet light is directed to selected elemental areas of the screen through the apertures of mask 12. If the light source is positioned, for example, to simulate the electron gun of the tube in process which is to excite the green phosphor material, the ultraviolet light will be confined to expose only those portions 11g oflayer 11 which overlie elemental areas of screen assigned to receive deposits of green phosphor. After this exposure step, portions 11b of layer 11 are similarly exposed, these constituting the portions of layer 11 that overlie elemental areas of screen 10 to receive deposits of blue phosphor. To achieve their exposure, it is only necessary to change the position of the light source in the exposure chamber so that it now simulates the electron gun of the tube intended to energize the blue phosphor dots. In a third position of that light source, where it simulates the red electron gun of the tube, a third set ofsmall portions llr of layer 11 are exposed and these portions overlie elemental areas of the screen intended to receive red phosphor. As a consequence of the multiple exposures, there are established in layer 11 interleaved sets of images of circular, elemental areas of the screen that are separated from one another and are to receive assigned ones of the phosphor materials.

While not essential, it is preferred that the exposed elemental areas of layer 11 be smaller in size than the apertures of the shadow mask as that mask is finally and permanently installed within the completed tube, The correct relative size may be realized in a variety of ways. The mask may be provided initially with a coating to temporarily close down the size of its apertures so that the dimensions of the phosphor dots determined by exposure through the shadow mask are properly related to the final size of the mask apertures. If closing down of the mask apertures by a coating technique is resorted to, the

' mask coating is removed after screening has taken place so that the final hole size of the mask is properly related to the phosphor dots.

A more attractive process which is currently used commercially is one in which the apertures originally formed in mash 12 have the size required for screening. After screening has taken place, the mask is re-etched to open up or enlarge its apertures to a desired final size. This has an advantage in precisely controlling the dimensions of the phosphor dots and also in attaining uniformity of size and configuration of the dots.

By whichever approach the selected portions 11g, 11b and llr of prpva layer 11 are exposed, the interleaved sets of images are next developed by removing all unexposed portions of photosensitive layer 11 producing the screen condition of FIG. 2. Inasmuch as the photosensitive material as applied to screen 10 is soluble in water and all exposed portions thereof have been rendered insoluble, washing the screen with water after the three exposure steps removes all of the unexposed portions of layer 11. The screen of FIG. 2 may be described as having clear deposits or dots of pva separated from one another by screen portions which are bare and are to receive a pigment or a material having light-absorbing capabilities.

The next step of the screening process constitutes depositing in the spaces between the elemental screen areas covered by the dots of clear pva a coating 13 of an inorganic pigment having light-absorbing capabilities and having the property that its adherence to screen 10 is substantially immune to attack by an active agent which may be employed to destroy the adherence of pva dots 11g, 11b and llr to the screen. While this light-absorbing material may be applied only to the areas surrounding the clear pva dots, it is far more convenient to apply the coating 13 over the entirety of the screen as indicated in FIG. 3 in which case the coating of light-absorbing material is also applied over the clear pva dots in the nature of an overcoat. Preferably, layer 13 is applied as a slurry and also preferably it is a colloid having in suspension a fine pulverulant material such as black iron oxide, powdered mica, molybdenum disulphide, manganous carbonate, ceramic black or graphite. Colloidal graphite is especially useful as the material of coating 13. The commercial form of colloidal graphite available under the tradename Aquadag" diluted to 3 per cent solid with deionized water is an acceptable slurry material. After the slurry coating 13 is applied, it is fixed by drying through the application of heat. Drying for a period of 5 minutes under a 750 watt heater, while rotating the panel at a rate of 5 to 8 r.p.m. and directing an air blower on the screen, has given satisfactory results.

It is next necessary to remove all of the coating materials from screen 10 except for the graphite coating 13 over those portions of the screen which intervene the dots llg, 11b, and llr of clear pva. This may be achieved by the application of a chemical stripper which reacts with pva to free or lift the clear pva dots off the panel. A suitable stripper is 30 per cent hydrogen peroxide and per cent water which may be poured onto the center of the screen and slurried to uniformly coat the screen. After a processing interval of about 30 seconds, the excess stripping solution is poured off and the screen is washed with a spray of deionized water for an inter val of 30 seconds to a minute at a pressure of 35 p.s.i. In this washing, the clear pva dots 11g, 11b and llr wash off the screen and take with them their individual overcoat of graphite. In this manner, a light-absorbing grille is produced on screen 10 having holes that are to receive deposits of phosphor material. For convenience, the elemental screen areas that are to receive phosphor deposits are designated 11g, 11b and llr in FIG. 4 which further illustrates that these elemental areas of the screen are totally surrounded by the light-absorbing grille, that is to say, by the residue of the colloidal graphite layer 13a. So far as the present invention is concerned, the processing up to this point is in the prior art and the improvement of the invention concerns the screening process by which the phosphor deposits designated 11g, 11b and llr" in FIG. 5 are made.

Screening apparatus with which the invention may be practiced is described in U.S, Pat. No. 3,319,556, issued to Joseph P. Fiore and in US. Pat. No. 3,3 19,759 issued to l-lajduk et al., both of which are assigned to the assignee of the present application. Briefly, to apply any given phosphor material, a charge of slurry including such material is dispensed onto the central portion of the screen in an amount exceeding that required to cover the screen with a slurry layer of a preselected thickness. The screen is then tilted slightly to distribute the slurry coating and form over the screen, including both the elemental screen areas desired to receive phosphor as well as the light-absorbing grille, a substantially uniform slurry layer of the desired thickness after which the excess slurry is removed. At this juncture, the slurry layer is fixed but without the application of heat so that all elemental areas of the screen are retained at the same uniform temperature. Fixing of the slurry layer is accomplished by evaporating a sufficient amount of the water constituent to increase the viscosity of the slurry material to a value that the layer, especially that which covers the elemental screen areas intended to receive phosphor, is not subject to migration but rather, is firmly secured over the entire screen area. After the slurry layer has been fixed, by evaporating I water, it is dried by the application of heat as with an infrared heater.

In practicing the invention with apparatus of the type illustrated in the above-identified patents, the screen with its previously applied grille having apertures to receive phosphor material is supported and rotated at a relatively slow rate of the order of 5 to 15 r.p.m. Heat from infrared heaters is applied to the screen to bring it to a uniform temperature'which exceeds the ambient or room temperature. It is necessary that the grille as well as the elemental areas of the screen circumscribed by the grille be brought to the same temperature and it has been found that they may be brought to a uniform temperature of 120 F. by subjecting the screen to an infrared heater of about 12 kw. for an interval of a minute to a minute and 30 seconds and then allowing the temperatures to equalize for 30 seconds to 1 minute.

After all portions of the screen have been brought to this relatively high temperature and with the screen rotating at a slow rate in a horizontal plane, a charge of pva slurry having, by way of illustration, green phosphor in suspension and being approximately at room temperature, is dispensed onto the center of the screen. The amount of the, charge exceeds that which is required to cover the entire inner surface of the screen with a slurry layer of a desired dry thickness of approximately 0.0005 inch. After the pva slurry has been dispensed, the screen is tilted slightly to an angle of approximately while its slow rotation is continued, in order to distribute the slurry and form over the screen a substantially uniform slurry layer of the desired thickness. Thereafter, the screen is tilted to an angle exceeding 90 and its rotation is increased to attain a balance between gravitational and centrifugal forces to the end that the excess slurry is dumped out of the screen into a catcher or container while the desired uniform layer remains in situ. The panel may be tiled, for example, to approximately 130 and its rotation increased into the range from 80 to 100 rpm. to dump the excess slurry.

After dumping, rotation of the screen is continued under the conditions of tilt and speed of rotation established during the dumping step and, importantly, these conditions are continued without further heating of the screen. In this continued rotation, some of the water component of the slurry evaporates, increasing the slurry viscosity and thereby fixing the slurry layer to all surface portions of the screen. Fixing of the slurry layer in this manner may be accomplished in from 1%2 to 2 minutes because of the assist attributable to preheating the panel before dispensing the slurry. This part of the process may, if desired, be speeded up in any fashion that increases the rate of evaporation provided it does not involve differential heating of the graphite coating and glass surface before the slurry layer has been fixed or set. For example, a stream of gas such as air at room temperature may be caused to flow overthe screen.

With the screen surface at a uniform temperature and the slurry layer thus fixed thereto, infrared heaters are energized to heat the screen and thoroughly dry the slurry layer preparatory to.exposure and development of so much of the layer as has been deposited in grille openings 11g. In this way, the green phosphor deposits 113" are made in the assigned ones of the openings ofgrille 13a. In similar processing steps, differing only in the location of the exposure source and the phosphor component of the slurry coating material, both the blue phosphor deposits 11b" and red phosphor deposits 11" are made to complete the formation of phosphor dot triads on the screen. From this point forward, including filming and aluminizing, the processing of the screen is conventional and need not be described.

The significant difference in the described screening process, compared with that heretofore practiced in the art, is in fixing or setting the phosphor slurry layer which, for the described process, occurs while all incremental areas of the screen are at the same temperature and with no application of heat, whereas previously the slurry layer was formed and then dried in conventional manner by the application of heat. This difference in setting the slurry layer is responsible for the formation of well packed phosphor deposits or the formation of porous deposits.

If the phosphor slurry is applied in the usual way over the grille and then set and dried by the application of heat as has been practiced heretofore in the art, an undesirable temperature gradient is established with the grille at a high temperature and-the elemental screen areas circumscribed by the grille at a lower temperature. This results from the fact that the graphite grille is essentially non-transmissive to infrared and, therefore, achieves a high temperature when exposed to infrared heaters. On the other hand, the incremental areas of the screen that are to receive phosphor and the phosphor slurry itself are relatively transmissive to infrared and therefore the incremental areas achieve a significantly lower temperature in the heating process. This temperature differential manifests itself in a migration of the slurry out of the holes of the grille and onto the surrounding grille surface. As a consequence, the grille accumulates a thicker slurry layer than the elemental screen areas intended to be coated with the phosphor in process. If the screen is exposed and developed in this condition, the resulting phosphor deposit is porous or at least lower in coating density than that on the graphite.

The described invention avoids this undesirable consequence of the prior art method of screening black-surround tubes. lt is to be emphasized that in practicing the invention the grille and the exposed elemental areas of the screen to be coated with phosphor are preferably heated and brought to the same uniform temperature before the layer of slurry including phosphor is deposited over them. While all portions of the screen are thus at a uniform temperature, the water constituent of the phosphor slurry is permitted to evaporate in part so that the viscosity is increased to fix the slurry layer. This avoids the difficulty of past practices by precluding the possibility of migration of slurry from the holes of the grille onto the grille surface. Of course, once the slurry layer has thus been fixed, the application of heat to accomplish complete drying of the layer, as required for the exposure step, does not destroy the uniformity of the slurry layer as between the elemental areas of the screen to receive the phosphor coating and thesurface of the grille. It is for this reason that subsequent exposure and developing of the slurry layer in the conventional way results in a compact deposit of phosphor and avoids the porous dots: that have been experienced in the past.

The curves of FIG. 6 show the variation of brightness with changes in the size holes formed in the grille. Curve A indicates how the brightness of the screen varies, theoretically, with the aperture size of the grille. Broken curve B represents measured brightness output of black-surround tubes with varying aperture sizes of the grille when those tubes have been screened in accordance with prior practices. It will be noted that for apertures in excess of 1 l mils the measured brightness is distinctly less than the theoretical value. At an aperture size of 13% mils which is used in commercial black-surround tubes the brightness is down 10 to 15 per cent or more. This results from the prior practice of heating the screen to dry the phosphor slurry layer without previously fixing the layer to avoid migration. Where the drying is delayed until such time that the slurry layershall have fixed itself over the entire screen surface, as taught by this invention, the measured brightness is essentially the same as that indicated by curve A and the brightness loss, as indicated by curve B, is recaptured.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. In the screening of a color cathode-ray tube in which a light-absorbing material is first applied to the screen in circumscribing relation to elemental areas of said screen which are to receive deposits of phosphor material, the improvement in applying phosphors to said elemental areas which comprises the following steps:

heating the screen to establish said elemental screen areas and said previously coated screen portions at a uniform temperature which exceeds ambient temperature; dispensing onto said screen a coating composition in an amount exceeding that required to cover said screen with a coating layer of a preselected thickness, said coating composition comprising at least a volatilizable liquid;

distributing said coating composition over said elemental screen areas and over said previously coated circumscribing screen portions to form a layer of coating composition having a substantially uniform temperature and a substantially uniform thickness of a desired preselected value and removing the excess of said coating composition;

evaporating a sufficient amount of said liquid, without the application of heat to said screen and while maintaining said elemental screen areas and said previously coated screen portions at a substantially uniform temperature, to increase the viscosity and fix said layer of coating composition to all surface portions of said screen;

and thereafter applying heat to dry said layer of coating composition.

2. The screening improvement in accordance with claim 1 in which said coating composition is applied to said screen at substantially ambient temperature during said dispensing step and in which said evaporating step takes place without the application of heat to said screen other than that preceding said dispensing step.

3. In the screening of a color cathode-ray tube in which a light-absorbing material is first applied to the screen in circumscribing relation to elemental areas of said screen which are to receive deposits of phosphor material, the improvement in applying phosphors to said elemental areas which comprises the following steps:

heating said screen and establishing said elemental screen areas and said previously coated screen portions at a uniform temperature which exceeds ambient temperature;

dispensing onto said screen a quantity of a coating composition, which is at a substantially lower temperature than the heated screen, and in an amount exceeding that required to cover said screen with a coating layer of a preselected thickness, said coating composition comprising a sensitized volatilizable liquid in which particles of phosphor material are suspended;

tilting said screen to an angle less than while rotating continuing the rotation of said screen at the angle of tilt and rate of rotation established in the last-described step, and without further heating said screen, to evaporate a sufficient amount of said liquid, while maintaining said elemental screen areas and said previously coated screen portions at a substantially uniform temperature, to increase the viscosity and fix said layer of coating composition to all surface portions of said screen;

and thereafter further heating said screen to dry said layer of coating composition.

Claims

2. The Screening improvement in accordance with claim 1 in which said coating composition is applied to said screen at substantially ambient temperature during said dispensing step and in which said evaporating step takes place without the application of heat to said screen other than that preceding said dispensing step.
3. In the screening of a color cathode-ray tube in which a light-absorbing material is first applied to the screen in circumscribing relation to elemental areas of said screen which are to receive deposits of phosphor material, the improvement in applying phosphors to said elemental areas which comprises the following steps: heating said screen and establishing said elemental screen areas and said previously coated screen portions at a uniform temperature which exceeds ambient temperature; dispensing onto said screen a quantity of a coating composition, which is at a substantially lower temperature than the heated screen, and in an amount exceeding that required to cover said screen with a coating layer of a preselected thickness, said coating composition comprising a sensitized volatilizable liquid in which particles of phosphor material are suspended; tilting said screen to an angle less than 90* while rotating said screen at a first rate to distribute said coating composition and to form over said elemental screen areas and over said previously coated circumscribing screen portions a substantially uniform layer of coating composition of said preselected thickness; thereafter, tilting said screen to an angle exceeding 90* and rotating said screen at a rate greater than said first rate to dump the excess of said coating composition while retaining said layer in situ; continuing the rotation of said screen at the angle of tilt and rate of rotation established in the last-described step, and without further heating said screen, to evaporate a sufficient amount of said liquid, while maintaining said elemental screen areas and said previously coated screen portions at a substantially uniform temperature, to increase the viscosity and fix said layer of coating composition to all surface portions of said screen; and thereafter further heating said screen to dry said layer of coating composition.