Method for metalizing a cathode ray tube screen

Abstract

This disclosure depicts methods and structures for applying a very thin layer of electrically conductive, light-reflective metal such as aluminum to the phosphor screen of a cathode ray tube. More particularly, there is disclosed the application of such a metal layer by the transfer of a metal layer formed on a substrate directly to a phosphor layer on the inner surface of a cathode ray tube faceplate. The metal layer is adhered to the phosphor layer by an adhering step which may involve the use of a pressure-sensitive adhesive. In one embodiment disclosed, the substrate is then stripped off; alternatively, the substrate may be removed by dissolution or volatilization. Remaining volatile substances are driven off in a baking operation. Other associated and alternative operations are depicted.

Description

Background of the Invention

This invention relates in general to the fabrication of phosphor screens for cathode ray tubes, and more particularly to improved methods and structures for applying a metal layer on the phosphor screen of a television cathode ray tube of a type having an envelope which includes a separate faceplate section. The metal layer, typically aluminum has the following primary functions. First, it serves as the high voltage accelerating anode for the cathode ray tube and acts as an electrically conductive layer for preventing the build-up of charge on the screen. Second, it reflects to the viewer light emitted rearwardly by the phosphor screen. Third, it acts as a physical barrier preventing negative ions from striking the phosphor screen.

It is standard practice in the fabrication of cathode ray tube screens to deposit a phosphor layer containing the phosphor material and a binder on the inner surface of the faceplate. Subsequently, a thin layer of aluminum is evaporated on the phosphor layer. Before the metal layer is deposited on the phosphor layer, an intermediate smoothing film is applied in order to improve the surface characteristics of the deposited aluminum layer.

The deposition of the thin metal layer, due to the nature of the vacuum deposition process, involves mounting the faceplate on a vacuum chamber, pumping the chamber down to a vacuum, heating a boat of aluminum and timing the evaporation to insure deposition of a metal layer having the appropriate thickness (typically 1500 A). The metal layer is desirably thick enough to reflect light emitted by the phosphor screen and yet thin enough to be transparent to the electron beam. The described evaporation process, particularly when set up on a high volume assembly line, is undesirably expensive. U.S. Pat.

Prior Art

This invention is directed to an improved cathode ray tube screen metalization process involving the transfer of a metal layer to the screen. It has been suggested in U.S. Pat. Nos. 2,734,013; 3,389,030; and 3,649,269 that a phosphor layer may be formed on the faceplate of a cathode ray tube by a transfer process. These patents discuss transfer processes in which a self-supporting web or decal containing a phosphor material and a binder is formed on a base and then subsequently transferred to a flat plate or cathode ray tube faceplate. Metalization of the transferred screen is achieved by conventional evaporation techniques after formation of the screen. U.S. Pat. No. 2,734,013 suggests as an alternative, without elaboration, that the ". . . light-reflecting layer may be applied . . . during fabrication of the decalcomania . . . "

None of these teachings are useful in solving the problem to which the present invention is addressed for at least the following reasons. The present invention involves the metalization of color phosphor screens, the phosphor patterns on which are, in the most common application, formed by photochemical processes which employ each tube's shadow mask as the mask for the phosphor pattern. A phosphor screen with a pre-formed phosphor pattern is thus not useful. Further, none of these patents deal with the metalization of pre-formed phosphor screens.

The brief suggestion in the U.S. Pat. No. 2,734,013 that the light-reflective layer may be transferred along with the phosphor layer is neither substantiated nor useful in the context of the present invention. The U.S. Pat. No. 2,734,013 suggests the feasibility of transferring a metal layer to a cathode ray tube faceplate, which layer is supported on a laminate comprising a layer of phosphor in a binder and a second film layer serving as a smooth base for the metal layer. The present invention is addressed to the much more difficult and dissimilar problem of transferring a very thin and fragile, unsupported layer of metal, typically only 1500 A thick, to a preformed patterned phosphor screen without tearing of the layer and with satisfactory uniformity and yield.

Metalization by direct transfer techniques has been known to be successfully tried only on small articles, as disclosed, e.g., in the article "Application of the Transfer Tape Technique in Electron Tubes," ADVANCES IN ELECTRON TUBE TECHNIQUES, Proceedings of the 6th National Conference, Sept. 1962.

Other Prior Art

U.S. Pat. No. 2,858,233

Objects of the Invention

It is a general object of this invention to provide improved methods and structures for metalizing the phosphor screen of a cathode ray tube.

It is a less general object to provide methods for metalizing cathode ray tube screens which are vastly more simple and economical than the prior art vacuum metalization methods.

It is yet another object to provide such metalization methods and structures which yield a metal layer having greater reflectivity than prior art methods and structures, and thus to provide metalization methods and structures which result in a greater luminous output from the processed cathode ray tube.

Brief Description of the Drawings

The features of the 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 conjunction with the accompanying drawings and in which:

FIGS. 1-8 show, in highly schematic fashion, a method and structure for metalizing a cathode ray tube screen in accordance with this invention; and

FIGS. 9-15 depict an alternative method for implementing the principles of this invention.

Description of the Preferred Embodiments

This invention has general applicability to the metalization of phosphor screens of cathode ray tubes of the types having envelopes including a discrete faceplate or front panel. In accordance with this invention, the phosphor screen of a cathode ray tube is metalized by a transfer process which involves forming a layer of metal to be applied to the screen on a substrate and subsequently transferring the metal layer to the phosphor screen. In a preferred method of practicing the invention, shown diagrammatically in FIGS. 1-8, the substrate is adapted to be stripped from the transferred metal layer.

Before engaging a discussion of the preferred screen metalization process, there will first be described a preferred method for forming a metalized web from which the metal layer is transferred. FIG. 1 illustrates a substrate 10. For reasons which will become clear as this description proceeds, the substrate 10 is preferably composed of a flexible and deformable material which may, for reasons stated below, be a thermoplastic material, such as a polyester, having a thickness approximately 0.0005 inch, however, it is contemplated that substrates composed of other materials such as polypropylene and shrinkable polyester may be used.

A release agent 12 is preferably employed to release from the substrate 10 layers subsequently deposited thereon. The release agent must be compatible with the operation and manufacture of the involved cathode ray tube and must provide a smooth base for successively deposited layers; it may be removable, as by dissolution or volatilization.

By way of example, it has been found that 4 .times. 10.sup..sup.-7 inch layer of evaporated sodium chloride works very satisfactorily. It is contemplated that other materials such as potassium chloride, or other salts may also be employed. Sodium chloride has been used previously as a release agent in an experimental image tube manufacturing process wherein a layer thereof was deposited upon a flat glass substrate, followed by evaporation of a layer of aluminum. The substrate was immersed in a solvent which dissolved the sodium chloride, permitting the aluminum layer to float free in the solvent. The aluminum layer was lifted from the solvent and deposited upon the faceplate of a prescreened image tube.

FIG. 2 shows the FIG. 1 substrate 10 and release agent 12, upon which is deposited a thin layer 14 of electrically conductive, light-reflective material such as aluminum. In successful reductions to practice of the present invention, a layer of aluminum approximately 1500 A thick was evaporated on the substrate 10 and release agent 12 in a conventional vacuum deposition chamber.

To cause the metal layer 14 to adhere to a phosphor layer on the inner surface of a cathode ray tube faceplate (to be described in detail below), a thin layer of adhesive 16 is deposited upon the metal layer 14 and cured. In accordance with this embodiment of the invention, the adhesive is preferably a pressure-sensitive adhesive such as K-396N or 86-2003, manufactured by National Starch & Chemical Corporation, which is capable of being converted to gaseous form if heated to temperatures above about 400.degree.C. By way of example, a 100-400 A layer of such adhesive may be employed as the adhesive 16.

The adhesive 16 preferably has a high flash point in the interest of safety; it also must be compatible with cathode ray tube manufacture and operation. It is desirably used in the smallest amount as possible which will cause satisfactory adhesion of the metal layer since it will ultimately have to be baked out through the transferred metal layer. In the preferred method of application the adhesive is sprayed upon the metal layer 14 so as to form a discontinuous layer comprising discrete adhesive drops spaced on the surface of the metal layer 14. It is contemplated that other adhesives than those described and other application methods may be employed to carry out the teachings of this invention.

FIG. 4 depicts a cathode ray tube faceplate 18 having disposed on an inner surface 20 thereof a phosphor layer 22 comprising a phosphor material held in a photosensitized binder. The phosphor layer 22 may be deposited by conventional slurry techniques and comprises, in a color cathode ray tube, e.g., successively-deposited layers of red-emissive, blue-emissive and green-emissive phosphor materials carried typically in a photosensitized binder of PVA (polyvinyl alcohol).

FIG. 5 illustrates a step wherein the web 24, comprising substrate 10, release agent 12, metal layer 14 and adhesive 16, is applied to the phosphor layer 22. In the schematic FIG. 5 illustration, the application is accomplished by first draping the web 24 over a press 26. In the FIG. 5 illustration, the press 26 is shown as comprising a base 28 having an upper surface 30 having generally the contour of the inner surface 20 of the faceplate 18. Disposed on the base 28 is a resilient cushion 32 which is somewhat thicker in the center than on the edges in order that the press will have a yieldable upper surface and in order that the press will cause the web 24 to engage the phosphor layer 22 initially in the center of the faceplate and thereafter to cause the web to be pressed against the phosphor layer 22 progressively outwardly from the faceplate center. By this technique, formation of bubbles under the web is precluded. A similar pressing technique is disclosed in the referent U.S. Pat. No. 3,389,030 in the manufacture of black and white cathode ray tubes. Other press structures and application techniques may be employed. It may be desirable, in order to effect a more rapid or more conforming application of the web 24 to the phosphor layer 22, to apply heat and/or air pressure to the web 24 as it is applied to the press or to the phosphor layer 22.

After the web 24 is adhered to the phosphor layer 22, the press 26 is removed, leaving on the faceplate a decal having the shape of the cathode ray tube screen. The invention is preferably employed for metalizing a phosphor screen of the "black surround" type described and claimed in U.S. Pat. No. 3,146,368 -- Fiore et al. A phosphor screen formed according to this patent has black material separating phosphor elements which emit different colored light. The black material typically extends beyond the electron-illuminated field and onto the sides of the faceplate. The decal preferably overlaps the black material in a screen of this type, precluding any need to form the decal to the exact shape of the electron-illuminated field. The decal may be formed by pre-cutting an outline of the decal configuration with perforations before transferring the decal, or, alternatively, the decal may be trimmed in situ. The shape of the metal layer deposited on the substrate can be determined, if desired, by evaporating the metal layer onto the substrate through a mask having an opening corresponding in configuration to the screen configuration. Alternatively, a pre-cut decal, rather than a continuous web, having the configuration of the screen may be employed.

In order to minimize breaking of the metal layer during the web draping operation, it may be desirable to pre-form or partially pre-form the substrate to the contour of the press or the faceplate before deposition of the metal layer thereon. So that the substrate may nevertheless be handled in roll form, pre-contoured decals may be formed at intervals on a substrate roll, the decals corresponding generally in size and configuration to the screen and having concentric circular or rectangular flutes or corrugations defining a flat bellows which is expansible out of the plane of the web without stretching thereof. When the decals are drawn over the press, the bellows will open and permit the decal to assume the shape of the press without excessive stretching thereof.

As shown in FIG. 6, the substrate 10 is then stripped away, leaving on the inner surface of the faceplate 18 the phosphor layer 22, the adhesive 16, the metal layer 14 and the release agent 12. It should be understood that FIG. 6 is schematic -- in practice, the substrate would come away from the faceplate with the press.

If the release agent is sodium chloride, for example, or some other composition which is capable of being dissolved, the release agent may be removed by a suitable solvent. See FIG. 7. If sodium chloride is used, the solvent may be water. The solvent, of course, will vary with the release agent used. Alternatively, if the release agent is sodium chloride, it may be desirable for reasons of economy to eliminate the release agent removal operation altogether. Tests have shown that the presence of sodium chloride in the tube does not result, upon electron bombardment, in poisoning of the electron guns.

FIG. 8 illustrates a baking operation for driving off the photosensitized binder from the phosphor layer, the adhesive layer 16 and, where the release agent may be of a nature as to be readily volatilized, the release agent. It is conventional in the manufacture of cathode ray tubes to include a "bake-out" operation, typically carried out at 400.degree.C or above during which the photosensitized binder and the afore-described smoothing layer or "film" deposited to form a base for the evaporated aluminum layer, are driven off. Thus, since a bake-out operation is required as a necessary step in the conventional manufacture of a cathode ray tube, the removal of the adhesive 16 can be achieved without the necessity of adding any special tube processing operations. Thus, by the use of the above-described metal transfer web, and by the above-described method, the phosphor screen of a cathode ray tube may be rapidly and economically metalized.

It has been found in a number of screens built and tested, that because the metal layer 14 is deposited upon a smooth surface, i.e., the prepared upper surface of the substrate 10, rather than on a relatively rough surface as in the case of conventional metalization of phosphor screens, the resulting metal layer is smoother than the metal layers deposited by conventional evaporation techniques. Tests have shown that in some cases, gains in brightness of the end product cathode ray tubes have been achieved.

FIGS. 9-15 portray in highly schematic form a second embodiment of the invention wherein the substrate comprising the base for a transfer web or decal is not strippable, but rather is adapted to be removed by dissolution or volatilization.

As above, before engaging a discussion of the FIGS. 9-15 screen metalization process, there will first be described a preferred method for forming the metalized decal or web from which the metal layer is transferred. FIG. 9 illustrates a substrate 42. For reasons which will become clear as this description proceeds, the substrate 42 is preferably composed of a flexible plastic material which may, for reasons stated below, be dissolved or volatilized. A suitable substrate material is an acrylic film having a thickness approximately 0.0005 to 0.0010 inch, however, it is contemplated that a substrate composed of other materials such as nitro-cellulose may be used.

FIG. 10 shows the FIG. 9 substrate 42, upon which is deposited a thin layer 44 of electrically conductive, light-reflective material such as aluminum. To cause the metal layer 44 to adhere to a phosphor layer on the inner surface of a cathode ray tube faceplate (to be described in detail below), a layer 46 of adhesive is deposited upon the metal layer 44 (see FIG. 10) or on the inner surface 48 of the faceplate 50. In accordance with this embodiment of the invention, the adhesive preferably takes the form of a thin film of ethyl silicate.

FIG. 11 depicts a cathode ray tube faceplate 50 having a novel flangeless configuration, on an inner surface 48 of which is disposed a phosphor layer 52 comprising a phosphor material held in a binder. The phosphor layer 52 may be as layer 22 described above.

FIG. 12 illustrates a step wherein a decal 54, comprising substrate 42, metal layer 44 and adhesive layer 46, is applied to the phosphor layer 52. As discussed above, the decal 54 is preferably formed to the shape and curvature of the inner surface 48 of the faceplate 50 to minimize wrinkling of the decal 54 upon transfer thereto and to minimize stretching of the metal layer 44.

In the schematic FIG. 12 illustration, the application is accomplished by first draping the decal 54 over a press 56 and securing it thereto. A tension band 57 is shown to schematically illustrate means for securing the decal 54. The press 56 and the decal application operation represented by FIG. 12 may be as described above with respect to FIG. 5.

After the decal 54 is adhered to the phosphor layer 52, the press 56 is removed, leaving on the faceplate 50 a decal 54 having the shape of the cathode ray tube screen.

The substrate 42 is then removed, preferably in this method embodiment by dissolution, as shown schematically in FIG. 14, leaving on the inner surface 48 of the faceplate 50 the phosphor layer 52, the adhesive layer 46 and the metal layer 44. If the substrate 42 comprises an acrylic, the solvent is preferably toluene. If the substrate 42 comprises nitro-cellulose, the solvent is preferably acetone and amyl acetate.

If the substrate 42 is of a composition such as acrylic which is readily volatilized rather than dissolved, the dissolution step shown in FIG. 14 would, of course, be eliminated. Rather, the faceplate would be baked, as shown schematically in FIG. 15 to volatilize and drive off the substrate.

The baking operation, represented in FIG. 15 by an oven 58, may be the conventional "bake-out" operation during which the phosphor binder and the afore-described smoothing layer or "film" deposited to form a base for the evaporated aluminum layer, are driven off. Thus, since a bake-out operation is required as a necessary step in the conventional manufacture of a cathode ray tube, the removal of the substrate 12 and the adhesive layer 16 can be achieved without the necessity of adding any special tube processing operations.

The invention is not limited to the particular details of construction of the embodiments depicted and other modifications and applications are contemplated. Certain changes may be made in the above-described methods and apparatus without departing from the true spirit and scope of the invention herein involved. For example, to minimize the possibility of blistering of the metal layer during the bake-out operation, the metal layer deposited upon the substrate may be caused to have tiny perforations which will serve ultimately as out-gassing openings for the materials volatilized under the metal layer. In the FIGS. 1-8 embodiment, rather than using a release agent, as described, satisfactory results may be obtainable by the use of a non-stick substrate, the surface of which inherently has low adherance to the metal layer. Alternatively, other forces than adhesion, e.g., electrostatic, may be employed to hold the metal layer upon the strippable substrate until the transfer of the metal layer is accomplished. Whereas in the above-described methods the binder in the phosphor layer to which the metal layer is transferred is described as a photosensitized binder, the invention is equally applicable to transferring a metal layer onto phosphor layers having non-sensitized organic binders or phosphor layers of other compositions. It is intended, therefore, that the subject matter of the above depiction shall be interpreted as illustrative and not in a limiting sense.

Claims

I claim:

1. A method for making a luminescent screen on the concave surface of a curved faceplate for a color cathode ray tube, comprising (not necessarily in the following order):

depositing on the inner surface of the cathode ray tube faceplate a phosphor screen comprising interleaved patterns of red-emissive, blue-emissive and green-emissive phosphor materials held in a photosensitized volatilizable binder;

providing a web comprising a flexible and deformable sheet substrate, a release agent deposited on the substrate and an ultra-thin layer of electrically conductive, light-reflective metal deposited on the release agent;

applying to the layer of a metal a pressure-sensitive adhesive which volatizes at a temperature not exceeding the volatization temperature of said binder;

using a press having a contour related to that of the concave surface of the faceplate, pressing the web against the inner surface of the faceplate by successive contact of adjacent web areas so as to adhere the light-reflective layer to the phosphor screen without forming air bubbles under the web;

stripping off the substrate; and

baking the faceplate at a temperature effective to volatilize said binder and said adhesive to thereby leave a patterned phosphor screen covered with said ultra-thin layer of metal.

2. The method defined by claim 1 wherein said substrate comprises a thermoplastic material and wherein said step of pressing the web includes heating the press and the web during the pressing operation to cause the web to assume the exact contour of the platen.