US 3922394 A
A ferrous-metal apertured mask for a cathode-ray tube is coated in the presence of water and oxygen with a film comprising an organic polymeric material, such as polyvinyl pyrrolidone (PVP), which is not insolubilized when ferric ions are present, and thereby avoids the formation of localized coating discontinuities (usually cometshaped) in the coating. The method is advantageously used in the meniscus-closure method for making an image-screen structure for a cathode-ray tube.
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Description (OCR text may contain errors)
Lozier States atent n91 [451 Nov. 25, 1975 METHOD FOR COATING FERROUS-METAL MASK FOR CATHODE-RAY TUBE  Inventor: Gerald Scott Lozier, Princeton, NJ.
 Assignee: RCA Corporation, New York, N.Y
 Filed: Nov. 9, 1973  Appl. No.: 414,322
 US. Cl. 427/247; 313/402; 427/388;
428/137  Int. Cl. B32B 3/10; B05D 3/02  Field of Search 117/99, 161 UA, 161 UN,
117/132 C, 33.5 C, 33.5 CM, 33.3; 96/361; 313/402; 427/247, 388; 428/137  References Cited UNITED STATES PATENTS 7 3,492,261 l/1970 Young et al. 117/132 C 3,568,486 3/1971 Rosenberg et al.. 117/132 C 3,574,013 Frantzen 117/99 UX 3,653,901 4/1972 Etter ll7/33.5 CM
3,723,154 3/1973 Oliver l17/33.3 3,736,137 5/1973 Kaplan ll7/99 UX 3,811,910 5/1974 Labana et al.. 117/99 X 3,811,926 5/1974 Smith ll7/33.5 C
Primary Examiner-Harry J. Gwinnell Attorney, Agent, or FirmG. H. Bruestle; L. Greenspan  ABSTRACT 4 Claims, No Drawings METHOD FOR COATING FERROUS-METAL MASK FOR CATHODE-RAY TUBE BACKGROUND AND SUMMARY OF THE INVENTION This invention relates to a method for coating a ferrous-metal surface in the presence of oxygen and water with an organic polymeric material.
The term ferrous-metal, as used herein, refers to a class of metal compositions which contain iron as a major constituent and exhibit corrosion or rusting when exposed to water and oxygen. Cold-rolled steel and chromiumiron alloys containing up to IO-Weightpercent chromium and the balance iron are exemplary of ferrous metals.
One process for preparing an image-screen structure for a color television picture tube includes coating a ferrous-metal (cold-rolled steel) apertured mask with a polyvinyl-alcohol composition to temporarily reduce the aperture sizes in the mask. The surface of the mask may have a black oxide coating formed therein. Such a process for temporarily reducing the aperture sizes is referred to as a meniscusclosure process and is described, for example, in US. patent applications Ser. Nos. 21 1,475 (now abandoned) and 21 1,476 (now US. Pat. No. 3,811,926 issued May 21, 1974) both filed Dec. 23, 1971 by B. K. Smith. During such a process, a meniscus or membrane is produced across each mask aperture, and then the meniscus is broken and the remaining coating material is consolidated into a coating of the desired thickness on the aperture wall.
In using this or similar processes, defects appear on the ferrous-metal surface which are called comets because they are usually comet-shaped. A comet includes a head or nodule of polymeric material which is tinted and is much less soluble or dispersible in warm water than the polymeric material in the initial coating composition. An attached tail extends from the head in the direction of flow of the coating material during the coating step. The tail is a thinner-than-normal coatmg.
Prior to coating, the mask surface may be cleaned so that, upon examination of the surface prior to coating, the surface is smooth and essentially free of particles and obstructions. Filtration of the coating composition just prior to coating removes any nodules and particles from the coating composition which could produce comets. Nevertheless, even with such extraordinary precautions, comets appear in the final coating causing unwanted variations in the thickness of the coating and in the amount of closure of the apertures.
It is believed that, during the coating step, localized areas of the mask surface are rapidly corroded in the presence of oxygen and water in the coating composition, with the formation of ferrous and ferric ions and hydrogen ions. This corrosion reaction is accelerated by chloride ions which are normally present in small amounts in the coating composition. The ferrous and ferric ions then insolubilize the soluble polyvinyl alcohol in the local area to form nodules which are the comet heads. The comet tails may be formed by a blocking by the nodule of the flow of coating material and/or by a decrease in viscosity of the coating material caused by a small decrease in the pH from the generated hydrogen ions.
It has been found that the formation of both comet heads and comet tails can be avoided by coating the mask with a film consisting essentially of polyvinyl pyrrolidone (PVP) or a copolymer thereof, which has an unexpected combination of properties giving it special utility in the novel method. First, PVP is not insolubil- DESCRIPTION OF THE PREFERRED EMBODIMENT The invention will now be described with respect to a particular method for temporarily reducing the maskaperture sizes in a meniscus-closure process for making an image-screen structure for a cathode-ray tube. It will be understood, however, that there are many variations that may be employed and still fall within the spirit of the invention.
Prepare a coating composition containing the following constituents in about the indicated proportions by weight (grams):
640 PVP (K-9O GAF Corp., New York NY.)
400 Ethylene glycol 24 Coloring Agent (Orange R dye, C.l. Direct Orange 8 CI. 22130) i 32 Wetting/dispersing agent (Gafac LO-529, GAF
Corp., New York, N. Y.)
1,000 Water It has been found desirable to stock the coating composition at near neutral pH. Just prior to use, the viscosity of the composition is adjusted by the addition of water to about 30 to 40 centipoises at 25C.
A dome-shaped ferrous-metal apertured mask is now coated on the convex surface thereof with the coating composition. One method for applying the coating is described in the above-cited patent application, Ser. No. 21 1,476, and need not be repeated in detail here. In this method, the mask is rotated about an axis tilted upward from the horizontal. A stream of liquid-coating composition is projected into an upward trajectory and into contact with the convex surface of the rotating mask substantially at the top of its trajectory. After about one rotation of the mask, the stream is removed and the rotation is increased to cause excess liquid to fly off the mask by centrifugal force. The retained liquid coats the convex surface and also produces a meniscus membrane that closes each aperture.
The central portions of the meniscuses or membranes are then opened to form temporary apertures smaller than the final-sized apertures of the mask. One method of opening the membranes is described in the abovecited patent application Ser. No. 21 1,475, and need not be repeated in detail here. In this method, the coated aperture mask is first heated at a slower rate to a lower temperature until the thickness of the membrane across each aperture is reduced to a thin meniscus. The coated mask is further heated at a faster rate and to a higher temperature until the membrane ruptures and the remaining material is consolidated into a coating of the desired thickness on the walls of the apertures. The surfaces of the wall coatings define the smaller temporary apertures.
The novel method employing a PVP coating composition produces a substantially uniformly thick coating over the surface of the mask that is free of comets. Prior methods employing polyvinyl-alcohol-coating compositions, particularly those that contain chloride ions, produce coatings on ferrous-metal surfaces which have many comets. It is believed that the uniformity of the coating thickness and the freedom from comets produced by the novel method provide more uniform films and membranes resulting in a greater uniformity in the sizes of the temporary apertures that are produced. Extraordinary process steps of precleaning and- /or prepassivating the mask prior to coating are eliminated, and also the additional equipments to carry out these procedures are eliminated.
An image-screen structure may then be produced using the mask with the smaller temporary apertures as a temporary master. The mask with the temporary apertures may be used to deposit phosphor elements of an image-screen structure by methods such as described, for example, in U.S. Pat. No. 3,406,068 to H. B. Law. The mask with the temporary apertures may also be used to deposit a light-absorbing matrix, such as the process described for example in U.S. Pat. No. 3,558,310 to E. E. Mayaud.
GENERAL CONSIDERATIONS A problem with the use of polyvinyl-alcohol (PVA) coating compositions for the temporary meniscus closure of an apertured mask is a random defect known as comets. The cause of the defect is believed to be localized premature gelling of the polyvinyl alcohol by ferrous and ferric ions from the corrosion of the mask due to the presence of oxygen and water during the coating. The corrosion occurs at breaks and/or poorly blackened areas of the mask especially where the coating composition has a relatively high chloride content. .Of several approaches taken to overcome the defects of comets, the use of polyvinyl pyrrolidone (PVP) offers advantages of not requiring more processing steps and- /or closer tolerances of the properties of the materials or of the processing steps used. Water-based coating compositions of polyvinyl pyrrolidone and/or its copolymers have a unique combination of properties that appear especially useful for meniscus closure of mask apertures. First, PVP coating compositions are not gelled (insolubilized) by ferrous or ferric ions, or at lower temperatures. Second, the viscosity of a PVP coating composition is not affected by small changes in pH. Third, PVP complexes with many dyes that are useful for opaquing the closure material to ultraviolet light. Fourth, PVP is commercially available in viscosity-molecular-weight ranges suitable for the meniscusclosure process. And, fifth, PVP is not toxic and is a nonallergic material.
PVP meniscus-closure coatings on ferrous-metal surfaces do not exhibit the comet problem. The PVP coatings are easier to remove by washing with water, and there are lower power requirements during drying as compared with similar polyvinyl-alcohol coatings. Finally, PVP may be used with a variety of plasticizers and inert polymeric materials for modifying the plasticizing and adhesion properties of the final coating.
PVP is a family of film-forming materials and not just a single material. The novel method may employ PVP or copolymers of PVP with other polymeric materials, particularly copolymers with vinyl acetate (PVP/VA). The PVP has a molecular weight of 100,000 to 500,000
but preferably about 200,000 to 300,000 and is present in the coating composition in concentrations of about 2 to 7 weight percent but preferably 3 to 5 weight percent. Above 7 percent, the membranes across the apertures may not rupture. Below 2 percent, the membrane ruptures but insufficient material is deposited on the aperture walls to provide a significant closure of the aperture.
In the preferred coating composition, compatible liquid vehicles such as methyl alcohol, ethyl alcohol, propyl alcohol, or butyl alcohol may be substituted in part for water. The mono alcohols may be substituted in part for water in the formulation to hasten drying. The poly alcohols may be substituted in part for water in various ratios to vary the action at high temperatures and to provide desired properties at these high temperatures. Since alcohols are more expensive than water and may create fire hazards, they are generally not used. Other high-boilin g liquids may be substituted for ethylene glycol in the preferred composition. Suitable high-boiling liquids include glycerine, glycol, and triethylene glycol. Other short-chain hydroxylated vehicles may also be substituted. The presence of many of the foregoing liquids increases the viscosity of the coating composition when substituted for water.
A series of hand-dispensed tests of various compositions of PVP/surfactant/dye with salt/ glycol ratios were run initially to determine a composition for large-batch evaluation. The initial hand-dispensed tests established. that PVP coatings were comet free, gave good aperture closure at about the same weight percent as PVA compositions at about the same viscosity, and required higher resin-to-glycol and resin-to-dye ratios as compared with PVA coating compositions. Also, during these initial PVP studies, several of the other more promising polymeric materials that had similar weightpercent-viscosity properties, that is, had a viscosity of about 40 centipoises at about 3.5 to 5.0 weight percent polymer in the coating composition, were surveyed. Samples of each of the following polymeric materials were found to produce useful results: PVP/vinyl acetate copolymers, and PVP of various molecular weights. A subsequent test with a PVP-coating composition on a pilot-production line established several additional advantages for PVP-coating compositions. A high humidity produced by introducing steam into the coating region reduced edge webbing and trailing-edge problems, and in many cases the partial holes at the mask edge were opened. A PVP coating on a mask is much easier to wash off the mask than a corresponding polyvinyl-alcohol coating. A PVP coating is less prone to webbing and cobwebs than a similar polyvinylalcohol coating. A PVP coating requires less power to dry than a corresponding polyvinyl-alcohol coating.
It was noted that PVP coatings frequently had a mottled appearance to the naked eye. This is attributed to their greater ease in dissolving or dispersing in water. In view of this, additional care should be taken during the coating, processing and storage to keep water droplets away from the dry coating. In addition, it was noted that PVP coatings tend to be brittle and sometimes exhibit poor adhesion. Both of these characteristics may be improved upon by the incorporation of plasticizers and/or copolymers. The coating composition preferably contains a dye which has the effect of absorbing ultraviolet light. This effectively makes the coating opaque to the sensitizing or actinic radiation during a subsequent image-screen-making process. Besides Orange R disclosed above in the Example, any of the following dyes may be substituted: Acid Yellow 38 CI. 25135, Rose Bengal Uvinul VS-49 (GAF Corp.) and Uvinul MS-40 (GAF Corp.). A dye may be present in amounts up to 1.0 weight percent of the coating composition. The presence of a dye usually increases the viscosity of the coating composition. The coating or film may be made opaque to light by including pigment particles, such as carbon, therein.
1. A method for reducing the sizes of apertures in a ferrous metal mask for use in manufacturing a cathoderay tube comprising coating at least one surface of said mask with a waterbased coating composition consisting 6 essentially of a polyvinyl pyrrolidone polymeric material present in said coating composition in concentrations of about 2 to 7 weight percent, and then drying said coating.
2. The method defined in claim 1 wherein said polyvinyl pyrrolidone has an average molecular weight of about 100,000 to 500,000.
3. The method defined in claim 1 wherein said composition has a viscosity of about 20 to 50 centipoises.
4. The method defined in claim 1 wherein said coating composition additionally contains a dye that is absorbing of ultraviolet light and complexes with said polyvinyl pyrrolidone polymeric material.