Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
1999-01-08
2002-08-06
Yoon, Tae H. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C528S488000, C528S499000, C528S501000, C524S832000, C524S845000, C523S340000, C523S342000
Reexamination Certificate
active
06429285
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is generally in the field of polymer emulsion compositions which are useful as processing aids in the manufacture of various products, and more particularly to compositions which are incorporated into a product during processing and subsequently removed by thermolytic means.
It is a common practice in the manufacture of cathode ray tubes (CRTs) to apply a reflective metal coating on the inside of the phosphor screen, where the metal coating reflects light from the phosphors through the front of the CRT, thereby improving screen brightness and avoiding undesirable light scattering by the CRT funnel assembly. The metal coating ordinarily is a thin layer of aluminum applied by vapor deposition.
In order to form a reasonably flat and reflective metal coating, it is necessary to form a smooth surface over the phosphor particles before application of the metal coating. Frequently, a smooth surface is achieved by applying a temporary polymer layer that is removed after application of the metal layer. Complete removal of the polymer layer, or substrate, is essential for a bright, stable, and high quality screen image.
In one method, a smooth surface can be formed by wetting the inner surface of the phosphor screen, applying an organic solution of a volatilizable polymer, and then evaporating the organic solvent to form a thin polymer film. After the film is dried, a metal, such as aluminum, is deposited onto the surface of the polymer film, and then the entire assembly is baked to volatilize the polymer. While the method gives satisfactory results, the required use of large volumes of organic solvents is undesirable, due to the risk of fire and worker or environmental exposure.
Consequently, the prevalent method in industry for forming the polymer substrate is to coat the dry phosphor screen with a water-based, i.e. aqueous, polymer emulsion at a temperature near or above its minimum filming temperature. Evaporation of the water yields a smooth polymer substrate, which is largely, but incompletely, volatilized by thermal treatment following metallization of the screen. U.S. Pat. No. 3,067,055 to Saulnier Jr., for example, discloses a general method for metallization of phosphor screens using aqueous polymer emulsions. U.S. Pat. No. 3,582,289 to Elmes; U.S. Pat. No. 3,582,390 to Saulnier; U.S. Pat. No. 4,123,563 to Mitobe et al.; U.S. Pat. No. 4,590,092 to Giancaterini et al.; U.S. Pat. No. 4,954,366 to Pezzulo et al.; and U.S. Pat. No. 5,178,906 to Patel et al. disclose additives which are used in concert with the aqueous emulsions to reduce blistering and/or to improve adhesion or other properties of the metallic layer.
The emulsions used for phosphor screen metallization typically are based on acrylic resins, since these polymers depolymerize or otherwise form volatile substances at temperatures below 450° C., the maximum temperature tolerated by the glass panel. Acrylic emulsion compositions, or lacquers, are available with a variety of film-forming temperatures, hardnesses, and particle size distributions, spanning the range of properties that are known to be useful in the metallization of CRT phosphor screens. A commonly used commercial acrylic emulsion is Rhoplex™ B-74 (Rohm and Haas Co., Philadelphia, Pa.), also known as Primal B-74. The use of combinations of acrylic emulsions to enhance the quality of the metallic layer is described in U.S. Pat. No. 4,284,662 to Matsuzaki et al. and U.S. Pat. No. 4,990,366 to Pezzulo et al.
While the use of known aqueous-based polymer emulsions does avoid the problems associated with use of organic solvent-based emulsions, the aqueous-based polymer emulsions available in the art present other disadvantages. It is generally observed that the emulsion-derived substrates do not completely bake out at temperatures up to 450° C., rather they leave a residue or ash, which requires repeated or prolonged bake-out cycles and/or oxygen supplementation in the baking atmosphere. These extra steps are undesirably costly, time-consuming, and raise additional safety issues. Furthermore, a certain quantity of residue may still remain on the phosphor glass, reducing screen brightness.
Various efforts to overcome these deficiencies are described in the art. For example, U.S. Pat. No. 5,145,511 to Patel et al. discloses a method for a combined panel bake/frit sealing cycle, which reduces the carbonaceous residue left behind on the phosphor screen by the emulsion. However, the proposed combined cycle is slower than the conventional cycle, and reduces only the carbonaceous residue, not the other residues that contribute to diminished CRT quality.
U.S. Pat. No. 4,339,475 to Hinosugi et al. describes a method in which the acrylic emulsion is deposited on the phosphor screen while the screen is still wet, in order to minimize the quantity of emulsion required and thereby reduce the quantity of residue after baking-out. However, this method is difficult to use since the miscibility of the acrylic emulsion with the underlying water layer facilitates the diffusion of polymer particles into the water layer. This process results in a polymer substrate of inadequate thickness and uniformity, thereby yielding a metallic layer with diminished reflectance.
U.S. Pat. No. 4,327,123 to Levine et al. describes the use of specific acrylic copolymer emulsions for phosphor screen metallization. However, the emulsions that worked suitably contained anionic surfactants, such as sodium dodecylsulfate, as well as residues from the potassium persulfate initiator. Levine disclosed that syntheses utilizing purely nonionic surfactants or no surfactants did not yield useful emulsions. The emulsions also contained inorganic initiators and/or their decomposition products. Accordingly, the emulsions do not significantly improve over other conventional materials, such as Rhoplex™ B-74, available from Rohm & Haas.
PCT application WO 96/17369 by Adebayo et al. discloses the use of an emulsion of a poly(hydroxyalkanoate) copolymer, poly-3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV or Biopol™), for metallization of a phosphor screen. However, due to its bacterial origin, PHBV emulsion contains biological residues that are nonvolatilizable and cause charring or discoloration after bake-out when this emulsion is used for CRT metallization. Accordingly, the existing emulsion techniques are unsuitable for use in producing CRTs for applications demanding superior brightness, such as high-definition television (HDTV).
It is therefore an object of this invention to provide aqueous polymer compositions for use in manufacturing processes with reduced levels of residue following their thermolytic removal.
It is another object of this invention to provide aqueous polymer emulsion compositions and methods of preparing and using these emulsions, which can be used to produce CRTs having enhanced luminosity.
SUMMARY OF THE INVENTION
Polymer emulsion compositions containing low levels of residue after thermolytic decomposition are provided. The emulsions are aqueous emulsions including polyhydroxyalkanoate and/or acrylic polymers, and can be prepared from commercially available emulsions which are treated to remove non-volatilizable components. The removal process preferably includes filtration and/or centrifugation techniques, and optionally can be conducted in the presence of additives known to be volatilizable under manufacturing conditions, such as in the making of cathode ray tubes (CRTs). Methods for preparing these emulsion compositions are described.
The polymer emulsions are useful in a variety of applications, especially in the metallization of phosphor screens of CRTs, where use of the emulsion yields enhanced CRT luminosity and provides improved ease of manufacture. In a preferred embodiment, the method for metallizing a phosphor screen includes the steps of (a) coating the phosphor screen with the polymer emulsion composition described herein; (b) drying the composition to form a polymer substrate; (c) applying a reflective metallic layer to the subst
Brennan Elaine M.
Horowitz Daniel M.
Metabolix Inc.
Yoon Tae H.
LandOfFree
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