Stock material or miscellaneous articles – Composite – Of quartz or glass
Reexamination Certificate
1998-06-29
2001-02-06
Speer, Timothy M. (Department: 1775)
Stock material or miscellaneous articles
Composite
Of quartz or glass
C428S428000, C501S011000, C501S014000, C501S015000, C501S022000, C065S032200, C065S032400, C313S364000
Reexamination Certificate
active
06183871
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to sealing glass pastes, and, in particular, to sealing glass pastes for cathode ray tubes (CRT). Cathode ray tubes are fabricated by sealing together a glass faceplate or panel supporting the phosphorescent display screen of the tube and a glass funnel having an electrically conductive interior coating which forms part of the electronic circuitry of the tube. To seal the components together a sealing glass paste is applied to the edges of one part (either the face panel or the funnel) in the form of a ribbon, which is then dried. The remaining part is then placed in contact with the dried paste and the assembly is heated to evaporate the solvent, burn out the binder and fuse the sealing glass.
2. Description of the Related Art
Wet coating techniques for sealing glass or ceramic parts have been proposed and generally have involved pastes having a sealing glass distributed throughout a binder/solvent system in which the solvent is an organic solvent. When the assembly has been formed it is then heated to evaporated the solvent, burnout the binder and fuse the glass. For example, U.S. Pat. No. 4,014,845 discloses a binder/solvent system which comprises a polymeric material which is a polymer containing oxygen and a monomer of an alkyl methacrylate and a solvent for the polymeric material. For sealing glasses which contain PbO and which melt and flow at low temperatures, preferably at below 475° C., such as PBO—B
2
O
3
—ZnO, the binder/solvent system which has been used commercially comprises nitrocellulose/amyl acetate or similar esters. Nitrocellulose has many disadvantages for use in such systems, but it has been tolerated because it does not cause the dielectric breakdown of the sealing glasses caused by reduction of PbO to Pb.
Other binders which have been tried in place of nitrocellulose do not prevent the reduction of PbO in the sealing glass to Pb metal during the sealing process. Thus, instead of a yellow seal being formed by the devitrified solder glass, a grey or grey-black seal is formed, which indicates the presence of metallic lead. Some of the discoloration may also be due to carbon residues formed on incomplete burn-out of the binder.
U.S. Pat. No. 3,973,975 discloses that the incorporation of Pb
3
O
4
in the sealing glasses containing PbO and the pastes made therewith, prevents the reduction of PbO to Pb. The Pb
3
O
4
is reduced to PbO, but the additional amount of PbO so formed does not affect the properties of the seal. The Pb
3
O
4
also acts to suppress the chemical reduction of the sealing glass by organic vapors during the sealing step.
U.S. Pat. No. 4,154,494 disclosed a method of preventing the chemical reduction of the devitrifiable solder glass by providing an oxygen-evolving agent with the bulb during the sealing step. The oxygen-containing compound is coated onto the funnel in an amount sufficient to suppress chemical reduction of the devitrifiable solder glass during sealing. Thus, the seal resists dielectric breakdown when very high voltages are subsequently applied.
SUMMARY OF THE INVENTION
We have now developed a sealing glass paste which overcomes the problems of the prior art and which comprises an aqueous binder system containing an oxidant, thus overcoming the problems which are encountered with volatile organic solvent system and with lead-containing sealing glasses.
Accordingly, the present invention provides a sealing glass paste which comprises at least one sealing glass frit powder and an aqueous binder which comprises an aqueous solution, dispersion or emulsion of a polymeric material and a water soluble oxidant which decomposes at a temperature of below 475° C., the polymeric material burning out or decomposing at a temperature of below 475° C., in the presence of the oxidant, to leave less than 2% of residues based on the total weight of the solids content of the binder.
DETAILED DESCRIPTION OF THE INVENTION PREFERRED EMBODIMENTS
The water soluble oxidant which is incorporated into the aqueous binder acts to minimize the chemical reduction of the sealing glass, i.e. if the sealing glass contains PbO, the reduction of the PbO to Pb will be minimized or prevented. The water soluble oxidant also assists in the burn-out of the binder during the sealing step and thus assists in preventing the formation of carbonaceous residues.
The water soluble oxidants which are used in the present invention decompose thermally at a temperature below 475° C., preferably at a temperature of below 450° C., thereby donating oxygen under the sealing conditions normally used to seal the components of a cathode ray tube.
Preferred oxidants for use in the present invention are the water soluble nitrates, perchlorates, permanganates and dichromates. Other oxidants which may also be used include the water soluble chromates, persulphates, halogenates (such as bromates, iodates, perbromates, hypochlorates, chorites and chlorates), peroxide salts, bismuthates, and transition metal high oxidation state oxides (such as Ag
2
O, AgCO3 molybdates and vanadates). Sodium potassium, lithium, ammonium or caesium nitrates are the most preferred oxidants for use in the invention.
The water soluble oxidant is generally contained in the aqueous binder in an amount of from 0.05% to 5% by weight, preferably 0.1% to 2% by weight, based on the aqueous binder.
The polymeric materials which may be used in the present invention include a cellulosic material such as methyl cellulose, ethyl cellulose, isopropyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methylhydroxyethyl cellulose, hydroxy-propylmethyl cellulose, carboxymethyl cellulose, carboxymethylhydroxyethyl cellulose, cyanoethyl-cellulose, cellulose acetate, cellulose sulphate or mixtures thereof; guar gum (a non-ionic poly-saccharide), xanthan gum (a polysaccharide), polyvinyl alcohol, polyacrylic acid, polymethacrylic acid, polyacrylate, polymethacrylate, polyvinyl pyrrolidone, polyethylene oxide, poly(alpha-methyl)styrene, polyethylenimine, polyacrylamides, polymeth-acrylamides, polyisopropy-acrylamides, poly-acrylonitrile, polymaleic anhydride, polymaleic acid, poly(ethylene glycol), poly(propylene glycol), polyvinyl acetate, poly(2-methoxyethoxy-ethylene), polyimino(1-oxytrimethylene), polyvinyl-pyridine, polyvinyl acetate, polyoxyethylenes, polymelamine-polyformaldehyde resins, polyurea-poly-formaldehyde resins, naturally occurring poly-saccharides, proteins or polypeptides. Mixtures or copolymers of the above may also be used.
Preferably a polymeric material is selected which leaves a low level of carbonaceous residue in order to keep the level of oxidant required in the aqueous binder to a minimum. The polymeric material may be water soluble and examples of water soluble polymers are the cellulose derivatives. Alternatively, the polymeric material may be in the form of a dispersion or emulsion and in this case it is generally necessary for the dispersion or emulsion to film form on drying.
It will be understood that the present invention may be used with polymeric materials which, in the absence of the oxidant, would give levels of residues of above 2% by weight. However, the present invention also envisages that polymeric materials which give lower levels of residues can be used in the invention, with the oxidant further reducing the level of residues and also reducing the problems of the reduction of lead oxide to lead in sealing glass pastes which are based on lead-containing sealing glasses.
Wetting agents may also be incorporated into the aqueous binder in an amount of up to about 5% by weight in order to improve the wetting of the frit glass powder after mixing. Suitable wetting agents include poly(ethylene glycol) alkyl ethers and silicone additives.
Dispersants may also be added to the aqueous binder formulation in order to improve the shelf life of the resultant glass frit paste. One type of dispersant is a surfactant, such as a polyphosphate, phosphate ester, metal silicate or metal carbonate which may be includ
Brewer Jason Robert
Lee Warren
Fry's Metals, Inc.
Mintz Levin Cohn Ferris Glovsky & Popeo P.C.
Speer Timothy M.
Young Bryant
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