SEALING GLASS SUSPENSION

Abstract: A sealing glass material comprising a sealing glass frit and a vehicle, the vehicle being an aqueous solution of a cellulosic polymer, and a method of preparing the sealing glass material.

Description: SEALING GLASS SUSPENSION FIELD OF THE INVENTION A suspension of a sealing glass frit in a vehicle free of volatile organic compounds. BACKGROUND OF THE INVENTION Glasses that melt at relatively low temperatures have long been used to join metal, glass and ceramic components by a fusion seal. Initially, such seals were vitreous in nature. Lead glasses, particularly lead zinc borates, were commonly used. The sealing art was revolutionized by discovery that such glasses could be crystallized in a controlled manner by thermal treatment. This combined the virtues of a low melting glass with a strong seal in the crystalline state. More recently, non-lead glasses, composed essentially of tin, zinc, and phosphorous oxides, have been proposed as a substitute for the traditional lead zinc borate glasses. United States Patent No. 5,246,890 (Aitken et al.) discloses examples of such phosphate glasses. In forming a seal with such sealing glasses, it has become customary to use the glass in a particle form known as glass frit. The glass frit is mixed with a vehicle and binder to form a suspension, or paste. This mixture is applied to a sealing surface, for example, by extrusion. Numerous organic vehicles and binders have been proposed. A combination of nitrocellulose and amyl acetate is commonly used in commercial cathode ray tube production. Numerous organic vehicles and binders have been proposed. A combination of nitrocellulose and amyl acetate is commonly used in commercial cathode ray tube production. Recently, environmental concerns have made it desirable to avoid the use of volatile organic compounds (VOCs) where possible. A prominent area of concern is sealing glass paste used in such diverse applications as LCD panels, cathode ray tubes, and lighting products. It is a purpose of the present invention to provide a sealing glass suspension that avoids the use of volatile organic compounds while meeting current process specifications for paste dispersibility, seal strength, electrical characteristics, and hermeticity in a seal. It is a further purpose to provide such a suspension having a smooth consistency. Another purpose is to provide a sealing glass material having Theological characteristics that provide good flow and easy dispensing. Still another purpose is to provide a sealing glass material that has acceptable electrical characteristics when fired to form a seal. In considering the present invention, it is necessary to distinguish between volatile and non-volatile organic compounds. The former characteristically vaporize at or near ambient temperature, thus entering the atmosphere in gaseous, organic form. The latter, which are acceptable, do not vaporize in organic form. Rather, the only material volatilized is water. SUMMARY OF THE INVENTION The present invention resides in part in a sealing glass material that is free of volatile organic compounds, and that comprises a sealing glass frit and an aqueous solution of a cellulosic polymer. The invention further resides in a fusion glass seal that is the fired product of the sealing glass material. BRTEF DESCRIPTION OF THE DRAWINGS The single FIGURE in the accompanying drawing is a graphical representation illustrating characteristics of the inventive materials. PRIOR ART Literature of possible relevance is listed and supplied separately. DESCRIPTION OF THE INVENTION The present invention arose from efforts to eliminate the need for volatile organics in producing a fusion sealing material. It was recognized that any substitute would necessarily have to be compatible with the glass frits currently available. Further, it would be necessary to maintain comparable rheological characteristics in order to permit application of the material with present procedures and equipment. We have found that the desired end can be accomplished by using a water-soluble, cellulosic binder with water or methanol as a solvent. While any soluble cellulosic binder may be employed, we have worked with a side chain functional type supplied by Alpha Metals (AMV). This material contains about 1.25% polymeric solids dispersed in water as solvent. A sealing paste is readily prepared by adding a low melting glass frit to the aqueous cellulosic vehicle. The term "low melting" glass frit is understood in the art to mean a mid-temperature sealing glass that softens to wet a sealing surface and form a seal therewith at a sealing temperature of about 400-450°C. The glass has a softening point on the order of 400-450°C. Commercial usage has grown up around lead borosilicate glasses, generally containing zinc oxide. Such glasses may consist essentially of 70-80% PbO, 10-14% ZnO, 6-12% B 2 0 3 , 0-3% Si0 2 and 0-3% BaO. A typical glass is Code 7590, available from Corning Incorporated and having a composition of about 75% PbO, 12% ZnO, 9% B 2 Oj, 2% Si0 2 and 2% BaO. Recently, non-lead glasses have been proposed. A family of SnO-ZnO-P 2 0s glasses is disclosed, for example, in the Aitken et al. patent referred to earlier. These glasses are lead-free and have compositions containing 25-50 mole % P 2 0, and SnO and ZnO in amounts such that the mole ratio of SnO:ZnO is in the range of 1 : 1 to 5: 1. They may further contain up to 20 mole % of modifying oxides including up to 5 mole % SiO j , up to 20 mole % B 2 0 3 , up to 5 mole % alkaline earth metal oxide, and up to 5 ole % A1 2 0 } ; also one or more crystallization promoters selected from 1 to 5 mole % zircon and or zirconia and 1-15 mole % R 2 0 and a seal adherence promoter selected from up to 5 mole % W0 3 , up to 5 mole % Mo0 3 , up to 0.10 mole % Ag metal and mixtures. The present invention has been developed for use with lead borosilicate glasses. However, it has also been shown to be effective for use with non-lead glasses, such as those disclosed in the Aitken et al. patent. A paste can be readily prepared by adding glass frit to the vehicle. The frit may be dispersed in the vehicle by mixing in a Hobart shaker, although a high speed mixer may be used to form a smoother paste. The ratio of glass frit to vehicle has been varied between 6: 1 and 10: 1 with a smooth paste resulting after mixing. It is then necessary to consider the rheological characteristics necessary for dispensing the paste onto a sealing surface. For dispensing onto a narrow surface, such as the rim of a CRT panel, it is common practice to extrude the paste through a small opening. This may be pressure applied through an orifice, a syringe, or an automatic dispenser. The rheological characteristics of a paste, that is, the viscosity or flow characteristics for dispensing purposes, are measured by employing a Rheometrics Dynamic analyzer. This is a parallel plate mechanism in which one of the plates is oscillated. A material sample is placed between the plates and viscosity is measured as the oscillation rate is increased. The single FIGURE in the accompanying drawing is a graphical representation of viscosity measurements made on the dynamic analyzer. Viscosity is plotted in log poises on the vertical axis; oscillatory frequency is plotted on the horizontal axis in radians/second (rad/s). Curve A displays the rheology of a paste prepared by mixing six parts of Code 7590 glass to one part of the cellulosic vehicle as received. It will be observed that the paste has a high value initially. This decreases as shear is applied, thus indicating a thinner, more fluid paste. Similar measurements were made on a current commercial paste in which the glass frit is dispersed in a vehicle of nitrocellulose dissolved in amyl acetate. This curve was substantially lower than curve A, thus indicating different viscosity characteristics. It was apparent that the material of curve A could not dispense properly using the practice employed for the present commercial material. This created a need to modify the paste in some manner to fit present application practice. Initially, attempts were made to dilute the paste by adding more vehicle. This addition of vehicle showed no significant effect. Rheology curves, corresponding to those shown as curve A, were obtained on the modified materials. They showed no significant change. Further study revealed that the desired lowering of the Theological curve of the paste of curve A can be achieved. Surprisingly, this is accomplished by prediluting the vehicle with solvents such as methanol or water. In effect, this reduces the ratio of polymeric solids in the vehicle to less than one part per hundred. Doubling the amount of solvent, which reduces the polymeric solids to about 0.6% of the vehicle, has been found to produce a paste having a rheology close to that of the present paste. Curve C is based on measurements on a paste prepared with a vehicle diluted with water. The amount of solvent in the vehicle was doubled so that the vehicle contained 0.6% solids in the form of cellulosic polymer. The paste prepared with this diluted vehicle was composed of six parts Code 7590 glass frit and one part of the diluted vehicle. A corresponding mixture in which the vehicle was diluted with methanol rather than water, provided a curve similar in shape to curve C, but slightly higher. Curve D is based on a paste containing ten parts glass frit to one part vehicle. In this case, the vehicle is diluted with methanol to provide 0.6% polymeric solids in the vehicle. A further consideration in producing a fusion seal is the color of the fired seal. A dark color indicates the possible presence of reduced lead. Consequently, a yellow or orange color is deemed desirable, and may be used as a qualitative process control. When a paste with a 6: 1 frit-vehicle ratio was fired, the color of the seal was a drab olive. Electrical testing showed acceptable properties. Nevertheless, it appeared necessary to allay fears by improving the seal color. It was found that higher frit-vehicle ratios gave an improved color. Also, various oxidants can be added in minor amount to improve color. These include peroxides, persulphates, and sulfates. We prefer to use potassium persulphate. We have found that the rheology of the paste can be further adjusted by the addition of a gelling agent to the vehicle before adding the glass frit. In particular, this permits modifying a material to have a higher viscosity at low shear, while still thinning out to a very low viscosity at higher shear. This is highly desirable as it allows easy dispensing of the paste while causing it to set up quickly so that it does not flow or drip after being deposited on a sealing surface. Thus, the rheology of the present paste can be tailored to closely match present practice. A material containing a clay gelling agent has been prepared in the following manner. 100 grams of vehicle, as received, was diluted with 100 grams of water. To this diluted vehicle was then added 7.7 grams of a clay-type rheology modifier. This modifier is available from Rheox, Inc. under the designation Bentone LT. The mixture was then heated at 38°C and mixed with a high-speed disperser for about 90 minutes to produce a homogeneous pregel blend. The sealing paste was then prepared by adding 14.28 grams of the pregel to 36.26 grams of the diluted vehicle. This was stirred rapidly for approximately 20 minutes. To this mixture, 500 grams of Code 7590 frit was then added in small additions. Following addition of the frit, the mixture was stirred to a creamy consistency preparatory to being used for sealing purposes. A series of tests were carried out to determine the effectiveness of the invention in conjunction with non-lead, SnO-ZnO-P 2 0 5 glass frits. A glass was selected having the following composition on a molar basis: 31.5% P 2 O s , 1.5% B 2 Oj, 14.6% ZnO, 51.2% SnO and 0.7% A1 2 0, and 0.5% WO,. Four blends of 74 grams glass frit, 6 grams alumina and 20 grams of zircon each were prepared. Two of the batches were mixed with the cellulose vehicle as received to form pastes. The frit:vehic!e was 4.8: 1 in one mixture and 6.7: 1 in the other. The other two blends were each mixed with vehicle diluted on a 1 : 1 basis with water to form pastes. In each paste the frit:vehicle ratio was 4.8: 1. The Theological characteristics of the four pastes thus produced were analyzed as described above employing the Rheometrics Dynamic analyzer. The two pastes containing vehicle as received produced viscosity curves comparable to those shown by curve A in the drawing. The other two pastes, prepared with the diluted vehicle produced viscosity curves comparable to those shown as curves C and D in the drawing. This demonstrated that the benefits of the invention were obtainable with both traditional lead borate frits and the newer non-lead frits.

Claims: WE CLAIM: 1. A sealing glass material comprising a sealing glass frit and a vehicle, the vehicle being an aqueous solution of a cellulosic polymer. 2. A sealing glass material in accordance with claim 1 wherein the glass frit is a lead borosilicate. 3. A sealing glass material in accordance with claim 1 wherein the glass frit consists essentially of, as calculated from the batch on an oxide basis in weight percent, 70-80% PbO, 10-14% ZnO, 6-12% B 2 0„ 0-3% Si0 2 and 0-3% BaO. 4. A sealing glass material in accordance with claim 3 wherein the glass frit consists essentially of 75% PbO, 12% ZnO, 9% B 2 0„ 2% Si0 2 and 2% BaO. 5. A sealing glass material in accordance with claim I wherein the glass frit is a non- lead, SnO-ZnO-P 2 O s glass. 6. A sealing glass material in accordance with claim 5 wherein the glass frit consists essentially of 25-50 mole % P 2 0, and SnO and ZnO in amounts such that the mole ratio is in the range of 1:1 to 5:1. 7. A sealing glass material in accordance with claim 1 wherein the material comprises the glass frit and the vehicle in a weight ratio between 5: 1 and 10: 1. 8. A sealing glass material in accordance with claim 1 wherein the cellulose porymer solids content in the vehicle is less than 1%. 9. A sealing glass material in accordance with claim 8 wherein the solids content in the vehicle is about 0.6%. 10. A sealing glass material in accordance with claim 1 which additionally contains a gelling agent. 11. A sealing glass material in accordance with claim 10, wherein the gelling agent is a clay. 12. A method of producing the sealing glass material of claim 1 which comprises preparing, as a vehicle, a solution of a water-soluble cellulosic polymer in which the solids content of the cellulosic polymer in the vehicle is less than 1%, and adding to the vehicle a sealing glass frit in an amount such that the frit-to-vehicle ratio is at least 6: 1 , but not over 12:1. 13. A method in accordance with claim 12 wherein an aqueous solution of a cellulosic polymer containing about 1.2% solids is diluted with water or methanol to a solids content less than 1%. 14. A method in accordance with claim 13 wherein the solids content is approximately halved by diluting with water or methanol. 15. A method in accordance with claim 13 wherein a pregel is prepared by adding a gelling agent to a portion of the diluted vehicle, dispersing the clay in the vehicle, adding the pregel to the vehicle with mixing, and adding the glass frit to the mixture of pregel and vehicle. 16. A method in accordance with claim 15 wherein the gelling agent is a clay.

Inventors: DOHN PAUL B, FEWKES EDWARD J, MORENA ROBERT, SMITH FRANCES M, WU LUNG-MING, DOHN, PAUL, B., FEWKES, EDWARD, J., MORENA, ROBERT, SMITH, FRANCES, M., WU, LUNG-MING
Applicants: CORNING INC, DOHN PAUL B, FEWKES EDWARD J, MORENA ROBERT, SMITH FRANCES M, WU LUNG MING, CORNING INCORPORATED, DOHN, PAUL, B., FEWKES, EDWARD, J., MORENA, ROBERT, SMITH, FRANCES, M., WU, LUNG-MING
Application year: 1997
Main classification: C03C8
IPC codes: C03C8