Coating apparatus – With heat exchange – drying – or non-coating gas or vapor...
Reexamination Certificate
2001-10-01
2004-01-27
Crispino, Richard (Department: 1734)
Coating apparatus
With heat exchange, drying, or non-coating gas or vapor...
C118S061000, C118S064000, C427S378000, C425S224000, C425S074000, C264S650000, C264S212000
Reexamination Certificate
active
06682598
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to apparatus and method for drying a coating containing a volatile liquid that has been applied to a support strip that is moved through the apparatus, and more specifically to a drying tunnel for a casting machine for casting ceramic slurry that forms self-supporting tape when dry.
BACKGROUND OF THE INVENTION
The most familiar example of a coating that is applied as a liquid and dried to a solid film is paint. A housepaint is typically formulated from mineral powders that are dispersed in a liquid, called a vehicle, that has a binder such as linseed oil dissolved in a solvent such as turpentine. When the paint is applied to a surface, the solvent evaporates into the atmosphere and leaves behind the mineral powders, or pigments, that are connected into a tough film by the binder, which also helps adhere the film to the substrate surface.
The electronics industry has long used a similar principle to form thin sheets of ceramic materials to use as the insulating portion of structures and devices used in electronic circuits.
U.S. Pat. No. 2,966,719, of J. L. Park, Jr., is the earliest disclosure of forming ceramic “tape” on a continuous support strip. U.S. Pat. No. 6,097,135, of Cappabianca, issued in 2001, discusses the art of “tape casting” from a modern perspective.
The constituents of the ceramic formulation are mixed into a “slurry”, or “slip”, by dispersing them into a vehicle that contains solvent, a binder, which is typically a polymer such as polyvinyl butyral, and handling aids such as wetting agent and “plasticizer”. The solvent should be one that dissolves all of the organic components, is not reactive with any of the components, either organic or inorganic, and has a high vapor pressure so that it dries quickly. In practice, the solvent is frequently a mixture of solvents of slightly differing properties.
The slurry is deposited on a support strip by a “doctor blade”, by curtain coating, or other applicator means. The support strip, with applied coating, is moved along a path that is long enough for the tape to dry by evaporation of the solvent before the tape is stripped from the support strip or is taken up on a storage reel still on the support strip. This dry product, called green tape, can be printed with conductive paste, punched or cut into desired shapes, and stacked and laminated into thick forms. The resulting structure is fired. During firing, the remaining binder and other organics are burned away and the inorganic components undergo melting, sintering, or chemical reaction to form a monolithic ceramic or glassy object.
Because the fired ceramic generally is required to be dense and pore-free for the sake of its electrical properties, a minimal amount of binder in the slurry is desirable. The more binder that must be burned to gas and released from the ceramic during firing, the more porosity and defects the fired article will have. Unfortunately, slurry with a low binder content is more likely to crack as it is dried to green tape.
In 1961, the greatest difficulty with Park's method was drying the tape quickly enough to manufacture the tape in a continuous strip without needing to provide a prohibitively long path for the tape to travel as the solvent dried. Park taught that drying could be accelerated by heating the slurry to a temperature less than the boiling point of the solvent used. “The factors of temperature and rate of drying are controlled by passing heated air in a countercurrent direction to the movement of the coated supporting tape through a drying chamber”. In 2001, Cappabianca teaches, “A major limiting factor of tape casting is the time required to dry the tape. The surface texture of the tape directly relates to the drying rates. This can cause the drying chamber to be as long as 35 meters depending on the drying rate”.
Cappabianca further states, “The evaporation rate is typically regulated by controlling the heating or air flow over the tape. The surface texture of a tape cast material directly relates to the drying rate. If the top surface dries too quickly, a skin may form over the tape resulting in defects. Some of the defects that arise by improper drying include bubbles in the tape, tape distortion, and tape cracking . . . The continuous caster's air flow normally starts at the exit of the drying chamber and flows in the opposite direction of the moving tape”.
Mistler and Twiname, in
Tape Casting: Theory and Practice
, say on page 104, “Casting machines are very simple in design, since they are really only elongated (in some cases) forced-air drying ovens”. On page 113, “Another factor in airflow requirements during the casting of any flammable solvent(s) in the presence of heat is safety. There are published regulations as to the volume of airflow that must be maintained to remain below the lower explosion limit (LEL)”. Page 115, “All fans used on or near the tape casting machine should be explosion-proof if volatile, flammable solvents are being used in the process”.
These references show that blowing heated air over the drying slurry in a countercurrent direction has two serious problems. Firstly, there is a grave safety and environmental hazard produced by mixing volatile (fast-drying) solvents with air. Tape casting operations are notorious for explosion and fire hazard, and workers are typically exposed to vapors of toxic chemicals. The means used to heat the air can add to the explosion danger. Solvent fumes that were once simply vented to the roof are now regulated and must be removed from the air by distillation, filtration, or ignition.
Air is typically blown over the slurry at a speed of 1 to 10 feet per second in order to maintain the solvent vapor at a concentration below the lower explosion limit. As a rule of thumb, the ratio of air mass to solvent vapor mass should be about one hundred. Keeping the concentration (partial pressure) of solvent vapor this low also increases the evaporation rate of the solvent from the surface of the slurry. This leads to rapid formation of the “skin” of binder on the upper surface of the slurry, as mentioned above by Cappabianca. The skin inhibits further evaporation and may cause cracks, wrinkles, wedging, and other defects in the tape.
Secondly, the electronics industry increasingly requires tape that is thicker, more uniform in thickness, and has fewer and smaller defects. At the same time, new materials are being introduced that are more difficult to cast thickly, such as ferrite for inductors. The usual method of using ferrite by the green tape process is to cast it into tapes less than 0.005″ in thickness, which are stacked together to form a layer of the required thickness. Attempting to cast the tape to the desired thickness with industry standard equipment can result in disastrous cracking and other defects.
To help decrease the defects induced by high airflow, many casting machines need to have baffles, louvers, and dampers to create local pockets of air with relatively higher concentration of solvent vapor when casting tapes thicker than about 0.010″ thickness. These devices are adjusted by trial and error and their interactions are not well understood.
Onur et al. (U.S. Pat. No. 5,212,877) teach that use of heated nitrogen instead of air to dry a coating such as magnetic tape avoids explosion and fire hazards, thus allowing the partial pressure of solvent vapor to be greater than the explosion limit in air, thereby moderating the evaporation of solvent and suppressing skin formation. Onur et al. teach discharging the nitrogen from nozzles above and below the support strip; the jets of gas float the strip in the stream to give quick, uniform drying. This method is not suitable for coatings as thick as green ceramic tape. It has been found that these thicker coatings tend to crack when heated from above. Also, rapid jets of gas impinging on the wet slurry would cause irregularity of the thickness because of the rheology of ceramic slurry.
Attempts have been made to use halogenated solvents such
Joly Ernest J.
Steinmueller Paul H.
Crispino Richard
Electronic Circuit Systems
Lazor Michelle Acevedo
Redman Mary Jo
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