Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Producing article having plural hollow channels
Reexamination Certificate
1999-11-23
2001-09-11
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Producing article having plural hollow channels
C264S631000, C264S638000, C264S656000
Reexamination Certificate
active
06287510
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an improved method of firing green structures containing organics. More particularly, the firing method involves slow heating in the early part of the firing cycle to maximize evaporation of volatile organic phases, fast heating in the mid part of the firing cycle to minimize differential heating within the body, and finally slower heating later in the cycle to minimize violence of chemical reactions.
BACKGROUND OF THE INVENTION
Powder mixtures having a cellulose ether binder are used in forming articles of various shapes. For example ceramic powder mixtures are formed into honeycombs which are used as substrates in catalytic and adsorption applications. The mixtures must be well blended and homogeneous in order for the resulting body to have good integrity in size and shape and uniform physical properties. The mixtures have organic additives in addition to the binders. These additives can be surfactants, lubricants, and dispersants and function as processing aids to enhance wetting thereby producing a uniform batch.
A major and ongoing need in extrusion of bodies from highly filled powder mixtures, especially multicellular bodies such as honeycombs is to extrude a stiffer body without causing proportional increase in pressures. The need is becoming increasingly critical as thinner walled higher cell density cellular structures are becoming more in demand for various applications. Thin walled products with current technology are extremely difficult to handle without causing shape distortion.
Rapid-setting characteristics are important for honeycomb substrates. If the cell walls of the honeycomb can be solidified quickly after forming, the dimension of the greenware will not be altered in subsequent cutting and handling steps. This is especially true for a fragile thin-walled or complex shaped product, or a product having a large frontal area.
More recently, attempts to extrude stiffer ceramic batches with the current batch components, i.e. cellulose ether binder, involving use of various organic materials in the forming mixture have been successful. One drawback of using organic materials, however, is that they have to be removed from the shaped green structure during the firing cycle. This generally results in cracking of the body possibly due to pressure build up inside the structure and/or differential heat produced by combustion of the organics in the early stages of firing.
The growing need for thinner webs (1-2 mil)/high density cellular products to be extruded to shape necessitates stiffer batches; and certain organics in the batch contribute to stiffening of the green structures. Therefore, a method to avoid the cracking during organic removal is highly desirable and would be a significant advancement in the art.
The present invention provides such a method.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, there is provided a method of forming an article that involves forming a mixture of components powder materials, organic binder, solvent for the binder, non-solvent with respect to at least the binder, the solvent, and the powder materials, wherein the non-solvent is lower in viscosity than the binder combined with the solvent, mixing and plasticizing, and shaping to form a green structure. The green structure is dried and fired to impart strength and form the product article. The firing is done by heating to a first temperature of less than 200° C. at a heating rate of no greater than 1° C. per minute, heating from the first temperature to a second temperature of 200° C. to 300° C. at a heating rate of 2° C. per minute to 20° C. per minute, heating from the second temperature to a third temperature of above 300° C. to about 500° C. at a heating rate of no greater than 1° C. per minute, and heating from the third temperature to a fourth temperature of greater than about 500° C.
DETAILED DESCRIPTION OF THE INVENTION
This invention relates to a method of making articles by shaping plasticized powder mixtures containing binder, solvent for the binder, a component in which at least the binder is not soluble (non-solvent), followed by firing. It is suitable for plasticized mixtures containing organics such as hydrocarbons.
This invention relates to an improved firing schedule for green articles produced from highly filled mixtures having organics such as hydrocarbons of various volatilization temperatures.
By highly filled mixtures is meant a high solid to liquid content in the mixture. For example, the powder material content in the mixture is typically at least about 45% by volume, and most typically at least about 55% by volume.
The Powder Material
Typical powders are inorganics such as ceramic, glass ceramic, glass, molecular sieve, metal, or combinations of these.
The invention is especially suitable for use with ceramic, particularly with cordierite and/or mullite-forming raw material powders.
By ceramic, glass ceramic and glass ceramic powders is meant those materials as well as their pre-fired precursors. By combinations is meant physical or chemical combinations, e.g., mixtures or composites. Examples of these powder materials are cordierite, mullite, clay, talc, zircon, zirconia, spinel, aluminas and their precursors, silicas and their precursors, silicates, aluminates, lithium aluminosilicates, alumina silica, feldspar, titania, fused silica, nitrides, carbides, borides, e.g., silicon carbide, silicon nitride, soda lime, aluminosilicate, borosilicate, soda barium borosilicate or mixtures of these, as well as others.
Especially suited are ceramic materials, such as those that yield cordierite, mullite, or mixtures of these on firing, some examples of such mixtures being about 2% to about 60% mullite, and about 30% to about 97% cordierite, with allowance for other phases, typically up to about 10% by weight. Some ceramic batch material compositions for forming cordierite that are especially suited to the practice of the present invention are those disclosed in U.S. Pat. No. 3,885,977 which is herein incorporated by reference as filed.
In accordance with a preferred embodiment, one composition which ultimately forms cordierite upon firing is as follows in percent by weight, although it is to be understood that the invention is not limited to such: about 33 to about 41, and most preferably about 34 to about 40 of aluminum oxide, about 46 to about 53 and most preferably about 48 to about 52 of silica, and about 11 to about 17 and most preferably about 12 to about 16 magnesium oxide.
The powders can be synthetically produced materials such as oxides, hydroxides, etc., or they can be naturally occurring minerals such as clays, talcs, or any combination of these. The invention is not limited to the types of powders or raw materials. These can be chosen depending on the properties desired in the body.
Some typical kinds of powder materials are given below. The particle size is given as median particle diameter by Sedigraph analysis, and the surface area is given as N
2
BET surface area.
Some types of clay are non-delaminated kaolinite raw clay, such as Hydrite MP™ clay, or Hydrite PX™ clay, delaminated kaolinite, such as KAOPAQUE-10™ (K10) clay, and calcined clay, such as Glomax LL. All of the above named materials are sold by Dry Branch Kaolin, Dry Branch, Georgia.
Some typical kinds of talc are those having a surface area of about 5-8 m
2
/g, such as supplied by Barretts Minerals, under the designation MB 96-67.
Some typical aluminas are coarse aluminas, for example, Alcan C-700 series, such as C-701™, or fine aluminas such as A-16SG from Alcoa.
One typical kind of silica is that having a particle size of about 9-11 micrometers, and a surface area of about 4-6 m
2
/g, such as IMSIL™ sold by Unimin Corporation.
In filter applications, such as in diesel particulate filters, it is customary to include a burnout agent in the mixture in an amount effective to subsequently obtain the porosity required for efficient filtering. A burnout agent is any particulate substance (not a binder) that burns out
Corning Incorporated
Fiorilla Christopher A.
Herzfeld L. Rita
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