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
1999-09-30
2001-03-27
Derrington, James (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
63, 63, 63, 63, C419S036000, C419S037000, C419S041000
Reexamination Certificate
active
06207101
ABSTRACT:
FIELD OF THE INVENTION
An organosilicon compound is included in formable plasticized powder mixtures that contain binder, solvent for binder, and a component in which at least the binder, its solvent, and the powder materials are insoluble, referred to as the non-solvent. The mixtures are shaped into green bodies and then fired. The organosilicon compound, which can be part of the non-solvent, or separate from the non-solvent, results in a number of advantages depending on how it is used. Among the advantages are increased stiffness and wet strength in the mixture and the green body, shape retention, less heat generation especially at the critical low temperature end of the firing cycle, less heat differential from outside to inside of the body, and significantly increased strength of the body through the low temperature profile of the firing schedule where binder burn-off occurs. On firing, the organosilicon compound converts to an inorganic binder that contributes to the strength and durability of the fired product. The fired bodies of this invention exhibit significantly reduced cracking.
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.
Prior rapid stiffening methods involve time-delayed stiffening using rapid set waxes as disclosed, for example in U.S. Pat. No. 5,568,652. These methods involve extrusion of soft batches. Historically, for highly filled ceramic mixtures, soft batches have lead to better extrusion quality. Attempts to extrude stiffer ceramic batches with the current batch components, i.e. cellulose ether binder, lowering the amount of water and/or additives such as sodium tallowate or sodium stearate have not been very successful because of the higher extrusion pressures resulting from collision of finer particles, and the abrasiveness of the materials involved.
The growing need for thinner webs (1-2 mil)/high density cellular products to be extruded to shape necessitates stiffening at the very instant the batch exits the die.
More recently, the above problems have been solved by forming stiff batches during plasticization increasing the binder to solvent ratio in the batch. This is done by partial removal of the solvent and supplementing the batch with a non-solvent, which is generally organic, to provide fluidity. However, the organics can pose problems during the firing of the green bodies, due to exothermic reactions that can cause cracking in the bodies, resulting in a weakened structure. This is especially true with multicellular structures such as honeycombs. In particular, very thin-walled structures are especially susceptible to cracking during firing.
The present invention fills the need for instantaneous stiffening of batches, which is especially beneficial for extrusion of thin walled honeycombs, and shape retention of extruded bodies at the very instant the batch exits the die, while at the same time providing for minimized exothermic reactions during the firing cycle, increasing the strength of the body during firing, and reducing cracking.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, there is provided a method of making a fired body that involves compounding powder, binder, aqueous solvent for the binder, surfactant, and non-solvent with respect to at least the binder, the solvent, and the powder, with the addition of an organosilicon compound. The non-solvent is lower in viscosity than the binder combined with the solvent, and the amount of solvent is less than the amount that would be used absent the non-solvent. The components are mixed and plasticized and then shaped into a green body, which is then fired to produce the product body. The organosilicon compound enhances the stiffness in the green body afforded by other non-solvents, and results in increased strength and reduced cracking in the fired body, among other advantages.
In accordance with another aspect of the invention, there is provided a body made by the above-described method.
DETAOLEd DESCRIPTION OF THE INVENTION
This invention relates to a method for forming and shaping stiff plasticized powder mixtures into green bodies that are subsequently fired. The mixtures contain a binder, solvent for the binder, surfactant, and a component in which at least the binder, its solvent, and the powder materials are essentially insoluble. This latter component is referred to as the non-solvent, although in actuality, there can be some solubility of the binder and the solvent in the non-solvent. The non-solvent is chosen to result in an increase in stiffness or wet green strength in both the mixture and in the green body that is eventually shaped. Furthermore, this increase in wet green strength occurs without proportional increases in forming pressure or mixing torque. Also, in extrusion of the above mixtures, the shape of the extrudate or green body is retained at the very instant it exits the die, with no time delay.
This invention relates to an approach using an organosilicon compound in the forming mixture. When used either as at least part of the non-solvent or separate from the non-solvent, the organosilicon compound enhances the stiffness and feedrate capability that are achieved as a result of having a non-solvent in the mixture. This material generates less heat during the firing cycle, so that cracking that would ordinarily occur due to rapid loss of the organics, is significantly reduced, and results possibly in reduced total volume of volatile species present. Furthermore, the fired product has greater strength due to conversion of the silicon species to silica which serves as an inorganic binder for the particles.
In historic batches, that is, batches of like composition but without the non-solvent, where there would be more solvent, a mixture or batch of a given composition can be made stiff by removing liquids. But extrusion of such stiff batches results in proportional increase in extrusion pressures and torque with enhanced flow defects such as e.g. swollen or deformed webs (in honeycombs). The method of the present invention enables forming e.g. extrusion of a stiff batch without adversely affecting performance such as pressures, torque, and the flow characteristics. Also, upon firing, crack-free bodies are produced.
The method involves forming a stiff batch instantaneously during the plasticization stage of mixing the batch. Stiff batches are formed during plasticization by increasing the binder to solvent ratio in the batch. This is done decreasing the amount of solvent that would be present in a historic batch, which contributes plasticity to the batch. The batch is supplemented with a component in which at least the binder, the solvent, and the pow
Beall Douglas M.
Chalasani Devi
Merkel Gregory Albert
Peng Y. Lisa
Corning Incorporated
Derrington James
Gheorghiu Anca C.
Herzfeld L. Rita
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