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
1999-04-20
2001-04-24
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, C264S669000, C264S670000
Reexamination Certificate
active
06221308
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of making articles by shaping plasticized powder mixtures containing binder, solvent for the binder, component in which the binder is not soluble (non-solvent), followed by firing. Cracking during firing is minimized or eliminated by use of non-solvent composed of special combination of low and high molecular weight organics. Such a combination results in very stiff extrudable batches, minimizes rapid loss of organics during extrusion, reduces the amount of organics entering the firing kiln, and reduces the exothermic load and soot formation of the kiln. As a result, it reduces the cracking of parts during firing.
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, and/or applying an external field such as an electrical, ultrasonic, or RF field at the die exit. All of 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 including more organic materials in the forming mixture such as disclosed in U.S. application Ser. Nos. 09/115,929, 09/116,144, and 60/095,292, among others. 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 weakening structure. This 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 forming of stiff 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 to reduce the possibility of firing crack formation.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, there is provided a method for producing a fired body that involves compounding the components of powder materials, binder, aqueous solvent for the binder, and non-solvent with respect to at least the solvent, binder, and powder materials. The non-solvent is made up of a high molecular weight organic portion having a molecular weight of greater than 200, and a low molecular weight organic portion having a molecular weight of up to 200. The components are mixed and plasticized to form a plasticized mixture which is then shaped to form a green body. The green body is then dried and fired.
In accordance with another aspect of the invention there is provided a fired body produced by the method described above.
DETAILED DESCRIPTION OF THE INVENTION
This invention relates to a method for forming and shaping stiff highly filled plasticized powder mixtures into green bodies that are subsequently fired. 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 invention is especially applicable to extrusion of multicellular structures such as honeycombs. This invention relates to a method of making articles by shaping plasticized powder mixtures containing binder, solvent for the binder, component in which the binder is not soluble (non-solvent), followed by firing. Cracking during the firing operation is minimized or eliminated by use of non-solvent composed of special compositions in the forming mixtures. The non-solvent component is made up of a high molecular weight organic portion, that is having a molecular weight of greater than about 200; and a low molecular weight organic portion, that is, having a molecular weight of up to about 200.
The combination of high and low molecular weight non-solvents according to this invention results in dual advantages of stiff extrudates that are strong; and a reduction in intensity of the exothermic reactions occurring during the early part of the firing (binder removal region) by minimizing the organics entering the kiln, reducing the exothermic load of the kiln. As a result, cracking in the body is reduced.
Using only light molecular weight oils such as mineral spirits does not result in good lubrication and stiffness, although the these oils can be driven off during drying. The green strength of the extrudate deteriorates especially when cell wall thickness increases.
Using only waxes poses another set of problems. The mixture must be heated during extrusion, and the extrudate must be quenched to obtain the desired stiffness. Because of the relatively small amount of waxes needed, firing of the extrudate does not pose much of a problem.
The combination of high and low molecular weight portions as the non-solvent offers a number of advantages: (1) a wide compositional window of the two portions of organics provides ideal extrusion mixture for lubrication and stiffness; (2) the low molecular weight portion is easily removed by evaporation such as during drying; (3) the high molecular weight organic, even in a lesser amount than has been used previously, provides adhesion between particles after drying, and facilitates handling of dried parts, and provides a temporary medium for the particles to sinter during the early firing stage. (A dried part with all of the organic removed is often weak and easily disintegrates); (4) the lesser amount of high molecular weight organic results in only a relatively low level of exothermic load. The green bodies can be fired successfully.
Historically, 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 pe
Corning Incorporated
Fiorilla Christopher A.
Gheorghiu Anca C.
Herzfeld L. Rita
LandOfFree
Method of making fired bodies does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of making fired bodies, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of making fired bodies will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2470369