Patent Application
20080022644
The present invention provides for the formation of a controlled pore size and reticulated channel morphology by the use of reticulated foam used a pore former in ceramic articles. According to certain embodiments, a bulk foam material is processed (e.g., ground, shredded or cut, for example) to a desired size, the resulting fragments are incorporated into ceramic batch, and a ceramic article is formed. One preferred method of forming the ceramic articles is by continuous or substantially continuous extrusion. One preferred ceramic article is a honeycomb shaped monolith used for use as a diesel particulate filter.
The reticulated pore former preferably has a three-dimensional structure. The reticulated pore former may be formed by grinding, grating, or shredding a block of flexible reticulated polymer foam at a temperature which is below the polymer's glass transition temperature. Typical polymer materials used to make the reticulated foam are either polyether or polyester urethane, for example. The resulting three-dimensional fragments (see
The reticulated pore formers of the present invention are typically obtained from comminuting a reticulated packing foam material, as shown in
The reticulated pore former is mixed into a powdered ceramic precursor dry batch. The powdered ceramic materials may be any material useful for forming a ceramic matrix material. The ceramic matrix may be selected from the group consisting of cordierite, aluminum titanate, silicon carbide, mullite, silicon nitride and other porous refractory materials. One suitable batch is that used to make cordierite (See Table 1 below) is mixed with up to 30% by volume of the final paste of the reticulated pore former of the present invention along with other processing aids, such as an organic binder and/or a surfactant and/or lubricant. The pore former is preferably mixed into dry batch, and then mixed with the liquids to form a wet batch. The wet batch is then plasticized by high shear mixing and subsequently compressed and de-aired. The plasticized mixture is then formed into a ceramic green body of any desired shape by any suitable ceramic method.
One especially suitable forming process is extrusion. In forming an extruded honeycomb article, the plasticized batch including the reticulated pore former may then be extruded, either by a ram process, single or twin screw extruder, through a honeycomb die to form a honeycomb article. The article may then be fired and plugged by conventional methods to form a diesel particulate filter. A diesel particulate filter includes a number of webs as shown in
The three dimensional nature of the foam fragment skeleton inhibits preferential alignment of the pore former along the flow direction during extrusion. During the mixing and extrusion steps, the structure of the reticulated pore former causes the pore former to tumble but maintains a random disposition with the struts pointing in random directions. Therefore, when the green body of the ceramic article is formed the pore former particles, if large enough, can span from one side of a web to the other. The green body is fired to form a fired ceramic article using a conventional ceramic firing cycle. The heat of the firing step will burn out the pore former leaving a reticulated channel through the web that allows exhaust gasses to flow from one side of a web to the other. Foams having various cell sizes and strut thicknesses are available from foam manufacturers such as Foamex and Crest Foam Industries. Reticulated cell sizes are typically reported in pores per inch (PPI). The higher the PPI value the thinner the dimensions of the struts, and closer packed the overall reticulated network is. For diesel filter applications it is preferred we use as fine a foam as possible with a size of 40-110 PPI, or even 80-110 PPI, or even 100 PPI or greater.
A green honeycomb extrudate including the reticulated pore former was examined under a stereo microscope.
An example of the benefit of reticulated pore formers of the present invention is shown in the following examples. The pore former is incorporated into a ceramic batch, the batch was extruded and fired and subsequently the pore size distribution was measured by mercury porosimetry.
In preparing the following examples, a bulk reticulated foam material was cryo-ground when the bulk foam was immersed in liquid nitrogen for 15-20 seconds and then placed in a food processor fixed with a fine blade grating plate. The size of the foam used was 110 open cell pores per linear inch (PPI). The fragments were then sifted through a coarse screen (10 mesh) and then a fine screen (80 mesh) to remove the very large (greater than 2 mm) and very small (less than 170 microns) fragments to segregate the preferred particle sizes (approximately 1900-200 microns).
A cordierite ceramic batch material was prepared with the composition shown in Table 1. The pore former, constituting approximately 30% by volume of the final dried green body, was mixed into the dry batch with a turbula mixer for 20 minutes. The liquids were added to the dry blend in a muller to mix and shear the batch into a plasticized batch for approximately 20 minutes. The plasticized batch was then loaded into a small hydraulic ram to be compressed and de-aired. The compressed, de-aired plasticized batch was extruded through a diesel honeycomb die having approximately 200 cells per square inch and a web thickness of 16 mils (0.406 mm) to form a green body honeycomb article. The extruded green body article was dried in a hot air oven at 90° C. for 3 days and then fired in a kiln with a schedule of 60° C./h up to 1400° C. where it was held for 15 h. The ware was then cooled at a rate of 200° C./h back to room temperature.
The cordierite honeycomb material was tested to determine pore size distribution.
The invention of this application has been described above both generically and with regard to specific embodiments. Although the invention has been set forth in what is believed to be the preferred embodiments, a wide variety of alternatives known to those of skill in the art can be selected within the generic disclosure. The invention is not otherwise limited, except for the recitation of the claims set forth below.
