Extrudable mixture for forming a porous block

Abstract

An extrudable mixture is provided for producing a highly porous substrate using an extrusion process. More particularly, the present invention enables fibers, such as organic, inorganic, glass, ceramic or metal fibers, to be mixed into a mass that when extruded and cured, forms a highly porous substrate. Depending on the particular mixture, the present invention enables substrate porosities of about 60% to about 90%, and enables process advantages at other porosities, as well. The extrudable mixture may use a wide variety of fibers and additives, and is adaptable to a wide variety of operating environments and applications. Fibers, which have an aspect ratio greater than 1, are selected according to substrate requirements, and are mixed with binders, pore-formers, extrusion aids, and fluid to form a homogeneous extrudable mass. The homogeneous mass is extruded into a green substrate. The more volatile material is preferentially removed from the green substrate, which allows the fibers to interconnect and contact. As the curing process continues, fiber to fiber bonds are formed to produce a structure having a substantially open pore network. The resulting porous substrate is useful in many applications, for example, as a substrate for a filter or catalyst host, or catalytic converter.

Description

RELATED APPLICATIONS [0001] This application claims priority to U.S. provisional patent application No. 60 / 737,237, filed Nov. 16, 2005, and entitled “System for Extruding a Porous Substrate”, which is incorporated herein in its entirety.BACKGROUND [0002] 1. Field [0003] The present invention relates generally to an extrusion processes for extruding a porous substrate, and in one particular implementation to an extrusion process for extruding a porous ceramic substrate. [0004] 2. Description of Related Art [0005] Many processes require rigid substrates for facilitating and supporting various processes. For example, substrates are used in filtering applications to filter particulate matter, separate different substances, or remove bacteria or germs from air. These substrates may be constructed to operate in air, exhaust gases or liquids, and may be manufactured to endure substantial environmental or chemical stresses. In another example, catalytic materials are deposited on the subst...

AI Technical Summary

Benefits of technology

[0013] Briefly, the present invention provides an extrudable mixture for producing a highly porous substrate using an extrusion process. More particularly, the present invention enables fibers, such as organic, inorganic, glass, ceramic or metal fibers, to be mixed into a mass that when extruded and cured, forms a highly porous substrate. Depending on the particular mixture, the present invention enables substrate porosities of about 60% to about 90%, and enables process advantages at other porosities, as well. The extrudable mixture may use a wide variety of fibers and additives, and is adaptable to a wide variety of operating environments and applications. Fibers, which have an aspect ratio greater than 1, are selected according to substrate requirements, and are mixed with binders, pore-formers, extrusion aids, and fluid to form a homogeneous extrudable mass. The homogeneous mass is extruded into a green substrate. The more volatile material is preferentially removed from the green substrate, which allows the fibers to interconnect and contact. As the curing process continues, fiber to fiber bonds are formed to produce a structure having a substantially open pore network. The resulting porous substrate is useful in many applications, for example, as a substrate for a filter or catalyst host, or catalytic converter.

[0016] Advantageously, the disclosed fiber extrusion system produces a substrate having high porosity, and having an open pore network that enables an associated high permeability, as well as having sufficient strength according to application needs. The fiber extrusion system also produces a substrate with sufficient cost effectiveness to enable widespread use of the resulting filters and catalytic converters. The extrusion system is easily scalable to mass production, and allows for flexible chemistries and constructions to support multitudes of applications. The present invention represents a pioneering use of fiber material in an extrudable mixture. This fibrous extrudable mixture enables extrusion of substrates with very high porosities, at a scalable production, and in a cost-effective manner. By enabling fibers to be used in the repeatable and robust extrusion process, the present invention enables mass production of filters and catalytic substrates for wide use throughout the world.

Problems solved by technology

However, this is not a particularly accurate characterization, as a substrate may be quite porous but still have limited permeability if the pores are not generally open and interconnected.

In this way, there are many voids and open spaces within the foam, but since the pores are not connected, the fluid or gas cannot flow from one side of the foam to the other.

However, extrusion of powdered ceramic material has reached a practical upper limit of porosity, and further increases in porosity appear to result in an unacceptably low strength.

For example, as porosity is increased beyond 60%, the extruded ceramic powder substrate has not proven strong enough to operate in the harsh environment of a diesel particulate filter.

In order to increase surface area, extruded ceramic powder substrates have tried to increase cell density, but the increase in cell density has resulted in an unacceptable back pressure to the engine.

Thus, the extruded ceramic powder substrate does not have sufficient strength at very high porosities, and also produces unacceptable back pressure when there is a need for increased surface area.

Accordingly, the extrusion of ceramic powder appears to have reached its practical utility limits.

However, the use of the pleated ceramic paper filters has been sporadic, and has not been widely adopted.

For example, pleated ceramic papers have not effectively been used in harsh environments.

Manufacturing the pleated ceramic papers requires the use of a paper making process that creates ceramic paper structures that are relatively weak, and do not appear to be cost-effective as compared to extruded filters.

Further, the formation of pleated ceramic papers allows very little flexibility in cell shape and density.

For example, it is difficult to create a paper pleated filter with large inlet channels and smaller outlet channels, which may be desirable in some filtering applications.

Accordingly, the use of pleated ceramic papers has not satisfied the requirement for higher porosity filter and catalytic substrates.

However, growing these crystals in-situ requires careful and accurate control of the curing process, making the process difficult to scale, relatively expensive, and prone to defects.

Further, this difficult process only gives a few more percentage points in porosity.

Finally, the process only grows a mullite type crystalline whisker, which limits the applicability of the substrate.

For example, mullite is known to have a large coefficient of thermal expansion, which makes crystalline mullite whiskers undesirable in many applications needing a wide operational temperature band and sharp temperature transitions.

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