Process for extruding a porous substrate

Abstract

A method is provided for extruding and curing a highly porous substrate. More particularly, the present invention enables fibers, such as organic, inorganic, glass, ceramic, polymer, or metal fibers, to be mixed and extruded to form 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, a catalyst host, a heat exchanger, a muffler, or a catalytic converter.

1 . A method of making a porous substrate, comprising: 
 mixing elongated ceramic-material fibers, binder material, and fluid into a homogeneous mass;    extruding the homogeneous mass into a green substrate; and    curing the green substrate into a porous substrate.    
   
   
       2 . The method according to  claim 1 , wherein the mixing step includes using less than about 40% by volume of ceramic-material fibers.  
   
   
       3 . The method according to  claim 1 , wherein the mixing step includes using less than about 20% by volume of ceramic-material fibers.  
   
   
       4 . The method according to  claim 1 , wherein the mixing step includes using a shear mixer.  
   
   
       5 . The method according to  claim 1 , wherein the mixing step includes mixing in a pore former to form the homogenous mass.  
   
   
       6 . The method according to  claim 1 , further including the step of selecting a plurality of binders, with each binder selected to have a different temperature at which it thermally degrades as compared to the other binder(s).  
   
   
       7 . The method according to  claim 1 , further including the step of selecting the binder to provide sufficient green strength to prevent deformation of the green substrate prior to curing.  
   
   
       8 . The method according to  claim 1 , further including the step of selecting the fibers so that substantially all the fibers, by volume, have an aspect ratio that exceeds 5.  
   
   
       9 . The method according to  claim 8 , wherein the ceramic-material includes fines or shot material.  
   
   
       10 . The method according to  claim 8 , wherein the ceramic-material is substantially free of fines or shot material.  
   
   
       11 . The method according to  claim 1 , wherein the curing step comprises forming bonds between intersecting fibers to form the porous substrate's structure.  
   
   
       12 . The method according to  claim 11 , wherein substantially all of the intersecting fibers are bonded.  
   
   
       13 . The method according to  claim 11 , wherein some of the intersecting fibers are not bonded.  
   
   
       14 . A method of making a porous substrate, comprising: 
 forming inorganic fibers, binder material, and a fluid into a homogeneous mass;    extruding the homogeneous mass into a green substrate; and    curing the green substrate into a porous substrate.    
   
   
       15 . The method according to  claim 14 , wherein the curing step comprises forming bonds between overlapping inorganic fibers to form the porous substrate's rigid structure.  
   
   
       16 . The method according to  claim 14 , wherein the bonds are solid state sintered bonds; liquid-assisted sintered bonds; or glass, glass-ceramic, or ceramic bonds.  
   
   
       17 . The method according to  claim 14 , wherein the curing step comprises burning off substantially all of the fluid and organic material.  
   
   
       18 . The method according to  claim 13 , wherein the inorganic fibers have formed bonds to create an open pore network.  
   
   
       19 . The method according to  claim 14 , wherein the extruding step further comprises pushing the extrudable mixture through a die.  
   
   
       20 . The method according to  claim 14 , wherein the extruding step further comprises pushing the extrudable mixture through a die using a piston or screw extruder.  
   
   
       21 . The method according to  claim 14 , wherein the extruding step is operated at room temperature or an elevated temperature.  
   
   
       22 . A method for making a porous substrate, comprising: 
 selecting a fiber material from Table 1 of  FIG. 6 ;    selecting a binder from Table 2 of  FIG. 6 ;    selecting a fluid from Table 4 of  FIG. 6 ;    processing the fiber material;    mixing the fiber material, binder, and fluid into a homogeneous mass;    adjusting the rheology of the homogenous mass to be extrudable;    extruding the homogenous mass into green substrates; and    curing the green substrates into the porous block.    
   
   
       23 . The method according to  claim 22 , wherein the processing step is at least partially performed in the mixing step, so that the mixing step cuts longer fibers into shorter fibers.  
   
   
       24 . The method according to  claim 22 , wherein the processing step further comprises the step of coating the fibers with organic material to assist in extrusion.  
   
   
       25 . The method according to  claim 22 , wherein the processing step includes making a slurry of the fiber material and fluid, and violently agitating the fiber material to cut longer fibers into shorter fibers.  
   
   
       26 . The method according to  claim 25 , wherein the slurry further includes a dispersant aid, an extrusion aid, or a strengthening aid.  
   
   
       27 . The method according to  claim 22  further including the steps of: 
 selecting additives selected from the group consisting of: pore formers, strengthening agents, opacifiers, extrusion aids, dispersants, pH modifiers, inorganic binders, clays, washcoat materials, and catalyst;    and mixing the additives into the homogenous mass.    
   
   
       28 . A process for curing a green substrate into a porous block, comprising: 
 removing fluid from a green substrate;    burning off organic material;    forming bonds between fibers; and    forming a fibrous open pore network in the substrate.    
   
   
       29 . The curing process according to  claim 18 , wherein the bonds are solid state sintered bonds; liquid-assisted sintered bonds; or glass, glass-ceramic, or ceramic bonds.  
   
   
       30 . The curing process according to  claim 28 , wherein, as organic material is burnt off, the fibers rearrange into an intersecting network.  
   
   
       31 . The curing process according to  claim 28 , further including using inorganic additive material to form a portion of the fibrous open pore network.  
   
   
       32 . A method for making a gradient porous substrate, comprising: forming a first extrudable mixture having a first mixture of fibers, additives, and fluid; 
 forming a second extrudable mixture having a second mixture of fibers, additives, and fluid;    arranging the first extrudable mixture adjacent to the second extrudable mixture in an extruder;    extruding the first and second extrudable mixture into a green substrate; and    curing the green substrate.    
   
   
       33 . The method according to  claim 32 , wherein the curing step comprises forming bonds between intersecting fibers to form the porous substrate's structure.  
   
   
       34 . The method according to  claim 33 , wherein at least some of the bonds are formed from one or more fibers from one extrudable mixture intersecting one or more fibers in the other extrudable mixture.  
   
   
       35 . The method according to  claim 33 , wherein the bonds are solid state sintered bonds; liquid-assisted sintered bonds; or glass, glass-ceramic, or ceramic bonds.  
   
   
       36 . The method according to  claim 32 , wherein the forming and arranging steps further comprise forming the first extrudable mixture into a cylinder shape, and arranging the second extrudable mixture as a concentric layer around the cylinder.  
   
   
       37 . The method according to  claim 32 , wherein the forming and arranging steps further comprise: 
 forming the first extrudable mixture into first pellets;    forming the second extrudable mixture into second pellets;    filling a tube with the first pellets;    surrounding the tube with second pellets; and    removing the tube.    
   
   
       38 . The method according to  claim 37 , further comprising the step of evacuating air from the pellets prior to extruding.  
   
   
       39 . The method according to  claim 32 , further comprising the steps of: 
 forming the first extrudate into a set of first discs;    forming the second extrudate a set of second discs; and    arranging the disks to form a cylinder of alternating first and second discs.    
   
   
       40 . A process for preparing fiber for use in an extruder, comprising: 
 placing bulk fibers in a liquid;    violently mixing the fibers and the liquid to chop the fibers; and    extracting most of the water from the mixture.    
   
   
       41 . The process according to  claim 40 , wherein the fibers are a ceramic fiber selected from the group listed in Table 1 of  FIG. 6 .  
   
   
       42 . The process according to  claim 40 , further including the step of adding a dispersant agent or bonding agent to the liquid.  
   
   
       43 . The process according to  claim 40 , wherein the extracting step further comprises pressing the fibers and liquid against a filter.  
   
   
       44 . The process according to  claim 40 , wherein the extracting step further comprises drying the fibers to remove most free liquid.