Method for improving the stiffness of extrudates

An improvement in a method of extruding a plasticized inorganic powder mixture having a plasticizing organic binder carried in an aqueous vehicle, by passing the mixture through an extruder and then through a die to produce an extrudate. The improvement involves homogeneously blending in the extruder as part of the mixture, carbon dioxide in the supercritical and/or liquid form, to lower the viscosity of the mixture and produce an extrudate that is stiffer in a shorter time than it would be, absent the carbon dioxide, without increasing the extrusion pressure.

This application claims the benefit of U.S. Provisional Application No. 
Ser. 60/019,402 filed Jun. 10, 1996, entitled METHOD FOR IMPROVING THE 
STIFFNESS OF EXTRUDATES, by Devi Chalasani, Ronald E. Johnson and 
Christopher J. Malarkey. 
This invention relates to a method of producing extruded structures from 
highly filled inorganic powder mixtures in which supercritical and/or 
liquid carbon dioxide is homogeneously blended as part of the mixture in 
the extruder. The carbon dioxide so utilized, serves as a diluent in the 
mixture in the extruder reducing the viscosity of the mixture, making it 
softer. As the extrudate exits the die, the carbon dioxide flashes off, 
leaving stiff extrudate. The extrudate is stiffer than would be possible 
without the carbon dioxide. Moreover the increase in stiffness is 
accomplished without increasing the extrusion pressure. 
BACKGROUND OF THE INVENTION 
Powder mixtures having a cellulose ether binder are used in forming 
articles of various shapes. For example ceramic or metal 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 shaped 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 higher pressures. The need is becoming 
increasingly critical as thinner walled 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. 
The present invention fills the need for rapid setting of extruded bodies 
which is especially beneficial for thin walled honeycombs. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the invention, there is provided an 
improvement in a method of extruding a plasticized inorganic powder 
mixture having a plasticizing organic binder carried in an aqueous 
vehicle, by passing the mixture through an extruder and then through a die 
to produce an extrudate. The improvement involves homogeneously blending 
in the extruder as part of the mixture, carbon dioxide in the 
supercritical and/or liquid form, to lower the viscosity of the mixture 
and produce an extrudate that is stiffer in a shorter time than it would 
be, absent the carbon dioxide, without increasing the extrusion pressure. 
DETAILED DESCRIPTION OF THE INVENTION 
This invention relates to a method for rapid stiffening of extrudates 
formed from highly filled plasticized inorganic powder mixtures having a 
plasticizing polymeric organic binder, such as certain cellulose ethers, 
carried in an aqueous vehicle. 
Carbon dioxide is blended in and becomes part of the extrusion mixture in 
the extruder. The carbon dioxide is in the form of either supercritical 
carbon dioxide or liquid carbon dioxide, or combinations of these forms. 
The environment of the mixture must be controlled under conditions of 
temperature and pressure suitable for maintaining the carbon dioxide in 
the desired form. For example, if supercritical carbon dioxide is used, 
the extruder must be maintained at about 88.degree. F. and about 1100 
PSIA, the critical temperature being about 87.8.degree. F., and critical 
pressure being about 1066.3 PSIA. Just below this temperature and 
pressure, carbon dioxide exists as liquid carbon dioxide, and at even 
lower temperatures and pressures it exists in the solid state. The 
physical states of carbon dioxide at various temperatures and pressures 
are given in a Temperature--Entropy Diagram, Form 6244 copyright 1974, by 
Liquid Carbonic Industries Corporation, Chicago, Ill. 
The Powder Material 
Typical powders are inorganics such as metal, ceramic, glass ceramic, 
glass, and molecular sieve, or combinations of these. 
The invention is especially suitable for use with metal powders. Metal 
powder mixtures generally have less vehicle than other, e.g. ceramic 
mixtures and the stiffening effects are therefore more pronounced than 
with mixtures having more vehicle. 
Any sinterable metal or metal composition can be used in the practice of 
the present invention. Especially suited are iron group metal, chromium, 
and aluminum compositions, with the preferred iron group metal being iron. 
Especially preferred is Fe, Al, and Cr. For example, Fe5-20Al5-40Cr, and 
Fe7-10Al10-20Cr powders with other possible additions are especially 
suited. Some typical compositions of metal powders are disclosed in U.S. 
Pat. Nos. 4,992,233, 4,758,272, and 5,427,601 which are herein 
incorporated by reference as filed. U.S. Pat. No. 4,992,233 relates to 
methods of producing porous sintered bodies made from metal powder 
compositions of Fe and Al with optional additions of Sn, Cu, and Cr. U.S. 
Pat. No. 5,427,601 relates to porous sintered bodies having a composition 
consisting essentially of in percent by weight about 5 to about 40 
chromium, about 2 to about 30 aluminum, 0 to about 5 of special metal, 0 
to about 4 of rare earth oxide additive and the balance being iron group 
metal, and unavoidable impurities such as eg., Mn or Mo, with the 
preferred iron group metal being iron. When rare earth oxide is present, 
the special metal is at least one of Y, lanthanides, Zr, Hf, Ti, Si, 
alkaline earth metal, B, Cu, and Sn. When no rare earth oxide is present, 
the special metal is at least one of Y, lanthanides, Zr, Hf, Ti, Si, and 
B, with optional additions of alkaline earths, Cu, and Sn. 
In general the metal and/or metal alloy powders and optionally rare earth 
oxide powders are mixed in amounts to result in the body having the 
desired composition. The starting metal powders are iron, cobalt, nickel, 
chromium, aluminum metals, and special metal powders, if they are to be 
used. The metal can be supplied in either the unalloyed form or alloyed 
with one or more of the other metals, or partially unalloyed and partially 
alloyed. Most typically, however, the iron, when added as the balance, is 
in the elemental form. The chromium can be elemental or alloyed with 
aluminum or iron. Chromium-aluminum alloy is preferable. Typically, the 
aluminum is supplied alloyed with iron and/or chromium for stability. Some 
typical alloy powders that can be used in formulating the mix to yield a 
body having some typical compositions of the present invention are 
Fe--Cr--Al--(Y, lanthanide series elements, Zr, Hf, or Cu) alloy powder, 
Cr--A--(Y, lanthanide series elements, Zr, Hf, or Cu) alloy powder, Fe--B, 
Fe--Si powder, etc. 
In general, the powder material is fine powder (in contrast to coarse 
grained materials) some components of which can either impart plasticity, 
such as clays, when mixed with a vehicle such as water, or which when 
combined with the organic binder can contribute to plasticity. 
By ceramic, glass ceramic and glass ceramic powders is meant those 
materials as well as their pre-fired precursors. By combinations is meant 
physical or chemical combinations, eg., mixtures or composites. Examples 
of these powder materials are cordierite, mullite, clay, talc, zircon, 
zirconia, spinel, aluminas and their precursors, silicas and their 
precursors, silicates, aluminates, lithium aluminosilicates, alumina 
silica, feldspar, titania, fused silica, nitrides, carbides, borides, eg., 
silicon carbide, silicon nitride, soda lime, aluminosilicate, 
borosilicate, soda barium borosilicate or mixtures of these, as well as 
others. 
Especially suited are ceramic materials, such as those that yield 
cordierite, mullite, or mixtures of these on firing, some examples of such 
mixtures being, for example, about 55% to about 60% mullite, and about 30% 
to about 45% cordierite, with allowance for other phases, typically up to 
about 10% by weight. Some ceramic batch material compositions for forming 
cordierite that are especially suited to the practice of the present 
invention are those disclosed in U.S. Pat. No. 3,885,977 which is herein 
incorporated by reference as filed.

In accordance with a preferred embodiment, one composition which ultimately 
forms cordierite upon firing is as follows in percent by weight, although 
it is to be understood that the invention is not limited to such: about 33 
to about 41, and most preferably about 34 to about 40 of aluminum oxide, 
about 46 to about 53 and most preferably about 48 to about 52 of silica, 
and about 11 to about 17 and most preferably about 12 to about 16 
magnesium oxide. 
The powders can be synthetically produced materials such as oxides, 
hydroxides, etc, or they can be naturally occurring minerals such as 
clays, talcs, or any combination of these. The invention is not limited to 
the types of powders or raw materials. These can be chosen depending on 
the properties desired in the body. 
Some typical kinds of powder materials are given below. The particle size 
is given as median particle diameter by Sedigraph analysis, and the 
surface area is given as N.sub.2 BET surface area. 
Some types of clay are non-delaminated kaolinite raw clay, having a 
particle size of about 7-9 micrometers, and a surface area of about 5-7 
m.sup.2 /g, such as Hydrite MP.TM., those having a particle size of about 
2-5 micrometers, and a surface area of about 10-14 m.sup.2 /g, such as 
Hydrite PX.TM., delaminated kaolinite having a particle size of about 1-3 
micrometers, and a surface area of about 13-17 m.sup.2 /g, such as 
KAOPAQUE-10.TM. (K10), calcined clay, having a particle size of about 1-3 
micrometers, and a surface area of about 6-8 m.sup.2 /g, such as Glomax 
LL. All of the above named materials are sold by Dry Branch Kaolin, Dry 
Branch, Ga. 
Some typical kinds of talc are those having a surface area of about 5-8 
m.sup.2 /g, such as supplied by Barretts Minerals, under the designation 
MB 96-67. 
Some typical aluminas are coarse aluminas, for example, Alcan C-700 series, 
such as those having a particle size of about 4-6 micrometers, and a 
surface area of about 0.5-1 m.sup.2 /g, eg., C-701.TM., fine alumina 
having a particle size of about 0.5-2 micrometers, and a surface area of 
about 8-11 m.sup.2 /g, such as A-16SG from Alcoa. 
One typical kind of silica is that having a particle size of about 9-11 
micrometers, and a surface area of about 4-6 m.sup.2 /g, such as IMSIL.TM. 
sold by Unimin Corporation. 
In filter applications, such as in diesel particulate filters, it is 
customary to include a burnout agent in the mixture in an amount effective 
to obtain the porosity required for efficient filtering. A burnout agent 
is any particulate substance (not a binder) that burns out of the green 
body in the firing step. Some types of burnout agents that can be used, 
although it is to be understood that the invention is not limited to 
these, are non-waxy organics that are solid at room temperature, elemental 
carbon, and combinations of these. Some examples are graphite, cellulose, 
flour, etc. Elemental particulate carbon is preferred. Graphite is 
especially preferred because it has the least adverse effect on the 
processing. In an extrusion process, for example, the rheology of the 
mixture is good when graphite is used. Typically, the amount of graphite 
is about 10% to about 30%, and more typically about 15% to about 30% by 
weight based on the powder material. 
Molecular sieves are crystalline substances having pores of size suitable 
for adsorbing molecules. The molecular sieve can be in the crystallized 
form or in the ammonium form or hydrogen form, or ion-exchanged with or 
impregnated with a cation. The molecular sieves can be provided in ion 
exchanged form or impregnated with cations either before forming into a 
body or after the product body has formed. The ion-exchange and 
impregnation methods are well known processes. Such treatments are within 
the scope of this invention. 
Some types of molecular sieves which are preferred for the practice of the 
present invention are carbon molecular sieves, zeolites, 
metallophosphates, silicoaluminophosphates, and combinations of these. 
Carbon molecular sieves have well defined micropores made out of carbon 
material. 
The molecular sieves that are especially suited to the invention are the 
zeolites. Some suitable zeolites are pentasil, such as ZSM-5, Y, such as 
ultrastable Y, beta, mordenite, X, such as 13X, or mixtures thereof. 
The invention is also suited for mixtures that contain activated carbon or 
carbon precursors, e.g. thermosetting resins, that can be later activated. 
The weight percents of the organic binder and vehicle are calculated as 
superadditions with respect to the non-organic solids by the following 
formula: 
##EQU1## 
The Organic Binder 
The organic binder contributes to the plasticity of the mixture for shaping 
into a body. The plasticizing organic binder according to the present 
invention refers to cellulose ether binders. Some typical organic binders 
according to the present invention are methylcellulose, ethylhydroxy 
ethylcellulose, hydroxybutyl methylcellulose, hydroxymethylcellulose, 
hydroxypropyl methylcellulose, hydroxyethyl methylcellulose, 
hydroxybutylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, 
sodium carboxy methylcellulose, and mixtures thereof. Methylcellulose 
and/or methylcellulose derivatives are especially suited as organic 
binders in the practice of the present invention with methylcellulose, 
hydroxypropyl methylcellulose, or combinations of these being preferred. 
Preferred sources of cellulose ethers are Methocel A4M, F4M, F240, and 
K75M from Dow Chemical Co. Methocel A4M is a methylcellulose binder having 
a thermal gel point of about 50.degree. C., and a gel strength of 5000 
g/cm.sup.2 (based on a 2% solution at 65.degree. C.). Methocel F4M, F240, 
and K75M are hydroxypropyl methylcellulose. Methocels F4M and F240 have 
thermal gel points of about 54.degree. C. Methocel K75M has a gel point of 
about 70.degree. C. (all based on a 2% solution in water). 
The organic binder makes up typically about 2-12% by weight, and more 
typically about 2-4% by weight of the mixture. 
The mixture can contain other additives such as surfactants, lubricants, 
dispersants, or other extrusion aids, usually up to about 4% by weight, 
typically about 1% to 4% by weight of the mixture. 
The aqueous vehicle content, which is typically water, can vary depending 
on the type of materials to impart optimum handling properties and 
compatibility with other components in the mixture. The vehicle content is 
less than it would be if the carbon dioxide were not used. For example, 
with water as a vehicle, the amount of water can be reduced by as much as 
15%. As a typical example, and it is to be understood that the present 
invention is not limited to these values, a water content of typically 
about 29% to about 32% by weight without the CO.sub.2 addition would be 
reduced to about 27% to 28% by weight or lower if feasible. 
The mixtures are highly filled. 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 extruder must be one in which the mixture components can be uniformly 
blended with the carbon dioxide. Thus two stage de-airing single auger 
extruder, or a twin screw mixer with a die assembly attached to the 
discharge end are suitable. In the latter, the proper screw elements are 
chosen according to material and other process conditions in order to 
build up sufficient pressure to force the batch material through the die. 
Extrusion temperatures typically range from room temperature to no higher 
than about 60.degree. C. 
The carbon dioxide can be introduced into the extruder in any form that is 
easy to handle. For example, dry ice can be introduced into the extruder. 
However, the mixture in the extruder must be maintained in the pressure 
and temperature range where supercritical and/or liquid carbon dioxide 
exists so that any carbon dioxide in the extruder regardless of how it was 
introduced, will convert to and be maintained as supercritical and/or 
liquid carbon dioxide. Supercritical and/or liquid carbon dioxide serves 
as a diluent to reduce viscosity to make a softer batch, resulting in 
lower extrusion pressures than would be possible for a similar batch 
without those forms of carbon dioxide. Softer mixtures of inorganic 
powders which can be abrasive, extend the life of the extrusion die, even 
with the more abrasive powders. Also, because the addition of carbon 
dioxide enables less water to be used, the drying time is reduced from 
what would be needed without the CO.sub.2 for a given system. At the same 
time there is a sharp increase in stiffness of the extrudate upon exiting 
the extrusion die due to the reduction in pressure as the carbon dioxide 
flashes off the extrudate passes from the extruder environment to ambient 
atmosphere. The resultant expansion of the carbon dioxide at this point 
can be controlled by venting at the die exit so as not to cause 
deformation of the extrudate due to too rapid release of gaseous carbon 
dioxide. 
The bodies according to the present invention can have any convenient size 
and shape and the invention is applicable to all processes in which powder 
mixtures having a cellulose ether binder are extruded and to the products 
made therefrom. However, the process is especially suited to production of 
cellular monolith bodies such as honeycombs. Cellular bodies find use in a 
number of applications such as catalyst carriers, electrically heated 
catalysts, filters such as diesel particulate filters, molten metal 
filters, regenerator cores, etc. 
Some examples of honeycombs produced by the process of the present 
invention, although it is to be understood that the invention is not 
limited to such, are those having about 94 cells/cm.sup.2 (about 600 
cells/in.sup.2), about 62 cells/cm.sup.2 (about 400 cells/in.sup.2), or 
about 47 cells/cm.sup.2 (about 300 cells/in.sup.2), those having about 31 
cells/cm.sup.2 (about 200 cells/in.sup.2), or those having about 15 
cells/cm.sup.2 (about 100 cells/in.sup.2). Typical wall thicknesses are 
for example, about 0.15 mm (about 6 mils) for about 62 cells/cm.sup.2 
(about 400 cells/in.sup.2) honeycombs. Wall (web) thicknesses range 
typically from about 0.1 to about 0.6 mm (about 4 to about 25 mils). The 
external size and shape of the body is controlled by the application, e.g. 
in automotive applications by engine size and space available for 
mounting, etc. Honeycombs having about 15 to about 30 cells/cm.sup.2 
(about 100 to about 200 cells/in.sup.2) and about 0.30 to about 0.64 mm 
(about 12 to about 25 mil) wall thicknesses are especially suited for 
diesel particulate filter applications. This invention is especially 
advantageous for honeycombs having very thin walls, e.g. .ltoreq.0.13 mm 
(5 mils). 
The extrudates can then be dried and fired according to known techniques 
except that drying times will be shorter due to less water in the 
extrudate. 
It should be understood that while the present invention has been described 
in detail with respect to certain illustrative and specific embodiments 
thereof, it should not be considered limited to such but may be used in 
other ways without departing from the spirit of the invention and the 
scope of the appended claims.