U.S. patent application number 14/886664 was filed with the patent office on 2016-02-11 for ceramic precursor batch composition and method of increasing ceramic precursor batch extrusion rate.
This patent application is currently assigned to Corning Incorporated. The applicant listed for this patent is Corning Incorporated. Invention is credited to Michael Edward DeRosa, Lung-Ming Wu.
Application Number | 20160039718 14/886664 |
Document ID | / |
Family ID | 40316901 |
Filed Date | 2016-02-11 |
United States Patent
Application |
20160039718 |
Kind Code |
A1 |
DeRosa; Michael Edward ; et
al. |
February 11, 2016 |
CERAMIC PRECURSOR BATCH COMPOSITION AND METHOD OF INCREASING
CERAMIC PRECURSOR BATCH EXTRUSION RATE
Abstract
A ceramic precursor batch composition comprising inorganic
ceramic-forming ingredients, a hydrophobically modified cellulose
ether binder having a molecular weight less than or equal to about
300,000 g/mole and an aqueous solvent is provided. The ceramic
precursor batch composition has a ratio of binder to aqueous
solvent of less than about 0.32. The ceramic precursor batch
composition may be used to increase the rate of extrusion of the
composition. A method for increasing a rate of extrusion of a
ceramic precursor batch composition is also disclosed.
Inventors: |
DeRosa; Michael Edward;
(Painted Post, NY) ; Wu; Lung-Ming; (US) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Corning Incorporated |
Corning |
NY |
US |
|
|
Assignee: |
Corning Incorporated
Corning
NY
|
Family ID: |
40316901 |
Appl. No.: |
14/886664 |
Filed: |
October 19, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12275007 |
Nov 20, 2008 |
9174879 |
|
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14886664 |
|
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61004996 |
Nov 30, 2007 |
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Current U.S.
Class: |
501/89 ; 501/1;
501/134 |
Current CPC
Class: |
C04B 35/6365 20130101;
C04B 2235/3222 20130101; C04B 2235/3463 20130101; C04B 2235/349
20130101; C04B 2235/3215 20130101; C04B 2235/3217 20130101; C04B
35/195 20130101; C04B 2235/3232 20130101; C04B 2235/3472 20130101;
C04B 2235/3244 20130101; C04B 2235/3873 20130101; C04B 2235/3817
20130101; C04B 35/478 20130101; C04B 2235/6021 20130101; C04B
2235/3826 20130101; C04B 2235/3248 20130101; C04B 2235/3852
20130101; C04B 2235/3804 20130101; C04B 2235/3481 20130101 |
International
Class: |
C04B 35/478 20060101
C04B035/478 |
Claims
1. A ceramic precursor batch composition, comprising: inorganic
ceramic-forming ingredients; a hydrophobically modified cellulose
ether binder having a molecular weight less than or equal to about
300,000 g/mole; an aqueous solvent; and wherein MC/W is less than
about 0.32, MC is a weight % of the hydrophobically modified
cellulose ether binder based on a 100% of the inorganic
ceramic-forming ingredients, and W is a weight % of water based on
the 100% of the inorganic ceramic-forming ingredients.
2. The ceramic paste composition of claim 1 wherein the
hydrophobically modified cellulose ether binder has a molecular
weight of from about 50,000 g/mole to about 300,000 g/mole.
3. The ceramic paste composition of claim 1 wherein the
hydrophobically modified cellulose ether binder has a molecular
weight of from about 100,000 g/mole to about 200,000 g/mole.
4. The ceramic precursor batch composition of claim 1 wherein the
hydrophobically modified cellulose ether binder has a molecular
weight of from about 200,000 g/mole.
5. The ceramic precursor batch composition of claim 1 wherein the
hydrophobically modified cellulose ether binder has a molecular
weight of from about 100,000 g/mole.
6. The ceramic precursor batch composition of claim 1 wherein the
aqueous solvent is water.
7. The ceramic precursor batch composition of claim 1 wherein MC/W
is less than about 0.22.
8. The ceramic precursor batch composition of claim 1 wherein the
hydrophobically modified cellulose ether binder comprises
methylcellulose, ethylhydroxy ethylcellulose, hydroxybutyl
methylcellulose, hydroxymethylcellulose, hydroxypropyl
methylcellulose, hydroxyethyl methylcellulose,
hydroxybutylcellulose, hydroxyethylcellulose,
hydroxypropylcellulose, sodium carboxy methylcellulose or mixtures
thereof.
9. The ceramic precursor batch composition of claim 1 wherein the
hydrophobically modified cellulose ether binder comprises at least
two hydrophobically modified cellulose ethers having different
molecular weights.
10. The ceramic precursor batch composition of claim 1 wherein the
hydrophobically modified cellulose ether binder comprises at least
two hydrophobically modified cellulose ethers having different
molecular weights and wherein the at least two hydrophobically
modified cellulose ethers have different hydrophobic groups,
different concentrations of the same hydrophobic group or both.
11. The ceramic precursor batch composition of claim 1 wherein the
ceramic precursor batch composition comprises aluminum
titanate-forming ingredients.
12. The ceramic precursor batch composition of claim 1 wherein
ceramic precursor batch composition comprises from about 3 wt % to
about 10 wt % of the hydrophobically modified cellulose ether
binder.
13. The ceramic precursor batch composition of claim 1 wherein the
composition further comprises 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, silicon carbide, silicon nitride, soda
lime, aluminosilicate, borosilicate, soda barium borosilicate or
mixtures of thereof.
Description
RELATED APPLICATIONS
[0001] This is a divisional application of U.S. patent application
Ser. No. 12/275,007 filed on Nov. 20, 2008, which claims priority
to and the benefit of U.S. Provisional Patent Application No.
61/004,996 filed on Nov. 30, 2007, both of which are relied upon
and hereby incorporated by reference for all purposes as if fully
set forth herein.
FIELD
[0002] The present invention relates generally to ceramic precursor
batch compositions, and particularly to ceramic precursor batch
compositions for use in extruding ceramic honeycombs.
BACKGROUND
[0003] Hydrophobically modified cellulose polymers such as
methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC) have
been used as binders in automotive substrate and diesel filter
ceramic precursor batch compositions. These polymers give the batch
the necessary plasticity and green strength in the forming and
drying stages to produce high quality honeycomb ware. However,
polymers such as MC and HPMC can undergo phase separation and
subsequent gelation at a characteristic temperature. At the right
temperature the polymer loses the water that surrounds the pendant
methoxy side groups. This loss of hydration exposes the methoxy
groups and enables hydrophobic associations to occur between the
methoxy substituents of neighboring chains. This leads to phase
separation and ultimately the build up of a long range network gel.
(Sarkar, N., J. Appl. Polym. Sci, 24, 1073-1087 (1979); Methocel
Cellulose Ethers Technical Handbook, Dow Chemical Co.; Li, L et
al., Langmuir, 18, 7291-7298 (2002)). When the binder undergoes
this thermal phase transition within a ceramic precursor batch, the
batch becomes stiffer and the extrusion pressure increases
significantly which can produce severe defects in the extruded
honeycomb structure.
[0004] The thermal transition behavior of polymers like MC and HPMC
can limit the extrusion process of numerous ceramic product lines.
For example, production will have to significantly increase the
extrusion feedrate of new diesel compositions such as aluminum
titanate (AT) and advanced cordierite (AC) in the next 1-2 years
due to increased demand for diesel products. However, the batch
temperature increases with feedrate due to increased shear heating
in the extruder. Ultimately, throughput reaches a limit as the
batch approaches the thermal transition temperature of the
binder.
SUMMARY
[0005] Accordingly, in light of the desire to increase federate, it
would be desirable to have a ceramic precursor batch composition
that allows for a greater extrusion feedrate. Such a ceramic
precursor batch may stiffen at higher temperatures without
sacrificing the properties of the final product such as, but not
limited to, strength.
[0006] One aspect of the invention is a ceramic precursor batch
composition comprising inorganic ceramic-forming ingredients, a
hydrophobically modified cellulose ether binder having a molecular
weight less than or equal to about 300,000 g/mole and an aqueous
solvent, wherein MC/W is less than about 0.32, MC is a weight % of
the hydrophobically modified cellulose ether binder based on a 100%
of the inorganic ceramic-forming ingredients, and W is a weight %
of aqueous solvent based on the 100% of the inorganic
ceramic-forming ingredients. There is an inverse linear
relationship between MC/W and the stiffening onset temperature.
Keeping MC/W less than about 0.32, or even less than 0.22, allows
for increased federates for the composition of the present
invention.
[0007] In another aspect, the present invention includes a method
for increasing a rate of extrusion of a ceramic precursor batch
composition, comprising the steps of providing inorganic
ceramic-forming ingredients, adding a hydrophobically modified
cellulose ether binder and water to the inorganic ceramic forming
ingredients wherein the hydrophobically modified cellulose ether
binder has a molecular weight of less than about 300,000 g/mole and
MC/W is less than about 0.32, wherein MC is a weight % of the
hydrophobically modified cellulose ether binder based on a 100% of
the inorganic ceramic-forming ingredients, and W is a weight % of
the water based on the 100% of the inorganic ceramic-forming
ingredients.
[0008] In yet another aspect, the present invention includes a
method for increasing a rate of extrusion of a ceramic precursor
batch composition, comprising the steps of providing an initial
ceramic precursor batch composition including inorganic
ceramic-forming ingredients, a high molecular weight
hydrophobically modified cellulose ether binder having a molecular
weight of greater than about 300,000 g/mole, and water,
substituting a low molecular weight hydrophobically modified
cellulose ether binder having a molecular weight of less than about
300,000 g/mole for the high molecular weight binder, and adjusting
a ratio of MC/W to be less than about 0.32 wherein MC is a weight %
of the hydrophobically modified cellulose ether binder based on a
100% of the inorganic ceramic-forming ingredients, and W is a
weight % of the water based on the 100% of the inorganic
ceramic-forming ingredients.
[0009] Additional features and advantages of the invention will be
set forth in the detailed description which follows, and in part
will be readily apparent to those skilled in the art from that
description or recognized by practicing the invention as described
herein, including the detailed description which follows, the
claims, as well as the appended drawings.
[0010] It is to be understood that both the foregoing general
description and the following detailed description present
embodiments of the invention, and are intended to provide an
overview or framework for understanding the nature and character of
the invention as it is claimed. The accompanying drawings are
included to provide a further understanding of the invention, and
are incorporated into and constitute a part of this specification.
The drawings illustrate various embodiments of the invention, and
together with the description serve to explain the principles and
operations of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a graphical illustration showing the relationship
between binder molecular weight and the stiffening onset
temperature, according to one embodiment of the present
invention;
[0012] FIG. 2 is a graphical illustration showing the relationship
between MC/W and the stiffening onset temperature, according to
another embodiment of the present invention;
[0013] FIG. 3 is a graphical illustration showing the relationship
between MC/W and the stiffening onset temperature for different
batch compositions, according to one embodiment of the present
invention;
[0014] FIG. 4 is a graphical illustration showing the relationship
between MC/W and the stiffening onset temperature for different
batch compositions, according to one embodiment of the invention;
and
[0015] FIG. 5 is a graphical illustration showing an example of a
capillary temperature sweep of an AT batch, according to the
present invention.
DETAILED DESCRIPTION
[0016] Broadly, the present invention provides a ceramic precursor
batch composition with a higher stiffening onset temperature,
allowing for greater extrusion federates without significant
increases in pressure. The composition may comprise inorganic
ceramic-forming ingredients, a hydrophobically modified cellulose
ether binder having a molecular weight less than or equal to about
300,000 g/mole and an aqueous solvent such as, but not limited to,
water. The ceramic precursor batch composition may have a MC/W less
than about 0.32 where MC is a weight % of the hydrophobically
modified cellulose ether binder based on a 100% of the inorganic
ceramic-forming ingredients, and W is a weight % of water based on
the 100% of the inorganic ceramic-forming ingredients. There is
also provided a method for increasing a rate of extrusion
(feedrate) of a ceramic precursor batch composition, comprising
using the ceramic precursor batch composition of the present
invention.
[0017] The ceramic precursor batch composition of the present
invention uses lower molecular weight hydrophobically modified
cellulose ether binders and a lower MC/W ratio to provide a batch
composition that has a higher stiffening onset temperature, a lower
pressure during extrusion and a greater feedrate than the ceramic
precursor batch compositions of the prior art using higher
molecular weight hydrophobically modified cellulose ethers. It has
been found that the MC/W ratio is inversely proportional to the
stiffening onset temperature of the ceramic precursor batch
composition. Additionally, it has also been found that lower
molecular weight hydrophobically modified cellulose ether binders
are more effectively hydrated, allowing for a lower MC/W ratio,
which results in a higher stiffening onset temperature. Therefore,
a low molecular weight binder may be substituted in a composition
with all other parameters being equal to obtain a higher stiffening
onset temperature. In contrast, the prior art attempts to solve the
problem of lower stiffening temperatures and lower feedrates by
either increasing the MC/W ratio, which can result in a weaker
green body, or by including additional ingredients. The present
invention does not rely on additional ingredients or increased
solvent amounts.
[0018] In accordance with the invention, the present invention for
a ceramic precursor batch composition includes inorganic
ceramic-forming ingredients, a hydrophobically modified cellulose
ether binder having a molecular weight less than or equal to about
300,000 g/mole and an aqueous solvent. The MC/W ratio is less than
about 0.32 where MC is a weight % of the hydrophobically modified
cellulose ether binder based on a 100% of the inorganic
ceramic-forming ingredients, and W is a weight % of aqueous solvent
based on the 100% of the inorganic ceramic-forming ingredients. It
will be appreciated that the weight percents of the binder,
solvent, and other additives are calculated as superadditions with
respect to the inorganic ceramic-forming ingredients by the
following formula:
weight of binder , solvent or other additives weight of inorganic
ceramic - forming ingredients + poreformer .times. 100
##EQU00001##
[0019] The inorganic ceramic-forming ingredients may be cordierite,
mullite, clay, talc, zircon, zirconia, spinel, aluminas and their
precursors, silicas and their precursors, silicates, aluminates,
lithium aluminosilicates, feldspar, titania, fused silica,
nitrides, carbides, borides, e.g., silicon carbide, silicon
nitride, soda lime, aluminosilicate, borosilicate, soda barium
borosilicate or combinations of these, as well as others.
Combinations of these materials may be physical or chemical
combinations, for example, mixtures or composites,
respectively.
[0020] In one exemplary embodiment, the inorganic ceramic-forming
ingredients may yield an aluminum-titanate ceramic material upon
firing. In another exemplary embodiment, the inorganic
ceramic-forming ingredients may be those that yield cordierite,
mullite, or mixtures of these on firing, some examples of such
mixtures being about 2% to about 60% mullite, and about 30% to
about 97% 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.
[0021] One composition, by way of a non-limiting example, 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 33-41, and most preferably about 34-40 of
aluminum oxide, about 46-53 and most preferably about 48-52 of
silica, and about 11-17 and most preferably about 12-16 magnesium
oxide.
[0022] In the practice of the present invention, the ceramic
precursor batch composition comprising the binder system and an
inorganic powder component consisting of a sinterable inorganic
particulate material, e.g., a ceramic powder material, can be
prepared by using the components in any desired amounts
selected.
[0023] The inorganic ceramic-forming ingredients 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.
[0024] The hydrophobically modified cellulose ether binder may be,
but not limited to, 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 cellulose products from Dow Chemical Co.
METHOCEL A4M cellulose is a methylcellulose. METHOCEL F4M, F240,
and K75M cellulose products are hydroxypropyl methylcellulose.
[0025] The properties of preferred cellulose ether binders such as
methylcellulose are water retention, water solubility, surface
activity or wetting ability, thickening of the mixture, dispersing
and lubricating of the inorganic particles, providing wet and dry
green strength to the green bodies, thermal gelation and
hydrophobic association in an aqueous environment. Cellulose ether
binders that promote hydrogen bonding interaction with the solvent
are desirable. Examples of substituent groups that maximize the
hydrogen bonding interaction with polar solvents e.g. water, are
hydroxypropyl and hydroxyethyl groups, and to a smaller extent
hydroxybutyl groups. While not wishing to be bound by theory, it is
believed that lower molecular weight cellulose ether binders, i.e.
less than or equal to about 300,000 g/mole, hydrate faster than
cellulose ether binders having a molecular weight greater than
300,000 g/mole. The faster hydration of the lower molecular weight
cellulose ether binders requires less water, allowing for a stiffer
and stronger composition.
[0026] The hydrophobically modified cellulose ether binders may
have a molecular weight of less than of equal to about 300,000
g/mole. In one exemplary embodiment the cellulose ether binder has
a molecular weight of from about 50,000 g/mole to about 300,000
g/mole. In another exemplary embodiment, the cellulose ether binder
has a molecular weight of from about 100,000 g/mole to about
200,000 g/mole. In a further exemplary embodiment the cellulose
ether binder has a molecular weight of from about 100,000 g/mole or
from about 200,000 g/mole. By way of comparison, commonly used
METHOCEL F220M and METHOCEL F40M (F240) have molecular weights of
374,450 g/mole and 309,500 g/mole respectively while METHOCELS F4M
and F50 have molecular weights of 178,850 g/mole and 75,650 g/mole
respectively.
[0027] As illustrated in FIG. 1, the stiffening onset temperature
(T.sub.onset) is inversely proportional to the molecular weight of
the cellulose ether binder. Historically the term batch "gelation
temperature" has been used to define the point at which the binder
undergoes its thermal phase transition. However, since polymers
such as methylcellulose and hydroxypropyl methylcellulose phase
separate and then subsequently gel, it's not clear which phenomenon
causes the batch viscosity to increase. Therefore, to avoid
confusion the more general term of "onset temperature"
(T.sub.onset) will be used herein to describe the temperature at
which the batch viscosity, or stiffness, begins to increase.
METHOCEL F220M, with the greatest molecular weight has the lowest
T.sub.onset while F50, with the lowest molecular weight has the
highest T.sub.onset. It will be appreciated that the higher the
T.sub.onset, the less likely stiffening will occur during
extrusion, causing increased pressures and decreased feedrates.
[0028] The hydrophobically modified cellulose ether binder may be a
combination of at least two different hydrophobically modified
cellulose ethers, where the average molecular weight of the
combination is less than or equal to about 300,000 g/mole.
Combining a higher molecular weight METHOCEL, such as F40M, with a
lower molecular weight cellulose ether may give a green body with
increased strength over using only a low molecular weight cellulose
ether such as METHOCEL F50. The combination of cellulose ethers may
comprise cellulose ethers having different molecular weights.
Alternatively, the combination of cellulose ethers may comprise
cellulose ethers having different hydrophobic groups, different
concentrations of the same hydrophobic group or other combinations.
Different hydrophobic groups may be, by way of non-limiting
example, hydroxyethyl or hydroxypropyl.
[0029] The hydrophobically modified cellulose ether binder makes
up, as a superaddition, typically about 1-10% by weight, and more
typically about 2-6% by weight of the inorganic ceramic-forming
material.
[0030] The solvent provides a medium for the binder to dissolve in
thus providing plasticity to the batch and wetting of the powders.
The solvent can be aqueous based, which are normally water or
water-miscible solvents; or organically based. Most useful are
aqueous based solvents which provide hydration of the binder and
powder particles. Typically, the amount of aqueous solvent is from
about 20% by weight to about 50% by weight.
[0031] It was unexpectedly found that there is a strong correlation
between the ratio of cellulose ether binder to aqueous solvent
(MC/W) and T.sub.onset wherein T.sub.onset decreases linearly with
increasing cellulose ether binder concentrations (FIGS. 2 and 3).
In contrast, there was correlation of T.sub.onset with the amount
of water present. FIG. 2 illustrates the relationship between MC/W
using METHOCEL F40M in water and aluminum-titanate ceramic-forming
materials. There is about a 20.degree. C. difference between the
highest MC/W ratio of about 0.34 to the lowest of about 0.20. FIG.
3 illustrates the relationship between MC/W using METHOCEL F40M in
water and different inorganic ceramic forming materials where CA-CF
are cordierite compositions comprising silica, alumina, magnesia
made from batches including clay, talc, silica and an alumina
forming source and an optional pore former, and AT is aluminum
titanate ceramic forming material. The T.sub.onset decreases with
increasing METHOCEL concentration across all seven ceramic
compositions tested. This result indicates that T.sub.onset is
primarily driven by the aqueous METHOCEL concentration and that
there is little influence from other batch parameters such as the
chemistry of the inorganic components, particle size distribution,
presence of oil lubricants, or surfactant type such as tall oil,
stearic acid, or Liga (sodium stearate). The fundamental aspect
that determines T.sub.onset appears to be the degree to which the
methylcellulose ether or hydroxypropyl methylcellulose ether
polymer chains are hydrated.
[0032] The correlation between MC/W and T.sub.onset is observed
with other METHOCEL binders having lower molecular weights than
F40M, as illustrated in FIG. 4. Three METHOCELS of different
molecular weights were used, F40M (M1), F4M (M2) and F50 (M3).
Different ceramic precursor materials were also used, such as
aluminum-titanate (AT) and cordierite (Cord). For all METHOCEL
binders, there is a 10-20.degree. C. increase in T.sub.onset when
MC/W is reduced.
[0033] In one exemplary embodiment, the MC/W ratio of the ceramic
precursor batch composition of the present invention is less than
0.32. In another exemplary embodiment, the MC/W is less than 0.27.
In a further exemplary embodiment, the MC/W is less than 0.22.
[0034] Decreasing MC/W results in increasing the amount of aqueous
solvent present. It will be appreciated that increasing the amount
of aqueous solvent present may increase the drying time of the
green bodies formed from a composition with a higher MC/W. However,
using a lower molecular weight cellulose ether binder can result in
an increased T.sub.onset without significantly lowering the MC/W
ratio.
[0035] The ceramic precursor batch composition of the present
invention may further comprise other additives such as surfactants,
oil lubricants and pore-forming material. Non-limiting examples of
surfactants that can be used in the practice of the present
invention are C.sub.8 to C.sub.22 fatty acids and/or their
derivatives. Additional surfactant components that can be used with
these fatty acids are C.sub.8 to C.sub.22 fatty esters, C.sub.8 to
C.sub.22 fatty alcohols, and combinations of these. Exemplary
surfactants are stearic, lauric, oleic, linoleic, palmitoleic
acids, and their derivatives, stearic acid in combination with
ammonium lauryl sulfate, and combinations of all of these. Most
preferred surfactants are lauric acid, stearic acid, oleic acid,
and combinations of these. The amount of surfactants typically may
be from about 0.5% by weight to about 2% by weight.
[0036] Non-limiting examples of oil lubricants are light mineral
oil, corn oil, high molecular weight polybutenes, polyol esters, a
blend of light mineral oil and wax emulsion, a blend of paraffin
wax in corn oil, and combinations of these. Typically, the amount
of oil lubricants may be from about 1% by weight to about 10% by
weight. In an exemplary embodiment, the oil lubricants are present
from about 3% by weight to about 6% by weight.
[0037] In filter applications, such as in diesel particulate
filters, it is customary to include a burnout poreformer in the
mixture in an amount effective to subsequently obtain the porosity
required for efficient filtering. A burnout poreformer 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 inorganic material. If a combination of graphite and
flour are used, the amount of burnout agent is typically form about
10% by weight to about 25% by weight with the graphite at 5% by
weight to 10% of each and the flour at 5% by weight to about 10% by
weight.
[0038] The present invention also provides a method for increasing
a rate of extrusion of a ceramic precursor batch composition,
comprising the steps of providing inorganic ceramic-forming
ingredients, adding a hydrophobically modified cellulose ether
binder and water to the inorganic ceramic forming ingredients
wherein the hydrophobically modified cellulose ether binder has a
molecular weight of less than about 300,000 g/mole and MC/W is less
than about 0.32, wherein MC is a weight % of the hydrophobically
modified cellulose ether binder based on a 100% of the inorganic
ceramic-forming ingredients, and W is a weight % of the water based
on the 100% of the inorganic ceramic-forming ingredients. The
inorganic materials, binder and water are mixed in a muller or plow
blade mixer. The water is added in an amount that is less than is
needed to plasticize the batch. With water as the solvent, the
water hydrates the binder and the powder particles. The surfactant
and/or oil lubricant, if desired, may then be added to the mix to
wet out the binder and powder particles.
[0039] The composition is then plasticized by shearing the wet mix
formed above in any suitable mixer in which the batch will be
plasticized, such as for example in a twin-screw extruder/mixer,
auger mixer, muller mixer, or double arm, etc. Extent of
plasticization is dependent on the concentration of the components
(binder, solvent, surfactant, oil lubricant and the inorganics),
temperature of the components, the amount of work put in to the
batch, the shear rate, and extrusion velocity. During
plasticization, the binder dissolves in the solvent and a gel is
formed. The gel that is formed is stiff because the system is very
solvent-deficient. The surfactant enables the binder-gel to adhere
to the powder particles.
[0040] In a further step, the composition is extruded to form green
bodies. Extrusion is done with devices that provide low to moderate
shear. For example hydraulic ram extrusion press, which is the
preferred device, or two stage de-airing single auger are low shear
devices. A single screw extruder is a moderate shear device. The
extrusion can be vertical or horizontal. Another example of forming
the green bodies is using the same plasticizing twin-screw extruder
as the forming extruder when appropriate forming dies are used, as
a single step process.
[0041] The bodies of this invention can have any convenient size
and shape and the invention is applicable to all processes in which
plastic powder mixtures are shaped. 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 catalytic, adsorption, electrically heated catalysts,
filters such as diesel particulate filters, molten metal filters,
regenerator cores, etc.
[0042] Generally honeycomb densities range from about 235
cells/cm.sup.2 (1500 cells/in.sup.2) to about 15 cells/cm.sup.2
(100 cells/in.sup.2). 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), or about 62
cells/cm.sup.2 (about 400 cells/in.sup.2) each having wall
thicknesses of about 0.1 mm (4 mils). Typical wall thicknesses are
from about 0.07 to about 0.6 mm (about 3 to about 25 mils),
although thicknesses of about 0.02-0.048 mm (1-2 mils) are possible
with better equipment. The method is especially suited for
extruding thin wall/high cell density honeycombs.
[0043] The extrudates can then be dried and fired according to
known techniques. The firing conditions of temperature and time
depend on the composition and size and geometry of the body, and
the invention is not limited to specific firing temperatures and
times. For example, in compositions which are primarily for forming
cordierite, the temperatures are typically from about 1300.degree.
C. to about 1450.degree. C., and the holding times at these
temperatures are from about 1 hour to about 6 hours. For mixtures
that are primarily for forming mullite, the temperatures are from
about 1400.degree. C. to about 1600.degree. C., and the holding
times at these temperatures are from about 1 hour to about 6 hours.
For cordierite-mullite forming mixtures which yield the previously
described cordierite-mullite compositions, the temperatures are
from about 1375.degree. C. to about 1425.degree. C. Firing times
depend on factors such as kinds and amounts of materials and nature
of equipment but typical total firing times are from about 20 hours
to about 80 hours.
EXAMPLES
[0044] The invention will be further clarified by the following
examples.
Example 1
[0045] Batch stiffening onset temperature (T.sub.one). Historically
the term batch "gelation temperature" has been used to define the
point at which the binder undergoes its thermal phase transition.
However, since polymers such as methylcellulose and hydroxypropyl
methylcellulose phase separate and then subsequently gel, it's not
clear which phenomenon causes the batch viscosity to increase.
Therefore, to avoid confusion we will use the more general term of
"onset temperature" (T.sub.onset) to describe the temperature at
which the batch viscosity, or stiffness, begins to increase.
[0046] The batch stiffening temperature of all batch samples was
measured by using the capillary temperature sweep method. A Malvern
RH7 capillary rheometer was used to extrude batch through two OEM
capillary dies made of tungsten carbide. One die has an L/d of 16
(1 mm diameter) while the other is an orifice die with an L/d of
0.25. The batch is extruded at a linear extrudate velocity of 12.7
mm/s at a temperature ramp rate of 1.degree. C./min. FIG. 5 shows
data from a typical temperature sweep test of AT.
[0047] Only the data from the orifice die is used to determine
T.sub.onset because this die almost always produces a flat baseline
pressure during the temperature ramp. This baseline pressure is
essential for how T.sub.onset is defined. Starting at five degrees
above the initial temperature of the scan, the pressure over the
next 15 degrees was averaged. The average pressure over this
15.degree. C. window is termed P.sub.avg. Pressure data from the
first five degrees was not used in order to avoid any pressure
start up effects which could alter P.sub.avg. The fifteen degree
window provides a sufficient amount of data to establish a baseline
pressure. Once P.sub.avg was obtained, a pressure that is 15%
higher than this value was calculated. T.sub.onset is taken to be
the temperature which is at a value 1.15 P.sub.avg. On an extruder
that is operating near the binder gel point, an increase in
extrusion pressure of 15% above a stable pressure is indicative of
a significant change in batch rheology related to the binder
transition.
[0048] The capillary temperature sweep method used to measure
T.sub.onset has several advantages over the conventional method of
using a Brabender mixer. First, the orifice die pressure has much
less noise and fluctuation than typical torque readings produced by
a Brabender. Second, the capillary method measures T.sub.onset of a
batch sample in a fixed shear history state. This can not be done
with the Brabender method because the Brabender shears the batch
during a temperature ramp and therefore the level of specific
mixing energy imparted to the material increases continuously
during the test.
Example 2
[0049] Effect of hydroxypropyl methylcellulose concentration on
T.sub.onset. A review of the literature shows that the findings are
mixed with regard to how the gel point of methyl cellulose and
hydroxypropyl methylcellulose solutions are affected by polymer
concentration. Some reports indicate that the gel temperature of
hydroxypropyl methylcellulose solutions decreases linearly with
increasing concentration up to 10 wt %. Unfortunately these results
have not been reproduced, even after significant efforts to do so.
In fact, the dynamic thermorheological results agree with those
reported in the prior art who saw no dependence of polymer
concentration on the gel points of E, F, and K-type HPMC
solutions.
[0050] Though there have been numerous studies on the gelation of
methyl cellulose and hydroxypropyl methylcellulose solutions, there
has been very little reported on how the concentration of methyl
cellulose and hydroxypropyl methylcellulose affects the stiffening
temperature of a highly filled ceramic batch.
[0051] A study with AT was conducted to determine how binder
concentration affects T.sub.onset in an actual ceramic batch
composition used in production. AT samples were prepared by twin
screw machine in four separate investigations. Two sets of tests
were conducted in the lab where we measured batch stiffening
temperature T.sub.onset of 13 mm diameter rods extruded directly
from a 34 mm twin screw machine. The other two sets of tests were
conducted on samples prepared on a 90 mm production twin screw
machine. The honeycomb ware samples from the 90 mm extruder were
compressed into blocks using a hydraulic press. 13 mm diameter rods
were cored from these blocks to load into the capillary rheometer
for temperature ramp testing.
[0052] Using the 90 mm extruder, F40M METHOCEL binder (Dow Chem.
Co.) was used in all samples and measured T.sub.onset at 3.5, 4.0
and 4.5% METHOCEL and two water calls at each METHOCEL level. As
the level of METHOCEL was reduced, it was observed that the batch
began to stiffen at a higher temperature.
[0053] T.sub.onset clearly showed a linear relationship with
METHOCEL to water ratio in the AT composition. However, production
samples of AT typically have a T.sub.onset value in the low 30's
.degree. C. which is much lower than compositions of cordierite.
T.sub.onset of CA was approximately 20.degree. C. higher than that
of AT even though both compositions use the exact same binder. The
only time such a large difference in stiffening temperature has
been observed is when two different binder chemistries have been
compared such as an A-type methycellulose versus a K-type
hydroxypropyl methylcellulose. It is hypothesized that the large
differences in T.sub.onset could be due to the differences in
METHOCEL concentration.
[0054] Temperature sweep data for a variety of auto and diesel
compositions were used to determine if binder concentration was
responsible for the differences in T.sub.onset. T.sub.onset values
for seven ceramic auto and diesel compositions were examined. All
samples contained F40M METHOCEL and were made with either the 34 mm
twin screw, 90 mm production twin screw, or were plasticized in a
Brabender mixer for 10-20 minutes to simulate the plasticizing step
that occurs in a twin screw machine. FIG. 3 shows a plot of
T.sub.onset for all ceramic compositions versus the METHOCEL/water
ratio.
[0055] The results in FIG. 3 show that there is a strong linear
correlation between T.sub.onset and the aqueous concentration of
binder in the batch. The batch stiffening temperature decreases
with increasing METHOCEL concentration across all seven ceramic
compositions tested. This result indicates that T.sub.onset is
primarily driven by the aqueous METHOCEL concentration and that
there is little influence from other batch parameters such as the
chemistry of the inorganic components, particle size distribution,
presence of oil lubricants, or surfactant type such as tall oil,
stearic acid, or Liga (sodium stearate). The fundamental aspect
that determines T.sub.onset appears to be the degree to which the
methyl cellulose and hydroxypropyl methylcellulose polymer chains
are hydrated.
Example 3
[0056] Effect of binder molecular weight on T.sub.onset. Another
secondary parameter that can impact T.sub.onset is the molecular
weight of the binder. There is one report in the prior art that
shows the impact of the binder molecular weight on the gelation
temperature of a ceramic batch. (Scheutz, J. E. Ceramic Bulletin,
65, 1556-1559 (1986). In this report a Brabender mixer was used to
measure the torque as a function of temperature of alumina batch
samples using K4M and K15M viscosity grade METHOCELS as the binders
at 2.5% and 5% loading. The results of the 2.5% binder test showed
that the lower viscosity (i.e. lower molecular weight) K4M binder
had a gelation temperature 14.degree. C. above that of the higher
molecular weight K15M binder. At 5% binder level, the K4M had a
gelation temperature 8.degree. C. higher than K15M. Since only two
molecular weights were used in this study, there is no way to
determine if there is a well-defined relationship between binder
molecular weight and gelation temperature. In addition to this, the
batch gelation temperatures were not compared on equal mixing
energy basis since mixing energy was not controlled.
[0057] The effect molecular weight has on T.sub.onset of an AT
batch was measured in a controlled experiment using four viscosity
grades of an F-type METHOCEL: F220M, F40M, F4M, and F50 with F220M
being the highest molecular weight and F50 being the lowest. A 4.5%
binder and 16% Emulsia T plus 2% additional water in each sample
were used. All samples were plasticized in the muller prior to
extruding rods for the capillary temperature sweep.
[0058] The results showed that the onset temperature increased with
decreasing binder molecular weight. T.sub.onset has a very
well-correlated dependence on molecular weight as is shown in FIG.
1.
* * * * *