Process Engineering
High-strength and
Corrosion-resistant Alumina
Tubes through Extrusion, Part 1
Abstract: During continuous operation under extreme temperatures and high pH value, e.g. in molten glass, thermal protection tubes are characterised by strong wear, the reduction of which was the aim of the presented development. The optimisation of numerous parameters also included the binding agent, which ultimately turned out to
be the decisive key for bringing the bulk density, the Al2O3-content, and the strength to a very high level for extruded alumina components in comparison. However, simultaneously it was possible to significantly reduce the deflection under high-temperature influence and chemical attack becoming apparent through glass level cut corrosion
and loss of wall thickness. It was not possible to extrude any tubes complying with the requirements specification
using different traditional cellulose ether binding agents. This could only be achieved by means of a new development, within the framework of which the exact requirements specification in the extrusion process and the final
application was taken into account. Exemplary action profiles of cellulose ethers in the extrusion process are demonstrated. For the first time the extrusion process enabled the production of parts showing properties (e.g. density,
strength, porosity) which could be realised so far only by isostatic pressing.
In Part 1 of the paper the development approach in general, the special requirements for the binding agent and
tests of different formulated pastes on the capillary rheometer are described.
In Part 2 [ceramic forum international cfi/Ber. DKG 89 (2012) No. 6–7] investigations using sintered samples, the
implementation of the pilot and operating tests as well as product and application tests will be presented.
Keywords: extrusion, alumina, binder, cellulose ether, corrosion resistance, high temperature corrosion, temperature stability
1 Introduction
Technical ceramic components
often are used where other materials
fail, under extreme conditions such
as high wear, high temperature, or
erosion, for example. Non-ceramic
materials fail quicker under such
conditions or are not suitable on the
basis of their worse material properties. In this case, the comparatively
high price of the ceramic components is circumstantial if these result in
significant improvements within the
used system. This first and foremost
pays off when using ceramic components in industrial applications
such as machines and production
processes where lost working days
result in high costs due to frequent
maintenance intervals or repair
work, because the entire production
normally must stand still when a partial process fails.
In the present case, information on
the development of a new class of
Roland Bayer
Dow Wolff Cellulosics GmbH
29699 Bomlitz, Germany
E-mail: rbayer@dow.com
www.dowwolffcellulosics.com
Mirco Lang
W. Haldenwanger Technische Keramik
GmbH & Co. KG
84478 Waldkraiburg, Germany
E-mail: lang@haldenwanger.de
www.haldenwanger.de
cfi/Ber. DKG 89 (2012) No. 5
high-strength, improved corrosionresistant alumina tubes to be used as
protective tube for thermocouples
and corona applications is provided.
When compared to similar products
already on the market (products
A–C), the new protective tubes
(product WH, Fig. 1) achieve higher
densities and strength values in
operational use, resulting in a longer
service life and increased productivity in operational use. The tubes
made by means of extrusion achieve
properties that could only be
achieved by isostatic pressing until
now. However, the extrusion process
provides for a significantly larger
selection of profiles. The use of a
new cellulose ether binding agent
adapted individually to the production circumstances, raw materials,
and recipe formed an essential condition for developing these products.
Fig. 1 Examples for possible geometries
and applications of the newly developed
high-strength, corrosion-resistant alumina
tube. Typical pairs of protective tubes for
temperature measurement in the
high-temperature range are shown in the
foreground; the combinations can be
designed completely or partially
in alumina.
Binder
Process
Quality
2 The selection of the
development approaches
In general, technical development
work in the ceramic industry aims at
one or several of the three following
objectives:
• Increase in quality
• Increase in productivity
• Reduction of the production costs.
Fig. 2 schematically shows that three
options are available for these ob-
Productivity
Costs
Ceramic raw materials
Fig. 2 In order to optimise the quality,
productivity, and costs in the operation
process, three technical options are
available: the optimisation of the process
(process engineering), the ceramic raw
materials, and the binding agent.
E 115
Process Engineering
jectives, whereby these options are
promising when used individually or
in combination:
1. selection of the suitable ceramic
raw materials with the suitable
design properties
2. process optimisation, i.e. the
enhancement of the procedural
design of the process, and
3. optimisation of the binding agent.
Normally, searching for the suitable
selection of ceramic raw materials is
the most familiar and first step for
the ceramicist. It ends with the availability of new and suitable ceramic
raw materials. Within the framework
of the day-to-day routine, the extent
of the second option, the procedural optimisation of the process,
often is limited by a given machine
and equipment park and by the performance of the aggregates specified thus. The third option, the
optimisation of the organic binding
agent(s), is an often disregarded
option which there is only little literature available for. However, this
third option provides in general for
the highest reserve regarding the
enhancement of existing ceramic
systems. Therefore, development
cooperation between the ceramic
industry and suitable binding agent
manufacturers seems natural. The
present case dealt with the increase
of the quality to an extremely high
level regarding chemical resistance
and temperature stability that could
hardly be achieved by extrusion
from a technical point of view up to
now. Densities, porosities, and
strength values as described in the
following were only possible by isostatic pressing up to now. However,
no small cross-sections can be produced by isostatic pressing. Of the
three options described above, the
first and the third, namely an optimisation of the ceramic raw materials and the optimisation of the polymer binding agent, were used in
order to achieve the desired properties.
tubes were to achieve a high level of
corrosion resistance significantly
exceeding current qualities. Accordingly, particular focus was on a very
high level of purity of the raw materials and, thus, on influencing the
grain boundary composition. The
ideal grain shape is achieved by
impurities being eliminated to the
greatest possible extent (e.g. NaCl).
This way, physical properties such as
temperature stability and resistance
against chemical attacks can be controlled. In the end, the sintered final
product must guarantee particularly
high temperature stability during
application and high levels of chemical resistance. Typical operating
conditions are 1800 °C in kilns or
1500 °C hot highly alkaline molten
glass, for example.
3 The development
3.1 The cellulose ether
binding agent
The development work had three
objectives: Initially, high strength
values were to be achieved. For this,
a fine, homogeneous structure with
minimum errors (pores) and high
density is required. Moreover, the
In contrast to plastic clay bodies,
many other, particularly synthetic
ceramic raw materials are characterised by little or no plasticity (ductility). If the recipe does not contain
any plastic clays or if these are not
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Process Engineering
desired due to technical reasons,
only organic binding agents can be
used for extruding complex moulded components for plastification, as
was also conducted in the present
case. These can be used to adjust the
properties of ceramic pastes listed in
the following to the desired size,
amongst others. Some of these
properties can be selected independently, others are coupled to
each other like the gears of a clock
mechanism:
• required water at constant stiffness
of the body (measured with the
penetrometer)
• paste consistency, paste viscositywater retention and homogeneity
of the body under high extrusion
pressures
• Newtonian or pseudoplastic rheological behaviour, thixotropy
• the profile stability of the paste
when escaping from the die (shape
retention)
• the required cohesion between the
particles
• the proper wall lubrication and the
extrusion pressure
• the escape rate of the paste from
the die (screw extruders)
• the resistance of the moist paste
against mechanical deformations,
without the formation of cracks
• the spring back (die swelling at the
nozzle opening)
• the ideal temperature window for
fault-free pasty processing
and many more.
Furthermore, the binding agent can
also be used to control some of the
properties of the extrudates upon
drying and burning, these mainly are:
• the content of salts and heavy
metals (via the purity of the binding agent)
• the propensity of the extrudate to
dry in a crack- and deformationfree manner
• the extent of shrinkage upon drying and burning
• the suitable de-binding temperature
• the strength upon burning.
In the present case, the desired
properties could not be achieved
with conventional cellulose ethers so
that a new development was re-
quired. In this, particular focus was
on three certain properties of the
cellulose ethers the importance of
which will be explained in more
detail in the following on the basis of
examples.
3.1.1 Highest possible
lubricating effect at the
lowest possible used quantity
of auxiliary process agents
A well lubricating ceramic paste can
be applied at a low extrusion pressure, resulting in lower energy consumption within the extruder and in
less wear on die and machine parts.
Good lubrication can easily be set by
using high quantities of different
organic auxiliary process agents,
which will result in a less dense structure, however. In the present case,
the used quantity of the cellulose
ether could be reduced by 30 % and
simultaneously the extrusion pressure could be reduced by 40 % with
a newly developed cellulose ether
binding agent when compared to a
conventional recipe.
Process Engineering
3.1.2 Highest possible purity
3.1.3 Largest possible
temperature window for
extrusion
Methylcelluloses (MC), hydroxypropyl methylcelluloses (HPMC), or
hydroxyethyl
methylcelluloses
(HEMC) in ceramic pastes are characterised by the phenomenon, at a
certain paste temperature, of thermal gelation (for MC) or precipitation (for HPMC and HEMC), above
which lubrication decreases abruptly
(regarding the differentiation between thermal gelation and thermal
precipitation [1]). This increases the
extrusion pressure. Simultaneously,
the paste loses its plastic properties
and embrittles; cracks and other
defects are the result. Therefore, it is
always important to extrude significantly below the gelation or precipitation temperatures during the
extrusion process. The level of the
precipitation or gelation temperatures is determined by numerous
influencing variables, two essential
variables of which are the degree of
substitution and the used quantity of
the cellulose ether.
Fig. 3 shows the drop of the gelation
temperature with an increase of the
cfi/Ber. DKG 89 (2012) No. 5
65
Extrusion pressure [bar]
Extrusion Pressure (bar)
60
2 pph
55
5 pph
50
45
40
2.5 pph
35
2 pph A4M
2.5 pph A4M
3 pph A4M
4 pph A4M
5 pph A4M
30
3 pph
25
20
10.0
4 pph
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
Temperature
of
extruded
profile
[°C]
Tem peratur
e of
extruded
prof
ile (°C )
Fig. 3 Illustration of the phenomenon of thermal gelation of a methylcellulose (Methocel™ A4M, rated viscosity 4000 mPa · s) at different used quantities in an extruded,
ceramic paste. The type of ceramic raw material is not important in this. The arrows
indicate the gelation temperature of the paste above which faults occur. (Recipe:
100 parts of cordierite Imerys 820M; 30,5 parts of water, as well as Methocel™ A4M in
the specified quantities, other bodies such as Al2O3 behave analogously); the used
quantity of the cellulose ether is indicated in pph (= parts per hundred) referring to
100 parts of cordierite in each case.
65
Extrusion pressure (bar)
Extrusion pressure [bar]
Cellulose ethers with different levels
of purity are available in the market.
From the ceramicist’s point of view,
the content of alkaline, alkaline
earth, and heavy metals plays the
largest role. The cellulose ether
manufacturer can influence the content of metal ions within the final
product by selecting its raw materials and via the production process.
It clearly differs regarding the product qualities offered on the market,
covering a range from technical
qualities to high-purity pharmaceutical qualities. The purity of a product
should be selected depending on
the profile of requirements. A too
high content of metal ions within
the final product may result in corrosion within the kiln, in high-temperature corrosion within the final
product, may have adverse effects
on the efficiency in catalysts, may
cause structure faults and disturbances in crystal phases, or affect the
ion conduction negatively. Foreign
ions may accumulate in the glass
phase between the crystallites and,
thus, they constitute either a decrease in strength of the structure or
a weak point for the high-temperature and other corrosion mechanisms.
2 pph K4M
60
2 pph F4M
55
2 pph H EM C ,D S (M e)1,83;
M S (H E)0,50;5700 m Pas
2 pph M TX 4000
50
2 pph A4M
45
40
35
30
25
887-1
20
15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
Extrusion temperature [°C in the extrudate]
Extrusion tem perature (°C in the extrudate)
Fig. 4 Dependence of the gelation temperature and/or the precipitation temperature
(see arrows) on the chemical substitution of different cellulose ether types (MC, HPMC,
and HEMC, all with a rated viscosity of 4000 mPa · s) when being used in the same
used quantity of 2 pph in each case (recipe as in Fig. 3). The arrows indicate the gelation and/or precipitation temperatures of the paste above which faults occur. There
are huge differences regarding the gelation temperature, but also regarding the pressure and the gradient of the straight line caused by temperature dependence of the viscosity of cellulose ethers in watery solution. The gelation temperature increases from
Methocel™ A4M (43 °C) via Methocel™ F4M (61 °C), Walocel™ MTX 4000 (63 °C),
and via a highly substituted HEMC (84 °C) up to Methocel™ K4M (86 °C).
used quantity of an MC (MethocelTM A4M) in an exemplary manner using an extrudable ceramic
paste. With an increasing used quantity the lubricating effect at low temperatures is enhanced significantly,
which is indicated by the decreasing
extrusion pressure. In the presented
recipe, the pressure minimum is
reached at approx. 4 pph (pph =
parts per hundred, parts cellulose
ether on 100 parts dry ceramic powder); higher used quantities result in
a slight increase of the extrusion
pressure. It is possible that this effect
is caused by the increasing adhesiveness of the paste. However, the
advantage of the good lubrication is
bought by a drop in the gelation
temperature. When using a quantity
of 2 pph, it still is 43 °C, when using
2,5 pph, it already is 35 °C, when
using 3 pph, it is 31 °C, when using
4 pph, it is 26 °C, and when using
5 pph, it already reaches room temperature (21 °C). Therefore, successful extrusion under the described
conditions with 5 pph Methocel
E 119
Process Engineering
Tab. 1 Chemical and physical properties of the cellulose ether binding agents
tested on the capillary rheometer (HPMC = hydroxypropyl methylcellulose,
HEMC = hydroxyethyl methylcellulose).
Cellulose Ether
Binding Agent
Product 1
Product 2
Product 3
Product 4
HPMC
HPMC
HPMC
HEMC
Newtonian
low
medium
high
Gelation,
flocculation
temperatures
medium
high
high
high
Chemical purity
technical
pure
pure
ultra-pure
Product class
Shear thinning
in solution/paste
Lefttpr
pressure,
high,
moisture
13,18
Lef
essure,hi
gh,m
oisture 13,
18%%
Right
pressure,
R
ightpr
essure,high,
high,moisture
m oisture 13,18
13,18%%
7
Lefttpr
pressure,
middle,
moisture
13,66
Lef
essure,m
iddle,m
oisture 13,
66%%
Right
pressure,
moisture
13,66
R
ightpr
essure,middle,
m iddle,m
oisture 13,
66%%
6
Lefttpr
pressure,
low,
moisture
Lef
essure,l
ow,m
oisture 13,54
13,54%%
Pressure
[MPa]
Pressur
e (M Pa)
Right
pressure,
R
ightpr
essure,low,
low,moisture
m oisture 13,54
13,54%%
5
Lefttpr
pressure,
newtonian,
moisture
13,29
Lef
essure,new
tonian,m
oisture 13,
29%%
Right
pressure,
moisture
13,29
R
ightpr
essure,newtonian,
new tonian,m
oisture 13,
29%%
4
3
2
1
0
0
20
40
60
80
100
Sam plnumber
e num ber
Sample
Fig. 5 Shear ramp profile of pastes of the four tested cellulose ethers on the
capillary rheometer. A shear ramp profile modelling the shear rates occurring
within the die was selected.
A4M could only be carried out at
temperatures significantly lower
than 20 °C. Using mixed ethers provides a remedy here, such as HEMC
or HPMC or mixtures of different cellulose ethers as have already been
described in the literature (e.g. [2]).
Fig. 4 shows the range of gelation
temperatures that may result subject
to the chemical substitution of the
cellulose ether at a used quantity of
2 pph in each case. The Methocel™
A4M curve is identical to the one of
Fig. 3 due to reasons of improved
comparability.
3.2 Tests of different cellulose
ether binding agents on the
capillary rheometer
Since the plastic and shear-thinning
properties of an Al2O3 extrusion
body essentially depend on the
selected binding agent, the development initially concentrated on this.
Initially, four Al2O3 pastes containing
four different cellulose ether binding
agents were subjected to tests on
the capillary rheometer. The wide
range of available cellulose ethers
E 120
was limited by selecting cellulose
ethers with different shear thinning,
chemical substitution, precipitation
temperature, and chemical purity
(Tab. 1). The shear thinning of the
tested cellulose ethers covered a
range from a virtually Newtonian
behaviour (virtually no shear thinning) via low up to high shear-thinning behaviour. A wide range of
products was tested regarding
chemical purity and precipitation
temperature as well. In order to
repeatedly obtain a body that can be
processed and extruded well despite
using cellulose ethers with very different rheological profiles, the content of binding agent and the addition of water must be adapted individually. For example, this may be
implemented applying an “isoviscous approach” [3, 4]. Within the
framework of the isoviscous approach, differently formulated ceramic pastes are brought to the same
paste viscosity. If a high cellulose
ether viscosity was used to plasticise
the ceramic paste, a lower used
quantity is required in order to
adjust a certain paste viscosity. If a
low cellulose ether viscosity was
used to plasticise the ceramic paste,
a higher used quantity is required in
order to adjust the same paste viscosity. Regarding the specified shear
rate of the used viscosimeter or
rheometer, all pastes have the same
viscosity; regarding lower or higher
shear rates, as they occur at other
points of the process (e.g. in the nozzle opening), these pastes show a
very different rheological behaviour,
however.
All paste recipes used in the following were adjusted virtually isoviscous
in accordance with the described
procedure. Regarding the rheological examination on the capillary
rheometer (Haake Rheo Cap T100,
Thermo Electron Corporation), a shear
ramp profile was selected (Fig. 5)
reflecting the typical shear rates during extrusion resulting from the feed
rate and the nozzle geometry.
The measured differential pressures
on the nozzles (“left” long nozzle
and “right” zero nozzle) were used
to calculate the viscosity subject to
the shear rate and/or shear force.
Wall sliding effects, as well as structure-viscous behaviour over wide
shear rate ranges not taken into
accounts within the framework of
the potency approach can be calculated using Δp of the two nozzles
(Bagley correction, Weißenberg-Rabinowitsch correction, pursuant to
[5]).
As explained, the shear viscosity η is
calculated subject to the rate drop
according to the differential pressure
measured on the two nozzles. The
nozzle geometry is also incorporated
into the calculation:
η=
τw
γ& w
mitwhere (1)
τw = Δp · d wall shear stress (2)
4·L
γw = 32 · V3 wall shear rate (3)
π·d
Δp = Δpmess – ΔpE
Δp is the differential pressure after Bagley
correction
V = volume flow
L = nozzle length
d = nozzle diameter
On the basis of the WeißenbergRabinowitsch correction, the determined wall shear rate is corrected in
order to take into account the Newtonian flow behaviour regarding the
calculated viscosity curves. The correction is implemented according to
the following approach:
cfi/Ber. DKG 89 (2012) No. 5
Process Engineering
ൌ ఊሶ
ఛೢ
where
m it (4)
1000
ଵ
shear
K orrigier
te Sc
ߛሶ ௞௢௥௥ ൌ ସ ߛሶ௪ ሺ͵ ൅ ܵሻcorrected
rate
ܵ ൌ
high,
%
high,moisture
m oisture13,18
13,18%
ೖ೚ೝೝ
ௗሺ୪୥ ௏ሶ ሻ
ௗሺ୪୥ ο௣ሻ
WeißenbergW eißenberg-R
Rabinowitsch
correction
The described corrections are stored
to the device software and were
taken into account regarding the
following analyses.
Observing the course of the pressing
pressure as function of the extrusion
velocity provides some indication
regarding the flowability and the
occurring pressures at the die escape
(Fig. 5). By means of grading the
feed rate in ramps it is possible to
observe velocity-dependent effects.
The curves illustrate the different
modes of action of the binding
agents in a virtually isoviscous test
set-up. For example, the course of
the curve of the Newtonian type
demonstrates a clear step profile
with distinct, level stages.
This is indicative of a homogeneous
plastic body with good extrusion
properties. In contrast to the aforementioned, the medium shear-thinning binding agent shows a poor
meas-ured result. The course of the
curve is edgy and not pronounced in
steps. Extrusion errors such as the
formation of surface flaws, turbulent
body flow, and string rupture must
be expected. The low and high
shear-thinning types demonstrate a
distinctive step profile in turn. The
absolute pressures at the outlet nozzle is indicative of the pressing pressure to be expected at the extruder.
The analysis shown in Fig. 6 illustrates the force required to deform
the body regarding the selected
shear rate profile. Thus, it is obvious
that the material with the high
shear-thinning binding agent has
the lowest yield point. The variant
with the Newtonian type has the
highest in contrast.
The graph in Fig. 7 can be used to
assess the shear-thinning behaviour
of the material. On the basis of the
cfi/Ber. DKG 89 (2012) No. 5
True
shear
viscosity
[Paty· (s]
True
Shear
Viscosi
Pa.s)
௞௢௥௥
middle,
%
m iddle,moisture
m oisture13,66
13,66%
low,
%
low,moisture
m oisture13,54
13,54%
newtonian,
%
new tonian,moisture
m oisture13,29
13,29%
100
10
1
10
Shear stress [kPa]
Shear Stress (kPa)
100
Fig. 6 Shear viscosity as function of the shear stress of the four different pastes.
high,
%
high,moisture
m oisture13,18
13,18%
1000
middle,
%
m iddle,moisture
m oisture13,66
13,66%
low,
%
low,moisture
m oisture13,54
13,54%
Trueshear
Shear
Viscosi
ty (·Pa.
True
viscosity
[Pa
s] s)
K
newtonian,
%
new tonian,moisture
m oisture13,29
13,29%
100
10
1
10
100
1000
Apparent
shear
rate
[se–1(]/s)
Apparent
Shear
R at
10000
Fig. 7 Shear viscosity as function of the shear rate of the four different pastes.
parallel course of the straight line,
the shear-thinning behaviour must
be deemed comparable. The Newtonian type sticks out due to a high
initial viscosity in comparison (cf.
Fig. 6).
References
[1] Bayer, R.; Knarr, M.; Thermal precipitation or gelling behaviour of dissolved methylcellulose (MC) derivatives – Behaviour in water and influence on the extrusion of ceramic
pastes. Part 1: Fundamentals of MCderivatives. J. Eur. Ceram. Soc. 32
(2012) 1007–1018
[2] Shy-Hsien Wu: Plastically deformable
metallic mixtures and their use. USPatent 5316577 (Corning)
[3] Bayer, R.: Die Steuerung der Prozesseigenschaften bei der Extrusion
keramischer Massen über die Pseudoplastizität
des
CelluloseetherBindemittels. Plenarvortrag auf der
DKG-Jahrestagung,
Hermsdorf,
23.3.2010
[4] Bayer, R.: Steering the ceramic’s extrusion process with the help of the
pseudoplastic properties of the cellulose ether binder. Plenarvortrag auf
der 11th Int. Conf. on Ceramic Processing Sci. (ICCPS 11), Zürich,
31.08.2010
[5] Schramm, G.: Einführung in die Rheologie und Rheometrie, 2nd Ed., Thermo Electron (Karlsruhe) GmbH, 2004
E 121