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Porous alumina through protein foamingЦconsolidation method effect of dispersant concentration on the physical properties.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2011; 6: 863–869
Published online 16 January 2011 in Wiley Online Library
(wileyonlinelibrary.com) DOI:10.1002/apj.526
Special theme research article
Porous alumina through protein foaming–consolidation
method: effect of dispersant concentration on the physical
properties
Ahmad Fadli,1,2 Iis Sopyan,1 * Maizirwan Mel3 and Zuraida Ahmad1
1
Department of Manufacturing and Materials Engineering, Faculty of Engineering, International Islamic University Malaysia, 50728 Kuala Lumpur,
Malaysia
2
Department of Chemical Engineering, Faculty of Engineering, Riau University, 28293 Pekanbaru, Indonesia
3
Department of Biotechnology Engineering, Faculty of Engineering, International Islamic University Malaysia, 50728 Kuala Lumpur, Malaysia
Received 2 November 2009; Revised 7 September 2010; Accepted 28 September 2010
ABSTRACT: The influence of dispersant concentration on the physical properties of porous alumina ceramics formed
by the protein foaming–consolidation method has been studied. Slurries of alumina powders, yolk, and dispersant
were prepared by rigorously stirring the mixture for 3 h with an alumina-to-yolk ratio of 1 : 1 in weight and dispersant
concentration of 0.01–0.05 wt.%. The resulting slip was poured into cylindrical shaped molds and followed by foaming
and consolidation through drying at 180 ◦ C for 1 h. The dried green bodies of the samples were then burned to remove
the pore-creating agent at 600 ◦ C for 1 h, followed by sintering at 1550 ◦ C for 2 h. The density of sintered alumina
ceramics could be controlled by varying the dispersant concentration of the slurry. Measurement of the average pore
size distribution showed that macropores of the sintered alumina porous bodies increased with decreasing density
and were found in the range of 54.4 ± 12 to 424.5 ± 25 µm. The compressive strength was 4.6 ± 0.8 MPa at 54.5%
porosity and it decreased significantly to 0.8 ± 0.1 MPa at 71.8% porosity. A shrinkage of 29.3 ± 2.7% was observed
for the sample prepared using the slurry without dispersant and it increased to 35.8 ± 0.9% when the dispersant of
0.03% was added.  2011 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: protein foaming–consolidation; ceramics; porosity; characterization
INTRODUCTION
Porous ceramics have great potential in many
applications in which chemical, thermal, or mechanical
stresses make metallic or polymeric materials unsuitable, as in thermally insulating applications, as filters,
membranes, and gas burner, or as bioceramics. The
requirements for the ceramic matrix and the pore structure may vary depending on the type of application. For
effective thermal insulation closed porosity is favorable,
whereas filters and membranes require open porosity.
In the bioceramics field, it is desirable to use porous
ceramic implants with certain porosity to promote integration with biological tissues.[1]
A number of routes such as polymeric foam impregnation, gel casting, and freeze drying have been used for
the consolidation of the powder suspensions into porous
bodies. In polymeric foam impregnation, a ceramic slip
is forced to penetrate a polymer; after drying, the polymer is removed by a burn-out operation and the ceramic
*Correspondence to: Iis Sopyan, Department of Manufacturing and
Materials Engineering, Faculty of Engineering, International Islamic
University Malaysia, 50728 Kuala Lumpur, Malaysia.
E-mail: sopyan@iiu.edu.my
 2011 Curtin University of Technology and John Wiley & Sons, Ltd.
Curtin University is a trademark of Curtin University of Technology
material is sintered, resulting in a material with open
porosity.[2] In gel casting, a ceramic powder suspension containing an organic monomer and a cross-linking
agent is foamed by using a blowing agent followed by
setting through in situ polymerization on the monomer.
Gel casting produces porous ceramics with a combination of open and closed pore microstructures having superior mechanical properties in a wide range of
porosities. However, the process has limitations with
respect to the control of pore size.[3] In the freeze drying process, controlled crystallization of ice is used for
consolidation of aqueous powder suspensions into bodies, and sublimation of the ice crystals under reduced
pressure resulted in a porous structure.[4]
Preparation of the trimodal pore ceramic structure
has been recently reported using the foaming and
starch consolidation method.[5] Different particles such
as poppy seeds, polystyrene, polyvinyl acetate (PVAC),
and wheat particles were used as a pore-forming
agent or template for producing porous ceramics.[6 – 9]
Proteins such as ovalbumin and bovine serum albumin have been used for the foaming and setting of
aqueous ceramic powder suspensions for preparation of
porous ceramics.[10,11] Lyckfeldt and Ferreira reported
864
A. FADLI et al.
a starch consolidation process for preparation of porous
ceramics.[12]
Porous alumina scaffolds have been applied to tissue
engineering. The scaffold is used for bone replacement
and orthopedic surgery because it facilitates fast cell
proliferation and has good biocompatibility, inertness,
and better chemical stability.[13 – 16] In order to fulfill
the different demands of many applications, various
techniques have been developed, allowing the design
and fabrication of porous alumina ceramics with tailored porosities, interconnectivities, mechanical properties, surface chemistry, and biocompatibility.[17]
Studies have shown that the rheology of the slip prior
to foaming is extremely important in determining the
structure of the resulting foam.[18] Generally, ceramic
slurries should contain a minimum amount of water
for molding because the shrinkage by drying decreases.
However, lower water content results in lower fluidity
of the slurries and it is necessary to use a dispersant to
maintain high fluidity.[19]
Recently, we have succeeded in developing a novel
method for the preparation of porous alumina with controllable physical properties using egg yolk as both
consolidating and foaming agent.[20] The advantage of
this method is that pore creation can be done at very
low temperatures (110–180 ◦ C)[20] compared with conventional methods (500–600 ◦ C).[6 – 9,12] In the direct
protein foaming method using egg white protein, pores
were generated via foaming by stirring or milling, followed by drying for consolidation or gelation,[10,11]
whereas in our work, both foaming and consolidation
occurred during drying. Therefore, control of porosity
can be achieved by controlling the composition of the
slurry in the earlier method and by composition of slurry
and drying temperature as well in our work. Egg yolk is
a complex association of lipids (33% by weight), proteins (17%), and water (50%).[21] This work suggested
that egg yolk delayed foaming during stirring. The lipid
phase in egg yolk would lessen the foaming capacity of
proteins in creating pores.[22]
In our earlier work, we have reported the effect
of time and temperature of foaming on the physical
properties of porous alumina.[23] A dispersant, Darvan
821A, was added to increase the slurry’s stability, thus
improving the foaming capacity and physical properties
of the porous alumina. This report presents the effect
of dispersant concentration on the physical properties
of sintered products.
EXPERIMENTAL
Materials
Commercial alumina powder (Sigma-Aldrich, Inc., St.
Louis, MO, USA), with a characteristic mean diameter of 0.25 µm (measured using a Malvern Instruments nanosizer, NanoS model) and a surface area
 2011 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
of 0.39 m2 /g (measured by N2 adsorption method on
a Quantachrome surface area analyzer, Autosorb-1
model), was used as raw material. The protein used
was yolk that was freshly isolated from chicken egg
procured from the local market. The egg yolk consisted of 25 wt.% proteins and 24 wt.% lipids as well
as 51 wt.% water [estimated gravimetrically by heating
at 1000 ◦ C at a heating rate of 10 ◦ C/min in a Perkin
Elmer TG/DTA (Pyris Diamond model)]. Darvan 821A
(40 wt.% aqueous solution of ammonium polyacrylate;
R.T. Vanderbilt, Norwalk, CT, USA) was selected as
the dispersant.
Preparation of ceramic body
Slurries were prepared by dispersing the alumina powder, yolk, and dispersant with an alumina-to-yolk weight
ratio of 1 : 1. The dispersant concentrations were varied
from 0.01 to 0.05 wt.% (to 100 wt.% alumina powder)
and one slurry was prepared without dispersant content.
The slurries were stirred in a glass beaker for 3 h at a
rate of 150 rpm. The slurries were cast in cylindrical
open stainless steel molds and heated in an air oven
(Memmert, 100–800 model) at 180 ◦ C for 1 h. Castor oil was used as lubricant for easy mold removal.
The dried samples were then heated in a SiC furnace
(Protherm, PLF 160/5 model) at a 10 ◦ C/min rate up
to 600 ◦ C for removal of the yolk and followed by
1 h holding. Then, heating was continued at a rate of
2 ◦ C/min up to 1550 ◦ C and ended with 2 h dwell time
at the temperature.
Determination of foaming capacity
The foaming capacity of slurries was evaluated by
measuring the change in volume of slurry as a function
of drying time. A volume of 10 mL of slurry contained
in a 100-mL glass measuring cylinder was placed in
a temperature-controlled air oven for 60 min at 180 ◦ C.
The change in slurry volume was monitored at specified
time intervals.
Characterization
The rheological property of the slurries was measured
in a ThermoHaake VT 550 viscotester with a measuring system of concentric cylinders using sensor type of
SV-DIN. All rheological measurements were conducted
at a shear rate from 10 to 700 s−1 at ambient temperature. The viscosity values were derived from shear
stress plots vs shear rates. The total porosity of the
as-sintered alumina ceramics was determined by the
apparent density and dimensions of the specimens. The
Asia-Pac. J. Chem. Eng. 2011; 6: 863–869
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering POROUS ALUMINA THROUGH PROTEIN FOAMING–CONSOLIDATION METHOD
apparent density of sintered samples obtained was measured using Archimedes principles in an Electronic densimeter (Alfa Mirage, MD300S model). The theoretical
density of fully densified alumina (3.98 g cm−3 ) was
used as reference to calculate the total volume fraction
of porosity.
Each sintered porous alumina sample was crosssectioned and coated with carbon before being subjected to analysis of surface morphology using scanning
electron microscopy (SEM) (JEOL, 5600 model) and
field emission scanning electron microscopy (FESEM)
(JEOL, JSM 6700 F model). The SEM measurement
was done on five different parts of each sample to
precisely evaluate the pore size, pore interconnection,
and grain morphology. The mechanical strength of the
porous bodies was measured using a universal testing
machine (Lloyd, LR10K plus model) through diametrical compression on samples of 3 : 2 height-to-diameter
ratios (15 mm in height and 10 mm in diameter). Five
scaffold specimens were used to determine the average
maximum compressive strength.
RESULTS AND DISCUSSION
Rheological properties
Figure 1 shows the rheological flow curves of slurries
with different dispersant concentrations. The viscosity
of the slurries was measured at various shear rates after
150 rpm stirring for 3 h at ambient temperature. The
viscosity of the slurries containing the same alumina-toyolk ratio decreased with increasing dispersant amount.
The viscosity of slurry with dispersant concentration of
0.03 wt.% and without dispersant content showed shear
thinning (pseudoplastic flow) behavior, i.e. apparent viscosity decreases with shear rate, whereas the slurry with
Figure 1. Rheological flow curves of slurries with different
dispersant concentrations.
 2011 Curtin University of Technology and John Wiley & Sons, Ltd.
0.05 wt.% dispersant concentration behaved as a Newtonian fluid. Pseudoplasticity in ceramic slurries usually arises because of the existence of an inter-particle
network, which undergoes a gradual breakdown with
increasing shear rate, causing the typically observed
decrease in viscosity of slurries. The viscosity value of
the slurries at high shear rate (700 s−1 ) was in the range
of 0.9–1.7 Pa s and it was still suitable for casting.
The measured flow curves for slurries were fitted with
a power law model[24] :
η = k γ n−1
(1)
where η is the viscosity of the slurry, γ the applied
shear rate, and k and n are the consistency factor
and non-Newtonian index, respectively. The extent of
pseudoplasticity could be due to the non-Newtonian
index, n, as defined by the power law model. The nonNewtonian index, n, is indicative of the inter-particle
network strength in the slurries. Highly shear thinning
slurries, with a strong inter-particle network, show a
rapid decrease in viscosity with an increase in shear rate,
corresponding to a lower value on the non-Newtonian
index, n, and slurries with weak or no inter-particle
network exhibit closer to Newtonian behavior with n
values approaching 1.0.[25]
The non-Newtonian index is shown in Fig. 2 as
a function of deviation in dispersant amount. The
calculated n index for slurry without dispersant content
is 0.51. Further, addition of dispersant up to 0.5 wt.%
results in increases in n index, which reaches the value
0.82, indicating behavior closer to Newtonian. On the
other hand, the calculated k parameter, which is a
consistency factor in the power law model, decreases
almost linearly with increasing dispersant concentration
in the ceramic slurry.
Figure 3 shows the influence of stirring time on the
viscosity of ceramic slurry at 0.01 and 0.04 wt.% dispersant at a shear rate of 10 s−1 . It can be seen that the
viscosity of 0.04 wt.% dispersant slurry is lower than
Figure 2. The parameters n and k as a function of
dispersant concentration in slurries.
Asia-Pac. J. Chem. Eng. 2011; 6: 863–869
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Figure 3. The viscosity of ceramic slurry vs stirring time
(shear rate = 10 s−1 ).
that of 0.01 wt.% dispersant. Properties of protein-based
suspensions such as stability, rheological behavior, and
texture are intimately related to interactions taking place
among adsorbed protein molecules and within the interfacial film.[26] The main chain of protein molecule is
characterized by covalent peptide bonds, while their
conformation is stabilized by weak, mostly noncovalent
bonds. Denaturation means loss of native structure and
biological activity of a protein through a breakdown
of the structure. Noncovalent bonds, such as hydrogen bonds, are broken and random coil or metastable
forms are formed. Denaturation is generally reversible
if the peptide chain is stabilized in its unfolded state,
which inhibits any intermolecular reactions. Irreversible
denaturation occurs when the unfolded peptide chain
is stabilized by interactions with other chains.[27] This
is the case of yolk, in which the denaturation exposes
amino acid side chains, which, in turn, may take part
in intermolecular interactions as a result of the mixing process. Figure 3 also shows that the viscosity of
0.01 wt.% dispersant is slightly increased from 8 to
10 Pa s when stirring time increases from 3 to 9 h,
but it increases significantly to 27 Pa s at 24 h stirring time. It can be inferred that, when the stirring time
of the slurry increases from 3 to 24 h, the polypeptide
chains of protein molecules become tangled to form a
three-dimensional gel network (coagulation) through the
formation of new hydrogen bonds between the chains,
hence the slurry viscosity is increased.[27]
On the other hand, increasing the stirring time from
3 to 24 h results in no significant changes in viscosity,
which reaches 4 Pa s for the alumina slurry containing 0.04 wt.% of dispersant. The viscosity trends of
0.01 and 0.04 wt.% of dispersant can be explained
from the Darvan 821 A dispersant composition. It contains 60 wt.% of water shifted rheological properties
of the slurry from shear thinning behavior to shear
independent flow behavior (Newtonian), hence the stirring time has no effect on viscosity of the slurry. In
 2011 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
Figure 4. Relationship between dispersant content in
slurries and volume increase.
other words, the composition has destabilized the particle supermolecular structure, leading yolk dispersions
in water to exhibit ‘weak’ gel properties.[26]
Foaming capacity
Figure 4 shows the effect of dispersant concentration
on the foaming capacity of the slurry after being dried
at 180 ◦ C for 1 h. The foaming capacity is evaluated in
terms of the volume ratio of the foamed slurry to the
original one.
The foaming capacity of the slurry without dispersant
content was lower than that with dispersant. As the
dispersant content was increased to 0.03 wt.%, the
volume of slurry was 2.5 times that of the original
one. It could be inferred that dispersant molecules will
decrease the viscosity of slurry (Fig. 1) and accelerate
transfer of proteins from the interior of the slurry toward
the newly created surface, thus decreasing the surface
tensions and increasing the foaming capacity.
Foaming by addition of a surfactant is a simple
process and has been described previously by Sevulpeda
and Binner.[28] Foam volume was found to reach a
maximum after 5 min. Several factors introduced in
the fabrication process can affect the cell size of the
foamed ceramics. These include rheology of the slip
prior to foaming, the foam volume, and also the type
of surfactant used. All of these mechanisms alter the
process of bubble formation and coalescence. Foaming
in the presence of the methylcellulose resulted in
foamed ceramics generated in a matter of minutes with
optimal pore and window sizes for osteoconduction.
Characterization of porous ceramics
During the preparation of green bodies, yolk constituents may have to function as both foaming and consolidating agents. The majority of proteins in yolk are
Asia-Pac. J. Chem. Eng. 2011; 6: 863–869
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering POROUS ALUMINA THROUGH PROTEIN FOAMING–CONSOLIDATION METHOD
organized into micellar and granular structures together
with polar and nonpolar lipid molecules. Yolk coagulation or gelation, as a result of heating, is an important functional property of the yolk in the preparation
and texture modification of ceramic products. Gel network structure formation by yolk is attributed to its
constituent protein denaturation, leading to molecular
interactions and the development of a hard and rubbery
structure.[26]
In the sintering process of green bodies, shrinkage
occurred. The linear shrinkage of samples at different dispersant concentrations is shown in Fig. 5. With
the increase of dispersant concentrations, the sintered
alumina shrunk increasingly. Shrinkage of the porous
alumina prepared from slurry with dispersant concentrations from 0.00 to 0.03% is in the range 29.3 ± 2.7
to 35.8 ± 0.9 vol.%, respectively. A substantial shrinkage occurred as the organic substances such as protein
and lipid were removed. The shrinkage increased with
increasing dispersant content.
Figure 6 shows the relative density of the sintered
alumina as a function of the dispersant concentration. It
can be seen that the relative densities were in the range
of 18 ± 2.8 to 45 ± 3.3%; it decreased with the dispersant concentrations of 0.00–0.05%. It could be considered that high dispersant concentration resulted in low
viscosity, which corresponded to high foaming capacity
and low relative density. So, the products with different
relative densities could be easily obtained and controlled
by means of the slurries with different dispersants.
Density played an important role in determining the
microstructure of the sintered porous alumina, as shown
in Fig. 7 and Table 1. It can be seen that an increase in
the pore size of alumina bodies occurred when the density decreased. The average pore size of sintered bodies
was in the range of 54.4 ± 12 to 424.5 ± 25 µm. It can
be explained that low viscosity of the slurry resulted in
Figure 6. Relative density of the sintered porous alumina
as a function of dispersant concentration.
Table 1. The average pore size of porous alumina
measured at various relative densities.
Relative density (%)
Pore size (µm)
45.4 ± 3.3
28.2 ± 2.2
20.9 ± 2.3
17.5 ± 2.8
54.4 ± 12
175.0 ± 07
319.5 ± 18
424.5 ± 25
 2011 Curtin University of Technology and John Wiley & Sons, Ltd.
6.1
4.0
7.5
7.5
high foaming capacity, thereby increasing the pore generation. Errors quoted are standard errors of the mean.
Porosity of the samples increased as the dispersant
concentration increased. Porosity of the samples prepared from the slurry without dispersant content and the
slurry with dispersant concentration of 0.03% was in the
range 54.5–71.8%. Generally, the strength of a porous
ceramic is strongly affected not only by the strength of
the ceramic wall (or strut) but also by the surface flaws
on the strut. In order to evaluate the mechanical properties of the samples, compressive strength tests were
conducted. The stress increased linearly with the elastic
response for all the fabricated samples and then dropped
rapidly owing to fast fracture. The compressive strength
was remarkably increased from 0.8 to 4.6 MPa, with
the porosity decreasing from 71.8 to 54.5%, as shown
in Fig. 8.
The dependence of compressive strength on porosity
can be described by several models. For highly porous
ceramic structures with interconnected pores, Gibson
and Ashby considered the macrofracture (crushing) of a
porous structure as a result of the bending microfracture
of the struts and derived the following formula[29] :
σ = σ0 (1 − P )3/2
Figure 5. Shrinkages of samples as a function of dispersant
concentration.
Error
(2)
where σ is the compressive strength of a porous
structure at porosity P and σ0 is the bending strength
of the strut material, which may contain micropores. It
is obvious from Eqn (2) that increasing porosity leads
to decreasing strength.
Asia-Pac. J. Chem. Eng. 2011; 6: 863–869
DOI: 10.1002/apj
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A. FADLI et al.
Asia-Pacific Journal of Chemical Engineering
Figure 7. SEM micrographs of porous alumina with relative densities of
(a) 45.4 ± 3.3%, (b) 28.2 ± 2.2%, (c) 20.9 ± 2.3%, and (d) 17.5 ± 2.8%.
Figure 8. Compressive strength of porous alumina as a
function of porosity.
Figure 9(a) shows the FESEM photomicrograph of
the porous alumina sample prepared at the dispersant
concentration of 0.05%. Interconnectivity of the pores
through windows is clearly seen in the micrographs.
The grains of porous alumina walls are irregular and
some small particles adsorb on the large grains as
shown in Fig. 9(b). There is more bonding area among
grains. It is well known that more bonding area usually
leads to higher strength between particles; consequently,
the fracture of alumina bodies mainly happens at the
particle boundaries. Hence, the increase of the grain’s
 2011 Curtin University of Technology and John Wiley & Sons, Ltd.
bonding area also has an important effect on the
improvement of sample strength.
Ceramic microcarrier is predicted to meet the special requirements of a microcarrier technique because
of its good mechanical properties as well as chemical
and thermal resistances. Moreover, in the application of
the cell culture using a stirred bioreactor, we need to
use floatable porous ceramics as a microcarrier media
for an optimum process of cell proliferation. We have
reported that porous alumina ceramics produced by the
current method are floatable as its density could reach
below 1.0 g cm−3 .[30] On account of its considerably
high compressive strength, we believe that our porous
alumina can be a potential candidate for floating microcarrier application, especially in a bioreactor cell culture. The future works should deal with biocompatibility
of our microcarriers. A group of investigators, however, proved that in vitro tests of porous alumina with
fibroblast exerted no acute cytotoxic effects, confirming the excellent biocompatibility of porous alumina
ceramics.[31]
CONCLUSIONS
The effect of dispersant concentrations on physical
properties of porous alumina was studied by discussing
five factors: rheological behavior, foaming capacity,
shrinkage, relative density, and strength.
Asia-Pac. J. Chem. Eng. 2011; 6: 863–869
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering POROUS ALUMINA THROUGH PROTEIN FOAMING–CONSOLIDATION METHOD
Figure 9. High connectivity of the porous alumina sample (a) and grain
structure of walls (b).
When the dispersant concentration increased from
0.00 to 0.05%, the flow behavior changed from pseudoplastic flow to a Newtonian fluid and the foaming capacity increased. With increasing dispersant concentration,
shrinkage occurred increasingly whereas relative density as well as compressive strength of sintered porous
alumina decreased. Porous alumina ceramics with 18 ±
3 to 45 ± 3% relative density and 54.5–71.8% porosity
as well as diametrical compressive strength of 0.8 ± 0.1
to 4.6 ± 0.8 MPa was obtained from slurries with dispersant concentration of 0.01–0.05%.
Shrinkage of the porous alumina increased from
29.3 ± 2.7 to 35.8 ± 0.9 vol.% with the addition of
dispersants from 0.00 to 0.03%. Macropores of the
sintered alumina increased with decreasing relative
density and were found in the range of 54.4 ± 12 to
424.5 ± 25 µm.
Acknowledgements
The authors would like to thank the faculty of Engineering and Research Management Center, International
Islamic University, Malaysia, for their financial support
(EDW B 0901-197) when the work was implemented.
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DOI: 10.1002/apj
869
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physical, porous, alumina, effect, properties, concentrations, protein, method, foamingцconsolidation, dispersal
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