INVESTIGADORES
CAMERUCCI Maria Andrea
congresos y reuniones científicas
Título:
Forming and characterization of mullite green bodies obtained by novel starch consolidation routes
Autor/es:
M.H. TALOU; M.L. SANDOVAL; R. MORENO; M.A. CAMERUCCI
Lugar:
Mohs
Reunión:
Congreso; Fifth International Conference on Shaping of Advanced Ceramics; 2013
Resumen:
The starch
consolidation forming method can be used in the manufacture of porous ceramics.
In this method, which is based on swelling and gelatinization properties of the
starch in aqueous suspension at temperature (55-80 °C), the starch granules
perform as a consolidator/binder of the green body as well as a pore former at
high temperature.
Commercially
available powders of mullite, and potato and corn starches were employed as raw
materials. Stable aqueous mullite/starch suspensions (0.25 starch volume
fraction of 40 vol.% total solid loading) were prepared by intensive mechanical
mixing and by homogenization in a ball mill for
6 h. The optimum stability conditions employed for preparing the aqueous
suspensions were determined in a previous work.
Taking into
account the problems associated with the use of native starches in the forming
of ceramic suspensions by direct consolidation method, and based on the premise
to develop porous bodies with homogeneous microstructures and adequate
mechanical properties, two shaping routes alternative to conventional
processing by starch direct consolidation were evaluated. One of them was
called Mixing Route (MR) and the other was denominated Soluble Route (SLR).
Both routes were proposed with the aim to increase the viscosity of aqueous
mullite/starch suspensions, and therefore, to prevent the segregation of
particles. Thus, chemical or physical modifications of the native starches were
performed.
In MR, a
mixture of ungelatinized native starch and a small portion (2.5 vol.% of the
total solid loading) of gelatinized native starch (80 °C,10 min in a
starch/water ratio of 1/10) was used by preparing the aqueous mullite/starch
suspension. Regarding SLR, a fraction of starch (2.5 vol.% of the total solids
in suspension) used in the aqueous mullite/starch suspension was a granular
cold-water-soluble (GCWS) starch. GCWS was prepared by heating an aqueous
suspension of the cassava native starch and a poliol (1,2-propanodiol) until 114 °C. Shear flow
properties of these aqueous suspensions were analyzed to obtain information on
their rheological behavior.
With both
routes, disks were formed by pouring the mullite/starch suspension at room
temperature directly into metallic molds and heating at 80 °C, 2 h. Once the
consolidation process was finished, samples were demoulded and dried (40 °C, 24 h). Green bodies
shaped by two routes and obtained before (MRbb and SLRbb)
and after (MRab and SLRab) burning out the starch, were
characterized by bulk density and apparent porosity measurements, and
microstructural analysis by SEM on the fracture surface.
Both routes
allowed minimize the segregation of mullite particles and starch granules and
to obtain green bodies without defects or surface deformation, and homogeneous
porous microstructure. The porosity was associated with tortuous cavities
distributed throughout the thickness whose sizes corresponded to those original
or swelled granules. An advanced gelatinization process occurred in all the samples.
Only in disks prepared by MR with potato starch a few starch granules with
integrity were observed in agreement with the high water retention capacity of
the potato gel. Moreover, the forming of mullite bodies by routes proposed did
not require the addition of others binders beside starch in order to increase
the suspension viscosity. Thus, the complications that might arise due to the
competition by superficial adsorption sites and rheological behaviour changes
of suspensions that should be evaluated were avoided.