Two alternative routes for starch consolidation of mullite green bodies

M. H. TALOU; M. A. CAMERUCCI

Madrid, España

Congreso; Shaping 4, Fourth International Conference on Shaping of Advanced Ceramics; 2009

Starch consolidation technique can be used in the manufacture of porous ceramics for thermalinsulations, gas burners, filters, catalyst supports, or bioceramic applications. This method of direct consolidation is based on swelling and gelatinization properties of starch granules in aqueous suspensions at temperatures between 55-80°C. The starch granules perform as bothconsolidator/binder of the green body and pore former at high temperature in the final body.In the present work, two alternative routes (labeled as CR and PGR) for starch consolidation of mullite green disks were studied with a view to develop green bodies with homogeneous microstructures. In CR route, disks were formed by pouring the mullite/starch aqueous suspension at room temperature directly into metallic molds and heating at different temperatures and times. In PGR route, disks were shaped by pouring pre-gelling mullite/starch suspensions into the molds and heating at the same experimental conditions. The pre-gelling process of the mullite/starch suspensions was carried out at 61-62°C, temperature range slightly lower then the onset gelatinization temperature of cassava starch (64.1°C).Commercial available powders of mullite (Baikowski, France) and cassava starch (Avebe S.A.,Argentine) were employed as raw materials. Cassava starch was used as consolidator agent of the mullite aqueous suspension and binder of the ceramic particles. Both cassava starch and mullite powder were characterized by several techniques: i.e. measurements of real density by Hepycnometry (Multipycnometer, Quantachrome, USA), particle size distribution by laser diffraction (Matersizer S, Malvern Instruments, UK), scanning electron microscopy, SEM (JEOL JSM-6460, Japan). The mullite powder presented a high purity level and fine particle size (D50 = 1.46 m). Mullite/starch aqueous suspensions (0.25 starch volume fraction of 40 vol.% total solid loading) were prepared by intensive mechanical mixing and homogenization in a ball mill. The optimum stability conditions of both mullite and mullite/starch aqueous suspensions were determined by measuring zeta potential (Zetasizer Nano ZS, Malvern Instruments, UK) and shear flow properties in controlled stress and controlled rate operating modes (Haake RS50, Thermo Electron Corp., Germany): 0.45 wt.% of dispersant (Dolapix CE-64, Zschimmer & Schwarz, Germany), pH 8.7 and ball milling for 6h. Furthermore, the viscoelastic behaviour (RDA-II, Rheometric Scientific, USA) of starch and mullite/starch aqueous suspensions as a function of temperature (30-95°C) and strain (0.1-625%) was studied in order to determine the consolidation experimental conditions used in each route. In both routes, CR and PGR, the thermal consolidation process of the mullite/starch suspension was carried out in an electric oven (Memmert UFP 400, Germany) at 70 and 80°C during different times (1 and 2 hours). Once consolidation process finished, samples were unmolded and dried in a ventilated oven at40°C for 24h. The green bodies obtained were characterized by bulk density measurements using the Archimedes´ method (immersion in mercury), apparent porosity and microstructural analysis by SEM (Jeol JSM-6460)/EDX (Genesis XM2-Sys) on external surface and fracture surface. The results were analyzed taken into account the behavior of the cassava starch in aqueous suspension at temperature (swelling and gelatinization process) and the characteristics of the gelatinized granules. The analysis of the green microstructures (mapping of aluminum, silicon and carbon) developed by both consolidation routes in function of the temperature and time of consolidation, allowed to evaluate mainly the homogeneity of the distribution of raw materials in the green bodies, outstanding aspect to be considered in the porous ceramic processing.