Two alternative routes for starch consolidation of mullite green bodies

The starch consolidation forming method can be used in the manufacture of porous ceramics. In this method, based on swelling and gelatinization properties of starch in aqueous suspension at temperature (55–80 °C), the starch granules perform as both consolidator/binder of the green body and pore former at high-temperature.Commercially available powders of mullite and cassava starch were employed as raw materials. 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.Two alternative forming routes of thermogelling mullite/starch aqueous suspensions—the Conventional Route (CR) and the Pre-Gelling Route (PGR)—were studied. With the CR, disks were formed by pouring the mullite/starch suspension at room temperature directly into metallic molds and heating at different temperatures (70 and 80 °C) and times (1 and 2 h). With the PGR, disks were shaped by pouring pre-gelled mullite/starch suspensions at 59 °C into heated molds and heating at the same experimental conditions. Once the consolidation process was finished, samples were removed of the mold and dried. Green bodies shaped by the two processing routes and obtained before (CRbb and PGRbb) and after (CRab and PGRab) burning out the starch, were characterized by bulk density and apparent porosity measurements and microstructural analysis by SEM/EDAX on the external and fracture surfaces. The homogeneity of the distribution of raw materials and pores, and the volume porosity were taken into account to establish the optimum consolidation conditions to be used in the preparation of mullite porous materials with homogeneous microstructures.