Freeze-gelcasting of aqueous alumina powder suspension using natural rubber latex

Abstract

A new and non-toxic aqueous alumina gelcasting system using natural rubber (NR) latex as a gel former has been studied. A gel of reasonable strength is achieved by freezing concentrated alumina-NR latex co-dispersions in a mould followed by mould removal and aging in acetone. Gelation is achieved by coagulation of latex particles due to the breakdown of the protein layer on their surface during ice crystal growth. The minimum concentration of rubber required to percolate and form a soft and stable gel is 8 wt % of alumina. Aging of the frozen body in acetone for solvent exchange results in strengthening of the alumina-rubber gel network by further coagulation leading to reasonably high strength (∼60 kPa). The frozen body passes through a semi-fluid state during aging in acetone which enables the gel to reorganize and fill the space created by melting of ice crystals and thereby preventing the occurrence of pores in the green ceramics. The exchange of water in the gelled body with acetone enables easy and faster gel drying at room temperature. The diametrical compressive strength of the green body increases from 0.35 to 2.14 MPa during annealing at 200 °C due to the cross-linking of rubber. The cross-linked rubber in the green body undergoes near-steady state burnout without creating any crack. Debinding followed by sintering of the gelcast bodies at 1550 °C produce ceramics of ∼96% density. The sintered ceramics show a dense microstructure with an average grain size of 1.3 μm. The gelcasting using the NR latex is capable of producing complex near-net-shapes.