Gelcasting of alumina using urea-formaldehyde II. Gelation and ceramic forming

A new aqueous gelcasting system for alumina was investigated. Concentrated (55 vol%) aqueous ceramic slurries dispersed using a mineral acid was gelled by pH controlled condensation polymerisation of methylolurea monomers derived from urea and formaldehyde. Gelation of the slurry occurred at ambient conditions during 30–80 min at pH 2.5–3. Humidity controlled drying of gelled bodies prepared using alumina of average particle size of 0.34 μm yielded green bodies with density of 57% (corrected for organic content) of theoretical density and strength (by diametrical compression test) of 0.35 MPa at polymer content ∼6% by weight of alumina. Debinderization of the green bodies was accomplished by pyrolysis in air below 350°C. The relative density of the bodies sintered at 1550°C for 2 h was 97% TD and the shrinkage during sintering was isotropic and averaged 14 to 16%. ©