Gelcasting of alumina using the hexamethylenediamine–paraformaldehyde monomer system

A new aqueous alumina gelcasting system using hexamethylenediamine (HMDA) and paraformaldehyde monomers has been studied. The 500 vol% aqueous alumina slurries ‘A’ and ‘B’ containing paraformaldehyde and HMDA, respectively, undergo gelation after thorough mixing of the two due to the polymerization of HMDA and formaldehyde. The gelation time of the slurries cast in a mold is in the range of 7–2.4 min at HMDA to formaldehyde mole ratio in the range of 1.1–1.5. The faster reaction between HMDA and formaldehyde prevents the formaldehyde emission during the processing. The minimum HMDA to formaldehyde mole ratio required for the formation of a mechanically stable gel is 1.1. The compressive strengths and Youngʹs modulus of the wet and dry alumina bodies increased with an increase in HMDA to formaldehyde mole ratio. Though the wet gelcast alumina bodies had low compressive strength (11.2–88.7 kPa) and Youngʹs modulus (0.17–5.9 MPa) the dried ones showed high strength (6–11.7 MPa) and Youngʹs modulus (209–364 MPa). The binder removal by slow heating to a temperature below 500 °C followed by sintering at 1600 °C produced alumina ceramics with ~97% of theoretical density.