Gelcasting of alumina using urea–formaldehyde III. Machinable green bodies by copolymerisation with acrylic acid

The present work is an attempt to improve the gel strength, green body strength and green machinability characteristics of gelcast alumina bodies by co-polymerisation of urea-formaldehyde (UF) with acrylic acid. Methylol urea and acrylic acid in the pre-mix solution undergo simultaneous polymerisation at 80°C. The polyacrylic acid stabilises the UF condensation products through hydrogen bonding and prevent precipitation, resulting in a transparent gel. The gel when heated at 110°C forms a tough polymer which gives a peak at 1738 cm−1 in the infrared spectrum indicative of crosslinking of the methylol urea condensation products and polyacrylic acid through ester linkage. The alumina slurries prepared in the premix solution, gels within 10–15 min at 80°C. The green strength of the gelcast bodies increases with acrylic content. A binder composition containing >50% acrylic acid shows diametrical compressive strength >7 MPa. The green bodies show excellent machinability and are subjected to lathing, milling and drilling using conventional tools and equipments. The green bodies during machining show chip resistance and machined body has good surface finish. A near steady state binder removal profile is achieved by heating the samples under both dynamic and isothermal conditions. The debinderised bodies on sintering gave up to 97% theoretical density.