Gelcasting of alumina using urea-formaldehyde III. Machinable green bodiesby copolymerisation with acrylic acid

The present work is an attempt to improve the gel strength, green body stre ngth and green machinability characteristics of gelcast alumina bodies by c o-polymerisation of urea-formaldehyde (UF) with acrylic acid. Methylol urea and acrylic acid in the pre-mix solution undergo simultaneous polymerisati on at 80 degreesC. The polyacrylic acid stabilises the UF condensation prod ucts through hydrogen bonding and prevent precipitation, resulting in a tra nsparent gel. The gel when heated at 110 degreesC forms a tough polymer whi ch gives a peak at 1738 cm(-1) in the infrared spectrum indicative of cross linking of the methylol urea condensation products and polyacrylic acid thr ough ester linkage. The alumina slurries prepared in the premix solution, g els within 10-15 min at 80 degreesC. The green strength of the gelcast bodi es increases with acrylic content. A binder composition containing > 50% ac rylic acid shows diametrical compressive strength > 7 MPa. The green bodies show excellent machinability and are subjected to lathing, milling and dri lling using conventional tools and equipments. The green bodies during mach ining show chip resistance and machined body has good surface finish. A nea r steady state binder removal profile is achieved by heating the samples un der both dynamic and iso-thermal conditions. The debinderised bodies on sin tering gave up to 97% theoretical density. (C) 2001 Elsevier Science Ltd an d Techna S.r.l. All rights reserved.