The use of poly (N-isopropylacrylamide) microgels as a multi-functional processing aid for aqueous alumina suspensions

Two temperature sensitive microgels of poly (N-isopropylacrylamide) were pr epared, one anionic in nature and the other cationic. The microgels were co ncentrated by centrifugation and the rheological properties of the microgel s measured as a function of temperature. The anionic microgel showed a tran sition from an elastic gel like structure to a liquid like structure at 32 degrees C, whilst the cationic microgel demonstrated the same transition, b ut at a slightly higher temperature of 34 degrees C. Both systems were comp letely reversible. A sub micron sized alumina powder was then mixed into th e microgels using an anionic dispersant, (Darvan 821A) to stabilise the pow der. The powder was added until the system was just reversible. The aim was to see which microgel behaved as the best processing aid for the alumina p articles, in terms of dispersion, gelation and adhesion after drying. The r esulting mixtures were freeze dried, sintered and investigated by electron microscopy. The best microgel / alumina system was the one with similarly c harged particles, i.e. the anionic microgel, whereas the system containing the oppositely charged cationic microgel particles flocculated, as would be expected from simple electrostatic theory. The presence of the inorganic p articles caused the sharp transition from elastic gel to viscous fluid to b roaden and the systems to behave viscoelastically over the whole temperatur e range. Sintered products made from the two systems were remarkably differ ent. With the cationic microgel the resulting structures crumbled, but with the anionic microgel robust structures were obtained. Hence it is feasible to use similarly charged microgels as processing aids for ceramics as long as both particles are not oppositely charged. (C) 2000 Elsevier Science Lt d. All rights reserved.