Study of decomposition behaviour of binders and the effect of binder type on strength and density of alumina samples

Organic binders are generally used during green shaping of ceramic bodies as these binders help in obtaining desired green density and defect minimization during pressing. The aim of the present study was two-fold. First was to study the decomposition behaviour of the three chosen binders (viz. PVA, Dextrin and Starch) through DSC/TG, FTIR and weight loss measurements in air. Second was incorporating these binders (in different amount) in reactive alumina (which was chosen as the base ceramic matrix) in different proportions. Subsequently these binder added powders were sintered in air at 1550oC and sintered pellets were characterized for apparent porosity, bulk density, strength and surface feature (through optical microscope).The strength and density measurements for both green and sintered samples were carried out and they showed varied trends. The green strength of starch added pellets was found to be least while PVA containing pellets had maximum green strength. In case of sintered pellets, starch added samples showed the most consistent set of strength values but its strength decreased with increase in binder concentration while the reverse occurred for PVA and dextrin containing pellets. Removal of binders is an important step during high temperature sintering process and under ideal conditions binder removal should be complete before consolidation of ceramic particles has started. When binder removal from sintered samples is incomplete, the sintered samples may contain defects such as black coring (due to incomplete burning of carbon which will be present as residual carbon below the surface), sealed pores, bloating, etc. Optical microscope imaging was done to observe the presence of such defects. These defects may lead to a variation in pore shape, size and distribution.