Response of granular powders to uniaxial loading and unloading

This study examines the effects of organic binder content and binder plasticity on the stress–strain response of a poly(vinyl alcohol) (PVA)-containing spray dried alumina powder (A16SG) in uniaxial compaction and ejection. Binder content ranged from 2.45 to 7.80 wt.% and the binder glass transition temperatures (Tg) ranged from −32°C to +35°C. As binder content increases, the compaction stress required to attain a specified density increases due to the greater amount of binder, which needs to be deformed and displaced. When pressing a powder, which contains a binder with a Tg much lower than the working temperature, the binder is easily deformed and high green densities can be achieved at moderate stresses. As binder Tg is increased, it becomes stiffer, requiring higher compaction stresses to achieve a given density. Pellet expansion after ejection occurs mainly in the axial direction and the total dimensional change is dominated by instantaneous springback. Increasing sample size resulted in less instantaneous springback and time-dependent relaxation. This effect may be due to a limited region of expansion in the pellets.