Response of granular powders to uniaxial loading and unloading

This study examines the effects of organic binder content and binder plasti city on the stress-strain response of a poly(vinyl alcohol) (PVA)-containin g spray dried alumina powder (A16SG) in uniaxial compaction and ejection. B inder content ranged from 2.45 to 7.80 Wt.% and the binder glass transition temperatures (T-g) ranged from - 32 degreesC to + 35 degreesC. As binder c ontent increases, the compaction stress required to attain a specified dens ity increases due to the greater amount of binder, which needs to be deform ed and displaced. When pressing a powder, which contains a binder with a T- g much lower than the working temperature, the binder is easily deformed an d high green densities can be achieved at moderate stresses. As binder T-g is increased, it becomes stiffer, requiring higher compaction stresses to a chieve a given density. Pellet expansion after ejection occurs mainly in th e axial direction and the total dimensional change is dominated by instanta neous springback. Increasing sample size resulted in less instantaneous spr ingback and time-dependent relaxation. This effect may be due to a limited region of expansion in the pellets. (C) 2001 Elsevier Science B.V. All righ ts reserved.