Experimental characterization of the behavior of granular visco-plastic and visco-elastic solids during compaction

The microscopic behavior of viscous materials under compaction was studied, with focus on the evolution of the pore structure with increasing pressure , at different strain rates. Granular polyethylene glycol (PEG) and high de nsity polyethylene (HDPE) were compacted with a compaction simulator up to different pressures, at two different strain rates. Compaction curves were constructed, and diametral strength tests were performed on the tablets. Sc anning electron microscopy (SEM) was used to characterize the microstructur e. PEG exhibited a visco-plastic behavior, as opposed to HDPE, which behave d visco-elastically. Observation of the pore structure revealed that PEG fr actured and developed rate-dependent permanent deformation, resulting in go od bonding and strong tablets. On the other hand, HDPE tablets contained la rge pores, even at high pressures, due to the considerable amount of spring back after ejection, and their diametral strength was low. For PEG, the out -of-die microstructure was strongly dependent on the strain rate, while the HDPE structure was almost independent of it. In both cases, the diametral strength and the in-die density were dependent on the strain rate. However, the effect of strain rate on diametral strength was considerably more evid ent for PEG than for HDPE, due to their different nature and to the resulti ng pore structure. (C) 2001 Kluwer Academic Publishers.