Investigations into the deformability and tensile strength of pellets

The fracture and deformation properties of soft and hard pellets, which were prepared as described by Lundqvist et al. (1997), have been studied by measuring individual pellets and groups of pellets subjected to compaction in a punch and die system. Uncompacted, hard pellets were found to be less brittle and less deformable than soft pellets. However, the soft pellets were found to fracture under the influence of low tabletting pressures, and laser light reflection measurements have shown that they are able to form a coherent network of deformable material in tablets at higher tabletting pressures. Hard pellets were more resistant to crack propagation, but cracks and flaws were formed if a threshold tabletting pressure of about 9 MPa was reached. A change in the surface and internal pellet structure appears therefore certain. However, even under the influence of high loads the pellets were unable to deform to such an extent that a coherent network of material was formed in the tablets. Hard pellets differed from soft pellets by a factor of about 5 in their tensile strength, whereas they differed by a factor of 2 in their shear strength. Tested in tension, hard pellets had a higher strength value than the soft pellets, while when tested in shear they were characterized by the lower strength value. While compacts made from soft pellets were found to reduce their volume considerably under load, compacts made from hard pellets decreased in volume only slightly by deformation, but initially by particle rearrangement. This is a further sign of the larger deformability of the soft pellets in comparison to the hard pellets studied. (C) 1998 Elsevier Science B.V. All rights reserved.