Stress relaxation test for the characterization of the viscoelasticity of pellets.

The characterization of the mechanical properties of single uncoated pellets was performed in order to verify if these parameters could be used to predict the pellets aptitude to be compressed or utilized differently. Different ratios of microcrystalline cellulose and lactose monohydrate were used for the preparation of four batches of pellets by an extrusion/spheronization process. The 0.6-0.71 mm pellet fraction was used for the tests. Crushing strength and stress relaxation tests were carried out on the single pellets. The first test provided information of both the mechanical strength and the fragmentation aptitude. The second test provided information about their deformation ability (viscous flow) and residual elasticity (stress relaxation modulus). The results obtained from these tests were then compared with those obtained from the Heckel analysis. An excellent consistency was discovered between the parameters obtained from both the stress relaxation and crushing strength tests on one side and the Heckel parameters on the other side. Tests performed on single pellets are very useful tools to predict their deformation and fragmentation aptitude under compression and can be used for early insight of the pellet aptitude to be compressed.