This thesis presents an experimental study designed for the characterization of the tabletting
behaviors of Starch 1500®, Starcap 1500®, LHPC LH-11 and LHPC LH-B1, chosen for their display
of self-lubrication properties, in the form of efficient stress transmission, low ejection forces
and anti-capping propensities, with two specific aims:
Firstly, the mechanisms underlying the tabletting performance of the materials were investigated.
Secondly, in order to develop a robust experimental characterization method, the value of
the experimental indicators used in this work was assessed.
The time dependence of plastic flow was evidenced through a strain rate sensitivity study: the
variations of a set of experimental parameters with the compaction rate were observed. On the
one hand, the axial (AER) and diametral (DER) tablet relaxations provided an indirect measure
of the elastic deformation of the powder bed. On the other hand, the Heckel Yield Stress (σyH),
Kawakita parameters a and b, and the maximum stress transmission coefficient (STCM), allowed
an evaluation of the extent of plastic flow.
Secondly, the initial (SRR0), final (SRRF) and average (SRRAv) stress relaxation rates, as well
as the proportion of stress lost during relaxation (ΔPM) of the four powders were quantified from
stress relaxation experiments. Also, a new approach based on linear combinations of exponential
decay functions, was proposed for the analysis of stress relaxation curves obtained for powder
beds.
Finally, the maximum ejection force and profiles, used in conjunction with the unejected compact
surface roughness and the final tablet shape served to elucidate the different relaxation phenomena
taking place during the unloading and ejection phases, as well as investigate die wall
friction. The mechanical strength of the final tablets allowed an estimation of the final particle
cohesion achieved.
The major finding of this work is that the tabletting behaviors of the powders result from the
specific balance between their degrees of plastic and elastic deformations. More precisely, plastic
flow governs the compact consolidation through interparticulate bonding, but also the strength
of the adhesive junctions formed between the tablet and the die wall, responsible for friction.
Axial elastic recovery of the tablet during unloading causes the weakening of interparticulate
bonds through their stretching, but also the cleavage of the compact-die wall plastic junctions
and thus a reduction in the friction force between them. The fine-tuning of these two phenomena
could lead to the formation of a tablet of satisfactory mechanical strength with minimal die wall
friction.