[learn_more caption=”Vincent Mazel”] Position: Assistant professor at the University of Bordeaux (France). Research performed at the institute of mechanics and engineering (I2M).
Field of research: compression of pharmaceutical powders and mechanical characterization of pharmaceutical compacts. Focus on the elastic properties of the compacts and on the problematic of failure.
Education:
Ph.D. 2006 : Materials Physics and Chemistry, with emphasis on the development of chemical imaging techniques for studying cultural heritage objects, Univ. Pierre et Marie Curie (Paris, France).
M.A. 2003: Material Science, Univ. Pierre et Marie Curie (Paris, France).
B.A. 2001: Fundamental Physics, Univ. Paris-Sud (France).[/learn_more] Virginie Busignies^1,2
Harona Diarra^1,2
Pierre Tchoreloff¹

1. Institut de Mécanique et d’Ingénierie (I2M, CNRS UMR 5295), Université de Bordeaux, 146 rue Léo Saignat, 33076 Bordeaux cedex, France.
2. Univ Paris-Sud. 5 rue Jean Baptiste Clément, 92290 Chatenay Malabry, France.

The elastic properties of pharmaceutical powders and compacts are of great interest to understand the complex phenomena that occur during the tableting process. Adverse phenomena like capping were linked to the elastic properties of the compact that can be evidenced during the unloading part of compaction cycle and the ejection phase. To model the elastic behavior of the compacts, linear elasticity based on Hook’s law has been used for a long time in the pharmaceutical field. According to this theory, the elastic behavior of the compacts can be modeled using two elastic parameters. Usually, Young’s modulus (E) and Poisson’s ratio (v) are considered but other parameters like the bulk modulus (K) or the shear modulus (G) could also be used. In the pharmaceutical literature Young’s modulus was mainly considered and less attention was focused on the determination of Poisson’s ratio which is most of the time considered as equal to 0.3. The reason for this is the complicated determination of Poisson’s ratio. In a first time, we developed a methodology to determine E and v. For this purpose, double compaction is performed on a compaction simulator. Precompression is used to form the compact, and the elastic properties are measured during the beginning of the main compaction. By measuring the axial and radial pressure and the powder bed thickness, E and v can be determined as a function of the porosity. These parameters were measured for different products. It was found that E but also v varied with the porosity and differed from one product to another.

The second step, was to study the correlation between the elastic parameters and the actual elastic recovery of the compacts. It was found that, contrary to what is usually used in the pharmaceutical literature, E was not well correlated with the elastic recovery. This was due to the variations of Poisson’s ratio from one product to another. On the contrary, K, which combines E and v, was well correlated with the elastic recovery of the compacts. K is thus the relevant parameters to study the elastic behavior of pharmaceutical compacts using the linear elastic theory.

Finally, on some products with very high elasticity like starch, it was clear that the linear elastic theory was not suited to represent the elastic behavior. An alternative model was necessary and, instead of E and v, it was found more suitable to use K and G. To model correctly the elastic behavior a variable K was used, which was dependent on the hydrostatic pressure. On the contrary, a constant G was found suitable in most of the cases.