14 Décembre – Thesis defense - Léo Desbois

14 h30 In videoconferencing

Mechanical behavior of pharmaceutical powders and tablets in compression : dependance on the strain rate.

The tablet is the most popular pharmaceutical form because of its many advantages. It is manufactured a die compaction process using powder grains whose size is of the order of 100µm. Unlike other fields where die compaction is used such as ceramics or metals, the production rates of pharmaceutical tablets are very high and the compression operation itself takes a few milliseconds. Although the die compaction process has been in use for over a century, a number of issues remain to be understood. It is for example well known that the compression speed has a significant impact on the final properties of the tablet but also on the appearance of certain defects such as capping. While this impact has been demonstrated in numerous studies, a detailed analysis of the underlying physical phenomena remains to be constructed. This type of phenomenon is designated by the term strain rate sensitivity, but different mechanisms participate in it, such as the friction between the die and the tablet or the viscoelasticity and viscoplasticity of the tablet. The objective of this research was to develop a better understanding and description of these three mechanisms through the establishment of experimental methodologies allowing them to be studied separately and the use of models, analytical and numerical, allowing their description.
The variation of the dynamic coefficient of friction as a function of the sliding speed was first studied. It has been possible to show that in the case of lubrication with magnesium stearate, this coefficient of friction varies logarithmically with the sliding speed. This type of behavior has been integrated into numerical simulation models. Then, an original protocol made it possible to characterize the viscoelastic behavior of the tablets. It was demonstrated that viscoelasticity was present in both the deviatoric and isostatic parts of the deformations. This behavior was modeled using Prony series by an analytical and numerical approach. This made it possible to show for certain materials the importance of viscoelasticity on the short characteristic times representative of the compression times. Finally, the viscoplastic part of the dependence on the strain rate has been studied in particular through the use of strain hardening type tests by varying the compression speeds and the relaxation times. These tests made it possible to characterize the visoplastic behavior of certain materials and to reveal certain phenomena, not described for pharmaceutical materials, which are similar to dynamic strain aging. Moreover, it appears that the speeds of compression accessible on the machine used (0.01mm/s - 100mm/s) are much higher than a speed which could make it possible to consider the plastic deformations as quasi-static.

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