Compression Mechanics of Powders and Granular Materials Probed by Force Distributions and a Micromechanically Based Compaction Equation
Abstract: The internal dynamics of powder systems under compression are as of yet not fully understood, and thus there is a necessity for approaches that can help in further clarifying and enhancing the level of understanding on this subject. To this end, the internal dynamics of powder systems under compression were probed by means of force distributions and a novel compaction equation.The determination of force distributions hinged on the use of carbon paper as a force sensor, where the imprints transferred from it onto white paper where converted through calibration into forces. Through analysis of these imprints, it was found that the absence of friction and bonding capacity between the particles composing the powder bed had no effect on how the applied load was transferred through the system. Additionally, it was found that pellet strength had a role to play in the homogeneity of force distributions, where, upon the occurrence of fracture, force distributions became less homogenous.A novel compaction equation was derived and tested on a series of systems composed of pellets with differing mechanical properties. The main value of the equation lay in its ability to predict compression behavior from single particle properties, and the agreement was especially good when a compact of zero porosity was formed.The utility of the equation was tested in two further studies, using a series of pharmaceutically relevant powder materials. It was established that the A parameter of the equation was a measure of the deformability of the powder material, much like the Heckel 1/K parameter, and can be used as a means to rank powders according to deformability, i.e. to establish plasticity scale. The equation also provided insights into the dominating compression mechanisms through an invariance that could be exploited to determine the point, at which the powder system became constrained, i.e. the end of rearrangement. Additionally, the robustness of the equation was demonstrated through fruitful analysis of a set of diverse materials.In summary, this thesis has provided insights and tools that can be translated into more efficient development and manufacturing of medicines in the form of tablets.
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