EPILOGUE TO THE PROJECT
The problem of compaction of powder mixtures and their properties has received attention over
the years by the academic and scientific community. We attempted in the project to explore
some ideas that will provide the scientific basis for the exploration of this problem. The most
important concept that became obvious in the early days of the project is the classification of
mixtures according the possibility and the type of interaction between their constituents:
- Non interacting mixtures. This is the simplest case in which neither the powder themselves
not the process of mixing affect the physical and chemical characteristics of the powders.
Even in this case the direct comparison between the individual constituents and the mixture is
no trivial. This is the problem that we have attempted to address using the discrete element
approach. We have presented this first order approach and have highlighted its weakness.
The problem here is that the DEM approach, even for single components, despite our
substantial effort (which was part of our project) is still relatively new and untested, in the
high density regime. Therefore utilizing it to more complex problems such as the one of the
mixtures is can be tricky.
- Directly interacting mixtures. This is a very broad category of problems because of the
possible type of interactions. To begin with we attempt to classify them into:
o Direct interactions, such as chemical reactions between the powders, physical interactions
such as those involving moisture transport, or local interdiffusion
o Indirect interactions, where the presence of one of the ingredients in the mixture alters the
properties or the behavior of the others. This interaction can take place in any of the
stages of the processing of the mixtures, for example during milling of the mixture or
during post compaction property evolution. We have attempted to demonstrate some of
this kind of behavior within the framework of the NaCl-Starch system, which had
received attention in the literature and was misinterpreted by studying it in the context of
the non-interacting mixtures.
In general, a mechanistic understanding of the ingredients and their interactions is of
paramount importance to the understanding of interacting mixtures.
In this report we present the results of the last year in particular:
(1) The work in the NaCl system, in terms of in depth understanding of the mechanisms and the
material interactions. The most important result is that understanding the mechanism that
controls strength in NaCl-X mixtures allows us to optimize their properties deliberate selection
of X with the appropriate properties (in this case matching of the elasticity modulus). NaCl can
be considered as a model material for soluble crystalline components in mixtures (e.g., APIs in
pharmaceutical formulations) because it may replicate their interaction with moisture.
(2) We employed the Discete Element Method (DEM) to explore the distribution of forces that
develop during compaction of powder mixtures as well as the residual stresses in the resulting
compacts. This work has clearly demonstrated the differences between a mixture of two
components and the individual components when compacted to the same pressure or the same
density.