Eric Serris

Acoustic emission of pharmaceutical powders during compaction

By measuring acoustic emission (A.E.), it is possible to understand and to identify many of the phenomena that occur during powder compaction of pharmaceutical products. Those phenomena are granular rearrangement, fragmentation, visco-plastic deformation of grains or granules. Other data, such as specific area and porosity evolution vs. compaction pressure, provide valuable information on domains where either fragmentation or visco-plastic deformation is important. As examples, brittle products like aspirin or saccharose produce a strong acoustic emission, whereas starch exhibiting a high visco-plastic behaviour produces a low acoustic emission.

Compression and Sintering of Powder Mixtures: Experiments and Modelling

In numerous industrial fields, materials are manufactured from powders. The most classical process consists in pouring the powder into a die and pressing in between two punches. The resulting component can be used in this state, as in the pharmaceutical industry. But, most often, for example in powder metallurgy or ceramic processing, it is submitted to a thermal treatment, called sintering, during which the particles are welded together. These two stages, compaction and sintering, have been extensively investigated in the last decades. The main physical mechanisms arising during both stages have been identified and more or less sophisticated models have been developed. However most of these studies have concerned single-component powders whereas industrial processes more frequently use mixtures of powders with different physical and mechanical. The interest of using mixtures instead of single-component powders may be to facilitate the compaction or the sintering or to create alloys or composite materials with outstanding features. Understanding and modelling the behavior of powder mixtures require taking into account the mechanical and chemical interactions between both phases. Concerning mechanical problems, in most cases, a simple law of mixture is not appropriate. Classical models of multiphase materials hardly give better results. Accounting for the granular nature of the material is thus absolutely necessary. Relevant issues are interparticle contacts, particle rearrangement, agglomeration, phase percolation. The compressibility of bimodal mixtures, for example, is mainly related with the ratio of the size of particles of one phase to the size of particles of the other phase. Chemical phenomena involve more or less complex phase transformations and chemical reactions. Such transformations or reactions usually occur during sintering but they can also be generated when the material is still in the powder state, for example during milling. Most research studies in the field of powder processing at the Federation concern powder mixtures. Three examples are presented in the following, illustrating recent research studies on the behavior of powder mixtures during compression or sintering.

Dry coating in a high shear mixer: Comparison of experimental results with DEM analysis of particle motions

Experimental dry coating of guest particles on the surface of host particles is performed by mechanical forces in a high shear mixer called “Cyclomix”. The studied system (a mixture of particles of sugar, “Suglet™” as host particles and magnesium stearate as guest particles) was chosen as a model one to achieve better understandings of the phenomena during mixing. To simulate the flow of host/guest particles in the mixer, the Discrete Element Method (DEM) was applied. Experimental results such as flowability and wettability can be explained by particles flows evolutions with different rotational speed or duration treatment inside the Cyclomix.