Investigation des mécanismes de liaison impliqués dans la cohésion des compacts a usage cosmétique

Synopsis In cosmetology eye shadows are often presented as pressed powders. The pressed powder studied contains about 80% of lamellar substances such as talc and titanised mica, pigments, preservatives and a liquid binder which is a mixture of lipophilic substances. Usually cohesion in compacts is obtained by establishing bonds between dry particles; on the contrary in the case of pressed powder in the presence of a liquid binder one can suppose that all particles are covered before compaction with a thin layer of liquid, so that the binding mechanism must be different to that for dry compacts. The aim of this study is to verify this hypothesis using (i) investigation of the energy of compaction by measuring the total energy balance during compaction; (ii) a resistance evaluation by a drop test; (iii) indentation hardness measurements. Among the different energy terms the network of compaction is representative of the formation of the bonds between particles and also of particle fragmentation or deformation occurring during compression. The results show that the network of compaction is influenced by the binder level, but the resistance is not proportional to the network. This indicates that the network of compaction, as it has been measured or calculated, is not able to predict the resistance of the compacts. However, a direct relation, independent of the binding level, has been found between network of compaction and indentation hardness. It means that indentation hardness is rather more indicative of the compact deformation under a low loading rate than of the resistance to rupture, which implies a high loading rate. However, the binder level is of paramount importance: the rupture resistance increases when the quantity of binder is increased. The rheological properties of the liquid binder are also important. The effect of a variation of the binder viscosity upon the compact cohesion is important, especially at low compaction pressures (<8 MPa); at higher pressures the viscosity influence is less drastic, suggesting a change of the banding mechanism. However, viscosity is only one aspect of the medogid properties of the binder. The rheological behaviwr is shown to be thixotropic so that shearing time aid relaxation time become an importnnt parameter for cohesion and resistance. This conclusion is of importance for the formulation. Those two facts throw light on the cohesion mechanism of pressed powder. The banding of the particles seems to be essentially due to the presence of the liquid binder. Capillarity forces or viscosity forces, depending on the binder rheology, am responsible for the properties exhibited by pressed powder. Finally, one can conclude that the pressed powders form damp powder-eompacted systems whose binding mechanism is very different from those involved in pharmaceutical compact cohesion, so that a good formulation must involve a complete study of the rheological properties of the binder.