(15e) Influence of mixing process characteristics for aqueous phase encapsulation by silica nanoparticles

Water-rich powders containing up to 97% (by weight) of water and still maintaining the same flow properties as a dry powder has been first prepared in a commercial blender rotating at 17000 rpm for 10 to 30 s. The solid phase consists of hydrophobized fumed silica, which are sintered nanosized particles of silicon dioxide treated with various hydrophobic agents. Preliminary experiments showed that the blending process can lead either to a powder or to a moss depending on the hydrophobicity of the silica particles. We studied the influence of the mixing characteristics (geometry and size of the stirrer, geometry of the tank, rotational speed and mixing time) for different mixing process (overhead stirrer, a Triaxe stirrer and small capacity industrial tank). The results showed that the quality of the final product depends on the global energy brought to the system. For highly hydrophobic silica, an energetic process is required to create micrometric liquid droplets and to bring the solid phase in close contact with the liquid phase. However for low hydrophobicity a vigorous and/or too long mixing process will force the solid phase inside the liquid phase and produce a foam. This study offers new advances for the comprehension of microencapsulation using a solid phase and may allow future developments of industrial products.