On the Origin of High Powder Cohesion after Milling: Micro-Scale Examination and Fundamental Approach to Reduced Cohesion

Micronization of powders leads to their very poor flow, packing, and dispersion, not to mention increased sticking tendency. Most of these problems are usually attributed to fine particle size, which results in higher cohesion as compared to particle weight. This is usually represented via granular Bond number, which increases with size reduction. However, in many cases, there is an additional factor, which further contributes to higher cohesion. This factor is related to the altered surface characteristics arising from micronization. Such behavior is usually prevalent in crystalline materials, where milling leads to exposing of higher surface energy sites or defects. This additional factor adds to the poor flow and handling of micronized powders. Unfortunately, this aspect is poorly investigated in literature and hence, poorly understood. In this work, fundamental investigation is conducted to shed light on this problem as well as its mitigation. Towards that goal, simultaneous micronization and surface coating of crystalline particles with nano silica is carried out to. The purpose is to examine the role of surface energetics and the influence of dry coating on both creation of nano-scale surface roughness and passivation/stabilization of high surface energy sites, and hence reduced cohesion. Pharmaceutical powders were used as test materials and a fluid energy mill was used to micronize the particles to several sizes under 30 µm with or without simultaneous nano-silica coating. Powder flow property and dispersibility were characterized using FT4 powder tester and Rodos/Helos laser diffraction particle sizer. Surface energy was characterized using a next generation Inverse Gas Chromatography instrument. Total surface energy as well as Lifshitz –van der Waals (LW) dispersion component of surface energy were measured as a function of milling intensity. In addition, surface energy heterogeneity was also examined, revealing the micro-scale changes that occur during milling of crystalline materials. Impact of dry coating with nano-silica on surface energetics and powder cohesion was investigated. It was found that dry coating leads to reduction in cohesion and granular Bond number, hence improved flowability due to both reduced LW dispersive component of surface energy and creating nano-scale surface roughness.