Mechanisms of Particle Interaction (Cont.)
The force relationship between all restrictive forces and the motivating force of gravity is what determines whether particles can move independently or whether they will exist as part of an agglomerate. In the latter case, bulk flow is influenced by the mass of the agglomerate and its relationship with surrounding agglomerates.
There is a common misconception that powders with small particle size have stronger cohesive forces, but this is not necessarily the case. Here, the gravitational forces acting on the particle are small because the individual particle mass is low, and hence the relative size of the cohesive force to the gravitation force is high, even though the absolute value of cohesive force could be low.
A powder consisting of mostly spherical particles will be optimised in terms of its dependence on particle shape. Therefore, changing properties like particle surface texture, extent of lubrication, moisture content, or the true density of the material will yield a strategy for optimising the properties of the bulk powder. A powder that is very cohesive may flow better if its free moisture content is increased, as the additional water on the surface of the particles acts as an electrical conductor and allows for some of the cohesive force to be dissipated. However, too much free moisture may result in capillary bonding between particles and flow will be compromised by the increased particle – particle adhesion.
The influence and importance of each mechanism also needs to be considered in regard to the process environment and the conditions to which the powder is being exposed. When powders are loosely packed, the cohesive forces are at their most influential, dictating the independence of each particle from one another. However, whenever the powder is consolidated, such as in a hopper or feeder, the frictional forces and those due to mechanical locking are far more dominant, due to the fact that the particles are forced together. Under these circumstances, the number of contact points, the contact pressure, and, depending on the compliance of the particles themselves, the contact area, will increase. Cohesion still exists, but only represents a fraction of the forces that restrict independent particle – particle movement.
Understanding the interplay between the mechanisms of particle interaction and the conditions imposed by the process or application is a key step in optimising a formulation or process. Cohesive powders can be processed efficiently if the process design and configuration is optimised for a cohesive powder. Indeed, a less cohesive powder may perform poorly in the same process. Efficient powder processing is not about engineering the least cohesive powder, but about optimising the characteristics of the powder for the process or application of interest.