Characterization of the grinding behaviour in a single particle impact device: studies on pharmaceutical powders.

The grinding behaviour of different materials can be described by the two material parameters fMat and Wm,min [Vogel, L., Peukert, W., 2003. Breakage behaviour of different materials-construction of a mastercurve for the breakage probability. Powder Technol. 129, 101-110]. fMat describes the resistance of particulate material against fracture in impact comminution, Wm,min characterizes the specific energy which a particle can take up without comminution. The material parameters are determined experimentally by single particle impact tests. This concept is also applicable to pharmaceutical powders, as will be shown in this work. A device is presented for the characterization of particles with sizes down to a few 10 microm. Particles are dispersed and accelerated in an air stream which is flowing against an impact plate. The impact velocity is controlled by the air flow. An LDA system enables the measurement of particle velocities. The results obtained with this jet mill are in accordance to those obtained from another single particle impact device used by Vogel and Peukert, in which the influence of fluid flow is completely avoided. Since the new device is especially designed for finer powders, it will allow a more detailed analysis of the material parameters at smaller particle sizes. Additionally, a new analysis method has been developed in order to determine the breakage probability not from sieve analysis but from laser light diffraction (LLD) data by using a population balance.