(38d) Air Permeability of Powders at Process Conditions

The powders are widely used in many industrial processes, e.g. pharmaceutical, food, battery, additive manufacturing. Many are the properties of the powders that influence the efficiency of the processes and the quality of the final product. Among these properties is the permeability, which can be defined as the capacity of a powder to transmit a fluid, usually air, through its bulk. For example, permeability affects: 1) the filling weight in dosator nozzle machines used for capsule filling (Faulhammer et al., 2014), 2) the effective dispersion and release of the inhalation mixture of dry powder inhalers (Shalash et al., 2018), 3) the capacity of the powder to get out from the silo during powder silo discharges (Hsiau et al., 2012).

Depending on the process conditions, the powder reaches specific packing fractions. The packing fraction, defined as the ratio between the bulk and the true density, is a measure of the powder densification. The permeability is strongly dependent on the packing fraction, as the larger the powder densification, the lower the permeability. On the contrary, the looser the powder, the easier the passage of the air through the particles and hence the larger the permeability. Therefore, it is important to measure the permeability at the same packing fractions induced to the powder by the process to properly correlate the permeability to the quality of the final product. In this work, a new measurement method is proposed. It combines the possibility of selecting the packing fraction in a wide range of densification conditions and the possibility of measuring the permeability in correspondence with the chosen packing fraction values. The instrument adopted is the GranuPack (Granutools, Belgium), a high resolution tapped density analyzer, by which it is possible to control the powder densification by applying the desired number of taps to the powder, contained in the permeability cell. At the bottom, the cell presents a porous frit to prevent the powder from entering the air supply pipe but allow the passage of the air for the permeability measurement. By alternating the application of taps and the permeability measurements, it is possible to have access to the evolution of the permeability with the packing fraction. Different lactose and abrasive powders, characterized by different shapes and sizes, have been tested and the evolution of permeability with the packing fraction has been obtained. Two parameters, that quantify the effect of densification on permeability, are proposed: the permeability ratio and the rate of variation of permeability. These parameters are directly related to the powder cohesiveness and hence can be used to complement the classical flowability indexes.

The new insights gathered from this approach give a better understanding of the material properties and allow the development of improved formulations for better processability.

References

Faulhammer, E., Llusa, M., Radeke, C., Scheibelhofer, O., Lawrence, S., Biserni, S., Calzolari, V., & Khinast, J. G. (2014). The effects of material attributes on capsule fill weight and weight variability in dosator nozzle machines. International Journal of Pharmaceutics, 471(1–2), 332–338. https://www.sciencedirect.com/science/article/pii/S0378517314004116

Hsiau, S., Liao, C., & Lee, J. H. (2012). The discharge of fine silica sand in a silo under different ambient air pressures. Physics of Fluids, 24(4). https://pubs.aip.org/aip/pof/article/24/4/043301/257578

Shalash, A. O., Khalafallah, N. M., Molokhia, A. M., & Elsayed, M. M. A. (2018). The Relationship Between the Permeability and the Performance of Carrier-Based Dry Powder Inhalation Mixtures: New Insights and Practical Guidance. AAPS PharmSciTech, 19(2), 912–922. https://doi.org/10.1208/S12249-017-0898-7