Assessing Flowability of Cohesive Powders from a Small Sample Quantity

Processing of fine and cohesive powders is very difficult and often marred by inconsistencies in powder flow behaviour which adversely affects plant reliability and productivity. Good examples are in pharmaceutical, fine chemicals and nuclear industries, where dosing and dispersion of small quantities of cohesive powders is technologically very challenging. For instance for drug delivery through lungs the functionality of dry powder inhalers is strongly dependant on flowability of weakly compacted bulk powders. Current test methods for assessing the flow behaviour of powders (unconfined compression and shear cell testing) require a relatively large amount (at least 100 g) of powders. This is undesirable for industries such as nuclear and pharmaceutical due to ionising radiation for the former and toxicity, cost of drugs and lack of availability at the early stages of development for the latter. Furthermore the traditional test methods are not suitable for testing very weak compacts and the packing may be different from that used in the common methods such as unconfined compression and shear cell. Therefore the flowability of the sample has to be assessed in its own container. This research project is formulated to evaluate the analysis of the deformation and flow behaviour of fine cohesive powders at small scales (typically a few cubic mm) and at very low loads by the indentation probe method. An integrated approach is proposed to achieve the goal, which includes experimental work to analyse the mechanical properties of the single particles and bulk flow behaviour and simulation work to relate the bulk behaviour to the properties of primary particles. The described procedure will establish a link between the microscopic and macroscopic behaviour of particle assemblies subjected to compression and will elucidate the circumstances under which the particles form a strong compact. In particular the deliverables are:-a correlation between powder flow function, based on the unconfined yield stress, and indentation characteristics for samples of a few cubic mm;-a validated test methodology for measurement of the bulk yield stress of small quantities of cohesive powders;-an understanding of powder deformation at low loads.