Assessing powder flowability at low stresses using ball indentation method: Evaluation of constraint factor

U Zafar,C Hare,A Hassanpour,M Ghadiri

doi:10.1016/j.powtec.2021.04.032

Abstract

Powders can exhibit different flow behaviour resulting from a combination of physical properties of the material and equipment design. Problems with powder flow are ubiquitous in process industry and become prominent when dealing with fine and cohesive powders. It is therefore of great importance to characterise the flowability of cohesive materials for better process control. Powder flowability is commonly assessed under relatively high preconsolidation loads using shear cell and uniaxial compression methods by which the unconfined yield strength (Y) is evaluated as a function of the applied load. However, these techniques are typically limited to applied stresses greater than 1 kPa and require a relatively large quantity of powder. To overcome these limitations, the recently developed Ball Indentation Method (BIM) is used in this work for assessing powder flow behaviour at low stress levels. The unconfined yield strength (Y) is inferred from the resistance to ball penetration into the surface of a powder bed, based on the method for measurement of hardness (H). This requires the flow resistance, represented by hardness, to be related to the unconfined yield strength by a proportionality factor termed the constraint factor, C, following the analogy with yield stress measurement in continuum solids, i.e. Y=H/C. The constraint factor for silanised glass ballotini, calcium carbonate, α-lactose monohydrate, Avicel and limestone is evaluated and reported here. It is shown that the unconfined yield strength inferred by this method correlates well with those from the uniaxial compression and shear cell measurement. The characterisation of the constraint factor makes it possible to use BIM for powder flowability testing at low stress levels and using a very small powder quantity. This is highly desirable for applications such as capsule filling, tableting and dry powder inhaler devices.

Highlights

The characterisation of bulk behaviour of cohesive powders is very important in processing of particulate solids, e.g. for reliable powder flow out of storage vessels
The tests are done under quasi-static conditions for all three test methods, i.e. shear cell, uniaxial compression test and Ball Indentation Method
The hardness is plotted against the preconsolidation stress, whilst the unconfined yield strength is a function of the major principal stress so the abscissa has two representations for the same numerical figures

Summary

Introduction

The characterisation of bulk behaviour of cohesive powders is very important in processing of particulate solids, e.g. for reliable powder flow out of storage vessels. There are various qualitative methods developed and frequently used for assessing powder flowability due to their simplicity These ring shear tester [6] and Brookfield powder tester [10]. The bulk cohesion and unconfined yield strength can only be measured reliably at applied loads typically larger than 1 kPa, and extension to lower stress levels are generally made by extrapolation. The yield locus of a powder is described by the non-linear Warren-Spring model [11], but most often a straight line is fitted to the yield locus These tests are generally not capable of handling measurements for applied loads much less than 1 kPa [12,13]

Methods

Results

Discussion

Conclusion