Umair Zafar

Assessment of Surface Caking of Powders Using the Ball Indentation Method

Powder caking is a ubiquitous problem, which could significantly decrease product quality and lead to economic losses. Hence it is important to know the conditions under which it occurs. The caking behaviour of three powder materials (PVP, HPC and CaHPO4) has been investigated by the ball indentation method (BIM) as affected by relative humidity (RH), temperature and time. The resistance to powder flow, as indicated by the hardness is measured by a ball indenting the powder bed surface. The surface hardness increases with increasing RH and temperature, indicating caking of the powder bed. Moreover, the temperature and RH show a coupled effect on powder caking. Irreversible caking is formed in PVP and HPC at 75% RH; the particles coalesce and the volume of powder bed is significantly reduced with time. However, the caking of CaHPO4 is reversible. To examine the caking mechanism of PVP and HPC, the critical glass transition RH is determined at 25°C and 45°C. The values are 63% and 53% RH for PVP and 61% and 50% RH for HPC, respectively. The glass transition moisture content in the ball indentation experiments is comparable with that determined by the dynamic vapor sorption measurement. BIM could be a fast and effective method for the assessment of powder surface caking.

Evaluation of a new dispersion technique for assessing triboelectric charging of powders

&NA; In a number of applications, especially in pharmaceutical drug development, there is often a very small powder quantity available for evaluating the manufacturability of new drugs. However, it is highly desirable to be able to quickly evaluate processing issues, and where possible using the smallest powder quantity. In the present work, a proprietary commercial powder dispersion device (the disperser of Malvern© Morphologi G3) is adapted to evaluate the triboelectric charging tendency. A very small powder quantity (as small as 0.1 mg) is dispersed by a pressure pulse of compressed gas such as air or nitrogen. This causes the particles to become air borne and collide with the containing walls, resulting in dispersion and leading to triboelectric charge transfer between the particles and the walls. In this work, the charging propensity of a number of materials is evaluated and the effect of particle surface functional groups on the tribo‐electric charge transfer is analysed. Model materials with a well‐defined shape (glass ballotini) but with different silane groups deposited on their surfaces as well as a number of organic crystalline particles (such as aspirin, &agr;‐lactose monohydrate and paracetamol) are tested. Following dispersion the particles move immediately to a Faraday cup placed directly underneath the disperser. Therefore, particle charge is measured with no decay. The method can differentiate charging of different polymorphs of the same material, different silane groups on the surfaces of glass ballotini and different crystal morphologies obtained from crystallisation from various solvents. Graphical abstract Schematic diagram of the dispersion setup for measurement of tribo‐electric charging of particles, used in this study. Figure. No caption available.