(473f) Characterization of Pharmaceutical Powder Flowability Using a Powder Flow Cell Coupled to an Air-Bearing Rheometer

The change in flow properties of powders at different stages of processing can greatly impact the quality of the final product. To ensure consistent outcomes, it is crucial to effectively characterize and comprehend the flow properties at each stage. Therefore, various powder characterization methods have been developed in literature and one needs to pick the one which mimics the state of powder in the process of interest to get the most meaningful information [1].

The focus of our work centers on a powder flow cell accessory mounted on an air-bearing rheometer which has a diverse array of testing methods [2]. In particular, Warren Spring Cohesion (WSC), Cohesion Strength (CS), and Dynamic Flow tests were explored on this device. The instrument is especially advantageous due to its sensitivity and ability to produce data at very low torque levels since it’s on an air-bearing platform. The WSC evaluates the flowability of a powder under a defined consolidated state (applied normal stress). The flowability of the powder is assessed by measuring the maximum torque registered by a submerged vane geometry, which is also the failure point of the sample and is called the WSC strength. Prior to the shear to failure step the penetration of the vaned geometry is also monitored (normal force evolution) which gives an indication of the penetration work for the said consolidated sample.

The CS test involves a powder preparation step which includes fluidizing the powder to erase any sample memory (loading effects) on the measurement. The Cohesion strength test describes the internal resistance of the powder to flow, and thereby is a measure of powder flowability under no load. This CS method can only be used with powders that can be fluidized. Comparability of different powders is gauged by probing similar stress states in the powder bed, which is ensured by measuring at the same position in the powder bed (i.e., using the same volume/mass). Data is normalized with aerated or loose density values when very dissimilar materials (having very different densities) are compared.

The dynamic flow cell testing allows the calculation of various dynamic flow parameters for quality control, such as basic flowability energy (BFE), stability index (SI), flow rate index (FRI) and specific energy (SE).The dynamic flow cell testing provides vast sensitivity in terms of determining flowability in both confined and unconfined states.

More than 10 pharmaceutical powders were tested using the above-mentioned tests. It was found that the WSC method is more applicable to materials that exhibit more of a cohesive behavior. When the pre-compaction of the powder bed was increased, materials that were categorized as cohesive demonstrated higher WSC values. The CS method finds its applicability in pharmaceutical manufacturing operations such as fluidized bed processes. Specific Energy, measured in the Dynamic flow test, showed much higher values for very cohesive powders.

In conclusion, these characterization studies must be analyzed in tandem to understand the flowability of Pharmaceutical Powders used in different manufacturing stages. By gathering all the data obtained from these techniques in an effective way, extensive material databases can be generated which are not prevalent in the industry. Using the powder flow cell coupled to an air-bearing rheometer provides users with useful information, while preserving standards of easy-to-use, precision, and repeatability.

[1] Schulze, Dietmar. "Powders and bulk solids." Behaviour, characterization, storage and flow. Springer22 (2008).

[2] Hassanpour, Ali, Colin Hare, and Massih Pasha, eds. Powder flow: Theory, characterisation and application. Royal Society of Chemistry, 2019.