Understanding flow properties of mannitol powder at a range of temperature and humidity

Finite Element Analysis of Extrinsic Parameters on the Onset of Tensile Strength in Powders

Most food, pharmaceutical, and chemical industries rely heavily on the supply of free-flowing powders that finds their application in raw materials, additives, and manufactured products. Improper storage conditions combined with environmental factors affect the free-flowing ability of powders. An undesirable transformation of these free-flowing powders into a coherent mass that resists flow is called caking. An important metric that can be used to measure the caking propensity of a material is the tensile strength, which is essentially the resistant stress needed to separate two layers of materials using an isostatic tensile strain. Even though several models have quantified the propensity of caking, the complex nature of interactions between the powder and its micro-environment makes the prediction of caking a challenging task. In the present work, the onset of tensile strength in isomalt with changes in temperature, relative humidity, and consolidation pressures using a shear cell was modeled using a finite element approach. The study found that at a consolidation pressure of 3 kPa and relative humidity of 85±0.1%, an increase in temperature by 5˚C increased the tensile strength of isomalt by a factor of 2. On the other hand, at a constant temperature of 25˚C, an increase in relative humidity from 85±0.1% to 86±0.1% registered an increase in tensile strength by 42.7%. This study also found that an increase in consolidation pressure from 3 kPa to 6 and 9 kPa increased the tensile strength by a factor of 1.79 and 2.54, respectively. The model had good agreement with the measurements and had an overall MAPE of 12.13%. This model can be applied to study the influence of extrinsic parameters on the propensity of caking during storage of bulk solids.

Evaluation of Time Consolidation Effect of Pharmaceutical Powders

PURPOSE

We aim to perform a systematic study of the time consolidation effect, i.e. the reduction of powder flowability resulting from at-rest storage, on a diverse array of pharmaceutical powders under different stress, humidity, and length of time.

METHODS

A ring shear cell-based methodology was employed. An instantaneous flow function was obtained, followed immediately by at-rest consolidation at precisely controlled humidity, stress, and duration. The consolidated powder was then subjected to shear-cell measurement. The difference in flowability between the immediate and consolidated specimens were attributed to the time consolidation effect.

RESULTS

Among the six excipients tested, three exhibited time consolidation at varying extents. Citric acid and starch underwent time consolidation only at high relative humidity (RH = 75%), promoted by vapor condensation and liquid bridge formation. For both materials, the flowability decreased with time, and the extent of time consolidation was not sensitive to the stress applied (0.4-2 kPa). Importantly, mannitol was found to time consolidate under both 50% and 75% RH. Given time, mannitol formed cake, giving rise to flow function below unity. Inverse gas chromatography analysis indicated that mannitol possesses high total surface energy among known pharmaceutical powders.

CONCLUSION

While time consolidation is prevalent among pharmaceutical powders, most can be mitigated by controlling the RH to below 75%. Notably, for materials possessing high surface energy, such as mannitol, time consolidation could occur at ambient humidity. Therefore, it is desirable to consider in-depth time consolidation evaluation for high surface energy powders, e.g. those subjected to milling or of amorphous nature.

Non-dispersive impact technology for powder flow characterization

This study developed a tester where the powder flow was characterized using a low sample mass (2 g) and impact instead of dispersion mechanism to mitigate test space constraint. An impact chamber was established where the test powder bed of seven lactose grades was weight-impacted to produce impact crater and ejecta, and imaged quantitatively to determine crater profiling signature (crater depth), regional topography (ejecta roughness), Otsu threshold (bed continuity) and edge segmentation (bed deformation). The Hausner ratio (HR) and Carr's index (CI) values of lactose, and their powder dispersion distance and surface area characteristics evaluated by gas-pressurized dispersibility test were examined as reference method. The crater signature profiling and regional topography were correlated to HR, CI, dispersive distance and surface area. A poorer powder flow was characterized by higher values of crater signature profiling, regional topography, HR, CI, and lower dispersive distance and surface area. The crater signature profiling and regional topography values were higher with smaller and rougher lactose particles that were cohesive. The powder impact flow is a viable non-dispersive approach to characterize powder flowability using a small sample mass and test space.