Pharmaceutical Technology - October 2021


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28 Pharmaceutical Technology APIs, EXCIPIENTS, AND MANUFACTURING eBOOK 2021 P h a r mTe c h . c o m Excipients ties were measured for five different commercial samples of untreated fumed silica, which is a f low aid used routinely in the pharmaceutical industry. Each of the samples was purchased from a different supplier but all had a similar D50 (median particle size) and particle size distribution. All properties were measured using standard test protocols for the instrument (FT4 Powder Rheometer, Freeman Technology) (6). Figure 3 highlights differences between the five supplies with respect to specific energy (SE), com- pressibility, and permeability. SE is a dynamic powder property that, like BFE, quantifies the f lowability of powders in a low stress state. How- ever, in contrast to BFE, SE is measured with an upward traverse of the powder rheometer blade that subjects the powder to a gentle, lifting action. Therefore, SE quantifies how a powder will be- have in an unconfined state, when f lowing under gravity, rather than under the forcing conditions applied in BFE measurement. Higher SE values are typically associated with a greater degree of mechanical interlocking and friction between particles in the powder, as a result of particle morphology—particle size, shape, and/or surface roughness. Sample 1 generates the highest SE value, which is almost double that of the lowest, Sample 4. This suggests that Sample 1 may be significantly more difficult to mix and disperse through a bulk sub- strate, which is an important characteristic for a f low aid. SE values provide a useful ranking of the supplies, in terms of this behavior. Compressibility is quantified in terms of per- centage change in volume (or density) as a func- tion of applied normal stress. The results show that Sample 5 is the most compressible material tested, while Samples 1, 2, and 3 have similar, much lower compressibility. Powders that are more cohesive typically exhibit high compress- ibilit y because of t heir abilit y to entrain air within an inefficiently packed powder bulk. As normal stress is increased, this air is forced out, significantly reducing volume. From a practi- cal perspective, powders that exhibit high com- pressibility are more susceptible to consolidation. These results indicate that Samples 1, 2, and 3 are less likely than Sample 4 and most of all Sample 5 to exhibit substantial consolidation during long- Figure 3. Specific energy (SE), compressibility, and permeability values for five commercial untreated fumed silica supplies show clear differences between the materials.

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