The effect of vessel material on granules produced in a high-shear mixer.

In this study the effect of different vessel wall materials on the granule size distributions obtained during high-shear granulation of different materials is investigated. The distributions obtained in glass and stainless steel vessels differ from those obtained in PMMA (polymethyl methacrylate) and PTFE (polytetrafluoroethylene) vessels. The high contact angle of PMMA forces all liquid immediately into the more easily wetted powder bed. In this vessel a fast liquid absorbing powder nucleates in the droplet controlled regime, leading to a narrow particle size distribution. In a vessel with a low contact angle (glass or stainless steel) a liquid layer can be formed on the wall surface. This liquid causes an inhomogeneous distribution of liquid over the powder bed; a broader granule size distribution is the result. With a powder that slowly absorbs liquid, local overwet areas can be created, resulting in large granules. This results in broader granule size distributions as well. In conclusion; the contact angle of the vessel material and the wetting rate of the powder used determine the granule growth process and the resulting granule size distribution.