Fluidized-Bed Coating with Sodium Sulfate and PVA-TiO2, 3. The Role of Tackiness and the Tack Stokes Number

In the first and second parts of this study [Hede, P. D.; Bach, P.; Jensen, A. D. Ind. Eng. Chem. Res. 2009, 48, 1893 and 1905], agglomeration tendencies were studied for two types of coatings: sodium sulfate and PVA-TiO2. Results showed that the agglomeration tendency is always lower for the salt coating process than for the polymer coating process, under similar process conditions, because of differences in stickiness, not because of differences in bulk viscosities. It was further shown that it is possible to optimize the PVA-TiO2 coating formulation and process to achieve a low tendency of agglomeration, similar to the salt coating process. With the PVA-TiO2, coating liquid layer thicknesses encountered during these fluid-bed coating processes, agglomeration seems to be governed primarily by liquid surface phenomena. A modification to the original viscous Stokes number is suggested in the present paper, which defines the Stokes number in terms of the work needed to reach maximum tack instead of the viscous dissipation energy. The new tack Stokes number correlates well with the observed levels of agglomeration and, as a promising feature, proportionality is observed between the agglomeration weight percentage and the difference between the new tack Stokes number and the critical tack Stokes number.