(162e) Modeling of Flow and Drying of Aqueous Polymer Coatings on Porous Pharmaceutical Tablets

Authors: 
Sørensen, E., University College London
Mazzei, L., University College London
The application of polymer coating films on the surface of tablet cores is crucial in the manufacture of solid dosage drugs. Understanding the coating film behaviour at the point of contact between the liquid solution and the porous tablet core can reduce the number of defective tablets being discarded and allow a better prediction of the optimal operating conditions for the coating process. In our work, to estimate the coating film attributes and uniformity, we developed a new mathematical model that defines and describes the spreading, absorption and evaporation of polymer solutions onto and from a tablet.

Several researchers (Weidner et al., 1995; Schwarz et al., 2001) have considered polymer coating film drying but have not accounted for changes in coating density owing to solvent evaporation. In our work, we considered that the film density increases as the solvent (water) evaporates from the surface of the tablet. The influence of solvent evaporation on important coating spreading parameters, such as the viscosity of the aqueous solution and the diffusion coefficient of the polymer in the solvent, was also investigated.

To better describe the behaviour of the film on the surface of the tablet we divided the overall process into three stages. For the first stage, we considered the coating solution impacting on the tablet surface and then spreading on the surface and absorbing into the tablet. We modelled the coating solution as a liquid of changing density and viscosity due to solvent (water) evaporation. The absorption during the first stage is affected by polymer particle retention in the tablet pores, the volume fraction of the latter not being necessarily uniform in the tablet. We assumed that the viscosity of the coating solution becomes infinite when the mass fraction of the polymer reaches a certain value (Weidner et al., 1995). At this point, the film spreading and absorption are hindered and the second stage commences. During the latter, we assumed that only solvent evaporation takes place from the tablet surface. This stage lasts until the polymer concentration reaches a new critical value, at which point a polymer particle crust develops at the surface of the coating film (Kadja and Bergeles, 2003). In the final stage, we considered the evaporation of the solvent (water) from beneath the coating crust (i.e., tablet porous core).

We derived an expression for the movement of the absorbed coating fluid inside the porous tablet based on a kinematic boundary condition of the free wetting front surface (Alleborn and Raszillier, 2004). The pressure and the vertical and radial velocities inside the porous substrate were calculated from the Laplace and Darcy equations, respectively. We also took into account the effect of the tablet porosity and permeability on the polymer particle retention.

We implemented our novel model in gPROMS, employing the Modelbuilder modelling platform (Process Systems Enterprise Ltd., 2015). The model estimated the water and polymer content inside the porous tablet core and the dry coating film position and height. The numerical results were in good agreement with experimental data taken from the literature (Schwartz et al. 2001; Möltgen et al. 2012). In future work, our approach could be extended to also predict the spreading of non-Newtonian coating liquids employed by the pharmaceutical industry as there are many different models that propose relationships between viscosity, stress and strain rate.

References:

Alleborn, N. and Raszillier, H. (2004) “Spreading and sorption of a droplet on a porous substrate", Chemical Engineering Science, 59(10), pp. 2071-2088.

Kadja, M., Bergeles, G., (2003) “Modelling of slurry droplet drying”, Applied Thermal Engineering, 23( 7) pp. 829-844.

Möltgen, C., Puchert, T., Menezes, J.C., Lochmann, D. and Reich, G. (2012) ‘A novel in-line NIR spectroscopy application for the monitoring of tablet film coating in an industrial scale process’, Talanta, 92, pp. 26–37.

Process Systems Enterprise Ltd. gPROMS User Guide. London, UK, December 2015.

Schwartz, L.W., Roy, R.V., Eley, R.R. and Petrash, S. (2001) “Dewetting patterns in a drying liquid film”, Journal of Colloid and Interface Science, 234(2), pp. 363-374.

Weidner, D.E., Schwartz, L.W. and Eley, R.R. (1996) “Role of surface tension gradients in correcting coating defects in corners”, Journal of Colloid and Interface Science, 179(1), pp. 66-75.