(162e) Modeling of Flow and Drying of Aqueous Polymer Coatings on Porous Pharmaceutical Tablets
- Conference: AIChE Annual Meeting
- Year: 2017
- Proceeding: 2017 AIChE Annual Meeting
- Group: Pharmaceutical Discovery, Development and Manufacturing Forum
Monday, October 30, 2017 - 1:58pm-2:20pm
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; MÃ¶ltgen 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.
Alleborn, N. and Raszillier, H. (2004) âSpreading and sorption of a droplet on a porous substrate", Chemical Engineering Science, 59(10), pp. 2071-2088.
Kadja, M., Bergeles, G., (2003) âModelling of slurry droplet dryingâ, Applied Thermal Engineering, 23( 7) pp. 829-844.
MÃ¶ltgen, C., Puchert, T., Menezes, J.C., Lochmann, D. and Reich, G. (2012) âA novel in-line NIR spectroscopy application for the monitoring of tablet film coating in an industrial scale processâ, Talanta, 92, pp. 26â37.
Process Systems Enterprise Ltd. gPROMS User Guide. London, UK, December 2015.
Schwartz, L.W., Roy, R.V., Eley, R.R. and Petrash, S. (2001) âDewetting patterns in a drying liquid filmâ, Journal of Colloid and Interface Science, 234(2), pp. 363-374.
Weidner, D.E., Schwartz, L.W. and Eley, R.R. (1996) âRole of surface tension gradients in correcting coating defects in cornersâ, Journal of Colloid and Interface Science, 179(1), pp. 66-75.