Modeling of multilayer drying of polymer films

Abstract

ABSTRACT:A model was developed to predict the drying behavior of multilayer polymer films on inert substrates. The model considers simultaneous heat and mass transfer controlled by complex thermodynamic and transport properties of polymer solutions. Key components of the model are the incorporation of the free volume theory to predict diffusivities in each polymer layer, the use of heat and mass transfer coefficients to describe complex transport phenomena in the gas phase, the incorporation of exact equilibrium boundary conditions at polymer-polymer interface, and the use of the Flory-Huggins theory to describe both liquid-liquid and vapor-liquid equilibria. The model can be applied to guide processing, product formulation, scale-up, and oven design. As an example, the model is applied to simulate the drying of a two-layer coating of poly vinyl acetate (in toluene) over polystyrene (also in toluene) on a polyester substrate.

Optimization of operating conditions in a single‐zone drier for two‐layer polymer coatings

“…The optimal heat‐transfer coefficients at any air temperature were higher and the residual solvent content was lower than those for the base case. Alsoy and Duda1 also showed that the residual solvent is lower if PS is in the lower layer. This is because of reduction of the commonly known “skinning” effect.…”

Drying of Films Formed by Ordered Poly(ethylene oxide)−Poly(propylene oxide) Block Copolymer Gels

“…The Flory-Huggins theory is widely used to describe the solvent activity in polymer systems 68 and has been utilized in the modeling of drying of polymer solutions. 9, 26,29,30,67 However, for the systems examined here, the Flory-Huggins equation is not adequate to describe the water activity. 39 Instead, we used a linear function to correlate the water vapor pressure and water concentration:…”

“…Since there is a large water vapor source (large amount of saturated aqueous salt solutions) in the sealed container where we kept the hydrogel film for drying, the water vapor pressure in contact with the Pluronic hydrogel film is considered constant at all times. In the case of variable film thickness (a function of water content in the film), the boundary condition at the film−air interface is obtained by a jump mass balance, ,,, where the term C (d h ( t )/d t ) accounts for the water flux due to the moving boundary: where h ( t ) is the thickness of the hydrogel film and is a function of drying time; k G is the mass transfer coefficient between the film surface and the adjacent air; p G is the water partial pressure at the film−air interface; and is the water pressure in the bulk air (constant at all times).…”

Nonlinear Model-Based Control of Thin-Film Drying for Continuous Pharmaceutical Manufacturing

“…Large enough concentration gradients indicate skin formation on the surface of the thin film, which is a common phenomenon in the drying of single-layer polymer films. 65 Figure 8 shows the evolution of the film temperature, which exhibits first-order dynamics in each of the drying zones and asymptotically increases to the hot air temperature in zone 3 as the cooling effect associated with vaporization of the solvent gradually diminishes due to lower solvent evaporation rates. Figure 8 indicates that the film temperature dynamics are significantly faster than the dynamics of the solvent concentration variations in the film (see Figure 5), which is a result of both the thermal diffusion in the thin film being much larger than the molecular diffusion (30 < Lewis number < 7 × 10 5 ) and the thermal convection dynamics being a few orders of magnitude faster than the mass convection dynamics (2 × 10 3 < k c /k m < 5 × 10 4 ).…”

“…However, as the drying proceeds, significant concentration gradients develop within the film, particularly in zone 3 (see Figure c), due to the strong diffusional resistance that severely slows the solvent removal rate. Large enough concentration gradients indicate skin formation on the surface of the thin film, which is a common phenomenon in the drying of single-layer polymer films …”