Drying of semicrystalline polymers: mathematical modeling and experimental characterization of poly(vinyl alcohol) films

Abstract

A mathematical model was developed to predict the drying mechanism of semicrystalline polymers involving multiple solvents. Since drying of semicrystalline polymers can be accompanied by changes in polymer degree of crystallinity, the model integrates crystallization kinetics and the Vrentas -Duda diffusion model to provide a better understanding of the mechanism. The model considers the effect of external conditions such as temperature, film shrinkage and diffusion and evaporation of multiple solvents during drying. Poly(vinyl alcohol) (PVA)/water/methanol was chosen as a test system. The drying kinetics of PVA films swollen in water and methanol were investigated using gravimetric techniques. The model predicts that higher temperatures, lower film thicknesses and lower methanol to water ratios increase the drying rate. The model predictions were compared with experimental data and showed good agreement. q

Experimental Studies and Modeling of the Drying Kinetics of Multicomponent Polymer Films

“…The drying mechanisms of semicrystalline polymers like PVA have been mathematically modeled in a few studies (13)(14)(15)(16)(17). These studies show that increasing the drying temperature and decreasing the molecular weight of the polymer can increase the rate of crystallization and the final crystallinity of the polymer.…”

“…The crystallization rate was inversely related to the molecular weight, which is explained by the inability of long polymer chains to fold themselves into crystals. Wong et al expanded on this theory, using the Flory-Huggins thermodynamic and Vrentas-Duda free-volume theories to form a consolidated crystallization kinetics and multi-solvent diffusion scheme (16,17). This model is used to study the effects of external factors like temperature, film folding, diffusion, and evaporation of different solvents (19)(20)(21).…”

Multi-Zone Drying Schemes for Lowering the Residual Solvent Content during Multi-Component Drying of Semicrystalline Polymers

“…The trapping of methanol at the rubbery region increased the overall residual solvent content in the system and may have an effect on the overall properties of the end product. Our simulation results indicated that the presence of water within the film helps the diffusion of methanol, [15,16] and this is due to an increase in free volume within the film by water. In addition, the presence of water lowers the rate of glassy skin formation and growth, thus allowing more methanol to be removed before it is trapped.…”

“…The mathematical models that we have previously developed integrate Vrentas-Duda free volume theory [20,21] with crystallization kinetics and glassy skin formation to paint a detailed picture of the concentration of multiple solvents and the development of crystallites within polymer films. [15,16] From our previous simulation results involving multiple solvent systems, the residual solvent trapped in the film may be higher than acceptable and desirable levels depending on drying conditions. We found that the interaction of the solvents within the film plays an important role in determining the rate of diffusion of individual solvents and the residual solvent content.…”

“…Glassy skin formation could further hinder the removal of solvents and may cause solvent trapping. We have developed mathematical models to describe these behaviors, [15,16] and this article focuses on application of these models to develop optimal drying schemes and verification of these theoretical results using experimental techniques.…”