Modeling of segmented polyurethane drying process

Biomedical segmented polyurethanes (SPU) are frequently processed by soluti on-casting techniques in order to obtain multilayer films. For processing p arameters setup empirical criteria is currently applied. However, the selec ted conditions can strongly affect the physico-chemical properties of the f inished articles and they must be accurately established In order to descri be the SPU coating formation, balances of heat, mass and momentum transfer must be considered. To study the drying process, two new medical-grade SPU solutions commercial ly available, Biospan(TM) and Chronoflex AR(TM), were dried at different te mperatures. Desorption curves and drying rate, both as a function of temper ature, were obtained from experimental data. The mathematical model to desc ribe this process take into account the heat and mass transfer and film shr inkage along the whole drying step. The diffusion coefficients and the cont rol mechanism were determined from the best experimental data fitting. To obtain the solvent concentration profile, residual solvent and film, thi ckness at any time, heat and mass transfer balances were numerically solved using an explicit finite-difference method. The formulations showed differ ent behavior For Biospan drying, performed in the range from 50 degrees C t o 80 degrees C, a two-stage process was observed The first one controlled b y mass transfer in the gas phase and the second one with mb control. When 4 0 degrees C drying temperature was applied a mb control was found along the whole process. This change could be attributed to the acrylic additive pre sent in the SPU matrix, which has a glass transition temperature value in t he range of the process temperature. Chronoflex samples showed a two-stage process in all cases and lower diffusion coefficients than Biospan samples.