Modeling of Segmented Polyurethane Drying Process

ABRAHAM,G.A.; CUADRADO,T.R.

INTERNATIONAL POLYMER PROCESSING

Carl Hanser Verlag

Lugar: Munich, Germany; Año: 1998 vol. 13 p. 369 - 378

0930-777X

Biomedical segmented polyurethanes (SPU) are frequently processed by solution-casting techniques in order to obtain multilayer films. For processing parameters setup empirical criteria is currently applied. However, the selected conditions can strongly affect the physico-chemical properties of the finished articles and they must be accurately established. In order to describe 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 commercially available, Biospan™ and Chronoflex AR™, were dried at different temperatures. Desorption curves and drying rate, both as a function of temperature, were obtained from experimental data. The mathematical model to describe this process take into account the heat and mass transfer and film shrinkage along the whole drying step. The diffusion coefficients and the control mechanism were determined from the best experimental data fitting. To obtain the solvent concentration profile, residual solvent and film thickness at any time, heat and mass transfer balances were numerically solved using an explicit finite-difference method. The formulations showed different behavior. For Biospan drying, performed in the range from 50°C to 80°C, a two-stage process was observed. The first one controlled by mass transfer in the gas phase and the second one with mix control. When 40°C drying temperature was applied, a mix control was found along the whole process. This change could be attributed to the acrylic additive present in the SPU matrix, which has a glass transition temperature value in the range of the process temperature. Chronoflex samples showed a two-stage process in all cases and lower diffusion coefficients than Biospan samples.