Modelling of the process of desorption of liquid by polymer beads, by considering diffusion and shrinkage

Abstract

Shrinkage of a polymer occurs during the process of desorption but the classical models, as well as the analytical solutions, are obtained by neglecting this change in the dimension of the polymer. A new model, based on a numerical method with finite differences, is built up by taking into account not only the transport of liquid through and out of the polymer but also the shrinkage of the polymer which follows this desorption. Various beads of ethylene (vinyl acetate) copolymers with various contents of vinyl acetate ranging from 14 to 40% are used for the purpose, because the capacity of absorption of liquid depends largely on the content of vinyl acetate. The presaturated beads are immersed in water, and the transport of aniline used as the liquid is described by a transient diffusion with constant diffusivity. Various results are obtained, viz. the kinetics of desorption, the profiles of concentration of liquid developed through the polymer, and the change in dimensions of the bead. Moreover, it is shown that the analytical solution cannot be used properly when the amount of liquid absorbed exceeds ca 15%.