Modelling and experimental characterization of unsaturated flow in absorbent and swelling porous media

The present study addresses unsaturated flow in swelling porous media. The investigated materials are made of super absorbent polymer granules (SAP) blended with cellulose fibres (fluff), relevant to absorbent hygiene products (AHP). A model is developed based on mass and momentum balance equations taking into account the morphological modifications due to the swelling of SAP particles during water absorption under saturated and/or unsaturated liquid conditions. Constitutive equations for the composite medium express the kinetics of water absorption, the hydraulic conductivity, the relative permeability and the hysteretic patterns upon imbibition and drainage. The model is validated and compared with results of experimental campaigns performed in unsaturated conditions using AHP samples with different SAP/fluff mass ratio (SFR). The systematic deviation between computed and experimental results in the very early stages of the tests is interpreted in the light of percolation theory, and model improvements are achieved implementing the finite-scale effects method.