Investigation of capillary impregnation for permeability prediction of fibrous reinforcements

In thermoplastic pultrusion process, unidirectional glass fibre bundles are very often used as reinforcement. This study was motivated by the industrial requirement to evolve from low viscosity thermoset resin processes to high viscosity thermoplastic polymers. The key parameters to control the impregnation process are the permeability of the fibrous reinforcement and the capillary pressure. The current study's objectives are threefold: (1) study the fibres arrangement by X-ray tomography, (2) determine the longitudinal and transversal permeabilities as well as capillary pressure to use them as input parameters of impregnation process in pultrusion and (3) carry out sensitivity analyses of the permeability measurements to the perturbation of fibre structure. To reach these purposes, two theoretical models were used to determine both axial and transversal tow permeability. The axial impregnation of aligned fibres was described by Washburn equation applied to 1D infiltration into the bundles. The in-plane impregnation was described by Darcy's law. The elliptical equation for highly anisotropic medium was resolved. Results show that permeability measurements are subjected to large variations, caused by perturbation of the fibre architecture during experiments. Besides, it was shown that the tensile force applied on fibres during pultrusion has a great effect on impregnation. Indeed, the permeability increases with the applied tension.