Résumé
With the use of Liquid Composite Moulding (LCM) for composite manufacturing in the transportation industry, the complexity of part shapes and quality requirements have increased tremendously. There is thus a strong need to accurately control and understand liquid flow through porous preforms in order to minimise void formation in composite parts. Capillary issues and wetting affinity between fibers and resin have to be considered to understand and simulate properly the resin flow during processing. Flax fibers are usually considered adequate bio-reinforcements for composites, thanks to their good mechanical properties and low density. Nevertheless their hydrophilic character makes them sensitive to moisture sorption and incompatible with hydrophobic resins. In this work, a thermal treatment aiming to modify the surface chemistry of flax fibers has been applied in order to make the fiber surface more hydrophobic and decrease the sensitivity to water sorption. Measurements on elementary fibers have been performed to quantify the effect of the thermal treatment on surface energy. On the other hand, the thermal treatment could also result in a decrease of the mechanical properties of flax fibers. The decrease of mechanical properties of yarns does not necessarily imply a decrease in the composite mechanical properties if the interfacial properties are enhanced. In order to evaluate effect of treatment on porosity formation and the Inter-Laminar Shear Strength (ILSS) of composites, panels reinforced by both untreated and treated flax fibers were manufactured by Vacuum Assisted Resin Transfer Molding (VARTM), with strictly controlled and identical process parameters. Quantification of voids and results of mechanical tests will be presented, highlighting the effect of thermal treatment on the mechanical behavior of flax yarns and flax reinforced composites.