Study and optimisation of flax fiber reinforced bio-sourced Epoxy resin composites interfacial adhesion and compatibility

Recent advances in the field of natural fibre reinforced composites, allowing to better understand and control their properties, the raise of environmental awareness amongst costumers and the need for low-weight, cost-effective materials in industries such as the automotive contribute to promising projections for the increase in demand of natural fibre reinforced composites. Flax fibres present the best combination of mechanical proprieties, whose specific values are comparable to those of glass fibres. However, the specific strength of composites made with untreated flax is still much lower than that of glass fibres composites, most probably due to poor load transfer. This indicates a need for interface improvement. Essentially controlled by the chemistry and morphology of the interface, interfacial compatibility and adhesion in fibre reinforced composites can be described in terms of physicochemical interactions, chemical bonding and mechanical interlocking. This work was mostly devoted to characterising the effects of chemical and mechanical surface properties of flax fibres as a means to optimise their adhesion and compatibility with the matrix. Fibres were either untreated, aminosilane-treated or washed via one of two different mild treatments with ethanol or water selected from the literature. Changes in physicochemical interactions were investigated using the Wilhelmy technique to assess fibres wettability. In parallel, X-ray photoelectric spectroscopy (XPS) analysis was performed to study surface chemical composition of untreated and aminosilane-treated fibres. The combined effect of those parameters and mechanical interlocking on interface mechanical properties due to fibres treatments were characterised via microbond tests. Single-fibre tensile tests were also performed, proving no significant degree of degradation due to storage conditions or handling of the fibres. Treatment with Bis(trimethoxysilylpropyl)amine was the only modification effective in improving the practical adhesion and was demonstrated to be strongly determined by chemical bonding, while, contrary to what was expected, washing methods seemed to worsen interface strength. Although the root causes of this phenomena may lay in the frictional stresses, it could not be assessed by the approach used.