Improving the mechanical properties of unidirectional flax fabrics reinforced epoxy composites by sodium bicarbonate treatment

Abstract

Despite their beneficial effects on the adhesion between natural fibers and polymeric matrices, chemical pretreatments have become less attractive since they are expensive and harmful to environment. The present paper deals with the effect of an eco-friendly and cost effective treatment, based on the use of a sodium bicarbonate solution, on the mechanical performances of flax reinforced epoxy composites. Materials and methods: A commercial epoxy resin and unidirectional flax fabrics (190 g/m2 areal density) were used as matrix and reinforcement, respectively. The fabrics were soaked in 5 and 10 wt% solutions of sodium bicarbonate for 5 days at 25°C, then dried at 105°C for 1 day. By using untreated and treated fibers, three epoxy laminates reinforced with four fabrics were manufactured through resin infusion. Quasi-static mechanical tests and dynamic mechanical tests were performed to evaluate the effect of the treatment on the mechanical performances of the composites. Moreover, morphological analysis of the fractured surfaces was performed through a scanning electron microscopy and the voids fraction of each laminate was evaluated by comparing the theoretical and real densities. Results: The results showed that all the properties increased with increasing the concentration of the sodium bicarbonate solution: i.e., the tensile strength and modulus of the 10% treated composites were 20% and ~45% higher than untreated composites. The improvements in flexural properties were equal to ~21% and 12.5% for strength and modulus, respectively. As concerns the DMTA results, the tanδ curves of all the laminates showed two peaks: the first one (at ~80°C) related to the epoxy glass transition and the second one at higher temperatures related to the fiber-matrix interphase. By increasing the solution concentration, a slight variation of the temperature and a significant decrease of the height of the second peak were found. Discussion: All the results are due to the beneficial effect of the treatment on the fiber-epoxy adhesion. This improved compatibility was confirmed by the SEM analysis of the fractured specimens: No pull-out phenomena affected the fiber-matrix interface and thin layers of matrix surround the fiber surface even after the composite failure.