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

Despite their beneficial effects on the adhesion between naturalfibers and polymeric matrices, chemical pretreatments have become lessattractive since they are expensive and harmful to environment. The presentpaper deals with the effect of an eco-friendly and cost effective treatment,based on the use of a sodium bicarbonate solution, on the mechanical performancesof flax reinforced epoxy composites.Materials and methods: A commercial epoxy resin and unidirectional flaxfabrics (190 g/m2 areal density) were used as matrix and reinforcement,respectively. The fabrics were soaked in 5 and 10 wt% solutions of sodiumbicarbonate for 5 days at 25°C, then dried at 105°C for 1 day. By using untreatedand treated fibers, three epoxy laminates reinforced with four fabricswere manufactured through resin infusion. Quasi-static mechanical tests anddynamic mechanical tests were performed to evaluate the effect of the treatmenton the mechanical performances of the composites.Moreover, morphological analysis of the fractured surfaces was performedthrough a scanning electron microscopy and the voids fraction of each laminatewas evaluated by comparing the theoretical and real densities.Results: The results showed that all the properties increased with increasingthe concentration of the sodium bicarbonate solution: i.e., the tensilestrength and modulus of the 10% treated composites were 20% and ~45%higher than untreated composites. The improvements in flexural propertieswere equal to ~21% and 12.5% for strength and modulus, respectively. Asconcerns the DMTA results, the tanδ curves of all the laminates showed twopeaks: the first one (at ~80°C) related to the epoxy glass transition and thesecond one at higher temperatures related to the fiber-matrix interphase. Byincreasing the solution concentration, a slight variation of the temperatureand 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 onthe fiber-epoxy adhesion. This improved compatibility was confirmed by theSEM analysis of the fractured specimens: No pull-out phenomena affectedthe fiber-matrix interface and thin layers of matrix surround the fiber surfaceeven after the composite failure.