Cyclic hygrothermal ageing of flax fibers' bundles and unidirectional flax/epoxy composite. Are bio-based reinforced composites so sensitive?

The use of vegetal fibers to reinforce polymeric matrix composites is challenging and goes with questions of durability issues, especially when exposed to damp conditions. The aim of this study is to quantify, up to 1 year, the impact of humidification/drying cycles, i.e. 3.5 days at 90% HR and 3.5 days at 40% HR, both at 55 degrees C, on the longitudinal mechanical properties of a unidirectional flax/epoxy composite. Then, by a multi-scale analysis, the objective is to identify the causes of mechanical properties evolutions. According to the results of this study, a cyclic hygrothermal ageing induces irreversible degradations of fibers (matrix-embedded or not) and interfaces within the composite. Indeed fiber/fiber and fiber/matrix debondings have been observed by morphological analyses and a loss of fibers hydrophilic components has been highlighted by chemical and microstructural characterizations. A plasticization of flax fibers has also been shown during this kind of ageing. All these modifications induce a drop of composite's moduli after the first cycles of exposure and a decrease of ultimate tensile stress and strain after long time exposure. But these mechanical properties evolutions are much lower than the values found in the literature. Decreases of about -10% for the modulus and about -14% for the ultimate tensile stress have been recorded after 1 year of exposure and 52 cycles of ageing. Thus, this study highlighted that a well manufactured flax/epoxy composite is resistant to a cyclic hygrothermal ageing. These results are therefore promising for the development of this type of bio-based materials on a larger scale.