Influence of Environmental Conditions on the Mechanical Properties of Flax Fiber Reinforced Thermoset Composites

A new type of flax fiber-reinforced bio-sourced thermoset composites is investigated to explore their usage in hot and humid conditions and is compared with flax fiber reinforced epoxy composites. Hygrothermal aging of composites is carried out for 40 days at 50 °C and 90%RH. The influence of moisture uptake of composites during aging is evaluated in terms of physical parameters (i.e., swelling and weight gain) and mechanical properties. The deterioration of the mechanical properties during aging is mainly attributed to the presence of flax fibers, especially the fiber-matrix interface. The evolution of flexural properties induced by aging is similar for both synthetic epoxy composites and bio-based benzoxazine composites and is described using an analytical model. The mechanical test results show that after 40 days of aging, flax/benzoxazine composites lose about 20% of their flexural properties whereas flax/epoxy composites lose about 32% of properties. The recovery of flexural properties after drying is higher for flax/benzoxazine composites than for flax/epoxy composites, which demonstrates the potential of this bio-sourced thermoset composite material.