The effect of wet-dry cycles on tensile properties of unidirectional flax fiber reinforced polymers

The impact of moisture is a critical research focus for natural fibres. There is a significant decrease in mechanical performance when fully saturated; however, this may not be representative of real-life applications. In this study, the tensile performance of flax fibre reinforced polymers (FFRP) is investigated when exposed to wet-dry cycles. Manufacturing method, fibre type and resin type were varied throughout this research program. Vacuum bag moulded specimens exhibited an increased strength of 19%, increased rupture strain of 105% and reduced tensile modulus of 29% when saturated; however, it reverted to its original state once dried. When exposed to wet-dry cycles, a statistically-significant reduction in mechanical properties began after 3 cycles, with a maximum reduction of 12% strength and 19% stiffness. Conversely, degradation for wet lay-up moulded specimens was delayed until 12 cycles. When epoxy was replaced with an epoxidized pine oil (EPO) blended resin, there was no mechanical difference throughout wet-dry cycles; however, FTIR showed signs of hydrolysis for both resins. Furthermore, the glass transition temperature of epoxy reduced from 87.2 degrees to 77.8 degrees C, whereas that of EPO increased from 65.6 degrees C to 70.2 degrees C. Glass-FRP was tested in parallel, and exhibited no change to performance across the entire conditioning protocol.