Durable mechanical properties of unidirectional flax fiber/phenolic composites under hydrothermal aging

Natural fibers show great potential in polymer composites reinforcement, it remains challenging to tackle the mechanical stability of the composites under moist/water condition for long-term outdoor applications. Herein, unidirectional flax fiber reinforced phenolic (FFRP) composites were developed to systematically study the effects of water absorption on their mechanical properties. Water absorption tests were conducted by immersing composite specimens into distilled water at three different temperatures (23, 37.8, and 60 °C) for a period up to 3 years. Water absorption curves indicate FFRP achieved adsorption equilibrium within 64 days, exhibiting diffusion coefficients ( D ) of 7.3 mm 2 /s and maximum moisture contents ( M m ) of 18.3%, which reveal the water absorption by FFRP composites in the initial stages followed Fickian behavior. After aging for 3 years, the mean values for tensile strength and Young's modulus were 57 MPa and 15 GPa, respectively, 77.7 and 47% lower, respectively, and the shear strength decreased by about 71.7% compared to the unaged specimens. The effects of hydrothermal aging on the tensile and interlaminar shear characteristics of the composites Long-term hydrothermal aging was found to weaken interlaminar shear characteristics of the composites, causing degradation/deformation of the flax fibers and degradation of the phenolic matrix. A hydrothermal aging mechanism defined as “swelling-debonding-matrix degradation-fiber degradation” at molecular scale is proposed for FFRP composites, promising to steer the future design and exploitation of natural fiber reinforced polymer composites to accommodate the prolonged adaptability at water conditions.