Water Uptake and Its Effects on Mechanical and Acoustic Properties of flax/polypropylene Composite

The effects of moisture on the mechanical and acoustic properties of flax fibers embedded in polypropylene (PP) were investigated in the present study. Flax/PP and hybrid flax-carbon/PP samples were immersed in water at controlled temperatures. The water intake over time in flax/PP was shown to approach Fickian behavior at room temperature while hybrids showed strong deviations. Through hybridization with 13% carbon fibers, water uptake in flax-carbon PP was reduced by 25%. For thin hybrid laminates, positioning carbon fibers at the outer edges caused no significant difference in the rate of moisture uptake. Micrographic analysis revealed that carbon fiber bundles are more susceptible to resin deficiency in flax-carbon/PP hybrids due to its densely packed thin fibers. The stiffness of moisture-saturated flax/PP samples was 50% lower than dry specimens, with 35% increase in failure strain. The tensile strength, however, was almost unaffected, and both, dry and wet samples, exhibited similar damage characteristics. The stiffness reduction affected the acoustic performance: the resonant frequency was reduced, and the STL were lowered in the stiffness-controlled frequency domain. To determine the unknown water uptake at saturation, an accelerated method was presented as an alternative time-efficient solution for designers.