Understanding the effect of moisture variation on the hygromechanical properties of porosity-controlled nonwoven biocomposites

This study investigates the evolution of hygromechanical properties of flax/PP nonwoven composites in a wide range of environmental Relative Humidity (RH) conditions from 10 to 98% RH. The influence of microstructure with various porosity content (= 5, 30, 50%) on the mechanical and hygroscopic behaviours is studied and compared to glass/PP composites as a baseline. No significant changes in the moisture content and mechanical properties of glass-fibre reinforced composites are observed. For flax fibre nonwoven composites, the porosity greatly impacts the kinetic of sorption with moisture saturation varying from 9 h to 15 days with decreasing voids. Hygroscopic expansion is hardly modified by the porosity content while anisotropic expansion is always observed. Tensile behaviour and properties are slightly changed over a range of 10-75%RH but negatively impacted between 75% and 98% RH. Interestingly, unlike the tangent tensile modulus and strain at rupture of flax/PP composites, the yield strength exhibits a non-monotonic trend with an optimal value over 50% RH; compressive stresses at the fibre/matrix interface induced by flax fibres hygroexpansion are proposed to explain this trend.