The anisotropic diffusion of water in Kevlar-epoxy composites

The diffusion of water into unidirectional Kevlar fibre reinforced epoxy resins was studied as a function of fibre orientation and, for unidirectional (0°) composites, as a function of volume fraction (V_f). As the angle increased from 0 to 90°, the diffusivity increased dramatically; i.e. as more and more fibre-ends were exposed to the shorter diffusion path, the diffusivity increased. The equilibrium weight gain of water (M∞) in the composites increased with the V_f of the fibre. M∞ of Kevlar fibre was calculated to be 4.9%. At a constant V_f, specimens of the same thickness and width but different lengths were used to determine D₂₂, the diffusion coefficient of the composite along the fibre, and D₂₂, the diffusion coefficient transverse to the fibre. The initial data for the percentage weight gain against the square root of time were non-linear, which was attributed to the anisotropy of the diffusion process. The anisotropy arises from the much higher value of D₁₁ as compared to D₂₂. As V_f increased from 0.37 to 0.59, D₁₁ increased from about 0.83 to about 4.2 × 10^-12m² sec^-1, whereas D₂₂ decreased from 0.21 to 0.033 × 10^-12 m2 sec^-1. Thus, the ratio D₁₁/D₂₂ increased from 3 to over 100 as U increased. The experimental sorption data could be fitted satisfactorily with these diffusion coefficients.