Effect of hygroscopic aging, filler ratio, and particle size on tensile and flexural properties of epoxy/kaolin composites

Abstract An experimental investigation was conducted to assess the effects of moisture uptake on the mechanical properties of epoxy/kaolin composites. The study varied two key material parameters: kaolin powder weight ratios (1 %, 3 %, and 5 %) and particle size (0–63 μm, 63–100 μm, and 100–125 μm). The evaluated mechanical characteristics included Young’s modulus, flexural modulus, maximum flexural and tensile stress. The results established that the total interfacial particle/matrix surface area is the primary factor governing moisture uptake and the evolution of mechanical properties. An increase in the weight ratio was found to correlate directly with a higher water absoption and a reduction in saturation time. Specifically, the composite with the highest filler content and smallest particle size (5 % WR, 0–63 μm) exhibited the most rapid water absorption, whereas the composite with the lowest filler content and largest particle size (1 % WR, 100–125 μm) demonstrated the most resistant behavior. A significant inversion in performance was observed between dry and saturated states. In the dry condition, the composite with the highest WR (5 % WR, 0–63 μm) displayed the most superior mechanical characteristics, while the composite with the lowest (1 % WR, 100–125 μm) exhibited the poorest. However, at saturation an inversion at a certain threshold of water uptake was observed. Microstructural analysis suggests that a greater weight ratio of fine particles creates a larger network of potential failure sites. These interfaces act as initiation points for microcracks via particle debonding, leading to localized stress concentrations. This phenomenon accelerates localized failure and promotes matrix plasticization, ultimately resulting in enhanced micro-debris formation leading to total failure.