Hybrid bio-filler effects on the performance of pearl shell and mahogany fruit shell-modified structural epoxy adhesives

This study investigates the performance of bio-derived fillers from pear shell (PS), mahogany fruit shell (MFS), and their hybrid as reinforcements in epoxy adhesive systems. Adhesives were fabricated with filler loadings of 0 - 30 wt%, and their tensile strength and mode-I fracture toughness were evaluated. The performance of adhesives as structural adhesives was assessed using aluminium single-lap joints. Fracture surface morphologies were examined via scanning electron microscopy. Results revealed that for single-filler systems, both tensile and fracture properties peaked at 15 wt%, beyond which agglomeration and property degradation occurred. In contrast, the hybrid filler system exhibited optimal mechanical performance at 20 wt%. Similarly, the mode-I fracture toughness of filled epoxy adhesives was significantly improved with increasing filler loading to 15–25 wt%, with an enhancement of up to 95%. Notably, the highest shear strength in bonded joints was also achieved at 20 wt% filler content. Microscopic analysis indicated that crack deflection and bifurcation were the dominant toughening mechanisms. Additionally, particle cleavage and pull-out were observed in PS- and MFS-filled epoxies, respectively. Overall, the findings demonstrate that PS and MFS fillers, individually and in combination, offer a sustainable and effective route to enhance the mechanical performance of epoxy adhesives.