Synchronous Enhancement of Expandable Graphite on the Bonding and Toughness Performances of Epoxy Adhesive

ABSTRACT This study systematically investigated the effects of micro‐scale expandable graphite (EG) on the bonding performance and impact toughness of an epoxy adhesive. The significant influence of EG content on adhesive properties was demonstrated by tensile shear tests, pendulum impact tests, scanning electron microscopy, metallographic microscopy, and finite element analysis using a representative volume element (RVE). The results indicated that EG particles synergistically enhanced both properties; however, the overall performance was highly dependent on the EG content. The addition of 0.1 wt.% EG improved the impact toughness by 105.8% and increased the tensile shear strength by 14.4%. At 0.5 wt.% EG, the tensile shear strength increased by 65.0%, while the impact toughness improved by 14.0%. However, when the EG content was further increased to 0.8, 1.0, and 2.0 wt.%, the excess particles tended to agglomerate and form isolated domains, leading to a decline in modification effectiveness. Considering the overall mechanical performance and processing feasibility, 0.1 wt.% EG was recommended as the optimal content. By correlating the failure morphologies of the bonded specimens, impact fracture micrographs, and RVE simulation results, this study elucidated the synchronous enhancement mechanism, linking the improved properties to interfacial pinning as well as crack deflection, bifurcation, and jumping.