Optimizing Rubber Particle Content in Epoxy Composites: Tensile Performance of Cores and Hemp Fabric Sandwich Structures.

Waste-tire accumulation poses major environmental and health risks through toxic pollutant release, fire hazards, and landfill burden, motivating routes that valorize scrap rubber into engineering materials. This study aimed to develop waste-tire rubber particle–reinforced epoxy composite cores and corresponding hemp fabric/epoxy skin sandwich structures, and to quantify how rubber particle content governs tensile response and failure mechanisms. Epoxy resin (Lapox B-11) and hardener (K-6) were mixed at a 10:1 ratio and reinforced with 60-mesh rubber particles at 0, 1, 3, 5, and 7 wt.% using hand stirring, then cast in a 200×200×10 mm aluminum mold and cured at room temperature to form cores. Sandwich panels were fabricated by compression molding epoxy-impregnated plain-woven hemp fabric (400 GSM) skins onto the cured cores under 50 kgf pressure and room-temperature curing. Tensile testing (ASTM D638, 5 mm/s) was conducted with four replicates per condition, and fracture surfaces were examined by SEM. Tensile strength and Young’s modulus increased with rubber addition up to an optimum of 3 wt.% and decreased at 5–7 wt.% for both cores and sandwiches. Specifically, the neat epoxy core (C0) achieved 27.90 MPa and 3.90 GPa, while the 3 wt.% core (C3) improved to 30.35 MPa and 4.24 GPa; the neat sandwich (SC0) showed 21.99 MPa and 0.987 GPa, rising to 25.59 MPa and 1.33 GPa at 3 wt.% (SC3). SEM revealed relatively uniform dispersion and fewer voids at ≤3 wt.% but pronounced agglomeration, interfacial gaps, and higher void density at 5–7 wt.%, which promoted stress concentrations and premature cracking; sandwich tensile performance was further limited by skin/interface defects (fibre pull-out, microcracking, air gaps, localized delamination). Overall, optimizing rubber content (~3 wt.%) and improving dispersion and skin quality are critical to reliably translate waste-tire rubber into lightweight, more sustainable epoxy-core and natural-fiber sandwich composites.