The growing demand for advanced insulating materials in high-voltage systems necessitates the development of polymer composites with high dielectric strength, thermal stability, and environmental sustainability. Although conventional epoxy resins are widely used in gas-insulated switchgears (GIS) and transmission lines (GITL), they suffer from limited breakdown strength and thermal endurance. In this study, a sustainable route was developed to enhance the performance of epoxy by incorporating gamma and alpha alumina (γ-Al₂O₃ and α-Al₂O₃) nanoparticles synthesized from recycled aluminum beverage cans. The recycled fillers exhibited high crystallinity, reaching 91.3 % for α-Al₂O₃, which enabled strong interfacial interactions with the epoxy matrix. The optimized nanocomposites exhibited remarkable improvements in both electrical and thermal characteristics. At 5 wt. %, γ-Al₂O₃ and α-Al₂O₃ nanocomposites achieved 46 % and 56 % higher breakdown strength, respectively, compared with neat epoxy. The onset degradation temperature increased from 341 °C for the neat epoxy to 383 and 408 °C, while the glass-transition temperature (Tg) rose from 93.5 °C to 117.6 and 121.4 °C for the γ-Al₂O₃ and α-phases, respectively. These enhancements confirm the strong interfacial bonding and restricted polymer mobility induced by the alumina nanoparticles, particularly in the α phase. Recycling-based synthesis reduces energy consumption and CO₂ emissions by 85–90 % compared to conventional primary-aluminum-based production routes. This substantial environmental benefit highlights the efficiency of the recycling approach and its potential for the large-scale, eco-efficient fabrication of alumina nanofillers. This study demonstrates a circular economy-based approach for developing epoxy insulation with enhanced dielectric and thermal performance while significantly reducing its environmental impact.