Vacuum Heat Treatment Regulates the Bamboo–Epoxy Interface: Balancing Mechanical Performance and Thermal Stability

The sustainable replacement of conventional synthetic-fiber composites requires renewable reinforcements that combine low density with high load-bearing capability. However, the hydrophilic nature of bamboo restricts effective stress transfer at the bamboo–epoxy interface, while excessive thermal modification can further compromise the structural integrity of the reinforcement. Here, a vacuum-thermal interfacial regulation strategy was employed to establish a favorable balance between interfacial strengthening and thermally induced structural degradation. Moderate treatment improved resin coverage and interfacial contact while preserving the bamboo structure, thereby promoting efficient stress transfer across the bamboo–epoxy interface. Owing to this synergistic interfacial regulation, the composite treated at 160 °C for 4 h achieved tensile and flexural strengths of 174.49 and 219.55 MPa, corresponding to improvements of 49.60% and 29.47%, respectively, over the untreated composite, while excessive treatment caused renewed interfacial defects and mechanical deterioration. Meanwhile, vacuum heat treatment also enhanced thermal stability, increasing T5% from 216.55 to 260.08 °C and the residual mass at 500 °C from 24.18% to 28.28%. These results reveal a processing-window mechanism governed by the competition between interfacial strengthening and thermal degradation, providing an effective route for developing high-performance and sustainable bamboo/epoxy composites.