Epoxy Latent Systems for Novel Hybrid 3D Printed Metal/CFs Reinforced Composite Joint Disassembly via Chemical Recycling

This study presents the chemical, thermal and mechanical characterization of two bio-based and fully-recyclable epoxy latent resins derived from pine oil, engineered to meet high-performance mechanical and thermal specifications for advanced composite applications. The characterized latent epoxy resins exhibited glass transition temperature (Tg) values ranging between 90 and 120 °C, flexural strength and modulus values within the range of 35–110 MPa and 2.5–3.2 GPa, respectively, making them suitable for structural applications. One of the latent epoxy resin was further employed in the fabrication of a hybrid joint comprising metal and CFs reinforced composites characterized by an average ILSS of 11.9 MPa and a maximum load of 1429.2 N, due to a good mechanical interlocking at the metal/CFs interface. Due to its latent curing behavior, the epoxy resin remains stable until exposed to elevated temperatures (≥ 80 °C), at which point cross-linking is initiated. This property affords an extended pot life and improved control during application and assembly. This feature makes the system especially attractive for hybrid manufacturing approaches, where the extended open time supports accurate positioning of fibers and metal inserts. Furthermore, the epoxy resin was cross-linked by using a cleavable amine hardener to achieve full recyclability, so enabling disassembly under mild acidic controlled conditions. This property facilitated the full recovery of constituent raw materials through a targeted chemical recycling process, achieving a 100% recovery yield. The proposed system offers a sustainable alternative to conventional thermoset-based hybrid metal/CFs composite manufacturing and end-of-life management. This class of hybrid joints can find industrial applications in aerospace and automotive sectors, in electronic and optical applications, in sports equipment and biomedical devices, for the manufacturing of next-generation structural solutions, with enhanced performance and versatility, by also supporting the global shift toward circular design and responsible material use.