Biobased Epoxidized Castor Oil Covalent Adaptable Networks Adhesives Reinforced with Recycled Carbon Fibers

The development of sustainable, smart, and functional adhesives is crucial in advancing environmentally friendly materials. In this study, a fully biobased adhesive based on Covalent Adaptable Networks (CANs) is presented, integrating epoxidized castor oil (ECO) as the bio-based monomer and a novel eugenol-based phosphate ester (EUGP) as the transesterification catalyst. The system was further enhanced with Recycled Carbon Fibers (RCF) as reinforcing filler. The curing process was thermally initiated using Ytterbium(III) trifluoromethanesulfonate (YTT), enabling efficient crosslinking even at room temperature over 24 h, permitting an eco-friendly production. Extensive characterizations were conducted. FT-IR confirmed high conversion rates, even with 20 phr of RCF, proving the curing efficiency. DSC revealed curing kinetics, and DMTA revealed a tunable glass transition temperature (0–10 ◦◦C) depending on the filler content. Rheological stress-relaxation tests demonstrated rapid stress relaxation (1000 s at 70 ◦◦C), confirming dynamic bond exchange and reprocessability. Contact angle analysis confirmed the presence of hydrophobic surfaces, which enhance moisture resistance. TGA indicated thermal stability up to 105 ◦◦C. Mechanical tests performed on different joined substrates (metals and ceramic composites) showed that both adhesive bulk and joint strength at room temperature increased with fiber loading. Rebonded joints retained 74–91% of their initial strength after two repair cycles. By exploiting transesterification chemistry, bio-based materials, and recycled conductive fillers, this study highlights a sustainable alternative to conventional adhesives. The results indicate that the analyzed fully biobased adhesive offers promising applications in flexible electronics, smart adhesives, and advanced composites, supporting the transition toward more environmentally responsible materials.