Development of Bio-Based and Recyclable Epoxy Adhesives by Modification with Thermoplastic Polymers

This paper deals with the design of novel epoxy adhesives by incorporating thermoplastic polymers such as polyetherimide (PEI) and poly(ε-caprolactone) (PCL) into a bio-based and recyclable epoxy resin, known as Polar Bear. The adhesives were characterized by their mechanical (quasi-static and dynamic) and rheological properties, thermal stability, and adhesion properties in single-lap joints tested at three different temperatures (i.e., −55 °C, 23 °C, 80 °C). The experimental results indicated that low PEI content substantially improved the mechanical performance and toughness of the adhesive, while preserving good processability. Nonetheless, exceeding 3% weight percentage adversely affected the adhesives’ mechanical resistance and workability. Conversely, while PCL addition enhanced the adhesives’ viscosity, it also decreased mechanical performance. However, its eco-friendliness offers potential for sustainable adhesive applications. It is worth noting that regardless of temperature, the modified adhesives consistently outperformed the commercial epoxy adhesive (DP-460), used as reference, in single-lap shear joint tests. Additionally, both PEI- and PCL-modified epoxy adhesives have demonstrated recyclability through a simple acid-based process, enabling joint disassembly and recycling of the adhesive into a thermoplastic polymer. Overall, the modified adhesives represent a promising eco-friendly, high-performance alternative for structural applications, aligning with sustainable and circular practices.