Sustainably Sourced Tannic Acid Enables Fast-Curing High-Strength Epoxy Adhesives with Increased Toughness

Simultaneously achieving high strength and toughness is a dilemma for most high-performance materials, especially solidified epoxy resins (ERs). Here, we used a sustainably sourced tannic acid (TA) molecule, a natural polyphenol hyperbranched polyester, as both a reinforcing and toughening agent for preparing high-strength epoxy adhesives with increased toughness. Benefiting from a highly branched structure and abundant active phenolic hydroxyl groups, TA molecules can be functionalized as multiple physiochemical cross-linking points for generating high-performance ERs. The optimal TA-modified ER (TA/ER-3) shows a high tensile strength of 112.2 ± 3.6 MPa and a high elongation at break of 3.7 ± 0.20%, illustrating increased tensile strength by 37.2% and elongation at break by 32.4% compared with pristine ERs, respectively. Combining scanning electron microscopy and dynamic mechanical analysis reveals that the TA molecules not only promote the curing and cross-linking reaction of epoxy resin but also possess good interface interaction with the matrix, which tune the cross-linking density and dissipate fracture energy through deformation, resulting in strengthening and toughening ERs. This work offers sustainably sourced additives for developing high-performance epoxy adhesives, which can be extended to reinforce other types of polymers or nanomaterials.