Thymol-Based Aromatic Diamine Hardener from Efficient Synthesis to High-Performance Fully Bio-Based Epoxy Thermosets and High-Strength Adhesives

Thymol-based bio-epoxy resins have recently garnered increased interest, but thymol-derived amine curing agents for epoxies remain little known. Herein starting from thymol-derived bisphenol E with 100% bio-based content, a bio-based aromatic diamine (DABP) is synthesized efficiently in 93% yield by attaching amino-substituted diphenyl ether moieties. DABP is used a hardener to cross-link two different epoxies, i.e., a conventional bisphenol A epoxy resin (E54) and a bio-based thymol bisphenol E (DHEP) one, while 4,4′-diaminodiphenyl ether (ODA) is used as a benchmarking hardener, with partially bio-based (DABP/E54), fully bio-based (DABP/DHEP), and fossil-based ODA/E54 epoxy formulations obtained. DABP is well compatible with both epoxies with improved processability and good curing reactivity. The DABP/E54, DABP/DHEP, and ODA/E54 thermosets exhibit Tg of 149, 179, and 182 °C, respectively, and high gel contents (>97%). Compared with ODA, DABP delivers lower absorption (1.7% vs 3.5%) in boiling water, much higher fracture toughness and energy (0.82 vs 0.69 MPa·m1/2 and 0.91 vs 0.32 kJ/m2), lower dielectric loss factor (0.009 vs 0.011 at 10 MHz), and higher adhesion strength (12.2 vs 10.1 MPa) to its cured E54 formulations. Furthermore, DABP/DHEP having more thymol building blocks demonstrates more outstanding performance (water absorption = 1.1%, adhesion strength = 26.8 MPa, and fracture energy = 1.13 kJ/m2), far better than those of ODA/E54. Taken together, DABP could be efficiently synthesized from bio-based and bulk feedstocks, be well compatible with epoxies, and endow the resultant thermosets with multiple advantages. In particular, the combination of DABP and DHEP delivers superior properties, exhibiting its promise as a structural adhesive.