Hyperbranched Thiol-Capped Polyurethane: A Toughening Curing Agent for High-Performance Epoxy Adhesives

Thermosetting epoxy resins are widely used in the fields of adhesives, coatings, electronic encapsulation, and aerospace. However, their inherent brittleness, due to their high cross-link density, has limited their application in advanced, high-precision fields. To address this, we designed and synthesized a series of thiol-terminated branched polyurethanes with varying molecular weights to act as curing agents for epoxy resins. This approach addresses issues like compatibility, dispersion, and optimal mixing ratio associated with adding separate toughening agents. By adjusting the molecular weight and cross-linking density of the curing agent, we optimized the curing rate, chain mobility, toughness, and adhesion of the resin. The cured epoxy adhesive demonstrated a lap shear strength of 27 MPa, a tensile strength of 47 MPa, and a fracture toughness of 4.1 MPa m^1/2, while also exhibiting broad substrate compatibility and excellent solvent resistance. Compared with conventional epoxy curing agents and commercially available polysulfide curing agents, the polyurethane-cured epoxy adhesive displays superior mechanical properties.