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

Area of Science:

• Polymer Science
• Materials Science

Background:

• Thermosetting epoxy resins offer excellent performance but suffer from inherent brittleness, limiting their use in high-precision applications.
• High cross-link density in epoxy resins contributes to their brittleness, hindering advanced material development.

Purpose of the Study:

• To design and synthesize novel thiol-terminated branched polyurethanes as curing agents for epoxy resins.
• To enhance the mechanical properties, specifically toughness and adhesion, of epoxy resins through tailored polyurethane curing agents.
• To overcome compatibility and dispersion challenges associated with traditional toughening agents.

Main Methods:

• Synthesis of thiol-terminated branched polyurethanes with controlled molecular weights.
• Utilizing these polyurethanes as curing agents for epoxy resins.
• Characterization of the mechanical properties (lap shear strength, tensile strength, fracture toughness) and solvent resistance of the cured epoxy adhesives.

Main Results:

• The developed polyurethane-cured epoxy adhesive achieved a lap shear strength of 27 MPa, tensile strength of 47 MPa, and fracture toughness of 4.1 MPa m^1/2.
• The new epoxy system demonstrated broad substrate compatibility and excellent solvent resistance.
• Superior mechanical properties were observed compared to conventional epoxy curing agents and commercial polysulfide curing agents.

Conclusions:

• Tailoring polyurethane curing agent molecular weight and cross-linking density effectively enhances epoxy resin toughness and adhesion.
• This approach provides a compatible and effective method for toughening epoxy resins without separate additives.
• The resulting polyurethane-cured epoxy adhesives exhibit superior performance, making them suitable for advanced applications.