Solvent-free urethane-based prepolymer as a versatile underwater adhesive material

High bonding strength and water resistance have always been the top priority in developing most reported manmade underwater adhesives; however, easy delivery, low solvent toxicity, and long working times are equally important for many applications, particularly for the development of sustainable and green bonding processes. In this study, we explored the use of a solvent-free urethane-based prepolymer that initially behaves like a pressuresensitive adhesive that works underwater, then cures for stronger permanent adhesion. These favorable properties are obtained by carefully balancing adhesive functionality, viscoelasticity, and inter-chain interactions. The resulting synthetic prepolymer adhesive material is a viscous semi-solid consisting of isocyanate-capped polymers bearing carboxyl and urethane groups as the potential sites of non-covalent and covalent interactions. Such a sticky adhesive can be easily deposited and has instant adhesion on various flat/curved substrates. It continues to strengthen over a curing time of nearly 2 days, reaching a comparable bonding strength of around 1.2 MPa and peel adhesion of around 2300 N m(-1) on aluminum sheets. The prolonged curing process allows for enough time for various applications, such as operating the underwater sealing of fractured items and bonding string to mussel shells. The easy delivery, eco-friendly nature, and initial adhesion of this adhesive provide great opportunities for application in engineering and aquaculture fields.