Robust\nSelf-Healing\nAdhesives Based on Dynamic Urethane\nExchange Reactions

Thermoset polyurethanes (PUs) have\nbeen successfully\nreprocessed\nas covalent adaptable networks (CANs) by catalyzing carbamate exchange.\nHere we extend bond exchange beyond the internal network cross-links\nto create a dynamic urethane adhesive. Interfacing PU CANs to substrates\nwith nucleophilic functional groups creates adhesives capable of reversible\ntranscarbamoylation with the substrate, which has not been demonstrated\npreviously by CAN adhesives. Two types of thermoset PU films were\nsynthesized, one containing the green carbamate exchange catalyst\nZr(tmhd)<sub>4</sub> and the other containing no catalyst. Although\notherwise identical in chemical and network properties, as indicated\nby FT-IR spectroscopy and dynamic mechanical thermal analysis (DMTA),\nthe film containing catalyst showed dynamic bond exchange behavior\nthrough stress relaxation analysis. When evaluated as an adhesive,\nthe CAN film exhibited self-healing properties and retained its adhesive\nstrength for five cycles, which is attributed to reversible covalent\nbonding to the glass substrate. This work expands industrially relevant\nCANs to structural adhesives and demonstrates their potential value\nin an application that presently employs PUs as single-use materials.