Assessing the mechanical behaviour of reusable polyurethane networks with graphene as internal nano-heater

Polyurethane adhesives with thermoreversible Diels Alder (DA) bonds offer a sustainable approach to improve recyclability of multilayer packaging. This study evaluates the effect of graphene, used as an internal heater to trigger bond breakage, on the mechanical properties by indentation and shear rheology. Particular attention is paid to their reversibility and cyclability upon thermal heating or IR irradiation. Graphene initially reduces the modulus by disrupting network formation resulting in reduced DA bond incorporation and crosslink density. The intrinsic properties of graphene counterbalance this effect at higher loadings. Rheology shows that graphene accelerates the retro-DA reaction during conventional thermal heating. Indentation tests indicate that IR exposure temporarily reduces the mechanical properties, with full recovery after 100 min. Both rheological and indentation methods show improved mechanical properties (modulus and hardness) after the initial retro-DA reaction, which stabilise after further thermal treatments and are related to the formation of a new network structure.