Recyclable Millable Polyurethane based on Enaminone Bonds With Upcycled Mechanical Performance

Thermoplastic polyurethane (TPU) exhibits re-processable properties, but the properties of TPU is deteriorated during the reprocessing for the oxidation and degradation of polymer chains. Meanwhile, although thermoset polyurethane exhibits excellent mechanical properties, it cannot be recycled for permanent crosslinking. Hence, it's still a challenge to obtain PU which exhibits the balance between the recyclability and mechanical properties. In this work, a new dynamic bond obtained from the reaction between enaminone and isocyanate is used to prepare re-processable millable polyurethane, and the morphology of network can be tuned via the dissociation of the cross-linked sites of PU. Interestingly, the cross-linked network can transform into a linear polymer by adding the amine which can be used when reacted with isocyanate to generate new re-crosslinked PU. This process can be carried out in a Haake mixer without any solvents. The mechanical properties of the re-crosslinked polyurethane can be tuned via the controlling of the amine and isocyanate addition, and the maximum tensile strength increase by 178.4% after processing for four times, realizing mechanical reinforcement after recycling. This kind of recycling achieves through one-step melting method in solvent-free conditions provides a feasible way to prepare recyclable PU with good mechanical performance and customizable properties.