Furan-based reversibly cross-linked unsaturated polyester resin displaying low flammability and potential for re-processability

Conventional thermosets are difficult to reprocess, recycle and reshape due to their permanent covalent cross-linking, and their flammability limits a wide range of applications. Herein a novel thermally switchable polymer, capable of transitioning from a thermo-reversibly self-healing polymer (re-PO-co-UPR) into an irreversibly cross-linked polymer (i-PO-co-UPR) with higher flame-retardancy, has been built through forming covalent adaptable networks (CANs) by Diels-Alder (DA) reaction between the unsaturated double bonds of polyester (UPR) and furan groups of the synthesized flame retardant cross-linker (POFA). Re-PO-co-UPR presents excellent self-healing properties and reprocess-ability owing to DA/rDA reactions which proceeds in the melt, with no need for any solvent, catalyst or additive. At higher temperatures, irreversible alkene addition reactions of UPR are triggered, forming irreversibly cross-linked conventional thermosets i-PO-co-UPR, which shows outstanding flame retardancy with 29.6% limiting oxygen index (LOI) and UL-94 V-0 rating. Such a convenient preparation and flexible structure design of CANs has successfully broken the boundary between thermoplastic and thermoset materials, so that they acquired self-healing properties and can be recycled through de-crosslinking process after being discarded. A prime example for the development of renewable thermoset materials has been presented.