Upcycling of Polyimide for the Preparation of a High-Performance Polyurethane Chain Extender

Polyimide (PI) is a polymer known for its high mechanical strength and excellent overall performance. However, PI is difficult to dissolve and melt, posing challenges for the recycling of PI waste, especially for cross-linked thermosetting PI. In this study, 4,4′-diaminodiphenyl ether (ODA) and N,N′,N″,N‴-1,2,4,5-tetra (hydroxyethyl) pyromellitamide (4HPA) monomers with high-purity were successfully prepared from waste PI films through an ammonolysis process. The chemical structure of the 4HPA monomer was thoroughly characterized. This kind of multifunctional monomer with four hydroxyl end groups enables its potential as a versatile cross-linking agent. This 4HPA monomer was subsequently employed as a chain extender in the synthesis of conventional polyurethane (PU) elastomers. It was found that with the addition of 6.75 wt % 4HPA, the obtained polyurethane (PU-4HPA-4) exhibited a tensile strength of 78.97 MPa and an elongation at break of 1267%. Moreover, the PU-4HPA samples demonstrated excellent transparency and heat resistance. Notably, the recycling process for PI in this work did not require the use of catalysts, and the PU prepared with the multifunctional monomer showed outstanding properties. This study offers insights into the recycling and reuse of thermosetting PI materials as well as other high-performance thermoplastics.