Functionalization and Repurposing of Polyolefins to Polyurethanes

(NOTE: Padden Award Symposium)

Post-use modification has shown promising results in upcycling polyolefins, removing the limitation of inertness, and improving the final physical properties of the recycled material while extending its useful lifetime. In this work, polypropylene (PP) was functionalized and upcycled to thermoset polyurethanes (PUs). First, a PP-based polyol was prepared through hydroxylation of maleic anhydride-grafted PP and subsequently cured with a diisocyanate to form a thermoset PU, that is reprocessable through the addition of a urethane exchange catalyst. The crystal structure (unit cell and lamellar structures) of the PP below and above the network T_g was preserved; nevertheless, the total fraction of PP that crystallized decreased compared to the PP predecessor due to barriers arising from the PU crosslinks. Additionally, the crystallization kinetics, studied by applying the Avrami and the Lauritzen-Hoffman models, showed no changes and moreover the nucleation constant (K_g) was not impacted by the PU crosslinks. At room temperature, the PU showed higher tensile modulus, due to the presence of PP crystals, with a trade-off in diminished ductility. Upon heating above the melting temperature of PP, the modulus decreased to a rubbery plateau, consistent with the presence of a rubbery network. The mechanical integrity of the PU was maintained through several reprocessing cycles due to the presence of a urethane exchange catalyst. This functionalization and upcycling route thus offers a promising alternative to repurposing PP waste.