Recovery of Polyol and Aromatic Amines from Rigid Polyurethane Foams via Ammonolysis

Polyurethane ranks as one of the most significant plastics globally in terms of production volume and economic value, serving a crucial role in modern society. Due to complexities in the chemical recycling of polyurethane, replacement rates in recycled materials tend to be low, and the recovery of the isocyanate-derived aromatic compounds is often neglected. While many recycling efforts try to address these shortcomings, they primarily focus on TDI-based flexible foams, as recycling rigid PU foams is more challenging. In this work, ammonolysis is reported as an effective method for the recycling of rigid PU foams using ammonia to efficiently produce aromatic amines and polyols, while enabling easy recovery of the excess ammonia. A quantitative ¹H NMR analysis method was developed for rigid PU materials, allowing for the identification and quantification of the polyol and isocyanate-derived products of rigid PU. For ammonolysis, volumetric productivities of up to 300 g·L^-1·h^-1 were obtained, surpassing current rigid PU recycling processes and highlighting its industrial significance. Kinetic studies provided mechanistic insight into PU depolymerization, with urea linkages splitting fastest, followed by carbamates and finally isocyanurates. Furthermore, solid-state NMR analysis demonstrated ammonia's hydrogen bonds disrupting effect in PU hard segments, presenting ammonolysis as an optimal method for recycling rigid PU foams, where the high content of hard segments makes degradation more challenging. The ammonolysis mixture could be purified into pure polyol and aromatic amine product fractions using two different separation methods. Finally, to highlight its industrial potential, our ammonolysis approach was demonstrated on various PU materials, extending the scope to end-of-life rigid PU waste materials. A comprehensive recycling process was applied, surpassing current rigid PU recycling efforts by reaching high isolated yields for isocyanate derivatives and polyol.