Brewing Sustainable Polyurethane Foams from Spent Grounds via Polyol Liquefaction

What happens to coffee grounds after the last cup is brewed? In this study, spent coffee grounds were reimagined as a renewable feedstock for sustainable polyurethane (PU) foams. Instead of being treated as waste, the grounds were converted into reactive bio-based polyols through a short polyol liquefaction process using a PEG400/glycerol co-solvent system. The resulting liquefied spent grounds exhibited suitable hydroxyl functionality and processability for PU foam preparation. Two-component PU foams were prepared using the liquefied bio-polyol with different isocyanate systems, including hexamethylene diisocyanate biuret trimer and polymeric methylene diphenyl diisocyanate (PMDI). The NCO/OH molar ratio was adjusted to tailor the hard-segment content, crosslinking structure, and cellular morphology of the foams. The incorporation of liquefied spent grounds not only introduced renewable carbon into the PU network but also influenced foam formation. Biomass-derived residues acted as reinforcing components and possible heterogeneous nucleation sites, contributing to improved cell stability and reduced foam anisotropy. The resulting PU foams exhibited low thermal conductivity, good solvent resistance, and enhanced compressive performance, especially in PMDI-based formulations. The properties analysis indicated that increasing the NCO/OH ratio promoted urethane and urea formation, enhanced hydrogen bonding, and improved the crosslinked network structure. These findings demonstrate that spent grounds can be upgraded from a daily waste stream into functional polymer foams with potential applications in thermal insulation, lightweight composites, and circular bio-based materials. This work presents a simple and promising “waste-to-polymer” strategy, transforming a familiar residue from everyday life into sustainable PU foam materials for the next generation of low-carbon polymer applications.