Cellulose Nanocrystals as Nucleating Agents in Lignin-Based Rigid Polyurethane Foams

Abstract This study investigates the incorporation of cellulose nanocrystals (CNC) into rigid polyurethane foams (RPUF) derived from lignin polyol. CNC promoted cellular refinement consistent with a heterogeneous nucleation mechanism, reducing average cell size by up to 18% and improving cellular uniformity, particularly at 0.2% and 0.4% concentrations. Fourier-transform infrared spectroscopy (FTIR) confirmed hydrogen bonding between CNC hydroxyl groups and the polyurethane matrix. Thermogravimetric analysis revealed a shift in the temperature at 10% mass loss from 272 °C (control) to 282–285 °C (0.2–0.4% wt % CNC), while DSC showed an increase in glass transition temperature (Tg) obtained by DSC from 31.5 °C for the neat RPUF to 58.2 °C for RPUF_0.2%. SEM micrographs confirmed more homogeneous cell distribution at low CNC loadings, whereas higher concentrations (0.6–0.8%) led to agglomeration and irregular cells. Apparent density increased by ∼7% at 0.4 wt % CNC, correlating with enhanced compressive strength (+12%) and storage modulus (+15%). The improvement in compressive performance was present after normalization by apparent density, indicating that the mechanical response was not exclusively density driven. Overall, CNC addition improved the structure and thermo-mechanical performance of lignin-based RPUF, being a sustainable route to high-performance foams for energy-efficient applications.