Preparation of Mechanically Robust Bio-Based Polyurethane Foams Using Depolymerized Native Lignin

Native lignin, a major structural polymer in lignocellulosic biomass, offers unique advantages as a feedstock for high-performance bio-based polymeric materials. In this work, depolymerized native lignin was utilized as a polyol in the synthesis of rigid bio-based polyurethane (PU) foams. Lignin in Pinus radiata wood was subjected to a pilot-scale hydrogenolysis process to obtain a depolymerized native lignin oil that was composed of monomers, dimers, and oligomers of predominant dihydroconiferyl alcohol and 4-propyl guaiacol. These lignin-derived polyols had low molecular weight, high hydroxyl group content, and good copolyol solubility, all of which were advantageous factors in preparing PU foams. The whole lignin oil, or fractions of the oil, was incorporated into PU foams by substituting sorbitol polyether polyol at 12.5–50 wt % to boost the bio-based content. Substituting with lignin-derived polyols led to foams with similar densities and thermal properties and substantially improved compression properties. The monomeric-/dimeric-rich fraction of the lignin oil, when incorporated at 33 or 50 wt % of the polyol, led to 100 and 61% improvements in compression modulus and strength in the resulting PU foams. The dimeric polyols in the lignin oil were principally responsible for enhancing the resistance of the foam to mechanical compression. These findings provide insights into how native lignin can be valorized via a lignin-first approach to produce rigid PU foam materials with an enhanced mechanical robustness and bio-based content.