The extraction of polyol for the synthesis of lignin-based polyurethane coatings - a review

Lignin, with its availability, aromatic structure, and abundant hydroxyl groups, presents a promising source of green bio-based polyols. The environmental implications of oil-derived polymeric materials have sparked interest in renewable biopolymers and composites from natural sources, including polyurethane composites. This review investigates optimal methods for lignin extraction for use in lignin-based polyurethane coatings, scrutinizes alternatives to oil-based polyols, compares existing chemical modification methods of lignin for use in polyurethane coatings and resins, and discusses the control of synthesis and final properties of lignin-based polyurethane, emphasizing the crucial role of the stoichiometric ratio of NCO groups in the isocyanate to the total OH groups in a lignin-based polyol. While numerous studies have focused on polyurethanes with and without isocyanate, this review describes the synthesis of lignin into bio-polyols and its potential as a substitute for petroleum-based polyols, highlighting organosolv and Kraft lignin as optimal choices for replacing oil-based polyols in polyurethanes due to their high purity..