Development of bio-based flame-retardant polyurethane

Polyurethane (PU) is a ubiquitous polymer in modern life across the world yet its inherent flammability and the severe toxicological impacts of traditional halogenated flame retardants (FRs) present critical safety and environmental challenges. This review comprehensively examines the framework shift from petrochemical-based, additive-reliant PUs toward inherently flame-retardant, bio-based polyurethanes and non-isocyanate polyurethanes (NIPUs). Herein we introduce an integrative "Structure-Property-Sustainability" framework that systematically connects molecular design, flame-retardant performance, material properties, industrial applicability, and lifecycle sustainability. We thoroughly analyze the molecular engineering of renewable feedstocks, such as lignin, chitosan, proteins, phytic acid and tannins, to promote robust condensed-phase charring and thermal shielding. We further explore advanced composite architectures, including layer-by-layer (LbL) nanocoatings and microencapsulation, designed to decouple fire performance from mechanical degradation. Furthermore, this review broadens its scope beyond conventional combustion performance to evaluate lifecycle impacts and techno-economic feasibility underscoring that sustainability hinges on chemical circularity in addition to the replacement of fossil resources with bio-based alternatives. By mapping the relationships between monomer structure, fire behaviour, and industrial scalability, this review delineates the critical research gaps and future trajectories required to transition bio-based FR PUs and NIPUs into commercially viable, next-generation sustainable materials.