Castor oil/DOPO-based waterborne polyurethane with flame retardancy-strength balance for microfiber leather

F lame- r etardant w aterborne p oly u rethane (FR-WPU) combines fire resistance with the environmental benefits of waterborne systems, making it attractive for advanced textiles, transportation, and construction applications. The traditional strategy relies on covalently embedding rigid, polar flame-retardant moieties into the PU backbone, which ensures durable flame resistance, but inevitably compromises chain mobility and mechanical robustness. Here, leveraging a macromolecular rigidity-flexibility synergy strategy, we have developed a novel bio-based FR-WPU by integrating flexible c astor o il (CO) segments with a rigid 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-derived flame-retardant chain extender. By precisely tuning the CO/DOPO ratio, we construct a balanced macromolecular architecture in which flexible chains dissipate stress and enhance compatibility, while rigid phosphorus-aromatic domains form thermally stable char barriers. The resulting FR-WPU achieves a l imiting o xygen i ndex (LOI) of 29.2% (well above the 27% threshold), along with excellent mechanical properties, including a tensile strength of 34.3 MPa (35.9% higher than that of CO-free PU) and an elongation at break of 347.9% (an 11.7% improvement). When applied to m icro f iber synthetic l eather (MFL), the MFL maintains robust flame retardancy (with an LOI of 28.1%). This work highlights rigidity-flexibility synergy as a universal design strategy to overcome the trade-off between macromolecular functionality and mechanical performance, paving the way for next-generation eco-friendly, high-performance polymeric materials.