Improving Thermal Stability and Combustion Behavior of Waterborne Polyurethane Using an Efficient Ternary Hybrid Flame Retardant by Dual Modification of Boron Nitride With Phytic Acid and Chitosan

To address the limitations of traditional binary flame retardant systems for waterborne polyurethane (WPU), this work proposes a ternary hybrid flame retardant design strategy: boron nitride (BN) particles coated with phytic acid (PA) and chitosan (CS) (BN@PA-CS) were fabricated via a facile, green modification strategy, to overcome poor dispersion and weak interfacial interaction from physical blending. The flame-retardant, mechanical and rheological behaviors of modified WPU were explored. The results show that, compared with pure waterborne polyurethane (WPU), the peak heat release rate (pHRR) of the BN@PA-CS7 sample decreased by 34.5%, the total heat release (THR) decreased by 45%, and the total smoke production (TSP) was reduced by up to 75%, demonstrating excellent smoke suppression performance. BN@PA-CS suppresses complete polymer oxidation combustion and toxic gas emission. Furthermore, a trade-off in mechanical properties: low filler loadings (1–2 wt%) improve tensile strength, yet higher contents (5–7 wt%) trigger property deterioration owing to filler aggregation; elongation at break decreases monotonically. Besides, complex viscosity of WPU is reduced. This work provides a strategy to develop biomass-derived flame retardants by constructing a ternary hybrid structure on inorganic particles, which offers new insights for balancing flame retardancy and comprehensive properties of polymer materials.