Fully bio-based phosphorus/nitrogen-functionalized cellulose nanocrystals for flame-retardant and mechanically strong...

The development of highly flame-retardant bio-based polyurethanes is critical for aligning material sustainability with public safety requirements. In this work, a fully bio-based flame retardant based on phosphorus/nitrogen-functionalized cellulose nanocrystals (CNC-PDA-PA) was synthesized through a three-step environmentally friendly pathway comprising periodate oxidation, Schiff base reaction, and phosphorylation, using renewable cellulose nanocrystal (CNC), 1,5-pentanediamine (PDA), and phytic acid (PA) as starting materials. The chemical structure, morphology, and thermal characteristics of the resulting flame retardant were systematically analyzed. A series of castor oil-based polyurethane (COPU) composites incorporating different CNC-PDA-PA loadings were then prepared via mechanical blending and solution casting. The effect of CNC-PDA-PA on the mechanical properties, thermal stability, combustion behavior, and associated flame-retardant mechanisms of the COPU composites was comprehensively examined. The results indicated that the integration of CNC-PDA-PA concurrently improved the mechanical properties and flame retardancy of COPU composites. Specifically, with the addition of 4 wt% CNC-PDA-PA, the tensile strength of the composite was elevated by 11.4% compared to neat COPU, while maintaining favorable flexibility, an outcome ascribed to the nano-reinforcement effect of CNC. Notably, at a 6 wt% loading of CNC-PDA-PA, the composite attained a UL-94 V-0 classification, together with substantial reductions in peak heat release rate (56.3%), total heat release (15.5%), and total smoke production (49.0%), reflecting superior flame-retardant efficacy and smoke suppression capacity via the synergistic action of condensed-phase char formation and gas-phase radical scavenging. This study introduces an innovative and efficient approach for producing high-performance, sustainable bio-based polyurethane composites.