Enhanced Flame Retardancy and Mechanical Properties of Waterborne Polyurethane via a DOPO-Based Reactive Flame Retardant

In response to growing demands for environmentally friendly flame-retardant materials, this study develops a novel waterborne polyurethane (WPU) by incorporating the reactive phosphaphenanthrene-based compound DOPO-HQ into the polymer backbone. The modified WPUs, denoted as P-WPUx, exhibit significantly enhanced flame retardancy without appreciably compromising mechanical performance. With only 3 wt % DOPO-HQ loading, P-WPU3 achieves an LOI of 35.35%, satisfies the UL-94 V-0 requirement, and retains mechanical robustness with 33.0 MPa tensile strength and 737.8% elongation. Comprehensive analyses confirm that DOPO-HQ operates via a dual-phase mechanism: scavenging free radicals within the gas phase while simultaneously facilitating carbonaceous layer development in the condensed phase. These findings underscore the potential of molecularly integrated phosphorus-containing modifiers in the design of high-performance, sustainable flame-retardant WPU materials.