A novel phosphorus-containing nitrogen oxide for enhancing fire safety and UV shielding properties of thermoplastic polyurethanes

The rapid development of emerging fields has put forward stricter requirements for the comprehensive properties of flame retardant thermoplastic polyurethane (TPU). Traditional phosphorus-nitrogen flame retardants needed high loading levels to achieve flame retardancy in TPU, resulting in significantly reduced mechanical performance and aging resistance of TPU composites. In this study, a novel flame retardant N-((diphenylphosphoryl)oxy)-P, P-diphenylphosphinic amide (NDPA), containing multiple flame retardant groups was successfully synthesized by introducing NO endowed with gas phase radical trapping capability while regulating P-C and P-O within a phosphorus-nitrogen flame retardant structure. Merely 2 wt% NDPA enabled TPU to achieve the UL-94 V-0 rating, with the limiting oxygen index (LOI) rising to 30.1 %. The flame retardant mechanism analysis showed that NDPA played an excellent free radical quenching effect in gas phase, and promoted the formation of expanded and high-quality shielding char layers in TPU composites, which significantly improved the flame retardancy of TPU composites. Therefore, the peak heat release rate and total heat release of TPU/NDPA2 composites were decreased by 49.7 % and 28.1 % compared with pure TPU. Meanwhile, due to the good compatibility and uniform dispersion of NDPA in the TPU matrix, the TPU/NDPA composites retained their original transparency and mechanical properties. Additionally, the conjugated aromatic structure of NDPA bestowed TPU composites with excellent UV shielding performance. This study demonstrated that the synthesized TPU/NDPA composites achieved the optimal balance of flame retardancy, transparency, UV shielding, and mechanical properties, providing an innovative scheme for the design of high-performance TPU composites.