Molecular design of different phosphorus-containing groups on the mechanism of fire retardancy, compatibility and mechanics of thermoplastic polyurethanes

The inherent flammability of thermoplastic polyurethane (TPU) severely limits its application in high-safety scenarios. Although phosphorus flame retardant TPU is regarded as an ideal alternative to halogen flame retardant TPU, poor matching between the structure of traditional phosphorus flame retardants and TPU causes low flame retardant efficiency and degraded comprehensive properties in TPU composites. To address these challenges, template-type flame retardants N,N′-(1,3-phenylene)bis( P , P -diphenylphosphinic amide) (NPPA) and tetraphenyl 1,3-phenylenebis(phosphoramidate) (TPPA) were synthesized via molecular structure design. For the first time, the influence of different phosphorus-containing groups (P C and P O) on the flame retardancy, compatibility and mechanical properties of TPU was systematically studied. Theoretical calculations and mechanism analyses indicated that NPPA primarily achieved efficient flame retardancy in TPU by gas phase phosphorus-containing radical quenching effect, while TPPA promoted dehydration and charring by phosphoric compounds in condensed phase. Compared with TPPA, NPPA containing P C exhibited higher flame retardant and UV shielding efficiency in TPU. Only 6 wt% enabled TPU/NPPA to obtain UL-94 V-0 rating with LOI of 29.7 %. PHRR and THR of TPU/NPPA6 and TPU/TPPA6 decreased by 39.6 %, 29.06 % and 54.4 %, 13.24 % in comparison with pure TPU. Since NPPA minimally disrupted the hydrogen bonds between TPU molecular chain segments, TPU/NPPA6 possessed excellent mechanical properties (26.2 MPa). However, uniform dispersion of TPPA in TPU matrix allowed TPU/TPPA to retain TPU transparency. This study provided structure-performance guidance between the structure of flame retardants and the comprehensive performance of materials including flame retardancy, mechanical properties and transparency for developing flame retardant TPU suitable for emerging fields like new energy and rail transit.