Polymer-based phosphoramidite flame retardant for TPU: Enhanced fire resistance with preserved transparency and mechanical properties

Abstract

In this work, a polymeric flame retardant (POP) containing a phosphoramidite bond was synthesized from piperazine and phenyl dichlorophosphate. The chemical structure of POP was confirmed using FTIR, TGA, DSC, ¹H NMR, ³¹P NMR, ¹³C NMR and PY-GC–MS. Adding POP to TPU significantly improved its flame retardancy while maintaining its mechanical properties and transparency. Only 3 wt% of POP reduced the peak heat release rate (PHRR) of the TPU composite by 50.1 %. With increasing POP content, the PHRR and total heat release (THR) of the TPU composites decreased further, and the char residue increased. At 15 % POP, the THR and PHRR of TPU decreased by 40.7 % and 60.2 %, with char residue reaching 21.6 %. Moreover, when the amount of POP was 30 %, the TPU composite reached a LOI value of 29.2 %, passing UL 94 V-0 level. The improved flame retardancy resulted from the prior concentrated emission of CO₂ in the gaseous phase and the creation of denser, more graphitized char residue within the condensed phase. Additionally, the excellent compatibility between POP and TPU ensured minimal impact on the mechanical properties and transparency. The tensile strength, elongation at break, and transmittance of TPU/15 % POP were 40.2 MPa, 525 %, and 81.5 %, nearly matching the performance of pure TPU.