Pentaerythritol derived phosphorous based bicyclic compounds as promising flame retardants for thermoplastic polyurethane films

Abstract Imparting flame retardancy without melt dripping in thermoplastic polyurethane (TPU) is a challenging task. In the current work, a series of pentaerythritol derived phosphorous based flame retardants (FRs labeled FR1, FR2, FR3, and FR4) were synthesized, and their suitability for TPU thin films (<100 micron) was explored. Vertical flammability tests revealed that a blend of FR2 and TPU at 2% phosphorous content exhibited excellent flame suppression and complete stoppage of melt dripping whereas samples prepared from FR1 showed reduced melt drips. In comparison to FR1 and FR2, FR3 and FR4 manifested excessive nonflamed melt drips, which helps in removal of heat from the combustion cycle and self‐extinguishment. The limiting oxygen index value of flame retardant (FRTPU) blends reached up to 33.5% compared to 19% for pure TPU. Thermogravimetric analysis and scanning electron microscopy studies suggest that blends of TPU with FR1 and FR2 display high char residue and condensed phase mechanism, whereas FR3 and FR4 blends show low char residue, working in the gas phase and by the removal of polymer from flame zone through melt dripping phenomena.