Flame-Retardant Polyurethane Based on Host–Guest Complexation: Inhibiting Incompatibility and Destruction of the Hydrogen-Bond Network

To meet long-term usage requirements, thermoplastic polyurethane (TPU) must exhibit comprehensive properties, including excellent mechanical strength, flame retardancy, transparency, and water resistance. Commonly used sodium tripolyphosphate (STP) flame retardants are incompatible with TPU, while phytic acid (PA) causes disruption of the internal hydrogen-bond network within TPU. This work presents a universal approach to overcoming both issues. An amphiphilic hyperbranched polymer backbone was synthesized, featuring a diphenylphosphine oxide (DPPO) hydrophobic shell and a hyperbranched polyethylenimine (HPEI) hydrophilic core. Leveraging host–guest interactions, STP or PA guest molecules were encapsulated within the backbone via phase transfer, yielding two flame-retardant complexes for TPU modification. Besides, the trace dark brown pigment in the PA aqueous solution could be excluded from the host–guest complex. The vertical burning rating reached the V-0 level, the limiting oxygen index (LOI) was 28.0, the water absorption rate of TPU decreased by approximately 35%, the tensile strength retention rate after aging remained around 85%, and the samples exhibited transparency. It is believable that this method is applicable to various phosphoric acid flame retardants, offering an effective approach for the flame-retardant modification of TPU.