Zn-doped carbon microspheres as synergist in intumescent flame-retardant thermoplastic polyurethane composites: Mechanism of char residues layer regulation

Despite thermoplastic polyurethane (TPU)/ammonium polyphosphate (APP) being an excellent intumescent flame-retardant TPU composite (IFR-TPU), it is still not a "fire-safety" TPU composite. In this work, the Zn-doped carbon microspheres (CMSs-Zn) were prepared and added into the TPU/APP composite as a synergist to fabricate an IFR-TPU composite with excellent fire safety performance (TPU/CMSs-Zn). Cone calorimeter tests exhibited the peak heat release rate (pHRR), total heat release (THR), peak smoke production rate (pSPR), total smoke release (TSR) of TPU/CMSs-Zn composites were decreased by 65.4 %, 50.7 %, 65.3 %, and 63.4 % respectively. Moreover, the smoke density tests showed that the luminous flux (LF) of TPU/CMSs-Zn increased from 1.4 % to 40.1 %. In addition, the investigation of the flame-retardant mechanism for TPU/CMSs-Zn revealed that CMSs-Zn amplifies the role of APP in condensed phase by participating in char formation and catalysis effect, thus forming a dense protective char residues layer. The char residues layer regulation mechanism can provide a novel strategy for facilitating fire-safety TPU composite.