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Thermal Degradation and Combustion Behaviors of Flame-Retardant Polypropylene/Thermoplastic Polyurethane Blends

The flame retardancy, thermal degradation, combustion behaviors and mechanical properties of a pure polypropylene (PP)/thermoplastic polyurethane (TPU) blend, and PP/TPU blends containing flame retardants, ammonium polyphosphate (APP), pentaerythritol (PER), and melamine cyanurate (MCA), as well as the surface-treated rectorite clay (REC), were investigated by the limiting oxygen index (LOI) test, vertical burning test (UL-94), thermogravimetric analysis (TGA), cone calorimetry, and mechanical testing machines. The results showed that the PP/TPU blend containing 25 wt% of the composite flame retardants, composed of APP/PER/MCA/REC (40/40/20/3 w/w/w/w), had the best flame retardancy and integrated mechanical properties: LOI value of 28.0% and UL-94 rating of V-0; the values of tensile strength, elongation at break, impact strength, and flexural strength were 21.3 MPa, 44%, 5.8 kJ m−2, and 28.8 MPa, respectively. Compared with the pure PP/TPU blend, the flame-retardant PP/TPU blend had increased fire performance index (FPI) of 26%, and its total heat release (THR), peak heat release rate (PHRR), and total smoke release (TSR) were decreased by 24%, 62%, and 13%, respectively. The results of TGA, cone calorimetry, and char morphology observation showed that the studied composite flame retardants displayed their flame-retardant effects via their partially earlier decomposition and the interactions between their components and TPU, forming a substantial and compact intumescent coating on the surface of the matrix.