A functional surface-modification agent was synthesized via a reaction between hexachlorocyclotriphosphazene and g-aminopropyl triethoxysilane. Ammonium polyphosphate (APP) was modified with this agent and then incorporated into a rigid polyurethane foam (RPUF). Fourier transform infrared spectroscopy, 1 H-NMR, and X-ray photoelectron spectroscopy were used to characterize the modified ammonium polyphosphate (M-APP). The results show that the dispersibility was improved and the particle size decreased after the modification. The limiting oxygen index and cone calorimetry test results show that M-APP enhanced the flameretardant properties of RPUF. The peak heat-release rate of polyurethane (PU)/20% M-APP decreased by 51.18% compared with that of PU-APP. The scanning electron microscopy results illustrate that M-APP facilitated the formation of intumescent and compact char. The excellent flame-retardant performance of M-APP resulted from the flame-inhibition and barrier effects, which were attributed to the phosphazene group and the intumescent char, respectively.
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