Ammonium Polyphosphate and Silicon-Containing Cyclotriphosphazene: Synergistic Effect in Flame-Retarded Polypropylene

A new synthetic non-halogen, flame-retardant cyclotriphosphazene containing the silicon functional group (APESP) was incorporated with ammonium polyphosphate (APP) to prepare flame-retarded polypropylene (PP). APESP/APP/PP composites were prepared by melt-blending and extrusion in a twin-screw extruder with different loading levels of APESP. The combustion and thermal behaviors were investigated based on the limiting oxygen index, UL-94, and cone calorimeter test as well as thermogravimetric analysis (TGA). Then, TGA coupled with Fourier transform infrared spectroscopy was used to probe the degradation mechanism of APESP/APP/PP composites. The results indicated that a good flame-retardant performance could be achieved by incorporating APESP with APP in PP. The synergistic effect between APESP and APP was observed and is discussed here. The mechanical properties and rheological behaviors of APESP/APP/PP composites indicated that APESP significantly contributed to the dispersion and distribution of APP in the PP matrix and remarkably improved the elongation at break. The morphology of APESP/APP/PP composites was investigated using scanning electron microscopy, and the profiles indicated that APESP enhanced the compatibility of the APP/PP system, which was verified by the blending energy and Flory-Huggins parameter (χ) values of APP, APESP, and PP in a multiple-scale simulation.