Synthesis of a novel branched cyclophosphazene-PEPA flame retardant and its application on polypropylene

A phosphorous–nitrogen containing cyclophosphazene derivative intumescent flame retardant with branched structures, named six(1-oxo-2,6,7-trioxa-1-phosphabicyclo[2,2,2]octane-4-methyl) cyclotriphosphazene (PEPAP), was synthesized by introducing caged phosphate to cyclotriphosphazene backbone. Then the PEPAP was applied to polypropylene (PP) by melt blending to improve the flame retardancy and thermal stability. Fourier transform infrared spectroscopy and proton nuclear magnetic resonance (1HNMR) were used to confirm the chemical structure of PEPAP. Thermal stability and flammability properties of PEPAP and PEPAP/PP composites were investigated by thermogravimetric analysis, limited oxygen index (LOI), vertical burning test (UL-94) and cone calorimetry tests. The flame-retardant mechanism was explored by thermogravimetric analysis/infrared spectrometry and thermogravimetry/mass spectrum analysis. The results showed that PEPAP had high thermal stability, and the addition of PEPAP greatly enhanced the carbonization process of PP. Also, the addition of PEPAP can dramatically improve the LOI value and UL 94 rating. For the 25% PEPAP/PP sample, the LOI value was increased to 29.4% and it can pass UL 94 V-0. The intumescent chars with an intact, multicellular structure formed from PEPAP/PP composites contain polyphosphoric acid, indicating a condensed flame-retardant mechanism of PEPAP.