A hyperbranched polyphosphate bisphenol-A ester flame retardant(HPPEA) was prepared by using phosphorus oxychloride and bisphenol-A.The structure and thermal stability of the product were characterized by Fourier transform infrared spectrometer(FTIR)

nuclear magnetic resonance spectrometer(1H-NMR

31P-NMR) and thermogravimetric analysis(TGA).Branching degree of HPPEA determined by 31P-NMR spectrogram was about 51%.HPPEA combined with melamine pyrophosphate(MPP) was incorporated into polyamide 6(PA6).The flame retardant and thermal properties of the flame retardant PA6(FR-PA6) were studied through limiting oxygen index(LOI) and UL-94 vertical burning and TGA tests.The results showed that the LOI for flame retardant PA6 was improved to 27.3 from 21.1 for PA6

and UL-94 V-0 rating was achieved with 20 wt% flame retardant.According to TGA results

the char yield of FR-PA6 at 600℃ was also increased to 16.4 wt% in air atmosphere and 13.6 wt% in N2.Combining with the study of kinetics of thermal degradation and complex viscosity of FR-PA6 samples

clearly synergistic effect existed between HPPEA and MPP

which improved cross linking and char forming of PA6.Furthermore

morphology of char residues of FR-PA6 samples was also investigated by SEM method.Honeycomb and compact char layer was found which effectively protects the internal structure

inhibits the heat transmission and reduces the fuel gas during combustion.