An Efficient Halogen-free Flame Retardant for Polyethylene: Piperazine-modified Ammonium Polyphosphates with Different Structures

In this study, piperazine-modified ammonium polyphosphates ( PA-APPs) with hierarchical structure were synthesized through ion exchange reaction. H-1 nuclear magnetic resonance ( H-1-NMR), Fourier transform infrared spectra ( FTIR), elemental analysis ( EA), and inductively coupled plasma atomic emission spectroscopy ( ICP-AES) confirmed that the PA-APPs with different structures were prepared successfully. Then these flame retardants were used alone as mono-component intumescent flame retardant for low-density polyethylene ( LDPE). Combustion tests demonstrated that the flame-retardant efficiency of PA-APP containing about 7 wt% carbon ( PA-APP(7)) was significantly higher than that of the other PA-APPs with more or less carbon. The flame-retarded LDPE system with 30 wt% PA-APP(7) passed the UL-94 V-0 rating, and had the oxygen index ( LOI) of 33.0%. Thermal analysis illustrated that the thermal decomposition behavior of PA-APP changed with incorporating different contents of PA. For all these PA-APPs, PA-APP7 showed higher thermal stability than the other PA-APP flame retardants. All the experimental results proved that PA-APP7 could reach the balance of an acid source, a blowing source, and a charring source as a mono-component intumescent flame retardant for LDPE. Further, it led to the formation of a compact intumescent char layer containing the structures of rich P-O-P,P-N-C, C=C, etc. during burning which in turn resulted in the excellent flame-retardant efficiency of PA-APP7.