An efficient halogen-free flame retardant for polyethylene: piperazinemodified ammonium polyphosphates with different structures

In this study, piperazine-modified ammonium polyphosphates (PA-APPs) with hierarchical structure were synthesized through ion exchange reaction. ¹H nuclear magnetic resonance (¹H-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 monocomponent intumescent flame retardant for low-density polyethylene (LDPE). Combustion tests demonstrated that the flameretardant efficiency of PA-APP containing about 7 wt% carbon (PA-APP₇) was significantly higher than that of the other PAAPPs with more or less carbon. The flame-retarded LDPE system with 30 wt% PA-APP₇ 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.