Synthesis of a novel phosphorus-nitrogen flame retardant and its application in epoxy resin

Abstract

A novel phosphorus-nitrogen flame retardant named as melamine phenyl phosphate (MAPPO) was synthesized successfully via the neutralization reaction between phenylphosphonic acid (PPOA) and melamine (MA). The chemical structure of MAPPO was characterized by Fourier transform infrared spectra (FT-IR), nuclear magnetic resonance (NMR) and element analysis (EA). MAPPO was introduced into epoxy resin by blending to improve the flame retardancy. Flame retardancy and combustion behavior of EP/MAPPO were investigated by limiting oxygen index (LOI) test, vertical burning (UL-94) test and cone calorimeter test. UL-94 and LOI tests results showed EP containing 18 wt% MAPPO passed the UL-94 V-0 rating and got a high LOI value of 33%. In the cone calorimeter test, compared with that of EP, the values of peak of heat release rate (HRR), total heat release (THR), peak of smoke production rate (PSPR) and total smoke production (TSP) of modified EP were reduced by 58.7%, 40%, 49% and 61.6%, respectively. By analyzing the volatile pyrolysis products of MAPPO, it was known that MAPPO mainly produced CO2, NH3, H2O and other nitrogen-containing compounds, which diluted the concentration of fuel gases and oxygen during combustion. Meanwhile, the char residue of EP/MAPPO system after combustion was also analyzed by scanning electron microscope (SEM), FT-IR and Raman tests, and the results showed MAPPO was able to promote the crosslinking of EP leading to the formation of compact char layer containing P–O–C, PO and CC, etc. In a word, the enhancement in flame retardancy was attributed to both dilution effect of non-combustible gases and barrier effect of compact char.