Preparation of Poly(phosphoric acid piperazine) and Its Application as an Effective Flame Retardant for Epoxy Resin

A phosphorus-nitrogen containing flame retardant additive of polyphosphoric acid piperazine, defined as PPAP, was synthesized by the salt-forming reaction between anhydrous piperazine and phosphoric acid, and the dehydration polymerization under heating in nitrogen atmosphere. Its chemical structure was well characterized by Fourier transform infrared (FTIR) spectroscopy, C-13 and P-31 solid-state nuclear magnetic resonance measurements. The synthesized PPAP and curing agent m-phenylenediamine were blended into epoxy resin (EP) to prepare flame retardant EP thermosets. The effects of PPAP on the fire retardancy and thermal degradation behavior of cured EP/PPAP composites were investigated by limiting oxygen index (LOI), vertical burning (UL-94), thermogravimetric analysis/infrared spectrometry (TG-IR) and cone calorimeter tests. The morphologies and chemical compositions of char residues for cured epoxy resin were investigated by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. The results demonstrated that the flame retardant EP thermosets successfully passed UL-94 V-0 flammability rating and the LOI value was as high as 30.8% when incorporating 5wt% PPAP into the EP thermosets. The TGA results indicated that the synthesized PPAP flame retardant additive possessed high thermal stability and excellent charring capability. Meanwhile, the incorporation of PPAP stimulated the epoxy resin matrix to decompose and charring ahead of time due to its catalytic decomposition effect, which led to a higher char yield at high temperature. The morphological structures and the analysis results of XPS for char residues of EP thermosets revealed that the introduction of PPAP benefited the formation of a sufficient, more compact and homogeneous char layer containing phosphorus-nitrogen flame retardant elements on the material surface during combustion. The formed char layer with high quality effectively prevented the heat transmission and diffusion, limited the production of combustible gases, and inhibited the emission of smoke, leading to the reduction of heat and smoke release.