Poly(piperazinyl phosphamide): a novel highly-efficient charring agent for an EVA/APP intumescent flame retardant system

Traditional charring agents have two obvious defects, which are low charring efficiency and bad interaction with the other flame-retardant components, leading to the unsatisfying efficiency of an intumescent flame retardant. To overcome the aforementioned disadvantages, a novel phosphorus and nitrogen compound, named poly(piperazinyl phosphamide) (PPPA), was synthesized through a one-step reaction. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) etc., confirmed that PPPA was prepared successfully. Thermogravimetric analysis (TGA) revealed that the char residues of PPPA at 600 degrees C under N-2 and air were 45.3 wt% and 51.6 wt%, respectively. Combustibility tests including oxygen index (OI), vertical burning test (UL-94) and cone calorimeter test (CC) further proved that the PPPA was a highly efficient charring agent from the viewpoint of its application. The OI value of the ethylene-vinyl acetate copolymer (EVA)/ammonium polyphosphate (APP)/PPPA composite reached 30.5%, and it passed the V-0 rating in the presence of only 5.5 wt% PPPA. The Py-GC/MS, FTIR and XPS tests demonstrated that abundant aromatic structures resulting from the decomposition of the PPPA, mainly consisting of C=C, C=N, P-N and P=O etc., facilitated the formation of a stable char layer. Moreover, APP could delay the fracture of the C-N bond in PPPA and catalyze the generation of the benzene ring, further enriching the char residue. So, both the highly-efficient charring capacity of PPPA itself and the synergetic effect resulting from APP and PPPA led to the excellent flame retardation of the EVA composite.