Influence of a novel P/N-containing oligomer on flame retardancy and thermal degradation of intumescent flame-retardant epoxy resin

In this work, a novel P/N-containing oligomer poly(piperazine phenylphosphamide) (BPOPA) was synthesized via the polymerization reaction of phenylphosphonic dichloride with piperazine. Its structure was characterized by Fourier transformation infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), elemental analysis (EA) and gel permeation chromatography (GPC). It was used to enhance the flame retardancy of epoxy resin/ammonium polyphosphate (APP). As expected, improvement in flame retardancy of EP/APP was observed. In detail, EP containing 10 wt% APP alone possessed a LOI value of 30.2% but UL-94 no rating. When both 7.5 wt% APP and 2.5 wt% BPOPA were added into EP, EP/7.5APP/2.5BPOPA passed a UL-94 V-0 rating with a LOI value of 33.1%. Moreover, EP/7.5APP/2.5BPOPA presented much lower values of the peak of heat release rate (PHRR) and total smoke production (TSP) compared with EP/10APP. The flame-retardant mechanism was analyzed by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis/infrared spectrometry (TG-IR). Results illustrated that EP/7.5APP/2.5BPOPA mainly functioned in condensed phase which was associated with the formation of more compact char layer.