Synthesis of a novel PEPA-substituted polyphosphoramide with high char residues and its performance as an intumescent flame retardant for epoxy resins

A novel polymeric intumescent flame retardant, poly(4,4′-diamino diphenyl sulfone 2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane-4-methanol (PEPA)-substituted phosphoramide) (PSA), has been synthesized through solution polycondensation. The product has been characterized by Fourier transform infrared spectroscopy (FTIR), 1H NMR, 13C NMR, 31P NMR, gel permeation chromatography (GPC) and thermogravimetric analysis (TGA). GPC and TGA suggested that PSA had sufficient molecular weight and high thermal stability, with a residual char of 66.3 wt.% at 700 °C. The performances of PSA and its mixtures with ammonium polyphosphate (APP) in enhancing the flame retardancy and thermal stability of epoxy resins have been evaluated by various methods. Differential scanning calorimetry (DSC) indicated that the incorporation of PSA and APP slightly increased the glass transition temperature of EP. The maximum LOI value of EP/PSA composites reached 32.0% and passed the UL 94 V-0 rating. An obvious synergistic effect between PSA and APP was observed by LOI, CONE tests, and TGA. Moreover, remarkable decreases in the peak heat release rate, total heat release, and total smoke release were observed when PSA was incorporated into EP. TGA data showed that the addition of PSA and APP greatly increased the amount of residual char and apparently reduced the amount of pyrolysis products during combustion. Additionally, the structure and morphology of the residual chars have been studied by FTIR, XPS, and SEM. The FTIR and XPS results illustrated that the flame retardancy of PSA and APP was mainly exerted in the condensed phase and could be attributed to the formation of residual char with aromatic structures bridged by P–O–C and P–O–P bonds.