Curing behavior, mechanical, and flame‐retardant properties of epoxy‐based composites filled by expandable graphite and ammonium polyphosphate

In this study, 4,4′‐Diaminodiphenyl methane (DDM) was exploited as hardener for diglycidyl ether of daidzein (DGED) and epoxy resin mixture (E‐D). The composites were produced by blending with different contents of ammonium polyphosphate (APP) and expandable graphite (EG) into E‐D resin, named E‐D/A‐E composites. Firstly, E‐D/A‐E blends were analyzed by Fourier transform infrared spectroscopy and DSC to evaluate their curing behavior. The TGA was performed on the E‐D/A‐E composites to analyze their thermal stability, and the thermal properties of the composites were enhanced by adding APP‐EG (A‐E) flame retardants. The mechanical properties and the fractured surfaces of the E‐D/A‐E composites were studied by tensile, impact tests and scanning electron microscopy. Finally, through torch burning test, limiting oxygen index (LOI), vertical burning (UL‐94) and micro‐calorimetry tests, the influence of A‐E flame retardants contents on flammability of composites were evaluated. The results showed that the UL‐94 V‐0 rating was obtained for the E‐D/A‐E composites and the composites exhibited a self‐extinguishing phenomenon. When A‐E flame retardants content reached 9.1 wt%, the LOI value reached 34.5%, and total heat release (Total HR) was reduced by 20.2% compared with E‐D/A‐E 1.