Impacts of multi-element flame retardants on flame retardancy, thermal stability, and pyrolysis behavior of epoxy resin

N-substituted bis(diphenylphosphanyl)amine RN(PPh 2 ) 2 (PNP; R = CH 2 CH 2 CH 2 Si(OEt) 3 ) and its mononuclear nickel(II) ethanedithiolate complexe RN(PPh 2 ) 2 Ni(SCH 2 CH 2 S) (PNS; R = CH 2 CH 2 CH 2 Si(OEt) 3 ) were prepared and used as multi-element flame retardants for epoxy resin (EP). Results of thermogravimetric analysis (TGA) revealed that the incorporation of PNP or PNS restrained the decomposition of EP and also improved the stability of char residues. Moreover, results of limited oxygen index (LOI), vertical burning test (UL-94), microscale combustion calorimeter (MCC), and cone calorimeter tests proved that both PNP and PNS endowed EP with good flame retardancy. Different chemical structures and action mechanisms of PNP and PNS resulted in different degrees of flame retardancy for EP. Although EP/PNP samples caused a little higher LOI values than those of EP/PNS ones, the incorporation of 7 wt% PNS endowed EP with a lower peak of heat release rate (PHRR) and total smoke production (TSP), thereby indicating better flame-retardant and smoke suppression effects. Furthermore, thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR) and pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) were employed to analyze the pyrolysis behavior of EPs and flame retardants . The morphologies of char layers for EPs were examined by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). In comparison to PNP, PNS promoted the formation of a stronger protective char barrier comprising by P/Si/S/Ni elements and also restrained the effects of flammable gases on EP.