Synergistic flame-retardant effect of expandable graphite and phosphorus-containing compounds for epoxy resin: Strong bonding of different carbon residues

In this paper, phosphorus-based compouds, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and hexa-phenoxy-cyclotriphosphazene (HPCP), and expandable graphite (EG) were adopted as flame retardants for epoxy resin (EP). The resulting EP composites were investigated using thermogravimeric analysis (TGA), limited oxygen index (LOI), vertical burning (UL94), cone calorimeter, pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), Fourier Transform Infrared (FTIR) and scanning electron microscope/energy dispersive X-ray (SEM-EDX). The results disclosed that EG affected the thermal decomposition process of EP composites and led to the earlier arisen pk-HRR and later lower HRR due to its high thermal conductivity and strong barrier effect. The further enhanced flame retardancy of EP composites containing both EG and phosphorus-containing compounds indicated the synergy between different flame retardant components. The morphology study showed that the residual char of EP/EG exhibited an intumescent but fluffy and wormlike structure with low adhesion. However, those of EP/EG/DOPO and EP/EG/HPCP revealed compact and tough structures composed of wormlike graphite and carbonized EP matrix. The strong interfacial bonding between these two different carbon residues strengthened the integrality and continuity of the intumescent char layers, leading to a stronger barrier effect in condensed phase. In addition, DOPO and HPCP provided flame retardant effect in gaseous phase for EG-containing epoxy resin.