Graphene platelets versus phosphorus compounds for elastomeric composites: flame retardancy, mechanical performance and mechanisms

Polymers are widely used but their flammability remains a serious issue causing fatalities and property damage. Herein we present an investigation into the effectiveness of graphene platelets (GnPs) to simultaneously improve the flame retardancy and mechanical properties of ethylene propylene diene monomer rubber (EPDM). EPDM was melt compounded respectively with GnPs and a commercial flame retardant (ammonium polyphosphate, APP) to produce two groups of composites. Although both fillers were well dispersed in EPDM, GnPs significantly improved the mechanical properties of EPDM whilst APP compromised some of the mechanical properties particularly at high fractions. This difference was attributed to the filler particle size and interfacial bonding. Through cone calorimetry testing, 21 wt% char yield was recorded for the EPDM/GnP composite at 12.0 vol%, in comparison with 8 wt% for the EPDM/APP composite. APP was able to lower the peak heat release rate (PHRR) and specific mass loss rate (MLR), but unfortunately it decreased the ignition time and fire performance index (FPI). By contrast, GnPs has been found to increase ignition time by 29% and FPI by 62%, while still achieved the same level of reductions in PHRR and specific MLR, demonstrating clear advantages over APP. During combustion the highly thermally stable GnPs bonded with the viscous, degraded EPDM macromolecules, forming a thick solid char layer which prevented the transport of heat and smoke, contributing to its superior flame retarding performance over APP.