Synergistic fire retardant effect between expandable graphite and ferrocene-based non-phosphorus polymer on polypropylene

The key point to enhance the flame retardancy of expandable graphite (EG) in polymeric materials is to weaken the excessive intumescence of EG during combustion. Herein, a kind of ferrocene-based polymer (PDPFDE) is introduced into the flame retardant polypropylene (PP/EG) system. The fire retardant performance of PP composites was investigated by limiting oxygen index (LOI), vertical burning (UL-94) and cone calorimeter (CC) tests. With 20 wt% EG and 5 wt% PDPFDE added, the sample (PP3) passed the UL-94 V-0 rating, and its LOI value increased from 17% to 28.8% compared with neat PP (PP0). In contrast to PP0, CC data showed that the PP3 had a significantly depressed the peak heat release rate (pHRR) by 82.7%, and the total heat release (THR) by 41.0%. The thermal stability of PP composites and the synergistic effect between EG and PDPFDE were also studied by TGA and XRD. SEM and Raman analysis of the char residues of PP composites after UL-94 tests indicate that PP3 can produce dense char residue with a high degree of graphitization. [Display omitted] •The excessive intumescence of EG in polypropylene during combustion can be prominently weakened by introducing PDPFDE.•There is an optimal loading ratio between EG and PDPFDE for producing a dense and high-quality residue char.•An obvious synergistic charring effect does exist between EG and PDPFDE to improve the fire retardancy of PP composites.•The synergistic charring mechanism is comprehensively discussed.•An appropriate amount of PDPFDE can reduce the smoke production rate and the CO yield.