A new strategy for constructing polypropylene composite foams with excellent ablation resistance and flame retardancy

In this work, we demonstrated a new method for rapidly forming a three-dimensional network porous char layer during the combustion of PP composite foam, based on the combination of intumescent flame retardant (IFR) with expandable graphite (EG) and carbon nanotubes (CNTs). The IFR was composed of pentaerythritol (PER) and ammonium polyphosphate (APP) with a mass ratio of 1:2. Compared to PP composite foams with 40% IFR (PP/40IFR), the PP/EG/CNTs/IFR composite foams we designed could preserve the original foam shape and show excellent flame retardancy after combustion due to the formation of a complete porous char layer. It was found that the EG aggregates violently expanded during combustion and offset the shrink of cellular structure, which can blocks the heat transfer from outside to inside and protect the inner material from the fire. At the same time, the CNTs could bind expanded EG aggregates tightly through the interaction between CNTs and EGs. Very interestingly, the IFR was found to form char residue on the surface of CNTs/expanded EG aggregates to not only improve the compactness of the porous char layer but also promote thermal oxidation resistance of this char layer. Owing to the synergistic effect of the IFR with expanded EG and CNTs, the flame retardancy of PP/EG/CNTs/IFR composite foams was dramatically improved, including delayed the flame spread and completely inhibited the droplet phenomenon, and reached the level of flame retardant HF-1 in the standard level combustion test. Moreover, the PP/10EG/5CNTs/25IFR composite foams prepared by this work also presented excellent ablation resistance in the case of a large-scale flame.