Metal–Organic Framework as an Efficient Synergist for Intumescent Flame Retardants against Highly Flammable Polypropylene

The intumescent flame retardant (IFR) technique has been considered as an alternative to halogen-based flame retardants for reducing the fire hazards of polymers, especially highly flammable polyolefins. However, the IFR technique possesses certain disadvantages, such as unsatisfactory flame retardant efficiency, high loading requirements, and toxic smoke release. In this study, zeolite imidazolate frameworks-8 (ZIF-8), a common type of metal–organic frameworks (MOFs), was incorporated into an IFR/polypropylene (IFR/PP) composite system as a synergist to further improve flame retardant efficiency and reduce the smoke emission. The results show that ZIF-8 and the IFR additives exhibit a strong synergistic effect between them for improvement of the formation and stability of the intumescent char layer, to prevent the intensive burning of PP. By adding only 2 wt % of ZIF-8, its limiting oxygen index value reached 31.2% and it obtained a UL-94 V-0 rating. From cone calorimeter test results, its peak heat release rate was further reduced by 44.1% and the total heat release at 700 s was reduced by 17.5% compared with the traditional IFR/PP system. The tendencies of fire spread and flashover were also reduced significantly. Furthermore, the addition of ZIF-8 could contribute to suppressing the smoke release as well as CO and CO2 production from burning IFR/PP composites. Based on the analysis of carbonaceous residue, we propose that the presence of ZIF-8 in the IFR formula inhibits the size growth of the carbonaceous microdomain during combustion, which could help to form more compact char layers and ultimately improve the flame retardant efficiency. This study provides new insights for developing more efficient IFR systems for polyolefins and reducing their smoke emissions during combustion.