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

The intumescentflame retardant (IFR) techniquehas been considered as an alternative to halogen-basedflameretardants for reducing thefire hazards of polymers, especiallyhighlyflammable polyolefins. However, the IFR techniquepossesses certain disadvantages, such as unsatisfactoryflameretardant efficiency, high loading requirements, and toxic smokerelease. In this study, zeolite imidazolate frameworks-8 (ZIF-8), acommon type of metal-organic frameworks (MOFs), wasincorporated into an IFR/polypropylene (IFR/PP) compositesystem as a synergist to further improveflame retardant efficiencyand reduce the smoke emission. The results show that ZIF-8 andthe IFR additives exhibit a strong synergistic effect between themfor improvement of the formation and stability of the intumescentchar layer, to prevent the intensive burning of PP. By adding only 2 wt % of ZIF-8, its limiting oxygen index value reached 31.2% andit obtained a UL-94 V-0 rating. From cone calorimeter test results, its peak heat release rate was further reduced by 44.1% and thetotal heat release at 700 s was reduced by 17.5% compared with the traditional IFR/PP system. The tendencies offire spread andflashover were also reduced significantly. Furthermore, the addition of ZIF-8 could contribute to suppressing the smoke release aswell as CO and CO2production from burning IFR/PP composites. Based on the analysis of carbonaceous residue, we propose thatthe presence of ZIF-8 in the IFR formula inhibits the size growth of the carbonaceous microdomain during combustion, which couldhelp to form more compact char layers and ultimately improve theflame retardant efficiency. This study provides new insights fordeveloping more efficient IFR systems for polyolefins and reducing their smoke emissions during combustion.

Automated summary

This study incorporates zeolite imidazolate frameworks-8 (ZIF-8), a type of metal-organic frameworks (MOFs), into an intumescent flame retardant (IFR)/polypropylene (PP) composite system to improve flame retardant efficiency and reduce smoke emission. The results demonstrate a synergistic effect between ZIF-8 and IFR additives, leading to the formation of a more stable intumescent char layer and preventing extensive burning of PP. Addition of only 2 wt% ZIF-8 significantly increases the limiting oxygen index value and achieves a UL-94 V-0 rating, while reducing heat release rate and fire spread tendencies. It is suggested that the presence of ZIF-8 inhibits carbonaceous microdomain growth during combustion, enabling the formation of more compact char layers and enhancing flame retardant efficiency.