Abstract: The use of intumescent flame retardants (IFRs) is considered an environmentally friendly and cost-effective strategy to suppress potential fire hazards from synthetic polymers. However, some conventional IFRs are neither efficient in developing a thermally stable char layer nor reducing the release of toxic byproducts during polymer combustion. In this work, we aim to discuss the effects of zeolitic imidazolate frameworks (ZIFs) on synergistically improving the flame retardancy behaviors in polypropylene (PP) composites, including thermal degradation (evolved gas analysis), free radical reactions in the gaseous phase (in-situ chemiluminescent image analysis), and carbonaceous structure in the condensed phase (micro-morphology and composition analysis). It is found that the transition metals in ZIFs can catalytically accelerate the crosslinking reaction at a lower initial temperature and decrease the amount of hydrocarbon volatiles in the gaseous phase. Once ignited, the embedded ZIFs can firstly bridge adjacent phosphorus chains in the polymer matrix to expand crosslinking degrees and then they are anchored in the developed N-doped phospho-carbonaceous networks after pyrolysis. As a result, more compact char residue structures are observed in the condensed phase for ZIF-reinforced composites. For example, by replacing 2 wt% of IFR with ZIF-67, the peak heat release rate, peak smoke production rate, and peak CO production rate are reduced by 69%, 80%, and 72%, respectively, when compared to the conventional composite. These results indicate an excellent solution to resolve inherent fire hazards associated with IFRs in polymers and achieve necessary efficiency for industrial applications. It also provides a new strategy for determining flammability characteristics and combustion mechanisms of polymer composites using in-situ chemiluminescence analysis.

Key words: Polymer flammability, Intumescent flame retardants, Zeolitic imidazolate frameworks, Thermal degradation, Free radical analysis, Flame retardant mechanism