Synthesis of ZIF-8 with encapsulated hexachlorocyclotriphosphazene and its quenching mechanism for flame-retardant epoxy resin

A novel type of zeolitic imidazolate framework (ZIF-8) encapsulated hexachlorocyclotriphosphazene (ZIF-8@HCCP) dodecahedral flame retardant was synthesized and incorporated into epoxy (EP) for reducing the fire hazard of EP. HCCP was successfully encapsulated into ZIF-8 by one-pot approach combining ZIF-8 synthesis and molecule encapsulation benefiting the small windows and large holes effect of ZIF-8. HCCP was interact with ZIF-8 by P–Cl–N. The thermal decomposition temperature of ZIF-8@HCCP was 550.9 °C, significantly higher than that of HCCP (105.6 °C). This encapsulation exhibits high efficiency in flame retardancy and smoke suppression. The peak heat release rate, smoke release rate and CO release rate of 4 phr ZIF-8@HCCP-incorporated EP sample decreased by 56.2%, 57.2% and 45.7% because of the “quenching mechanism”. This mechanism was proposed in which the structure of ZIF-8 collapsed following the rapid release of flame retardant exhibiting instantaneous flame-retardant behavior. Additionally, EP corresponded with ZIF-8@HCCP have good mechanical properties. This work provides a promising advance in designing good-thermal stability flame retardants for improving the fire safety of EP. [Display omitted] •Novel ZIF-8 encapsulated HCCP flame retardant was designed and was incorporated into epoxy resin to improve its fire safety.•The thermal stability of ZIF-8@HCCP composites could be controlled by dominating of encapsulating quantity.•ZIF-8@HCCP exhibited temperature-responsive transient release.•The flame retardancy and smoke suppression of the composites were improved dramatically.•Quenching flame-retardant mechanism was proposed and confirmed.