Bimetallic organic framework-derived nanocages loaded with phosphorus-containing compound for flame-retardant epoxy composites

Small-molecule phosphorus-containing flame retardants typically exhibit low thermal stability and are prone to precipitate in polymers. In this study, cobalt‑nickel layered double hydroxide (CoNi-LDH) with a hollow mesoporous structure was synthesized using ZIF-67 (zeolitic imidazolate framework) as a self-template. Subsequently, dimethyl methylphosphonate (DMMP)-loaded CoNi-LDH (P@CoNi-LDH) was prepared, utilizing DMMP (as a small molecule of phosphorus-containing flame retardant) as a guest component and immobilizing DMMP in its channels and cavities. This innovative loading structure addresses the limitations of small molecule flame retardants, and the synergistic use of two different types of flame retardants achieves satisfactory flame retardancy. The conclusions indicated that the epoxy (EP) composites blended with 5 wt% P@CoNi-LDH (sample EP-5 %P@CoNi-LDH) received a vertical burning V-1 rating and a limiting oxygen index (LOI) value of 31.2 %. The peak heat release rate, peak smoke production rate, and peak production rate of toxic gas carbon monoxide in EP-5 %P@CoNi-LDH were reduced by 43.9 %, 31.1 %, and 46.5 %, respectively, compared to pure EP. Moreover, the hollow mesoporous structure of CoNi-LDH enhanced the interfacial interaction with EP, positively influencing various mechanical properties and thermal stability. Specifically, the impact strength and flexural strength of EP-1 %P@CoNi-LDH increased by 54.8 % and 21.4 %, respectively. This work presents a feasible method for the synthesis of flame retardant carriers with hollow mesoporous structures generated from bimetallic-organic frameworks.