DOPO-functionalized Fe-based metal–organic framework and its synergistic flame retardant effect with microencapsulated ammonium polyphosphate in epoxy composites

Abstract

A novel hybrid flame-retardant system, named MOF-DOPO, was developed by chemically grafting 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) to an Fe-based metal–organic framework (Fe-MOF-NH₂). The combination of MOF-DOPO and urea–melamine–formaldehyde microencapsulated ammonium polyphosphate (APP@UMF) markedly enhanced the fire resistance of epoxy resin. Comprehensive analyses indicated a synergistic flame-retardant mechanism between MOF-DOPO and APP@UMF operating in both gas and condensed phases. In the gas phase, phosphorus-centered radicals from DOPO effectively captured flame-propagating radicals, while nonflammable gases (NH₃, CO₂, H₂O) diluted and cooled the combustion zone. In the condensed phase, a compact intumescent char reinforced by thermally stable iron phosphate species (Fe₂P₂O₇, Fe₂P₄O₁₂, Fe(PO₃)₂) served as an efficient physical barrier to heat and mass transfer. Moreover, the formation of the spherical particulate network of iron phosphate species covering the surface of the burning material imparted a “lotus-leaf effect”, minimizing flame impingement and retarding further degradation. Additionally, the incorporation of MOF-DOPO significantly mitigated the mechanical deterioration induced by APP@UMF. This study provides a feasible and effective strategy for developing epoxy nanocomposites with superior flame retardancy and good mechanical properties.