Improving the flame retardancy of sustainable lignin-based epoxy resins using phosphorus/nitrogen treated cobalt metal-organic frameworks

Lignin and glyoxal are potential sustainable substitutes for preparing lignin-based epoxy resins but exhibit poor flammability; although lignin incorporation enhances the charring capacity of thermosetting materials, intense burning occurs on ignition. A metal-organic framework (P-MOF) comprising cobalt and phosphorus/nitrogen components was therefore synthesized to improve fire safety. Limiting oxygen index (LOI) and vertical burning tests (UL-94) indicate excellent flame retardancy, with a higher LOI value and V-0 rating level achieved after P-MOF addition. Cone calorimeter tests investigating gaseous inhibition of epoxy matrixes reveal that heat release and smoke production dramatically decrease on addition of P-MOF because of fire quenching (phosphorus organic component) and smoke suppression (cobalt component). Synergy between lignin and P-MOF components suppressed combustion of condensed phases, producing more char residues with a higher degree of graphitization. A mechanism for the flame retardancy of the epoxy composite is proposed, which highlights the promise of P-MOF additives for reducing fire hazards due to cobalt-catalyzed CO oxidation during epoxy burning. • Formaldehyde-free, bio-based epoxide polymers were fabricated via cross-linking. • Charring capacity was promoted via lignin addition. • A P/N-modified Co-MOF was fabricated as an effective flame retardant. • Degradation of the organic P/N component conferred excellent fire quenching. • Inorganic Co-MOF component suppressed CO production and promoted charring.