Bio-based flame retardancy for epoxy-coated wood: Improved fire performance and quantitative fire-risk evaluation

This study addresses the growing need for quantitative fire-risk evaluation of wood products, given the increasing use of wood in buildings and concerns regarding the persistence and regulatory implications of some conventional flame retardants. We investigate bio-derived phytic acid (PA) as an epoxy-coating additive for spruce and benchmark its performance against ammonium polyphosphate (APP), aiming to propose a coating strategy based on eco-friendly flame retardants. Four systems—spruce, epoxy-coated spruce, epoxy-coated spruce with APP, and epoxy-coated spruce with PA—were prepared and tested using cone calorimetry. To translate the measured fire-response indicators into quantitative risk levels, we adopt two complementary indices: the Fire Growth Rate Index (FIGRA), representing early-stage fire growth, and a Python-based Relative Fire Risk Score (RFRS), which integrates multiple risk-relevant factors into a single comparative score. Epoxy intensified burning compared with uncoated spruce, increasing both peak and total heat release. APP improved ignition delay and reduced cumulative burning and material consumption but exhibited a pronounced early peak, implying limited protection during the most critical growth stage. In contrast, PA delivered the most effective overall mitigation, with substantially lower cumulative heat release and mass loss and a moderated, delayed peak. PA suppressed the acceleration of heat release by delaying the initial fire spread and ignition, yielding a low FIGRA, and achieved the best RFRS among all systems. Overall, the results highlight PA as a promising naturally derived flame retardant and demonstrate a practical framework for quantified fire-risk evaluation of coated wood products.