Effect of an intumescent flame-retardant coating on the pyrolysis and combustion characteristics of UV-aged wood

Whether intumescent flame-retardant coatings retain their effectiveness on ultraviolet (UV)-aged wood remains unclear because photoaging alters the surface structure of the substrate. This study elucidated the thermal degradation pathway and fire-protective mechanism of an intumescent coating applied to UV-aged pine using thermogravimetry, isoconversional kinetics, kinetic model fitting, cone calorimetry, and char morphology analysis. The coating initiated dehydration and charring at lower temperatures but markedly delayed the main decomposition stage. Compared with UV-aged uncoated wood, the T _50% increased by 49.36–69.67 ℃, while the char residue increased from 5.43–6.99% to 14.34–17.37%. The average apparent activation energy increased by 29.98 and 27.00 kJ·mol ^− 1 according to the Kissinger–Akahira–Sunose and Flynn–Wall–Ozawa methods, respectively. More importantly, the dominant kinetic mechanism shifted from a reaction-order model to an Avrami–Erofeev nucleation-and-growth model, revealing a transition from rapid degradation to controlled char formation. At 45 kW·m ^− 2 , the peak heat release rate, peak mass loss rate, and total smoke production decreased by 50.44%, 44.52%, and 72.60%, respectively. Notably, the coated UV-aged wood formed a denser and more continuous honeycomb-like char layer than the coated unaged wood and exhibited the lowest fire growth tendency. These findings demonstrate that the coating remains effective after UV aging by redirecting wood pyrolysis toward condensed-phase charring and constructing a stable heat- and mass-transfer barrier.