Enhancing fire resistance in pine wood through DMDHEU resin and phosphate-nitrogen flame retardant synergies

Abstract Wood modification with 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) has been widely demonstrated to enhance the biological durability and dimension stability of wood, thus it serves a predestined technology to equip wooden products for outdoor applications. However, such technology hardly affects the fire performance of wood, a parameter which gains increasing relevance in the construction sector. Recently, the combination of curing resins with flame retardants has been proposed as a potential approach to enhance the fire performance of wood in a permanent way, hence providing a durable fire protection system. Scots pine sapwood ( Pinus sylvestris L.) was impregnated with aqueous compositions of DMDHEU, a commercially available phosphate-nitrogen based fire retardant (adapted for wooden applications) and combinations thereof. The fixation of the flame retardant was assessed on the basis of a cold water leaching process according to EN 84 (2020). The fire performance of the treated wood was assessed through thermal gravimetry analysis (TGA), a Bunsen burner ignition test and mass loss calorimeter. Both wood treated with pure flame retardant and combination of a flame retardant and DMDHEU, demonstrated improvements in thermal stability, reduced flammability and decreased heat release after treatment. Notably, the THR 600s of wood treated with DMDHEU in combination with the flame retardant was reduced by 89.3% compared to wood treated solely with DMDHEU. However, after the leaching process, the flame retardant happened to leach out almost completely, which was assessed on the basis of a loss in its efficacy related to fire protection. On the contrary, DMDHEU seemed to partially fix the flame retardant inside the wood matrix, which was derived from a persistent enhancement in fire performance, even after wooden specimens passed a cold water leaching.