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

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.