Design of flame retardants for wood-based intumescent coatings for outdoor applications

This study presents the synthesis and evaluation of silane-based flame retardants (FRs) for intumescent coatings designed to enhance the fire resistance of wood for outdoor applications. Four silane-based FRs were synthesized: DEPA-APTMS (a silane bearing a diethyl phosphate group as acid source), Mel-PTMS (a melamine-functionalized silane as gas donor), and two dual-function silanes, 2DEPA-Taz-APTMS and 2DEPA-Mel-PTMS, incorporating triazine or melamine structures and two phosphate units. Among these, 2DEPA-Taz-APTMS demonstrated the most promising fire performance in combination with standard intumescent components and was selected for further formulation studies. Fire shaft and cone calorimeter tests confirmed that formulations containing 2DEPA-Taz-APTMS and PU binder 6 achieved the best fire protection, with low fire growth rates and simulated building material class B. Gas-phase analysis revealed reduced emission of combustible species and confirmed ammonia release from gas donors. Char characterization demonstrated the formation of stable, phosphorus-rich protective layers with siloxane structures. Additionally, natural weathering tests showed that coatings with silane-based FRs and binder 6 maintained structural integrity and durability.