A Composite Coating of Microencapsulated Ammonium Phosphate in Urea-Formaldehyde Resin Enhances Flame-Retardant and Mechanical Properties of Wood

Although ammonium polyphosphate (APP) is an effective flame-retardant, its poor compatibility with the matrix material often leads to inferior mechanical properties. We addressed this limitation by combining APP with diammonium phosphate (ADP) to create a synergistic flame-retardant system and encapsulated this system in a polymeric material. We used 3-aminopropyltriethoxysilane (KH550) and β-cyclodextrin (β-CD) as cross-linking and char-forming agents, respectively, to prepare a core–shell-structured flame retardant (denoted as APDKC) via microencapsulation technology. The final flame-retardant coating (UF-APDKC) was formulated by mixing APDKC with urea-formaldehyde (UF) resin. This approach enhances the compatibility of APP with the UF resin and significantly improves the mechanical and flame-retardant properties of the coating. The impact strength of natural wood increased from 6 to 14 J/m2 after coating with UF-APDKC, and the limiting oxygen index of the UF-APDKC-coated wood reached 32%, meeting the UL-94 V-0 flammability rating. Moreover, the heat release rate and total heat release of the UF-APDKC-coated wood decreased by 82.86 and 91.47%, respectively, compared with those of natural wood; the residue char yield of the UF-APDKC-coated wood at 800 °C was 28.95 wt %. This work provides a strategy for developing an effective composite flame-retardant.