A waterborne flame-retardant coating integrating self-healing and self-catalytic properties with boric acid as a mediator

Flame-retardant coating treatment is a simple and effective fire prevention strategy. However, the development of functional waterborne flame-retardant coatings that combine self-healing capability with high flame-retardant efficiency remains challenging. In this study, a self-healing waterborne coating featuring boron‑phosphorus synergistic flame‑retardant is developed. The coating is fabricated by incorporating ammonium polyphosphate (APP) as a flame retardant, piperazine and tartaric acid as carbonized crosslinking agents into a self-healing film-forming system composed of polyvinyl alcohol (PVA), tapioca starch, and boric acid. The formation of dynamic borate ester bonds between boric acid and PVA imparts self-healing ability to the coating. Furthermore, the synergistic interaction between boric acid and APP promotes the in-situ generation of BPO₄ as a catalytic species during combustion, which accelerates carbonization and cross-linking reactions among PVA, piperazine and tartaric acid, thereby significantly enhancing the flame-retardant efficiency of the coating. The coating was applied to highly flammable rigid polyurethane foam (RPUF) to evaluate its flame-retardant performance. The test results indicate that a 64 μm-thick coating increases the limiting oxygen index (LOI) of RPUF from 18.9% to 54.3%, achieving a V -0 rating in vertical burning tests. The total heat release and total smoke production are reduced by 21% and 20%, respectively, indicating a substantial improvement in fire safety performance. Additionally, benefiting from the abundant hydroxyl groups in the system, the coating exhibited strong adhesion to polar substrates, effectively improving its durability. This study presents a rationally designed waterborne flame-retardant coating that integrates both self-healing and self-catalytic functionalities through formulation optimization, offering new insights into the development of advanced functional flame-retardant coatings.