Waterborne Polydopamine/Ammonium Polyphosphate Composite Coating for Flame Retardant Modification of Rigid Polyurethane Foam

ABSTRACT Rigid polyurethane foam (RPUF) is widely recognized as an ideal building material due to its low thermal conductivity and excellent compressive strength. However, achieving flame retardancy in RPUF typically requires high loadings of flame retardants, which can compromise the mechanical properties of the foam. In this study, a two‐component waterborne flame‐retardant coating (PA x ), composed of ammonium polyphosphate (APP) and polydopamine (PDA), is developed for the flame‐retardant modification of RPUF. Experimental results indicate that surface modification with a minimal amount of PDA (1:75, mass ratio of PDA to APP) improves the dispersibility of APP in aqueous solution, resulting in a stable waterborne coating. With the aid of PDA mediated adhesion, APP firmly adheres to the surface of RPUF, forming a dense and stable flame‐retardant layer. The PA x ‐coated foam (RPUF‐PA 15 ) demonstrates excellent flame‐retardant performance, achieving a limiting oxygen index (LOI) of 50.9% and meeting the UL‐94 V‐0 rating. Additionally, the compressive strength of RPUF‐PA 15 (733.5 kPa) is 6.4% higher than pristine RPUF (689.3 kPa). In conclusion, an efficient waterborne flame‐retardant coating has been successfully developed, achieving efficient flame retardancy for RPUF and providing a solid foundation for the development of such coatings.