Boron Phosphates In Situ Generated From Bilayer Coating Approach for Flame‐Retardant Applications

Abstract Intumescent flame‐retardant coatings offer excellent passive fire protection for flammable materials and steel. However, a key challenge is the conflict between increased melt viscosity from additive catalyst and chemical foaming behavior. Herein, “interfacial autocatalytic” strategy‐guided design of a double dopant‐free epoxy coating is reported with a 100 µm boron‐rich upper layer and a 300 µm phosphorus‐rich bottom layer. This coating generates BPO₄ catalyst in situ at the interface during combustion, synergistically catalyzing charcoal formation with the expansion process. BPO₄, with its better lattice arrangement, is able to form a dense ceramic layer of 5.5 µm ceramic layer from a stack of 10–40 nm nanoparticles, encapsulating an underlying 1.8 cm intumescent char layer. The coating demonstrates the superior thermal insulation with the heat‐resistant time for up to 50 min, a 271% delay in ignition time, an ultra‐low fire growth rate with 58% reduction, and a 33% reduction in total heat release. In addition to high transparency, this coating presents easy self‐healing, good mechanical properties, and water resistance. This autocatalytic strategy in a confined space‐time introduces a new method for enhancing catalytic charring flame retardancy.