Multifunctional PN synergistic epoxy coatings for polycarbonate: Simultaneous enhancement in fire safety, scratch resistance, adhesion, and UV shielding

Surface vulnerabilities and inherent flammability critically restrict the broader application of polycarbonate, yet strategies integrating fire safety with surface durability remain scarce. Herein, we designed a phosphorus‑nitrogen synergistic epoxy coating via the incorporation of a synthesized melamine cyanurate and resorcinol bis(diphenyl phosphate) hybrid alongside lignin. Modulating the RDP/MCA ratio strengthenend the coating network and promoted early char formation. Advanced morphological and spectroscopic studies revealed a dual flame inhibition pathway: the condensed phase forms a compact, graphitized char barrier phosphorus-rich char barrier suppressing heat and mass transfer, whereas the gas phase releases nonflammable gases and phosphorus-containing radicals to dilute combustible volatiles and quench flame propagation. Consequently, coated polycarbonte (PC/M-R-2.5) achieved a UL-94 V-0 rating, a limiting oxygen index of 29.0%, and a 46.8% reduction in peak heat release rate compared with neat polycarbonate, alongside effective thermal shielding in a butane blowtorch test. Furthermore, the coating exhibited 3H pencil hardness, Grade 1 adhesion, and strong UV shielding, while retaining its hardness and adhesion after 120 h of accelerated UV exposure, as well as maintaining its UV-shielding capability after Taber abrasion testing. This work elucidates composition-dependent flame-retardant mechanisms and provides a practical surface engineering strategy for developing multifunctional protective coatings. Novelty statement In this manuscript, we report a phosphorus‑nitrogen synergistic epoxy coating for polycarbonate that is built on a newly synthesized MCA-RDP hybrid flame retardant derived from melamine cyanurate and resorcinol bis(diphenyl phosphate). Unlike previous coating studies that mainly emphasize either flame retardancy or surface hardness, this work systematically regulates the RDP/MCA ratio to tailor the crosslinked coating structure, char-forming behavior, and gas-phase inhibition simultaneously. The incorporation of lignin further strengthens the condensed-phase charring process and contributes to UV shielding. As a result, the optimized coated polycarbonate achieves a UL-94 V-0 rating, an LOI of 29.0%, a reduced peak heat release rate, and an improved pencil hardness of 3H, while maintaining excellent adhesion. More importantly, a clear structure-property-mechanism relationship is established through combined FTIR, XPS, SEM, Raman, TGA, and TG-FTIR analyses, demonstrating how composition-controlled P/N synergy governs both fire safety and surface protection in a single coating system. We believe that this manuscript presents novel insights and substantial interest, making it well-suited for publication in Progress in Organic Coatings .