Single-additive-driven multifunctionality in waterborne polyurethane coatings for flame retardancy, water repellency, and UV resistance

Waterborne polyurethane (WPU) coatings are widely used in building and infrastructure applications owing to their environmental friendliness and balanced mechanical properties. However, simultaneously achieving durable flame retardancy, water repellency, and UV resistance in waterborne systems remains challenging due to the intrinsic hydrophilicity of WPU and the migration of conventional flame retardants. Here, we propose a single-additive strategy to construct multifunctional WPU coatings by encapsulating a P N flame retardant within a tailored silane coupling agent and a polydimethylsiloxane (PDMS) shell. This design forms a stable core-shell architecture and establishes an organosilicon-mediated P-N-Si synergistic system, imparting high flame retardancy (UL-94 V-0 rating and a limiting oxygen index >33%) while enhancing interfacial compatibility and structural stability within the coating matrix. Surface enrichment of hydrophobic siloxane segments lowers surface energy and limits water penetration, thereby suppressing additive leaching during prolonged water immersion. The resulting coating maintains a water contact angle above 110° after 96 h of immersion while preserves UL-94 V-0 performance. In addition, the coating demonstrates strong adhesion, UV resistance, and abrasion durability. This work establishes a practical single-additive design strategy for multifunctional WPU coatings and provides a scalable route toward durable protective materials for demanding service environments.