UV-cured phosphorus–nitrogen flame-retardant coatings for rigid polyurethane foam with enhanced flame retardancy and water resistance

A UV-cured P/N/Si hybrid network enables durable flame retardancy in RPUF. • Organic–inorganic crosslinking suppresses water-induced coating deterioration. • The coating retains a UL-94 V-0 rating after 14 d of water immersion. • Flame retardancy persists after 31 d, with an LOI of 26.0%. • The hybrid network delays thermal degradation and promotes stable char. To improve the fire safety of rigid polyurethane foam (RPUF), a phosphonic-acid-containing reactive monomer (TP) was synthesized and used with phytic acid to prepare a UV-cured flame-retardant coating (TA). TA increased the limiting oxygen index (LOI) of RPUF from 19.0% to 55.0%, achieved a UL-94 V-0 rating, and reduced the peak heat release rate by 83.0%. However, TA exhibited poor water resistance: after only 0.5 h of water immersion, its mass loss reached 64.7%, the LOI sharply decreased to 22.3%, and the V-0 rating was lost. To improve durability, polyurethane acrylate, pentaerythritol triacrylate, and SiO 2 were introduced to construct an organic–inorganic hybrid network. The optimized P 3 P 27 S 3 T 4 coating exhibited an initial LOI of 44.5% and, after 0.5 h of immersion, retained an LOI of 33.3% and a V-0 rating with only 16.2% mass loss. It maintained a V-0 rating after 14 d and an LOI of 26.0% after 31 d. Moreover, its maximum mass-loss-rate temperature and residue at 800 °C reached 423.22 °C and 30.98 wt%, respectively. These results demonstrate that the hybrid network substantially improves water resistance while maintaining effective flame retardancy and thermal stability.