Encapsulated Water Imparts Unprecedented Flame Retardancy to Cross-Linked Polystyrene Foams

Water is an optimal flame retardant owing to high vaporization and specific heat, greenness and low-cost. The challenge, however, lies in how to integrate water into the matrix materials, and how to ensure its retention. To address the challenge, we developed cross-linked polystyrene foams (cPSs) that contain water within closed pores. These composite cPSs achieved a flame retardancy rating above V-0 in the UL-94 test when the water volume fraction in the ablation layer exceeded 33%. Notably, a representative composite, cPSs-M2, demonstrated significant improvements: the peak heat release rate (PHRR) and total heat release (THR) were reduced by 55.4 and 31.1%, respectively. Additionally, the time to ignition (TTI) and flame performance index (FPI) increased by over 10 and 25 times, respectively. Water evaporation is further and significantly delayed by flame-retardant coating and salts incorporation. The estimated service life of the surface-coated composite cPSs is projected to exceed eight years.