Novel self-healing and recyclable fire-retardant polyvinyl alcohol/borax hydrogel coatings for the fire safety of rigid polyurethane foam.

Rigid polyurethane foam (RPUF) has attracted great attention as an insulation material, but its inherent flammability restricts its practical application. Developing a sustainable fire-retardant strategy that can improve its fire safety is particularly desirable and challenging. Herein, novel fire-retardant hydrogel coatings based on polyvinyl alcohol (PVA) and borax are proposed and applied in RPUF, and the self-healing, recyclability and flame retardant properties of the coatings are investigated. The dynamic and reversible cross-linked networks based on the borate ester bonds and hydrogen bonds endow the hydrogels with excellent repairability, recyclability, and elasticity. Compared with a neat RUPF, the coated RPUF exhibited improved fire-retardant properties without the inherent advantages being influenced and can be reflected by the 8% increase in the limiting oxygen index (LOI), 20% reduction in total heat release (THR), and 25% decrease in total smoke production (TSP) with the coatings, along with a rapid self-quenching behavior. The novel hydrogel coatings provide a new strategy for the development of flame-retardant coatings, demonstrating the potential of the next generation of self-healing hydrogel coatings to reduce the fire risk of the RPUF.