Flame-Retardant Polyurethane Foams: One-Pot, Bioinspired Silica Nanoparticle Coating

The replacement of halogen-free flame retardants, driven by health concerns, has sparked a large demand for new "green" flame-retardant alternatives. Inspired by the natural flame-retardant properties of Cladophora sp. algae coated with silica diatoms, a silica sol–gel method has been employed to improve the fire resistance of common, open-cell polyurethane (PU) foams. The Stöber process with components 2-propanol, water, tetraethyl orthosilicate (TEOS), and ammonium hydroxide was employed for silica nanoparticle synthesis on the inside walls and struts of PU foam. Upon ignition, the treated foams briefly burn, followed by formation of a propagating char front that leads to self-extinguishment. Most importantly, the coating of silica nanoparticles prevents dripping of flaming residues seen in common untreated PU foams. Microcomputerized tomography of silica-treated foam after burning reveals that char formation is confined to the outer edges of the bulk foam. Via cone calorimetry, the peak heat release rate of a 0.5 M TEOS foam was reported as dropping from 560 to 262 kW/m2, relative to untreated foam. These results, coupled with the ease of application of the silica coatings, suggest a viable and scalable approach to the mitigation of burning of common open-cell PU foams.