Flame retardancy of clay–sodium silicate composite coatings on wood for construction purposes

Abstract

Composites of clay minerals mixed with sodium silicate as new inorganic flame retardants for wood have been investigated. Vermiculite beads were exfoliated by an ultra-sonication method and characterized by scanning electron microscopy, Fourier transform infrared absorption spectroscopy and thermogravimetry. The fire properties of wood samples coated with vermiculite and various vermiculite–sodium silicate composites were tested by a cone calorimeter. When the samples were exposed to the heat flux during the test, formation of a solid foamy layer was observed and composite coatings were effective in delaying of ignition coupled with decreasing the heat release rate and the total heat release. Synergy between the low thermal conductivity of the foamy layer and the release of water molecules from the composites improved the flame retardancy of these composite coatings. Among the coatings, multiple layers of sodium silicate and vermiculite exhibited the best flame retardancy in terms of maximum ignition time and minimum heat release rate.