Alkali silicate particles as intumescing agents in fire protective coatings

This work explores the solid self-expanding properties of alkali silicate particles with focus on their potential as intumescing agents in fire protective coatings. The applications of hot-stage microscopy together with TGA, FTIR, XRD, and SEM were used to map the hydration chemistry and thermal behaviour. Herein, the synthesized lithium-, sodium-, and potassium silicates were investigated with varying SiO₂/M₂O molar ratios and under different curing humidity. A correlation between the expansion potential, the SiO₂/M₂O molar ratio, and curing humidity was revealed with increasing expansion for lower molar ratios (3.1-7.4) and higher curing humidity (35-90% RH). While previous studies focus on the intumescence effect of solid-bound H₂O at higher temperatures, the influence of a softening of the silicate matrix is emphasized in this research. Here, a timely balanced softening with subsequent H₂O release is a key factor contributing to increased expansion potential. The combination of hot-stage microscopy and TGA suggested an intumescence mechanism of initial shrinkage, sphere formation, expansion due to viscoelastic melt formation and H₂O evolution, and a second shrinkage before an eventual melting of the particles. Notably, the sodium silicate particles with a SiO₂/M₂O molar ratio of 3.4 exhibited a uniform, solid relative expansion of up to 550%, which was maintained until a second shrinkage at 650 ºC and a later melting at 950 ºC. The observed expansion potential of alkali silicate particles emerges as promising and more sustainable candidates as intumescing agents in fire protective coatings.