Inorganic intumescing alkali silicate particles

New generation intumescent coatings apply self-intumescing particles to induce expansion in a controlled manner. Various studies have investigated alkali silicate-based coatings due to their inherent expansion with the release of H2O. The presented work investigated the self-intumescing concept of alkali silicates as particles. A thorough study of inorganic alkali silicate particles as potential intumescent ingredients in passive fire protection systems was carried out by applying HSDM, TGA, FTIR, XRD, and SEM. Herein, particles of lithium-, sodium-, and potassium silicates were synthesized with varying silica-to-alkali oxide molar ratios and curing humidity. Subsequently, the hydration chemistry and thermal behavior were examined. The results revealed increased expansion potential with decreasing SiO2/M2O molar ratio (M = Na, Li, or K) in the range 3.1–7.4 and increasing curing humidity in the range 35, 50, and 90% RH. Previous studies have emphasized the effect of solid-bound H2O to induce intumescence. In this study, the influence of the softening of the silicate matrix and H2O release is highlighted to allow increased expansion potential. Here, the coupling of thermal analysis with microscopy suggests an intumescence mechanism in the order of 1) initial shrinkage due to loss of free and physical bound H2O, 2) sphere formation to minimize surface energy, 3) expansion as a result of H2O evolution and viscoelastic melt formation, and 4) second shrinkage due to sintering before 5) melting. The sodium alkali silicate particles with a SiO2/M2O molar ratio of 3.4 exhibited a timely softening of the silicate matrix from about 155 °C to provide a distinct spherical particle followed by ionic H2O evolution. As a result, hereof, hollow, spherical particles with relative solid expansions of up to 550% were obtained and maintained until a second shrinkage at 650 °C with a subsequent melting at 950 °C. In comparison, the imbalance in viscoelastic melt formation and H2O release of lithium- and potassium silicate particles resulted in less expansion. Considering the expansion potential and thermal behavior of the alkali silicate particles, it is suggested as a possible, sustainable intumescence ingredient.