Formulation optimization of a silicon-based fire protective coating in terms of intumescent alkali silicate particles

Alkali silicate particles suggested efficient expansion capacity due to water release, making them potential candidates as expandable agents in intumescent coatings. Different types of particles with different alkali cations (Na-, K- or Li-) and varied SiO₂/Na₂O molar ratios were incorporated into a silicone binder and their fire protection performance was investigated. Among the tested alkali cations, the Na-based system exhibited the best performance with a critical time of 64.3 minutes reaching a failure temperature of the steel of 550 °C, tested under UL 1709 fire scenario. This was attributed to its efficient expansion ratio, while a lower degree of intumescence led to the unsatisfactory performance of K- and Li-based systems. Additionally, the melting behaviour of K-based silicate particles and their pronounced catalytic effect on silicone decomposition were unfavourable for effective heat insulation of the composed coating. An optimal SiO₂/Na₂O molar ratio was found to be 4.2 among a lower molar ratio of 3.5 and a higher molar ratio of 4.9. By optimizing the particle size of sodium silicate particles to 125–212 µm, the critical time was further prolonged to 78.6 minutes. These findings underscore the adverse effects of excessive expansion, which results in the formation of large pores that facilitate heat transfer. This conclusion stems from a detailed structural analysis of the selected expanded coatings utilizing X-ray microcomputed tomography (µ-CT).