Intumescent coatings: Comprehensive advances with a focus on inorganic-dominant systems

Intumescent coatings are widely employed as passive fire protection systems for construction materials. When exposed to high temperatures, an intumescent coating undergoes swelling and a sequence of chemical reactions, forming a thermal barrier that slows down the pace at which the underlying substrate's temperature increases. This review presents the development of intumescent coatings, including insights from published literature, patents and commercially available products. Intumescent coatings currently in use predominately consist of organic systems, and so we elucidate their primary constituents and working mechanisms herein. This review focuses especially on inorganic intumescent coatings, mainly alkali silicate-, phosphate- and silicone-based coatings, and it covers the reported formulations and their working mechanisms by addressing their distinct advantages and related challenges. Alkali silicate-based coatings, including geopolymers, exhibit inherent intumescence facilitated by matrix softening and water release, culminating in a ceramifying process that yields protective ceramic layers. Their exceptional intumescent capacity makes them promising candidates for intumescent coatings, but weathering sensitivity and difficulties in implementation restrict their further development. Silicone, known for its high thermal and oxidative stability, serves as a reliable fireproofing material, and when utilised as a binder, it can expand by incorporating expandable agents such as expandable graphite, organic compounds and hydrated alkali silicate particles. Certain additives play a crucial role in enhancing the thermal stability and mechanical strength of silicone, thereby mitigating its inevitable decomposition at high temperatures through the formation of silica, which lacks mechanical strength. Geopolymers and silicone-based intumescent coatings provide enhanced thermal stability and mechanical strength, thus offering an alternative solution for protecting construction materials from fire.