Blowing agents and solid-expandable particles in fire protective coatings: A review

Blowing agents are critical to the fire protection performance of intumescent coatings, dictating their ability to expand and form an effective thermal barrier. Despite decades of research on intumescent coatings, the mechanistic understanding and systematic evaluation of blowing agents have often been neglected. This review covers their decomposition, gas-evolution, and interactions with acid and carbon sources, emphasizing how blowing agents govern expansion kinetics, char morphology, and fire resistance. It summarizes the current knowledge on blowing agents employed in intumescent systems, spanning nitrogen-rich compounds such as melamine urea, glycine, and guanidine, as well as solid-expandable materials including expandable graphite, alkali silicates, and minerals. While melamine remains the benchmark due to its thermal compatibility and high gas yield, its recent toxicity classification has necessitated the search for new blowing agents. Comparative analysis reveals emerging alternatives, such as bio-based compounds (e.g., phytic acid, glycine) and inorganic particles (e.g., expandable graphite, alkali silicates), which offer promising routes toward safer, low-toxicity, and sustainable formulations.