Fire-resistant and thermal insulation coatings, under high-temperature environments, undergo a series of complex physicochemical changes to construct a multi-layered protective barrier. The core mechanisms can be summarized into four dimensions: structural phase transformation, endothermic reaction, gas dilution, and the formation of the insulation layer. When exposed to flames or high temperatures, the polymer matrix in the coating first softens and melts. This process is accompanied by a sharp drop in viscosity, providing a flowability basis for subsequent expansion and foaming. Simultaneously, the foaming agent in the expanding components (such as nitrogen-containing compounds) begins to decompose, releasing non-flammable gases such as nitrogen and ammonia. These gases form dense bubbles in the molten matrix, driving the coating volume to expand rapidly, ultimately forming a honeycomb or sponge-like porous carbonized layer.
The formation of the carbonized layer is the result of the synergistic effect of physical structure and chemical changes. Under high temperatures, the carbon source in the coating (such as pentaerythritol, starch, and other hydroxyl compounds) undergoes a dehydration and carbonization reaction catalyzed by an acid source (such as ammonium polyphosphate), generating a stable carbon skeleton. This process not only consumes a large amount of heat energy and lowers the ambient temperature but also provides orderly diffusion channels for gases, preventing coating cracking due to excessive gas escape. The porosity of the carbonized layer typically exceeds 80%, and its interior is filled with inactive gases, forming a highly efficient thermal insulation barrier that significantly reduces heat transfer efficiency. Experiments show that the thermal conductivity of a high-quality fire-resistant and thermal insulation coating can be reduced to below 0.1 W/(m·K), only 1/50th that of steel, significantly delaying heat transfer to the substrate.
At the chemical level, the decomposition reaction of inorganic fillers (such as aluminum hydroxide and zinc borate) is a key step in the endothermic cooling process. Taking aluminum hydroxide as an example, it undergoes a dehydration reaction at 200-300℃, absorbing a large amount of heat energy and lowering the combustion system temperature by 80-120℃. The resulting alumina and carbonized material together form a dense coating layer, further isolating oxygen from contact with combustibles. Furthermore, halogenated flame retardants (such as decabromodiphenyl ether) release reactive gases like HBr at high temperatures, which neutralize OH· free radicals, interrupting the chain combustion reaction and reducing flame propagation speed by 50%-70%. Nitrogen-phosphorus flame retardants, on the other hand, produce inert gases like NH₃ and H₂O from their decomposition, which dilute the oxygen concentration to below 15%, lower than the critical value of 18% for wood combustion, thus inhibiting flaming combustion.
It is worth noting that the reaction mechanisms of the coatings differ across different temperature ranges. In the 200-300℃ range, the coating primarily undergoes endothermic decomposition and expansion foaming, forming a preliminary insulation layer. When the temperature exceeds 400℃, silicate materials begin to melt, forming a glaze-like protective film on the substrate surface. Its viscosity increases exponentially with temperature, effectively providing the substrate with a "liquid protective armor." When the temperature surpasses 600℃, some silicate matrices may crack due to sintering, significantly reducing the coating's fire resistance limit. Nanocomposite technology is then needed to extend the diffusion path of pyrolysis gases to maintain insulation performance.
From an engineering application perspective, these physicochemical changes collectively endow the fire-resistant and thermal insulation coating with both "active defense" and "passive isolation" capabilities. It not only reduces ambient temperature through endothermic reactions but also constructs a physical barrier through the expanding carbonized layer and glaze film, while simultaneously diluting oxygen concentration with non-combustible gases, forming a three-dimensional protective network. This synergistic effect of multiple mechanisms enables the fire-resistant and thermal insulation coating to exhibit excellent protective effects on steel structures, concrete, and other substrates, providing reliable assurance for building safety.
