When non-expansive fire retardant coatings are exposed to fire, their core mechanism of action is to form a glaze-like protective layer that isolates oxygen and some heat transfer. However, the heat insulation performance of this protective layer has significant limitations. The fundamental reason is that the physical structure and material properties of the glaze layer determine its high thermal conductivity, making it difficult to effectively block heat penetration in high-temperature environments. These coatings typically use inorganic materials as a matrix, such as silicates, borates, or phosphates. These components melt at high temperatures to form a dense glaze surface. However, although the glaze surface appears continuous and closed, its internal microstructure still contains numerous grain boundaries or micropores. These defects become channels for heat transfer, allowing heat to rapidly penetrate the coating via solid-state conduction and directly affect the protected substrate.
The high thermal conductivity of the glaze-like protective layer is one of the key factors limiting its heat insulation performance. Compared to the loose, porous charred layer formed by intumescent flame-retardant coatings upon exposure to fire, the glaze structure of non-intumescent coatings is denser. However, this density does not translate into insulation advantages; instead, the inherent thermal conductivity of the material exacerbates heat transfer. For example, the glaze layer of an inorganic salt matrix may exhibit a glass-like physical state at high temperatures. While the thermal conductivity of glass is lower than that of metal, it is still significantly higher than that of still air or a charred foam layer. Therefore, although the glaze layer can isolate oxygen, it cannot significantly reduce the heat flow rate through air gaps or a low-thermal-conductivity carbonaceous skeleton like the foam layer of intumescent coatings, causing the protected substrate to heat up rapidly in a fire.
Furthermore, the formation process of the glaze protective layer involves a dynamic equilibrium issue, further weakening its insulation stability. In the early stages of a fire, the coating surface begins to melt and gradually cover the substrate, but this process takes time. During this period, parts of the substrate may already be exposed to high temperatures, leading to a decrease in structural strength. Meanwhile, the glaze layer may sag or crack under sustained high temperatures, especially when the coating thickness is uneven or there is stress concentration on the substrate surface. The formation of cracks directly damages the integrity of the glaze layer, allowing heat to penetrate rapidly through the cracks. This dynamic failure mode makes it difficult to maintain the thermal insulation effect of non-expansive fire retardant coatings in the long term, particularly in scenarios with rapid fire spread or high heat radiation intensity, where its limitations are more pronounced.
From a material composition perspective, the formulation design of non-expansive fire retardant coatings also limits the optimization space for their thermal insulation performance. To ensure the glaze layer's meltability and adhesion at high temperatures, a high proportion of inorganic binders and fillers, such as sodium silicate, alumina, or talc, is typically added to the coating. While these components can improve the coating's fire resistance time and mechanical strength, they significantly increase the material's heat capacity and density, leading to an increase in the amount of heat absorbed per unit volume, which in turn exacerbates heat transfer to the substrate. In contrast, intumescent coatings, by introducing foaming agents and carbon sources, form a lightweight charred layer upon exposure to fire. Their low density and high porosity make them a more efficient thermal barrier. Non-expansive coatings, due to inherent material limitations, cannot achieve similar insulation effects through formulation adjustments.
Environmental factors in real-world applications also amplify the shortcomings of non-expansive fire retardant coatings in terms of insulation performance. For example, in fire protection of steel structures, non-expansive coatings require increased coating thickness to extend their fire resistance limit. However, thicker coatings are more prone to peeling, powdering, or delamination when exposed to prolonged ultraviolet radiation, humidity changes, or mechanical vibration. These problems not only weaken the coating's insulation performance but may also lead to premature failure of the steel structure in a fire due to localized protective failure. Furthermore, the application process for non-expansive coatings is more demanding. Improper substrate preparation or insufficient coating curing will further reduce the density and adhesion of the glaze-like protective layer, thus exacerbating the degradation of insulation performance.
While non-expansive fire retardant coatings have limitations in terms of thermal insulation performance, their unique advantages still make them irreplaceable in certain fields. For example, their glaze-like protective layer's high oxygen barrier properties make them suitable for scenarios with stringent oxidation prevention requirements, such as fire protection for chemical equipment or storage tanks. Furthermore, non-expansive coatings generally have better weather resistance and chemical stability, enabling long-term use in outdoor or corrosive environments, while intumescent coatings, due to their organic components, are prone to losing their expansion properties due to UV aging or solvent corrosion. Therefore, when selecting flame-retardant coatings, a comprehensive balance must be struck based on the specific application's fire protection requirements, environmental conditions, and cost factors. Non-expansive coatings remain a reasonable choice for scenarios with moderate thermal insulation requirements but also needing to consider durability and oxidation prevention.
