The expansion ratio of indoor intumescent steel structure fire retardant coating is one of its core performance indicators, directly affecting the coating's thermal insulation protection effect on the steel structure during a fire. This indicator is influenced by multiple factors, including coating formulation design, raw material selection, construction technology, and environmental conditions. Its mechanism of action requires comprehensive analysis from the perspectives of materials science, thermodynamics, and chemical reaction kinetics.
The film-forming substances in the indoor intumescent steel structure fire retardant coating are fundamental to determining the expansion behavior. The type and molecular structure of the resin matrix directly affect the melting characteristics and viscoelasticity of the coating when heated. For example, epoxy resins, due to their high crosslinking density, have a relatively high viscosity in the initial melting stage, requiring adjustments to the curing agent ratio or the introduction of flexible segments to optimize their expansion fluidity; while acrylic emulsions, due to their better molecular chain flexibility, more easily form a uniform bubble structure at the same temperature. Furthermore, the interfacial compatibility between the resin and the filler is also crucial. If the difference in their coefficients of thermal expansion is too large, it may lead to interfacial delamination during expansion, affecting the integrity of the char layer.
The foaming agent, as the core driving component of the expansion process, directly determines the expansion ratio through its decomposition temperature and gas production efficiency. Common physical foaming agents (such as expandable microspheres) require uniform gas release within a specific temperature range, while the decomposition reaction of chemical foaming agents (such as aminosulfonates) needs to be synchronized with the resin melting process. If the foaming agent decomposes too early, gas may escape, causing cell collapse; if it decomposes too late, sufficient expansion pressure cannot be formed. Therefore, differential scanning calorimetry (DSC) is needed to precisely control the compatibility between the foaming agent and the resin, ensuring a high degree of overlap between gas release and coating softening stages.
The addition of inorganic fillers has a bidirectional regulatory effect on the expansion ratio. Appropriate amounts of calcium carbonate or aluminum hydroxide can fill resin gaps, forming uniform bubble nuclei and improving expansion density; however, excessive addition leads to increased resin crosslinking and higher melt viscosity, which in turn inhibits bubble growth. For example, when the magnesium oxide content is below 8%, it can fill voids, making it difficult for gas to diffuse rapidly during expansion and improving expansion capacity; however, when the content exceeds 12%, excessive melt formed by low-melting-point glass powder may lead to dripping, damaging the internal cell structure. Furthermore, the water vapor released when fillers containing water of crystallization (such as zinc borate hydrate) decompose upon heating can dilute combustible gases and promote cell stability; however, its addition must be controlled to avoid a decrease in the uniformity of the carbon layer pore size.
The impact of application techniques on the expansion ratio is often underestimated. Coating thickness must be strictly controlled within the standard range; too thin a layer will prevent the expansion layer from forming an effective thermal barrier, while too thick a layer may cause cracking due to internal stress concentration. Spraying pressure and speed must be uniform; otherwise, uneven coating density will occur locally, affecting expansion synchronization. Curing conditions are equally critical; excessive humidity may cause filler hydration, reducing resin crosslinking efficiency; excessively low temperatures may delay the curing reaction, preventing the coating from reaching its optimal expansion state before a fire.
Among environmental factors, temperature and humidity have a significant impact on the long-term stability of the expansion ratio. High-temperature and high-humidity environments may accelerate resin hydrolysis, reducing the adhesion between the coating and the steel structure; while low-temperature and dry environments may cause filler crystallization, affecting the decomposition efficiency of the foaming agent. In addition, ultraviolet radiation may cause resin photoaging, resulting in microcracks in the coating before a fire, reducing expansion density.
Formula optimization requires orthogonal experiments to balance the effects of each component. For example, when adjusting the amount of foaming agent, the proportion of flame retardant must be optimized simultaneously to avoid increased resin brittleness due to excessive flame retardant elements. When introducing new nanofillers, their dispersibility with the resin must be verified to prevent agglomeration from affecting the uniformity of expansion. In practical engineering, the compatibility of indoor intumescent steel structure fire retardant coating with anti-rust primer must also be considered to avoid the expansion layer peeling off due to insufficient interlayer adhesion.
The expansion ratio of indoor intumescent steel structure fire retardant coating is a comprehensive reflection of material design, construction control, and environmental adaptability. By precisely controlling the interaction of the resin matrix, foaming agent, and filler, combined with scientific construction techniques and environmental management, a dense and stable expanded char layer can be rapidly formed in a fire, providing long-term thermal insulation protection for the steel structure. This process not only requires in-depth exploration of materials science but also relies on continuous optimization and verification in engineering practice.
