How does the carbon layer structure formed during the expansion process of intumescent fire retardant coating achieve efficient heat insulation and oxygen barrier?
Publish Time: 2025-12-17
In modern building fire protection systems, steel structures are widely used due to their high strength, ease of construction, and high space utilization. However, steel heats up rapidly in a fire, and its load-bearing capacity drops sharply when the temperature exceeds 540℃, easily leading to structural collapse. To address this safety hazard, intumescent fire retardant coating was developed. Its core mechanism lies in the fact that upon exposure to fire, the coating rapidly expands and foams, forming a dense, porous, low-thermal-conductivity carbonaceous insulation layer. This effectively isolates heat transfer and oxygen supply, significantly delaying the temperature rise of steel components and ensuring the structural stability of the building during a fire.1. Porous Carbon Layer: Constructing a Physical Barrier with Low Thermal ConductivityIntumescent fire retardant coatings are in a normal coating state at room temperature, but once exposed to flames or high temperatures, their internal chemical components are activated. First, the acid source decomposes upon heating to produce phosphoric acid, which catalyzes the dehydration and carbonization of the charcoal agent. Simultaneously, the gas source releases a large amount of non-combustible gas. These gases form countless tiny bubbles within the molten coating, causing the coating volume to expand 5 to 50 times, eventually solidifying into a honeycomb-like or porous foamed carbon layer. This structure is filled with closed or semi-closed micropores, trapping air and hindering convection. Furthermore, the carbon skeleton itself has extremely low thermal conductivity, far lower than that of steel. Therefore, this carbon layer acts like a "thermal insulation blanket," significantly reducing heat conduction and radiation from the flame to the steel substrate.2. Dense Surface Layer: Effectively Blocks the Exchange of Oxygen and Combustible GasesAlthough the carbon layer is porous internally, its outer surface forms a relatively dense carbonized shell at high temperatures. This shell not only has high mechanical strength, resisting flame erosion and slight vibrations, but more importantly, it significantly hinders the diffusion of external oxygen inward and the escape of internal combustible decomposition products. Since combustion is a continuous reaction between fuel and oxygen, isolating oxygen is equivalent to breaking the combustion chain reaction. Meanwhile, the non-combustible gases released from the coating further dilute the oxygen concentration near the char layer, creating a localized inert atmosphere that inhibits the exothermic oxidation of the steel surface, thus slowing down the temperature rise rate from the source.3. Synergistic Components: A Dual Chemical and Physical Protection MechanismThe high efficiency of intumescent coatings also stems from their "three-in-one" chemical formulation design:An acid source provides a catalytic environment to promote rapid char formation; a charring agent forms a continuous char skeleton to support the expansion structure; a gas source generates a large amount of gas, driving expansion and filling pores. The precise matching of these three components ensures the rapid formation of a complete, uniform, and strongly adherent char layer in the early stages of a fire. An imbalance in any component can lead to insufficient expansion, cracking or detachment of the char layer, resulting in loss of protection. Furthermore, some high-end products also add nanofillers (such as nano-silica) or ceramic precursors to form a ceramicized network at high temperatures, further enhancing the thermal stability and ablation resistance of the char layer.4. Structural Adaptability in Practical ApplicationsAccording to the building's fire resistance rating requirements, the thickness of the expanded char layer and its insulation performance can be controlled by adjusting the coating thickness. For example, thicker coatings are needed for floor slabs or beams with a Class I fire resistance rating to form a more durable heat barrier; while columns in single-story factory buildings can use thinner coatings to meet lower fire resistance ratings. This flexibility allows intumescent fire retardant coatings to precisely match the fire protection requirements of different steel components, balancing safety and economy.The char layer formed by intumescent fire retardant coating, seemingly light and fluffy, is actually an intelligent fire barrier integrating materials science, thermodynamics, and combustion chemistry. It uses a "soft yet strong" approach to construct a life-saving line of defense in the face of flames. It is precisely this self-igniting, highly efficient heat insulation, and oxygen-barrier flame-retardant property that makes it an indispensable "invisible armor" for modern steel structure buildings, silently safeguarding the building's safety baseline.
