Expansion type fireproof coating is the core material for passive fire protection of steel structures, building components, power facilities, etc. It relies on multi-stage synergistic reactions of “dehydration esterification, melt foaming, and cross-linking into carbon” at high temperatures to form a loose and porous, low thermal conductivity expansion carbon layer, effectively blocking heat transfer, suppressing substrate temperature rise, and delaying mechanical failure of steel structures. It is widely used in fields such as building fire protection, rail transit, and industrial equipment. The complete expansion fireproof system consists of three core components: acid source, carbon source, and foaming agent. The foaming agent, as the functional core of gas expansion, directly determines the coating foaming ratio, uniformity of foam cell structure, thickness of carbon layer, and thermal insulation stability. It is a key component in regulating the final fire resistance limit of fireproof coatings. Compared with acid source dehydration catalysis and carbon source skeleton carbonization mechanism, the thermal decomposition timing, gas production rate, gas type, and compatibility with melt rheology of the foaming system have a more sensitive impact on the quality of carbon layer formation, and are also the key and difficult points for optimizing the formulation and upgrading the performance of expansion fireproof coatings.
Traditional expansion type fireproof coatings commonly suffer from problems such as mismatched foaming timing, uneven pore size, loose and prone to collapse carbon layer, premature gas release, and poor weather resistance, resulting in unstable actual fire resistance efficiency, weak high-temperature carbon layer erosion resistance, and severe degradation of long-term outdoor service performance. To clarify the mechanism of action, existing defects, and modification directions of the foaming system, this article reviews the mainstream types of foaming agents, foaming synergy mechanisms, foaming failure mechanisms, modification technologies, and the latest research progress of expandable fireproof coatings. It analyzes the compatibility rules between foaming systems and resin matrices, toughening aids, and inorganic fillers, summarizes current technological bottlenecks, and looks forward to the development trends of high-performance, weather resistant, and long-term stable foaming systems, providing theoretical references for the formulation design and engineering application of new expandable fireproof coatings.
The ternary synergistic system and basic principle of foaming for 1 expansion type fireproof coating
The fire resistance of expansive fireproof coatings depends on the precise temperature window matching and dynamic synergistic reaction of acid source, carbon source, and foaming agent. The excellent performance of a single component cannot achieve high-quality expansion into carbon. The acid source is mainly ammonium polyphosphate (APP), which decomposes when heated in the low temperature range of 200-300 ℃, releasing strong acidic substances such as polyphosphate and pyrophosphate. It catalyzes the dehydration esterification reaction of carbon source polyol compounds, forming viscous molten carbonaceous precursors; The carbon source is mainly composed of pentaerythritol, starch, and polyol resin, which are dehydrated and crosslinked under acidic catalysis to construct a high-temperature stable carbon skeleton precursor, providing matrix support for bubble growth and carbon layer shaping; The foaming agent serves as the gas producing core of the system, which decomposes upon heating within the matching temperature range, releasing a large amount of inert and flame retardant gases such as NH3, N ₂, CO ₂, H ₂ O, etc., causing the molten viscous carbonaceous precursor to undergo volume expansion, foaming and pore formation, and ultimately forming a porous honeycomb shaped insulation carbon layer through high-temperature cross-linking and solidification.
The complete process of foaming into carbon is divided into four stages of temporal coupling: the first stage is low-temperature softening and melting, where the resin matrix and carbon source form a highly viscous and elastic melt under the catalysis of an acid source; The second stage is uniform gas nucleation, where the foaming agent decomposes precisely to produce inert gas, forming dense bubble nuclei inside the melt; The third stage is the growth and expansion of bubbles, where the melt stretches and expands under gas pressure, causing the pore walls to elongate and thicken, resulting in a doubling of the coating volume; The fourth stage is the solidification and shaping of the carbon layer, where the system temperature continues to rise, the carbon skeleton crosslinks and solidifies, the pore structure locks and shapes, and a stable thermal insulation carbon layer is formed. Research has shown that the core prerequisite for an excellent fire prevention system is a high degree of matching between the foaming gas production temperature, melt viscosity range, and carbonization solidification temperature. Any timing misalignment can lead to foaming defects and fire failure.
Classification and Performance Characteristics of 2 Mainstream Foaming Agents
According to the chemical composition and foaming mechanism, expansion type fireproof coating foaming agents are mainly divided into three categories: nitrogen-containing organic foaming agents, inorganic expansion foaming agents, and composite modified foaming agents. Different types of foaming agents have significant differences in thermal decomposition range, gas production efficiency, foam cell regulation ability, and carbon layer adaptability, and are suitable for fireproof coating systems under different working conditions.
2.1 Nitrogen containing organic foaming agent
Nitrogen containing organic foaming agents are the most widely used mainstream foaming systems in water-based expansion fireproof coatings, with melamine, melamine, and urea as typical representatives. They have the advantages of moderate decomposition temperature, large gas production, fine bubbles, and good compatibility with ternary systems. The thermal decomposition range of melamine is concentrated between 220-300 ℃, which highly overlaps with the APP acid source catalysis and pentaerythritol carbonization temperature. The thermal stability is moderate and the gas production is uniform. The decomposition products are mainly composed of NH3 and N ₂ inert gases, with no toxic or harmful by-products. It can stably promote the foaming of the melt and form a honeycomb carbon layer with uniform pore structure. It is the preferred foaming agent for universal fireproof coatings. However, pure melamine has shortcomings such as high hydrophilicity, easy moisture absorption failure during long-term storage, and rapid high-temperature gas production rate, which can lead to problems such as early gas release and insufficient foaming ratio.
Dicyandiamide has a slightly higher decomposition temperature than melamine, stronger thermal stability, and a gentle gas production rate, which can effectively avoid bubble rupture and carbon layer collapse caused by instantaneous boiling gas production. It is suitable for high-temperature working conditions and thick coating fireproof coatings; Urea has a low foaming temperature, fast gas production, and low cost, but it has poor thermal stability, is prone to premature decomposition at low temperatures, and has serious gas leakage. Its foaming effect is poor when used alone, and it is often used as an auxiliary compound foaming component. Overall, nitrogen-containing organic foaming agents have fine foaming and high expansion ratio, but their water resistance and thermal stability are relatively weak, and the stability of the system needs to be optimized through modification and compounding.
2.2 Inorganic expansion foaming agent
Represented by expandable graphite (EG), it is a new type of functional inorganic foam reinforcement material. Expanded graphite has a layered intercalation structure, with acidic intercalation objects wrapped between layers. Upon heating, the interlayer material rapidly vaporizes and expands, causing the graphite layers to peel off and expand, achieving physical volume expansion. It has a dual function of foaming and pore formation, as well as carbon layer reinforcement. Its expansion temperature range is wide, expansion ratio is high, and high-temperature thermal stability is excellent. The formed graphite carbon layer is dense and hard, with strong resistance to flame erosion, which can significantly improve the high-temperature structural stability of the carbon layer and compensate for the defects of organic foaming agent carbon layer being soft and prone to collapse.
However, expandable graphite has obvious shortcomings: expansion is a physical delamination mechanism, there is no sustained gas foaming effect, and the use of foam cells alone is sparse and has limited thermal insulation performance; At the same time, graphite appears black and can damage the appearance and color of decorative fireproof coatings. The surface of the paint film is rough and the covering power is poor. It is only suitable for non decorative fireproof systems of industrial steel structures and cannot be applied to light colored, high aesthetic indoor decorative fireproof coatings.
2.3 Composite modified foaming agent
Single foaming agents generally have performance shortcomings, making it difficult to simultaneously meet the requirements of high expansion ratio, high carbon layer strength, and high weather stability. Therefore, composite foaming systems have become the mainstream development direction. Through the combination of melamine melamine, organic foaming agent expandable graphite, and microcapsule modified foaming agent, complementary advantages can be achieved. Organic compounding can accurately regulate the timing of gas production, while balancing the fineness and stability of foaming; The organic-inorganic compound can achieve efficient foaming and carbon layer reinforcement simultaneously, solving the inherent contradiction of traditional systems that “high foaming results in soft carbon layers, while hard carbon layers result in low expansion”.
Core matching mechanism and performance influence law of foaming system
The quality of the foaming system does not depend on the single gas production capacity, but on the dynamic matching accuracy of the thermal decomposition timing, melt viscosity, and char formation rate. This is the core mechanism that determines the microstructure and fire resistance limit of the char layer.
3.1 Temperature Window Matching Mechanism
The ideal foaming process needs to meet the following requirements: acid source first decomposes and catalyzes the softening of the melt → foaming agent synchronously produces gas for foaming → later carbon layer cross-linking and solidification to lock the pores. If the decomposition temperature of the foaming agent is too low, the coating has not yet formed an effective molten viscous melt, and the gas quickly dissipates, unable to form a bubble structure, ultimately resulting in low expansion ratio and no obvious foaming layer; If the decomposition temperature of the foaming agent is too high, the melt has already crosslinked and solidified, and the viscosity has sharply increased. Gas cannot promote the expansion of the melt, making it difficult for bubbles to grow, resulting in rigid foaming and almost no expansion effect. A large number of experiments have shown that the peak temperature of foaming agent decomposition needs to lag behind the acid source catalytic temperature by 20-50 ℃, and completely coincide with the optimal viscosity range of the melt (10 ³ -10 ⁶ Pa · s), in order to achieve the optimal foaming effect.
3.2 Gas production rate and pore structure regulation mechanism
The gas production rate of foaming agent directly determines the morphology of foam pores and thermal insulation performance. If the gas production rate is too fast, a large number of bubbles will instantly form, merge with each other, and break through the wall to connect, forming a large pore size and through-hole defect carbon layer. Heat is easily transferred through convection in the pores, resulting in a significant decrease in thermal insulation performance; The gas production rate is too slow, the number of bubble nucleation is small, the pores are sparse, the thickness of the carbon layer is insufficient, and the thermal insulation ability is weak. The uniform and gentle stepped gas production can generate a fine, closed cell, and uniform honeycomb structure, with thick pore walls and continuous structure, which can block heat conduction and convection to the greatest extent possible, and has the best fire resistance performance.
3.3 Performance balance law of foaming agent dosage
There is a strict optimal range for the amount of foaming agent added, showing a significant bidirectional balancing law. At low dosage, there is insufficient gas production, few bubbles, low coating expansion ratio, thin carbon layer, and insufficient thermal insulation protection ability; As the dosage increases, the foaming is sufficient, the carbon layer thickens, and the fire resistance limit continues to improve; When the dosage exceeds the critical threshold, excessive gas continues to impact the cell walls, causing cell rupture, collapse, and connectivity. The carbon layer becomes loose and porous, and the mechanical strength decreases significantly. Under flame erosion, it is highly prone to pulverization and detachment, which in turn leads to a decline in fire resistance. Excessive residual nitrogen-containing foaming agents can enhance the hydrophilicity of the paint film, reduce the water and moisture resistance of the coating, and affect its long-term service stability.
Typical Failure Mechanisms of Foam Systems
In engineering applications, the fire failure of expansion fireproof coatings is mostly not due to the failure of flame retardant components, but to the structural failure of the carbon layer caused by the imbalance of the foam system matching, which can be mainly divided into four typical mechanisms.
Firstly, timing mismatch failure. The foaming, melting, and carbonization reactions are not synchronized, and the gas escapes prematurely or lags behind, resulting in no effective expansion of the carbon layer. Secondly, the pore structure fails. Uneven gas production results in mixed large and small pores, multiple through holes, discontinuous carbon layer, and extremely poor thermal barrier ability. Thirdly, the mechanical failure of the carbon layer. Excessive foaming leads to thin pore walls, loose carbon layers, collapse and pulverization under high-temperature flame erosion, and loss of protective function. Fourth, storage aging and failure. Long term storage of hydrophilic foaming agents leads to moisture absorption and aggregation, decreased dispersibility, disordered gas production upon heating, significant deterioration of foaming performance, and insufficient stability of coating shelf life.
5. Foam system modification and collaborative optimization technology
In response to the defects of traditional foaming systems such as timing mismatch, uneven foam pores, weak carbon layer, and poor weather resistance, the current mainstream modification technologies in the industry mainly include microcapsule coating modification, multi-component compound coordination, and additive coupling regulation.
5.1 Microcapsule encapsulation modification technology
Microcapsule encapsulation is the core technology for improving the stability of foaming agents and accurately controlling the timing of gas production. Surface coating of foaming agents such as melamine and melamine with shell materials such as polyurethane, polyurea, and silica can isolate water vapor, inhibit moisture absorption and aggregation, and significantly improve storage stability; At the same time, the shell layer can delay the thermal decomposition temperature of the foaming agent, smooth the gas production rate, and achieve precise matching of foaming timing. Related studies have shown that after coating modification, the foaming agent has a wider gas production window, more uniform step gas production, significantly improved pore size and carbon layer continuity, and can increase the fire resistance limit by 15% to 25%. At the same time, it effectively improves the water resistance and aging resistance of the coating.
5.2 Multi component compound foaming synergistic technology
By adopting the system of “organic multi-element foaming compound+inorganic reinforcement synergy”, the performance shortcomings of a single foaming agent can be overcome. The combination of melamine and melamine can achieve stepwise continuous gas production, with sufficient gas production in the early stage and continuous gas replenishment in the later stage, avoiding foaming faults; Introducing a small amount of expandable graphite can strengthen the carbon layer skeleton without damaging its appearance, enhance its high-temperature resistance to erosion and collapse, and achieve a two-way balance of “high expansion rate+high carbon layer strength”. This composite system is currently the mainstream formulation scheme for high-performance industrial fireproof coatings.
5.3 Coupling foaming control technology with additives
The rheological agents, toughening agents, and foam stabilizing agents in the coating system can indirectly regulate the foaming effect. The HEUR polyurethane rheological thickening system can optimize the high-temperature melt viscoelasticity of coatings, avoiding bubble rupture caused by too thin melt and foaming obstruction caused by too thick melt; Moderate stabilizing agents can enhance the strength of bubble liquid film, suppress bubble coalescence and bursting, and ensure uniform and stable bubble pores. However, excessive use of flexible additives can reduce the high-temperature viscosity of the melt, resulting in excessive foaming and a soft carbon layer. There is a clear balance between additives and foaming, and precise matching of dosage is required.
6 existing technological bottlenecks
At present, there are still multiple technical bottlenecks in the foaming system of expandable fireproof coatings that urgently need to be overcome. Firstly, there is an inherent balance contradiction in performance: high expansion ratio and high mechanical strength of the carbon layer are naturally opposed. The more fully foamed the carbon layer is, the looser it becomes. Strengthening the carbon layer limits foaming and makes it difficult to optimize synchronously. Secondly, insufficient weather stability: Most organic foaming agents are hydrophilic and easily hygroscopic, and are prone to failure after long-term outdoor service, leading to a decline in foaming performance. Thirdly, the formula has low fault tolerance: The foaming system is highly sensitive to resin matrix, additives, fillers, and moisture content, and even minor formula fluctuations can cause timing mismatches and performance fluctuations in foaming. Fourth, uneven foaming of thick film coating: The temperature difference between the inside and outside of the thick film coating is large, and the foaming of the surface and bottom layers is not synchronized, which can easily lead to local carbon layer defects and affect the overall fire stability.
7 Development Trends and Prospects
With the upgrading of fire protection standards and the increasing demand for high-end industrial coatings, the expansion type fireproof coating foam system is gradually developing towards precise timing control, weather resistance and long-term stability, strong and tough carbon layer synergy, and green, low toxicity and environmental protection. Firstly, functional modified foaming agents have become mainstream. Microcapsule encapsulation and hydrophobic modified foaming agents can effectively solve the problems of moisture absorption failure and timing mismatch, achieving precise and controllable foaming process. Secondly, the organic-inorganic synergistic foaming reinforcement system continues to upgrade, balancing high expansion insulation with high mechanical carbon layer stability, and breaking the traditional performance balance contradiction. Thirdly, intelligent formula regulation is gradually becoming popular. Through rheological matching and thermal decomposition kinetics fitting, the ternary system ratio is accurately optimized to improve formula tolerance and batch stability. Fourthly, green bio based foaming agents and low toxicity environmentally friendly composite foaming systems are gradually replacing traditional high hydrophilic and high residue foaming components, adapting to the development trend of green coatings.
8 Conclusion
The foaming system is the core functional system that determines the quality of carbon foaming and fire insulation performance of expandable water-based fireproof coatings. The type of foaming agent, thermal decomposition timing, gas production characteristics, and compatibility with the matrix melt directly regulate the microstructure of the foam pores, morphology of the carbon layer, and high-temperature protection performance. Traditional single organic foaming agents have defects such as timing mismatch, uneven pore size, weak carbon layer, and poor weather resistance. Through techniques such as microcapsule modification, multi-component compounding, and synergistic regulation of additives, the foaming kinetics process can be effectively optimized to balance the balance between expansion ratio and carbon layer strength, greatly improving the fire stability and long-term service performance of coatings. In the future, a new type of foam system that is precise, controllable, weather resistant, stable, green and efficient will become the core research and development direction of high-performance expandable fireproof coatings, providing important support for the upgrading of passive fire protection technology for steel structures.
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Post time: Sep-09-2026