Current Advances in Flame-Retardant Performance of Tunnel Intumescent Fireproof Coatings: A Review

Abstract

As building safety standards keep escalating, research on intumescent fireproof coatings has garnered growing attention. Among them, tunnels, with their enclosed configuration and relatively high accident occurrence rate, impose higher demands on the environmental friendliness, durability, and thermal stability of fireproof coatings. At present, intumescent fireproof coatings have been extensively applied in tunnels; however, a comprehensive and in-depth overview of intumescent fireproof coatings and their application in tunnels is still lacking. This paper summarizes the fire prevention mechanism of intumescent fireproof coatings, the intumescent fireproof system, the impact of functional fillers on the fire resistance performance of intumescent fireproof coatings, and the application of intumescent fireproof coatings in tunnels. Additionally, we present the synergistic effect of the combined use of different functional fillers. Finally, some key challenges regarding the use of intumescent fireproof coatings in tunnel environments are put forward, along with prospects and opportunities.

1. Introduction

Fireproof paint is a simple and effective fire prevention method that has been widely used in industrial projects since the 20th century [1]. In recent years, advancements in nanomaterials and coating technologies have significantly improved the fireproof performance of fireproof paint. Fireproof paint can be categorized into two types: intumescent and non-intumescent, based on the state of the coating after heating. Compared to non-intumescent coatings, intumescent coatings have superior fireproof performance, thin coatings, convenient construction, and are gradually becoming the main choice of tunnel fireproof materials. Intumescent coatings can be described as a mixture that expands and foams into a highly porous, thick, and thermally stable carbon layer when exposed to fire [2,3]. Traditional expansion systems consist of carbon sources, acid sources, and foaming agents [4,5,6]. In the event of a fire, intumescent fire-resistant coatings generate a carbon layer. This carbonaceous honeycomb/porous residue acts as a barrier against heat, air, and pyrolysis products, ultimately safeguarding tunnel structures from fire propagation and thereby reducing losses due to fires [7]. The coordination of decomposition temperatures among the components is a critical factor. Discrepancies in decomposition temperatures between the carbon source and acid source, or premature gas source decomposition, can hinder effective expansion and flame-retardant properties. Additionally, using water-based dispersion media and film-forming materials presents challenges for IFR systems, including uniform dispersion, compatibility, and water resistance [8].

Tunnels are one of the applications of fireproof paint. Compared with other modes of transportation, tunnel traffic has a lower accident probability but faces a higher risk of fire. This is due to the unique narrow and enclosed tube-like structure of tunnels, which limits the diffusion of high temperatures and toxic gases [9,10,11]. Once a fire occurs, the internal temperature of the tunnel rapidly rises, and large amounts of toxic gases that are difficult to diffuse are produced, causing harm to personnel [12]. In addition, although reinforced concrete is a non-combustible material, its fire resistance is relatively poor. For example, in high-strength concrete, the strength begins to decrease at 380 °C, and the compressive strength loss can reach 40% at 450 °C; when the temperature reaches 600 °C, about 75% of the compressive strength is lost. Practice has proved that if a fire lasts for more than 60 min in a tunnel, the overall structure will suffer serious damage. Therefore, it can be said that tunnel fire accidents are among the most difficult to deal with and cause the greatest economic losses [13].

When a fire occurs, the fire-resistant materials used in the tunnel must ensure the integrity and stability of the reinforced concrete structure. In most cases, the fire-resistant coating is the only barrier between the tunnel structure and the fire source, so it must effectively resist the damage caused by high temperatures during the fire process, reduce the heat exchange between the high-temperature gas and the tunnel structure, and prevent the spread of flames [14,15]. When subjected to fire, the expanding fireproof coating generates a carbon layer that effectively impedes heat transfer, isolates combustible materials, and reduces the release of organic compounds [16]. This process consequently diminishes the generation of smoke and harmful gases. Such mechanisms serve to protect tunnel structures, ensuring their stability and safety while providing critical time for personnel evacuation and rescue operations. Furthermore, this approach minimizes both the damage and repair costs associated with fire incidents. Additionally, it contributes to reduced maintenance downtime, establishing itself as a preferred method for fire protection in tunnel environments [12].

Currently, many reviews have emphasized the application of expandable fire-resistant coatings in steel structures [17], wood [18], and numerous polymer scenarios [19,20]. However, there is still a lack of a comprehensive review in the application scenario of tunnels, a special structure. This paper will introduce the various components of expandable flame-retardant coatings, including expandable flame-retardant systems, film matrix, and functional fillers, and discuss their impact on the overall performance of the coatings, such as flame retardancy, smoke suppression, thermal insulation, and mechanical properties. Additionally, the typical expandable flame-retardant mechanism and the fire characteristics of tunnels will be discussed, followed by the application of expandable flame-retardant coatings in tunnels. Finally, the paper will explore the major challenges facing expandable flame-retardant coatings in the field of flame retardancy and offer suggestions for their future development and opportunities.

2. Flame-Retardant Mechanism of Intumescent Coating

Combustion must be ensured by the simultaneous presence of three elements: a combustion-supporting agent (such as oxygen or oxidant in the air), a source of ignition (such as a flame or high temperature), and a combustible material, all in contact with each other. If any one of the conditions is missing or the three cannot directly contact each other, combustion cannot occur or there is a phenomenon of instantaneous extinction of combustion. To prevent combustion from proceeding, one can use this principle to separate one of the elements.

The traditional flame-retardant mechanisms are summarized as shown in

Table 1. Flame-retardant mechanism of intumescent coating.

Flame-Retardant Mechanism	Flame-Retardant Process
Non-flammable gas dilution [21]	The additives produce inert gases when heated, which dilute oxygen and combustible concentrations.
Thermal quenching [22]	Additive endothermic degradation to reduce or maintain the surface temperature of the substrate.
Physical dilution [23]	A large amount of inorganic fillers are incorporated into the paint, resulting in a reduction in combustibles in the paint.
Chemical interaction [24]	Some flame retardants produce free radicals when heated, which can interfere with combustible-phase combustion.
Insulating barriers [25]	The insulation carbon layer generated by the coating after combustion reduces the diffusion of heat to the substrate, as well as the diffusion of oxygen and combustibles.

. However, after differentiating, the main flame-retardant mechanisms can be divided into two main types: the condensed-phase flame-retardant mechanism and the gas-phase flame-retardant mechanism.

2.1. Condensed-Phase Flame-Retardant Mechanism

The condensed phase plays a dominant role in the intumescent flame retardant, exerting the primary flame-retardant effect. Its flame-retardant mechanism involves the formation of an incombustible porous carbonaceous layer on the surface of the substrate during combustion, effectively isolating oxygen and high temperatures to achieve flame retardancy. The process of forming the carbonaceous layer is as follows:

• At lower temperatures, the dehydrating agent decomposes to generate inorganic acids (such as phosphoric acid or polyphosphoric acid), which subsequently undergo esterification reactions with polyols.
• When the temperature exceeds that at which the dehydrating agent decomposes to produce inorganic acids, amines within the system catalyze and promote esterification reactions between inorganic acids and polyols.
• Simultaneously with esterification reactions, foaming agents thermally decompose to produce non-flammable gases, primarily NO and NH₃. These non-flammable gases cause expansion and foaming of the molten system as they escape. Additionally, esters formed from reactions between inorganic acids and polyols dehydrate into carbon, further contributing to expansion and foaming.
• After a period of esterification reaction has elapsed, the flame-retardant system begins to solidify, forming a porous foam-like carbon layer covering the substrate surface that effectively isolates oxygen and high temperatures for achieving flame retardancy.

2.2. Gas-Phase Flame-Retardant Mechanism

In the fireproof paint formula, additives are added that release large amounts of inert gases when decomposed. The production of large amounts of inert gases can reduce the oxygen content, which is the gas shielding effect, making it difficult for oxygen to support combustion and improving the fireproof effect [26]. For example, polymers treated with triphenylphosphine oxide generate gaseous products upon thermal decomposition, which contain PO free radicals after being analyzed by mass spectrometry. The PO· free radicals capture H, causing the concentration of hydrogen atoms in the flame to decrease. Free radicals in the gas phase also collide with carbon particles in the foam-forming layer, then synthesize stable molecules, causing the chain reaction to be interrupted, thus achieving the goal of flame retardancy. In addition, certain additives do not undergo chemical reactions under high-temperature conditions, but merely release water vapor. Water vapor can lower the concentration of combustible materials in the air and thus serve as a flame retardant and control the spread of the fire. It is worth noting that the two flame-retardant mechanisms mentioned above are not mutually exclusive and usually occur simultaneously to work together.

In fact, intumescent fireproofing coatings are composed of a variety of compounds, each with their own unique function. These compounds undergo a sequence of chemical reactions and physical changes to produce an intumescent carbon layer. If there is too much time between the two steps or if the steps are not performed in the correct order, the coating will not expand.

The mechanism of condensed-phase flame retardancy mainly acts on the surface or interior of solid materials. By altering the physical or chemical properties of the materials, it inhibits the combustion process. Specific methods include forming a carbonized layer, reducing heat conduction, and isolating oxygen. At high temperatures, flame retardants react with the materials to generate non-combustible carbonized layers or dense insulating layers, providing protection and preventing the flame from penetrating into the interior of the materials. For instance, intumescent flame retardants can expand at high temperatures and form porous carbon layers, effectively blocking heat transfer and oxygen diffusion.

The gas-phase flame-retardant mechanism mainly acts on the gas-phase environment in the combustion zone, usually interacting with the flame or combustion products in the gaseous state. It inhibits the combustion process by releasing gases that suppress combustion (such as free radical scavengers), interfering with the progress of the combustion chain reaction. Flame retardants evaporate or decompose at high temperatures, generating gases that prevent the reaction between oxygen and fuel, thereby suppressing the spread of the flame. For example, halogen- and phosphorus-based flame retardants can release substances such as hydrogen halides or phosphate esters to capture free radicals produced during combustion and interrupt the combustion chain reaction.

In high-temperature and large-scale fire environments, when the flame-retardant target is steel, wood, concrete, and other materials with high requirements for strength and fire resistance, the formation of a carbonized layer can effectively block heat conduction. At this time, the focus should be on the condensed-phase flame-retardant mechanism of the coating. However, in situations where rapid flame suppression and interference with the combustion chain reaction are needed, especially for thin-layer materials or applications that require efficient and rapid response in the early stage of a fire, more attention should be paid to the gas-phase mechanism of the coating. It should be noted that the two flame-retardant mechanisms are not carried out separately, but at the same time. The two flame-retardant mechanisms are shown in Figure 1.

3. Composition and Performance of Intumescent Coatings

3.1. Binder

The binder’s role is to bind the various components of the paint together, and the solidified paint forms a uniform and solid coating adhering to the substrate. The binder is the main film-forming substance in the fireproof paint, and its type will affect the performance of the paint. Some binders can also serve as carbon sources. The binder has a decisive effect on the physical and chemical properties of the paint and the quality of the carbon layer formed, so choosing the right binder is very important [27]. Depending on the properties of the binder substance, it can be divided into inorganic compound type and organic compound type. Water glass, silicate, and silica sol are commonly used inorganic compound binders; some organic compound binders can play a role in flame-retardant synergism, including styrene–acrylic, polypropylene, epoxy resin, amino resin, polyvinyl acetate, and pure acrylic. However, some organic compound binders emit large amounts of smoke and toxic gases when burned [28,29]. Due to the closed nature of the tunnel environment, these toxic gases can greatly endanger the lives of passengers and workers, and currently the most widely used binder for intumescent coatings is organic compound binder. In addition to its basic function, the binder must ensure good adhesion between the fireproof coating and the substrate.

When research on fireproof coatings was in its infancy, traditional film-forming binders such as acrylic emulsions and polypropylene were commonly used. However, the resulting fireproof coatings exhibited poor weather resistance, low adhesion, uneven foaming of the carbon layer, and a tendency to peel off. These issues significantly diminished the fire-retardant effectiveness of the coatings. As research progressed, fatty emulsions gained prominence due to their high environmental friendliness, corrosion resistance, and superior film strength. Consequently, they have gradually become widely utilized and hold great potential for various applications. In addition, the final performance of the coating is largely influenced by the degree of curing [30,31]. Some related studies have indicated that the addition of nanoparticles can lead to different effects on the curing process of composite coatings [32,33]. Therefore, in order to better analyze the flame-retardant performance of composite coatings, it is necessary to test the degree of curing of the coatings. The degree of curing can be characterized by a curing index (CI), which effectively represents the curing state of composite coatings.

The measurement of the CI usually relies on monitoring the physical and chemical changes during the curing process. Commonly used methods for measuring the CI include Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analysis (DMA), Fourier Transform Infrared Spectroscopy (FTIR), viscosity measurement, etc. Taking DSC as an example, its basic principle is to observe the exothermic or endothermic conditions during the curing process by measuring the heat flow difference between the sample and the reference substance. Curing reactions are usually accompanied by heat release (i.e., exothermic reactions), so the degree of curing can be estimated by measuring the heat release of the sample. Different materials (such as different types of resins, hardeners, or solvents) exhibit different thermal effects (the amount and rate of exothermic or endothermic reactions) during the curing process. The heat flow curves of these reactions can provide key information such as the initial temperature, peak temperature, and reaction heat of the curing reaction.

The structure of polymers significantly affects the rate and extent of curing reactions. For instance, polymers with higher molecular weights or those that are highly cross-linked tend to cure more slowly and release more heat; in contrast, low-molecular-weight polymers react more quickly, and their CI increases more rapidly. Resins with a higher number of functional groups (such as epoxy resins) typically have higher reactivity, more pronounced thermal effects during curing, and their CI is also easier to measure.

The curing reaction of copolymers is usually rather complex because the reaction rates and curing characteristics of different monomers vary. In DSC measurements, the heat flow during the curing process may show multiple peaks, which represent the curing reactions of different monomers. The changes in the CI of copolymers are closely related to the reactivity of each monomer, the mixing ratio, and the reaction mechanism.

The fire-resistant performance of coatings composed of different base materials varies. Researchers have compared two types of coatings based on acrylic resin emulsion and vinyl acetate-based branched emulsion as binders. It was found that the adhesion strength of the coating using vinyl acetate-based resin emulsion as a binder can reach up to 1.412 MPa, approximately 1.5 times higher than that of the coating using acrylic resin emulsion as a binder. Furthermore, the fire resistance limit of the coating using vinyl acetate-based resin emulsion as a binder is stronger [34]. Jiang [35] utilized 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) to modify epoxy resin molecules and obtained halogen-free intumescent high-residue epoxy resin (EP-DOPO). Partial replacement of EP with EP-DOPO significantly improves the smoke density and carbon layer strength in addition to enhancing the flexibility and density of the carbon layer in epoxy intumescent fire-resistant coatings.

3.2. Intumescent Flame-Retardant System

A dehydration carbon catalyst, carbonizing agent, and foaming agent are the three main constituents of intumescent flame-retardant systems. These systems play a significant role in fireproof coatings, with the ratio of the carbonizing agent, dehydrating agent, and foaming agent having a substantial impact on the fireproof performance of such coatings [36,37,38]. Upon heating, the dehydrating agent undergoes decomposition to release inorganic acid, which promotes dehydration and cross-linking of carbon sources to form carbon. Simultaneously, the foaming agent releases non-flammable gases that facilitate the formation of an intumescent protective carbon layer. This layer acts as a physical barrier hindering heat transfer to the surface of combustible materials [39,40].

By adjusting the proportions of carbonizing agents, dehydrating agents, and foaming agents according to other types and ratios of formulations in paints or coatings, it is possible to identify optimal ratios for achieving effective flame retardancy. Furthermore, compounds constituting paints can be substituted by one or more alternative compounds to enhance expansion efficiency and fire resistance properties [41]. The most widely used IFR is composed of APP, PER, and MEL, as depicted in Figure 2. The reaction kinetics and interactions of these components determine the overall fire resistance performance of the coating [42].

3.2.1. Dehydrating Agent

Desiccants are typically inorganic acids or substances capable of decomposing into acids between 100 and 250 °C. Their decomposition temperature must be lower than the thermal decomposition temperature of the composite substrate. Furthermore, to ensure the dehydration of carbon-containing compounds, the acid must decompose prior to any other compounds within the coating [20,43]. Common acidic compounds include APP, zinc borate, organic esters, phosphates, amide salts, or amine salts. These compounds decompose within the temperature range of 100 to 250 °C, which is lower than the thermal decomposition temperature of most organic resins used in composite materials.

The type of acid can also have an impact on the performance of the coating, and the use of organic amides or amines can catalyze acid hydrolysis reactions [44]. Ammonium phosphate and ammonium dihydrogen phosphate, due to their high water solubility, can reduce the water resistance of fire-retardant coatings. Therefore, polyphosphate ammonium (APP) is commonly chosen as the dehydration catalyst for water-based ultra-thin intumescent coatings. APP serves as both an acid source and a foaming agent. Upon heating, APP decomposes to produce phosphoric acid or polyphosphate with strong dehydrating properties. It undergoes an esterification reaction with polyols to dehydrate and form carbon, generating a dense carbonized layer with a three-dimensional structure. Additionally, at high temperatures, APP decomposes into CO₂, N₂, NH_3, and other non-flammable gases that lower the concentration of combustible gases and slow down the combustion process [45,46,47,48]. Modifying the APP can enhance its compatibility and thermal stability with the binder, while reducing water solubility. Currently, the main modification methods include surfactant modification [49], microencapsulation [50], and MEL modification [51]. When encapsulated with MEL–formaldehyde resin, the water solubility of the APP is significantly reduced at 25 °C, decreasing from 8.2% to 0.2% compared to non-encapsulated APP [52]. Wu [53,54,55] used MEL, formaldehyde, urea, etc., as the coating materials to prepare microcapsules of APP and found that its water resistance and flame retardancy had significantly improved.

3.2.2. Carbonizing Agent

The carbonizing agent provides a source of carbon for the expansion layer, typically in the form of high-carbon content polyhydroxyl compounds. Under the catalytic action of dehydration, a porous carbonized layer is formed, using substances such as starch, glucose, pentaerythritol (PER), dipentaerythritol (DPER), and tripentaerythritol (TPER) [56]. The decomposition temperature, carbon content, and number of hydroxyl groups in the char-forming agent have an impact on the flame-retardant effect of fireproof coatings. A higher carbon content leads to a faster rate of carbonization, while a greater number of hydroxyl groups results in a quicker dehydration rate and improved flame-retardant effect. However, an increased presence of hydroxyl groups can lead to higher water solubility. PER is the most utilized char-forming agent; however, its structure contains four hydroxyl groups, resulting in high water solubility that causes additive migration and reduces the flame-retardant performance of fireproof coatings. Consequently, derivatives such as DPER and TPER are often employed as substitutes for PER [57]. DPER and TPER contribute to mitigating the tendency of high-temperature cracking in the expansion layer, thereby enhancing its wind resistance. This is particularly beneficial for fireproof coatings used in cylindrical steel tower-type structures. In the DPER and TPER structures, hydroxyl groups are still present, indicating that they retain a certain degree of water solubility. In response to this, Peng proposed a novel carbonizing agent bis (2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane-1-oxo-4-hydroxymethyl) phenylphosphonate (BCPPO), the synthesis process of which is illustrated in Figure 3. BCPPO does not contain hydroxyl groups and exhibits hydrophobic properties. Furthermore, it demonstrated good flame retardancy in experiments [58].

3.2.3. Blowing Agent

When the blowing agent is heated, it decomposes to produce non-flammable gases such as NH₃, H₂O, CO_2, and hydrogen halide, causing the molten layer to expand and form a certain height of expanded layer. The thermal decomposition temperature of the foaming agent is the most important factor affecting its effectiveness. It is generally required to be slightly lower than the thermal decomposition temperature of the base resin. If the decomposition temperature is too low, a large amount of gas escapes before charring, which fails to achieve the expansion effect; if the decomposition temperature is too high, gas will lift or blow off the charred layer, affecting the structure of the expanded layer. Amine compounds are commonly chosen as foaming agents for intumescent fire-retardant coatings, such as MEL, dicyandiamide, ammonium phosphate salts, and chlorinated paraffin wax. Among them, MEL exhibits excellent flame retardancy and versatility [6]. MEL plays a crucial role in the expansion process, and the multi-ring structure formed during the self-condensation of MEL enhances the stability of carbon and improves the thermal insulation performance of the carbon layer [59]. Furthermore, MEL also possesses advantages such as low toxicity and low smoke density [60], cost-effectiveness [61], environmental safety [62], and corrosion resistance. MEL derivatives (salts formed with organic or inorganic acids, such as phosphoric acid, boric acid, uric acid, pyrophosphate/polyphosphate) exhibit excellent flame-retardant performance and versatility in use due to their ability to adopt multiple modes of flame retardation [63].

3.3. Fireproof Filler

Fillers make up a small proportion of fireproof coatings, yet they play a crucial role. Due to differences in composition and physical form, fillers have varied effects. Inorganic fillers, as an important component of intumescent coatings, serve to support and enhance the strength of the carbonized layer. The addition of certain fillers in small amounts can significantly improve the thermal stability and smoke suppression effect of fireproof coatings while also enhancing the efficiency of the intumescent flame-retardant system [64,65].

3.3.1. Kaolinite

Kaolinite is a fine-grained clay mineral with a two-layer crystal structure composed of silicon tetrahedra and aluminum octahedra, as illustrated in Figure 4.

The most reactive functional groups in kaolinite are hydroxyl groups, which can participate in numerous chemical reactions and ion exchange processes [66,67,68]. Kaolinite can enhance the swelling properties of paint [69], improve the carbon layer structure, and form a ceramic-like barrier on the surface of materials to reduce thermal conductivity efficiency [70]. Kaolinite, as a filler, can enhance the homogeneity and rigidity of the carbon layer. It can also fill cracks in the carbon layer to restrict the overflow of inert gases [71]. Kaolinite and TiO₂ exhibit significant synergistic effects. When used together, they can enhance the adhesion, mechanical properties, and thermal stability of coatings. The addition of kaolinite promotes the dehydration and esterification of carbon, increasing the height of the carbon layer and facilitating the formation of more crystalline TiO₂ within the carbon layer, thereby improving its strength and density. The interaction between TiO₂ and kaolin inhibits their aggregation and promotes uniform dispersion throughout the coating system, thereby increasing the residual mass of the coating after combustion and enhancing the fire resistance of the carbon layer [72]. The addition of kaolin and zirconia-containing ceramic fibers forms a good synergistic effect with the intumescent flame-retardant system. The defoaming effect of kaolin and zirconia-containing ceramic fibers is balanced with the foaming speed of zirconia-containing ceramic fibers, making the carbon layer not only have a high expansion ratio but also have a uniform and dense internal structure. In addition, the low thermal conductivity of zirconia-containing ceramic fibers reduces the transfer of external heat to the substrate during the initial expansion of the coating, jointly enhancing the fire resistance limit of the coating. The addition of zirconia-containing ceramic fibers significantly enhances the strength of the intumescent carbon layer, mainly due to its excellent high-temperature stability and fibrous structure. These fibers interweave with each other, not only making the powder more closely combined but also interlacing in the carbon layer to provide support. However, when the amount of zirconia-containing ceramic fibers is excessive, the expansion performance of the coating is inhibited, the expansion ratio decreases, and thus the fire resistance performance declines [73]. Additionally, kaolinite demonstrates certain synergistic effects with aluminum hydroxide and aluminum oxide. The water vapor released when aluminum hydroxide decomposes upon heating can promote the expansion of kaolin, thereby accelerating the formation of the expansion layer, creating a thicker carbon layer and delaying the temperature rise in the coating. The presence of kaolin helps to enhance the stability of aluminum hydroxide at high temperatures, preventing its premature decomposition or failure, and thus improving the overall flame-retardant effect [74]. The thermal behavior of the coating and its SEM image when kaolin is used in combination with other fillers is shown in Figure 5.

In paint, kaolinite tends to aggregate due to Van der Waals forces and hydrogen bonding, which hinders its full performance [75]. At present, the modification of kaolinite is typically achieved through the intercalation of organic molecules, essentially transforming kaolin into a single-layer mineral to produce a high-performance nano-composite coating [76]. Currently, dimethyl sulfoxide (DMSO), dodecylamine, and thiourea (TU) are commonly used to delaminate kaolinite [77,78,79,80].

3.3.2. Titanium Dioxide (TiO₂)

TiO₂ is the most used inorganic filler due to its chemical inertness at room temperature, non-toxic nature, and ability to impart opacity, brightness, and whiteness to paints. When fire-resistant paint containing APP and TiO₂ undergoes full expansion, the internal layer of the expanded material turns black while the surface remains white. The white substance on the surface is decomposed phosphoric acid from APP reacting with TiO₂ to form a cross-linked structure of titanium pyrophosphate [81]. In this reaction, TiO₂ acts as a catalyst that accelerates the decomposition of APP [82,83], thereby enhancing the structure and strength of the expanded layer, inhibiting oxidative decomposition of internal carbonaceous materials during late-stage combustion, increasing the residue weight, and improving the fire resistance performance. As a filler, TiO₂ offers advantages such as uniform and slow expansion at high temperatures, a high carbon layer expansion ratio after expansion, and high porosity. Thirumal Mariappan [84] noted that excessive use of TiO₂ does not improve the thermal insulation performance in paint; instead, it leads to more severe suppression of carbon layer expansion [85]. This view was also confirmed by Mogens Hinge [86], who indicated that excessive TiO₂ increases the potential permeability in viscous paint formulations, leading to excessive cross-linking during heating, resulting in a significant reduction in carbonization.

Furthermore, TiO₂ exhibits synergistic effects with various compounds. When combined with natural preservatives such as zinc borate, it not only enhances flame retardancy but also improves corrosion resistance, water resistance, and adhesion [87,88,89]. Additionally, the joint use of TiO₂ with bentonite [90], RHA (rice husk ash) [91], Al(OH)₃ [92], and Mg(OH)₂ [93] can reduce smoke production to varying degrees and increase the residual weight. This combination also enhances the anti-oxidation performance of coatings and strengthens adhesion between the coating and substrate. The time–temperature curve and thermogravimetric analysis curve when TiO₂ works together with other fillers are shown in Figure 6.

When TiO₂ and zinc borate are used together as fillers, the acidic gas produced by zinc borate at high temperatures can react with the surface of TiO₂ to form a harder and more stable protective layer. This layer structure effectively blocks heat transfer, enhancing the flame retardancy of the coating. Moreover, the reaction process is endothermic, which further reduces the flame temperature. Additionally, the acidic gas released by zinc borate can form an expanded protective layer, further isolating the heat source.

The combination of the high thermal stability of TiO₂ and the expansibility of bentonite forms a multi-layer protective structure during a fire. When heated, bentonite expands, increasing the thermal conduction path and reducing the speed of heat transfer. The hardness and adhesiveness of both materials work together to enhance the overall strength and high-temperature resistance of the fireproof coating, improving its fire resistance. This mechanism not only enhances the fireproof effect of the coating but also prevents it from peeling off or being damaged during a fire.

The synergistic mechanism of TiO₂ and Al(OH)₃ is similar to that of Mg(OH)₂. Firstly, Al(OH)₃ and Mg(OH)₂ absorb heat and decompose at high temperatures to release water vapor, lowering the temperature, and their residues accumulate to form a protective layer, enhancing the flame-retardant effect. When TiO₂ and Al(OH)₃ are used together as fillers, they can generate a denser and more uniform carbon layer structure, further improving the flame-retardant performance. Secondly, due to the low solubility of Al(OH)₃ in water, its addition can slow down the penetration and migration of moisture, thereby enhancing the water resistance of the coating, but it may lead to a decrease in the coating’s durability. On the other hand, the addition of Mg(OH)₂ increases the bonding strength between the coating and the metal interface, but it also reduces the water resistance of the coating.

The structure of TiO₂ also has a certain influence on the fire resistance performance of intumescent coatings. Titanium dioxide can be divided into anatase and rutile types, as shown in Figure 7. Gu [94] et al. conducted a comparative study of rutile-type TiO₂ and anatase-type TiO₂, finding that compared with anatase-type TiO₂, rutile-type TiO₂ exhibits better dispersion in coatings and improved thermal stability. Additionally, at high temperatures, rutile-type TiO₂ transforms into anatase-type TiO₂. It was also noted that the melt flow rate (MFR) is closely related to the carbonization process; achieving high-quality carbon layers requires appropriate MFR. Furthermore, TiO₂ is commonly used in transparent coatings, where the dispersion of nanoparticles within the resin matrix is a critical factor for maintaining coating transparency. This can be achieved through chemical grafting and surface modification methods [95,96].

3.3.3. Expansible Graphite (EG)

EG is achieved by exposing flake graphite to concentrated sulfuric acid combined with other strong oxidizing agents such as nitric acid or potassium permanganate. Its structure consists of hexagonal network plane layered structures composed of carbon elements, as shown in Figure 8. EG retains the high-temperature resistance and corrosion resistance characteristics of graphite, while also possessing unique properties not found in natural graphite, including flexibility, compressibility, elasticity, impermeability, porosity, and looseness [97].

EG expands when heated, and due to this property, it is used as a synergistic co-expansion agent in fire-retardant coating systems and as an excellent halogen-free physical expansion flame retardant. In comparison with traditional expansion coatings, coatings containing expanded graphite exhibit greater carbon expansion capacity [98]. During heating, paints containing EG interact synergistically with P-C-N expansion systems, resulting in rapid paint expansion. The main component of the intumescent material is carbon, which can withstand high temperatures and increase the residual weight, achieving fire-retardant effects [99].

Electron microscopy scans of the intumescent coatings containing EG after combustion reveal a “worm-like” structure that wraps within the carbon layer and serves as a framework, which enhances the expansion rate and strength of the carbon layer, as shown in Figure 9 [100,101].

In coatings, EG generally manifests four effects. The first is the “popcorn effect”, in which the rapid expansion of EG due to heat absorption occurs [103]. The second effect is known as the “candlewick effect”, where combustible impurities form a continuous flame propagation channel, thereby accelerating the combustion process [104]. The third effect is termed the “labyrinth effect”, which inhibits flame diffusion within the substrate and direct diffusion, leading to an increase in sustained flame duration [105]. Lastly, there is the “barrier effect”, characterized by the formation of non-combustible carbon layers and particles that inhibit heat and mass transfer while diluting volatile compound concentrations.

The flame-retardant properties of EG primarily stem from its expansion. When added in equivalent amounts, EG with smaller particle sizes exhibits better dispersibility within coating systems, allowing it to more effectively fulfill its fire-resistant role [106]. However, a further reduction in the particle size results in increased EG particles and overall expanded body volume. This excessive dispersion makes it challenging to achieve uniformity and consequently diminishes the flame-retardant performance [107,108,109].

While EG’s barrier function is predominantly driven by the “popcorn effect”, this phenomenon may lead to discontinuous carbon layer formation [110]. Tan [111] also emphasized that EG’s expansion plays a critical role in hindering heat transfer. Although EG demonstrates good fire resistance perpendicular to substrates, components dominated by EG may not necessarily provide optimal fire resistance horizontally.

The synergistic interaction of EG with other compounds is also one of its important characteristics. When used as a filler in combination with MoSi₂ [112], zirconium silicate [113], sericite [114], and dolomite [115,116], it exhibits good synergy, which can to some extent enhance the density and thermal stability of the carbon layer, as well as strengthen the heat resistance, fire resistance, and overall mechanical properties of the coating. The time–temperature curve and thermogravimetric analysis curve when EG works together with other fillers are shown in Figure 10.

When MoSi₂ and EG are used together as fillers, MoSi₂ will generate SiO₂ at high temperatures, thereby forming a ceramic-like material covering the carbon layer. This ceramic-like material not only enhances the anti-oxidation performance but also protects the residual carbon of EG from thermal oxidation. The expansibility of EG will further increase the thickness of the carbon layer, thereby improving the thermal insulation performance of the coating. However, MoSi₂ will to some extent inhibit the expansion of the carbon layer, so the proportion of the two should be reasonably adjusted when used together.

Zirconium silicate exhibits excellent high-temperature resistance. It can enhance the expansion of the carbon layer, improve its microstructure, form a porous sheet-like network, reduce the cracks in the carbon layer, slow down the heat transfer rate, and thereby increase the degradation temperature and residual mass of the coating. When zirconium silicate is used in combination with EG, it can more effectively protect the substrate.

The lamellar structure of sericite reduces the friction coefficient of the coating, decreases plastic deformation, and prevents the generation and expansion of cracks, thereby enhancing wear resistance. Sericite promotes the formation of a dense layer network structure in the coating, improving compactness, impermeability, and strength, and thus enhancing water resistance. In fireproof coatings, sericite flakes are dispersed to delay heat conduction; however, excessive sericite can inhibit expansion and foaming, reducing fire resistance. When EG is used in conjunction with sericite, the “worm-like” structure formed by EG upon heating interpenetrates the carbonized layer, enhancing the fibrous effect, while sericite also improves the heat resistance, corrosion resistance, and mechanical properties of the coating.

Dolomite has a ring structure composed of magnesium ions and calcium ions. When this ring structure breaks, thermal degradation occurs, and then magnesium ions play an active role in the expansion process. This delayed thermal degradation provides better thermal insulation during combustion and enhances the barrier performance of heat transfer. In addition, the presence of dolomite increases the residual weight, further improving the fire resistance of the coating.

3.3.4. Carbon Nanotube (CNT)

Carbon nanotubes can be broadly classified into two categories: small-diameter (1–2 nm) single-walled carbon nanotubes (SWNTs) and large-diameter (10–100 nm) multi-walled carbon nanotubes (MWNTs). These materials exhibit excellent mechanical properties, maintaining their structural integrity when subjected to bending or torsion, with only localized flattening occurring at the point of deformation. Studies have indicated that CNTs serve as an ideal fire-retardant filler [117]. When ignited, they form a protective network structure that enhances the strength of the carbon layer, reduces voids and cracks, improves the density, and increases fire resistance [118], while also decreasing smoke production [119]. However, some researchers argue that when used as a standalone filler on steel substrates, MWNTs may lead to the formation of cracks and holes in the carbon layer without improving the fire resistance of coatings [120].

Although carbon nanotubes have a good flame-retardant effect, their flame-retardant performance when used alone is still limited. Compared with traditional flame-retardant additives (such as phosphates and nitrogen-based flame retardants), the flame-retardant effect of carbon nanotubes usually needs to be significantly enhanced by combining with other flame retardants. Due to their nanoscale size, carbon nanotubes may pose certain potential risks to the environment and human health during production and processing. Inhaling suspended carbon nanotubes may have adverse effects on the respiratory system. Therefore, strict control of their exposure and emissions is necessary during the handling of carbon nanotubes. The cost of carbon nanotubes is typically between several hundred and several thousand dollars per kilogram, with the specific price being influenced by the production process, purity, and functionalization treatment. The high production cost makes it face certain economic pressure in some commercial applications. For flame-retardant materials with low-cost requirements, carbon nanotubes may not be suitable for large-scale application.

CNTs are highly prone to agglomeration and difficult to disperse, requiring modification for enhanced dispersibility when used in fire-retardant coatings. Currently, the main methods for obtaining flame-retardant functionalized CNTs include the following three approaches: Firstly, surface modification of CNTs using coupling agents to improve their dispersibility. Common coupling agents include epoxy, amino, silane, etc. They combine with the surface of carbon nanotubes through the formation of chemical bonds or physical adsorption, improving the interface bonding between CNTs and polymers, reducing the agglomeration of CNTs, and ensuring their uniform distribution in the coating. Secondly, covalent bonding of organic flame retardants with CNTs; it is noteworthy that the surface treatment of CNTs is necessary to form a covalent bond between the organic flame retardant and CNTs. Typically, pristine CNTs can be hydroxylated using potassium hydroxide and ethanol [121] or carboxylated through an acid oxidation procedure [122]. After surface treatment, the CNTs with active functional groups can be grafted onto the organic flame retardant. By covalent bonding, organic flame retardants can be directly fixed on the surface of carbon nanotubes, which can effectively prevent the volatilization or precipitation of flame retardants during use, and also enhance the compatibility between carbon nanotubes and the polymer matrix. Thirdly, enhancement in flame-retardant performance by hybridizing CNTs with inorganic particles.

Carbon nanotubes can also exhibit synergistic effects with various fillers. The combination of multi-walled carbon nanotubes (MWCNTs) with fly ash has been shown to enhance the fire resistance and mechanical properties of coatings. The fibrous structure of MWCNTs can enhance the strength of the carbon layer and slow down the diffusion of oxygen. The addition of fly ash forms a fly ash barrier on the surface of the coating, effectively preventing the spread of fire and helping to suppress the combustion process. [123]. Yang [124] developed a novel inorganic flame retardant (BP@Si/CNTs) by combining CNTs with boron nitride, which significantly improves the thermal stability and flame retardancy of the carbon layer. The combination of graphene and carbon nanotubes can more effectively prevent the escape of non-flammable gases, thereby improving the expansion of the carbon layer. Its flame-retardant mechanism is shown in Figure 11a, and the time–temperature curve and thermogravimetric analysis curve are shown in Figure 11b. Additionally, an increase in the graphene content contributes to improved mechanical properties of coatings [109,125]. It is important to note that sometimes, CNTs may exhibit antagonistic effects with other flame-retardant additives instead of synergistic effects [126,127]. Some studies have suggested that combining CNTs with organophosphate flame retardants may lead to decreased flame-retardant performance due to reduced char yield and deterioration in char quality. Moreover, excessive CNTs in the coating can lead to uneven distribution, causing severe entanglement and increasing melt viscosity, thereby interfering with the expansion process of the coating. During combustion, the intumescent carbon layer may crack, which will result in a deterioration in fire resistance. In conclusion, for the flame-retardant system composed of intumescent flame retardants and CNTs, there should be an optimal concentration ratio of intumescent flame retardants to CNTs to ensure that the flame-retardant performance of the material is optimal.

3.3.5. Other Flame Retardants

In addition to the aforementioned fillers, common inorganic fillers include talc, mica, etc. Generally, plate-like fillers such as EG and glass flakes exhibit a superior shielding effect, significantly improving the paint’s water resistance and corrosion resistance. Fiber-type fillers can enhance the strength of the carbonized layer and prevent cracking of the expansion layer [128,129].

Currently, there is extensive research on noncompounds. Noncompounds can enhance the fire-resistant paint’s oxidation resistance at high temperatures, mechanical properties, flame retardance, and smoke toxicity suppression performance. The synergistic fire-retardant mechanism of nanofillers is attributed to their promotion of thicker and stronger carbon layer formation in the condensed phase effectively preventing heat transfer and mass loss.

The enhanced fire-retardant efficiency of nanofillers in swelling systems is related to their geometric shape and composition. It has been reported that layered nanofillers demonstrate superior fire-retardant efficiency compared with tubular or spherical nanofillers [130].

3.3.6. Environmentally Friendly Fillers

Although the intumescent fireproof coating has good fireproof performance, it may cause some harm to the environment in the process of production, use, and disposal. Specifically, some intumescent fireproofing coatings contain chemical components such as organic solvents, flame retardants, and plasticizers, which may release harmful gases during production and coating, as well as under high-temperature conditions, leading to air pollution and endangering human health. In addition, the production process of intumescent fireproof coatings often requires a large amount of chemicals, energy, and water resources, increasing the environmental burden; the lack of effective waste gas, wastewater, and solid waste treatment measures will further increase the risk of water, soil, and air pollution. In the use stage, with the aging of the building, the intumescent fire-retardant paint may flake or be removed. If the waste paint is not properly recycled and treated, the harmful chemicals contained in it, such as heavy metals and flame retardants, may penetrate into the soil and water, causing environmental pollution. It is important to note that the materials used in some intumescent fireproofing coatings are non-degradable, which means that they persist in the environment for a long time, increasing the environmental burden and potentially having a persistent negative impact on the ecosystem. In view of this, the development of environmentally friendly fireproof coatings has become a top priority [131,132].

At present, the fillers used in environmentally friendly fireproof coatings mainly include inorganic flame retardants (such as expanded graphite, aluminum hydroxide, borates, etc.), natural minerals (such as fluorine-containing minerals, bauxite, etc.), and plant-based flame retardants (such as natural resins and amino acids extracted from plants). In addition, there are some sustainable fillers, such as natural resins and amino acids, extracted from plants. These flame retardants are derived from nature and have degradability and environmental friendliness. Although the use of plant extracts as additives can improve the performance of the coating, there are also certain limitations, especially their instability. Due to the biodegradability of plant extracts, this limits their storage and long-term use [133]. Some scholars have used waste from the high-quality cork stopper manufacturing industry as fillers to make fireproof coatings, which can provide effective protection for buildings for more than three hours. Cork is a natural insulating material obtained from the bark of the cork oak tree and is one of the few materials with a negative carbon footprint. Its unique honeycomb structure makes it a natural carbon sink, helping to reduce the level of carbon dioxide in the atmosphere. Even when cork is processed into final products, this carbon storage capacity is still retained. In addition to excellent fire resistance, cork also has excellent heat insulation and sound insulation effects, which help maintain stable temperature and humidity levels, thereby reducing energy consumption and improving indoor comfort, providing many benefits for sustainable development and a circular economy [134,135].

3.4. Performance of Intumescent Coating

To thoroughly evaluate intumescent fireproof coatings, one must understand several key indicators that determine their effectiveness and reliability in high-temperature environments. Firstly, the expansion ratio indicates how much the coating expands under high temperatures, affecting its heat insulation. The Limiting Oxygen Index (LOI) measures the coating’s fire resistance, reflecting its safety in a fire. Time to Ignition (TTI) measures how quickly the coating ignites under high temperatures, which is crucial for assessing its fire resistance. Peak Heat Release Rate (PHRR) describes the maximum heat released by the coating during a fire, directly affecting its protective ability. Total Heat Release (THR) measures the total heat released by the coating during combustion, affecting its fireproof performance. Additionally, the coating’s durability in different environmental conditions is crucial. This includes its stability against ultraviolet rays and moisture. By considering these indicators, one can evaluate the fireproof effectiveness of intumescent coatings in practical applications and various environments.

Table 2. Performance of intumescent coating.

References	Binder	Fillers	Performance
[2]	Acrylic resin	TiO2:Al(OH)3:CES (1:1:1)	Weight losses: 68.7%, Static Immersion Test weight loss rates: 14.69%, Adhesion strength: 0.272 MPa.
[27]	Poly(vinyl acetate)-co-poly(vinyl ester)	Rutile titania (16 wt.%)	Swelling rates: 0.37 mm/s, Expansion factors: 60, SMOGRA: 27 m2/s2.
		Kaolinite (16 wt.%)	Expansion factors: 7,
		Barite (16 wt.%)	Swelling rates: 0.59 mm/s. Expansion factors: 85, SMOGRA: 16 m2/s2.
[30]	PP	ZHS (1 wt.%)	LOI: 32% ↑, UL-94 rating: V-0, TTI: 40 s, PHRR: 193 kW/m2, THR: 75 MJ/m2.
		ZnO (1 wt.%)	LOI: 28.5%, UL-94 rating: V-1.
		SnO2 (1 wt.%)	LOI: 26.5%, UL-94 rating: V-1.
		ZS (1 wt.%)	LOI: 30%, UL-94 rating: V-0.
[31]	EP (curing time: 6 h)	EG (9 wt.%)	TTI: 10 s, PHRR: 152 kW/m2, THR: 110 MJ/m2.
		HNTs (9 wt.%)	TTI: 5 s, PHRR: 969 kW/m2, THR: 110 MJ/m2.
	EP (curing time: 8 h)	EG (9 wt.%)	TTI: 5 s, PHRR: 321 kW/m2, THR: 109 MJ/m2.
		HNTs (9 wt.%)	TTI: 10 s, PHRR: 928 kW/m2, THR: 82 MJ/m2.
[37]	BPA	GWF:RWF (1:1), Fiber length: 12 mm	Weight loss (800 °C): 14.08%, Expansion ratio: 4.35.
[39]	PLA	Starch (7 wt.%)	LOI: 37.3%, UL-94 rating: V-0.
[55]	PP	MCAPP (15 wt.%)	LOI: 34.5%, UL-94 rating: V-0.
[58]	PP	BCPPO (6 wt.%)	LOI: 30.3%, UL-94 rating: V-0.
[69]	BPA	Kaolin (5 wt.%)	After a 60 min fire test, the temperature behind the steel plate reached 237 °C, Expansion ratio: 15, Weathering resistance.
[71]	EP	EG (7.9 wt.%), Kaolin (7 wt.%)	Mass loss: 68.37%, Expansion ratio: 20.59.
		EG:TlO2 (1:3)	Total mass loss: −67.09%, Expansion ratio: 10.41.
[75]	PP	Kaolinite (1 wt.%)	LOI: 27%, PHRR: 474 ± 17 kW/m2, TTI: 17 ± 1 s, AHRR: 397 ± 9 kW/m2, AMLR: 0.080 ± 0.003 g/s.
		E-Kaol (1 wt.%)	LOI: 28%, PHRR: 19 ± 1 kW/m2, TTI: 19 ± 1 s, AMLR: 0.073 ± 0.002 g/s, AHRR: 369 ± 7 kW/m2.
[77]	PP	E-Kaol (1.5 wt.%)	LOI: 35.5 ± 0.2%, UL-94 rating: V-0, THR: 122 ± 1 MJ/m2, TTI: 18 ± 1 s, Smoke emission.
		N-Kaol (1.5 wt.%)	LOI: 34.5 ± 0.2%, UL-94 rating: V-0, THR: 120 ± 1 MJ/m2, TTI19 ± 1 s, Smoke emission.
[84]	VAC	TiO2 (12.7 wt.%)	Residual weights: 42.9%, Waterproofness.
[91]	VAC	TiO2:Al(OH)3:RHA:Eggshell (1:1:1:1)	Weight loss: 57.69%, TTI: 161.5 s, PHRR: 35 kW/m2.
[108]	Acrylic resin	TiO2:Al(OH)3:MMT:Graphene:CNTs (8:8:4:3:1)	Residual weights: 40.54%, PHRR: 32.29 kW·m−2, THR: 1.13 MJ/m2, TTI: 143 s ↓, FRI: 15.02.
[111]	Acrylic resin	EG:MoSi2 (5:9)	Residual weights: 39.92%, Carbon layer structure improved.
[112]	EP	EG (5.8 wt.%), Zirconium silicate (5 wt.%)	Expansion ratio 24, Residue weights: 38.2%, Smoke emission.
[116]	EPDM	EG (10 wt.%)	LOI: 30.4% ↑, UL-94: V-0, Residue weights: 43.9%, TTI: 70 s, PHRR: 197 kW/m2, THR: 77.6 MJ/m2, TTPHRR: 115 s, FIGRA: 1.7 kW/(m2·s), FRI: 6.52 ↑; TSP: 16.3 m3.
[126]	EP	GO@M-CNTs hybrids (1.5 wt.%)	Expansion ratio: 10.79, Residue weights: 28.8%.
[129]	EP	Kaolin:TiO2:GF (2:5:3)	Expansion ratio: 14.4, Equilibrium temperature of coating: 253 ± 1 °C, Residue weights: 40.8%.

summarizes the fireproof coating performance from several studies.

Intumescent coatings perform better at high temperatures by expanding to form an insulating layer. This layer effectively separates flames from high heat. These coatings are especially suitable for environments that require high fire resistance ratings. However, they are often more expensive and require careful application and maintenance. In contrast, non-intumescent coatings offer a wider range of applications. They are simpler to apply and generally cost less. However, their performance at high temperatures is less effective compared to expanding coatings. Therefore, it is crucial to choose the right type of fireproof coating based on the specific application requirements and environmental conditions.

Table 3. Comparison of intumescent and non-intumescent fireproof coatings.

	Intumescent Coating	Non-Intumescent Coating
Fire-resistant mechanism	It expands when heated, forming an insulating layer that separates the high temperature from the substrate.	The fire resistance performance is enhanced through the application of flame retardants and the suppression of flame propagation.
Fire resistance	The protective layer formed by expansion can provide effective thermal insulation and typically exhibits high fire resistance.	The primary reliance is on the flame-retardant properties of the chemical composition, which typically offers less protection against high temperatures compared to intumescent types.
Operating temperature range	It has a remarkable effect at high temperatures and is suitable for fire prevention in high-temperature environments.	Strong adaptability to temperature changes, but poor performance at extreme high temperatures.
Coating thickness	The thick layer formed after expansion helps in insulation, and a thicker coating is usually required.	There is no need to expand the formation of a thick layer, the coating thickness is relatively thin.
Maintenance and durability	Due to the presence of the intumescent layer, regular inspection and maintenance may be required during long-term use.	Usually durable but may need to be replaced in extreme conditions.
Fire-resistant mechanism	It expands when heated, forming an insulating layer that separates the high temperature from the substrate.	The fire resistance performance is enhanced through the application of flame retardants and the suppression of flame propagation.

provides a comparison of intumescent and non-intumescent fireproof coatings.

4. Application of Intumescent Fireproof Coating in Tunnel Fires

4.1. Characteristics of Tunnel Fires

Tunnels have the characteristic of being narrow and enclosed, which makes it difficult for temperature and smoke to dissipate effectively in case of a fire. This can lead to problems such as high temperatures, thick smoke, and low visibility, which can have a negative impact on the structural safety, personnel safety, and evacuation of the tunnel [136]. Tunnel fires are mainly caused by vehicle collisions, and when they occur, they can cause severe traffic congestion, making rescue and evacuation difficult and prolonging the time needed for disposal. In addition, the burning of automotive fuel can cause secondary combustion, igniting other vehicles and causing the fire to spread. The unique environment in which tunnels are located and the role they play make tunnel fires have the following characteristics.

• Suddenness: Tunnel fires occur suddenly, propagate rapidly, and are challenging to contain. They are primarily triggered by vehicle malfunctions, leading to short circuits or traffic accidents within the tunnel.
• Evacuation difficulty: The traffic congestion is severe, making it difficult to clear the roads in a timely manner and ensure the normal operation of traffic. The tunnels are narrow and the road surface is narrow, and there is a large flow of vehicles. In case of a fire, it is difficult to evacuate vehicles in time, which can easily cause congestion. In addition, there are more fuels and combustible materials on vehicles, which can cause the fire to spread rapidly, further complicating the evacuation efforts.
• Poor ventilation: The intense fire in the tunnel generated thick smoke, hindering extinguishment, as shown in Figure 12. In just 10 min, the temperature could soar to 1000 °C, accompanied by high levels of smoke and toxic gases. Limited space and poor ventilation made it challenging to clear the smoke promptly, delaying firefighting and rescue efforts and posing risks to passengers, firefighters, and rescue personnel.
• Rescue difficulty: The rescue operation and evacuation are challenging due to the limited entrances and exits in the tunnel, narrow space, and closed environment. Inhalation of fire gases, smoke, and toxic gases can lead to suffocation, making them the primary cause of about 80% of fire-related deaths [137,138,139].

4.2. Temperature Distribution in a Fire Scene

Tunnel fires generally have a fast heating rate, high maximum temperature, and long duration, which are distinct from building fires. Moreover, the temperature distribution patterns in different tunnel fire situations are also different. For the distribution of tunnel temperature during a fire, many tunnel fire experiments have been conducted in various countries. There are five different fire standard temperature curves for tunnels in different countries around the world, including the ISO 834 standard time–temperature curve (fiber-based) [140], hydrocarbon fire curve, RWS curve, Runehamer curve, and RABT curve.

The study of ISO 834 curves dates to the 1940s and is based on the burning rate of building materials (fiber-based) and the fire temperature–time curve in indoor building environments.

The traffic in the tunnel is heavy, there are a lot of fuels and compounds, and the combustion rate of fuels and compounds is much higher than that of fiber materials. Therefore, for fire resistance tests due to fuel combustion, another form of fire curve—hydrocarbon fire curve (HC)—is needed. The HC fire curve is based on a standardized fire and is feasible for fuel and petroleum fuel combustion. HC_inc is used to simulate more severe fire conditions and is obtained by multiplying the HC curve by a scaling factor.

Based on research conducted at the TNO laboratory in the Netherlands, the RWS curve was developed, which assumes that in the worst fire scenario, 300 MW of latent heat value fuel or a tanker truck will continue to burn for 120 min. This curve simulates the burning of a tanker truck in a tunnel, with an initial rapid temperature rise followed by a gradual decrease as the fuel supply decreases. This typically occurs in relatively enclosed spaces where heat cannot or is only minimally dissipated, such as tunnels.

The Runehamer curve was obtained from four heavy truck fire experiments conducted by Norway in the Runehamer tunnel, and the curve can be considered a standard curve formed by combining the HC and RWS curves.

Germany developed the RABT curve through a series of experiments (such as the Eureka project) to meet certain special requirements. The curve can raise the temperature to 1200 °C within 5 min, which is faster than the hydrocarbon curve, which only reaches 1150 °C after 60 min. The RABT curve has a shorter duration at 1200 °C than other curves, and the temperature drops rapidly in the following 30 min. The five curves mentioned above are shown in Figure 12.

Many scholars have conducted a series of studies on the development of and changes in temperature fields in tunnels during fires and have achieved rich results. The application of tunnel fire curves varies from country to country, but there are similar characteristics. (1) Tunnel fires heat up quickly, usually reaching the highest temperature within 2–15 min; (2) the average maximum temperature reaches 1200 °C; and (3) the duration of high temperature in tunnel fires is generally not less than 2 h. To understand the fire behavior of highway tunnels,

Table 4. Summary of past research work on tunnel fire.

References	Findings	Remarks
[141]	It was observed that the cross section of a tunnel affects the heat release rates.	Experimental study
[142]	It was observed that the tunnel geometry and ventilation rates also affect peak HRR in tunnel fires.	Experimental study
[143]	It was found that the tunnel aspect ratio also affects the smoke temperature distribution in tunnel fires.	Experimental study
[144]	Critical ventilation velocity increases with heat release rates in tunnel fires.	Theoretical and experimental
[145]	This report highlights the effects of the ventilation system on peak HRR.	Technical study
[146]	This research highlighted the effect of tunnel geometry on peak HRR in HGV fire in tunnels.	Theoretical and experimental
[147,148,149,150]	Research shows the influence of longitudinal ventilation systems on fires in tunnels	Experimental study

lists some research findings by scholars.

4.3. Application of Intumescent Fireproof Paint

After a fire, the temperature in the tunnel fire zone can quickly rise to 1000 °C, causing the reinforced concrete that makes up the tunnel structure to immediately reach the critical failure threshold and lead to structural damage. Fire protection measures aim to prevent concrete cracking and reduce heat transfer from the fire source to the interior.

Using fire-resistant coatings in tunnels offers several benefits. These coatings can quickly reduce flammability, inhibit rapid fire spread, prevent rust and corrosion, and maintain structural integrity during a fire. They are also cost-effective, easy to apply, and water-resistant.

Currently, intumescent coatings are widely used on the concrete or steel surfaces of tunnels, as well as on the fire protection of cables and pipelines. Additionally, most intumescent coatings contain organic substances, and these coatings may release volatile organic compounds during construction and curing, which could potentially have an impact on the environment and the health of construction workers, so appropriate protective measures should be taken.

In recent years, a special type of wide-network structure “inorganic resin” known as amorphous aluminum phosphate has been developed for the preparation of heat-resistant adhesives and lightweight insulating porous ceramics [151,152,153,154,155,156]. Upon exposure to fire, it decomposes, foams, and expands. Although its foaming and expansion effect is not as pronounced as that of organic polymers, its decomposition products exhibit high thermal stability and low thermal conductivity, thereby providing certain thermal insulation effects. Importantly, its primary decomposition product is water without the release of toxic substances. Furthermore, amorphous aluminum phosphate demonstrates an extremely high bonding strength, which can enhance the practicality of the coating. Wang et al. [157] utilized an amorphous aluminum phosphate-based emulsion as the matrix and employed tetrammonium phosphate, nitric acid, and oxalic acid as foaming agents. Boron carbide (B₄C), SiO_2, and Al₂O₃ are used as fillers to prepare an inorganic intumescent coating. When subjected to a butane flame, the coating exhibited complete non-flammability, with a back surface temperature of only 140 °C after 60 min, demonstrating excellent thermal insulation performance. However, due to the presence of foaming agents, it released nitrogen-containing smoke (such as N₂O and NO₂) when exposed to fire, resulting in a smoke density of 64.73%, which limited its application in tunnels. The time–temperature curve and thermogravimetric analysis curve of this coating are shown in Figure 13a, and the photo of the coating resisting flames is shown in Figure 13b. Inspired by the study, Cai [158] modified the coating by incorporating nanosilica, hollow silica microspheres, hollow glass microspheres, and boron carbide to develop a novel amorphous aluminum phosphate-based flame-retardant coating. Testing revealed that when the ratio of glass microspheres to boron carbide was 3:1, the coating exhibited optimal fire resistance performance. When subjected to a butane flame at approximately 1200–1300 °C for 10 min, the maximum back surface temperature reached 226 °C, gradually decreasing to 175 °C after 60 min.

Many scholars conducted fire resistance tests on tunnels coated with fireproof paint. Wang [159] found that the rebinding strength of different parts of the tunnel without fireproof paint protection decreased to varying degrees after the fire test, with the greatest decrease occurring at the crown and shoulder. They also divided the influence of fire on concrete temperature into four stages. The first stage is the delay stage, in which the temperature rise in the concrete has a lag, which depends on the specific location. In the second stage, the temperature rises rapidly. The third stage is the dehydration and boiling water stage, in which the temperature gradually rises but is below 100 °C. In the fourth stage, the aggregate will be heated again, causing a rapid temperature rise in a continuous fire. The fireproof concrete tunnel lining with a fireproof coating has strong thermal inertia and can avoid serious temperature rise and potential peeling in a fire. In addition, the fireproof coating reduces the temperature and heating rate of concrete, delaying the deformation of the tunnel, but even with the protection of a fireproof coating, the outer concrete of the tunnel will crack in a fire, with the maximum crack width reaching 1.86 mm. These cracks mainly form in the early stages of the fireproof test and gradually increase with the prolongation of the fire time, and water vapor continues to leak out of the cracks (see Figure 14). The fireproof coating also has a significant protective effect on the local components of the prefabricated frame tunnel joints [160,161].

5. Conclusions

This paper summarizes the fire-resistant mechanism of intumescent coatings, the intumescent fire-resistant system, its application in tunnels, and the influence of functional fillers on the fire-resistant properties of the obtained coatings. In addition, we introduce the synergistic effect of using different functional fillers together.

Currently, research on intumescent coatings mainly focuses on the combination, substitution, and modification of various additives. While the basic chemical properties of additives are well known, the formation of carbon layers in intumescent coatings is a complex process that requires consideration of factors such as thermal conductivity, viscosity, kinetic parameters, and chemical composition. In addition, the morphology and structure of fillers also affect the fire-resistant properties of the coating; for example, plate-shaped fillers can prevent the escape of inert gases to increase the expansion height and extend the length of the infiltration path of oxygen and combustible gases into the substrate. Fiber-shaped fillers can improve the mechanical strength and other mechanical properties of the coating.

Intumescent coatings have been widely used in tunnels, but due to the tunnel’s unique closed structure, there are still some urgent problems to be solved. For example, the durability, environmental protection, and stability of the coatings in different environmental conditions still need to be further verified and optimized. In addition, the construction process, cost-effectiveness, and long-term performance monitoring of the coatings in actual applications also need to be strengthened. The actual application effects of the intumescent coatings in tunnels lack validation by full-scale experiments. Since the intumescent coatings are mainly based on organic substrates, they produce toxic gases during construction or combustion, which is one of the main limiting factors for their application in tunnels. Overall, environmentally friendly formulations are still the direction of development for fireproof coatings.