A Comprehensive Review on Intumescent Coatings: Formulation, Manufacturing Methods, Research Development, and Issues

Abstract

Fire has been proven to threaten human lives and buildings significantly. Extensive research is being conducted globally to reduce fire risks, particularly in high-rise buildings that incorporate steel for structural support, timber for decorative elements, and cladding for insulation. Traditional passive fireproofing materials, such as concrete coverings, gypsum boards, and cementitious coatings, often lack aesthetic appeal. Intumescent coatings offer a promising solution to this issue. These coatings require a thin layer on the substrate to protect from fire, and the thin layer expands up to many times its original thickness when exposed to fire, forming an insulating char that acts as a barrier between fire and the substrate. This barrier prevents the steel from reaching critical temperature and helps maintain its integrity during a fire incident. Hence, intumescent coatings are a great choice for passive fire protection of load-bearing steel, wooden structures, timber, and cementitious buildings. Although some research articles discuss intumescent coating types, application methods, fabrication processes, cost-effectiveness, bonding performance, toxicity, and various uses, a comprehensive study encompassing all these topics still needs to be conducted. This review paper explores different types of intumescent coatings, their formulation and manufacturing methods, their application processes, and their use on various substrates. It also covers the key intumescent coating materials and their interactions during fire. Challenges and issues, such as fire protection time, char-forming temperature, and toxicity, are discussed.

1. Introduction

Fire is a significant danger to humans and buildings [1]. Throughout history, recurrent fire incidents have tragically taken numerous lives, such as the incidents at the Lacrosse Building in Melbourne (2014), Grenfell Tower in London (2017), Neo Tower in Melbourne (2019) [2,3], Abbco Tower in the United Arab Emirates (2020) [4], Torre dei Moro in Italy (2021) [5], Twin Parks North West in the United States (2022) [6], The Kimpton in Hong Kong (2023) [7], and Botanik Torre Flora in Brazil (2024) [8]. These tragic events emphasise the significant importance of improving the fire resistance of building materials [2,3]. Timber is a common choice as a building material because of its aesthetic look, easy processing, and minimal environmental impact. However, wood has high flammability and vulnerability to fire. Timber structures are highly combustible and prone to collapse in the fire, posing a significant threat to both lives and property [9,10]. Steel is another material commonly used in the building industry. Structural steels are widely employed for their high strength, reduced weight, good ductility, and appealing design in civil and industrial construction [11]. Steel structures also provide easy construction, rapid erection, and efficient fabrication. However, steel structures show vulnerability in a fire, and at 600 °C, mild steel loses more than 60% of its room-temperature yield strength, a substantial reduction from standard structural steel [12]. High temperatures can cause the collapse of steel structures. So it is important to use a fire-protecting material on the substrate of building components to prevent the fire spreading and the building’s failure [12].

Applying flame-retardant coatings on the substrate has become increasingly popular day by day. Flame-retardant intumescent coatings have extensive applications in the fields of shipboard [13], transportation infrastructure [14], industrial structures [15], and high-rise buildings [16]. These flame-retardant coatings play a vital role in slowing down the ignition and spread of fires [17]. Researchers have shown interest in fire-retardant coatings in recent years, especially those that are highly efficient, less toxic, and environmentally friendly [18]. In the current market, there are two main categories of fire-retardant coatings: non-intumescent fire-retardant coatings and intumescent fire-retardant coatings [19,20]. Non-intumescent coatings are used as decorative and architectural coatings to minimise the rate of flame spread and smoke generation on combustible substrates. They range from A to C, which rates their ability to mitigate contributions to fire and smoke. The flame spread rate depends on the substrate type, the coating formulation, and thickness. Thicker coatings or certain substrates can affect the flame propagation rate, affecting the overall fire resistance [20]. On the other hand, fire-retardant intumescent coatings are highly effective; flame-retardant coating works by expanding and creating a dense, porous char layer when exposed to fire. The char layer provides excellent insulation, preventing excessive temperature increase and restricting oxygen access to the substrate. The char layer protects the substrate from fire [21]. The protective char barrier allows safe occupant evacuation by delaying the critical temperature of the substrate during a fire [22]. Intumescent coatings were invented in the 1980s and commercially introduced in the market from the 1990s. Over the last 20–25 years, intumescent coatings have continued to improve in terms of fire resistance, with some products offering fire ratings ranging from 30 min to 4 h. A fire rating is the amount of time a material can resist fire before it fails. Advanced formulations have improved durability, led to faster drying times, and provided better resistance to the environmental factors of intumescent coatings [23].

The fire resistance of intumescent coatings mainly depends on the char’s formation, swelling rate, thickness, and density [24]. These inherent char properties depend on the compositions/matrix used in the intumescent coating. Generally, intumescent coatings consist of a polymeric matrix added with organic and inorganic components [24,25,26]. Intumescent coatings typically contain three key ingredients: an acid source, a char former, and a blowing agent. These three components interact with each other and form a char layer that expands during a fire [27]. Additionally, to enhance the fire-resistant property of intumescent coatings, fundamental fillers such as dolomite clay [28], titanium oxide [29], boric acid [30], kaolin [31], and montmorillonite [32] are often added in varied concentrations. Pigments and binders are also essential ingredients in intumescent coatings [33,34]. The accurate dosages and mixing procedure of these fundamental components are essential factors in establishing coating functionality [34]. Intumescent coatings seem like pigmented, thin layers at room temperature, and the coating is applied with dry film thickness (DFT) ranging from a few millimetres for slender coatings (commonly solvent-based or waterborne) to several centimetres for thicker coatings (typically epoxy-based) [35]. When the intumescent coatings encounter heat, these coatings undergo several chemical reactions and a transformative expansion, evolving into substantial, low-density, and thermally insulating porous chars. The char acts as a shield by preventing the underlying substrate from attaining elevated temperatures [36]. In this process, the intumescent coatings swell up many times to their original thickness [37].

Extensive research has been conducted on intumescent fire-retardant coatings, and numerous studies are available in this field. However, current research predominantly focuses on the composition and analysis of different intumescent coatings [16,38]. While some research articles address the intumescent coating types [38,39,40], application or fabrication procedure [41], bonding performance [42,43], toxicity [44], and diverse applications [41,45,46,47], a comprehensive article covering all these aspects is notably lacking. Therefore, further research is required to provide comprehensive information on intumescent coating, which offers vast opportunities for exploration and significant advancement in various sectors related to fire safety.

This review paper endeavours to bridge existing gaps in intumescent coating by providing a comprehensive analysis of various types of intumescent fire-retardant coatings developed and their application in different fields. This article covers working mechanisms, manufacturing approaches, testing methods, cost-effectiveness, durability, weather performance, adhesion capabilities, challenges, and other crucial aspects of intumescent coatings. This research aims to provide in-depth information to understand the fire resistance of intumescent fire-retardant coatings. This review is intended to be a useful resource that reveals the complexities of intumescent coatings and encourages further research and developments in this crucial field.

2. Research Methodology

A comprehensive research study is conducted using a systematic literature review. Several steps have been followed to explore the topic of intumescent coatings, as shown in Figure 1. In Steps 1 and 2, keywords such as intumescent coatings, fire safety, flame retardant, water-based intumescent coating, solvent-based intumescent coating, epoxy-based intumescent coating, hybrid intumescent coating, application of intumescent coating, etc., are defined for searching. These keywords are used in Step 3 to search multiple online platforms, including ScienceDirect, Web of Science, Springer, and Google Scholar. The initial search result yielded approximately 13,400 papers from 1994 to 2024. In Step 4, this large number of papers was screened by checking the title, keywords, and abstract, which reduced the number of papers to 630. A detailed review of the full content of 630 papers was conducted in Step 5 to ensure relevance, and a secondary screening was completed, resulting in 213 papers. It is worth mentioning that 73 out of 213 peer-reviewed papers were published between 2020 and 2024, and 173 papers were published between 2010 and 2024. In addition to these articles (213), additional research reports and web links have been referred to in this paper to discuss the material types, properties, etc. In the final step, i.e., Step 6, data extraction and synthesis were carried out to ensure a comprehensive and organised literature review on intumescent coatings, thus providing a strong knowledge base about intumescent coatings and their fire protection applications.

3. Fundamentals of Intumescent Coating

3.1. Chemical Composition

The effectiveness of the intumescent coating depends on its ingredients and their proper combination [35]. As shown in Figure 2, four ingredients are usually used in the intumescent coating: the acid source, a carbonaceous compound, foaming compounds, and binding resins or a binder [48]. An acid source reduces flammability and improves hardness, adhesion, corrosion resistance, etc. A carbonaceous compound/char source material is used as the source of chars, gases, and water. The blowing agent is used to generate gas bubbles during char formation. The binder is used to form a uniform foam layer in an intumescent coating. Other ingredients like pigments, deformers, thickeners, surfactants, additives, etc., are also used to formulate intumescent coatings. These components are selected based on the coating’s function. Each ingredient has a selective role, and their careful combination ensures that the coating works effectively [16,49]. Ammonium polyphosphate (APP) as the acid donor [50,51], pentaerythritol (PER) or expandable graphite (EG) as the carbon source [51,52], and melamine (MEL) as the blowing agent are frequent components of a composition of intumescent coating [51]. Titanium dioxide (TiO₂) serves as a filler and a white pigment [53], while polyvinyl acetate (PVAC) and acrylic resin are frequently employed as binders [54,55]. The details of each component are discussed below.

Acid source: Acid source plays a vital role in the intumescence process by breaking down and releasing a mineral acid. The released acid helps remove water molecules from the carbon source through a dehydration reaction and facilitates the formation of a stable and protective char layer [16]. As an acid source, a non-volatile acid salt is mainly used, for example, ammonium phosphate, sulphuric, or phosphoric acid [48]. Ammonium phosphate (APP) is commonly used; its good thermal stability (>200 °C) and versatility are the main reasons for its popularity. APP decomposes into polyphosphoric acid and ammonia in the fire [35]. Phosphorus-containing fragments play an important role in the coating; they not only reduce flammability but also improve adhesion, corrosion resistance, and other valuable properties [56]. As APP has poor hydrolytic stability, other acid sources, including monoammonium phosphate, diammonium phosphate, and various melamine-based compounds, are now commonly used [35].

• Ammonium polyphosphate (APP):

The released phosphoric acid from APP plays a crucial role in the char formation. It reacts with the polyol and releases carbonaceous char [16]. Additionally, the phosphoric acid reacts with the inorganic fillers and produces cementitious materials which improve the char integrity and fire resistance properties [57].

Carbonaceous compound or char source: A range of hydroxyl-containing hydrocarbons are preferably chosen for carbonaceous compounds [16]. The carbonaceous compound, which is present in the intumescent coating, decomposes with acid in the fire and forms chars, gases, and water. The potentiality of carbon sources depends on their carbon content and the number of hydroxyl groups [35,58]. Polyfunctional alcohols and carbohydrates are commonly used as carbonaceous compounds, such as PER and starches. Monopentaerythritol (PER) is less effective than dipentaerythritol (DPER) because of its increased water solubility, particularly after washing or in humid circumstances [35].

• Monopentaerythritol (PER):

Researchers have shown keen interest in fire retardant formulations incorporating expandable graphite (EG) in recent years, which is a material composed of tightly bound carbon atoms arranged in layers [59,60]. EG works as an effective carbonisation agent, and it is also an environmentally friendly intumescent system [61]. EG has other outstanding abilities, including suppressing flammable gases and reducing flame spread [62]. Some recent research shows that using a low amount of graphene (0.2–0.7 wt.%) can significantly strengthen the materials [62,63,64]. The graphite helps to create a dense and uniform layer of chars when it burns with other materials. This process slows down the spread of the fire [63]. The mixture of carbon nanotubes and graphene sheets forms a network, which works great at turning the protective char during combustion into composite materials [64].

Foaming compound or blowing agent: In flame-retardant intumescent coatings, a crucial role is assigned to the porophores, which are well-known as gas-forming agent. It enables chars to form. In high thermal conditions, gas formation occurs and the evaporation of substances with low molecular weight [16,35,48,65]. It breaks down and creates ammonia, carbon dioxide, and other substances. The gases produced create a coating foam that makes a thick, protective, fire-resistant layer. Two types of blowing agents are used: one activates at low temperatures, like dicyandiamide, urea, glycine, MEL, or guanidine, and another activates at high temperatures, such as chlorinated paraffin (CP). It is important that the blowing agent decomposes at the right temperature, which is after the melt forms and before gelation occurs [35]. Melamine is a well-recognised gas-foaming agent that involves ammonium phosphate [16,35]. However, expandable graphite (EG) is also an excellent choice as a blowing agent in intumescent coatings. Due to its exfoliated structure intercalated with acid agents like H₂SO₄ or HNO₃, EG releases gases at high temperatures, expanding perpendicular to the carbon layers [16].

• Melamine:

Binding resins or binders: The main purpose of binding resins or binders is to form a uniform foam layer in intumescent coatings. Two types of resins are used: thermoplastic resins and thermosetting resins. Thermoplastic binders are commonly used in waterborne and solvent-based coatings; on the other hand, thermosetting binders are used in solventless coatings [35]. For foam expansion to be effective, the binder must remain flexible, not become overly hard, and melt at lower temperatures, notably over 200 °C [35,48]. The selection of binders also relies on the coating’s expected durability and the environmental conditions it will encounter [16]. Vinyl acetate dispersion is commonly used in water-based intumescent coatings [57,66]. However, acrylic resin and epoxy resin are also used as binders [67,68].

Various Pigments: Including pigments in the intumescent coatings leads to the creation of dense, uniform cells and a compact char structure, which helps to enhance fire protection. Titanium dioxide (TiO₂) is a pigment in the intumescent coating that often exhibits a white expanded foam exterior. In the interaction between APP and TiO₂, titanium pyrophosphate is created primarily [69]. It is also believed that molybdenum trioxide (MoO₃) and ferric oxide (Fe₂O₃) can enhance both the outer and inner surface structure of the residual char and boost the thermal stability of APP–PER–MEL intumescent coatings [70].

Other additives and flame-retardant fillers: Various inorganic fillers and additives play a crucial role in intumescent coating. They are mainly used to enhance the char residue at high temperatures and improve fire protection. The right mix of these flame-retardant fillers can affect the performance of the intumescent coating. For example, magnesium hydroxide is now commonly used as a fire-retardant filler. It improves the bonding strength of intumescent coatings on metal surfaces [71]. The effectiveness of the intumescent coating depends on the amount of flame retardant needed for fire safety. The amount of flame retardant that must be added to achieve the expected level of fire safety can range from less than 1%, which is for highly effective flame retardants, up to more than 50% for inorganic fillers [72]. Researchers working on intumescent coatings have explored different types of fillers. Aluminium trihydrate (ATH) is used in the intumescent coating to improve flame retardancy by influencing fire properties like limiting oxygen index (LOI), time of ignition (t_ignition), total heat evolved (THE), and peak heat release rate (PHRR). Smaller particles of ATH perform better, enhancing char stability and strength during combustion [73]. The addition of alumina (Al₂O₃) and talc (Mg₃Si₄O₁₀(OH)₂) in the intumescent coating improves the formation void, which helps to reduce heat transfer during fire and enhance fire resistance [74]. Kaolin clay has a significant effect on char expansion in the intumescent coating. The addition of 5% kaolin clay showed a 228% increase in char expansion compared to the control [75]. Kaolin clay also helps to enhance fire retardant properties and reduce heat transfer. Additionally, it greatly affects maintaining the char’s structure and effectiveness [75]. Moderate amounts of silica can improve flame retardancy and smoke suppression by enhancing thermal stability and forming a compact char layer [76]. Currently, extensive research is being conducted on nano-scale additives [16,77]. The addition of nano-scale additives enhances the fire protection performance of intumescent coatings by improving thermal shielding, water resistance, and char formation. Nano-clays (e.g., montmorillonite, OMMT), carbon nanotubes (CNTs), metals (silica, aluminum, titanium), silsesquioxane, and layered double hydroxides (LDHs) retain their effectiveness in different formulations of intumescent coatings [16].

3.2. Intumescent Process

The process of intumescence, which involves reaction, expansion, and the formation of a char in intumescent coatings, can be delineated through distinct stages, as shown in Figure 3. Numerous research studies highlight the identification of the following distinct phases in intumescent coating [16,25,78].

Before heating (pre-heating zone): Before the heat exposure, the intumescent coating looks like a conventional paint and has a smooth and uniform appearance, as shown in Figure 3a. In addition to maintaining the material’s aesthetic qualities, this guarantees a smooth integration with architectural and design features, making it a desirable fire safety solution without sacrificing aesthetic appeal [41].

Thermally induced decomposition (melting zone): When the coating is exposed to a heating source, the coating absorbs a large amount of thermal energy and reaches the critical temperature where a significant transformation happens. At this juncture, the inorganic acid source undergoes a thermal decomposition at temperatures of 100 °C to 250 °C [78,79]. This results in the surface melting and converting into a viscous fluid, as shown in Figure 3b. During this stage, various chemical reactions take place, breaking down the coating’s components and generating gas products [78], such as ammonium polyphosphate, which break down into phosphoric acid and ammonia gas. The released phosphoric acid reacts with the polyol and creates a complex compound [16]. The melting zone prepares the material for the swelling and charring phases by acting as the base for the intumescent reaction.

Swelling (reaction zone): After the activation of the blowing agent, the coating undergoes a reaction by absorbing heat from the substrate and decomposes to release a significant amount of incombustible and combustible gases at around 300 °C to 350 °C temperature [80,81], as shown in Figure 3c. These gas bubbles become entrapped in the surface of the substrate within the coating, which causes the molten matrix [82,83]. In order to construct the protective char barrier, the swelling process is necessary because the expanded char structure serves as an insulating layer that lessens heat transfer to the substrate. The swelling process continues until the underlying coating layer is fully consumed. However, the swelling process can be stopped if the heating process is interrupted [83].

Char formation (charring zone): The intumescent interface slowly moves from outside to inside the material, leaving a residual char behind, as shown in Figure 3d. Char is a formation of a low-density, low-thermal-conductivity porous medium that serves as a thermal barrier for the substrate [82,84]. In this stage, the material can be divided into three independent layers: the virgin layer, the intumescent layer, and the char layer, as shown in Figure 4. The resulting char layer is black in colour and highly carbonous, functioning as an insulating layer. The quality and durability of the char layer determine the overall effectiveness of the intumescent coating [41,82].

Char deformation (char structure changes): For typical foam material, the force rises until it reaches the destructive point and remains steady until complete destruction occurs. In the case of char foam, a distinct behaviour is observed. The force increases after the initial break, which occurs when the char breaks, as shown in Figure 3e. This is followed by the continuous compression of the underlying pyrolysis zone, and the black and compact char structure changes into white, dusty foam, as shown in Figure 3f [85]. This change occurs as the char structure progressively degrades due to thermal and mechanical stress. The char loses density and releases trapped gases as it weakens with heat exposure after initially being stiff and insulating.

3.3. Working Mechanism and Chemical Reaction Steps

When intumescent coatings encounter a fire, the main vital elements of the intumescent coatings are an acid donor, a carbon or char source, and a blowing or foaming agent, which take part in the reaction. At around 200 °C temperature, the chemical reaction began within the intumescent coating [56,86]. When the fire begins and keeps on going, the temperature of the intumescent coating makes the intumescent layer continue. When the temperature rises above 250 °C in a fire, an acid source (such as APP) releases acid [30]. The acid reacts with polyol and makes a tough, fire-resistant substance, which is called carbonaceous char. At the same time, the blowing agent (melamine) also releases gases like NH₃, CO₂ and H₂O, which take part in the foam formation and make the char layer. Every carbon (C) and nitrogen (N) molecule present in the layer undergoes oxidation, leaving behind an insulating white foam structure that is made of titanium phosphates [16,86]. Studies show that at higher temperatures, APP crosslinks and forms structures like P–O–P and P–N–P [86,87]. At the high temperature, organic compounds evaporate. After that evaporation of the organic compound, inorganic filler (titanium dioxide (TiO₂)) reacts with the remaining phosphoric acid and forms a ceramic structure [54]. Melamine and polyphosphoric acid react to form dipolyphosphate and melamine polyphosphate. This procedure uses esters of cyclic phosphoric acid as blowing agents. The low decomposition temperature esters produced by glycerol as a char former improve the fire resistance of the coating [69]. Additionally, during burning, compounds like APP and melamine release radicals that help slow down the fire by reacting with and neutralising the harmful chemicals that fuel the flames. This process makes the material more fire-resistant [88]. The char formation steps for the intumescent coating are illustrated below.

4. Manufacturing Methods

There are various types of intumescent coatings available in the market. Based on the binding agent and manufacturing methods, intumescent coatings can be classified into four categories, such as (i) water-based intumescent coating, (ii) solvent-based intumescent coating, (iii) epoxy-based intumescent coating, and (iv) hybrid intumescent coating. The main purpose of each of these four coatings is to improve the fire resistance, weather durability, and safety of building materials [89,90,91,92]. The ultimate focus of each intumescent coating is to reduce emissions of smoke, volatile organic compounds (VOC), and toxic gases and provide safer and more environmentally friendly products for end-users [93]. Figure 5 shows the types of each intumescent coating, and details are discussed in the subjections below.

Each category has four main components: An acid source (APP), a blowing agent (MEL), a carbon source (PER or EG), and fillers. The chemical reactions occur between the fillers and the acid source. Those reactions encourage the development of a homogeneous foamed structure with appropriate thermal properties, such as lower heat conductivity and reduced emissivity at the surface [50,51,94]. The homogeneous foam structure is called an intumescent char layer that exhibits a shielding effect and effectively isolates the transmission of air and heat. The ultimate purpose of the char is to safeguard the structure from potential fire damage [95]. However, the intumescent char layer is prone to easy destruction in a fire, leading to inadequate fire resistance and making it inappropriate for industrial applications [95,96]. To solve this issue, a significant number of synergistic flame-retardant additives are required to achieve satisfactory performance [97,98]. Additionally, using inorganic fillers can enhance fire retardancy performance because they can modify the chemical and physical properties of the intumescent char during combustion. Silica, montmorillonite, talc, and calcium carbonate are most commonly employed as inorganic fillers. Notably, calcium carbonate is the most extensively used inorganic filler due to its cost-effectiveness, high thermal stability, and availability [98].

4.1. Water-Based Intumescent Coating

Water-based intumescent coatings are low in cost and more eco-friendly than other options. They have less chemical odour. But they take longer to dry in humid and cold environments [99]. They can give better protection against water, oil, and other chemicals with the help of a primer. Using a primer before applying the water-based intumescent coating improves the adhesion [14]. Water-based intumescent coatings are low in toxicity and VOC emissions and have a low environmental impact [90]. They are suitable for indoor and outdoor applications in all weather conditions [14]. Low dry film thickness (DFT) is common for water-based intumescent coatings. They also earn a “green” label because of their low VOC emission content in the environment [99].

The formulation process (Figure 6) for water-based intumescent coatings varies depending on the specific materials used in each formulation. However, the dispenser is commonly used at room temperature at different rotating speeds. Various percentages of pigment (titanium dioxide) and a dispersing agent are added to the dispenser to achieve proper dispersion and deionised water. This mixing process is carried out at high-speed rotation and at room temperature [57,66]. After a certain time, three fire-retardant agents (APP, PER, and MEL) are added slowly to prevent agglomeration. Lastly, the binder is added to the mixture [66].

4.2. Solvent-Based Intumescent Coating

Solvent-based coatings are used in semi-exposed environments and undergo various tests to determine their resilience against varying weather conditions and temperature changes. Compared with water-based intumescent coatings, they are more resistant to weather conditions [67]. Solvent-based intumescent coatings are suitable for protecting metal, plastics, and concrete surfaces. They can adhere to various substrates, making them suitable for indoor and outdoor use. They come in two different forms: a powder coating and a liquid coating. Powder coating is applied with a coater gun, and liquid coating is sprayed with an atomiser [14]. They dry faster and leave a smoother surface than water-based intumescent coatings. However, they release higher volatile organic compound (VOC) contents than other intumescent coatings [99].The solvent-based coating preparation process (shown in Figure 7) can differ depending on the materials used in the formulation. Solvent-based intumescent coatings are prepared by blending three flame-retardant additives (APP, MEL, and PER) with an acrylic binder [91]. This mixture is then combined with different fillers in a dispenser and dispersed at high speed until the mixture is homogenous [91,100]. After the coating preparation, the mixture is applied to the sample plate and layered until the expected dry film thickness is attained. After drying, the coating is ready for characterisation [91,100].

4.3. Epoxy-Based Intumescent Coating

Epoxy-based intumescent coatings are suitable for application in corrosive or harsher environments, including in offshore industries and chemical sectors. These coatings can give exceptional hydrocarbon fire protection, which is the speciality of epoxy-based intumescent coatings [67,99]. Epoxy-based intumescent coatings also perform well in terms of protection and durability. Besides that, they can be easily applied and removed without damaging the surface [89]. They offer high water resistance, oil resistance, and are scratching proof [89]. Epoxy-based intumescent coatings come in two parts which need to be mixed to form a sturdy and thick coating [67].

The epoxy-based coatings are prepared using a dispenser stirred at 6000 rpm for 1 h [101]. The coating process (shown in Figure 8) starts with heating a base material in a stirring container at 60 °C for 30 min. The stirring process continues until a homogeneous mixture is achieved. Fillers and additives are preheated and dried for 24 h. The preheated additives and fillers were added to the homogeneous mixture and stirred at high speed until the mixture became uniform. The curing agent is slowly introduced into the mixture and stirred [102]. The coating is then applied onto steel plates until reaching the required thickness. After drying, the thickness of the coating is measured using a digital vernier calliper [101,102].

4.4. Hybrid Intumescent Coating

Hybrid intumescent coatings are newer inventions which are distinguished by their composition of organic/inorganic hybrid resin. These coatings contain the best features, which are strong and stick well, resisting wear and tear. Their applications are easy, completed with just one spray, and can be applied to different surfaces [103]. Hybrid intumescent coatings are designed for high-traffic areas requiring frequent and swift repairs. They prevent infiltration and peeling. Besides, they can be used for their glossy texture and durability [89]. The synthesis of hybridised intumescent flame retardant (HIFR) (shown in Figure 9) involves many steps. At first, the expandable graphite, POCl₃, and 1,4-dioxane are mixed in a reactor for 3 h at 60 °C, followed by adding PER and heating at 100 °C. At this stage, 1,3,5-Triglycidyl isocyanurate (TGIC) and SnCl₂ are added to the reactor and maintained at 100 °C for 6 h. Hybridised intumescent flame retardant (hIFR) is obtained after filtration and drying [92]. The compound 2,2-bis(4-cyanatophenyl) isopropylidene (CE) is modified by adding CE to the hIFR at 150 °C for 3 h. As a result, a prepolymer (hIFR/CE) is obtained, followed by curing and post-curing. The synthesis material is assessed using various measurements, including FTIR, Raman spectra, XRD patterns, SEM-EDS observations, HAADF-STEM imaging, DSC, TG analysis, LOI values, flammability tests, etc., to analyse the properties and performances of the synthesised materials [92].

5. Fire Testing Methods and Standards

Numerous national and international organisations worldwide have established testing standards and methods to assess the flammability of polymeric materials. These testing approaches cover a range of fire-related properties: ignition [66,104], flame spread [105], heat release [55], and smoke production [106]. Five types of testing methods are commonly used: the ignitability test (or UL-94), the flame spread test, the limiting oxygen index (LOI), the heat release test (cone calorimeter), and the smoke test [107,108,109].

5.1. Ignitability Tests (Or UL94)

The UL-94, also known as ASTM D3801, is a horizontal burning ignitability test (shown in Figure 10) used widely for assessing the flammability of polymeric solids coated with an intumescent coating [66,110,111]. It is developed by Underwriters Laboratory Inc. (UL) and is a common method for checking a product’s fire safety standards [66,104]. In this test, a small intumescent coated sample is vertically or horizontally positioned above a cotton bed and subjected to a Bunsen burner-type flame for 10 s. The time after removing the burner is recorded, which represents the duration the sample remains aflame. A V-0 rating indicates that the flame stops within 10 s without the cotton igniting. A V-1 rating indicates that it stops within 30 s. A V-2 rating is assigned if the cotton catches fire [66,104,110]. The rating criteria used in the UL-94 standards are summarised in

Table 1. UL-94 standards for substrate ratings [112].

Criteria	V-0 (Lowest Flammability)	V-1	V-2 (Highest Flammability)
After flame time (t) (in secs)	<10 s	<30 s	<30 s
Whether the flame reached the top of the specimen	No	No	Yes
Cotton indicator (whether dripping occurred)	No	No	Yes

. Materials that fail to meet these standard criteria may get no ratings (NR) or undergo another test called the horizontal burning test. The UL-94 test is essential for materials because it helps to understand how various materials perform in fires, making it easier to design materials that can meet the V-0 rating [104,110,112].

5.2. Flame Spread Tests

There are several international standards for flame spread testing to evaluate materials’ ignition and flame spread properties. The ASTM E1321-97a is one of them, also known as the Lateral Ignition and Flame Spread Test (LIFT) [105]. It has two major protocols: one is to determine the ignition parameters of the sample materials, and another is to obtain the lateral flame spread properties [114]. The BS476: Part 7 [115] was developed to understand fire spread through hallways. This test was designed in response to unwanted incidents where the flame spread rapidly on the walls, and it is used to identify how various materials behave in the fire [114]. The ASTM E84-99 or ANSI/NFPA 255-2000 is a standard test method for analysing surface burning characteristics of different building materials, also known as the “tunnel test” [114,116]. It was developed at Underwriter’s Laboratories and used to identify the growth stage of a fire. It has great use in the regional and local building-code authorities in the USA [114]. ISO 9705: 1993(E) [117] is an international standard that reveals surface materials’ reaction-to-fire properties. It was developed based on room scenarios introduced in the 1950s, and it measures heat release using the oxygen consumption method [114]. Figure 11 shows a flame spread set-up. Samples are placed on the Kaowool insulation secured with high-temperature Loctite epoxy, exposed to a propane burner, and placed under quiescent conditions. The mass loss rate was recorded at 1 Hz, averaged, and smoothed after 4–6 test repetitions for analysis [118].

5.3. Limiting Oxygen Index

The limiting oxygen index (LOI), also known as ASTM D2863 or ISO 4589 (as shown in Figure 12), is mainly used to assess the flammability of polymeric materials and to evaluate the efficiency of fire-retardant materials [104,111,119]. Since intumescent coatings contain polymeric compounds like vinyl acetate copolymer [57,66], it is important to consider conducting the Limiting Oxygen Index (LOI) test. The LOI test determines the lowest oxygen concentration needed to support candle-like downward flame combustion. A minimum LOI number of 26 is required for a material to be considered self-extinguishing [107].

In this test, a material sample coated with an intumescent coating is placed vertically in a glass tube. A small flame is applied at the top with a nitrogen/oxygen mixture flowing in the bottom of the tubular system, and the coated bar is ignited, allowing the flame to burn downward into the unheated coating [71,120]. This test continues for approximately 3 min or until 5 cm of the specimen is consumed, determining the minimum level of oxygen in nitrogen required to sustain material combustion. The higher the oxygen needed and the greater the LOI value, the more fire-resistant the material [104]. This method is inexpensive because the equipment is affordable and requires a small sample size. Therefore, it is suitable for the research and development stage [107].

5.4. Heat Release Tests (Cone Calorimeter)

The cone calorimeter (Figure 13) is a well-known method in fire safety engineering to determine the heat release rate (HRR), mass loss rate (MLR), and the effective heat of combustion (EHC) of a material [121]. The oxygen consumption is measured in the cone calorimeter testing, and the oxygen consumption rate is converted into heat release rate. It cannot provide accurate toxicity data under ventilated conditions. When test samples are exposed to a heat flow of up to 25 or 100 kW/m², the sides of the samples are observed carefully. The char growth is observed on the exposed side, and the thermal stability is observed on the unexposed side [55]. Therefore, it can measure concentrations of carbon dioxide and carbon monoxide with the gas sensors. However, the cone calorimeter must be calibrated before the test to ensure its accuracy [55,121,122]. The cone calorimeter has gained popularity recently, and significant research has been noticed. It has been used to evaluate the performance of various manufactured products, spanning polymers, furniture, wall lining materials, prison mattresses, and electric cables. The cone calorimeter meets the requirements given by several industrial assessment standards, including ASTM E1354, ISO 5660, ASTM E1474, and ASTM D6113 [123].

5.5. Smoke Tests

To investigate smoke emission characteristics, a smoke chamber is used, compatible with the National Bureau of Standards (NBS) specifications for non-flaming settings as outlined in ASTM E662. This smoke chamber analyses smoke characteristics through the percentage transmittance (T%) of light and provides dynamic data on smoke intensity. This equipment is designed and operated to evaluate smoke production under the Polymer Laboratories Instrument System SN-2400, offering a proper understanding of smoke behaviour in various environments [106].

5.6. Full-Scale Fire Test

To investigate the real-life fire resistance of intumescent coatings, full-scale fire tests are preferred, using standards like EN 13381-8 [41], ASTM E119 [125], BS 476 [126], AS 1530.4 [127], and ISO 834 [128]. The EN 13381-8 test standard is used for evaluating the effectiveness of applied fire protection systems on steel beams or steel columns and evaluating their thermal protection [41]. ASTM E119 is a fire test standard for building constructions and structural elements such as walls, partitions, columns, floors, roofs, beams, steel substrates, etc. [129,130]. The BS 476 fire test is a British standard and generally used on flat materials, composites, or assemblies [131]. The test involved exposing intumescent-coated steel to a small flame for 20 min, with an additional 2 kW of irradiance applied from the third to the final minute [126]. The Australian standard AS 1530.4 is utilised to determine fire resistance ratings and evaluate the insulation performance, structural integrity, and load-bearing capacity of the structural elements [127,128]. The ISO 834 test standard is designed for intumescent coatings to assess the fire resistance of structural steel elements protected by these coatings [132].

At the beginning of the full-scale fire testing process, steel structure members, such as columns, beams, or steel plates, are coated with varying intumescent coating thickness (1500–2500 µm) [41]. After drying, these specimens are then exposed to a controlled fire in a furnace or other testing apparatus to evaluate their performance [41,128,133]. The test evaluates the adhesion, thermal properties, and fire resistance of the intumescent coating by continuously monitoring the temperature of the steel or other substrate [41,128]. The intumescent coating protects the steel substrate by preventing it from reaching the critical temperature and provides insulation to preserve the steel’s load-bearing capacity during the fire [41].

5.7. Others

The Bunsen burner test (shown in Figure 14) is commonly used [22,102,134] to evaluate the temperature response of coating specimens [71,91,126,135]. This test focuses on the char formation and the reaction of intumescent coatings. Additionally, this test visualises the comparison of temperature evolution between bare steel plates and single-side-coated steel plates, using two handheld J-type thermocouples with digital panel indicators. It is also used to characterise some properties of the formed char [135,136]. For this test, coatings are applied on one side of the specimen, and the process is repeated until achieving a dry film thickness of 1.5 ± 0.2 mm. After drying the coating at room temperature, approximately 1000 °C is applied to the vertically mounted coated specimen [71]. The distance between the coating and the burner should be maintained at 7–10 cm [57,137]. This testing method provides insights into the coating’s protective capabilities under a high temperature [57,71].

6. Research and Development on Intumescent Coating

This literature review on the research and development of intumescent coatings is summarised based on the category of research conducted in the field of application, flame-retardant coating development, bonding performance, corrosion, durability, fire resistance, and cost-effectiveness. For example, Section 6.1, “Application of Intumescent Coating”, focuses solely on the application of intumescent coatings on steel, timber, and concrete, and Section 6.2, “Flame-Retardant Coating Development”, discusses various types of coating development, the procedures involved, key ingredients used by different researchers, and their significant findings. The details of each section are discussed in the below sections.

6.1. Application of Intumescent Coating

Intumescent coatings are commonly used on steel and timber structures to improve their fire-resistant properties. Steel can handle temperatures of up to 550 °C before collapsing in a fire, and timber is prone to catching fire quickly. Intumescent coatings help improve the fire resistance of both materials by creating a protective layer that delays damage and enhances overall safety.

6.1.1. Structural Steel

Steel structures are commonly used in building construction, but they face a major challenge in preserving their structural integrity during and after a fire. Consequently, fires in buildings lead to a significant number of fatalities each year [37]. During a fire, steel frames experience two simultaneous phenomena: an increase in temperature within the steel frame due to heat transfer and changes in mechanical properties due to thermal expansion and material softening [139]. Traditionally, thermal barriers, such as concrete or gypsum, are widely used to prevent steel from reaching critical temperatures, typically 550–600 °C, according to Eurocode guidelines [140]. These concrete and gypsum solutions are aesthetically unappealing. Intumescent coatings are excellent alternatives, offering protection while preserving the visual elegance of slender and light structures [37]. Various experiments have proven the effectiveness of intumescent coatings on steel members [37,135,141,142,143,144]. Nevertheless, applying these coatings necessitates careful consideration of their features because of the thin cross-sections and high surface curvature of tension rods. Intumescent coatings are widely accepted worldwide as a primary solution for safeguarding steel against fire (shown in Figure 15) [145].

Intumescent coatings expand mainly upward when exposed to fire, which is great for flat steel. However, some fire tests showed that the coating can crack on curved surfaces and edges due to limited sideway expansion [148,149]. This can result in a reduction in thermal protection and a rapid rise in the temperature of the steel structure. On-site application is required for intumescent coatings on curved, intricate steel tension rod system components. Applying an intumescent coating with the same dry film thickness (DFT) for tension rods and connections is necessary for extra strength during the fire. In a fire, load-bearing capacity reserves may be activated by using the same DFT for tension rods and connection components [145]. Intumescent coatings are now widely recognised and accepted as a primary fire protection system for steel structures. Even with extensive research, some areas still require more understanding. These include numerical modelling approaches, verifying the coatings certification in furnaces, and examining the effects of weathering and ageing [150]. Further research is required to better understand and improve intumescent coatings for corrosion resistance.

6.1.2. Timber

Timber is a traditional construction material that is crucial in the modern construction industry. It is used for diverse purposes, crafting beams, joists, columns, cords, roofs, walls, and floors, available in forms like plywood, panels, and boards [151]. Light timber frame construction has recently gained worldwide popularity in dwelling buildings because of its environmentally friendly nature, cost-effectiveness, ease of construction, and excellent resistance to impact [134]. However, timber may not match the strength of concrete and steel; its lighter weight makes it a better competitor and an excellent choice for constructing long-span and tall structures. In addition, the low weight of timber makes it ideal for seismic-resistant construction, providing greater resistance to the effects of seismic shaking [151]. The application of intumescent coating on timber offers several advantages. Past researchers have proved that intumescent coatings could improve fire-resistance performance on wooden surfaces through various experimental evidence. Timber treated with intumescent coating exhibits a delayed ignition time, reduced heat release rates (both total and peak) during combustion, and reduced flame spread and smoke emissions [152]. However, intumescent coatings designed for timber surfaces need to be labelled with a BAL-29 rating in accordance with AS 3959:2009. A BAL-29 rating indicates the suitability of the coating in areas prone to bushfires [55]. Researchers have conducted various tests on timber samples coated with intumescent materials in laboratory-scale experiments [46,153,154].

For example, Lucherini et al. [152] investigated the burning behaviour of intumescent coatings on timber with different thicknesses, as shown in Figure 16. They also examined the ignition time and temperature. In addition, the study looked at characteristics such as depth and swell, particularly for Australian softwood. Vermiculite and perlite are recognised for their flame-retardant properties, offering low density, high porosity, and thermal insulation, making them excellent additives for heat insulation. They absorb moisture from the environment, which contributes to extending fire duration. Yin et al. [134] experimented to create a flame-retardant filler that includes aluminium hydroxide, calcium silicate, magnesium hydroxide, chicken eggshell powder, and calcium carbonate. This experiment aimed to develop a prototype for a fire-resistant timber door, assessing properties such as fire protection, mechanical strength, and the chemical characteristics of intumescent flame-retardant coatings. Researchers have been trying to develop a high-efficiency and environmentally friendly intumescent fire-retardant coating in various aspects. Modifying the decanoic/palmitic eutectic mixture (DPEM) is a successful example of an eco-friendly and efficient intumescent fire-retardant coating [155].

6.1.3. Concrete Material

Concrete is a versatile material widely used in construction. However, high temperatures can cause dehydration, causing cracks and stresses that may lead to explosive failures. Intumescent coatings made from organic and mineral binders protect concrete from spalling [16,103,156,157], as shown in Figure 17. Intumescent coatings swell and form a protective foam layer when exposed to the fire; the layer insulates the concrete and keeps it from reaching the critical temperature. Sodium silicate-based intumescent coatings have good adhesive properties for concrete [157]. However, they tend to lose their swelling ability with time. Song et al. [158] investigated intumescent coating performance on concrete-filled steel tubular (CFST) columns in standard fire circumstances up to 180 min. Three types of intumescent fire coating were tested on CFST columns. The specimen showed minimal deformation and maintained temperatures below the critical failure level at 350 °C. Intumescent coatings were effective and cohesive and formed protective layers without concrete spalling. Fibre-reinforced epoxy intumescent coatings (FRIC) provided lateral confinement to concrete cylinders, significantly enhancing axial load capacity and deformability. Triantafyllidis et al. [159] proved that the FRIC-wrapped specimens’ peak strength and ultimate axial strain increased by 42% and 480% from the average peak strength and ultimate axial strain, which indicates the effectiveness of the coating.

6.2. Flame-Retardant Coating Development

The development of fire-retardant coatings depends on selecting raw ingredients and their percentages for specific performance goals. Table 2 presents an overview of the key ingredients and fillers commonly used in the recent development of intumescent coatings, highlighting the range of percentages for each material, from the lowest to the highest, as explored in prior research. Additionally, the table outlines each component’s decomposition temperatures and chemical structures, which is critical for understanding their thermal behaviour in fire protection applications. Table 3 summarises the key compositions used in the different types of intumescent coatings, providing insights into the char activation temperatures and highlighting the advantages and limitations of the coatings examined. This comprehensive comparison underscores the performance characteristics of different formulations and their potential for improving fire resistance in various structural applications.

Raw material selection needs to align with the intended purpose and functionality of the coating. Additionally, the preparation for waterborne fire-retardant coating [151] and epoxy-based coating [160] are different. The waterborne intumescent coating preparation process involves the mixing and grinding of various components [161,162]. For example, Liu et al. [161] prepared an intumescent coating, including a waterborne polyurethane emulsion modified with epoxy resin, fire-retardant additives (including PER, MEL, etc.), and water. The mixture was agitated in an agate mortar for 1 h. The grinding process ensures that all ingredients mix well, creating a homogeneous coating. The intumescent coating showed a fire-resistance time of around 100 min for steel plates [161]. Yin et al. [134] conducted a project that involved synthesising water-based intumescent paint using a high-speed disperser mixer at room temperature for 30 min to achieve a homogeneous coating. The coating formulation included flame-retardant additives (APP, PER, MEL), flame-retardant fillers, and a water-based polymer binder. Aluminium hydroxide, magnesium hydroxide, calcium silicate, chicken eggshell (CES) powder, calcium carbonate, titanium dioxide, etc., were used as a flame-retardant filler, and vinyl acetate (VA) copolymer emulsion was used as a water-based polymer binder. The resulting intumescent coating exhibited an equilibrium temperature of 217 °C. It formed the thickest char layer at 600 °C, and the char layer had a higher thermal insulation property. The char layer could withstand a maximum weight load of 650 g, demonstrating higher char strength [134]. Zhao et al. [102] prepared an epoxy-based flame-retardant coating where a designated quantity of E−51 is placed in a 250 mL beaker and heated to 60 °C with stirring in a water bath for 30 min. Additionally, flame-retardant additives (APP and Cu₂O) were preheated at 80 °C for 24 h, with varying mass ratios. Then, the flame-retardant additives were systematically added to the epoxy, and the mixture was stirred at a high speed for 10 min until it became homogeneous. Stirring continues for three more minutes under the high-speed disperser as curing agent 593 is carefully added to the blend at a mass ratio of 4/1 of E-51 to curing agent 593. After the preparation, the intumescent coatings are put through various tests, such as the Bunsen burner test, furnace test, SEM analysis, adhesion strength test, freeze/thaw cycle test, and the TGA test, to assess their fire resistance and physical properties [161]. According to the fire test results, the steel plates’ backside temperature dropped with increasing coating thickness. The backside temperature for the 2 mm thick coating was 259.7 °C, providing the best protection. Although the 3 mm coating further reduced the temperature to 236.5 °C, the improvement was minimal compared to the 2 mm coating [102]. De Sá et al. [163] prepared an environmentally friendly intumescent coating with vegetable compounds such as coffee husk and ginger powder. They used coffee husk and ginger powder as carbon sources. To prepare the intumescent coating, they dispersed the Araldite 488 N40 epoxy resin for a few minutes to achieve homogenisation. Then, other components such as triphenyl phosphate (TPP), zinc phosphate (ZnP), boric acid, melamine, TiO₂, coffee husk (milled to 250 μm), and ginger powder were added. ZnP was used in place of TiO₂ and TPP in some formulations. After adding all the components, the mixture was stirred constantly for another half an hour at 3000 rpm to guarantee thorough mixing and uniformity. After 15 min of flame exposure, the coating exhibited a drop in the metal substrate temperature, stabilising at about 100 °C. The fire resistance performance was greatly enhanced by the use of ginger powder as the carbon source [163]. Baena et al. [55] prepared an intumescent coating by mixing resin, Disperbyk-199, and distilled water at 400 rpm for 5 min. APP and TiO₂ were also incorporated into the homogenised solution for another 5 min. EG was added and mixed for another 5 min. They tested the coating performance on timber. The timber surface was cleaned with dampened tissue with distilled water and dried at room temperature for 60 min. The coating was applied at a dry film thickness (DFT) of 400 μm. The coating’s low mass loss of 4.63% made it the most economical fire protection for timber. It is suitable for wood exposed to BAL-29 conditions because of its great flame retardancy with a lower peak heat release rate (p-HRR) and total heat release (THR) [55].

Table 2. Different key materials of intumescent coating with their usage range and char formatting temperature.

Source	Materials	%	Decomposition Temperature	Chemical Structure	References
Acid source	Ammonium polyphosphate (APP)	5.76–45	250–450 °C		[55,164,165]
Carbon source	Pentaerythritol (PER)	6.5–13	187–189 °C		[166,167,168]
	Expandable graphite (EG)	5.5–45	150–300 °C		[55,169,170]
Blowing agent	Melamine (MEL)	5.5–15	250 °C		[70,171,172]
Binders	Vinyl acetate copolymer	45–50	180–380 °C		[57,134,173,174,175]
	Acrylic resin	20–60	-	-	[55,176]
	Epoxy resin	39.80–70.57	-	-	[164,177]
Fillers	Mg(OH)2	0.5–20	350 °C		[178,179,180]
	Al(OH)3 (ATH)	2.5–7.4	260–400 °C		[71,151,181]
	Alumina	0.1–5	700–1200 °C		[182,183]
	Cenosphere	1–10	-	-	[57]
	Boric acid	0–11.76	500–650 °C		[55,171,184]
Pigment	TiO2	2–10	-		[55,185]

Table 2. Different key materials of intumescent coating with their usage range and char formatting temperature.

Source	Materials	%	Decomposition Temperature	Chemical Structure	References
Acid source	Ammonium polyphosphate (APP)	5.76–45	250–450 °C		[55,164,165]
Carbon source	Pentaerythritol (PER)	6.5–13	187–189 °C		[166,167,168]
	Expandable graphite (EG)	5.5–45	150–300 °C		[55,169,170]
Blowing agent	Melamine (MEL)	5.5–15	250 °C		[70,171,172]
Binders	Vinyl acetate copolymer	45–50	180–380 °C		[57,134,173,174,175]
	Acrylic resin	20–60	-	-	[55,176]
	Epoxy resin	39.80–70.57	-	-	[164,177]
Fillers	Mg(OH)2	0.5–20	350 °C		[178,179,180]
	Al(OH)3 (ATH)	2.5–7.4	260–400 °C		[71,151,181]
	Alumina	0.1–5	700–1200 °C		[182,183]
	Cenosphere	1–10	-	-	[57]
	Boric acid	0–11.76	500–650 °C		[55,171,184]
Pigment	TiO2	2–10	-		[55,185]

Table 3. Different types of intumescent coating composition and properties comparison.

Name	Composition of Intumescent Coating	Activation Temperature	Advantages	Limitations	Ref.
Water-based coating (Type 1)	Vinyl acetate/vinyl ester dispersion, APP, PER, polyols, MEL, rutile TiO2, pigment, vermiculite (Ver), celite, ATH, deionised water, bondox T-80 (dispersing agents), texanol (coalescing agents), and tylose (thickener).	Below 250 °C	Showed effective heat insulation, which is crucial for protecting structural materials in the event of fire.	After time passed, the coatings showed signs of deterioration where distinct differences among samples are noted.	[66]
Water-based coating (Type 2)	Cenosphere (Grade CIL30), PER, MEL, boric acid (BA), TiO2, rutile R-902), APP 422, and binder (ethylene vinyl acetate copolymer and Vinappas LL3112).	At 330 °C	The addition of cenospheres improves fire protection by enhancing char expansion, forming a protective layer, and increasing thermal stability.	Incorporating cenospheres may increase formulation complexity and cost.	[57]
Water-based coating (Type 3)	Hollow glass microspheres (HGMs), MPP (melamine polyphosphate), starch, water-based acrylic emulsion (HS-6121), PVP (polyvinylpyrrolidone)—K90 grade, aluminium isopropoxide (AIP), CaCO3, mica flakes, defoamer (DP-633), 3-aminopropyltriethoxysilane, anhydrous ethanol, isopropanol, and deionised water.	At 410.1 °C	HGMs@Al2O3 composite microspheres of the coating: reduces thermal conductivity, enhancing the coating’s thermal insulation properties.	The coating formulation is its complexity, involving multiple steps, which may lead to increased production costs.	[186]
Solvent-based coating (Type 1)	acrylic resin (binder), APP phase II, MEL, PER, (Mg(OH)2), and titanium dioxide (TiO2).	Relatively at 188 °C	Mg(OH)2 and TiO2 improved the coating’s fire protection and foam structure, yielding a thicker char layer.	This coating has less water resistance.	[67]
Solvent-based coating (Type 2)	Epoxy resin (binder), APP phase II, MEL, PER, Mg(OH)2, and TiO2.	Relatively at 285 °C	Higher epoxy content improved adhesion strength and water resistance, maximising bonding to the metal surface.	Char formation is not uniform and porous, causing heat transfer.	[67]
Solvent-based coating (Type 3)	APP, MEL, PER, acrylic resin (binder), TiO2, nano-CES, expandable graphite, zinc borate, and calcium silicate.	At 190 °C	The ingredient cost is low.	Multiple flame retardants are required in this formulation.	[187]
Epoxy-based coating (Type 1)	Epichlorohydrin based epoxy resin, cycloaliphatic polyamine-based hardener (CeTePox 1393 H, 93 g/eq), Exolit AP 750, zinc borate (ZB 467), boric acid, and MEL.	At 800 °C	Fire-protection properties of the coating increase with the proper proportion of ingredients.	Antagonistic effects observed with ZB complicate the formulation despite increased char yield.	[68]
Epoxy-based coating (Type 2)	Epoxy resin (NPEL-128), polyamide amine (hardener H-2310), APP, expandable graphite (EG), boric acid, MEL, basalt chopped strands, ethanol, and mild steel plates S355.	_	Ethanol as a dispersing agent enhances the dispersion of basalt that helps to achieve high thermal stability, insulation, and fire protection.	Ethanol is flammable.	[130]
Hybrid-based coating (Type 1)	2,2′-bis(4-cyanatophenyl) isopropylidene, expandable graphite (EG), POCl3, and PER, 1,3,5-triglycidyl isocyanurate (TGIC).	At 700 °C	This coating offers improved resistance to high temperatures, effectively slowing down the spread of fire.	The production process may be costly.	[92]
Hybrid-based coating (Type 2)	4,4′-diaminodiphenyl ether (ODA), 4,4′-diaminodiphenylmethane (DDM), hexachlorocyclotriphosphazene (HCCP), anhydrous ethanol, MEL, pyridine, acetone, APP-II, and epoxy resin (EP or DGEBA, E-44).	_	PZMA@APP works great to enhance flame retardancy and the mechanical performance of EP composites.	PZMA@APP is a complex synthesis and incorporation process that may increase production costs and limit scalability.	[188]

Note: APP—ammonium polyphosphate, MEL—melamine, PER—pentaerythritol, and TiO₂—titanium dioxide.

Table 3. Different types of intumescent coating composition and properties comparison.

Name	Composition of Intumescent Coating	Activation Temperature	Advantages	Limitations	Ref.
Water-based coating (Type 1)	Vinyl acetate/vinyl ester dispersion, APP, PER, polyols, MEL, rutile TiO2, pigment, vermiculite (Ver), celite, ATH, deionised water, bondox T-80 (dispersing agents), texanol (coalescing agents), and tylose (thickener).	Below 250 °C	Showed effective heat insulation, which is crucial for protecting structural materials in the event of fire.	After time passed, the coatings showed signs of deterioration where distinct differences among samples are noted.	[66]
Water-based coating (Type 2)	Cenosphere (Grade CIL30), PER, MEL, boric acid (BA), TiO2, rutile R-902), APP 422, and binder (ethylene vinyl acetate copolymer and Vinappas LL3112).	At 330 °C	The addition of cenospheres improves fire protection by enhancing char expansion, forming a protective layer, and increasing thermal stability.	Incorporating cenospheres may increase formulation complexity and cost.	[57]
Water-based coating (Type 3)	Hollow glass microspheres (HGMs), MPP (melamine polyphosphate), starch, water-based acrylic emulsion (HS-6121), PVP (polyvinylpyrrolidone)—K90 grade, aluminium isopropoxide (AIP), CaCO3, mica flakes, defoamer (DP-633), 3-aminopropyltriethoxysilane, anhydrous ethanol, isopropanol, and deionised water.	At 410.1 °C	HGMs@Al2O3 composite microspheres of the coating: reduces thermal conductivity, enhancing the coating’s thermal insulation properties.	The coating formulation is its complexity, involving multiple steps, which may lead to increased production costs.	[186]
Solvent-based coating (Type 1)	acrylic resin (binder), APP phase II, MEL, PER, (Mg(OH)2), and titanium dioxide (TiO2).	Relatively at 188 °C	Mg(OH)2 and TiO2 improved the coating’s fire protection and foam structure, yielding a thicker char layer.	This coating has less water resistance.	[67]
Solvent-based coating (Type 2)	Epoxy resin (binder), APP phase II, MEL, PER, Mg(OH)2, and TiO2.	Relatively at 285 °C	Higher epoxy content improved adhesion strength and water resistance, maximising bonding to the metal surface.	Char formation is not uniform and porous, causing heat transfer.	[67]
Solvent-based coating (Type 3)	APP, MEL, PER, acrylic resin (binder), TiO2, nano-CES, expandable graphite, zinc borate, and calcium silicate.	At 190 °C	The ingredient cost is low.	Multiple flame retardants are required in this formulation.	[187]
Epoxy-based coating (Type 1)	Epichlorohydrin based epoxy resin, cycloaliphatic polyamine-based hardener (CeTePox 1393 H, 93 g/eq), Exolit AP 750, zinc borate (ZB 467), boric acid, and MEL.	At 800 °C	Fire-protection properties of the coating increase with the proper proportion of ingredients.	Antagonistic effects observed with ZB complicate the formulation despite increased char yield.	[68]
Epoxy-based coating (Type 2)	Epoxy resin (NPEL-128), polyamide amine (hardener H-2310), APP, expandable graphite (EG), boric acid, MEL, basalt chopped strands, ethanol, and mild steel plates S355.	_	Ethanol as a dispersing agent enhances the dispersion of basalt that helps to achieve high thermal stability, insulation, and fire protection.	Ethanol is flammable.	[130]
Hybrid-based coating (Type 1)	2,2′-bis(4-cyanatophenyl) isopropylidene, expandable graphite (EG), POCl3, and PER, 1,3,5-triglycidyl isocyanurate (TGIC).	At 700 °C	This coating offers improved resistance to high temperatures, effectively slowing down the spread of fire.	The production process may be costly.	[92]
Hybrid-based coating (Type 2)	4,4′-diaminodiphenyl ether (ODA), 4,4′-diaminodiphenylmethane (DDM), hexachlorocyclotriphosphazene (HCCP), anhydrous ethanol, MEL, pyridine, acetone, APP-II, and epoxy resin (EP or DGEBA, E-44).	_	PZMA@APP works great to enhance flame retardancy and the mechanical performance of EP composites.	PZMA@APP is a complex synthesis and incorporation process that may increase production costs and limit scalability.	[188]

Note: APP—ammonium polyphosphate, MEL—melamine, PER—pentaerythritol, and TiO₂—titanium dioxide.

Nasirzadeh et al. [66] prepared an intumescent coating by mixing the pigment (TiO₂) and dispersing agent in deionised water with a high-speed dispenser. Then, other fire-retardant agents (APP, PER, MEL) and fillers (vermiculite, celite, ATH) were added to the mixer to ensure uniformity. Lastly, a water-based vinyl acetate dispersion resin and coalescing agents were then added to complete the formulation. The prepared coating was applied on the pre-treated low-carbon steel plates (150 × 100 × 1 mm) with an average dry film thickness of 400 ± 10 μm, and the samples were cured at 20 °C and 50% relative humidity for a week. The torch flame test revealed that the best insulation was provided by intumescent coatings with 3% filler, especially vermiculite-based filler, which maintained temperatures below 550 °C for 30 min. Due to excessive expansion, performance was lowered by a higher filler content of 6%. In the thermogravimetric analysis (TGA), the vermiculite-based intumescent coating had the highest thermal resistance, with degradation starting above 600 °C and a final residual weight of slightly over 40–45% at 800 °C [66]. Yew et al. [67] prepared two intumescent coatings using acrylic resin and epoxy resin as a binder. Both intumescent coatings were prepared in the same way by combining a binder (acrylic resin or epoxy resin) and a variety of flame-retardant agents, including ammonium polyphosphate phase II, melamine, pentaerythritol, magnesium hydroxide, aluminium hydroxide, and titanium dioxide. These components were mixed with binder using a high-speed dispenser mixer at 3000 rpm. The prepared intumescent coatings were applied on a steel plate by a gun sprayer, and after drying, a fire test was conducted. Both provided superior fire protection; one coating had the thickest layer (25.7 mm) and the lowest equilibrium temperature (188 °C). And the other one also performed well, with a 22.5 mm layer and 201 °C equilibrium temperature. The TGA results showed varying thermal behaviours, with the coating containing TiO₂ and Mg(OH)₂ exhibiting the highest residual weight (28.07 wt.% at 750 °C) and superior fire resistance. Intumescence occurred at different temperatures for acrylic-based coatings (250–620 °C) and epoxy-based coatings (100–715 °C) [67]. Yasir et al. [130] prepared an intumescent coating by combining flame retardants such as ammonium polyphosphate (APP), expandable graphite (EG), boric acid, melamine (MEL), and basalt chopped strands in the epoxy resin (NPEL-128) with a polyamide amine hardener (H-2310). Finally, a dispersing agent (ethanol) was incorporated to improve the dispersion of fillers. The fire test showed that the coating had great fire-resistant properties, with a backside temperature of 189 °C after 60 min and a dense, crack-free char. In the TGA result, the intumescent coating formulation showed the residual weight (38%) and improved thermal stability. The degradation occurred in four stages, with the addition of basalt fibres slowing down the degradation process and enhancing fire resistance. Qiu et al. [188] prepared an intumescent coating using epoxy resin (diglycidyl ether of bisphenol A, DGEBA) with a polyamine hardener (4,4′-diaminodiphenylmethane, DDM). To enhance fire resistance, a hybrid filler, PZMA@APP, was synthesised by reacting melamine, 4,4′-diaminodiphenyl ether (ODA), hexachlorocyclotriphosphazene (HCCP), and ammonium polyphosphate (APP). The PZMA@APP was then incorporated into the epoxy resin to produce the desired fire-retardant properties. The formulated coating showed enhanced flame retardancy and thermal stability. The TGA result demonstrated greater char residue and thermal stability, whereas SEM analysis demonstrated improved interfacial adhesion. The composites received a UL-94 V-0 rating, and the LOI value rose to 29%. At 10 percent weight of PZMA@APP, the cone calorimeter tests demonstrated a reduction of up to 75.6% in peak heat release and 65.9% in total heat release. Furthermore, less smoke was produced, improving fire safety [188].

6.3. Adhesive or Bonding Performance

Bonding or adhesion between the intumescent coating and the substrate is a significant area of research [42]. The char layer of the intumescent coating acts as a physical barrier, preventing the substrate from rapidly heating and preserving the operational properties of building construction. Therefore, the char layer must be stuck on the substrate under fire or weather conditions [189]. The quality of a flame-retardant coating depends on its adhesion and adhesion strength [34,190]. Additionally, adhesion strength depends on a few factors, such as wetting ability, surface tension, chemical character, and the surface roughness of the substrate. The coating must be absorbed by the substrate for good adhesion compatibility, enhancing interface interaction between the coating and the substrate and ensuring durability [42]. The coating thickness, suitable surface roughness, and the use of an appropriate primer can significantly impact the improvement of adhesion strength. The overall performance of intumescent coating, including adhesion bonding performance, depends on the generic types of primer and its thickness [42,191]. However, metal surfaces like steel do not absorb intumescent coating and have a smooth, non-porous texture. Magnesium hydroxide plays a vital role in enhancing the adhesion and strength of intumescent coating on metal surfaces [71]. Additionally, inhibitive pigments like zinc phosphate can play a vital role in enhancing the adhesion between the coating and the substrate and preventing surface corrosion [192,193]. Additionally, a coating’s adhesion performance is influenced by the interface region’s atomic bonding structure, elastic modulus, fracture toughness, thickness, and purity [71]. However, significant research in this field is limited. Weak adhesion can lead to the separation of the coating from the substrate in a fire, which can damage the substrate.

6.4. Corrosion and Chemical Resistance Performance

Flame retardant coatings offer fire protection for steel structures, but some cannot protect the steel structure against aggressive external environments [191]. The key ingredient of flame-retardant coating, ammonium polyphosphate (APP), undergoes hydrolysis and produces ammonium ions (NH₄⁺) and pyrophosphate under a corrosive environment; this weakens thermal insulation, compromising structural safety in fires [194,195]. The ammonium ions (NH₄⁺) and chlorine ions (Cl⁻) in the hydrochloric acid solution act as a catalyst for the hydrolysis process, resulting in the formation of ammonium chloride (NH₄Cl) [196]. With time, APP consumption gradually increases, its diffusion speed decreases, and corrosion continues. Finally, the interaction between hydrochloric acid and APP at the coating surface is responsible for a size reduction in APP particle size [194]. The primer can be a solution for a corrosive environment, containing anti-corrosive pigments that reduce the corrosion rates. The primers also have great wetting properties, ensuring excellent bonding between the coating and substrate [191]. However, it will increase the total cost.

The intumescent coatings are chemically reactive and may lose their fire protection properties over time with prolonged exposure to environmental conditions [197]. Researchers have conducted various studies [35,198,199,200,201] and accelerated ageing experiments to explain and understand the long-term performance of intumescent coatings. Exposure to environmental elements, especially salty water, can affect the fire protection of intumescent coatings. Two general methods are used to improve durability: incorporating particles as modifiers into intumescent coatings [35,195,202,203,204,205] and applying a topcoat to the surface of the coating. The application of topcoat is commonly used [206,207].

6.5. Durability and Weather Performance

The usage of intumescent coatings on exterior surfaces faces exposure to humidity, temperature changes, sunlight, and other weathering factors. These factors can impact the long-term performance and durability of the coatings. UV rays can cause colour fading, and weathering can also result in surface erosion, gloss loss, and other effects [197,208]. Polymer bonding can be broken in weathering due to material degradation [197,198,208]. Generally, flame-retardant coatings contain a corrosion prevention layer to protect the substrate from corrosion. The effectiveness of the protective layer is the primary factor that determines how long-lasting these coatings are overall. The conventional intumescent coating requires an additional layer of weathering protection, known as a topcoat, to enhance durability [143,208]. The durability and weathering performance of intumescent coatings are evaluated through artificial accelerated laboratory weathering, natural weathering, and accelerated natural weathering. The artificially accelerated laboratory weathering test simulates environmental conditions such as UV exposure and humidity in controlled chambers. In the natural weathering test, the coating is exposed to a real outdoor environment, like sunlight and rain. Lastly, accelerated natural weathering speeds up this exposure using intensified factors. The natural weathering test is the most reliable for assessing long-term performance [198].

6.6. Fire Resistance

The fire resistance of intumescent coatings depends on the fire and thermal resistance properties of the expanded char layer when exposed to fire [25]. At the activation temperature, these coatings undergo chemical reactions that release gases. The gases trap the coating surface and cause the coating to swell, creating a thick insulating barrier on the substrate [209]. High-quality intumescent coatings can withstand prolonged exposure to high temperatures without significant degradation, ensuring continuous protection during a fire event [50]. The effectiveness of intumescent coatings depends on their thermal stability and their capacity to delay heat transfer to the substrate in a fire [25,50,90,210]. The addition of intumescent flame-retardant additives for long-term environmental stability can cause poor fire resistance, as reported in numerous studies [40,195,202,203,205,211,212]. Wang et al. [203] demonstrated that after 500 h of water immersion, acrylic-based APP–PER–melamine coatings exhibited a 34.3% reduction in fire resistance and char layer thickness. Adding 8.5 wt.% of expandable graphite maintained fire resistance, and nano-SiO₂ improved fire resistance compared to coatings without silica [203]. In another experiment, Wang et al. [202] showed that adding more particles, such as layered double hydroxides and TiO₂, to intumescent coatings enhanced a material’s ability to withstand weathering and, consequently, its fire resistance. During char formation, gas trapping properly in the highly viscous melt is essential for achieving homogeneous char and a high swelling ratio, which improves the fire resistance performance of intumescent coatings [136].

6.7. Cost-Effectiveness

The cost-effectiveness of a material involves several considerations. Intumescent coatings have higher costs than cementitious materials, ranging from 4 USD to 12 USD per square foot [213]. However, the application procedure of intumescent coatings is quite simple, leading to low labour costs compared to cementitious materials [213]. Additionally, their application procedure is direct and prevents the formation of gaps susceptible to moisture-induced corrosion, possibly saving money on upkeep and repairs in the long run. On the other hand, cementitious coatings are initially less expensive due to their inexpensive materials, but they have higher labour costs [213,214]. Cementitious materials tend to separate when dried; delamination is the key issue. The gaps between the cementitious coating materials and the substrate can allow moisture to penetrate and cause corrosion. This type of corrosion scope can increase long-term maintenance expenses. However, intumescent coatings help to prevent these gaps and provide better production with low maintenance costs. Although intumescent coatings may require a larger initial investment, they will be more cost-effective than cementitious coatings due to their lower labour costs and potential maintenance savings [213,214,215,216].

7. Challenges and Issues

7.1. Fire Protection Time and Flame Spread

Most of the intumescent coatings could offer 60 min of fire resistance to the substrate in the past. Recent advancements in the application and production of intumescent coating technologies have drastically reduced coating thicknesses, making intumescent coating paints competitive in the 120 min fire resistance market [81]. Due to modern research, an increasing number of intumescent coatings can give remarkable fire protection from 120 to 180 min [217]. This development has completely changed the field of fire prevention engineering, especially for buildings that must withstand 120 min of fire, such as high-rise structures [80]. Different intumescent coatings possess varying chemical compositions and fabrication methods, which can substantially affect their fire protection time and fire resistance [218]. Additionally, dry film thickness (DFT) can influence fire protection time [219]. Although intumescent coatings can provide fire protection from 60 to 180 min. Some intumescent coatings have a fire spread issue at the early stage of coating; burning before proper char formation is one of the key issues for some intumescent coatings [138,218].

7.2. Char Activation Temperature

Intumescent coatings are used for decorative and fire-retardant purposes and initially behave as inert paint under normal temperatures without fire. However, when exposed to fire, they undergo chemical reactions that cause swelling. The swelling is a protective and insulating layer that acts as fire resistance and impedes fire spread. Usually, at 250 °C, intumescent coatings begin to swell, and at a temperature range of 300 °C to 350 °C, the blowing agent is activated to produce a char that strengthens their ability to withstand fire [80]. The exact temperature range between 200 °C and 300 °C at which swelling begins depends on the makeup of the coating’s constituent elements [37]. Char formation mainly depends on the char activation temperature at which the intumescent coating starts to swell. The char activation temperature at which the char occurs for the intumescent coating is critical for different substrates, as each material has its own ignition or critical temperature. For instance, wood ignites at 250 °C, while the critical temperature of steel is around 600 °C. So, lower char activation temperature intumescent coatings are preferable for wooden structures or open-cavity barriers. On the other hand, for steel or concrete applications, higher char activation temperature intumescent coatings are more suitable. The char activation temperature of intumescent coatings varies from 180 to 800 °C (Table 3). Figure 18 shows these different intumescent coatings, categorised by their base materials and activation temperatures.

The activation temperature range is from 188 °C to 800 °C, indicating the temperature at which each coating begins to expand and form a fire protective char layer. Water-based intumescent coatings (Type 1, Type 2, and Type 3) exhibit activation temperatures of 250 °C, 330 °C, and 410.1 °C [57,66,186]. Conversely, solvent-based intumescent coatings (Type 1, Type 2, and Type 3) activate at 188 °C, 285 °C, and 190 °C [67,187]. Epoxy-based coatings (Type 1) exhibit the highest activation temperature at 800 °C [68]. On the other hand, hybrid-based coatings demonstrate a slightly lower activation temperature of 700 °C compared to the epoxy-based coatings [92]. However, different types of coating surely have different char formation temperature ranges, so further research is required for further investigation to understand the trade-offs, including potential reductions in fire resistance time when opting for lower activation temperatures.

7.3. Toxicity

Three primary methods—thermal volatilisation analysis, tubular furnace, and cone calorimeter—are frequently employed to identify gases produced during the thermal decomposition of intumescent coatings. Fourier Transform Infrared (FTIR) analysis is particularly effective for detailed examination of gases during thermal degradation or fire. By continuously sampling the emitted compounds, these techniques can detect a range of gases generated during a fire. Toxic gases such as hydrogen cyanide, nitrogen oxides, carbon monoxide, and carbon dioxide pose serious risks to human health [220,221,222,223]. Some intumescent coatings employ organic binders that emit harmful toxic gases during thermal decomposition [224]. Heisterkamp et al. [225] investigated intumescent coatings’ toxicity by a combination of leaching and ecotoxicological tests. They found that Zn, Ba, Mg, and organic compounds such as phenol and benzaldehyde were present in significant amounts. Silicone-based coatings release fewer toxic gases during degradation compared to traditional organic-based coatings. However, they release some oligomers of silicone and aromatic compounds such as benzene [226]. Halogenated flame retardants are widely recognised for their high effectiveness in the construction industry. However, when they burn during a fire, they emit toxic gases that are hazardous to the environment and human health [227,228,229]. Further research is required to minimise the toxicity level.

8. Conclusions and Recommendations

This study conducted a comprehensive systematic review of existing research on various intumescent coatings, their formulation, manufacturing methods, and their suitability for different applications. In addition, their chemical composition and reaction mechanisms under fire exposure have also been discussed. The findings revealed several points.

• Chemical formulation greatly controls the fire resistance performance of the intumescent coating where the acid donor, carbon or char source, and foaming/blowing agent play a crucial role. Further studies are needed to fully understand the potential of intumescent coatings, optimise their formulations for efficient char activation temperature with reduced flame hazard, and ensure their successful integration into comprehensive fire protection strategies for building materials.
• The manufacturing methods for conventional intumescent coatings, such as water-based, solvent-based, and epoxy-based intumescent coatings, are almost similar, whereas the process is more complex and involves more steps in the case of hybrid intumescent coatings, though they exhibit better performance.
• The fire spread due to the burning of some intumescent coatings at the early stage of fire exposure still poses a great threat by providing a gateway for a secondary fire hazard source. Smoke toxicity for some intumescent coatings can be another hazard. Silicone-based coatings release fewer toxic gases during degradation compared to traditional organic-based coatings. Further research can be conducted to minimise the toxicity level of intumescent coatings. This can be improved by using appropriate acid donors, flame retardant fillers, and binding materials.
• The corrosion and durability of intumescent coatings exposed to corrosive and wet environmental conditions are one of the key issues, which can be improved by applying an appropriate topcoat to the surface of the coating or by incorporating particles as modifiers into intumescent coatings.
• Extensive research has been conducted on the steel and timber structures using intumescent coatings. However, there is limited research on concrete using intumescent coatings. Further research can be conducted to improve the fire resistance and spalling issue of concrete.
• Intumescent coatings can also be explored for additional materials like plastics, solid aluminium panels, ACP (aluminium composite panel) cladding, and cementitious materials for high-rise buildings. A significant study is also required on the environmental aspect of sustainable application.