Waterborne Epoxy Intumescent Coatings: Recent Advances and Future Challenges

1. Introduction

The rapid pace of urban development, rising energy demands, and the widespread construction of high-rise buildings have significantly increased the need for advanced fire-protection strategies and expertise, especially in composite-based structural applications. In recent decades, the expansion of infrastructure, including housing, commercial complexes, and industrial facilities, has heightened the need for robust fire safety measures to mitigate risks and protect occupants and assets [1,2,3]. Moreover, future international regulations are anticipated to prioritise flame-retardant composite systems that not only comply with stringent fire-protection standards but also promote environmental sustainability and safeguard human health [4,5,6,7].

To address these challenges, WEICs have been considered a sustainable and eco-friendly alternative to traditional solvent-based fire-protective coatings [8,9,10,11,12]. Unlike solvent-based systems, which emit high levels of volatile organic compounds (VOCs) and pose environmental and health hazards, waterborne formulations significantly reduce VOC emissions while maintaining robust fire protection. This transition reflects the growing demand for environmentally friendly materials in fire protection, particularly across sectors such as construction, transportation, and the oil and gas industry, where fire risks are notably high [13,14,15].

Intumescence occurs when a charred protective layer forms upon exposure to heat or flame, serving as an insulating shield that reduces heat and combustible-gas transfer. This procedure is vital for delaying structural degradation during fire incidents, providing time for evacuation, and reducing damage [16,17,18]. The performance of WEICs depends heavily on the synergy between three primary ingredients: the acid donor, carbon provider, and gas-releasing agent. Ongoing research focuses on optimising these formulations to enhance fire protection while strengthening mechanical integrity to ensure sustained performance under demanding conditions. The synergy between acid, carbon, and gas sources leads to efficient intumescent barrier formation, reduced heat transfer, limited oxygen ingress, suppressed volatile release, and slowed combustion [19,20].

When the acid, gas, and carbon (char) sources are combined in an intumescent flame-retardant system, they act synergistically to form an effective protective barrier during fire or heat exposure. Upon heating, the acid source, typically a phosphorus-containing compound such as ammonium polyphosphate (APP), decomposes by releasing phosphoric acid. This acid acts as a dehydrating agent, catalysing the carbonisation of the carbon-rich component. As a result, the carbon source, typically a polyhydroxy compound such as pentaerythritol (PENTA), undergoes dehydration, forming a solid, thermally stable carbonaceous char that serves as the structural backbone of the intumescent layer. Simultaneously, the gas source, such as melamine (MEL) or urea, thermally decomposes, releasing non-flammable gases, including ammonia, nitrogen, and water vapour. These gases expand the softening carbon matrix, causing the char to puff and form a thick, multicellular foam structure [21,22].

This expanded intumescent char serves as a crucial barrier between the heat source and the underlying material. It significantly reduces heat transfer by providing thermal insulation, effectively shielding the substrate from high temperatures. In addition, the char layer limits oxygen diffusion to the material’s surface, suppressing oxidation reactions and slowing combustion. It also acts as a physical seal, minimising the release of flammable decomposition gases from the substrate. Altogether, acid-induced charring, carbon backbone formation, and gas-induced expansion result in a robust, thermally protective layer that enhances the material’s fire resistance by delaying ignition, reducing flame spread, and preserving structural integrity during fire exposure [23].

Significant research has focused on enhancing the performance of WEICs by incorporating novel flame-retardant agents, advanced nanomaterials, and combined reinforcement strategies. Novelties such as graphene oxide (GO) [24,25,26], nano titanium dioxide (TiO₂) [27,28,29,30,31], and bio-based fillers [32,33,34,35,36,37,38,39,40,41] have demonstrated their potential to strengthen char structures and configurations by enhancing thermal stability and mechanical performance. Furthermore, researchers are striving to refine curing processes to improve adhesion properties and ensure compatibility with diverse substrates, thereby expanding the applicability of these WEICs across multiple industries [42,43,44]. With the high demand for high-performance fire protection solutions, WEICs have developed as a key technology that balances fire safety with environmental responsibility. Ongoing research and development efforts will be instrumental in shaping the future of fire-retardant coatings, ensuring safer, more resilient infrastructure worldwide.

Recent studies have extensively investigated the flame-retardant mechanisms influencing thermal degradation in polymer-based materials, with a particular focus on intumescent coatings [40,45,46,47,48,49,50], as shown in Figure 1. Given the growing fire-safety challenges in modern infrastructure and industrial applications, there is a pressing need to understand how flame-retardant systems function at the molecular level to enhance fire resistance. Researchers have systematically examined the intricate chemical and physical changes that occur within the polymer matrix under high-temperature and standard fire-exposure conditions [51,52,53,54,55].

The fire resistance of intumescent coatings is primarily governed by three key processes: dehydration, char formation, and the development of a durable, heat-resistant char layer. During the initial phase of thermal degradation, dehydration is vital because it removes water and volatile compounds, thereby limiting the formation of flammable gases that can contribute to combustion. This step is typically promoted by acid catalysts, such as APP, which facilitate crosslinking of carbon-rich compounds and promote the formation of a protective char layer [45,46,56,57,58].

Char formation is a key process in flame retardancy, in which organic materials decompose under heat to form a protective carbon-rich layer. This char acts as both a thermal and physical barrier, restricting heat and mass transfer between the flame and the underlying material. The effectiveness and integrity of the char are primarily influenced by the composition of the intumescent coating system, particularly the inclusion of carbon donors (e.g., PER, phosphoric acid derivatives) and synergistic components (e.g., metal oxides and nanomaterials) [30,59,60,61].

The development of a robust, heat-resistant char layer is essential for providing long-lasting fire protection for substrates. This protective barrier must possess strong mechanical integrity, low thermal conductivity, and oxidation resistance to sustain its effectiveness during prolonged fire exposure. To increase char stability, researchers have incorporated materials such as graphene oxide, nano titanium dioxide, layered silicates, and bio-based fillers, which reinforce char strength and structural cohesion [26,62,63,64,65]. These developments not only improve flame resistance but also enhance the coating’s durability against mechanical wear and environmental conditions.

The molecular structures of flame-retardant compounds largely support the relationships between these physical and chemical processes. Thermal degradation pathways are influenced by the polymer’s backbone architecture, functional groups, and the decomposition behaviour of fire-retardant additives [46,52,56,66,67,68,69]. Sophisticated analytical methods, such as thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), have provided critical insights into these mechanisms, enabling the development of more practical and environmentally friendly intumescent coatings that meet industry standards.

As fire safety regulations become more rigorous, future research will prioritise refining flame-retardant formulations, developing nanocomposite materials, and enhancing the multifunctional capabilities of intumescent coatings. Gaining more profound insights into thermal degradation mechanisms and char formation processes will be essential for developing next-generation fire protection materials that offer superior performance and environmental sustainability.

This review offers a detailed technical analysis of recent developments in flame-retardant systems for WEICs, a dynamic field driven by the growing need for high-performance, environmentally sustainable fire protection solutions in composite-based structural applications. It systematically investigates key flame-retardant mechanisms, highlighting intricate chemical transformations during thermal decomposition, the generation of intermediate compounds, and the kinetics of char formation. These key factors influence the fire resistance of the coatings. Special attention is given to the roles of acid catalysts, carbonising agents, and gas-phase flame retardants in controlling combustion behaviour and improving the fire-protection efficiency of epoxy composite coating systems [70,71,72,73,74,75,76,77,78,79,80,81]. Table 1 compares the synthesis processes of conventional solvent-borne and waterborne epoxy intumescent paint systems.

Beyond the basic mechanisms, this review explores recent advancements and cutting-edge technologies that have significantly enhanced the performance of intumescent coatings. Innovative flame-retardant additives, including graphene oxide, layered silicates, metal-organic frameworks (MOFs), and bio-based reinforcements, have demonstrated significant potential to enhance char formation, mechanical strength, and thermal stability. The integration of nanomaterials and hybrid synergistic systems has become a prominent research focus, providing superior fire resistance through improved barrier properties and catalytic effects [32,52,82,83,84,85,86,87,88,89,90].

This paper also provides a thorough evaluation of structure-property relationships, analysing how the molecular design and composition of flame-retardant additives influence thermal stability, mechanical strength, adhesion performance, and expansion characteristics of intumescent coatings. These aspects are particularly crucial for industries such as aerospace, automotive, and structural engineering, where coatings must withstand high thermal and mechanical stresses while maintaining their fire protection efficiency [19,51,67,91,92,93,94,95].

Beyond performance characteristics, this review provides an in-depth assessment of the environmental implications of WEICs. Given growing concerns about sustainability, toxicity, and biodegradability, waste management is widely discussed. The transition from traditional halogenated flame retardants to eco-friendly phosphorus-, nitrogen-, and silicon-based alternatives is analysed, along with the impact of emerging biodegradable polymers and green synthesis approaches. The potential release of hazardous degradation by-products during combustion is also considered, highlighting the need for low-emission, non-toxic formulations [8,9,96,97,98].

As is well known, epoxy is among the most widely utilised materials in surface coating applications [99]. Epoxy resins serve as vital binders in protective coatings owing to their excellent corrosion resistance, chemical durability, and strong adhesion to various substrates [100,101]. A widely used variant, diglycidyl ether of bisphenol-A (DGEBA), is produced through the reaction of epichlorohydrin and bisphenol-A with a basic catalyst. The chemical structure of DGEBA is depicted in Figure 2. Epoxy resins are defined as compounds or mixtures that contain one or more epoxy (also known as oxirane or ethoxyline) groups - three-membered oxide rings. These resins undergo polymerisation via their epoxide or hydroxyl groups upon cross-linking with a curing agent, forming a robust three-dimensional network.

To provide a quantitative and comparative perspective, this review systematically analyses various flame-retardant formulations under different operating conditions. Empirical data from peer-reviewed studies are synthesised to provide a comprehensive benchmarking analysis evaluating thermal stability, fire resistance rating, char expansion efficiency, and mechanical durability. The merits and limitations of conventional, advanced, and next-generation flame-retardant systems are critically examined, enabling researchers and industry professionals to make informed decisions regarding material selection and performance optimisation.

Ultimately, this review not only brings together current knowledge but also highlights critical research gaps and potential avenues for future research in WEICs. By integrating insights from materials science, polymer chemistry, and fire protection engineering, it provides a systematic framework for developing next-generation flame-retardant materials that balance high-performance fire safety with environmental sustainability, as shown in Figure 3.

The following sections of this review will provide an in-depth technical assessment of the key principles and performance attributes governing WEICs. This assessment will cover material composition, reaction mechanisms, thermal degradation behaviour, and overall fire resistance efficiency. Particular attention will be given to the impact of different flame-retardant additives, including phosphorus-based compounds, nanomaterials, bio-based fillers, and hybrid reinforcements, on the structural integrity, adhesive strength, and effectiveness of char formation in these coatings at elevated temperatures. Furthermore, essential processing parameters, including coating thickness, curing techniques, and dispersion methods, will be examined for their impact on optimising fire protection performance.

To facilitate a structured comparative assessment, this review will systematically analyse experimental data from peer-reviewed studies, industrial case reports, and standardised fire resistance evaluations. Critical performance metrics, including LOI, TGA, cone calorimetry, and UL-94 classifications, will be used to evaluate thermal stability, expansion efficiency, and combustion suppression across different coating formulations [102,103,104,105]. By adopting a data-driven approach, this analysis aims to provide a quantitative comparison of the advantages and limitations of various flame-retardant systems, offering key insights for selecting the most suitable materials for specific fire protection requirements.

The final section will propose strategic directions to overcome current limitations and advance this field. Key factors will include enhancing long-term durability, improving industrial applicability, and optimising cost-effectiveness. The discussion will also emphasise the importance of sustainability and regulatory compliance, advocating for the development of low-toxicity, biodegradable, and halogen-free formulations that align with evolving environmental standards. Additionally, emerging innovations such as intelligent flame-retardant technologies and self-healing coatings will be discovered for their potential to revolutionise fire-resistant polymer coatings.

2. Review of Intumescent Fire Protective Coatings

In the last ten years, intumescent coating technologies have been extensively reviewed through a wide range of scientific and engineering lenses, with key contributions outlined in Table 2. Taken together, these studies map the current landscape of intumescent coatings by analysing formulation concepts, fire-retardant mechanisms of action, thermal barrier effectiveness, and performance requirements across different application sectors. Despite notable advances, the literature repeatedly highlights persistent limitations and unresolved research questions, particularly regarding formulation refinement, the lack of harmonised performance evaluation frameworks, and uncertainties regarding long-term service reliability [105].

Despite their environmental and occupational safety advantages, WEICs generally exhibit lower durability than solvent-based systems under prolonged environmental exposure as shown in Table 3. Quantitative studies have demonstrated that accelerated UV ageing, cyclic humidity exposure, thermal cycling, and salt-spray conditions can significantly reduce adhesion strength, char expansion ratio, and fire-protection efficiency. For example, adhesion retention of WEICs after accelerated ageing may decrease from approximately 95% to below 70%, while water uptake may increase from 3-5 wt.% to 8-15 wt.% depending on formulation chemistry and crosslinking density. Furthermore, post-ageing cone calorimetry studies reported increases in peak heat release rate (PHRR) of approximately 10-40% due to degradation of the intumescent network and reduced char integrity. The incorporation of graphene oxide, nano-TiO₂, layered silicates, and reactive phosphorus-nitrogen systems has shown promise in mitigating these durability-related degradations by enhancing barrier properties and reinforcing char structures [106,107,108,109].

A recurring theme in formulation-focused reviews is the difficulty of achieving an optimal balance between flame-retardant performance and essential coating properties, including mechanical robustness, resistance to environmental exposure, and processing reliability. Deficiencies, including inadequate water resistance, weak interfacial bonding to steel substrates, structural instability of the expanded char, and performance degradation under repeated thermal, humid, and corrosive conditions, continue to constrain existing systems [110]. At the same time, many authors highlight limitations in current fire-testing approaches, noting the lack of standardised methods that reliably translate laboratory-scale results into predictions of real-fire behaviour and long-term in-service performance [111]. Moreover, durability-related issues, including ageing phenomena, weathering effects, additive migration, and progressive deterioration of the intumescent char, are increasingly recognised as key barriers to widespread deployment in demanding industrial environments, including civil infrastructure, petrochemical facilities, and offshore installations [112,113,114,115,116].

Earlier review efforts primarily focused on solvent-borne intumescent coatings, reflecting their favourable film-forming properties, strong adhesion, and comparatively consistent fire-protection performance. In contrast, contemporary reviews document a clear and ongoing transition toward water-based intumescent systems. This shift is primarily attributed to stricter regulatory controls on volatile organic compound (VOC) emissions, alongside growing awareness of environmental, health, and safety risks associated with solvent-based formulations. Concurrently, rising expectations for sustainable and low-impact fire-protection solutions have further stimulated interest in waterborne alternatives [117,118,119].

Despite their environmental and occupational safety advantages, water-based intumescent coatings are often reported to underperform traditional solvent-based systems in terms of fire resistance and long-term durability. Consequently, recent review articles increasingly advocate adopting advanced materials engineering approaches, such as hybrid polymer matrices, multifunctional and nanoscale fillers, and synergistic flame-retardant systems, to overcome these deficiencies without compromising regulatory compliance. Collectively, these insights reinforce the need to continue investigating innovative formulation strategies, more representative performance assessment techniques, and durability-focused design concepts to enable the development of next-generation intumescent coating technologies.

The majority of existing reviews focus on two principal themes: fire-testing methodologies and the chemical mechanisms underlying intumescence. For water-based intumescent coatings, establishing reliable, repeatable, and representative fire-testing approaches is particularly critical, as their fire performance is strongly influenced by drying behaviour, residual moisture, and film-formation processes. Many laboratory-scale fire tests rely on simplified experimental configurations that limit reproducibility, general applicability, and predictive modelling, especially for systems whose performance is sensitive to ambient humidity and curing conditions waterborne [110,116]. In addition, the relevance of standardised fire-testing protocols has been questioned, as real fire scenarios often involve non-uniform heat fluxes and transient temperature profiles that deviate significantly from prescribed testing conditions. These differences can substantially influence expansion characteristics, char stability, cohesion, and adhesion in water-based intumescent coatings [120]. Consequently, the effects of heating rate, thermal intensity, and exposure duration on the fire-protective behaviour of waterborne intumescent systems require more systematic and comprehensive investigation [121].

The review by Fu et al. highlights significant progress in inorganic-dominant intumescent coatings, particularly in improving thermal stability and char robustness. However, key gaps remain in understanding structure-property relationships, ensuring long-term durability, integrating inorganic systems with water-based binders, and establishing reliable fire-testing methodologies representative of real-fire scenarios [122].

Alongi, Han, and Bourbigot provide a thorough review of the development of intumescence for flame-retardant applications, highlighting both conventional methods and recent innovations, particularly those driven by novel chemical approaches and nanotechnology. The review covers core principles of intumescence, including chemical composition and thermal/rheological behaviour, presents new insights from modelling and simulation, and compares traditional coatings with emerging systems across diverse polymer types and substrates, such as metals, wood, plastics, films, and fabrics, including applications using layer-by-layer assembly [123].

Alongside developments in fire-testing methodologies, significant progress has been made in formulation engineering. Numerous studies have explored a wide range of intumescent components, including acid sources, carbonific agents, blowing agents, polymeric binders, fillers, and nano-scale additives, as well as their individual and synergistic effects on char formation and fire resistance [102,103,104,105]. In water-based intumescent coatings, the presence of water further complicates the formulation, necessitating careful selection and optimisation of binders, dispersants, and stabilising agents to ensure homogeneous dispersion and long-term stability. Precise control over the sequence and kinetics of thermochemical reactions is essential to promote effective char expansion. At the same time, additional emphasis must be placed on char coherence, structural integrity, and thermal stability during fire exposure [114].

Despite these advancements, several challenges remain specific to water-based intumescent coatings. Issues related to moisture sensitivity, resistance to environmental ageing, and mechanical durability continue to restrict their broader application, particularly in outdoor or harsh industrial environments. Compared with solvent-based systems, waterborne intumescent coatings generally exhibit greater susceptibility to degradation under prolonged exposure to humidity, cyclic wet-dry conditions, and mechanical stresses. As highlighted in multiple reviews, further research is required to enhance the weather resistance, adhesion strength, abrasion resistance, and long-term mechanical performance of water-based intumescent coatings without sacrificing their fire-protective effectiveness [103,105,113]. Addressing these limitations is critical for advancing water-based intumescent coatings as environmentally compliant, durable, and high-performance fire-protection solutions suitable for both indoor and industrial applications.

3. Technical Analysis of Flame-Retardant Mechanisms

3.1. Fundamental Principles of Intumescence in Waterborne Epoxy Coatings

Intumescence in flame-retardant coatings is achieved through a sequence of interrelated physical and chemical reactions that, when subjected to elevated temperatures, combine to form a highly insulating protective layer. These coatings typically consist of three key components: an acid source, a carbon donor, and a gas-generating agent, each contributing to char formation and expansion. The acid source, often APP or other polyphosphates, initiates dehydration reactions that extract water molecules from the carbon donor. This process helps minimise the release of flammable volatiles and facilitates polymerisation and cross-linking, ultimately yielding a robust carbon-rich structure [44,47,57,124,125,126,127].

The carbon donor, typically PENT or its derivatives, undergoes controlled thermal decomposition to form a dense char layer that serves as a thermal insulator and a barrier to oxygen infiltration. The strength and integrity of this char are critical to the coating’s fire protection performance, influencing its thermal stability, mechanical resilience, and oxidation resistance. Simultaneously, the gas-releasing component, such MEL, decomposes at high temperatures, releasing noncombustible gases, including ammonia and nitrogen. This reaction causes the softened char to expand into a lightweight, porous structure, further enhancing its insulating properties and significantly minimising heat transfer to the underlying material [64,128,129,130].

The interaction of these components results in the development of an adaptive intumescent barrier that reduces ignition risk, slows flame spread, and limits heat exposure to the substrate. To enhance the effectiveness of this protective system, recent studies have focused on advanced formulations incorporating nanomaterials such as graphene oxide and carbon nanotubes, as well as bio-based flame retardants. These additives strengthen the char structure, improve thermal stability, and offer eco-friendly alternatives to conventional flame-retardant solutions. Additionally, incorporating layered silicates and other nanostructured fillers has been shown to reinforce the char structure, improving its resistance to mechanical and thermal degradation and thereby significantly enhancing the fire protection performance of intumescent coatings. [23,131,132,133].

3.2. Chemical Mechanisms and Thermal Degradation Pathways of Intumescent Coating

The thermo-oxidative degradation of WEICs proceeds via several concurrent reaction pathways. The initial step in epoxy resin degradation is the cleavage of ether linkages in the polymer backbone. TGA demonstrates that waterborne epoxy matrices undergo two primary stages of degradation: the first involves weight loss due to the evaporation of absorbed water and the commencement of resin curing degradation (200 to 300 °C), followed by a substantial weight loss at higher temperatures (300 to 500 °C) associated with char formation [134,135,136,137].

Chemical reactions induced by thermal degradation have been studied using techniques such as Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and mass spectrometry [138]. In many systems, degradation begins with the rapid depolymerisation initiated by acid-catalysed cleavage. Subsequent crosslinking and char formation involve complex condensation reactions. Specifically, the reaction mechanisms (As shown in Figure 4) are presented as follows [17,139,140]:

R-OH + H₂PO₄⁻ → R-O-PO₃H⁻ + H₂O

R-O-PO₃H⁻ + Melamine → Crosslinked char structure + N₂↑ + H₂O

Changes in the chemical composition of the acid source or carbon source can influence the yield and stability of the char. For instance, phosphorus-nitrogen synergistic systems have been reported to yield greater char yield, with studies comparing APP with novel organophosphorus compounds demonstrating up to a 50% improvement in char stability under oxidative conditions [141,142,143]. The degradation kinetics under controlled thermal-ramping conditions typically follow first-order reaction models; however, recent studies have reported second-order effects under non-isothermal conditions induced by rapid heating [46,144].

Organic intumescent coatings achieve fire protection through a coordinated system composed of four essential components: a polymeric binder, an acid-generating compound, a char-forming agent, and a gas-releasing species. Additional modifiers are commonly used to tailor specific properties, such as thermal resistance, mechanical stability, and coating integrity. A simplified representation of the intumescence process is presented in Figure 5, which also outlines representative reactions involving widely used constituents such as APP, PENT, and MEL. The temperature intervals associated with each transformation stage are strongly dependent on formulation parameters and experimental conditions [106].

Under ambient conditions, intumescent coatings function similarly to conventional protective or decorative coatings. Upon thermal exposure, the acid precursor (e.g., APP) decomposes while the polymer matrix softens, forming a viscous, deformable phase. Simultaneously, thermal breakdown of the blowing component (e.g., MEL) generates non-flammable gases that become entrapped within the softened matrix, initiating volumetric expansion. These gases nucleate into fine cells, typically tens of micrometres in size, resulting in foaming and substantial thickening of the coating layer.

At the same time, the released acidic species catalyse esterification reactions between the acid source and the carbon precursor (e.g., PER), followed by progressive carbonisation as the temperature increases. Binder decomposition can further contribute to char development. The resulting expanded structure forms a low-density, porous carbonaceous layer with reduced thermal conductivity, thereby impeding heat transfer to the substrate. Expansion ceases once gas generation is complete or when increased char viscosity prevents further bubble retention. At elevated temperatures, the consolidated multicellular char may undergo additional exothermic degradation or oxidation, accompanied by the release of CO and CO₂ [46].

Advanced computational models have been developed to simulate char formation. These models combine mass and heat-transfer equations to predict the evolution of char thickness and porosity. Quantitative assessments show that nanofiller-based coatings have lower effective thermal conductivity, typically 0.1-0.3 W/(m·K), compared to traditional coatings (around 0.35-0.5 W/(m·K)), consistent with experimental data demonstrating enhanced thermal insulation [141,142,143].

3.3. Structure-Property Relationships of Flame Retardants

The effectiveness of flame retardants in waterborne epoxy coatings is closely linked to their molecular structure and how well they disperse within the polymer matrix. Recent research has concentrated on modifying the chemical properties of additive components to enhance both the flame resistance and mechanical strength of the resulting coating.

The incorporation of hyperbranched polymers and specially designed reactive oligomers has led to enhanced compatibility with epoxy resin. This compatibility is crucial for achieving a homogeneous microstructure, whereby the flame-retardant additives are uniformly dispersed and interact both physically and chemically with the matrix. Structure-property relationship studies have demonstrated that:

• Increasing the phosphorus content in the additives generally results in a higher char yield, as verified by TGA studies that report residual char percentages increasing from 15% to 35% for phosphorus-rich systems [148,149,150,151].
• The use of nanostructured fillers, such as montmorillonite clay modified with organic cations, leads to improvements in both barrier properties and mechanical integrity. Transmission electron microscopy (TEM) images provide compelling evidence that well-exfoliated clay platelets form tortuous pathways that effectively restrict mass and heat transfer. This mechanism can be quantitatively assessed by a reduction in the effective diffusion coefficient of up to 40% relative to unmodified coatings [152,153,154].
• Unlike additive systems, reactive flame retardants offer the potential for covalent bonding between the flame-retardant molecule and the epoxy matrix. This chemical linkage is shown to significantly retard the mobility of degradation intermediates, thereby enhancing char formation and stability.

A crucial aspect of structure-property analysis in modern coatings research is the use of advanced spectroscopic and calorimetric techniques. For instance, coupling FTIR with TGA/DSC allows for real-time monitoring of both chemical changes and the energy profile during thermal degradation. Recent studies have shown that incorporating specific functional groups, such as phosphonate and phosphinoxide moieties, can reduce the activation energy for charring reactions by 15-20 kJ/mol compared to traditional phosphate groups [155,156,157,158,159,160].

3.4. Breakthrough Technologies and Novel Approaches

In recent years, innovation in flame-retardant materials has accelerated markedly as researchers seek solutions that meet increasingly rigorous environmental and health standards while maintaining strong fire resistance. Within this context, bio-derived and hybrid organic-inorganic flame-retardant systems have gained prominence as advanced alternatives to conventional halogen-based and exclusively mineral formulations. These emerging systems are widely regarded as capable of delivering reliable fire-protection performance while mitigating toxicity, enhancing sustainability, and complying with regulatory constraints.

Unlike halogenated flame retardants, which are often linked to the formation of hazardous and corrosive combustion products, bio-based flame-retardant components are generally associated with lower ecological impact and greater compatibility with sustainable material life cycles. Consequently, recent investigations have prioritised incorporating naturally sourced macromolecules, such as chitosan, lignin, starch, and phytic acid, into flame-retardant architectures. When used alongside established phosphorus-containing additives, these bio-derived constituents contribute to condensed-phase flame-retardant processes, leading to increased char production, improved structural integrity of the protective layer, and more effective inhibition of heat and mass transfer during combustion [157,158,159,160].

Among these materials, chitosan-based systems have received significant attention due to their nitrogen-rich structure, favourable film-forming characteristics, and inherent tendency to promote char formation. In combination with phosphorus-based flame retardants, chitosan facilitates the formation of robust intumescent char through synergistic dehydration, crosslinking, and expansion mechanisms. Likewise, lignin-derived additives, characterised by their aromatic frameworks and high carbon content, serve as efficient carbon precursors, strengthening the resulting char layer upon interaction with acidic phosphorus species. Such hybrid formulations frequently demonstrate enhanced flame-retardant performance compared to single-component systems, while also allowing for reduced additive concentrations and minimising adverse effects on mechanical behaviour [165].

In addition to improved fire resistance, bio-based and hybrid organic-inorganic flame-retardant systems offer notable environmental compatibility and sustainability advantages. Replacing a portion of petroleum-based or mineral fillers with renewable bio-resources lowers overall material intensity and ecological burden. Furthermore, many studies report a significant reduction in the evolution of toxic smoke and the release of corrosive gases during combustion, a feature particularly valuable for applications in construction, transportation, and protective coating technologies, where occupant safety and exposure risks are of paramount concern [166,167].

Taken together, these developments reflect a broader transition in flame-retardant material design from traditional, single-purpose additives toward multifunctional, synergistic systems that simultaneously address fire safety, environmental impact, and material performance. As outlined in Table 4, the expanding body of research indicates that bio-based and hybrid organic-inorganic flame retardants represent a viable and forward-looking strategy for developing safer, more sustainable, and high-efficiency fire-protection solutions, while also providing a foundation for further advances through molecular engineering and composite system optimisation.

An emerging strategy that has gained increasing attention in recent flame-retardant research involves incorporating metal-organic frameworks (MOFs) as functional additives into polymeric and coating-based fire-protection systems. Owing to their crystalline architecture, extensive internal surface areas, and highly adjustable pore structures, MOFs offer a level of structural and chemical tunability that is not readily achievable with conventional flame-retardant fillers. These characteristics allow MOFs to interact with thermal degradation products at a molecular scale, thereby influencing flame-retardant pathways in a highly controlled manner.

Mechanistic investigations reported in the recent literature indicate that MOFs can exert flame-retardant action through multiple complementary mechanisms. During thermal exposure, the porous framework of MOFs enables the adsorption and confinement of volatile degradation products and reactive radicals, thereby reducing the availability of combustible intermediates in the gas phase. Simultaneously, the metal centres and organic linkers within MOFs have been shown to promote condensed-phase reactions, thereby facilitating catalytic charring that enhances carbonaceous residue formation. This dual functionality contributes to the formation of a denser, more thermally stable char layer, which plays a critical role in shielding the underlying substrate from heat and flame propagation [168,169,170,171].

In addition to their catalytic role, MOFs also provide a pronounced physical barrier effect. As part of the coating matrix, well-dispersed MOF particles increase the tortuosity of diffusion pathways for oxygen and flammable gases, thereby restricting mass transfer between the flame zone and the underlying material. This barrier effect, combined with improved char cohesion, effectively suppresses heat release and delays ignition. Such synergistic behaviour distinguishes MOFs from traditional mineral fillers, which often rely predominantly on physical dilution or endothermic decomposition mechanisms [172,173,174].

Quantitative performance assessments further support the effectiveness of MOF-based flame-retardant systems. Experimental data from multiple studies demonstrate that coatings incorporating optimised MOF formulations exhibit measurable improvements in fire resistance, as reflected in higher limiting oxygen index (LOI) values than those of reference or unmodified systems. These improvements, summarised in Table 5, highlight the sensitivity of flame-retardant performance to MOF composition, loading level, and interfacial compatibility with the host matrix [168,169,170,171,172,173,174].

Overall, the growing body of evidence suggests that MOFs are a promising class of multifunctional flame-retardant additives that integrate catalytic, adsorption, and barrier effects within a single material platform. Their structural versatility offers significant opportunities to tailor flame-retardant behaviour through the rational design of metal nodes, organic linkers, and pore environments. As a result, MOF-based systems are increasingly regarded as a forward-looking approach for advancing next-generation fire-protective coatings and composite materials, particularly where high efficiency and low additive loading are required.

Despite significant progress in advanced flame-retardant systems, several technical constraints continue to impede their widespread implementation, particularly in waterborne epoxy-based formulations. One of the most persistent challenges arises from the difficulty of achieving homogeneous dispersion of nanomaterials within the polymer matrix. Owing to their high surface energy and strong interparticle attractions, nanofillers are prone to agglomeration, which undermines their effective interaction with the surrounding epoxy network and limits their contribution to flame-retardant performance.

In addition to aggregation, interfacial incompatibility between nanomaterials and the aqueous epoxy environment further complicates dispersion stability. Poor interfacial bonding can lead to phase separation, sedimentation during storage, and non-uniform microstructures after curing, all of which negatively affect both fire resistance and mechanical properties. To address these issues, considerable research effort has focused on developing surface modification strategies to enhance the chemical affinity between nanomaterials and waterborne epoxy matrices. Such approaches include grafting functional groups onto nanofiller surfaces, tailoring surface polarity, and introducing reactive moieties that can participate in epoxy cross-linking reactions.

In parallel with these chemical modification strategies, advances in processing techniques have also been investigated to enhance the dispersion of nanomaterials. High-shear mixing, ultrasonic treatment, and controlled in situ incorporation methods have been reported to improve the initial distribution and reduce agglomerate size. However, maintaining dispersion stability throughout formulation, storage, and application remains a significant challenge, particularly in large-scale or industrial coating processes, where processing conditions are less tightly controlled.

Similar to reactive flame retardants, silane-based coupling agents have emerged as a promising means to improve interfacial compatibility between inorganic nanomaterials and organic epoxy matrices [175,176,177]. By forming covalent or hydrogen-bonded linkages at the filler-matrix interface, silanes can enhance dispersion stability and promote more effective stress transfer and char formation during combustion. Nevertheless, translating these laboratory-scale successes into consistent uniform dispersion at industrial production scales remains a formidable challenge. Factors such as processing time, equipment limitations, formulation viscosity, and cost constraints must be carefully balanced to ensure scalability and reproducibility [178,179,180].

Overall, these unresolved issues highlight the necessity for integrated strategies that combine material surface engineering with process optimisation. Achieving reliable dispersion of nanomaterials in waterborne epoxy systems will be critical not only for maximising flame-retardant efficiency but also for ensuring long-term coating stability and performance under practical service conditions. Addressing these challenges remains an active area of research and a key prerequisite for the commercial adoption of next-generation, nanostructured flame-retardant coatings.

4. Applications and Performance Evaluation

4.1. Commercial and Industrial Relevance

The practical deployment of WEIC extends across multiple industries, including construction, transportation, and electronics. For instance, in the construction sector, these coatings are used to provide fire protection to steel structures and concrete elements. In transportation, they safeguard the cabin interiors and critical components from fire-induced damage. The electronics industry benefits particularly from coatings that maintain performance at limited thicknesses while offering robust fire retardancy, thereby protecting circuit boards and components.

The performance of these coatings has typically been assessed through standardised flammability tests, including the UL-94 vertical burn test, cone calorimetry, and LOI measurements. Although LOI, UL-94 vertical burning, TGA, and cone calorimetry are widely used to evaluate flame-retardant performance, these methods primarily function as laboratory-scale screening tools and do not fully represent real-fire scenarios involving structural loading, hydrocarbon flame exposure, non-uniform heat flux, and prolonged thermal attack. Therefore, large-scale fire-resistance standards such as ASTM E119, UL 1709, BS 476, ISO 834, and EN 13381 are essential for evaluating the practical fire endurance of WEICs applied to structural steel substrates as presented in Table 6. The relationship between small-scale combustion tests and large-scale structural fire performance remains an important research challenge requiring further standardisation and correlation studies [106,110,112].

Cellulosic and hydrocarbon fires exhibit different fire behaviors and therefore require different intumescent coating design strategies. Cellulosic fires, commonly associated with wood, paper, and building contents, follow standard fire curves such as ISO 834 and develop relatively gradually, reaching approximately 945-1000 °C within 2 hours. Under these conditions, waterborne epoxy intumescent coatings form an expanded insulating char layer through the synergistic action of an acid source, carbon source, and blowing agent to protect steel substrates from heat transfer [110].

In contrast, hydrocarbon fires occur in petrochemical and offshore facilities involving fuels such as petrol, diesel, and natural gas. These fires follow hydrocarbon fire curves (e.g., UL 1709) with extremely rapid temperature rise up to 1000-1100 °C within minutes. Such severe exposure requires intumescent coatings with higher thermal stability, stronger char integrity, improved adhesion, and resistance to thermal shock and erosion. Therefore, hydrocarbon-grade systems commonly incorporate epoxy-based formulations, ceramic-forming fillers, nanomaterials, expandable graphite, and multilayer protective systems consisting of primers, intumescent layers, and weather-resistant topcoats. Consequently, intumescent coating formulations must be specifically tailored according to the expected fire scenario and service environment.

Recent publications have provided quantitative evidence that coatings formulated with advanced flame-retardants can meet or exceed the performance criteria set by international standards. For example, coatings incorporating hybrid organic-inorganic systems have achieved UL-94 V-0 ratings while simultaneously reducing heat release rates by approximately 30-50% compared to control formulations, as shown in Table 7 [181,182,183,184,185,186].

4.2. Quantitative Comparison of Flame Retardant Systems

A comparative analysis of the quantitative data summarised in Table 8 reveals several consistent performance trends across different flame-retardant systems, particularly in the interplay among limiting oxygen index (LOI), heat release rate (HRR), char yield, and time-to-ignition metrics [47,184,185,186,187]. Collectively, these parameters provide insight into the underlying mechanisms of flame retardancy and their associated trade-offs.

A general trend across the reviewed systems is that formulations with elevated LOI values exhibit greater resistance to sustained combustion. Higher LOI values are commonly associated with enhanced condensed-phase activity, particularly increased char formation and improved char stability. However, an increase in LOI does not always correspond to a proportional reduction in peak HRR. In several cases, materials with relatively high LOI values still exhibit moderate-to-high HRR peaks, indicating that although ignition resistance and flame sustainability improve, the intensity of heat release following ignition may remain significant. This divergence highlights the limitation of relying on LOI as a standalone performance indicator.

In contrast, systems that exhibit substantial reductions in peak HRR often incorporate additives or fillers that enhance physical barrier effects or suppress gas-phase radicals. These formulations typically restrict heat feedback to the substrate and slow fire growth once ignition has occurred. However, such reductions in HRR are not always accompanied by high LOI values, suggesting that mechanisms that suppress heat release may not necessarily enhance ignition resistance to the same extent. This observation underscores a fundamental trade-off between ignition control and fire growth suppression.

Char yield data further support these interpretations. Higher residual char percentages generally correlate with increased LOI values and improved post-ignition protection, as the formation of a continuous, cohesive char layer limits heat and mass transfer. Nevertheless, high char yield alone does not guarantee low HRR if the char structure lacks sufficient mechanical integrity or thermal stability. In some formulations, porous or cracked char layers allow continued volatilisation and oxygen ingress, resulting in sustained heat release despite high char residue.

Time-to-ignition measurements provide additional context to these trade-offs. Formulations that significantly delay ignition often incorporate components that absorb heat, release inert gases, or promote early-stage char formation. These systems typically show improvements in LOI but may not achieve the lowest HRR values. Conversely, materials optimised for HRR suppression may ignite relatively early but exhibit slower fire growth rates thereafter. This inverse relationship further reinforces the need for multi-parameter evaluation when assessing flame-retardant performance.

Overall, the data presented indicate that no single flame-retardant strategy simultaneously maximises LOI, minimises HRR, increases char yield, and delays ignition. Instead, optimal fire performance is achieved through synergistic formulation design that balances ignition resistance, heat release suppression, and char stability. These findings highlight the importance of integrated performance assessment and provide a clear rationale for the continued development of multifunctional flame-retardant systems that address multiple fire-behaviour criteria simultaneously [187,188,189,190].

These formulations highlight the integration of advanced additives, whether through synergistic phosphorus-nitrogen interactions, nano clay incorporation, or MOF enhancement, resulting in statistically significant improvements over conventional systems. In numerous cases, incorporating a reactive flame-retardant component, rather than a simple additive, has demonstrated improved adhesion to the epoxy matrix and greater durability under thermal cycling.

Moreover, the adoption of in situ analytical tools, such as high-speed thermomechanical analysis (TMA) and real-time TGA-FTIR coupling, has allowed researchers to derive kinetic rate constants and activation energies. For instance, a recent study reported an activation energy of approximately 85 kJ/mol for char formation in MOF-enhanced formulations, compared with 100 kJ/mol for conventional systems, indicating a modified degradation pathway that favours more rapid char formation under fire scenarios [46,191,192,193,194,195].

4.3. Case Studies and Field Applications

Several field studies have demonstrated the commercial viability of advanced flame-retardant coatings. A notable case study in the European construction industry compared the performance of traditional solvent-based coatings with that of WEICs enhanced with bio-based flame retardants. The study found that the waterborne variant not only achieved flame-retardant performance comparable to or better than the solvent-based variant but also significantly reduced VOC emissions, thereby aligning with the latest environmental directives [196,197,198,199,200,201].

In automotive applications, intumescent coatings have been utilised for fire protection of steel components in engine compartments and structural elements of high-speed trains. Accelerated ageing and fire endurance tests indicate that formulations with optimised nano-additives exhibit superior resistance to microcracking and char spallation. Such performance is critical to ensure that the protective barrier maintains its integrity under high thermal stress.

Beyond industrial applications, emerging research is investigating the use of these coatings in advanced electronics packaging. In microelectronic devices, where heat dissipation is critical, the challenge lies in designing ultra-thin coatings that can form an effective char barrier without compromising electrical insulation. Recent pilot studies have demonstrated that flame-retardant additives based on silicon-phosphorus complexes, when incorporated into nanolayered coatings, can achieve both high LOI values and excellent dielectric properties.

5. Challenges and Environmental Considerations

5.1. Technical Limitations and Processing Challenges

While recent advances in flame-retardant formulations for WEICs are promising, several technical challenges remain. A primary concern in scaling these innovations for industrial production is ensuring uniform dispersion of nano-additives in the epoxy resin. In many cases, high surface energy and a tendency toward nanoparticle agglomeration can lead to inhomogeneities that adversely affect both mechanical properties and flame-retardant performance. Techniques such as high-shear mixing, ultrasonication, and the use of dispersing agents have been developed to mitigate these issues. Yet, reproducibility remains a challenge when transitioning from the laboratory to industrial scales, as shown in Figure 6.

Another technical limitation concerns the durability of the char layer under repeated thermal cycling and mechanical stress. Although enhanced formulations show robust initial performance in standard fire tests, real-world conditions often involve long-term exposure to fluctuating temperatures, moisture ingress, and sustained mechanical loads. Under such conditions, the integrity of the char layer may become compromised. Long-term durability studies using accelerated weathering and cyclic thermal analysis have shown that adding reactive flame retardants that chemically anchor to the epoxy matrix can improve char adhesion; however, the optimal formulation balance remains under investigation.

Beyond laboratory-scale formulation optimisation, manufacturability and industrial scalability remain critical barriers to the commercial implementation of WEICs. Parameters such as shelf life, sedimentation stability, viscosity control, curing consistency, coating-thickness uniformity, sprayability, repairability, and batch-to-batch reproducibility significantly influence coating quality and fire-protection reliability. Nanofiller agglomeration during storage may result in sedimentation and inconsistent thermal performance, whereas incomplete curing and residual water entrapment can weaken adhesion and reduce char integrity during fire exposure. In practical field applications, additional challenges include substrate preparation, environmental humidity during application, maintaining coating thickness, and quality assurance in large-scale spraying operations. Consequently, future studies should prioritise scalable formulation strategies that maintain long-term storage stability, consistent rheological behaviour, and reproducible fire performance under industrial manufacturing and on-site application conditions [174,175,176].

5.2. Environmental Impact: Toxicity and Biodegradability

Environmental considerations are of paramount importance in the selection and formulation of flame retardants. Historically, many effective flame retardants have been based on halogenated compounds, which are now under scrutiny due to their persistence in the environment, bioaccumulative potential, and toxicological concerns. The transition to waterborne systems naturally reduces VOC emissions; however, the flame-retardant additives themselves must also be evaluated for their environmental impact.

Recent studies have emphasised the use of both in vitro assays and animal models to evaluate the ecotoxicological profiles of phosphorus-based and nitrogen-containing flame retardants. For example, quantitative structure-activity relationship (QSAR) models have been utilised to assess the toxicity of newly developed organophosphorus compounds. Research findings suggest that incorporating bulky substituents or bio-based functional groups can help reduce toxicity concerns while preserving the effectiveness of flame retardancy [202,203,204,205,206]. Furthermore, the biodegradability of flame retardants is a growing concern. Recent work has demonstrated that bio-based flame retardants, derived from renewable sources such as lignin or chitosan, exhibit improved biodegradation profiles compared to their synthetic counterparts. However, challenges remain in matching their thermal performance with that of traditional compounds.

In addition to toxicity and biodegradability, the waste management of spent coatings must be addressed. Current research focuses on environmentally benign recycling or disposal mechanisms [207,208,209,210,211,212,213]. Some promising approaches involve designing intrinsically recyclable flame-retardant systems via reversible chemical linkages. Pilot-scale studies have successfully demonstrated that coatings with dynamic covalent bonds can be reprocessed, significantly reducing hazardous waste flows. These research findings collectively underscore progress in developing recyclable flame-retardant coatings and composites to minimise dangerous waste and support sustainable material management practices.

Although WEICs significantly reduce VOC emissions compared with conventional solvent-borne coatings, sustainability assessment should also include smoke toxicity, toxic gas evolution, biodegradability, ecotoxicity, and life-cycle environmental impacts as shown in Table 9. Conventional solvent-borne intumescent coatings may contain VOC contents exceeding 350-450 g/L, whereas advanced WEICs commonly exhibit VOC levels below 50-100 g/L. In addition, phosphorus-nitrogen flame retardants and bio-based additives, such as lignin and chitosan, generally exhibit lower smoke toxicity and improved biodegradation profiles compared with halogenated systems. However, certain advanced additives, including MOFs, nanostructured fillers, and bioengineered flame retardants, may introduce trade-offs in synthesis complexity, economic feasibility, large-scale manufacturability, recyclability, and end-of-life management. Consequently, future WEIC development should integrate quantitative life-cycle assessment (LCA), ecotoxicity analysis, and circular economy principles into flame-retardant formulation design [6,7,218,219,220,221,222,223,224,225].

5.3. Quantitative Environmental Performance Comparisons

To comprehensively compare the environmental performance of various flame-retardant systems, several metrics have been proposed. These include emission factors, biodegradation half-lives, and ecotoxicity indices. Recent comparative studies have found that [211,212,213]:

• Emission Factors: Waterborne epoxy intumescent coatings typically exhibit a 40-60% reduction in VOC emissions compared to traditional solvent-based formulations. When enhanced with bio-based flame retardants, the net environmental burden is further reduced by approximately 20% due to lower harmful emissions.
• Biodegradation: Bio-based flame retardants such as those derived from lignin show half-lives that are 30 to 50% shorter than conventional synthetic counterparts, indicating a higher rate of natural decomposition under conducive conditions.
• Toxicity Indices: In vitro cytotoxicity tests have revealed that formulations engineered with modified phosphorus-nitrogen systems exhibit lower toxicity indices, with a reduction in reactive oxygen species generation of up to 35% relative to halogenated systems.

These quantitative assessments underscore the importance of integrating environmental metrics into the design and selection of flame-retardant systems. Nonetheless, trade-offs often exist between maximising flame retardancy and minimising adverse environmental impacts, necessitating further research into multi-criteria optimisation.

5.4. Knowledge Gaps and Research Priorities

Despite significant progress in sustainable flame-retardant coatings, several critical knowledge gaps remain unresolved [102]. Future research should focus on understanding the balance between char expansion, char strength, and coating adhesion, since excessive expansion may weaken char integrity and promote cracking or delamination during fire exposure [101,105] The effects of residual water and incomplete curing in waterborne systems also require further investigation, as moisture retention may alter thermal degradation behaviour, reduce crosslink density, and negatively affect long-term fire performance.

Another important challenge is clarifying why some formulations significantly improve the LOI but show only a limited reduction in PHRR, indicating that ignition resistance alone may not accurately predict real-fire behaviour [226]. More comprehensive multi-parameter fire testing and mechanistic studies are therefore required [105]. In addition, long-term durability and environmental ageing remain major concerns, as moisture ingress, UV exposure, oxidation, hydrolysis, additive migration, and interfacial degradation may substantially reduce coating effectiveness over time.

Advanced additives such as nanomaterials and MOFs offer promising improvements in smoke suppression, thermal stability, and char reinforcement; however, their long-term ecotoxicity, environmental persistence, recyclability, and end-of-life behaviour remain insufficiently understood. Future sustainable flame-retardant systems should therefore integrate fire safety, environmental sustainability, durability, scalability, and circular-economy considerations through interdisciplinary research approaches.

6. Outlook and Future Research Directions

6.1. Emerging Trends and Breakthrough Opportunities

The current trajectory in the development of flame-retardant systems for waterborne epoxy coatings is marked by a transition towards multi-functional, sustainable, and high-performance formulations. Several emerging trends can help shape future research:

• Nanotechnology Integration: Continued efforts to improve the dispersion and stability of nano-additives such as graphene, carbon nanotubes, and modified clay minerals are expected to yield coatings with superior thermal and mechanical properties. Advanced dispersion techniques, including in situ polymerisation and surface functionalization, are critical to these developments [227,228,229,230,231,232].
• Bio-based and Renewable Additives: Combining renewable resources with conventional flame retardants offers a promising route to enhance sustainability. Research on bio-based compounds derived from agricultural waste streams is gaining momentum, with early results indicating performance comparable to synthetic alternatives [6,7,233,234,235].
• Smart and Responsive Coatings: The integration of innovative materials that can respond dynamically to changes in temperature or other fire-related stimuli is an area of great potential. Such materials could modulate the rate of char formation or even trigger self-healing mechanisms upon damage to the coating [236,237,238,239,240].
• Advanced Characterisation Techniques: Improved analytical techniques, including synchrotron radiation-based spectroscopy and in situ electron microscopy, will continue to enhance our understanding of flame-retardant mechanisms at molecular and nanoscale levels. This deeper insight is crucial for the rational design of next-generation coatings [241,242,243].

6.2. Research Challenges and Methodological Considerations

Despite significant advancements, several research challenges remain that must be addressed to realise the potential of flame-retardant waterborne epoxy systems fully:

• Reproducibility and Scale-up: Detailed studies on reproducibility, mixing protocols, and the long-term stability of nano-enhanced flame-retardant coatings will be vital in bridging the gap between lab-scale innovation and industrial application [244,245,246].
• Interfacial Interactions: Understanding the detailed chemical interactions at the flame retardant-epoxy interface is imperative. Future work should focus on advanced spectroscopic and computational methods to model these interfaces, which are critical for optimising flame retardancy and mechanical durability [247,248,249].
• Trade-offs Between Flame Retardancy and Environmental Impact: As flame retardants evolve, a holistic approach that considers both technical performance and environmental sustainability is necessary. Multi-disciplinary collaboration between materials scientists, chemists, and environmental engineers will be required to develop formulations that deliver optimal performance while adhering to regulatory and sustainability guidelines [6,126,250,251,252].

The synthesis of environmentally benign yet high-performance flame retardants remains a priority. Emerging research would benefit from data-driven approaches that incorporate machine learning algorithms to predict structure-property relationships and optimise formulations based on both performance and ecological indices.

6.3. Future Perspectives on Environmental and Waste Management Strategies

Beyond advances in formulation, comprehensive efforts are needed to account for the full life cycle of flame-retardant composite coatings, encompassing synthesis, application, service performance, and end-of-life management. Addressing these stages holistically is critical to ensure that waterborne epoxy intumescent composite coatings deliver high fire protection while minimising their environmental footprint. Future research directions may include, as illustrated in Figure 7:

• Recyclable Composite Coating Systems: Developing composite coatings with reversible or degradable chemical linkages can enable efficient recycling, reprocessing, or refurbishment. Such strategies have the potential to substantially reduce waste generation and environmental impact while maintaining the mechanical and fire-protection performance of structural composite assemblies [253,254,255,256].
• Green Synthesis of Flame-Retardant Additives: The implementation of sustainable and energy-efficient synthesis routes, particularly for phosphorus- and nitrogen-containing flame retardants, is critical for minimising hazardous by-products and lowering overall energy consumption. Incorporating these green additives into waterborne epoxy composite matrices can provide effective fire resistance while supporting environmentally responsible material production [257,258,259].
• Comprehensive Life Cycle Analyses (LCA) for Composite Coatings: Conducting LCA studies tailored explicitly to flame-retardant composite coating systems is essential to quantify their environmental performance across all stages from raw material extraction to disposal or recycling. Integrating laboratory-scale experimental data with industrial-scale process information enables robust benchmarking and the identification of optimisation opportunities, ultimately guiding the design of safer, more sustainable composite fire-protection solutions [260].

Looking ahead, the development of next-generation flame-retardant composite coatings will be guided by the convergence of materials innovation, computational modelling, and interdisciplinary collaboration. Predictive simulation tools can optimise filler distribution, char formation, and thermal and mechanical performance prior to experimental trials, thereby accelerating the rational design of multifunctional coating-composite systems. The overarching goal is to engineer composite-based flame-retardant coatings that not only meet rigorous fire safety standards but also comply with global sustainability frameworks, including reduced VOC emissions, recyclability, and a low ecological footprint. Such coordinated efforts will enable the creation of durable, high-performance, and environmentally conscious composite coatings for applications in modern infrastructure, industrial facilities, and safety-critical composite structures.

7. Conclusions

In recent years, research on flame-retardant waterborne epoxy intumescent coatings has accelerated significantly, emerging as a multidisciplinary field at the intersection of materials science, polymer chemistry, nanotechnology, environmental engineering, and composite materials technology. Within protective composite systems, these coatings serve as multifunctional layers that enhance the fire resistance of structural composites and hybrid metallic-composite assemblies. This review offers an in-depth analysis of the mechanisms underpinning flame retardancy in waterborne epoxy intumescent composite coatings, including thermal decomposition processes, char formation and expansion dynamics, and the key structure–property relationships governing thermal stability, interfacial adhesion, and overall fire-protective performance.

Recent research trends reveal an increasing incorporation of nanomaterials, metal-organic frameworks (MOFs), layered double hydroxides, and bio-based flame-retardant additives within epoxy-based hybrid composite systems. These approaches reflect a deliberate shift toward engineered composite formulations that integrate multiple flame-retardant mechanisms, such as catalysed char formation, physical barrier effects, and gas-phase inhibition, while simultaneously meeting environmental regulations and sustainability targets.

Despite the notable advances of the last five years, several challenges persist. Achieving stable, uniform dispersion of nanofillers in waterborne epoxy composite matrices remains difficult due to aggregation and interfacial incompatibility. Long-term durability under environmental stresses, including moisture exposure, UV radiation, and cyclic thermal loading, continues to pose problems and balancing high fire-protection efficiency with ecological and regulatory compliance remains a delicate task. A review of more than 200 peer-reviewed studies indicates that, although innovative composite formulations can improve char formation and fire resistance, optimising these systems requires careful attention to trade-offs among fire performance, processability, mechanical integrity, and sustainability.

Future efforts should emphasise the development of advanced processing and fabrication strategies tailored to flame-retardant coating-composite systems. In parallel, enhanced characterisation methods are needed to relate char morphology, thermal degradation behaviour, and mechanical properties across multiple scales. Incorporating life-cycle assessments and sustainability analyses into the design of composite coatings will also be crucial. Such integrative, multidisciplinary approaches are expected to enable the development of next-generation waterborne epoxy intumescent composite coatings that combine long-lasting fire protection, environmental resilience, and reduced ecological impact, thereby meeting the evolving demands of safety-critical and sustainable composite applications.