Flame-Retardant Polyurea Coatings: Mechanisms, Strategies, and Multifunctional Enhancements

Abstract

The imperative for high-performance protective materials has catalyzed the rapid evolution of polyurea (PUA) coatings, widely recognized for their mechanical robustness, chemical resistance, and rapid-curing properties. However, their inherent flammability and harmful combustion byproducts pose significant challenges for safe use in applications where fire safety is a critical concern. In response, significant efforts focus on improving the fire resistance of PUA materials through chemical modifications and the use of functional additives. The review highlights progress in developing flame-retardant approaches for PUA coatings, placing particular emphasis on the underlying combustion mechanisms and the combined action of condensed-phase, gas-phase, and interrupted heat feedback pathways. Particular emphasis is placed on phosphorus-based, intumescent, and nano-enabled flame retardants, as well as hybrid systems incorporating two-dimensional nanomaterials and metal–organic frameworks, with a focus on exploring their synergistic effects in enhancing thermal stability, reducing smoke production, and maintaining mechanical integrity. By evaluating current strategies and recent progress, this work identifies key challenges and outlines future directions for the development of high-performance and fire-safe PUA coatings. These insights aim to guide the design of next-generation protective materials that meet the growing demand for safety and sustainability in advanced engineering applications.

1. Introduction

Growing requirements in engineering and structural applications have driven the continuous development of advanced protective coatings [1,2,3,4]. Among these, polyurea (PUA) elastomers have gained prominence in industrial coatings, valued for their exceptional mechanical strength, chemical resistance, and rapid curing properties [5,6,7,8,9,10]. Formed through the reaction of isocyanates with amine compounds, polyurea coatings exhibit superior adhesion, flexibility, and durability, making them indispensable in construction, transportation, and military applications [10,11,12]. The global demand for high-performance protective coatings has driven significant growth in the polyurea market, projected to reach USD 1481 million by 2025, up from USD 885 million in 2020, reflecting a compound annual growth rate (CAGR) of 10.8% [13]. Their seamless and highly protective nature has positioned them as a preferred choice for demanding environments requiring long-term performance under extreme conditions.

Owing to its intrinsic chemical structure and entirely organic backbone, polyurea demonstrates poor resistance to combustion, with a limiting oxygen index (LOI) typically below 20% [14]. This inherent flammability severely restricts its applicability in environments requiring high fire safety standards. Moreover, under fire conditions, polyurea thermally decomposes, releasing substantial smoke and harmful volatile compounds, which exacerbate the risks associated with fire incidents by impairing visibility and increasing toxicity [15,16]. These safety concerns have become a major obstacle to the broader adoption of polyurea in applications where stringent fire regulations are in place.

The rising global emphasis on fire safety, driven by increasing fire-related incidents and stricter environmental regulations, has intensified research efforts aimed at developing flame-retardant polyurea coatings that preserve its desirable mechanical and chemical properties while significantly improving thermal stability and fire resistance [16,17,18]. Traditional flame-retardant strategies, such as halogenated additives, have been widely criticized due to their environmental persistence and adverse impacts on ecological and human safety. As a result, researchers are exploring alternative flame-retardant systems, including phosphorus-containing, nitrogen-containing, and silicon-containing compounds, along with synergistic nanocomposite reinforcements, to enhance the flame-retardant performance of polyurea while mitigating smoke toxicity and maintaining structural integrity [13,19,20,21]. Such progress contributes to the development of next-generation PUA coatings, designed to address the increasing need for high-performance, fire-resistant materials in construction, transportation, and industrial applications.

Polyurea (PUA) is frequently employed in various advanced applications owing to its distinctive molecular architecture and superior performance attributes that set it apart from conventional polyurethane systems. Unlike polyurethanes, which are synthesized from the reaction between isocyanates and polyols, PUAs are formed through the reaction of isocyanates with amine-terminated compounds. This structural difference results in faster reaction kinetics, higher crosslinking density, and improved phase separation, leading to enhanced thermal stability and mechanical strength. Additionally, PUA exhibits excellent chemical resistance, water resistance, and durability under harsh environmental conditions. These properties make PUA an ideal candidate for applications requiring high-performance coatings, adhesives, and protective materials, where traditional polyurethanes may fall short in terms of longevity or resistance to degradation.

The use of polyurea in coatings is distinguished by its solvent-free nature and the absence of volatile organic compounds (VOCs), offering an environmentally friendly alternative to conventional coating systems [22,23]. This unique advantage enables efficient processing through two-component spraying technology, wherein diisocyanate and diamine rapidly react to form a seamless, high-performance protective layer. Since its development in the 1990s, spray application has become the predominant method for PUA, owing to its rapid curing time, strong adhesion to various substrates, and adaptability to complex surfaces. Advancements in spraying technology have further optimized the application process, allowing for precise control over coating thickness and uniformity while maintaining excellent mechanical properties [24,25,26]. The capability of PUA to be applied in challenging environments, such as high humidity and low temperatures, has expanded their use across various industries. As formulation strategies advance and flame-retardant performance improves, PUA coatings are increasingly recognized as essential components in the development of future high-performance protective materials.

Despite considerable progress related to flame-retardant PUA systems, a comprehensive review that consolidates recent advancements remains limited. There is a growing need for a systematic evaluation of developments over the past decade, particularly as researchers have investigated diverse approaches such as structural tuning and the use of functional additives. This review presents a thorough analysis of current advancements in PUA materials engineered for fire safety, emphasizing mechanisms of fire resistance and recent innovations in their structural design. Furthermore, a wide range of enhancement approaches is analyzed, covering both traditional flame retardants and emerging bio-based and nanomaterial-integrated technologies aimed at improving thermal stability and mechanical properties and suppressing smoke release. In addition to highlighting recent progress, this work identifies key challenges that still need to be addressed.

2. Flame-Retarding Mechanisms for PUA Coatings

Polyurea coatings, recognized for their superior mechanical properties and versatility, have become increasingly important in advancing fire-resistant materials [27,28,29]. These coatings are derived from the reaction of isocyanates and amines, resulting in high-performance polymeric films with excellent abrasion resistance, flexibility, and environmental stability. These characteristics, coupled with their inherent ability to be modified for enhanced flame retardancy, have established polyurea as a highly promising material across diverse fire-resistant uses. Especially within industrial and construction applications, PUA coatings provide essential protection to structures and materials against fire hazards.

2.1. The Combustion Properties of PUA Coatings

PUA inherently possesses limited fire resistance and is classified as Class E according to the European EN 13501 standard, indicating its low level of fire protection [30]. The organic composition and the presence of oxygen in its molecular structure make it prone to sustaining combustion once ignited. The presence of oxygen in the molecular structure of PUA can influence its fire behavior by facilitating the formation of reactive oxygen-containing radicals during thermal decomposition, such as hydroxyl (•OH) and carbonyl (C=O) species. These radicals can accelerate the combustion process by enhancing the breakdown of the polymer backbone and promoting sustained oxidative reactions in the gas phase. However, it is important to note that oxygen content is only one factor. The overall flammability also depends on molecular architecture, the availability of flammable volatiles, and the presence or absence of flame-retardant functionalities. This vulnerability arises as the polymer begins to decompose at elevated temperatures, causing critical molecular linkages to break and making it difficult to halt the combustion process [31]. Nevertheless, the combustion properties of PUA can be significantly enhanced through the addition of flame-retardant agents. Once combined with the PUA material, these additives improve its fire performance, slowing ignition and reducing overall flammability [32,33]. Ma et al. [32] reported that incorporating 3 wt% ZnO@MOF@PZS into PUA significantly enhanced its flame retardancy and reduced smoke toxicity. Cone calorimeter tests revealed that the peak heat release rate, total heat release, and total CO production were reduced by 28.30%, 19.59%, and 36.65%, respectively. Pan et al. [33] prepared a ternary flame retardant (DE@ZIF@ILs) to enhance compatibility with PUA and improve its fire safety. With the addition of DE@ZIF@ILs, PUA showed significant reductions in peak heat release rate (49.7%), total heat release (45.3%), peak smoke production rate (53.0%), and total smoke production (41.4%), confirming its effective flame-retardant and smoke-suppressing performance. This modification has expanded the applicability of PUA coatings, particularly in fire-sensitive environments. For instance, polyurea coatings are increasingly employed in roof coatings, where they satisfy the external fire exposure requirements of the B-Roof T1-4 classification, especially in specific climate zones. The enhanced fire resistance achieved by adding flame-retardant agents expands the applicability of PUA coatings in critical fire protection applications.

2.2. The Mechanism of FRs in PUA Coatings

The flame-retardant mechanism in PUA coatings is closely related to their unique molecular structure, which features alternating soft and hard segments formed by the rapid reaction of isocyanates and amines. As illustrated in Figure 1, flame retardant mechanisms in PUA coatings primarily function through two separate phases: the gas phase and the condensed phase, both of which are essential in reducing fire hazards [34,35,36,37]. Flame retardants emit volatile compounds that disrupt combustion by interrupting radical chain reactions, which helps to slow the spread of fire and decrease the amount of heat generated in the gas phase. This mechanism effectively delays ignition and decreases the overall flammability of the material. Meanwhile, certain additives promote the creation of a protective carbonaceous residue that shields the polymer beneath in the condensed phase, reducing thermal conduction and slowing material decomposition.

2.2.1. Gas-Phase Flame Retardant Mechanism

The effectiveness of FRs within polyurea coatings is largely attributed to their ability to disrupt the combustion process, especially through the disruption of radical chain reactions occurring during burning. During thermal decomposition, flame retardants or their breakdown products emit active species that deactivate critical radicals responsible for sustaining flame propagation [38,39,40,41]. By inhibiting these radicals, the exothermic reactions occurring within the flame are suppressed, thereby reducing heat generation and slowing combustion. This suppression mechanism not only prevents further ignition but also facilitates a cooling effect within the system, helping to mitigate thermal runaway. The overall efficiency of flame inhibition varies according to the specific flame retardant, as halogenated compounds, phosphorus-based additives, and nitrogen-based additives each exhibit unique radical-scavenging capabilities.

2.2.2. Condensed-Phase Flame Retardant Mechanism

Flame retardants could enhance fire resistance by promoting the formation of a stable carbon-rich residue that shields the polymer from further degradation [42,43]. This char layer forms through complex chemical processes such as cyclization and dehydrogenation, which convert the polymer into a heat-resistant structure during thermal exposure [44]. The resulting carbon structure serves as a protective shield, reducing thermal exposure and blocking the movement of oxygen and flammable volatiles that fuel the combustion process. By reducing fuel availability and slowing thermal decomposition, this mechanism significantly improves the flame-retardant performance of PUA materials. The effectiveness of condensed-phase flame retardancy depends on the specific formulation used, where elements like phosphorus-based agents, intumescent components, and inorganic fillers are crucial for promoting robust and stable char formation [34,35,45]. As summarized in Figure 2, Co₃O₄@OZC-2 exhibits catalytic activity due to its transition metal content and open-wall structure, both contributing to the development of a dense char layer. Additionally, its multi-level pore design delays the escape of volatile compounds during matrix breakdown, while the unique cage-like architecture effectively traps toxic gases and smoke particles through physical and chemical adsorption, forcing them to follow a more complex “labyrinth” pathway [15,46]. Understanding these processes is essential for designing advanced polyurea-based coatings with enhanced fire protection performance.

2.2.3. Interrupted Heat Exchange Flame Retardant Mechanism

The interrupted heat exchange mechanism is a critical flame retardant pathway in PUA coatings, effectively helping to limit energy feedback to the burning surface and thereby slowing fire spread [47,48]. This mechanism works by restricting heat flow back into the material, which slows thermal breakdown and reduces the release of additional combustible volatiles. A key aspect of this process involves the use of flame-retardant additives that undergo endothermic decomposition at high temperatures, absorbing thermal energy and emitting non-combustible gases that help cool the system. This not only cools the surrounding material but also dilutes combustible gases, lowering their concentration below the critical threshold required for sustained ignition.

2.2.4. Synergistic Flame-Retardant Mechanism

The enhanced fire resistance of PUA coatings results from the combined action of multiple flame retardants that work together to enhance fire resistance beyond the sum of their individual effects [49,50,51]. This approach typically integrates additives with distinct modes of action. For example, phosphorus-based compounds may promote char formation while simultaneously releasing phosphorus species that help suppress combustion, while nitrogen-based additives can further reinforce this effect by promoting intumescence or enhancing char stability. Additionally, metal-containing flame retardants, such as metal oxides and bimetallic oxides, can promote char formation and enhance thermal stability [52,53,54]. The synergistic flame-retardant mechanism by Sun et al. highlights the synergistic effect of ammonium polyphosphate (APP) and kaolinite (K0) in polyurea (PUA) composites [29]. During thermal decomposition, APP reacts with the charring-foaming agent (CFA) to release nonflammable gases such as CO₂, NH₃, and H₂O, which dilute combustible volatiles and inhibit flame propagation. Phosphorus-containing radicals from APP act as scavengers for H• and OH• radicals, suppressing the combustion chain reaction. The enhanced gas release and thermal stability of the PI-1.0K0 system lead to a higher LOI. Meanwhile, K0 reinforces the char by forming a surface barrier that insulates against heat and oxygen. Together, these gas-phase and condensed-phase actions significantly improve the flame retardancy of the PUA composites.

3. Innovative Approaches in the Development of Flame-Retardant PUA Coatings

3.1. Additive-Type Flame Retardants

Additive-type flame retardants are essential for improving the fire resistance of PUA coatings and are commonly introduced into the polymer through physical techniques like melt blending or solvent blending. Among these, the solution-based method is the most commonly used approach in PUA formulations [55]. These techniques ensure flame-retardant agents are evenly distributed while preserving the polymer’s original chemical structure, allowing compatibility with a wide range of formulations. Additionally, they provide a straightforward and scalable means of integrating flame retardants without requiring complex chemical modifications.

The presence of flame retardants can have a noticeable impact on the curing behavior and functional properties of PUA. Since PUA cures rapidly through the reaction between isocyanates and amine-terminated components, the introduction of FRs may influence this process depending on their chemical structure and physical form. Reactive FRs containing functional groups such as hydroxyl or amine may participate in the curing reaction, potentially altering the crosslinking density and network structure. Non-reactive FRs, especially those in particulate form, can interfere with component mobility or affect the local reaction environment, which may slow down the curing rate or cause uneven curing. In addition to curing, FRs can influence other functional properties of PUA, such as mechanical strength, flexibility, adhesion, and environmental resistance. Adding flame retardant agents can cause phase separation, weaken adhesion, or negatively affect key mechanical characteristics like tensile strength and flexibility. Therefore, optimizing dispersion methods, choosing compatible flame retardants, and enhancing compatibility with the polymer are essential to achieving both effective fire resistance and reliable mechanical properties in flame-retardant PUA coatings.

3.1.1. Phosphorus-Containing Flame Retardants

Additives such as melamine polyphosphate (MPP) [56], ammonium polyphosphate (APP) [50], 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) [57], red phosphorus, and microencapsulated ammonium polyphosphate (MCAPP) [58] are widely used to improve the fire resistance of PUA coatings. The phosphorus-containing flame retardants contribute to fire resistance by reducing heat release, inhibiting flame propagation, and forming protective insulating layers. The continuous development of phosphorus-based flame retardants, including hybrid phosphorus-nitrogen compounds and reactive phosphate monomers, further enhances the flame resistance of PUA coatings without compromising their mechanical strength and overall functionality.

Yang et al. [59] developed a red phosphorus-based hybrid flame retardant featuring in situ growth and even distribution of ZIF-67 nanoparticles (Figure 3). When incorporated into a polyurea (PUA) matrix, the resulting 2D porous RP@ZIF exhibited outstanding flame-retardant performance. Specifically, incorporating 5 wt% RP@ZIF significantly enhanced fire safety by reducing the peak heat release rate (pHRR) and total heat release (THR) by 48.0% and 54.2%, respectively. Furthermore, the peak smoke production rate (PSPR) and total smoke production (TSP) decreased by 20.4% and 22.2%, respectively, demonstrating improved smoke suppression capabilities. Beyond its fire-resistant capabilities, the composite preserved excellent mechanical performance, reaching an elongation at break of 308.02%. However, a significant limitation is that a single phosphorus-based additive often lacks sufficient effectiveness to provide adequate fire resistance.

Phosphorus-containing flame retardants exhibit diverse chemical behaviors and flame-retardant mechanisms, largely governed by the oxidation state of the phosphorus atom [60]. Common oxidation states include −3, 0, +1, +3, and +5, each contributing differently to fire resistance. In polyurea systems, Thirumal et al. [61] investigated how additives containing phosphite, phosphate, and phosphine oxide influenced thermal stability and potential chemical interactions in PUA, with the phosphorus content uniformly maintained at 1.5 wt.%. Triphenylphosphite was found to significantly reduce the flammability of epoxy resins, whereas triphenylphosphate was more effective in reducing the combustion intensity of polyurea. However, the incorporation of triphenylphosphite into the PUA matrix resulted in poor flame resistance, likely due to its plasticizing effect.

3.1.2. Intumescent Flame Retardants

To overcome the limitations of single phosphorus-containing FRs, researchers often combine them with other synergistic flame retardants or modify their chemical structures to enhance their effectiveness. Among various strategies, intumescent systems stand out due to their low toxicity and outstanding fire protection capabilities [62]. IFRs function by promoting char formation during combustion, effectively limiting heat transfer, slowing flame spread, and minimizing the release of harmful gases. Typically, IFR systems are generally composed of synergistic components that dehydrate, promote carbon structure development, and release gases under heat, resulting in an expanded, thermally insulating char layer upon exposure to heat.

Ammonium polyphosphate (APP) serves as an effective acid source that promotes the dehydration of char-forming agents, thereby increasing char yield and enhancing its structural stability [63,64]. Furthermore, recent research suggests that integrating clay-based additives into IFR systems can markedly improve their flame-retardant performance. Additives such as halloysite, montmorillonite, and layered double hydroxides have demonstrated synergistic effects with IFRs, reinforcing char integrity and strengthening thermal barrier properties. Sun et al. [29] investigated the incorporation of kaolinite (K₀) into PUA composites, aiming to enhance their resistance to combustion through synergistic interaction with conventional char-forming components. The incorporation of 1.0 wt% K0 (alongside 24 wt% IFR) significantly boosted the flame resistance of the material, increasing the LOI value to 36.0% and reducing THR by 16.0%. The PUA/IFR/K₀ composite also exhibited enhanced mechanical performance, as evidenced by a 36.9% increase in elongation at break compared to the PUA/IFR system.

3.1.3. Two-Dimensional Nanomaterials and Other Functional Nanoparticles

Two-dimensional nanomaterials and other functional nanoparticles are increasingly utilized to boost the functionality and effectiveness of PUA coatings. Materials like graphene oxide [65,66], layered double hydroxides (LDHs) [67,68], montmorillonite [51,69,70], and MXenes [71,72,73] exhibit exceptional mechanical strength and resistance to heat and gas permeation, attributed to their distinctive layered morphology and extensive surface area. When uniformly dispersed in the polyurea matrix, these nanosheets can create efficient physical barriers that hinder the transmission of heat, gases, and degradation products, contributing to improved flame resistance and durability.

In flame-retardant systems, these nanomaterials are often used in combination with other additives to enhance overall performance through synergistic effects [49,74,75,76]. For example, their combination with phosphorus-containing or intumescent flame retardants has shown promising results, where the nanoparticles can promote char formation, stabilize the protective carbonaceous layer, and reduce smoke release. Lin et al. [7] significantly enhanced the fire safety of PUA coatings by incorporating modified ammonium polyphosphate (MAPP), graphene oxide (GO), and Mg-Al layered double hydroxides (LDH) into the PUA composite. The combination of MAPP with two-dimensional nanofillers led to a 31.0% reduction in THR and a 33.4% decrease in pHRR, underscoring their synergistic role in markedly enhancing the thermal resistance and flame-retardant efficiency of PUA coatings. Moreover, tuning the proportion between LDH and GO within the range of 1:1 to 2:1 resulted in PUA composites with enhanced resistance to heat, improved flame-retardant behavior, and well-balanced impact performance. However, exceeding the optimal GO content led to diminished fire resistance, underscoring the importance of maintaining a well-balanced filler composition for optimal effectiveness.

Similarly, incorporating metal–organic frameworks (MOFs) or metal oxide nanoparticles (e.g., MgO, ZnO, TiO₂) into PUA coatings can further improve their ability to withstand combustion, reduce smoke generation, and strengthen their structural integrity [77,78]. Ma et al. [32] reported a significant enhancement in the fire safety of PUA by incorporating 3 wt% ZnO@MOF@PZS, which was synthesized through hydrothermal and polycondensation methods using ZnO nanoflowers as both a template and zinc source. Compared to pristine PUA, the addition of this hybrid additive led to a 28.30% reduction in pHRR, a 19.59% decrease in THR, and a 36.65% decline in total CO production (TCO). This improvement is mainly attributed to ZnO@MOF@PZS promoting the development of a dense, stable residue that effectively restricts the movement of thermal energy and combustible substances during combustion. Xie et al. [79] introduced an innovative approach to improve the fire safety of PUA by incorporating flame-retardant porous liquids (PLs) derived from a defect-engineered Co-LDH@ZIF-67 (d-LDH@ZIF) heterostructure, as shown in Figure 4. With the addition of 20 wt% d-LDH@ZIF porous liquids (PLs-d-L@Z) to the PUA matrix, the limiting oxygen index (LOI) rose to 24.2%, and the composite attained a V-0 rating in the UL-94 vertical burning test.

3.1.4. Bio-Based Flame Retardants

Recently, bio-based flame retardants have gained considerable interest as sustainable and eco-friendly substitutes for traditional halogenated or synthetic flame retardants [80,81,82,83,84]. Sourced from materials like lignin, phytic acid, tannins, cellulose, chitosan, and proteins, these substances naturally contain flame-retardant elements such as phosphorus, nitrogen, and aromatic groups that encourage char formation and reduce heat and mass transfer during burning. Despite their promise, bio-based flame retardants still face several limitations that hinder their widespread application. These include relatively low thermal stability, poor compatibility with polymer matrices, and processing challenges such as moisture sensitivity or limited reactivity. Moreover, the efficiency of flame retardancy often does not match that of conventional systems without significant chemical modification. To address these challenges, recent studies have focused on advanced modification techniques such as phosphorylation, ammoniation, and surface grafting, which enhance both the thermal stability and compatibility of bio-based FRs with host polymers. Additionally, the integration of bio-based FRs with nanomaterials or their incorporation into intumescent systems has shown great potential in improving flame-retardant efficiency while maintaining mechanical properties. These innovations highlight the growing potential of bio-based flame retardants, although further research is needed to optimize their performance, scalability, and cost-effectiveness for practical applications.

Phytic acid (PA), a saturated cyclic acid found abundantly in seeds and grains, is recognized as a promising bio-based flame retardant [85,86,87,88]. Rich in phosphorus (approximately 28% by weight), PA can serve as an effective phosphorus source for flame-retardant formulations. As depicted in Figure 5, bio-based melamine-phytate nanoflakes were integrated with metakaolinite (Mul-K0), resulting in marked improvements in both fire resistance and mechanical strength of PUA coatings when used alongside intumescent flame retardants [89]. Compared to neat PUA, the optimized composite (PI-1.0MK0) exhibited markedly improved fire safety, with a highest fire performance index (FPI) of 0.109 m²·s/kW and a fire risk index (FRI) of 8.758. Additionally, PI-1.0MK0 showed a 28.2% improvement in strength and a 181.2% enhancement in flexibility compared to the K0/IFR/PUA composite, due to better interfacial compatibility and dispersion resulting from the expanded interlayer spacing of Mul-K0.

Moreover, adding bio-derived flame retardants alongside nanomaterials or reactive flame-retardant monomers can enhance the coating’s strength, heat resistance, and protective capabilities [87,90,91,92]. As research progresses, developing versatile bio-based flame retardants specifically for polyurea systems is anticipated to produce advanced coatings that meet environmental standards and regulatory demands. Meng et al. [93] developed a novel bio-based nano-additive (CPN@MMT) by exfoliating montmorillonite (MMT) through chitosan-assisted ball milling, followed by co-assembly with phytic acid and urea. When incorporated into PUA coatings, the CPN@MMT nanohybrid significantly enhanced flame retardancy. Specifically, the PUA/CPN@MMT composite exhibited a 34.11% reduction in pHRR, a 44.64% decrease in TSP, and a 39.88% reduction in total CO production. These enhancements resulted from the well-dispersed nanohybrids promoting dense char development and providing effective catalytic activity and barrier protection. This not only suppressed heat diffusion and volatile release but also markedly enhanced the overall fire safety of the PUA system, demonstrating the potential of bio-based nanomaterials in multifunctional protective coatings.

3.2. Reactive-Type Flame Retardant

Reactive-type flame retardants chemically bond to the polymer backbone during synthesis instead of being physically mixed into the material [94]. In PUA coatings, flame retardants are commonly incorporated by chemically modifying isocyanate or amine components with flame-retardant groups containing elements such as phosphorus, nitrogen, or silicon. Integrating flame-retardant groups directly within the polyurea structure improves fire resistance while preventing common problems seen with additive-type retardants, such as migration, leaching, and deterioration of mechanical performance.

3.2.1. Phosphorus-Containing Polyols

Phosphorus-containing polyols function as reactive additives, enhancing both the structural strength and flame protection of PUA coatings [95]. These compounds are synthesized by incorporating phosphorus-based functional groups such as phosphate, phosphonate, or phosphinate into polyol molecules [96]. When added to polyurea formulations, they react with isocyanates during the curing process, becoming chemically bonded within the polymer network. This covalent integration ensures even distribution, reduces migration, and improves durability over time. Phosphorus elements encourage the development of a protective residue during combustion that limits heat and gas flow, helping to decrease flammability. Besides boosting fire resistance, phosphorus-containing polyols can strengthen the coating’s durability and toughness while maintaining good compatibility with other components. Their multifunctional nature makes them a promising solution for developing safe, high-performance polyurea coatings used in construction, transportation, and industrial protection.

Zhao et al. [28] demonstrated that the incorporation of only 2.22 wt% of a phosphate-containing polyol (OP550) to the PUA significantly improved its fire safety, allowing the material (PUA-2) to pass the UL-94 V-0 test with self-extinguishing properties and minimized flaming drips. In addition to its flame-retardant function, OP550 also contributed to improved mechanical performance, with PUA-2 exhibiting a tensile strength of 15.4 MPa, an elongation at break of 1287.5%, and a tearing strength of 65.4 N·mm⁻¹. These findings demonstrate that even at low concentrations, OP550 significantly enhances both the fire safety and durability of PUAs, supporting their use in demanding, high-performance applications.

3.2.2. Reactive Flame-Retardant Additives

Covalently bonding flame-retardant groups into the polymer network during synthesis has gained attention as a promising approach to improve PUA’s fire safety while preserving its mechanical and chemical properties. Among reactive flame retardants, aromatic dianhydride compounds have shown promising results. These compounds incorporate phosphorus, nitrogen, or aromatic groups within the polyurea backbone by reacting with isocyanates or amino groups. By incorporating such reactive flame retardants into the polyurea backbone, the resulting PUAs exhibit durable flame-retardant performance with minimal impact on coating integrity and long-term stability. In addition, reactive systems avoid the migration and leaching problems associated with traditional additive flame retardants, making them suitable for environmentally sensitive applications.

Liang et al. [97] explored how aromatic dianhydrides (ArDAs) influence the thermal decomposition of PUA, as shown in Figure 6, focusing on their role in modifying degradation pathways. Their results showed that among the three ArDAs examined, BTDA reacted most strongly with the modified isocyanate, indicating possible bonding interactions between the filler surface and the PUA matrix. The incorporation of ArDAs notably enhanced the flame-retardant performance of PUA while preserving its mechanical integrity. Specifically, when 20 wt% of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) was added, the pHRR, THR, total smoke release (TSR), and total CO release (TCO) of PUA were reduced by 70.6%, 32.5%, 44.7%, and 58.8%, respectively. Meanwhile, the char residue increased by 405%, indicating a substantial improvement in thermal stability and the capacity to develop protective residue. Notably, these enhancements were achieved with minimal impact on mechanical properties, highlighting the promise of aromatic dianhydrides as environmentally friendly reactive additives for high-performance PUA systems.

3.2.3. In Situ Polymerization Incorporating Nanoparticles

Widely used in the synthesis and functional modification of PUA coatings, in situ polymerization enables accurate tuning of the molecular structure and overall performance of the material [98]. In this process, the polymerization reaction occurs directly at the application site or within the target matrix, enabling uniform distribution of functional components and strong interfacial interactions. For polyurea systems, in situ polymerization typically involves the rapid reaction between isocyanate prepolymers and amine-terminated curing agents, which proceeds without the need for catalysts and results in fast gelation and curing under ambient or controlled conditions.

This approach offers significant benefits for incorporating functional additives such as nanoparticles by enabling their uniform dispersion and chemical integration throughout the polymer matrix during synthesis. For example, flame-retardant nanomaterials added during in situ polymerization become covalently bonded within the polyurea network, enhancing thermal stability, promoting char formation, and reducing smoke generation while maintaining mechanical performance.

Qian et al. [99] demonstrated that employing in situ polymerization significantly improves both the flame resistance and mechanical properties of PUA composites reinforced with surface-modified graphene oxide (rGO). To address the inherent aggregation of rGO in polymer matrices, rGO was functionalized with silicon-based organic/inorganic flame retardants via a sol–gel method, producing FGO-1 (1.9 nm thick) and FGO-2 (4.9 nm thick). At a low loading of 1 wt%, both functionalized graphenes were introduced via an in situ polymerization process, resulting in significantly improved dispersion. Notably, the tensile strength of the PUA/FGO-1 composite increased by 17.7%, whereas PUA/FGO-2 achieved a remarkable 31.4% enhancement. Zhang et al. [17] designed a multifunctional reactive additive, MX-SP-NH₂, by grafting SPDPC onto MXene (Ti₃C₂Tₓ) and subsequently modifying it with PDA. This additive was incorporated into polyurea (PUA) at a loading of just 1.0 wt%, where it participated in the crosslinking process of the polymer matrix. As a result, the mechanical properties of PUA were greatly enhanced, with tensile strength, elongation at break, and toughness increasing by 184.3%, 126.1%, and 436.3%, respectively. Moreover, the material exhibited enhanced fire resistance, evidenced by notable decreases of 31.7%, 28.9%, and 61.5% in pHRR, SPR, and CO production rates. These findings demonstrate that MX-SP-NH₂ effectively enhances the structural integrity and fire resistance of PUA, highlighting its strong potential for use in flexible electronics.

4. Multifunctional Performance of Flame-Retardant PUA

Flame-retardant polyurea coatings have found broad applications across various industries due to their exceptional thermal stability, rapid curing, high mechanical strength, and resistance to extreme environmental conditions [8,100,101]. These coatings are particularly valued due to their seamless durability, which effectively hinders flame spread, reduces smoke output, and suppresses the release of toxic gases. In the construction industry, flame-retardant PUA coatings are commonly used as protective layers for steel structures, wood, insulation panels, and concrete surfaces, effectively improving fire resistance and reducing flame spread in buildings. These coatings are particularly beneficial in high-risk areas such as walls, ceilings, and floors, where enhancing resistance to fire propagation can significantly improve occupant safety. Additionally, flame-retardant polyurea is applied to tunnel linings, bridges, and subway systems, offering enhanced fire protection and structural integrity under high-temperature conditions. In tunnels and underground facilities, where escape routes and fire safety are of paramount importance, these coatings provide an extra layer of protection, allowing for safer evacuations and reduced fire damage to infrastructure.

Flame-retardant PUA coatings exhibit a unique integration of mechanical robustness and chemical durability, rendering them highly effective in environments requiring both impact resistance and corrosion protection. The intrinsic elasticity and tensile strength of polyurea enable it to absorb and dissipate mechanical energy, thereby mitigating structural damage under dynamic loading conditions. Concurrently, the formation of a dense, hydrophobic barrier imparts excellent resistance to water ingress, chemical attack, and environmental degradation. The incorporation of flame-retardant components further extends their functional scope, enabling these coatings to meet stringent fire safety standards without compromising structural integrity. These combined attributes make flame-retardant polyurea coatings well-suited for infrastructure, transportation, and marine applications where simultaneous protection against mechanical impact, corrosive exposure, and fire hazards is critical.

As shown in Figure 7, Pan et al. [102] demonstrated that incorporating triphenyl phosphate-modified Co-based isomers into polyurea results in enhanced interfacial compatibility and filler dispersion, as evidenced by SEM images showing more uniformly embedded fillers and blurred interfacial gaps compared to binary polyurea/Co-based isomer blends. The improved dispersion is attributed to TPP functionalization, which enhances interfacial compatibility between the fillers and the polymer, leading to a more uniform distribution throughout the material. Additionally, the use of CoCo-LDH increases surface roughness, strengthening interfacial adhesion. While conventional flame retardants often compromise mechanical performance due to poor dispersion, PUA/m-CBCP@LDH and PUA/CBC-P composites exhibited notable enhancements in strength and flexibility, primarily resulting from stronger filler–matrix interactions and the improved compatibility of TPP-modified fillers with the polyurea network.

Meng et al. [93] prepared bio-based nano-additives by exfoliating montmorillonite using chitosan, followed by constructing a nanohybrid structure (CPN@MMT, see Figure 8) through the integration of phytic acid and urea. This additive demonstrated excellent compatibility and dispersibility in the PUA matrix, effectively enhancing the mechanical strength, impact resistance, corrosion protection, and fire safety of PUA. The enhancement in impact resistance observed in PUA/CPN@MMT coatings can be attributed to three primary mechanisms: (i) reversible rupture and recombination of hydrogen bonds between urea groups enable effective absorption; (ii) the microphase-separated structure of polyurea allows energy dissipation through compaction of hard segments under stress; and (iii) CPN@MMT enhances hard segment rigidity, accelerates unloading wave propagation, and improves energy transfer through strong interfacial bonding and uniform dispersion, resulting in better impact energy absorption.

Flame-retardant polyurea coatings serve as multifunctional protective layers, improving thermal stability while providing autonomous self-healing, which makes them strong candidates for next-generation fire-safe materials. By integrating dynamic covalent bonds, hydrogen bonding networks, or microencapsulated healing agents into the polyurea matrix, these coatings can spontaneously repair microcracks and surface damage caused by thermal stress or mechanical impact, thereby preserving their flame-retardant performance over prolonged use. This intrinsic healing ability significantly extends the service life of coated substrates, reduces maintenance costs, and enhances structural reliability, particularly in high-risk environments such as aerospace, construction, and transportation. The synergy between flame retardancy and self-healing in polyurea systems represents a critical advance toward intelligent, durable, and sustainable fire protection technologies.

Recent advancements in PUAs have demonstrated significant potential for multifunctional protective applications, particularly through the integration of flame retardancy and autonomous self-healing [103,104]. Kuan et al. [105] reported the development of polyaspartic PUA nanocomposites reinforced with isocyanate-functionalized graphene nanoplatelets (IP-GNPs), achieving not only remarkable mechanical enhancement but also intrinsic repairability. At an ultralow loading of 0.05 vol% IP-GNPs, the nanocomposite exhibited a 108.21% increase in tensile strength over pristine PUA, alongside excellent chemical resistance. Crucially, driven by reversible hydrogen bonding, the material achieved a self-healing efficiency of up to 80.10% after mild thermal activation at 60 °C, enabling recovery of mechanical integrity post-damage. The formation of a continuous IP-GNP network further contributed to enhanced thermal conductivity, indirectly supporting fire safety by promoting efficient heat dissipation. This synergistic integration of mechanical robustness, thermal management, and self-healing within a flame-retardant PUA matrix underscores its promise for durable and intelligent coatings in demanding environments.

5. Concluding Remarks and Recommendations for Future Works

Flame-retardant polyurea (PUA) coatings have attracted increasing attention due to their rapid curing, high mechanical strength, chemical resistance, and excellent adhesion properties. However, integrating flame retardants into PUA systems presents unique challenges that differ from those in other polymers. These include the potential interference of flame-retardant additives with the fast-curing kinetics of PUA, changes in the phase separation between hard and soft segments, and the need to maintain a delicate balance between flame retardancy and the intrinsic physical properties of the coating.

Future research should focus on the molecular-level design of reactive flame retardants that can chemically bond with PUA chains without disrupting the curing behavior or phase morphology. There is also a need to explore multifunctional flame retardant systems that not only improve fire resistance but also enhance properties such as flexibility, weatherability, and durability. Furthermore, in situ characterization of the curing process in the presence of flame retardants would provide valuable insights into reaction mechanisms and structure–property relationships. Developing environmentally friendly and non-toxic flame retardants tailored to the unique chemistry of PUA is another key direction for sustainable development in this field.

Despite these promising developments, a number of persistent challenges remain. Many current flame-retardant systems, especially those relying on physically blended additives, can compromise the mechanical integrity, adhesion, or elasticity of PUA coatings. Inadequate compatibility and poor dispersion of flame retardants within the polymer matrix often result in phase separation and inconsistent performance. Environmental concerns also remain significant, as some flame-retardant formulations continue to rely on halogenated or non-degradable compounds that may pose health and ecological risks. Addressing these issues through targeted, chemistry-specific innovation will be essential to fully realize the potential of flame-retardant PUA materials.

To address these challenges and further the development of high-performance, fire-resistant PUA coatings, several research directions are recommended for future work.

First, the development of green and sustainable flame retardants tailored for polyurea systems is critically important. Future research should prioritize the creation of bio-based, non-toxic, and environmentally friendly flame-retardant compounds that are compatible with the unique chemical structure and fast-curing nature of PUA. These next-generation flame retardants should not only deliver excellent fire resistance and thermal stability but also preserve or enhance the mechanical strength, elasticity, and protective functions inherent to PUA coatings. Integrating such sustainable materials will support environmental objectives while meeting the performance demands of advanced PUA applications.

Second, the design of advanced nanocomposite flame-retardant systems remains a highly promising direction for polyurea coatings. Nanomaterials such as MXene, graphene oxide, layered silicates, and MOFs possess unique structural features and functional properties that can be leveraged to improve both flame retardancy and mechanical reinforcement in PUA matrices. When properly surface-modified and uniformly dispersed within the phase-separated morphology of PUA, these nanofillers can synergistically enhance thermal stability, barrier performance, and durability. The exploration of hybrid or functionalized nanocomposites offers significant potential to realize multifunctional flame-retardant PUA coatings with superior overall performance.

Third, advancing a deeper mechanistic understanding of flame retardancy in polyurea systems is essential to guide rational material design. Detailed investigations into thermal degradation pathways, flame-retardant action modes, and interfacial interactions between flame retardants and the polyurea matrix at the molecular level are necessary. Employing advanced in situ characterization techniques combined with computational modeling will enable visualization of decomposition and reaction processes specific to PUA under fire conditions. These insights will facilitate targeted optimization of flame-retardant formulations that maximize fire safety without compromising the unique mechanical and curing properties of polyurea coatings.

In conclusion, flame-retardant polyurea coatings occupy a crucial position where high performance, safety, and environmental sustainability converge. These coatings have become integral to modern industries that require reliable protective materials with exceptional flame resistance. As research and technological advancements continue, it is anticipated that future innovations will lead to the creation of even more efficient and safer coatings. These next-generation solutions will not only enhance protective properties but also be designed with a focus on minimizing environmental impact, thereby offering sustainable alternatives to traditional materials. Such advancements are vital for addressing the evolving challenges faced by industries, ensuring that flame-retardant coatings can provide both superior protection and environmental responsibility in the years to come.