Recent Advances in MXene-Based Flame Retardants for Enhancing Fire Safety in Thermoplastic Resins

Abstract

Thermoplastic resins are indispensable materials in various applications, including automotive, electronics, packaging, and construction, owing to their superior mechanical strength, ease of processing, and versatility. However, their inherent flammability poses a significant challenge to fire safety, limiting their broader use in environments demanding stringent flame-retardant performance. MXenes, a newly emerging class of two-dimensional (2D) nanomaterials, have recently attracted significant attention for their application in flame-retardant polymer composites because of their multilayered structure and exceptional versatility. This review comprehensively explores the state-of-the-art advancements in flame-retardant thermoplastic resins, focusing on the use of MXenes and MXene-based materials to meet high-performance requirements, including mechanical strength, thermal stability, flame retardancy, electromagnetic interference (EMI) properties, and multifunctionality. Furthermore, the modification approaches of MXene-based flame retardants, the preparation of MXene-based thermoplastic resin composites, and the mechanisms of action for different matrices are also discussed in this review. Finally, this review discusses the challenges and potential developments for MXene-based thermoplastic composites, offering insights into the practical applications and growing demands for flame-retardant materials.

1. Introduction

Since the mid-20th century, plastics have become a key transformative material of modern civilization. Starting from modest origins, global plastic production has experienced a dramatic increase, driven by their remarkable versatility and broad range of applications across various industries [1,2]. By 2010, global production had surged to an estimated 200 million tons annually, marking a milestone in the evolution of synthetic polymers [3]. This rapid expansion highlights the indispensable role of plastics in modern society, while also intensifying concerns about their environmental impact and sustainability. The issues of the 21st century require innovative strategies to balance the benefits of plastic production with its far-reaching consequences.

The widespread adoption of plastics can be attributed to their unparalleled advantages, including lightweight properties, durability, and exceptional versatility [4,5,6]. These characteristics have positioned plastics as indispensable materials from packaging and construction to healthcare and electronics. Their ability to be tailored to specific applications has further cemented their role as a cornerstone of modern innovation and functionality. However, thermoplastic resins present a considerable fire hazard because of the intrinsic high flammability and low thermal stability, which are influenced by the specific structure of polymers [7,8,9,10,11,12,13,14]. These properties make them prone to ignition and rapid flame spread, raising substantial safety concerns in their widespread applications. Addressing and reducing these fire risks remain key challenges, motivating the development of novel materials and flame-retardant technologies to enhance their fire resistance.

Flame-retardant technologies are essential for improving the fire resistance of thermoplastic resins, addressing their inherent susceptibility to ignition and flame spread [15,16,17,18]. These technologies typically involve the incorporation of flame-retardant additives, like halogenated compounds, phosphorus-based agents, and intumescent systems, which act to suppress combustion, reduce heat release, and minimize smoke generation. Additionally, the development of polymer blends, coatings, and nanomaterial-based solutions has shown promise in improving the thermal stability and fire safety of thermoplastics. Recently, two-dimensional (2D) layered nanomaterials, including graphene [19,20], boron nitride [21,22,23], molybdenum disulfide [24,25,26,27], and MXene [28,29,30,31], have emerged as highly effective solutions for enhancing the fire resistance of polymer composites. Their unique structural properties, high surface area, and efficiency at relatively low loading levels make them particularly valuable [32]. Furthermore, 2D nanomaterials are capable of forming stable, heat-resistant structures at the polymer interface, enhancing flame-retardant properties without compromising mechanical performance. These advancements collectively contribute to the development of thermoplastic resins with significantly improved fire safety, addressing the growing need for safer materials in diverse applications.

MXene is increasingly regarded as a highly promising reinforcement material for polymer composites, distinguishing itself from other two-dimensional materials across a broad range of applications, including energy storage [33,34,35,36], electromagnetic interference (EMI) shielding [37,38,39,40,41], sensors [42,43,44], photocatalysis [45,46,47], and other fields [48,49,50]. The previous review introduced MXenes and MXene-based materials utilized in thermoplastic polyurethane (TPU) nanomaterials [31]. Generally, MXenes are made up of transition metal carbides, nitrides, or carbonitrides, following the general chemical formula M_n+1X_nT_x, where M refers to transition metal atoms, such as titanium (Ti), niobium (Nb), or tantalum (Ta). Additionally, X is either carbon (C) or nitrogen (N) and n can be 1, 2, or 3, indicating the number of metal layers in the structure. The surface of MXenes is often functionalized with groups like –O (oxygen), –OH (hydroxy), or –F (fluorine) (denoted as T_x), which can be used to modify their chemical properties for various applications [51,52,53,54]. This unique material exhibits a combination of properties, including hydrophilicity, excellent colloidal stability in aqueous solutions, high electrical conductivity, superior thermal stability, large surface area, and ease of functionalization. Additionally, it can improve the mechanical properties of polymer composites, making it highly versatile for various advanced applications. Ling et al. reported a 200% increase in tensile strength for MXene/PVA composites compared to pure PVA films [55]. Zhi et al. observed a 20% increase in tensile strength when 0.5 wt% MXene was incorporated into polyurethane nanocomposites [56]. Sheng et al. observed a 47.1% enhancement in tensile strength of TPU nanocomposites, with just 0.5 wt% PEG-modified MXene [57]. Importantly, the early transition metal compounds in MXenes exhibit significant catalytic properties, which are crucial in decreasing smoke release and promoting the development of stable chars. These compounds interfere with the combustion process, effectively minimizing the emission of harmful gasses and enhancing the material’s fire resistance. The catalytic effect is a key factor in the exceptional flame-retardant performance of MXenes, underscoring their increasing ability to improve the safety of polymer-based materials [58,59]. The ability of MXene to suppress harmful smoke emissions during combustion helps reduce environmental damage, promotes the creation of more sustainable and safer materials, and directly contributes to the protection of human lives and property.

This review provides a comprehensive analysis of recent advancements in MXene research, with a focus on their synthesis, modification, and the development of MXene/thermoplastic composites. It examines how these innovations and the underlying mechanisms in different polymer matrices impact the properties and applications of MXene-based polymers. Our previous review specifically examined thermoplastic polyurethane as a type of thermoplastic polymer [31]. However, this review extends the discussion to explore how these advancements and mechanisms influence the properties and applications of MXene-based thermoplastic polymer materials more broadly. Additionally, the distinctive advantages of MXenes in the development of multifunctional and high-performance flame-retardant nanocomposites are highlighted. This paper also highlights the challenges encountered by MXene/thermoplastic polymer composites and outlines future development directions, offering valuable insights for advancing MXene-based composites with outstanding properties.

2. Synthesis and Modification of MXenes

More than 30 distinct MXenes have been synthesized through the selective etching of elements from the IIIA or IVA groups, including aluminum (Al), gallium (Ga), silicon (Si), and germanium (Ge), which are eliminated during the etching process [60,61,62]. This approach enables the development of diverse MXene materials and expands their potential applications across a range of high-end fields. Recently, their remarkable potential to improve the fire safety of thermoplastic resin composites has gained increasing recognition. This advancement highlights the growing versatility of MXenes, positioning them as an effective approach to enhance the fire safety and other properties of composite materials.

2.1. Synthesis of Pristine MXenes

2.1.1. Fluoride-Containing Etching Methods

The synthesis of MXenes widely employs fluoride-containing etching methods, which enable the selective removal of aluminum or other metallic elements from their parent carbide or nitride precursors [63,64]. The methods typically utilize hydrofluoric acid (HF) or fluoride salts such as sodium fluoride (NaF) in combination with a mild acid, such as hydrochloric acid (HCl), to etch the metal layers and expose the two-dimensional (2D) MXene structure. In 2011, Naguib et al. [65] utilized hydrofluoric acid (HF) as an etchant to selectively etch Ti₃AlC₂, resulting in the formation of Ti₃C₂T_x, where T_x refers to surface groups such as F, O, or OH. This Ti₃C₂T_x material was named MXene to highlight its structural and morphological similarities to graphene.

Wang’s group [66] pioneered the investigation of MXene’s flame-retardant potential in polymer materials by preparing a polyvinyl alcohol (PVA)/Ti₃C₂T_x composite film using a simple aqueous casting method. The composite demonstrated remarkable thermal stability, retaining 30.5 wt% of its weight at 800 °C, and exhibited a 15 °C increase in onset decomposition temperature. Notably, when exposed to an alcohol flame, the composite film showed no dripping during combustion and retained its original shape after flame treatment. The findings confirm the efficiency of MXene nanosheets in improving the thermal stability and antidripping characteristics of polymer materials.

2.1.2. Fluoride-Free Methods

Fluoride-free methods for MXene synthesis have gained attention as safer and more sustainable alternatives to traditional fluoride-based techniques [67,68,69,70]. These approaches eliminate the use of hazardous fluorine-containing chemicals, reducing environmental and health risks while enabling the tailored synthesis of MXenes. Among these methods, the electrochemical etching strategy selectively removes A-site elements using an electrochemical cell, enabling precise control over the etching process and surface functionalization. Yang and colleagues [71] demonstrated the electrochemical etching of Ti₃AlC₂ using a solution containing 1 M ammonium chloride (NH₄Cl) and 0.2 M tetramethylammonium hydroxide as the electrolyte to produce single- or double-layer Ti₃C₂T_x. Meanwhile, the molten salt method relies on the ability of Lewis acids in their molten state to act as strong electron-accepting ligands, which thermodynamically react with the A-site elements, facilitating their selective removal and resulting in the formation of MXenes. Li et al. [72] employed a top-down approach to prepare Ti₃C₂Cl₂ and Ti₂CCl₂ through a replacement reaction involving molten ZnCl₂, followed by exfoliation (see Figure 1). In this process, Ti₃ZnC₂ and Ti₂ZnC were first formed by the exchange of Zn from molten ZnCl₂ with Al in the MAX-phase precursors. Subsequently, Ti₃C₂Cl₂ and Ti₂CCl₂ were obtained through further exfoliation.

2.2. Modification of MXenes

The modification of MXenes is a critical strategy to improve their properties and broaden their applications [73,74]. Given the distinctive characteristics including high electrical conductivity, large surface area, and surface functional groups like –OH, –O, and –F, MXenes can be tailored through surface functionalization, chemical modification, and hybridization. Surface functionalization, achieved by introducing organic molecules or nanoparticles, can enhance dispersion, mechanical strength, catalytic activity, and flame retardancy. Specifically, functionalizing MXenes with flame-retardant groups or metal nanoparticles can substantially improve their ability to suppress flammability and minimize smoke emissions. Additionally, the intercalation of flame-retardant agents into MXene layers can further improve their thermal stability and resistance to fire. MXene-based composites exhibit enhanced flame-retardant performance over traditional fillers due to their ability to create stable, heat-resistant char layers during combustion. These modifications, along with the inherent catalytic effects of MXenes on smoke suppression, significantly enhance their effectiveness in boosting the flame retardancy of thermoplastic composites.

2.2.1. Non-Covalent Modification

Non-covalent modification of MXene-based flame retardants (FRs) involves the application of physical interactions to improve their flame-retardant properties while preserving the inherent structure and chemistry of MXenes [75]. Among these approaches, hydrogen bonding, electrostatic interaction, and cationic modification are the most commonly used methods to improve the dispersion, thermal stability, and fire-suppressing efficiency of MXenes in composite materials.

One effective non-covalent modification method is hydrogen bonding, which occurs between the MXene surface and flame-retardant molecules containing hydrogen-bonding groups. MXenes naturally possess hydroxyl (–OH) and oxygen (–O) functional groups on their surface, allowing them to interact with organic or inorganic FRs that contain hydrogen-bonding sites. This interaction enhances the dispersion of the flame-retardant agents on the MXene surface and improves the overall flame-retardant performance. Extensive hydrogen bonding was established between the delaminated Ti₃C₂T_x flakes and aramid nanofiber (ANF), contributing to the desired mechanical properties of the MXene/ANF composite paper [76]. Additionally, MXene could interact with PVA through hydrogen bonding, leading to significant improvements in both EMI shielding and flame-retardant properties [77]. Xue et al. [78] pioneered the design of an MXene–PPDA flame retardant, leveraging the strong hydrogen bonding between the amino groups in PPDA and the terminal groups on the surfaces of MXene sheets. The incorporation of as little as 1.0 wt% MXene–PPDA into the PLA composite resulted in a UL-94 V-0 rating and a 22.2% reduction in peak heat release rate (pHRR).

Electrostatic interactions are another widely used strategy to modify MXenes for flame retardancy [79,80]. Because of the negatively charged surface, MXenes could interact with positively charged flame-retardant molecules, such as ammonium salts or other charged flame-retardant agents. This electrostatic attraction promotes a strong attachment of these agents to the MXene surface, improving the stability and dispersion of the flame retardants. This result is a more effective flame retardant that improves the thermal stability of polymer composites while also enhancing the materials’ ability to suppress smoke and reduce fire hazards. Yu et al. [81] modified MXene by functionalizing it with protonated 3-amino-propylheptaisobutyl-polyhedral oligomeric silsesquioxane (POS-HCl) through electrostatic interactions. The POSS-functionalized Ti₃C₂T_x resulted in a 54.4% reduction in peak CO production rate and a 39.1% decrease in pHRR for polystyrene (PS) composites. As illustrated in Figure 2, Yuen et al. [82] successfully synthesized a boron dipyrromethene (BODIPY)-modified MXene hybrid by exploiting the electrostatic interactions between negatively charged MXene and protonated BODIPY. Remarkably, the incorporation of just 0.5 wt% of the hybrid BODIPY-MXene flame retardant into acrylonitrile–butadiene–styrene (ABS) effectively decreased the release of toxic hazards when burning.

Cationic modification involves introducing positively charged species to the surface of MXenes. This is possible by using cationic surfactants or salts that attach to the MXene surface. Adding cationic groups enhances the affinity of MXenes for flame-retardant agents, particularly those that have anionic or neutral charges. This modification enhances the interaction between the MXene and the FRs, thereby improving the flame resistance of the composites. As depicted in Figure 3, Yu’s study [83] utilized cetyltrimethylammonium bromide (CTAB) and tetrabutylphosphine chloride (TBPC) to functionalize MXene, thereby controlling its sheet size, shape, and surface polarity. The cationic modifiers with long alkyl chains formed strong interactions with TPU. Notably, the TBPC-modified MXene demonstrated significantly higher flame-retardant efficiency in the TPU matrix compared to pristine MXene, attributed to the presence of flame-retardant elements in TBPC.

2.2.2. Covalent Modification

Covalent modification of MXene-based FRs presents a viable approach to improve the stability and performance of these materials [29,84]. In contrast with non-covalent bonding, which primarily relies on physical interactions like van der Waals forces, covalent functionalization involves the creation of strong chemical bonds between MXene nanosheets and functional groups. This process leads to a more stable chemical structure, ensuring better integration with polymer matrices and improved compatibility. Covalent modification enables the tailoring of MXenes’ surface properties, allowing for the introduction of various functional groups, such as hydroxyl, amine, or silane, which can further enhance their flame-retardant properties. These functional groups improve the dispersion of MXenes in the polymer, strengthen interfacial bonding, and facilitate the formation of protective char layers during combustion. Moreover, covalent functionalization can enhance the overall performance of MXene-based composites, improving their ability to hinder flame propagation, reduce smoke emissions, and increase the material’s durability under extreme conditions. Therefore, covalent modification plays a key role in advancing the progress of MXene-based flame retardants for high-performance, fire-resistant polymer composites.

As shown in Figure 4, Jiang et al. [85] functionalized delaminated MXene (d-MXene) nanosheets with a vinylic double bond using the modifier 2-Isocyanatoethyl methacrylate. The functionalized MXene was subsequently grafted onto the molecular chains of a UV-curable intumescent flame retardant (IFR), resulting in the formation of a hybrid IFR/MXene flame-retardant coating. To improve the interfacial compatibility of MXene with PLA, a novel MXene-DOPO flame retardant was synthesized by exploiting the covalent bonding between the reactive groups of DOPO and the –OH groups on the MXene sheets [86]. The synergistic effect between MXene and DOPO led to a PLA composite with a UL-94 V-0 rating, alongside significant reductions in pHRR (33.7%) and THR (47%).

3. Fabrication of MXene-Reinforced Polymer Composites

The process involves incorporating MXene nanosheets into polymer matrices, improving their mechanical, thermal, and flame retardancy in the fabrication of MXene-reinforced polymer composites. Typically, the process begins with the exfoliation of MXenes, often through selective etching or delamination, to produce individual nanosheets with high surface area and functionality. These MXenes are then introduced into the polymer matrix through various methods, such as ex situ blending, melt blending, or in situ polymerization.

During the fabrication of MXene-based thermoplastic polymers, several types of bonds can form between the MXenes and polymer chains, significantly influencing the properties of the final material [33,87]. These bonds include hydrogen bonds, van der Waals interactions, covalent bonds, and electrostatic interactions, which depend on both the surface characteristics of the MXenes and the nature of the thermoplastic polymer. In in situ polymerization, covalent bonds can form if functional groups on the MXenes react with the growing polymer chains, creating stronger interactions and better integration between the MXenes and the polymer matrix. Electrostatic interactions may also play a role, particularly with charged MXenes interacting with polar regions of the polymer. These bonding mechanisms ensure enhanced mechanical, thermal, and fire safety in the final composite material.

3.1. Ex Situ Blending

The reinforcing material, such as MXene nanosheets, is first dispersed in a solvent or other medium before being incorporated into the polymer matrix. Ex situ blending is a widely used method for producing polymer composites [88]. In this method, the reinforcement is prepared and treated externally to the polymer matrix, which is why it is referred to as ex situ blending. The process generally starts with the dispersion of the reinforcement, such as MXenes, in an appropriate solvent or dispersing medium to achieve a uniform distribution. This is often achieved using sonication or other techniques to ensure the exfoliation of nanosheets or fibers into individual particles, which helps to prevent agglomeration and ensures a consistent dispersion. Once the reinforcement is adequately dispersed, it is mixed with the polymer, either in a solution or molten form. When a solvent is involved, it is subsequently removed through evaporation or drying, resulting in the final composite material.

The dispersion of MXenes is a crucial step for their integration into various applications, and selecting the right solvent is key to maintaining the stability and functionality of the dispersion. Water is often the preferred solvent due to the hydrophilic nature of MXenes, which allows them to disperse effectively without additional surfactants. For MXenes with more complex surface chemistry or when high solvent polarity is required, polar aprotic solvents like dimethyl sulfoxide and N-Methyl-2-pyrrolidone are commonly employed. Gogotsi et al. [89] identified several organic solvents, including N,N-dimethylformamide, N-methyl-2-pyrrolidone, propylene carbonate, and ethanol, where Ti₃C₂T_x can form dispersions that exhibit long-term stability. The reinforcing material, such as MXene nanosheets, is then dispersed into this solution, often through techniques such as sonication or stirring, to achieve uniform distribution of the filler throughout the polymer matrix. This step is crucial to prevent agglomeration and ensure optimal interaction between the filler and the polymer. As illustrated in Figure 5, MXene and PVA are thoroughly mixed under ultrasonic and stirring conditions. The mixture is then poured into a prepared mold, followed by solvent removal to produce a composite material with excellent dispersibility [66]. In Figure 6, it is demonstrated that MXene can form hydrogen bonds with TPU chains during dispersion in solution, effectively enhancing the distribution of the modified MXene [90].

3.2. Melt Blending

In the melt blending method, the polymer matrix and reinforcement materials are physically combined in their molten state using high-shear mixers, extruders, or kneaders. This solvent-free method is both environmentally friendly and scalable, making it an ideal choice for fabricating MXene/thermoplastic polymer nanocomposites such as polypropylene [91], polystyrene, thermoplastic polyurethane [57], ultrahigh-molecular-weight polyethylene [92], and polylactic acid [78]. It enables effective incorporation and even dispersion of reinforcements within the polymers without requiring solvents.

Huang et al. [93] thoroughly explored the combined effects of MXene nanosheets in improving the flame retardancy of PLA/IFRs composites through melt blending. During combustion, Ti₃C₂ MXene nanosheets were oxidized, resulting in the formation of anatase TiO₂ particles. These particles exhibited a significant catalytic effect, promoting char formation and cross-linking within the PLA/IFRs system. This process led to the development of uniform and compact barrier layers, effectively hindering the further spread of combustion. In a similar approach, Ti₃C₂T_x/polypropylene (PP) nanocomposites were prepared using the melt blending method to investigate their mechanical and thermal properties [91,94].

3.3. In Situ Polymerization

A fabrication technique for polymer composites, in situ polymerization incorporates reinforcement materials directly into the polymer as the polymerization process takes place. In this method, monomers and reinforcements are combined, and polymerization is triggered, enabling the polymer to form around the dispersed reinforcement. This strategy promotes strong interfacial bonding between the reinforcing material and the polymer, leading to improved mechanical, thermal, and functional properties of the composite. Zhang et al. [95] synthesized Ti₃C₂/polyacrylamide (PAM) hydrogels by in situ free radical polymerization, resulting in composites that exhibited outstanding mechanical properties and improved drug release performance. Iqbal et al. [96] incorporated electrically conductive MXene and semiconducting ZnO nanoparticles into an insulating PMMA polymer matrix through in situ bulk polymerization, aided by strong sonication, which ensured the homogeneous distribution of the nanofillers and achieved desirable thermal and dielectric properties.

3.4. Other Methods

Besides the methods mentioned above, MXene-based coatings have attracted considerable interest because of their outstanding properties. These coatings are produced by incorporating MXene nanosheets into various polymers, thereby improving the functional characteristics of the base material. As depicted in Figure 7, Zhou et al. [97] applied MXene and AgNW onto a polycarbonate (PC) film in sequence, followed by transferring and partially embedding the materials into an ultra-thin PVA film using a spin coating technique. The film was then hot-pressed to improve interfacial adhesion, resulting in the creation of the MXene/AgNW-PVA film.

The strong interfacial interactions between MXenes and polymer matrices are crucial for tightly binding MXenes to the polymers, significantly enhancing the overall performance of MXene/thermoplastic polymer composites. These interactions contribute to improved mechanical strength and other functional properties. However, the dispersity of MXenes within the polymer matrix is equally critical. Achieving uniform dispersion ensures effective load transfer, prevents agglomeration, and maximizes the interfacial bonding surface area, thereby enhancing the properties of polymer composites. Therefore, further investigation into the differences among various synthesis methods and continued efforts to improve MXene dispersion in polymers are essential for advancing the field.

4. Flame-Retardant Mechanism

Thermoplastics are polymers that can be repeatedly softened upon heating and hardened upon cooling. Their thermal degradation behavior is characterized by the breakdown of polymer chains at elevated temperatures, typically involving bond scission and the formation of volatile degradation products. As the temperature rises, the polymer chains undergo chain scission, releasing monomers or smaller oligomers and forming free radicals that can further propagate the degradation process [98]. This can result in the development of cross-linked structures or the production of volatile gasses such as carbon monoxide (CO), carbon dioxide (CO₂), and various organic compounds. The bonding characteristics of thermoplastics are inherent in their molecular structure. For instance, polymers like polystyrene (PS) or polypropylene feature long chains of carbon atoms forming the backbone, with each carbon atom bonded to hydrogen atoms (C-H). These carbon–carbon (C-C) and carbon–hydrogen (C-H) bonds are relatively weak compared to those found in thermosets or crosslinked polymers, making thermoplastics more vulnerable to heat-induced degradation [99]. These structural features highlight the importance of additives, which can modify the thermal behavior of polymers and enhance their resistance to degradation.

The unique properties of MXene and its thermal decomposition behavior are intricately linked to the flame-retardant mechanism of MXene-reinforced polymers [100,101]. These properties are closely intertwined, as the way MXenes interact with heat and undergo structural changes during thermal degradation directly influence their ability to improve the fire resistance of the polymer composites. The thermal stability and decomposition products of MXenes help develop protective char layers, while their interaction within the polymer matrix can modify heat transfer and combustion processes [102,103]. Research by Liu et al. [19] and Huang et al. [93] has emphasized the significant catalytic effects of titanium-based substances, such as TiO₂ particles, which are generated during the thermal decomposition of MXene. These TiO₂ particles play a vital role in enhancing flame retardancy through several mechanisms. Acting as catalysts, they promote the thermal degradation and cross-linking of the polymer matrix, thereby facilitating the development of protective chars. The chars act as physical barriers, effectively limiting the transfer of heat, oxygen, and combustible gasses to the flame zone. Furthermore, TiO₂ particles facilitate the decomposition of volatile and flammable compounds, decreasing smoke production and the emission of harmful byproducts during combustion. MXene’s layered structure further strengthens its barrier properties, hindering the migration of flammable components. Collectively, these effects highlight the versatile role of MXene in enhancing the flame-retardant performance of thermoplastic composites, positioning it as a promising material for fire-resistant applications.

4.1. Barrier Effect of MXene and Catalytic Effect of TiO₂

With its unique two-dimensional layered structure, MXene forms a compact and dense protective layer under high-temperature conditions. The char layer serves as a physical barrier, effectively preventing the transfer of heat, oxygen, and flammable gasses to the combustion zone. This barrier effect of MXene is primarily attributed to its high surface area and the stability of its oxide or hydroxide layers, which form during thermal decomposition. This reduces the rate of combustion, limits the spread of fire, and minimizes smoke and toxic gas release, making MXene-reinforced composites highly effective in fire protection applications. Additionally, the catalytic effect of TiO₂ is pivotal in various chemical processes, including its role in flame retardancy, according to a previous study [104]. TiO₂ is well known for its strong Lewis acid properties, which make it an efficient solid acid catalyst. This property allows TiO₂ to facilitate diverse chemical reactions by providing active catalytic sites. In polymer matrices, TiO₂ serves as a catalyst for thermal degradation and the formation of graphitized chars. These chars function as efficient barriers, reducing heat transfer and limiting the release of flammable gasses.

He et al. [105] incorporated Ti₃C₂ MXene, modified with positively charged poly(diallyldimethylammonium chloride) (PDDA), into a polyurethane (TPU) matrix. The inclusion of 3 wt% PDDA-modified MXene resulted in a 50% reduction in pHRR and a 47% decrease in total smoke production (TSP). As shown in Figure 8, the physical barrier effect of MXene effectively suppressed heat and mass transfer. At elevated temperatures, Ti₃C₂T_x gradually transforms into TiO₂. The resulting anatase TiO₂ particles exhibit excellent catalytic properties, promoting the formation of a chemical char. As a result, a high-quality flame-retardant layer is formed in the condensed phase, combining the physical blocking effect of Ti₃C₂T_x with the catalytic action of TiO₂.

4.2. Other Mechanisms

In addition to the high-temperature flame-retardant mechanisms, another significant concern in fire safety is the emission of harmful gasses, such as smoke, carbon monoxide (CO), and carbon dioxide (CO₂), which are produced during the thermal degradation of polymer materials [106,107]. These gasses present immediate health hazards, while also leading to environmental pollution and decreasing visibility in fire situations. As a result, smoke suppression has become a key factor in evaluating the effectiveness of flame-retardant materials. Xie et al. [29] reported that the PCS-modified MXene significantly reduced the production of CO and CO₂ in TPU by 52.1% and 60.7%, respectively. This effect can be attributed to two main factors: firstly, the high aspect ratio and lamellar structure of the modified MXene nanosheets obstruct the emission of volatile compounds by increasing their diffusion paths; secondly, the dense char formed from the MXene-based system acts as a protective layer, hindering polymer degradation and consequently suppressing smoke production. Similarly, Yu et al. [81] developed an MXene/POSS system to reduce smoke generation during PS combustion, with the mechanisms primarily involving adsorption, catalysis, and the “tortuous path” effect.

5. Concluding Remarks and Future Prospects

In summary, this review has summarized the state-of-the-art research progress on MXene-based flame retardants, focusing on the unique properties and mechanisms through which MXenes enhance the flame retardancy, mechanical performance, and smoke suppression of polymer composites. This review also covered various modification strategies employed to optimize the performance of MXene-based flame retardants, including non-covalent modification and covalent modification with other flame-retardant materials to further improve their efficiency and compatibility with different polymers. Additionally, the preparation methods for MXene-based thermoplastic polymer composites were discussed, highlighting key approaches such as ex situ blending, melt blending, and in situ polymerization that facilitate the even dispersion of MXenes throughout the polymer structure. The mechanisms of action for MXene-based flame retardants in different matrices were also elaborated upon, revealing how MXenes interact with polymer chains during thermal decomposition to form protective char layers, suppress smoke and toxic gas emissions, and promote catalytic effects.

MXene/thermoplastic polymer composites face several challenges that hinder their widespread application. (1) Poor dispersion is a significant issue, as the strong van der Waals forces between MXene nanosheets lead to their aggregation, leading to uneven distribution within the polymer matrix. This results in compromised composite performance [108]. (2) Interfacial compatibility is another challenge, as the hydrophilic nature of MXenes makes them poorly compatible with nonpolar polymers, resulting in weak adhesion between the MXene and polymer phases [109]. This weak interfacial interaction can negatively impact the properties of the polymer composites. (3) The oxidation and degradation of MXenes is also a concern, as they are vulnerable to moisture and oxygen, resulting in a loss in their fire resistance and mechanical properties over time, particularly under harsh environmental conditions [110]. (4) Additionally, the processing of MXene/thermoplastic polymer composites can be complex and costly, as special techniques are required to address the dispersion and stability issues, making large-scale production more challenging.

Future efforts in the development of MXene/thermoplastic polymer composites should focus on overcoming the existing challenges to enhance their performance and broaden their applications. One key area is improving the distribution of MXene nanosheets within the polymer matrix, which can be achieved through surface functionalization, the use of dispersing agents, or advanced processing methods [111]. Strengthening the interfacial interactions between MXenes and nonpolar polymers is also crucial and can be addressed through the development of effective coupling agents or the incorporation of functional groups to improve adhesion. Additionally, efforts should be directed at increasing the environmental stability of MXenes by developing oxidation-resistant materials or protective coatings to maintain the long-term performance of composites under harsh conditions. Lastly, simplifying the manufacturing process by optimizing techniques and exploring new polymer matrices that better accommodate MXenes will be essential for the scalability and affordability of these composites. Meanwhile, an exciting future direction for MXene in flame-retardant materials is its potential use in fire detection and early warning systems [112,113]. Addressing these challenges will enable future research to fully realize the potential of MXene/thermoplastic polymer composites for various applications.

Author Contributions

Conceptualization, L.X. and W.W.; methodology, Y.Y. and Q.G; software, Q.G; validation, Y.Y., Q.G. and L.X.; formal analysis, Y.Y. and L.X.; investigation, Y.Y. and W.W.; resources, Q.G; writing—original draft preparation, Y.Y.; writing—review and editing, L.X. and W.W.; visualization, Q.G.; supervision, W.W.; project administration, Y.Y.; funding acquisition, Y.Y. and W.W. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflicts of interest. The authors declare that they do not have any competing financial interests or personal relationships that might influence their work.

Footnotes

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Figures

Figure 1. A method for synthesizing a series of Zn-based MAX phases and Cl-terminated MXenes originating from the replacement reaction. Reproduced with permission from Ref. [72], Copyright 2019, American Chemical Society.

Figure 2. Schematic preparation route of BODIPY-MXene nanosheets. Reproduced with permission from Ref. [82], Copyright 2021, Elsevier.

Figure 3. Illustration for preparation of (a) modified Ti3C2 ultra-thin nanosheets with cationic modifiers, (b) TPU-modified Ti3C2 nanocomposites, and (c) the chemical structures of cationic modifiers. Reproduced with permission from Ref. [83], Copyright 2019, Elsevier.

Figure 4. Schematic illustration of the fabrication procedure of surface-modified MXene nanosheets. Reproduced with permission from Ref. [85], Copyright 2020, Elsevier.

Figure 5. Schematic illustration of the process for (a) etching and delamination of MXene nanosheets and (b) preparation of PVA/MXene film. Reproduced with permission from Ref. [66], Copyright 2019, Elsevier.

Figure 6. Schematic diagrams for (a) synthesis of Ti3C2Tx@MCA nanohybrid and (c) preparation of TPU/Ti3C2Tx@MCA nanocomposites; (b) digital photographs of (nano)additive dispersion. Reproduced with permission from Ref. [90], Copyright 2020, Elsevier.

Figure 7. (a) Schematic of the fabrication for transparent MXene/AgNWx-PVA films. (b) Photograph of MXene/AgNW632-PVA film. AFM images of (c) MXene-PC, (d) MXene/AgNW948-PC, and (e) MXene/AgNW948-PVA films. (f) Cross-sectional SEM image of MXene/AgNW948-PVA film. (g) Transmittance spectra of MXene/AgNWx-PVA films. (h) Plot of transmittance (550 nm) versus sheet resistance of MXene/AgNWx-PVA films. Reproduced with permission from Ref. [97], Copyright 2020, American Chemical Society.

Figure 8. Scheme of proposed flame-retardant mechanism for f-Ti3C2 in TPU composites. Reproduced with permission from Ref. [105], Copyright 2019, Elsevier.

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